Synergism of Toughening Mechanisms in Whisker-Reinforced Ceramic-Matrix Composites

The improved fracture resistance of whisker-reinforced ceramic-matrix composites involves more than one energy-absorbing mechanism. The possible mechanisms are reviewed and a micromechanical model evaluating the relative contributions to the overall toughness is presented. The mechanisms involve microcracking, load transfer, bridging, and crack deflection. The synergism of these mechanisms is examined using an energy release rate balance equation. The basic assumption of the proposed model is that the load transfer between the matrix and the whiskers is due to Coulomb friction. The model has been applied to an Al₂O₃/SiC whisker composite and shows reasonable agreement with reported experimental results. The role of the thermal residual stresses is also examined in light of the frictional load transfer assumption.     

Effect of Processing Temperature on the Morphology of Silicon Carbide Whiskers

Two kinds of SiC whiskers were annealed at temperatures similar to those for the processing of ceramic matrix composites. The morphology and structure of the as-received and postannealed whiskers were investigated by SEM, TEM, and XRD, and the influence of processing temperature on the mechanical properties of ceramic matrix composites was discussed.     

Structure evolution of conductive polymer composites revealed by solvent vapor induced time-dependent percolation

The authors of this paper synthesized a series of amphiphilic triblock copolymers of polystyrene-b-poly(ethylene glycol)-b-polystyrene (PS-PEG-PS) having different PEG/PS ratios with nearly identical molecular weights of the entire copolymers. The interfacial interactions in the composites consisting of carbon black and the copolymers can thus be tailored. When these conducting composites are exposed to certain solvent vapors, their electrical resistances greatly increase, showing the gas sensitivity. The present work indicated that this switching behavior is controlled by the structural relaxation of the composites because matrix swelling acts as the main mechanism. The response time has been correlated with absolute temperature by Arrhenius equation, and the estimated activation energy reflects mobility of the fillers involved in the solvent induced expansion of the surrounding polymer. Therefore, by using the gas sensibility of the conductive composites, the structure evolution of the composite materials in solid state and the effect of filler/matrix interfacial interaction on the relaxation property of the matrix polymer has been inspected. It was found that lower activation energy represents stronger interfacial interaction in case good solvent of the matrix was used for the test.     

水泥基压电复合材料的制备及极化工艺研究

采用压制成型法，首次以快硬早强的硫铝酸盐水泥为基体制备了水泥基压电复合材料。详细阐述了水泥基压电复合材料的制备过程；通过实验确定了适宜的极化条件，分析讨论了极化过程中出现漏电流的原因，并提出了一些初步的解决途径。结果表明，对硫铝酸盐水泥基压电复合材料来说，适宜的极化工艺参数：极化电场强度为4kV／mm，极化时间为45min，极化温度应低于130℃。     

Electromagnetic properties of ferroelectric-ferrite ceramic composites

The NiFe₂O₄ and Pb(Zr_0.52Ti_0.48)O₃ ceramic composites are prepared by conventional solid state reaction method. The microstructural and morphological properties of the composites are studied by X-ray diffraction analysis and scanning electron microscopy. Results indicate that the ferrite and piezoelectric phases can co-exist in the composites. The relationship between the dielectric constant and frequency (40 Hz–40 MHz) as well as temperature is also investigated. It is noted that the transition temperature of the composites is about 380 °C. A double electric hysteresis loop is observed in our composites. The magnetic properties are relatively weak according to the magnetic hysteresis loop. With increase in the content of the ferrite phase, the permeability increases. The electromagnetic properties of the as-prepared composites are tunable according to the content of the NiFe₂O₄ phase.     

Electrooptic studies on polymer‐dispersed liquid‐crystal composite films. III. Poly(methyl methacrylate‐co‐butyl acrylate)/E7 and poly(methyl methacrylate‐co‐butyl acrylate)/E8 composites

Composite films composed of poly(methyl methacrylate‐co‐butyl acrylate) (PMMABA) and nematic‐type liquid crystals E7 and E8 (commercial products from E. Merck, Darmstadt, Germany) were prepared through solvent casting in chloroform. The morphology and electrooptic responses were studied. Scanning electron microscopy observations showed that the liquid‐crystal phase (E7 or E8), as larger, elongated, interconnected cavities, was continuously embedded in a spongelike PMMABA matrix. At a specific level of the liquid‐crystal (E7 or E8) loading (30/70 wt %), the effects of the voltage, temperature, and frequency of an applied alternating‐current electric field on the transmittance of the composite films were measured with a He–Ne laser (wavelength = 632.8 nm). The results were interpreted in terms of the aggregation structure, interfacial interaction, and solubility of the liquid crystal in the matrix polymer. The results indicated that, under these experimental conditions, the output could be controlled to a desired level by the selection of suitable liquid crystals to prepare polymer‐dispersed liquid‐crystal, electrooptic, active composite films with a response time of the order of only milliseconds or less. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Swelling properties of chemically modified oil palm empty fruit bunch based polyurethane composites

In this study, the diffusion of various types of solvent in oil palm empty fruit bunch/polyurethane composites, produced from chemically modified empty fruit bunches, was investigated. The solubility parameters and polymer–solvent interaction parameters of the produced composites were determined. The void contents of the composites were also determined before swelling tests to eliminate the free solvent present in the system. From the results obtained, we found that the diffusion of the solvents was dependent on the compatible group available and the voids present in the system. The solubility parameters of the empty fruit bunch/polyurethane composites with different degrees of chemical modification were 11.6 and 11.7 (cal/cm^?3)^1/2. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A comparative study of short nylon fiber—Natural rubber composites prepared from dry rubber and latex masterbatch

A novel method for the preparation of short nylon fiber–natural rubber composites was developed in which short fibers chopped to approximately 6 mm were incorporated in the latex stage and processed into sheet form. By this method, mixing cycle time was reduced without compromising the fiber dispersion. Fiber breakage during mixing was reduced. The new composites when compounded with a dry bonding system based on hexamethylenetetramine, resorcinol and hydrated silica (HRH) showed improved modulus, tensile strength and abrasion resistance compared to conventional composites. Tear strength, resilience, and compression set were similar to the conventional composites. SEM analysis indicated better interaction between matrix and fibers in the case of latex master batch. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal and mechanical properties of epoxy composites reinforced by a natural hydrophobic sand

If a low weight percentage of crude fine fillers can improve properties of polymer materials directly without complicated chemical treatment process involved, it will be significant for many industrial applications. Our previous study indicated that a kind of Cancun natural sand could be an effective filler material for polymer composites. In this current work, the epoxy composites reinforced by this kind of natural sand particles were prepared and thermal and mechanical properties of the composites containing up to 5 wt % of the sand particles were characterized. Results showed that the highest flexural strength appears in the epoxy composite containing 1 wt % sand particles. A damage model was used to interpret the flexural properties, which showed an acceptable agreement with the experimental results. The glass transition temperature, high temperature storage modulus, and dimensional stability of the sand/epoxy composites monotonically increased with the addition of the sand particles. The sand particle/epoxy composites also displayed a noticeable enhancement in thermal conductivity. Theoretical analysis showed that in addition to conduction, other heat transport mechanisms played roles in the improved heat transmission through the composites. As a natural porous micron-scale material, Cancun sand has the potential for applications in cost-effective composites with enhanced mechanical and thermal properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008