A unique ultrasonic-based technique has been developed to measure temperature profiles in materials used in thermal protection systems (TPS). The technology requires measurements of the thermal expansion coefficient and the ultrasonic velocity for these materials as a function of temperature in order to determine the variation of ultrasonic propagation speed with temperature. Generally, this is done by slowly heating materials to a set temperature so that the samples are isothermal.
The material characterization technique requires uniform temperature within the material to eliminate spatial temperature gradients that can affect the calibration of the technique. This proves to be difficult for TPS materials that are designed to have very low thermal transport characteristics, and therefore introduction of heating of the sample surface either through conduction or convection does not readily propagate into the interior. Some TPS materials, such as the Phenolic Impregnated Carbon Ablator (PICA) that uses a carbon substrate, are electrically conductive. By passing a regulated current through the volume of the material, one can generate very rapid volumetric heating, producing samples that are isothermal at elevated temperatures. Simultaneous with this resistive heating, the samples can be configured so that the variation in ultrasonic time-of-flight over a fixed distance can be measured. This configuration can be used to measure the elastic modulus and the velocity expansion coefficient of materials. Joule heating of electrically conducting TPS is ideally suited for rapidly measuring the elastic modulus and velocity expansion coefficient over an extended temperature range from ambient to 1300 °C. It is also useful for quantitative measurement of thermally induced property changes prior to ablation.
The measurement technique has been applied to PICA, the material used on the TPS for the Mars Science Laboratory. Other carbon-containing TPS that are electrically conductive, such as carbon weaves, other carbon phenolic material architectures, and carbon/carbon composites, are potential candidates. The method is applicable to metals; however, most metals have higher thermal transport characteristics so the more conventional conduction or convention heating is applicable.