S. Turenne

Numerical Simulation of Performance and Thermomechanical Behavior of Thermoelectric Modules with Segmented Bismuth-Telluride-Based Legs

The approach of using segmented legs to build thermoelectric (TE) modules can enhance the performance of TE generators. This approach is based on the selection of materials for different segments that are optimized in terms of their TE properties with respect to the temperature range to which they are exposed during module operation. For this purpose, by carefully controlling the chemical composition of ternary and quaternary bismuth-telluride-based alloys, we have optimized the figure of merit ZT of p-type and n-type alloys implemented by a powder technology approach. The alloys were prepared by mechanical alloying followed by hot extrusion, and their mechanical and TE properties were fully characterized as a function of temperature, which gave us a solid database for simulation of modules containing these materials. Finite-element numerical simulation was applied to evaluate the impact of TE materials properties on the level of mechanical stresses generated by thermal gradients in modules made of segmented legs. Keeping the same total length of two-segment p- and n-type legs, the relative length of each segment was varied to obtain an 8% relative increase of generated electrical power compared with homogeneous legs of the same total length. Under these conditions, the presence of solder interface between the two segments and between the segments and the copper conductors of the module concentrates plastic strain, leading to a significant reduction of the stress level in the TE materials compared with that resulting from using nonsegmented legs. Leg segmentation not only leads to improved TE performance but could also significantly modify the maximum values and distribution of thermomechanical stresses in the modules, depending on how it is realized. The study presents how this numerical simulation tool can be used to optimize the design of segmented modules.

Influence of particle-matrix interface, temperature, and agglomeration on heat conduction in dispersions

A combination of the effective medium and the phonon approaches is used to investigate heat conduction in heterogeneous media composed of a homogeneous matrix in which spherical particles of micro and nanosizes are dispersed. In particular, we explore the effect of different types of scattering on the particle-matrix interface, temperature dependence of the effective heat conduction coefficient, and the effect of various degrees of agglomeration of the particles. Predictions calculated explicitly for Si nanoparticles dispersed in Ge matrix agree with available Monte Carlo simulations. Our predictions show that the higher is the temperature the lower is the heat conductivity and the smaller is the influence of the details of the particle-matrix interactions. As for the influence of the agglomeration, we predict both decrease and increase of the heat conduction depending on the degree of the agglomeration.

Production of thermoelectric materials by mechanical alloying - extrusion process

The extrusion process offers one of the greatest prospects for the industrial production of Bi/sub 2/Te/sub 3/-based thermoelectric alloys. When coupled with mechanical alloying, this process promises substantial cost savings because of its ability to deliver net shape components, leading to a reduction in the so-called kerf losses and higher material yields. It is also well known that materials produced using one of the various powder processing routes have greater mechanical strength when compared to conventionally grown alloys, leading to an improved reliability. We have produced both p-type (Bi/sub 2-x/Sb/sub x/Te/sub 3/) and n-type (Bi/sub 2/Te/sub 3-y/Se/sub y/) alloys using mechanical alloying and extrusion and have studied the structure, composition and thermoelectric properties of these alloys. By carefully controlling every step of the process we have found that it is possible to produce rods of thermoelectric material, for both p- and n-type alloys, which have good properties. Figures of merit of up to 3.35 /spl times/ 10/sup -3/ K/sup -1/ for p-type alloys and 2.8 /spl times/ 10/sup -3/ K/sup -1/ for n-type alloys have been obtained. A fivefold improvement in the mechanical properties over conventional melt grown material has also been observed.

Characterization of NiTi shape memory alloys using dual kriging interpolation

Abstract A large number of industrial applications could benefit from the remarkable properties of shape memory alloys (SMA). The development of a general material law is the first important step before reliable design calculations of shape memory devices can be carried out. This paper presents a new phenomenological constitutive law based on dual kriging, which is a powerful mathematical tool used here as interpolation method to simulate the macroscopic mechanical behavior of shape memory alloys. From a set of experimental strain–temperature curves at constant loads, two deformation surfaces are constructed in the stress, strain and temperature space which describe the cooling and heating behaviors of the material for any stress. The response of a specimen subjected to complex thermomechanical loading can be calculated by dual kriging form a general 3-dimensional parametric solid constructed inside the hysteretic domain delimited by the main cooling and heating deformation surfaces. This approach presents the advantage of yielding immediately the explicit equation of any partial cycle inside the main hysteretic domain, thus yielding a general material law for shape memory alloys. Preliminary validation for a set of simple examples demonstrates the potential of this new model that includes in a single formulation superelasticity, rubber-like behavior and shape memory effect.

Effective heat conduction in dispersion of wires

We derive a formula for the heat conductivity coefficient of dispersions of wires in a homogeneous matrix. Such formula is particularly useful for thermoelectric applications. The method used to derive this type of formula in Behrang et al. [J. Appl. Phys. 114, 014305 (2013)] for spherical particles is adapted to generally oriented wires of a finite length. Both diffuse and specular scatterings on the wire-matrix interface are considered. The results obtained previously from numerical solutions of the phonon kinetic equation under the assumption of diffuse scattering agree with predictions based on the formula.

Evaluation of friction conditions in powder compaction for admixed and die wall lubrication

For the purpose of modelling powder compaction, a quantitative evaluation of the friction conditions at the die wall interface is required. Using an instrumented compaction die, the friction at the interface between powder compact and die wall was evaluated through an empirical parameter called the slide coefficient, obtained from the applied pressure/transmitted pressure ratio. For powder compaction modelling, knowledge of the friction coefficient at the interface, however, is much more useful but its measurement is difficult in real experimental conditions. A procedure based on the evaluation of the radial stress/axial stress ratio leads to the quantitative determination of the friction coefficient in admixed zinc stearate added to iron powder with and without die wall lubrication. The resulting decreasing friction coefficient at the end of compaction gives a more uniform density characterised by low loss of pressure through the compact and lower ejection energy per unit volume of the compact.

Effective heat conduction in hybrid sphere & wire nanodispersions

Heat conductivity of dispersions can be modified by varying shapes of dispersed particles and also by making hybrid dispersions containing particles of different shapes and sizes. Spheres and their agglomerates that we have investigated previously are replaced in this paper by spheres and wires. The method used to derive the formulas for the overall effective heat conductivity is based on the Maxwell homogenization (adapted to hybrid dispersions) followed by a mesoscopic analysis in which heat transfer is regarded as transport of phonons. The mesoscopic formulation provides then also a setting for investigation the role of particle-matrix nanoscale interfaces.

Increase in the density of states in n-type extruded (Bi(1−x)Sbx)2(Te(1−y)Sey)3 thermoelectric alloys

The concentration and doping of n-type doped (Bi(1−x)Sbx)2(Te(1−y)Sey)3 thermoelectric alloys produced by powder metallurgy followed by hot extrusion are varied in order to optimize their performance for the generation of electricity. The material is polycrystalline and strongly textured, with an undetermined volumetric fraction of nanoscale subgrains, and its thermoelectric properties are optimal along the extrusion direction. Within the composition range 0 ≤ x, y ≤ 0.1 the quaternary (Bi0.97Sb0.03)2(Te0.93Se0.07)3 shows the highest temperature-averaged dimensionless figure of merit ZT for applications where TC = 295 K and TH = 420 K. This average ZT is further optimized for values of carrier concentrations close to n = 3.4 × 1019 cm−3. The introduction of substitution elements constituting these quaternary alloys leads to an increase in the electronic equivalent density of states compared with Bi2Te3. This increase has a direct impact on the Seebeck coefficient, the electronic contribution to the thermal conductivity and the carrier mobility.

Impact of binders on viscosity of low-pressure powder injection molded Inconel 718 superalloy

Rheological behavior of powder-binder mixture has a direct impact on the successful mold filling for parts obtained from powder injection molding. In this study, the impact of binders on rheological properties of feedstocks was investigated. The experiments were conducted on several feedstocks obtained by mixing of Inconel 718 powder with wax-based binder systems. Their rheological and thermal properties were investigated using rotational rheometry and differential scanning calorimetry techniques, respectively. It was demonstrated that a large amount of ethylene vinyl acetate (EVA) should be added to paraffin wax (PW) to produce a thickening effect of the mixture. At low shear rate, the mixing of high-viscosity paraffin waxes with a low-viscosity PW produces a similar thickening effect denoted with the EVA. In addition, the feedstocks containing only PW demonstrate a pseudoplastic behavior. It was also shown that an addition of only 1 vol% of stearic acid (SA) in PW generates an important decrease in viscosity, and further increases of this constituent induce no effect on rheological behavior. From a rheological perspective, the best candidate feedstocks are the mixtures containing PW and SA while feedstocks based on PW, beeswax or containing a small amount of EVA could be also considered as good.

Low viscosity feedstocks for powder injection moulding

Abstract Powder injection moulding (PIM) carried out with the use of low viscosity feedstocks offers numerous benefits for manufacturing small complex shape parts. Unlike typical high pressure metal injection moulding (HPIM) viscous feedstocks, soft tooling can be employed for prototyping and small volume manufacturing. Compared to HPIM, there are very few studies on the rheology of low viscosity feedstocks. The objective of this paper is to clearly determine, using a statistical method, optimal models which define viscosity as a function of three parameters: shear rate, temperature and solid loading for low viscosity feedstocks. With the statistical method employed, it was found that the models of Herschel–Bulkley, Arrhenius, and Maron and Pierce can be used respectively to effectively model each of the three parameters stated previously. Moreover, the combination of these three models in one global model is proposed to predict the combined effect of the three parameters on low viscosity PIM feedstocks.