Hypersonic vehicles encounter hostile service environments of thermal/mechanical/chemical coupling, so thermal protection materials are crucial and essential. Ceramizable composites have recently ...attracted intensive interest due to their ability to provide large-area thermal protection for hypersonic vehicles. In this work, a novel ceramizable composite of quartz fiber/benzoxazine resin modified with fused SiOsub.2 and h-BN was fabricated using a prepreg compression molding technique. The effects of the fused SiOsub.2 and h-BN contents on the thermal, mechanical, and ablative properties of the ceramizable composite were systematically investigated. The ceramizable composite with an optimized amount of fused SiOsub.2 and h-BN exhibited superb thermal stability, with a peak degradation temperature and residue yield at 1400 °C of 533.2 °C and 71.5%, respectively. Moreover, the modified ceramizable composite exhibited excellent load-bearing capacity with a flexural strength of 402.2 MPa and superior ablation resistance with a linear ablation rate of 0.0147 mm/s at a heat flux of 4.2 MW/msup.2, which was significantly better than the pristine quartz fiber/benzoxazine resin composite. In addition, possible ablation mechanisms were revealed based on the microstructure analysis, phase transformation, chemical bonding states, and the degree of graphitization of the ceramized products. The readily oxidized pyrolytic carbon (PyC) and the SiOsub.2 with a relatively low melting point were converted in situ into refractory carbide. Thus, a robust thermal protective barrier with SiC as the skeleton and borosilicate glass as the matrix protected the composite from severe thermochemical erosion and thermomechanical denudation.
The trends of miniaturization, lightweight, and high integration in electronics have brought serious issues in heat dissipation and electromagnetic compatibility and also limited the simultaneous use ...of thermally conductive and microwave absorption materials. Therefore, it is imperative to design materials that possess those dual functions. In this work, one‐pot method is used to anchor zeolitic imidazolate framework ZIF‐67 coated with polydopamine (PDA) on boron nitride (BN) to obtain BN@ZIF‐67@PDA. The pyrolysis product BN@Co‐C@C is used as heterostructured thermally conductive/microwave absorption fillers and blended with polyethylene terephthalate (PET) to prepare BN@Co‐C@C/PET composites. When the mass ratio of BN to ZIF‐67@PDA is 7.5:1 and the mass fraction of BN7.5@Co‐C@C is 45 wt%, the BN7.5@Co‐C@C/PET composites exhibit excellent thermal conductivities and microwave absorption performances. The thermal conductivity coefficient is 5.37 W m−1 K−1, which is 35.8 times higher than that of PET (0.15 W m−1 K−1), and also higher than that of 45 wt% (BN7.5/Co‐C@C)/PET composites (4.03 W m−1 K−1) prepared by directly mixing. The minimum reflection loss of 45 wt% BN7.5@Co‐C@C/PET composites are −63.1 dB at 4.72 GHz, and the corresponding effective absorption bandwidth is 1.28 GHz (4.08–5.36 GHz), achieving excellent microwave absorption performance at C band.
Utilizing the super adhesion of PDA and the “forward deposition‐reverse growth” pattern, the prepared heterostructured BN@Co‐C@C provides rich heterostructured interfaces between BN and Co‐C@C on the premise of the good stacking of BN, which helps realize the simultaneous enhancements of thermal conductivities and wave absorption performances for the obtained BN@Co‐C@C/PET composites.
Hexagonal boron nitride (h‐BN) is a layered inorganic synthetic crystal exhibiting high temperature stability and high thermal conductivity. As a ceramic material it has been widely used for thermal ...management, heat shielding, lubrication, and as a filler material for structural composites. Recent scientific advances in isolating atomically thin monolayers from layered van der Waals crystals to study their unique properties has propelled research interest in mono/few layered h‐BN as a wide bandgap insulating support for nanoscale electronics, tunnel barriers, communications, neutron detectors, optics, sensing, novel separations, quantum emission from defects, among others. Realizing these futuristic applications hinges on scalable cost‐effective high‐quality h‐BN synthesis. Here, the authors review scalable approaches of high‐quality mono/multilayer h‐BN synthesis, discuss the challenges and opportunities for each method, and contextualize their relevance to emerging applications. Maintaining a stoichiometric balance B:N = 1 as the atoms incorporate into the growing layered crystal and maintaining stacking order between layers during multi‐layer synthesis emerge as some of the main challenges for h‐BN synthesis and the development of processes to address these aspects can inform and guide the synthesis of other layered materials with more than one constituent element. Finally, the authors contextualize h‐BN synthesis efforts along with quality requirements for emerging applications via a technological roadmap.
Scalable approaches of high‐quality mono/multilayer hexagonal boron nitride (h‐BN) synthesis are reviewed, the challenges and opportunities for each method are discussed, and their relevance to emerging applications is contextualized. Maintaining stoichiometric balance B:N = 1 in the growing crystal and enabling stacking order between layers emerge as the main challenges. Advances in these aspects will inform/guide the synthesis of other 2D materials with >1 constituent element.
In this paper, g-C.sub.3N.sub.4/SnO.sub.2:Sb composite photocatalysts were fabricated by in situ loading Sb-doped SnO.sub.2 (SnO.sub.2:Sb) nanoparticles on graphitic carbon nitride (g-C.sub.3N.sub.4) ...nanosheets via a facile hydrothermal method. The synthesized g-C.sub.3N.sub.4/SnO.sub.2:Sb composites delivered enhanced visible light photocatalytic performance for degradation of rhodamine B in comparison with g-C.sub.3N.sub.4/SnO.sub.2 composites without doping Sb. Various techniques including XRD, SEM, TEM, FTIR, XPS, PL and electrochemical method were employed to demonstrate the successful fabrication of g-C.sub.3N.sub.4/SnO.sub.2:Sb composite and to investigate the enhanced mechanism of photocatalytic activity. The improvement of visible light absorption and the promotion of separation efficiency and interfacial transfer of photogenerated carriers induced by Sb doping were responsible for the enhancement of photocatalytic activity. This study provides a simple and convenient method to synthesize a visible light responsive catalyst with promising performance for the potential application in environmental protection.
Fire retardant coatings have been proven effective at reducing the heat release rate (HRR) of structural materials during burning; yet effective methods for increasing the ignition temperature and ...delay time prior to burning are rarely reported. Herein, a strong, fire‐resistant wood structural material is developed by combining a densification treatment with an anisotropic thermally conductive flame‐retardant coating of hexagonal boron nitride (h‐BN) nanosheets to produce BN‐densified wood. The thermal management properties created by the BN coating provide fast, in‐plane thermal diffusion, slowing the conduction of heat through the densified wood, which improves the material's ignition properties. Compared with densified wood without the BN coating, a 41 °C enhancement in ignition temperature (Tig), a twofold increase in ignition delay time (tig), and a 25% decrease in the maximum HRR of BN‐densified wood can be achieved. As a proof of concept for scalability, the pieces of the BN‐densified wood are fabricated with a length larger than 25 cm, width greater than 15 cm, and thickness more than 7 mm. The improved thermal management, fire resistance, mechanical strength, and scalable production of BN‐densified wood position it as a promising structural material for safe and energy‐efficient buildings.
An anisotropic thermally conductive hexagonal boron nitride (h‐BN) coating strategy is demonstrated to increase the ignition temperature and delay time prior to burning for densified wood. The BN‐densified wood demonstrates enhanced fire resistance, outstanding mechanical properties, and scalable production, holding great potential for safe and energy‐efficient construction applications.
The morphology and dimension of the conductive filament formed in a memristive device are strongly influenced by the thickness of its switching medium layer. Aggressive scaling of this active layer ...thickness is critical toward reducing the operating current, voltage, and energy consumption in filamentary‐type memristors. Previously, the thickness of this filament layer has been limited to above a few nanometers due to processing constraints, making it challenging to further suppress the on‐state current and the switching voltage. Here, the formation of conductive filaments in a material medium with sub‐nanometer thickness formed through the oxidation of atomically thin two‐dimensional boron nitride is studied. The resulting memristive device exhibits sub‐nanometer filamentary switching with sub‐pA operation current and femtojoule per bit energy consumption. Furthermore, by confining the filament to the atomic scale, current switching characteristics are observed that are distinct from that in thicker medium due to the profoundly different atomic kinetics. The filament morphology in such an aggressively scaled memristive device is also theoretically explored. These ultralow energy devices are promising for realizing femtojoule and sub‐femtojoule electronic computation, which can be attractive for applications in a wide range of electronics systems that desire ultralow power operation.
A nonvolatile memristive device with a sub‐nanometer thick switching layer, sub‐picoampere operating current, and femtojoule per bit energy consumption is demonstrated. The ultrathin medium layer is formed through the oxidation of atomically thin hexagonal boron nitride. Due to the atomic‐scale confinement of the filament length, current switching characteristics disparate from that in a thicker medium are observed resulting from the distinct ionic kinetics.
Hexagonal boron nitride (h‐BN) has emerged as a strong candidate for two‐dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal ...stability, and chemical inertness. Super‐thin h‐BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti‐corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h‐BN followed by a comprehensive account of state‐of‐the‐art synthesis strategies for 2D h‐BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal‐mechanical‐chemical stability of 2D h‐BN, various potential applications of these structures are described.
h‐BN is one of the most promising inorganic materials of this century, with possible applications ranging from aerospace to medicine. It has emerged as an exotic 2D material in the post‐graphene era, owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. An encyclopedic view of the structure, properties, synthesis, and applications of h‐BN is provided.
We investigate defect properties in hexagonal boron nitride (hBN) which is attracting much attention as a single photon emitter. Using first-principles calculations, we find that nitrogen-vacancy ...defect (Vsub.N) has a lower energy structure in Csub.1h symmetry in 1- charge state than the previously known Dsub.3h symmetry structure. Noting that carbon has one more valence electron than boron species, our finding naturally points to the correspondence between Vsub.N and Vsub.NCsub.B defects with one charge state difference between them, which is indeed confirmed by the similarity of atomic symmetries, density of states, and excitation energies. Since Vsub.NCsub.B is considered as a promising candidate for the source of single photon emission, our study suggests Vsub.N as another important candidate worth attention, with its simpler form without the incorporation of foreign elements into the host material.
![A Novel Fused SiO[sub.2] an...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/2073-4360/small "A Novel Fused SiO[sub.2] and h-BN Modified Quartz Fiber/Benzoxazine Resin Ceramizable Composite with Excellent Flexural Strength and Ablation Resistance")
PDF
