Study On Preparation Of Porous Ceramics Made From TBA/H₂O Based Slurries By Freeze-Casting Process

This paper studied on preparation of porous ceramics by freeze-casting formingprocess of TBA/H₂O based ceramics powder suspensions, in which TBA/H₂O mixturewith different TBA concentration acted as cosolvent in slurry. The physical andproperties of TBA/H₂O binary system were studied. The liquid-solid（γls）andsolid-solid（γss）interface tension between the cosolvent and its crystals with ceramicssubstrates （polished SiC and Al₂O₃）, as well as their differential value Δγ weremeasured and calculated. Based on the research, the interaction between poresmorphology in oriented porous SiC ceramics and the interface tension wasdemonstrated. Furthermore, nano porous SiO2monolithic insulation was preparedfrom TBA/H₂O based nano SiO2powder slurry by freeze-casting and dry-pressing,subsequently.In this paper, an optical method based on Young-Good-Girifalco–Fowkesformula was used for measuring γls，γssand Δγ. The results shows that Δγ hasmaximum value with TBA concentration of0,30,60and90%, respectively, and theminimum value with TBA concentration of20,50,70and100%, respectively.By observation of microstructure of SiC porous ceramics fabricated byfreeze-casting of TBA/H₂O slurries, it was found that laminar and columnar orientedpores were obtained with different TBA concentration, and the two kinds of porescorresponding with the maximum points and the minimum points of Δγ, respectively.The reason for these phenomena was that when liquid-solid interface transfer tosolid-solid interface during freeze process, more energy were need with higher Δγ, solaminar crystals with small specific surface area were preferred to be formed, whileon the contrary, columnar pores with large specific surface area were easily to beformed with lower Δγ.Based on the former results, a novel method for fabricating nano pores SiO2monolithic insulation was presented by combined freeze-casting with dry-pressingprocess. Nano-sized SiO2powder suspensions with TBA concentration of15,20,40,70%, were selected as the slurries for freeze-casting. The microstructure of these samples showed different regulation with SiC porous ceramics due to the existing ofH+ions and the hydrolyzaion of nano-sized SiO2powders. In this case, slurries withTBA concentration of40and70%were suitable for making porous SiO2monolithwith finer pores. The slurry with TBA=40%, solid loading of13wt%, and with theaddition of2wt%glass fibers was finally chosen for preparation of SiO2porousmonolith by considering the integrative properties of finest pore morphology andsmallest volume changing during freezing process. After pressing of the freeze-castedsamples by pressure of15Mpa, a nano porous SiO2monolithic insulation wasfabricated with density of0.4g/cm3, mean pores of29.32nm, and specific surface areaof265.63m2/g, respectively. The insulation ability evaluated by back side temperaturemeasurment from is higher than Alumina silicates paper （thermal conductivity:0.08W/（m·K）（700oC）） with same thickness.