The Preparation And Application Of High Tc Dc-Squid In Low-Field Nmr

Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) are widely used in condensed matter physics, analytical chemistry, biology and medicine, etc. Now ultra low field (ULF) NMR/MRI technologies have attracted scientists’attention due to their simplicity and cost effectiveness. Superconducting quantum interference device (SQUID) magnetometer has an irreplaceable role in measuring very low magnetic signals, it is the most sensitive devices for detecting magnetic field. The main task of this dissertation is to study the preparation of high temperature superconducting thin films, and the application of dc superconducting quantum interference devices (dc-SQUID) in the ultra-low-field NMR and MRI.The first chapter introduces the Josephson effect and RSJ model, as well as the theory of the dc-SQUID.The second chapter describes the principle and manufacturing conditions of high temperature superconducting YBCO thin films, and furthermore has a simple understanding of measuring the quality of the superconducting film and an overlooking of film lithography technologies.The third chapter elaborates the application of high-temperature dc-SQUID in the low-field NMR. This chapter starts from the history and characteristics of the low-field NMR, then introduces the phenomenon of nuclear magnetic resonance free induction decay (FID) signal and spin-echo signal and their obtaining principle, and the entire experiment measuring devices and circuit analysis. The preliminary experimental results show that the high SNR can be obtained from FID signals, and on this basis we obtain the T1curve of pure water and magnetic nano particles of different concentrations, their T1-contrast imaging is also obtained. Finally, it is studied and discussed of finding the optimal frequency in indirect coupling measurement method. The forth chapter first introduces magnetic resonance imaging(directly back projection imaging and Fourier imaging) principle in details, and then we explore the laboratory imaging conditions through the one-dimensional projection of water in the small round bottles, in order to try to get the win-win choice of spatial resolution and signal-to-noise(S/N) ratio. Finally, we show the imaging result of a number of different shapes of water samples in the directly back projection imaging method, which both have a high resolution, at last white radish is used as a sample, we obtain the two-dimensional MRI images in the same way.