Thomas M. Hartnett

Phonon‐defect scattering in high thermal conductivity diamond films

We have investigated the thermal conductivity of large diamond samples grown by both hot filament and microwave plasma assisted chemical vapor deposition in order to study in detail the processes limiting heat conduction in this system. For samples containing nearly 100% diamond material and no apparent defects, the thermal conductivity is consistent with that expected for polycrystalline diamond with a given crystallite size. In films prepared by the hot filament technique, we observe an additional scattering of phonons near 60 K, which we attribute to either a resonant phonon‐defect interaction, or a crossover from geometrical to Rayleigh phonon‐defect scattering.

Recent advances in spinel optical ceramic

New military requirements have reinvigorated the need for transparent magnesium aluminate (MgAl2O4) spinel. Surmet has developed a process that yields high quality transparent spinel at production scale. Several issues related to the extreme requirements of processing ultrafine spinel powders are described. Transmission data is presented for a significant dataset of parts made by this process. More recently, the process has been expanded to include a capability for producing domes for the Joint Common Missile program. Domes at nominal 6” and 7” diameter have been successfully fabricated. Despite early challenges related to the forming portion of the process, a repeatable capability for these domes has been demonstrated. Several challenges remain in spinel processing in order to support additional military requirements. In particular, the strength of the material needs further improvement. Also, improvements in optical quality with regard to inclusions are needed.

Improved thermal conductivity in isotopically enriched chemical vapor deposited diamond

The thermal conductivity κ of an isotopically enriched (0.055% 13C) polycrystalline diamond plate made by chemical vapor deposition (CVD) has been measured for heat flowing in a direction either parallel (κ∥) or perpendicular (κ⊥) to the plane of the plate. The room‐temperature conductivities (κ∥=21.8 and κ⊥=26 W cm−1 K−1) are higher than for any CVD diamond previously reported, and the κ⊥ value is higher than the best gem‐quality single crystal with the natural abundance (1.1%) of 13C. Analysis of the temperature dependence of κ∥ reveals that the point‐defect scattering of phonons is in fact significantly lower than expected for the natural abundance of 13C and that it is responsible for the improved conductivity. The observed anisotropy κ∥/κ⊥=0.84 at room temperature is associated with the anisotropic grain structure.

Current and Future Development of Calcium Lanthanum Sulfide

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.

Recent advances in ALON optical ceramic

Aluminum Oxynitride (ALONTM Optical Ceramic) is a transparent ceramic material which combines transparency from the UV to the MWIR with excellent mechanical properties. ALON’s optical and mechanical properties are isotropic by virtue of its cubic crystalline structure. Consequently, ALON is transparent in its polycrystalline form and can be made by conventional powder processing techniques. This combination of properties and manufacturability make ALON suitable for a range of applications from IR windows, domes and lenses to transparent armor. The technology for producing transparent ALON was developed at Raytheon and has been transferred to Surmet Corporation where it is currently in production. Surmet is currently selling ALON into a number of military (e.g., windows and domes) and commercial (e.g., supermarket scanner windows) applications. The capability to manufacture large ALON windows for both sensor window and armor applications is in place. ALON windows up to 20x30 inches have been fabricated. In addition, the capability to shape and polish these large and curved windows is being developed and demonstrated at Surmet. Complex shapes, both hyper-hemispherical and conformal, are also under development and will be described.

Optical properties of ALON (aluminum oxynitride)

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.

Laser damage resistant anti-reflection microstructures in Raytheon ceramic YAG, sapphire, ALON, and quartz

A study of the laser induced damage threshold (LiDT) of anti-reflection (AR) microstructures (ARMs) built in the end facets of metal ion doped yttrium aluminum garnet (YAG) laser gain material, has been conducted. Test samples of undoped and ytterbium-doped polycrystalline YAG produced by Raytheon Company were processed with ARMs in one surface and subjected to standardized pulsed LiDT testing at the near-infrared (NIR) wavelength of 1064nm. As received YAG samples with a simple commercial polish were also submitted to the damage tests for comparison, along with YAG samples that were treated with a single layer thin-film AR coating designed for maximum transmission at 1064nm. Additional samples of single crystal sapphire and quartz, and polycrystalline ALONTM windows were prepared with thin-film AR coatings and ARMs textures to expand the 1064nm laser damage testing to other important NIR transmitting materials. It was found that the pulsed laser damage resistance of ARMs textured ceramic YAG windows is 11 J/cm2, a value that is 43% higher than untreated ceramic YAG windows, suggesting that ARMs fabrication removed residual sub-surface damage, a factor that has been shown to be important for increasing the damage resistance of an optic. This conclusion is also supported by the high damage threshold values found with the single layer AR coatings on ceramic YAG where the coatings may have shielded the sub-surface polishing damage. Testing results for the highly polished sapphire windows also support the notion that better surface preparation produces higher damage resistance. The damage threshold for untreated sapphire windows exceeded 32 J/cm2 for one sample with an average of 27.5 J/cm2 for the two samples tested. The ARMs-treated sapphire windows had similar damage thresholds as the untreated material, averaging 24.9 J/cm2, a value 1.5 to 2 times higher than the damage threshold of the thin film AR coated sapphire windows.

Recent advances in aluminum oxynitride (ALON) optical ceramic

Aluminum Oxynitride or ALON optical ceramic is transparent material, developed and patented by Raytheon, which is very similar to sapphire, being comprised mostly of Al2O3 with a small amount of additional nitrogen. This nitrogen addition has the effect of producing a cubic material whose optical and mechanical properties are isotropic. Importantly, this means that it can be produced by powder processing methods, which are scalable to larger sizes, and at lower prices than can be achieved by the single crystal growth techniques that are used to grow sapphire. Furthermore, its isotropic properties make it much easier to grind and polish than sapphire. Recently, the interest in ALON optical ceramic has grown substantially following impressive results in ballistic testing. Ballistic laminates, containing ALON layers, have demonstrated protection against armor piercing rounds, at half the areal density and thickness of conventional ballistic laminates. ALON plates as large as 14x20in are being produced, under Air Force funding, for evaluation as IR windows and transparent armor, using conventional powder processing techniques. The production processes themselves are now being scaled to produce large pieces and large quantities of ALON optical ceramic.

Diamond for high heat flux applications

In polycrystalline CVD diamond of useful macroscopic dimensions, which may be considered for high heat flux applications, thermal conductivity parameters are largely determined by grain size resulting from growth morphology, defects and impurities in the material. Thermal conductivity has been measured in a number of state-of-the-art diamond samples, by the steady state technique, over the temperature range 6 to 400 K. The results are presented, and discussed in terms of microstructural differences between samples. At approximately 30 K, a departure from normal Debye type behavior is observed as a lowering of the predicted conductivity. At higher temperatures, this departure becomes less significant so that above approximately 350 K, where only Umklapp processes contribute to phonon scattering, the measured thermal conductivity is close to that predicted by the model and in good agreement with reference data for natural type IIa single crystal diamond. To account for the observed temperature dependence of conductivity, an additional phonon scattering term is used which may be described as Rayleigh scattering at low temperature by defects of 0.7 to 1.3 nm in size.

Polycrystalline Diamond for Infrared Optical Applications Prepared by the Microwave Plasma and Hot Filament Chemical Vapor Deposition Techniques

Abstract Absorption in the 8-12 μm spectral region in diamond films is dominated by single phonon effects due to departures from perfect crystallinity in the deposits. Conditions which maximize crystalline perfection must therefore be chosen to deposit diamond having the highest infrared (IR) transparency. Diamond having promising IR transmission has been deposited by both the hot filament and microwave plasma techniques. Quantitative measurement of IR absorption in polycrystalline diamond requires improvements in surface preparation techniques to reduce scatter and an understanding of the magnitude of surface absorption effects.