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The Electron Emission Characteristics of Aluminum, Molybdenum and Carbon Nanotubes Studied by Field Emission and Photoemission.

Description: The electron emission characteristics of aluminum, molybdenum and carbon nanotubes were studied. The experiments were setup to study the emission behavior as a function of temperature and exposure to oxygen. Changes in the surface work function as a result of thermal annealing were monitored with low energy ultra-violet photoelectron spectroscopy for flat samples while field emission energy distributions were used on tip samples. The change in the field emission from fabricated single tips exposed to oxygen while in operation was measured using simultaneous Fowler-Nordheim plots and electron energy distributions. From the results a mechanism for the degradation in the emission was concluded. Thermal experiments on molybdenum and aluminum showed that these two materials can be reduced at elevated temperatures, while carbon nanotubes on the other hand show effects of oxidation. To purely reduce molybdenum a temperature in excess of 750 ºC is required. This temperature exceeds that allowed by current display device technology. Aluminum on the other hand shows reduction at a much lower temperature of at least 125 ºC; however, its extreme reactivity towards oxygen containing species produces re-oxidation. It is believed that this reduction is due to the outward diffusion of aluminum atoms through the oxide. Carbon nanotubes on the other hand show signs of oxidation as they are heated above 700 ºC. In this case the elevated temperatures cause the opening of the end caps allowing the uptake of water. Oxygen exposure experiments indicate that degradation in field emission is two-fold and is ultimately dependent on the emission current at which the tip is operated. At low emission currents the degradation is exclusively due to oxidation. At high emission currents ion bombardment results in the degradation of the emitter. In between the two extremes, molybdenum tips are capable of stable emission.
Date: December 2002
Creator: Sosa, Edward Delarosa
Partner: UNT Libraries

The approach curve method for large anode-cathode distances

Description: An important technique used to characterize field emission is the measurement of the emitted current against electric field (IxE). In this work we discuss a procedure for obtaining IxE data based on multiple approach curves. We show that the simulated features obtained for an idealized uniform surface matches available experimental data for small anode-cathode distances, while for large distances the simulation predicts a departure from the linear regime. We also discuss the shape of the approach curves for large anode-cathode distances for a cathode made of carbon nanotubes.
Date: September 20, 2003
Creator: Mammana, Victor P.; Monteiro, Othon R. & Fonseca, Leo R.C.
Partner: UNT Libraries Government Documents Department

Field-emission cathode micro-electro-mechanical system technology for sensors, diagnostics, and microelectronics

Description: We have investigated field-emission devices for applications in sensors, diagnostics, and micro-electronics. Field-emission devices micromachined from silicon and molybdenum have been fabricated and tested. Experimental results have shown that the presence of nitrogen gas molecules for pressures up to 1 mtorr has only a moderate impact on device emission performance, and has no long term effect on device operation. These results demonstrate that field-emission cathodes can potentially be used in sensors applications where gas ionization by election impact is desirable, or other applications in which the field-emission device is exposed to less than ideal vacuum conditions.
Date: March 30, 1998
Creator: Morse, J.D.; Koo, J.C.; Graff, R.T.; Jankowski, A.F. & Hayes, J.P.
Partner: UNT Libraries Government Documents Department

DC Field Emission Studies on Nb

Description: The DC field emission studies in TJNAF aim to further explore the causes of field emission from Niobium surfaces and enhance our understanding of various preparation techniques for the purpose of improving field emission suppression. Field emitters on Nb samples are located by a DC field emission scanning apparatus in ultra high vacuum, then individually studied under SEM with EDS for characterization. Results at above 100 MV/m from samples prepared by BCP, and by electropolishing are presented and compared.
Date: September 1, 2001
Creator: Wang, T.; Reece, C. & Sundelin, R.
Partner: UNT Libraries Government Documents Department

The Effects of Residual Gases on the Field Emission Properties of ZnO, GaN, ZnS Nanostructures, and the Effects of Light on the Resistivity of Graphene

Description: In this dissertation, I present that at a vacuum of 3×10-7 Torr, residual O2, CO2, H2 and Ar exposure do not significantly degrade the field emission (FE) properties of ZnO nanorods, but N2 exposure significantly does. I propose that this could be due to the dissociation of N2 into atomic nitrogen species and the reaction of such species with ZnO. I also present the effects of O2, CO2, H2O, N2, H2, and Ar residual gas exposure on the FE properties of GaN and ZnS nanostructure. A brief review of growth of ZnO, GaN and ZnS is provided. In addition, Cs deposition on GaN nanostructures at ultra-high vacuum results in 30% decrease in turn-on voltage and 60% in work function. The improvement in FE properties could be due to a Cs-induced space-charge layer at the surface that reduces the barrier for FE and lowers the work function. I describe a new phenomenon, in which the resistivity of CVD-grown graphene increases to a higher saturated value under light exposure, and depends on the wavelength of the light—the shorter the wavelength, the higher the resistivity. First-principle calculations and theoretical analysis based on density functional theory show that (1) a water molecule close to a graphene defect is easier to be split than that of the case of no defect existing and (2) there are a series of meta-stable partially disassociated states for an interfacial water molecule. Calculated disassociation energies are from 2.5 eV to 4.6 eV, that match the experimental observation range of light wavelength from visible to 254 nm UV light under which the resistivity of CVD-grown graphene is increased.
Date: May 2014
Creator: Mo, Yudong
Partner: UNT Libraries

The Physics of Gaseous Exposures on Active Field Emission Microcathode Arrays

Description: The interaction of active molybdenum field emission microcathode arrays with oxygen, water, carbon dioxide, methane, hydrogen and helium gases was studied. Experiments were setup to measure the emission characteristics as a function of gas exposures. The resulting changes in the surface work function of the tips were determined from the Fowler-Nordheim plots. The kinetics of the FEA-gas interaction were studied by observing the ion species originating from the array during and after gas exposures with a high resolution quadrupole mass spectrometer. With the work function data and the mass spectrometry information, the mechanisms responsible for emission degradation and subsequent device recovery after exposures have been determined. The data obtained was used in estimating the device lifetimes under various vacuum environments. Also it was found that the gas exposure effects are similar in dc and pulsed modes of operation of the arrays, thus permitting the use of dc mode testing as an effective acceleration method in establishing the device lifetimes under various vacuum conditions. The vacuum conditions required for the long term emission current stability and reliability of vacuum microelectronic devices employing FEAs are established. Exposure of Mo field emitter arrays to oxygen bearing species like oxygen, water and carbon dioxide resulted in serious emission current degradation. Whereas, exposure to methane and hydrogen caused a significant increase in emission current. The control of residual gases like 02, C02 and H20 in the vacuum envelope is essential for the emission current stability and long term reliability of vacuum microelectronic devices employing field emission microcathode technology.
Date: September 1996
Creator: Chalamala, Babu Reddy
Partner: UNT Libraries

Amorphous Diamond Flat Panel Displays - Final Report of ER-LTR CRADA project with SI Diamond Technology

Description: The objective of this project was to determine why diamond-based films are unusually efficient electron emitters (field emission cathodes) at room temperature. Efficient cathodes based on diamond are being developed by SI Diamond Technology (SIDT) as components for bright, sunlight-readable, flat panel displays. When the project started, it was known that only a small fraction (<1%) of the cathode area is active in electron emission and that the emission sites themselves are sub-micron in size. The critical challenge of this project was to develop new microcharacterization methods capable of examining known emission sites. The research team used a combination of cathode emission imaging (developed at SIDT), micro-Raman spectroscopy (LBNL), and electron microscopy and spectroscopy (National Center for Electron Microscopy, LBNL) to examine the properties of known emission sites. The most significant accomplishment of the project was the development at LBNL of a very high resolution scanning probe that, for the first time, measured simultaneously the topography and electrical characteristics of single emission sites. The increased understanding of the emission mechanism helped SIDT to develop a new cathode material,''nano-diamond,'' which they have incorporated into their Field Emission Picture Element (FEPix) product. SIDT is developing large-format flat panel displays based on these picture elements that will be brighter and more efficient than existing outdoor displays such as Jumbotrons. The energy saving that will be realized if field emission displays are introduced commercially is in line with the energy conservation mission of DOE. The unique characterization tools developed in this project (particularly the new scanning microscopy method) are being used in ongoing BES-funded basic research.
Date: May 8, 1998
Creator: Ager III, Joel W.
Partner: UNT Libraries Government Documents Department

Rf Gun with High-Current Density Field Emission Cathode

Description: High current-density field emission from an array of carbon nanotubes, with field-emission-transistor control, and with secondary electron channel multiplication in a ceramic facing structure, have been combined in a cold cathode for rf guns and diode guns. Electrodynamic and space-charge flow simulations were conducted to specify the cathode configuration and range of emission current density from the field emission cold cathode. Design of this cathode has been made for installation and testing in an existing S-band 2-1/2 cell rf gun. With emission control and modulation, and with current density in the range of 0.1-1 kA/cm2, this cathode could provide performance and long-life not enjoyed by other currently-available cathodes
Date: December 19, 2005
Creator: Hirshfield, Jay L.
Partner: UNT Libraries Government Documents Department

Improved Performance of JLab 7-Cell Cavities by Electropolishing

Description: The great majority of experience in niobium SRF cavity processing at Jefferson Lab is with BCP etching. This has been used on CEBAF cavities and others totalling over 600 in number. With improved process quality control, field emission is now largely controlled and other factors limit performance. All of the prototype cavities developed for the 12 GeV upgrade, although meeting minimum requirements, have demonstrated a Q-drop in the 17?23 MV/m range that is not remedied by 120 C bake. Most of these cavities received >250 micron removal by BCP etch. Two of these cavities have been electropolished using the protocol under development within ILC R&D activities. The first such cavity was transformed from Q = 3×109 at 17 MV/m to quench from 1×1010 at 35 MV/m. The details of this and subsequent electropolished JLab 7-cell cavities will be reported.
Date: May 1, 2009
Creator: Charles Reece, Rongli Geng, Anthony Crawford
Partner: UNT Libraries Government Documents Department

Fastest Electropolishing Technique on Niobium for Particle Accelerators

Description: Field emission on the inner surfaces of niobium (Nb) superconducting radio frequency (SRF) cavities is still one of the major obstacles for reaching high accelerating gradients for SRF community. Our previous experimental results [1] seemed to imply that the threshold of field emission was related to the thickness of Nb surface oxide layers. In this contribution, a more detailed study on the influences of the surface oxide layers on the field emission on Nb surfaces will be reported. By anodization technique, the thickness of the surface pentoxide layer was artificially fabricated from 3nm up to 460nm. A home-made scanning field emission microscope (SFEM) was employed to perform the scans on the surfaces. Emitters were characterized using a scanning electron microscope together with an energy dispersive x-ray analyzer. The experimental results could be understood by a simple model calculation based on classic electromagnetic theory as shown in Ref.1. Possibly implications for Nb SRF cavity applications from this study will be discussed.
Date: September 1, 2011
Creator: A.T. Wu, S. Jin, R.A. Rimmer, X.Y. Lu, K. Zhao
Partner: UNT Libraries Government Documents Department

Superconducting Resonators Development for the FRIB and ReA Linacs at MSU: Recent Achievements and Future Goals

Description: The superconducting driver and post-accelerator linacs of the FRIB project, the large scale radioactive beam facility under construction at MSU, require the construction of about 400 low-{beta} Quarter-wave (QWR) and Half-wave resonators (HWR) with four different optimum velocities. 1st and 2nd generation prototypes of {beta}{sub 0} = 0.041 and 0.085 QWRs and {beta}{sub 0} = 0.53 HWRs have been built and tested, and have more than fulfilled the FRIB and ReA design goals. The present cavity surface preparation at MSU allowed production of low-{beta} cavities nearly free from field emission. The first two cryostats of {beta}{sub 0} = 0.041 QWRs are now in operation in the ReA3 linac. A 3rd generation design of the FRIB resonators allowed to further improve the cavity parameters, reducing the peak magnetic field in operation and increasing the possible operation gradient, with consequent reduction of the number of required resonators. The construction of the cavities for FRIB, which includes three phases for each cavity type (development, pre-production and production runs) has started. Cavity design, construction, treatment and performance will be described and discussed.
Date: July 1, 2012
Creator: Facco, A; Binkowski, J; Compton, C; Crisp, J L; Dubbs, L J; Elliot, K et al.
Partner: UNT Libraries Government Documents Department

High power tests of dressed supconducting 1.3 GHz RF cavities

Description: A single-cavity test cryostat is used to conduct pulsed high power RF tests of superconducting 1.3 GHz RF cavities at 2 K. The cavities under test are welded inside individual helium vessels and are outfitted ('dressed') with a fundamental power coupler, higher-order mode couplers, magnetic shielding, a blade tuner, and piezoelectric tuners. The cavity performance is evaluated in terms of accelerating gradient, unloaded quality factor, and field emission, and the functionality of the auxiliary components is verified. Test results from the first set of dressed cavities are presented here.
Date: March 1, 2011
Creator: Hocker, A.; Harms, E.R.; Lunin, A.; Sukhanov, A. & /Fermilab
Partner: UNT Libraries Government Documents Department

Test results of [beta]=0.64, 700 MHZ, 5-cell elliptical cavities

Description: Six 5-cell superconducting elliptical cavities have been fabricated at LANL and in industry and tested at LANL and TJNAF. The APT (Accelerator Production of Tritium) specification requires Qo > 5 x 109 at an accelerating field of 5 MV/m. So far, the results of vertical tests have shown maximum accelerating fields of 12 MV/m (peak surface field of 41 MV/m and 835 Oe ) and maximum Q0 of 3.6 x 1010 at 2 K (2.1 x 1010 at Eacc = 5 MV/m). The present limitations are available input power, field emission and quench. With a new 600 W amplifier and a mapping system for surface temperature and x-ray radiation, we are trying to identify and localize the emission/quench source(s) to further improve the cavity performance.
Date: January 1, 2001
Creator: Chan, K. D. (Kwok-Chi D.); Edwards, R. L. (Randall L.); Gentzlinger, R.C. (Robert C.); Haynes, W. B. (William B.); Kelley, J. P. (John Patrick); Krawczyk, F. L. (Frank L.) et al.
Partner: UNT Libraries Government Documents Department

The Effects of Cesium Deposition and Gas Exposure on the Field Emission Properties of Single Wall and Multiwall Carbon Nanotubes

Description: The effects of Cs deposition on the field emission (FE) properties of single-walled carbon nanotube (SWNT) bundles were studied. In addition, a comparative study was made on the effects of O2, Ar and H2 gases on the field emission properties of SWNT bundles and multiwall carbon nanotubes (MWNTs). We observed that Cs deposition decreases the turn-on field for FE by a factor of 2.1 - 2.9 and increases the FE current by 6 orders of magnitude. After Cs deposition, the FE current versus voltage (I-V) curves showed non-Fowler-Nordheim behavior at large currents consistent with tunneling from adsorbate states. At lower currents, the ratio of the slope of the FE I-V curves before and after Cs deposition was approximately 2.1. Exposure to N2 does not decrease the FE current, while exposure to O2 decreases the FE current. Our results show that cesiated SWNT bundles have great potential as economical and reliable vacuum electron sources. We find that H2 and Ar gases do not significantly affect the FE properties of SWNTs or MWNTs. O2 temporarily reduces the FE current and increases the turn-on voltage of SWNTs. Full recovery of these properties occurred after operation in UHV. The higher operating voltages in an O2 environment caused a permanent decrease of FE current and increase in turn-on field of MWNTs. The ratios of the slopes before and after O2 exposure were approximately 1.04 and 0.82 for SWNTs and MWNTs, respectively. SWNTs compared to MWNTs would appear to make more economical and reliable vacuum electron sources.
Date: May 2002
Creator: Wadhawan, Atul
Partner: UNT Libraries

EM-PIC simulations of e-beam interaction with field emitted ions from bremsstrahlung targets

Description: We investigate electron beam defocusing caused by field emitted ions from the bremsstrahlung target of a radiography machine using fully electromagnetic particle-in-cell simulations. This possibly deleterious effect is relevant to both current radiography machines (FXR) and machines being built (DARHT-2) or planned (AHF). A simple theory of the acceleration of ions desorbed from the heated target, and subsequent beam defocusing due to partial charge neutralization is in reasonable agreement with the more detailed simulations. For parameters corresponding to FXR (I{sub b}=2.3 kA, {epsilon}{sub b}=16 MeV), simulations assuming space-charge-limited emission of protons predict prompt beam defocusing. Time integrated spot-size measurement, however, is dominated by early-time small spot brightness, and so is not a sensitive diagnostic. Comparisons are made to available FXR data. We also investigate use of a recessed target geometry to mitigate field emitted ion acceleration; only modest improvements are predicted.
Date: August 13, 1998
Creator: Rambo, P. W., LLNL
Partner: UNT Libraries Government Documents Department

Deposition of field emissions cathodes over large areas

Description: Field emission cathodes (FECs) with characteristics of cold emission, low voltage operation, high current density and microscopic size meet the requirements for an electron source for use in vacuum microelectronics. Deposition efforts have focused on evaporation techniques, as electron beam, to produce the size and shape of cathode required for efficient operation. After two decades of development, the convention for FEC synthesis involves coating with very high tolerances for thickness uniformity using a planetary substrate fixture and a long source-to-substrate distance. A further reduction in the operating voltage results by increasing the density of emitters through a reduction of cathode size and spacing. In addition, the objective of scaling the substrate size from small to large areas has compounded the manufacturing requirements to a point beyond that which is obtainable through modifications to the conventional FEC deposition process. We have been successful in a new alternative approach to design, assemble and operate a system that enables FEC synthesis over large areas through the control of deposition source divergence and step-and-repeat substrate handling.
Date: April 3, 1997
Creator: Jankowski, A.F. & Hayes, J.P.
Partner: UNT Libraries Government Documents Department

Low temperature growth of ultrananocrystalline diamond on glass substrates for field emission applications.

Description: Recent studies of field emission from diamond have focused on the feasibility of growing diamond films on glass substrates, which are the preferred choice for cost-effective, large area flat panel displays. However, diamond growth on glass requires temperatures {le} 500 C, which is much lower than the temperature needed for growing conventional microwave plasma chemical vapor deposition (CVD) diamond films. In addition, it is desirable to minimize the deposition time for cost-effective processing. The authors have grown ultrananocrystalline diamond (UNCD) films using a unique microwave plasma technique that involves CH{sub 4}-Ar gas mixtures, as opposed to the conventional CH{sub 4}-H{sub 2} plasma CVD method. The growth species in the CH{sub 4}-Ar CVD method are C{sub 2} dimers, resulting in lower activation energies and consequently the ability to grow diamond at lower temperatures than conventional CVD diamond processes. For the work discussed here, the UNCD films were grown with plasma-enhanced chemical vapor deposition (PECVD) at low temperatures on glass substrates coated with Ti thin films. The turn-on field was as low as 3 V/{mu}m for a film grown at 500 C with a gas chemistry of 1%CH{sub 4}/99%/Ar at 100 Torr, and 7 V/{mu}m for a film grown at 350 C. UV Raman spectroscopy revealed the presence of high quality diamond in the films.
Date: January 17, 2000
Creator: Corrigan, T. D.; Krauss, A. R.; Gruen, D. M.; Auciello, O. & Chang, R. P. H.
Partner: UNT Libraries Government Documents Department

Cryogenic Testing of the RF Input Waveguide for the CEBAF Upgrade Cryomodule

Description: In order to support the planned CEBAF upgrade at the Jefferson Lab a new cryomodule has been designed. A key component of the new cryomodule is the upgraded RF input waveguide, which may couple as much as 10 kW of Radio Frequency (RF) power to Superconducting Radio Frequency (SRF) cavities contained in a bath of superfluid helium. The coupler consists of a straight copper plated stainless steel waveguide as a thermal transition between 2K and 300K and one ceramic window at the warm end of the waveguide. The waveguide interior shares a common vacuum with the cavity. Initial testing of the upgraded coupler has been conducted at Jefferson Lab in a representative cryomodule. During testing, data was obtained regarding waveguide temperature profiles as well as coupler arcing and multipacting.Predicted temperature profiles were used to determine the optimum location of the 50K heat intercept on the waveguide, and were found to be comparable to the actual measured profiles.The coupler was found to be free of multipacting up to 1.7 kW. No arcing occurred during multiple eight-hour runs in heavy field emission with typical radiation levels of 0.5 to 1.0 R/hr outside the cryostat.
Date: June 1, 2001
Creator: Breth, M.; Drury, M.; Getz, R.; Hiatt, T.; Phillips, L.; Preble, J. et al.
Partner: UNT Libraries Government Documents Department

Field emission and growth of fullerene nanotubes

Description: Efforts to control the growth of individual carbon nanotubes from nanotube seed crystals have led to a characterization of their field-induced electron emission behavior. The application of a bias voltage in the growth apparatus was motivated by the prolific formation of nanotubes in the carbon are growth method, in which the electric field appears to play a central role. The authors report here the ability to achieve various tube tip configurations by the controlled application of voltage, heat and chemicals to an individual nanotube, and that these states are well characterized by the emission currents they induce.
Date: November 1994
Creator: Rinzler, A. G.; Hafner, J. H.; Nilolaev, P.; Colbert, D. T. & Smalley, R. E.
Partner: UNT Libraries Government Documents Department

Light emission phenomena in superconducting niobium cavities

Description: During the investigation of field emission limitations of superconducting niobium cavities, a CCD camera was inserted at the end of the beam pipe on a single-cell 1500 MHz cavity. When operating the cavity in field emission, glowing filaments of light were observed trapped by RF fields in closed-orbit trajectories. These filaments were traveling at frequencies much lower than the oscillating RF fields and formed various patterns of light for up to several seconds. This experiment was then repeated on a production CEBAF five-cell cavity with similar results. Events from both experiments were captured on video tape and are presented in this paper along with a discussion of the possible origin of these types of light patterns and the plans to further investigate the phenomena.
Date: March 1, 1999
Creator: Delayen, J.R. & Mammosser, J.
Partner: UNT Libraries Government Documents Department

The effect of aspect ratio and sp2/sp3 content on the field emission properties of carbon films grown by Ns-spiked PECVD

Description: The authors have deposited carbon films from mixtures of methane and N{sub 2} using Plasma Enhanced Chemical Vapor Deposition. By changing the percentage of N{sub 2} in the feed gas, they were able to produce films that have various aspect ratios and sp{sup 2}/sp{sup 3} contents. The film with the highest field emission contains spears of aspect ratio of 10:1. They also found that in their sp{sup 3}-rich films, higher sp{sup 2} content enhanced field emission. This is ascribed to improved charge transport to the field emission sites.
Date: April 1998
Creator: Tong, W.; Felter, T. E.; Pan, L. S.; Anders, S.; Cossy-Facre, A. & Stammler, T.
Partner: UNT Libraries Government Documents Department

A long pulse high-power diode based on a microelectronic emitter

Description: Microelectronic cathode emitter technology being developed at Sandia for supplying continuous low current for flat panel displays appears to be a promising technology for providing high currents when operated in a pulsed, higher voltage mode. If currents in excess of one amp per square centimeter could be produced for tens of microseconds at several kilohertz repetition rate, important applications in such as large volume food or waste sterilization in situ detection, and high power microwave production could be achieved. A testbed was built to perform the experiments. The desired current densities have been demonstrated using small emitter arrays.
Date: November 1, 1995
Creator: Marder, B.; Clark, C.; Walko, R. & Fleming, J.
Partner: UNT Libraries Government Documents Department