Difference between revisions of "Gallium" - New World Encyclopedia

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{{Elementbox_header | number=31 | symbol=Ga | name=gallium | left=[[zinc]] | right=[[germanium]] | above=[[aluminium|Al]] | below=[[indium|In]] | color1=#cccccc | color2=black }}
 
{{Elementbox_header | number=31 | symbol=Ga | name=gallium | left=[[zinc]] | right=[[germanium]] | above=[[aluminium|Al]] | below=[[indium|In]] | color1=#cccccc | color2=black }}
 
{{Elementbox_series | [[poor metal]]s }}
 
{{Elementbox_series | [[poor metal]]s }}
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{{Elementbox_footer | color1=#cccccc | color2=black }}
  
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'''Gallium''' ([[International Phonetic Alphabet|IPA]]: {{IPA|/ˈgaliəm/}}) is a [[chemical element]] that has the symbol '''Ga''' and [[atomic number]] 31. A rare, soft silvery metallic [[poor metal]], gallium is a brittle solid at low temperatures but liquefies slightly above room temperature and indeed will melt in the hand. It occurs in trace amounts in [[bauxite]] (an [[aluminum]] ore) and [[zinc]] ores. An important application is in the compound [[gallium arsenide]], used as a [[semiconductor]], most notably in [[light-emitting diode]]s (LEDs).
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'''Gallium''' (chemical symbol '''Ga,''' [[atomic number]] 31) is a rare, soft, silvery [[metal]]. It is a brittle solid at low temperatures, but it liquefies slightly above room temperature and melts in the hand. It is one of only a few materials that expands when freezing (like [[water]]), and its liquid form has a higher [[density]] than the solid form (like water). Gallium occurs in trace amounts in [[bauxite]] (an [[aluminum]] ore) and [[zinc]] ores.
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{{toc}}
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Gallium is most commonly used in the form of the compound gallium(III) arsenide, which is a [[semiconductor]] useful for [[integrated circuit]]s, [[light-emitting diode]]s (LEDs), and [[laser diode]]s. The nitride and phosphide of gallium are also valuable semiconductor materials, and gallium itself is used as a [[dopant]] in semiconductors. In addition, this metal is a component in low-melting temperature [[alloy]]s, and its alloy with [[indium]] and [[tin]] is used in medical thermometers to replace [[mercury (element)|mercury]]. Also, gallium can wet (coat) glass to create brilliant [[mirror]]s.
  
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== Occurrence ==
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== Occurrence and isolation ==
  
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Gallium does not exist in free form in nature, nor are there any gallium-rich minerals that might serve as primary sources of extraction of the element or its compounds. Gallium is rather found and extracted as a trace component in [[bauxite]], [[coal]], [[diaspore]], [[germanite]], and [[sphalerite]]. The [[United States Geological Survey|USGS]] estimates gallium reserves based on 50 ppm by weight concentration in known reserves of bauxite and zinc ores.  Some [[flue]] [[dust]]s from burning coal have been shown to contain as much as 1.5 percent gallium.
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Gallium does not exist in free form in nature, nor are there any gallium-rich minerals that might serve as primary sources of extraction of the element or its compounds. Rather, gallium is extracted as a trace component from [[bauxite]], [[coal]], [[diaspore]], [[germanite]], and [[sphalerite]]. Some [[flue]] [[dust]]s from burning coal have been shown to contain as much as 1.5 percent gallium.
  
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Most gallium is extracted from the crude [[aluminium hydroxide]] solution of the [[Bayer process]] for producing alumina and aluminum. A [[mercury (element)|mercury]] cell [[electrolysis]] and [[hydrolysis]] of the [[amalgam]] with [[sodium hydroxide]] leads to sodium gallate. Electrolysis then gives gallium metal. For [[semiconductor]] use, further purification is carried out using [[zone melting]], or else single crystal extraction from a melt ([[Czochralski process]]). Purities of 99.9999% are routinely achieved and commercially widely available.
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Most gallium is extracted from the crude [[aluminum hydroxide]] solution of the [[Bayer process]] for producing alumina and aluminum. A [[mercury (element)|mercury]] cell [[electrolysis]] and [[hydrolysis]] of the [[amalgam]] with [[sodium hydroxide]] leads to sodium gallate. Electrolysis then gives gallium metal. For [[semiconductor]] use, further purification is carried out using [[zone melting]], or else single crystal extraction from a melt ([[Czochralski process]]). Purities of 99.9999 percent are routinely achieved and widely available commercially.
  
 
== History ==
 
== History ==
  
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Gallium ([[Latin]] ''Gallia'' meaning France; also ''gallus'', meaning "rooster") was discovered [[spectroscopy|spectroscopically]] by [[Lecoq de Boisbaudran]] in [[1875]] by its characteristic spectrum (two [[violet (color)|violet]] lines) in an examination of a [[zinc blende]] from the [[Pyrenees]]. Before its discovery, most of its properties had been predicted and described by [[Dmitri Mendeleev]] (who called the hypothetical element ''[[Mendeleev's predicted elements|eka-aluminium]]'') on the basis of its  position in his [[periodic table]]. Later, in 1875, Boisbaudran obtained the free metal through the [[electrolysis]] of its [[hydroxide]] in [[potassium hydroxide|KOH]] solution. He named the element "gallia" after his native land of [[France]] and, in one of those multilingual [[pun]]s so beloved of men of science of the early [[19th century]], after himself, as 'Lecoq' = the [[rooster]], and [[Latin]] for rooster is "gallus".
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Before gallium was discovered, the element and many of its properties had been predicted and described by [[Dmitri Mendeleev]], on the basis of its position in the [[periodic table]]. Mendeleev called the hypothetical element ''[[Mendeleev's predicted elements|eka-aluminum]].''
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In 1875, [[Lecoq de Boisbaudran]] discovered gallium by the technique known as [[spectroscopy]]. When examining a sample of [[zinc blende]] from the [[Pyrenees]], he noticed two unique violet lines in its spectrum, indicative of a previously unknown element. Later, he obtained the free metal by the [[electrolysis]] of its [[hydroxide]] in [[potassium hydroxide|KOH]] solution. He named the element "gallia" after his native land of [[France]]; also, in one of those multilingual [[pun]]s so beloved of men of science of the early nineteenth century, he named it after himself—''Lecoq'' means "the rooster" in French, and [[Latin]] for rooster is ''gallus.''
  
 
== Notable characteristics ==
 
== Notable characteristics ==
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[[Image:Gallium1_640x480.jpg|left|thumb|100px|Very pure gallium metal has a brilliant silvery color.]] Elemental gallium is not found in nature, but it is easily obtained by [[smelting]].
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[[Image:Gallium1_640x480.jpg|left|thumb|100px|Very pure gallium metal has a brilliant silvery color.]]
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In the [[periodic table]], gallium lies in group 13 (former group 3A), between [[aluminum]] and [[indium]], and in the same group as [[thallium]]. Consequently, its properties resemble those of these three elements. In addition, it is situated in period 4, between [[zinc]] and [[germanium]]. It is also said to be one of the "poor metals"—elements located between the [[transition metal]]s and metalloids in the periodic table.
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High-purity, metallic gallium has a brilliant, silvery color. By contrast, like most metals, finely divided gallium loses its luster—powdered gallium appears gray. The solid form fractures [[Conchoidal fracture|conchoid]]ally, like [[glass]]. When liquid gallium solidifies, it expands by 3.1 percent. Thus, its liquid state has a higher [[density]] than the solid state—a property characteristic of only a few materials like water and [[bismuth]]. Also, given the property of expansion during solidification, gallium is not stored in either glass or metal containers to prevent the container from rupturing when the element freezes.
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Gallium also diffuses into the [[crystal]] lattice of most other metals. This is another reason why it is important to keep gallium away from metal containers such as steel or aluminum. Gallium easily alloys with many other metals, and it was used in small quantities in the core of the first [[atomic bomb]] to help stabilize the [[plutonium]] crystal structure.
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Given its [[melting point]] of 30°C, the metal readily melts in the hand. Also, the liquid form has a strong tendency to [[supercool]] below its melting point, and it needs to be [[seed crystal|seeded]] for solidification to begin. Gallium is one of the metals—along with [[cesium]], [[francium]], and [[mercury (element)|mercury]])—that is liquid at or near normal room temperature. It can therefore be used in metal-in-glass high-temperature [[thermometer]]s. It is also notable for having one of the largest liquid ranges for a metal, and (unlike mercury) for having a low [[vapor pressure]] at high temperatures.
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Unlike mercury, liquid gallium wets (coats) glass and skin, making it mechanically more difficult to handle, although it is substantially less toxic and requires far fewer precautions. For this reason, as well as the metal contamination and freezing expansion problems noted above, samples of gallium metal are usually supplied in polyethylene packets within other containers.
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Gallium does not [[crystal]]lize into any of the simple crystal structures. The stable phase under normal conditions is [[orthorhombic]], with eight [[atom]]s in the conventional [[unit cell]]. Each atom has only one nearest neighbor (at a distance of 244 picometers) and six other neighbors within an additional 39-picometer radius. The bonding between nearest neighbors has [[covalent bond|covalent]] character. Also, the element has many stable and [[metastable]] phases, depending on the temperature and pressure conditions.
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 +
High-purity gallium is attacked slowly by [[mineral acid]]s.
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=== Isotopes ===
  
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In the [[periodic table]], gallium lies in group 13 (former group 3A), between [[aluminum]] and [[indium]], and in the same group as [[thallium]]. Consequently, its properties resemble those of these three elements. In addition, it is situated in period 4, between [[zinc]] and [[germanium]]. It is also said to be one of the "poor metals"—elements located between the [[transition metal]]s and metalloids in the periodic table.
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Many [[isotope]]s of gallium are known, ranging from <sup>56</sup>Ga to <sup>86</sup>Ga. Among them, there are two stable isotopes: <sup>69</sup>Ga and <sup>71</sup>Ga, at relative abundances estimated at 60.11 percent and 39.89 percent, respectively. The radioisotopes, by contrast, have extremely short [[half-life|half-lives]].
  
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Very pure gallium metal has a brilliant silvery color and its solid metal fractures [[Conchoidal fracture|conchoid]]ally like [[glass]]. Gallium metal expands by 3.1 percent when it solidifies, and therefore storage in either glass or metal containers is avoided, due to the possiblity of container rupture with freezing. Gallium shares the higher-density liquid state with only a few materials like water and [[bismuth]].
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== Compounds ==
  
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Gallium also [[Liquid metal embrittlement|attacks]] most other metals by [[diffusion|diffusing]] into their metal [[crystal structure|lattice]] — another reason why it is important to keep gallium away from metal containers such as steel or aluminum. Gallium metal easily alloys with many metals, and was used in small quantities in the core of the first atomic bomb to help stabilize the plutonium crystal structure.
+
Gallium can form a number of compounds. Some of them are mentioned below.
  
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The [[melting point]] temperature of 30 °C allows the metal to be melted in the hand. This metal has a strong tendency to [[supercool]] below its [[melting point]], thus necessitating [[seed crystal|seeding]] in order to solidify. Gallium is one of the metals (with [[caesium]], [[francium]] and [[mercury (element)|mercury]]) which are liquid at or near normal room temperature, and can therefore be used in metal-in-glass high-temperature [[thermometer]]s. It is also notable for having one of the largest liquid ranges for a metal, and (unlike mercury) for having a low [[vapor pressure]] at high temperatures. Unlike mercury, liquid gallium metal wets glass and skin, making it mechanically more difficult to handle (even though it is substantially less toxic and requires far fewer precautions). For this reason as well as the metal contamination problem and freezing-expansion problems noted above, samples of gallium metal are usually supplied in polyethylene packets within other containers.
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* '''Gallium(III) arsenide''' (GaAs): It is an important [[semiconductor]], used for such devices as [[microwave]]-frequency [[integrated circuit]]s (Monolithic Microwave Integrated Circuits, or [[MMIC]]s), [[infrared]] [[light-emitting diode]]s (LEDs), [[laser diode]]s, and [[solar cell]]s. Some of its electronic properties are superior to those of [[silicon]]. For instance, GaAs devices can function at higher frequencies (above 250 gigahertz), generating less noise, and can be operated at higher power levels. Also, they have a direct band gap, so they can be used to emit light.
  
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Gallium does not [[crystal]]lize in any of the simple [[crystal structure]]s. The stable phase under normal conditions is [[orthorhombic]] with 8 atoms in the conventional [[unit cell]]. Each atom has only one nearest neighbor (at a distance of 244 [[picometre|pm]]) and six other neighbors within additional 39 pm. Many stable and [[metastable]] phases are found as function of temperature and pressure.
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* '''Gallium(III) hydroxide''' (Ga(OH)<sub>3</sub>): This is the normal [[mineral]] form of gallium in the [[Earth]]'s crust. It does not occur as a discrete mineral, but gallium replaces [[aluminum]] in [[ore]]s such as [[bauxite]]. Gallium hydroxide is [[amphoteric]], that is, it can behave as an [[acid]] as well as a [[base (chemistry)|base]]. In strongly acidic conditions, the [[Ion (physics)|ion]] Ga<sup>3+</sup> is formed; in strongly basic conditions, Ga(OH)<sub>4</sub><sup>-</sup>, is formed.
  
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The bonding between the nearest neighbors is found to be of [[covalent]] character, hence Ga<sub>2</sub> [[dimer]]s are seen as the fundamental building blocks of the crystal. The compound with [[arsenic]], [[gallium(III) arsenide|gallium arsenide]] is a [[semiconductor]] commonly used in [[light-emitting diode]]s.
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* '''Gallium(III) nitride''' (GaN): This hard, mechanically stable material is a binary [[semiconductor]] with a wide, direct band gap. It is used in [[optoelectronic]] devices such as high-brightness, blue LEDs and blue [[laser diode]]s. Its sensitivity to [[ionizing radiation]] is low, making it a suitable material for [[solar cell]] arrays for [[satellite]]s. It is being investigated for use in high-frequency, high-power [[transistor]]s that can operate at high temperatures.
  
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High-purity gallium is attacked slowly by [[mineral acid]]s.
+
* '''Gallium(III) phosphide''' (GaP): This solid, crystalline material has the appearance of pale orange pieces. It is odorless and insoluble in water, and it melts at 1,480°C. It is a [[semiconductor]] with an indirect band gap (2.26 [[electronvolt]]). [[Sulfur]] or [[tellurium]] may be added as [[doping (semiconductors)|dopant]]s to turn gallium phosphide into an n-type semiconductor; or [[zinc]] may be added as a dopant to prepare a p-type semiconductor. GaP is used for the manufacture of low- and standard-brightness red, orange, and green LEDs.
  
 
== Applications ==
 
== Applications ==
  
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As a component of the semiconductor [[Gallium(III) arsenide|Gallium arsenide]], the most common application for gallium is analog [[integrated circuit]]s, with the second largest use being [[optoelectronic]] devices (mostly [[laser diode]]s and [[LED|light-emitting diodes]].)
+
Gallium, its [[alloy]]s, and its compounds have many applications. Some of them are listed below.
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* The most common applications of gallium are in the form of the [[semiconductor]] gallium(III) arsenide. This compound is used mainly for analog [[integrated circuit]]s, and also for [[optoelectronic]] devices such as [[LED]]s and [[laser diode]]s.
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Other uses include:
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* Gallium is widely used as a [[dopant]] in semiconductors, to produce solid-state devices such as [[transistor]]s.
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*Since gallium [[wetting|wets]] glass or [[porcelain]], gallium can be used to create brilliant [[mirror]]s.
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* Given that gallium can wet [[glass]] or [[porcelain]], it can be used to create brilliant [[mirror]]s.
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*Used widely as a [[dopant]] to [[doping (semiconductor)|dope]] [[semiconductor]]s and produce solid-state devices like [[transistor]]s.
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* Gallium readily alloys with most metals, and has been used as a component in low-melting alloys.
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*Gallium readily [[alloy]]s with most metals, and has been used as a component in [[fusible alloy|low-melting alloys]]. The [[plutonium]] used in [[Nuclear weapon design#Pit|nuclear weapon pits]] is machined by alloying with gallium to stabilize the [[allotropy|allotrope]]s of plutonium. Much research is being devoted to gallium alloys as substitutes for mercury [[dental amalgam]]s, but such compounds have yet to see wide acceptance. 
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* The [[plutonium]] used in nuclear weapon pits is machined by alloying with gallium to stabilize the [[allotropy|allotrope]]s of plutonium.
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*Gallium added in quantities up to 2% in common [[solder]]s can aid wetting and flow characteristics.
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* When added in quantities up to 2 percent in common [[solder]]s, gallium can aid wetting and flow characteristics.
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*Gallium is used in some high temperature thermometers.
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* Gallium is used in some high-temperature [[thermometer]]s.
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*A [[eutectic]] alloy of gallium, [[indium]], and [[tin]], is widely available in medical thermometers (fever thermometers), replacing problematic mercury. This alloy, with the trade name ''[[Galinstan]]'', has a freezing point of &minus;20 °C.
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* An alloy of gallium, [[indium]], and [[tin]] (trade name ''Galinstan'') is widely available in medical thermometers (fever thermometers), replacing problematic [[mercury (element)|mercury]]. This alloy has a freezing point of &minus;20°C.
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*[[Magnesium]] [[gallate]] containing impurities (such as Mn<sup>+2</sup>), is beginning to be used in [[ultraviolet]]-activated [[phosphor]] powder.
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* [[Magnesium]] gallate, containing impurities such as Mn<sup>+2</sup>, is beginning to be used in [[ultraviolet]]-activated [[phosphor]] powder.
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*It has been suggested that a liquid gallium-[[tin]] alloy could be used to cool computer chips in place of water. As it conducts heat approximately 65 times better than water it can make a comparable [[coolant]]. [http://www.newscientist.com/article.ns?id=dn7348]
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*Gallium [[salt]]s, such as gallium [[citrate]] or gallium [[nitrate]] containing a radioactive [[isotope]] of gallium, have been used in [[nuclear medicine]] imaging. This use, however, has largely been replaced by FDG [[Positron emission tomography|PET]] scans.
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*Gallium, however has a [[heat capacity]] of .37 J/(g K), meaning that it requires .37 [[Joule]] to raise the temperature of 1 [[gram]] of it, by 1 degree [[Kelvin]]. Gallium's density is 5.91 g per cubic centimeter, whereas water is 1 g/cm<sup>3</sup>. For the same volume of water versus gallium, the gallium will become just about twice as hot as the water would, when used in a cooling device. (For '''Gallium''') .37 J/(g K) X 5.91 g/cm<sup>3</sup> = '''2.2''' J/(cm<sup>3</sup> K) (For '''Water''') 4.184 J/(g K) X 1 g/cm<sup>3</sup> = '''4.184''' J/(cm<sup>3</sup> K)
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* Much research is being devoted to gallium alloys as substitutes for mercury [[dental amalgam]]s, but such compounds have yet to gain wide acceptance.
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*Gallium [[salt]]s such as gallium [[citrate]] and gallium [[nitrate]] are used as [[radiopharmaceutical]] agents in [[nuclear medicine]] imaging. (The form or salt is not important, since it is the free dissolved gallium ion Ga<sup>+3</sup> which is active). For these applications, a [[radionuclide|radioactive isotope]] such as <sup>67</sup>Ga is used. The body handles Ga<sup>+3</sup> in many ways as though it were iron, and thus it is bound (and concentrates) in areas of inflammation, such as infection, and also areas of rapid cell division. This allows such sites to be imaged by nuclear scan techniques. See [[gallium scan]]. This use has largely been replaced by FDG [[PET]] scan.
+
* Gallium is the rarest component of new [[photovoltaic]] compounds (such as copper indium gallium selenium sulphide or Cu(In,Ga)(Se,S)<sub>2</sub>, announced by [[South Africa]]n researchers) for use in solar panels as an alternative to [[crystalline silicon]], which is currently in short supply.
−
*Gallium is the rarest component of new [[photovoltaic]] compounds (such as copper indium gallium selenium sulphide or Cu(In,Ga)(Se,S)2, recently announced by South African researchers) for use in solar panels as an alternative to [[crystalline silicon]], which is currently in short supply.
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* It has been suggested that a liquid gallium-[[tin]] alloy could be used to cool [[computer]] chips in place of water. As it conducts heat approximately 65 times better than water, it can make a comparable [[coolant]].<ref>Will Knight. [http://www.newscientist.com/article.ns?id=dn7348 Hot chips chilled with liquid metal.] NewScientist.com. Retrieved April 6, 2007.</ref>
  
 
== Precautions ==
 
== Precautions ==
  
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While not considered toxic, the data about gallium is inconclusive. Some sources suggest that it may cause [[dermatitis]] from prolonged exposure; other tests have not caused a positive reaction. Like most metals, finely divided gallium loses its luster. Powdered gallium appears gray. When gallium is handled with bare hands, the extremely fine dispersion of liquid gallium droplets which results from wetting skin with the metal may appear as a gray skin stain.
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Gallium is not considered toxic, but the data about its effects are inconclusive. Some sources suggest that it may cause [[dermatitis]] from prolonged exposure; other tests have not caused a positive reaction. When the element is handled with bare hands, the skin acquires a gray stain from an extremely fine dispersion of liquid gallium droplets.
  
 
==See also==
 
==See also==
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* [[Metal]]
 
* [[Metal]]
 
* [[Periodic table]]
 
* [[Periodic table]]
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* [[:category:Gallium compounds|Gallium compounds]]
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 +
== Notes ==
 +
<references/>
  
 
== References ==
 
== References ==
  
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*[http://periodic.lanl.gov/elements/31.html Los Alamos National Laboratory &ndash; Gallium]
+
*[http://www.webelements.com/webelements/elements/text/Ga/key.html Gallium.] WebElements. Retrieved October 22, 2022.
−
*[http://www.webelements.com/webelements/elements/text/Ga/key.html Webelements: detailed information on gallium]
 
  
 
== External links ==
 
== External links ==
 +
All links retrieved April 17, 2024.
 +
 +
*[http://www.webelements.com/webelements/elements/text/Ga/index.html WebElements.com &ndash; information on gallium]
 +
*[http://www.acialloys.com/msds/ga.html Material Safety Data Sheet]
 +
*[http://www.lenntech.com/Periodic-chart-elements/Ga-en.htm Information on gallium]
 +
*[http://minerals.usgs.gov/minerals/pubs/commodity/gallium/index.html Environmental effects of gallium]
 +
* [http://www.ioffe.ru/SVA/NSM/Semicond/GaAs/index.html Properties of gallium arsenide]
 +
* [http://www.ioffe.rssi.ru/SVA/NSM/Semicond/GaN/index.html Properties of gallium nitride]
 +
* [http://www.ioffe.rssi.ru/SVA/NSM/Semicond/GaP/index.html Properties of gallium phosphide]
  
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*[http://www.webelements.com/webelements/elements/text/Ga/index.html WebElements.com &ndash; textbook information on gallium]
 
−
*[http://www.pniok.de/ga.htm Picture in the Element collection from Heinrich Pniok]
 
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*[[Material safety data sheet]] at [http://www.acialloys.com/msds/ga.html acialloys.com]
 
−
*[http://www.lenntech.com/Periodic-chart-elements/Ga-en.htm www.lenntech.com &ndash; textbook information regarding gallium]
 
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*[http://minerals.usgs.gov/minerals/pubs/commodity/gallium/index.html environmental effects of gallium]
 
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]
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[[Category:Chemical elements]]
 
[[Category:Chemical elements]]
  
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{{credit|83642167}}
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{{credit5|Gallium|83642167|Gallium(III)_arsenide|81645319|Gallium(III)_hydroxide|63863706|Gallium(III)_nitride|80840721|Gallium(III)_phosphide|82081836}}

Latest revision as of 04:06, 18 April 2024

31 zinc ← gallium → germanium
Al
↑
Ga
↓
In
Ga-TableImage.png
periodic table
General
Name, Symbol, Number gallium, Ga, 31
Chemical series poor metals
Group, Period, Block 13, 4, p
Appearance silvery white
Typical (melted blob) Crystallized
Atomic mass 69.723(1) g/mol
Electron configuration [Ar] 3d10 4s2 4p1
Electrons per shell 2, 8, 18, 3
Physical properties
Phase solid
Density (near r.t.) 5.91 g/cm³
Liquid density at m.p. 6.095 g/cm³
Melting point 302.9146 K
(29.7646 °C, 85.5763 °F)
Boiling point 2477 K
(2204 °C, 3999 °F)
Heat of fusion 5.59 kJ/mol
Heat of vaporization 254 kJ/mol
Heat capacity (25 °C) 25.86 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 1310 1448 1620 1838 2125 2518
Atomic properties
Crystal structure orthorhombic
Oxidation states 3
(amphoteric oxide)
Electronegativity 1.81 (Pauling scale)
Ionization energies
(more) 		1st: 578.8 kJ/mol
2nd: 1979.3 kJ/mol
3rd: 2963 kJ/mol
Atomic radius 130 pm
Atomic radius (calc.) 136 pm
Covalent radius 126 pm
Van der Waals radius 187 pm
Miscellaneous
Magnetic ordering no data
Thermal conductivity (300 K) 40.6 W/(m·K)
Speed of sound (thin rod) (20 °C) 2740 m/s
Mohs hardness 1.5
Brinell hardness 60 MPa
CAS registry number 7440-55-3
Notable isotopes
Main article: Isotopes of gallium
iso NA half-life DM DE (MeV) DP
69Ga 60.11% Ga is stable with 38 neutrons
71Ga 39.89% Ga is stable with 40 neutrons

Gallium (chemical symbol Ga, atomic number 31) is a rare, soft, silvery metal. It is a brittle solid at low temperatures, but it liquefies slightly above room temperature and melts in the hand. It is one of only a few materials that expands when freezing (like water), and its liquid form has a higher density than the solid form (like water). Gallium occurs in trace amounts in bauxite (an aluminum ore) and zinc ores.

Gallium is most commonly used in the form of the compound gallium(III) arsenide, which is a semiconductor useful for integrated circuits, light-emitting diodes (LEDs), and laser diodes. The nitride and phosphide of gallium are also valuable semiconductor materials, and gallium itself is used as a dopant in semiconductors. In addition, this metal is a component in low-melting temperature alloys, and its alloy with indium and tin is used in medical thermometers to replace mercury. Also, gallium can wet (coat) glass to create brilliant mirrors.

Occurrence and isolation

Gallium does not exist in free form in nature, nor are there any gallium-rich minerals that might serve as primary sources of extraction of the element or its compounds. Rather, gallium is extracted as a trace component from bauxite, coal, diaspore, germanite, and sphalerite. Some flue dusts from burning coal have been shown to contain as much as 1.5 percent gallium.

Most gallium is extracted from the crude aluminum hydroxide solution of the Bayer process for producing alumina and aluminum. A mercury cell electrolysis and hydrolysis of the amalgam with sodium hydroxide leads to sodium gallate. Electrolysis then gives gallium metal. For semiconductor use, further purification is carried out using zone melting, or else single crystal extraction from a melt (Czochralski process). Purities of 99.9999 percent are routinely achieved and widely available commercially.

History

Before gallium was discovered, the element and many of its properties had been predicted and described by Dmitri Mendeleev, on the basis of its position in the periodic table. Mendeleev called the hypothetical element eka-aluminum.

In 1875, Lecoq de Boisbaudran discovered gallium by the technique known as spectroscopy. When examining a sample of zinc blende from the Pyrenees, he noticed two unique violet lines in its spectrum, indicative of a previously unknown element. Later, he obtained the free metal by the electrolysis of its hydroxide in KOH solution. He named the element "gallia" after his native land of France; also, in one of those multilingual puns so beloved of men of science of the early nineteenth century, he named it after himself—Lecoq means "the rooster" in French, and Latin for rooster is gallus.

Notable characteristics

Very pure gallium metal has a brilliant silvery color.

In the periodic table, gallium lies in group 13 (former group 3A), between aluminum and indium, and in the same group as thallium. Consequently, its properties resemble those of these three elements. In addition, it is situated in period 4, between zinc and germanium. It is also said to be one of the "poor metals"—elements located between the transition metals and metalloids in the periodic table.

High-purity, metallic gallium has a brilliant, silvery color. By contrast, like most metals, finely divided gallium loses its luster—powdered gallium appears gray. The solid form fractures conchoidally, like glass. When liquid gallium solidifies, it expands by 3.1 percent. Thus, its liquid state has a higher density than the solid state—a property characteristic of only a few materials like water and bismuth. Also, given the property of expansion during solidification, gallium is not stored in either glass or metal containers to prevent the container from rupturing when the element freezes.

Gallium also diffuses into the crystal lattice of most other metals. This is another reason why it is important to keep gallium away from metal containers such as steel or aluminum. Gallium easily alloys with many other metals, and it was used in small quantities in the core of the first atomic bomb to help stabilize the plutonium crystal structure.

Given its melting point of 30°C, the metal readily melts in the hand. Also, the liquid form has a strong tendency to supercool below its melting point, and it needs to be seeded for solidification to begin. Gallium is one of the metals—along with cesium, francium, and mercury)—that is liquid at or near normal room temperature. It can therefore be used in metal-in-glass high-temperature thermometers. It is also notable for having one of the largest liquid ranges for a metal, and (unlike mercury) for having a low vapor pressure at high temperatures.

Unlike mercury, liquid gallium wets (coats) glass and skin, making it mechanically more difficult to handle, although it is substantially less toxic and requires far fewer precautions. For this reason, as well as the metal contamination and freezing expansion problems noted above, samples of gallium metal are usually supplied in polyethylene packets within other containers.

Gallium does not crystallize into any of the simple crystal structures. The stable phase under normal conditions is orthorhombic, with eight atoms in the conventional unit cell. Each atom has only one nearest neighbor (at a distance of 244 picometers) and six other neighbors within an additional 39-picometer radius. The bonding between nearest neighbors has covalent character. Also, the element has many stable and metastable phases, depending on the temperature and pressure conditions.

High-purity gallium is attacked slowly by mineral acids.

Isotopes

Many isotopes of gallium are known, ranging from 56Ga to 86Ga. Among them, there are two stable isotopes: 69Ga and 71Ga, at relative abundances estimated at 60.11 percent and 39.89 percent, respectively. The radioisotopes, by contrast, have extremely short half-lives.

Compounds

Gallium can form a number of compounds. Some of them are mentioned below.

  • Gallium(III) arsenide (GaAs): It is an important semiconductor, used for such devices as microwave-frequency integrated circuits (Monolithic Microwave Integrated Circuits, or MMICs), infrared light-emitting diodes (LEDs), laser diodes, and solar cells. Some of its electronic properties are superior to those of silicon. For instance, GaAs devices can function at higher frequencies (above 250 gigahertz), generating less noise, and can be operated at higher power levels. Also, they have a direct band gap, so they can be used to emit light.
  • Gallium(III) hydroxide (Ga(OH)3): This is the normal mineral form of gallium in the Earth's crust. It does not occur as a discrete mineral, but gallium replaces aluminum in ores such as bauxite. Gallium hydroxide is amphoteric, that is, it can behave as an acid as well as a base. In strongly acidic conditions, the ion Ga3+ is formed; in strongly basic conditions, Ga(OH)4-, is formed.
  • Gallium(III) nitride (GaN): This hard, mechanically stable material is a binary semiconductor with a wide, direct band gap. It is used in optoelectronic devices such as high-brightness, blue LEDs and blue laser diodes. Its sensitivity to ionizing radiation is low, making it a suitable material for solar cell arrays for satellites. It is being investigated for use in high-frequency, high-power transistors that can operate at high temperatures.
  • Gallium(III) phosphide (GaP): This solid, crystalline material has the appearance of pale orange pieces. It is odorless and insoluble in water, and it melts at 1,480°C. It is a semiconductor with an indirect band gap (2.26 electronvolt). Sulfur or tellurium may be added as dopants to turn gallium phosphide into an n-type semiconductor; or zinc may be added as a dopant to prepare a p-type semiconductor. GaP is used for the manufacture of low- and standard-brightness red, orange, and green LEDs.

Applications

Gallium, its alloys, and its compounds have many applications. Some of them are listed below.

  • The most common applications of gallium are in the form of the semiconductor gallium(III) arsenide. This compound is used mainly for analog integrated circuits, and also for optoelectronic devices such as LEDs and laser diodes.
  • Gallium is widely used as a dopant in semiconductors, to produce solid-state devices such as transistors.
  • Given that gallium can wet glass or porcelain, it can be used to create brilliant mirrors.
  • Gallium readily alloys with most metals, and has been used as a component in low-melting alloys.
  • The plutonium used in nuclear weapon pits is machined by alloying with gallium to stabilize the allotropes of plutonium.
  • When added in quantities up to 2 percent in common solders, gallium can aid wetting and flow characteristics.
  • Gallium is used in some high-temperature thermometers.
  • An alloy of gallium, indium, and tin (trade name Galinstan) is widely available in medical thermometers (fever thermometers), replacing problematic mercury. This alloy has a freezing point of −20°C.
  • Magnesium gallate, containing impurities such as Mn+2, is beginning to be used in ultraviolet-activated phosphor powder.
  • Gallium salts, such as gallium citrate or gallium nitrate containing a radioactive isotope of gallium, have been used in nuclear medicine imaging. This use, however, has largely been replaced by FDG PET scans.
  • Much research is being devoted to gallium alloys as substitutes for mercury dental amalgams, but such compounds have yet to gain wide acceptance.
  • Gallium is the rarest component of new photovoltaic compounds (such as copper indium gallium selenium sulphide or Cu(In,Ga)(Se,S)2, announced by South African researchers) for use in solar panels as an alternative to crystalline silicon, which is currently in short supply.
  • It has been suggested that a liquid gallium-tin alloy could be used to cool computer chips in place of water. As it conducts heat approximately 65 times better than water, it can make a comparable coolant.[1]

Precautions

Gallium is not considered toxic, but the data about its effects are inconclusive. Some sources suggest that it may cause dermatitis from prolonged exposure; other tests have not caused a positive reaction. When the element is handled with bare hands, the skin acquires a gray stain from an extremely fine dispersion of liquid gallium droplets.

See also

Notes

  1. ↑ Will Knight. Hot chips chilled with liquid metal. NewScientist.com. Retrieved April 6, 2007.

References
ISBN links support NWE through referral fees

  • Gallium. WebElements. Retrieved October 22, 2022.

External links

All links retrieved April 17, 2024.

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