US3788721A - Electrically conductive components - Google Patents

Abstract

Metal elements, usually of refractory metal, for electrical devices such as filaments for incandescent lamps, discharge lamp cathodes, grids or heaters for thermionic valves and radiation elements for electric heaters are made by forming an etch resist pattern on a metal web, etching the web, removing the resist and bending the etched product into compact form. The element produced has a substantially rectangular section which provides a high surface area to cross-sectional area ratio, leading to high thermal and light emission efficiency.

Description

United States Patent 1191 Vause Jan. 29, 1974 ELECTRICALLY CONDUCTIVE COMPONENTS [75] Inventor: Arthur Samuel Vause, London,

England [73] Assignee: Thorn Electrical Industries, Limited, London, England 22 Filed: July 26,197]

21 Appl.No.: 165,516

[30] Foreign Application Priority Data Dec. 15, 1970 Great Britain 59,567/70 [52] US. Cl 316/17, 29/25.17, 29/624, 313/315 [51] Int. Cl.H01j 9/02 [58] Field of Search 29/625-627, 602,

[56] References Cited UNITED STATES PATENTS 2,666,254 1/1954 Eisler 29/602 2,722,621 11/1955 Schenan 29/602 2,773,239 12/1956 Parker 29/602 UX 3,089,106 5/1963 Soaty 29/602 3,216,089 ll/l965 Dettman 29/630 B 1,013,157 5/1909 Hadaway 1,674,488 6/1928 Tang 1,697,607 l/1929 Maxson....

2,655,582 10/1953 Kirby 338/207 X OTHER PUBLICATIONS Durand, Metals Handbook, Vol. 3, Chemical Machining, 1967, pp. 240244.

Primary Examiner-Charles W. Lanham Assistant Examiner-Joseph A. Walkowski Attorney, Agent, or Firm-Robert F. OConnell; Dike, Bronstein, Roberts & Cushman [57] ABSTRACT Metal elements, usually of refractory metal, for electrical devices such as filaments for incandescent lamps, discharge lamp cathodes, grids or heaters for thermionic valves and radiation elements for electric heaters are made by forming an etch resist pattern on a metal web, etching the web, removing the resist and bending the etched product into compact form. The element produced has a substantially rectangular section which provides a high surface area to crosssectional area ratio, leading to high thermal and light emission efficiency.

' 7 Claims, 11 Drawing Figures ymminmzs 1914 3,788,721

SHEET 1 BF 3 ELECTRICALLY CONDUCTIVE COMPONENTS The present invention relates to electrically conductive components for electrical devices and methods of making them. Examples of such components include filaments for electric incandescent lamps, discharge lamp cathodes, radiation elements for electric heaters and grids and heaters for'thermionic valves.

In accordance with the present invention there is provided a method of making an electrically conductive component for an electrical device which comprises etching away a metal web having etch-resistant material on both faces thereof disposed in such manner as to protect the portions corresponding to the component it is desired to produce, with at least one face of the web having the etch-resistant material disposed in a pattern correspondingto the components it is desired to produce, and removing the etch-resistant material from both faces pf the component thus produced. Initially the portions of the component produced lie in the simple surface of the workpiece web. In order to improve the operating effeciency of thermal or incandescent elements made in accordance with the invention, the component produce by etching the web can be bent into compact form in which portions of the element are brought closer together. Preferably, both faces of the web have the etch-resistant material disposed in a pattern corresponding to the component it is desired to produce, the patterns being in overlying relationship to one another.

The method of the invention is particularly suitable for automated manufacture of electrical components, and may be applied to the manufacture of components from metal webs in the form of foil, ribbon or sheet whereby continuous or batch production can be employed as the circumstances require.

A further advantage is that changes in the shape of the component produced can readily be made merely by applying a different pattern of etch-resistant material. It is preferred to employ a photographic method for providing the etch resistant pattern or patterns on the web, whereby design changes can be made by amendment of the master drawing of the pattern, or substitution of a different drawing.

Where the component is to be used as a thermal or incandescent element it will normally be produced in elongate form, and be made of a refractory metal. It will be appreciated that the component produced by etching a web will be a rectangular (which term is intended to include square) cross-section. Thermal or incandescent elements produced in accordance with the invention have the advantage, as compared with known .elements of circular section wire, of having a greater ratio of surface area .to cross-sectional area, thereby providing proportionalincrease in emmitted radiation and therefore in light output.

For example, comparing equal cross-sectional areas in circular and rectangular forms the surface of a rectangular section (of 4:1 side ratio) exceeds that of the circular section by more than 40percent.

In a further aspect of the invention there is provided an electrical device having a heat or light emissive element of substantially rectangular section.

The invention also provides an electrically conductive component for an electrical device comprising an elongate substantially rectangular section member of refractory metal bent into a compact form.

Where the component is to be employed as a thermal or incandescent element it is usually bent so as to have a plurality of sections inclined to the general direction in which the filament extends. Sections of the element may be parallel to one another and perpendicular to the axis of the filament.

The present invention will now be more fully described, by way of example only, with reference to the accompanying drawings in which:

FIGS. 1, 2 and 3 show tungsten webs having lamp filaments defined therein by surrounding etched away regions;

FIGS. la and 1b show the filament of FIG. 1 bent into alternative compact forms;

FIGS. 2a and 3a shown the filaments of FIGS. 2 and 3 respectively bent into compact forms;

FIG. 3b shows fragmentary views of the filament of FIG. 3a provided with tabs for engaging filament supports;

FIGS. 4 and 5 show lamp filaments produced by etching a tungsten web; and

FIG. 4a shows the filament of FIG. 4 bent into compact form.

In an example of the method of the invention, tungsten foil of 0.005 inches thickness is initially degreased in liquid or vapour phase with for example trichlorethylene. The foil is dipped in a solution of photo-resist material for example Kodak I(.M.E.R. diluted with an equal part of K.M.E.R. thinner and with dipping and withdrawl being carried at a controlled rate of 6 in/min. After coating, the foil is allowed to dry for 5 minutes and is then stored at 120C for 10 minutes.

A twin pair of mask profiles corresponding to the component it is desired to produce, for example a lamp filament as shown in any Figure of the accompanying drawings, is placed in intimate contact with each opposing face of the resist-coated foil. The profiles, which may be sealed photographic images, may be prepared by scaling-down by a photographic method a sharply defined and accurately dimensioned master drawing.

The foil with the mask profiles is irradiated with ultraviolet rich light from fluorescent lamp, high intensity metal vapour discharge lamps, carbon arcs or pulsed xenon lamps. Pulsed xenon lamps may be preferred as the exposure time is short and this helps to reduce undercut at the subsequently etched surfaces.

The exposed photo-resist is developed in xylol, typically for between I to 3 minutes. After development the photo-resist image is washed in water and baked for 10 minutes at 120C.

The etching step may be carried out chemically or electrochemically (deplating). We prefer chemical etching with a mixed agneous solution of potassium ferricyanide, e.g. 48 oz. per gallon, and sodium hydroxide, eg, 7 oz. per gallon, at C for 10 minutes, and with this etching method we have found that very little undercut is obtained. The etchant solution may be spray applied.

The photo-resist can be removed from both sides of the component produced by mechanical treatment such as scrubbing or by washing.

It will be appreciated that the patterns of etch resist material can be provided on the foil by known methods other than photo-resist techniques.

We have found that tungsten sheet materials of thicknesses down to 0.001 in. can be satisfactorily processed by the method described above and, where the component is a lamp filament having sections inclined to the general direction in which the filament extends, with a separation between adjacents'ections as little as 0.0005

Referring to FIG. 1, a lamp filament 1 is defined in a piece oftungsten foil 2 by surrounding etched away regions of the foil, the whole being produced by the method described above. Before use, the ends of the filament l are severed from the remainder of thefoil 2.

FIG. 1a shows one way in which the filament 1 can be bent into compact form to give thermal operating efficiency be reducing cooling losses when employed as a filament in a gas-containing incandescent lamp.

In FIG. lb the filament 1 is bent into a compact form effecting a compromise between thermal operating efficiency and increasedsource brightness.

Whereas known methods of filament manufacture require the manufacturer to hold large stocks of tungsten wires of varying diameters and stocks of winding mandrels, the method of the invention avoids the need for mandrels and offers flexibility in the choice of dimensions of the filament which can be obtained from tungsten sheet of a single thickness, since the width of the filament can be varied at will. Furthermore, the cost of providing and maintaining high speed precision machinery required for known filament winding techniques is avoided. In lamp manufacture, the cost of shaping filament tails and positioning them relative to the filament axes is frequently greater than the cost of the filament itself and requires precision jigging or skilled operations. In the method of the invention the filaments can be provided with tails of any desired profile more simply.

The filaments of the invention can be produced with integral fuse protection sections at their ends. The fuse section can be of reduced-cross-section appropriate to the circuit protection requirements and at a distance from the incandescent region of the filament suited to the particular lamp design.

In known coil-wound filaments, filament failure through burn-out has been attributed to the presence of severe thermal gradients betwen adjacent turns. To reduce the likelihood of such failure the practice of winding filaments with varying pitch between adjacent turns has been advocated to even out the temperature over the whole of the filament length. FIG. 2 illustrates how such variation can be applied in the present invention, where a filament 3 has sections inclined to the general direction in which the filament 3 extends and the spacing of adjacent sections near the ends of the filament 3 is smaller than the spacing intermediate the ends. FIG. 2a shows how varying pitch between the sections results when the filament 3 is bent into compact form.

In known methods in which wire is wound on a mandrel, the space occupied by the mandrel results in a relatively large hollow core in the formed filament. The invention avoids the need for a mandrel, allowing greater scope in designing the filament to achieve optimum performance. FIGS. 3 and 3a illustrate how the separation between portions of a filament 6 can be minimised within the total space occupied. In FIG. 3a the filament 6 is bent to achieve a close approximation to a solid light source.

In forming a filament by the method of the invention, the etch-resistant material may be so disposed as to produce a filament which is-elongate and has regularly spaced tab portions projecting laterally therefrom, the tab in use serving to engage supports for the filament. In FIG. 3b there are shown tab portions 7 which can be employed as support-engaging portions for the filament 6. By employing tab portions 7, undesirable mechanical stresses and localised cooling, which could occur where the supports contact the filament, are avoided.

FIGS. 4 and 4a show respectively alamp filament 8 of the invention when the planar form and when bent into compact form. The form of filament shown in FIGS. 4 and 4a provides a basis from which a close approximation to uniformly bright spherical source, suited to many optical applications, may be achieved.

FIG. shows alamp filament 9 suitable for digital display formed by the method of the invention. The

- lamp comprises seven segments having individualintegral terminals 11 through which the segments can be energised singly or in combination.

In lamp manufacture it is usually advantageous to employ filaments of metal having selected crystalline characteristics, as for example in gas filled high performance lamps where filaments of tungsten having well-developed and interlocking tungsten crystals are desirable. In the method of the invention, the crystal structure of the initial metal web can be determined by etching the surface of the web and identifying the crystal boundaries. The regions of the web from which it is desired to form the filaments can then be selected, whereby it is possible to produce filaments having any pre-selected crystal structure.

The rectangular section of lamp filaments in accordance with the invention facilitates their attachment to components within lamps for example leads, shields, refectors and insulating bridges, since they are stronger than conventional circular section filaments, and can be more precisely located. In particular their use in connection with the filament supports described in our co-pending United Kingdom Patent Application No. 18294/ offers significant advantages.

The use of filaments in accordance with the invention may simplify lamp manufacture where rectangular section tails of the lamps are employed as lead-in wires and are sealed directly through the envelope wall.

Where fused silica envelopes are employed the filaments may have portions of their tails subjected to rolling or chemical etching to reduce their thickness and cross-section to values permitting them to be sealed hermetically through the envelope wall. Filaments in accordance with the invention operating at temperatures of l,500C and upwards, of for example tantalum molybdenum or tungsten, may be sealed directly through glass or glass-ceramic envelopes of matching expansion, for example boro and alumina silicate glasses.

Heater filaments for valves or low temperature lamp filaments in accordance with the invention, of for example nichrome or nickel iron alloy, can be sealed directly through soft glass (lime soda) envelopes.

Although the invention has been described mainly as applied to incandescent lamp filaments, it will be apparent that the invention can be readily applied to discharge lamp electrodes for low and high pressure lamps, heating elements for domestic electrical heaters, heaters and grids for thermionic valves and cathode ray tubes.

In particular, the advantages of the invention, where applied to electrodes for low pressure fluorescent tubs,

where flat tape-like electrodes of close pitch interweaving construction can be employed, and for high pressure discharge lamps, where thin sheet enclosures hav ing precisely dimensioned and spaced apertures surrounding an emissive material core can be employed, are comparable to those obtained with incadescent filaments lamps.

I claim: 1. A method of manufacturing a gas-filled electric incandescent lamp comprising the steps of:

providing a lamp envelope; preparing a filament therefor by a method which includes the steps of providing etch resistant material coatings on both faces of a refractory metal web, at least one of said coatings having the form of a pattern corresponding to the component it is desired to make; etching said web to etch away metal from areas thereof unprotected by said coatings; and removing said coatings; positioning said filament within said envelope; sealing said filament within said envelope; and gasfilling said envelope.

2. A method according to claim 1 wherein both of said coatings have the form of a pattern corresponding to the component it is desired to make, said patterns being disposed in overlying relationship.

3. A method according to claim 1 wherein said pattern coating is provided on said web by a photographic method.

4. A method according to claim 1 wherein said pattern has an elongate form with regularly spaced tab portions projecting laterally therefrom.

5. A method according to claim 1 wherein said web is of tungsten.

6. A method according to claim 5 wherein said web is etched with a solution of potassium ferrocyanide and sodium hydroxide.

7. A method according to claim 1 and further including the step of bending said etched web to bring closely adjacent one another portions of said etched web which are spaced apart along said etched web, whereby said etched web is brought into compact form before being positioned in said envelope.

* =l =l l

Claims (6)

1. 2. A method according to claim 1 wherein both of said coatings have the form of a pattern corresponding to the component it is desired to make, said patterns being disposed in overlying relationship.
2. 3. A method according to claim 1 wherein said pattern coating is provided on said web by a photographic method.
3. 4. A method according to claim 1 wherein said pattern has an elongate form with regularly spaced tab portions projecting laterally therefrom.
4. 5. A method according to claim 1 wherein said web is of tungsten.
5. 6. A method according to claim 5 wherein said web is etched with a solution of potassium ferrocyanide and sodium hydroxide.
6. 7. A method according to claim 1 and further including the step of bending said etched web to bring closely adjacent one another portions of said etched web which are spaced apart along said etched web, whereby said etched web is brought into compact form before being positioned in said envelope.

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