Electromagnetic absorbing properties of graphene–polymer composite shields

A graphene-based composite, consisting of a thermosetting polymeric matrix filled with multilayer graphene microsheets (MLGs), is developed for application in thin radar absorbing materials. An innovative simulation model is proposed for the calculation of the effective permittivity and electrical conductivity of the composite, and used for the electromagnetic design of thin radar absorbing screens. The model takes into account the effects of the MLG morphology and of the fabrication process on the effective electromagnetic properties of the composite. Experimental tests demonstrate the validity of the proposed approach and the accuracy of the developed simulation models, which allow to understand the interaction mechanism between the incident electromagnetic field radiation and the MLG-based composite. Two dielectric Salisbury screen prototypes with resonant frequency at 12 GHz or 12.5 GHz and total thickness of 1.8 mm and 1.7 mm, respectively, are fabricated and tested. The results and technique proposed represent a simple and effective approach to produce thin absorbing screens for application in stealth technology or electromagnetic interference suppression.     

Preparation of SiCf/Si–O–C composites by impregnation pyrolysis of polysiloxane and its electromagnetic wave absorption properties

High-temperature structural electromagnetic wave (EMW) absorption materials are increasing in demand because they can simultaneously possess the functions of mechanical load-bearing, heatproof, and EMW absorption. Herein, SiC_f/Si–O–C composites were prepared by precursor impregnation pyrolysis using continuous SiC fibers needled felt as reinforcement and polysiloxane as a precursor, respectively. The phase composition, microstructure, complex permittivity, and EMW absorption properties of SiC_f/Si–O–C composites after annealing at various temperatures were investigated. The annealing at 1400–1500°C affects positively the EMW absorption performance of the composites, because the β-SiC microcrystals and SiC nanowires were generated by the activation of carbothermal reduction reaction in the composites, and the aspect ratio of SiC nanowires increased with the rise of temperature. The composites exhibit optimal EMW absorption performance, with the effective absorption bandwidth covering the entire X-band and the minimum reflection loss (RL_min) of −32.8 dB at 4.0 mm when the annealing temperature is raised to 1500°C. This is because that the impedance matching is improved as the rising of ε′ and decreasing of ε″ due to the conversion of free carbon in the composite into SiC nanowires.     

Improved tribological properties of polyimide composites by micro–nano reinforcement

The hydroxylate carbon nanotubes (CNTs) were grafted by chemical method on the surface of the oxidized carbon fibers (CF) to improve the mechanical and tribological properties of polyimide (PI). The microstructure and fracture surface of the polyimide composites indicated that CF–CNTs hybrid as a multiscale reinforcement can distribute into the PI matrix homogeneously. Tribo-tests further showed that CF–CNTs hybrid had a better effect on hardness increment, impact strength enhancement, friction reduction, and wear resistance. Compared to the neat PI, the friction coefficient and wear rate of CF–CNTs/PI composite deceased by 23.2 and 55.9%, respectively. In particular, the loading capacity and high speed resistance of CF–CNTs/PI composite were greatly improved. The corresponding wear mechanisms were also discussed by observing the worn surface of the PI composites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47900.     

Dielectric and electromagnetic wave absorption properties of reduced graphene oxide/barium aluminosilicate glass–ceramic composites

BaAl₂Si₂O₈ (BAS) glass–ceramic powders were prepared by sol–gel method. Graphene oxide (GO)/BAS mixture powders were prepared by a simple mixing process of GO and BAS. Dense and uniform reduced graphene oxide (RGO)/BAS composites were fabricated by the hot-pressing of GO/BAS, which was accompanied by the in-situ thermal reduction of GO. Microstructure, phase composition, dielectric and electromagnetic wave (EM) absorption properties of RGO/BAS were investigated. The results reveal that RGO can promote the hexacelsian-to-celsian phase transformation of BAS. In the frequency range from 8 GHz to 12 GHz, the complex permittivity of RGO/BAS increases with increasing RGO content. The composite with 1.5 wt% of RGO shows good EM absorbing ability. When the sample thickness is 2.1 mm, the minimum reflection coefficient (RC) reaches −33 dB, and the effective absorption bandwidth is more than 3.1 GHz.     

In situ synthesis,physical and mechanical properties of ZrB2–ZrC–WB composites

In this study, two composition ZrB₂–ZrC–WB composites were synthesized by reactive hot-pressing of Zr + B₄C + WC powder mixtures at 1900 °C. The microstructure of the resulting composites was characterized by a combination of scanning electron microscopy and X-ray diffraction. It is seen that highly-dense ZrB₂–ZrC–WB composites with a homogenous fine-microstructure were obtained after the sintering. The mechanical behavior of the composites was evaluated using by testing under four-point bend testing at room and high temperatures. The results show that the high-temperature strength of the ZrB₂–ZrC–WB composites was substantially improved, compared to ZrB₂–ZrC-based composites without WB. In addition, the elastic properties, electrical conductivity, hardness and fracture toughness of the composites were measured at room temperature. The results reveal that these properties were comparable to those of ZrB₂–ZrC-based composites without WB.     

High-temperature properties of 2.5D SiO2f/SiO2 composites by sol–gel

2.5D SiO_2f/SiO₂ composites were fabricated by sol–gel process. The mechanical and fracture behavior of SiO_2f/SiO₂ composites under higher temperature were discussed. The oxidation behavior at 1200 °C and 1500 °C was investigated. The results showed that SiO_2f/SiO₂ composites had high flexural strength, and the fracture mechanism was a combination of brittle and ductile fracture. After higher temperature oxidation, the fracture mechanism changed to typical brittle/sudden fracture. For long time usage at higher temperature, it was necessary to stabilize SiO₂ fibers and SiO₂ matrix of SiO_2f/SiO₂ composites.     

Electrical properties of low-temperature-fired ferrite–dielectric composites

The effects of the BaO·(Nd_0.8Bi_0.2)₂O₃·4TiO₂ (BNBT) to NiCuZn ferrite ratio and addition of Bi₂O₃–B₂O₃–SiO₂–ZnO (BBSZ) glass on the sintering behavior, microstructure evolution, dielectric and magnetic properties of BNBT–NiCuZn ferrite composites were investigated in developing low-temperature-fired composites for high frequency electromagnetic interference (EMI) devices. The results indicate that these composites can be densified at 900 °C and exhibit superior dielectric and magnetic properties with the addition of BBSZ glass. The dielectric system used in the ferrite–dielectric composites reported in the previous studies mostly belong to the ferroelectricity group, which are not suitable for use in the high frequency range (>800 MHz) due to the selfresonance frequency limit. In this study, the dielectric constant remains nearly a constant over a wide range of frequencies (100 MHz to 1 GHz) and the magnetic resonance frequencies are larger than 100 MHz for the BNBT + BBSZ glass–NiCuZn ferrite composites. Therefore, the BNBT + BBSZ glass–NiCuZn ferrite composites can be a good candidate material for high frequency EMI device applications.     

Quasistatic and dynamic properties of 1–3 composites made by soft molding

The development of the soft mold process allows for the preparation of fine scale 1–3 composites with PZT rods of different size, shape and spacing, which can be used as ultrasonic transducers for frequencies ⩾5 MHz. [Gebhardt, S., Schönecker, A., Steinhausen, R., Hauke, T., Seifert, W. & Beige, H., Fine scale 1-3 composites fabricated by the soft mold process: preparation and modeling. Ferroelectrics, 241 (2000) 67]. By this method, composites with a square, hexagonal and non regular arrangement of PZT rods of different shape have been fabricated and characterized by measuring their quasistatic and dynamic properties. The experimental results were compared with data from finite element method (FEM) modeling and analytical solutions. The vibrational characteristics of the composites were strongly influenced by the rod geometry and the rod arrangement. To evaluate the 1–3 composite performance, modal analysis and modeling of the impedance spectrum were carried out using the FEM package ANSYS.     

Reinforced properties of ethylene–propylene–diene monomer composites by vinyltrimethoxysiloxane functionalised barium titanate

In this study, non-conductive ethylene–propylene–diene monomer (EPDM)/barium titanate (BaTiO₃) composites with high dielectric constant and low dielectric loss are prepared. Fourier transform infrared (FT-IR) spectra show the chemical adherence of vinyltrimethoxysiloxane oligomer (SG-Si6490) to the surface of BaTiO₃ particles. Functionalised BaTiO₃ particles have better compatibility with EPDM matrix and promote the cure properties of EPDM composites. It is found that when the content of BaTiO₃ increases to 40?vol.-%, the resistivity, rheological, dielectric and mechanical properties of EPDM/BaTiO₃ composites change drastically. The dielectric constant of EPDM with 50?vol.-% BaTiO₃ at 10?MHz is 15, which is 7.5 times higher than that of EPDM control. Meanwhile, the volume resistivity results show EPDM with 50?vol.-% BaTiO₃ is still non-conductive. As for mechanical properties, the tensile and tear strength of EPDM control increase from 1.45?MPa and 8.73?kN?m^?1 to 10.02?MPa (about seven times higher) and 24.65?kN?m^?1 (about three times higher), respectively.     

Effect of silicon carbide dispersion on the microwave absorbing properties of silicon carbide-epoxy composites in 2–40 GHz

In this study, silicon carbide powders were manufactured successfully by the method of preheating combustion synthesis in nitrogen atmosphere where it was introduced into an epoxy resin to produce a microwave absorber. The structure of the silicon carbide was characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Composite based on the various loadings of silicon carbide and epoxy resin specimens were prepared and the reflection losses of these composite samples were studied using the free space method. Based on the microwave measurements, microwave absorber specimens of silicon carbide with thermal plastic resin at frequencies between 2 and 18 and 18–40 GHz could be obtained from a matching thickness of 2.0 mm by controlling the content of silicon carbide.     

Fabrication and properties of cordierite based glass/AlN composites by sol–gel and pressureless sintering

In this study, the effect of AlN content on the crystallization behavior of cordierite based glass, was firstly investigated. Results show that μ-cordierite appeared in the composites with high AlN content even at high temperatures, which implied that the AlN may broad the crystallization temperature range of μ-cordierite and depress the transformation of μ→α-cordierite. The crystallization temperature of α-cordierite was about 980 °C for the pure glass and the temperature increased with AlN content for composites, but the crystallization temperature of μ-cordierite had reverse trend. The composites owned excellent bending strength when the AlN content was 20 wt%. With increasing of AlN content, the dielectric loss was increased which was caused mainly by the structural loss and the appearance of μ-cordierite, but the dielectric constant had crosscurrent. It was observed that the composites were beneficial in producing LTCC material which can be highlighted with high strength, low shrinkage and good dielectric properties at 1 MHz.     

Friction and wear properties of C/C–SiC braking composites

Two series of C/C–SiC composites were fabricated via precursor infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) using porous C/C composites with different original densities as preforms, respectively. The tribological characteristics of C/C–SiC braking composites were investigated by means of MM-1000 type of friction testing machine. The friction and wear behaviors of the two series of composites were compared and the factors that influence the friction and wear properties of C/C–SiC composites were discussed. Results show that the friction and wear properties relate close-knit to the content of SiC and porosity. As the original preform density increasing, the content of SiC and porosity decrease, and then the friction coefficient increases obviously, the braking time and the wear rate both decrease. Preparation techniques play an important role in the tribological properties of C/C–SiC composites. Compared with PIP process, the samples from CVI have a little higher friction coefficient, shorter braking time and higher wear rate.     

Preparation of Si–diamond–SiC composites by in-situ reactive sintering and their thermal properties

This article described a novel method of preparation of Si–diamond–SiC composites by in-situ reactive spark plasma sintering (SPS) process. The relative packing density of Si–diamond–SiC composite was 98.5% or higher in a volume fraction range of diamond between 20% and 60%. Si–diamond–SiC composites containing 60 vol% diamond particles yielded a thermal conductivity of 392 W/m K, higher than 95% the theoretical thermal conductivity calculated by Maxwell–Eucken's equation. Coefficients of thermal expansion (CTEs) of the composites are lower than the values of theoretical models, indicating strong bonding between the diamond particle and the Si matrix in the composite. The microstructures of these materials were studied by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). As a result of reaction between diamond and silicon, SiC phase formed.     

Investigation of microstructure and mechanical properties of Cu/Ti–B–SiCp hybrid composites

In this study, Cu/Ti–B-SiC_p hybrid composite materials were produced by powder metallurgy method using three different sintering temperatures (950, 1000, 1050 °C). The optimum sintering temperature of Cu main matrix composites reinforced with Ti–B-SiC_p reinforcement materials at 2-4-6-8 wt.% were determined and their microstructure and mechanical properties were investigated. As a result of microstructure studies, it was determined that reinforcement elements have a homogeneous interface in the main matrix. The hardness of the produced composites was determined by the Brinell hardness method. The highest hardness value (77.74 HB) was determined in the sample with 6 wt% reinforcement ratio. In the tensile and three point bending tests, maximum strength values (112.96 MPa, 37.76 MPa) were found in samples with a reinforcement ratio of 4 wt%. It was determined that increasing reinforcement ratios and sintering temperature made a positive contribution to the hybrid composite materials produced.     

Effects of SiC on the microstructures and mechanical properties of B4C–SiC–rGO composites prepared using spark plasma sintering

In this study, B₄C–SiC–rGO composites with different SiC contents were prepared by spark plasma sintering at 1800 °C for 5 min under a uniaxial pressure of 50 MPa. The effects of SiC on the microstructures and mechanical properties of the B₄C–SiC–rGO composites were investigated. The optimal values for flexural strength (545.25 ± 23 MPa) and fracture toughness (5.72 ± 0.13 MPa·m^1/2) were obtained simultaneously when 15 wt.% SiC was added to 5 wt.%–GO reinforced B₄C composites (BS15G5). It was found that SiC and rGO inhibited the grain growth of B₄C and improved the mechanical properties of the B₄C–SiC–rGO composites. The clear and narrow grain boundaries of rGO–B₄C and rGO–SiC, as well as the semi-coherent B₄C–SiC interface, indicated strong interface compatibility. The twin structures of SiC and B₄C observed in the composites improved their fracture toughness. Crack deflection and crack bridging caused by the SiC grains as well as rGO bridging and rGO pull-out were observed on the crack propagation path.     

Renewable antioxidant properties of suspensible chitosan–polypyrrole composites

Conductive polymers have the ability to capture radicals and have become in focus for antioxidant applications of food packaging or biomedical applications. Unfortunately, the conducting polymers such as polypyrrole are difficult to suspense in solution after chemical or electrochemical polymerization. Chitosan, as a natural polymer from chitin, can be dissolved in diluted acetic acid solutions. In the present study, composites suspensible in diluted acetic acid solutions have been produced by the chemical polymerization of pyrrole in chitosan solution using ammonium persulfate (APS) as the oxidant. FTIR and UV–Vis measurements did identify an attachment of polypyrrole to chitosan.In order to optimize the activity and stability of the composites, the ratios of APS: polypyrrole: chitosan were analyzed. The chitosan–polypyrrole composites were formed as membranes (coatings); impedance measurements indicated their conductivity to be in the range of 10^?3–10^?7 S cm^?1. The antioxidant (radical scavenger activity) properties were determined by the di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH) assay. The radical scavenger activity of the composites was found renewable by means of electrochemical cycling.     

Processing–microstructure–properties relationships of MoSi2–SiC composites

Silicon carbide green bodies with and without carbon-fibre reinforcement have been infiltrated with MoSi₂–Si–X in order to produce high-temperature resistant materials. X is Cr, Ti, Al or B respectively. By adding silicon and one of these components to MoSi₂ the melting point is lowered dramatically. The composites therefore could be gained by melt infiltration at max. 1600 °C. During infiltration the additives react within the infiltrated body with carbon or silicon to form high-temperature resistant carbides or silicides. Thermodynamic calculations have been performed to analyse the reactions during infiltration. The infiltration parameters have been studied with respect to the resulting microstructure and properties. By fitting the amount of additives to the quantity of carbon in the SiC-body (or vice versa) no decrease in strength could be observed up to 1500 °C. The fracture toughness can be increased by the use of high-modulus carbon fibres. The most promising X-element for a high-temperature resistant material is titanium.     

Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

C_f–Si₃N₄ sandwich composites were prepared by gelcasting using α-Si₃N₄ powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of C_f–Si₃N₄ sandwich composites were investigated. SEM and EDS analysis results reveal that the SiC interphase could effectively overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700 °C. The investigation of microwave absorbing property reveals that, compared with the Si₃N₄ ceramics, both the real (ε?$\epsilon ?$) and imaginary (ε??$\epsilon ??$) permittivity of C_f–Si₃N₄ sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the C_f–Si₃N₄ sandwich composites are significantly enhanced compared with pure Si₃N₄ ceramic, and the reflection loss gradually decreases from −3.5 dB to −14.4 dB with the increase of frequency, while the pure Si₃N₄ ceramic keeps at −0.1 dB. Particularly, the relationship between permittivity of C_f–Si₃N₄ sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both ε?$\epsilon ?$ and ωε??$\omega \epsilon ??$ are inversely proportional to the frequency square ω²${\omega }^2$, and the predicted results agree quite well with the measured data.     

Effect of TiN concentration on microstructure and properties of Ni/W–TiN composites obtained by pulse current electrodeposition

In this study, Ni/W–TiN composites were fabricated by the pulse current electrodeposition (PCE) method. The effects of TiN concentration on the microstructure, microhardness, and wear properties of the resulting composites were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), microhardness tester, and friction wear testing. Among the four obtained composites, Ni/W–TiN composite prepared at 8 g/L showed the densest and finest surface structure. The TiN contents in obtained Ni/W–TiN composites at 8 and 16 g/L were estimated to 8.1 and 5.4 wt%, respectively. The average Ni/W grain diameter in Ni/W–TiN composite obtained at 8 g/L TiN was recorded as 84.7 nm. The protrusion and depression heights of the composite deposited at 8 g/L were 81.8 and 45.4 nm, respectively. This composite also processed an average microhardness of 897.6 HV, with only a few shallow and narrow scratches on its worn surface, demonstrating its prominent wear resistance when compared to the other three composites.