Low Group Velocity Research Articles

Ultralow lattice thermal conductivity in K2AuBi and its thermoelectric properties

Thermoelectric materials are best known for their prowess to transform the environment’s waste heat into electricity. In an endeavor to explore new thermoelectric prospects, in the present study, we investigate K2AuBi using density functional theory-based first-principles simulations. From our simulations, we find an intrinsically low lattice thermal conductivity of 0.43 W m−1 K−1 at 300 K in K2AuBi. Based on our detailed analysis, we find the reasons for such a low value of lattice thermal conductivity as, low phonon group velocities, short phonon lifetimes, anharmonicity in the lattice vibrations, and significant mean square displacements of K and Au atoms. The large mean square displacements hint at weak bonding and anharmonicity in the lattice vibrations, favoring more phonons scattering. We also find that the vibrations of K-atoms can be related to rattlers, conducive to low lattice thermal conductivity. Our simulations predict a high value, ∼784 μV K−1, of Seebeck coefficient at 700 K on account of the large density of states in the vicinity of Fermi level. Combining our computed lattice thermal conductivity with electrical transport properties, we obtain a high figure of merit, ZT∼ 1.04, at 700 K in K2AuBi.

Theoretical investigation into potential thermoelectric material Monolayer InI with direct bandgap and ultralow lattice thermal conductivity

Monolayer metal iodides are garnering widespread attention due to their ultralow lattice thermal conductivity and their potential applications in the field of thermoelectrics. Here, we utilized first-principles calculations and Boltzmann transport theory to thoroughly investigate the thermoelectric behavior of monolayer InI. Results indicate that this material functions as a direct bandgap semiconductor and features an exceptionally low lattice thermal conductivity of 0.27 W m−1K−1 at 300 K, a result of its low group velocities and short phonon lifetimes. Furthermore, transport coefficients were calculated with enhanced precision through the Wannier interpolation method, incorporating the HSE06 hybrid functional and spin-orbit coupling (SOC) effects. Remarkably, the maximum thermoelectric figure of merit (ZT) values for monolayer InI demonstrate substantial growth with increased temperatures, surging from 0.31 at 300 K to 1.20 at 450 K for p-type, and escalating from 0.26 at 300 K to 0.72 at 450 K for n-type. These promising low-temperature thermoelectric properties exhibited by monolayer InI suggest its suitability for integration into the production of thermoelectric devices.

Ultralow thermal conductivity and high thermoelectric performance induced by dimensionality reduction in chain-like compounds X2PtSe2 (X=K, rb)

Balancing low lattice thermal conductivity (κL) with a high power factor is a major challenge in the research of thermoelectric (TE) materials. In this paper, we report on a class of chain-like compounds, X2PtSe2(X = K, Rb), characterized by strong lattice anharmonicity, weak bonds, and low phonon group velocities. Using first-principles calculations in conjunction with self-consistent phonon theory and the Boltzmann transport equation, we systematically investigate the thermal and electrical transport properties of X2PtSe2(X = K, Rb). At room temperature, the κL of K2PtSe2 and Rb2PtSe2 in the direction perpendicular to the Pt–Se chains is only 0.15 and 0.10 Wm−1K−1, respectively. Additionally, due to the low-dimensional nature of the Pt–Se chains, these compounds exhibit a ‘pudding-mold type’ band structure, contributing to a TE power factor. At 300K, under n-type doping, K2PtSe2 and Rb2PtSe2 achieve TE figures of merit (ZT) values of 1.66 and 1.67, respectively, in the direction perpendicular to the Pt–Se chains. At 500K, the ZT value of Rb2PtSe2 approaches 4, far surpassing traditional TE materials. These results indicate that the chain-like compounds X2PtSe2(X = K, Rb) exhibit excellent TE performance, achieving energy conversion efficiencies comparable to commercial transducer devices.

All-Inorganic Halide Perovskites Boost High-Ranged Figure-of-Merit in Bi0.4Sb1.6Te3 for Thermoelectric Cooling and Low-Grade Heat Recovery.

The all-inorganic halide perovskite CsPbX3 (X = Cl, Br, or I) offers various advantages, such as tunable electronic structure and high carrier mobility. However, its potential application in thermoelectric materials remains underexplored. In this study, we propose a simple yet effective method to synthesize a CsPbX3/Bi0.4Sb1.6Te3 (BST) nanocomposite by sintering a uniformly mixed raw powder. The intrinsic excitation of the BST system is suppressed by exploiting the rich phase structure and tunable electrical transport properties of CsPbX3, and the thermoelectric properties were synergistically optimized. Notably, for CsPbI3, its phase-transition-induced dislocation arrays together with low group velocities drastically reduce thermal conductivity. As a result, the composite achieves an ultrahigh average figure-of-merit (ZT) of 1.4 from 298 to 523 K. The two-pair TE module demonstrates a superior conversion efficiency of 7.3%. This study expands the potential applications of inorganic halide perovskites, into thermoelectrics.

Loose bonding induced ultralow lattice thermal conductivity of a metallic crystal KNaRb

In the domain of metallic systems, electronic thermal conductivity typically governs heat transfer, while lattice (phononic) thermal conductivity (LTC) remains non-negligible magnitude. This study introduces the exceptional metallic material, cubic half-Heusler-type KNaRb, via phonon Boltzmann transport equation (BTE) resolution and first-principles calculations. KNaRb exhibits an ultralow LTC of 0.114 W/mK at room temperature, comparable to the lowest reported for metallic materials, contributing merely 0.66 % to overall thermal transport. Analysis attributes KNaRb's low LTC to low group velocity and strong anharmonicity, originating from its loosely bonded electronic structure. Acoustic modes, primarily from Rb atoms, dominate thermal transport. Examination of mean square displacement and potential energy profiles reveals significant movement of loosely bonded Rb atoms within KNaRb, acting as intrinsic "rattlers," inducing pronounced phonon anharmonicity and ultrashort lifetimes. This study advances understanding of heat conduction in metals, offering insights into materials with extremely low lattice thermal conductivity for potential future applications.

High‐Performance CaMg2Bi2‐Based Thermoelectric Materials Driven by Lattice Softening and Orbital Alignment via Cadmium Doping

AbstractZintl compounds such as n‐type Mg3(Sb,Bi)2 show promising thermoelectric applications benefiting from their high valley degeneracy and low lattice thermal conductivity. However, the heavier p‐type AMg2X2 (A = Ca, and Yb; X = Bi and Sb) Zintl counterparts even exhibit a higher κlat due to strong chemical bonding. Reducing κlat of AMg2X2 is an important route for improving thermoelectric performance. Herein, it is found that Cd doping at the Mg site in CaMg2Bi2 can weaken intralayer covalent bonds and soften acoustic phonons, as well as fill the optical phonon gap. These effects result in large atomic displacement, low phonon group velocity, and strong lattice vibration anharmonicity. Doping 10% Cd leads to a reduction of 56% in the κlat of CaMg2Bi2. Moreover, Cd doping promotes orbital alignment and thus increases the density‐of‐states effective mass and Seebeck coefficient. Eventually, in conjunction with carrier concentration optimization by Na doping and band structure engineering by Ba doping, a high ZT of ≈1.3 at 873 K in (Ca0.85Ba0.15)0.995Na0.005Mg1.85Cd0.15Bi2 sample is realized. This work highlights the significant role of manipulating chemical bonding in suppressing phonon propagation of semiconductors.

High-throughput screening of potentially ductile and low thermal conductivity ABX3 (X = S, Se, Te) thermoelectric perovskites

Chalcogenide perovskites, renowned for their low lattice thermal conductivity, have emerged as promising candidates for thermoelectric applications. Hence, we leveraged first-principles high-throughput calculations to investigate the electrical and thermal transport properties, as well as the ductility, of the chalcogenide perovskites ABX3 (X = S, Se, Te). Candidates with 30 combinations were initially screened by bandgap screening (Eg > 0.1 eV), stability assessment (Born–Huang criterion), and ductility evaluation (Pugh's ratio: G/K < 0.571, the ratio of shear modulus G to bulk modulus K) from the MatHub-3d database (176 ABX3 crystal structures, 32 kinds of space groups, and number of atoms Natom < 40). Intriguingly, weaker chemical bonding between the A and X site atom pairs gives rise to a higher ductility in the screened quasi-ductile perovskites. Furthermore, it should be noted that the low phonon group velocities confirmed the low lattice thermal conductivity of the materials. In consequence, the identification of quasi-ductile thermoelectrics, characterized by six n-type and six p-type candidates with ZT > 0.3 at 300 K, stands as the most promising candidates for application in thermoelectrics.

Phonon transport properties of Janus Pb2 XAs (X = P, Sb, and Bi) monolayers: A DFT study

Grasping the underlying mechanisms behind the low lattice thermal conductivity of materials is essential for the efficient design and development of high-performance thermoelectric materials and thermal barrier coating materials. In this paper, we present a first-principles calculations of the phonon transport properties of Janus Pb2PAs and Pb2SbAs monolayers. Both materials possess low lattice thermal conductivity, at least two orders of magnitude lower than graphene and h-BN. The room temperature thermal conductivity of Pb2SbAs (0.91 W/mK) is only a quarter of that of Pb2PAs (3.88 W/mK). We analyze in depth the bonding, lattice dynamics, and phonon mode level information of these materials. Ultimately, it is determined that the synergistic effect of low group velocity due to weak bonding and strong phonon anharmonicity is the fundamental cause of the intrinsic low thermal conductivity in these Janus structures. Relative regular residual analysis further indicates that the four-phonon processes are limited in Pb2PAs and Pb2SbAs, and the three-phonon scattering is sufficient to describe their anharmonicity. In this study, the thermal transport properties of Janus Pb2PAs and Pb2SbAs monolayers are illuminated based on fundamental physical mechanisms, and the low lattice thermal conductivity endows them with the potential applications in the field of thermal barriers and thermoelectrics.

Intrinsic ultralow lattice thermal conductivity in lead-free halide perovskites Cs3Bi2X9 (X = Br, I).

Lead-free halide perovskites have recently garnered significant attention due to their rich structural diversity and exceptionally ultralow lattice thermal conductivity (κL). Here, we employ first-principles calculations in conjunction with self-consistent phonon theory and Boltzmann transport equations to investigate the crystal structure, electronic structure, mechanical properties, and κLs of two typical vacancy-ordered halide perovskites, denoted with the general formula Cs3Bi2X9 (X = Br, I). Ultralow κLs of 0.401 and 0.262 W mK-1 at 300 K are predicted for Cs3Bi2Br9 and Cs3Bi2I9, respectively. Our findings reveal that the ultralow κLs are mainly associated with the Cs rattling-like motion, vibrations of halide polyhedral frameworks, and strong scattering in the acoustic and low-frequency optical phonon branches. The structural analysis indicates that these phonon dynamic properties are closely relevant to the bonding hierarchy. The presence of the extended Bi-X antibonding states at the valence band maximum contributes to the soft elastic lattice and low phonon group velocities. Compared to Cs3Bi2Br9, the face-sharing feature and weaker bond strength in Cs3Bi2I9 lead to a softer elasticity modulus and stronger anharmonicity. Additionally, we demonstrate the presence of wave-like κC in Cs3Bi2X9 by evaluating the coherent contribution. Our work provides the physical microscopic mechanisms of the wave-like κC in two typical lead-free halide perovskites, which are beneficial to designing intrinsic materials with the feature of ultralow κL.

A first-principles study on the promising thermoelectric properties of SnX (X = S, Se, Te) compounds.

SnSe has emerged as an outstanding thermoelectric material due to its exceptional performance. In this study, first-principles calculations are employed to investigate the thermoelectric properties of materials within the SnX family, where X can be either S, Se, or Te. Initially, we assessed the stability of SnX (X = S, Se, Te). We found that SnS exhibits better mechanical and thermal stability than SnSe and SnTe. We then conduct phonon and electronic transport analysis. Following the general rule that heavier atoms have lower thermal conductivity, SnTe demonstrates lower thermal conductivity due to its low group velocity compared with SnS and SnSe. Regarding electrical transport properties, the band gaps for SnS, SnSe, and SnTe are 0.56, 0.54, and 0.35 eV, respectively. Notably, the small band gap and higher degeneracy in its band valleys for SnTe make it more effective for achieving a high power factor. The maximum ZT values are determined to be 1.41, 1.41, and 1.87 for SnS, SnSe, and SnTe, respectively. Remarkably, ZTmax of SnTe exceeds that of SnSe by 32.6%. Overall, the results clearly demonstrate that SnTe exhibits superior thermoelectric properties compared to SnSe and SnS. This study provides valuable insights into the electronic structure, thermal conductivity, and mechanical and thermal stability of materials within the SnSe family, such as SnS or SnTe, without the need for extensive and costly experimental work.

Low lattice thermal conductivity of novel NiVSn with 19 valence electron count contrasted to 18 of NiVAl and NiAlSb Half-Heusler compounds

In the current study, the novel pristine compounds of both VEC 18 (NiVAl & NiAlSb) and VEC 19 (NiVSn) materials have been synthesized employing arc-melting and hot-pressed at 1073 K. The measured carrier concentration for all the compounds was found ∿ 1020-1021 cm−3. Further, NiAlSb, NiVAl, and NiVSn half-Heusler (HH) compounds exhibited n-type semiconductor behaviour, with maximum Seebeck coefficients (S) of −50.2 μV/K, −40.5 μV/K, and −14.5 μV/K at 723 K. As a result of the measured S and electrical conductivity, the estimated power factor for NiVSn (0.20 mWm−1K−2), NiAlSb (0.41 mWm−1K−2), and NiVAl (0.65 mWm−1K−2) at 723 K. The NiVSn lattice thermal conductivity (κl) was reduced to ∿ 79.80 % and 76.71 % as compared to NiAlSb and NiVAl compounds. Despite having reduced κl, low zT exhibited for NiVSn compound mainly due to nominal S. The reduced κl was mainly attributed to weak chemical bonding results from lower group velocity of phonon, leading to a higher anharmonicity effect in the lattice. Therefore, NiVSn of VEC 19-based compound can be considered as a potential candidate material for TE applications other than traditional 18-VEC HH compounds. The thermoelectric performance of the novel HH compounds is suitably corroborated with the structural and microstructural investigations.

On enhancing the noise-reduction performance of the acoustic lined duct utilizing the phase-modulating metasurface

This work proposes a noise-reduction structure that integrates phase-modulating metasurface (PMM) with acoustic liners (ALs) to enhance the narrow band absorption performance of a duct with relatively small length-diameter ratio. The PMM manipulates the wavefront by introducing different transmission phase shifts based on an array of Helmholtz resonators, so that the spinning wave within the duct can be generated. Compared with the plane wave, the generated spinning wave has a lower group velocity, which results in a greater traveling distance over the ALs in the duct. The optimization design is performed to determine the final structural parameters of the PMM, which is based on the predictions of the amplitude and phase shift of the acoustic wave at the outlet of the PMM using the theory of passive phased array. With the manipulation of the PMM, the incident plane wave is modulated into a spinning wave, and then enters into the acoustic liner duct (ALD), whose structural parameters are optimized by maximizing the transmission loss using the mode-matching technique. Finally, the noise-reduction performance of this combined structure is evaluated by numerical simulations in the presence of grazing flow. The results demonstrate that, compared with the traditional ALD, the proposed structure exhibits a significant increase in transmission loss within the considered frequency band, especially near the peak frequency of the narrow band noise.

Simultaneous guidance of electromagnetic and elastic waves via glide symmetry phoxonic crystal waveguides

A phoxonic crystal waveguide with the glide symmetry is designed, in which both electromagnetic and elastic waves can propagate along the glide plane at the same time. Due to the glide symmetry, the bands of the phoxonic crystal super-cell degenerate in pairs at the boundary of the Brillouin zone. This is the so-called band-sticking effect and it causes the appearance of gapless guided-modes. By adjusting the magnitude of the glide dislocation the edge bandgaps, the bandgap of the guided-modes at the boundary of the Brillouin zone, can be further adjusted. The photonic and phononic guided-modes can then possess only one mode for a certain frequency with relatively low group velocities, achieving single-mode guided-bands with relatively flat dispersion relationship. In addition, there exists acousto–optic interaction in the cavity constructed by the glide plane. The proposed waveguide has potential applications in the design of novel optomechanical devices.

Thermoelectric figure-of-merit of metastable crystalline ST12 germanium allotrope

Computational science has facilitated significant improvement in identifying stable crystal systems that are good thermoelectric materials with a high figure-of-merit (ZT). One approach for further discontinuous improvement is to explore the metastable phase of the crystals, particularly because there is an increasing need for thermoelectric materials operating at room temperature in energy harvesting applications. Here, using first-principles calculations, we find that the single-crystalline metastable ST12 germanium allotrope has an much higher ZT than DC8 stable phase. The optimal ZT for p- and n-type doping is 0.22 and 0.27 at room temperature, respectively, and the values reach 0.56 and 0.87 at 500 K. The electrical and thermal conductivities are higher and significantly lower than those of stable germanium, respectively. This is attributed to the smaller carrier effective mass and denser phonon bands with a lower group velocity and larger scattering phase space. The Seebeck coefficient remains high owing to the adequate bandgap and large derivative of the electron density of states with respect to the Fermi energy at the band edges. This work demonstrates that the metastable ST12 germanium allotrope, which has been recently synthesized, is a promising candidate to realize improvement in thermoelectric materials, and this case study encourages future exploration of metastable materials.

Exploring the ultra-low lattice thermal conductivity of a two-dimensional compound Tl2Se3

Two-dimensional (2D) materials have attracted extensive attention in the field of thermoelectric (TE) materials, where heat transport properties play an important role in determining the performance of TE devices. In this paper, we investigate the thermal transport properties of novel 2D Tl2Se3 by means of first-principles approach combined with the Boltzmann transport equation (BTE). The small phonon sound group speed and strong anharmonicity are responsible for an ultra-low kl of 0.28 W/mK of 2D Tl2Se3 at room temperature. Meanwhile, the lattice thermal conductivity gradually decreases with the increase of temperature, indicating its potential value at high-temperature. The anti-bonding orbital filling between the Se-4p and Tl-6p states leads to the weak chemical bonds using the crystal orbital Hamiltonian population (COHP) analysis, which results in low phonon group velocities. Also, due to the strong acoustic-optical coupling in 2D Tl2Se3, the scattering processes of different acoustic modes is further studied to analyze how kl is suppressed. These findings demonstrate that 2D Tl2Se3 has potential applications in TE devices, which will motivate Tl2Se3 based experimental investigations.

Intrinsically low lattice thermal conductivity in layered Mn3Si2Te6

The ferrimagnetic nodal-line semiconductor Mn3Si2Te6 has recently received much attention due to its colossal angular magnetoresistance (Seo et al 2021 Nature 599 581). The magnetic and electronic properties of Mn3Si2Te6 have been extensively studied. Meanwhile, a recent experiment showed that Mn3Si2Te6 has a low in-plane lattice thermal conductivity, which implies its potential applications in thermoelectricity. Here, we have investigated phonon dispersion and lattice thermal conductivity of Mn3Si2Te6 by the first-principles calculations and the Peierls–Boltzmann transport equation. It is found that the lattice thermal conductivities of Mn3Si2Te6 are quite low, which are 1.33 and 0.96 Wm−1K−1 along the a and c axes at 300 K, respectively. A significant contribution (>90%) to the thermal conductivity comes from the acoustic phonons and low-frequency optical phonons linked to the vibration of Te atoms. Meanwhile, it is found that such low thermal conductivities of Mn3Si2Te6 are a consequence of the low group velocities and relatively short phonon lifetimes, which are intrinsically derived from the quite complex crystal structure, heavy Te atoms, and relatively weak chemical bonding. Our work not only explains the origin of the intrinsically low thermal conductivity of Mn3Si2Te6 but also could be helpful to the study on the thermal conductivity of other similar layered magnetic materials.

Thermal conductivity at finite temperature and electronic structure of the ultra-wide band gap fluorinated 2D GaN

Passivation makes 2D hexagonal structure more stable than the planar variant. Surface fluorinated monolayer of GaN have been found to have ultra-wide band gap and have promising applications in optoelectronic conversion devices. In this work, using theoretical method, we have explored the thermal conductivity as well as the electronic structure of F–GaN. It has a low thermal conductivity of 7.67 W (mK)−1 due to the low group velocity and short phonon lifetime. The calculated direct band gap value is 4.63 eV, which could be modulated by strain and biaxial strain is found to more effective. Attractively, direct band gap can be maintained under tensile strain. Breakdown of symmetry by uniaxial strain lifts the band degeneracy of the VBM, which will lead to polarized light emission. The in-depth analysis shows that Ga–F as well as N–F bonds are strongly ionic, which is responsible for its low thermal conductivity and ultra-wide band gap.

Application of linear electron Bernstein current drive models in reactor-relevant spherical tokamaks

Electron Bernstein current drive (EBCD) systems in spherical tokamaks are sensitive to plasma and launch conditions, and therefore require large parametric scans to optimise their design. One particular bottleneck in the simulation workflow is quasilinear modelling of current drive efficiency. Linear adjoint models are an attractive alternative, offering a ∼103 × speed-up compared to quasilinear codes. While linear models are well-tested and commonly used for electron cyclotron current drive (ECCD), they have seen little use in EBCD modelling. In this work, variants of the linear model are applied to EBCD and compared to quasilinear results in a reactor-relevant plasma, i.e. Spherical Tokamak for Energy Production (STEP). This comparison reveals it is important to accurately model the collision operator and finite Larmor radius effects in the linear model. When done properly, good agreement is found with quasilinear calculations, at least for normalised minor radii ρ < 0.7 and at low power densities. The power density threshold for quasilinear effects during EBCD is found to be significantly lower than that of ECCD. This is attributed to the much lower group velocity of the electron Bernstein wave (EBW). Thus, the linear model is only valid for EBCD modelling at low power densities (e.g. ≲ 1 MW launched EBW power in STEP). This may be satisfied in present-day experimental devices, but certainly not in reactors targeting non-inductive operation.

Ultralow lattice thermal conductivity and thermoelectric performance of twisted Graphene/Boron Nitride heterostructure through strain engineering

We designed and investigated the electronic, mechanical, and thermoelectric properties of Graphene/hexagonal Boron Nitride (Gr/h-BN) heterostructure at various twisting angles based on the Ab-initio simulation. The structural stability was studied at optimized rotation angles (ϕ) = 0∘, 16.10∘, 21.79∘, 38.21∘, 43.90∘ and 60∘. The heterostructure shows semiconducting nature at ϕ=0∘, 21.79∘ and 38.21∘. These twisted heterostructures have demonstrated extraordinary mechanical properties such as Young’s modulus and bulk modulus. Using the semiclassical Boltzmann transport theory, it is observed that the Seebeck coefficient, electric conductivity, and power factor at ϕ=0∘, 21.79∘, 38.21∘, and 60∘ are much higher than the values measured at ϕ=16.10∘ and 43.90∘. Moreover, at ϕ=60∘, the Power Factor for the n-type dopants can reach 1.37 × 1011 W/msK2. The lattice thermal conductivity at room temperature is found to be very low for ϕ=16.10∘, 21.79∘, 43.90∘ and 38.21∘ rotation angles. An ultralow lattice thermal conductivity with a value of 0.095 W/mK at 300K has been observed for 21.79∘ rotation angle, which is lower than other rotation angles because of very low group velocity (22.1 km/s) and short phonon lifetime (∼0.12 ps). The high thermoelectric performance results from an ultralow thermal conductivity arising due to the strong lattice anharmonicity. The present observations can offer significant impact on the design of high performance thermoelectric materials based on twisted van der Waals heterostructure (vdWH).

Low lattice thermal conductivity of two-dimensional monolayers of BxN (x = 2, 3, and 5) induced by complex bonding networks: A comparative study with h-BN

It is well known that different bonding networks could bring a wide variety of physical properties to the materials although they hold analogous element and structure features. In this paper, we present a first-principles calculation about the lattice dynamics and phonon transport properties of two-dimensional (2D) boron-rich material BxN (x = 2, 3, and 5). The calculations show that besides the obvious differences in electrical properties compared to h-BN, the special bonding environment in these boron-rich materials also results in quite different phonon behaviors, where their thermal conductivity is at least one order of magnitude smaller than that of h-BN. By performing bonding and lattice dynamics analysis, we reveal that such dramatic reduction of conductivity is attributed to the synergistic effect of weak bond strength (low phonon group velocity) and complex bonding network (strong phonon scattering). Relative regular residual analysis further indicates that the four-phonon process is limited in BxN, and the three-phonon scattering is sufficient to describe their anharmonicity. Starting from the basic physical mechanism, the present study sheds light on the thermal transport properties of 2D boron-rich BxN compounds, which could provide useful insight for their widespread applications in thermal management.