Mechanical and Thermal Analysis of Duroplastic Matrix Composites over a Range of Temperatures.

It is commonly acknowledged that polymer composites in service are often subjected to not only intricate mechanical loads but also harsh environmental conditions. The mechanical and thermal properties of five particular composites are explored here. The composites are composed of laminates of glass cloth type "E" sheet infilled with a duroplastic matrix. This is a thermoset polymer-epoxy resin with different molecular weights. The composites were fabricated by IZOERG company, which is based in Poland. The final articles were 1.5 mm thick by 60 cm long and 30 cm wide, with the glass layers arranged parallel to the thickness. Young's modulus and tensile strength were measured at room temperature. Using the thermal analysis of dynamic mechanical properties (DMTA), the values of the storage modulus and the loss modulus were determined, and the damping factor was used to determine the glass transition temperature (Tg). It was revealed that the nature of changes in the storage modulus, loss modulus, and damping factor of composite materials depends on the type of epoxy resin used. Thermal expansion is a crucial parameter when choosing a material for application in cryogenic conditions. Thanks to the TMA method, thermal expansion coefficients for composite materials were determined. The results show that the highest value of the coefficient of thermal expansion leads the laminate EP_4_2 based on brominated epoxy resin cured with novolac P. Duroplastic composites were characterized at cryogenic temperatures, and the results are interesting for developing cryogenic applications, including electric motors, generators, magnets, and other devices.

Influence of the filler distribution on PDMS-graphene based nanocomposites selected properties.

Insufficient homogeneity is one of the pressing problems in nanocomposites' production as it largely impairs the properties of materials with relatively high filler concentration. Within this work, it is demonstrated how selected mixing techniques (magnetic mixer stirring, calendaring and microfluidization) affect filler distribution in poly(dimethylsiloxane)-graphene based nanocomposites and, consequently, their properties. The differences were assessed via imaging and thermal techniques, i.a. Raman spectroscopy, differential scanning calorimetry and thermogravimetry. As microfluidization proved to provide the best homogenization, it was used to prepare nanocomposites of different filler concentration, whose structural and thermal properties were investigated. The results show that the concentration of graphene significantly affects polymer chain mobility, grain sizes, defect density and cross-linking level. Both factors considered in this work considerably influence thermal stability and other features which are crucial for application in electronics, EMI shielding, thermal interface materials etc.

Structural changes across thermodynamic maxima in supercooled liquid tellurium: A water-like scenario.

Liquid polymorphism is an intriguing phenomenon that has been found in a few single-component systems, the most famous being water. By supercooling liquid Te to more than 130 K below its melting point and performing simultaneous small-angle and wide-angle X-ray scattering measurements, we observe clear maxima in its thermodynamic response functions around 615 K, suggesting the possible existence of liquid polymorphism. A close look at the underlying structural evolution shows the development of intermediate-range order upon cooling, most strongly around the thermodynamic maxima, which we attribute to bond-orientational ordering. The striking similarities between our results and those of water, despite the lack of hydrogen-bonding and tetrahedrality in Te, indicate that water-like anomalies may be a general phenomenon among liquid systems with competing bond- and density-ordering.

Variation of cation distribution with temperature and its consequences on thermal expansion for Ca3Eu2(BO3)4.

The structure of calcium europium orthoborate, Ca3Eu2(BO3)4, was determined using high-resolution powder X-ray diffraction data collected at the ID22 beamline (ESRF) under ambient conditions, as well as at high temperature. Rietveld refinement allowed determination of the lattice constants and structural details, including the Ca/Eu ratios at the three cationic sites and their evolution with temperature. Clear thermal expansion anisotropy was found, and slope changes of lattice-constant dependencies on temperature were observed at 923 K. Above this temperature the changes in occupation of the Ca/Eu sites occur, exhibiting a tendency towards a more uniform Eu distribution over the three Ca/Eu sites. Possible structural origins of the observed thermal expansion anisotropy are discussed.

Crystallization during bending of a Pd-based metallic glass detected by x-ray microscopy.

Without the availability of slip systems and dislocation glide as in crystalline materials, metallic glasses resist irreversible deformation to elastic strains of 2% or more before undergoing heterogeneous plastic flow via the formation of shear bands. Observation of crystallite formation under compressive load was previously obtained by transmission electron microscopy. In this Letter, we present results of nondestructive x-ray diffraction microprofiling of the section of a bent glassy Pd40Cu30Ni10P20 ribbon in transmission using a synchrotron microbeam. Crystallization was clearly detected but only on the compression side of the neutral fiber. The experimental results and crystal nucleation frequency analysis are consistent with massive nucleation in shear bands forming under compressive stress but mainly for metallic glasses that show a large supercooled liquid temperature range ΔT=T(x)-T(g) between glass transition at T(g) and crystallization at T(x). The phenomenon is sensitively dependent on the volume change that accompanies crystallization in the supercooled liquid temperature range where the much larger liquid-state thermal expansion coefficient significantly increases the specific volume difference between the liquid and crystalline states. The results are also consistent with the many reports of extensive strain to fracture of metallic glasses under compressive load but not under tension.