Sugar-Derived Isotropic Nanoscale Polycrystalline Graphite Capable of Considerable Plastic Deformation

Sugar-Derived Nanocrystalline Graphite Matrix C/C Composites with Excellent Ablative Resistance at 3000 °C

Sugars are renewable resources essential to human life, but they are rarely used as raw materials for the industrial production of carbon-based materials, especially for the preparation of carbon fiber-reinforced carbon-matrix (C/C) composites, which are extremely useful for the semiconductor and aerospace sectors. Herein, a method utilizing sugar-derived carbon to replace petrochemicals as dense matrix to preparing C/C composites is reported. The matrix from sugar-derived C/C (S-C/C) composites has a nanocrystalline graphite structure that is highly thermally stable and effectively bonded to the carbon fibers. The mechanical properties of the S-C/C composite are comparable to those prepared from petrochemical sources; significantly, it exhibits a linear ablation rate of 0.03 mm s-1 after 200 s of ablation at 3000 °C in 10 MW m-2 heat flux. This new class of S-C/C is promising for use in a broad range of fields, ranging from semiconductor to aerospace.

Ductile amorphous boron nitride microribbons

The broad applications of ceramic materials in functional devices are often limited by their intrinsic brittleness. Amorphous boron nitride (a-BN), as a promising ceramic has shown high thermal stability and excellent dielectric properties that can be applied to microfabricated aerogel and nano dielectric layers, while its mechanical properties at small scales are yet to be studied. Here we report synthesized a-BN microribbons can have a uniform elongation at a breaking strain of more than 50% upon tension, exhibiting outstanding ductility. Such a-BN microribbons with lengths ranging from tens to hundreds of micro-meters were prepared via the small molecule precursors sol-gel method. Through in situ uniaxial tensile measurements, we demonstrated that a-BN microribbons also display a surprising flaw-tolerance behaviour. Combining high-resolution atomic characterization with molecular dynamics simulations, we reveal that the large tensile plasticity of a-BN originates from the topological deformation induced multiple energy-dissipation mechanisms including unfolding and reorientation of local curly h-BN layers and their interlayer debonding, slippage as well as the intralayer tearing. Our findings provide new insights to develop ductile amorphous covalent-bonded materials for emerging applications.

Visible Light Locking in Mineral-Based Composite Phase Change Materials Enabling High Photothermal Conversion and Storage

Fully stimulating the capacity of light-driven phase change materials (PCMs) for efficient capture, conversion, and storage solar energy requires an ingenious combination of PCMs, supporting structural materials, and photothermal materials, therefore motivating the synergistic effects between the components. Herein, this work thoroughly explores the interaction forces between PCMs and supporting structural materials and the synergy between PCMs and photothermal materials in photothermal conversion. Rejoicingly, when capitalizing on the prepared directional channel structure of hierarchically porous composite aerogel (PEPG) as a supporting structural material, a superior paraffin wax (PW) encapsulation rate of 85.11% is achieved, and the prepared PEPG2-PW has a high phase change enthalpy of 182.9 J/g. The van der Waals force and Lewis acid-base action between PEPG and PW molecules reveal the excellent stabilities of PEPG-PW. More importantly, the PEPG2-PW has an ultrahigh photothermal conversion efficiency of 95.2% under 1 sun irradiation and durability. Most importantly, the COMSOL Multiphysics software calculations demonstrate that transparent PW can anchor sunlight on the surface of graphite nanoplates, converting it into heat by enhancing the loss of graphite backbone lattice vibrations, and the accumulated heat is then stored in molten PW. This work provides some design principles for high-efficiency solar-thermal conversion materials.

Eco-Friendly Preparation of Carbon-Bonded Carbon Fiber Based on Glucose-Polyacrylamide Hydrogel Derived Carbon as Binder

Lightweight, high-temperature-resistant carbon-bonded carbon fiber (CBCF) composites with excellent thermal insulation properties are desirable materials for thermal protection systems in military and aerospace applications. Here, glucose was introduced into the polyacrylamide hydrogel to form the glucose-polyacrylamide (Glu-PAM) hydrogel. The CBCF composites were prepared using the Glu-PAM hydrogel as a brand-new binder, and the synergistic effect between glucose and acrylamide was investigated. The results showed the Glu-PAM hydrogel could limit the foaming of glucose and enhance the carbon yield of glucose. Meanwhile, the dopamine-modified chopped carbon fiber could be uniformly mixed by high-speed shearing to form a slurry with the Glu-PAM hydrogel. Finally, the slurry was successfully extruded and molded to prepare CBCF composites with a density of 0.158~0.390 g cm^-3 and excellent thermal insulation performance and good mechanical properties. The compressive strength of CBCF composites with a density of 0.158 g cm^-3 in the Z direction is 0.18 MPa, and the thermal conductivity in the Z direction at 25 °C and 1200 °C is 0.10 W m^-1 k^-1 and 0.20 W m^-1 k^-1, respectively. This study provided an efficient, environment-friendly, and cost-effective strategy for the preparation of CBCF composites.

Analysis of Formation Mechanisms of Sugar-Derived Dense Carbons via Hydrogel Carbonization Method

Four kinds of sugar (glucose, fructose, sucrose, and maltose) were selected as carbon precursors, and corresponding dense carbon products were prepared using a novel hydrogel carbonization method. The carbonization processes of sugar-polyacrylamide (sugar-PAM) hydrogels were studied in detail. The molecular structures in the raw materials were analyzed by proton nuclear magnetic resonance spectroscopy (¹H NMR). Samples prepared at different temperatures were characterized by thermogravimetry analysis (TGA) and Fourier-transform infrared (FTIR) spectroscopy. The morphology and microstructure of sugar-derived carbons were confirmed by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The results indicated that the sugar solution was surrounded by PAM with a three-dimensional network structure and formed hydrogels in the initial stage. The sugar solution was considered to be separated into nanocapsules. In each nanocapsule, sugar molecules could be limited within the hydrogel via walls formed by PAM chains. The hydroxyl group in the sugar molecules connected with PAM by the hydrogen bond and intermolecular force, which can strengthen the entire hydrogel system. The self-generated pressure of hydrogel constrains the foam of sugar during the heat treatment. Finally, dense carbon materials with low graphitization instead of porous structure were prepared at 1200 °C.