Transparent magnesium aluminate spinel (MgAl2O4) has been developed as an optical ceramic for a variety of applications, including as windows. As a broadband, ultraviolet (UV) thru midwave infrared (MIR) material, it has been developed for windows and has many desirable properties compared with alternative infrared glasses and other transparent ceramics. Current efforts to advance high strength spinel manufacturing processes have demonstrated progress toward large format windows. Although low-absorption spinel, specifically in the near-infrared (NIR) has been demonstrated previously, additional processing is required for new, large-scale spinel manufacturing processes to decrease the effects of impurities near one-micron wavelengths. In this work we present recent results that show measured absorption near 1 μm is reduced by annealing, which reduces effects of trace impurities. Experimental results from photo-thermal common-path interferometer measurements are reported.
CeraNova will present a review of its work to address processing, properties, and manufacturability challenges for multiple transparent ceramic materials. The high strength and high optical quality of CeraNova’s materials provide significant performance advantages for systems operating in the most demanding environments. CeraNova’s proprietary methods deliver controlled micro-structure and high phase purity for several material systems. CeraNova is developing scaled-up processing, advanced manufacturing, and quality management systems for improved yields and reduced costs. CeraNova is also expanding its materials and product offerings to address the needs of advanced systems such as hypersonic vehicles and directed energy weapons systems.
The refractive index of fully dense, infrared-transparent polycrystalline alumina (PCA) with a mean grain size of ∼0.6 μm is reported for the wavelength range 0.85 to 5.0 μm over the temperature range T=296 to 498 K. The temperature-dependent Sellmeier equation is n2−1=(A+B[T2−To2])λ2/[λ2−(λ1+C[T2−To2])2]+Dλ2/(λ2−λ22), where λ is expressed in μm, To=295.15 K, A=2.07156, B=6.273×10−8, λ1=0.091293, C=−1.9516×10−8, D=5.62675, λ2=18.5533, and the root-mean square deviation from measurements is 0.0002. This paper describes how to predict the refractive index of fully dense isotropic PCA with randomly oriented grains using the ordinary and extraordinary refractive indices (no and ne) of sapphire spatially averaged over the surface of a hemisphere. The refractive index of alumina at 296 and 470 K agrees within ±0.0002 with the predicted values. Similarly, the ordinary and extraordinary optical constants ko and ke are used to predict the absorption coefficient of alumina. The refractive indices no and ne of sapphire grown at Rubicon Technologies by the Kyropoulos method were measured at 295 K and agree with published Sellmeier equations for sapphire grown by other methods within ±0.0002.
Transparent ceramics are finding increasing use in optical applications with demanding operating conditions. Polycrystalline ceramics provide a unique combination of mechanical, dielectric and optical properties for sensor window applications that were previously not possible. The mechanical strength of CeraNova’s transparent alumina and spinel was measured by an equibiaxial strength test method. The results of the tests and their analysis, included those at elevated temperatures for transparent alumina, will be presented.
Samples of fine-grain, transparent polycrystalline alumina (CeraNova Corp) and multispectral zinc sulfide (Cleartran) were tested to determine mechanical strength and slow crack growth parameters. Mechanical strength measurements of coupons were fit to a Weibull equation that describes the material strength and its distribution. Slow crack growth parameters were calculated using the procedure set forth by Weiderhorn.1 This paper describes the derivation of Weibull and slow crack growth parameters from strength measurements over a range of stress rates and how these parameters are used to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.
The refractive index of polycrystalline α-alumina prisms with an average grain size of 0.6 μm is reported for the wavelength range 0.9 to 5.0 and the temperature range 293 to 498K. Results agree within 0.0002 with the refractive index predicted for randomly oriented grains of single-crystal aluminum oxide. This paper provides tutorial background on the behavior of birefringent materials and explains how the refractive index of polycrystalline alumina can be predicted from the ordinary and extraordinary refractive indices of sapphire. The refractive index of polycrystalline alumina is described by
where wavelength λ is expressed in μm, To = 295.15 K, A = 2.07156, B = 6.273× 10-8, λ1 = 0.091293, C = –1.9516 × 10-8, D = 5.62675, and λ2 = 18.5533. The slope dn/dT varies with λ and T, but has the approximate value 1.4 × 10-5 K-1 in the range 296–498 K.
Transparent ceramics are finding applications in demanding optical applications were traditional mineral salts and amorphous materials are limited and single crystals are not practical. Polycrystalline ceramics offer a unique combination of mechanical, electrical and optical properties that allow window and dome applications and possibilities that were previously not possible. Transparent ceramics are being developed for use in a number of applications with each material possessing a distinctive set of properties that address a particular application. The current status of CeraNova’s fine grain transparent ceramic programs for dome and window applications will be presented with emphasis on their exceptional material properties for specific applications.
Transparent ceramics are finding applications is demanding optical applications were traditional mineral salts and
amorphous materials are limited and single crystals are not practical. Polycrystalline ceramics offer a unique
combination of mechanical, electrical and optical properties that allow window and dome applications and
possibilities that were previously not possible. Transparent ceramics are being developed for use in a number of
applications with each material possessing a distinctive set of properties that address a particular application. The
current status of CeraNova's fine grain transparent ceramic programs for dome and window applications will be
presented with emphasis on their exceptional properties for specific applications.
CeraNova's transparent polycrystalline alumina (CeraLuminTM)a has sub-micron grain size (300-500nm) and high
transmittance in the mid-wave infrared (>85% in the 3-5µm MWIR region). The fine, uniform grain size imparts
high hardness, high strength, and high thermal shock resistance. Polycrystalline alumina is a viable alternative to
sapphire for domes, particularly for aerodynamic shapes which are readily fabricated by powder processing. Both
hemispheric and ogive domes (sub-scale and full-size) have been successfully molded and densified to transparency.
Hemispheric domes have been optically finished. Current efforts include a focus on scale-up, fabrication, and
metrology of aerodynamic domes. This paper presents recent analyses of microstructure, optical properties, and
mechanical properties.
Polycrystalline alumina (PCA) has great potential for providing performance comparable to or better than single-crystal sapphire, yet offers the opportunity for low-cost powder based manufacturing. CeraNova has demonstrated transparent PCA, by processing the material to simultaneously achieve 100% density and sub-micron grain size. CeraNova PCA displays low scatter in the infrared, with high transmittance (>85%) in the 3.0-4.0μm region, comparable to sapphire. In addition, the sub-micron grain size leads to high hardness, high strength and high thermal shock resistance. This fine-grain PCA is a viable sapphire replacement for dome applications, including those that require aerodynamic shapes readily possible by powder processing. Such shapes present not only processing challenges, but also surface finishing issues. Results from a current program to address these issues in creating an ogive dome of PCA are discussed.
Lead Zirconate Titanate (PZT) active fibers, from 80 to 250 micrometers in diameter, are produced for the AFOSR/DARPA funded Active Fiber Composites Consortium (AFCC) Program and commercial customers. CeraNova has developed a proprietary ceramics-based technology to produce PZT mono-filaments of the required purity, composition, straightness, and piezoelectric properties for use in active fiber composite structures. CeraNova's process begins with the extrusion of continuous lengths of mono-filament precursor fiber from a plasticized mix of PZT-5A powder. The care that must be taken to avoid mix contamination is described using illustrations form problems experiences with extruder wear and metallic contamination. Corrective actions are described and example microstructures are shown. The consequences of inadequate lead control are also shown. Sintered mono- filament mechanical strength and piezoelectric properties data approach bulk values but the validity of such a benchmark is questioned based on variable correlation with composite performance measures. Comb-like ceramic preform structures are shown that are being developed to minimize process and handling costs while maintaining the required mono-filament straightness necessary for composite fabrication. Lastly, actuation performance data are presented for composite structures fabricated and tested by Continuum Control Corporation. Free strain actuation in excess of 2000 microstrain are observed.
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