冷冻速率对冻干法制备BN多孔陶瓷显微结构的影响

Effect of freezing rate on microstructure of porous BN ceramics fabricated by a freeze-drying route

为研究利用冻干法制备氮化硼多孔陶瓷时冻干过程中冷冻速率的影响,以分析纯六方氮化硼(h-BN)为原料,分析纯氧化硼为烧结助剂,去离子水为冷冻溶剂配制浆料,再将分散均匀、稳定的浆料通过冷冻速率分别为1.03、1.77和34.7℃·min^(-1)的三种冷冻干燥方式制备成冻干坯体,最后在1 500℃的氮气气氛下保温3 h热处理后制得BN多孔陶瓷。通过观察冻干试样的显微结构以及检测烧后多孔陶瓷试样的气孔率及体积密度,探讨了冷冻速率对冰晶生长行为以及多孔材料显微结构的影响。结果表明,通过调整浆料冷冻过程中的冷冻速率,可以实现对BN多孔陶瓷材料显微结构的控制。当冷冻速率较小(1.03℃·min^(-1))时,冰晶成核的位置随机分布,得到试样的气孔分布不均匀。随着冷冻速率的增大,试样的孔结构发生变化:当冷冻速率为1.77℃·min^(-1)时,冰晶的成核速率接近冰晶的生长速率,得到分布均匀的直通孔结构,孔径约为50μm;当冷冻速率增加至34.7℃·min^(-1)时,冰晶的成核速率超过冰晶生长速率,从而得到分布均匀的竹节状孔结构,孔径细小,约为10μm。而且材料的热导率随着多孔结构的引入会大大降低。

In order to clarify the effect of freezing rate during the preparation of porous BN ceramics by a freeze-drying route,the slurries were fabricated using analytically reagent （AR） h-BN as starting material, AR B2O3 as sintering aid,and deionized water as solvent. The slurries were frozen at different freezing rates of 1.03,1.77 and 34.7 ℃·min^-1 ,and subsequently sintered in N2 for 3 h at 1 500 ℃ to produce porous BN ceramics. The microstructure of the green bodies was observed and the porosity and bulk density of the sintered porous ceramics were measured to clarify the effect of freezing rate on the growth behavior of the ice crystals and the microstructure of porous ceramics prepared by freeze-drying process. The results demonstrate that the microstructure of the resultant porous BN ceramics can be controlled by adjusting the freezing rate. When the freezing rate is 1.03℃·min^-1 ,the location of ice nucleation distributes randomly, the pores are disorderedly distributed. As the freezing rate increases,the pore structure changes. When the freezing rate increases to 1.77℃·min^-1, the ice nucleation rate was closed to the rate of ice crystal growth. The pore structure is changed to uniform aligned pore channels,the pore size is about 50 pm;after the freezing rate reaches to 34.7℃·min^-1,the ice nucleation rate is larger than the growing rate of ice crystall Homogeneous bamboo-like pores are obtaiiqed and the pore size reduces to 10 μm. Thermal conductivity of BN porous ceramics decreases because of the presence of porous structure.