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.
Electric current output or scintillation light from solid-state inorganic materials under ionizing radiation is very useful
for nuclear and radiation detection. Direct electric current measurements in semiconductors or ionic crystals provide
high resolution spectroscopy and imaging capability even though there are scalability and cost issues. In contrast,
inorganic scintillation materials utilizing photons generated by incident radiation have been developed for many decades
and provide better scalability and lower cost. Ceramic materials offer compelling advantages including large size,
mechanical strength, and homogeneity. In this work, we review current status of advanced radiation detection materials
and introduce our efforts in the development of ceramic scintillator materials, mainly for gamma ray detection.
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.
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.
Aluminum oxynitride or ALON is a transparent polycrystalline ceramic material having high strength (380 MPa) and hardness (1950 kg/mm2). The transmission range of ALON extends from 0.2 micrometer in the UV through the visible to 6.0 micrometer in the infrared. This material is made by conventional powder processing and sintering a powder compact to full density and optical transparency. Powder compacts of near net shape and size are made by conventional dry pressing, by slip casting, and by injection molding methods. This gives the material great latitude in size and shape capabilities not afforded by materials formed by single crystal growth methods. Intrinsic transparency extending from ultraviolet wavelengths (UV) to mid-infrared wavelengths (MID-IR) and low levels of optical scatter have been achieved. In this paper recent measurements of the spectral dependence of forward optical scatter, the spectral emittance from room temperature to 1200 degrees Celsius, and the index of refraction (n) of ALON are presented. Literature values for the changes in refractive index with temperature (dn/dT) are compared.
Aluminum oxynitride (ALON) is a polycrystalline ceramic material useful for windows, domes, and other optical elements. It is transparent in the visible to mid-wavelength infrared. Data is presented on the affects of impacts with sand and water particles on polished surfaces. Velocities up to 690 m/sec were used.
LWIR windows are exposed to harsh conditions during high speed flight. These include high speed rain drop impact, sand abrasion, corrosion, and aerothermodynamic load. With the possible exception of diamond, there are no LWIR transparent window/dome materials which can withstand these various environments. Rain erosion protective (REP) and oxide based abrasion resistant/oxidation resistant durable antireflection (DAR) coatings have been developed for LWIR applications. These coatings have demonstrated a substantial degree of raindrop impact protection (i.e., damage threshold velocities of approximately Mach 1, for 2 mm equivalent waterdrop impacts at normal incidence). The combination of REP + DAR coating have also demonstrated excellent resistance to sand abrasion in simulated flight environments. The high degree of abrasion resistance makes the DAR coatings applicable to ground based systems, using ZnS and ZnSe, windows as well. An additional advantage of the Raytheon REP + DAR combination is that they are transparent from the visible to the LWIR (8 to 12 mm), making them suitable for applications requiring broadband transparency. Furthermore, the DAR coatings have protected ZnS substrates from oxidation at temperatures up to 1000 degree(s)C. The combination of ZnS REP coating and DAR coating are ideally suited for protecting high speed LWIR missiles from rain and sand damage during captive carry, as well as protecting the dome/window from oxidation during high speed flight. Data are presented to demonstrate the rain/sand and oxidation protection provided by these coatings. The REP and DAR coatings have been scaled up to coat windows and domes for far infrared applications.
The amount and type of damage a sample receives during waterdrop impact experiments depends not only on the size and impact velocity of the waterdrop, but also on the microstructure of the underlying substrate, and its impact history. The geometry of the ring fractures resulting from single impacts is strongly affected by the morphology (i.e. grain size and orientation) of the substrate material. Furthermore, repeated impacts on or near previously impacted sites will create damage which depends not only on the morphology of the substrate material, but also on nature of the previous damage. Impact resistance refers to a previously unimpacted samples ability to withstand damage from individual waterdrop impacts. Durability refers to a samples ability to withstand extended exposures to high speed rain fields. Rain protective coatings can be applied to substrates to significantly enhance their survivability. Coating have been shown to increase a substrate's Damage Threshold Velocity (DTV) and to significantly reduce the cumulative damage that samples receive during prolonged exposures to high speed rain fields.
The development of novel, highly durable coatings useful in protecting infrared materials against abrasion and high speed raindrop impact damage is reviewed. Key materials properties including fracture toughness, elastic modulus, hardness as well as internal stress in the film and substrate are discussed. Significant levels of protection have been demonstrated by several investigators.
The biaxial modulus and residual (post deposition) stress of polycrystalline diamond (PCD) films deposited by microwave plasma CVD is determined using the bulge test technique. This method involves measuring the deflection of a circular membrane under an applied differential pressure. A calibration parameter for the bulge test is determined by evaluating the biaxial modulus of a silicon specimen standard. The film is characterized using X-Ray diffraction. Preliminary results yield a biaxial modulus value of 960 GPa for the PCD film. 1. INTRODUCFION Diamond inherently exhibits several unique physical and optical properties enabling its use in various applicalions. Its large bandgap (5. 45 evi) and covalently bonded aliphatic sp hybridized carbon atoms arranged t. eirahedrally result in an optical transparency in wavelength from 220 to 2500 nm and wavelengths above 6000 nm (beyond mid-infrared). Diamond is extremely hard highly thermally conductive and has both low coefficient of thermal expansion as well as high elastic modulus. This combination of properties results in diamond being extremely resistant to thermal shock as is characterized by the thermal shock resistance figure of merit R: (1v)nc R Ea where v equals the Poisson ratio a is the fracture strength ic is the thermal conductivity E the elastic (Young''s) modulus and a is the coefficient of thermal expansion2. Similarly the damage velocity threshold v1 for brittle materials impacted by water drops has been has been related to the
Thin films were deposited onto ZnS as a means of strengthening and toughening. Fracture strength increases of
up to 40% have been observed. These mechanical property enhancements were found to be related to the
compressive stress in the deposited films. Rain erosion resistance is also enhanced through the application of
compressively stressed thin film coatings.
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