Patterned crystal growth and heat wave generation in hydrogels

The crystallization of metastable liquid phase change materials releases stored energy as latent heat upon nucleation and may therefore provide a triggerable means of activating downstream processes that respond to changes in temperature. In this work, we describe a strategy for controlling the fast, exothermic crystallization of sodium acetate from a metastable aqueous solution into trihydrate crystals within a polyacrylamide hydrogel whose polymerization state has been patterned using photomasks. A comprehensive experimental study of crystal shapes, crystal growth front velocities and evolving thermal profiles showed that rapid growth of long needle-like crystals through unpolymerized solutions produced peak temperatures of up to 45˚C, while slower-crystallizing polymerized solutions produced polycrystalline composites and peaked at 30˚C due to lower rates of heat release relative to dissipation in these regions. This temperature difference in the propagating heat waves, which we describe using a proposed analytical model, enables the use of this strategy to selectively activate thermoresponsive processes in predefined areas.

Control of Void Formation in 4H-SiC Solution Growth

Void formation in 4H-SiC crystals grown from solution has been investigated by secondary ion mass spectrometry and Raman scattering. It becomes clear that ambient Ar gas is filled in voids and the solvent (Si) partially remains. The result indicates that Ar dissolved in the solvent vaporizes and forms bubbles. The trapped bubbles at the crystal growth front are considered to be incorporated in the growing crystal as voids. We also have developed following methods for suppression of the void formation; (1) dipping seed crystals so that the growth front faces upward, (2) growth under He atmosphere, and (3) the high temperature treatment of the solvent before crystal growth.

Effect of Low Frequency Magnetic Field on SiC Solution Growth

We investigated numerically fluid dynamics and carbon transport in a 2 inches SiC solution growth with the presence of alternative magnetic fields. Buoyancy and Marangoni convection are taken into account. Our numerical results revealed that the magnetic field parameters have a strong impact on the melt convection. We also propose a solution to increase the mass transfer at the crystal growth front.

Aspects of Dislocation Behavior in SiC

A review is presented of the current understanding of the dislocation configurations observed in PVT-grown 4H- and 6H-SiC boules and CVD-grown 4H-SiC homoepitaxial layers. In both PVT-grown boules and CVD-grown epilayers, dislocation configurations are classified according to whether they are growth dislocations, i.e., formed during growth via the replication of dislocations which thread the moving crystal growth front, or result from deformation processes (under either mechanical or electrical stress) immediately following growth, during post growth cooling, i.e., behind the crystal growth front or during device operation. Possible formation mechanisms of growth defects in the PVT grown boules, such as axial screw dislocations and threading edge dislocation walls are proposed. Similarly, possible origins of growth defect configurations in CVD-grown epilayers, such as Frank faults bounded by Frank partials, BPDs and TEDs, are also discussed. In a similar way, the origins of BPD configurations resulting from relaxation of thermal stresses during post-growth cooling of the PVT boules are discussed. Finally, the susceptibility of BPD configurations replicated into CVD grown epilayers from the substrate towards Recombination Enhanced Dislocation Glide (REDG) is discussed.

High SiC Growth Rate Obtained by Vapour-Liquid-Solid Mechanism

The growth of 3C-SiC polycrystal and 6H-SiC homoepitaxial layers from Metal-Si alloys is carried out as function of temperature and propane partial pressure. Based on the vapourliquid- solid mechanism, we present a new configuration for the growth of SiC which could allow first to simplify the liquid handling at high temperature and second to precisely control the crystal growth front. 3C-SiC crystals exhibiting well-faceted morphology are obtained at 1100-1200°C with outstanding deposition rates, varying from 1 to 1.5 mm/h in Ti-Si melt. At 1200-1300°C, thick homoepitaxial 6H-SiC layers were successfully obtained in Co-Si melts, with growth rates up to 200 ,m/h. Details on the experiments will be given and the potentialities of such process for the growth of bulk crystals will be discussed..

Analysis on the Ceasing Mechanism of the YBCO Crystal Growth during Melting Growth Process by Unidirectional Solidification

The unidirectional solidification technology by a zone melting method was performed to obtain the large single domain YBCO. The interface morphology and chemical composition at the growth front of the YBCO crystal were investigated in order to make clear the growth characteristic of the YBCO crystal during melting growth by unidirectional solidification. It was found that YBCO crystal would cease growing when yttrium was depleted in the liquid phase at the YBCO crystal growth front. For maintaining the continuous growth of YBCO crystal, compositions of Y, Ba and Cu in raw samples have to be adjusted so as to make yttrium rich in the liquid phase at the YBCO crystal growth front during the melting growth process. It is very useful for the study on the mechanism of the YBCO crystal growth.

Lattice-plane curvature and small-angle grain boundaries in SiC bulk crystals

SiC crystals grown by the physical vapour transport process along the [001] direction show a curvature of the crystal growth front in correspondence with the shape of the isotherms. A large radius for the curvature of the isotherms enhances the formation of an extended facet. Under the facet, the lattice planes are flat with a high crystal quality as expressed by rocking-curve half widths of 0.022°. In the non-faceted region, the lattice planes become bent, following the shape of the isotherms with a radius of typically 0.5 to 0.8 m and an increased rocking-curve half width of 0.3°. A reduction of the growth rate from 300 µm h−1to 70 µm h−1does not affect this behaviour significantly. The lattice-plane curvature and the development of the facet are predominantly affected by the shape of the isotherms. For crystals grown in the [015] direction, the lattice planes adjust only in a one-dimensional manner to the isotherms. In all cases, the lattice-plane curvature results from the formation of a high density of small-angle grain boundaries. They are generated by the condensation of dislocations with Burgers vectors in theabplane.