Boule lengths

Boule lengths are also likely to increase. More powder may be introduced into the growth chamber, however, thermal considerations become an issue as the boules increase too much in length. Even if the surface is kept at constant distance in the chamber, the distance from the source will vary as the material is depleted and the growth conditions change. 

As soon as the substance catches fire the source of heat is withdrawn and observation is made as to whether the combustion continues in the part of the trough.which was not heated. If the comhustion does continue, the times necessary to reach.the 50, lOQcm, etc marks are recorded and the velocity of propagation of the flame in cm per sec, is calculated. If the time is below 5 sec for the entire length.of the trough, it is advisable to use a chronograph, such as of Le Boul-lange described in VoL 3 of Encycl, p C307-R... 

In 1978, Tairov and Tsvetkov [24] reported a sublimation technique to produce SiC boules for device application. They produced an 8 mm diameter by 8 mm long boule of SiC on a seed crystal placed within a graphite crucible. In a further study, growth of SiC boules up to 14 mm diameter and 18 mm in length were obtained [15]. 

Hence the silicon plays the role of a carbon-transporting agent. Water-clear boules of the pure 6H poly type having diameters > 50 mm and lengths > 60 mm have been grown by this technique. It is believed that by the year 2001 the size of the boules will have increased to 15 cm in diameter. 

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