Crystal Phases of Nickel as Related to Adhesion of Thermionic Coatings and to Emission Phenomena

For conventional oxide cathodes the possibility of forming micro-separations between the nickel base and the coating during the activation phase is demonstrated in a large-screen thermionic emission microscope. When using work-hardened nickel, shearing forces necessarily occurring during recrystallization owing to crystal growth and grain boundary expansion must lead to disruption of the base-to-coating contact. Severe limitations to emission current by increased resistance, next to interface resistance, are imminent. A preventive measure of pro-heating all cathode sleeves at 1200°c is recommended. Artificial activation of the polycrystalline nickel base by a thin-film barium layer leads to the qualitative observation of anomalous emission phenomena as caused by the anisotropy of different crystal planes. Emission reversal with decreasing temperature, crystal memory shapes, preferential etchability of higher work function planes, recovery after apparent barium exhaustion and after hot oxygen poisoning are among the observations. Theoretical efforts for reconciling apparent contradictions seem essential since thin-film activation appears to be the ultimate mechanism of electron omission in any thermionic cathode structure.