A self-consistent kinetic model of the effect of striation of low-pressure discharges in inert gases

A self-consistent kinetic model is used to treat the effect of striation of the positive column of gas discharge in low-pressure inert gases. The model is based on the solution of the kinetic Boltzmann equation for the electron energy distribution function, of the unsteady-state equation of drift and diffusion for ions, and of the Poisson equation for electric field. Spatial distributions of the electron and ion density are obtained, as well as of the electric field in the positive column of striated discharge. The model is used to explain the kinetic mechanism of origination of the effect of striation in low-pressure inert gases. The obtained distributions are independent of the initial conditions, and the solution gives a self-consistent “resonance” length of strata and the value and form of modulation of electric field.     

Numerical modeling of field-assisted ion-exchanged channel waveguides by the explicit consideration of space-charge buildup

A numerical model explicitly considering the space-charge density evolved both under the mask and in the region of optical structure formation was used to predict the profiles of Ag concentration during field-assisted Ag(+)-Na(+) ion exchange channel waveguide fabrication. The influence of the unequal values of diffusion constants and mobilities of incoming and outgoing ions, the value of a correlation factor (Haven ratio), and particularly space-charge density induced during the ion exchange, on the resulting profiles of Ag concentration was analyzed and discussed. It was shown that the incorporation into the numerical model of a small quantity of highly mobile ions other than exclusively Ag(+) and Na(+) may considerably affect the range and shape of calculated Ag profiles in the multicomponent glass. The Poisson equation was used to predict the electric field spread evolution in the glass substrate. The results of the numerical analysis were verified by the experimental data of Ag concentration in a channel waveguide fabricated using a field-assisted process.     

The role of hole mobility in scintillator proportionality

Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient characteristic of electron tracks in scintillators and other radiation detector materials. As expected, the lower mobility carrier (typically the hole) controls the ambipolar diffusion. Carrier separation when electron and hole mobilities are unequal produces a built-in radial electric field near the track analogous to an n-intrinsic semiconductor junction. The diffusion is shown to have significant effects on both the low dE/dx and high dE/dx ends of electron light-yield curves and their respective contributions to nonproportionality. In CsI:Tl, it is shown that electron confinement toward the end of the track accentuates high-order quenching such as Auger recombination or dipole-dipole transfer, while in HPGe extremely rapid (<1 fs) dilution of carrier concentration by radial diffusion renders Auger quenching negligible. Separation of geminate carriers is accentuated in the beginning of the track if electron and hole mobilities are widely unequal as in CsI:Tl, leading to bimolecular recombination of trapped carriers by slower thermal hopping routes as the favored channel at low dE/dx.     

Electron transport in xenon-hydrogen gas mixtures

We present the results of numerical solution of the Boltzmann equation in a two-term approximation taking into account both elastic and inelastic electron collisions in Xe-H₂ gas mixtures. Using the obtained electron energy distribution functions, the electron transport properties (drift velocities, mobilities, mean and characteristic energies, diffusion coefficients) are calculated for the E/N parameter (the electric field strength to gas density ratio) ranging within several townsends. A similarity rule is derived for the properties of Xe-H₂ mixtures of various densities, which allows the electron transport coefficients to be determined for the mixtures with small (below 4%) hydrogen content.     

A study of the dynamic properties of photo-induced charge carriers at nanoporous TiO(2)/conductive substrate interfaces by the transient photovoltage technique

Two interfaces with opposite orientations of the built-in electric field, nanoporous TiO(2)?film/ FTO electrode and nanoporous TiO(2) film/semitransparent Pt substrate, were constructed. The separation and transport of photo-induced charge carriers in the two systems of nanoporous TiO(2)/conductive substrate were studied by the transient photovoltage technique. Various transient photovoltage responses were obtained when the laser beam was incident from the surface of the nanoporous TiO(2) film (top illumination) or the TiO(2)/substrate interface (bottom illumination). A linear and a logarithmic dependence of the photovoltage amplitude on the excitation level were observed for top illumination and bottom illumination, respectively. The results indicate that diffusion is the major way for the separation of charge carriers in the nanoporous TiO(2) film and that the excess carriers were separated by drift under the built-in electric field at the TiO(2)/substrate interfaces.     

Theory of the field-assisted photomagnetoelectric effect in thin films: Experiment on n-type GaAs and applications

The field-assisted photomagnetoelectric (FAPME) effect is the generalization of the classical PME effect obtained by the application of a (strong) longitudinal electric field: photogenerated carriers are magnetoconcentrated by the Lorentz force and consequently the current-voltage (I(V)) law becomes non-linear. The theory of the FAPME effect is proposed for thin semiconductor layers, where the recombination rate at the film-substrate interface is a key parameter. The roles of other important factors (electric and magnetic fields, lifetime, dimensions) are also systematically investigated. Experimental data for the open-circuit voltage, the photomagnetoresistance and the I(V) characteristics were obtained for n-type GaAs films 1.5 μm thick in magnetic and electric fields of up to 13 T and 1.5 x 10⁴ V m^-1 respectively. A satisfactory agreement with the theory is found, in spite of some inherent limitations (film non-uniformity, influence of the semi-insulating GaAs substrate). Complementary magnetotransport measurements were performed without photoexcitation in the temperature range 77–300 K. These numerous and independent experiments are simultaneously exploited for the assessment of several basic properties of GaAs: the lifetime, the surface recombination velocities, the Hall and drift mobilities and the scattering mechanisms.     

Characterization of CuInTe2 crystals

A single crystal of CuInTe₂ (CIT) was prepared from melt by using a vertical Bridgman technique. The crystal was characterized by X-ray diffraction. Electrical conductivity, Hall effect and thermoelectric power (TEP) measurements were performed over a wide range of temperature. From these measurements, various physical parameters such as carrier mobilities, effective masses of charge carriers, diffusion coefficient, relaxation time, and diffusion length for both majority and minority carriers were estimated. In addition, the energy gap was determined as 1.057 eV and the type of conduction was observed to be p-type as indicated from the positive sign of both the Hall coefficient and thermoelectric power.     

Distribution of nonequilibrium charge carriers in a nonlinear thin-film capacitor

The process of capacitance relaxation between stationary states determined by different values of the bias voltage has been measured and analyzed for a nonlinear thin-film BSTO capacitor. A method used to measure the dynamic capacitance-voltage characteristics is described. The diffusion coefficients and mobilities of nonequilibrium charge carriers and the region of their spatial localization are determined.     

Electrosmotic mobility and conductivity in columns for capillary electrochromatography.

Columns employed so far in capillary electrochromatography (CEC) contain both a packed and an open segment with concomitant changes of the electric field strength and the flow velocity at the interface of the two segments in such duplex columns. To take this into account in measuring, processing, and interpreting CEC data, a framework is presented for the evaluation of the conductivity ratio and the interstitial electrosmotic flow (EOF) mobility and their usage as tools for characterizing CEC columns. This is illustrated by experimental data obtained from measurement of the current and the EOF in capillary columns packed with different stationary phases. The current data yielded the ratio of the conductivities of the packed and open segments that has been shown to be useful for the evaluation of the porosity and tortuosity. It is assumed that these important packing characteristics are the same for the flow of current and for the flow of the bulk mobile phase in the CEC column. The EOF mobility in such duplex columns is defined in two different ways. The apparent mobility, which is widely reported at present, is obtained from the length of packed segment, the migration time, and the overall electric field strength. On the other hand, the actual mobility is obtained after taking into account the porosity and tortuosity of the packing as well. Thus, the actual mobility is made independent of the porosity and tortuosity and therefore can be useful to estimate the zeta potential for characterizing the packing surface. Measurements of both the apparent and actual electrosmotic mobilities for a number of different columns have shown that the apparent and actual mobilities are significantly different in their magnitude. For this reason, it is recommended that, instead of the apparent EOF mobility, the actual mobility is used for the characterization of the packing in CEC columns.     

Fluorescence correlation spectroscopy for rapid multicomponent analysis in a capillary electrophoresis system

We describe a new technique for performing multicomponent analysis using a combination of capillary electrophoresis (CE) and fluorescence correlation spectroscopy (FCS), which we refer to as CE/FCS. FCS is a highly sensitive and rapid optical technique that is often used to perform multicomponent analysis in static solutions based on the different diffusion times of the analyte species through the detection region of a tightly focused laser beam. In CE/FCS, transit times are measured for a mixture of analytes continuously flowing through a microcapillary in the presence of an electric field. Application of an electric field between the inlet and outlet of the capillary alters the transit times, depending on the magnitude and polarity of the applied field and the electrophoretic mobilities of the analytes. Multicomponent analysis is accomplished without the need to perform a chemical separation, due to the different electrophoretic mobilities of the analytes. This technique is particularly applicable to ultradilute solutions of analyte. We have used CE/FCS to analyze subnanomolar aqueous solutions containing mixtures of Rhodamine 6G (R6G) and R6G-labeled deoxycytosine triphosphate nucleotides. Under these conditions, fewer than two molecules were typically present in the detection region at a time. The relative concentrations of the analytes were determined with uncertainties of ～10%. Like diffusional FCS, this technique is highly sensitive and rapid. Concentration detection limits are below 10(-)(11) M, and analysis times are tens of seconds or less. However, CE/FCS does not require the diffusion coefficients of the analytes to be significantly different and can, therefore, be applied to multicomponent analysis of systems that would be difficult or impossible to study by diffusional FCS.     

A positive column of electric discharge in a transverse magnetic field

The influence of a transverse magnetic field on the characteristics of the positive column of electric discharge was investigated in both the diffusion and nondiffusion approximations. All solutions in the diffusion approximation were derived in analytical form. It is shown that with the enhancement of the magnetic field induction, the concentration distributions of the plasma and particle fluxes to insulated walls become nonsymmetric, the concentration maximum displaces in the direction of the magnetic force action, and the ion flux concurrent with this force may exceed substantially the ion flux in the opposite direction. The dependences of discharge parameters on the induction value are defined for helium. It is demonstrated that there is a maximum value of induction which bounds from above the range of magnetic fields, where the stationary state of the positive column is possible. In the range allowed for the stationary state, a single induction value is matched by two different modes of the positive column varying in values of the electron energy, drift speed, and electric-field strength. By means of the transverse magnetic field, it is possible to vary the electron energy within wide limits (from a few electronvolts to several hundred electronvolts).     

Impurity ions in liquid helium

A technique for introducing a wide variety of positive ions into liquid helium is described and used to investigate positive atomic ions of K, Rb, Cs, Ca, Sr, and Ba. The zero-field mobilities of the impurity ions were measured in He II in the temperature range 1.2 < T < 1.4 K. The dependence of mobility on ion mass alone was found directly for Ca⁺ ions by investigating two isotopes. Measurements were made of the ion drift velocities in high electric fields, where nucleation of quantized vortex rings occurs. All the data and its analysis are consistent with the assumptions that all the impurity ions are singly charged, that the size of the cluster surrounding the ion is significantly different for the different ions, and that the ion-roton cross section is approximately geometric. A possible alternative model for the structure of the ion complex is discussed and a mechanism is suggested which may account for the observed variations in ionic radii.     

Peculiarities of the photovoltaic properties of films based on photoconducting polymer and organic dye in samples with free surfaces and between electric contacts

Composite films based on a carbazolyl-containing oligomer with polymethine dye additives exhibit a change in the magnitude and sign of the photovoltaic response on the passage from the samples with free surfaces to sandwich structures. It is concluded that the photovoltaic effect in the former case is determined by the diffusion of positive charge carriers possessing higher mobility, while in the latter case this effect is controlled by a significant drift of carriers in the electric field created by different work functions of the charge-collecting electrodes. It is shown that the photovoltaic effect also takes place in a nonphotoconducting polymer (polyvinyl ethylal). However, neither the sign nor magnitude of the effect in this case change on the passage from the samples with free surfaces to sandwich structures.     

A nonlocal model of spatially nonuniform positive column of DC discharge

A self-consistent kinetic model is given of the positive column of low-pressure low-current dc discharge in neon in a tube of circular cross section, which is nonuniform in both the radial and longitudinal directions. The model is based on the solution of kinetic equation for the electron distribution function in nonlocal approximation, of drift equation for ions, of diffusion equation for metastable ions, and of Poisson equation for electric field. Calculations are performed for the case where the longitudinal nonuniformity of discharge is caused by a jumpwise variation of the discharge tube radius.     

真空断路器弧后鞘层发展过程仿真分析

以真空断路器弧后鞘层发展过程为对象,采用等离子体流体力学模型,求解电子、离子密度和平均电子能量的漂移扩散方程和耦合电场的泊松方程,引入粒子间碰撞反应,仿真分析其弧后介质恢复变化和鞘层发展阶段电子和离子的空间分布、密度分布以及间隙电势分布。采用对比分析法,研究不同初始条件对鞘层发展的影响,结果表明:在保持其余参数不变的条件下,暂态恢复电压(TRV)斜率和间隙轴向长度均与鞘层发展速度呈正相关,初始等离子体密度和弧后金属蒸气压强均与鞘层发展速度呈负相关。     

Ion velocity distribution function in intrinsic gas plasma under conditions of resonance recharging as a main process. Theory

An analytical solution is found for the kinetic Boltzmann equation for ions in natural gas at the arbitrary electric field strength in plasma under conditions where the dominating process is resonance recharging. The ion distribution function in the strong field coincides with the function obtained by authors earlier, and in the weak field it is close to the Maxwell’s function with the temperature of atoms. For the ambipolar field of arbitrary value, the dependence of the cross-section of resonance recharging on the relative velocity is taken into account. It is shown that the ion velocity distribution function essentially differs from the Maxwell’s and is determined by two parameters. Good agreement between the results of calculations of the drift velocity of He⁺ ions in He, Ar⁺ in Ar and the reduced mobility of N₂ ⁺ in N₂ and known experimental data is obtained in a wide range of reduced electric field values. The analytical calculations of the average energy of ions at different values of the reduced electric field are compared with the results of the numerical simulation using the Monte Carlo method performed by other authors.     

Formation of stable stacking zones in a flow stream for sample immobilization in microfluidic systems

Electrocapture is a multifunctional microfluidic tool that can be used for concentration, sample cleanup, multistep reactions, and separation of biomolecules. Herein, we investigate the mechanisms underlying the electrocapture principle. A microfluidic electrocapture device was found to be capable of generating regions of different electric field, which are maintained in the flow by electric and hydrodynamic forces, with the zones of lower electric field strength upstream of those with higher strength. In addition to detection of the local electric fields by direct measurements, the existence of the zones was observed by the capture of a solution containing Coomassie and myoglobin. The two molecules were captured at different spots in a steady-state manner and were released (separated) at different electric fields. Considering these observations and the experimental values for the electric field strengths, flow velocities, and electrophoretic mobilities of DNA, proteins, and peptides, it is concluded that the macromolecules are captured between the field zones by a stacking mechanism.     

Potential of New Superconductivity Produced by Electrostatic Field and Diffusion Current in Semiconductor

This paper proposes a unique device that uses a constant-current source to demonstrate a new superconductivity concept. This device is considerably different from conventional superconductors, and is based on the idea that the voltage that produces Joule heating can be in proportion to the voltage derived from the line integral of an internal electric field employing a condenser when the current is supplied to a doped semiconductor by a current source. In this case, the charge-carrier concentration is spatially nonuniform. The concentration gradient of the current source leads to diffusion of the charge carriers, and the motion of these carriers contributes to the current density. An electric field is not needed to move the charge carriers, because they move by diffusion and not by drift. Because the voltage associated with Joule heating is proportional to the voltage of the internal electric field, the total voltage in the semiconductor is zero; however, the current carried from the current source prevents the total current from being zero. We show that this property of the device results in a superconducting state arising from the diffusion-current state. In our theoretical analysis, we demonstrate that two electrons in the device form a pair and that Bose?CEinstein condensation of all pairs is produced. From this result, we derive the existence of a superconducting current without voltage. Furthermore, we have developed an experimental setup and confirmed zero electric resistance and energy emission from the semiconductor. Therefore, we conclude that a new type of superconductivity had been achieved. This phenomenon was observed multiple times and thus can be reproduced.     

Evidence for a non-thermal microwave effect in the sintering of partially stabilized zirconia

A hybrid furnace, which allows the simultaneous application of microwave and radiant energy, has been used to investigate the sintering of partially stabilized zirconia (doped with 3 mol % yttria). Microwave-enhanced sintering is clearly demonstrated with densification occurring at a lower temperature when a high-frequency electric field is applied. By considering the variation of electric field strength with furnace temperature, this enhancement is shown to be non-thermal in nature, being dependent on the electric field strength and not the power density (heating) of the microwaves. This dependence on electric field is consistent with an additional driving force term in the equation which describes the diffusion of vacancies through the material during sintering.