US 2997979 A
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Aug. 29, 1961 Filed Sept. 15. 1958 L TASSARA APPARATUS FOR APPLYING METALLIC FILM TO ELECTRICAL COMPONENTS AND THE LIKE 2 Sheets-Sheet 1 INVENTOR.
Aug. 29, 1961 SSARA 2,997,979
L, TA APPARATUS FOR APPLYING METALLIC FILM TO ELECTRICAL COMPONENTS AND THE LIKE Filed Sept. 15, 1958 2 Sheets-Sheet 2 INVENTOR. [0/6/ 72/55/2214 rates Patent 2,997,979 APPARATUS FUR APPLYING METALLIC FILM T0 ELECTRECAL COMPONENTS AND THE LIKE Luigi 'lfassara, Via Olmetto 3, Milan, Italy Filed Sept. 15,1958, Ser. No. 761,061 Claims. (Cl. 11849) This invention relates to apparatus for applying a metallic film to a supporting body. In particular, the invention relates to apparatus for depositing an even, resistive, metallic film on the cylindrical surface of a ceramic rod to form a resistor.
In making such resistors it is desirable that the film be as uniform as possible over the surface of the ceramic rod. The advantages of a uniform film are twofold: In the first place, uniformity of the films on each of a group of ceramic rods coated simultaneously permits the resistance value of each of the films to be determined by measuring the resistance of only one. This is desirable in the manufacture of all resistors but is especially desirable in the manufacture of precision resistors. In the second place, if a coated rod is to be further treated, such as by grinding away selected portions of the film thereon in order to increase the resistance of the remainder, uniformity of the film insures that there will be no poorly coated areas which could result in complete breaks in the resistive element unless the grinding were done with special care.
Basically, the machine includes a source of metallic vapor, such as an electrically heated crucible containing the metal to be vaporized; a carrier, or turret, on which a group of supporting members may be mounted so as to be moved past the crucible; and a stationary apertured shield interposed between the crucible and the path followed by the moving members. More than one crucible may be proivded, particularly if thick metallic films are to be formed, and, if so, the shield must have an aperture for each crucible, the shape of each aperture being such as to permit a predetermined amount of vapor to be directed to difierent parts of the supporting members. For example, if, as will usually be the case, the supporting members are ceramic rods, the aperture or apertures in the shield will usually be shaped so as to permit an equal amount of vapor to reach each elemental sub-area of the whole cylindrical surface of each rod. Under some conditions the apertures may be shaped to permit a greater amount of vapor to reach the middle of each rod than the ends, or vice versa, but in any case, the film must be uriform around the circumference of each rod. In order to obtain the desired uniformity of coating, the rods are rotated, in addition to being moved past the apertures, so that all parts of the circumference will be exposed uniformily to the vapor. In accordance with standard metal film formation, the vaporization and condensation are normally carried out in a vacuum.
The invention will be described more explicitly in the following specification together with the drawings in which:
FIG. 1 shows a resistor made according to the invenion, parts being broken away to show a partial cross iectional view thereof;
MG. 2 shows a partial cross sectional View of one emodiment of the invention; and
FIG. 3 shows a plan view of the embodiment of FIG. 7
As a preliminary to describing the machine which comrises the invention, one embodiment of an end product nade by the machine will first be discussed. This is the esistor in FIG. 1. The resistor comprises a supporting nember 11 in the form of a hollow rod of insulating material such as ceramic. Each end of the rod 11 is :oated with a conductive layer 12 of some material,
such as silver, which has a very low resistance and which serves to make contact with the resistive coating 13 on the outer cylindrical surface of rod 11. In the resistor shown wire leads 14 are inserted into the ends of the rod and are held in place by any convenient means, such as by crimping the end of the wire and by filling up the end of the rod with solder 16. The resistor is usually coated with an insulating material 17 to protect the resistive layer and to prevent changes in resistance due to humidity.
The present invention is concerned with a machine for depositing the metallic coating 13 on the resistor in FIG. 1 or on similar articles. FIG. 2 is a cross sectional view of the machine. In the particular embodiment shown three ceramic rods 11a, 11b and 11c are mounted on a mandrel 21 which has a wheel 22 attached to one end. The mandrel is supported from a turret 23 by a pair of hangers 24 md 26 which fit the mandrel loosely and allow it to turn easily. A second mandrel 21a, shown on the other side of turret 23, carries only a single ceramic rod 11d which is used as a measuring standard in order to gauge the resistance of the coating which will be applied to each of the rods 11. While only two mandrels have been shown in FIG. 2 it will be understood that ordinarily a large number of them may be suspended from turret 23, which is attached to an axle 27.
The metallic vapor to be deposited upon the rods 11 comes from metal 28 heated in a refractory crucible 29. I have found that an alloy containing about nickel and about 20% chromium forms a very satisfactory resistive film. This crucible is heated by an electric current which passes through two supporting electrodes 31. Between the crucible 29 and the rods 11 is a stationary, horizontal plate 32 supported by a base 33 and provided with an opening, or aperture, 34 immediately over the crucible 29. The shape of this opening is an essential part of the invention and will be more fully described in connection with FIG. 3. The outer periphery of the plate forms a track 36 which frictionally engages the wheels on each of the mandrels.
The axle 27 is supported by a bearing 37 in which it is free to turn. As the axle 27 turns, the mandrels supported from it each pass across the opening 34 in the plate 32. Furthermore, since the plate 32 is stationary, the frictional engagement between each of the wheels 22 and the track 36 causes the wheels to turn the mandrels 21 and rods 11 supported thereon. In this way as each mandrel and its attached rods passes across the opening 34 and is exposed to the metallic vapors rising from the crucible 31, the rods are turned so as to expose all portions of the cylindrical surface thereof to the metallic vapor in order to produce an even coating on each of the rods.
In accordance with standard metallic vapor deposition processes, the process of condensing vapor on the ceramic rods 11 is carried out in an evacuated housing 38. This raises the problem of supplying mechanical force to turn the axle 27 which is inside the evacuated housing 38. One way in which this may be done is to attach a magnet 39 to the end of axle 27 and to attach a second magnet 41 to motor 42 by means of suitable gears 43. The motor, gears and magnet 41 may be supported by an arm 44 which can swivel aside to permit the housing 38 to be removed for loading and unloading the rods 11 from the turret 23. The purpose of magnets 39 and 41 is to provide interlinking magnetic fields so that as the outside magnet 41 is rotated by the motor 42, the inside magnet 39, will rotate along with it and will thereby rotate the axle 27.
FIG. 3 shows a plan view of the stationary plate 32 so as to give a better idea of the configuration of the aperture 34. This aperture is in the form of a four-sided figure, each of the sides being curvilinear. Sides 46 and 47 are concentric with the axle 27 and are spaced apart radially by a suflicient distance to permit metallic vapor fromv the crucible 29 toradiate to the outermost end of rod 11;; and the innermost end of rod 11a. In order to obtain an even metallic coating on all of the rods supported by mandrel 21 or on all parts of a single long rod which could alternately be supported by mandrel 21, it is preferable to place the crucible 29 midway between the sides 46 and 47. The other two sides 48 and 49 are also curved in such a way as to direct an even coating on each of the rods 11a, 11b, and 110. The reason for the curvature of sides 48 and 49 is that the center of rod 11b passes most directly over the crucible '29 and thus is exposed to the greatest density of metallic vapor while the outer end of rod 11a and the inner end of rod 11c are farthest removed from the crucible 29 and so are exposed to a much less dense cloud of vapor. Because of the variation in density of the vapor to which various points along the rod 11a 11b and 110 are exposed, sides 48 and 49 of the aperture 34 must be curved so as to permit the center of rod 11b to be exposed to vapor for the shortest time and to permit the outer end of rod 11a and the inner end of rod lie to be exposed for the longest time. Thus, the points of side 48 and 49 which are closest together are those points which intercept the central are 51 which passes over the crucible 29. Although in the embodiment shown, crucible 29 is midway between the two sides 46 and 47, it should be noted that if for some reason it were desirable to do so, the crucible 31 could be moved either inward toward side 46 or outward toward side 47 and the shape of sides 48 and 49 could be varied accordingly.
The evenness with which the coating may be applied to rods which are at dififerent radial distances from the axle 27 makes it possible to use only a single rod as a standard and to measure the resistance of this rod in order to control the deposition of metal on a large number of rods disposed about the axle 27. FIG. 2 shows one form of contact means for connecting to the coating on rod 11d. One end of rod lid is held in place by a conductive stop 52 which makes good electrical contact both with the conductive coating deposited on the end of the rod and with the mandrel 21a. The conductive coating at the other end of rod 11d is insulated from contact with the mandrel 21a. Instead, an insulated brush 53 suspended from the turret 28 connects with the other end of rod lid. A sliding electrical contact is provided in the form of a trolley 54 which rides on the upper end of brush 53 and which is connected to electrical measuring equipment (not shown) outside of the evacuated chamher. In this way it is relatively simple for an operator to observe the resistance measured across rod 11d and to halt the machine when the resistance of this rod becomes sufiiciently low to indicate that a thick enough metallic coating has been deposited on rod 11d. Because of the evenness of coatings on all of the rods, it is unnecessary to measure each of them individually or even to measure rods at different radial distances from axle 27.
Although this invention has been described in connection with a particular embodiment, variations within the scope of the following claims will readily occur to those skilled in the art.
According to an important feature of this invention, within evacuated housing 38 there are provided electric heating elements 60, e.g. heating resistances electrically connected in any known manner to an electric source (not shown). The heat generated by said elements promotes difiusion of the depositing resistive layer 13 in the mycroporous ceramic structure of the rods 11 whereby an intimate bond is obtained.
To obtain a better diffusion, the rods after their removal from the housing 38 can be subjected toa further;
ea r at What is claimed is:
1. Apparatus for depositing a metallic film simultaneously on a plurality of cylindrical insulators, said apparatus comprising: an electricaT heated refractory crucible containing metal to be deposited on said insulators; a rotatable axle; a turret supported and rotated by said axle; a plurality of pairs of arms extending downward from said turret; a plurality of mandrels supporting said insulators over said crucible, each of said mandrels extending in a radial direction from said axle and supported by a pair of said arms; a wheel on each of said mandrels; a track engaging each said wheel to turn each of said mandrels about its own axis as said turret is rotated by said axle; and a fixed shield plate perpendicular to said axle and located between said crucible and said mandrels, said plate having an aperture near the edge to permit vaporized metal to pass along straight lines from said crucible tothe surface of each of said insulators in turn as the rotation of said turret about said axle causes said insulators to pass directly over said aperture, the shape of said aperture being defined by an inner arcuate side and an outer arcuate side, both said sides being substantially concentric with said axle, and two curved sides joining the ends of said arcuate sides, the curvature of said curved sides being such as to place their arcuately proximal points on an arcuate line concentric with said axis and passing through said crucible.
2. The machine of claim 1 in which said crucible is substantially equidistant from said inner and outer arcuate sides.
3. The apparatus of claim 1 in which said curved sides are mirror images of each other and are equidistant from said crucible and are convexly curved so that the central portion of each of said curved sides extends into said aperture to direct an equal thickness of metallic film to all areas of the cylindrical surface of each of said insulators.
4. Apparatus for depositing a metallic film on each of a plurality of hollow cylindrical insulators, said apparatus comprising: an electrically heated refractory crucible containing metal to be deposited on the cylindricl surface of said insulators; a rotating vertical axle; a turret supported and rotated by said axle; a plurality of pairs ofarms extending downward from said turret; a plurality of mandrels each comprising a rod extending through and supporting a plurality of said hollow insulators, each of said mandrels extending substantially horizontally and radially out from said axle and being supported by a pair of said arms; a wheel on each of said mandrels; a substantially horizontal circular track fri'ctionally engaging all the wheels to turn each of said mandrels and the supported insulators about its own axis as said mandrels are rotated about said vertical axle; and a horizontal shield plate between said mandrels and said crucible, said plate having an aperture directly over said crucible to permit vaporized metal to radiate upward along straight lines to strike the cylindrical surface of each of said'insulatorsas the rotation of said mandrels about said vertical axis causes said insulators to pass across said aperture, the shape of said aperture being defined by four curvilinear sides, two of said sides being circularly arcuate and concentric with said vertical axle and the remaining two sides being mirror images of each other and joining the ends of said arcuate sides, said crucible being located substantially midway between said arcuate sides and substantially midway between said remaining two sides, the curvature of said remaining two sides being such as to place their proximal points on a circle concentric with said axle and passing directly over the center of said crucible and to direct an equal amount of metallic vapor to all areas of the cylindrical surface of each of said insulators.
5. The-machine of claim 4 including electrical contact means rotatable'with said mandrels about said verticalaxleand making contact with the ends of'only one of said insulators to measure the electrical resistance of said insulator.
References Cited in the file of this patent UNITED STATES PATENTS McLeod Oct. 28, 1941 Sukumlyn Dec. 30, 1947 Johnson et a1. Sept. 12, 1950 Van Leer et al. Dec. 5, 1950 Dimmick et a1 Mar. 31, 1953 10