Thomas E. Felter

ENERGETIC PROCESSING OF INTERSTELLAR SILICATE GRAINS BY COSMIC RAYS

While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively ‘‘low’’ energy, heavy-ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single-crystal synthetic forsterite (Mg2SiO4) amorphizes when irradiated by 10 MeV Xe ions at large enoughfluences.Usingmodeling,weextrapolatetheseresultstoshowthat0.1Y5.0GeVheavy-ioncosmicrayscan rapidly (� 70 Myr) amorphize crystalline silicate grains ejected by stars into the interstellar medium. Subject headingg cosmic rays — dust, extinction Online material: color figures

Characterization of novel powder and thin film RGB phosphors for field emission display application

The spectral response, brightness, and outgassing characteristics of new, low-voltage phosphors for application in field emission flat panel displays, are presented. The tested phosphor materials include combustion synthesized powders and thin films prepared by rf diode or magnetron sputtering, laser ablation, and molecular beam epitaxy. These cathodoluminescent materials are tested with e-beam excitation at currents up to 50 μA within the 200–2000 V (e.g., “low-voltage”) and 3–8 kV (e.g., “medium-voltage”) ranges. The spectral coordinates are compared to commercial low-voltage phosphors. Phosphor outgassing, as a function of time, is measured with a residual gas analyzer at fixed 50 μA beam current in the low-voltage range. We find that levels of outgassing stabilize to low values after the first few hours of excitation. The desorption rates measured for powder phosphor layers with different thicknesses are compared to desorption from thin films.

Effects of potassium and lithium metal deposition on the emission characteristics of Spindt-type thin-film field emission microcathode arrays

The effects of K and Li deposition on the emission characteristics of Spindt‐type thin‐film field emission microcathode arrays [Spindt et al. J. Appl. Phys. 47, 5248 (1976)] (TFFEMC) with Mo tips are discussed. The amount by which each alkali metal lowered the effective work function of a TFFEMC is reported and compared to results of similar measurements performed with a clean Mo(100) surface. In addition, it is shown that the emission current is enhanced during exposure to K flux, even after a saturation coverage has been established. These results are interpreted with a model based on an excess K coverage produced during dosing, which enhances the overall emission of the array.

Laser damage probability studies of fused silica modified by MeV ion implantation

Energetic ions in the MeV regime have pronounced effects on the stress-state and geometry of fused silica. In particular, Polman and co-workers have shown that 4 MeV xenon ions cause substantial changes in thin films and microspheres of fused silica. For example, 2 μm wide trenches in thin films can be partially closed and microspheres substantially distorted. In our study, we investigate implantation into bulk silica and the subsequent response to high intensity ultra violet light. Specifically, we compare the damage threshold of fused silica to intense ultra violet light at 355 nm before and after room temperature ion bombardment and find little change despite clear alteration of the stress-state in the glass. We have also performed molecular dynamics simulations in order to understand the underlying effects that lead to obscuration of optics under laser and ion irradiation.

Electronic structure of chromia aerogels from soft x-ray absorption spectroscopy

The electronic structure of ultralow density nanoporous chromia monoliths (aerogels) is studied by soft x-ray absorption near-edge structure (XANES) spectroscopy. These aerogels are prepared by the epoxide sol-gel method. High-resolution O K edge and Cr L2,3 edge XANES spectra of aerogels (before and after thermal annealing at 550°C) are compared with spectra of microcrystalline α‐Cr2O3 and orthorhombic CrO3 powders, α‐Cr2O3 (0001) and (11¯02) single crystals, and an unrelaxed amorphous phase of full-density Cr2O3. Spectra are interpreted based on the results of crystal-field multiplet calculations (for Cr L2,3 edge) and ab initio spin-polarized density functional theory calculations with and without a Hubbard-type on-site Coulomb repulsion (for O K edge). Results show that the electronic structure of as-prepared aerogels, with a large fraction of undercoordinated surface atoms, is significantly different from that of bulk α‐Cr2O3, CrO3, or amorphous Cr2O3. Thermal treatment transforms the as-prepared amo...

Fabrication and testing of vertical metal edge emitters with well defined gate to emitter separation

Vertical metal edge emitter arrays with well defined gate to emitter separations have been fabricated. The emitter to gate spacing is determined by the thickness of a deposited layer which can also serve as a current limiting resistor. Current limiting resistors can also be formed by a self‐aligned etch of the underlying substrate. Parts with 300 nm emitter to gate spacing created using a chemical mechanical polishing based process begin to emit at as low as 60 V. While those created using a reactive ion etching process with 200 nm emitter to gate spacing begin to emit at 40 V. Emission stability is good and dc current densities of up to 3 A/cm2 from 100 emitter arrays on a close packed 5 μm pitch have been demonstrated.

Cathodoluminescent Field Emission Flat Panel Display Prototype Built Using Arrays of Diamond-Coated Silicon Tips

Vacuum-sealed field-emission non-addressable flat panel display prototypes are manufactured and tested. The single-color 50×45×3 mm displays with emissive area 17×17 mm use ungated diode configuration based on arrays of diamond-coated silicon tips. Brightness of 100–200 candelas per square meter is achieved at 1500 volts.

Thin Film Energy‐Controlled Variable Color Cathodoluminescent Screens

A new technique for obtaining energy-variable multicolor cathodoluminescent display screens is presented. This technique employs radio frequency ion plasma sputtering of one thin-film phosphor material over a dissimilar phosphor substrate. The technique employed differs from the old Penetron screen in that the substrate and the secondary phosphor layer are single-crystal materials. This results in higher resolution and allows the use of lower energy beam excitation. The experiments presented use Y 2 0 3 :Eu (red) deposited on a Y 3 Al 5 0 12 -Tb,Ce (green) substrate with excitation in the low killovolt range. The method demonstrated, however, is extendable to a full red-green-blue triplet composed of a wide variety of materials. The screens are particularly applicable to small, low-power formats, such as avionics and instrumentation displays.

Nanoporous Cu–C composites based on carbon-nanotube aerogels

Current synthesis methods of nanoporous Cu–C composites offer limited control of the material composition, structure, and properties, particularly for large Cu loadings of ≳20 wt%. Here, we describe two related approaches to realize novel nanoporous Cu–C composites based on the templating of recently developed carbon-nanotube aerogels (CNT-CAs). Our first approach involves the trapping of Cu nanoparticles while CNT-CAs undergo gelation. This method yields nanofoams with relatively high densities of ≳65 mg cm−3 for Cu loadings of ≳10 wt%. Our second approach overcomes this limitation by filling the pores of undoped CNT-CA monoliths with an aqueous solution of CuSO4 followed by (i) freeze-drying to remove water and (ii) thermal decomposition of CuSO4. With this approach, we demonstrate Cu–C composites with a C matrix density of ∼25 mg cm−3 and Cu loadings of up to 70 wt%. These versatile methods could be extended to fabricate other nanoporous metal–carbon composite materials geared for specific applications.

High-power laser interaction with low-density C–Cu foams

We study the propagation of high-power laser beams in micro-structured carbon foams by monitoring the x-ray output from deliberately introduced Cu content. In particular, we characterize this phenomenon measuring absolute time-resolved x-ray yields, time-resolved x-ray imaging, and x-ray spectroscopy. New experimental results for C–Cu foams show a faster heat front velocity than simulation that assumed homogeneous plasma. We suggest the foam micro-structure may explain this trend.