REVERSIBLE DISPROPORTIONATION OF ZRCO UNDER HIGH-TEMPERATURE AND HYDROGEN PRESSURE

Chemical behavior of an intermetallic compound ZrCo with hydrogen at high temperature region was studied with respect to application utilizing the reversible hydrogenation. While ZrCo compound reacts with hydrogen to form ZrCoH(0-3) below 400 degrees C, the disproportionation reaction, 2ZrCo + H-2 --> ZrH2 + ZrCo2 occurs above 400 degrees C under hydrogen pressure higher than the equilibrium decomposition pressure of ZrCoH(0-3). X-ray diffraction analysis showed that the reaction is completed in 5 h at 500 degrees C and it takes more than 40 h at 400 degrees C. No sign of generation of ZrH2 + ZrCo2 phase was observed by the X-ray diffraction in 10 h at 400 degrees C, however. The disproportionated product completely returned to single-phase ZrCo when evacuated at or above 500 degrees C for several hours, thus this reaction is reversible. Kinetics of the disproportionation reaction was expressed by an Avrami equation R = 1 - exp{-(t/tau)(n)}. The reaction speed was expressed by an Arrhenius form. For practical application of ZrCo, the results suggest some operational precaution and limitation, and recovery treatment when the material is suspected to have lost some capacity by disproportionation reaction.