Enchancment in viscoelastic properties of flake-shaped iron based magnetorheological fluid using ferrofluid

Correlating the Dipolar Interactions Induced Magneto-Viscoelasticity and Thermal Conductivity Enhancements in Nanomagnetic Fluids

The effective thermal management of electronic system holds the key to maximize their performance. The recent miniaturization trends require a cooling system with high heat flux capacity, localized cooling, and active control. Nanomagnetic fluids (NMFs) based cooling systems have the ability to meet the current demand of the cooling system for the miniaturized electronic system. However, the thermal characteristics of NMFs have a long way to go before the internal mechanisms are well understood. This review mainly focuses on the three aspects to establish a correlation between the thermal and rheological properties of the NMFs. First, the background, stability, and factors affecting the properties of the NMFs are discussed. Second, the ferrohydrodynamic equations are introduced for the NMFs to explain the rheological behavior and relaxation mechanism. Finally, different theoretical and experimental models are summarized that explain the thermal characteristics of the NMFs. Thermal characteristics of the NMFs are significantly affected by the morphology and composition of the magnetic nanoparticles (MNPs) in NMFs as well as the type of carrier liquids and surface functionalization that also influences the rheological properties. Thus, understanding the correlation between the thermal characteristics of the NMFs and rheological properties helps develop cooling systems with improved performance.

Facile synthesized zinc oxide nanorod film humidity sensor based on variation in optical transmissivity

Variation in the transmitted light intensity from metal oxide thin films with moisture content provides a great opportunity to use them for humidity sensing. Herein, we have developed a novel and simple humidity sensor based on ZnO nanorod (ZNR) thin films which work as transmission-based sensing elements in an in-house fabricated sensing setup. The ZNR sensing element shows excellent linear sensing performance in the relative humidity (RH) range 10-90% and does not show any hysteresis. A maximum change in optical power of ∼95 μW is observed with the change in RH in the range 10-90%, for the sample with the smallest crystallite size (ZNR1) and highest pore diameter of the ZNR film. Also, a maximum sensitivity of 1.104 μW/% RH is observed for the ZNR1 sample which drops to 0.604 μW/% RH for the highest crystallite size sample (ZNR4). The presence of oxygen vacancies and the micro-porous nature of the film allow the absorption of water vapour on the film which deflects light at different angles that vary with the moisture content. The experimental results suggest that the ZNR film with a smaller crystallite size and larger pore diameter is more sensitive for humidity measurements. Further, an improved sensing performance is perceived in ZNRs because of the larger surface area of the nanorods. The ZNR based sensing elements do not suffer from ageing effects and exhibit high repeatability (88.74%). Further, the humidity sensor has a response time of 62 seconds and recovery time of 100 seconds which can be considered as a fairly quick response.

Facile Synthesis, Static, and Dynamic Magnetic Characteristics of Varying Size Double-Surfactant-Coated Mesoscopic Magnetic Nanoparticles Dispersed Stable Aqueous Magnetic Fluids

The present work reports the synthesis of a stable aqueous magnetic fluid (AMF) by dispersing double-surfactant-coated Fe3O4 magnetic nanoparticles (MNPs) in water using a facile ambient scalable wet chemical route. MNPs do not disperse well in water, resulting in low stability. This was improved by dispersing double-surfactant (oleic acid and sodium oleate)-coated MNPs in water, where cross-linking between the surfactants improves the stability of the AMFs. The stability was probed by rheological measurements and all the AMF samples showed a good long-term stability and stability against a gradient magnetic field. Further, the microwave spin resonance behavior of AMFs was studied in detail by corroborating the experimental results obtained from the ferromagnetic resonance (FMR) technique to theoretical predictions by appropriate fittings. A broad spectrum was perceived for AMFs which indicates strong ferromagnetic characteristics. The resonance field shifted to higher magnetic field values with the decrease in particle size as larger-size MNPs magnetize and demagnetize more easily since their magnetic spins can align in the field direction more definitely. The FMR spectra was fitted to obtain various spin resonance parameters. The asymmetric shapes of the FMR spectra were observed with a decrease in particle sizes, which indicates an increase in relaxation time. The relaxation time increased with a decrease in particle sizes (sample A to D) from 37.2779 ps to 42.8301 ps. Further, a detailed investigation of the structural, morphological, and dc magnetic properties of the AMF samples was performed. Room temperature dc magnetic measurements confirmed the superparamagnetic (SPM) characteristics of the AMF and the M-H plot for each sample was fitted with a Langevin function to obtain the domain magnetization, permeability, and hydrodynamic diameter of the MNPs. The saturation magnetization and coercivity of the AMF samples increased with the increase in dispersed MNPs' size of the samples. The improvement in the stability and magnetic characteristics makes AMFs suitable candidates for various biomedical applications such as drug delivery, magnetic fluid hyperthermia, and biomedicines.

The Effect of Sr-CoFe2O4 Nanoparticles with Different Particles Sized as Additives in CIP-Based Magnetorheological Fluid

This study investigated the effect of adding strontium (Sr)-doped cobalt ferrite (CoFe₂O₄) nanoparticles in carbonyl iron particle (CIP)-based magnetorheological fluids (MRFs). Sr-CoFe₂O₄ nanoparticles were fabricated at different particle sizes using co-precipitation at calcination temperatures of 300 and 400 °C. Field emission scanning electron microscopy (FESEM) was used to evaluate the morphology of the Sr-CoFe₂O₄ nanoparticles, which were found to be spherical. The average grain sizes were 71-91 nm and 118-157 nm for nanoparticles that had been calcinated at 300 and 400 °C, respectively. As such, higher calcination temperatures were found to produce larger-sized Sr-CoFe₂O₄ nanoparticles. To investigate the rheological effects that Sr-CoFe₂O₄ nanoparticles have on CIP-based MRF, three MRF samples were prepared: (1) CIP-based MRF without nanoparticle additives (CIP-based MRF), (2) CIP-based MRF with Sr-CoFe₂O₄ nanoparticles calcinated at 300 °C (MRF CIP+Sr-CoFe₂O₄-T300), and (3) CIP-based MRF with Sr-CoFe₂O₄ nanoparticles calcinated at 400 °C (MRF CIP+Sr-CoFe₂O₄-T400). The rheological properties of these MRF samples were then observed at room temperature using a rheometer with a parallel plate at a gap of 1 mm. Dispersion stability tests were also performed to determine the sedimentation ratio of the three CIP-based MRF samples.

Strong and Stable Magnetorheological Fluids Based on Flaky Sendust-Co0.4Fe0.4Ni0.2 Nanocomposite Particles

The magnetorheological (MR) performance of suspensions based on magnetic (flaky Sendust (FS))-magnetic (Co_0.4Fe_0.4Ni_0.2) nanocomposite particles was investigated by using a vibrating sample magnetometer and a rotational rheometer. Flaky Sendust@Co_0.4Fe_0.4Ni_0.2 nanocomposite particles were fabricated through wet chemical synthesis of Co_0.4Fe_0.4Ni_0.2 on the surface of FS. The density of the resultant FS@Co_0.4Fe_0.4Ni_0.2 was less than that of FS due to the pore/void formation in the composite particles. Because of the high saturation magnetization of Co_0.4Fe_0.4Ni_0.2 (165 emu/g), FS@Co_0.4Fe_0.4Ni_0.2 (145 emu/g) exhibited greater magnetization than bare FS (130 emu/g), which resulted in the good performance of FS@Co_0.4Fe_0.4Ni_0.2-based MR fluids: the suspension exhibited a remarkably high yield stress, almost one order greater than that of MR fluids based on hierarchically structured (HS) Fe₃O₄ particles. In addition, the high drag coefficient of FS@Co_0.4Fe_0.4Ni_0.2 in the liquid medium, in conjunction with its lower density, resulted in a substantially improved long-term stability, better than that of Co_0.4Fe_0.4Ni_0.2- or FS-based suspensions. Although the density of the FS@Co_0.4Fe_0.4Ni_0.2 nanoparticles is higher than that of HS-Fe₃O₄ particles, their stability is much better than the stability of HS-Fe₃O₄ particle's suspension. Manufactured magnetic-magnetic nanocomposite particles provide a feasible MR suspension of high MR performance and long-term stability.

Spin dynamics investigations of multifunctional ambient scalable Fe3O4 surface decorated ZnO magnetic nanocomposite using FMR

Microwave spin resonance behavior of the Fe3O4 surface decorated ZnO nanocomposites (FZNC) has been investigated by ferromagnetic resonance (FMR). Modified hydrothermal method has been adopted to fabricate FZNC samples with Fe3O4 nanoparticles chains were used as seeds in the uniform magnetic field to decorate them on the surface of the ZnO nanoparticles in a unique configuration. Spin dynamics investigation confirms the transition of ZnO from diamagnetic to ferromagnetic as the sharp FMR spectra converts to the broad spectra with Fe3O4 nanoparticles incorporation. A single broad FMR spectra confirms that no isolated Fe3+ or Zn2+ ions exist which is also in agreement with XRD confirming suitable composite formation. Further, the increase in Fe3O4 concentration leads to decrease in g-value which is resulting from the internal field enhancement due to magnetic ordering. Also, various spin resonance parameters were calculated for the FZNC which provides a detail information about the magnetic ordering, exchange coupling and anisotropy. Elemental analysis confirms the presence of Fe and Zn simultaneously and transmission electron microscopy (TEM) image show the presence of Fe3O4 on the grain boundaries of ZnO which has been confirmed by taking high-resolution TEM and electron diffraction patterns on both sides of the interface. These unique structural configuration of the FZNC has tremendous potential in various magneto-optoelectronic, spintronics and electro-chemical applications.

Microwave spin resonance investigation on the effect of the post-processing annealing of CoFe2O4 nanoparticles

A novel investigation on the finite-size effects on the spin resonance properties of cobalt ferrite (CoFe₂O₄) nanoparticles has been performed using a room temperature ferromagnetic resonance (FMR) technique. A single broad spectrum was obtained for the CoFe₂O₄ nanoparticle samples, which indicated that all the samples were showing ferromagnetic characteristics. An asymmetric FMR line shape with a hefty trailing section was obtained due to the high magneto-crystalline anisotropy in CoFe₂O₄ nanoparticles, which changed with the size distribution. The resonance field for the samples shifted to a higher value due to the increase in the magneto-crystalline anisotropy in the CoFe₂O₄ nanoparticles with an increase in size. A systematic change in the resonance field and line width was observed with the change in the size distribution of the particles. Initially, it decreased with an increase in the size of the particles and increased after the critical size range. The critical size range is the imprint of the shift of the magnetic domain from a single domain to multi domain. The line width increased at higher annealing temperatures due to the enhancement in the dipole-dipole interaction, which led to a higher spin concentration as well as magneto-crystalline anisotropy. Furthermore, the saturation magnetization (M _s) as well as 'M _r/M _s' increased from 37.7 to 71.4 emu g^-1 and 0.06 to 0.31, respectively. The highest coercivity (750.9 Oe) and anisotropy constant (4.62 × 10⁴ erg cm^-3) were found for the sample annealed at 700 °C, which can be corroborated by the literature as the critical annealing temperature at which CoFe₂O₄ nanoparticles shift from single domain nanoparticles to multi-domain nanoparticles. Post-processing annealing is critical in advanced processing techniques and spin dynamics plays a vital role in various interdisciplinary areas of applications.

Temperature selectivity for single phase hydrothermal synthesis of PEG-400 coated magnetite nanoparticles

Temperature plays a vital role in the hydrothermal synthesis of the nanoparticles. Herein, we have provided a very detailed spin dynamic investigation on the varying size Fe₃O₄ nanoparticles using FMR technique.

The Effect of Particle Shapes on the Field-Dependent Rheological Properties of Magnetorheological Greases

The transient response of magnetorheological (MR) materials, in general, is very important for design consideration in MR-based devices. Better response to magnetic fields is beneficial for a better response rate to the electrical current applied in the electromagnetic coil. As a result, MR-based devices would have a high response to external stimuli. In this work, the principal characteristics of magnetorheological greases (MRGs) which have two different particle shapes are experimentally investigated. One type of particle distributed in the grease medium is conventional spherical-shaped carbonyl iron (CI) particles, while the other is plate-like CI particles made using a high-energy rotary ball mill from spherical CI particles. A set of bidisperse MRG samples are firstly prepared by adjusting the weight percentage of the plate-like CI particles and mixing with the spherical CI particles. Subsequently, three important properties of MRGs in terms of their practical application are measured and compared between the two different particle shapes. The field-dependent apparent viscoelastic properties of the prepared MRG samples are measured, followed by the field-dependent storage and loss moduli using an oscillatory shear rheometer. In addition, the transient response time, which indicates the speed in the actuating period of MRGs, is measured by changing the strain amplitude. Then, a comparative assessment on the three properties are undertaken between two different particle shapes by presenting the corresponding results in the same plot. It is shown that the bidisperse MRG with plate-like CI particles exhibits an increase in the initial apparent viscosity as well as stiffness property compared to the MRG with spherical particles only.