An alloy is a mixture or metallic solid solution composed of two or more elements.[1] Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal (heat treatment) history. Alloys usually have different properties from those of the component element .Alloy constituents are usually measured by mass. Alloys are usually classified as substitutional or interstitial alloys, depending on the atomic arrangement that forms the alloy. They can be further classified as homogeneous, consisting of a single phase, heterogeneous, consisting of two or more phases, or intermetallic, where there is no distinct boundary between phases.

Alloy

An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resultant material has metallic properties. An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. The result is a metallic substance with properties different from those of its components.

Alloys are usually designed to have properties that are more desirable than those of their components. For instance, steel is stronger than iron, one of its main elements, and brass is more durable than copper, but more attractive than zinc.

Unlike pure metals, many alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. Special alloys can be designed with a single melting point, however, and these are called eutectic mixtures.

Sometimes an alloy is just named for the base metal, as 14 karat gold is an alloy of gold with other elements. The same holds for silver used in jewellery, and aluminium used structurally.

Alloys include:

Alloys are used more extensively than pure metals because they can be engineered to have specific

properties.

Ads by Google

Alloys of iron and carbon include cast iron and steels; brass and bronze are important alloys of

copper; amalgams are alloys that contain mercury; and chromium is an important additive in stainless

steel.

Ads by Google

For example, they may be poorer conductors of heat and electricity, harder, or more resistant to

corrosion.

Ads by Google

Alloying a metal is done by combining it with one or more other metals or non-metals that often enhance its properties. For example, steel is stronger than iron, its primary element. The physical properties, such as density, reactivity, Young's modulus, and electrical and thermal conductivity, of an alloy may not differ greatly from those of its elements, but engineering properties such as tensile strength [2] and shear strength may be substantially different from those of the constituent materials. This is sometimes a result of the sizes of the atoms in the alloy, because larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy resist deformation. Sometimes alloys may exhibit marked differences in behavior even when small amounts of one element occur. For example, impurities in semi-conducting ferromagnetic alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura.[3][4] Some alloys are made by melting and mixing two or more metals. Bronze, an alloy of copper and tin, was the first alloy discovered, during the prehistoric period now known as the bronze age; it was harder than pure copper and originally used to make tools and weapons, but was later superseded by metals and alloys with better properties. In later times bronze has been used for ornaments, bells, statues, and bearings. Brass is an alloy made from copper and zinc.

Unlike pure metals, most alloys do not have a single melting point, but a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and the temperature when melting is just complete is called the liquidus. However, for most alloys there is a particular proportion of constituents (in rare cases two)—the eutectic mixture—which gives the alloy a unique melting point.

[edit] Terminology

The use of alloys by humans started with the use of meteoric iron, a naturally occurring alloy of nickel and iron. As no metallurgic processes were used to separate iron from nickel, the alloy was used as it was.[7] Meteoric iron could be forged from a red heat to make objects such as tools, weapons, and nails. In many cultures it was shaped by cold hammering into knives and arrowheads. They were often used as anvils. Meteoric iron was very rare and valuable, and difficult for ancient people to work.[8]

Iron is usually found as iron ore on Earth, except for one deposit of native iron in Greenland, which was used by the Inuit people.[9] Native copper, however, was found worldwide, along with silver, gold and platinum, which were also used to make tools, jewelry, and other objects since Neolithic times. Copper was the hardest of these metals, and the most widely

distributed. It became one of the most important metals to the ancients. Eventually, humans learned to smelt metals such as copper and tin from ore, and, around 2500 BC, began alloying the two metals to form bronze, which is much harder than its ingredients. Tin was rare, however, being found mostly in Great Britain. In the Middle East, people began alloying copper with zinc to form brass.[10] Ancient civilizations took into account the mixture and the various properties it produced, such as hardness, toughness and melting point, under various conditions of temperature and work hardening, developing much of the information contained in modern alloy constitution diagrams.[11]

The first known smelting of iron began in Anatolia, around 1800 BC. Called the bloomery process, it produced very soft but ductile wrought iron and, by 800 BC, the technology had spread to Europe. Pig iron, a very hard but brittle alloy of iron and carbon, was being produced in China as early as 1200 BC, but did not arrive in Europe until the Middle Ages. These metals found little practical use until the introduction of crucible steel around 300 BC. These steels were of poor quality, and the introduction of pattern welding, around the 1st century AD, sought to balance the extreme properties of the alloys by laminating them, to create a tougher metal.[11]

Mercury had been smelted from cinnabar for thousands of years. Mercury dissolves many metals, such as gold, silver, and tin, to form amalgams (an alloy in a soft paste, or liquid form at ambient temperature). Amalgams have been used since 200 BC in China for plating objects with precious metals, called gilding, such as armor and mirrors. The ancient Romans often used mercury-tin amalgams for gilding their armor. The amalgam was applied as a paste and then heated until the mercury vaporized, leaving the gold, silver, or tin behind.[12] Mercury was often used in mining, to extract precious metals like gold and silver from their ores.[13]

Many ancient civilizations alloyed metals for purely aesthetic purposes. In ancient Egypt and Mycenae, gold was often alloyed with copper to produce red-gold, or iron to produce a bright burgundy-gold. Silver was often found alloyed with gold. These metals were also used to strengthen each other, for more practical purposes. Quite often, precious metals were alloyed with less valuable substances as a means to deceive buyers.[14] Around 250 BC, Archimedes was commissioned by the king to find a way to check the purity of the gold in a crown, leading to the famous bath-house shouting of "Eureka!" upon the discovery of Archimedes' principle.[15]

While the use of iron started to become more widespread around 1200 BC, mainly because of interruptions in the trade routes for tin, the metal is much softer than bronze. However, very small amounts of steel, (an alloy of iron and around 1% carbon), was always a byproduct of the bloomery process. The ability to modify the hardness of steel by heat treatment had been known since 1100 BC, and the rare material was valued for use in tool and weapon making. Because the ancients could not produce temperatures high enough to melt iron fully, the production of steel in decent quantities did not occur until the introduction of blister steel during the Middle Ages. This method introduced carbon by heating wrought iron in charcoal for long periods of time, but the penetration of carbon was not very deep, so the alloy was not homogeneous. In 1740, Benjamin Huntsman began melting blister steel in a crucible to even out the carbon content, creating the first process for the mass production of tool steel. Huntsman's process was used for manufacturing tool steel until the early 1900s.[16]

With the introduction of the blast furnace to Europe in the Middle Ages, pig iron was able to be produced in much higher volumes than wrought iron. Because pig iron could be melted,

people began to develop processes of reducing the carbon in the liquid pig iron to create steel. Puddling was introduced during the 1700s, where molten pig iron was stirred while exposed to the air, to remove the carbon by oxidation. In 1858, Sir Henry Bessemer developed a process of steel making by blowing hot air through liquid pig iron to reduce the carbon content. The Bessemer process was able to produce the first large scale manufacture of steel. Once the Bessemer process began to gain widespread use, other alloys of steel began to follow, such as mangalloy, an alloy of steel and manganese, which exhibits extreme hardness and toughness.[17]

In 1906, precipitation hardening alloys were discovered by Alfred Wilm. Precipitation hardening alloys, such as certain alloys of aluminium, titanium, and copper, are heat-treatable alloys that soften when quenched (cooled quickly), and then harden over time. After quenching a ternary alloy of aluminium, copper, and magnesium, Wilm discovered that the alloy increased in hardness when left to age at room temperature. Although an explanation for the phenomenon was not provided until 1919, duralumin was one of the first "age hardening" alloys to be used, and was soon followed by many others. These alloys became widely used in many forms of industry, including the construction of modern aircraft.[18]

[edit] See also

A polymer is a large molecule (macromolecule) composed of repeating structural units. These sub-units are typically connected by covalent chemical bonds. Although the term polymer is sometimes taken to refer to plastics, it actually encompasses a large class of compounds comprising both natural and synthetic materials with a wide variety of properties.

Because of the extraordinary range of properties of polymeric materials,[2] they play an essential and ubiquitous role in everyday life.[3] This role ranges from familiar synthetic plastics and elastomers to natural biopolymers such as nucleic acids and proteins that are essential for life.

Natural polymeric materials such as shellac, amber, and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose, which is the main constituent of wood and paper. The list of synthetic polymers includes synthetic rubber, Bakelite, neoprene, nylon, PVC, polystyrene, polyethylene, polypropylene, polyacrylonitrile, PVB, silicone, and many more.

Most commonly, the continuously linked backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. A simple example is polyethylene ('polythene' in British English), whose repeating unit is based on ethylene monomer. However, other structures do exist; for example, elements such as silicon form familiar materials such as silicones, examples being Silly Putty and waterproof plumbing sealant. Oxygen is also commonly present in polymer backbones, such as those of polyethylene glycol, polysaccharides (in glycosidic bonds), and DNA (in phosphodiester bonds).

Polymers are studied in the fields of polymer chemistry, polymer physics, and polymer science.

Contents [hide] 

Brand Name PolymerCharacteristic

propertiesUses

BakelitePhenol-formaldehyde resin

High electric, heat and chemical resistance

Insulation of wires, manufacturing sockets, electrical devices, breakpads, etc. Bakelite

Kevlar Para-aramid fibre High tensile strengthManufacturing armour, sports and musical equipments. Used in the field of cryogenics

Twaron Para-aramidHeat resistant and strong fibre

Bullet-proof body armor, helmets, brake pads, ropes, cables and optical fibre cables, etc. and as an asbestos substitute

MylarPolyethylene terephthalate film

High strength and stiffness, less permeable to gases, almost reflects light completely

Food packaging, transparent covering over paper, reflector for rollsigns and solar cooking stoves

Neoprene Polychloroprene Chemically inert

Manufacturing gaskets, corrosion resistant coatings, waterproof seat covers, substitute for corks and latex

Nylon Insulation of wires, manufacturing sockets, electrical devices, breakpads

Polyamide

Silky, thermoplastic and resistant to biological and chemical agents

Stockings, fabrics, toothbrushes. Molded nylon is used in making machine screws, gears etcNylon. Nylon

Nomex Meta-aramid polymer

Excellent thermal, chemical, and radiation resistance, rigid, durable and fireproof.

Hood of firefighter's mask, electrical lamination of circuit boards and transformer cores and in Thermal Micrometeoroid Garment

Orlon Polyacrylonitrile (PAN) Wool-like, resistant to chemicals, oils, 

Used for making clothes and fabrics like sweaters, hats, yarns, 

moths and sunlightrugs, etc., and as a precursor of carbon fibres

Rilsan Polyamide 11 & 12 Bioplastic

Used in high-performance applications such as sports shoes, electronic device components, automotive fuel lines, pneumatic airbrake tubing, oil and gas flexible pipes and control fluid umbilicals, and catheters.

Technora Copolyamid

High tensile strength, resistance to corrosion, heat, chemicals and saltwater

Used for manufacturing optical fiber cables, umbilical cables, drumheads, automotive industry, ropes, wire ropes and cables

TeflonPolytetrafluoroethylene (PTFE)

Very low coefficient of friction, excellent dielectric properties, high melting, chemically inert

Plain bearings, gears, non-stick panss, etc. due to its low friction. Used as a tubing for highly corrosive chemicals.

Ultem Polyimide

Heat,flame and solvent resistant. Has high dielectric strength

Used in medical and chemical instrumentation, also in guitar picks

Vectran aromatic polyester

High thermal and chemical stability. Golden color. Has high strength, low creep, and is moisture resistant

Used as reinforcing fibres for ropes, cables, sailcloth. Also used in manufacturing badminton strings, bike tires and in electronics applications. Is the key component of a line of inflatable spacecraft developed by Bigelow Aerospace

VitonPolytetrafluoroethylene (PTFE)

Elastomer

Depends on the grade of the polymer. Viton B is used in chemical process plants and gaskets.

Zylonpoly-p-phenylene-2,6-benzobisoxazole (PBO)

Very high tensile strength and thermal stability

Used in tennis racquets, table tennis blades, body armor, etc.

Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct within the finished structure.

A common example of a composite would be disc brake pads, which consist of hard ceramic particles embedded in soft metal matrix. Another example is found in shower stalls and bathtubs which are made of fibreglass. Imitation granite and cultured marble sinks and countertops are also widely used. The most advanced examples perform routinely on spacecraft in demanding environments.

Wattle and daub is one of the oldest man-made composite materials, at over 6000 years old.[1] Concrete is also a composite material, and is used more than any other man-made material in the world.[2] As of 2006, about 7.5 billion cubic metres of concrete are made each year—more than one cubic metre for every person on Earth.[3]

Contents [hide] 

Properties

An individual structural glass fiber is both stiff and strong in tension and compression—that is, along its axis. Although it might be assumed that the fiber is weak in compression, it is actually only the long aspect ratio of the fiber which makes it seem so; i.e., because a typical fiber is long and narrow, it buckles easily. On the other hand, the glass fiber is weak in shear—that is, across its axis. Therefore if a collection of fibers can be arranged permanently in a preferred direction within a material, and if the fibers can be prevented from buckling in compression, then that material will become preferentially strong in that direction.

Furthermore, by laying multiple layers of fiber on top of one another, with each layer oriented in various preferred directions, the stiffness and strength properties of the overall material can be controlled in an efficient manner. In the case of fiberglass, it is the plastic matrix which permanently constrains the structural glass fibers to directions chosen by the designer. With chopped strand mat, this directionality is essentially an entire two dimensional plane; with woven fabrics or unidirectional layers, directionality of stiffness and strength can be more precisely controlled within the plane.

A fiberglass component is typically of a thin "shell" construction, sometimes filled on the inside with structural foam, as in the case of surfboards. The component may be of nearly arbitrary shape, limited only by the complexity and tolerances of the mold used for manufacturing the shell.

MaterialSpecific gravity

Tensile strength MPa (ksi)

Compressive strength MPa (ksi)

Polyester resin (unreinforced)[4] 1.28 55 (7.98) 140 (20.3)

Polyester and Chopped Strand Mat Laminate 30% E-glass[4]

1.4 100 (14.5) 150 (21.8)

Polyester and Woven Rovings Laminate 45% E-glass[4]

1.6 250 (36.3) 150 (21.8)

Polyester and Satin Weave Cloth Laminate 55% E-glass[4]

1.7 300 (43.5) 250 (36.3)

Polyester and Continuous Rovings Laminate 70% E-glass[4]

1.9 800 (116) 350 (50.8)

E-Glass Epoxy composite[5] 1.99 1,770 (257)

S-Glass Epoxy composite[5] 1.95 2,358 (342)

There are both advantages and disadvantages to photochromic lenses. The main advantage is that they will darken into a sunglass tint when exposed to U.V. thus removing the need to carry a separate pair of sunglasses for protection against harmful U.V. rays from the sun.

The main disadvantage of photochromic lenses is they do not adjust immediately. It could take up to two minutes for the lenses to completely change from light to dark or vice versa. Another disadvantage for some users is that they will not darken when worn inside vehicles (glass absorbs virtually 100% of UV light). Since they do not darken inside vehicles, they may not be adequate as driving glasses. Another disadvantage is that they become extremely dark when they get cold. This is a big disadvantage for skiers, who need their glasses to lighten when light is dim and flat.

[edit] References

The glass version of these lenses achieve their photochromic properties through the embedding of microcrystalline silver halides (usually silver chloride), or molecules in a glass substrate. Plastic photochromic lenses rely on organic photochromic molecules (for example oxazines and naphthopyrans) to achieve the reversible darkening effect. The reason these lenses darken in sunlight but not indoors under artificial light, is that room light does not contain the UV (short wavelength light) found in sunlight. Automobile windows also block UV so these lenses would darken less in a car. Lenses that darken in response to visible (rather than UV) light would avoid these issues, but they are not feasible for most

applications. In order to respond to light, it is necessary to absorb it, thus the glass could not be made to be clear in its low-light state. This correctly implies photochromic lenses are not entirely transparent, specifically they filter out UV light. This does not represent a problem, because the human eye does not see in the UV spectrum.

With the photochromic material dispersed in the glass substrate, the degree of darkening depends on the thickness of glass, which poses problems with variable-thickness lenses in prescription glasses. With plastic lenses, the material is typically embedded into the surface layer of the plastic in a uniform thickness of up to 150 µm.

Typically, photochromic lenses darken substantially in response to UV light in less than one minute, and then continue to darken very slightly over the next fifteen minutes.[1] The lenses fade back to clear along a similar pattern. The lenses will begin to clear as soon as they are away from UV light, and will be noticeably lighter within two minutes and mostly clear within five minutes. However, it normally takes more than fifteen minutes for the lenses to completely fade to their non-exposed state. A study by the Institute of Ophthalmology at the University College London has suggested that even in dark conditions photochromic lenses can absorb up to 20% of ambient light.[citation needed]

Because photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather. Conversely, photochromic lenses will get very dark in cold weather conditions, which makes them more suitable for snow skiers than beachgoers while outside. Once inside, away from the triggering UV light, the cold lenses take longer to regain their transparency than warm lenses.

A number of sunglass manufacturers/retailers (Intercast, Oakley, Serengeti Eyewear, Persol to name a few) offer products that use photochromism to make lenses that go from a dark to a darker state. Because these products are tinted in the bleached state, they are typically used only outdoors and are not considered general-purpose lenses

Other applications of ceramics

Knife blades: the blade of a ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.

Vehicle ceramic brake discs are resistant to abrasion at high temperatures.

Advanced composite ceramic and metal matrices have been designed for most modern armoured fighting vehicles because they offer superior penetrating resistance against shaped charges such as high explosive antitank (HEAT) rounds and kinetic energy penetrators.

Ceramics such as alumina and boron carbide have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small arms protective inserts (SAPIs). Similar material is used to protect cockpits of some military airplanes, because of the low weight of the material.

Ceramic balls can be used to replace steel in ball bearings. Their higher hardness means they are much less susceptible to wear and can offer more than triple lifetimes. They also deform less under load, meaning they have less contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings, which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. In some cases, their electricity-insulating properties may also be valuable in bearings. The two major drawbacks to using ceramics are a significantly higher cost and susceptibility to damage under shock loads.

In the early 1980s, Toyota researched production of an adiabatic engine using ceramic components in the hot gas area. The ceramics would have allowed temperatures of over 3000°F (1650°C). The expected advantages would have lighter materials, no or reduced cooling system, and hence a major weight reduction. The expected increase of fuel efficiency of the engine (caused by the higher temperature, as shown by Carnot's theorem) could not be verified experimentally; it was found that the heat transfer on the hot ceramic cylinder walls is higher than the transfer to a cooler metal wall. Obviously the cooler gas film on the metal surface works as a thermal insulator. Thus, despite all of these desirable properties, such engines have not succeeded in production because of costs for the ceramic components and the limited advantages. (Small imperfections in the ceramic material with its low fracture toughness lead to cracks, which can lead to potentially dangerous equipment failure.) Such engines are possible in laboratory settings, but mass production is not feasible with current technology.[citation needed]

Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.

Recent advances have been made in ceramics which include bioceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants coated with these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions so are of great interest for gene delivery and tissue engineering scaffolds. Most hydroxyapatite ceramics are very porous and lack mechanical strength, and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong, fully dense nano crystalline hydroxyapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic, but naturally occurring, bone mineral. Ultimately, these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.

High-tech ceramic is used in watchmaking for producing watch cases. The material is valued by watchmakers for its light weight, scratch resistance, durability and smooth touch. IWC is one of the brands that initiated the use of ceramic in watchmaking. The case of the IWC 2007 Top Gun edition of the Pilot's Watch double chronograph is crafted in black ceramic

ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling.[1] Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous (e.g., a 

glass). Because most common ceramics are crystalline, the definition of ceramic is often restricted to inorganic crystalline materials, as opposed to the noncrystalline glasses

Types of ceramic products

For convenience, ceramic products are usually divided into four sectors; these are shown below with some examples:

Structural, including bricks, pipes, floor and roof tiles Refractories   , such as kiln linings, gas fire radiants, steel and glass making crucibles Whitewares, including tableware, cookware, wall tiles, pottery products and sanitary ware Technical, is also known as engineering, advanced, special, and in Japan, fine ceramics. Such 

items include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, biomedical implants, coatings of jet engine turbine blades, ceramic disk brake, missile nose cones, bearing (mechanical). Frequently, the raw materials do not include clays.[6]

[edit] Examples of whiteware ceramics

Earthenware   , which is often made from clay, quartz and feldspar. Stoneware    Porcelain   , which is often made from kaolin Bone china   

[edit] Classification of technical ceramics

Technical ceramics can also be classified into three distinct material categories:

Oxides   : alumina, beryllia, ceria, zirconia Nonoxides: carbide, boride, nitride, silicide Composite materials   : particulate reinforced, fiber reinforced, combinations of oxides and 

nonoxides.

Each one of these classes can develop unique material properties because ceramics tend to be crystalline

Glass is an amorphous (non-crystalline) solid material. Glasses are typically brittle and optically transparent.

The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica (SiO2) plus Na2O, CaO, and several minor additives. Often, the term glass is used in a restricted sense to refer to this specific use.

In science, however, the term glass is usually defined in a much wider sense, including every solid that possesses a non-crystalline (i.e., amorphous) structure and that exhibits a glass transition when heated towards the liquid state. In this wider sense, glasses can be made of quite different classes of materials: metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers. For many applications (bottles, eyewear) polymer glasses (acrylic glass, polycarbonate, polyethylene terephthalate) are a lighter alternative to traditional silica glassesWhile fused quartz (primarily composed of SiO2) is used for some special

applications, it is not very common due to its high glass transition temperature of over 1200 °C (2192 °F).[3] Normally, other substances are added to simplify processing. One is sodium carbonate (Na2CO3), which lowers the glass transition temperature. However, the soda makes the glass water soluble, which is usually undesirable, so lime (calcium oxide [CaO], generally obtained from limestone), some magnesium oxide (MgO) and aluminium oxide (Al2O3) are added to provide for a better chemical durability. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass.[4] Soda-lime glasses account for about 90% of manufactured glass.

Most common glass has other ingredients added to change its properties. Lead glass or flint glass is more 'brilliant' because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eye glasses.[citation needed] Iron can be incorporated into glass to absorb infrared energy, for example in heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths.[5]

1. Fused silica glass, vitreous silica glass: silica (SiO2). Has very low thermal expansion, is very hard and resists high temperatures (1000–1500 ºC). It is also the most resistant against weathering (alkali ions leaching out of the glass, while staining it). It is used for high temperature applications such as furnace tubes, melting crucibles, etc.

2. Soda-lime-silica glass, window glass: silica 72% + sodium oxide (Na2O) 14.2% + magnesia (MgO) 2.5% + lime (CaO) 10.0% + alumina (Al2O3) 0.6%. Is transparent, easily formed and most suitable for window glass. It has a high thermal expansion and poor resistance to heat (500–600 ºC). Used for windows, containers, light bulbs, tableware.

3. Sodium borosilicate glass, Pyrex: silica 81% + boric oxide (B2O3) 12% + soda (Na2O) 4.5% + alumina (Al2O3) 2.0%. Stands heat expansion much better than window glass. Used for chemical glassware, cooking glass, car head lamps, etc. Borosilicate glasses (e.g. Pyrex) have as main constituents silica and boron oxide. They have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10–6/°C[6] as compared to about 9×10−6/°C for a typical soda-lime glass[7]), making them more dimensionally stable. The lower CTE also makes them less subject to stress caused by thermal expansion, thus less vulnerable to cracking from thermal shock. They are commonly used for reagent bottles, optical components and household cookware.

4. Lead-oxide glass, crystal glass: silica 59% + soda (Na2O) 2.0% + lead oxide (PbO) 25% + potassium oxide (K2O) 12% + alumina 0.4% + zinc oxide (ZnO) 1.5%. Has a high refractive index, making the look of glassware more brilliant (crystal glass). It also has a high elasticity, making glassware 'ring'. It is also more workable in the factory, but cannot stand heating very well.

5. Aluminosilicate glass: silica 57% + alumina 16% + boric oxide (B2O3) 4.0% + barium oxide (BaO) 6.0% + magnesia 7.0% + lime 10%. Extensively used for fiberglass, used for making glass-reinforced plastics (boats, fishing rods, etc.). Also for halogen bulb glass.

6. Oxide glass: alumina 90% + germanium oxide (GeO2) 10%. Extremely clear glass, used for fiber-optic wave guides in communication networks. Light loses only 5% of its intensity through 1km of glass fiber.[8]

Another common glass ingredient is "cullet" (recycled glass). The recycled glass saves on raw materials and energy; however, impurities in the cullet can lead to product and equipment failure. Borosilicate glass has a very low thermal expansion coefficient (3.3 x 10−6/K),[4] about one-third that of ordinary glass. This reduces material stresses caused by

temperature gradients which makes borosilicate the most suitable kind of glass for certain applications (see below).

The softening point (temperature at which viscosity is approximately poise) of type 7740 Pyrex is 820 °C (1,510 °F).[5]

Borosilicate glass is less dense than ordinary glass.

While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subject to rapid or uneven temperature variations. When broken, borosilicate glass tends to crack into large pieces rather than shattering (it will snap rather than splinter).

Optically, borosilicate glasses are crown glasses with low dispersion (Abbe numbers around 65) and relatively low refractive indices (1.51–1.54 across the visible range).

[edit] Usage

Fining agents such as sodium sulfate, sodium chloride, or antimony oxide may be added to reduce the number of air bubbles in the glass mixture.[4] Glass batch calculation is the method by which the correct raw material mixture is determined to achieve the desired glass composition

Uses

[edit] Fertilizer

Approximately 83% (as of 2004) of ammonia is used as fertilizers either as its salts or as solutions. When applied to soil, it helps provide increased yields of crops such as corn and wheat. Consuming more than 1% of all man-made power, the production of ammonia is a significant component of the world energy budget.[7]

[edit] Precursor to nitrogenous compounds

Ammonia is directly or indirectly the precursor to most nitrogen-containing compounds. Virtually all synthetic nitrogen compounds are derived from ammonia. An important derivative is nitric acid. This key material is generated via the Ostwald process by oxidation of ammonia with air over a platinum catalyst at 700–850 °C, ~9 atm. Nitric oxide is an intermediate in this conversion:[39]

NH3 + 2 O2 → HNO3 + H2O

Nitric acid is used for the production of fertilizers, explosives, and many organonitrogen compounds.

[edit] Cleaner

Household ammonia is a solution of NH3 in water (i.e., ammonium hydroxide) used as a general purpose cleaner for many surfaces. Because ammonia results in a relatively streak-free shine, one of its most common uses is to clean glass, porcelain and stainless steel. It is also frequently used for cleaning ovens and soaking items to loosen baked-on grime. Household ammonia ranges in concentration from 5 to 10 weight percent ammonia.

[edit] Fermentation

Solutions of ammonia ranging from 16% to 25% are used in the fermentation industry as a source of nitrogen for microorganisms and to adjust pH during fermentation.

[edit] Antimicrobial agent for food products

As early as in 1895, it was known that ammonia was “strongly antiseptic .. it requires 1.4 grams per litre to preserve beef tea.”[40] Anhydrous ammonia has been shown effective as an antimicrobial agent for animal feed [41] and is currently used commercially to reduce or eliminate microbial contamination of beef.[42][43][44] The New York Times reported in October, 2009 on an American company, Beef Products Inc., which turns fatty beef trimmings, averaging between 50 and 70 percent fat, into seven million pounds per week of lean finely textured beef by removing the fat using heat and centrifugation, then disinfecting the lean product with ammonia; the process was rated by the US Department of Agriculture as effective and safe on the basis of a study (financed by Beef Products) that found that the treatment reduces E. coli to undetectable levels.[45] Further investigation by The New York Times published in December, 2009 revealed safety concerns about the process as well as consumer complaints about the taste and smell of beef treated at optimal levels of ammonia.[46]The following week, the newspaper published an editorial, "More Perils of Ground Meat", reiterating the concerns posed in the news article. Several days later, the editorial was appended with a retraction, stating it had incorrectly claimed there had been two recalls of ground meat because of this process, and "No meat produced by Beef Products Inc. has been linked to any illnesses or outbreaks." [47]

[edit] Minor and emerging uses

[edit] Refrigeration – R717

Because of its favourable vaporization properties, ammonia is an attractive refrigerant.[7] It was commonly used prior to the popularisation of chlorofluorocarbons (Freons). Anhydrous ammonia is widely used in industrial refrigeration applications and hockey rinks because of its high energy efficiency and low cost. The Kalina cycle, which is of growing importance to geothermal power plants, depends on the wide boiling range of the ammonia-water mixture. Ammonia is used less frequently in commercial applications, such as in grocery store freezer cases and refrigerated displays due to its toxicity.

[edit] For remediation of gaseous emissions

Ammonia is used to scrub SO2 from the burning of fossil fuels, and the resulting product is converted to ammonium sulfate for use as fertilizer. Ammonia neutralizes the nitrogen oxides (NOx) pollutants emitted by diesel engines. This technology, called SCR (selective catalytic

reduction), relies on a vanadia-based catalyst.[48]

Ammonia may be used to mitigate gaseous spills of phosgene.[49]

[edit] As a fuel

Drawing of an Ammoniacal Gas Engine Streetcar in New Orleans (1871) by Alfred Rudolph Waud

The X-15 aircraft used ammonia as one component fuel of its rocket engine

Ammonia was used during World War II to power buses in Belgium, and in engine and solar energy applications prior to 1900. Liquid ammonia was used as the fuel of the rocket airplane, the X-15. Although not as powerful as other fuels, it left no soot in the reusable rocket engine and its density approximately matches the density of the oxidizer, liquid oxygen, which simplified the aircraft's design.

Ammonia has been proposed as a practical alternative to fossil fuel for internal combustion engines.[50] The calorific value of ammonia is 22.5 MJ/kg (9690 BTU/lb), which is about half that of diesel. In a normal engine, in which the water vapour is not condensed, the calorific value of ammonia will be about 21% less than this figure. It can be used in existing engines with only minor modifications to carburettors/injectors.

To meet these demands, significant capital would be required to increase present production levels. Although the second most produced chemical, the scale of ammonia production is a small fraction of world petroleum usage. It could be manufactured from renewable energy

sources, as well as coal or nuclear power. It is, however, significantly less efficient than batteries.[citation needed] The 60 MW Rjukan dam in Telemark, Norway produced ammonia via electrolysis of water for many years from 1913 producing fertilizer for much of Europe. If produced from coal, the CO2 can be readily sequestered [50][51] (the combustion products are nitrogen and water). In 1981 a Canadian company converted a 1981 Chevrolet Impala to operate using ammonia as fuel.[52][53]

Ammonia engines or ammonia motors, using ammonia as a working fluid, have been proposed and occasionally used.[54] The principle is similar to that used in a fireless locomotive, but with ammonia as the working fluid, instead of steam or compressed air. Ammonia engines were used experimentally in the 19th century by Goldsworthy Gurney in the UK and in streetcars in New Orleans in the USA.

[edit] As a stimulant

Anti-meth sign on tank of anhydrous ammonia, Otley, Iowa. Anhydrous ammonia is a common farm fertilizer that is also a critical ingredient in making methamphetamine. In 2005, Iowa state used grant money to give out thousands of locks to prevent criminals from getting into the tanks.[55]

Ammonia has found significant use in various sports – particularly the strength sports of weightlifting and Olympic weightlifting as a respiratory stimulant.[citation needed] Ammonia is commonly used in the illegal manufacture of methamphetamine through a Birch reduction,[56] the Birch method of making methamphetamine is dangerous because the alkali metal and liquid ammonia are both extremely reactive, and the temperature of liquid ammonia makes it susceptible to explosive boiling when reactants are added.

[edit] Textile

Liquid ammonia is used for treatment of cotton materials, give a properties like mercerisation using alkalies. In particular, it is used for prewashing of wool.[57]

[edit] Lifting gas

At standard temperature and pressure, ammonia is less dense than atmosphere, and has approximately 60% of the lifting power of hydrogen or helium. Ammonia has sometimes been used to fill weather balloons as a lifting gas. Because of its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquefied aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).

[edit] Woodworking

Ammonia has been used to darken quartersawn white oak in Arts & Crafts and Mission-style furniture. Ammonia fumes react with the natural tannins in the wood and cause it to change colours.[58]

[edit] Properties

Ammonia is a colourless gas with a characteristic pungent smell. It is lighter than air, its density being 0.589 times that of air. It is easily liquefied due to the strong hydrogen bonding between molecules; the liquid boils at −33.3 °C, and freezes at −77.7 °C to white crystals.[14] The crystal symmetry is cubic, Pearson symbol cP16, space group P213 No.198, lattice constant 0.5125 nm.[23] Liquid ammonia possesses strong ionising powers reflecting its high ε of 22. Liquid ammonia has a very high standard enthalpy change of vaporization (23.35 kJ/mol, cf. water 40.65 kJ/mol, methane 8.19 kJ/mol, phosphine 14.6 kJ/mol) and can therefore be used in laboratories in uninsulated vessels without additional refrigeration.

It is miscible with water. Ammonia in an aqueous solution can be expelled by boiling. The aqueous solution of ammonia is basic. The maximum concentration of ammonia in water (a saturated solution) has a density of 0.880 g/cm3 and is often known as '.880 ammonia'. Ammonia does not burn readily or sustain combustion, except under narrow fuel-to-air mixtures of 15–25% air. When mixed with oxygen, it burns with a pale yellowish-green flame. At high temperature and in the presence of a suitable catalyst, ammonia is decomposed into its constituent elements. Ignition occurs when chlorine is passed into ammonia, forming nitrogen and hydrogen chloride; if chlorine is present in excess, then the highly explosive nitrogen trichloride (NCl3) is also formed.

The ammonia molecule readily undergoes nitrogen inversion at room temperature; a useful analogy is an umbrella turning itself inside out in a strong wind. The energy barrier to this inversion is 24.7 kJ/mol, and the resonance frequency is 23.79 GHz, corresponding to microwave radiation of a wavelength of 1.260 cm. The absorption at this frequency was the first microwave spectrum to be observed.[24]

Ammonia may be conveniently deodorized by reacting it with either sodium bicarbonate or acetic acid. Both of these reactions form an odourless ammonium salt.

[edit] Basicity

One of the most characteristic properties of ammonia is its basicity. It combines with acids to form salts; thus with hydrochloric acid it forms ammonium chloride (sal-ammoniac); with nitric acid, ammonium nitrate, etc. However, perfectly dry ammonia will not combine with perfectly dry hydrogen chloride; moisture is necessary to bring about the reaction.[25] As a demonstration experiment, opened bottles of concentrated ammonia and hydrochloric acid produce clouds of ammonium chloride, which seem to appear “out of nothing” as the salt forms where the two diffusing clouds of molecules meet, somewhere between the two bottles.

NH3 + HCl → NH4Cl

The salts produced by the action of ammonia on acids are known as the ammonium salts and all contain the ammonium ion (NH4

+). Dilute aqueous ammonia can be applied on the skin to lessen the effects of acidic animal venoms, such as from insects and jellyfish.

The basicity of ammonia also is the basis of its toxicity and its use as a cleaner.

By creating a solution with a pH much higher than a neutral water solution, proteins (enzymes) will denaturate, leading to cell damage, death of the cell, and eventually death of the organism.

Dirt often consists of fats and oils, which are sparingly soluble in water. Ammonia brings them into aqueous solution. The remaining water, also containing excess ammonia, will evaporate completely, leaving a clean surface.

[edit] Acidity

Although ammonia is well known as a weak base, it can also act as an extremely weak acid. It is a protic substance and is capable of formation of amides (which contain the NH2

− ion). For example, lithium dissolves into liquid ammonia to give a solution of lithium amide:

2 Li + 2 NH3 → 2 LiNH2 + H2

[edit] Self-dissociation

Like water, ammonia undergoes molecular autoionisation to form its acid and base conjugates:

2 NH3 (l)  NH+4 (aq) + NH−2 (aq)

At standard pressure and temperature, K=[NH+4][NH−2] = 10−30

[edit] Combustion

The combustion of ammonia to nitrogen and water is exothermic:

4 NH3 + 3 O2 → 2 N2 + 6 H2O (g) (Δ   H   º  r = –1267.20 kJ/mol)

The standard enthalpy change of combustion, ΔHºc, expressed per mole of ammonia and with condensation of the water formed, is −382.81 kJ/mol. Dinitrogen is the thermodynamic product of combustion: all nitrogen oxides are unstable with respect to nitrogen and oxygen, which is the principle behind the catalytic converter. However, nitrogen oxides can be formed as kinetic products in the presence of appropriate catalysts, a reaction of great industrial importance in the production of nitric acid:

4 NH3 + 5 O2 → 4 NO + 6 H2O

A subsequent reaction leads to water and NO2

2 NO + O2 → 2 NO2

The combustion of ammonia in air is very difficult in the absence of a catalyst (such as platinum gauze), as the temperature of the flame is usually lower than the ignition temperature of the ammonia-air mixture. The flammable range of ammonia in air is 16–25%.[26]

[edit] Formation of other compounds

In organic chemistry, ammonia can act as a nucleophile in substitution reactions. Amines can be formed by the reaction of ammonia with alkyl halides, although the resulting –NH2 group is also nucleophilic and secondary and tertiary amines are often formed as byproducts. An excess of ammonia helps minimise multiple substitution, and neutralises the hydrogen halide formed. Methylamine is prepared commercially by the reaction of ammonia with chloromethane, and the reaction of ammonia with 2-bromopropanoic acid has been used to prepare racemic alanine in 70% yield. Ethanolamine is prepared

The process

Flow diagram for the Haber Bosch process

By far the major source of the hydrogen required for the Haber-Bosch process is methane from natural gas, obtained through a heterogeneous catalytic process, which requires far less external energy than the process used initially by Bosch at BASF: the electrolysis of water. Far less commonly, in some countries, coal is used as the source of hydrogen through a process called coal gasification. The source of the hydrogen is of no consequence in the Haber-Bosch process.

[edit] Synthesis gas preparationMain article: Hydrogen production

The methane is first cleaned, mainly to remove sulfur oxide and hydrogen sulfide impurities that would poison the catalysts.

The clean methane is then reacted with steam over a catalyst of nickel oxide. This is called steam reforming:

CH4 + H2O → CO + 3 H2

Secondary reforming then takes place with the addition of air to convert the methane that did not react during steam reforming:

2 CH4 + O2 → 2 CO + 4 H2

CH4 + 2 O2 → CO2 + 2 H2O

Then the water gas shift reaction yields more hydrogen from CO and steam:

CO + H2O → CO2 + H2

The gas mixture is now passed into a methanator[9] which converts most of the remaining CO into methane for recycling:

CO + 3 H2 → CH4 + H2O

This last step is necessary as carbon monoxide poisons the catalyst. (Note, this reaction is the reverse of steam reforming). The overall reaction so far turns methane and steam into carbon dioxide, steam, and hydrogen.

[edit] Ammonia synthesis – Haber process

The final stage, which is the actual Haber process, is the synthesis of ammonia using an iron catalyst promoted with K2O, CaO and Al2O3:[citation needed]

N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g)   (ΔH = −92.22 kJ·mol−1)

This is done at 15–25 MPa (150–250 bar) and between 300 and 550 °C, as the gases are passed over four beds of catalyst, with cooling between each pass so as to maintain a reasonable equilibrium constant. On each pass only about 15% conversion occurs, but any unreacted gases are recycled, and eventually an overall conversion of 97% is achieved.[citation

needed]

The steam reforming, shift conversion, carbon dioxide removal, and methanation steps each operate at absolute pressures of about 2.5–3.5 MPa (25–35 bar), and the ammonia synthesis loop operates at absolute pressures ranging from 6–18 MPa (59–178 atm), depending upon which proprietary design is used.[citation needed]

[edit] Reaction rate and equilibrium

There are two opposing considerations in this synthesis: the position of the equilibrium and the rate of reaction. At room temperature, the reaction is slow and the obvious solution is to raise the temperature. This may increase the rate of the reaction but, since the reaction is exothermic, it also has the effect, according to Le Chatelier's principle, of favouring the reverse reaction and thus reducing the amount of product, given by:

As the temperature increases, the equilibrium is shifted and hence, the amount of product drops dramatically according to the Van't Hoff equation. Thus one might suppose that a low temperature is to be used and some other means to increase rate. However, the catalyst itself requires a temperature of at least 400 °C to be efficient.

Pressure is the obvious choice to favour the forward reaction because there are 4 moles of reactant for every 2 moles of product (see entropy), and the pressure used (around 200 atm) alters the equilibrium concentrations to give a profitable yield.

Economically, though, pressure is an expensive commodity. Pipes and reaction vessels need to be strengthened, valves more rigorous, and there are safety considerations of working at 200 atm. In addition, running pumps and compressors takes considerable energy. Thus the compromise used gives a single pass yield of around 15%.

Another way to increase the yield of the reaction would be to remove the product (i.e. ammonia gas) from the system. In practice, gaseous ammonia is not removed from the reactor itself, since the temperature is too high; but it is removed from the equilibrium mixture of gases leaving the reaction vessel. The hot gases are cooled enough, whilst maintaining a high pressure, for the ammonia to condense and be removed as liquid. Unreacted hydrogen and nitrogen gases are then returned to the reaction vessel to undergo further reaction.

[edit] Catalysts

The catalyst has no effect on the position of chemical equilibrium; rather, it provides an alternative pathway with lower activation energy and hence increases the reaction rate, while remaining chemically unchanged at the end of the reaction. The first Haber–Bosch reaction chambers used osmium and ruthenium as catalysts. However, under Bosch's direction in 1909, the BASF researcher Alwin Mittasch discovered a much less expensive iron-based catalyst that is still used today. Part of the industrial production now takes place with a ruthenium rather than an iron catalyst (the KAAP process), because this more active catalyst allows reduced operating pressures.

In industrial practice, the iron catalyst is prepared by exposing a mass of magnetite, an iron oxide, to the hot hydrogen feedstock. This reduces some of the magnetite to metallic iron, removing oxygen in the process. However, the catalyst maintains most of its bulk volume during the reduction, and so the result is a highly porous material whose large surface area aids its effectiveness as a catalyst. Other minor components of the catalyst include calcium and aluminium oxides, which support the porous iron catalyst and help it maintain its surface area over time, and potassium, which increases the electron density of the catalyst and so improves its activity.

The reaction mechanism, involving the heterogeneous catalyst, is believed to be as follows:

1. N2 (g) → N2 (adsorbed)2. N2 (adsorbed) → 2 N (adsorbed)

Variation in Keq for the equilibriumN2 (g) + 3H2 (g)  2NH3 (g)

as a function of temperature[10]

Temperature (°C) Keq

300 4.34 x 10−3

400 1.64 x 10−4

450 4.51 x 10−5

500 1.45 x 10−5

550 5.38 x 10−6

600 2.25 x 10−6

3. H2(g) → H2 (adsorbed)4. H2 (adsorbed) → 2 H (adsorbed)5. N (adsorbed) + 3 H(adsorbed)→ NH3 (adsorbed)6. NH3 (adsorbed) → NH3 (g)

Reaction 5 occurs in three steps, forming NH, NH2, and then NH3. Experimental evidence points to reaction 2 as being the slow, rate-determining step.

A major contributor to the elucidation of this mechanism is Gerhard Ertl.