Formation, characterization and magnetic properties of carbon-encapsulated iron carbide nanoparticles

Fe₃C nanoparticles encapsulated in carbon shell with a size range of 20–50 nm were obtained in large scale by reacting anhydrous FeCl₃, hexamethylenetetramine and metal Na in an autoclave at 650 °C. Magnetization measurements show that the as-obtained materials display superparamagnetic properties at room temperature. A possible formation mechanism of the core–shell nano-structures was discussed.     

Catalytic activity and magnetic properties of barium hexaferrite prepared from barite ore

Barium hexaferrite (BaFe₁₂O₁₉) has traditionally been used in permanent magnets and more recently used for high density magnetic recording. The classical ceramic method for the preparation of barium hexaferrite consists of firing mixture of chemical grade iron oxide and barium carbonate at high temperature. In this paper a mixture of chemical grade hematite, barium oxide and predetermined mixtures of iron oxide ore and barite ore containing variable amounts of coke were used to prepare barium hexaferrite (BaFe₁₂O₁₉) as a permanent magnetic material. The mixtures were mixed in a ball mill and fired for 20 h in a tube furnace at different temperatures (1100, 1150, 1200 and 1250 °C). XRD, magnetic properties, porosity measurements and catalytic activity were used for characterization of the produced ferrite. The results of experiments showed that the optimum conditions for the preparation of barium hexaferrite are found at 1200 °C for the mixture of chemical grade hematite and barium oxide. It was also found that the barium hexaferrite can be prepared from the iron and barite ores at 1200 °C. The addition of coke enhanced the yield of barium hexaferrite and improved its physicochemical properties. Samples prepared from ores with coke% = 0 show the most acidic active sites, they show a higher catalytic activity towards H₂O₂ decomposition. With addition of coke the catalytic activity decreases due to the poisoning effect of carbon on the available active site.     

Effect of processing conditions on the structure and collective magnetic properties of flowerlike nickel nanostructures

To acquiring more insights into the relationship between the morphology and magnetic properties of magnetic nanocrystallites, uniform flowerlike Ni nanostructures with different branch lengths were fabricated via a simple solvothermal reduction route based on a series of comparative experiments. The formation mechanism of the Ni flowers was proposed. Moreover, the magnetic properties of the products were evaluated. Results indicate that the morphology of the Ni particles strongly depends on reaction temperature, and the branch length of the flowerlike Ni particles strongly depends on the initial concentration of Ni²⁺ ions. The flowerlike Ni particles with a longer branch length show higher coercivity value, which may be attributed to the peculiar microstructure. We believe the present research may provide an ideal example for the synthesis of assembled magnetic nanostructures with controllable morphology and magnetic properties.     

Effect of PbO-SiO2 and PbO-B2O3 flux systems on the crystalline and magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite prepared from the mixed powders

Lead borate and lead silicate were added to lower the sintering temperature of a Ni_0.5Zn_0.5Fe₂O₄ ferrite prepared from the blend of two types of powders and to homogenize the grain size. 5PbO·SiO₂ and 5PbO·B₂O₃ flux systems were added to lower the sintering temperature and diminish the magnetic loss at high frequencies. The ferrites were studied by bulk density, scanning electron microscopy and impedance analysis. It was found that the addition of PbO markedly accelerated the grain growth, while SiO₂ and B₂O₃ were found to be effective to obstruct the movement of grain boundaries and to minimize the grain size. Doping with PbO in the mixed powders appropriately increased the densification and initial permeability. The ferrite doped with 1% of 5PbO·SiO₂ possessed the lowest loss tangent (tgδ) in the range of 5 M-40 MHz and the highest threshold frequency.     

Preparation and magnetic properties of spindle porous iron nanoparticles

Spindle porous iron nanoparticles were firstly synthesized by reducing the pre-synthesized hematite (α-Fe₂O₃) spindle particles with hydrogen gas. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption/desorption isotherms and vibrating sample magnetometry (VSM). A lattice shrinkage mechanism was employed to explain the formation process of the porous structure, and the adsorbed phosphate was proposed as a protective shell in the reduction process. N₂ adsorption/desorption result showed a Brunauer-Emmett-Teller (BET) surface area of 29.7 m²/g and a continuous pore size distribution from 2 nm to 100 nm. The magnetic hysteresis loop of the synthesized iron particles showed a saturation magnetization of 84.65 emu/g and a coercivity of 442.36 Oe at room temperature.     

Microwave-hydrothermal synthesis of γ-Fe2O3 nanoparticles and their magnetic properties

Nanosized γ-Fe₂O₃ is synthesized by the microwave-hydrothermal method. Powder X-ray diffraction and transmission electron microscopic studies showed that the average particle size is 10 nm. Magnetic studies reveal that the γ-Fe₂O₃ nanoparticles are superparamagnetic at room temperature, with a superparamagnetic blocking temperature of 200 K. The magnetic characteristics of the nanoparticles indicate their strongly interacting nature.     

The enhanced coercivity for the magnetite/silica nanocomposite at room temperature

Magnetite/silica nanocomposite was synthesized by a facile solvothermal processing at 150 °C for about 10 h. X-ray diffraction (XRD) analysis revealed the effect of annealing on the crystallinity of silica. Transmission electron microscopy (TEM) images showed the good dispersion of magnetite in the silica matrix. Magnetic properties of the nanocomposite were characterized by vibration sample magnetometer (VSM), and the enhanced coercivity was explained by the intrinsic anisotropy of the particles enhanced by the interparticle dipolar fields.     

Synthesis, photoluminescence and magnetic properties of barium vanadate nanoflowers

The flower-shaped barium vanadate has been obtained by the composite hydroxide mediated (CHM) method from V₂O₅ and BaCl₂ at 200 °C for 13 h. XRD and XPS spectrum of the as-synthesized sample indicate it is hexagonal Ba₃V₂O₈ with small amount of Ba₃VO_4.8 coexistence. Scan electron microscope and transmission electron microscope display that the flower-shaped crystals are composed of nanosheets with thickness of ∼20 nm. The UV-visible spectrum shows that the barium vanadate sample has two optical gaps (3.85 eV and 3.12 eV). Photoluminescence spectrum of the barium vanadate flowers exhibits a visible light emission centered at 492 and 525 nm which might be attributed to VO₄ tetrahedron with T_d symmetry in Ba₃V₂O₈. The ferromagnetic behavior of the barium vanadate nanoflowers has been found with saturation magnetization of about 83.50 × 10⁻³ emu/g, coercivity of 18.89 Oe and remnant magnetization of 4.63 × 10⁻³ emu/g, which is mainly due to the presence of a non-orthovanadate phase with spin S = 1/2.     

High T2-relaxation and biocompatible magnetic nanofluids with poly(amino acid) coating

A facile one-pot method has been developed to prepare poly(amino acid) functionalized, water-stable, biocompatible, and superparamagnetic iron oxide nanoparticles (NPs) with small diameters of ∼10 nm. The obtained biocompatible magnetic nanoparticles capped with polyaspartic acid (PASP) exhibit a relatively high saturation magnetization (57.1 emu/g) and a much strong magnetic resonance (MR) T₂ relaxation effect with the transverse relaxivity coefficient (r₂) as high as 302.6 s⁻¹ mM⁻¹. Interestingly, the as-prepared Fe₃O₄@PASP NPs are highly stable in aqueous solution and demonstrate the property of magnetic nanofluids. The high T₂ effect, good water-stability, superparamagnetization, biocompatibility and bioconjugatability render the as-synthesized Fe₃O₄@PASP NPs great desirable for bioapplications such as magnetic resonance imaging (MRI), bioseparation, targeted drug delivery, and so on.     

Tuning of magnetic parameters in cobalt-polystyrene nanocomposites by reduction cycling

Cobalt nanoparticles were prepared by a reduction process inside polymer pores using CoSO₄·7H₂O and NaBH₄. A porous polymer network (sulphonated polystyrene) was chosen, as the template for the synthesis of elementary cobalt as high surface area cobalt nanoparticles are prone to oxidation. The preliminary studies reveal that the cobalt is first formed with an oxide protective layer outside and upon repeating the reduction cycles, inner pores of the polymers are opened which enhanced the yield of metallic cobalt. These high surface area cobalt nanoparticles embedded in a polymer are ideal for the synthesis of carbon nanotubes as cobalt can act as a catalyst for the nanotube synthesis. The concentration of cobalt can be tuned in this technique by repeating the cycling process.     

EuCuOSe: Crystal structure refinement, electrical and magnetic properties

The europium copper oxyselenide EuCuOSe has been prepared by reacting Eu, CuO and Se in the ratio 1:1:1 at 1123 K for a period of 10 days in sealed quartz ampoule. The structure has been determined by single-crystal X-ray methods. The compound crystallizes tetragonal in the space group P4/nmm (no. 129) with two formula units in the cell with dimensions a = 393.65(8) pm and c = 871.80(17) pm. The structure is composed of double layers separated by copper atoms, which are tetrahedrally coordinated to Se²⁻ anions. According to the resistivity measurements, EuCuOSe is a semiconductor. The magnetic susceptibility data shows the typical non-Curie-Weiss behavior of the ⁷F_J states of Eu in the 4f⁶ configuration.     

A comprehensive study of soft magnetic materials based on FeSi spheres and polymeric resin modified by silica nanorods

A novel soft magnetic composite (SMC) based on spherical FeSi particles precisely covered by hybrid phenolic resin was designed. The hybrid resin including silica nano-rods chemically incorporated into the phenolic polymer matrix was prepared by the modified sol–gel method. A chemical bridge connecting silica nano-rods with the base polymeric net was verified by FTIR, ¹³C and ²⁹Si NMR spectroscopy, whereas the shape and size of silica nano-rods were determined by TEM. It is shown that the modification of polymeric resin by silica nano-rods generally leads to the improved thermal and mechanical properties of the final samples. The hybrid resin serves as a perfect insulating coating deposited on FeSi particles and the core–shell particles can be further compacted by standard powder metallurgy methods in order to prepare final samples for mechanical, electric and magnetic testing. SEM images evidence negligible porosity, uniform distribution of the hybrid resin around FeSi particles, as well as, dimensional shape stability of the final samples after thermal treatment. The hardness, flexural strength and density of the final samples are comparable to the sintered SMCs, but they simultaneously exhibit much higher specific resistivity along with only slightly lower coercivity and permeability.     

Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles

CoO nanoparticles in the 38-93 nm range have been prepared by thermal decomposition. The particles were characterized to be pyramid shape with a hexagonal close-packed structure. Their anomalous magnetic behavior includes: (i) vanishing of antiferromagnetic transition around 300 K; (ii) creation of hysteresis below a blocking temperature of 6-11 K; (iii) presence of relatively large moments and coercivities accompany with specific loop shifts at 5 K; and (iv) appearance of an additional small peak located in low field in the electron spin resonance spectrum. Further, the present results provide evidence for the existence of uncompensated surface spins. The coercivity and exchange bias decrease with increasing particle size, indicating a distinct size effect. These observations can be explained by the multisublattice model, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the total CoO particle.     

Low temperature synthesis of nanocrystalline lithium ferrite by a modified citrate gel precursor method

Single phase nanocrystalline lithium ferrite is synthesized by a modified citrate gel precursor technique. Ferrite nanoparticles of average size of 8 nm, obtained after calcination of the citrate gel made by the usual method at 450 °C, show superparamagnetic behavior. However, small amounts of -Fe₂O₃ is formed as an impurity phase due to the initial formation of some -Fe₂O₃ phase. On the other hand, when the pH of the mixed solution is increased to 7 after the addition of ammonia solution, a lower calcination temperature of 200 °C is sufficient for the formation of single phase lithium ferrite nanoparticles of size 30 nm. No impurity phases are detected when the nanocrystalline powders are calcined at higher temperatures. The magnetic properties of the ferrite nanoparticles of different sizes obtained by calcining the powders at different temperatures are studied.     

The influence of parameters of chemical synthesis on the optical properties of CdS nanocrystals

A systematic analysis is presented on the influence of parameters of synthesis on the formation and optical properties of CdS nanocrystals prepared using dimethyl formamide as solvent and stabilizing agent. The parameters considered are the molar ratios of cadmium to sulfur, initial concentration of Cd²⁺ and S²⁻ at a fixed molar ratio and atmospheric conditions. Optical absorption and photoluminescence measurements were carried out during the synthesis to monitor the changes in optical properties. The results clearly indicate that the nanocrystals size could be tuned by varying the molar ratio of the starting materials. Ambient conditions are found to play a vital role on the stability and optical properties of the nanocrystals whereas the concentration of the starting materials had little influence on the optical absorption and luminescence properties. These results are expected to help in developing a better understanding of the mechanisms of solution growth and size stabilization of semiconductor nanocrystals during chemical synthesis.     

The affect of densification and dehydroxylation on the mechanical properties of stoichiometric hydroxyapatite bioceramics

This paper reports the effects of processing densification on the mechanical properties of hydroxyapatite bioceramics. Densification of synthetic hydroxyapatite is conducted in the range 1000-1300 °C. X-ray diffraction and SEM microscopy are used to check the microstructure transformations. Vickers hardness, toughness and Young's modulus are analyzed versus the density and grain size. The sintering temperature and the particle size influence strongly the densification and the resulting mechanical properties. In addition, the critical sintering temperature appears around 1200 °C and the declined strength at the temperature up to 1200 °C is found sensitive to the dehydroxylation process of hydroxyapatite.     

Metallic cobalt microcrystals with flowerlike architectures: Synthesis, growth mechanism and magnetic properties

Well-defined, three-dimensional (3D) flowerlike metallic Co microcrystals with several radiating hexagonal-tapered petals assembled by particles size of 150-250 nm were fabricated via a facile hydrothermal reduction route under a fixed basic condition. The morphology and structure of the products were characterized by scanning electronic microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (XRD). The probable formation mechanism of the flowerlike Co microcrystals was discussed based on the experimental results. Magnetic properties of Co microcrystals were investigated by a commercial Physical Properties Measurement System (PPMS). The flowerlike products exhibited ferromagnetic characteristics with a saturation magnetization of 128.1 emu/g and a coercivity of 232.5 Oe at room temperature. Compared to the coercivity value of bulk Co, the products displayed a remarkable enhanced value due to their special morphology.     

Studies on initial permeability and loss factor in Ni-Zn ferrites synthesized by oxalate precursors

Ni_XZn_1−XFe₂O₄ ferrites with (X = 0.28-0.40 in step of 0.2) have been synthesized by oxalate precursor method and investigated for their, initial permeability and loss factor measurements. Initial permeability has been observed to increase with the increase in Ni²⁺ up to X = 0.32, beyond which it decreases. The variation of initial permeability has been explained by considering the factors such as grain size, saturation magnetization and anisotropy constant. Thermal variation of initial permeability reveals a peak height in μ_i-T curves which tends to increase with increase in Ni²⁺ content. μ_i-T curves also exhibit thermal hysteresis, which reveals the inverse relationship between the difference in heating and cooling curves at which hysteresis falls between Hopkinson peak and T_c with value of initial permeability. Loss factor values are small which is attributed to high density of the samples and processing techniques.     

Magnetic properties and photoabsorption of the Mn-doped CeO2 nanorods

Mn-doped CeO₂ nanorods have been prepared from CeO₂ particles through a facile composite-hydroxide-mediated (CHM) approach. The analysis from X-ray photoelectron spectroscopy indicates that the manganese doped in CeO₂ exists as Mn²⁺. The magnetic measurement of the Mn-doped CeO₂ nanorods exhibits an enhanced ferromagnetic property at room temperature with a remanence magnetization (Mr) of 1.36 × 10⁻³ emu/g and coercivity (Hc) of 22 Oe. Comparative UV-visible spectra reveal the shift of the absorption peak of the CeO₂ from ultraviolet region to visible light region after being doped with Mn. The room temperature ferromagnetic properties and light absorption of the Mn-doped CeO₂ nanorods would have potential applications in photocatalysis and building of photovoltaic devices.     

Ultrasonic-assisted synthesis and magnetic studies of iron oxide/MCM-41 nanocomposite

Iron oxide nanoparticles were stabilized within the pores of mesoporous silica MCM-41 amino-functionalized by a sonochemical method. Formation of iron oxide nanoparticles inside the mesoporous channels of amino-functionalized MCM-41 was realized by wet impregnation using iron nitrate, followed by calcinations at 550 °C in air. The effect of functionalization level on structural and magnetic properties of obtained nanocomposites was studied. The resulting materials were characterized by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy and selected area electron diffraction (HRTEM and SAED), vibrating sample and superconducting quantum interface magnetometers (VSM and SQUID) and nitrogen adsorption–desorption isotherms measurements. The HRTEM images reveal that the most of the iron oxide nanoparticles were dispersed inside the mesopores of silica matrix and the pore diameter of the amino-functionalized MCM-41 matrix dictates the particle size of iron oxide nanoparticles. The obtained material possesses mesoporous structure and interesting magnetic properties. Saturation magnetization value of magnetic iron oxide nanopatricles stabilized in MCM-41 amino-functionalized by in situ sonochemical synthesis was 1.84 emu g⁻¹. An important finding is that obtained magnetic nanocomposite materials exhibit enhanced magnetic properties than those of iron oxide/MCM-41 nanocomposite obtained by conventional method. The described method is providing a rather short preparation time and a narrow size distribution of iron oxide nanoparticles.