Harman method

Direct measurement of the figure of merit ZT using the Harman method

The thermoelectric figure of merit ZT can be determined either through calculations based on individual measurements of Electrical Conductivity, Seebeck Coefficient, and Thermal Conductivity, or through direct measurement methods such as the Harman method.

a) Schematic setup for direct ZT measurement of thermoelectric materials using the Harman method. b) Measuring curve of a Harman measurement with ohmic voltage drop UR and occurring thermal voltage Uth with applied measuring current I.

In the Harman method, a thermoelectric sample, typically in the form of a cylinder or rod, is electrically contacted on its two end faces and with two additional contacts on one of the side faces. To ensure measurement accuracy, it is crucial to establish adiabatic conditions by minimizing heat exchange between the sample and the environment. Typically conducted under vacuum conditions, the sample may be free-standing or affixed to a temperature-controlled base plate, and a heat shield is often employed to reduce radiation-induced heat exchange.

The measurement begins by applying a current (I0) between the contacts on the end faces. This causes one transition between the sample and electrode to heat up while the other cools due to the Peltier effect. Adiabatic conditions result in a temperature gradient along the sample, determined by the applied current and the thermoelectric properties of the sample. Voltage probes on the side surface of the sample measure both the ohmic component of the voltage drop (UR) immediately after current application and the thermal voltage (Uth) in the steady-state. The thermoelectric figure of merit ZT can then be directly calculated from the ratio of these two voltages.

The accuracy of the Harman method hinges on maintaining adiabatic conditions and minimizing contact resistance between the sample and current electrodes. Consequently, the maximum operating temperature of the Harman setup is typically limited to 200°C to 300°C.

Which properties are evaluated?

The Harman method allows for the direct measurement of ZT, offering lower error rates under favorable experimental conditions compared to the conventional approach of calculating ZT from separate measurements conducted by instruments like the LSR-3 and LFA machine. Additionally, the Harman method enables the calculation of thermal conductivity as a material property, providing a cost-effective solution for assessing the thermal transport properties of thermoelectric samples using standard platforms like the Linseis LSR-3.

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