de Martín Lilian, Bouwman Wim G, van Ommen J Ruud
Department of Chemical Engineering and ‡Department of Radiation, Science and Technology, Delft University of Technology , Delft, The Netherlands.
Langmuir. 2014 Oct 28;30(42):12696-702. doi: 10.1021/la502987e. Epub 2014 Oct 14.
We show that fluidized nanoparticle agglomerates are hierarchical fractal structures with three fractal dimensions: one characterizing sintered aggregates formed during nanoparticle synthesis, one that is also found in stored agglomerates and represents unbroken agglomerates, and one describing the large agglomerates broken during fluidization. This has been possible by using spin-echo small-angle neutron scattering-a relatively novel technique that, for the first time, allowed to characterize in situ the structure of fluidized nanoparticle agglomerates from 21 nm to ∼20 μm. The results show that serial agglomeration mechanisms in the gas phase can generate nanoparticle clusters with different fractal dimensions, contradicting the common approach that considers fluidized nanoparticle agglomerates as single fractals, in analogy to the agglomerates formed by micron-sized particles. This work has important implications for the fluidization field but also has a wider impact. Current studies deal with the formation and properties of clusters where the building blocks are particles and the structure can be characterized by only one fractal dimension. However, fluidized nanoparticle agglomerates are low-dimensional clusters formed by higher-dimensional clusters that are formed by low-dimensional clusters. This multifractality demands a new type of multiscale model able to capture the interplay between different scales.
我们表明，流化纳米颗粒团聚体是具有三个分形维数的分层分形结构：一个表征纳米颗粒合成过程中形成的烧结聚集体，一个在储存的团聚体中也能发现且代表未破碎的团聚体，还有一个描述流化过程中破碎的大团聚体。这是通过使用自旋回波小角中子散射实现的——这是一种相对新颖的技术，首次能够原位表征从21纳米到约20微米的流化纳米颗粒团聚体的结构。结果表明，气相中的连续团聚机制可以产生具有不同分形维数的纳米颗粒簇，这与将流化纳米颗粒团聚体视为单一分形的常见方法相矛盾，后者类似于由微米级颗粒形成的团聚体。这项工作对流化领域具有重要意义，而且影响更为广泛。目前的研究涉及以颗粒为构建单元且结构仅能用一个分形维数表征的簇的形成和性质。然而，流化纳米颗粒团聚体是由低维簇形成的高维簇构成的低维簇。这种多重分形性需要一种新型的多尺度模型来捕捉不同尺度之间的相互作用。
