No abstract

Impedance spectroscopy has been shown as a promising method to characterize thermoelectric (TE) materials and devices. In particular, the possibility to determine the thermal conductivity λ, electrical conductivity σ, and the dimensionless figure of merit ZT of a TE element, if the Seebeck coefficient S is known, has been reported, although so far for a high-performance TE material (Bi2Te3) at room temperature. Here, we demonstrate the capability of this approach at temperatures up to 250 °C and for a material with modest TE properties. Moreover, we compare the results obtained with values from commercial equipment and quantify the precision and accuracy of the method. This is achieved by measuring the impedance response of a skutterudite material contacted by Cu contacts. The method shows excellent precision (random errors < 4.5% for all properties) and very good agreement with the results from commercial equipment (<4% for λ, between 4% and 6% for σ, and <8% for ZT), which proves its suitability to accurately characterize bulk TE materials. Especially, the capability to provide λ with good accuracy represents a useful alternative to the laser flash method, which typically exhibits higher errors and requires the measurement of additional properties (density and specific heat), which are not necessarily needed to obtain the ZT.

Impedance spectroscopy is a useful method for the characterization of thermoelectric (TE) modules. It can determine with high accuracy the module's dimensionless figure of merit (zT) as well as the average TE properties of the module's thermoelements. Interpretation of impedance results requires the use of a theoretical model (equivalent circuit), which provides the desired device parameters after a fitting is performed to the experimental results. Here, we extend the currently available equivalent circuit, only valid for adiabatic conditions, to account for the effect of convection at the outer surface of the module ceramic plates, which is the part of the device where convection is more prominent. This is performed by solving the heat equation in the frequency domain including convection heat losses. As a result, a new element (convection resistance) appears in the developed equivalent circuit, which starts to influence at mid-low frequencies, causing a decrease of the typically observed semicircle in the impedance spectrum. If this effect is not taken into account, an underestimation of the zT occurs when measurements are performed under room conditions. The theoretical model is validated by experimental measurements performed in a commercial module with and without vacuum. Interestingly, the use of the new equivalent circuit allows the determination of the convection heat transfer coefficient (h), if the module's Seebeck coefficient is known, and an impedance measurement in vacuum is performed, opening up the possibility to develop TE modules as h sensors. On the other hand, if h is known, all the properties of the module (zT, ohmic (internal) resistance, average Seebeck coefficient and average thermal conductivity of the thermoelements and thermal conductivity of the ceramics) can be obtained from one impedance measurement in vacuum and another measurement under room conditions.

The Harman method is used extensively for the characterization of the dimensionless figure of merit ZT of thermoelectric (TE) materials and devices. However, its accuracy has often been questioned, since in many cases there are relatively high errors associated with the method. The impedance spectroscopy technique, which has recently been shown as a suitable tool to also characterize TE materials and devices, has some similarities with the Harman method, and can also directly provide ZT. In order to obtain reliable measurements in both methods, there are some common critical points that must be taken into account, such as for example the requirement of fully adiabatic conditions, and a negligible Joule effect. In this study, we have evaluated the effect of different experimental conditions in the accuracy of both methods using a sample with known TE properties. Our analysis has led to the identification of different sources of errors and other issues that have not been clearly identified to date that can lead to inaccurate results. Namely, the need of a homogeneous Peltier effect at the junctions, problems arising from the use of Ag paint, and the selection of the right value for the current perturbation applied to the system. These problems and sources of error need to be identified and carefully considered if accurate results are to be obtained.

Skutterudite-based thermoelectric materials are promising candidates for waste heat recovery applications at intermediate temperatures (300-500 °C) owing to their high dimensionless figure of merit and power factor. Recently, several researchers have reported the high performance of skutterudite-based thermoelectric devices obtained by optimizing the crystal structure and microstructure of skutterudite materials and developing metallization layers for device fabrication. Despite extensive research efforts toward maximizing the power density and thermoelectric conversion efficiency of skutterudite-based devices, the thermoelectric properties of such devices after fabrication remain largely unknown. Here, we systematically investigated the factors that affect the thermoelectric properties of skutterudite-based devices within the range of practical operating temperatures (23-450 °C). We successfully prepared a two-couple skutterudite-based device with titanium metallization layers on both sides of the thermoelectric legs and characterized it using scanning and transmission electron microscopy and specific contact resistance measurements. Impedance spectroscopy measurements of the two-couple skutterudite-based device revealed the figure of merit of the device and enabled the extraction of three key thermoelectric parameters (Seebeck coefficient, thermal conductivity, and electrical conductivity). The impedance spectra and extracted parameters depended strongly on the measurement temperature and were mainly attributable to the thermoelectric properties of skutterudite materials. These observations demonstrate the interplay between the properties of thermoelectric materials and devices and can aid in directing future research on thermoelectric device fabrication.