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

Evaluation of true interlamellar spacing from microstructural observations

A method for evaluating true interlamellar spacing from micrographs is proposed for a multidomained lamellar structure. The microstructure of these materials is assumed to be composed of many domains with the lamellae aligned roughly parallel to each other within each domain and with the domains themselves randomly oriented relative to one another. An explicit expression for the distribution of apparent interlamellar spacing is derived assuming that the distribution of the true interlamellar spacing is Gaussian. The average interlamellar spacing is close to the peak interlamellar spacing observed in the distribution. The theoretical distributions are compared with experimental ones obtained by analyzing micrographs of PbTe–Sb2Te3 lamellar composites

Zone Leveling Crystal Growth of Thermoelectric PbTe Alloys with Sb_(2)Te_3 Widmanstätten Precipitates

Unidirectional solidification of PbTe-rich alloys in the pseudobinary PbTe-Sb_(2)Te_3 system using the zone leveling technique enables the production of large regions of homogeneous solid solutions for the formation of precipitate nanocomposites as compared with Bridgman solidification. (PbTe)_(0.940)(Sb_(2)Te_3)_(0.060) and (PbTe)_(0.952)(Sb_(2)Te_3)_(0.048) alloys were successfully grown using (PbTe)_(0.4)(Sb_(2)Te_3)_(0.6) and (PbTe)_(0.461)(Sb_(2)Te_3)_(0.539) as seed alloys, respectively, with 1 mm h^(–1) withdrawal velocity. In the unidirectionally solidified regions of both alloys, Widmanstatten precipitates are formed due to the decrease in solubility of Sb_(2)Te_3 in PbTe. To determine the compositions of the seed alloys for the zone leveling experiments, the solute distribution in solidification in the PbTe-richer part of the pseudobinary PbTe-Sb_(2)Te_3 system has been examined from the concentration profiles in the samples unidirectionally solidified by the Bridgman method

Magnetism and Electron Transport in Magnetoresistive Lanthanum Calcium Manganite

It is the goal of this thesis to understand the physical properties associated with the large negative magnetoresistance found in lanthanum calcium manganite. Such large magnetoresistances have been reported that this material is being considered for use as a magnetic field sensor. However, there are many variables such as temperature, magnetic field, chemical composition and processing that greatly influence the magnitude of the magnetoresistance. After introducing the problem in Chapter 1, Chapters 2 and 3 describe the materials synthesis and physical property measurements used in this work. In Chapter 4, the intrinsic magnetic and electron transport properties of lanthanum calcium manganite are distinguished from those that depend largely on the chemical synthesis and processing. Chemical substitution of lanthanum by gadolinium, discussed in Chapter 5, not only induces ferrimagnetism, but also dramatically alters the electron transport because of slight structural changes. The physical mechanisms and empirical relationships found among the resistivity, magnetoresistance and magnetism in Chapters 3 and 4 are studied in greater depth in Chapters 6 and 7 and compared with theoretical predictions. This analysis provides a useful method for predicting the magnetoresistance as a function of temperature, magnetic field and transition temperature. The related perovskite, strontium ruthenate, proves to be a model compound for the study of metallic ferromagnets. The results of this work is presented in two appendices, and compared with the manganite results throughout the text

Phonon engineering through crystal chemistry

Mitigation of the global energy crisis requires tailoring the thermal conductivity of materials. Low thermal conductivity is critical in a broad range of energy conversion technologies, including thermoelectrics and thermal barrier coatings. Here, we review the chemical trends and explore the origins of low thermal conductivity in crystalline materials. A unifying feature in the latest materials is the incorporation of structural complexity to decrease the phonon velocity and increase scattering. With this understanding, strategies for combining these mechanisms can be formulated for designing new materials with exceptionally low thermal conductivity

Control of defects for optimizing performance in thermoelectric alloys

Processing influences many aspects of thermal and electrical transport properties in materials. Non-equilibrium microstructures and defects can strongly scatter phonons and electrons in many ways. Even the equilibrium defects and disorder inherent in materials has a profound effect on transport properties. Using near-equilibrium alloys we can combine our atomic level understanding of point defects (using ab initio methods) with phase relations determined from equilibrium thermodynamics to predict alloy structures at varying composition and temperature. Then using simple but effective models for semiconductors, the thermal and electrical transport properties can be explained and improvements predicted. Defects are important because achieving the maximum performance of a typical thermoelectric semiconductor requires optimization of the carrier concentration, which is entirely controlled by defects. Here we will discuss the chemical control afforded by extrinsic (impurity atom) defects. Ab initio methods have become extremely powerful but knowing which charge states to include is poorly defined [1]. Examples discussed in the field of thermoelectrics include the lead chalcogenides such as PbSe and AZn2Sb2 where A is any of the isovalent +2 element Ca, Yb, Sr, Eu (Figure). While all these A element donate the same 2 electrons to the valence band, the electronegativity of the A cation determines the vacancy defect concentration and therefore carrier concentration [2]. Complexing substitutional and interstitial defects are found in skutterudites and their control can also be used to optimize these excellent thermoelectric materials [3]. Defects from alloys also reduce lattice thermal conductivity but this benefit to thermoelectric performance must also be weighed against the detriment of the reduction in charge carrier mobility [4]. Please click Additional Files below to see the full abstract

Rapid consolidation of powdered materials by induction hot pressing

A rapid hot press system in which the heat is supplied by RF induction to rapidly consolidate thermoelectric materials is described. Use of RF induction heating enables rapid heating and consolidation of powdered materials over a wide temperature range. Such rapid consolidation in nanomaterials is typically performed by spark plasma sintering (SPS) which can be much more expensive. Details of the system design, instrumentation, and performance using a thermoelectric material as an example are reported. The Seebeck coefficient, electrical resistivity, and thermal diffusivity of thermoelectric PbTe material pressed at an optimized temperature and time in this system are shown to agree with material consolidated under typical consolidation parameters

Zintl Chemistry for Designing High Efficiency Thermoelectric Materials

Zintl phases and related compounds are promising thermoelectric materials; for instance, high zT has been found in Yb_(14)MnSb_(11), clathrates, and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors

Effective thermal conductivity of polycrystalline materials with randomly oriented superlattice grains

A model has been established for the effective thermal conductivity of a bulk polycrystal made of randomly oriented superlattice grains with anisotropic thermal conductivity. The in-plane and cross-plane thermal conductivities of each superlattice grain are combined using an analytical averaging rule that is verified using finite element methods. The superlattice conductivities are calculated using frequency dependent solutions of the Boltzmann transport equation, which capture greater thermal conductivity reductions as compared to the simpler gray medium approximation. The model is applied to a PbTe/Sb_2Te_3 nanobulk material to investigate the effects of period, specularity, and temperature. The calculations show that the effective thermal conductivity of the polycrystal is most sensitive to the in-plane conductivity of each superlattice grain, which is generally four to five times larger than the cross-plane conductivity of a grain. The model is compared to experimental measurements of the same system for periods ranging from 287 to 1590 nm and temperatures from 300 to 500 K. The comparison suggests that the effective specularity increases with increasing annealing temperature and shows that these samples are in a mixed regime where both Umklapp and boundary scattering are important