Lanthanum hexaboride LaB

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... 

When heated, some surfaces emit copious amounts of electrons (thermoelectron emission). Tungsten and thoriated tungsten are common examples but lanthanum hexaboride (LaB ) is an interesting material in that, at a temperature of 1700 C, it has an electron emission of >20 A/cm, which is much higher than that of tungsten at the same temperature. Hot surfaces of these materials are used as electron sources in some ion and plasma sources. 

Single-walled carbon nemo tubes (SWCNs) have received wide interest for a variety of applications. Its self-aggregation problem has been an obstacle for its use in composite materials. Shim et al. have incorporated SWCNs into nanocomposite films using a layer-by-layer (LBL) method. SWCNs in an LBL film were imaged by a conventional JEOL 3011 TEM with a 300 kV lanthanum hexaboride (LaBs) thermoelectric emission gun. The contrast of SWCN is low under conventional TEM due to its small thickness, all carbon composition, and diameters around 1-2 nm. The SWCNs were dispersed by sodium PSS in aqueous solution or Nation in ethanol solution. LVEM images of the dispersed SWCNs show them to be... 

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. 

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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