ADVANCING MATERIALS SERIES: HISTORY OF COATING MATERIALS NEWS & MORE
By: David Sanchez, Applications Engineer, Materion Advanced Materials
Thorium Fluoride Alternatives | The Choice is Yours
Coating Materials News Volume 5, Issue 1 | Originally printed by Cerac® January-March, 1995
With Coating Materials News & More coming a long way since its first issue with the Cerac® company in 1995, here is a technical look back at a popular article that still resonates with customers today who are looking for non-radioactive material alternatives. This highly technical newsletter was the precursor to inspiring production of other materials such as Ytterbium Fluoride (YbF3), higher purity Yttrium Fluoride (YF3), Cerium Fluoride (CeF3), Barium Fluoride (BaF2), and helped to improve supply chain issues with Thorium Fluoride (ThF4). We continue to generate interest and demand for this information even 26 years later, and expect the new CMN & More to uncover more opportunities for materials the industry so desperately needs.
Please feel free reach out to our very own David Sanchez, Applications Engineer if there is a future topic you’d like to see discussed in the new Coating Materials News & More at David.Sanchez@Materion.com or follow David on LinkedIn.
Long wavelength infrared coatings designed to operate in the region above 10 µm require a low refractive index material with low absorption at these wavelengths. The low index layers are combined with high index materials, such as germanium or zinc sulfide or zinc selenide, to produce anti-reflection coatings, bandpass filters or high reflecting coatings. Applications include medical and high energy laser optics for industrial and military purposes. In passive applications, such as imaging or remote sensing, high efficiency is required because the optical system is generally energy starved. In high energy applications such as with the CO2 laser, high damage thresholds are required. Absorption in the film layers imposes the limit in both applications: in one case contributing to system low signal to noise ratio and in the second case resulting in system failure.
The only material that has been found to provide low absorption and high laser power damage thresholds is thorium fluoride, ThF4. This compound has been used since the 1970s. Thorium fluoride provides a refractive index near 1.35, absorption <0.1% and sufficient hardness and low stress to be used in thicknesses of a quarter wave (QW at 10.6 µm=1960 nm). The disadvantage that ThF4 possesses is being an alpha particle emitter and, therefore, is considered a hazardous substance, especially to lung tissue. Environmental health regulators have imposed strict processing and handling conditions on its use, and many government customers have disallowed its use in the presence of personnel.
CERAC (now known as Materion Advanced Materials) operates an ongoing research program for developing non-radioactive alternates to ThF4 for the above applications [1, 2]. Investigation of the evaporation behaviors, optical properties, and durabilities of deposited films of several compounds and new materials has resulted in a choice among alternates that provide specific advantages. Among the alternates are materials that offer substantial improvements in environmental durability, evaporation ease, long wavelength transmission and mechanical stress. As with ThF4, no single material provides all of the above properties. In many applications, one property is weighted preferentially over the others. For this reason, CERAC (Materion Advanced Materials) makes several materials available for the designer/coater to apply to his or her requirements.
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