WO2007023308A1

WO2007023308A1 - Silver wire

Info

Publication number: WO2007023308A1
Application number: PCT/GB2006/050171
Authority: WO
Inventors: Graham John Bratton
Original assignee: Middlesex Silver Co. Limited
Priority date: 2005-08-23
Filing date: 2006-06-28
Publication date: 2007-03-01
Also published as: GB0517166D0

Abstract

This invention relates to silver alloy fibres or wire of sufficiently small diameter that they have properties similar to conventional textile fibres, to structures e.g. woven, non-woven or knitted products from said fibres and to their manufacture. The fibres may be of Ag Cu Ge alloy and of diameter less than 200 m. The invention relates to articles e.g woven, non-woven, knitted, needle-punched twisted, linked or braided articles comprising fibres as aforesaid either alone or combination with other e.g. natural or synthetic organic fibres for a variety of applications including filtration or catalyst support, formation of composite materials and formation of textile products.

Description

SILVER WIRE

FIELD OF THE INVENTION

This invention relates to silver fibres or wire of sufficiently small diameter that they have properties similar to conventional textile fibres, to structures e.g. woven, non- woven or knitted products from said fibres and to their manufacture.

BACKGROUND TO THE INVENTION

The literature on production of silver wire is relatively sparse. For example US Patent 6627149 (Tayama) discloses the production of silver wire of relatively large diameter and high purity for use in recording or image transmission applications.

The literature concerning woven structures based on silver is sparse. Such woven structures have mainly been based on strips, strands or filaments braided together (US-A-240096, Crane, US-A-253587 Crane and US-A-5203182 Wiriath). However, US-A-2708788 (Cassman et al) discloses silver mesh or foil through which material is to be evaporated during the manufacture of television tubes, the mesh being tautened by depositing gold thereon and alloying the silver and gold to bring about shrinkage of the mesh. US-A-5122185 discloses mesh of precious metal used as so- called "getters" in recovery of platinum from a gas stream from the oxidation of ammonia. The mesh is preferably of pure palladium, but may also be an alloy of palladium with one or more metals selected from nickel, cobalt, platinum, ruthenium, iridium, gold, silver and copper.

It is known to knit metal wires or fibers e.g. as in US-A-2274684 (Goodloe), but existing knitted metal fabrics are predominantly of ferrous alloys. US-A-5118813 (Fairey et al; Johnson Matthey) discloses weft-knitted fabrics consisting essentially of interlocking loops of fibers of precious metal selected from platinum group metals, gold, silver and alloys thereof using circular or flat-bed knitting machines, with platinum or platinum alloys for use as catalyst gauzes being preferred. Fairey et al found that wires of platinum alloy or of metals with similar mechanical properties could not be knitted effectively and that attempts to do so resulted in fiber breakage and machine jams because the tensile strength of the metal fibers was insufficient to withstand the factional forces in the knitting process. The solution disclosed was to feed the metal fiber with a supplementary fiber that acted as a lubricant, the supplementary fiber preferably being in the form of a multi-strand rather than a monofilament and the strands surrounding the metal wire to minimize metal-to-metal contact. After knitting, the supplementary fiber may be removed by dissolving in a solvent or by pyrolysis. WO 92/02301 (Heywood) discloses warp-knit fabric of platinum, palladium or rhodium wires e.g. using tricot, raschel or jacquard knitting to give catalyst gauzes that are more flexible or open than woven gauzes and that are less likely to warp under thermal stress. Knitting is facilitated either by lubricating the wire with a lubricant such as starch or wax or by feeding a supplementary fiber. A particular structure of fine mesh warp-knit fabric based on wires of noble metal and for use as a catalyst is disclosed in US-A- 6089051 (Gorywoda et al). None of the above references discloses or suggests forming knitted structures based on fine silver or on a silver alloy, and our experience is that

Patent GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche) discloses a novel silver alloy that maintains the properties of hardness and lustre inherent in Ag-Cu alloys while reducing problems resulting from the tendency of the copper content to oxidize. The alloys are ternary Ag-Cu-Ge alloys containing at least 92.5 wt% Ag, 0.5-3 wt% Ge and the balance, apart from impurities, copper. The alloys are stainless in ambient air during conventional production, transformation and finishing operations, are easily deformable when cold, easily brazed and do not give rise to significant shrinkage on casting. They also exhibit superior ductility and tensile strength. Germanium is stated to exert a protective function that was responsible for the advantageous combination of properties exhibited by the new alloys, and is in solid solution in both the silver and the copper phases. The microstructure of the alloy is constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver in copper which itself contains a few inter-metallic CuGe phase dispersoids. The germanium in the copper-rich phase was said to inhibit surface oxidation of that phase by forming a thin GeO and/or GeO₂ protective coating which prevented the appearance of firestain during brazing and flame annealing. Furthermore the development of tarnish was appreciably delayed by the addition of germanium, the surface turned slightly yellow rather than black and tarnish products were easily removed by ordinary tap water.

Patents US-A-6168071 and EP-B-0729398 (Johns) disclosed a silver/germanium alloy which comprised a silver content of at least 77 wt % and a germanium content of between 0.4 and 7%, the remainder principally being copper apart from any impurities, which alloy contained elemental boron as a grain refiner at a concentration of greater than 0 ppm and less than 20ppm. The boron content of the alloy could be achieved by providing the boron in a master copper/boron alloy having 2 wt % elemental boron. It was reported that such low concentrations of boron surprisingly provided excellent grain refining in a silver/germanium alloy, imparting greater strength and ductility to the alloy compared with a silver/germanium alloy without boron. Argentium (Trade Mark) sterling comprises Ag 92.5 wt % and Ge 1.2 wt %, the balance being copper and about 4 ppm boron as grain refiner. The Society of American Silversmiths maintains a website for commercial embodiments of the above-mentioned alloys known as Argentium (Trade Mark) at the web address http://www.silyersmitMng.com/largentium.htm.

US-A-6726877 (Eccles) discloses inter alia an allegedly fire scale resistant, work hardenable jewellery silver alloy composition comprising 81-95.409 wt % Ag, 0.5-6 wt% Cu, 0.05-5 wt% Zn, 0.02-2 wt% Si, 0.01-2 wt % by weight B, 0.01-1.5 wt% In and 0.01 -no more than 2.0 wt% Ge. The germanium content is alleged to result in alloys having work hardening characteristics of a kind exhibited by conventional 0.925 silver alloys, together with the firestain resistance of allegedly firestain resistant alloys known prior to June 1994. Amounts of Ge in the alloy of from about 0.04 to 2.0 wt% are alleged to provide modified work hardening properties relative to alloys of the firestain resistant kind not including germanium, but the hardening performance is not linear with increasing germanium nor is the hardening linear with degree of work. The Zn content of the alloy has a bearing on the colour of the alloy as well as functioning as a reducing agent for silver and copper oxides and is preferably 2.0-4.0 wt%. The Si content of the alloy is preferably adjusted relative to the proportion of Zn used and is preferably 0.15 to 0.2 wt%. Precipitation hardening following annealing is not disclosed, and there is no disclosure or suggestion that the problems of distortion and damage to soldered joints in nearly finished work made of this alloy can be avoided.

By way of background, US-A-4810308 (Leach & Garner) discloses a hardenable silver alloy comprising not less than 90% silver; not less than 2.0% copper; and at least one metal selected from the group consisting of lithium, tin and antimony. The silver alloy can also contain up to 0.5% by weight of bismuth. Preferably, the metals comprising the alloy are combined and heated to a temperature not less than 1250- 1400°F (676-760°C) e.g. for about 2 hours to anneal the alloy into a solid solution, a temperature of 1350° (732°C) being used in the Examples. The annealed alloy is then quickly cooled to ambient temperature by quenching. It can then be age hardened by reheating to 300-700°F (149-371°C) for a predetermined time followed by cooling of the age hardened alloy to ambient temperature. The age-hardened alloy demonstrates hardness substantially greater that that of traditional sterling silver, typically 100 ITVN (Vickers Hardness Number), and can being returned by elevated temperatures to a relatively soft state. The disclosure of US-A-4869757 (Leach & Garner) is similar. In both cases the disclosed annealing temperature is higher than that of Argentium, and neither reference discloses firestain or tarnish-resistant alloys. The inventor is not aware of the process disclosed in these patents being used for commercial production, and again there is no disclosure or suggestion that hardening can be achieved in nearly finished work.

A silver alloy called Steralite is said to be covered by US-A-05817195 and 5882441. US-A-6726877 and to exhibit high tarnish and corrosion resistance. The alloy of US-A-5817195 (Davitz) contains 90-92.5 wt % Ag, 5.75-5.5 wt % Zn, 0.25 to less than 1 wt % Cu, 0.25-0.5 wt % Ni, 0.1-0.25 wt % Si and 0.0-0.5 wt % In. The alloy of

US-A-5882441 (Davitz) contains 90-94 wt % Ag, 3.5-7.35 wt % Zn, 1-3 wt % Cu and

0.1-2.5 wt % Si. A similar high zinc low copper alloy is disclosed in US-A-4973446 (Bernhard) and is said to exhibit reduced firestain, reduced porosity and reduced grain scale. SUMMARY OF THE INVENTION

This invention is based on the realization that silver alloys containing germanium in an amount effective to impart tarnish and firestain resistance can be made into fibres or wires sufficiently fine that they will bend and drape in a similar way to conventional textile fibres of e.g. cotton, polyester or polyamide and can be formed into woven, non-woven, needled, knitted or braided products in similar ways to textile fibres. The fibres that can be made according to the invention may be similar in size to or smaller than a human hair or even of sub-micron size e.g. for use in HEPA filers and the like.

The invention therefore provides wire or fibres of AgCuGe alloy of diameter less than 200 μm.

The wire or fibres are typically of diameter 1-100 μm, more typically about 15-

50 μm, e.g. 1-30 μm .

DESCRIPTION OF PREFERRED EMBODIMENTS

Wire or fibres in accordance with the invention may have a variety of shapes, they may be circular, elliptical, oval or somewhat rectangular as when shaved from curved foil.

Significantly improved ductility, tensile strength and thermal and electrical resistance are possessed by at least some embodiments of the fibres, and it is expected that they can be formed into sintered or otherwise thermally bonded structures. In particular, if the fibres are formed into a mat or other generally planar structure, they may be resistance-heated by current applied across the planar faces in an inert atmosphere. In that case, the current will flow through the points of contact between the fibres, which owing to the slightly electrically resistive nature of germanium-containing silver alloys will become selectively heated and give a thermally bonded structure. Fibres of the above size range may be made by conventional wire drawing techniques. Fibres of diameter about 25-100 μm may also be made by a process which comprises forming a coil of thin metal foil of the desired thickness and shaving the edge of the coil to form the desired fibres (see e.g. EP- A-0319959).

The alloys that may be formed into wire of the above diameter include silver/germanium alloys having an Ag content of at least 77% by weight, a Ge content of between 0.5 and 3% by weight, the remainder being copper apart from any incidental ingredients and impurities, which alloy contains boron as a grain refiner. If desired, the germanium content may be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected. The weight ratio of germanium to incidental ingredient elements may range from 100: 0 to 80: 20, preferably from 100: 0 to 60: 40. The term "incidental ingredients" permits the ingredient to have ancillary functionality within the alloy e.g. to improve colour or as-moulded appearance, and includes the metals or metalloids Si, Zn, Sn or In in amounts appropriate for "deox".

The alloys that may be formed into wire according to the invention include coinage grade, 800-grade (including 830 and 850 grades and the like) and standard Sterling silver and an alloy of silver containing an amount of germanium that is effective to reduce firestain and/or tarnishing. The ternary Ag-Cu-Ge alloys and quaternary Ag-Cu-Zn-Ge alloys that can suitably be made by the method of the present invention are those having a silver content of at least 80%, and most preferably at least 92.5%, by weight of the alloy, up to a maximum of no more than 98%, preferably no more than 97%. The germanium content of the Ag-Cu-(Zn)-Ge alloys should be at least 0.1%, preferably at least 0.5%, more preferably at least 1.1%, and most preferably at least 1.5%, by weight of the alloy, up to a maximum of preferably no more than 6.5%, more preferably no more than 4%. Silicon, in particular, may be added to silver alloys e.g. in an amount of up to 0.5 wt %, typically 0.5-3 wt %, more usually 0.1-0.2 wt%, and is conveniently provided in the form of a copper-silicon master alloy containing e.g. about 10 wt% Si..

Boron is incorporated into precious metal alloys as an oxygen scavanger or in the case of silver alloys additionally or alternatively as a grain refiner. It may be added e.g. to molten silver alloy as copper boron master alloy or by bubbling a gaseous borane e.g. diborane into the alloy in admixture with a non-reactive gas such as argon, by introducing into the alloy a borane which is solid at ambient temperatures e.g. decaborane B₁₀H₁₄ (m.p 100°C, b.p. 213°C), or by adding an alkylated borane e.g. triethylborane or tri-«-butyl borane, although the latter reagents are spontaneously combustible and require care in handling. The boron may also be added as a metal borohydride, e.g. a borohydride of an alkali metal, a pseudo-alkali metal or an alkaline earth metal, e.g. lithium borohydride. Sodium borohydride is especially preferred because it is widely available commercially and can be obtained in the form of relatively large pellets which are convenient to handle during precious metal melting operations. The boron is advantageously solid e.g. a metal borohydride or a higher borane such as decaborane, and is in the form of pellets or granules which are advantageously wrapped in a layer or foil of precious metal and plunged as a group into the molten metal. It has surprisingly been found that when adding a borane or borohydride that more than 20 ppm can be incorporated into a silver alloy without the development of boron hard spots.

Forming silver or germanium-containing silver into wire of the required diameter may be carried out using conventional wire-manufacturing processes. The metal is cast to form ingots which are rolled in a roiling mill to form wire rod. The resulting rod is drawn successively through a series of dies of progressively reducing diameter to give the required size. Drawing may be in single block machines, or the wire may be drawn on continuous wire-drawing machines having a series of ides through which the wire passes in a continuous manner. Lubrication may be provided as necessary. Fibres of the above size range may be made by conventional wire drawing techniques. Fibres of diameter about 25-100 μm may also be made by a process which comprises forming a coil of thin metal foil of the desired thickness e.g 1-200 μm and shaving the edge of the coil to form the desired fibres (see e.g. EP-A-0319959) which have a generally rectangular cross-section and a constant and well-controlled cross- section.. The fibres may also be made by a bundle-drawing process in which a bundle of fibres is placed within a base metal tube, the resulting assembly is reduced in diameter one or more times by rolling and/or passage through a die, after which the base metal is leached away to release the fibres.

At the final step, and in the case of wire drawing as required at intermediate steps, the wire may be annealed to restore ductility. Preferably this step is carried out in an atmosphere which is not too reducing or is mildly oxidizing. The corrosion resistance of the present AgCuCe alloys depends on the presence of oxide films, and these are reduced by e.g. an atmosphere of 50% hydrogen, 50% nitrogen with some loss of corrosion resistance. At each stage, it is desirable that the annealing atmosphere should be inert gas, generally nitrogen, with less than 10% of hydrogen, typically 3-10%, preferably about 3-5%. If the furnace atmosphere is cracked ammonia, it is preferred that the hydrogen content should be not more than the above indicated range.

We have found that it is possible to have mildly oxidising conditions, i.e. temperatures and oxygen partial pressures, which allow the Ag-Cu- (Zn)-Ge alloys to be processed such that Ge will react to formGeθ2 without Cu forming Cu₂O. However, restrictions on the maximum processing temperature and time at temperature arise from the normal commercial annealing temperature and time used for producing silver- copper alloys such as Sterling silver, typically about 625°C or650°C. We have established that Ag-Cu (Zn)-Ge alloys can be processed even at annealing temperatures such as 625°C and650°C to selectively oxidise Ge toGeθ2, by using a controlled atmosphere. Preferably, the atmosphere is a wet selectively oxidizing atmosphere. By 'wet' in this context is meant an atmosphere containing moisture (H₂O), such that the atmosphere exhibits a dew point of at least +1°C, preferably at least +25°C, more preferably at least +40°C. Preferably, the dew point falls within the range from +1°C to +80°C, more preferably in the range from +2°C to+50°C. The dew point may be defined as the temperature to which an atmosphere containing water vapor must be cooled in order for saturation to occur, whereby further cooling below the dew point temperature results in the formation of dew. A more comprehensive definition is given in "Handbook of Chemistry and Physics", 65^th Edition (1985-85), CRC Press Inc., USA, page F-75. We prefer that the selectively oxidizing atmosphere comprises hydrogen and moisture, for example an atmosphere of nitrogen, hydrogen and water vapor, such as a 95% nitrogen/5% hydrogen gas mixture (v/v) containing water vapor, or a furnace atmosphere of nitrogen, hydrogen, carbon monoxide, carbon dioxide, methane, and water vapor.

In practice, it is preferred to produce the wet selectively oxidizing atmosphere by controlling the addition of water vapor to a substantially dry inert or dry reducing furnace atmosphere, for example to a furnace atmosphere of predominantly nitrogen or nitrogen and hydrogen, and typically comprising nitrogen, hydrogen, carbon monoxide, carbon dioxide and methane. The dew point in the furnace can be measured by conventional means such as a dew point meter or probe in the furnace, and the gas mixing ratios adjusted accordingly in order to control the selectively oxidizing atmosphere.

As explained above, the annealing of the wire should be carried out under the selectively oxidizing atmosphere. If, as is usual, the annealing is carried out as successive annealing steps, for example with intervening drawing steps, then at least the final annealing step should be carried out under the selectively oxidizing atmosphere. Preferably one or more of the annealing steps preceding the final annealing step could be conducted under a reducing atmosphere. However, we prefer that all of the annealing steps are carried out under a selectively oxidizing atmosphere.

The annealing of the wire may be carried out at a temperature in the range from 400°C to750°C, typically in the range from 400°C to700°C, preferably in the range from 500°C to675°C, more preferably in the range from 600°C to 650°C, and in particular at about 625°C. The annealing is suitably carried out for a total period in the range of from 5 minutes, at the higher annealing temperatures, to 5 hours, at the lower annealing temperatures, and preferably in the range from 15 minutes to 2 hours.

A further improvement in tarnish resistance may be obtained by heating the wire post production, i.e. after the alloy has been drawn and annealed to provide a finished wire. Heating may be in an air or steam atmosphere at a temperature in the range from

40°C to 220°C, preferably in the range from 50°C to200°C, more preferably in the range from 60°C tol80°C. Preferably, the post-production heat treatment is carried out for a period in the range from 1 minute to 24 hours, preferably in the range from 10 minutes to 4 hours. Thus, the germanium oxide protective coating may be further developed within the surface of the alloy. Advantageously, this post-production treatment further enhances the alloy protection against tarnishing, which is particularly important for fine wire because of its high surface area relative to its mass.

Silver wire according to the invention may be provided in the form of continuous filaments, continuous bundles, chopped fibres, continuous or spun yarns, blended spun yarns, sintered or unsintered webs, sintered media e.g. for filters, knitted or woven fabrics, braids and needle-punched felts in which a scrim may be provided also of silver wire or of organic or other metal. The present silver wire may be incorporated into slivers (bundles of staple fibres) or yarn wholly of silver fibres or containing e.g. 0.1-50% silver fibres e.g. 1-30% silver fibres and the balance natural or synthetic organic fibres, and may be used for a variety of applications e.g. anti-static, electrical heater, electro-magnetic shielding or anti-microbial applications. Other fibres with which the present silver wire may be blended include continuous or staple fibres of synthetic materials including polyamide, polyimide, polyester, polyalkylene (e.g. polypropylene), acrylic, cellulose and modified celluloses e.g. viscose, cotton, wool, jute, hemp and the like. The fibres may be incorporated into backing materials e.g. for carpets. They may also be incorporated, as yarns wholly or partly of metal fibres, into garments e.g. for protective clothing or into iashion garments. They may be formed into circular or flat knitted fabrics, warp knitted fabrics, sleeves, tapes, needle punched or other felts, twisted or braided cordage or ropes and chains. Silver wire according to the invention either alone or in admixture with other metallic or natural or synthetic organic fibres or filaments may be formed into porous media e.g. three-dimensional non-woven structures. For filtration or catalyst support applications, the silver wire may typically have diameter 2-80 μm in deep bed media and 5-60 μm in monolayer or oligo-layer media. For HEPA media, thinner silver wire may be required e.g. 0.01-0.5μm.

For example, the silver wire of the invention may be formed into a non-woven high porosity matrix of sintered metal fibres which exhibits high gas permeability, or into a layer which may be pleated. The sintered metal fibres may be formed into media having a plurality of layers e.g. 1-3 layers optionally with an internal or superficial support mesh or scrim for a variety of filtration and other applications including catalysts, gas-solid and/or gas/liquid filtration and/or odour removal (typically using fibre sizes of 0.1-10 μm) and liquid/solid filtration (typically at sizes of 2-40μm). Because of the high porosity achievable, filter media made using fibres according to the invention may exhibit a relatively low pressure drop. They may also be used at e.g. 0.25-2 vol% as fillers for resin or plastics structures to give electrically conductive composite materials. They may be used as such or incorporated as minor components into textile products e.g. into bandaging to provide antibacterial properties. The wire can also be used as a connector wire in electronics applications.

Claims

1. Fibres of AgCuGe alloy of diameter less than 200 μm.

2. The fibres of claim 1 of diameter 1-100 μm.

3. The fibres of claim 1 of diameter about 15-50 μm.

4. The fibres of any preceding claim, which are of a ternary alloy of silver, copper and germanium.

5. The fibres of claim 1, 2 or 3, which are of a ternary alloy comprising, apart from impurities, up to 0.5 % incidental ingredients, and up to 1000 ppm boron as grain refiner, of 80-98% silver, 0.5-3% germanium and 1-19.9% copper, by weight of the alloy.

6. The fibres of any preceding claim, which are of a ternary alloy consisting, apart from impurities and up to 0.5 % incidental ingredients, of 92.5-98% silver, 0.5-3% germanium, and 1-7.2% copper, by weight of the alloy, together with 1-1000 ppm boron as grain refiner.

7. The fibres of claim 6, wherein the ternary alloy consists, apart from impurities and uo to 0.5% of incidental ingredients of 92.5-96% silver, 1-2% germanium, and 1- 7% copper, by weight of the alloy, together with up to 100 ppm boron as grain refiner.

8. The fibres of claim 1, which are of a ternary alloy which comprises 92.5-92.7 wt% Ag, 6.1-6.3 wt% Cu, about 1.2 wt% Ge and up to 100 ppm boron as grain refiner.

9. The fibres of any preceding claim, wherein said alloy contains up to 0.5 wt% Si as incidental ingredient.

10. The fibres of claim 9, wherein said alloy contains 0.05 - 0.2 wt% Si as incidental ingredient.

11. Fibres of AgCuGe alloy which are of sufficiently small diameter to drape in similar manner to conventional textile fibres.

12. Fibres of any preceding claim, obtained by a manufacturing process involving successive reduction steps at least one of which is carried out in an inert as atmosphere containing not more than 10 vol% hydrogen.

13. The fibres of claim 12, wherein said atmosphere comprises water or another oxygen-containing gas.

14. A woven, non- woven, knitted, needle-punched twisted, linked or braided article comprising fibres as defined in any of claims 1-13.

15. A thermally bonded structure formed of fibres as claimed in any of claims 1-13.