• Proceedings of the Korea Association of Crystal Growth Conference
• /
• 1997.10a
• /
• pp.75-80
• /
• 1997

Ethanol을 용매로 사용한 iron(III) Nitrate(Fe(NO$_3$)$_3$.9$H_2O$) 용액의 가수분해 반응에 의해 $\alpha$-Fe$_2$O$_3$미분말을 합성하였다. 가수분해 정도를 알아보기 위해 pH 변화를 측정하였으며, 반응온도와 iron(III) nitrate의 농도를 변화시켜 각각이 입자크기에 미치는 영향을 관찰하였다. 하소 후의 XRD pattern을 관찰하여 $\alpha$-Fe$_2$O$_3$결정상의 생성을 확인하였으며, 미세구조 관찰 결과 얻어진 $\alpha$-Fe$_2$O$_3$분말은 50nm이하의 미세한 입자임이 확인되었다.

• Journal of Magnetics
• /
• v.15 no.1
• /
• pp.21-24
• /
• 2010

A new synthetic method for the formation of uniform $\alpha-Fe_2O_3$ nanoparticles was reported and their magnetic properties were investigated. The sonochemical synthesis and the subsequent take-off technique resulted in spherical shaped $\alpha-Fe_2O_3$ nanoparticles with an average diameter of 60 nm. The temperature- and applied magnetic field-dependent magnetization of the $\alpha-Fe_2O_3$ nanoparticles was explained by the sum of two contributions, i.e., the Morin transition and superparamagnetism, because the critical size for superparamagnetism was within the size variation of the nanoparticles.

• Korean Chemical Engineering Research
• /
• v.56 no.6
• /
• pp.819-825
• /
• 2018

Porous nanofibers comprising hollow ${\alpha}-Fe_2O_3$ nanospheres were prepared by applying both template method and Kirkendall diffusion effect to electrospinning process. During heat-treatment processes, the solid Fe nano-metals formed by initial heat-treatment in the carbon matrix were converted into the hollow structured ${\alpha}-Fe_2O_3$ nanospheres. In particular, PS nanobeads added in the spinning solution were decomposed and formed numerous channels in the composite, which served as a good pathway for Kirkendall diffusion gas. The resulting porous nanofibers comprising hollow ${\alpha}-Fe_2O_3$ nanospheres were applied as an anode material for lithium-ion batteries. The discharge capacities of the nanofibers for the 30th cycle at a high current density of $1.0A\;g^{-1}$ was $776mA\;h\;g^{-1}$. The good lithium ion storage property was attributed to the synergetic effects of the hollow ${\alpha}-Fe_2O_3$ nanospheres and the interstitial nanovoids between the nanospheres. The synthetic method proposed in this study could be applied to the preparation of porous nanofibers comprising hollow nanospheres with various composition for various applications, including energy storage.

• Clean Technology
• /
• v.20 no.4
• /
• pp.375-382
• /
• 2014

This work investigated the plasma-catalytic decomposition of isopropyl alcohol (IPA) and the behavior of the byproduct compounds over monolith-supported metal oxide catalysts. Iron oxide ($Fe_2O_3$) or copper oxide (CuO) was loaded on a monolithic porous ${\alpha}-Al_2O_3$ support, which was placed inside the coaxial electrodes of plasma reactor. The IPA decomposition efficiency itself hardly depended on the presence and type of metal oxides because the rate of plasma-induced decomposition was so fast, but the behavior of byproduct formation was largely affected by them. The concentrations of the unwanted byproducts, including acetone, formaldehyde, acetaldehyde, methane, carbon monoxide, etc., were in order of $Fe_2O_3/{\alpha}-Al_2O_3$ < $CuO/{\alpha}-Al_2O_3$ < ${\alpha}-Al_2O_3$ from low to high. Under the condition (flow rate: $1L\;min^{-1}$; IPA concentration: 5,000 ppm; $O_2$ content: 10%; discharge power: 47 W), the selectivity towards $CO_2$ was about 40, 80 and 95% for ${\alpha}-Al_2O_3$, $CuO/{\alpha}-Al_2O_3$ and $Fe_2O_3/{\alpha}-Al_2O_3$, respectively, indicating that $Fe_2O_3/{\alpha}-Al_2O_3$ is the most effective for plasma-catalytic oxidation of IPA. Unlike plasma-alone processes in which tar-like products formed from volatile organic compounds are deposited, the present plasma-catalyst hybrid system did not exhibit such a phenomenon, thus retaining the original catalytic activity.

• Journal of the Korean Ceramic Society
• /
• v.38 no.12
• /
• pp.1115-1122
• /
• 2001

The tendency of the miximum brightness of $Y_2$O$_3$:Eu phosphor with various activator concentration, by different surface treatment methods as well as different exciting energies were investigated. The surface treatment methods were the adsorption method used $\alpha$-Fe$_2$O$_3$powder prepared emulsion-drying process and the precipitation method used FeSO$_4$/ethanol. Eu concentration of maximum brightness of $Y_2$O$_3$:Eu phosphor prepared by solid-solid state was changed with various exciting energies. The concentrations were 0.02 mol at VUV(147 nm) as well as 400 V and 0.03 mol at 5 kV. The phosphor treated both by adsorption method and precipitation method showed decreasing luminescent intensity with increasing amount of $\alpha$-Fe$_2$O$_3$, and the methods are chosen by exciting energy. Adsorption method was effective in a low voltage and VUV(147nm) region, and precipitation method was effective in the high voltage region.

• Journal of the Korean Magnetics Society
• /
• v.5 no.4
• /
• pp.281-286
• /
• 1995

In order to study the effect of additives $SiO_{2}$ on the magnetic properties of hexaferrite sheet magnet, we used X-ray diffractometer, Mossbauer spectrometer, and VSM magnetometer. We have prepared $Ba_{0.25}Sr_{0.75}Fe_{12}O_{19}$ green sheets by the Dr. Blade method. Most of samples have a magnetoplurnbite crystal structure of typical M-type hexaferrite. The lattice parameters are found not to be affected by the addition of $SiO_{2}$. ${\alpha}-Fe_{2}O_{3}$ phase develops above $SiO_{2}$ 2.0 wt.%. Isomer shifts indicate that the valence of Fe ions is trivalent. Curie temperatures decrease slightly with increasing $SiO_{2}$ concentrations. It means that the $Si^{4+}$ subsitution for 12k-site $Fe^{3+}$ has an effect on the superexchange interactions Fe-O-Fe, which change the distance and the angle between cations and anions. It was suggested that ${\alpha}-Fe_{2}O_{3}$ phase results from the excessive Fe produced by subsituting $Si^{4+}$ for $Fe^{3+}$. Based upon the results of $Ba_{0.25}Sr_{0.75}Fe_{12}O_{19}$ added with $SiO_{2}$, we concluded that $H_{c}$, $M_{s}$ and $M_{r}$ depend more strongly on the microstructure chracteristics than on the cation substitution.

• Journal of the Korean Ceramic Society
• /
• v.54 no.2
• /
• pp.137-140
• /
• 2017

A $TiO_2$ nanofilm was deposited on ${\alpha}-Fe_2O_3$ nanopowders using the atomic layer deposition method. The $TiO_2$ film was prepared at $300^{\circ}C$ using $Ti(N(CH_3)_2)_4$ and $H_2O$ as the precursor and reactant gas, respectively. The thickness and composition of the $TiO_2$ surface were characterized by TEM and EDS measurements. The TEM results showed that the growth rate of the film was about $0.12{\AA}/cycle$. The EDS and SAED analyses showed the presence of titanium oxide on the surface of the ${\alpha}-Fe_2O_3$ nanopowders, confirming the deposition of the $TiO_2$ nanofilm. The Zeta potential and sedimentation test results showed that the dispersibility of the coated nanopowders was higher than that of the uncoated nanopowders. This is attributed to the electrostatic repulsion between the $TiO_2$-coated layers on the surface of the ${\alpha}-Fe_2O_3$ nanopowders. The results revealed that the $TiO_2$-coated layers modified the surface characteristics of the ${\alpha}-Fe_2O_3$ nanopowders and improved their dispersibility.

• Proceedings of the KIEE Conference
• /
• 1990.07a
• /
• pp.227-230
• /
• 1990

The addition of titanium has come to produce an increase in the conductivity of ${\alpha}-Fe_2O_3$ and has been shown NTC ( negative temperature coefficient ) characteristics. Titanium enters the ${\alpha}-Fe_2O_3$ lattice substitutionally as $Ti^{4+}$,thus producing an $Fe^{2+}$ and maintaining the average charge per cation at three. Thus the $Fe^{2+}$ acts as a donor center with respect to the surrounding $Fe^{3+}$ ions. The sintering temperature, compacting pressure and sintering tire have an effect on the electrical properties. C-V and other properties have been measured on polycrystalline samples of ${\alpha}-Fe_2O_3$ containing small deviations from stoichiometry and small amounts of added Titanium. This measurment was made in the course of an investigation of the NTC mechanism in oxides whose cations have a partially filled d-level. C-V and frequency properties have been applied to the measurement of the trap barrier properties at the grain boundary. The double Schottky barrier at the grain boundary is the major cause of the NTC mechanism in NTC thermistor of ${\alpha}-Fe_2O_3$ containing N-type impurity.

• Proceedings of the Korea Association of Crystal Growth Conference
• /
• 1997.06a
• /
• pp.79-85
• /
• 1997

Fe$_2$O$_3$ thin films were prepared on $Al_2$O$_3$ substrate by PECVD(Plasma-Enhanced Chemical Vapor Deposition) process. The phase transformation of iron oxide film was determined as the substrate temperature and reduction-oxidation process. $\alpha$-Fe$_2$O$_3$ was stable in deposition temperature ranges of 80~15$0^{\circ}C$. Fe$_3$O$_4$ phase was obtained by the reduction process of $\alpha$-Fe$_2$O$_3$ phase in H$_2$ ambient. Fe$_3$O$_4$ phase was transformed into a ${\gamma}$-Fe$_2$O$_3$ thin film under controlled oxidation conditions at 280~30$0^{\circ}C$.

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