EP1815032A2

EP1815032A2 - Fabric structure comprising silver-germanium-copper alloy

Info

Publication number: EP1815032A2
Application number: EP05803591A
Authority: EP
Inventors: Peter Gamon Johns
Original assignee: Middlesex Silver Co Ltd
Current assignee: Middlesex Silver Co Ltd
Priority date: 2004-11-15
Filing date: 2005-11-14
Publication date: 2007-08-08
Also published as: CA2587350A1; JP2008519914A; EA011065B1; WO2006051338A2; CN101087893A; GB0425152D0; US20080128054A1; AU2005303541A1; WO2006051338A3; EA200701058A1

Abstract

A woven, braided or knitted fabric structure comprises wires of silver alloy, preferably a precipitation-hardenable Ag Cu Ge alloy. The process for making a fabric structure may comprise providing silver wire having a temper of more than fully soft but less than half hardness, forming said wire into said structure and heating the structure to precipitation harden the wire.

Description

FABRIC STRUCTURE

FIELD OF THE INVENTION

This invention relates to fabric structures based on silver wires, which may comprise the whole or part of said structures.

BACKGROUND TO THE INVENTION

The literature on production of silver wire is relatively sparse. For example US

Patent 6627149 (Tayama et al.) 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 also sparse. Such woven structures have mainly been based on strips, strands or filaments braided together, see US-A-240096 (Crane), US-A-253587 (Crane) and US-A-5203182 (Wiriath). However, US-A-2708788 (Cassman et at) 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 (Hochella) 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 fibres e.g. as in US-A-2274684 (Goodloe), but existing knitted metal fabrics are predominantly of ferrous alloys. US-A-5188813 (Fairey et al; Johnson Matthey) discloses weft-knitted fabrics consisting essentially of interlocking loops of fibres 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 fibre breakage and machine jams because the tensile strength of the metal fibres was insufficient to withstand the frictional forces in the knitting process. The solution disclosed was to feed the metal fibre with a supplementary fibre that acted as a lubricant, the supplementary fibre preferably being in the form of a multi-strand rather than a monofilament and the strands surrounding the metal wire to minimise metal-to-metal contact. After knitting, the supplementary fibre 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 fibre. 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 at). 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 standard Sterling silver has insufficient tensile strength for effective machine knitting.

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 oxidise. 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 intermetallic 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 (Johns) 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

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 (Eagar et al.\ 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 1250-1400⁰F (676-760⁰C) 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 300-700⁰F (149-371⁰C) 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 HVN (Vickers Hardness Number), and can being returned by elevated temperatures to a relatively soft state. The disclosure of US-A-4869757 (Eagar et al.\ 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-5817195 (Davitz),

5882441 (Davitz) 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 et at) and is said to exhibit reduced firestain, reduced porosity and reduced grain scale. SUMMARY OF THE INVENTION

It has now been found that silver wire can be machine-formed into fabric structures by processes such as weaving, knitting or braiding and that sufficient strength can be imparted to the wire for machine-forming if the wire is work hardened from its fully annealed state prior to fabric forming, while permitting the further work-hardening that takes place in the fabric-forming process and still permitting the development of further hardness by precipitation hardening. Argentium wire and other silver/copper/germanium alloy wires, in particular, have a particularly desirable combination of physical properties that permits them to be knitted or otherwise formed into fabric or cable structures or into braided cord structures.

The invention provides a fabric structure comprising wires of silver alloy which may be knitted, woven, braided, crocheted or otherwise formed and which may comprise wholly, predominantly or partially silver fibres.

The invention also provides a process for making a fabric structure as aforesaid which comprises providing silver wire having a temper of more than fully soft but less than half hardness, forming said wire into the fabric structure, and heating the structure to precipitation harden the wire.

In a further aspect, the invention provides a fabric structure (e.g. a structure formed by knitting, crocheting or otherwise assembling interlocking loops of wire) comprising (as the totality of the filaments or yarns in said structure or as some of the filaments or yarns in said structure) wires of silver alloy having a grain structure refined by incorporation into molten silver alloy from which said wire is formed of a decomposable boron compound. DESCRIPTION OF PREFERRED FEATURES

Alloys for forming wire

The wire used to form the present structures may be any work and precipitation- hardenable grade of silver, but is preferably an alloy of silver, copper and germanium e.g. an alloy that consists, apart from impurities and any grain refiner, of 80-96% silver, 0.1-5% germanium and 1-19.9% copper, by weight of the alloy. Sterling grade alloys of the above type may comprise apart from impurities and grain refiner, 92.5-98% silver, 0.3-3% germanium, and 1-7.2% copper, by weight of the alloy, together with 1-200 ppm e.g. 1-40 ppm boron as grain refiner. A particularly preferred group of such alloys consists, apart from impurities and grain refiner, of 92.5-96% silver, 0.5-2% germanium, and 1-7% copper, by weight of the alloy, together with 1-40 ppm boron as grain refiner. The alloy may further comprise zinc, preferably in a ratio, by weight, to the copper of no more than 1:1. Thus the alloy may comprise 81-95.49 wt % Ag, 0.5-6 wt% Cu, 0.05-5 wt% Zn, 0.02-2 wt% Si, 0.01-2 wt % by weight B, optionally 0.01-1.5 wt% In, optionally 0.25-6 wt% Sn and 0.01 -no more than 2.0 wt% Ge.

The alloy from which the present wire is formed may contain one or more incidental ingredients known per se in the production of silver alloys in amounts (e.g. in total up to 0.5 wt%) that are not detrimental to the mechanical strength, tarnish resistance and other properties of the material. Cadmium may also be added in similar amounts although its use is presently not preferred. Tin may be beneficial, typically in an amount of 0.5 wt%. Indium may be added in small quantities e.g. as a grain refiner and to improve the wetability of the alloy. Other possible incidental ingredient elements selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Si, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected. Grain refinement of the alloys

Boron may be incorporated into silver alloys used to make wire for the present purposes as a grain refiner. It may be added e.g. to molten silver alloy as copper/boron master alloy, 2 wt% B. However, it has recently been found that alloys having improved mechanical properties (including e.g. tensile strength) may be made by introducing the boron into the alloy as a boron compound selected from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof. The use of wire made from molten silver treated with decomposable boron compounds as aforesaid is advantageous for the present invention insofar as the mechanical properties thereof are more consistent and the strength may be higher both prior to formation of the fabric structure and after oven-heating of the fabric structure to effect hardening. In some embodiments, silver grain-refined by means of a decomposable boron compound is detectable e.g. on electron micrograph examination because of its fine grain structure.

The boron compound may be introduced into molten silver alloy in the gas phase, advantageously in admixture with a carrier gas which assists in creating a stirring action in the molten alloy and dispersing the boron content of the gas mixture into said alloy. Suitable carrier gases include, for example, hydrogen, nitrogen and argon. The gaseous boron compound and the carrier gas may be introduced from above into a vessel containing molten silver e.g. a crucible in a silver-melting furnace, a casting ladle or a tundish using a metallurgical lance which may be a elongated tubular body of refractory material e.g. graphite or may be a metal tube clad in refractory material and is immersed at its lower end in the molten metal. The lance is preferably of sufficient length to permit injection of the gaseous boron compound and carrier gas deep into the molten silver alloy. Alternatively the boron-containing gas may be introduced into the molten silver from the side or from below e.g. using a gas-permeable bubbling plug or a submerged injection nozzle. For example, Rautomead International of Dundee, Scotland manufacture horizontal continuous casting machines in the RMK series for the continuous casting of semi-finished products in silver. The alloy to be heated is placed in a solid graphite crucible, protected by an inert gas atmosphere which may for example be oxygen-free nitrogen containing < 5 ppm oxygen and < 2 ppm moisture and is heated by electrical resistance heating using graphite blocks. Such furnaces have a built-in facility for bubbling inert gas through the melt. Addition of small quantities of thermally decomposable boron-containing gas to the inert gas being bubbled through the melt readily provides a desired few ppm or few tens of ppm boron content The introduction of the boron compound into the alloy as a dilute gas stream over an period of time, the carrier gas of the gas stream serving to stir the molten metal or alloy, rather than in one or more relatively large quantities is believed to be favourable from the standpoint of avoiding development in the metal or alloy of boron hard spots. Compounds which may be introduced into molten silver or alloys thereof in this way include boron trifluoride, diborane or trimethylboron which are available in pressurised cylinders diluted with hydrogen, argon, nitrogen or helium, diborane being preferred because apart from the boron, the only other element is introduced into the alloy is hydrogen. A yet further possibility is to bubble carrier gas through the molten silver to effect stirring thereof and to add a solid boron compound e.g. NaBH₄ or NaBF₄ into the fluidized gas stream as a finely divided powder which forms an aerosol.

A boron compound may also be introduced into the molten silver alloy in the liquid phase, either as such or in an inert organic solvent. Compounds which may be introduced in this way include alkylboranes or alkoxy-alkyl boranes such as triethylborane, tripropylborane, tri-n-butylborane and methoxydiethylborane which for safe handling may be dissolved in hexane or THF. The liquid boron compound may be filled and sealed into containers of silver or of copper foil resembling a capsule or sachet using known liquid/capsule or liquid/sachet filling machinery and using a protective atmosphere to give filled capsules sachets or other small containers typically of capacity 0.5-5 ml, more typically about 1-1.5 ml. The filled capsules or sachets in appropriate number may then be plunged individually or as one or more groups into the molten silver or alloy thereof. A yet further possibility is to atomize the liquid boron- containing compound into a stream of carrier gas which is used to stir the molten silver as described above. The droplets may take the form of an aerosol in the carrier gas stream, or they may become vaporised therein. Preferably the boron compound is introduced into the molten silver alloy in the solid phase, e.g. using a solid borane e.g. decaborane B_1OH₁₄ (m.p. 100⁰C, b.p. 213°C). However, the boron is preferably added in the form of either a boron-containing metal hydride or a boron-containing metal fluoride. When a boron-containing metal hydride is used, suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a boron-containing metal fluoride is used, sodium is the preferred metal. Most preferred is sodium borohydride, NaBH₄ which has a molecular weight of 37.85 and contains 28.75% boron.

Boron can be added to molten silver alloy both on first melting and at intervals during storage of the alloy in the molten state and subsequent to make up for boron loss if the alloy is held in the molten state for a period of time, as in a continuous casting process for grain.

It has surprisingly been found that when adding a decomposable boron compound such as a borane or borohydride that more than 20 ppm can be incorporated into a silver alloy without the development of boron hard spots. This is advantageous because boron is rapidly lost from molten silver: according to one experiment the content of boron in molten silver decaying with a half-life of about 2 minutes. The mechanism for this decay is not clear, but it may be an oxidative process. It is therefore desirable to incorporate more than 20 ppm boron into an alloy as first cast, and amounts of e.g. up to 50 ppm, typically up to 80 ppm, and in some instances up to 800 or even 1000 ppm may be incorporated. Thus there could be produced silver casting grain containing about 40 ppm boron. Owing to boron loss during subsequent re-melting and formation of wire, the boron content of the finished wire may be closer to 1-20 ppm, but the ability to achieve relatively high initial boron concentrations means that improved consistency and improved mechanical properties may be achieved.

Forming wire from the alloys

Forming germanium-containing silver into wire for forming into fabric according to the invention of may be carried out using conventional wire-manufacturing processes. In certain embodiments of the invention 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 guides through which the wire passes in a continuous manner. Lubrication may be provided as necessary.

At the final step, and 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 tarnish 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 during annelaing, i.e. temperatures and oxygen partial pressures, which allow the Ag-Cu-(Zn)- Ge alloys to be processed such that Ge will react to formGe0₂ 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 or 650⁰C. We have established that Ag-Cu-(Zn)-Ge alloys can be processed even at annealing temperatures such as 625°C and 650⁰C to selectively oxidise Ge toGeO₂, by using a controlled atmosphere. Preferably, the annealing 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 +40⁰C. Preferably, the dew point falls within the range from +1°C to +80⁰C, 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 vapour 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", 65th Ed. (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 vapour, such as a 95% nitrogen/5% hydrogen gas mixture (v/v) containing water vapour, or a furnace atmosphere of nitrogen, hydrogen, carbon monoxide, carbon dioxide, methane, and water vapour.

In practice, it is preferred to produce the wet selectively oxidizing annealing atmosphere by controlling the addition of water vapour 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, in some embodiments of the invention, the annealing of the wire is 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. In further embodiments of the invention, one or more of the annealing steps preceding the final annealing step is conducted under a reducing atmosphere. However, in other embodiments of the invention, all of the annealing steps are carried out under a selectively oxidizing atmosphere.

In embodiments of the invention, the annealing of the wire is carried out at a temperature in the range from 400⁰C to750°C, typically in the range from 400⁰C to

700⁰C, preferably in the range from 500⁰C to 675°C, more preferably in the range from 600⁰C to 650⁰C, and in particular at about 625°C. In embodiments of the invention, the annealing is 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 40⁰C to 220⁰C, preferably in the range from 50⁰C to 200⁰C, more preferably in the range from 60⁰C to 180⁰C. 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.

The structures of the invention may consist wholly or principally of silver wire, or silver wire may be a minor component e.g. when incorporated into bandages to take advantage of the antibacterial properties of the silver. Wire is a solid section other than strip, and may be furnished in a coil on a spool or reel. The wire used to make the present woven structures may be of circular cross section, but other sections may be employed, e.g. oval, polygonal, strip or flat wire depending on the appearance desired for the finished chain. The wire will typically be of circular section. It may be of diameter or size 0.05 - 2.0 mm, typically 0.1- 1 mm. The wire may be single stranded or may comprise a plurality of strands twisted together.

Wire hardness for forming fabric structures

Prior to formation of the present structures, the wire of the invention should preferably be more than fully soft but less than half hard. These expressions have well- understood meanings in the jewellery trade. In jewellery wire, hardness or malleability is graded soft or dead soft, quarter hard, half-hard, hard, and spring hard. Numbers instead of names can also designate wire hardness. The numbering system, which goes from zero to 10 or more, is based on the number of times wire has been drawn though progressively smaller holes in a drawplate. Each increment in the number designates a doubling of the preceding number. Soft or dead soft wire is as annealed, has not subsequently been drawn through a plate and has a number of zero. It is malleable and can be bent easily by hand into a myriad of shapes but does not hold its shape under stress. Quarter-hard wire has been drawn through a single plate, half-hard wire has been drawn twice and hard wire has been drawn through four times. The wire used to form the present structures is preferably quarter hard, which imparts the necessary bending and breaking strength for machine weaving or machine knitting but leaves enough material in solid solution for both work hardening during weaving or knitting and for subsequent precipitation hardening.

Structures that can be formed from the wire

The wire may be weft-knit on a flat-bed or circular knitting machine to produce e.g. a single layer tricot stitch structure, or double-layer structures, or more open net- like structures which may be tubular or may be flat sheets. In particular, single-layer tubular cable-like structures based on a single layer or on two layers may be used as a substitute for conventional chains in the manufacture of jewellery such as bracelets and necklaces and has the advantage of attractive appearance and lightness. The wire may also be warp-knit. The wire may further be formed into braided cable structures e.g. by twisting together a plurality of single filaments of silver to form plied yarns which are then braided, see e.g. US-A-4170921 and US-A6070434 (Fig. 6) e.g. to form a jacket of braided silver surrounding core which may be of silver, another metal or e.g. plastics filaments. A further possibility is to form the wire into a crocheted structure "Crochet" as used herein means a process of making needlework comprising looped stitches formed from a single thread or filament e.g. of silver/copper/germanium alloy using a hooked needle and includes both formation of a foundation row which may be useful per se as a jewellery chain and making plain or open-work fabric structure from successive rows of stitching. Lace and band-type structures can be made.

Embodiments of the invention for knitting or crocheting further employ a sacrificial thread placed substantially parallel and adjacent to the silver alloy wire during the operations involved in knitting or crocheting and fed simultaneously with it. The sacrificial thread can be formed of any suitable material which can be removed after the knitted structure is formed. For example, suitable materials for the sacrificial thread can include cotton, readily soluble metal and natural or synthetic polymers, including polyamides, polyesters, cellulosic fibres, acrylic styrene polymers, PVA and other vinyl polymers, aliginate, and the like. Multistrand fibres or threads and monofilament fibres or threads may be used. One of the advantages of a sacrificial thread is to provide a spacer to control spacing in the knitted fibre structure. Thus, the thickness of the sacrificial thread can be used as one way to increase or decrease the volume of space between adjacent portions of the knitted wire. Typically, the sacrificial thread can have a diameter which is about the same as the wire. As mentioned above, it may be desirable to decompose or dissolve the sacrificial thread, and the selection of sacrificial thread is conveniently made to permit easy decomposition or dissolution after the fabric structure has been formed. Most organic fibres, for example, may be pyrolysed and/or oxidised to leave little or no residue, or a strong acid such as sulphuric or nitric acid may be used.. In addition or as an alternative to a sacrificial thread there may be used a lubricant e.g. starch to reduce the friction in the knitting or crocheting process.

After formation of a knitted, braided, crocheted or woven structure, it may be subjected to a precipitation hardening treatment by heating in a furnace to e.g. about 300⁰C for about 30-45 minutes followed by gradual cooling. A surprising difference in properties exists between conventional Sterling silver alloys and other Ag-Cu binary alloys on the one hand and Ag-Cu-Ge silver alloys on the other hand, in which gradual cooling of the binary Sterling-type alloys results in coarse precipitates and little precipitation hardening, whereas gradual cooling of Ag-Cu-Ge alloys results in fine precipitates and useful precipitation hardening, particularly where the silver alloy contains an effective amount of grain refiner. Furthermore, the addition of germanium to sterling silver changes the thermal conductivity of the silver alloy, compared to standard sterling silver. The International Annealed Copper Scale (IACS) is a measure of conductivity in metals. On this scale the value of copper is 100%, pure silver is 106%, and standard sterling silver 96%, while a sterling alloy containing 1.1% germanium has a conductivity of 56%. The significance of this is that the Argentium sterling and other germanium-containing silver alloys do not dissipate heat as quickly as standard sterling silver or their non-germanium-containing equivalents, a piece will take longer to cool, and precipitation hardening to a commercially useful level (preferably to Vickers hardness 110 or above, more preferably to 115 or above) can take place during natural air cooling or during slow controlled air cooling. A number of Ag-Cu-Ge-Zn alloys grain refined with boron using copper boron master alloy or using a decomposable boron compound also exhibit precipitation hardening under the conditions indicated above.

The present structures can be used for making wearable articles e.g. chain, bracelets, necklaces, earrings, key-chains and the like. Silver wire may be incorporated, in embodiments of the invention, into a variety of additional structures e.g. for use in catalysis or water treatment. Thus it may be incorporated into backing material e.g. for carpets, as a minor component into woven or knitted garments e.g. for protective clothing or into fashion garments, into circular or flat knitted general textile fabrics, warp knitted fabrics, sleeves, tapes, needle punched or other felts, and twisted or braided cordage or ropes. Silver wire 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 e.g. for filtration (e.g. of water where the anti¬ bacterial qualities of silver may be an advantage) or catalyst support applications. It may be incorporated as a component of bandaging on account of its antibacterial properties. In additional embodiments, silver wire 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 and liquid/solid filtration. Because of the high porosity achievable, filter media made using fibres according to the invention may exhibit a relatively low pressure drop. They may be used as such or incorporated as minor components into textile products e.g. into bandaging to provide antibacterial properties.

Claims

1. A fabric structure comprising wires of silver alloy.

2. The structure of claim 1, wherein the alloy is an alloy of silver, copper and germanium.

3. The structure of claim 2, wherein the alloy consists, apart from impurities and any grain refiner, of 80-96% silver, 0.1-5% germanium and 1-19.9% copper, by weight of the alloy.

4. The structure of claim 3, wherein the alloy comprises apart from impurities and grain refiner, of 92.5-98% silver, 0.3-3% germanium, and 1-7.2% copper, by weight of the alloy, together with 1-40 ppm boron as grain refiner.

5. The structure of claim 4, wherein the alloy consists, apart from impurities and grain refiner, of 92.5-96% silver, 0.5-2% germanium, and 1-7% copper, by weight of the alloy, together with 1-40 ppm boron as grain refiner.

6. The structure of any preceding claim, wherein said alloy further comprises zinc.

7. The structure of claim 6, wherein the zinc is present in a ratio, by weight, to the copper of no more than 1:1.

8. The structure of any preceding claim, wherein said alloy comprises 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, optionally 0.01-1.5 wt% In, optionally 0.25-6 wt% Sn and 0.01-no more than 2.0 wt% Ge.

9. The structure of any preceding claim, consisting essentially of silver wire.

10. The structure of any preceding claim, wherein the wire is of diameter 0.05 - 2.0 mm.

11. The structure of claim 10, wherein the wire is of diameter 0.1- 1 mm.

12. The structure of any preceding claim, wherein the silver wire is single stranded.

13. The structure of any of claims 1-12, wherein the silver wire comprises a plurality of strands.

14. The structure of any preceding claim, which is woven.

15. The structure of any of claims 1-13, which is knitted.

16. The structure of claim 15, which comprises a single layer.

17. The structure of claim 15, which comprises two or more layers of loops knitted together.

18. The structure of claim 15, 16 or 17 which is weft knitted.

19. The structure of claim 15, 16 or 17 which is warp knitted.

20. The structure of any of claims 15-19, which is tubular or cable-like.

21. The structure of any of claims 15-19, which is a flat sheet.

22. The structure of any preceding claim, obtainable by forming quarter-hard wire.

23. The structure of any preceding claim, precipitation hardened after said structure has been formed.

24. The structure of claim 23, precipitation hardened by heating to about 300⁰C for about 30 minutes.

25. A process for making a fabric structure which comprises providing silver wire having a temper of more than fully soft but less than half hardness, forming said wire into said structure and heating the structure to precipitation harden the wire.

26. The process of claim 25, wherein the wire prior to knitting is quarter hard.

27. The process of claim 25 or 26, wherein the fabric structure is formed by knitting the wire.

28. The process of claim 27, wherein the structure is formed by weft knitting.

29. The process of claim 27, wherein the structure is formed by warp knitting.

30. The process of any of claims 25-29, wherein the wire is of a precipitation hardenable Ag Cu Ge alloy containing at least 80 wt% Ag.

31. The process of claim 30, wherein the alloy an amount of boron effective as a grain refiner and upto 20ppm.

32. A fabric structure comprising wires of silver alloy having a grain structure refined by incorporation into molten silver alloy from which said wire is formed of a decomposable boron compound.

33. The structure of claim 32, wherein said decomposable boron compound is sodium borohydride.

34. The structure of claim 32 or 33, formed by machine knitting.