Preparation And Thermal Stability Of Zirconia Composite Membranes

Owing to their good chemical and thermal stability as well as other propertiessuch as catalysis and permselectivity for oxygen, zirconia membranes are currentlyattracting considerable interest. Sol-gel is one of the most commonly used methods tofabricate zirconia membranes. Because the metal alkoxides is expensive, theinorganic alkoxides is often used as the raw material to prepare zirconia membranes.The phase formation at high temperature is another property of zirconia membranes,which will affect the integraty , so stabilizing agent must be added to solve thisproblem. The aim of this work is to use inorganic zirconia salt as the precursor toprepare zirconia composite membranes, meanwhile, the stability of the membranes athigh temperature will be discussed systematically. The defect-free zirconia composite membranes were prepared by sol-gel methodon the alumina substrate. In this process, zirconium nitrate and silicon dioxide wereused as the precursor and stabilizing agent respectively. The thermal stability, phasetransformation, surface morphology and pore size distribution of the compositemembranes were investigated by using thermo-gravimetric analysis and differentialthermal analysis (TG-DTA), scanning electron microscopy (SEM), transmissionelectron microscopes (TEM), X-ray diffraction (XRD) and Gas Adsorption Analyzer(BET), etc. The defect-free and smooth membranes we prepared have a narrow pore sizedistribution, the average pore size of which is about 8 nm. Zirconium nitrate was moresuitable to be the precursor than zirconium oxychloride in order to get stable zirconyloxalate sol with uniformly-distributed particles. Pure zirconia was prone to betransformed from tetragonal phase to monoclinic phase at 500～550oC, while thecrystal transformation of zirconia was prohibited by the interaction between silica andzirconia, leading to the increase in crystallization temperature of silica. Between500～900oC, only the tetragonal zirconia existed, the size of which did not changewith temperature. It is found in the experiments of gas permeation that Knudsendiffusion controls in the composite membranes, which has a weak discrimination ofhydrogen and nitrogen. As the medium-pore ceramic membranes, it can be used as thetransition layer in the supported dense membranes aimed at modifying the substratesurface in order to achieve an appropriate pore size as well as gas separation ability.