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NANO-SIZED COMPOSITE MATERIALS WITH HIGH PERFORMANCE
Niihara N.,Choa H.Y.,Sekino T. 한국분말야금학회 1996 한국분말야금학회 학술대회논문집 Vol.1996 No.1
Ceramic based nanocomposite, in which nano-sized ceramics and metals were dispersed within matrix grains and/or at grain boundaries, were successfully fabricated in the ceramic/cerarnic and ceramic/metal composite systems such as /SiC, /, MgO/SiC, mullite/SiC, /SiC, /B, /W, /Mo, /Ni and /Mo systems. In these systems, the ceramiclceramic composites were fabricated from homogeneously mixed powders, powders with thin coatings of the second phases and amorphous precursor composite powders by usual powder metallurgical methods. The ceramiclmetal nanocomposites were prepared by combination of H2 reduction of metal oxides in the early stage of sinterings and usual powder metallurgical processes. The transmission electron microscopic observation for the /SiC nanocomposite indicated that the second phases less than 70nm were mainly located within matrix grains and the larger particles were dispersed at the grain boundaries. The similar observation was also identified for other cerarnic/ceramic and ceramiclmetal nanocornposites. The striking findings in these nanocomposites were that mechanical properties were significantly improved by the nano-sized dispersion from 5 to 10 vol% even at high temperatures. For example, the improvement in hcture strength by 2 to 5 times and in creep resistance by 2 to 4 orders was observed not only for the ceramidceramic nanocomposites but also for the ceramiclmetal nanocomposites with only 5~01%se cond phase. The newly developed silicon nitride/boron nitride nanocomposites, in which nano-sized hexagonal BN particulates with low Young's modulus and fracture strength were dispersed mainly within matrix grains, gave also the strong improvement in fracture strength and thermal shock fracture resistance. In presentation, the process-rnicro/nanostructure-properties relationship will be presented in detail. The special emphasis will be placed on the understanding of the roles of nano-sized dispersions on mechanical properties.
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.
Synthesis of well-aligned carbon nanotubes with open tips
Liu, B.C.,Lee, T.J.,Jung, S.I.,Park, C.Y.,Choa, Y.H.,Lee, C.J. Elsevier 2005 Carbon Vol.43 No.7
<P><B>Abstract</B></P><P>Large amounts of well-aligned carbon nanotubes (CNTs) with open tips have been produced by pyrolysis of iron(II) phthalocyanine. The aligned CNTs have an average length about 10μm and diameters ranging from 92 to 229nm. Some of produced CNTs showed Y-junction structure due to the self-joint growth of two neighboring CNTs. The well-aligned CNTs indicated a bamboo-shaped multiwalled structure and fairly good crystallinity. The aligned CNTs follow tip growth mechanism.</P>
Choa Y.H.,Kim B.H.,Jeong Y.K.,Chae K.W.,T.Nakayama,T. Kusunose,T.Sekino,K. Niibara 한국분말야금학회 2001 한국분말재료학회지 (KPMI) Vol.8 No.3
MgO based nanocomposite powder including ferromagnetic iron particle dispersions, which can be available for the magnetic and catalytic applications, was fabricated by the spray pyrolysis process using ultra-sonic atomizer and reduction processes. Liquid source was prepared from iron (Fe)-nitrate, as a source of Fe nano-dispersion, and magnesium (Mg)-nitrate, as a source of MgO materials, with pure water solvent. After the chamber were heated to given temperatures (500~), the mist of liquid droplets generated by ultrasonic atomizer carried into the chamber by a carrier gas of air, and the ist was decomposed into Fe-oxide and MgO nano-powder. The obtained powders were reduced by hydrogen atmosphere at 600~. The reduction behavior was investigated by thermal gravity and hygrometry. After reduction, the aggregated sub-micron Fe/MgO powders were obtained, and each aggregated powder composed of nano-sized Fe/MgO materials. By the difference of the chamber temperature, the particle size of Fe and MgO was changed in a few 10 nm levels. Also, the nano-porous Fe-MgO sub-micron powders were obtained. Through this preparation process and the evaluation of phase and microstructure, it was concluded that the Fe/MgO nanocomposite powders with high surface area and the higher coercive force were successfully fabricated.
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.
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.
Mechanical properties of Au thin film for application in MEMS/NENS using microtensile test
한승우,H.W. Lee,H.J. Lee,J.Y. Kim,J.H. Kim,C.S. Oh,S.H. Choa 한국물리학회 2006 Current Applied Physics Vol.6 No.1l
In the case of MEMS and NEMS, the mechanical properties of their materials are closely related to their performance and reliability.The measurement of the mechanical properties of thin lm for MEMS/NEMS is dicult because of the small sizes, forces, and displace-ments involved as well as the need for special techniques to prepare and handle the specimens.In general, micromechanical tests involve three main issues; specimen preparation, force application and measurement, and displace-ment or strain measurement. The microtensile testing system was developed. The grip ends of the specimen are glued to the grips attachedto a piezoelectrically actuated loading system. Strain is measured directly on the specimen with laser interferometry named ISDG.sputtering process, of which thickness is 1l m. For strain measurement the markers on the specimen was made by focused ion beam(FIB). Using the microtensile test, stressstrain curve, elastic modulus and ultimate tensile strength of Au thin lm was obtained.The results were compared with those of nanoindentation test.