US2408822A - Electrical discharge device - Google Patents

Description

Patented Oct. 8, 1946 ELECTRICAL DISCHARGE DEVICE Hubert E. Tania, Jr., Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application July 30, 1942, Serial No. 452,834

8 Claims. (Cl. 250-27-5) The present invention relates to electrical discharge devices. It is particularly concerned with, and has as its principal object, the treatment of the exterior surfaces of electrical discharge devices of the type desired to have an accurately predeterminable breakdown voltage, to prevent surface electrical leakage under varying and extreme atmospheric conditions.

- Other objects of my invention will become apparent from the following description of my invention taken in connection with the accompanying drawing in which Fig. 1 is a sectional view of a discharge device or tube of the type with which the present invention is concerned and Fig. 2 shows, in curve form, the results of tests to determine the amount of electrical leakage on the surfaces of untreated tubes and tubes treated in accordance with my invention.

It is well known that with many electrical discharge devices the presence of moisture, usually in the form of humidity in th atmosphere, may cause endless annoyance in the form of electrical leakage paths over the surfaces 01 insulating materials. In some instances, as where the device is desired to have an accurately predetermined and controlled breakdown voltage, success or failure of a project depends mainly on eliminating this source of trouble. Due to the fact that in many cases space is at a premium, one cannot always build suitable heated enclosures or evacuated chambers to dispel J the troublesome moisture because of the complications and expense involved.

A discharge device of this type is shown in Fig. 1. This device, which is more fully described and claimed in the copending application of Kenneth H. Kingdon and Elliott J. Lawton, B. N. 414,710, filed October 13, 1941, and assigned to the same assignee as the present invention, comprises a pair of glass cylinders I and 2 which are separated by a metal ring 3 sealed between them and which are closed at their respective extremities by means of transverse headers Land 5. The glass and metal parts are sealed together in vacuum-tight relation. The upper metal header 4, which forms the anode electrode, is provided centrally with a sealed-oi! metal tubulation 6 adapted to be used during fabrication of the tube for evacuating it and for introducing a suitable gaseous filling, such, for example, as argon or neon at a pressure of a few microns to a few millimeters of mercury. When sealed, the tubulation provides means for attaching a terminal '1 used to connect the anode l to a suitable po- 2 as a control grid. Control potential is applied to the mesh through terminal conductor 9 connected externally to the ring 3.

The header 5, in addition to. serving as a closure .member and base for the tube, further provides a support or composite cathode structure having a filamentary part It formed of an uncoated metal, such as tungsten, capable of being maintained at a temperature of effective thermionic emission without excessiv vaporization and an auxiliary part H to be described more fully at a later point. The filament is mounted between a bracket I 2 secured to header 5 and a relatively rigid support rod it which is insulatingly sealed through the header by means of a glass-to-metal seal indicated at ll. A terminal connection for ton application, the heating current is passed through the filament it by the application of potential between the support rod l3 and the header 5. As a consequence, the filament is maintained in emissive condition, although the resulting electron supply is relatively limited because of the small size of the wire. As long as the potential of the grid 8 is maintained below a predetermined level, no current is permitted to ilow to the anode 4. However, as soon as the grid potential exceeds this value, current flow begins, and ionization of the gas illling of the tube ensues. Some of the positive ions thus'produced in the discharge space are drawn to the negatively charged cathode surfaces, including' the insulated surfaces of the part II, and produce intense local gradients on the anodized surface. These gradients quickly become so great as to initiate cold cathode emission. This emission tends to increase rapidly in a cumulative fashion so that as soon as it is once initiated, an intense arc strikes almost immediately to the affected region of the cathode part II. The abundant electron emission thus realized completes the ionization of the discharge space and thus leads to complete breakdown of the tube.

The results obtained under the conditions specifled in the foregoing are extremely consistent and are reproducible with different tubes of similar design. This is a consequence of the fact 3 that initiation of the discharge depends almost entirely upon emission from the tungsten filament i and, since the characteristics of such filaments vary very little from sample to sample, one tube may be expected to perform in substantially the same manner as another.

However, when tubes ofthis type wherein the conductive parts are separated by externally exposed insulating members are. subjected to extreme atmospheric conditions of temperature and humidity, accurate operation of the tube is often impossible due to leakage paths resulting from the presence of imperceptible amounts of moisture on the surfaces of the insulating members. For example, for certain applications, the circumstances under which the tubes must function properly may be quite exacting. The temperature range over which the tubes are e pected to perform satisfactorily may spread from -60 C. to +50 C. with accompanying degrees of moisture in the atmosphere up to 100 per cent relative humidity. This means that some precautions must be taken to protect the tube against moisture. The moistureproofing means must of course meet the same requirements as the tube itself.

The present invention is based on my discovery that the effect of moisture on the operation of electrical discharge devices of the above-mew tioned type, i. e., surface electrical leakage, may be completely eliminated without the use of space-c0nsuming heated or evacuated enclosures or similar protective measures by treating the surface with organo-silicon derivatives which are in themselves water-repellent or which become so on contact with the atmosphere. I prefer to coat the entire outer surface of the device with the oily, resinous product obtained by hydrolyzing a methyldihalogenosilane of the formula CHaSiHX:

wherein X represents a halogen atom, preferably a. bromine or chlorine atom or a mixture of low boiling partially methylated and halogenated silanes consisting substantially of methyldihalogenosilane. The preparation of the hydrolysis product, which is a heat-hardenable, oily or resinous liquid, is more fully described in the copending application of Francis J. Norton, 8. N. 452,885, filed concurrently herewith, now Patent No. 2,386,259, issued October 9, 1945, and assigned to the same assignee as the present invention, and broadly covering the application of the oily hydrolysis product in waterproofing fibrous and similar water-non-repellent materials. An alternative treatment for tubes intended for use under humidity and temperature conditions normally encountered in temperature climates comprises treating the tubes with an organo-silicon halide in the vapor state, in a manner more fully described and claimed in the applications of Winton I. Patnode, S. N. 365,983, filed November 16, 1940 (now Patent 2,306,222), and S. N. 433,327, filed March 4, 1942, as a continuation-in-part of the earlier application. These applications, both of which were assigned to the same assignee as the present invention cover the treatment of non-water-repellent bodies with vapors or solutions of organo-silicon halides to render said bodies water-repellent. Illustrative examples of such organo-silicon halides are the alkyl silicon halides (e. g., ethyl, propyl, butyl, etc., silicon halides), the aryl silicon halides (e. g., phenyl silicon halides, etc.) arallwl silicon halides (e. g., phenylmethyl silicon halides, etc.), alkaryl silicon halides (e. g. tolyl silicon halides, etc.) and compounds such, for example, as (CH:):HSiCl and similar alkyl, aryl, etc., halosilanes, specifically chlorosiianes. Preferably the organo-silicon halide is a methyl silicon halide such as methyltrichlorosilane (methyl silicon trichloride), dimethyldichlorosilane (dimethyl silicon dichloride), or a mixture comprising one or both of these compounds. After application, the hill-- ides are believed to hydrolyze by reaction with moisture in the air or on the tube surface to form hydrolysis products of the same general type as those formed by use of a previously hydrolyzed methyldichlorosilane. Thicker or more corrosion-resistant films are secured by use of the coating comprising the hydrolysis products of methyldihalogenosilane.

Preferably the tubes for either vapor or resin treatment are initially cleaned by immersing them for 10 minutes in a concentrated alcoholic solution of potassium hydroxide. The tubes are then washed in running tap water for about 30 minutes with a final rinse in distilled water.

In treating tubes by the vapor process an effective leak-proof coating is obtained only when the moisture content of the atmosphere at the time of exposure to the vapors is in excess of about 30 per cent and then only when the tube has been previously washed with some hydrophilic liquid. such as ethyl alcohol, acetone, or the like. Apparently, the small amount of moisture that collects on the surface of a tube which has been treated with a hydrophilic liquid and thereafter vapor-treated in a humid atmosphere facilitates the hydrolysis of the organo-silicon halides to form a water-repellent coating capable of effectively preventing surface leakage. About a 15- second exposure of the tube to the vapor is sumcient to produce the desired coating When the resinous silicone hydrolysis product is used, it may be applied by any suitable means. For example, the tubes may be dipped into a solution of the hydrolysis product in an inert organic solvent such as toluene, carbon tetrachloride, benzene, ether, liquid aliphatic hydrocarbons, or the like. The'concentration of the coating solution may be as low as about 1 per cent. The upper limit of the concentration depends primarily on the desired viscosity of the solution. I have found that the best results, both in so far as the thickness and the efiective moisture-resistance of the resultant film are concerned, are obtained by using coating solutions containing from 20 to 40, preferably about 35 per cent, of the silicone resin. The coated tubes are airdried for a short time to evaporate most of the solvent and then baked at an elevated temperature to harden the coating. In most cases a 30- minute bake at 130 C. is sufficient.

Tubes coated with the hydrolysis product have operated perfectly when stored in an atmosphere having a per cent relative humidity at 50 C. for 48 hours and thereafter tested at 100 per cent relative humidity both at 55" C. and at room temperature. Tubes have also been cooled to 50 C. and frosted by suspending the cooled tube in a 100 per cent relative humidity chamber at room temperature. The frosted tubes were immediately tested for surface leakage by intermittently recording the grid control voltage as the temperature of the tubes increased through the frost melting phase and up to room temperature. The tubes showed'no signs of surface leakage. The tubes operated under perfect grid control even at the time that the frost melted to form amyriad ofsmalldropletsevenlydistributed over the tube surface. During this period the surface of the tube appeared to the unaided eye to be completely frosted. However, observation'under a glass revealed the fact that each tiny droplet was actually completely isolated one from another, thus preventing the formation of any continuous leakage path. Untreated tubes will not operate under satisfactory grid control at a relative humidity above 50 per cent at room temperature.

The very thin, tightly adherent, water-repellent surface illm formed when a preconditioned tube is brought into contact with the vapors of an organo-silicon halide, preferably a methyl silicon chloride, or mixtures of methyl silicon chlorides, will effectively prevent electrical leakage under ordinary atmospheric conditions. For

example, a tube which had been treated with the vapor of a mixture of methyl silicon chlorides consisting chiefly of methyltrichlorosilane and dimethyldichlorosilane operated satisfactorily when suspended at room temperature in 100 per cent relative humidity for 19 hours. Another tube treated with vapors of the same mixture of methyl silicon chlorides continued to function properly and normally for over 16 hours while suspended in an atmosphere having 100 per cent relative humidity at 50 C. However, if the vapor-treated tubes are subjected to such extreme conditions for several days, the metal parts corrode and electrical leakage develops.

The results of surface leakage tests on treated and untreated tubes are shown in Fig. 2 of the ac ompanyin drawing. The curves were obtained by measuring the resistance in megohms of a V4 inch path on the glass surface of the tube under an applied potential of 500 volts D. C. and plotting these values against time of exposure of the tube to various temperature and humidity conditions.

The apparatus used for these tests was not capable of measuring resistances greater than 55,000 megohms which therefore represents themaximum resistance value that could be recorded and plotted.

Tube A, which was untreated, was cooled to 80' C. while suspended in an atmosphere having a 100 per cent relative humidity, and immediately tested at room temperature. After about 1 minute the frost melted. The resistance dropped to 0.4 megohm within the next twenty seconds. Tube B was also an untreated tube. It was tested at room temperature in an atmosphere having a 100 per cent relative humidity. Within 10 minutes the resistance dropped to about 210 megohms. The maximum recordable resistance values were obtained under similar conditions with tubes and D which were coated with the resinous hydrolyzed methyldichlorosilane. Tube C was tested under the same conditions as tube A. Tube D was tested under the same conditions as tube B except that the temperature was varied from room temperature to 55' C. during the test. These tests on treated tubes have been run for as long as 96 hours. At the end of this time. no measurable decrease in the resistance has been noted. I

The organo-silicon coating materials I employ in preparing the leak-proof coatings appear to be unique in their property of eifectively protecting the tube over a wide range of temperatures and under varying conditions of humidity. Well .known waterproofing materials such as phenolaldehyde condensation products, asphaltic compounds, P lystyrene, alkyd resins, and various waxes, such as parailin, have been tried but have been found in general to be ineffective or at the best only partially effective, in preventing corrosion and electrical leakage under normal or extreme atmosphere conditions.

Although I have described my invention in its application to a three-element discharge device, it is to be understood that it is equally applicable to any discharge device comprising two or more elements separated by an insulating member a surface of which is exposed to the atmosphere.

whatlclaimasnewanddesiretosecurehy Letters Patent of the United States is:

Llnadischargedeviceofatypedesiredto have a predeterminable breakdown voltage, the combination which comprises conductive parts separated by externally exposed insulating members and a water-repellent coating on the externally exposed insulating members, said coating comprising a hydrolyzed organo-silicon halide.

2. In a discharge device of a type desired to have a predeterminable breakdown voltage, the combination which comprises conductive parts separated by externally exposed insulating members and a water-repellent coating on the externally exposed insulating members, said coating comprising a hydrolyzed methyl silicon halide.

3. In a discharge device of a type desired to have a predeterminable breakdown voltage, the combination which comprises conductive parts separated by externally exposed insulating members and a water-repellent coating on the externally exposed insulating members, said coating comprising the heat-hardened hydrolysis product of a mixture of low boiling methylhalogenosilanes consisting essentially of methyldihalogenosilane.

4. In a discharge device of a type desired to have a predeterminable breakdown voltage, the combination -which com-prises conductive parts separated by externally exposed insulating mem- 45 bers and a water-repellent coating on the externally exposed surfaces of said device, said coating comprising a baked film of the product of hydrolysis of a methyldihalogenosilane.

5. A discharge device comprising conductive parts separated by an insulating member, the

externally exposed surface portions of which have been rendered water-repellent by treatment with vapors or an organo-silicon halide.

6. In a discharge device of the type desired to 55 have a predetermined breakdown voltage, the

combination which comprises conductive parts separated by an insulating member having a portion of its surface exposed to the atmosphere and a coating on at least the exposed surfaces of said insulating member, said coating consisting of the product of hydrolysis of methyldichlorosilane.

'7. A discharge device comprising at least two conductive parts separated by an insulating member having an externally exposed surface 5 portion coated with a water-repellent film consisting of the cured resinous product of hydrolysis of methyldichlorosilane.

8. A discharge device comprising at least two conductive parts separated by an insulating member having a water-repellent, externally exposed surface portion obtained by treating the externally exposed surface portion of said member with a methyl-silicon halide in vapor form.

HUBERT n. TANIS, Jx.

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