In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, zT.Advice for improving the data quality in Seebeck coefficient, electrical resistivity, and thermal conductivity (from flash diffusivity and heat capacity) measurements are given together with methods for identifying possible erroneous data. Measurement of the Hall coefficient and calculation of the charge carrier concentration and mobility is also included due to its importance for understanding materials. It is not intended to be a complete record or comparison of all the different techniques employed in thermoelectrics. Rather, by providing an overview of common techniques and their inherent difficulties it is an aid to new researchers or students in the field. The focus is mainly on high temperature measurements but low temperature techniques are also briefly discussed. Measurement guide for authors and reviewersMeasurements should always be repeatable on the same sample, and on new samples produced in the manner described. Thermoelectric effects are steady-state effects so any time dependence or hysteresis is indication that phenomena outside thermoelectric effects are at play. Materials with chemical oxidants/ reductants incorporated are likely to contain unstable internal voltages not due to thermoelectric effects. Unconventional samples or measurement methods deserve reexamination of assumptions. AccuracyTrue accuracy is not represented by a single heating curve from one sample, even with error bars representing instrument precision. Showing heating and cooling data and multiple samples gives a better indication of measurement variability for a typical type of sample. Anisotropy, cracks and inhomogeneities can lead to large variation in measurements. One unusual data point or sample outside the trend, particularly at temperatures just prior to decomposition, usually indicates a problem in sample or measurement. Unusual resultsTypical thermoelectric materials behave like heavily doped semiconductors with thermopower (absolute value of Seebeck coefficient) of less than 300 mV K À1 , resistivity of 0.1-10 mU cm, and are optimized when electronic contribution to the thermal conductivity is about 1/2 the total thermal conductivity. Extraordinary results should be checked by extra means. Unusual results can be caused by bad contacts, thermocouples that have broken, chemically reacted, or simply dried out of calibration. Exceptional resultsReported values of zT > 1 or in unexpected materials receive extra attention from reviewers who may ask for additional conrmation. Convincing measurements may need to be performed on the same sample along the same direction and be repeatable with other samples and measurement methods. There is no official record keeping for claimed or veried zT values. Several papers, patents and press releases have claimed extraordinarily high zT but most have been forgotten over time and likely resul...

Study of laser megajoule calorimeter's thermal behaviour for energy measurement uncertainty optimisation Rev. Sci. Instrum. 84, 014902 (2013) Impedance spectroscopy on ceramic materials at high temperatures, considering stray fields and electromagnetic noise Rev. Sci. Instrum. 84, 015118 (2013) An apparatus for concurrent measurement of thermoelectric material parameters Rev. Sci. Instrum. 84, 013907 (2013) Casimir probe based upon metallized high Q SiN nanomembrane resonator Rev. Sci. Instrum. 84, 015115 (2013) Four-electrode impedance spectrometer for investigation of solid ion conductors Rev. Sci. Instrum. 84, 013902 (2013) Additional information on Rev. Sci. Instrum. The implementation of the van der Pauw (VDP) technique for combined high temperature measurement of the electrical resistivity and Hall coefficient is described. The VDP method is convenient for use since it accepts sample geometries compatible with other measurements. The technique is simple to use and can be used with samples showing a broad range of shapes and physical properties, from near insulators to metals. Three instruments utilizing the VDP method for measurement of heavily doped semiconductors, such as thermoelectrics, are discussed.

While thermoelectric materials can be used for solid state cooling, waste heat recovery, and solar electricity generation, low values of the thermoelectric figure of merit, zT, have led to an efficiency too low for widespread use. Thermoelectric effects are characterized by the Seebeck coefficient or thermopower, which is related to the entropy associated with charge transport. For example, coupling spin entropy with the presence of charge carriers has enabled the enhancement of zT in cobalt oxides. We demonstrate that the coupling of a continuous phase transition to carrier transport in Cu2Se over a broad (360–410 K) temperature range results in a dramatic peak in thermopower, an increase in phonon and electron scattering, and a corresponding doubling of zT (to 0.7 at 406 K), and a similar but larger increase over a wider temperature range in the zT of Cu1.97Ag.03Se (almost 1.0 at 400 K). The use of structural entropy for enhanced thermopower could lead to new engineering approaches for thermoelectric materials with high zT and new green applications for thermoelectrics.

The unique properties of graphene, transition-metal dichalcogenides and other two-dimensional (2D) materials have boosted interest in layered coordination solids. In particular, 2D materials that behave as both conductors and magnets could find applications in quantum magnetoelectronics and spintronics. Here, we report the synthesis of CrCl(pyrazine), an air-stable layered solid, by reaction of CrCl with pyrazine (pyz). This compound displays a ferrimagnetic order below ∼55 K, reflecting the presence of strong magnetic interactions. Electrical conductivity measurements demonstrate that CrCl(pyz) reaches a conductivity of 32 mS cm at room temperature, which operates through a 2D hopping-based transport mechanism. These properties are induced by the redox-activity of the pyrazine ligand, which leads to a smearing of the Cr 3d and pyrazine π states. We suggest that the combination of redox-active ligands and reducing paramagnetic metal ions represents a general approach towards tuneable 2D materials that consist of charge-neutral layers and exhibit both long-range magnetic order and high electronic conductivity.

The crystal structure uniquely imparts the specific properties of a material, and thus provides the starting point for any quantitative understanding of thermoelectric properties. Cu 2Àx Se is an intensely studied high performing, non-toxic and cheap thermoelectric material, and here for the first time, the average structure of -Cu 2Àx Se is reported based on analysis of multitemperature single-crystal X-ray diffraction data. It consists of Se-Cu layers with additional copper between every alternate layer. The structural changes during the peculiar zT enhancing phase transition mainly consist of changes in the inter-layer distance coupled with subtle Cu migration. Just prior to the transition the structure exhibits strong negative thermal expansion due to the reordering of Cu atoms, when approached from low temperatures. The phase transition is fully reversible and group-subgroup symmetry relations are derived that relate the low-temperature -phase to the high-temperature -phase. Weak superstructure reflections are observed and a possible Cu ordering is proposed. The structural rearrangement may have a significant impact on the band structure and the Cu rearrangement may also be linked to an entropy increase. Both factors potentially contribute to the extraordinary zT enhancement across the phase transition.