Block structure and residual stresses in tubular sapphire single crystals grown by the Stepanov method

The formation of the block structure and residual stresses in sapphire single-crystal tubes grown from the melt by the Stepanov method has been studied. The distribution of the difference in the residual stress tensor components ($\sigma_\varphi$–$\sigma_r$) in a thick-walled [0001] sapphire tube has been measured by the conoscopy method. It has been shown that circumferential tensile $\sigma_\varphi$ and compressive stresses act on the outer and inner tube surfaces, respectively. The maximum stress is $\sim$ 20 MPa. It has been demonstrated that the development of the block structure begins from the outer tube surface; as the crystal length increases during growth, blocks propagate over the entire cross section.