X-ray absorption and optical spectroscopy studies of (Mg1−x_{1-x}Alx_x)B2_2

X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$ for \emph{x} = 0.0,0.1, 0.2, 0.3, and 0.4. The important role of B 2\emph{p} $\sigma$ hole states to superconductivity has been identified, and the decrease in the hole carrier number is \emph{quantitatively} determined. The rate of the decrease in the hole concentration agree well with the theoretical calculations. On the other hand,while the evolution of the electronic structure is gradual through the doping range, $T_c$ suppression is most significant at \emph{x} = 0.4. These results suggest that the superstructure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$, in addition to the $\sigma$ holes, can affect the lattice dynamics and contributes to the $T_c$ suppression effect. Other possible explanations like the topological change of the $\sigma$ band Fermi surface are also discussed.Comment: 17 pages, 5 figures. Phys. Rev. B, in pres

Unlocking microwatt power: enhanced performance of Fe–V–Al thin films in thermoelectric microgenerators

Microwatt power output was obtained in thermoelectric microgenerators based on cost-effective and non-toxic Fe–V–Al thin films deposited by a DC magnetron co-sputtering process. A maximum electrical power of almost 5 μW at a temperature difference of 134 K was measured. This result leads to a maximum power density of 58.5 ± 6 mW cm−2, which is among the highest values obtained by a microdevice. Contrary to what is observed for other thermoelectric materials like Bi2Te3 or PEDOT composites, the performances of the present devices, assembled with junctions between Fe–V–Al and aluminum electrodes, are weakly impacted by their contact resistance. These results are very encouraging for the development of new architectures based on low-resistance Fe–V–Al thin films to power autonomous sensors in the Internet of Thing domain

Effect of Nanostructuring on the Thermoelectric Properties of Co0.97Pd0.03Sb3

International audienceWe synthesized n-type CeO2/Co0.97Pd0.03Sb3 composites with nanometric grain sizes (200 nm to 300 nm) by spark plasma sintering in order to promote phonon scattering at grain boundaries. Powdered samples were initially obtained by ball milling Co0.97Pd0.03Sb3 together with x vol.% (x = 0, 0.5, 1, 2) of CeO2 nanoparticles. This additive slows down the grain size growth of the skutterudite matrix which occurs during sintering, thereby contributing to phonon scattering. The nanostructured samples display reduced Hall electron concentration compared with that of the reference Co0.97Pd0.03Sb3 because of Fe contamination by the steel balls and vials. However, the electronic transport properties are nearly identical to those of Co0.98Pd0.02Sb3, which allows for comparison with this latter compound. The lattice thermal conductivity is strongly decreased in nano-Co0.97Pd0.03Sb12 (-40% at 300 K). This results in an enhanced (+32%) ZT value peaking at 0.65 at 650 K in nano-Co0.97Pd0.03Sb12 + 2% CeO2 when compared with micro-Co0.98Pd0.02Sb3

Crystal structure and valence behavior of YbPd_xGa_11-x

The ternary phase YbPdxGa11-x has been synthesized from the elements by high frequency argon melting. An extended homogeneous region at 600 degrees C has been established from X-ray powder data for 2.3 less than or equal to x less than or equal to 3.4. The crystal structure of YbPd3Ga8 has been derived from X-ray single crystal counter data: partially ordered BaHg11-type, space group Pm3m(O-h(1), No. 221), a= 8.4346(5)Angstrom:R-p = 0.054 for 176 independent reflections with I>2 sigma. YbPdxGa11-x samples are temperature independent Pauli-type paramagnets for x less than or equal to 3. Increasing the palladium content to x>3 leads to an enhanced f-d electron interaction, and a variation of the magnetic moment due to valence instabilities (4f(14)-->4f(13)) is established using X-ray absorption spectroscopy

Recherche de nouveaux matériaux thermoélectriques par criblage ab-initio de composés intermétalliques ternaires

International audienceDans le but de trouver de nouveaux matériaux thermoélectriques, une approche de criblage sur un large ensemble large de composés ternaires par des méthodes de calculs des propriétés électroniques basées sur la théorie de la fonctionnelle de la densité (DFT) a été mise en oeuvre. Cette approche a permis d’examiner 570 compositions possibles d’intermétalliques ternaires TMX: T un métal de transition des colonnes Ti, V, Cr, une terre rare ou un alcalino-terreux, M un élément de la première ligne des métaux de transition et X un élément sp (Al, Si, P, Sn et Sb). Le processus de criblage se fait en plusieurs étapes dans le but d’identifier les composés stables et ceux qui sont semi-conducteurs.Dans un premier temps, une étude systématique des bases de données cristallographiques a permis d’identifier quatre types structuraux prédominants pour la stœchiométrie « 1 :1 :1 » : TiNiSi (Pnma, 62), MgAgAs (F-43m, 216), BeZrSi(P63/mmc, 194) and ZrNiAl (P-62m, 189). Toutes les compositions possibles (13 T * 9 M * 5 X = 570) ont été envisagées et calculées par DFT pour les 4 prototypes structuraux. Ces calculs ont permis de prédire pour chaque composition si elle est stable thermodynamiquement, et si oui quel est le prototype structural le plus favorable. Ensuite, la structure électronique de chaque composé a été analysée afin d’éliminer les TMX qui sont métalliques pour ne garder que les semi-conducteurs. Ce travail de criblage a ainsi permis d’identifier 472 composés stables parmi lesquels 21 sont prédits comme étant non-métalliques

Looking for new semiconductors among the TMX intermetallic using high-throughput calculations

International audienceNovel high-performance materials are needed for developing thermoelectric devices. Oneway to answer this need is to screen a large number of compounds by first-principlescalculations and to synthesize the most promising compounds.We focus our investigation on the ternary intermetallic compounds T-M-X, with T a transitionmetal, a rare earth or an alkaline earth metal, M an element from the first line of thetransition metal and X, a metalloid. For each prototype, all the possible T-M-X combinationsare investigated by DFT calculations. Among all these candidates, those which arepredicted as thermodynamically unstable and metallic are dismissed. For the mostpromising compounds, predicted as stable and semiconductor, complementary calculationsas the phonon band structure and additional BoltzTrap electronic transport calculations arecarried out to better assess their thermoelectric properties. After those theoretical steps,experimental investigations are performed to confirm calculations.The study of the equimolar TMX in four different prototypes lead to the investigation of 2280possible configurations. Then our screening process allowed us to highlight more than 45possible new semiconductors. Among these, new compositions such as HfCoP, SrCuSb orTaFeSb were predicted to be stable and non-metallic. Complementary calculations allowedto estimate their lattice thermal conductivity and Seebeck coefficient

Mesostructured thermoelectric Co1−yMySb3 (M = Ni, Pd) skutterudites

International audiencen-type skutterudites Co0.97Pd0.03Sb3 + nano-CeO2 and Co0.94Ni0.06Sb3 + nano-CeO2 nanocomposites were synthesized by ball milling and spark plasma sintering in order to promote phonon scattering at the grain boundaries. Addition of nano-CeO2 slows down the grain size growth of the skutterudite matrix which occurs during sintering, leading to grain sizes as small as respectively 220 and 170 nm in Co0.97Pd0.03Sb3 + 2% CeO2 and Co0.94Ni0.06Sb3 + 3% CeO2. Subsequently, the lattice thermal conductivity is reduced by up to 47% at 300 K. An experimental value of ~110 nm can be estimated for the bulk phonon mean free path in both compounds. In Co0.94Ni0.06Sb3 + 3% CeO2, an improved ZT (~0.7) is obtained at 700 K. Annealing Co0.97Pd0.03Sb3 + 2% CeO2 at 825 K leads to a slightly increased grain size (240 nm), which remains stable on a long term (95 days)

Ground State Properties in the TiNiSi structure-type

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High thermoelectric figure of merit in mesostructured In0.25Co4Sb12 n-type skutterudite

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