Nanocrystalline spinel zinc-substituted cobalt ferrite thick film an efficient ethanol sensor

This study considered Zn-substituted cobalt ferrite (Zn_xCo_1-xFe₂O₄ (x = 0.0–1.0) (ZCF)) thick films structural, morphological, and electrical properties; and gas sensing performance. The ZCF thick film sensor was screen printed on a glass substrate and tested for different analyte gases, including H₂, H₂S, CO₂, Cl₂, NH₃, LPG, and C₂H₅OH. We used X-ray photoelectron spectrometry to investigate composition, chemical state, iron/cobalt or zinc ratio, and cation distribution within Zn-substituted cobalt spinel ferrite tetrahedral and octahedral sites without impurities. FESEM and HR-TEM confirmed grain dimensions between 0.13 and 0.23 μm and porous, nearly spherical to flake-like morphology for the ZCF samples. Sample DC resistivity reduced with increasing temperature, confirming semiconductor nature. Thick film ZCF composition achieved highest the gas response and selectivity to 100 ppm ethanol at room temperature (30 °C). Overall results confirmed that flake-like ZCF sensors could be effective ethanol gas sensors.     

铁酸盐的制备、表征及其催化性能的研究

采用空气氧化湿法制备了含Zn、 Ni、 Co的尖晶石型铁酸盐。运用XRD、IR、UV-DRS和TPR等方法对所制备的样品进行了表征。研究了铁酸盐的化学组成、结构和氧化还原性质。考察了铁酸盐对二氧化碳选择性氧化乙苯制苯乙烯的催化性能,并讨论了催化剂的结构、组成对反应活性的影响以及二氧化碳的作用。     

Review on augmentation in photocatalytic activity of CoFe2O4 via heterojunction formation for photocatalysis of organic pollutants in water

In previous years, cobalt ferrite has gained huge consideration in the field of semiconductor photocatalysis for waste water treatment. Cobalt ferrite and its derivatives own tunable magnetic properties which results in higher absorption capability in comparison with other photocatalyst semiconductors. In the current review, a brief overview of CoFe₂O₄ as a semiconductor photocatalyst is presented and ferromagnetic behaviour of CoFe₂O₄ is also discussed. Few drawbacks such as agglomeration, photocorrosion and recombination rate of electrons-holes are also discussed. For the enhancement of photocatalytic action of cobalt ferrite, the role of cobalt ferrite with type I, type II, direct Z-scheme, solid state Z-scheme heterojunctions, Schottky and p-n heterojunctions based on different heterostructures were also discussed. In conclusive outlook formation of cobalt ferrite based heterojunctions is best approach for the enhancement of photocatalytic performance. This is because heterojunction formation enhanced the rate of charge separation and thus reduced the electron–hole recombination. Herein, this review highlights the CoFe₂O₄ based heterojunctions for the photodegradation of noxious organic pollutants in water. Furthermore, the future expectations and challenges in exploiting CoFe₂O₄ nanocomposites for water treatment, also discussed in precise conclusion of this review.     

Magnetic interactions in Cu-substituted manganese ferrites

A series of Mn_1−xCu_xFe₂O₄, with x=0, 0.25, 0.50, 0.75 and 1.0, spinel ferrites were prepared by standard ceramic method, to study the effect of compositional variation on magnetic susceptibility, saturation magnetization (M_s), Curie temperature (T_c) and magnetic moments (μ_B). The Curie temperatures have been evaluated by measuring the ac susceptibility using the mutual inductance technique. On increasing Cu contents from 0.0 to 0.50, the saturation magnetization increases while the Curie temperature decreases. On further increase in Cu contents, x>0.50 a decreasing trend in M_s is exhibited while T_c continues to decrease. This effect can be partially related to the low magnetic moments of Cu⁺² ions. The dominant interaction in all ferrite samples is A-B interaction which is due to the negative values of the characteristic temperature θ(K) showing that the magnetic ordering is antiferromagnetic. The Y-K angle increases gradually with increasing copper contents and extrapolates to 90° for CuFe₂O₄. From the computation of Y-K angles for Mn_1−xCu_xFe₂O₄, it can be concluded that the mixed copper ferrites exhibit a non-collinearity of the Y-K type while MnFe₂O₄ shows a Neel type of ordering.     

On the effects of Ga3+ and La3+ ions in MgCu ferrite: Humidity‐sensitive electrical conduction

Pure and gallium or lanthanum substituted MgCu ferrites, Mg_0.5Cu_0.5Fe_2‐xM_xO₄ (with x = 0 or 0.2 and M = La or Ga) have been prepared by solid state reaction. Sintering experiments were carried out at different temperatures between 900 and 1100°C. The phase composition and lattice parameters were determined by XRD, while the effect of Ga and La substitutions on the granular structure was studied by SEM. Experimental results revealed that the densification behaviour and some physical properties are in close relation with the structural changes of pure ferrite caused by the presence of La and Ga substitutions. The gallium containing compound, Mg_0.5Cu_0.5Fe_1.8Ga_0.2O₄, is monophasic and contains a great number of pores, whereas the lanthanum containing compound, Mg_0.5Cu_0.5Fe_1.8La_0.2O₄, is biphasic and exhibits a high density. The humidity characteristics show that the gallium ion enhances the humidity sensitivity of the MgCu ferrite sintered at 1000°C. This interesting effect is promising for the future of the Ga‐substituted MgCu ferrite to be used as sensitive material for fabrication of ceramic humidity sensors. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mössbauer and magnetic study of Mn2+- and Cr3+-substituted spinel magnesioferrites of the composition Mg1−xMnxFe2−2xCr2xO4

Chromium and manganese co-substituted spinel magnesioferrites of the composition Mg_1?x Mn _x Fe_2?2x Cr_2x O₄ (x?=?0.0, 0.1, 0.2, 0.3, and 0.5) were investigated with X-ray diffraction (XRD), Mössbauer spectroscopy and magnetic measurements. The cation distribution inferred suggests that Mn²⁺ and Cr³⁺ ions dominantly occupy the A- and B-sites respectively. The gradual decrease of the hyperfine fields and Curie temperatures with increasing x reflects a gradual weakening in the AB exchange interaction. Mössbauer data of the sample with x = 0.5 is suggestive of cation clustering and/or superparamagnetism. The magnetization data is suggestive of Yafet-Kittel-type canted magnetism.     

Structural,textural, morphological,magnetic and electromagnetic study of Cu-doped NiZn ferrite synthesized by pilot-scale combustion for RAM application

The synthesis of the Ni_0.5-xZn_0.5-xCu_2xFe₂O₄ (x = 0; 0.10 and 0.15) ferrite with the differential of pilot-scale production by the combustion reaction method was investigated for RAM application purposes. Combustion temperatures ranging from 682 °C to 738 °C were observed. All ferrites were sintered at 1200 °C for 1 h. A comprehensive study of the influence of substitution with Cu²⁺ in a partial and proportional way to the Ni²⁺ and Zn²⁺ ions, doping mode little reported in the literature, and also of the sintering process over the structural, textural, morphological, magnetic and electromagnetic properties of NiZnCu ferrites was performed. The XRD patterns of the ferrites as synthesized revealed the formation of the cubic structure of the inverse spinel as majoritary phase, and traces of hematite and zinc oxide as segregated phases. After sintering, it was proven the single-phase formation of cubic spinel ferrite structure. The introduction of Cu led to a reduction in the lattice parameter, whose values ranged from 8.337 to 8.385 Å. The EDX results confirm the composition of oxides. The textural and morphological analyses confirmed the densest characteristic, with increase of particle size and reducing of surface area and pore volume after Cu-doping. All ferrites showed characteristics of soft ferrimagnetic material, where the increase in Cu content contributed to a slight reduction in saturation magnetization, whose values were of ~22–29 emu/g for the as synthesized ferrites and ~71–85 emu/g for the sintered ones. The best result of electromagnetic absorption in X-band was presented by the sintered ferrite with 0.3 mol of Cu, reaching an attenuation of 99.8% at 11.5 GHz frequency, thus confirming the efficiency of the pilot-scale combustion synthesis in obtaining a ferrite with great potential for RAM application.     

Catalytic properties of ferrites in oxidation reactions

Catalytic activities of ferrites MFe₂O₄ (M = Cu, Co, Ni, Mg, and Zn) and M¹ _0.5M² _{0 .5}Fe₂O₄ (M¹ = CU; M² = Co, Zn, and Mg) in oxidation of CO and ethylbenzene were investigated, and their dependences on the cation nature were established. Higher activities were observed for catalysts containing ions with variable valence (Cu, Co, and Ni). A correlation between catalytic and adsorption properties of ferrites was found.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 49–52, January, 1996.     

The magnetic properties of aligned M hexa-ferrite fibres

Aligned and random fibres of strontium hexaferrite (SrM, SrFe₁₂O₁₉) and barium hexaferrite (BaM, BaFe₁₂O₁₉) were manufactured by blow spinning from an aqueous inorganic sol–gel precursor, which was then fired to give the hexagonal ferrite fibre. Their magnetic properties were studied by VSM, investigating the evolution of these properties with firing and measurement temperature, and in particular the effects of fibre alignment. It has been predicted that aligned ferrite fibres will demonstrate an enhanced magnetisation along the axis of alignment with respect to perpendicular to the axis, and this has been demonstrated here for the first time. The optimum firing temperature was 1000 °C, at which point they still had submicron grains. In BaM random fibres M_s=63.8 emu g⁻¹ and H_c=428.1 kA m⁻¹, and in SrM random fibres M_s=63.3 emu g⁻¹ and H_c=452.8 kA m⁻¹, high values for polycrystalline materials. Fibres aligned parallel to the applied field had saturation magnetisation (M_s) values equal to those of the random fibres, whilst fibres aligned perpendicular to the field had M_s values 62% and 75% lower, for BaM and SrM, respectively. There was no change in coercivity (H_c) between random or aligned fibres of any orientation, and fibres aligned 45° and parallel to H appeared identical. Therefore, properties along the axis of alignment were superior when compared to measurements perpendicular to the axis of alignment, giving a directionality to the magnetisation in an otherwise randomly oriented ferrite material.