Rabiu Musah

Effects of Oil-Based Nanofluid on a Stretching Surface with Variable Suction and Thermal Conductivity

The combined effect of suction and thermal conductivity on the boundary layer flow of oil–based nanofluid over a porous stretching surface has been investigated. Similarity techniques were employed in transforming the governing partial differential equations into a coupled third order ordinary differential equations. The higher third order ordinary differential equations were then reduced into a system of first order ordinary differential equations and solved numerically using the fourth order Runge-Kutta algorithm with a shooting method. The results were presented in tabular and graphically forms for various controlling parameters. It was found that increasing the thermal conductivities of the base fluid (oil) and nanoparticle size (CuO) of the nanofluid did not affect the velocity boundary layer thickness but depreciates with suction and permeability. The suction parameter and thermal conductivity of the base fluid also made the thermal boundary layer thinner.

Hot electrons injection in carbon nanotubes under the influence of quasi-static ac-field ☆

Abstract The theory of hot electrons injection in carbon nanotubes (CNTs) where both dc electric field (Ez), and a quasi-static ac field exist simultaneously (i.e. when the frequency ω of ac field is much less than the scattering frequency v ( ω ⪡ v or ω τ ⪡ 1 , v = τ − 1 ) where τ is relaxation time) is studied. The investigation is done theoretically by solving semi-classical Boltzmann transport equation with and without the presence of the hot electrons source to derive the current densities. Plots of the normalized current density versus dc field (Ez) applied along the axis of the CNTs in the presence and absence of hot electrons reveal ohmic conductivity initially and finally negative differential conductivity (NDC) provided ω τ ⪡ 1 (i.e. quasi- static case). With strong enough axial injection of the hot electrons, there is a switch from NDC to positive differential conductivity (PDC) about E z ≥ 75 kV / cm and E z ≥ 140 kV / cm for a zigzag CNT and an armchair CNT respectively. Thus, the most important tough problem for NDC region which is the space charge instabilities can be suppressed due to the switch from the NDC behaviour to the PDC behaviour predicting a potential generation of terahertz radiations whose applications are relevance in current-day technology, industry, and research.

High frequency conductivity of hot electrons in carbon nanotubes

Abstract High frequency conductivity of hot electrons in undoped single walled achiral Carbon Nanotubes (CNTs) under the influence of ac–dc driven fields was considered. We investigated semi-classically Boltzmanns transport equation with and without the presence of the hot electrons’ source by deriving the current densities in CNTs. Plots of the normalized current density versus frequency of ac-field revealed an increase in both the minimum and maximum peaks of normalized current density at lower frequencies as a result of a strong injection of hot electrons. The applied ac-field plays a twofold role of suppressing the space-charge instability in CNTs and simultaneously pumping an energy for lower frequency generation and amplification of THz radiations. These have enormous promising applications in very different areas of science and technology.

Terahertz generation and amplification in graphene nanoribbons in multi-frequency electric fields

Abstract We study theoretically a multi-frequency response of electrons in confined graphene subject to DC–AC driven fields. We explore the possibility for using graphene nanoribbons (GNRs) to generate and amplify terahertz (THz) radiations in electric field domainless regime. We discover two main important schemes of generation; when the frequencies are commensurate, THz generation is due to wave mixing and when they are non-commensurate, a single strong field suppresses space charge instability and any weak signals can get amplified.

Combined Effects of Variable Viscosity and Thermal Conductivity on Dissipative Flow of Oil-Based Nanofluid over a Permeable Vertical Surface

Abstract: The combined effect of variable viscosity and thermal conductivity on dissipative flow of oil-based nanofluid over a permeable vertical plate with suction has been studied. The governing partial differential equations have been transformed into a coupled third-order ordinary differential equations using similarity techniques. The resulting third-order ordinary differential equations were then reduced into a system of first-order ordinary differential equations and solved numerically using the fourth-order Runge-Kutta algorithm with a shooting method. The results revealed that both viscosity and thermal conductivities of CuO oil-based nanofluid enhances the intensity of the skin friction coefficient and the rate of heat transfer at the surface of the plate. Furthermore, the thermal boundary layer thickness is weakened by the viscosity of CuO oil-based nanofluid, the Prandtl number, the suction parameter, the permeability of the medium and the thermal Grashof number