Modelling nucleation in wet granulation

Nucleation is the first stage in any granulation process where binder liquid first comes into contact with the powder. This paper investigates the nucleation process where binder liquid is added to a fine powder with a spray nozzle. The dimensionless spray flux approach of Hapgood et al. (Powder Technol. 141 (2004) 20) is extended to account for nonuniform spray patterns and allow for overlap of nuclei granules rather than spray drops. A dimensionless nuclei distribution function which describes the effects of the design and operating parameters of the nucleation process (binder spray characteristics, the nucleation area ratio between droplets and nuclei and the powder bed velocity) on the fractional surface area coverage of nuclei on a moving powder bed is developed. From this starting point, a Monte Carlo nucleation model that simulates full nuclei size distributions as a function of the design and operating parameters that were implemented in the dimensionless nuclei distribution function is developed. The nucleation model was then used to investigate the effects of the design and operating parameters on the formed nuclei size distributions and to correlate these effects to changes of the dimensionless nuclei distribution function. Model simulations also showed that it is possible to predict nuclei size distributions beyond the drop controlled nucleation regime in Hapgood's nucleation regime map. Qualitative comparison of model simulations and experimental nucleation data showed similar shapes of the nuclei size distributions. In its current form, the nucleation model can replace the nucleation term in one-dimensional population balance models describing wet granulation processes. Implementation of more sophisticated nucleation kinetics can make the model applicable to multi-dimensional population balance models.

Modeling of adsorption and nucleation in infinite cylindrical pores by two-dimensional density functional theory

In this paper, we present an analysis of argon adsorption in cylindrical pores having amorphous silica structure by means of a nonlocal density functional theory (NLDFT). In the modeling, we account for the radial and longitudinal density distributions, which allow us to consider the interface between the liquidlike and vaporlike fluids separated by a hemispherical meniscus in the canonical ensemble. The Helmholtz free energy of the meniscus was determined as a function of pore diameter. The canonical NLDFT simulations show the details of density rearrangement at the vaporlike and liquidlike spinodal points. The limits of stability of the smallest bridge and the smallest bubble were also determined with the canonical NLDFT. The energy of nucleation as a function of the bulk pressure and the pore diameter was determined with the grand canonical NLDFT using an additional external potential field. It was shown that the experimentally observed reversibility of argon adsorption isotherms at its boiling point up to the pore diameter of 4 nm is possible if the potential barrier of 22kT is overcome due to density fluctuations.

Heterogeneous nucleation of Mg-Al alloys

Grain size is one of the most important microstructural characteristics determining the mechanical properties and therefore the service performance of polycrystalline materials. Heterogeneous nucleation involves the addition or in situ formation of potent nuclei in the system to promote nucleation events, leading to a fine grain structure. This paper reports experimental results using graphite and SiC as potential grain refining agents to form in situ nuclei for Mg in Mg-Al alloys, and demonstrates the key role of Al4C3 in grain refilling this important alloy system. This insight will contribute to the design and development of the most cost effective, eco-friendly grain refining agents for Mg-Al alloys. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The secondary nucleation threshold and crystal growth of alpha-glucose monohydrate in aqueous solution

Investigation of the secondary nucleation threshold (SNT) of alpha-glucose monohydrate was conducted in aqueous solutions in agitated batch systems for the temperature range 10 to 40 degrees C. The width of the SNT decreased as the induction time increased and was found to be temperature independent when supersaturation was based on the absolute concentration driving force. Nonnucleating seeded batch bulk crystallizations of this sugar were performed isothermally in the same temperature range as the SNT experiments, and within the SNT region to avoid nucleation. The growth kinetics were found to be linearly dependent on the supersaturation of total glucose in the system when the mutarotation reaction is not rate limiting. The growth rate constant increases with increasing temperature and follows an Arrhenius relationship with an activation energy of 50 +/- 2 kJ/mol. alpha-Glucose monohydrate shows significant crystal growth rate dispersion (GRD). For the seeds used, the 95% range of growth rates was within a factor of 6 for seeds with a narrow particle size distribution, and 8 for seeds with a wider distribution that was used at 25 degrees C. The results will be used to model the significance of the mutarotation reaction on the overall crystallization rate of D-glucose in industrial crystallization.

Effect of solute on the growth rate and the constitutional undercooling ahead of the advancing interface during solidification of an alloy and the implications for nucleation

A framework is presented for modeling the nucleation in the constitutionally supercooled liquid ahead of the advancing solid/liquid interface. The effects of temperature gradient, imposed velocity, slope of liquidus, and initial concentration have been taken into account in this model by considering the effect of interface retardation, which is caused by solute buildup at the interface. Furthermore, the effect of solute concentration on the chemical driving force for nucleation has been considered in this model. The model is used for describing the nucleation of Al-Si and Al-Cu alloys. It was found that the solute of Si has a significant impact on the chemical driving force for nucleation in AI-Si alloys whereas Cu has almost no effect in Al-Cu alloys.

Synthesis and expression of a gene encoding a 48-residue repeat in the Pseudomonas syringae ice nucleation protein

The aim of this work was to construct short analogues of the repetitive water-binding domain of the Pseudomonas syringae ice nucleation protein, InaZ. Structural analysis of these analogues might provide data pertaining to the protein-water contacts that underlie ice nucleation. An artificial gene coding for a 48-mer repeat sequence from InaZ was synthesized from four oligodeoxyribonucleotides and ligated into the expression vector, pGEX2T. The recombinant vector was cloned in Escherichia coli and a glutathione S-transferase fusion protein obtained. This fusion protein displayed a low level of ice-nucleating activity when tested by a droplet freezing assay. The fusion protein could be cleaved with thrombin, providing a means for future recovery of the 48-mer peptide in amounts suitable for structural analysis by nuclear magnetic resonance spectroscopy.

Intragranular nucleation of austenite

The mechanism of intragranular nucleation of austenite in a duplex stainless steel (Fe-23.1 Cr-6.1 Ni-3.1 Mo-O.165 N-0.017 C, wt.%) weld metal and heat-affected zone (HAZ) has been examined. In the weld metal the acicular austenite is found to nucleate intragranularly on inclusions and subsequent plates form sympathetically resulting in a fine interlocked microstructure. Austenite plates adopt the Kurdjumov-Sachs orientation relationship with the ferrite matrix and grow with diffusion-controlled mechanism as evident from partitioning of substitutional alloying elements. At least one set of fine intrinsic dislocations on the austenite/ferrite interphase interfaces is observed suggesting that the boundaries are semi-coherent. The high cooling rates involved in the HAZ result in a supersaturated ferrite matrix where precipitation of intragranular austenite occurs as a result of reheating associated with subsequent passes. Austenite particles in the HAZ nucleate preferentially away from gain boundary austenite allotriomorphs indicating that intragranular precipitation is favoured by the supersaturated matrix.

Initial nucleation study and new technique for sublimation growth of AlN on SiC substrate

Single crystal platelets of AlN were successfully grown on 6H-SiC(0001) by a novel technique designed to suppress SiC decomposition, promote two-dimensional growth, and eliminate cracking in the AlN. X-ray diffractometry and synchrotron white beam X-ray topography demonstrate that the final AlN single crystal is of high structural quality.