Void distribution and susceptibility differences in ceramic materials using MRI
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William M. Ritchey Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078 (Received 8 August 1991; accepted 28 October 1992)
Magnetic Resonance Imaging (MRI) is applied to porous ceramic materials to study structural properties. In ceramics, processing differences create inhomogeneous binder distribution in the materials which can cause the formation of regions with differing densities and voids. These defects can be observed with MRI using solvent permeation. Fractional porosity obtained by using image intensity measurements and weight gain due to solvent permeation can be correlated. Dark regions in the image are due to defects such as closed voids, pockets of binder, or agglomerates. Defects such as voids or agglomerates usually have different magnetic susceptibilities. This difference causes artifacts in the image. By exploiting the increase in signal loss using a gradient-echo sequence, apparent enhancement of voids in ceramics is achieved.
I. INTRODUCTION Magnetic Resonance Imaging (MRI) has been a valuable tool in the medical field1 and is useful in the nondestructive, noninvasive evaluation of patients. Within the last several years, an interest in applying MRI to materials has developed. Materials science should benefit considerably from this type of nondestructive analysis. Several groups have already applied MRI to materials to improve existing methods of analysis. Studies of elastomers,2 water flow in porous materials,3 and diffusion in rocks4 and wood5 are among the applications that have appeared in the literature. Magic angle spinning (MAS),6 used for materials with modest dipolar broadening (less than about 10 KHz), and multiple pulse techniques,7 used for materials with larger dipolar broadening, have been applied to the imaging of solid materials. All these applications show the increased interest in materials imaging over the last decade. The usual method of analysis for ceramics is x-ray analysis. This method suffers from two drawbacks. First, the typical resolution is approximately 100 /itn although several groups have done better. Second, these high density fired ceramics have high x-ray absorption which limits this technique in void determination to relatively small specimens. This is one of the advantages of MRI. It allows for the analysis of bore-hole uniformity in the fired ceramic in a nondestructive manner by imaging the solvent that enters into the bore hole. Another major advantage of MRI over other techniques is the noninvasive evaluation of void distribution and defects in the green-state ceramic. Susceptibility differences between two different materials in a sample have caused problems that have hinJ. Mater. Res., Vol. 8, No. 3, Mar 1993
dered MRI in the medical field. Susceptibility artifacts in spin-echo sequences have been observed and previously studied.8'9 In spin-echo sequences, positional displacements, also known as chemical shift type misregistration, are observed at susceptibility interfaces. This is often viewed as a bright crescent-shape
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