High-Performance Aluminum Mirrors for Far-Ultraviolet Applications

NASA’s innovative passivation process consists of exposing in-situ Physical Vapor Deposited oxide-free AI samples in a high or ultra-high vacuum chamber at ambient temperature to a low-pressure reactive gas of xenon difluoride (XeF2) immediately after depositing the AI layer. In this chemisorption process, the XeF2 is adsorbed on the surface of the AI layer, and due to the affinity of Al to fluorine molecules, the weakly-bound Xe dissociates and is pumped out of the chamber. A thin AlF3 overcoat layer is produced as a result. Other embodiments of the invention include the use of alternative processing gasses for the passivation treatment, or the utilization of a plasma source to increase the ion/atom ratio of the incident fluorine species resulting after XeF2 dissociation. Similarly, different mirror substrates materials with suitable surface characteristics in the FUV could be employed. In addition to the improved FUV reflectance, environmental stability, and maintained efficiency at higher wavelengths of the resulting Al mirrors, this NASA process has several unique features. Firstly, the entire process is carried out at ambient temperature, eliminating the need for high-temperature fluoride deposition. Secondly, the process is highly scalable, limited only by the size of the coating chamber where the passivation of XeF2 is carried out. Finally, the process can be manipulated to conform to any geometry, enabling its use for curved optics. While NASA originally developed the passivation of oxide-free aluminum coatings to realize reflectivity enhancement in the Far-Ultraviolet for the Large UV/Optical/IR Surveyor (LUVOIR), it may also be useful to companies that manufacture ground-based or space-based optical systems with sensitivity to the FUV spectrum. Examples include optics for space telescopes with reflective elements, vacuum-ultraviolet (VUV) plasma analysis tools, photolithography instrumentation, and wafer inspection tools.

The ALD-Enhanced Far-to-Mid IR Camera Coating is fabricated by first applying a conductively loaded epoxy binder ~500 microns thick onto a conductive metal substrate (e.g., Cu, Al). This serves to provide high absorptance and low reflectance at the longest wavelength of interest, as well as to provide a mechanical buffer layer to reduce coating stress. Borosilicate glass microspheres are coated with a thin film metal via ALD, essentially turning the microspheres into resonators. That film is optically thin in the far infrared and approximates a resistive (~200 ohms per square) coating. Light trapped in the borosilicate glass microspheres is reflected back and forth within the glass–at each contact point, the light is attenuated by 50%. A monolayer of thin metal film-coated borosilicate glass microspheres is applied to the epoxy binder and cured, forming a robust mechanical structure that can be grounded to prevent deep dielectric charging by ionizing radiation in space. Once cured, the far-to-mid IR absorber structure can be coated with a traditional ~20-to-50 microns “black” absorptive paint to enhance the absorption band at short wavelengths, or a “white” diffusive paint to reject optical radiation. At this thickness and broad tolerance, the longwave response of the coating is preserved. Tailoring the electromagnetic properties of the coating layers and geometry enables realization of a broad band absorption response where the mass required per unit area has been minimized. While NASA originally developed the ALD-Enhanced Far-to-Mid IR Camera Coating for the Stratospheric Observatory for Infrared Astronomy mission, its robustness, absorptive qualities, and optical performance make it a significant addition to IR and terahertz imaging systems. The IR camera coating is at Technology Readiness Level (TRL) 3 (experimental proof-of-concept) and is available for patent licensing.

These materials, which are composed of highly optically transmissive materials, are engineered to provide near-perfect reflection of the full solar spectrum in space. The materials are finely divided such that they scatter and reflect the incoming radiation from the UV down into the mid-IR and are also coated in some fashion with silver to extend the reflectance down into the far IR region of the solar spectrum. With this near-perfect reflectance of the complete solar spectrum, the scientists envision use of these materials for maintaining cryogenic temperatures for extended periods of time in space. The materials have also been developed into highly flexible, moisture resistant selective surface paint.The use and storage of cryogenics fluids is critical to many space operations, and while there are thermal control coatings in use today for spacecraft, none can provide this near-perfect reflection required for long-term maintenance of cryogenic temperatures.

In order to obtain real-time temperature measurements with this technology, the phosphor can either be applied as a coating onto the surface of the object for temperature mapping, or incorporated onto a sensor and attached to the end of a fiber-optic probe for local temperature measurements. Next, the object is exposed to a pulsed light source, which causes the temperature-sensing phosphor to produce an ultra-bright broadband emission. An algorithm is used to evaluate post-pulse emission decay time, which is then converted to a precise temperature reading. State-of-the-art temperature sensing phosphors suffer from thermal quenching, or unacceptable loss of signal intensity as temperature increases. Alternative high-temperature measurement systems such as thermocouples and pyrometers, offer only spot measurements. Moreover, thermocouples suffer from attachment issues and electromagnetic interference, while pyrometers frequently suffer from reflected radiation interference and unknown surface emissivity. Cr:GdAlO3, which retains ultra-bright emission intensity to temperatures well above 1000°C, provides an ideal solution to all of these issues with a few added benefits. It has a perovskite structure that is both non-reactive and stable in high-temperature environments. Furthermore, it possesses a favorable electron energy level spacing that enables this sensor to maintain stronger signal intensity than its competitors. Due to the broad absorption and emission bands for Cr:GdAlO3 there is considerable flexibility in the choice of excitation and emission wavelength detection bands. The combination of these factors makes this technology a clear choice for luminescence-based optical high-temperature sensing for a variety of industries from aerospace to manufacturing.

NASA’s PLED thermal inspection system consists of an array of high- powered LED chips configured to deliver controlled pulses of visible light. The system includes 8 LED chip arrays, mounted on an aluminum heat sink and housed in a hood configuration. The inspection hood is specially designed with filters to prevent internal reflections. The LEDs are powered by regulated power supplies and controlled via a computer interface that synchronizes heat pulses with an infrared camera. An acrylic filter is placed over the LEDs to block residual infrared radiation, ensuring that only visible light reaches the target surface. The system’s infrared camera, operating in the mid-wave infrared (MWIR) range does not detect the visible light and captures the transient thermal response of the material, allowing for precise defect detection. By eliminating the need for high-intensity broadband infrared sources, the PLED system provides a cleaner and more accurate thermal response, particularly for unpainted metals and additively manufactured (AM) components. Performance validation of the PLED system has demonstrated significant advantages over traditional flash thermography. In tests on aluminum samples with material loss and AM Ti-6Al-4V metal specimens, the PLED system successfully detected defects with superior contrast and no heat source reflections. Principal Component Analysis (PCA) applied to PLED inspection data revealed clearer defect indications compared to flash-based methods, which introduced unwanted artifacts due to transient reflections. Additionally, the PLED system enabled quantitative thermal diffusivity measurements, offering a new approach to single-sided material characterization. NASA's PLED thermal inspection technology is available for patent licensing. Potential applications include corrosion detection in aerospace components, quality control of AM metal parts, structural health monitoring of industrial materials, and more.