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28 novembre 2011 1 28 /11 /novembre /2011 03:05

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases). Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide. Sony VAIO VGN-NW11S/S Battery

Nearly all of this argon is radiogenic argon-40 derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, being the preferred argon isotope produced by stellar nucleosynthesis in supernovas.Sony VAIO VGN-NW11S/T Battery

The name "argon" is derived from the Greek word ????? meaning "lazy" or "the inactive one", a reference to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements.Sony VAIO VGN-NW11Z/S Battery

Its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.

Argon is produced industrially by the fractional distillation of liquid air. Argon is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily non-reactive substances become reactive;Sony VAIO VGN-NW11Z/T Battery

for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning. Argon gas also has uses in incandescent and fluorescent lighting, and other types of gas discharge tubes. Argon makes a distinctive blue-green gas laser.Sony VAIO VGN-NW21EF/S Battery

Argon has approximately the same solubility in water as oxygen gas and is 2.5 times more soluble in water than nitrogen gas. Argon is colorless, odorless, and nontoxic as a solid, liquid, and gas. Argon is inert under most conditions and forms no confirmed stable compounds at room temperature.Sony VAIO VGN-NW21JF Battery

Although argon is a noble gas, it has been found to have the capability of forming some compounds. For example, the creation of argon fluorohydride (HArF), a marginally stable compound of argon with fluorine and hydrogen, was reported by researchers at the University of Helsinki in 2000.Sony VAIO VGN-NW21MF Battery

Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules. Argon-containing ions and excited state complexes, such as ArH+and ArF, respectively, are known to exist. Sony VAIO VGN-NW21MF/W Battery

Theoretical calculations have predicted several argon compounds that should be stable, but for which no synthesis routes are currently known.

Argon (?????, Greek meaning "inactive", in reference to its chemical inactivity) Sony VAIO VGN-NW21ZF Battery

was suspected to be present in air by Henry Cavendish in 1785 but was not isolated until 1894 by Lord Rayleigh and Sir William Ramsay in Scotland in an experiment in which they removed all of theoxygen, carbon dioxide, water and nitrogen from a sample of clean air.Sony VAIO VGN-NW31EF/W Battery

They had determined that nitrogen produced from chemical compounds was one-half percent lighter than nitrogen from the atmosphere. The difference seemed insignificant, but it was important enough to attract their attention for many months. They concluded that there was another gas in the air mixed in with the nitrogen.Sony VAIO VGN-NW31JF Battery

Argon was also encountered in 1882 through independent research of H. F. Newall and W.N. Hartley. Each observed new lines in the color spectrum of air but were unable to identify the element responsible for the lines. Argon became the first member of the noble gases to be discovered. The symbol for argon is now Ar, but up until 1957 it was A.Sony VAIO VGN-NW320F/B Battery

Occurrence

Argon constitutes 0.934% by volume and 1.28% by mass of the Earth's atmosphere, and air is the primary raw material used by industry to produce purified argon products. Argon is isolated from air by fractionation, most commonly by cryogenic fractional distillation, a process that also produces purified nitrogen, oxygen, neon, krypton and xenon. Sony VAIO VGN-NW320F/TC Battery

The main isotopes of argon found on Earth are 40Ar (99.6%), 36Ar (0.34%), and 38Ar (0.06%). Naturally occurring 40K with a half-life of 1.25×109 years, decays to stable 40Ar (11.2%) byelectron capture or positron emission, and also to stable 40Ca (88.8%) via beta decay. Sony VAIO VGN-NW35E Battery

These properties and ratios are used to determine the age of rocks by the method of K-Ar dating.

In the Earth's atmosphere, 39Ar is made by cosmic ray activity, primarily with 40Ar. In the subsurface environment, it is also produced through neutron capture by 39K or alpha emission bycalcium. Sony VAIO VGN-NW380F/S Battery

37Ar is created from the neutron spallation of 40Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days .

Argon is notable in that its isotopic composition varies greatly between different locations in the solar system. Where the major source of argon is the decay of 40K in rocks, 40Ar will be the dominant isotope, as it is on earth. Sony VAIO VGN-NW380F/T Battery

Argon produced directly by stellar nucleosynthesis, in contrast, is dominated by the alpha process nuclide, 36Ar. Correspondingly, solar argon contains 84.6% 36Ar based on solar wind measurements.

The predominance of radiogenic 40Ar is responsible for the fact that the standard atomic weight of terrestrial argon is greater than that of the next element, potassium. Sony VAIO VGN-NW50JB Battery

This was puzzling at the time when argon was discovered, since Mendeleev had placed the elements in his periodic table in order of atomic weight, although the inertness of argon implies that it must be placed before the reactive alkali metal potassium.Sony VAIO VGN-NW51FB/N Battery

Henry Moseley later solved this problem by showing that the periodic table is actually arranged in order of atomic number. (See History of the periodic table).

The much greater atmospheric abundance of argon relative to the other noble gases is also due to the presence of radiogenic40Ar.Sony VAIO VGN-NW51FB/W Battery

Primordial 36Ar has an abundance of only 31.5 ppmv (= 9340 ppmv x 0.337%), comparable to that of neon (18.18 ppmv).

The Martian atmosphere contains 1.6% of 40Ar and 5 ppm of 36Ar. The Mariner space probe fly-by of the planet Mercury in 1973 found that Mercury has a very thin atmosphere with 70% argon, Sony VAIO VGN-NW70JB Battery

believed to result from releases of the gas as a decay product from radioactive materials on the planet. In 2005, the Huygens probe also discovered the presence of 40Ar on Titan, the largest moon of Saturn.

Argon’s complete octet of electrons indicates full s and p subshells. This full outer energy level makes argon very stable and extremely resistant to bonding with other elements. Sony VAIO VGN-NW71FB/N Battery

Before 1962, argon and the other noble gases were considered to be chemically inert and unable to form compounds; however, compounds of the heavier noble gases have since been synthesized. In August 2000, the first argon compounds were formed by researchers at the University of Helsinki. Sony VAIO VGN-NW71FB/W Battery

By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride, argon fluorohydride (HArF) was formed. It is stable up to 40 kelvin (?233 °C). TheArCF2+2 metastable dication was also observed.

Industrial

Argon is produced industrially by the fractional distillation of liquid air in a cryogenic air separation unit;Sony VAIO VGN-NW91FS Battery

a process that separates liquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K and liquid oxygen, which boils at 90.2 K. About 700,000 tonnes of argon are produced worldwide every year.

In radioactive decays

40Ar, the most abundant isotope of argon, is produced by the decay of 40K with a half-life of 1.25×109 years by electron capture or positron emission.Sony VAIO VGN-NW91GS Battery

Because of this, it is used in potassium-argon dating to determine the age of rocks.

There are several different reasons argon is used in particular applications:

  • An inert gas is needed. In particular, argon is the cheapest alternative when nitrogen is not sufficiently inert.
  • Low thermal conductivity is required.
  • The electronic properties (ionization and/or the emission spectrum) are necessary.Sony VAIO VGN-NW91VS Battery

Other noble gases would probably work as well in most of these applications, but argon is by far the cheapest. Argon is inexpensive since it is a byproduct of the production of liquid oxygen and liquid nitrogen from a cryogenic air separation unit, both of which are used on a large industrial scale. Sony VAIO VGN-SR11M Battery

The other noble gases (except helium) are produced this way as well, but argon is the most plentiful since it has the highest concentration in the atmosphere. The bulk of argon applications arise simply because it is inert and relatively cheap.Sony VAIO VGN-SR11MR Battery

Industrial processes

Argon is used in some high-temperature industrial processes, where ordinarily non-reactive substances become reactive. For example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning.Sony VAIO VGN-SR140D Battery

For some of these processes, the presence of nitrogen or oxygen gases might cause defects within the material. Argon is used in various types of arc welding such as gas metal arc welding and gas tungsten arc welding, as well as in the processing of titanium and other reactive elements.Sony VAIO VGN-SR140D/B Battery

An argon atmosphere is also used for growing crystals of silicon and germanium.

Argon is an asphyxiant in the poultry industry, either for mass culling following disease outbreaks, or as a means of slaughter more humane than the electric bath. Sony VAIO VGN-SR140D/P Battery

Argon's relatively high density causes it to remain close to the ground during gassing. Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen within the dead bird, argon also enhances shelf life.

Argon is sometimes used for extinguishing fires where damage to equipment is to be avoided.Sony VAIO VGN-SR140D/S Battery

Scientific research

Argon is used, primarily in liquid form, as the target for direct dark matter searches. The interaction of a hypothetical WIMP particle with the argon nucleus produces scintillation light that is then detected by photomultiplier tubes.Sony VAIO VGN-SR140E Battery

Two-phase detectors also use argon gas to detect the ionized electrons produced during the WIMP-nucleus scattering. As with most other liquefied noble gases, argon has a high scintillation lightyield (~ 51 photons / keV[20]), is transparent to its own scintillation light, and is relatively easy to purify. Sony VAIO VGN-SR140E/B Battery

Compared to xenon, argon is cheaper and has a distinct scintillation time profile which allows the separation of electronic recoils from nuclear recoils. On the other hand, its intrinsic gamma-ray background is larger due to 39Ar contamination, unless one uses underground argon sources with a low level of radioactivity.Sony VAIO VGN-SR140E/P Battery

Dark matter detectors currently operating with liquid argon include WArP, ArDM, microCLEAN and DEAP-I.

Preservative

Argon is used to displace oxygen- and moisture-containing air in packaging material to extend the shelf-lives of the contents (argon has the European food additive code of E938).Sony VAIO VGN-SR140E/S Battery

Aerial oxidation, hydrolysis, and other chemical reactions which degrade the products are retarded or prevented entirely. Bottles of high-purity chemicals and certain pharmaceutical products are available in sealed bottles or ampoules packed in argon.Sony VAIO VGN-SR140N/S Battery

In wine making, argon is used to top-off barrels to avoid the aerial oxidation ofethanol to acetic acid during the aging process.

Argon is also available in aerosol-type cans, which may be used to preserve compounds such as varnish, polyurethane, paint, etc. for storage after opening.Sony VAIO VGN-SR165E/B Battery

Since 2001, the American National Archives stores important national documents such as the Declaration of Independence and theConstitution within argon-filled cases to retard their degradation. Using argon reduces gas leakage, compared with the helium used in the preceding five decades.Sony VAIO VGN-SR165E/P Battery

Laboratory equipment

Argon may be used as the inert gas within Schlenk lines and gloveboxes. The use of argon over comparatively less expensive nitrogen is preferred where nitrogen may react with the experimental reagents or apparatus.Sony VAIO VGN-SR165E/S Battery

Argon may be used as the carrier gas in gas chromatography and in electrospray ionization mass spectrometry; it is the gas of choice for the plasma used in ICP spectroscopy. Argon is preferred for the sputter coating of specimens for scanning electron microscopy. Argon ions are also used for sputtering in microelectronics.Sony VAIO VGN-SR190EBJ Battery

Medical use

Cryosurgery procedures such as cryoablation use liquefied argon to destroy cancer cells. In surgery it is used in a procedure called "argon enhanced coagulation" which is a form of argon plasma beam electrosurgery. The procedure carries a risk of producinggas embolism in the patient and has resulted in the death of one person via this type of accident.Sony VAIO VGN-SR190EBQ Battery

Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects. It has also been used experimentally to replace nitrogen in the breathing or decompression mix, to speed the elimination of dissolved nitrogen from the blood. Sony VAIO VGN-SR190EEJ/C Battery

Lighting

Incandescent lights are filled with argon, to preserve the filaments at high temperature from oxidation. It is used for the specific way it ionizes and emits light, such as in plasma globes and calorimetry in experimental particle physics. Gas-discharge lamps filled with argon provide blue light. Argon is also used for the creation of blue and green laser light.Sony VAIO VGN-SR190NAB Battery

Miscellaneous uses

It is used for thermal insulation in energy efficient windows. Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity. Sony VAIO VGN-SR190NBB Battery

Compressed argon is allowed to expand, to cool the seeker heads of the AIM-9 Sidewinder missile, and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure.

Argon-39, with a half-life of 269 years, has been used for a number of applications, primarily ice core and ground water dating. Also,potassium-argon dating is used in dating igneous rocks.Sony VAIO VGN-SR190NDB Battery

Safety

Although argon is non-toxic, it does not satisfy the body's need for oxygen and is thus an asphyxiant. Argon is 25% more dense than air and is considered highly dangerous in closed areas. It is also difficult to detect because it is colorless, odorless, and tasteless.Sony VAIO VGN-SR190NEB Battery

In confined spaces, it is known to result in death due to asphyxiation. A 1994 incident in which a man was asphyxiated after entering an argon filled section of oil pipe under construction inAlaska highlights the dangers of argon tank leakage in confined spaces, and emphasizes the need for proper use, storage and handling.Sony VAIO VGN-SR190NGB Battery

Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less commonly carbon dioxide and nitrogen. Sony VAIO VGN-SR190PAB Battery

A common theme among these techniques is the use of a high vacuum to remove air, and the use of an inert gas: preferably argon, but often nitrogen.

The two most common types of air-free technique involve the use of a glovebox and a Schlenk line. In both methods, glassware (often Schlenk tubes) are pre-dried in ovens prior to use.Sony VAIO VGN-SR190PCB Battery

They may be flame-dried to remove adsorbed water. Prior to coming into an inert atmosphere, vessels are further dried by purge-and-refill — the vessel is subjected to a vacuum to remove gases and water, and then refilled with inert gas. This cycle is usually repeated three times or the vacuum is applied for an extended period of time. Sony VAIO VGN-SR190PFB Battery

One of the differences between the use of a glovebox and a Schlenk line is where the purge-and-refill cycle is applied. When using a glovebox the purge-and-refill is applied to an airlock attached to the glovebox, commonly called the "port" or "ante-chamber". Sony VAIO VGN-SR19VN Battery

In contrast when using a Schlenk line the purge-and-refill is applied directly to the reaction vessel through a hose or ground glass joint that is connected to the manifold.

The most straightforward type of air-free technique is the use of a glovebox. A glove bag uses the same idea, but is usually a poorer substitute because it is more difficult to purge, and less well sealed. Sony VAIO VGN-SR19VN Battery

Inventive ways of accessing items beyond the reach of the gloves exist, such as the use of tongs and strings. The main drawbacks to using a glovebox are the cost of the glovebox itself, and limited dexterity wearing the gloves.

In the glovebox, conventional laboratory equipment can often be set up and manipulated, despite the need to handle the apparatus with the gloves. Sony VAIO VGN-SR19VRN Battery

By providing a sealed but recirculating atmosphere of the inert gas, the glove box necessitates few other precautions. Cross contamination of samples due to poor technique is also problematic, especially where a glovebox is shared between workers using differing reagents, volatile ones in particular.Sony VAIO VGN-SR19XN Battery

Two styles have evolved in the use of gloveboxes for synthetic chemistry. In a more conservative mode, they are used solely to store, weigh, and transfer air-sensitive reagents. Reactions are thereafter carried out using Schlenk techniques. The gloveboxes are thus only used for the most air-sensitive stages in an experiment. Sony VAIO VGN-SR19XN Battery

In their more liberal use, gloveboxes are used for the entire synthetic operations including reactions in solvents, work-up, and preparation of samples for spectroscopy.

Not all reagents and solvents are acceptable for use in the glovebox, although again, different laboratories adopt different cultures.Sony VAIO VGN-SR210J/S Battery

The "box atmosphere" is usually continuously deoxygenated over a copper catalyst. Certain volatile chemicals such as halogenated compounds and especially strongly coordinating species such as phosphines and thiols can be problematic because they irreversibly poison the copper catalyst. Sony VAIO VGN-SR21M/S Battery

Because of this problem, many experimentalists choose to handle such compounds using Schlenk techniques. In the more liberal use of gloveboxes, it is accepted that the copper catalyst will require more frequent replacement but this cost is considered to be an acceptable trade-off for the efficiency of conducting an entire synthesis within a protected environment.Sony VAIO VGN-SR21RM/H Battery

The other main technique for the preparation and handing of air-sensitive compounds are associated with the use of a Schlenk line. The main techniques include:

  • counterflow additions, where air-stable reagents are added to the reaction vessel against a flow of inert gas.Sony VAIO VGN-SR21RM/S Battery
  • the use of syringes and rubber septa to transfer liquids and solutions
  • cannula transfer, where liquids or solutions of air-sensitive reagents are transferred between different vessels stoppered with septa using a long thin tube known as a cannula. Liquid flow is achieved via vacuum or inert gas pressure.Sony VAIO VGN-SR220J/B Battery

Glassware are usually connected via tightly-fitting and greased ground glass joints. Round bends of glass tubing with ground glass joints may be used to adjust the orientation of various vessels. Filtrations may be accomplished by dedicated equipment.Sony VAIO VGN-SR220J/H Battery

Commercially available purified inert gas (argon or nitrogen) is adequate for most purposes. However, for certain applications, it is necessary to further remove water and oxygen. This additional purification can be accomplished by piping the inert gas line through a heated column of copper catalyst, which converts the oxygen to copper oxide.Sony VAIO VGN-SR240J/B Battery

Water is removed by piping the gas through a column of desiccant such as phosphorus pentoxide or molecular sieves.

Air- and water-free solvents are also necessary. If high-purity solvents are available in nitrogen-purged Winchesters, they can be brought directly into the glovebox. Sony VAIO VGN-SR240N/B Battery

For use with Schlenk technique, they can be quickly poured into Schlenk flasks charged with molecular sieves, and degassed. More typically, solvent is dispensed directly from a still or solvent purification column.Sony VAIO VGN-SR290JTH Battery

Degassing

Two procedures for degassing are common. The first is known as freeze-pump-thaw — the solvent is frozen under liquid nitrogen, and a vacuum is applied. Thereafter, the stopcock is closed and the solvent is thawed in warm water, allowing trapped bubbles of gas to escape.Sony VAIO VGN-SR290JTJ Battery

The second procedure is to simply subject the solvent to a vacuum. Stirring or mechanical agitation using an ultrasonicator is useful. Dissolved gases evolve first; once the solvent starts to evaporate, noted by condensation outside the flask walls, the flask is refilled with inert gas. Both procedures are repeated three times.Sony VAIO VGN-SR290JTQ Battery

Drying

Solvents are traditionally purified by distillation over an appropriate desiccant under an inert atmosphere. The main problem with the use of sodium as a desiccant (below its melting point) is associated with the slow rate of reaction between a solid and a solution. When however, the desiccant is soluble, the speed of drying is much higher.Sony VAIO VGN-SR290JVB/C Battery

Benzophenone is often used to generate such a soluble drying agent. An advantage to this application is the intense blue color of the ketyl radical anion. Thus, sodium/benzophenone can be used as an indicator of air-free and moisture-free conditions in the purification of solvents by distillation. Sony VAIO VGN-SR290JVH/C Battery

However, distillation stills are fire hazards and are increasingly being replaced by alternative solvent-drying systems. Particularly popular is the filtration of deoxygenated solvents through columns filled with activated alumina.Sony VAIO VGN-SR290NTB Battery

Drying of solids can be brought about by storing the solid over a drying agent such as phosphorus pentoxide (P2O5) or silica gel, storing in a drying oven/vacuum-drying oven, heating under a high vacuum or in a drying pistol, or to remove trace amounts of water, simply storing the solid in a glove box that has a dry atmosphere.Sony VAIO VGN-SR29VN/S Battery

Alternatives

Both these techniques require rather expensive equipment and can be time consuming. Where air-free requirements are not stringent, other techniques can be used. For example, using a sacrificial excess of a reagent that reacts with water/oxygen can be used. Sony VAIO VGN-SR29XN/S Battery

The sacrificial excess in effect "dries" the reaction by reacting with the water (e.g. in the solvent). However, this method is only suitable where the impurities produced in this reaction are not in turn detrimental to the desired product of the reaction or can be easily removed. Sony VAIO VGN-SR2RVN/S Battery

Typically, reactions using such a sacrificial excess are only effective when doing reactions on a reasonably large scale such that this by-reaction is negligible compared to the desired product reaction. For example, when preparing Grignard reagents, magnesium (the cheapest reagent) Sony VAIO VGN-SR390NAB Battery

is often used in excess, which reacts to remove trace water, either by reacting directly with water to give magnesium hydroxide or via the in situ formation of the Grignard reagent which in turn reacts with water (e.g. R-Mg-Hal + H2O ? HO-Mg-Hal + R-H). Sony VAIO VGN-SR390NAH Battery

To maintain the resultant "dry" environment it is usually sufficient to connect a guard tube filled with calcium chloride to the reflux condenser to slow moisture re-entering the reaction over time, or connect an inert gas line.

Drying can also be achieved by the use of in situ desiccants such as molecular sieves, or the use of azeotropic distillation techniques e.g. with a Dean-Stark apparatus.Sony VAIO VGN-SR41M/P Battery

Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, most notably gas metal arc welding and gas tungsten arc welding(GMAW and GTAW, more popularly known as MIG and TIG, respectively). Their purpose is to protect the weld area from atmospheric gases, such as oxygen, nitrogen,carbon dioxide, and water vapour. Sony VAIO VGN-SR41M/S Battery

Depending on the materials being welded, these atmospheric gases can reduce the quality of the weld or make the welding process more difficult to use. Other arc welding processes use other methods of protecting the weld from the atmosphere as well – shielded metal arc welding, for example, Sony VAIO VGN-SR49VN/H Battery

uses an electrode covered in a flux that produces carbon dioxide when consumed, a semi-inert gas that is an acceptable shielding gas for welding steel.

Improper choice of a welding gas can lead to a porous and weak weld, or to excessive spatter; the latter, while not affecting the weld itself, causes loss of productivity due to the manpower needed to remove the scattered drops.Sony VAIO VGN-SR51B/P Battery

Common shielding gases

Shielding gases fall into two categories—inert or semi-inert. Only two of the noble gases, helium and argon, are cost effective enough to be used in welding. These inert gases are used in gas tungsten arc welding, and also in gas metal arc welding for the welding of non-ferrous metals. Sony VAIO VGN-SR51B/S Battery

Pure argon and helium are used only for some nonferrous metals. Semi-inert shielding gases, or active shield gases, include carbon dioxide, oxygen, nitrogen, and hydrogen. Most of these gases, in large quantities, would damage the weld, but when used in small, controlled quantities, can improve weld characteristics.Sony VAIO VGN-SR51MF Battery

Properties

The important properties of shielding gases are their thermal conductivity and heat transfer properties, their density relative to air, and how easy they undergo ionization. Gases heavier than air (e.g. argon) blanket the weld and require lower flow rates than gases lighter than air (e.g. helium). Sony VAIO VGN-SR51MF/P Battery

Heat transfer is important for heating the weld around the arc. Ionizability influences how easy the arc starts, and how high voltage is required. Shielding gases can be used pure, or as a blend of two or three gases. In laser welding, the shielding gas has an additional role, preventing formation of a cloud of plasma above the weld, absorbing significant fraction of the laser energy. Sony VAIO VGN-SR51MF/S Battery

This is important for CO2 lasers; Nd:YAG lasers show lower tendency to form such plasma. Helium plays this role best due to its high ionization potential; the gas can absorb high amount of energy before becoming ionized.

Helium is lighter than air; larger flow rates are required. It is an inert gas, not reacting with the molten metals.Sony VAIO VGN-SR51MF/W Battery

Its thermal conductivity is high. It is not easy to ionize, requiring higher voltage to start the arc. Due to higher ionization potential it produces hotter arc at higher voltage, provides wide deep bead; this is an advantage for aluminium, magnesium, and copper alloys. Other gases are often added. Sony VAIO VGN-SR51MR Battery

Blends of helium with addition of 5–10% of argon and 2–5% of carbon dioxide ("tri-mix") can be used for welding of stainless steel. Used also for aluminium and other non-ferrous metals, especially for thicker welds. In comparison with argon, helium provides more energy-rich but less stable arc. Sony VAIO VGN-SR59VG Battery

Helium and carbon dioxide were the first shielding gases used, since the beginning of World War 2. Helium is used as a shield gas in laser welding for carbon dioxide lasers. Helium is more expensive than argon and requires higher flow rates, so despite its advantages it may not be a cost-effective choice for higher-volume production.Sony VAIO VGN-SR59VG/H Battery

Pure helium is not used for steel, as it then provides erratic arc and encourages spatter.

Argon is heavier than air; lower flow rates are needed to blanket the weld. It is an inert gas, not reacting with the molten metals.Sony VAIO VGN-SR70B/S Battery

It has low thermal conductivity. It ionizes easily, providing a stable arc with an excellent current path and high current density. It produces a very narrow arc cone and narrow penetration profile. It is often used as pure when welding aluminium and other nonferrous metals, though other gases can be added; pure argon does not provide sufficient penetration for welding steel. Sony VAIO VGN-SR72B/P Battery

A blend of argon with 25–50% of helium is used for some nonferrous metals, as helium improves heat transfer into the base material and makes the molten metal more fluid. An oxidizing component (oxygen, carbon dioxide) is usually added to stabilize the arc for welding of steels; without it the arc control can be difficult as the arc tends to stray. Sony VAIO VGN-SR72B/S Battery

In industrial gas business it is known as "the big A". Argon is used as a shield gas in laser welding for Nd:YAG lasers.

Carbon dioxide has good heat transfer properties; it dissociates in the weld and recombines in contact with the colder metal. Produces very deep weld but somewhat unstable arc and, due to its reactivity, intense spatter. Sony VAIO VGN-SR73JB/S Battery

Due to the presence of dissociated oxygen, the weld zone has oxidizing properties, producing more slag. Carbon dioxide can be used as pure (only for short-circuiting), or in a mixture with 5 to 25% argon, sometimes up to 50% (also for spray transfer); the argon addition inhibits sputtering. Sony VAIO VGN-SR74FB/S Battery

Increasing percentage of carbon dioxide increases the width and depth of the weld penetration. For welding of stainless steels where carbon content control is required, an argon-helium blend with 1–2% of carbon dioxide can be used. "Trimix" blends of argon – oxygen – carbon dioxide are more common in United Kingdom, while argon-carbon dioxide blends are more common in the USA.Sony VAIO VGN-SR90FS Battery

In comparison with argon-carbon dioxide mixture, for steel welding, pure carbon dioxide increases spatter and the arc is less stable.Pure carbon dioxide provides deep weld penetration and is very cheap. Pure carbon dioxide usage is limited to short circuit and globular transfer welding. Sony VAIO VGN-SR90NS Battery

It has high spatter and deep penetration. Provides good mechanical properties. Can be used for carbon steel. Has high production of smoke and fumes. It is very cheap.

Oxygen is used in small amounts as an addition to other gases; typically as 2–5% addition to argon.Sony VAIO VGN-SR90S Battery

It enhances arc stability and reduces the surface tension of the molten metal, increasing wetting of the solid metal. It is used for spray transfer welding of mild carbon steels, low alloy and stainless steels. Its presence increases the amount of slag. Argon-oxygen (Ar-O2) blends are often being replaced with argon-carbon dioxide ones. Sony VAIO VGN-SR90US Battery

Argon-carbon dioxide-oxygen blends are also used. Oxygen causes oxidation of the weld, so it is not suitable for welding aluminium, magnesium, copper, and some exotic metals. Increased oxygen makes the shielding gas oxidize the electrode, which can lead to porosity in the deposit if the electrode does not contain sufficient deoxidizers. Sony VAIO VGN-SR91NS Battery

Excessive oxygen, especially when used in application for which it is not prescribed, can lead to brittleness in the heat affected zone. Argon-oxygen blends with 1–2% oxygen are used for austenitic stainless steel where argon-CO2 can not be used due to required low content of carbon in the weld; the weld has a tough oxide coating and may require cleaning.Sony VAIO VGN-SR91PS Battery

Nitrogen is used for welding of some stainless steels. It increases the weld penetration and enhances arc stability. It however can cause porosity in carbon steels. Argon-carbon dioxide-nitrogen blends can be used. Pure nitrogen is also used, or can be blended with 10% of hydrogen, depending on application.Sony VAIO VGN-SR91S Battery

Blends with nitrogen content are used to weld nitrogen-containing alloys (up to 0.5% nitrogen increases mechanical properties and resistance to pitting corrosion) to prevent loss of nitrogen from the metal. Nitrogen can be used in some cases of laser welding; it is almost as good as helium for plasma formation suppression but can cause embrittlement of some steels.Sony VAIO VGN-SR91US Battery

Hydrogen is used for welding of nickel and some stainless steels, especially thicker pieces. It improves the molten metal fluidity, and enhances cleanness of the surface. It can however cause hydrogen embrittlement of many alloys and especially carbon steel, so its application is usually limited only to some stainless steels. Sony VAIO VGN-SR92NS Battery

It is added to argon in amounts typically under 10%. It can be added to argon-carbon dioxide blends to counteract the oxidizing effects of carbon dioxide. Its addition narrows the arc and increases the arc temperature, leading to better weld penetration. In higher concentrations (up to 25% hydrogen), it may be used for welding conductive materials such as copper. Sony VAIO VGN-SR92PS Battery

However, it should not be used on steel, aluminum or magnesium because it can cause porosity and hydrogen embrittlement.

Nitric oxide addition serves to reduce production of ozone. It can also stabilize the arc when welding aluminium and high-alloyed stainless steel.Sony VAIO VGN-SR92S Battery

Other gases can be used for special applications, pure

or as blend additives; e.g. sulfur hexafluoride or dichlorodifluoromethane.

Sulfur hexafluoride can be added to shield gas for aluminium welding to bind hydrogen in the weld area to reduce weld porosity.Sony VAIO VGN-SR92US Battery

Dichlorodifluoromethane with argon can be used for protective atmosphere for melting of aluminium-lithium alloys. It reduces the content of hydrogen in the aluminium weld, preventing the associated porosity.

Common mixes

  • Argon-carbon dioxide
    • C-50 (50% argon/50% CO2) is used for short arc welding of pipes,
    • C-40 (60% argon/40% CO2) is used for some flux-cored arc welding cases. Better weld penetration than C-25.Sony VAIO VGN-SR93DS Battery
    • C-25 (75% argon/25% CO2) is commonly used by hobbyists and in small-scale production. Limited to short circuit and globular transfer welding. Common for short-circuit gas metal arc welding of low carbon steel.
    • C-20 (80% argon/20% CO2) is used for short-circuiting and spray transfer of carbon steel.Sony VAIO VGN-SR93JS Batery
    • C-15 (85% argon/15% CO2) is common in production environment for carbon and low alloy steels. Has lower spatter and good weld penetration, suitable for thicker plates and steel significantly covered with mill scale. Suitable for short circuit, globular, pulse and spray transfer welding. Maximum productivity for thin metals in short-circuiting mode; has lower tendency to burn through than higher-CO2 mixes and has suitably high deposition rates.Sony VAIO VGN-SR93PS Batttery
    • C-10 (90% argon/10% CO2) is common in production environment. Has low spatter and good weld penetration, though lower than C-15 one; suitable for many steels. Same applications as 85/15 mix. Sufficient for ferritic stainless steels.Sony VAIO VGN-SR93YS Battery
    • C-5 (95% argon/5% CO2) is used for pulse spray transfer and short-circuiting of low alloy steel. Has better tolerance for mill scale and better puddle control than argon-oxygen, though less than C-10. Less heat than C-10.[11] Sufficient for ferritic stainless steels. Similar performance to argon with 1% oxygen.Sony VAIO VGN-SR94FS Battery
  • Argon-oxygen
    • O-5 (95% argon/5% oxygen) is the most common gas for general carbon steel welding. Higher oxygen content allows higher speed of welding. More than 5% oxygen makes the shielding gas oxidize the electrode, which can lead to porosity in the deposit if the electrode does not contain sufficient deoxidizers.Sony VAIO VGN-SR94GS Battery
    • O-2 (98% argon/2% oxygen) is used for spray arc on stainless steel, carbon steels, and low alloy steels. Better wetting than O-1. Weld is darker and more oxidized than with O-1. The addition of 2% oxygen encourages spray transfer, which is critical for spray-arc and pulsed spray-arc GMAW.Sony VAIO VGN-SR94HS Battery
    • O-1 (99% argon/1% oxygen) is used for stainless steels. Oxygen stabilizes the arc.
  • Argon-helium
    • A-25 (75% argon/25% helium) is used for nonferrous base when higher heat input and good weld appearance are needed.Sony VAIO VGN-SR94VS Battery
    • A-50 (50% argon/50% helium) is used for nonferrous metals thinner than 0.75 inch for high-speed mechanized welding.
    • A-75 (25% argon/75% helium) is used for mechanized welding of thick aluminium. Reduces weld porosity in copper.
  • Argon-hydrogen
    • H-2 (98% argon/2% hydrogen)
    • H-5 (95% argon/5% hydrogen)Sony VAIO VGN-SZ35B/B Battery
    • H-10 (80% argon/20% hydrogen)
    • H-35 (65% argon/35% hydrogen)[13]
  • Others
    • Argon with 25–35% helium and 1–2% CO2 provides high productivity and good welds on austenitic stainless steels. Can be used for joining stainless steel to carbon steel.Sony VAIO VGN-SZ54B/B Battery
    • Argon-CO2 with 1–2% hydrogen provides a reducing atmosphere that lowers amount of oxide on the weld surface, improves wetting and penetration. Good for austenitic stainless steels.
    • Argon with 2–5% nitrogen and 2–5% CO2 in short-circuiting yields good weld shape and color and increases welding speed. For spray and pulsed spray transfer it is nearly equivalent to other trimixes. Sony VAIO VGN-SZ55B/B Battery
    • When joining stainless to carbon steels in presence of nitrogen, care has to be taken to ensure the proper weld microstructure. Nitrogen increases arc stability and penetration and reduces distortion of the welded part. In duplex stainless steels assists in maintaining proper nitrogen content.Sony VAIO VGN-SZ55GN/B Battery
    • 85–95% helium with 5–10% argon and 2–5% CO2 is an industry standard for short-circuit welding of carbon steel.
    • Argon – carbon dioxide – oxygen
    • Argon–helium–hydrogen
    • Argon – helium – hydrogen – carbon dioxideSony VAIO VGN-SZ56 Battery

Applications

The applications of shielding gases are limited primarily by the cost of the gas, the cost of the equipment, and by the location of the welding. Some shielding gases, like argon, are expensive, limiting its use.Sony VAIO VGN-SZ561N Battery

The equipment used for the delivery of the gas is also an added cost, and as a result, processes like shielded metal arc welding, which require less expensive equipment, might be preferred in certain situations. Finally, because atmospheric movements can cause the dispersion of the shielding gas around the weld, Sony VAIO VGN-SZ562N Battery

welding processes that require shielding gases are often only done indoors, where the environment is stable and atmospheric gases can be effectively prevented from entering the weld area.

The desirable rate of gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode being utilized.Sony VAIO VGN-SZ57N Battery

Welding flat surfaces requires higher flow than welding grooved materials, since the gas is dispersed more quickly. Faster welding speeds, in general, mean that more gas needs to be supplied to provide adequate coverage. Additionally, higher current requires greater flow, and generally, more helium is required to provide adequate coverage than argon. Sony VAIO VGN-SZ58GN Battery

Perhaps most importantly, the four primary variations of GMAW have differing shielding gas flow requirements—for the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min (20 ft3/h) is generally suitable, while for globular transfer, around 15 L/min (30 ft3/h) is preferred. Sony VAIO VGN-SZ58GN/C Battery

The spray transfer variation normally requires more because of its higher heat input and thus larger weld pool; along the lines of 20–25 L/min (40–50 ft3/h).

A van der Waals molecule is a stable cluster consisting of two or more molecules held together by van der Waals forces or by hydrogen bonds. Sony VAIO VGN-SZ58N Battery

The name originated in the beginning of the 1970s when stable molecular clusters were regularly observed in molecular beam microwave spectroscopy.

In (supersonic) molecular beams temperatures are very low (usually less than 5 K). At these low temperatures van der Waals (vdW) molecules are stable and can be investigated by microwave and far-infrared spectroscopy.Sony VAIO VGN-SZ61MN/B Battery

Also in cold equilibrium gases vdW molecules are formed, albeit in small, temperature dependent, concentrations. Rotational and vibrational transitions in vdW molecules have indeed been observed in gases, mainly by UV and IR spectroscopy.Sony VAIO VGN-SZ61VN/X Battery

Van der Waals molecules are usually very non-rigid and different versions are separated by low energy barriers, so that tunneling splittings, observable in far-infrared spectra, are relatively large. Thus, in the far-infrared one may observe intermolecular vibrations, rotations, and tunneling motions of vdW molecules (VRT spectroscopy). Sony VAIO VGN-SZ61WN/C Battery

The VRT spectroscopic study of vdW molecules is one of the most direct routes to the understanding of intermolecular forces.

Argon flash, also known as argon bomb, argon flash bomb, argon candle, and argon light source, is a single-use source of very short and extremely bright flashes of light. Sony VAIO VGN-SZ640N/B Battery

The light is generated by a shock wave in argon or, less commonly, other noble gas. The shock wave is usually produced by an explosion. Argon flash devices are almost exclusively used for photographing explosions and shock waves.

Although krypton and xenon can be also used; argon is favorable because of its low cost.Sony VAIO VGN-SZ645P3 Battery

The light generated by an explosion is produced primarily by compression heating of the surrounding air. Replacement of the air with a noble gas considerably increases the light output; with molecular gases, the energy is consumed partially by dissociation and other processes, while noble gases are monoatomic and can only undergo ionization; the ionized gas then produces the light. Sony VAIO VGN-SZ650N/C Battery

The low specific heat capacity of noble gases allows heating to higher temperatures, yielding brighter emission.[1] Flashtubes are filled with noble gases for the same reason.

Typical argon flash devices consist of an argon-filled cardboard or plastic tube with a transparent window on one end and an explosive charge on the other end. Many explosives can be used; Sony VAIO VGN-SZ660N/C Battery

Composition B, PETN, RDX, and plastic bonded explosives are just a few examples.

The device consists of a vessel filled with argon and a solid explosive charge. The explosion generates a shock wave, which heats the gas to very high temperature (over 104 K; published values vary between 15,000 K to 30,000 K with the best values around 25,000 K[1]). Sony VAIO VGN-SZ670N/C Battery

The gas becomes incandescent and emits a flash of intense visible and ultraviolet black body radiation. The emission for the temperature range is highest between 97–193 nm, but usually only the visible and near-ultraviolet ranges are exploited.

To achieve emission, the layer of at least one or two optical depths of the gas has to be compressed to sufficient temperature. Sony VAIO VGN-SZ680ND Battery

The light intensity rises to full magnitude in about 0.1 microsecond. For about 0.5 microsecond the shock wave front instabilities are sufficient to create significant striations in the produced light; this effect diminishes as the thickness of the compressed layer increases. Sony VAIO VGN-NS51B/L Battery

Only about 75 micrometers thick layer of the gas is responsible for the light emission. The shock wave reflects after reaching the window at the end of the tube; this yields a short increase of light intensity. The intensity then fades

The amount of explosive can control the intensity of the shock wave and therefore of the flash.Sony VAIO VGN-NS51B/P Battery

The intensity of the flash can be increased and its duration decreased by reflecting the shock wave by a suitable obstacle; a foil or a curved glass can be used. The duration of the flash is about as long as the explosion itself, depending on the construction of the lamp, between 0.1 and 100 microsecond.Sony VAIO VGN-NS51B/W Battery

The duration is dependent on the length of the shockwave path through the gas, which is propoportional to the length of the tube; it was shown that each centimeter of the path of shock wave through the argon medium is equivalent to 2 microseconds.Sony VAIO VGN-NS52JB/L Battery

Argon flash is a standard procedure for high speed photography, especially for photographing explosions, or less commonly for use in high altitude test vehicles. Sony VAIO VGN-NS52JB/P Battery

The photography of explosions and shock waves is made easy by the fact that the detonation of the argon flash lamp charge can be accurately timed relative to the test specimen explosion and the light intensity can overpower the light generated by the explosion itself. The formation of shock waves during explosions of shaped charges can be imaged this way.Sony VAIO VGN-NS52JB/W Battery

As the amount of released radiant energy is fairly high, significant heating of the illuminated object can occur. Especially in case of high explosives this has to be taken into account.Sony VAIO VGN-NS70B/W Battery

Super Radiant Light (SRL) sources are an alternative to argon flash. An electron beam source delivers brief and intense pulse of electrons to suitable crystals (e.g.cadmium sulfide). Flash times in nanosecond to picosecond range are achievable. Pulsed lasers are another alternative.Sony VAIO VGN-NS71B/W Battery,Sony VAIO VGN-NS72JB/W Battery,Sony VAIO VGN-NS90HS Battery,Sony VAIO VGN-NS92JS Battery,Sony VAIO VGN-NS92XS Battery

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