Pulsed DC Voltage Research Articles

Kinetic aspects of superfast consolidation of silicon nitride based ceramics by spark plasma sintering

A newly developed novel rapid densification process named Spark Plasma Sintering (SPS) has been successfuly applied to compact silicon nitride based ceramics. In this process the precursor powders are pressed uniaxially in a carbon die, and an on–off pulsed DC voltage is applied simultaneously. The current passes through the carbon pressure die as well as the sample, so that the sample is heated both from the outside and the inside, which allows very fast heating rates to be applied (up to 600°C min−1). In addition to heat and pressure this process utilises the self-heating action caused by spark discharges between the particles, which occurs in the initial stage of the current–voltage pulse. These processes promote material transfer and allow the preparation of fully dense compacts of silicon nitride based ceramics within a few minutes. In this presentation, the phase transformation sequences during the initial stages of the sintering process, as well as the consolidation mechanisms are discussed from a kinetic point of view for Si3N4, β-sialon, and α–β-sialon composite materials.

Deposition of duplex Al 2O 3/DLC coatings on Al alloys for tribological applications using a combined micro-arc oxidation and plasma-immersion ion implantation technique

Micro-arc discharge oxidation (MDO) is a cost-effective plasma electrolytic process which can be used to improve the wear resistance of aluminium alloy parts by creating a hard thick alumina coating on the component surface. However, for sliding wear applications, such alumina coatings often exhibit relatively high friction coefficients against many counterface materials. Therefore, a duplex treatment, combining a load-supporting MDO alumina layer with a low friction diamond-like carbon (DLC) coating, produced by a modified plasma-immersion ion implantation (PI 3) process, has been investigated. PI 3 provides a flexible method of implanting ions into complex-shaped parts using a low temperature, low voltage plasma onto which high voltage pulses are superimposed. It can also be used to enhance the adhesion and growth characteristics of films formed under plasma conditions. In this work, a weakly-ionized, hot-filament supported low-voltage argon–acetylene plasma (with C 2H 2/Ar ratios from 1.0 to 0.15) was used, in combination with a low-frequency dc pulse voltage PI 3 system (in this case 100 μs, 5 kV pulses at 850 Hz) to deposit a low friction DLC top layer onto MDO-treated Al alloy coupons. Microhardness measurements and pin-on-disc sliding wear tests were performed to evaluate the mechanical and tribological properties. Ball-on-plate impact tests were also carried out to assess coating layer adhesion/cohesion. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc tests and crater scars from impact tests. The work demonstrates that a hard and uniform DLC coating, with good adhesion and a low coefficient of friction, can be successfully deposited on top of an alumina intermediate layer, which provides excellent load support; such that the coating can withstand much higher contact stresses than would normally be the case with aluminium-alloy substrate materials. The C 2H 2/Ar ratio significantly influences the interfacial adhesion between the DLC and alumina layers, but has no significant effect on coating hardness. It is suggested that the C 2H 2/Ar ratio should be selected in the range of 0.25–0.35 to obtain a hard a-C:H carbon film with low-hydrogen-content and excellent adhesion. The investigations indicate that a duplex combination of micro-arc oxidation and PI 3 represents a promising technique for surface modification of Al-alloys for tribological applications in which high contact loads are anticipated.

An Experimental Investigation into the Motion of a Single Drop in a Pulsed DC Electric Field

It has been shown that the separation of high volume fraction water-in-oil dispersions is improved by the application of a pulsed DC electric field. It is thought that the improvement in the rate of separation could be due to the formation of dipoles in the aqueous drops, caused by the action of the electric field, but it is also possible that the drops acquire a charge from the continuous phase. At present there is a lack of fundamental experimental results to support these theories. By studying the motion of a single drop in a uniform pulsed DC field, it is possible to make some deductions about the charging mechanism of the drop. A de-ionized water drop, with a diameter of 100 μ to 400 μ, was positioned in n-dodecane between a pair of parallel plate electrodes, the positive electrode being insulated. The drop motion was recorded using a high-speed video camera. Image analysis enabled the x, y co-ordinates of the drop centre in a sequence of images to be obtained and hence the drop trajectory. The pulsed DC voltage applied across the electrodes was also recorded and could be superimposed onto the drop trajectory to enable any correlation between drop motion and the field applied to be observed. The relationship between the drop motion and the continuous phase conductivity was studied and it was found that as the conductivity was increased, the amplitude of the drop motion decreased. This decrease in drop motion was due to a reduction of the field strength in the n-dodecane as its conductivity was increased, relative to that of the insulation coating. Experiments without the insulation coating are shown to confirm this explanation.

Resonant converter in high power factor, high voltage DC applications

The performance of the high-frequency high-order series resonant converter operating in high power factor mode is presented. The pulsating DC voltage obtained from a single phase uncontrolled diode bridge rectifier is applied to the high-frequency full-bridge configuration of the resonant converter. Such an AC–DC converter operates at a high power factor without any active control of the input line current. High frequency (HF) transformer nonidealities (i.e. leakage inductance and magnetising inductance) are incorporated for analysis of the basic operation of circuit. This configuration is well-suited for higher output voltage applications such as high voltage DC power supplies. The lower order harmonics present in input line current are minimised by removing the gating pulses to the HF switches for a predetermined duration known as modulating window. A useful analytical technique, based on classical complex AC circuit analysis is suggested for designing the high-order series resonant tank circuit. The output voltage is regulated by independently using variable frequency control and duty ratio control. This AC–DC converter is simulated using TUTSIM and PSPICE packages, and an experimental prototype unit is developed using high frequency switches (MOSFETs).

Physiological stress effects of continuous‐ and pulsed‐DC electroshock on juvenile bull trout

Juvenile bull trout Salvelinus confluentus exposed to continuous‐ or pulsed‐DC electroshock exhibited rapid elevations in plasma cortisol and glucose, but plasma chloride did not change. In a 1‐h experiment using 240 V at 1·4 A of 60‐Hz pulsed DC (voltage gradient 0·81 V cm−1), which proved lethal, plasma cortisol and glucose rose significantly within 15 min of a 10‐s electroshock. Plasma cortisol reached a peak level of 156 ± 18 ng ml−1 at 45 min and then decreased, whereas plasma glucose reached its highest level of 179 ± 7·5mg dl−1 at 1 h. In a 24‐h experiment using lower dosages, plasma cortisol increased from 6·1‐16 ng ml−1 to peak levels of 155–161 ng ml−1 in 1 h in response to a 10‐s electroshock of continuous (130 V, 0·5 A, 1·45 V cm−1) or pulsed (120 V, 0·5 A, 60 Hz, 0·55 V cm−1) DC. Although plasma concentrations declined thereafter, levels remained above control values at 24 h. Plasma glucose was elevated from 60–65 to 120–134 mg dl−1 after 1h by both electroshock treatments and remained near or above those levels for the 24‐h duration. Plasma cortisol and glucose levels were much higher in electroshocked bull trout at 1 h compared with those in fish 1 h after receiving a 30‐s handling stressor (cortisol, 90 ± 12 ng ml−1; glucose, 82 ± 6·1 mg dl−1). The results indicate that both continuous and pulsed DC were more stressful to juvenile bull trout than handling and that recovery, at least for pulsed DC, may take longer than 24 h.

Deformation of Anodic Surface Induced by Applying Electric Field in Metal/Thin Insulator/Metal Stacking Structure

The metal (Al)/insulator ( SiO2 and/or MgO)/metal (Au) stacking structure with thin (5–20 nm) insulating layers has been fabricated. The electrically insulating state of this as-formed structure abruptly changes to the conductive state upon application of pulsed dc voltage over a certain threshold voltage (V t), which is measured as a function of the thickness of insulator layers. The topographies of anodic Au surfaces before and after application of the voltage have been observed using the scanning tunneling microscope (STM). It is confirmed that, by applying voltage, the anodic Au surface deforms to form metallic paths penetrating into the insulator layer.

Electron Swarm Parameters Measured Using Photoelectrons Induced by a Pulse Laser

Electron swarm parameters such as drift velocity and ionization coefficient are measured using photoelectrons induced by a pulse laser. In order to measure parameters in a wide region of E/p, a pulsed dc voltage synchronized with the pulse laser is applied between electrodes with a pure configuration to keep the electric field uniform. Dynamic behavior of electrons is observed as a transient waveform of an induced current. From the analysis of the waveform, the drift velocity and the ionization coefficient in helium gas can be obtained at E/p up to 500 and 4000 V/cm·Torr, respectively. Further experiments are performed using an actual discharge tube or a gas mixture of halogen and helium. The results are discussed in comparison with those obtained with a pure configuration of electrodes.

Large-area YBCO thin film deposition using linear hollow cathode discharge sputtering

We report on a sputtering device based on a linear hollow cathode with a rectangular-shaped discharge cross-section of 100 × 20 mm that allows YBCO deposition on large area substrates. Due to the hollow cathode target arrangement the substrate is in an off-axis position. The sputter source is powered by a low frequency (0.5 Hz) pulsed DC voltage with a pulse width of 1 s. This relatively simple sputtering device does not need any auxiliary magnetic field. The sputtering gas is directly fed through the cathode slot thus facilitating the material transport towards the substrate. During the deposition the substrate is moved perpendicularly to the cathode slot in order to obtain uniformly thin films over large areas up to 3″ in diameter. YBCO thin films sputtered on (100) MgO and (001) YSZ buffered r-plane sapphire have been analysed by means of post growth in situ XPS, conventional RBS, and visible and FT-NIR Raman spectroscopy.

Rapid Consolidation of Nanophase Al2O3 and an Al2O3/Al2TiO5 Composite

ABSTRACTA rapid consolidation technique has been utilized in producing single phase AI2O3 in less than 10 minutes at 1400°C resulting in a grain size less than 500 nm. TiO2 has been added in hopes of obtaining Al2O3/Al2TiO5 nanocomposites in sintering times less than 30 minutes. The sintering process involves resistance heating of a graphite die containing the powder at heating rates of about 10 °C/s. The resistance heating step is preceded by a preparatory step consisting of DC voltage pulses applied across a prepressed powder compact. The retention of the nanostructure is attributed to the rapid heating rate although the possible effect of the DC pulses are also discussed. An Al2O3/Al2TiO5 composite has been produced during a short anneal immediately following sintering of an Al2O3/TiO2 nanocomposite. Substantial grain growth has been observed to occur during the transformation taking the composite to the microcrystalline regime.

A Method for Determination of Rotational Viscosity and Pretilt Angle from Transient Current in Twisted Nematic Liquid Crystal Cells

A method for the determination of rotational viscosity γ1 and pretilt angle θ° in twisted nematic liquid crystal cells is proposed. The method is based on the analysis of transient current of a cell excited by a dc voltage pulse. The accuracy of γ1 and θ° values determined by the method is demonstrated by means of numerical calculation of exact basic equations. Experimental results for a ZLI2293 (Merck) cell are presented, and the γ1 values obtained are found to be in excellent agreement with values in the literature.

Transient and DC electroluminescence of some new conjugated polymers

Electroluminescence (EL) of thin and thick polymer films sandwiched between ITO and aluminum electrode was studied under DC or rectangular pulsed voltage. Results are reported for two conjugated ladder type modifications of PPP (having similar band structure properties as PPV) and a non-conjugated polymer consisting of peryelene units. Thermal annealing improves the device properties. The luminescence versus current density follows a power law with exponents between 1.3 and 2 depending on the substance. We have observed a new type of transient EL in organic LED devices with low capacity, namely, sharp peaks of luminescence at the falling and rising edge of the square wave of the applied voltage. The threshold voltage for this type of transient EL is significantly smaller compared to the forward bias of the DC luminescence.

Pulsed axial activation in the ion trap: A new method for performing tandem mass spectroscopy (MS/MS)

AbstractA short DC voltage pulse is applied to one endcap of a quadrupole ion‐trap mass spectrometer to activate trapped ions. High internal‐energy deposition is achieved as evidenced by 91+/92+ peak ratios in excess of 20 for dissociation of the n‐butylbenzene molecular ion.1 The amount of internal energy can be controlled directly by vaarying the amplitude of the pulse. Dissociation efficiencies are less than those typically observed for collision‐induced dissociation in the ion trap and range from 50% at low internal energy deposition to 5% at maximum internal energy deposition.

Interspecific electrofusion of protoplasts between Aspergillus oryzae and Aspergillus sojae.

An α-amylase hyper producer A. oryzae and a protease hyper producer A. sojae were electrofused to breed more excellent koji-molds. Fusion between the yellow-spored, pant mutant or the yellow-spored, met mutant of A. oryzae and the white-spored, bio mutant or the white-spored, ade mutant of A. sojae produced heterokaryons with a frequency of 2 × 10 –5 per protoplast pair. The optimal parameters for the fusion of Aspergillus protoplasts were 1.1 MHz AC frequency, 400 V/cm AC voltage, 4kV/cm DC pulse voltage, 60 μsec pulse duration, and 2 or 3 pulses. In this combination of the fusion-parents, all the fusants with stable green conidia formed an alkaline proteinase with the mobility of A. oryzae-type, and none of them formed any intermediate type. The productivity and molecular specificity of proteinases behaved independently from each other in the processes of fusion and haploidization.

プロトプラスト電気融合法による焼酎酵母の改良

The electrofusion technique was applied to the breeding of shochu yeast. An auxotrophic strain (lys) obtained from Saccharomyces cerevisiae MK 021, a diploid strain used for shochu making, and anauxotrophic strain (ade) obtained from Saccharomyces cerevisiae Kyokai No.9, a diploid strain, usedfor sake making, were fused by the electrofusion technique. The optimal parameters for the fusion of Saccharomyces protoplasts were: 2 MHz AC frequency, 400 V/cm AC voltage, 6 kV/cm DC pulse voltage, 60 psec duration, and 2 pulses. Some of the fusants showed good growth and fermentation, and produced larger amounts of aroma components, when compared to the original strains. Spore segregation test was carried out to obtain more stable strains. Spores from a fusant strain, F-1 having the highest sporulation ability were isolated by micromanipulation, and their fermentation rate and formation of aroma components were investigated using rice or barley koji. Of the selected segregants, F-1-2, was found to show especially good growth and to produce much aroma components such as n-propyl alcohol, isobutyl alcohol, isoamyl alcohol and isoamyl acetate.

Epithelial and subepithelial contributions to transmural electrical resistance of intact rat jejunum, in vitro.

Epithelial and subepithelial resistance of rat jejunum was measured in vitro by two independent methods. (i) Transepithelial AC impedance data were interpreted in terms of a simple parallel RpCp element (representing the epithelial cell layer) in series with an ohmic resistor RS (representing the subepithelial layers). (ii) In separate experiments, the tip of a microelectrode was positioned between epithelium and subepithelial layers and the respective resistances were obtained from DC-pulse voltage divider ratios between both structures. The total tissue resistance as measured in conventional Ussing-chamber experiments (49 +/- 4 Ohm X cm2, mean of both methods) was formed to 81 +/- 6% (40 +/- 3 Ohm X cm2) by subepithelial layers and to only 19 +/- 3% (9 +/- 1 Ohm X cm2) by the epithelial cell line. We conclude that rat jejunum is more conductive than assumed so far. In in vitro flux studies on intact jejunal sheets a pronounced back-diffusion of absorbed substances will lead to an underestimation of the true net transport capacity of this structure. This error averages about fivefold and will be found likewise in conventional short-circuit measurements.

High Speed Colour Switching Element Utilizing Ferroelectric Liquid Crystal

Characteristics of ferroelctric liquid crystals mixed with dichroic dyes are studied by the application of a dc voltage and voltage pulse trains to understand the fundamental processes as a high speed colour switching element. The guesthost effect and its combined effect with the transient light scattering are observed in ferroelectric liquid crystals. The response speed does not change by the introduction of dichroic dyes. The contrast as the optical switching element of the transient scattering mode has been found to increase by the introduction of dichroic dyes.

Discovery of dc switching of a bistable boundary layer liquid crystal display

A liquid crystal display based on bistable boundary layer configurations has been shown previously [Cheng et al., Appl. Phys. Lett. 40, 1007 (1982)] to have memory, adequate contrast, low-voltage operation, and a bistability that is relatively insensitive to changes in material properties, cell parameters, and temperature. We report here the discovery that a short, low voltage dc pulse switches the display into one or the other of its bistable states, depending on the polarity of the pulse. Experiments suggest that a storage display of this type having n lines of pels can be rewritten in (50+20n) milliseconds using voltages of less than 2 V. Consequently, a large area, low power, nonrefreshed display is possible.

A cryogenic microwave switch

A cryogenic microwave switch has been developed which has an isolation greater than 70 db, a throughput of 0.99, and a rise time of a few nanoseconds. The switch consists of an H-plane T in which one arm contains a short and a sapphire tube containing He. The short is fixed so as to isolate the other two arms when the He is not excited. The high degree of isolation is a direct consequence of the superconducting properties of the waveguide. The tube of He is located a quarter guide wavelength from the T so that when a plasma discharge is created in it, the other two arms become tightly coupled. Plasma rise times of order several nanoseconds have been achieved by applying a high voltage dc pulse across the tube. The switch has been used to release microwave energy stored in a superconducting resonator, creating high power microwave pulses. Here we will discuss the design parameters of the switch and its observed performance.

Determination of semiconductor surface properties using surface acoustic waves

Surface properties of semiconductors may be determined by measuring the change in attenuation constant, acoustoelectric voltage, and convolution output of a semiconductor on a lithium niobate acoustic surface wave convolver structure. The change is caused by the application of a dc pulse voltage. Experimental results are presented for both p- and n-type silicon. The method is contactless, sensitive, and requires no sample preparation except polishing of one side. Also it is possible to study the variations of the semiconductor surface states along the direction of propagation of the surface wave by proper modification of this method.

Optimum design of electron beam-semiconductor linear low-pass amplifiers—Part II: Output capabilities

This paper presents a simplest case analysis of the peak quasi-static output capability of a linear lumped-target beam-semiconductor amplifier. The basic assumptions in the response analysis of Part I [1] are used, together with the premise that the semiconductor velocity-field characteristic exhibits strong saturation at fields well below avalanche breakdown. The analysis applies to the ultimate pulsed output at low duty cycle since thermal effects are not considered. The fundamental current and voltage limitations on target output are discussed separately in limiting cases, and it is shown that the linear output capability is determined jointly by the drift-region width, bias field profile, and avalanche multiplication constraint. The dynamic electric field behavior is calculated for the general case where voltage and current effects appear simultaneously, and it is shown how the output capability can be maximized for a given target area and load impedance by proper choice of drift-region width and doping level. The ultimate pulsed dc output current, voltage, and power levels are evaluated for semiconductor targets in terms of the target area, load impedance, carrier drift velocity, semiconductor band gap, and avalanche multiplication constraint. Detailed numerical predictions are given for silicon targets. The usefulness of the quasi, static analysis is discussed, and it is shown that the results obtained apply with minor modification to the case of step-function excitation. In conjunction with the results of Part I, the combined gain, rise time, and pulsed output power performance boundaries are established for the basic lumped-target beam-semiconductor amplifier. The feasibility of a subnanosecond-rise-time linear amplifier with 50-dB gain and a multikilowatt pulsed output level is shown. Various semiconductor materials are evaluated for application in a beam-semiconductor amplifier by examination of an overall figure of merit comprising gain, rise time, and peak output capability, and it is demonstrated that Si, Ge, and GaAs are nearly equal in overall merit. In conclusion, several novel devices are described that illustrate the adaptability of the electron beam-semiconductor concept to multiple-signal amplifiers, fast waveform sampling, and traveling-wave amplification.