US3900755A

US3900755A - Arc suppressing coating for metal-dielectric interface surfaces

Info

Publication number: US3900755A
Application number: US266003A
Authority: US
Inventors: Lawrence H Tisdale; Leonard Lesensky
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1972-06-26
Filing date: 1972-06-26
Publication date: 1975-08-19
Anticipated expiration: 1992-08-19

Abstract

Adjacent surfaces of contacting electrically conductive and nonconductive bodies are coated with a relatively thick film of an oxide of a dielectric material to substantially reduce high electric fields generated in the presence of DC or RF energy. Stable nonconducting oxides of such materials as alumina (Al2O3), silicon and high-k titanites are deposited on adjacent surfaces in the interface region where the high fields associated with electrical energy at high power levels tend to initiate damaging arcs. Applicable embodiments include electron discharge devices operating in the microwave energy spectrum as well as high voltage electrical equipment such as insulators subject to generation of substantial electrostatic fields.

Description

United States Patent Tisdale et al.

[451 Aug. 19, 1975 [54] ARC SUPPRESSING CQATING FOR 1448,4[3 6/l969 Preist .r 3l3/IU6 METAL D|ELECTR|C INTERFACE 1671987 6/l972 OKeeffe l l7/2l2 SURFACES OTHER PUBLlCATlONS [75] Inventors: Lawrence H. Tisdale, Wakefield; POWeIL VEPOY A ID v Wil y & S0nS- 1966.

Leonard Lesensky, Lexington, both pof Mass.

Prinmrv liraminer-Michael F. Es osito [73] Asmgnee: Rythflm Company Lexington Attorney, Age!!! or FirmEdgar Rost; Joseph D.

Mass Pannone; Milton D. Bartlett [22] Filed: June 26, I972 21 1 Appl. No.: 266,003 [57] ABSTRACT Adjacent surfaces of contacting electrically conductive and nonconductive bodies are coated with a rela- [52] 313/212; 1 17/2] 7/215; tively thick film of an oxide of a dielectric material to 117/230; 333/98 R substantially reduce high electric fields generated in 15] 1 "3 Cl Hmp 7/06 the presence of DC or RF energy. Stable nonconduct- [58] held of Search 313/106 107; 333/98 R; ing oxides of such materials as alumina (Al- 0 sili- 315/5. 39.53; ll7/230. 2l2. 1 con and high-k titanites are deposited on adjacent surfaces in the interface region where the high fields as- I56] References cued sociated with electrical energy at high power levels UNITED STATES PATENTS tend to initiate damaging arcs. Applicable embodi- 1,926,846 9/l933 Giard ll7/23U ments include electron discharge devices operating in 2.21 .55 9/l940 Veenemans 3 l 3/107 the microwave energy spectrum as well as high voltage 31147441 H953 B fill/I64 electrical equipment such as insulators subject to gen- 295HJ 04 lI/l9 Badger 315/539 eration of substantial electrostatic fields.

3.274025 9/1966 Ostis l l7/2 l 7 3345535 lO/l967 Johnson .l 331/98 3 Claims. 4 Drawing Figures ARC SUPPRESSING COATING FOR METAL-DIELECTRIC INTERFACE SURFACES BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to electron discharge devices and means for suppressing electrical arcs particularly in the interface region of electrically conductive and nonconductive surfaces exposed to vacuum.

2. Descriptiomof the Prior Art Bodies of dielectric material selected from the group including alumina and beryllia have been utilized in the microwave art for the transmission of electromagnetic energy and to provide vacuum seals in high power electron discharge devices due to the high thermal shock handling capabilities. Magnetrons. klystrons, crossed field amplifiers. as well as oscillators and traveling wave tubes. are exemplary electron discharge devices employing such dielectric bodies. Typically. the dielectric body is metallized around the peripheral edge and is then brazed in recessed sockets or to flat plates of electrically conductive materials. such as oxygen-free copper. The surfaces of the dielectric bodies exposed to the vacuum are subject to damaging electrical arcs due to the build up of the electrical fields in the presence of electromagnetic energy at high power levels handled by the applicable devices. A limitation. therefore. has retarded the advancement of the state-of-the-art due to destructive heating leading to punctures or fractures of the dielectric bodies with resultant destruction of the vacuum condition. High power window failures in electron discharge devices have been described in detail in an article entitled. Some High-Power Window Failures" by .I. R. M. Vaughan. IRE Transactions on Electron Deviccs. July l96l pps. 302-308. Particularly, the high voltages associated with electrostatic charging on the dielectric surfaces in the presence of RF or DC fields were observed to contribute to the failures. The arcs also tend to initiate in the region of juxtaposed conductive and nonconductive bodies.

Attempts to solve the foregoing problem include the deposition of a metallic-type conductive material such as titanium on both the dielectric window members as well as adjacent metallic supporting walls to provide a coefficient of secondary electron emission of approximately unity on the applicable surfaces. Examples of such prior art efforts may be found in U.S. Pat. No. 3.448.4l 3. issued June 3. I969. to D. H. Preist et al. A conflict exists. however. in that such prior art coatings result in very high electrical resistivity which can advcrsely affect the electrical transmission characteristics of the windows and walls. Electrical arcs leading to eatastrophic failure of evacuated electron discharge devices or other equipment. therefore. remain a serious problem requiring solution.

SUMMARY OF THE INVENTION In accordance with the present invention contacting electrically conductive and nonconductive bodies exposed to electromagnetic energy are coated with nonconducting oxide materials having a high dielectric constant to suppress the high electrical fields capable of initiating damaging arcs. Selected oxides are principally of such materials as alumina. silicon or high-k tita nates which remain stable after bake-out temperature conditions utilized in the evacuating of the applicable devices such as. for example. 600C. The term non conducting" is defined for the purposes of the present invention as a material that conducts electricity only in a very small degree. The term semiconducting as used in the specification refers to a material whose electrical conductivity is between that of a conductor and that of an insulator. The term microwave is defined as that portion of the electromagnetic energy spectrum having wavelengths in the order of 1 meter to l millimeter and frequencies in excess of 300 MHz.

The coating for suppression of electrical ares may be deposited on the electrically conductive and nonconductive surfaces with appropriate masking by any of the following techniques:

a. chemical vapor deposition;

b. evaporation of the oxide source in presence of a low pressure oxygen;

c. evaporation of the oxide in high vacuum;

d. sputtering of the metal followed by controlled oxi dation of the coating;

e. RF or DC sputtering from a target composed of the desired oxide.

Coating thicknesses averaging between 1,000 and 2,000 Angstrom units of the selected oxides have been found to substantially suppress electrical arcing in the interface area by reducing build up of the electrostatic on fields the vacuum side in the presence of RF or DC electrical energy. The high dielectric constant value and high dielectric strength of the nonconducting oxide coatings in the critical interface areas permit operation at higher power levels and prolong the operational life of the devices.

A copending patent application relates to electron multipactor suppression utilizing semiconducting oxides of silicon, copper. cobalt. chromium, iron, manganese or nickel on dielectric substrates. Ser. No. 266.020. now U.S. Pat. No. 3.829.985 filed by Lawrence H. Tisdale and is assigned to the assignee of the present invention. This teaching provides an alternative embodiment of the present invention in that a twolayer structure can now be provided on dielectric bodies with adjacent metallic surfaces with the first nonconducting oxide layer providing for suppression of electrical arcs and superimposed semiconducting oxide coating to effectively suppress electron multipactoring.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention will be readily understood after consideration of the following description of a preferred embodiment and reference to the accompanying drawings. wherein:

FIG. I is a cross-sectional view of a dielectric window assembly for high power microwave devices with the view taken along the line l-l in FIG. 2;

FIG. 2 is an isometric view of a high power crossed field amplifier embodying the invention;

FIG. 3 is a fragmentary view. partially in cross section and partly in elevation. of a helix slow wave support structure for high power traveling wave devices; and

FIG. 4 is a fragmentary cross-sectional view of an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings in particular FIGS. 1 and 2 a crossed field amplifier I0 is shown of the type utilized in pulsed type operation to provide up to 3 megawatts peak power when driven by an RF signal of approximately 500 kilowatts. The average power output for such devices is typically kilowatts or higher. The device shown is of the externally mounted magnet type with the

U-shaped members

12 and 14. An evacuated metallic envelope 16 houses the internal components including an anode structure and a cold cathode. A cathode support and external lead assembly 18 provides for the application of the anode-cathode electrical voltages in the range of from 40 to 50 kilovolts. The electromagnetic energy is coupled into and from the anode structure, for example, a slow wave delay line, by input and output rectangular access waveguides l9 and 20 sealed at their outer ends by energy

permeable window assemblies

22 and 24. The device is provided with forced fluid cooling coupled through

conduits

26 and 28 in each of the window assemblies to handle the very high RF energy.

Window assembly 24 is shown in greater detail in FIG. 1. Conventionally, dielectric materials with an electromagnetic energy permeability and a high thermal shock capability are selected from the group including alumina, beryllia or boron nitride. The window materials are conventionally of a circular configuration and such a dielectric body 30 is shown sealed within a circular metallic waveguide support body 32 by means of known metallizing and brazing techniques. These techniques include the provision ofa metallic interface coating along the peripheral edge of the window member 30. A solder ring is then mounted in the contact area and after brazing

forms fillets

33, 35 joining the two bodies. Such fillets on the vacuum side of the assembly initiate the damaging electrical arcs in the presence of the intense electromagnetic energy fields.

Hollow passageway 34 provides for the forced circulation of a cooling medium introduced through

tubular adaptors

36 and 38 with threaded

portions

40 and 42 to engage

conduits

26 and 28. The waveguide body 32 is provided at one end with a flange 44 for securing the assembly to the rectangular access waveguides or the flange may be affixed directly to a tube envelope to enclose an access opening. The opposing end of the waveguide body is provided with a thicker waveguide mounting flange 46 of the type conventionally used in transmission systems for coupling the electromagnetic energy from power supplies and utilization loads.

In accordance with the invention a relatively thick coating 48 is deposited on the adjacent electrically conductive and nonconductivc surfaces in the interface region where dielectric body 30 joins the waveguide walls 32. Coatings having a thickness in the range of about 1,000 to 2,000 Angstrom units of a nonconducting oxide material will effectively suppress arcing. A nonconducting oxide material selected from the group including silicon, alumina and any of the high-k titanates is coated on the applicable surfaces by any of the various methods previously enumerated. A metallic or nonmetallic mask member 50 is applied to the central area of the dielectric body member 30 where the deposition of the nonconducting oxide coating is not desired.

One exemplary method of applying the oxide coating will now be described. RF sputtering in a vacuum utilizing a target was found to be highly successful in the application of coatings of silicon monoxide with thicknesses in accordance with the invention. The window assembly including waveguide 32 and dielectric body 30 together with mask member 50 is first cleaned by conventional techniques and then is mounted in the RF sputtering system which is evacuated to a pressure of approximately 2 X l0'6 torr or less. High purity argon is introduced into the chamber and the pressure adjusted to approximately 5- 6 millitorr. A screen at anode potential is disposed between the target source and the window assembly to provide a uniform RF field to assure uniform sputtering. The required sputtering times for the 1,000 to 2,000 Angstrom coatings is determined on the basis of measurements by an interferometer prior to establishing the manufacturing specifications. The bombardment by the argon gas ions of the target releases the oxide material which is deposited on the contacting surfaces of the applicable bodies. DC sputtering may also be utilized in the practice of the invention.

In FIG. 3 another application ofthe invention is illustrated involving a large number of dielectric supports for mounting a helix type slow wave structure in traveling wave tubes. Each of the helix turns 52 has spaced recessed sockets 54 along the circumferential edge. Dielectric pins 56 have

metallic posts

58 and 60 of a highly conductive material such as copper secured at opposing ends by the known metallizing techniques. Pins 56 are fabricated from materials selected from the group including berryllium oxide, boron nitride or any other high thermal shock-resistant dielectric materials. Copper post 60 is provided with a hollow passageway 62 to relieve the stresses during the brazing operation. A channel member 64 and conduit 66 are disposed adjacent to the metallic envelope 68 forming the outer shell of the overall traveling wave device and post 58. The conduit 66 provides means for the forced circulation of fluid media during operation as well as means for providing a stress to exert radial forces on the helix structure to insure effieient thermal contact with the envelope. in such arrangements the interface surfaces between the dielectric material and adjacent conductive walls of the post and socket are subject to damag ing arcing since high electromagnetic fields are conducted by the helix structure.

ln this embodiment a nonconducting oxide coating 70 is deposited on the recess socket walls of the helix turns 52 as well as the copper posts 60. Alternatively, due to the excellent insulating properties of the nonconductive oxide coating the helix turns can be provided with shallower socket recesses or simply have a locating hole arrangement for the proper centering of the support pins 56. In other traveling wave devices where the helix is supported by elongated rods extending lengthwise the coating also can be readily applied.

With the arc suppressing capabilities of the nonconducting oxide coatings of the described dielectric materials on the adjacent electrically conductive and nonconductive contacting surfaces, it is also possible to combine the features of the present invention with those of the referenced eopending application relating to electron multipaetor suppression. In FIG. 4 such a combination is disclosed with the interface surfaces illustratcd. An electrically conductive member 72 and a nonconductive dielectric member 74 are joined by the metallizing techniques. A nonconducting oxide layer 76 is first deposited on the adjacent surfaces in the interface region utilizing the masking techniques previously enumerated. Then a semiconducting oxide coat ing 78 of the materials described in the referenced application is superimposed over the first layer 76 .is well as the previously masked region in the central portion of the dielectric body 74. In this manner the combined suppression of electron multipactoring as well as reduction of the electric field strengths will measurably increase the power handling capabilities of the applicable devices by substantially reducing the principal causes of thermal shock.

Other applications of the described nonconducting oxide coating including high voltage bushings and electrodes which are required to stand-off high values of DC and AC electrical energy. The cathode assembly 18 in the previously described embodiment in FIG. 2 has ceramic bushing members and metallic conducting members to couple the 4050 kilovolts voltages to the anode and cathode. The utilization of the nonconducting oxide layers in the interface regions in the vacuum will effectively reduce the electric field strengths which lead to arcing. The described coating material can also be applied to devices or equipment not operated in an evacuated atmosphere where the build up of electrostatic fields on the dielectric body materials is a serious problem. Further applications, variations and modifications will be evident to those skilled in the art. It is intended, therefore, that the foregoing description of the invention and illustrative embodiments be considered broadly and not in a limiting sense.

What is claimed is:

1. An electromagnetic energy electron discharge device comprising:

an evacuated envelope having an access opening;

a high power window assembly for sealing said access opening including a body of a dielectric material permeable to electromagnetic wave energy and hermetically joined to a support body of a metallic material; and

a coating of an oxide of a dielectric material selected from the group including silicon, alumina and titanate disposed solely on the interface surfaces of said joined bodies to be exposed to vacuum to substantially reduce surface electric field strengths in the interface regions;

said coating having a thickness averaging in the range of about l,000-2,000 Angstrom units.

2. The device according to claim 1 and a second coating of a semiconducting oxide material selected from the group including chromium, cobalt, copper, iron, manganese and nickel deposited over said dielectric oxide coating and said dielectric body to substantially suppress electron multipactor phenomena.

3. A traveling wave electron discharge device comprising:

an evacuated envelope;

a metallic helix slow wave energy propagating structure disposed along the longitudinal axis of said envelope;

a support body of a dielectric material disposed in thermal contact with said helix; and

a coating of an oxide of a dielectric material selected from the group including silicon, alumina and titanatc disposed solely on the interface surfaces of said support body and said helix to substantially reduce surface electric field strengths and suppress electrical arcing; said coating having a thickness averaging in the range of about 1000-2000 Angstrom units.

Claims

1. AN ELECTROMAGNETIC ENERGY ELELECTION DISCHARGE DEVICE COMPRISING: AN EVACUATED ENVELOP HAVING AN ACCESS OPENING, A HIGH POWER WINDOW ASSEMBLY FOR SEALING SAID ACCESS OPENING INCLUDING A BODY OF A DIELECTRIC MATERIAL PERMEABLE TO ELECTROMAGNETIC WAVE ENERGY AND HERMETICALLY JOINTED TO A SUPPORT BODY OF A METALLIC MATERIAL, AND A COATING OF AN OXIDE OF A DIELECTRIC MATERIAL SELECTED FROM THE GROUP INCLUDING SILICON, ALUMINA AND TITANATE ISPOSED SOLELY ON THE INTERFACE SURFACES OF SAID JOINTED BODIES TO BE EXPOSED TO VACUUM TO SUBSTANTIALLY REDUCED SURFACE ELECTRIC FIELD STRENGTHS IN THE INTERFACE REGIONS, SAID COATING HAVING A THICKNESS AVERAGING IN THE RANGE OF ABOUT 1,000-2,000 ANGSTROM UNITS.

3. A traveling wave electron discharge device comprising: an evacuated envelope; a metallic helix slow wave energy propagating structure disposed along the longitudinal axis of said envelope; a support body of a dielectric material disposed in thermal contact with said helix; and a coating of an oxide of a dielectric material selected from the group including silicon, alumina and titanate disposed solely on the interface surfaces of said support body and said helix to substantially reduce surface electric field strengths and suppress electrical arcing; said coating having a thickness averaging in the range of about 1000-2000 Angstrom units.