The effect of crucible bottom deformation on the quality of Nd:GGG crystals grown by Czochralski method

In this paper, we report the growth of neodymium doped Gadolinium Gallium Garnet (Nd: GGG) crystal using Czochralski (CZ) method, and study the effects of crucible bottom deformation and thermal insulator thickness on the growth process and crystal quality. Garnet structure and <111> crystallography orientation of the crystal were confirmed by the X‐Ray diffraction (XRD) analysis. Macroscopic defects, residual stresses, quality, and homogeneity of the crystals were investigated by means of parallel plane polariscope and laser fizeau interferometer respectively and the results compared together. Experimental observations show that the crucible bottom deformation from flat to convex, and decreasing the thickness of zirconia insulator under the crucible result in the formation of lateral cores and increasing the crystal inhomogeneity and tensions, leading to the decrease of the crystal quality. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimal standby activation in m-out-of-n systems: A preventive approach

This article investigates an optimal preventive standby activation policy for an $m$-out-of-$n$ redundant system. We assume that for such a system, which starts operating at time $t=0$, a standby component will be activated at either the failure time of the system or at a predetermined time $\tau$, whichever occurs first. We first obtain the system reliability function under this switching policy as a function of $\tau$. Then, we investigate the optimal switching time $\tau$ so that the mean time to failure of the system is maximized. The results indicate that the existence of an optimal value of $\tau$ depends on the lifetime of the standby redundancy and its virtual age in the standby state. Some illustrative examples are presented to examine the theoretical outcomes.