Relaxation Measurement Method And Application Based On Dynamic Nuclear Polarization Enhancement

Low field nuclear magnetic resonance(NMR)is widely used in oil exploitation,medical treatment,food and medicine.Especially in the field of oil exploitation,low field NMR can reduce the influence of internal gradient caused by the difference of magnetic susceptibility between solid and liquid in porous materials,which has great advantages over high field NMR.The relaxation and diffusion characteristics were mainly used in low field NMR to study the microstructure information of materials.The two-dimensional relaxation method can obtain the distribution of components with similar relaxation time in porous materials,which plays a great role in low field NMR analysis.However,due to the problems of the low signal-to-noise ratio in low field and waiting time of 5Ti,the two-dimensional T1-T2 relaxation method by conventional inversion recovery is very time-consuming,which limits its application in rapid detection and fluid recognition.Dynamic nuclear polarization(DNP)can greatly improve NMR signal-to-noise ratio,which provides a new possibility for the development and application of low field NMR method.The DNP process requires the participation of unpaired electrons,which provided by free radicals(polarizers).The commonly used free radicals are exogenous small molecular free radicals,such as TEMPO.The NMR sensitivity can be greatly improved by the introduction of exogenous free radicals.An efficient test and analysis method are provided by DNP using exogenous free radicals,for the structural characterization of porous materials and the study of internal hydrodynamics.However,the original relaxation characteristics will be inevitably changed by exogenous free radicals.As an endogenous polarizer,nanoparticles have relatively little effect on the system and can modify the surface wettability of materials.It is expected to improve NMR signal-tonoise ratio in combination with ODNP.Nanodiamond is one of the nanoparticles.However,due to the limitation of the polarization efficiency of nanodiamond in liquid,the relationship between the surface properties of nanodiamond and polarization enhancement has not been systematically studied,which is not conducive to the development of experiments.Focusing on the time-consuming problem of conventional T1-T2 experiment and the new nanoparticle polarizer,the following studies were carried out:(1)Development of electron-nuclear double resonance probe at ultra-low field of 7.5 mT.In this paper,an electron-nuclear double resonance DNP probe based on slit cavity is designed,simulated and developed.Using the signal source and RF switch,the electronic signal transmission of 210 MHz and the synchronous control of electronic and nuclear signals in ultra-low field are realized.On the electromagnet,nearly-130 times proton polarization enhancement is achieved by using ODNP method.(2)One dimensional relaxation method based on dynamic nuclear polarization.Both the depolarization time method and polarization buildup time method of ODNP are useful for Ti time measurement of systems containing free radicals.In this paper,two methods of measuring one-dimensional Ti time by ODNP are explored.Ti of single component samples and two-component samples with different relaxation times are studied.Combined with the selective enhancement experiment of OMRI,the importance of measuring Ti time by ODNP is emphasized,which can supplement the results of OMRI.OMRI technology can be combined with the measurement of Ti by ODNP,which is useful to study the characterization systems containing free radicals.(3)Research on fast two-dimensional relaxation method.In this paper,the methods of ONDP measuring Ti(depolarization time method and polarization build time)are introduced into the two-dimensional relaxation methods,and a fast two-dimensional T1-T2 method using damaged module to achieve zero waiting time is proposed,which containing Depolarization-T1-T2 and Buildup-T1-T2.The effectiveness of ODNP fast two-dimensional relaxation method is verified in a single system and two systems.Compared with the conventional inversion recovery T1-T2 method,the ODNP fast twodimensional T1-T2 method improves the time efficiency by nearly 40 times on the basis of higher resolution and accuracy.(4)Application of fast two-dimensional relaxation method in porous materials.Using ODNP fast two-dimensional relaxation method,in the porous material model including free water and confined water system,the distribution of free water and confined water is quickly and accurately distinguished within 90s.In the porous material model of oil-water mixture,the rapid and accurate identification of oil-water is successfully realized.From the two-dimensional T1-T2 relaxation spectrum,the wetting characteristics of porous material surface are indirectly reflected by the value of T1/T2.In addition,the fast two-dimensional relaxation method of ODNP can be used to observe the phase transition process of thermosensitive hydrogels quickly and accurately.(5)Study on the polarization properties of nanodiamond as a new polarization polarizer.The surface properties of nanodiamond were systematically studied by Raman spectroscopy,ESR and SEM.The possibility of nanodiamond as polarization polarizer was determined.Then,using nanodiamond as polarizing agent,the polarization enhancement of nanodiamond suspensions with different nanoparticle sizes and concentrations was studied.The maximum polarization enhancement of protons was-22.5 times in the current liquid environment using nanodiamond as polarizing agent.It is found that the particle size of nanoparticles is negatively correlated with polarization enhancement,and the specific proportion of sp2 hybrid on diamond surface,nanoparticle concentration and microwave power are positively correlated with polarization enhancement.Based on the above experimental results,a method to optimize the properties of nanoparticle polarizer is proposed,which provides a development direction for further improving the polarization enhancement.