Correction of the Electrical and Thermal Extrinsic Effects in Thermoelectric Measurements by the Harman Method

Abstract

Although the Harman method evaluates the thermoelectric figure-of-merit in a rapid and simple fashion, the accuracy of this method is affected by several electrical and thermal extrinsic factors that have not been thoroughly investigated. Here, we study the relevant extrinsic effects and a correction scheme for them. A finite element model simulates the electrical potential and temperature fields of a sample, and enables the detailed analysis of electrical and thermal transport. The model predicts that the measurement strongly depends on the materials, sample geometries, and contact resistance of the electrodes. To verify the model, we measure the thermoelectric properties of Bi2-Te3 based alloys with systematically varied sample geometries and either with a point or a surface current source. By comparing the model and experimental data, we understand how the measurement conditions determine the extrinsic effects, and, furthermore, able to extract the intrinsic thermoelectric properties. A correction scheme is proposed to eliminate the associated extrinsic effects for an accurate evaluation. This work will help the Harman method be more consistent and accurate and contribute to the development of thermoelectric materials.

Harman Measurements for Thermoelectric Materials and Modules under Non-Adiabatic Conditions

“…Unlike the separate measurements2 of α , ρ , and k , the Harman method requires a single apparatus and a single sample preparation, hence essentially involves smaller uncertainties in the measurements3. However, the accuracy of the Harman method has often been questioned, as it is highly sensitive to the heat transfer from or to the sample and the electrical contact resistance3456789. Without correcting the parasitic thermal effects (PTEs) in the Harman method, Z measured for a single material could vary more than 50% among the samples with different dimensions or different electrical contact configurations368.…”

“…Even with thin electric wires (diameters of few tens of μm), the heat flows via the wires reduce the temperature gradient across the specimen as well as DC voltage response368. Simply reducing the thermal conductance of the electric wires would not completely solve the problem, as it mostly leads to the increases of the electrical resistance and also Joule heating of the wires.…”

“…Simply reducing the thermal conductance of the electric wires would not completely solve the problem, as it mostly leads to the increases of the electrical resistance and also Joule heating of the wires. Radiative heat transfer considerably increases at high temperature, if the sample and the surrounding are at different temperatures3479. Thus, the use of radiation shield is helpful to lower the error due to the radiation below few percentages47.…”

“…There have been various efforts to correct the PTEs involved in the Harman method3468910. Heat transfer models that account for the TE effects and the PTEs were presented3468910.…”

“…Heat transfer models that account for the TE effects and the PTEs were presented3468910. Models predict that a sample with smaller thermal conductance, i.e.…”