Abstract
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.
Highlights
This paper reports the refractive index of infrared-transparent polycrystalline alumina (PCA) in the wavelength range 0.85 to 5.0 μm from 296 to 498 K
Ordinary light rays passing through an axially symmetric crystal such as sapphire (α-Al2O3) are polarized with their electric field oscillating perpendicular to the symmetry axis (c-axis) of the crystal
1 2 no þ neðφÞ, where no is the ordinary refractive index of sapphire and neðφÞ is the spatial average of the extraordinary refractive index over the surface of a hemisphere
Summary
This paper reports the refractive index of infrared-transparent polycrystalline alumina (PCA) in the wavelength range 0.85 to 5.0 μm from 296 to 498 K. 1960s, General Electric Co. commercialized translucent alumina called Lucalox for high-pressure sodium lamp envelopes Hayashi et al. achieved translucency with a grain size of ∼1 μm in alumina prepared by injection molding, sintering at 1260°C, hot isostatic pressing at 1260°C at 101 MPa, and annealing in vacuum at 1150°C. Mizuta et al. prepared translucent alumina with grain sizes of 0.7 to 15 μm by slip casting, sintering at 1240°C, and hot isostatic pressing at 1050°C to 1400°C at 150 MPa. In-line transmittance at an unstated visible wavelength was 30% to 46%, but decreased with decreasing grain size, suggesting that a grain-boundary phase was responsible for some scatter. Parish et al. made PCA with a grain size of 0.5 μm and near-theoretical transmittance at a wavelength of 4 μm and a thickness of 2 mm. Bernard-Granger et al. reported more processing details, as well as thermal, mechanical, and optical properties of transparent PCA with a grain size of 0.5 to 0.6 μm and moderate visible transparency
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