Mixed-phase effect of a high Seebeck coefficient and low electrical resistivity in Ag ₂ S

Abstract

Abstract The improvement of thermoelectric performance is very challenging, because the thermoelectric properties, the Seebeck coefficient, electrical resistivity, and thermal conductivity are generally correlated with each other via a common physical quantity known as carrier concentration. In this study, we successfully achieved separate control of the thermoelectric properties of polycrystalline Ag 2 S using spatial-phase separation into low- and high-temperature phases using a configuration based on bottom heating and top measurement. We experimentally confirmed that the Seebeck coefficient was determined by the low-temperature phase at the top surface but the electrical resistivity was dominated by the high-temperature phase lying below the low-temperature phase. As a result, a high Seebeck coefficient ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mo>−</mml:mo> <mml:mn>650</mml:mn> <mml:mi>μ</mml:mi> <mml:mrow> <mml:mtext>V</mml:mtext> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>K</mml:mtext> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ) and low electrical resistivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>2</mml:mn> <mml:mrow> <mml:mtext>m</mml:mtext> <mml:mi mathvariant="normal">Ω</mml:mi> <mml:mtext> </mml:mtext> <mml:mtext>cm</mml:mtext> </mml:mrow> </mml:math> ) were simultaneously observed over a broad temperature range (390 ∼ 440 K). This experimental discovery suggests a new concept for thermoelectric materials and devices.