A facile method to synthesize magnetic polymer nanospheres with multifunctional groups

Magnetic poly(styrene methyl methacrylate)/Fe₃O₄ nanospheres with ester groups were prepared by a modified one-step mini-emulsion polymerization in the presence of Fe₃O₄ ferrofluids. The effects of monomer dose, surfactant content, ferrofluid concentration and initiator content on the particle characteristics such as the size, morphology and magnetic properties were investigated by Fourier-transform infrared spectroscopy, transmission electron microscopy, thermogravimetric analysis and vibrating sample magnetometer. The results indicated that magnetic nanospheres were superparamagnetic with high saturation magnetization of 51.0 emu/g and corresponding magnetite content of 61.5 wt%. Subsequently, magnetic nanospheres with carboxyl and amino groups were also obtained by hydrolysis and ammonolysis reaction. These magnetic nanospheres with multifunctional groups have biomedical applications.     

Synthesis and magnetic properties of aligned carbon nanotubes by microwave-assisted pyrolysis of acetylene

Aligned carbon nanotubes with high quality were synthesized at low temperature by microwave-assisted pyrolysis of acetylene in nitrogen atmosphere. The morphology and structure of the products were characterized by field-emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The results indicated that the ACNTs with high crystallinity are densely packed, and some Fe₃C nanoparticles are encapsulated in all parts of carbon tubes. In addition, the effect of the reaction temperature on the morphologies of the CNTs was also studied in detail. Magnetic measurements showed that the Fe₃C-filled ACNTs display ferromagnetic properties at room temperature, and can be easily manipulated by an external magnetic field.     

Preparation and application of magnetic poly(styrene-glycidyl methacrylate) microspheres

A novel method for the preparation of polymer-based, magnetic microspheres is proposed. Pre-made, poly(styrene-glycidyl methacrylate) (PS-GMA) particles of micron size were swollen by a mixture of N-methyl-2-pyrrolidone and water, and then incubated with superparamagnetic nanoparticles. The nanoparticles were allowed to diffuse into polymer microspheres during the incubation, became entrapped and made the polymer microspheres superparamagnetic. These magnetic PS-GMA microspheres were chemically modified and then coupled with single-stranded oligonucleotides as probes for DNA hybridization. The immobilized probes showed repeatable capture of target oligonucleotides.     

Preparation of novel magnetic fluorescent nanospheres by sonochemical method

This study focuses on the preparation and characterization of magnetic fluorescent nanospheres (MFNs). MFNs are prepared using a sonochemical method in the presence of hydrophobic Fe₃O₄ nanopaticles, and then the nanospheres are modified with Rhodamine B through an electrostatic interaction. The properties of MFNs are characterized using transmission electron microscopy, Fourier-transform infrared, thermogravimetric analysis, vibration sample magnetometry, and fluorescence emission spectrum. The results indicate that the superparamagnetic nanospheres have a particle size of 160 nm, high saturation magnetization of 54 emu/g, and significant fluorescence. MFNs possess potential in medical imaging, drug targeting, and catalysis. The possible formation mechanism of MFNs is discussed.     

Study on immobilization of lipase onto magnetic microspheres with epoxy groups

Magnetic microspheres were synthesized by the suspension polymerization of glycidyl methacrylate (GMA), methacrylic acid (MAA) and divinyl benzene (DVB) in the presence of oleic acid-coated Fe₃O₄ nanoparticles. Triacylglycerol lipase from porcine pancreas was covalently immobilized on the magnetic microspheres via the active epoxy groups with the activity yield up to 63% (±2.3%) and enzyme loading of 39 (±0.5) mg/g supports. The resulting immobilized lipase had higher optimum temperature compared with those of free lipase and exhibited better thermal, broader pH stability and excellent reusability. Furthermore, the catalyzed capability of immobilized lipase was also investigated by catalyzing synthesis of hexyl acetate and the esterification conversion rate reached to 83% (±2.5%) after 12 h in nonaqueous solvent.     

Structural characterization and magnetic properties of superparamagnetic zinc ferrite nanoparticles synthesized by the coprecipitation method

Single phase zinc ferrite (ZnFe₂O₄) nanoparticles have been prepared by the coprecipitation method without any subsequent calcination. The effects of precipitation temperature in the range 20–80 °C on the structural and the magnetic properties of zinc ferrite nanoparticles were investigated. The crystallite size, microstructure and magnetic properties of the prepared nanoparticles were studied using X-ray diffraction (XRD), Fourier transmission infrared spectrum, transmission electron microscope (TEM), energy dispersive X-ray spectrometer and vibrating sample magnetometer. The XRD results showed that the coprecipitated nanoparticles were single phase zinc ferrite with mixture of normal and inverse spinel structures. Furthermore, ZnFe₂O₄ nanoparticles have the crystallite size in the range 5–10 nm, as confirmed by TEM. The magnetic measurements exhibited that the zinc ferrite nanoparticles synthesized at 40 °C were superparamagnetic with the maximum magnetization of 7.3 emu/g at 10 kOe.     

Fast fabrication of epoxy-functionalized magnetic polymer core-shell microspheres using glycidyl methacrylate as monomer via photo-initiated miniemulsion polymerization

Epoxy-functionalized magnetic polymer microspheres with high magnetic responsiveness were prepared through a one-step photo-initiated miniemulsion polymerization. This kind of core-shell particle can be synthesized successfully with hydrophilic monomer, glycidyl methacrylate, as a single monomer by a 10 min ultraviolet irradiation, and phase separation induced eccentric cores. The morphology of the particles was examined with a transmission electron microscope and a scanning electron microscope. The incorporation of magnetic particles was characterized with X-ray diffraction. The magnetic content of the microspheres was detected by both vibrating sample magnetometer and thermo-gravimetric analyzer.     

A novel approach to preparing magnetic protein microspheres with core-shell structure

Magnetic protein microspheres with core-shell structure were prepared through a novel approach based on the sonochemical method and the emulsion solvent evaporation method. The microspheres are composed of the oleic acid and undecylenic acid modified Fe₃O₄ cores and coated with globular bovine serum albumin (BSA). Under an optimized condition, up to 57.8 wt% of approximately 10 nm superparamagnetic Fe₃O₄ nanoparticles could be uniformly encapsulated into the BSA microspheres with the diameter of approximately 160 nm and the high saturation magnetization of 38.5 emu/g, besides of the abundant functional groups. The possible formation mechanism of magnetic microspheres was discussed in detail.     

A comparative study of magnetic transferability of superparamagnetic nanoparticles

The aspect of magnetic transferability was established using an automated magnetic particle transfer workstation. Maghemite (γ-Fe₂O₃) nanoparticles were synthesized via conventional co-precipitation procedure. Their transferability was determined in addition to several commercial nanoparticles that ranged in diameter, surface functionality, and composition. Transmission and scanning electron micrographs and infrared spectrum, respectively, provided size and surface information on the synthesized particles for comparison to commercially available magnetic nanoparticles.     

Preparation and characterization of magnetic P(St-co-MAA-co-AM) microspheres

In this study, magnetic polymer-coated microspheres were prepared by the microemulsion polymerization of styrene (St), methacrylic acid (MAA), acryamide (AM) in the presence of emulsifiers with the size of 1–5 μm. The magnetic material (i.e. Fe₃O₄) coated with oleic acid used in the preparation of the microspheres was synthesized in a classical co-precipitation procedure. The morphological and magnetic properties of the microspheres were investigated by different techniques (i.e. TEM, TGA, optical microscopy, vibrating sample magnetometer). The results indicated that the magnetic microspheres were superparamagnetic, well shaped spheres, mono-dispersed with abundant functional groups on the surfaces of the magnetic microspheres and good thermal stability. The microspheres could be linked well with the avidin and FITC antibody.     

Density limits imposed by the microstructure of magnetic recording media

The fundamental limit of magnetic recording density on conventional media is set by the grain size. Once this grain size limit is reached, only a reduction of the grain size allows an increased SNR and thus an increased areal density. It is shown that, whilst maintaining thermal stability, scaling demands that the required anisotropy energy density K is proportional to the areal density, or the square of the areal density if the medium thickness reaches the critical thickness (A is the exchange stiffness of the material). Recording onto materials with such a high anisotropy requires some form of a write-assist. It is furthermore shown that the grain size limit cannot be obtained with intergranular exchange present, and six different requirements are listed that constitute ideal media. An alternative path for increasing areal density of magnetic recording is to use patterned media, where each bit contains only one grain. In this case, written-in errors dominate system performance and the maximum achievable areal density is estimated to be about 6 Tbit/in². Patterned media need to exhibit narrow distributions of their physical and structural properties with standard deviations of the order of 5% or less.     

Effects of excessive grain growth on the magnetic and mechanical properties of hot-deformed NdFeB magnets

The magnetic and mechanical properties of rare-earth magnets hot-deformed at temperature range 750-950 °C have been investigated. The grains tended to grow excessively from dozens of nanometers to several microns at the temperatures above 850 °C. The alignment of grains was disrupted by the hot deformation at the high temperatures. The Nd-rich phase was extruded at the temperatures which are higher than 850 °C. The Nd-rich phase extrusion resulted in the reduction of density by 1% and the reduction of remanence from 1.42 to 0.72 T. The reduction of grain boundaries caused by flat platelet-shaped grains changing to spherical grains and the weak binding strength among large grains of Nd₂Fe₁₄B phase may be the main reasons for the low mechanical strength of hot-deformed magnets.     

Synthesis and characterization of magnetic poly(glycidyl methacrylate) microspheres

Magnetic nanoparticles encapsulated in poly(glycidyl methacrylate) microspheres were prepared and their detailed structural and magnetic characteristics given. Iron oxide nanoparticles were obtained by chemical coprecipitation of Fe(II) and Fe(III) salts and stabilized with dextran, (carboxymethyl)dextran or tetramethylammonium hydroxide. The microspheres were prepared by emulsion or dispersion polymerization of glycidyl methacrylate in the presence of ferrofluid. The microspheres were uniform both in shape and usually also in size; their size distribution was narrow. All the magnetic parameters confirm superparamagnetic nature of the microspheres. Blocking temperature was not observed, suggesting the absence of magnetic interactions at low temperatures. This is most probably caused by complete encapsulation and the absence of agglomeration. Such microspheres can be used in biomedical applications.     

Changes of microstructure and magnetic properties of Nd-Fe-B sintered magnets by doping Al-Cu

The microstructural and magnetic properties of Al_100−xCu_x (15at%≤x≤45 at%) doped Nd-Fe-B magnets were studied. The distribution and alloying effects of Cu or Al on the intergranular microstructure were investigated by thermodynamic analysis, differential scanning calorimetery and microscopy techniques. It was observed that when the Cu content of Al_100xCu_x exceeds to 25 at%, the (Pr, Nd)Cu and CuAl₂ phases form in these magnets. The formation of (Pr, Nd)Cu phase depends on the negative formation enthalpy of (Pr, Nd)Cu and the exclusive distribution of Cu in the intergranular regions. The eutectic reaction between (Pr, Nd)Cu phase and (Pr, Nd) occurs at 480 °C, which forms the liquid phase that dissolves the (Pr, Nd)₂Fe₁₄B surface irregularities and thus increases the quantities of (Pr, Nd)-rich phase at the grain boundaries. These changes benefit the grain boundary microstructure, especially the distribution of (Pr, Nd)-rich phase, which effectively improves the intrinsic coercivity _iH_c due to the decreases of exchange coupling between the (Pr, Nd)₂Fe₁₄B grains.     

Heat treatment effects on microstructure and magnetic properties of Mn-Zn ferrite powders

Mn-Zn ferrite powders (Mn_0.5Zn_0.5Fe₂O₄) were prepared by the nitrate-citrate auto-combustion method and subsequently annealed in air or argon. The effects of heat treatment temperature on crystalline phases formation, microstructure and magnetic properties of Mn-Zn ferrite were investigated by X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy and vibrating sample magnetometer. Ferrites decomposed to Fe₂O₃ and Mn₂O₃ after annealing above 550 °C in air, and had poor magnetic properties. However, Fe₂O₃ and Mn₂O₃ were dissolved after ferrites annealing above 1100 °C. Moreover, the 1200 °C annealed sample showed pure ferrite phase, larger saturation magnetization (M_s=48.15 emu g⁻¹) and lower coercivity (H_c=51 Oe) compared with the auto-combusted ferrite powder (M_s=44.32 emu g⁻¹, H_c=70 Oe). The 600 °C air annealed sample had the largest saturation magnetization (M_s=56.37 emu g⁻¹) and the lowest coercivity (H_c=32 Oe) due to the presence of pure ferrite spinel phase, its microstructure and crystalline size.     

Effect of surface modification on dielectric and magnetic properties of metal powder/polymer nanocomposites

Metal nanopowder (Co and Fe)/polymer composites, both with and without surface modification by behenic acid, were fabricated and their dielectric and magnetic properties were measured at 1 GHz to study the effect of surface modification on the electromagnetic properties. The relative permittivity and the real part of the permeability of the composites with surface modified powders were higher than those with unmodified powders. Related dielectric losses remained at almost the same level, but magnetic losses were somewhat increased. The increase of relative permittivity could result from the increased volume fraction of interphase with a slightly higher relative permittivity at the particle/polymer interface than that of the bulk polymer. The increase in the real part of the permeability may be caused by suppression of the induced demagnetizing field due to suppression of eddy currents by a better particle distribution and a decrease in effective agglomerate size because of the surface modification.     

Preparation, and magnetic and electromagnetic properties of La-doped strontium ferrite films

SrLa_xFe_12−xO₁₉ films (x=0-1.0) with large magneto-crystalline anisotropy were synthesized on SiO₂ substrate by sol-gel and self-propagating high-temperature synthesis technique. The films were characterized by various experimental techniques including X-ray diffraction analysis, Field Emission Scanning Electron Microscope, Atomic Force Microscopy, Vibrating Sample Magnetometry and vector network analyzer. The results show that La ions completely enter into strontium ferrite lattice without changing the ferrite appearance; its grain size is approximately 40-80 nm, its length is 100 nm; the magnetoplumbite structure is proved through testing a concertina form of the crystal grain; the maximum coercivity is 5986 Oe at x=0.2; La-doped films possess a wider microwave absorption frequency range with better gross loss angle tangent (tan δ>0.1), from 9 to 10.5 GHz at x=0.2, where the maximum value of tan δ reaches 0.2709. The La-doped films reach smaller nanometer size, better magnetic properties and microwave absorption properties with the doping of lanthanum.     

Effects of In-substitution on the microstructure and magnetic properties of Bi-CVG ferrite with low temperature sintering

[Y_1.05Bi_0.75Ca_1.2](Fe_4.4−xIn_xV_0.6)O₁₂(In_x:Bi-CVG) ferrite material has been prepared successfully by a solid-state reaction method. The effects of In³⁺ substitution and sintering temperatures on the bulk density, microstructure and magnetic properties are performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), materials automatic test system (MATS) and microwave ferrite parameters meter. The results show that In³⁺ can lower the sintering temperatures and enhance the magnetic properties of Bi-CVG ferrite. Besides, all sintered specimens with different In³⁺ contents show a single garnet crystal structure. The specimen of [Y_1.05Bi_0.75Ca_1.2](Fe₄In_0.4V_0.6)O₁₂ sintered at 1075 °C shows homogenous distribution of grain size and densified microstructures. The ferromagnetic resonance linewidth (ΔH) has an increase with In³⁺ contents. Additionally, the sample has the optimum magnetic properties: ρ=5.23 g/cm³, B_r=31.3 mT, H_c=378.8 A/m, 4πM_s=506.2×10⁻⁴ T.     

Facile fabrication of novel cyclomatrix-type polyphosphazene nanotubes with active hydroxyl groups via an in situ template approach

Novel polyphosphazene nanotubes with active hydroxyl groups were fabricated via an in situ template approach under ultrasonic irradiation. SEM and TEM results indicated that the nanotubes were uniform with length of several micrometers, inner diameter of ca. 20 nm and outer diameter of 60-80 nm. FTIR spectra revealed that the content of the hydroxyl groups on the nanotube surface was dependent on the feed ratio of hexachlorocyclotriphosphazene (HCCP) to 4,4′-sulfonyldiphenol. The successful esterification of polymer nanotubes with benzoxy chloride demonstrated the high reactivity of the hydroxyl groups. The method employed here might provide a simple and effective way to prepare functional nanotubes used for biological applications.