Water¹H NMR Technique to Analyse the Porous Structure of Ceramics

Abstract

Traditional silicatic ceramic materials used for floor and wall tiles, are characterized by a relevant presence of pores in their microstructure, and they are classified on the basis of their porosity. [1] The deep knowledge of the microstructure of ceramic materials is fundamental in controlling and adjusting the different steps of the production and the parameters affecting the final properties and performance of these ceramic products. [2] Nowadays, the study and the control of the porosity in ceramic tiles is based on conventional techniques, more or less approximate, such as the determination by weight measurements of the amount of water absorbed by a reference volume, the analysis of images, the use of Scanning Electron Microscopy (SEM) and of Mercury Intrusion Porosimetry (MIP). These methods are destructive, and the possibility of finding a non-invasive technique could represent a real innovation. From this point of view, Nuclear Magnetic Resonance (NMR) Imaging (MRI) and Relaxometry (MRR) experiments offer a unique opportunity to study different kinds of samples in several fields, such as medical, biological, biochemical, and also in material science. [3][4][5][6][7] It is a common practice to relate a distribution of relaxation times (multi-exponential relaxation) to a distribution of pore sizes, based on the assumption that the surfaces are uniformly effective in causing relaxation and that water molecules within a pore are mixed by diffusion in a local relaxation time (fast diffusion regime). We verified that the fast diffusion regime applies by noting that the diffusion distance (2 DT 1 ) 0.5 is greater than relevant pore dimensions shown by SEM images. In the case of an elongated pore, it is the smallest dimension that is relevant for relaxation and diffusion.MRI technique allows one to obtain images in which the contrast, i.e. the difference of brightness in adjacent regions, is related to NMR parameters, which are intrinsic properties of the sample. [8] The contrast depends on the parameters of the pulse sequence used for the imaging experiment, i.e. flip angle, repetition time (TR), echo time (TE). Useful NMR parameters are the longitudinal relaxation time T 1 , the transverse relaxation time T 2 and the signal density S 0 . Proton density NMR images can map the distribution of water across the sample and the concentration of water is strictly related to the porosity of the sample, [9] but the knowledge of NMR relaxation times of water protons is required.In this work, the relaxation times of water protons absorbed in ceramic silicatic samples characterized by different microstructures, have been measured by MRR, and the results compared with those of morphological studies obtained by MRI, SEM and MIP. While SEM shows both open and close porosity, NMR, MIP and gravimetric methods gives only connected porosity. Experimental MaterialsIn order to obtain ceramic products with different microstructures, two different types of ceramics were chosen: a low porosity ceramic, used for porcelain stoneware t...

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