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Thermal conduction was investigated in titanium-chromium oxide natural superlattices with an ordered arrangement of crystallographic shear planes, which were nearly pristine interfaces for thermal phonons. Thermal conductivity exhibited a clear maximum value at around 50 K, which indicates interface roughness as small as 20 pm based on the calculation of thermal conductivity by a modified Debye-Callaway model. With an ordered arrangement of nearly pristine interfaces at nanoscale, the wave contribution to phonon transport was estimated to be predominant at lower temperatures (less than 100 K) and as much as 30 % even at room temperature. We found that the interface roughness greatly influences the thermal conduction in natural superlattice systems, which may lead to variations in the reported value of thermal conductivity. We have demonstrated that the wave contribution of thermal conductivity become apparent in bulk crystal having nano scale periodic structure with pristine interfaces.

We have investigated thermal conduction in bulk titanium oxide natural superlattices with crystallographic shear (CS) structures, in which dense planar faults are introduced with different periodicities, prepared by reductive annealing of rutile TiO2 and crystal growth by the floating zone method. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) revealed that (132)rutile and (121)rutile CS planes with interspacings of 2.7 and 1.0 nm were introduced in the mother rutile structure. Time-domain thermoreflectance (TDTR) revealed that the thermal conductivity decreased by the introduction of CS planes, but that the decrease is not monotonic with increasing density of CS planes. Calculation of the thermal conductivity and the mean free path for phonons revealed that a crossover from incoherent to coherent thermal conduction took place, and coherent interfaces with nanoscale periodicity were formed as thermodynamically stable phases in bulk titanium oxide natural superlattices., Available online 11 October 2021.

For solution growth of silicon carbide, it is significant to understanding the evolutionary mechanism of step bunching. This study infers that solute’s incorporation into steps and transport together determines step bunching progress. The occurrence of step bunching is due to the depletion of solute in the region with high step density, caused by a high step kinetic coefficient. On the other hand, by promoting the transport of the solute in the solution, the step speed becomes uniform, thereby the step bunching can be prevented. Furthermore, we proposed a non-dimensional Damköhler number for crystal growth in step-flow mode. It correlates incorporation rates with bulk diffusion rates and can build a phase map of growth rates and step bunching stability. Several solvents are located in the phase map, demonstrating the possible usage of the phase map as a pointer for solvent designing.

The macroscopic distribution of fluid flows, which affect the quality of final products for various kinds of materials, is often difficult to describe in mathematical formulae and hinders the implementation of empirical knowledge in scaling up. In the present study, the characteristics of the flow distribution in silicon carbide (SiC) solution growth are described by using the position of the saddle point and the solution growth conditions are optimized by computational fluid dynamics simulation, machine learning, and a genetic algorithm. As a result, the candidates of the optimal condition for the solution growth of 6-in. SiC crystals are successfully obtained from the empirical knowledge gained from 3-in. crystal growth, by adding the topological description to the objective function. The present design of the objective function using the topological description can possibly be applied to other crystal growth or materials processing problems and to overcome scale-up difficulties, which can facilitate the rapid development of functional materials such as SiC wafers for power device applications.