亲疏水性SiO2复合有机树脂包覆对FeSiCr磁粉芯性能的影响

研究了亲疏水性SiO₂复合有机树脂对FeSiCr磁粉芯性能的影响以及退火热处理对磁粉芯磁性能的影响。将FeSiCr合金粉末经磷化处理后,分别采用疏水性、亲水性两种不同类型的SiO₂复合有机树脂对粉体进行绝缘包覆,经压制成型并固化制备成FeSiCr磁粉芯。结果表明,采用偶联改性亲水性SiO₂复合有机树脂的包覆效果明显优于疏水性SiO₂复合有机树脂包覆。随着SiO₂添加量的增加,磁粉芯的体电阻率和矫顽力增加,而密度、饱和磁化强度及磁导率降低;采用质量分数0.5%疏水性SiO₂复合有机树脂包覆工艺制备的FeSiCr磁粉芯退火后综合性能最优,在20 mT,100 kHz测试条件下,磁损耗仅为49.84 kW/m³,相比于采用纯树脂包覆工艺制备的磁粉芯损耗下降了16.9%。退火热处理能够有效提高FeSiCr磁粉芯的磁性能。     

亲疏水性SiO2复合有机树脂包覆对FeSiCr磁粉芯性能的影响

研究了亲疏水性SiO₂复合有机树脂对FeSiCr磁粉芯性能的影响以及退火热处理对磁粉芯磁性能的影响。将FeSiCr合金粉末经磷化处理后,分别采用疏水性、亲水性两种不同类型的SiO₂复合有机树脂对粉体进行绝缘包覆,经压制成型并固化制备成FeSiCr磁粉芯。结果表明,采用偶联改性亲水性SiO₂复合有机树脂的包覆效果明显优于疏水性SiO₂复合有机树脂包覆。随着SiO₂添加量的增加,磁粉芯的体电阻率和矫顽力增加,而密度、饱和磁化强度及磁导率降低;采用质量分数0.5%疏水性SiO₂复合有机树脂包覆工艺制备的FeSiCr磁粉芯退火后综合性能最优,在20 mT,100 kHz测试条件下,磁损耗仅为49.84 kW/m³,相比于采用纯树脂包覆工艺制备的磁粉芯损耗下降了16.9%。退火热处理能够有效提高FeSiCr磁粉芯的磁性能。     

Application of RF varactor using Ba(x)Sr(1-x)TiO3/TiO2/HR-Si substrate for reconfigurable radio

In this paper, the potential feasibility of integrating Ba_xSr_1-xTiO₃ (BST) films into Si wafer by adopting tunable interdigital capacitor (IDC) with TiO₂ thin film buffer layer and a RF tunable active bandpass filter (BPF) using BST based capacitor are proposed. TiO₂ as a buffer layer is grown onto Si substrate by atomic layer deposition (ALD) and the interdigital capacitor on BST(500 nm)/TiO₂(50 nm)/HR-Si is fabricated. BST interdigital tunable capacitor integrated on HR-Si substrate with high tunability and low loss tangent are characterized for their microwave performances. BST/TiO₂/HR-Si IDC shows much enhanced tunability values of 40% and commutation quality factor (CQF) of 56.71. A resonator consists of an active capacitance circuit together with a BST varactor. The active capacitor is made of a field effect transistor (FET) that exhibits negative resistance as well as capacitance. The measured second order active BPF shows bandwidth of 110 MHz, insertion loss of about 1 dB at the 1.81 GHz center frequency and tuning frequency of 230 MHz (1.81-2.04 GHz).     

Single-Junction Thermal Voltage Converters With Reduced Uncertainties at Frequencies up to 1 MHz

This paper presents a new realization of standards for ac–dc difference at voltages ranging from 1 to 4 V and frequencies of up to 1 MHz. This work has mainly been focused on improving the determination of the components of ac–dc difference that most significantly contributed to the uncertainty of the previous realization. These components are the ac–dc difference due to skin effect, transmission line effect, and the interaction between parasitic inductances and capacitances, which is described here as the L/C effect. As a result, combined $1 - sigma$ uncertainties as low as 4.3 $muhbox{V/V}$ at 1 MHz have been achieved.      

Precision interferometric frequency measurement of the 674 nm2S1/2-2D5/2 transition in asingle cold Sr+ ion

The narrow linewidth 674 nm ²S_1/2-² D_5/2 transition in a cold Sr⁺ ion confined within an RF Paul trap has been probed using an optically narrowed 674 nm diode laser offset-locked to a second diode laser stabilized to a high-finesse ultra-low-expansion reference cavity. The transition frequency has been measured by means of interferometric comparison with an iodine-stabilized 633 nm He-Ne reference standard. A preliminary value for the ²S_1/2-²D_5/2 transition line center is 444 779 045 (9) MHz, limited by residual micromotion and low magnetic field Zeeman splitting     

The study of transient electromagnetic fields in a multi-turn,voltage-excited laminated core reactor

A combined explicit and implicit finite-different scheme is used for analyzing transient electromagnetic fields in a multiturn, voltage-excited reactor with a laminated core of nonlinear magnetic property. In the analysis, the reactor is divided into iron, air and winding regions. By applying a step function and a time-variant sinusoidal voltage source, the time-dependent magnetic vector potential function is obtained with the aid of B-μ table. The transient responses of the current and inductance are given. The computed results are compared with those of an ideal R-L circuit, and an excellent agreement between the light-load result and the ideal value is found. The nonlinear effects on currents, flux distributions, inductances, and magnetic vector potentials due to the saturated iron core can be found in a heavy load condition     

$hbox{Mg}_{1-{rm x}}hbox{Co}_{rm x}hbox{Fe}_{1.98}hbox{O}_{4}$ Ceramics With Promising Magnetodielectric Properties for Antenna Miniaturization

We have studied the effects of concentration of cobalt and sintering temperature on the complex relative permittivity and permeability of MgFe_1.98O₄ ferrite ceramics. All samples were doped with 3 wt% Bi₂O₃ to improve the sintering properties of the Mg_1-xCo_xFe_1.98O₄ ceramics and thus attain a low dielectric loss tangent. X-ray diffraction data indicated that the ferrite samples all have a single-phase spinel structure and that there was no reaction between the ferrite and Bi₂O₃. In terms of microstructure development and grain growth, the effect of Bi₂O₃ is much more pronounced than that of Co. At a given concentration of Co, the magnetic properties can be fairly explained by a magnetic circuit model. The relationship between static permeability and the concentration of Co can be understood by using the single-ion model combined with the local uniaxial anisotropic model. Among the samples we prepared, Mg_0.96Co_0.04Fe_1.98O₄ sintered at 1000degC for 2 h has almost equal real permeability and permittivity-both approximately 10 - as well as sufficiently low dielectric and magnetic loss tangent over a range 3 to 30 MHz [the high-frequency (HF) band]. These properties that should be useful for miniaturization of HF antennas.     

Dielectric spectroscopy of Nano BaTiO/sub 3/ confined in MCM-41 mesoporous molecular sieve materials

Dielectric properties of barium titanate (BaTiO₃) particles, synthesized directly in the pores of MCM-41 materials, have been investigated in the frequency range from 20 Hz to 1 MHz for temperature intervals from 100 K to 500 K. The dielectric spectra of BaTiO₃ confined in these molecular sieves were compared with the results obtained from the investigation of pure MCM-41 materials. Obtained results confirmed successful incorporation of BaTiO₃ into porous matrix, but no phase transition from paraelectric to ferroelectric phase was observed due to the particle size being smaller than the critical size. Also, the overall dielectric response of investigated materials is strongly influenced by adsorbed water molecules     

Study of a microwave ferrite resonator antenna, based on a ferrimagnetic composite (Gd3Fe5O12)GdIGX-(Y3Fe5O12)YIG1-X

The authors report a study on microwave properties of a ferrimagnetic ceramic composite (Gd₃Fe₅O₁₂)GdIG_X-(Y₃Fe₅O₁₂)YIG_1-X. The magnetic and dielectric measurements at microwave frequency were made through the Hakki?Coleman?s procedure and through the excitation of a cylindrical ferrite resonator antenna (FRA). The hysteresis loops of the GdIG_XYIG_1-X ferrimagnetic composite were obtained. The numerical simulation study was performed using Ansoft?s High Frequency Structure Simulator (HFSS) software. The agreement between the experimental data and the simulated results is very good. These results demonstrated that, by varying the material composition of the resonator, ferrimagnetic composite GdIG_XYIG_1-X could operate as a magneto-dielectric antenna with bandwidth in the range of 7.5--11% and in the frequency range of 4.4--5.3 GHz. Additionally, all FRAs presented a low negative value of T_f, which indicates suitability for microwave applications such as mobile communication systems or satellite broadcast systems.     

Half-thickness inversion layer high-frequency ultrasonic transducers using LiNbO/sub 3/ single crystal

Half-thickness inversion layer high-frequency ultrasonic transducers were fabricated using lithium niobate (LiNbO₃) single crystal plate. The transducers developed for this study used a 36deg rotated Y-cut LiNbO₃ thin plate with an active element thickness of 115 mum. The designed center frequency was in the range of 30 to 60 MHz. Half-thickness inversion layer was formed after the sample was annealed at a high temperature, and it is shown that the inversion layer thickness can be controlled by the temperature. Silver powder/epoxy composite and parylene were used as acoustic matching layers. A lossy silver epoxy was used as the backing material. Using an analytical method, the electrical impedance for different inversion layer ratios was determined. The measured resonant frequency was consistent with the modeled data. Even-order higher frequency broadband ultrasonic transducers with a center frequency at 60 MHz were obtained using half-thickness inversion layer of LiNbO₃ single crystal     

Dielectric and magnetoelectric properties of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 − x)PbZr0.8Ti0.2O3 composites

Ceramic composites of Ni_0.8Co_0.1Cu_0.1Fe₂O₄ and lead–zirconate–titanate (PZT) were prepared using conventional solid state reaction method. The presence of constituent phases in composites was confirmed by X-ray diffraction (XRD). The variation of dielectric constant with frequency (100 Hz–1 MHz) and temperature has been studied. The variation of loss tangent (tan δ) with temperature (at frequency 1 kHz) has also been studied. The magnetoelectric (ME) output was measured as a function of dc magnetic field. The maximum value of ME output (625 mV/cm) was observed for 25% ferrite + 75% ferroelectric phase. The maximum ME response can be explained in terms of the content of ferrite, permittivity of dielectric material and the intensity of magnetic field. The ME response of these composites was observed to be linear within low dc magnetic field. These composites may form the basis for the development of magnetic sensors and transducers for use in solid state microelectronics and microwave devices.     

A magnetic head for 150 MHz, high density recording

A multilayered magnetic head that can read and write at 150 MHz on metal particle tape with a coercivity of 120 kA/m (1500 Oe) has been developed. Ten 2-μm layers of Fe₆₈Ru₈Ga₇ Si₁₇ alloy, with 100 nm of SiO₂ used as spacer, form the magnetic-core thickness and the track width. The head was tested in a rotary recording system at a relative head-to-tape speed of 73 m/s. At a linear density of 4000 fc/mm (100 kfc) and 150 MHz, the measured single frequency signal to 300-kHz-slot noise was 33 dB (RMS-RMS). The measured frequency response curve agrees with theory and indicates a head-to-tape spacing of 70 nm at high speed. The read efficiency of the head decreases from 37% at low frequency to 15% at 150 MHz     

Field modulated microwave absorption in high Tcsuperconductors

Modified electron spin resonance (ESR) techniques were used to measure low-field (<200 Oe) microwave absorption of high-T _c cuprates YBa₂Cu₃O_7-x. A series of experiments on polycrystalline and single-crystal samples in which the angle between the DC and the modulating magnetic fields was varied showed that the microwave absorption obeys results predicted from thermodynamic considerations of fluxoids. The absorption takes place in the normal regions created by flux penetration at low fields in these samples. The change in magnetically modulated microwave absorption on passing through T_c is explained by the decrease in absorption that occurs when part of the sample becomes superconducting and by the rapid variation of the penetration depth near T_c. The technique is an extremely sensitive method of detecting superconductivity in very small samples and for studying the superconducting state. Possible applications such as microwave attenuators, magnetic memories, and gaussmeters are discussed     

Magnetoimpedance effect at various temperatures for manganite La0.7Ca0.3MnO3−δ

In the present work, the AC magnetoimpedance effect in La_0.7Ca_0.3MnO_3−δ at various temperatures are investigated. The peak of the metal–insulator transition occurs in the temperature dependence of impedance. Negative magnetoimpedance effect in the La_0.7Ca_0.3MnO_3−δ is obtained at frequencies f≤10 MHz. In the magnetoimpedance effect of manganites, the magnetic field not only decreases the permeability μ_t, but also reduces the resistivity ρ by aligning the local spins and varying the transfer integral t_ij. The AC magnetoimpedance participated by the DC colossal magnetoresistance (CMR) in manganites, should be connected with the combined effects of double exchange interaction, electron–phonon coupling and skin effect.     

Unusual high-frequency behavior of some amorphous metallic-alloy tape-wound magnetic cores

The behavior of several samples of amorphous metallic-alloy tape-wound magnetic cores excited under sine-wave voltage conditions in the 1 kHz to 100 kHz frequency range is investigated. These tape-wound cores made by Magnetics, Inc. from two different Allied Corporation amorphous metallic-glass alloys, METGLAS® alloy 2605S-3 (composition Fe79B16Si5) and METGLAS® alloy 2605SC (composition Fe81B13.5Si3.5C2), are studied along with Magnetics, Inc. F ferrite cores. Measurements of the induced-voltage and exciting-current waveforms are used to obtain dynamic (ac) core characteristic loops of flux density B versus net ampere turns per unit magnetic path length Npip/lm, as well as core loss. The behavior of one of the amorphous metallic-glass alloys (Alloy 2605S-3) at flux-density levels in the range of 0.5 T and 1 T is found to lead to core characteristic loops with strange shapes and to unusual variations in the plots of energy loss per unit volume per cycle versus frequency. The second amorphous alloy (Alloy 2605SC) shows similar unusual properties but only at high flux-density levels close to saturation, above 1.25 T. The unexpected behavior observed in these two alloys is due to magnetoelastic resonances of the toroidally shaped cores. By modeling the tape-wound core as a thin ring, calculations are made for the resonant frequencies corresponding to the mode of extensional vibrations which agree well with experimentally measured results.     

High performances boron doped γ-Fe2O3particles prepared from oxalic precursors

γ-Fe₂O₃ particles (L≈0.15 to 0.2 μm, L/D≈2 to 5) with coercivity up to 400 Oe have been prepared from oxalic precursors. To preserve the particle form, the oxalates were doped with boron. The effects due to this element was studied during the different thermal treatments which effect the pseudomorphous transformation of oxalate particles into spinel ferrite. ESCA spectroscopy, and X-ray measurements could point out the presence of a Fe₃BO₅ phase at the surface of γ-Fe₂O₃ particles. In spite of the important mass losses (H₂O, CO, CO₂) and the important crystallographic structure modifications occurring during the transformation of oxalate into magnetic oxide, γ-Fe₂O ₃ particles obtained by this method have very good textural characteristics. Boron-doped γ-Fe₂O₃ particles were used to make 3.5-in diskettes with standard formulation. In these conditions, media properties were comparable to trading diskettes properties     

Free-space measurement of complex permittivity and complexpermeability of magnetic materials at microwave frequencies

A free-space measurement system operating in the 8.2-40-GHz frequency range is used to measure the reflection and transmission coefficients, S₁₁ and S₂₁, of planar samples. The complex electric permittivity and the magnetic permeability are calculated from the measured values of S₁₁ and S₂₁. The measurement system consists of transmit and receive horn lens antennas, a network analyzer, mode transitions, and a computer. Diffraction effects at the edges of the sample are minimized by using spot-focusing lens antennas. Errors due to multiple reflections between antennas via the surface of the sample are corrected by using a free-space TRL (thru, reflect, line) calibration technique. For thin, flexible samples, the sample had to be sandwiched between two half-wavelength (at mid-band) quartz plates to eliminate sagging. Results are reported in the frequency range of 8.6-13.4 GHz for materials such as Teflon, sodium borosilicate glass, and microwave-absorbing materials     

Polar nanodomains and relaxor behaviour in (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 crystals with x = 0.3–0.5

Phase transitions and dielectric properties of the (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ crystals with x = 0.3–0.5 are studied. The solid solutions in this composition range are shown to be relaxor ferroelectrics. The crystals with low x demonstrate a diffused maximum in the temperature dependences of the dielectric permittivity at T_m. T_m varies with frequency according to the Vogel–Fulcher law. The polarizing microscopy investigations reveal a first-order phase transition from the relaxor phase to the low-temperature ferroelectric phase at T_C, which is several degrees below T_m. The permittivity peak in the crystals with x = 0.5 is sharp, and T_m is equal to T_C and does not depend on frequency, as is typical of the transition from a ferroelectric to an ordinary paraelectric phase. Nevertheless, the relaxor, but not the paraelectric, phase is observed at T > T_m. This conclusion is confirmed by the observation of the temperature behaviour of complex dielectric permittivity at T > T_m, which is typical of relaxors and related to the existence of polar nanodomains.     

Surface impedance studies of the high-Tc oxidesuperconductors

The authors briefly review results of measurements of the multimeter-wave surface impedance of ceramic, thin-film and single-crystal samples of the high-T_c oxide superconductors. The observed losses and temperature dependences do not agree with BCS (Bardeen-Cooper-Schriefer) theory. The authors discuss recent measurements of single-crystal B₂CaSr₂Cu ₂O_x and the effect of the application of a perpendicular magnetic field. They consider the possibilities of intrinsic and defect mechanisms to explain the observations. In particular, they discuss the effects of weakly coupled granular films on surface impedance. It is concluded that better samples are necessary to resolve the issue of the source of the large values of the surface impedance observed thus far     

复合磁性催化剂β-MnO2/MnxZn1-xFe2O4的制备与表征

采用化学共沉淀法制备了以Mn_xZn_1-x Fe₂O₄为磁性基体的β-MnO₂/Mn_xZn_1-xFe₂O₄复合磁性催化剂,利用XRD、SEM、FTIR和超导量子干涉仪对复合磁性催化剂的结构和性能进行了表征,以罗丹明B （RhB）为模拟污染物,研究了β-MnO₂/Mn_xZn_1-xFe₂O₄的催化活性,并考察了其稳定性。结果表明,球状的β-MnO₂与块状的磁性基体Mn_xZn_1-xFe₂O₄成功复合,且制备的β-MnO₂/Mn_xZn_1-xFe₂O₄复合磁性催化剂具有良好的催化性能和磁学性能。当Mn_xZn_1-xFe₂O₄与β-MnO₂的质量比为20：100时,在2 mL含量为30%的H₂O₂作用下,1 h内β-MnO₂/Mn_xZn_1-xFe₂O₄复合磁性催化剂对100 mL浓度为10 mg/L的RhB降解率（93.9%）远高于纯β-MnO₂（33.7%）;在磁场作用下,β-MnO₂/Mn_xZn_1-xFe₂O₄复合磁性催化剂的回收率为89%,经过5次循环利用之后其对RhB的降解率仍达76%。