Low-Dose STEM Characterisation of Nanoscale Calcium Carbonate Particles

This thesis forms a framework for the reliable identification and quantification by transmission electron microscopy (both conventional and scanning) of discrete particle crystallinity in nominally amorphous, nanoscale calcium carbonate particles used industrially as fuel detergents. The foundations of this framework are built on understanding and quantifying the progressive radiolytic damage of ~50 nm calcite nanoparticles. The degradation pathway of calcite involves disruption of the crystal lattice, the formation of pores, and the transformation from CaCO3 to CaO and CO2. The damage threshold for the loss of lattice integrity at 300 kV in CTEM was determined to be 2.7x107 e-nm-2.
Microscope operating conditions were optimised to maximise the extracted analytical information, whilst minimising the extent of electron beam damage. It is shown that STEM offers significant benefits over CTEM, however only in the presence of hydrocarbon contamination, increasing the fluence threshold for the detection of irradiation-induced faults in the calcite lattice to over 1.8×108 e-nm-2 for 300 kV STEM in wide-angle bright field operation.
These conditions were used to characterise and quantify the incidence of discrete crystallinity in nominally amorphous ~5 nm fuel detergent particles, which were found to crystallise following ~3x107 e-nm-2 at 300 kV CTEM irradiation. Quantification of discrete crystallinity was achieved by automated imaging, and random sampling. This revealed a low incidence of inherent vaterite crystallinity of 1 in 5000 particles which was not detected using conventional laboratory based analytical techniques such as X-ray diffraction and Fourier Transform Infrared spectroscopy.
In addition, STEM imaging was used to observe the deliberate, solvent-induced destabilisation of the amorphous fuel detergent particles, demonstrating agglomeration, aggregation, and eventual crystallisation into vaterite over 28 days. This was again detected by STEM prior to conventional laboratory analytical techniques, making a compelling case for low-dose STEM imaging to be used as a form of process control for the synthesis of the fuel detergent particles.