Jean-Claude Tedenac

Nickel-substituted skutterudites: synthesis, structural and electrical properties

Abstract Based on AB 3 (A=Co, Ir, Rh and B=P, As and Sb) compounds, filled skutterudites (MA 4 B 12 ; M=rare earth) are promising new thermoelectric elements. Increasing their figure of merit requires realizing substitutions to give these compounds semiconducting character with a carrier concentration around 10 19 cm −3 and decreasing their thermal conductivity. In this framework, many published works have been devoted to CeFe 4 Sb 12 which, although metallic due to the trivalent character of the cerium ion, is the starting compound for numerous substitutions. For Ce y Fe 4− x Ni x Sb 12 , the substitution of nickel for iron gives semiconducting compounds. In this paper, we present our results on these compounds synthesized by the thermal treatment usually applied to obtain CeFe 4 Sb 12 .

Mechanical alloying of a new promising thermoelectric material, Sb3Zn4

We report in this paper a study of the mechanical alloying (MA) process for a high-performance p-type thermoelectric material, Sb3Zn4. Three samples belonging to the homogeneity range of Sb3Zn4 and one with a slightly higher content in zinc have been prepared by MA. From systematic analyses of the latter one, the mechanism of formation for Sb3Zn4 has been studied. The powders have been characterised by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. The results lead us to conclude that MA powder of Sb3Zn4 is produced by solid state diffusion. With the experimental conditions used in this work, single phase material can not be obtained directly from MA, its formation requires a further heating at low temperatures. Since this heating occurs in the industrial processes for solidification and net-shaping to thermoelectric devices, MA seems to be a good alternative method of preparation for Sb3Zn4.

Joining of nickel monoaluminide to a superalloy substrate by high pressure self-propagating high-temperature synthesis

A parallelepipedic nickel monoaluminide (NiAl) has been synthesised by self-propagating high-temperature synthesis (SHS) from an equimolar mixture of nickel and aluminium powders and simultaneously joined to a superalloy substrate with the same shape. By heating the joining couple (compact (Ni+Al)/superalloy substrate) to approximately 920 K, an exothermic reaction synthesis, Ni+Al→NiAl, spontaneously starts and propagates along the powder compact. The temperature of the compact quickly rises owing to the heat of the reaction and reaches the melting temperature of NiAl (1950 K) in adiabatic conditions. The heat transfer provides the superficial melting of the substrate which wetted the contact surfaces and partially dissolved the NiAl compound. In such conditions, an aluminium-rich nickel base superalloy has been produced at the interface. The SHS joining process can be referred to as exothermic welding. This process involves different metallurgical phenomena such as fusion, dissolution, diffusion and solidification.

Phase stability and physical properties of Ta 5 Si 3 compounds from first-principles calculations

We present a study of the thermodynamic and physical properties of Ta5Si3 compounds by means of density functional theory based calculations. Among the three different structures (D8m, D8l, D88), the D8l structure (Cr5B3-prototype) is the low temperature phase with a high formation enthalpy of -449.20kJ/mol, the D8m structure (W5Si3-prototype) is the high temperature phase with a formation enthalpy of -419.36kJ/mol, and the D88 structure (Mn5Si3-prototype) is a metastable phase. The optimized lattice constants of the different Ta5Si3 compounds are also in good agreement with the experimental data. The electronic density of states (DOS) and the bonding charge density have also been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. The elastic constants of the D8m and D8l structures have been calculated together with the different moduli. Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion and the Gruneisen parameter have also been obtained in the present work. The transformation temperature (2303.7K) between the D8m and the D8l structures has been predicted by means of the Gibbs energy, and this predicted temperature (2303.7K) is close to the experimental value (2433.5K).

Comparative studies between the growth characteristics of Bi2Te3 thin films deposited on SiO2, Si(100) and Si(111)

Thin films of Bi2Te3 on SiO2, Si(100) and Si(111) were deposited using a hot wall epitaxy (HWE) system. The growth conditions were optimized by the criterion of the highest mobility. XRD and SEM analysis show that the films obtained were stoichiometric and present a good morphology. No difference was observed between layers deposited on Si(100) and on Si(111): the layers were not sensitive to the initial orientation of silicon. The electrical measurements performed at room temperature show that the quality of layers deposited on SiO2 was better than the quality of layers deposited on Si(100) and Si(111). The figure of merit obtained in the case of the SiO2 substrate was Z = 1.9 × 10−3K−1, which is close to those reported for the monocrystal. The study of the first growing stage shows that the difference obtained between the substrates can be explained by the degree of reactivity of their surfaces.

Physical properties of thermoelectric zinc antimonide using first-principles calculations

We report first-principles calculations of the structural, electronic, elastic, and vibrational properties of the semiconducting orthorhombic ZnSb compound. We study also the intrinsic point defects in order to eventually improve the thermoelectric properties of this already very promising thermoelectric material. Concerning the electronic properties, in addition to the band structure, we show that the Zn (Sb) crystallographically equivalent atoms are not exactly equivalent from the electronic point of view. Lattice dynamics, elastic, and thermodynamic properties are found to be in good agreement with the experiments and they confirm the nonequivalency of the zinc and antimony atoms from the vibrational point of view. The calculated elastic properties show a relatively weak anisotropy and the hardest direction is the y direction. We observe the presence of low energy modes involving both Zn and Sb atoms at about 5-6 meV, similar to what has been found in Zn4Sb3, and we suggest that the interactions of these modes with acoustic phonons could explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the most stable defects, and this explains the intrinsic p-type conductivity of ZnSb.

Mechanical alloying and sintering of lead telluride

Abstract Lead telluride, appropriately doped, has promising good thermoelectric properties. It has often been utilised since the 1960s in thermogenerators. Conventional solidification was used in this case. Now, for industrial applications, we must reduce the cost of fabrication, meanwhile thermoelectric properties must be enhanced. In this study, we develop thermoelectric materials for a new process, mechanical alloying, which permits us to obtain smaller grain size powders. Moreover, we have succeeded in sintering these powders.

Thermoelectric properties of Bi2Te3 material obtained by the ultrarapid quenching process route

Abstract Bismuth telluride materials were fabricated by ultrarapid quenching. Foils are obtained with a thickness varying from 10 to 60 μm. The thermoelectric properties were determined measuring electrical resistivity, Seebeck coefficient and Hall coefficient. The influence of quenching temperature and heat treatment on the Seebeck coefficient was studied. The variation of thermoelectric properties with temperature was also studied. N-type degenerated materials were obtained with a carrier concentration of 10 27 m −3 .

Studies of the phase equilibria in the Ag-Sn-Zn system

Abstract The isothermal section of the equilibrium phase diagram Ag–Sn–Zn has been constructed at 380°C using optical and scanning electron microscopy, X-ray analyses, microhardness measurements and differential scanning calorimetry. The ternary β phase (solid solution range of the binary AgZn β phase) contains up to ≈13 at.% Sn, the ϵ phase about 0.2 at.% Sn. The solubility of Zn in the Ag–Sn-based intermetallic phases is determined for the first time. The microhardness of the γ phase (≈450 MPa) is the highest in the system Ag–Sn–Zn. A linear dependence of the microhardness versus mol fraction of silver has been revealed for the ϵ phase.

Prospective investigations of orthorhombic ZnGeN2: synthesis, lattice dynamics and optical properties

Abstract We have reported the synthesis of orthorhombic ZnGeN2 with extremely intense photoluminescence signal in the visible portion of the spectrum. The first analysis of the lattice dynamics of this crystalline phase has been proposed by using both infra-red and micro-Raman spectroscopy.