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STATUS OF THORIUM FLUORIDE REPLACEMENTS

Caption: Depiction of coatings with the subtractive and additive combinations of colors produced by transmission through dichroic filters designed for the visible region.  (Credit: S. Pellicori). 

Non-Radioactive Coating Materials for the Infrared
Thorium fluoride (ThF4) has been the favored low-index IR coating material since its investigation and introduction 45 years ago. However, the radioactivity of Thorium (Th) led to forbidding the use of ThF4 by the 1980’s. With that loss, subsequent studies of different fluoride compounds were conducted by scholarly groups and coating companies who sought an alternate material that would exhibit ThF4 transparency, physical durability, and affordable cost. Those who work in the spectral range above ~11 µm are already aware of suitable candidate replacements. [1, 2, 3, 4]. Although several substitute IR materials have been identified with comparable properties, none have equaled ThF4. This article reviews the current industry and Materion’s status and progress in the pursuit.
 
Thorium Fluoride Properties: Desired and Not Desired
All optical coatings that are designed to provide specific functions such as reduction of surface reflection, isolation of a pass band, or definition of LW and SW separation edges, require the combination of low-index and high-index material layers. For IR wavelengths longer than ~7 µm, suitable materials are few in number as limited by absorption.  In this region, Oxide compounds are eliminated from consideration for not meeting the highest efficiency and LDT applications. This leaves fluoride compounds for the low-index choices and zinc sulfide or zinc selenide and Ge for the high-index layers. Thus, optical coatings functioning in the thermal IR (“LWIR”) are composed of a fluoride compound and a high-index material as cited above. The materials on this short list of candidates are deposited using thermal evaporation sources such as resistance- heating or E-beam evaporation. (While sputtering and ion-beam deposition is possible, those processes are not generally used for IR coatings). As is true of all fluoride compounds, the replacements have been found to require special deposition process-dependent preparation and specific deposition parameters to provide optimal properties. 

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