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**Provisional chapter**

## **Manganese-Zinc Spinel Ferrite Nanoparticles and Ferrofluids Ferrofluids**

**Manganese-Zinc Spinel Ferrite Nanoparticles and** 

Rajender Singh and Gadipelly Thirupathi Rajender Singh and Gadipelly Thirupathi Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66522

#### **Abstract**

The nanoparticles and ferrofluids of spinel ferrites are useful in bio-sensors, transducers, storage devices, optical devices, and so on. The Mn-Zn ferrite (MZF) is generalized soft spinel ferrite having high saturation magnetization at low applied magnetic field. This chapter covers the synthesis of nanoparticles of various sizes and compositions of Mn1-*<sup>x</sup>* Zn*<sup>x</sup>* Fe<sup>2</sup> O4 with *x* = 0–1 by co-precipitation method. The structural and magnetic properties of the nanoparticles are discussed. The ferrofluids of superparamagnetic and ferromagnetic MZF nanoparticles were synthesized. The magneto-viscosity of ferrofluids with the dispersion of nanoparticles in different colloidal was studied. The Herschel-Bulkley model is applied to analyse the data for low viscosity ferrofluids.

**Keywords:** spinel, Mn Zn ferrite, nanoparticles, ferrofluids

## **1. Introduction**

Ferrites are ceramic materials with magnetic properties. According to the crystal structure, the ferrites are mainly classified as spinels, hexaferrites, garnets and perovskites. Spinel is the first class among these ferrites having mineral structure with significant magnetic behaviour. The nanoparticles and ferrofluids of spinel ferrites are useful in bio-sensors, transducers, storage devices, energy conversion devices, heat absorbers and generators, shock absorbers, lubricants, magneto-optical devices, and so on.

Among all ferrite types, the spinel ferrites are easy to form with control on the size of their nanoparticles. The Mn-Zn ferrite (MZF) is generalized soft spinel ferrite having high magnetization, and it saturates at low applied magnetic field. In this work, the optimization of the synthesis procedures was done to obtain stable nanoparticles and ferrofluids of Mn-Zn spinel ferrites. The structural and magnetic properties of the nanoparticles and magneto-viscosity of

the MZF ferrofluids were studied to understand the correlation between physical properties of nanoparticles and flow behaviour of ferrofluid in applied magnetic field.

## **2. Synthesis of nanoparticles of spinel ferrite**

There are several methods for the synthesis of ferrite nanoparticles, that is, photo synthesis, microemulsion, sol-gel, hydrothermal, ball-milling, co-precipitation, catalyst-based methods and so on discussed in the literature [1–4].

In this work, the nanoparticles of Mn1-*<sup>x</sup>* Zn*<sup>x</sup>* Fe<sup>2</sup> O4 (MZF) with *x* = 0–1 were synthesized by soft chemical approach of co-precipitation method [5–10]. The metal salts with high solubility around room temperature, that is, MnCl<sup>2</sup> , ZnSO<sup>4</sup> and FeNO<sup>3</sup> , were chosen for the synthesis. The metal salts in stoichiometric mole ratio were dissolved in distilled water, and the mixture was heated at 353 K. The size of nanoparticles was controlled by controlling moles of OH ions in the solution (*n*) according to the following chemical formula:

$$\text{(1-x)}\,\text{Mn}^{2+} + \text{xZn}^{2+} + 2\,\text{Fe}^{3+} + \text{nOH} \rightarrow \text{Mn}\_{1\cdot\text{x}}\,\text{Zn}\_{\text{x}}\,\text{Fe}\_{2}\,\text{O}\_{4} + \text{H}\_{2}\,\text{O}\_{4}$$

The solution was washed with distilled water until the pH = 7 was reached and the slurry was heated at 373 K to get the MZF nanoparticles. The MZF nanoparticles of different compositions in Mn1-*<sup>x</sup>* Zn*<sup>x</sup>* Fe<sup>2</sup> O4 with *x* = 0, 0.25, 0.5, 0.75 and 1 were synthesized.

## **3. Synthesis of the ferrofluids of spinel ferrite**

The properties of ferrofluid depend on the hydrodynamic distribution of magnetic nanoparticles. The following ferrofluids were synthesized using MZF nanoparticles:


Finding suitable colloidal is necessary for the stability of ferrofluid in various technological applications. A number of colloidal, with their properties listed in **Table 1**, were used for synthesizing different MZF ferrofluids using methods given the literature [11–15].

The synthesized (MZF) ferrofluids are listed in **Table 2**. These ferrofluids were studied for their magneto-viscosity properties. The table has three categories of ferrofluids:



**Table 1.** Properties of colloidal used for making ferrofluids.

the MZF ferrofluids were studied to understand the correlation between physical properties

There are several methods for the synthesis of ferrite nanoparticles, that is, photo synthesis, microemulsion, sol-gel, hydrothermal, ball-milling, co-precipitation, catalyst-based methods

chemical approach of co-precipitation method [5–10]. The metal salts with high solubility

The metal salts in stoichiometric mole ratio were dissolved in distilled water, and the mixture was heated at 353 K. The size of nanoparticles was controlled by controlling moles of OH-

The solution was washed with distilled water until the pH = 7 was reached and the slurry was heated at 373 K to get the MZF nanoparticles. The MZF nanoparticles of different composi-

O4 with *x* = 0, 0.25, 0.5, 0.75 and 1 were synthesized.

The properties of ferrofluid depend on the hydrodynamic distribution of magnetic nanopar-

(i) Ferrofluids with the dispersion of superparamagnetic (SPM) nanoparticles (no surface

(ii) Ferrofluids with the dispersion of ferromagnetic nanoparticles in different colloidal with

Finding suitable colloidal is necessary for the stability of ferrofluid in various technological applications. A number of colloidal, with their properties listed in **Table 1**, were used for syn-

The synthesized (MZF) ferrofluids are listed in **Table 2**. These ferrofluids were studied for

(ii) Water, kerosene and toluene-based ferrofluids synthesized with surfactant-coated Mn-

O4

and FeNO<sup>3</sup>

, ZnSO<sup>4</sup>

O4 (MZF) with *x* = 0–1 were synthesized by soft

, were chosen for the synthesis.

O4

O4

nanoparticles in paraffin.

nanoparticles.

nanoparticles in wa-

ions

of nanoparticles and flow behaviour of ferrofluid in applied magnetic field.

Zn*<sup>x</sup>* Fe<sup>2</sup>

(1-x) Mn2+ + x Zn2+ + 2 Fe3+ + n OH- → Mn1-x Znx Fe2 O 4 +H<sup>2</sup> O

ticles. The following ferrofluids were synthesized using MZF nanoparticles:

thesizing different MZF ferrofluids using methods given the literature [11–15].

their magneto-viscosity properties. The table has three categories of ferrofluids:

(i) Ethylene glycol-based ferrofluids synthesized from SPM Mn0.75Zn0.25Fe<sup>2</sup>

(iii) The ferrofluids synthesized with surfactant-coated Mn0.75Zn0.25Fe<sup>2</sup>

ter, kerosene, toluene and paraffin and Mn0.9Zn0.1Fe<sup>2</sup>

**2. Synthesis of nanoparticles of spinel ferrite**

in the solution (*n*) according to the following chemical formula:

**3. Synthesis of the ferrofluids of spinel ferrite**

suitable surface coating of the nanoparticles.

and so on discussed in the literature [1–4].

around room temperature, that is, MnCl<sup>2</sup>

In this work, the nanoparticles of Mn1-*<sup>x</sup>*

140 Magnetic Spinels- Synthesis, Properties and Applications

tions in Mn1-*<sup>x</sup>*

Zn*<sup>x</sup>* Fe<sup>2</sup>

coating) in ethylene glycol.

ferrite (MF) nanoparticles.


**Table 2.** Different MZF-based ferrofluids investigated in this work.

## **4. Structural and magnetic properties of spinel ferrites nanoparticles**

The chemical formula of spinel ferrite is generally expressed, where 'Me' represents a divalent metal ion (e.g. Fe2+, Ni2+, Mn2+, Mg2+, Co2+, Cu2+, etc.) and '*i*' is inversion parameter which is 0 for normal spinel and 1 for inverse spinel. The inverse parameter varies from 0 to 1 for mixed spinel. The ionic radii of cation, crystal field effect, ionic charge, and so on are the main factors for the cation distribution in the nanoparticles of spinel ferrites. In the cubic close-packed arrangement of the spinel structure, the unit cell has 32 octahedral sites (B-site) and 64 tetrahedral sites (B-site), out of which only 16 octahedral sites and eight tetrahedral sites are occupied.

The Mn-Zn ferrite nanoparticles of various sizes and compositions were synthesized. Their structural properties were studied by X-ray diffraction (XRD) analysis. The morphological and microstructure of these nanoparticles was analysed using transmission electron microscopy (TEM). The magnetic properties of the nanoparticles were studied by measuring magnetization as a function of temperature and applied magnetic field, *M*(*T,H*) and temperature-dependent ferromagnetic resonance (FMR) spectra.

The MZF nanoparticles of various sizes were synthesized by varying metal ions to hydroxide ratio (*r* = Me/OH- ) by co-precipitation method [16]. The value of *r* was varied from 0.375 to 0.17. The XRD patterns of all the samples (**Figure 1**, left) are analysed using Rietveld refinement method. The structural refinements are fitted with a single-phase spinel structure

**Figure 1.** XRD pattern (left) and magnetization as a function of temperature at 100 Oe (right) of MZF nanoparticles for different Zn content.


The MZF nanoparticles of various sizes were synthesized by varying metal ions to hydrox-

to 0.17. The XRD patterns of all the samples (**Figure 1**, left) are analysed using Rietveld refinement method. The structural refinements are fitted with a single-phase spinel structure

**Figure 1.** XRD pattern (left) and magnetization as a function of temperature at 100 Oe (right) of MZF nanoparticles for

) by co-precipitation method [16]. The value of *r* was varied from 0.375

ide ratio (*r* = Me/OH-

142 Magnetic Spinels- Synthesis, Properties and Applications

different Zn content.

**Table 3.** Listed here are lattice parameter (a) and crystallite size D determined from XRD for various compositions of Mn1-*<sup>x</sup>* Zn*<sup>x</sup>* Fe<sup>2</sup> O4 with *x* = 0–1 (MZF) nanoparticles.

having space group Fd 3 " m by Fullprof suite program for the XRD pattern [17, 18]. The structural parameters (lattice parameter (a), crystallite size, strain, etc.) are extracted from the fits. The atomic position coordinates of A-, B- and O-sites are (1/8, 1/8, 1/8), (1/2, 1/2, 1/2) and (1/4+*u*, 1/4+*u*, 1/4+*u*), respectively. Here, *u* is the oxygen position coordinate shift parameter. The crystallite size and strain are calculated from Williamson-Hall plot for XRD pattern (see **Table 3**). Since the ionic radii of Mn2+ is comparable in coordination of octahedral and tetrahedral sites, so Mn-ferrite (MF) shows more inverse spinel ferrite.

The lattice parameter of MZF nanoparticles with *x* = 0, 0.5, 0.75 and 1 decreases with an increase in the size of the nanoparticle. This indicates the lattice contraction due to reduction of lattice disorder with an increase in nanoparticle size. The nanoparticles have several types of disorders such as oxygen deficiency, lattice disorders, dangling bonds, and so on. It is pointed out that the lattice expansion takes place with a decrease in nanoparticle size [19]. However, in the present study the lattice parameter of MZF nanoparticles with *x* = 0.25 increases initially when the particle size increases from 7 to 10 nm and then decreases with further increase in particle size. The nanoparticles of 7-nm size have impurity phase (Fe<sup>3</sup> O4 phase due to orthorhombic structure). The secondary impurity phase shares the lattice. So, the lattice parameter is lower for these nanoparticles. But in general, the resultant unit cell volume from both the phases will be higher.

The variation of crystallite size of MZF nanoparticles with Zn content is shown in **Figure 2** (left). The drastic decrease in crystallite size occurs at *x* = 0.25. This is attributed to the fact that when Zn2+ ions are substituted in Mn-ferrite, it causes a change from mixed spinel to normal spinel structure. The Zn2+ ions prefer to occupy tetrahedral site due to its stable valence

**Figure 2.** The variation of crystallite size (left) and lattice parameter (right) with Zn content of MZF nanoparticles.

**Figure 3.** The TEM micrographs of ZnFe<sup>2</sup> O4 nanoparticles of crystallite size 5 nm.

and ion size. The decrease in crystallite size is marginal with further increase in Zn content. **Figure 3** (right) shows the variation of lattice parameter with Zn content in MZF nanoparticle. The lattice parameter decreases when *x* increases from 0 to 0.5 and then increases when Zn content increases from 0.5 to 1. This is due to the transition from mixed spinel to normal spinel phase as *x* increases from 0 to 0.5. The MZF nanoparticles again return to mixed spinel phase when *x* increases from 0.5 to 1. This is attributed to the nanophase formation due to the existence of lattice disorders in the system of nanoparticles.

The temperature dependence of the magnetization, *M*(*T*), of Mn-ferrite and Mn0.75Zn0.25Fe<sup>2</sup> O4 nanoparticles measured at *H* = 100 Oe (see **Figure 1**, right) shows the bifurcation (irreversible) temperature (*T*irr). The *T*irr-value is high (>350 K) for Mn-ferrite nanoparticles. The irreversibility also depends on the crystalline size. The *T*irr deceases when Zn content *x* = 0.25 is added in Mn-ferrite nanoparticles. This indicates the decrease in ferromagnetic interactions due to drastic decrease in size with Zn addition and the change in cation distribution which is influenced by the structural changes. With further increase in Zn content, the crystallite size becomes smaller than the critical superparamagnetic size. A single superparamagnetic particle or a system of mono-dispersed nanoparticles shows overlapped-blocking temperature (*T*B) and irreversible temperature in *M*(*T*) plots. If the size distribution of nanoparticles is taken into account, each particle size will show a *T*B-value. The resultant will be a distribution in *T*Bvalue. The distribution of particle size and magnetic anisotropy is explained by thermomagnetic plots in earlier reports [20–22]. The *T*B-value also depends on the magnetic field used for FC-ZFC mode and the measurement time. This is due to the slow relaxation of spins near the blocked state. The size distribution can be found from Neel's model. This model explains the relaxation of non-interacting single- domain nanoparticles experiencing uniaxial anisotropy. The uniaxial anisotropy gives a double well potential in two directions of spin alignment. The wells are separated by an energy barrier *E*B to overcome thermal activation with the relaxation time [23]. The relaxation time is: *τ* = *τ*<sup>0</sup> exp(*EB*/*kB T* ) whereas the 1/τ<sup>0</sup> is attempt frequency. In the small field limit: *EB* = *Keff V* where the *K*eff is the effective anisotropy constant and *V* is the particle volume. The volume distribution *f*(*V*) can be estimated from *M*ZFC plot (the thermo-

magnetic plot in zero-field-cooled mode). Then, the *M*ZFC can be written in terms of distribu-

tion of *T*B, that is, *f*(*T*B) as follows:

and ion size. The decrease in crystallite size is marginal with further increase in Zn content. **Figure 3** (right) shows the variation of lattice parameter with Zn content in MZF nanoparticle. The lattice parameter decreases when *x* increases from 0 to 0.5 and then increases when Zn content increases from 0.5 to 1. This is due to the transition from mixed spinel to normal spinel phase as *x* increases from 0 to 0.5. The MZF nanoparticles again return to mixed spinel phase when *x* increases from 0.5 to 1. This is attributed to the nanophase formation due to the exis-

nanoparticles of crystallite size 5 nm.

The temperature dependence of the magnetization, *M*(*T*), of Mn-ferrite and Mn0.75Zn0.25Fe<sup>2</sup>

nanoparticles measured at *H* = 100 Oe (see **Figure 1**, right) shows the bifurcation (irreversible) temperature (*T*irr). The *T*irr-value is high (>350 K) for Mn-ferrite nanoparticles. The irreversibility also depends on the crystalline size. The *T*irr deceases when Zn content *x* = 0.25 is added in Mn-ferrite nanoparticles. This indicates the decrease in ferromagnetic interactions due to drastic decrease in size with Zn addition and the change in cation distribution which is influenced by the structural changes. With further increase in Zn content, the crystallite size becomes smaller than the critical superparamagnetic size. A single superparamagnetic particle or a system of mono-dispersed nanoparticles shows overlapped-blocking temperature (*T*B) and irreversible temperature in *M*(*T*) plots. If the size distribution of nanoparticles is taken

O4

tence of lattice disorders in the system of nanoparticles.

O4

**Figure 3.** The TEM micrographs of ZnFe<sup>2</sup>

144 Magnetic Spinels- Synthesis, Properties and Applications

$$\mathcal{m}\_{ZFC}(T) = \frac{H \, m\_s^2(T)}{3 \, k\_{\text{g}} \, T} \int\_0^r f(T\_{\text{g}}) \, d \, T\_{\text{g}}$$

The equation implies to *f*(*TB* ) *<sup>α</sup>* \_\_\_d dT (*<sup>T</sup> mZFC*(*<sup>T</sup>* ) )

This equation is modified into the following by considering the log-normal size distribution *f*(*D*) [21]:

$$f(D\_{\cdot})\_{\cdot} = f\_{0}(1/T\_{\,\,B}^{2})\frac{d}{dT}(T\,m\_{ZFC}(T\,))$$

The decrease of *T*B is observed in *M*(*T*) plots of superparamagnetic MZF nanoparticles (**Figure 1**, right) with increase in Zn content from 0.5 to 1. This is due to the decrease in size and creation of short-range magnetic order in the nanoparticles. The *T*B-value shift is attributed to the magnetic cluster formation and its size distribution [24–28]. The *T*B-value and *T*irr will overlap for the small-sized nanoparticles, whereas they are distinctly different for the large-sized particles. This is attributed to the existence of narrow-size distribution in the smaller nanoparticles compared to large nanoparticles [29–31]. The *M*(*T*) results are in good agreement with the size distributions obtained from TEM data analysis. The uniform particle size distribution is observed in ZnFe<sup>2</sup> O4 nanoparticles of crystallite size 5 nm from the transmission electron microscope (TEM) micrograph analysis as shown in **Figure 3**.

**Figure 4** (left) shows the magnetic hysteresis loops *M*(*H*) plots of MZF nanoparticles at 5 and 325 K. These plots show soft ferrimagnetic behaviour for Mn-ferrite and Mn0.75Zn0.25Fe<sup>2</sup> O4 nanoparticles [32]. The coercivity is more for the Mn0.75Zn0.25Fe<sup>2</sup> O4 nanoparticles compared to Mn-ferrite at 5 and 325 K. This indicates that the domain size is more effective to slow domain wall motion in Mn0.75Zn0.25Fe<sup>2</sup> O4 nanoparticles due to non-magnetic Zn inclusion in Mn-ferrite. But there is not much change in saturation magnetization with *x* = 0.25 doping. Here, the surface-to-volume ratio plays more prominent role as the crystallite size decreases from 104 to 11 nm when Zn content varies from 0 to 0.25. Apart from this, Mn-ferrite is cubic spinel ferrite with a partial inverse spinel structure. Inter-sublattice super-exchange interactions of the cations on the (A–B) are much stronger than the (A–A) and (B–B) intra-sublattice exchange interactions. So, the cation distribution plays a major role on the magnetic properties of Mn-ferrite nanoparticles. The Zn

**Figure 4.** The M(H) plots at 5 and 325 K (left) and the FMR spectra (right) of MZF nanoparticles with different Zn content.

doping in Mn-ferrite decreases the crystallite size due to structural changes towards normal spinel structure. But the net magnetization is not reduced due to a decrease in the A-sublattice magnetization. So, the ferrimagnetism does not show much change in saturation magnetization.

The *M*(*H*) plots of MZF nanoparticles with Zn content of 0.5, 0.75 and 1 having an average crystallite size of 5 nm show zero coercivity at 325 K and small coercivity at 5 K. This is because spins below blocking temperature do not relax. This can be attributed to spin canting and surface spin disorder in the nanoparticles [33, 34]. The coercivity at 5 K decreases with an increase in Zn content from 0.5 to 1. So, the decrease in blocking temperature can be expected with increasing Zn content. The shape of magnetization curve also depends on the measurement time and Neel relaxation of the nanoparticles. The *M*(*H*) plots show the change in *M*<sup>S</sup> which is due to the influence of the cationic stoichiometry and occupancy of cations in specific sites. In addition, random canting of particle surface spins and non-saturation effects due to a random distribution of particles are also responsible for the shape of *M*(*H*) plots. The ferromagnetic resonance spectra shown in **Figure 4** (right) confirm the increase in the ferromagnetic component as Zn content is decreased in the MZF. This is indicated by progressive shifting of shoulder peak towards lower field value with a decrease in Zn content in MZF.

## **5. Investigation of magnetic properties of spinel ferrites based on Mössbauer studies**

Mössbauer measurements on MZF nanoparticles were carried out at room temperature in transmission geometry using 57Fe nuclei. Mössbauer spectrum of Mn-ferrite nanoparticles

**Figure 5.** The Mössbauer spectra of Mn-ferrite and Mn0.75Zn0.25Fe<sup>2</sup> O4 nanoparticles.

**Figure 4.** The M(H) plots at 5 and 325 K (left) and the FMR spectra (right) of MZF nanoparticles with different Zn content.

146 Magnetic Spinels- Synthesis, Properties and Applications

with crystallite size of 104 nm is resolved into two sextets and one doublet (**Figure 5**). This system is soft ferromagnetic at room temperature. Two sextets come from two different Fe co-ordinations.

**Figure 6.** The Mössbauer spectra of MZF nanoparticles.


**Table 4.** Parameters determined from the analysis of the Mössbauer spectra are listed here.

Sextet 1 has the hyperfine field value of 478 kOe and the absence of quadrupole splitting indicates that the Fe site is octahedral coordinated in cubic symmetry with 3+ valence state. Sextet 2 has a hyperfine field value of 452 kOe with quadrupole splitting of 0.009 mm/s due to 3+ valence state of Fe in tetrahedral coordination [35–37]. In addition, the doublet of relative area of around 22% is observed and this is due to the existence of small-sized magnetic particles showing superparamagnetic behaviour.

When 25 mole % of Zn (*x* = 0.25) is introduced in Mn-ferrite nanoparticles (with the crystallite size of 11 nm), the Mössbauer spectra are fitted with resolved one sextet and one doublet with a relative area of 67 and 33%, respectively. The sextet has a hyperfine field value of 385 Oe with quadrupole splitting of 0.055 mm/s. This indicates the possibility of tetrahedral coordination of Fe3+ ions. Mössbauer spectra of MZF nanoparticles (*x* = 0.5, 0.75 and 1) showed single-doublet pattern shown in **Figure 6** for the crystallite size of 5, 4 and 9 nm, respectively. This indicates that the nanoparticles are small-sized magnetic particles showing superparamagnetic behaviour at room temperature. Mössbauer parameters of all the samples investigated here are listed in **Table 4**.

## **6. Magneto-viscosity of ferrofluids of spinel ferrites**

with crystallite size of 104 nm is resolved into two sextets and one doublet (**Figure 5**). This system is soft ferromagnetic at room temperature. Two sextets come from two different Fe

co-ordinations.

148 Magnetic Spinels- Synthesis, Properties and Applications

**Figure 6.** The Mössbauer spectra of MZF nanoparticles.

MnFe<sup>2</sup>

Mn0.75Zn0.25Fe<sup>2</sup>

Mn0.5Zn0.5Fe<sup>2</sup>

Mn0.25Zn0.75Fe<sup>2</sup>

ZnFe<sup>2</sup>

**Sample Sub-spectrum Mössbauer parameters**

*H***hf (kOe) Quadrupole** 

**shift (Δ***E***q) (mm/s)**

Sextet-2 (B-site) 452 0.009 0.262 0.86 58 Doublet (SPM) – 0.691 0.223 0.55 22

O4 Sextet 385 0.055 0.129 1.86 67 Doublet (SPM) – 0.684 0.234 0.65 33

O4 Doublet (SPM) – 0.633 0.222 0.65 100

O4 Doublet (SPM) – 0.541 0.225 0.46 100

O4 Doublet (SPM) – 0.553 0.231 0.53 100

**Table 4.** Parameters determined from the analysis of the Mössbauer spectra are listed here.

O4 Sextet-1 (A-site) 478 0.002 0.195 0.40 20

**Isomer shift** *δ* **(mm/s)**

**Line width WV (mm/s)**

**Relative intensity (%)** When the magnetic nanoparticles are suspended in colloidal, there are three main forces acting on the particles, that is, internal magnetic force, surface tension of the fluid and gravitational force. The ferrofluids show spikes when the fluid is subjected to the magnetic field. The chain-like or spot-like structure formation depends on these forces. The magneto-viscosity of the ferrofluid mainly depends on the magnetic properties of the nanoparticles used in the colloidal. The ferrofluids of superparamagnetic and ferromagnetic nanoparticles were chosen

**Figure 7.** Flow curves at different applied fields (left) and magneto-viscosity curves at different shear rates (right) of MZFE ferrofluid.

for the study of magneto-viscosity. The surfactant coating to the nanoparticles increases the stability of the ferrofluid.

#### **6.1. Magneto-viscosity of ferromagnetic nanoparticles-dispersed ferrofluid without surfactant coating**

The Mn0.75Zn0.25Fe<sup>2</sup> O4 nanoparticles of 3-nm size were dispersed in ethylene glycol to prepare MZFE ferrofluid. The volume ratio of nanoparticles to the colloidal is 1:4. The superparamagnetic nature of the nanoparticles is confirmed by magnetization studies. **Figure 8** shows shear viscosity versus shear flow plot at various magnetic field values. The viscosity decreases with an increase in shear rate at zero magnetic field. It shows a shear-thinning effect at low shear rate and Newtonian behaviour over a wide range of shear rate. This can be explained by considering that at low shear rate the aggregates or clusters of nanoparticles offer high resistance to fluid flow leading to high viscosity. With increases in shear rate, the aggregates break into smaller units leading to fluid flow with low viscosity [38]. The interaction between aggregates or clusters increases when magnetic field is applied and aligns them in the direction of magnetic field forming chain-like structures which offer high resistance to the fluid flow leading to an increase in viscosity. Magneto-viscosity of other ferrofluids also shows similar behaviour [39, 40].

**Figure 8.** Viscosity as a function of shear rate at different applied magnetic fields (left) and viscosity as a function of applied magnetic field at a shear rate of 10 S–1 (right) of water-based (MFW), kerosene-based (MFK) and toluene-based (MFT) Mn-ferrite ferrofluids.

Odenbach [41] studied the magneto-viscous effect at various shear rates in a commercial magnetite-based ferrofluid and explained it in terms of interactions between the particles in the agglomerations aligned in straight chains and not due to the interaction between the chains. **Figure 7** shows irreversible nonlinear behaviour in *η* versus applied magnetic field plots at the steady shear rates of 1 and 50 s–1. The irreversibility of magneto-viscosity curve is larger at low shear rate similar to the earlier reports [42]. The field dependence of viscosity indicates the particle size distribution and formation of chains with an increase in magnetic field. With a decrease in field, the viscosity increases due to the magnetic interactions and mechanical alignment of particles and slow loss of aggregation of particles. Then, the system tries to retain its initial state. The open hysteresis loop is due to unbroken chains and aggregates present in the system [43]. Odenbach [41] and Rhodesa [43] explained magnetic influence on the viscous properties of ferrofluid in terms of four forces acting on the magnetic particles in the ferrofluid. These forces arise from liquid media, internal magnetic field between magnetic particles, gravitational field and the applied magnetic field. The four regions in the plots of **Figure 7** are according to the limitations of these forces. Apart from this, the irreversibility is due to the relaxation of the nanoparticles in ferrofluids. The Brownian and Neel relaxations play a role for the observed behaviour of SPM nanoparticles in liquid media [13, 44].

#### **6.2. Magneto-viscosity of surfactant-coated ferromagnetic nanoparticles-dispersed ferrofluids**

It is quite interesting and more specific to study the magneto-viscosity of magnetic nanoparticles in less viscous colloidal such as water, kerosene and toluene. The ferromagnetic MF and MZF nanoparticles are used in different colloidal media with suitable surfactant coating for the preparation of MF and MZF ferrofluids. The surfactants have hydrophobic and hydrophilic ends. The hydrophilic end will be on the nanoparticle surface and hydrophobic end will overcome the formation of agglomerates of the nanoparticles. This property increases the stability of the ferrofluids.

#### *6.2.1. Magneto-viscosity of Mn-ferrite ferrofluids*

for the study of magneto-viscosity. The surfactant coating to the nanoparticles increases the

MZFE ferrofluid. The volume ratio of nanoparticles to the colloidal is 1:4. The superparamagnetic nature of the nanoparticles is confirmed by magnetization studies. **Figure 8** shows shear viscosity versus shear flow plot at various magnetic field values. The viscosity decreases with an increase in shear rate at zero magnetic field. It shows a shear-thinning effect at low shear rate and Newtonian behaviour over a wide range of shear rate. This can be explained by considering that at low shear rate the aggregates or clusters of nanoparticles offer high resistance to fluid flow leading to high viscosity. With increases in shear rate, the aggregates break into smaller units leading to fluid flow with low viscosity [38]. The interaction between aggregates or clusters increases when magnetic field is applied and aligns them in the direction of magnetic field forming chain-like structures which offer high resistance to the fluid flow leading to an increase in viscosity. Magneto-viscosity of other ferrofluids also shows similar behaviour [39, 40].

**Figure 8.** Viscosity as a function of shear rate at different applied magnetic fields (left) and viscosity as a function of applied magnetic field at a shear rate of 10 S–1 (right) of water-based (MFW), kerosene-based (MFK) and toluene-based

nanoparticles of 3-nm size were dispersed in ethylene glycol to prepare

**6.1. Magneto-viscosity of ferromagnetic nanoparticles-dispersed ferrofluid without** 

stability of the ferrofluid.

O4

150 Magnetic Spinels- Synthesis, Properties and Applications

**surfactant coating**

The Mn0.75Zn0.25Fe<sup>2</sup>

(MFT) Mn-ferrite ferrofluids.

The magneto-viscosity of Mn-ferrite ferrofluid is investigated in flow field (flow curves) and in external magnetic field (magneto-viscosity plots). Mn-ferrite ferrofluids (i.e. MFW, MFK and MFT) were synthesized using Mn-ferrite nanoparticles suspension in different colloidal, that is, water, kerosene and toluene, respectively. The flow behaviour is studied for these ferrofluids in various applied magnetic fields. The respective volume ratio of Mn-ferrite nanoparticles, surfactant and colloidal (water, kerosene and toluene for respective ferrofluids) is taken as 1:0.5:1.5 for the preparation of Mn-ferrite ferrofluids. The MF nanoparticles are coated with tetramethyl ammonia (TMA) for the water-based MF ferrofluid (MFW). The kerosene-based ferrofluid (MFK) and toluene-based ferrofluid (MFT) are synthesized by using oleic acid-coated nanoparticles.

#### *6.2.2. Flow curves of Mn-ferrite ferrofluids*

**Figure 8** (left) shows the flow curves of MF ferrofluid at various applied magnetic fields in the range from 0 to 1.33 T. The MFW ferrofluid shows non-Newtonian behaviour at lower shear rates for zero magnetic field. The behaviour changes to Newtonian behaviour with increasing shear rate. The TMA coating of the nanoparticles exhibits more hydrophobic nature of the nanoparticles, leading to decrease in friction between the layers of the ferrofluid. So the power law behaviour cannot be observed in zero field magneto-viscosity plot for the waterbased ferrofluid. For all the range of shear rates, it follows single behaviour, that is, power law behaviour without any discrepancy.

$$f(D\_{\\_})\_{\\_=0} = f\_0(1/T\_{\\_}^2) \frac{d}{dT} (T \, m\_{ZFC}(T) \,)^2$$

Here, *K* is the consistency coefficient and the exponent *n* is the power law index.

Similar behaviour is observed in the MFK and MFT ferrofluids but the magnetic response is more. This is because the MFK and MFT are synthesized using FM nanoparticles in less viscous colloidal. So the hydrodynamic force is less compared to MFW ferrofluid. The very small *n-*value (0.1–0.05) indicates the high shear-thinning behaviour as reported in an earlier work [45]. With increase in applied field, *n-*value decreases indicating that the viscosity is affected more by magnetic field at low shear rates than at high shear rates [46, 47]. This is due to the competition between the flow field and the applied magnetic field. In this process, at higher shear rates (shear rate greater than 200 s–1), the flow field dominates, whereas applied field dominates at lower shear rates. These ferrofluids showed completely non-Newtonian behaviour whereas the SPM nanoparticles-based ferrofluids (MZFE1) show the change in behaviour from non-Newtonian to Newtonian with an increase in shear rate similar to that reported in our earlier work [48, 49]. Above the applied field of 0.3 T, the magnetic nanoparticles (magnetic nano-dipoles) orient completely in the field direction with long-chain formation in the fluid.

#### *6.2.3. Magneto-viscosity of Mn-ferrite ferrofluids*

The magneto-viscosity plots at a shear rate of 10 s–1 (**Figure 8**, right) show a rapid increase with an increase in magnetic field initially, followed by its saturation at higher fields, with low hysteresis when the applied field is decreased to zero. Since the ferromagnetic particles are dispersed in less viscous fluid, as the field increases the nanoparticles try to rotate in the field direction [50–52]. Above the applied field of 0.3 T, the viscosity saturates. Because of long-chain formation along the field direction, the viscosity reaches the maximum possible value at the given shear rate. Magneto-viscosity at different shear rates is described in Refs. [53–56]. This behaviour is similar to magnetization plots of nanoparticles as a function of magnetic field.

#### *6.2.4. Magneto-viscosity of Mn-Zn ferrite ferrofluids*

Mn-Zn ferrite ferrofluids were synthesized using the MZF nanoparticles suspension in different colloidal, that is, water, kerosene, toluene and paraffin, respectively. The respective volume ratio of Mn-Zn ferrite nanoparticles, oleic acid and colloidal (water, kerosene, toluene or paraffin for respective ferrofluids) is taken as 1:0.5:1.5 for the preparation of Mn-Zn ferrite ferrofluids.

#### *6.2.5. Flow curves of Mn-Zn ferrite ferrofluids*

The flow behaviour is studied using flow curves shown in **Figure 9** (left)) for Mn0.75Zn0.25 Fe<sup>2</sup> O4 ferrofluids in different applied magnetic fields. The fluid shows power law behaviour with

rates for zero magnetic field. The behaviour changes to Newtonian behaviour with increasing shear rate. The TMA coating of the nanoparticles exhibits more hydrophobic nature of the nanoparticles, leading to decrease in friction between the layers of the ferrofluid. So the power law behaviour cannot be observed in zero field magneto-viscosity plot for the waterbased ferrofluid. For all the range of shear rates, it follows single behaviour, that is, power law

dT(*<sup>T</sup> mZFC*(*<sup>T</sup>* ) )

Similar behaviour is observed in the MFK and MFT ferrofluids but the magnetic response is more. This is because the MFK and MFT are synthesized using FM nanoparticles in less viscous colloidal. So the hydrodynamic force is less compared to MFW ferrofluid. The very small *n-*value (0.1–0.05) indicates the high shear-thinning behaviour as reported in an earlier work [45]. With increase in applied field, *n-*value decreases indicating that the viscosity is affected more by magnetic field at low shear rates than at high shear rates [46, 47]. This is due to the competition between the flow field and the applied magnetic field. In this process, at higher shear rates (shear rate greater than 200 s–1), the flow field dominates, whereas applied field dominates at lower shear rates. These ferrofluids showed completely non-Newtonian behaviour whereas the SPM nanoparticles-based ferrofluids (MZFE1) show the change in behaviour from non-Newtonian to Newtonian with an increase in shear rate similar to that reported in our earlier work [48, 49]. Above the applied field of 0.3 T, the magnetic nanoparticles (magnetic nano-dipoles) orient completely in the field direction with long-chain formation in the fluid.

The magneto-viscosity plots at a shear rate of 10 s–1 (**Figure 8**, right) show a rapid increase with an increase in magnetic field initially, followed by its saturation at higher fields, with low hysteresis when the applied field is decreased to zero. Since the ferromagnetic particles are dispersed in less viscous fluid, as the field increases the nanoparticles try to rotate in the field direction [50–52]. Above the applied field of 0.3 T, the viscosity saturates. Because of long-chain formation along the field direction, the viscosity reaches the maximum possible value at the given shear rate. Magneto-viscosity at different shear rates is described in Refs. [53–56]. This behaviour is similar to magnetization plots of nanoparticles as a function of magnetic field.

Mn-Zn ferrite ferrofluids were synthesized using the MZF nanoparticles suspension in different colloidal, that is, water, kerosene, toluene and paraffin, respectively. The respective volume ratio of Mn-Zn ferrite nanoparticles, oleic acid and colloidal (water, kerosene, toluene or paraffin for respective ferrofluids) is taken as 1:0.5:1.5 for the preparation of Mn-Zn ferrite ferrofluids.

The flow behaviour is studied using flow curves shown in **Figure 9** (left)) for Mn0.75Zn0.25 Fe<sup>2</sup>

ferrofluids in different applied magnetic fields. The fluid shows power law behaviour with

O4

0 (1/*TB* <sup>2</sup> ) \_\_\_d

Here, *K* is the consistency coefficient and the exponent *n* is the power law index.

behaviour without any discrepancy. *f*(*D* ) = *f*

152 Magnetic Spinels- Synthesis, Properties and Applications

*6.2.3. Magneto-viscosity of Mn-ferrite ferrofluids*

*6.2.4. Magneto-viscosity of Mn-Zn ferrite ferrofluids*

*6.2.5. Flow curves of Mn-Zn ferrite ferrofluids*

**Figure 9.** Viscosity as a function of shear rate at different applied magnetic fields (left) and viscosity as a function of applied magnetic field at a shear rate of 10 S–1 (right) of MZF ferrofluids synthesized with various colloidal.

small *n-*value between 0.1 and 0.04 for various magnetic fields in the range of 0–1.33 T indicating non-Newtonian behaviour with higher shear thinning [45]. The viscosity versus shear rate plot show the change from non-Newtonian to Newtonian behaviour at higher shear rates (shear rate greater than 300 s-1), as we described in our earlier work [48]. The fluid viscosity behaviour is accounted to only the applied magnetic field. So for all the range of shear rate, it follows single power law behaviour without any discrepancy. The non-dimensional parameter, Mason number (*Ma*), explains the flow behaviour. Similar to electro-rheological fluids, the 'Ma' is the ratio of shear forces or hydrodynamic forces (*F*H) to the magnetic forces (*F*M), that is, *Ma* = *F*H/*F*M. The viscosity as a function of Mason number also follows the power law behaviour. The critical *Ma*-value determines the transition from magnetization to hydrodynamic control of the suspension structure [57].

#### *6.2.6. Magneto-viscosity plots of Mn-Zn ferrite ferrofluids*

The viscosity versus applied magnetic field at a shear rate of 10 S–1 is shown in **Figure 9** (right). The magneto-viscosity plots show a rapid increase with an increase in magnetic field initially. The viscosity is saturated at higher fields (around 0.2 T) and show low hysteresis when the applied field is decreased to zero. This behaviour is similar to the magnetization plots of the nanoparticles as a function of magnetic field.

#### *6.2.7. Magneto-viscosity of paraffin-based ferrofluids*

Paraffin is more stable colloidal compared with other colloidals used for the synthesis of ferrofluids. The dispersion of FM nanoparticles in paraffin gives fine control of viscosity in magnetic field. This study is quite interesting and useful for ferrofluid applications. Similar to the synthesis of other ferrofluids, the paraffin-based Mn0.75Zn0.25Fe<sup>2</sup> O4 (MZFP1) and Mn0.9Zn0.1 Fe<sup>2</sup> O4 (MZFP2) ferrofluids are synthesized using oleic acid-coated nanoparticles. The magneto-viscosity plots of MZFP1 and MZFP2 ferrofluids are shown in **Figure 10** at the shear rate of 1 and 10 s–1.

The comparison between the magneto-viscosity of these ferrofluids is as follows:


**Figure 10.** Viscosity as a function of applied magnetic field at different shear rates of paraffin-based ferrofluids of composition Mn0.75Zn0.25Fe<sup>2</sup> O4 (MZFP1) (left) and Mn0.9Zn0.1 Fe<sup>2</sup> O4 (MZFP2) (right).


#### *6.2.8. The Herschel-Bulkley behaviour in ferrofluids*

*6.2.6. Magneto-viscosity plots of Mn-Zn ferrite ferrofluids*

nanoparticles as a function of magnetic field.

154 Magnetic Spinels- Synthesis, Properties and Applications

*6.2.7. Magneto-viscosity of paraffin-based ferrofluids*

Fe<sup>2</sup> O4

of 1 and 10 s–1.

magnetic field.

composition Mn0.75Zn0.25Fe<sup>2</sup>

O4

the synthesis of other ferrofluids, the paraffin-based Mn0.75Zn0.25Fe<sup>2</sup>

The comparison between the magneto-viscosity of these ferrofluids is as follows:

magnetic field whereas sharp increment is observed in MZFP1 ferrofluid.

The viscosity versus applied magnetic field at a shear rate of 10 S–1 is shown in **Figure 9** (right). The magneto-viscosity plots show a rapid increase with an increase in magnetic field initially. The viscosity is saturated at higher fields (around 0.2 T) and show low hysteresis when the applied field is decreased to zero. This behaviour is similar to the magnetization plots of the

Paraffin is more stable colloidal compared with other colloidals used for the synthesis of ferrofluids. The dispersion of FM nanoparticles in paraffin gives fine control of viscosity in magnetic field. This study is quite interesting and useful for ferrofluid applications. Similar to

 (MZFP2) ferrofluids are synthesized using oleic acid-coated nanoparticles. The magneto-viscosity plots of MZFP1 and MZFP2 ferrofluids are shown in **Figure 10** at the shear rate

(i) The gradual increment of viscosity is observed in MZFP2 ferrofluid with an increase in

(ii) When the magnetic field is decreased, the magneto-viscosity plots of MZFP1 ferrofluid show less hysteresis and the plot is not relaxing to initial position at zero

**Figure 10.** Viscosity as a function of applied magnetic field at different shear rates of paraffin-based ferrofluids of

O4

(MZFP2) (right).

(MZFP1) (left) and Mn0.9Zn0.1 Fe<sup>2</sup>

O4 (MZFP1) and Mn0.9Zn0.1

The MF nanoparticles dispersed in less viscous colloidal (toluene) are free to move in the fluid and so it is easy to study the structural changes in spot-like and chain-like structures as a function of applied magnetic field. MFT ferrofluid is chosen to explain the Herschel-Bulkley (H-B) behaviour in ferrofluids. Shear stress (*τ*) as a function of shear rate (*γ*") plots is shown in **Figure 11**.

The H-B model combines the power law model with the yield stress as per the equation, *τ* = *τ*<sup>0</sup> + *K γ*˙ *<sup>n</sup>*

Here, *τ*<sup>0</sup> is the yield stress. The yield stress can be determined from the plots using H-B model.

Apart from the magnetic behaviour of magnetic particles and the colloidal behaviour, the dosage of surfactant and the concentration of the magnetic nanoparticles in ferrofluids play a major role for the change of rheological behaviour at lower shear rates. This influences the flow field interaction in the ferrofluid. With the application of field, the shear stress versus shear rate plots show the H-B fluid behaviour. It is also observed as the magnetic field increases, the shear stress increases. The yield stress was determined by the extrapolation of the plots using the B-H model. The plots shifted upward due to the drastic change in yield stress. It increases from 4 to 27 Pa as magnetic field increases from 0 to 0.0717 T. The deviation from H-B model fit can be observed in shear stress (*τ*) versus shear rate plot at 1.33 T applied magnetic field [58]. As the applied field increases, the formation of chain-like and spot-like structure takes

**Figure 11.** Shear stress as a function of shear rate, whereas solid line is H-B model fit for applied fields 0.000 (a), 0.0061 (b) and 0.0717 T (c) of MFT ferrofluid.

place [59]. The fluid remains as H-B fluid in the presence of small number of chains formed in the low field region with the increase in magnetic field. The strong- and long-chain formation occurs with the increase in magnetic field further. The magnetic field force dominates the other forces present in the fluid, as the magnetic field is increased leading to deviation from H-B fluid behaviour.

#### **7. Conclusions**

The nanoparticles of various sizes and compositions Mn1-*<sup>x</sup>* Zn*<sup>x</sup>* Fe<sup>2</sup> O4 with *x* = 0–1 were synthesized by co-precipitation method. The detailed structural and magnetic properties of MZF nanoparticles were carried out. The lattice parameter decreases with an increase in Zn content reaching a minimum value at *x* = 0.5 followed by an increasing trend with further increase in Zn content. The saturation magnetization and blocking temperature increase with a decrease in Zn content. The ferrofluids based on superparamagnetic and ferromagnetic MZF nanoparticles were synthesized. The magneto-viscosity of ferrofluids with the dispersion of nanoparticles in different colloidal was studied. The viscosity versus shear rate plot in applied magnetic field follows single power law behaviour without any discrepancy. The data on ferrofluids based on MF nanoparticles dispersed in less viscous colloidal (toluene) are analysed in view of Herschel-Bulkley model.

## **Author details**

Rajender Singh\* and Gadipelly Thirupathi

\*Address all correspondence to: rsinghsp@gmail.com

School of Physics, University of Hyderabad, Hyderabad, India

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place [59]. The fluid remains as H-B fluid in the presence of small number of chains formed in the low field region with the increase in magnetic field. The strong- and long-chain formation occurs with the increase in magnetic field further. The magnetic field force dominates the other forces present in the fluid, as the magnetic field is increased leading to deviation from

by co-precipitation method. The detailed structural and magnetic properties of MZF nanoparticles were carried out. The lattice parameter decreases with an increase in Zn content reaching a minimum value at *x* = 0.5 followed by an increasing trend with further increase in Zn content. The saturation magnetization and blocking temperature increase with a decrease in Zn content. The ferrofluids based on superparamagnetic and ferromagnetic MZF nanoparticles were synthesized. The magneto-viscosity of ferrofluids with the dispersion of nanoparticles in different colloidal was studied. The viscosity versus shear rate plot in applied magnetic field follows single power law behaviour without any discrepancy. The data on ferrofluids based on MF nanoparticles dispersed in less viscous colloidal (toluene) are analysed in view of Herschel-Bulkley model.

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#### **Mechanochemical Synthesis of Water-Based Magnetite Magnetic Fluids Mechanochemical Synthesis of Water-Based Magnetite Magnetic Fluids**

#### Tomohiro Iwasaki Tomohiro Iwasaki

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66075

#### **Abstract**

Magnetite, Fe3O4 (FeIIO-FeIII2O3), is a member of the spinel group as well as a common ferrite with a cubic inverse spinel structure. Aqueous colloidal solutions of Fe3O4, i.e. water-based Fe3O4 magnetic fluids, have attracted substantial attention in biomedical applications, such as drug delivery, magnetic resonance imaging, and magnetic hyperthermia. In this study, to readily prepare water-based magnetic fluids with biocompatible dispersant-coated Fe3O4 nanoparticles that are stably dispersed in water medium, a mechanochemical synthesis method was developed. In this method, an iron-free citric acid solution is milled in a tumbling ball mill with steel balls at room temperature, reducing the production costs and environmental impacts. The initial gas phase in the milling vessel is air, and pressure is varied to control the formation of Fe3O4 nanoparticles. Although no iron species are contained in the starting solution, Fe3O4 nanoparticles form in the solution according to the reaction mechanism based on the oxidation-reduction processes of the corrosion of steel. At the same time, the Fe3O4 nanoparticle surface is modified with citrate ions, resulting in a stable dispersion. The magnetic fluids prepared using this mechanochemical method possess good induction heating properties in an alternating current magnetic field.

**Keywords:** magnetite nanoparticles, inverse spinel, magnetic fluid, mechanochemical reaction, oxidation-reduction, hyperthermia

#### **1. Introduction**

Magnetic fluids are colloidal solutions containing magnetic nanoparticles stably dispersed in a liquid medium so that the entire fluid behaves like a ferromagnet. Moreover, no solid-liquid

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

separation occurs, even under centrifugal force fields [1]. Therefore, magnetic fluids are widely used in industrial products, such as rotating-axis seal material, lubricants, and liquid damper. In magnetic fluids, iron-based oxide nanoparticles are often employed as the magnetic material. In particular, magnetic fluids that consist of superparamagnetic magnetite (Fe3O4) nanoparticles are used in biomedical and environmental fields, such as magnetic resonance imaging (MRI) contrast agents [2], magnetic hyperthermia in cancer therapy [3], drug delivery systems (DDS) [4], and so on, due to their high performance, low toxicity, and low environmental impact. The appropriate size of the Fe3O4 nanoparticles depends on their application. For example, when used for MRI, hyperthermia, and DDS, the particle size is required to be below 30 nm. Appropriate surface modification of the Fe3O4 nanoparticles can bind them to biomolecules as a result of the large surface area, leading to good bio-circulation [5]. In addition, the enhanced permeability and retention (EPR) effect can selectively accumulate Fe3O4 nanoparticles into cancer tissue [6]. Accordingly, the sizes of primary particles and aggregates must be controlled, and good stable dispersion is needed. Therefore, many studies on controlling the particle size as well as dispersion and aggregation using additives and organic solvents have been conducted. Wen et al. [7] prepared Fe3O4 nanoparticles with a size of 4–5 nm in an organic solvent by adding sodium oleate as a surfactant. Zhang et al. [8] synthesized Fe3O4 nanoparticles with good dispersibility in an organic solvent using surface modification with polystyrene. Frimpong et al. [9] controlled the sizes of the primary particle and aggregate using citric acid. Elisa de Sousa et al. [10] obtained citrate-adsorbed Fe3O4 nanoparticles that were dispersed under neutral conditions. In Fe3O4 nanoparticles that are used in vivo, biocompatible molecules, such as dextran, polyethylene glycol, polyvinyl alcohol, citric acid, polyacrylic acid, and phospholipid, are frequently employed as additives. In particular, citric acid is non-toxic and can form ultrafine primary particles. Furthermore, citrate ions can adsorb onto Fe3O4 nanoparticles, leading to aggregation inhibition due tothe steric hindrance and electrostatic repulsion. Therefore, citric acid is widely used as an anti-aggregation agent.

Magnetic fluids were originally developed by Papell of the United States National Aeronautics and Space Administration (NASA) in 1965 for position control of liquid fuel under zero gravity [11]. The initially developed magnetic fluid was prepared by ball-mill grinding of Fe3O4 grains in kerosene containing oleic acid for several hundred hours, followed by the removal of coarse particles by centrifugal separation. At present, various methods for synthesizing Fe3O4 nanoparticles using chemical reactions in gas, liquid, and solid phases have been developed. Among them, liquid phase synthesis has been actively studied because the component and concentration of the reactants can be controlled fairly easily and the formation reaction can progress, even under moderate conditions. In particular, coprecipitation methods that produce Fe3O4 by adding a base as a precipitant to a solution containing Fe2+ ions and Fe3+ ions are industrially employed because they can easily provide homogeneous Fe3O4 nanoparticles with smaller primary sizes [12]. The thermal decomposition method is also frequently used. Jeyadevan et al. [13] synthesized Fe3O4 nanoparticles that are suitable for magnetic hyperthermia via thermal decomposition of iron pentacarbonyl in oleic acid-containing dioctyl ether. Sun and Zeng [14] prepared mono-dispersed superparamagneticFe3O4 nanoparticles using iron acetylacetonate, oleic acid, and amine oleate. As a result, conventional methods can provide magnetic fluids that are desirable for various applications. For the previously mentioned biomedical applications, water-based Fe3O4 magnetic fluids are suitable. However, in many cases, the conventional methods require iron salt, base, and organic solvents, requiring the removal of unnecessary components from the product to clean water-based magnetic fluids. This may increase the environmental impact and production cost. As a result, an innovative process is required to more readily prepare water-based Fe3O4 magnetic fluids without any additional operations.

To meet the demand, we developed a new mechanochemical process with an iron-free aqueous solution containing citric acid (CA) as a reaction accelerator and anti-aggregation agent milled at room temperature with a ball mill using steel balls, resulting in the production of waterbased Fe3O4 magnetic fluid [15]. The formation of crystalline Fe3O4 nanoparticles in this process may consist of several steps, as follows: (1) corrosion of steel balls, (2) oxidation of released ferrous ions, (3) reduction of ferric ions, and (4) formation and crystal growth of Fe3O4. Therefore, the mechanochemical effect can enhance the formation and crystallization of Fe3O4. Furthermore, this method does not use iron salts as iron sources or a base as the precipitant because iron ions and hydroxide ions form during the corrosion, which can provide a waterbased Fe3O4 magnetic fluid without any post-operations, such as the removal of unnecessary ions and solvent displacement.

This paper presents the properties of Fe3O4 magnetic fluids prepared by this mechanochemical process and a detailed analysis of the reaction mechanism based on changes in the composition of gas, liquid, and solid phases in the formation of Fe3O4 as well as ferrous, ferric, and hydroxide ions. Furthermore, the formation reaction of Fe3O4 is kinetically analysed under different gas phase conditions and mechanical energy fields.

## **2. Experimental**

separation occurs, even under centrifugal force fields [1]. Therefore, magnetic fluids are widely used in industrial products, such as rotating-axis seal material, lubricants, and liquid damper. In magnetic fluids, iron-based oxide nanoparticles are often employed as the magnetic material. In particular, magnetic fluids that consist of superparamagnetic magnetite (Fe3O4) nanoparticles are used in biomedical and environmental fields, such as magnetic resonance imaging (MRI) contrast agents [2], magnetic hyperthermia in cancer therapy [3], drug delivery systems (DDS) [4], and so on, due to their high performance, low toxicity, and low environmental impact. The appropriate size of the Fe3O4 nanoparticles depends on their application. For example, when used for MRI, hyperthermia, and DDS, the particle size is required to be below 30 nm. Appropriate surface modification of the Fe3O4 nanoparticles can bind them to biomolecules as a result of the large surface area, leading to good bio-circulation [5]. In addition, the enhanced permeability and retention (EPR) effect can selectively accumulate Fe3O4 nanoparticles into cancer tissue [6]. Accordingly, the sizes of primary particles and aggregates must be controlled, and good stable dispersion is needed. Therefore, many studies on controlling the particle size as well as dispersion and aggregation using additives and organic solvents have been conducted. Wen et al. [7] prepared Fe3O4 nanoparticles with a size of 4–5 nm in an organic solvent by adding sodium oleate as a surfactant. Zhang et al. [8] synthesized Fe3O4 nanoparticles with good dispersibility in an organic solvent using surface modification with polystyrene. Frimpong et al. [9] controlled the sizes of the primary particle and aggregate using citric acid. Elisa de Sousa et al. [10] obtained citrate-adsorbed Fe3O4 nanoparticles that were dispersed under neutral conditions. In Fe3O4 nanoparticles that are used in vivo, biocompatible molecules, such as dextran, polyethylene glycol, polyvinyl alcohol, citric acid, polyacrylic acid, and phospholipid, are frequently employed as additives. In particular, citric acid is non-toxic and can form ultrafine primary particles. Furthermore, citrate ions can adsorb onto Fe3O4 nanoparticles, leading to aggregation inhibition due tothe steric hindrance and electrostatic

162 Magnetic Spinels- Synthesis, Properties and Applications

repulsion. Therefore, citric acid is widely used as an anti-aggregation agent.

Magnetic fluids were originally developed by Papell of the United States National Aeronautics and Space Administration (NASA) in 1965 for position control of liquid fuel under zero gravity [11]. The initially developed magnetic fluid was prepared by ball-mill grinding of Fe3O4 grains in kerosene containing oleic acid for several hundred hours, followed by the removal of coarse particles by centrifugal separation. At present, various methods for synthesizing Fe3O4 nanoparticles using chemical reactions in gas, liquid, and solid phases have been developed. Among them, liquid phase synthesis has been actively studied because the component and concentration of the reactants can be controlled fairly easily and the formation reaction can progress, even under moderate conditions. In particular, coprecipitation methods that produce Fe3O4 by adding a base as a precipitant to a solution containing Fe2+ ions and Fe3+ ions are industrially employed because they can easily provide homogeneous Fe3O4 nanoparticles with smaller primary sizes [12]. The thermal decomposition method is also frequently used. Jeyadevan et al. [13] synthesized Fe3O4 nanoparticles that are suitable for magnetic hyperthermia via thermal decomposition of iron pentacarbonyl in oleic acid-containing dioctyl ether. Sun and Zeng [14] prepared mono-dispersed superparamagneticFe3O4 nanoparticles using iron acetylacetonate, oleic acid, and amine oleate. As a result, conventional methods can provide magnetic fluids that are desirable for various applications. For the previously

All chemicals used in this work were purchased from Wako Pure Chemical Industries and were used without further purification. Some preliminary experiments determined that the CA concentration in the starting solution was desirable to be 5 mmol/L, and a single Fe3O4 phase was finally obtained. Ninety millilitres of the CA solution (pH = 2.7) was placed in a tumbling ball mill consisting of a Teflon-lined gas-tight vessel (capacity 500 mL, diameter 90 mm) and carbon steel balls (Fe > 99 mass%, diameter 3 mm). The charged volume of the balls (includes the voids among balls) was 40% of the vessel capacity. The initial gas phase in the vessel was air at atmospheric pressure. The solution was milled at room temperature for anappropriate time period. The rotational speed of the vessel was 140 rpm, corresponding to the theoretically determined critical rotational speed. After milling, the fluid was removed from the vessel and characterized. The weight loss of balls during milling was also measured.

The particle size distribution and zeta potential of samples were determined using a particle size analyser (Malvern Zetasizer Nano ZS), and the phase evolution was evaluated using a powder X-ray diffractometer (Rigaku RINT-1500) after drying the sample. The average crystallite size was determined using Scherrer's equation for the diffraction peak from the Fe3O4 (311) plane at 2θ = 35.4°. The FT-IR spectrum was measured using a Fourier transform infrared spectrophotometer (Shimadzu IRAffinity-1). The magnetic properties (i.e. magnetization-magnetic field hysteretic cycle) were analysed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL) at room temperature over a magnetic field range of −10 kOe to +10 kOe. The magnetic hyperthermia-related induction heating properties of the fluid were evaluated by an alternating magnetic field generator consisting of a radio frequency power source (Thamway T162-5723A), an impedance matching box (Thamway T020-5723F), and a solenoid coil (inner diameter 70 mm) with 21 copper tube (outer diameter of 4 mm and inner diameter of 3 mm) turns [13]. Cooling water flowed inside the copper tube. A proper amount (approximately 1.0 g) of fluid was charged in a glass tube with a diameter of 16 mm, and the test tube was placed in the coil centre. The temperature increase in the fluid from 37°C (corresponding to normal body temperature) in an alternating magnetic field was measured with an optical fibre thermometer (FISO Technologies FTI-10 equipped with FOT-L-NS-967). The frequency and amplitude of the magnetic field were 600 kHz and 3 kA/m, respectively.

In the analysis of the reaction mechanism, 0.5 mL of the gas in the vessel was sampled with a syringe after milling, and the gas component was analysed by a gas chromatograph (Shimadzu GC-8A). The pH of the resultant fluid was measured with a pH meter (Horiba D-21 equipped with 9625-10D electrode), and the iron(II) and iron(III) concentrations were determined by a colorimetric method with 1,10-phenanthroline and a spectrophotometer (JASCO Ubest V-530).

The influence of gas phase conditions in the vessel on the Fe3O4 formation was studied. The 5 mmol/L CA solution was placed in the vessel; then, compressed air was charged into the vessel before milling. The total pressure in the gas phase was varied from 1 atm (atmospheric pressure) to 6 atm, corresponding to initial oxygen partial pressures of 0.21–1.26 atm. The milling time was 24 h at total pressures lower than 2.5 atm but 48 h at higher than 3 atm. The pressure in the vessel before and after milling was measured, and the oxygen consumption was determined from the pressure change. Furthermore, the rotational speed of the vessel was varied from 0 to 140 rpm at a total pressure of 1 atm, which can alter the intensity of the mechanical energy field. Based on the obtained results, the kinetics of the Fe3O4 formation reaction were investigated.

Lastly, scaleup of the process was examined. In this investigation, a Teflon-lined milling vessel with a capacity of 2.6 L (diameter 150 mm) was used. The charged volume of the steel balls (diameter 3 mm) was 40% of the vessel capacity, and 476 mL of the 5 mmol/L CA solution was placed in the vessel and milled for 24 h. The gas phase was air at atmospheric pressure. The rotational speed of the large vessel was adjusted so that either the Froude number or the peripheral velocity in both vessels agreed with each other.

## **3. Results and discussion**

## **3.1. Formation of Fe3O4 magnetic fluids**

**Figures 1** and **2** show the X-ray diffraction (XRD) patterns and average crystallite sizes of solid products obtained after milling the CA solution, respectively. At milling times of less than 1.5 h, broad peaks that were attributed to ferrihydrite (Fe5O8H 4H2O), which is an amorphous or low crystalline oxyhydroxide [16–18], were observed in the XRD patterns. After 2 h, formation and crystal growth of Fe3O4 occurred, and milling for more than 18 h provided relatively high crystalline Fe3O4. However, most of the Fe3O4 particles settled in the fluid due to aggregation. To improve the dispersion, the total citrate concentration of fluids was increased by adding anhydrous CA and trisodium citrate dehydrate to the fluid obtained by milling for 18 h; and then the zeta potential was measured. The pH of the fluids was kept constant at approximately 8 in all fluids by adding proper amounts of CA and trisodium citrate. **Figure 3** illustrates the change in the zeta potential with the changing citrate concentration. The absolute value of the zeta potential increased with the increasing citrate concentration. When the citrate concentration was 14 mmol/L, corresponding to an iron/citrate molar ratio of approximately 3 in the fluid, the zeta potential was less than −40 mV, leading to good dispersion. It was inferred that an increase in the citrate concentration can improve the dispersion.

**Figure 1.** Effect of the milling time on the Fe3O4 phase evolution.

infrared spectrophotometer (Shimadzu IRAffinity-1). The magnetic properties (i.e. magnetization-magnetic field hysteretic cycle) were analysed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL) at room temperature over a magnetic field range of −10 kOe to +10 kOe. The magnetic hyperthermia-related induction heating properties of the fluid were evaluated by an alternating magnetic field generator consisting of a radio frequency power source (Thamway T162-5723A), an impedance matching box (Thamway T020-5723F), and a solenoid coil (inner diameter 70 mm) with 21 copper tube (outer diameter of 4 mm and inner diameter of 3 mm) turns [13]. Cooling water flowed inside the copper tube. A proper amount (approximately 1.0 g) of fluid was charged in a glass tube with a diameter of 16 mm, and the test tube was placed in the coil centre. The temperature increase in the fluid from 37°C (corresponding to normal body temperature) in an alternating magnetic field was measured with an optical fibre thermometer (FISO Technologies FTI-10 equipped with FOT-L-NS-967). The frequency and amplitude of the magnetic

In the analysis of the reaction mechanism, 0.5 mL of the gas in the vessel was sampled with a syringe after milling, and the gas component was analysed by a gas chromatograph (Shimadzu GC-8A). The pH of the resultant fluid was measured with a pH meter (Horiba D-21 equipped with 9625-10D electrode), and the iron(II) and iron(III) concentrations were determined by a colorimetric method with 1,10-phenanthroline and a spectrophotometer (JASCO Ubest V-530). The influence of gas phase conditions in the vessel on the Fe3O4 formation was studied. The 5 mmol/L CA solution was placed in the vessel; then, compressed air was charged into the vessel before milling. The total pressure in the gas phase was varied from 1 atm (atmospheric pressure) to 6 atm, corresponding to initial oxygen partial pressures of 0.21–1.26 atm. The milling time was 24 h at total pressures lower than 2.5 atm but 48 h at higher than 3 atm. The pressure in the vessel before and after milling was measured, and the oxygen consumption was determined from the pressure change. Furthermore, the rotational speed of the vessel was varied from 0 to 140 rpm at a total pressure of 1 atm, which can alter the intensity of the mechanical energy field. Based on the obtained results, the kinetics of the Fe3O4 forma-

Lastly, scaleup of the process was examined. In this investigation, a Teflon-lined milling vessel with a capacity of 2.6 L (diameter 150 mm) was used. The charged volume of the steel balls (diameter 3 mm) was 40% of the vessel capacity, and 476 mL of the 5 mmol/L CA solution was placed in the vessel and milled for 24 h. The gas phase was air at atmospheric pressure. The rotational speed of the large vessel was adjusted so that either the Froude number or the

**Figures 1** and **2** show the X-ray diffraction (XRD) patterns and average crystallite sizes of solid products obtained after milling the CA solution, respectively. At milling times of less

field were 600 kHz and 3 kA/m, respectively.

164 Magnetic Spinels- Synthesis, Properties and Applications

tion reaction were investigated.

**3. Results and discussion**

**3.1. Formation of Fe3O4 magnetic fluids**

peripheral velocity in both vessels agreed with each other.

Using the fluid obtained at a milling time of 24 h, fluid with a total citrate concentration of 27 mmol/L was prepared and characterized. As shown in **Figure 4**, this fluid had good dispersion even after 2 weeks. Furthermore, when a permanent magnet was placed beside the glass bottle, the fluid was attracted to magnet, suggesting that the Fe3O4 nanoparticles have superparamagnetic properties. **Figure 5** shows the particle size distribution, magnetizationmagnetic field curve, and FT-IR spectrum of the solid product obtained by drying the fluid after magnetic separation. The median diameter was 7.3 nm, while the crystallite size was 8.8 nm. The primary particle size was almost the same as the crystallite size, suggesting that the obtained Fe3O4 nanoparticles were monocrystalline. The saturation magnetization was 27 emu/g, which was much lower than that of bulk Fe3O4 (92 emu/g) due to the smaller particle size. The residual magnetization was approximately zero, and the coercivity was very low, indicating superparamagnetism. Additionally, some strong absorption bands in the FT-IR spectrum were observed, which were attributed to citrate ions, indicating that the Fe3O4 nanoparticle surface was modified by citrate ions. **Figure 6** illustrates a temperature adjustment of the fluid within 43 ± 0.5°C, corresponding to a typical temperature range in hyperthermia treatments, in an on-off-controlled alternating magnetic field. This fluid was found to exhibit good magnetic hyperthermia properties. In addition, the temperature was successfully controlled within the temperature range, suggesting that the fluid can be used in hyperthermia therapies.

**Figure 2.** Change in the average crystallite size of Fe3O4 with milling time.

**Figure 3.** Effect of citrate concentration on the zeta potential of the fluid obtained with milling time of 18 h.

**Figure 4.** A fluid with a total citrate concentration of 27 mmol/L after 2 weeks.

nanoparticle surface was modified by citrate ions. **Figure 6** illustrates a temperature adjustment of the fluid within 43 ± 0.5°C, corresponding to a typical temperature range in hyperthermia treatments, in an on-off-controlled alternating magnetic field. This fluid was found to exhibit good magnetic hyperthermia properties. In addition, the temperature was successfully controlled within the temperature range, suggesting that the fluid can be used in

hyperthermia therapies.

166 Magnetic Spinels- Synthesis, Properties and Applications

**Figure 2.** Change in the average crystallite size of Fe3O4 with milling time.

**Figure 3.** Effect of citrate concentration on the zeta potential of the fluid obtained with milling time of 18 h.

**Figure 5.** (a) Particle size distribution, (b) magnetization-magnetic field hysteresis cycle, and (c) FT-IR spectrum of the solid product.

**Figure 6.** Temperature adjustment of the fluid in an on-off-controlled alternating magnetic field.

#### **3.2. Reaction mechanism**

**Figure 7** shows the changes in the weight loss of balls and the iron concentration of the fluid with the milling time. With increasing milling time, the mass of the balls decreased and the iron concentrations increased, implying that ferrous (Fe2+) ions were released from steel balls during milling due to corrosion, as expressed by Eq. (1).

**Figure 7.** Change in the (a) weight loss of balls and (b) iron concentration with milling time at the initial stages.

$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2\text{e}^- \tag{1}$$

**Figure 8** shows the variation in the pH of fluids with the milling time. The fluid was acidic before milling and then immediately neutral. In general, free electrons (e− ) generated simultaneously with Fe2+ ion release are received by oxygen molecules (O2) in a solution under acidic conditions, according to a cathode reaction expressed by Eq. (2).

**Figure 8.** Change in the pH with milling time at the initial stages.

$$\text{H}\_2 + 4\text{H}^+ + 4\text{e}^- \rightarrow 2\text{H}\_2\text{O} \tag{2}$$

As hydrogen (H+ ) ions in the solution are consumed, the solution pH increases and the solution becomes neutral. Under neutral conditions, the following cathode reaction occurs and hydroxide (OH− ) ions form.

$$\text{H}\_2\text{O}\_2 + 2\text{H}\_2\text{O} + 4\text{e}^- \rightarrow 4\text{OH}^- \tag{3}$$

On the other hand, released Fe2+ ions react with the OH− ions formed according to Eq. (3), producing ferrous hydroxide (Fe(OH)2) by Eq. (4).

$$\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}\_2 \tag{4}$$

As seen in **Figure 7**, most Fe(OH)2 is immediately oxidized by dissolved O2; then, ferric hydroxide (Fe(OH)3) forms according to Eq. (5).

$$4\text{Fe(OH)}\_{2} + 2\text{H}\_{2}\text{O} + \text{O}\_{2} \rightarrow 4\text{Fe(OH)}\_{3} \tag{5}$$

Fe(OH)3 can be transformed to ferrihydrite (Fe5O8H 4H2O) as below:

$$\text{\textbullet Fe(OH)}\_{3} \rightarrow \text{Fe}\_{5}\text{O}\_{8}\text{H} \cdot 4\text{H}\_{2}\text{O} + \text{\textbullet H}\_{2}\text{O} \tag{6}$$

From Eqs. (1)–(6), Eq. (7) and Eq. (8) are obtained.

**3.2. Reaction mechanism**

168 Magnetic Spinels- Synthesis, Properties and Applications

during milling due to corrosion, as expressed by Eq. (1).

**Figure 7** shows the changes in the weight loss of balls and the iron concentration of the fluid with the milling time. With increasing milling time, the mass of the balls decreased and the iron concentrations increased, implying that ferrous (Fe2+) ions were released from steel balls

**Figure 7.** Change in the (a) weight loss of balls and (b) iron concentration with milling time at the initial stages.

before milling and then immediately neutral. In general, free electrons (e−

conditions, according to a cathode reaction expressed by Eq. (2).

**Figure 8.** Change in the pH with milling time at the initial stages.

**Figure 8** shows the variation in the pH of fluids with the milling time. The fluid was acidic

taneously with Fe2+ ion release are received by oxygen molecules (O2) in a solution under acidic

<sup>2</sup> Fe Fe 2e ® + + - (1)

) generated simul-

$$2\text{ Fe} + \text{O}\_2 + 2\text{H}\_2\text{O} \rightarrow 2\text{Fe(OH)}\_2\tag{7}$$

$$2\text{\textbulletFe(OH)}\_{2} + 5\text{O}\_{2} \rightarrow 4\text{Fe}\_{5}\text{O}\_{8}\text{H} \cdot 4\text{H}\_{2}\text{O} + 2\text{H}\_{2}\text{O} \tag{8}$$

Eqs. (7) and (8) include O2 as a reactant. Therefore, by analysing the dissolved O2 concentration, the validity of the proposed reaction mechanism was confirmed. The dissolved O2 concentration was estimated from the O2 partial pressure in the gas phase using Henry's law:

$$p = H\mathbf{x} \tag{9}$$

where *x* is the molar fraction of O2 in a liquid phase, *p* is the O2 partial pressure in a gas phase in equilibrium with the liquid phase, and *H* is the Henry constant (= 4.38 × 104 atm at 25°C [19]). In this process, the dissolved O2 concentration can always vary during milling. However, the liquid phase is well mixed with the gas phase by milling, resulting in a relatively large gasliquid interfacial area. Therefore, when the dissolved O2 is consumed, O2 can immediately be supplied from the gas phase. Accordingly, it can be assumed that the liquid and gas phases are always in equilibrium with each other. **Figure 9** shows the gas phase composition during milling. The open circles in this figure indicate the estimated values that were calculated using Eqs. (7) and (8) from the iron concentrations shown in **Figure 6**. It was confirmed that the volume of nitrogen (N2) gas was almost constant and that hydrogen (H2) gas evolution hardly occurred. In contrast, the O2 gas was completely consumed within 2 h. Furthermore, the experimental data of the O2 gas volume mostly agreed with the calculated values. The results demonstrate the validity of the reaction mechanism according to Eqs. (1)–(8).

**Figure 9.** Change of the gas volumes in the vessel with milling time at the initial stages.

As seen in **Figure 8**, after 2 h, O2 was absent from the vessel and the pH was almost constant at 7.8. **Figure 10** shows the change in the Fe2+ and Fe3+ ion concentrations with a milling time of up to 24 h. Fe3+ ions were reduced after 2 h. These results indicate that the free electrons formed at the steel corrosion are incorporated into Fe5O8H 4H2O, namely Fe3+ ions are reduced, according to Eq. (10) instead of Eq. (3).

$$\text{Fe}\_8\text{O}\_8\text{H}\cdot4\text{H}\_2\text{O} + \text{3H}\_2\text{O} + \text{5e}^- \rightarrow \text{5Fe}\text{(OH)}\_2 + \text{5OH}^-\tag{10}$$

Although this chemical reaction produces OH− ions, the OH− ions can be completely consumed to form Fe(OH)2 according to Eq. (4), which keeps the pH constant. In the absence of dissolved O2, the formed Fe(OH)2 can react with Fe5O8H 4H2O without being oxidized, resulting in the formation of Fe3O4 according to Eq. (11).

$$\text{2Fe}\_3\text{O}\_8\text{H} \cdot 4\text{H}\_2\text{O} + \text{5Fe(OH)}\_2 \rightarrow \text{5Fe}\_3\text{O}\_4 + \text{14H}\_2\text{O} \tag{11}$$

Consequently, the overall reaction under low O2 conditions can be described by Eq. (12).

$$\text{1.5Fe} + 8\text{Fe}\_5\text{O}\_8\text{H} \cdot 4\text{H}\_2\text{O} \rightarrow \text{1.5Fe}\_3\text{O}\_4 + \text{36H}\_2\text{O} \tag{12}$$

**Figure 10.** Change in iron concentrations with a milling time of up to 24 h.

In this process, the dissolved O2 concentration can always vary during milling. However, the liquid phase is well mixed with the gas phase by milling, resulting in a relatively large gasliquid interfacial area. Therefore, when the dissolved O2 is consumed, O2 can immediately be supplied from the gas phase. Accordingly, it can be assumed that the liquid and gas phases are always in equilibrium with each other. **Figure 9** shows the gas phase composition during milling. The open circles in this figure indicate the estimated values that were calculated using Eqs. (7) and (8) from the iron concentrations shown in **Figure 6**. It was confirmed that the volume of nitrogen (N2) gas was almost constant and that hydrogen (H2) gas evolution hardly occurred. In contrast, the O2 gas was completely consumed within 2 h. Furthermore, the experimental data of the O2 gas volume mostly agreed with the calculated values. The results

demonstrate the validity of the reaction mechanism according to Eqs. (1)–(8).

**Figure 9.** Change of the gas volumes in the vessel with milling time at the initial stages.

duced, according to Eq. (10) instead of Eq. (3).

170 Magnetic Spinels- Synthesis, Properties and Applications

Although this chemical reaction produces OH−

in the formation of Fe3O4 according to Eq. (11).

As seen in **Figure 8**, after 2 h, O2 was absent from the vessel and the pH was almost constant at 7.8. **Figure 10** shows the change in the Fe2+ and Fe3+ ion concentrations with a milling time of up to 24 h. Fe3+ ions were reduced after 2 h. These results indicate that the free electrons formed at the steel corrosion are incorporated into Fe5O8H 4H2O, namely Fe3+ ions are re-

to form Fe(OH)2 according to Eq. (4), which keeps the pH constant. In the absence of dissolved O2, the formed Fe(OH)2 can react with Fe5O8H 4H2O without being oxidized, resulting

Consequently, the overall reaction under low O2 conditions can be described by Eq. (12).

58 2 2 ( )<sup>2</sup> Fe O H 4H O 3H O 5e 5Fe OH 5OH - - × + +® + (10)

58 2 ( ) 34 2 <sup>2</sup> 2Fe O H 4H O 5Fe OH 5Fe O 14H O ×+ ® + (11)

ions can be completely consumed

ions, the OH−

**Figure 11** illustrates the evolution of H2 during milling. The formation of H2 was noticed for long milling times. As seen in **Figure 10**, the iron concentrations slightly increased even after 6 h. Thus, the following reduction reaction may occur, resulting in H2 evolution.

**Figure 11.** Evolution of H2 during milling.

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow 2\text{OH}^- + \text{H}\_2\tag{13}$$

OH− ions thus formed react with Fe2+ ions, resulting in the formation of Fe(OH)2. According to Eq. (4), Fe(OH)2 can transform to Fe3O4 according to Eq. (14).

$$\text{C}3\text{Fe}(\text{OH})\_2 \rightarrow \text{Fe}\_3\text{O}\_4 + 2\text{H}\_2\text{O} + \text{H}\_2\tag{14}$$

From Eqs. (1), (4), (13), and (14), Eq. (15) is derived as the overall reaction for the formation of Fe3O4:

$$\text{^2Be} + 4\text{H}\_2\text{O} \rightarrow \text{Fe}\_3\text{O}\_4 + 4\text{H}\_2\tag{15}$$

In summary, it has been established that a magnetic fluid consisting of well-dispersed superparamagnetic Fe3O4 nanoparticles can be formed by milling a CA solution with steel balls, according to the following reaction mechanism.


#### **3.3. Effect of the oxygen partial pressure**

**Figures 12**–**14** illustrate the XRD pattern of the obtained solid products, pH after milling, and internal pressure change of the vessel, respectively, under various initial total pressures in the vessel. Regardless of the internal total pressure, crystalline Fe3O4 was obtained, and the pH after milling was approximately 9. As seen in **Figure 14**, the internal pressure change was in agreement with the theoretical values (shown by a broken line) that had been calculated by assuming that O2 in the vessel was completely consumed during milling, suggesting that under an initial oxygen partial pressure of less than 1.26 atm, O2 was used in the Fe3O4 formation process.

From Eqs. (1), (3)–(6), and (11), the overall reaction for Fe3O4 formation under neutral and high O2 conditions is expressed by Eq. (16).

Mechanochemical Synthesis of Water-Based Magnetite Magnetic Fluids http://dx.doi.org/10.5772/66075 173

$$\text{2Fe} + \text{2O}\_2 \rightarrow \text{Fe}\_3\text{O}\_4\tag{16}$$

**Figure 12.** Effect of the initial total pressure on Fe3O4 phase evolution.

2 2 2H O 2e 2OH H - - +® + (13)

( ) 34 2 2 <sup>2</sup> 3Fe OH Fe O 2H O H ® ++ (14)

2 34 2 3Fe 4H O Fe O 4H +® + (15)

ions are produced from O2 and H2O and

ions and H2 are produced via reduction of

ions, and Fe3O4 and H2 are produced from Fe(OH)2.

ions thus formed react with Fe2+ ions, resulting in the formation of Fe(OH)2. According to

From Eqs. (1), (4), (13), and (14), Eq. (15) is derived as the overall reaction for the formation of

In summary, it has been established that a magnetic fluid consisting of well-dispersed superparamagnetic Fe3O4 nanoparticles can be formed by milling a CA solution with steel balls,

**1.** Under high O2 conditions, milling the solution can result in an immediate mass transfer of O2 from the gas phase to the liquid phase. Fe2+ ions are released from the steel balls. At

**2.** Under low O2 conditions, Fe5O8H 4H2O is reduced to Fe(OH)2, and the reaction between

**Figures 12**–**14** illustrate the XRD pattern of the obtained solid products, pH after milling, and internal pressure change of the vessel, respectively, under various initial total pressures in the vessel. Regardless of the internal total pressure, crystalline Fe3O4 was obtained, and the pH after milling was approximately 9. As seen in **Figure 14**, the internal pressure change was in agreement with the theoretical values (shown by a broken line) that had been calculated by assuming that O2 in the vessel was completely consumed during milling, suggesting that under an initial oxygen partial pressure of less than 1.26 atm, O2 was used in the Fe3O4 formation

From Eqs. (1), (3)–(6), and (11), the overall reaction for Fe3O4 formation under neutral and

then react with Fe2+ ions, resulting in the formation of Fe(OH)2. Fe(OH)2 is oxidized by

Eq. (4), Fe(OH)2 can transform to Fe3O4 according to Eq. (14).

172 Magnetic Spinels- Synthesis, Properties and Applications

according to the following reaction mechanism.

dissolved O2; then, Fe5O8H 4H2O forms.

Fe(OH)2 and Fe5O8H 4H2O produces Fe3O4.

**3.** After the consumption of Fe5O8H 4H2O, OH−

H2O. Fe(OH)2 forms from Fe2+ and OH−

**3.3. Effect of the oxygen partial pressure**

high O2 conditions is expressed by Eq. (16).

the same time, using formed free electrons, OH−

OH−

Fe3O4:

process.

**Figure 13.** Change in the pH after milling with the initial O2 partial pressure.

**Figure 15** shows the iron concentration of fluids estimated based on the internal pressure change, as shown in **Figure 14** with Eq. (16). The iron concentration increased with increasing initial O2 partial pressure, indicating that the iron concentration can be controlled by the initial O2 partial pressure.

**Figure 14.** Change in the pressure after milling with the initial O2 partial pressure.

**Figure 15.** Change in the estimated iron concentration with the initial O2 partial pressure.

#### **3.4. Kinetics of the reaction for the formation of Fe3O4**

Based on the concentrations of Fe2+ and Fe3+ ions during milling, the Fe3O4 formation reaction was kinetically analysed. **Figure 16** shows the change in the O2 partial pressure and iron concentrations with the milling time in the initial stages at an initial total pressure of 1 atm. A monotonous decrease in the O2 partial pressure with increased milling time was observed. Because the dissolved O2 concentration is proportional to the O2 partial pressure, the rate of reactions related to the dissolved O2 concentration, as shown by Eqs. (7) and (8), can also vary depending on the milling time. However, at less than 1.5 h, O2 was consumed at a constant rate, and the rates of Fe2+ and Fe3+ ion formation were almost constant regardless of the milling time, as seen in **Figure 7**b. The results imply that the reactions are independent of the dissolved O2 concentration. Therefore, the reaction rates of Eqs. (7) and (8) can be described by a zero-order model. This model is effective because O2 quickly dissolves into the solution due to vigorous gas-liquid mixing by milling and milling accelerates the corrosion of steel due to the improvement in the diffusion rate of O2 to the steel surface. Accordingly, the Fe3O4 formation process may be the oxidation-reduction reaction control, and both the dissolution of O2 from the gas phase to the liquid phase and diffusion rate of O2 in the liquid phase can be much faster than the rate of the oxidation-reduction reaction.

**Figure 16.** Changes in the O2 partial pressure with milling time at an initial total pressure of 1 atm.

Using the data shown in **Figure 16**, the rates of Fe2+ and Fe3+ ion formation were calculated. Based on Eqs. (7) and (8), the O2 consumption rate was determined to be approximately 0.40 µmol/s, which nearly agreed with the experimental data (0.41 µmol/s) shown in **Figure 16**. This result suggests that the consumed O2 was spent on the release and oxidation of Fe2+ ions.

Eqs. (7) and (8) are rewritten as follows:

**Figure 14.** Change in the pressure after milling with the initial O2 partial pressure.

174 Magnetic Spinels- Synthesis, Properties and Applications

**Figure 15.** Change in the estimated iron concentration with the initial O2 partial pressure.

Based on the concentrations of Fe2+ and Fe3+ ions during milling, the Fe3O4 formation reaction was kinetically analysed. **Figure 16** shows the change in the O2 partial pressure and iron concentrations with the milling time in the initial stages at an initial total pressure of 1 atm. A monotonous decrease in the O2 partial pressure with increased milling time was observed. Because the dissolved O2 concentration is proportional to the O2 partial pressure, the rate of reactions related to the dissolved O2 concentration, as shown by Eqs. (7) and (8), can also vary depending on the milling time. However, at less than 1.5 h, O2 was consumed at a constant rate, and the rates of Fe2+ and Fe3+ ion formation were almost constant regardless of the milling

**3.4. Kinetics of the reaction for the formation of Fe3O4**

$$\text{Fe} + \frac{1}{2}\text{O}\_2 + \text{H}\_2\text{O} \rightarrow \text{Fe(OH)}\_2\tag{17}$$

$$\text{Fe(OH)}\_{2} + \frac{1}{4}\text{O}\_{2} \rightarrow \frac{1}{5}\text{Fe}\_{5}\text{O}\_{8}\text{H} \cdot 4\text{H}\_{2}\text{O} + \frac{1}{10}\text{H}\_{2}\text{O} \tag{18}$$

Assuming that the reaction rates of Eqs. (17) and (18), *r*1 and *r*2, can be described by a zeroorder model and expressed as follows:

$$r\_1 = k\_1 \tag{19}$$

$$r\_2 = k\_2 \tag{20}$$

Here *k*1 and *k*2 are the rate constants of Eqs. (17) and (18), respectively. As a result, the rate of Fe(OH)2 formation reaction, *r*, can be expressed by Eq. (21).

$$r = r\_1 - r\_2 = k\_1 - k\_2 = K \tag{21}$$

*K* indicates the rate constant of the overall Fe(OH)2 formation reaction.

**Figure 17** shows the change in the O2 partial pressure with a milling time of up to 24 h. Regardless of the milling time, the rate of O2 consumption was constant under all of the initial total pressures. **Figure 18** illustrates the rate constant *K* calculated using Eqs. (19)–(21) as a function of the initial O2 partial pressure. *K* was found to decrease with the increasing initial O2 partial pressure. As shown in **Figure 19**, when the initial O2 partial pressure was relatively high, goethite (α-FeOOH) and Fe5O8H 4H2O formed as intermediates. In general, the rate of steel corrosion is affected by the mass fraction of iron oxyhydroxides and iron oxides in a corrosion product on the steel. In particular, when a high level of α-FeOOH is contained in the corrosion product, the corrosion rate can decrease because α-FeOOH prevents O2 from penetrating into the steel surface [20]. At high initial O2 partial pressures, Fe5O8H 4H2O can make a phase transition to α-FeOOH, and a dense corrosion product layer may form on the steel surface. This inhibits the mass transfer of O2, resulting in a decrease in the rate of Fe(OH)2 formation reaction.

**Figure 17.** Change in the O2 partial pressure with a milling time of up to 24 h.

Next, the effect of the vessel rotational speed on the O2 consumption was studied in atmospheric pressure. **Figure 20** depicts the change in the O2 partial pressure with the milling time. The O2 consumption rate was almost the same, regardless of the rotational speed. **Figure 21** illustrates the relationship between the rotational speed and *K* calculated assuming that the O2 consumption can be expressed by a zero-order equation. At rotational speeds higher than 10 rpm, *K* was almost constant, suggesting that the milling generated a sufficient level of energy required for the mass transfer and consumption of O2, even at low rotational speeds.

**Figure 18.** Change in the rate constant *K* with an initial O2 partial pressure.

1 1

Fe(OH)2 formation reaction, *r*, can be expressed by Eq. (21).

176 Magnetic Spinels- Synthesis, Properties and Applications

**Figure 17.** Change in the O2 partial pressure with a milling time of up to 24 h.

Fe(OH)2 formation reaction.

*K* indicates the rate constant of the overall Fe(OH)2 formation reaction.

Here *k*1 and *k*2 are the rate constants of Eqs. (17) and (18), respectively. As a result, the rate of

**Figure 17** shows the change in the O2 partial pressure with a milling time of up to 24 h. Regardless of the milling time, the rate of O2 consumption was constant under all of the initial total pressures. **Figure 18** illustrates the rate constant *K* calculated using Eqs. (19)–(21) as a function of the initial O2 partial pressure. *K* was found to decrease with the increasing initial O2 partial pressure. As shown in **Figure 19**, when the initial O2 partial pressure was relatively high, goethite (α-FeOOH) and Fe5O8H 4H2O formed as intermediates. In general, the rate of steel corrosion is affected by the mass fraction of iron oxyhydroxides and iron oxides in a corrosion product on the steel. In particular, when a high level of α-FeOOH is contained in the corrosion product, the corrosion rate can decrease because α-FeOOH prevents O2 from penetrating into the steel surface [20]. At high initial O2 partial pressures, Fe5O8H 4H2O can make a phase transition to α-FeOOH, and a dense corrosion product layer may form on the steel surface. This inhibits the mass transfer of O2, resulting in a decrease in the rate of

*r k* = (19)

2 2 *r k* = (20)

12 1 2 *rrr k k K* =- = - = (21)

**Figure 19.** XRD patterns of a solid product obtained at an initial total pressure of 3 atm.

**Figure 20.** Effect of the rotational speed on the O2 partial pressure after milling.

**Figure 21.** Effect of the vessel rotational speed on the rate constant *K*.

#### **3.5. Scaleup of the process**

Using a milling vessel with a capacity of 2.6 L, which was approximately 5 times as large as the small vessel used in the above investigations, the scaleup of the process was studied based on two cases, constant Froude number and constant peripheral velocity of the vessels. The rotational speed of 140 rpm for the small vessel corresponds to 108.4 rpm for the large vessel at a constant Froude number (= 0.0500) and 87.7 rpm at a constant peripheral velocity (= 0.735 m/s). **Figures 22**–**24** show the XRD pattern of solid products, average crystallite size, and pH after milling, respectively. The results demonstrate that the large scale process can fabricate a similar fluid, suggesting that scale-up of the process can be successful based on either the Froude number or peripheral velocity of the vessel.

Mechanochemical Synthesis of Water-Based Magnetite Magnetic Fluids http://dx.doi.org/10.5772/66075 179

**Figure 22.** XRD patterns of solid products obtained in both processes.

**Figure 20.** Effect of the rotational speed on the O2 partial pressure after milling.

178 Magnetic Spinels- Synthesis, Properties and Applications

**Figure 21.** Effect of the vessel rotational speed on the rate constant *K*.

either the Froude number or peripheral velocity of the vessel.

Using a milling vessel with a capacity of 2.6 L, which was approximately 5 times as large as the small vessel used in the above investigations, the scaleup of the process was studied based on two cases, constant Froude number and constant peripheral velocity of the vessels. The rotational speed of 140 rpm for the small vessel corresponds to 108.4 rpm for the large vessel at a constant Froude number (= 0.0500) and 87.7 rpm at a constant peripheral velocity (= 0.735 m/s). **Figures 22**–**24** show the XRD pattern of solid products, average crystallite size, and pH after milling, respectively. The results demonstrate that the large scale process can fabricate a similar fluid, suggesting that scale-up of the process can be successful based on

**3.5. Scaleup of the process**

**Figure 23.** Comparison of the average crystallite size of solid products obtained in both processes.

**Figure 24.** Comparison of the final pH of fluids obtained in both processes.

## **4. Conclusions**

This study analysed in detail a new mechanochemical process for readily synthesizing waterbased magnetic Fe3O4 fluids. Major conclusions are summarized as follows:


## **Acknowledgements**

This work was financially supported by JSPS KAKENHI Grant Number JP24686090. The author thanks Mr. Tsukasa Kagawa for his supports during the experiments.

## **Author details**

Tomohiro Iwasaki

Address all correspondence to: iwasaki@chemeng.osakafu-u.ac.jp

Osaka Prefecture University, Sakai, Japan

## **References**


[3] Lima-Tenório MK, Gómez Pineda EA, Ahmad NM, Fessi H, Elaissari E. Magnetic nanoparticles: In vivo cancer diagnosis and therapy. Int J Pharm. 205;493:313– 327.

**4. Conclusions**

vessel.

reaction mechanism.

180 Magnetic Spinels- Synthesis, Properties and Applications

**Acknowledgements**

**Author details**

Tomohiro Iwasaki

**References**

837.

1996;14:391–402.

peripheral velocity of the vessel.

Osaka Prefecture University, Sakai, Japan

This study analysed in detail a new mechanochemical process for readily synthesizing water-

**a.** The Fe3O4 formation mechanism in this process has been clarified, which can be described by several oxidation-reduction reactions, such as the corrosion of steel, oxidation of Fe(OH)2, and reduction of Fe5O8H 4H2O. Additionally, O2 plays an important role in the

**b.** The Fe3O4 concentration of fluids can be controlled by the initial O2 partial pressure in the

**c.** Kinetic analysis of the process clarified the effects of the dissolved O2 and the vessel rotational speed on the rate of the Fe(OH)2 formation reaction. In the process, the reaction

**d.** Scaleup of the process can be successful by considering either the Froude number or

This work was financially supported by JSPS KAKENHI Grant Number JP24686090. The

[1] Mitamura Y. Medical applications of magnetic fluids. J Jpn Soc Prec Eng. 2006;72:834–

[2] Tiefenauer LX, Tschirky A, Kühne G, Andres RY. In vivo evaluation of magnetite nanoparticles for use as a tumor contrast agent in MRI. Magn Reson Imaging.

based magnetic Fe3O4 fluids. Major conclusions are summarized as follows:

rate of O2 consumption can be expressed by a zero-order equation.

author thanks Mr. Tsukasa Kagawa for his supports during the experiments.

Address all correspondence to: iwasaki@chemeng.osakafu-u.ac.jp

