<title>Thermo-optic, thermo-mechanical, and electromagnetic effects in IR windows and domes, and the rationale for GaAs, GaP, and diamond</title>

Abstract

Electro-optical (EO) systems, the platforms they perform on, and their missions continue to place increasing requirements on the infrared windows and domes associated with these systems. Supersonic flight, observability, EMI shielding, sensor range, multispectral sensors, environmental degradation resistance (sand and rain erosion resistance), and affordability are some of the requirements that are rendering most current IR window and dome technologies inadequate. Texas Instruments (TI), through both IR&D and DoD contract work, has been developing enabling IR materials technology to address these critical needs. Specifically, our work on CVD diamond, gallium arsenide (GaAs), gallium phosphide (GaP), and polymers is described and compared with othe IR window and dome materials. Trade studies involving thermal shock, transmission, absorption, emission, strength, durability, protective coatings, EMI shielding, transmitted wavefront distortion, and material status are presented. High-speed IR domes for use with 8- to 12 micrometers sensors on Mach 3 or greater missiles will generally require diamond; slower missiles could use ZnS, GaAs, and possibly GaP. For Mach 3 or greater missile systems with IR sensors operating in the 3- to 5-micrometers range, GaP is the most promising material, with its higher thermal shock resistance and lower absorption and, therefore, lower emission at elevated temperatures at 3 to 5 micrometers than sapphire, spinel, yttria, ALON, Si or ZnS. For multispectral use (3 to 5 and 8 to 12 micrometers ) at supersonic speeds, GaP and multispectral ZnS are the candidates; low supersonic use could include GaAs; and subsonic use could include ZnSe. For IR windows and domes where EMI shielding is required, GaAs offers the highest shielding of any window with or without metal grids. Si, Ge, and GaP offer bulk electrical conductivity like GaAs, but, because of the intrinsic behavior of the carriers, cannot offer the same level of shielding. Durability is a growing concern on all IR windows and domes. Various coatings for rain and sand erosion including diamond, BP, and TI GaP, amorphous carbon and novel Ti IR polymers are discussed.