Diagnostics of pulsed vacuum arc discharges by optical emission spectroscopy and electrostatic double-probe measurements

In this work the study of plasma characteristics by means of two of the most common techniques employed by the scientific community dedicated to the experimental plasma research is presented. The plasma was generated in a vacuum reaction chamber which was filled with hydrogen gas. Inside the chamber, two opposite electrodes were placed: the cathode, which was formed by a target of highly oriented pirolitic carbon and the anode. The electron temperature T_e and the electron density n_e were measured by using optical emission spectroscopy and electrostatic double probe, obtaining very close values for each case. T_e was calculated as approximately 0.7 eV and n_e of the order of 10¹³ cm⁻³. The optical emission measurement allows one to identify the substances that are in the plasma like C I, C II, C III, H I and H₂ and some possible reactions. The double-probe technique showed the plasma potential of about 24 V. The characteristic curve of the double probe exhibited oscillational plasma instabilities which could be attributed to the charge density variation or other factors, such as the employed AC signal and the geometric probe.     

How “Musical” Are High-T c Superconductors and What Can Empirical Rules Tell About Their Origin?

It is shown that parameters such as optimal T _c for cuprate superconductors or details of their doping curves can be organized on phenomenological rules. Accordingly, T _c in a range between a kink and the optimum scale linearly with the number of effective holes h _e, according to T _e = h _e T _c ^e, with T _c ^e = 600 K. Effective holes are composed of the difference between holes in the Cu—O bonds in the CuO₂ planes, h _p, and holes in the Cu—O bonds with the c axis or apical O, h _e, according to h _e = h _p? h_c = fh _p. The deleterious effect of the apical O manifests itself in three levels, depending on the basic modes of its coordination of the CuO₂ planes in zero, one, or two sheets (according to factor f = 1, 2/3, ¹/₂). The values of h _p at T _c optimum tend to rational fractions, ranging from 1/6 to 1/3, and are determined by lattice pressure. This “musical” or harmonic T _c matrix, originating from two structurally determined factors, groups optimal T _c into families. Knight shift data, establishing h _p, bear out the general assumptions. Some flexibility in the range within families is observed. This flexibility indicates the operation of more complex influences from structural detail, such as the varying distance of Cu to the apical O. The existence of ranges within optimal T _c families indicate a somewhat ‘tunable’ rather than a strict “musical” T _c-level scheme with measured intervals. The details of the doping curves are similarly organized. These phenomenological rules suggest the operation of bond ordering effects. Arguments for the actual nature of the bond orderings are presented in terms of local pairs of doped bonds in trijugate positions. These quantitative concepts can be expanded to other characteristic features in the doping curves of cuprates and other high-T _c materials such as C or B containing systems, providing a universal frame for explaining high-T _c superconductivity in bond ordering terms.     

Control of the electron temperature by varying the resonance zone width in ECR plasma

The electron temperature (T_e) in an electron cyclotron resonance plasma is clarified to depend on the spatial profiles of the microwave-power absorption by both the electromagnetic-waves measurement and the simulation of microwave power absorption. It is found that T_e is controlled by varying the magnetic field configuration and/or the microwave frequency since the power absorption profile is influenced by the effective resonance width. In fact, T_e is observed to decrease with decreasing the magnetic field gradient at the resonance point for N₂, Ar and O₂/Ar plasma.     

Characterization of SiNx/a-Si:H crystalline silicon surface passivation under UV light exposure

One of the most promising solution for crystalline silicon surface passivation in solar cell fabrication consists in a low temperature (< 400 °C) Plasma Enhanced Chemical Vapor Deposition of a double layer composed by intrinsic hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (SiN_x). Due to the high amount of hydrogen in the gas mixture during the double layer deposition, the passivation process results particularly useful in case of multi-crystalline silicon substrates in which hydrogenation of grain boundaries is very needed. In turn the presence of hydrogen inside both amorphous layers can induce metastability effects. To evaluate these effects we have investigated the stability of the silicon surface passivation obtained by the double layer under ultraviolet light exposure. In particular we have verified that this double layer is effective to passivate both p- and n-type crystalline silicon surface by measuring minority carrier high lifetime, via photoconductance-decay. To get better inside the passivation mechanisms, strongly connected to the charge laying inside the SiN_x layer, we have collected the Infrared spectra of the SiN_x/a-Si:H/c-Si structures and we have monitored the capacitance-voltage profiles of Al/SiN_x/a-Si:H/c-Si Metal Insulator Semiconductor structures at different stages of UltraViolet (UV) light exposure. Finally we have verified the stability of the double passivation layer applied to the front side of solar cell devices by measuring their photovoltaic parameters during the UV light exposure.     

Modelling of a thermal insulation system based on the coldest temperature conditions by using artificial neural networks to determine performance of building for wall types in Turkey

In formation of building external envelope, as two important criteria, climatic data and wall types must be taken into consideration. In the selection of wall type, the thickness of thermal insulation layer (d_i) must be calculated. As a new approach, this study proposes determining the thermal insulation layer by using artificial neural network (ANN) technique. In this technique five different wall types in four different climatic regions in Turkey have been selected. The ANN was trained and tested by using MATLAB toolbox on a personal computer. As ANN input parameters, U_w, T_e,Met, T_e,TSE, R_wt, and q_TSE were used, while d_i was the output parameter. It was found that the maximum mean absolute percentage error (MRE, %) is less than 7.658%. R² (%) for the training data were found ranging about from 99.68 to 99.98 and R² for the testing data varied between 97.55 and 99.96. These results show that ANN model can be used as a reliable modeling method of d_i studies.     

Effect of matrix on the dry friction coefficient of unidirectional fiber-reinforced composite

The coefficients of dry rest (μ₀) and sliding (μ _s ) friction on a polished disk made of quenched steel have been measured for various polymer matrices and fibers and for a composite unidirectionally reinforced with poly(amidobenzimidazole) (PABI) fibers. It is established that μ₀ > μ _s for matrices and fibers with glass transition temperatures T _g below room temperature T _R , otherwise μ₀ ≈ μ _s . This effect is explained by a sharp growth in the plasticity of polymers at T _g , which leads to an increase in the polymer-steel contact area. For a composite with T _g > T _R , the dry friction coefficients obey the relation μ₀ ≈ μ _s ≈ μ _e /C, where μ _e is the coefficients of sliding friction of PABI fibers and C is their concentration. For T _g < T _R (plastic matrix), the friction coefficients of the composite and matrix are close because the latter cannot hold the fiber ends during friction. As a result, they are bent and aligned along the matrix surface or embedded in the surface layer.     

Magnetic and electrical behaviour of La0.67Ba0.33Mn1 − xFexO3 perovskites

(La_0.67Ba_0.33)Mn_1 ? xFe_xO₃ manganites compounds have been prepared by doping up to 20% of Fe at the Mn site. As previously reported in the literature paramagnetic (PM) to ferromagnetic (FM) phase transition has been observed in materials with low Fe doping (≤ 10%). In our x = 0 and x = 0.05 compounds the Curie temperature (T_C) is close to room temperature. Above 10% Fe amount, specimens exhibit a glass magnetic behaviour with a spin- or cluster-like freezing process that can be related to a loss of ferromagnetic double exchange interaction. Below 10% of Fe³⁺ doping electrical-resistivity measurement shows metal – semiconductor transition with a maximum peak of resistivity (ρ_max) at a temperature T_P close to T_C. Above 10% of Fe³⁺ doping amount the materials exhibit only semiconductor behaviour. Both T_C and T_P decrease with doping rates with an increasing difference in temperature (T_P being lower than T_C). Results are consistent with a reduction of the number of available hopping sites for the Mn e_g(↑) electron due to substitution of Mn³⁺ by Fe³⁺ that suppress the double exchange (DE) interactions.     

Effect of magnetic barrier on the plasma parameters in a Trimix-M galatea

The parameters of plasma trapped in a Trimix-M galatea with increased values of the magnetic barrier and the energy of a hydrogen plasma bunch injected in the trap have been determined. For a barrier magnetic field of B _b ～ 0.1 T, the plasma confinement time in the trap is τ_p ≈ 300 μs (which agrees with estimates obtained using formulas describing the classical transfer), the maximum electron density is n _e ～ 5 × 10¹³ cm^?3, and the electron and ion temperatures are T _e ≈ 20 eV and T _i ～ 2T _e, respectively. The energy of trapped plasma is ～110 J, and the ratio of the gaskinetic to magnetic pressure in the plasma is β₀ ～ 0.2.     

Time and space resolved Langmuir probe measurements of a pulsed vacuum arc plasma

The time and space evolution of pulsed vacuum arc plasma parameters have been measured using a single cylindrical Langmuir probe in a free expansion cup. Electron density n_e, effective electron temperature T_eff and electron energy distribution function (EEDF) are derived from the I-V curves using Druyvesteyn method. Results show that during the discharge time, the electron density n_e is between 0.27 and 1.82 × 10¹⁸ m⁻³ and the effective electron temperature T_eff is between 6.14 and 14.72 eV, both of which decrease as a function of the discharge time. The electron energy distribution function (EEDF) is no-Maxwellian since the high-energy electrons depart from the Maxwellian distribution. Due to the plasma expansion, the electron density n_e decreases as increase of the expansion distance, but the effective electron temperature T_eff is weakly dependent on the distance in the free expansion cup.     

Effect of nonideality on the collisional recombination coefficient in an ultracold plasma

The effect of nonideality on the dependence of the collisional recombination coefficient α in an ultracold plasma on the temperature T _e and density n _e of electrons is discussed. It has been shown that the dependence of the recombination coefficient on the density and temperature α ? n _e ² T _e ^?9/2 characteristic of a weakly nonideal plasma is transformed to α ? n _e ^1.2 T _e ^?2 when the ratio of the potential energy to the kinetic energy of an electron (nonideality parameter) reaches unity. As the nonideality parameter increases, the temperature dependence disappears and the density dependence becomes n _e ^1/2 . The existing theoretical results for various physical models have been analyzed and compared to the experimental data.     

An accurate equation for the V−T characteristic of GaAs diode thermometers in the 4–300 K range

GaAs diode thermometers have been demonstrated to be reliable and reproducible transducers in both the liquid nitrogen and helium range, but so far no equation has been available for interpolating the voltage-temperature characteristic, at constant current, with an accuracy better than 1 K.An equation is developed: V = A + Blog_e (T/T₁ + 1) + C(log_e(T/T₁ + 1)² + …] which determines the V?T characteristic with a voltage accuracy of 0.01%, corresponding to 0.1 K on the whole 4–300 K range.     

On the theory of recombination of ultracold plasma

Results are given of the solution of an algebroid set of kinetic differential equations of populations of states of excited atoms in recombining plasma. Homogeneous nonequilibrium two-temperature hydrogen plasma with initial electron temperatures T _e0 = 1, 6, 13.33 K and initial density n _e0 = (1–2.7) × 10⁹ cm^?3 is considered. Comparison is made with theoretical and experimental data. The question is considered of the validity of the formulas for the probabilities of quenching an excited atom by electron impact in ultracold plasma (T _e0 < 100 K). The calculation is performed for weakly nonideal (γ _e0 < 1) and nonideal (γ _e0 ≥ 1) plasma. Dependences are obtained of the concentration of excited atoms and free electrons, the temperature of free electrons, and the nonideality parameter on the time and on the distribution of excited atoms with respect to the main quantum number.     

Fluctuations of the Order Parameter's Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c ≤ T _c ≤ T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Anomalous resistivity of sodium-lithium-niobium-tantalum ceramics

The electric resistance R of the samples of a Na_0.88Li_0.12Nb_0.5Ta_0.5O₃ ferroelectric ceramic composition with the electron conductivity increased by thermal reduction in vacuum exhibit anomalous temperature variation of the posistor type in the temperature region of 350–400°C, where the nonreduced samples exhibit transition to the state with superionic conductivity. The R(T) anomaly was observed during the electrical measurements both in air and in vacuum. The effect depends on the degree of material reduction and the measurement frequency, reaching 4–5 orders of magnitude for the dc measurements in samples with maximum conductivity. It is suggested that the R(T) anomaly is localized in the near-electrode layer and is related to a mutual influence of the electron and ion transport processes.     

A High-Temperature,Thermal Non-equilibirum Thermochemical Model for Polytetrafluoroethylene

A comprehensive thermochemical model for polytetrafluoroethylene (PTFE), also known as Teflon^®, is developed for use with computational fluid dynamic and magnetohydrodynamic computer codes. The model computes the thermodynamic properties of PTFE for a temperature range of 500 K to 580 230 K (50 eV) and extends to density values as low as 10^?8 kg · m^?3. The 23 equation nonlinear system produced under the assumptions of ideal gas and two-temperature local thermodynamic equilibrium (LTE) was solved numerically using the Newton–Raphson method. The extended thermochemical model is verified for both the composition and thermodynamic properties by comparisons to existing thermochemical models in the literature. These comparisons verify the model for the available, yet limited, temperature and density ranges. The properties display expected trends such as an increase in the degree of ionization with decreasing density, while almost independent of the electron to heavy-particle temperature ratio (θ _e/h = T _e/T _h). The specific internal energy adheres to a fairly predictable curve, i.e., the specific internal energy is linear as the mixture stays at a fairly constant composition over some T _e range. However, over the T _e range where reactions occur, it was observed that such variation shows a steeper positive slope that represents energy deposition to the internal modes of the gas as opposed to heating. That is, the density is the main factor in deviations from one curve to the next while θ had a slight effect. Likewise, for the specific internal energy, the density had the greatest impact.     

Nonequilibrium state of highly ionized helium plasma at atmospheric pressure

A spectral study of arc helium plasma of atmospheric pressure with the electron density n _e ≤ 10¹⁷ cm^?3 and the ionization degree of ～0.25 is performed. The character of the population of HeI levels with the excitation energy of 20.96–24.04 eV, which indicates infeasibility of the Boltzmann law with the electron temperature T _e for describing the relative population of excited HeI states, is experimentally determined. Due to the experimentally revealed unfitness of the local thermodynamic equilibrium (LTE) model for estimating T _e , mixed contours of the HeI 1083 line were divided into the Lorentz and Gaussian components. As a result, both the Stark effect constant that is poorly studied for this line and the temperature of heavy particles T _g in the plasma were estimated based on the Doppler component of its Gaussian half-width. As the arc current varied from 200 to 400 A, T _g increased from 20000 to 25 500 K, while the increase in the ionization temperature was, in this case, negligible (from 20000 to 21000 K).     

Melting and Evaporation of Monolayer H2-D2 Mixtures Adsorbed on D2-Plated Graphite

We report heat capacity measurements on second layer mixtures of H ₂ -D ₂ adsorbed on exfoliated graphite plated with one monolayer of D ₂ , for 3K ₂ concentration, between 0 and 1, across the three-phase melting region (T _m ) and the two-phase liquid-vapor coexistence region through its high temperature boundary (T _e ). Deviations from a linear dependence of T _m and T _e on x have been found. An estimate of the degree of fractionation is presented.     

Effect of Cu excess on three-stage CuGaSe2 thin films using in-situ process controls

The objective of this work is to study the effect of Cu excess and compare the growth mechanism of CuGaSe₂ (CGS) films co-evaporated using a bilayer and a three-stage process and evaluate the consequences of the latter for CGS on transparent back contacts. CGS thin films are prepared by co-evaporation in a three-stage process onto Mo/soda-lime substrates and onto FTO. In contrast to the bilayer process, Cu-Se phases are only observed on the surface at the end of the second stage, e₂. This allows to work with a broader Cu-excess window. Atomic ratios (Cu/Ga)e₂ of around 1.3 at the end of deposition phase 2 in the three-stage process show the better device efficiencies due to a larger grain formation. Increasing the Cu content leads to a slight decrease of the grain size and voids are observed in the film, reducing the FF of the device. The CGS morphology and the solar cells efficiency are dominated by the Cu excess more than by the T_substrate between 535 °C and 500 °C. Similar results are obtained for CGS on FTO: (Cu/Ga)e₂ ∼ 1.3 as best composition at T_substrate = 500 °C.     

Design of an enhanced 1 GeV electron neutrino beam

An enhanced ν_e beam would be useful for experiments that have been proposed to investigate the time evolution of a ν_e beam or to test the universality of ν_e and ν_μ interactions. We have performed detailed calculations to maximize the flux of electron neutrinos through a detector while minimizing the more copious muon neutrinos produced at 30 GeV proton accelerators. The ν_e beam is formed from the semi-leptonic decay modes of the neutral kaon. Muon neutrinos generated by decays of charged pions and kaons are suppressed by a dipole sweeping magnet. The ν_e/ν_μ ratio is enhanced from its usual value of $\text{～ 1}\text{1000}$ to a value of $\text{～ 1}\text{2}$, albeit at a low flux. We find with this design a typical flux of 1.5 × 10⁷ν_e/m²s. We find that a high magnetic field (≥ 40 kG) is essential to achieve this ν_e/ν_μ enhancement. Although the use of collimators and/or plugs inside the magnet reduces the ν_μ flux, the ν_e flux is also diminished so that there is little beneficial effect on the ν_e/ν_μ ratio. Magnetic focusing horns and quadrupole beams do not enhance the ν_e/ν_μ ratio. The accuracy of the energy dependence of the calculation, as well as the absolute normalization of the fluxes, is determined by a subsidiary calculation of the ν_μ yield from the magnetic horn focused beam at Brookhaven National Laboratory. This calculation is the first to our knowledge to agree well with the ν_μ yield as measured in the BNL seven foot bubble chamber.     

Observation of Re-entrant Resistance in NbN/NbO/Co Trilayer

Transport measurements on a niobium nitride (NbN) film covered with a ferromagnetic cobalt layer are reported here. The sample shows a superconducting transition (T _SC) at 6.5?K. In the superconducting state, a dip in resistance is observed at a temperature referred to as T _MIN. Below T _MIN, resistance reappears, with a magnitude of about 1% of the normal-state resistance. The observed resistance is found to decrease on increasing the applied current?(I). A?possible reason for re-entrant resistance might be the vortex dynamics in NbN superconducting layer due to the stray fields created by the Co layer. Further confirmation of the observed behavior is obtained from the plot of resistance versus probe current measured at various temperatures in close vicinity of?T _SC.