Electroconvection between coaxial cylinders of arbitrary ratio subjected to strong unipolar injection

This study deals with the electroconvection phenomenon that takes place in a dielectric liquid layer placed between two annular electrodes and subjected to the action of an electric field.The full set of governing equations describing the combined Electro-Hydro-Dynamic (EHD) flow is directly solved with the finite volume method.The development of electroconvective motion is investigated in details in the case of strong injection when the emitter electrode is the inner cylinder.We first examine the stability of such flow. As in the plane–plane configuration we have highlighted the existence of a linear and non-linear critical electric Rayleigh numbers giving rise to a hysteresis loop. The flow structure under the effect of the electric field is analyzed and the distribution of electric charge density is presented. In particular we investigate the effect of various parameters involved in this configuration, such as the radius ratio, injection strength and electric Rayleigh number.A multicellular convective pattern is particularly observed and it is shown that the number of cells is increased when the annular gap is narrower. Finally a significant increase of the rotational speed of the vortex with the electric Rayleigh number is recorded.     

Investigation of electroconvective flow regimes in liquid dielectrics possessing unipolar injection conductivity

Isothermal electroconvection in a dielectric liquid arising in a plane-parallel electrode system due to unipolar injection of charges from the cathode is considered. Stationary flow regimes formed after the crisis of the equilibrium layer stability loss are numerically modeled. Variations of initial conditions yield two flow regimes providing comparatively higher volume charge densities in the first case near the cathode and in the second case near the anode. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 3–7, June, 2008.     

Electroconvection under injection from cathode and heating from above

We study the electroconvection that appears in a nonuniformly heated, poorly conducting liquid in a parallel-plate horizontal capacitor due to the action of an external static electric field on the charge injected from the cathode. It is shown that the heating of the layer from above prevents steady-state convection and that, unlike the isothermal situation, electroconvection can appear in the oscillatory manner as a result of direct Hopf bifurcation. The effect of the heating intensity, the intensity of charge injection from the cathode, and the charge mobility on the thresholds of oscillatory and monotonic electroconvection is analyzed and the characteristic scales and frequencies of critical perturbations are determined. The nonlinear wave and steady-state regimes of the 2D convective structures formed in the poorly conducting liquid under the action of thermogravitational and injection mechanisms of convection are analyzed. The domains of existence of standing, traveling, and modulated waves are determined.     

Electric convection of low-conductivity liquid in a horizontal capacitor in the presence of unipolar charge injection

Electric convection of low-conductivity liquid in a horizontal plane capacitor is analyzed with allowance for unipolar charge injection. Dynamics of charge transfer through stationary isothermal liquid in the presence of modulated electric field is studied. Effect of modulation amplitude and frequency on the spatiotemporal distribution of charge density and electric potential is considered. Nonlinear electric convection of nonisothermal low-conductivity liquid is studied in the presence of static electric field in a 2D system. Hysteresis transitions between two different (with respect to intensity) regimes of electric convection are analyzed.     

Dynamics of radiation-induced conductivity in the presence of spatial charge

The previously proposed creation-annihilation model of electric conductivity (CAMEC) was applied to the case of radiation-induced conductivity (RIC) in dielectrics. The numerical calculations for Teflon-like material were performed. The results show that if radiation-induced conductivity is less than 10⁻¹³ S/m, the RIC has enough time to attain very low values before it nearly compensates the spatial, implanted charge. In general, application of the one-dimensional CAMEC to the case of RIC qualitatively reproduces the experimentally observed radiation effects on the lifetime of electrets.     

On phonon-wind-driven spin injection and the conductivity mismatch problem

Spin injection from a ferromagnetic metal into a semiconductor driven by a constant phonon flow is considered. It is shown that diffusive (as opposed to electrical) spin injection gives rise to a highly spin-polarized current through the semiconductor. In other words, phonon-wind-driven injection under certain conditions eliminates the conductivity mismatch responsible for reduced spin polarization of the electric current injected from a ferromagnetic metal into a semiconductor.     

Dynamical thermal conductivity of bilayer graphene in the presence of bias voltage

We study dynamical thermal conductivity of doped biased bilayer graphene for both AA and AB-stacking in the context of tight binding model Hamiltonian. The effects of bias voltage and chemical potential on the behavior of dynamical thermal conductivity are discussed for different stacking of bilayer graphene. Green's function approach has been implemented to find the behavior of thermal conductivity of bilayer graphene within linear response theory. We have found that thermal conductivity decreases with chemical potential for different values of temperature and frequency. Also thermal conductivity of AB stacked bilayer graphene versus bias voltage includes a peak for each value of chemical potential. Furthermore we study the frequency dependence of thermal conductivity of AA stacked bilayer graphene for different values of temperature and bias voltage.     

Electroconvection of a poorly conducting fluid in a steady electric field

The emergence of electroconvection and nonlinear flows of a nonisothermal poorly conducting fluid in a steady electric field of a horizontal capacitor is investigated. The main mechanism of charge formation in the fluids under investigation is electric conduction due to the temperature dependence of the charge mobility. The Galerkin method is used for obtaining a system of nonlinear differential equations for the amplitudes of spatial modes; on the basis of this system, steady-state, periodic, and random nonlinear regimes of the fluid flow are investigated. The scenarios of transition to chaotic flows (via intermittence and a subharmonic cascade) are discovered. The interrelation between the electroconductive and thermogravitational mechanisms of convection is analyzed, and hysteretic transitions between monotonic and oscillatory regimes are studied. Qualitative comparison with the experiment is carried out.     

Numerical study of the heat transfer and electro-thermo-convective flow patterns in dielectric liquid layer subjected to unipolar injection

In this article we study the electro-thermal convection in a dielectric liquid layer placed between two electrodes and subjected to the simultaneous action of an electric field and a thermal gradient. The full set of equations describing the electro-thermo-convective phenomena is directly solved using a finite volume method. We first heat the liquid from below at time t = 0, wait for the thermal steady state and then inject the electric charges by applying the electric potential. The development of the electro-convective motion is analysed in detail in two cases: 1) strong injection from the lower electrode, 2) strong injection from the upper one. We also study the heat transfer enhancement due to electro-convection. The evolution in time of the Nusselt number Nu for different combinations of the two usual non-dimensional parameters associated to the electro-thermo-convection phenomena (Rayleigh number Ra and the electrical parameter T) is also given and analysed.     

Structural characteristics and residual stresses in oxide films produced on Ti by pulsed unipolar plasma electrolytic oxidation

Oxide films, 7–10 µm thick, were produced on commercially pure titanium by plasma electrolytic oxidation in a sodium orthophosphate electrolyte using a pulsed unipolar current with frequency (f) and duty cycle (δ) varying within f = 0.1–10 kHz and δ = 0.8–0.2, respectively. The coatings comprised a mixture of an amorphous phase with nanocrystalline anatase and rutile phases, where the relative rutile content range was 17–25 wt%. Incorporation of phosphorus from the electrolyte into the coating in the form of PO₂ ^–, PO₃ ^2– and PO₄ ^3–, as demonstrated by EDX and FT-IR analyses, contributed to the formation of the amorphous phase. Residual stresses associated with the crystalline coating phase constituents were evaluated using the X-ray diffraction sin² ψ method. It was found that, depending on the treatment parameters, internal direct and shear stresses in anatase ranged from–205 (±17) to–431 (±27) MPa and from–98 (±6) to–145 (±10) MPa, respectively, whereas the rutile structure is comparatively stress-free.     

Electroconvection in nematic liquid crystals: comparison between experimental results and the hydrodynamic model

We describe experimental observations illustrating the extraordinary sensitivity of pattern formation in electroconvection in nematic liquid crystals to changes in magnitude and sign of dynamic transport coefficients triggered by weak fluctuations from a lower temperature nematic-smectic A phase transition. These results are compared to predictions of a hydrodynamic model (HM). We find two patterns at onset not predicted by HM: a rectangular pattern of short normal rolls, and, at lower temperatures, localized patterns coexisting with the subcritical state.     

Absolute negative conductivity in zig-zag carbon nanotubes in the presence of a magnetic field

The volt-ampere and gauss-ampere characteristics of zig-zag carbon nanotubes are calculated at low temperatures based on the average electron method. The characteristics obtained are analyzed as functions of the magnetic field strength. It is revealed that a constant electric field can arise spontaneously in carbon nanotubes upon application of an alternating high-frequency electric field. This effect can be due to the nonequilibrium electron subsystem of the carbon nanotubes.     

Absence of residual quasiparticle conductivity in the underdoped cuprate YBa2Cu4O8

We report measurements of the in-plane thermal conductivity kappa of the stoichiometric underdoped cuprate YBa2Cu4O8 (Y124) below 1 K. kappa(T) is shown to follow a simple phononic T3 dependence at the lowest temperature T for both current directions, with a negligible linear quasiparticle contribution. This observation is in marked contrast with behavior reported in optimally doped cuprates, and implies that extended zero-energy (or low-energy) quasiparticles are absent in Y124.     

Efficiency of homonuclear Hartmann-Hahn and COSY-type mixing sequences in the presence of scalar and residual dipolar couplings

In the presence of scalar (J) and residual dipolar (D) couplings, the transfer efficiency of homonuclear Hartmann-Hahn and COSY-type mixing depends on the ratio D/J and on the mixing sequence. This dependence is analyzed theoretically and the results are confirmed experimentally. At least two different mixing sequences are required to yield good transfer efficiencies for all ratios D/J. In contrast to COSY-type experiments, homonuclear Hartmann-Hahn sequences can provide efficient transfer even if the sum of D and J is zero, i.e., if the coupling vanishes in the weak coupling limit.     

On two-dimensional finite amplitude electro-convection in a dielectric liquid induced by a strong unipolar injection

The hydrodynamic stability of a dielectric liquid subjected to strong unipolar injection is numerically investigated. We determined the linear criterion T_c (T being the electric Rayleigh number) and finite amplitude one T_f over a wide range of the mobility parameter M. A noticeable discrepancy is shown for T_f between our numerical prediction and the value predicted by stability analysis, which is due to the velocity field used in stability analysis. Recent studies revealed a transition of the flow structure from one cell to two with an increase in T. We demonstrate that this transition results in a new subcritical bifurcation.     

The electrical conductivity properties of polythiophene/TiO2 nanocomposites prepared in the presence of surfactants

A composite of polythiophene (PT) and nano-titanium dioxide (TiO₂), possessing core–shell structure, was synthesized via oxidative polymerization of thiophene using FeCl₃ in the presence of three different surfactants: anionic, cationic, and nonionic. The morphology of the obtained composite materials was investigated by SEM, proving the core–shell structure of the prepared nanocomposite. The formation and thermal stability of the PT onto TiO₂ nanoparticles were confirmed by FTIR and TGA analyses, respectively. XRD data show all of composite materials were amorphous structures. The electrical properties of the nanocomposites were investigated in the presence of surfactant materials, and the best semiconductor property was observed for PT/TiO₂-anionic system. This difference in the conductivity has been attributed to differences in the stability of the composites.