Phonon Conductivity Research Articles

Thermal Conductivity of poly-p-phenylene-2,6-benzobisoxazole Film along the In-Plane Axis in the 10–300 K Temperature Range

The thermal conductivities of compression molded thin films of poly-p-phenylene-2,6-benzobisoxazole (PBO) were measured in directions along an in-plane axis in the 10–300 K temperature range by a steady-state heat flow method, with interest in the use of the material for superconductivity applications. The thermal conductivities of the PBO films increased from 0.3 W/mK to 9.0 W/mK with increasing temperature from 10 K to 300 K and these were much higher than those of polyimide films, epoxy resin and glass fiber reinforced plastics at all temperatures. The 9.0 W/mK at 300 K was 60% of that of stainless steel (SUS304). It was 6 W/mK at 150 K, which was half that of SUS304 and was 3.3 W/mK at 77 K, which was 33% of that of SUS304. The thermal conductivities of the PBO films were lower than those of a cloth of high strength ultrahigh molecular weight polyethylene fiber reinforced plastics in the 30 K–180 K temperature range and were almost equivalent to its values in the 180 K–300 K temperature range. The main contribution to the thermal conduction in the PBO films was from thermal phonon conduction along the molecular chains. Although many kinds of high thermal conductivity polymeric materials have been prepared by a uni-directional drawing process or by adding high thermal conductive additives, the PBO film showed high thermal conductivity without a uni-directional drawing process or high thermal conductive additive.

Direct SPD-processing to achieve high-ZT skutterudites

For mass fabrication of thermoelectric generators a fast, cheap and simple method is required to produce high ZT leg material. It is well known that severe plastic deformation (SPD) enhances the density of defects and dislocations, and also refines the grains to nano-size. These are the essential key parameters for low thermal phonon conductivity. Consequently and as a first example, SPD via high-pressure torsion (HPT) at elevated temperatures in protective gas atmosphere was used to directly consolidate and plastically deform commercial p-type skutterudite powder, DDyFe3CoSb12, (DD stands for didymium) into a dense thermoelectric solid. The HPT-sample exhibited a high figure of merit, ZT > 1.3 at 773 K, much higher than that of the hot-pressed reference sample (HP) from the same powder. The ZT achieved is comparable with ZTs of corresponding high energy ball milled and hot pressed samples (HBM-HP) as well as of ball milled, hot pressed plus HPT processed skutterudites (BM-HP-HPT). The thermoelectric efficiency is even 4% higher. Synchrotron measurements were performed at temperatures from 300 to 825 K in order to evaluate the changes in grain size and dislocation density of the CP-HPT samples before, during and after annealing. SEM and TEM images served to give a better insight into the temperature dependent diversification of the new material. Due to grain refinement and enhanced dislocation density, the CP-HPT material exhibits higher hardness values than the reference sample. The elastic moduli are, within the error bar, the same.With large high-pressure torsion facilities at hand, a new, fast and therefore cheap mass production of thermoelectric leg material directly from powders can be envisaged. This new production technique furthermore warrants high figures of merit but excludes time-consuming ball milling and hot-pressing steps.

Single-Mode Phononic Wire.

Photons and electrons transmit information to form complex systems and networks. Phonons on the other hand, the quanta of mechanical motion, are often considered only as carriers of thermal energy. Nonetheless, their flow can also be molded in fabricated nanoscale circuits. We design and experimentally demonstrate wires for phonons by patterning the surface of a silicon chip. Our device eliminates all but one channel of phonon conduction, allowing coherent phonon transport over millimeter length scales. We characterize the phononic wire optically, by coupling it strongly to an optomechanical transducer. The phononic wire enables new ways to manipulate information and energy on a chip. In particular, our result is an important step towards realizing on-chip phonon networks, in which quantum information is transmitted between nodes via phonons.

Discovery of Emergent Photon and Monopoles in a Quantum Spin Liquid

Quantum spin liquid (QSL) is an exotic quantum phase of matter whose ground state is quantum-mechanically entangled without any magnetic ordering. A central issue concerns emergent excitations that characterize QSLs, which are hypothetically associated with quasiparticle fractionalization and topological order. Here we report highly unusual heat conduction generated by the spin degrees of freedom in a QSL state of the pyrochlore magnet Pr$_2$Zr$_2$O$_7$, which hosts spin-ice correlations with strong quantum fluctuations. The thermal conductivity in high temperature regime exhibits a two-gap behavior, which is consistent with the gapped excitations of magnetic ($M$-) and electric monopoles ($E$-particles). At very low temperatures below 200\,mK, the thermal conductivity unexpectedly shows a dramatic enhancement, which well exceeds purely phononic conductivity, demonstrating the presence of highly mobile spin excitations. This new type of excitations can be attributed to emergent photons ($\nu$-particle), coherent gapless spin excitations in a spin-ice manifold.

Phonon Conductivity of Nanoparticles Embedded in Dielectric Material

A theory of the thermal conductivity has been developed for nanomaterials made by embedding nanoparticles in a host dielectric material. The phonon dispersion relation has been calculated using the transfer matrix method in the long‐range approximation, where the phonon wavelength is larger than the size of the nanoparticle. We found that these nanomaterials have two phonon branches known as ‐phonon and ‐phonon branches. For both phonon branches, the density of states and the phonon velocity are calculated. The thermal conductivity is evaluated with the Kubo formalism and the Green's function method for both ‐phonon and ‐phonon branches. It is also found that the density of states, phonon velocity and thermal conductivity for both phonon branches depend on the size of the nanoparticles, spacing between nanoparticles, and the phonon refractive index of the nanoparticles and the host material. In the long wave approximation, expressions of the phonon conductivity, the density of states and the phonon velocity have very simple forms which can be used by experimentalists to explain their experiments or plan new experiments. We have also applied our theory to explain the experimental thermal conductivity data of silica‐resin, alumina‐resin, AlN‐resin and CaO‐polyethylene nanomaterials. A good agreement between theory and experiments is achieved. Our results furthermore illustrate that one can fabricate new types of nanomaterials with high and low thermal conductivity by adjusting the refractive index contrast between nanoparticles and the host material. These are very novel and interesting properties and they can be used to fabricate new types of thermal devices.

Phononic thermal conductivity in silicene: the role of vacancy defects and boundary scattering

We calculate the thermal conductivity of free-standing silicene using the phonon Boltzmann transport equation within the relaxation time approximation. In this calculation, we investigate the effects of sample size and different scattering mechanisms such as phonon–phonon, phonon-boundary, phonon-isotope and phonon-vacancy defect. We obtain some similar results to earlier works using a different model and provide a more detailed analysis of the phonon conduction behavior and various mode contributions. We show that the dominant contribution to the thermal conductivity of silicene, which originates from the in-plane acoustic branches, is about 70% at room temperature and this contribution becomes larger by considering vacancy defects. Our results indicate that while the thermal conductivity of silicene is significantly suppressed by the vacancy defects, the effect of isotopes on the phononic transport is small. Our calculations demonstrate that by removing only one of every 400 silicon atoms, a substantial reduction of about 58% in thermal conductivity is achieved. Furthermore, we find that the phonon-boundary scattering is important in defectless and small-size silicene samples, especially at low temperatures.

Effect of 2D MoS2 and Graphene interfaces with CoSb3 nanoparticles in enhancing thermoelectric properties of 2D MoS2-CoSb3 and Graphene-CoSb3 nanocomposites

The approach of incorporating a secondary phase in the bulk thermoelectric (TE) material has proved to be beneficial for enhancing the thermoelectric performance. We have investigated the effect of the presence of two-dimensional (2D) materials (MoS2 or graphene) on the structural, electrical, and thermoelectric properties of CoSb3 nanocomposite, in which CoSb3 nanoparticles of sizes 20–50 nm are uniformly anchored on the surface of 2D-sheets of MoS2 or graphene. The presence of 2D nanosheets enhances TE power factor and figure of merit (ZT). Inclusion of graphene in CoSb3 causes large enhancement in power factor as a result of significantly high electrical conductivity and appreciable Seebeck coefficient. 2D graphene seems to work by providing extra carrier conduction channels along with a low interfacial potential barrier for charge transport. Homogeneously dispersed 2D-sheets of MoS2 in CoSb3 seem to cause interfacial modulation of charge carrier effective mass assisted by relatively larger interfacial barrier to result in significantly larger Seebeck coefficient and highly suppressed phonon conductivity, much more than graphene. The ZT value in both nanocomposites gets significantly enhanced in the entire studied temperature range of 300–700 K, the gain increasing with temperature over the CoSb3. Whereas CoSb3/graphene nanocomposites exhibit unusually high ZT at higher temperatures (550–700 K), the CoSb3/2D-MoS2 nanocomposites exhibit better performance (over graphene) in near room temperature range. The present study provides a possible strategy to enhance the conversion efficiency of various TE materials and has significant potential for waste heat recovery applications in various temperature ranges.

Research for waterborne polyurethane/composites with heat transfer performance: a review

Waterborne polyurethanes (WPUs) have been pushing toward more multifunctional directions developing which is due to the more extensively utilized in the world. Obviously, electronic devices are becoming more miniaturization and intelligentialize, accompanying many problems (e.g., heat dissipating and heat aging) to resolve. Organic polymers are to be good substitute for metal substrate used in devices because of their excellent performances. However, due to the most of polymers are poor conductor of heat, so many researchers utilize the hybrid method to enhance thermal conductivity of composites, through the pathway formed by fillers connection to build heat networks for phonon conduction. In this study, we do some reviews of WPU/composites on heat conducting. We put a focus on the enhancement of thermal conductivity through the compatibility and feasibility of hybrid methods, which includes the physical and chemical modification with non-covalent and covalent bond interactions. Plus, referring to laboratory previous study of heat conduction, on which pure WPU possessed with short-range, long-range ordering and microcrystal structures can facilitate the phonon conduction with the improvement of thermal conductivity.

Impact of thermally dead volume on phonon conduction along silicon nanoladders.

Thermal conduction in complex periodic nanostructures remains a key area of open questions and research, and a particularly provocative and challenging detail is the impact of nanoscale material volumes that do not lie along the optimal line of sight for conduction. Here, we experimentally study thermal transport in silicon nanoladders, which feature two orthogonal heat conduction paths: unobstructed line-of-sight channels in the axial direction and interconnecting bridges between them. The nanoladders feature an array of rectangular holes in a 10 μm long straight beam with a 970 nm wide and 75 nm thick cross-section. We vary the pitch of these holes from 200 nm to 1100 nm to modulate the contribution of bridges to the net transport of heat in the axial direction. The effective thermal conductivity, corresponding to reduced heat flux, decreases from ∼45 W m-1 K-1 to ∼31 W m-1 K-1 with decreasing pitch. By solving the Boltzmann transport equation using phonon mean free paths taken from ab initio calculations, we model thermal transport in the nanoladders, and experimental results show excellent agreement with the predictions to within ∼11%. A combination of experiments and calculations shows that with decreasing pitch, thermal transport in nanoladders approaches the counterpart in a straight beam equivalent to the line-of-sight channels, indicating that the bridges constitute a thermally dead volume. This study suggests that ballistic effects are dictated by the line-of-sight channels, providing key insights into thermal conduction in nanostructured metamaterials.

Thermal conductivity of PrRh4.8B2, a layered boride compound

Transmission electron microscopy (TEM), specific heat, and thermal conductivity measurements were carried out for PrRh4.8B2 single crystals. PrRh4.8B2 takes an interesting layered crystal structure, where PrRh3B2 blocks are separated by metal Rh honeycomb layers. The existence of the Rh layers enhances the ferrimagnetic transition temperature. From area-selective picosecond thermoreflectance, the cross-plane thermal conductivity of PrRh4.8B2 is estimated to be 1.39 Wm−1 K−1, much lower than other layered borides like AlB2. Judging from the fine crystal structures obtained by TEM, phonon conduction is considered to be depressed due to random dispersion of Rh vacancy sites and two-dimensional nature of PrRh4.8B2.

Thermal conductivity of type-I, type-II, and type-VIII pristine silicon clathrates: A first-principles study

By applying first-principles Peierls-Boltzmann transport calculations, the lattice thermal conductivities of type-I, type-II, and type-VIII silicon clathrates are investigated. The lattice thermal conductivity, spectral lattice thermal conductivity, and lattice thermal conductivity versus mean-free path (MFP) of phonons, relaxation times, and the anharmonicity of selected Si clathrates are calculated and compared. The microscopic origin of the slight difference in the lattice thermal conductivity of Si-I/II/VIII clathrates is discussed extensively. Our calculations uncover that the impact of phonon group velocity on the lattice thermal conductivity is far from enough to explain the discrepancies in ${\ensuremath{\kappa}}_{\text{lat}}$ of studied Si clathrates and it mainly is a consequence of the shorter lifetimes of the acoustic modes in type-VIII structure. Similarity between phase space for three-phonon anharmonic scattering processes of studied silicon clathrates highlights the influence of three-phonon coupling strength and the further calculation of the frequency-resolved final state spectra re-emphasizes that the subtle differences arise from the stronger three-phonon interaction coefficients.

Anharmonic, dimensionality and size effects in phonon transport

We have developed and employed a numerically efficient semi- ab initio theory, based on density-functional and relaxation-time schemes, to examine anharmonic, dimensionality and size effects in phonon transport in three- and two-dimensional solids of different crystal symmetries. Our method uses third- and fourth-order terms in crystal Hamiltonian expressed in terms of a temperature-dependent Grüneisen’s constant. All input to numerical calculations are generated from phonon calculations based on the density-functional perturbation theory. It is found that four-phonon processes make important and measurable contribution to lattice thermal resistivity above the Debye temperature. From our numerical results for bulk Si, bulk Ge, bulk MoS2 and monolayer MoS2 we find that the sample length dependence of phonon conductivity is significantly stronger in low-dimensional solids.

Correction to Extremely Low Lattice Thermal Conductivity and Point Defect Scattering of Phonons in Ag-doped (SnSe)1–x(SnS)x Compounds

Correction to Extremely Low Lattice Thermal Conductivity and Point Defect Scattering of Phonons in Ag-doped (SnSe)_1–<i>x</i>(SnS)_<i>x</i> Compounds

Phonon transport in nanocomposites

The transport properties of phonons in a nanocomposite have been investigated in this paper. In our model, the nanocomposite consists of nanoparticles embedded in a host dielectric material. The phonon dispersion relation of the nanocomposite is characterized in terms of a defined phonon refractive index and a filling factor parameter. This phonon refractive index depends on its velocity whereas the filling factor is determined by the diameter of the nanoparticles. Consequently, at zero filling, only the host material is present. Our model calculations also include point defects such as impurities, vacancies and the mass and size variations of nanoparticles. The phonon conductivity is determined by employing the Kubo formulism. We also report our calculated results for the phonon relaxation rates due to point defects and boundaries scattering where the latter is due to phonons interacting with the surface boundaries of nanoparticles and the host material. The thermal conductivity, dispersion relation, velocity, density-of-states and relaxation rates of phonons are determined by the filling factor and phonon refractive indices of the nanoparticles. There is a good agreement between our numerical results and the data obtained experimentally for the conductivity of AlN-polyimide nanocomposite. Our results also demonstrate that as the filling factor is increased the phonon conductivity also increases. Possible applications are new types of nanocomposites with variable thermal conductivity by adjusting the filling factor and refractive indices of the nanoparticle and host material. These nanocomposites may be important components in the fabrication of thermal nanodevices used for heating and cooling.

A discrete unified gas kinetic scheme for phonon Boltzmann transport equation accounting for phonon dispersion and polarization

Different from the gray model for phonon transport, the non-gray model takes account of the dispersion and polarization of phonons. Under the consideration of the real dispersion curve, the phonon transport in micro and nanoscale devices is intractable multiscale problem. On the basis of previous work about gray model for phonon transport by a discrete unified gas kinetic scheme (DUGKS), we extend the DUGKS to solving non-gray transport determined by the frequency-dependent phonon Boltzmann equation. The extension is straightforward due to the intrinsic multiscale property of the DUGKS. Four classic test cases, cross-plane heat conduction, in-plane heat conduction, one-dimensional transient thermal grating problem and two-dimensional steady phonon conduction are used to validate our scheme. Numerical results show that the present scheme can accurately capture ballistic-diffusive transport phenomenon in a wide range. This method may provide a powerful numerical tool for a deep research into nanoscale and microscale heat transport.

Optimizing the thermoelectric performance of graphene nano-ribbons without degrading the electronic properties

The enhancement of thermoelectric figure of merit ZT requires to either increase the power factor or reduce the phonon conductance, or even both. In graphene, the high phonon thermal conductivity is the main factor limiting the thermoelectric conversion. The common strategy to enhance ZT is therefore to introduce phonon scatterers to suppress the phonon conductance while retaining high electrical conductance and Seebeck coefficient. Although thermoelectric performance is eventually enhanced, all studies based on this strategy show a significant reduction of the electrical conductance. In this study we demonstrate that appropriate sources of disorder, including isotopes and vacancies at lowest electron density positions, can be used as phonon scatterers to reduce the phonon conductance in graphene ribbons without degrading the electrical conductance, particularly in the low-energy region which is the most important range for device operation. By means of atomistic calculations we show that the natural electronic properties of graphene ribbons can be fully preserved while their thermoelectric efficiency is strongly enhanced. For ribbons of width M = 5 dimer lines, room-temperature ZT is enhanced from less than 0.26 to more than 2.5. This study is likely to set the milestones of a new generation of nano-devices with dual electronic/thermoelectric functionalities.

Phonon conduction in silicon nanobeams

Despite extensive studies on thermal transport in thin silicon films, there has been little work studying the thermal conductivity of single-crystal rectangular, cross-sectional nanobeams that are commonly used in many applications such as nanoelectronics (FinFETs), nano-electromechanical systems, and nanophotonics. Here, we report experimental data on the thermal conductivity of silicon nanobeams of a thickness of ∼78 nm and widths of ∼65 nm, 170 nm, 270 nm, 470 nm, and 970 nm. The experimental data agree well (within ∼9%) with the predictions of a thermal conductivity model that uses a combination of bulk mean free paths obtained from ab initio calculations and a suppression function derived from the kinetic theory. This work quantifies the impact of nanobeam aspect ratios on thermal transport and establishes a criterion to differentiate between thin films and beams in studying thermal transport. The thermal conductivity of a 78 nm × 65 nm nanobeam is ∼32 W m−1 K−1, which is roughly a factor of two smaller than that of a 78 nm thick film.

Impact of grain boundary characteristics on lattice thermal conductivity: A kinetic theory study on ZnO

Grain boundaries are natural interfaces present in polycrystalline materials and have an important role in transport properties. In this work, the impact of grain boundary crystallographic mismatch, local impurity modulation, and spacing on lattice thermal conductivity is examined from the kinetic theory approach, with ZnO as a case study. We employ a dislocation-based model to describe the grain boundary scatterings of phonons, in which structural characteristics of grain boundaries are explicitly built-in and grain boundary scattering time depends on phonon frequency. This is in contrast to the gray model or the commonly used Casimir limit, which is blind to both grain boundary features and phonon frequency. We show that the lattice thermal conductivity generally decreases with grain boundary misorientation angle, and this dependence is significant for small grain boundary spacing while it tends to diminish for a large one. Intriguingly, our results show that local grain boundary chemistry can affect even more substantially than the crystallographic misfit on phonon relaxation time and interfacial thermal (Kapitza) resistance. Our results suggest new opportunities in tuning lattice thermal conductivity besides the nanostructure engineering approach, and demonstrates the synergetic effects of grain boundary characteristics on phonon conduction in polycrystals.

Thermostat Influence on the Structural Development and Material Removal during Abrasion of Nanocrystalline Ferrite.

We consider a nanomachining process of hard, abrasive particles grinding on the rough surface of a polycrystalline ferritic work piece. Using extensive large-scale molecular dynamics (MD) simulations, we show that the mode of thermostating, i.e., the way that the heat generated through deformation and friction is removed from the system, has crucial impact on tribological and materials related phenomena. By adopting an electron-phonon coupling approach to parametrize the thermostat of the system, thus including the electronic contribution to the thermal conductivity of iron, we can reproduce the experimentally measured values that yield realistic temperature gradients in the work piece. We compare these results to those obtained by assuming the two extreme cases of only phononic heat conduction and instantaneous removal of the heat generated in the machining interface. Our discussion of the differences between these three cases reveals that although the average shear stress is virtually temperature independent up to a normal pressure of approximately 1 GPa, the grain and chip morphology as well as most relevant quantities depend heavily on the mode of thermostating beyond a normal pressure of 0.4 GPa. These pronounced differences can be explained by the thermally activated processes that guide the reaction of the Fe lattice to the external mechanical and thermal loads caused by nanomachining.

Influence of phonon focusing on the Knudsen flow of phonon gas in single-crystal nanofilms of spintronic materials

Influence of the anisotropy of elastic energy on the phonon transport has been investigated in single- crystal nanofilms of Fe, Cu, MgO, InSb, and GaAs materials used for spintronic instruments and devices in the Knudsen flow regime of phonon gas. The dependences of the lattice thermal conductivity and lengths of free paths of phonons for all acoustic modes on the geometric parameters of the films have been considered for low temperatures with the dominance of the diffuse scattering of phonons at the boundaries. Physical aspects of the propagation of phonon modes in the films have been analyzed. It has been shown that the anisotropy of phonon transport in single-crystal films is due to the features of the propagation of phonon modes in elastically anisotropic films with a different relationship of the geometric parameters. The directions of heat flow and orientations of the film planes that yield the maximum and minimum thermal conductivity of phonons in film planes have been determined.