ABSTRACT
The motivations of the present work are to investigate the exfoliation of montmorillonite within a linear low-density polyethylene matrix and to control its orientation during the cast extrusion process. The first part is focused on the exfoliation of the montmorillonite through the melt extrusion process. The accuracy and relevance of each method used to determine the exfoliation state of montmorillonite have been examined, thanks to X-ray diffraction, transmission electronic microscopy, and rheology. All these methods have presented limitations, but the combination of all leads to a better estimation of the exfoliation state. Finally, the orientation of the montmorillonite is quantified systematically by X-ray texture analysis and correlated with process parameters to discern which one is affecting their orientation. The results have demonstrated an enhancement of the "in-plane" orientation of the montmorillonite with the exfoliation, especially at high concentration and when combined with cast extrusion. Finally, in the multi-nano-layer polymer film configuration, the reduction of the individual layers 29 nm thickness leads to some orientation improvements. However, these improvements are almost at the same level as the concentration effect in a monolayer system. This work gives an overview of all the parameters needed to achieve a significant organo-modified montmorillonite "in-plane" orientation.
ABSTRACT
The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu-Se and Zn-Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells.
