In-situ Catalytic Synthesis Of SiC Whiskers （SiC_w） And Their Reinforced Low Carbon Al₂O₃-C Refractories

Because of their excellent mechanical properties,SiC whiskers（SiC_w）are widely used in advanced refractory materials,ceramic-based reinforcement materials,grinding and polishing materials,special coatings and aerospace materials.In this thesis,SiC_wwere synthesized by in-situ catalysis using Si powder,nano carbon black,flake graphite and expanded graphite as raw materials,Al powder as antioxidant,and Co₂O₃,Cr₂O₃and Al F₃·3H₂O as catalysts,respectively.Based on the thermodynamic analysis of in-situ synthesis process,the effects of carbon source type,catalyst dosage,mole ratio of Si/C and reaction temperature on the synthesis process of SiC_w were investigated.In terms of density functional theory,the formation mechanism of in-situ catalytic synthesis of SiC_w was discussed.On this basis,SiC_w-reinforced low-carbon Al₂O₃-C refractories were prepared via in-situ catalysis method,and the effects of catalyst type on the microstructure and room-temperature mechanical properties of as-prepared low-carbon Al₂O₃-C refractories were investigated.The results indicated that:（1）The optimum process condition for the in-situ catalytic synthesis of SiC_w using Si powder,nano carbon black,flake graphite and expanded graphite as raw materials,and Co₂O₃ as catalyst was that SiC_w with the average diameter of 50 nm were synthesized at 1673 K/2 h with nano carbon black as raw material,the mole ratio of Si/C=1:1,4 wt%Co₂O₃,and 1.5%Al powder.The growth process of SiC_w was mainly controlled by VLS growth mechanism.The addition of Co₂O₃ was first converted to Co in the reaction system,and then Co reacted with Si O to form silicon cobalt alloy,and finally silicon cobalt alloy was reduced to Co under the action of CO and C.The cyclic reaction promoted the formation of SiC_w.（2）SiC_w were synthesized via in-situ catalysis using Si powder and nano carbon black as raw materials,and Cr₂O₃ as a catalyst.When the content of Cr₂O₃ was 3 wt%,lots of SiC_w with average diameter of 50-120 nm were synthesized in the samples with the mole ratio of Si/C=1:1 at 1673 K/2 h.The amount of SiC_w in the sample were better than those of synthesized with Co₂O₃.The growth process of SiC_w was mainly controlled by the VS growth mechanism.The results of thermodynamic calculations and first principles calculations showed that the interactions and charge transfer effects existed between Cr₂O₃ and CO（g）,Si O（g）molecules,which weaked the bonding strength of C-O and Si-O bonds and promoted the formation of SiC_w.Meanwhile,Cr₃C₂was formed through Cr₂O₃ reacted with CO,and then Cr₃C₂ reacted with Si O to form Cr₂O₃.The cycle reaction further promoted the formation of SiC_w.（3）SiC_w were synthesized via in-situ catalysis using Si powder and nano carbon black as raw materials,and Al F₃·3H₂O as a catalyst fired at 1273-1773 K for 2 h.When the addition of Al F₃·3H₂O was 2 wt%,the highest amount of SiC_w with the average diameter of 90 nm were synthesized in the sample with the mole ratio of Si/C=1.2:1and the content of 1.5%Al powder fired at 1673 K/2 h.The growth process of SiC_wwas mainly controlled via VS growth mechanism.The addition of Al F₃·3H₂O was superior to reacting with the generated Si O to form Si F₄.The formed Si F₄ reacted with Al to form Al F₃,which promoted the formation of SiC_w.（4）The CMOR of as-prepared SiC_w-reinforced low carbon Al₂O₃-C refractories via in-situ catalysis using slab corundum,Si powder andα-Al₂O₃ powders as the base materials,N220-type nano carbon black as the carbon source,Al powders as the antioxidant,and 0.2 wt%Co₂O₃,0.15 wt%Cr₂O₃and 0.1 wt%Al F₃·3H₂O as the catalysts were respectively 18.4 MPa,19.1 MPa and 15.9 MPa,which were higher35.3%,40.4%and 16.9%than those of the samples prepared without catalyst.