Scientists use a variety of instruments to develop the materials used when building for spacecraft. Protective measures must be taken to prevent damage to sensitive instrumentation within the spacecraft and the people it carries. The protective coating on the surface of a spacecraft requires three important properties: reflectance, emissivity and diffusion.
Reflectance sends thermal energy away from the craft to keep it cool. The color white tends to reflect the light, which is why most spacecraft coatings are typically white.
Thermal emissivity is the property that sends visible and infrared energy (heat) away from an object. Heat generated by the craft (from instruments, controls or people), energy not fully reflected by coatings, or the thermal energy caused by atmospheric friction, needs to efficiently exit the surface of the protective material to prevent an intolerable buildup of heat.
Diffusion (directional bias) distributes thermal energy in several directions so that dangerous heat is not concentrated on a particular surface.
These properties ensure the craft remains cool. The thermal coating must also operate over a wide range of exposure geometries. For example, a craft orbiting Earth will be in extreme heat conditions from the sun and under extreme cold when the craft moves behind Earth (or another planet). Surfaces face many directions as the spacecraft orbits and rotates on its own axes.
The International Space Station (ISS) uses a lightweight ceramic material, labeled Z93, to protect itself. This material has been used as a viable protective material for more than 10 years. Z93 is a zinc oxide pigment in a potassium silicate binder. The material appears as a white ceramic with a microporous structure that diffuses radiation. Nonetheless, spacecraft developers continue to evaluate other thermal control coatings, especially as these craft enter new environments.
Typically, scientists use an integrating sphere to measure the diffuse reflectance properties of thermal control coatings. NASA used this technique in the 1990s to evaluate the Z93 material. One company, Surface Optics Corporation has a long history of contributing to NASA missions, most notably supplying coatings to space flight hardware aboard the Kepler, NuStar, and Chandra space explorers. The company has developed a Hemispherical Directional Reflectometer (HDR) that greatly improves evaluating thermal control coatings. The instrument uses an imaging hemiellipsoid to illuminate the sample using a blackbody source. A movable overhead mirror captures the energy reflected at various angle and directs a collimated beam into a Fourier Transform Infrared (FTIR) spectrometer.
They used our Thermo Scientific Nicolet iS50 FTIR spectrometer to complete its HDR instrument. Among other benefits, the Nicolet FTIR spectrometer has access to the widest range of radiation wavelengths available on an FTIR instrument. Together, the reflectometer and spectrometer provide full scattered transmittance data into large angles up to a hemisphere with a solid angle of 2π steradians. From there, FTIR software provides a spectral profile of the reflectance. The HDR is also useful in evaluating materials for energy conservation such as low-e glass and military hardware protection.
In order to demonstrate the optical measurement capabilities of the SOC-100 HDR, Surface Optics tapped Michael Bradley, one of our FTIR product managers, to collaborate on publication of its study of the Z93 material. These results were published recently in Spectroscopy Magazine. The findings reveal why the Z93 coating has ably protected the ISS for many years, while pointing to avenues for further development of thermal control materials.
The Nicolet iS50 FTIR Spectrometer is an invaluable tool for advanced materials research with the power to acquire data utilizing Attenuated Total Reflectance (ATR) in the both the mid- and far-IR regions, as well as FT-Raman and near-IR spectroscopy – without having to manually change accessories or components.
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