Tsutomu Iida

Thermoelectric and electrical transport properties of Mg2Si multi-doped with Sb, Al and Zn

Enhanced thermoelectric and electrical transport properties of Mg2Si-based thermoelectric materials have been achieved by multi-doping with Sb, Al and Zn. Results on the investigation of the electrical transport and thermoelectric properties of multi-doped samples prepared using the spark plasma sintering technique are reported. Synchrotron radiation powder X-ray diffraction was used to characterize the structures of the doped samples. The electrical transport properties were determined from mid-infrared reflectivities, Hall effect and conventional quasi-four probe conductivity measurements. Using the electron concentrations (N) determined from the Hall coefficients, the effective masses (m*) were calculated from the frequency of the plasma edge (ωP) of the infrared reflectivities. The thermoelectric performance and thermoelectric figure of merits (ZT) in the temperature range of 300 K to 900 K of the doped Mg2Si compounds were calculated from the measured temperature dependent electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ). A maximum ZT of 0.964 was found for Sb0.5%Zn0.5% doped Mg2Si at 880 K. This value is comparable to those of PbTe based thermoelectric materials.

Stress Analysis and Output Power Measurement of an n-Mg2Si Thermoelectric Power Generator with an Unconventional Structure

We examine the mechanical stability of an unconventional Mg2Si thermoelectric generator (TEG) structure. In this structure, the angle θ between the thermoelectric (TE) chips and the heat sink is less than 90°. We examined the tolerance to an external force of various Mg2Si TEG structures using a finite-element method (FEM) with the ANSYS code. The output power of the TEGs was also measured. First, for the FEM analysis, the mechanical properties of sintered Mg2Si TE chips, such as the bending strength and Young’s modulus, were measured. Then, two-dimensional (2D) TEG models with various values of θ (90°, 75°, 60°, 45°, 30°, 15°, and 0°) were constructed in ANSYS. The x and y axes were defined as being in the horizontal and vertical directions of the substrate, respectively. In the analysis, the maximum tensile stress in the chip when a constant load was applied to the TEG model in the x direction was determined. Based on the analytical results, an appropriate structure was selected and a module fabricated. For the TEG fabrication, eight TE chips, each with dimensions of 3xa0mmxa0×xa03xa0mmxa0×xa010xa0mm and consisting of Sb-doped n-Mg2Si prepared by a plasma-activated sintering process, were assembled such that two chips were connected in parallel, and four pairs of these were connected in series on a footprint of 46xa0mmxa0×xa012xa0mm. The measured power generation characteristics and temperature distribution with temperature differences between 873xa0K and 373xa0K are discussed.

Selection and Evaluation of Thermal Interface Materials for Reduction of the Thermal Contact Resistance of Thermoelectric Generators

A variety of thermal interface materials (TIMs) were investigated to find a suitable TIM for improving the performance of thermoelectric power generators (TEGs) operating in the medium-temperature range (600–900xa0K). The thermal resistance at the thermal interface between which the TIM was inserted was evaluated. The TIMs were chosen on the basis of their thermal stability when used with TEGs operating at medium temperatures, their electrical insulating properties, their thermal conductivity, and their thickness. The results suggest that the boron nitride (BN)-based ceramic coating, Whity Paint, and the polyurethane-based sheet, TSU700-H, are suitable TIMs for the heat source and heat sink sides, respectively, of the TEG. Use of these effectively enhances TEG performance because they reduce the thermal contact resistance at the thermal interface.

Examination of a Thermally Viable Structure for an Unconventional Uni-Leg Mg2Si Thermoelectric Power Generator

We have fabricated an unconventional uni-leg structure thermoelectric generator (TEG) element using quad thermoelectric (TE) chips of Sb-doped n-Mg2Si, which were prepared by a plasma-activated sintering process. The power curve characteristics, the effect of aging up to 500xa0h, and the thermal gradients at several points on the module were investigated. The observed maximum output power with the heat source at 975xa0K and the heat sink at 345xa0K was 341xa0mW, from which the ΔT for the TE chip was calculated to be about 333xa0K. In aging testing in air ambient, a remarkable feature of the results was that there was no notable change from the initial resistance of the TEG module for as long as 500xa0h. The thermal distribution for the fabricated uni-leg TEG element was analyzed by finite-element modeling using ANSYS software. To tune the calculation parameters of ANSYS, such as the thermal conductance properties of the corresponding coupled materials in the module, precise measurements of the temperature at various probe points on the module were made. Then, meticulous verification between the measured temperature values and the results calculated by ANSYS was carried out to optimize the parameters.

Formation of Ni electrodes on sintered N-type Mg2Si using monobloc sintering and electroless plating methods

Ni was examined as an electrode for n-type Mg2Si. To form the Ni electrodes, two methods, monobloc sintering and a modified electroless Ni (EN) plating method, were adopted. Although the process of EN plating to Mg2Si was formerly believed to be untenable owing to degradation of the matrix during the acidity processes, a promising modified EN plating method was developed. Regarding the adhesive properties, no obvious damage was observed in either Ni/Mg2Si sample after more than 200 h. The observed electrical resistances of the EN plated and monobloc-sintered samples were comparable, with values of 10.2 and 10.5 mΩ, respectively. In addition, the results of the output power measurements gave similar values of 120.9 mW and 120.7 mW for the EN plated and sintered Ni electrodes, respectively.

First-Principles Study on Structural and Thermoelectric Properties of Al- and Sb-Doped Mg2Si

We theoretically investigate the structural and thermoelectric properties of magnesium silicide (Mg2Si) incorporating Al or Sb atoms as impurities using first-principles calculations. We optimized the structural properties through variable-cell relaxation using a pseudopotential method based on density functional theory. The result indicates that the lattice constant can be affected by the insertion of impurity atoms into the system, mainly because the ionic radii of these impurities differ from those of the matrix constituents Mg and Si. We then estimate, on the basis of the optimized structures, the site preferences of the impurity atoms using a formation energy calculation. The result shows a nontrivial concentration-dependence of the site occupation, such that Al tends to go into the Si, Mg, and interstitial sites with comparable formation energies at low doping levels (<2 at.%); it can start to substitute for the Mg sites preferentially at higher doping levels (<4 at.%). Sb, on the other hand, shows a strong preference for the Si sites at all impurity concentrations. Furthermore, we obtain the temperature-dependence of the thermoelectromotive force (Seebeck coefficient) of the Al- and Sb-doped Mg2Si using the full-potential linearized augmented-plane-wave method and the Boltzmann transport equation.

Theoretical analysis of structure and formation energy of impurity-doped Mg2Si: Comparison of first-principles codes for material properties

We theoretically investigate the impurity doping effects on the structural parameters such as lattice constant, atomic positions, and site preferences of impurity dopants for Al-doped magnesium silicide (Mg2Si) crystal using the first-principles calculation methods. We present comparison between several codes: ABCAP, Quantum Espresso, and Machikaneyama2002 (Akai KKR), which are based on the full-potential linearized augmented plane-wave method, the pseudopotential method, and KKR/GGA Greens function method, respectively. As a result, any codes used in the present study exhibit qualitative consistency both in the dependence of the lattice constants on the doping concentration and the energetic preference of the Al atom for the following sites; substitutional Si and Mg sites, and interstitial 4b site; in particular, ABCAP, which is based on the all-electron full-potential method, and Quantum Espresso, which is a code of the pseudopotential method, produce closely-resemble calculation results. We also discuss the effects of local atomic displacement owing to the presence of impurities to the structural parameters of a bulk. Using the analytical method considering the local atomic displacement, moreover, we evaluate the formation energy of Na- and B-doped systems as examples of p-type doping in order to examine the possilbility of realizing p-type Mg2Si.

Development of a Thermal Buffering Device to Cope with Temperature Fluctuations for a Thermoelectric Power Generator

To stabilize the heat input to a thermoelectric generator (TEG) and protect it from large temperature fluctuations, a thermal buffering device (TBD) was fabricated and examined using a typical Bi-Te TEG module and a brand-new Mg2Si TEG module. The TBD comprises two adjoining heat storage containers, each containing different alloys, which can be optimized for the temperature range of the TEG. The combination of two alloys in series diminishes the thermal fluctuations, stabilizing the heat input to the TEG module. This is achieved by having two metallic materials with large enthalpies of fusion that can be placed between the heat source and the TEG. The combination of the two alloys can be optimized for the temperature ranges of Bi-Te, Pb-Te, or Co-Sb. For the Bi-Te TEG, 15Al-85Zn and 30Sn-70Zn alloys were used for the heat source side and the TEG side, respectively. The corresponding alloys for the Mg2Si TEG were 20Ni-80Al and 7Si-93Al. With the use of a TBD, the Bi-Te TEG exhibited no notable damage even in the rather high temperature range beyond ∼573xa0K. For the Mg2Si TEG, no operational damage of the Mg2Si TEG module was observed even with a temperature of 1020xa0K.

Fabrication of large sintered pellets of Sb-doped N-type Mg2Si using a plasma activated sintering method

Sb is known to be the most stable n-type dopant in Mg2Si, whereas the reproducibility and sintering scalabity of Sb-doped Mg2Si has proved to be difficult. We have developed a metallic binder for the Sb-doped Mg2Si sintering process. A remarkable benefit of the binder is that it enables us to reproducibly fabricate large sintered Sb-doped Mg2Si pellets up to 30 mm in diameter with no internal cracks. For binder contents of 3 to 7 wt%, the observed power factor and thermal conductivity were identical to that of conventional Sb-doped samples, while excess binder degraded the TE properties. The binder mixture also plays a part in increasing the yield in the production of TE legs, up to 82% for 5 wt% Ni incorporation, as well as improving the hardness of the sintered sample. Incorporation of 5 wt% Ni binder affected an increase in the ZT value up to 0.97.Sb is known to be the most stable n-type dopant in Mg2Si, whereas the reproducibility and sintering scalabity of Sb-doped Mg2Si has proved to be difficult. We have developed a metallic binder for the Sb-doped Mg2Si sintering process. A remarkable benefit of the binder is that it enables us to reproducibly fabricate large sintered Sb-doped Mg2Si pellets up to 30 mm in diameter with no internal cracks. For binder contents of 3 to 7 wt%, the observed power factor and thermal conductivity were identical to that of conventional Sb-doped samples, while excess binder degraded the TE properties. The binder mixture also plays a part in increasing the yield in the production of TE legs, up to 82% for 5 wt% Ni incorporation, as well as improving the hardness of the sintered sample. Incorporation of 5 wt% Ni binder affected an increase in the ZT value up to 0.97.

Mechanical properties of Mg2Si with metallic binders

The mechanical properties of Sb-doped Mg2Si were measured by various mechanical methods, and the effects of four incorporated metallic binders (Ni, Cu, Zn, and Al) on the mechanical properties were investigated. The measured Youngs modulus, bending strength, Vickers hardness, and fracture toughness of Mg2Si without binders were 105 GPa, 57 MPa, 700, and 1.2 MPa m1/2, respectively. These values, except for Vickers hardness, were improved by the addition of the metallic binders. Moreover, the thermoelectric performance of Mg2Si was also slightly improved by the incorporation of the metallic binders. These results suggest that the addition of a metallic binder to Mg2Si is effective in improving its mechanical properties, in addition to improving its thermoelectric property and sinterability.