采用热裂解聚合物前驱体法制备出了具有竹节结构的Si B C N纳米材料。扫描电子显微镜(SEM)和透射电子显微镜(TEM)结果表明样品具有特殊的竹节状(叠杯状)形貌,电子散射能谱(EDX)证实了样品组分为Si、B、C、N。通过微区喇曼光谱仪研究了...采用热裂解聚合物前驱体法制备出了具有竹节结构的Si B C N纳米材料。扫描电子显微镜(SEM)和透射电子显微镜(TEM)结果表明样品具有特殊的竹节状(叠杯状)形貌,电子散射能谱(EDX)证实了样品组分为Si、B、C、N。通过微区喇曼光谱仪研究了样品在488nm激光激发下从84～290K的变温发射特性,在490～800nm观察到位于580,620nm附近两个较强发射峰和740nm附近一个弱的发射峰。变温实验说明相应发射峰与材料禁带中形成的杂质能级有关。展开更多
Thermal expansion behaviors of some precursor-derived amorphous Si-C-N and Si-B-C-N ceramics, which were shaped by plastic forming after crosslink, were studied. To complete the shrinkage and densification, after ther...Thermal expansion behaviors of some precursor-derived amorphous Si-C-N and Si-B-C-N ceramics, which were shaped by plastic forming after crosslink, were studied. To complete the shrinkage and densification, after thermolysis specimens were heat treated at a temperature of 1400℃ for 10 h in nitrogen atmosphere. The thermal expansion coefficient of VT50-derived amorphous Si-C-N ceramic increases from 1.98×10-6/K at 400℃ to 3.09×10-6/K at 1000℃, of NCP200-derived amorphous Si-C-N ceramic increases from 2.35×10-6/K at 400℃ to 3.45×10-6/K at 1000℃, and of T2-l-derived amorphous Si-B-C-N ceramic increases from 2.08×10-6/K at 400℃ to 3.18×10-6/K at 1000℃. No glass transition for these amorphous ceramic materials was detected, indicating that as-thermolyzed precursor-derived Si-(B-)C-N ceramic materials are amorphous solids, but not glasses.展开更多
In the past twenty years,Si-B-C-N ceramic has attracted wide attention due to its special structure and outstanding properties.The ceramic generally has an amorphous or a nano-crystalline structure,and has excellent s...In the past twenty years,Si-B-C-N ceramic has attracted wide attention due to its special structure and outstanding properties.The ceramic generally has an amorphous or a nano-crystalline structure,and has excellent structural stability,oxidation resistance,creep resistance and high-temperature mechanical properties,etc.Thus,Si-B-C-N ceramic attracts many researchers and finds potential applications in transportation,aerocraft,energy,information,microelectronics and environment,etc.Much work has been carried out on its raw materials,preparation processes,structural evolution,phase equilibrium and high-temperature properties.In recent years,many researchers focus on its new preparation methods,the preparation of dense ceramic sample with large dimensions,ceramic matrix composites reinforced by carbon fiber or SiC whisker,or components with various applications.Research on Si-B-C-N ceramic will develop our insight into the relationship between structures and properties of ceramics,and will be helpful to the development of novel high-performance ceramics.This paper reviews the preparation processes,general microstructures,mechanical,chemical,electrical and optical properties,and potential applications of Si-B-C-N ceramic,as well as its matrix composites.展开更多
Polyorganoborosilazane((B[C_(2)H_(4)-Si(CH_(3))NH]_(3))n)was synthesized via monomer route from a single-source precursor and thermolyzed at 1300℃in argon atmosphere.The as-thermolyzed Si-B-C-N ceramic was characteri...Polyorganoborosilazane((B[C_(2)H_(4)-Si(CH_(3))NH]_(3))n)was synthesized via monomer route from a single-source precursor and thermolyzed at 1300℃in argon atmosphere.The as-thermolyzed Si-B-C-N ceramic was characterized using X-ray diffraction(XRD)and Raman spectroscopy.The crystallization behavior of silicon carbide in the as-thermolyzed amorphous Si-B-C-N matrix was understood by XRD studies,and the crystallite size calculated using Scherrer equation was found to increase from 2 nm to 8 nm with increase in dwelling time.Concomitantly,Raman spectroscopy was used to characterize the free carbon present in the as-thermolyzed ceramic.The peak positions,intensities and full width at half maximum(FWHM)of D and G bands in the Raman spectra were used to study and understand the structural disorder of the free carbon.The G peak shift towards 1600 cm-1 indicated the decrease in cluster size of the free carbon.The cluster diameter of the free carbon calculated using TK(Tuinstra and Koenl)equation was found to decrease from 6.2 nm to 5.4 nm with increase in dwelling time,indicating increase in structural disorder.展开更多
Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved e...Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000℃ followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200℃ and dominate at 1500℃. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400℃. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si3N4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si3N4-like clusters at 1200–1400℃. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si3N4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si3N4-like clusters as well as from the excessive oxygen to form the stable SiO2.展开更多
Superhard materials are solids whose Vickers hardness is beyond 40 GPa. They have wide applications in industry such as cutting and polishing tools, wear-resistant coatings. Most preparations of superhard materials ar...Superhard materials are solids whose Vickers hardness is beyond 40 GPa. They have wide applications in industry such as cutting and polishing tools, wear-resistant coatings. Most preparations of superhard materials are conducted under extreme pressure and temperature conditions, not only for scientific investigations, but also for the practical applications. In this paper, we would introduce the recent progress on the design and preparations of novel superhard materials, mainly on nanopolycrystalline diamond, B–C–N superhard solid solutions, and cubic-Si3N4/diamond nanocomposites prepared under ultrahigh pressure and high temperature(HPHT), using multi-anvil apparatus based on the hinged-type cubic press. Bulk materials of all these superhard phases have been successfully synthesized and are systematically tested. We emphasize that ultra-HPHT method plays an important role in the scientific research and industrial production of superhard materials. It provides the driving forces for the light elements forming novel superhard phases as well as the way for sintering high-density nanosuperhard materials.展开更多
文摘采用热裂解聚合物前驱体法制备出了具有竹节结构的Si B C N纳米材料。扫描电子显微镜(SEM)和透射电子显微镜(TEM)结果表明样品具有特殊的竹节状(叠杯状)形貌,电子散射能谱(EDX)证实了样品组分为Si、B、C、N。通过微区喇曼光谱仪研究了样品在488nm激光激发下从84～290K的变温发射特性,在490～800nm观察到位于580,620nm附近两个较强发射峰和740nm附近一个弱的发射峰。变温实验说明相应发射峰与材料禁带中形成的杂质能级有关。
文摘Thermal expansion behaviors of some precursor-derived amorphous Si-C-N and Si-B-C-N ceramics, which were shaped by plastic forming after crosslink, were studied. To complete the shrinkage and densification, after thermolysis specimens were heat treated at a temperature of 1400℃ for 10 h in nitrogen atmosphere. The thermal expansion coefficient of VT50-derived amorphous Si-C-N ceramic increases from 1.98×10-6/K at 400℃ to 3.09×10-6/K at 1000℃, of NCP200-derived amorphous Si-C-N ceramic increases from 2.35×10-6/K at 400℃ to 3.45×10-6/K at 1000℃, and of T2-l-derived amorphous Si-B-C-N ceramic increases from 2.08×10-6/K at 400℃ to 3.18×10-6/K at 1000℃. No glass transition for these amorphous ceramic materials was detected, indicating that as-thermolyzed precursor-derived Si-(B-)C-N ceramic materials are amorphous solids, but not glasses.
基金the National Natural Science Foundation of China(Nos.51072041,50902031 and 51021002)the National Science Foundation for Distinguished Young Scholars of China(No.51225203).
文摘In the past twenty years,Si-B-C-N ceramic has attracted wide attention due to its special structure and outstanding properties.The ceramic generally has an amorphous or a nano-crystalline structure,and has excellent structural stability,oxidation resistance,creep resistance and high-temperature mechanical properties,etc.Thus,Si-B-C-N ceramic attracts many researchers and finds potential applications in transportation,aerocraft,energy,information,microelectronics and environment,etc.Much work has been carried out on its raw materials,preparation processes,structural evolution,phase equilibrium and high-temperature properties.In recent years,many researchers focus on its new preparation methods,the preparation of dense ceramic sample with large dimensions,ceramic matrix composites reinforced by carbon fiber or SiC whisker,or components with various applications.Research on Si-B-C-N ceramic will develop our insight into the relationship between structures and properties of ceramics,and will be helpful to the development of novel high-performance ceramics.This paper reviews the preparation processes,general microstructures,mechanical,chemical,electrical and optical properties,and potential applications of Si-B-C-N ceramic,as well as its matrix composites.
基金We gratefully acknowledge the financial support from the Vikram Sarabhai Space Centre,Thiruvananthapuram through ISRO-IITM cell(Project No.ICSR/ISRO-IITM/MET/08-09/122/RAVK).
文摘Polyorganoborosilazane((B[C_(2)H_(4)-Si(CH_(3))NH]_(3))n)was synthesized via monomer route from a single-source precursor and thermolyzed at 1300℃in argon atmosphere.The as-thermolyzed Si-B-C-N ceramic was characterized using X-ray diffraction(XRD)and Raman spectroscopy.The crystallization behavior of silicon carbide in the as-thermolyzed amorphous Si-B-C-N matrix was understood by XRD studies,and the crystallite size calculated using Scherrer equation was found to increase from 2 nm to 8 nm with increase in dwelling time.Concomitantly,Raman spectroscopy was used to characterize the free carbon present in the as-thermolyzed ceramic.The peak positions,intensities and full width at half maximum(FWHM)of D and G bands in the Raman spectra were used to study and understand the structural disorder of the free carbon.The G peak shift towards 1600 cm-1 indicated the decrease in cluster size of the free carbon.The cluster diameter of the free carbon calculated using TK(Tuinstra and Koenl)equation was found to decrease from 6.2 nm to 5.4 nm with increase in dwelling time,indicating increase in structural disorder.
基金financially supported by National Natural Science Foundation of China (Grant Nos. 51172255 and 51532006)
文摘Nucleation behavior of amorphous Si–B–C–N ceramics derived from boron-modified polyvinylsilazane procusors was systematically investigated by transmission electron microscopy(TEM) combined with spatially-resolved electron energy-loss spectroscopy(EELS) analysis. The ceramics were pyrolyzed at1000℃ followed by further annealing in N2, and SiC nano-crystallites start to emerge at 1200℃ and dominate at 1500℃. Observed by high-angle annular dark-field imaging, bright and dark clusters were revealed as universal nano-structured features in ceramic matrices before and after nucleation, and the growth of cluster size saturated before reaching 5 nm at 1400℃. EELS analysis demonstrated the gradual development of bonding structures successively into SiC, graphetic BNCxand Si3N4 phases, as well as a constant presence of unexpected oxygen in the matrices. Furthermore, EELS profiling revealed the bright SiC clusters and less bright Si3N4-like clusters at 1200–1400℃. Since the amorphous matrix has already phase separated into SiCN and carbon clusters, another phase separation of SiCN into SiC and Si3N4-like clusters might occur by annealing to accompany their nucleation and growth, albeit one crystallized and another remained in amorphous structure. Hinderance of the cluster growth and further crystallization was owing to the formation of BNCxlayers that developed between SiC and Si3N4-like clusters as well as from the excessive oxygen to form the stable SiO2.
基金supported by the National Basic Research Program of China(2011CB808200)the National Natural Science Foundation of China(11027405)
文摘Superhard materials are solids whose Vickers hardness is beyond 40 GPa. They have wide applications in industry such as cutting and polishing tools, wear-resistant coatings. Most preparations of superhard materials are conducted under extreme pressure and temperature conditions, not only for scientific investigations, but also for the practical applications. In this paper, we would introduce the recent progress on the design and preparations of novel superhard materials, mainly on nanopolycrystalline diamond, B–C–N superhard solid solutions, and cubic-Si3N4/diamond nanocomposites prepared under ultrahigh pressure and high temperature(HPHT), using multi-anvil apparatus based on the hinged-type cubic press. Bulk materials of all these superhard phases have been successfully synthesized and are systematically tested. We emphasize that ultra-HPHT method plays an important role in the scientific research and industrial production of superhard materials. It provides the driving forces for the light elements forming novel superhard phases as well as the way for sintering high-density nanosuperhard materials.
