• Corrosion Science and Technology
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• v.3 no.4
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• pp.140-147
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• 2004

High temperature oxidation of Fe-19Cr and Fe-19Cr-0.2Ti alloys is studied at 1173-1373 K in 16.5 kPa $O_2$ - balances $N_2$ atmosphere aimed at clarifying the effect of titanium addition. Oxidation rate of Fe-19Cr alloy was accelerated with titanium. For both alloys chromium rich $(Fe,\;Cr)_2O_3$ was formed as a major oxidation product. On Fe-19Cr-0.2Ti alloy, a thin layer composed of spinel type oxide and titanium oxide was also formed and an internal oxidation of titanium was observed. Titanium was concentrated at the oxide surface and internal oxidation zone but a small amount of titanium was also found in the intermediate corundum type $(Fe,\;Cr)_2O_3$ layer. Crystals of corundum type $(Fe,\;Cr)_2O_3$ formed on Fe-19Cr alloy are coarse but that formed on Fe-19Cr -0.2Ti alloys were fine and columnar. Reason for the difference in oxidation kinetics and crystal structure will be discussed relating to the distribution of aliovalent titanium in corundum type $(Fe,\;Cr)_2O_3$ oxide layer.

• Journal of Magnetics
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• v.4 no.1
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• pp.33-37
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• 1999

Unidirectional magnetic anisotropy field ($H_an$) was investigated for thin films of $CdCr{2-2x}In_{2X}Se_4 (0$\leq$x$\leq$0.2). This anisotropy originates from the microscopic anisotropic Dzyaloshinskii-Moriya (DM) interaction which arise from the spin-orbit scattering of the conduction electrons by the nonmagnetic impurities. This interaction maintains the remanent magnetization in the direction of the initial applied field. Then the single easy direction of the magnetization is parallel to the direction of the magnetic field. The anisotropy produced by field cooling is unidirectional I.e. the spins system deeps some memory of the cooling field direction. The chalcogenide spinel of$ CdCr_{2-2x}In){2X}Se_4$belongs to the class of the magnetic semiconductors. The magnetic disordered state is obtained when ferromagnetic structure is diluted by In. Then we have the mixed phase characterised by coexistence the magnetic long range ordering (IFN-infinite ferromagnetic network) and the spin glass order (Fc-finite clusters). The total magnetic anisotropy energy depends on the state of magnetic ordering. In our study we concentrated on the magnetic state with reentrant transition and spin glass state. The polycrystalline $ CdCr_{2-2x}In){2X}Se_4$ thin films were obtained by rf sputtering technique. We applied the ferromagnetic resonance (FMR) and M-H loop techniques for determining the temperature composition dependencies of Han. From the experimental data, we have found that Han decreases almost linearly when temperature is increased and in the low temperature is about three times bigger at SG state with comparison to the state with REE.

• Bulletin of the Korean Chemical Society
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• v.30 no.8
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• pp.1738-1742
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• 2009

Nanocrystalline Zn$Fe_2O_4$ oxide-semiconductor with spinel structure was synthesized by the polymerized complex (PC) method and investigated for its photocatalytic and photoelectric properties. The observation of a highly pure phase and a lower crystallization temperature in Zn$Fe_2O_4$ made by PC method is in total contrast to that was observed in Zn$Fe_2O_4$ prepared by the conventional solid-state reaction (SSR) method. The band gap of the nanocrystalline Zn$Fe_2O_4$ determined by UV-DRS was 1.90 eV (653 nm). The photocatalytic activity of Zn$Fe_2O_4$ prepared by PC method as investigated by the photo-decomposition of isopropyl alcohol (IPA) under visible light (${\geq}$ 420 nm) was much higher than that of the Zn$Fe_2O_4$ prepared by SSR as well as Ti$O_{2-x}N_x$. High photocatalytic activity of Zn$Fe_2O_4$ prepared by PC method was mainly due to its surface area, crystallinity and the dispersity of platinum metal over Zn$Fe_2O_4$.

• Journal of the Korean Magnetics Society
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• v.17 no.6
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• pp.238-241
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• 2007

Ni-Cu-Zn ferrites were prepared by the co-precipitation. Physical properties and Microwave absorbing properties were investigated in Ni-Cu-Zn ferrite for the aim of microwave absorbers. From the analysis of X-ray diffraction patterns, we can see that all the particles have only a single phase spinel structure. The loss factor was maximum at sintering temperature $1100^{\circ}C$. The initial permeability of sintered ferrite obtained was an average of 50. We found that the $(Ni_{0.7}Cu_{0.2}Zn_{0.1}O)_{1.02}(Fe_2O_3)_{0.98}$ can be used in ferrite rubber composite microwave absorber when sintering temperature at $1100^{\circ}C$.

• Journal of Magnetics
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• v.7 no.2
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• pp.25-28
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• 2002

Slowly cooled $Cu_xFe_{3-x}O_4$ ($\chi$=0.1, 0.2) have been investigated over a temperature range from 82 to 700 K using the M$\ddot{o}$ssbauer technique. X-ray diffraction shows that these have a single-phase cubic spinel structure of lattice parameters $\alpha$=8.396 and 8.398${\AA}$, respectively. Since Cu ions prefer B (octahedral) sites to A (tetrahedral) sites, the ionic distribution is $(Fe)_A[Fe_{2-x}Cu_x]_BO_4$. M$\ddot{o}$ssbauer spectra consisted of two sets of 6-line pattern from. A site in ferric state and B site in ferrous-ferric state. Intensity ratio of B to A subspectra is 1.0 at 82 K and increases to 2.0 at 700 K with increasing temperature. After annealing the samples under vacuum at $450^circ{C}$ for a half hour, x-ray diffraction patterns have the peaks of magnetite- and hematite-phase. Lattice constants of magnetite-phase are 8.395 and 8.392 ${\AA}$ smaller than 8.396 and 8.398 ${\AA}$ before annealing, respectively. M$\ddot{o}$ssbauer spectra reveal the conventional magnetite pattern with the additional hematite pattern. Intensity ratios of B to A subspectra fur magnetite-phase become 1.9-2.0 over all temperature ranges and Cu ions are distributed over A and B sites randomly. Ratios of hematite to total intensity in M$\ddot{o}$ssbauer spectra for $\chi$= 0.1 and $\chi$= 0.2 are 10 and 21%, respectively. These hematite ratios may be due to annealing under vacuum at $450^circ{C}$, which transforms $Cu^{2+}$ ionic states into $Cu^{1+}$. Verwey temperatures far $\chi$= 0.1 and $\chi$= 0.2 are $123\pm2$ K and $128\pm2$ K.

• The Journal of Engineering Research
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• v.1 no.1
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• pp.15-22
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• 1997

The crystallization behaviours of cordierite gel derived from sol-gel method and glass prepared from conventional melting method with or without $TiO_2$ as nucleants are compared. The densification temperature of gel is $810^{\circ}C$ and its chemical structure identified by IR analysis is same as that of glass melted by conventional method. The beginning crystallization temperature of gel is $965^{\circ}C$ lower than that of melted glass with 10wt% $TiO_2$, which is $978^{\circ}C$. The crystalline phases developed from gel during heat treatment are identified as spinel, $\beta$-quartz solid solution and $\alpha$-cordierite crystal and crystalline phases in case of glass are (Mg,Al)TiOn and $\beta$-quartz solid solution and $\alpha$-cordierite crystal, respectively. The crystallization in melted glass with nucleants occurs through bulk crystallization and in case of that without nucleants surface crystallization occurs, while the crystallization in gel is internal crystallization from interface between particles formed after densification.

• Journal of the Korean institute of surface engineering
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• v.44 no.3
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• pp.82-88
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• 2011

A uniform chromium-free conversion coating treated with an alkaline phosphate- permanganate solution was formed on the AZ 31 magnesium alloy. The effect of acid pickling on the morphology and on the corrosion resistance of the alkaline phosphate-permanganate conversion coating was investigated. The chemical composition and phase structure of conversion coating layer were determined via optical microscopy, SEM, EDS, XPS and XRD. Results show that the conversion coatings are relatively uniform and continuous, with thickness 1.8 to $2.4\;{\mu}m$. The alkaline phosphate-permanganate conversion coating was mainly composed of elements Mg, O, P, Al and Mn. The conversion-coated layers were stable compounds of magnesium oxide and spinel ($MgAl_2O_4$). These compounds were excellent inhibitors to corrosion. The electrochemical corrosion behaviors of coatings in 3.5 wt.% NaCl solutions were evaluated by electrochemical impedance spectroscopy, potentiodynamic polarization technique. EIS results showed a polarization resistance of $0.1\;k{\Omega}$ for the untreated Mg and $16\;k{\Omega}$ for the alkaline phosphate-permanganate conversion treatment sample, giving an improvement of about 160 times. The results of the electrochemical measurements demonstrated that the corrosion resistance of the AZ 31 magnesium alloy was improved by the alkaline phosphate-permanganate conversion treatment.

• Ceramist
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• v.21 no.4
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• pp.388-405
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• 2018

In this paper, we review about current development of electrode materials for Li-ion batteries and catalysts for fuel cells. We scrutinized various electrode materials for cathode and anode in Li-ion batteries, which include the materials currently being used in the industry and candidates with high energy density. While layered, spinel, olivine, and rock-salt type inorganic electrode materials were introduced as the cathode materials, the Li metal, graphite, Li-alloying metal, and oxide compound have been discussed for the application to the anode materials. In the development of fuel cell catalysts, the catalyst structures classified according to the catalyst composition and surface structure, such as Pt-based metal nanoparticles, non-Pt catalysts, and carbon-based materials, were discussed in detail. Moreover, various support materials used to maximize the active surface area of fuel cell catalysts were explained. New electrode materials and catalysts with both high electrochemical performance and stability can be developed based on the thorough understanding of earlier studied electrode materials and catalysts.

• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.778-781
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• 1995

The mixed ferrite $Ni_{x}Co_{1-x}Fe_{2}O_{4}$ have been investigated by X-ray and $M\"{o}ssbauer$ spectoscpy. From the results of X-ray diffraction measurement the structure for this system is spinel, and the lattice constant is in accord with Vegard's law. $M\"{o}ssbauer$ spectra of $Ni_{x}Co_{1-x}Fe_{2}O_{4}$ have been taken at various temperature ranging from 13 to 800 K. The isomer shifts indicate that the valence states of the irons at both A(tetrahedral) and B(octahedral) sithe are found to be in ferric high-spin states. The variation of magnetic hyperfine fields at the A and B sites are explained on the basis on A-B and B-B supertransferred hyperfine interactions. It is found that Debye temperatures for the A and B sites of $CoFe_{2}O_{4}$ and $NiFe_{2}O_{4}$ are found to be ${\theta}_{A}=734{\pm}5K,\;{\theta}_{B}=248{\pm}5K,\;and\;{\theta}_{A}=378{\pm}5K,\;{\theta}_{B}=357{\pm}5K$, respectively. Atomic migration of $Ni_{0.3}Co_{0.7}Fe_{2}O_{4}$ starts near 450 K and increases rapidly with increasing temperature to such a degree that 61 % of the ferric ions at the A site have moved over to the B site by 700 K.

• Journal of the Korean Magnetics Society
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• v.18 no.4
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• pp.154-158
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• 2008

Magnetite($Fe_3O_4$) is currently one of key materials for applications in magnetic storage and many bioinspired applications because bulk $Fe_3O_4$ has a high Curie temperature($Tc={\sim}850K$) and nearly full spin polarization at room temperature(RT). In this work, $Fe_3O_4$ nanoparticles with different sizes of 12 to 15 nm were prepared in a well-controlled manner by a nonhydrolytic synthetic method. Here, we report the significant intergrain magneto-resistance(MR) of ${\sim}2%$ at RT in $Fe_3O_4$ nanoparticle pellets. The tunneling conductance was also investigated based on the Brinkman model, as well. Our results show clearly that the surface or interfacial property of the particles plays a crucial role in the MR effect.