ZnS-based materials have a long history of use as x-ray luminescent materials. ZnS was one of the first discovered scintillators and is reported to have one of the highest scintillator efficiencies. The use of ZnS for high energy luminescence has been thus far limited to thin powder screens, such as ZnS:Ag which is used for detecting alpha radiation, due to opacity to its scintillation light, primarily due to scattering. ZnS in bulk form (chemical vapor deposited, powder processed, and single crystal) has high transmission and low scattering compared to powder screens. In this paper, the performance of single crystalline ZnS is evaluated for low energy x-ray (<10 keV) imaging. For these applications, a scintillator needs to be thick enough to absorb the incoming x-rays and to provide sufficient gain, but thin enough to allow for a good spatial resolution. The scintillators also need to have a good radiation hardness, a fast decay time, and low levels of afterglow. We present a trade study which compares the calculated scintillation gain and absolute efficiency for low energy x-rays (<10 keV) comparing thin (<100 μm) ZnS to CsI:Tl, Bi<sub>4</sub>Ge<sub>3</sub>O<sub>12</sub> (BGO), and Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>:Ce (YAG:Ce). The study also gives insight into the spatial resolution of these scintillators. Further, photoluminescence (PL) and PL excitation (PLE) of several undoped ZnS single crystals is compared to their Radioluminescence (RL) spectra. It was found that the ZnS emission wavelength varies on the excitation source energy.
A large body of literature was reviewed with the aim of identifying binary and ternary systems for producing long-wave infrared transmitting glass-ceramics for window applications. Known optical and physical property data was summarized for many ternary sulfides as well as their constituent binary sulfides. Some phosphide and arsenide chalcopyrite structures were reviewed as well. Where available, data on the transmission range, energy gap, refractive index, and hardness were tabulated. Several glass-forming systems were identified containing Ga<sub>2</sub>S<sub>3</sub>, GeS<sub>2</sub>, or As<sub>2</sub>S<sub>3</sub>.
Zinc sulfide has shown unequaled utility for infrared windows that require a combination of long-wavelength infrared transparency, mechanical durability, and elevated-temperature performance. This book reviews the physical properties of chemical vapor deposited ZnS and their relationship to the CVD process that produced them. An in-depth look at the material microstructure is included, along with a discussion of the material's optical properties. Finally, because the CVD process itself is central to the development of this material, a brief history is presented.
It is often useful to obtain custom glasses that meet particular requirements of refractive index and dispersion for highend
optical design and applications. In the case of infrared glasses, limited experimental data are available due to
difficulties in processing of these glasses and also measuring refractive indices accurately. This paper proposes
methods to estimate refractive index and dispersion as a function of composition for selected infrared-transmitting
glasses. Methods for refractive index determination are reviewed and evaluated, including Gladstone-Dale, Wemple-
DiDomenico single oscillator, Optical basicity, and Lorentz-Lorenz total polarizability. Various estimates for a set of
PbO-Bi<sub>2</sub>O<sub>3</sub>-Ga<sub>2</sub>O<sub>3</sub> (heavy metal oxide) and As-S (chalcogenide) glasses will be compared with measured values of
index and dispersion.
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.
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.
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 °C 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 °C for 1 hour and at 750 °C for 2 hours to assess the relative effects of temperature and metal on the recrystallization. At 900 °C recrystallization proceeded in the bulk even without metal, but was accelerated by certain metals. At 750 °C, 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.
A model has been created based on scattering that describes the λ<sup>-2</sup> dependence of the extinction in bulk samples of
CVD ZnS. The model is a version of surface scattering from internal surfaces of layers with different refractive index.
The form of the model was inspired by observation of a lamellar nanostructure in CVD ZnS composed of alternating
layers of thickness on the order of 10 to 100 nm. The scattering model produces a family of solutions which depends on
the difference in refractive index (Δn), the layer thickness, and the roughness. Reasonable ratios of roughness to layer
thickness require Δn for CVD ZnS with higher values than can be explained solely by the Δn between sphalerite and
wurtzite phases of ZnS. Other evidence suggests a substantial oxygen component in CVD ZnS that could result in the
lower refractive index Zn(O,S) necessary for the model. Differences in transmission for CVD ZnS, elemental ZnS, and
multispectral ZnS can be explained simply by a different magnitude of Δn between the layers. Absolute transmission is
modeled satisfactorily from the band edge to 10 μm using this approach. Extracted Δn's from transmission
measurements of various samples correlate well with measured hexagonality from x-ray diffraction.
Polycrystalline Yttrium Aluminum Garnet (YAG) is being considered as an attractive material candidate for IR transparent missile domes and reconnaissance windows, due to its superior optical clarity and mechanical properties compared to the incumbent material choices. YAG possesses a very uniform index of refraction with minimal variation. Its fracture strength, hardness, and toughness also rank high among various other optically transparent materials and can be optimized further through grain size minimization. Polycrystalline YAG has been in development for several years at Raytheon for laser gain and IR transparency applications. Recent advances in optical loss characterization and optimization, scale-up efforts, and the fabrication of non-planar geometries such as hemispherical domes will be presented. In addition, the YAG material trade study conducted to date on thermal, optical, mechanical properties are discussed.
The materials community realized that zinc sulfide (ZnS) was an important optical material for infrared windows over
forty years ago. Chemical vapor deposition (CVD) quickly became the method of choice for producing large ZnS
windows and domes. In addition to the development initiated in the United States, several international efforts for
understanding the processing and properties of CVD ZnS are notable. This paper summarizes the published history of
non-U.S. CVD ZnS development including the significant efforts in the United Kingdom, the former Soviet Union,
Israel, China, and Japan.