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Research in copper selenide thermoelectric (TE) alloys has raised the possibility of a significant enhancement of the TE figure-of-merit ZT when the Seebeck coefficient is affected by a concurrent phase transformation. This ZT increase has also been related to a radical reduction of the thermal conductivity evaluated by transient laser flash thermal diffusivity measurements. In contrast, steady-state Harman-based measurements do not support a significant ZT increase only a modest one, because the thermal conductivity instead of decreasing goes through a sharp maximum as it approaches the critical phase transformation temperature of 407 K. The nature of this sharp increase of heat transfer has not been related to the well-known electronic or phononic contributions. Below the critical temperature, when the alloy is exposed to a steady-state temperature gradient, an additional heat transfer phenomenon takes place, induced by the ongoing gradual phase transition. We show that the enthalpy associated to the α to β and β to α phase transformations can lead to heat flow in the direction of the temperature gradient above and beyond conventional heat conduction. This unconventional heat transfer mechanism disappears when the temperature rises above the critical temperature where only a stable β phase remains. We propose a model of such a heat transport which leads to the sharp maximum of the related thermal conductivity. Numerical results obtained from the model compare favorably to the experimentally measured thermal conductivity. [ABSTRACT FROM AUTHOR]

Magnesium silicide is a very promising thermoelectric material for applications in the temperature range of 500-800 K, and is of particular interest for large-scale applications because its constituents are non-toxic, inexpensive and very abundant in the Earth’s crust. Although the hot extrusion (HE) method to compact powders has long been considered for thermoelectric applications because it lends itself easily to large-scale industrial applications, advances to obtain Mg2Si by HE are still difficult to implement. We present the transformations undergone by Mg2Si powders during the nascent HE as well as the modifications of the structural, thermal and electronic properties of the compacted solid. MoS2 particles (2 at.%) are added to the starting Mg2Si:Sb (0.5 at.%) powders which play the role of solid lubricant during this process at 873 K. Samples are extracted from different areas of the die along the extrusion direction and separately characterized, describing the transformations of the material through different stages of the nascent extrusion. X-ray diffraction reveals the expected structure for all samples without any significant texturing. The increase in grain size along the HE direction towards the exit has been determined from analysis of scanning electron microscopy observations. The thermoelectric properties have been characterized using the Harman method between 300 K and 700 K, giving Seebeck coefficients which vary between − 200  μV K−1and − 215 μV K−1 at 700 K. The thermal (λ) and electrical (σ) conductivity decrease as the sample progresses in the extrusion process, and in the case of λ can be accounted for by the increase of sample porosity. The highest figure-of-merit ZT is to be found for the sample extracted from the exit of the die. It increases with temperature reaching a maximum value of 0.32 at 700 K, the highest temperature we could attain experimentally. [ABSTRACT FROM AUTHOR]

Some materials such as Cu2−xSe, Cu1.97Ag0.03Se, and SnSe have attracted attention by demonstrating a significant enhancement of their thermoelectric performance, which is associated with a phase transition. This phenomenon, observed in a limited temperature (T) interval, results in sharp changes of the Seebeck coefficient (S), the electrical resistivity (ρ), and the thermal conductivity (κ), which may render the correct evaluation of the dimensionless figure of merit (ZT) difficult. We report the thermoelectric properties of a polycrystalline Cu2−xSe sample which is known to undergo a phase transition near 410 K, containing a mixture of α- and β-phases at room temperature, as determined by x-ray diffraction measurements. We have used a Harman-based setup (TEMTE Inc.), which assures the direct measurement of ZT at all temperatures, including the phase transition region. This approach ensures that κ(T) is determined under steady-state conditions at any given temperature, including points arbitrarily close to the transition temperature which cannot be guaranteed by previously used techniques such as laser flash. We have observed a sharp maximum for κ(T) near 410 K, similar to the reported specific heat variation, with a ZT peak value of 0.2 at 400 K. The expected gain in ZT related to the phase transition is reduced because the increase in S is counterbalanced by the increase in κ(T). Thus, our detailed assessment of the temperature variation of the individual thermoelectric properties accurately evaluates the performance enhancement associated to a structural phase transition and helps to elucidate this complex phenomenon. [ABSTRACT FROM AUTHOR]

We present a comparison of different surface preparation techniques for hot-extruded (BiSb)(TeSe) alloys prior to Ni plating and their consequences on adhesion. The surface preparation is carried out by a relatively fast and well-controlled electrochemical etching procedure that provides morphologies with homogeneous flat surfaces on a scale of tens of micrometers. This procedure is not optimal for Ni metallization since it does not ensure enough roughness on the surface. Applying wet chemical etching with hypochlorite and nitric acid after the electrochemical etching changes the morphology, and the surface roughness increases, as evidenced by scanning electron microscopy and atomic force microscopy. Tensile tests carried out on n-type and p-type 1-mm-thick specimens, covered with a 12- μm Ni layer followed by Sn-Pb soldered joints, confirm a significant improvement in the ultimate adhesion strength, with fracture occurring within the thermoelectric material. Use of this two-step surface treatment, after slicing and prior to metallization, leads to maximum adhesion strength of around 38 MPa for either type of thermoelectric material. These results are consistent with the morphological changes observed by scanning electron microscopy on the surface after chemical etching. [ABSTRACT FROM AUTHOR]