(SiC-Si3N4)w/SiBCN composite ceramics with tunable electromagnetic properties

The reasons for high toughness in Al2O3/SiC whisker/ZrO2 ceramic composite were elucidated by each toughening mechanims such as crack deflection and whisker pull out.The condition required for SiC whisker to obtain ceramic composite was analysed by fracture mechanics and was found that diamater of SiC whisker must be lowered under a critical size. Toughening mechanisms by SiC whisker such as crack deflection and whisker pull out were also discussed. Importance of diameter, length and strength of SiC whisker was made clear. On the other hand contributions of UP ZrO2 to high toughness were also explained by grain refining and transformation toughening. Finally, additional effect of each toughening mechanism, which is called multiple toughening, was shown to be important to obtain high toughness ceramic composite.

The influence of gradients in hardness and elastic properties at interfaces of dissimilar materials in laminated metallic composites (LMCs) on fatigue crack propagation is investigated experimentally for three different LMC systems: Al/Al-LMCs with dissimilar yield stress and Al/Steel-LMCs as well as Al/Ti/Steel-LMCs with dissimilar yield stress and Young’s modulus, respectively. The damage tolerant fatigue behavior in Al/Al-LMCs with an alternating layer structure is enhanced significantly compared to constituent monolithic materials. The prevalent toughening mechanisms at the interfaces are identified by microscopical methods and synchrotron X-ray computed tomography. For the soft/hard transition, crack deflection mechanisms at the vicinity of the interface are observed, whereas crack bifurcation mechanisms can be seen for the hard/soft transition. The crack propagation in Al/Steel-LMCs was studied conducting in-situ scanning electron microscope (SEM) experiments in the respective low cycle fatigue (LCF) and high cycle fatigue (HCF) regimes of the laminate. The enhanced resistance against crack propagation in the LCF regime is attributed to the prevalent stress redistribution, crack deflection, and crack bridging mechanisms. The fatigue properties of different Al/Ti/Steel-LMC systems show the potential of LMCs in terms of an appropriate selection of constituents in combination with an optimized architecture. The results are also discussed under the aspect of tailored lightweight applications subjected to cyclic loading.

This study aims to design and manufacture a low-density hybrid corrugated core sandwich structure, employing a nickel-coated glass fiber (NCF)/polyether ether ketone (PEEK) thermoplastic matrix, which demonstrates exceptional radar absorption performance and lightning resistance within the C~Ku (4-18 GHz) band. To modify the electromagnetic (EM) characteristics of glass fibers (GF), a nickel electroless plating technique was employed to plate the surface of the GF with nickel. The complex permittivity (ε = ε' - jε'') of the GF/PEEK and NCF/PEEK composites was measured using a free space measurement (FSM) technique within the C~X-band (5.8-12.4 GHz) to assess their EM properties. The measurement results revealed that at 10 GHz, the ε of GF/PEEK was 4.69 - j0.079, whereas the ε of NCF/PEEK increased to 18.50 - j15.32, indicating improved dielectric properties. To design a broadband radar absorbing structure (RAS) with exceptional microwave absorption performance, the optimal shape of the corrugated core was determined through the utilization of CST Microwave Studio. The designed RAS demonstrated absorption performance exceeding 90% within the C~Ku band (4-18 GHz). Additionally, to improve the lightning protection effectiveness in RAS designs, a polyimide (PI) film coated with patterned silver paste was applied to the facesheet of corrugated core sandwich panel. Through lightning experiments, it was observed that the lightning protection effect was enhanced in the presence of the dedicated lightning strike protection film. The outcomes of this study indicate that the developed corrugated core sandwich structure for RAS holds potential for applications in stealth technology.

Composite ceramics of B4C containing up to 30vol% SiC whisker were fabricated by hot pressing at 2000°-2200°C under 30MPa for 30min. The mechanical properties (bending strength, fracture toughness, hardness, elastic modules) of the hot pressed bodies were measured. Fully dense B4C-SiC whisker composite ceramics were obtained at the whisker contents up to 30vol% at a sintering temperature above 2100°C. These composites had maximum strength at the whisker content of 10 or 20vol% depending upon the hot pressing temperature. The composite containing 10vol% SiC whisker, fired at 2100°C, had the highest strength (about 830MPa), which was 40% larger than that of monolithic B4C. Slight increase of fracture toughness were obtained in the composites because there was no pull-out of whiskers and no crack deflection in the fracture process. The strength and fracture toughness of the composites containing whiskers with a small diameter were lower than those of the composites with a large diameter, since the whiskers with a small diameter could not maintain their original morphology after sintering. The hardness and elastic modulus were 30GPa and 430GPa, respectively, independent of whisker addition.

This paper studied the effect of graphene oxide nano‐platelets (GOPs) on the fracture behaviour of epoxy adhesive‐bonded joints, experimentally and numerically. The double cantilever beam specimens were tested and the fracture energies and resistance curves were determined for the unreinforced and reinforced adhesives. A maximum improvement of 69% in the fracture energy was achieved with the addition of 0.3 wt% of GOPs into the adhesive layer. Moreover, the fracture surfaces were examined using the scanning electron microscopy technique to find out the toughening mechanisms involved in this improvement and mechanisms of GOP debonding and crack deflection were observed. Furthermore, a cohesive zone model was utilised to numerically study the effect of GOPs on the fracture behaviour of the adhesive. The numerical analyses showed that the damage process zone length was also influenced by GOPs. The longest damage process zone was obtained for the toughened adhesive with 0.3 wt% of GOPs.

Fabrication of radar absorbing structure (RAS) using GFR-nano composite and spring-back compensation of hybrid composite RAS shells

Polymer‐derived ceramics (PDCs), such as SiOC, SiCN, SiBCN, and SiC are considered the best candidates for designing high‐performance microwave absorber due to their controllable structure, homogeneous element composition at atom level, tunable electromagnetic and electrochemical properties. Herein, Fe ions doped silicon oxycarbide (Fe ions‐SiOC) ceramics have been successfully fabricated via solvothermal method. The electromagnetic absorption performances of the nanocomposites prove to be controllable via tailoring Fe ion contents. The Fe ions effectively enhance both the interfacial polarization of amorphous SiOC, and the dielectric properties of the nanocomposites but barely effect magnetic properties of the nanocomposite. As for 0.16 mol/L‐SiOC ceramics annealed at 1450°C, the effective absorption bandwidth as high as 2.00 GHz and reflection loss of −59.60 dB at 5.40 GHz with the thickness of 4.55 mm are obtained. Such work opens up a novel and simple route to scale up the PDC‐based materials with broadband and excellent microwave absorbing performances.

Although microstructure optimization is an effective strategy to improve and regulate electromagnetic wave (EMW) absorption properties, preparing microwave absorbents with enhanced EMW absorbing performance and tuned absorption band by a simple method remains challenging. Herein, ZnIn2S4/reduced graphene oxide (rGO) composites with flower-like and cloud-like morphologies were fabricated by a convenient hydrothermal method. The ZnIn2S4/rGO composites with different morphologies realize efficient EMW absorption and tunable absorption bands, covering a wide frequency range. The flower-like structure has an optimal reflection loss (RL) of up to -49.2 dB with a maximum effective absorption bandwidth (EAB) of 5.7 GHz, and its main absorption peaks are concentrated in the C and Ku bands. The minimal RL of the cloud-like structure can reach -36.3 dB, and the absorption peak shifts to the junction of X and Ku bands. The distinguished EMW absorption capacity originates from the uniquely optimized microstructure, complementary effect of ZnIn2S4 and rGO in dielectric constant, and synergy of various loss mechanisms, such as interfacial polarization, dipole polarization, conductive loss, and multiple reflections. This study proposes a guide for the structural optimization of an ideal EMW absorber to achieve efficient and tunable EMW absorption performance.

Tunable strength of SiCf/β-Yb2Si2O7 interface for different requirements in SiCf/SiC CMC: Inspiration from model composite investigation

Grain boundary devitrification was carried out on three Ca α-sialon ceramics with different grain sizes and morphologies and various amounts of grain boundary glass. The devitrified product was gehlenite in all samples, indicating that the crystallization of the Ca oxynitride glass was accompanied by a volume reduction. The volume reduction upon crystallization and the thermal expansion mismatch between the devitrified product and α-sialon grains would result in tensile residual stresses located at multi-grain junctions. These residual tensile stresses were expected to promote the crack deflection and bridging mechanism and thus to improve the material toughness. However, indentation fracture toughness measurement and scanning electron microscope study showed that there was no significant difference in fracture toughness and the fracture mode in present samples prior to and post heat treatment. This may be attributed to a change in the chemistry of the residual glass as a result of the grain boundary devitrification, which could enhance the bonding strength between the adjacent α-sialon grains. The enhanced bonding strength could have to some degree hindered the crack deflection and bridging mechanism.

The optical properties of canted antiferromagnets are discussed from the perspective of magnetic polaritons and field tunable electromagnetic (EM) properties. Canted antiferromagnets have a weak magnetization. Their EM properties can be affected by externally applied magnetic fields as well as electric fields, if the material is multiferroic. A theoretical approach that is useful for describing the THz frequency EM response and properties of a large variety of canted spin systems is outlined here. The properties of two example systems are discussed: an antiferromagnet in which spin canting is induced through an externally applied magnetic field; and an antiferromagnetic multiferroic wherein the magnetic sublattices are canted through an internal coupling to a spontaneous dielectric polarization.

Preparation and properties of Ta fiber reinforced high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-SiC composite ceramics

Au nano-tetradecapods with well-defined morphology and controllable branches are synthesized, and their tunable optical and electromagnetic properties are systematically investigated.

Conductivity-dependent dielectric properties and microwave absorption of Al-doped SiC whiskers

Porous SiC-Si2N2O-Si3N4 composite ceramics with excellent EMW absorption properties prepared by gelcasting and carbonthermal reduction