Conductivity and space charge in LDPE containing nano- and micro-sized ZnO particles

DC conductivity and ac impedance measurements were made in air and in vacuum on samples of low density polyethylene to which nano-sized and micro-sized ZnO particles and a dispersant had been added. The samples were 150-200 mum thick. The temperature range was 30-70degC. The temperature dependence of the vacuum dc conductivity in samples containing the dispersant and 10% w/w nanosized ZnO followed an Arrhenius relationship closely, the conductivity being 1-2 orders of magnitude lower than that of a sample containing dispersant only. The addition of 10% w/w microsized ZnO had very little effect on the dc conductivity. The ac measurements were made in the frequency range 10 mHz-1 MHz. Addition of nanoparticles increased the ac conductivity at higher frequencies but decreased it at lower frequencies, the cross-over frequency increasing with increasing temperature. The real part of the relative permittivity of samples with nanoparticles was increased relative to that of samples containing dispersant only, at all temperatures, but the corresponding values in samples with microparticles were unchanged, within experimental error. Space charge profiles were obtained using the laser-intensity-modulation-method (LIMM). Space charge densities of order 300 Cm^-3 were measured in the bulk near the electrodes, several hours after poling at field strengths around 30 kV/mm.     

Particle-in-cell simulations on a single gap gas discharge

In order to help understand the mechanisms of gas discharges at a low pressure and high voltage, simulations on the initial phase of a single gap gas discharge in the case of atomic hydrogen has been carried out, based on the models of the explosive electron emission and neutral gas ionization, by using the self-consistent, 2-1/2 dimensional particle-in-cell W MAGIC 2DT code. The motion patterns of the charged particles during the initial process of the gas discharge are obtained at a hydrogen pressure of 1.33/spl times/10/sup -2/ Pa and an applied voltage of 10 kV. The profiles of phase space, the statistic histograms of velocities for the charged particles in the gap, the net charge density along the symmetrical axis direction, as well as the effects of applied voltage in the range of 10-40 kV and gas species (e.g. H/sub 2/, N/sub 2/ and Ar) on the characteristics of gas discharges are presented. Results also show that some simulations are in good accordance with existing experimental data.     

Surface switching characteristics of variable permittivity dielectrics

Flashover voltage, lifetimes, and switch performance of insulators utilizing square thin and thick film electrodes were examined to determine the viability of using thin electrodes for reliable surface discharge switching. Gold, silver, and platinum were sputtered (0.25 /spl mu/m) and screen printed (15 /spl mu/m) onto Al/sub 2/O/sub 3/, TiO/sub 2/, and modified BaTiO/sub 3/ (MBT), then tested in air at 10/sup 5/ Pa, under vacuum (10/sup -3/ torr), and while immersed in an insulating fluid, SF-2 (manufactured by 3M). For the measured range of 0.5 to 3 mm in air, the flashover voltage for all three insulators was found to have a linear dependence on the electrode separation distance with 15 /spl mu/m thick screen printed electrodes and a square root dependence with 0.25 /spl mu/m thick sputtered electrodes. Delay times of approximately 20 ns with a corresponding jitter of 6 ns were observed across all three insulators under triggered flashover. Insulators in air with sputtered electrodes had lifetimes of approximately 5 flashovers for dc flashover and 40 for triggered flashover. Screen printed TiO/sub 2/ and MBT had dc lifetimes of approximately 10 flashovers in air, and 3 flashovers in vacuum and SF-2. Screen printed TiO/sub 2/ and MBT had triggered lifetimes of greater than 200 flashovers in air, and <3 flashovers in vacuum and SF-2. Screen printed Al/sub 2/O/sub 3/ had dc and triggered lifetimes of greater than 200 flashovers in air, vacuum and SF-2. Insulator failure during dc flashover was determined to be due to the formation of a conductive channel between the anode and cathode. Formation of the channel was attributed to insulator thermal and dielectric properties and the presence of vaporized electrode species in the gap region during flashover.     

Correlation between dielectric properties and sintering temperatures of polycrystalline CaCu/sub 3/Ti/sub 4/O/sub 12/

Dielectric properties of polycrystalline CaCu/sub 3/Ti/sub 4/O/sub 12/ (CCTO) pellets sintered in the temperature range 1000-1200/spl deg/C were evaluated with impedance spectroscopy at frequency range of 10/sup 2/ to 10/sup 7/ Hz from 90 K to 294 K. A correlation has been established between the pair values of low frequency limit dielectric constant and the total resistivity and the sintering temperature. For example, the sample sintered at 1100/spl deg/C demonstrates higher value of low frequency limit dielectric constant and lower value of total resistivity, while the sample sintered at 1000/spl deg/C demonstrates lower values of low frequency limit dielectric constant and higher value of total resistivity. This correlation has been successfully explained by relating with the difference in grain size and grain volume resistivities of these two polycrystalline CCTO samples. Further, it is suggested that donor doping of oxygen vacancies Vo' and Vo" may be the reason to cause the difference in the grain volume resistivities of these two samples.     

The impact of LOC structures on 670-nm (Al)GaInP high-power lasers

We investigate the potential of large optical cavity (LOC)-laser structures for AlGaInP high-power lasers. For that we study large series of broad area lasers with varying waveguide widths to obtain statistically relevant data. We study in detail I/sub th/, /spl alpha//sub i/, /spl eta//sub i/, and P/sub max/, and analyze above-threshold behavior including temperature stability and leakage current. We got as expected for LOC structures minimal /spl alpha//sub i//spl les/1 cm/sup -1/ resulting in /spl eta//sup d/=1.1 W/A for 64/spl times/2000 /spl mu/m/sup 2/ uncoated devices. We obtain total output powers /spl ges/3.2 W (qCW) and /spl ges/1.5 W (CW) at 20/spl deg/C.     

Threshold voltage instabilities in high-/spl kappa/ gate dielectric stacks

Over recent years, there has been increasing research and development efforts to replace SiO/sub 2/ with high dielectric constant (high-/spl kappa/) materials such as HfO/sub 2/, HfSiO, and Al/sub 2/O/sub 3/. An important transistor reliability issue is the threshold voltage stability under prolonged stressing. In these materials, threshold voltage is observed to shift with stressing time and conditions, thereby giving rise to threshold voltage instabilities. In this paper, we review various causes of threshold voltage instability: charge trapping under positive bias stressing, positive charge creation under negative bias stressing (NBTI), hot-carrier stressing, de-trapping and transient charge trapping effects in high-/spl kappa/ gate dielectric stacks. Experimental and modeling studies for these threshold voltage instabilities are reviewed.     

Induction charging of granular materials in an electric field

It is well known that particles can be freely levitated in an electric field due to the charge induced on the particles by the external field. The charge depends upon the electric field strength and particle properties. This paper studies some of these factors to investigate the fundamentals of induction charging for granular materials. An experimental apparatus was set up to collect the levitated particles in a filter contained in a Faraday pail and the charge-to-mass (Q/M) ratio was obtained based on the charge and mass measurements for the samples in the filter. Furthermore, the particle size distribution was measured and analyzed by laser diffraction and microscopy and the surface mean diameter (D/sub s/) and volume mean diameter (D/sub v/) were obtained. In these experiments irregular shaped Al/sub 2/O/sub 3/ particles and spherical glass beads with a size range of 42-390 /spl mu/m were used and tested at different electric field strengths. By combining the results of Q/M, D/sub s/ and D/sub v/, the charge per particle was calculated and the results compared with theoretical values. It was confirmed that the particle charge is dependent upon the electric field strength and the particle properties of size, shape, density, resistivity, and adhesive force.     

Dielectric and conductive spectra of the composite of barium titanate and LiClO/sub 4/-doped polyethylene oxide

Dielectric and conductive frequency spectra in a 10 mHz-10 GHz range have been measured for a composite consisting of barium titanate (BaTiO/sub 3/) inclusions dispersed in a LiClO/sub 4/-doped polyethylene oxide (Li-PEO) matrix with volume fraction /spl Phi/ = 0-40%. Pure Li-PEO behaves as a dielectric showing a segmental-mode dielectric relaxation at high frequencies (dielectric regime) and transfers to an ionic conductor below 10 MHz (conductive regime). BaTiO/sub 3/ is a ferroelectric having a very large dielectric permittivity and spontaneous polarization. The introduction of BaTiO/sub 3/ into Li-PEO caused a rapid increase in permittivity in the dielectric regime. In the conductive regime, the composite exhibited an additional relaxation at a frequency related to the ratio of DC conductivity of Li-PEO and the permittivity of BaTiO/sub 3/. This relaxation was attributed to accumulation of dissociated Li/sup +/ and ClO4/sup o/ns at the inclusion/matrix interface which resulted in an increase of effective permittivity and a decrease of effective conductivity. Quantitative analyses based on mixing laws for the two-phase spherical dispersion system have shown that the Bruggeman equation accurately predicted the /spl Phi/-dependence of the effective permittivity over the entire frequency range. Regarding the effective conductivity, it predicted values lower than the observed. We attributed this discrepancy to the spontaneous polarization of BaTiO/sub 3/, which induced ion trapping to reduce the DC conductivity of Li-PEO matrix.     

Comparative study on the spectroscopic properties of Nd:GdVO/sub 4/ and Nd:YVO/sub 4/ with hybrid process

We have proposed a hybrid procedure for determining spectroscopic parameters for uniaxial solid-state laser crystals. Using our procedure, the spectroscopic properties of Nd:GdVO/sub 4/ were evaluated and compared to those of Nd:YVO/sub 4/. As a result, the peaks of absorption and stimulated emission cross sections of Nd:GdVO/sub 4/ in /spl pi/-polarization were determined to be 2.6 and 10.3/spl times/10/sup -19/ cm/sup 2/, respectively, and were smaller than those of Nd:YVO/sub 4/. On the other hand, the fluorescence lifetime of 1 at% Nd:GdVO/sub 4/ was evaluated to be 83.4 /spl mu/s, and was similar to 84.1 /spl mu/s of 1 at% Nd:YVO/sub 4/. Therefore, the product of stimulated emission cross section and fluorescence lifetime (/spl sigma//sub em//spl tau//sub f/ product) of Nd:GdVO/sub 4/ was smaller than that of Nd:YVO/sub 4/ under 1 at% of Nd/sup 3+/ doping concentration. The radiative lifetime of spontaneous emission of Nd:GdVO/sub 4/ was 168 /spl mu/s and was 1.9 times longer than that of Nd:YVO/sub 4/. Because of the low value of radiative quantum efficiency of Nd:GdVO/sub 4/ (50%), careful cavity design is required for creating a well performing solid-state laser with Nd:GdVO/sub 4/, based on the larger /spl sigma//sub em//spl tau//sub f/ product rather than the /spl sigma//sub em//spl tau//sub f/ product of Nd:YAG.     

Improving withstand voltage by roughening the surface of an insulating spacer used in vacuum

This paper describes a simple and reliable method of improving the surface insulation strength of a spacer used in vacuum. The method is to roughen the spacer surface to an average roughness R/sub a/ higher than 1 or 2 /spl mu/m. The material of the spacer examined is SiO/sub 2/, PMMA, PTFE or Al/sub 2/O/sub 3/ and their shape is a right cylinder with 10 mm in height and 54 mm in diameter. The spacer is subjected to a ramped DC voltage and its surface charging is observed by using an electrostatic probe embedded in the cathode. It has been found that R/sub a/ decisively affects the charging, which decreases as R/sub a/ increases. Increasing R/sub a/ larger than about 2 /spl mu/m suppresses the charging until a higher applied voltage is reached, thus improving the insulation property.     

Flashover voltage of alumina ceramic contaminated by metal vapor in vacuum interrupters

We investigated the influence of metal vapor contamination of ceramic surfaces on flashover voltage (FOV) in vacuum. First, disk shape alumina (Al/sub 2/O/sub 3/) ceramics with surface resistivity (/spl rho/) of 10/sup 2/-10/sup 15/ /spl Omega/ were produced using deposition phenomena of metal vapor emitted from CuCr contacts. The impulse FOV for the ceramics decreased, as /spl rho/ reduced; FOV, the conditioning effect on FOV, and the scattering of FOV decreased when /spl rho/ was below 10/sup 12/ /spl Omega/. Therefore, the criterion value /spl rho/, which maintains excellent flashover performances of ceramic surface, is 10/sup 12/ /spl Omega/. Second, experimental vacuum interrupters (VIs) were produced to measure breakdown voltage before and after forty short-circuit current switchings with 20-40 kA/sub rms/ and were disassembled to measure the /spl rho/ of their inner ceramic surface. In a VI, which has inside diameters at both ends of the main shield much larger than the contact diameter, /spl rho/ was reduced to 10/sup 4/ /spl Omega/, further decreasing breakdown voltage between terminals.     

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.     

Use of zeolite filter in portable equipment for recovering SF/sub 6/ in SF/sub 6//N/sub 2/ mixtures

We propose using zeolite filter in portable processing equipment to separate and recover SF/sub 6/ from SF/sub 6//N/sub 2/ mixtures. Testing of a prototype filter has shown that SF/sub 6/ is completely adsorbed by the filter, and the exhaust gas from the filter is pure N/sub 2/ (i.e., at an undetectable level of SF/sub 6/ measured by a gas chromatograph). The SF/sub 6/ fraction of the sorbate phase (solid phase) has been evaluated from the tests, and it is a high fraction as expected. For example, 70% sorbate corresponding to processing a mixture of 5% SF/sub 6/, and 83% sorbate corresponding to processing a mixture containing 10% SF/sub 6/. The process of releasing the SF/sub 6/ from the filter and renewing the filter by thermal swing adsorption (TSA) technique has also been investigated to determine a commercial operating temperature of 150/spl deg/C. The feasibility of applying this new concept in a portable SF/sub 6//N/sub 2/ mixture processing equipment is discussed and compared with an alternative system that the authors developed previously which was based on pressure swing adsorption (PSA) technique and also using zeolite.     

DC conduction and electrical breakdown of MgO/LDPE nanocomposite

To understand basic electric properties of nano-sized magnesium oxide (MgO) / low-density polyethylene (LDPE) nanocomposite under DC voltage application, the volume resistivity, the space charge distribution and the breakdown strength were investigated. By the addition of nano-sized MgO filler, both the DC breakdown strength and the volume resistivity of LDPE increased. At the average DC electric field of about 85 kV/mm and more, a positive packet space charge was observed in LDPE without MgO nano-filler, whereas a little homogeneous space charge was observed in MgO/LDPE nanocomposite material at the front of electrode. From these results, it is confirmed that the addition of MgO nano-filler leads to the improvement of DC electrical insulating properties of LDPE.     

Conduction band-edge States associated with the removal of d-state degeneracies by the Jahn-Teller effect

X-ray absorption spectroscopy (XAS) is used to study band edge electronic structure of high-/spl kappa/ transition metal (TM) and trivalent lanthanide rare earth (RE) oxide gate dielectrics. The lowest conduction band d/sup */-states in TiO/sub 2/, ZrO/sub 2/ and HfO/sub 2/ are correlated with: 1) features in the O K/sub 1/ edge, and 2) transitions from occupied Ti 2p, Zr 3p and Hf 4p states to empty Ti 3d-, Zr 4d-, and Hf 5d-states, respectively. The relative energies of d-state features indicate that the respective optical bandgaps, E/sub opt/ (or equivalently, E/sub g/), and conduction band offset energy with respect to Si, E/sub B/, scale monotonically with the d-state energies of the TM/RE atoms. The multiplicity of d-state features in the Ti L/sub 2,3/ spectrum of TiO/sub 2/, and in the derivative of the O K/sub 1/ spectra for ZrO/sub 2/ and HfO/sub 2/ indicate a removal of d-state degeneracies that results from a static Jahn-Teller effect in these nanocrystalline thin film oxides. Similar removals of d-state degeneracies are demonstrated for complex TM/RE oxides including Zr and Hf titanates, and La, Gd and Dy scandates. Analysis of XAS and band edge spectra indicate an additional band edge state that is assigned Jahn-Teller distortions at internal grain boundaries. These band edges defect states are electronically active in photoconductivity (PC), internal photoemission (IPE), and act as bulk traps in metal oxide semiconductor (MOS) devices, contributing to asymmetries in tunneling and Frenkel-Poole transport that have important consequences for performance and reliability in advanced Si devices.     

Evaluation of PTCa/PEKK composites for acoustic emission detection

The electromechanical properties of calcium modified lead titanate/poly(ether ke-tone ketone) (PTCa/PEKK) composites and their ability after poling to detect acoustic emission (AE) when mounted on carbon fiber reinforced composite (CFRC) panels is reported. They have been characterized by measurements of conductance, G, permittivity, /spl epsi/, pyroelectric, p, and piezoelectric, d/sub 33/, coefficients for different ceramic volume fractions. In addition, the electromechanical coupling coefficient, k/sub t/, the elastic compliance coefficient, s/sub 33/, the acoustic impedance, Z/sub a/, and the quality factor coefficient, Q/sub m/ have been calculated from the measurement of the complex impedance (Z) of the composite samples acting as free resonating transducers. It is demonstrated that in a square array they can be used to locate AE sources with good accuracy and that they will also detect AE caused by the delamination of a CFRC beam.     

Toepler's spark law in a GIS with compressed SF/sub 6/-N/sub 2/ mixture

This paper deals with the experimental measurements and analysis of the formative time lags to breakdown and an estimation of the Toepler's constant for gas gaps, under the application of 50 Hz AC voltage. The experiments were carried out in a 145 kV gas insulated system (GIS) bus duct with pure N/sub 2/, pure SF/sub 6/ and SF/sub 6/-N/sub 2/ mixture as insulating media. The formative time lags to breakdown in the gas gaps were measured using a fast response capacitive sensor. Toepler's spark law has been used to explain the breakdown phenomenon in the GIS and the values of Toepler's constant (k/sub t/), which gives an estimation of the formative time lags, were determined. Results show that the formative time lags vary inversely with gas pressure and the gas mixture concentrations for two gaps studied (0.46 mm and 0.61 mm). In the case of another gap (0.20 mm), the variation in the formative time lags with pressure as well as SE, concentration in the mixture has been found to be negligibly small between gas mixtures, although significant variation can be seen between pure SF/sub 6/ and pure N/sub 2/. Toepler's constant, k/sub t/, increases with gas pressure as well as SF/sub 6/ concentration in the mixture for the gaps studied. Hence, k/sub t/ is a function of the gas pressure and the concentrations of SF/sub 6/ in the gas mixture for the above-mentioned gaps.     

Continuous monitoring of SF/sub 6/ degradation in high voltage switchgear using Raman scattering

In high voltage SF/sub 6/ insulated switchgear, the level of SF/sub 6/ concentration needs to be monitored regularly because of its degradation by switching arcs and on-going partial discharges or coronas. In the work reported here, measurements of SF/sub 6/ dissociation rate following 75 /spl mu/A positive and negative coronas have been made using a mass spectrometer and Raman spectroscopy. The two sets of data correlate well, the rates of degradation of SF/sub 6/ and production of gaseous by-products being shown to be proportional to the charge transported by the corona. SF/sub 6/ provides a particularly strong Raman signal at 773.5 cm/sup -1/, and the results demonstrate the feasibility of using Raman scattering to monitor SF/sub 6/ degradation in high-voltage switchgear.     

The effect of electric field strength on the induction charge of freely levitating particles

Many industrial processes such as electrostatic separation, fluidization, and coating rely upon induction charging of fine particles. This paper considers the effects of electric field strength on the magnitude of the induction charge on freely levitating particles. The charging time and charge on a freely levitating particle depend on a number of properties, mainly the electric field strength, particle size, density, and resistivity. A charging model showing the dependence upon the electric field strength is presented and analyzed, along with a model of the levitation process. A high-speed digital imaging system was used to measure individual particle motion during levitation. Using these data along with the developed models, it was possible to determine the charge on the particle. Semiconductive particles with a mass mean diameter (MMD) of 156 /spl mu/m were used in these experiments and tested at electric fields of 6.8, 8.5, 15, and 21 kV/cm, respectively. In addition, some experiments using particles 97-/spl mu/m and 412-/spl mu/m MMD at an electric field of 15 kV/cm were carried out to confirm the results obtained for the 156-/spl mu/m particles. It was found that the particle charge was dependent upon both the charging time and electric field strength. In particular, for high electric fields the particle did not achieve its saturation charge before liftoff occurred. This shows that higher electric field strength is not necessarily the optimum condition for levitation of semiconductive particles.     

1.3-/spl mu/m AlGaInAs strain compensated MQW-buried-heterostructure lasers for uncooled 10-gb/s operation

We have successfully fabricated 1.3-/spl mu/m AlGaInAs strain-compensated multiple-quantum-well (MQW) buried-heterostructure (BH) lasers by narrow-stripe selective metalorganic vapor-phase epitaxy. Based on the optimization of AlGaInAs strain compensated MQW and the Al-oxidation-free BH process, we obtained a low-threshold current of 12.5 mA and a relaxation frequency of more than 10 GHz at 85/spl deg/C for Fabry-Perot lasers. For distributed feedback lasers, we demonstrated a 10-Gb/s operation and transmission of over 16 Km for a single mode fiber at 100/spl deg/C. Furthermore, a record-low 25.8-mA/sub p-p/ modulation current for a 10-Gb/s modulation at 100/spl deg/C was demonstrated with shorter cavity and high grating-coupling coefficient. A median life of more than 1/spl times/10/sup 5/ h at 85/spl deg/C was estimated after an aging test of over 5000 h for these lasers. These superior characteristics at high temperatures were achieved by the combination of the high differential gain of AlGaInAs strain compensated MQW and the BH structure.