The Design And Applications Research Of Unilateral NMR Sensor With A Constant Gradient Field

Nuclear magnetic resonance(NMR) technology has been widely used in geology, medicine, chemistry, biology and other fields, for its unique advantages of exploring materials’ micro structures and performances. However, in general the traditional NMR instruments are heavy and expensive, and have a enclosed structure which limited the maximum size of the test object. These features to a great extent hindered the engineering application of NMR technology.In view of the huge potential application of the cheap portable NMR equipments, this study designed a low-field miniature unilateral NMR sensor. The sensor is not only small volume, low cost. But also the sensitive area(ROI) is located in the out of magnet, there is no limit to the volume of the test object.With the theory of electromagnetic field, a set of design and optimization methods of unilateral magnet and radio frequency(RF) coil of unilateral NMR sensor was put forward in this paper. The content includes the following parts:① The design of unilateral magnet structure. This design belongs to the inverse problem of the static magnetic field. The magnetic field distribution in ROI was given in advance, and then, the second-order finite element method was adopted to calculate the static magnetic field of the permanent magnet. A unilateral magnet, consisting of five magnet boxes which were placed along an elliptical arc, was designed. Basing on this structure, passive shimming method was further employed to improve the uniformity of magnetic field on the target surface within the ROI; The optimal magnet structure, which is composed of five identical cuboid permanent magnet blocks and two passive shimming sheets, generates a highly uniform 0B field in a 10mm×10mm lateral area. Along the two orthogonal lines in this lateral area, the measured 0B strength variation is less than 338 ppm. Vertically, the measured 0B magnitude decreases from 144.387 m T to 130.603 m T, with a constant gradient of 4.6 T/m from 0.15 mm to 3.15 mm above the magnet surface.② The design of RF coil. To improve the signal-to-noise ratio(SNR) of unilateral NMR systems, the planar rectangular spiral coil was optimized based on the parameters of spatial uniformity and SNR. As a result, the optimal RF coil not only has a best SNR in the target surface, but also has a uniform RF field area which is enough to cover the ROI.③ The performance tests of the unilateral NMR sensor. The following performance tests were conducted on the optimal unilateral NMR sensor: 1) the SNR test of the RF coils: in this experiment, the value of SNR was represented by the ratio of the NMR signal amplitude to the noise signal amplitude. The simulation result would be verified by comparing the measured signals of the different RF coils; 2) The measurement of the static magnetic field strength: this measurement adopted a NMR echo signal analysis method based on FFT. The field uniformity on the horizontal target plane and the vertical gradient of the unilateral magnet were measured accurately; 3) The measurement of transverse relaxation time(2T):the quality of the measured transverse relaxation signal of a piece of rubber was used to test the performance of the unilateral NMR sensor; 4) The measurement of the vertical resolution; 5) One-dimensional profiling experiment: two sandwich-like samples consisted of oiled cotton papers and cover glasses were employed. One dimensional frequency distributions of the samples were measured by NMR experiment. The differences between the real thickness and the calculated value were found. The above performance tests fully proved the good performances of the unilateral NMR sensor.In this paper, the sensor was applied to assess the aging status of the sheds of silicone rubber insulators. This application was conducted on the basis of the previous studies of our research team. However, this study further explored the aging law of the sheds of silicone rubber insulators in depth direction. One-dimensional profiling measuring method on depth direction and data processing method were proposed. The measuring depths into samples were precisely controlled through a one-dimensional longitudinal motion stepper motor. The sheds were measured layer by layer along the depth direction, and 2long meanT? in each 50μm thickness layer was extracted by a band-pass filtering method based on Fast Fourier Transform(FFT). The experimental results have demonstrated that the composite insulator with a longer service time is characterized by a more serious and deeper aging in depth direction. This study is great significant to better understand the aging law of composite insulators in depth direction, and to evaluate the service life of composite insulators and the quality of the insulator material scientifically, etc.