【作者】 何炜；

【导师】 陈初升；

【作者基本信息】 中国科学技术大学 ， 材料学， 2014， 博士

【摘要】 致密陶瓷氧分离膜有望将氧气生产成本降低30%左右,其构成的反应器集氧分离和化学反应于一体,更是具有很好的经济性。膜反应器应用不仅要求材料具有可观的氧渗透能力,而且能在高温、氧化性气氛和还原性气氛下都能保持结构和化学稳定。已有的研究表明：单相氧离子电子混合传导材料的氧渗透速率较高,但其稳定性无法满足要求。双相复合混合传导材料具有很好的稳定性,但其氧渗透速率偏低,通过将其制成非对称平板结构,减小致密氧分离层的厚度,可以提高其氧渗透能力。平板结构的一大优点是能利用日趋成熟的固体氧化物燃料电池堆的组建技术构建膜组件,便于氧分离膜技术的应用。本博士论文拟研究双相复合透氧膜的材料组成、相转化流延非对称结构平板膜制备方法、透氧性能和膜反应器应用。第一章简要介绍了陶瓷透氧膜的背景,原理以及相转化制备非对称膜工艺第二章研究了单相材料La0.8Sr0.2CrxFe1-xO3-δ(LSCrF)的稳定性和透氧性能。LSCrF在稀释的氢气气氛下失重0.5%,对应于晶格氧空位的增加。LSCrF粉体在950℃纯氢气氛下退火30h后,发生部分分解。热力学计算获得LSCrF的分解氧分压,在950℃为6.3×10-28atm。将致密片状样品LSCrF的一侧暴露在空气气氛中,另一侧采用CO吹扫,测得950℃的氧渗透速率为0.37mL·cm-2·min-1。氧渗透实验揭示该材料存在氧离子电导,950℃的氧离子电导率约为0.01S/cm,只有Zr0.84Y0.16O1.92(YSZ)的10%左右。氧渗透实验结束后,与CO接触的膜表面形成了多孔疏松层。本章还研究了由LSCrF与YSZ构成的双相复合材料,发现氧渗透速率略小于单相LSCrF,但是其稳定性得到明显改进。第三章研究了Zr0.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ (YSZ-LSCrF)非对称结构平板膜的制备方法和氧渗透性能。我们将相转化流延新方法用于YSZ-LSCrF复合材料的制备,所制得的平板膜为非对称结构,由一个厚度120gm的致密层和厚度880μm的多孔支撑层组成。将平板膜的多孔支撑层暴露在空气中,致密层用He气吹扫,在900℃时测得氧渗透速率为0.14mL·cm-2·min-1。采用CO作为吹扫气,氧渗透速率升高到0.41mL·cm-2·min-1,这是因为渗透的氧与CO反应,导致渗透侧的氧分压大幅降低,膜两侧的氧分压差增大；在致密层表面涂覆一层同质多孔层,氧渗透速率达到1.57mL·cm-2·min-1。第四章研究了基于YSZ-LSCrF非对称平板膜的甲烷部分氧化(POM)反应。采用的平板膜面积为4.5×4.5cm2,在其致密层表面涂覆了Ni/LSCrF催化剂。将膜的多孔侧暴露在空气中,在致密侧引入甲烷,与渗透的氧发生POM反应。在875℃、甲烷通入速率为30mL/min时,甲烷转化率达92.3%,CO和H2的选择性分别为92.2%和93.9%,H2/CO比例为2.0。膜反应器具有优异的POM反应性能,有望用作SOFC的前端燃料重整器,在天然气制氢和费托工艺制液体燃料中也有良好的应用前景。第五章研究了Zr0.84Y0.16O1.92-La0.8Sr0.2MnO3.δ(YSZ-LSM)双相复合平板膜的制备方法和氧渗透性能。采用相转化流延成型方法制备了非对称结构平板膜,其致密层厚度为150μm,多孔支撑体厚度为850μm。将平板膜的多孔支撑层暴露在空气中,致密层用He气吹扫,在900℃时测得氧渗透速率为0.26mL·cm-2·min-1,而采用常规双层流延方法制备的非对称平板膜的透氧速率只有0.05mL·cm-2·min-1。两种膜的透氧性能的差别很可能与其多孔支撑层的孔结构有关。两种膜的致密层厚度一样,多孔支撑层的厚度也差别不大,但是孔结构有显著差别。对于相转化法的平板膜,其支撑层含有沿厚度方向生长的指状孔,孔径为数十微米,有利于气体分子的传输。而对于常规双层流延法制备的平板膜,孔径为是10μm左右,孔的趋向随机,不利于气体分子的传输。第六章研究了Ce0.8Sm0.2O1.9-La0.8Sr0.2MnO3.δ(SDC-LSM)双相复合平板膜的制备和氧渗透性能。采用相转化流延成型方法制备了非对称结构平板膜,其致密层厚度为150μm,多孔支撑体厚度为850gm。将平板膜的多孔支撑层暴露在空气中,致密层用He气吹扫,在900。C时氧渗透速率0.19mL·cm-2·min-1,比同样温度下的YSZ-LSM要大得多(0.07mL·cm-2·min-1)。SDC-LSM非对称平板膜在中低温具有相当可观的透氧速率,在纯氧制备方面有良好的应用前景。第七章对本论文进行了总结,并对非对称平板膜的相转化流延制备方法的优化、结构表征、膜组件制备和应用等后续研究提出了建议。更多还原

【Abstract】 The dense ceramic oxygen-permeable membranes hold promise to reduce the oxygen production cost by30%. The chemical reactors made from the membranes also hold huge economic potential through integration of oxygen separation and chemical reactions into a single space. For successful applications, the membranes are required to possess both high oxygen permeability and satisfactory stability under stringent operation conditions. It has been found that some perovskite transition metal oxides exhibit mixed oxide ionic and electronic conductivity and thus allow oxygen to permeate through, but the stability of these single-phase membranes is problematic especially for the membrane reactor applications. It has also been found that composite membranes made from an oxygen-ionic conductor and an electronic conductor demonstrate much better stability, but their oxygen permeability is poorer. The oxygen permeation performance of the dual-phase composite membranes can be improved significantly by fabrication of the membranes into an asymmetric planar structure in which a thin and dense oxygen separation layer is mechanically supported by a thick and porous layer. This planar membrane structure has another advantage in the membrane module can be built using the existing techniques developed for construction solid oxide fuel cells. This thesis is intended to investigate the dual-phase composite membrane materials, phase-inversion tape casting and oxygen permeation property of the supported membranes and their applications in membrane reactors.Charter1introduces the background of the dense ceramic oxygen-permeable membranes. A new variant of tape-casting method involving phase-inversion is also introduced for preparation of supported ceramic membranes.In Charter2, the stability and oxygen permeability of La0.8Sr0.2CrxFeo.5O3-δ (LSCrF) and Zro.84Y0.16O1.92-Lao.8Sro.2Cr0.5FeO.503.s (YSZ-LSCrF) are investigated. LSCrF powder exposed to diluted hydrogen was found to loss a weight of only-0.5%, corresponding to the formation of oxygen vacancies in the lattice.LSCrF powder exposed to flowing concentrated hydrogen for30h was found to decompose partially. The decomposition oxygen partial pressure of LSCrF was calculated to be6.3x10-28atm from the thermodynamic data. The stability of LSCrF under an oxygen chemical potential gradient was also examined by exposing a disk-shaped dense sample to air at one side and to reducing atmosphere (CO) at the other side at elevated temperatures. A thin, porous layer was formed on the low oxygen pressure side (CO). An oxygen permeation flux of0.37mL·cm-2·min-1was observed at950℃under given air/CO gradient. The occurrence of oxygen permeation revealed the presence of mixed oxygen ionic and electronic conductivity. The oxygen ionic conductivity was estimated to be-0.01S/cm at950℃, which is less10%of YSZ. The composite membrane made of LSCrF and YSZ exhibited smaller oxygen permeabiliy than the single-phase LSCrF but the stability was significantly improved.In Charter3, phase-inversion tape casting and oxygen permeation properties of Zro.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ (YSZ-LSCrF) are investigated. A supported planar membrane was prepared by phase-inversion tape-casting technique. The as-prepared membrane consists of a dense layer of thickness120μm supported by a porous layer of thickness880μm. The membrane with this asymmetric structure showed an improved oxygen permeation performance. An oxygen permeation flux of0.14mL·cm-2·min-1was obtained at900℃by exposing the porous support layer side to air and the dense layer to flowing helium. And the oxygen permeation flux was increased to0.41mL·cm-2·min-1when CO was used as sweep gas. Modification of permeate side of the membrane with a porous YSZ-LSCrF layer further raised the oxygen permeation flux to1.57mL·cm-2·min-1.In Charter4, the applicability of the YSZ-LSCrF membrane in partial oxidation of methane (POM) reaction is examined. The POM reaction was facilitated at elevated temperature with the porous side of the membrane exposed to air and the dense side methane. The membrane used had an area of4.5x4.5cm2, and to catalyze the POM reaction, the dense side of the membrane was coated with Ni-LSCrF catalyst. At875℃and CH4feed rate of30ml/min, the CH4conversion was92.3%, CO selectivity92.2%and H2selectivity93.9%, the H2/CO ratio2.0. The membrane reactor demonstrates excellent POM performance, and thus may find applications in methane pre-reforming for solid oxide fuel cells and conversion of methane to synthesis gas (a H2and CO mixture) required for production of hydrogen and liquid fuels.In Charter5, the fabrication method and oxygen permeation properties of the supported planar membranes composite with Zr0.84Y0.16O1.92-La0.8Sr0.2Mn03-δ (YSZ-LSM) dual phase are investigated. A supported planar membrane was prepared by phase-inversion tape casting technology, a dense layer with thickness of150μm supported on a thick porous layer with thickness of850μm. When the support layer was exposed to air and the dense layer was swept by helium, an oxygen permeation flux as large as0.26mL cm-2min-1was obtained at900℃, which is much larger than the supported membrane (0.05mL cm-2 min-1) prepared by conventional tape casting of the same composition and thickness. This may be due to the gas transport properties of the two support layers, the porous layer prepared by phase-inversion tape casting contains finger-like pores along the thickness direction, with an average diameter of a few tens of micrometers, and showed higher gas transport property. The pore of the other sample had a size of10micrometers, but pores were randomly distributed, the pore paths were tortuous, also unfavorable for gas transport.In Charter6, the phase-inversion tape casting and oxygen permeation properties of Ce0.8Sm0.2O1.9-La0.8Sr0.2MnO3-δ (SDC-LSM) composite planar membrane are reported. A supported planar membrane was prepared by phase-inversion tape casting technology, a dense layer with thickness of150μm supported on a thick porous layer with thickness of850μm.When the support layer was exposed to air and the dense layer was swept by helium, an oxygen permeation flux as large as0.188mL cm-2min-1was obtained at800℃, which is much larger than the supported YSZ-LSM membrane (0.07mL·cm-2·min-1).The supported planar SDC-LSM membrane shows appreciable oxygen permeability at intermediate temperatures, thus holds promises for application in production of oxygen.In Chapter7, the summary of this thesis is presented, and further research needs are identified including the optimization of the phase-inversion tape casting, characterization of the as-formed membrane,and the fabrication of membrane module.更多还原