Anisotropic Thermal Conductivity Research Articles

Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al2Te5 monolayers.

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency is still a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al2Te3 and Al2Te5 monolayers. While the former forms a straightforward covalent Al-Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al2Te5, a square planar chain (SPC) known as polytelluride [Te3]2- is neutralized by the covalently bonded [Al2Te2]2+ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, a feature that induces significant anharmonicity in Al2Te5 monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al2Te5 monolayer. The calculated κL values of 1.8 and 0.5 W m-1 K-1 along the a- and b-axes at 600 K are one order of magnitude lower than those of Al2Te3, and even lower than monolayers that contain heavy cations like Bi2Te3. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al2Te5, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (zT). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al2Te5. The pioneering zT of Al2Te5 compared to that of Al2Te3 is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.

Anisotropy of Electrical and Thermal Conductivity in High-Density Graphite Foils

Flexible graphite foils with varying thicknesses (S = 282 ± 5 μm, M = 494 ± 7 μm, L = 746 ± 8 μm) and an initial density of 0.70 g/cm3 were obtained using the nitrate method. The specific electrical and thermal conductivity of these foils were investigated. As the density increased from 0.70 g/cm3 to 1.75 g/cm3, the specific electrical conductivity increased from 69 to 192 kS/m and the thermal conductivity increased from 109 to 326 W/(m·K) due to the rolling of graphite foils. The study showed that conductivity and anisotropy depend on the shape, orientation, and contact area of thermally expanded graphite (TEG) mesoparticles (mesostructural factor), and the crystal structure of nanocrystallites (nanostructural factor). A proposed mesostructural model explained these increases, with denser foils showing elongated, narrowed TEG particles and larger contact areas, confirmed by electron microscopy results. For graphite foils 200 and 750 μm thick, increased density led to a larger coherent scattering region, likely due to the rotation of graphite mesoparticles under mechanical action, while thinner foils (<200 μm) with densities > 1.7 g/cm3 showed increased plastic deformation, indicated by a sharp reduction in the coherent scattering region size. This was also evident from the decrease in misorientation angles with increasing density. Rolling reduced nanocrystallite misorientation angles along the rolling direction compared to the transverse direction (TD) (for 1.75 g/cm3 density ΔMA = 1.2° (S), 2.6° (M), and 2.4° (L)), explaining the observed anisotropy in the electrical and mechanical properties of the rolled graphite foils. X-ray analysis confirmed the preferred nanocrystallite orientation and anisotropy coefficients (A) using Kearns parameters, which aligned well with experimental measurements (for L series foils calculated as: A0.70 = 1.05, A1.30 = 1.10, and A1.75 = 1.16). These calculated values corresponded well with the experimental measurements of specific electrical conductivity, where the anisotropy coefficient changed from 1.00 to 1.16 and mechanical properties varied from 0.98 to 1.13.

Ultralow lattice thermal conductivity in type-I Dirac MBene TiB2

MBenes, the emergent novel two-dimensional family of transition metal borides have recently attracted remarkable attention. Transport studies of such two-dimensional structures are very rare and are of sparking interest. In this paper Using Boltzmann transport theory with ab-initio inputs from density functional theory, we examined the transport in TiB2 MBene system, which is highly dependent on number of layers. We have shown that the addition of an extra layer (as in bilayer BL) destroys the formation of type-I Dirac state by introducing the positional change and tilt to the Dirac cones, thereby imparting the type-II Weyl metallic character in contrast to Dirac-semimetallic character in monolayer ML. Such non-trivial electronic ordering significantly impacts the transport behavior. We further show that the anisotropic room temperature lattice thermal conductivity κ L for ML (BL) is observed to be 0.41 (0.52) and 2.00 (2.04) W m−1 K−1 for x and y directions, respectively, while the high temperature κ L (ML 0.13 W m−1 K−1 and BL 0.21 W m−1 K−1 at 900 K in x direction) achieves ultralow values. Our analysis reveals that such values are attributed to enhanced anharmonic phonon scattering, enhanced weighted phase space and co-existence of electronic and phononic Dirac states. We have further calculated the electronic transport coefficients for TiB2 MBene, where the layer dependent competing behavior is observed at lower temperatures. Our results further unravels the layer dependent thermoelectric performance, where ML is shown to have promising room-temperature thermoelectric figure of merit (ZT) as 1.71 compared to 0.38 for BL.

A hybrid thermo-mechanical phase-field model for anisotropic brittle fracture

The phase-field modeling approach is integrated with the isogeometric-meshfree approach to investigate the coupled thermo-mechanical fracture behaviors of composite materials, accounting for material anisotropy. The present approach is formulated within a thermodynamic framework, exploring the effects of both thermal and mechanical factors on crack evolution. The crack driving force in composites is determined by the collective contributions from both fibers and the matrix. Moreover, a hybrid computational framework, enhanced with an adaptive mesh refinement technique, is proposed for the efficient implementation of thermo-mechanical fracture. Finally, simulations of the thermal deformation and fracture processes in both homogeneous and composite materials are implemented to verify the present approach. The impact of the weak material anisotropy, such as anisotropic thermal conductivity and material strength, along with the strong anisotropy coefficient on fracture patterns in orthotropic composites is also investigated. The simulation results demonstrate that the developed approach can predict complex failure patterns in composites under thermo-mechanical loading, thus offering insights for the design of composites.

Flow and heat transfer characteristics of hydrocarbon fuel in lightweight C/SiC regenerative cooling combustor

To meet the requirements of higher thermal protection ability and lower structural weight of regenerative cooling structures in advanced hypersonic vehicles, carbon-fiber-reinforced silicon carbide (C/SiC) composites with high permittable service temperatures and low densities are promising for application in thermal protection systems. In this study, four cylindrical regenerative cooling configurations applying C/SiC composites for a designated scramjet engine combustor are proposed, and the thermal protection performance under the influence of the cooling equivalence ratio (0.4∼1.0) and anisotropic thermal conductivity is studied. The results show that the C/SiC composites can significantly improve the heat resistance of the entire regenerative cooling structure owing to their relatively low thermal conductivity, which reduces the total heat flux transferred to the inner structure. The thermal protection ability can be maintained, whereas the coolant flow rate is reduced by 50%. C/SiC composites with higher thermal conductivities in the circumferential direction are beneficial for uniform temperature distribution. A flow distribution deterioration phenomenon caused by the thermal cracking of the coolant and the increasing flow resistance in the cooling channels can be observed under certain conditions, leading to a mass flow rate difference of 2.5 times among the cooling channels. Moreover, introducing C/SiC composites into the regenerative cooling structure design can significantly reduce the total structural weight by over 50% compared to traditional superalloys.

Numerical Study on Effect of Flow Field Configuration on Air-Breathing Proton Exchange Membrane Fuel Stacks

Air-breathing proton exchange membrane fuel cells (PEMFCs) show enormous potential in small and portable applications because of their brief construction time without the need for gas supply, humidification and cooling devices. In the current work, a 3D multiphase model of single air-breathing PEMFCs is developed by considering the contact resistance between the gas diffusion layer and bipolar plate and the anisotropic thermal conduction and electric conductive in the through-plane and in-plane directions. The 3D model presents good grid independence and agreement with the experimental polarization curve. The single PEMFC with the best open area ratio of 55% achieves the maximum peak power density of 179.3 mW cm−2. For the fuel cell stack with 10 single fuel cells, the application of the anode window flow field is beneficial to improve the stack peak power density compared to the anode serpentine flow field. The developed model is capable of providing assistance in designing high-performance air-breathing PEMFC stacks.

Study on Thermal Metamaterial for Uniform Surface Temperature Distribution

Abstract This paper proposes a thermal metamaterial based on the theory of thermodynamics transformation, which can achieve uniform temperature distribution. The required anisotropic thermal conductivity is theoretically derived. This thermal metamaterial structure is achieved by alternating aluminum and polyimide of different lengths. The temperature distribution under the condition of different convective heat transfer coefficients was simulated and compared with pure aluminum. The simulation and experimental results show that the metamaterial significantly reduced the non-uniformity of the temperature distribution.

Anisotropic Fourier Heat Conduction and phonon Boltzmann transport equation based simulation of time domain thermo-reflectance experiments

The anisotropic Fourier Heat Conduction Equation (FHCE) and the multidimensional phonon Boltzmann Transport Equation (BTE) were solved numerically in cylindrical coordinates and in time domain to simulate a Time Domain Thermo-Reflectance (TDTR) experimental silicon/aluminum substrate/transducer setup. The out-of-phase response of the probe laser was predicted at various beam offset distances for a pump laser pulse frequency of 80 MHz and modulation frequency of 10 MHz and compared against experimental measurements for a silicon substrate. The isotropic FHCE was also solved for comparison. Results show that the isotropic FHCE with bulk thermal conductivity of 145 W/m/K significantly underpredicts the out-of-phase temperature difference, particularly at smaller beam offsets. With an isotropic thermal conductivity of 105 W/m/K, the computed results match experimental data at smaller beam offsets well, but overpredicts the experimental data at larger beam offsets. An almost-perfect match is obtained by using an anisotropic thermal conductivity wherein the radial (in-plane) thermal conductivity is set to 85 W/m/K and the axial (through-plane) conductivity is set to 130 W/m/K. The multidimensional frequency and polarization dependent phonon BTE is next solved. The BTE results for the out-of-phase temperature difference match experimental observations well at small and intermediate beam offsets, but overpredicts the experimental data at larger beam offsets. FHCE results are fitted to the BTE predictions, and the extracted (best fit) thermal conductivity is found to be 110 W/m/K.

Dielectric, electrical and thermal properties of Sodium Molybdate-hexagonal Boron Nitride composites enabled by cold sintering

Advances in 5G and 6G communication technologies and their applications are in part limited by the capabilities of current electronic packaging materials, as expanded bandwidth is a pressing need for novel dielectric substrates capable of co-firing into packages and devices, characterized by low dielectric loss and enhanced thermal conductivity. This investigation provides further characterization in the dielectric, electrical and thermal conductivity properties over previously reported cold sintered composite of Sodium Molybdate Na2Mo2O7 (NMO) with hexagonal Boron Nitride (hBN), such as relative permittivity (εr) and dielectric loss (tan δ) values at high frequencies of 75–110 GHz, electrical resistivity (ρ) fitting to percolation theory, Weibull statistical analysis of electrical breakdown strength (Eb) and its anisotropic thermal conductivity (κ) influenced by the filler's crystal structure. The cold sintered composites were systematically characterized with respect to filler volume fraction, temperature, and frequency. The findings in this analysis position engineered composites as a promising alternative for microwave substrate materials, with using a densification method that limits interactions and maximizes densification, hence the demonstration with cold sintering.

Improved thermal properties of phase change composites reinforced by Cu nanoparticles modified anisotropic carbon rolls

The scalability of phase change materials (PCMs) for industrial applications is impeded by issues such as leakage during phase transition, suboptimal thermal conductivity, and insufficient photothermal absorption. This research introduces advanced phase change composites (PCCs) reinforced by Cu nanoparticle-enriched porous carbon rolls. These rolls are derived from cotton cloth sheets and are serviced as a scaffold to embed Cu nanoparticles, substantially enhancing interlayer thermal transport. The PCCs are synthesized through a vacuum impregnation process, utilizing paraffin wax as the PCM, yielding a composite with augmented thermal conductivity and improved structural integrity. This innovation elevates anisotropic thermal conductivity values, achieving axial and radial conductivities of 3.4 W/(m·K) and 2 W/(m·K), respectively. The PCCs exhibit advanced solar-thermal-electric energy conversion performance, positioning them as impactful materials for applications in solar energy collection, thermal energy storage, and thermal management.

Vertically aligned carbon fiber/polydimethylsiloxane composites prepared by lyophilization with exceptional thermal conductivity and electromagnetic interference shielding performance

Composites with high thermal conductivity and exceptional electromagnetic interference (EMI) shielding properties are crucial for ensuring the long-term stability of electronic devices. One-dimensional carbon fibers (CFs) stand out as promising materials due to their excellent thermal and electrical conductivity. However, the anisotropic nature of CFs limits the overall electrical and thermal conductivity of the composite, thus impeding its practical application. To address this limitation, we investigated the effect of different freezing methods on CF orientation using the ice template method. Subsequently, CF-150/PDMS and CF-250/PDMS composites with varying average lengths of CFs were prepared using the unidirectional freezing technique. We studied the influence of CF content and average length on the anisotropic thermal conductivity and EMI shielding properties of the composites. The CF-250/PDMS composite, with a CF content of 15.71 vol%, exhibits a remarkable thermal conductivity of 9.07 W·m−1·K−1 and a low thermal resistance of 2.61 °C·cm2‧W−1. The thermal conductive mechanism of CF/PDMS composites was simulated by thermal conduction model and finite element analysis. Furthermore, it demonstrates longitudinal electromagnetic interference shielding effectiveness (EMI SE) reaches up to 39.66 dB, while the transverse EMI SE reaches up to 52.55 dB. The vertically aligned CF/PDMS composites exhibit significant potential for a range of applications in thermal management and EMI shielding.

Spatially resolved lock-in micro-thermography (SR-LIT): A tensor analysis-enhanced method for anisotropic thermal characterization

While high-throughput (HT) computations have streamlined the discovery of promising new materials, experimental characterization remains challenging and time-consuming. One significant bottleneck is the lack of an HT thermal characterization technique capable of analyzing advanced materials exhibiting varying surface roughness and in-plane anisotropy. To tackle these challenges, we introduce spatially resolved lock-in micro-thermography, an innovative technique enhanced by tensor analysis for optical thermal characterization. Our comprehensive analysis and experimental findings showcase notable advancements: We present a novel tensor-based methodology that surpasses the limitations of vector-based analysis prevalent in existing techniques, significantly enhancing the characterization of arbitrary in-plane anisotropic thermal conductivity tensors. On the instrumental side, we introduce a straightforward camera-based detection system that, when combined with the tensor-based methodology, enables HT thermal measurements. This technique requires minimal sample preparation and enables the determination of the entire in-plane thermal conductivity tensor with a single data acquisition lasting under 40 s, demonstrating a time efficiency over 90 times superior to state-of-the-art HT thermology. Additionally, our method accommodates millimeter-sized samples with poor surface finish, tolerating surface roughness up to 3.5 μm. These features highlight an innovative approach to realizing HT and accurate thermal characterization across various research areas and real-world applications.

Anisotropy Reversal of Thermal Conductivity in Silicon Nanowire Networks Driven by Quasi-Ballistic Phonon Transport.

Nanostructured semiconductors promise functional thermal management for microelectronics and thermoelectrics through a rich design capability. However, experimental studies on anisotropic in-plane thermal conduction remain limited, despite the demand for directional heat dissipation. Here, inspired by an oriental wave pattern, a periodic network of bent wires, we investigate anisotropic in-plane thermal conduction in nanoscale silicon phononic crystals with the thermally dead volume. We observed the anisotropy reversal of the material thermal conductivity from 1.2 at 300 K to 0.8 at 4 K, with the reversal temperature of 80 K mediated by the transition from a diffusive to a quasi-ballistic regime. Our Monte Carlo simulations revealed that the backflow of the directional phonons induces the anisotropy reversal, showing that the quasi-ballistic phonon transport introduces preferential thermal conduction channels with anomalous temperature dependence. Accordingly, the anisotropy of the effective thermal conductivity varied from 2.7 to 5.0 in the range of 4-300 K, indicating an anisotropic heat manipulation capability. Our findings demonstrate that the design of nanowire networks enables the directional thermal management of electronic devices.

A molecular dynamics simulation study of thermal transport in hydrazinium cyclo-pentazolate

Cyclo-pentazolate salts (CPSs) as a new type of high-energy-density materials (HEDMs) with high nitrogen content have attracted considerable research attention. In contrast to the extensive studies on their energy properties, the thermal transport process in CPSs has been less studied which relates closely to the thermal safety of this material. Concerning the hydrazinium cyclo-pentazolate (HCP), we conduct a computational study to estimate the thermal conductivity of HCP by means of the non-equilibrium molecular dynamics (NEMD) simulation. To achieve that, we have customized interaction parameters based on the default OPLS force field for the HCP, as benchmarked by its crystal structure. Our simulations have revealed surprisingly anisotropic thermal conductivity for the HCP, while the thermal conductivity becomes highest roughly in the direction perpendicular to cyclo-N 5− with its value notably higher than common high explosives. By modulating the interaction parameters within the HCP molecule, we have further captured a dominant role of the interaction between cyclo-N 5− in regulating the thermal conductivity of the HCP. The attractive Lennard–Jones (LJ) potential may restrict the relative motion between cyclo-N 5− , which forms a long-range order thus enhances thermal transport in the direction perpendicular to cyclo-N 5− . Our simulations result on the effect of (cyclo-N 5− )-(cyclo-N 5− ) interaction provide insights to engineer thermal transport in CPSs at the molecular level.

Mesoscopic combinatorial design of anisotropic graphitic carbon with ordered porous frameworks for thermoelectric conversion

The incorporation of carbon materials can effectively enhance the performance of thermoelectric materials. By optimizing the thermoelectric properties of the carbon scaffold, it is possible to further improve the thermoelectric performance of the prepared carbon-based composites. To elevate the thermoelectric performance of the carbon scaffold, this work introduces a novel mesoscopic design strategy, utilizing natural flake graphite to create a three-dimensional ordered porous framework. Highly controllable anisotropic conductivity and thermal conductivity of the porous graphite skeleton were achieved, by combining sizable sodium chlorine templates (37.5–250 μm) with a natural flake graphite matrix (50–150 μm). A porous graphite with high electrical conductivity and low thermal conductivity along the vertical axis was obtained by carefully regulating the parameters. The sample demonstrated exceptional thermoelectric conversion performance, achieving a thermoelectric power factor of 12.2 μW m−1 K−2 and a figure of merit value of 28.3 × 10−4. Furthermore, the thermoelectric property of the samples could be greatly improved by adding a small amount of ferric chloride, with a thermoelectric power factor reaching 41.9 × 10−4 and a thermoelectric figure of merit reaching 81.4 μW m−1 K−2. This work provides an effective scheme for the mesoscopic regulation of porous graphite carbon structures and offers a new strategy for enhancing the performance of thermoelectric generators.

Anisotropy of Thermal Conductivity and P-wave Velocity in Sedimentary Soft Rocks and their Relationships with Porosity

Anisotropy of Thermal Conductivity and P-wave Velocity in Sedimentary Soft Rocks and their Relationships with Porosity

Unleashing the Potential of Boron Nitride Spheres for High‐Performance Thermal Management

AbstractHighly integrated and miniaturized electronic devices require advanced thermal management techniques to improve reliability and performance. Thanks to their high thermal conductivity and electrical insulation, boron nitride nanosheets (BNNSs) are commomly used as fillers to construct thermally conductive polymer composites for heat dissipation. However, the BNNS reinforced composites exhibit anisotropic thermal conductivity due to the anisotropic structure of BNNSs. Micro‐sized boron nitride spheres (BNSs) with isotropic thermal conductivity are considered one of the best solutions. Nevertheless, precisely measuring the thermal conductivity of BNSs remains a challenge, limiting the understanding of the thermal transport mechanism. Herein, we have successfully estimated the thermal conductivity of BNSs using the laser flash method. Factors influencing BNSs’ thermal conductivity, including precursor, polymer binder and sintering temperature, are also investigated. Under optimized conditions, BNSs exhibit high, isotropic thermal conductivity of 37.2±2.5 W/(m ⋅ K), and the BNS pellet outperforms its h‐BN counterpart in heat dissipation for an LED light. This superiority is attributed to outstanding heat transfer performance in the cross‐plane direction, in addition to high in‐plane thermal conductivity. This study provides a feasible method to estimate the thermal conductivity of spherical materials and highlights promising boron nitride materials with isotropic thermal conductivity for heat dissipation in advanced electronics.

Group III (In/Ga)-V (P/As)-VI (S/Se) Monolayers: A New Class of Auxetic Semiconductors with Highly Anisotropic Electronic/Optical/Mechanical/Thermal Properties.

We present a theoretical design of a class of 2D semiconducting materials, namely, group III (In/Ga)-V (P/As)-VI (S/Se) monolayers, whose global-minimum structures are predicted based on the particle swarm optimization method. Electronic structure calculations suggest that all group III-V-VI monolayers exhibit quasi-direct semiconducting characteristics with desirable band gaps ranging from 1.76 to 2.86 eV (HSE06 functional). Moreover, most group III-V-VI monolayers possess highly anisotropic carrier mobilities with large anisotropic ratios (3.4-6 for electrons, 2.2-25 for holes). G0W0+BSE calculations suggest that these monolayers show high optical anisotropy and relatively large exciton binding energies (0.33-0.75 eV), comparable to that (0.5 eV) of MoS2 monolayer. In particular, the GaPS monolayer manifests strikingly anisotropic I-V curves with a large ON/OFF ratio of ∼105 (106 for the GaPS bilayer) and anisotropic lattice thermal conductivity. Furthermore, the GaPS monolayer is predicted to exhibit both in-plane and out-of-plane negative Poisson ratios (NPRs) and prominent anisotropic Young moduli.

Unraveling High Thermal Conductivity with In-Plane Anisotropy Observed in Suspended SiP2.

The anisotropic thermal transport properties of low-symmetry two-dimensional materials play an important role in understanding heat dissipation and optimizing thermal management in integrated devices. Examples of efficient energy dissipation and enhanced power sustainability have been demonstrated in nanodevices based on materials with anisotropic thermal transport properties. However, the exploration of materials with high thermal conductivity and strong in-plane anisotropy remains challenging. Herein, we demonstrate the observation of anisotropic in-plane thermal conductivities of few-layer SiP2 based on the micro-Raman thermometry method. For suspended SiP2 nanoflake, the thermal conductivity parallel to P-P chain direction (κ∥b) can reach 131 W m-1 K-1 and perpendicular to P-P chain direction (κ⊥b) is 89 W m-1 K-1 at room temperature, resulting in a significant anisotropic ratio (κ∥b/κ⊥b) of 1.47. Note that such a large anisotropic ratio mainly results from the higher phonon group velocity along the P-P chain direction. We also found that the thermal conductivity can be effectively modulated by increasing the SiP2 thickness, reaching a value as high as 202 W m-1 K-1 (120 W m-1 K-1) for κ∥b (κ⊥b) at 111 nm thickness, which is the highest among layered anisotropic phosphide materials. Notably, the anisotropic ratio always remains at a high level between 1.47 and 1.68, regardless of the variation of SiP2 thickness. Our observation provides a new platform to verify the fundamental theory of thermal transport and a crucial guidance for designing efficient thermal management schemes of anisotropic electronic devices.

Thermally Conductive Polymer Composites with Hexagonal Boron Nitride for Medical Device Thermal Management

Polymer composites with hexagonal boron nitride (hBN) have the potential to meet the heat dissipation and electrical insulation requirements of electronic and medical devices. In this study, hBN/polymer composites were fabricated with thermoplastic polyurethane (TPU) and epoxy as matrix materials. Three hBN powder types (BN1, BN2, BN3) with different platelet sizes and degrees of agglomeration were used. BN1 (with average size of 1 μm) and BN2 (with average size of 12 μm) mainly contained hBN platelets, while BN3 (with average size of 20 μm) contained both platelets and agglomerates of platelets. BN2 gave the highest thermal conductivity, while BN1 gave the lowest thermal conductivity. Thermal simulations were performed for a medical device with a limit for the maximum surface temperature. For the encapsulation of this device, several material combinations were simulated, using anisotropic thermal conductivity data. This included encapsulations with inner layers of commercial thermally and electrically conductive materials and an outer layer of the best thermally conductive (and electrically insulating) hBN/TPU composite fabricated by the authors.