YTTRIUM ALUMINIUM GARNET (Y₃ Al₅ O₁₂)

DESCRIPTION

Yttrium aluminium garnets (YAG) are structural analogues of natural garnets, but excel it in hardness, transparence and dimension of flawless zone. They cut and polish like natural garnets and are quite pretty, especially in clear and light colours. Synthetic garnets, including YAG, are a very popular faceting material.

Due to high dispersion, YAG crystals create a diamond play-of-color. Moreover, artificial garnets come in a much wider range of colors than their naturally formed counterparts. Yttrium-aluminum garnet crystals are colorless when grown without additives, but doping with rare-earth elements is enough to give the crystals a wide spectrum of colors, including exclusive hues. For instance, when doped with neodymium or erbium, they get pink or purple shades result, chromium, thulium and vanadium give a green colour, manganese – red. If cobalt is used as a dopant, the crystals get a blue shade. When doped with neodymium the YAl-Garnets are useful as the lasing medium in lasers.

We provide high quality optically homogenious YAG of several colours with different dimensions and specifications for use in jewellery, as well as in industrial, medical and scientific fields.

PROPERTIES

Chemical composition: Y₃Al₅O₁₂

Hardness (Mohs' scale): 8.5
Density: 4.55 (average)
Refractive Index: 1.832
Dispersion: 0.028
Crystal Structure: cubic

Cleavage: Imperfect
Fracture: Conchoidal

The mechanical and chemical stability of YAG is similar to that of sapphire, but YAG is not birefringent. This particular feature is important for some optical applications.

HISTORY

Yttrium aluminuim and some other synthetic garnets first appeared relatively recently, in the middle of 1960's.

SYNTHESIS

YAG crystals may be grown by different methods, but the most widely used for commercial production of YAG are Czochralsky technique, also known as vertical pulling method, and a floating-zone method.

The Czochralski process, or crystal pulling technique, is a method of crystal growth used to obtain crystals (especially rod-shaped single crystals), metals and salts. The process starts with melting the materials of appropriate composition in a crucible. A seed crystal mounted on a rod is immersed into the melt where it starts to rotate. The crystal's rod is then pulled upwards slowly and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single crystal from the melt.

To make yttrium-aluminum crystals with the structure of garnet, an iridium crucible lined with zirconium oxide is used. The process is carried out in nitrogen or argon atmosphere at a normal pressure. Aluminum oxide and yttrium oxide are mixed in a ratio of 3:5; the mixture is calcined at a temperature of 1500-1700°C for approximately 24 hours, and thereafter melted at a temperature of 1950°C. Then the seed crystal is put into the melt and the temperature is lowered by pulling the reaction vessel slowly away from the heat source until crystallization process begins. Then the rotated seed crystal is pulled out of the melt.

For the quality of resulted crystals, the correlation between the pulling rate and the rotating rate appears to be crucially important since this correlation determines the quality of crystals' surface. With the pulling rate of 0.5 – 5 mm/hour and the rotation rate of 5 – 20 revolutions per minute the synthesized crystals have flat, almost flawless surface. Both figures change if additives or dopants are present in the melt.

The floating-zone method has been used to produce single crystals of high-melting-point oxides since 1960's. Various types of heaters were used, including induction, resistance, radiational, electron-beam and plasma sources, since the method of heating is very important for this technique. The floating-zone technique is crucible-free, hence the possibility of contamination from crucibles is eliminated: the purity of the resulting crystal will be at least as good as that of the initial polycrystalline starting material.

The floating zone furnace is equipped with a sealed quartz chamber, which allows the growth of single-crystals to be conducted in a wide range of carefully controlled atmospheres and pressures. The required powdered starting compounds are pressed into the shape of a rod, which is sintered in preparation for use in the floating-zone furnace. The sintered feed rod is suspended from the upper shaft of the furnace on a platinum wire. Another rod, a seed rod, is held directly below the feed rod. Heat from the heating source is then focused on a narrow 'floating zone' between the rods to produce molten material adjoining the rods. The rods are rotated in opposite directions. Growth occurs at the boundary between the seed rod and the molten zone, where the microscopic preference of the cooling melt is to grow with the same symmetry as the seed crystal. The equipment must be regularly monitored to ensure that the shafts are rotating and descending at suitable speeds for the promotion of crystal growth, and that the crystal is developing along a desirable growth axis.

YAG crystals grown by the floating-zone method can be up to 6 mm thick and 80 mm long. Crystallization rate is about 5 mm/hour, the feed rod is sintered at a temperature of about 1400°C.

However, the biggest YAG crystals were grown by a modified floating-zone method, where a tungsten dish containing the starting compounds (aluminium and yttrium oxides) and possible dopants is slowly pulled under a heating element. A narrow melted zone created by the heat crystallizes. This modification was invented by a Russian scientist, professor X.S.Bagdasarov.

COLOURS AVAILABLE

            Blue

            Green

            Pink

            Yellow

            White

Garnet varieties similar to demantoid garnet and tanzanite in color and play-of-color are available. Other colors are available on customers' request.

USES

Yttrium Aluminum Garnet (YAG) is an important material used

·        as a gemstone

·        in laser applications

·        for coating electronic devices

·        as substrates in calculators and computers

·        as optical material for UV and IR optics

·        for spacecrafts and submarine illuminators

·        other structural applications at high temperatures