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1.
Maghemite nanoparticles covalently coated with polyethylene glycol are investigated with respect to different loss processes in magnetic AC-fields. Transmission electron microscopy reveals a narrow size distribution which may be well approximated by a normal distribution (mean diameter 15.3 nm and distribution width 4.9 nm). Aqueous ferrofluids were characterised by DC-magnetometry, by measuring susceptibility spectra for a frequency range 20 Hz to 1 MHz and by calorimetric measurements of specific loss power (SLP) at 330 and 410 kHz for field amplitudes up to 11.7 kA/m. Extremely high values of SLP in the order of 600 W/g result for 400 kHz and 11 kA/m. In addition to liquid ferrofluids measurements were performed with suspensions in gel in order to elucidate the role of Brownian relaxation. The measured susceptibility spectra may be well reproduced by a model using a superposition of Néel and Brown loss processes under consideration of the observed narrow normal size distribution. In this way the observed very high specific heating power may be well understood. Results are discussed with respect to further optimisation of SLP for medical as well as technical RF-heating applications.  相似文献   

2.
We report on the frequency and field dependent complex magnetic susceptibility measurements of a kerosene-based magnetic fluid with iron oxide nanoparticles, stabilized with oleic acid, in the frequency range 0.1-6 GHz and over the polarising field range of 0-168.4 kA/m.By increasing polarising field, H, a subsidiary loss-peak clearly occurs in the vicinity of the ferromagnetic resonance peak, from which it remains distinct even in strong polarising fields of 168.4 kA/m. This is in contrast to other reported cases in which the intra-well relaxation process is manifested only as a shoulder of the resonance peak, which vanishes in polarising fields larger than that of 100 kA/m.The results of the XRD analysis connected to the anisotropy field results confirm that the investigated sample contains particles of magnetite and of the tetragonal phase of maghemite.Taking into account the characteristics of our sample, the theoretical analysis revealed that the intra-well relaxation process of the small particles of the tetragonal phase of maghemite may be responsible for the subsidiary loss peak of the investigated magnetic fluid.  相似文献   

3.
Suspensions of bacterial magnetosomes are investigated with respect to magnetic losses. Mean core diameter of about 30 nm results from TEM, XRD and magnetic analysis. Specific loss power is determined from hysteresis loops, susceptibility spectra and calorimetry with a maximum value of 960 W/g at 410 kHz and field amplitude 10 kA/m. Results may be understood by relaxation effects on hysteresis.  相似文献   

4.
Ferrofluids based on two types of hybrid particles Fe3O4/β-cyclodextrin were prepared: Using monodomain (below 60 nm) magnetite nanoparticles with (A) non-superparamagnetic (non-SPM) behaviour and (B) with superparamagnetic (SPM) behaviour. We found a strong dependence of the hybrid particles’ magnetic properties on their size and homogeneity. In both types of ferrofluids we observed hyperthermia upon applying an ac electromagnetic field with frequency 40 kHz and amplitude 30 kA/m. The maximal ΔТ upon irradiation with duration of about 12 min for the non-SPM particles was 12 °C, while for the SPM ones it was 3.5 °C.  相似文献   

5.
For a variety of magnetically based biomedical applications, it is advantageous to use sedimentation stable suspensions of relatively large (d>20 nm) magnetic core-shell nanoparticles. Water-based suspensions of multicore nanoparticles were prepared by coating of the particles (synthesized by means of a modified alkaline precipitation method) with a carboxymethyldextran shell. The resulting ferrofluids were structurally and magnetically characterized. It was found that these fluids show a specific heating power of about 60 W/g (f=400 kHz, H=10 kA/m). This value was increased up to 330 W/g by a simple fractionation method based on centrifugation. Finally, the cellular uptake of the multicore nanoparticles was demonstrated.  相似文献   

6.
Tumor-targeted magnetic hyperthermia has recently attracted much attention.Magnetic nanoparticles(NPs) are heat mediator nanoprobes in magnetic hyperthermia for cancer treatment.In this paper,single cubic spinel structural Zn0.3Fe2.7O4 magnetic NPs with sizes of 14 nm-20 nm were synthesized,followed by coating with SiO2 shell.The SLP value of Zn0.3Fe2.7O4/SiO2 NPs below 20 nm changes non-monotonically with the concentration of solution under the alternating current(AC) magnetic field of 430 kHz and 27 kA/m.SLP values of all Zn0.3Fe2.7O4/SiO2 NPs appear a peak value with change of solution concentration.The solution concentrations with optimal SLP value decrease with increasing magnetic core size.This work can give guidance to the better prediction and control of the magnetic hyperthermia performance of materials in clinical applications.  相似文献   

7.
Development of a new soft ferrite core for power applications   总被引:2,自引:0,他引:2  
Manganese-substituted nickel–zinc ferrites have been investigated as power core materials for applications in switched-mode-power supplies. High frequency operation of these power supplies requires high performance cores with low power losses. The main contributors to the power loss are eddy current loss, hysteresis loss and residual loss. The ferrites have been synthesized by the citrate precursor technique and their electromagnetic properties such as resistivity, permeability, saturation magnetization and Curie temperature studied. A power loss of 500 mW/cc could be obtained at a frequency of 500 kHz, flux density of 50 mT and temperature 100 °C.  相似文献   

8.
A magnetite ferrofluid coated by dextran with a high specific absorption rate (SAR) of 75 W/g in an AC field of 55 kHz and 200 Oe was prepared by the gel crystallization method with ultrasonic treatment. For comparison, uncoated magnetite particles with a mean diameter of 50 nm were also fabricated. Several possible mechanisms such as Brownian, Neel and diffusion relaxation processes on heating effects and their influence on SAR are discussed. Several factors which can increase the value of SAR were discussed, including dextran coating, ultrasonic treatment, proper particle size and the presence of defects and disorder in the particles.  相似文献   

9.
This paper reports on the frequency dependence of the magnetic and electric power dissipation in a magnetic fluid sample, in the microwave frequency range (0.5 to 8GHz), at various values of the static magnetic field (0 to 167.8kA/m). The computation of the power dissipation relies on the experimental values measured for the complex dielectric permittivity, ɛ = ɛ′ - iɛ″, and the complex magnetic permeability, μ = μ′ - iμ″, over the same frequency range. The results show that the magnetic power dissipation is much larger than the electric one for the investigated sample. At a specific frequency, f (Hz) , the power dissipation, p, depends on the external magnetic field, and exhibits a maximum. The result obtained suggests the possibility of controlling the energy absorption in the microwave range by means of the application of an external magnetic field.  相似文献   

10.
Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.  相似文献   

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