Conduction anisotropy in carbon nanowall layers obtained by a low-pressure plasma jet

Carbon nanowalls (CNW, vertical graphenes) consisting of inter-connected vertical carbon sheets built from small graphene domains with their c-axis parallel to the substrate are promising materials for new electrical devices. Nevertheless, their electrical properties are not sufficiently known [1]. In this study we studied the behavior of the load carriers in plasma deposited CNW. The carbon layers were grown by Plasma Enhanced Chemical Vapor Deposition by injecting H2, and C2H2 (acetylene) in an Ar radiofrequency plasma jet [2]. SEM and Raman techniques were used to characterize the material. A special electrical cell was designed, consisting of a three-layer sandwich built on silicon wafer, having as base a Pt electrode, in the middle the CNW layer, and on top, as upper electrode, a gold disc. The upper gold electrode was deposited by magnetron sputtering at angled incidence (30 degrees to vertical) thus preventing the deep penetration of the metal in the pores of the CNW layer, thus preventing the short-circuit with the Pt electrode. The I-V curves were measured directly between the Pt and Au, across the CNW layer, without and with a magnetic field applied parallel to the substrate (in plane with the c-axis of the sheets). The measurements, revealed us the type of conduction and the mobility of carriers along, and perpendicular on the carbon sheets. The results indicated for CNW a semiconductor of p-type, whose conductivities parallel and perpendicular to the sheets planes are different.

Pore scale evaluation of thermal conduction anisotropy in granular porous media using Lattice Boltzmann method

Purpose Thermal conduction anisotropy, which is defined by the dependency of thermal conductivity on direction, is an important parameter in many engineering and research studies such as the design of nuclear waste depositional sites. In this context, the authors aim to investigate the effect of grain shape in thermal conduction anisotropy using pore scale modeling that utilizes real shapes of grains, pores and throats to characterize petrophysical properties of a porous medium. Design/methodology/approach The authors generalize the swelling circle approach to generate porous media composed of randomly arranged but regularly oriented elliptical grains at various grain ratios and porosities. Unlike previous studies that use fitting parameters to capture the effect of grain–grain thermal contact resistance, the authors apply roughness to grains’ surface. The authors utilize Lattice Boltzmann method to solve steady state heat conduction through medium. Findings Based on the results, when the temperature field is not parallel to either major or minor axes of grains, the overall heat flux vector makes a “deviation angle” with the temperature field. Deviation angle increases by augmenting the ratio of thermal conductivities of solid to fluid and the aspect ratios of grains. In addition, the authors show that porosity and surface roughness can considerably change the anisotropic properties of a porous medium whose grains are elliptical in shape. Originality/value The authors developed an algorithm for generation of non-circular-based porous medium with a novel approach to include grain surface roughness. In previous studies, the effect of grain contacts has been simulated using fitting parameters, whereas in this work, the authors impose the roughness based on the its fractal geometry.