Luminescence-Based Temperature Mapping and Sensing Systems

Solid state power conversion devices, such as thermoelectrics, depend upon temperature gradients for their operation. For example, aeronautic gas turbine engines maintain the necessary temperature gradients throughout their systems due to the enthalpic processes of combustion, which offers the possibility of generating electrical power for use in primary and secondary electrical systems in the aircraft. However, until now thermoelectric materials have not been able to withstand the combination of high temperatures and oxidative environments present in gas turbine engines. Glenn's innovation overcomes these limitations by using a doped oxide pyrochlore (crystal compound) semiconductor as the thermoelectric material. The material has a low thermal conductivity, which allows it to maintain a thermal gradient and sufficient electrical conductivity to produce an electromotive force. The pyrochlore allows the thermoelectric material to be present within a gas turbine engine, converting heat directly into electricity and functioning at high temperatures without oxidizing in air. Glenn's innovative thermoelectric material permits the benefits of solid-state power conversion devices to improve fuel efficiencies for a broader range of applications than has ever been possible. This innovation is in the early stages of development, and Glenn welcomes opportunities for co-development.

The thin film sensor’s principal advantage lies in its potential to take high frequency temperature measurements from the surface of a reentering spacecraft while simultaneously withstanding the high temperature and oxidizing environment encountered. This data provides engineers with operational phase measurements used to refine the spacecraft’s operational envelope and track flight hardware behavior in addition to providing high frequency temperature measurements that can inform the physics of a boundary layer. Mismatches in coefficients of thermal expansion (CTE) are expected in TPS-based sensor applications because the metallic materials used for temperature sensing have thermal expansion rates that differ from the rates of the substrate and coating materials in the TPS. At high temperatures during reentry, this mismatch in CTE can create a significant strain differential between the metallic sensor, sensor leads, and the materials to which the sensor and leads are bonded. High frequency response temperature measurements on the surface of entry spacecraft are not currently possible above ~700 F with existing measurement capabilities. This shortcoming is primarily due to the need for robust sensor behavior at temperatures of several thousand degrees F. The sensor design of this technology preserves the integrity of sensor components while enhancing its high temperature functionality. The thin film temperature sensor has a technology readiness level (TRL) 5 (Component and/or breadboard validation in relevant environment) and is now available for patent licensing. Please note that NASA does not manufacture products itself for commercial sale.

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.

This NASA technology transforms a conventional infrared (IR) imaging system into a multi-wavelength imaging pyrometer using a tunable optical filter. The actively tunable optical filter is based on an exotic phase-change material (PCM) which exhibits a large reversible refractive index shift through an applied energetic stimulus. This change is non-volatile, and no additional energy is required to maintain its state once set. The filter is placed between the scene and the imaging sensor and switched between user selected center-wavelengths to create a series of single-wavelength, monochromatic, two-dimensional images. At the pixel level, the intensity values of these monochromatic images represent the wavelength-dependent, blackbody energy emitted by the object due to its temperature. Ratioing the measured spectral irradiance for each wavelength yields emissivity-independent temperature data at each pixel. The filter’s Center Wavelength (CWL) and Full Width Half Maximum (FWHM), which are related to the quality factor (Q) of the filter, are actively tunable on the order of nanoseconds-microseconds (GHz-MHz). This behavior is electronically controlled and can be operated time-sequentially (on a nanosecond time scale) in the control electronics, a capability not possible with conventional optical filtering technologies.

This innovative NASA process begins with the deposition of an Al layer onto an optically smooth glass substrate using a high-vacuum PVD system. Unlike conventional methods where the Al surface is immediately exposed to air or coated with LiF, this new technique entails an initial exposure of the fresh Al film to XeF2 gas. This step chemically passivates the Al surface, forming a thin (~2.5-3.2 nm) protective layer of aluminum fluoride (AlF3). This barrier prevents oxidation and contamination of the aluminum before the LiF layer is applied, preserving its reflectivity. Following the XeF2 treatment, a flash-evaporated LiF overcoat is deposited onto the Al surface using a conventional PVD process. Deposition is performed at a high rate to increase the density of the LiF layer, enhancing its environmental stability and optical performance. Immediately after the LiF deposition, the mirror undergoes a second XeF2 exposure, further passivating the surface. This dual XeF2 treatment is key to the improved durability of the mirrors, as it effectively seals the interfaces and prevents degradation over time. One of the most significant advantages of this process is that it is performed entirely at room temperature, eliminating the need for high-temperature deposition techniques conventionally used for such coatings. Extensive testing of these new mirrors has demonstrated their performance and durability. Testing of prototypes optimized for 121.6 nm demonstrated 92.6% reflectivity, surpassing all previously reported values for Al/LiF coatings. Long-term environmental testing has shown that these mirrors maintain their high reflectance even after months of storage in moderate humidity conditions. Further stress testing in environments with 50-60% humidity for three weeks resulted in a reflectance reduction of about 2%, demonstrating high environmental stability. This NASA invention is available for patent licensing to industry.