Showing papers in "Journal of Physics D in 1995"

Theory of the magnetomechanical effect

Abstract: This study investigated a model theory of the changes in magnetization that a ferromagnetic material undergoes when subjected to an applied uniaxial stress. The description of these effects is shown to be totally different from the description of the changes in the hysteresis curve under a series of constant applied stresses. The main mechanism in the proposed model theory is the unpinning of domain walls by the application of stress, which allows the walls to move and causes a change in the magnetization. This change in magnetization reduces the displacement from the anhysteretic magnetization. In addition, the anhysteretic magnetization itself is changed by the application of stress via the magnetoelastic coupling. It is shown that the effect can be described by an equation in which the rate of change of magnetization with elastic energy is proportional to the displacement of the magnetization from the anhysteretic magnetization. This is termed the 'law of approach'. This law seems to apply when the starting condition of the material is on a major hysteresis loop.

X-ray plane-wave topography observation of the phase contrast from a non-crystalline object

Abstract: A technique for observation of the inner structure of objects is proposed The sensitivity to variations in direction of the primary pseudo-plane beam which occurs on boundaries between media with slight differences in density values is impressive Two types of contrasts are pointed out: 'area contrast' caused by slight deviation of the primary beam and 'boundary contrast' Basic principles and experimental results are presented

Laser-induced breakdown spectroscopy of steel samples using multiple Q-switch Nd:YAG laser pulses

Abstract: Single, double and multiple Q-switch Nd:YAG laser pulses are used to vaporize material from solid steel samples and to induce a plasma. The material ablation of different pulse bursts, emission intensities of iron lines and electron temperatures and densities are determined. Material ablation is found to increase with multiple pulses compared to single pulses of fixed total energy as well as electron temperatures and densities. Line intensities can be increased by a factor of about two using double pulses. Quantitative microchemical analysis of low-alloy steel is performed with single and double pulses. The analytical performance is improved by the double-pulse technique.

A model of the cathode region of atmospheric pressure arcs

Abstract: The paper deals with calculation of parameters in the near-cathode plasma layer, on the cathode surface and in the body of a cathode in high-pressure arc discharges. These parameters can be calculated independently of the arc column if the heat flux coming from the column to the edge of the near-cathode layer does not play a decisive role in the energy balance of the layer, which, according to the estimates presented, is a likely case. The physics of the near-cathode layer is reconsidered in view of major contradictions that have appeared in the literature recently, in particular with regard to the role of the near-cathode space charge sheath. A model of a near-cathode layer is developed that is based on a multifluid description of the plasma and takes into account multiply charged ions. The model is employed to calculate parameters of the layer as functions of the voltage drop in the layer and of the local value of the surface temperature. By means of these data, an approximate asymptotic theory of arc spots is extended to cathode spots in high-pressure plasmas. Calculated spot parameters are presented for the following combinations cathode/plasma: tungsten/argon, thoriated-tungsten/argon, thoriated-tungsten/nitrogen, and zirconium/nitrogen. The obtained results agree with the recent measurements of the spot temperature.

Synthesis of plasma-polymerized tetraethoxysilane and hexamethyldisiloxane films prepared by atmospheric pressure glow discharge

Abstract: The synthesis of plasma-polymerized tetraethoxysilane (TEOS) and hexamethyldisiloxane (HMDSO) thin films by atmospheric pressure glow (APG) discharge was investigated and the characteristics of this method were compared with those of low-pressure plasma-enhanced CVD (LPPECVD) methods. The obtained films had smooth surfaces, no pinholes and uniform thicknesses. The qualities of the films analysed by FTIR and XPS, and emission spectra during the reaction, were very similar to those observed in the LPPECVD. The relationship between the substrate temperature and the deposition rate was strongly dependent on the presence of oxygen; the deposition rate increased with increases in the substrate temperature in the presence of oxygen. This behaviour is uniquely characteristic of the APG discharge process. This is because the gas-phase reaction of the monomer decompositions is enhanced by atomic oxygen, and the reaction rates in the gas phase increase as the gas-phase temperature increases.

Coulomb solids and low-frequency fluctuations in RF dusty plasmas

Abstract: It is pointed out that the approximately 12 Hz fluctuations measured in a dusty plasma in the paper by Chu et al. (1994) are very likely the first laboratory observation of the type of wave known as dust-acoustic.

An introduction to methods of periodic poling for second-harmonic generation

Abstract: Second-harmonic generation (SHG) can be produced by phase matching using the birefringence of nonlinear crystals via the modal dispersion in the case of optical waveguides. Such an approach limits the range of frequencies which can be doubled and also the choice of the nonlinear coefficients. One solution to both problems is to modify the crystal so as to have regions of periodic domain polarity. Whilst this approach does not allow a perfect phase match between the fundamental and harmonic, it nevertheless can be entirely constructive throughout the interaction length of the material and is termed quasi-phase matching (QPM). Periodic modulation of the nonlinear coefficient along the direction of propagation can achieve conversion efficiencies up to 20 times greater than with previous methods. Candidates of interest for quasi-phase-matching are wide band gap inorganic crystals such as LiNbO3, LiTaO3 and KTP, and also organic materials if they are transparent, stable against optical damage and have large nonlinear coefficients. To achieve QPM a variety of methods are being tried in order to invert domains periodically, either during the crystal growth phase, or subsequently by altering the lattice of the crystal. For inorganic ferroelectrics most effort has been concentrated on domain inversion in LiNbO3 and LiTaO3. Techniques have included application of pulsed electric fields, fields generated during electron bombardment, thermal pulsing or chemically driven movement of lithium. Many of the methods are semi-empirical in that the mechanisms by which the lattice re-structures are poorly understood. This review will therefore not only list the methods that are currently being used, but also comment on the underlying physical processes which allow, or prevent, the re-structuring of the lattice and the domain walls, whilst preserving the non-centrosymmetric characteristics of the lattice. An understanding of mechanisms is valuable for related poling applications in other crystals and it is further noted that many amorphous systems, including glasses used for optical fibre communication, may be stimulated to show periodic structural changes although the usage precedes the knowledge of the mechanisms. The commercial applications and research possibilities for efficient SHG guarantee that this topic area will continue to be central to photonics for a considerable time.

On the use of actinometry to measure the dissociation in O2 DC glow discharges: determination of the wall recombination probability

Abstract: A study has been performed to re-investigate the actinometric technique used to determine the absolute concentration of O atoms in DC O2 flowing glow discharges for pressures ranging from 036 to 2 Torr and discharge currents ranging from 5 to 80 mA in Pyrex tubes of three different diameters (16, 7 and 4 mm) Actinometric measurements using O(3P-3S) 844 nm, O(5P-5S) 777 nm and Ar(2p1-1S2) 750 nm transitions are compared to VUV absorption spectrometry The choice of the excitation cross sections for the calculations of atomic excitation rates as a function of the reduced electric field using a Boltzmann code and the contribution of the quenching processes of the excited states are discussed The dissociation ratio (O)/(O2) can be determined from the ratio of intensities I844/I750 by the relation (O)/(O2)=C3P2p1 I844/I750 We have found that C3P2p1 remains constant (C3P2p1=26*10-3) throughout the range of experimental conditions investigated The recombination probability gamma of the O atoms at the wall is calculated and correlated to the wall temperature of the Pyrex tubes The variation of the recombination probability as a function of the wall temperature is fitted by the relation gamma =094exp(-1780/Twall) for 300

Development of pulsed dielectric breakdown in liquids

Abstract: A model is presented for the development of breakdown in liquids subjected to uniform high amplitude electric field stresses with duration of microseconds or less. In this model, breakdown proceeds in four stages: (i) formation of a low density site (nucleation) in the liquid near an electrode, (ii) growth and expansion of this site until the local density is reduced below a critical density for electron impact ionization to take place, (iii) growth of an electron avalanche and its transformation into an ionizing front, and (iv) propagation of the ionization front via a sequence of processes occurring in the region ahead of the front; namely, heating by electron injection, lowering of the liquid density and avalanche growth and retardation. Expressions for the duration of each of these stages are derived and their behaviour with pressure and field strength analysed for cathode initiated breakdown. From this, a critical energy criterion for breakdown is obtained. Comparison is made with experimental results for water, salt solutions, and the liquid noble gases and for pulse durations in the microsecond and sub-microsecond time scales. This model serves to elucidate the dynamics of pulsed breakdown of liquids.

Effect of PMMA addition on the solution crystallization of the alpha and beta phases of poly(vinylidene fluoride) (PVDF)

Abstract: This study determined the effect of PMMA addition on the crystalline forms ( alpha and beta phases) of PVDF, crystallized from solution. The changes in the content of the two phases due to addition of different concentrations of PMMA at different crystallization temperatures were detected by IR spectroscopy. DSC analysis revealed that the melting temperature for both phases and the crystallization temperature of the alpha -phase decreased slowly with the increase of PMMA concentration. It was also observed that small concentrations of PMMA (10 and 15 wt%) favoured the beta -phase crystallization.

Non-equilibrium coupled kinetics in stationary N2-O2 discharges

Abstract: A detailed study of the coupled electron and heavy-particle kinetics in a low-pressure stationary N2-O2 discharge is carried out. The model is based on the self-consistent solutions to the Boltzmann equation coupled to the rate balance equations for the vibrationally excited molecules N2(X1 Sigma g+,v) and O2(X3 Sigma g-,v'), NO(X2 Pi r) molecules and N(4S) and O(3P) atoms. It is shown that the vibrational distribution of N2(X,v) plays a central role in the whole problem, affecting considerably the predicted concentrations of NO molecules and N atoms, whereas the concentration of O atoms is practically independent of both vibrational distributions. In particular, it is shown that, in the case of a rate coefficient of about 10-13 cm3 s-1 for the reaction N2(X,v)+O to NO+N, the N2(X,v) molecules are strongly de-excited by vibrational-translational energy exchange processes associated with N2-N collisions. In contrast, in the case of a higher value for this rate coefficient, the N2(X,v) molecules are efficiently destroyed by this mechanism. The contributions of the different processes to the total production of NO, N and O are evaluated and compared.

Time-resolved observation of gas-dynamic discontinuities arising during excimer laser ablation and their interpretation

Abstract: Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.

Parallel-beam coupling into channel-cut monochromators using curved graded multilayers

Abstract: Parabolically bent multilayers with laterally graded period were applied as condensing reflectors to convert divergent X-rays from laboratory X-ray sources into a parallel beam. Two different modes of coupling such a collimated beam into multi-reflection channel-cut monochromators for high-resolution X-ray diffractometry were tested. (i) Parallel coupling (the scattering vector of the mirror reflection is in the plane of the scattering vectors of the monochromator and the sample) enables one to exploit a wider solid angle range of the X-ray source and to gain nearly two orders of magnitude in intensity. (ii) Crossed coupling (the scattering vector of the mirror reflection is perpendicular to the scattering vectors of the monochromator and the sample) delivers a beam with much reduced vertical divergence. This eliminates the line broadening in rocking curve measurements even for strongly tilted samples.

Efficiency of a Joule-Brayton engine at maximum power density

Abstract: A new kind of power analysis is conducted on a reversible Joule-Brayton cycle. Although many performance analyses have been carried out resulting in famous efficiencies (Carnot, Curzon-Ahlborn), most do not consider the sizes of the engines. In the studies of Curzon and Ahlborn and others, researchers utilized the thermal efficiency at maximum power as an efficiency standard for practical heat engines. In this paper, instead of just maximizing power for certain cycle parameters, the power density defined as the ratio of power to the maximum specific volume in the cycle, is maximised. Therefore the effects of the engine sizes were included in the analysis. The result showed a new type of efficiency at the maximum power density which is always greater than that at the maximum power (Curzon-Ahlborn efficiency). Evaluations show that design parameters at the maximum power density lead to smaller and more efficient Joule-Brayton engines.

Automatic determination of complex constants of piezoelectric lossy materials in the radial mode

Abstract: An iterative automatic method is described for the characterization of lossy piezoelectric materials in the radial resonance mode based on the use of the more general expression for the complex admittance. From the experimental data of Y at four adequately selected frequencies, the constants of the material are determined with the necessary accuracy to reproduce the piezoelectric behaviour of the sample around resonance. The IEEE-176 Standard procedure has been automatized for the initial estimation of the real parts of the elastic constants. The method is applicable even to those materials in which said standard does not allow one to determine the piezoelectric constants.

The conditions required for the propagation of a cathode-directed positive streamer in air

Abstract: The properties of the cathode-directed positive streamer discharge, propagating in a uniform electric field, have been investigated. Precautions have been taken to keep the effects of space charge, due to branching, to a minimum, and conditions under which streamers possess minimum initial energy have been obtained. In this way, a threshold field of 440 kV m-1 for the propagation of a single streamer has been determined. The velocity of streamers in the uniform field has been studied. This rises linearly with the electric field and the results confirm that space-charge effects are negligible. Comparison is made with previously published work.

Charge relaxation process dominates contact charging of a particle in atmospheric conditions

Abstract: A new model is proposed for a mechanism dominating the charge generated on a particle due to impact or contact with metal plate. In this model, the impact/contact charging of a particle is determined as the remaining charge by the charge relaxation process due to gaseous discharge in atmospheric conditions; here, some difficulties of the simple condenser model are avoided. The model prediction is in good agreement with the `equilibrium charges` given as the result of impact charging experiments. Furthermore, the impact charging experiments conducted in Ar gas indicated that the impact charging characteristics changed corresponding to the breakdown limit potential of the environmental gas; this was also predicted quantitatively by the model. As a result of the proposal of this new model, hereafter in the case of treating charge generating phenomena due to impact or contact, in atmospheric conditions at least, consideration of the charge relaxation process due to gaseous discharge should be essential.

Some considerations about the analysis of thermostimulated depolarization peaks

Abstract: The dielectric relaxation mode associated with the liquid-glass transition of a fluorinated co-polymer, P(VDF-TRFE) 75/25 mol.%, has been resolved experimentally by the technique of fractional polarizations, and a discrete distribution of relaxation times has been obtained. This distribution shows a compensation pattern characteristic of the glass transition/relaxation of polymers. Attention has been paid to the effect of the experimental set-up on the obtained activation parameters by using polarization windows Delta Tp of 0, 5 and 10 degrees C. It was shown that the activation energy increased as the polarization window was reduced. The latter also had a slight influence on the compensation parameters. This behaviour has been explained in terms of an increase in the width of the distribution of relaxation times associated with elementary peaks as Delta Tp increased. Thus, the Bucci-Fieschi-Guidi (BFG) analysis based on monokinetic relaxations is no longer valid as this distribution broadens. Overall, the use of Delta Tp=5 degrees C as polarization window constituted the best compromise between validity of the BFG analysis and the experimental resolution of the depolarization current. The errors in the compensation temperature have been estimated and a test has been applied in order to reject the hypothesis of a compensation pattern due to a propagation of experimental errors. Thus, the compensation phenomena described true kinetic effects, independently from the experimental set-up. It was shown that data pertaining to different methods of analysis could also describe a compensation law, which was actually statistical in nature.

Dynamics of laser-induced plume expansion into an ambient gas during film deposition

Abstract: The interaction between laser-induced plasma plume and gas background during film deposition is studied theoretically. The gas dynamic model has been developed to describe the temporal and spatial evolution of the plume expanding into a gas under a pressure of typically a few tens of pascals. The model is based upon the generation of a spherical plasma cloud whose expansion is described in a two-temperature approximation by use of the Euler equations. The dynamics of laser ablation of YBa2Cu3O7-x superconductor in an oxygen environment have been analysed using this model. The calculations show that the plume does not stop upon reaching the maximum propagation distance but moves repeatedly back and forth up to 200 mu s after ablation. Two types of shock waves determine the interaction dynamics: series of primary (external) waves which propagate through the ambient gas, and a secondary shock wave which is formed in the plume region and executes a periodic motion. The conversion of cloud ionization energy into gas dynamic flow during expansion approximately doubles the kinetic energy of the plasma, but has no significant effect on the plume propagation distance. The results are in quantitative agreement with the previously observed oscillation behaviour of the plume.

The effect of plasma formation on beam focusing in deep penetration welding with CO2 lasers

Abstract: This paper deals with the absorption and defocusing of a CO2 laser beam by the laser-induced plasma plume in deep penetration welding. To derive the `effective` intensity distribution in the focal plane theoretically, the laser beam propagation through the plasma plume is calculated by solving the paraxial wave equation with a finite-difference scheme. Corresponding to experimental results, documented in the literature, the properties of the plasma plume (spatial temperature distribution and shielding gas content) are pre-set within the calculation. Parametric studies demonstrate that the intensity at the focus is reduced due to the defocusing effect of the plasma plume, mainly, and only to a minor extent due to absorption within the plume. Because of refraction within the plume, the intensity distribution in the focal plane is dependent on the plasma`s size, position and temperature. On studying the dependency of the optical properties on plasma temperature and shielding gas composition, it is found that, by applying a shielding gas mixture of He and Ar in the ratio 3:1, the variation of the focal diameter with plasma temperature can be significantly reduced. This shielding gas mixture, therefore, is recommended for enhancing process stability when welding with high-power CO2 lasers.

Two-dimensional simulation of the positive streamer in N2 between parallel-plate electrodes

Abstract: We report results of a two-dimensional simulation (three-dimensional, cylindrically symmetric) of positive streamer propagation in atmospheric pressure nitrogen between parallel-plate electrodes. The streamer is initiated at the anode. The most important features of the streamer dynamics are (i) high radial field around the streamer head (about 100 kV cm-1) which leads to significant expansion of the head in the radial direction and (ii) the formation of a secondary wave which propagates along the streamer channel towards the cathode and increases channel conductivity.

Fast photoresponse and the related change of crystallite barriers for ZnO films deposited by RF sputtering

Abstract: Hexagonal ZnO films were deposited on a glass substrate at room temperature by RF sputtering a ZnO target using a mixture of oxygen and argon. Polycrystalline ZnO films prepared in the presence of excess oxygen or at a high oxygen pressure tend to have crystallites with the orientation of the c-axis parallel to the substrate and a large and relatively fast photoresponse. The time constants of the photoresponse can be much reduced by doping the surface layer with nitrogen. This fast photoresponse of the resulting films is related to the changes of barrier heights of grain boundaries due to photodesorption of the chemisorbed oxygen on the grain boundaries when the films are irradiated with uv light.

Experimental and theoretical investigation of a N2-O2 DC flowing glow discharge

Abstract: A study has been performed to investigate the kinetics of production of N atoms and NO molecules in DC N2-O2 flowing glow discharges at pressure rho =2.0 Torr for discharge currents I=15, 30, 80 mA. Measurements of the plasma parameters, reduced electric field E/N, electronic density ne, as well as of the gas temperature Tg, the vibrational temperature Tnu of N2 molecules and the concentration of N atoms and NO molecules have been carried out over the whole range of oxygen percentages (0-100%). This set of measurements is compared to the results of a one-dimensional model including a detailed vibrational kinetics, a large number of chemical reactions and the thermal balance of the discharge.

Electrical characterization of the rectifying contact between aluminium and electrodeposited poly(3-methylthiophene)

Abstract: A detailed investigation both of the DC and of the AC electrical properties of the Schottky barrier formed between aluminium and electrodeposited poly(3-methylthiophene) is reported. The devices show rectification ratios up to 2*104 which can be increased further after post-metal annealing. The reverse characteristics of the devices follow predictions based on the image-force lowering of the Schottky barrier, from which the doping density can be estimated. As the forward voltage increases, the device current is limited by the bulk resistance of the polymer with some evidence for injection limitation at the gold counter-electrode at high bias. In the bulk-limited regime, the device current is thermally activated near room temperature with an activation energy in the range 0.2-0.3 eV. Below about 150 K the device current is almost independent of temperature. Capacitance voltage plots obtained at frequencies well below the device relaxation frequency indicate the presence of two distinct acceptor states. A set of shallow acceptor states are active in forward bias and are believed to determine the bulk conductivity of the polymer. A set of deeper acceptors are active only for very small forward voltages and for all reverse voltages, namely when band banding causes the Fermi energy to cross these states. The density of these deeper states is approximately an order of magnitude greater than that of the shallow states. Evidence is presented also for the influence of fabrication conditions on the formation of an insulating interfacial layer at the rectifying interface. The presence of such a layer leads to inversion at the polymer surface and a modification of the I-V characteristics.

The incorporation of metal clusters into thin organic dye layers as a method for producing strongly absorbing composite layers: an oscillator model approach to resonant metal cluster absorption

Abstract: We present data on the NIR/VIS/UV optical constants of thin composite layers, built up from a copper phthalocyanine matrix with embedded metal clusters. The metal clusters (copper, gold and silver) have diameters on a nanometre scale and act as strong absorption centres. In particular, local plasmon resonances and metal interband transitions could be observed from a fit of the experimentally determined dielectric functions by means of a Lorentzian multi-oscillator model. Average dipole transition matrix elements with respect to a single metal cluster have been estimated from the oscillator parameters.

Surface temperature measurements for tungsten-based cathodes of high-current free-burning arcs

Abstract: Measured temperature profiles for various oxide-tungsten cathodes and for pure tungsten cathodes are presented for high-current arcs burning in argon at atmospheric pressure. Temperature profiles are also presented for thoriated tungsten cathodes with different cathode cone angles, are currents and composition of the gas provided to the arc. Evidence is also presented that the temperature and the behaviour of the cathode are sensitive to the oxygen concentration in the argon.

The dynamics of arc cathode spots in vacuum

Abstract: High-speed emission pictures of arc cathode spots in vacuum have been obtained in a spectrally narrowed range. It has been found that the total radiation was delayed with respect to the arc current by 8.5 ns for Ti cathodes because of the limited transition probability of excited states. This effect sets lower bounds to spatial and temporal resolution. By selecting a spectral range with prevailing continuum radiation, the resolution could be improved down to 5 mu m and 2 ns, respectively. In this way a typical size of the spot plasma of about 10 mu m was observed for nanosecond discharges as well as for durations up to 100 mu s. The spots showed random displacements with a diffusion constant D=4*10-4 m2 s-1. The spot brightness was subjected to periodical fluctuations with discrete intervals Tn, n=0-3. The measured values obey the relation Tn/T0=(D/4 kappa )n, with kappa being the thermal diffusivity of the cathode material. The smallest time T0 is assumed to be the elementary spot lifetime tau s. It shows a distribution according to a 'survival law' with an average value of 14.2 ns for Ti. These results agree with studies of laser absorption pictures and surface craters. They are in line with explosive-spot models. The occurrence of fluctuations at discrete intervals Tn is explained by heat accumulation at the cathode surface during the random walking of the spots.

Melt pool dynamics during laser welding

Abstract: The dynamics of the melt pool and keyhole was investigated during CO2 laser welding using high-speed video photography and the laser reflectometer technique. A low-power argon laser beam, focused on the weld pool, provided illumination to obtain a direct image of the weld pool surface. The near-surface plasma emission background was decreased by using a narrow-bandwidth interference filter centred at the argon laser wavelength (514 nm). A variation in the shape of the keyhole opening with a characteristic frequency higher than 1 kHz was observed both during spot welding and during welding with a moving beam. For the case of spot welding with a 20 ms laser pulse, long-wavelength (about 1 mm) oscillations of the weld pool were observed with a frequency during the laser pulse and the first 5 ms after the laser pulse in the range 200-500 Hz. In the time interval starting at 25 ms and ending at approximately 40 ms from the beginning of the laser pulse, the long-wave oscillation frequency increased up to 1.3 kHz. The solidification time was determined to be approximately equal to the pulse duration for the spot welding. Surface deformation during cooling was also observed. This information is used to develop a model illustrating the dynamics of the post-pulse weld pool.

White-beam synchrotron topographic studies of defects in 6H-SiC single crystals

Abstract: Synchrotron white-beam X-ray topography studies, in conjunction with Nomarski optical microscopy, have been carried out on 6H-SiC single crystals grown by the sublimation physical vapour transport technique. Two kinds of dislocations were observed using topography: dislocations exhibiting bimodal images of various widths and with line directions approximately parallel to the (0001) axis and dislocations confined to the basal plane, which appear to have emanated from the former dislocations. The larger bimodal image width dislocations were found to have hollow cores, known as 'micropipes'. Detailed contrast analysis of topographic images obtained in transmission and back-reflection geometries establishes that 'micropipes' are Frank-type hollow-core screw dislocations with Burgers vectors typically equal to 3-7 times the c lattice parameter. X-ray topography also revealed many line defects approximately parallel to the (0001) axis that were determined to be screw dislocations with Burgers vectors equal to the c lattice parameter and there were no discernible 'micropipes' associated with these latter screw dislocations.

An ion-neutral species collision model for particle simulation of glow discharge

Abstract: An ion-neutral species collision model with charge exchange is developed for use in particle simulation of a glow discharge based on an extension of the theory of Langevin (1905) and Hasse (1926). The validity of the model is checked by Monte Carlo calculation of drift velocity for He+-He, Ne+-Ne, Ar+-Ar and Kr+-Kr collisions. The results show good agreement with the experimental data and the solution by the moment method, especially for heavy gases. By proper choice of the cut-off value of the dimensionless impact parameter in the model, it is applicable to a collision of ions of energy 1 keV.