Slikanje porazdelitve magnetnih delcev z uporabo šibkih magnetnih polj.

Malec, Jan (author), Arčon, Denis (mentor)

Slikanje z magnetnimi delci je metoda, pri kateri uporabljamo selekcijska magnetna polja za določanje prostorske gostote magnetnih nanodelcev z nelinearnim odzivom magnetizacije na zunanja polja. Ključni problemi, ki jih je bilo potrebno rešiti za uspešno izvedbo, so sinteza nanodelcev z velikim magnetnim momentom ter nizkim poljem nasičenja, razumevanje relaksacijskih pojavov, izdelava vzbujevalnih in sprejemnih tuljav ter optimizacija algoritmov za rekonstrukcijo slike. Pri raziskovanju smo pregledali tri vzorce, ki so jih sintetizirali na odseku za fiziko materialov Instituta Jožef Stefan in ocenili njihovo primernost za slikanje. Dva vzorca sta vsebovala vodni suspenziji mešanice železovih oksidov Fe2O3 in Fe3O4 z znanima porazdelitvama velikosti nanodelcev (9 +- 2) nm in (11 +- 2) nm. Tretji vzorec je vseboval suhe nanodelce brez podane velikosti. Njihovo porazdelitev velikosti smo z uporabo elektronskega mikroskopa določili na (54 +- 15) nm. Magnetne lastnosti vseh treh vzorcev smo določili z uporabo vibracijskega magnetometra in magnetometra na osnovi superprevodnega kvantnega interferometra. Delci v suspenzijah so se na zunanje magnetno polje odzivali kot popolni superparamagneti, suhi delci pa so pri sobni temperaturi kazali neničelno remanentno polje velikosti (414 +- 4) A/m. Iz meritev magnetizacije suhega vzorca v magnetnem polju in ob odsotnosti magnetnega polja lahko razberemo, da pride pri temperaturi 125 K do hitre spremembe magnetizacije, ki smo jo pripisali Verweyevemu prehodu. Za potrebe slikanja smo sestavili napravo, ki je vsebovala par tuljav v Maxwellovi konfiguraciji, s katerimi smo ustvarili statično gradientno polje, par manjših tuljav, s katerimi smo ustvarili izmenično, vzbujevalno polje, sprejemno tuljavo ter kompenzacijski tuljavi, s pomočjo katerih smo signal delcev ločili od signala vzbujanja. Signal smo zajemali z ojačevalnikom s fazno vpeto zanko. Večjo ponovljivost rezultatov smo dosegli tako, da smo motorizirali premikanje delcev, podatke pa zajemali z osebnim računalnikom. S postavitvijo smo lahko posneli osnovno sliko in tako pokazali, da princip slikanja deluje. Dosegli smo ločljivost velikostnega reda 1 mm. Glavno omejitev pri slikanju je v našem primeru predstavljala velikost magnetnega gradienta, saj magnetizacija delcev v območju slikanja ni dosegla nasičene vrednosti.

• Language: Slovenian
• Keywords: superparamagnetizem, slikanje magnetnih delcev, nanodelci, magnetit, Verweyev prehod, polje nasičenosti, prostorska ločljivost
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2019
• PID: 20.500.12556/RUL-106109
• COBISS.SI-ID: 3291236
• Publication date in RUL: 28.01.2019

Secondary language

• Language: English
• Title: Imaging of a magnetic particle distribution using weak magnetic fields

Abstract

Magnetic particle imaging (MPI) is a method that uses spatially selective magnetic fields to measure the concentration of magnetic nanoparticles with non-linear response to applied magnetic fields. The key problems that must be solved to achieve imaging are synthesis of nanoparticles with high magnetic moment and low saturation field, understanding of relaxation effects, manufacture of generating and receiving coils and optimization of image reconstruction algorithms. We have studied three samples of magnetic nanoparticles produced by the department of material science of Institute Jožef Stefan and determined their characteristics regarding MPI imaging. Two of the samples were composed of water suspensions of iron oxide with known size distributions of (9 +- 2) nm and (11 +- 2) nm. The size distribution of the third sample containing dry particles was determined using a scanning electron microscope to (54 +- 15) nm. The magnetic properties of the samples were determined using a vibrational magnetometer and a magnetometer based on superconducting quantum interference device. The particles in both suspensions exhibited an ideal superparamagnetic response to the outside magnetic field, while the dry particles showed a remanence of (414 +- 4) A/m. By looking at the temperature dependent magnetization of the dry particles measured in presence of an external magnetic field and in absence of an external magnetic field we can identify a phase transition around 125 K. The phase transition was identified as a Verwey transition. The particles were imaged using a device that we assembled using a pair of Maxwell coils generating a time-independent magnetic gradient, a par of extraction coils used to create a sinusoidal extraction field, a receive coil used to acquire the signal, and two compensation coils used to separate the particle signal from the extraction signal. The signal was acquired using a detector with a phase locked loop. Motorized sample movement and digital data acquisition were set up to make the measurements more reproducible. We have demonstrated the effect of spatial selection and showed that it is possible to image objects with simple geometry with a spatial resolution of approximately 1 mm.

• Keywords: superparamagnetism, magnetic particle imaging, nanoparticles, magnetite, Verwey transition, saturation field, spatial resolution