Calorimetrically measured enthalpies for the reaction of Hf with H(D) 2 (g)
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I.
INTRODUCTION
H A F N I U M , like Zr and Ti, has two allotropic structures, hcp and bcc, but the latter is stable only at relatively high temperatures in comparison to its stability in the Zr or Ti systems. Hf-H has not been investigated as often as the other two group 4B metal-H systems. A eutectoid transition has not as yet been observed in the Hf-H phase diagram up to 1200 K, tll although Tada t21 claims from calculations based on his pressurecomposition-temperature (p-c-T) data that it occurs at about 1000 K. Edwards and Veleckis tl~ have carried out the most thorough p-c-T investigation, and much of the existing phase diagram has been derived from their results (Figure 1). More recently, Hashino and cow o r k e r s [3,4] have measured isotherms up to 1273 K and have also employed X-ray diffraction to help in characterizing the phase diagram at elevated temperatures; in their work, there is no indication of a eutectoid reaction up to 1273 K in contrast to the contention of Tada. Hashino and co-workers emphasized that, according to their observations, there is a significant difference between H f and the other members of the group 4B metal-H systems with regard the sloping of the pressures in the two-phase coexistence region, the "plateau pressures." Their isotherms for Hf-H consist of a single plateau which is quite sloped whereas in the same temperature range, which is above the eutectoid temperatures of the Ti- and Zr-H systems, they find that these systems exhibit several plateaus which do not slope much. They found from X-ray diffraction that the (a + 6 ) / 6 boundary compositions (Figure 1) are greater than those reported by Edwards and Veleckis who equated only the horizontal pressure plateau region with the two-phase region. W. LUO, formerly Graduate Student, Department of Chemistry, University of Vermont, is Materials Engineer, Duracell Worldwide Technology Center, Needham, MA 02194. TED B. FLANAGAN, Professor of Chemistry, and J.D. CLEWLEY, Research Assistant Professor, are with the University of Vermont, Burlington, VT 05405. P. DANTZER, Director of Research, is with CNRS-URA, 0446ISMa 91405, Orsay, France. Manuscript submitted February 16, 1993. METALLURGICAL
TRANSACTIONS A
The reaction which occurs in the plateau region is 1 1 2 H2 (g) + - b-a
1 HfHa(a) ~ - b-a
HfHb(6)
[1]
a and b are the phase boundary compositions of the a and 6 phases, and these both change markedly with temperature (Figure 1). Edwards and Veleckis ~q report a value of - 6 5 . 5 k J / m o l H for reaction [1] at an elevated temperature, e . g . , 1000 K, where the 6 phase has an H content corresponding to r = 0.44 according to the results of Kimura et al.[4] No other workers have reported an enthalpy value for Reaction [1]. The primary purpose of this research is to determine AH~_,~ (=AHplat) for Reaction [1] at both moderate and high temperatures for comparison with the other m e m bers of the group 4B metals. II.
EXPERIMENTAL
The element Hf, which was employed for the calorimetry at 334 K, was obtained from Teledyn
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