Dendritic Polycrystalline Research Articles

A phase-field simulation of the solidification process under compression

Semi-solid metal forming processes cover a wide variety of technologies in manufacturing, generally leading to non-dendritic microstructures typified by rosette-like or spherical grains. With the contemporary experimental methods, researchers still cannot completely understand the formation of such microstructures. However, as a process coupled with dendritic growth, deformation and flow, the microstructural evolution during the semi-solid deformation has been scarcely simulated. With the vector-valued phase-field method, two-phase flow model and an innovative design of boundary conditions, the current research numerically compressed a single dendrite into a less dendritic polycrystalline structure. The resulting microstructure was consistent with the metallographic analysis and the simulation enables a clear observation of the whole process: the ripening transformed the highly branched shape into a less dendritic one; the subsequent deformation crushed the dendrite into a stubby structure; eventually, the deformation caused the boundaries inside this structure, dividing the solid into multiple grains. The analysis on the boundaries inside the deformed solid offered further insight into the formation of non-dendritic structures. These boundaries showed a tendency towards melting with increasing misorientation, demonstrating the possibility of the grain boundary wetting, which was considered as the source of the fragmentation when the dendrite undergoes deformation, but is hard to validate in situ even with the cutting-edge X-ray imaging technique.

Thermoelectric Nanowires Based on Bismuth Telluride

Bismuth antimony telluride (BixSb2-xTe3) and bismuth tellurium selenide (Bi2Te3-xSex) nanowires, witha 60nmdiameter, have been potentiostatically electrodeposited from perchloric baths using anodic alumina membrane. For n-type nanowires, gradual selenium concentration studies have established which one gives the closest stoichiometry to Bi2Te2.7Se0.3. Seebeck coefficients reached-70μV/K after annealing. For p-type nanowires, some samples show Seebeck coefficients higher than +160μV/K. Nanowires morphology and stoichiometry were investigated by SEM and EDX, while structure and phases were determined by XRD.First electrical conductivity measurements exhibit a high resistivity for p-type nanowires.The problem of contact resistance between nanowires and metals is revealed through the resistance measurement of single and assembled nanowires.This work reveals the strong dependence of nanowires chemical composition and morphology on deposition potential. For the low deposition over-potential, the deposition rate is fairly slow and nanowires are highly oriented with a compact cylindrical aspect; whereas for higher deposition over-potentials, fast growth rate leads to dendritic polycrystalline structures.

Weak superconductivity in the surface layer of a bulk single-crystal boron-doped diamond

We have grown and investigated bulk single-crystal heavily boron-doped diamonds possessing superconductivity with . Only the surface layer with the thickness less than showed the degenerate semiconductor behavior with transition to the superconducting state, while the bulk of the crystal was a typical doped semiconductor. The morphology of the surface layer is dendritic polycrystalline with an average boron content of 2.5–2.9 at.%. The typical Josephson junction current-voltage characteristic was observed. The degenerate semiconductor-superconductor transition as in single-crystal high-temperature superconductors and the structural data analysis of the surface layer indicate the two-dimensional character of superconductivity, and the actual superconducting structure is a set of few-nanometer thick boron carbide layers embedded in a diamond structure.

Preparation of a Preplated Bismuth Film on Pt Electrode and its Application for Determination of Trace Aluminum(III) by Adsorptive Stripping Voltammetry

AbstractAdsorptive stripping voltammetry (AdSV) was successfully explored for the determination of aluminum(III) in sodium carbonate solution (1 M) on a preplated bismuth film electrode (BiFE). Microscopic investigation displays that the morphology of the preplated bismuth film on Pt electrode relates to the plating conditions. The resulted perfect dendritic polycrystalline BiFE demonstrates better performance than the flake single crystal BiFE in AdSV determination of Al(III) in the presence of Eriochrome blue black R (EBBR). Under the selected conditions, the limit of detection(3σ) for Al(III) on flake and dendrite BiFE are 1.02×10−8 M and 5.6×10−10 M, respectively, and the current variation ratio of the coexistent ions(CaII, VV, SiIV, MgII, FeII and FeIII) is less than ±0.700 % in all cases.

Effect of electron irradiation on nanogroove-networked single-crystalline and dendritic polycrystalline platinum nanosheets prepared from lyotropic surfactant liquid-crystal templates

Using transmission electron microscopy (TEM) electron irradiation effects were studied for nanogroove-network structured single-crystalline and dendritic polycrystalline Pt nanosheets 50–60 nm in size. These two nanosheets with nearly the same average groove-width or dendritic spacing of 1.3–1.4 nm were prepared from the mixed and single surfactant liquid crystalline templates, respectively. On exposure to electron beam for 20 min at the acceleration voltage of 200 kV, the nanogrooved nanosheets were morphologically little affected, but the dendritic ones were transformed into less branched polycrystalline structures with spacings distributed around ∼1.7 nm. The shape transformation of the latter occurred by the combined mechanism of segmental migration and atomic diffusion. These observations indicate that the nanogrooved Pt nanosheets are highly stabilized by the grooved but crystallographically continuous Pt framework, leading to their extremely high thermo-resistance, in marked contrast to the polycrystalline dendritic structures constructed of crystallographically discontinuous linkages of nanoblocks.

Prediction of the conformation of ice-nucleation protein by conformational energy calculation.

The active conformation of an ice-nucleation protein, whose major portion consists of a long polypeptide segment of nearly repetitive octapeptides, is predicted by the analyses of conformational energy and the mechanism of crystal growth. The protein ideally has an exact octapeptide repetition and is assumed to have a helical conformation. The present study searched for low-energy helical conformations and each of the obtained low-energy conformations examined as to whether it has a surface structure that can promote crystal formation. Two conformations obtained were good candidates for an ice nucleus. Both were found to have on their surfaces an arrangement of hydrogen-bonding sites, which fits well with those of hydrogen bonds in hexagonal ice crystal. Further, one of the two conformations had a hexagonal conformational symmetry consistent with the hexagonal ice crystal structure. The other conformation had a pentagonal conformational symmetry that could enable the growth of an ice crystal--dendritic polycrystalline snow crystal--which grows on metastable cubic ice.

Thin film crystal growth of Si on fused silica: Effects of growth front dynamics on crystallography

Reflective high speed video microphotography was used to study the advance of the resolidification front during CO2 laser melting of encapsulated polycrystalline silicon thin film structures on bulk fused silica substrates. Prepatterned silicon films were laser recrystallized, removed from the substrate, and then examined by transmission electron microscopy. The crystal structure which results from seeded and unseeded growth and from various liquid-solid growth front shapes was characterized. Supercooling of the melt and subsequent dendritic polycrystalline regrowth is observed in cases where nucleation occurs in isolated molten regions. Transmission electron micrographs demonstrate the necessity of providing controlled seeding conditions, such as regrowth through a narrow neck of silicon. A flat liquid-solid interface supports a [111] growth direction whereas concave thermal profiles cause twinning or low angle grain boundaries in the films.