Lanthanum hexaboride cathodes

Lanthanum hexaboride cathodes fall short of the required brightness for a round beam to compete with a shaped beam only producing lxl06 A/cm2.ster. (25 kV) with adequate reliability for unattended production applications (40). In summary, highest throughput in production is obtained with shaped beams. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

The cathode emits electrons that are accelerated towards the anode with a defined voltage, typically 50-30,000 V. There are basically two types of electrodes thermionic cathodes (tungsten or LaBs (lanthanum hexaboride)) and field emission cathodes. The Wehnelt cylinder controls the current density and brightness of the electron beam. Brightness is defined as current per unit area normal to the given direction, per unit solid angle, and a criterion for beam quality. 

Figure 12.1 shows the electron beam generated on the cathode, typically a tungsten filament (W) or lanthanum hexaboride (LaBs), which is heated by an electric current. By thermionic effect, it creates an electron beam that is accelerated by an... 

Thermionic cathodes consist of a directly heated tungsten hairpin cathode at = 2500 -3000 K, or an indirectly heated pointed rod of lanthanum or cerium hexaboride (LaB, CeB(,) at 1400 - 2000 K. The electrons must overcome the work function of 4.5 eV (W) or 2.7 eV (LaBfe) by thermal activation (Fig. 78, curve a). Between the cathode at the potential -V and the grounded anode, a negatively biased Wehnelt electrode forms a crossover of diameter 20-50 pm (W) or 10-20 pm (LaBe) as an effective electron source. The emitted electrons show an energy spread A = 1 - 2 eV (W) or 0.5- 1 eV (LaBft). A measure of the quality of an electron gun is the axial gun brightness [i ... 

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