Finding Tungsten
Tungsten is often found in conjunction with other metals such as wolframite (the element in which tungsten was first discovered whose name means "devourer of tin") and scheelite. Around the world, China is the most common place that tungsten is found and mined, though other locales such as Austria, Bolivia and Portugal also have mines that produce respectable amounts of this metal for use in light bulbs, rockets, and in the case of some first world countries, spacecraft.

Refining the Tungsten
Once the ores containing tungsten have been mined from the earth, a process developed by those who discovered tungsten (the Elhuyar brothers Juan and Fausto who were chemists in the late 1700s) is used to purify the metal. The ore that contains the tungsten is crushed, cleaned and then treated with various alkalis so that tungsten trioxide is formed. The tungsten trioxide is then heated using either carbon or hydrogen gas, which then separates the tungsten metal from either the water vapor or the carbon dioxide that forms, depending on which gas was used in the heating and refining process.

The Final Step
Pure tungsten, which is often a whitish or a gray color once it's separated from the ore it was mined in, is in a state referred to as powered tungsten. If necessary, powered tungsten can be used right away without any further modifications. However, if there is no pressing need for the tungsten right then, it will be shaped into bars to be stored or transported. Given that tungsten is a relatively malleable and ductile metal, this process isn't a very difficult one.

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Background

Osmium has the highest optical emission rate of all metals. Consequently, after Edison’s carbon filaments, it was used at the beginning of the lamp industry. Osmium’s big disadvantage is its high vapour pressure, resulting in a short lamp life. Tungsten withstands considerably higher temperatures than osmium and has a very low vapour pressure, resulting in more luminosity combined with a long lifetime.

Wire Properties

Tungsten wire possesses characteristics that have provided it with a unique place in the origin and growth of the lamp industry. The lamp industry represents the largest commercial application of tungsten wire. It is used in this application because it displays excellent creep resistance at elevated temperatures. Tungsten is an attractive lamp filament material because it has an extremely high melting temperature (~3680 K) and a low vapour pressure at high temperatures. Tungsten is also intrinsically brittle and, initially, this prevented the manufacture of tungsten wire. However, at the beginning of this century William Coolidge, working at the General Electric Company, pursued the idea of deforming tungsten at elevated temperatures in order to make small diameter tungsten wire. Two important findings of his work were, first, to develop a method to work a powder metallurgy ingot down to wire by using deformation at elevated temperatures; and, second, to produce a ductile material from this deformation. Today, the ability to handle tungsten wire and coil filaments without breakage is the backbone of the whole incandescent lamp industry.

Processing

The initial stages of thermomechanical processing of sintered tungsten ingot are usually performed by rolling and / or swaging. These operations allow large deformations at relatively high temperatures and during the initial stages of deformation the ingot reaches full density. By working the tungsten at elevated temperatures, the tungsten is maintained well above the ductile to brittle transition temperature but below the recrystallisation temperature. At various points during this deformation, anneals must be applied, or the tungsten will become overworked and begin to fracture. Finally, wire drawing is used to reduce the tungsten to its final desired diameter. At this point, the microstructure consists of fibres which have very high aspect ratios: they act like fibres in a rope and provide bend ductility.

It was not until the advent of transmission electron microscopy that potassium was located in small bubbles in the tungsten. It is these potassium bubbles which provide the wire with its unique high temperature creep resistance. Potassium is essentially insoluble in the tungsten. The bubbles are first formed from the doped powder in the ingot during sintering. During thermomechanical processing, these initial bubbles are drawn out into tubes. When the wire is annealed, these tubes break up to form the rows of bubbles.

Once wire drawing is complete, the tungsten can be coiled into a filament and recrystallised. When the wire is recrystallised, the grain boundaries interact with the potassium bubble rows as the boundaries migrate, giving rise to an interlocking grain structure.

‘Bamboo’ Structure

Recrystallized pure tungsten wire forms a bamboo structure: the grains occupy the entire wire diameter, and the boundaries are essentially perpendicular to the wire axis. At elevated temperatures, under the stress produced by gravity, these boundaries would slide past one another by diffusion and produce a rapid failure. However, when potassium is present in the wire, the interlocking grain structure reduces the rate of grain boundary sliding and extends the filament life. These bubbles continue to pin the grain boundaries at the temperatures of lamp operation, and thus maintain a stable microstructure during the life of the lamp.

Applications

Tungsten is used in many different types of incandescent lamps. The most common types are the general household lamps, automotive lamps, and reflector lamps for floodlight or projector applications. There are also many thousands of speciality lamps, which have a broad range of applications, such as audio-visual projectors, fibre-optical systems, video camera lights, airport runway markers, photocopiers, medical and scientific instruments, and stage or studio systems.

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The quality of a cutting tool can determine whether cuts will be smooth and clean or difficult and sloppy. A metal cutting tool's quality begins with the hardness and durability of its metal. According to the Society of Manufacturing Engineers, "carbide and carbide-coated tools cut about three to five times faster than high-speed steels" and "higher tungsten content increases wear resistance." There are tungsten carbide cutting tools for a wide variety of applications and, once you understand the breadth of their possible uses, you'll be able to choose one for professional, amateur or even industrial projects.

Circular Saw Blades
Circular blades surrounded by sharp, tungsten carbide-tipped teeth can be found in factories for manufacturing, carpenters' tool kits for milling---and in the home, because do-it-yourselfers can buy them at hardware stores. These circular saw blades are sized to fit not only in industrial machinery but also in standard table saws and hand-held circular saws. The most common tungsten carbide circular saw blades are not solid tungsten carbide; their body is typically tempered steel and only the cutting edges of the teeth are coated with tungsten carbide. A variation of the toothed saw blade is the grinding "wheel" or abrasive blade---still circular but without teeth. Their edge is coated with tungsten carbide and used to exert abrasive force as they grind through stone, ceramics and metal.

Reciprocating Saw Blades
Tungsten carbide coats the edges of blades for reciprocating saws, such as jigsaws and demolition reciprocal saws. The back-and-forth sawing motion of these types of saws is well-suited for abrasion, and their coating of tungsten carbide grit allows them to grind through metals, ceramics and masonry. Applications include removing cast iron pipes and metal fasteners during demolition or cutting access holes through stone and tile during remodeling.

Drill Bit Tools
Drill bits may be solid tungsten carbide or tipped with it to provide the speed, accuracy and wear resistance required by machine shops and high-precision manufacturing applications. Construction professionals and amateurs buy similar bits that attach to hand power drills, drill presses and hobbyist rotary tools. Precision drill bit and grinding tools with tungsten carbide tips might be used for milling or sculpting metal as well as decorative carving of stone and precious gemstones. As materials scientist Dale Wittmer stated in a Southern Illinois University publication, "Tungsten carbide tools have been the standard for over 50 years in the mining industry."

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Properties

Tungsten is practically the only material used for electron emitters. Although other, more electropositive, metals would yield higher emission rates, the advantage of tungsten is its extremely low vapour pressure even at high temperatures.

This property is also important for electrical contact materials. While more conductive metals like copper or silver evaporate (erode) under the conditions of an electric arc, tungsten withstands these.

Applications

The following list illustrates some of the resulting tungsten applications:

•         Directly heated cathodes or heater coils for indirectly heated cathodes in cathode ray tubes for TV sets or computer displays, X-ray tubes, electron tubes, klystrons, magnetrons for microwave ovens;

•         Thoria or rare earth oxide alloyed rod electrodes for inert gas welding (TIG welding electrodes), as well as High Intensity Discharge (HID) lamps;

•         Tungsten disks for substrate of high power semiconductor rectifying devices;

•         Sintered pure tungsten electrical contacts

•         Sintered tungsten-copper or tungsten-silver electrical contacts for high-voltage breakers

•         Infiltrated tungsten-copper or tungsten-silver contacts, made from very coarse (50-150 µm) tungsten metal powder

•         High temperature furnace parts such as tungsten heating coils, reflectors and structural material

•         Tungsten / tungsten-rhenium thermocouples for measuring the temperature in such furnaces

•         X-ray tubes for medical use are not only equipped with a tungsten emitter coil, but also with a rotary (or static) anode made of tungsten or tungsten/ tungsten-rhenium. Important here are not only the low vapour pressure, but also tungsten’s good heat conductivity and the wavelength of the resulting X-rays

•         Calcium and/or magnesium tungstate is the phosphor in intensifying screens used with the X-ray photo films. These phosphors convert X-rays into visible (blue) light, resulting in a smaller X-ray charge for the patient

•         Modern business machines, such as photocopiers, facsimile machines, laser printers and air cleaners are equipped with tungsten charger wires. Not only drawn tungsten wire, but also electro-polished, gold-plated or platinum clad tungsten wires are used for this application

•         Ceramic circuit boards are printed by means of a metal paste. After printing, the metallic layer is sintered to form the conductor, and tungsten is a commonly used metal for this purpose

•         Modern computer processors like the 586 series and higher, generate a heat output per square centimetre similar to that of a household cooker surface. Tungsten-copper heat sinks and the processor fan remove the heat

In modern microelectronics, the advantages of tungsten are its good electrical and thermal conductivity, and its thermal expansion coefficient is almost the same as that of silicon. Tungsten plays an essential role, as the following examples show:

•         Ultra high purity tungsten, tungsten silicide and tungsten-titanium PVD sputtering targets are used in VLSI and ULSI DRAM chip technology. A W-Ti layer on a wafer acts as a diffusion barrier, while W and WSi layers function as electrical conductor materials.

•         In liquid crystal display (LCD) technology, ultra high purity molybdenum-tungsten alloy targets are now used instead of molybdenum-tantalum, resulting in improved definition of LCD panels.

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