THERMAL DECOMPOSITION OF THE INTERMETALLIDE HYDRIDES ZrNiH3_ x AND ZrzNiHs_ x V. Zh. Ratushnaya, V. A. Lavrenko, A. M. Kalinichenko, V. B. Stanislavskii, and V. Zh. Shemet UDC 541.124.7.128 As is known, intermetallide hydrides are widely used as hydrogen accumulators [1]. In addition, many of them are catalysts in organic synthesis reactions [2]. The catalytic activity of such catalysts is largely determined by the condition of their surface formed under the influence of liberated hydrogen. However, in the literature there is only limited data on the kinetics and mechanism of decomposition of ternary hydrides. According to [3] the Zr--Ni system is capable of accumulating a significant quantity of hydrogen with high rates of absorption and desorption of hydrogen. It has also been established that volume diffusion of hydrogen atoms influences the kinetics of this process. The purpose of this work was a comparative investigation of the thermal decomposition of zirconium- and nickel-base intermetallide hydrides with different crystalline structures. The system considered contains Zr2NiH5_ x with a tetragonal lattice and the isostructural compounds Zr-NiH t_x and ZrNiH3_ x with different forms of filling of the interstices (orthorhombic lattice). It should be noted that in the Zr--Ni--H structure there are five forms of interstices possible for hydrogen atoms and the surroundings of the interstices may consist both of similar and of different atoms in various combinations. The features of kinetic desorption of hydrogen from ZrNiH3_ x and Zr2NiH 5_x in a helium atmosphere and also the nuclear magnetic resonance spectra of these hydride systems were studied. The differential scanning calorimetry curves of the process of decomposition of the hydrides were obtained on a DuPont 1090 thermal analyzer under nonisothermal conditions with a heating rate of the hydride powders of 5 deg/min. The proton magnetic resonance spectra were recorded on an RYa-2301 radio spectrometer. The ZrNiH3_x and Zr2NiHs_ x powders were prepared in accordance with the method of [4]. Data on the composition and certain properties of the specimens is given in Table 1. The normal pulse method [5] was used for measurement of the time of spin-lattice relaxation of a proton T 1 in the 293 to 373"K temperature range. In accordance with general relationships between T 1 and the diffusion rate of hydrogen atoms [6] the data presented (Table 1) indicates significant mobility of the hydrogen atoms in the given structures. For example, in ZrNiH3_ x hydride the proton has two times of relaxation while in Zr2NiH 5_x one. Such a phenomenon, which is also observed in other hydride systems [7, 8], may be explained by the nonequivalent position of the hydrogen atoms located in different voids of the crystalline lattice. The proton magnetic resonance spectra for specimens of both hydrides (Fig. 1) forms a superposition of two lines of different width, approximately 1.65 and 4.90 G for ZrNiH3_ x and 1.7 and 5.8 G, respectively, for Zr2NiH5_ x. The presence of narrow lines in the nuclear magnetic resonance spectra is apparently related to processes of atomic exchange between mobile and locally captured hydrogen atoms. The mechanism of this phenomenon is in the stage of study at present. The different time of relaxation of protons in ZrNiH3_ x must influence the features of the kinetics of its thermal decomposition. Using the method of [9] the apparent activation energy E a and the formal order of the reaction n for various tempera- ture ranges of the process of thermal decomposition of hydrides were determined. The values of the kinetic parameters obtained from the differential scanning calorimetry curves are shown in Table 2. Although the values of the activation energy of thermal desorption of ZrNiH3_ x are comparatively close, nevertheless at lower temperatures E a is less than at higher temperatures. This has also been observed for other hydride systems [10]. It has been proposed [7, 8] that the different time of spin-lattice relax- ation is related to the two forms of movement of hydrogen atoms. The lower value of T1 corresponds to the local jump between the closely located interstitial positions of hydrogen and the higher to volume diffusion of hydrogen atoms in the lattice of the hydride. It should be noted that the character of the structure of the compound plays a very important role in the complex and Institute of Problems of Material Science, Academy of Sciences of the Ukrainian SSR. Translated from Poroshkovaya Metallurgiya, No. 2(326), pp. 69-72, February, 1990. Original article submitted July 12, 1988. 150 0038-5735/90/2902-0150512.50 9 Plenum Publishing Corporation