Stephen Dongmin Kang

Charge-transport model for conducting polymers

The growing technological importance of conducting polymers makes the fundamental understanding of their charge transport extremely important for materials and process design. Various hopping and mobility edge transport mechanisms have been proposed, but their experimental verification is limited to poor conductors. Now that advanced organic and polymer semiconductors have shown high conductivity approaching that of metals, the transport mechanism should be discernible by modelling the transport like a semiconductor with a transport edge and a transport parameter s. Here we analyse the electrical conductivity and Seebeck coefficient together and determine that most polymers (except possibly PEDOT:tosylate) have s = 3 and thermally activated conductivity, whereas s = 1 and itinerant conductivity is typically found in crystalline semiconductors and metals. The different transport in polymers may result from the percolation of charge carriers from conducting ordered regions through poorly conducting disordered regions, consistent with what has been expected from structural studies.

Dislocation strain as the mechanism of phonon scattering at grain boundaries

Thermal conductivities of polycrystalline thermoelectric materials are satisfactorily calculated by replacing the commonly used Casimir model (freqeuncy-independent) with grain boundary dislocation strain model (frequency-dependent) of Klemens. It is demonstrated that the grain boundaries are better described as a collection of dislocations rather than perfectly scattering interfaces.

YCuTe2: a member of a new class of thermoelectric materials with CuTe4-based layered structure

Intrinsically doped samples of YCuTe2 were prepared by solid state reaction of the elements. Based on the differential scanning calorimetry and the high temperature X-ray diffraction analyses, YCuTe2 exhibits a first order phase transition at ∼440 K from a low-temperature-phase crystallizing in the space group Pm1 to a high-temperature-phase in P. Above the phase transition temperature, partially ordered Cu atoms become completely disordered in the crystal structure. Small increases to the Cu content are observed to favour the formation of the high temperature phase. We find no indication of superionic Cu ions as for binary copper chalcogenides (e.g., Cu2Se or Cu2Te). All investigated samples exhibit very low thermal conductivities (as low as ∼0.5 W m−1 K−1 at 800 K) due to highly disordered Cu atoms. Electronic structure calculations are employed to better understand the high thermoelectric efficiency for YCuTe2. The maximum thermoelectric figure of merit, zT, is measured to be ∼0.75 at 780 K for Y0.96Cu1.08Te2, which is promising for mid-temperature thermoelectric applications.

Apparent critical phenomena in the superionic phase transition of Cu2-xSe

The superionic phase transition of Cu_(2-x)Se accompanies drastic changes in transport properties. The Seebeck coefficient increases sharply while the electrical conductivity and thermal diffusivity drops. Such behavior has previously been attributed to critical phenomena under the assumption of a continuous phase transition. However, applying Landaus criteria suggests that the transition should be first order. Using the phase diagram that is consistent with a first order transition, we show that the observed transport properties and heat capacity curves can be accounted for and modeled with good agreement. The apparent critical phenomena is shown to be a result of compositional degree-of-freedom. Understanding of the phase transition allows to explain the enhancement in the thermoelectric figure-of-merit that is accompanied with the transition.

Band engineering in Mg3Sb2 by alloying with Mg3Bi2 for enhanced thermoelectric performance

Mg_3Sb_2–Mg_3Bi_2 alloys show excellent thermoelectric properties. The benefit of alloying has been attributed to the reduction in lattice thermal conductivity. However, Mg_3Bi_2-alloying may also be expected to significantly change the electronic structure. By comparatively modeling the transport properties of n- and p-type Mg_3Sb_2–Mg_3Bi_2 and also Mg_3Bi_2-alloyed and non-alloyed samples, we elucidate the origin of the highest zT composition where electronic properties account for about 50% of the improvement. We find that Mg_3Bi_2 alloying increases the weighted mobility while reducing the band gap. The reduced band gap is found not to compromise the thermoelectric performance for a small amount of Mg_3Bi_2 because the peak zT in unalloyed Mg_3Sb_2 is at a temperature higher than the stable range for the material. By quantifying the electronic influence of Mg_3Bi_2 alloying, we model the optimum Mg_3Bi_2 content for thermoelectrics to be in the range of 20–30%, consistent with the most commonly reported composition Mg_3Sb_(1.5)Bi_(0.5).

Enhancement of average thermoelectric figure of merit by increasing the grain-size of Mg3.2Sb1.5Bi0.49Te0.01

Zintl compound n-type Mg3(Sb,Bi)2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700u2009K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720u2009K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0u2009μm to 7.8u2009μm, the Hall mobility below 500u2009K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔTu2009=u2009420u2009K), which is quite high for a polycrystalline pristine material.

Optimization principles and the figure of merit for triboelectric generators

Maximizing output power density with design parameters. Energy harvesting with triboelectric nanogenerators is a burgeoning field, with a growing portfolio of creative application schemes attracting much interest. Although power generation capabilities and its optimization are one of the most important subjects, a satisfactory elemental model that illustrates the basic principles and sets the optimization guideline remains elusive. We use a simple model to clarify how the energy generation mechanism is electrostatic induction but with a time-varying character that makes the optimal matching for power generation more restrictive. By combining multiple parameters into dimensionless variables, we pinpoint the optimum condition with only two independent parameters, leading to predictions of the maximum limit of power density, which allows us to derive the triboelectric material and device figure of merit. We reveal the importance of optimizing device capacitance, not only load resistance, and minimizing the impact of parasitic capacitance. Optimized capacitances can lead to an overall increase in power density of more than 10 times.

Improving the thermoelectric performance in Mg3+xSb1.5Bi0.49Te0.01 by reducing excess Mg

The thermoelectric performance of Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01) was improved by reducing the amount of excess Mg (x = 0.01-0.2). A 20% reduction in effective lattice thermal conductivity at 600 K was observed by decreasing the nominal xfrom 0.2 to 0.01 in Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01), leading to a 20% improvement in the figure-of-merit zT. Since materials with different amounts of Mg have similar electronic properties, the enhancement is attributed primarily to the reduction in thermal conductivity. It is known that excess Mg is required to make n-type Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01); however, too much excess Mg in the material increases the thermal conductivity and is therefore detrimental for the overall thermoelectric performance of the material.

A practical field guide to thermoelectrics: Fundamentals, synthesis, and characterization

The study of thermoelectric materials spans condensed matter physics, materials science and engineering, and solid-state chemistry. The diversity of the participants and the inherent complexity of the topic mean that it is difficult, if not impossible, for a researcher to be fluent in all aspects of the field. This review, which grew out of a one-week summer school for graduate students, aims to provide an introduction and practical guidance for selected conceptual, synthetic, and characterization approaches and to craft a common umbrella of language, theory, and experimental practice for those engaged in the field of thermoelectric materials. This review does not attempt to cover all major aspects of thermoelectric materials research or review state-of-the-art thermoelectric materials. Rather, the topics discussed herein reflect the expertise and experience of the authors. We begin by discussing a universal approach to modeling electronic transport using Landauer theory. The core sections of the review are focused on bulk inorganic materials and include a discussion of effective strategies for powder and single crystal synthesis, the use of national synchrotron sources to characterize crystalline materials, error analysis, and modeling of transport data using an effective mass model, and characterization of phonon behavior using inelastic neutron scattering and ultrasonic speed of sound measurements. The final core section discusses the challenges faced when synthesizing carbon-based samples and the measuring or interpretation of their transport properties. We conclude this review with a brief discussion of some of the grand challenges and opportunities that remain to be addressed in the study of thermoelectrics.