Boris Kozinsky

Massachusetts Institute of Technology, Cambridge, MA, United States

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Publications (28)72.66 Total impact

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    ABSTRACT: We report a peak dimensionless figure-of-merit (ZT) of 1 at 700 °C in a nanostructured p-type Nb0.6Ti0.4FeSb0.95Sn0.05 composition. Even though the power factor of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition is improved by 25%, in comparison to the previously reported p-type Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, the ZT value is not increased due to a higher thermal conductivity. However, the higher power factor of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition led to a 15% increase in the power output of a thermoelectric device in comparison to a device made from the previous best material Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. The n-type material used to make the unicouple device is the best reported nanostructured Hf0.25Zr0.75NiSn0.99Sb0.01 composition with the lowest hafnium (Hf) content. Both the p- and n-type nanostructured samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. Moreover, the raw material cost of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition is more than six times lower compared to the cost of the previous best p-type Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. This cost reduction is crucial for these materials to be used in large-scale quantities for vehicle and industrial waste heat recovery applications.
    Energy & Environmental Science 10/2014; · 11.65 Impact Factor
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    ABSTRACT: The ab initio $GW$ method is considered as the most accurate approach for calculating the band gaps of semiconductors and insulators. Yet its application to transition metal oxides (TMOs) has been hindered by the failure of traditional approximations developed for conventional semiconductors. In this work, we examine the effects of these approximations on the values of band gaps for ZnO, Cu$_2$O, and TiO$_2$. In particular, we explore the origin of the differences between the two widely used plasmon-pole models. Based on the comparison of our results with the experimental data and previously published calculations, we discuss which approximations are suitable for TMOs and why.
    Journal of Physics Condensed Matter 07/2014; 26(47). · 2.22 Impact Factor
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    ABSTRACT: We screen a large chemical space of perovskite alloys for systems with optimal properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host an MPB and suggests a few others that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems.
    Physical Review B 03/2014; 89(13). · 3.66 Impact Factor
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    ABSTRACT: We present a first-principles study of the temperature- and density-dependent intrinsic electrical resistivity of graphene. We use density-functional theory and density-functional perturbation theory together with very accurate Wannier interpolations to compute all electronic and vibrational properties and electron-phonon coupling matrix elements; the phonon-limited resistivity is then calculated within a Boltzmann-transport approach. An effective tight-binding model, validated against first-principles results, is also used to study the role of electron-electron interactions at the level of many-body perturbation theory. The results found are in excellent agreement with recent experimental data on graphene samples at high carrier densities and elucidate the role of the different phonon modes in limiting electron mobility. Moreover, we find that the resistivity arising from scattering with transverse acoustic phonons is 2.5 times higher than that from longitudinal acoustic phonons. Last, high-energy, optical, and zone-boundary phonons contribute as much as acoustic phonons to the intrinsic electrical resistivity even at room temperature and become dominant at higher temperatures.
    Nano Letters 02/2014; · 13.03 Impact Factor
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    ABSTRACT: We screen a large chemical space of perovskite alloys for systems with the right properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end-points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host a MPB, and suggests a few new ones that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems.
    09/2013;
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    ABSTRACT: We present a new code to evaluate thermoelectric and electronic transport properties of extended systems with a maximally-localized Wannier function basis set. The semiclassical Boltzmann transport equations for the homogeneous infinite system are solved in the constant relaxation-time approximation and band energies and band derivatives are obtained via Wannier interpolations. Thanks to the exponential localization of the Wannier functions obtained, very high accuracy in the Brillouin zone integrals can be achieved with very moderate computational costs. Moreover, the analytical expression for the band derivatives in the Wannier basis resolves any issues that may occur when evaluating derivatives near band crossings. The code is tested on binary and ternary skutterudites CoSb_3 and CoGe_{3/2}S_{3/2}.
    Computer Physics Communications 05/2013; · 2.41 Impact Factor
  • Daehyun Wee, Boris Kozinsky, Marco Fornari
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    ABSTRACT: 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 engineering pnictogens in skutterudites may have significant impacts on the properties of filler vibrations.
    Journal of the Physical Society of Japan 01/2013; 82(1):4602-. · 2.09 Impact Factor
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    ABSTRACT: Experimental thermal conductivity of bulk materials are often modeled using Debye approximation together with functional forms of relaxation time with fitting parameters. While such models can fit the temperature dependence of thermal conductivity of bulk materials, the Debye approximation leads to large error in the actual phonon mean free path, and consequently, the predictions of the thermal conductivity of the nanostructured materials using the same relaxation time are not correct even after considering additional size effect on the mean free path. We investigate phonon mean free path distribution inside fully unfilled (Co4Sb12) and fully filled (LaFe4Sb12) bulk skutterudites by fitting their thermal conductivity to analytical models which employ different phonon dispersions. We show that theoretical thermal conductivity predictions of the nanostructured samples are in agreement with the experimental data obtained for samples of different grain sizes only when the full phonon dispersion is considered.
    Journal of Applied Physics 08/2012; 112(4). · 2.21 Impact Factor
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    ABSTRACT: Systematic discovery of materials with optimized properties based on first principles methodologies requires well-defined descriptors, in addition to the automation infrastructure for calculations and data analysis. We have designed a set of computationally affordable descriptors for enhanced piezoelectric performances and analyzed the chemical space for oxides with the perovskite structure. Our results include phase stability for the most promising compositions and ad hoc interpolation schemes that have been exploited to identify 49 alloys with favorable properties.
    02/2012;
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    ABSTRACT: A whole spectra of intriguing physical properties appears in conventional materials when structural features reach nanoscale. Since thermal conductivity is controlled by the heat carriers' mean free paths, it becomes of paramount importance to understand and engineer the role of alloying and nanostructuring on transport coefficients. First-principles calculations often provide accurate microscopic parameters, but at significant computational cost even for ideal, perfect systems. We present a hybrid classical-quantum method to compute thermal conductivity from both harmonic and anharmonic terms using Boltzmann transport formalism. We combine first-principles calculations of harmonic terms and force-field calculations of third-order and fourth-order force constant. Results for SiGe will be discussed to show the validity of approach. We also discuss the effects of nanostructuring by introducing boundary scattering contributions, as well as mechanisms of filler rattling in thermoelectric skutterudites.
    02/2012;
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    ABSTRACT: First principles calculations are used to investigate electronic band structure and vibrational spectra of pnictogen substituted ternary skutterudites. We compare the results with the prototypical binary composition CoSb$_3$ to identify the effects of substitutions on the Sb site, and evaluate the potential of ternary skutterudites for thermoelectric applications. Electronic transport coefficients are computed within the Boltzmann transport formalism assuming a constant relaxation time, using a new methodology based on maximally localized Wannier function interpolation. Our results point to a large sensitivity of the electronic transport coefficients to carrier concentration and to scattering mechanisms associated with the enhanced polarity. The ionic character of the bonds is used to explain the detrimental effect on the thermoelectric properties.
    Physical review. B, Condensed matter 12/2011; 85(24). · 3.66 Impact Factor
  • Physical review. B, Condensed matter 10/2011; · 3.66 Impact Factor
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    ABSTRACT: We present a large-scale density functional theory (DFT) investigation of the ABO3 chemical space in the perovskite crystal structure, with the aim of identifying those that are relevant for forming piezoelectric materials. Screening criteria on the DFT results are used to select 49 compositions, which can be seen as the fundamental building blocks from which to create alloys with potentially good piezoelectric performance. This screening finds all the alloy end points used in three well-known high-performance piezoelectrics. The energy differences between different structural distortions, deformation, coupling between the displacement of the A and B sites, spontaneous polarization, Born effective charges, and stability is analyzed in each composition. We discuss the features that cause the high piezoelectric performance of the well-known piezoelectric lead zirconate titanate (PZT), and investigate to what extent these features occur in other compositions. We demonstrate how our results can be useful in the design of isovalent alloys with high piezoelectric performance.
    Physical Review B 07/2011; 84(1):014103. · 3.66 Impact Factor
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    ABSTRACT: We present a nearly exhaustive density functional theory (DFT) survey over the chemical space of perovskite compounds on ABO3 form, with the aim of identifying alloy end points for new piezoelectric materials. Our screening criteria on the DFT results selects 85 relevant compounds, among which all well known alloy end points for high performance piezoelectrics are present. We analyze the compounds with respect to macroscopic polarization, born effective charges, and energy differences between different structure distortions. We discuss the energy features that cause the high piezoelectric performance of the well known piezoelectric lead zirconate titanate (PZT), and to what extent these features are rare among the found compounds. The results are used to discuss relevant isovalent alloys of the selected compounds.
    03/2011;
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    ABSTRACT: One of the most effective strategies to improve the thermoelectric figure of merit in skutterudites is to reduce thermal conductivity via alloying, filling, or nanostructuring. The latter is most effective when the dimension of the domains is comparable in size to the mean free path of the dominant heat-conducting phonons. In bulk, pristine semiconductors and insulators thermal conductivity and phonons' mean-free paths can nowadays be calculated fully from first-principles from the anharmonic terms in the ionic displacements. We show here our results for the lattice thermal conductivity of several compounds with the skutterudite structure, obtained from the Boltzmann transport equation using phonon lifetimes determined from density functional calculations. We will also discuss the effect of fourth-order terms, albeit as obtained using phenomenological approaches. Last, we comment on the interplay between the different length scales for the nanostructured domains and the relevant heat-carrying phonons.
    03/2011;
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    ABSTRACT: The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1   μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity.
    Physical Review Letters 01/2011; 106(4):045901. · 7.73 Impact Factor
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    ABSTRACT: The discovery of outstanding piezoelectric performance in perovkiste niobates alloys has pointed to these materials as viable lead-free substitutes for enviromentally sound transducers and actuators. Similarly to PZT (the solid solution between PbTiO3 and PbZrO3) the large electromechanical coupling in niobates has been originally linked to a morphotropic phase boundary (MPB) that involves a tetragonal phase formed at room temperature when 1-7% of Li is inserted in KxNa1-xNbO3 (KNN) with x ˜0.5. More recently an alternative explanation based on polymorphic phase transition (PPT) was proposed that equally justifies the enhanced piezoelectric constants and points to lack of stability of the electromechanical response as a function of temperature. We have performed first principles density functional calculations to characterize the role of the different chemical components and the local structure near Li atoms. We will discuss Li-Na interaction as the key mechanism that lead to PPT and we will point to specific consequences of the local structure in KNN on the temperature dependence.
    03/2010;
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    ABSTRACT: The possible application of pnictogen substituted ternary skutterudites in thermoelectric devices is currently limited due to their relatively large electrical resistivity. These materials, however, exhibit thermal conductivity of the order 0.7-2.2 k/Wm-1K-1 at room temperature and may be amenable to optimization. In skutterudites at high temperatures the thermal transport is primarily dominated by anharmonic interactions of phonons. The precise evaluation of such interactions from first principles is currently a formidable task. We have analyzed the third order phonon-phonon scattering mechanisms, that arise from the anharmonicity of the interatomic potentials, and use a standard Boltzmann transport approach to derive the thermal conductivity. In our methodology we have combined first principles approaches and phenomenological interatomic potentials. We specifically studied the transport properties of CoGe3/2S3/2, CoGe3/2Te3/2 and CoSn3/2Te3/2 and provide comparison to the parental binary CoSb3 . We validate our approach by testing the approximations in simpler systems where full evaluation of anharmonic force constants from first- principles calculations is possible.
    03/2010;
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    ABSTRACT: We use ab initio computations to investigate the effect of filler ions on the properties of CoSb3 skutterudites. We analyze global and local structural effects of filling, using the Ba-filled system as an example. We show that the deformation of Sb network induced by the filler affects primarily nearest neighboring Sb sites around the filler site as the soft Sb rings accommodate the distortion. Rearrangement of Sb atoms affects the electronic band structure and we clarify the effect of this local strain on the band gap. We compute the phonon dispersions and identify the filler-dominated modes from the lowest-frequency optical modes at Γ. Their weak dispersion across the Brillouin zone indicates that they are localized and a force-constant analysis shows that the filler vibration is strongly coupled with nearby Sb atoms.
    Physical review. B, Condensed matter 01/2010; 81(4). · 3.66 Impact Factor
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    ABSTRACT: We study from first-principles ternary skutterudites derived from CoSb3, where the pnictogen is substituted with elements from the IVB and VIB groups. We focus on CoGe3/2S3/2, CoGe3/2Te3/2 and CoSn3/2Te3/2, and compute the structure, electronic structure and vibrational properties from density-functional and density-functional perturbation theory. Since the direct evaluation of transport quantities in the relaxation-time approximation is computationally demanding, we use maximally-localized Wannier functions (MLWFs) for accurate integrations of operators across the Brillouin zone. This MLWFs basis leads to a very efficient and well-conditioned scheme to calculate the thermoelectric transport coefficients and to disentangle and identify the contribution of single bands. In addition, it provides a detailed, transferable picture of bonding in these complex materials.
    03/2009;