Reduction Of Phonon Thermal Conductivity Research Articles

Phonon thermal transport in polycrystalline graphene:Effects of grain, vacancy and strain

The phonon thermal conductivity of polycrystalline graphene nanoribbons has been calculated by non-equilibrium molecular dynamics method. The effects of grain size and boundary orientation, vacancy defect and strain on the phonon thermal conductivity of polycrystalline graphene nanoribbons have been studied. It was found that for polycrystalline graphene nanoribbons with the same average grain size, the phonon thermal conductivity increased with the increase of nanoribbon length. At the same size of nanoribbons, the phonon thermal conductivity increases with the increase of grain size in polycrystalline graphene nanoribbons. Grain boundary orientation has also an important impact on the phonon thermal conductivity. In addition, the phonon thermal conductivity of polycrystalline graphene decreases with the increase of tensile strain/compressive strain because the strain induces the change of structure and mechanical properties, which affects the phonon transport behavior. Compared with the pristine graphene, the vacancy defects lead to the suppression of some phonon modes of polycrystalline graphene and thus the reduction of phonon thermal conductivity due to the enhancement of phonon boundary scattering and phonon defect scattering. Our results are helpful to understand the physical mechanism of phonon thermal transport in polycrystalline graphene, and provides an important guidance for the regulation of phonon thermal conductivity of polycrystalline graphene.

Intrinsic thermoelectric properties in biphenylene nanoribbons and effect of lattice defects

Intrinsic thermoelectric properties in biphenylene nanoribbons (BNRs) and effect of lattice defects are investigated by employing the nonequilibrium Green’s function. A comparative analysis on intrinsic thermoelectric properties between graphene nanoribbons (GNRs) and BNRs is performed. Results show that although the Seebeck coefficient of BNRs is lower than that of the corresponding GNRs, BNRs exhibit better thermoelectric properties than the corresponding GNRs due to the lower phonon thermal conductance of BNRs. It is also shown that the lattice defects can enhance the maximum ZT (ZTmax) in armchair BNRs (ABNRs) and zigzag BNRs (ZGNRs), where the ZTmax can be up to 1.1 and 0.4 at T = 300 K, respectively. Moreover, such the enhancement in defective ZBNRs is more remarkable than that in defective ABNRs. Theoretical analysis reveals that this enhancement of the ZTmax originates from the substantial reduction of phonon thermal conductance due to the strong backscattering by the highly localized phonon states around the lattice defects.

Thermoelectric and electron heat rectification properties of quantum dot superlattice nanowire arrays

Heat engines made of quantum dot (QD) superlattice nanowires (SLNWs) offer promising applications in energy harvesting due to the reduction in phonon thermal conductivity. In solid state electrical generators (refrigerators), one needs to generate (remove) a large amount of charge current (heat current). Consequently, a high QD SLNW density is required for realistic applications. This study theoretically investigated the properties of power factor and electron heat rectification for an SLNW array under the transition from a one-dimensional system to a two-dimensional system. The SLNW arrays show the functionality of heat diodes, which is mainly attributed to a transmission coefficient with a temperature-bias direction dependent characteristic.

Alkyl group functionalization-induced phonon thermal conductivity attenuation in graphene nanoribbons**Project supported by the National Natural Science Foundation of China (Grant No. 11504418), China Scholarship Council Scholarship Program (Grant No. 201706425053), and the Fundamental Research Funds for the Central Universities of China (Grant No. 2015XKMS075).

We calculated the room-temperature phonon thermal conductivity and phonon spectrum of alkyl group-functionalized zigzag graphene nanoribbons (ZGNRs) with molecular dynamics simulations. The increase in both chain length and concentration of alkyl groups caused remarkable reduction of phonon thermal conductivity in functionalized ZGNRs. Phonon spectra analysis showed that functionalization of ZGNR with alkyl functional groups induced phonon–structural defect scattering, thus leading to the reduction of phonon thermal conductivity of ZGNR. Our study showed that surface functionalization is an effective routine to tune the phonon thermal conductivity of GNRs, which is useful in graphene thermal-related applications.

Low cross-plane thermal conductivity of sub-1 µm polycrystalline silicon thin films for thermoelectric applications

Silicon thin films with sub-1 µm thickness are of technological significance in many electronic and energy conversion devices. While a number of previous works have investigated the in-plane thermal conductivity of silicon films, less reported is their cross-plane thermal conductivity, particularly at sub-1 µm thickness. Here, we report on the cross-plane thermal conductivity of sub-1 µm, undoped, polycrystalline silicon films (75 and 246 nm) with columnar grain structure at room temperature. Using an optical pump-probe thermoreflectance metrology and a semi-classical phonon transport model, we find the cross-plane thermal conductivity to be 4.5 ± 0.8 and 12.5 ± 2.3 W m−1 K−1 for the 75 and 246 nm films, respectively. To the best of our knowledge, the cross-plane thermal conductivities we report here are the lowest ever reported for undoped polycrystalline silicon films with columnar grain structure, which can be ascribed to the impact of increased phonon scattering by a higher density of defects as compared to previous data. Given that a significant reduction in phonon thermal conductivity is required to enhance the thermoelectric energy conversion efficiency, our findings may be promising particularly for thermoelectric energy conversion applications.

Optimizing thermoelectric efficiency of superlattice nanowires at room temperature

It is known that the figure of merit (ZT) of thin nanowires can be significantly enhanced at room temperature due to the reduction of phonon thermal conductance arising from the increase of boundary scattering of phonons. It is expected that the phonon thermal conductance of nanowires filled with quantum dots (QDs) will be further reduced. Here we consider a superlattice nanowire (SLNW) modeled by a linear chain of strongly coupled QDs connected to electrodes. We study the dependence of ZT on the QD energy level (E0) (relative to the Fermi level EF in the electrodes), inter-dot coupling strength (tc), tunneling rate (Γ), and temperature T in order to optimize the design. It is found that at room temperature the maximum power factor occurs when (E0 − EF)∕kBT ≈ 2.4 and Γ = tc, a result almost independent of the number of QDs in SLNW as long as tc∕kBT < 0.5. By using reasonable physical parameters we show that thin SLNW with cross-sectional width near 3 nm has a potential to achieve ZT ≥ 3.

Decompose the electron and phonon thermal transport of intermetallic compounds NiAl and Ni3Al by first-principles calculations

In order to develop a better understanding of the thermal conduction of the intermetallic compounds NiAl and Ni3Al, detailed electron-phonon scattering was investigated by first-principles calculations. The phonon thermal conductivity of NiAl and Ni3Al was calculated by considering phonon-phonon and electron-phonon scatterings. It is found that electron-phonon coupling has a strong effect on the phonon thermal conductivity. At a temperature of 100K, the electron-phonon coupling will induce 55% and 75% reduction in phonon thermal conductivity of NiAl and Ni3Al, respectively. Such an effect decreases with increasing temperature. The electron thermal conductivity was also predicted by considering electron-phonon scattering. We find that though the electrons dominate the thermal transport in NiAl and Ni3Al at high temperatures, the phonons have significant contribution to the total thermal conductivity in the sub-room temperature range. Moreover, we find that at room temperature phonons with mean free paths between 1 and 100nm are the dominant contributors to the thermal conductivity, while the mean free paths of dominant electrons are 1–20nm.

Microstructure and its influence on thermoelectric properties of hot-extruded Bi-Sb-Te bulk materials

The microstructural features and relationships among extrusion temperature, microstructure, and thermoelectric/mechanical properties of hot-extruded Bi0.4Sb1.6Te3 materials have been investigated. The microstructure was characterized by fine and equiaxed grains with many twin boundaries and preferred orientation of {0001}. A twin pair shared a certain {0001} basal plane as the twin plane and had an orientation relationship of 60° or 180° rotation about c-axis. The sample extruded at higher temperature exhibited higher fractions of small-angle grain boundaries and twin boundaries due to formation of Te-rich and Sb-rich phases. The higher fraction of twin boundaries caused a reduction in phonon thermal conductivity.

Thermoelectric properties of EuZn2Sb2 Zintl compounds: zT enhancement through Yb substitution for Eu

AZn2Sb2 (A: Ca, Sr, Eu, Yb) Zintl compounds have shown excellent thermoelectric performance peaking around 700 K. Among these materials, EuZn2Sb2 and YbZn2Sb2 compounds have the highest carrier mobility, making them particularly interesting. We present the thermoelectric properties of EuδZn2Sb2 (δ = 0.9, 1.0, 1.1) and Eu1-xYbxZn2Sb2 compounds (x = 0, 0.1, 0.25, 0.5, 0.75, 1) from 373 to 773 K. Our interest is in studying whether introducing A-site vacancies increases the hole carrier concentration with decreasing Eu content or with increasing Yb substitution, which could lead to optimized power factor at high temperatures. The electrical conductivity showed a non-monotonic change caused by carrier mobility scattering. The thermal conductivity decreased through deviation of the Eu content and through Eu-Yb substitution; in particular, the phonon contribution at 773 K reduced gradually from 1.1 W m−1 K−1 (x = 0) to 0.42 W m−1 K−1 (x = 1.0) with increasing Yb fraction and reached the predicted minimum lattice thermal conductivity. The dimensionless figure of merit increased from 0.58 to 0.68 at 773 K through a 10% Yb substitution, i.e. Eu0.9Yb0.1Zn2Sb2, mainly due to the reduction of phonon thermal conductivity without lowering the power factor.

Enhancement of thermoelectric performance by reducing phonon thermal conductance in multiple core-shell nanowires.

The thermoelectric properties of multiple core-shell nanowires are investigated by using nonequilibrium Green's function method and molecular dynamics simulations. The results show that the thermoelectric performance of multiple core-shell NWs can be improved observably with the increase of shell number compared with the single component NWs due to the significant reduction of phonon thermal conductance. The ZT value of multiple core-shell NWs can reach three times greater than that of the single component GaSb NWs at room temperature. Moreover, the ZT values of both the core-shell NWs and single component NWs are increased with the increasing temperature, but the ZT value of core-shell NWs increases more slowly than that of single component NWs. These results show that the single component NWs is suitable as thermoelectric material at much high temperature, but the multiple core-shell NWs is more suitable as thermoelectric material at room temperature.

Phonon transport in an initially twisted polyvinyl acetate nanofiber

A reduction in phonon thermal conductivity enhances thermoelectric performance. One method to accomplish this is by applying mechanical stress to a nanostructure. We consider an initially twisted 20 nm polyvinyl acetate nanofiber undergoing torsion. Our analytical method uses the continuum approach of Mooney's model. Torsion modifies the phonon dispersion. Angles of twist between 45° and 70° reduce the averaged phonon group velocity and the phonon thermal conductivity but when these angles are increased further, this conductivity increases. This suggests a phonon engineering approach to tune the thermal conductivity of nanomaterials.

Origin of reduction in phonon thermal conductivity of microporous solids

Porous structures have strong tunable size effects due to increased surface area. Size effects on phonon thermal conductivity have been observed in porous materials with periodic voids on the order of microns. This letter explores the origin of this size effect on phonon thermal conductivity observed in periodic microporous membranes. Pore-edge boundary scattering of low frequency phonons explains the temperature trends in the thermal conductivity; further reduction in thermal conductivity is explained by the porosity.

Nanostructures and enhanced thermoelectric properties in Ce-filled skutterudite bulk materials

The nanostructures induced by uniaxial compression in Ce0.29Fe1.40Co2.60Sb11.24 bulk thermoelectric material is reported. High-resolution transmission electron microscope images reveal that the nanostructures consist of Ce0.31Fe1.38Co2.62Sb11.56 crystals and Fe0.34Co0.66Sb1.99 minicrystal/noncrystal with grain sizes about 5–20nm. It is shown that the Seebeck coefficient is increased by 21%, the phonon thermal conductivity is reduced by 19%, while the electrical conductivity rises slightly at 300K. This leads to a surprising increase in the power factor by 51%. The significant increase of Seebeck coefficient and the remarkable reduction of phonon thermal conductivity are believed to be due to quantum effect and size effect of the nanostructures, respectively.