Carbides of Group 4 and 5 Metals as Functional Materials for Hydrogen Energy

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Carbides of group 4 and 5 metals have unique physical and chemical properties such as refractoriness, extreme hardness and high chemical stability. Hence, systematic studies on the interactions between these carbides and hydrogen isotopes may lead to the development of new functional materials for handling of hydrogen isotopes. From these viewpoints, hydrogen absorption, desorption, diffusion and permeation were examined for these carbides in the present study.Hydrogen absorption and desorption by powder specimens of TiC and ZrC were examined in the range of temperature 773 to 973 K and H_2 pressure 1 Pa to 50 kPa. Reversible hydrogen absorption-desorption was observed for these carbides, although TiN and ZrN having the same crystal structure as carbides did not react with hydrogen. Such significant difference between carbides and nitrides was ascribed to the presence of carbon vacancy in the former. The equilibrium hydrogen concentration was 0.1-1 mol% in TiC and 0.01 - 0.1 mol% in ZrC under the present conditions; the solubility of hydrogen in the carbides showed much weaker dependence on H_2 pressure than that expected from Sieverts' law. The diffusivity of hydrogen in TiC was also evaluated.The hydrogen permeation through the carbides of group 5 metals was investigated at 573 - 773 K by examining the hydrogen absorption by sheet type specimens of group 5 metals with carbides layers. The thickness of carbide layers ranged 100 nm to several μm. Rapid hydrogen absorption was observed if Pd coating was prepared on the specimen surfaces over the carbide layers, although the absorption rate without Pd coating was significantly smaller. These observations indicated that the carbides of group 5 metals have rather high permeability for hydrogen, although their surfaces are sensitive to poisoning effects by impurities in hydrogen gas such as water vapor.