Every Cloud Has a Silver Lining Repair

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1. INTRODUCTION

Silver (Ag) is a shining white metal which occurs in nature as in ores and as a free metal. It is an element in group 11 of transition metals in the periodic table with a mass number of 108 and atomic number of 47. Silver is a rare but naturally occurring metal, often found deposited as a mineral ore in association with other elements. The ores are smelted to extract silver and use for different applications. Silver found to be the metal with highest electrical conductivity. This led the use of silver in high-tech electric equipment. And its tensile strength can be improved with by forming alloys to suit for different application such as dental fillings and replacement for some internal parts of our body. With the developments in science and technology, more uses of silver has been discovered which are around as. Pure silver is converted to other compounds and used to make photographic film. It is utilized in electrical and electronic equipment including specialized batteries. It is also practiced as a medicine for some diseases. For making control rods in nuclear reactors silver rods are applied as it absorbs neutrons. In some chemical reaction it is being used as catalyst. And its endless uses continue. Silver forms both soluble and insoluble salts by reacting with other non-metals. Ions of silver are formed by dissolving in water or leach from the soil. The global biogeochemical movements of silver are characterized by releases to the atmosphere, water, and land by natural and manmade sources, long-range transport of fine particles in the atmosphere, wet and dry deposition, and sorption to soils and sediments. It is a mildly toxic element to our health. As silver has played an important role in the past, it has become a significantly material in today’s life and will continue to be a vital element for future generation.

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2. DISCOVERY OF SILVER.
No one actually knows exactly who discovered the chemical element silver or when and where it was discovered, but there is evidence that it was known to the ancients. Silver was probably first discovered after gold and copper. Archaeologists have found silver objects dating to about 3400 B.C. in Egypt. The earliest known large-size mines were those of Cappadocia in eastern Anatolia. The best-known of the ancient mines were located at the Laurium silver-lead deposit in Greece and were actively mined from 500 BC to AD 100. About 2500 B.C. the Chaldeans used a mining process called "cupellation" to extract silver from lead-silver ores. Today the most common silver ores found are argentite (Ag 2S); cerargyrite, or "horn silver" (AgCl); proustite (3Ag 2S ○ As2S3); and pyrargyrite (Ag2S ○ Sb2S3). A significant discovery of silver ore occurred in 1492, and from 1500 to 1800 Bolivia, Peru and Mexico supplied 85% of the world's silver. Rapidly developing technology from 1876 to 1920 contributed to exploitation of new worldwide silver supplies from Australia, Central America and Europe to Canada, the U.S. and Africa. Today, Peru is the world’s biggest silver mining country. In 2008, global silver mine production reached a record high value of 680.9 Moz due to the increase in production in Bolivia, Russia and Peru. Other leading countries in silver mining are Mexico, China, Australia and Chile. Due to its unique properties, demand of silver is increasing.

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3. PHYSICAL PROPERTIES SILVER
3.1. Introduction

Silver is a soft white, lustrous metal that is extremely malleable, ductile, and has the highest electrical and thermal conductivities of all metals. It is relatively a stable element which does not react with moist air or dry oxygen and does not react readily with organic acids and bases.

3.2. Conductivity
Silver has the highest electrical conductivity of all the other metals. As a result it is considered as futures interconnect material for ultra large scale integrated (ULSI) circuit technology.

In integrated circuit technologies, with the increase in multilayer metallization the performance limiting resistive and capacitive signal delays increase accordingly. To solve this problem, a low resistive metal has to be used. Since silver has the lowest resistivity of all the metal, it suit for the job.

Fig.3.1. Resistivity of pure metals

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The electrical resistivity of metal films usually decreases during an annealing treatment. This is because surface morphology of thin film metal affects the electrical resistivity and surface morphology changes during annealing.

Fig.3.2. Resistivity (ρ) of a 50-nm-thick silver film independence on the annealing temperature T. In the thin metal film, rough surfaces occur at the initial stage of agglomeration process which begins at the grain boundary. During the initial stage of agglomeration more source of conducting electrons are provided through the thin films thus decreasing electrical resistivity. Void and island formation occurs in the final stage which cause the reduction of conducting area thus prevent further decrease of resistivity of silver.

Electron migration another fear identified as a primary failure mode of interconnects lines used in semiconductor-integrated circuits. Silver as the most conductive metal, has a better electromigration resistance when compare with aluminum but need to be further improved since copper has a better electronmigration resistance.

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3.3. Strength
Most commercially used silver has a tensile strength of 170 MPa in annealed condition and when it is 50% cold worked, tensile strength increases to 296 MPa. Commercially pure silver has 74 GPa of modulus of elasticity. 3.3.1. Silver in Amalgams An important application of silver’s strength is alloying with tin to form silver-tin amalgams used for dental filling. When amalgam is used for dental filling, it is subjected to compression. Therefore to choose the appropriate different composition of amalgams is subjected to compression strength at different temperature.

Alloy A B C D E F G H

Composition and Homogenization of Silver-Tin Alloys Homogenization Tin Composition Temperature Time (%) (°C) (Days) 4.00 852 1(hr) 8.00 765 4 12.00 550 7 16.00 672 3 20.00 624 4 24.00 392 14 26.85 392 14 30.85 204 10

Phase α α α+β β β+γ β+γ γ γ+Sn

Table 3.1: Different composition of silver-tin alloy and its homogenization with phase. The most successful amalgam alloys are formed at 25-27 percent range of silver-tin system. Ag3Sn intermetallic compound occurs within this range containing silver, 73.15 weight percent, and tin, 26.83 weight percent.

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Fig.3.3. Relationship of amalgam compressive strength to alloy composition. ∆−−∆=0.5 minute; о—о=1.0 minute; □−−□=2.0 minutes; ●−−●=3.0 minutes. The results show that compressive strength increased with increase in trituration time, until a maximum strength is obtained that was not changed by further trituration. From this it can be considered that increased trituration increases bond strength in amalgamation. When the composite has α and β phases, it shows less strength than others. And if β and γ phases were combined in same alloy, the strength increased with the addition of γ. The γ phase alloy resulted in the strongest amalgam, probably due to less volume of matrix and greater bond strength.

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3.4. Phase diagram (copper-silver system)
In copper-silver system, three phase regions are found: α, β and liquid. The α phase is a solid solution rich in copper with silver as the absolute component and an FCC crystal structure. The β phase solid also has an FCC structure, but copper is the solute. Pure copper and pure silver are also considered to be α and β phases, respectively.

Fig.3.4. copper–silver binary phases diagram.

In this phase diagram, the solubility of each of these solid phases is limited. Any temperature below line BEG, only a limited concentration of silver will dissolve in copper (for the α phase), and similarly for copper in silver (for the β phase). The solubility for α phase in the region labeled as CBA, increase with increase in temperature up to a maximum point at B (8.0 wt% Ag at 779°C) and decrease back to zero at meting temperature of pure copper, point A (1085°C). The line separating the region α and α+β is termed as solvus line and the line between α and α+L is termed as solidus line. Both these lines also exist for β
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phase, HG and GF, respectively as shown in the figure and the maximum solubility of β copper occur at point G (8.8 wt% Cu at 779 °C). As silver is added to copper, the temperature at which the alloys become totally liquid decreases along the liquidus line AE; decreasing the melting temperature of copper with addition of silver. Introduction of copper also reduces the melting temperature of silver at line the liquidus line FE. The point at these liquidus lines meet is known as invariant point (E). Upon cooling a liquid phase is transformed into the two solid and phases at the temperature TE and the opposite reaction occurs upon heating. This is called eutectic reaction.

Important properties of silver Description Melting point Thermal conductivity Coefficient of thermal expansion Specific heat Specific gravity Elastic modulus Tensile strength (annealed) Electrical resistivity (commercially pure) Density 962 4.29 18.6 x 10 0.237 10.5 71 152 1.59 10.5 GPa Mpa µΩ-m g/cc
-6

Value °C

Units

W/cm °C cm/cm °C J/g mol

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4. CHEMICAL PROPERTIES OF CLOUD AND SILVER
4.1. Introduction
Clouds are condensed water vapor in air. But clouds are not an inert mixture. Particles that facilitate cloud formation have various chemical compositions and the atmospheric gases around the droplet also can modify its chemical composition. Silver is a transition metal. It has 47 protons and the electron configuration is [Kr]4d105s1. The number of neutrons varies with different isotopes of silver. The two isotopes of silver that exist are Ag2+. It does not react with air or pure water in normal condition. Silver reacts with halides and acids to form its salts. It also acts as a catalyst in some chemical reactions.
107 109

Ag and

Ag. It exhibit three oxidation

states, Ag (I), Ag (II) and Ag (III) and most common ions found are Ag+ and

4.2. Cloud Formation
Cloud is formed from the water vapor that evaporates from sea, river lakes and other sources. These vapors condense and form droplets or ice crystals. Particles that allow the cloud’s formation are called Cloud Condensation Nuclei (CCN), which have various chemical compositions according to their origin (from human or natural source). Since clouds are surrounded by atmospheric gases, it can also affect the chemical composition of cloud.

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Fig. 4.1. Four main processes occur within the cloud droplet: the CCN composition and size can change after the droplet has evaporated (1); dissolution of soluble content of the particle (2) and aqueous reactions inside the water droplet (3); transfers between atmospheric gases and liquid phase (4). 4.2.1. The particle inside the droplet The water soluble fraction of an aerosol governs its capacity to grow into a droplet. The chemical composition of the particles that act as CCN determines the initial composition of the cloud droplet, as its soluble content dissolves in the condense water. 4.2.2. The atmospheric gases around the droplet Whether a chemical species stay in the gas phase or is absorbed in the water droplet is estimated by considering the Henry’s law equilibrium: A (aq) = HA PA, where A (aq) is the aqueous phase concentration (mol/L), P A is the partial pressure of A in the gas phase (atm), and H A is the Henry’s law coefficient of the gas considered. Some species go back to the gas phase and migrate away from the drop; others, once captured, remain associated with the aqueous phase unless total evaporation occurs.
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4.2.3. Reactions inside the droplet. Hundreds of chemical reactions take place in the droplet. They are effective in changing the acidity of the precipitation, which may lead to acid rain. The main chemical species that are involved in acid rain are sulfuric and nitric acids All this complex chemistry in which a cloud is involved modifies the cloud itself and the atmosphere around the cloud.

4.3. Reactions of Silver
Silver does not react with clean air or water under normal condition. 4.3.1. Reaction with halides. Aqueous silver reacts with elements of group VII to form its salt. Ag+(aq) + X-(aq) = AgX(s) By reacting with silver and halides, various precipitate will be formed. Ion present Cl¯ Brˉ Iˉ Observation White precipitate Very pale cream precipitate Very pale yellow precipitate

Table. 4.1. Different colors silver halogens.

4.3.2. Reaction with acids. Silver does not react with dilute sulphuric acid. But it dissolves in hot concentrated sulphuric acid and produces its sulphate and hydrogen gas. H2SO4 (aq) + Ag(s) = AgSO4(s) + H2 (g) Silver metal also dissolves in dilute or concentrated nitric acid to give silver nitrate and nitrogen dioxide. This method is use in manufacture of silver nitrate. Ag(s) + 2HNO3 (aq) → AgNO3 (aq) + NO2 (g) + H2O (l)

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4.3.3. Reactions of Silver Nitrate Silver nitrate reacts with hydroxide of group 1 metals to from solid silver hydroxide and aqueous metal nitrate. AgNO3 (aq) + NaOH (aq) = AgOH (s) + NaNO3 (aq) When copper metal is dipped into a solution of silver nitrate, AgNO 3, silver crystals and a blue-green solution of copper nitrate is formed. Cu(s) + 2 AgNO3 (aq) → Cu (NO3)2(aq) + 2 Ag (s) Similar reaction takes place when metals of lead or zinc are used. 4.3.4. Reaction with water. Silver sulphides present in soil reacts with water and produce silver metal. 3 Ag2S + 2 H2O = 6 Ag + 2 H2S + SO2 4.3.5. Decomposing of silver nitrate On heating, silver nitrate decomposes to give silver metal. 2AgNO3 →2Ag +2NO2 +O2 4.3.6. Reaction with organic materials. Disilver Acetylide Silver ions react with acetylene in solution to yield the acetylide compound, Ag2C2. 2Ag(NH3)2NO3(aq) + C2H2(aq) = Ag2C2(cr) + 2NH4NO3(aq) + 2NH3(aq) Silver Phenylacetylide Phenylacetylide reacts with disilver acetylide to from silver phenylacetylide. PhC≡CPh(s) + Ag2C2(s) = 2PhC≡CAg(s)

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Silver cyanide Potassium cyanide reacts with silver nitrate to give a white precipitate of silver cyanide. KCN + AgNO3 = AgCN + KNO3 Solid silver fulminate reacts with nitrogen gas to give solid silver cyanide and dinitrogen oxide. AgCNO(s) + N2(g) = AgCN(s) + N2O(g) Solid silver cyanide reacts with ammonia gas to produce complex of ammonium silver cyanide. AgCN(s) + NH3(g) = AgCN•NH3(s) Alkylsilver complex Alkylsilver complexes can be formed by the reaction of silver nitrate with tetraalkyllead in alcoholic solution at a low temperature. AgNO3 + R4Pb = AgR + R3PbNO3 The yellow precipitates of the formula RAg (R = methyl, ethyl, propyl ...) were stable at -80 °C for several hours, but decompose to metallic silver and gaseous hydrocarbons upon warming to room temperature. 4AgCnH2n+1 = 4Ag0 + CnH2n+2 + (CnH2n+1)2 + CnH2n

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Properties of silver atom Description Density Atomic radius Atomic Volume Covalent Radius Ionic Radius Specific Heat (@20°C) Fusion Heat Evaporation Heat First Ionizing Energy Oxidation States: Lattice Structure: Lattice Constant (Å) 10.5 144 10.3 134 89 (+2e) 126 (+1e) 0.237 11.95 254.1 730.5 2, 1 Face-Centered Cubic 4.090 J/g mol kJ/mol kJ/mol kJ/mol Value g/cc pm cc/mol pm Units

Table 4.2. Properties of silver atom and silver compound.

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5.

PRODUCTION OF SILVER

5.1. Introduction
Slag dumps in Asia Minor and on islands in the Aegean Sea indicate that humans learned to separate silver from lead by about 2000 BC and ever since used it for jewelry and medium of exchange. Silver is produced from different ores and other silver bearing compounds. Few ore bodies contain sufficient silver to justify extraction for the silver alone. Three-fourths of all silver is produced as a byproduct of Pb, Zn, Cu, and Au mining. The most common silver-bearing compounds are sulfides and chlorides.

5.2. Extraction of silver from lead ore using thiourea leaching solution
Silver can be extracted from silver lead ore by using thiourea [SC(NH2)2] leaching from a pretreated Ag-Pb ore in an ammonia carbonate solution. The process of pretreatment is carried out in a covered reactor under pure oxygen pressure. During leaching with thiourea, silver is leached in to its solution. Silver is then refined from this solution. During leaching with thiourea, still some silver is not leached. This solid is treated with Fluorosilicate solution which leaches the lead into solution, leaving silver in it.

Fig.5.1. Principle flowsheet for processingof Ag−Pb ore.
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5.3. Extraction from silver sulfate by electrolysis
The silver ore content subjected to leaching with HNO3. The nitric acid nitric oxide formed is removed and solution is heated until dryness. This perocedure is repeated with distilled water to obtain a neutral pH. NH 3 is added to form silver ion complexes. Then the solution is treated with Na2SO3 and NaH2PO4. The pH of solution is then decrease to 4.0 by adding H3PO4 solution followed by Na2S2O3 powder and stirred. Temperature of solution is increased to 50 – 60 °C and keeps it to leach for 4 – 6 hours. The solution is then transferred to electrolytic cell. Electrolysis is carried out using platinum electrode and silver that deposit on the anode is collected. Overall reactions During leaching Ag2S + 4S2O2-3 + 2SO2-3 + 6H+ → 2Ag(S2O3)3-2 + 3S + 3H2O During electrolysis 2Ag(S2O3)3-2(aq) + eAg(s) + S2O32-(aq)

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6. APPLICATION OF SILVER AND SILVER COMPOUNDS
6.1. Introduction
Silver is best known as precious metal used as jewelry and decoration in important places. This is because of it is a rare metal which is chemically stable in normal condition and shiny color. Other than as jewelry, there are many more uses of silver.

6.2. Silver coins
Silver has been used as a medium of exchange since earliest recorded history, but silver coins – those minted by an authority either public or private – came on the scene in the eastern Mediterranean during 550 BC. By 269 BC, the Roman Empire adopted silver as part of its standard coinage and it was used throughout the trading world. Today, other than collector's silver coins, silver bullion coins are popular among people who desire to be cautious against currency inflation or store of value. Bullion coins are coins struck from precious metal and kept as a store of value or an investment, rather than used in day-to-day commerce. Description Canadian Silver Maple Leaf Mexican Silver Libertad Chinese Silver Panda American Silver Eagle Australian Silver Kookaburra Australian Silver Kangaroo British Silver Britannia Austrian Silver Vienna Philharmonic Russian George the Victorious Year of introduction 1988 1982 1983 1986 1990 1993 1997 2008 2009 % purity 99.99% 99.90% 99.90% 99.90% 99.90% 99.90% 95.80% 99.90% 99.90%

Table 6.1. World’s most common silver bullions
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6.3. Photography
Silver-based photography is based on light striking sensitive silver-halide crystals suspended on a film. Through use of chemical ‘developers’ the differences in light intensity form negative images which can then be processed into paper pictures by using silver-imbedded paper. Approximately 5,000 color photographs can be taken using one ounce of silver.

6.4. Battery
Rechargeable and disposable batteries are manufactured with silver alloys as the cathode or negative side. Silver is an expensive material to use in this application, but it has higher efficiency when compare to batteries with other metals. The most common of these batteries is the small button-shaped silver-oxide cell (used in cameras, toys, hearing aids, watches and calculators) which is approximately 35 percent silver by weight. In a silver-oxide cell, silver-oxide is used as the positive electrode and zinc as the negative electrode with an alkaline electrolyte (usually sodium hydroxide or potassium hydroxide). Inside the cell, silver ions in silver oxide reduce to solid silver at the cathode and zinc oxidizes from metal to its ion at the anode. The following reaction takes place in the cell. Zn + Ag2O
KOH/NaOH

ZnO + 2Ag

6.1. A typical silver oxide round cell.

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Due to environmental and safety concerns, silver-oxide batteries are beginning to replace lithium-ion batteries in mobile phones and laptop computers. Silver-zinc batteries feature a water-based chemistry and contain no lithium or flammable liquids.

6.5. Bearing
Steel ball bearings electroplated with silver have greater fatigue strength and load carrying capacity than any other type. These bearings are used in continuous, heavy-duty applications such as in jet engines. Because steel has a poor coefficient of friction, placing a layer of silver between the steel ball bearing and the housing reduces friction between the two, increasing the performance and longevity of the engine. Despite high internal temperatures, silver-coated bearings provide superior performance and a critical margin of safety for engines. Even in the event of an oil pump failure, silver-plated bearings provide enough lubrication to allow a safe engine shut-down before more serious damage can occur.

6.6. Brazing and Soldering
The joining of two parts facilitated by introducing a second, low melting metal between them is called brazing, when done at temperature above 600 °C and soldering when done at a lower temperature. During this process the parts do not melt and produce strong, leak-tight, corrosion-resistant joint with high tensile strength, ductility and thermal conductivity. Silver brazing alloys are used widely in applications ranging from air-conditioning and refrigeration to electric power distribution. It is also used in the automobile and aerospace industries. Silver-tin solders are used for bonding copper pipe, where they not only eliminate the use of harmful lead-based solders, but also provide the piping with silver’s natural antibacterial action. In refrigerator, silver-based bonding materials are used to provide the ductility required for constant changes in temperature of the cooling tubes.

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6.7. Catalyst
A catalyst is a substance that allows or facilitates a chemical process chemically without involve in the reaction. Silver, because of its unique chemical properties, has become an increasingly important catalyst for many industrial processes. Every year over 700,000 kg of silver is being used in catalytic reactions and approximately 90% of these are used in the production of ethylene oxide from ethylene. Ethylene oxide is the foundation for flexible plastics such as polyester textiles, used to make all types of clothing and a variety of specialty fabrics. It is also used for molded items such as insulating handles for stoves, key tops for computers, electrical control knobs, domestic appliance components, and electrical connector housings. About 25% of ethylene oxide production is used to manufacture antifreeze coolant for automobiles and other vehicles. Formaldehyde, a chemical produced from methanol, is the building block of solid plastics including adhesives, laminating resins for construction plywood and particle board. Formaldehyde also helps to produce finishes for paper and electronic equipment, textiles, surface coatings that resist heat and scratches, dinnerware and buttons, casings for appliances, handles and knobs, packaging materials, automotive parts, thermal and electrical insulating materials, toys and many other products Silver is a recognized powerful oxidizer. Metallurgists have long known the unique affinity of silver with oxygen. Molten silver will hold ten times its volume in oxygen. On freezing, the contraction of silver vigorously ejects the oxygen; a dangerous activity known as spitting. Not all oxygen is ejected; much is retained in the silver lattice as well as adhered to its surface.

6.8. Electronics
Because of silver’s excellent electrical conductivity, it finds many applications in electronics from printed circuit boards to switches and TV screens.

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Silver membrane switches, which require only a light touch, are used in buttons on televisions, telephones, microwave ovens, children’s toys and computer keyboards. These switches are highly reliable and last for millions of on/off cycles. Silver is also used in conventional switches likes those used for controlling room lights. For printed circuit boards, used in consumer items from mobile phones to computers, silver-based inks and films are applied to composite boards to create electrical pathways. In similar fashion, silver-based inks produce so-called RFID tags (radio frequency identification) antennas used in hundreds of millions of products to prevent theft and allow easy inventory control. Silver is also used to coat Compact Disks (CDs) and Digital Video Disks (DVDs). In addition, silver is employed in Plasma Display Panels used in television sets and monitors.

6.9. Medical Applications
Silver’s anti-bacterial powers have been known for centuries and have been tested and proven scientifically. For example, the ancient knew that water, wine or vinegar kept in silver vessels stayed fresh during long sea voyages. Scientists have discovered that silver interrupts a bacteria cell’s ability to form chemical bonds essential to its survival. These bonds produce the cell’s physical structure so bacteria in the presence of silver literally fall apart. Cells in humans and other animals have thick walls and are not disturbed by silver. Therefore, silver prevents bacteria growth but is harmless to humans. One of the most important uses of silver as a biocide is in hospitals and other health care facilities because they grapple with MRSA (Methicillin-resistant Staphylococcus aureus), a type of life-threatening Staph germ. MRSA is resistant to almost all chemical antibiotics; therefore many hospitals are employing silverimbedded equipment including surgical tools, catheters, needles, stethoscopes, furniture, door handles and even paper files.

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One of the most promising applications is in silver-imbedded bandages for burn and wound victims. The silver ions help prevent infection but also speed healing because the body doesn’t have to focus its energy on fighting infection. Currently we’re seeing a surge of applications for silver-based biocides in all areas: industrial, commercial and consumer. New products are being introduced almost daily. The newest trend is the use of nano-silver particles to deliver silver ions.

6.10. Mirror and Coatings
When polished, silver offers nearly a perfect reflectivity which makes it ideal for energy-efficient windows. An extremely thin, transparent coating of silver on window panes reflects the hot, summer sun and deflects inward the room’s own internal heat. So-called ‘low E squared’ double pane windows reflect away almost 95 percent of the hot rays of the sun, offering household energy savings. Silver halide crystals melted into the glass can change light transmission from 96 percent to 22 percent in less than 60 seconds and block at least 97 percent of the sun's ultraviolet rays. Another increasing use of silver is in paints. Silver ions offer an anti-bacterial shield that keeps the coating germ and fungus free. This is particularly important in health care facilities, jails, schools, food and beverage factories and other places in which bacteria growth can be dangerous to health.

6.11. Solar Energy
As the price of fossil fuels rises, scientists and engineers are more interested in solar cells to produce electricity. Silver paste is used in 90% of all crystalline silicon photovoltaic cells, which are the most common types of solar cell. Photovoltaic systems are simple and provide immediately useful power with no pollution.

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Silver is used in another way to generate electricity by reflecting and concentrating solar energy onto collectors containing salts which are used to run generators. In California, for example, 1,926 silver-coated mirrors reflect solar heat onto black-coated stainless steel tubes at 300-foot tower. This heats the tubes and the nitrate salt inside them to over 1050 ° F. The scalding hot salt is then piped to boilers, turning water to steam which drives steam turbines that run electric generators. They generate electricity to power 10,000 homes.

6.12. Water Purification
As silver ions prevent bacteria and algae growth, silver is employing in purification systems in hospitals, small community water systems pools etc. by using silver in purification process, it has replaces the traditional methods of germ-killing methods that employ harsh and dangerous chemicals. In building water supply systems, silver ions can destroy Legionnaires ’ disease, which is caused by bacteria building up in pipes, connections and water tanks. In pools and spas, silver ions, usually held in canisters within filters, are activated by water to spread a biocide blanket to all components, keeping the water pure and disease free. With several different methods of water purification in the tube, often including a charcoal filter, silver’s role is to prevent the growth of bacteria and fungi that could overwhelm the system and render it useless.

6.13. Making Rain
Sometimes in cloudy days, it does rain. This happen because the droplets are two small and light to fall under gravity and they are stable, that is to say they do not become large enough to fall. Rain falls when the droplets freeze to ice so that it will be a stable nucleus which grows by attracting water vapors from surrounding droplets. The ice particle then grows to a size which gravity can pull it down, and it falls as rain.

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The difficult stage in rain formation is getting the ice to nucleate it. If the water droplets are clean, then the any nucleation catalysts. In this situation the only nucleation agent is ice which can be formed at a very low temperature (about -40 °C). Industrial dusts and smokes also do the nucleation.

Fig. 6.2. Rain falls when the water droplets in clouds turn to ice. When dust particles are present they can catalyze nucleation at temperatures quite close to 0°C. This is why there is often heavy rainfall downwind of factory chimneys.

Crystal of ice and crystal of silver has hexagonal structure with similar lattice constants (H20; a = 0.452 nm, AgI; a = 0.458 nm). If silver iodide is put into super cooled water, it can act as nucleation agent. More ice can grow on it easily, at a low under cooling. This makes silver iodide a perfect material for artificial rain maker. In artificial rainmaking silver iodide, in the form of a very fine powder of crystals, is either dusted into the cloud from a plane flying above it, or is shot into it with a rocket from below. The powder “seeds” ice crystals which grow and start to fall, taking the silver iodide with them. But when it starts to fall, it leaves tiny fragments of ice to seed next lot of crystals, thus self-catalyzing.

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7. SILVER IN ENVIRONMENT

7.1. Introduction
Silver and its ions are considered mildly toxic. Higher concentration of silver in may cause danger to life in the environment. Seawater contains approximately 2-100 ppt of silver, and the surface concentration may be even lower. River water generally contains approximately 0.3-1 ppb of silver. The phytoplankton concentration is 0.1-1 ppm (dry mass), leading to a 104-105 bio concentration factor in seawater. In oyster tissue concentrations of approximately 890 ppm (dry mass) were found. Dissolved in water silver mainly occurs as Ag+ (aq), and in seawater as AgCl2ˉ (aq). Under normal conditions silver is water insoluble. This also applies to a number of silver compounds, such as silver sulphide. For example, silver chloride has a water solubility of 0.1 mg/L, maximum. Silver nitrate has a water solubility of 2450 g/L. Silver fluorides are generally water soluble, but other silver halogens are not.

7.2. Source of Silver
The main source of silver contamination of water is silver thiosulfate complexes in photographic developing solutions that photofinishers discard directly to sewers. During waste treatment, most of the silver thiosulfate complexes are converted to insoluble silver sulfide and forms some metallic silver. Silver on suspended matter and in colloidal forms and insoluble salts ultimately settles out in the sediments. At the water treatment plant, most of the silver is precipitated after treatment with lime or adsorbed after treatment with alum flocculant. Chlorination converts some silver to silver chloride or to a soluble silver chloride complex. Aerobic biodegradation of a photoprocessing wastewater containing 1.85 mg total silver/liter did not adversely affect the activated sludge process.

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When fresh sludge and aerobically digested sludge solids is subjected to leaching procedures, the silver concentration increases in water. Forms of silver in atmospheric emissions are mostly silver sulfide, silver sulfate (Ag2SO4), silver carbonate (Ag2CO3), silver halides, and metallic silver. About 50% of the silver released into the atmosphere from industrial operations is transported more than 100 km and is eventually deposited in precipitation. Emissions of silver from coal-fired power plants may lead to accumulations in nearby soils. Silver in soils is largely immobilized by precipitation to insoluble salts and by complexation or adsorption to organic matter, clays, and manganese and iron oxides.

7.3. Effects to Environment
Sensitive marine algae accumulates silver from water containing as little as 2 µg silver/liter (as silver nitrate) to whole-cell burdens as high as 58 mg silver/kg dry weight. Uptake of silver by phytoplankton is rapid but proportional to silver concentration, and inversely proportional to water salinity. Silver once integrated with phytoplankton does not lose even if the salinity increases since it is tightly bound to the cell membrane. The ability to accumulate dissolved silver varies widely between species. Silver accumulation is higher in marine species through dissolved silver ions. At concentrations normally encountered in the environment, food-chain biomagnifications of silver in aquatic systems is unlikely. Silver inhibits enzymes for the phosphorus, sulfur, and nitrogen cycles of nitrifying bacteria in soil thus effecting the plant growth. The concentration of silver also can affect the germination process of the seeds. In some birds, increasing silver concentration reduce their growth rate. In wild mammal, silver accumulates in liver and kidneys which result in damaging the kidney.

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7.4. Health Effects of Silver
Silver is not a dietary requirement for humans. The body of an adult contains approximately 2 mg of silver. From a daily intake of 20-80 μg silver, approximately 10% is absorbed which are not health threatening. However, a higher concentration of silver ions may be toxic due to its high affinity for sulhur hydryl and amino groups thus the process of complexation occurs with amino acids, nucleic acid and other compounds in the body. Silver that ends up in the body is generally deposited in connective tissue, skin and eyes and causes a gray to black colouring. Within 50 years, one is able to accumulate approximately 9 mg of silver. Silver oxide is harmful upon swallowing, because it irritates the eyes, respiratory tract and skin. Silver nitrate is much more harmful, because it is a strong oxidant. It causes corrosion and at oral uptake it leads to vomiting, dizziness and diarrhoea.

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8. SILVER AND ISLAM
8.1. Introduction
Silver as precious metal can be used in different application. It is used as jewellery, sanitary applications and some fixtures. In Islam, these applications are given different judgments. Therefore being a Muslim one have to follow the path shown in Islam. In this section, I try to explain those ruling in light of Quran and sunnah of Messenger of Allaah (peace and blessings of Allaah be upon him).

8.2. Silver Jewellery
Silver and other kinds of jewelleries are allowed to Muslim women as adornment. But for men there are limitations in using jewelleries. Wearing gold is haraam for men. Men are permissible to wear rings of silver and there is nothing wrong with it. Anas (may Allaah be pleased with him) said: “When the Prophet (peace and blessings of Allaah be upon him) wanted to send a letter to the Romans, he was told that they would not read any letter unless it had a seal on it, so he took a ring of silver. It is as if I can see it shining on his hand, and engraved on it were the words ‘Muhammad the Messenger of Allaah.’” (Narrated by al-Bukhaari, 5537; Muslim 2092). Shaykh al-Islam (Ibn Taymiyah) was asked about men wearing silver, and he said: “As for rings of silver, this is permissible according to the consensus of the imaams, for it was reported that the Prophet (peace and blessings of Allaah be upon him) had a ring of silver and that his companions wore rings. This is unlike gold rings, which are forbidden according to the consensus of the four imaams. It was reported that the Prophet (peace and blessings of Allaah be upon him) forbade that. As for wearing silver, there is no general statement that it is forbidden, and no one has the right to say that it is forbidden if there is no shar’i evidence (daleel) to that effect. As the Sunnah permits wearing silver rings, this is evidence that wearing silver is permissible…”
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Wearing chains and adornments on the wrist and neck, and on the ears, is an imitation of women, as this is something that is only for women and there is no report which says that it is permissible for men to wear chains made of silver. So it is not allowable for men to wear bracelets, earrings, anklets, or chains. Ibn ‘Abbaas (may Allaah be pleased with him) reported that “The Messenger of Allaah (peace and blessings of Allaah be upon him) cursed men who make themselves look like women and women who make themselves look like men.” (Reported by al-Bukhaari, 5435).

8.3. Silver as a Valuable
Silver has been mentioned in Qur’an as a valuable and medium of exchange. Our Prophet Muhammad (sallalahu ‘alaihi wa sallam) also explains the status of silver while establishing the basics of Islamic economic system. Abī Sa’īd al-Khudri reported Allah’s Messenger as saying: “Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt. (When a transaction is) like for like, payment being made on the spot, then if anyone gives more or asks for more, he has dealt in Ribā, the receiver and the giver being equally guilty.” (Sahīh, Muslim) The above Hadīth of Prophet Muhammad (sallalahu ‘alaihi wa sallam) has very clearly established ‘money’ in Islam to be either precious metals such as gold and silver, or other commodities such as wheat, barley, dates and salt which are commodities of regular consumption as food but which have a shelf-life. In Qur’an, silver was referred as Dirham and gold as Dinar.

“And they sold him for a few measly Dirhams and they did so because they considered him to be of little worth.” (Yūsuf, 12:20) In this verse of the Qur’ān Allah Most High has referred to ‘money’ as ‘silver’ coins (Dirhams).
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There are other verses of the Qur’an that also refer to gold and silver as wealth.

“Beautified for mankind is love of the joys (that come) from women and offspring; and stored-up heaps of gold and silver (i.e., heaps of Dinars and Dirhams), and horses branded (with their mark), and cattle and land. That is comfort of the life of the world. Allah! With Him is a more excellent abode.” (Āle ‘Imrān, 3:14)

“O ye who believe! Lo! Many of the (Jewish) rabbis and the (Christian) monks devour the wealth of mankind wantonly and debar (men) from the way of Allah. They who hoard up gold and silver and spend it not (this would obviously be in the context of use of gold and silver as money) in the way of Allah, unto them give tidings (O Muhammad) of a painful doom.” (Al-Tauba, 9:34)

"And if you wish to have (one) wife in place of another and you have given one of them a Qintār (i.e., a treasure of gold and silver coins), then take not from it anything; would you take it by slandering (her) and (doing her) manifest wrong?” (Al-Nisā, 4:20)

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The Qur’an had revealed the amazing news that gold and silver would keep up their status as objects of great value in the next world as well.

“Their raiment will be fine green silk embroidered in gold. Bracelets of silver will they wear. Their Lord will quench their thirst with a drink (sparkling) with purity.” [This verse, as well as others that follow, reveal that gold and silver will remain precious and valuable in the hereafter as well.] (Al -Insān, 76:21)

8.4. Silver Vessels
Vessels of gold and silver are forbidden according to the texts of Islam and according to scholarly consensus. It was proven that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do not drink from vessels of gold and silver; do not eat from plates of gold and silver, because they are for them in this world and for you in the Hereafter.” (Saheeh, agreed upon; from the hadeeth of Hudhayfah may Allaah be pleased with him). And it was proven that he (peace and blessings of Allaah be upon him) said: “Whoever eats and drinks from vessels of gold and silver, is taking fire from Hell into his belly.” (Saheeh, agreed upon; from the hadeeth of Umm Salamah, may Allaah be pleased with him; this version was narrated by Muslim). It is not permissible to use gold and silver as vessels, whether for eating or drinking. All of that is prohibited according to the hadeeth of the Messenger of Allaah (peace and blessings of Allaah be upon him). It is not permissible to use them in spoons, coffee cups or teacups; all of that is forbidden, because these are kinds of vessels.

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8.5. Zakaah on Silver
The scholars are agreed that zakaah is obligatory on gold and silver jewellery if it is a kind of adornment that it is “haraam” forbidden to use, or if it is prepared for trading etc. But if it is a permissible kind of jewellery that is intended to be used or loaned, such as a silver ring or women’s jewellery, or things that it is permitted to use to adorn weapons, then there is a difference of scholarly opinion as to whether zakaah is obligatory on these things.

“And those who hoard up gold and silver (Al-Kanz: the money, the Zakaah of which has not been paid) and spend them not in the way of Allaah, announce unto them a painful torment” (al-Tawbah 9:34) Al-Qurtubi said, in his tafseer of this aayah: “Ibn ‘Umar explained the meaning of this in Saheeh al-Bukhaari, when a Bedouin said to him, ‘Tell me about the aayah (interpretation of the meaning), “And those who hoard up gold and silver.”’ Ibn ‘Umar said: ‘(It means) the one who stockpiles them and does not pay zakaah on them – woe to him. This was before (the order to pay) zakaah on them was revealed, and when it was revealed, Allaah made it a means of purification of wealth.’” (Narrated by al-Bukhaari, 2/111, ta’leeqan; 5/204, also ta’leeqan. Ibn Maajah, 1/569-570, no. 1787. Al-Bayhaqi, 4/82). Zakat is obligatory on    The free, adult sane Muslim, When he possesses the nisab with complete possession, and A [lunar] year has passed over it.

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Zakat Nisab 1. There is no charity [obligatory] on any [silver] less than 200 dirhams. 2. Then, if it is 200 dirhams, and a [lunar] years passes over it, 5 dirhams are due for it. 3. There is nothing due on the excess until it reaches 40 dirhams, and then 1 dirham is due for it. [Similarly] for every 40 dirhams, there is 1 dirham . Abu Yusuf and Muhammad said: Whatever exceeds 200 [dirhams] its zakat is in proportion. 4. If the silver is dominant in silver coins, then their ruling is that of silver. But, if alloy is dominant then their ruling is that of trade goods, and its reaching nisab is taken into account.

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9.

CONCLUSION

Silver is precious metal with long history. With its discovery unknown, silver had been used by ancient Egyptians during the time Pharaoh for decorations and jewelries. Silver is an important metal not because of its price or its brightness. It is important to us due to different application its can be used and the achievement we have made in science and technology with the help of silver. It made our life easy and comfortable. Because of its unique properties, silver is most preferred metal for high-tech industries. Due to high conductivity, it is preferred to use in electronics and heat transmitting appliance. Due to the unique chemical and biological properties, it is perfect for medical functions. With these properties, scientists and researchers are still trying to find more about silver.

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