Kasper A. Borup

Measurement of the electrical resistivity and Hall coefficient at high temperatures

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.

Measuring thermoelectric transport properties of materials

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.

Phase transition enhanced thermoelectric figure-of-merit in copper chalcogenides

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 Cu 2Se 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 Cu 1.97 Ag .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.

Enhanced thermoelectric properties of Mg2Si by addition of TiO2 nanoparticles

The effects on the thermoelectric properties of Mg2Si when adding TiO2 nanoparticles have been evaluated experimentally. A batch of Mg2Si was prepared through direct solid state reaction and divided into portions which were mechanically mixed with different amounts of TiO2 nanoparticles ranging from 0.5 to 3 vol% and subsequently sintered to disks. All materials showed n-type conduction and the absolute value of the Seebeck coefficient was reduced with increasing amount of TiO2 added, while the electrical resistivity was greatly reduced. The thermal conductivity was surprisingly little affected by the addition of the nanoparticles. An optimum value of the thermoelectric figure-of-merit ZT = TS2 sigma/k was found for the addition of 1 vol% TiO2, showing almost three times higher ZT value than that of the pure Mg2Si. Larger TiO2 additions resulted in lower ZT values and with 3 vol% added TiO2 the ZT was comparable to the pure Mg2Si. The sintering process resulted in reduction or chemical reaction of all TiO2 to TiSi2 and possibly elemental titanium as well as reduced TiOx. The increased electrical conductivity and the decreased Seebeck coefficient were found due to an increased charge carrier concentration, likely caused by the included compounds or titanium-doping of the Mg2Si matrix. The low observed effect on the thermal conductivity of the composites may be explained by the relatively higher thermal conductivity of the included compounds, counter-balancing the expected increased grain boundary scattering. Alternatively, the introduction of compounds does not significantly increase the concentration of scattering grain boundaries.

Phase Separation and Bulk p‐n Transition in Single Crystals of Bi2Te2Se Topological Insulator

Bismuth chalcogenide (Bi 2 Ch 3 ) alloys are among the most extensively studied and commonly used thermoelectric materials. [ 1 ] They are also currently of great interest in condensedmatter physics as prototypical three-dimensional topological insulators (TIs), due to the existence of stable and topologically protected Dirac like states on the surface. [ 2 , 3 ] Shubnikov– de Haas and weak-fi eld Hall anomalies show a substantially enhanced surface current and mobility of the surface states over bulk Bi 2 Te 3 values. [ 4 ] However, it is challenging to investigate the charge-transport characteristics of the surface states directly, as the charge transport is dominated by the bulk properties. [ 3–5 ] Most studies of TIs in the Bi 2 Ch 3 family have focused on binary Bi 2 Se 3 and Bi 2 Te 3 systems. Both compounds are semiconductors, but they display high bulk conductivities due to intrinsic defects. [ 4–6 ] A conversion from nto p-type conduction in Bi 2 Te 3 thin fi lms is observed when the growth condition changes. [ 5b ] Recently, the ternary compound Bi 2 Te 2 Se has been suggested to be the best material for studies of the surface transport due to its large bulk resistivity. [ 3 , 6–8 ] However, controlling the bulk conductivity is diffi cult, even for Bi 2 Te 2 Se, due to unintentional doping by crystal defects. [ 6 , 8 ] Here we show that the diffi culty of making high-quality Bi 2 Te 2 Se single crystals originates from the internal features of the specifi c solid-state composition and phase separation in Bi 2 Te 2 Se. Bi 2 Ch 3 has the tetradymite-type rhombohedral structure, which can be described by the R-3m space group. The lattice can be regarded as a hexagonal layered structure in which the layers stack in the sequence of Ch 1 -Bi-Ch 2 -Bi-Ch 1 , where Ch is Te or Se in the present study. Each unit cell consists of three of these fi velayer groups, which are connected by bonds with a high degree of van der Waals character between the Ch 1 -Ch 1 layers. Early phase-diagram studies showed that Bi 2 Te x Se y ( x + y = 3) compounds form continuous solid solutions at temperatures above 500 ° C; however, the crystal structures tend to be ordered at

Crystal structure across the β to α phase transition in thermoelectric Cu2−xSe

The average structure of β-Cu2−xSe is reported based on analysis of multi-temperature single-crystal X-ray diffraction data, and structural changes, including a large negative thermal expansion, across the β to α phase transition are discussed. The structural model also describes well high-resolution synchrotron powder X-ray diffraction data.

“Glass-like” thermal conductivity gradually induced in thermoelectric Sr8Ga16Ge30 clathrate by off-centered guest atoms

The origin of the “glass-like” plateau in thermal conductivity of inorganic type I clathrates has been debated for more than a decade. Here, it is demonstrated that the low temperature thermal conductivity of Sr8Ga16Ge30 can be controlled by the synthesis method: A flux-grown sample has a “glass-like” plateau in thermal conductivity at low temperature, while a zone-melted sample instead has a crystalline peak. A combination of flux-growth and zone-melting produces an intermediate thermal conductivity. In a comprehensive study of three single crystal samples, it is shown by neutron diffraction that the transition from crystalline peak to “glass-like” plateau is related to an increase in Sr guest atom off-centering distance from 0.24 A to 0.43 A. By modifying ab initio calculated force constants for the guest atom to an isotropic model, we reproduce both measured heat capacity and inelastic neutron scattering data. The transition from peak to plateau in the thermal conductivity can be modeled by a combined ...