Low Electronic Thermal Conductivity Research Articles

DFT study of the half‐metallic variant perovskites A2OsX6 (A = Rb/Cs; X = Cl/Br) for spintronic and thermoelectric applications

AbstractWe present our results of the spin‐polarized calculations on the structural, magneto‐electronic, thermodynamic, and thermoelectric properties of vacancy‐ordered double perovskites A2OsX6 (A = Rb/Cs; X = Cl/Br). We utilized the Wu‐Cohen generalized gradient approximation (Wu‐GGA) and the mBJ scheme to determine a more reliable electronic structure. The compounds exhibit negative formation energy, suitable tolerance factor, and a stable phonon spectrum, indicating their stability. The compounds show half‐metallicity, acting as semiconductors with direct band gaps between 2 and 3 eV in the spin‐up orientation while metallic in the spin‐down. Each compound shows a total spin magnetic moment of 2.00 μB per formula unit, with Os‐t2g states contributing the most (~1.5 μB). The computed thermoelectric coefficient indicates the usability of these compounds across a wide temperature range (200–800 K) with high electrical conductivity and low electronic thermal conductivity. The compounds exhibit high Seebeck coefficient and figure of merit (ZT), making them suitable for thermoelectric applications. With ferromagnetic and half‐metallic characteristics, these compounds could be promising candidates for spintronics, thermoelectronic, and data storage applications.

Thermoelectric Properties of HfSe2 Monolayer

In this work detailed investigation of thermoelectric properties of HfSe2 monolayer is investigated employing Boltzmann transport formalism. We present numerical calculations of thermoelectric properties namely, electrical conductivity, σ, electronic thermal conductivity, Ke , diffusion thermopower, Sd and thermoelectric figure of merit, ZT , considering the scattering of electrons from acoustic phonons and charged impurities. We find that HfSe2 monolayer possess extremely low electronic thermal conductivity and relatively high thermopower (~ 100µV/K). Our calculations show that thermoelectric figure of merit is more than one at higher temperatures thus making HfSe2 monolayer suitable candidate for thermoelectric applications.

Decreasing the Carrier Concentration of ZrNiSn: An Opposite Way to the Best N-Type Half-Heusler Thermoelectrics.

N-type ZrNiSn-based alloys reach a record thermoelectric figure of merit zT ≈1.2 by increasing the carrier concentration to 4-5 × 1020 cm-3 . In this work, It is reported that a comparable zT can also be realized in trace Ru-doped ZrNiSn-based alloy at even lower temperature by decreasing the carrier concentration. Compared to the previously reported Co doping, the doping of Ru results in a more effective reduction in carrier concentration, and thus higher Seebeck coefficient, lower electronic thermal conductivity, and enhanced thermoelectric performance. The electronic specific heat coefficient of the ZrNi1- x Rux Sn sample remains constant with increasing Ru content, indicating no obvious change in the density of states effective mass. Theoretical calculations show that the doping of Ru has negligible effect on the bottom of conduction band. The lattice thermal conductivity is further reduced by alloying Ti and Hf at the Zr site, and the bipolar diffusion is suppressed by doping of 0.5 at.% Sb. As a result, Ti0.25 Zr0.5 Hf0.25 Ni0.99 Ru0.01 Sn0.995 Sb0.005 reaches not only a zT value of 1.1 at 773 K but also a record average zT value of 0.8 in 300 to 873 K, demonstrating the effectiveness of trace Ru doping on boosting the thermoelectric performance of ZrNiSn-based alloys.

Thermoelectric properties of C2P4 monolayer: A first principle study

We have theoretically explored the electronic and thermoelectric properties of the C2P4 monolayer with the interface of density functional theory and semi-classical transport theory. Our calculation shows a high Seebeck coefficient and low electronic thermal conductivity in the vicinity of zero chemical potential (μ = 0), resulting in a good power factor (PF) and a high figure of merit (ZT). More particularly, the electronic figure of merit (ZTe) exhibits two high peak values around μ = 0 due to the significant contribution of thermoelectric parameters. Furthermore, ZTe decreases by increasing the temperature, giving a peak value of 0.98 in the negative chemical potential (μ), whereas, for μ > 0, the peak value increases slightly with temperature. Additionally, the ZTe peak value is robust against ±10% of uni- and biaxial strains at room temperature. To make our calculation more realistic, we add phonon contributions to the thermal conductivity in pristine C2P4 and calculate the total ZT. We have found that phonon contribution dominates at low temperatures, and the ZT peak is reduced to 0.78. These optimal thermoelectric parameters of the C2P4 monolayer may be suitable for demonstrating the feasibility of a good thermoelectric material.

High thermoelectric power factor in LaVO3/KTaO3 heterostructure

Perovskite oxides are well-known for exhibiting semiconducting properties and widely used for electronic applications. The combination of two different perovskite oxides form heterojunction interface that tailors the charge transfer mechanism. Here, the thermoelectric(TE) electrical transport properties are investigated of pervoskite oxide heterostructure (HS) L a V O 3 / K T a O 3 ( L V O / K T O ) using Boltzmann transport equation (BTE) for symmetric and asymmetric termination in [001] orientation. The electron–phonon scattering and defect scattering due to oxygen vacancies are incorporated with BTE. A strong electron–phonon scattering is observed. The effect of oxygen defect concentration on electrical transport is also studied. These HS exhibits high electrical conductivity 2 . 9 × 1 0 4 S/m at room temperature due to n-type conducting interface and highly dispersive degenerate conduction bands. Besides moderate Seebeck coefficient, a relatively high thermoelectric power factor of 222 μ V m − 1 K − 2 for symmetric termination HS is obtained at 300 K as compared to bulk K T a O 3 and S r T i O 3 . The high power factor and low electronic thermal conductivity (0.19 Wm − 1 K − 1 ) designate L V O / K T O HS as an efficient material for photothermoelectric-based detectors and photovoltaic applications. • Electrical transport is predicted using Boltzmann transport theory. • Defect scattering due to oxygen vacancies is implemented. • Electron phonon scattering dominant at all temperatures. • High electrical conductivity of 2.9 x 10 4 S/m is obtained at 300 K. • Enhancement in power factor compared to bulk KTO and STO.

Enhanced thermoelectric perfromance in cubic form of SnSe stabilized through enformatingly alloying AgSbTe2

Metastable cubic SnSe has received extensive attention in terms of theoretical predictions since orthorhombic SnSe was experimentally developed as one of the most promising thermoelectric candidates in recent years. In this work, we successfully stabilized the cubic SnSe through AgSbTe2 alloying, after which the thermoelectric transport properties were significantly optimized by simultaneously suppressing lattice and electronic thermal conductivities via introducing Pb. The maximum power factor of (SnSe)0.6(AgSbTe2)0.4 reached to 13 μWcm−1K−2 at 723 K, with a high average power factor of ∼ 12 μWcm−1K−2 at 300–773 K. Based on 40% AgSbTe2 alloying, the strong phonon-defect scattering and low electronic thermal conductivity caused by Pb alloying contribute greatly to the reduction of total thermal conductivity, while the excellent electrical transport properties were substantially maintained. Finally, the peak ZT value reached to 1.20 and a high average ZT value of ∼ 0.90 over 300–773 K were achieved in the cubic phase (SnSe)0.6(AgSbTe2)0.4 with 10% Pb alloyed.

Optimized thermoelectric performance in thin (Bi2Se3)1−x(Bi2Te3)x alloyed films

Herein, thin films of (Bi2Se3)1−x(Bi2Te3)x were synthesized by thermal evaporation in high vacuum using highly crystalline bulk samples of Bi2Se3 alloyed with Bi2Te3. Preparation and characterization together with probing the thermoelectric properties of thin (Bi2Se3)1−x(Bi2Te3)x films are discussed in this article. Crystal structure, surface morphology, roughness and lattice features of the deposited films were probed via XRD, SEM and HRTEM techniques, which confirmed the perfect crystallinity and the nano-scalability of the prepared thin films. Thermoelectric measurements were carried out for the as-deposited films within a temperature range of 300–473 K. Seebeck coefficient of the studied samples is about two times larger than that of the previously reported bulk samples. The highest power factor was recorded at 131 μW/m K2 at 473 K. The high value of the power factor shows that the materials under the study are promising for applications such as resource recovery of waste and also as nanomaterials for environmental applications. Very low electronic thermal conductivity was obtained due to the small electrical conductivity and due to the scattering of carriers by the tiny grains constituting the prepared films.

Enhancing the room temperature thermoelectric performance of n-type Bismuth-telluride-based polycrystalline materials by low-angle grain boundaries

Bi2Te3-based thermoelectric materials are widely used in solid-state refrigeration near room temperature. However, the room temperature figure of merit (zT) of n-type Bi2Te3-based polycrystals produced by once sintering is always lower than 0.8. Herein, low-angle grain boundaries (LAGBs) are introduced in n-type Bi2Te2.7+xSe0.3 by a simple step-hot-pressing procedure (once sintering) and well characterized by scanning transmission electron microscopy. LAGBs consist of dislocation arrays that can effectively scatter the medium-frequency phonons and thus suppress the lattice thermal conductivity. Although LAGBs also serve as scattering centers of low-energy electrons and deteriorate the carrier mobility, they could contribute to the enhanced Seebeck coefficient owing to the increased scattering factor. Overall, the samples with LAGBs own lower electronic thermal conductivity at the same power factor level. Finally, a high room temperature zT of 0.94 is obtained in n-type Bi2Te2.7Se0.3, which is comparable to those produced by multiple-time sintering. Moreover, step-hot-pressing is also found to be effective in promoting the room temperature zT of p-type Bi2Te3-based polycrystals. This work puts forward a new and simple method to construct LAGBs in Bi2Te3-based alloys that enhances their zTs, and sheds light on the underlying mechanisms about how the LAGB affects the electrical and thermal transport properties.

Outstanding performance of Si-bilayer graphene using first-principle computation: Significant effects of Si atoms configuration

We theoretically investigate silicon doped bilayer graphene, Si-BLG, with different stoichiometry of Si-BLG structures. The dangling bonds of C-Si atoms are found at low concentration ratio of Si atoms inducing sp3-hybridization of buckled pattern in the structure. The Si-BLG with dangling bonds revel strong mechanical response at low strain of a uniaxial load, and the fracture strain is seen at low strain ratio. The sp3-hybridization forms a small bandgap which induces an intermediate thermal and optical response of the system. In contrast, at higher Si concentration ratio, the Young modulus and fracture strain are increased comparing to the low Si concentration ratio. This is due to presence of high number of C-Si bonds which have a high tolerant under uniaxial load. In addition, a relatively larger bandgap or an overlap between valence and conduction bands are found depending on the Si configurations. In the presence of gaped Si-BLG, the thermal and optical response are high. We thus obtain a high Seebeck coefficient and a figure of merit with low electronic thermal conductivity which are useful for thermoelectric nanodevices. Furthermore, an enhancement of optical response is obtained with a redshift in the visible range for the Si-BLG structures.

The study of structural, electronic and thermoelectric properties of Ca1−xYbxZn2Sb2 (x = 0, 0.25, 0.5, 0.75, 1) Zintl compounds

Based on the electronic structure, the physical properties of [Formula: see text] ([Formula: see text], 0.25, 0.5, 0.75, 1) Zintl compounds are studied. The transport properties can be significantly changed by varying the composition [Formula: see text]. The materials under study are more metallic with increasing [Formula: see text] and behaves like a semiconductor when [Formula: see text] decreases. It is found that [Formula: see text] exhibits a larger thermopower magnitude ([Formula: see text] at [Formula: see text] and the Seebeck coefficient decreases as [Formula: see text] increases. The calculated figure of merit factor of [Formula: see text] is found to be low, this is explained by the fact that its structure is very compact and its bandgap is small which lead to high electrical and thermal conductivity due to high carrier concentration ([Formula: see text] at [Formula: see text]). On other hand a narrow-gap (0.46 eV for [Formula: see text]), provides a balance between a high Seebeck coefficient and low electronic thermal conductivity, with a slight increase in the carrier concentration when the temperature increases ([Formula: see text] at 600 K). As a consequence, [Formula: see text] compound is predicted to have good performance for thermoelectric applications. The electrical [Formula: see text] and the thermal [Formula: see text] conductivity for [Formula: see text] compound in both directions (along [Formula: see text] and [Formula: see text]-axes) are calculated. It is obtained that [Formula: see text] is 120% of [Formula: see text] at high-temperature, whereas [Formula: see text] Seebeck coefficient was higher than [Formula: see text] especially at [Formula: see text] ([Formula: see text]. The large value of [Formula: see text] showed that the transport is dominated by zz-axis.

Strain dependent electronic transport of pristine Si and Ge nanowires

We have studied strain governed electronic structure and transport properties of Si nanowire (SiNW) and Ge nanowire (GeNW) utilizing generalized gradient approximation (GGA) of density functional theory (DFT). The SiNW and GeNW have a direct bandgap of 1.699 eV and 1.491 eV, respectively, at Γ point. The bandgap of SiNW for the intrinsic and uniaxial strain of −6% to 2%, and bandgap of GeNW for a compressive strain of 2% and 4%, is more than 1.5 eV; therefore, these Si/GeNW has potential applications in optoelectronic devices. The work function of Si/GeNW reduced drastically on applying compressing strain. For all the value of compressive strain, electrical and electronic thermal conductivity is higher than the relaxed SiNW; contradictory for tensile strain electrical, and electronic thermal conductivity is less than the relaxed SiNW. While for the all strained GeNW, the electrical and electronic thermal conductivity reduces drastically compared to strain-free GeNW. All strained GeNW have very low electronic thermal conductivity and a very high Seebeck coefficient as compared to strain-free GeNW. The figure of merit is high for SiNW, and it is greater than 1 in the range of 300 K to 600 K for an intrinsic tensile strain of 4%. GeNW for an intrinsic and uniaxial compressive strain of 8%, the value of ZTe is near one at 300 K and remains almost constant up to 800 K. High figure of merit of SiNW, very high Seebeck coefficient of GeNW and very low thermal conductivity of GeNW encourage using Si/GeNW as thermoelectric devices but is not limited to the thermoelectric generator or heat pump.

Anomalous electronic and thermoelectric transport properties in cubic Rb3AuO antiperovskite

We use first-principles calculations combined with self-consistent phonon (SCP) theory, electron-phonon ($e\ensuremath{-}ph$) coupling, and the Boltzmann transport equation (BTE) to investigate the electronic and thermoelectric transport properties in the cubic ${\mathrm{Rb}}_{3}\mathrm{AuO}$ antiperovskite with strongly cubic and quartic lattice anharmonicity. The combination of SCP theory and Wannier-Fourier interpolation is used to calculate the $e\ensuremath{-}ph$ coupling due to the failure of density functional perturbation theory in solving $e\ensuremath{-}ph$ matrix elements for a strongly anharmonic crystal. Our results exhibit that a high electron mobility ${\ensuremath{\mu}}_{e}$, e.g., $\ensuremath{\sim}454\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}$ at $300\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ from the iterative BTE solution, and a relatively weak temperature dependence of ${\ensuremath{\mu}}_{e}\ensuremath{\propto}{T}^{\ensuremath{-}1.1}$ are obtained in the $n$-type cubic ${\mathrm{Rb}}_{3}\mathrm{AuO}$. We demonstrate that the coupling between electrons and polar optical phonons is responsible for the good charge transport, which, along with the high thermopower deriving from a light and threefold degenerate conduction-band pocket around the $X$ point, leads to a very high power factor reaching $5.5\phantom{\rule{4pt}{0ex}}\mathrm{mW}/{\mathrm{mK}}^{2}$ at $800\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Meanwhile, a low lattice thermal conductivity and an undersized Lorenz number that signifies a resulting low electron thermal conductivity are also detected. As a result, a good thermoelectric performance with a figure of merit $ZT\ensuremath{\sim}1.02$ at $300\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and an anomalously high $ZT\ensuremath{\sim}3.02$ at $800\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ is captured in the $n$-type cubic ${\mathrm{Rb}}_{3}\mathrm{AuO}$. This finding breaks the long-term record of $ZT<3$ in most of the reported bulk thermoelectric materials to date, and illustrates that the cubic ${\mathrm{Rb}}_{3}\mathrm{AuO}$ is an excellent candidate for thermoelectric applications.

(Invited) Thermoelectric Energy Conversion and Thermal Memristance and Switching

Heat and charge are fundamental energy carrying modes relevant to nearly all dynamic properties of interest in engineering, physics, and modern energy technologies. Here, I will discuss the design and understanding of transport properties in complex nanoscale systems where the dimensional constraints are changed systematically to reveal fundamental principles of heat and charge transport. The first part of this talk will focus on heat and charge transport in polymeric and hybrid nanomaterials, revealing the physical origins of scaling laws for charge transport in these materials. The second portion of my talk will discuss heat and charge transport in a different class of hybrid materials, the ever-popular perovskites. Here, I will briefly discuss possible origins for the ultra-low thermal conductivity observed in nanoscale samples, and new measurement approaches for resistive bulk perovskites. The final portion of my talk will briefly highlight strong deviations from the Wiedemann-Franz law in materials with electronic phase transitions, and possible applications of this phenomenology for novel devices exhibiting, for example, thermal rectification and thermal memristance. Selected references: 1. Organic electronics: one model to rule them all, Nature Materials (2017) 2. Polymer morphology dominates over energy-dependent scattering in inorganic-organic hybrid thermoelectrics, Nature Communications (2018) 3. Ultralow thermal conductivity in all inorganic halide perovskites, PNAS (2017) 4. Phonon dispersion lifetimes of organic-inorganic hybrid perovskite CH3NH3PbI3 from Inelastic X-ray scattering (2018, in review). 5. Anomalously low electronic thermal conductivity in metallic vanadium dioxide, Science (2017) 6. Solid-state thermal rectification and negative differential thermal resistance using junctions of phase transition materials, Phys. Rev. Applied (2018)

Compton profile of VO2 across the metal-insulator transition: Evidence of a non-Fermi liquid metal

Many-body diffusion Monte Carlo is used to obtain the first-principles momentum distribution and Compton profile of vanadium dioxide. Our results for the Compton profile are in good agreement with the experimental values, and we show that good qualitative agreement in the scaled Compton profile difference across the monoclinic to rutile phase transition depends on an accurate description of electron correlation. The electron momentum distribution enables new insights into the metal-insulator phase transition. For example, the probability for electron scattering in the proximity of the Fermi surface (forward scattering) is suppressed in the vanadium chain direction (rutile $c$ axis) but enhanced in perpendicular directions. However, along the $c$ axis we observe an increase at $\ensuremath{\sim}2{k}_{\mathrm{F}}$ in the momentum distribution, which is characteristic for Friedel oscillations (backscattering). Our analysis of the momentum distribution supports experimentally observed anisotropies and provides an explanation for the anomalously low electronic thermal conductivity observed recently in the metallic phase [S. Lee et al., Science 355, 371 (2017)]. Moreover, our results indicate non-Fermi liquid behavior as well as quasi-one-dimensional Friedel oscillations in the metallic rutile phase, which is reminiscent of a Tomanaga-Luttinger liquid with impurities.

Preparation of monophasic titanium sub-oxides of Magnéli phase with enhanced thermoelectric performance

Titanium sub-oxides of Magnéli phase are considered as earth-abundant, non-toxic materials for thermoelectric application at high temperature, but very limited investigation has been reported mainly owing to the difficulty lying in obtaining monophasic material. Herein we describe a facile strategy to prepare monophasic Ti9O17 and Ti4O7 powder, in which the single Ti9O17 phase is obtained for the first time. After spark plasma sintering, the thermoelectric properties of monophasic Ti9O17 and Ti4O7 ceramics are studied comparatively. Despite the one magnitude lower electrical conductivity, Ti9O17 shows higher power factor compared with Ti4O7 due to the large Seebeck coefficient. Moreover, with the low electronic thermal conductivity, sub-micron scaled grain size and nano-scaled crystallographic shear plane, the Ti9O17 ceramic exhibits even lower thermal conductivity compared to Ti4O7, leading to a ZT of 0.16 at 764K, which is the best thermoelectric performance among reported Magnéli phases.

Anomalously low electronic thermal conductivity in metallic vanadium dioxide.

In electrically conductive solids, the Wiedemann-Franz law requires the electronic contribution to thermal conductivity to be proportional to electrical conductivity. Violations of the Wiedemann-Franz law are typically an indication of unconventional quasiparticle dynamics, such as inelastic scattering, or hydrodynamic collective motion of charge carriers, typically pronounced only at cryogenic temperatures. We report an order-of-magnitude breakdown of the Wiedemann-Franz law at high temperatures ranging from 240 to 340 kelvin in metallic vanadium dioxide in the vicinity of its metal-insulator transition. Different from previously established mechanisms, the unusually low electronic thermal conductivity is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently.

Effect of X on the transport properties of skutterudites LaFe4X12 (X = P, As and Sb) compounds

We have investigated the influence of substituting P by As and As by Sb on the thermoelectric properties of LaFe4X12 (X = P, As and Sb) compounds. It has been found that substituting P → As → Sb show significant influence on the bands dispersions, bond lengths, effective masses, the carriers mobility and hence the transport properties. Replacing P by As cause significant increases in the carriers concentration with increasing the temperatures, while replacing As by Sb exhibit insignificant increase in the carriers concentration of LaFe4Sb12 with respect to LaFe4As12. It is clear that LaFe4As12 show the highest electrical conductivity among the other compounds along the temperature range. Calculations show that LaFe4Sb12 exhibit low electronic thermal conductivity in the temperature range between 50 and 600 K, then above 650 K the LaFe4P12 compound show the lower electronic thermal conductivity. The Seebeck coefficient of LaFe4X12 (X = P, As and Sb) compounds increases with increasing the temperature. Moving from P → As → Sb cause significant increases in the Seebeck coefficient and LaFe4Sb12 compound exhibit the highest Seebeck coefficient along whole temperature scale. The power factor of LaFe4X12 (X = P, As and Sb) compounds increases with increasing the temperature and LaFe4Sb12 compound show the highest power factor along the whole temperature range.

Femtosecond Laser Pulse Train Effect on Optical Characteristics and Nonequilibrium Heat Transfer in Thin Metal Films

The objective of this study is to numerically investigate the electron-phonon interactions and the nonequilibrium energy transfer in metal thin films irradiated by ultrashort pulse train lasers. In particular, the temporal and spatial variations in the optical properties during laser irradiation are discussed; the influence of the number of pulses per train and the pulse separation time are also examined. The present study uses the well-established two-temperature model to describe laser-solid interactions and the quantum approach to determine various properties such as electron heat capacity, electron thermal conductivity, collision frequencies, reflectivity, and absorption rates. It is found that as the number of pulses per pulse train increases, the nonequilibrium state between electrons and phonons gradually disappears because of the energy relaxation and the low electron thermal conductivity. The results show that the electron-electron and electron-phonon collision frequencies vary significantly with the number of pulses per train and the separation time per pulse, and that they considerably affect the reflectivity and absorption rate, in turn leading to a change in the ablation mechanism of thin metal films for pulse train laser heating. [doi:10.2320/matertrans.M2010009]

Low-temperature lattice thermal conductivity of heavily deformed metallic alloys

Lattice thermal conductivity in Cu-Al alloys is calculated for the first time within a two-dimensional formalism involving the scattering of phonons by strain fields of finite spatial extent around randomly oriented dislocations of edge and screw character. For the finite strain fields, scattering widths indicate that long-wavelength phonons scatter very weakly. In addition to phonon scattering by the strain field around the dislocation, phonon-electron scattering is included to obtain the overall phonon relaxation rate. Comparison is made between the calculated results and measurements on the commercial alloy Evanohm. Evanohm has a very high residual resistivity and thus possesses a low electronic thermal conductivity. Therefore, the phonon contribution to the thermal conductivity may be unambiguously identified for this alloy unlike the situation encountered for most other alloys at very low temperatures. The calculated lattice thermal conductivity is found to depart from the usual ${T}^{2}$ dependence in general agreement with the measurements on Evanohm.