Residual DC Voltage Research Articles

Controlled doping rates of graphene oxide in aluminum for high electrical performance and oxygen reduction reaction

The brush coating method allows the advantage of being able to produce thin films simply and quickly. This study introduces a method for thin-film production by brushing a solution containing graphene doped in Al2O3 using the sol–gel method. Graphene oxide (GO) is suitable for semiconductors with bandgap values of about 1.7 eV at room temperature, and the characteristics of the thin films were analyzed according to the doping ratio. First, X-ray photoelectron spectroscopy measurements were obtained to analyze the chemical composition of the thin-film surface. Since graphene is a carbon isomer, the characteristic of oxygen vacancies was confirmed by the increasing C–C bond intensity with increasing GO doping concentration. In addition, Raman analysis was performed to analyze the concentration of molecular groups of compounds for defects in graphene. Thereafter, through atomic force microscopy measurements, as the graphene doping ratio increased, the average roughness increased from 1.785 to 33.67, the residual DC voltage also increased by about 48.42 %, and the polar anchoring energy also increased by about 16.33 %. In addition, response-time–transmittance measurements were performed to measure the electro-optical properties of the thin film, and excellent Vth and stable response speed were obtained. Additionally, the validity of the results was supported through bandgap analysis. Finally, the degree of alignment of liquid–crystal molecules on the film surface was confirmed by polarized optical microscopy and pretilt angle measurements, and the suitability of the thin film for display devices was shown via transmittance measurements. As a result, GO:Al2O3 hybrid thin film is an excellent candidate for use as an alignment film for solar energy, secondary batteries, and next-generation liquid crystal displays.

High electrical characteristics through graphene oxide doping process on physicochemically reformed inorganic thin films.

This study investigated the improvement of the electro-optical properties of a liquid crystal (LC) cell fabricated through brush coating using graphene oxide (GO) doping. The physical deformation of the surface was analyzed using atomic force microscopy. The size of the groove increased as the GO dopant concentration increased, but the direction of the groove along the brush direction was maintained. X-ray photoelectron spectroscopy analysis confirmed that the number of C-C and O-Sn bonds increased as the GO concentration increased. Since the van der Waals force on the surface increases as the number of O-metal bonds increases, we were able to determine why the anchoring energy of the LC alignment layer increased. This was confirmed by residual DC voltage and anchoring energy measurements that were later performed. As the GO concentration increased, the width of the hysteresis curve decreased, indicating that the residual DC voltage decreased. Additionally, the 15% GO-doped sample exhibited a significant increase in its anchoring energy up to 1.34 × 10-3J/m2, which is similar to that of rubbed polyimide. It also secured a high level of electro-optical properties and demonstrated potential as a next-generation thin-film display despite being produced via a simple brush-coating process.

P‐94: Enhancement of the reliability of TFT‐LCD displays by focusing on study of RDC

In this paper, the test of residual DC voltage was carried out on the test cells with different combinations of liquid crystal (LC), polyimide alignment film (PI) and color resist flat layer (PFA). Combined with the circuit simulation results, the residual DC voltage was comprehensively analyzed, it is its characteristics of "fast accumulation" and "slow release" that lead to the decrease in the reliability of TFT‐LCD display. By analyzing the characteristic parameters of related materials, we have given the optimal combination of materials to reduce the residual DC voltage. As various application scenarios require higher and higher panel performance and their material combinations become more and more complex, the research results in this article will be of great significance for improving the reliability of TFT‐LCD displays.

57.3: Improvement of Image Sticking for High Standard Automotive Application

As the rapid growth of LCD market, higher level of image quality is required. Image sticking is a phenomenon that a faint outline of previously displayed image remains visible on the screen as the frame is refreshed and it is an unvarying problem as the birth of LCD display. Furthermore, it has been a critical issue for high standard automotive application. In essence, the image sticking arises from the residual electric field (DC) in liquid crystal which caused by the ordered arrangement of ionic charges or dielectric polarization under the external DC offset voltage. In this paper, we will summarize the mechanism of the the residual DC voltage generation and image sticking. And an optimum Vcom setting method minimizing external DC offset voltage is introduced to improve image sticking.

Vertical Liquid Crystal Alignment of Comb-Like Alkyl Hydroxybenzoate-Substituted Polystyrene

We investigated the liquid crystal (LC) alignment behaviors of butyl hydroxybenzoate-substituted polystyrene (PBHB#; # = 20, 40, 60, 80, and 100, where # indicates the molar fraction of butyl hydroxybenzoate in the side chain), methyl hydroxybenzoate-substituted polystyrene (PMHB100), and ethyl hydroxybenzoate-substituted polystyrene (PEHB100). Generally, LC cells made employing polymer films having longer alkyl groups in the side chain show vertical LC alignment. For instance, a LC cell fabricated with the PMHB100 film showed random planar LC alignment, while the LC cells made from the PEHB100 and PBHB100 films exhibited vertical LC alignment. Moreover, LC cells prepared from a polymer film having a higher molar content of butyl hydroxybenzoate in the side chain exhibited vertical LC alignment. The observed vertical LC alignment behaviors are closely related to the surface energy of these polymer films. For instance, vertical LC alignment was observed when the surface energy of the polymer film was less than ~43.86 mJ/m2, which could result from the nonpolar alkyl groups of the alkyl hydroxybenzoate moiety. The LC cell prepared from PBHB100 as the LC alignment layer showed good electro-optical characteristics such as voltage holding ratio, residual DC voltage, and alignment stability at 200 °C.

P‐47: Improvement of Image Sticking for High Standard Automotive Application

As the rapid growth of LCD market, higher level of image quality is required. Image sticking is a phenomenon that a faint outline of previously displayed image remains visible on the screen as the frame is refreshed and it is an unvarying problem as the birth of LCD display. Furthermore, it has been a critical issue for high standard automotive application. In essence, the image sticking arises from the residual electric field (DC) in liquid crystal which caused by the ordered arrangement of ionic charges or dielectric polarization under the external DC offset voltage. In this paper, we will summarize the mechanism of the the residual DC voltage generation and image sticking. And an optimum Vcom setting method minimizing external DC offset voltage is introduced to improve image sticking.

Surface charge accumulation on 500kV cone-type GIS spacer under residual DC voltage

In recent years, GIS compact design has gradually been recognized and adopted by the industry. In the trend of the compact design, the insulation performance of the cone-type spacer is getting more and more attention by GIS manufacturers and grid operators and maintenance departments. In particular, under the action of DC residual pressure, the insulation properties of cone-type spacer become the focus of attention of all parties. Based on the field equipment operating conditions, the experimental study of DC residual voltage for the true cone-type spacer insulation performance is significant. In order to obtain the proposed pre-stressed time of DC voltage and distributions of surface charge, for the insulation test of 500 kV cone-type spacer under lightning impulse with DC voltage superimposed. A full scale surface charge measurement platform is build up for 500 kV GIS spacer. Then, long-term DC voltage experiments were carried out to obtain the distributions of surface charge on spacer in stationary state under different voltages. Moreover, the saturation time of surface charge accumulation under different DC electric stress is calculated and discussed. The study results show that (1) the distribution of surface charge on spacer is not uniform. It can be observed that on most areas of 500 kV spacer concave surface is dominated by the electric conduction through the volume with positive charges. (2) The time constant of surface charge accumulation decreases with the residual DC voltage amplitude, it can be estimated that the saturation time of surface charge accumulation on 500 kV cone-type spacer is about 23 h for the residual DC voltage reach to 1.0 p.u. The study results are thought to be helpful to the insulation test of 500 kV conetype spacer under impulse voltage with a superimposed residual DC voltage.

Surface modified solution-derived lanthanum-doped zinc oxide film for nematic liquid crystal system with free residual DC voltage

The liquid crystal (LC) alignment characteristics of the solution-derived lanthanum-doped zinc oxide (La:ZnO) film with ion-beam (IB) irradiation were investigated as a function of curing temperature. The best LC alignment state with uniform and homogeneous alignment was achieved at a curing temperature of 300 °C. To observe the effect of the IB irradiation on the La:ZnO film, physical and chemical surface analyses were conducted. Atomic-force microscopy revealed that surface roughness was increased, and the leptokurtic surface was changed to a platykurtic surface after the IB irradiation. X-ray photoelectron spectroscopy showed the breakage of the metal–oxide bonds, which induced an increase of oxygen vacancies. This caused van der Waals force, which strongly anchored LC molecules to the La:ZnO film. These results indicated that uniform and homogeneous LC alignment with the solution-derived La:ZnO films was induced by physical and chemical surface modification due to the IB irradiation. Moreover, residual DC was characterized by a capacitance-voltage hysteresis curve and nearly zero hysteresis was achieved. Therefore, IB irradiation is a useful method to achieve uniform and homogenous LC alignment, and using it with the solution-derived La:ZnO film exhibited potential for high-quality LC applications.

Renewable, Eugenol-Modified Polystyrene Layer for Liquid Crystal Orientation.

We synthesized a series of plant-based and renewable, eugenol-modified polystyrene (PEUG#) (# = 20, 40, 60, 80, and 100, in which # is the molar content of the eugenol moiety in the side group). Eugenol is extracted from clove oil. We used polymer modification reactions to determine the liquid crystal (LC) orientation properties of the polymer films. In general, the LC cells fabricated using the polymer films with a higher molar content of eugenol side groups exhibited vertical LC orientation behavior. The vertical orientation behavior was well correlated with the surface energy value of the polymer films. The vertical LC orientation could be formed due to the low polar surface energy value on the polymer film generated by the nonpolar carbon group. Electro-optical performances (e.g., voltage holding ratio (VHR), residual DC voltage (R-DC), and thermal orientation stabilities) were good enough to be observed for LC cells using PEUG100 polymer as an eco-friendly LC orientation material.

Liquid crystal aligning capabilities on surface-reformed indium-doped zinc oxide films via ion-beam exposure

ABSTRACTWe investigated the characteristics of a solution-processed indium-doped zinc oxide (In:ZnO) film formed via ion-beam (IB) irradiation as a liquid crystal (LC) alignment layer. The In:ZnO film was deposited using solution processing and cured at various temperatures. Uniform LC alignment was observed at all curing temperatures in cross-polarised optical microscopy images. A regular pre-tilt angle supported these results and showed homogeneous LC alignment. Several surface analyses were conducted to evaluate the effect of IB irradiation on the In:ZnO film surface. X-ray diffraction analysis showed an amorphous structure both before and after IB irradiation, and physical surface reformation was observed using atomic force microscopy. Root mean square surface roughness was reduced and a smooth surface was achieved after IB irradiation. X-ray photoelectron spectroscopy was used to detect chemical surface reformation. It was found that the IB irradiation broke the metal-oxide bonds and increased the occurrence of oxygen vacancies, which affected the van der Waals forces between the LC molecules and the In:ZnO film surface. Electrical performance was observed to identify the possibility of using the In:ZnO film in LC applications. Enhanced electro-optical performance was measured and zero residual DC voltage which was verified using a capacitance-voltage curve was achieved.

Vertical alignment of liquid crystal on tocopherol-substituted polystyrene films

ABSTRACTWe synthesised a series of vitamin-based and renewable tocopherol-substituted polystyrene (PTOC#, # = 20, 40, 60, 80 and 100), where # is the molar content of tocopherol moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a higher molar content of tocopherol side group showed vertical LC alignment behaviour. The vertical alignment (VA) behaviour was well correlated with the surface energy value of these polymer films. For example, VA was observed when the surface energy values of the polymer were smaller than about 35.22 mJ/m2 generated by the nonpolar tocopherol moiety having long and bulky carbon groups. Good electro-optical characteristics, such as voltage holding ratio and residual DC voltage, and aligning stabilities at 200°C and ultraviolet irradiation of 10 J/cm2 were observed for the LC cells fabricated using PTOC100 as a LC alignment layer. Therefore, it was first found that the renewable tocopherol-based materials can produce an eco-friendly vertical LC alignment system.

Study on insulation characteristics of GIS under combined voltage of DC and lightning impulse

Operating experience shows that residual DC voltage remains in GIS bus bar for a long time after opening of disconnectors. This DC voltage will not only cause accumulation of charges and contaminants on the insulator surface, but will also easily lead to motion of free metallic particles in SF6 gas gap, weakening the insulation ability of GIS. At the time of disconnectors reclosing, impulse voltage could be generated in the bus bar and superimpose on the pre-existing DC voltage. As a result, GIS will be under combined voltage of DC and impulse. This paper experimentally studies the insulation characteristics of GIS under combined voltage of DC and lightning impulse. It is found that, for non-contaminated insulators, when DC and LI voltage are of the same polarity, flashover voltage under combined voltage is nearly the same as the flashover voltage under LI alone. However, when they are of the opposite polarity, flashover voltage decreases with increasing DC voltage. In cases there are metallic particle or powder around insulator, pre-stressed DC voltage will cause adhesion of contaminants on the insulator surface and lead to decrease of flashover voltage. Specially, for the particle-contaminated insulator, it is found that the surface charge accumulation caused by DC voltage will have big influence on the flashover voltage, and the most critical condition is when DC and LI voltage are of the opposite polarity. In addition, the breakdown characteristics of SF6 gas gap with free metallic particle are investigated. It is found that under pre-stressed DC voltage, particle will do standing motion or bouncing motion. And breakdown voltage becomes lowest when particle is doing standing motion. The results of the experiments show that residual DC voltage in GIS could deteriorate its insulation ability and the combined voltage of DC and impulse is found to be a critical condition for GIS insulation. Moreover, the combined voltage is shown to be more sensitive to detect some insulation defects in GIS than applying impulse voltage alone, especially for free metallic particles, which are commonly found in GIS but up to now no effective detection method is proposed. Therefore combined voltage of DC and impulse could be considered to serve as a supplement of field test for GIS.

Free residual DC voltage for nematic liquid crystals on solution-derived lanthanum tin oxide film

ABSTRACTWe present the liquid crystal (LC) alignment properties of solution-derived lanthanum tin oxide (LaSnO) films cured at various temperatures and exposed to ion-beam (IB) irradiation. Using a solution process, LaSnO films were deposited on the indium-tin-oxide glass substrates and IB irradiation was used as an alignment method. Homogeneous and uniform LC alignment was achieved and observed by cross-polarised optical microscopy. Pre-tilt angle results with low standard deviation supported the notion of uniform LC alignment. The LaSnO film cured at 300°C showed nearly zero capacitance–voltage hysteresis. The change of the surface morphology of the LaSnO film due to IB irradiation was observed by atomic force microscopy. The effects of IB irradiation on the LC alignment layer were further demonstrated by X-ray photoelectron spectroscopy. The strong IB irradiation broke the metal–oxide bonds present, which in turn induced an increased number of oxygen vacancies on the whole surface. Uniform LC alignment was attributed to surface reformation and van der Waals forces.

Liquid Crystal Alignment Behaviors on Comb-Like Fluorinated Polystyrene Films

ABSTRACTWe synthesized a series of fluorinated polystyrene (PCMSF#, # = 50, 80, and 100), where # is the molar content of 1H,1H,2H,2H-perfluorodecanethiolate moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a higher molar content of fluorinated side group showed vertical LC alignment behavior with a pretilt angle of about 90°. The vertical alignment behavior was well correlated with the surface energy value and fluorine atomic percent of these polymer films. For example, vertical alignment was observed when the surface energy values of the polymer were smaller than about 8.77 mJ/m2 generated by the nonpolar and long fluorocarbon groups. Good electro-optical characteristics, such as voltage holding ratio (VHR) and residual DC voltage (R-DC), and aligning stabilities at 250°C were observed for the LC cells fabricated using PCMSF100 as a LC alignment layer.

Photoalignment Behavior on Polystyrene Films Containing Coumarin Moieties

We synthesized a series of polystyrene derivatives containing coumarin side groups, poly(6-(4-vinylbenzyloxy)coumarin)(P6COU#), where # is the molar content of 6-hydroxycoumarin using polymer modification reaction, in order to study the effect of the 6-hydroxycoumarin side groups on the liquid crystal (LC) alignment properties. The LC alignment behavior of these polymers was investigated using photoalignment method. The LC cells made from photoirradiated P6COU# films exhibited homogeneous planar LC alignment with a pretilt angle of approximately 0°. We found that LC aligning ability of the LC cells made from photoirradiated P6COU# films was affected by the molar content of 6-hydroxycoumarin side groups. For example, the azimuthal anchoring energy of a P6COU80 compared to P6COU25 is about 1 × 10−5 J/m2 and 5 × 10−7 J/m2, respectively. The electro-optical characteristics of the LC cells fabricated with P6COU80 films such as voltage holding ratio and residual DC voltage, threshold voltage, driving voltage, and response time were as good as those fabricated from rubbed polyimide films, the most commonly used LC alignment layers.

Behavior of metallic particles in GIS under DC voltage

The paper investigated the behavior and partial discharge (PD) characteristics of a metallic particle under residual dc voltage, using the setting conditions and length of the particle, the applied dc voltage, and other factors as parameters and a gas insulated switchgear (GIS) bus bar model equivalent to that of an actual 300 kV GIS. A metallic particle repeated reciprocating movement at a relatively high frequency of about 5 times per second between the high-voltage conductor and the sheath when the electric field at the bottom surface of the tank exceeded its lift-off electric field. It emerged that, in the process of this reciprocating movement, PD occurred at the moment when the particle collided with the high-voltage conductor. In addition, when a particle collided with the electrode, the electrical charges moved and the residual dc voltage was damped. Conversely, where the sheath side was insulation-coated, the particle did not move at the normal operating voltage level, and even though it moved if vibration was applied, its movement stopped relatively soon. If a metallic particle exists in GIS and moves, there is concern that the insulating performance may decline significantly. When a metallic particle moves under dc voltage, a relatively stable PD is generated and consequently, for example, the PD measurement is considered an effective way to detect a particle. It is also considered effective to take physical measures, such as using an insulating sheath at the bottom surface of the tank to restrain the behavior of particles.

Electric conductivity characteristics of FRP and epoxy insulators for GIS under DC voltage

Now that gas insulated switchgear (GIS) for ac systems is becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. The insulating portions for which the influence of dc voltage must be taken into consideration are solid insulators, such as insulating spacers. Under dc voltage, since the electric field distribution in an insulator differs from that under ac or impulse voltage and is governed by the resistance characteristics, clarifying its characteristics is crucial to study the GIS dc insulation design. As a solid insulator, focusing on fiber-reinforced plastics (FRP) used for GIS, for example, insulating rods, as well as partially treated epoxy resin; this paper experimentally investigated the bulk and surface electric conductivity under dc voltage, using the electric field, temperature, and other factors as parameters. As a result, the bulk electric conductivity of FRP in an edgewise direction exceeded that in the penetrating direction by one digit. It emerged that the electric conductivity of an insulating material with orientation like FRP varied depending on its direction. It was also found that, despite the fact the bulk and surface conductivity depended on the electric field for both FRP and epoxy resin, the variation width was relatively narrow within the range of the actual GIS operating electric field. The bulk and surface electric conductivity were also temperature-dependent, which meant the variation width was relatively wide. Furthermore, the surface electric conductivity was measured in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas and in the air to investigate the influence of the ambient atmosphere, whereupon it emerged that the electric conductivity was higher in air due to the adherence of moisture. As mentioned above, the electric conductivity of an insulator varies due to various factors, such as the influence of the material orientation, electric field, temperature, and moisture. Consequently, the electric field distribution inside the insulator also changes, meaning these electric conductivity characteristics must be taken into consideration to study the GIS dc insulation characteristics.

Insulation characteristics of GIS epoxy insulators with non-uniform surface resistance under DC voltage

Now that gas-insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. For GIS insulating epoxy spacer, the present paper experimentally and analytically studied the influence of spacer surface condition on dc withstand voltage performance. In specific terms, initially, an insulating spacer model with an area of non-uniform resistivity present in the surface layer was created and the dc voltage breakdown characteristics were obtained. As a result, it was clarified that the time delay to breakdown increased with lowering the applied voltage, or in other words, even if a low voltage was applied, breakdown may occur after an extended period. Subsequently, using the same non-uniform resistivity model, the change in the electric field distribution over time under dc voltage was investigated through transient electric field analysis. Consequently, it was found that the electric field distribution varied from a capacitive to a resistive field and the maximum electric field was generated in the boundary between the high- and low-resistivity areas. It was further clarified, based on these breakdown characteristics and electric field analytical results, that the presence of an area of nonuniform resistivity on the insulating spacer creepage surface caused electric field concentration over time and breakdown occurred if the electric field in the creepage surface direction exceeded 40 kV/mm or so. Moreover, the breakdown characteristics were investigated where a lightning impulse voltage was superimposed over a dc voltage. Consequently, the creepage breakdown electric field where a dc voltage was applied alone was almost identical to that where the superimposed voltage was applied. It means that the application of the superimposed voltage may potentially allow the insulating spacer withstand voltage performance to be evaluated by a shorter time test for a dc voltage alone or the dc voltage with a superimposed VFTO.

Liquid crystal alignment behaviours on poly(methyl methacrylate) having polyhedral oligomeric silsesquioxane groups

A series of poly(methyl methacrylate) derivatives containing polyhedral oligomeric silsesquioxane (POSS) groups (MCP#) were synthesised via free radical polymerisation (FRP) using methacryl isobutyl POSS (MA-POSS) and methyl methacrylate as monomers to investigate liquid crystal (LC) alignment property of these polymer films. The LC cells made from the films of the polymers having 100 mol% of MA-POSS units (MCP100) showed vertical LC alignment having a pretilt angle of about 90°. The vertical LC alignment behaviour on the MCP100 film was ascribed to the very hydrophobic MCP100 surface having the surface energy value smaller than about 23 mJ/m2 generated by the nonpolar bulky POSS group. Good electro-optical characteristics, such as voltage holding ratio (VHR) and residual DC voltage (R-DC), were observed for the LC cells fabricated using MCP100 as a LC alignment layer.

Image sticking in liquid crystal displays with lateral electric fields

We propose a kinetic model to account for the nonuniform adsorption and desorption processes in fringe field switching (FFS) and in-plane-switching liquid crystal displays. An equation is proposed to describe the generation mechanism of residual DC voltage and good agreements with experiment are obtained. Based on this model, the mechanisms underlying the formation and relaxation processes of residual DC voltage as well as their dependences on offset DC voltage and temperature are investigated. Moreover, the residual DC voltages of FFS cells employing positive and negative dielectric anisotropy LCs are compared and the physics responsible for the observed difference is explained.