High Current Field Emission from ZrC with Differing Cone Angles
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569 Mat. Res. Soc. Symp. Proc. Vol. 558 © 2000 Materials Research Society
EXPERIMENTAL Single emitter fabrication starts with single crystal material grown via a floatingzone refining technique.5 Sintered stock with a calculated excess of carbon was used to generate single crystal rods with a bulk stoichiometry of approximately 1.0 (0.98 as 6 verified by chemical analysis) . Refinement was done using a (100) oriented seed crystal to eventually generate (100) oriented field emitters. These refined single crystal rods were centerless ground to a 0.75-mm diameter, cut to length, and usually mounted on ceramic bases using a Vogel type mount. In this mounting method the emitter shank is held between two pyrolitic carbon blocks by spring tension supplied by Mo/Re posts. The emitter tips were formed through electrochemical etching in a 10% perchloric/90% acetic acid solution. The SEM was used to ascertain the emitter radius prior to mounting, evacuation and testing. Emitters are generally tested in a standard field emission microscope (FEM) tubes, available from reference 6. After thermal cleaning, to 2100 K for ZrC, I(V) and high current measurements are taken. Etched emitters made as described above generally have small emitter-cone half angles. Though the end-form radius may vary, they typically have half angles which average 12.5 degrees (see Fig 1).
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Fig. 1: SEM micrographs of end-form geometry of two typical tips. The first (a) has been electrochemically etched as described in the text. The second (b) has been "in-situ formed" and shows a larger emitter-cone half angle while retaining the small emitter radius. RESULTS Ungated emitters We have recently developed a new and unique processing method especially suited for transition metal carbides. This method can be used in-situ and can even rejuvenate arced emitters. There are several beneficial outcomes obtainable through the use of this "in-situ forming" method which are derived from the increased emitter-cone half angle (see Fig. 1). Emitters formed by this method have average emitter-cone half angles of 32.5 degrees, roughly 3-times that of the etched emitters. The significant benefits derived from this type of emitter are summarized by the following mechanisms:
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First, this larger emitter-cone angle translates into slightly better emission confinement. This can readily be seen from viewing Fig. 1. The field lines close to the emitter generally follow the apex/shank geometry. For these larger emitter-cone angles the emission angle is slightly compressed over emitters having a smaller emitter-cone angle. This fact seems to be born out by emission image observations.
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Second, these emitters generally seem to have smaller emitter radii, thus lowering the extraction voltage needed to obtain any given emission current. This too can be seen through viewing several SEM emitter end-form images.
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The third benefit is that the larger cone angle emitters seem capable of delivering, on average, larger total currents. While not knowing
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