Improving Thermoelectric Performance Of Pulverized P-Type Bi2te3 Via A Grain Boundary Engineering Approach

It has been previously demonstrated that the hydrothermal treatment of polycrystalline p-Bi2Te3 by alkali metal salt XBH4 (X = Na or Rb) solution resulted in lower thermal conductivity for Na-treatment and higher power factor for Rb-treatment due to a thin layer formed at the grain boundary. One step further, we herein report the results of an approach combining both Na- and Rb-treatment with various molar ratios. It is found that a similar grain boundary layer is formed with a compositional gradient along the depth direction. Hall coefficient measurements have shown that the grain boundary phase introduced new carriers into the system and thereby compensated for the loss in mobility. With the Seebeck coefficient largely retained, the electrical conductivity to thermal conductivity ratio has been optimized by varying the Na:Rb ratio in the starting solution. As a result, the Na:Rb similar to 1:2 ratio yields the best dimensionless figure of merit ZT value similar to 0.92 at 350 K, comparable with that of the start-of-the art p-type Bi2Te3 commercial ingot.