Excellent thermomagnetic power generation for harvesting waste heat via a second-order ferromagnetic transition.

Thermomagnetic generation (TMG), a promising technology to convert low-grade waste heat to electricity, utilizes high performance TMG materials. However, the drawbacks of large hysteresis, poor mechanical properties and inadequate service life hinder the practical applications. For the first time, we evaluated the effect of different phase transitions on the TMG performance by systematically comparing the TMG performance of three typical Heusler alloys with similar composition but different phase transitions. Ni2Mn1.4In0.6 exhibits second-order magnetic transition (SOMT) from the ferromagnetic (FM) to paramagnetic (PM) state around TC = 316 K without thermal hysteresis. It presents highly comprehensive TMG performance, which is not only better than those of other two Heusler alloys with different phase transitions, but also better than those of most typical TMG materials. The maximum power density (1752.3 mW m-3), cost index (2.78 µW per €), and power generation index PGI (8.91 × 10-4) of Ni2Mn1.4In0.6 are 1-5, 1-4, and 1-7 orders of magnitude higher than those of most typical reported materials, respectively. In addition, Ni2Mn1.4In0.6 with SOMT also shows some advantages that first-order magnetic transition (FOMT) materials do not have, such as zero hysteresis and a long-term service life. In contrast to the short lifetime of a few minutes for the materials with FOMT, Ni2Mn1.4In0.6 with SOMT can serve for one month or even longer with excellent cycling stability. Consequently, we conclude that the SOMT Ni2Mn1.4In0.6 Heusler alloy with good TMG performance as well as zero hysteresis and long service life can be a better candidate than FOMT materials for practical applications of TMG.

Rapid multiple property determination from bulk materials libraries prepared from chemically synthesized powders.

A variety of high-performance materials are utilized in electrical, electronic, and mechanical systems. Such systems account for a significant fraction of the world's electricity consumption. The next generation of such systems urgently require new material compositions which possess a better combination of both structural and functional properties. Only accelerated methodologies can rapidly determine the required multiple property set. Hence, a range of iron-cobalt-nickel ternary alloy composition powders were chemically synthesized. Compositionally graded bulk materials libraries were prepared by spark plasma sintering of these powders. A multiple property set of the crystal structure, magnetic, mechanical, and electrical properties were determined for a range of compositions. This property set revealed that a good combination of magnetic and mechanical properties can be obtained from Fe50Co40Ni10, high electrical resistivity from Fe54Co17Ni29 and high saturation magnetization as well as high hardness from Fe57Co29Ni14. Thus, this multiple property library, developed by accelerated methodologies, can be utilized to identify new ternary compositions satisfying diverse property sets relevant to next generation systems.

Optimal ferrofluids for magnetic cooling devices.

Superior passive cooling technologies are urgently required to tackle device overheating, consequent performance degradation, and service life reduction. Magnetic cooling, governed by the thermomagnetic convection of a ferrofluid, is a promising emerging passive heat transfer technology to meet these challenges. Hence, we studied the performance metrics, non-dimensional parameters, and thermomagnetic cooling performance of various ferrite and metal-based ferrofluids. The magnetic pressure, friction factor, power transfer, and exergy loss were determined to predict the performance of such cooling devices. We also investigated the significance of the magnetic properties of the nanoparticles used in the ferrofluid on cooling performance. γ-Fe2O3, Fe3O4, and CoFe2O4 nanoparticles exhibited superior cooling performance among ferrite-based ferrofluids. FeCo nanoparticles had the best cooling performance for the case of metallic ferrofluids. The saturation magnetization of the magnetic nanoparticles is found to be a significant parameter to enhance heat transfer and heat load cooling. These results can be used to select the optimum magnetic nanoparticle-based ferrofluid for a specific magnetic cooling device application.

Influence of Cr Substitution and Temperature on Hierarchical Phase Decomposition in the AlCoFeNi High Entropy Alloy.

While the AlCoFeNi high entropy alloy exhibits a single ordered B2 phase at high temperature, both the substitution of ferromagnetic Co with antiferromagnetic Cr, and lower annealing temperatures lead to a tendency for this system to decompose into a two-phase mixture of ordered B2 and disordered BCC solid solution. The length scale of this decomposition is determined by the combination of composition and annealing temperature, as demonstrated in this investigation by comparing and contrasting AlCoFeNi with the AlCo0.5Cr0.5FeNi alloy. The resulting phase stability has been rationalized based on solution thermodynamic predictions. Additionally, it is shown that replacement of Co by Cr in the AlCoFeNi alloy resulted in a substantial reduction in saturation magnetization and increase in coercivity. The microhardness is also strongly influenced by the composition and the length scale of B2 + BCC decomposition in these high entropy alloys.

Label-Free Alignment of Nonmagnetic Particles in a Small Uniform Magnetic Field.

Label-free manipulation of biological entities can minimize damage, increase viability and improve efficiency of subsequent analysis. Understanding the mechanism of interaction between magnetic and nonmagnetic particles in an inverse ferrofluid can provide a mechanism of label-free manipulation of such entities in a uniform magnetic field. The magnetic force, induced by relative magnetic susceptibility difference between nonmagnetic particles and surrounding magnetic particles as well as particle-particle interaction were studied. Label-free alignment of nonmagnetic particles can be achieved by higher magnetic field strength (Ba), smaller particle spacing (R), larger particle size (rp1), and higher relative magnetic permeability difference between particle and the surrounding fluid (Rµr). Rµr can be used to predict the direction of the magnetic force between both magnetic and nonmagnetic particles. A sandwich structure, containing alternate layers of magnetic and nonmagnetic particle chains, was studied. This work can be used for manipulation of nonmagnetic particles in lab-on-a-chip applications.

High energy product chemically synthesized exchange coupled Nd2Fe14B/α-Fe magnetic powders.

The excellent hard magnetic properties of Nd2Fe14B based magnets have an enormous range of technological applications. Exchange-coupled Nd2Fe14B/α-Fe magnets were chemically synthesized by a microwave assisted combustion process to produce mixed oxides, followed by a reduction diffusion process to form magnetic nano-composite powder. This synthesis technique offers an inexpensive and facile platform to produce exchange coupled hard magnets. The size dependent magnetic properties were investigated. The formation mechanisms of the oxide powders and the reduction diffusion mechanism were identified. The microwave power was found to play a crucial role in determining the crystallite size. The coercivity of the powder increased with increasing particle size. Room temperature coercivity (Hc) values greater than 9 kOe and magnetization of 110 emu g-1 was obtained in particles with a mean size of ∼62 nm. An energy product of 5.2 MGOe was obtained, which is the highest reported value for chemically synthesized hard magnetic Nd2Fe14B/α-Fe powders.

Mechanochemical synthesis of high coercivity Nd2(Fe,Co)14B magnetic particles.

With increasing demand for magnets in energy conversion systems, the quest for the development and understanding of novel processing routes to produce permanent magnets has become urgent. We report a novel mechanochemical process for the synthesis of Nd2(Fe,Co)14B magnetic particles with a high coercivity of 12.4 kOe. This process involves the reduction of neodymium oxide, iron oxide, cobalt oxide and boron anhydride in the presence of a calcium reducing agent and a CaO diluent. The formation mechanism of Nd2(Fe,Co)14B changed with increasing CaO content, and the average crystal size of the Nd2(Fe,Co)14B particles also increased, resulting in an increase in the coercivity values. The reaction mechanism during milling was revealed through a study of the phase transformations as a function of milling time. It was found that unlike self-propagating reactions, this reduction reaction during milling requires continuous input of mechanical energy to reach a steady state.

Magnetic Janus particles synthesized using droplet micro-magnetofluidic techniques for protein detection.

Magnetic droplets on a microfluidic platform can act as micro-robots, providing wireless, remote, and programmable control. This field of droplet micro-magnetofluidics (DMMF) is useful for droplet merging, mixing and synthesis of Janus structures. Specifically, magnetic Janus particles (MJP) are useful for protein and DNA detection as well as magnetically controlled bioprinting. However, synthesis of MJP with control of the functional phases is a challenge. Hence, we developed a high flow rate, surfactant-free, wash-less method to synthesize MJP by integration of DMMF with hybrid magnetic fields. The effects of the flow rate, flow rate ratio, and hybrid magnetic field on the magnetic component of the Janus droplets and the MJP were investigated. It was found that the magnetization, particle size, and phase distribution inside MJP could be readily tuned by the flow rates and the magnetic field. The magnetic component in the MJP could be concentrated after mixing at flow rate ratio values less than 7.5 and flow rates less than 3 ml h-1. The experimental results and our simulations are in good agreement. The synthesized magnetic-fluorescent Janus particles were used for protein detection, with BSA as a model protein.

Droplet Merging on a Lab-on-a-Chip Platform by Uniform Magnetic Fields.

Droplet microfluidics offers a range of Lab-on-a-chip (LoC) applications. However, wireless and programmable manipulation of such droplets is a challenge. We address this challenge by experimental and modelling studies of uniform magnetic field induced merging of ferrofluid based droplets. Control of droplet velocity and merging was achieved through uniform magnetic field and flow rate ratio. Conditions for droplet merging with respect to droplet velocity were studied. Merging and mixing of colour dye + magnetite composite droplets was demonstrated. Our experimental and numerical results are in good agreement. These studies are useful for wireless and programmable droplet merging as well as mixing relevant to biosensing, bioassay, microfluidic-based synthesis, reaction kinetics, and magnetochemistry.

Magnetocaloric Properties of Fe-Ni-Cr Nanoparticles for Active Cooling.

Low cost, earth abundant, rare earth free magnetocaloric nanoparticles have attracted an enormous amount of attention for green, energy efficient, active near room temperature thermal management. Hence, we investigated the magnetocaloric properties of transition metal based (Fe70Ni30)100-xCrx (x = 1, 3, 5, 6 and 7) nanoparticles. The influence of Cr additions on the Curie temperature (TC) was studied. Only 5% of Cr can reduce the TC from ~438 K to 258 K. These alloys exhibit broad entropy v/s temperature curves, which is useful to enhance relative cooling power (RCP). For a field change of 5 T, the RCP for (Fe70Ni30)99Cr1 nanoparticles was found to be 548 J-kg-1. Tunable TCin broad range, good RCP, low cost, high corrosion resistance and earth abundance make these nanoparticles suitable for low-grade waste heat recovery as well as near room temperature active cooling applications.

Magnetic Trapping of Bacteria at Low Magnetic Fields.

A suspension of non-magnetic entities in a ferrofluid is referred to as an inverse ferrofluid. Current research to trap non-magnetic entities in an inverse ferrofluid focuses on using large permanent magnets to generate high magnetic field gradients, which seriously limits Lab-on-a-Chip applications. On the other hand, in this work, trapping of non-magnetic entities, e.g., bacteria in a uniform external magnetic field was studied with a novel chip design. An inverse ferrofluid flows in a channel and a non-magnetic island is placed in the middle of this channel. The magnetic field was distorted by this island due to the magnetic susceptibility difference between this island and the surrounding ferrofluid, resulting in magnetic forces applied on the non-magnetic entities. Both the ferromagnetic particles and the non-magnetic entities, e.g., bacteria were attracted towards the island, and subsequently accumulate in different regions. The alignment of the ferrimagnetic particles and optical transparency of the ferrofluid was greatly enhanced by the bacteria at low applied magnetic fields. This work is applicable to lab-on-a-chip based detection and trapping of non-magnetic entities bacteria and cells.

Facile production of monodisperse nanoparticles on a liquid surface.

The emergence of monodispersity during particle growth on a liquid substrate was investigated both by experimental methods and by computer simulation. Monodispersity arises through a novel mechanism (termed "shared coarsening"), associated with the spatial distribution of the particles; smaller particles are simultaneously consumed by several larger particles. Particle monodispersity was predicted by kinetic Monte Carlo simulation for suitable substrate adsorption probability and adatom diffusion length conditions. High particle monodispersity is predicted for low adsorption probability and low/intermediate diffusion length values. Experimentally, the formation of uniformly sized copper nanoparticles by physical vapor deposition on a liquid substrate was demonstrated. These results demonstrate, by experiment and simulation, the facile production of monodisperse particles on liquid substrates.

Magnetic Field Triggered Multicycle Damage Sensing and Self Healing.

Multifunctional materials inspired by biological structures have attracted great interest, e.g. for wearable/ flexible "skin" and smart coatings. A current challenge in this area is to develop an artificial material which mimics biological skin by simultaneously displaying color change on damage as well as self healing of the damaged region. Here we report, for the first time, the development of a damage sensing and self healing magnet-polymer composite (Magpol), which actively responds to an external magnetic field. We incorporated reversible sensing using mechanochromic molecules in a shape memory thermoplastic matrix. Exposure to an alternating magnetic field (AMF) triggers shape recovery and facilitates damage repair. Magpol exhibited a linear strain response upto 150% strain and complete recovery after healing. We have demonstrated the use of this concept in a reusable biomedical device i.e., coated guidewires. Our findings offer a new synergistic method to bestow multifunctionality for applications ranging from medical device coatings to adaptive wing structures.

Low-temperature synthesis and nanomagnetism of large-area alpha-Fe2O3 nanobelts.

Large-area one dimensional (1D) alpha-Fe2O3 nanostructures were grown on iron substrates by catalyst-free thermal oxidation process at low temperatures in air. The structure characterization revealed that the nanostructures are single crystalline alpha-Fe2O3. Two kinds of alpha-Fe2O3 nanostructures, nanobelts and nanoflakes, were obtained due to the different growth temperature range. A surface diffusion mechanism is proposed to account for the nanobelts and nanoflakes growth. The Morin temperature T(M) of pure 1D alpha-Fe2O3 nanostructures is 121 K, which is far below their bulk counterparts. The coercive field depends on temperature, and takes values 471 Oe at 5 K and about 260 Oe when the temperature is greater than T(M), respectively.

Magnetic glass in shape memory alloy: Ni45Co5Mn38Sn12.

The first order martensitic transition in the ferromagnetic shape memory alloy Ni(45)Co(5)Mn(38)Sn(12) is also a magnetic transition and has a large field induced effect. While cooling in the presence of a field this first order magnetic martensite transition is kinetically arrested. Depending on the cooling field, a fraction of the arrested ferromagnetic austenite phase persists down to the lowest temperature as a magnetic glassy state, similar to the one observed in various intermetallic alloys and in half doped manganites. A detailed investigation of this first order ferromagnetic austenite (FM-A) to low magnetization martensite (LM-M) state transition as a function of temperature and field has been carried out by magnetization measurements. Extensive cooling and heating in unequal field (CHUF) measurements and a novel field cooled protocol for isothermal MH measurements (FC-MH) are utilized to investigate the glass like arrested states and show a reverse martensite transition. Finally, we determine a field-temperature (HT) phase diagram of Ni(45)Co(5)Mn(38)Sn(12) from various magnetization measurements which brings out the regions where thermodynamic and metastable states coexist in the HT space, clearly depicting this system as a 'magnetic glass'.

Exchange bias effect in partially oxidized amorphous Fe-Ni-B based metallic glass nanostructures.

The magnetic properties of amorphous Fe-Ni-B based metallic glass nanostructures were investigated. The nanostructures underwent a spin-glass transition at temperatures below 100 K and revealed an irreversible temperature following the linear de Almeida-Thouless dependence. When the nanostructures were cooled below 25 K in a magnetic field, they exhibited an exchange bias effect with enhanced coercivity. The observed onset of exchange bias is associated with the coexistence of the spin-glass phase along with the appearance of another spin-glass phase formed by oxidation of the structurally disordered surface layer, displaying a distinct training effect and cooling field dependence. The latter showed a maximum in exchange bias field and coercivity, which is probably due to competing multiple equivalent spin configurations at the boundary between the two spin-glass phases.

Nonlinear deformation of a ferrofluid droplet in a uniform magnetic field.

This paper reports experimental and numerical results of the deformation of a ferrofluid droplet on a superhydrophobic surface under the effect of a uniform magnetic field. A water-based ferrofluid droplet surrounded by immiscible mineral oil was stretched by a magnetic field parallel to the substrate surface. The results show that an increasing flux density increases the droplet width and decreases the droplet height. A numerical model was established to study the equilibrium shape of the ferrofluid droplet. The governing equations for physical fields, including the magnetic field, are solved by the finite volume method. The interface between the two immiscible liquids was tracked by the level-set method. Nonlinear magnetization was implemented in the model. Comparison between experimental and numerical results shows that the numerical model can predict well the nonlinear deformation of a ferrofluid droplet in a uniform magnetic field.

Insights into the mechanism of magnetic particle assisted gene delivery.

In magnetic particle assisted gene delivery DNA is complexed with polymer-coated aggregated magnetic nanoparticles (AMNPs) to effect transfection. In vitro studies based on COS-7 cells were carried out using pEGFP-N1 and pMIR-REPORT-complexed, polyethylenimine (PEI)-coated iron oxide magnetic nanoparticles (MNPs). PEI-coated AMNPs (PEI-AMNPs) with average individual particle diameters of 8, 16 and 30 nm were synthesized. Normal, reverse and retention magnetic transfection experiments and cell wounding assays were performed. Our results show that the optimum magnetic field yields maximum transfection efficiency with good viability. The results of the normal, reverse and retention magnetic transfection experiments show that the highest transfection efficiency was achieved in normal magnetic transfection mode due to clustering of the PEI-AMNPs on the cells. Cell wounding assay results suggest that the mechanism of magnetic transfection is endocytosis rather than cell wounding.

Synthesis of Co/Co3O4 nanocomposite particles relevant to magnetic field processing.

Co/Co3O4 nanocomposite particles of various morphologies were synthesized by the reverse micelle technique. Equiaxed, rod and faceted crystals with rectangular, pentagonal and hexagonal cross sections were observed. Annealing resulted in the formation of a composite of cobalt oxide (Co3O4) and fcc cobalt (Co). Removal of boron residues from the final product was established by surface characterization. Magnetic moment of these nanocomposite particles is relevant to magnetic field processing.

Microwave synthesis of noncentrosymmetric BaTiO3 truncated nanocubes for charge storage applications.

Truncated nanocubes of barium titanate (BT) were synthesized using a rapid, facile microwave-assisted hydrothermal route. Stoichiometric composition of pellets of nanocube BT powders was prepared by two-stage microwave process. Characterization by powder XRD, Rietveld refinement, SEM, TEM, and dielectric and polarization measurements was performed. X-ray diffraction revealed a polymorphic transformation from cubic Pm3̅m to tetragonal P4mm after 15 min of microwave irradiation, arising from titanium displacement along the c-axis. Secondary electron images were examined for nanocube BT synthesis and annealed at different timings. Transmission electron microscopy showed a narrow particle size distribution with an average size of 70 ± 9 nm. The remanence and saturation polarization were 15.5 ± 1.6 and 19.3 ± 1.2 µC/cm(2), respectively. A charge storage density of 925 ± 47 nF/cm(2) was obtained; Pt/BT/Pt multilayer ceramic capacitor stack had an average leakage current density of 5.78 ± 0.46 × 10(-8) A/cm(2) at ±2 V. The significance of this study shows an inexpensive and facile processing platform for synthesis of high-k dielectric for charge storage applications.