Litong Zhang

Graphene-wrapped ZnO hollow spheres with enhanced electromagnetic wave absorption properties

Graphene-wrapped ZnO hollow spheres were synthesized by a two-step process, which combined a hydrothermal reaction with surface modification. The experimental results show that reduced graphene oxide sheets adhere entirely to the surface of the ZnO hollow spheres consisting of nanoparticles. The unique structure effectively decreases the density of the composite without sacrificing the contact between graphene and the nanoparticles. Different mass ratios of graphene to ZnO hollow spheres mixed in a paraffin wax matrix (50 wt%) were prepared to investigate the electromagnetic wave absorption properties in the X-band region. When the mass ratio of graphene oxide to ZnO is 12 : 88, the composite exhibits a maximum absorption of −45.05 dB at 9.7 GHz with a sample thickness of only 2.2 mm. The fundamental mechanism based on electrical conductivity and the polarization between the graphene sheets and ZnO nanoparticles is discussed. The hierarchical structure of graphene-wrapped ZnO hollow spheres exhibits a promising designable approach to lightweight electromagnetic wave absorbing materials.

Electromagnetic properties of Si–C–N based ceramics and composites

Abstract Besides the excellent high-temperature mechanical properties, Si3N4 and SiC based ceramics containing insulating or electrically conductive phase are attractive for their tunable dielectric properties, which may vary from electromagnetic (EM) wave transparent to absorption and shielding. Consequently, SiC, Si3N4, SiON, SiBN, SiBC, SiCN and SiBCN ceramics have attracted extensive interest in recent years. SiO2, Si3N4, Si3N4–SiO2, Si3N4–BN, and Si3N4–SiO2–BN are promising EM wave transparent materials for applications in microelectronic packaging, microwave transparent reaction chamber, radome and antenna window. C, SiC, SiC–C, Si3N4–C and Si3N4–SiC are potential EM wave shielding materials, which can be used as electronic packaging of highly integrated circuits, and be used in wireless communication system, telecommunication base stations and the other electronic devices. Si3N4–SiBC, Si3N4–SiCN and Si3N4–SiBCN are attractive EM wave absorbing materials for potential applications in amplifier, accelerator, microwave heating, anechoic chambers, stealth aircraft and ship. Other potential harsh environment or high-temperature applications will also benefit from the Si–C–N ceramic system. The concept of hybrid structure and EM metamaterials (MMS) opens up new avenues in developing EM wave absorption materials. The key developments and future challenges in this field are summarised. The main issues regarding permittivity of high-temperature structural ceramics are discussed, with an emphasis on the EM wave transparent, shielding and absorbing mechanisms that are responsible for the EM wave properties.

Ti3C2 MXenes with Modified Surface for High-Performance Electromagnetic Absorption and Shielding in the X-Band

Electromagnetic (EM) absorbing and shielding composites with tunable absorbing behaviors based on Ti3C2 MXenes are fabricated via HF etching and annealing treatment. Localized sandwich structure without sacrificing the original layered morphology is realized, which is responsible for the enhancement of EM absorbing capability in the X-band. The composite with 50 wt % annealed MXenes exhibits a minimum reflection loss of -48.4 dB at 11.6 GHz, because of the formation of TiO2 nanocrystals and amorphous carbon. Moreover, superior shielding effectiveness with high absorption effectiveness is achieved. The total and absorbing shielding effectiveness of Ti3C2 MXenes in a wax matrix with a thickness of only 1 mm reach values of 76.1 and 67.3 dB, while those of annealed Ti3C2 MXenes/wax composites are 32 and 24.2 dB, respectively. Considering the promising performance of Ti3C2 MXenes with the modified surface, this work is expected to open the door for the expanded applications of MXenes family in EM absorbing and shielding fields.

Microstructure and mechanical properties of three-dimensional carbon/silicon carbide composites fabricated by chemical vapor infiltration

Abstract Three-dimensional carbon/silicon carbide composites were fabricated by chemical vapor infiltration, and the microstructure and mechanical properties were investigated. For the composites ( C SiC ) with no pyrolytic carbon interfacial layer, the mechanical properties (flexural strength, flexural elastic modulus, shear strength and fracture toughness) are increased with density of the composites. High density ( p = 2.1 g cm −3 ) C SiC composites exhibit high fracture toughness (16.5 MPa m 1 2 ) but brittle fracture behavior because of strong bonding between the fiber/matrix. Low density composites show non-catastrophic failure mode with bundle pull-out. The composites (C/PyC/SiC) with pyrolytic carbon interfacial layer exhibit good mechanical properties and a typical failure behavior involving fiber pull-out and brittle fracture of sub-bundle. Microstructural observations and theoretical analysis reveal that the tortuosity and bottleneck effect of the pores and large molar mass of reactant agent (methyltrichlorosilane) are three key issues to hinder the densification of composites. Cracks formed in the SiC matrix by thermal stress have two influences on the mechanical properties of the composites: to decrease mechanical properties and have some contribution on toughness and failure behavior by deflecting cracks.

Carbon/silicon carbide composites prepared by chemical vapor infiltration combined with silicon melt infiltration

In order to reduce processing costs and improve the thermal stability of three-dimensional carbon fiber-reinforced silicon carbide composites, a chemical vapor infiltration combined with silicon melt infiltration method was developed for fabricating composites. According to the size of the pores in the preform, chemical vapor infiltration (CVI) and silicon melt infiltration (SMI) were mainly used to infiltrate small pores between fibers in a bundle and large pores between bundles, respectively. In the chemical vapor infiltration process, a pyrolytic carbon interfacial layer and a silicon carbide barrier layer were deposited on the surface of the carbon fiber. Then the pre-coated preform was infiltrated with pitch which was pyrolysed to form a porous carbon matrix in the pores. Finally, the preform was infiltrated with silicon melt to obtain composites. The influence of the interface thickness on the mechanical properties and the failure behavior of the composites were investigated, and the optimum thickness of the pyrolytic carbon layer was obtained. Experimental results also revealed that CVI+SMI composites exhibited good thermal stability of the mechanical properties and failure behaviors after the composites were annealed at high temperatures.

Mechanical properties of 3D fiber reinforced C/SiC composites

Abstract High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0–2.1 g cm−3 after the three dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. At elevated temperatures (1300 and 1600°C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m1/2 and 12.0 kJ·m−2, respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23.3. The value of dynamic fracture toughness was 62 kJ·m−2 measured by Charpy impact tests.

Carbon Hollow Microspheres with a Designable Mesoporous Shell for High-Performance Electromagnetic Wave Absorption

In this work, mesoporous carbon hollow microspheres (PCHMs) with designable mesoporous shell and interior void are constructed by a facile in situ stöber templating approach and a pyrolysis-etching process. The PCHMs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, Raman spectroscopy, and nitrogen adsorption and desorption system. A uniform mesoporous shell (pore size 4.7 nm) with a thickness of 55 nm and a cavity size of 345 nm is realized. The composite of paraffin mixed with 20 wt % PCHMs exhibits a minimum reflection coefficient (RCmin) of -84 dB at 8.2 GHz with a sample thickness of 3.9 mm and an effective absorption bandwidth (EAB) of 4.8 GHz below -10 dB (>90% electromagnetic wave is attenuated). Moreover, the composite of phenolic resin mixed with 20 wt % PCHMs exhibits an ultrawide EAB of 8 GHz below -10 dB with a thinner thickness of 2.15 mm. Such excellent electromagnetic wave absorption properties are ascribed to the large carbon-air interface in the mesoporous shell and interior void, which is favorable for the matching of characteristic impedance as compared with carbon hollow microspheres and carbon solid microspheres. Considering the excellent performance of PCHMs, we believe the as-fabricated PCHMs can be promising candidates as highly effective microwave absorbers, and the design philosophy can be extended to other spherical absorbers.

Preparation of an oxidation protection coating for c/c composites by low pressure chemical vapor deposition

Preparation defects in a SiC coating on C/C substrates are considered to be the controlling factor which decreases the oxidation protection life at the temperature of preparing. To decrease the number of such defects, a low-pressure chemical vapor deposition (LPCVD) method, instead of general CVD was used, and the substrates were suspended, rather than supported. Multi-deposition was employed to seal the preparation defects at the edges of the substrates produced by the suspension. The SiC coating, which was of a high quality, had a smaller crack width and a better interfacial binding. Every layer in the multi-layer coating was uniform and smooth, and the layer thickness could be easily controlled. The oxidation tests indicated that the preparation defects in the coating could not be sealed by multi-deposition as the gaps between the multi-layers acted as channels for oxygen diffusion towards the defects in the inner layer. The weight loss decreased as the multi-deposition times were increased. An analysis showed that no matter how many layers the coating consisted of, failure always began from the preparation defects in the inner layer which were located at the edges of substrates, and a cavity would be formed beneath the inner layer by direct oxidation of the C/C. The oxidation process in C/C substrates with a multi-layer coating was controlled by the rate of oxygen diffusion along the interlayer gaps which were thinner and longer than the defects. The more the multi-deposition times, the larger the length of the gaps, and the smaller the oxygen transport rate.

Carbon Nanotube–Multilayered Graphene Edge Plane Core–Shell Hybrid Foams for Ultrahigh‐Performance Electromagnetic‐Interference Shielding

Materials with an ultralow density and ultrahigh electromagnetic-interference (EMI)-shielding performance are highly desirable in fields of aerospace, portable electronics, and so on. Theoretical work predicts that 3D carbon nanotube (CNT)/graphene hybrids are one of the most promising lightweight EMI shielding materials, owing to their unique nanostructures and extraordinary electronic properties. Herein, for the first time, a lightweight, flexible, and conductive CNT-multilayered graphene edge plane (MLGEP) core-shell hybrid foam is fabricated using chemical vapor deposition. MLGEPs are seamlessly grown on the CNTs, and the hybrid foam exhibits excellent EMI shielding effectiveness which exceeds 38.4 or 47.5 dB in X-band at 1.6 mm, while the density is merely 0.0058 or 0.0089 g cm-3 , respectively, which far surpasses the best values of reported carbon-based composite materials. The grafted MLGEPs on CNTs can obviously enhance the penetration losses of microwaves in foams, leading to a greatly improved EMI shielding performance. In addition, the CNT-MLGEP hybrids also exhibit a great potential as nano-reinforcements for fabricating high-strength polymer-based composites. The results provide an alternative approach to fully explore the potentials of CNT and graphene, for developing advanced multifunctional materials.

Effect of glass sealing on the oxidation behavior of three dimensional C/SiC composites in air

Abstract A borosilicate glass which consisted of 40–45 mol.% B2O3 and 55–60 mol.% SiO2 was used as a sealant for the C/SiC composites with and without a Si–Zr coating, and the oxidation behavior of the C/SiC composites in air was investigated. The glass increased greatly the oxidation resistance by decreasing the maximum weight loss and the cracking temperature range below 1000°C as well as by lowering the transition temperature. The effective sealing temperature of the glass was from 700 to 900°C. The glass began losing its sealing ability very rapidly above 900°C, and lost this ability completely above 1000°C. After oxidation for 20 h in an air atmosphere with a large temperature gradient from 350 to 1300°C, the strengths of the composite with both the Si–Zr coating and the glass sealant were maintained over the whole temperature range. The composite with the sealant only lost strength mostly at high temperature and that with only the coating lost almost all its strength at low temperature, and both of the composites could not be used in environments with a temperature gradient. The sealing temperature range of the glass was narrower than the cracking temperature range of the composite. No matter how the composition was changed, the borosilicate glass could not seal over the whole cracking temperature range of the composite at the same time for long-term use.