Evaluation of Electrical Contact Material Stability on Mercuric Iodide *
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EVALUATION OF ELECTRICAL CONTACT MATERIAL STABILITY ON MERCURIC IODIDE* A. Y. Cheng EG&G Energy Measurements, Santa Barbara Operations, Goleta, CA 93117 ABSTRACT Mercuric iodide detectors are leading candidates for room-temperature radiation detection applications. The inherently reactive nature of mercuric iodide limits the number of materials suitable for fabrication of electrical contacts. The theoretical stabilities of elemental contact materials on mercuric iodide were evaluated at 25°C. Additionally, the stabilities of transparent conductive compounds, for photodetector applications, were studied. Calculations were based on Gibbs free energy data, estimates and a series of hypothesized reactions with mercuric iodide. Leading candidate materials were identified and compared to experimental results.
INTRODUCTION Mercuric iodide detectors are leading candidates for room-temperature radiation detection applications. 1 The inherently reactive nature of HgI2 limits the number of materials suitable for fabrication of electrical contacts. Unfortunately, many times adhesion is promoted by a reaction between the contact material and the HgI2 . Hence, some of the desired properties of the contact material are contradictory. A theoretical evaluation of contact material stability with respect to HgI 2 can be used to predict and narrow the list of candidate contact materials before actual, time consuming, experiments are carried out. The stability of elemental contact materials, from Z= 1 to 92, at 298°K and one atmosphere pressure, were examined. Transparent, oxide-type, conductors, for use in photodetector applications, were also evaluated. THERMODYNAMIC CALCULATIONS Determination of thermodynamic stability can be accomplished by calculating the Gibbs 0 free energy of reaction, AG rxn, based on a series of hypothesized reactions of the contact material with HgI 2 . Utilizing either known or estimated values of the Gibbs free energy of formation of the possible reactants, AG~fR, and products, AG0fp, the AG.rxn can be expressed as.
ACjr,=Z VpAGp-y vRAGR(1 P
R
where vp and vR are the stoichiometric coefficients of product P and reactant R, respectively. Note that AG~f's are used instead of the chemical potentials, Wi's. for calculations at 2980 K and I
* This work was performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC08-88-NV10617. Note: By acceptance of this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a nonexclusive royalty-free license in and to any copyright covering this paper. Reference to a company or product name does not imply approval or recommendation of the product by the U.S. Department of Energy to the exclusion of others that may be suitable. Mat. Res. Soc. Symp. Proc. Vol. 302. 01993 Materials Research Society
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atm. For direct comparison of the magnitude of reactions, the equilibrium constant for each 0 reaction, K, can be calculated from the AG rxn values by: AG 0rxn = RT In(K)
(2)
0 Stable contact materials would
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