Patent GB-289509-A (GB)
Title: Improvements in and relating to electron-emitting cathodes, and in the methods of manufacture thereof
IPC: H01J9/06
Filed: 1927-01-24
Abstract: 289,509. Sinclair, J. R. Jan. 24, 1927. Vacuum tubes.-A filament is coated with electron-emitting material by the combined action of cataphoresis and electrolysis obtained by immersing the filament in a bath containing the coating material in suspension or in a colloidal state, and connecting the filament to the positive and the bath to the negative terminal of a source of supply, the filament being moved continuously. The bath may contain the coating material in suspension or in a colloidal state in a fluid to which is added a solution of an alkaline compound of one or more of the coating materials; for example barium peroxide suspended in alcohol to which a 1 per cent solution of barium hydroxide in water or tetrachlorethane is added. The function of the electrolyte is to split up the alkali. Barium oxide and hydroxide in suspension in the proportion of 98 :2 by weight, or barium and strontium oxides and barium hydroxide suspended in acetone in the proportions by weight of 60:30:10 may be used. Alternatively the materials may be deposited in a colloidal state. For this purpose barium peroxide is ground in a ball mill in the presence of a cold saturated solution of barium hydroxide in water. The powder is digested with dilute barium hydroxide solution to which casein is added, and the density of the solution increased until the casein is peptized. A mixture of peroxides of barium, strontium and calcium in the proportions of 2 : 1 : 0.25 by weight is suitable. The coating is subsequently reduced to monoxide by heating to 900' C. in a vacuum. Oxides of the rare earth metals may be deposited by the same process. For example thoria and ceria in the proportion of 3 : 1 by weight are ground in a ball mill in the presence of a cold saturated solution of barium hydroxide in water, and the powder is digested with barium hydroxide solution having a strength of not less than 30 grams per litre of water. After an hour the liquid is decanted and the powder again ground. A second digestion with the decanted liquid is made and casein added, and the density increased until the casein is peptized. Several oxides may be deposited from a single bath, or bath may be arranged in series, the oxides being deposited in succession. In the apparatus shown in Fig. 1, a filament R is wound from a reel D, passed under porcelain rollers B in a metal bath A, over a roller E, under a grooved roller F, and on to a motordriven reel K. The rollers D, E are connected through brushes with the positive terminal, and the bath with the negative terminal of the generator C. A protective coating is applied to the coated filament by causing a solution of celluloid in amylacetate and acetone to drip from a tank G either by gravity or air pressure on to the roller F, the coating being dried by a jet h of hot air. In the apparatus shown in Fig. 2, a detachable metal liner b<3> is mounted in a porcelain container A<1>. The spindles of the rollers B pass through slots in the liner, and are mounted in slots in the container A<1> The liquid is circulated by a fan P. A jacket a<3> has baffles a<8>, a<9> to direct the flow of cooling water around the container.