• Journal of the Korean Ceramic Society
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• v.25 no.3
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• pp.193-200
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• 1988

The starting materials were aluminum hydroxide prepared by precipitation method at the conditions of pH values; 7, 9, 10 and 11. The properties of alumina powder on heat-treatment were studied. After dehydrating structural water from amorphous aluminum hydroxide, the first formed phase was amorphous alumina and its specific surface are was decreased. The specific surface area was increased by dehydration of structural water from aluminum hydroxides except amorphous aluminum hydroxide. The specific surface area was increased with increase of the ratio of A1OOH to $A1(OH)_3$ in the region of transition aluminas. The rate of transition from aluminum hydroxide to alpha alumina occurred in the order of 7, 10, 9 and 11 of pH values. The morphology of alpha alumina powders was skeleton particles remaining outer shape of aluminum hydroxide. Both the elevation of heat-treatment temperature and the transition toalpha alumina decreased specific surface area and brought about the growth of particles.

• Journal of the Korean Society for Heat Treatment
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• v.29 no.2
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• pp.51-55
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• 2016

The P/M processing of titanium aluminide using amorphous TiAl is developed by which it is possible to overcome inherent fabricability problems and to obtain a fine microstructure. A high quality amorphous TiAl powder produced by reaction ball milling shows clear glass transition far below a temperature at the onset of crystallization in differential scanning calorimetry above a heating rate of 0.05 K/s. We obtained a fully dense compact of amorphous TiAl powders, encapsulated in a vacuumed can, via viscous flow by hot isostatic pressing (HIP). Isothermally annealing of HIP'ed amorphous compact under a pressure of 196 MPa shows a progressive growth of ${\gamma}-TiAl$ phase with ${\alpha}2$ ($Ti_3Al$), which is characterized by increasing sharpness of X-ray peaks with temperature. Fully dense HIP'ed compact of titanium aluminide TiAl shows a high hardness of 505 Hv, suggesting strengthening mechanisms by sub-micron sized grain of ${\gamma}-TiAl$ and particle-dispersion by second phase constituent, ${\alpha}2$.

• Korean Journal of Materials Research
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• v.30 no.11
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• pp.589-594
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• 2020

Many electronic applications require magnetic materials with high permeability and frequency properties. We improve the magnetic permeability of soft magnetic powder by controlling the shape magnetic anisotropy of the powders and through the preparation of amorphous nanoparticles. For this purpose, the effect of the shape magnetic anisotropy of amorphous Fe-B-P nanoparticles is observed through a magnetic field and the frequency characteristics and permeability of these amorphous nanoparticles are observed. These characteristics are investigated by analyzing the composition of particles, crystal structure, microstructure, magnetic properties, and permeability of particles. The composition, crystal structure, and microstructure of the particles are analyzed using inductively coupled plasma optical emission spectrometry-, X-ray diffraction, scanning electron microscopy and focused ion beam analysis. The saturation magnetization and permeability are measured using a vibrating sample magnetometer and an LCR meter, respectively. It is confirmed that the shape magnetic anisotropy of the particles influences the permeability. Finally, the permeability and frequency characteristics of the amorphous Fe-B-P nanoparticles are improved.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.4
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• pp.165-170
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• 2018

W, Fe, and carbon powders were mechanical alloyed to produce $W_{35}Fe_{43}C_{22}$ ternary alloy powders containing nanocrystal W embedded within amorphous matrix. When the powder samples were heated to the primary crystallization temperature of $735^{\circ}C$, most parts of their amorphous region were fully crystallized to [W,Fe]-rich $M_6C$ carbides. Interestingly, a little portion of the carbides changes to stoichiometric line compounds ($M_{12}C$ and $W_6Fe_7$) and a solution phase (Fe-rich bcc), and remaining parts of the crystallites were amorphized again. The resulting microstructure was retained even by cyclic heating between room temperature of $1,200^{\circ}C$, and thus we found that the amorphous structure can be irreversibly formed at above glass transition temperature.

• Journal of Magnetics
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• v.7 no.4
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• pp.143-146
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• 2002

Mechanical milling technique is considered to be a useful way of processing the fine Nd-Fe-B-type powder with high coercivity. In the present study, phase evolution of the $Nd_{15}(Fe_{1-x}Co_{x})_{77}B_{8}$ (x=0-0.6) alloys during the high energy mechanical milling and annealing was investigated. The effect of Co-substitution on the crystallization of the mechanically milled $Nd_{15}(Fe_{1-x}Co_{x})_{77}B_{8}$ amorphous material was examined. The Nd-Fe-B-type alloys can be amorphized completely by a high-energy mechanical milling. On annealing of the amorphous material, fine $\alpha$-Fe crystallites form first from the amorphous. These fine $\alpha$-Fe crystallites reacts with the remaining amorphous afterwards, leading to crystallization to $Nd_2Fe_{14}$B phase. The Co-substitution for Fe in $Nd_{15}(Fe_{1-x}Co_{x})_{77}B_{8}$ ($\mu$x=0∼0.6) alloys lower significantly the crystallization temperature of the amorphous phase to the $Nd_2Fe_{14}$B phase. The mechanically milled and annealed $Nd_{15}Fe_{77}B_8$ alloy without Co-substitution exhibits consistently better magnetic properties with respect to the alloys with Co-substitution.

• Journal of Powder Materials
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• v.13 no.4 s.57
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• pp.285-289
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• 2006

Co-Fe-Ni-B-Si-Cr based amorphous strips containing nitrogen were manufactured via melt spinning, and then devitrified by crystallization treatment at the various annealing temperatures of $300^{\circ}C{\sim}540^{\circ}C$ for up to 30 minutes in an inert gas $(N_2)$ atmosphere. The microstructures were examined by using XRD and TEM and the magnetic properties were measured by using VSM and B-H meter. Among the alloys, the amorphous ribbons of $Co_{72.6}Fe_{9.8}Ni_{5.5}B_{2.4}Si_{7.1}Cr_{2.6}$ containing 121 ppm of nitrogen showed relatively high saturation magnetization. The alloy ribbons crystallized at $540^{\circ}C$ showed that the grain size of $Co_{72.6}Fe_{9.8}Ni_{5.5}B_{2.4}Si_{7.1}Cr_{2.6}$ alloy containing 121 ppm of nitrogen was about f nm, which exhibited paramagnetic behavior. The formation of nano-grain structure was attributed to the finely dispersed Fe4N particles and the solid-solutionized nitrogen atoms in the matrix. Accordingly, it can be concluded that the nano-grain structure of 5nm in size could reduce the core loss within the normally applied magnetic field of 300A/m at 10kHz.

• Korean Journal of Materials Research
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• v.25 no.12
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• pp.659-665
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• 2015

The self-propagating high temperature synthesis approach was applied to synthesize amorphous boron nano-powders in argon atmospheres. For this purpose, we investigated the characteristics of a thermally induced combustion wave in the $B_2O_3+{\alpha}Mg$ system(${\alpha}=1.0-8.0$) in an argon atmospheres. In this study, the exothermic nature of the $B_2O_3-Mg$ reaction was investigated using thermodynamic calculations. Experimental study was conducted based on the calculation data and the SHS products consisting of crystalline boron and other compounds were obtained starting with a different initial molar ratio of Mg. It was found that the $B_2O_3$ and Mg reaction system produced a high combustion temperature with a rapid combustion reaction. In order to regulate the combustion reaction, NaCl, $Na_2B_4O_7$ and $H_3BO_3$ additives were investigated as diluents. In an experimental study, it was found that all diluents effectively stabilized the reaction regime. The final product of the $B_2O_3+{\alpha}Mg$ system with 0.5 mole $Na_2B_4O_7$ was identified to be amorphous boron nano-powders(< 100 nm).

• Journal of the Korean Ceramic Society
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• v.32 no.6
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• pp.710-718
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• 1995

The new forming method, Pressureless Powder Packing Forming Method was applied to the manufacturing of reaction sintered SiC. After the experiments of vibratory powder packing and binder infiltration, the abrasive SiC powder of which mean size is 45${\mu}{\textrm}{m}$ was selected to this forming method. Uniform green bodies with porosity of 45% and narrow pore size distribution could be formed by this new forming method. Also, complex or varied cross-sectional shapes could be easily manufactured through the silicone rubber mould used in this forming method. Maximum 15 wt% amorphous carbon was penetrated into green body by multi impregnation-carbonization cycles. And reaction-bonded SiC was manufactured by infiltration of SiC-carbon shaped bodies with liquid silicon.

• Journal of the Korean Ceramic Society
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• v.33 no.6
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• pp.693-699
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• 1996

YIG precursor powder was obtained by homogeneous precipitation in chloride salt solution by thermal decom-position of urea. It was found that ferric ions precipitated prior to yttrium ions. The precipitate was minute and spherical in shape. The precipitate formed consisted of the mixture of amorphous and ferric oxyhydroxide. Crystallization of YIG was proceeded by solid state reaction of intermediate YFeO3 and Fe2O3 in the temperature range of 85$0^{\circ}C$ to 140$0^{\circ}C$. Single phase of YIG was obtained by heat-treatment of the powder at 140$0^{\circ}C$ for 6 hrs in air. The powder calcined was molded into pellets and sintered in air. The maximum density of 4,92 g/cm3(95.1% of theoretical density) was obtainable for the pellet sintered at 145$0^{\circ}C$ using the powder calcined at 90$0^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.37 no.2
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• pp.181-185
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• 2000

A new technique capable of accelerating the crystallization of ferrite powder at low temperature is developed. Effects of the ultrasonic waves on the crystallization were studied for ferrite powders prepared using the co-precipitation method. The crystallization of the ferrite powders exposed to the ultrasonic waves were characterized by the XRD. The amorphous ferrite powders prepared using the co-precipitation method were crystallized as a result of the exposure to the ultrasonic waves for 5h and the crystallization of the ferrite powders became more enhanced in proportion to the time exposed. The ferrite powder exposed to the ultrasonic waves for 25h had higher crystallinity a larger specific surface area than the ferrite powder calcined at 500$^{\circ}C$ for 2h.