• Composites Research
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• v.32 no.3
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• pp.141-147
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• 2019

In this study, micro-sized and nano-sized pure aluminum (Al) powders were compressed by unidirectional pressure at room temperature. Although neither type of Al bulk was heated, they had a high relative density and improved mechanical properties. The microstructural analysis showed a difference in the process of densification according to particle size, and the mechanical properties were measured by the Vickers hardness test and the nano indentation test. The Vickers hardness of micro Al and nano Al fabricated in this study was five to eight times that of ordinary Al. The grain refinement effect was considered to be one of the strengthening factors, and the Hall-Petch equation was introduced to analyze the improved hardness caused by grain size reduction. In addition, the effect of particle size and dispersion of aluminum oxide in the bulk were additionally considered. Based on these results, the present study facilitates the examination of the effect of particle size on the mechanical properties of compacted bulk fabricated by the powder metallurgy method and suggests the possible way to improve the mechanical properties of nano-crystalline powders.

• Tribology and Lubricants
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• v.36 no.3
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• pp.124-132
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• 2020

The effect of the stoichiometry on the sliding wear properties of NiAl coatings has been investigated. Three different powder mixtures with the compositions of Ni-50at%Al, Ni-54at%Al and Ni-42at%Al were diepressed respectively, and which were subsequently coated on mild steel through combustion synthesis in an induction heating system. Sliding wear behavior of the coatings was examined against an alloyed tool steel using a pin-on-disc type sliding wear test machine. As results, it could be seen that powder mixture(Ni-54at%Al) with displaying Al-rich deviations from the stoichiometry of NiAl(Ni-50at%Al) was promoted the most the synthetic reactivity. The microstructure of the coating layer with the compositions of Ni-54at%Al exhibits the porous NiAl single phase structure. However, the microstructure of the coating layer of the compositions of Ni-42at%Al exhibits the denser multi-phase structure containing several intermediate phases in addition to NiAl. Densification of the coating layer was enhanced by increasing the reacting temperature. On the other hand, the wear properties of the coating layers showed that the wear mode at speeds of around 1 m/s was severe wear, regardless of the stoichiometry and reacting temperature. However, wear properties of coating layer with the compositions of Ni-42at%Al were superior to those of coating layer with the compositions of Ni-54at%Al. This would be attributed by the fact that coating layer with the compositions of Ni-42at%Al develops little void and much intermediate phases with high strength.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.27 no.6
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• pp.372-376
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• 2014

We investigated the effect of excess CuO on the sintering behavior, ferroelectric, and piezoelectric properties of lead-free $Bi_{0.5}(Na_{0.82}K_{0.18})_{0.5}TiO_3$ (BNKT) ceramics. The addition of excess CuO was found to greatly contribute to the densification and grain growth, however, excess CuO over 3 mol% was precipitated at grain boundaries after sintering. BNKT with 1~2 mol% CuO in excess sintered at $975^{\circ}C$ showed piezoelectric properties comparable to those of unmodified BNKT sintered at $1,175^{\circ}C$. These results seem meaningful for its application to low cost multilayer actuators (MLAs) because low firing ceramics make it possible to apply less expensive base metals to the inner electrode of MLAs.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.19 no.2
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• pp.145-152
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• 2006

The densification behavior and electrical conductivity of $Ce_{0.8}Gd_{0.2}O_{1.9}$ ceramics were investigated with the strontium gallate concentration ranging from 0 to $5\;mol\%$. Both the sintered density and grain size were found to increase rapidly up to $0.5\;mol\%$ $Sr_2Ga_2O_5$, and then to decrease with further addition. Dense $Ce_{0.8}Gd_{0.2}O_{1.9}$ ceramics with $97\%$ of the theoretical density could be obtained for $0.5\;mol\%$ $Sr_2Ga_2O_5$-added specimen sintered at $1250^{\circ}C$ for 5 h, whereas pure $Ce_{0.8}Gd_{0.2}O_{1.9}$ ceramics needed to be sintered at $1550^{\circ}C$ in order to obtain an equivalent theoretical density, Electrical conductivity was measured as a function of dopant content, over the temperature range of $350\;-\;600^{\circ}C$ in air. Total conductivity of $0.5\;mol\%$ $Sr_2Ga_2O_5$-added specimen showed the maximum conductivity of $2.37{\times}10^{-2}{{\Omega}-1}{\cdot}cm^{-1}$ at $500^{\circ}C$, The addition of strontium gallate was found to promote the sintering properties and electrical conductivities of $Gd_2O_3$-doped $CeO_2$.

• Journal of the Korean Ceramic Society
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• v.44 no.8
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• pp.451-456
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• 2007

This paper presents results and observations obtained from a study of sintering behavior and crystallization in $Li_2O-Al_2O_3-SiO_2$ (LAS) Glass by screen printing method. The variable experimental conditions were determined carefully by Thermal-Mechanical Analyzer (TMA), Differential Thermal Analyzer (DTA) for setting the optimum transparent sintering conditions in LAS glass system, $10.5Li_2O-14.7Al_2O_3-58.1SiO_2-16.7B_2O_3(wt%)$, such as glass-ceramics which usually have low crystallization temperatures. Crystallization glasses generated during sintering was observed from diffraction patterns by X-Ray Diffraction (XRD), transmittance by UV-Vis spectrometer. Finally, the optimum sintering condition of LAS glass and the relation between factors and results in several sintering conditions were given by using Response Surface Methodology (RSM). From this study, we confirmed that crystallization interrupted densification during glass powder sintering. Furthermore, we observed that main effect of factors in glass powder sintering with concurrent crystallization depended on experimental conditions from main effects plot by MINTAB-14.

• Composites Research
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• v.16 no.3
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• pp.1-8
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• 2003

Densification occurs by the inelastic flow of the matrix materials during the consolidation processes at high temperature for MMCs, and the results depend on many process conditions such as applied pressure, temperature and volume fraction of fiber and matrix materials. This is particularly important in titanium matrix composites since material failure may occur by either the applied conditions or microstructural parameters through the processes, and thus a generic model based on micro-mechanical approaches enabling the evolution of density over time to be predicted has been developed. The mode developed is then implemented into FEM so that practical process simulation has been carried out. Further the experimental investigation of the consolidation behavior of SiC/Ti-6Al-4V composites using vacuum hot pressing has been performed, and the results obtained are compared with the model predictions.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.11
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• pp.979-986
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• 2003

Sintering behavior and electrical properties of CeO$_2$ system were investigated as a function of the amount of Gd:$_2$O$_3$, and Sm$_2$O$_3$, addition. Doped CeO$_2$ consisted of a homogeneous solid solution of the cubic fluorite structure within the amount of addition from 0 mol% to 15 mol%. Grain growth rate of Gd$_2$O$_3$-doped CeO$_2$ was much smaller than that of pure CeO$_2$, while densification rate was considerably larger. Thus doped CeO$_2$ showed a higher density than pure CeO$_2$. The electrical conductivity of Ce$_1$-$_{x}$Sm$_{x}$O$_1$-$_{x}$/2 was increased up to x = 0.2. However, with further increasing dopant concentrations, the magnitude of the conductivity was found to decrease remarkably. The ionic conductivity value obtained at $700^{\circ}C$ for 10 mol% Sm$_2$O$_3$-doped CeO$_2$ electrolyte was 4.6${\times}$10$^{-2}$ S$.$$cm^{-1}$ /.EX> /.

• Nuclear Engineering and Technology
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• v.35 no.1
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• pp.14-24
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• 2003

This work investigates the effects of Cr$_2$O$_3$ and oxygen potential on grain growth and densification of UO$_2$ pellets. Powder mixtures of UO$_2$ and 0.03-0.4wt% Cr$_2$O$_3$ were pressed and sintered in 3 different gas atmospheres: the $H_2O$-to-H$_2$ ratios were 5$\times$10$^{-4}$ , 1$\times$10$^{-2}$ and 3$\times$10$^{-2}$ In the first gas atmosphere the Cr$_2$O$_3$ contents below 0.2 wt% have an insignificant effect on grain size, but the Cr$_2$O$_3$ contents more than 0.3 wt% promote grain growth in the inner zone of a pellet but not in the outer zone. In both the second and third atmospheres, the grain size increases with the Cr$_2$O$_3$ content. With the same level of Cr$_2$O$_3$ content the grain size is larger in the second atmosphere than in the third. Sintering behavior and developed microstructure are discussed in terms of the reduction of C$r^2$O$^3$ to Cr, the dissolution of C$r^2$O$^3$ in UO$_2$, and liquid phase sintering.

• Advanced Composite Materials
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• v.17 no.3
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• pp.259-275
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• 2008

This study aims to optimize the mechanical and electrical properties of electrically conductive polymer composites (CPCs) for use as a material of bipolar plates for PEM fuel cells. The thin CPCs consisting of conductive fillers and polymer resin were fabricated by a preform molding technique. Expanded graphite (EG), flake-type graphite (FG) and carbon fiber (CF) were used as conductive fillers. This study tested two types of CPCs, EG/FG filled CPCs and EG/CF filled CPCs, to optimize the material properties. First, the characteristics of EG/FG filled CPCs were investigated according to the FG ratio for 7 and $100{\mu}m$ sized FG. CPCs using $100{\mu}m$ FG showed optimal material properties at 60 wt% FG ratio, which were an electrical conductivity of 390 S/cm and flexural strength of 51 MPa. The particle size was an important parameter to change the mechanical and electrical behaviors. The flexural strength was sensitive to the particle size due to the different levels of densification. The electrical conductivity also showed size-dependent behavior because of the different contributions to the conductive network. Meanwhile, the material properties of EG/CF filled CPCs was also optimized according to the CF ratio, and the optimized electrical conductivity and flexural strength were 290 S/cm and 58 MPa, respectively. The electrical conductivity of this case decreased similarly to the EG/FG filled case. On the other hand, the behavior of the flexural strength was more complicated than the EG/FG filled case, and the reason was attributed to the interaction between the strengthening effect of CF and the deterioration of voids.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.49-49
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• 2008

ZnO-based varistors have been widely used for voltage stabilization or transient surge suppression in electric power systems and electronic circuits. Recently, It has reported that the varistor behavior with nonlinear coefficient of 6~17 in Mn-doped ZnO. In this study we have chosen the composition of ZnO-$TeO_2-Mn_3O_4$ (ZTM) system to the purpose of whether varistor behavior appeared in doped ZnO by the solid state sintering or not. We investigated the sintering and electric properties of 0.5~3.0 at% Mn doped ZnO-1.0 at% $TeO_2$ system. Electrical properties, such as current-voltage (I-V), capacitance-voltage (C-V), and impedance spectroscopy were conducted. $TeO_2$ itself melts at $732^{\circ}C$ in air but forms the $ZnTeO_3$ phase with ZnO as increasing temperature and therefore retards the densification of ZnO to $1000^{\circ}C$. The average grain size of sintered samples was at about $3{\mu}m$ and decreased with increasing Mn contents. It was found that a good varistor characteristics were developed in ZTM system sintered at $1100^{\circ}C$ (nonlinear coefficient $\alpha$ ~ 60). The results of C-V characteristics such as barrier height ($\Theta$), donor density ($N_d$), depletion layer (W), and interface state density ($N_t$) in ZTM ceramics were $4\times10^{17}cm^{-3}$, 0.7 V, 40 nm, and $1.6\times10^{12}cm^{-2}$, respectively. It will be discussed about the stability and homogeneity of grain boundaries using distribution parameter ($\alpha$) simulated with the Z(T)"-logf plots in ZTM system.