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lthough discovered in 1875 by Lecoq de BoisA baudran, gallium has remained until recently a curiosity, principally noted for its rarity. With the
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present annual production exceeding one ton, however, gallium can no longer be comidered rare. The increased production has also decreased the price, recently comparable with gold, to a value approaching that of silver. Despite about 3800 published articles dealing with gallium and its compounds (Z), many of their properties are virtually unknown. In view of the increased interest in gallium, it is useful to recall the principal properties of gallium and its compounds, m a t ofwhich are important in the production of semiconductor materials and some of which have yet to find use
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Gallium P I E R R E
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BRETEQUE
Progressive lowering of the price m y permit the unique properties of gallium to be more widely utilized PROPERTIES OF GALLIUM
Atomic Number Atomic Weight lsotooes GaS9
Ga" q - 4 ~ Gravity
Solid
5.907120' C.1 5.904 129.6' C.1
Liquid
6.095 129.8' C.1 5.905 1300" C.1 ~.
~
19.16 cal./g.
Heat of Vaporization
1014 caf./g.
Specific Heat Solid
0.0887 cal./g. 115' C.1
C.
Liquid
0.0977 cal./g. 1300 c.1
C.
Vaoor Pressure
1 1 m o c.1
c.1
I11400 11300" C.1
11500~c.1
12030~c.1 Thermal Conductivity I300 c.1 Electrochemical Properties Standard Oxidation Potentia Electrochemical Equivalents g./Al Ah/g 54
I X lO-'rnrn. Hg 1 X 10-2mm. Hg 1 X 10-'mm. Hg 1 100
mm. Hg mm. Hg
0.074.09 Units
According to geochemists gallium is about as abundant
as lead but is greatly scattered. It is not found in a unitary ore but is always associated with other metals from which it is separated by by-product refining. Despite the low concentrations (0.0034.10%), the chief sources of gallium are the bauxites. In alumina production, e.g., gallium is found in the aluminate extracts in concentrations as high as 0.1 to 0.2 gram of gallium per liter of extract. A number of processes (7,3,4,6,9) are used to recover gallium from the extracts. If all the gallium thus available were recovered, the annual world production would be about 1000 tons. Gallium is also obtained in small amounts as a by-product in zinc refining where metallic gallium is obtained by electrolysis of aqueous sodium gallate extracts in purities of 99%. Further physical and chemical refinement permits achieving purities of up to 99.9999% that required for Scmiconductor production. Physical Properties
Expansion on Freezing 3.2% ~- ~. Melting Point 29.789 c. ~_______ Boiling Point 2400' C. 1approx.l Heot of Fusion
NabnaI State and Extraction
CGS
-0.56 volt
0.87 1.15
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Its melting point of 29.8' C. classifies gallium with mercury and cesium as the lowest melting metals. The boiling point of 2400' C. (8) gives gallium the greatest liquid range of all the elements and, at 1000" C., the vapor pressure is only 0.001 mm. of mercury. Except for bismuth, gallium is the only element which expands on cooling, the volume increase during solidification being 3.2%. Solid gallium has a blue cast and crystallizes in the orthorhombic system, often with single crystals weighing several kilogram. T o the eye, liquid gallium is indistinguishable from mercury hut has a greater viscosity and a specific gravity of about 6. While solid gallium is only about as hard as aluminum, it is easily fractured and can be readily "smeared" l i e lead. When slowly cooled from well above its melting point, liquid gallium easily can be subcooled to well helow 0' C. The anisotropy of gallium crystals is exemplified by the maximum/
AUTHOR Pinre de la Brctcque is Dircctor of Researchfor the Societe Francaue pour Plndushe de PAluminum, Marseih (France), a subndiary company of Swiss Aluminum Ltd.
minimum ratio of about 7 for electrical and thermal conductivities, the highest value for all elements. Liquid gallium, unlike mercury, wets most surfaces. Exceptions are graphite and, to some extent, Teflon. Wetting is a function of the purity of gallium and handling is made easier by adding dilute hydrochloric acid to prevent wetting. Chemical Properties
The standard electromotive potential of gallium is -0.56 volt, ranking it with zinc and iron, slightly more noble than aluminum. When cold, an invisible layer of oxide prevents gallium from tarnishing in air but a temperature of 1200’ C. is necessary to prepare Gaz03 by direct oxidation. Metallic gallium is not reactive at low temperatures with other metals but is very active at high temperatures. Nonetheless, even at low temperatures, gallium rubbed on aluminum will produce brittleness in the aluminum due to diffusion of gallium into the grain boundaries of the aluminum. Below 100’ C. water does not react with gallium, even at elevated pressures. Pure gallium is only slightly attacked by inorganic acids except for aqua regia which readily attacks the metal. Nitric acid tends to disperse liquid gallium into fine droplets while hydrochloric acid tends to agglomerate them rapidly. Alkali solutions (KOH and NaOH) readily dissolve hot gallium and the halogens, especially chlorine and fluorine, actively attack it. Although gallium is more electronegative than aluminum, the difference in potential is small and reflects chemical similarities with aluminum except for the greater stability of the gallate solutions over the corresponding aluminates. Thus, stable gallate solutions in the molecular ratio NazO/CrasO or K 2 0 / G a z 0 3= 1 can be prepared (5) in concentrations of over 800 grams of gallium per liter of solution. Compounds
Many gallium compounds, some of them commercially available, have been described in the literature. Greatest interest is in the oxides and salts of inorganic acids: Ga203 (hydrated and unhydrated), GaC12, GaC13, GaBra, GaF3, GaFo(NH4)3.3H30, Ga2(S04)3 (hydrated and unhydrated), GaZS2, and Ga&. These compounds differ little from the corresponding aluminum compounds. The same is not true for the semiconductors GaAs and GaP or for the superconductor GaV3. Utilization
As previously indicated, the chief use for gallium a t present is in semiconductor manufacture. GaAs and GaP are used as diodes, thermistors, transistors, andlasers. The advantage of gallium in these applications is its high temperature resistance to property change. Other actual or proposed uses are: HEATTRANSFER MEDIUM. In the area of atomic energy, gallium and its alloys are valuable as heat
transfer media although high cost and corrosive character (70) are undesirable. Also in this connection, gallium has been suggested for use in making PVT measurements at high temperature (7). LIQUIDSEALS. Gallium is occasionally used in making liquid seals for high temperature applications. HIGH PRESSURES.Gallium has been used to obtain high pressures by utilizing its property of expansion on solidification. HIGH TEMPERATURE LUBRICANTS. Liquid gallium, mixed with a solid filler to form a paste, has been proposed as a lubricant for vacuum applications. The low vapor pressure is an advantage. LOW TEMPERATURE SOLDERS.Gallium alloys, e.g., gallium-indium, are now in use in making electrical connections on semiconductors at low temperatures. “AMALGAM” TYPEALLOYS.The utilization of gallium “amalgams” with Au, Ni Sn, Cd, Zn, and Bi are proposed for several uses. SUPERCONDUCTORS. Among the compounds under investigation is GaV3, which is desirable because it retains its properties in dense magnetic fields. OTHER ALLOYS. The high cost of gallium restricts its use to those alloys where it is present in small amounts. Gallium acts as a hardener for aluminum and magnesium, the chief alloys investigated thus far. MAGNETIC COMPOUNDS. Some gallium compounds of the “soft ferrite” type, crystallizing in the spinel or garnet systems, are suggested for R F choke coils. Among the particular compounds mentioned for this purpose are spinels containing Ba, Ga, Fe, 0 or Mn, Ga, Fe, 0. Garnet type compounds suggested include those containing Y , Ga, 0. Applications of the garnet type compounds require very strict control of purity (77, 12). “Magnetic ferrites,” such as Cu2MnGa are also being considered. ORGANIC REACTIONS. Certain gallium compounds have been studied for their utility in organic synthesis. The more important are:
+
Gallium halides, particularly for reactions of the FriedelCrafts type or for the polymerization of vinyl esters; gallium chloride has the advantage of being soluble in a number of organic liquids. Certain gallium hydrides. Certain organometallic compounds. MEDICINE,BIOCHEMISTRY,AGRICULTURE. Gallium is slightly toxic but its influence on microbes and as an agricultural trace mineral have not been found interesting. REFERENCES (1) Bcja, M. (to Pechiney), French Patent 969,033 (Dec. 1946). (2) Breteque, de la. P., “Gallium,” Societe Francaise pour 1’Industrie de I’Aluminium, Marseille (1962, 1963). (3) Breteque, de la, P. (to Alusuisse), U. S. Patent 2,793,179 (June 1955) (4) Ibid., 2,798,845 (June 1955). (5) Breteque, de la, P., Bull. Sot. Chim. Frence, 2364 (1961). (6) Brown, R. W. (to Alcoa), U. S. Patent 2,582,377 (Apr. 1947). (7) Chem Eng News, No. 27,41 (1959). (8) Cochran, C. N., Foster, L. M., J.Eldctrochm. Soc. 109, 144 (1962). (9) Frary, F. C. (to Alcoa),U. S. Patent 2,582,576 (Apr. 1947). (10) Jaffee, R. I., U.S.A.E.C. Rep., AECD 3317 (1949). (11) Nielsen, J. W., J . A@. Phys. 31, 5 1 (1960). (12) Ibtd., 33, 1370 (1962).
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