Technology development for application of impregnated dispenser-type inserts in hollow cathodes. Barium calcium aluminate impregnant materials: Interaction with sea-level ambient atmospheres and decomposition behaviors of reaction products

摘要

It has long been recognized that aluminate-type impregnant materials, as used in dispenser cathodes, react with certain constituents of the sea level ambient atmosphere. Several potentially damaging consequences of such reactions have been noted. One is 'blooming', or the appearance of small amount of impregnant-derived reaction products on the cathode surface, typically at pore ends, resulting in a hazy appearance, and which, in the extreme, as a result of significant volume expansion may crack the surrounding cathode matrix. A second issue is the possibility of detrimental chemical reactions involving the evolving decomposition products which can occur during heating of the cathode subsequent to exposure. The details of the chemistry involved have not been the subject of comprehensive investigation, and the historical level of knowledge has been largely empirically derived. It is widely believed that hydrolysis of the barium calcium aluminate (BCA) impregnant as a result of exposure to atmospheric water vapor is the critical chemical process. Studies to systematically determine the effects of exposure of the commonly used 4BaO . 1CaO . 1Al2O3 composition (4:1:1) have been undertaken. In addition the decomposition behavior of the reaction products has been characterized. A number of analytical tools, including thermogravimetric analysis (TGA), differential thermal analysis (DTA) and x-ray diffraction (XRD) have been applied to characterize the reactions occurring between the impregnant material and sea level ambient atmosphere and during heating of the impregnant material after atmospheric exposure. Substantial weight losses, over the temperature range from room temperature to 1050 C, have been observed for material exposed to room temperature (21-23 C) ambient atmospheres with dewpoints ranging from 5 C to saturation, and at atmospheric carbon dioxide vapor pressure, as well as restricted carbon dioxide exposure, for times ranging from one hour to a few hundred hours. Evidence of reaction with not only water vapor, but also with atmospheric carbon dioxide has been obtained. Substantial formation of barium hydroxide octahydrate is noted along with barium carbonate and calcium hydroxide. For materials exposed over extended time periods weight losses approaching 40 percent are observed indicating the extensive level of reaction which may occur.