Magnetic Shielding Materials Research Articles

Measuring thermal and electrical performances of additively manufactured magnetic shielding material: an active thermography approach

ABSTRACT The thermal and electrical responses of additive manufactured specimens were analysed for a additive manufactured steel magnetic shield as a case study. The analysis was based on the evidence that variations in the thermal properties of a material can be measured as a phase delay in thermal diffusion through the material bulk. The signal post-processing was performed, and the results were presented in a phase diagram. The results showed that after heat treatment, the slope of the phase diagram changed to less steep, indicating an increase in thermal diffusivity and hence thermal conductivity. The electrical conductivity was predicted using the thermal conductivity and the Weidemann-Franz law and validated by experimental measurements of the electrical conductivity. The same approach was applied to predict the electrical conductivity in the magnetic shielding, taking into consideration the scaling of the density due to porosity. The results showed that the thermographic non-destructive full field non-contact approach can be used to evaluate the electrical properties of a component and that the heat-treated specimens show better thermal diffusivity and hence thermal and electrical conductivity.

Shielding Design for High-Frequency Wireless Power Transfer System for EV Charging With Self-Resonant Coils

This paper provides a complete coil and shielding design solution for a 6.6 kW electric vehicle wireless power transfer system based on compact self-resonant (SR) coils. Due to the presence of a conductive vehicle body above the receiver coil, vertical fringing flux must be shielded in any electric vehicle wireless charger. Using high-frequency SR coils, parasitic capacitance introduced by standard shielding design approaches degrades the quality factor of the coils. In this work, the high-frequency parasitic capacitance introduced by the magnetic and conductive shielding materials is analyzed in detail, and a shielding geometry optimization method is proposed. Using ferrite and an additional dielectric spacer, prototype aluminum-backed SR coils are fabricated to validate the modeling. The total thicknesses of the transmitter and receiver coils are only 11.4 mm and 7.4 mm, respectively. The prototype coils achieve 92.3% DC-DC efficiency and 7.1 kW/dm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> volumetric power density. This paper demonstrates the first 6.6 kW WPT system for EV charging using compact megahertz-self-resonant coils including a complete magnetic and conductive shielding implementation.

A Magnetic Compensation System Composed of Biplanar Coils Avoiding Coupling Effect of Magnetic Shielding

Magnetic compensation coils are often used to improve the effectiveness of a magnetically shielded room (MSR), but a distortion of the field is caused by the shielding layer consisting of high-permeability material, which leads to a reduction in compensation capacity. To solve this problem, a new method is presented in this article based on a target-field method, an image method, and particle swarm optimization. By this way, a magnetic compensation system composed of biplanar coils is designed and fabricated to compensate the background field for a wearable magnetoencephalography device inside the MSR, and the coupling effect between the magnetic shielding material and the compensation field is effectively avoided. Experimental data show that these compensation coils produce uniform fields and field gradients with the inhomogeneity of less than 3.8%, consistent with the simulation results. In addition, this system realizes a real-time compensation to the residual field and field gradient in the MSR.

Magnetic Shielding for Electric Car Power Cables

In this article, the effect of different magnetic shielding materials for electric car power cables has been studied using finite element analysis (FEA), verified by experimental results. Bearing in mind the complex spectrum modulation, typically using pulsewidth modulation (PWM), a magnetic field shielding up to 2 kHz is required. Therefore, magnetic shielding was studied for frequencies of 2 kHz, 50 Hz, and dc field. The employed magnetic shielding materials were high permeability layers, such as permalloy, as well as composites based on resin matrix including flakes or powders from nanocrystalline ribbons, such as nanocrystalline FINAMET ribbons. Magnetic shielding was studied for cable illustrating good shielding ability using a combination of Cu and permalloy sheets for frequencies up to 2 kHz. Furthermore, magnetic shielding was studied for the case of floor shielding, where permalloy appeared to be an acceptable shielding solution.

Magnetic Field Analysis and Optimization of the Gauge of Hybrid Maglev Needles

Compared with the traditional needle driving method, hybrid maglev needle driving is a new weft knitting machine technology, which alleviates the problems of noise, heat, and needle breakage. However, in the structure of needle arrays, magnetic disturbance between permanent magnet knitting needles leads to unstable needle control. Therefore, this paper attempts to solve this problem through a performance analysis of hybrid maglev needle driving. Based on the structure, the magnetic force distribution model of permanent magnet knitting needles is established. Aiming at the magnetic interference between magnetic arrays, a magnetic shielding material, silicon steel with a high permeability, is proposed to optimize the driving structure of a magnetic levitation needle array. Through simulation and experimental analysis, the influence of different silicon steel thicknesses on magnetic field shielding is analyzed. It is concluded that the optimal value of a silicon steel sheet is 1 mm and that the optimal gauge of hybrid maglev knitting needles is 8 mm. Finally, compared with the theoretical and simulation analysis, the experimental results have indicated that the proposed optimized structure of the gauge of hybrid maglev knitting needles is correct and effective.

Effect of Magnetic Shielding on Levitation Characteristics of Superconducting Gravimeter

The superconducting gravimeter is currently the most accurate instrument in relative gravity measurement, so the high-precision calculation and analysis of superconducting magnetic levitation system is crucial to the development work of superconducting gravimeter. Shielding material can weaken the influence of external magnetic field source fluctuation on the internal levitation magnetic field and provide a stable electromagnetic environment for the magnetic levitation system. But at the same time, shielding material will also have an impact on the internal suspended magnetic field itself, which will lead to large deviations in the study of electromagnetic characteristics of the magnetic levitation system. This article establishes and optimizes the simulation calculation model of a superconducting gravimeter magnetic levitation system based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> method, and studies the effect of magnetic shielding on the magnetic field, levitation force, and magnetic force gradient of the internal levitation system. After that, the simulation calculation results are compared with the experimental results, and the simulation calculation error of superconducting levitation force and magnetic gradient is less than 1%, which verifies the correctness of the optimized simulation model. This study is an important guide for the accurate calculation of the electromagnetic properties of all superconducting systems containing magnetic shielding materials, such as superconducting gravimeter, superconducting gravity gradiometer, etc.

A Self-Tuning LCC/SP System for Electric Vehicle Wireless Charging against Large Self- and Mutual Inductance Variations

An LCC/SP self-tuning wireless charging system is proposed herein for use in a wireless charging test bench. With different dislocations in addition to changes in the coil self-inductance and mutual inductance caused by different secondary magnetic shielding materials, the system can ensure that the high power factor of the primary side remains unchanged without changing the circuit topology. Based on this normalized detuning LCC/SP circuit model, a switch-controlled capacitor (SCC) self-tuning method based on PI control is proposed. The control scheme employs only two MOSFETs and capacitors, without WIFI communication or parameter identification. A 2 kW experimental device was set up in the laboratory, and experimental verification was carried out with large-scale dislocations and different secondary magnetic shielding materials. The experimental results confirm that the system can maintain a high power factor (&gt;0.9) under a system mutual inductance variation of 47.7% and secondary coil self-inductance variation of 12%, and that it can be applied in electric vehicle wireless chargers or high-power wireless charger test benches.

Ultra-broadband near-field magnetic shielding realized by the Halbach-like structure

With the great developments in electronic communication technology and miniaturized electromagnetic devices, near-field magnetic shielding has attracted much attention. However, for the widely used natural magnetic shielding materials, metal and ferrite, they have the unique limitations of large Ohmic loss and heavy weight, respectively. Although a compromise solution of the shielding layer may resort to the composite structure with metal and ferrite slabs, practical magnetic shielding with broadband, high efficiency, and ultra-thinness has remained a great challenge. In this work, inspired by the effective magnetic flux path established by the Halbach array, which is constructed by stacking permanent magnet in diverse directions, we propose a physical mechanism of local magnetic moment control in artificial structures, called the “Halbach-like structure.” We demonstrate the highly efficient and ultra-broadband near-field magnetic shielding in the Halbach-like structure with patterned metal and ferrite structures. By ingeniously designing the local magnetic moment, our structure not only provides an effective method for realizing high performance magnetic shielding but also paves the way to the other near-field controls, such as the wireless power transfer, wireless communications, and magnetic resonance imaging.

Measurement of noise of current source by pump-probe atomic magnetometer

&lt;sec&gt;The stable and reproducible magnetic field generated by a precision current source and a coil is usually used to calibrate the sensitivity of an atomic magnetometer. The noise of the current source directly determines the noise of the magnetic field. Therefore a highly sensitive atomic magnetometer can be used to measure the noise of the current source.&lt;/sec&gt;&lt;sec&gt;In this paper, a pump-probe atomic magnetometer is used to measure and estimate the noises of two current sources in a wide range. Firstly, in order to suppress the drift of magnetic field, which is caused by the drift of the current source or the gradual change of the magnetization of magnetic shielding materials, a method of implementing the magnetic compensation by using a precision source B2912A with small current is proposed and realized. The experimental results show that the magnetic compensation significantly suppresses the drift of magnetic field and reduces the amplitude of the power spectral density of magnetic field values to less than 0.1 Hz, but have little effect on the amplitude of the power spectral density of magnetic field values more than 0.1 Hz. Secondly, the relationship between the sensitivity of the pump-probe atomic magnetometer and the noises of two current sources in a wide range is respectively verified experimentally. When the magnetic field varies from 100 nT to 10000 nT, the sensitivity of the pump-probe atomic magnetometer increases stepwise from 0.2 pT/Hz&lt;sup&gt;1/2&lt;/sup&gt; to 15 pT/Hz&lt;sup&gt;1/2&lt;/sup&gt; by using a precision source B2912A to generate the magnetic field, while the magnetometer sensitivity is always about 20 pT/Hz&lt;sup&gt;1/2&lt;/sup&gt; by using a DC power analyzer N6705B to generate the magnetic field. When the magnetic field increases from 5000 nT to 6000 nT, the current resolution of B2912A changes from 100 nA to 1 μA, leading the peak to peak of the measured magnetic field to change from 23 pT to 230 pT. In the same transformation process of the magnetic field, the current resolution of N6705B is always about 2 μA, causing the peak to peak of the measured magnetic field to maintain at 300 pT. The experimental results show that the sensitivity of the pump-probe atomic magnetometer is limited by the noise of the magnetic field, thus the current noise can be estimated by the sensitivity of the pump-probe atomic magnetometer. When the magnetic field is set to 5000 nT, the current of B2912A or N6705B supplied to the coil is 94.8 mA, while the noise of B2912A or N6705B is 22.70 nA/Hz&lt;sup&gt;1/2&lt;/sup&gt; or 0.39 μA/Hz&lt;sup&gt;1/2&lt;/sup&gt;, respectively. The value of the current noise is about 20% of the value of the current resolution, which will be given a more reasonable explanation by combining the data processing process and the calibration details of current source in the future.&lt;/sec&gt;&lt;sec&gt;Our research is of great significance in calibrating the sensitivity of magnetic sensor, developing the high-precision current sources, and co-developing the magnetic induction metrology and current metrology.&lt;/sec&gt;

Feasibility Study of High-Efficiency Cooling of High-Temperature Superconducting Coils by Magnetic Refrigeration

As a high-efficiency cooling technology for high-temperature superconducting coils, we have begun research and development to examine the feasibility of a cooling assist technology that maintains a cryogenic state by combining the magnetic force generated by the superconducting coil with magnetic refrigeration technology. Magnetic refrigeration requires the magneto caloric material to change the magnetic field. In many cases, the magnetic field change is obtained by moving the magnetic source, but moving the superconducting coil is not a good idea. Although it is possible to generate a change in the magnetic field by turning on and off the power supply of the high-temperature superconducting coil, it is unlikely to be established as a system that assists cooling of the superconducting coil because the coil generates heat due to AC loss. Therefore, it was considered that the magnetic field change can be obtained if the magnetic force generated from the superconducting coil can be controlled by the magnetic shield. As a verification of the principle, it was clarified experimentally that a magnetic field change can be obtained by repeatedly inserting and removing the magnetic shield into and from the gap between the magnetic field generation source and the magneto caloric material, and the temperature change can be extracted by the magneto caloric effect. In the experiment, the temperature change obtained when a magnetic shield was inserted into the air gap was measured by a simple test device using an iron-based magneto caloric material having high magneto caloric effect performance at room temperature and a permanent magnet. The principle verification confirmation test was performed using several types of magnetic shielding materials such as Permalloy bulks and the electrical steel sheets. In addition, numerical analysis is performed on the magnetic shielding effect, and the shielding effect is improved by increasing the thickness of the shielding material and the possibility of using high temperature superconductors as magnetic shielding materials were also analyzed. In this study, we report the possibility of cooling the high-temperature superconducting coil with high efficiency by combining the magnetic field created by the superconducting coil and magnetic refrigeration.

Design of Biplanar Coils for Degrading Residual Field in Magnetic Shielding Room

The uniform field and field gradient coils can be utilized for nulling the background field inside a magnetic shielding room (MSR), but the desired field produced by the coils is inevitably distorted due to the coupling effect of the magnetic shielding material. To overcome this problem, an improved method is proposed based on the image method and target field method (TFM). Directed by this method, a series of biplanar coils (BCs) are designed to produce the uniform fields and field gradients for an active compensation in an MSR. Then, their finite element models and practical experiment are established to verify the effectiveness. Compared with the conventional TFM, the proposed method shows significant effects for both uniform field coils and field gradient coils. Uniformities of the $\boldsymbol {B_{x}}$ and $\boldsymbol {B_{z}}$ increase by 79.9% and 72.5%, and the $\boldsymbol {dB_{x}/dz}$ and $\boldsymbol {dB_{z}/dz}$ improve by 89.6% and 63.2%, respectively. It demonstrates that this method is adopted to design the BCs fixed in MSR and holds great promise for the active compensation of environmental magnetic field in the measurement of magnetoencephalogram (MEG).

Local Accumulation of Ferromagnetic Particles by Controlling Frequency of Rotating Magnetic Field

A novel cancer treatment with low side effect and less invasiveness by blocking newborn blood vessels around the diseased part was proposed. Ferromagnetic particles administered into the body are accumulated in the newborn blood vessels using a rotating magnetic field (RMF), and then are aggregated to block the blood vessels. We proposed a system consisting of four superconducting magnets and a cylindrical magnetic shielding material with a slit to apply the RMF to the human body. The purpose of this study was to accumulate particles locally within a spherical range of 10 mm in diameter. The results of fundamental experiments with neodymium magnets and particle trajectory calculations showed that the range of particle accumulation could be controlled by adjusting the frequency of the RMF.

Flexible Fe3O4/graphene foam/poly dimethylsiloxane composite for high-performance electromagnetic interference shielding

The combination of electronic and magnetic shielding materials has been attracted considerable attentions because of the high-performance electromagnetic interference (EMI) shielding effectiveness (SE) produced by the synergetic effects between them. Here, the innovative structural Fe3O4/graphene foam (GF) composite is assembled by magnetic Fe3O4 nanoparticles anchoring onto the highly electronic conductive 3D GF. The electrical conductivity of the flexible Fe3O4/GF/poly dimethylsiloxane (PDMS) composite reaches up to 2.5 S cm−1. The EMI SE of Fe3O4/GF/PDMS composite (~1.0 mm) increases from ~26.6 dB for GF/PDMS composite to ~32.4 dB in the frequency range of 8.2–12.4 GHz, which is attributed to the synergistic effect between Fe3O4 nanoparticles and GF. Furthermore, after repeatedly bending for 10000 cycles, the EMI SE of the Fe3O4/GF/PDMS composite still reaches up to 29.4 dB.

DEVELOPMENT OF MAGNETICALLY SHIELDED ROOM COMPRISED OF COBALT-BASED AMORPHOUS ALLOY

Paying attention to cobalt-based amorphous alloy as magnetic shielding material to replace permalloy, the authors attempted to modify materials suitable for magnetic shielding by heat treatment. Furthermore, magnetic shaking technique was applied. As a result, incremental magnetic permeability could be increased to about 59.5 times that of permalloy. Next, the magnetically shielded room using cobalt-based amorphous alloy was developed. It was confirmed that shielding performance was greatly improved as a result of fabrication of magnetic shield modules, coupling them to fabricate magnetic shield panels, and adding them as magnetic shielding layers to an established magnetically shielded room.

Study of Shielding Ratio of Cylindrical Ferrite Enclosure With Gaps and Holes

A stable magnetic field environment is one of the key factors to realize the ultrasensitive magnetic field detection. Ferrite materials, which possess both high electrical resistivity and high permeability, are ideal magnetic shielding materials. Gaps and holes usually exist in the large size magnetic shield which effects shielding effectiveness. In this paper, the variation of the DC magnetic shielding ratio, for different gaps and access hole dimensions, is quantificationally investigated using the finite element method. The shielding ratio in the simulation is close to the theoretical calculation for the closed shield when the thickness of gaps is very small. Our study indicates that gaps and holes separately reduced the shielding effectiveness, and the coexistence of gaps and holes aggravated the reduction of shielding effectiveness. Compared with the shielding ratio of the ferrite enclosure without gaps and holes for the known size shield, the transverse and longitudinal direction of the shielding ratio increase by 9% and 274% with gaps (thickness = 0.1 mm) and holes (diameter = 25 mm), respectively. The simulation results are partially verified with experimentally using a cylindrical ferrite shield made of Zn0.17Mn0.74Fe2.06O4 and Zn0.33Mn0.58Fe2.06O4. The difference between the experimental and simulation results is less than 10%.

Magnetic Shielding Analysis for Arrayed Eddy Current Testing

Arrayed eddy current testing (AECT) has been widely applied in the field of crack detection, displacement measurement and many other fields due to its non-destructive and high efficiency. But the coupling interference of AECT exists objectively and leads to the detection error directly. In this paper, a magnetic shielding layer is proposed to reduce the coupling interference. The relationships between electromagnetic parameters of magnetic shielding material and coupled magnetic field are discussed theoretically. Furthermore, the simulation and experiments are carried out, the results are consistent. It shows that the application of magnetic shielding layer in AECT can reduce the mutual inductance of probe coils and increase the output voltage. Additionally, the most important factor to select material of magnetic shielding layer is the conductivity. Thus, the magnetic shielding layer with large conductivity will be suitable for AECT to promote the testing performance.

The Phase Diagram and Exotic Magnetostrictive Behaviors in Spinel Oxide Co(Fe1-xAlx)2O4 System.

We report the magnetic and magnetostrictive behaviors of the pseudobinary ferrimagnetic spinel oxide system (1−x)CoFe2O4–xCoAl2O4 [Co(Fe1−xAlx)2O4], with one end-member being the ferrimagnetic CoFe2O4 and the other end-member being CoAl2O4 that is paramagnetic above 9.8 K. The temperature spectra of magnetization and magnetic susceptibility were employed to detect the magnetic transition temperatures and to determine the phase diagram of this system. Composition dependent and temperature dependent magnetostrictive behaviors reveal an exotic phase boundary that separates two ferrimagnetic states: At room temperature and under small magnetic fields (∼500 Oe), Fe-rich compositions exhibit negative magnetostriction while the Al-rich compositions exhibit positive magnetostriction though the values are small (<10 ppm). Moreover, the compositions around this phase boundary at room temperature (x = 0.35, 0.4, 0.45, 0.5) exhibit near-zero magnetostriction and enhanced magnetic susceptibility, which may be promising in the applications for magnetic cores, current sensors, or magnetic shielding materials.

Transition of magnetism in graphene coated with metal nanoparticles

The unique two-dimensional structure and very high surface area of graphene results in amazing properties and makes it an ideal substrate for the chemical adsorption of many types of metal nanoparticles (NPs). Here, we demonstrate a novel approach for synthesizing multi-functional graphene-magnetic nanoparticles (GNPs) hybrids. The hybrids exhibit a combination of features, including excellent processability, superparamagnetism, electrical conductivity and high chemical reactivity. The synthesis of graphene by Cu or Ni reduction of exfoliated graphene oxide results in the removal of oxygen functionalities of the graphene oxide. The process is scalable, green and efficiently enables the controllable production of GNPs. The GNPs have great potential for a variety of applications, including as materials for magnetic resonance imaging, microwave absorption and electromagnetic interference shielding.

Analysis of Power Supply Heating Effect during High Temperature Experiments Based on the Electromagnetic Steel Teeming Technology

AbstractFor further lowering inclusions and improving the quality of steel, a new electromagnetic steel-teeming technology based on electromagnetic induction heating was proposed. To assess the proposed technology, an experimental platform that imitates the actual production condition of steelmakers was established. High temperature experiments were performed to investigate the melting length of Fe-C alloy under different power and frequency conditions. The heating effect was analyzed, and the method of magnetic shielding to reduce the power loss of power supply was put forward. The results show that when the power is 40 kW and frequency is 25 kHz, the melting length of the Fe-C alloy is 89.2 mm in 120 s, which meets the requirements of steel teeming. In addition, when magnetic shielding material is installed under the induction coil, the power loss is reduced by about 64 %, effectively improving the heating effect of power supply.

Low-carbon steel ultra-high-vacuum Schottky emitter electron gun with double O-rings for axis adjustment

With the aim to create a simpler structure and reduce the production cost of an existing Schottky emitter-scanning electron microscope (SE-SEM), the authors have built and tested a double-O-ring electron gun which is also compatible with ultrahigh vacuum (UHV). Specifically, the gun and column of the SEM consist of low-carbon steel, of which the magnetic shielding effect is greater than that of stainless steel, allowing magnification of ×200 000 in the adapted SEM base without additional magnetic shielding material, such as permalloy or mu-metal. The position of the electron gun can be adjusted along the horizontal axis while maintaining the UHV condition. Excellent beam current stability with less than 1% variation for more than 1 h was noted. Therefore, the authors anticipate that the double-O-ring electron gun and column of low-carbon steel together represent an inexpensive and uncomplicated SE-SEM compared to existing types.