US1829237A - Heater element and method of making it - Google Patents

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:06. COMPOSITIONS, I

COATING 0R PLASTiG Cross Reference A R Oct. 27, 1931. RUQEN 1,829,237 O BEA BER ELEMENT AND METHOD'OF MAKING IT Filed Oct. 11. 1928 anomtoz MMUEL use/y Patented Oct. 27, 1931 UNITED STATES HEATER ELEMENT AND METHOD OF MAKING IT Application filed October 11, 1928. Serial No. 311,952.

This invention relates to heater elements and methods of making heater elements for indirectly heatin cathodes in electron emission tubes. An 0%]' ect of this invention is the rovision of a heater element that will afford ong life and a high degree of thermal conduction and radiation to the cathode heater elements in electron emission tubes operating from an alternating current circuit.

'rtr an comm, a m;

insulation. of the desired properties is employed, by which the heat is uniformly distributed over its length to the cathode; this insuresa long life for that member. The heater or resistance wire, suchastungsten, is coated with a layer of finely powdered aluminum oxigg that has previously been calc1ne an mlxed with a small percentage of:

sodipp; sfiicate. .When this material is prop- The problem of suitable insulation for the er y me on the wire it is heated in a remember used for indirectly heating cathodes in electron emission tubes involves several important factors; they are, the vapor pressure of the coating, which should be a mini- 7 mum, so as not to affect the pure electron discharge from the cathode surface; the thermal conductivity of the insulation, which should be as high as possible to maintain at a minimum the temperature gradient between the base heater surface and the exterior radiating or contacting surface; the thermal emissivlty of the insulation which should be high in order to obtain maximum radiation; and the insulation resistance which should be as high as practicable, especially at the maximum operating temperatures. The insulation element should be of uniform thickness and should be closely and firmly adherent to the heater base.

Employed in the devices of the prior art to insulate the heater element from the cathode, an insulating tube or other rigid body is used, having spaces provided in it through which the heater wire is extended. In practice this has been usually in the form of a quartz or porcelain tube having bores of small cross section extending through it, for containing the heater wire of such refractory material as tungsten. The inherent drawback to this type of heater is that as the current is initially applied there is an excessive current flow, due to the contact of the heater wire with this relatively large mass of the insulating body; and as the wire contacts with the insulating member only in spots, those parts of the heater wire not so in contact are excessively heated, causing rapid deteriora tion.

By the terms of my invention a heater element, preferably flexible, with an electrical yolati lized, and the aluminum oxide formmg a crystalline layer of corundum. The: resultant crystalline. laye a hi h thermal conductivity and electrical insu a--';

' alundu m", does not hav tli e proper composi- Harm especially it does not adhere sufliciently to the base to withstand vibration or thermo-mechanical action.

The coating is preferably prepared by grinding the calcined aluminum oxide with a sodium sili ie sp'll'utlon Having a specific gravity of f6?) until iiii proper fineness of material is obtained. The surface of the resistance wire base should be previously oxidized by heating for a short period in air, after which the oxide mixture is applied in three evenly distributed coatings, each being allowed to dry before the next is added. The completely coated wire .is then slowly passed through a chamber maintained at 1000 C.. and then slowly c La-d. The final 'c'baTirYgis very tough and has a density of about double its initial density. Y

I have found that when this insulation is used in ahigh vacuum device there is a negligible evolution of gas or vapor from it; this permits the application of the heater to uses in which high voltages between the elements are employed.

In a preferred form of construction, .the coated heater wire is suspended in and in close surface contact with a nickel cylinder ei ijoracr cc ducm atmos here to about 195 650., all of the undeslragle bmdef material Be'm' g theni ment for an indirectly heated cathode of an as the cathode, the exterior surface of which latter member is coated with an alkaline earth oxide for eflicient thermo-electronic emission. So employed it has been found that the average insulation resistance between the cathode and the heater element is in the order of 31negohms. This Value varies with the extent and degree of pressure of contact and the size of the crystals of the coating.

Reference is ma de accompanying H electrical discharge evice which comprises heating said element to form 'an oxide coating thereon, applying a thin coatin of a mixture of Qwdered aluminum oxide andvso- :dium silicate, allowing said mixture to dry,

and heating said element in a reducing atmosphere to about 1000 C.

'In testimony whereof SAMUEL RUBEN, has

signed his name to this specification this 10th day of October, 1928.

a 5' SAMUEL RUBEN. r

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