New, more durable windows and domes will be required for future 3-5 micron IR systems. Various oxide and oxynitride crystalline materials are candidates for these new requirements, including sapphire, spinel, ALON, yttria, and MgO. These materials are compared with respect to optical properties, durability, and fabrication costs. Two extrinsic properties, thermal conductivity and fracture strength, have strong effects on thermal shock resistance. Birefringent polycrystalline materials will not have adequate optical resolution for future systems.

Lanthana-doped yttria has a number of favorable intrinsic properties for infrared window applications. La203 is a sintering aid which makes possible pore-free transparent polycrystalline near-net-shape domes and windows by a unique transient second solid-phase sintering technique. This same second phase can be retained by special anneals to impart mechanical toughening by a second-phase crack-deflection mechanism. Transmission electron microscopy has been utilized to characterize crack deflection and the complex second phase nucleation and growth process. Reduced absorption coefficients have been achieved through improved processing, 0H- removal, and stoichiometry adjustments. The trade-off between mechanical and optical properties has been quantified, and a region of promising compromise identified.

Infrared transparent mullite (3A12O3•2Si02) was formed at 1250°C through densification of monolithic gels. The gels were prepared through colloidal mixing of boehmite (A100H, aluminum monohydroxide) and tetraethoxysilane (TEOS). After four hours at 1250°, monolithic gels densified to 98% of the theoretical density (TD). Densification was promoted by viscous phase deformation of silica (Si02) during sintering. The extent of densification was found to be dependent upon the extent of agglomeration of boehmite powder in suspension. Transparency in the infrared region was obtained as a result of mullite formation in the sintered product. The microstuctural evolution during densification and the concomitant spectral transmittances were characterized.

AlF3-based fluoride glasses were developed. The glass systems AlF3-YF3-RF2, AlF3-YF3-ZrF4- RF2 (RF2-alkali earth fluoride) have been studied in detail. The relationship between physical properties and chemical composition have been established. The structure of AlF3- based fluoride glasses were studied by Raman and infrared spectroscopy and new structural model has been proposed.

Large diameter, sapphire crystals are grown using the heat exchanger method (HEM). In order to address current applications of sapphire 30 kg, 25 cm diameter boules have been put into production. The feasibility of growth of (0001) orientation boules has been demonstrated. This allows for larger sapphire components for zero birefringence optics applications. The HEM has been adapted as an "investment casting" technique for growing complicated sapphire components directly from the melt.

The Edge-defined, Film-fed, Growth (EFG) process was chosen by the authors for continuous growth of 10 mil (250 micron) optical quality sapphire filament. Sapphire filament is capable of withstanding temperatures above the melting point of conventional fiber optics, while also resisting corrosion, and providing spectral transmission into the infrared range. The lowest transmission loss measured approached 10 dB/meter. While the EFG process has been used to grow continuous lengths of structural quality filament exceeding 100 feet, the longest continuous optical filament grown in this study was approximately 16 feet.

The characterization of the bulk absorption and scattering properties of sapphire, spinel, and yttria, as a function of temperature, is accomplished using a Bomem DA3.02 Fourier transform spectrometer and a heated cell. The measurements are performed between 2 μm and 20 μm and from room temperatures to 775 K. The ratio of the spectra of two samples of different thicknesses is taken to eliminate mutual surface effects and to provide a direct measure of the bulk extinction coefficient. The absorption coefficient data are used to determine parameters in a mulitphoton absorption model. The model has proven valid up to the melting temperature of the material. Therefore, an accurate means of interpolating and extrapolating the data is obtained. A comprehensive characterization of the material's absorption properties with frequency and temperature is realized.

Six advanced MWIR transmissive materials have been selected for a comprehensive investigation to measure and compare their optical and mechanical properties. The six materials are: sapphire; A1ON (5A1N*9A1203); cubic zirconia (Zr02*9.4%mo Y203); spinel (MgO*A1203) fabricated by three processes; lanthana-doped yttria (Y203*9%La203) fabricated by three processes; and yttria (Y203). The following experiments have been performed on samples of each material: -thermal variation of transmittance -thermal variation of fracture strength - color center absorption A summary of the measured data is presented for each material.

A study is reported of the changes in the mechanical properties of chemically vapour deposited ZnS produced as a result of subsequent hot isostatic pressing. Particular emphasis is placed on the microstructural modification produced and the enhancement of fracture toughness achieved. An assessment of the resistance of the material to water jet impact has also been carried out demonstrating a slight improvement in the threshold velocity for damage in the HIPped material. The results are discussed with reference to crack extension theory.

Transparent samples of CaLa2S4 have been produced with good optical imaging characteristics and no significant impurity absorption bands between the intrinsic absorption edges. However, the optical transmittance of CaLa2S4 is still not adequate due to scattering and broadband absorption. Processing studies to improve the optical transmittance have concentrated on powder sulfurization, milling, and sintering. The best optical quality samples achieved to date have been fabricated using powder consisting primarily of CaSO4 and La2O2S, milled with burundum media, and sintered at 1150°C. The rain erosion resistance of CaLa2S4 has been shown to be substantially better than that of ZnS.

Calcium lanthanum sulphide optical ceramic has been identified as a potential 8-12 μm infrared window material. However, since there is a solid solution region in the phase diagram between CaLa2S4 and La2S3 other compositions from this region may also be of interest. The most promising synthesis route, also used in the present work, appears to be that of sintering a pure sulphide powder to closed porosity followed by hot isostatic pressing to achieve full density. A mixed oxide precursor powder has been made by the evaporative decomposition of solution (EDS) synthesis route in which a mixed nitrate solution was sprayed through a hot furnace. The mixed oxide powder was then fired in an H2S containing gas to synthesise a very fine sulphide powder of a number of compositions in the CaLa2S4 - La2S3 phase diagram. The evolution of the powder synthesis and ceramic processing techniques has enabled a continued improvement in ceramic quality. For example in 1983 a dark brown CaLa2S4 ceramic partially transmitting in the visible and IR but showing extrinsic S03= and SO4= absorptions was prepared by hot pressing followed by annealing in H2S. By 1985 extrinsic absorption free CaS 45 La2S3 55 mole % material transmitting in the visible and IR but showing visible and near IR scatter has been synthesised by sintering and hot isostatic pressing. In 1986 a range of compositions in the CaLa2S4 - La2S3 phase diagram have been prepared in a similar manner.

Chalcohalide glasses, which are mixtures of chalcogenides and halides, have recently received attentions as a new family of IR-transmitting materials. A systematic study on Ge-based chalcohalide glasses such as Ge-S-I, Ge-Se-I and Ge-S-Br was performed to elucidate the effect of halogen atoms on properties. The ability to control the refractive index of these glasses together with their long IR transmittance, good water durability offer some advantages to practical applications.

The elastic modulus, hardness, and fracture energy were measured for 'simple' chalcogenide glasses (e.g., Se and As2S3) and for more complicated glasses (e.g., Ge33As12Se55 and 0.3PbSe - 0.7Ge1.5As0.5Se3). Although the more complicated glasses tended to have improved mechanical properties (i.e., higher elastic modulus and hardness), this was not always the case, i.e. fracture energy varied. Further investigation is necessary to clarify the effect of composition on mechanical properties. The stress corrosion susceptibility, n, of As2S3 infrared fibers and bulk glass was compared and shown to be similar for two different techniques of measurement; Slow crack growth measurements on bulk glass using the constant double cantilever beam determined n to be 17. Strength - stressing rate experiments on fibers determined n to be between 11 and 17 depending on processing. Fracture surface analysis of these fibers and other bulk glasses showed that we can expect different fracture characteristics for chalcogenide glasses than for silicates possibly due to the existence of viscoelastic effects during fracture.

The sol-gel process is a chemical approach to making optical materials at low temperatures. Through hydrolysis and condensation reactions, a metal alkoxide such as tetraethyl orthosilicate (TEOS) is converted largely to high surface area silica gel. The ratios of the components in the TEOS-water-alcohol solution determine the geometry of the gel preform. The preform may be a fiber, a supported thin film or a rigid monolithic shape. The bulk density of the preform is typically half that of conventional fused silica. In all cases, the microporosity is interconnected and the average pore size is generally smaller than 10 nm. Consequently, the material is transparent to visible light. In the case of fibers, an inexpensive plastic fiber can be used to establish the shape and diameter of the gel preform. Then the plastic can be sacrificed at a temperature below 600°C to leave behind a porous silica fiber. This type of fiber is being considered for light transmission over short distances.

Materials with high toughness often have fibrous microstructures or are made as fibrous composites. One possible route to new infrared-transmitting materials is through sulfide analogs of those silicates known to have fibrous structures. SiS2 itself occurs as fibrous crystals isostructural with the rare oxide polymorph, silica-W. However, it is extremely reactive with oxygen and water. An investigation is underway of compound formation in the systems CaS-SiS2, MgS-SiS2, and NaS-Al2S3-SiS2 to search for such fibrous silicate analogs as CaSiS3 (thiowollastonite), MgSiS3 (thioenstatite), CaMgSi2S6 (thiodiopside), and NaA1Si2S6 (thiojadeite). A second phase of the investigation is to construct crystal chemical analogs with less chemical reactivity: e.g. Ge for Si, rare earths for Al, and the larger alkaline earths (Ba, Sr) for the more reactive smaller ones (Ca, Mg).

Synthesis of infrared (IR) windows for use in the 8-14 µm "atmospheric window" has been investigated. Materials for this application require corrosion resistance, high hardness and strength with good thermal shock resistance. Oxides, which might have the required mechanical properties because of their strong M-0 bonds, especially when M is a light metal, absorb in this region; a more massive anion is needed.

Organometallic reagents are being examined for the low-temperature preparation of metal sulfide powders which are desired as precursors to 8-12 µm infrared-transmitting optical ceramics. The most studied system is the reaction of diethylzinc with hydrogen sulfide in toluene solution at or near room temperature. Electron microscopy and X-ray powder diffraction data show that the white ZnS product consists of 0.1 μm agglomerates with crystallite sizes of ~50 Å. The product is predominantly β-ZnS (cubic structure), which contains residual hydrocarbon due to unreacted zinc alkyl groups (determined by acid hydrolysis and gas chromatography). In order to optimize the reaction, several experimental parameters have been varied including the nature of the alkyl group on zinc, the method of addition of reagents, temperature, solvent, and concentration of reactants. The reaction has also been extended to organometallic complexes of Al and Mg and also to a mixed system of Zn and Al.

Single crystals of ZnSiP2, ZnGeP2 and ZnGeP1.8As.2 have been grown by several techniques and their electronic and optical properties compared. For ZnSiP2 there are marked absorption bands at 10 and 11.5 μm, and at 13 μm for ZnGeP2. Upon substitution of 10 mole percent of arsenic for phosphorus , the latter band is red-shifted by 0.3 μm. This composition represents the limit of substitution of arsenic for phosphorus in this structure.

There is a requirement for new materials which are transparent in the 8-12 µm range and have superior thermal shock resistance, mechanical strength, toughness and thermal stability compared to those materials which are presently available. In oxide systems, the SiAlON family of ceramics and glasses is known to have particularly good thermal shock resistance and mechanical properties, thus GeGaSP analogs of the SiAlON ceramics should be a promising system to investigate for new materials. Preliminary results of an experimental study are reported.

Experiments were conducted on the deposition of diamond films from hydrogen and methane mixtures on silicon substrates by a microwave plasma enhanced chemical vapor deposition technique. Thin films of diamond were obtained. The crystallinity of the films, as determined by electron diffraction, can be controlled and changed from polycrystalline to amorphous. The chemical vapor deposition method of growing diamonds is discussed in a historical perspective in relationship to the optical properties of natural diamonds and to CVD diamond films.

Improved fracture toughness by as much as a factor of two in infrared window materials has been observed after the deposition of thin compressive surface films by reactive ion beam techniques. The relationship between film stress, film thickness, substrate properties, and observed fracture toughness are being investigated, using Vickers indentation to determine mechanical properties and a bending plate capacitance method to independently determine film stress. Comparisons are made to a theoretical model developed by Lawn and Fuller.

The effect of point defects on absorption in transparent polycrystalline lanthana-doped yttria was investigated by measuring the absorption coefficient as a function of temperature and oxygen partial pressure P02 at two wavelengths (0.357 and 3.39 μm). The composition of the specimens was controlled by preannealing at 1400°C in appropriate oxygen pressures ranging from 10-16 to 10-5 atm. The technique used for determining the absorption coefficient consisted of measuring the transmittance of two samples of different thicknesses. The absorption at 0.357 μm showed a sharp increase with temperature beginning at about 1000°C. The magnitude of this increase was a function of P02. A broad minimum was found in the high-temperature absorption at 1400°C, ranging from 10-13 to 10-10 atm P02; this represents the stoichiometry range. Initial absorption measurements in the infrared (3.39 μm) indicated much less temperature dependence, with little or no absorption increase above 1000°C at 3 x 10-13 atm P02 and only a small increase at 2 x 10-3 atm P02. Also reported in this paper are other optical properties, including refractive index, temperature coefficient of refractive index, and scattering in lanthana-doped yttria.

The origin of scattered light, particularly near-angle scattered light, and its effect on optical resolution have been analyzed. Also, an instrument is described that can measure near-angle scatter. An application of Rayleigh and Mie scattering theories shows that a few large defects cause a high level of near-angle scatter, which leads to a loss of resolution. The degradation in resolution is severe when trying to resolve a bright object near a faint object. A relatively simple apparatus has been built that can measure near-angle scatter at angles as small as 0.03 degrees and at levels of 10-6 peak intensity.

For infrared optics the use of poly- versus monocrystalline Germanium is often questioned. Although MTF seems marginally sensitive to distinguish between both types of material, detailed transmission and absorption measurements indicate optical effects correlated with grain boundaries. In this paper the results of recent measurements on currently available material are presented. Special attention is paid to analyse optical phenomena at grain boundaries as well as their origin. Finally, a definition of "optical" monocrystalline Germanium based on crystallographic data will be given.

Oxide fibers were shown to undergo permanent deformation under a stress of 1.7 G Pa at a temperature 350°C below Tq . Similar experiments were performed on ZBLAN (ZrF4-BaF2-LaF3- AlF3-NaF)glass fibers which have a much lower Tq (268°C) than oxide glasses. These fluorozirconate fibers undergo deformation undeg relatively low stress and at temperatures ranging from room temperature to 100°C. It is suggested that the bending may well be related to the large specific volume of the fibers compared to the bulk glass.