Semiclassical Boltzmann Transport Theory Research Articles

Dynamical stability, electronic and thermoelectric properties of quaternary ZnFeTiSi Heusler compound

We investigated the dynamical stability, electronic and thermoelectric properties of the ZnFeTiSi Heusler compound by combining the first-principles calculations and semi-classical Boltzmann transport theory. The phonon dispersion indicates the dynamical stability and the calculated formation energy is negative which confirm the stability of ZnFeTiSi in the Heusler structure. The calculated electronic structures show that ZnFeTiSi is a semiconductor with an indirect band gap of about 0.573 eV using GGA and 0.643 eV by mBJ-GGA potentials at equilibrium lattice parameter (5.90 Å). Seebeck coefficient, electrical conductivity and electronic thermal conductivity were calculated to describe the thermoelectric properties of the ZnFeTiSi compound. It is found that it exhibits high Seebeck coefficient and power factor, making it promising for future thermoelectric applications.

Ab initio study on anisotropic thermoelectric transport in ternary pnictide KZnP

Strongly anisotropic bands near the Fermi level via band structure engineering have been proposed to enhance thermoelectric performance in functional materials. Recent works exhibit the presence of flat-and-dispersive-band-like strong anisotropy in a class of ternary transition metal pnictides. Taking KZnP as a representative example, here we investigate the thermoelectric properties of this class of materials based on first-principles calculations and semiclassical Boltzmann transport theory. Strikingly, the calculated lattice thermal conductivity shows a strong anisotropy with a small value of about 5.24 (2.58) W m−1K−1 along the in-plane (out-of-plane) lattice direction at room temperature. Based on the electron relaxation time calculated from intensive ab initio electron–phonon interactions, a thermoelectric figure of merit zT of 0.32 is predicted for n-type doped KZnP, about two times lower than the value estimated by the constant relaxation time approximation from deformation potential theory. Nanostructures with the characteristic length shorter than 13 nm can reduce the by 40%, enhancing zT to 0.52 along the c axis direction. This work supports that KZnP is a potential candidate for thermoelectric applications.

Temperature-dependent optical and electrical properties of bulk Ti2AlC and two-dimensional MXenes from first-principles

The accurate representation of optical and electrical properties of MAX phase ternary carbide Ti2AlC and its derivative two-dimensional MXenes at finite temperature is crucial to render their applications in electrodes and photocatalysis. With the assumption of empirical electron scattering rate, however, previous works fails to accurately predict their optical and electrical properties. In this work, we start from calculating the electron-phonon interactions to accurately obtain electron scattering rate and then use them to obtain optical and electrical properties of Ti2AlC and two-dimensional MXenes from fully first-principles without empirical parameters. By combining the electron-phonon interaction with semiclassical Boltzmann transport theory, the electronic resistivity at finite temperature was calculated. And the dielectric function was computed by superimposing the intraband and interband transition, with the electron scattering rate obtained from solving electron-phonon coupling. The calculated resistivity and dielectric function of Ti2AlC were compared with available experimental results in literature, and demonstrate an overall good agreement. The calculated electrical and optical properties of MXenes show strong dependence on the surface terminated groups. When terminated with F or OH, the 2D MXenes shows metallic properties with good conducting ability, while Ti2CO2 exhibits semiconducting property. The results obtained from this work help to advance 2D MXenes in the applications such as transparent electrodes and photocatalysis.

Structural, optoelectronic and thermoelectric properties of antiperovskite compounds Ae3PbS (Ae = Ca, Sr and Ba): A first principles study

In the last years, alkaline-earth based antiperovskite compounds with small semiconductor band gap have been proven to be promising candidate for optoelectronic and thermoelectric applications. In this work, the structural, electronic, optical and thermoelectric properties of Ae3PbS (Ae = Ca, Sr and Ba) compounds have been predicted using first principles calculations based on the full-potential linearized augmented plane-wave (FP-LAPW) method and semiclassical Boltzmann transport theory. Exchange-correlation effect is treated with the generalized gradient approximation with Perdew–Burke–Ernzerhof scheme (GGA-PBE) and Tran–Blaha modified Becke–Johnson exchange potential. The lattice constant of considered materials increases as Ae goes in order from Ca to Ba and the hardness slightly decreases in this order. Ca3PbS and Sr3PbS are semiconductor with direct band gap of 0.199 eV and 0.116 eV, respectively, while Ba3PbS is nearly metallic. Important optical responses of studied antiperovskites are found in the visible and ultraviolet energy range. Finally, the thermoelectric properties including Seebeck coefficient, electrical conductivity, thermal conductivity, power factor and figure of merit are calculated. Obtained results show that Ca3PbS and Sr3PbS could be candidate for applications in thermoelectric generators at low and moderate temperatures due to their high figure of merit values.

Semiclassical Boltzmann transport theory of few-layer black phosphorus in various phases

Black phosphorus (BP), a two-dimensional (2D) van der Waals layered material composed of phosphorus atoms, has been one of the most actively studied 2D materials in recent years due to its tunable energy band gap (tunable even to a negative value) and its highly anisotropic electronic structure. Depending on the sign of the band gap tuning parameter, few-layer BP can be in a gapped insulator phase, gapless Dirac semimetal phase, or gapless semi-Dirac transition point between the two phases. Using the fully anisotropic multiband Boltzmann transport theory, we systematically study the dc conductivity of few-layer BP as a function of the carrier density and temperature by varying the band gap tuning parameter, and determine the characteristic density and temperature dependence corresponding to each phase.

Predicted Janus monolayer ZrSSe with enhanced n-type thermoelectric properties compared with monolayer [formula omitted

In analogy to transition-metal dichalcogenide (TMD) monolayers, which have wide applications in photoelectricity, piezoelectricity and thermoelectricity, Janus MoSSe monolayer has been successfully synthesized by substituting the top S atomic layer in MoS2 by Se atoms. In this work, Janus monolayer ZrSSe is proposed by ab initio calculations. For the electron part, the generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is used as exchange-correlation potential, while GGA for lattice part. Calculated results show that the ZrSSe monolayer is dynamically, mechanically and thermally stable, which exhibits mechanical flexibility due to small Young’s modulus. It is found that ZrSSe monolayer is an indirect-gap semiconductor with band gap of 0.60 eV. The electronic and phonon transports of ZrSSe monolayer are investigated by semiclassical Boltzmann transport theory. In n-type doping, the ZTe between ZrSSe and ZrS2 monolayers is almost the same due to similar outlines of conduction bands. The p-type ZTe of ZrSSe monolayer is lower than that of ZrS2 monolayer, which is due to larger spin-orbit splitting for ZrSSe than ZrS2 monolayer. The room-temperature sheet thermal conductance is 33.6 WK-1 for ZrSSe monolayer, which is lower than 47.8 WK-1 of ZrS2 monolayer. Compared to ZrS2 monolayer, the low sheet thermal conductance of ZrSSe monolayer is mainly due to small group velocities and short phonon lifetimes of ZA mode. Considering their ZTe and lattice thermal conductivities, the ZrSSe monolayer may have better n-type thermoelectric performance than ZrS2 monolayer. These results can stimulate further experimental works to synthesize ZrSSe monolayer.

Theoretical study of electronic structure, thermoelectric and thermodynamic properties of 2H-AgAlO2

Electronic structure of AgAlO2 have been investigated using first principles calculations based on the full-potential linearized plane-wave method as implemented in WIEN2k package. Generalized gradient approximation (PBESol) and Tran-Blaha modified Becke-Johnson exchange potential (mBJ) are adopted for the treatment of exchange-correlation effects. Calculated electronic properties show the considerable improvement of results with mBJ potential while the strong hybridization between O-2p and Ag-4d states is revealed. From the band structure obtained with mBJ potential, the thermoelectric properties are calculated using semi-classical Boltzmann transport theory. Obtained results suggest that AgAlO2 is a good thermoelectric p-type compound with figure of merit value close to unity at charge-carrier concentration about 1017 (cm−3). Finally, the thermodynamic properties including bulk modulus, isochoric and isobaric heat capacities, thermal expansion and Debye temperature of AgAlO2 also are calculated using quasi-harmonic Debye model and the obtained results are discussed in detail.

Heat and charge transport in bulk semiconductors with interstitial defects

Interstitial defects are inevitably present in doped semiconductors that enable modern-day electronic, optoelectronic or thermoelectric technologies. Understanding of stability of interstitials and their bonding mechanisms in the silicon lattice was accomplished only recently with the advent of first-principles modeling techniques, supported by powerful experimental methods. However, much less attention has been paid to the effect of different naturally occurring interstitials on the thermal and electrical properties of silicon. In this work, we present a systematic study of the variability of heat and charge transport properties of bulk silicon, in the presence of randomly distributed interstitial defects (Si, Ge, C and Li). We find through atomistic lattice dynamics and molecular dynamics modeling studies that, interstitial defects scatter heat-carrying phonons to suppress thermal transport-1.56% of randomly distributed Ge and Li interstitials reduce the thermal conductivity of silicon by $\sim$ 30 and 34 times, respectively. Using first principles density functional theory and semi-classical Boltzmann transport theory, we compute electronic transport coefficients of bulk Si with 1.56% Ge, C, Si and Li interstitials, in hexagonal, tetrahedral, split-interstitial and bond-centered sites. We demonstrate that hexagonal-Si and hexagonal-Ge interstitials minimally impact charge transport. To complete the study, we predict the thermoelectric property of an experimentally realizable bulk Si sample that contains Ge interstitials in different symmetry sites. Our research establishes a direct relationship between the variability of structures dictated by fabrication processes and heat and charge transport properties of silicon. The relationship provides guidance to accurately estimate performance of Si-based materials for various technological applications.

Thermoelectric response of ZrNiSn and ZrNiPb Half-Heuslers: Applicability of semi-classical Boltzmann transport theory

Abstract We investigated the thermoelectric transport mechanism of ZrNiSn and ZrNiPb Half-Heuslers within the Boltzmann transport theory, under constant relaxation time approximation. The prediction of band structure behavior along the high symmetry Brillouin zone and density of states are used to establish relations among various transport coefficients like Seebeck coefficient, electrical conductivity, thermal conductivity and power factor which have been found to be in good agreement with the experimental results. The Seebeck coefficient variation show the n-type behavior of heat carriers. The electrical and thermal conductivity show a semiconducting nature of bands along the Fermi level. The lattice part of thermal conductivity show an exponential decreasing trend along the high temperatures. Power factor variation and figure of merit show the heavier element doping of ZrNiSn results in descent increase in efficiency triggering its applicability. The overall measurements show that semi classical Boltzmann transport theory has well behaved potential in predicting the transport properties of the compounds.

Effects of transport direction and carrier concentration on the thermoelectric properties of AgIn5Te8: A first-principles study

The synthesis of AgIn5Te8 is realized and the thermoelectric properties dependent on temperature is examined. However, the mechanism remains unclear and the synergistic effect of the temperature and the carrier concentration on thermoelectric properties is not explored. Here, the electronic transport properties are calculated by using the first-principles density functional theory combined with the semi-classical Boltzmann transport theory. The relaxation time is estimated by the deformation potential theory. The lattice thermal conductivity is evaluated by the Slack’s model, and the result is in good agreement with the experimental value. High anisotropic behavior is identified for the thermal and electronic transport properties, which supports the experimental observation. By tuning the carrier concentration and temperature, a maximum thermoelectric figure of merit of 2.28 can be achieved for p-type AgIn5Te8 in xx direction. The present results provide insight into the thermoelectric properties of AgIn5Te8 and a guide to prepare thermoelectric materials with AgIn5Te8.

Strain induced valley degeneracy: a route to the enhancement of thermoelectric properties of monolayer WS2.

Two-dimensional transition metal dichalcogenides show great potential as promising thermoelectric materials due to their lower dimensionality, the unique density of states and quantum confinement of carriers. Here the effects of mechanical strain on the thermoelectric performances of monolayer WS2 have been investigated using density functional theory associated with semiclassical Boltzmann transport theory. The variation of the Seebeck coefficient and band gap with applied strain has followed the same type of trend. For n-type material the relaxation time scaled power factor (S2σ/τ) increases by the application of compressive strain whereas for p-type material it increases with the application of tensile strain due to valley degeneracy. A 77% increase in the power factor has been observed for the n-type material by the application of uniaxial compressive strain. A decrease in lattice thermal conductivity with the increase in temperature causes an almost 40% increase in ZT product under applied uniaxial compressive strain. From the study, it is observed that uniaxial compressive strain is more effective among all types of strain to enhance the thermoelectric performance of monolayer WS2. Such strain induced enhancement of thermoelectric properties in monolayer WS2 could open a new window for the fabrication of high-quality thermoelectric devices.

Electronic, optical and thermoelectric properties of Fe2ZrP compound determined via first-principles calculations.

In this study, based on the density functional theory and semi-classical Boltzmann transport theory, we investigated the structural, thermoelectric, optical and phononic properties of the Fe2ZrP compound. The results of the electronic band structure analysis indicate that Fe2ZrP is an indirect band gap semiconductor in the spin-down state with the band gap of 0.48 eV. Thermoelectric properties in the temperature range of 300–800 K were calculated. Fe2ZrP exhibits the high Seebeck coefficient of 512 μV K−1 at room temperature along with the huge power factor of 19.21 × 1011 W m−1 K−2 s−1 at 800 K, suggesting Fe2ZrP as a potential thermoelectric material. The Seebeck coefficient decreased with an increase in temperature, and the highest value was obtained for p-type doped Fe2ZrP when the optimum carrier concentration was 0.22 × 1023 cm−3; the n-type doped Fe2ZrP had high electrical conductivity than the p-type doped Fe2ZrP. Thermal conductivity increased with an increase in chemical potential. Optical calculations illustrated that there was a threshold in the imaginary dielectric function for the spin-down channel. Spin-dependent optical calculations showed that the intraband contributions affected only the spin-up optical spectra due to the free-electron effects. Generally, the results confirmed that the intraband contribution had the main role in the optical spectra in the low energy infra-red and visible ranges of light. We also presented the phononic properties and found that these materials were dynamically stable.

Improved thermoelectric property of Ti0.75HfMo0.25CrGe by doping Ti2CrGe Heusler alloy with Hf and Mo: Confirmation of entropy “gene” in thermoelectric materials design

The electronic structure, thermoelectric properties, and thermodynamic entropy of Ti2CrGe-doped Ti0.75HfMo0.25CrGe were investigated using first-principles calculations in combination with the semi-classical Boltzmann transport theory and a common thermodynamic formalism. The band structure was half-metallic with a narrow gap of 0.02 eV in the spin-down channel and metallic character in the spin-up channel. The calculated thermoelectric transport properties revealed that Ti0.75HfMo0.25CrGe exhibited a larger thermoelectric figure of merit ZT with a lower lattice thermal conductivity than its prototype alloy Ti2CrGe. In particular, the entropy of Ti0.75HfMo0.25CrGe was larger than that of Ti2CrGe in the temperature range of 0–1000 K. These results indicate that increasing the entropy is an effective approach for the design of high-performance thermoelectric materials and confirm the entropy “gene” in thermoelectric materials.

Theoretical investigation of electronic structure and thermoelectric properties of MX2 (M=Zr, Hf; X=S, Se) van der Waals heterostructures

In this paper, van der Waals heterostructures consisting of MX2 (M = Zr, Hf and X = S, Se) monolayers are modeled. The favorable stacking and stability of the modeled monolayer heterostructures are confirmed through binding energy and phonon dispersion calculations. After confirming stability, the electronic and thermoelectric properties of these compounds are explored using the first-principles calculations combined with semiclassical Boltzmann transport theory. It is found that type-II band alignment in ZrS2HfSe2 facilitates charge separation for optoelectronics and solar energy conversion. All studied heterostructures show remarkably higher electrical conductivity than corresponding monolayers, responsible for large power factor values, especially at 1200 K. These findings indicate that the creation of van der Waals heterostructures from MX2 may be promising for efficient optoelectronic and thermoelectric devices.

High Thermoelectric Figure of Merit via Tunable Valley Convergence Coupled Low Thermal Conductivity in AIIBIVC2V Chalcopyrites

Development of efficient thermoelectric materials requires a designing approach that leads to excellent electronic and phononic transport properties. Using first-principles density functional theory and semiclassical Boltzmann transport theory, we report unprecedented enhancement in electronic transport properties of AIIBIVC2V (group II = Be, Mg, Zn, and Cd; group IV = Si, Ge, and Sn; and group V = P and As) chalcopyrites via isoelectronic substitution. Multiple valleys in conduction bands, present in these compounds, are tuned to converge by substitution of group IV dopant. Additionally, this substitution improves the convergence of valence bands, which is found to have a direct correlation with the tetragonal distortion of these chalcopyrites. Furthermore, several chalcopyrite compounds with heavy elements such as Zn, Cd, and As possess low phonon group velocities and large Gruneisen parameters that lead to low lattice thermal conductivity. Combination of optimized electronic transport properties and lo...

The origin of Mooij correlations in disordered metals

Sufficiently disordered metals display systematic deviations from the behavior predicted by semi-classical Boltzmann transport theory. Here the scattering events from impurities or thermal excitations can no longer be considered as additive-independent processes, as asserted by Matthiessen’s rule following from this picture. In the intermediate region between the regime of good conduction and that of insulation, one typically finds a change of sign of the temperature coefficient of resistivity, even at elevated temperature spanning ambient conditions, a phenomenology that was first identified by Mooij in 1973. Traditional weak coupling approaches to identify relevant corrections to the Boltzmann picture focused on long-distance interference effects such as “weak localization”, which are especially important in low dimensions (1D and 2D) and close to the zero-temperature limit. Here we formulate a strong-coupling approach to tackle the interplay of strong disorder and lattice deformations (phonons) in bulk three-dimensional metals at high temperatures. We identify a polaronic mechanism of strong disorder renormalization, which describes how a lattice locally responds to the relevant impurity potential. This mechanism, which quantitatively captures the Mooij regime, is physically distinct and unrelated to Anderson localization, but realizes early seminal ideas of Anderson himself, concerning the interplay of disorder and lattice deformations.

Computational mining of the pressure effect on thermodynamic and thermoelectric properties of cubic Ca2Si

Adjusting external pressure has been emerging as one of the most powerful method to unravel the physical and chemical origin of the materials. In this work, we have investigated the pressure effect on thermodynamic and thermoelectric properties of the cubic phase Ca2Si systematically based on density functional theory within quasi-harmonic approximation and semi-classical Boltzmann transport theory. Our results not only shed light on the origin of the stability of cubic Ca2Si at all the pressures but also provide thermodynamic and thermoelectric maps for cubic Ca2Si under various external pressures. By combining the Seebeck coefficient decreasing and electronic conductivity increasing, the pressure independent power factor of cubic Ca2Si have been highlighted as well. We believe that our findings will facilitate the pressure related applications of Ca2Si in extensive fields.

Thermoelectric properties of the novel cubic structured silicon monochalcogenides: A first-principles study

The low-cost and non-toxic candidates of the Group-IV monochalcogenide family have attracted significant attention in recent years for large-scale thermoelectric applications. We conduct comprehensive investigations of the thermoelectric response of relatively inexpensive and less toxic cubic structured Si-monochalcogenides (π-SiS, π-SiSe, and π-SiTe) for renewable energy applications. The full-potential linearized-augmented-plus-local-orbital method within density functional theory has been adopted to calculate the ground state energies, whereas the semi-classical Boltzmann transport theory has been used for the calculations of thermoelectric properties. The Si-monochalcogenides in cubic phase demonstrate large values of thermopowers that amounts to 1740.0 μV/K, 1405.0 μV/K, and 771.92 μV/K of the π-SiS, π-SiSe, and π-SiTe respectively at 300 K. The thermopowers show an insignificant response to increase in temperature which is beneficial for the high-temperature thermoelectric applications of these materials. The optimal values of thermoelectric power factors of the cubic structured Si-chalcogenides occur at attainable doping levels and have been originated from the joint contribution of moderate electrical conductivities and thermopowers. These materials demonstrate the figure of merit values approaching unity and have shown a trivial response to the temperature gradient. Moreover, the occurrence of the optimal values of thermoelectric coefficients for electrons doped regime suggests the n-type doping as an easy option for enhancing the thermoelectric performance of these materials. Our investigations show that the Si-monochalcogenides in cubic phase feature interesting thermoelectric performance and can be used as a suitable replacement for the toxic and expensive binary chalcogenides for thermoelectric applications.

FeTaSb and FeMnTiSb as promising thermoelectric materials: An ab initio approach

Thermoelectricity in principle provides a pathway to put waste heat to good use. Motivated by this we investigate thermal and electrical transport properties of two new Fe-based Heusler alloys, FeTaSb and FeMnTiSb, by a first principles approach and semiclassical Boltzmann transport theory within the constant relaxation-time approximation. We find a high power factor of \textit{p}-doped FeTaSb, competitive with best performing Heusler alloy FeNbSb at 1100 K. The obtained power factor of \textit{n}-doped FeMnTiSb at room temperature is higher than that of both FeNbSb and FeTaSb. Remarkably, FeMnTiSb can be used for both \textit{n}-type and \textit{p}-type legs in a thermoelectric module. The Seebeck coefficients of the two proposed systems are in line with those of earlier reported Heusler alloys. We also provide conservative estimates of the figure of merit for the two systems. Overall, our findings suggest a high temperature thermoelectric potential of FeTaSb while the low cost FeMnTiSb is a viable room temperature thermoelectric candidate material.

Electronic and thermoelectric properties of the layered BaFAgCh (Ch = S, Se and Te): First-principles study

By using the full potential linearized augmented plane wave (FP-LAPW) method, the electronic properties of the layered BaAgChF (Ch = S, Se, Te) were investigated. Both the standard GGA and the TB-mBJ potential were used to model the exchange-correlation potential. To evaluate the spin-orbit coupling (SOC) effect, both the scalar relativistic and full relativistic calculations were performed. The SOC effect is found to be not negligible in the title compounds. The FP-LAPW band structure and the semi-classical Boltzmann transport theory were used to study the charge-carrier concentration and temperature dependences of the thermoelectric parameters, including Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit. Our results show that the values of the thermoelectric parameters of the p-type compounds are larger than that of the n-type ones. The optimal p-type doping concentrations and temperatures that yield the maximum values of the figure of merit of the title compounds were calculated. These are important parameters to guide experimental works.