Low-temperature Catalytic Preparation And Study Of High Temperature Properties Of ?-SiC Bonded SiC Refractories

?-SiC bonded SiC refractories(self bonded SiC refractories)are especially promising for iron and steel and non-ferrous industry because of their good mechanical properties,excellent thermal shock resistance and chemical stability.However,the traditional self bonded SiC refractory suffers the high preparation temperature and poor sinterability.In this thesis,3C-SiC(Cubic SiC,?-SiC)were firstly synthesized via in-situ catalytic method using expanded graphite and Si powders as starting materials,and Fe,Co and Ni nanoparticles as catalysts and Isobam-104 as the protective agent.The effects of reaction temperature,kinds and contents of catalysts,content of the protective agent and molar ratio of expanded graphite/Si on the synthesis of 3C-SiC were investigated.Based on the First-principles calculations,the catalytic mechanisms of Fe,Co and Ni nanoparticles and the growth mechanisms of 3C-SiC whiskers were proposed.Secondly,by using expanded graphite and tetraethyl orthosilicate(TEOS)as raw materials,and Fe,Co and Ni nanaoparticles as catalysts,the 3C-SiC and 3C-SiC/multilayer graphene composite powders were also prepared via catalytic carbothermal reduction reaction.The effects of reaction temperature,kinds and contents of catalysts and molar ratio of expanded graphite/SiO2 on the preparation of 3C-SiC and SiC/multilayer graphene composite powders were studied.The water wettability and the applications of as-prepared 3C-SiC/multilayer graphene composite powders in structural ceramics and refractories are as well as explored.Finally,self bonded SiC refractories were catalytically prepared using Fe,Co and Ni nanoparticles as catalysts,and the mechanical properties and the high temperature performance of as-prepared referactories were investigated.The results indicated that:(1)3C-SiC was catalytically synthesized with lwt%Fe,3wt%Co or Ni nanoparticles in diameter of less than 20nm at 1573K for 3h in flow Ar using expanded graphite and Si powders as starting materials;for comparison,the content of 3C-SiC was only 50wt%in the sample without any catalysts.First-principles calculations suggested that metal nanoparticals accelerated the 3C-SiC formation via weakening the bond strength in C=C dimer,and CO and SiO molecules absorbed onto the metal catalysts.(2)Under the condition of expanded graphite/SiO2 molar ratio of 3:1 and 3.5:1,the 3C-SiC and 3C-SiC/multilayer graphene composite powders were formed via catalytically carbothermal reduction method with 0.5wt%Fe,1wt%Co or Ni catalysts after 3h firing at 1673K in flow Ar.For comparison,the content of 3C-SiC was only 9wt%in the sample without any catalysts under identical conditions.When the expanded graphite was excessive,the product was the composite powders of SiC and multilayer graphene,and the thickness of the in-situ formed multilayer graphene in the composite powders was 3.5nm.As well as the water wettability of the composite powders was significantly improved comparing to expanded graphite and the properties of 3C-SiC/multilayer graphene composite ceramics and 3C-SiC/multilayer graphene bonded SiC refractories prepared by adding catalysts are better than those without any catalysts,and the multilayer graphene has no agglomeration.(3)Self-bonded SiC refractories were catalytically prepared by using black SiC grain,expanded graphite and Si powders as raw materials after 3h firing at 1473?1573K.The introduction of catalysts reduced the synthesis temperature by about 100K and promoted the formation of 3C-SiC whiskers which acted as an excellent bonding phase for self-bonded SiC refractories.The mechanical properties of as-prepared self-bonded SiC refractories with 1wt%Fe catalysts and 15wt%3C-SiC bonding phase prepared at 1573K for 3h were 2 times that of without any catalysts;and similar results were also obtained for the cases of Ni and Co catalysts.(4)The hot modulus of rupture and the modulus of elasticity of self bonded SiC refractories prepared with catalysts were twice as high as those of without catalysts.The apparent oxidation activation energy of the initial and middle stages of the specimens with catalysts was about 1.5 times as high as that without any catalysts.The residual modulus of rupture ratio of self-bonded SiC refractories with catalysts was more than 2 times higher than that without catalysts when ?T=1075K.The incorporation of catalysts not only reduced the reaction temperature but also facilitated the in-situ formation of 3C-SiC whiskers which interlinked and connected each other in the matrix of as-prepared self-bonded refractories and improved the high temperature properties of self-bonded SiC refractories.