DEVELOPMENT OF MEASUREMENT PROCEDURES FOR DETERMINING THE MAGNETIC LOSSES OF MAGNETIC FLUIDS

Magnetne tekočine sodijo v kategorijo nano-materialov in so po definiciji stabilne koloidne disperzije magnetnih nanodelcev, trajno suspendiranih v nosilni tekočini. Kot takšne imajo določene specifične lastnosti, ki jih s pridom izkoriščajo na različnih področjih, kot na primer v mehaniki za rotacijsko tesnjenje, mehansko blaženje, odvajanje toplote in tudi v biomedicini za ciljno doziranje zdravilnih učinkovin, kontrastni agenti pri slikanju z magnetno resonanco, hipertermiji, itd. Ne glede na aplikacijo pa je za varno in učinkovito rabo potrebno dobro poznavanje fizikalnih lastnosti materiala. V doktorski disertaciji smo obravnavali nekatere lastnosti magnetnih tekočin s poudarkom na obnašanju v izmeničnem magnetnem polju. V tem primeru se magnetni delci oziroma vektor magnetizacije delcev poravnajo s smerjo magnetnega polja, kar opisujeta pojava Brownove in Néelove relaksacije. Obračanje magnetnih domen znotraj materiala opisujemo kot magnetne izgube, katerih posledica je povišanje temperature tekočine. Z višanjem amplitude in frekvence magnetnega polja so izgube intenzivnejše. Ravno ta pojav, ki ga imenujemo hipertermija, se izkorišča v medicini in predstavlja alternativno metodo zdravljenja rakavih tkiv, kjer z vbrizganjem magnetne tekočine v tkivo in njeni izpostavitvi visokofrekvenčnemu magnetnemu polju dosežemo njihovo termično uničenje. Za uspešno izvajanje takšnega zdravljenja pa je bistvenega pomena karakterizacija magnetnih izgub magnetne tekočine. Glavni namen doktorske disertacije je izgradnja merilnega sistema, ki omogoča merjenje parametrov za določitev magnetnih izgub magnetih tekočin. Merilni sistem je zasnovan tako, da z uporabo kalorimetrične metode na osnovi povišane temperature tekočine določi parameter izgub SAR (specific absorption rate). V tem primeru gre za izboljšanje uveljavljene metode, kjer je poudarek na homogenosti magnetnega polja ter na boljši toplotni izolaciji vzorca in okolice. Druga metoda za določitev magnetnih izgub pa je metoda magnetnega merjenja, kjer lahko na osnovi površine histerezne zanke določimo specifične moči izgub SPL (specific power loss). Bistvo metode sta dve merilni tuljavici okoli merjenega vzorca, ki sta bili predhodno umerjeni v magnetni normali in merita inducirani napetosti, na osnovi katerih lahko izračunamo magnetno poljsko jakost v sistemu ter gostoto magnetnega pretoka vzorca. Ta dva signala definirata histerezno zanko, katere površina je proporcionalna magnetnim izgubam. Za komercialno dostopen vzorec magnetne tekočine je izvedena karakterizacija izgub po obeh metodah, kjer so izgube podane v odvisnosti od amplitude in frekvence magnetne poljske jakosti, določena pa je tudi njihova temperaturna odvisnost. V disertaciji je predstavljena tudi analiza magnetnega polja merilnega sistema s pomočjo metode končnih elementov (MKE), v istem sistemu pa je izvedena še termična analiza, kjer temperaturno odvisne izgube povzročijo časovno spremembo temperature vzorca, rezultati obeh analiz pa so skladni z meritvami.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 Neel’s relaxation. Rotating magnetic domains, within the material, are recognized as magnetic losses that among others result in an increase of fluids’ 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 on determination 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 analysisresults of both analyses are consistent with measurements