Showing papers on "Liquid dielectric published in 1996"

Nanostructure multilayer dielectric materials for capacitors and insulators

Abstract: A capacitor is formed of at least two metal conductors having a multilayer dielectric and opposite dielectric-conductor interface layers in between. The multilayer dielectric includes many alternating layers of amorphous zirconium oxide (ZrO 2 ) and alumina (Al 2 O 3 ). The dielectric-conductor interface layers are engineered for increased voltage breakdown and extended service life. The local interfacial work function is increased to reduce charge injection and thus increase breakdown voltage. Proper material choices can prevent electrochemical reactions and diffusion between the conductor and dielectric. Physical vapor deposition is used to deposit the zirconium oxide (ZrO 2 ) and alumina (Al 2 O 3 ) in alternating layers to form a nano-laminate.

Electrostatic spraying: a novel technique for preparation of polymer coatings on electrodes.

Abstract: A liquid flow emerging from a tip or a thin tube under the influence of a strong electric field will, due to charging of the dielectric liquid, break up into small droplets. Thus, if a polymer material is dissolved in the liquid, this electrodeposition technique can be utilized for producing polymer coatings on electrodes. The method was applied for in situ formation of ultrathin (∼3000 A) cellulose acetate (CA) phase inversion membranes on glassy carbon electrodes. The purpose of the membrane was to protect the electrode surface from fouling by macromolecular species. The spraying liquid consisted of CA, acetone, and aqueous magnesium perchlorate as pore former, and the spraying voltage was 14 kV. Profilometric measurements showed that the thickness of the spray-cast membranes was much more uniform than that of similar membranes formed by solvent casting. By using cadmium and lead as test analytes and differential pulse anodic stripping voltammetry as detection method, it was found that the membranes pre...

The temperature of a plasma used in electrical discharge machining

Abstract: The temperature in an electrical discharge machining (EDM) plasma was determined from measurements of relative intensities of the 411.85 and 413.29 nm Fe I spectral lines. The time-dependence of the plasma temperature was measured for individual electrical machining pulses. Under our experimental conditions (electrodes of C6(+) and M1E(-), cosmetic paraffin as the dielectric liquid) the measured plasma temperature ranged from 8000 to 10 000 K for different current pulses. We observed higher temperature values during the initial phase of the discharge. Temporal characteristics of discharge electric parameters, light emission and the plasma temperature are presented.

Electrohydrodynamic behaviour of single spherical or cylindrical conducting particles in an insulating liquid subjected to a uniform DC field

Abstract: The author presents a theoretical study of the electrohydrodynamic motion of single conducting particles (spheres or cylinders) in an insulating fluid between horizontal plane-parallel electrodes subjected to a DC voltage. These particles, when in contact with the bottom electrode, acquire a charge and, after `lift-off' proceed to bounce up and down. The laws of particle velocity and the current - voltage relationships are established for the three possible hydrodynamic r?gimes of motion (viscous, intermediate and inertial) in the bulk according to the Reynolds number of particles. The phenomena taking place in the vicinity of the electrodes (field-enhancement and microdischarges) are analysed and quantified via numerical computations performed with a charge-simulation program. An experimental investigation has been performed with large (millimetre) steel particles in an insulating oil. Charges and velocities deduced from measured lift-off voltages and transient currents are in good agreement with the predicted values. The apparent charge transferred in the microdischarges is in accord with the numerical calculations.

High voltage insulation performance of cryogenic liquids for superconducting power apparatus

Abstract: This paper describes high voltage insulation technology of cryogenic liquids for superconducting power apparatus. The insulation of a superconducting transformer is classified into several insulation components. Moreover, we investigate fundamental insulation characteristics peculiar to the superconducting and cryogenic circumstances: area and volume effects on breakdown strength, V-t characteristics, quench induced dynamic breakdown characteristics and so on. Finally, we systematize the electrical insulation of cryogenic liquids, and propose a flow chart for the practical insulation design of superconducting power apparatus.

Measurement of Energy Distribution in Continuous EDM Process.

Abstract: A clear understanding of the energy distribution in the EDM process is very important for the explanation of some phenomena which cannot be explained well up to now. In this paper, methods and results of measuring the energies distributed into the tool electrode, workpiece and dielectric in the continuous EDM-sinking process are described. The energies distributed into the tool electrode and workpiece are obtained by means of measuring the temperatures and removal amounts of the electrode and workpiece, and the energy distributed into dielectric fluid is obtained by measuring the temperature rise of the dielectric fluid after flowing through the discharge gap. It is found from the experimental results that the energy distributed into the tool electrode is greater than that into the workpiece independently of the pulse duration and that the energy distributed the dielectric is quite small compared with the energies into the electrodes and the workpiece.

Hermetically sealed, non-venting electrical apparatus with dielectric fluid having defined chemical composition

Abstract: An electrical apparatus (10), such as a transformer, includes an expandable internal chamber (12) that is non-venting and completely and permanently sealed from the ambient environment. The chamber houses a core and coil assembly (11) or other current-carrying conductor and is completely filled with dieletric fluid (40) having a pressure less than one atmosphere. The enclosure walls (12) are flexible and are permitted to bow inwardly and outwardly as the volume of the dielectric fluid changes due to thermal expansion and contraction. A method of processing the dielectric fluid and filling and sealing the transformer at sub-atmospheric pressure is also disclosed.

Kerr electro-optic field mapping and charge dynamics in impurity-doped transformer oil

Abstract: We measured dc electric field distribution in transformer oil mixed with different impurities and additives so as to simulate practical degradation conditions of transformer oil. The electric field in the oil doped with asphalt was reduced near the cathode and enhanced near the anode. On the other hand, electric field in oil with copper oleate was enhanced near both electrodes and reduced at the center between the electrodes. The space charge density was estimated from the measured electric field profile using a 1-dimensional form of Gauss law; negative ions occurred more than positive ions by 5 to 40 pC/cm/sup 3/ in the oil with asphalt from 10 to 30 ppm. It was also found that heterocharges with 100 to 300 pC/cm/sup 3/ existed near both electrodes in the oil with copper oleate. Consequently, the electric field and the charge distribution in the oil proved to change depending on the kind and content of impurities or additives and thus depending on the degradation of the oil.

Properties of streamers in transformer oil

Abstract: The electrical characteristics of creeping discharges and single creeping streamers in transformer oil first are compared with those of streamers developing in the liquid bulk. The distribution of electric potential along the channel of a single negative creeping streamer is determined using a capacitive probe technique. Then the distribution of the space charge associated with each streamer channel is discussed and the electric field around the channels is estimated. A strong correlation between the mean potential gradient and the capacitance of the streamer channels is found. The different results and considerations tend to support the hypothesis of the same basic physical mechanism for both creeping discharges and streamers developing in the bulk.

Size effect and statistical characteristics of dc and pulsed breakdown of liquid helium

Abstract: Breakdown time lag in liquid helium is measured over a wide range of electrode sizes and pulsed electric field strengths. The breakdown time lag and dc breakdown strength are statistically analyzed by using the Weibull distribution function and weak link theory. It is found that the time lag depends on both electrical stress and the electrode surface area stressed above a critical level. It is supposed that breakdown triggering electrons are generated by field emission phenomena at small protrusion tips on the cathode surface. In higher external electric fields, a less sharper protrusion emits initial electrons with a shorter time lag and may become responsible for liquid breakdown. A theoretical equation is proposed to predict the electrode size and electrical stress dependency of the breakdown time lag, based on Fowler and Nordheim theory. It is shown that the equation is consistent with the Weibull distribution function under multiple stress of electric field and stressing time.

Method for repetitive measurement of the electrostatic charging tendency of liquid dielectrics

Abstract: The construction of an apparatus for the measurement of the electrostatic charging tendency (ECT) of a dielectric liquid is described. A sealed reciprocating piston arrangement permits a liquid to be repetitively passed through a charge separating element. The use of a gold element is shown to minimize the effects of moisture and control temporal effects. The system is evaluated by testing samples of transformer oil drawn from the field. In general, the new system ranks the oils in the same way as a conventional assessment against a paper filter, although anomalies have been found which are an impediment to adoption.

Influence of rise time on the dielectric behavior of stator insulation materials

Abstract: New rotating machine power supplies (inverters) produce voltage shapes whose characteristics are very different from the classical a.c ones. Up to now, their impacts' on stator insulation materials are not clearly understood. The most harmful parameters have been identified. These are the waveform (square or pulse like), the rise time (>kv//spl mu/s) and the high frequency solicitation (kHz). These stresses have been recreated in laboratory on two kind of samples representative of turn to ground insulation: PET and PEN. In a first step, these materials have been studied before ageing using dielectric spectroscopy from 20 Hz to 1 MHz and for temperature ranging between LN2 and 200/spl deg/C. Different relaxation phenomena are identified. Aging is then undertaken for 1000 hours in different conditions. After aging, different characterization have been undertaken to understand the induced changes (dielectric spectroscopy measurements, surface potential measurements, breakdown measurements...). The main conclusion of this work is that aging induced changes are directly related to the voltage rise time values. Reasons of such a behavior will be discussed and a physical interpretation will be given.

Precise high resolution non-contact method to measure dielectric homogeneity

Abstract: An apparatus and method for measuring the dielectric constant or permittivity at a plurality of positions on a test specimen are provided. Accordingly, the present invention enables the spatial variance of the dielectric constant, i.e., the dielectric homogeneity, of the test specimen to be measured. In particular, high resolution dielectric homogeneity measurements are performed at low frequency (100 kHz to 10 MHz) using a specially designed guarded electrode assembly and a specially selected high permittivity matching liquid. The guarded electrode assembly comprises a guarded electrode surrounded by a guard electrode which is supported over a test specimen resting on a planar electrode. The test specimen and the planar electrode are immersed in a dielectric fluid. The guarded electrode as well as the guard electrode, which extends into the dielectric fluid, are translated across the test specimen while the capacitance between the guarded electrode and the planar electrode is measured. The present invention is designed so that the number of required measurements are reduced compared to other techniques for measuring permittivity. The present invention also overcomes problems associated with prior art methods such as long processing times, low accuracy, and low resolution. Accordingly, the present invention permits rapid accurate high resolution measurements of dielectric homogeneity at a significant cost reduction in contrast to prior art methods.

Arcing Resistance of High Fire Point Dielectric Liquids

Abstract: High fire point (HFP) dielectric liquids have been in use since the demise of Askarels (PCB/TCB) in the mid 1970s. Having fire points of at least 300/spl deg/C, their main application has been in the role of transformer impregnants particularly in units located in, on, or near buildings. This study was aimed at evaluating the suitability of such liquids for application in arcing environments. At present, most HFP liquids are produced from one of three different chemical bases. These are: (a) polyol ester; (b) high molecular weight hydrocarbon (HMWH); and (c) dimethyl silicone. Samples of these liquids were evaluated in a typical oil switch primarily utilized in capacitor switching applications. Although not an HFP liquid, transformer oil was included in the test matrix to provide a base line to which the other liquids could be compared. Each liquid was exposed to 250 cycles of switching 8000 V at 50 A. The results show that both polyol ester and HMWHs compare favorably to transformer oil in this application. However, the dielectric breakdown value of dimethyl silicone was so rapidly degraded that its use under such conditions requires considerable caution. Further analysis of the particulate arc by-products formed in the dimethyl silicone revealed that this material had an extremely high dielectric constant. Through computer modeling, it was shown that the great disparity in dielectric constants between the fluid and the particulate would greatly intensify the electrical stress on the fluid and cause dielectric breakdown.

Method and apparatus for manufacturing radio frequency board with curved surface

Abstract: In a first step, a conductive layer a1 is formed on a three-dimensionally curved upper surface of a main mold. In a second step, a peripheral surface mold is attached to the outer peripheral surface of the main mold, and a liquid dielectric of an age hardening type is cast onto the conductor layer, and hardened. In a third step, the peripheral surface mold is detached from the main mold, and an unnecessary portion of the hardened dielectric is cut to thereby form a dielectric layer. In a fourth step, another conductor layer is formed on the dielectric layer. A method comprising these steps enables a dielectric board with a voluntary three-dimensionally curved surface to be manufactured with high accuracy.

Effect of streaming potential on heat transfer in transformer oil

Abstract: The results of an experimental study of the effects of streaming electrification on the heat transfer of transformer oil (Voltesso 35) in Plexiglas and paper/pressboard pipe models are reported. The magnitude and the polarity of the charge in the oil entering the test section was varied by using two different filters (Millipore cellulose and Sartorius fiber glass) in the upstream of the oil flow. An enhancement in heat transfer was observed with both types of filters in the Plexiglas model and with cellulose filter in the paper/pressboard model; whereas a reduction in heat transfer was observed with fiber glass filter in paper/pressboard model under similar flow rates and temperatures. With uncharged oil, heat transfer was either inhibited or no enhancement was noticed in both models. The results have been discussed considering the temperature gradients, the flow conditions and the electrical forces due to static charge in the liquid.

The effect of electrode shape, gap and moisture on dielectric breakdown of transformer oil

Abstract: Dielectric liquids have been used as an insulator for more than a century. The initiation of electrical breakdown and the role various types of effects have on it have been investigated for the last 30-40 years. In this study the variation of the breakdown voltage of transformer oil chosen as a liquid insulator with both electrode shape and gap has been examined. For this purpose, homogeneous and inhomogeneous fields with 5 pairs of electrodes which are at different standards have been created. The effects on electrical breakdown voltage of moisture content in the oil have also been investigated.

Electric discharge machining method and electric discharge machining fluid

Abstract: In order to form a mirror-finish surface on a workpiece (3), a machining fluid (921) obtained by blending a titanium carbide powder into an oil-based dielectric fluid which is supplied to a machining gap formed between the workpiece and a tool electrode (1, 2). The fine powder has an average particle size of about 0.5-1 μm, and is supplied to the dielectric fluid in a range of about 8-50 g/l. Alternatively, the electric discharge machining fluid includes a mixture of an oil-based dielectric with a silicide powder such as TiSi 2 , VSi 2 , CrSi 2 , ZrSi 2 , TaSi 2 , NbSi 2 , MnSi 2 , MoSi 2 , FeSi 2 , CoSi 2 , WSi 2 , or NiSi 2 . The silicide powder has an average particle size of about 0.5-15 μm, and is present in the dielectric fluid in a range of about 3-50 g/l.

Computer analysis and observation of streamer growth at a dielectric interface

Abstract: A simulation study of streamer growth within a dielectric liquid at a solid interface was undertaken. Using the finite element method with computer graphics, the shape of the growing streamer was determined and plotted. In addition, the changing voltage and electric field values within the dielectric were calculated, and simulated images of these parameters produced. The behavior of the streamer is based on the formation of a localized gas phase discharge and the growth of the resulting plasma within the liquid. The model determines an inception site and predicts a growth path that depends on the electric field immediately surrounding the streamer tip. Two liquids, n-hexane and water, both with an interface, were investigated using the simulation model. The effect of the choice of the liquid and the solid dielectric, in addition to the geometry, was explored. For mixed solid and liquid insulation, the results show that streamer growth depends on the permittivity mismatch between the two materials. Experimental findings are presented that show streamer growth for different interfacial conditions. A good correlation is observed between the theoretical and experimental work.

Nanosecond pre-breakdown and breakdown phenomena in water: influence of pressure, conductivity, and ionic sheath formation

Abstract: The influence of hydrostatic pressure (/spl les/400 atm), liquid conductivity, and ionic sheath formation on the dielectric breakdown of water (distilled and ultrasonically treated, or non-distilled) and sodium chloride solutions (0.001-1.0 molar) subjected to sub-microsecond electric fields has been investigated. The prebreakdown current-voltage relationship is found to be nearly linear for the fields considered and therefore described by a resistance, R. For constant concentration, R does not extrapolate to zero as the gap width, d, goes to zero, suggesting the presence of a high resistivity sheath near the electrodes. Estimates for the sheath parameters (electric field, E/sub s/, width, d/sub l/, formation time, t/sub l/, and ion density, n/sub s/) are obtained. The time lag to breakdown, /spl tau//sub bd/, increases with increasing pressure and is independent of liquid conductivity. For the parameters investigated, the input power is primarily due to electron field emission currents whose magnitude is a function of the field in the ionic sheath. These findings have relevance to the ongoing discussion concerning 'thermal' vs. 'electronic' mechanisms for dielectric breakdown in liquids.

Breaking arc in dielectric liquids

Abstract: The purpose of this paper is to report the effect of dielectric liquids on breaking arc phenomena in detail. The experiments were carried out for Cu vs. Cu electrodes in several kinds of dielectric liquids such as distilled water, methanol, n-hexane, and gasoline. The following can be obtained as findings in the switching tests with an inductive circuit (L=20 mH, load current=0.5-1.2 A). It was found that the dielectric constant of liquids affected the arc discharge types. In distilled water and methanol (which have high dielectric constant) showering arc was observed under the test conditions. On the other hand, in n-hexane and gasoline (which have low dielectric constant) steady arc occurred in air. The relation between dielectric constant and arc discharge types is discussed with a simplified model. Also, surface morphology was dependent on liquids. The contact surfaces in n-hexane and gasoline were smoother than those in distilled water and methanol, and much smoother than those in air with the same breaking current. It is considered to be due to the carbon (or carbonaceous material) covering the contact surfaces observed by the EPMA analyses. Furthermore, contact resistance in n-hexane and gasoline stay low during the test.

Liquid encapsulated MCM package-thermal demonstration

Abstract: This experimental study was undertaken to demonstrate the capability of liquid-encapsulated system in its simplest form of implementation to cool a multichip module (MCM) package. The MCM package was made of kovar (dimension: 58/spl times/35/spl times/11 mm). Two silicon thermal test chips were bonded to an FR-4 board and housed inside the metal package. Tests were performed on both dry (no liquid filled) and liquid-filled packages, In the liquid-filled situation, either a pure dielectric liquid or a dielectric liquid mixture was employed. The MCM package was externally cooled by either free-air or forced-air (1.0 to 2.54 m/s of air flow). Heat transfer history from single-phase, through nucleate boiling, to film boiling was documented. The overall improvement for the liquid-filled package was 2-4 times compared to the dry package, due to the superior thermal properties of dielectric liquids compared to air. The maximum power dissipation in the liquid-filled package at 2.54 m/s external air flow was 18 W (based on junction temperature maximum of 125/spl deg/C). In the liquid-filled package, both the single-phase and boiling heat transfer were enhanced by varying the external boundary conditions from free-air to forced-air. The total power dissipation limit in the liquid-filled package is strongly influenced by the ability to condense the vapor in the package. By changing the boundary conditions from free-air to forced-air (2.54 m/s), the condenser (top lid) efficiency is raised, thus raising the maximum power dissipation by 2.5 times. Results also indicated that for any given external boundary condition, the allowable power dissipation did not vary much under different liquid conditions: nondegassed liquid, degassed liquid at slightly above ambient pressure, and degassed liquid at ambient pressure. When the system was sealed, increasing power in the package created large liquid pressure and raised its boiling point. The dies remained in the single phase regime and no boiling occurred.

Method and device for evaluating thermal impedance of packaged semiconductor constituting element

Abstract: PROBLEM TO BE SOLVED: To measure the thermal impedance of a packaged semiconductor chip having especially high circuit integration density with high accuracy and in excellent reproducibility by providing the subject apparatus with a constant temperature fluid bath, providing the bath with a submerged jet-impulse cooling mechanism and shocking the surface of a package with liquid jet. SOLUTION: A constant-temperature fluid bath 12 is filled with dielectric fluid 13 such as silicone oil. When a temperature-sensitive electric parameter is calibrated, control is performed so that the entire body of a semiconductor chip and a package 14 is kept at the liquid temperature of about ±0.1 deg.C. The semiconductor chip 14 is immersed into the dielectric liquid 13 and surrounded by a liquid jet-impulse cooling mechanism 15. For a plurality of the nozzles of the cooling mechanism 15, the flow speed, which can obtain a plurality of the high-speed jet streams at the package surface, is imparted by a circulating pump 16. The liquid temperature is measured by a temperature sensor 17 using, e.g. a platinum thermometer. Thus, the electric power cycle and the thermal impedance can be evaluated by using the same apparatus.

Imploding and exploding circular EHD solitary waves propagating onto an insulating dielectric liquid layer

Abstract: We present an extensive experimental study on an EHD phenomenon discovered recently: a wave created and propagating at the surface of an insulating liquid film deposed on a polymer sheet lying itself on a metallic electrode, when switching off the corona effect produced by a point facing the layers onto the metallic plate. If now we modify the experiment by having a pin-hole drilled through the sheet at the center, a second wave simultaneously appears, and this one driving inward. Video recording and image analysis combine to characterize the phenomenon in terms of the relevant parameters. From recordings, the wave velocities have been measured: they decline with wave propagations, the decay decreasing with the charging voltage level. The effects of point-plane separation and of liquid viscosity are also investigated. In addition, the main features of corona pulsing during both the precharging phase and the subsequent wave-discharging phase have been examined. A rough analysis giving an explanation of the basic phenomenon is also presented.

Electrohydrodynamic instabilities in dielectric liquids induced by corona discharge

Abstract: Corona discharge from a needle is used to induce electrohydrodynamic (EHD) instabilities in a flat thin layer of dielectric liquid. We encounter two types of instabilities. The first one appears only for very insulating liquids and it is linked to the space charge induced in the liquid. This instability happens to be the classical EHD instability induced by unipolar injection. At higher voltages we observe a second instability in highly insulating liquids that is also observable for more conducting liquids. This second instability is due to the existence of a charged free surface.