Optical and thermal properties of spinel with revised (increased) absorption at 4 to 5 μm wavelengths and comparison with sapphire

Daniel C Harris,Michael Bass,Nicholas D Haynes,Linda F Johnson,Tod Lewis,David E Zelmon,Giorgio Turri,Robert Seaver

doi:10.1117/1.oe.52.8.087113

Abstract

Infrared absorption of high-quality, commercial, polycrystalline MgAl 2 O 4 spinel is ∼40% greater in the range of 3.8 to 5.0 μm than the value predicted by the computer code OPTIMATR®, which has been used for window and dome design for more than 20 years. As a result, spinel and a -plane sapphire windows designed to support the same external pressure with the same probability of survival have approximately the same infrared absorptance in the range 3.8 to 5.0 μm. c -Plane sapphire has greater absorptance than spinel in the range 3.8 to 5.0 μm. Spinel has two weak absorption bands near 1.8 and 3.0 μm. At 1.064 μm, the laser calorimetric absorption coefficient of spinel is 10 to 50 times greater than that of sapphire. New measurements of specific heat capacity, thermal expansion, thermal conductivity, elastic constants, and refractive index (including dn/dT ) of spinel are reported.

Highlights

Spinel, and aluminum oxynitride (ALON) are the most durable, commercially available midwave infrared (3 to 5 μm) window materials that are candidates for applications requiring environmental durability, such as sand and rain erosion resistance or the ability to operate at elevated temperature.[1]
This paper presents the results of laser calorimetry, as well as new measurements of spectral transmittance, specific heat capacity, refractive index, thermal expansion, thermal conductivity, Young’s modulus, and Poisson’s ratio of spinel
We find that a-plane sapphire has equivalent absorptance to spinel 2 whose characteristic strength is 150 Mean absorptance sc (MPa) and whose Weibull modulus is 6

Summary

Introduction

Spinel, and aluminum oxynitride (ALON) are the most durable, commercially available midwave infrared (3 to 5 μm) window materials that are candidates for applications requiring environmental durability, such as sand and rain erosion resistance or the ability to operate at elevated temperature.[1]. For applications requiring maximum transmittance or minimum optical emittance in the 4 to 5 μm region, it appears from Fig. 1 that spinel would be the best choice if it is the same thickness as sapphire and has sufficient mechanical strength. Sapphire is mechanically stronger than spinel,[1] so a thinner sapphire window might be optically superior to a thicker spinel window and still meet mechanical requirements. Of the two commonly available crystal orientations of sapphire, a-plane material has less absorption than c-plane material.[3] a-plane sapphire has a refractive index anisotropy of 0.001 in the plane of the window, whereas c-plane sapphire has an isotropic refractive index in the plane of the window. Optical anisotropy might be an issue for some applications.[4] a-Plane sapphire is currently available in larger sizes than c-plane material, so the choice of crystal plane can be dictated by the required window size

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