Enhanced rectification effect in silver chalcogenide-based thermal diode by using precipitation/dissolution of Ag impurity across the structure phase transition

For developing high-performance composite-type thermal diodes, this study focuses on silver chalcogenides, which undergo structural phase transitions in the temperature range of 350 K to 473 K, accompanied by a significant stepwise change in thermal conductivity. Ag_{2 + <i>x</i>}Te_0.9S_0.1 (<i>x</i> = 0, 0.01, 0.02, 0.025, 0.03, 0.035, 0.04, and 0.05) and Ag₂S_{1 – <i>y</i>}Se_<i>y</i> (<i>y</i> = 0.35, 0.375, 0.4, 0.425, and 0.45) samples were synthesized with precisely controlled compositions, and their temperature-dependent thermal conductivity across the phase transition was studied with the composition dependence. Ag₂Te_0.9S_0.1 exhibits a stepwise decrease in thermal conductivity with transitioning from the low-temperature phase (LTP) to the high-temperature phase (HTP), and this behavior was further enhanced by adding excess Ag. The added silver precipitated in the LTP and dissolved into the HTP of Ag₂Te_0.9S_0.1, resulting in a maximum thermal conductivity change (<i>κ</i>_LTP / <i>κ</i>_HTP) of 2.7-fold with the phase transition at <i>x</i> = 0.025. On the other hand, the Ag₂S_{1 – <i>y</i>}Se_<i>y</i> samples exhibited a stepwise increase in thermal conductivity with transitioning from the LTP to the HTP, and the maximum thermal conductivity change of <i>κ</i>_HTP / <i>κ</i>_LTP = 5 was observed at <i>y</i> = 0.4. A composite thermal diode was fabricated using Ag_2.025Te_0.9S_0.1 and Ag₂S_0.6Se_0.4 with the length ratio of Ag_2.025Te_0.9S_0.1: Ag₂S_0.6Se_0.4 = 47:53 and, consequently, exhibited <i>TRR</i> = 3.3 when it was placed between heat reservoirs maintained at <i>T</i>_H = 412 K and <i>T</i>_L = 300 K. This <i>TRR</i> value is the largest ever reported for all-solid-state composite thermal diodes. This work achieves record-high thermal rectification ratio of 3.3 in all-solid-state composite thermal diodes by enhancing thermal conductivity changes through precipitation/dissolution of silver impurities across phase transitions in silver chalcogenides.