Effects of SnO2 addition on the magnetic properties of manganese zinc ferrites

SnO₂ was added to high-permeability MnZn ferrites and MnZn ferrites for high-frequency power supplies. The effects of the SnO₂ addition were studied. Sn⁴⁺ ions can dissolve into the spinel lattice and form stable Fe²⁺–Sn⁴⁺ pairs and hence can compensate the magneto-crystalline anisotropy constant K₁ and improve the initial permeability effectively. The initial permeability of ferrites is also improved as abnormal grain growth caused by ion vacancy is controlled with SnO₂ doping. In addition, the SnO₂ doping also leads to a decrease in the relative loss factor and an increase in density. The power loss and minimum power loss temperature decrease with SnO₂ doping.     

Synthesis of nanocrystalline spinel CoFe2O4 via a polymer-pyrolysis route

Nanocrystalline CoFe₂O₄ spinel ferrites were synthesized via the pyrolysis of polyacrylate salt precursors prepared by in situ polymerization of metal salts and acrylic acid. The pyrolytic behaviors of the polymeric precursors were analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). The structural characteristics of the calcined products were obtained by powder X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscope (TEM). The results revealed that cobalt ferrites had nano-sized morphology and good crystallinity even if calcined at moderate temperature like 500 °C for 3 h. The average size of nanocrystalline cobalt ferrites ranged from 20 to 30 nm with a narrow size distribution, while the particle size increased with the increase of the calcination temperature. Magnetic properties were obtained at room temperature using a vibrating sample magnetometer. The samples exhibited hysteresis loop typical of magnetic behaviors, indicating that the presence of an ordered magnetic structure could exist in the mixed spinel system. The as-calcined cobalt ferrites at 500 °C exhibited the highest magnetization value of 77.4 emu/g at 10 kOe, while the highest remanence and coercivity of 35.6 emu/g and 1445 Oe, respectively, for those calcined at 700 °C were obtained.     

Technique under compressive stress for determining the magneto-elastic effects in ferrites

A method for determining the magneto-elastic effects in spinel ferrites at high frequencies is presented. To investigate the high-frequency properties of different ferrite materials under stress, a reflection/transmission cell had been designed. In the measurement technique described here, we use a cell which based on a modified reflection/transmission line. This setup allows measurement of permeability under stress up to 20 MPa from 1 MHz to 10 GHz. Specific retrieval procedures have been developed to take into account the modifications in the line geometry. In this study, the permeability properties of spinel ferrites versus frequency between 1 MHz and 6 GHz as a function of the applied stress are showed. The two most important contributions of the permeability are the wall bulging and the gyro-magnetism which have a different behavior versus the applied stress.     

Dielectric loss,conductivity relaxation process and magnetic properties of Mg substituted Ni–Cu ferrites

The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni–Cu ferrites with general formula Ni_0.5Cu_0.5−xMg_xFe₂O₄ (0.0≤x≤0.5) have been investigated as a function of frequency, temperature and composition. ac resistivity of all the samples decreases with increase in the frequency exhibiting normal ferrimagnetic behavior. The frequency dependence of dielectric loss tangent showed a maximum in between 10 Hz and 1 kHz in all the ferrites. The conductivity relaxation of the charge carriers was examined using the electrical modulus formulism, and the results indicate the presence of the non-Debye type of relaxation in the prepared ferrites. Similar values of activation energies for dc conduction and for conductivity relaxation reveal that the mechanism of electrical conduction and dielectric polarization is the same in these ferrites. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates that the distribution of relaxation time is temperature independent. The saturation magnetization and coercivity as calculated from the hysteresis loop measurement show striking dependence on composition.     

Effects of composition and sintering temperature on properties of NiZn and NiCuZn ferrites

Effects of composition and sintering temperature on grain size, porosity and magnetic properties of the NiZn and NiCuZn ferrites were investigated. It was found that the lowest power loss could be obtained with the equimolar composition for both NiZn and NiCuZn ferrites, which could be attributed to the lowest porosity. A slight deficiency or excess of Fe₂O₃ content had no pronounced influence on saturation magnetic flux density (B_s) in our testing range. However, a slight excess of Fe₂O₃ was effective to improve the initial permeability, which could be attributed to decrease of the magnetocrystalline anisotropy. With the increase of sintering temperature, the initial permeability and power loss of the NiZn and NiCuZn ferrites had different development trend, which could be explained by the different variation trend of the grain size and porosity. Power losses of the NiCuZn ferrite samples were lower than that of the NiZn ferrite samples at any sintering temperature. Synthetically, the NiCuZn ferrites had a better performance than the NiZn ferrites in power field use.     

Magnetic disaccommodation in Sr hexagonal ferrites with X-phase (2SrO·15Fe2O3) initial composition

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of X-phase (2SrO·15Fe₂O₃) in the temperature range between 80 and 500 K. The time decay of the initial permeability after sample demagnetization is represented by means of isochronal disaccommodation curves which show the presence of different disaccommodation processes whose maxima lie at 380, 300 and 160 K (resp. A, B and D peaks). This behaviour is explained regarding the spectra corresponding to barium ferrites in order to ascribe the different relaxation processes found for ionic transitions in the cationic sites within the hexagonal structure.     

Improvement in electrical and magnetic properties of mixed Mg-Al-Mn ferrite system synthesized by citrate precursor technique

In the present work, mixed magnesium-manganese ferrites of composition Mg_0.9Mn_0.1Al_0.3Co_zFe_1.7−zO₄ where z=0.3, 0.5 and 0.7 have been synthesized by the citrate precursor technique. X-ray diffraction patterns of the samples confirmed the formation of single-phase spinel structure. The ferrites have been investigated for their electric and magnetic properties such as dc resistivity, Curie temperature, saturation magnetization, initial permeability and relative loss factor (RLF). Fairly constant value of initial permeability over a wide frequency range (0.1-20 MHz) and low values of the relative loss factor of the order of 10⁻⁴-10⁻⁵, in the frequency range 0.1-30 MHz, are the cardinal achievements of the present investigation. In addition to this, initial permeability was found to increase with an increase in temperature while RLF was observed to be low at these temperatures. The dc resistivity and Curie temperature were found to increase with an increase in cobalt content. The mechanisms contributing to these results are discussed in detail in this paper.     

Ni-Mg,Mg-Mn和Ni-Zn三种铁氧体在3厘米波段的法拉第旋转及衰耗

Faraday rotation and loss characteristics at 9200 me and at room temperature of Ni-Mg, Mg-Mn and Ni-Zn ferrite rods and disks placed in a circular waveguide are measured as a function of applied longitudinal DC magnetic field.Of particular interest are the results obtained with a new type of Ni-Mg ferrites, since very little data on Ni-Mg ferrites have been published in the literature. In this paper sufficient data are given on Faraday rotation and loss characteristics of a complete range of nickel-magnesium ferrites with composition varying from pure nickel to pure magnesium. Experimental results at 9200 me indicate that for the sintering temperature of 1300℃, the best composition range for nickel-magnesium ferrites would be Ni_0.7Mg_0.3Fe₂O₄-Ni_0.8Mg_0.2Fe₂O₄. In future, nickel-magnesium ferrites, due to their very low loss, may find practical applications in the microwave field.Comparison information as to the relative merits of these three useful microwave ferrites are given in the form of curves and tables, and their results are discussed in some detail.This paper also gives a summary of the linear theory on Faraday rotation of a linearly polarized wave propagating in ferrite-loaded waveguide which is longitudinally magnetized.     

Study of dielectric behaviour of Mn-Zn nano ferrites

Dielectric properties of Mn_0.4Zn_0.6Fe₂O₄ ferrites synthesised by co-precipitation method have been investigated as a function of frequency (up to 30 MHz) at different temperatures. Dispersion in dielectric constant has been observed between temperatures 450-500 K. DC resistivity was found to increase up to 100 times greater than those for the samples prepared by the conventional ceramic methods. Resistivity variation with temperature is also reported in the present work. The particle size is calculated using Scherrer equation for Lorentzian peak, which comes out between 9 and 19 nm. Possible mechanisms contributing to these processes have been discussed.     

Effects of Ta2O5 addition on the microstructure and temperature dependence of magnetic properties of MnZn ferrites

MnZn ferrites with the chemical formula Mn_0.68Zn_0.25Fe_2.07O₄ have been prepared by the conventional ceramic technique. Toroidal cores were sintered at 1350 °C for 4 h in N₂/O₂ atmosphere with 4% oxygen. Then the influence of Ta₂O₅ addition on the microstructure and temperature dependence of magnetic properties of MnZn ferrites was investigated by characterizing the fracture surface micrograph and measuring the magnetic properties over a temperature ranging from 25 to 120 °C. The results show that, when the Ta₂O₅ concentration is not more than 0.04wt%, the grain size has a slight increase with the increase of Ta₂O₅ concentration, the temperature of secondary maximum peak in the curve of initial permeability versus temperature and the lowest power loss shift to lower temperature. However, excessive Ta₂O₅ concentration (>0.04wt%) results in the exaggerated grain growth and porosity increase, which make the initial permeability and saturation magnetic flux density decrease and the power loss increase at room temperature. Furthermore, the temperature of secondary maximum peak in the curve of initial permeability versus temperature and the lowest power loss shift to about 100 °C.     

Effect of magnesium substitution on dielectric and magnetic properties of Ni-Zn ferrite

The dielectric and magnetic properties of Mg incorporated Ni-Zn spinel ferrites have been investigated. Ni_0.5−xZn_0.5Mg_xFe₂O₄ ferrites have been prepared by sol-gel auto-combustion technique. The as prepared ferrites were annealed at 673, 873 and 1073 K. The X-ray diffraction studies reveal the spinel structure of annealed ferrites. The TEM results are in agreement with XRD results. FTIR study has also been carried out to get insight into the structure of these ferrites. The dielectric measurements show that the dielectric constant (ε′), dielectric loss (tan δ) and conductivity (σ_ac) increase on incorporation of Mg in the Ni-Zn ferrite. ε′, tan δ and σ_ac also show dependence on temperature, frequency of external applied electric field and microstructure of the samples. The magnetic moment measurements reveal that the saturation magnetization (M_s) increases and coercivity (H_c) decreases with the increase in concentration of Mg²⁺ ions. M_s and H_c also show dependence on the annealing temperature.     

Ultrasonic characterization of heavy metal TeO2–WO3–PbO glasses below room temperature

The longitudinal ultrasonic attenuation measurements have been made using pulse echo method at fundamental frequencies of 2, 4, 6 and 8 MHz in 20WO₃–(80−x) TeO₂–xPbO ternary tellurite glasses (x=10, 12.5, 15, 17.5 and 20 mol%) in the temperature range 160–280 K. The results showed the presence of a broad peak which shifts to higher temperature with increasing frequency. The ultrasonic attenuation peaks suggest that the experimental behavior is controlled by thermally activated structural relaxations. The internal friction, acoustic activation energy, deformation potential, relaxation strength, number of loss centers and density of state have been calculated both as a function of temperature and PbO content. The acoustic activation energy was found to decrease from 0.156 to 0.135 eV with the increase of PbO content. The results showed that both the number of loss centers and their activation energy decrease with the atomic ring size. An increase in the density of state is observed with addition of PbO content at the same frequency in the whole range of temperature which is associated with structural units formed when PbO is added.     

Microwave-absorbing properties of NiCoZn spinel ferrites

In this paper, the microwave-absorbing properties of (Ni_1−x−yCo_xZn_y)Fe₂O₄ spinel ferrites have been investigated within the frequency range of 0.5–14 GHz. There are two kinds of resonance peaks observed in the permeability spectra: domain-wall resonances at lower frequency and spin-rotation resonances at higher frequency. The reflection loss (RL) calculations show that the prepared NiCoZn spinel ferrites are good electromagnetic (EM) wave absorbers in microwave range. In terms of the absorbing frequency band (AFB) and the matching thickness (t_m), (Ni_0.407Co_0.207Zn_0.386)Fe₂O₄ shows the best performances: t_m=3.15 mm and the AFB is 8.64–11.2 GHz. Decreasing the weight ratio of NiCoZn ferrites in ferrites/wax composites, the matching thickness decreases and the AFB shifts to higher frequencies. Compared with the absorbers with single-layer ferrites, the absorbers with double-layers ferrites have better absorbing performances, such as a thinner matching thickness and a wider EM wave AFB.     

High permeability and low power loss of Ti and Zn substitution lithium ferrite in high frequency range

The effect of Zn and Ti on the magnetic, power loss and structural properties of Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ ferrites (x=0.0 to 0.30 in step of 0.05)+0.5 wt% Bi₂O_3, prepared by standard ceramic technique, has been investigated. Complex permeability (μ*=μ′−jμ″) has been analyzed at room temperature in frequency range from 1 to 10³ MHz. It was found an enhancement in permeability with Ti and Zn concentration in Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ and exhibits the maximum value 106 for x=0.20 sample. Complex permeability of these ferrites exhibits stable frequency response up to 7 MHz beyond which the real part decreases sharply and imaginary part increases to have a peak at the relaxation frequency. Power loss measurements have been carried out in induction condition (B=10 mT) in frequency range of 50 kHz to 3 MHz. Power loss has been found to be quite low with the substitution of Ti and Zn in lithium ferrite.     

HL-2M 装置电子回旋共振加热高压电源系统的研制

为满足 HL-2M 装置电子回旋共振加热、低杂波电流驱动、中性束注入等二级加热系统的需要,设 计了多套基于脉冲步进技术的大功率高压电源,为速调管、回旋管及中性束、离子源等提供高压。重点针对一套 用于电子回旋系统的高压电源进行了分析。该高压电源设计了多副边绕组的高压隔离变压器、全固态高压调制器 和电源控制系统,并得出了适合本电源控制的控制算法,易于调整高压的输出幅值以及高压的上升下降时间。最 后给出实验结果,以验证电源的保护能力、电源控制算法的可操作性及实用性。试验证明,此套电源不仅满足对 负载的快速保护的要求,其电源工作的稳定度等其它参数也满足设计要求。      

Losses from rotational hysteresis and magnetic anisotropy in single crystals of barium and barium-cobalt hexaferrites

The rotational-hysteresis loss as a function of field has been examined for single crystals of the hexaferrites BaM and Co₂W. Curves for K₁(T) for BaM and K₃(T) for Co₂W have been recorded. The domain structure has been examined on specimens with residual magnetization. The rotational-hysteresis loss for BaM is of the type usual for ferrites, having a negative temperature coefficient below 120 °C, but above this there is an anomalous positive coefficient, which is due to diffusion of Co ions in the rotating field. The initial increase in the loss is related to the dynamics of the domain structure. Heat treatment influences the temperature dependence of the anisotropy constants and hence the rotational hysteresis loss.     

Enhancement of AC-losses of magnetic nanoparticles for heating applications

Aqueous ferrofluids of maghemite nanoparticles coated with carboxydextran were investigated with respect to their specific loss power (SLP) in dependence on frequency and field amplitude of magnetic AC-fields. In order to elucidate the effect of the size distribution on SLP fluid fractions with different mean particle core size were prepared by a magnetic separation procedure from the original ferrofluid. Structural characterisation by means of TEM and XRD as well as reconstruction of core size distributions from magnetisation curves reveals that the narrow size distributions of the fractions cover a range of mean core sizes from about 8 up to 20 nm. Spectra of the complex susceptibility were measured for a frequency range of 20 Hz to 1 MHz. From the imaginary part of the susceptibility the specific loss power is calculated in dependence on frequency. The results are compared with calorimetrical measurements performed in dependence on field amplitude up to 11 kA/m at 410 kHz. A very high specific loss power in the order of 400 W per gram maghemite was found at 410 kHz and 11 kA/m for the fluid fraction having the largest mean core diameter. A deviation from linear response behaviour is found for this sample showing a power law field dependence of the specific loss power SLPH^2.5. In addition to liquid suspensions measurements were performed with particles immobilised in mannitol or gel in order to elucidate the role of Brownian relaxation. The experimentally found dependence of SLP on the mean particle core diameter may be understood in the frame of the Debye dispersion model. Results are discussed with respect to applications of ferrofluids in RF-magnetic hyperthermia.     

Synthesis, electrical and electromechanical properties of a tungsten-bronze ceramic oxide: Pb0.68K0.64Nb2O6

A tungsten-bronze ceramic oxide, Pb_0.68K_0.64Nb₂O₆, has been prepared by a standard solid-state reaction technique. Compound formation and phase identification has been confirmed by X-ray diffraction (XRD) studies. The dielectric permittivity and the loss tangent of the sample have been measured in a frequency range 45 Hz–5 MHz and a temperature range 35–590 °C. Electrical properties of the material were studied using an impedance spectroscopic technique. Detailed analysis of the impedance spectrum suggested that the electrical properties of the material are strongly temperature dependent. The Nyquist plots clearly showed the presence of bulk and grain boundary effect in the compound. The imaginary part of modulus at different temperatures shows a relaxation peak and its position shifts to higher frequency with increase in temperature. This suggests a temperature-dependent relaxation. The frequency dependent ac conductivity at different temperatures indicated that the conduction process is thermally activated process.     

Development of low loss Mn–Zn ferrite working at frequency higher than 3 MHz

With the advance of modern electronic technology, there has been a critical need for Mn–Zn ferrites with even higher permeability and even lower power loss at higher frequencies. In this study, ferrite with extremely low losses than conventional ferrite materials at high frequency was developed employing a conventional ceramic powder processing technique. As a result, the core loss at 3 MHz, 10 mT and 100 °C is around 300 kW/m³, and its cutoff frequency is 4 MHz. Furthermore, the electromagnetic characteristics and the microstructure of this new DMR50 material are also discussed.     

Comparison of in vitro and in vivo acoustic response of a novel 50:50 PLGA contrast agent

A comparison between in vitro and in vivo experiments conducted to investigate the acoustic properties of a novel, 1.2 μm diameter poly(lactic-co-glycolic acid) (50:50) (PLGA) ultrasound contrast agent, the development of which was described previously by us, is presented. A pulse-echo setup was used to determine enhancement in vitro. Additional in vitro studies further characterized the hollow microcapsules, including resonance frequency from attenuation measurements (from 2.25 to 15 MHz) and temperature effects (25 °C vs. 37 °C). In vivo, four rabbits received intravenous injections of the agent (dose range: 0.005–0.13 ml/kg). Quantitative in vivo dose–responses were calculated off-line using spectral power analysis of audio Doppler signals acquired from a custom-made 10 MHz cuff transducer placed around the surgically exposed distal aorta. This frequency was chosen since the very shallow scanning depths encountered in rabbits, in particular for the cuff transducer placed directly around the vessel, necessitates the use of high frequency imaging devices with sufficient spatial resolution to enable meaningful measurements. For qualitative assessments, two rabbits were imaged pre- and post-contrast administration (dose: 0.1 ml/kg) in power Doppler mode. Significant acoustic enhancements (up to 24 dB) were reported both in vitro and in vivo. Moreover, the rabbits did not show any adverse side effects from multiple injections (>20) of the agent. Measured in vitro resonance frequency between 3.09 and 3.49 MHz was lower than predicted for a similar sized free bubble, potentially due to capsule wall structure. Minimal loss of signal (4 dB) was observed at 25 °C over 20 min of insonation at 5 MHz but at 37 °C the signal dropped close to base line within the first 5 min. This temperature sensitivity could be due to loss of capsule integrity (and hence loss of gas). Potential causes include increased hydrolysis or polymer softening and increased water uptake by the shell at temperatures closer to the glass transition temperature (T_g).