Translated Abstract
Porous silicon carbide ceramics have low density, high strength, high permeability, corrosion resistance, oxidation resistance, thermal shock resistance, and high temperature resistant, all of these advantages make it widely used in the area of high-temperature catalytic filtration and load bearing. Recrystallized silicon carbide (RSiC) ceramics have excellent high-temperature mechanical properties, good oxidation resistance, high thermal conductivity and low thermal expansion coefficient, because of no shrinkage during sintering, high purity of products and no second phase at the grain boundaries. It is one of the best candidate materials for diesel particulate filter (DPF). However, the higher sintering temperature usually between 2200oC and 2450°C, which has large energy consumption, high production cost and high equipment requirement, which limits its large scale application. It can lower the sintering temperature with the nano-sized SiC additives.
In the present study, we took the nano carbon black, carbon fiber and carbon nanotubes as solid phase carbon source, respectively, using SiO powder as vapor phase silicon source, low-dimension nano SiC/Si3N4 materials were prepared in the atmosphere of Ar/N2 by vapor-solid reaction. The effects of different carbon sources and reaction conditions on the crystal structure and microstructure of SiC/ Si3N4 were studied, and the growth process of nanomaterials was also discussed. On this basis, a approach combination of vapor-solid (VS) reaction and subsequent recrystallization sintering was developed for preparing porous silicon carbide ceramics. At first, micro-size SiC particles were used as skeleton and adding nano carbon black with different content, nano-size SiC particles were in-situ synthesized by vapor-solid reaction between silicon monoxide gas and nano carbon black. After that, pure porous SiC with enlarged neck areas were fabricated through the evaporation-condensation process of nano-size SiC. The effect of particle size, nanoparticle content and sintering conditions on the microstructure and mechanical properties of porous SiC ceramics was investigated. The final goal was to realize the control of the materials microstructure and properties and obtain the SiC ceramics with the optimum matching of the porosity and mechanical properties.
The results showed that, low-dimensional nano SiC/Si3N4 have been successfully synthesized at 1700-1750ºC under Ar/N2 atmosphere by vapor-solid reaction, using SiO powders as the silicon source, and different carbon materials as the carbon sources. In Ar atmosphere, when carbon black as the carbon source, the reaction product was 70-90 nm round granular SiC; while carbon fiber as the carbon source, the product retained the morphology of carbon fiber macroscopically, and the microstructure was composed of nano-sized SiC particles; nanoscale rods and whiskers were obtained using carbon nanotube (CNTs) as the carbon source, and the grain size increased with prolonging the holding time of the reaction. However, in N2 atmosphere, carbon black transformed into Si3N4 particles and belts after 1 h at 1750°C, and became zigzag Si3N4 belts after 3 h; using carbon fiber as the carbon source, the reaction product were Si3N4 fiber which made of columnar grains; nevertheless CNTs changed into Si3N4 belts with smooth surface and flat edges after holding at 1750°C for 3 h. The transformation from carbon to nano-sized SiC/Si3N4 reflected a shape memory effect, and the growth process was attributed to the VS mechanism;
Using micron SiC, carbon black and SiO as raw materials, the pre-sintered body containing nano-sized SiC was prepared by VS reaction at 1700 °C for 2 h in Ar atmosphere, and then recrystallized at 1850-2000°C, high purity porous SiC ceramics with controllable microstructure were successfully prepared. The results showed that, the sintering necks between SiC grains (sintering neck size/micron SiC grain size, d/d0) determined the flexural strength of porous materials. After sintered at 1950°C for 2 h , with increasing the weight content of nano-sized SiC from 10% to 20%, d/d0 increased from 73% to 99% , and the flexural strength of porous SiC increased from 45.1 MPa to 74.5 MPa. With the further increase of nano SiC to 30%, d/d0 changed little(97%), and the flexural strength reduced to 62.3 MPa. During sintering process, evaporation-condensation process of nano SiC was dominate due to its high vapor pressure. Consequently, large amount of nano SiC addition led to much mass transport to the neck area, resulted in an enlarged neck regions and therefore high strength. The pore size and porosity of porous SiC ceramics can be effectively controlled by changing the initial micron SiC particle size. With the decrease of micron SiC particle size from 14 μm to 3.5 μm, the pore size decreases and the flexural strength increases. Beside it, the flexural strength increase with the sintering temperature due to the enlarged neck regions. While under the same temperature, with increasing the holding time, the d/d0 and flexural strength of porous SiC materials increase rapidly, but after reaching the highest value, which was unchanged or slightly decreased. However when the temperature is higher than 2100°C, the partial volatilization and decomposition of the SiC resulting in a reduction in the flexural strength of the material. The 20-P3.5 sample showed optimum microstructure and properties after heating at 2000oC for 1 h in the Ar atmosphere with a pressure of 1 atm. Under this condition, the average value of the necking area was 15.91 μm2, the d/d0 was 100%, the porosity and the flexural strength of the samples were 42% and 75 MPa, respectively.
Translated Keyword
[Crystallization-process, Glass-ceramics, Heat treatment, Lithium disilicate, Orthogonal test]
Corresponding authors email
