Field emission properties of microscopic particles of graphite, sulfur, and MoS2 deposited on nonemitting Nb surfaces under ultrahigh vacuum conditions are described. Both graphite and MoS2 particles are shown to emit strongly, with field enhancement factors of the order of 100, in contrast to sulfur particles, which emit only weakly. The emission characteristics are measured locally on the individual sites, and emitting particles are further characterized by microfocus Auger spectroscopy and scanning electron microscopy. Their behavior upon heat treatment was studied, with the main result that both for graphite and MoS2 particles the emission disappears after heating to 1200 °C. The properties of the artificially created emitters are compared with those of naturally occurring field emitters.
We present results made with a new apparatus for the study of localized field emission (FE) sites on broad-area cathodes. The apparatus is centered around a UHV field emission scanning microscope, consisting of a micromanipulator allowing precision cathode motion, a rotatable holder for broad and microtip anodes, and a fast high-voltage regulator allowing constant-current measurement of a distribution of both weakly and strongly emitting sites. Also included are an in situ scanning electron microscope, and a facility for in situ microfocus Auger analysis as well as scanning Auger microscopy. Samples can be high-temperature annealed without removal from UHV. These tools have been used to study the effects of heat treatment (HT) up to 2000 °C on the FE from nonanodized and anodized Nb cathodes. We find that HT at 800–900 °C increases the density of field emitters at a given electric field. HT at T>1000 °C reduces the density, while at T≥1400 °C a drastic decrease of the emission occurs. We have repeatedly obtained surfaces of cm2 size which do not emit at 100 MV/m. If such an emission-free surface is heat treated again at 800 °C, new emitters appear. Typically, the physical size of the field emitters is a few μm, although in some cases a larger particle was found and in others no feature was seen at 0.5-μm resolution. A large variety of elements is associated with these particles. We discuss in particular the nature and origin of three types of emitters: (1) sulfur containing particles, (2) carbon particles, both of which were identified as new emitters after a 800 °C HT, and (3) FE sites which are especially resistant against HT.
Nous presentons une etude de l'emission de champ augmentde sur des surfaces de niobium. Notre experience a ete effectuee dans un "Vacuum Generators Escalabn, equip8 d'un microscope Auger B balayage, d'un microscope B emission de champ et dfun systeme de chauffage in situ de la cathode. Nous observons que la densiti de sites dr6mission augmente syst6matiquement lors de traitements thermiques entre 650°C et 900°C. Par contre il est possible d'obtenir des surfaces sans emission (I < 40 nA) jusqu'i des champs de 100 MV/m a l'aide de traitements thermiques a 1400°C et au-dessus. Un nouveau traitement thermique a environ 800°C conduit A une nouvelle augmentation du nombre des sites. Cette emission est trhs souvent like a des particules contenant du carbone ou, du souffre et du molybdhne.The study of enhanced field emission from broad area niobium cathodes, and the characterization of individual sites are presented. This experiment was built[l] in a UHV surface analysis system which included a SEM, a scanning Auger microscopy facility and also an in situ heat treatment unit where the samples could be annealed up to 2000°C. Upon heat treatment of the niobium samples, it is observed that the density of field emitting sites increases with the heat treatment temperature up to 900°C whereafter the density decreases sharply. After heat treatments at 1400°C and higher we have been able to obtain, consistently, emission free surfaces at 100 MV/m. Further heat treatments of the emission free sample show a maxima in the density of field emitting sites at 800°C. These sites were either activated or created during the heat treatment. Typical examples of the activated and created sites are presented.
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