THERMOELASTIC STRESSES IN RIBBONS AND TUBES GROWN FROM THE MELT BY THE STEPANOV METHOD

Ribbons and tubes grown from the melt by the Stepanov technique have a wide range of technical applications. Sapphire ribbons are used as su bstrates in microelectronics and sapphire tubes are used as gas-discha rge balloons in laser engineering, fine chemical technology and high-v acuum equipment. Practice has shown that misorientation angles of smal l-angle boundaries in sapphire crystals should not exceed several degr ees because an increase in the misorientation angles between blocks dr astically lowers the strength and worsens the dielectric properties of these crystals. One of the main mechanisms of formation of the block structure of melt-grown crystals, including shaped sapphire crystals, is dislocation polygonization that begins when the dislocation density exceeds a certain critical value. In turn, dislocations are formed un der deformations due to thermal stresses. Calculations of thermal fiel ds in crystals and the corresponding thermoelastic stress fields can b e used as an input to improve and optimize the growth process. The dep endence of thermoelastic stresses in ribbons and tubes on the technolo gical parameters has been calculated.