The secondary nucleation of magnesium sulfate due to the action of fluid forces on a seed crystal was studied by introducing the hypothesized nuclei so generated into a region of high supersaturation. The number of nuclei observed is strongly dependent on the supersaturation at the seed and the severity of shear forces. Classical nucleation theory is used to interpret the results. These results are consistent with other experimental observations described in the recent literature on contact nucleation. The growth and nucleation process inferred is similar to that illustrated by Clontz and McCabe (1972) SCOPEThe objective of this work was the experimental demregion of relatively large supersaturation had to exist onstration that fluid mechanical forces could effect the within the nucleating chamber. The magnesium sulfate generation of secondary nuclei. This objective was system was studied and the, experimental results were achieved only insofar as nuclei were generated but a analyzed using classical nucleation models. CONCLUSIONS AND SIGNIFICANCEThe interpretation Powers (1963) has reported experiments in which nucleation occurred when a sucrose crystal was held on a rotating glass rod in a supersaturated solution. However, both La1 et al. (1969) and Clontz and McCabe (1972) have suggested that fluid shear alone could not produce breeding with MgS04. 7Hz0 at a AT < 4"C, in the region of supersaturations where needle breeding did not occur and where the crystals grow in a nondendritic form. It may have been that the degree of supersaturation was, in fact, too low so that nuclei or embryos which were formed dissolved; also the magnitude of the shear was not considered. In this work we hypothesize that secondary embryonic species can be produced by fluid shear at a growing crystal surface. We suggest that these potentially may have been present in previous work (La1 et al., 1969; Clontz and McCabe, 1972) but may have dissolved because of insufficient level of supersaturation and thus not observed.According to the Gibbs-Thomson equation the critical size varies inversely as the supersaturation: at a given supersaturation, all particles smaller than the critical size will dissolve, and only those larger will survive. The higher the supersaturation, the smaller the critical size, and hence, the greater the number of particles finally surviving. This has been called the survival theory (Garabedian and Strickland-Constable, 1972; StricklandConstable, 1972). According to the hypothesis in this work, fluid forces at a growing crystal surface can cause the formation of potential secondary nuclei. If the supersaturation in which these develop is great enough, then the critical size required is very small. In the work to be described here, these potential nuclei are visualized as formed due to fluid forces at a growing crystal surface, they are introduced quickly into a region of great supersaturation and there they exist as particles for which the sizes are greater than the critical size associated with this region. ...
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We demonstrate that the crystal orientation of single-crystal silicon layers may be changed in selected areas from one orientation to another by an amorphization/templated recrystallization (ATR) process, and then introduce ATR as an alternative approach for fabricating planar hybrid orientation substrates with surface regions of (100)- and (110)-oriented Si. The ATR technique, applied to a starting substrate comprising a thin (50–200 nm) overlayer of (100) or (110) Si on a (110) or (100) Si handle wafer, consists of two process steps: (i) Si+ or Ge+ ion implantation to create an amorphous silicon (a-Si) layer extending from the top of the overlayer to a depth below the overlayer/handle wafer interface, and (ii) a thermal anneal to produce the handle-wafer-templated epitaxial recrystallization of the a-Si layer. Regions exposed to the ATR process assume the orientation of the handle wafer while regions not exposed to the ATR process retain their original orientation. The practicality of this approach is demonstrated with the fabrication of a planar hybrid orientation substrate comprising (100) and (110) Si regions separated by SiO2-filled trenches.
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