Boride Cathodes

Abstract

The thermionic emission properties of the borides of the alkaline-earth and rare-earth metals and thorium have been investigated. These compounds all have the same formula MB6 and the same crystal structure consisting of a three-dimensional boron framework in whose interlattice spaces the metal atoms are embedded. The valence electrons of the metal atoms are not accepted by the B6 complex, thus giving rise to the presence of free electrons which impart a metallic character to these compounds. This, together with the strong bonds between the boron atoms in the framework, produces a series of compounds which have high electrical conductivities and high thermal and chemical stabilities—ideal properties for a cathode material. When this structure is heated to a sufficiently high temperature, the metal atoms at the surface evaporate away. They are, however, immediately replaced by diffusion of metal atoms from the underlying cells. The boron frame work does not evaporate but remains intact. This process gives a mechanism for constantly maintaining an active cathode surface. Thermionic emission measurements made on these materials show the rare-earth metal borides to be superior to the others. The highest emission was obtained from lanthanum boride. Its emission constants for the Dushman equation were φ=2.66 volts and A=29 amps/cm2/degK2. This is higher than the emission normally obtained from thoria. Lanthanum boride has a relatively low evaporation rate corresponding to a latent heat of evaporation of 169 kilocalories per mole. If the hexaborides are operated at high temperature in contact with the refractory metals, boron diffuses into their metal lattices forming interstitial boron alloys with them. When this occurs, the boron framework which holds the alkaline-earth or rare-earth metal atoms collapses, permitting the latter to evaporate. However, the hexaboride cathodes may be operated at high temperatures in contact with tantalum carbide or graphite. Lanthanum boride cathodes are especially useful in applications where high current densities are required. They are also suitable for high voltage applications because they stand up well under positive ion bombardment. Since they are atmospherically stable and activate easily, they have found wide use in experimental demountable systems.

Development of Hollow Cathodes for Space Electric Propulsion at Sitael

“…The dependence of the results on the geometry can thus be assessed to target the definition of the main dimensions of the cathode, i.e., emitter inner and outer diameters, emitter length, orifice diameter, and orifice length. The work function of LaB 6 considered in the computation is 2.66 eV, along with a Richardson constant of 29 A·cm −2 ·K −1 , in accordance with the values reported by Lafferty [9]. The cathode lifetime is estimated on the basis of an evaporation model at the computed emitter surface temperature, where the lifetime is described as the time to halve the emitter initial mass.…”

“…A LaB 6 emitter does not require lengthy activation procedures and has a low sensitivity to contaminants and air exposure, increasing the cathode reliability. The main disadvantage of LaB 6 with respect to dispenser emitters is the higher work function, which is between 2.4 and 2.7 eV for polycrystalline LaB 6 cathodes [9,10], compared to about 2.1 eV for dispenser cathodes [4]. This property directly translates into operating temperatures of over 1600 • C for LaB 6 emitters compared to over 1000 • C for dispenser emitters, to deliver current density in the order of 10 5 A/m 2 .…”

Back-bombardment compensation in microwave thermionic electron guns

“…7 and 8, limiting the pulse length to 5 μs, the numerical model shows that by using a laser prepulse which deposits 329 mJ (average fluence of F ¼ 4.65 J=cm 2 ) of heat onto the cathode surface over 7 μs duration just before the rf pulse enables pulses up to 29 μs, illustrated in Fig. 9, while keeping the current in same range as we now operate and maintaining the surface temperature of the cathode below 2287 K-well below the melting point of LaB 6 of 2483 K [20]. This is nearly a factor of 6 longer than the 5 μs pulses we currently operate with.…”

“…This improved model was fit to simultaneous gun current and cathode temperature data via the work function of the LaB 6 cathode (with measured values in the literature between 2.4 [19] and 2.66 eV [20]), resulting in a final fit value of 2.485 eV, and via the shunt resistance in the circuit model of the cavity which was used to match the change in current due to beam loading. A value of 29 A=cm 2 was used for the Richardson constant [20]. Results of the model fit to data are shown for short time scales in Fig.…”

“…The basic formulation of this one-dimensional numerical model is described in a reference [15], with the back-bombardment power as a function of depth into the cathode calculated using PARMELA [16], though we have made several improvements: the model of the current emitted by the gun now accounts for space charge limited emission at low fields (Child's law) [17], transitioning to the field assisted thermionic emission, or Schottky equation, when the current becomes thermionically limited [18], and the accelerating field is now appropriately reduced due to beam loading at higher currents using a shunt resistance in the circuit model of the cavity [7]. This improved model was fit to simultaneous gun current and cathode temperature data via the work function of the LaB 6 cathode (with measured values in the literature between 2.4 [19] and 2.66 eV [20]), resulting in a final fit value of 2.485 eV, and via the shunt resistance in the circuit model of the cavity which was used to match the change in current due to beam loading. A value of 29 A=cm 2 was used for the Richardson constant [20].…”