Preparation And Properties Of Unfired SiC-Si₃N₄Composite Refractories

In this doctoral dissertation, we aimed at coping with the critical problems (e.g.,high cost, high energy consumption and unstable quality) which were raised during thepreparation process and the high temperature sintering process in the traditionalpreparation technique of the widely used Si3N4-SiC composite refractories. Quartz andrutile were used as the raw materials to synthesize the non-oxide composite powdersincluding Si3N4and TiCN. We advanced a novel unfired technology to prepare theSiC-Si3N4composite refractories using the synthesized TiCN, Si3N4and SiC as thestarting materials. The preparation parameters and the properties optimization of theunfired SiC-Si3N4composite refractories were investigated in detail.The phase transformation of quartz and rutile during the carbothermal reductionnitridation (CRN) process were studied. The optimal experimental parameters forsynthesizing high purity β-Si3N4powder were carbon content of stoichiometric content,temperature of1600°C for3h and Fe2O3content of5%. The optimum parameters forsynthesizing TiCN-Si3N4by CRN process were the mass ratio between rutile and quartzof1∶9, carbon addition of stoichiometric content and temperature of1600°C for3h.The preparation parameters and the properties optimization of the unfiredSiC-Si3N4composite refractories were studied. The unfired SiC-Si3N4refractories with50wt%Si3N4had best comprehensive properties, with density of2.31g·cm-3, roomtemperature flexural strength of7.41MPa, good cryolite erosion resistance and gooderosion wear resistance. The erosion fractal dimensions of the unfired refractoriesdecreased from1.0568to1.0105with the increase of the TiCN-Si3N4content, so theslag erosion resistance of the unfired SiC-TiCN-Si3N4refractories was improvedobviously. On the one hand, TiCN increased the viscosity of slag and reduced thepenetration of slag. On the other hand, the oxidation products of TiCN and Si3N4wereTiO2and SiO2, which could react with slag to generate a high viscosity phase. The highviscosity phase concentrated in the reaction layer between the slag and the refractory,could block the invasion of the slag effectively.The mechanism for acquiring high strength of the unfired SiC-Si3N4composite refractories was also investigated. Below150°C, the strength of the unfired refractoriescombined with silicon powder-phenolic resin was obtained through the cross-linkinghardening of phenolic resin. With the increase of temperature, the density of the unfiredrefractories decreased.600~700°C, the high temperature pyrolysis and oxidation of thephenolic resin caused the loss of the strength of the unfired refractories. At800°C, aminimum flexural strength was obtained.900~1400°C, the internal oxidation sinteringrole of the unfired refractories was strengthened, so the flexural strength increased. At1400°C, the grains in the unfired refractories interlocked and overlapped each other toform the crystallization body with high strength, so flexural strength increased.At room temperature, the high strength of the unfired refractories combined withaluminate cement-phenolic resin was obtained through the hydration of cement and thecross-linking hardening of phenolic resin. With the increase of temperature, the lowtemperature hydration mineral gradually transformed into high temperature hydrationmineral, so the strength decreased. At800°C, the hydration calcium aluminatetransformed into secondary CA and CA2completely, the cementation of cement got lostand caused the formation of the pore, so a minimum flexural strength was obtained.Above1100°C, the flexural strength of the unfired refractories increased remarkablydue to the liquid phase sintering and in-situ formation of mullite whisker, which hadeffect of in situ self-healing/self strengthening and toughening. At1400°C, hightemperature flexural strength of the unfired refractories was up to48.83MPa. We foundthe relationship between flexural strength and temperature at150~800°C and800~1600°C.The above research results can provide theoretical basis and technical support todevelop high performance unfired SiC-Si3N4refractories with low cost and independentintellectual property rights. They also have important significance for promoting theresearch and development of a new generation of high performance refractory materials.