Abstract
SummaryFor over half a century, the development of thermoelectric materials has based on the dimensionless figure of merit , assuming that the efficiency is mainly determined by this single parameter. Here, we show that the thermoelectric conversion efficiency is determined by three independent parameters, , τ, and β, which we call the three thermoelectric degrees of freedom (DoFs). is the well-defined mean of the traditional under nonzero temperature differences. The two additional parameters τ and β are gradients of material properties and crucial to evaluating the heat current altered by nonzero Thomson heat and asymmetric Joule heat escape. Each parameter is a figure of merit. Therefore, increasing one of the three DoFs leads to higher efficiency. Our finding explains why the single-parameter theory is inaccurate. Further, it suggests an alternative direction in material discovery and device design in thermoelectrics, such as high τ and β, beyond .
Highlights
A thermoelectric device converts heat into electricity by generating a voltage in the thermoelectric leg placed between high-temperature Th and low-temperature Tc regions through the Seebeck effect (Rowe (2005); Goupil (2015); Snyder and Toberer (2008))
For over half a century, the development of thermoelectric materials has based on the dimensionless figure of merit zT, assuming that the efficiency is mainly determined by this single parameter
When the thermoelectric properties are constant, the maximum thermoelectric efficiency hmax of a thermoelectric material leg can be determined by the single parameter zT (Ioffe (1957)) defined as a2 zT : = T; rk where the Seebeck coefficient a, electrical resistivity r, and thermal conductivity k are three thermoelectric properties and T is the absolute temperature
Summary
For over half a century, the development of thermoelectric materials has based on the dimensionless figure of merit zT , assuming that the efficiency is mainly determined by this single parameter. We show that the thermoelectric conversion efficiency is determined by three independent parameters, Zgen, t, and b, which we call the three thermoelectric degrees of freedom (DoFs). The two additional parameters t and b are gradients of material properties and crucial to evaluating the heat current altered by nonzero Thomson heat and asymmetric Joule heat escape. Each parameter is a figure of merit. Our finding explains why the single-parameter theory is inaccurate. It suggests an alternative direction in material discovery and device design in thermoelectrics, such as high t and b, beyond zT
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