Key Engineering Materials Vols. 512-515

Abstract: Diamond/Si/C porous preform was prepared with phenolic resin, Si, Diamond and graphite. Subsequent reactive vapor infiltration (RVI) of gaseous silicon at 1600 °C for 2 h in vacuum atmosphere resulted in the formation of a compact Diamond/SiC composite. The influence factors of perform porosity were investigated, including process pressure and raw materials. Density and microstructure of Diamond/SiC composites were also discussed. The results showed that the open porosity of preform was mainly influenced by the pressing pressure. It can also be affected by the content, morphology and the particle size of diamond. The preform with open porosity higher than 20% after RVI treatment can obtain Diamond/SiC composites with high density. However, over-high porosity of the perform will cause a mass of unreacted Si exists in the composite which is unfavorable for its application. In order to reduce the content of residual Si, open porosity of preform should be no higher than 40%.

Abstract: In recent years, with the development of the stealthy and anti-stealthy system, the electromagnetic pollution increasing, microwave absorbing materials with thermal resistance and wide absorption band have been the one of the hotpots in functional materials. Silicon Carbon as a semiconductor material having unique mechanical properties and high temperature stability, is one of the preferred absorbing materials. In this paper, we used silicon carbide, barium titanate and barium ferrite as the raw materials, which belongs to the absorbing mechanisms of resistance loss, dielectric loss and magnetic loss, respectively. The composites were sintered by hot pressed sintering method, and characterizated by XRD, DTA-TG and SEM techniques. XRD results showed that Ba₂TiSi₂O₈ was turn up in the sintering composite materials, and the SEM-EDS photograph was clearly observed that Ba₂TiSi₂O₈ whisker were equally distributed on the surface of silicon carbide. we can assumed that the SiC matrix complex absorbers may be promising candidate for microwave absorption materials with strong-absorption, light-weight, wide-band properties.

Abstract: Aluminum Oxide microwave dielectric ceramics exhibit good microwave dielectric characteristics and suitable be used in the microwave devices. In this letter, using screen-printing technique and Ag/Pd paste, a novel dual-mode 2.4 GHz microstrip bandpass filter with U-shaped defected ground structure is screen printed on the Aluminum Oxide (Al₂O₃, relative dielectric constant=9.8) microwave dielectric ceramic substrate. In addition, a pair of input/output microstrip lines are designed to be 50 Ω and coupled to a λ/4×λ/4 square loop resonator. And in the ground plane, a U-shaped defected ground structure is used toλ excite the degenerate modes, which can be considered as a perturbation element to control the odd-mode frequency and -3 dB bandwidth of the bandpass filter. Finally, the coupling effects between these degenerate modes are presented and detailed investigated.

Abstract: Nano-Cordierite powders used for high frequency chip inductors (MLCIs) were prepared by the sol-emulsion-gel method. The samples derived from these powders could be sintered at 900°C to be cordierite ceramics having a low dielectric constant and dielectric loss (ε=3.18, tgδ=0.00011; 1GHz). Effects of precursor concentration, treating temperature, and [H2O]/[Si] ratio of this material were studied. The pseudo-ternary phase diagram of sol/TritonX-100/n-hexanol/cy- clohexane was constructed and the influences of each variable on the phase diagram were discussed. Results showed that the optimal precursor concentration, treating temperature, and [H2O]/[Si] ratio for qualifying the gelation of the material were 1.5 mol/L, 45°C, and 4, respectively. Results also showed that for qualifying cordierite powders, the use of TritonX-100 as a surfactant together with n-hexanol as a co-surfactant could dissolve more magnesium nitrate and aluminium nitrate, and obtain a wider W/O micro-emulsion region than when other reagents were used. The samples derived from this method can be co-fired with a silver electrode, an ideal dielectric material for high frequency MLCIs.

Abstract: Microwave diffraction behavior of microwave absorbing ceramics was investigated by numerical calculation, and effective electro-magnetic parameters of the ceramics was obtained by comparing the diffraction behavior with that of homogeneous material. Then, the calculated results were compared with the results of effective medium models, aiming to investigate the applicability of these models. The obvious difference between these results were pointed out, also discussed is the possible reason for this difference.

Abstract: In order to improve absorbing properties of material effectively, enhance the ability of attenuating electromagnetic wave, and at the same time, make the impedance matching with air to the most extent, this paper mixed ferrite magnetism microwave-absorbing agents with semiconductor particulates. We got the effective EM parameter of the mixture using effective EM parameter model, and then analyzed absorption properties. As the result, it figured out the cubage proportion and conductance of semiconductor particulates based on the resistance matching principle. Through calculating the reflection ratio、impedance matching characteristic and attenuation coefficient of the material, it is proved that mixing ferrite magnetism microwave-absorbing agents with semiconductor particulate which has less permittivity could reduce real complex permittivity and increase imaginary complex permittivity of the composite material. It was able to enhance the ability of attenuating electromagnetic wave, improve absorbing properties, and meanwhile do not destroy the impedance matching characteristic with air.

Abstract: In order to compare the absorbing properties of different microwave-absorbing agents and their mixture, this paper has selected two typical microwave-absorbing agents which respectively are carbon fiber resistance microwave-absorbing agents and ferrite magnetism microwave-absorbing agents, and took them as monolayer absorbing materials. Then used MG formula to calculate the effective electromagnetic parameter of their mixture under different proportion at X band, and finally analyzed their absorbing properties separately. Then through contrasting the impedance matching characteristic and attenuation coefficient of the materials, it is proved that adding carbon fiber in ferrite can not improve absorption properties. Although the ability of attenuating electromagnetic wave would be enhanced when resistance materials and magnetism materials are blend with each other, doping will destroy the materials’ impedance matching characteristic because most resistance materials have high permittivity. It makes reflection strengthen and can not improve absorbing properties.

Abstract: Microstructure control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the nowaday challenges for increasing capacitive volumetric efficiency and high voltage dielectric properties. In this paper, the ultrafine-grained BaTiO3 dielectric ceramics with uniform grain size distribution were prepared by chemical coating approach. The doping effects of rare-earth element (Ho) on the microstructures and dielectric properties of the ceramics were investigated. The addition of Ho in nano BaTiO3-based ceramics was found to improve the temperature coefficient of capacitance (TCC) performance, which could be attributed to a significant reduction in the grain size. Moreover, the effects of two-step sintering (TSS) method on the microstructures and dielectric properties, compared with conventional sintering (CS) method, were also discussed. The dielectric constant of ceramics sintered by the TSS method was higher than that of ceramics prepared by CS method. The results reveal that the adoption of TSS method could control the grain size and suppress the abnormal grain growth more effectively, which improves the TCC characteristics.

Abstract: BaTiO₃/ which has good dielectric properties, they were widely used in ceramic capacitors, thermistors, etc. In this study, BaTiO₃/ ceramics prepared by sol-gel powder, then mixed in the organic polymer polyether imide (PEI) and dispersant (surface active agents) and silane coupling agent (Silane Coupling Agent) of the composite material on flexible substrates, And analysis of its properties and electrical properties. Increase by a ceramic powder, to explore the impact of the substrate. In the physical analysis is used XRD, SEM to measure the intensity of crystalline phase and surface uniformity of the electrical measurement using HP4294 measuring dielectric constant and dielectric loss.

Abstract: High density BaTiO₃ ceramics have been prepared by the addition of CuO-Bi₂O₃-B₂O₃ mixed oxide. The results indicate that the sintering temperature of BaTiO₃ bulk can be decreased by about 300°C and high dielectric behavior can be remained. The CuO-Bi₂O₃-B₂O₃ additives can increase the breakdown strength, and leads to an obvious enhancement of the energy density, which is nearly 2 times of pure BaTiO₃.