Silicon (Si) clathrate compounds are composed of host Si atoms organized in three-dimensional frameworks and guest atoms enclosed in the Si cage-like frameworks. In the present study, single ...crystals of Si clathrates containing Na atoms, type-I (Na8Si46) and type-II (Na24Si136), were synthesized by using a Na-Sn flux. The single crystals having facets grew up to a few millimeters in size by evaporating Na from a Na-Si-Sn solution at 723–873 K. This synthesis procedure enabled selective synthesis of type-I and -II clathrates by controlling the heating temperature. One of the obtained single crystals was used as a seed for the following crystal growth of the type-II clathrate. The octahedral single crystal surrounded by {111} facets with a size of 3 mm was obtained on the seed crystal. Novel ternary clathrate, Na8(Ga/Si)46, and quaternary clathrates, (Sr/Na)8(Ga/Si)46, were synthesized by using Na-Ga-Sn composite fluxes.
Twin boundary formation at grain boundaries in multicrystalline Si during directional solidification was investigated by in situ observation of the crystal/melt interface. It was clearly shown that a ...twin boundary was formed on the {111} facet of grain-boundary groove at the crystal/melt interface. The large amount of undercooling in the melt at grain-boundary grooves promoted rapid crystallization and twin boundary formation.
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The top surface of a crystal–melt interface was observed directly during directional solidification of polycrystalline GaSb at a constant cooling rate, and the effect of {111}Σ3 twin boundary ...formation on the rate of grain growth parallel to the 〈111〉 direction was investigated. A {111}Σ3 twin boundary was generated by the decomposition of another {111}Σ3 twin boundary with formation of a Σ9 grain boundary. The growth of the crystal–melt interface in the direction parallel to the B direction was stalled during the nucleation and propagation of the new {111}Σ3 twin boundary. The growth rate of the crystal–melt interface after twin formation was approximately half of that before. This is considered to be due to {111} polarity reversal by twinning. The dangling bond density on the {111}A plane of GaSb is almost ten times of that on the {111}B plane, and the dangling bond density is related to the attachment energy of atoms of the plane; therefore, the growth rate in the A direction would be significantly lower than that in the B direction. These results indicate different growth rates in opposite 〈111〉 directions and a significant effect of the polarity on the competition of grain growth during the directional solidification of polycrystalline GaSb.
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Eutectic growth of Ni-Si alloy with a composition of Ni44Si56 was observed in situ during directional solidification. Lamellar structure was evident behind the solidifying interface, which indicates ...that the two intermetallic phases, NiSi and NiSi2, generated a regular eutectic. The lamellae spacing was approximately 9.8 μm at a growth rate of 149.6 μm s−1. The phase boundaries migrated rapidly during the cooling stage and the lamellae enlarged significantly, which suggests that the lamellae spacing measured after the solidification may not be accurate for obtaining the relationship between the growth rate and the lamellae spacing. The in situ observation provides a promising way to obtain the exact eutectic spacing of Ni-Si eutectics.
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•Facet formation during solidification of pure Sb was directly observed.•The faceted solid–liquid interface of pure Sb is bounded by {11¯02}•Mullins-Sekerka instability was not identified on the ...Sb{11¯02}•The α factor in the Jackson model does not explicitly predict the growth form.
In contrast to metals and semiconductors, few studies have focused on the faceting behavior of semimetals, and the atomic configuration of the solid–liquid interface is still incompletely understood. In this study, to obtain insight into the growth dynamics of semimetals, we directly observed solid–liquid interfaces during directional solidification of pure antimony. Morphological transformation of the solid–liquid interface depends on the direction of solidification: a zig-zag shaped solid–liquid interface is formed after the onset of instability in the direction of solidification normal to {312¯925}, while a curved interface becomes planar and no zig-zag facet formation is identified in the direction of solidification normal to {811¯324}. By combining in-situ observation and ex-situ electron-backscattered diffraction techniques, we showed that zig-zag shaped faces formed after the onset of instability at the solid–liquid interface are bounded by equivalent {11¯02} planes. The fact that the slowest growth kinetics occurs on the {11¯02} plane was also confirmed by measuring the growth velocity of the solid–liquid interface and the rate of temperature decrease in an observation furnace. Calculations based on thermal diffusion equations show that the Mullins-Sekerka instability does not occur on the {11¯02} plane, indicating that the growing {11¯02} plane at the solid–liquid interface is faceted.
A novel synthesis technique, called the high-pressure diffusion control (HPDC) method, was developed in this study. The method combined the high-pressure synthesis using a cubic anvil apparatus and ...an anisotropic diffusion control technique; the electrical processing in high-pressure and high-temperature environments of up to 4 GPa and over 1000 °C is enabled by simultaneously adjusting the temperature, pressure, and voltage. This nonequilibrium state is effective in creating metastable materials. The developed novel technique was applied to polycrystalline NaAlB14 with a boron covalent framework. Although electronic conduction is dominant in this material and no Na-ion conduction is observed even at high temperatures, the HPDC method successfully extracted Na ions by utilizing the difference in bond strength between Na and B, creating the metastable material AlB14 while maintaining its basic crystal structure. During the decrease in the Na concentration, applying a high pressure compressed the sample according to the volume change and maintained good contact at the intergrain boundary in the polycrystalline sample, promoting Na-ion diffusion. The Na extraction functioned as electron carrier modulation and significantly reduced the electrical resistivity. The developed HPDC method is expected to be applicable to various compounds with a difference in the bond strength between constituent elements and has the potential to open up new avenues in the inorganic synthesis of polycrystalline metastable materials with dense sintered states and modulate their physical properties.
In this work, crystal–melt interfaces were directly observed during the directional solidification of molten GaSb having a stoichiometric composition, and the formation of dendrite structures was ...studied. With increases in growth velocity, the planar crystal–melt interface became unstable and dendrite growth was initiated. Dendrites having {111} facets were found to grow in either the 〈110〉 or 〈112〉 directions, and a pair of twin boundaries was identified at the center of a dendrite. The results show that the growth of dendrites in GaSb is associated with nucleation at twin-related reentrant corners.
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Single crystals of the type I clathrate Na8Si46 were synthesized by the evaporation of Na from a Na–Si–Sn solution (Na:Si:Sn = 6:2:1 molar ratio) at 723–873 K under an Ar atmosphere of 105 Pa. In the ...mixture of type I and type II clathrates prepared by heating at 773 K for 72 h, type I single crystals with sizes of up to 5 mm were found. A single phase of single crystals of the Na24Si136 type-II clathrate having a {111} crystal facet of about 2 mm on one side was obtained by heating at 873 K for 9 h.
The grain boundary groove at the crystal/melt interface in pure Cu was observed in situ to measure the grain boundary energy at the melting temperature. It was observed that the shape of the groove ...changed upon solidification even though the planar crystal/melt interface was maintained. The crystal/melt interface was kept in equilibrium through precise control of the temperature, which enabled the measurement of the grain boundary energy on the basis of the shape of the equilibrated groove. The low-angle grain boundary energy at the melting temperature of Cu was obtained.
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