Preparation and Performance of Si3N4 Hollow Microspheres by the Template Method and Carbothermal Reduction Nitridation

Dual metal synergistic modulation of boron nitride for high-temperature wave-transparent metamaterials

Electromagnetic metamaterials have demonstrated immense potential in the development of novel high-temperature wave-transparent materials, yet the requirements of their intricate structural design and strict stability pose dual challenges, particularly in high-speed radome applications. A strategy involving the synergistic modulation of boron nitride (BN) by dual metallic elements of Ca and Al (0.5Ca-0.5Al-BN) was proposed in this study, which elegantly integrates the advantages of metamaterial-like split ring resonator (SRR) features and h-BN's oxidation resistance enhancement. The highest wave transmittance at room temperature reaches 0.96 at 2-18 GHz. Notably, Al elements play a pivotal dual role in: (1) facilitating the solid solution of Ca to optimize the formation of metamaterial-like structures and (2) generating an amorphous Al2O3 protective layer to preferentially defend against surface oxidation. This further prevents the breakdown of metamaterial characteristics at high temperatures, thereby striking a dual balance between the preservation of metamaterial-like structures and the high temperature stability of BN. Notably, 0.5Ca-0.5Al-BN retains its metamaterial-like characteristics, with a low permittivity not exceeding 2 even after exposure to 1500 °C oxidation. The corresponding wave transmission rate remains above 0.7 in most frequency bands at incidence angles of 0°, 10°, and 30°, ensuring superior wave-transparent properties. Furthermore, 0.5Ca-0.5Al-BN exhibits great hydrophobicity, benefiting resistance to rain and snow erosion. By integrating the merits between fundamental materials and metamaterials, this work transcends the limitations of conventional metamaterial design and offers fresh insights and empirical support for developing high-speed aircraft radome materials.

Preparation and application of hemostatic microspheres containing biological macromolecules and others

Bleeding from uncontrollable wounds can be fatal, and the body's clotting mechanisms are unable to control bleeding in a timely and effective manner in emergencies such as battlefields and traffic accidents. For irregular and inaccessible wounds, hemostatic materials are needed to intervene to stop bleeding. Hemostatic microspheres are promising for hemostasis, as their unique structural features can promote coagulation. There is a wide choice of materials for the preparation of microspheres, and the modification of natural macromolecular materials such as chitosan to enhance the hemostatic properties and make up for the deficiencies of synthetic macromolecular materials makes the hemostatic microspheres multifunctional and expands the application fields of hemostatic microspheres. Here, we focus on the hemostatic mechanism of different materials and the preparation methods of microspheres, and introduce the modification methods, related properties and applications (in cancer therapy) for the structural characteristics of hemostatic microspheres. Finally, we discuss the future trends of hemostatic microspheres and research opportunities for developing the next generation of hemostatic microsphere materials.

Application of Biomedical Microspheres in Wound Healing

Tissue injury, one of the most common traumatic injuries in daily life, easily leads to secondary wound infections. To promote wound healing and reduce scarring, various kinds of wound dressings, such as gauze, bandages, sponges, patches, and microspheres, have been developed for wound healing. Among them, microsphere-based tissue dressings have attracted increasing attention due to the advantage of easy to fabricate, excellent physicochemical performance and superior drug release ability. In this review, we first introduced the common methods for microspheres preparation, such as emulsification-solvent method, electrospray method, microfluidic technology as well as phase separation methods. Next, we summarized the common biomaterials for the fabrication of the microspheres including natural polymers and synthetic polymers. Then, we presented the application of the various microspheres from different processing methods in wound healing and other applications. Finally, we analyzed the limitations and discussed the future development direction of microspheres in the future.

1D CNT-Expanded 3D Carbon Foam/Si3N4 Sandwich Heterostructure: Utilizing the Polarization Compensation Effect for Keeping Stable Electromagnetic Absorption Performance at Elevated Temperature

Modern electromagnetic (EM) absorbing materials (EAMs) are experiencing a revolution triggered by advanced information technology. Simultaneously, the diverse harsh EM application scenarios entail a more stringent appeal of practicability to EAMs, especially under high-temperature conditions. Therefore, exploring EAMs with both excellent absorbing performance and practicability at elevated temperatures is necessary. Herein, a novel 3D porous carbon foam/carbon nanotubes@Si₃N₄ (CF/CNTs@Si₃N₄) heterostructure was constructed by the chemical vapor infiltration process. The optimally grown 1D CNTs embedded in 3D CF/Si₃N₄ are utilized to provide abundant nanointerface coupling effects to compensate for the excessive increase in the conductive loss during rising temperature to realize a self-adjustment in response to high temperature. A high-efficiency EM absorption over a wide temperature range from 25 to 480 °C was achieved (with a ≥90% absorbing ratio covering the whole X-band). In addition, the Si₃N₄ coating can improve the thermal stability of the carbon matrix and maintain the tailored inner structure. Multiple investigations into other environmental adaptabilities also exhibited the application perspective of such a heterostructure. This work points out a new strategy for preparing designable, efficient, and high-temperature applicable EAMs, promoting the diverse development of electronic devices.

Formation of Ultra-High Temperature Ceramic Hollow Microspheres as Promising Lightweight Thermal Insulation Materials via a Molten Salt-Assisted Template Method

Ultra-high temperature ceramics (UHTCs) have become a vital candidate material system for thermal protection systems in aerospace applications. However, high thermal conductivity and high density are the main obstacles to the application of UHTCs. It is a promising solution to prepare porous UHTCs using UHTC hollow microspheres (HMs) as a pore-forming agent. In this work, UHTC (ZrC, TiC, and HfC) HMs are successfully synthesized using carbon hollow microspheres (CHMs) as a template to react with metal powders in molten salt. The diameter of ZrC HMs is about 1 μm and the wall thickness is about 100 nm. The density of each microsphere and the volume fraction of ZrC are 3.36 g/cm³ and 48.42 vol %, respectively. The morphology, microstructure, and phase composition of the obtained ZrC HMs were characterized. The formation mechanism of the UHTC HMs was discussed. Porous ZrC ceramics were prepared using ZrC HMs as a pore-forming agent. The density and thermal conductivity of the porous ZrC ceramics are 3.12 g/cm³ and 1.82 W/(m·K), respectively, which are 53.64 and 91.12% lower than the density and thermal conductivity of dense ZrC ceramics, respectively. The results indicated that ZrC HMs are promising as pore-forming agents or a matrix for lightweight thermal insulation and high-temperature resistance applications in ultra-high temperature environments.

Synthesis and carbothermal nitridation mechanism of ultra-long single crystal α-Si3N4 nanobelts

One-dimensional Si₃N₄ nanostructures are desirable for constructing nanoscale electric and optoelectronic devices due to their peculiar morphologies. Herein, a facile and environmentally friendly catalyst-free method is proposed to synthesize ultra-long single crystal α-Si₃N₄ nanobelts via carbothermal nitridation of carbon nanotubes at 1750 °C. The obtained α-Si₃N₄ nanobelts with a flat surface (thickness of ∼150 nm, length of several millimeters) exhibited an extremely high aspect ratio, perfect crystal structure, and high specific surface area of 7.34-10.09 m² g^-1. In addition, the width was increased from approximately 80 nm to 8 μm by increasing the holding time from 1 to 3 h. The nanobelt formation was governed by the vapor-solid (VS) reaction between SiO vapor, N₂ and carbon nanotubes, and the vapor-vapor reaction between SiO, CO and N₂. The former was responsible for the initial nucleation and successive base-growth of α-Si₃N₄ nanotubes. The latter additionally contributed to the nanorod and subsequent proto-nanobelt formation and to the growth of the nanobelts. During high-temperature annealing at 1750 °C, the original Si₃N₄ nanotubes gradually transformed into nanorods, and, finally, nanobelts with stable shapes as a result of surface energy minimization.