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

4族和5族金属碳化物具有耐火、极高硬度和高化学稳定性等独特的物理化学性质。因此，系统地研究这些碳化物与氢同位素之间的相互作用可能会导致处理氢同位素的新功能材料的发展。从这些角度出发，对这些碳化物的吸氢、解吸、扩散和渗透进行了研究。在温度为773 ~ 973 K， H_2压力为1 Pa ~ 50 kPa的条件下，对TiC和ZrC粉末试样的吸氢和解吸进行了研究。尽管与碳化物具有相同晶体结构的TiN和ZrN不与氢发生反应，但这些碳化物的氢吸收-解吸是可逆的。碳化物与氮化物之间的这种显著差异归因于碳化物中碳空位的存在。在此条件下，TiC的平衡氢浓度为0.1 ~ 0.1 mol%， ZrC的平衡氢浓度为0.01 ~ 0.1 mol%；氢在碳化物中的溶解度对H_2压力的依赖性明显弱于Sieverts定律。并对氢在TiC中的扩散系数进行了评价。在573 ~ 773 K温度下，通过对带有碳化物层的5族金属板型试样的吸氢实验，研究了氢在5族金属碳化物中的渗透行为。碳化物层厚度从100 nm到几μm不等。如果在样品表面的碳化物层上制备了Pd涂层，则可以观察到快速的氢吸收，尽管没有Pd涂层的吸收率明显小得多。这些观察结果表明，5族金属的碳化物对氢具有相当高的渗透性，尽管它们的表面对氢气中的杂质（如水蒸气）的中毒作用很敏感。

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.