Scandate Cathodes Research Articles

Calculation of diode charateristics and proposed characterization of cathode emission capability

It is rather common practice to characterize the emission capability of thermionic cathodes by means of the zero-field current density, j 0( T), as determined from the Schottky plot by means of extrapolation towards zero diode voltage. It will be shown that the Schottky part of the characteristics (“saturation” region) - taking into account both space charge and initial velocities - can be accurately represented by an analytic expression. However, for most practical cathodes it shows considerable deviations from experiments. This is due to the fact that the Schottky effect is overruled by other effects, such as TF emission, emission from pores, patch effects and so on. Since the relative contributions of these effects are different for different types of cathodes, their comparison on the basis of j 0 values obtained from the Schottky extrapolation is questionable. This means, for instance, that the ranking of different types of dispernser cathodes - such as the W-Os alloy matrix and the scandate cathode - is not really known. Therefore, another method for determining j 0, based on the above mentioned calculations, will be discussed. This gives rise to a proposal for characterizing the emission capability of a cathode.

The characterization of the surfaces of tungsten-based dispenser cathodes

The “CD”, “M” and scandate cathodes all have a better thermionic emission performance than the “B” type of impregnated tungsten based dispenser cathode. Modern surface analysis techniques (AES, ISS, SIMS and SLEEP) have been used to examine the surfaces of the four cathode types. The cathode surfaces have been studied in their active state, during reactivation following a sputter cleaning of their surfaces and whilst poisoned by, and reactivating from, an exposure to 10 L O 2. The results are discussed in terms of surface models which would be consistent with the observed data. The differences in the surfaces of the different cathode types are examined and the value of the various surface analysis techniques discussed.

Work function and gas desorption studies during turn-on of various shelf-stored cathodes

A comparative study has been made of the oxide-type coated powder cathode (CPC), and impregnated types B, M, tungstate and scandate cathodes in their ability to rapidly turn on after extended periods of shelf life. Particular performance objectives were to attain pulsed emission levels in the 4–5 A/cm 2 region within a few seconds after shelf lives corresponding up to several years. Preliminary surface studies indicate that all cathodes significantly poison within a few hours or days after being shut down in a typical sealed-off tube environment. Therefore, apart from bringing the cathode up to thermionic emission temperature, the kinetics of the turn-on step must also involve the kinetics of reactivation. Comparative data are presented of the work function variation with reactivation temperature for the various cathodes, thus showing the degree of poisoning, the thermal reactivation characteristics and the work function-temperature relationship in their respective active states. The results show that in the active state, the CPC has the highest emission, the M and scandate impregnated cathodes are somewhat lower in emission, i.e. ∼50 K higher temperature for the same emission, while the type-B impregnated cathode requires ∼100 K more. The shelf-life poisoning among the impregnated cathodes, on the other hand, was greatest for the M cathodes, a bit less for the scandate and least for the type B. For typical fast turn-on reactivation schedules, the CPC reactivated below its normal operating temperature of ∼1250 K but all impregnated cathodes required activation temperatures of ∼1500 K and above (i.e., about 200 K above their operating temperatures). Studies were also made on the pick-up and desorption of various gases present in the shelf-life vacuum ambient. The data demonstrated a direct correlation between the chemical reactivity that the different cathodes had with the residual ambient gases and the corresponding extent to which these cathodes poisoned during shelf life. While a study of the shelf-life poisoning and reactivation mechanisms for the impregnated-type cathodes is still in progress, initial results have been completed for the oxide-type cathodes and these are presented elsewhere in this issue.

Activation and early life of a pressed barium scandate cathode

Activation and early life of a pressed barium scandate cathode have been investigated, correlating thermionic emission properties with elemental composition and distribution, as determined with scanning Auger microscopy. It is shown that such cathodes after activation have a current density J ≈ 10 A/cm 2 at 950°C which is due to activated scandium oxide (Sc 2O 3) regions in the surface.