Heating Characteristics of Transformer Oil-Based Magnetic Fluids of Different Magnetic Particle Concentrations

The heating ability of mineral oil-based magnetic fluids with different magnetic particle concentrations is studied. The calorimetric measurements were carried out in an alternating magnetic field of 500 A · m⁻¹ to 2500 A · m⁻¹ amplitude and of 1500 kHz frequency. The revealed H ⁿ law-type dependence of the temperature increase rate, (dT/dt) _t=0, on the amplitude of the magnetic field indicates the presence of superparamagnetic and partially ferromagnetic particles in the tested samples since n > 2. The specific absorption rate (SAR) defined as the rate of energy absorption per unit mass increases with a decrease of the volume fraction of the dispersed phase. This can be explained by the formation of aggregates in the samples with a higher concentration of magnetic particles.     

Thermophysical and Magnetic Properties of Carbon Beads Containing Cobalt Nanocrystallites

Magnetic Co-beads were fabricated in the course of a three-step procedure comprising preparation of a metal–acrylamide complex, followed by frontal polymerization and finally pyrolysis of the polymer. The composites obtained were composed of cobalt nanocrystallites stabilized in a carbon matrix built of disordered graphite. The crystallite size, material morphology, fraction of the magnetic component, and thus the magnetic properties can be tailored by a proper choice of the processing variables. The samples were subjected to an alternating magnetic field of different strengths (H = 0 to 5 kA · m⁻¹) at a frequency of f = 500 kHz. From the calorimetric measurements, we concluded that the relaxation processes dominate in the heat generation mechanism for the beads pyrolyzed at 773 K. For the beads pyrolyzed at 1073 K, significant values of magnetic properties, such as the coercive force and remanence give substantial contribution to the energy losses for hysteresis. The specific absorption coefficient (SAR) related to the cobalt mass unit for the 1073 K pyrolyzed beads (SAR = 1340   W ·g^-1 _cobalt){({\it SAR} = 1340 \, \, {\rm W} \cdot {\rm g}^{-1 }_{\rm cobalt})} is in very good conformity with the results obtained by other authors. The effective density power loss, caused by eddy currents, can be neglected for heating processes applied in magnetic hyperthermia. The Co-beads can potentially be applied for hyperthermia treatment.     

AC Magnetic Technique to Measure Specific Absorption Rate of Magnetic Nanoparticles

The electrodynamic method is applied to determine the specific absorption rate (SAR) of an assembly of superparamagnetic nanoparticles as a function of frequency and magnetic field amplitude. The home made frequency-adjustable electromagnet is used to create a nearly uniform magnetic field in a core gap of a volume 1×3×3 cm³ in the frequency range f=10–150 kHz and for magnetic field amplitudes up to H ₀=250 Oe. Two oppositely connected pick-up coils are used to record the electromotive force signal (EMF) generated by magnetic nanoparticles. By integrating the EMF signal one can determine the low-frequency hysteresis loops of the assembly and the assembly SAR. Using this method the measurement of SAR has been carried out for magnetite nanoparticles with an average diameter D=25 nm. The electrodynamic method is shown to be capable of measuring a small amount of magnetic nanoparticles, up to 5×10^?5 g, dispersed in a solid matrix. The maximal SAR ～?80 W/g has been obtained for the magnetite nanoparticle assembly investigated.     

Magnetic Properties of YBCO Single-Crystal Grown on Y2O3 Layer by a Cold Top-Seeding Method

In this work, single-crystal samples 20 mm in diameter were grown by a cold top-seeding method. In order to study the effect of an Y₂O₃ buffer layer, a compacted precursor was located in a crucible on a buffer layer of freely poured Y₂O₃ powder. The YBCO bulk samples were carefully prepared in the same chemical composition. In order to examine the homogeneity of the samples, rectangular specimens were cut from a sample. The structural orientation of the specimens was measured by a X-ray diffractrometer (XRD). The microstructure was observed with a Scanning Electron Microscope (SEM). The magnetic properties were measured using a Vibrating Sample Magnetometer (VSM) at 77 K with the applied magnetic field parallel to the c-axis. As a result only (00l) peaks were observed in X-ray diffraction data, indicating that all specimens are highly oriented with the c-axis perpendicular to the top surface. Also the cut samples exhibited very fine Y211 inclusion distributed in the bulk sample observed in SEM micrographs. The critical current density J _c (A/cm²) has been estimated by the extended Bean model. The maximum value of the critical current density at 77 K is 5.2×10⁴ A/cm² in a self-field. The trapped magnetic field measurements of the samples were performed by using Hall probe scanning device with static field of 0.5 T at 77 K. The single-domain sample exhibits a trapped field of 1140 G at 77 K.     

Magnetic PNIPA hydrogels for hyperthermia applications in cancer therapy

Temperature-sensitive Poly (N-isopropylacrylamide), PNIPA gels were synthesized with micron-sized iron and iron oxide (Fe₃O₄) particles to investigate their viability for combined hyperthermia and drug release applications. The ultimate goal is to combine hyperthermia and triggered drug release. Induction heating of the magnetic hydrogels with varying concentration of magnetic powder was conducted at a frequency of 375 kHz for magnetic field strength varying from 1.7 kA/m to 2.5 kA/m. It was observed that the maximum temperature induced in the magnetic hydrogels increased with the concentration of magnetic particles and magnetic field strength. The PNIPA gel underwent a collapse transition at 34 °C. The best combination was found for the PNIPA–Fe₃O₄ system, 2.5 wt.% Fe₃O₄ in PNIPA–Fe₃O₄ system took 260 s to be heated to 45 °C under a magnetic field strength of 1.7 kA/m and the specific absorption rate (SAR) was found to be 1.83. SAR of iron oxide was found to be higher than the SAR of iron.     

Magnetoelectric performance of cylindrical Ni–lead zirconate titanate–Ni laminated composite synthesized by electroless deposition

The cylindrical Ni–lead zirconate titanate (PZT)–Ni laminated composites with various magnetostrictive–piezoelectric phase thickness ratios were synthesized by electroless deposition. The influences of the bias magnetic field (H _dc) and the ac magnetic field frequency (f) on magnetoelectric (ME) effect are discussed. It is seen that the ME voltage coefficient depends strongly on H _dc and f. The ME voltage coefficient and electromechanical resonance frequency increase as the magnetostrictive–piezoelectric phase thickness ratio increases. The calculated resonant frequency increases with the magnetostrictive–piezoelectric phase thickness ratio, which agrees well with the experimental results. The maximum ME voltage coefficient of the cylindrical Ni–PZT–Ni laminated composite is 3.256 V cm⁻¹ Oe⁻¹, which is much higher than that of the plate laminated composite with the same magnetostrictive–piezoelectric phase thickness ratio. Electroless deposition is an efficient method to prepare ME laminated composites with complex structures. Proper resonant frequency and stronger ME effect can be obtained by optimizing the structure.     

Electric-field-induced dielectric response and magnetization in nano–microscale lead-free multiferroic composite

A new kind of nano/microscale lead-free multiferroic composite with a formal of 0.4CoFe₂O₄–0.6[0.948(K_0.5Na_0.5)NbO₃–0.052LiSbO₃] was prepared. The electric-field-induced dielectric (EID) and magnetization (EIM) behaviors were systemically investigated for this 0–3-type structured multiferroic ME composite. Significant electric-field-induced effects were obtained with a high tunability and a large EIM coefficient, which the former has a value of K = 3.35% under the low dc electric-field of 2 kV/cm and the later is in excess of 2.15 × 10⁻¹⁰ s/m at a low frequency of 1.0 kHz with a low magnetic bias field of 2000 Oe for this composite. It is also proved that its EIM behavior is strongly dependent on the bias magnetic field H _Bias and the frequency of driving electric-field. The measured EIM has an excellent linear relationship with the applied ac electric-field with amplitude varying from 7.1 to 73.6 V/cm. The electric-field-induced effects indicate that the 0–3-type particulate ME composites not only have physical interest but also have potential practical use.     

Thermophysical and Magnetic Properties of Carbon Beads Containing Nickel Nanocrystallites

Ferromagnetic and superparamagnetic nickel nanocrystallites, stabilized in a carbon matrix, were prepared by a three-step procedure including formation of a Ni acrylamide complex, followed by frontal polymerization and pyrolysis of the polymer at various temperatures. It was found that the procedure applied enables fabrication of magnetic beads containing metallic nanocrystallites embedded in a carbon matrix. The size of the crystallites, their morphology, volume fraction, and magnetic properties can be tailored by the pyrolysis temperature. The size of the crystallites affects their behavior in an external magnetic field, i.e., a heating process is the most effective for a sample pyrolyzed at 873 K. The revealed H ⁿ-type dependence of the temperature increase rate, (dT/dt) _t=0, on the amplitude of the magnetic field indicates the presence of both superparamagnetic and ferromagnetic particles in all the samples studied since n > 2. For the superparamagnetic particles, the heating mechanism is associated with Néel relaxation. For the lower values of the magnetic field amplitude, H <  H ₀, the relaxation losses dominate whereas for the opposite case, H >  H ₀, the magnetic hysteresis is the main source of thermal energy losses. The composites containing magnetic Ni nanocrystallites entrapped in a carbon matrix can be potentially applied for hyperthermia treatment.     

In vitro heat generation by ferrimagnetic maghemite microspheres for hyperthermic treatment of cancer under an alternating magnetic field

Ferrimagnetic materials can be expected to be useful as thermo seeds for hyperthermic treatment of cancer, especially where the cancer is located in deep parts of body, as they can generate heat by magnetic hysteretic loss when they are placed in an alternating magnetic field. Recently, it was reported that ferrimagnetic maghemite (γ-Fe₂O₃) microspheres 20–30 μm in diameter prepared in aqueous solution can show excellent heat generating ability. However, these microspheres have many cracks on their surfaces. In this study, the preparation conditions for the microspheres was further optimized in order to obtain crack-free ferrimagnetic microspheres, and the in vitro heat generation of the obtained microspheres was measured in an agar phantom under an alternating magnetic field. Crack-free γ-Fe₂O₃ microspheres 20–30 μm in diameter were obtained successfully. Their saturation magnetization and coercive force were 68 emu g⁻¹ and 198 Oe, respectively. Their heat generation under an alternating magnetic field of 300 Oe at 100 kHz was estimated to be 42 W g⁻¹. The microspheres showed in vitro heat generation when they were dispersed in an agar phantom and placed under an alternating magnetic field. It is believed that these microspheres may be useful for the in situ hyperthermic treatment of cancer.     

Transport Properties under Magnetic Field and J c (H) Peak Effect on Single-Crystal Bi-2212 Whiskers

In this study, 50 nm–18 mm in length superconducting single-crystal Bi-2212 whiskers have been fabricated using glass-ceramic BSCCO material. The microstructure, transport and magnetic properties were characterized by XRD, SEM-EDX, R–T, I–V and M–H analysis. Highly c-axis oriented single-crystal whiskers were produced without any grain boundaries in any crystallographic direction. T _c and T _zero were obtained to be 94.8 K and 92.8 K, respectively, and decreased with increasing the magnetic field. Magnetic properties, M–H curves of the whiskers, were investigated at 3 different constant temperatures up to 5 T. Symmetric hysteresis loops for all temperature and field cases were obtained and maximum J _c ^mag value was calculated to be 4.58×10⁶ A cm⁻² at 10 K for 0 T but dropped to 1.61×10⁶ A cm⁻² at 30 K for 0 T. Transport critical current density, J _c ^trans, of the whiskers in low magnetic field (between 0 and 270 Oe) and in a wide temperature region (5–75 K) was investigated. It was obtained that initially J _c ^trans increased with increasing the magnetic field but then started to decrease with further increase on the magnetic field at all temperature values. We have investigated this problem in terms of the Bulk pinning model, the Geometrical barrier model and the Bean–Livingston surface barrier model. We have noticed that all these models were not fitted well to our data. However, we have modified the Bean–Livingstone model by adding a new parameter. It was found that this modified formula fitted well to the data obtained in this work and can give successful explanations to the other results obtained by other groups.     

Magnet–PNIPA hydrogels for bioengineering applications

Temperature-sensitive Poly (N-isopropylacrylamide), PNIPA gels were synthesized with micron-sized iron and iron oxide (Fe₃O₄) particles to investigate their viability for hyperthermia applications. Induction heating of the magnetic hydrogels with varying concentration of magnetic powder was conducted at a frequency of 375  kHz for magnetic field strength varying from 1.7 kA/m (21 Oe) to 2.5 kA/m (31.4 Oe). It was observed that the maximum temperature induced in the magnetic hydrogels increased with the concentration of magnetic particles and magnetic field strength. The PNIPA gel underwent a collapse transition at 34 °C. It was found that a 2.5 wt.% Fe₃O₄ in PNIPA composite took 260 s to be heated to 45 °C under a magnetic field strength of 1.7 kA/m, the specific absorption rate (SAR) was found to be 1.83. SAR of iron oxide was found to be higher than the SAR of iron.     

On the Applicability of the Magnetic Susceptibility Models for the Magnetic Superconductor Ru-1212

The applicability of theoretical models for the ac susceptibility measurements of polycrystalline RuSr₂GdCu₂O₈ superconductor has been examined within the temperature range between 8–50 K, ac magnetic field 0.5–25 G, and frequency 20–12500 Hz. In general, a reasonable qualitative agreement between theory and experiment was attained. An evident and detectable asymmetry was observed within the Cole–Cole polar plots with a peak enhancement for both theoretical and experimental data. The modified critical state models are found to generate much better explanation of the ac susceptibility measured data than Bean’s model. For fields above 20 G, the results are agreed roughly with the Bean critical state model, while below 20 G, the Kim–Anderson model is more suitable to account of the magnetic performance. The temperature and field amplitude dependencies of the flux-creep exponent, n, were extracted from the real part of susceptibility, χ′, dependence on frequency. The flux-creep exponent was found to decrease with both temperature and ac field amplitude in accordance to a power-law of the form: n(T,H)=n ₀(H)T ^−s(H). Such dependence might be an indication of a crossover to flux-creep bundles regime.     

Magnetic cluster state in anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

It is established that the magnetic state of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite represents the spin-glass state of the cluster. The magnetic field at the beginning (H < 10 kOe) leads to the fragmentation of ferromagnetic clusters, then (H > 10 kOe) leads to the transition to a ferromagnetic state of an antiferromagnetic matrix and to increase in the degree of polarization of local spins of manganese. It is determined that the freezing temperature of magnetic moments varies as T _f = 65 − 6H ^0.21. The causes and mechanism of the magnetic phase separation are discussed.     

Magnetic ordering in Cu-Zn ferrite

The variation of magnetization with temperature of the Zn _x Cu_1–x Fe₂O₄ system has been obtained between 300 K and the Néel temperature at a constant magnetic field of 5.57×10⁵ A m^–1 for x=0 to 0.8. The observations indicate the existence of a Yafet-Kittel (Y-K) type of magnetic ordering in the mixed ferrites. A molecular field analysis of the Y-K spin-ordering using a three-sublattice model is shown to explain the experimental data satisfactorily. For the sake of verification, Néel temperatures of Cu-Zn ferrites were also determined from Mössbauer studies.     

Influence of Sm substitution on the phase,microstructure and microwave dielectric properties of SrLa<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript>

New dielectric ceramics in the SrLa_4−xSm_xTi₅O₁₇ (0 ≤ x ≤ 4) composition series were prepared through a solid state mixed oxide route to investigate the effect of Sm⁺³ substitution for La⁺³ on the phase, microstructure and microwave dielectric properties. At x = 0–3, all the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1500–1580 °C. At x = 4, a mixture of Sm₂Ti₂O₇ and SrTiO₃ formed. The maximum Sm⁺³-containing single phase ceramics, SrLaSm₃Ti₅O₁₇, exhibited relative permittivity (ε_r) = 42.6, temperature coefficient of resonant frequency (τ _f ) = −96 ppm/^oC and quality factor (Q _u f _o ) = 7332 GHz. An analysis of results presented here indicates that SrLa_4−xSm_xTi₅O₁₇ ceramics, exhibiting τ _f  ~ 0 and ε_r ~ 53 could be achieved at x ~ 1.4 but at the cost of decrease in Q _u f _o .     

Influence of Zr doping on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

Pure and Zr-substituted CaCu₃(Ti_1−x Zr _x )₄O₁₂ (x = 0, 0.01, 0.02, 0.03) ceramics were prepared by the Pechini method. X-ray powder diffraction analysis indicated the formation of single-phase compound, and all the diffraction peaks were completely indexed by the body-centered cubic perovskite-related structure. The effects of Zr⁴⁺ ion substituting partially Ti⁴⁺ ion on the dielectric properties were investigated in frequency range between 100 Hz and 1 GHz. The low frequency (f ≤ 10⁵ Hz) dielectric constant decreases with Zr substitution and the high frequency (f ≥ 10⁷ Hz) dielectric constant is unchanged. Interestingly, a low-frequency relaxation was observed at room temperature through Zr substitution. The observed dielectric properties in Zr-substituted samples were discussed using the internal barrier layer capacitor model. A corresponding equivalent circuit was adopted to explain the dielectric dispersion. The characteristic frequency of low-frequency relaxation rises due to the decrease of the resistivity of grain boundary with Zr substitution, which is likely responsible for the large low-frequency response at room temperature.     

A critical assessment of the standard molar gibbs free energy of formation of NiWO4

Three independent studies have been reported on the free energy of formation of NiWO₄. Results of these measurements are analyzed by the “third-law” method, using thermal functions for NiWO₄ derived from both low and high temperature heat capacity measurements. Values for the standard molar enthalpy of formation of NiWO₄ at 298·15 K obtained from “third-law” analysis are compared with direct calorimetric determinations. Only one set of free energy measurements is found to be compatible with calorimetric enthalpies of formation. The selected value for Δ_f H _m ⁰ (NiWO₄, cr, 298·15 K) is the average of the three calorimetric measurements, using both high temperature solution and combustion techniques, and the compatible free energy determination. A new set of evaluated data for NiWO₄ is presented.     

Enzymatic preparation of hollow magnetite microspheres for hyperthermic treatment of cancer

Ferrimagnetic materials can be expected to be useful as thermal seeds for hyperthermic treatment of cancer, especially where the cancer is located in deep parts of body, as they can generate heat by magnetic hysteretic loss when they are placed in an alternating magnetic field. In this study, hollow magnetite (Fe₃O₄) particles were prepared using an enzymatic reaction of urease. A hollow particle was obtained by using a Pasteur pipette. The particle was 500 μm in size and was composed of Fe₃O₄. Its saturation magnetization and coercive force were 57 emu⋅g⁻¹ and 183 Oe, respectively. Its heat generation under an alternating magnetic field of 300 Oe at 100 kHz was estimated to be 45 W⋅g⁻¹. Microspheres 30 μm in diameter were also successfully obtained by using a spray gun.     

Non-destructive evaluation of defects in ferromagnetic plates using a sensitive magnetic sensor based on second harmonic response of superconducting Bi1.6Pb0.4Sr4Ca2Cu3O10+δ pellet

The characteristics of a magnetic sensor, based on the non-linear electromagnetic response of the weak links present in polycrystalline BPSCCO superconductor are reported. The second harmonic response of the sensor in an alternating magnetic field at 40 kHz and at 77 K being a strong linear function of low d.c. field is utilized for magnetic field sensing. The noise limited resolution of the sensor is found to be 3.16 x 10⁻⁹ T/√Hz forH _a.c.= 16 G and frequency 40 kHz. The magnetic sensor has been applied for nondestructive detection of various types of flaws in ferromagnetic plates and also for detection of small magnetic inclusions in a non-magnetic matrix. Our results suggest that the 2f response based BPSCCO superconductive magnetometer has potential for its application in the area of non-destructive evaluation of defects in ferromagnetic materials.     

Dielectric and ferromagnetic properties of BaTiO<Subscript>3</Subscript>/Ni<Subscript>0.93</Subscript>Co<Subscript>0.02</Subscript>Cu<Subscript>0.05</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> composites

xBaTiO₃ + (1 − x)Ni_0.93Co_0.02Cu_0.05Fe₂O₄ (x = 0.5, 0.6, 0.7, 0.8) composites with ferroelectric–ferromagnetic characteristics were synthesized by the ceramic sintering technique. The presence of constituent phases in the composites was confirmed by X-ray diffraction studies. The average grain size was calculated by using a scanning electron micrograph. The dielectric characteristics were studied in the 100 kHz to 15 MHz. The dielectric constant changed higher with ferroelectric content increasing; and it was constant in this frequency range. The relation of dielectric constant with temperature was researched at 1, 10, 100 kHz. The Curie temperature would be higher with frequency increasing. The hysteresis behavior was studied to understand the magnetic properties such as saturation magnetization (M _s). The composites were a typical soft magnetic character with low coercive force. Both the ferroelectric and ferromagnetic phases preserve their basic properties in the bulk composite, thus these composites are good candidates as magnetoelectric materials.