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Thermoelectric properties of silicon hexaboride prepared by spark plasma sintering method
KwangBoShim,KyoungHunKim,J.Matsushita,K.Niihara,KeunHoAuh,DaeWoongLee 한양대학교 세라믹연구소 2002 Journal of Ceramic Processing Research Vol.3 No.3
Silicon hexaboride is attractive for various industrial applications because of its high temperature capability, high hardness and excellent electrical conductivity, in particular it is a promising material for use as a thermoelectric semiconductor at high temperature. We have used a spark plasma sintering technique to produce silicon hexaboride ceramics. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured and the effect of additives, phase composition and microstructure on the thermoelectric properties were discussed. The approximate value of Z (figure of merit value) of the SPS specimen reached about 9.6×10-6/K at 1273 K. The thermoelectric properties (Z) of the SPS specimen are improved in comparison with the specimen by hot-pressing. The effect of the addition of lanthanum and boron on thermoelectric properties of SiB6 were also evaluated.
Promotion of grain growth in NiFe<sub>2</sub>O<sub>4</sub> by annealing with oleic acid
Kurosawa, R.,Suzuki, T.,Nakayama, T.,Suematsu, H.,Niihara, K.,Jeong, Y.K.,Kim, K.H. Elsevier 2012 CURRENT APPLIED PHYSICS Vol.12 No.suppl2
For investigation of the grain-size dependence on magnetization, NiFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles were heated at 440 <SUP>o</SUP>C, 600 <SUP>o</SUP>C, 800 <SUP>o</SUP>C, and 1000 <SUP>o</SUP>C with and without oleic acid. It was recognized that the median diameter (D<SUB>50</SUB>) increased from 5.9 to 145.9 nm by the heat treatments from 440 to 1000 <SUP>o</SUP>C. In addition, it was confirmed that NiFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles after the heat treatment with oleic acid at 600 <SUP>o</SUP>C have higher grain growth rate than that after only heat treatment without oleic acid. It was deduced that NiFe<SUB>2</SUB>O<SUB>4</SUB> was dissolved in oleic acid, so that sintering speed became fast. From these results, it was concluded that the formally observed saturation magnetization increase in heat treated NiFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles with oleic acid was caused partly by grain growth enhancement with the presence of oleic acid.
Alumina/SiC 나노복합재료에서의 잔류 열응력 완화거동에 관한 연구
Y. H. Choa,K.Niihara,T. Ohji,J. P. Singh 한국분말야금학회 2002 한국분말야금학회 학술대회논문집 Vol.2002 No.1
Plastic deformation was observed by TEM around the intragranular SiC particles in the matrix for nanocomposite system. The dislocations are generated at selected planes and there is a tendency for the dislocations to form a subgrain boundary structure with low-angel grain boundaries and networks. In this study, dislocation generated in the matrix during cooling down from sintering temperatures by the highly localized thermal stresses within and/or around SiC particles caused from the thermal expansion mismatch between matrix and SiC particle was observed. In monolithic and microcomposite system. These phenomena is closely related to the plastic relaxation of the elastic stress and strain energy associated with both thermal misfitting inclusions and creep behaviors. The plastic relaxation behavior was explained by combination of yield stress and internal stress.
Choa,Y.H.,Niihara,K. 국립경상대학교 공과대학 부설 첨단소재연구소 1998 尖端素材 Vol.8 No.-
Plastic deformation as thermal residual stress relaxation was observed by TEM around interface of intragranular SiC particles in the Al₂O₃matrix for Al₂O₃/SiC nanocompo site system. The dislocations are generated at selected planes and there is a tendency far the dislocations to form a subgrain boundary structure with low-angle grain bound aries and networks. In this experiment, dislocation generated in the Al₂O₃matrix during down from sintering temperatures by the highly localized thermal stresses within and/or around SiC particles caused from the thermal expansion mismatch between Al₂O₃matrix and SiC particle was observed. In monolithic Al₂O₃and Al₂O₃/SiC microcomposite system, however, the dislocation formation was not observed compared to Al₂O₃/SiC nanocompo site system. These phenomena is closely related to the plastic relaxation of the elastic stress and strain energy associated with both thermal misfitting inclusions and creep behaviors. The plastic relaxation behavior was explained by combination of yield stress and internal stress.
이광민,정우남,양상선,K. Niihara 한국분말야금학회 2002 한국분말재료학회지 (KPMI) Vol.9 No.1
In the present study of IGC (Inert Gas Condensation) evaporation-condensation processing study, the effects of IGC convection gas on the crystallographic structure, size and shape of tin oxide nanoparticles were investigated. In addition, the phase transformation of tin oxide nanoparticles was studied after heat treatment. IGC processing was conducted at 1000℃ for 1 hr. The mixture gas of oxygen and helium was used as a convection gas. Metastable tetragonal SnO nanoparticles were obtained at a lower convection gas pressure, whereas amorphous tin oxide nanoparticles were obtained at a higher one. The formation of amorphous phase could be explained by the rapid quenching of the vaporized atoms. The resultant nanoparticles size was about 10 nm with a rounded shape. The tin oxide nanoparticles prepared by IGC were almost transformed to the stable tetragonal SnO₂ after heat treatment.
Self-Sealing of Nanoporous Low Dielectric Constant Patterns Fabricated by Nanoimprint Lithography
Ro, Hyun Wook,Peng, Huagen,Niihara, Ken-ichi,Lee, Hae-Jeong,Lin, Eric K.,Karim, Alamgir,Gidley, David W.,Jinnai, Hiroshi,Yoon, Do Y.,Soles, Christopher L. WILEY-VCH Verlag 2008 ADVANCED MATERIALS Vol.20 No.10
<B>Graphic Abstract</B> <P>The cross-sectional TEM image shows that line-space patterns can be directly imprinted, with high fidelity, into highly porous spin-on organosilicate materials. This publication quantifies how the porosity and distribution of pores within the patterns are affected by the nanoimprint lithography processes, including evidence for a densified pattern surface. <img src='wiley_img/09359648-2008-20-10-ADMA200701994-content.gif' alt='wiley_img/09359648-2008-20-10-ADMA200701994-content'> </P>
Boron Nitride Dispersed Nanocomposites with High Thermal Shock Resistance
Kusunose, T.,Sekino, T.,Choa, Y.H.,Nakayama, T.,Niihara, K. The Korean Powder Metallurgy Institute 2001 한국분말재료학회지 (KPMI) Vol.8 No.3
The microstructure and mechanical properties of $Si_3N_4/BN $nanocomposites synthesized by chemical processing were investigated. The nanocomposites containing 15 vol% hexagonal BN (h-BN) were fabricated by hot-pressing $\alpha-Si_3N_4$powders covered with turbostratic BN (t-BN). The t-BN coating on $\alpha-Si_3N_4$particles was prepared by heating $\alpha-Si_3N_4$ particles covered with a mixture of boric acid and urea in hydrogen gas. TEM observations of this nanocomposite revealed that nano-sized h-BN particles were homogeneously dispersed within $Si_3N_4$grains as well as at grain boundaries. The strength and thermal shock resistance were significantly improved in comparison with the $Si_3N_4/BN$ microcomposites.