A white metal (symbol Ag), silver is very malleable and ductile, and is classed with the precious metals. It occurs in the native state, and also combined with sulfur and chlorine. Copper, lead, and zinc ores frequently contain silver; about 70% of the production of silver is a by-product of the refining of these metals.
Silver is the whitest of all the metals and takes a high polish, but easily tarnishes in the air because of the formation of a silver sulfide. It has the highest electrical and heat conductivity: 108% IACS relative to 100% for the copper standard and about 422 W/mK, respectively. Cold work reduces conductivity slightly. The specific gravity is 10.7, and the melting point is 962°C. When heated above the boiling point (2163°C), it passes off as a green vapor. It is soluble in nitric acid and in hot sulfuric acid. The tensile strength of cast silver is 282 MPa, with Brinell hardness 59. The metal is marketed on a troy-ounce value.
Pure silver has the highest thermal and electrical conductivity of any metal, as well as the highest optical reflectivity. Next to gold it is the most ductile and most malleable of any metal. Silver can be hammered into sheet 0.01 mm thick or drawn out in wire so fine 120 m would weigh only 1 g. Classified as one of the most corrosion-resistant metals, silver, under ordinary conditions, will not be affected by caustics or corrosive elements, unless hydrogen sulfide is present, causing silver sulfide to form. Silver will dissolve rapidly in nitric acid and more slowly in hot concentrated sulfuric acid. Unless oxidizing agents are present, the action of diluted or cold solutions of sulfuric acid is negligible. Organic acids generally do not attack the metal and caustic alkalies have but a slight effect on pure silver.
Although silver tarnishes quickly in the presence of sulfur and sulfur-bearing compounds, it oxidizes slowly in air and the oxide decomposes at a relatively low temperature.
Classification
Silver is classified by grades in parts per thousand based on the silver content (impurities are reported in parts per hundred). Commercial grades are Fine Silver and High Fine Silver. As ordinarily supplied, fine silver contains at least 999.0 parts silver per 1000, and may go as high as 999.3 parts per 1000. Any of the common base metals may be present, although copper is usually the major impurity. Any silver of higher purity than commercial fine contains its purity in its description, i.e., 999.7 High Fine Silver. The purest silver obtainable in quantity is 999.9 plus; the impurities are less than 0.01 part per 1000. Fine silver may also contain small percentages of oxygen or hydrogen; deoxidized silver is available for applications where these elements may be a detriment.
Fabricability
Silver can be cold-worked, extruded, rolled, swaged, and drawn. It can be cold-rolled or cold-drawn drastically between anneals, and can be annealed at relatively low temperatures. To prevent oxidation when casting by conventional methods, silver should be protected by a layer of charcoal or by melting under neutral or reducing gas. Deoxidation by adding lithium or phosphorus can be obtained leaving a residual content of 0.01% max. The excellent ductility of silver makes it readily workable hot or cold.
Molten silver will absorb approximately 20 times its own volume of oxygen. Most of this oxygen is given up when the silver solidifies in cooling, but care should be taken in melting and casting because any oxygen left in the cast bars will cause cracking when they are fabricated and the castings may have blow holes.
Galling, seizing of the tool, and surface tearing are problems encountered when machining fine silver. This can be somewhat alleviated by using material cold-worked as much as possible.
Joining
Fine silver can be soldered without difficulty using tin-lead solders. Boron-silver filler metal can be used in brazing, and welding can be done by resistance methods and by atomic hydrogen or inert-gas shielded arc processes. A range of 204 to 427°C is recommended for annealing, with best strength and ductility achieved between 371 and 427°C. Little additional softening occurs at higher temperatures, which may induce welding of adjacent surfaces. The lighter the gauge, the lower should be the annealing temperature.
Applications of Alloys and Compounds
Because silver is a very soft metal, it is not normally used industrially in a pure state, but is alloyed with a hardener, usually copper. Sterling silver is the name given to a standard high-grade alloy containing a minimum of 925 parts in 1000 of silver. It is used for the best tableware, jewelry, and electrical contacts. This alloy of 7.5% copper work-hardens and requires annealing between roll passes. Silver can also be hardened by alloying with other elements.
The standard types of commercial silver are fine silver, sterling silver, and coin silver. Fine silver is at least 99.9% pure, and is used for plating, making chemicals, and for parts produced by powder metallurgy. Coin silver is usually an alloy of 90% silver and 10% copper, but when actually used for coins the composition and weight of the coin are designated by law. Silver and gold are the only two metals that fulfill all the requirements for coinage. The so-called coins made from other metals are really official tokens, corresponding to paper money, and are not true coins. Coin silver has a Vickers hardness of 148 compared with a hardness of 76 for hard-rolled pure silver. It is also used for silverware, ornaments, plating, for alloying with gold, and for electric contacts. Silver is not an industrial metal in the ordinary sense. It derives its coinage value from its intrinsic aesthetic value for jewelry and plate, and in all civilized countries silver is a controlled metal.
Silver powder, 99.9% purity, for use in coatings, integrated circuits, and other electrical and electronic applications, is produced in several forms. Amorphous powder is made by chemical reduction and comes in particle sizes of 0.9 to 15 |m. Powder made electrolytically is in dendritic crystals with particle sizes from 10 to 200 | m. Atomized powder has spherical particles and may be as fine as 400 mesh. Silver-clad powder for electric contacts is a copper powder coated with silver to economize on silver. Silver flake is in the form of laminar platelets and is particularly useful for conductive and reflective coatings and circuitry. The tiny flat plates are deposited in overlapping layers permitting a metal weight saving of as much as 30% without reduction in electrical properties.
Nickel-coated silver powder, for contacts and other parts made by powder metallurgy, comes in grades with V4, V2, 1, and 2% nickel by weight.
The porous silver comes in sheets in standard porosity grades from 2 to 55 |im. It is used for chemical filtering.
Silver plating is sometimes done with a silver-tin alloy containing 20 to 40 parts silver and the remainder tin. It gives a plate having the appearance of silver but with better wear resistance. Silver plates have good reflectivity at high wavelengths, but reflectivity falls off at about 350 nm, and is zero at 3000, so that it is not used for heat reflectors.
Silver-clad sheet, made of a cheaper non-ferrous sheet with a coating of silver rolled on, is used for food-processing equipment. It is resistant to organic acids but not to products containing sulfur. Silver-clad steel, used for machinery bearings, shims, and reflectors, is made with pure silver bonded to the billet of steel and then rolled. For bearings, the silver is 0.025 to 0.889 cm thick, but for reflectors the silver is only 0.003 to 0.008 cm thick. Silver-clad stainless steel is stainless-steel sheet with a thin layer of silver rolled on one side for electrical conductivity.
Silver iodide is a pale-yellow powder of the composition AgI, best known for its use as a nucleating agent and for seeding rain clouds. Silver nitrate, formerly known as lunar caustic, is a colorless, crystalline, poisonous, and corrosive material of the composition AgNO3. It is used for silvering mirrors, for silver plating, in indelible inks, in medicine, and for making other silver chemicals. The high-purity material is made by dissolving silver in nitric acid, evaporating the solution, and crystallizing the nitrate, then re-dissolving the crystals in distilled water and recrystallizing. It is an active oxidizing agent. Silver chloride, AgCl, is a white granular powder used in silver-plating solutions. This salt of silver and other halogen compounds of silver, especially silver bromide, AgBr, are used for photographic plates and films. Silver chloride is used in the preparation of yellow glazes, purple of Cassius, and silver lusters. A yellowish-silver luster is obtained by mixing silver chloride with three times its weight of clay and ochre and sufficient water to form a paste.
Silver chloride crystals in sizes up to 4.5 kg are grown synthetically. The crystals are cubic, and can be heated and pressed into sheets. The specific gravity is 5.56, index of refraction 2.071, and melting point of 455°C. They are slightly soluble in water and soluble in alkalies. The crystals transmit more than 80% of the wavelengths from 50 to 200 | m.
Silver sulfide, Ag2S, is a gray-black, heavy powder used for inlaying in metal work. It changes its crystal structure at about 179°C, with a drop in electrical resistivity, and is also used for self-resetting circuit breakers. Silver potassium cyanide, KAg(CN)2, is a white, crystalline, poisonous solid used for silver-plating solutions. Silver tungstate, Ag 2WO4, silver manganate, AgMnO4, and other silver compounds are produced in high-purity grades for electronic and chemical uses.
Silver nitrate, AgNO3, with a melting point of 212°C, decomposes at 444°C and is soluble, corrosive, and poisonous. It is prepared by the action of nitric acid on metallic silver. Silver nitrate is the most convenient method of introducing silver into a glass; a solution of the compound is poured over the batch.
The photosensitive halides used in photography, the cyanides used in electroplating, and most of the minor silver salts are prepared from silver nitrate.
In advanced ceramic applications, silver is unsurpassed as a conductor of heat and electricity. Silver is used in conductive coatings for capacitors, printed wiring, and printed circuits on titanites, glass-bonded mica, steatite, alumina, porcelain, glass, and other ceramic bodies. These coatings also are used to metallize ceramic parts to serve as hermetically sealed enclosures, becoming integral sections of coils, transformers, semiconductors, and monolithic and integrated circuits.
Two types of conductive coatings can be used on ceramic parts: those that are fired on and those that are baked on or air dried. The fired-on type contains, in addition to silver powder, a finely divided low-melting glass powder, temporary organic binder, and liquid solvents in formulations with direct soldering properties and others suitable for electroplating, both possessing excellent adhesion and electrical conductivity. The baked-on and air-dry types contain, in addition to silver powder, a permanent organic binder and liquid solvents. These preparations have somewhat less adhesion, electrical conductivity, and solderability than the fired-on type, but can be electroplated if desired. The air-dry type is used when it is not desirable to subject the base material to elevated firing temperatures.
Any of the above silver compositions are available in a variety of vehicles suitable for application by squeegee, brushing, dipping, spraying, bonding wheel, roller coating, etc.
Firing temperatures for direct-solder silver preparations range from 677 to 788°C. Silver compositions to be copper plated are fired at 1200 to 1250F. The firing cycle used with these temperatures will vary from 10 min to 6 h, depending on the time required to equalize the temperature of the furnace charge.
A 62% Sn-36% Pb-2% Ag solder is generally used with the direct-solder silver compositions. It is recommended that this solder be used at a temperature of 213 to 219°C. Soldering to the plated silver coating is less critical and 50% Sn-50% Pb or 40% Sn-60% Pb, as well as other soft solders, are being used with good results.
The air-dried silver compositions will, as the designation implies, air dry at room temperature in approximately 16 h. This drying time can be shortened by subjecting the coating to temperatures of 60 to 93°C for 10 to 30 min. The baked-on preparations must be cured at a minimum temperature of 149°C for 5 to 16 h. The time may be shortened to 1 h by raising the temperature to 301°C.
The same soft solders and techniques as recommended for the fired-on coatings may be used for the electroplated air-dried and baked-on preparations. It is extremely difficult to solder to air-dried or baked-on coatings without first electroplating.
The surface conductivity of the fired silver coating is far better than that of the air-dried or baked-on coating. Fired coatings have a surface electrical square resistance of approximately 0.01 Q while the surface electrical square resistance of air-dried or baked-on is about 1 Q.
Usually Alloyed
Because pure silver is so soft, it is usually alloyed with other metals for strength and durability. The most common alloying metal is copper, which imparts hardness and strength without appreciably changing the desirable characteristics of silver. Sterling silver has applications in manufacturing processes. For example, sterling silver plus lithium has been used in the aircraft industry for brazing honeycomb sections. Other silver-copper alloys are coin silver, 90.0% silver, 10.0% copper, and the silver-copper eutectic, 72% silver, 28% copper. This latter alloy has the highest combination of strength, hardness, and electrical properties of any of the silver alloys.
Silver braze filler metals are widely used for joining virtually all ferrous and nonferrous metals, with the exception of aluminum, magnesium, and some other lower-melting-point metals. Whereas pure silver melts at 960°C, silver alloys, with compositions of 10 to 85% silver (the alloying metals are copper, zinc, cadmium, and/or other base metals), have melting points of 618 to 960°C. These alloys have ductility and malleability and can be rolled into sheet or drawn into wire of very small diameter. They may be employed in all brazing processes and are generally free flowing when molten. Recommended joint clearances are 0.05 to 0.13 mm when used with flux. Whereas fluxes are usually required, zinc and cadmium-free alloys can be brazed in a vacuum or in reducing or inert atmospheres without flux. Joints made with silver brazing alloys are strong, ductile, and highly resistant to shock and vibration. With proper design there is no difficulty in obtaining joint strength equal to or greater than that of the metals joined. The strongest joints have but a few thousandths of an inch of the alloy as bonding material. Typical joints made with silver brazing alloys, giving the greatest degree of safety, are scarf, lap, and butt joints.
For electrical contacts, silver is combined with a number of other metals, which increase hardness and reduce the tendency to sulfide tarnishing. Silver-cadmium, for example, is extensively used for contacts, with the cadmium ranging from 10 to 15%. The advantages of these alloys are resistance to sticking or welding, more uniform wear, and a decreased tendency for metal transfer.
Alloys used for contact and spring purposes are the silver-magnesium-nickel series (99.5% silver), which are used where electrical contacts are to be joined by brazing without loss of hardness, in miniature electron tubes for spring clips where high thermal conductivity is essential, and for instruments and relay springs requiring good electrical conductivity at high temperatures. These are unique, oxidation-hardening alloys. Before being hardened, the silver-magnesium-nickel alloy can be worked by standard procedures. After hardening in an oxidizing atmosphere, the room-temperature tensile properties are similar to those of hard rolled sterling silver or coin silver.
Gold and palladium are also combined with silver for contact use because they reduce welding and tarnishing and, to some extent, increase hardness.
When certain base metals do not combine with silver by conventional methods, powder metal processes are employed. This is particularly true of silver-iron, silver-nickel, silver-graphite, silver-tungsten, etc. These alloys are used in electrical contacts because of the desirable conductivity of silver and the mechanical properties of the base metals. They can be pressed, sintered, and rolled into sheet and wire that is ductile and suitable for forming into contacts by heading or stamping operations.
Other silver products are those produced chemically — powder, flake, oxide, nitrate, and paint. Silver powder and flake are composed of large amounts of silver with 0.03 or 0.04% copper and traces of lead, iron, and other volatiles.
Silver Paints
These are used as conductive coatings that are pigmented with metallic silver flake or powder and bonding agents that are specially selected for the type of base material to which they are applied. These coatings are used to make conductive surfaces on such materials as ceramics, glass, quartz, mica, plastics, and paper, as well as on some metals. They are used for making printed circuits, resistor and capacitor terminals, and in miniature electrical instruments and equipment. Silver paints fall into two classifications: (1) fired-on types for base materials that can withstand temperatures in the 399 to 927°C range, and (2) air-dried or baked-on types for organic base materials that are dried at temperatures ranging from 21.1 to 427°C. The bonding agent in the fired-on type of coating is a powdered glass frit, whereas in the air-dried or baked-on type of coating organic resins are used. The viscosity and drying rate of each type varies, depending on the method of application, such as spraying, dipping, brushing, roller coating, or screen stenciling.
Silver-Type Batteries
Because they are six times lighter and five times smaller than other batteries of similar capacity, silver-zinc batteries have found wide use in guided missiles, telemetering equipment, and guidance control circuits and mechanisms. Where longer life and ruggedness are more important than the weight, silver-cadmium rechargeable batteries are specified. Where sea-water activation is required, silver chloride-magnesium couples are used. Another silver type of battery is the solid electrolyte type made with silver, silver iodide, and vanadium pentoxide. This battery, designed for low-current applications, weighs less than 1 oz and has almost unlimited shelf life.
