• International Journal of Precision Engineering and Manufacturing
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• 제5권1호
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• pp.19-26
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• 2004

The growth of solid particles in TiC-XC-2vo1.% and TiC-XC-30vo1.% Ni alloys, (where X=Zr, W or Mo) was fitted to the equation of the form $d^3$-${do}^3$=Kt during the liquid phase sintering at 1,673K. Also, the grain growth behavior decreased markedly with the addition of ${MO}_2$C or WC and increased with the addition of zrC. The contiguity was greater in the alloys with a smaller growth rate constant and especially, decreased by increasing the Ni content in the TiC-${MO}_2$C-Ni alloy. In addition, the effect of the addition of carbide on the grain growth of 2 vo1.% Ni alloys was found to be similar to that of 30vo1.% Ni alloys. Consequently, the grain growth mechanism cannot be explained by the usual solution / reprecipitation process, but can be explained in terms of a new growth velocity equation, which includes the effects of contiguous carbide grain boundaries in restricting the overall grain growth, as well as the area of the solid / liquid interface in the alloy.

• 전기의세계
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• 제31권6호
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• pp.437-444
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• 1982

Although several kinds of conduction mechanism of PTC thermistor have been reported, there were few satisfying results. In this paper, the reported conduction mechanism theories were scrutinized and analyzed by using the experimental results. PTC thermistors for this study were manufactured by adding Sb$\_$2/O$\_$3/, AI$\_$2/O$\_$3/, TiO$\_$2/, and SiO$\_$2/ to BaTiO$\_$3/, and by sintering it at different temperatures. In order to analyze the conduction mechanism, R-T characteristics and its frequency dependence of specimens were measured. And also, the structures of specimens were studied. Especially this paper emphasized the explanation of the resistivity characteristics as the grain growth state of PTC thermistor specimens with respect to soaking temperature. According to the results, the resistivity of PTC thermistor whose grain was formed by semiconducting, was independent to the grain size at room temperature. For small and uniform grain size, the slope of the resistivity near the Curie temperature and the resistivity above the Curie temperature became greater and PTCR effect was improved.

• 한국재료학회지
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• 제19권11호
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• pp.588-595
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• 2009

The knowledge of grain growth of carbide particles is very important for manufacturing micrograined cemented carbides. In the present study, continuous and discontinuous grain growth in WC-Co and WC-VC-Co cemented carbides is investigated using the Monte Carlo computer simulation technique. The Ostwald ripening process (solution/re-precipitation) and the grain boundary migration process are assumed in the simulation as the grain growth mechanism. The effects of liquid phase fraction, grain boundary energy and implanted coarse grain are examined. At higher liquid phase content, mass transfer via solid/liquid interfaces plays a major role in grain growth. Growth rate of the implanted grain was higher than that of the matrix grains through solution/re-precipitation and coalescence with neighboring grains. The results of these simulations qualitatively agree with experimental ones and suggest that distribution of liquid phase and carbide particle/carbide grain boundary energy as well as contamination by coarse grain are important factors controlling discontinuous grain growth in WC-Co and WC-VC-Co cemented carbides. The contamination by coarse grains must by avoided in the manufacturing process of fine grain cemented carbides, especially with low Co.

• Bulletin of the Korean Chemical Society
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• 제33권8호
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• pp.2483-2488
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• 2012

The X-ray line broadening technique was used to calculate the grain size of MgO at 1023, 1123, 1223 K respectively either in $CO_2$ or during the thermal decomposition of magnesites in air as well as in vacuum. By referring to the conventional grain growth equation, $D^n=kt$, the activation energy and pre-exponential factor for the process in air are gained as 125.8 kJ/mol and $1.56{\times}10^8\;nm^4/s$, respectively. Ranman spectroscopy was employed to study the surface structure of MgO obtained during calcination of magnesite, by which the mechanism of grain growth was analyzed and discussed. It is suggested that a kind of highly reactive MgO is produced during the thermal decomposition of magnesites, which is exactly the reason why the activation energy of the grain growth during the thermal decomposition of magnesite is lower than that of bulk diffusion or surface diffusion.

• 열처리공학회지
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• 제5권1호
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• pp.13-22
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• 1992

The thermal stability of duplex high Mn-steel structure have been investigated using 15%Mn~1.0~2.4%C steels which are composed of ${\gamma}$-and ${\theta}$-phases in the range of temperature from 900 to $1100^{\circ}C$, and time from 50 to 300h. The results are as follows ; 1) The grain growth in single-phase region proceeds by grain boundary migration and the relation between mean radius $\bar{r}$ and annealing time t is described as follows ; $\bar{r}^2-{\bar{r}_0}^2=k_0{\cdot}t$ 2) The grain growth of duplex, (${\gamma}+{\theta}$), strucrure is slower than that single phase because the chemical composition of ${\gamma}$-and ${\theta}$-phases differs esch others. 3) The grain of (${\gamma}+{\theta}$) duplex structure grow slowly in a mode of Ostwald ripening. Because grain boundaries of ${\gamma}$-phase migrate under a restriction of pinning by ${\theta}$-phases. 4) In the duplex structures. the dispersed structures change to the dual-structures, as the volume fraction of the dispersed second-phase increase. Consequently, the growth-law, which is controlled by boundary-diffusion change to that of the volume diffusion-mechanism.

• 한국세라믹학회지
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• 제32권5호
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• pp.587-593
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• 1995

Abnomal grain growth behavior of BaTiO3 ceramics was investigated with addition of seed grains. It was foudn that the nucleation rate of abnormal grain was constant and growth of abnormal grain was linearly increased with sitnering time, regardless of amount of seed grains. These facts were also confirmed by fitting of the volume fraction of abnormal grain vs. sintering time using Avrami type equation (n=4). It was suggested that seed grains did not change the nucleation rate or growth mechanism of abnormal grain but increase the number of abnormal grains at initial stage of sintering and then it led to fine microstructure of BaTiO3 ceramics.

• 한국재료학회지
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• 제12권10호
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• pp.825-830
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• 2002

The growth rate of solid grains in TiC-XC-2vol% and TiC-XC-30vo1% Ni cermets, where X=Zr, W or Mo, was fitted to an equation of the form $d^3$-$do^3$=Kt. The grain growth behavior during liquid phase sintering at 1673K decreased markedly with addition of $Mo_2$C or WC and increased with addition of ZrC. The contiguity ratio was greater in the alloys with smaller growth rate constant and decreased with increasing Ni content in the $TiC-Mo_2$C-Ni cermet. The grain growth mechanism could be explained by the effect of contiguous grain boundaries in restricting the overall grain growth.

• 대한기계학회논문집A
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• 제20권4호
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• pp.1177-1185
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• 1996

The crack growth under creep condition is one of the major damage mechanisms which determines remaining life of the component operating at high temperatures. In this paper, the creep crack growth by grain boundary cavitation is studied, which is frequently observed failure mechanism for creep brittle materials. As a result of diffusive growth of creep cavities, it is shown that the crack-tip stress field is modified from the original stress distribution by the amount of singularity attenuation parameter which is function of crack growth rate and material properties. Also, the stress relaxation at crack-tip results in the extension of cavitating area by the load dump effect to meet the macroscopic force equilibrium conditdion.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.62-63
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• 2006

Both densification and grain growth are driven by the reduction of the interfacial area, kinetics of which depends strongly on the interface structure. Abnormal grain coarsening in the system of singular solid/liquid interface such as WC-Co alloys was explained by the growth mechanism of 2-dimensional nucleation. Based on this concept, the marked inhibition of coarsening of WC grains by VC addition can be approached by the increase in the step free energy, which increases the barrier of 2-dimensional nucleation. The activated sintering in tungsten powders can be approached by the interface structure change induced by the addition of a small amount of nickel.

• Interaction and multiscale mechanics
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• 제1권3호
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• pp.369-379
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• 2008

A combined stochastic diffusion and mean-field model is developed for a systematic study of the grain growth in a pure single-phase polycrystalline material. A corresponding Fokker-Planck continuity equation is formulated, and the interplay/competition of stochastic and curvature-driven mechanisms is investigated. Finite difference results show that the stochastic diffusion coefficient has a strong effect on the growth of small grains in the early stage in both two-dimensional columnar and three-dimensional grain systems, and the corresponding growth exponents are ~0.33 and ~0.25, respectively. With the increase in grain size, the deterministic curvature-driven mechanism becomes dominant and the growth exponent is close to 0.5. The transition ranges between these two mechanisms are about 2-26 and 2-15 nm with boundary energy of 0.01-1 J $m^{-2}$ in two- and three-dimensional systems, respectively. The grain size distribution of a three-dimensional system changes dramatically with increasing time, while it changes a little in a two-dimensional system. The grain size distribution from the combined model is consistent with experimental data available.