Eric Alleno

Invited Article: A round robin test of the uncertainty on the measurement of the thermoelectric dimensionless figure of merit of Co0.97Ni0.03Sb3

A round robin test aiming at measuring the high-temperature thermoelectric properties was carried out by a group of European (mainly French) laboratories (labs). Polycrystalline skutterudite Co0.97Ni0.03Sb3 was characterized by Seebeck coefficient (8 labs), electrical resistivity (9 labs), thermal diffusivity (6 labs), mass volume density (6 labs), and specific heat (6 labs) measurements. These data were statistically processed to determine the uncertainty on all these measured quantities as a function of temperature and combined to obtain an overall uncertainty on the thermal conductivity (product of thermal diffusivity by density and by specific heat) and on the thermoelectric figure of merit ZT. An increase with temperature of all these uncertainties is observed, in agreement with growing difficulties to measure these quantities when temperature increases. The uncertainties on the electrical resistivity and thermal diffusivity are most likely dominated by the uncertainty on the sample dimensions. The temperature-averaged (300-700 K) relative standard uncertainties at the confidence level of 68% amount to 6%, 8%, 11%, and 19% for the Seebeck coefficient, electrical resistivity, thermal conductivity, and figure of merit ZT, respectively. Thermal conductivity measurements appear as the least accurate. The moderate value of the temperature-averaged relative expanded (confidence level of 95%) uncertainty of 17% on the mean of ZT is essential in establishing Co0.97Ni0.03Sb3 as a high temperature standard n-type thermoelectric material.

A comprehensive study of the crystallization of Cu–As–Te glasses: microstructure and thermoelectric properties

We report a thorough experimental study on the microstructure, thermal behavior and thermoelectric properties of the amorphous composition Cu15As30Te55 and the glass–ceramics related-compounds synthesized by using the Spark Plasma Sintering (SPS) technique. Varying the conditions of the SPS process enables the synthesis of composite glassy-crystalline samples with different crystal/glass ratios. Such treatments result in complex microstructures composed of large glassy domains where nanocrystals of the metastable β-As2Te3 phase are embedded. These domains are separated by regions of the dendritic crystalline phase surrounded by a Cu-rich glassy matrix. The presence of β-As2Te3, confirmed by both powder X-ray diffraction and scanning electron microscopy, suggests that pressure and/or internal stresses play an important role in stabilizing this phase. This conclusion is further supported by neutron thermodiffraction experiments revealing a sharp crossover from the β-As2Te3 to the stable α-As2Te3 phase at temperatures below that of the SPS treatment. Transport properties measurements show that the presence of a crystalline fraction significantly lowers the electrical resistivity by four orders of magnitude. However, the probable intrinsic n-type behavior of β-As2Te3 has a detrimental influence on the thermopower values. Even though the partial crystallization of the glassy matrix leads to an increase in the thermal conductivity, the measured values remain on the order of 1 W m−1 K−1 at 300 K. Besides an overall increase in the dimensionless figure of merit ZT, our results demonstrate that the partial crystallization of an amorphous matrix is an efficient tool to tune the electrical resistivity over several orders of magnitude while maintaining low thermal conductivity values.

Polymorphism in Thermoelectric As2Te3.

Metastable β-As2Te3 (R3̅m, a = 4.047 Å and c = 29.492 Å at 300 K) is isostructural to layered Bi2Te3 and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As2Te3 leads to multiphase samples, while β-As2Te3 could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, β-As2Te3 reconstructively transforms into stable α-As2Te3 (C2/m, a = 14.337 Å, b = 4.015 Å, c = 9.887 Å, and β = 95.06°) at 480 K. This β → α transformation can be seen as the displacement of part of the As atoms from their As2Te3 layers into the van der Waals bonding interspace. Upon cooling, β-As2Te3 displacively transforms in two steps below T(S1) = 205-210 K and T(S2) = 193-197 K into a new β-As2Te3 allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic β-As2Te3 crystal structure (P2(1)/m, a = 6.982 Å, b = 16.187 Å, c = 10.232 Å, β = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by β-As2Te3 is accompanied by a 4-fold modulation along its b axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the β-phase being more stable than the β-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As2Te3 layers in all three structures.

Effective medium theory based modeling of the thermoelectric properties of composites: comparison between predictions and experiments in the glass-crystal composite system Si10As15Te75–Bi0.4Sb1.6Te3

We report on the theoretical predictions of the effective medium theory (EMT) and its generalized version taking into account percolation theory (GEMT) on the thermoelectric properties of composites based on Landauer and Sonntags equations. The results were tested experimentally on composites composed of the glassy phase Si10As15Te75 and the crystalline phase Bi0.4Sb1.7Te3. The evolution of the electrical resistivity and thermal conductivity with the fraction of crystalline phase matches very well the experimental data, although the GEMT model fails to predict the thermopower. A better agreement between theory and experiment could be obtained by combining the principles of the GEMT and the Webman–Jortner–Cohen models. Despite the fact that the GEMT model originally predicts the possibility to optimize the dimensionless figure of merit ZT of composites by adjusting the fraction and the values of the transport properties of each phase, the new model developed rules out any beneficial influence on the ZT values. These results confirm within a different framework the early conclusions of Bergman regarding the impossibility of improving the ZT values using multi-phased materials.

Thermoelectric Properties of the alpha-As2Te3 Crystalline Phase

As2Te3 exists in two crystallographic configurations: α- and β-As2Te3, of which only the latter crystallizes in the same rhombohedral structure-type as Bi2Te3. While β-As2Te3 shows interesting thermoelectric (TE) properties which can be adjusted through alloying, the transport properties of the monoclinic phase α-As2Te3, more thermodynamically stable than the β-phase at room temperature, has not yet been studied thoroughly. We report here on the samples preparation by powder metallurgy, and on the microstructural characterization of polycrystalline α-As2Te3. Preliminary results on the electrical and thermal properties measured between 5xa0K and 523xa0K are also reported. Transport properties measurements were performed both along and perpendicular to the pressing direction indicating that the transport properties demonstrate some degree of anisotropy. Remarkably, low thermal conductivity values (below 1xa0Wxa0m−1xa0K−1 above 300xa0K) were measured suggesting that this compound may be an interesting platform to design novel TE materials with high efficiency.

Stabilization of Metastable Thermoelectric Crystalline Phases by Tuning the Glass Composition in the Cu–As–Te System

Recrystallization of amorphous compounds can lead to the stabilization of metastable crystalline phases, which offers an interesting way to unveil novel binary or ternary compounds and control the transport properties of the obtained glass ceramics. Here, we report on a systematic study of the Cu-As-Te glassy system and show that under specific synthesis conditions using the spark-plasma-sintering technique, the α-As2Te3 and β-As2Te3 binary phases and the previously unreported AsTe3 phase can be selectively crystallized within an amorphous matrix. The microstructures and transport properties of three different glass ceramics, each of them containing one of these phases with roughly the same crystalline fraction (∼30% in volume), were investigated in detail by means of X-ray diffraction, scanning electron microscopy, neutron thermodiffraction, Raman scattering (experimental and lattice-dynamics calculations), and transport-property measurements. The physical properties of the glass ceramics are compared with those of both the parent glasses and the pure crystalline phases that could be successfully synthesized. SEM images coupled with Raman spectroscopy evidence a coast-to-island or dendriticlike microstructure with microsized crystallites. The presence of the crystallized phase results in a significant decrease in the electrical resistivity while maintaining the thermal conductivity to low values. This study demonstrates that new compounds with interesting transport properties can be obtained by recrystallization, which in turn provides a tuning parameter for the transport properties of the parent glasses.

Low-Temperature Transport Properties of Bi-Substitutedβ-As2Te3 Compounds

Abstractβ-As2Te3 belongs to the family of Bi2Te3-based alloys, a well-known class of efficient thermoelectric materials around room temperature. As2Te3 exists in two allotropic configurations: α- and β-As2Te3, of which only the latter crystallizes in the same rhombohedral structure as Bi2Te3. Herein, we report on substitution of Bi for As in the As2−xBixTe3 system with xxa0=xa00.0, 0.015, 0.025, and 0.035. These samples have been characterized by x-ray diffraction and scanning electron microscopy. The transport properties have been measured at low temperatures (5xa0K to 300xa0K) in both directions, parallel and perpendicular to the pressing direction. The results are compared with those obtained in previous study on samples substituted by Sn. Compared with Sn, Bi allows for a clear decrease in electrical resistivity while maintaining the thermal conductivity below 1xa0W/(mxa0K) over the whole temperature range. As a result, a comparable peak ZT value near 0.2 was obtained at room temperature.

Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α-As2Te3–xSex (0 ≤ x ≤ 1.5)

We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5-523 K) of the solid solution α-As2Te3-xSex (0 ≤ x ≤ 1.5). All of the polycrystalline compounds α-As2Te3-xSex crystallize isostructurally in the monoclinic space group C2/m (No. 12, Z = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing x up to x = 1.5 leaves unchanged both the p-type character and semiconducting nature of α-As2Te3. The electrical resistivity and thermopower gradually increase with x as a result of the progressive increase in the band gap energy. Despite the fact that α-As2Te3 exhibits very low lattice thermal conductivity κL, the substitution of Se for Te further lowers κL to 0.35 W m-1 K-1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point-defect scattering.

Synthesis of nanocrystalline filled skutterudites by mechanical alloying

We report on the preparation of nanocrystalline filled skutterudites in the series CeyFe4-xCoxSb12 by mechanical alloying. X-ray diffraction and energy dispersive X-ray spectroscopy reveal that skutterudites can be prepared with an homogeneity range larger than what can be obtained by powder metallurgy (PM). Interestingly, no after-milling thermal treatment is required to prepare essentially single phase samples (~90-95%). This is an important difference with skutterudites prepared by PM which only form after long term annealing below the peritectic decomposition temperature. Differential thermal analysis shows that grain growth occurs above ~350degC and that the peritectic decomposition temperature is similar to PM- (i.e. 750degC for x = 1). Grain sizes are in the 12-30 nm range as deduced from Rietveld refinements of X-ray diffraction data and scanning electron microscopy. Cerium is trivalent in nanocrystalline filled skutterudites, as it is in samples prepared by PM. Both p-type and n-type skutterudites can be obtained but values of the Seebeck coefficient are of the same order of magnitude as in PM samples only for p-type skutterudites

Looking for new thermoelectric materials among TMX intermetallics using high-throughput calculations

Within 4 different crystal structures, 2280 ternary intermetallic configurations have been investigated via high-throughput density functional theory calculations in order to discover new semiconducting materials. The screening is restricted to intermetallics with the equimolar composition TMX, where T is a transition metal from the Ti, V, Cr columns, Sr, Ba, Y and La, M an element from the first line of transition metals and X a sp elements (Al, P, Si, Sn and Sb), i.e. to a list of 24 possible elements. Since the calculations are done combinatorically, every possible ternary composition is considered, even those not reported in the literature. All these TMX configurations are investigated in the 4 most reported structure-types: TiNiSi, MgAgAs, BeZrSi and ZrNiAl. With an excellent agreement between calculations and literature for the reported stable phases, we identify 472 possible stable compounds among which 21 are predicted as non-metallic. Among these 21 compositions, 4 could be considered as new semiconductors.