Highly uniform single-sided portable NMR sensor and its application in assessing the aging level of silicone rubber insulators

Abstract

This study presents a high-performance single-sided portable nuclear magnetic resonance (NMR) sensor. The magnet array, which is composed of five identical cuboid permanent magnet blocks and two passive shimming sheets, generates a highly uniform B0 field in a 10 × 10 mm 2 lateral area. Along the two orthogonal lines in this lateral area, the measured B0 strength variation is less than 338 ppm. Vertically, the measured B0 magnitude decreases from 144.387 mT to 130.603 mT, with a constant gradient of 4.6 T/m from 0.15 mm to 3.15 mm above the magnet surface. A planar radiofrequency coil is optimized according to the signal-to-noise ratio level of the coil. A one-dimensional profiling experiment on an oil film shows that a vertical resolution of approximately 1.7 μm can be achieved. As an application, the silicone rubber insulators are measured using the NMR sensor. Results demonstrate that the silicone rubber insulators with different service times can be distinguished through the transverse relaxation time T2 of the silicone rubber insulators. Z. Xu et al. / Highly uniform single-sided portable NMR sensor and its applicationoptimization design on magnets should be considered to improve the homogeneity of the B 0 field in the lateral plane.Furthermore, the different magnets presented by several groups are reported. Chang et al. [7,8] introduced the following two single-sided magnet structures: one has a gradient of 10.2 T/m constructed by a single, disc-shaped permanent magnet, which is polarized in a radial direction; the other is a small Halbach magnet, which has a relatively constant gradient of 14.1 T/m between 0 and 2 mm at the top of the magnet. Perlo et al. [9] introduced the NMR-MOUSE, which has a constant gradient of 2.5 T/m. Juan et al.[10] reported a single-sided three-magnet array that has an extended constant magnetic field gradient. Raich and Blumler [11] presented a "sandwich" magnet based on the Halbach layout. This magnet has a gradient strength of roughly 0.3 T/m, and a uniformity of ∼700 ppm is observed for a 18 × 18 × 30 mm 3 water sample. Although uniformity is favorable, the magnet is not single-sided. Maxime Van Landeghem et al.[12] described a shimming approach to make the B 0 field uniform in the lateral section. The magnetic field strength (210 mT) varies by approximately 0.25 mT over a 10 × 10 mm 2 lateral slice, and the uniformity is about 1,200 ppm. Perlo et al.[13] presented a microscopic resolution single-sided NMR sensor, the magnetic field of which is approximately 410 mT, and the variation of the strength is less than 0.2 mT in a range of 10 × 10 mm 2 at a depth of 10 mm. The uniformity of this magnet is approximately 487 ppm, and to the best of our knowledge, this magnet has the best uniformity obtained from a single-sided NMR.In the recent two decades, silicone rubber insulators have been widely used in power transmission systems because of their low cost, low weight, high mechanical strength, and excellent hydrophobicity. However, the superior performance of silicone rubber insu...

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