Ralph Korenstein

Anisotropy in Structural and Optical Properties of Chemical Vapor Deposited ZnS

Significant anisotropy in as-deposited CVD ZnS at several length scales has been demonstrated through investigation of structural and optical properties. Compressive strength of cylinders of CVD ZnS oriented in the growth direction is ~50% higher than cylinders taken perpendicular to the growth direction. Lattice parameter measurements of mandrel side (first-to-grow) material is ~0.4% smaller than growth side (last-to-grow) material in a cored sample representing ~500 hours of CVD growth, indicating significant strain along the growth direction. X-ray diffraction also shows evidence of preferred orientations for hexagonality which differ depending on position in the growth history. In crosssection, the cored sample shows several large bands which are correlated with different degrees of infrared absorption and BTDF scattering. However, no universal trend is found that applies to the whole length from the mandrel to the growth side regarding optical properties. The extinction in the visible and infrared is lower for measurements perpendicular to the growth axis than parallel to it, possibly due to scattering from the growth bands.

Optical properties of durable oxide coatings for infrared applications

There is a continuing need for durable and protective coatings for long wavelength infrared (LWIR) windows and domes as a result of the environmental and mechanical vulnerability of most LWIS transparent materials. Diamond coatings would be ideal except for the fact that relatively high deposition temperatures are required to deposit films having low optical absorption. Diamond-like carbon films deposited at low temperatures are typically too absorbing or highly stressed. Certain transition metal oxide films can be used successfully for many applications, are very durable and can be deposited by traditional thin film deposition methods. In this study, Y2O3, ZrO2 and HfO2 films are deposited and characterized, in particulara their absorption coefficients as a function of wavelength are derived at wavelengths in the LWIR. Durable oxide coatings are deposited over full-size LWIR windows.

Variability in Chemical Vapor Deposited Zinc Sulfide: Assessment of Legacy and International CVD ZnS Materials

Samples of CVD ZnS from the United States, Germany, Israel, and China were evaluated using x-ray diffraction, transmission and Raman spectroscopy, and biaxial flexure testing. Visible and near-infrared scattering, 6 μm absorption, and ultraviolet cut-on edge varied substantially in tested materials. Transmission cut-on (ultraviolet edge) blue-shifts with annealing and correlates with visible color but not the 6 μm absorption. Raman scattering for CVD ZnS, presented here for the first time, was similar for all ZnS tested. Crystallographic hexagonality and texture was determined and correlated qualitatively with optical scattering. All CVD ZnS tested with biaxial flexure exhibit similar fracture strength values and Weibull moduli. This survey suggests that despite over 30 years of production as an infrared window, the optical properties of CVD ZnS and their variability still defy easy explanation.

Diamond-coated ZnS for improved erosion resistance

Durable coatings are used to improve the erosion resistance of high performance optical materials such ZnS. Diamond is the hardest and stiffest of all LWIR transparent materials and would make an excellent protective coating for ZnS. Direct deposition of diamond on ZnS by microwave plasma CVD has proved to be very difficult. Atomic hydrogen used in the diamond deposition process attacks and destroys ZnS very rapidly. In order to protect ZnS during the diamond deposition process protective IR transparent interlayers were developed. These layers encapsulate the ZnS and provide a nucleating surface for diamond deposition. Two different methods of nucleating diamond on these interlayers were developed to produce fully dense diamond films several microns thick. The sand erosion resistance of diamond coated ZnS was found to improve when the diamond was deposited on patterned ZnS substrates.

Microwave mediated synthesis of ZnS spherical nanoparticles for IR optical ceramics

The existing material choice for long-wave infrared (LWIR) and semi-active laser domes is multispectral zinc sulfide (ZnS), made by chemical vapor deposition. An alternative route to make more erosion-resistant ZnS could be through hot pressing ZnS nanoparticles into small-grain material. We have attempted to produce ZnS nanoparticles both by microwave and microwave-hydrothermal methods. Microwave route produced ultrahigh purity, homogeneous, well dispersed, and uniformly spherical ZnS nanoparticles. Microwave-hydrothermal route produced equiaxed cubic-faceted nanoparticles. The powder X-ray diffraction patterns of ZnS shows the presence of broad reflections corresponding to the (1 1 1), (2 2 0), and (3 1 1) planes of the cubic crystalline ZnS material. The domain size of the particles estimated from the Debye-Scherrer formula for the main reflection (111) gives a value of 2.9 and 2.5 for the microwave and microwave-hydrothermal methods respectively.