Yujiro Nakatani

Mechanical Aspects of Structural Optimization in a Bi-Te Thermoelectric Module for Power Generation

The thermal stress and strain occurring on a Bi-Te thermoelectric module subjected to variable thermal conditions were estimated based on three-dimensional elastic-plastic finite element method (FEM) analysis. The analysis showed that mechanical integrity of the interface between a Bi-Te thermoelement and electrodes of Al and Mo coatings formed by atmospheric plasma spraying was significantly reduced and that shear strain rose to 0.6∼1.1% in the vicinity of the interface. Furthermore, to estimate the sensitivity of configurational parameters of the module to the thermal strain, statistical sensitivity analysis based on the design of experiment (DoE) and response surface method (RSM) was conducted. The statistical analysis revealed that the thickness of electrode coatings of Al and Mo affected the thermal strain and that the thinner Al coating and the thicker Mo coating reduced the thermal strain. In this study, a thermal fatigue test machine was newly developed with a view to verifying these analytical studies.

Analytical method for estimating the thermal expansion coefficient of metals at high temperature

In this paper, we propose an analytical method for estimating the thermal expansion coefficient (TEC) of metals at high-temperature ranges. Although the conventional method based on quasiharmonic approximation (QHA) shows good results at low temperatures, anharmonic effects caused by large-amplitude thermal vibrations reduces its accuracy at high temperatures. Molecular dynamics (MD) naturally includes the anharmonic effect. However, since the computational cost of MD is relatively high, in order to make an interatomic potential capable of reproducing TEC, an analytical method is essential. In our method, analytical formulation of the radial distribution function (RDF) at finite temperature realizes the estimation of the TEC. Each peak of the RDF is approximated by the Gaussian distribution. The average and variance of the Gaussian distribution are formulated by decomposing the fluctuation of interatomic distance into independent elastic waves. We incorporated two significant anharmonic effects into the method. One is the increase in the averaged interatomic distance caused by large amplitude vibration. The second is the variation in the frequency of elastic waves. As a result, the TECs of fcc and bcc crystals estimated by our method show good agreement with those of MD. Our method enables us to make an interatomic potential that reproduces the TEC at high temperature. We developed the GEAM potential for nickel. The TEC of the fitted potential showed good agreement with experimental data from room temperature to 1000?K. As compared with the original potential, it was found that the third derivative of the wide-range curve was modified, while the zeroth, first and second derivatives were unchanged. This result supports the conventional theory of solid state physics. We believe our analytical method and developed interatomic potential will contribute to future high-temperature material development.