Improvement of Low‐Temperature zT in a Mg 3 Sb 2 –Mg 3 Bi 2 Solid Solution via Mg‐Vapor Annealing

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n‐Type Mg 3 Sb 2 ‐based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain‐boundary effect has been limited due to loss of Mg, which hinders a sample's n‐type dopability. A Mg‐vapor anneal processing step that grows a sample's grain size and preserves its n‐type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon‐scattering‐dominated T −3/2 trend over a large temperature range, further supporting the conclusion that the temperature‐activated mobility in Mg 3 Sb 2 ‐based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm 2 V −1 s −1 in annealed 800 °C sintered Mg 3 + δ Sb 1.49 Bi 0.5 Te 0.01 , the highest ever reported for Mg 3 Sb 2 ‐based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n‐type Bi 2 Te 3 ) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.