Search

• Layer-by-layer scraping can produce a powerful shearing effect for UHPE/GNPs gel. • Strong shearing induced dense stacking and ordered alignment of GNPs and PE chains. • The dense and ordered structure drastically enhanced in-plane TC up to 28.88 W/mK. • The solar-thermal conversion ability was also improved by the ordered structure. The simultaneous achievement of ordered alignment for fillers and polymer matrix is highly desired in polymer-based thermally conductive composites, but it is still extremely challenging. Herein, we put forward a layer-by-layer scraping (LbLS) method to simultaneously arrange ultra-high molecular weight polyethylene (UHPE) chains and graphene nanoplates (GNP) into a highly oriented structure. When UHPE/GNP gel suffering the strong shearing effect during LbLS process, the inner UHPE chains and GNP were dragged and orderly arranged along shearing direction, and these oriented structures were retained after rapidly drying by distillation. After layer-by-layer stacking, nacre-like UHPE/GNP films with tightly layered structure were obtained. By adjusting the scraping gap from 200 to 50 μm, the oriented structure can be effectively reinforced with the Herman orientation factors f increasing from 0.83 to 0.88 for GNP and 0.75 to 0.82 for UHPE chains. The highly ordered structure can form a tight and oriented thermal conduction network with low interfacial thermal resistance in plane. The LbLS composite film thus significantly enhanced the in-plane thermal conductivity up to 28.88 W/mK, compared with traditional solution casting film (9.63 W/mK). Moreover, the highly oriented structure can significantly increase the solar-thermal conversion performance (58.1 °C@100 mW/cm2) by improving its light absorption ability comparing to that of solution casting film. Besides, the mechanical strength and toughness that depend on the filler and chain alignment were effectively increased by the oriented structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ammonium polyphosphate functionalized BNNS (BNNS@APP) was prepared by one-step wet ball milling method. • The macromolecular APP play the role of organic "binders" to construct 3D BNNS skeleton. • The 3D BNNS@APP skeleton with a highly oriented structure was obtained by unidirectional freezing. • The EP/BNNS@APP composite presented excellent thermal conductivity and flame retardancy. Three-dimensional (3D) boron nitride skeleton has a promising prospect in improving the thermal conductivity and flame retardancy of polymer composites, yet the problems of interface thermal resistance and combustion collapse in the skeleton caused by organic adhesives still need to be solved. In this work, macromolecular flame retardancy, ammonium polyphosphate (APP) with high polymerization degree, was firstly used to modify and assemble boron nitride nanosheets (BNNS) into 3D thermally conductive skeleton, in which APP with rich amino groups can result in the strong grafting modification to reduce the interfacial thermal resistance in BNNS skeleton, meanwhile, the catalytic carbonization of APP can reinforce the 3D skeleton barrier without collapse during combustion. Typically, one-step wet ball-milling with assisted APP was used to exfoliate and modify BNNS, followed by unidirectional freezing to achieve 3D BNNS@APP skeleton with highly oriented structure. As expected, the constructing 3D skeleton with low interfacial thermal resistance can effectively form a heat transfer path in polymer matrix. The obtained epoxy-based composite showed a high thermal conductivity of 3.28 W/mK at 9.28 vol% BNNS@APP. Meanwhile, the producing 3D skeleton barrier during combustion induced the significant improvement in flame retardancy, with heat release rate (HRR) and total heat release (THR) decreasing by 68.3% and 52.7%, respectively. This work provides a new adhesive choice in constructing 3D thermal conductive and flame retardant skeleton, which exhibits high potential in thermal management application. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbon welding is used to reduce filler-to-filler ITR in 3D graphene skeleton. • Introducing the graphene skeleton into PCM improves its thermal conductivity. • The graphene skeleton can avoid the leakage of PCM during solid–liquid phase change. • The graphene skeleton endows PCM with solar-to-heat conversion capacity. Advanced phase change materials (PCMs) with three-dimensional (3D) thermal conductive skeletons reveal the promising prospect in the thermal management of lithium battery. However, the high filler-to-filler interfacial thermal resistance (ITR) arising from the weak interfacial contact connection in 3D skeleton is still challenging. Herein, we demonstrate a "carbon welding" strategy for reducing the filler-to-filler ITR in 3D graphene skeleton by achieving the lattice connection between contact graphene. Typically, the ordered 3D graphene skeleton was constructed by unidirectionally ice template assembling graphene nanoplates (GNP) with the assistance of poly (amic acid) (PAA). Imidization and carbonization treatments were employed to weld the adjacent GNP in 3D skeleton. The similar lattice structure of carbonized polyimide (PI) and graphene can result in the significant reduction of phonon scattering and ITR at these contact areas. After impregnation with polyvinyl alcohol (PEG), the high-performance PCMs with high-efficient phonon transmission expressway were obtained. As expected, the prepared composites reveal the high thermal conductivities with a maximum value of 7.032 W m−1 K−1 at ~11.6 vol% GNP, which is more than two-fold than that of the composite with uncarbonized skeleton. Finite element simulation and nonlinear model analyses confirm that the reduced filler-to-filler ITR in skeleton is the main reason for the improving thermal conductivity. In addition, the presence of 3D graphene skeleton can effectively avoid the leakage during solid–liquid phase change, and significantly improve the shape stability of the PCMs. At the same time, the graphene skeleton can endow the PCM with an excellent solar-to-heat conversion performance, which ensure a wide range of application in actual environment. [ABSTRACT FROM AUTHOR]

• 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% BNNS 2 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 (h BN) 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 h BN 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 (BNNS 1 and BNNS 2) 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 (R b) caused by phonon scattering, leading to an enhancement (312.4% and 397.0%) in the TC of EP/BNNS composites at 30 wt% BNNS 1 and BNNS 2 , respectively. [ABSTRACT FROM AUTHOR]