New nano-structured thermoelectric (TE) materials have been developed and fabricated that have much higher conversion efficiencies than the state-of-the-art (SOTA) bulk thermoelectrics. In these new quantum well (QW) materials, the carrier and barrier materials (in this case SiGe and Si) are confined in alternating layers less than 10 nm thick, and this confinement has been shown to result in greatly improved TE properties (Seebeck coefficient, electrical resistivity and thermal conductivity) leading to higher TE Figure of Merit, ZT, conversion efficiencies and Coefficient of Performance (COP) for cooling applications than for SOTA thermoelectrics. From the most recent QW test data, ZTs greater than 3 at room temperature have been obtained which constitutes a significant improvement over the SOTA bulk thermoelectrics which have ZTs less than 1. QW materials have the best measured TE power factor (Seebeck coefficient squared divided by electrical resistivity) and, combined with low thermal conductivity substrates, should provide very high efficiency TE modules. The QW TE materials with ZTs greater than 3 lead to conversion efficiencies greater than 20 percent, which allows for much wider commercial applications, particularly in the applications such as the waste-heat recovery from truck engines, refrigeration, and air conditioning, where the SOTA bulk TE modules were shown to be technically feasible but economically unjustified due to low conversion efficiencies. With higher efficiency QW materials, these applications become economically attractive. The above mentioned QW TE ZTs include the effect of the substrate which degrades the overall performance, and a new test technique was developed that eliminates the effect of the substrate and for just the QW films, ZTs greater than 6 have been measured. This illustrated the importance of using a low thermal conductivity substrate in order to achieve good TE performance. In a recent QW test, a conversion efficiency corresponding to 62 percent of the Carnot efficiency was measured and this is believed to be the highest such value ever measured for a TE material. For power generation applications, QW TE generators can be designed for capacities ranging from milliwatts to kilowatts and for cooling applications with capacities ranging from watts to several tons of refrigeration. The paper discusses the effects of the thermal and electrical contact resistances and of substrate thermal conductivity on the TE performance, the status of the prototype QW TE generators and coolers being designed and fabricated, and the latest test results.
The Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of SRM-735
