Ceramic membranes thin membrane fabrication

In this case study, a zirconia-alumina membrane has been developed using the sol-gel technique with and without support.6-7 The porous ceramic was prepared to fabricate the membrane support. A thin film of aluminum and zirconium were formed on the porous ceramic support. Unsupported membrane was also prepared. The unsupported membrane was not strong enough to hold a high-pressure gradient it was very fragile and not useful... 

We have successfully developed a new inorganic ceramic membrane coated with zirconium and alumina. A thin film of alumina and zirconia unsupported membrane was also fabricated. The successful method developed was the sol-gel technique. 

Dr. Hui has worked on various projects, including chemical sensors, solid oxide fuel cells, magnetic materials, gas separation membranes, nanostruc-tured materials, thin film fabrication, and protective coatings for metals. He has more than 80 research publications, one worldwide patent, and one U.S. patent (pending). He is currently leading and involved in several projects for the development of metal-supported solid oxide fuel cells (SOFCs), ceramic nanomaterials as catalyst supports for high-temperature PEM fuel cells, protective ceramic coatings on metallic substrates, ceramic electrode materials for batteries, and ceramic proton conductors. Dr. Hui is also an active member of the Electrochemical Society and the American Ceramic Society. 

The most common methods for manufacturing thin metal membranes include rolled foil, drawn tubes, and films deposited onto porous substrates (ceramic or sintered metal). Usually, electroless plating or electrolytic plating are the methods used to deposit the permselective metal onto the porous substrates although vapor deposition methods have been the subject of much research effort However, to date, vapor deposition methods have not proven to be a superior membrane fabrication method. There are pros and cons to each of these methods, but commercial membrane modules have only succeeded using rolled foil and drawn tubular membranes. 

Ceramic thin films, sensors, nanoscale materials, multi-functional ceramic composites, optical fibers, ceramic membranes and many other products can be manufactured by the sol-gel process [1-3]. The major applications of sol-gel processing are in ceramic industry for fabrication of oxide ceramics and glasses. Several studies have been reported on the preparation of supported catalysts and zeolite granular particles using the sol-gel technique [4-10]. Sol-gel derived inorganic thin films and membranes have recently attracted attentions from both academia and industry [11-13]. Only limited studies have been carried out on the sol-gel fabrication of adsorbents for industrial separation or purification purposes. 

The sol-gel process involves the transition of a system from a liquid "sol" (mostly colloidal) into a solid "gel" phase (11). By applying this methodology, it is possible to fabricate ceramic or glass materials in a wide variety of forms ultrafine or spherical-shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. 

Ceramic and semiconductor thin films have been prepared by a number of methods including chemical vapor deposition (CVD), spray-coating, and sol-gel techniques. In the present work, the sol-gel method was chosen to prepare uniform, thin films of titanium oxides on palladium Titanium oxide was chosen because of its versatility as a support material and also because the sol-gel synthesis of titania films has been clearly described by Takahashi and co-workers (22). The procedure utilized herein follows the work of Takahashi, but is modified to take advantage of the hydrogen permeability of the palladium substrate. Our objective was to develop a reliable procedure for the fabrication of thin titania films on palladium, and then to evaluate the performance of the resulting metalloceramic membranes for hydrogen transport and ethylene hydrogenation for comparison to the pure palladium membrane results. 

As yet, more work is also required to gain insight in the role of the ceramic microstructure in the performance values of membranes, and to evaluate different processing routes for the fabrication of perovskite thin films. 

The whole study of this research has been divided into two parts preparation of porous substrate and deposition of thin palladium membrane. This paper reveals only the first one, i.e. the fabrication of porous ceramic tubes by extrusion method. Early ceramic supports for palladium membrane were made of AI2O3 In recent work, we attempted to examine the properties of two kinds of ceramic materials, AI2O3 and YSZ (yittria stabilized zirconia). The extrusion of those ceramic materials was carried out by mixing with additives in various portions. After that, they were sintered at temperature between 1200 - 1450 °C in order to investigate the effect of sintering temperature on pore size and porosity of porous support. The mechanical strength was also inspected to clarify the most appropriate sintering temperature for each ceramics support. 

For ill-designed composite membranes, for example, formed by depositing palladium onto substrates which it does not wet, surface tension will force the thin film to contract and ball up if the palladium atoms acquire sufficient surface mobility. Pinholes may form as a prelude to complete de-wetting, or pinholes may remain from the initial fabrication if the palladium did not fully wet its substrate. Kinetics of de-wetting is accelerated at elevated temperature and in the presence of adsorbates such as CO, which increase surface mobility of Pd. If molten metals do not wet ceramics, they will be expelled from ceramic pores. During sintering of cermets, Pd and other metals will not adhere to the ceramic phase, if the metal and ceramic do not wet. 

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