Investigations on the Nucleation Thermodynamics of RbTiOXO4 (X = P or As) Crystals Grown from High Temperature Solution

The nucleation thermodynamics of RbTiOXO₄ (where X = P or As) family crystals crystallizing from high temperature solution using the phosphate and tungstate solvents have been studied. Using the regular solution model and classical nucleation theory the nucleation thermodynamical parameters like interfacial energy, chemical potential, free energy change, critical energy barrier and radius of critical nucleus have been calculated which leads to better understanding of the nucleation process. Comparative study has also been made to investigate the metastable zone width of the above family crystals grown from different fluxes.     

Zirconia‐Alumina Nanoparticles Prepared by Laser Evaporation: Powder Characterisation by TEM and 27Al MAS NMR

Contrary to predictions of the ZrO₂‐Al₂O₃ phase diagram powders of solid solutions of alumina in zirconia with up to 40 mol% Al₂O₃ are described in the recent literature. Up to now the interpretations of the microstructure of these materials are still inconsistent. Therefore we reinvestigated nanopowders prepared by laser evaporation starting from a mixture of 55 mol% ZrO₂ and 45 mol% Al₂O₃ by analytical high resolution transmission electron microscopy (TEM) and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). The results reveal that in case of this preparation route a heterogeneous powder system is formed consisting dominantly of nanocrystallites of zirconia with an incorporation of, obviously, only a very small amount of alumina, and that the crystallites are surrounded by an amorphous layer of predominantly alumina. No indication was found that Al³⁺ ions occupy seven‐ or eight‐fold co‐ordinated sites of Zr⁴⁺ ions. A simple kinetic model for the formation of the nanoparticles' microstructure is given.     

FCC铝纳米压痕的多尺度模拟

采用准连续介质方法模拟面心立方(FCC)铝单晶薄膜在纳米压痕下产生的变形过程.分别用四种不同的压头宽度,得出载荷-位移响应曲线和应变能变化曲线,发现压头宽度越大,晶体产生塑性变形的临界载荷越大;临界载荷的大小和采用能量理论预测的大小基本一致;模拟过程中,观察到位错成核现象,了解到载荷-位移响应曲线的突降是由位错成核现象所引起,四种情况中压头载荷的降幅大致相同;最后分析了模型在原子层次下的变形机理.     

Temperature Imaging in an Unsteady Propane-Air Counterflow Diffusion Flame Subjected to Low Frequency Oscillations

Current emphasis in non-premixed turbulent combustion research is focused on the effects of hydrodynamic unsteadiness and transient effects in these flames. To address these effects, measurements of the temperature field in unsteady propane-air flames were made using planar laser-induced fluorescence of the hydroxyl radical to ascertain the effect of fluctuating hydrodynamics on flame temperature. Planar temperature measurements were made at four temporal locations within the 25 Hz velocity fluctuation as a function of initial steady strain rate and forcing amplitude. Results show that temperature in the propane flame is rather insensitive to initial strain rate for these weakly strained flames due to the balance between decreased heat release rate and reduced radiative losses resulting from diminished soot production as the strain rate is increased. The temperature is significantly influenced by the imposed velocity oscillation, however, which causes large fluctuations in the instantaneous strain rate. A decoupling of peak flame temperature and maximum PAH and excited CH concentration indicates significant transient effects resulting from the unsteady flow field even at these low oscillation frequencies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Particle formation under monomer‐starved conditions in the semibatch emulsion polymerization of styrene. I. Experimental

Particle formation and particle growth compete in the course of an emulsion polymerization reaction. Any variation in the rate of particle growth, therefore, will result in an opposite effect on the rate of particle formation. The particle formation in a semibatch emulsion polymerization of styrene under monomer‐starved conditions was studied. The semibatch emulsion polymerization reactions were started by the monomer being fed at a low rate to a reaction vessel containing deionized water, an emulsifier, and an initiator. The number of polymer particles increased with a decreasing monomer feed rate. A much larger number of particles (within 1–2 orders of magnitude) than that generally expected from a conventional batch emulsion polymerization was obtained. The results showed a higher dependence of the number of polymer particles on the emulsifier and initiator concentrations compared with that for a batch emulsion polymerization. The size distribution of the particles was characterized by a positive skewness due to the declining rate of the growth of particles during the nucleation stage. A routine for monomer partitioning among the polymer phase, the aqueous phase, and micelles was developed. The results showed that particle formation most likely occurred under monomer‐starved conditions. A small average radical number was obtained because of the formation of a large number of polymer particles, so the kinetics of the system could be explained by a zero–one system. The particle size distribution of the latexes broadened with time as a result of stochastic broadening associated with zero–one systems. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3940–3952, 2001     

Growth and characterization of a novel organic NLO crystal: 4 – ethoxy benzaldehyde – n – methyl 4 – stilbazolium tosylate

The organic material 4‐Ethoxybenzaldehyde‐N‐methyl 4‐Stilbazolium Tosylate (EBST) is a new NLO material and new derivative in Stilbazolium Tosylate family. In this work we synthesized the EBST, the derivative of DAST. By slow evaporation method, we have grown the EBST crystal. Powder XRD confirms the crystalline property, the lattice parameters are calculated from single crystal XRD data and the molecular structure also revealed. The crystal system is found as monoclinic. The crystalline perfection is assessed by the high‐resolution X‐ray diffractometry. A single and reasonably sharp peak observed in the diffraction curve indicates that the quality of the crystal is quite good without having any internal structural grain boundaries. The FTIR and proton NMR study confirm the presence of functional groups. From the UV – Vis Far IR absorption spectra the good transparency is revealed. The Kurtz Perry SHG test confirms the NLO property of the EBST crystal grown and it is 11 times greater than urea. The melting point of the grown crystal is found to be 237°C from the DSC curve. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Probabilistic approach to protein crystal nucleation

Simple probabilistic model for step‐wise growth of polymers is used for making some parallels with the nucleation of protein crystals. Although the considerations are made within 1D case, this approximation still shows some important peculiarities of protein crystal nucleation and growth. Thus, the present approach turns out to be useful for interpretation of some important experimental results regarding these processes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations on nucleation,thermodynamical parameters and growth of Benzimidazole crystals from low temperature solution

Metastable zonewidth and induction period measurements of Benzimidazole (BMZ) are presented. The nucleation parameters such as interfacial tension, radius of the critical nucleus and critical free energy change have been calculated for the solution grown Benzimidazole (BMZ) single crystals at different supersaturation ratios. The grown BMZ crystals were characterized by the differential scanning calorimetric studies for analysis of thermal properties. The dielectric behaviour of the crystal was studied at different temperature and frequency. The laser damage threshold studies show that BMZ crystal has higher laser damage threshold. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Protein structural codes and nucleation sites for protein folding

One of the long-standing controversial arguments in protein folding is Levinthal's paradox. We have recently proposed a new nucleation hypothesis and shown that the nucleation residues are the most conserved sequences in protein. To avoid the complicated effect of tertiary interactions, we limit our search for structural codes to the nucleation residues. Starting with the hypotheses of secondary structure nucleation and conservation of residues important for folding, we have analysed 762 folds classified as unique by SCOP. Segments of 17 residues around the top 20% conserved amino acids are analysed, resulting in approximately 100 clusters each for the main secondary structure classes of helix, sheet and coil. Helical clusters have the longest correlation range, coils the shortest (four residues). Strong specific sequence-structure correlation is observed for coil but not for helix and sheet, suggesting a mapping relationship between the sequence and the structure for coil. We propose that the central sequences in these clusters form `structural codes', a useful basis set for identifying nucleation sites, protein fragments stable in isolation, and secondary structural patterns in proteins (particularly turns and loops).