Mass synthesis of nanocrystalline spinel ferrites by a polymer-pyrolysis route

Nanocrystalline MeFe₂O₄ (Me = Mn, Ni and Zn) spinel ferrites have been synthesized by polymer-pyrolysis method. The pyrolysis behaviors of the polymeric precursors prepared via in situ polymerization of metal salts and acrylic acid are analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). Then, the structural characteristics of the products are studied by powder X-ray diffraction (XRD), infrared spectroscopy (IR), transmission electron microscope (TEM) and electron diffraction (ED) pattern. The results revealed that the spinel ferrites have nano-sized morphology and good crystallinity even if calcined at moderate temperature like 500 °C for 3 h. The average sizes of nanocrystalline spinel ferrites range from 10 to 30 nm with narrow size distributions. Magnetic measurements at room temperature show that Mn, Ni and Zn ferrites with the small coercivity and remanence exhibit soft magnetic behaviors. The spinel ferrites (MnFe₂O₄ and NiFe₂O₄) obtained here show higher saturation magnetization than the corresponding spinel ferrites produced by other methods such as conventional ceramic and wet chemical route.     

Effect of Metal Substitution on the Magnetic Properties of Spinel Ferrites Synthesized from Zinc-Bearing Dust

This paper provides a facile way for comprehensive utilization of multi-metal-associated and intractable zinc-bearing dust to synthesize multi-metal-doped spinel ferrites. The effects of preparation conditions including mass ratios of zinc-bearing dust to MgO addition and calcination temperature on the purity of as-prepared samples were investigated in detail, as well as the effect of metal substitution on the magnetic properties of as-synthesized spinel ferrites. The results show that single-phase spinel ferrite could be obtained when the mixture with the mass ratio of zinc-bearing dust to MgO addition of 1:0.20 was calcined at 1000 ^°C for 2 h. The saturation magnetization M_s values for Mg-substituted, Ni-substituted, and Mn-substituted ferrites are 43.2, 47.6, and 54.3 emu g^??1, respectively, which can be explained by Neel’s two sub-lattice collinear models and the cation distribution between the tetrahedron (A) sites and octahedral (B) sites of the as-prepared samples. Notably, the heavy metals leaching from the as-prepared samples are very limited compared with zinc-bearing dust. In this way, hazardous zinc-bearing dust could be comprehensively utilized to synthesize spinel ferrites with excellent magnetic properties.     

Mixed metal oxalate hydrazinates as compound precursors to spinel ferrites

Mixed metal oxalate hydrazinates MFe₂(C₂O₄)₃(N₂H₄)₆ where M = Mn, Co, Ni, Zn have been prepared, characterised and investigated as compound precursors to spinel ferrites, MFe₂O₄. These precursors decompose exothermically at very low temperatures (130°–250°C) to yield ferrites of large surface area. These compound precursors exhibit autocatalytic behaviour, ie. once ignited combustion is self sustained. Characterisation of the precursors and the combustion products has been done using chemical analysis, infrared and Mössbauer spectra and X-ray diffraction.     

Preferred orientation of NiAl2O4 spinel grown on sapphire

The orientation relationships between the spinel nickel aluminate (NiAl₂O₄) and the aluminum oxide single crystal from which it is grown have been investigated. A variety of sapphire surface orientations were used. The nickel source was primarily nickel vapor, but nickel metal was also used. The large grained spinel layer growing in contact with the sapphire is highly textured, and a thinner fine grained layer on its outer surface is only lightly textured. Two principal textures were observed: for sapphire surfaces oriented near the basal plane, the {111} spinel is parallel to the (0001) sapphire; for surfaces oriented away from the basal plane the {111} spinel is parallel to the {1120} sapphire. In both cases the directions 〈110〉 spinel and 〈1010〉 sapphire are aligned. Considerable twinning and several secondary textures have also been seen.     

A comparative study on the optical limiting properties of different nano spinel ferrites with Z-scan technique

We report the optical limiting properties of five different spinel ferrites, NiFe₂O₄, Ni_0.5Zn_0.5Fe₂O₄, ZnFe₂O₄, Ni_0.5Co_0.5Fe₂O₄, and CoFe₂O₄ with an average particle grain size of 8 nm. The optical limiting properties are investigated using the open aperture Z-scan technique. The obtained nonlinearity fits to a two-photon like absorption process. Except for NiFe₂O₄, the observed nonlinearity has contributions from excited state absorption. The optical limiting response is also studied against particle size and the nonlinearity is found to increase with increasing particle size within the range of our investigations. On comparing the optical limiting properties, ZnFe₂O₄ is found to be a better candidate for the optical limiting applications. To the best of our knowledge, this is the first report where the optical limiting properties of spinel ferrites are compared.     

Preparation and characterisation of chromium-doped cobalt oxide spinel thin films

Thin films of cobalt-based oxide spinel were prepared by pulsed spray evaporation chemical vapour deposition (PSE-CVD), and doping with chromium was systematically investigated up to a Cr/Co ratio of 0.096, corresponding to a stoichiometry of Co_3−x Cr _x O₄ with x = 0.00–0.26. The effect of doping concentration on the structure, assessed by X-ray diffraction and Raman scattering, and on the optical and electrical properties of the oxide films was investigated. Single-phase spinel could be preserved for stoichiometries below x = 0.22. The influence of Cr-doping on the band gap energies and on the electrical conductivity was determined, and the obtained results were exploited to discuss the cationic site occupations. The influence of Cr doping was complemented by the investigation of the surface catalytic reactivity towards the oxidation of dichloromethane.     

Infrared emission properties of RE (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) and Mn co-doped Co0.6Zn0.4Fe2O4 ferrites

The RE (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) and Mn ions co-doped Co-Zn ferrites were prepared by solid-state reaction method. X-ray diffraction, infrared spectra, X-ray photoelectron spectroscopy, Mössbauer spectroscopy and infrared emission measurement (IRE-2) were employed to investigate the effect of the substitution RE³⁺ and Mn ions for Fe³⁺ ones on the spinel structure, the chemical homogeneity, and the infrared emission properties of the Co-Zn ferrites. The substitution leads to non-monotonous change of the lattice parameters and infrared emissivity properties, which is mainly attributed to the partial cation exchange among the spinel structure of Co-Zn ferrites. The infrared emission properties of Co-Zn ferrites seem to be greatly influenced by the co-doped of RE³⁺ and Mn ions—maxima values were 0.96-0.97, found for LaF, NdF and GdF, respectively.     

Mössbauer Studies in the Mechanochemistry of Spinel Ferrites

Mössbauer spectroscopy can provide information on a large variety of mechanochemical processes in solids. In this overview, selected examples are presented of Mössbauer studies of homogeneous and heterogeneous mechanochemical reactions in spinel ferrites. Several interesting features are involved in the work, e.g., mechanically induced cation redistributions, changes of nearest-neighbor configurations, superparamagnetic relaxation, spin-canting effects, as well as formation and redox reactions. The considerable literature on mechanically treated spinel ferrites is reviewed. Despite numerous efforts, the understanding of the nonequilibrium mechanochemical processes in spinel ferrites is considered to be far from complete.     

Epitaxial growth of tungsten nanoparticles on alumina and spinel surfaces

Isolated tungsten nanoparticles (α-W and β-W phase) were synthesized and epitaxially grown on alumina and spinel particle surfaces with an average tungsten size of ≤20?nm for a low tungsten content (of ≤1.5?vol%). Using tungsten (VI) ethoxide alcoholic solutions, tungsten trioxide hydrated precursors were attached to a ceramic grains surface as a nanoparticle coating. High-resolution transmission electron microscopy (HRTEM) micrographs showed epitaxial interfaces between alumina, spinel and metallic tungsten. This epitaxial growth is assumed to be due to the effect of water vapour on the sublimation of ortho-tungstic acid during the reduction process in a hydrogen atmosphere. The planes involved in the epitaxy were found to be [Formula: see text] and [Formula: see text].     

Morphology and structure of ZnCr2O4 spinel crystallites

Computer simulation techniques have been used to predict the crystal morphology of the spinel ZnCr₂O₄. In agreement with experiment, crystallites are predicted to be essentially octahedral with the {111} surface dominating the structure. However, surfaces for materials with the spinel structure are highly complex and stabilized only by the formation of surface defects. This leads to a large number of different possible surface structures.     

Synthesis of nanocrystalline nickel and zinc ferrites by microemulsion technique

Abstract

To use the unique properties of nanocrystalline ferrites in magneto-optical devices, it is important that they have a narrow particle size distribution. The present paper illustrates the potential of the microemulsion technique to synthesise different nanocrystalline ferrites (nickel ferrites, zinc ferrites) with such a narrow particle size distribution. The results of preliminary characterisation by field emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction are presented, confirming the nanocrystalline and spinel crystallographic structure of nickel ferrites.     

Mechanochemical synthesis of stoichiometric MgFe2O4 spinel

The influence of long-term milling of a mixture of (1) MgO and α-Fe₂O₃, (2) MgCO₃, and α-Fe₂O₃, and (3) Mg(OH)₂ and α-Fe₂O₃ powders in a planetary ball mill on the reaction synthesis of nanosized MgFe₂O₄ ferrites was studied. Mechanochemical reaction leading to formation of the MgFe₂O₄ spinel phase was followed by electron microscopy, (SEM and TEM), X-ray diffraction and magnetization measurements. The spinel phase was observed first in cases (1) and (2) after 5 h of milling, and its formation was observed in all cases after 10 h. The synthesized MgFe₂O₄ ferrite has a nanocrystalline structure with a crystallite size of about 11, 10, and 12 nm, respectively for cases (1)–(3). Magnetic measurements after 10 h of milling show magnetization values of 19.8 J/(Tkg), 23.5 J/(Tkg) and 13.8 J/(Tkg), respectively for the cases (1)–(3).     

Rate effects on the mechanical response of magnesium aluminate spinel

The uniaxial compressive, biaxial flexural strength and fracture toughness of a polycrystalline transparent MgAl₂O₄ spinel were characterized over a wide range of loading rates. The flexural tests were carried out by means of ring-on-ring equibiaxial bending, while the fracture toughness was determined by four-point bending on samples with Chevron notch (CN) configuration. The surface crack (SC) method was also attempted in determining the fracture toughness. Quasi-static experiments were conducted on a servohydraulic testing machine, while the high-rate experiments were performed on a modified Kolsky bar. Results showed that both the failure strength and fracture toughness of the spinel were rate sensitive. Edge beveling in sample preparation did not affect the ring-on-ring flexural strength significantly, and the failure initiation sites were found to be inside the loading ring area regardless of edge conditions. Fracture toughness tests following ASTM standard were largely affected by the inherent coarse microstructure of this material.     

Catalytic behaviour of NiAl2O4 spinel upon hydrogen treatment

The effect of reduction on the catalytic activity and selectivity of nickel aluminate spinel has been reported using the 2-propanol decomposition reaction. The unreduced catalyst showed only dehydration activity. When reduced in hydrogen at different temperatures starting from 200 °C, dehydrogenation activity was observed. After reduction at 450 °C, the activity completely transformed into dehydrogenation, while the spinel structure remained unchanged. The unreduced and reduced catalysts were characterized by X-ray diffraction, temperature-programmed reduction, diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The mechanism for the decomposition is discussed in terms of the coordination of aluminium ion and reducibility of nickel.     

Non-stoichiometric NiZn ferrite by sol-gel processing

Non-stoichiometric Ni_0.5Zn_0.5Fe_2−xO_4−3/2x ferrites over a wide range of x = 0∼0.8 are synthesized by a sol-gel processing. Phase evolution, crystal structure and crystallite size of spinel ferrites are dependent on annealing temperature and the amount of Fe deficiency. The crystallite size of spinel increases with annealing temperature and grows faster in stoichiometric ferrites than that of non-stoichiometric. Fe deficiency results in the partial reduction of spinel ferrite to zincite ZnO. XRD indicates that the crystallization temperature of ZnO is increased to about 700 °C. Zincite reduces the number of ferrite crystallites and disfavors the growth of spinel ferrites. The lattice parameters decrease with Fe deficiency and are insensitive to the variation in composition in the samples annealed at lower temperature due to the segregation of ZnO and lattice expansion in the ultrafine crystallites.     

Comparison of preparation routes of spinel catalyst for alkaline fuel cells

MnCo₂O₄ has been used as a catalyst for oxygen reduction reaction (ORR) in alkaline fuel cells due to easier production and lower costs compared to noble metals. A novel method using a microwave-assisted route of synthesis in the presence of amorphous carbon was developed resulting in MnCo₂O₄ with particle sizes <30 nm. For comparison, spinel prepared in a conventional oven with or without carbon was also studied. The surface area, carbon content, and chemical and morphological properties of the catalyst materials were analyzed. The catalytic activity towards oxygen reduction reaction was measured in a half-cell in 6 M KOH and by microagglomerate method. Compared to the catalysts prepared in a conventional oven, microwave-assisted route resulted in higher specific surface areas, higher current densities and stability, as well as lower apparent activation energies and Tafel slopes.     

Wetting behaviour of Cu based alloys on spinel substrates in pyrometallurgical context

Metal droplet losses in slags are an important issue in copper industry. One significant aspect that promotes the entrainment of metal droplets in the slag is their attachment to spinel solids. In the present study, the wetting behaviour of copper alloys on spinel substrates has been investigated in the presence and absence of a slag phase. At first, the attachment was investigated using a synthetic slag containing spinel particles. Microstructural analysis of quenched slag reveals the presence of microdroplets sticking onto a surface of the spinel particles. Second, the metal–spinel interaction was investigated using the sessile drop technique. Wetting angle measurements were performed between Cu–Ag alloys and MgAl₂O₄ substrates. A non-wetting behaviour between the alloys and substrates was observed. The results suggest that the oxygen partial pressure and the amount of Ag in the alloy both influence the wetting behaviour.     

Templated epitaxial coatings on magnesium aluminate spinel using the sol-gel method

Polycrystalline magnesium aluminate (MgAl₂O₄) spinel has attractive properties for a range of applications including radomes, windows, and ballistic protection. A wet chemical approach using solutions of magnesium nitrate hexahydrate Mg(NO₃)₂ · 6H₂O in 2-methoxy-ethanol (2-MOE) and aluminum nitrate nonahydrate (Al(NO₃)₃ · 9H₂O) in ethylene glycol, was developed for spin coating on coarse-grain, polycrystalline spinel substrates. The coated substrates were subjected to isothermal heat treatments in the temperature range 1000–1400 °C, and subsequently examined using low voltage scanning electron microscopy, EBSD (electron back scattered diffraction), and transmission electron microscopy. The results showed that for annealing at 1200–1400 °C, the coatings were converted to crystalline MgAl₂O₄ which was epitaxial with the substrate grains. Heat treatment at lower temperatures, however, resulted in porous, fine-grained polycrystalline coatings. Thermal faceting of the grain surfaces was observed to occur. The observations suggest that faceting occurs preferentially on {100} and {110} planes. The morphology of the faceting was discussed in terms of the reported relative surface energy values for the low index planes in MgAl₂O₄ spinel. Finally, the influence of the coating process on spinel substrates which had been lightly abraded prior to spin coating was investigated.     

少量钴掺杂对Mn-Zn铁氧体结构和磁性质的影响(英文)

采用溶胶凝胶自燃烧法制备了钴掺杂的Mn-Zn铁氧体,研究了少量钴掺杂对其结构和磁性质的影响。利用X射线衍射仪对制备的粉末测试发现,Mn0.4Zn0.6-xCoxFe2O4系列铁氧体都具有纯净的尖晶石结构。晶格常数和饱和磁化强度都随着钴替代量的增加而增加。μi-T线具有两个磁导率最大的峰值并且磁导率的第二峰值随着钴的增加向低温移动。磁导率的第二峰值的温度可以通过改变钴含量来调节。基于铁氧体中磁晶各向异性补偿模型,阐明了少量钴掺杂对铁氧体磁性质影响的作用机理。     

Cellulose-precursor synthesis of nanocrystalline Co0.5Cu0.5Fe2O4 spinel ferrites

Nanocrystalline Cu_0.5Co_0.5Fe₂O₄ powders were prepared via a metal-cellulose precursor synthetic route. Cellulose was used as a fuel and a dispersing agent. The resulting precursors were calcined in the temperature range of 450–600 °C. The phase development of the samples was determined by using Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD). The field-dependent magnetizations of the nanopowders were measured by vibrating sample magnetometer (VSM). All XRD patterns are of a spinel ferrite with cubic symmetry. Microstructure of the ferrites showed irregular shapes and uniform particles with agglomeration. From XRD data, the crystallite sizes are in range of 16–42 nm. Saturation magnetization and coercivity increased with increasing calcining temperature due to enhancement of crystallinity and reduction of oxygen vacancies.