High Voltage Field Emission Research Articles

Particle-in-cell modeling of the nanosecond field emission driven discharge in pressurized hydrogen

The high-voltage field-emission driven nanosecond discharge in pressurized hydrogen is studied using the one-dimensional Particle-in-Cell Monte Carlo collision model. It is obtained that the main part of the field-emitted electrons becomes runaway in the thin cathode sheath. These runaway electrons propagate the entire cathode-anode gap, creating rather dense (∼1012 cm−3) seeding plasma electrons. In addition, these electrons initiate a streamer propagating through this background plasma with a speed ∼30% of the speed of light. Such a high streamer speed allows the self-acceleration mechanism of runaway electrons present between the streamer head and the anode to be realized. As a consequence, the energy of runaway electrons exceeds the cathode-anode gap voltage. In addition, the influence of the field emission switching-off time is analyzed. It is obtained that this time significantly influences the discharge dynamics.

Evaluation of niobium as candidate electrode material for dc high voltage photoelectron guns

The field emission characteristics of niobium electrodes were compared to those of stainless steel electrodes using a DC high voltage field emission test apparatus. A total of eight electrodes were evaluated: two 304 stainless steel electrodes polished to mirrorlike finish with diamond grit and six niobium electrodes (two single-crystal, two large-grain, and two fine-grain) that were chemically polished using a buffered-chemical acid solution. Upon the first application of high voltage, the best large-grain and single-crystal niobium electrodes performed better than the best stainless steel electrodes, exhibiting less field emission at comparable voltage and field strength. In all cases, field emission from electrodes (stainless steel and/or niobium) could be significantly reduced and sometimes completely eliminated, by introducing krypton gas into the vacuum chamber while the electrode was biased at high voltage. Of all the electrodes tested, a large-grain niobium electrode performed the best, exhibiting no measurable field emission ($<10\text{ }\text{ }\mathrm{pA}$) at 225 kV with 20 mm cathode/anode gap, corresponding to a field strength of $18.7\text{ }\text{ }\mathrm{MV}/\mathrm{m}$.

Deterioration of ZnS-type and Eu3+-activated phosphors for high-voltage field-emission displays

Deterioration of luminescence efficiency η of blue- and green-emitting ZnS-type phosphors, and red-emitting Eu3+-activated phosphors has been studied for high-voltage field emission displays (FEDs). In the blue-emitting ZnS:Ag,Al(Cl) phosphor (P22B and P55) case, the deterioration of η depends mainly on the crystallinity of the host crystal within the electron penetration depth d. It was found that a high-voltage FED tube having a longer lifetime was successfully prepared by using a crystalline-improved ZnS:Ag,Al phosphor. While, in the green-phosphor case, in order to optimize the donor–acceptor (D–A) pair density in ZnS:Cu,Al (P22G) for a suitable color gamut in FEDs, various ZnS:Cu,Al phosphors with low defect density were prepared by using a suitable firing method with a surface treatment like an improved ZnS:Ag,Al phosphor. It was found that the deterioration of improved ZnS:Cu,Al phosphors depends mainly on the D–A pair density in ZnS crystal. Furthermore, in the red-phosphor case, deterioration of Y2O2S:Eu3+ phosphor (P22R) for color-TV has been studied in comparison with a Y2O3:Eu3+(P56) for projection CRTs. It was found that a Y2O2S:Eu3+ phosphor shows a longer luminescence lifetime than a Y2O3:Eu3+ phosphor in high-voltage FEDs. These evidences suggest that the factors influencing the deterioration of luminescence efficiency in ZnS-type and Eu3+-activated phosphors for high-voltage FEDs are the crystallinity of the host crystal, the density of luminescence centers (i.e., susceptibility to the electron irradiation-induced lattice-defects), and the host material (i.e., susceptibility to surface chemical stability).

Blue-emitting ZnS:Ag,Al phosphors with low defect density for high-voltage field-emission displays

In recent years, deterioration of luminescence efficiency of blue-emitting ZnS:Ag,Al powder phosphors has been investigated for high-voltage (e.g., 6–10 kV) field-emission displays (FEDs). It has been found by cross-sectional transmission electron microscopy that the degree of crystallinity of ZnS host crystal within the electron penetration depth is one of the important factors in order to obtain ZnS:Ag,Al phosphors having a longer lifetime of luminescence efficiency. ZnS:Ag,Al phosphors with lower defect density N0 as compared with a P22B ZnS:Ag,Al phosphor for color-TV use have been developed by using a suitable firing method with subsequent annealing and surface treatments. As a result, ZnS:Ag,Al phosphors with lower N0 were successfully prepared, and also FED tubes having a longer lifetime in blue luminescence were made by using ZnS:Ag,Al phosphors with lower N0. In addition, it was revealed that both the initial luminescence efficiency and the thermal quenching point of ZnS:Ag,Al phosphors with lower N0 are considerably higher than those of a P22B ZnS:Ag,Al phosphor.

Characterization of a high voltage flat panel display unit using nanotube-based emitters

Details are given of an experimental investigation carried out to study the field electron emission characteristics of a field emission flat panel display unit using a carbon nanotube-epoxy composite as electron emission material. These include: (i) dependence of direct emission current–voltage characteristic on vacuum gap spacing, (ii) the variation of the proportion of emission current passing through an aperture hole of a gate electrode with changing structural parameters of the device, and (iii) the uniformity and display characteristics of a typical display unit. Our findings indicate that it is very likely for one to produce a near-market prototype high voltage field emission flat panel display, if more sophisticated fabrication and assembly technique is adapted.

Cross-sectional transmission electron microscopy investigation of the dead layer of ZnS:Ag,Al phosphors in field emission displays

The dead surface layer of blue-emitting ZnS:Ag,Al phosphor with Al metallized thin film in high-voltage field emission displays (FEDs) has been investigated by means of cross-sectional transmission electron microscopy. From these observations, it was found that electron irradiation at 6 keV excitation causes the decomposition of ZnS and the subsequent evolution of sulfur in the topmost surface layer ∼30 nm, and also causes the formation of lattice defects within the electron penetration depth of ∼300 nm in a life-end stage. When this evidence was taken into account, it was estimated that the decomposition rate of ZnS and the formation rate of lattice defects depend mainly on the degree of crystallinity and the atomic-scale surface roughness of ZnS phosphor particles. Necessary characteristics of ZnS:Ag,Al phosphors for longer luminescence lifetime in FEDs were suggested in this work.

Integration of high voltage field emission display followed by macro- and nanostructural analysis on microtip

For resolving electron beam focusing and electric field breakdown between the anode and cathode plates in a high voltage field emission display (FED), the metal mesh grids are adapted in one body with spacers. The spacer charging mechanism is modeled in the real panel domain and confirmed by experiment. We analyze the morphology and the chemical composition modifications of the microtips in the narrow vacuum gap of a 5.2 in. FED panel, and infer possible degradation mechanisms in a FED device from our experimental data. The degree of oxidation formed on the microtip surface during fabrication and device operation is studied. Gas aging method for suppressing oxidation on the microtips and stabilizing phosphor is implemented for our 5.2 in. high voltage FED.

Structural and process characterization of high voltage operated field emission displays with focus electrodes

Focus electrodes are engaged to improve focusing characteristics of electron beams in 5.2 in. high voltage field emission displays. It is observed experimentally that the electron beam is collimated fairly well on an optimal condition, which is also compared with simulation results. The layout of ceramic spacers is optimized experimentally as well as by simulations. We uniquely develop a carrier gas of Ar and 1.5% H2, which is effective in prohibiting deterioration of device performance during thermal sealing. A luminance in excess of 400 cd/m2 is achieved at an anode voltage of 5000 V. A video running full color image is successfully demonstrated.

Highly efficient cathodoluminescent phosphors and screens for mid- and high-voltage field emission displays

The commercialization of field emission displays (FEDs) has been delayed despite optimistic promises in the mid 1990’s. Apparently, more time is needed to identify optimum solutions to the complex problems associated with fabrication, assembly, and operation of the displays. One of the reasons for this delay is that a majority of the development effort has been focused on the back plate with the traditional cold cathodes or other emitting matrix structures. As a result, the development of light-emitting anodes, i.e., the front phosphor plate or screen, was based on high-voltage cathode ray tube (CRT) phosphors [L. E. Tannas, Jr., Flat-Panel Displays and CRTs (Van Nostrand Reinhold Company, New York, 1985); V. L. Gerus, Fizicheskiye osnovy ELP (Fizmat Literatura-Nauka, 1993), p. 352]. According to published data, the FED screens are deposited with the photolithography techniques for RGB phosphor screen deposition used in CRTs. In addition, nonoptimized processing is used for aluminizing these standard high-voltage phosphor screens, again using the technology developed for CRTs. This yields a screen luminance of less than 150 cd/m2 and an unsatisfactory RGB luminous screen efficiency of about 1 lm/W at accelerating voltages less than 5 kV. Transfer of known CRT technologies and materials provided some economy of time in the short run to get FED programs started. However, in the case of novel FED device development, the past experience should be examined carefully and used wisely. The FED screens should have significantly better emission and power parameters, as well as maintenance [R. O. Petersen, Inf. Disp. 13, 22 (1997)]. As compared to other novel and traditional flat displays, FED displays, in our opinion, have the following advantages. A high excitation power density from 50 to 250 mW/cm2 can be achieved without activator quenching or device damage. The extremely high concentration of secondary cathodoluminescent excitation, 1022–1024 pairs/cm3 s, does not cause heat and electric breakdowns. High efficiency, over 10%, is standard as compared to low-efficiency and frequently nonradiative one-particle processes in Xe, Kr plasma, or organic semiconductor based displays. An instantaneous pixel pulse luminance of 106–107 cd/m2 allows a variety of addressing schemes of the display including HDTV applications. The phosphor’s luminescent decay times can be adjusted to enable image frequency generation of 50–100 Hz. This eliminates image flicker and RGB temporal defects on the FED screen. Adjustable linear or band emitters can be selected to provide a range of colors to match traditional PAL, SECAM, and NTSC standards, as well as HDTV systems. In this short review, we will present some of our recent results. The work covers new territory in the field of phosphor screen fabrication and maintenance. In particular, we report on the fabrication of highly efficient FED screens. We estimate their light output efficiency as a function of aluminum layer thickness and density, and discuss critical parameters associated with phosphor layer adhesion.

Characterisation of an improved two-dimensional p22 12 1 crystal from bovine rhodopsin

Dialysis of rhodopsin isolated from bovine rod outer segments resulted in the formation of a new two-dimensional crystal form suitable for electron crystallography. The crystals obtained were tubular or single layers and showed p 22 12 1symmetry ( a=60.6(±0.8) Å, b=86.3(±1.6) Å). For the first time the size and order of the crystals allowed us to take electron diffraction patterns showing spots to a resolution of about 3.5 Å. Images were recorded at liquid nitrogen temperature using a high voltage field emission electron microscope. Out of a large number of images 20 crystalline areas were selected and processed with the MRC image processing software. A projection structure of bovine rhodopsin to 5 Å resolution was calculated using amplitudes and phases extracted from these images. The achieved resolution exceeds the resolution of all previously obtained structures of frog, bovine and squid rhodopsin crystals. In this map small differences are observed compared to the previous maps. Helix 5 seems to be even more highly tilted and between the arc-shaped feature and helix 5 a peak is present suggesting that helix 3 is prolonging this feature towards helix 5. These observations are in agreement with the latest model for the three-dimensional arrangement of rhodopsin. The resolution achieved as well as the availability of electron diffraction data suggest that there is a good possibility to collect data from tilted crystals and calculate an improved three-dimensional structure of rhodopsin.