Thermally resistive thermoelectrics

Think. Really think about all the various electronic devices you take for granted. Electronics like the central processing unit (CPU) of your computers and graphic cards, which you so desperately need to enjoy your games and movies, and also to work efficiently. Think about all of these. What would you do without these?

Unfortunately, electronic devices all undergo heating with usage. This heating could damage your electronics. Thus, thermoelectric materials are used to help cool down these electronic items so that you can continue to obtain maximum enjoyment and efficiency from them.

The efficiency of a thermoelectric material at a finite temperature depends on its Seebeck coefficient, electrical conductivity and thermal conductivity. For an effective thermoelectric material, the material should have good electrical and poor thermal conductivity. Low thermal conductivity is achieved by having a shorter phonon lifetime. A phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in a solid or a liquid. 

Materials with a larger Gruneisen parameter exhibit shorter phonon lifetime. The Gruneisen parameter determines what is known as the anharmonicity of the thermoelectric material. Anharmonicity is the property of a material that does not exhibit harmonic oscillation. The oscillation of a pendulum is an example of an anharmonic oscillation. In such an oscillation, the restoring force is not proportional to the angular position.

Bismuth telluride (Bi2Te3) and bismuth selenide (Bi2Se3) have received a lot of attention as thermoelectric materials. Bismuth plays a vital role due to its low lattice thermal conductivity and low-frequency phonon band, primarily caused by soft localized vibrations, low sound velocity and strong anharmonic interaction of bismuth atoms.

To determine the efficiency of thermoelectric materials, measuring their thermal conductivity is important. The thermal conductivity can be measured using a number of experimental techniques like the 3-ω method, laser flash analyser, Wiedemann-Franz law etc. Unfortunately, all these methods have their own limitations.

The authors of this paper which include Mr. Vipin K. E., Dr. Soumendra Kumar Das, and Prof. Prahallad Padhan from the Department of Physics, Nanoscale Physics Laboratory, Indian Institute of Technology Madras, Chennai, India (Prof. Prahallad Padhan is also from the Functional Oxides Research Group, Indian Institute of Technology Madras, Chennai, India), have adopted a non-contact measurement technique to calculate the lattice thermal conductivity from the temperature and laser power-dependent Raman spectra of the bismuth selenide (Bi2Se3) nanocrystals with truncated hexagon plate morphology stabilized in a hexagonal crystal structure.

Raman spectroscopy refers to a spectroscopic technique that is used to determine the vibrational modes of molecules. Temperature and laser power-dependent Raman spectroscopy is an important tool for determining the lattice thermal conductivity and phonon lifetimes.

The authors have estimated the phonon lifetime, the Gruneisen parameter, and the lattice thermal conductivity of various vibrational frequency modes, which suggest the crucial role of the anharmonicity and acoustic-optical phonon scattering for the reduced lattice thermal conductivity of the bismuth selenide. The anharmonicity and acoustic-optical phonon scattering in thermoelectric materials can be introduced by alloying, nanostructuring, resonant bonding, rattling atoms, and bonding inhomogeneity in the main crystal structure of the thermoelectric materials to achieve low lattice thermal conductivity.

The non-contact measurement method provides an alternative route to estimate the lattice thermal conductivity and opens up exciting opportunities to address the anharmonic effects in low dimensions of various thermoelectric materials.

Prof. A. Sundaresan, Chair of the Chemistry & Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research. Bengaluru, India, gave his analysis of the work done by the authors with the following comments: “It is challenging to achieve low thermal conductivity, high electrical conductivity, and a large Seebeck coefficient for high ZT thermoelectric materials.  The study introduces a non-contact measurement technique and calculated lattice thermal conductivity (kL) for Bi2Se3 nanocrystals. The results, based on temperature and power-dependent Raman spectra, align with theoretical predictions and demonstrate the effectiveness of the three-phonon process. The investigation also reveals the role of anharmonicity and acoustic-optical phonon scattering in reducing kL.”

Article by Akshay Anantharaman
Click here for the original link to the paper


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