• Journal of the Korean Magnetics Society
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• v.2 no.1
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• pp.8-14
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• 1992

To improve the coercivity of ${\gamma}-Fe_{2}O_{3}$ particles. We have made cobalt ferrite epitaxial ${\gamma}-Fe_{2}O_{3}$ particles by crystallizing cobalt ferrite on the surface of ${\gamma}-Fe_{2}O_{3}$ particles. The coercivity of $Co_{x}Fe_{3-x}O_{4}$ which is conform to coating layer showed the most superior characteristic when the value of x was 1. On the formation of cobalt epitaxial ${\gamma}-Fe_{2}O_{3}$ particles, controlling reaction atmosphere as oxidation at. mosphere after non-oxidation atmosphere was the optimum condition to prepare cobalt epitaxial ${\gamma}-Fe_{2}O_{3}$ particles with superior magnetic properties. Epitaxial growth of cobalt ferrite on the surface of ${\gamma}-Fe_{2}O_{3}$ particles was accomplished at $90^{\circ}C$ for 30 minutes.

• Journal of Sensor Science and Technology
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• v.8 no.2
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• pp.171-176
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• 1999

${\gamma}-Fe_2O_3$ thin films on $Al_2O_3$ substrate were prepared by the oxidation of $Fe_3O_4$ thin films processed by PECVD(Plasma-Enhanced Chemical Vapor Deposition) technique. The phase transformation of ${\gamma}-Fe_2O_3$ thin films was mainly controlled by the substrate temperature and oxidation process of $Fe_3O_4$ phase. $Fe_3O_4$ phase was obtained at the deposition temperature of $200{\sim}300^{\circ}C$. $Fe_3O_4$ phase could be transformed into ${\gamma}-Fe_2O_3$ phase under controlled oxidation at $280{\sim}300^{\circ}C$. $Fe_3O_4$ and ${\gamma}-Fe_2O_3$ obtained by oxidation of $Fe_3O_4$ phase had the same spinel structure and were coexisted. The oxidized ${\gamma}-Fe_2O_3$ thin film on $Al_2O_3$ substrate showed a porous island structure.

• Journal of the Korean Magnetics Society
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• v.3 no.2
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• pp.108-114
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• 1993

This paper presents a study on the effects of silica coating in the production of ${\gamma}-Fe_{2}O_{3}$ powders suitable for magnetic recording media. Emphasis has been put on investigating the relationship between the powder characteristics and the effects of silica coating in the heat-treatment stage of ${\gamma}-Fe_{2}O_{3}$ production. After we prepared non-coated ${\gamma}-Fe_{2}O_{3}$ and silica coated ${\gamma}-Fe_{2}O_{3}$ with coating water glass on the surface of goethite and heattreatment process, we compared and investigated powder characteristics. As silica coated layer played a role of preventing the powders from overreduction to metal iron and rapid oxidation, silica coated ${\gamma}-Fe_{2}O_{3}$ showed superior magnetization value due to inhibiting t!1e adulteration of ${\alpha}-Fe_{2}O_{3}$ into the final product. When silica coated layer acted as a sintering restrainer, silica coated ${\gamma}-Fe_{2}O_{3}$ showed high coercivity and specific sur-face area due to good acicularity.

• Journal of Powder Materials
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• v.9 no.5
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• pp.341-345
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• 2002

Nanoparticles of $Fe_2O_3$ with a mean particle size of 4-30 nm have been prepared by a pulsed wire evaporation method, and its structural and magnetic properties were studied by SQUID magnetometer and Mossbauer spectroscopy. From the main peak intensity of XRD and absorption rate of Mossbauer spectrum, the amounts of $\gamma-Fe_2O_3$ and $\alpha-Fe_2O_3$ in as-prepared sample are about 70% and 30%, respectively. The coercivity (53 Oe) and the saturation magnetization (14 emu/g) are about 20% of those of the bulk $\gamma-Fe_2O_3$. The low value of coercivity and saturation magnetization indicate that the $\gamma-Fe_2O_3$ phase nearly shows the spin glass-like behavior. Analysis of the set of Mossbauer spectrum indicates a distribution of magnetic hyperfine fields due to the particle size distribution yielding 20 nm of average particle size. The magnetic hyperfine parameters are consistent with values reported of bulk $\gamma-Fe_2O_3$ and $alpha-Fe_2O_3$. A quadrupole line on the center of spectrum represents of superparamagnetic phase of $\gamma-Fe_2O_3$ with a mean particle size of 7 nm or below.

• Journal of the Korean Ceramic Society
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• v.23 no.5
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• pp.1-8
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• 1986

This paper is concerned with a gas sensor composed of semi-conducting ${\gamma}$-$Fe_2O_3$ ceramics made by oxidizing $Fe_2O_3$ sintered body. Acicular $\alpha$-FeOOH powder prepared by precipitation of $FeSO_4$.$7H_2O$ solution was transformed to $FeSO_4$ sintered at 700$^{\circ}$-850$^{\circ}$C for 1 hr. and then oxidized to ${\gamma}$-$Fe_2O_3$ The gas sensitive properties of ${\gamma}$-$Fe_2O_3$ ceramic bodies based on the lectrical resistance change was measured in 0.5-2 vol% $H_2$ and $C_2$ $H_2$ gas at 35$0^{\circ}C$ The specific surface area of sintered specimen largely dependent on the sintering temperature and grain shape directly affected the gas sensitive pro-perties of ${\gamma}$-$Fe_2O_3$gas sensor. Specimens having larger specific surface area showed better sensitivity which means the electrical resistance change due to oxidation and reduction process occurs on ly at the surface of grains microscopically in the ${\gamma}$-$Fe_2O_3$ceramics. Micropores made in $Fe_2O_3$ powder during dehydration of $\alpha$-FeOOH can not prompt the gas sensitive properties of sintered ${\gamma}$-$Fe_2O_3$ because they are sintered or closed in the grains during sintering process and dose not affect the specific surface area of sintered body.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.424-425
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• 2006

The effects of reaction temperature and precursor concentration on the microstructure and magnetic properties of ${\gamma}-Fe_2O_3$ nanoparticles synthesized as final products of iron acetylacetonate in chemical vapor condensation (CVC) were investigated. Pure ${\gamma}-Fe_2O_3$ phase was obtained at temperature above $900^{\circ}C$ and crystallite size of ${\gamma}-Fe_2O_3$ nanoparticles decreased with lowering precursor concentration. Also, the coercivity decreases with decreasing crystallite size of nanopowder. The lowest coercivity was 7.8 Oe, which was obtained from the ${\gamma}-Fe_2O_3$ nanopowder sample synthesized at precursor concentration of 0.3M. Then, the crystallite size of ${\gamma}-Fe_2O_3$ nanoparticles was 8.8 nm.

• Journal of the Korean Magnetics Society
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• v.20 no.5
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• pp.196-200
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• 2010

$\gamma-Fe_2O_3$ nanoparticles have been prepared by a sol-gel method. The structural and magnetic properties have been investigated by XRD, VSM and Mossbauer spectroscopy. $\gamma-Fe_2O_3$ powder annealed at $150^{\circ}C$ has a spinel structure and superparamagnetical behavior. The estimated size of superparammagnetic $\gamma-Fe_2O_3$ nanoparticle is around 7 nm. The hyperfine fields at $-261^{\circ}C$ for the A and B patterns were found to be 503 and 485 kOe, respectively. The blocking temperature ($T_B$) of superparammagnetic $\gamma-Fe_2O_3$ nanoparticle is about $-183^{\circ}C$. The magnetic anisotropy constant of $\gamma-Fe_2O_3$ nanoparticle was calculated to be $1.6{\times}10^6ergs/cm^3$. $\gamma-Fe_2O_3$ nanoparticle annealed at $150^{\circ}C$ can be a candidate for biomedicine applications as magnetic carriers.

• Journal of the Korean Ceramic Society
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• v.28 no.2
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• pp.119-129
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• 1991

$\gamma$-FeOOH suitable for magnetic recording media was synthesized using waste acid which is the by-product of the iron works factory. Effects of concentration of the acid and alkali and reaction temperature on the particle properties of $\gamma$-FeOOH and magnetic properties of $\gamma$-$Fe_2O_3$ were studied. $\gamma$-FeOOH single phase was formed below 1M of acid concentration with 1M of alkali concentration and at 0.4M of acid concentration with 4M of alkali concentration. While the width of acicular particle was increased, the length of acicular particle was decreased with diluting acid concentration. The magnetic properties of the $\gamma$-$Fe_2O_3$ improved with increasing acid concentration. $\gamma$-FeOOH single phase was formed in the temperature range of 30 to $80^{\circ}C$. The length of the particle was decreased with increasing temperature. $\gamma$-Fe2O3 produced from dehydration of $\gamma$-FeOOH showed bad magnetic properties due to the presence of many pores in the particle. But with successive reduction and oxidation of $\gamma$-$Fe_2O_3$ produced from dehydration of $\gamma$-FeOOH, $\gamma$-$Fe_2O_3$ showed good magnetic properties.

• Journal of the Korean Ceramic Society
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• v.29 no.12
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• pp.942-948
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• 1992

Ultrafine iron oxide powder, {{{{ gamma }}-Fe2O3 and $\alpha$-Fe2O3, were prepared by the thermal decomposition of organometallic compounds. The formation process of powder includes the thermal decomposition and oxidation of the organometallic precursors, Fe(N2H3COO)2(N2H4)2 (A) and N2H5Fe(N2H3COO)3.H2O (B). The organometallic precursors, A and B, were synthesized by the reaction of ferrous ion with hydrazinocarboxylic acid, and characterized by quantitative analysis and infrared spectroscopy. The mechanistic study for the thermal decomposition was performed by DAT-TG. The iron oxide powder was obtained by the heat treatment of the precursors at 20$0^{\circ}C$ and $600^{\circ}C$ for half an hour in air. The phases of the resulting product were proved {{{{ gamma }}-Fe2O3 and $\alpha$-Fe2O3 respectively. The particle shape was equiaxial and the particle size was less than 0.1 ${\mu}{\textrm}{m}$. Magnetic properties of the {{{{ gamma }}-Fe2O3 powder obtained from A and B was 234 Oe of coercivity, 64.26 emu/g of saturation magnetization, 23.59 emu/g of remanent magnetization and 24.1 Oe, 47.27 emu/g, 3.118 emu/g respectively. The value of $\alpha$-Fe2O3 powder was 1.494 Oe, 0.4862 emu/g, 0.1832 emu/g and 1,276 Oe, 0.4854 emu/g, 0.1856 emu/g respectively.

• Journal of the Korean Magnetics Society
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• v.24 no.5
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• pp.135-139
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• 2014

Iron-oxide nanopowders were synthesized by a pulsed wire evaporation (PWE) in various ambient gas conditions. SEM measurement indicates that the spherical iron nanoparticles are about 50 nm in diameter. The phase analysis for the produced iron-oxide powders was systematically investigated by using $M\ddot{o}ssbauer$ spectra and the results show that classified phases of $Fe_2O_3$ and $Fe_3O_4$ can be controlled by regulating the oxygen concentration in the mixed gas during the PWE process. A quadrupole line on the center of $M\ddot{o}ssbauer$ spectrum represents the superparamagnetic phase of 12 % from ${\gamma}-Fe_2O_3$ phase.