**3. Results**

The characteristics of synthesized ferrites are given in **Table 1** and **Figures 1**–**5**. It has been found that all synthesized ferrites, with the exception of the spray-drying method, are nanocrystalline stoichiometric single-phase powders (**Figure 1**) with a SSA in the wide range of 30–55 m<sup>2</sup> /g depending on the synthesis method and calculated (average) particle size of 20–40 nm (**Table 1**, **Figure 2**). The crystallite size of these ferrites is also in the range of 10–40 nm. During the spraydrying process high-dispersity nanoparticles, mainly consisting of cobalt or nickel ferrite, iron hydroxide FeO(OH), and X-ray amorphous part of the sample [38] were obtained. The SSA of these samples was in the range of 80–90 m<sup>2</sup> /g (**Table 1**), but the calculated average particle size was 13–15 nm [38]. In this process, pellets of up to 10 μm were obtained (**Figure 3**).

by the plasma synthesis is the most extensive (20–100 nm) with individual particles up to

The samples obtained at the optimal synthesis conditions were very clean because any other additional phase (usually magnetite, maghemite, hematite or other metal oxides) was not found by

200 nm. Plasma-derived particles are spherical.

**Figure 1.** XRD pattern of ferrite nanopowders.

**Sample SSA,** 

Average particle size calculated from SSA.

**Table 1.** Properties of synthesized ferrite nanopowders.

p.a.—partially amorphous; *Ms*

CoFe2 O4

CoFe2 O4

CoFe2 O4

CoFe2 O4

NiFe2 O4

NiFe2 O4

NiFe2 O4

NiFe2 O4

\*

**m2 /g** **d50, nm\***

(plasma) 29 39 40 CoFe2

(combust.) 37 31 20 CoFe2

(hydrotherm.) 54 21 10–12 CoFe2

(plasma) 29 38 40 NiFe2

(combust.) 43 26 10 NiFe2

(hydrotherm.) 42 26 22 NiFe2

(spray) 85 13 — p.a. NiFe2

—saturation magnetization; *Mr*

(spray) 84 14 — p.a. CoFe2

**Crystallite size,** 

**Phase composition** *Ms*

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained…

O4 ,

O4 ,

—remanent magnetization; *Hc*

FeO(OH)

FeO(OH)

**, emu/g**

O4 75.4 32.0 780

O4 53.4 20.3 1170

O4 50.1 12.6 390

O4 44.2 10.0 74

O4 21.4 2.3 81

O4 39.0 2.6 23

*Mr* **, emu/g**

http://dx.doi.org/10.5772/intechopen.76809

— — —

— — —

*—*coercivity

*Hc* **, Oe** 101

**nm**

The finer particles were obtained in the spray-drying process, hydrothermal and sol-gel selfpropagating combustion synthesis, but the distribution of the particle size of ferrites obtained The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained… http://dx.doi.org/10.5772/intechopen.76809 101


\* Average particle size calculated from SSA.

**B.** for spraying the hydroxide mixture with the spray-drying method, the pelleting machine was used developed by RTU Institute of Inorganic Chemistry. Main parameters of the

Technological equipment developed by the Institute of Inorganic Chemistry of the Riga Technical University [35] was used for the production of ferrites by means of high-frequency (HF) plasma chemical synthesis. Commercial metals and metal oxides (Ni, Co, NiO, CoO and FeO) powders were evaporated in HF plasma to obtain ferrites. All raw materials in stoichiometric ratios (to

5800–6200 K. After evaporation of the raw materials, the vapor was cooled very quickly with the cooling gas (air) and the product condensed on the filter in the form of nanosized ferrite particles. Ferrite nanopowders for sintering were prepared as follows: the ferrite nanopowder samples were mechanically mixed for 1 h in a planetary mill with 3% by weight of stearic acid

Stearic acid was used for better pressing. After mixing, the samples were dried in an oven at 80°C and sieved through a 200 μm sieve. For sintering without pressure samples were pressed (200 MPa) as tablets with a diameter of 12 mm and a height of 4–6 mm. Stearic acid was burned out at 600°C. Samples were sintered at 900–1300°C in an air atmosphere at a rate

All samples were analyzed using the X-ray diffractometer Advance 8 (Bruker AXS). The size of the crystallites was calculated using the Scherer's equation. The magnetic properties of the synthesized ferrites were analyzed using vibrating sample magnetometry (VSM Lake Shore Cryotronics, Inc., Model 7404 VSM). The SSA was measured using the BET single point method. The size and morphology of the particles as well as the microstructure of the sintered material were studied using transmission electron microscope JEM-100S (JEOL) and a scanning electron microscope Mira/Tescan and Tescan Lyra-3 on the fracture surfaces. The density and open porosity of the sintered samples were determined by the Archimedes method.

The characteristics of synthesized ferrites are given in **Table 1** and **Figures 1**–**5**. It has been found that all synthesized ferrites, with the exception of the spray-drying method, are nanocrystalline stoichiometric single-phase powders (**Figure 1**) with a SSA in the wide range of 30–55 m<sup>2</sup>

depending on the synthesis method and calculated (average) particle size of 20–40 nm (**Table 1**, **Figure 2**). The crystallite size of these ferrites is also in the range of 10–40 nm. During the spraydrying process high-dispersity nanoparticles, mainly consisting of cobalt or nickel ferrite, iron hydroxide FeO(OH), and X-ray amorphous part of the sample [38] were obtained. The SSA of

The finer particles were obtained in the spray-drying process, hydrothermal and sol-gel selfpropagating combustion synthesis, but the distribution of the particle size of ferrites obtained

was 13–15 nm [38]. In this process, pellets of up to 10 μm were obtained (**Figure 3**).

) were injected into nitrogen plasma at an average temperature of

ball material) using isopropanol as a dispersing medium.

/g (**Table 1**), but the calculated average particle size

/h, tempera-

/g

suspension spray: hot air temperature and consumption of 370°C and 24 m3

ture in evaporating chamber 120–130°C.

O4

container, ZrO2

these samples was in the range of 80–90 m<sup>2</sup>

of 10°C/min in an oven LHT-08/18 (Nabertherm GmbH) for 2 h.

obtain NiFe2

100 Powder Technology

(400 rpm, ZrO2

**3. Results**

O4

and CoFe2

p.a.—partially amorphous; *Ms* —saturation magnetization; *Mr* —remanent magnetization; *Hc —*coercivity

**Table 1.** Properties of synthesized ferrite nanopowders.

**Figure 1.** XRD pattern of ferrite nanopowders.

by the plasma synthesis is the most extensive (20–100 nm) with individual particles up to 200 nm. Plasma-derived particles are spherical.

The samples obtained at the optimal synthesis conditions were very clean because any other additional phase (usually magnetite, maghemite, hematite or other metal oxides) was not found by

**Figure 2.** The electron microscope image of CoFe2 O4 (A, C) and NiFe<sup>2</sup> O4 (B, D) obtained by the plasma synthesis (A), self-combustion (B), hydrothermal (C) and spray (D) methods.

**Figure 3.** The electron microscope (SEM) images of spray-dried NiFe<sup>2</sup> O4 at different enlargements.

the X-ray analysis. By analyzing samples of ferrites produced by different methods, slight differences in relative intensity and width of reflexes, indicating differences in crystallite size (**Figure 1**), can be seen in the X-ray images. The self-combustion and hydrothermal synthesis methods give nanopowders with a lower crystallite size than those obtained by plasma synthesis (**Table 1**).

The magnetic properties of the nanoparticles obtained by the plasma synthesis process (**Table 1**, **Figure 4**) are very close to those of the standard dense material (CoFe2 O4 magnetic saturation values are 80 emu/g and NiFe<sup>2</sup> O4 50 emu/g [40]). In contrast, the samples prepared by selfcombustion and hydrothermal method have different magnetic properties than those obtained by plasma synthesis. This is probably due to difference in the size of nanoparticles obtained by plasma, self-combustion and hydrothermal synthesis. The products obtained by the spray-drying method have magnetic properties only after heat treatment at 400–450°C at 550°C, the saturation magnetization of nickel ferrite is 16.9 emu/g, while for the cobalt ferrite is 51.3 emu/g.

Another interesting feature of nanoparticles synthesized in this study is their magnetic behavior, that is, although for all synthesized powders the particle size is below the critical size of a single domain (about 70 nm [41]), quasi-supermagnetic behavior is observed only for plasma-

**Figure 4.** Magnetic properties of ferrites synthesized by the spray-drying (1) at 450°C, sol-gel self-combustion (2) and

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained…

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103

O4

mentioned. Synthesizing at 200°C for 1 h, the product contains also FeO(OH) in addition to the basic phase (**Figure 5**). The product has weak magnetic properties (**Table 2**). Experiments

O4

hydrothermal synthesis can be

nanopowder prepared at: 1–200°C, 1 h; 2–230°C, 1 h;

synthesized NiFe2

3–250°C, 1 h.

O4

hydrothermal (3) method and in plasma (4).

nanoparticles.

**Figure 5.** XRD pattern of the hydrothermally synthesized CoFe<sup>2</sup>

As an example of the impact of synthesis parameters, CoFe<sup>2</sup>

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained… http://dx.doi.org/10.5772/intechopen.76809 103

**Figure 4.** Magnetic properties of ferrites synthesized by the spray-drying (1) at 450°C, sol-gel self-combustion (2) and hydrothermal (3) method and in plasma (4).

**Figure 2.** The electron microscope image of CoFe2

102 Powder Technology

self-combustion (B), hydrothermal (C) and spray (D) methods.

**Figure 3.** The electron microscope (SEM) images of spray-dried NiFe<sup>2</sup>

values are 80 emu/g and NiFe<sup>2</sup>

O4

(A, C) and NiFe<sup>2</sup>

O4

O4

the X-ray analysis. By analyzing samples of ferrites produced by different methods, slight differences in relative intensity and width of reflexes, indicating differences in crystallite size (**Figure 1**), can be seen in the X-ray images. The self-combustion and hydrothermal synthesis methods give nanopowders with a lower crystallite size than those obtained by plasma synthesis (**Table 1**).

The magnetic properties of the nanoparticles obtained by the plasma synthesis process (**Table 1**,

combustion and hydrothermal method have different magnetic properties than those obtained by plasma synthesis. This is probably due to difference in the size of nanoparticles obtained by plasma, self-combustion and hydrothermal synthesis. The products obtained by the spray-drying method have magnetic properties only after heat treatment at 400–450°C at 550°C, the saturation magnetization of nickel ferrite is 16.9 emu/g, while for the cobalt ferrite is 51.3 emu/g.

**Figure 4**) are very close to those of the standard dense material (CoFe2

O4

at different enlargements.

50 emu/g [40]). In contrast, the samples prepared by self-

O4

magnetic saturation

(B, D) obtained by the plasma synthesis (A),

**Figure 5.** XRD pattern of the hydrothermally synthesized CoFe<sup>2</sup> O4 nanopowder prepared at: 1–200°C, 1 h; 2–230°C, 1 h; 3–250°C, 1 h.

Another interesting feature of nanoparticles synthesized in this study is their magnetic behavior, that is, although for all synthesized powders the particle size is below the critical size of a single domain (about 70 nm [41]), quasi-supermagnetic behavior is observed only for plasmasynthesized NiFe2 O4 nanoparticles.

As an example of the impact of synthesis parameters, CoFe<sup>2</sup> O4 hydrothermal synthesis can be mentioned. Synthesizing at 200°C for 1 h, the product contains also FeO(OH) in addition to the basic phase (**Figure 5**). The product has weak magnetic properties (**Table 2**). Experiments


**Table 2.** Characteristics of CoFe2 O4 nanopowders prepared hydrothermally.

have shown that the optimum synthesis temperature, when the pure one-phase product is formed, is from 230°C. Increasing the processing temperatures (up to 250°C) and time (up to 3 h) does not significantly affect the size of the specific surface area and crystallite. Increasing of the synthesis temperature and hydrothermal treatment time results in a small increase in magnetic characteristics (saturation magnetization Ms , remanent magnetization Mr and coercivity H<sup>c</sup> ) (**Table 2**).

After thermal treatment at higher temperatures, ferrite nanopowders synthesized by selfcombustion, hydrothermal and spray-drying method, tend to decrease their SSA, but the particle size and crystallite size increase (**Figure 6**). This trend can be explained by the fact that the particles recrystallize and grow at higher temperatures, so the specific surface decreases. With the increase of crystallite size, the saturation magnetization and remanent magnetization of ferrites increase (**Tables 3** and **4**, **Figures 7** and **8**). For example, after thermal treatment of CoFe2 O4 obtained by self-combustion and hydrothermal method at 800°C and more, the saturation magnetization increases to 80 and 72 emu/g, respectively.

The spray-dried powder after the synthesis and granulation is partially amorphous and contains a small amount of FeO(OH). After heat treatment, starting from 400 to 450°C, a stoichio-

combust. Raw powder 53.4 20.3 1170

hydrotherm. Raw powder 50.0 10.2 495

spray Raw powder — — —

O4

increases, respectively, from 6 and 15 emu/g (at 450°C) to 40 and 77 emu/g (at 950°C) (**Tables 3**

The relative density of samples before sintering was of 51–52% for plasma synthesized products and of 31–33% for products obtained by other methods. This shows that the ferrite nanopowders obtained by these methods are more difficult to compress because their par-

Nanosized ferrite powders were sinteredat 900–1300°C. The density of ferrites after the heat

The sintering process of plasma synthesis products is the fastest compared with all investigated nanopowders: they have a high density at 900°C, but above 1000°C, the density is

to achieve high density. Although the sintering temperature of the ferrites obtained by the

O4

or CoFe2

) of the NiFe2

synthesized by the sol-gel self-combustion, hydrothermal and spray-drying

O4

**, emu/g Mr**

 55.0 21.7 1190 76.1 39.3 1350 79.9 35.7 930 79.8 31.3 980

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained…

400 50.1 12.6 390 600 62.8 22.4 760 800 71.6 28.9 875

350 — — — 51.3 14.7 649 61.1 22.3 878 76.8 34.1 1067

**, emu/g Hc**

http://dx.doi.org/10.5772/intechopen.76809

**, Oe**

105

, which increases with the increase of the pro-

O4

/g (at 950°C) (**Figure 6**). The crystallite size at 350°C is 4

ferrites synthesized by other methods have a relatively

ferrites require the temperature of 1200°C or higher

) (**Figure 9**) was formed,

O4

ferrites

and CoFe2

metric, single-phase nanocrystalline powder (NiFe2

methods after thermal treatment (2 h at different temperatures).

**Samples Heating temperature, °C Ms**

cessing temperature. The saturation magnetization (Ms

ticles are finer than ferrite powders synthesized in plasma.

O4

O4

O4

O4

and CoFe2

with SSA from 100 (at 350°C) to 20 m<sup>2</sup>

and 6 nm, respectively for NiFe<sup>2</sup>

**Table 3.** Magnetic properties of CoFe2

treatment is shown in **Table 5**.

approaching already 100%. CoFe2

high density at 1100°C, while NiFe2

and **4**, **Figure 8**).

CoFe2 O4

CoFe2 O4

CoFe2 O4

**Figure 6.** Specific surface area (SSA) and crystallite size comparison depending on temperature for NiFe<sup>2</sup> O4 and CoFe2 O4 synthesized by the sol-gel self-combustion (A), the hydrothermal (B) and spray-drying (C) method.

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained… http://dx.doi.org/10.5772/intechopen.76809 105


**Table 3.** Magnetic properties of CoFe2 O4 synthesized by the sol-gel self-combustion, hydrothermal and spray-drying methods after thermal treatment (2 h at different temperatures).

The spray-dried powder after the synthesis and granulation is partially amorphous and contains a small amount of FeO(OH). After heat treatment, starting from 400 to 450°C, a stoichiometric, single-phase nanocrystalline powder (NiFe2 O4 or CoFe2 O4 ) (**Figure 9**) was formed, with SSA from 100 (at 350°C) to 20 m<sup>2</sup> /g (at 950°C) (**Figure 6**). The crystallite size at 350°C is 4 and 6 nm, respectively for NiFe<sup>2</sup> O4 and CoFe2 O4 , which increases with the increase of the processing temperature. The saturation magnetization (Ms ) of the NiFe2 O4 and CoFe2 O4 ferrites increases, respectively, from 6 and 15 emu/g (at 450°C) to 40 and 77 emu/g (at 950°C) (**Tables 3** and **4**, **Figure 8**).

The relative density of samples before sintering was of 51–52% for plasma synthesized products and of 31–33% for products obtained by other methods. This shows that the ferrite nanopowders obtained by these methods are more difficult to compress because their particles are finer than ferrite powders synthesized in plasma.

Nanosized ferrite powders were sinteredat 900–1300°C. The density of ferrites after the heat treatment is shown in **Table 5**.

The sintering process of plasma synthesis products is the fastest compared with all investigated nanopowders: they have a high density at 900°C, but above 1000°C, the density is approaching already 100%. CoFe2 O4 ferrites synthesized by other methods have a relatively high density at 1100°C, while NiFe2 O4 ferrites require the temperature of 1200°C or higher to achieve high density. Although the sintering temperature of the ferrites obtained by the

**Figure 6.** Specific surface area (SSA) and crystallite size comparison depending on temperature for NiFe<sup>2</sup>

have shown that the optimum synthesis temperature, when the pure one-phase product is formed, is from 230°C. Increasing the processing temperatures (up to 250°C) and time (up to 3 h) does not significantly affect the size of the specific surface area and crystallite. Increasing of the synthesis temperature and hydrothermal treatment time results in a small increase in

nanopowders prepared hydrothermally.

After thermal treatment at higher temperatures, ferrite nanopowders synthesized by selfcombustion, hydrothermal and spray-drying method, tend to decrease their SSA, but the particle size and crystallite size increase (**Figure 6**). This trend can be explained by the fact that the particles recrystallize and grow at higher temperatures, so the specific surface decreases. With the increase of crystallite size, the saturation magnetization and remanent magnetization of ferrites increase (**Tables 3** and **4**, **Figures 7** and **8**). For example, after thermal treatment

obtained by self-combustion and hydrothermal method at 800°C and more, the

, remanent magnetization Mr

**XRD phases Magnetic properties Ms , emu/g**

O4, FeO(OH) 13.2 2.6 105

O4 — — —

O4 50.0 10.2 494

O4 58.9 17.8 643

O4 57.3 17.3 566

O4 59.8 16.8 574

**Mr**

**, emu/g Hc**

**, Oe**

magnetic characteristics (saturation magnetization Ms

civity H<sup>c</sup>

\*

Calculated from SSA.

**Table 2.** Characteristics of CoFe2

**No Mode, °C /h**

104 Powder Technology

**SSA, m2**

**/g d50, nm\* Crystallite size, nm**

1 200/1 58 20 ~10 CoFe2

2 200/3 59 19 13–14 CoFe2

3 230/1 63 18 10–13 CoFe2

4 230/3 55 21 15–16 CoFe2

5 250/1 61 19 12–13 CoFe2

6 250/3 62 18 10–12 CoFe2

O4

of CoFe2

O4

) (**Table 2**).

synthesized by the sol-gel self-combustion (A), the hydrothermal (B) and spray-drying (C) method.

saturation magnetization increases to 80 and 72 emu/g, respectively.

O4

and CoFe2

and coer-

O4


**Table 4.** Magnetic properties of NiFe2 O4 synthesized by the sol-gel self-combustion, hydrothermal and spray-drying methods after thermal treatment (2 h at different temperatures).

The crystallite size grows slightly during sintering: from 70 to 80 nm at 1100°C to 120–

ferrite nanopowders.

O4

O4

and NiFe2

O4

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained…

to 13 nm in the raw powder to 75 nm (1000°C) and 150 nm (sintered at 1200°C). The grain size of samples sintered at 1200°C, obtained from self-combustion, hydrothermal and spraydried powders, does not exceed 1 to 6 μm (**Figure 10**). As a result of high sintering activity,

Compared with the ferrite nanopowders, ceramic materials have a higher saturation magnetization (**Figure 11**) and lower coercivity. This could be explained by the increase in grain size and crystallite size. An increase in the temperature of the sintering results in the increase of the grain size and magnetization for all ferrite materials, while coercivity decreases (**Table 6**).

O4

after thermal treatment at 450 (1), 650 (2) and 950

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107

O4

varies from 10

and 10–30 μm

140 nm at 1300°C. For example, the crystallite size of hydrothermal CoFe<sup>2</sup>

and NiFe2

O4

the grain size of plasma-synthesized ferrite outweighs: 10–15 μm for NiFe<sup>2</sup>

for CoFe2

O4 .

**Figure 8.** The magnetic properties of samples of CoFe2

(3)°C prepared by the spray-drying method.

**Figure 9.** XRD pattern of spray-dried CoFe<sup>2</sup>

**Figure 7.** The magnetic properties of the sample CoFe2 O4 prepared by the hydrothermal synthesis (A) after thermal treatment at 400°C (1), 600°C (2) and 800°C (3), NiFe<sup>2</sup> O4 prepared by the self-combustion synthesis (B) after thermal treatment at 450°C (4), 650°C (5) and 850°C (6).

spray method is slightly higher, they could be the most promising on the technological point of view among all these nanopowders because they are flowing and can be pressed without further treatment.

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained… http://dx.doi.org/10.5772/intechopen.76809 107

**Figure 8.** The magnetic properties of samples of CoFe2 O4 and NiFe2 O4 after thermal treatment at 450 (1), 650 (2) and 950 (3)°C prepared by the spray-drying method.

**Figure 9.** XRD pattern of spray-dried CoFe<sup>2</sup> O4 and NiFe2 O4 ferrite nanopowders.

**Figure 7.** The magnetic properties of the sample CoFe2

**Samples Heating temperature, °C Ms**

O4

methods after thermal treatment (2 h at different temperatures).

combust. Raw powder 29.0 6.0 120

hydrotherm. Raw powder 37.4 2.6 23

spray Raw powder — — —

NiFe2 O4

106 Powder Technology

NiFe2 O4

NiFe2 O4

treatment at 400°C (1), 600°C (2) and 800°C (3), NiFe<sup>2</sup>

treatment at 450°C (4), 650°C (5) and 850°C (6).

**Table 4.** Magnetic properties of NiFe2

further treatment.

O4

spray method is slightly higher, they could be the most promising on the technological point of view among all these nanopowders because they are flowing and can be pressed without

O4

prepared by the hydrothermal synthesis (A) after thermal

prepared by the self-combustion synthesis (B) after thermal

**, emu/g Mr**

 31.4 4.8 130 37.4 9.1 200 45.2 14.8 145 47.4 15.0 135

400 36.7 3.8 34 600 40.2 5.2 55 800 42.6 5.0 70

350 — — — 16.9 1.1 57 21.6 4.5 214 40.0 8.6 151

synthesized by the sol-gel self-combustion, hydrothermal and spray-drying

**, emu/g Hc**

**, Oe**

The crystallite size grows slightly during sintering: from 70 to 80 nm at 1100°C to 120– 140 nm at 1300°C. For example, the crystallite size of hydrothermal CoFe<sup>2</sup> O4 varies from 10 to 13 nm in the raw powder to 75 nm (1000°C) and 150 nm (sintered at 1200°C). The grain size of samples sintered at 1200°C, obtained from self-combustion, hydrothermal and spraydried powders, does not exceed 1 to 6 μm (**Figure 10**). As a result of high sintering activity, the grain size of plasma-synthesized ferrite outweighs: 10–15 μm for NiFe<sup>2</sup> O4 and 10–30 μm for CoFe2 O4 .

Compared with the ferrite nanopowders, ceramic materials have a higher saturation magnetization (**Figure 11**) and lower coercivity. This could be explained by the increase in grain size and crystallite size. An increase in the temperature of the sintering results in the increase of the grain size and magnetization for all ferrite materials, while coercivity decreases (**Table 6**).


**Table 5.** The relative density and open porosity of ferrites depending on sintering temperature (after 2 h sintering).

The magnetic properties of the samples sintered at 1200°C are almost the same regardless

ceramics after 2 h sintering.

Single-phase nickel and cobalt ferrite nanopowders can be successfully synthesized by the chemical sol-gel self-propagating combustion and co-precipitation method combined with

O4 is of

of the method of extracting ferrite powders: saturation magnetization for CoFe2

O4 .

O4

80–84 emu/g and 46–48 emu/g for NiFe<sup>2</sup>

**Table 6.** Magnetic properties of CoFe2

**Figure 11.** Magnetic properties of CoFe2

**Heating temperature, °C CoFe2**

Self-combustion

Hydrothermal

Spray

Plasma

hydrothermal, 2—spray-drying, 3—combustion, 4—plasma.

**Ms**

O4

and NiFe2

O4

**, emu/g Mr**

(a) and NiFe2

O4

**O4 NiFe2**

**, emu/g Hc**

1200 82.6 6.9 190 — — —

1100 77.0 20.7 493 40.4 6.5 102 1200 81.3 14.1 169 46.0 1.7 15

1100 74.6 15.3 427 48.0 3.0 35 1300 73.8 5.9 187 47.0 2.4 11

1100 81.8 14.0 258 45.7 3.8 35 1200 83.6 8.0 110 46.3 0.7 11

(B) ferrite, sintered at 1200°C from different powders: 1—

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**O4**

**, emu/g Mr**

**, emu/g Hc**

**, Oe**

109

**, Oe Ms**

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained…

**4. Conclusions**

**Figure 10.** Typical SEM image of NiFe<sup>2</sup> O4 (a, D) and CoFe<sup>2</sup> O4 (B, C, E) ceramics sintered at 1200°C 2 h. The powders are prepared by hydrothermal (A, B), sol-gel self-propagating combustion (C), spray-drying (D) and plasma (E) methods.

The Synthesis and Characterization of Nickel and Cobalt Ferrite Nanopowders Obtained… http://dx.doi.org/10.5772/intechopen.76809 109

**Figure 11.** Magnetic properties of CoFe2 O4 (a) and NiFe2 O4 (B) ferrite, sintered at 1200°C from different powders: 1 hydrothermal, 2—spray-drying, 3—combustion, 4—plasma.


**Table 6.** Magnetic properties of CoFe2 O4 and NiFe2 O4 ceramics after 2 h sintering.

The magnetic properties of the samples sintered at 1200°C are almost the same regardless of the method of extracting ferrite powders: saturation magnetization for CoFe2 O4 is of 80–84 emu/g and 46–48 emu/g for NiFe<sup>2</sup> O4 .

#### **4. Conclusions**

**Figure 10.** Typical SEM image of NiFe<sup>2</sup>

**Sample Sintering temperature, °C**

CoFe2 O4

108 Powder Technology

CoFe2 O4

CoFe2 O4

CoFe2 O4

NiFe2 O4

NiFe2 O4

NiFe2 O4

NiFe2 O4

ρ—density; Pop—open porosity.

**900 1000 1100 1200 1300**

(plasma) 82.6 16.0 97.0 0.2 98.5 0.1 97.9 0 — —

(combust.) — — 65.7 33.4 78.3 21.6 93.4 3.1 — —

(hydrotherm.) — — 81.3 14.2 94.3 0.8 95.0 0.1 — —

(spray) — — 62.3 35.5 90.0 8.8 90.8 4.7 95.1 0.7

(plasma) 87.9 12.1 99.4 0.2 99.9 0.1 100.0 0 — —

(combust.) — — 72.4 25.5 87.7 9.4 96.1 1.6 — —

(hydrotherm.) — — — — 79.1 19.8 85.8 12.0 — —

(spray) — — 52.2 44.0 69.5 27.6 85.3 12.1 90.7 7.1

**Table 5.** The relative density and open porosity of ferrites depending on sintering temperature (after 2 h sintering).

**ρ, % Pop., % ρ, % Pop., % ρ, % Pop., % ρ, % Pop., % ρ, % Pop., %**

O4

(a, D) and CoFe<sup>2</sup>

O4

prepared by hydrothermal (A, B), sol-gel self-propagating combustion (C), spray-drying (D) and plasma (E) methods.

(B, C, E) ceramics sintered at 1200°C 2 h. The powders are

Single-phase nickel and cobalt ferrite nanopowders can be successfully synthesized by the chemical sol-gel self-propagating combustion and co-precipitation method combined with hydrothermal synthesis or spray-drying method as well as high-frequency plasma synthesis. The magnetic properties of synthesized ferrite powders depend on their synthesis method.

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Fe2 O4

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Comparing the methods for obtaining ferrite nanopowders described earlier, we can say that plasma synthesis currently is the most productive method resulting in the highest magnetic properties (75 emu/g for CoFe2 O4 and 44 emu/g for NiFe2 O4 ). The disadvantage of this method is the presence of particles exceeding the size of 100 nm in a product that is not acceptable in all applications.

The chemical sol-gel self-propagating combustion and hydrothermal synthesis methods enables the production of smaller particles (SSA. = 35–55 m<sup>2</sup> /g; average particle size 20–30 nm) with less explicited magnetic properties (50–55 emu/g for CoFe<sup>2</sup> O4 and 20–40 emu/g for NiFe2 O4 ) after synthesis, which can be increased after heat treatment at temperatures up to 800°C. The lack of these methods is a time-consuming process of filtering nanoparticles.

The filtration process can be bypassed by the spray-drying method. Here, the smallest particles of the powder (SSA = 80–90 m<sup>2</sup> /g, average particle size 10–15 nm) are obtained, but due to the low processing temperatures, they have no explicited magnetic properties. Magnetic properties are observed after additional treatment starting at 400–450°C. However, the granular product is well suited for automated pressing processes for production of ceramic materials.

Sintered materials have higher magnetic properties than nanopowders. Magnetic properties of samples sintered at 1200°C are almost the same regardless of the method of obtaining ferrite powders: the saturation magnetization of CoFe2 O4 is 80–84 emu/g and 46–48 emu/g for NiFe<sup>2</sup> O4 .
