Study Of Method For Obtaining The Size Distribution Of Magnetic Nanoparticles

This thesis aims to obtain the size distribution of magnetic nanoparticles(MNPs) by using the magnetization curves. The magnetization curves of MNPs follows Langevin function and contains information about the particle size distribution. The magnetization curves can be gained from homemade device and fitted by optimization Algorithm. Thus the size distribution of MNPs can be obtained. The research on the size distribution of MNPs is of great significance to its application such as magnetic particle imaging and thermotherapy.Firstly, the particle size measurement model can be established by assuming that the size of MNPs obeys lognormal distribution and discreting the magnetization curves. In ferrofluid, monomer particles and polymer particles are in a state of dynamic equilibrium, and all contribute to the magnetization of MNPs in the presence of an external magnetic field. So the size distribution of MNPs can also be detected when polymer particles exist.Secondly, the magnetization curve of MNPs can be accurately gained from the homemade device under triangular-wave magnetic field. This device includes three functional modules: generating triangular-wave magnetic fields, measuring the magnetization curve and data processing. The triangular-wave magnetic fields can be generated by applying pre- edited excitation waveform and PID controller. Also it can be detected by measuring the voltage on sampling resistor. The magnetization of MNPs can be detected by using two reverse tandem detecting coil. Data processing module includes inhibition of background magnetic field, reconstruction of magnetization, averaging the data, obtaining the magnetization curve of MNPs.Finally, the size distribution of MNPs can be obtained by magnetization curve fitting with optimization algorithm. The most commonly used method for nonlinear curve fitting is L-M algorithm. L-M algorithm is a kind of local optimization algorithm, so the size distribution of MNPs can't be accurately obtained. To improve the accuracy of particle size measurement, adopting global optimization algorithm—Global Search. The result is ideal, even when the initial point is far away from the optimal solution or the concentration is unknown.The system and method for obtaining the size distribution of MNPs under triangular-wave magnetic field is proved by using SOR-20 and SOR-30 samples.