US 1927812 A
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Description (OCR text may contain errors)
Sept. 19, I933. H, H S I Q 1,927,812
PHOTO ELECTRI C TUBE Filed Nov. 27. 1931 Inventor: Hugh E.Thomson,
Patented Sept. 19, 1933 UNITED STATES PHOTO-ELECTRIC TUBE nugh E. Thomson, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application November 27, 1931 Serial No. 577,423
The present invention relates to photo-electric devices and in particular to a novel method of treating the light-sensitive surface.
An object of the present invention is to improve the sensitivity of photo-electric tubes. This object is attained, in accordance with my invention, by subjecting the light-sensitive surface to a glow discharge in the presence of an inert gas.
The invention will be more fully understood when reference is made to the following description and the acompanying drawing which illustrates two types of photo-electric tubes now in common use. Fig. 1 shows a photo-electric tube having a' cathode of double-plate construction r while in \Fig. 2, the cathode takes the form of a single semi-cylindrical member. Both of these types employ cathodes physically separate from the envelope but it is to be understood that my invention applies equally well to photo-electric 20 tubes in which the light-sensitive surface is formed'as a coating on the interior of the envelope.
Referring to Fig. 1,-nu neral 1 designates an evacuated envelope which preferably is filled with inert gas, for example argon. The envelope terminates in a reentrant stem 2 from which the electrodes are supported in the usual manner. For reasons which will appear presently, the envelope is made preferably of lime glass and the stem 2 preferably of lead-glass. The bottom of the envelope is sealed in a standard base 3. The envelope contains a cathode consisting of two plate members 4 disposed at an angle to one another and coated with a light-sensitive substance in a manner described hereinafter. The cathode may be made of any suitable metal but nickel and copper are preferred, since these metals lend themselves to being silver-plated, as will be exu plained later, and also to being formed as comparatively thin and rugged plates. The envelope also contains an anode which consists of an upright rod or wire 5, preferably of nickel, and equidistantly arranged between the plate members 4. The electrodes are supported invany suitable manner from the stem 2 and connections made to terminals 6 of which four are, shown to conform to a standard radio tube socket, but only two actually are used for contact purposes. Resting on top of the rod-like anode 5 and out of contact with the cathode, there is positioned a capsule 7 which contains a compound capable of evolving light- I sensitive material upon being heated. The photo-electrictube shown in Fig.2 is similar to that illustrated in Fig. 1 except that the cathode 4 takes the form of a semi-cylinder arranged concentrically with respect to the rod-like anode.
The first step after forming the cathode into the desired shape is to coat the same with silver, and this may be conveniently accomplished by immersing the copper member in an electrolytic bath containing silver or by evaporation in the well understood manner. Both sides of the copper member are coated by this process but it will be understood that only the inner side of the cathode is normally activated by light and for this reason only this side need have the silver plating. The silver-plated member is then introduced into the envelope together with the cooperating rod electrode with the capsule at tached.
The envelope may now be placed on the pump, exhausted and at the same time, an oven is lowered over the envelope and heated to approximately 350 C. for a "bake-out". When it is determined that all of the deleterious gases such as water vapor and the like have been removed, the tube is allowed to cool to room temperature at which time the pump connection is closed and oxygen in small doses admitted 'until a pressure of about 250 to 300 microns Hg is reached.
The silver-plated member 4 is then oxidized While the oxidation step may be performed by merely heating the tube in an oven to a relatively high temperature for a period depending upon 55 the degree of oxidation required, I prefer to perform this step by an electrical method as disclosed and claimed in the Charlton application Serial No. 578,323, filed Dec. 1 1931, and entitled Photoelectric tubes.' 90
According to the Charlton process, the lightsensitive cathode is immersed in a glow discharge produced by applying a relatively high voltage between the rod 5 and the plate member 4. The voltage may be obtained from any suitable source 'su'ch as a high frequency induction spark coil or a high voltage battery but preferably from a high voltage rectifier (not shown) energized by low voltagealternating current. As we stated is probably obtained by an increased depth of oxide layer caused by the electrical treatment.
When the desired degree of oxidation has been reached which, in the case of the electrical method, may be controlled conveniently and ace curately by simply regulating the applied voltage and the time during which the voltage is applied, the excess oxygen is pumped out and a highly light-sensitive material introduced into the envelope. This material may consist of caesium, rubidium or other reactive metal and is contained in the capsule 7 which is shown as mounted at the upper end of the rod anode and which may be supported at any other part of the electrode of the electrode structure and envelope.
. change in color.
structure. Pellets of dichromate of caesium and a suitable reducing agent, such as silicon, advantageously are used for this purpose.
The capsule may be flashed by means of current induced by a high frequency coil, whereupon the alkali metal is released and condenses on the silver-plated member 4 as well as other portions The tube is then heated, preferably by lowering the oven, for a short time, in order to revaporize the caesium which has condensed on the envelope wall and to cause the same tore-condense on the The length of time of the second bakeout depends not only upon the amount of silver oxide present on the member 4, and on the amount of free caesium released at the capsule, but also on the bake-out temperature. A temperature of about 275 C. has been found to be satisfactory for this purpose in which case the bake-out may extend for about 3 to 8 minutes.
During this time and for a short while thereafter, the envelope is pumped until as much of the excess alkali metal as is possible, has been removed from the envelope. Some of the excess alkali metal will combine chemically with the lead glass stem and thus be permanently removed from the light-sensitive surface and from any other part of the electrode or envelope structure. The purpose of removing the excess alkali is to endow the tube with uniformity of operation, as is stated in the Bainbridge Patent No. 1,901,577,
granted March 14, 1933, in which claims to this feature have been made. Following this treatment, argon or other inert gas at a pressure of about 125 microns is admitted and theenvelope sealed off.
In accordance with my invention, I propose further to sensitize the light-sensitive surface by subjecting the tube to a second glow discharge andfor this purpose, a direct current voltage of about 400 to 500 may be used with the anode 5 positive as in the prior glowing operation. While I prefer direct current for this additional sensitization treatment, it will be understood that alternating current derived from any suitable source may also be employed. The. time during which the glow discharge is produced is best determined by experiment and, in general, it will be found that,.in the case of caesium, tubes, the envelope as well as the glow discharge undergoes a. marked Whereas the color initially is pinkish, it later becomes purplish or blue. The glow discharge is also found to increase in intensity during this period. When one or more of these indications is reached, the treatment ordinarily is stopped. The time of treatment usually extends for about 3-10 seconds and after having been determined by operating tubes treated in this manner under practical conditions, the treatment will be found to be reproducible in so far as specific sensitivity of the cathode is concerned. It has been found that the sensitivity of gasfilled tubes is greatly enhanced by the second glow treatment. I do not desire to be limited to any particular theory but believe that the addiional glow treatment causes a re-arrangement of the cathode surface, i. e., of the various particles comprising this surface, in such a way as to cause the surface to respond more readily to light. The precise manner in which this re-arrangement is effected is not completely understood at this time but it is a fact that tubes in which the lightsensitive cathode has been subjected to a glow dischargein argon or other inert gas, is several times more sensitive as measured in microamperes per lumen than gas-filled tubes containing inert gas in which the glow treatment has not been given.
After this treatment, the argon may be left within the envelope so that amplification by reason of the presence of gas may be obtained in the usual manner, in addition to the added sensitivity afforded by the glow treatment. If the requirements of the gas tube are such that only a relatively low pressure of argon is required within the envelope, the glow treatment can still be made but the voltage in this case may be increased. If desired, the argon may be removed entirely after the glow treatment and the photoelectric tube sealed as a high vacuum device.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. In the art of manufacturing photo-electric tubes which includes the steps of coating a foundation member with material which is readily oxidized, oxidizing said material and depositing thereon a layer of light-sensitive material, the further steps of introducing an inert gas into the tube and immersing the foundation member as an electrode in a glow discharge whereby the lightsensitive surface is further sensitized.
2. In the art. of manufacturing photo-electric tubes containing a light-sensitive cathode and an anode, said art including the steps of oxidizing the light-sensitive cathode, and depositing thereon a layer of light-sensitive material, the further steps of introducing an inert gas into the tube and applying a relatively high voltage between the cathode and anode, said voltage being sufficient to cause a rearrangement of the cathode surface to provide additional sensitization.
3. In the art of manufacturing photo-electric tubes containing a light-sensitive cathode and an anode, said art including the steps of oxidizing the cathode, and depositing thereon a layer of alkali material, the further steps of introducing argon into the tube and applying direct current voltage between the cathode and anode with the positive terminal of the voltage supply connected to the anode and the negative terminal to the cathode, said voltage being sufficient tocause a glow discharge between .the electrodes whereby the sensitivity of the cathode surface is enhanced.
' HUGH E. THOMSON.