Daehyun Wee

Prediction of high thermoelectric potential in AMN2 layered nitrides: electronic structure, phonons, and anharmonic effects

Band structures, electronic transport coefficients, harmonic and anharmonic vibrational properties of novel layered nitrides have been studied to evaluate the potential for thermoelectric applications. Using first principles theoretical methods we predict that AMN2 compounds with A = Ca, Sr, and Ba, and M = Ti, Zr, Hf may exhibit Seebeck coefficients in excess of 150 μV K−1 and good electrical conductivities. The phonon dispersions indicate the presence of low lying optic modes that can lead to low thermal conductivity. The analysis of the mode resolved Gruneisen parameter points to large anharmonicity. In addition, we show that the A-site substitution controls the degeneracies at the top of the valence band and the anisotropy of the Seebeck tensors.

Achieving Maximum Power in Thermoelectric Generation with Simple Power Electronics

A thermoelectric generator typically delivers a relatively low power output, and hence it is of great practical importance to determine a design and operating condition close to those which can provide the maximum attainable power. To maintain a favorable condition for the maximum power output, power electronics circuits are usually applied. One of the simplest methods is to control the operating voltage at half the open-circuit voltage, assuming that the typical impedance-matching condition, in which the load and internal resistances are matched, yields the maximum power output. However, recent investigations have shown that, when external thermal resistances exist between the thermoelectric modules and thermal reservoirs, the impedance-matching condition is not identical to the condition for the maximum power output. In this article, it is argued that, although the impedance-matching condition is not the condition for maximum power output, the maximum power is still achievable when the operating voltage is kept at half the open-circuit voltage. More precisely, it is shown that the typical V–I curve for thermoelectric generators must show approximately linear behavior, which justifies the use of a simple strategy in thermoelectric power generation applications. The conditions for the validity of the approximation are mathematically discussed, supported by a few examples. Experimental evidence at room temperature is also provided.

Thermoelectric properties of pnictogen-substituted skutterudites with alkaline-earth fillers using first-principles calculations

First-principles calculations have been performed to investigate electronic band structures, vibrational characters, and related transport properties of pnictogen-substituted skutterudites filled with alkaline-earth elements ( MxCo4A6B6, where M = Ca, Sr, or Ba, A = Ge or Sn, B = Se or Te, and x = 0.5 or 1). Electronic transport properties related to thermoelectricity, including the Seebeck coefficient and the electrical conductivity, are computed by using the Boltzmann transport formalism within the constant-relaxation-time approximation. The results are compared against the corresponding properties of the unfilled pnictogen-substituted ternary skutterudites ( CoA1.5B1.5) to identify the effects of filling to estimate the potential for thermoelectric applications. The changes in the ionic character of the interatomic bonding between the Group 14 (A) and Group 16 (B) elements, which was suspected to be a major scattering source in unfilled pnictogen-substituted ternary skutterudites, are probed by analyzi...

Frequency of Filler Vibrations in CoSb3 Skutterudites: A Mechanical Interpretation

A mechanical interpretation of the frequency trend observed in Ca-, Sr-, and Ba-filled CoSb3 skutterudites is presented. Relevant vibrational frequencies computed at the zone center are presented for fully filled, half-filled, and unfilled systems. The frequency of the filler vibrations increases as the mass of the filler atom increases, which is a counterintuitive trend that is difficult to explain within the classical “rattler” concept. As an alternative theory, we propose the interpretation of the filler vibrations as modified Sb ring vibrations instead. The energetically degenerate Sb ring vibrations in unfilled CoSb3 split into two separate groups of vibrations through the mechanical interaction introduced by fillers, and one of the group forms the filler vibrations. A one-dimensional mass-spring model is also presented for illustrative purposes. The frequency trend of the ab initio phonons at the zone center is reproduced by the model, substantiating our interpretation. The result suggests that engi...

Transient Thermal Response of Heat Sinks and Its Implication on Power Control Strategies

Thermoelectric generation has relatively low efficiency and power output compared with other types of energy production. Therefore, it is necessary to optimize the operating conditions of the thermoelectric system in the applied research phase. In this study, we aim to suggest the optimal operating conditions and develop a strategy to obtain maximum power output in a thermoelectric generator. First, we develop a theoretical framework for optimal control of operating voltage to get the maximum power output. Then, to maintain the optimal conditions, the temperature variation of the heat sink, which is controlled by current and voltage in the circuit, will be discussed by modeling the heat transfer of a heat sink under real-world conditions. Because of the importance of voltage control, we suggest the optimal voltage control strategy.