The rare earth hexaboride compounds exhibit an excellent combination of low work function and low volatility. In fact, lanthanum hexaboride (LaB6) is in wide-spread everyday use in a variety of fine-focused electron beam instruments. LaB6 cathodes are ideal for many small spot size applications such as SEM, TEM, sterilization, surface analysis and metrology, and for high current applications such as lithography, x-ray sources, small space-craft thrusters, and even electron-beam welders.
The unique properties of lanthanum hexaboride crystals provide stable electron emitting media with work functions near 2.70 eV. The low work function yields higher currents at lower cathode temperatures than tungsten, which means greater brightness, or current at the beam focus, and longer LaB6 cathode life. Typically, LaB6 cathodes exhibit
10 times the brightness and 50 times the service life of tungsten cathodes. In electron microscope applications, these characteristics translate to more beam current in a smaller spot at the sample, improved resolution, and less frequent cathode replacement.
We grow and fabricate our own high quality, single-crystal materials using a well-defined process called “Inert Gas Arc Float Zone Refining.” An electric arc melts a pressed-powder stick of LaB6 in a controlled atmosphere of purified inert gas, allowing the liquid-phase zone to freeze onto a selected-orientation seed crystal as the arc is moved along the stick. The finished crystal assumes the desired orientation of the seed with less than 30 parts per million by weight metal impurities. Correct melt zone temperature and process speed minimize excessive boron evaporation to achieve the optimum ratio of metal to boron atoms in the grown crystal. Our floating zone refining technique has been perfected over the years to produce some of the purest material available. This coupled with our stringent and exacting composition requirements produces both the purest and the lowest work function LaB6 available anywhere in the world.
LaB6 cathodes have modest vacuum requirements and long shelf life, and need only to be brought up to operating temperature to provide emission, eliminating the activation procedure required of dispenser cathodes. They can provide long-term, stable, operation at current densities up to 20 A/cm², and may be fabricated in a variety of shapes and with many different heating and mounting configurations. LaB6 cathodes are the materials of choice for high current cathodes in a variety of advanced and custom applications.
Coefficient of Thermal Expansion
Effective Work Function (100) at 1800 K
Absolute Work Function (100) at 300 K
Spectral Emissivity at 0.65 microns
Evaporation rate at 1800 K (UHV)
Orientation limit for specific orientation
Pyrolitic block mount resistance @ 1800 K
Clamping pressure of SMVM
Yield of finished cathodes
ppm by wt.
α x 10^6
2.2 x 10^-9
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McMinnville, OR USA
Transition Metal Carbides
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