| Composite palladium membranes for hydrogen separation had been studied widely in recent years. Current composite membrane's substrates included porous ceramics, which had low mechanical properties and thermal expansion coefficient, and porous metals which were easy to interdiffuse with palladium. As a burgeoning inorganic porous material , porous TiAl alloy had excellent material properties and potentially could be a substrate for composite palladium membrane. It was the first time in this paper to synthesize the pd/porous TiAl alloy composite membrane. The paper discussed the fabrication process of porous TiAl alloy firstly, including the choice of alloying component, sintered mode and surface treatments after sintering. Then, the pore structure properties and the corrosion resistance in hot hydrochloric acid solution of porous TiAl alloy were further investigated by various advanced testing methods. The Pd/porous TiAl alloy composite membrane was synthesized by electroless plating, and heat treatment was also carried out in vacuum atmosphere. Performances such as the structure of palladium membrane and the hydrogen separation effect were investigated thoroughly and detailedly before and after heat treatment. More details were given as follows.Porous TiAl alloy with 35wt%Al is fabricated with titanium and aluminum elemental powders by reactive synthesis. The pore structure is optimized by the confined sintering procedure. The maximum pore size decreases to 2-3μm from 8-12μm., the pore size distribution turns unimodal from bimmdal and the range of pore size distribution is more narrow after optimization.The corrosion resistance in hot hydrochloric acid solution of porous TiAl alloy discs made by unconfined sintering was investigated. The open porosity increases 7.72%, the maximum pore size increases 0.88%, the gas permeability decreases to 90%, and the surface topography changes scarcely after 50 hours cyclical corrosion at 90℃with pH=2. The stability of pore structure of porous TiAl alloy is obviously better than the one of porous Ti, porous Ni and porous 316L stainless steel. The kinetic equation for the cyclical corrosion of porous TiAl alloy at 90℃with PH=2 was determined to be△m2 =5.43×10-9·t-2.15×10-8. After 50 hours corrosion, the weight loss of TiAl alloy is only 4.9×10-4g·m-2 which is 14.75%, 5.55% and 0.29% of porous Ti, porous 316L stainless steel and porous Ni respectively. The pore structure stability of porous TiAl alloy is improved slightly when pH value changed from 2 to 3. The kinetic equation when pH=3 was determined to be△m2 =1.84×10-9·t-5.28×10(-9).The smoothness and cleanliness of porous TiAl alloy disc fabricated by the confined sintering is greatly improved after surface treatments. The palladium membrane is deposited on the surface of porous TiAl alloy substrate after surface treatments by electroless plating. The optimum processing parameters of electroless plating are that, concentration of PdCl2 is 3.5 g/L, concentration of N2H4·H2O is 0.6ml/L, concentration of Na2EDTA is 86g/L, concentration of NH3·H2O is 550ml/L, temperature of water bath is 70℃, and reaction time is 2hours. Palladium membrane obtained by this process is uniform, dense and pure, and the thickness is about 7μm.Based on the dynamic observation about the process of electroless plating and reasonable presumption, the growth model of palladium membrane is builded. Palladium particles grow to multilayer membrane under the monolayer growth model on the surface plate of porous TiAl alloy, and bridge over pores at suitable pore size.The hydrogen separation result of Pd/porous TiAl alloy composite membrane is tested by single-gas method at 350-500℃. when the pressure difference is 0.02-0.18MPa, the average hydrogen permeance ofcomposite membrane is 1.6×10-6 mol.m-2 .s-1 .Pa-1 and the H2/N2 selectivity factor is 86-102 at 500℃. The hydrogen pressure exponent for composite membrane is close to 1 and deviates from Sievert's law which indicates that the rate-controlling step of hydrogen permeation is not the bulk diffusion of atomic hydrogen but the surface processes of hydrogen. The activation energy of hydrogen permeation for the membrane prepared here is 24.78KJ/mol, and the hydrogen permeation equation is J = 6.596×10-5exp(-2.980×103/T)(Pr-Pp).Heat treatment results show that no interdiffusion exists between Pd and Ti or Al element below 5 50℃(contain) for 50hurs. The tightness of palladium membrane, the binding force both between composite membrane's interface and between palladium membrane's internal layers, and the resistance to thermal shocks is improved after heat treatment. The hydrogen separation performance of composite membrane after heated treatment at 500℃is tested, too. At 500℃and the pressure difference of 0.02-0.18MPa, the average hydrogen permeance of composite membrane is as high as 5.1×0-6 mol.m-2.s-1.Pa-1 and the H2/N2 selectivity factor is 323-400. The hydrogen pressure exponent for composite membrane is still close to 1. The activation energy of hydrogen permeation for the composite membrane here is 30.03KJ/mol, and the hydrogen permeation equation is J = 5.809×0-4dxp(-3.612×03/T)(Pr-Pp) . The hydrogen permeation flux keeps unchanged within the long operating time of 108hours at 500℃and the hydrogen pressure difference of 0.1MPa. KEY WORDS TiAl alloy, porous material, palladium membrane, composite membrane, electroless plating, hydrogen separation... |
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