Development of tools for investigating process-structure-property relationships in particle formulations and their applications
Process technologies for nanoparticles represent an unprecedented challenge and opportunity. This opportunity can be tapped by continued innovation brought through research at and across the process scale, particle scale, and perhaps right down to the molecular and atomic scale. Particle formulation products can be simple dispersions of well-structured and shaped particles, thin films, or formulated products of complex hierarchical particle assemblies. The ability to construct and control the process-structure and structure-property relations is necessary if particle processing knowledge is to springboard into next generation products through scalable and sustainable manufacturing. This consequently will lead to far-reaching societal and environmental benefits and advancement of chemical engineering sciences. This thesis is concerned with enabling this ability to characterize and explain process-structure-property relations. Apart from model systems, the particle systems I have studied here include materials for energy conversion and storage such as fuel cell catalyst inks and battery slurries. Despite the great importance of ink formulation for the high-performance electrode layers in fuel cells and batteries, their optimization is largely empirical. Moreover, the literature consists of contradictory data, partly because of the huge parameter space (e.g., particle selection, solvent selection, fabrication process) and partly due to the lack of approaches to determine the surface properties of particles for quality control and their effect on subsequent process and solvent selection. Further complexity arises from the application-related high solids concentrations and the use of binders/additives resulting in a multicomponent system. This work presents the methods to characterize such systems comprehensively, embracing the complexity of the formulation. I prepared nanoparticle dispersions and performed particle characterization experiments such as the evaluation of surface properties and rheology. The aim here is to investigate the systems at their technical concentrations, without altering the sample integrity. I use a comparatively new analytical technique for the measurement of such complex multi-material particle systems, namely analytical centrifugation. Further, I propose and developed novel data processing workflows using the characterization measurement data. Among these data processing methods, first I describe transmittograms that allow facile visualizations for settling assemblages. I show how they can be used to draw information about the particles in suspension from sedimentation dynamics and rationalize observations made regarding agglomeration phenomena and particle size. Secondly, I describe stability trajectories, which provide a quantitative framework for assessing the heterogeneity in the formulation. Together, by using these procedures, I demonstrate how temporal and spatial patterns can be extracted from them to bridge the process-structure-property relationships. Finally, I propose and describe a combinatorics-based approach, which provides a deterministic framework for the evaluation of the Hansen solubility parameters of particles. Overall, these procedures and tools equip formulators and facilitate the study and development of particle-based formulations.