Dendrite growth processes of silicon and germanium from highly undercooled melts

The crystal growth behavior of a semiconductor from a very highly undercooled melt is expected to be different from that of a metal. In the present experiment, highly pure undoped Si and Ge were undercooled by an electromagnetic levitation method, and their crystal growth velocities (V) were measured as a function of undercooling (ΔT). The value of V increased with ΔT, and V=26 m/s was observed at ΔT=260 K for Si. This result corresponds well with the predicted value based on the dendrite growth theory. The growth behaviors of Si and Ge were found to be thermally controlled in the measured range of undercooling. The microstructures of samples solidified from undercooled liquid were investigated, and the amount of dendrites immediately after recalescence increased with undercooling. The dendrite growth was also observed by a high-speed camera.     

The brittle-to-ductile transition in silicon

In order to better understand the brittle-to-ductile transition in silicon, details of different experimental and numerical studies are discussed. In the concept of elastical stress intensity factors, experiments with four-point-bending bars are not directly comparable with experiments done with tapered-double-cantilever-beam samples. The internal elastic and plastic conditions of the samples are different due to large differences of the load levels applied in both cases. A smooth transition from cleavage to semi-brittle fracture with small, relatively homogeneous plastic zones precedes a sharp transition to general plasticity. The fracture toughness between both transitions is independent of temperature, yielding a plateau region. Numerical simulations qualitatively show the same results. The plateau region is a direct consequence of the shielding dynamics, the sharp transition is a consequence of the assumed emission configuration of the dislocations. The simulations do not rely on a heterogeneous distribution of dislocation sources.     

Direct observation of the separation process by the chromato-videoscope

Separation process in a liquid chromatographic column were visualized and analyzed by a developed chromato-videoscope. The migration aspects were evaluated with successively obtained densitograms. In reversed-phase chromatography, the band widths of each solute band were almost equal fore both weakly and strongly retained solutes when compared at the same column position (the same migration distance in the column). In the gradient elution mode, the position of the solute band showed that the migration velocity of the solute band changed gradually according to changes in solvent composition. The drug trapping process to BSA- coated ODS packings for direct injection of biological fluid was also observed. In the absence of BSA from the sample solution, the drug molecules were trapped in a narrow band. However, at higher BSA concentrations in the sample solution, a broader band shape was observed. This band broadening shows how the drug molecules were retained on the protein.     

Direct observation of protein-ligand interaction kinetics

Internal dynamics on the micro- to millisecond time scale have a strong influence on the affinity and specificity with which a protein binds ligands. This time scale is accessible through relaxation dispersion measurements using NMR. By studying the dynamics of a protein with different concentrations of a ligand, one can determine the dynamic effects induced by the ligand. Here we have studied slow internal dynamics of the N-terminal src homology 2 domain of phosphatidylinositide 3-kinase to probe the role of individual residues for the interaction with a tyrosine-phosphorylated binding sequence from polyoma middle T antigen. While slow dynamic motion was restricted to a few residues in the free SH2 and in the SH2 complex, motion was significantly enhanced by adding even small amounts of ligand. Kinetic rates induced by ligand binding varied between 300 and 2000 s(-1). High rates reflected direct interactions with the ligand or rearrangements caused by ligand binding. Large differences in rates were observed for residues adjacent in the primary sequence reflecting their individual roles in ligand interaction. However, rates were similar for residues involved in the same side chain interactions, reflecting concerted motions during ligand binding. For a subset of residues, exchange must involve structural intermediates which play a crucial role in high-affinity ligand binding. This analysis supports a new view of the dynamics of individual sites of a protein during ligand interaction.     

Nucleation in undercooled Ag-Ge alloys

Experiments with bulk samples of Ag-Ge alloys have shown that the silver solid solution will undercool ∼11°C belowT _e in the presence of primary germanium. These data agree with the results of Sundquist and Mondolfo.1 On the other hand, a modified single crystal substrate method similar to that used by Wagner2 has shown that large droplets (0.25 to 2.5g) of hypereutectic Ag-Ge alloy melted on a single crystal of germanium undercooled to a maximum of 56°C belowT _e. This result is closer to Wagner’s data and in apparent conflict with the bulk sample results and those of Sundquist and Mondolfo.1 An explanation is proposed for this discrepancy. Formerly with the Thayer School of Engineering, Dartmouth College, Hanover, N. H.     

Criterion for silicon formation in transition metal-silicon diffusion couples

In this paper, a criterion for silicon formation in metal-silicon diffusion couples, based on the rate of change of free energy, referred to here as the free energy degradation rate (FEDR), has been developed from a kinetic model for silicon formation. In the kinetic model, silicon formation is divided into three steps: diffusion of the predominant diffuser (or moving reactant) to the reactive interface, followed by release of the less mobile species (non-moving reactant) from its lattice and intermixing with the moving reactant, and finally formation and growth of the silicon phase. It has been shown that the free energy change due to silicon formation in a diffusion couple can be determined by examining the free energy change of the reaction region (or reactive interface) located between the growing silicon and the non-moving reactant phase. The free energy degradation rate per unit area of a given reaction region can be expressed as a sum of three contributions, each corresponding to one of the three steps. Each term is a product of a thermodynamic flux and a driving force. These fluxes and driving forces have been examined individually; by analyzing how they change with time, it is shown that when a number of possible reactions compete with one another in a reaction region, there always exists a reaction that will result in the largest FEDR in this region. It is also shown that the largest FEDR leads the system to a relative minimum free energy state that is most stable compared with any other energy state at a given instant.Based on these results, a criterion for silicon reactions has been proposed. During silicon reaction in a reaction region of a metal-Si diffusion couple, there are always a number of possible reactions competing with one another. The reactions which result in the largest FEDR will actually occur. This criterion combined with the new kinetic model has been successfully applied to predict silicon formation in 15 metal-Si systems.