**Abstract**

Ferrite-based nanoparticles, namely, bismuth ferrite (BiFeO3) and calcium ferrite (CaFe4O7), have been synthesized via sol-gel and chemically dissolved method, respectively, employing hematite (α-Fe2O3) as the Fe3+ ion source. Firstly, α-Fe2O3 nanoparticles were prepared from natural iron sand containing mostly magnetite (Fe3O4) phase through coprecipitation technique continued by sintering process at 800°C for 2 h. Higher BiFeO3 phase content was achieved after Bi-Fe gel being annealed at 650°C for 1 h in air atmosphere. Furthermore, major phase of CaFe4O7 was formed with molar ratio of Fe3+/Ca2+ = 6 and sintering temperature of 800°C for 3 h. Interestingly, the powders with dominant CaFe4O7 phase, known as calcium biferrite, exhibit higher ferromagnetism at room temperature. The magnetic properties of the calcium biferrite are comparable to those of barium hexaferrite which can be applied for radar-absorbing material. Meanwhile, BiFeO3 powders also show weak room temperature ferromagnetism. It has also demonstrated that Ni doping in the bismuth ferrite (BiFe1−*x*Ni*x*O3 with *x* = 0.1) nanoparticles results in enhancement of the magnetic properties. Moreover, a ferroelectric hysteresis loop and a trend of frequency dependence of the dielectric constant have been observed, which were enhanced by Pb doping (Bi1−*y*Pb*y*FeO3 with *y* = 0.1). These results suggest a multiferroic behavior in the BiFeO3 nanoparticles.

**Keywords:** bismuth ferrite, calcium ferrite, iron sand, multiferroic, nanoparticles, precipitation, sol-gel

#### **1. Introduction**

Development of functional nanomaterials for scientific and industrial applications is very crucial for advanced technologies. The use of natural resources as the starting compounds for producing nanomaterials is currently developing. Many researchers are exploring natural materials and even waste biomass applied as a functional material that has a high selling value for various specific applications. For example, the use of silica sand from Tanah Laut, Kalimantan, Indonesia, as a raw material for manufacturing pure SiO2, zircon, and zirconia with high phase purity and crystallite size in nanometer range was reported [1]. Moreover, natural iron sand exploration as a starting material has been shown to produce magnetite (Fe3O4) nanoparticles as magnetic coating, magnetic fluid (ferrofluid), and magnetic gel (ferrogel) for radar-absorbing materials, biomedical applications, and tissue engineering, respectively [2–5].

Fe3O4 is one of the magnetic particles that can be obtained from natural iron sand after conducting the separation technique from its impurities by mechanical and chemical processes. In nature, iron sand consists of more than 90 wt% of Fe3O4 particles. Generally, Fe3O4 has been synthesized using commercial raw materials, such as FeCl2.4H2O and FeCl3.6H2O [6]. The commonly used synthesis methods are sol-gel, hydrothermal, and coprecipitation techniques [7–9]. Because Fe3O4 nanoparticles tend to agglomerate among particles, the addition of surfactants or templates has been widely applied to produce homogeneous nanoparticles with certain sizes and morphologies [10–14]. Research on preparing Fe3O4 nanoparticles from iron sand has been the main topic for the past few years. The use of doping, for example, doping Mn and Zn, on Fe3O4 makes it superparamagnetic so that it can be applied in biomedicine applications [15–18].

Hematite (α-Fe2O3) is the most stable iron oxides at high temperatures. α-Fe2O3 is commonly obtained from iron rust which is one of the dominant corrosion products of iron metal or iron alloys. In general, α-Fe2O3 nanoparticles have been successfully prepared by several methods, namely, hydrothermal [19] and coprecipitation technique [20], using commercial raw materials, such as Fe(NO3)3·9H2O and FeCl3.6H2O, respectively. It is found that the concentration of Fe3+ ions used in the preparation of α-Fe2O3 nanoparticles may influence the particle size and morphology, as well as the optical bandgap [20]. α-Fe2O3 nanoparticles with particle size of 8 nm possess superparamagnetic properties with relatively high magnetization at room temperature [21]. Therefore, it is possible to be applied for biomedical and spintronic applications. Moreover, Liu et al. have successfully prepared porous Fe2O3 nanorods with particle size of ~10 nm and pore sizes in the range of 5–50 nm. These porous Fe2O3 nanorods exhibit excellent photocatalytic properties [22].

In the field of environmental engineering, α-Fe2O3 nanoparticles can be synthesized from hydrated ferric chloride and ferrous sulfate salt solution through chemical coprecipitation method and calcination process at relatively high temperature of 500°C [23]. In addition, a simple chemically coprecipitation method has been employed to obtain Fe3O4 nanoparticles using HCl and NH4OH as dissolving and precipitating agent, respectively [3]. Some researchers have investigated the transformation from Fe3O4 to α-Fe2O3 phase through oxidation process of Fe2+ to Fe3+ ions [24]. It is noted that Fe3O4 nanoparticles could be transformed into maghemite (γ-Fe2O3) and hematite (α-Fe2O3) via dry oxidation process at temperature range between 350 and 400°C and 600 and 800°C, respectively [25]. Focusing on the use of natural resources as raw materials for synthesizing functional materials, in this chapter, α-Fe2O3 nanoparticles were synthesized from natural iron sand through chemical coprecipitation method followed by sintering process at temperature of 800°C. Then, the obtained α-Fe2O3 nanoparticles were utilized as one of the raw materials for preparing calcium ferrite (Ca-ferrites) and bismuth ferrite (BiFeO3) nanoparticles as potential materials for radar-absorbing and data storage materials, respectively. The physical characterizations for all obtained ferrite-based nanoparticles include elemental and phase identification, particle morphology, and magnetic and electrical properties.

Based on the phase diagram of CaO-Fe2O3 system [26, 27], it is known that there are three main phases of calcium ferrite compounds and those are 2CaO.Fe2O3 (Ca2Fe2O5), CaO.Fe2O3 (CaFe2O4), and CaO.2Fe2O3 (CaFe4O7). It is possible that the reaction between CaO and Fe2O3 results in other unstable calcium ferrite phases, such as CaFe12O19. In addition, Boyanov [28] has pointed out that the mixture of

**35**

limestone.

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source*

and Fe3+ ions as the precursors and also the atmospheric condition [29].

CaCO3-Fe2O3 after thermal treatment has produced various types of calcium ferrite compounds consisting of ~50% CaO.2Fe2O3, ~20% CaO.Fe2O3, ~8% 2CaO.Fe2O3, and other ferrite products. The formation of calcium ferrite compounds depends on the kinetics of chemical reaction at the boundary between the phases and oxide diffusion during the reaction affected by the concentration ratio of the existing Ca2+

Calcium ferrite compounds exhibit soft ferromagnetism, and, therefore, it can be used for radar-absorbing materials in the calcium ferrite/graphite nanocomposites [30]. In this case, calcium ferrite nanoparticles have magnetic properties that are comparable to barium ferrite (BaO.6Fe2O3) and strontium ferrite (SrO.6Fe2O3) known as M-type hexaferrite for microwave-absorbing applications. In order to be used for this application and also for biomedical applications as targeted drug delivery, calcium ferrite should exhibit superparamagnetic behavior [31]. Compared with the other ferrites, such as MFe2O4 (M = Zn, Mn, Ni, and Cu), CaFe2O4 is one of the biocompatible materials and environmentally friendly due to the use of calcium rather than heavy metals. Moreover, Ca2Fe2O5 with the brownmillerite structure has a specific application as p-type thermoelectric device [32]. This is due to the fact that this compound has interesting electrical properties [33, 34]. Oxygen deficiencies in the Ca2Fe2O5 crystals may enhance the electrochemical activity [35]. On the other hand, CaFe4O7 has not been explored yet regarding its magnetic properties. In contrast to the other calcium ferrites, in this chapter, CaFe4O7 nanoparticles were prepared by mixing Fe2O3 from natural iron sand and CaCO3 from natural

Bismuth ferrite (BiFeO3) is one of multiferroic system showing a magnetic-electric coupling at room temperature. Multiferroic material has perovskite structure with chemical formula ABO3. The type of A and B sites, the cation nonstoichiometry, and the presence of oxygen vacancies may have an impact on the structural, electronic, and magnetic properties [36]. BiFeO3 crystallizes in a distorted rhombohedral perovskite with space group R3c [37]. It has high Curie temperature and Néel temperature of 1100 and 640 K, respectively [38]. It is difficult to obtain a pure phase of BiFeO3 because the kinetics of phase formation leads to the formation of secondary phases, such as Bi25FeO40 (sillenite) and Bi2Fe4O9 (mullite). Various techniques have been reported to prepare single phase of BiFeO3, and those are chemical coprecipitation [39], hydrothermal [40], and sol-gel methods [41–43]. The ideas of those techniques are to achieve a single phase of BiFeO3 with a simple route, low temperature, and cost-effectiveness. Wang et al. have found that the formation of BiFeO3 phase starts at 425°C with impurity phases about 30% by the low-heating temperature solid-state precursor method [44, 45]. Further calcination from 450 to 550°C results in a pure BiFeO3 phase without any impurity phases. However, impurity phase of Bi2Fe4O9 has been detected in the powder calcined at above 650°C. Moreover, BiFeO3 nanoparticles synthesized by microwave-assisted sol-gel method at calcination temperature of 450°C exhibit a pure phase of BiFeO3 structure with particle size of 40 nm and no detected secondary phase [46].

Magnetic and dielectric properties of BiFeO3 nanoparticles are determined by the introduction of doping and particle size influenced by the synthesis method, temperature, and duration of calcination. It has been found that all magnetic parameters, such as saturation magnetization, enhance with decreasing particle size [43]. BiFeO3 nanoparticles with the size below 100 nm have weak ferromagnetism at room temperature. This ferromagnetic behavior in the nanoparticles is due to the presence of oxygen vacancies in BiFeO3 system [41, 47]. Enhancement of magnetic as well as dielectric properties in BiFeO3 can be achieved by adding doping of Mn, Ni, Pb, Ti, Sr, and Zn [48–56]. Up to the present, there have been various studies examining the doping effects of BiFeO3 nanoparticles with numerous advanced

*DOI: http://dx.doi.org/10.5772/intechopen.88027*

CaCO3-Fe2O3 after thermal treatment has produced various types of calcium ferrite compounds consisting of ~50% CaO.2Fe2O3, ~20% CaO.Fe2O3, ~8% 2CaO.Fe2O3, and other ferrite products. The formation of calcium ferrite compounds depends on the kinetics of chemical reaction at the boundary between the phases and oxide diffusion during the reaction affected by the concentration ratio of the existing Ca2+ and Fe3+ ions as the precursors and also the atmospheric condition [29].

Calcium ferrite compounds exhibit soft ferromagnetism, and, therefore, it can be used for radar-absorbing materials in the calcium ferrite/graphite nanocomposites [30]. In this case, calcium ferrite nanoparticles have magnetic properties that are comparable to barium ferrite (BaO.6Fe2O3) and strontium ferrite (SrO.6Fe2O3) known as M-type hexaferrite for microwave-absorbing applications. In order to be used for this application and also for biomedical applications as targeted drug delivery, calcium ferrite should exhibit superparamagnetic behavior [31]. Compared with the other ferrites, such as MFe2O4 (M = Zn, Mn, Ni, and Cu), CaFe2O4 is one of the biocompatible materials and environmentally friendly due to the use of calcium rather than heavy metals. Moreover, Ca2Fe2O5 with the brownmillerite structure has a specific application as p-type thermoelectric device [32]. This is due to the fact that this compound has interesting electrical properties [33, 34]. Oxygen deficiencies in the Ca2Fe2O5 crystals may enhance the electrochemical activity [35]. On the other hand, CaFe4O7 has not been explored yet regarding its magnetic properties. In contrast to the other calcium ferrites, in this chapter, CaFe4O7 nanoparticles were prepared by mixing Fe2O3 from natural iron sand and CaCO3 from natural limestone.

Bismuth ferrite (BiFeO3) is one of multiferroic system showing a magnetic-electric coupling at room temperature. Multiferroic material has perovskite structure with chemical formula ABO3. The type of A and B sites, the cation nonstoichiometry, and the presence of oxygen vacancies may have an impact on the structural, electronic, and magnetic properties [36]. BiFeO3 crystallizes in a distorted rhombohedral perovskite with space group R3c [37]. It has high Curie temperature and Néel temperature of 1100 and 640 K, respectively [38]. It is difficult to obtain a pure phase of BiFeO3 because the kinetics of phase formation leads to the formation of secondary phases, such as Bi25FeO40 (sillenite) and Bi2Fe4O9 (mullite). Various techniques have been reported to prepare single phase of BiFeO3, and those are chemical coprecipitation [39], hydrothermal [40], and sol-gel methods [41–43]. The ideas of those techniques are to achieve a single phase of BiFeO3 with a simple route, low temperature, and cost-effectiveness. Wang et al. have found that the formation of BiFeO3 phase starts at 425°C with impurity phases about 30% by the low-heating temperature solid-state precursor method [44, 45]. Further calcination from 450 to 550°C results in a pure BiFeO3 phase without any impurity phases. However, impurity phase of Bi2Fe4O9 has been detected in the powder calcined at above 650°C. Moreover, BiFeO3 nanoparticles synthesized by microwave-assisted sol-gel method at calcination temperature of 450°C exhibit a pure phase of BiFeO3 structure with particle size of 40 nm and no detected secondary phase [46].

Magnetic and dielectric properties of BiFeO3 nanoparticles are determined by the introduction of doping and particle size influenced by the synthesis method, temperature, and duration of calcination. It has been found that all magnetic parameters, such as saturation magnetization, enhance with decreasing particle size [43]. BiFeO3 nanoparticles with the size below 100 nm have weak ferromagnetism at room temperature. This ferromagnetic behavior in the nanoparticles is due to the presence of oxygen vacancies in BiFeO3 system [41, 47]. Enhancement of magnetic as well as dielectric properties in BiFeO3 can be achieved by adding doping of Mn, Ni, Pb, Ti, Sr, and Zn [48–56]. Up to the present, there have been various studies examining the doping effects of BiFeO3 nanoparticles with numerous advanced

*Nanocrystalline Materials*

tissue engineering, respectively [2–5].

applied in biomedicine applications [15–18].

iron sand exploration as a starting material has been shown to produce magnetite (Fe3O4) nanoparticles as magnetic coating, magnetic fluid (ferrofluid), and magnetic gel (ferrogel) for radar-absorbing materials, biomedical applications, and

Fe3O4 is one of the magnetic particles that can be obtained from natural iron sand after conducting the separation technique from its impurities by mechanical and chemical processes. In nature, iron sand consists of more than 90 wt% of Fe3O4 particles. Generally, Fe3O4 has been synthesized using commercial raw materials, such as FeCl2.4H2O and FeCl3.6H2O [6]. The commonly used synthesis methods are sol-gel, hydrothermal, and coprecipitation techniques [7–9]. Because Fe3O4 nanoparticles tend to agglomerate among particles, the addition of surfactants or templates has been widely applied to produce homogeneous nanoparticles with certain sizes and morphologies [10–14]. Research on preparing Fe3O4 nanoparticles from iron sand has been the main topic for the past few years. The use of doping, for example, doping Mn and Zn, on Fe3O4 makes it superparamagnetic so that it can be

Hematite (α-Fe2O3) is the most stable iron oxides at high temperatures. α-Fe2O3 is commonly obtained from iron rust which is one of the dominant corrosion products of iron metal or iron alloys. In general, α-Fe2O3 nanoparticles have been successfully prepared by several methods, namely, hydrothermal [19] and coprecipitation technique [20], using commercial raw materials, such as Fe(NO3)3·9H2O and FeCl3.6H2O, respectively. It is found that the concentration of Fe3+ ions used in the preparation of α-Fe2O3 nanoparticles may influence the particle size and morphology, as well as the optical bandgap [20]. α-Fe2O3 nanoparticles with particle size of 8 nm possess superparamagnetic properties with relatively high magnetization at room temperature [21]. Therefore, it is possible to be applied for biomedical and spintronic applications. Moreover, Liu et al. have successfully prepared porous Fe2O3 nanorods with particle size of ~10 nm and pore sizes in the range of 5–50 nm. These porous Fe2O3 nanorods exhibit excellent photocatalytic properties [22].

In the field of environmental engineering, α-Fe2O3 nanoparticles can be synthesized from hydrated ferric chloride and ferrous sulfate salt solution through chemical coprecipitation method and calcination process at relatively high temperature of 500°C [23]. In addition, a simple chemically coprecipitation method has been employed to obtain Fe3O4 nanoparticles using HCl and NH4OH as dissolving and precipitating agent, respectively [3]. Some researchers have investigated the transformation from Fe3O4 to α-Fe2O3 phase through oxidation process of Fe2+ to Fe3+ ions [24]. It is noted that Fe3O4 nanoparticles could be transformed into maghemite (γ-Fe2O3) and hematite (α-Fe2O3) via dry oxidation process at temperature range between 350 and 400°C and 600 and 800°C, respectively [25]. Focusing on the use of natural resources as raw materials for synthesizing functional materials, in this chapter, α-Fe2O3 nanoparticles were synthesized from natural iron sand through chemical coprecipitation method followed by sintering process at temperature of 800°C. Then, the obtained α-Fe2O3 nanoparticles were utilized as one of the raw materials for preparing calcium ferrite (Ca-ferrites) and bismuth ferrite (BiFeO3) nanoparticles as potential materials for radar-absorbing and data storage materials, respectively. The physical characterizations for all obtained ferrite-based nanoparticles include elemental and phase identification, particle morphology, and

Based on the phase diagram of CaO-Fe2O3 system [26, 27], it is known that there

are three main phases of calcium ferrite compounds and those are 2CaO.Fe2O3 (Ca2Fe2O5), CaO.Fe2O3 (CaFe2O4), and CaO.2Fe2O3 (CaFe4O7). It is possible that the reaction between CaO and Fe2O3 results in other unstable calcium ferrite phases, such as CaFe12O19. In addition, Boyanov [28] has pointed out that the mixture of

**34**

magnetic and electrical properties.

techniques to improve their performance. In the case of the enhancing magnetization induced by doping, it has been suggested that this is probably due to increasing distortion of local structure, increasing the effect of Dzyaloshinskii-Moriya (DM) interaction, distortion of Fe and O bonding, destruction of spin cycloid structure, and the presence of impurity phase in the BiFeO3 systems [53, 57]. Besides affecting the magnetic properties, introduction of doping in BiFeO3 leads to the improvement of dielectric and ferroelectric properties [50, 58, 59]. Yuan et al. [54] have found that a sufficient amount of Sr/Pb doping can improve the magnetic properties as well as high-frequency dielectric properties.

In addition, the dielectric properties of pure BiFeO3 phase strongly depend on the atmospheric condition during the powder synthesis. Liu et al. [60] have found a higher spontaneous polarization and lower breakdown field based on polarizationelectrical field (P-E) hysteresis loops in the samples annealed in H2 and N2 atmospheres. In this chapter, BiFeO3 nanoparticles were synthesized by sol-gel method using natural iron sand as one of the raw materials and calcined in air atmosphere. Then, the ferroelectric and the dielectric properties were intensively investigated in the Pb- and Ni-doped BiFeO3 nanoparticles.
