Optimization of the parameters of a carbon nanotube-based field-emission cathode

A procedure for optimizing a field-emission cathode based on carbon nanotubes (CNTs) is developed. An array of identical equidistant vertical CNTs is considered. The optimization procedure takes into account the effect of screening of an electric field by neighboring nanotubes by solving a Laplace equation and the thermal instability of nanotubes, which limits the emission current density of a nanotube, by solving a heat conduction equation. The relation between the emission current and the applied voltage is described by the Fowler-Nordheim relationship containing the CNT tip temperature as a parameter. Upon optimization, the optimum distance between CNTs that ensures the maximum emission current density is calculated. The calculation results demonstrate that this parameter depends substantially on both the applied voltage and the nanotube geometry. These dependences are weakly sensitive to the choice of the transport coefficients (thermal conductivity, electrical conductivity) of nanotubes.     

Photochemical properties of photosystem 1 immobilized in a mesoporous semiconductor matrix

The pigment-protein complex of photosystem 1 (PS1) isolated from cyanobacterium Synechocystis sp. PCC 6803 has been adsorbed on a solid mesoporous film made from TiO₂ nanoparticles. was on The TiO₂ film supported on a glass substrate with a surface area of 1 cm² adsorbs up to 0.045 nmol of PS1. PS1 molecules are distributed in the pores of the mesoporous support. Immobilization has an insignificant effect on the optical and photochemical properties of PS1. A reversible photoinduced EPR signal from the oxidized primary electron donor P700 of immobilized PS1 has been detected. It has been shown by photoelectrochemical methods that the photoexcitation of PS1 results in electron injection from PS1 to the conduction band of TiO₂.     

Some aspects of causal & neutral equations used in modelling

The problem that motivates the considerations here is the construction of mathematical models of natural phenomena that depend upon past states. The paper divides naturally into two parts: in the first, we expound the inter-connection between ordinary differential equations, delay-differential equations, neutral delay-differential equations and integral equations (with emphasis on certain linear cases). As we show, this leads to a natural hierarchy of model complexity when such equations are used in mathematical and computational modelling, and to the possibility of reformulating problems either to facilitate their numerical solution or to provide mathematical insight, or both. Volterra integral equations include as special cases the others we consider. In the second part, we develop some practical and theoretical consequences of results given in the first part. In particular, we consider various approaches to the definition of an adjoint, we establish (notably, in the context of sensitivity analysis for neutral delay-differential equations) rôles for well-defined adjoints and ‘quasi-adjoints’, and we explore relationships between sensitivity analysis, the variation of parameters formulae, the fundamental solution and adjoints.     

Formation of the <Superscript>178<Emphasis Type="Italic">m</Emphasis>2</Superscript>Hf isomer in a natural mixture of hafnium isotopes irradiated by an intense γ quantum beam (experiment)

The results of an experiment on the production of the ^178m2Hf isotope by exposing a natural mixture of hafnium isotopes to an intense bremsstrahlung beam are reported. The sample under investigation was exposed to an electron beam with an energy of 30 MeV and current 200 μ A for 500 h. The spectrometric analysis of the irradiated sample using radiochemical methods for the separation of isotopes did not reveal ^178m2 Hf lines.     

Force acting on a particle in unsteady flow of a pseudoplastic fluid

The accelerated flow of a pseudoplastic fluid around a quiescent sphere at Reynolds numbers Re = 0–200 and dimensionless acceleration Ga = 10–10⁴ is studied by numerical simulation. It is shown that the analytical expression of the added mass force for an ideal fluid is appropriate for a pseudoplastic fluid. An expression for calculating the hereditary Basset force for a pseudoplastic fluid is proposed.     

Aqueous suspensions of single-wall carbon nanotubes: Degree of aggregation into bundles and optical properties

Aqueous suspensions of nanotubes, as well as the structure and optical properties of their aggregates (bundles), are studied by spectroscopy and high-resolution electron microscopy. The structure of nanoparticles is controlled by varying the ultrasonication time during preparation of suspensions. It is found that the defectiveness of nanotubes increases with decreasing bundle size. A correlation is shown to take place between the suspension preparation regime, the structure of nanoparticles, and the relaxation of the photoexcitation energy of their electronic shells. It is found that the efficiency of photoexcitation energy conversion into heat increases with increasing degree of aggregation of nanotubes into bundles.     

Percolation Transition Under Thermal Reduction of Graphene Oxide

The conductivity of thermally reduced graphene oxide (GO) measured at various reduction stages shows that the samples demonstrate transition to the conducting state within a narrow range of annealing temperatures (150-170 °С). The conductivity in this temperature region increases by about five orders of magnitude to indicate that the percolation mechanism is responsible for this transition. According to this mechanism, the above temperature range is associated with formation of a certain number of conductive channels composed of GO fragments reduced to the conducting state. Thus, it is a new type of percolation transition, since increased conductivity is a result of increased conductivity of the particles due to thermal treatment rather than a result of increased concentration of conducting particles. A further temperature growth is associated with increased number of conducting GO fragments and the number of conducting channels, and some smoother conductivity increase. The XPS spectra of partially reduced GO samples indicate correlation between increased conductivity of GO fragments and plasmon oscillations revealed as XPS peaks. The current-voltage characteristics measured for the samples of partially reduced GO indicate a non-ohmic conductivity which manifests as increased conductivity versus voltage dependence. This is due to the fact that electrical resistance of the percolation channels is the sum of resistances of conductive GO fragments and contact resistances which depend on the voltage drop at the contact and, therefore, on the voltage applied.     

Manifestation of resonance dipole-dipole interaction in spectra of low-temperature molecular mixtures of C2F6 in CF4 and CF4 in C2F6

We have measured IR absorption spectra of solutions C₂F₆ in CF₄ (T = 178 K) and CF₄ in C₂F₆ (T = 173 K) in the overtone range of the spectrum. We have studied how the resonance dipole-dipole interaction affects the formation of contours of bands that correspond to transitions to states involving the vibrations ν₁₀(C₂F₆) and ν₃(CF₄), which are strong in the dipole absorption. For the system C₂F₆ in liquid CF₄, the state ν₂ + ν₁₀(C₂F₆) resonantly interacts with the state ν₂(C₂F₆) + ν₃(CF₄), and, for the system CF₄ in liquid C₂F₆, the state ν₁ + ν₃(CF₄) resonantly interacts with the state ν₁(CF₄) + ν₁₀(C₂F₆). The contours of the bands ν₂ + ν₁₀ (C₂F₆) in the spectrum of the mixture with CF₄ and of the bands ν₁ + ν₃(CF₄) in the spectrum of the mixture with C₂F₆ have been calculated.