SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> ferrites and hard magnetic PVA nanocomposite: investigation of magnetization,coecivity and remanence

In this work, various morphologies of SrFe₁₂O₁₉ (SrFe) nanostructures were synthesized via a simple sol–gel method. The effect of concentration, temperature and various surfactants on the morphology and particle size of the magnetic seeds was investigated. The prepared magnetic products were characterized by X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy techniques. Alternating gradient force magnetometry reveals that the samples exhibit hard magnetic property with the coercivity up to 5300 Oe. Strontium ferrite was added to poly vinyl alcohol to prepare the magnetic polymeric matrix thin film nanocomposites. The saturation magnetization and coercivity decreased due to agglomeration of magnetic nanoparticles in polymer matrix.     

Preparation of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> · 6SrB<Subscript>2</Subscript>O<Subscript>4</Subscript> magnetic composites by spray pyrolysis

Spray pyrolysis has been used to produce X-ray amorphous precursors with the nominal composition SrFe₁₂O₁₉ · 6SrB₂O₄ in the form of spherical particles 0.3 to 2 μm in diameter. Heat treatment of the precursors at temperatures from 650 to 900°C has produced platelike strontium hexaferrite particles embedded in a SrB₂O₄ matrix. With increasing annealing temperature, the average dimensions of the hexaferrite particles increase from 80 × 20 to 450 × 100 nm and the coercivity of the material rises from 240 to 440 kA/m.     

Preparation and microwave absorption properties of nanosized Ni/SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> magnetic powder

A new type of Ni-coated strontium ferrite powder was prepared with electroless plating enhanced by ultrasonic wave at room temperature. The morphology, crystal structure and microwave absorption properties of the Ni-coated powder were investigated with field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), EDS and vector network analyzer. The results show that the powder possesses excellent microwave absorption properties. The maximum microwave loss of the composite powder reaches −41.3 dB. The bandwidth with the loss above −10 dB reaches 8 GHz.     

Facile synthesis of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanoparticles and its photocatalyst application

The strontium hexaferrite (SrFe₁₂O₁₉) nanoparticles have been successfully synthesized by co-precipitation route. The effect of various parameters such as calcination temperature and chelating agents were screened to achieve optimum condition. Different chelating agents such as amino acids (proline, alanine, aspartic acid) and surfactants (SDBS, PVP, and EDTA) were used. Compared with the amino acids, the surfactants increase the particle size and the best result was observed for alanine. The SrFe₁₂O₁₉ nanoparticles showed enhanced photocatalytic activity in the degradation of methyl orange under visible light irradiation (λ?>?400 nm). The degradation rates of the methyl orange were measured to be as high as 95% in 220 min. The nanoparticles were also characterized by several techniques including FT-IR, XRD, SEM, and VSM. The VSM measurement showed a saturation magnetization value (Ms) of 32 emu/g. The SEM images proposed that the particles are almost spherical with an average particle size of 90 nm.     

Functionalization of electrospun magnetically separable TiO<Subscript>2</Subscript>-coated SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers: strongly effective photocatalyst and magnetic separation

Magnetically separable TiO₂-coated SrFe₁₂O₁₉ electrospun nanofibers were obtained successfully by means of sol–gel, electrospinning, and coating technology, followed by heat treatment at 550–650 °C for 3 h. The average diameter of the electrospun fibers was 500–600 nm. The fibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). The optimized calcining temperature was determined by XRD and the analysis of decolorizing efficiency of methylene blue (MB) under UV–vis irradiation. The photocatalytic activity of the TiO₂-coated SrFe₁₂O₁₉ fibers was investigated using ultraviolet–visible absorbance by following the photooxidative decomposition of a model pollutant dye solution, MB in a photochemical reactor. In contrast to pure TiO₂ fibers, the TiO₂-coated SrFe₁₂O₁₉ fibers have higher absorption in 250–750 nm wavelength regions. The presence of SrFe₁₂O₁₉ not only broadened the response region of visible-light, but also enhanced the absorbance for UV light. The decolorizing efficiency of MB under UV–vis irradiation was up to 98.19%, which was a little higher than that of Degussa P25 (97.68%). Furthermore, these fibers could be recollected easily with a magnet in a photocatalytic process and had effectively avoided secondary pollution of treated water.     

The effect of nano sized SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> additions on the magnetic properties of chromium-doped strontium-hexaferrite ceramics

Strontium hexaferrite (SrFe₁₂O₁₉) which crystallizes in the hexagonal system and has a uniaxial magnetoplumbite structure, displays distinctive magnetic characteristics, good chemical stability, good tribological properties and a weak temperature dependent coercivity at about room temperature. In the present work the synthesis conditions for the solid-state preparation of the chromium-doped hexaferrite SrCr_0.3Fe_11.7O₁₉ were optimised and the effect on the magnetic properties of this compound of additions of nanosized SrFe₁₂O₁₉ was studied. The nanosized SrFe₁₂O₁₉ additive, synthesized by the citrate–nitrate reaction, was substituted in varying amounts for a commercial calcium silicate borate sintering additive mixture. A combination of 0.75 wt% of nano-sized SrFe₁₂O₁₉ with 0.75 wt% of the commercial additive increases the intrinsic coercivity, remanence magnetization and rectangularity ratio and results in superior magnetic properties than obtained with 1.5 wt% of nanosized SrFe₁₂O₁₉ or the commercial sintering additive alone.     

Thermodynamic properties of SrAl<Subscript>12</Subscript>O<Subscript>19</Subscript> and SrAl<Subscript>4</Subscript>O<Subscript>7</Subscript>

Strontium aluminates are important compounds with interesting properties such as long-duration phosphorescence and elastico-deformation luminescence. They have potential application in flexible light emitting panels. Since there are serious discrepancies in available thermodynamic data for these compounds, a redetermination of their Gibbs energies of formation was undertaken using solid-state electrochemical cells incorporating single-crystal SrF₂ as the electrolyte in the temperature range from 1000 to 1300 K. However, the measurements were restricted to SrAl₁₂O₁₉ and SrAl₄O₇ because of the formation of strontium oxyfluoride phase between SrAl₂O₄ and SrF₂. For the reactions, SrO + 6 Al₂O₃ → SrAl₁₂O₁₉, ΔG ^o/J mol^?1 (± 280) = ?83386 ? 25.744 (T/K), and SrO + 2Al₂O₃ → SrAl₄O₇, ΔG ^o/J mol^?1 (± 240) = ?80187 ? 25.376 (T/K). The high entropy of SrAl₄O₇ and SrAl₁₂O₁₉ can be partly related to their complex structures. The results of this study are consistent with calorimetric data on enthalpy of formation of other Sr-rich aluminates and indicate only marginal stability for SrAl₄O₇ relative to its neighbours, SrAl₁₂O₁₉ and SrAl₂O₄. The thermodynamic data explain the difficulty in direct synthesis of phase pure SrAl₄O₇ and the formation of SrAl₂O₄ as the initial ternary phase when reacting SrO and Al₂O₃ or crystallizing from amorphous state, irrespective of composition.     

Reaction kinetic,magnetic and microwave absorption studies of SrFe<Subscript>11.2</Subscript>Ni <Subscript>0.8</Subscript>O<Subscript>19</Subscript> hexaferrite nanoparticles

Nickel substituted strontium hexaferrite, SrFe_11.2Ni _0.8O₁₉ nanoparticles having super paramagnetic nature were synthesised by co-precipitation of chloride salts using 7.5 M sodium hydroxide solution. The resulting precursors were heat treated (HT) at 900 and 1,200 °C for 4 h in nitrogen atmosphere. During heat treatment, transformation proceeds as a constant rate of nucleation and three dimensional growth with activation energy of 183.724 kJ/mole. The hysteresis loops showed an increase in saturation magnetization from 1.045 to 65.188 emu/g with increasing HT temperatures. The ‘as-synthesised’ particles have size in the range of 20–25 nm with spherical and needle shapes. Further, these spherical and needle shaped nanoparticles tend to change their morphology to hexagonal plates with increase in HT temperatures. The effect of such a systematic morphological transformation of nanoparticles on dielectric (complex permitivity and permeability) and microwave absorption properties were estimated in X band (8.2–12.2 GHz). The maximum reflection loss of the composite powder reaches −24.92 dB at the thickness of 2.2 mm which suits its application in RADAR absorbing materials.     

Sol–gel synthesis of Ce-substituted BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>

Ce-substituted BaFe₁₂O₁₉ (BaCe _x Fe_12−x O₁₉, x = 0, 0.01, 0.03, 0.05) was prepared by citrate sol–gel method. The thermal decomposition process of precursor was investigated by TG-DSC. The phase composition of the BaCe _x Fe_12−x O₁₉ was characterized by X-ray powder diffraction analysis (XRD) which reveals that the BaCe _x Fe_12−x O₁₉ crystallizes in a hexagonal structure. The lattice parameter of BaCe _x Fe_12−x O₁₉ increases slightly when Ce was substituted into BaFe₁₂O₁₉. The average crystallite size calculated from the XRD line broadening is about 30–33 nm and no intermediate phases are detected in the XRD patterns. The transmission electron microscope (TEM) analysis indicates that the particles of samples obtained are the rod-like morphology.     

Microstructural and Magnetic Features of SrFe<Subscript>12</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>19</Subscript> Materials Synthesized from Different Fuels by Sol-Gel Auto-Combustion Method

The nanocrystalline SrFe₁₂ O ₁₉ materials were prepared by a sol-gel auto-combustion method using different fuels such as citric acid, dextrose, aniline, and hexamine. The combustion product obtained from all the fuels except from that of aniline show a single phase of SrFe₁₂ O ₁₉ materials upon annealing at 1000 ^°C/2 h. The combustion product obtained from aniline as fuel shows SrFe₁₂ O ₁₉ as the main phase with α-Fe₂ O ₃ as impurity. No notable change in lattice parameters is observed due to variation in fuels for SrFe₁₂ O ₁₉ materials. With a little change in the NIR relative reflectance (72–85 %) on fuels, the different SrFe₁₂ O ₁₉ materials display high NIR reflectance in the wavelength range, 1500–2500 nm. The photoluminescence emission spectra of SrFe₁₂ O ₁₉ materials reveal a broad emission peak at ～350 nm which is reminiscent to the Ba-based hexaferrite, BaFe₁₂ O ₁₉. The FESEM images expose quite dissimilar morphology for the various fuels used in the synthesis of SrFe₁₂ O ₁₉ materials. Hysteresis loops for all the nanocrystalline SrFe₁₂ O ₁₉ materials observed under the applied field of ±1.5 T at room temperature exhibit hard ferromagnetic property. The SrFe₁₂ O ₁₉ materials produced from glycine and aniline as fuels exhibit highest and lowest M _s values of 61.3 and 50.5 emu/g, respectively.     

Synthesis of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> composites with core/shell/shell nano-structure and evaluation of their photo-catalytic efficiency for degradation of methylene blue

The SrFe₁₂O₁₉/SiO₂/TiO₂ nanostructures with hard magnetic core were successfully synthesized through the facile and efficient wet chemical processes. At first, nanocrystalline strontium hexaferrite (SrFe₁₂O₁₉) powder was prepared using a new co-precipitation route in ethanol/water media. In the next step, SrFe₁₂O₁₉/SiO₂ composites were produced by well-known Stöber method using tetraethyl orthosilicate as precursor. Finally titania was coated on SrFe₁₂O₁₉/SiO₂ composite particles using titanium n-butoxide precursor. The core/shell/shell nanostructures have been characterized by means of X-ray diffraction, vibrating sample magnetometer, Fourier transform infrared spectra, field emission scanning electron microscopy, and transmission electron microscopy equipped with an energy-dispersive X-ray spectroscopy detector. The catalytic activity of SrFe₁₂O₁₉/SiO₂/TiO₂ composites has been investigated in the degradation of methylene blue dye under UV illumination. The results indicated that the obtained SrFe₁₂O₁₉/SiO₂/TiO₂ composite has photo-catalytic properties and can be retrieved by magnetic separation. The photo-degradation of methylene blue dye was about 80% in the presence of photo-catalyst powder at irradiation time of 180 min. Recycled composite particles could be used again.     

The stability of BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanoparticles in polar solvents

We have studied suspensions of hard-magnetic BaFe₁₂O₁₉ particles in water, ethanol and 1-butanol. The surfaces of these particles were previously modified with the surfactant dodecylbenzylsulphonic acid. The stabilities of the suspensions were estimated from their saturation concentrations and zeta potentials. We found that the 1-butanol suspensions were more stable than the ethanol-based suspensions and much more stable than the water-based suspensions. We analyzed the suspensions and the dispersed particles using gravimetry, conductometry and transmission electron microscopy, measured their zeta potentials, and calculated the interparticle-attraction interaction energies due to the van der Waals and magnetic dipole–dipole forces. The magnitudes of the attraction energies varied significantly with the particles’ sizes and the separation distances between the particles, and we found that the contribution of the van der Waals attraction energy can be neglected with respect to the magnetic dipole–dipole attraction. The observed differences in the stability of the suspensions were explained on the basis of the calculated electrostatic and steric repulsion energies. Electrosteric stabilization was possible in the 1-butanol and the ethanol for particles with radii and thicknesses up to 15 nm, while a too small electrostatic repulsion and the absence of steric repulsion in the water resulted in rapid agglomeration.     

Stability of the Fe<Subscript>12</Subscript>O<Subscript>12</Subscript> cluster

Superexchange effects play an important role in the determination of crystal structures; however, there has been much less reported on how they determine the stability of clusters. Using evolutionary search strategies and DFT+U (density functional theory with the Hubbard U correction) calculations, we investigate the global minimum-energy structures of Fe₁₂O_n clusters. Among predicted Fe₁₂O_n clusters, a cage-shaped Fe₁₂O₁₂ cluster with unexpected stability was observed. In addition, the bare Fe₁₂O₁₂ cluster is shown to possess an extremely large energy gap (2.00 eV), which is greater than that of C₆₀, Au₂₀ and Al₁₃?clusters. Using a Heisenberg model, we traced the origin of the unexpected stability of the bare Fe₁₂O₁₂ cluster to magnetic competition between the nearest-neighbor exchange constant J₁ and the next-nearest neighbor exchange constant J₂ that was induced by the superexchange interactions. The bare Fe₁₂O₁₂ cluster is thus a unique molecule that is stable and chemically inert.

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Preparation and characterization of mesoporous Co<Subscript>3</Subscript>O<Subscript>4</Subscript> electrode material

Mesoporous electrode materials of spinel Co₃O₄ were synthesized by hydrothermal method using polyethylene glycol-6000 (PEG-6000) as dispersant and subsequent calcination at different temperatures in air. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nitrogen adsorption and electrochemical measurements. The results showed that the dispersant PEG-6000 had a distinct effect to control the porosity, particle size and homogeneity of Co₃O₄; the calcination temperature had a significant influence on the crystal structure, surface area, porosity and morphology, and indeed electrochemical performance. The resultant Co₃O₄ sample calcinated at 350° C possessed a narrow mesopore distribution around 4 nm and exhibited excellent electrochemical performance. It had the specific capacitance as high as 348.7 F/g, increased by 21.5% over that of pure Co₃O₄ and showed good cyclical charge–discharge stability.     

EPR and optical investigation of ultraviolet-emitting Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> garnet

Gd₃Ga₅O₁₂ garnet was prepared by a solution combustion method and characterized using powder X-ray diffraction, electron paramagnetic resonance (EPR) and optical spectroscopic techniques. EPR spectrum of the samples at ambient and low temperatures exhibited resonance signal at g?≈?2 attributed to Gd³⁺ ions disposed in an octahedral symmetry. The optical absorption spectrum showed a band centered at 274 nm attributed to ⁸S_7/2→⁶I_J transition of the Gd³⁺ ions. The excitation spectrum showed a maximum at 273 nm along with two relatively weaker peaks at 276 and 279 nm. These three excitation bands were assigned to the transitions ⁸S_7/2→⁶I_J. The emission spectrum showed two peaks centered at 311 and 306 nm that were assigned to the transitions ⁶P_7/2→⁸S_7/2 and ⁶P_5/2→⁸S_7/2 of Gd³⁺, respectively. To evaluate the phosphor performance, several magnetic and thermodynamic parameters for the system were calculated adopting standard procedure.     

Microwave absorption properties of substituted BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/TiO<Subscript>2</Subscript> nanocomposite multilayer film

The nanocomposite multilayer film (NCMF) of substituted BaFe₁₂O₁₉ and TiO₂ was prepared using sol-gel method. BaFe_10.5Al_1.5O₁₉, BaFe_10.1Al_1.9O₁₉ and BaFe_11.4Cr_0.6O₁₉ with different absorption frequencies were selected to fabricate the multilayer film with TiO₂. The morphology, crystalline structure and microwave absorption property of the NCMF were investigated with an atomic force microscopy, X-ray diffraction and vector network analyses. The results show that the NCMF is uniform without microcracks and it is an ideal microwave attenuation material with a broad frequency range. Its maximum loss efficiency is about −40 dB. The frequency range with the loss above −10 dB is more than 7 GHz. The multilayer film assembles the achievements of each layer film. Moreover, the compounding of ferrites with TiO₂ is helpful to absorb more microwave energy. TiO₂ particles block the growth of ferrite grains and make most of their size within nanometers. TiO₂ also improves the dielectric loss efficiency of the NCMF.     

Synthesis and structure of the CsSmP<Subscript>4</Subscript>O<Subscript>12</Subscript> cyclotetraphosphate (cubic CsNdP<Subscript>4</Subscript>O<Subscript>12</Subscript> structure)

CsSmP₄O₁₂ crystals have been prepared at 300°C in molten polyphosphoric acids containing Cs, Mg, and Sm cations, and their crystal structure has been determined: sp. gr. I \(\bar 4\) 3d, a = 15.1225(8) Å, Z = 12, CsNdP₄O₁₂ structure.     

Preparation and characterization of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu phosphors by combustion process

Europium-doped yttrium phosphors were successfully synthesized by combustion process using yttrium nitrate hexahydrate [Y(NO₃)₃·6H₂O], europium nitrate [Eu(NO₃)₃·6H₂O], and carbohydrazide [CO(N₂H₃)₂]. The process takes only a few minutes to obtain Y₂O₃:Eu phosphors. The resultant powders annealed at different temperature and were investigated by X-ray diffractometer (XRD), differential thermal analyzer/thermogravimeter (DTA/TG), Fourier transform infrared spectroscopy (FTIR), photoluminescence excitation and emission spectra (PL), and transmission electron microscopy (TEM). The results showed that the formation temperature of Y₂O₃ phase is significant low, compared to solid-state reaction route of constituent oxides. For luminescence property, the emission intensity of Y₂O₃:Eu phosphors synthesized by combustion process increases steadily with increasing europium amount from 1 mol% to 5 mol%.     

Preparation and characterization of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/Ag composite magnetic nanoparticles

Fe₃O₄ magnetic nanoparticles being used as seeding materials, Ag⁺ ions on the Fe₃O₄ magnetic nanoparticles reduced to the metal form by tartaric acid using heated treatment. Thus, Fe₃O₄/Ag composite core-shell magnetic nanoparticles were synthesized. The products were characterized by transmission electron microscope (TEM) and x-ray diffraction (XRD). Both TEM and XRD results showed that the Ag nanoparticles were well distributed on the surface of Fe₃O₄ magnetic nanoparticles. The size for Fe₃O₄/Ag composite magnetic nanoparticles which were spherical shape was ≃40 nm. Furthermore, the magnetic properties of samples were characterized on a vibrating sample magnetometer. Under optimal conditions, Fe₃O₄/Ag composite nanoparticles showed higher magnetism than pure Fe₃O₄ nanoparticles. The text was submitted by the authors in English.