US3295006A - Unannealed nickel screen grid mesh for pickup tubes - Google Patents

Description

Dec. 27, 1966 J. G. ZIEDONIS 3,29

UNANNEALED NICKEL SCREEN GRID MESH FOR PICKUP TUBES Filed Feb. :5, 1964 I NVENTOR. JIM/5 & Z /00/V/5 OMKJW United States Patent 3,295,0il6 UNANNEALED NICKEL SCREEN GRED MESH FGR PICKUP TUBES Janis G. Ziedonis, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Feb. 3, 1964, Ser. No. 342,089 6 Claims. (Cl. 313-269) This invention relates to pickup tubes and in particu velope and facing the electron gun positioned in the other end portion of the envelope. The target electrode includes a transparent conductive coating or signal electrode on the gun side of the transparent support member, and a photoconductor comprising a planar deposit of photoconductive material on the transparent conductive coating. Photoconductive materials are materials which undergo a change in their electrical conductivity in response to incident radiations. These materials have a relatively high electrical resistance when in the dark and a relatively high electrical conductivity when exposed to light or other radiations of a selected frequency.

Closely spaced from the exposed planar surface of the photoconductive material in the direction of the electron gun, is a fine mesh screen electrode. The fine mesh screen electrode is substantially planar and is positioned during manufacture of the tube so as to lie substantially parallel to the exposed planar surface of the photoconductive material. The preservation of such parallel disposition of the mesh screen is desirable during operation of the tube so that the screen may perform its function without adverse effects on the output of the tube. Such function involves creating a uniform field between the target and mesh for causing the electron beam to be perpendicular to the target on striking it.

Adverse effects are likely to arise if the mesh screen is caused to vibrate with appreciable amplitude during operation of the tube. Such amplitude vibrations may be of the resonant type excited by relatively small mechanical shocks applied to the tube during operation.

The relative movement between the target and mesh involved in such vibrations results in variation of the effective capacitance between the mesh and target. This variation of the effective capacitance is reflected in a corresponding variation in the picture signal output level.

A contributing fact-or to the problem of mesh vibration has been the belief heretofore that the composition of the mesh should be restricted to a non-magnetic material such as copper and that the mesh should be heated to a relatively high temperature during processing. A prior practice of making an assembly including a mesh mounted on a support ring, has involved the following steps. A copper mesh was first loosely extended across the support ring and then welded thereto without attempting to tighten the mesh mechanically. For accomplishing a tightening of the mesh on the support ring and also for cleaning the mesh, the assembly was then heated in a furnace to about 780 C. However, in spite of the attempts to tighten the mesh by heating, the mesh having the foregoing composition and processed as indicated, was characterized by appreciable vibration during operation of a tube in which it was incorporated.

This is believed due to the fact that copper begins to stress relieve at a relatively low temperature, i.e., about 200 C. Therefore, heating the copper to a temperature of about 780 C. as required in prior methods, fully stress-relieved or annealed the copper, thereby enlarging its crystals and causing a reduction in its internal damping property.

Accordingly, it is an object of the invention to provide an improved pickup tube.

Another object is to provide a pickup tube having an improved mesh screen in which vibration is minimized.

A further object is to provide an improved method of making such a mesh screen assembly.

The foregoing objects are realized in a pickup tube having a mesh screen assembly using a screen which is unannealed and stretched across the supporting ring prior to being welded thereto. The subsequent welding of the mesh to the ring completes the mesh-ring assembly. Heating of the assembly is deliberately avoided.

Further, instead of copper as heretofore employed, the mesh screen is made of a material such as nickel. Although this material is magnetic, applicant has found that the magnetic character of the metal is not objectionable.

This preference for a material such as nickel is a consequence of the fact that the annealing temperature of this metal is much higher than the normal tube processing' and application temperatures. Stretching the screen mesh across the support ring prior to welding the screen mesh to the support ring and without annealing the screen assembly has a significant bearing on screen vibration. It is believed that screens made according to this invention provide internal damping of the mesh assembly, thus considerably reducing the decay time of vibration.

A vibration of the mesh screen at a relatively small amplitude or a vibration having a relatively fast amplitude decay, is free from any appreciable harmful effects in the output of the tube. Applicant has found that his improved mesh screen is characterized by such a fast decay of vibration amplitude that it is not detectible after a period of about 0.1 second on a monitor of a camera. Applicants mesh screen is therefore of appreciably greater advantage than prior copper mesh screens.

In the drawing which illustrates an embodiment of the invention,

FIG. 1 is a side view, partly in section, of a vidicon type of pickup tube in which the invention is used;

FIG. 2 shows a sectional view of a mesh screen assembly employed in the tube of FIG. 1;

FIG. 3 shows a jig in cross-section, employed in carrying out one step of the method of the invention; and

FIG. 4 shows in cross-section the jig of FIG. 3 as well as an additional jig employed in another step of the method of the invention.

Referring to the drawing, in FIG. 1 there is shown a photoconductive type pickup tube 10 which is conventional except for the improvements embodied therein in accordance with this invention as will appear hereinafter. The tube 10 comprises an elongated evacuated envelope 12 having in one end thereof an electron gun 14 for producing an electron beam. The electron gun 14 includes a cathode electrode 16, a cup-shaped control electrode 18, a first accelerating electrode 20, a second accelerating electrode 21 and a final accelerating electrode 22. The end of the accelerating electrode 22 remote from the gun 14 is terminated by an apertured fine mesh screen electrode 24. The electrode assembly forming the gun 14 is supported by lead-in pins 25 that extend through a stem 26 that forms an end of the envelope 12 and the spring spacer elements 23. The stem 26 also includes an exhaust tabulation 27.

The other end of the envelope 12 includes a lighttransparent faceplate 28 made of a material such as glass and having a target electrode 29 on the inner face thereof.

The target electrode includes a layer 30 of photoconductive material such as porous antimony trisulfide deposited over a conducting transparent layer or signal electrode 32. The conducting layer 32 may consist of tin oxide. The screen electrode 24 is relatively closely spaced with respect to the target electrode 29, i.e., .050 inch.

As shown in more detail in FIG. 2, the mesh screen electrode 24 is mounted in stretched condition across a circular mounting ring 40. The mesh screen electrode is made of a metal in which the internal stresses thereof are preserved during processing and operation of tube 10. Such metal may be nickel in relatively pure form, such as electrolytic nickel. The ring 40 is made of a metal that is characterized by the several properties of being non-magnetic and substantially free from oxidation, and contributing to the formation of a good braze or weld. One material that applicant has found suitable for the ring 40 is an alloy known under the trade name of Nichrome. This alloy consists of nickel, iron and chromium. The mesh screen electrode 24 is fixed to the outwardly extending flange 42 of the mounting ring 40 by suitable means such as a body of brazing material 44 made of Nichrome.

The assembly comprising the screen electrode 24 fixed to the support ring 40, is mounted on the end of the final accelerating electrode 22 adjacent to the target electrode 29 (FIG. 1). The mounting of the screen assembly on electrode 22 is accomplished by a forced fit only. The mounting is facilitated by means of embossments 43 extending inwardly from the electrode 22 (FIG. 2). Three or more embossments may be employed. The limited area contacts provided by the embossments absorb the entire of a moderate force applied to the screen and ring assembly when a portion of the assembly is telescoped into one end of the electrode 22, as shown in FIG. 2. This results in a relatively high pressure per unit area between the embossments 43 and the ring support 40. Such high pressure is adequate to prevent displacement of the screen and ring assembly from the electrode 22 during operation of the tube 10. This type of mounting is advantageous from several standpoints. It contributes to economy in tube fabrication and avoids harmful effects from heat required in welding or brazing.

The assembly comprising the screen 24 and the support ring 40 is made in a convenient manner by utilizing jigs shown in FIGS. 3 and 4. The jig shown in FIG. 3 comprises two metal rings 46, 48 adapted to be clamped together by spring clamps 50 fixed to the outer periphery of ring 48 and urged into recesses 52 in the outer periphery of ring 46. A screen workpiece 54 positioned between jig rings 46, 48 is held firmly when the clamps are seated in the recesses 52. To assure a coplanar disposition of the central portion of the screen workpiece 54 with the portion thereof engaged by the rings 46, 48, the jig includes a support 56 having a relatively thin annular portion 58 on which the ring 48 is adapted to rest. The central portion 60 of support 56 is sufficient thick so that its upper surface, as viewed in FIG. 3, is in the plane of the upper surface of the ring 48 when this ring rests on the annular portion 58. In this way the screen workpiece 54, when laid over the ring 48 and the thick portion 60 of the support, is disposed in a single plane. It is found, however, that after the support 56 is removed subsequent to clamping the screen workpiece 54 by the rings 46, 48, a slight dropping displacement of the central portion of the screen workpiece occurs. Such displacement is objectionable in the completed screen electrode inthat when this electrode is incorporated in the tube the displacement will adversely affect a desired uniformity in spacing between the screen electrode and the target electrode 29.

To avoid such displacement of the screen workpiece 54 prior to affixing thereof to the flange 42 of the support ring 40, the jig shown in FIG. 4 is employed. This jig comprises a base 62 from which a cylinder 64 extends upwardly. The cylinder 64 has an inner diameter for loosely receiving therein the cylindrical portion 66 of the screen support ring 40, and supportingly engages the under-side of flange 42 of the support ring. When the screen workpiece 54 while clamped between the rings 46, 48, is laid over the flange 42 of the support ring, and the rings 46, 48 are relieved of support, the combined Weight of the rings which may be grams, is suflicient to stretch the screen workpiece 54 a desirable amount, so that the entire portion of the screen workpiece within the circle defined by the flange 42 is in one plane when supported solely by this flange It has been found that a stretching which produces a frequency response, of, for example 5000 -c.p.s., results in a satisfactory screen.

While the screen workpiece 54 is so supported by the flange 42, a relatively thin brazing washer is placed over the portion of the workpiece engaged by the flange 42. The brazing washer 68 may be made of Nichrome. Successive portions of the brazing washer 68 are heated to the melting temperature of the washer i.e. 1400 C., for brazing the screen workpiece 54 to the flange 42. The heating may be accomplished electrically by means of an elect-rode 70 connected to a suitable electric power supply 72 as shown in FIG. 4. To avoid weld splash, the power density used is about 10 Watt seconds.

The localized character of the heating and the appreciably large thermal reservoir provided by the support ring 40 results in the avoidance of any harmful heating of the screen workpiece 54 during the brazing operation,

After the brazing operation has been completed, the screen workpiece 54 is released from the clamping rings 46, 48 and the portion of the screen extending radially outward from the flange 42 is suitably removed. After such removal the assembly comprising the screen grid 24 and support ring 40 is completed.

The advantage of using nickel, or similar metal, as the composition of the mesh screen will appear from the following.

Nickel has an annealing or stress relieving temperature of about 620 C. This temperature is appreciably above that reached by a pickup tube either during processing or operation. The highest processing temperature of the tube is about 350 C. The operating temperature is appreciably lower. Thus a mesh screen made of nickel remains free from stress relief and is characterized by a higher number of dislocations (various lattice imperfections) and relatively small crystal structure.

The different lattice imperfections are very important in re-establishing equilibrium or steady-state conditions in a solid such as a screen mesh. Such imperfections are either point, or line defects. Vacant lattice sites and interstitial atoms are the principal point defects involved in migration phenomena through the material. The principal line defects are the dislocations. These dislocations arise during crystal growth in manufacturing the mesh. These dislocations are important because of their significance in plastic behavior of crystals and crystal boundaries.

By exposing the mesh to extremely high temperatures the number of dislocations in the mesh material is reduced and the crystal structure therein is considerably enlarged. Such reduction in the number of crystal dislocations is undesirable for the purposes of the present invention. This is because a high number of dislocation is necessary for high mechanical energy losses in the material.

The greater shock absorbing characteristic of unannealed material is believed to be due to the fact that external impact energy doe irreversible work on the dislocations and hence dissipate energy. Also the smaller crystal size contributes to higher internal damping. This may be explained in terms of the increased crystal boundary area defined by the smaller crystals. This increased crystal boundary area will increase the internal energy loss through crystal boundary slip. In an annealed maten'al having relatively few dislocations, movement of the dislocations in the material are few and the impact energy cannot be dissipated fully in such movements. The only way that the impact energy applied to a body of annealed material can be relieved is by movements of the entire body in the form of relatively high amplitude vibrations.

One technique practiced by applicant in making the mesh screen workpiece 54 has involved the following steps. A coating of Wax one-sixteenth of an inch thick is applied to a smooth glass substrate. By means of a ruling engine horizontal and vertical grooves are provided in the wax coating. The grooves are of sutficient depth to expose portions of the glass substrate. In one example 1000 horizontal grooves and 1000 vertical grooves were provided per square inch of area ruled, A smaller number of grooves may be provided as desired. Thereafter palladium is sputtered over the grooved surface, to substantially fill the grooves with palladium. This also resulted in the formation of an unwanted thin palladium coating on the remaining Wax portions of the grooved surface. Palladium was selected as the best material upon which to deposit nickel to form a nickel screen in accord ance with the invention, because of the ease with which palladium can be removed after it has served its purpose. Thus after the palladium has been sputtered upon the grooved surface the relatively thin coating thereof formed on the wax is easily removed after the coated substrate is immersed in deionized water. One way in which removal of the thin coating may be accomplished is by merely moving the hand of an operator lightly across the coated surface. The relatively thin coating of palladium on the waxed portions of the grooved surface is thus easily removed while the palladium in the grooves is shielded by the wax and remains in the form of a palladium matrix. The glass substrate having the palladium matrix thereon is next subjected to an electroplating operation for depositing a layer of nickel over the palladium. The bath used in the electroplating operation may include nickel sulfamate in a suitable solvent, into which the substrate having the palladium matrix thereon is immersed. Since all portions of the palladium matrix are electrically interconnected, as a consequence of the integral structure of the matrix, one portion thereof adjacent to the edge of the substrate is electrically connected, as by an alligator clip, to the negative side of a current supply. The positive side of the current supply is connected to an anode, preferably made of a nickel-carbon alloy, immersed in the plating bath. A 60 ampere current supply may be employed.

The substrate with the palladium matrix thereon is permitted to remain in the bath until a nickel coating having a thickness of about 0.2 mil is deposited upon the palladium matrix. Thereafter the substrate having thereon the palladium matrix coated with nickel, is removed from the plating bath and placed in a bath of deionized water. The matrix or screen of nickel-coated palladium is then pulled manually from the substrate. The palladium is then removed from the nickel screen as by rubbing. The resultant nickel screen constitutes the screen workpiece 54 shown in FIG. 3. The screen so processed has a transparency of 68%.

It will be noted in FIG. 4 that the screen workpiece 54 includes an edge portion extending beyond the circle of the sup-port ring 40. This edge portion forms no part of the active portion constituting the screen 24 shown in FIG. 2. Therefore the edge portion of the screen 54 adversely affected by the engagement of the palladium matrix by the alligator contact during the plating operation, is removed after the screen 24 is fixed to the ring 40.

Constructing a screen in accordance with this invention results in screen elect-rode for a camera tube in which the period of vibration of the screen (the decay time) is appreciably reduced thus providing improved operation of the tube in which the screen is used. An improved meth- 0d of making such a screen electrode is also provided.

What is claimed is:

1. A pickup tube having:

(a) a planar screen grid,

(b) said screen grid including a tautly supported perforated planar metal mesh structure,

(c) the metal of said structure being unannealed and having an annealing temperature higher than the processing and operating temperatures of said tube,

(d) said structure being uncoated,

(e) whereby said screen grid is characterized by a relatively high internal damping of vibrations.

2. A pickup tube having:

(a) a planar screen grid,

(b) said screen grid comprising a metal ring and a metal mesh screen structure tautly supported across said ring,

(c) said planar metal screen consisting of bare unannealed nickel having an annealing temperature higher than the processing and operating temperatures of said tube,

(d) whereby said structure is adapted to dampen vibrations thereof.

3. A pickup tube having:

(a) a planar target,

(b) a planar mesh screen grid adjacent and parallel to said target,

(1) said screen grid including a screen portion of uncoated unannealed metal,

(2) said metal having an annealing temperature higher than the processing and operating temperatures of said tube,

(3) whereby vibrations of said screen portion are damped for preserving a substantially constant predetermined spacing between said target and grid.

4. A pickup tube having:

(a) a target,

(b) a screen grid closely spaced from said target,

(1) said screen grid including a metal support ring,

(2) a perforated metal mesh structure tautly supported across said ring, said metal mesh structure being free of any coating,

(3) said metal structure consisting of unannealed nickel having an annealing temperature higher than the processing and operating temperatures of said tube,

(4) whereby vibrations of said metal structure are damped for preserving constant the relatively close spacing between said grid and target.

5. A pickup tube having:

(a) an elongated envelope,

(b) a planar target in one end portion of said envelope,

(c) an electron gun in the other end portion of said envelope, and

(d) a planar screen electrode intermediate said target and electron gun,

(1) said screen electrode being parallel and adja cent to said target and including an uncoated mesh screen portion consisting of an unannealed metal,

(2) said metal having an annealing temperature substantially higher than the highest temperature to which said tube is subjected during processing and operation.

6. A pickup tube having:

(a) an elongated envelope,

(b) a planar target in one end portion of said envelope,

(c) an electron gun in the other end portion of said envelope, and

(d) a planar mesh screen electrode intermediate said target and electron gun, (1) said screen electrode and said target electrode being in two relatively closely spaced and paralr lel planes,

(2) said screen electrode being made of unannealed and uncoated metal having an annealing temperature substantially higher than the processing and operating temperature of said tube,

(3) whereby high amplitude vibrations of said screen electrode are damped and said close spacing between said target and screen electrode is preserved constant.

References Cited by the Examiner UNITED STATES PATENTS 2,538,836 1/1951 Jensen 31389 8 2,547,638 4/1951 Gardner. 2,738,436 3/1956 Zaphiropoulos. 2,878,540 3/ 1959 Willner 2925.14 2,901,649 8/ 1959 Knight 31368 2,926,419 3/ 1960 Harris 313-348 X 2,930,718 3/1960 Coflin. 2,951,962 9/1960 Miller et a1. 313-89 2,979,633 4/1961 Harris 31389 3,042,992 7/ 1962 Weissfioch 2925 .14

0 JOHN W. HUCKERT, Primary Examiner.

DAVID J. GALVIN, A. J. JAMES, J. D. KA-LLAM,

' Assistant Examiners.

Claims (1)

1. A PICKUP TUBE HAVING: (A) A PLANAR SCREEN GRID, (B) SAID SCREEN GRID INCLUDING A TAUTLY SUPPORTED PERFORATED PLANAR METAL MESH STRUCTURE, (C) THE METAL OF SAID STRUCTURE BEING UNANNEALED AND HAVING AN ANNEALING TEMPERATURE HIGHER THAN THE

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