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.
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<p> </p> 𝑛𝑛<sup>2</sup> − 1 = (A+B [𝑇𝑇<sup>2</sup>−𝑇𝑇<sup>2</sup><sub>0</sub>]) +Dλ<sup>2</sup> /λ2−(λ<sub>1</sub>+C [𝑇𝑇<sup>2</sup>−𝑇𝑇<sup>2</sup><sub>0</sub>])<sup>2</sup> + λ<sup>2</sup>−λ<sub>2</sub><sup>2</sup> <p> </p>where wavelength λ is expressed in μm, T<sub>o</sub> = 295.15 K, A = 2.07156, B = 6.273× 10<sup>-8</sup>, λ<sub>1</sub> = 0.091293, C = –1.9516 × 10<sup>-8</sup>, D = 5.62675, and λ<sub>2</sub> = 18.5533. The slope dn/dT varies with λ and T, but has the approximate value 1.4 × 10<sup>-5</sup> K<sup>-1</sup> in the range 296–498 K.
Materials and Systems Research, Inc. is developing a material with a low coefficient of thermal expansion
(CTE) that could be used in an infrared-transparent window. The material is derived from a solid solution of
Al<sub>2</sub>(WO<sub>4</sub>)<sub>3</sub>, which has positive thermal expansion, and Sc2(WO4)3 with a negative thermal expansion. An
optimum composition of Al 0.5 Sc1.5(WO<sub>4</sub>)<sub>3</sub> was identified by synthesizing solid solutions, Al<sub>2-x</sub>Scx(WO<sub>4</sub>)<sub>3</sub>, by a
solid-state route with compositions ranging from x = 0 to 2.0. A single orthorhombic phase was obtained at all
compositions. A composition corresponding to x = 1.5 had a low CTE value of -0.15 x 10<sup>-6</sup>/oC in the
temperature range, 25-700ºC. A low temperature solution combustion process was developed for this
optimum composition resulting in a single phase powder with a surface area of ~ 14 m<sup>2</sup>/g and average particle
size (as determined from surface area) of 91 nm. Preliminary densification experiments via dry uniaxial
pressing and pressureless sintering at 1100°C for 2 hours resulted in a sintered compact 97.5% in density and
submicron grain size.
Grain-boundary scattering due to intrinsic birefringence limits the optical transmittance of polycrystalline alumina
(PCA). Smallest grain size and highest density are desired for maximum real in-line transmittance (RIT). Grain size
versus density or sintering path plots were employed to compare different colloidal routes for fabricating green bodies
followed by pressureless sintering and hot-isostatic pressing. Compacts fabricated by colloidal pressing showed superior
density at similar grain size as compared to slip-cast compacts. The real in-line transmittance of the PCA was measured
over a range of wavelengths (0.19-10 μm). The compacts fabricated by colloidal pressing showed higher transmittance
as compared to slip-cast specimens. The measured transmittance was still, however, slightly lower than the theoretical
values predicted by the grain-boundary scattering model of Apetz and van Bruggen. The grain-size dependence of RIT
was analyzed using a model that combined grain-boundary scattering and scattering by isolated grain-boundary pores.
Grain boundaries scatter light in polycrystalline materials consisting of birefringent crystals. Apetz and van Bruggen
developed a model for grain-boundary scattering based on Rayleigh-Gans-Debye light-scattering theory and
demonstrated its application to polycrystalline alumina. This paper reports the measurements of in-line transmittance in
polycrystalline magnesium fluoride with different grain sizes and compares the results with the grain-boundary
scattering model. Good agreement was obtained between the model predictions and the measured data for grain sizes
varying between 0.2 and 2.3 μm for light in the wavelength range, 0.633-5.5 μm.