Highly Multifunctional Performances of Boron Nitride Nanosheets/Polydimethylsiloxane Composite Foams

Although this kind of hexagonal boron nitride (h-BN)-filled polydimethylsiloxane (PDMS) multifunctional composite foam has been greatly expected, its development is still relatively slow as a result of the limitation of synthetic challenge. In this work, a new foaming process of BNNSs-PDMS, alcohol, and water three-phase emulsion system is employed to synthesize a series of high-quality BNNSs/PDMS composite foams (BSFs) filled with highly functional and uniformly distributed BNNSs. As a result of well-bonded interfaces between the BNNSs and PDMS, enhanced multiple functions of BSFs appeared. The BSFs can show complete resilience at a compressive strain of 90% and only 3.99% irreversible deformation after 100,000 compressing-releasing hyperelastic cycles at a strain of 60%. On the basis of their outstanding shape-memory properties, the maximum voltage value of compression-driven piezo-triboelectric (CDPT) responses of the BSFs is up to ∼20 V. Depending on the remarkable super-elastic and CDPT performances, the BSFs can be used for sensitive sensing of temperature difference and electromechanical responses. Also, in the range of 12-40 GHz, the BSF materials display ultralow dielectric constants between 1.1 and 1.4 with proper dielectric loss tangent values of <0.3 and exhibit an enhanced and broadened sound adsorption capacity ranging from 500 to 6500 Hz. Although BSFs have high porosities of >65%, their thermal conductivities can still reach up to 0.407 ± 0.039 W m^-1 K^-1. Moreover, the BSF materials display favorable thermal stability, obviously reduced coefficient of thermal expansion, and good flame retardancy. All of these properties render the BSFs as a new category of excellent multifunctional material.

Cadmium-Rich Plant Powder/PAN/PU Foams with Low Thermal Conductivity

Treating and utilizing heavy metal enriched plants have become growing problems. In this work, a series of composite foams were made from the powder of Cadmium-rich plant, polyacrylonitrile (PAN) and polyurethane (PU). Test results indicated that the addition of plant powder can not only increase the specific surface area, but also improve the apparent density and thermal stability of the foams. Besides, compared with the foam without plant powder, the powder-added foams exhibited a decreasing trend for thermal conductivity, and the minimum was 0.048 w/(m·k), which indicated that the addition of plant powder can help to enhance the thermal insulation of composite foam. More importantly, the results of leaching experiment showed that the leaching rate of heavy metal cadmium in the composite foam with 50% plant powder content was as low as 0.14% after being immersed in the acidic (pH = 3) solution for 5 days, which implies that the foam materials are very safe. This study provides a new way to realize high value-added resource utilization of heavy metal-enriched plants.

Facile Design of Three-Dimensional Nitrogen-Doped Reduced Graphene Oxide/Multi-Walled Carbon Nanotube Composite Foams as Lightweight and Highly Efficient Microwave Absorbers

Graphene foams with three-dimensional (3D) network structure, high porosity, and ultralow density have been regarded as lightweight microwave absorption materials. Herein, nitrogen-doped reduced graphene oxide/multi-walled carbon nanotube composite foams were prepared through a two-step strategy of hydrothermal self-assembly and subsequent high-temperature calcination. Morphology analysis indicated that the 3D networks were composed of overlapped flaky reduced graphene oxide. In addition, the influences of nitrogen doping, calcination temperature, and filler ratios on microwave absorption of composite foams were explored. Results manifested that the microwave absorption of composite foams was remarkably improved with the calcination temperature increased. Dramatically, it was noteworthy that the composite foam obtained under 600 °C calcination (bulk density of ∼10.8 mg/cm³) with an 8 wt % mass filler ratio presented the strongest microwave absorption of -69.6 dB at 12.5 GHz and broadest absorption bandwidth achieved 4.3 GHz (13.2-17.5 GHz) at an extremely low matching thickness equal to 1.5 mm. Moreover, the microwave absorption performance could be conveniently adjusted through modifying the thicknesses, filler ratios, and calcination temperature. The excellent microwave absorption performance of as-prepared composite foams was greatly derived from a well-constructed 3D network structure, significant nitrogen doping, enhanced polarization relaxation, and improved conduction loss. This work proposed a new strategy for fabricating graphene-based composites with a 3D network structure as high-efficiency microwave absorbers.