The formation of multiple parallel twin boundaries at grain boundary grooves during Si solidification was investigated using an in situ observation system. Twin boundaries were repeatedly generated ...at a crystal/melt interface from a step-like grain boundary during solidification. The growth velocity of the micro-facet at the grain boundary groove accelerated and decelerated, which resulted in the nucleation of a twin and the development of a step-like structure at the grain boundary. The undercooling for multiple parallel twin formation as estimated based on experimental observation and the result was further discussed by comparison with previous studies.
The generation of a {112}Σ3 grain boundary (GB) was observed in situ from the decomposition of a Σ9 GB during directional solidification of multicrystalline Si. A faceted groove formed at the ...junction of the solid/melt interface and the {112}Σ3 GB. This mechanism is different from that for the growth of {111}Σ3 GBs, for which no groove formed at the interface. If the growth rates for the adjacent facets of the groove are the same, the GB can grow in a straight manner along the {112} plane. The present results suggest that kinetics can give rise to high-energy GBs during solidification.
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Si0.7Ge0.3 was prepared by rapid cooling (∼330 °C/min) to get the fine grain structure for high thermoelectric performance. The same material was prepared under slow cooling (1 °C/min) for ...comparative analysis. The results were out of our expectation as the EBSD pattern revealed almost same grain structure of both samples. To know the reason for the similar grain structures, crystal growth of Si0.7Ge0.3 sample was in-situ observed under rapid and slow cooling. During rapid cooling, initially grown fine dendrites were completely re-melted and recrystallized on further cooling. No re-melting was occurred in Si under rapid cooling. Due to the growth of Si-rich Si1-xGex dendrites at high temperature the melt becomes Ge richer thereby initially grown fine dendrites were re-melted to reach equilibrium liquidus composition. Moreover, the similar grain structures of Si1-xGex under rapid cooling as well as slow cooling was originated from the re-melting of initially grown fine dendrites and recrystallization at relatively low temperature.
•Si1-xGex was prepared under rapid (330 °C/min) and slow cooling (1 °C/min) processes.•EBSD of both samples show same grain structures despite of different cooling rates.•To understand the grain structure formation, Si1-xGex growth was in-situ observed.•Re-melted was observed in Si1-xGex, while Si was not re-melted under rapid cooling.•Same grain structures were attributed due to re-melting and recrystallization.
Impurity partitioning at grain boundaries (GBs) during polycrystalline colloidal crystallization has been investigated via direct observation. Polycrystalline grains have a partitioning behavior ...similar to that of single colloidal crystals, which follows the Burton, Prim, and Slichter (BPS) model. We have found that impurities segregate at GBs during colloidal crystal growth, and the impurity concentration at GBs (C GB) for various misorientation angles (θ) between adjacent grains and growth rates (V) has been investigated. C GB was found to increase with either increasing θ or V, and also when the size of the impurity is close to that of the host colloid particle. In situ observations reveal that impurities incorporated into GBs are supplied mostly from the impurities segregated at the solid–liquid interface, and C GB and the growth rate show a BPS-like relationship.
Type-II Na24Si136 clathrate octahedral single crystals surrounded by {111} facets were grown by evaporating Na from a molten mixture of Na4Si4 and Na9Sn4 at 823 K for 12 h. One of the obtained single ...crystals was used as a seed for the following single crystal growth of the type-II clathrate using the same method. The single crystal grown on the seed maintained the octahedral shape. The weight of the crystal grown with the seed was increased from 0.6 to 30.4 mg by repeating the seeded growth and was proportional to the surface area of the seed crystal.
Dislocations in Si multicrystals strongly affect the efficiency of solar cells, and are usually generated from random grain boundaries during crystal growth. The low density of random grain ...boundaries and the coherency of random grain boundaries are very important in suppressing dislocations. Controlling arrangement of dendrite crystals grown along the bottom of ingots is effective for decreasing the density of random grain boundaries and for improving the coherency of random grain boundaries. A method of controlling thermal conductivity under crucibles to control the arrangement of dendrite crystals was proposed. Graphite plates with different thermal conductivities were used all over the bottom surface of crucibles. Two types of graphite plates, one with a line-shaped highly cooled part and the other with a ring-shaped one, were used. Using the graphite plates, the distribution of dendrite crystals was well arranged, and dendrite crystals were controlled to be fairly parallel to each other.
The effect of grain boundary characteristics on the crystal/melt interface morphology during the unidirectional solidification of multicrystalline Si was studied by in situ observations. It was shown ...that sharp and smooth grooves were formed at the crystal/melt interfaces at Σ27 and random grain boundaries, whereas no grooves were formed at Σ3 {111} twin boundaries. We explain the reasons why the grooves form at grain boundaries and discuss the impurity segregation at the grain boundary grooves on the crystal/melt interface.
We propose a new concept of growing a polycrystalline Si ingot suitable for solar cells by casting based on the directional growth behavior of polycrystalline Si investigated using an in situ ...observation system. The cooling conditions for the Si melt were crucial for controlling growth in the initial stage. At a high cooling rate, dendrite growth occurred along the crucible wall. This growth mechanism was found to be useful for obtaining a polycrystalline structure with large oriented grains. In fact, using the dendrites grown in the initial stage of casting, a polycrystalline Si ingot with large oriented grains was obtained. The solar cell properties of such structure-controlled polycrystalline Si were as good as those of single-crystalline Si.
The instability of a crystal–melt interface during the directional growth of pure antimony was studied using an in-situ observation technique. The morphology of the crystal–melt interface was planar ...at a low growth velocity, while the interface transitioned to wavy and then zig-zag faceted at 60 μm s−1. Calculating the thermal field at the crystal–melt interface using a diffusion equation showed that the temperature gradient in the melt becomes negative when the moving velocity of the interface is higher than 52 μm s−1. These results demonstrate that perturbations introduced into the planar interface are amplified by a negative temperature gradient and these amplified perturbations result in zig-zag facets.
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