Synthetic Crystal

Synthetic Crystal

 

(also, man-made crystal), any of the crystals grown artificially either in the laboratory or in industry. Of the total number of synthetic crystals, approximately 10,000 are inorganic substances, and some of these do not occur in nature. Organic synthetic crystals, however, are far more numerous, numbering in the hundreds of thousands; these crystals are of various compositions and, in general, do not occur in nature. On the other hand, of the 3,000 crystals that make up the variety of natural minerals, it has been possible to grow only a few hundred, of which only 20–30 are of practical importance (see Table 1). This low number is explained by the complexity of the crystallization processes and the technical difficulties involved in precise maintenance of the conditions necessary for the growth of single crystals.

The first attempts to synthesize crystals were made in the 16th and 17th centuries; they consisted of the recrystallization of water-soluble crystalline substances encountered in crystalline form in nature (sulfates, halides). Later, knowledge of the composition of natural minerals brought attempts to synthesize minerals from powders using roasting techniques. The first small synthetic crystals were obtained by this method. In the early 20th century, E. S. Fedorov and G. V. Vul’f studied the synthesis of crystals, investigated the conditions of crystallization of water-soluble compounds, and improved the crystallization apparatus. Subsequently, A. V. Shubnikov worked out general principles for the formation of crystals from aqueous solutions (Seignette salt, potassium dihydrogen phosphate) and melts (single-component and multicomponent systems). Under his direction, the first factory for the synthesis of crystals was set up.

Synthetic crystals of quartz are obtained under hydrothermal conditions. Small crystals (seeds) of various crystallographic directions are cut from natural quartz crystals. Although quartz is very common in nature, supply does not meet the demands of technology. Furthermore, natural quartz contains many impurities. Synthetic crystals of quartz of up to 15 kg are grown in autoclaves over a period of many months; the growth period for high-purity crystals (large flawless quartz crystals used in optical devices) can reach several years.

Table 1. The most common synthetic crystals
NameChemical formulaMethod of growingAverage crystal sizeUse
QuartzSiO2Hydrothermal1–15 kg,
300 × 200 × 150 mm
Piezoelectric transducers, jewelry, optical instruments
CorundumAl2 O3Verneuil and Czochralski methods, zone meltingRods of diameter 20–40 mm, length up to 2 m; plates 200 × 300 × 30 mmInstrument-making and watchmaking industries, jewelry
GermaniumGeCzochralski method100 g to 10 kg, cylinders 200 mm × 500 mmSemiconductor devices
SiliconSiCzochralski method100 g to 10 kg, cylinders 200 mm × 500 mmSemiconductor devices
HalidesKCI, NaCICzochralski method1 to 25 kg, 100 × 100 × 600 mmScintillators
SeignettesaltKNaC4 H4 O6–4H2 OCrystallization from solution1–40 kg, 500 × 500 × 300 mmPiezoelectric elements
Potassium dihydrogen phosphateKH2 PO4Crystallization from solution1–40 kg, 500 × 500 × 300 mmPiezoelectric elements
Yttrium aluminum garnetY3 AI5 O12Czochralski method, zone melting40 × 40 × 150 mm 30 × 200 × 150 mmLasers, jewelry
Yttrium iron garnetY3 Fe5 O12Crystallization from solutions of melts30 × 30 × 30 mmRadioacoustics, electronics
Gadolinium gallium garnetGd3 Ga5 O12Czochralski method20 × 30 × 100 mmSubstrates for magnetic films
DiamondCCrystallization at ultrahigh pressures0.1–3 mmAbrasives
Lithium niobateLiNbO3Czochralski method10 × 10 × 100 mmPiezoelectric and ferroelectric elements
NaphthaleneC10H8Kyropoulos methodBlocks of several kilogramsScintillation instruments
Potassium biphthalateC8 H5 O4 KCrystallization from aqueous solutions40 × 100 × 100 mmX-ray analyzers, nonlinear optics
CalciteCaCO3Hydrothermal10 × 30 × 30 mmOptical devices
Cadmium sulfideCdSGrowth from gaseous phaseRods 20 × 20 × 100 mmSemiconductor devices
Zinc sulfideZnSGrowth from gaseous phaseRods 20 × 20 × 100mmSemiconductor devices
Gallium arsenideGaAsGas transport reactionsRods 20 × 20 × 100 mmSemiconductor devices
Gallium phosphideGaPGas transport reactionsRods 20 × 20 × 100 mmSemiconductor devices
Molybdates of rareearth elementsY2(MoO4)3Combined Czochralski method10 × 10 × 100 mmLasers
Zirconium dioxideZrO2Induction heating in a cold containerBlocks of approximately 2 kg, columnar crystals 100 × 10 × 50 mmJewelry
Hafnium dioxideHfO2Induction heating in a cold containerBlocks of approximately 2 kg; columnar crystals 100 × 10 × 50 mmJewelry
Calcium tungstateCaWO4Induction heating in a cold container10 × 10 × 100 mmLasers
Yttrium aluminateYAIO3Czochralski method10 × 10 × 100 mmLasers
Aluminum (tubes of various cross section)AIStepanov methodlength 103mm, diameter 3–200 mmMetallurgy

Because geometrically regular crystals are often associated with gems, the efforts of many scientists have been directed toward the synthesis of, for example, diamonds, rubies, aquamarines and sapphires. Synthetic ruby crystals (minute dark crimson crystals) were obtained in the early 19th century from solutions in melts of potash and sodium carbonate. Later, at the end of the century, the French scientist Verneuil invented a special apparatus, subsequently improved, for producing synthetic crystals of ruby. Here, Al2 O3 powder with an additive of a few percent Cr2 O3 is fed continuously into the zone of a furnace where hydrogen is burning in oxygen. Drops of the molten mass then fall onto a cooler segment of the seed crystal and immediately crystallize. In the USSR, apparatus operate according to the system of S. K. Popov, which makes possible the production of synthetic crystals of ruby in the shape of rods with a diameter from 20 to 40 mm and a length of up to 2 m. These crystals are used in lasers and yarn carriers and in the glass used in space instruments. A large fraction of the synthetic crystals of ruby is used in the watchmaking industry, although the major consumer is the jewelry industry. The addition of impurities in the form of the salts of Ti, Co, and Ni to Al2 O3 permits the production of synthetic crystals having colors similar to those of such natural gems as sapphires, topazes, and aquamarines.

Synthetic diamond crystals were obtained in the 1950’s from graphite powder mixed with Ni. The mixture was pressed into small (2–3 cm) disks, which were then heated to 2000°-3000°C at pressures of 100,000–200,000 atmospheres. Under these conditions, graphite is converted into diamond. The size of synthetic crystals of diamond is of the order of tenths of a millimeter, although under special conditions it is possible to obtain crystals of up to 2–3 mm. The diamond industry in the USSR was established mainly to meet the requirements of drilling technology. Synthetic crystals of diamond able to compete with the natural diamonds used in jewelry have so far been obtained in only small quantities.

The 1950’s saw the development of the industry for the synthesis of organic crystals. These crystals, which include those of naphthalene, stilbene, tolan, and anthracene, are used in scintillation counters. The synthesis is carried out mainly by the Czochralski method. In size, the crystals are comparable to large inorganic (water-soluble) crystals. The most widely used semiconductor crystals (Ge, Si, Ga, As) do not occur in nature. All are grown from melts; they are cylindrically shaped, with a diameter ranging from 10 to 20 cm and a length ranging from 30 to 50 cm.

Synthetic crystals of iron garnets and emeralds are grown under laboratory conditions from solutions of the melts. However, these methods have not yet been developed industrially. Studies are currently under way on the industrial production of synthetic gemstones from yttrium aluminum garnets (garnetites) and zirconium and hafnium dioxides (flanites). These synthetic crystals have a wide range of colors and resemble emeralds, topazes, and diamonds because of their high light refraction.

REFERENCES

Fedorov, E. S. “Protsess kristallizatsii.” Priroda, December 1915.
Vul’f, G. V. Kristally, ikh obrazovanie, vidistroenie. Moscow, 1917.
Shubnikov, A. V. Kak rastut kristally. Moscow-Leningrad, 1935.
Ansheles, O. M., V. B. Tatarskii, and A. A. Shternberg. Skorostnoe vyrashchivanie odnorodnykh kristallov iz rastvorov. [Leningrad] 1945.
Popov, S. K. “Novyi proizvodstvennyi metod vyrashchivaniia kristallov korunda.” Izv. AN SSSR: Seriia fizicheskaia, 1946, vol. 10, nos. 5–6.
Shternberg, A. A. Kristally v prirode i tekhnike. Moscow, 1961.
“Usloviia rosta i real’naia struktura kvartsa.” In IV Vsesoiuznoe soveshchanie po rostu kristallov. Yerevan, 1972. Part 2, p. 186.
Mil’vidskii, M. G., and V. B. Osvenskii. “Poluchenie sovershennykh monokristallov poluprovodnikov pri kristallizatsii iz rasplava.” Ibid. Part 2, p. 50.
Bagdasarov, Kh. S. “Problemy sinteza krupnykh tugoplavkikh opticheskikh monokristallov.” Ibid. Part 2, p. 6.
Timofeeva, V. A., and I. B. Dokhnovskii. “Vyrashchivanie ittrievo-zhelezistykh granatov iz rastvorov-rasplavov na tochechnykh zatravkakh v dinamicheskom rezhime.” Krislallografiia, 1971, vol. 16, issue 3, p. 616.
Iakovlev, Iu. M., and S. Sh. Gendelev. Monokristally ferritov v radioelektronike. Moscow, 1975.

V. A. TIMOFEEVA

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