Mapping Thermal Conductivity at the Atomic Scale: A Step toward the Thermal Design of Materials

Ge2Sb2Te5 alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge2Sb2Te5 alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge2Sb2Te5 alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge2Sb2Te5 alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge2Sb2Te5 alloy accounts for the thermal conduction mechanism.

The use of polypropylene (PP)/elastomer blend in HVDC cable insulation is widespread due to its eco-friendliness. However, the low thermal conductivity of this material limits its application due to the temperature gradient caused by Joule heat from the current in the cable conductor. To address this issue, boron nitride (BN) nanosheets with high thermal conductivity and excellent insulation performance were added to the PP/propylene-based elastomer (PBE) blend. Threedimensional (3D) thermal conductive pathways were created by a self-support and pressure-reinforced method. Thermal and electrical conductivities were measured at 30, 60, and 90°C. The results showed that the addition of BN improved the thermal conductivity and reduced the electrical conductivity of the PP/PBE blend. As the temperature increased, the thermal conductivity of the PP/PBE/BN nanocomposite sample decreased. However, samples with the 3D structure exhibited better thermal conductivity performance at high temperatures. The electrical conductivity increased with the temperature, and the electrical conductivity dependence on temperature was reduced by the introduction of BN. It is suggested that the technique of incorporating the 3D thermal conduction pathways established by thermal conductive nanoparticles into the PP/elastomer blend is advantageous in enhancing its thermal conductivity and insulation performance.

We investigate the thermal conduction properties of a one-dimensional lattice of atoms carrying classical spins and coupled vibrationally. The spin degrees of freedom interact via a classical Heisenberg interaction, while the vibrational degrees of freedom are coupled through nearest-neighbor linear as well as nonlinear forces. The thermal conductivity in spin-phonon systems has both a phononic as well as a magnetic contribution. We use extensive numerical simulations and evaluate the magnetic and phononic thermal current correlation functions as well as the combined thermal conductivity coefficient. We employ two distinct numerical approaches: The first is based on the linear response theory and proceeds through an evaluation of the energy current correlation function using the Green-Kubo formula. The second is through a simulation of the stochastic baths and a subsequent direct numerical evaluation of the magnetic and phononic heat currents. We find an anomalous thermal conductivity when the spins are coupled to a harmonic acoustic phonon chain. However, when the harmonic phonon chain contains, additionally, an optical mode, we find that the thermal conductivity is normal for a certain regime of on-site force parameters, while it becomes anomalous when the on-site frequency becomes larger than a certain value. Coupling thus to a harmonic system with an optical mode provides a case of tunable conductivity that switches from being diffusive to ballistic as a function of structural model parameters or of the temperature. When the spins are coupled to anharmonic chains, we find an anomalous conductivity when the phonon chain is acoustic, for instance, in the Fermi-Past-Ulam case, or a normal one when the nonlinearity is of optic type. For the cases analyzed, we provide quantitative information on the exponent characterizing the power law decay of the energy current correlation function and determine size and temperature dependencies of the conductivity coefficient. Finally, we also address the dependence of the thermal conductivity of spin-phonon chains on an externally applied magnetic field and find that in the harmonic case it generally increases with the field.

Surface phonon polaritons are coupled states of polar atomic vibrations (optical phonons) and electromagnetic waves that are known to enhance near-field thermal radiation from polar materials. Theory suggests that surface phonon polaritons could also increase the thermal conductivity of nanoscale materials under certain conditions. This concept, however, is difficult to demonstrate experimentally, especially at room temperature. Here we design, predict, and demonstrate a different approach to observe thermal energy conduction by surface phonon polaritons – that is, a surface phonon polariton crystal. Packed beds of silicon dioxide nanoparticles have ultralow thermal conductivity at room temperature and large internal surface area, such that, when several water molecules are adsorbed on the nanoparticles to increase the effective relative permittivity of their surrounding medium (a mix of air and water), thermal energy conduction from surface phonon polaritons can dominate that generated by phonons. Using this material, we resolve experimentally a surface phonon polariton thermal conductivity that is as high as 1.7 times the phonon value near room temperature. Long-range coupling of surface phonon polaritons in the ordered nanoparticle bed, analogous to the extension of phonons in an atomic crystal, enable this enhancement. In contrast to expectations, the surface phonon polaritons dictate the total thermal conductivity of the material due to an apparent quenching of phonon conduction. The effects of nanoparticle diameter and surrounding medium on thermal conductivity data are in excellent agreement with theory, providing strong evidence for significant thermal energy conduction by surface phonon polaritons. We anticipate that the theoretical, and practical, framework established here will enable heat conduction by surface phonon polaritons to be explored in detail experimentally, to build on recent interesting theoretical predictions, and to potentially provide an alternative route to engineer thermally conductive dielectric materials for thermal management.

<p indent="0mm">In the 5G era, the effective thermal management has become more demanding due to the ever-rising integration of electronic devices. High thermal conductivity materials play a crucial role in the field of thermal management, for example, thermal interface materials (TIMs) are used to fill the gap between the electronic chip and the heat sink to improve thermal transfer. In recent years, polymers have become a popular choice for thermal conductive materials because of their light, economical and excellent insulation and processability. To improve the thermal conductivity of materials, inorganic fillers with high thermal conductivity are generally composited with polymers. With the merits of high thermal conductivity, desirable chemical stability, carbon nanotubes (CNTs) are considered to have broad application potential in thermal conductive composites. Simple composition methods failed to increase thermal conductivity of composite materials to expected levels due to the large interfacial thermal resistance between CNTs and polymers and the disorderly distribution of CNTs in polymers. Therefore, reasonable design of polymer composites filled with CNTs is the key to achieving high thermal conductivity. This review mainly introduces the application of CNTs in thermal conductive polymer composites. Based on the existing theoretical research on thermal conductivity of composites and the application of molecular dynamics, more feasible strategies for improving the thermal conductivity of polymer composites filled with CNTs have been proposed. The approaches that improve the thermal conductivity of composites are mainly introduced from three aspects. (1) The intrinsic thermal conductivity of CNTs is an important factor affecting the thermal conductivity of polymer-based composites. CNTs and their macroscopic bulk materials both have excellent thermal conductivity. However, the thermal conductivity test results of the macroscopic materials of CNTs (such as CNTs fibers, arrays, and films) showed that the thermal conductivity of the macroscopic materials of CNTs was much smaller than that of single CNTs due to impurities, defects and inter-tube contact thermal resistance. Studies have shown that purification of CNTs and reduction of intertubular contact thermal resistance can improve the intrinsic thermal conductivity of CNTs. (2) From a microscopic perspective, phonons, the quantized energy of lattice vibration, are the main mechanism of heat conduction in most carbon fillers and polymers. Phonon scattering occurs in the process of phonon transfer, including the scattering between phonons and the scattering at the interface caused by defects and impurities, resulting in thermal resistance. The bonding strength of fillers and polymer interfaces are the crucial factors affecting the transmission of phonons. Hence, the thermal conductivity of composites could be effectively enhanced by surface treatment of CNTs, including covalent functionalization and non-covalent functionalization. Studies have shown that the functionalization can enhance the interfacial interaction between CNTs and polymers, while improving the dispersion of CNTs in polymers. (3) According to the thermal conduction network theory, the key to improving the thermal conductivity is whether the fillers can form a large number of continuous thermal conduction paths in the polymers and maintain a stable existence. However, high CNTs content usually affects the comprehensive properties of composites. To solve this problem, the arrangement and distribution of CNTs in the polymer should be improved to construct more heat conduction pathways, which can achieve high thermal conductivity at a low filling content. Here we introduce some effective methods, including the synergy effect, field orientation and the construction of 3D network structures. In this review, the characteristics and improvement effects of different technical approaches are summarized, which provides a reference for the research and application of CNT-filled polymer-based composites with high thermal conductivity. Finally, the future development prospects of carbon nanomaterial-filled polymer composites are discussed from perspectives of theoretical research, experimental design and engineering application.

The unique properties of the Ge1Sb4Te7 alloy as a chalcogenide make it a good candidate for application in phase-change random access memory as well as thermoelectric materials. The thermal and electrical conductivity of the Ge1Sb4Te7 alloy play an important role in both applications. This work aims to determine the thermal conductivity and electrical resistivity of the Ge1Sb4Te7 alloy as a function of temperature and to discuss the thermal conduction mechanism. Thermal conductivity and electrical resistivity were measured from room temperature to 778 K using the hot strip method and the four-terminal method, respectively. The thermal conductivity of the Ge1Sb4Te7 alloy shows an interesting temperature dependence: it decreases up to about 600 K, and then increases with increasing temperature. The electrical resistivity shows a monotonic increase with increasing temperature. Through a discussion of the thermal conductivity results together with electrical resistivity results, it is proposed that electronic thermal conductivity dominates the thermal conductivity, while the bipolar diffusion contributes to the increase in the thermal conductivity at higher temperatures. The resonance bonding existing in this chalcogenide alloy accounts for the low lattice thermal conductivity.

Filler free technology for enhanced thermally conductive optically transparent polymeric materials using low thermally conductive organic linkers

Chapter 26 - Thermal and Electrical Conductivities

Results of measurements of the thermal and electrical conductivities of polycarbonate samples as a function of chopped pitch-based carbon fiber concentration are reported. The fibers, roughly 500 μm long, were found to be strongly oriented in the composite leading to a highly anisotropic thermal conductivity. Longitudinal thermal con ductivities comparable to that of metallic alloys are obtained in the direction of the fibers, while in the transverse direction only a small increase in the thermal conductivity is observed. We proposed a new model which is a derivation of equations used for con tinuous fibers composites. This model is quite well in accordance with the linear de pendence of the longitudinal thermal conductivity on the fiber concentration of our data. Taking into account the effect of the polymeric matrix, the importance of contact resis tances between fibers and their maximum packing as well as the qualitative differences be tween thermal and electrical conduction is discussed.

Constraining the heat flow across the core-mantle boundary (CMB) is crucial for understanding the thermal history of Earth&amp;#8217;s mantle and the core. The primary mechanism governing heat transfer at the CMB is conduction, with lattice vibration (lattice thermal conductivity) commonly considered to be the dominant mechanism of thermal conduction in the lower mantle. However, there are large uncertainties in current estimates of lattice thermal conductivity of mantle material under CMB condition, due to the influence from mineral composition and the post-perovskite phase transition (e.g., Hsieh et al., 2018 PNAS; Ohta et al., 2017 EPSL). On the other hand, the role of radiative contribution (radiative thermal conductivity) remains less well understood. Several recent studies have attempted to measure the radiative thermal conductivity of bridgmanite and pyrolitic materials under lower mantle conditions, but the resulting experimental data have yielded divergent estimations for the radiative thermal conductivity of average mantle material at CMB conditions, ranging from 0.35 W/(m K) to 4.2 W/(m K) (Lobanov et al., 2020 EPSL; Murakami et al., 2022 EPSL). Adopting the highest estimate could result in an approximate 50% increase in the estimated bulk thermal conductivity compared to conventionally assumed values. &amp;#160; To address the implications of these thermal conductivity uncertainties on mantle convection, we have incorporated variable thermal conductivities into a global thermochemical geodynamic model, StagYY. The simulations use a 2D spherical annulus geometry and extend over a 4.5 Gyr timespan. The geodynamic model includes parameterized core cooling, heat-producing elements partitioning, and crust formation, but it does not include an initial primordial reservoir at CMB. Preliminary findings from our study reveal that the relationship between thermal conductivity and CMB heat flux is not always straightforward. For models with stagnant-lid tectonics, higher thermal conductivity leads to higher CMB heat flux in the initial 1 Gyr and lower CMB heat flux at 4.5 Gyr. However, in models with mobile-lid tectonics, the CMB heat flux also increases with higher thermal conductivity in the first 1 Gyr, but CMB heat flux varies more and becomes unrelated to thermal conductivity at 4.5 Gyr. In summary, deep mantle thermal conductivity has little effect on the present-day CMB heat flux due to plate tectonics on Earth. Varying thermal conductivity mainly influences the amount of core cooling, particularly in early planetary evolution.&amp;#160;

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Polymers are widely used in applications due to their diverse and controllable properties in many physical domains. However, polymers have not historically been used in applications for which a high thermal conductivity is required as bulk polymers are typically thermal insulators. However, research in recent decades on a handful of highly oriented or semi-crystalline polymers has shown the potential for dramatically increased uniaxial thermal conductivity by factors exceeding 100. This dramatic increase in thermal conductivity is because heat is conducted by atomic vibrations along the covalently bonded polymer backbone rather than across chains by weak van der Waals bonds as in unoriented polymers. While it is known that polymers can be processed to yield these properties, much remains unknown about the microscopic transport properties of atomic vibrations in these materials and the true upper limits to thermal conductivity. In this thesis, we address these knowledge gaps by using a combination of simulations and experiments to investigate thermal conduction in semi-crystalline and crystalline polymers. First, we present molecular dynamics simulations of a perfect polymer crystal, polynorbornene. While polymer crystals studied typically exhibit substantially enhanced thermal conductivities above those of the amorphous form, polynorbornene exhibits a glass-like thermal conductivity of less than 1 Wm-1K-1 even as a perfect crystal. This unusual behavior occurs despite the polymer satisfying many of the conventional criteria for high thermal conductivity. Using our simulations, we show that the origin of this unusual behavior is excessively anharmonic bonds and a complex unit cell. Second, we move to experimental studies of thermal transport in polymers. A key requirement to perform materials science is a method to routinely and easily characterize the property of interest in diverse samples. For polymers, this property is typically the in-plane thermal conductivity. This property turns out to be surprisingly difficult to measure using conventional thermal characterization methods. In this work, we adapt transient grating spectroscopy (TG), a well-known method in the chemistry community, to perform in-plane thermal conductivity measurements of polymer films. TG can resolve the in-plane thermal anisotropy of a sample without any physical contact and at tunable length scales, a substantial advance in capability over all prior characterization methods. We extend the application of TG to probe sub-µm length scales, and we successfully apply the technique to numerous poor quality polymer samples as well as thin films. Finally, we exploit the capability of TG to probe thermal conduction over sub-µm length scales to provide the first experimentally resolved microscopic transport properties of atomic vibrations in semi-crystalline polyethylene (PE). Despite the intense interest over decades in PE due to its high intrinsic thermal conductivity, no experimental measurement has yet been able to directly probe the heat-carrying phonons, leading to many questions about the relevant scattering mechanisms and absolute upper limits of thermal conductivity in real samples. Using TG, we present the first observation of quasi-ballistic thermal transport at sub-µm length scales, from which we obtain the phonon mean free path spectra of a semi-crystalline PE sample. Further, we pair these results with Small-Angle X-ray Scattering measurements to show that thermal phonons propagate ballistically within and across nanocrystalline domains, contrary to the conventional viewpoint. These results provide an unprecedented microscopic view of thermal transport in polymer crystals that was previously experimentally inaccessible.

Heat transfer and thermoregulation within single cells revealed by transient plasmonic imaging

Thermal Conductivity and Lattice Vibrational Modes

Recent studies have revealed that the symmetry of interparticle potential plays an important role in the one-dimensional thermal conduction problem. Here we demonstrate that, by introducing strain into the Fermi–Pasta–Ulam-β lattice, the interparticle potential can be converted from symmetric to asymmetric, which leads to a change of the asymptotic decaying behavior of the heat current autocorrelation function. More specifically, such a change in the symmetry of the potential induces a fast decaying stage, in which the heat current autocorrelation function decays faster than power-law manners or in a power-law manner but faster than ~t−1, in the transient stage. The duration of the fast decaying stage increases with increasing strain ratio and decreasing of the temperature. As a result, the thermal conductivity calculated following the Green–Kubo formula may show a truncation-time independent behavior, suggesting a system-size independent thermal conductivity.