Preparation of spinel ferrites from citrate precursor route—A comparative study

Spinel ferrites MFe₂O₄ (M = Mn, Co, Ni, Cu) have been prepared from the thermolysis of transition metal bis(citrato)ferrates(III), M₃[Fe(C₆H₅O₇)₂]₂·xH₂O. Various physico-chemical studies, i.e. TG–DTG–DSC, XRD and Mössbauer spectroscopy have been employed for the investigation of the mode of decomposition and characterization of intermediates/products formed. After dehydration the anhydrous precursor undergoes an oxidative decomposition to yield α-Fe₂O₃ and respective metal oxide. Finally spinel ferrites, MFe₂O₄, are formed as a result of a solid-state reaction between the oxides at a much lower temperature (310–440 °C) and in less time as compared to that of the conventional ceramic method. SEM studies show these ferrites to be of nanosized. Ferrites obtained from the thermolysis of transition metal ferricitrate precursors show higher values of saturation magnetization than those got from respective ferrimalonate precursors, thus designating the former as novel materials to operate at high frequencies.     

Preparation of cobalt ferrite from the thermolysis of cobalt tris(malonato)ferrate(III)trihydrate precursor

Thermal decomposition of cobalt tris(malonato)ferrate(III)trihydrate precursor, Co₃[Fe(CH₂C₂O₄)₃]·3H₂O has been investigated from ambient temperature to 600 °C in static air atmosphere using various physico-chemical techniques, i.e. TG–DTG–DSC, XRD, Mössbauer and IR spectroscopic techniques. The precursor undergoes dehydration and decomposition simultaneously to yield cobalt malonate and iron(II) malonate intermediates at 205 °C. At higher temperature (325 °C) these intermediate species undergo exothermic decomposition to yield CoO and α-Fe₂O₃, respectively. Finally cobalt ferrite, CoFe₂O₄, has been obtained as a result of solid–solid reaction between Fe₂O₃ and CoO at a temperature (380 °C) much lower than that of ceramic method. SEM analysis of the final thermolysis product reveals the formation of monodisperse cobalt ferrite nano-particles with an average particle size of 45 nm. Magnetic studies show that these particles have a saturation magnetization of 3095 G and Curie temperature of 504 °C. Lower magnitude of these parameters as compared to the bulk values is attributed to the smaller particle size.     

Gadolinium ferrite nanoparticles: Synthesis and morphological,structural and magnetic properties

In this study we report on the successful synthesis of Gd_xFe_3−xO₄ nanoparticles with nominal Gd-content (x) in the range 0.00≤x≤0.50. The effect of the nominal Gd-content on morphological, structural and magnetic properties was investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and Mössbauer spectroscopy. We found the actual inclusion of Gd³⁺ ions into cubic ferrite structure lower than the nominal values, though no extra phase was observed in the whole range of our investigation. Moreover, from Mössbauer data we found evidences of Gd³⁺ ions replacing both Fe³⁺ and Fe²⁺ ions, the latter leading to iron vacancies in the cubic ferrite crystal structure. As the nominal Gd-content, the lattice parameter and the average crystallite size increases monotonically. We found that in the same range of nominal Gd-content the lattice parameter decreases with the increase of iron vacancy content.     

Synthesis of CuGa2O4 nanoparticles by precursor and self-propagating combustion methods

Copper gallate spinels, CuGa₂O₄, have been synthesized by two wet chemical routes: precursor method and self-propagating combustion involving a glycine-nitrate system. All complex precursors have been characterized by chemical analysis, infrared spectroscopy (IR), ultraviolet visible spectroscopy (UV–vis), electron paramagnetic resonance spectroscopy (EPR), thermal analysis and scanning electron microscopy (SEM). The copper gallate spinel oxides have been further investigated by X-ray diffraction (XRD), SEM, IR, UV–vis, magnetic measurements and EPR. The crystallite size of the copper gallate was found about 280 Å.     

Physico-chemical investigation on mixed alkali metal ferrites prepared by solution combustion method – A comparative study

Mixed alkali metal nanoferrites of the compositions M_0.5−X/2Zn_XMn_0.05Fe_2.45−X/2O₄ (M = Li, Na and K), where x varies from 0→0.5 in steps of 0.1, have been prepared by solution combustion method. Powder X-ray diffraction analysis for all the samples show the formation of single phase cubic spinel structure. The lattice parameter increases linearly with Zn content, which is attributed to ionic size differences of the cations involved. Both X-ray as well as experimental densities show upward trend with increasing ‘x’ due to increase in the molecular weight of the ferrite composition. Mössbauer spectra display the superimposition of paramagnetic doublet over ferrimagnetic sextet with increasing diamagnetic ‘Zn’ content. The key magnetic properties of the ferrite obtained, such as saturation magnetization and Curie temperature have also been studied. The combustion method used for the synthesis is a rapid approach for direct conversion of the stoichiometric reactant solutions into fine nanoparticles of ferrite product at a temperature (600 °C) much lower than that of the conventional ceramic method. Scanning electron micrographs confirm the formation of nanosized ferrites.     

Relaxation dynamics,phase pattern in the vicinity of the Curie temperature,Fe valent state and the Mössbauer effect in PFN ceramics

High-density lead ferroniobate, PbFe_1/2Nb_1/2O₃ (PFN), was prepared by the conventional ceramic technology. Its dielectric properties, phase pattern in the vicinity of transition into the polar phase, and the x-ray electron and Mössbauer spectra were studied. The relaxation dynamics discovered at the temperature exceeding the Curie temperature at the frequencies of 3×10⁻²–10⁵ Hz is described in detail. Near T_C, the following sequence of phase transitions was established: rhombohedral (T<368 K)→pseudocubic (368 K<T<387 K)→cubic (T>387 K). It is shown that in both the ferroelectric and paraelectric phases the Fe ions are, mainly, in a high-spin valent state Fe³⁺ in the octahedral environment.     

Mössbauer effect study of the Fe-substituted NASICON

Fe-substituted NASICON was prepared by co-precipitation method and the content of iron at different oxidation states was estimated by Mössbauer spectroscopy (MS). Compounds with different phase composition were obtained depending on the preparation procedure. In the case of samples sintered in reducing atmosphere, NASICON containing di- and trivalent iron was detected. The lattice constants were calculated by Rietveld method. Higher values of the unit cell parameters for Na₃Zr_1−yFe_1−y²⁺Fe_2y³⁺P₃O₁₂ compared to the structural analogue γ-Na₃Fe₂P₃O₁₂ indicated the partial replacement of smaller Fe³⁺ ions by bigger Fe²⁺ and Zr⁴⁺ in the NASICON lattice. The validation of Mössbauer spectroscopy as the useful tool in the calculation of iron content was also performed. The sintering in air led to the multiphase product, which was identified by XRD and MS.     

Size-controlled synthesis of spinel nickel ferrite nanorods by thermal decomposition of a bimetallic Fe/Ni-MOF

In this work, size-controlled synthesis of nickel ferrite nanoparticles was achieved by the calcination of a bimetallic (Fe/Ni) metal-organic framework (MOF). The bimetallic MOF (Fe₂Ni-MIL-88B) itself was prepared by a two-step route. The first step involved synthesis of the secondary building unit (SBU) by reacting stoichiometric amounts of Ni and Fe precursors with acetic acid. A ligand substitution reaction (terephthalate replaces acetate) in the SBU leads to the formation of the MOF, which was characterized by PXRD, FTIR, SEM and TEM. Afterwards, the MOF was calcined under air atmosphere to obtain nickel ferrite nanorods. PXRD analysis confirmed the spinel structure of the nickel ferrites while electron microscopic analysis (SEM, TEM) revealed their nanorod-like morphology. By increasing the calcination temperature from 600 to 1000 °C, particle size increased from 16 to 32 nm. Oxidation of benzyl alcohol was used as a model test reaction to probe the applicability of spinel nickel ferrite nanorods for catalysis. Interestingly, the largest nanorods exhibited the highest activity (86% conversion), thus demonstrating the potential of spinel ferrites in catalyzing oxidation reactions.     

Direct and facile synthesis of Li–Sr–Zn ferrites via low temperature solution combustion route

Li–Sr–Zn nanoferrites i.e. Li_0.25Sr_0.5–xZn_xFe_2.25O₄, where ‘x’ varies from 0–0.5, have been synthesized using a low temperature solution combustion method which proves to be an efficient and economical technique for synthesizing these type of nanoferrites. The as-synthesized nanoferrites have cubic spinel structure as characterized by X-ray powder diffraction. Powder XRD and TEM (Transmission Electron Microscopy) characterization also evidence that the crystallite and particle size are in close agreement to each other. Mössbauer spectroscopy studies demonstrate that there is a gradual transition from ferrimagnetic to superparamagnetic character, which is also supported by the saturation magnetization and coercivity values. At room temperature, the nanoferrites were found to be superparamagnetic with negligible coercivities approaching towards zero while saturation magnetization values were found to be in the range 6.87–30.10 emu g⁻¹. The frequency dependent dielectric constant and loss values are in accordance with Koop׳s model. These nanoferrites show great potential in high density recordings, magnetic nanodevices and biomagnetic applications.     

Sintering behaviour of cobalt ferrite ceramic

Pure cobalt ferrite ceramic powder was prepared using standard solid-state ceramic processing. Uniaxially pressed pure cobalt ferrite discs, sintered under isothermal ramp rate and single dwell time conditions, yielded a maximum theoretical density (%D_th) of <90%. Discs made from finer particle size powder yielded a %D_th of 91.5%. Based on dilatometry analysis, a sintering profile comprising non-isothermal sintering, and two-step sintering was devised, yielding discs with %D_th of 96%. Cylindrical rods of pure cobalt ferrite were cold iso-statically pressed and sintered according to the revised sintering profile. Pycnometry analysis was used to quantify the percentages of open and closed pores in the rods after sintering.     

Silica coated ferrite nanoparticles: Influence of citrate functionalization procedure on final particle morphology

In this paper, magnetic nanoparticles (Fe₃O₄ and NiFe₂O₄) were coated with a biocompatible silica shell via hydrolysis and condensation of tetraethyl orthosilicate (TEOS) by the Stöber process. Magnetic nanoparticles, prepared by chemical co-precipitation from iron and nickel salts, were functionalized with citric acid, in order to provide their deagglomeration and to enable their coating with silica. The parameters of the functionalization procedure were varied (concentration–pH and type of treatment), in order to examine if and how this particular step of preparation affects the final morphology of the core-shell particles. Transmission electron microscopy, zeta potential and particle size measurements revealed that the morphology and the size of obtained core shell particles depend significantly on the core particle size, and thus on the parameters of the functionalization step.     

Synthesis and characterization of pure single phase Ni-Zn ferrite nanopowders by oxalate based precursor method

Series of single-phase Ni_1 − xZn_xFe₂O₄ (x = 0.20, 0.35, 0.50 and 0.60) nanopowders with average particle size of ∼ 35 nm have been synthesized by using oxalate based precursor method. Precursor powders were synthesized by reacting aqueous solutions of metal nitrates and oxalic acid by using different total metal ions: oxalic acid molar ratios and then evaporating them to dryness. Pure, single-phase Ni-Zn ferrite nanopowder was formed by calcining the precursor with total metal ion: oxalic acid ratio of 1:0.125 at a temperature of 850 °C. The synthesized nanopowders were characterized by using X-ray diffraction, Thermo-gravimetric and Differential Scanning Calorimetric analysis, Transmission Electron Microscope and Scanning Electron Microscope. Room temperature DC resistivity of the nanopowders was measured with respect to temperature by the two-probe method and was of the order of ∼ 10⁷ Ωcm. Room temperature saturation magnetization was measured by using Vibrating Sample Magnetometer and it varied between 34-49 emu/g depending on the composition. This aqueous solution based method provides a simple and cost-effective route to synthesize single phase, Ni-Zn ferrite nanopowders.     

Structural and magnetic properties of NiFe2−xBixO4 (x = 0, 0.1, 0.15) nanoparticles prepared via sol-gel method

NiFe_2−xBi_xO₄ (x = 0, 0.1, 0.15) nanopowders were synthesized via sol-gel method. The precursor gels were calcined at 773 K in air for 1 h to obtain the pure nanostructured NiFe_2−xBi_xO₄ spinel phase. The crystal structure and magnetic properties of the substituted spinel series of NiFe_2−xBi_xO₄ have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy, transmission electron microscopy and alternating gradient force magnetometry. Mössbauer spectroscopic measurements revealed that Bi³⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite, i.e., the crystal-chemical formula of the as-prepared nanoparticles may be written as: (Fe)[NiFe_1−xBi_x]O₄ (x = 0, 0.1, 0.15), where parentheses and square brackets enclose cations on sites of tetrahedral and octahedral coordination, respectively. Selective area electron diffraction studies provided evidence that the samples of the NiFe_2−xBi_xO₄ series, independently of x, exhibit the cubic spinel structure. The values of the saturation magnetization and the coercive field of NiFe_2−xBi_xO₄ nanoparticles were found to decrease with increasing degree of bismuth substitution.     

Magnetic and catalytic properties of copper ferrite nanopowders prepared by a microwave-induced combustion process

Copper ferrite nanopowders were successfully synthesized by a microwave-induced combustion process using copper nitrate, iron nitrate, and urea. The process only took a few minutes to obtain CuFe₂O₄ nanopowders. The resultant powders were investigated by XRD, SEM, VSM, and surface area measurement. The results revealed that the CuFe₂O₄ powders showed that the average particle size ranged from 300 to 600 nm. Also, it possessed a saturation magnetization of 21.16 emu/g, and an intrinsic coercive force of 600.84 Oe, whereas, upon annealing at 800 °C for 1 h. The CuFe₂O₄ powders specific surface area was 5.60 m²/g. Moreover, these copper ferrite magnetic nanopowders also acted as a catalyst for the oxidation of 2,3,6-trimethylphenol to synthesize 2,3,5-trimethylhydrogenquinone and 2,3,5-trimethyl-1,4-benzoquinone for the first time. On the basis of experimental evidence, a rational reaction mechanism is proposed to explain the results satisfactorily.     

Characterization of submicrometer-sized NiZn ferrite prepared by spark plasma sintering

High-density submicrometer-sized Ni_0.5Zn_0.5Fe₂O₄ ferrite ceramics were prepared by spark plasma sintering in conjunction with sufficient high energy ball milling. They were evaluated by different characterization techniques such as X-ray diffraction, scanning electron microscopy, and dielectric and magnetic measurements. All samples prepared at sintering temperatures ranging from 850 to 925 °C exhibit a single spinel phase and their relative densities and grain sizes range from 90% to 99% and ~100 nm to ~300 nm, respectively. The dielectric constant increases with decreasing grain size until ~250 nm, and then decreases dramatically with further decreasing grain size. The saturation magnetization increases continuously with increasing grain size/density but the magnetic coercivity decreases. The highest dielectric constant and saturation magnetization at room temperature are approximately 1.0×10⁵ and 84.4 emu/g, respectively, while the lowest magnetic coercivity is only around 15 Oe. These outstanding properties may be associated with high density and uniform microstructure created by spark plasma sintering. Therefore, the spark plasma sintering is a promising technique for fabricating high-quality NiZn ferrites with high saturation magnetization and low coercivity.     

Solution combustion synthesis of bioceramic calcium phosphates by single and mixed fuels—A comparative study

Calcium phosphate based bioceramics have been synthesized by a modified combustion synthetic route using both citric acid and succinic acid separately and in mixture as fuels and nitrate and nitric acid as oxidants. Calcium nitrate and diammonium hydrogen phosphate were used as calcium and phosphate sources. The effects of citric acid to succinic acid ratio on the phase formation have been investigated. The precursors and the calcined products have been characterized by powder X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy. Succinic acid has been used as a fuel for the first time to synthesize hydroxyapatite.     

Synthesis of nanostructured material by mechanical milling and study on structural property modifications in Ni0.5Zn0.5Fe2O4

Mechanical milling induced structural property modifications in Ni_0.5Zn_0.5Fe₂O₄ spinel ferrite have been studied. We have observed two interesting phenomena (i) “temperature diffuse scattering” due to displacement of atoms from their mean position and (ii) appearance and gradual evolution of (4 2 0) plane in X-ray diffraction profiles and increase in average percentage disagreement between observed and calculated intensity ratios value, due to “preferred grain orientation.” Both these effects get prominent with milling time. The X-ray diffraction line intensity calculations revealed large B-site occupancy of Zn²⁺-ions, mainly due to modified synthesis procedure employed. The grain orientation factor increases from 10.6% to 18.1% on milling. It is found that milling has marked influence on various parameters: lattice constant, grain size, stress–strain, surface area and energy.     

Chromium substituted copper ferrites via gluconate precursor route

CuFe_2−xCr_xO₄ spinel (0≤x≤2) powders were synthesized by a soft chemistry method—the gluconate multimetallic complex precursor route. The complex precursors were characterized by elemental chemical analysis, infrared (IR) and ultraviolet–visible (UV–vis) spectroscopy, thermal analysis and Mössbauer spectroscopy. The oxide powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), IR, Raman and Mössbauer spectroscopy. It was shown that the structure, morphology and magnetic properties of the obtained spinel powders depend on the concentration of Cr³⁺ ion. The XRD of the chromium substituted copper ferrite powders calcined at 700 °C/1 h indicated the formation of a cubic spinel type structure for x=0.5, 1.0 and a tetragonal structure for x=0, 0.2, 2. The crystallite size ranged from 19 nm to 39 nm. The Mössbauer spectroscopy revealed the site occupancy of iron ions, relative abundance and internal hyperfine magnetic fields in both tetrahedral and cubic CuFe_2−xCr_xO₄ spinels.     

Development of hard/soft ferrite nanocomposite for enhanced microwave absorption

Nickel and zinc substituted strontium hexaferrite, SrFe₁₁Zn_0.5Ni_0.5O₁₉ (SrFe₁₂O₁₉/NiFe₂O₄/ZnFe₂O₄) nanoparticles having super paramagnetic nature are synthesized by co-precipitation of chloride salts using 7.5 M sodium hydroxide solution. The resulting precursors are heat treated (HT) at 900 and 1200 °C for 4 h in nitrogen atmosphere. During heat treatment, transformation proceeds as a constant rate of nucleation and three dimensional growth with an activation energy of 176.79 kJ/mol. The hysteresis loops show an increase in saturation magnetization from 1.042 to 59.789 emu/g with increasing HT temperatures. The ‘as-synthesized’ particles with spherical and needle shapes have size in the range of 20–25 nm. Further, these spherical and needle shaped nanoparticles tend to change their morphology to hexagonal plate and pyramidal shapes with increase in HT temperatures. The effect of such a systematic morphological transformation of nanoparticles on dielectric (complex permittivity and permeability) and microwave absorption properties are estimated in X band (8.2–12.2 GHz). The maximum reflection loss of the composite reaches −29.62 dB (99% power attenuation) at 10.21 GHz which suits its application in RADAR absorbing materials.     

Effect of sintering regimes on the microstructure and magnetic properties of LiTiZn ferrite ceramics

In present work, the influence of sintering regimes on the microstructure, saturation magnetization, density and porosity, the grain size, the Curie point, and the temperature dependence of the initial permeability of LiTiZn ferrite ceramics was investigated. Ceramics was prepared by a standard ceramic technique. The formation of a single-phase cubic spinel structure was confirmed by XRD analysis. The Curie point was determined from both the temperature dependences of the initial permeability and the method of thermogravimetric measurements in a magnetic field. Density/porosity and the grain size, the Curie point and magnetization are sensitive to the sintering regime. The initial permeability of ferrite decreases with sintering temperature (in the range of 1010–1150?°С) and grain size increasing that contradicts the generally accepted Globus and Smith-Wijn theories. A possible reason of such behavior is the formation of intragranular pores growing with the increase in the sintering temperature and inhibiting the domain wall motion inside the grain. These results correspond to the porosity of the investigated ferrite ceramic samples, which grows with sintering temperature increasing.The non-stoichiometry arising due to evaporation of lithium and zinc oxides at temperature above 1010?°C affects the initial permeability. In this work, a qualitative assessment of the defective state of ferrite samples obtained under various sintering regimes was given.