High Intrinsic Thermal Conductivity Research Articles

A Polymer Film with Very High Seebeck Coefficient and Overall Thermoelectric Properties by Secondary Doping, Dedoping Engineering and Ionic Energy Filtering

AbstractThermoelectric (TE) materials are significant for sustainable development because they can be used to harvest waste heat into electricity. Organic TE materials have unique merits including high mechanical flexibility, low cost, and low intrinsic thermal conductivity. But their TE properties particularly the Seebeck coefficient are notably lower than the inorganic counterparts. Here, a poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) film is reported with a very high Seebeck coefficient and figure of merit (zT) at room temperature through secondary doping, dedoping engineering, and ionic energy filtering. The PEDOT:PSS films are successively treated with acid, base, and vitamin C that is a reductant, and then coated with 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA) that is an ionic liquid. This can exhibit a record‐high Seebeck coefficient and power factor of 111 µV K−1 and 1285 µW m−1 K−2, respectively, and the corresponding zT value is 1.05. This is the highest zT value for polymers and polymer composites, and it is comparable to that of the best inorganic TE materials.

PEDOT:PSS Films With Very High Thermoelectric Properties Through Water‐Swollen Assisted Reduction With a Tetrakis(dimethylamino)ethylene Solution

AbstractThermoelectric (TE) materials have attracted significant attention because they can be used to directly harvest waste heat into electricity. Organic TE materials like poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have advantages including high mechanical flexibility, low cost, and low intrinsic thermal conductivity. However, their TE properties are notably inferior to their inorganic counterparts. Here, PEDOT:PSS films with very high thermoelectric properties through the sequential treatments with H2SO4, water, and ethanol solution of tetrakis(dimethylamino)ethylene (TDAE) are reported for the first time. The PEDOT:PSS films can exhibit a Seebeck coefficient of 58.2 µV K−1 and electrical conductivity of 1552 S cm−1, and the corresponding power factor is 526 µW m−1 K−2. The water treatment prior to the TDAE solution treatment is crucial for the high TE properties of PEDOT:PSS films. Without the water treatment, the control PEDOT:PSS films exhibit a Seebeck coefficient of only 32.4 µV K−1 and electrical conductivity of 1124 S cm−1, and the corresponding power factor is only 118 µW m−1 K−2. The effect of the water treatment on the Seebeck coefficient is attributed to the swelling of PEDOT:PSS films, which facilitates the penetration of TDAE from solution into the polymer films and thus the reduction of PEDOT:PSS.

Excellent thermoelectric performance of Fe2NbAl alloy induced by strong crystal anharmonicity and high band degeneracy

Full-Heusler alloys with earth-abundant elements exhibit high mechanical strength and favorable electrical transport behavior, but their high intrinsic lattice thermal conductivity limits potential thermoelectric application. Here, the thermoelectric transport properties of Fe-based Full-Heusler Fe2MAl (M = V, Nb, Ta) alloys are comprehensively investigated utilizing density functional theory. The results suggest that Fe2NbAl exhibits exceptionally low lattice thermal conductivity due to low phonon velocities and weakly bound Nb atoms. In Fe2NbAl, the underbonding of the Nb atoms leads large Grüneisen parameters and high anharmonic scattering rates of low-frequency acoustic phonon. Meanwhile, the high band degeneracy and large electrical conductivity lead to a maximum p-type power factor of 255.6 μW·K−2·cm−1 at 900 K. The combination of low lattice thermal conductivity and favorable electrical transport properties leads a maximum p-type dimensionless figure of merit of 1.7. Our work indicates Fe2NbAl, as a low-cost, environmentally friendly, is a potential high-performance p-type thermoelectric material.

Flexible hexagonal boron nitride@liquid metal/polydimethylsiloxane composites with excellent thermal conductivity and superior mechanical properties

AbstractHexagonal boron nitride (h‐BN) has a layered lattice structure, high intrinsic thermal conductivity, and good chemical stability, and it has a promising applications in thermal management materials. However, poor interfacial compatibility and dispersion of BN flakes in the polymer matrix cause thermal contact resistance and gaps, directly impairing the performance of the composites and limiting their thermal conduction applications. Herein, the P‐BN@LM/PDMS thermally conductive composites with superior mechanical properties were fabricated by incorporating dopamine and liquid metal (LM) into BN flakes. The dopamine self‐polymerizes to form polydopamine (PDA) on the BN surface, denoted as P‐BN, which effectively improves interface compatibility and dispersion. The LM attaches to the functionalized P‐BN surface by mechanical grinding, acting as a bridge between adjacent BN flakes to enhance the integrity of the thermal conductive network within the composites. The coordination between P‐BN and LM increases interface strength, effectively improving mechanical performance. Hence, the P‐BN@LM/PDMS composites exhibit excellent thermal conductivity (4.0 W/mK) and an enhancement factor of 1373%, which is 2.22 times that of BN/PDMS composites with the same 30 wt% loading. Additionally, the composite shows superior tensile strength (2.11 MPa) and elongation at break (121%), representing 441% and 203% improvements compared with pure polydimethylsiloxane (PDMS). The P‐BN@LM/PDMS composites, with significant advantages in thermal conductivity and mechanical properties, are promising for future applications as flexible thermal interface materials in thermal management.Highlights The P‐BN@LM/PDMS composites exhibit excellent thermal conductivity (4.0 W/mK) and an enhancement factor of 1373%, making them promising for future applications as flexible thermal interface materials in thermal management. The P‐BN@LM/PDMS composites present a superior tensile strength (2.11 MPa) and elongation at break (121%). PDA formed by the self‐polymerization of dopamine on the BN surface effectively enhances interfacial compatibility and dispersion. LM attaches to the functionalized BN surface through mechanical grinding, acting as a bridge between adjacent BN flakes, thus enhancing the integrity of the thermal conductive network within the composites.

Multi‐layer graphene nanosheets bridging binary aluminium oxide for the synergistic enhancement of thermal conductivity and electrical insulation of silicone resin composite

AbstractThe flexible polymer composites usually require high intrinsic thermal conductivity fillers for the applications in thermal management, such as the well‐known graphene, which can, in turn, compromise their electrical insulation properties. Here, the authors developed the silicone resin (SR) composites filled with multi‐layer graphene nanosheets (MGN) and binary alumina (Al2O3), and the microstructure of composites exhibits the dominant skeleton of large‐sized Al2O3 filler and branching of small‐sized Al2O3 fillers features by the size matching effect of optimal binary Al2O3. The introduction of supplementary MGN fillers further increases the face‐to‐face contact probability and bridges the isolated binary Al2O3, which contributes to the high thermal conductivity of 3.0 W·m−1·K−1 under these synergistic effects. The dense and connected ‘Al2O3‐MGN‐Al2O3’ thermal conductive pathway is successfully built, as supported by the numerical simulation and Agari model fitting. Meanwhile, the electrical conductivity caused by MGN can be blocked by the surrounding insulation Al2O3 filler network, exhibiting a high volume resistivity of 1.7 × 1015 Ω · cm. Moreover, the Al2O3/MGN/SR composite films effectively transfer the heat and diminish the hot‐spot temperature by 53.5°C in cooling light emitting diode chip module application.

Modulating cross-linked network structure of epoxy resin blends towards concurrently high intrinsic thermal conductivity and dielectric properties

Modulating cross-linked network structure of epoxy resin blends towards concurrently high intrinsic thermal conductivity and dielectric properties

Tutorial: AI-assisted exploration and active design of polymers with high intrinsic thermal conductivity

Designing polymers with high intrinsic thermal conductivity (TC) is critically important for the thermal management of organic electronics and photonics. However, this is a challenging task owing to the diversity of the chemical space and the barriers to advanced synthetic experiments/characterization techniques for polymers. In this Tutorial, the fundamentals and implementation of combining classical molecular dynamics simulation and machine learning (ML) for the development of polymers with high TC are comprehensively introduced. We begin by describing the core components of a universal ML framework, involving polymer data sets, property calculators, feature engineering, and informatics algorithms. Then, the process of constructing interpretable regression algorithms for TC prediction is introduced, aiming to extract the underlying relationships between microstructures and TCs for polymers. We also explore the design of sequence-ordered polymers with high TC using lightweight and mainstream active learning algorithms. Lastly, we conclude by addressing the current limitations and suggesting potential avenues for future research on this topic.

Damping and mechanical behavior of graphene nanoplatelets reinforced Al-30Zn alloy matrix bioinspired laminated composites by two steps flake powder metallurgy

Damping and mechanical properties are often incompatible in metallic materials. In this paper, bio-inspired laminated graphene nanoplatelets (GNPs) reinforced Al-30Zn alloy matrix composites with an outstanding combination of damping-mechanical properties are prepared using a flake powder metallurgy (FPM) process. The increased grain/phase boundary owing to grain refinement improves the intrinsic damping properties of the composites, while the lamellar grains promote the oriented alignment and crack deflection of GNPs. The multilayered GNPs with large specific surface area experienced repeated stretching and compression during cyclic bending deformation, leading to intense interfacial frictional slip, and GNPs' high intrinsic thermal conductivity also helped to dissipate heat quickly, which promoted mechanical energy dissipation. The improvement in mechanical properties is mainly attributed to grain refinement, the load transfer effect of GNPs and their interaction with dislocations. The Al-30Zn-0.5GNPs composites showed the best combination of damping properties and mechanical properties.

AI-assisted inverse design of sequence-ordered high intrinsic thermal conductivity polymers

Artificial intelligence (AI) promotes the polymer design paradigm from a traditional trial-and-error approach to a data-driven style. Achieving high thermal conductivity (TC) for intrinsic polymers is urgent because of their importance in the thermal management of many industrial applications such as microelectronic devices and integrated circuits. In this work, we have proposed a robust AI-assisted workflow for the inverse design of high TC polymers. By using 1144 polymers with known computational TCs, we construct a surrogate deep neural network model for TC prediction and extract a polymer-unit library with 32 sequences. Two state-of-the-art multi-objective optimization algorithms of unified non-dominated sorting genetic algorithm III (U-NSGA-III) and q-noisy expected hypervolume improvement (qNEHVI) are employed for sequence-ordered polymer design with both high TC and synthetic possibility. For triblock polymer design, the result indicates that qNHEVI is capable of exploring a diversity of optimal polymers at the Pareto front, but the uncertainty in Quasi-Monte Carlo sampling makes the trials costly. The performance of U-NSGA-III is affected by the initial random structures and usually falls into a locally optimal solution, but it takes fewer attempts with lower costs. 20 parallel U-NSGA-III runs are conducted to design the pentablock polymers with high TC, and half of the candidates among 1921 generated polymers achieve the targets (TC > 0.4 W m−1K−1 and SA < 3.0). Ultimately, we check the TC of 50 promising polymers through molecular dynamics simulations and reveal the intrinsic connections between microstructures and TCs. Our developed AI-assisted inverse design approach for polymers is flexible and universal, and can be extended to the design of polymers with other target properties.

Electric-field-aligned liquid crystal polymer for doubling anisotropic thermal conductivity

High thermal conductive polymers have become more important because equipment requires high performance, high-energy density, and high integration. There are different strategies to make high thermal conductive polymers, among which is the synthesis of polymers in the liquid crystal phase. However, the thermal conductivity of such material is rarely beyond 1 W m−1 K−1 because of the disordered molecular directionality. The disordered directionality between crystal zones limits the thermal conductivity in a specific direction. Here, we show a method for unifying the direction of crystal zones by applying an external electric field on the liquid crystal monomers. Meanwhile, by exposing the transparent equipment and specially designed photopolymerisable monomer in UV light, the liquid crystal monomer is in situ polymerised into a liquid crystal polymer with a high intrinsic thermal conductivity of 1.02 W m−1 K−1. The molecular alignment was characterised and resulted in the resultant high conductivity.

Low dielectric constant and highly intrinsic thermal conductivity fluorine‐containing epoxy resins with ordered liquid crystal structures

AbstractEpoxy resins with a high dielectric constant and low intrinsic thermal conductivity coefficient cannot meet the current application requirements of advanced electronic and electrical equipment. Therefore, novel fluorine‐containing liquid crystal epoxy compounds (TFSAEy) with fluorinated groups, biphenyl units, and flexible alkyl chains are first synthesized via amidation and esterification reactions. Then, 4,4′‐diaminodiphenylmethane (DDM) is used as a curing agent to prepare the corresponding fluorine‐containing liquid crystal epoxy resins. The obtained dielectric constant (ε) and dielectric loss (tan δ) values of TFSAEy/DDM at 1 MHz are 2.54 and 0.025, respectively, which are significantly lower than those of conventional epoxy resins (E‐51/DDM, 3.52 and 0.038). Additionally, the intrinsic thermal conductivity coefficient (λ) of TFSAEy/DDM is 0.36 W/(m·K), 71.4% higher than that of E‐51/DDM (0.21 W/(m·K)). Meanwhile, the corresponding elastic modulus, hardness, glass transition temperature, and heat resistance index of TFSAEy/DDM are 5.73 GPa, 0.35 GPa, 213.5°C, and 188.7°C, respectively, all superior to those of E‐51/DDM (3.68 GPa, 0.27 GPa, 107.2°C, and 174.8°C), presenting potential application in high‐heating electronic component packaging and printed circuit boards.

Phonon Thermal Transport at Interfaces of a Graphene/Vertically Aligned Carbon Nanotubes/Hexagonal Boron Nitride Sandwiched Heterostructure

Molecular dynamics simulation is used to calculate the interfacial thermal resistance of a graphene/carbon nanotubes/hexagonal boron nitride (Gr/CNTs/hBN) sandwiched heterostructure, in which vertically aligned carbon nanotube (VACNT) arrays are covalently bonded to graphene and hexagonal boron nitride layers. We find that the interfacial thermal resistance (ITR) of the Gr/VACNT/hBN sandwiched heterostructure is one to two orders of magnitude smaller than the ITR of a Gr/hBN van der Waals heterostructure with the same plane size. It is observed that covalent bonding effectively enhances the phonon coupling between Gr and hBN layers, resulting in an increase in the overlap factor of phonon density of states between Gr and hBN, thus reducing the ITR of Gr and hBN. In addition, the chirality, size (diameter and length), and packing density of sandwich-layer VACNTs have an important influence on the ITR of the heterostructure. Under the same CNT diameter and length, the ITR of the sandwiched heterostructure with armchair-shaped VACNTs is higher than that of the sandwiched heterostructure with zigzag-shaped VACNTs due to the different chemical bonding of chiral CNTs with Gr and hBN. When the armchair-shaped CNT diameter increases or the length decreases, the ITR of the sandwiched heterostructure tends to decrease. Moreover, the increase in the VACNT packing density also leads to a continuous decrease in the ITR of the sandwiched heterostructure, attributed to the extremely high intrinsic thermal conductivity of CNTs and the increase of out-of-plane heat transfer channels. This work may be helpful for understanding the mechanism for ITR in multilayer vertical heterostructures, and provides theoretical guidance for a new strategy to regulate the interlayer thermal resistance of heterostructures by optimizing the design of sandwich layer thermal interface materials.

Forced convection heat transfer inside graphene aerogel

Ultralight heat-dissipating materials are essential for the effective thermal management of mobile devices. Graphene aerogel is a highly porous material made of thermally conductive carbon nanomaterial. Owing to aerogel's highly porous structure and graphene's high intrinsic thermal conductivity, graphene aerogel has a very low density and could be a promising ultralight thermal management material. In this study, forced convective heat transfer inside graphene aerogel is experimentally examined, and the effect of the aerogel microstructure on the heat and fluid flow characteristics of the graphene aerogel is discussed. It is established that graphene aerogel has a better convective heat transfer coefficient compared to conventional porous media. Furthermore, methods to improve the heat and fluid flow inside the aerogel are discussed. This work sheds light on the effective thermal management of various devices where lightness is critical.

Design of efficient thermal conductive epoxy resin composites via highspeed transport pathways of heterogeneous compatible carbon framework

Thermal interface materials are crucial for addressing the hot issues of a rapid increase in thermal density in narrow and limited service spaces. Flexible and designable epoxy resin (EP) based composites are competitive choice yet lacks desirable thermal conductivity (∼0.2 W m−1 K−1) and mechanical properties (tensile strength: ∼19.6 MPa). Herein, EP-based composites with a reinforced three-dimensional (3D) interconnected carbon material architecture were prepared by in-situ growing 1D carbon nanotubes (CNTs) on the surface of 2D carbon fiber braid (CFB) and infiltrating matrix EP. CNTs not only promote the wettability between carbon fibers inside CFB and EP but also observably bridge the adjacent carbon fibers. The analysis of numerical models reveals the prominent contribution of 3D CFB/CNTs network to a significant increase in thermal conductivity. Non-equilibrium molecular dynamics (NEMD) indicates the high intrinsic thermal conductivity of CNTs in both systems: single CNT model and CNT/Ni model. The coupling behavior of high-frequency phonons at the interface contributes to the in-plane thermal transport. The in-plane thermal conductivity of 7.86 W m−1 K−1 and the through-plane thermal conductivity of 5.85 W m−1 K−1 are obtained in the composites with 23.2 wt% hybrid fillers, increased by 3830 % compared to neat EP. The tensile (58.93 MPa) and compressive strength (138.83 MPa) are also enhanced, meeting practical demands. These properties even perform no obvious changes after 100 cycles of bending. The stable and reliable EP-based composites with outstanding comprehensive performance designed by this work have enormous application potential in the advanced heat dissipation system.

Investigations on thermal contact resistance between filled polymer composites and solids using micro thermography

This article reports the use of a new measurement technique based on micro thermography for determining the thermal contact resistances (TCRs) between filled polymers and solids. The thermal conductivity of polymers can be significantly increased by using thermally conductive fillers. For numerous applications, not only is a high intrinsic thermal conductivity required but also a good thermal transfer between the filled polymer and an adjacent solid surface. The physical principles of thermal transport when considering this type of contact have not yet been investigated in detail, and only a few experimental results are available. The most common measurement techniques determine a macroscopic resistance and project it onto the contacting surface. However, the heterogeneous microstructure of a filled polymer causes the TCR to be a volumetric phenomenon in the overall boundary region. The utilized IR camera system takes thermal images with a spatial resolution of less than per pixel. The new method resolves the TCRs spatially and gives new insights into the microscale effects on the particle level. In addition to the common zero-gap extrapolation for the extraction of TCRs, we propose another evaluation method that considers all microscale effects of the boundary layers and evaluates TCR as a volumetric phenomenon. For the first systematic study, samples consisting of two aluminum substrates and a filled epoxy polymer were prepared and investigated. We studied the effects of filler size, filler material, filler volume fraction, and surface structure, focusing on monomodally filled polymers with filler amounts between and . The obtained results and the uncertainties of the new method are discussed within this paper.

AgCl Addition to Chalcopyrite Compound for Ultra-Low Thermal Conductivity in Realizing High ZT Thermoelectric Materials.

Optimizing the performance of thermoelectric materials by reducing its thermal conductivity is crucial to enhance its thermoelectric efficiency. Novel thermoelectric materials like the CuGaTe2 compound are hindered by high intrinsic thermal conductivity, which negatively impacts its thermoelectric performance. In this paper, we report that the introduction of AgCl by the solid-phase melting method will influence the thermal conductivity of CuGaTe2. The generated multiple scattering mechanisms are expected to reduce the lattice thermal conductivity while maintaining sufficient good electrical properties. The experimental results were supported by first-principles calculations confirming that the doping of the Ag will decrease the elastic constants, bulk modulus, and shear modulus of CuGaTe2, which makes the mean sound velocity and Debye temperature of Ag-doped samples lower than those of CuGaTe2, indicating the lower lattice thermal conductivity. In addition, the Cl elements within the CuGaTe2 matrix escaping during the sintering process will create holes of various sizes within the sample. These combined effects of holes and impurities will induce phonon scattering, which further reduces the lattice thermal conductivity. Based on our findings, we conclude that the introduction of AgCl into CuGaTe2 has shown a lower thermal conductivity without compromising the electrical performance, resulting in an ultra-high ZT value of 1.4 in the (CuGaTe2)0.96(AgCl)0.04 sample at 823 K.

Revealing the promising near-room-temperature thermoelectric performance in Ag2Se single crystals

With high carrier mobility and intrinsic low lattice thermal conductivity, Ag2Se compounds have attracted increasing attention for thermoelectric application near room temperature. Due to its phase transition at ∼406 K and resulting thermal volume expansion, the growth and thermoelectric properties of large-sized Ag2Se single crystals have seldom been reported so far. In this work, the vertical Bridgeman method was used for growing bulk Ag2Se single crystal, with an orientation preference along low-symmetric (201) plane. The Hall mobility as high as 2 000 cm2/(V·s) and weak electron-phonon coupling contributes to a high electronic quality BE of ∼7.0 in near-room-temperature β-Ag2Se single crystals, which is superior to the high-temperature phase α-Ag2Se. The observed low lattice thermal conductivity of 0.8 W/(m·K) at 300 K is due to the low group speeds and strong anharmonicity. A promising peak zT of 0.66 at 375 K and an average zT of 0.65 at 300–375 K were realized in β-Ag2Se crystals. The low Vickers hardness and good ductile properties were confirmed by experiment and theoretical analysis. This work not only synthesized large-sized and highly-orientated Ag2Se crystals, but also revealed its great potential of thermoelectric performance and mechanical properties for various applications near room temperature.

High Thermal Conductivity of Sandwich-Structured Flexible Thermal Interface Materials.

Thermal interfaces are vital for effective thermal management in modern electronics, especially in the emerging fields of flexible electronics and soft robotics that impose requirements for interface materials to be soft and flexible in addition to having high thermal performance. Here, a novel sandwich-structured thermal interface material (TIM) is developed that simultaneously possesses record-low thermal resistance and high flexibility. Frequency-domain thermoreflectance (FDTR)is employed to investigate the overall thermal performance of the sandwich structure. As the core of this sandwich, a vertically aligned coppernanowire (CuNW) array preserves its high intrinsic thermal conductivity, which is further enhanced by 60% via a thick 3D graphene (3DG) coating. The thin copper layers on the top and bottom play the critical roles in protecting the nanowires during device assembly. Through the bottom-up fabrication process, excellent contacts between the graphene-coated CuNWs and the top/bottom layer are realized, leading to minimal interfacial resistance. In total, the thermal resistance of the sandwich is determined as low as ~0.23mm2 KW-1 . This work investigates a new generation of flexible thermal interface materials with an ultralow thermal resistance, which therefore renders the great promise for advanced thermal management in a wide variety of electronics.

High-throughput screening of amorphous polymers with high intrinsic thermal conductivity via automated physical feature engineering

An interpretable informatics algorithm-assisted framework has been proposed and applied for the exploration of high thermal conductivity polymers by integrating automated physical feature engineering and symbolic regression.

Nanoarchitectonics of graphene oxide with functionalized cellulose nanocrystals achieving simultaneous dual connections and defect repair through catalytic graphitization for high thermal conductivity

Graphene, with an extremely high intrinsic thermal conductivity of 5300 W/mK, is of special interest to meet thermal requirements in future wearable and high-power electronic devices. Generally, microscopic graphene sheets can be built into a macroscopic material by using graphene oxide (GO) as the precursor. However, both the defects on the basal plane of GO sheets and the lack of connections at their boundaries induce massive scattering of phonons and a low thermal conductivity. Herein, we solve the problem through the nanoarchitectonics of GO with nano Fe3O4 functionalized cellulose nanocrystals (Fe–CNC). During the assembly process, Fe–CNC were compounded with GO homogeneously and formed a composite GO/Fe–CNC film with an aligned lamellar structure. A subsequent thermal reduction at 1500 °C resulted in the generation of Fe/Fe3C nanoparticles in the matrix of the reduced GO/Fe–CNC. During this process, the reduced GO (R-GO) sheets were dually connected by the Fe/Fe3C nanoparticles as well as the carbonized cellulose nanocrystals. Furthermore, the defects on the R-GO are repaired by Fe through a catalytic graphitization process. Thus, the scatterings of phonons at the adjacent sheet boundaries and on the basal plane of the R-GO sheets were reduced. A flexible and light weight composite film of reduced GO/Fe–CNC with a thermal conductivity of 1958.14 W/mK was obtained. The thermal conductivity is much higher than that of a pure R-GO film (388.72 W/mK) prepared under the same conditions. This assembly process achieved dual connections and defect repairs simultaneously and it provides an avenue for the design of graphene-based films with a high thermal conductivity.