Matrix thermal expansivity influence

Crosslinked low-density polyethylene foams with a closedcell structure were investigated using differential scanning calorimetry, scanning electron microscopy, density, and thermal expansion measurements. At room temperature, the coefficient of thermal expansion decreased as the density increased. This was attributed to the influence of gas expansion within the cells. At a given material density, the expansion increased as the cell size became smaller. At higher temperatures, the relationship between thermal expansion and density was more complex, due to physical transitions in the matrix polymer. Materials with high density and thick cell walls were concluded to be the best for low expansion applications. 16 refs. 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

The consolidation mechanism of many of these ceramic composites involves a thin glassy phase linking the matrix grains, the reinforcement and the crystalline portion of the sintering additive.14 The thermal and mechanical properties of all these phases are quite different, which will therefore influence the behaviour of the composite. Although the coefficient of thermal expansion... 

Processing conditions required to attain desirable composite properties can be defined more easily if the factors controlling composite microstructure are understood. Such factors include type of precursors employed and the composite s processing history. The microstructure of the matrix may contribute to the performance of the fibers and influence the properties of the composite. Reviewed are experiments to determine matrix micro-structural features, how microstructural variations are achieved, and ways in which thermal expansion and fracture behavior relate to microstructure. 

Thermal Expansivity (AL/L). Experiments were performed to determine what influence microstructure has on the thermal expansion characteristics of three-dimensional (3D) C-C composites prepared with two types of coal tar pitch as matrix precursors. 

Thermal residual strains are generated in composite materials on the microscale and macroscale. The former arise from a mismatch in the coefficients of thermal expansion of the fibre and the matrix. This effect is magnified by the presence of fibre bundles, where the radial stresses can change from compressive to tensile and influence interfacial failure under... 

The difference in thermal expansion between matrix and fibre reinforcement is an important parameter influencing the mechanical behaviour of the composites because it determines the residual stress distribution after fabrication [ 105]. In glass matrix composites, these residual stresses can be measured by photoelastic techniques [127]. 

Yates et al [135] determined the influence of the fiber volume fraction using HTS fiber in a DLS35I/BF34OO matrix at 90-500 K. The linear thermal expansion coefficients of the fiber at room temperature were found consistent with —10 x 10 K-i

Yates B, McCalla BA, Phillips LN, Kingston-Lee DM, Rogers KF, The thermal expansion of carbon fibre reinforced plastics, Part 5. The influence of matrix curing characteristics, J Mater Sci, 14, 1207-1217, 1979. 

The factors, which influence the permeability or mass transport, are the following chemical composition of the polymer matrix and its free volume. In fact, crystallinity, molecular orientation, and physical aging in turn influence the free volume of a polymer matrix. In addition, porosity and voids, like free volume, offer sites into which molecules can absorb and are far less of a barrier to transport than solid polymer. Temperature also affects permeability and diffusion properties of small molecules in polymers. With increased temperature, the mobility of molecular chains (in polymer) increases and thermal expansion leads to reduced density therefore, the free volume in the system will increase. External tensile stress applied is expected to increase free volume and open up internal voids or crazes, providing additional sites into which molecules can absorb. Of course, there may be unquantified internal residual stresses, arising from processing, present in the polymers. It is well established that the properties of materials... 

To examine the influence of other parameters on thermal expansion behaviour, the thermal expansion of 2-D carbon-cloth-reinforced C/C composites with different porosity was measured at low temperatures. The C/C composites were reinforced with 35 V/o 8-harness satin-carbon cloth (sigrafil GDS 8/30) and have porosites of 18 and 15 V/o, respectively. The matrix precursor was CT pitch, modified with elemental sulfur and heat treated to lOOO C. 

Organic/inorganic nanocomposites prepared by in situ polymerization methods have received extensive attention in recent years. Unlike microscale fillers, nanoscale fillers can offer excellent properties to a polymer matrix. Nanosized filler, with a few weight percent in the reinforced polymer nanocomposites, strongly influences the macroscopic properties of the polymer. The resultant polymer nanocomposites can significantly improve some of their properties, such as higher heat distortion temperatures, enhanced flame resistance, increased modulus, better barrier properties, reduced thermal expansion coefficient, and altered electronic and optical properties. 

Pure pressure effects have been accounted for by the Laird and Skinner theory [4]. This theory can be conveniently expanded to also include the thermal expansion of the matrix and its influence on the dye molecule, for details see Refs. [20, 21]. In analogy to the inhomogeneous distribution (Eq. 1), the temperature-pressure kernel, i.e. the probability that... 

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