Corrosion of polycrystalline ZnSe by alternating high voltage in a saline solution
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Corrosion of polycrystalline ZnSe by alternating high voltage in a saline solution Joseph King Department of Engineering, Harvey Mudd College, Claremont, California 91711 (Received 7 September 1990; accepted 27 December 1990) Prolonged exposure of chemical vapor deposited, polycrystalline ZnSe to high ac voltages in the presence of a 1 molal NaCl solution induces severe mechanical damage. The damage takes the form of defects which originate at the surface and grow intergranularly into the bulk with a bush-like morphology. Neither exposure to the salt solution in the absence of the high voltage nor low voltage dc electrolytic exposure produces the defects. The damage may be associated with an intergranular hydride phase or grain boundary impurity segregations present or formed during the environmental exposure.
ZnSe is an ionic solid used in applications requiring transparency in the mid-IR. Bulk polycrystalline material is typically prepared by the chemical vapor deposition from Zn and H2Se. Electron induced beam current and cathodoluminescence has previously established a grain boundary impurity segregation or a hydride phase present in this material.1 Insufficient work has been reported to determine the extent of this segregation within or between material production lots. If the segregation is universally present, it apparently has little effect on the mechanical behavior of polycrystalline ZnSe. Previous studies on the fracture behavior of CVD ZnSe report low fracture toughness levels typical of ionic solids.2'3 They further report a reduction of the fracture toughness and an increase in the slow crack growth rate resulting from mechanical testing in the presence of distilled water. This sensitivity to water under slow strain rate conditions is also typical of ionic solids. However, both studies report that crack propagation, with or without water, was found to be transgranular. The initial purpose of this investigation was to study the structural and chemical changes in polyethylene (PE) which may precede water tree initiation and growth.4 ZnSe was intended to provide an IR window for optical sampling in the attenuated total reflection configuration. In the course of this investigation, the ZnSe was exposed to an unusual combination of high voltage and a saline solution for prolonged periods. The response of the material to this aggressive environment is reported in this communication. The configuration used in this investigation is shown in Fig. 1. The housing used to contain the 1 molal NaCl solution was machined from Macor®. J. Mater. Res., Vol. 6, No. 4, Apr 1991 http://journals.cambridge.org
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1.5 mm PE Sheet
3 mm ZnSe Crystal FIG. 1. Cross-sectional view of experimental setup.
Very thin cuts 1.4 mm deep were pressed into the 1.5 mm thick PE sheet to provide water tree initiation sites. The cuts were uniform in depth and allowed the electrolyte to penetrate to within 100 ^.m of the ZnSe crystal prior to high voltage application. The ZnSe internal reflection elements, purchased t
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