NMR techniques: A non-destructive analysis to follow microstructural changes induced in ceramics

Magnetic resonance relaxation (MRR) and magnetic resonance imaging (MRI) of water 1H nuclei confined in high-surface-to-volume ratio systems are important tools to investigate pore-space properties such as connected porosity and pore-size distribution in a non-destructive way. In the present work, MRR and MRI techniques were applied to study the microstructure of ceramic materials used for the production of tiles, with different compositions and fired at different temperatures, in terms of porosity distribution and pore-size distribution. Relaxation analysis of samples saturated with water gave a clear characterisation of the pore space of the materials, with high resolution power. Actually, the decay of the nuclear magnetization was turned into pore-size distribution, the relaxation rates depending on local surface-to-volume ratios. MRI has been applied to visualize in a non-destructive way internal sections of ceramics saturated with water, in order to check the spatial homogeneity. It was also possible to evaluate parameters connected to the local structure in the inside regions. The effects of different firing temperatures and compositions were distinguished. The results were compared with microstructural studies carried out by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). SEM analysis confirmed the outcomes of MRR and MRI, and the comparison with MIP made it clear that a higher temperature allows the formation of larger pores connected by smaller channels. MRR and MRI proved to be particularly useful to assess, in a non-destructive way, the effects of different fluxes on the final microstructures.