M.R. Halse

Capillary Imbibition and Pore Characterisation in Cement Pastes

The kinetics of capillary imbibition in ordinary Portland cement pastes has been studied experimentally and theoretically. Nuclear magnetic resonance stray field imaging (STRAFI) has been used to record water concentration profiles for various ingress times. The profiles follow a √t law and thus a master curve can be formed using the Boltzmann transformation. The distribution of pore sizes within the sample as measured by NMR cryoporometry shows a prominent peak at 100 Å. A computer model of the pore structure was developed consisting of a lattice of interconnecting pores with a size distribution consistent with the cryoporometry results. The Hagen–Poiseuille law was used to describe the kinetics of the water in this pore structure. The best agreement between the computer simulations and the experimental master curve was obtained by using a narrower range of pore sizes than indicated by the cryoporometry results.

NMR imaging of solids using large oscillating field gradients

A method of imaging short T2 materials is demonstrated that makes use of a large sinusoidally time-varying magnetic field gradient. The technique is experimentally verified by imaging both rubber and PTFE samples. A multiple pulse scheme is also described that is shown to provide a signal-to-noise improvement with no time penalty.

Two-dimensional fourier transform NMR imaging of solids using large oscillating field gradients

Abstract We demonstrate a simple gradient echo technique for the NMR imaging of short T 2 materials in which large oscillating field gradients are used to provide adequate resolution. The technique involves the use of oscillating gradients with frequencies ω and ω/2 and, with suitable correction for nonlinear sampling, is shown to be capable of covering k space in a manner similar to phase and frequency encoding in standard 2DFT. An acceptable two-dimensional 19 F image of a 6 mm diameter phantom of PTFE ( T 2 δ60μs) of 4 mm length was obtained in 26 min.

DIFFUSION AND MOLECULAR MOBILITY IN MICROPOROUS MEDIA: APPLICATIONS TO RUBBER AND ZEOLITE 4A POWDERS

Broad-line magnetic resonance imaging using large oscillating field gradients has become a very useful and robust tool in the study of systems with a T2 of order 0.1-1 ms. Its use is illustrated with two examples involving microporous media: the ingress of decane into lightly cross-linked rubber and water transport in compacted zeolite 4A powder beds.

A study of the diffusion of decane into lightly cross-linked natural rubber by broad-line MRI

The diffusion coefficient of decane in a lightly cross-linked peroxide vulcanizate of natural rubber (polyisoprene) has been measured by broad-line proton magnetic resonance imaging (MRI). The value of the diffusion coefficient deduced from the MRI results is higher than that obtained from conventional mass-uptake measurements. The discrepancy can be removed by assuming that the diffusion coefficient of decane increases exponentially with concentration. Proton MRI has been used for the parallel study of both the diffusing solvent and the mobility of the polymer matrix. This was achieved by exploiting the wide differences in the proton NMR relaxation times of the two components. Broad-line imaging techniques using resonant oscillating field gradients were used to overcome the short T-2 ( similar to 1 ms) of the rubber. The results show a good correlation between the ingress of the solvent and the softening of the rubber matrix.

Slice selection methods for the NMR imaging of solid materials

Three methods of achieving slice selection in short T2 materials are investigated. Two of the methods involve storage of the magnetization during gradient switching, while the third employs a three-dimensional reconstruction technique. Each method is tested experimentally by imaging suitably shaped rubber samples, and the results are compared.

Ingress of water into zeolite 4A powder plugs

One dimensional profiles of the concentration of water absorbed from vapour diffusing into compacted type 4A zeolite powder have been obtained by broadline NMR imaging. After an induction period of approximately 6 h, a region of full hydration advances linearly with time into the zeolite plug. This behaviour is typical of Case II diffusion. A simple numerical simulation gives good agreement with the experimental results.

NMR studies of molecular mobility and diffusion in porous systems.

The uptake, partitioning, and release of ingredients such as water, oil, surfactant, and ions are important factors to understand and control in the design and manufacture of detergent and personal products. Although conventional pulse NMR (PNMR) spectroscopy continues to be used to analyse bulk molecular mobility and phase composition, more recently MR imaging techniques have created unique opportunities for gaining spatial information about these processes in ways that are noninvasive and potentially quantitative. This paper describes the evaluation of MRI and associated PNMR techniques to study transport in three relevant cases: ion diffusion (e.g., fluoride) in concentrated dispersions, oil transport through powders, and water ingress into porous powders (zeolite). Results are presented to illustrate the potential of multiple pulse and gradient echo MRI methods for dealing with the short T2 scenarios that represent a common problem in quantitative imaging of water in solid-containing composites involving, for instance, zeolite, or silica. Pore-size characterisation results are also presented.