Key Engineering Materials Vol. 799

Abstract: Graphene has been employed as electrode materials in various electrochemical biosensors due to its excellent electrical, mechanical, thermal and optical properties. In the present study, Chemical Vapor Deposited (CVD) and epitaxial graphene on SiC were examined as material for electrochemical biosensing application. The surface of both types of graphene were characterized using Raman spectroscopy as well as with Scanning Electron Microscopy (SEM). As the key point for the comparison, the impedance spectroscopy measurements of different graphene films using deionized water and saline 0.9% NaCl solution were performed as well. The method of impedance measurements applied to graphene films expands the range of possibilities for using this material as sensitive biosensors. Based on the comparative tests results, it is possible to draw the first simple conclusions about the advantages of CVD or epitaxial graphene. Based on the results of impedance spectroscopy, it is possible to draw a simple conclusion – single layer graphene has the higher sensitivity.

Abstract: Lithium-sulfur (Li-S) batteries are promising as a next generation energy-storage device because their energy density is higher than that of current Li-ion devices. Alumina nanofibers coated with graphene is electrochemically active material with tunable graphene flakes and surface area. Combination of this material with sulfur leads to an improved initial discharge capacity and cycle stability, probably due to improved electrical and ionic transport during electrochemical reactions. Based on this understanding, the resulting graphene sulfur composite showed high and stable specific capacities up to ∼900 mAh/g after 50 cycles, representing a promising cathode material for rechargeable lithium batteries with high energy density.

Abstract: Treatment of graphene/SiC dies in inorganic electrolytes (KOH, KCl and Na₂SO₄) is discussed. An electrochemical method based on the cyclic voltammetry in a conventional three-electrode cell with Ag/AgCl reference electrode, a platinum counter electrode, and the graphene/SiC dies as working electrode (anode) is used for the treatment. It was observed either partial oxidation of graphene or its complete dissolution with the formation of CO₂. The treatment performed resulted in the deterioration of the graphene films and change of the graphene-die resistivity depending on the range of the scanning potential applied to the graphene/SiC dies.

Abstract: In the present paper, a mathematical model has been constructed in order to describe the impact response of a linear Kirchhoff-Love plate made of viscoelastic auxetic material possessing fractional viscosity. Auxetic’s Poisson’s ratio is a time-dependent value changing from negative to positive magnitudes with time. In the case of a linear plate, the solution out of the contact domain is found through the Green function, and within the contact zone via the modified Hertz contact theory. Integral equations for the contact force and local indentation have been obtained.

Abstract: Contact modeling could be widely used for different machine elements normal contact pressure calculations and wear simulations. However, classical contact models as for example Hertz contact models have many assumptions (contact bodies are elastic, the contact between bodies is ellipse-shaped, contact is frictionless and non-conforming). In conditions, when analytical calculations cannot be performed and experimental research is economically inexpedient, numerical methods have been applied for solving such engineering tasks. Contact stiffness parameters appear to be one of the most influential factors during finite element modeling of contact. Contact stiffness factors are usually selected according to finite element analysis software recommendations. More precise analysis of contact stiffness parameters is often required for finite element modeling of contact.

Abstract: The paper proposes an approach to study the mechanism of deformation of epoxy polymers based on the methods of fractal calculations. The advantages of the method for determining the fractality index of the deformation curves of polymer samples using the minimum coverage method are shown. An algorithm for the quantitative determination of the location of the "critical" points of the deformation curves of polymer samples under tension is proposed. It is shown that the use of the developed methodology for the fractal analysis of time series on the basis of a set of points of deformation curves of epoxy polymer samples under tension, provides valuable information about the processes occurring in the structure of composite materials affected by mechanical loads and various aggressive factors.

Abstract: Vibration of an axially loaded viscoelastic nanobeam is analyzed in this study. Viscoelasticity of the nanobeam is modeled as a Kelvin-Voigt material. Equation of motion and boundary conditions for viscoelastic nanobeam are provided with help of Eringen’s Nonlocal Elasticity Theory. Initial conditions are used in solution of governing equation of motion. Damping effect of the viscoelastic nanobeam structure is investigated. Nonlocal effect on natural frequency and damping of nanobeam and critical buckling load is obtained.

Abstract: The higher order Haar wavelet method (HOHWM) introduced recently by workgroup is utilized for vibration analysis of nanobeams. The results obtained are compared with widely used Haar wavelet method. It has been observed that the absolute error has been reduced several magnitudes depending on number of collocation points used and the numerical rate of convergence was improved from two to four. These results are obtained in the case of the simplest higher order approach where expansion parameter k is equal to one. The complexity issues of the HOHWM are discussed.

Abstract: 3D printing of plastic materials is very popular nowadays, while printing of wear resistant hard materials is still an issue. Gradient or functionally graded structures are providing improved performance in impact-abrasive application (tunnelling, geothermal drilling, mining, etc.) through the optimal positioning of areas providing high wear resistance and high resistance against impacts. However, printing of such structures by widely used powder bed selective laser sintering leads to the high consumption of raw materials (powders) that cannot be used again for next printing. A method to overcome this problem is explored in current work. The cermets were produced from following powders: (1) commercial, (2) obtained by disintegration of cutting tool inserts (recycled) and (3) reused recycled ones. Tungsten and cobalt used for the production of the majority of cermets are critical raw materials and their consumption should be reduced while the rate of recycling needs to be increased. The surfaces were studied with the help of optical microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The hardness and wear rate of samples in abrasive conditions were measured.

Abstract: In this study the authors investigate the possibility of using Selective Laser Melting (SLM) to manufacture the porous element (stack) of a thermoacoustic engine. Two stacks, provided with different internal geometries but with the same hydraulic radii, have been produced: one with parallel plates and one with a new, oblique pin array configuration. The pin array stacks provide smaller amount of viscous losses during the thermal to acoustic power conversion when working fluids with Pr < 1, e.g. air. Furthermore, the oblique configuration is designed to reduce the heat transmission from the hot to the cold surface of the stack, thus to achieve more easily the temperature gradient for the onset of the thermoacoustic phenomenon. In general, the pin array configurations are difficult to manufacture with conventional technologies. However, the SLM provides precise control of the features of the printed object, thus allowing to objectively compare the behavior of the two stacks. To this aim, the temperature gradient between the hot and cold faces of the stacks and the corresponding sound pressure levels measured at 15 cm from the open end of the engine are measured and compared.