Thermoelectric materials, which allow solid-state conversion of thermal to electrical energy, have a major energy application for vehicular engine waste heat recovery. The Department of Energy (DOE) is funding a thermoelectric materials program with the goal of increasing gasoline mileage by 10% by 2011. This increase would result from the conversion to electric power of heat lost through the radiator and exhaust system, which can run either an electric motor or accessories (e.g., air conditioning). However, at present the best conversion devices yield only a 3% increase. The availability of higher conversion efficiency thermoelectric materials will play a significant role in moving towards the DOE goal.
High conversion efficiency thermoelectric devices require materials that possess large figures of merit, ZT, which is equal to S2s T/k, where S is the Seebeck coefficient, s the electrical conductivity, k the thermal conductivity, and T the absolute temperature. Functional Properties Group researchers have demonstrated the utility of using combinatorial thin film libraries to enable rapid measurement of the Seebeck coefficient as a function of composition (M. Otani et al, Applied Physics Letters 91, 132102, 2007). Automotive leaders such as GM and Honda have expressed much interest in NIST’s combinatorial approach. However, to obtain ZT, the thermal conductivity of the film must be measured. This is a notoriously difficult measurement to perform because the mass of the substrate is far greater than that of the film.
To address this measurement need, Functional Properties Group researchers have developed a scanning frequency domain thermoreflectance measurement system that can rapidly and locally (10 micrometer spot size) measure the thermal conductivity of combinatorial (composition-spread) films. The sample, a thermoelectric film coated with a thin molybdenum layer, is locally heated by an intensity-modulated (1 MHz) laser; the thermal response of the film is detected by the reflected beam of a second (probe) laser. Evaluation of the phase lag between the thermoreflectance and the heating laser signals enables one to determine the thermal effusivity b, equal to (k cr)1/2, where c is the specific heat, and r the density. A thermal effusivity calibration curve was obtained from bulk samples of SiO2, SrTiO3, LaAlO3, Al2O3, and Si. Using this calibration, and applying a two-layer thermal mathematical model, the thermal conductivity of an yttria (Y2O3)-containing ternary oxide film was then determined. The measured value, 11.96 J(Kms)-1, is within about 10% of the reported value of 12.87 J(Kms)-1.