Thermal Transport Performance Research Articles

Large improvement of thermal transport and mechanical performance of polyvinyl alcohol composites based on interface enhanced by SiO2 nanoparticle-modified-hexagonal boron nitride

The interface plays a key role in determining the properties of polymer composites. However, chemical inertness of hexagonal boron nitride (h-BN) makes its surface modification troublesome. In this study, we, for the first time, used SiO2 as the modified molecular and investigated the effects of exfoliated h-BN and SiO2 nanoparticles-modified-exfoliated h-BN (SiO2@exfoliated h-BN) on the storage modulus, the glass transition temperature, the filler dispersion and the thermal conductivity of polyvinyl alcohol (PVA) composites prepared by a solution mixture method. The results showed that the SiO2 modification enhanced the interface interaction and improved the performance of the PVA composites. Based on the enhanced interface interaction by SiO2 modification and the aligned fillers in PVA by vacuum filtration, the high thermal conductivity of 13.88 W/m·K was realized at 15.2 wt% SiO2@exfoliated h-BN loading. Meanwhile, the tensile strength was also largely improved to 156 MPa, compared with 47 MPa of pure PVA. This study broadens the surface modification method of h-BN and provides a valuable reference for fabrication of high-performance thermal interface materials.

Near-infrared light triggered shape memory and self-healable polyurethane/functionalized graphene oxide composites containing diselenide bonds

In this work, polyurethane/functionalized graphene oxide composites containing diselenide bonds were fabricated by covalently incorporating functionalized graphene oxide (FGO) into hydroxyl-terminated polyurethane prepolymers via in-situ polymerization. It was found that the isocyanate terminated molecules were covalently attached onto the graphene oxide surface, and the FGO could act as a multifunctional reinforcer in mechanical and photo-thermal properties of polyurethane composites. Significant enhancements of mechanical, thermal transport, and crystallization performances of composites were observed with the incorporation of 2 wt% of FGO. Due to the photo-thermal and crystallization-induced effects of FGO as well as the exchangeable characteristic of diselenide bonds, the PID-FGO2 composite exhibited both shape memory and repeatable self-healing functions upon exposure to near infrared (NIR) irradiation. In particular, the introduction of diselenide bonds could greatly improve the healing efficiency from ∼45% to ∼90%, and the healing efficiencies were maintained above 75% even after five healing cycles. Such favorable healing is acquired by the photo-thermal response, shape memory effect, and diselenide exchange reaction simultaneously, and the shape memory effect is of great importance for assisting the healing process. We expect that this work could provide a viable avenue for multifunctional composite materials.

Liquid-liquid phase separation heat transfer in advanced micro structure

An experimental heat transfer study using triethylamine (TEA)-water solution as a coolant in a Piranha Pin Fin (PPF) structure is reported here. The triethylamine-water solution undergoes phase separation when heated to a temperature over 18.2 °C at a TEA mass fraction of 32.1% (i.e., critical composition). This separation process was proven to enhance heat transfer in the plain channel geometry. With the aid of the PPF structure, the TEA-water solution was able to dissipate a heat flux of up to 500 W/cm2 at a mass flux of 600 kg/m2s and to keep the surface temperature below 80 °C. The phase separation of a TEA-water mixture in the PPF structure yields about 1.8 times greater heat transfer coefficient compared to a corresponding homogeneous mixture flow. Flow boiling was achieved during experiments when the heat flux exceeded 600 W/cm2. The effects of different configurations of PPF were also studied, and it is shown that the PPF configuration with fewer and smaller fins provides the best performance enhancement over water flow at the same mass flux. Two flow conditions, channel flow and extraction flow, were examined. The TEA-rich phase was extracted during the extraction flow experiment and the remaining water-rich phase results in favorable thermal transport performance. A string-shaped flow pattern is observed due to both shear force and the flow disturbance provided by the PPFs. The system exhibits reduced pressure drop as the fluid viscosities decrease after phase separation.

Iodine nanoparticle-enhancing electrical and thermal transport for carbon nanotube fibers

The excellent electrical and thermal transport properties of individual carbon nanotubes (CNTs) have allowed them to stand out in nanoenergy and nanoelectronics applications. However, scaling them up from nanometer-sized tubes to micrometer assembled fibers generally leads to considerable electrical and thermal transport deterioration. In pursuit of a new strategy for boosting thermal and electrical transport, we experimentally confirmed that the intercalation of iodine (I) molecules can significantly raise the inter-tube interfacial electrical and thermal transport (IET and ITT), and thus boost the overall electrical and thermal transport performance of the fibers. The underlying mechanisms from an energy carrier standpoint are also explored and discussed. It is demonstrated that the introduced I molecules can invoke electron extraction from CNTs, which enhances the IET. The formed (I3)− and (I5)− polyiodide chains act as additional heat transfer channels and induce low-frequency phonons of the interfacial carbon atoms, and the intercalation also improves the matching level of the vibrational density of states at the inter-tube interface. These three-fold effects contribute to the ITT. Our findings are one step forward on the path towards advanced CNT-based functional materials with high electrical and thermal transport.

Low melt alloy enhanced solid-liquid phase change organic sugar alcohol for solar thermal energy storage

In this work, the thermo-physical behavior along with the energy storage performance of low melting alloy (LMA) mixed with d-Mannitol (DM)–a solid-liquid PCM, were experimentally investigated. LMA is an alloy consisting of Tin (59%), Bismuth (32%), Zinc (4%) and Indium (5%). Composites with different mass loadings of LMA (0.1% and 0.5%) were prepared using a low energy ball mill. A maximum thermal conductivity enhancement of ~8% and ~24% was found with the use of 0.1 wt% and 0.5 wt% LMA, respectively. The effect on melting point due to the addition of LMA particles was relatively small, however the change in solidification temperature indicates that the nucleating capabilities of LMA helped reduce the sub-cooling temperature, during the discharging process. 350 repetitive melting/freezing cycles were conducted to study the effect of thermal cycling on the PCM, and though the results showed a decrease in enthalpies, the base PCM still had a very high latent heat of 225 kJ/kg. Further, the kinetics of crystallization (non-isothermal) showed a dramatic increase in the crystallization rate of the composite, after the addition of the LMA particles. The structural, thermal and chemical stability performance of the PCM were studied using XRD, TGA and FTIR techniques respectively. While the XRD spectra revealed that the PCM was structurally stable, the FTIR data showed that no new chemical bonds were formed in the composite PCM. Moreover, the prepared composite retained thermal stability up to a temperature of <300 °C. The energy storage and discharging characteristics of the PCM composites were studied using an experimental setup. The results showed that the time taken for the completion of one cycle was reduced ~25% for 0.5 wt% DM-LMA composites. From the results obtained, the potential for using DM-LMA as a new PCM composite is evident. Thus, with an enhanced thermal transport, energy storage and release performance, DM-LMA is a promising candidate for efficient storage, recovery and utilization of solar heat and industrial waste heat applications.

Microstructural and thermal property evolution of reaction bonded silicon carbide (RBSC)

Temperature dependent thermal conductivity of reaction bonded silicon carbide (RBSC) from 300 K to 1073 K is evaluated. The thermal conductivity of 80 vol% and 90 vol% SiC RBSC is measured to be 185.7 W/m·K and 211.4 W/m·K at room temperature and decreases to 51.46 W/m·K and 55.77 W/m·K at 1073 K, respectively. Thermal transport behavior of RBSC at elevated temperatures suggests that a structural mechanism plausibly associated with phase transformation occurs in the composite system. TEM in-situ heating test is employed to investigate the RBSC phase evolution. Results indicate the existence of minor Si amorphous phase near SiC/Si interface, and the amorphous Si phase transformation to crystalline Si could start at a relatively low temperature. The Si phase transformation improves the thermal transport performance of the RBSC system.

Parametric study of enriched gadolinium in burnable neutron poison fuel rods for Angra-2

Evaluation of gadolinium enrichment in Gadolinia (Gd2O3) burnable poison fuel rods was performed for a model of PWR Angra-2 reactor fuel assembly. A parametric study of the effect of 155Gd enrichment on neutronic performance was investigated during burn-up for a proposed very low gadolinium design (VLowGad-FA) having 1% of Gadolinia in weight fraction. Low fraction gadolinium designs enable the Angra-2 reactor to use its licensed limit of 235U to its maximum potential, that is 4.25%, with better reactivity behavior for some cases. Thermal transport performance of irradiated fuel was followed through estimation of inter-assembly peak pin power during burn-up. Results showed that Gd enriched design would behave very similar to LowGad assembly in terms of controlling local core power. It was confirmed in the neutronic context that low gadolinium design provides fuel cycle economic benefits while supporting longer cycle operation. It was estimated savings of 18 effective full power days if LowGad (2.0% Gadolinia, 4.25% 235U) is used in place of refGad (7.0% Gadolinia, 2.90% 235U). Respect the proposed design, savings are up to 41 effective full power hours if the burnable poison fuel rod VLowGad (1% Gadolinia, 99% 155Gd) is used in place of the commercial LowGad. The break-even point for Gd-155 enrichment cost for VLow155Gd99 fuel assembly was calculated to be 371 US$/gram-Gd-155 based in year 2018 prices of MWe for Angra-2.

Hot-pressing induced alignment of boron nitride in polyurethane for composite films with thermal conductivity over 50 Wm−1 K−1

Flexible insulating composites with superior thermal conductivity (usually below 5 W m−1 K−1 for the previously-reported composites) are highly desirable for further miniaturization of high-power electronics and portable devices. Herein hexagonal boron nitride (hBN)/thermoplastic polyurethane (TPU) composites were readily fabricated by solution ball-milling of hBN and TPU in N,N′-dimethylformamide followed by precipitation in ethanol. The coagulated solids were then processed by hot-pressing to produce TPU composite films with well-dispersed hBN platelets. The amount of hBN in the TPU matrix was loaded as high as 95 wt% while retaining mechanical flexibility. The in-plane thermal conductivity variations' trend with hBN loadings exhibited a sudden jump happened with the loading level over 40 wt% in the polymer matrix. Intriguingly, the TPU composite film with 95 wt% hBN exhibits an in-plane thermal conductivity of up to 50.3 W m−1 K−1 (a 264-fold increase compared to pure TPU), due to the hot-compression induced alignment of hBN platelets and strong interactions in/with the TPU matrix. The corresponding out-of-plane thermal conductivity is also as high as 6.9 W m−1 K−1. The hBN/TPU composite films were further employed as heat spreaders for light-emitting diodes, resulting in a significant decrease by about 40 °C in the set-point and the saturation temperatures. Such superior thermal transport performance has rarely been reported in the polymer composites. The proposed method provides a facile and novel strategy for the scalable fabrication of high-performance polymer composites containing two dimensional layered materials.

Anisotropic thermal conductive properties of cigarette filter-templated graphene/epoxy composites.

Herein, a cigarette filter-templated graphene/epoxy composite was prepared with enhanced thermal conductive properties. The through-plane thermal conductivity of the epoxy composite was up to 1.2 W mK−1, which was 4 times that of it in the in-plane (0.298 W mK−1) after only 5 filtration cycles. The thermal conductive anisotropy and improvement in the through-plane thermal conductivity of the epoxy composite were attributed to the particular structure of cigarette filter-templated graphene in the epoxy matrix. The unique structure formed effective conductive pathways in the composite to improve the thermal transportation properties. The excellent thermal transportation properties allow the epoxy composite to be used as an efficient heat dissipation material for thermal management applications.

Coincident modulation of lattice and electron thermal transport performance in MXenes via surface functionalization.

Efficiently modulating the thermal transport performance of materials including MXenes is highly desired as heat transfer is critical in a wide range of applications. However, the design principles for MXenes to achieve optimized thermal conductivity are not yet understood. Herein we highlight that the thermal conductivity modulation can be achieved by altering the surface fuctionalization, which also exhibits unexpected coincident effects on both the lattice and electron contribution to thermal transport. Our results indicate that the functionalization of O significantly decreases both the lattice and electron thermal conductivities of Ti2C MXenes because O will induce not only a shorter phonon relaxation time but also a metal-semiconductor transition, showing great potential for applications including thermoelectrics. In contrast to O, after being functionalized by F or OH both the lattice and electron thermal conductivities are increased, which will improve heat dissipation in electronics and batteries. Our findings will provide a fundamental guideline to the design of MXene-based devices with optimal thermal transport performance.

Effects of heat flux, mass flux and two-phase inlet quality on flow boiling in a vertical superhydrophilic microchannel

Saturated flow boiling experiments were conducted to investigate the influence of surface wettability on the hydraulic and thermal transport performance in a large width-to-height aspect ratio, one-sided heated rectangular microchannel with deionized water as the working fluid. The contact angles of the bare silicon wafer surface and superhydrophilic surface after deposited by a thin film of 100-nm-thickness silicon dioxide were 65° ± 3° and less than 5° respectively, both of which were utilized as heated surfaces of the microchannel. Parametric experimental studies were carried out with the inlet vapor quality varied from 0.03 to 0.1 and the wall heat fluxes spanned from 4 W/cm2 to 20 W/cm2, at various mass fluxes ranging from 120 to 360 kg/m2 s. High speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux, mass flux and two phase inlet quality on the local heat transfer coefficient, averaged heat transfer performance, two phase flow structure and pressure drop characteristics for surfaces with distinct surface wettability characteristics in the microchannel. Experimental results showed that the local heat transfer coefficient decreased first until approaching a minimum value and then increased towards the exit along the flow direction. Severe heat transfer deterioration was obtained for the bared silicon wafer surface with increased inlet vapor quality and heat flux, resulting from the local dryout phenomenon as can be verified by the flow visualization. While the heat transfer performance of the superhydrophilic surface was relatively constant due to continuous and uniform distribution of the thin liquid film on the heated surface during annular flow dominance and subsequent delay to partial dryout occurrence, which outperformed the untreated hydrophilic surface without additional pressure drop penalty.

Thermal Conductivity Study on a Working Li-Ion Battery Cell

Lithium-ion batteries have been popular powering mobile electronics and electric vehicles owing to its high energy density and good electrochemical performance. However, thermal transport performance, especially in cross-plane direction, is not on a par with electrochemical ones. Poor thermal transport may result in overheating and even thermal runaway while there’s still a lack of methods studying battery electrode thermal conductivity under working condition1. Previous efforts provide measurement on either dry battery electrodes or reassembled electrodes with electrolyte but not being able to charge and discharge2, 3. Current study demonstrates a new method measuring thermal conductivity of a working battery cell through flash diffusivity technique. By minimizing contact resistance between electrode sample and holder, the work can contribute to comprehensive study of a working battery cell for better thermal performance.1. V. Vishwakarma, C. Waghela, Z. Wei, R. Prasher, S. C. Nagpure, J. Li, F. Liu, C. Daniel, and A. Jain, Journal of Power Sources, 300 123-131 (2015).2. H. Maleki, S. A. Hallaj, J. R. Selman, R. B. Dinwiddie, and H. Wang, Journal of The Electrochemical Society, 146 (3), 947-954 (1999).3. S. C. Nagpure, R. Dinwiddie, S. S. Babu, G. Rizzoni, B. Bhushan, and T. Frech, Journal of Power Sources, 195 (3), 872-876 (2010).

Effect of different sizes of graphene on thermal transport performance of graphene paper

Abstract As a two dimensional materials, graphene attracts great attention as heat dissipation material due to its excellent thermal transport property. Herein, three kinds of graphene papers were fabricated with three different thickness graphene nanoplatelets (GNP) via simple vacuum filtration method. The effects of the different size of GNP on the thermal conductivity of graphene papers are investigated systematically. The in-plane thermal conductivity of GNP-7 (the thickness of GNP approximately 7 nm) paper achieves 149.2 Wm -1 K -1 , is about 7 times compared to that of GNP-3 (3 nm) and GNP-5 (5 nm), which indicates that thermal conductivity of graphene film increased with increasing the thickness of GNP. Furthermore, the in-plane thermal conductivity of GNP-7 can increase by 25% and raise up to 187.4 Wm -1 K -1 after cold-compaction as cold-compaction will further tighten the loose stacked layers, making the paper more anisotropic in heat conduction. The excellent heat conductive properties of the paper are expected to use as efficient heat spreader for thermal management applications.

Multiple thermal spin transport performances of graphene nanoribbon heterojuction co-doped with Nitrogen and Boron

Graphene nanoribbon is a popular material in spintronics owing to its unique electronic properties. Here, we propose a novel spin caloritronics device based on zigzag graphene nanoribbon (ZGNR), which is a heterojunction consisting of a pure single-hydrogen-terminated ZGNR and one doped with nitrogen and boron. Using the density functional theory combined with the non-equilibrium Green’s function, we investigate the thermal spin transport properties of the heterojunction under different magnetic configurations only by a temperature gradient without an external gate or bias voltage. Our results indicate that thermally-induced spin polarized currents can be tuned by switching the magnetic configurations, resulting in a perfect thermal colossal magnetoresistance effect. The heterojunctions with different magnetic configurations exhibit a variety of excellent transport characteristics, including the spin-Seebeck effect, the spin-filtering effect, the temperature switching effect, the negative differential thermal resistance effect and the spin-Seebeck diode feature, which makes the heterojunction a promising candidate for high-efficiently multifunctional spin caloritronic applications.

Enhanced thermal transport performance for poly(vinylidene fluoride) composites with superfullerene

High thermal conductive polymer composites have recently attracted much attention, along with the quick development to the electronic devices toward higher speed. The addition of high thermal conductive fillers is an efficient method to solve this problem. Here, we introduced superfullerene (SF), a novel zero-dimensional carbon-based filler, and incorporated into PVDF by a solution method. The effects of SF filler on the thermal conductivity of PVDF composites were systematically investigated. It was found that PVDF composites exhibited an improvement in thermal conductivity at a low SF loading. PVDF composites with only 5 wt% SF filler present the thermal conductivity value of 0.365 Wm-1K-1, which is as much as 121 % enhancement in comparison with that of neat PVDF. In view of the excellent thermal transport performance, the composites may enable some applications in thermal management in the future.

Surface-restrained growth of vertically aligned carbon nanotube arrays with excellent thermal transport performance.

A vertically aligned carbon nanotube (VACNT) array is a promising candidate for a high-performance thermal interface material in high-power microprocessors due to its excellent thermal transport property. However, its rough and entangled free tips always cause poor interfacial contact, which results in serious contact resistance dominating the total thermal resistance. Here, we employed a thin carbon cover to restrain the disorderly growth of the free tips of a VACNT array. As a result, all the free tips are seamlessly connected by this thin carbon cover and the top surface of the array is smoothed. This unique structure guarantees the participation of all the carbon nanotubes in the array in the heat transport. Consequently the VACNT array grown on a Cu substrate shows a record low thermal resistance of 0.8 mm2 K W-1 including the two-sided contact resistances, which is 4 times lower than the best result previously reported. Remarkably, the VACNT array can be easily peeled away from the Cu substrate and act as a thermal pad with excellent flexibility, adhesive ability and heat transport capability. As a result the CNT array with a thin carbon cover shows great potential for use as a high-performance flexible thermal interface material.

A unified description of thermal transport performance in disordered organic semiconductors

Thermal transport in disordered organic semiconductors fundamentally differs from that in inorganic materials and is determined by the charge carriers and phonons in localized states. Understanding thermal transport performance of organic semiconductors is important for a fundamental description of energy flow and then a better design of organic thermoelectric devices. We present a unified theoretical model to describe the thermal transport performance of the disordered organic semiconductors based on hopping transport theory. The proposed model predicts that the contribution from phonon to the thermal conductivity is larger than that from charge carrier in the disordered organic semiconductors. Moreover, the proposed model can well interpret the thermal transport feature of the organic semiconductors by combining the disorder, temperature, and carrier concentration. Simulation results imply that thermal conductivity in the disordered organic semiconductors could be strongly affected under large electric field, high carrier and dopant concentration.

Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing Five-Membered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole.

Five-membered 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocycle-based copolymers as active layer have long been ignored in solution-processable n-channel polymer field-effect transistors (PFETs) despite the long history of using OZ or TZ derivatives as the electron-injecting materials in organic light-emitting devices and their favorable electron affinities. Herein, we first report the synthesis and PFETs performance of two n-channel conjugated polymers bearing OZ- or TZ-based acceptor moieties, i.e., PNOZ and PNTZ, where simple thiophene units are utilized as the weak donors and additional alkylated-naphthalenediimides units are used as the second acceptors. A comparative study has been performed to reveal the effect of different heterocyclic acceptors on thermal properties, electronic properties, ordering structures, and carrier transport performance of the target polymers. It is found that both polymers possess low-lying LUMO values below -4.0 eV, indicating high electron affinity for both heterocycle-based polymers. Because of strong polarizable ability of sulfur atom in TZ heterocycle, PNTZ exhibits a red shift in maximal absorption and stronger molecular aggregation even in the diluted chlorobenzene solution as compared to the OZ-containing PNOZ. Surface morphological study reveals that a nodule-like surface with a rough surface morphology is observed clearly for PNOZ films, whereas PNTZ films display highly uniform surface morphology with well interconnected fiber-like polycrystalline grains. Investigation of PFETs performance indicates that both polymers afford air-stable n-channel transport characteristics. The uniform morphological structure and compact π-π stacking endow PNTZ with a high electron mobility of 0.36 cm2 V-1 s-1, much higher than that of PNOZ (0.026 cm2 V-1 s-1). These results manifest the feasibility in improving electron-transporting property simply by tuning heteroatom substitutes in n-channel polymers; further demostrate that TZ derivatives possess much superior potential for developing high-performance n-channel polymers compared to OZ derivatives.

Microplasma Processed Ultrathin Boron Nitride Nanosheets for Polymer Nanocomposites with Enhanced Thermal Transport Performance.

This Research Article reports on the enhancement of the thermal transport properties of nanocomposite materials containing hexagonal boron nitride in poly(vinyl alcohol) through room-temperature atmospheric pressure direct-current microplasma processing. Results show that the microplasma treatment leads to exfoliation of the hexagonal boron nitride in isopropyl alcohol, reducing the number of stacks from >30 to a few or single layers. The thermal diffusivity of the resulting nanocomposites reaches 8.5 mm(2) s(-1), 50 times greater than blank poly(vinyl alcohol) and twice that of nanocomposites containing nonplasma treated boron nitride nanosheets. From TEM analysis, we observe much less aggregation of the nanosheets after plasma processing along with indications of an amorphous carbon interfacial layer, which may contribute to stable dispersion of boron nitride nanosheets in the resulting plasma treated colloids.

Improvement of the thermal transport performance of a poly(vinylidene fluoride) composite film including silver nanowire

ABSTRACTThe miniaturization trend of electronic devices requires that components have a high heat dissipation in industrial applications and in daily life. In this context, a highly thermally conductive film was fabricated with silver nanowire (AgNW) and poly(vinylidene fluoride) (PVDF) with a bar‐coating method. The thermal transport performance and mechanism of the AgNW/PVDF composite film were investigated. The through‐plane and in‐plane thermal conductivity of the AgNW/PVDF composite film reached 0.31 and 1.61 W m−1 K−1, respectively; these values far exceeded those of the pristine PVDF film. The experiment illustrated that the thermally conductive pathways formed successfully in the PVDF substrate with the addition of AgNW, and the heat tended to transfer along the thermally conductive pathway rather than along the PVDF substrate. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43554.