Vance Scott Robinson

Work function reduction of graphitic nanofibers by potassium intercalation

Materials with low work functions hold great potential for improving the performance of thermionic energy converters and other thermionic emission devices. Thermionic electron energy distributions (TEEDs) of graphitic carbon nanofibers (GCNFs) with and without intercalated potassium are used to characterize performance under realistic operating conditions. TEEDs of intercalated GCNFs at temperatures of 600 and 700 °C reveal an effective work function of 2.2 eV, a reduction of 2.5 eV from the work function of the GCNF without intercalate. In addition, consistent with other published work, a narrowing of the electron energy spectrum’s width occurs with intercalation. This narrower energy distribution may indicate emission from hybridized carbon–potassium states.

Synthesis and thermionic emission properties of graphitic carbon nanofibres supported on Si wafers or carbon felt

Preparation procedures and thermionic emission properties of graphitic carbon nanofibres (GCNFs) supported on Si wafer or commercial carbon felt supports are reported. GCNF/native-oxide Si wafer, GCNF/oxidized Si wafer, GCNF/Ni-coated Si wafer and GCNF/carbon felt nanocomposites are obtained by growing GCNFs from growth catalyst nanoparticles supported on these supports. Narrow herringbone GCNF/SiO2/carbon felt mats are prepared from growth catalyst nanoparticles supported on fumed silica flakes. Due to weak GCNF-to-support binding in GCNF/Si wafer mats, GCNF/carbon felt mats and GCNF/SiO2/carbon felt mats, mechanical loss of the GCNF component is facile. However, carbothermal reduction of GCNF/SiO2/carbon felt nanocomposites affords mechanically robust GCNF/SiC/carbon felt mats. Thermionic electron energy distribution profiles recorded for these new nanofibre compositions indicate classic free-electron emission with estimated work functions (4.25–4.91 eV) slightly lower than those observed for un-doped graphite or carbon nanotubes. Electron energy distributions along the low energy leading region of the profiles display a cascade of emission peaks equally spaced by ca 0.014 eV, tentatively attributed to electron emission from localized GCNF edge sites.

Experimental demonstration of a dynamic bowtie for region-based CT fluence optimization

Technology development in Computed Tomography (CT) is driven by clinical needs, for example the need for image quality sufficient for the clinical task, and the need to obtain the required image quality using the lowest possible radiation dose to the patient. One approach to manage dose without compromising image quality is to spatially vary the X-ray flux such that regions of high interest receive more radiation while regions of low interest or regions sensitive to radiation receive less dose. If the region of interest (ROI) is centered at the CT system’s axis of rotation, a simple stationary bowtie mounted between the X-ray tube and the patient is sufficient to reduce the X-ray flux outside the central region. If the ROI is off center, then a dynamic bowtie that can track the ROI as the gantry rotates is preferred. We experimentally demonstrated the dynamic bowtie using a design that is relatively simple, low cost, requires no auxiliary power supply, and can be retrofitted to an existing clinical CT scanner. We installed our prototype dynamic bowtie on a clinical CT scanner, and we scanned a phantom with a pre-selected off-center ROI. The dynamic bowtie reduced the X-ray intensity outside the targeted ROI tenfold. As a result, the reconstructed image shows significantly lower noise within the dynamic bowtie ROI compared to regions outside it. Our preliminary results suggest that a dynamic bowtie could be an effective solution for further reducing CT radiation dose.

Pilot study assessing the impact of platelet activation electric stimulation protocols on hematopoietic and mesenchymal stem cell proliferation

Recent research has shown that pulsed electric fields can successfully activate platelets ex-vivo; activation means here the release of growth factors and clotting. Typically, platelets are in a complex biological matrix, such as platelet rich plasma (PRP), which contains a variety of cell types. While specific electric pulses can activate the platelets, the impact of electric simulation on other cell types is an open question. The pilot study presented here focuses on evaluating electric pulse effects on hematopoietic and mesenchymal stem cells when they are in a complex biological matrix also containing platelets. Experimental results indicate that stem cell proliferation at two weeks post treatment can be tuned as a function of electrical parameters. We demonstrate in this pilot study that stem cell proliferation can be either low (via conductive coupling, 8.5 kV/cm electric field amplitude) or high (via capacitive coupling, 2.5 kV/cm electric field amplitude) two weeks after stimulation, despite these two electric pulse delivery mechanisms inducing roughly similar growth factor release and immediate cell viability post treatment. These observations may open up additional ways of tuning electric pulse delivery for platelet activation and other biomedical applications.

Image-based characterization of microfocus x-ray target failure

X-ray targets in microfocus x-ray tubes fail primarily due to sublimation and evaporation of tungsten while exposed to the electron beam. The temperature at the point of impact of the electron beam depends on the beam energy (200-300 kV), the beam current (<10 mA), the cross section (<1 mm) and the intensity profile. In order to preserve the target for a reasonable lifetime, temperatures at the spot do not typically exceed 2500 C. As tungsten evaporates from the surface of the target, the surface starts to pit and this can affect the x-ray production in multiple ways: the photon flux decreases, the heel effect is enhanced, the effective spot size changes shape and/or size. Indirectly, the target damage incurred over time or due to intense use will undermine the image quality by reducing image contrast, changing the resolution or degrading the signal to noise ratio. A detailed description of how x-ray target damage is incurred and the potential impact on image quality is reviewed in detail. Experimental results showing the target damage and associated loss of image quality are discussed.

A case for ZnO nanowire field emitter arrays in advanced x-ray source applications

Reviewing current efforts in X-ray source miniaturization reveals a broad spectrum of applications: Portable and/or remote nondestructive evaluation, high throughput protein crystallography, invasive radiotherapy, monitoring fluid flow and particulate generation in situ, and portable radiography devices for battle-front or large scale disaster triage scenarios. For the most part, all of these applications are being addressed with a top-down approach aimed at improving portability, weight and size. That is, the existing system or a critical sub-component is shrunk in some manner in order to miniaturize the overall package. In parallel to top-down x-ray source miniaturization, more recent efforts leverage field emission and semiconductor device fabrication techniques to achieve small scale x-ray sources via a bottom-up approach where phenomena effective at a micro/nanoscale are coordinated for macro-scale effect. The bottom-up approach holds potential to address all the applications previously mentioned but its entitlement extends into new applications with much more ground-breaking potential. One such bottom-up application is the distributed x-ray source platform. In the medical space, using an array of microscale x-ray sources instead of a single source promises significant reductions in patient dose as well as smaller feature detectability and fewer image artifacts. Cold cathode field emitters are ideal for this application because they can be gated electrostatically or via photonic excitation, they do not generate excessive heat like other common electron emitters, they have higher brightness and they are relatively compact. This document describes how ZnO nanowire field emitter arrays are well suited for distributed x-ray source applications because they hold promise in each of the following critical areas: emission stability, simple scalable fabrication, performance, radiation resistance and photonic coupling.

THERMAL analysis of high power x-ray target: scaling effects

High resolution x-ray imaging systems require small focal spots ranging from 1 μm to 1 mm. In NDE applications, the demand for small spot sizes for high spatial resolution conflicts with the need for increased x-ray flux for faster scan times. In this paper, a finite element model is developed to compute the temperature of a stationary x-ray target exposed to micrometer-sized high power (10’s to 100’s of watts) electron beams. Such extremely high power densities at the focal spot are the limiting factor in both performance and life of many x-ray imaging system. This model is used to demonstrate the effect of focal spot size – diameter, on the heat dissipation capability. As the spot size reduces, a higher power density may be sustained by the target. This effect is explained by increased lateral heat conduction. The peak temperature of a small focal spot also becomes more sensitive to the current density distribution of the incident electron beam. The relationship of the peak power and electron beam profile, volumetric power deposition into the x-ray target and focal spot aspect ratio are discussed. Some experimental data demonstrating such scaling effects is included. General design rules for higher-flux capable targets leveraging these scaling effects are also proposed.

Electron Beam Characterization via Infrared Pyrometry of Thin Tungsten Foil

A method is explored for characterizing the energy distribution of an electron beam in vacuum. A thin (125 μm) tungsten foil intercepts the beam causing its temperature to rise. An infrared camera images the opposite side of the foil to map the temperature distribution. To the extent the thermal conduction can be neglected, the temperature distribution serves as a representation of the energy distribution of the incident electron beam. Design of experiments (DOE) methodology is used to develop an equation for predicting the shape of the temperature distribution based on the potential of a single electrostatic lens (−1000 V to 0 V) and that of the tungsten foil (0 V to 2000 V). An assessment of this technique’s utility for future electron beam characterization experiments is included.Copyright

Simulation of Electron Beam Heating of a Cu Thin Film on a Si Substrate

Electron beam interaction with thin films is a critical phenomenon in applications such as nanostructure fabrication, surface treatment and curing, surface sterilization, scanning electron microscopy, and electron beam lithography. Unlike bulk solids, thin films whose thickness is on the same order of magnitude as the penetration depth require consideration of interface effects: namely resistance to heat transfer and more electron scattering. In such cases, the energy deposition profile, the location of interfaces and the associated change in material properties must be accounted for. In this paper we describe the thermal simulation of a thin copper film (0.5 μm, 1 μm, 2μm) on a Si substrate irradiated by an electron beam (10 keV, 20 keV, 40 keV). We explore the effect of the interface position relative to the electron range and the local heating effects associated with continuous or long pulse beams.Copyright

Thermionic Emission Energy Distributions From Nanocrystalline Diamond

Thermionic electron emission provides a means of direct energy conversion. Some of the benefits of thermionic power generation include: compactness, scalability, stability, lack of moving parts, and applicability to microscale devices. Taking advantage of these benefits requires the analysis and subsequent manufacturing of materials that emit electrons efficiently and at reasonable temperatures (<1000 °C). We report here on a study being performed to characterize the emission properties of such materials, namely, nanocrystalline diamond with hydrogen and oxygen termination. A hemispherical energy analyzer is used to measure the electron energy distribution from this nanostructured material at elevated temperatures. The effective work function and the presence of regions of differing work functions are determined. Measurements of thermionic emission energy distributions (TEEDs) at temperatures ranging from 573 to 778 °C are presented. The TEEDs show an intriguing development of multiple peaks at higher temperatures, possibly indicating instability in the emitter’s surface chemistry.Copyright