The Effect of Metal on the Formation of Multispectral Zinc Sulfide John McCloy, *a Ralph Korenstein b a Raytheon Missile Systems, 1151 E. Hermans Road, Tucson, AZ, USA 85734 b Raytheon Company, 350 Lowell Street, Andover, MA, USA 01810 ABSTRACT A series of experiments was performed to ascertain the effects of various metals on the structure and properties of hot isostatic pressed (HIPped) chemical vapor deposited (CVD) ZnS. Samples were HIPped without metal and with Fe, Co, Ni, Cu, Pd, Ag, Pt, and Au foils. It was found that metals promote recrystallization of CVD ZnS to a greater or lesser extent. A processing temperature of 750 ºC for 16 hours was chosen to assess the effect of the metals, since HIPping without metal under these conditions does not recrystallize ZnS. Fe and Co have little or no effect on recrystallization, Ni and Au have moderate effect, and Pt, Pd, Ag, and Cu have the greatest effect. Ag and Cu, however, seem to have problems with indiffusion of the metal. Recrystallization is correlated with improvement in transmission characteristic of multispectral ZnS. “Interrupted HIP” experiments were conducted at 900 ºC for 1 hour and at 750 ºC for 2 hours to assess the relative effects of temperature and metal on the recrystallization. At 900 ºC recrystallization proceeded in the bulk even without metal, but was accelerated by certain metals. At 750 ºC, recrystallization took place only on the surface in contact with certain metals and not in the bulk. The role of contact of the metal to the ZnS surface was further explored by comparing Pt HIP experiments with foil fully in contact, foil with air gap, and sputtered metal Pt. Some possible mechanisms for the role of the metal in promoting recrystallization of ZnS are suggested. Keywords: Zinc sulfide, chemical vapor deposition, IR window, multi-spectral ZnS 1. INTRODUCTION & EXPERIMENTAL In late 1979 Aldinger and Werdecker from the German company W.C. Hereaus GmbH applied for a patent entitled “Zinc Sulfide Bodies for Optical Purposes” which was granted in the US at the end of 1981 as patent # 4,303,635. This patent described the use of a hot isostatic pressing (HIPping) step after chemical vapor deposition (CVD) of zinc sulfide to remove the absorption around 6 μm and slightly improve transmission at 0.4 to 0.5 μm, at the expense of transmission in the 0.7 to 4.0 μm region. It had been found early on by Raytheon that annealing tended to remove the 6 μm absorption and produce colorless material. 1 Around the same time period, while experimenting with HIPping, Raytheon produced a superior grade of ZnS, which was eventually trademarked as Multi-spectral® zinc sulfide. This material was highly transparent in the visible, colorless, and termed “water clear,” owing to a significant reduction of scatter in the visible region. Raytheon filed a patent application on December 29, 1980 to protect their Multi-spectral® zinc sulfide process. On July 31, 1990, after almost a decade, Willingham and Pappis of Raytheon were granted US patent # 4,944,900 for the HIP process on CVD zinc sulfide. The Raytheon process was notable over the German process in that it involved wrapping the yellow zinc sulfide (or zinc selenide) parts in platinum foil and subjecting them to a HIP at 5,000 to 30,000 psi (34 – 207 MPa) pressures. The platinum foil was originally thought to prevent contamination or chemical reaction with the container (typically graphite) and to act as a getter for outgassing agents. 2 Recently, new evidence has come to light that suggests a substantially more active role for the platinum in the HIPping process. In the current study a series of experiments was undertaken to assess the role of the platinum foil in the HIP process and explore alternative metals. Foils of Fe, Co, Ni, Cu, Pd, Ag, Pt, Au, and C (grafoil), with thicknesses 0.001-0.004,” depending on the metal, were obtained for testing. The hot isostatic press (Autoclave Engineers Model #30P) consisted of a graphite inner heater and a sample fixture. CVD ZnS coupons were carefully covered on both sides with a layer of foil. When multiple samples were placed in a run, they were separated from each other by sapphire isolators. In several * Present address: Battelle Pacific Northwest National Laboratory, john.mccloy@pnl.gov Window and Dome Technologies and Materials XI, edited by Randal W. Tustison, Proc. of SPIE Vol. 7302, 73020N · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.819461 Proc. of SPIE Vol. 7302 73020N-1