Еffect of graphene filler oxidation on the thermal destruction of epoxy-graphene composites

The participation of the electronic subsystem of graphene nanoparticles in heat transfer on the interfaphase surface with epoxy polymer, its participation in the thermodestruction processes of epoxy matrix and the concentration interval of the subsystem's influence on the thermal destruction of the polymer matrix are investigated. For such purpose, epoxy resin composites with oxidized and non-oxidized graphene nanoparticles have been used.The particles were obtained by electrochemical method and those are characterized by the same dispersion and analogical of defect spectra. The particles have the same crystal structure, however in composites with oxidized graphene, the participation of the electronic subsystem in thermophysical processes on the interfacial surface is blocked by the atomic layer of adsorbed oxygen. Сomposites of epoxy resin filled with the same particles of nonoxidized and oxidized nanoparticles in the filler content 0.0, 1.0, 2.0, and 5.0 wt%. The multilayered graphene particles were studied by X-ray diffraction analysis (XRD) and Raman spectroscopy (RS) methods. It was shown that the graphene particles are the 2D dimensional structures with about of 100 layers. Desorption curves of epoxy and its composites have been obtained using a programmable thermal desorption mass-spectroscopic (TDMS) technique for fragments with 15≤ m/z ≤108 and temperature interval 35 - 800 оС. The activation energy of desorption was determined from the Wigner-Polanyi equation as 35 - 150 kJ/mol, temperature and mass dependences of the quantity of desorbed atomic fragments have been calculated. It were established the graphene electron subsystem takes part in polymer structure thermodestruction for epoxy composites with nonoxidized graphene enhancing their heat resistance at graphene content С ≤ 1 wt%. With increasing filler content, the thermodestruction behavior in pristine epoxy and its composites with nonoxidized and oxidized graphene is analogical. The thermodestruction characterizes by the stepwise variations in the desorption intensity of atomic fragments. The electron subsystem of graphene particles does not participate in the heat resistance variations.

FECOCRNIMOTIW HIGH-ENTROPIC COATINGS AND THEIR PROPERTIES

The objects of study were high-entropy coatings of the composition FeCoCrNiMoTiW made by mechanical alloying. It is shown that the hardness of most stainless steels is 1.5-2 times less than high-entropy coatings, and the dry friction coefficients are in the range of 0.08-0.16. Such a difference in the coefficients of friction for high-entropy coatings is due to their nanostructural feature and the manifestation of the dimensional dependence of their properties. Theoretically, we consider the question of the response of the electron subsystem in high-entropy alloys to an external action during friction from the standpoint of nonequilibrium statistical thermodynamics. As a result, it was shown that the coefficient of friction of the coating decreases with the use of a high-entropy alloy and with a decrease in the surface energy of the coating.

On the target surface temperature during dc magnetron sputtering

New simple method for target surface temperature (TST) measurements is successfully developed and described in detail. Along with temperature measurements we measured also the emissivity of each of targets by the method specially developed for this aim. The measurements demonstrate that the surface temperature of the targets prepared from Cu, Mo, Nb discs may substantially (up to ∼300 °C) exceed the temperature of the volume of the target. The definition of a “target surface temperature” is given. It is supposed that the thickness of the surface layer that appears on a target subjected to the ion bombardment is equal or close to the penetration depth of ions bombarding the target. The physical model explaining the formation of the surface layer is suggested. The main idea of the model is that the Ar ions bombarding the target may effectively transfer their kinetic energy mostly to the ionic subsystem rather than to the electron subsystem of a target. Due to very low mobility of metal ions within the layer the thermal conductivity of the layer is substantially lower compared to the rest target volume. As a result the temperature of the layer is higher than that of the rest part of the target.