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Synthesis and magnetic properties of nanostructured spinel ferrites using a glycine–nitrate process
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Journal of Magnetism and Magnetic Materials 284 (2004) 206–214 www.elsevier.com/locate/jmmm
Synthesis and magnetic properties of nanostructured spinel ferrites using a glycine–nitrate process Nobuyuki Kikukawa, Makoto Takemori, Yoshinobu Nagano, Masami Sugasawa, Satoru Kobayashi National Institute of Advanced Industrial Science and Technology (AIST), Research Institute for Green Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan Received 17 February 2004; received in revised form 18 June 2004 Available online 23 July 2004
Abstract To prepare zinc-substituted spinel-type ferrite fine particles of M1xZnxFe2O4 (M=Mg, Mn, Co, Ni, Cu, (Li, Fe) x=0–1) with good crystallinity and stoichiometry, the authors investigated a glycine–nitrate process. The product powder was an agglomerate of fine particles whose typical diameter was several tens of nanometers. X-ray diffraction patterns revealed that the produced particles were mono-phase in almost all reaction systems. Energy-dispersive X-ray spectroscopy microanalysis of the product particles (Mn–Zn–Fe–O) revealed that the distributions of Mn/Fe ratio and Zn/Fe ratio were highly sharp both within the agglomerate and between agglomerates. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.y; 75.50.Gg; 81.20.Ka Keywords: Combustion synthesis; Fine particles; Magnetism; Spinel ferrite; Crystallinity; Stoichiometry
1. Introduction Magnetic fine particles have attracted much interest in many fields [1–5]. One application of magnetic materials utilizes their heating effects, i.e. magnetic hysteresis energy losses. For example, there are a considerable number of works on Corresponding author. Tel: +81-298618490; fax: +81-
298618459. E-mail address: [email protected] (N. Kikukawa).
hyperthermia [6,7], which is one of the promising approaches in cancer therapy. In this application, the magnetic materials used are fine particles with coercive forces suitable for use with applied alternating magnetic fields. We are planning to apply the heating effects of magnetic materials to a new environmental application, which will be described in the near future. In this study, we attempted to prepare zincsubstituted spinel-type ferrite fine particles with a coercive force and Curie point that will work with
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.06.039
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the new application. Such particles would have to be several tens of nanometers in diameter, and their zinc contents must be controlled. These characteristics are necessary because superparamagnetic particles (typically smaller than 10 nm) do not show magnetic hysteresis around room temperature. In general, the coercive forces of magnetic particles have their maximum when the diameter is between 20 and 50 nm [8]. Moreover, since zinc content strongly affects the Curie point in spinel-type ferrite, the stoichiometry in each fine particle is important. Various wet-methods [9–19] have been reported for preparing spinel-type ferrite fine particles, such as coprecipitation methods [9,10], hydrothermal synthesis [11,12], microemulsion synthesis [13–15], and sol–gel methods [16]. In addition, various drymethods [20–25] including grinding [21], mechanical alloying [22], and thermal plasma methods [23,24] have been employed. Beside these methods, there have been several attempts using auto-ignited combustion reactions [26–29]. Among them, a glycine–nitrate process (GNP) [29–31] is also applicable, which is a very simple, low-cost, fast method for preparing well-crystallized double oxides in about 30 min from preparing the precursor solution to obtaining final products. The production speed of the combustion synthesis seems to be as high as that of microwave-hydrothermal synthesis [12]. However, the apparatus needed for the former is far less expensive than that of the latter. Since, there have been only a few studies on the preparation of spinel-type ferrites by GNP [29], we investigated the synthesis and the properties of various zinc-substituted spinel-type ferrites (M1xZnxFe2O4, M=Mg, Mn, Co, Ni, Cu, (Li, Fe) x=0–1) using this method. In the GNP, the amino acid, glycine, prevents selective precipitation from the precursor solution, and it serves as fuel for the combustion reaction, being oxidized by the nitrate ions [30e]. The main controllable processing variable is the glycine-to-nitrate ion (G/N) ratio, which affects the flame temperature, the combustion velocity, and the product morphology and composition.
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The peak flame temperature is said to be typically obtained at a G/N ratio that corresponds to complete combustion, producing H2O, CO2, and N2 as the waste gases, with no atmospheric oxygen required [30e]. The G/N ratio that corresponds to complete combustion is called the stoichiometric ratio [30e]. For the spinel-type ferrite synthesis from tri-valent iron nitrate (Fe(NO3)3) and divalent metal nitrate (M(NO3)2), the stoichiometric G/N molar ratio is 59; as shown in the following equation: 18FeðNO3 Þ3 þ 9MðNO3 Þ2 þ 40NH2 CH2 COOH ! 9MFe2 O4 þ 56N2 þ 80CO2 þ 100H2 O The optimum powder product is not always obtained at the stoichiometric G/N ratio [30e]. In this study, we changed the G/N ratio from 0.17 to 0.83 and examined the effects of altering the G/N ratio on the morphology, crystal structure, and magnetic properties. 2. Experimental In an appropriate ratio, analytical-grade nitrates Fe(NO3)3 9H2O, Zn(NO3)2 6H2O, Mg(NO3)2 6H2O, Mn(NO3)2 6H2O, Co(NO3)2 6H2O, Ni(NO3)2 6H2O, Cu(NO3)2 3H2O, LiNO3 and glycine (H2NCH2COOH) were dissolved in distilled water to obtain the precursor solution. The total concentration of the metal ions was approximately 1 mol/l. About 50 ml of the precursor solution was put into a two-liter stainless-steel beaker on a hot plate within a draft chamber. The beaker was covered with a stainless-steel mesh screen. The hot plate was set on ‘‘high,’’ and the solution was simply allowed to boil until it ignited. Within several tens seconds, the combustion reaction produced brownto-black, porous products that were easily ground to powder. The product powder was characterized by an X-ray diffractometer (XRD, Rigaku RU-300), a transmission electron microscope (TEM, Philips CM-30) equipped with an energy-dispersive X-ray spectrometer (EDX), and a vibrating sample magnetometer (VSM, Riken Denshi BHV-55).
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3. Results and discussion 3.1. Morphology and uniformity of composition
BET specific area [m2/g] and crystallite size [nm]
The typical TEM images of the product powder of Mn0.5Zn0.5 Fe2O4 are shown in Figs. 1(a) (low magnification) and (b) (high magnification). The G/N ratio of the precursor was 0.44. From the figures, one can see that the product powder consists of agglomerates of fine primary particles. The size distribution of primary particles in Fig. 1b showed lognormal distribution and the average particle size was 51 nm. However, it should be noted that, under current experimental conditions, there were some variations in the crystallinity of primary particles; sometimes highly grown crystal-
lites were observed and sometimes amorphous-like particles were also observed. Nevertheless, Figs. 1a and b are the most frequently observed makeup of the products. The crystallite size deduced from XRD peak width is considered to show the average figure. As shown in Fig. 2, the crystallite size and the BET specific surface area (SBET) of the powder produced from the precursor of Co:Fe=1:2 were dependent on the G/N ratio. The crystallite size was the maximum and SBET was the minimum at a G/N ratio of 59; which corresponds to the stoichiometric ratio. The crystallite sizes show fairly good agreement with the TEM diameters. However, when we calculated the average diameter from SBET with the assumption of perfect dispersion of primary particles, the average diameters were several times larger than the corresponding crystallite sizes, which implies that the primary particles were somewhat necked together during the GNP reaction. Another example of a TEM image is shown in Fig. 3. From the figure, it seems that the agglomerate was generated through the process where the primary particles gathered and necked to each other, rather than through crystallization from liquid melt. Since the zinc contents of the zinc-substituted spinel-type ferrite strongly affect the Curie point, the stoichiometry of each particle is important. Therefore, we measured the Mn/Fe and Zn/Fe 50 40 30
BET Crystallite size
20 10 0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
G/N ratio [-]
Fig. 1. Typical TEM images of product powder of Mn0.5Zn0.5Fe2O4. (a) low magnification, (b) high magnification. Run No. 16, G/N ratio: 0.44.
Fig. 2. Dependency of the BET specific surface areas and the crystallite sizes of the powder produced from the precursor of Co:Fe=1:2 upon glycine/nitrate ratio. circle: BET specific surface area, square: crystallite size deduced from the peak width of the XRD pattern.
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Fig. 3. TEM image of the powder produced from the precursor of Mn:Zn:Fe=1:1:4 with a G/N ratio of 0.67 (run no. 51).
ratios of individual particles corresponding to Fig. 1 by EDX microanalysis. Fig. 4. shows the results of the distributions between agglomerates (a) and between primary particles within an agglomerate (b). The results of that of coprecipitated powder are shown in (c) for comparison. As the standard deviations are small enough compared with those of coprecipitated powder, we can conclude that the GNP yielded stoichiometric primary particles and agglomerates. 3.2. Effects of the G/N ratio on crystal structure All precursors with various G/N ratios examined were found to auto-ignite. The yields were almost 100%. A higher G/N ratio often resulted in a mixed phase with ferrite and a small portion of low-valent phases such as FeO and/or metals or alloys (in the case of Ni, Cu, Co and their zincsubstituted systems), and a lower G/N ratio resulted in a higher oxidized phase of Mn3O4 (in the case of Mn and Mn–Zn systems with G/N ratio less than 0.3). Generally, the precursors with a G/N ratio less than about 0.5 generated monophase spinel-type ferrites. The XRD patterns of some examples are shown in Fig. 5. In the case of copper, the products were always a mixture of cubic type spinel phase and various Cu-containing phases such as CuFeO2, Cu2O, CuO, and Cu metal, as shown in Fig. 6a. We
Fig. 4. Histograms of Mn/Fe and Zn/Fe ratios (a) between agglomerates, (b) within agglomerates, and (c) of co-precipitated particles calcined at 873 K, measured through EDX. The initial metal composition ratio was Mn:Zn:Fe=1:1:4.
added ammonium nitrate, which acts as both fuel and oxidant and is said to raise the flame temperature [30], to the precursor solution. Although a small fraction of CuO remained, the product became nearly the mono-phase of cuprospinel, as shown in Fig. 6b, when the molar ratio of ammonium nitrate to nitrate ion and G/N ratio were 0.5 and 0.33, respectively. 3.3. Effects of the G/N ratio on magnetic properties Figs. 7a and b show the dependency of magnetic properties on the G/N ratio in the cases of
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Fig. 5. Examples of the XRD patterns of synthesized M1xZnxFe2O4 (M=Mg, Mn, Co, Ni, x=0, 0.3, 0.5) particles. The marks indicate peaks of spinel-type ferrites.
Co–Fe–O, Mn–Zn–Fe–O and Cu–Zn–Fe–O. The saturation magnetizations (Fig. 7a) show their maximum at around a G/N ratio of 0.4–0.5, just below the stoichiometric ratio of 59: The initial increase of the saturation magnetization can be attributed to the improved crystallinity, because the crystallite size gradually increases with the G/ N ratio (see Fig. 2). When the G/N ratio is greater than about 0.5, as the product powder becomes the mixture of magnetic spinel phase and other phases such as FeO and alloys, the saturation magnetization tends to decrease gradually other than in the cases of Co–Fe–O, in which coexisting cubic Co phase is ferromagnetic.
Fig. 6. XRD patterns in the case of the Cu–Zn–Fe–O system. (a) The product powder with a different G/N ratio. (b) The product powder synthesized with ammonium nitrate.
On the other hand, the coercive forces (Fig. 7b) show rather complicated dependency on the G/N ratio. The dependency of the crystallite size of spinel-type ferrite phases on the G/N ratio (Fig. 2) is considered to affect mainly. The low coercive forces at the lower G/N ratio should be attributed to superparamagnetism. CoFe2O4 is known to have very small superparamagnetic critical size of 14 nm [32] and the first two points in Fig. 7b were less than the critical size (see Fig. 2). Since another ferrites have larger critical sizes [32], the coercive
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forces of Mn0.5Zn0.5Fe2O4 (Fig. 7b), and others such as MgFe2O4 (not shown) gradually increased with the G/N ratio. However, Cu–Zn–Fe–O system showed more complicated dependency. As mentioned above, the GNP products of
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Cu–Zn–Fe–O system always consisted of mixed Cu-containing crystal phases and we do not have any quantitative phase composition data yet. Although, we think the coexisting magnetic and/ or nonmagnetic phases may affect this dependency, the detail of the Cu–Zn–Fe–O system is a future subject to be solved. Table 1 summarizes the magnetic properties of produced MFe2O4 (M=Mg, Mn, Ni, Co, Cu, (Li, Fe)) with a G/N ratio of around 0.4. The literature values [33] are also included in the table for comparison. Since the Curie points and the lattice constants show fairly good agreement, we believe the purities of magnetic crystal phase are high. Regarding saturation magnetization, GNP powder had slightly lower values than the literature ones, probably because the crystallites have not fully grown during the GNP. Coercive force will be important in our planned application using heating effect of magnetic materials. Fig. 8a shows the magnetic hysteresis curves in a magnetic field of 100 Oe of Mn–Zn ferrite synthesized by the GNP (run no.G31; solid line) and by the conventional ceramic method (commercial powder; broken line). We preliminarily investigated the heating behaviors of these two Mn–Zn ferrite powders under a 56-kHz AC magnetic field of 100 Oe. Fig. 8b shows a preliminary result. The GNP-synthesized powder showed a far steeper rise than that of the
Fig. 7. Dependency of magnetic properties on G/N ratio at the magnetic field of 5 kOe: (a) saturation magnetization, (b) coercive force. Circle: Co–Fe–O (Co:Fe=1:2), square: Mn–Zn–Fe–O (Mn:Zn:Fe=1:1:4), triangle: Cu–Zn–Fe–O (Cu:Zn:Fe=0.7:0.3:2).
Table 1 Magnetic properties of synthesized spinel-type ferrite particles Run No.
LiFe5O8 MgFe2O4 MnFe2O4 CoFe2O4 NiFe2O4 CuFe2O4 ZnFe2O4
G66 G34 G24 G53 G08 G25 G49
Experimental results
Literature
Lattice constant (nm)
Saturation magnetization (emu/g)
Coercive force (Oe)
Curie point Lattice constant (K) (nm)a
Saturation magnetization (emu/g)b
Curie point (K)b
0.8339 0.8391 0.8499 0.8386 0.8349 0.8382 0.8435
57 32 72 79 42 49 7.3
155 72 89 885 183 149 19
903 696 625 797 868 749 —
69 31 112 94 56 30 —
903 713 573 793 858 728 —
0.8333 0.83873 0.8499 0.83919 0.8339 0.8369 0.84411
(17-114) (36-398) (10-319) (22-1086) (10-325) (25-283) (22-1012)
Saturation magnetization and coercive force were measured at the magnetic field of 15 kOe and at room temperature. a Powder diffraction file (PDF). b Ref. [33].
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glycine–nitrate process (GNP) and drew the following conclusions: (1) We successfully synthesized mono-phase spinel-type ferrites when the G/N ratio was less than about 0.5 for the systems including Mg, Mg–Zn, Co, Co–Zn, Ni, Ni–Zn, (Li, Fe), and (Li, Fe)–Zn, and the ratio between 0.3 and 0.5 for Mn, and Mn–Zn systems. (2) TEM observation showed that the product powder consisted of agglomerates of primary particles with a typical diameter was about 50 nm for a G/N ratioof around 0.4. (3) EDX microanalysis of the product particles revealed that the distributions of the elemental ratios were very sharp both within an agglomerate and between agglomerates.
Fig. 8. Comparison of synthesized Mn–Zn ferrite powder with a commercial powder: (a) magnetic hysteresis curves in a magnetic field of 100 Oe at room temperature, (b) preliminary measurements of temperature vs. time curves under an AC magnetic field (56 kHz, 100 Oe). Solid line: synthesized Mn0.5Zn0.5Fe2O4 powder (run no. G31, Curie point: 501 K). Broken line: commercial Mn–Zn ferrite powder (Curie point: 523 K).
commercial powder, which reflects the larger hysteresis-loop area (see Fig. 8a). We are now conducting a study on the heating behavior of various GNP-synthesized ferrites under an extensive AC magnetic field
4. Conclusion In order to prepare zinc-substituted spinel-type ferrite fine particles of M1xZnxFe2O4 (M=Mg, Mn, Co, Ni, Cu, (Li, Fe) x=0–1) with good crystallinity and stoichiometry, we investigated a
The GNP was found to be a rapid, low-cost method for synthesizing spinel-type ferrites. Although the nanometer-sized primary crystallites were always somewhat necked together, the fact will not affect our planning application using heating effects of magnetic hysteresis energy losses, since the coercive force does not depend on the agglomerate size but on the crystallite size. Therefore, we conclude that the GNP is one of the suitable methods for high-speed preparation of various spinel-type ferrites.
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Journal of Magnetism and Magnetic Materials 284 (2004) 206–214 www.elsevier.com/locate/jmmm
Synthesis and magnetic properties of nanostructured spinel ferrites using a glycine–nitrate process Nobuyuki Kikukawa, Makoto Takemori, Yoshinobu Nagano, Masami Sugasawa, Satoru Kobayashi National Institute of Advanced Industrial Science and Technology (AIST), Research Institute for Green Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan Received 17 February 2004; received in revised form 18 June 2004 Available online 23 July 2004
Abstract To prepare zinc-substituted spinel-type ferrite fine particles of M1xZnxFe2O4 (M=Mg, Mn, Co, Ni, Cu, (Li, Fe) x=0–1) with good crystallinity and stoichiometry, the authors investigated a glycine–nitrate process. The product powder was an agglomerate of fine particles whose typical diameter was several tens of nanometers. X-ray diffraction patterns revealed that the produced particles were mono-phase in almost all reaction systems. Energy-dispersive X-ray spectroscopy microanalysis of the product particles (Mn–Zn–Fe–O) revealed that the distributions of Mn/Fe ratio and Zn/Fe ratio were highly sharp both within the agglomerate and between agglomerates. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.y; 75.50.Gg; 81.20.Ka Keywords: Combustion synthesis; Fine particles; Magnetism; Spinel ferrite; Crystallinity; Stoichiometry
1. Introduction Magnetic fine particles have attracted much interest in many fields [1–5]. One application of magnetic materials utilizes their heating effects, i.e. magnetic hysteresis energy losses. For example, there are a considerable number of works on Corresponding author. Tel: +81-298618490; fax: +81-
298618459. E-mail address: [email protected] (N. Kikukawa).
hyperthermia [6,7], which is one of the promising approaches in cancer therapy. In this application, the magnetic materials used are fine particles with coercive forces suitable for use with applied alternating magnetic fields. We are planning to apply the heating effects of magnetic materials to a new environmental application, which will be described in the near future. In this study, we attempted to prepare zincsubstituted spinel-type ferrite fine particles with a coercive force and Curie point that will work with
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.06.039
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the new application. Such particles would have to be several tens of nanometers in diameter, and their zinc contents must be controlled. These characteristics are necessary because superparamagnetic particles (typically smaller than 10 nm) do not show magnetic hysteresis around room temperature. In general, the coercive forces of magnetic particles have their maximum when the diameter is between 20 and 50 nm [8]. Moreover, since zinc content strongly affects the Curie point in spinel-type ferrite, the stoichiometry in each fine particle is important. Various wet-methods [9–19] have been reported for preparing spinel-type ferrite fine particles, such as coprecipitation methods [9,10], hydrothermal synthesis [11,12], microemulsion synthesis [13–15], and sol–gel methods [16]. In addition, various drymethods [20–25] including grinding [21], mechanical alloying [22], and thermal plasma methods [23,24] have been employed. Beside these methods, there have been several attempts using auto-ignited combustion reactions [26–29]. Among them, a glycine–nitrate process (GNP) [29–31] is also applicable, which is a very simple, low-cost, fast method for preparing well-crystallized double oxides in about 30 min from preparing the precursor solution to obtaining final products. The production speed of the combustion synthesis seems to be as high as that of microwave-hydrothermal synthesis [12]. However, the apparatus needed for the former is far less expensive than that of the latter. Since, there have been only a few studies on the preparation of spinel-type ferrites by GNP [29], we investigated the synthesis and the properties of various zinc-substituted spinel-type ferrites (M1xZnxFe2O4, M=Mg, Mn, Co, Ni, Cu, (Li, Fe) x=0–1) using this method. In the GNP, the amino acid, glycine, prevents selective precipitation from the precursor solution, and it serves as fuel for the combustion reaction, being oxidized by the nitrate ions [30e]. The main controllable processing variable is the glycine-to-nitrate ion (G/N) ratio, which affects the flame temperature, the combustion velocity, and the product morphology and composition.
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The peak flame temperature is said to be typically obtained at a G/N ratio that corresponds to complete combustion, producing H2O, CO2, and N2 as the waste gases, with no atmospheric oxygen required [30e]. The G/N ratio that corresponds to complete combustion is called the stoichiometric ratio [30e]. For the spinel-type ferrite synthesis from tri-valent iron nitrate (Fe(NO3)3) and divalent metal nitrate (M(NO3)2), the stoichiometric G/N molar ratio is 59; as shown in the following equation: 18FeðNO3 Þ3 þ 9MðNO3 Þ2 þ 40NH2 CH2 COOH ! 9MFe2 O4 þ 56N2 þ 80CO2 þ 100H2 O The optimum powder product is not always obtained at the stoichiometric G/N ratio [30e]. In this study, we changed the G/N ratio from 0.17 to 0.83 and examined the effects of altering the G/N ratio on the morphology, crystal structure, and magnetic properties. 2. Experimental In an appropriate ratio, analytical-grade nitrates Fe(NO3)3 9H2O, Zn(NO3)2 6H2O, Mg(NO3)2 6H2O, Mn(NO3)2 6H2O, Co(NO3)2 6H2O, Ni(NO3)2 6H2O, Cu(NO3)2 3H2O, LiNO3 and glycine (H2NCH2COOH) were dissolved in distilled water to obtain the precursor solution. The total concentration of the metal ions was approximately 1 mol/l. About 50 ml of the precursor solution was put into a two-liter stainless-steel beaker on a hot plate within a draft chamber. The beaker was covered with a stainless-steel mesh screen. The hot plate was set on ‘‘high,’’ and the solution was simply allowed to boil until it ignited. Within several tens seconds, the combustion reaction produced brownto-black, porous products that were easily ground to powder. The product powder was characterized by an X-ray diffractometer (XRD, Rigaku RU-300), a transmission electron microscope (TEM, Philips CM-30) equipped with an energy-dispersive X-ray spectrometer (EDX), and a vibrating sample magnetometer (VSM, Riken Denshi BHV-55).
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3. Results and discussion 3.1. Morphology and uniformity of composition
BET specific area [m2/g] and crystallite size [nm]
The typical TEM images of the product powder of Mn0.5Zn0.5 Fe2O4 are shown in Figs. 1(a) (low magnification) and (b) (high magnification). The G/N ratio of the precursor was 0.44. From the figures, one can see that the product powder consists of agglomerates of fine primary particles. The size distribution of primary particles in Fig. 1b showed lognormal distribution and the average particle size was 51 nm. However, it should be noted that, under current experimental conditions, there were some variations in the crystallinity of primary particles; sometimes highly grown crystal-
lites were observed and sometimes amorphous-like particles were also observed. Nevertheless, Figs. 1a and b are the most frequently observed makeup of the products. The crystallite size deduced from XRD peak width is considered to show the average figure. As shown in Fig. 2, the crystallite size and the BET specific surface area (SBET) of the powder produced from the precursor of Co:Fe=1:2 were dependent on the G/N ratio. The crystallite size was the maximum and SBET was the minimum at a G/N ratio of 59; which corresponds to the stoichiometric ratio. The crystallite sizes show fairly good agreement with the TEM diameters. However, when we calculated the average diameter from SBET with the assumption of perfect dispersion of primary particles, the average diameters were several times larger than the corresponding crystallite sizes, which implies that the primary particles were somewhat necked together during the GNP reaction. Another example of a TEM image is shown in Fig. 3. From the figure, it seems that the agglomerate was generated through the process where the primary particles gathered and necked to each other, rather than through crystallization from liquid melt. Since the zinc contents of the zinc-substituted spinel-type ferrite strongly affect the Curie point, the stoichiometry of each particle is important. Therefore, we measured the Mn/Fe and Zn/Fe 50 40 30
BET Crystallite size
20 10 0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
G/N ratio [-]
Fig. 1. Typical TEM images of product powder of Mn0.5Zn0.5Fe2O4. (a) low magnification, (b) high magnification. Run No. 16, G/N ratio: 0.44.
Fig. 2. Dependency of the BET specific surface areas and the crystallite sizes of the powder produced from the precursor of Co:Fe=1:2 upon glycine/nitrate ratio. circle: BET specific surface area, square: crystallite size deduced from the peak width of the XRD pattern.
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Fig. 3. TEM image of the powder produced from the precursor of Mn:Zn:Fe=1:1:4 with a G/N ratio of 0.67 (run no. 51).
ratios of individual particles corresponding to Fig. 1 by EDX microanalysis. Fig. 4. shows the results of the distributions between agglomerates (a) and between primary particles within an agglomerate (b). The results of that of coprecipitated powder are shown in (c) for comparison. As the standard deviations are small enough compared with those of coprecipitated powder, we can conclude that the GNP yielded stoichiometric primary particles and agglomerates. 3.2. Effects of the G/N ratio on crystal structure All precursors with various G/N ratios examined were found to auto-ignite. The yields were almost 100%. A higher G/N ratio often resulted in a mixed phase with ferrite and a small portion of low-valent phases such as FeO and/or metals or alloys (in the case of Ni, Cu, Co and their zincsubstituted systems), and a lower G/N ratio resulted in a higher oxidized phase of Mn3O4 (in the case of Mn and Mn–Zn systems with G/N ratio less than 0.3). Generally, the precursors with a G/N ratio less than about 0.5 generated monophase spinel-type ferrites. The XRD patterns of some examples are shown in Fig. 5. In the case of copper, the products were always a mixture of cubic type spinel phase and various Cu-containing phases such as CuFeO2, Cu2O, CuO, and Cu metal, as shown in Fig. 6a. We
Fig. 4. Histograms of Mn/Fe and Zn/Fe ratios (a) between agglomerates, (b) within agglomerates, and (c) of co-precipitated particles calcined at 873 K, measured through EDX. The initial metal composition ratio was Mn:Zn:Fe=1:1:4.
added ammonium nitrate, which acts as both fuel and oxidant and is said to raise the flame temperature [30], to the precursor solution. Although a small fraction of CuO remained, the product became nearly the mono-phase of cuprospinel, as shown in Fig. 6b, when the molar ratio of ammonium nitrate to nitrate ion and G/N ratio were 0.5 and 0.33, respectively. 3.3. Effects of the G/N ratio on magnetic properties Figs. 7a and b show the dependency of magnetic properties on the G/N ratio in the cases of
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Fig. 5. Examples of the XRD patterns of synthesized M1xZnxFe2O4 (M=Mg, Mn, Co, Ni, x=0, 0.3, 0.5) particles. The marks indicate peaks of spinel-type ferrites.
Co–Fe–O, Mn–Zn–Fe–O and Cu–Zn–Fe–O. The saturation magnetizations (Fig. 7a) show their maximum at around a G/N ratio of 0.4–0.5, just below the stoichiometric ratio of 59: The initial increase of the saturation magnetization can be attributed to the improved crystallinity, because the crystallite size gradually increases with the G/ N ratio (see Fig. 2). When the G/N ratio is greater than about 0.5, as the product powder becomes the mixture of magnetic spinel phase and other phases such as FeO and alloys, the saturation magnetization tends to decrease gradually other than in the cases of Co–Fe–O, in which coexisting cubic Co phase is ferromagnetic.
Fig. 6. XRD patterns in the case of the Cu–Zn–Fe–O system. (a) The product powder with a different G/N ratio. (b) The product powder synthesized with ammonium nitrate.
On the other hand, the coercive forces (Fig. 7b) show rather complicated dependency on the G/N ratio. The dependency of the crystallite size of spinel-type ferrite phases on the G/N ratio (Fig. 2) is considered to affect mainly. The low coercive forces at the lower G/N ratio should be attributed to superparamagnetism. CoFe2O4 is known to have very small superparamagnetic critical size of 14 nm [32] and the first two points in Fig. 7b were less than the critical size (see Fig. 2). Since another ferrites have larger critical sizes [32], the coercive
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forces of Mn0.5Zn0.5Fe2O4 (Fig. 7b), and others such as MgFe2O4 (not shown) gradually increased with the G/N ratio. However, Cu–Zn–Fe–O system showed more complicated dependency. As mentioned above, the GNP products of
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Cu–Zn–Fe–O system always consisted of mixed Cu-containing crystal phases and we do not have any quantitative phase composition data yet. Although, we think the coexisting magnetic and/ or nonmagnetic phases may affect this dependency, the detail of the Cu–Zn–Fe–O system is a future subject to be solved. Table 1 summarizes the magnetic properties of produced MFe2O4 (M=Mg, Mn, Ni, Co, Cu, (Li, Fe)) with a G/N ratio of around 0.4. The literature values [33] are also included in the table for comparison. Since the Curie points and the lattice constants show fairly good agreement, we believe the purities of magnetic crystal phase are high. Regarding saturation magnetization, GNP powder had slightly lower values than the literature ones, probably because the crystallites have not fully grown during the GNP. Coercive force will be important in our planned application using heating effect of magnetic materials. Fig. 8a shows the magnetic hysteresis curves in a magnetic field of 100 Oe of Mn–Zn ferrite synthesized by the GNP (run no.G31; solid line) and by the conventional ceramic method (commercial powder; broken line). We preliminarily investigated the heating behaviors of these two Mn–Zn ferrite powders under a 56-kHz AC magnetic field of 100 Oe. Fig. 8b shows a preliminary result. The GNP-synthesized powder showed a far steeper rise than that of the
Fig. 7. Dependency of magnetic properties on G/N ratio at the magnetic field of 5 kOe: (a) saturation magnetization, (b) coercive force. Circle: Co–Fe–O (Co:Fe=1:2), square: Mn–Zn–Fe–O (Mn:Zn:Fe=1:1:4), triangle: Cu–Zn–Fe–O (Cu:Zn:Fe=0.7:0.3:2).
Table 1 Magnetic properties of synthesized spinel-type ferrite particles Run No.
LiFe5O8 MgFe2O4 MnFe2O4 CoFe2O4 NiFe2O4 CuFe2O4 ZnFe2O4
G66 G34 G24 G53 G08 G25 G49
Experimental results
Literature
Lattice constant (nm)
Saturation magnetization (emu/g)
Coercive force (Oe)
Curie point Lattice constant (K) (nm)a
Saturation magnetization (emu/g)b
Curie point (K)b
0.8339 0.8391 0.8499 0.8386 0.8349 0.8382 0.8435
57 32 72 79 42 49 7.3
155 72 89 885 183 149 19
903 696 625 797 868 749 —
69 31 112 94 56 30 —
903 713 573 793 858 728 —
0.8333 0.83873 0.8499 0.83919 0.8339 0.8369 0.84411
(17-114) (36-398) (10-319) (22-1086) (10-325) (25-283) (22-1012)
Saturation magnetization and coercive force were measured at the magnetic field of 15 kOe and at room temperature. a Powder diffraction file (PDF). b Ref. [33].
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glycine–nitrate process (GNP) and drew the following conclusions: (1) We successfully synthesized mono-phase spinel-type ferrites when the G/N ratio was less than about 0.5 for the systems including Mg, Mg–Zn, Co, Co–Zn, Ni, Ni–Zn, (Li, Fe), and (Li, Fe)–Zn, and the ratio between 0.3 and 0.5 for Mn, and Mn–Zn systems. (2) TEM observation showed that the product powder consisted of agglomerates of primary particles with a typical diameter was about 50 nm for a G/N ratioof around 0.4. (3) EDX microanalysis of the product particles revealed that the distributions of the elemental ratios were very sharp both within an agglomerate and between agglomerates.
Fig. 8. Comparison of synthesized Mn–Zn ferrite powder with a commercial powder: (a) magnetic hysteresis curves in a magnetic field of 100 Oe at room temperature, (b) preliminary measurements of temperature vs. time curves under an AC magnetic field (56 kHz, 100 Oe). Solid line: synthesized Mn0.5Zn0.5Fe2O4 powder (run no. G31, Curie point: 501 K). Broken line: commercial Mn–Zn ferrite powder (Curie point: 523 K).
commercial powder, which reflects the larger hysteresis-loop area (see Fig. 8a). We are now conducting a study on the heating behavior of various GNP-synthesized ferrites under an extensive AC magnetic field
4. Conclusion In order to prepare zinc-substituted spinel-type ferrite fine particles of M1xZnxFe2O4 (M=Mg, Mn, Co, Ni, Cu, (Li, Fe) x=0–1) with good crystallinity and stoichiometry, we investigated a
The GNP was found to be a rapid, low-cost method for synthesizing spinel-type ferrites. Although the nanometer-sized primary crystallites were always somewhat necked together, the fact will not affect our planning application using heating effects of magnetic hysteresis energy losses, since the coercive force does not depend on the agglomerate size but on the crystallite size. Therefore, we conclude that the GNP is one of the suitable methods for high-speed preparation of various spinel-type ferrites.
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