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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2016)

Thermoelectric materials, which enable direct conversion between thermal and electrical energy, provide an alternative for power generation and refrigeration. The key parameter that defines the efficiency of thermoelectric materials is the ‘dimensionless figure of merit’ ZT, which is composed of the Seebeck coefficient, electrical conductivity and total thermal conductivity respectively. Ideally, to achieve high ZT both the Seebeck coefficient and electrical conductivity should be large, while total thermal conductivity must be minimized. In this thesis, first-principles calculations of the Seebeck coefficient, lattice thermal conductivity and electrical conductivity are performed to study mechanisms and factors that gives rise to high ZT.

One effective way to enhance ZT is through direct reduction of lattice thermal conductivity. We perform calculation and analysis of lattice thermal conductivity for thermoelectric materials by solving the Boltzmann transport equation iteratively in the framework of perturbation theory. The second- and third-order interatomic force constants are extracted using the recently developed CSLD (compressive sensing lattice dynamics) method. Afterwards, we evaluate opportunities to achieve further reduction of lattice thermal conductivity. Our first study of ternary zinc-blende-based mineral compounds famatinite (Cu3SbS4) and permingeatite (Cu3SbSe4) shows that optical modes in these two compounds contribute a sizable portion of the total lattice thermal conductivity and thus cannot be neglected. Due to the fact that phonon modes with mean free paths larger than 10 nm carry about 80% of the heat, nanostructuring, which reduces the mean free path, is a promising way to reduce the lattice thermal conductivity by reducing the characteristic length. In addition, our simple alloying model including mass disorder reproduces experimental findings that forming solid solutions rapidly decreases the lattice thermal conductivity. An alternative way to reduce lattice thermal conductivity is to introduce guest atoms in host cage structures. Our study of type-I Si clathrates containing guest atoms Na and Ba shows that Na tends to form incoherent localized phonon mode while Ba coherently couples with the host cages. The low lattice thermal conductivities of Na- and Ba-filled Si clathrates should be attributed to the dramatic reductions in both phonon lifetime and group velocity. Analysis of phonon scattering process reveals that localized modes can be effectively emitted and absorbed, thus dramatically enhancing overall scattering rates.

Another widely adopted approach to achieve high ZT is through maintaining a high power factor. To accurately determine the Seebeck coefficient and electrical conductivity, we estimate carrier lifetime due to electron-phonon interaction under relaxation time approximation using the electron-phonon Wannier interpolation technique. Our study of noble metals Cu and Ag shows that their positive Seebeck coefficients can be mostly attributed to the negative energy dependence of carrier lifetime. In contrast to the previous study of positive Seebeck in Li, which is due to the deviation of electronic behavior from that in free electron model, it is the nontrivial energy dependence of electron-phonon interaction vertex that leads to the positive Seebeck coefficient. Intermetallic compound B20-type CoSi has drawn considerable attention due to its exceptionally high power factor and large Seebeck coefficient. Our study shows that the large negative Seebeck coefficient of the pristine CoSi is mostly due to the strong energy dependence of carrier lifetime, which together with the high electrical conductivity leads to the high power factor. For heat transport, both electron-phonon and phonon-phonon interactions contribute significantly to phonon scattering at temperatures lower than 200 K. While at temperatures higher than 300 K, phonon-phonon interaction dominates over electron-phonon interaction. Based on the optimized power factor with properly adjusted carrier concentration, we predict that the maximum ZTs at 300 and 600 K are about 0.11 and 0.25 respectively without further reducing the total thermal conductivity.

Known good thermoelectric materials often are comprised of elements that are in low abundance, toxic and require careful doping and complex synthesis procedures. High performance thermoelectricity has been reported in earth-abundant compounds based on natural mineral tetrahedrite (Cu12Sb4S13). Our first-principles electronic structure calculations of Cu12Sb4S13 show that Cu atoms are all in the monovalent state, creating two free hole states per formula unit of the pristine compound. Optimal thermoelectric performance can be achieved via electron doping. Substituting transition metals on Cu 12d sites does the job. Detailed analysis shows that Zn and Fe substitutions tend to fill the empty hole states, while Ni substitution introduces an additional hole to the valence band by forming ferromagnetic configuration. Experimentally observed extremely low lattice thermal conductivity can be attributed to the out-of-plane vibrations of the three-fold Cu ions. This is further verified by the large Gruneisen parameter calculated.

    Cover page: First-Principles Studies of Phonons and Electrons in Bulk Thermoelectrics
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2014)
    First-principles prediction of lattice thermal conductivity κ(L) of strongly anharmonic crystals is a long-standing challenge in solid-state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics. Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Nonintuitively, high accuracy is achieved when the model is trained on first-principles forces in quasirandom atomic configurations. The method is demonstrated for Si, NaCl, and Cu(12)Sb(4)S(13), an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature κ(L) to values near the amorphous limit.
      Cover page: Lattice Anharmonicity and Thermal Conductivity from Compressive Sensing of First-Principles Calculations
      • Article
      • Peer Reviewed
      LBL Publications (2021)
      Understanding how to optimize electronic band structures for thermoelectrics is a topic of long-standing interest in the community. Prior models have been limited to simplified bands and/or scattering models. In this study, we apply more rigorous scattering treatments to more realistic model band structures—upward-parabolic bands that inflect to an inverted-parabolic behavior—including cases of multiple bands. In contrast to common descriptors (e.g., quality factor and complexity factor), the degree to which multiple pockets improve thermoelectric performance is bounded by interband scattering and the relative shapes of the bands. We establish that extremely anisotropic “flat-and-dispersive” bands, although best-performing in theory, may not represent a promising design strategy in practice. Critically, we determine optimum bandwidth, dependent on temperature and lattice thermal conductivity, from perfect transport cutoffs that can in theory significantly boost zT beyond the values attainable through intrinsic band structures alone. Our analysis should be widely useful as the thermoelectric research community eyes zT > 3.
        Cover page: Optimal band structure for thermoelectrics with realistic scattering and bands
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2015)
        We report a more than 30% increase in the thermoelectric figure of merit of Ni-doped tetrahedrite (Cu12Sb4S13) by the addition of Zn. We show that this enhancement is due to the combination of two effects: (1) a tuning of the Fermi energy to optimize the Seebeck coefficient and thermoelectric power factor and (2) a reduction in thermal conductivity due to decreases in both the electronic and lattice thermal conductivities. Unlike tetrahedrites doped solely with Zn, which can become electrically insulating at high Zn concentrations, Ni doped samples remain conducting due to the existence of an additional valence band from spin-split Ni states. By adding Zn to these Ni-doped materials, electrons can fill the holes in the valence band, allowing for tuning of the thermoelectric properties. Tetrahedrites optimally doped with both Ni and Zn have thermal conductivity approaching theoretical minimum values, which helps boost the thermoelectric figure of merit zT in these compounds above unity.
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        Cover page: Increasing the Thermoelectric Figure of Merit of Tetrahedrites by Co-Doping with Nickel and Zinc
        • Article
        • Peer Reviewed
        LBL Publications (2021)
        Band convergence is considered a clear benefit to thermoelectric performance because it increases the charge carrier concentration for a given Fermi level, which typically enhances charge conductivity while preserving the Seebeck coefficient. However, this advantage hinges on the assumption that interband scattering of carriers is weak or insignificant. With first-principles treatment of electron-phonon scattering in the CaMg2Sb2-CaZn2Sb2 Zintl system and full Heusler Sr2SbAu, we demonstrate that the benefit of band convergence can be intrinsically negated by interband scattering depending on the manner in which bands converge. In the Zintl alloy, band convergence does not improve weighted mobility or the density-of-states effective mass. We trace the underlying reason to the fact that the bands converge at a one k-point, which induces strong interband scattering of both the deformation-potential and the polar-optical kinds. The case contrasts with band convergence at distant k-points (as in the full Heusler), which better preserves the single-band scattering behavior thereby successfully leading to improved performance. Therefore, we suggest that band convergence as thermoelectric design principle is best suited to cases in which it occurs at distant k-points.
          Cover page: When band convergence is not beneficial for thermoelectrics
          • Article
          • Peer Reviewed
          UC Santa Cruz Previously Published Works (2022)
          Non-alcoholic fatty liver disease (NAFLD) is currently related to a heavy socioeconomic burden and increased incidence. Since obesity is the most prevalent risk factor for NAFLD, weight loss is an effective therapeutic solution. Bariatric surgery (BS), which can achieve long-term weight loss, improves the overall health of patients with NAFLD. The two most common surgeries are the Roux-en-Y gastric bypass and sleeve gastrectomy. The gut-liver axis is the complex network of cross-talking between the gut, its microbiome, and the liver. The gut microbiome, involved in the homeostasis of the gut-liver axis, is believed to play a significant role in the pathogenesis of NAFLD and the metabolic improvement after BS. Alterations in the gut microbiome in NAFLD have been confirmed compared to that in healthy individuals. The mechanisms linking the gut microbiome to NAFLD have been proposed, including increased intestinal permeability, higher energy intake, and other pathophysiological alterations. Interestingly, several correlation studies suggested that the gut microbial signatures after BS become more similar to those of lean, healthy controls than that of patients with NAFLD. The resolution of NAFLD after BS is related to changes in the gut microbiome and its metabolites. However, confirming a causal link remains challenging. This review summarizes characteristics of the gut microbiome in patients with NAFLD before and after BS and accumulates existing evidence about the underlying mechanisms of the gut microbiome.
            Cover page: Gut microbiome and microbial metabolites in NAFLD and after bariatric surgery: Correlation and causality.
            • Article
            • Peer Reviewed
            LBL Publications (2020)
            We report a first-principles density-functional study of electron-phonon interactions in and thermoelectric transport properties of the full Heusler compounds Sr2BiAu and Sr2SbAu. Our results show that ultrahigh intrinsic bulk thermoelectric performance across a wide range of temperatures is physically possible and point to the presence of multiply degenerate and highly dispersive carrier pockets as the key factor for achieving this. Sr2BiAu, which features ten energy-aligned low-effective-mass pockets (six along Γ-X and four at L), is predicted to deliver n-type zT=0.4-4.9 at T=100-700 K. Comparison with the previously investigated compound Ba2BiAu shows that the additional L pockets in Sr2BiAu significantly increase its low-temperature power factor to a maximum value of 12 mW m-1 K-2 near T=300 K. However, at high temperatures the power factor of Sr2BiAu drops below that of Ba2BiAu because the L states are heavier and subject to strong scattering by phonon deformation, as opposed to the lighter Γ-X states, which are limited by polar-optical scattering. Sr2SbAu is predicted to deliver a lower n-type zT=3.4 at T=750 K due to appreciable misalignment between the L and Γ-X carrier pockets, generally heavier scattering, and a slightly higher lattice thermal conductivity. Soft acoustic modes, which are responsible for the low lattice thermal conductivity, also increase the vibrational entropy and high-temperature stability of these Heusler compounds, suggesting that their experimental synthesis may be feasible. The dominant intrinsic defects are found to be Au vacancies, which drive the Fermi level towards the conduction band and work in favor of n-doping.
              Cover page: High Thermoelectric Performance and Defect Energetics of Multipocketed Full Heusler Compounds
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              • Peer Reviewed
              UCLA Previously Published Works (2016)
              Epitaxial heterostructures with precisely controlled composition and electronic modulation are of central importance for electronics, optoelectronics, thermoelectrics, and catalysis. In general, epitaxial material growth requires identical or nearly identical crystal structures with small misfit in lattice symmetry and parameters and is typically achieved by vapor-phase depositions in vacuum. We report a scalable solution-phase growth of symmetry-mismatched PbSe/Bi2Se3 epitaxial heterostructures by using two-dimensional (2D) Bi2Se3 nanoplates as soft templates. The dangling bond-free surface of 2D Bi2Se3 nanoplates guides the growth of PbSe crystal without requiring a one-to-one match in the atomic structure, which exerts minimal restriction on the epitaxial layer. With a layered structure and weak van der Waals interlayer interaction, the interface layer in the 2D Bi2Se3 nanoplates can deform to accommodate incoming layer, thus functioning as a soft template for symmetry-mismatched epitaxial growth of cubic PbSe crystal on rhombohedral Bi2Se3 nanoplates. We show that a solution chemistry approach can be readily used for the synthesis of gram-scale PbSe/Bi2Se3 epitaxial heterostructures, in which the square PbSe (001) layer forms on the trigonal/hexagonal (0001) plane of Bi2Se3 nanoplates. We further show that the resulted PbSe/Bi2Se3 heterostructures can be readily processed into bulk pellet with considerably suppressed thermal conductivity (0.30 W/m·K at room temperature) while retaining respectable electrical conductivity, together delivering a thermoelectric figure of merit ZT three times higher than that of the pristine Bi2Se3 nanoplates at 575 K. Our study demonstrates a unique epitaxy mode enabled by the 2D nanocrystal soft template via an affordable and scalable solution chemistry approach. It opens up new opportunities for the creation of diverse epitaxial heterostructures with highly disparate structures and functions.
                Cover page: Scalable solution-phase epitaxial growth of symmetry-mismatched heterostructures on two-dimensional crystal soft template.
                • Article
                • Peer Reviewed
                Chemical Sciences (2018)
                Hybrid organic-inorganic perovskites are emerging semiconductors for cheap and efficient photovoltaics and light-emitting devices. Different from conventional inorganic semiconductors, hybrid perovskites consist of coexisting organic and inorganic sub-lattices, which present disparate atomic masses and bond strengths. The nanoscopic interpenetration of these disparate components, which lack strong electronic and vibrational coupling, presents fundamental challenges to the understanding of charge and heat dissipation. Here we study phonon population and equilibration processes in methylammonium lead iodide (MAPbI3) by transiently probing the vibrational modes of the organic sub-lattice following above-bandgap optical excitation. We observe inter-sub-lattice thermal equilibration on timescales ranging from hundreds of picoseconds to a couple of nanoseconds. As supported by a two-temperature model based on first-principles calculations, the slow thermal equilibration is attributable to the sequential phonon populations of the inorganic and organic sub-lattices, respectively. The observed long-lasting thermal non-equilibrium offers insights into thermal transport and heat management of the emergent hybrid material class.
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