Thermodynamic Properties of Dilute Solutions of Hydrogen in Glassy PD. 80 SI. 20
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B.H.
Kear,
B.C.
Giessen,
and M. Cohen,
243
editors
THERMODYNAMIC PROPERTIES OF DILUTE SOLUTIONS OF HYDROGEN IN GLASSY PD.
8 0 SI. 2 0*
R.S. FINOCCHIARO, C.L. TSAI AND B.C. GIESSEN Department of Chemistry and Materials Science Division, Institute of Chemical Analysis, Northeastern University, Boston, Massachusetts 02115, U.S.A. ABSTRACT Equilibrium vapor pressures of dilute solutions of hydrogen in glassy Pd. 8 0 Si. 2 0 have been measured from 10-90C and at hydrogen pressures P of 1-100 torr. Under these conditions the ratio of hydrogen to alloy, x as determined by a volumetric method, reaches a maximum value of 0.0070. Over this range of x, the system exhibits a positive deviation from Sieverts' Law; isotherms were analyzed in terms of Lacher's modified statistical mechanical theory of hydrogen in palladium. The data were used to calculate relative partial molar enthalpies, excess entropies, and excess free energies for the formation of the solid solutions. The thermodynamic properties were found to vary with hydrogen content over the composition range studied. INTRODUCTION Metallic glass systems have been the subject of intense investigation in the last decade by virtue of their interesting thermal, mechanical, magnetic, and electrical properties [1]. Recently various groups have investigated the interaction of selected metallic glass systems with hydrogen [2-6]. However, there is currently no literature on the thermodynamic properties of dilute solutions of hydrogen dissolved in glassy metal systems [7]. The present paper reports a study of the thermodynamic properties of hydrogen dissolved in glassy Pd. 8 0 Si. 2 0. EXPERIMENTAL Palladium-silicon alloys of the appropriate composition were prepared from palladium shot (99.9% pure; Strem Chemical Co.) and silicon (semiconductor grade). The alloy was prepared by arc melting on a water-cooled hearth under argon atmosphere. Typical sample masses ranged from 2-5 grams and weight losses of up to 0.15% were tolerated, corresponding to maximum compositional changes of +0.5%Si. In order to render the alloy glassy, the samples were melt-spun under vacuum in an inside-the-wheel melt spinleer [8]. The quenching rate is assumed to be of the order of 105 to 10i K sec- 1 . The ribbons obtained were typically 25pm thick, 1mm wide and I meter long. 300-500 mg of ribbons were cut into strips approximately 3 cm long and loaded into a sample chamber. The sample chamber was then attached to the low pressure Sieverts' type hydriding appratus shown in Figure 1, and pumped down to 10- 5 torr. The pressurecomposition isotherms were then determined as described in the following: *Communication No.
115 from the Institute of Chemical Analysis.
244
VOLUME
Mn
SAMPLE CHAMBER
Fig. 1.
Low pressure SievertsO type hydriding apparatus
Loading of the sample with hydrogen gas was done in several steps. First the hydrogen gas (99.995% pure by volume; Matheson Gas Co.) was loaded into the dosing volume (%l5cc); its pressure was measured using an MKS Baratron 0-100 torr differential p
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