Measurement, modelling, and closed-loop control of crystal shape distribution： Literature review and future perspectives

Crystal morphology is known to be of great importance to the end-use properties of crystal products, and to affect down-stream processing such as filtration and drying. However, it has been previously regarded as too challenging to achieve automatic closed-loop control. Previous work has focused on controlling the crystal size distribution, where the size of a crystal is often defined as the diameter of a sphere that has the same volume as the crystal. This paper reviews the new advances in morphological population balance models for modelling and simulating the crystal shape distribution （CShD）, measuring and estimating crystal facet growth kinetics, and two- and three-dimensional imaging for on-line characterisation of the crystal morphology and CShD. A framework is presented that integrates the various components to achieve the ultimate objective of model-based closed-loop control of the CShD. The knowledge gaps and challenges that require further research are also identified.

Diffusion-reaction compromise the polymorphs of precipitated calcium carbonate

Diffusion is seldom considered by chemists and materialists in the preparation of materials while it plays an important role in the field of chemical engineering. If we look at crystallization at the atomic level, crystal growth in a solution starts from the diffusion of ions to the growing surface followed by the incorporation of ions into its lattice. Diffusion can be a rate determining step for the growth of crystals. In this paper, we take the crystallization of calcium carbonate as an example to illustrate the microscopic processes of diffusion and reaction and their compromising influence on the morphology of the crystals produced. The diffusion effect is studied in a specially designed three-cell reactor. Experiments show that a decrease of diffusion leads to retardation of supersaturation and the formation of a continuous concen- tration gradient in the reaction cell, thus promoting the formation of cubic calcite particles. The reaction rate is regulated by temperature. Increase of reaction rate favors the formation of needle-like aragonite particles. When diffusion and reaction play joint roles in the reaction system, their compromise dominates the formation of products, leading to a mixture of cubic and needle-like particles with a controllable ratio. Since diffusion and reaction are universal factors in the preparation of materials, the finding of this paper could be helpful in the controlled synthesis of other materials.

Tadpole-shaped Au nano-particles fabricated by laser fragmentation of Au nano-spheres in liquid

Tadpole-shaped Au nano-particles with controllable tails are successfully fabricated by simply using laser fragmentation of separated Au nano-spheres in liquid. The optimum laser power densities（1.5–3 GW∕cm-2） can enable part of the individual Au nano-sphere to be re-melted, released, and ultra-rapidly recondensed/crystallized on the outside of the original region. We find that the length of the tail in a tadpole-shaped Au nano-particle significantly increases from about 10 to 25 nm by increasing the laser power density. Benefiting from the unique structural features, the localized surface plasmon resonance（LSPR） absorption spectra of the tadpole-shaped Au nano-particles become broader by increasing the tail length. Moreover, the LSPR absorption band also exhibits a noticeable red shift from about 520 to 650 nm. Our results provide a convenient and valuable strategy to fabricated novel anisotropic-shaped nano-structures with fascinating properties.