Construction of Single-sided Mobile NMR Apparatus
Date Issued
2009
Date
2009
Author(s)
Chang, Wei-Hao
Abstract
Single-sided NMR is technique which emerges recently. Its principle is to use permanent magnets to generate a static magnetic field B0 and a radiofrequency (RF) coil to generate an RF magnetic field B1; B0 and B1 fields are made perpendicular to each other in some space outside a single-sided NMR apparatus, where a sensitive volume of the apparatus is generated. The NMR signal of an object situated in the sensitive volume of the apparatus can be acquired by the RF excitation and signal detection of the RF coil. In this architecture, the experimental object does not need to be encompassed by the magnet. Consequently, the single-sided NMR apparatus can be utilized to measure the NMR signal of an experimental object with arbitrary dimensions, attaining a totally non-destructive inspection. Many researchers proposed various designs of the single-sided NMR apparatus recently; however, most reported apparatuses have relatively large sizes and heavy weights, decreasing the portability of the apparatus. Meanwhile, the ability to collect an NMR signal in one shot is not reported for most apparatuses, which reflects the insufficient signal-to-noise ratio of the most apparatuses. his research presents two new designs of the single-sided mobile NMR apparatus. One design employs a Halbach magnet and the other design uses a bar magnet with transverse magnetization. Each constructed apparatus can be held in one’s palm, weighs less than 2 KG and the production cost is low (less than 15000 NTD). An experimental object can be measured after it is placed on the apparatus top; consequently the apparatus has an open structure. Each apparatus can measure an NMR signal of a pencil eraser block in one shot and demonstrates the adequate SNR of the apparatus. Each apparatus can measure the NMR relaxation parameters of an experimental object, as well as perform one-dimensional imaging of a thin sample with a thickness of about several hundreds of microns by utilizing the static magnetic field gradient of > 10 T/m directly above the apparatus top center. The single-sided mobile NMR apparatus with the bar-magnet design also can measure the self-diffusion coefficients of liquids. Applications of the apparatuses include evaluating the proton mobility of a wood plate after undergoing thermal damage, monitoring the hardening process of epoxy putties, monitoring the moisture absorption of a cracker and measuring the fat content of milk powders. We also employ numerical software simulations to find several possible modifications of the proposed apparatus, including halving the magnet along its magnetized direction and adding shim magnets. These approaches can adjust the static magnetic field profiles above the apparatus to generate better spatial resolution for one-dimensional imaging or a longer detection distance compared with the unmodified apparatus.ompared with most previously published single-sided NMR apparatuses, the proposed apparatuses in this research have relatively small dimensions and light weights and provide a higher SNR; consequently the proposed apparatuses have better portability and can reduce the experimental time. Several application experiments demonstrate that the proposed apparatuses can inspect the status of the polymer or food samples by accessing their relaxation parameters or echo signal amplitudes; therefore, the proposed apparatuses have the potential to conduct quality control of polymers and foods. We expect that the modification approaches presented in the final chapter can enhance the performance of the apparatus for non-destructive inspection.
Subjects
single-sided NMR
mobile NMR
unilateral NMR
Type
thesis
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