Characterization and modeling of nanocomposite thermoelectric materials system bismuth antimony telluride ((Biy̳Sb1̳-̳y̳)2̳Te3̳) as a function of temperature and magnetic field

[摘要] Furthermore, we see that the point defect contribution (A;;) reaches the highest value when both the nanoparticles and nanoinclusions prepared via melt spinning are present in the nanocomposite samples (e.g. the 40% sample), similar to the alloying effect on the thermal conductivity. In general, it is desirable to increase the values of A;; and B;;, resulting in a decrease in the KL values. However, care needs to be taken to ensure that the phonon parameters are independent of the electron parameters so that no adverse effect on ZT would result. The determination of L is also carried out based on my electron model findings. We observe that L is isotropic. Moreover, L for each sample investigated reaches the same value of 2.44x10-8 W-Ohm/K2 as T -> OK (completely degenerate limit of (+_ )2). Furthermore, the higher the hole concentration the sample has, the higher its C value is at a given temperature. Lastly, I find that a lower f leads to higher ZT values at 297K for the BC nanocomposite samples measured in the _L direction. On the other hand, a lower f leads to lower ZT values at 297K for the NTU nanocomposite samples measured in both the // and _L directions. From the magnetic field studies on the Ingot and on the 40% samples, few important facts are demonstrated. First, an applied magnetic field can be used to effectively increase the ZT of (BiySbpy)2Te3 , especially at temperatures below 200K. Use of a magnetic field might theoretically extend the effective temperature ranges over which (BiySb-y)2Te3 materials can be used for refrigeration. Second, the data under various applied B fields allow me to confidently calculate the C value below the temperature ranges where a plateau has occurred. Third, the data under various applied B fields serve as an important guideline for both validating any electron model and extrapolating values for L above the plateau occurrence temperatures. As a result, this allows me to get some insights into the temperature dependence of L (see Figure 4-14). Fourth, from the magnetic field dependent transport studies on our samples, we observe that the applied B field pushes away the holes more effectively in the Ingot // than the holes in the nanocomposite samples. We also find that the VvtIplateau values obtained under the magnetic field study serve as a more realistic and practical limit for KL. Lastly, from the magnetic field-dependent studies, we find that having point defects as the dominant scattering mechanism for the carriers results in an increase in ZT under an applied magnetic field. It would be extremely useful if one can make a sample such that the point defect dominant regime is extended to higher temperatures, resulting in a shift of the increase in the ZT ratio regime to a temperature range closer to room temperature (300K). However, care needs to be taken to ensure that such modifications would result in an increase in the ZT values under an applied magnetic field.

[发布机构] Massachusetts Institute of Technology