Novel Sr-Zn-Co hexagonal ferrite nano-rods by wood-template chemical solution synthesis

Novel one-dimensional structured SrZnCoFe₁₆O₂₇ hexagonal ferrites were successfully prepared by using the nature pine-wood template assisted chemical solution method. The diameters of the w-type hexagonal ferrite nano-rods obtained were in the range of 500 ± 50 nm and the length attained was 5-6 μm. The one-dimensional structured SrZnCoFe₁₆O₂₇ hexagonal ferrite exhibited magnetop lumbite structure and presented hard magnetic properties. The magnetization value of the rod-like ferrites was much higher than that of the bulk hexagonal ferrites. The reasonable explanation of the improvement in the magnetization value was the shape effect. To our knowledge, only a few papers have reported the successful preparation of 1D structured Ba/Sr hexagonal ferrite and this novel SrZnCoFe₁₆O₂₇ hexagonal ferrite can be potentially applied in magnetic material industries.     

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.     

Low-temperature sintering of Z-type hexagonal ferrite by addition of fluorine containing glass powder

In order to reduce the sintering temperature of Ba₃Co₂Fe₂₄O₄₁ (Co₂Z), fluorine containing glass powder was added as a sintering aid to ferrite powder with a Co₂Z stoichiometric composition prepared by a solid-state reaction, and dense sintered specimens could be obtained at 1000°C in air. The densification was achieved by liquid-phase sintering which was induced by the melting of the additive glass at 800°C. The main crystalline phase was Co₂Z, and spinel ferrite appeared as the impurity phase. By sintering in a sealed container, the densification was accelerated still more, and in addition to spinel ferrite, Ba-M also appeared as the impurity phase. The Ba-M contained some Co instead of Fe, and grew to discontinuously large hexagonal plate-like grains. In a fluorine and/or fluorides rich atmosphere, Co₂Z was discomposed to Ba-M and spinel ferrite, and large hexagonal plate-like grains appeared. These results suggest that fluorine and/or fluorides evaporated from the additive glass decomposed Co₂Z to Ba-M and spinel ferrite, and induced the discontinuously grain growth of Ba-M. The initial permeability was lower than that of the specimen with no additive glass but remained almost constant in the frequency regions up to 1 GHz.     

High frequency magnetic mechanism of Ni-substituted Co2Z hexagonal ferrite

The magnetic properties, especially the high frequency magnetic mechanism, of Ni-substituted Co₂Z hexagonal ferrite were studied. The polycrystalline Z-type hexagonal ferrite of Ba₃Ni_xCo_2−xFe₂₄O₄₁ (0 ≤ x ≤ 2) were prepared by solid-state reaction. The results indicate that Ni-substituted Co₂Z samples all exhibit typical soft magnetic character. Substitution of Ni for Co will turn the planar magnetocrystalline anisotropy of Co₂Z to uniaxial anisotropy when x ≥ 1, so that the permeability drops dramatically and domain wall resonance appears in the frequency spectra. With the rise of Ni amount or sintering temperature, domain wall resonance strengthens gradually.     

A simple approach for the synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocrystals

By using Co²⁺ and Co³⁺ salts, and freshly extracted ovalbumin, Co₃O₄ nanocrystals have been synthesized successfully. The pH of the solution was self-regulated for the hydrolysis of metal ions as the ovalbumin-water mixture was highly basic. Water soluble ovalbumin proteins served as a perfect matrix for entrapment of Co²⁺ and Co³⁺ ions thus forming a gel. Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Co₃O₄. The crystallite size obtained by XRD line profile fitting was 45 ± 8 nm and particle size estimated from the SEM was in the range 20 nm-2 μm. EPR results show a very good fit to literature reports for nanocrystals in the size range of 8–17 nm. Even though the overall particle size is quite large and its distribution is quite wide EPR results confirm nanocrystalline nature of the particles obtained. Presented route is simple, cost effective, and environmentally friendly.     

Mechanical response of nanocrystalline steel obtained by mechanical attrition

The mechanical properties of bulk specimens of nanocrystalline 0.55% C steel with a grain size of 30 nm and a relative density higher than 97% have been determined. Samples were obtained by cold compaction and warm sintering at 425 °C of nanocrystalline powders obtained by mechanical attrition in a planetary ball mill. In both processes an Ar protective atmosphere was used in order to avoid oxygen contamination. X-ray diffraction (XRD) and Transmission electron microscopy (TEM) analysis indicated that a volume-averaged grain size of 30 nm is maintained after the warm consolidation processes. TEM studies also showed equiaxed ferrite with no dislocations inside the grains. However, the grain size distribution was no homogeneous as large grains of 100 nm were observed. An average hardness of 8.5 GPa was obtained, in good agreement with other bulk specimens of nanocrystalline Fe or eutectoid carbon steel prepared by other authors. Compression tests of bulk specimens at a strain rate of 10⁻⁴ s⁻¹ showed a compression strength near 2,500 MPa with an absolute lack of ductility. Nanoindentation measurements at room temperature provided a strain rate sensitivity parameter of 0.012, indicating that the deformation mechanism is somehow governed by diffusion mechanisms.     

Magnetic Properties of ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanoparticles Synthesized by Starch-Assisted Sol–Gel Auto-combustion Method

In this paper, ZnFe₂O₄ spinel ferrite nanoparticles with different grain sizes at different annealing temperatures have been synthesized using the starch-assisted sol–gel auto-combustion method. The synthesized nanoparticles were characterized by conventional powder X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and vibrating sample magnetometer. The X-ray diffraction (XRD) patterns demonstrated that the ZnFe₂O₄ nanoparticles consist of single-phase spinel structure with crystallite sizes 4.81, 8.72, 12.06, 29.32, and 72.60 nm annealed at 400, 600, 800, 1000, and 1200 °C, respectively. Field emission scanning electron microscopy reveals that particles are of spherical morphology at lower annealing temperature and hexagonal-like morphology at higher temperature. An infrared spectroscopy study shows the presence of two principal absorption bands in the frequency range around 525 cm^?1 (ν ₁) and around 350 cm^?1 (ν ₂), which indicate the presence of tetrahedral and octahedral group complexes, respectively, within the spinel ferrite nanoparticles. Raman spectroscopy study also indicated the change in octahedral and tetrahedral site-related Raman modes in zinc ferrite nanoparticles with change of particle size. The nanocrystalline ZnFe₂O₄ samples (4.81, 8.72, 12.06, 29.32 nm) show ferrimagnetic behavior, and bulk sample (72.60 nm) shows paramagnetic behavior. This change in magnetic behavior is due to change of cation distribution in ZnFe₂O₄ nanoparticles with decrease of particle size.     

The effect of heat treatment temperature on the microstructure and magnetic properties of Ba2Co2Fe12O22 (Co2Y) prepared by sol–gel method

Ferroxplana type hexagonal ferrite with composition Ba₂Co₂Fe₁₂O₂₂ (Co₂Y) was prepared by polymeric sol–gel method from the aqueous solution of their corresponding metal salts. All the samples were characterized by using X-ray diffraction and scanning electron microscopic technique. The formation and the microstructure of the Ba₂Co₂Fe₁₂O₂₂ was studied as a function of heat treatment temperature and it was observed that well defined hexagonal plate-like fine particles with particle sizes between 130 and 400 nm were formed at relatively lower temperature (900 °C) as compared to conventional solid state reaction method (1100 °C). Magnetic properties of the samples were also studied as a function of heat treatment temperature and it showed large saturation magnetization (M_s) ranging from 30.9 to 47.6 emu/g and a wide range of coercivity (130∼1566 Oe) depending on the heat treatment temperature. The very high saturation magnetization at relatively lower temperature (say 800 °C) arises due to the presence of several high magnetic impurities, such as BaFe₁₂O₁₉ and CoFe₂O₄. The saturation magnetization increases with increasing heat treatment temperature above 900 °C, while the coercivity showed a reversed order.     

Electrical and dielectric properties of nano crystalline Ni–Co spinel ferrites

Nanocrystalline samples of Ni_xCo₁–_xFe₂O₄, where x = 1, 0.8, 0.6, 0.4, 0.2 and 0, were synthesized by chemical co-precipitation method. The spinel cubic phase formation of Ni–Co ferrite samples was confirmed by X-ray diffraction (XRD) data analysis. All the Bragg lines observed in XRD pattern belong to cubic spinel structure of ferrite. Scanning Electron Microscopy (SEM) technique was used to study the surface morphology of the Ni–Co ferrite samples. Nanocrystalline size of Ni–Co ferrite series was observed in SEM images. Pellets of Ni–Co ferrite were used to study the electrical and dielectric properties. The resistivity measurements were carried out on the samples in the temperature range 300–900 K. Ferrimagnetic to paramagnetic transition temperature (T_c) for all samples was noted from resistivity data. The activation energy below and above T_c was calculated. The dielectric constant (ɛ′) measurements with increasing temperature show two peaks in the temperature range of measurements for all samples under investigation. The peaks observed show frequency and compositional dependences as a function of temperature. Electrical and dielectric properties of nanocrystalline Ni_xCo₁–_xFe₂O₄ samples show unusual behavior in temperature range of 500–750 K. To our knowledge, nobody has discussed such anomalies for nanocrystalline Ni_xCo₁–_xFe₂O₄ at high temperature. Here, we discuss the mechanism responsible for electrical and dielectric behavior of nanocrystalline Ni_xCo₁–_xFe₂O₄ samples.     

Magnetic properties of nanocrystalline ɛ-Fe3N and Co4N phases synthesized by newer precursor route

Nanocrystalline ɛ-Fe₃N and Co₄N nitride phases are synthesized first time by using tris(1,2-diaminoethane)iron(II) chloride and tris(1,2-diaminoethane)cobalt(III) chloride precursors, respectively. To prepare ɛ-Fe₃N and Co₄N nitride phases, the synthesized precursors were mixed with urea in 1:12 ratio and heat treated at various temperatures in the range of 450–900 °C under the ultrapure nitrogen gas atmosphere. The precursors are confirmed by FT-IR study. The ɛ-Fe₃N phase crystallizes in hexagonal structure with unit cell parameters, a = 4.76 Å and c = 4.41 Å. The Co₄N phase crystallizes in face centred cubic (fcc) structure with unit cell parameters, a = 3.55 Å. The estimated crystallite size for ɛ-Fe₃N and Co₄N phases are 29 nm and 22 nm, respectively. The scanning electron microscopy (SEM) studies confirm the nanocrystalline nature of the materials. The values of saturation magnetization for ɛ-Fe₃N and Co₄N phases are found to be 28.1 emu/g and 123.6 emu/g, respectively. The reduction of magnetic moments in ultrafine materials compared to bulk materials have been explained by spin pairing effect, lattice expansion, superparamagnetic behaviour and canted spin structures at the surface of the particles.     

Effects of nanocrystalline ferrite particles on densification and magnetic properties of the NiCuZn ferrites

Effects of nanocrystalline ferrite particles addition on densification behavior and magnetic properties of the NiCuZn ferrites were investigated. It was confirmed that nanocrystalline ferrite particles enhanced densification of the samples obviously. The reason was attributed to the nanocrystalline particles, which spread around the micron-sized ferrite particles, increased contacting area and inter-diffusion of the particles. When the amount of nanocrystalline particles addition reached to 30 wt%, the samples obtained an approximate densification behavior as the 1.5 wt% Bi₂O₃ added samples. Due to relatively bigger grain size, higher sintering density and no different chemical composition sintering aids added, the sample with 30wt% nanocrystalline ferrite particles got the highest permeability and relatively high Q-factor when sintered at 900.     

Experimental and modeling studies on phase stability of nanocrystalline magnetic Sm2Co7

In contrast to the conventional polycrystalline low-Co Sm–Co alloys that have very weak permanent magnetic properties, the Sm₂Co₇ alloy has been found to have fairly promising permanent magnetic performance when its grain size is reduced to the nanoscale. It was discovered that the crystal structure of the nanocrystalline Sm₂Co₇ has a strong nanograin-size-dependent stability. The rhombohedral structure of Sm₂Co₇ phase which is metastable at temperatures lower than 1435 K in conventional polycrystalline system can exist stably at room temperature in the nanocrystalline system. To understand the phase stability of the nanocrystalline Sm₂Co₇, the experimental and nanothermodynamic analyses were combined to describe quantitatively the phase transformation behavior of Sm₂Co₇ on the nanoscale. The results are important for the development of nanostructured Sm–Co permanent magnets.     

Cobalt oxide (Co3O4) nanorings prepared from hexagonal β-Co(OH)2 nanosheets

Hexagonal cobalt hydroxide (β-Co(OH)₂) nanosheets over a size range from 100 nm to 1 μm were synthesized using a very simple hydrothermal route with cobalt naphthenate as the cobalt source. Additionally, hexagonal cobalt oxide (Co₃O₄) nanorings over a size range from 100 nm to 1 μm consisting of cubic nanocrystals were obtained via a hydrothermal method using as-prepared β-Co(OH)₂ nanosheets as the precursors. A probable mechanism of formation of the hexagonal Co₃O₄ nanorings is proposed on the basis of time-dependent experimental results.     

Microwave sintering of nanocrystalline γ-Al2O3

The densification and phase transformation behavior of gas condensation synthesized nanocrystalline γ-A1₂O₃ sintered with microwave radiation has been studied. The polymorphic nucleation and growth phase transformations which occurred as the material was heated through the temperature range of 800–1300°C present significant obstacles in the achievement of specimens which possess high bulk densities. These phase transformations are accompanied by a change in particle morphology, crystallite size, and surface area. Alumina derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or δ alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.     

Barium hexaferrite nanoparticles: Synthesis and magnetic properties

Carbon combustion synthesis is applied to rapid and energy efficient fabrication of crystalline barium hexaferrite nanoparticles with the average particle size of 50-100 nm. In this method, the exothermic oxidation of carbon nanoparticles with an average size of 5 nm with a surface area of 80 m²/g generates a self-propagating thermal wave with maximum temperatures of up to 1000 °C. The thermal front rapidly propagates through the mixture of solid reactants converting it to the hexagonal barium ferrite. Carbon is not incorporated in the product and is emitted from the reaction zone as a gaseous CO₂. The activation energy for carbon combustion synthesis of BaFe₁₂O₁₉ was estimated to be 98 kJ/mol. A complete conversion to hexagonal barium ferrite is obtained for carbon concentration exceeding 11 wt.%. The magnetic properties H_c∼3000 Oe and M_s∼50.3 emu/g of the compact sintered ferrites compare well with those produced by other synthesis methods.     

Direct nanocrystalline hydroxyapatite formation on titanium from ultrasonated electrochemical bath at physiological pH

An electrochemical method of producing nanocrystalline hydroxyapatite coatings on titanium surface is reported. The bath contained Ca(NO₃)₂ and NH₄H₂PO₄ in the molar ratio 1.67:1. The electrolyte was maintained at physiological pH and was ultrasonically agitated throughout the time of electrolysis. Coatings were deposited for 30 min at 10 and 15 mA/cm² and contained mono hydroxyapatite phase whose crystal sizes were lower than 30 nm. These sizes are comparable to the size of the bone hydroxyapatite crystals. Small globules of hydroxyapatite covered the coating surface completely. Fourier transformed infra-red spectroscopy (FT-IR) studies showed that the coatings contained large amounts of hydroxide and phosphate groups to enable the formation of hydroxyapatite. The coatings had a roughness (R_a) of about 0.3 μm and water contact angles of about 49°. Ultrasonic agitation promoted the formation of nanocrystalline structure which will help in better attachment of bone tissues to the implant surface.     

Synthesis of hexagonal β-Co(OH)2 nano-platelets with high catalytic activity via a low-temperature precipitation method

Large-scale β-Co(OH)₂ hexagonal nano-platelets have been synthesized successfully at 70 °C using only CoCl₂ and NaOH as reactants. The physicochemical features of the β-Co(OH)₂ hexagonal nano-platelets were characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) and nitrogen absorption–desorption. Results showed that the β-Co(OH)₂ obtained consisted of thin, regular, hexagonal nano-platelets with diameters and thicknesses of 50–300 nm and 30–80 nm, respectively. A transformation from β-Co(OH)₂ to Co₃O₄ was observed in the temperature range 160–220 °C. The average pore-size and the Brunauer–Emmet–Teller (BET) surface area of the synthesized β-Co(OH)₂ were 16.76 nm and 29.45 m²/g. The prepared product had a significant effect on the catalytic decomposition of ozone.     

Diffusion distribution of 57Co atoms in nanocrystalline Co79Nb14B7 alloy

Distribution of ⁵⁷Co atoms in a nanocrystalline Co₇₉Nb₁₄B₇ alloy after annealing at different temperatures was studied by means of ⁵⁷Co emission Mössbauer spectroscopy. After annealing below the recrystallization temperature, the diffusing ⁵⁷Co atoms were found in all phases expected to be present in the nanocrystalline alloy. They do not prefer the intergranular phase. Paths of diffusing atoms are probably close to interfaces between crystallites (grain boundaries) and/or intergranular phase. With increasing annealing temperature, when the nanocrystalline state transforms into a coarse polycrystal, the ⁵⁷Co atoms occupy regular sites in the bulk Co and Co₃B phases.     

Polymer composite P3HT:Eu doped La2O3 nanoparticles as a down-converter material to improve the solar spectrum energy

Europium-doped La₂O₃ nanocrystalline powders with sizes in the range of 50-200 nm have been obtained by the modified sol-gel Pechini method. These nanocrystals have been deagglomerated using sonication for 3 h and have been dispersed into a semiconductor P3HT polymeric matrix. We studied and analysed the spectroscopic properties of the trivalent europium in the hexagonal La₂O₃ nanocrystals dispersed in the polymer. The luminescence spectrum of Eu³⁺ in these nanocrystals is dominated by the ⁵D₀ → ⁷F₂ transition with a maximum intensity peak located at 626 nm. We observed that P3HT absorbs part of the light emitted by the nanoparticles. These properties look promising for using this material as a down-converter material in solar cells.