Large Reduction In Thermal Conductivity Research Articles

Enhanced thermoelectric performance of TiS2 via large thermal conductivity reduction by solid solution alloying with TiSe2

Enhanced thermoelectric performance of TiS2 via large thermal conductivity reduction by solid solution alloying with TiSe2

Effect of four-phonon scattering on the intrinsic thermal conductivity of penta-graphene

Penta-graphene (PG), as a novel and stable pentagonal structure, has attracted considerable attention due to its large band gap, ultrahigh strength, negative Poisson's ratio, which indicates promising potential in nanoelectronics and nanomechanics. Thermal conductivity is a key physical quantity for the potential applications of PG based nano-devices. However, previous studies focused on the thermal conductivity of PG dominated by the well-known cubic anharmonic effect, and the quartic anharmonicity, although probably significant, was still neglected. In this work, we study the effect of four-phonon scattering on the thermal transport in PG through using first-principles calculations and solving the Boltzmann transport equation (BTE). We find that the intrinsic lattice thermal conductivity of PG is 182.1 W/mK at room temperature after including four-phonon scattering, which is reduced by 73.5 % as compared to the obtained results with only three-phonon scattering. Meanwhile, the relative contribution of the flexural acoustic (ZA) branch is reduced from 60.5 % to 32.5 % when four-phonon scattering is included. Thus, the large reduction in thermal conductivity can be ascribed to the large four-phonon scattering rates, which are completely comparable with those of three-phonon processes in the acoustic phonon frequency range even at 300 K. It is well believed that this finding is not a special case but should exist in other 2D materials with similar phonon features. Furthermore, the validity of the sum of three- and four-phonon scattering rates has been verified by the results based on the MD simulations. Our finding provides critical revisit and valuable insight to the exact thermal conductivity value of PG.

Super-suppression of long phonon mean-free-paths in nano-engineered Si due to heat current anticorrelations

The ability to minimize the thermal conductivity of dielectrics with minimal structural intervention that could affect electrical properties is an important capability for engineering thermoelectric efficiency in low-cost materials such as Si. We recently reported the discovery of special arrangements for nanoscale pores in Si that produce a particularly large reduction in thermal conductivity accompanied by strongly anticorrelated heat current fluctuations [1] – a phenomenon that is missed by the diffuse adiabatic boundary conditions conventionally used in Boltzmann transport models. This manuscript presents the results of molecular dynamics simulations and a Monte Carlo ray tracing model that teases apart this phenomenon to reveal that special pore layouts elastically backscatter long-wavelength heat-carrying phonons. This means that heat carriage by a phonon before scattering is undone by the scattered phonon, resulting in an effective mean-free-path that is significantly shorter than the geometric line-of-sight due to the pores. This effect is particularly noticeable for the long-wavelength, long mean-free-path phonons whose transport is impeded drastically more than is expected purely from the usual considerations of scattering defined by the distance between defects. This “super-suppression” of the mean-free-path below the characteristic length scale of the nanostructuring offers a route for minimizing thermal conductivity with minimal structural impact, while the stronger impact on long wavelengths offers possibilities for the design of band-pass phonon filtering. Moreover, the ray tracing model developed in this paper shows that different forms of correlated scattering imprint a unique signature in the heat current autocorrelation function that could be used as a diagnostic in other nanostructured systems.

Phonon resonant effect in silicon membranes with different crystallographic orientations

Engineering low-frequency phonon transport in nanostructures with the phonon resonant mechanism has become an important research direction. On the basis of non-equilibrium molecular dynamics simulations, the thermal transport in pristine and resonant Si-membranes bounded with {100}, {110} and {111} facets is investigated. It is found that the creation of surfaces can introduce anisotropic thermal transport due to the lattice symmetry breaking. Besides, ballistic phonon transport is found in pristine membranes with lengths up to 500 nm at low-frequencies with a critical frequency mainly dependent on the crystallographic orientation. Moreover, although surface resonances can dramatically reduce the thermal conductivity of all membranes, the resonant effect strongly relies on membrane orientation. Among the three studied membrane orientations, the resonant effect is maximized in the {111}-membrane, where the thermal conductivity is tuned from the largest one to the smallest one among the three membrane types by resonant pillars. The large thermal conductivity reduction in the {111}-membranes by resonances originated from the reduced spectral heat flux between 3 and 12 THz. Furthermore, the resonant coupling strength can be tuned by the interface vacancy between resonant pillars and the base material, which can enhance phonon transport at an intermediate frequency range. Our work provides further insights on thermal transport engineering by phonon resonances and could be useful for thermal conductivity engineering with surface orientations and resonances.

(Invited) Reduced Thermal Conductivity of Tcnq and F4-TCNQ Infiltrated HKUST-1 Due to Suppression of Linker Flexural Phonon Modes

MOFs are of great interest for applications in gas storage, separation, catalysis and more recently, thermoelectrics (TE) due to their extremely high porosity (surface area), large chemical and mechanical tailorability defined by the choice of the metal node, the organic ligand, and infiltrated guest molecules, and low thermal conductivities. In particular, new emergent properties, such as electronic conductivity and energy transfer, have been achieved by infiltrating MOF pores with specialized ‘guest’ molecules. The thermal transport in infiltrated MOFs is critically important for understanding the efficiency of both gas adsorption and thermoelectric applications. Understanding the impacts on thermal transport upon MOF infiltration with an electrically conductive guest molecule is critical for realizing an efficient TE material. This work experimentally investigates the impacts on the thermal properties of HKUST-1 MOFs infiltrated with charge accepting TCNQ and F4-TCNQ guest molecules. We provide experimental evidence paired with Molecular Dynamics (MD) results to describe the thermal transport mechanism as a loading-dependent structural change within the MOF. Since HKUST-1 exhibits negative thermal expansion (NTE), we note the unusual phenomena that the material softens as it becomes denser with increasing temperatures. We show this is achieved by which the ligands take on a contorted configuration as they absorb more energy at higher temperatures. Upon infiltration, the MOF softens, however the softening (and therefore reduction in phonon group velocity) cannot be completely responsible for the large reduction in thermal conductivity that is experimentally observed (~71% at room temperature). Spectral Energy Density (SED) and MD results indicate that localized phonon modes from the adsorbates act as additional scattering mechanisms to reduce the thermal conductivity. Further, experimental evidence of the existence of low frequency ZA-like flexural modes (usually found only in 2D materials) within the highly porous 3D MOF structure likely contributes to a large portion of the thermal conductivity, and is suppressed upon infiltration with a guest molecule, causing the large reduction in thermal conductivity.

Influence of stacking disorder on cross-plane thermal transport properties in TMPS3 (TM = Mn, Ni, Fe)

We investigated the thermal transport properties of magnetic van der Waals materials, TMPS3 (TM = Mn, Ni, and Fe), using the time-domain thermoreflectance technique. We determined the cross-plane thermal conductivity, which turns out to be relatively low, i.e., about 1 W m−1 K−1 for all TMPS3 investigated. When compared with previous results of graphite and transition metal dichalcogenides (TMDs), thermal conductivity becomes smaller as it goes from graphite to TMDs to TMPS3, and the difference is larger at low temperature, e.g., around 50 K. From the Callaway model analysis, we could attribute the large thermal conductivity reduction for TMPS3, particularly at low temperature, to the phonon scattering from the boundary. We actually confirmed the existence of the large population of the stacking faults with the cross-sectional transmission electron microscopy image of MnPS3. This suggests that intrinsic or extrinsic stacking faults formed in van der Waals materials and their heterostructures can play an important role in reducing the cross-plane thermal conductivity as a source of the boundary scattering.

Reduction of thermal conductivity in ferroelectric SrTiO3 thin films

Bulk ${\mathrm{SrTiO}}_{3}$ is a quantum paraelectric in which an antiferrodistortive distortion below $\ensuremath{\approx}105$ K and quantum fluctuations at low temperature preclude the stabilization of a long-range ferroelectric state. However, biaxial mechanical stress, impurity doping, and Sr nonstoichiometry, among other mechanisms, are able to stabilize a ferroelectric or relaxor ferroelectric state at room temperature, which develops into a longer-range ferroelectric state below 250 K. In this paper, we show that epitaxial ${\mathrm{SrTiO}}_{3}$ thin films grown under tensile strain on ${\mathrm{DyScO}}_{3}$ exhibit a large reduction of thermal conductivity, of $\ensuremath{\approx}60%$ at room temperature, with respect to identical strain-free or compressed films. The thermal conductivity shows a further reduction below 250 K, a temperature concurrent with the peak in the dielectric constant [J. H. Haeni et al., Nature (London) 430, 758 (2004)]. These results suggest that strain gradients in the relaxor and ferroelectric phase of ${\mathrm{SrTiO}}_{3}$ are very effective phonon scatterers, limiting the thermal transport in this material.

Impact of irradiation induced dislocation loops on thermal conductivity in ceramics

AbstractExperimental work aimed at understanding the role of dislocation loops in limiting phonon mediated thermal transport in ceramics is presented. Faulted dislocation loops, having diameters of a few nanometers, were introduced by irradiating a polycrystalline cerium dioxide sample with 1.6 MeV protons at 700°C. XRD analysis indicated that irradiated samples retained their crystalline structure and exhibit very little lattice expansion suggesting a low concentration of point defects. Further microstructure characterization using transmission electron microscopy revealed that interstitial type faulted dislocation loops were primarily created as expected for these irradiation conditions. Thermal conductivity of the damaged layer was measured using a modulated thermoreflectance approach. Analysis of the experimental data using the classical Klemens‐Callaway approach reveals that the conductivity reduction is primarily due to dislocation loops, while point defects and voids play only a minor role. These results provide experimental confirmation that faulted loops offer a unique arrangement for displaced atoms that leads to an unusually large reduction of thermal conductivity.

Electrical and thermal transport properties of S- and Te-doped InSe alloys

Metal chalcogenide materials, including indium selenide (InSe), have attracted attention as potential thermoelectric materials due to their intrinsically low thermal conductivity, which is due to their layered structure with weak atomic bonding. However, their low electrical conductivity should be increased for application as thermoelectric materials. In this study, we examined the electrical and thermal transport properties of 7% S- or Te-doped InSe polycrystalline alloys (InSe0.93S0.07 and InSe0.93Te0.07). These anion substitutions increased the electrical conductivity of InSe, while the enhancement was larger for the Te-doped InSe with a smaller bandgap. The negative Seebeck coefficient did not change significantly upon doping, resulting in enhancement of the power factor. Doping caused a relatively large reduction of thermal conductivity due to additional point defect phonon scattering. Consequently, the thermoelectric figure of merit (zT) increased by 62% and 230% to a maximum of 0.13 and 0.28 at 735 K for InSe0.93S0.07 and InSe0.93Te0.07, respectively, which offers the possibility of InSe-based thermoelectric materials. Cation doping could increase the zT of S/Te-doped InSe further.

Giant reduction and tunability of the thermal conductivity of carbon nanotubes through low-frequency resonant modes

Manipulating thermal transport by designing materials with control of their properties over the entire spectral range of vibrational frequencies would provide a unique path to create solids with designer thermal conductivities. Traditional routes of nanostructuring to reduce the vibrational thermal conductivity of solids typically target narrow bands of the vibrational energy spectrum, which is often based on the characteristic dimensions of the nanostructure. In this work, we demonstrate the ability to simultaneously impact the phonon transport of both high- and low-frequency modes by creating defects that act as both high-frequency phonon scattering sites while coherently manipulating low-frequency waves via resonance with the long wavelength phonons. We use atomistic simulations to identify fullerenes functionalized on the sidewalls of carbon nanotubes (CNT) as efficient phonon blocks realized through localized resonances that appear due to hybridization between the modes of the fullerene and the underlying CNT. We show that with a large surface coverage and high periodicity in the inclusion of the covalently bonded fullerenes, the thermal conductivity of individual CNTs can be lowered by more than an order of magnitude, thus providing a large tunability in the thermal transport across these materials. We prescribe the large reduction in thermal conductivity to a combination of resonant phonon localization effects leading to phonon band anticrossings and vibrational scattering effects due to the inclusion of the strongly bonded fullerene molecules.

Tunable thermal conductivity in mesoporous silicon by slight porosity change

We report the thermal conductivity of photoelectrochemically synthesized mesoporous silicon (MPS), with ∼20-nm diameter pores and 52%–58% porosity. The thermal conductivity of MPS samples with a thickness of a few microns was measured using the three omega (3ω) differential technique. We experimentally demonstrated that the thermal conductivity of MPS varies between 3 and 7 W/m K at room temperature and is dependent on the photoelectrochemical etching times used during the MPS synthesis, which induces a slight change in the MPS porosity. Calculations were conducted using the Boltzmann transport equation in the relaxation time approximation, with the results suggesting that the large thermal conductivity reduction in the MPSs was not entirely explained by the pore boundary scattering. Our findings indicate that elastic softening in the mesoporous structure may be responsible for the reduction in the thermal conductivity.

Effects of Mn substitution on thermoelectric properties of CuIn1−xMnxTe2**Project supported by the National Natural Science Foundation of China (Nos. 51632005 and 51371194) and National Basic Research Program of China (Grant No. 2013CB632500).

CuIn1 − xMnxTe2 samples have been synthesized by a melt-annealing method. The x-ray powder diffraction (XRD) analysis shows that the CuIn1−xMnxTe2 samples crystallize in the chalcopyrite phase. Mn doping can effectively optimize the electrical properties and accordingly improve the power factor. The room temperature electrical conductivity of doped CuInTe2 increases by several orders of magnitude due to substituting In with Mn. In addition, a large reduction in thermal conductivity is achieved through the enhanced phonon scattering via Mn-related point defects and precipitates. Therefore, an enhanced average ZT value up to 0.34 is achieved for sample CuIn0.925Mn0.075Te2, which is 41% higher than that of the pristine CuInTe2.

Thermal conductivity reduction of tungsten plasma facing material due to helium plasma irradiation in PISCES using the improved 3-omega method

The near-surface region of plasma facing material (PFM) plays an important role in thermal management of fusion reactors. In this work, we measured thermal conductivity of tungsten (W) surface layers damaged by He plasma in PISCES at UCSD. We studied the damage effect on both bulk, and thin film, W. We observed that the surface morphology of both bulk and thin film was altered after exposure to He plasma with the fluence of 1 × 1026 m−2 (bulk) and 2 × 1024 m−2 (thin film). Transmission electron microscopy (TEM) analysis reveals that the depth of the irradiation damaged layer was approximately 20 nm on the bulk W exposed to He plasma at 773 K for 2000 s. In order to measure the thermal conductivity of this exceedingly thin damaged layer in the bulk W, we adopted the well-established ‘3-omega’ method and employed novel nanofabrication techniques to improve the measurement sensitivity. For the damaged W thin film sample, we measured the reduction in electrical conductivity and used the Wiedemann-Franz (W-F) law to extract the thermal conductivity. Results from both measurements show that thermal conductivity in the damaged layers was reduced by at least ∼80% compared to that of undamaged W. This large reduction in thermal conductivity can be attributed to the scattering of electrons, the dominant heat carriers in W, caused by defects introduced by He plasma irradiation.

Thermoelectric Properties of Silicon Germanium Alloy Nanocomposite Fabricated by Mechanical Alloying and Spark Plasma Sintering

High dense p-type Si95Ge5 doped with nanoSi70Ge30B5particles thermoelectric materials were firstly fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) method. The effects of different nanoSi70Ge30B5 doping content on the thermoelectric and phase properties were studied. A dimensionless thermoelectric figure-of-merit (ZT) of 0.47 at 710K in p-type nanocomposite bulk silicon germanium (SiGe) alloys is achieved. The enhancement of ZT is due to a large reduction of thermal conductivity caused by the increased grain boundaries of the numerous nanograins that effectively scatter long wavelength phonons.

Large reduction in thermal conductivity for SiGe alloy nanowire wrapped with a Ge nanoparticle-embedded SiO2 shell

We demonstrate silicon germanium (SiGe) alloy nanowires (NWs) with Ge nanoparticles (GeNPs) embedded in a SiO2 shell as a material for decreasing thermal conductivity. During thermal oxidation of SiGe NWs to form SiGe–SiO2 core–shell structures, Ge atoms were diffused into the SiO2 shell to relax the strain in the SiGe core, and agglomerated as a few nanometer-sized particles. This structure leads to a large reduction in thermal conductivity due to the GeNP–phonon interaction, while electrical conductivity is sustained because the core of the SiGe alloy NW provides a current path for the charged carriers. The thermal conductivity of the SiGe alloy NWs wrapped with a GeNP-embedded SiO2 shell is 0.41 W m−1 K−1 at 300 K.

Raising the Thermoelectric Performance of Fe3CoSb12 Skutterudites via Nd Filling and In-Situ Nanostructuring.

p-type skutterudites NdxFe3CoSb12 with x equaling 0.8, 0.85, 0.9, 0.95, 1.0 have been synthesized by solid state reaction followed by spark plasma sintering. The influence of Nd filling on electrical and thermal transport properties has been investigated in the Nd-filled skutterudite compounds in the temperature range from room temperature to 800 K. It was found that the Seebeck coefficient is drastically enhanced via filling Nd in p-Type skutterudites as well as the corresponding power factor although electrical conductivity is reduced. In addition, a large reduction in thermal conductivity is achieved by Nd fillers through rattling effect along with the in-situ nanostructured precipitate through scattering phonons with much wider frequency. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of p-type skutterudites.

Effect of Al substitution on Thermoelectric Performance of CuInTe2 compounds

ABSTRACTThermoelectric CuIn1-xAlxTe2 compounds (x=0, 0.05, 0.1, 0.15, 0.50) have been synthesized by solid state reaction followed by spark plasma sintering. The influence of Al substitution on electrical and thermal transport properties has been investigated in the CuInTe2 compounds. It was found that the Seebeck coefficient and electrical conductivity is reduced by isovalent replacement of In with Al. Our first principle calculation indicates Al substitution leads to the widen band gap, the reduction in the number of degeneracy of valence band and the effective mass. Furthermore, a large reduction in thermal conductivity is achieved through the enhanced phonon scattering via point defect as well as the nano-sized particles observed between grain boundaries and on the grain surface. In spite of the reduced charge transport properties, an improved figure-of- merit ZT is achieved, reaching 0.8 at 800 K, 33% higher in comparison to the pure CuInTe2 compound.

Thermal Transport in Graphene Oxide – From Ballistic Extreme to Amorphous Limit

Graphene oxide is being used in energy, optical, electronic and sensor devices due to its unique properties. However, unlike its counterpart – graphene – the thermal transport properties of graphene oxide remain unknown. In this work, we use large-scale molecular dynamics simulations with reactive potentials to systematically study the role of oxygen adatoms on the thermal transport in graphene oxide. For pristine graphene, highly ballistic thermal transport is observed. As the oxygen coverage increases, the thermal conductivity is significantly reduced. An oxygen coverage of 5% can reduce the graphene thermal conductivity by ~90% and a coverage of 20% lower it to ~8.8 W/mK. This value is even lower than the calculated amorphous limit (~11.6 W/mK for graphene), which is usually regarded as the minimal possible thermal conductivity of a solid. Analyses show that the large reduction in thermal conductivity is due to the significantly enhanced phonon scattering induced by the oxygen defects which introduce dramatic structural deformations. These results provide important insight to the thermal transport physics in graphene oxide and offer valuable information for the design of graphene oxide-based materials and devices.

Flexible Thermoelectric Devices Based on Indium Phosphide Nanowire Networks on Copper

ABSTRACTMore than 50% of total input energy is wasted as heat in various industrial processes. If we could harness a small fraction of the waste heat while satisfying the economic demands of cost versus performance, then thermoelectric (TE) power generation could bring substantial positive impacts. To meet these demands single-crystal semiconductor nanowire networks have been investigated as a method to achieve advanced TE devices because of their predicted large reduction in thermal conductivity and increase in power factor.To further our goal of developing practical and economical TE devices, we designed and developed a material platform that combined a semiconductor nanowire network and a semiconductor thin film integrated directly on a mechanically flexible metallic substrate. We assessed the potential of this platform by using indium phosphide (InP) nanowire networks and a doped poly-silicon (poly-Si) thin film combined on copper sheets. InP nanowires were grown by metal organic chemical vapor deposition (MOCVD). In the nanowire network, InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. Subsequently, plasma-assisted CVD was utilized to form a poly-Si thin film to prevent electrical shorting when an ohmic copper top contact was made. An additional facet to this design is the utilization of multiple materials to address the various temperature ranges at which each material is most efficient at heat-to-energy conversion. The utilization of multiple materials could enable the enhancement of total power generation for a given temperature gradient. We investigated the use of poly-Si thin films combined with InP nanowires to enhance TE properties. TE power production and challenges of a large area nanowire device on a flexible metallic substrate were presented.

Anisotropic charge and heat conduction through arrays of parallel elliptic cylinders in a continuous medium

Arrays of circular pores in silicon can exhibit a phononic bandgap when the lattice constant is smaller than the phonon scattering length, and so have become of interest for use as thermoelectric materials, due to the large reduction in thermal conductivity that this bandgap can cause. The reduction in electrical conductivity is expected to be less, because the lattice constant of these arrays is engineered to be much larger than the electron scattering length. As a result, electron transport through the effective medium is well described by the diffusion equation, and the Seebeck coefficient is expected to increase. In this paper, we develop an expression for the purely diffusive thermal (or electrical) conductivity of a composite comprised of square or hexagonal arrays of parallel circular or elliptic cylinders of one material in a continuum of a second material. The transport parallel to the cylinders is straightforward, so we consider the transport in the two principal directions normal to the cylinders, using a self-consistent local field calculation based on the point dipole approximation. There are two limiting cases: large negative contrast (e.g., pores in a conductor) and large positive contrast (conducting pillars in air). In the large negative contrast case, the transport is only slightly affected parallel to the major axis of the elliptic cylinders but can be significantly affected parallel to the minor axis, even in the limit of zero volume fraction of pores. The positive contrast case is just the opposite: the transport is only slightly affected parallel to the minor axis of the pillars but can be significantly affected parallel to the major axis, even in the limit of zero volume fraction of pillars. The analytical results are compared to extensive FEA calculations obtained using Comsol™ and the agreement is generally very good, provided the cylinders are sufficiently small compared to the lattice constant.