High field conduction and prebreakdown phenomena in dielectric liquids

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (/spl sim/ MV/cm), and of propagation (requiring low electric field, /spl sim/ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data.     

Electrical conduction of thin 1,4-cis-polybutadiene films

This work contains results on the internal structure and dc conduction of 1,4-cispolybutadiene. X-ray and thermally stimulated depolarization (TSD) studies have been a source of information on phase transitions in the investigated material. The conduction measurements have been performed on thin films of thickness varying from 1 to 10 μm. Gold and aluminum have been applied as electrodes. The temperature range changed from 13 to 325 K. The investigated material was 1,4-cis polybutadiene of the Philips Petroleum Co, It contained 96% cis and 4% of the trans form of polybutadiene. It was observed that the internal structure of the polymer had a strong influence on its electrical properties. Electric conduction resulted from non-activated or thermally activated hopping with some contribution of the Poole-Frenkel effect. Electrode contact phenomena, such as thermionic emission and field emission (tunneling), are very likely responsible for charge carrier injection into the investigated material. The obtained activation energies range from 0.002 to N 0.3 eV     

The optical constant of ZnSe/SiO2 composite thin films investigated by spectroscopic ellipsometers

In this paper, ZnSe/SiO₂ composite thin films was prepared by sol–gel process. X-ray diffraction results indicate the phase structure of ZnSe particles embedded in SiO₂ composite thin films is sphalerite (cubic ZnS). The dependence of ellipsometric angle ψ with wavelength λ of ZnSe/SiO₂ composite thin films was investigated through spectroscopic ellipsometers. The optical constant, thickness, porosity and the concentration of ZnSe/SiO₂ composite thin films were fitted according to Maxwell-Garnett effective medium theory. The thickness of ZnSe/SiO₂ composite thin films was measured through surface profile.     

Nanoscale phenomena of gallium-doped ZnO thin films on sapphire substrates

Gallium-doped ZnO (1.2 at. %) thin films with various thicknesses were deposited on sapphire (001) substrates at 500^∘C using a pulsed laser deposition (PLD) technique. The thin films with different thicknesses (20, 40, 100, 200, 400, and 600 nm, respectively) were obtained by changing the deposition time. An x-ray diffractometer (XRD) was used to investigate the structural properties of the thin films. All of the thin films had a preferred (002) orientation. However, the thin films with 20 and 40 nm thicknesses were of low crystallinity. With increasing thickness the (002) peak increased greatly, and the full width at half maximum (FWHM) values were calculated by using omega scans. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used to investigate the nanoscale phenomena and the surface morphologies of the thin films. The surface roughness increased as the thickness increased. The thin film with 20 nm thickness was very smooth, and no nucleation center could be observed. However, the thin film with thickness over 100 nm showed nucleation. The nucleation center varied with increasing thickness. A spectrometer was used to investigate the luminescent properties of the thin films. It was found that all of the thin films showed near band edge emissions and no deep-level emissions were observed. A blueshift was also observed due to the Burstein-Moss effect.     

Characterization of conduction in PZT thin films produced by laser ablation deposition

Abstract

Both direct current (d.c.) and alternating current (a.c.) conductivity measurements were undertaken on lead zirconate titanate (PZT) films synthesized by laser ablation deposition. Direct current (I) displayed an initial time dependence of the form I ∝ t^?γ (γ ∝ 0.5–1.0). The possible reasons for this time dependence are discussed. At lower temperatures, the a.c. electrical conductivity shows a frequency dependence of the form σ ∝ ω′ which is explained as electrical charge hopping. At higher temperatures, the d.c. component of electrical conductivity becomes dominant, and is accompanied by a strong low frequency dispersion of the dielectric constant. The results are compared to published data on conductivity in SrTiO₃, and discussed in terms of the latest theories for dielectric response of materials.     

Electric field dependence of piezoelectric coefficient in ferroelectric thin films

A simple model is developed to explain the hysteretic electric field dependence of the piezoelectric coefficient in ferroelectric thin films. The nonlinear susceptibility and hysteretic electrical polarization behavior can explain the piezoelectric hysteresis characteristics. An empirical model introducing a weighting factor is utilized to represent the electric field dependence of the nonlinear susceptibility. Experimentally, the magnitude of the maximum of the piezoelectric coefficient measured in the backward direction of the piezoelectric hysteresis loop is usually larger than that of the maximum measured in the forward direction. The proposed model shows that the weighting factor in modeling the nonlinear susceptibility may account for this observed phenomenon. The nonlinear susceptibility may also explain the peaked shape observed in piezoelectric coeffieint-electric field hysteresis loops. The approach in this work does not require the “hard ferroelectric approximation” of the polarization characteristics, and provides a method of predicting the piezoresponse from measured capacitance and polarization properties of the ferroelectric thin film.     

Electrical conduction in fluoropolymer films

Isothermal charging, discharging and transport currents in fluoropolymer films are measured in the temperature range of 50 to 200°C at electric fields to 40 MV/m. The currents are measured as a function of time elapsed after the application of steady state voltages in the range of 1 to 10⁴ s. Charging currents increase with time to 10³ s, depending upon the electric field and temperature of the polymer. For longer duration the currents decrease with time. Discharging currents decrease monotonically with time and the discharge current-time characteristics are dissimilar to charging current-time characteristics. Transport currents which are derived by subtracting the discharging currents from charging currents are shown to arise due to Schottky emission assisted by the electric field. At shorter time intervals (<10³ s), it is suggested that the electric field at the cathode increases due to heterocharges and gives rise to currents increasing with time, while for longer times, gradual filling of the traps results in a decrease of the current. Low frequency dielectric loss factors are evaluated from the discharging current measurements     

F2 laser deposition of CdTe microcrystallites-dopedfluoropolymer thin films

Crystalline thin films of polytetrafluoroethylene (PTFE) were deposited by F₂ laser (157 nm) ablation in 200 mTorr Ar gas atmosphere. Combining this PTFE thin-film process with CdTe microcrystallites synthesis in sizes of 3-7 nm via KrF laser (248 nm) ablation, CdTe microcrystallites-doped PTFE thin films were fabricated. The X-ray photoemission spectra show that the main architecture of PTFE and CdTe are maintained in the doped films. CdTe microcrystallites doped in PTFE matrix show an absorption edge shift toward higher energy and a third-order optical nonlinearity, which are induced by the quantum size effect     

Dielectric study of thin films of Ta2O5 andZrO2

Electronic conduction in sputtered Ta₂O₅ and ZrO₂ thin films have been studied using impedance spectroscopy, isothermal transient ionic current, and current-voltage measurements. The dielectric properties of Ta₂O₅ were shown to be sensitively dependent on deposition parameters with two different frequency responses: a flat loss behavior with very low DC conductivity, or a relaxation peak together with a somewhat higher DC conductivity. ZrO₂ has different dielectric properties when fresh, i.e. newly deposited, or aged. A fresh sample arbitrarily can show two different behaviors, consisting of a DC conductivity with a relaxation peak superimposed on it. The DC conductivity shows either of two different values. The aged sample has a lower permittivity and DC conductivity, and the relaxation peak is found at much lower frequencies. Fresh samples of ZrO₂ also show switching behavior     

Very high field phenomena in dielectrics

Understanding of very high field phenomena in polymeric dielectric is reviewed, based on the concepts of field-dependent conductivity and space charge limited field, which leads to the conclusion that the electric field distribution can be predicted with reasonable accuracy under high field conditions. Other phenomena, such as photon emission, cannot be predicted reliably, probably because the concepts underlying the entire theory, that of "traps" or "impurity levels" within the band gap, are not understood, i.e., while the theory is written in terms of such concepts, no convincing physical basis for these phenomena has been provided. This issue is discussed and some suggestions possible directions for future research are suggested.     

Interfacial phenomena in composite high voltage insulation

This paper deals with two different types of composite insulating materials for HV outdoor insulation technology. For outdoor applications fiberglass reinforced polymer (FRP) is used mainly in transmission line insulators (composite long rods), and as power apparatus housings (composite hollow core insulators). During the last decade mineral filled polymer (MFP) was found to be very suitable for outdoor insulation in the medium and HV ranges. To meet the outdoor demands, long-term stability and durability against environmental stresses are necessary. Composite insulation consists of more than one dielectric component, and linking at least two different kinds of materials leads to interface problems. These regions always appear as weak material structures that can be attacked by various aging mechanisms. The long-term performance and aging resistance are determined by the interface quality. One can distinguish four principal kinds of interface: microscopic, e.g. those between fillers and matrix components; macroscopic, e.g. those between glass fiber rod and polymeric shielding material; internal, in the insulation bulk; and external, solid surface against a liquid or gaseous phase. Today the use of composite insulating materials in HV technology is state of the art. They offer a wide range of superior properties for indoor as well as for outdoor applications. Further improvements should focus on the hydrophobicity and on the long term resistance of the external interface and the stability of the internal interface     

铜锌锡硫薄膜太阳电池研究进展

Cu2ZnSnS4(CZTS)薄膜太阳电池的组成元素在地球上含量丰富,安全无毒,非常适合用来发展高效、廉价的太阳电池。目前,CZTS薄膜太阳电池的光电转换效率已经超过12%。总结了近年来CZTS薄膜太阳电池的研究进展情况,指出非真空工艺制备CZTS吸收层的可控性更强,更有利于提高电池性能。最后提出了优化CZTS薄膜及太阳电池性能的关键技术问题,并展望其未来发展趋势。     

Mocvd growth and characterization of PbTiO3 thin films on Pt/Ti/SiO2/Si substrates

Abstract

PbTiO₃ thin films were grown by Metalorganic Chemical Vapor Deposition (MOCVD) on 6′′ platinized Si wafers. In a first stage of investigation, the quality of the platinum surface was correlated with the substrate temperature and the composition of the atmosphere of the warming-up procedure in the reactor. The Pt reflectivity and the surface roughness were clearly modified by higher temperatures and by oxygen environments, though no clear diffusion mechanisms could be observed with Auger. In a second stage of investigation, the growth of PbTiO₃ was studied. Using a standard PbTiO₃ growth procedure, we studied the influence of substrate temperature and gas phase composition. Below 650°C, PbO appears as the main phase; at 650°C, PbTiO₃ and PbO coexist; and at 700°C, only PbTiO₃ is detected. The morphologies were examined by SEM / AFM and the roughness at the interface was studied by ellipsometry.     

Characterization of the CeO2 thin films for insulation layer and Pt/SrBi2Ta2O9/CeO2/Si MFISFET structure

Abstract

CeO₂ and SrBi₂Ta₂O₉ (SBT) thin films for MFISFET (metal-fcrroelectrics-insulator-semiconductor field effect transistor) were deposited by rf sputtering and pulsed laser deposition method, respectively. The effects of oxygen partial pressure during deposition for CeO₂ films were investigated. The oxygen partial pressure significantly affected the preferred orientation, grain size and electrical properties of CeO₂ films. The CeO₂ thin films with a (200) preferred orientation were deposited on Si(100) substrates at 600°C. The films deposited under the oxygen partial pressure of 50 % showed the best C-V characteristics among those under various conditions. The leakage current density of films showed order of the 10^?7～10^?8 A/cm² at 100 kV/cm. The SBT thin films on CeO₂/Si substrate showed dense microstructure of polycrystalline phase. From the C-V characteristics of MFIS structure composed of the SBT film annealed at 800°C, the memory window width was 0.9 V at ±5 V. The leakage current density of Pt/SBT/CeO₂/Si structure annealed at 800°C was 4×10^?7 A/cm² at 5 V.     

Dielectric properties of ZnNb2 O 6 -TiO 2 mixture thin films

ZnNb₂O₆-TiO₂ mixture thin films with multilayer structures were fabricated via a sol-gel spin coating process. TiO₂ layers were deposited on the pre-crystallized ZnNb₂O₆ layers in order to suppress the formation of the ixiolite phase which always forms in the bulk system. The phase constitution of the thin films, confirmed by X-ray diffraction (XRD), could be controlled by the annealing temperatures, which, in turn, influenced the dielectric properties of the thin films. TiO₂ layers crystallized as the anatase phase and then transformed to the rutile phase at temperatures higher than 725^∘C. Dielectric constants of the mixture thin films, measured at 1 MHz with an MIM (metal-insulator-metal) structure, increased from 27 to 41 with dielectric losses below 0.005 as the annealing temperature increased from 700^∘C to 900^∘C. The increase in the dielectric constants was understood to originate from the increasing amounts of the rutile phase. Temperature coefficients of capacitance (TCC) were also measured between 25^∘C and 125^∘C, which showed a decreasing manner from positive values to negative values with increasing annealing temperatures. When annealed at 850^∘C, the TCC of the thin films could be tuned to be approximately 0 ppm/^oC with dielectric constant and dielectric loss of 36 and 0.002, respectively.     

Electrical and optical properties of metal-insulator-transition VO2 thin films

Vanadium dioxide thin films were deposited using in situ pulsed laser deposition technique on a-, c-, and r-plane Al₂O_3, and MgO(100) substrates. Microstructure of the films was varied between epitaxial VO₂, polycrystalline VO₂, mixtures of VO₂ and V₆O₁₃ with VO₂ as the main phase, and mixtures of VO₂ and V₆O₁₃ with V₆O₁₃ as the main phase by controlling the deposition oxygen partial pressure. Detailed XRD, SPM, and FESEM measurements were performed in order to analyze the structure of the films. Resistivity as a function of temperature, current–voltage characteristics in electric current induced Joule heating transition process, and the optical transmittance both in insulator and metal states were measured. MIT effect led to 10³–10⁵ change in resistivity with varying transition temperature and hysteresis loop widths. The largest and the steepest transitions were found in the films with polycrystalline microstructure deposited at higher oxygen pressures. Epitaxial films had 2.5 times higher insulator state conductivity than polycrystalline films, which lead to a clearly smaller switching powers in MIT effect generated by Joule heating. However, the optical properties in both states were not considerable affected by conductivity or microstructure properties. The relationships between the microstructure, electrical and optical properties, as well as MIT switching effect together its dynamics in the films are discussed.     

Cracking of low temperature solution deposited CeO2 thin films

Crystalline CeO₂ films grown in aqueous solutions at 45^∘C on glass slides formed by the island growth mode. The film had a refractive index of 1.83 and indicated that the film had a porosity of 41.3%, which significantly lowered the elastic stiffness of the film. The film cracked only after drying in a mud crack pattern when it reached a critical thickness. This indicated that the film cracked under tensile stress due to the accumulation of tensile strains generated from grain coalescence during growth, thermal expansion mismatch during cooling and capillary stress during drying.     

Co2Z软磁铁氧体薄膜的磁性能

首先用球磨法制备了BaxCo2Fe24O41铁氧体粉料,烧结后制得块体材料,然后以其为靶材,用射频磁控溅射的方法在单晶硅基片上制备了膜厚为100nm的铁氧体薄膜.实验表明,沉积态的薄膜为非晶态结构,经过高温热处理后形成了较好的磁铅石结构.研究了成分、热处理温度、氧分压对薄膜结构和磁性能的影响.通过对热处理温度和氧分压的...     

Study of nanocrystal TiO2 thin films by thermal annealing

The amorphous films were annealed in a wide temperature range (250–1000^∘C) and film properties of TiO₂ thin films were studied. Nano-sized anatase polycrystallites had been induced by thermal annealing for the films annealed at and above 300^∘C as confirmed by X-ray diffraction. Strong LO-phonon Raman modes, especially B_1g (395 cm⁻¹) and E _g (636 cm⁻¹) in Raman spectra and the absorption peak at 436 cm⁻¹ in absorbance spectra by Fourier transform infrared spectroscopy also indicated the existence of anatase phase in crystalline thin films. In addition, with the increase of the annealing temperature, the wettability of the film surface was enhanced as shown by the decrease of water contact angle from over 90^∘ to less than 40^∘. Moreover, upon UV laser irradiation on film surface, the water contact angle saturated at 10^∘ indicative of a highly hydrophilic surface for all the films, which arose from the dissociative adsorption of water molecules on the defect sits of the surface generated by the photocatalysis reactions of TiO₂. This behavior makes the film a good potential candidate for self-clean coatings.