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Polymeric composites with boron nitride nanosheets (BNNs), which are thermally conductive yet electrically insulating, have been pursued for a variety of technological applications, especially those for thermal management in electronic devices and systems. Highlighted in this review are recent advances in the effort to improve in-plane thermal transport performance in polymer/BNNs composites and also the growing research activities aimed at composites of enhanced cross-plane or isotropic thermal conductivity, for which various filler alignment strategies during composite fabrication have been explored. Also highlighted and discussed are some significant challenges and major opportunities for further advances in the development of thermally conductive composite materials and their mechanistic understandings. [ABSTRACT FROM AUTHOR]

With the rapid development of high-power-density electronic devices, interface thermal resistance has become a critical barrier for effective heat management in high-performance electronic products. Therefore, there is an urgent demand for advanced thermal interface materials (TIMs) with high cross-plane thermal conductivity and excellent compressibility to withstand increasingly complex operating conditions. To achieve this aim, a promising strategy involves vertically arranging highly thermoconductive graphene on polymers. However, with the currently available methods, achieving a balance between low interfacial thermal resistance, bidirectional high thermal conductivity, and large-scale production is challenging. Herein, we prepared a graphene framework with continuous filler structures in in-plane and cross-plane directions by bonding corrugated graphene to planar graphene paper. The interface interaction between the graphene paper framework and polymer matrix was enhanced via surface functionalization to reduce the interface thermal resistance. The resulting three-dimensional thermal framework endows the polymer composite material with a cross-plane thermal conductivity of 14.4 W·m−1·K−1 and in-plane thermal conductivity of 130 W·m−1·K−1 when the thermal filler loading is 10.1 wt%, with a thermal conductivity enhancement per 1 wt% filler loading of 831%, outperforming various graphene structures as fillers. Given its high thermal conductivity, low contact thermal resistance, and low compressive modulus, the developed highly thermoconductive composite material demonstrates superior performance in TIM testing compared with TFLEX-700, an advanced commercial TIM, effectively solving the interfacial heat transfer issues in electronic systems. This novel filler structure framework also provides a solution for achieving a balance between efficient thermal management and ease of processing. [ABSTRACT FROM AUTHOR]

Heat transfer in layered metal-dielectric structures is considered theoretically based on an analytical solution of the Boltzmann transfer equation for the phonon distribution function. Taking into account the size effect, the problem of effective cross-plane thermal conductivity of structures containing two metal layers is analyzed in detail. If the thickness of the metal layers is less than the phonon mean free path, interlayer heat transfer is carried out predominantly by phonons, and the effective cross-plane thermal conductivity is determined by the reflection of phonons from the metal/dielectric interfaces. In the opposite case of thick metal layers, the effective cross-plane thermal conductivity is determined both by the thermal conductivity of the metal layers and by the thermal resistance of the dielectric layers. The results obtained are generalized to multilayer structures and superlattices. [ABSTRACT FROM AUTHOR]

Aluminum scandium nitride (AlScN) has been receiving increasing interest for radio frequency microelectromechanical systems because of their higher achievable bandwidths owing to the larger piezoelectric response of AlScN compared to AlN. However, alloying scandium (Sc) with aluminum nitride (AlN) significantly lowers the thermal conductivity of AlScN due to phonon alloy scattering. Self-heating in AlScN devices potentially limits power handling, constrains the maximum transmission rate, and ultimately leads to thermal failure. We grew plasma-assisted molecular beam epitaxy (PAMBE) AlScN on AlN-Al2O3 and GaN-Al2O3 substrates, and compared the cross-plane thermal conductivity to current work on AlScN grown on Si substrates.

Thermal transport properties in acceptor‐doped thiophene‐based polymer films such as poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) and poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT) are investigated using the cross‐plane 3‐omega method. A bilayer structure consisting of polymer semiconductors sequentially doped with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and poly(methylmethacrylate) (PMMA) on a Si substrate is originally prepared by spin coating and employed to prevent leakage current from a heater wire during the 3‐omega measurements. Nondoped PBTTT thin film reveals an intrinsic cross‐plane thermal conductivity of 0.49 W m−1 K−1, which is higher than that for nondoped P3HT film (0.20 W m−1 K−1). X‐ray diffraction (XRD) analysis suggests that higher crystallinity of PBTTT films may result in higher thermal conductivity. The intrinsic thermal conductivity of P3HT and the interfacial thermal resistance between PMMA and P3HT are slightly increased at a low doping concentration (0.01 mg mL−1) and decreased at a higher doping concentration of 0.1 mg mL−1, whereas F4TCNQ doping reduces these values monotonously in the case of PBTTT layers. Furthermore, excess doping occurs only for P3HT and increments both intrinsic thermal conductivity and thermal resistance at PMMA/P3HT interface. The observed trend in the cross‐plane thermal transport caused by the sequential doping can be attributed to the differences between P3HT and PBTTT in their crystallinity and effective doping levels. [ABSTRACT FROM AUTHOR]

Aluminum scandium nitride (AlScN) has been receiving increasing interest for radio frequency microelectromechanical systems because of their higher achievable bandwidths owing to the larger piezoelectric response of AlScN compared to AlN. However, alloying scandium (Sc) with aluminum nitride (AlN) significantly lowers the thermal conductivity of AlScN due to phonon alloy scattering. Self-heating in AlScN devices potentially limits power handling, constrains the maximum transmission rate, and ultimately leads to thermal failure. We grew plasma-assisted molecular beam epitaxy (PAMBE) AlScN on AlN-Al2O3 and GaN-Al2O3 substrates, and compared the cross-plane thermal conductivity to current work on AlScN grown on Si substrates. AlScN grown on AlN-Al2O3 and GaN-Al2O3 substrates achieve a better lattice match and a comparable thermal conductivity to AlScN grown on Si substrates, but with significantly thinner films. [ABSTRACT FROM AUTHOR]

Polymeric composites with boron nitride nanosheets (BNNs), which are thermally conductive yet electrically insulating, have been pursued for a variety of technological applications, especially those for thermal management in electronic devices and systems. Highlighted in this review are recent advances in the effort to improve in-plane thermal transport performance in polymer/BNNs composites and also the growing research activities aimed at composites of enhanced cross-plane or isotropic thermal conductivity, for which various filler alignment strategies during composite fabrication have been explored. Also highlighted and discussed are some significant challenges and major opportunities for further advances in the development of thermally conductive composite materials and their mechanistic understandings.

Efficient thermal management of modern electronics requires the use of thin films with highly anisotropic thermal conductivity. Such films enable the effective dissipation of excess heat along one direction while simultaneously providing thermal insulation along the perpendicular direction. This study employs non-equilibrium molecular dynamics to investigate the thermal conductivity of bilayer graphene (BLG) sheets, examining both in-plane and cross-plane thermal conductivities. The in-plane thermal conductivity of 10 nm × 10 nm BLG with zigzag and armchair edges at room temperature is found to be around 204 W/m·K and 124 W/m·K, respectively. The in-plane thermal conductivity of BLG increases with sheet length. BLG with zigzag edges consistently exhibits 30–40% higher thermal conductivity than BLG with armchair edges. In addition, increasing temperature from 300 K to 600 K decreases the in-plane thermal conductivity of a 10 nm × 10 nm zigzag BLG by about 34%. Similarly, the application of a 12.5% tensile strain induces a 51% reduction in its thermal conductivity compared to the strain-free values. Armchair configurations exhibit similar responses to variations in temperature and strain, but with less sensitivity. Furthermore, the cross-plane thermal conductivity of BLG at 300 K is estimated to be 0.05 W/m·K, significantly lower than the in-plane results. The cross-plane thermal conductance of BLG decreases with increasing temperatures, specifically, at 600 K, its value is almost 16% of that observed at 300 K. [ABSTRACT FROM AUTHOR]

With the extensive use of lithium-ion cells, cell thermal safety has been widely concerned and investigated. The thermal conductivity of the cell is one of the important thermal parameters of the battery thermal management system. This work provides rapid and nondestructive measurement of the cross-plane thermal conductivity based on the alternating positive and negative pulse (APNP) test, and this method is verified by the heating plate method. The results show that when the duration of APNP pulse is less than 72 s, the error of cross-plane thermal conductivity is almost within ± 2%, and it indicates that reducing the pulse duration can improve the cross-plane thermal conductivity performance. Additionally, with the cell temperature increasing, the thermal conductivity decreases gradually. This study highlights that the heat dissipation scheme of the cell needs to be adjusted to cope with the deterioration of thermal conductivity as the cell temperature increases. [ABSTRACT FROM AUTHOR]

Improved thermoelectric (TE) property involves low thermal conductivity (k) but high electrical conductivity (σ ) and Seebeck coefficient (S). Experiment has confirmed that interfacial polarization electric (IPE) field (~ 1 MV/cm) of GaN/InxGa1−xN/GaN superlattices (SLs) enhances both S and σ . In this work, role of IPE field on thermal boundary resistance (TBR) and in-plane (kip) as well as cross-plane thermal conductivities (kcp) of indium-rich GaN/InxGa1−xN /GaN SLs (x ≥ 0.7) are explored theoretically. IPE field influences lattice vibrations on account of the inverse piezoelectric effect, resulting in modification of elastic and phonon properties of the SLs. Our results show that TBR is enhanced (2.10–5.30 × 10–9 m2 KW−1) due to unequal changes in phonon velocity and specific heat on both sides of the interface leading to enhanced interface scattering, decreased phonon transmission, and more mismatches of acoustic properties. This caused reduction in kip and kcp of the SL under the action of IPE field. Room temperature (RT) kip in the presence (absence) of IPE field of GaN (10 nm)/InxGa1−xN (5 nm) SL are 8.204(9.402) and 9.312(10.564) Wm−1 K−1 respectively, for x = 0.7 and 0.9, whereas RT kcp for the same x are 4.871(6.012) and 6.083(7.327) Wm−1 K−1 exhibiting more than 20% reduction and are in good agreement with available experimental results of similar type of SLs. This work demonstrates that desired value of k can be achieved by tailoring polarization mechanism of nitride SLs for optimum TE power production at RT and above. [ABSTRACT FROM AUTHOR]

Superior thermoelectric property requires high electrical conductivity (σ ), Seebeck coefficient (S) but low thermal conductivity (k). GaN/InxGa1-xN/GaN superlattice (SL) has a strong interfacial polarization electric (IPE) field ~ 1 MV/cm at hetero-interfaces. Experiments reported that IPE field enhances S and σ of the SL. In this paper, IPE field on thermal boundary resistance (TBR) and cross-plane thermal conductivity (kcp) of the SL is explored for indium contents x = 0.1 and 0.3 theoretically. IPE field revises phonon velocity causing enhanced boundary/interface scattering. Our result shows that TBR is enhanced (2.10–5. 30) ✕ 10–9 m2 KW−1 due to unequal changes in specific heat and phonon velocity on both sides of interface leading to decreased phonon transmission and acoustic properties of material become more dissimilar under IPE field. This reduced kcp. For GaN (10 nm)/InxGa1-xN (5 nm) SL, room-temperature (RT) kcp in the presence (absence) of IPE field is 4.652 (5.720) and 4.282 (5.221) Wm−1 K−1, respectively, for x = 0.1 and 0.3 proving more than 20% reduction. This work demonstrates that desired value of k can be achieved by tailoring interfacial polarization field of SL for optimum power production at RT and above. [ABSTRACT FROM AUTHOR]

Abstract Thermoelectric multilayer thin films used in nanoscale energy conversion have been receiving increasing attention in both academic research and industrial applications. Thermal transport across multilayer interface plays a key role in improving thermoelectric conversion efficiency. In this study, the cross-plane thermal conductivities of nano-constructed Sb2Te3/(Cu, Ag, Au, Pt) thermoelectric multilayer thin films have been measured using time-domain thermoreflectance method. The interface morphology features of multilayer thin film samples were characterized by using scanning and transmission electron microscopes. The effects of interface microstructure on the cross-plane thermal conductivities of the multilayer thin films have been extensively examined and the thermal transfer mechanism has been explored. The results indicated that electron–phonon coupling occurred at the semiconductor/metal interface that strongly affected the cross-plane thermal conductivity. By appropriately optimizing the period thickness of the metal layer, the cross-plane thermal conductivity can be effectively reduced, thereby improving the thermoelectric conversion efficiency. This work presents both experimental and theoretical understanding of the thermal transport properties of Sb2Te3/metal multilayer thin film junctions with important implications for exploring a novel approach to improving the thermoelectric conversion efficiency.

We developed a picosecond transient thermoreflectance (ps-TTR) system for thermal property characterization, using a low-repetition-rate picosecond pulsed laser (1064 nm) as the heating source and a 532 nm CW laser as the probe. Low-repetition-rate pump eliminates the complication from thermal accumulation effect. Without the need of a mechanical delay stage, this ps-TTR system can measure the thermal decay curve from 500 ps up to 1 ms. Three groups of samples are tested: bulk crystals (glass, Si, GaAs, and sapphire); MoS2 thin films (157 ~ 900 nm thickness); InGaAs random alloy and GaAs/InAs digital alloy (short period superlattices). Analysis of the thermoreflectance signals shows that this ps-TTR system is able to measure both thermal conductivity and interface conductance in nanostructures. The measured thermal conductivity values in bulk crystals, MoS2 thin films, and InGaAs random alloy are all consistent with literature values. Cross-plane thermal conductivity in MoS2 thin films does not show obvious thickness dependence. Thermal conductivities of GaAs/InAs digital alloys are smaller than InGaAs random alloy, due to the efficient scattering at interfaces. We also discuss the advantages and disadvantages of this newly developed ps-TTR system comparing with the popular time-domain thermoreflectance system. [ABSTRACT FROM AUTHOR]

Abstract Nanoscale oxide resistive switching structures have attracted widespread attention in connection with future logic, memory, neuromorphic computing, and storage structure applications. Resistive switching effects are usually assumed to be caused by conducting filaments (CFs), which are formed by metal diffusion, and their breaking across the insulating oxide between the metal electrodes. Understanding thermal transport in CF containing metal/oxide based structures is critical to determine whether these filaments exist. This paper reports phonon transport in Cu diffused aluminum oxide (AO) in a Cu/AO/Si resistive switching structure where Cu based CFs have formed locally as a result of electric breakdown. This was studied by measuring cross-plane thermal conductivity on CF containing AO layers from 100 to 500 K. Our results suggest that conferring thermal properties upon non-thermoelectric materials as a result of CF formation enables thermal conductivity to be controlled by CF density. Therefore, the proposed methodology, including a local probing method, will also help understand other physical properties of metal/oxide based resistive switching structures in the future. Graphical abstract Image 1 Highlights • We examined phonon transport of conducting filaments (CFs)-containing AO layer in AO-based resistive switching structure. • CF-containing AO layer exhibits 1.3–2.2 times higher thermal conductivity than that of unprocessed sample. • The density of the CFs plays a strong effect in enhancement of thermal conductivity. [ABSTRACT FROM AUTHOR]

The effect of twisted angle over the wider span on the impact-resistant mechanical properties and thermal conductivity properties has been explored. [Display omitted] • The in-plane and cross-plane thermal conductivity of bilayer graphene at the twist angle of 0°–60° was studied. • The influence of the twist angle on the impact properties of double-layer and triple-layer graphene was explored. • The variation of the ballistic resistance with the twist angle between the layers was analyzed. The twist as a new degree of freedom induces several angle dependent properties in twisted graphene, such as electronic and optical properties. However, the ballistic resistance and thermal conductivity of twisted graphene remain not well understood and experimentally challenging. Therefore, the effect of interlayer twist angle on the thermal conductivity and ballistic resistance of bilayer graphene (BLG) has been explored using molecular dynamics simulation herein. Results show that the thermal conductivity of graphene is very sensitive to the twist angle of the interlayer plane. As the twist angle increases from 0° to 60°, the in-plane thermal conductivity of twisted bilayer graphene (t-BLG) first decreases and then increases with a periodic interval of 30°, exhibiting an asymmetric double V-shaped curve. And the cross-plane thermal conductivity of t-BLG gradually first decreases from 0° to 30° and then increases from 30° to 60°, also showing an asymmetric V-shaped curve. In contrast, the interlayer twist angle has a limited influence on the ballistic resistance of t-BLG. With the change of the twist angle, the ballistic limit velocity does not change much, and the fluctuation range is less than 1.56%; the relative kinetic energy also changes little with the fluctuation less than 4%. Such a slight difference is insufficient to make a significant change in its ballistic resistance and its impact resistance is mainly affected by the number of membrane layers. Each additional layer increases its ballistic limit speed by about 8%. This study will provide some theoretical guidance for the study of heat dissipation in the anti-ballistic process of twisted graphene and the design of advanced protective materials. [ABSTRACT FROM AUTHOR]

As modern electronics are developed towards miniaturisation, high-degree integration and intelligentisation, a large amount of heat will be generated during the operation of devices. How to efficiently remove needless heat is becoming more and more crucial for the lifetime and performance of electronic devices. Many efforts have been made to improve the thermal conductivity of polymer composites, which is an important component of electronics. Herein, the authors report on preparation of boron nitride micosphere/epoxy composites. The cross-plane thermal conductivity of the resultant composites is up to 1.03 Wm^‒1K^‒1. This is attributed to the thermally conductive network formed by the peeled hexagonal boron nitride flakes. Thanks to the superior thermal stability of boron nitride micosphere, the boron nitride micosphere/epoxy composite shows a decreased coefficient of thermal expansion (53.47 ppm/K) and an increased glass transition temperature (147.2°C) compared with the pure epoxy resin. In addition, the boron nitride micosphere/epoxy composite exhibits a lower dielectric constant compared with that of the hexagonal boron nitride/epoxy composite. This strategy can potentially pave the way for the design and fabrication of materials with high cross-plane thermal conductivity and lower dielectric properties.

We measured the lattice temperature distribution, the cross-plane thermal conductivity , and the thermal boundary resistance (TBR) of the As quantum cascade lasers (QCLs) operating at 2.83 THz in the heat sink temperature range 45-300 K. This information was extracted from the analysis of microprobe band-to-band photoluminescence in QCLs operating in continuous wave. Both and TBR decrease monotonically at increasing temperature, the main influence on arising from the high density of interfaces. [ABSTRACT FROM PUBLISHER]

In this work, it is theoretically investigated that interfacial polarization electric (IPE) field of GaN/In x Ga 1-x N superlattice (SL) can be exploited for further reduction of thermal conductivity (k). IPE field modifies SL's elastic constant and phonon velocity as a result of inverse piezoelectric effect. This enhances interfacial scattering and thermal boundary resistance due to unequal changes in specific heat and phonon velocity leading to decreased phonon transmission and more mismatches of acoustic properties causing reduction in in-plane (k ip) and cross-plane (k cp) thermal conductivities. Room temperature k ip in the presence (absence) of IPE field of GaN(10 nm)/In x Ga 1-x N(5 nm) SL are 7.807(8.921), 7.350(8.355), 7.018(8.090), 8.204(9.402) and 9.312(10.564) Wm−1K−1 respectively, for x = 0.1, 0.3, 0.5, 0.7 and 0.9; whereas k cp for the same x are 4.652(5.710), 4.282(5.221), 4.081(5.185), 4.871(6.012) and 6.083(7.327) Wm−1K−1 demonstrating more than 20% reduction. It reveals that desired k can be achieved by tailoring electric field of nitride SL for better thermoelectric improvements. • Interfacial electric field is exploited to reduce thermal conductivity of GaN/In x Ga 1-x N/GaN SLs. • Electric field modifies thermal parameters and phonon properties of SL via inverse piezoelectric effect. • TBR is enhanced from 2.10 to 5.30 × 10−9 m2KW−1 closer to reported value of InGaAs SL. • k cp in presence of field shows more than 20% reduction due to enhanced TBR. • k cp in presence(absence) of field for x = 0.1 and 0.9 are 4.652 (5.710) and 6.083(7.327) Wm−1K−1. [ABSTRACT FROM AUTHOR]

A novel method involving the effect of thermal contact resistance (TCR) was proposed using a plane heat source smaller than the measured samples for improving measurement accuracy of the simultaneous determination of in-plane and cross-plane thermal conductivities and the volumetric heat capacity of anisotropic materials. The heat transfer during the measurement process was mathematically modeled in a 3D Cartesian coordinate system. The temperature distribution inside the sample was analytically derived by applying Laplace transform and the variables separation method. A multiparameter estimation algorithm was developed on the basis of the sensitivity analysis of the parameters to simultaneously estimate the measured parameters. The correctness of the algorithm was verified by performing simulation experiments. The thermophysical parameters of insulating materials were experimentally measured using the proposed method at different temperatures and pressures. Fiber glass and ceramic insulation materials were tested at room temperature. The measured results showed that the relative error was 1.6% less than the standard value and proved the accuracy of the proposed method. The TCRs measured at different pressures were compared with those obtained using the steady-state method, and the maximum deviation was 8.5%. The thermal conductivity obtained with the contact thermal resistance was smaller than that without the thermal resistance. The measurement results for the anisotropic silica aerogels at different temperatures and pressures revealed that the thermal conductivity and thermal contact conductance increased as temperature and pressure increased. [ABSTRACT FROM AUTHOR]

Thermoelectric multilayer thin films used in nanoscale energy conversion have been receiving increasing attention in both academic research and industrial applications. Thermal transport across multilayer interface plays a key role in improving thermoelectric conversion efficiency. In this study, the cross-plane thermal conductivities of nano-constructed Sb2Te3/(Cu, Ag, Au, Pt) thermoelectric multilayer thin films have been measured using time-domain thermoreflectance method. The interface morphology features of multilayer thin film samples were characterized by using scanning and transmission electron microscopes. The effects of interface microstructure on the cross-plane thermal conductivities of the multilayer thin films have been extensively examined and the thermal transfer mechanism has been explored. The results indicated that electron-phonon coupling occurred at the semiconductor/metal interface that strongly affected the cross-plane thermal conductivity. By appropriately optimizing the period thickness of the metal layer, the cross-plane thermal conductivity can be effectively reduced, thereby improving the thermoelectric conversion efficiency. This work presents both experimental and theoretical understanding of the thermal transport properties of Sb2Te3/metal multilayer thin film junctions with important implications for exploring a novel approach to improving the thermoelectric conversion efficiency. [ABSTRACT FROM AUTHOR]