Porous ultra-high temperature ceramics(UHTCs) are potential candidates as high-temperature thermal insulation materials. However, high thermal conductivity is the main obstacle to the application of porous UHTCs. In o...Porous ultra-high temperature ceramics(UHTCs) are potential candidates as high-temperature thermal insulation materials. However, high thermal conductivity is the main obstacle to the application of porous UHTCs. In order to address this problem, herein, a new method combining in-situ reaction and partial sintering has been developed for preparing porous Zr C and Hf C with low conductivity. In this process, porous Zr C and Hf C are directly obtained from ZrO2/C and HfO2/C green bodies without adding any pore-forming agents. The release of reaction gas can not only increase the porosity but also block the shrinkage. The asprepared porous Zr C and Hf C exhibit homogeneous porous microstructure with grain sizes in the range of 300–600 nm and 200–500 nm, high porosity of 68.74% and 77.82%, low room temperature thermal conductivity of 1.12 and 1.01 W·m-1 K-1, and compressive strength of 8.28 and 5.51 MPa, respectively.These features render porous Zr C and Hf C promising as light-weight thermal insulation materials for ultrahigh temperature applications. Furthermore, the feasibility of this method has been demonstrated and porous Nb C, Ta C as well as Ti C have been prepared by this method.展开更多
The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase di...The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase diagram of the Ti-C system at 0 K was obtained by elucidating formation enthalpies as a function of compositions, and their mechanical and metallic properties of titanium carbides were investigated sys- tematically. We also discussed the relation of titanium concentration to the both mechanical and metallic properties of titanium carbides. It has been found that the average valence electron density and tractil-ity improved at higher concentrations of titanium, while the degree of covalent bonding directionality decreased. To this effect, the hardness of titanium carbide decreases as the content of titanium increases. Our results indicated that the titanium content significantly affected the metallic properties of the Ti-C system.展开更多
On the basis of the experimental work of adhesion(W)data,the adhesion between transition metal car- bides and pure liquid metals which do not react with carbides is studied.In view of great scattering of the ex- perim...On the basis of the experimental work of adhesion(W)data,the adhesion between transition metal car- bides and pure liquid metals which do not react with carbides is studied.In view of great scattering of the ex- perimental values of W,a critical analysis of these results is performed.The selected W values for 9 copper/carbide systems and 6 metal/TiC systems are used to discuss the various suggestions concerning the mechanism of adhesion and to evidence the role of the valence electrons of the both carbide and metal on the interactions between metals and carbides.The interactions between a metal and a carbide are essentially metal- lic interactions,resulting from the overlapping of the valence electrons at the metal/carbide interface.展开更多
研究M C与M n+1A C n(M=Sc,Ti,V,Cr,Mn;A=Al,Si,P,S;n=1,2,3)结构的稳定性与电子特征有利于探究三元层状结构M n+1A C n稳定性的内在原因和设计新型M n+1A C n结构.第一性原理计算研究表明,M-3d与C-2p轨道间的电子转移对M C与M n+1A C ...研究M C与M n+1A C n(M=Sc,Ti,V,Cr,Mn;A=Al,Si,P,S;n=1,2,3)结构的稳定性与电子特征有利于探究三元层状结构M n+1A C n稳定性的内在原因和设计新型M n+1A C n结构.第一性原理计算研究表明,M-3d与C-2p轨道间的电子转移对M C与M n+1A C n的形成焓有较大影响.供电子能力较强的前过渡金属可以形成稳定的M C结构.计算结果显示,M C结构是缺电子体系,其趋向于与具有一定供电子能力的MA结构结合形成M n+1A C n.与M2PC和M2SC相比,M2Al C和M2Si C可以更为容易地被分离成二维M2C结构.展开更多
We have extensively explored the ground-state structure of RuC using the particle swarm optimization algorithm for crystal structural prediction. A hexagonal I：t-3m structure has been proposed ms the best candidate, ...We have extensively explored the ground-state structure of RuC using the particle swarm optimization algorithm for crystal structural prediction. A hexagonal I：t-3m structure has been proposed ms the best candidate, which is energetically more favorable than the previously proposed zinc blend structure. The R-3m-RuC possesses alternative stacking of double hexagonal close-packed Ru atom layers and C atom layers, and it is dynamically stable evidenced by the calculation of phonon dispersion. The calculated large bulk modulus, shear modulus, and elastic constant C44 reveal that it is an ultra-incompressible and hard material. The evidence of strong covalent bonding of Ru C, which plays an important role to form a hard material, is manifested by the partial densities of states analysis.展开更多
基金financially supported by the National Natural Science Foundation of China under Grant Nos. 51672064 and U1435206the Beijing Municipal Science & Technology Commission under Grant No. D161100002416001
文摘Porous ultra-high temperature ceramics(UHTCs) are potential candidates as high-temperature thermal insulation materials. However, high thermal conductivity is the main obstacle to the application of porous UHTCs. In order to address this problem, herein, a new method combining in-situ reaction and partial sintering has been developed for preparing porous Zr C and Hf C with low conductivity. In this process, porous Zr C and Hf C are directly obtained from ZrO2/C and HfO2/C green bodies without adding any pore-forming agents. The release of reaction gas can not only increase the porosity but also block the shrinkage. The asprepared porous Zr C and Hf C exhibit homogeneous porous microstructure with grain sizes in the range of 300–600 nm and 200–500 nm, high porosity of 68.74% and 77.82%, low room temperature thermal conductivity of 1.12 and 1.01 W·m-1 K-1, and compressive strength of 8.28 and 5.51 MPa, respectively.These features render porous Zr C and Hf C promising as light-weight thermal insulation materials for ultrahigh temperature applications. Furthermore, the feasibility of this method has been demonstrated and porous Nb C, Ta C as well as Ti C have been prepared by this method.
基金supported by the National Key Research and Development Program of China(Nos.2016YFB0700505,and2016YFB0701401)
文摘The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase diagram of the Ti-C system at 0 K was obtained by elucidating formation enthalpies as a function of compositions, and their mechanical and metallic properties of titanium carbides were investigated sys- tematically. We also discussed the relation of titanium concentration to the both mechanical and metallic properties of titanium carbides. It has been found that the average valence electron density and tractil-ity improved at higher concentrations of titanium, while the degree of covalent bonding directionality decreased. To this effect, the hardness of titanium carbide decreases as the content of titanium increases. Our results indicated that the titanium content significantly affected the metallic properties of the Ti-C system.
文摘On the basis of the experimental work of adhesion(W)data,the adhesion between transition metal car- bides and pure liquid metals which do not react with carbides is studied.In view of great scattering of the ex- perimental values of W,a critical analysis of these results is performed.The selected W values for 9 copper/carbide systems and 6 metal/TiC systems are used to discuss the various suggestions concerning the mechanism of adhesion and to evidence the role of the valence electrons of the both carbide and metal on the interactions between metals and carbides.The interactions between a metal and a carbide are essentially metal- lic interactions,resulting from the overlapping of the valence electrons at the metal/carbide interface.
文摘研究M C与M n+1A C n(M=Sc,Ti,V,Cr,Mn;A=Al,Si,P,S;n=1,2,3)结构的稳定性与电子特征有利于探究三元层状结构M n+1A C n稳定性的内在原因和设计新型M n+1A C n结构.第一性原理计算研究表明,M-3d与C-2p轨道间的电子转移对M C与M n+1A C n的形成焓有较大影响.供电子能力较强的前过渡金属可以形成稳定的M C结构.计算结果显示,M C结构是缺电子体系,其趋向于与具有一定供电子能力的MA结构结合形成M n+1A C n.与M2PC和M2SC相比,M2Al C和M2Si C可以更为容易地被分离成二维M2C结构.
基金Project supported by the Natural Science Basic Research Plan of Shaanxi Province of China (Grant No. 2012JQ1005)the Key Research Foundation of Baoji University of Arts and Sciences, China (Grant Nos. ZK1032, ZK11060, ZK11061, and ZK11146)the Fund from the Phytochemistry Key Laboratory of Shaanxi Province, China (Grant No. 11JS008)
文摘We have extensively explored the ground-state structure of RuC using the particle swarm optimization algorithm for crystal structural prediction. A hexagonal I：t-3m structure has been proposed ms the best candidate, which is energetically more favorable than the previously proposed zinc blend structure. The R-3m-RuC possesses alternative stacking of double hexagonal close-packed Ru atom layers and C atom layers, and it is dynamically stable evidenced by the calculation of phonon dispersion. The calculated large bulk modulus, shear modulus, and elastic constant C44 reveal that it is an ultra-incompressible and hard material. The evidence of strong covalent bonding of Ru C, which plays an important role to form a hard material, is manifested by the partial densities of states analysis.
