Preparation and temperature-resistance characteristics of novel dense SiAlCN ceramics

The compact green bodies, prepared via a novel solid-liquid mixing method of precursors, were successfully pyrolyzed to obtain the dense bulk SiAlCN ceramics at 1000 °C. It can be seen from their SEM that they have uniform and dense microstructure, indicating that this method can be used to prepare bulk ceramics. In order to verify that they can be used as sensor heads, their temperature-resistance characteristics and repeatability were tested. The results show that the conductive mechanism belongs to Arrhenius's Tailed-State and Extended-State in the temperature range of 500–650 °C and 650–930 °C, respectively. And it shows that SiAlCN ceramics can be used as the sensor heads for high-temperature sensors.     

Facile synthesis of a carbon-rich SiAlCN precursor and investigation of its structural evolution during the polymer-ceramic conversion process

A liquid carbon-rich SiAlCN precursor is facilely synthetized by hydrosilylation between liquid polyaluminocarbosilane (LPACS) and 1,3,5,7-tetravinyl- 1,3,5,7-tetramethylcyclotetrasilazane {[CH₃(CH₂CH₂)SiNH]₄} (TeVSZ). The structural evolution during the polymer-ceramic conversion process is investigated by various methods. The results show that the main cured mechanism is β-addition on hydrosilylation, although α-addition on hydrosilylation, polymerization of vinyl groups and dehydrocoupling reaction between N–H bonds also occur during the cured process. During the pyrolysis process, dehydrogenation and dehydrocarbonation condensation reactions, transamination reactions occur, leading to formation of a three-dimensional network inorganic structure at 400–800 °C, where part of Al–O bonds convert to Al–N bonds. Then the network inorganic structure undergoes demixing and separation into amorphous SiAlCN(O) phase, where the amorphous turbostratic free carbon phase also form at 800–1200 °C. With demixing and decomposition of the amorphous carbon-rich SiAlCN(O) phase, the crystalline β-SiC and graphitic carbon start to form at about 1400 °C, the crystalline sizes of them both enlarge with increasing temperature. However, the crystal growth of β-SiC is distinctly inhibited due to existence of the rich carbon phase, tiny amounts of Al₂O₃ and AlN. In addition, a small amount of AlN can promote the formation of α-SiC at 1800 °C.     

SiAlCN型PDC陶瓷的研究进展

综述了SiAlCN型PDC（Polymer Derived Ceramics）陶瓷的制备、性能和应用。SiAlCN陶瓷有四类制备方法：粉末混合型：聚硅氮烷陶瓷前驱体与氧化铝粉末直接混合;粉末溶解型：含铝化合物粉末溶解于聚硅氮烷前驱体溶液中;单源前驱体型：铝原子通过适当的含铝化合物接枝在聚硅氮烷主链上,生成一种单源陶瓷前驱体聚铝硅氮烷;聚合物混合型：两种聚合物即聚硅氮烷与含铝聚合物共混;然后交联裂解制备陶瓷。与无Al的Si/C和Si/C/N体系相比,SiAlCN陶瓷具有优异的抗蠕变性、更好的抗氧化性和耐腐蚀性以及更好的导热性。因此,聚合物衍生的硅铝碳氮化物(SiAlCN)陶瓷是在高温和恶劣环境中应用很有潜力的材料。     

Chemistry of a series of aluminum-modified polysilazanes: Synthesis,pyrolysis behaviour and microstructural evolution

A series of polyaluminosilazanes was synthesized by reaction of dimethylethylamine alane complex with polysilazanes. Starting from different dichlorosilanes, polysilazanes were obtained by means of ammonolysis. The latter reacted with the alane complex and led to solid polyalumino(carbo)silazanes that can be described as “custom-made” polymers since all the steps were monitored and controlled (from the choice of the molecular precursor to the polymerization and the functionalization with aluminum) in order to study the effect of the introduction of Al on the different reactive sites of the Si- backbone in the polysilazanes. Detailed information on the polymer structures were obtained by FTIR and multinuclear solid state NMR spectroscopies. The transition from the polymeric state to the ceramic inorganic state was investigated by means of solid-state NMR spectroscopy and finally the structural evolution of the final ceramics by high angle XRD, Raman spectroscopy and TEM imaging. It is noted that the polymer-derived SiAlCN ceramics are X-ray amorphous even at high temperature under nitrogen with some local crystallization. Upon further heating these materials tend to further crystallize into thermodynamically stable phases at a given chemical composition, such as SiC, Si₃N₄, free C, or AlN. Finally, the present study shows that the amorphous-to-crystalline transition is closely related to the nature and structure of the preceramic polymer.     

Microstructure and fracture toughness of SiAlCN ceramics toughened by SiCw or GNPs

Mechanical alloying and spark plasma sintering (SPS) were used to prepare dense SiAlCN ceramic and SiAlCN ceramic toughened by SiC whiskers (SiC_w) or graphene nanoplatelets (GNPs). The influences of different reinforcements on the microstructure and fracture toughness were investigated. The SiAlCN ceramic exhibited a fracture toughness of 4.4 MPa m^1/2 and the fracture characteristics of grain bridging, alternative intergranular and transgranular fracture. The fracture toughness of SiC_w/SiAlCN ceramic increased to 5.8 MPa m^1/2 and toughening mechanisms were crack deflection, SiC_w bridging and pull-out. The fracture toughness of GNP/SiAlCN ceramic increased significantly, which was up to 6.6 MPa m^1/2. GNPs played an important role in grain refinement, which resulted in the smallest grain size. Multiple toughening mechanisms, including crack deflection, crack branch, GNP bridging and pull-out could be found. The better toughening effect could be attributed to the larger specific surface area of GNPs and the appropriate interface bonding between GNPs and matrix.