Agglomeration applying high intensity mixing (mechano-fusion)
An integrated approach towards the synthesis of tailored heteroaggregates combining experiments with 2D and 3D structural characterization via image analysis and stochastic modeling
Project Leader :
Prof. Dr.-Ing. Urs Peuker
TU Bergakademie Freiberg
Prof. Dr. Volker Schmidt
University of Ulm
The specific properties of hetero-agglomerates are determined by the particle-particle contacts within the agglomerates. The number of contacts, as well as the interaction between different materials at these contact points, play a crucial role in determining macroscopic application properties. Examples include transport properties, such as electrical, ionic, or thermal conductivity.
The project applies the process step of mechano-fusion, which is positioned between grinding and high-energy mixing, to produce hetero-agglomerates from various primary particles (see Fig. 1, left). The mechanism of agglomerate synthesis relies on dynamic de-agglomeration and re-agglomeration processes, where the high shear and pressure forces in the machine create stable particle-particle contacts within the agglomerate. This process allows for a micro- or nanoparticle coating (guest particles) to be applied to a larger carrier particle. By considering the disperse and material-specific factors of the primary particles, defined agglomerates can be controlled and produced. Examples of various hetero-agglomerate structures can be seen in the scanning electron microscope (SEM) images in Fig. 2.
During the first funding period, the characterization of the hetero-agglomerates was carried out using various methods, including 3D micro-computed tomography (µ-CT). At the single-particle level, the arrangement of the guest particles in particle system B from Fig. 2 was quantified. Characteristic metrics, such as surface coverage, number of contacts, and coating thickness, were used for this purpose [1]. An intensive parameter study of the mechano-fusion process parameters revealed that, in the case of polymeric guest particles and harder, oxide-based carrier particles, significant deformation of the guest particles can occur. This deformation was resolved using high-resolution atomic force microscopy (AFM) in topography mode. By applying an ellipsoid fit to the extracted topography data of a single guest particle, the deformation was quantified [2]. Further investigations during the first funding period included a correlative approach to the characterization of hetero-agglomerates through relatively low-resolution µ-CT and high-resolution SEM. Additionally, the change in the flow behavior of hetero-agglomerates in relation to process parameters was studied, with an aim to correlate it with structural property metrics. Hetero-agglomerates from mixed systems (two different types of guest particles, see Fig. 2, particle system D) were also generated and are to be characterized using nano-CT. Furthermore, a tool was developed in the first funding period [3], which adapts a 3D hetero-agglomerate model using a stereological, neural network-based method to 2D-STEM data. The training of the neural networks was based on simulated 2D-STEM images. The model enables the generation of 3D hetero-agglomerates that are statistically similar to those observed in 2D-STEM images. These can then be used for morphological studies and numerical simulations. For example, assuming an appropriate agglomerate model, core 3D sizes and the average heterogeneous contact number can be determined from 2D projections, which would otherwise suffer from significant stereological issues using conventional methods.
In the second funding period, the design of hetero-agglomerates now aims to precisely understand both the mixing state and the contact geometry to derive a holistic structure-property relationship. This allows for the functionality of the nano- and particle-technological applications of the hetero-agglomerate material to be linked with the structural properties of the hetero-agglomerate itself, more specifically the contacts within the hetero-agglomerate.
The scientific work is therefore focused on the hetero-contacts themselves, which are generated through the high-intensity mixing, collision, and shearing in the mechano-fusion process. The analysis methods will resolve relevant properties at both the micro (atomic) and meso (guest particle) levels of the contacts. Further analyses with AFM, besides providing geometry data, will also offer insights into various material properties at the contact points. In particular, Kelvin probe force microscopy (KP-AFM) will provide insights into the local conductivity at the contact points. Regarding the analysis of hetero-agglomerates using imaging techniques, foundational studies were conducted during the first funding period to develop a workflow for 3D analysis of hetero-agglomerates. Particles with sizes of approximately 30-40 µm as carrier particles and 3-5 µm as guest particles were used. In the second funding period, the size range for some experiments will be reduced by about one-tenth, using carrier particles of 5-10 µm and guest particles of approximately 500-800 nm. To achieve the higher resolution required for 3D analysis of the coatings on these smaller particles, synchrotron tomography with a voxel size of about 100 nm will be employed. This ensures that the workflow already developed can also be used for the particles in the second funding period. Other measurement techniques, such as wide-angle light scattering [4], will also be used in the second funding period to analyze hetero-agglomerates.
Quantifying the functionality dependent on hetero-contacts includes both macroscopic functionality. For example, in energy materials, the percolation of hetero-agglomerates within the particle bed influences its electrical conductivity. Another structural factor affecting conductivity is the resistance at the contact points between carrier and guest particles. Since various carrier particle materials exhibit low conductivity, KP-AFM measurements will focus on investigating the conductive properties in the meso region at the contact points to determine the local distribution of conductivity. It is assumed that the mechano-fusion process affects the resistance at the contact points and reduces the electron barrier between the two materials of the hetero-agglomerates.
To derive structure-property relationships, image data obtained from various imaging techniques must be preprocessed and segmented. An already developed technique for super-resolution of 2D image data will be extended to increase the resolution of µ-CT image data, making fine structures visible in 3D that would otherwise only be observable through more complex measurement techniques. These data will then be segmented by particle and phase to characterize the 3D morphology of hetero-agglomerates. Various structural descriptors will be calculated, such as phase volume fractions, fractal dimension, sphericity, and average coating thickness.
Digital twins of real hetero-agglomerates will be generated and used as input for numerical simulations to derive quantitative relationships between the structural descriptors and the macroscopic functionality of the hetero-agglomerates. These relationships will ultimately be used to optimize material development. Process parameters can thus be specifically adjusted to achieve the desired 3D morphologies of the hetero-agglomerates with the desired functionality. The process parameters found in this way will then be experimentally validated.
Publications from FP 1: