US3531678A

US3531678A - Heater with boron nitride coating

Info

Publication number: US3531678A
Application number: US739399A
Authority: US
Inventors: Frank J Schiavone
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1968-06-24
Filing date: 1968-06-24
Publication date: 1970-09-29
Anticipated expiration: 1987-09-29

Description

Sept. 29, 1970 F. J. SCHIAVONE HEATER WITH BORON NITRIDE COATING Filed June 24, 1968 I6 I I 22 F/G 3 I INVE/VT'OI? FRANK .1. SCH/A VO/VE United States Patent ()1 iice 3,531,678 Patented Sept. 29, 1970 US. Cl. 313-637 3 Claims ABSTRACT OF THE DISCLOSURE A heater for devices such as electron discharge devices comprising a wire or similar substrate having thereon an electrically insulating thermally conductive coating of boron nitride.

BACKGROUND OF THE INVENTION This invention relates to heater elements such as filament wires of electron discharge devices or the like, and has particular reference to heater wires having deposited thereon a coating of boron nitride, preferably isotropic in nature, for providing the heater wire with improved mechanical and electrical properties.

In the manufacture of heaters such as filament wires of electron tubes and the like, it has been necessary to coat the filament with a suitable insulating material which will protect the filament while preventing the occurrence of electrical shorts under adverse conditions of use. Filaments of tungsten are commonly used and have been provided with coatings of alumina formed by sintering alumina powder to the surface of the tungsten wire at high temperatures.

However, serious problems are encountered when using alumina as the protective coating since there exists a considerable difference in thermal expansion between tungsten and alumina. Alumina has an expansion coefficient of 8.512.5 l C., while that of tungsten is 4.4 l0 C. Therefore, separation often occurs during use of a heater embodying an alumina-coated tungsten wire.

It is essential, of course, that the coating be highly transmissive to the heat generated by the filament so that, in an electron tube, for example, the filament may be utilized efficiently to heat an adjacent electron emitter to the point of electron emission.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a novel heater structure which comprises a tungsten filament having thereon a coating of boron nitrile, preferably isotropic in nature, which coating effectively insulates the filament wire, elficiently conducts heat from the filament wire to the ambient, and has a thermal expansion coefiicient which closely matches that of the underlying tungsten.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of a coated heater embodying the invention, showing the coating partially broken away;

FIG. 2 is a sectional view taken on line 22 of FIG. 1; and

FIG. 3 is an axial sectional view of a cathode structure embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to the drawing, wherein like characters of reference designate like parts throughout the several views, there is shown in FIGS. 1 and 2 a heater element such as a tungsten wire 12 having thereon a coating 14 of boron nitride. The heater may be of any other selected configuration, rod, sphere, disc, or the like, and is suitably connected to a source 16 of filament power SP whereby it may be heated to temperatures as high as 1500 C., for example.

One example of actual use of a heater element of a type embodying this invention is shown in FIG. 3 wherein a coated filament wire 18 coiled in a predetermined configuration is disposed within a hollow cylindrical metal cathode 20 which is provided on its outer surface with a coating 22 of electron emitting material. The cathode 20 may be nickel, and the electron emitting material may be barium-strontium oxide, for example. The ends of the filament wire 18 are mounted upon

respective leads

24 and 26 whereby connection may be made to a suitable source of filament power. Thus, when the filament wire is heated by flow of current therethrough, the generated heat is transmitted through the boron nitride coating into the ambient, whereupon the cathode 2 0 will become heated to a temperature level which will cause electrons to be emitted from coating 22. A cathode structure such as illustrated in FIG. 3 is a commonly used component in many types of electron discharge devices.

The boron nitride coating 14 is an excellent insulator and prevents shorting which may occur, for example, when vibration, mechanical shock, or other influence causes the heater wire 18 to move into physical contact with the cathode cylinder 20. Boron nitride also is an excellent conductor of heat and, in fact, is better in this respect than alumina. Thus, more eflicient transmission of heat to the cathode cylinder is achieved.

Furthermore, tungsten and boron nitride have very similar coefiicients of expansion, being 4.4X10- C. for tungsten and 4.0 10 C. for boron nitride. This is a decided improvement over alumina which has a coefficient of expansion of 8.5l2.5 l0- C. Thus, under severe conditions of use, continued expansion and contraction of the tungsten and boron nitride will not produce undesirable separation as often occurs when alumina is used as the insulating coating.

The coating may be relatively thin because of the extreme density of the boron nitride material, a satisfac tory thickness being in the range of about .0O1.006 inch, for example.

The boron nitride layer or coating 14 may be deposited upon the heater element 12 by any of the known methods which result in the production of anisotropic boron nitride, such as, for example, the method disclosed in US. Pat. No. 2,824,787 wherein, briefly, a borate ester such as methyl or ethyl borate and ammonia are reacted at a temperature between 850 C. and 900 C. and at atmospheric pressure to produce a white, finely dispersed powder which is thereafter treated with ammonia at a temperature between 800 C. and 1300 C. to produce powdered boron nitride. Such a powder may be applied under heat and pressure to a heater element to form a coating thereon.

Another known method, as taught by Basche in US. Pat. No. 3,152,006, results in the production of a solid boron nitride coating on a substrate by vapor-phase reaction between ammonia and boron trichloride at a temperature between about 1450 C. and 2300 C. at a pressure below about 50 mm. of mercury.

Such coatings are anisotropic, having been found to possess excellent thermal conductivity, electrical resistance, and other characteristics which, however, are better in one direction than in another due to the anisotropicity of the material. Therefore, to obtain optimum results when applying an anisotropic boron nitride coating on a heater wire, best results are obtained when the coating is so applied that the direction of maximum thermal conductivity is perpendicular to the surface of the coating.

Even better results are obtained, however, by applying an isotropic boron nitride coating to the heater element. This may be done by a method disclosed in pending U.S. patent application Ser. No. 582,686 now abandoned and assigned to the same assignee as the present invention, which teaches the production of isotropic boron nitride by the reaction of ammonia gas with an organic boron compound such as trimethyl borate, dimethyl boric acid, triethyl borate, trimethyl triborine trioxane or tetramethoxyborine, or others, in a vapor deposition furnace at an elevated temperature in the range of about 1200.2300= C. at a pressure of less than about 100 torr, whereby when the gaseous reactant agents are co-mingled and then directed upon a substrate, such as heater element 12, for a selected length of time, they will form on the heater element a solid isotropic boron nitride coating of a thickness primarily dependent upon the time cycle.

The electrical resistivity of the isotropic boron nitride coating 14 ranges from about 10 ohm/ cm. at room temperature to about 10 ohm/cm. at 1350 0, making it one of the best high temperature electrically insulating materials known. Furthermore, because of its isotropic characteristics it can be easily, efficiently and permanently sealed to the metal heater wire, and can be readily made in various sizes and shapes and can be of practically any configuration, regardless of the radius of curvature employed.

The heat during the process of depositing the isotropic boron nitride layer can be generated by an external source or may be generated within the heater element by impressing a suitable voltage to the element during the deposition process.

Additional details of the methods of applying the boron nitride coating 14 to the heater element 12, and of the characteristics of the coating may be obtained from the above-mentioned patents and application. The methods do not constitute a part of this invention.

It is to be understood that various modifications and changes in the invention disclosed herein may be made by those skilled in the art without departing from the spirit of the invention as expressed in the appended claims.

I claim:

1. A heater element comprising a tungsten substrate having thereon a coating of isotropic boron nitride, the direction of maximum thermal conductivity of the coating being perpendicular to the surface thereof.

2. A cathode structure comprising an electron emissive member, and a heater element adjacent said member for heating the member to a temperature level at which electrons are emitted therefrom, said heater element comprising a tungsten substrate having thereon a coating of isotropic boron nitride, the direction of maximum thermal conductivity of the coating being perpendicular to the surface thereof.

3. In an electron discharge device, a cathode structure comprising a hollow metal cathode cylinder having a layer of electron emissive material on the outer surface thereof, a filament wire within the cathode cylinder for heating the cathode cylinder and emissive material to a temperature at which electrons are emitted by the material, an insulating heat transmitting coating of isotropic boron nitride on said filament wire, the direction of maximum thermal conductivity of the coating being perpendicular to the surface thereof, and means for connecting said filament wire to a source of power external to the device.

References Cited UNITED STATES PATENTS 3,119,897 1/1964 Coper 313337 X 3,152,006 10/1964 Basche 117-106 3,178,308 4/1965 Oxley et al. 117l06 3,259,783 7/1966 Norris et a1. 313--337 3,273,005 9/1966 Lafierty 313-337 X 3,321,337 5/1967 Patterson ll7l06 X JAMES D. KALLAM, Primary Examiner A. J. JAMES, Assistant Examiner U.S. Cl. X.R.