Low-cost Fabrication Investigation Of Mullite/Ceria-based Inorganic Ceramic Membranes

Inorganic ceramic membranes are applied in environmental protection, chemical, energy,food, and pharmaceutical industry due to their unique advantages especially such as goodhigh temperature resistance, good acid and alkali resistance and high mechanical strength.However, the applications of ceramic membranes are limited by high preparation cost, singleperformance, etc. In this dissertation work, some efforts were focused on trying to solve thehigh-cost problems during the fabrication of inorganic ceramic membranes. By usinglow-cost raw materials and sinterability-promoting additive, the preparation costs of porousmullite ceramic membrane supports and dense ceria-based ceramic membrane electrolyteswere effectively reduced. The work in this dissertation can be briefly described as thefollowing two aspects:1. Low-cost fabrication of porous mullite ceramic membrane supportsAs a classic ceramic material, mullite (3A12O3-2SiO2) is widely applied in refractorymaterials, structural ceramics and electronic industry because of its advantages, such as lowthermal expansion co-efficient, good acid and alkali resistance. Porous mullite ceramicsupports were successfully fabricated via recycling of industrial solid waste coal gangue andcheap mineral bauxite, and the dynamic sintering behaviors, phase evolution, shrinkage,porosity and pore size, gas permeation flux, microstructure and mechanical property weresystematically studied in this dissertation work. During the sintering process, secondarymullitization reaction mainly occurs from1100to1200oC, finally ends at1400oC with84.7wt.%mullite. A unique volume-expansion stage is observed at temperatures increasing from1276to1481oC. During this stage open porosity and mechanical strength increases anddecreases, respectively, with the increasing temperature. The SEM images show that mullitecrystals grow gradually with increasing temperature from rod-like into blocky-likemorphologies, which causes the volume-expansion stage. Mechanical strength of the porousmullite ceramic membrane supports is enhanced due to the interlocking structure betweenmullite crystals. Nitrogen gas flux test shows that under0.1-0.4MPa trans-membranepressure, the mullite ceramics sintered at1400oC have the highest nitrogen gas fluxes, whichis due to a high open porosity and large average pore size. Pore-forming agent is added to obtain mullite membrane supports with sufficient porosity and acceptable mechanicalstrength. The relationship between porosity and mechanical strength can be fitted using aparabolic equation KBFS(P)=173.91(1–2.97×10-3P–2.15×10-4P2). Mullite ceramicmembrane supports show good performance in the acid and alkali resistance test with masslosses of1.25and13.38g m-2after20h corrosion in acid and alkali solutions, respectively.2. Low-cost fabrication of Sm-doped CeO2ceramic thick-membrane electrolytes and theeffect of CuO doping on sinterability, mechanical and electrical propertiesCompared with traditional solid oxide fuel cell (SOFC) electrolyte yttria stabilized zirconia(YSZ), Sm-doped CeO2(SDC) materials have high oxygen–ionic conductivity inintermeadiate and low operating temperature. But high densification temperature and weakmechanical strength limit their applications. In this dissertation work, Ce0.80Sm0.20O1.9·(CuO)x(CSCO，x=0,0.5,1.0,2.0and5.0mol%, labeled as CSO, CSCO-0.5, CSCO-1, CSCO-2andCSCO-5, respectively) were prepared with CuO as the sinterability-promoting additive usingpolyvinyl alcohol (PVA) assisted combustion method, and their variations in theirsinterability, mechanical strength and electrical property were studied. XRD results show thatonly CeO2-based cubic fluorite phase is observed in CSCO powders synthesized by PVAassisted combustion method after900oC calcination, and lattice parameter increases with thedoping amount of CuO. Doping with CuO can effectively promote the densification of CSOand significantly lower the sintering temperature~500oC due to the liquid phase sinteringprocess of CSCO. Because of the enhancement of bond strength between grains makingfracture transformation from intergranular to transgranular, the mechanical strength ofCSCO-1sintered at900oC is as high as197±12MPa, even higher than that of CSO sinteredat1400oC (197±12MPa). But there is no obvious change of mechanical strength when theamounts of CuO are2and5mol%. Compared with CSO, CSCO-1has lower activationenergy and the total conductivities of CSCO-1between800-550oC improve significantly,because the smaller size of CSCO-1increases numbers of the grain boundaries and facilitatesthe movement of oxygen vacancies across the grain boundaries.