Porous graphene-based electrodes: Advances in electrochemical sensing of environmental contaminants

Extravagant toxins release at an uncontrollable scale due to the continuous embarking of organic and inorganic contaminants has become a severe threat to the ecosystem’s healthy sustainability. The timely monitoring and determination of the environmental contaminants are crucial to take proper steps for environmental remediation. Among advanced nanomaterials, graphene is one of the extensively explored electrode materials for sensing environmental toxins. However, the graphene sheets' agglomeration due to weak wander Waals forces and the π–π interactions compromise its unique inherent features. The integration of graphene into porous structures can facilitate in exploiting its intrinsic properties. Graphene porous network offers highly conductive multiplexed pathways with a well-defined porous structure that provides a better diffusion of the electrolyte along with analytes to encounter the active electrode material. The pores in the 2D sheet or 3D architecture of graphene provide extensive active sites for various analytes' interaction. Rationally designed porous graphene-based materials and nanocomposites are promising advanced electrode materials for trace level quantification of environmental toxins. Herein, we critically discuss the advances in developing the porous graphene and its composites-based electrodes for electrochemical sensing of the environmental toxins.     

Preparation of silica nanowires using porous silicon as Si source

This very paper is focusing on the preparation of silica nano-wires via annealing porous silicon wafer at 1200 °C in H₂ atmosphere and without the assistant metal catalysts. X-ray diffraction, X-ray energy dispersion spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and selected area diffraction technology have been employed for characterizing the structures, the morphology and the chemical components of the nano-wires prepared, respectively. It is found that the diameter and the length of the nano-wires were about 100 nm and tens micron, respectively. Meanwhile, it is also necessary to be pointed out that silica NWs only formed in the cracks of porous wafers, where the stress induced both by the electro-chemical etching procedure for the porous silicon preparation and nanowires growth procedure is believed to be lower than that at the center of the island. Therefore, a stress-driven mechanism for the NWs growth model is proposed to explain these findings.     

Preparation of cis-1,1,1,4,4,4-hexafluorobut-2-ene by cis-selective semi-hydrogenation of perfluoro-2-butyne

Cis-1,1,1,4,4,4-hexafluorobut-2-ene has a zero ozone depletion potential (ODP), low global warming potential (GWP) and non-flammable properties, so it is believed to be a potential foam expansion agent. For the synthetic process of cis-1,1,1,4,4,4-hexafluorobut-2-ene, the process catalysts are the key factors for its yield and cost. In this paper, the catalysts of palladium attached to porous aluminum fluoride, to active carbon, to Al₂O₃, and the blends of palladium and bismuth to AlF₃ used to prepare cis-1,1,1,4,4,4-hexafluorobut-2-ene by cis-selective semi-hydrogenation of perfluoro-2-butyne were investigated. The performance of above-mentioned catalysts was compared in reaction process. The experimental results indicate that the additive of bismuth to palladium catalyst is useful for improving the activity and selectivity compared to Pd/C and Pd/Al₂O₃. The role of bismuth in the synthetic process is discussed based on the experimental results and theory analysis.     

Surface Treatment and Biomimetic Mineralization of Porous Microspheres Fabricated by Calcium Gluconate-g-Poly(D,L-lactide)

Porous hybrid microspheres were fabricated by the synthesized calcium gluconate-g-poly(D,L-lactide) (CG-g-PDLLA) composites.These hybrid microspheres were treated with an alkaline solution for different period of time to control the amount of generated carboxylate groups and remained CG on the surface.The microspheres were then incubated in a supersaturated simulated body fluid (1,5 SBF) solution for different time to investigate their biomimetic mineralization behavior.The depositions were found to have a fine cluster morphology,a similar crystal structure and chemical structure to natural hydroxyapatite,and a medium Ca/P of approximately 1.30.The effect of surface treating time on the structure and mineralization behavior of these microspheres has been discussed in detail.The results indicate that the nucleation and growth of apatite on the surface are influenced by the induced carboxylate groups and the remained CG.The hybrid CG-g-PDLLA microspheres have the potential as a novel alternative in bone tissue engineering.     

Onset of Buoyancy-driven Instability in Porous Media Melted from Below

When a nonhomogeneous solid is melting from below, convection may be induced in a thermally–unstable melt layer. In this study, the onset of buoyancy-driven convection during time-dependent melting is investigated by using similarly transformed disturbance equations. The critical Darcy–Rayleigh numbers based on the melt-layer thickness, Ra _H,c, are found numerically for various conditions. For small superheats, the present predictions show that Ra _H,c is located between 27.1 and 4π ² and it approaches the well-known results of the original Horton–Rogers–Lapwood problem. However, for high superheats, it is dependent on the phase change rate λ and the relation of Ra _H,c λ = 25.89 is shown.     

Motion of a porous sphere in a spherical container

The creeping motion of a porous sphere at the instant it passes the center of a spherical container has been investigated. The Brinkman's model for the flow inside the porous sphere and the Stokes equation for the flow in the spherical container were used to study the motion. The stream function (and thus the velocity) and pressure (both for the flow inside the porous sphere and inside the spherical container) are calculated. The drag force experienced by the porous spherical particle and wall correction factor is determined. To cite this article: D. Srinivasacharya, C. R. Mecanique 333 (2005).     

Bounds on convective heat transport in a rotating porous layer

Using the background field variational method, bounds on convective heat transport in a rotating porous layer heated from below are derived from the primitive equations. The enhancement of heat transport beyond the minimal conduction value (the Nusselt number Nu) is bounded in terms of the dimensionless temperature difference across the layer (the Rayleigh number Ra) according to

This rigorous upper bound shows that rotation has a retarding effect on convective heat transport.     

NUMERICAL SIMULATIONS OF HYDRODYNAMIC BEHAVIORS IN CONICAL SPOUTED BEDS

The axial and radial distributions of static pressures and vertical particle velocities of conical spouted beds have been simulated and compared with experimental data. Simulation results show that, among all factors investigated, the Actual Pressure Gradient (the APG term) in conical spouted beds, introduced as the default gravity term plus an empirical axial solid phase source term, has the most significant influence on static pressure profiles, followed by the restitution coefficient and frictional viscosity, while other factors almost have no effect. Apart from the solid bulk viscosity, almost all other factors affect the radial distribution of the axial particle velocity, although the influence of the APG term is less significant. For complex systems such as conical spouted beds where a fluidized spout region and a defluidized annulus region co-exist, the new term introduced in this work can improve the CFD simulation. Furthermore, for other systems with the Actual Pressure Gradient different from either fluidized beds or packed beds, the new approach can also be applied.     

Wall Region Porosity Distributions for Packed Beds of Uniform Spheres with Modified and Unmodified Walls

A mathematical derivation of the porosity (local void fraction) distribution near the walls of packed beds of uniform spheres is presented. This investigation supports the study of methods of reducing or eliminating the so-called wall effect, or bypass flow, which accompanies the increase in porosity when spheres come in contact with a solid boundary. Limiting the amount of bypass flow is important in certain applications such as in packed bed nuclear reactors where bypass flow allows some coolant to avoid the high power density core region. Four basic porosity distributions are determined. The first investigates the case where spheres against a flat wall are packed in the tightest possible packing density. This density is then reduced by changing the sphere spacing until the minimum porosity matches that obtained experimentally. In the other cases, the effect of various ways of embedding spheres in the wall on the wall region porosity is examined. By partially embedding spheres in the wall, the porosity at the wall is reduced and the most direct cause of the bypass flow is thereby eliminated. The porosity is found by evaluating the ratio of the solid area to total area in a plane which is parallel to the wall. The local porosity is derived as a function of distance from the wall in the region within one-half a sphere diameter from the wall. The average porosity of the wall region is also calculated. This research has application to flow situations such as packed bed chemical reactors, pebble bed nuclear reactors and flow in packed beds.     

Lie-group method of solution for steady two-dimensional boundary-layer stagnation-point flow towards a heated stretching sheet placed in a porous medium

The boundary-layer equations for two-dimensional steady flow of an incompressible, viscous fluid near a stagnation point at a heated stretching sheet placed in a porous medium are considered. We apply Lie-group method for determining symmetry reductions of partial differential equations. Lie-group method starts out with a general infinitesimal group of transformations under which the given partial differential equations are invariant. The determining equations are a set of linear differential equations, the solution of which gives the transformation function or the infinitesimals of the dependent and independent variables. After the group has been determined, a solution to the given partial differential equations may be found from the invariant surface condition such that its solution leads to similarity variables that reduce the number of independent variables of the system. The effect of the velocity parameter λ, which is the ratio of the external free stream velocity to the stretching surface velocity, permeability parameter of the porous medium k ₁, and Prandtl number Pr on the horizontal and transverse velocities, temperature profiles, surface heat flux and the wall shear stress, has been studied.