|Publication number||US3607475 A|
|Publication date||Sep 21, 1971|
|Filing date||Oct 24, 1967|
|Priority date||Oct 24, 1966|
|Also published as||DE1615121A1, DE1615121B2|
|Publication number||US 3607475 A, US 3607475A, US-A-3607475, US3607475 A, US3607475A|
|Inventors||Nils Gustav Schrewelius|
|Original Assignee||Kanthal Ab|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (13), Classifications (18)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent  Inventor Nils Gustav Schrewelius Hallstahammer, Sweden [2i Appl. No. 677,569  Filed Oct. 24, 1967  Patented Sept.2l,l97l  Assignee Alrtlebolaget Kanthal Hallstahlmmar, Sweden  Priority Oct. 24, 1966  Sweden  14564-66  METHOD OF MANUFACTURING ELECTRICAL RESISTANCE ELEMENTS AND ELEMENTS MANUFACTURED BY THE METHOD 10 Claims, 2 Drawing Figs.
52 u.s.c| 156/6, 15612191 1i,219/s 3,3 31357, 13/25 [5 l] Int. Cl ..C23f 17/00, H0lj 1/02  Field of Search l56/2, 6, 8, ll,l8;29/6ll;2l9/553;3l3/357;13/25  References Cited UNITED STATES PATENTS 2,432,800 l2/l947 Reichold 156/8 X 3,174,920 3/1965 Post OTHER REFERENCES Chemical Milling for Thin Complex Parts. Metal industry 8 Aug. 1963 Vol. 103 P. I88
Primary ExaminerJacob H. Steinberg Atromey Fidler, Bradley, Patnaude & Lazo ABSTRACT: An electric resistance element of molybdenum disilicide has a thin incandescent zone whose dimensions are formed by chemical decomposition, dissolution or corrosion.
METHOD OF MANUFACTURING ELECTRICAL RESISTANCE ELEMENTS AND ELEMENTS MANUFACTURED BY THE METHOD The present invention relates to a method of manufacturing electrical resistance elements substantially consisting of molybdenum disilicide and having an incandescent zone and at least one thicker terminal. Such electrical resistance elements have been known for a long time.
The natural brittleness of the material in cold condition has limited the use of molybdenum disilicide substantially to elements having a rather large cross sectional area. As an example of such elements the proven type of element sold under the registered trade mark of KANTHAL SUPER may be mentioned, said element consisting of an incandescent portion in U-shape and having straight terminals. The incandescent portion may have a diameter of about 6 mm. an the terminals which are generally connected to the incandescent portion by resistance butt welding, may have a diameter of about 12 mm. ln these thick terminals the temperature will be low even though the incandescent portion has a temperature of about l,600 to l,700 C., and thereby it becomes possible to con nect them with contacts of aluminum, for instance.
When trying to make resistance elements of the type mentioned but having considerably smaller dimensions, such as diameters of the magnitude 1 mm. for the incandescent portion, it has in practice been found very difficult to weld on the terminals as the material is hard to handle owing to its great brittleness. Therefore, in the case of thin elements of molybdenum disilicide it has been necessary to choose another way, viz, not to use a welded-on thicker terminal. Instead, the whole element is designed as an incandescent portion soldered to air or liquid cooled contacts. Owing to the strongly positive temperature coefficient of the element material for the electrical resistance, it may be possible to obtain rather cold contacts in such elements.
- These known thin elements which are soldered to contacts have, however, proved troublesome in use. The majority of metallic materials, with the exception of gold and silver, react with the element material at high temperatures, and because of the difierences in heat expansion coefficient the soldered joints do not become durable, particularly if the elements are used intermittently which, as a rule, is the case.
The object of the present invention is to remedy the above difficulties in a simple and cheap way, and a thin molybdenum disilicide element with at least one thicker terminal is characterized, according to the invention, substantially in that at least the incandescent portion of the element has been given its final dimension by removing certain portions of an element blank by a chemical removal process such as chemical dissolution, decomposing or corrosion.
The simplest way of carrying out the method according to the invention is to submerge a U-shaped rod blank in an acid mixture for such a long period of time and to such depth that the acid corrodes away the amount of material required to give the incandescent portion the desired final diameter and length.
The chemically dissoluting, decomposing or corroding agent which has proven most useful in this connection is an acid mixture containing hydrofluoric acid, nitric acid and, if desired, water.
Resistance elements which are in the known way built up of a powder metallurgical composition of molybdenum disilicide and a glass rich in SiO are dissolved in said acid mixture as the molybdenum disilicide and the glass component are dissolved at approximately the same rate. Thereby a smooth surface and a controllable cross sectional area of the portion treated with acid are obtained. However, resistance elements substantially consisting of molybdenum disilicide may also contain other substances as additions in order to modify different properties.
. An example of such an addition is finely powdered silicon car bide which is then substituted for the glass component in the above example. The glass as well as the silicon carbide prevents the grain growth of the molybdenum disilicide at high temperature and also gives the material other advantageous properties, such as plastic workability in heat, for instance. However, the silicon carbide component in such an element is not dissolved at all or only dissolved very slowly in the acid mixture mentioned, but nevertheless it is possible according to the invention to produce elements with thin incandescent portions by chemical treatment provided that the molybdenum disilicide is the main component; unattacked silicon carbide present in rather insignificant amounts will automatically fall off from the goods dissolved. Also other additions and modifications of the composition may be permitted according to the invention, such as a modification of the MoSi, component by adding as alloy components small amounts of other metals which do not influence the chemical solubility.
The interface between air and liquid in an acid bath for chemical dissolution as per above has proved to have a disadvantageous influence on the smoothness of the corroded portion. Therefore, it has proved convenient to keep the element in motion during the treatment to avoid a reduced diameter of the portion located at the interface. Another way of avoiding the problems with the interface between air and liquid is to coat those portions of the element blank which are not to be corroded with a layer of an acidproof material, such as plastics, the element blank being submerged in the acid bath to such a depth that the interface air-liquid will be within the plastics-coated area.
Thus, the method according to the invention presupposes that there is no intercrystalline corrosion or any other type of attack interiorly of the element. Therefore, it is important that the bodies containing molybdenum disilicide are free of pores.
The porosity should be less than 2 percent, preferably below 0.5-1 percent. To obtain such low porosity it is convenient to use low melting glass as binding agent for the molybdenum disilicide.
The invention is illustrated by the following practical example:
A straight homogenous rod of 4.6 mm. diameter and having a porosity less than 1 percent consisting of a fine-grained powder metallurgical dense body containing 90 percent by weight of MoSi and 10 percent by weight of a glass rich in SiO,, which has been produced by pressing and sintering MoSi and bentonite was bent in heat into U-shape, the legs having a length of 50 mm., and the space between the legs being 15 mm. This U-shaped blank was half submerged in a cold mixture of 50 parts of volume of concentrated nitric acid, 50 parts by volume of concentrated hydrofluoric acid and l0 parts by volume of water. The element was kept submerged in this mixture for 2 minutes, while being slightly moved, and was then carefully rinsed in water. After the treatment, an incandescent zone having the diameter 2.3 mm., had been formed. The element was provided with sprayed-on contact layers of aluminum, whereupon it was connected in a current circuit and permitted operating temperatures of 1,600 C. and more in the incandescent zone, while the thicker, uncorroded terminals had a temperature of less than 200 C. in the contact layers.
By the new method it is possible to produce miniature elements in an economical and practical way. This method is useful for the manufacture of very small elements intended for gas igniters, for instance, and having a smaller diameter than 1 mm. for the incandescent zone, such as 0.4 mm., and terminals of 1 to 2 mm. The advantage of the method according to the invention is, inter alia, that it is possible to accurately control the diameter and that the difficult bending in heat of very thin incandescent zones may be dispensed with. Thus. it
is possible to shape a small element having a leg length of 50 mm., for instance, and a space between the legs of only 3 mm., by starting from a straight rod with adiameter of 1.5 mm.,
. bending it in heat into a U with the desired leg space and then is even possible to make elements with incandescent zones having diameters of 0.1 to 0.2 mm.
As it is not practicable to make wires of molybdenum disilicide by drawing, the new method offers possibilities of making such thin dimensions as could not previously be manufactured with the desirable accuracy of measures.
Also, the method offers, possibilities of automatizing the production, as it is easy to make such arrangements that the resistance of the element blank may be caused, during the acid treatment, to control the moment at which the blank is removed from the acid bath.
The method may also be applied such that a blank of molybdenum disilicide in the form of a tube or a plate, for instance,
is coated with a material resistant to the acid mixture, such as plastics, for instance, and that a desired pattern is drawn on the surface so that a selective dissolution of the material can then take place in accordance with particular wishes. For instance, it is possible in this way to make from a very thin plate a flat resistance coil with broader terminals. Such flat elements may be used in cigarette lighters, for instance.
Two embodiments of elements according to the invention are shown in the accompanying drawing wherein FIG. 1 is a perspective view of a hairpin-shaped element and FIG. 2 is a plan view of an element formed as a flat spiral. The elements in FIG. 1 which consists essentially of MoSi has two terminals 11, 12 with flame sprayed aluminiumcontacts 11A, 12A and of a diameter of about 1 mm. and a length of about 60 mm. each. The terminals 11, 12 are parallel to each other and spaced apart about 3-10 mm. The bent incandescent zone 13 which is formed from the same rodshaped blank as the terminals 11, 12 has had its diameter reduced down to about 0.4 mm. by chemical corrosion as discussed above.
The element 14 in FIG. 2 has been formed with an incandescent zone 15 and two wider terminal portions 16, 17 by corroding away a spiral shaped portion to form a spiral slot 18 in a flat, plate-shaped blank, essentially of MoSi in the above-described manner.
A plurality of modifications and alterations of details are possible within the scope of the invention idea.
I claim: 1. A method of manufacturing an electrical resistance element having a thin incandescent zone, comprising the steps of forming a blank of said resistance element from molybdenum disilicide and a glass rich in silicon dioxide to have a porosity lower than 2 percent and having at least one electrode portion conforming substantially to the electrode portion of said resistance element and having an incandescent zone thicker than the incandescent zone of said electrical resistance element, I
bringing said incandescent portion of said blank into contact with a chemical agent capable of removing molybdenum disilicide for a sufficient time to uniformly thin said incandescent portion while said electrode is protected from said chemical agent; and
separating said incandescent portion from said chemical agent before sufficient material has been removed from said incandescent portion to form any aperture in said incandescent portion, whereby the desired dimensions ofcomprising nitric acid and hydrofluoric acid into contact with said incandescent portion.
3. A method in accordance with claim 2 in which said step of bringing a chemical agent into contact with said incandescent portion comprises the step of bringing an acid mixture consisting essentially of 50 parts by volume of concentrated nitric acid and 50 parts by volume of concentrated hydrofluoric acid into contact with said incandescent ortion.
4. A method in accordance with claim in which said step of bringing said chemical agent into contact with said incandescent portion comprises the step of submerging said incandescent portion into said chemical agent while maintaining said electrode portion free from said chemical agent.
5. A method in accordance with claim 4 in which said step of forming said electrical resistance blank comprises the step of coating said electrode portion with a masking substance that is not affected by said chemical agent, whereby the interface between the chemical agent and the atmosphere is prevented from creating a discontinuity in said electrical resistance element.
6. A method in accordance with claim 4 in which said electrical resistance blank is moved with respect to said chemical agent while submerged therein, whereby said interface between said chemical agent and the atmosphere is prevented from causing a discontinuity in said electrical resistance element.
7. A method according to claim 1 in which said step of forming said electrical resistance blank includes the step of forming said electrical resistance blank from a material consisting of at least 60 percent by volume of molybdenum disilicide and at most 40 percent by volume of ceramic additions.
8. A method in accordance with claim 1 in which:
said step of forming said electrical resistance blank includes the step of forming a straight rod of approximately 5 millimeters in diameter with a porosity less than 1 percent of a fine-grained powder metallurgical body containing percent by weight of molybdenum disilicide and 10 percent by weight of a glass rich in silicone dioxide which has been produced by sintering molybdenum disilicide and bentonite and the step of bending said rod while heated into a U-shape with the legs of the U having a length of approximately 50 millimeters and with a space between the legs of approximately 15 millimeters; said step of bringing a chemical agent into contact with said incandescent portion includes the step of submerging the U-shaped blank half way into a cold mixture of 50'parts by volume of concentrated nitric acid, 50 parts by volume of concentrated hydrofluoric acid and [0 parts by volume of water. for approximately two minutes while being moved; and a said step of separating said'incandescent portion and said chemical agent includes the steps of removing said U- shaped blank from said cold mixture and rinsing with water. Q 1 9. An electrical resistance elementformed in accordance with the method of claim 1 and having a rodlike cross section in which the diameter of the incandescent zone is less than 6 millimeters.
10. An electrical resistance element formed in accordance with the method of claim 1 and having a rodlike cross section in which the diameter of the incandescent zone is less than i millimeter and the diameter of the electrode portion is more than 1 millimeter.
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|US20050236399 *||Mar 6, 2003||Oct 27, 2005||Sandvik Ab||Method of marking a heating element of the molybdenum silicide type and a heating element|
|US20050252909 *||Mar 7, 2003||Nov 17, 2005||Jan Andersson||Electrical heating resistance element|
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|U.S. Classification||216/101, 219/553, 216/109, 29/611, 313/357|
|International Classification||H05B3/12, C23F1/00, H01B1/14, H05B3/14|
|Cooperative Classification||H05B3/12, H05B3/148, H01B1/14, H05B2203/018, C23F1/00|
|European Classification||H01B1/14, H05B3/14S, C23F1/00, H05B3/12|