Studies in the Ba-Ca-Al-O System Chemical Modification of Aluminum Alkoxide by Propionic Acid
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STUDIES IN THE Ba-Ca-AI-O SYSTEM CHEMICAL MODIFICATION OF ALUMINUM ALKOXIDE BY PROPIONIC ACID V. HEBERT*, S. VILMINOT* and D. BRION * I.P.C.M.S., Groupe des Mat6rlaux Inorganlques, EHICS, 1 rue Blalse Pascal, 67008 Strasbourg Cedex, France 1*Thomson Tubes Electroniques, 2 rue Lat6cobre, Velizy-Villacoublay, France
ABSTRACT Barium and calcium aluminate powders have been elaborated using chemical methods like coprecipitation, alkoxide polymerization and metalloorganic decomposition. These powders have been successfully used for impregnation of porous tungsten cathodes for use in electronic tubes. Spectroscopic studies on the modification of aluminum s-butoxide reveal that carboxylic acid substitutes OR groups forming preferentially bidentate chelating complexes.
INTRODUCTION Dispenser cathodes for use in electronic tubes are currently elaborated by immersion In a melted oxide mixture of a porous tungsten element. Tungsten powder, with a mean grain size around 4 um, is isostatically pressed into bars and subsequently sintered at 2200 °C under hydrogen atmosphere. In these conditions, the final porosity equals 18 %.The impregnated cathods are obtained by partial immersion in a melting of barium and calcium aluminate, the liquid entering the porosity by capillarity. The cathods are then chemically cleaned, to completely remove the surface aluminates, dried at 100lC and conserved in a nitrogen atmosphere in sealed pockets before use. Different aluminates have been checked and typical compositions are referred as 4.1.1. (4BaO lCaO - 1 Al O3 ) and 5.3.2. (5BaO - 3CaO - 2AI 2 03). Barium is the emitting element, aluminum being introduced in order to limit Ba evaporation and calcium to lower the extraction energy. The aluminates are currently prepared by solid state reaction from a mixture of BaCO 3, CaCO 3 and AI2 03 in stoichiometric proportions. One of the main problems encountered with the cathods elaborated from these powders is that the electronic emission stops before all the barium available inside the pores has been consumed. This observation leads to the possible existence of heterogeneities within the impregnant powder. Chemical routes, like oxalate coprecipitation, alkoxide polymerization and metalloorganic decomposition, have therefore been developed in order to improve the homogeneity, to obtain a more reactive powder and to lower the melting point. In a second part, we have considered the modification of aluminum alkoxide by propionic acid in order to get more insight into the elaboration process from metalloorganic decomposition.
POWDER ELABORATION Three different routes have been used for the elaboration of barium and calcium aluminates: oxalate coprecipitation, alkoxide polymerisation and metalloorganic decomposition.
Oxalate coorecioitation Whereas calcium oxalate is nearly insoluble in water, the barium compound is slightly soluble in acidic solutions. Addition of oxalic acid in an aluminum salt solution does not yield precipitation. It therefore appears necessary to adjust the pH value in order compl
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