US1988525A - Photo-electric tube - Google Patents

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Jan. 22, 1935. w. F. TEDHAM ET AL PHOTO ELECTRIC TUBE Filed Nov. 20, 1931 INVENTORSI T Vi'lZiamFrancis Tedham, Geoffrgybef'nard Daren EBQMM Q A THE/R ATTORNEY.

Patented Jan. 22,. 1935 UNITED STATES PATENT OFFICE PHOTO ELECTRIC TUBE Britain Application November 20, 1931, Serial No. 576,242 In Great Britain November 21, 1930 1 Claim. (Cl. 250-275) The present invention relates to photoelectric tubes and particularly to cathodes or light-sensitive electrodes therefor of the type comprising an alkali metal deposited on the oxidized surface of a metal support or backing. This metal sup-.

port or backing may be deposited on the inner surface of the envelope or glass bulb'of the tube, or on a support within the tube.

It has been proposed, in the preparation of cathodes of this type, to oxidize the surface of the metal backing onto which the alkali metal is deposited, by exposing said surface to oxygen at a low pressure, such as lto 2 mm. of mercury, and heating in an exhaust oven to a temperature of 360 C. It has also been proposed to effect this oxidation by glowing the bulb or envelope within which the metallic backing and oxygen are contained by applying a high frequency current thereto from a spark coil. The discharge produced by the spark coil, however, is of an intermittent and irregular character and results in uneven oxidation of the metal surface.

The object of the present invention is to provide an improvedprocess for the oxidation of the metal surface whereby a uniformly oxidized surface is obtained, which lends itself to the production of an eflicient and highly sensitive cathode.

In accordance with the present invention, the surface of the metallic backing of the cathode of a photoelectric tube is oxidized prior to the deposition of the light-sensitive material thereon by applying a continuous wave, high frequency discharge thereto in the presence of a low pressure of oxygen. The continuous wave, high frequency discharge may be produced by a coil connected across the metal backing and an electrode within the envelope, for example, the anode of the tube, and coupled to a high frequency valve oscillator giving a constant voltage output. The voltage required is low, varying from 850 to 1,500 volts R. M. S., in contradistinction to the high voltage employed with the spark coil discharge.

In the preferred form of the invention, the metallic backing consists of silver, and it has been found that in this instance the most satisfactory result is obtained if, for an area of approximately tube prepared in accordance with the present invention, part of the envelope thereof being broken away; and

Fig. 2 shows diagrammatically a circuit arrangement for applying a continuous wave, high frequency discharge to the metal backing or support of the cathode of a. photoelectric tube, in accordance with the present invention.

Referring to Fig. 1, a bulb 1, consisting of a cylindrical portion 2 and a spherical portion 3 with an area available for the deposition of the metallic backing of about 80 sq. cm., is used. A silver bead (not shown) is placed in a tungsten spiral 4 welded between the ends of two leads 5 and 6 which are carried by a stem 7 sealed into the cylindrical portion 2 of the bulb 1 in such a manner that the spiral 4 is contained within the spherical portion 3 of the bulb 1., The two leads 5 and 6 also carry two circular discs 8 and 9. The disc 9 is made of metal, and projects from the end of the supporting lead 5 so as to screen a part of the spherical portion of the bulb from the silver bead. The other disc.8 is made of insulating material, and is mounted across the leads 5 and 6 between the stem 7 and the silver bead.

The bulb 1 is connected to a pump, evacuated and baked in a vacuum for 30 minutes at 500 C. and, following this, f,or 10 minutes at 350? C. After the bulb has cooled down, the tungsten spiral 4 is caused to glow by passing a direct current therethrough and the silver bead thus vaporized. In this manner, a coating of silver is deposited on the inner surface of the bulb 1. The disc 8 prevents the deposition of silver on the stem 7, and the disc 9 prevents deposition on a portion 10 of the spherical part 3 of the bulb l, a window being thus formed in the known manner. The wire 11, which extends through the portion 3 of the bulb, is in connection with the silver coating, and serves as the cathode lead.

Oxygen is now admitted to the bulb to a pressure of 0.20 mm. of mercury and absorbed in the silver layer until the pressure falls to 0.14 mm., the closed volume connected with and including the bulb being 1000 cc., and the temperature at which the pressure is measured being about 20 C. -The absorption of the oxygen or oxidation of the silver coating is effected by passing a glow discharge at about 1000 kilocycles between the metal disc 9 and the spiral 4 as one electrode, and the silver coating as the other electrode. The discharge is produced by a coil 12 connected across these two electrodes, as shown in Figure 2, the coil 12 being coupled to a suitable high frequency valve oscillator which gives a constant voltage output of the order of 1000 volts R. M. S. High frequency oscillations are produced across the tuned oscillating circuit consisting of a coil 13 and a variable condenser 14 connected as shown in the plate circuit of a thermionic valve 15. The anode of the 'valve 15 is connected through a high frequency choke'l6 and a lamp 1'7 to a source of potential of the order of 1000 volts. The coil 12 is inductively coupled with the coil 13. One end of the coil 12 is connected to the lead 5 and the other end may be connected to the lead 11 or to a metal ring close to or touching the outer surface of the part of the bulb 1 which is coated on the inside with silver.

A uniform glow is thereby produced in the bulb 1, the oxidation process being maintained for about 30 seconds. In thismanner, a uniform layer of silver oxide of a deep gold color is obtained.

The excess oxygen is pumped out, and caesium distilled in from a side tube connected to the bulb and containing a mixture of calcium turnings and caesium bromide powder. Alternatively, the side tube may contain caesium metal in a sealed glass capsule. The top of the latter is made extremely thin, and can be broken by the dropping of a metal ball thereon, the action of the ball being controlled externally by a magnet. The capsule and ball are placed in the side tube prior to the latter being sealed onto the bulb. After a certain amount. of caesium has been in- 1,oss,52s I troduced into the bulb, the latter is baked at C. to C. for three minutes. This process of introducing caesium and baking at 155 C. to 165 C. is repeated, and after each baking, the tube is cooled and the sensitivity measured. When the sensitivity has risen to 15 microamperes per lumen from a gas filled lamp, no further caesium is added, but the baking process is continued for about ten minutes, causing the sensitivity to rise to about 30 microamperes per lumen. Continuation of the baking process may cause the sensitivity to increase still more. The sensitivity of the tube may be measured with a gas-filled lamp as a light source, the temperature of the filament being 2700" K. The tube may now be sealed oil, or a filling of inert gas, such as xenon or krypton, supplied and the tube then sealed off.

We claim as our invention:

The method of oxidizing the surface of a silver electrode which comprises exposing the electrode to oxygen at a pressure of substantially .20 millimeters of mercury, measured at 20 degrees centigrade, the ratio of the total volume of oxygen, measured in cubic centimeters, to the surface area of the electrode, measured in square centimeters, being of the order of subjecting the electrode to the action'of a glow discharge and maintaining the discharge until the pressure of the oxygen has fallen to substantially .14 millimeters of mercury.

FRANCIS TEDHAM. GEOFFREY BERNARD BAKER.

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