Elemental Zinc Sulfide (eZnS®) Provides a Clear View for Missile Systems' Tri-mode Seekers
Raytheon's recently revived elemental zinc sulfide manufacturing capability can potentially benefit many current infrared (IR) imaging systems and, with further process improvements, ensure high quality and low cost dome solutions for Raytheon's advanced missile systems that use tri-mode seeker technology.
The requirement that missile IR domes used in advanced seekers be highly transmitting across a wide range of wavelengths while also retaining high mechanical durability is challenging for either of the two commercially available grades of zinc sulfide (ZnS). Though chemical vapor deposited zinc sulfide (CVD-ZnS) exhibits good mechanical strength and adequate transmission at mid and long wavelengths in the infrared (MWIR and LWIR), transmission at shorter wavelengths (near infrared [NIR] and visible) is insufficient for multimode applications. However, by further processing CVD-ZnS at high temperatures and pressures, the crystalline micro-structure is transformed, leading to a dramatic increase in transparency across the entire spectral range. This transformed, highly transparent grade of ZnS is referred to as Multispectral grade®. It was invented at Raytheon in 1992 (U.S. Patent No. 5126081) and is used today in high-resolution infrared imaging systems. Unfortunately, this post-CVD treatment also reduces the material's strength and hardness.
A few years later, Raytheon began work on a third variant of ZnS to reduce the environmental health and safety risks of growing ZnS from gaseous precursors like hydrogen sulfide gas. This material was coined or since it is elemental zinc sulfidee or eZnS, since it is synthesized directly from the elemental constituents zinc and sulfur. This new material has the strength and hardness of CVD-ZnS. Surprisingly, however, the transparency of eZnS is significantly higher than the previous CVD material, particularly in the near infrared (NIR). The transmittance of all three variants of ZnS is illustrated in Figure 1. Note that the narrow absorption band evident in this figure at six micrometers in wavelength does not affect either the MWIR or LWIR transmittance in eZnS. While the beneficial properties of this new material were recognized, eZnS was produced for only a brief time. However, the need for a strong, multispectral infrared material remained, and in fact intensified with the advent of multimode seekers.
Reviving the Capability
Early in 2009, a Raytheon team of scientists and engineers, including engineers who were instrumental in the original process, began the recovery of the eZnS process that was first demonstrated almost fifteen years earlier. This team designed and supervised the installation and process prove-in of a new eZnS manufacturing facility (Figure 2).
This was done to meet the urgent need for a durable, multispectral infrared transparent material. The effort was part of Raytheon's collaboration with the Aviation and Missile Research Development and Engineering Center (AMRDEC), through the Army Manufacturing Technology (ManTech) Program Office. The focus of this ManTech effort was the implementation of advanced manufacturing and process improvement methods to reduce the cost of multimode missile seeker components. An added benefit is an increased defense industry production capability for a critical infrared optical material. For this work, the Raytheon/AMRDEC team was awarded the 2011 Defense Manufacturing Excellence Award for Large Business by the National Center for Manufacturing Sciences.
The new facility began operation in the spring of 2009, adding yet another chapter to Raytheon's record of infrared materials innovation. Using a 10-factor, reduced-matrix design of experiments, the Raytheon team was able to increase the eZnS deposition rate while maintaining the high optical quality of the material, thus driving down the production cycle time for domes by over 30 percent. Elemental ZnS missile domes produced using the new process are shown in Figure 3.
ZnS dome post-processing improvements were also identified during detailed value-stream analyses. Cycle-time reduction opportunities were found in the milling, grinding and polishing processes, which, if implemented even as early as low rate initial production (LRIP), could additionally reduce dome fabrication costs. Improvements to the anti-reflective coating process to address durability of the ZnS domes in the harsh environments of Army and Air Force theaters are also underway (Figure 4).
These efforts by Raytheon demonstrate a continued commitment to advancing the state of the art in multimode missile seeker technology through advanced manufacturing methods and processes. Thus, the legacy that began over forty years ago with the deposition of the first infrared missile dome using the CVD process continues. The results of this work will provide a capability to produce affordable multimode windows and domes for the new generation of sensors for missiles, munitions and surveillance systems.
Acknowledgement: This work is supported in part by the Army Manufacturing Technology Program. The author would like to thank Anthony Haynes of AMRDEC for his support during the ManTech effort.
Approved for Public Release FN5859 (23May12)
Teresa J. Clement
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