Fluorine-containing polyimide (PI) with low dielectric constant (ε) value was firstly synthesized by a polycondensation reaction of 4, 4-(hexafluoroisopropyl) diphthalic anhydride (6FDA), 1, 3, ...4-triphenyldiether diamine (APB) and 1, 3-bis(3-aminopropyl) tetramethyldisiloxane (GAPD). γ-glycidoxypropyltrimethoxysilane (KH-560) and aminopropylisobutyl polyhedral oligomeric silsesquioxane (NH2-POSS) were synchronously performed to functionalize the surface of boron nitride (f-BN) fillers, which were then utilized as thermally conductive fillers to fabricate the corresponding f-BN/PI composites. NH2-POSS was successfully grafted on the surface of BN fillers. All the f-BN/PI composites presented better thermal conductivities, dielectric and thermal properties than those of BN/PI composites at the same addition of BN fillers. The obtained thermal conductivity coefficient (λ) of the f-BN/PI composites with 30 wt% f-BN was 0.71 W/mK, higher than that of BN/PI composites with 30 wt% BN (λ of 0.69 W/mK). Modified Hashin-Shtrikman model demonstrated that f-BN possessed relatively lower thermal resistance with PI matrix. Meantime, the corresponding ε and dielectric loss tangent (tanδ) value of the f-BN/PI composites was decreased to 3.32 and 0.004, respectively, lower than that of BN/PI composites with 30 wt% BN (ε of 3.77 and tanδ of 0.007). Moreover, the corresponding heat resistance index (THRI) and glass transition temperature (Tg) of the f-BN/PI composites was further enhanced to 280.2 °C and 251.7 °C, respectively.
Fabricating thermally conductive yet electrical insulated composite films faces dilemmas of ineffective exfoliation of boron nitride (BN) platelets, unsatisfactory thermal conductivity (TC) and poor ...anisotropy ratio. Herein, few-layered and functionalized boron nitride nanosheets (BNNSs) were effectively exfoliated from BN platelets via eco-friendly biomolecule-assisted exfoliation. Then, BNNS/ethylene-vinyl acetate copolymer (EVA) composite films with the laminated structure were achieved by the green and scalable vacuum-assisted self-assembly. The as-prepared BNNS/EVA composite film showed superior in-plane TC of 13.2 W/mK and strong anisotropy ratio of ~2500% at the BNNS loading of 50 wt%. It was mainly ascribed that the highly oriented BNNSs formed effectively thermally conductive pathways for heat transfer. Additionally, the oxygen-containing functional groups of BNNSs improved interfacial interaction with the EVA matrix and reduced phonon scattering. Thermal interface resistance of the 50 wt% BNNS/EVA film was reduced by 68% compared to the 50 wt% BN/EVA counterpart. Furthermore, the BNNS/EVA films exhibited an attractive flexibility and TC reliability. The retention ratio of in-plane TC was 98% after repetitive bending, 95% after repeated tensile test, and 97% after heating/cooling cycles. The obtained results offer valuable fundamentals to fabricate high-performance thermally conductive polymer composites as advanced thermal management materials.
Display omitted
•BNNS was exfoliated and flame retardant functionalized by one-step ball-milling.•Functionalized BNNS showed a potential in preparing highly flame-retardant PTCs.•Both flame ...retardancy and thermal conductivity of EP were enhanced by adding BNNS.•The PHRR of composite with 5 wt% BNNS2 decrease by 60.9% when comparing to pure EP.
Boron nitride nanosheet (BNNS) reveals a huge potential in preparing highly flame-retardant polymer-based thermal conductive composite, but is limited by its difficult exfoliation and functionalization. Here, hexagonal boron nitride (hBN) was simultaneously exfoliated and flame-retardant functionalized into BNNS via one-step ball milling process based on the synergetic effect of mechanical shear and chemical peeling of ammonium phosphate and sodium hydroxide. Then the epoxy (EP)-based composites containing hBN or BNNS were prepared by solution blending and program-controlled curing. The possible mechanochemical reaction mechanisms were proposed according to the incorporation of density functional theory (DFT) calculations and chemical structure characteristics. As one of potential applications, the obtained flame-retardant functionalized BNNS (BNNS1 and BNNS2) were used as multifunctional additives for fabricating high-performance EP-based thermal conductive composites with excellent flame retardancy. As expected, the obtained EP-based composites containing only 5 wt% BNNS exhibited a superior flame retardancy with a dramatic decrease in the values of peak heat release rate (PHRR), the total heat release (THR), the smoke production rate (SPR) and the total smoke production (TSP) corresponding to 60.9%, 35.7%, 44.3% and 38.8% reductions, respectively, compared to neat EP. The dramatical enhancement in flame retardancy was mainly attributed to the catalytic charring effect and physical barrier action of flame-retardant functionalized BNNS, led to the formation of a compact and robust char layer during combustion to protect the underlying polymer. Simultaneously, due to uniform dispersion and strong interfacial adhesion, the incorporation of BNNS not only increased the thermal conduction paths by increasing specific surface area, but also reduced the interfacial thermal resistance (Rb) caused by phonon scattering, leading to an enhancement (312.4% and 397.0%) in the TC of EP/BNNS composites at 30 wt% BNNS1 and BNNS2, respectively.
Display omitted
•“Line-Plane”-like hetero-structured thermally conductive GO@MWCNTs fillers are prepared by electrostatic self-assembly.•The λ of GO@MWCNTs/PDMS composites contain 20 wt% of GO@MWCNTs ...reaches 2.10 W/(m·K).•Thermally conductive GO@MWCNTs/PDMS composites present good thermal conduction stability.
Graphite oxide (GO) and cetyltrimethylammonium bromide (CTAB) modified multi-walled carbon nanotubes (m-MWCNTs) are utilized to fabricate “Line-Plane”-like hetero-structured thermally conductive GO@MWCNTs fillers by electrostatic self-assembly, which are then introduced into polydimethylsiloxane (PDMS) to fabricate thermally conductive GO@MWCNTs/PDMS composites. When the mass ratio of GO to m-MWCNTs is 2:1, GO@MWCNTs fillers have optimal morphologies and thermal conductivity contribution. When the mass fraction of GO@MWCNTs is 20 wt%, the thermal conductivity coefficient (λ) of GO@MWCNTs/PDMS composites reaches 2.10 W/(m·K), 950% higher than that of pure PDMS (0.20 W/(m·K)), which is also superior to the λ of MWCNTs/PDMS (0.68 W/(m·K)), GO/PDMS (1.59 W/(m·K)) and (GO/MWCNTs)/PDMS (1.28 W/(m·K)) composites with the same amount of single or hybrid thermally conductive fillers. Meantime, the GO@MWCNTs/PDMS composites also present good thermal conduction stability (average λ after 15 heating-cooling cycles in the temperature of 21 to 100°C is 2.14 W/(m·K)) and thermal stability (heat resistance index is 249.3°C).
Liquid crystal epoxy resin presents high intrinsic thermal conductivity coefficient (λ). However, the complex molecular structure design and tedious synthesis process severely limit its rapid ...development and further industrial application. In this work, a kind of liquid crystal epoxy (LCE) based on biphenyl mesomorphic unit is synthesized from 4,4′-biphenol, triethylene glycol, and epichlorohydrin. Curing agent of 4,4′-diaminodiphenyl methane (DDM) and boron nitride (BN) fillers are both performed to prepare the intrinsic highly thermally conductive liquid crystal epoxy resin (LCER) and BN/LCER thermally conductive composites via casting method. LCE has been successfully synthesized with expected structure, presenting nematic liquid crystal with range of 135–165 °C. LCER shows liquid crystal property with intrinsic λ up to 0.51 W/mK, about 3 times higher than that of general bisphenol A epoxy resin (E−51, 0.19 W/mK). Simultaneously, LCER has good thermal stability with heat resistance index (THRI) being 183.9 °C. In addition, the λ values of the BN/LCER thermally conductive composites increase with the increasing loading of BN fillers. When the content of BN fillers is 30 wt%, the λ value of BN/LCER thermally conductive composites is 1.02 W/mK, twice as much as that of pure LCER, also much higher than that of 30 wt% BN/E−51 composites (0.52 W/mK).
Thermally conductive polymer nanocomposites are enticing candidates for not only thermal managements in electronics but also functional components in emerging thermal energy storage and conversion ...systems and intelligent devices. A high thermal conductivity (k) depends largely on the ordered assembly of high-k fillers in the composites. In the past decades, various templating assembly techniques have been developed to rationally construct nanoscale fillers into three-dimensional (3D) interconnected structures, further improving the k of composites compared to conventional methods. Herein, recent advances are summarized in developing thermally conductive polymer composites based on self-templating, sacrificial templating, foam-templating, ice-templating and template-directed chemical vapor deposition techniques. These unique templating methods to fabricate 3D interconnected fillers in the form of segregated, cellular, lamellar, and radially aligned structures are reviewed, and their correlations to the k of composites are thoroughly probed. Moreover, multiscale structural design strategies combined with different templating methods to further improve the k of composites are highlighted. This review offers a constructive guidance to fabricate next-generation thermally conductive polymer composites for diverse thermal energy applications.
Display omitted
Recently, conductive composites have been used in flexible electronic devices and have attracted attention. The integration of self-healing, high sensitivity, large tensile strength, ...environmental stability, and easy recyclability into conductive composites is very desirable yet challenging. Hence, a conductive composite as a flexible strain sensor with a self-healing and recyclability is facilely developed, with a polyurethane (PU) elastomer bearing dynamic boronic ester as the polymer matrix and carbon nanotubes (CNTs) as a conductive filler. Due to the dynamic boronic ester bond and hydrogen bond, the prepared polyurethane conductive composite has good self-healing and mechanical properties. It not only has a high healing efficiency of 78 % but also has a tensile strength of 15.4 MPa and an elongation at break of 420 %. In addition, the prepared conductive composite has high conductivity (0.57 mS/cm) and sensitivity. As a wearable sensor, it can identify human activities in all directions, such as elbow and finger bending, speaking, and facial changes. Consequently, the polyurethane conductive composite prepared in this study exhibited wonderful application potential in wearable electronic devices such as self-healing strain sensors.
Silicon carbide nanowires (SiCnws) were in-situ grown on boron nitride nanosheets (BNNS) to construct a novel kind of nest-like hetero-structured BNNS@SiCnws thermally conductive fillers from natural ...bamboo leaves and tetraethyl orthosilicate (TEOS) by means of ultrasonic impregnation, sol-gel followed by carbothermic reduction. Then, the thermally conductive & electrically insulating BNNS@SiCnws/epoxy composites were prepared via blending-casting method. When the amount of BNNS@SiCnws-II (65/35, wt/wt) was 20 wt%, BNNS@SiCnws/epoxy composites presented the optimal overall performances. Thermal conductivity coefficient (λ) of BNNS@SiCnws-II (20 wt%)/epoxy composites increased from 0.22 W/mK of pure epoxy matrix to 1.17 W/mK, higher than that of SiCnws/epoxy (0.72 W/mK), BNNS/epoxy (0.82 W/mK) and (BNNS/SiCnws)/epoxy composites (direct mixing BNNS/SiCnws, 65/35, wt/wt, 0.76 W/mK) with the same filler concentration of 20 wt%. Meanwhile, BNNS@SiCnws/epoxy composites presented excellent heat transfer/heat dissipation efficiency, due to synergistic effect of the “line-to-surface” hetero-structure of SiCnws and BNNS, which could significantly improve the formation probability of the thermally conductive paths. Furthermore, the BNNS@SiCnws/epoxy composites possessed favorable electrical insulation, thermostability and ideal mechanical properties. Furthermore, the related surface & volume resistivities, the electric breakdown strength, the glass-transition temperature, the heat resistant index, the flexural strength as well as the impact strength of BNNS@SiCnws-II (20 wt%)/epoxy composites reached to be 3.7 × 1015 Ω, 5.17 × 1015 Ω·cm, 22.1 kV/mm, 126.7 °C, 185.5 °C, 75.7 MPa and 8.2 kJ/m2, respectively.
-The epoxy composites with 20 wt% nest-like hetero-structured BNNS@SiCnws fillers present outstanding thermal conductivity of 1.17 W/mK, higher than that of pure epoxy (0.22 W/mK) and that of BNNS/SiCnws (mixing 65/35, wt/wt)/epoxy composites (0.76 W/mK).-BNNS@SiCnws/epoxy composites present outstanding heat transfer/heat dissipation efficiency, due to synergistic thermal conductive effect of the“line-to-surface” hetero-structure of BNNS@SiCnws.-BNNS@SiCnws/epoxy composites possessed favorable electrical insulation, thermostability and ideal mechanical properties.
Due to their extraordinary properties, boron nitride nanosheets (BNNSs) have great promise for many applications. However, the difficulty of their efficient preparation and their poor dispersibility ...in liquids are the current factors that limit this. A simple yet efficient sugar‐assisted mechanochemical exfoliation (SAMCE) method is developed here to simultaneously achieve their exfoliation and functionalization. This method has a high actual exfoliation yield of 87.3%, and the resultant BNNSs are covalently grafted with sugar (sucrose) molecules, and are well dispersed in both water and organic liquids. A new mechanical force–induced exfoliation and chemical grafting mechanism is proposed based on experimental and density functional theory investigations. Thanks to the good dispersibility of the nanosheets, flexible and transparent BNNS/poly(vinyl alcohol) (PVA) composite films with multifunctionality is fabricated. Compared to pure PVA films, the composite films have a remarkably improved tensile strength and thermal dissipation capability. Noteworthy, they are flame retardant and can effectively block light from the deep blue to the UV region. This SAMCE production method has proven to be highly efficient, green, low cost, and scalable, and is extended to the exfoliation and functionalization of other two‐dimensional (2D) materials including MoS2, WS2, and graphite.
Sucrose‐grafted BN nanosheets (sucrose‐g‐BNNSs) are produced by a simple yet efficient sucrose‐assisted mechanochemical exfoliation process, and easily dispersed in polar liquids. Compared to pure poly(vinyl alcohol) (PVA), the sucrose‐g‐BNNS/PVA composites show remarkably improved tensile strength, thermal dissipation and flame‐retardancy. This method also works for the simultaneous exfoliation and functionalization of many other two‐dimensional (2D) materials.
Fused Filament Fabrication (FFF) can promote high aspect-ratio fillers to align in specific direction due to deposition-induced effect, so it is widely used in the preparation of thermal conductive ...composites. Though many studies about FFF have explored the influence of filler types, content, shape, and orientation on thermal conductivity during FFF, the size effect of filler is neglected. In this work, TPU/BN composites with 30 wt% h-BN were successfully printed by Fused Filament Fabrication. The size effect of h-BN on orientation, morphology, thermal conductivity, and other properties were studied. It is found that large-sized fillers are more prone to orientation and mutual contact during the FFF printing process to form thermal conductive paths. An increase in filler size results in an increase in thermal conductivity along the printing direction, which is up to 2.58 W m−1 K−1. The prepared TPU/BN composite not only has high thermal conductivity but also ensures good flexibility.
Display omitted
•Larger size h-BN is more prone to orientation during FFF.•Large size h-BN can better improve thermal conductivity of printed composite.•Oriented h-BN distributes differently around and inside the printed layers.
