Studies On Preparation Of SiC Based Ceramic Foam Filter Material By PSD And FGC Processes

Because of its open three-dimensional network skeleton structure, which has the advantages as high porosity, pore size distribution, specific surface area, thermal conductivity, good adaptability of temperature, corrosion resistance and anti-oxidation, foam ceramic has been widely used in the field of high temperature metal melt filtering and purifying. SiC has well thermal shock resistance and chemical stability, making it the preferred material for the filter of nonferrous super alloys and ferrous alloy. Based on the comparison and summary of advanced research results of domestic and foreign about ceramic foam, the SiC ceramic foam was prepared in laboratory in this paper by the method of organic foam impregnation process (PSD) and foaming gel process (FGC), combining with the methods of organic foam surface treatment, sintering system optimization, cerium phosphate and yttrium oxide toughening. The properties of the prepared material were characterized.1) Effect of silica sol on rheological behavior of slurry in the preparing SiC foam ceramics by PSD method was investigated, and the preparation technology of slurry on the basis of the study was optimized.①Solid content of SiC size mixture was fixed at55vol%. After silica sol was added, the trend of RSH value decreases with the increase of pH value. Under the conditions of8wt%of silica sol and10.3of pH, RSH value reaches the bottom and the effect of dispersant was the best. The optimum solid to liquid ratio was fixed at55-60vol%;②The milling time-viscosity curves was shown that good thixotropy and fluidity is obtained when the milling time was fixed at60-90min.2) Treatment of organic polyurethane foam surface in the solution of NaOH, CMC, SDS and silica sol could improve the roughness of surface and coating properties.①The polyurethane foam was hydrolysised at60℃for40min in15wt%NaOH solution, which increased the roughness of hole reinforcement surface and removed the hole bandfilm effectively, simultaneously ensuring polyurethane foam elasticity. ①Soaking of CMC, SDS and silica sol could improve the hydrophilic of polyurethane foam surface effectively, enhancing the sizing performance. The best effect of improvement was achieved with35wt%of silica sol, and foam unit weight hanging pulp reached30.7g/g.③Optimized slurry discharge extrusion ratio was1/5in terms of the solid content of mixed slurry was60vol%.3) Optimization of sintering curve was formulated. The bending strength of sintered at1550℃,1650℃,1750℃and1850℃were1.826MPa,2.434MPa,3.175MPa and3.713MPa, respectively; Fracture tough-ness reaches2.21MPa-m1/2,2.77MPa-m1/2,3.23MPa-m1/2and3.56MPa-m1/2. The increasing rate of flexural strength and fracture tough-ness reached maximum at1550℃～1650℃. At1750℃, the optimized bending strength and fracture toughness curve increased18.8%and42.9%respectively compared with industrial curve. Analysis of XRD and SEM was shown that the strengthen effect of matrix is mainly due to the densification reunion of alumina, full phase of SiO2, densification effect of liquid phase and oxidation effect of SiC.4) Using CePO4as a modifier, the fracture toughness of products was improved when the flexural strength is not affected dramatically.￤ith rising temperature, hydrothermal synthesis of amorphous products of Ce1-x/3(PO4)1-X(HPO4)X.nH2O turns to m-CePO4after a series of phase transition.②Adding certain amount of CePO4powder in SiC mixed slurry at1200℃, bending strength and fracture toughness increased with the increase of sintering ending temperature, and the largest increasing rate was obtained at1300～1400℃. At the same sintering temperature, the fracture toughness increased firstly with the increass of CePO4content and then decreased after reaching the top value.③Adding110℃dry CePO4powder, the flexural strength and fracture toughness of sintered body increased with the increase of sintering ending temperature. The increasing rate of flexural strength at.1200～1400℃was largest and the largest increasing rate of fracture toughness happened at1200～1300℃. Compared with sintered powder at the same temperature, the bending strength and fracture toughness were improved. Actually, the increasing rate of bending strength of sintered body was largest at1200℃and smallest at1500℃, i.e. the higher sintering temperature caused the smaller difference about intensity; the fracture toughness got maximum increasing at1300℃.5) In the preparation of SiC ceramic foam with FGC process, the effect on the " AM-MBAM" redox system of slurry rheological properties by sodium lignosulphonate dispersant was investigated, and the effect of pH value, initiator concentration and initial temperature on the polymerization reaction efficiency were determined.①The viscosity and thixotropic slurry was in line with the requirements for injection molding with0.20-0.25wt%of sodium lignosulphonate dispersant content. In terms of slurry solid content of70vol%, zeta potential reached maximum with0.25wt%of dispersant and11.5of pH.②Gel strength was very low in Alkali system. In acid condition, it caused ceramic aggregate network deterioration and reduced the body strength. When the pH value was changed between3and10, the catalytic lag caused mild gel that was the most favor of the form of even green body.③It was difficult to trigger widespread gel reaction and curing time was long and green body strength was low because of initiator and the free base amount was too small. The high content of initiator caused fast heating speed, short reaction time and rapid solidification, which is not conducive to the slurry injection molding and is easy to damage structure of bubble network.④Effect of initial temperature on the polymerization efficiency were studied with0.3wt%initiator concentration slurry and8.23of pH. Effect of initial temperature on the rate of decomposition of initiator, monomer diffusion and growth chain cage etc. were analyzed. Combined with the body photos, the optimized initial temperature was determined.⑤The porosity rate of sintered specimen reached85.7%when stirring at35r/s. At the same time, the bulk density is increased, meeting the requirements of the filter.6) Various factors were studied for rheological properties of SiC, which used yttrium oxide and alumina as sintering agent, gelatin as gelling agent, THAM as dispersant, Tween80as foaming agent.①The effect of dispersion and viscosity reducing was achieved when the content of TMAH reached1.52wt%. The addition of alumina and yttria could make isoelectric point of SiC slurry shift to the basic direction.②The curve of viscosity shear rate was shown that the higher solid content and the higher the same shear rate viscosity was, the less flow of slurry was. When the solid content is low, the liquidity was improved with the viscosity decreased, and the strength of network of ceramic foam hole band was weaken.③The gelatin content-slurry viscosity curve was shown that the higher the content of gelatin was, the poorer viscosity curve smoothness and fluidity were.④The stirring speed had significant influence on the ceramic foam type, pore size distribution and porosity. In the mixing rate of30r/s and40r/s respectively, the foam ceramics were prepared with76%and90%of open pores,200μm and120μm of average pore size, and50～800μ m and25～400μ m of pore size distribution range.⑤The alumina and yttria can make the base of high temperature phase changed, producing large amounts of liquid phase and acicular crystals of mullite, which promoted the matrix densification sintering. With too much high of sintering temperature, the large amounts of low viscosity liquid accelerated the oxidation of SiC surface and generation of gas which is more difficult to discharge. These conditions are beneficial to the growth of coarse columnar crystals, which decreased the mechanical properties of matrix.