Papers by Keyword: Complex Permeability

Abstract: The existing low-frequency magnetic parameter test is mainly based on impedance analysis,Its sample needs to be prepared into a ring,the hard materials are difficult to process into rings for testing. Small piece of material cannot be tested using impedance analysis because it is too small to be cut into a ring for testing. In this paper, the principle of impedance analysis and the basic theory of strip line be used, an empirical formula for the complex permeability of the magnetic bulk thick film material in the 0.1Ghz~0.6GHz frequency band is obtained through design and test by design simulation and physical fixture, it makes the testing of the complex permeability of thick film materials easier due to it avoid damage and ring cutting before testing.

Abstract: Three-dimensional flower-like nickel-ferrite composites were successfully synthesized via decomposition of the nickel-iron alkoxide precursors by varying the ratio of iron and nickel ion in the solution. The overall morphology and size of the particles had no obvious change compared with the precursors. The high-frequency absorption properties of the composites were investigated in a frequency range of 2-18 GHz. The reflection loss and bandwidth varied with the nickel ratio and thicknesses of the compositions, showing valuable prospect in high-frequency wave attenuation. The wave absorbing mechanism was also discussed, which could be attributed to the dielectric loss, magnetic loss, and the synergetic effect.

Abstract: The phase structure, and electrical and magnetic properties of La_0.7Sr_0.3MnO₃ (LSMO)-x Ag (x is the mole ratio, x=0, 0.3, 0.5) composite were investigated. It is found that the sample with x=0 is single phase; the samples with x=0.3 and 0.5 present three phase composite structure of the manganese oxide and Ag. With the increasing of Ag content, the grain size of the samples increases and the grain boundaries transition from fully faceted to partially faceted. The permittivity of spectrum (10 MHz - 1 GHz) and the theoretical simulation reveal that the plasma frequency f_p increase with Ag content, due to the increasing of free electron concentration, which is further supported by the enhancement of conductivity. While for the permeability (μ_r'), the μ_r' decrease with the increasing of Ag content at low frequency range (f < 20 MHz), while at the relative high frequency range (f > 300 MHz), the μ_r' increased with Ag content. Therefore, the introduction of elemental Ag resulted in a higher μ_r' at the relative high frequency range.

Abstract: Nanocrystalline Mn_1-xZn_xFe₂O₄ (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) ferrites have been successfully synthesised using microwave–hydrothermal method for high frequency applications. The nanopowders were characterised using X-ray diffraction (XRD) and sintered using microwave furnace at 900°C and the total time taken for sintering is 30 min. The frequency dependence of real and imaginary part of permeability were measured in the range 1 MHz to 1.8 GHz. The saturation magnetisation and coercive force were obtained using a vibration sample magnetometer (VSM) in the field of 1.5 T. The temperature dependence of initial permeability (μ_i) was measured in the temperature range of 300K to 600K at 10 kHz. The high values of permeability and saturation magnetization enables these materials to be the potential candidates for a number of applications, for example, in transformers, choke coils, noise filters and recording heads.

Abstract: In this paper, an improved coaxial measurement system with a newly designed coaxial fixture is presented. The electromagnetic parameters of samples are retrieved from the scattering parameters measured by a vector network analyzer (VNA) at microwave frequencies. The measurements of air and PTFE in the range of 1~6 GHz were carried out to verify the reliability and the accuracy of this measurement system. By using frequency-sweep and data processing techniques, the multiplicity of roots is eliminated. The results show that this system can effectively be applied to measure the material electromagnetic parameters of absorbing materials with high accuracy in a wide frequency band.

Abstract: Mn_0.7Zn_0.3Fe₂O₄ is synthesized by sintering the nanosize precursor with sintering aids, which is synthesized by the coprecipitation method. The crystal growth of Mn_0.7Zn_0.3Fe₂O₄ is controlled by the amount of sintering aids. Complex permeability is explained by the Maxwell-Garnett (MG) effective medium model. The ferromagnetic resonance frequency more than 1 GHz can be explained by the shape anisotropy under the sintering process of the Mn_0.7Zn_0.3Fe₂O₄ particles. These results suggest possibility of Mn_0.7Zn_0.3Fe₂O₄ as a high frequency device material.

Abstract: Magnetic composites, especially ferrite composites, are of great interest for embedded inductor applications. In this paper, the Ni-Zn ferrite particles (Ni1-xZnxFe2O4, x=0.2～0.8) with different zinc contents were synthesized via chemical coprecipitation method followed by modification with γ-glycidoxypropyl trimethoxysilane (KH-560). The particles were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The results show that the prepared Ni1-xZnxFe2O4(x=0.2～0.7) have good spinel structures, higher saturation magnetization (35.18～77.69 emu/g) and smaller hysteresis hoops, while Ni0.2Zn0.8Fe2O4 grains exhibit some paramagnetic behaviors, such as almost zero hysteresis and non-saturated magnetization. Next Ni1-xZnxFe2O4 magnetic/epoxy composites with different volume fraction of ferrite were prepared and their magnetic performances at high frequencies were characterized by an Agilent E4991A impedance analyzer (USA). It is found that with zinc content in Ni1-xZnxFe2O4 increasing from 0.2 to 0.7, the real part of the complex permeability (μ′) of these composites increase first and then decrease with the frequency increasing gradually from 10 MHz to 1 GHz. Of all, the epoxy composites with filler of Ni0.6Zn0.4Fe2O4 or Ni0.5Zn0.5Fe2O4 ferrite show good frequency stability, and the composites including Ni0.4Zn0.6Fe2O4 ferrite have the highest permeability, and the maximal value at the frequency of 100 MHz is 5.55 when the volume faction is 42.75%. The imaginary part of the complex permeability (μ′′) of all magnetic composites is low For the Ni0.2Zn0.8Fe2O4/epoxy composites, they have very low real permeability (μ′～1) and imaginary permeability (μ″≤0.2).

Abstract: The polycrystalline Yttrium Iron Garnet (YIG) powder with the chemical formula Y3Fe5O12 has been synthesized by using High Energy Ball Milling technique. The effect of various preparation parameters on the crystallinity, morphology and complex permeability of YIG, which includes milling time and annealing temperature were studied respectively by using XRD, SEM and Impedance Material Analyzer. The frequency dependence of complex permeability namely real permeability, µ’ and magnetic loss, µ’’ were measured at room temperature for samples sintered from 600°C to 1400°C, in the frequency range 10 MHz to 1 GHz. The results showed that milling time plays a role in determining the crystallinity of the milled powder where higher milling time results in better crystallinity due to high reactivity of the particles. From complex permeability measurement, it was observed that the initial permeability and magnetic loss increased with increasing grain size. The permeability values increased with annealing temperature and the absolute values of permeability decreased after attaining the natural resonance frequency of the material.

Abstract: The effect of the quenched speed on the microstructure and electromagnetic properties of melt spun Nd3Fe₆₈Co₁₈B₁₁ nanocomposites was investigated. XRD spectra showed that only α-Fe phase can be obtained for both quenched samples. The complex permeability () and permittivity () within 2-18GHz were measured by vector network analysis. By virtue of quenched speed increased, the flake-like Nd₃Fe₆₈Co₁₈B₁₁ decreased the value of permittivity while that of permeability slightly changed. According to transmission line theory, the reflection loss was calculated. With thin matching thickness of 1.5mm, The Nd₃Fe₆₈Co₁₈B₁₁ composites realized the optimum reflection loss RL=-5.3dB at 3.6GHz and RL=-3.5dB at 3.9GHz for quenched speed 30m/s and 20m/s, respectively. It showed that it is possible to obtain good microwave absorbing properties with appreciated quenched speed for as spun Nd-Fe-B nanocomposites.

Abstract: Nowadays human are exposed to an environment filling with electromagnetic waves over a wide frequency range. The electromagnetic properties of microwave absorbing materials plays an important role in the performance of civilian electromagnetic interference (EMI) shielding at low frequency range and military stealth technology at high frequency one. The electromagnetic properties include complex permittivity and permeability and its combination determines the electromagnetic wave absorption ability of a material. Complex permittivity and permeability can be measured by three different methods, i.e., free-space method, coaxial/waveguide method, and resonant cavity perturbation method. The first one requires a large space, expensive equipment, and not suitable for academic usage. In the present study, using coaxial/waveguide and resonant cavity perturbation methods, electromagnetic characteristics of iron-based microwave absorbing materials will be obtained and its microwave absorption performance will be investigated. In addition, a comparison between the measurements by these two methods will be addressed.