In this study, a frequency tuner system is developed for generating variable frequency magnetic fields for magnetic hyperthermia applications. The tuning device contains three specially designed phase lock loop devices that drive a resonant inverter working in the frequency band of 180–525 kHz. This tuner system can be adapted for other resonant inverters employed in the studies of ferrofluids with superparamagnetic nanoparticles. The performance of the whole system is also examined. Our findings were in agreement with the theoretical expectations of phase locking and frequency tuning. The system is tested for samples of a solid magnetic material of cylindrical shape and ferrofluids with differing concentrations of powdered magnetite. The observations indicate significant frequency changes of the magnetic field due to heating of the samples. These frequency variations can be a source of errors, which should not be neglected in experiments determining the specific absorption rate or power dissipated density.

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An AC magnetometer system is developed to determine the specific absorption rate (SAR) of ferrofluids designed to work in the range of interest for magnetic hyperthermia. The experimental setup contains a set of configurable RL coil sensors (resistance plus inductance) to obtain the inner area of the dynamic hysteresis loops of colloidal dispersions, which are stimulated using a multi-range alternating magnetic field generator. This magnetometer is suitable for covering the frequency band of 100-450 kHz, and special considerations concerning sample size and placement inside the magnetic field region are taken into account. The performance of the system is tested using a ferrofluid of water-dispersed iron oxide nanoparticles. The SAR determined with the developed system is compared with that obtained using the typical calorimetric procedure. The observations are consistent with both kinds of measurement, and also coincide with the results for other previously reported experimental systems.

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An experimental setup designed to determine the Curie temperature T c of solid materials is presented. The main idea is based on the experimental frequency tracking of a resonant inverter circuit, which is controlled by a phase lock loop (PLL) device. When a ferromagnetic metallic piece is placed inside the resonant coil, the effective impedance is modified due to its magnetic permeability variations caused by heating. Hence, the PLL frequency and the temperature of the sample are simultaneously recorded to determine the magnetic transition point. Later, the Curie temperature of powdered and solid pieces of pure nickel, stainless steel and MnZn-ferrite are measured. In addition, a sample of magnetite nanoparticles is analysed. Discrepancies lower than 2.3% of the known T c values are observed for the powdered samples, and similar results are obtained with the solid samples. The ferrite did not completely reach magnetic transition, differing by up to 30% from the reference value.

In this study, an industrial process is proposed to produce Fe 3 O 4 nanoparticles with a high specific absorption rate. The analysis was focused on the theoretical study of the dynamic performance in a continuous stirred tank reactor system, using the chemical kinetics of the reaction, which was obtained experimentally in the present work. For this purpose, nanoparticles with different sizes were prepared varying the reaction time by the thermal decomposition method. Subsequently, their physical and chemical properties were characterized by X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, transmission electron microscopy, and magnetic measurements. The results obtained suggested that this material could be used in magnetic hyperthermia; for this reason, the nanoparticles were functionalized with DMSA through a binder exchange reaction. Afterward, the specific absorption rate of the ferrofluid was determined under an applied AC magnetic field (17.5 kA/m, 153 kHz), obtaining a value of 109.75 W/g. To produce these nanoparticles industrially, the use of two CSTRs connected in series was considered because one was required for the nucleation zone and another for the growth zone. The heating in each of the reactors was carried out by a set of electrical resistances regulated by a feedback control system, using PI controllers, which were tuned minimizing the IAE through the stochastic method of simulated annealing. The preheating in each stream that fed each reactor and the cooling in the output stream of the second reactor were through heat exchangers. The results indicate that the proposed process is appropriate because the dynamic responses in both reactors do not present higher oscillations when the temperature is disturbed in each of the currents that feed to the reactors. Also, the stabilization time in the temperature of the CSTRs is not greater than 10 min, which leads to a reasonable control of the particle size during its production, since the size is a function of the reaction time and temperature. Therefore, these Fe 3 O 4 nanoparticles are suitable to be applied in magnetic hyperthermia and produced by the proposed process.

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