Field emission from laser-processed niobium (110) single crystals

Niobium is a technologically important material, which is typically used for superconducting radio-frequency applications. Superconductive cavities made of niobium require contamination-free and smooth surfaces to ensure the best performance of a particle accelerator. Interior surfaces of niobium cavities are usually obtained by centrifugal barrel polishing, buffered chemical polishing, and electropolishing or a combination of these methods. However, a standard inspection of the inner cavity surface after the treatment still shows the presence of sharp features limiting cavity performance. Laser polishing is a potential alternative or can be used as an additional final step for a more efficient surface treatment toward higher electric field gradients. In the present study, a chemically polished (110)-oriented single crystal niobium surface was processed with a focused pulsed laser with a pulse duration of 3.5 ns and a repetition rate of 10 Hz. The laser fluence and the number of pulses were varied in the range from 0.68 up to 4.27  J/cm^{2} and from 20 up to 200, respectively. The magnitude of laser-induced surface structures and boiling traces was systematically studied by means of scanning electron microscopy, atomic force microscopy, and optical profilometry. Finally, the local field emission behavior was investigated and correlated with the observed surface modifications. Typical current-field characteristics and a field enhancement statistic of laser-processed areas are presented in the study. The surface processing with a rather low laser fluence of 0.68  J/cm^{2} yielded high onset electric fields of 650–940  MV/m with field enhancement factors below 10. The processing with a higher laser fluence and/or a higher number of pulses resulted in boiledlike structures emitting at 70–190  MV/m

Role of height and contact interface of CNT microstructures on Si for high current field emission cathodes

Regular arrays of vertically aligned microstructures consisting of entangled carbon nanotubes (CNTs) of different height and contact interface were grown on Si substrates with a bimetallic catalyst by water-assisted chemical vapor deposition. The arrays of high and wide CNT blocks (150–300 μm, 50–140 μm square) showed the ability to reach high stable field emission (FE) currents per block up to 300 μA due to the presence of multiple CNT emitters. However, significant outgrowth of the CNTs and limited mechanical stiffness of such blocks led to a limited FE homogeneity and alignment of the emitters. For the arrays of small rounded CNT bundles (∼5 μm, 20 μm diameter), well-aligned and highly efficient FE with maximum currents up to 40 μA per CNT bundle have been achieved. Unusual I-V curves with current saturation, strong activation effects and glowing spots just before destruction have been observed and are discussed by means of band structure considerations

A national vegetation database for South Africa

Field emission (FE) properties of unstructured and patch-structured cathodes with randomly distributed and vertically aligned copper nanocones (Cu-NCs) are reported. The cones of ~28 μm height, 2.4 μm base diameter and 95–220 nm tip radius were fabricated by electrochemical deposition of Cu into conical channels of heavy-ion-irradiated and asymmetrically etched polycarbonate membranes. FE measurements of the unstructured cathodes with slowly-grown Cu-NCs of high number density (107 cm−2) and excellent mechanical stability yield stable currents up to 280 μA from an emission spot of 30 μm. For the structured cathodes with a triangular patch array of less dense Cu-NCs (<106 cm−2), well-aligned FE with ~90% efficiency is reproducibly achieved. A trade-off between low onset field (~22 V/μm) for sparsely grown Cu-NCs with sharp tips and high current limit (~100 μA) for densely grown ones with broader tips is observed. Possibilities for further optimization of such field emitters for cold cathode applications are discussed

Field Emission Cathodes Based on Structured Carbon Nanotube Arrays

Field emission properties of the structured carbon nanotube cathodes were investigated by field emission scanningnbsp microscopy, scanning electron microscopy and integral field emission measurements with luminescence screen. The carbon nanotube arrays were synthesized by the atmospheric pressure floating catalyst chemical vapour deposition method under the high temperature pyrolysis of ferrocene/xylene solution. Varying arrays of carbon nanotube columns and blocks were fabricated on Si, SiO2 and porous anodic alumina substrates.nbsp Well-aligned field emission from nearly 100% of the patches at electric field lt10 V/microm in direct current and pulsed mode integrally and locally was observed. High current capabilities up to mA currents for structured carbon nanotube cathodes were achieved. Integral field emission measurements with luminescence screen and processing under N2 and O2 exposures of up to 3times10minus5 mbar demonstrated homogeneous current distribution and long-term stability of the structured carbon nanotube cathodes.nbs