Magnetic Core Size Research Articles

Applying Stochastic Langevin Function with Coupled Brownian-Néel Relaxations to Study the Dynamic Magnetizations of Nanoparticle Tracers in Magnetic Particle Imaging

Abstract Magnetic particle imaging (MPI) is a new imaging technique that utilizes biologically safe iron oxide nanoparticle tracers for medical imaging. Although in the preclinical stage, MPI has shown strong potential for applications such as cell tracking, angiography, cancer imaging, etc. With the ever-growing research interest in MPI and the increasing desire to apply it in clinical medical imaging, the design of tracers, MPI systems, image reconstruction algorithms, and other related components has become increasingly critical. To date, most of the theoretical studies in MPI rely on the static Langevin function to describe the magnetization responses of superparamagnetic iron oxide nanoparticle (SPION) tracers. However, under a fast-changing excitation field (usually tens of kHz), the magnetic relaxation time of SPION tracers is no longer negligible, thus, the static Langevin function is inaccurate in explaining the tracers’ magnetic signals in MPI. Herein, we apply a stochastic Langevin function with coupled Brownian-Néel relaxation models to study the dynamic magnetization responses of SPION tracers in MPI. The time domain magnetization responses (M-t curve), dynamic magnetization-field hysteresis loops (M-H curve), and point spread functions (PSF) are modeled for different SPION tracer designs with varying physical and magnetic properties. Our results show that larger magnetic core sizes reduce the MPI spatial resolution. Conversely, thicker non-magnetic coatings on tracers do not significantly affect the spatial resolution. The increased anisotropy diminishes the MPI resolution, and a higher saturation magnetization favors higher MPI resolution.

Effect of tracer size distribution on magnetic particle imaging performance

Abstract Magnetic particle imaging (MPI) is a tracer-based tomographic imaging technique utilized in applications such as lung perfusion imaging, cancer diagnosis, stem cell tracking, etc. The goal of translating MPI to clinical use has prompted studies on further improving the spatial-temporal resolutions of MPI through various methods, including image reconstruction algorithm, scanning trajectory design, magnetic field profile design, and tracer design. Iron oxide magnetic nanoparticles (MNPs) are favored for MPI and magnetic resonance imaging (MRI) over other materials due to their high biocompatibility, low cost, and ease of preparation and surface modification. For core–shell MNPs, the tracers’ magnetic core size and non-magnetic coating layer characteristics can significantly affect MPI signals through dynamic magnetization relaxations. Most works to date have assumed an ensemble of MNP tracers with identical sizes, ignoring that artificially synthesized MNPs typically follow a log-normal size distribution, which can deviate theoretical results from experimental data. In this work, we first characterize the size distributions of four commercially available iron oxide MNP products and then model the collective magnetic responses of these MNPs for MPI applications. For an ensemble of MNP tracers with size standard deviations of σ, we applied a stochastic Langevin model to study the effect of size distribution on MPI imaging performance. Under an alternating magnetic field (AMF), i.e., the excitation field in MPI, we collected the time domain dynamic magnetizations (M-t curves), magnetization-field hysteresis loops (M-H curves), point-spread functions (PSFs), and higher harmonics from these MNP tracers. The intrinsic MPI spatial resolution, which is related to the full width at half maximum (FWHM) of the PSF profile, along with the higher harmonics, serve as metrics to provide insights into how the size distribution of MNP tracers affects MPI performance.

ANALYSIS OF A HYBRID CURRENT MODULATED DUAL ACTIVE BRIDGE PHOTOVOLTAIC DC/DC CONVERTER FOR UNIFORM IRRADIATION CONDITIONS

The energy of Photovoltaic (PV) modules can be connected to DC grid with high step-up ratio power electronics converters. Generally, two-stage isolated boost-based converters are used for this purpose. However, these types of converters consist of a high number of semiconductors which must withstand high voltage levels, and they utilize input inductance which has high value and large magnetic core size. To solve these problems, single-stage conversion-based Hybrid Current Modulated (HCM) isolated Dual Active Bridge (DAB) converter is proposed in the literature. However, this converter is designed for considering partial shading (PS), and PS rarely occurs in PV farms. In this study, the design methodology of DAB PV DC/DC converter is given for uniform irradiation (UI) conditions that is more suitable for PV farms. In addition, UI and PS designs of HCM DAB PV DC/DC converter are compared to each other. Analyses show that UI-based design has 3.88% smaller RMS2 current, 3.56% smaller peak current and 2.35% smaller core size than the PS-based design. Hence, UI-based design can be more efficient, cheaper, and smaller in size.

Influence of coating on peroxidase-like activity of magnetic nanoparticles

Chitosan stabilized iron oxide magnetic nanoparticles (Chit-MNPs) were prepared with the theoretical Chit/Fe3O4 weight ratios equal to 0.5, 1, 2, 3 w/w and characterized by different techniques. The effect of various Chit/Fe3O4 weight ratios on the samples’ hydrodynamic diameter, zeta potential, and isoelectric point was studied by the dynamic light-scattering method. Using TEM analysis, the average magnetic core size was found to be ranging from 6 nm to 9 nm. The obtained results are comparable to those from magnetic measurements. The magnetic measurements confirmed successful chitosan coating on MNPs by means of the decrease in saturation magnetizations from 69.2 emu·g−1 for unmodified MNPs to 66.6, 65.9, 65.5 and 65.2 emu·g−1 for Chit-MNPs at different ratios of Chit/MNPs 0.5, 1, 2, and 3. TGA was used to quantify the amount of chitosan adsorbed on MNPs surface – the max. amount was 0.045 mg chitosan/mg MNPs. The peroxidase-like catalytic activity of unmodified MNPs and chitosan Chit-MNPs in present of a substrate such as N,N-diethyl-p-phenylenediamine sulfate salt (DPD) and hydrogen peroxide was studied. The results confirmed the peroxidase-like activity of samples, – they catalytically oxidise substrate DPD by H2O2 to produce a typical colour reaction. The chitosan coating was found to slightly decrease catalytic activity compared to MNPs at pH 7.4. Despite the fact, these findings can be useful for creating sets of MNPs and Chit-MNPs samples possessing predefined peroxidase-like activity and could be applied for chromogenic reactions in the future.

Synthesis of iron oxide nanocubes and their incorporation with luminescent lanthanide (Ln = Tb/ Eu)-Terephthalate complex in silica nanospheres using a reverse microemulsion method

Luminescent magnetic nanocomposites are of interest because of their promising applications in the biomedical field. This article reports the preparation of silica-coated lanthanide terephthalate complexes, Ln-TA (Ln = Tb and Eu) containing luminescent magnetic nanocomposites. Cubic-shaped iron oxide nanoparticles were synthesized through the thermal decomposition of the iron-oleate complex. Then, polymerization of tetraethyl ortho silicate in a water-in-oil reverse microemulsion in the presence of previously prepared iron oxide nanocubes and a terbium (Tb3+)/europium (Eu3+)-terephthalate (TA) complex was performed to obtain luminescent magnetic Fe3O4@Ln-TA (Ln = Tb, Eu)/SiO2 nanocomposites. The size (8.9 nm) and cubic shape of magnetic core were determined via transmission electron microscopy and the superparamagnetic behavior using a vibrating sample magnetometer. Energy dispersive X-ray analysis and inductively coupled plasma optical emission spectroscopy found that the luminescent Tb-TA and Eu-TA complexes were incorporated in the magnetic silica nanocomposites. The nanocomposites, Fe3O4@Ln-TA/SiO2, exhibited distinct luminescent green and red emission peaks and showed promise as a T1 contrast agent for magnetic resonance imaging.

Cobalt Ferrite Nanoparticles Capped with Perchloric Acid for Life-Science Application

Among the modern oncological therapies, one of the most promising is based on tumor hyperthermia with magnetic nanoparticles resulting from the crystallization of iron and cobalt oxides. We synthesized core–shell magnetic nanoparticles of perchlorate-CoxFe3−xO4 (x = 0; 0.5; 1.0) via the co-precipitation method and stabilized them in aqueous suspensions. Fine granulation of the dispersed ferrophase was revealed by Transmission Electron Microscopy and Dynamical Light Scattering, with FTIR data detailing the surface-interaction phenomena. X-ray diffractometry revealed specific crystallization features of inverse spinel lattice, providing crystallite size and lattice parameters dependent on the cobalt content. The results of the Vibrating Sample Magnetometry investigations indicated that cobalt doping has reduced the magnetic core size and increased the nanoparticle dimension, which could be the result of crystallization defects at the nanoparticle surface related to the presence of cobalt ions. A mathematical model was applied with a focus on the quantitative description of the temperature distribution around magnetic nanoparticles. Further development of our research will consider new cobalt ferrite nanoparticles with new cobalt contents and different organic coatings to contribute to their biocompatibility and stability in aqueous suspensions, as required by administration in living organisms.

An E-Core Based Integrated Coupled Inductor for Interleaved Boost Converter

Interleaving in a boost converter is beneficial for lowering input and output current ripples through ripple cancellation due to phase-shift between channel currents. However, interleaving does not affect the channel current ripple. The channel current comprises circulating Differential Mode (DM) current and Common Mode (CM) boost current, whose ripples constitute the total channel current ripple. Inverse coupling between channel inductors effectively lowers channel current ripple while maintaining the same input and output current ripples. However, with a single inverse coupled inductor, its leakage inductance, which serves as a boost inductor, depends on the winding arrangement and is challenging to balance in both channels. To overcome this, the inverse coupled inductor can be implemented as a cascade of inverse (DM inductance) and direct (CM inductance) coupled inductors. The DM and CM currents and their ripples then depend on CM and DM inductances, respectively. Nonetheless, this approach results in increased size and count of magnetic cores. In this paper, an Integrated Magnetic Structure (IMS), based on a gapped EE-core, is proposed that combines both CM and DM inductances in a single core. The CM and DM inductances are independent and depend on separate winding turns. A reluctance model is derived, and a design procedure is developed where core parameters are expressed in terms of converter parameters. Finally, the proposed IMS concept is validated through a 300 W, 100 V to 168 V prototype, switching at 70 kHz.

Controllable pore size of super-hydrophobic magnetic core-shell nanospheres with dendritic architecture and their pore-dependent performances in oil/water separation

The fundamental properties of porous materials strongly rely on their pore sizes. Pore-size-dependent performances play significant roles in their applications like catalysis, adsorption, delivery, and so forth. In this work, a feasible pore-expanding approach has been successfully explored to control the pore size of magnetic core–shell nanospheres with dendritic architecture (Fe3O4@DMSNs). The size can be adjusted from ca. 10 nm to 25 nm and further to 60 nm in an unprecedented wide range. Further, a concise modification route has been adopted to endow Fe3O4@DMSNs particles with super-hydrophobic abilities by the usage of a long-chain silane (Fe3O4@DMSNs-OTES). The as-prepared nanospheres with magnetic response function can not only form magnetic liquid marbles (MLMs) easily, but also separate n-hexane in oil–water mixtures or oil-in-water emulsions rapidly and efficiently. However, the above three particles display certain differences in their basic properties (like pore volume) and application performances (like oil removal efficiency). Most importantly, Fe3O4@DMSNs-OTES with 60 nm pores exhibit the best super-hydrophobicity in contact angles of H2O and MLMs, Fe3O4@DMSNs-OTES with 25 nm pores possess the maximum adsorption capacity, while the ones with 10 nm pores show the quickest adsorption rate (removal efficiency). The relationship between the performances and the pore size has been thoroughly revealed and well deciphered by wettability theory (e.g., Cassie-Baxter equation) and surface tension theory. In a word, all Fe3O4@DMSNs-OTES hold promising potentials as candidates for the practical wastewater purification on the basis of their characteristics.

Inversely Coupled Inductors With Small Volume and Reduced Power Loss for Switching Converters

The letter presents a patent of lateral coupled inductor structure and its applications in single-phase and multiphase DC-DC converters. The CPUs, GPUs and FPGAs in the latest servers and artificial intelligence systems are powered by multiphase DC-DC switching converters and require high current, small size, fast transient response and high efficiency. This single-turn lateral coupled-inductor structure reduces magnetic core size through inverse coupling and minimizes inductor DCR power loss for high-current applications. The coupled inductor can be embedded inside a motherboard or a power module. The letter illustrates different implementations of the patent and provides solutions to the artificial intelligence systems that require high current and small volume of the power converters.

The effect of size, shape, coating and functionalization on nuclear relaxation properties in iron oxide core-shell nanoparticles: a brief review of the situation.

In this perspective article, we present a short selection of some of the most significant case studies on magnetic nanoparticles for potential applications in nanomedicine, mainly magnetic resonance. For almost 10 years, our research activity focused on the comprehension of the physical mechanisms on the basis of the nuclear relaxation of magnetic nanoparticles in the presence of magnetic fields; taking advantage of the insights gathered over this time span, we report on the dependence of the relaxation behaviour on the chemico-physical properties of magnetic nanoparticles and discuss them in full detail. In particular, a critical review is carried out on the correlations between their efficiency as contrast agents in magnetic resonance imaging and the magnetic core of magnetic nanoparticles (mainly iron oxides), their size and shape, and the coating and solvent used for making them biocompatible and well dispersible in physiological media. Finally, the heuristic model proposed by Roch and coworkers is presented, as it was extensively adopted to describe most of the experimental data sets. The large amount of data analyzed allowed us to highlight both the advantages and limitations of the model.

Magnetic field and core size dependent opto-electronic properties of an impurity in CdS/ZnS core/shell quantum dot

Effects due to magnetic field and the inner core size on the opto-electronic characters of impurity in the CdS/ZnS core/shell quantum dot are investigated. The magnetic field dependent ground state energies and the transition energies of donor impurity in the CdS/ZnS core/shell quantum dot are computed. The oscillator strength, linear, nonlinear and total absorption coefficient and the changes of refractive index are investigated for various values of magnetic field. The results show that the energy difference due to the ground and first excited states is much more sensitive to the strong confinement and the magnetic field. The maximum of absorption peak and the changes due to refractive index are found to enhance with the inclusion of impurity. These properties can be applied in different potential applications based on the intraband optical transitions.

Battery Energy Storage System With Interleaving Structure of Dual-Active-Bridge Converter and Non-Isolated DC-to-DC Converter With Wide Input and Output Voltage

We propose a circuit topology suitable as a battery charge/discharge tester with a DAB converter and a non-isolated dc-dc converter as a module structure. The module structure can be configured to have a wide input and output voltage range because it is easy to expand. DAB converters are used for bidirectional power transfer and galvanic isolation. By controlling the phase difference of the voltages across the DAB to be almost identical, the generation of circulating current due to the phase difference is minimized, and the ZVS is guaranteed. In addition, by switching at a phase angle that flattens the DAB switch current, the peak current is reduced to diminish the switch’s conduction loss, lowering the switch current rating, and reducing the magnetic core size. The non-isolated dc-dc converter is used to control the battery output voltage. Through the interleaved structure, good dynamic characteristics and current ripple can be reduced, so the effect of improving the battery lifecycle can be expected. We present four series-parallel structures of the proposed converter and compare them with the case of generating an output voltage only with a DAB converter. Analyze the characteristics of automatic voltage balancing due to the difference between the input capacitors’ initial voltage and capacitance connected in series. We analyze the features of the proposed system through simulation and verify the technical feasibility and excellence of the proposed battery charge/discharge tester through charge/discharge experiments using supercapacitors instead of batteries.

Ripple Improvement and Steady State Analysis of a Boost Converter with Higher Step-up Ratio

In this paper, a ripple improved DC-DC boost converter is proposed by using a dual-winding directly coupled coupled-inductor operating in continuous conduction mode (CCM). The proposed converter achieves larger step-up conversion ratio than the conventional boost converter by using a diode-capacitor cell. The cell consists of two crossly connected identical capacitors and two diodes placed in parallel. An inductor is required to be placed in the output-end to oppose the sudden change in load current due to series-parallel configuration of the identical capacitors of the cell. Thus, the converter requires two inductors to achieve the voltage gain. These two inductors are replaced by a dual-winding directly coupled coupled-inductor. This modification in the circuit improves the ripple in input and inductor currents and, ripple in output voltage without affecting the conversion ratio. Apart from that, the reduction of size of magnetic core also reduces the cost and bulkiness of the proposed converter.

Improved hybrid current modulation for bidirectional power flow of single-stage dual active bridge AC/DC converter

In this paper, an improved hybrid current modulation (iHCM) method for bidirectional power flow operation of dual active bridge (DAB) ac/dc converter is proposed. In this method, trapezoidal (TZM) and triangular (TRM) current modulations are performed together in a grid period to obtain distortionless grid current with forming minimum RMS current in the transformer of DAB. This study put forward a detailed analysis of the mode changing instants (between TRM and TZM), RMS and peak current values of the transformer and required magnetic core sizes. Furthermore, the iHCM is compared in analysis and simulations by the variable switching frequency dual phase shift modulation (VSF DPSM) method that have small RMS current advantage in the vicinity of nominal output power. According to the analysis, the iHCM, unlike the VSF DPSM, can operate over a wide range of transformer turn ratio (or over the wide range of ac peak voltage/reflected dc voltage, λ) without increasing the size of transformer, and can provide small RMS currents at light loads as it has no nonactive (circulating) current. The theoretical analysis is validated with a downsized 250 W experimental prototype.

Properties of single- and multi-core magnetic nanoparticles assessed by magnetic susceptibility measurements

Characterization of structural and magnetic properties of MNP ensembles is crucial in tailoring their performance for biomedical applications. In this work, we evaluate the size distribution of magnetic cores in single- and multi-core nanoparticles in water suspension using transmission electron microscopy (TEM) and static magnetization curve, with the geometrical core size ranges from 8.3 to 40.2 nm. A reasonable core size derived from the magnetization curve is obtained in comparison to the geometrical size from TEM image. The magnetic moment distribution possibly reveals that the reduction of effective magnetic core size is due to the magnetization degradation in cores. The hydrodynamic size and average anisotropy energy ratio obtained from the AC susceptibility response of MNPs from 5 Hz to 100 kHz are also evaluated. The complex distribution of relaxation time is constructed by applying a non-negative least square method to an AC susceptibility model that incorporates the inter- and intra-potential-well contributions. It is found that a log-normal distribution might not be adequate to represent the hydrodynamic size distribution reconstructed from the AC susceptibility responses of the suspended samples. It is demonstrated that the AC susceptibility model used in this study can be used to fairly estimate the average anisotropy energy ratio for MNP ensembles dominated by thermally blocked particles. Moreover, it can be suggested that besides the geometrical core size, the degree of core aggregation also plays an important role in determining the anisotropy energy ratio and effective magnetic moment.

Dual-Active-Bridge Isolated DC–DC Converter With Variable Inductor for Wide Load Range Operation

This article explores the use of a variable inductor as a reactive element for energy transfer in a dual-active-bridge (DAB) converter. By using a controlled variable inductor, the optimal switching region in the operation of the phase-shift DAB can be extended, and thus, high efficiencies can be achieved over wider load ranges compared to the traditional solution. Moreover, the combined use of the variable inductance together with the phase shift, as two control parameters, allows for the linearization of the DAB converter transfer function, which improves the stability and the performance of the controller. And finally, due to the controlled saturation of the device, it is feasible to reduce the magnetic core size, yielding a size optimization of the full converter. This article develops a circuit-based model to simulate the converter system, aiming to study the proposed improvements. Furthermore, a 2-kW SiC-based DAB converter prototype is constructed, including a controlled variable inductor. The experimental results presented in this article validate the studies carried out for the different operation conditions.

Temperature and field dependences of transverse relaxivity of Co–Zn ferrite nanoparticles coated with silica: The role of magnetic properties and different regimes

The transverse relaxivity r2, characterizing the performance of negative contrast agents like magnetic nanoparticles in T2-weighted MRI, depends on multiple factors, most importantly on magnetic properties and the size of the particles, including both the size of magnetic cores and the effective size of whole particles present as individual entities in the suspension. The current study probes the transverse relaxivity of ferrimagnetic CoxZnyFe3-x-yO4 nanoparticles whose behavior varies from the blocked state, across the superparamagnetic regime to the vicinity of the paramagnetic state, while other parameters, namely the size of the magnetic cores (≈10 nm), the size of whole colloidally stable particles, and their geometry (≈30 nm-sized clusters of crystallites uniformly coated with ≈12 nm thick silica shell) are kept the same. Among the samples studied, the silica-coated clusters of Co0·66Zn0·47Fe1·87O4 crystallites show high values of r2 = 534 s−1 mmol(Me3O4)−1 L at room temperature in the magnetic field of 1.5 T. Importantly, the field and temperature dependences of transverse relaxivity are discussed in relation to relevant theoretical models, i.e., motional averaging and static dephasing regimes. The increase in r2 in higher magnetic fields is related to the notable paraprocess in the employed cores, whereas the gradual decrease of r2 with increasing temperature is dominated by the significant increase of the self-diffusion coefficient of water.

Size effects on the magnetic behavior of γ-Fe2O3 core/SiO2 shell nanoparticle assemblies

Theeffect of core size on the magnetic behavior of nanoparticle assemblies of γ-Fe2O3 core/SiO2 shell morphology is investigated. Long-range magnetostatic interactions are probed in two highly monodispersed experimental test systems of spherical nanoparticles with core diameters of 10 nm and 12.5 nm, and a shell thickness varying from 0 nm (bare particles) to ~50 nm. Zero-Field-Cooled magnetization curves are calculated by employing the Monte Carlo simulation technique in a mesoscopic-scale model for the assembly, assuming spin collinearity and coherent spin-reversal mechanisms. Simulation results reproduce the trend in the behavior of the Zero-Field-Cooled magnetization versus T curves in good qualitative agreement with the experimental findings. They also demonstrate that the increase of the magnetic core size results in a shift of the maximum magnetization peak, Tmax, to higher temperatures due to enhanced dipolar coupling. The results shed light on how interparticle distance and magnetic core size influence the value of Tmax through collective behavior and its transition to a single-particle superparamagnetic blocking temperature, TB, as the assembly becomes magnetically diluted with increasing shell thickness.

Tuning dipolar effects on magnetic hyperthermia of Zn0.3Fe2.7O4/SiO2 nanoparticles by silica shell

Effects of magnetic dipole interactions on specific loss power (SLP) for Zn0.3Fe2.7O4/SiO2 core/shell nanoparticles (NPs) with identical magnetic core size and varying non-magnetic shell thicknesses were systematically investigated. In the case of thin silica shell, NPs form chain-like structures due to strong dipole interactions between NPs. The chain formation induces additional shape anisotropy. With increasing thickness of the silica shell, the dipole interaction and therefore the anisotropy induced by the dipole interactions decrease. Meanwhile, the hydrodynamic size of the NPs shows a counterintuitive reduction with increasing shell thickness, also due to the decrease in interactions. The magnetic anisotropy and hydrodynamic size affect the SLP through changes in the Néel and Brownian relaxation rates, leading to different field-dependent SLP values for NPs with thin and thick shells.

Effects of dipolar interactions on the magnetic hyperthermia of Zn0.3Fe2.7O4 nanoparticles with different sizes* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51771124, 51571146, and 51701130).

Tumor-targeted magnetic hyperthermia has recently attracted much attention. Magnetic nanoparticles (NPs) are heat mediator nanoprobes in magnetic hyperthermia for cancer treatment. In this paper, single cubic spinel structural Zn0.3Fe2.7O4 magnetic NPs with sizes of 14 nm–20 nm were synthesized, followed by coating with SiO2 shell. The SLP value of Zn0.3Fe2.7O4/SiO2 NPs below 20 nm changes non-monotonically with the concentration of solution under the alternating current (AC) magnetic field of 430 kHz and 27 kA/m. SLP values of all Zn0.3Fe2.7O4/SiO2 NPs appear a peak value with change of solution concentration. The solution concentrations with optimal SLP value decrease with increasing magnetic core size. This work can give guidance to the better prediction and control of the magnetic hyperthermia performance of materials in clinical applications.