Research

We are specifically interested in Bulk Thermoelectric materials for Power Generation in the intermediate temperature range (300 K - 700 K). Some of our research activities are listed below:

(A) Material Property Improvement by Tuning Electrical & Thermal Properties

Material systems which are currently being investigated are solid solutions of Mg₂Si-Mg₂Sn, ZnSb-CdSb and Mg₃Sb₂-Mg₃Bi₂. Solid solutions are known to possess better thermoelectric properties compared to the end members. This is due to the possibility for tuning both the electrical and thermal properties by varying the chemical composition. Our work involves improving TE properties in these solid solutions using a combination of conventional techniques and material specific effects identified from our research.

Some of the recent activities are given below

• · Incorporation of embedded nano-precipitates in Mg₂Si_0.3Sn_0.7 for zT improvement

n-type Mg₂Si_1-xSn_x solid solutions have high TE performance due to electronic band convergence effect coupled with enhanced phonon scattering due to alloying. In this work, further reduction of the lattice thermal conductivity was attempted by scattering of the mid wavelength phonons. This was carried out by incorporation of embedded nano-precipitates in the Mg₂Si_0.3Sn_0.7 matrix which resulted in a record peak zT value in silicide based TE materials.

• · Bond Anharmonicity and its Influence on lattice thermal conductivity in Zn_1-xCd_xSb solid solutions

Solid solution formation results in lowering of lattice thermal conductivity (k_L) due to additional scattering of phonons from mass and strain disorder. However, experimental studies in ZnSb-CdSb solid solutions indicate k_L values which are lower than the predicted alloy scattering limit. Detailed theoretical investigations indicate significant contribution to phonon scattering from anharmonic bonds which are present in such electron-poor semiconductors

(B) Modelling of Electronic Band Structure of TE materials

Band structure modelling is a topic of interest in TE research as it provides valuable information regarding microscopic material parameters and also for predicting high performance compositions. The modelling work carried out in our group can be subdivided into (i) Solution based approach and (ii) Fitting based optimization approach. The Solution approach involves determined mathematical problems where band models (single/multiband) are chosen in a way to satisfy the solvability criterion at every step. While this is a powerful tool which is commonly used for its predictive ability, it is based on certain assumptions which are necessarily always valid. To overcome this problem, we also adopt a Fitting based optimization approach which can handle multiple parameters at the same time.

Some of the modelling related activities are given below:

• Predicting high performance compositions in p-type Mg₂Si-Mg₂Sn solid solutions

The search for the best p-type Mg2Si_1-xSn_x (x=0-1) involves finding both the best Si to Sn ratio (x) and the optimal doping concentration (n). A theoretical study for this was performed using semi-classical Boltzmann Transport Equation and Single Parabolic Band (SPB) modelling. The obtained data was plotted as 2D maps of n vs x at various temperatures to identify the suitable compositions.

• Studying Band Convergence in Mg2Sn using Multiband Fitting technique

A multiband fitting technique has been developed to study temperature dependent changes in the electronic band structure of semiconductors. This technique was used to probe the effect of doping in Mg₂Sn and reveal changes in the conduction band edge not reported earlier in this material.

(C) Contacting Studies and TE Device Fabrication

Our research also involves developing suitable electrodes (contacting) and lab scale TE modules from the in-house developed materials. Some activities related to this are provided below

• Multilayer approach for contacting of TE solid solutions

Contacting of TE materials is typically sensitive to changes in the chemical composition due to associated thermal expansion coefficient mismatch with the diffusion barrier layer. To overcome this problem, a multilayer approach has been attempted with each layer having different functionalities. In this process, an intermediate layer which is a mixture of diffusion barrier material and the TE material is provided. Additionally, a top metallic layer facilitates the joining with the metal interconnects.

• Mg₂Si based TE module for Power Generation

A low-cost TE module consisting of both n & p legs made of Mg₂Si_0.3Sn_0.7 was fabricated. Electroding of the legs were carried out using the multi-layer approach. Measurements indicate a decent power density of 0.5 W/cm² and conversion efficiency of 5%.

(D) Thermoelectric Measurement and Instrumentation

The uncertainty associated with measurement of TE properties at elevated temperatures can be quite high. Even with the best measurement practices, the cumulative error in zT can be as high as 20%. Thus, at times it is difficult to distinguish between real advancements and erroneous data. Further, high uncertainties in measured TE properties affect the modelling output leading to false predictions. Thus, improving TE measurement methodologies and practices is a field of research in itself. We are currently studying ways to improve the accuracy of the Seebeck coefficient data and discussed below

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• Multi-Probe method for accurate measurement of Seebeck Coefficient

The uncertainty in S measurement depends mainly on the nature of the thermal coupling of the specimen to the surrounding. Currently, we are developing a method in which using multiple measurement probes it is possible to quantify the error in the measured S value.