Transport property analysis method for thermoelectric materials: material quality factor and the effective mass model

Thermoelectric semiconducting materials are often evaluated by their figure-of-merit, zT. However, by using zT as the metric for showing improvements, it is not immediately clear whether the improvement is from an enhancement of the inherent material property or from optimization of the carrier concentration. Here, we review the quality factor approach which allows one to separate these two contributions even without Hall measurements. We introduce practical methods that can be used without numerical integration. We discuss the underlying effective mass model behind this method and show how it can be further advanced to study complex band structures using the Seebeck effective mass. We thereby dispel the common misconception that the usefulness of effective band models is limited to single parabolic band materials.Comment: 5 pages, 3 figure

Charge Transport Analysis Using the Seebeck Coefficient-Conductivity Relation

Charge transport properties like electrical conductivity or the Seebeck coefficient are defined phenomenologically from near-equilibrium thermodynamics, while the analysis or modeling of them often involves a physical model based on mechanistic principles. In other words, physical models connect microscopic and physical parameters to phenomenological and experimental properties. One of the challenges is that the complexity of solid state requires many physical parameters, whereas the measurable properties which help to determine those parameters are limited. The interrelations of measured properties are very important to overcome this challenge, but this aspect is not well recognized in conventional analysis themes. In this thesis, the concept of using a phenomenological transport function is devised to help combine a collection of measurements into an intermediate level of phenomenology, relevant for Fermion transport but not dependent on a particular physical model. This phenomenological transport function can be determined by examining the electrical conductivity, the Seebeck coefficient, and potentially the Lorenz number. Because the phenomenological transport function combines information from a set of multiple measurable properties, a direct comparison to the transport function of a physical model serves as a strong test for the model. Particular usefulness comes from extracting transport functions from the Seebeck coefficient-conductivity relation, especially in doped semiconductors. This approach is applied to contrast CeO2-x and n-type SrTiO3 as narrow and dispersive transport function materials, each consistent with polaron and band conduction, respectively. In band conductors such as SrTiO3 and Mg3Sb2, the approach is used to test and refute previous claims about the scattering mechanism and find consistency with deformation potential scattering in both cases. In conducting polymers, which do not resemble any other type of conventional conductors, the Seebeck-conductivity relation reveals a qualitative disagreement with the commonly cited Mott's models. For the case of Cu2Se, a peculiar band conductor which shows anomalies in the Hall measurement of the high temperature phase and also in other transport properties at the phase transition, the transport function approach is applied as a workaround for modeling. On the practical side, for thermoelectric applications, the transport function approach is used to characterize material quality factors for both majority carrier conduction and bipolar conduction. Finally, experimental efforts for improving the accuracy and applicability of Seebeck measurements is discussed.</p

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

Thermopower-conductivity relation for distinguishing transport mechanisms: Polaron hopping in CeO_2 and band conduction in SrTiO_3

The charge transport mechanism in a solid is often inferred by observing very simple features like the temperature dependency of electrical conductivity or resistivity. However, comparing complicated physical models to such simple signatures leaves much ambiguity. Because models generally have more parameters than the types of measurements available, inconsistencies can long go unrecognized until the interrelation between different measurements is closely examined. We show that a simple investigation of the thermopower-conductivity relation allows one to phenomenologically characterize transport from experiments; the phenomenologically determined transport function can be compared to physical models to distinguish transport mechanisms and straightforwardly point out inconsistencies in literature models. We highlight two example cases, ceria and strontium titanate, to show that our analysis method can clarify whether the transport mechanism is through hopping or delocalized states. We question previous suggestions about the scattering mechanism in SrTiO_3 and suggest deformation potential scattering on elongated Fermi surfaces as the origin of high-temperature T^2 resistivity

Optimization principles and the figure of merit for triboelectric generators

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

Exceptional thermoelectric performance in Mg_3Sb_(0.6)Bi_(1.4) for low-grade waste heat recovery

Bi_2Te_3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300–500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg_3Sb_(0.6)Bi_(1.4), with a thermoelectric figure-of-merit zT of 1.0–1.2 at 400–500 K, finally surpasses n-type Bi_2Te_3. This exceptional performance is achieved by tuning the alloy composition of Mg_3(Sb_(1−x)Bi_x)_2. The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg_3Bi_2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg_3Sb_(0.6)Bi_(1.4), after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg_3Sb_(0.6)Bi_(1.4) is a significant advancement towards sustainable heat recovery technology

Effect of Two-Dimensional Crystal Orbitals on Fermi Surfaces and Electron Transport in Three-Dimensional Perovskite Oxides

Perovskite oxides are candidate materials in catalysis, fuel cells, thermoelectrics, and electronics, where electronic transport is vital to their use. While the fundamental transport properties of these materials have been heavily studied, there are still key features that are not well understood, including the temperature‐squared behavior of their resistivities. Standard transport models fail to account for this atypical property because Fermi surfaces of many perovskite oxides are low‐dimensional and distinct from traditional semiconductors. In this work, the low‐dimensional Fermi surfaces of perovskite oxides are chemically interpreted in terms of two‐dimensional crystal orbitals that form the conduction bands. Using SrTiO_3 as a case study, the d/p‐hybridization that creates these low‐dimensional electronic structures is reviewed and connected to its fundamentally different electronic properties. A low‐dimensional band model explains several experimental transport properties, including the temperature and carrier‐density dependence of the effective mass, the carrier‐density dependence of scattering, and the temperature dependence of resistivity. This work highlights how chemical bonding influences semiconductor transport

Optimization principles and the figure of merit for triboelectric generators

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

Thermopower-conductivity relation for distinguishing transport mechanisms: Polaron hopping in CeO_2 and band conduction in SrTiO_3

The charge transport mechanism in a solid is often inferred by observing very simple features like the temperature dependency of electrical conductivity or resistivity. However, comparing complicated physical models to such simple signatures leaves much ambiguity. Because models generally have more parameters than the types of measurements available, inconsistencies can long go unrecognized until the interrelation between different measurements is closely examined. We show that a simple investigation of the thermopower-conductivity relation allows one to phenomenologically characterize transport from experiments; the phenomenologically determined transport function can be compared to physical models to distinguish transport mechanisms and straightforwardly point out inconsistencies in literature models. We highlight two example cases, ceria and strontium titanate, to show that our analysis method can clarify whether the transport mechanism is through hopping or delocalized states. We question previous suggestions about the scattering mechanism in SrTiO_3 and suggest deformation potential scattering on elongated Fermi surfaces as the origin of high-temperature T^2 resistivity

Exceptional thermoelectric performance in Mg_3Sb_(0.6)Bi_(1.4) for low-grade waste heat recovery

Bi_2Te_3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300–500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg_3Sb_(0.6)Bi_(1.4), with a thermoelectric figure-of-merit zT of 1.0–1.2 at 400–500 K, finally surpasses n-type Bi_2Te_3. This exceptional performance is achieved by tuning the alloy composition of Mg_3(Sb_(1−x)Bi_x)_2. The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg_3Bi_2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg_3Sb_(0.6)Bi_(1.4), after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg_3Sb_(0.6)Bi_(1.4) is a significant advancement towards sustainable heat recovery technology