Intensification of the freeze drying process by the control of both freezing and primary drying steps

The problem of optimization of freeze-drying cycles is addressed, with emphasis in both freezing and primary drying steps. In particular, this study shows that the control of the nucleation event produces more uniform batches (as ice nucleation is induced in all the vials of batch almost at the same time and temperature) and allows a marked reduction in the duration of the optimized cycle (if compared to cycles carried out with conventional stochastic nucleation

Cholesterol nucleation time in gallbladder bile of patients with solitary or multiple cholesterol gallstones

Patients with multiple cholesterol gallbladder stones have been found to be at a higher risk for the recurrence of gallstones after successful nonsurgical treatment than those with a solitary stone. Cholesterol gallstone recurrence, like primary gallstone formation, probably involves a triple defect with supersaturation, abnormally rapid nucleation of cholesterol in bile and altered gallbladder motor function. We investigated whether the increased recurrence rate of patients with multiple stones might be caused by more rapid nucleation. Therefore the time required for cholesterol monohydrate crystals to appear in ultracentrifuged bile of patients with solitary (n = 71) or multiple (n = 42) cholesterol gallstones was determined. The cholesterol nucleation time was significantly (p 4 days) nucleation time. However, no difference in the cholesterol saturation index was found between the bile samples from patients with solitary stones and the bile samples from patients with multiple stones (1.55 ± 0.65 vs. 1.54 ± 0.59, mean ± S.D., respectively). The more rapid cholesterol nucleation in gallbladder bile may, therefore, be the major risk factor causing the higher percentage of stone recurrence in patients with multiple cholesterol stones as compared with patients with solitary cholesterol stones

Monte Carlo simulation of recrystallization

A Monte Carlo computer simulation technique, in which a continuum system is modeled employing a discrete lattice, has been applied to the problem of recrystallization. Primary recrystallization is modeled under conditions where the degree of stored energy is varied and nucleation occurs homogeneously (without regard for position in the microstructure). The nucleation rate is chosen as site saturated. Temporal evolution of the simulated microstructures is analyzed to provide the time dependence of the recrystallized volume fraction and grain sizes. The recrystallized volume fraction shows sigmoidal variations with time. The data are approximately fit by the Johnson-Mehl-Avrami equation with the expected exponents, however significant deviations are observed for both small and large recrystallized volume fractions. Under constant rate nucleation conditions, the propensity for irregular grain shapes is decreased and the density of two sided grains increases

Explaining global surface aerosol number concentrations in terms of primary emissions and particle formation

We use observations of total particle number concentration at 36 worldwide sites and a global aerosol model to quantify the primary and secondary sources of particle number. We show that emissions of primary particles can reasonably reproduce the spatial pattern of observed condensation nuclei (CN) (R2=0.51) but fail to explain the observed seasonal cycle at many sites (R2=0.1). The modeled CN concentration in the free troposphere is biased low (normalised mean bias, NMB=−88%) unless a secondary source of particles is included, for example from binary homogeneous nucleation of sulfuric acid and water (NMB=−25%). Simulated CN concentrations in the continental boundary layer (BL) are also biased low (NMB=−74%) unless the number emission of anthropogenic primary particles is increased or an empirical BL particle formation mechanism based on sulfuric acid is used. We find that the seasonal CN cycle observed at continental BL sites is better simulated by including a BL particle formation mechanism (R2=0.3) than by increasing the number emission from primary anthropogenic sources (R2=0.18). Using sensitivity tests we derive optimum rate coefficients for this nucleation mechanism, which agree with values derived from detailed case studies at individual sites

A glass transition scenario based on heterogeneities and entropy barriers

We propose a scenario for the glass transition based on the cooperative nature of nucleation processes and entropic effects. The main point is the relation between the off-equilibrium energy dissipation and nucleation processes in off-equilibrium supercooled liquids which leads to a natural definition of the complexity. From the absence of coarsening growth we can derive an entropy based fluctuation formula which relates the free energy dissipation rate in the glass with the nucleation rate of the largest cooperative regions. As by-product we obtain a new phenomenological relation between the largest relaxation time in the supercooled liquid phase and an effective temperature. This differs from the Adam-Gibbs relation in that predicts no divergence of the primary relaxation time at the Kauzmann temperature and the existence of a crossover from fragile to strong behavior.Comment: 8th International Workshop on Disordered Systems, Andalo (Trento), Italy, 12-15 March 200

Effect of Inoculation on the Primary Auetenite and Graphite-Austenite Eutectic in Gray Cast Iron

Nucleation and growth during solidification of the primary austenite were investigated, paying attention to the degree of undercooling for nucleation, the degree of nucleation and the growth morphology of austenite. The experiment was conducted on hypo-eutectic iron of Fe-C system inoculated with ferro-silicon or calcium-silicide and held at the molten state. Inoculation effect and fading phenomenon were observed in the nucleation of graphite eutectic with Fe-Si or Ca-Si addition, in agreement with the previous studies. Nucleation of primary austenite is promoted by adding Ca-Si, but is hindered by adding Fe-Si. Inoculation effect on the solidification of primary austenite differs from that on the graphite eutectic

Effects of Ti and Nb on the Nucleation of Primary Austenite in Hypoeutectic Cast Iron of Fe-C System

Experiments were conducted to observe how nucleation and growth of primary austenite (r) of Fe-C system cast iron change with the addition of titanium (Ti) and niobium (Nb), which form carbides by reacting with carbon (C) in the melt. The results obtained were discussed both from the structural observation of carbides and thermodynamics concerning the variation of the amount of carbides formed(TiCor NbC) and the elements dissolved in the melt during cooling. The atomic concentrations of Ti and Nb in the melt were nearly equal. The degree of undercooling before nucleation and the amount of nucleation (number of austenite grains) increase by increasing the Ti content. By increasing the Ti content the growth morphology of the primary r tends to become endogeneous, i.e. the nuclei formedover the whole area of the melt grow as equiaxed crystals. Nucleation and growth morphology are scarcely affected by the addition of Nb. Ti carbides are formed in sequence over the range from a high temperature, 1,500°C, to eutectic　temperature. Most of the Nb carbides are formed at considerably higher temeprature than liquidus and are scarcely formed at temperature near that of nucleation of the primary austenite. Consequently the amount of C atoms taken away by Ti from the melt at the liquidus is extremely larger than that by Nb. It was concluded that the factor governing the nucleation of r is not the carbides or their amount formed before the nucleation of r, but the action of the elements forming carbides while taking C out of the melt. The newly formed surfaces of the carbides are the most effective sites for nucleation, since that part is most enriched in Fe. It was also concluded that the addition of Nb hardly promotes the nucleation, because it takes little C atoms out of the melt at the nucleation temperature, while the addition of Ti promotes the nucleation by taking a large quantity of C atoms out of the melt at the nucleation temperature of the primary r