Razvoj merilnih postopkov za določitev magnetih izgub magnetnih tekočin : doktorska disertacija

slovenian

doktorska dela

2011

Univerza v Mariboru, Fakulteta za elektrotehniko računalništvo in informatiko

Magnetic fluids fall into the category of nano-materials and are, by definition, a stable colloidal dispersion of magnetic nanoparticles permanently suspended in a carrier liquid. As such, they are distinguished by certain specific characteristics which are effectively utilized in various fields such as mechanics as rotary seals, mechanical dampers, heat conductors and in biomedicine to target the dosage of medicinal substances, contrast agents for magnetic resonance imaging, magnetic fluid hyperthermia, etc. For safe and effective use of the material a good knowledge of its physical properties is required, regardless of the applications. In this thesis, some properties of magnetic fluids with an emphasis on its behavior in the alternating magnetic field are discussed. In this case, the magnetic particles or its magnetization vector align with the direction of magnetic field, which describes the Brownian and Neelćs relaxation. Rotating magnetic domains, within the material, are recognized as magnetic losses that among others result in an increase of fluidsć temperature that intensify with increasing amplitude and frequency of magnetic field. This exact property is used in the application of medical hyperthermia, which represents an alternative method for cancer treatment, where the magnetic fluid is injected into the tumor tissue and heated by means of magnetic field to achieve thermal destruction of tumor. For successful implementation of such treatment the characterization of magnetic losses of magnetic fluids represents the essential role. The main purpose of this dissertation is to build a measurement system that enables the measurement of crucial parameters for determining the losses of magnetic fluid. The measurement system is designed in a way that allows the determination of loss parameter SAR (Specific Absorption Rate) using a calorimetric method. In this case, we deal with the improvement of established method, where the emphasis is on the homogeneity of the magnetic field and a better thermal insulation of the sample and its surroundings. Second method for determining the magnetic losses that is tackled in this dissertation is method of magnetic measurement. It is based ondetermination of the hysteresis loops area for determination of the parameter of SPL (specific power loss). The essential parts of this method are two pickup coils wound around the measured sample. They are calibrated in the magnetic normal and therefore enable the calculation of magnetic field strength in the system and the magnetic flux density of the measured sample out of measured induced voltages. These two variables define the hysteresis loop whose area is proportional to magnetic losses. Characterization of magnetic losses is carried out by both methods for commercially available sample of magnetic fluid, where the results are given as a function of amplitude and frequency magnetic field. This thesis also presents a magnetic field analysis of the measurement system using the finite element method (FEM)and thermal field analysis; results of both analyses are consistent with measurements.

07.08.2023