Rodolphe Decourt

Reversible switching between p- and n-type conduction in the semiconductor Ag10Te4Br3

Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag(10)Te(4)Br(3) is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 microV K(-1), in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.

Large thermoelectric power factors and impact of texturing on the thermal conductivity in polycrystalline SnSe

Single crystals of SnSe have been reported to have very high thermoelectric efficiencies with a maximum figure merit zT = 2.5. This outstanding performance is due to ultralow thermal conductivities. We report on the synthesis of highly textured polycrystalline SnSe ingots with large single-crystal magnitude power factors, S2/ρ = 0.2–0.4 mW m−1 K−2 between 300–600 K, increasing to 0.9 mW m−1 K−2 at 800 K, and bulk thermal conductivity values κ300K = 1.5 W m−1 K−1. However, small SnSe ingots, which were measured in their entirety, were found to have a substantially reduced κ300K = 0.6 W m−1 K−1. Microscopy and diffraction revealed two distinct types of texturing within the hot-pressed ingots. In the interior, large coherent domains of SnSe platelets with a ∼45° orientation with respect to the pressing direction are found, while the platelets are preferentially oriented at 90° to the pressing direction at the top and bottom of the ingots. Fitting the κ(T) data suggests an increase in defect scattering for the smaller ingots, which is in keeping with the presence of regions of structural disorder due to the change in texturing. Combining the measured S2/ρ with the bulk ingot κ values yields zT = 1.1 at 873 K.

Purification of molybdenum, growth and characterization of medium volume ZnMoO4 crystals for the LUMINEU program

The LUMINEU program aims at performing a pilot experiment on neutrinoless double beta decay of 100Mo using radiopure ZnMoO4 crystals operated as cryogenic scintillating bolometers. Growth of high quality radiopure crystals is a complex task, since there are no commercial molybdenum compounds available with the required level of purity and radioactive contamination. This paper discusses approaches to purify molybdenum and synthesize compounds for high quality radiopure ZnMoO4 crystal growth. A combination of a double sublimation (with addition of zinc molybdate) with subsequent recrystallization in aqueous solutions (using zinc molybdate as a collector) was used. Zinc molybdate crystals up to 1.5 kg were grown by the low-thermal-gradient Czochralski technique; their optical, luminescent, diamagnetic, thermal and bolometric properties were tested.

Glass-like thermal conductivity in SrTiO3 thermoelectrics induced by A-site vacancies

The introduction of A-site vacancies in SrTiO3 results in a glass-like thermal conductivity while Nb substituted samples maintains good electrical conductivity. This unexpected result brings SrTiO3 one step closer to being a high-performing phonon-glass electron-crystal thermoelectric material.

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.

Development and underground test of radiopure ZnMoO4 scintillating bolometers for the LUMINEU 0 nu 2 beta project

The LUMINEU (Luminescent Underground Molybdenum Investigation for NEUtrino mass and nature) project envisages a high-sensitivity search for neutrinoless double beta (0 nu 2 beta) decay of Mo-100 with the help of scintillating bolometers based on zinc molybdate (ZnMoO4) crystals. One of the crucial points for the successful performance of this experiment is the development of a protocol for producing high quality large mass ZnMoO4 crystal scintillators with extremely high internal radiopurity. Here we report a significant progress in the development of large volume ZnMoO4 crystalline boules (with mass up to 1 kg) from deeply purified materials. We present and discuss the results achieved with two ZnMoO4 samples (with mass of about 0.3 kg each): one is a precursor of the LUMINEU project, while the other one was produced in the framework of LUMINEU with an improved purification / crystallization procedure. The two crystals were measured deep underground as scintillating bolometers in the EDELWEISS dilution refrigerator at the Laboratoire Souterrain de Modane (France) protected by a rock overburden corresponding to 4800 m w.e. The results indicate that both tested crystals are highly radiopure. However, the advanced LUMINEU sample shows a clear improvement with respect to the precursor, exhibiting only a trace internal contamination related with Po-210 at the level of 1 mBq/kg, while the activity of Ra-226 and Th-228 is below 0.005 mBq/kg. This demonstrates that the LUMINEU purification and crystal-growth procedures are very efficient and leads to radiopurity levels which exceedingly satisfy not only the LUMINEU goals but also the requirements of a next-generation 0 nu 2 beta experiment.

Flux growth at 1230 °C of cubic Tb2O3 single crystals and characterization of their optical and magnetic properties

In this work, we present the first crystal growth of cubic Tb2O3 single crystals by a controlled atmosphere flux method which uses a heavy metal free solvent working at less than half the melting temperature of this sesquioxide. Cubic millimeter-sized crystals extracted from as-grown boules are phase (powder XRD) and chemically (GDMS) pure and exhibit a Verdet constant in the visible and near-infrared spectral ranges, which is at least three times higher than that of a commercial Tb3Ga5O12 (TGG) crystal. The 1.36 mm thick crystals display a transmission coefficient higher than 77% over the 525 nm–1.38 μm spectral range. The absorption spectrum, magnetic susceptibility and specific heat measurements on the single crystals confirm the absence of detectable Tb4+ cations and other impurities.

Structure and properties of alkali cobalt double oxides A(0.6)CoO(2) (A = Li, Na, and K).

Lamellar A(x)CoO(2) cobalt double oxides with A = Li, Na, and K (x approximately 0.6) have been synthesized and their chemical (alkali content, oxidation state, and structure) and physical (resistivity, thermopower, magnetization, and specific heat) properties have been studied. All the three materials exhibit strong electron correlation emphasized by their behavior ranging from Fermi liquid to spin-polarized system. Our results show that both the dimensionality of the interactions and the nature of the alkali play a determining role on the properties.

Effect of co-substitution of Mn and Al on thermoelectric properties of chromium disilicide

CrSi2 was earlier reported to be an interesting thermoelectric material for high temperature applications because of its high oxidation resistance and good mechanical properties. In order to enhance its figure of merit, Mn at Cr site and Al at Si site were substituted into CrSi2. Our results indicate that Cr1−xMnxSi2−xAlx solid solutions exhibit significantly lower thermal conductivity and a higher figure of merit than CrSi2.

Growth and spectroscopic properties of 6Li- and 10B-enriched crystals for heat-scintillation cryogenic bolometers used in the rare events searches

We present the crystal growth of centimeter-sized 6Li6Eu(10BO3)3 and Li6(Eu,Gd)(BO3)3 single crystals by combined Czochralski and Kyropoulos methods, initiated on specifically oriented seeds, with a heretofore unexplored concentration range for heat-scintillation cryogenic bolometers application. The crystal structure and lattice thermal expansion of the former, obtained for the first time by single crystal X-ray diffraction (XRD), and the spectroscopic characterizations together with a consistent set of related thermodynamical properties measurements (magnetic susceptibility, specific heat), also unknown to date, are discussed. Finally, preliminary scintillation measurements at low temperature between 300 and 1100 nm under X-ray excitation are shown.