Lanthanum hexaboride electron

The Lanthanum Hexaboride Electron Emission Source and the Vacuum Environment, Amray Technical Bulletin 112-277, Amray, Bedford, Mass., 1986, pp. 11-18. 

Figure 2. Diffraction camera for single-molecule electron diffraction. A Lanthanum hexaboride electron source is used. The laser and associated optics is rotated after each data readout for a new molecular beam orientation. Organic molecules are picked up within liquid helium droplets to form a molecular beam traversing the electron beam.

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. 

XPS spectra were obtained using a Perkin-Elmer Physical Electronics (PHI) 555 electron spectrometer equipped with a double pass cylindrical mirror analyzer (CMA) and 04-500 dual anode x-ray source. The x-ray source used a combination magnesium-silicon anode, with collimation by a shotgun-type collimator (1.). AES/SAM spectra and photomicrographs were obtained with a Perkin-Elmer PHI 610 Scanning Auger Microprobe, which uses a single pass CMA with coaxial lanthanum hexaboride (LaBe) electron gun. 

Electron sources used in EBL exposure tools are similar to those used in conventional electron microscopes. They can be divided into two main groups— thermionic or field emission—depending on the way in which they emit electrons. The sources that rely on the emission of electrons from a material that is heated to a temperature at which electrons are emitted from the surface are referred to as thermionic sources. These sources are fabricated from materials such as tungsten, thoriated tungsten, or lanthanum hexaboride (LaBe). ... 

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. 

A second thermionic emission source uses lanthanum hexaboride (LaBe). This has a much lower work function than tungsten and so will emit electrons when heated to only 1,800 K (tungsten operates at -2,500 K). LaBe is reactive at its operating temperature, but the emitter is a direct replacement for a tungsten filament, requiring only very minor alteration of the instrument. Cerium hexaboride is an alternative source material of the same type. 

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... 

From this we can expect that lanthanum hexaboride would be applicable as a (photo) electron emitter both as oxidised thin layer and bulk material surely by using energies up to 5 eV, however if a careful treatment of the layer is provided values as low as 4 eV or even 3.8 eV might be achieved. 

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. 

Europium hexaboride is a semiconductor (Eg — 0.38 eV) but becomes a metallic conductor if 1 % of the europium atoms is replaced by lanthanum. The boron nets can accommodate some of the valence electrons from the lanthanum. The compound is metallically conducting because of the excess of valence electrons that go into interstitials. The hexaborides have a high melting point and a high strength. They are hard and stiff because of the strong covalent bonds in the lattice. 

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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