Discovery of Metallic Grain Boundary Resistance in the High Temperature Thermoelectric La3Te4

Added electrical resistance from grain boundaries is major limitation in the thermoelectric performance of many materials. This boundary resistance is typically observed as a large increase in resistivity at low temperature that decays exponentially at higher temperatures like the resistivity of an insulator or intrinsic semiconductor. As a result, engineering studies to mitigate boundary resistance have had most impact on low temperature thermoelectrics like Mg2Sb3 or improving the average zT of fine-grained, mid-temperature thermoelectrics like half-Heuslers. With much less influence at high temperature, there would be little expectation to improve the high temperature performance of thermoelectrics through mitigating electrical boundary resistance. In this work, however, we demonstrate that it is necessary to consider grain boundary resistance even in high temperature thermoelectrics by improving the thermoelectric performance at temperatures up to 1000 C in the thermoelectric La3Te4 by increasing grain size. In contrast with previous reports of grain boundary resistance in other thermoelectric materials, this improved performance is largest at high rather than low temperatures. This is a result of a new form of boundary resistance not previously reported in thermoelectric materials: uncharged or metallic boundary resistance. We observe a boundary resistance that increases linearly with temperature as expected in metals instead of the exponentially decaying resistance expected for charged boundaries. With both this new form of boundary resistance and the example of improved high temperature performance in La3Te4, we have shown that grain boundary engineering is still needed to optimize thermoelectric performance even for high temperature applications