US1874542A - Insulating coating for filaments and method of applying the same - Google Patents

Description

Aug. 1932. J. KAUL 1,874,542 INSULATING COATING FOR FILAMENTS AND METHOD OF APPLYING THE SAME Filed Sept. 24. 1929 INVENTOR Jo m/ KczuZ BY Hi5 ATTORNEY Patented Aug. 1932 UNITED STATES PATENT OFFICE JOHN RAUL, O]? IRVIIIGTON, NEW JERSEY, ASSIGNOB TO ARCTUBUS RADIO TUBE COH- Pm, OF NEWARK, NEW JERSEY, A CORPORATION OF DELAWARE INSULATING COATING FOB IILAKENTS AND METHOD OF APPLYIITG THE Application fled September 24, 1829. Serial No. 894,823.

This invention relates to coatings for filamentary wires and especially insulating coatings used on the filaments of vacuum tubes.

It is generally desirable in vacuum tubes to place the filament as nearly as possible to the other elements of the tube in order to take advantage of all of the heat radiated from the filament or the electron emission where a filamentary cathode is used. Especially in a heater type tube is it necessary to have the heating element as close to the cathode as possible so as to conserve the heat energy of the filament and reduce to a minimum the time required to bring the cathode up to electron u emitting temperature when the current is ofi the binder.

turned on. Where a bare filament is positioned in close proximity to the cathode any movement of the filament with respect to the cathode mayjcause the two to touch and thereby short circuit the tube. It has been found advantageous to provide some insulating means between the cathode and filament to prevent such short circuiting and one way of providing such an insulation between the two elements is to coat it upon the filament itself. Such an insulating coating for filaments has been provided by mixing an insulating oxide as for instance, aluminum or magnesium oxide with a suitable binder coating this mixture upon the filaments and heating to drive There are certain disadvantages which follow from this method of insulation, the first being that it is extremely difficult to drive off all of the binder and in some instances where a little of one binder remains the conductivity of the insulation is increased thereby causing break down at relatively low temperatures. Another disadvantage of this type of insulation is that the particles of insulation materials are not in a molecular state but, even though ground in a ball mill and made exceptionally fine, tend to loosen up and absorb moisture so that the finished coating has a tendency to crumble off of the wireand geneous, extremely compact surface tightly associated with the metal base.

Another object of the invention is to provide 2. hard, compact insulating coating for filaments capable of standing the temperatures of tube operation and manipulating of the filament before it is positioned in the tube without the coating crumbling or breakin down electrically under the potentials use Another object of the invention is to provide a method of attaching the oxide of a metal to a wire without any of the disadvantages arising from the use of binders.

Other objects and objects relating to methods of preparing the various ingredients of the coating and applying the coating to the wire will be apparent as the description of the invention proceeds.

Referring to the accompanying drawing, the figure therein illustrates in general a coated filament in accordance with my invention.

The specific showing in the drawing illus trates a filament 1 upon which is applied a coating 2. Thefilament comprises a length of wire of a suitable character to be heated by passing a current therethrough. The coatin is preferably an oxide as will now be more ully described. I 7

Among the oxides which form the best insulating material for use in vacuum tubes are those of aluminum, beryllium, magnesium, calcium, chromium, and titanium. Of these metals I prefer to use beryllium or aluminum because the compounds are commercially obtainable and as good results have been obtained with aluminum the method of obtaining a coating of aluminum oxide will be described.

I first prepare a saturated solution of aluminum nitrate and apply any desired number of even coatings of this solution upon the wire which is preferably heated after each coating has been applied to dry the material in place. The action of heat on the aluminum nitrate breaks it up into aluminum oxide,

nitric oxide, and water, the last two being driven oil in va or form leaving a coating of aluminum oxi e in its molecular state. I have used this coating on a tungsten filament ess of the tube and cause the aluminum exit e to adhere very closely to the tungsten wire.

I have found that the degree of saturation of the aluminum nitrate solution is important as this determines the number of applications or layers necessary for acoatingoi the desired thickness and to a certain extent the character of the coating after it is applied. If

vthe solution is weak, a large number of applications will be needed to give the necessary cross section. I And if the solution is. a supersaturated solution there is a tendency vfor the aluminum nitrate to crystallize and cause lumps and uneven ridges on the wire surface.

A saturated solution'having a specific gravity of 1.4 at 27 C. has been found to give good results. With1a6 mil wire I- have found that about coatings of this solution are necessary to raise the diameter to from 13 to 15 mils.

To aluminum nitrate solution I addyfrom 3 to 10% by weight of'silicon dioxide'which melts below the filament temperature during the exhausting of the vacuum tube, thereby forming themeans'to integrate the aluminum oxide more closely to the wire. It is important to get enough of the silicon dioxide to melt the coatingso that it closely adheres to the wire and not enough to lower the dielectric.- o

After a filament is inserted in a vacuum tube'the tube is exhausted at which time the filament is heated to a. temperature in the neighborhood of 600" to 900 C. 'or toa dull red heat. I have found that this heat-' ing of the filament where aluminum nitrate has been coated thereon without any silicon dioxide causes a softening of the coating which renders it crumbly and undesirable to be used in a vacuum tube, and although I do not know the cause of this softening I have found that the addition of the silicon dioxide prevents it and hardens the coating into one that is extremely difficult to chip or crumble.

While I have described the invention in connection with aluminum any of the metals listed above may be used, and while I have specified silicon dioxide as preferable to mix with the material applied to the wire, any metallic oxide which is a good insulator and has a meltingv point at or below a dull red hea-t may be used for this purpose}:

l he seen that I have forming a metallic The .e, inexpensive, and produces coating for the filament which inble or break ofi when the filahent in desired shape' for positiening in a vacuum tuhe, nor will it crumble or break down when the filameiia is in actual service thereby greatly lengthening-the life of the vacuum tubes sin the invention, especially those of t e9. 1' t Modifications e the in without depemting from th nticn may be made spirit thereoi and I do not desire to limit the invention to crystallization temperature of ther ungsten" of said Wire, heating said wire after each layer of the solution hasbeen applied to reduce the aluminumnitrate to aluminum 'ox ide, and subsequently raising the temperature of -said wire to a point between the melting point of the first mentioned oxide and the crystallization temperature of said wire.

2. The method of coating a tungsten wire with a hard, homogeneous insulating coat ing which comprises applying layers of a, solution of aluminum nitrate and a small-percentage of silicon dioxide to the wire, heating t ewire after each successive application to chemically change the aluminum nitrate to aluminum oxide, and subsequently heating the wire to a temperature above the lmelting point of silicon dioxide but below the crystallization temperature of tungsten.

3. The method of forming a metallic oxide ona filamentary wire which comprises coating'the wire with a plurality of layers of a saturated solution of aluminum nitrate having a specific gravity of 1.4 at27 Chmixed with from 3 to 10% by weight of silicon dioxide, heating the wire after each successive layer has been applied to reduce the aluminum nitrate to aluminum oxide, and subsequently heating the wire to a temperature above the melting point of silicon dioxide but below the crystallizationpoint of said filamentary wire.

' In testimony whereof, I afix my signature.

' JOHN KAUL.

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