Oxygen evolution during the formation and sintering of nickel-manganese oxide spinels for thermistor applications

Nickel-manganese spinel, prepared from 20 wt % NiO and 80 wt % Mn₂O₃, forms at 950°C by the intermediate formation of Mn₃O₄ with evolution of oxygen, determined by mass-spectrometric evolved gas analysis. On heating to higher temperatures, further oxygen is evolved resulting in pore formation and bloating of pressed sintered samples and anomalies in their densification and electrical properties. Thermodynamic considerations and X-ray diffraction intensity measurements suggest that all the Mn²⁺ is located in the tetrahedral spinel sites, this cation configuration remaining unchanged by higher-temperature treatments such as sintering.     

Crystallographic and electrical study of the chromium substituted ferrous zinc copper ferrites

The structural and electrical properties of the oxidic spinel Zn_0·5Cu_0·5Fe _x Cr_2−x O₄ (x=0·8, 0·9, 1·0, 1·1) have been investigated through X-ray diffraction, electrical resistivity, and thermoelectric measurements. X-ray diffraction data showed formation of single spinel phase with cubic structure. D.C. resistivity measurements from room temperature to 850 K was carried out and activation energy for all the compositions evaluated. Thermoelectric measurements showedp-type semiconducting nature in all the samples.     

Preparation and characterization of indium doped CdS0.2Se0.8 thin films by spray pyrolysis

The CdS_0.2Se_0.8 and indium doped CdS_0.2Se_0.8 thin films have been deposited onto the amorphous glass and fluorine doped tin oxide coated glass substrates by spray pyrolysis. The doping concentration of indium has been optimized by photoelectrochemical characterization technique. The structural, surface morphological, optical and electrical properties of CdS_0.2Se_0.8 and indium doped CdS_0.2Se_0.8 thin films have been studied. X-ray diffraction studies reveal that the films are polycrystalline in nature with hexagonal crystal structure. Scanning electron microscopy studies reveal that the grains are uniform with uneven spherically shaped, distributed over the entire substrate surface. The complete surface morphology has been changed after doping. In optical studies, the transition of the deposited films is found to be direct allowed with optical energy gaps decreasing from 1.91 to 1.67 eV with indium doping. Semiconducting behavior has been observed from resistivity measurements. The thermoelectric power measurements reveal that the films exhibit n-type conductivity.     

Electrochromic and photoelectrochemical behavior of thin WO3 films prepared from quantized colloidal particles

The electrochromic, photochromic and photoelectrochemical properties of thin semiconductor WO₃ films, prepared from quantum-size colloids, have been examined. These films have been found to exhibit reversible electrochromic and photochromic behavior (blue coloration). The trapped electrons in the particulate films are thought to be the major species responsible for the blue coloration. The photoelectrochemical measurements have shown a very low conversion efficiency of WO₃ particulate films. This is most probably due to the efficient trapping of electrons by the lattice defects in the film.     

Effect of sintering temperature on the electrical properties of Mn (1 %) added MgCuZn ferrites by microwave sintering method

Mg_0.446Cu_0:144Mn_0.01Zn_0.4Fe₂O₄ samples are sintered at various sintering temperature using a multimode cavity of 2.45 GHz microwave oven. The XRD results reveal that all the samples are single phase with spinel structure. The grain size increases with increasing sintering temperature. The electrical resistivity decreases and the dielectric constant increases with increasing sintering temperature. Initial permeability increases with increasing sintering temperature. The results suggest that sintering temperature has a prominent effect on the electric properties.     

Preparation and electrical properties of monophase cubic spinel,Mn1.5Co0.95Ni0.55O4, derived from rock salt type oxide

The purpose of this study was to prepare a sintered body consisting of monophase cubic spinel type oxide, Mn_1.5Co_0.95Ni_0.55O₄, and to evaluate its electrical properties. It was found that cooling from 1400 to 1000 °C in nitrogen did not affect the preservation of the sintered rock salt type oxide formed at 1400 °C. A crack free sintered body of monophase cubic spinel may be obtained by heat treatment at 1000 °C in air, using a specimen cooled from 1400 °C at a rate of 500 °C min ^–1. A heat treatment time in air at 1000 °C of more than 48 h was required to convert the rock salt type structure into a perfect cubic spinel structure. The electrical conductivity, , of the sintered cubic spinel oxide synthesized in this work was found to be stable at 100 and 200 °C in air and at 100, 200 and 300 °C in nitrogen. The sintered spinel oxide was a p-type semiconductor, based on small polaron hopping conduction. The intrinsic hole concentration, n, was estimated to be constant, with a value of 1.6–1.8×10²⁸m^–3. The mobility, , increased exponentially with increasing annealing temperature in both atmospheres, suggesting that the change in is dependent on .     

From tin oxalate to (Fe, Co, Nb)-doped SnO2: Sintering behavior, microstructural and electrical features

Highly reactive SnO₂-doped nanocrystalline powders with average particle size of 20 nm and specific surface areas above 30 m²/g were obtained through the polymeric precursor method with tin oxalate (SnC₂O₄) as a chloride-free precursor for SnO₂. Powders and sintered discs were characterized by means of X-ray powder diffraction (XRD), SEM and TEM. The influence of Co and Fe on the microstructure development and on the electrical properties of dense SnO₂-based ceramics was studied. Co and Fe species were found to decrease in more than 70 °C the sintering temperature of SnO₂ with respect to mixed oxide procedures. Secondary phases enriched in Co and Fe were detected and identified in sintered samples with XRD. Current-voltage curves were registered for electrical characterization. Doping with iron increased the electrical breakdown field and a nonlinearity coefficient of 20 was achieved.     

M?ssbauer and Electrical Studies of Mn x Co1?x Fe2O4 Compounds Prepared via Glycothermal Route

Manganese-cobalt ferrite powders (Mn _x Co_1?x Fe₂O₄ with x varying from 0.0 to?0.6) have been produced by glycothermal process from pure metal chlorides. Single phase cubic spinel structure and nanophase structure of the as-synthesized samples were confirmed by X-ray diffraction (XRD) and by transmission electron microscope (TEM). The results show that the produced powders have grain sizes in the range 7 to 13?nm. Fe-57 M?ssbauer spectra for as-synthesized and for annealed samples at 700???C (in Ar atmosphere for 1?hour) are found to be similar. No significant changes in the spectra are observed across the composition range studied. The variations of grain sizes, lattice parameters, and M?ssbauer parameters as a function of composition have also been investigated. Bulk samples in the form of pellets were also produced from the as-synthesized compounds for resistivity measurements. The temperature dependence of the electrical resistivity for the samples sintered at 1050???C were studied using the four-probe method, from room temperature to about 110???C in a PID controlled oven. Two possible mechanisms for resistivity involving T ^?1/2 and T ^?1 dependences are investigated.     

Structural and Electrical Properties of Bismuth Ferrite Ceramics Sintered in Different Atmospheres

Bismuth ferrite (BiFeO₃) ceramics were synthesized by the solid-state reaction method followed by rapid liquid phase sintering. The effect of sintering atmosphere (N₂, air and O₂) on the structure and electrical properties of BiFeO₃ multiferroic ceramics were investigated. XRD analysis revealed that N₂ sintering was effective in reducing impurity phases and improving the crystallization behavior. XPS analysis showed that fewer Fe²⁺ ions but more oxygen vacancies were involved in the N₂ sintered ceramics. The SEM investigations suggested that the grain size of the BiFeO₃ ceramics sintered in nitrogen are larger than those sintered in air and O₂. Electrical measurements revealed that the ceramics sintered in N₂ showed lower leakage current, superior dielectric and ferroelectric properties.     

Effect of Sintering Temperature on the Microstructure,Electrical, and Magnetic Properties of Bi 0 . 8 5 Eu 0 . 1 5 FeO 3 Ceramics

Polycrystalline Bi _0.85Eu _0.15FeO ₃ ceramics were synthesized by solid-state reaction method with rapid liquid phase sintering process at various sintering temperatures. The dependence of structural, microstructural, electrical, and magnetic properties on sintering temperature was systematically investigated. X-ray diffraction measurements reveal that single perovskite phase is developed in Bi _0.85Eu _0.15FeO ₃ ceramics sintered at 850 and 870^° C, while secondary phases can be detected in the samples sintered at 890^° C due to the volatilization of Bi ³⁺ ions, and the crystallinity increases with increasing sintering temperature from 850 to 890 °C. The scanning electron microscopy investigation has suggested that the grain size increases with increasing sintering temperature from 855 t o 890^° C; while the pore size decreases with increasing sintering temperature from 850 to 870^° C and then increases with a further increase of sintering temperature. The electrical and magnetic measurements show that the leakage current, dielectric constant, dielectric loss, and magnetic properties are strongly dependent on the sintering temperature. The Bi _0.85Eu _0.15FeO ₃ ceramics sintered at 870^° C have the lower leakage current, higher dielectric constant, and lower dielectric loss. The room temperature magnetization increases with increasing sintering temperature from 850 to 890 °C. The possible reason for all the above observations was discussed.     

Sintering behaviour of MgAl2O4—a prospective anode material

Awareness in environmental hygiene and escalation in the energy cost in electrometallurgical industries have necessitated a reauditing of the production technologies, more so in the case of aluminium as it is a highly energy intensive process. Metal aluminates are of interest due to their technological applications in aggressive environments. The preparation of the metal aluminates usually involves the solid state reaction (SSR) of the corresponding metal oxides or sulphates at high temperatures. The present investigation is the preparation of MgAl₂O₄ using the metal oxides by solid state synthesis. The compacts were sintered at high temperatures and their properties such as particle size, density, porosity, hardness, ac electrical conductivity and dielectric behaviour were evaluated. The phase formation and structural properties have been ascertained by XRD, FTIR, TG/differential thermal analysis (DTA) and scanning electron micrograph (SEM) studies. The sinterability characteristics of the materials strongly dependent on the temperature of the processing. The XRD patterns confirm the phase formation of MgAl₂O₄. The FTIR spectra show the structural details of the synthesized compound. From the SEM micrographs, it is revealed that the agglomeration process progresses with the increasing temperature. The ac electrical conductivity and dielectric behaviour of aluminates strongly depend upon sintering time, temperature and spinel formation.     

Structural and Electrical Properties of Ni-Co Nanoferrites Prepared by Co-precipitation Route

A series of Ni_1?x Co _x Fe₂O₄ (x=0.1, 0.2, 0.3, 0.4, 0.5) spinel ferrites have been synthesized successfully using the chemical co-precipitation route. The materials were characterized by X-rays powder diffractometry (XRD) and the electrical properties. The obtained crystallite size variation was within 15 to 33 nm using the Scherrer formula. The dc electrical resistivity was measured as a function of temperature. It is noticed that ?? _dc increases with a rise in temperature. The dielectric measurements were carried out at room temperature as a function of frequency and composition (x). The dielectric constant (????) and dielectric loss tangent (tan???) showed a decreasing trend with increasing field frequency. The ac electrical conductivity is calculated from the dielectric measurements; it increases with the rise in frequency.     

Substitutional and defect doping effects on the photoelectrochemical properties of Fe2O3

Fe₂O₃ samples have been prepared using both substitutional and defect doping techniques. Defect doped samples were sintered in air at 1350° C for 20 hours, reduced in a H₂ atmosphere for various times at 300° C, and finally reoxidized at 700–900° C for short periods. Resulting resistivities ranged from .01 to 5000 ohm-cm. Such electrodes were found to have good photoelectrochemical properties, and to be quite sensitive to chemical etching. Substitutional doping of Fe₂O₃ with Si, Ca, Nb, Cu, Ru, Mg, and Zr was performed, with Si and Cu providing the best results. The effects of quenching time and temperature on the properties of these electrodes are discussed.     

Effect of copper content on optostructural, morphological and photoelectrochemical properties of MoBi2−x Cu x Se4 thin films

In the present investigation we have reported a facile chemical route for the deposition of MoBi_2?x Cu _x Se₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) thin films at room temperature by using a simple and self-organised arrested precipitation technique. The deposited samples were characterised for their structural, morphological, optical and photoelectrochemical properties. X-ray diffraction patterns revealed that, undoped MoBi₂Se₅ shows a rhombohedral crystal structure, while mixed rhombohedral and orthorhombic crystal structures were observed with shifting of diffraction peaks after copper doping. The scanning electron microscopy and transmission electron microscopy images revealed that the surface morphology was improved with copper content. Compositional analysis of all samples was carried out by using energy dispersive X-ray spectroscopy. The direct band gap energy of all the samples estimated from absorbance spectra varies from 1.26 to 1.60 eV. The photoelectrochemical properties of all samples were studied in I^?/I₃ ^? redox electrolyte which demonstrated that the electrical conductivity was transformed from n-type to p-type after copper doping and photoelectrochemical response of p-type MoBi_2?x Cu _x Se₄ thin film electrode was improved with increasing copper content. The mechanism of change in the type of electrical conductivity and augmentation in photoelectrochemical response after copper doping are discussed.     

Physicochemical properties of fine-particle ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> prepared by spray pyrolysis of nitrate solutions

Fine zinc ferrite (ZnFe₂O₄) powders uniform in morphology have been prepared by spray pyrolysis of nitrate solutions. Examination by scanning electron microscopy showed that the powders consisted of micron- and submicron-sized polycrystalline spherical particles. The ZnFe₂O₄ sample prepared by pyrolysis at 1000°C had paramagnetic properties. Its crystal structure was refined by the Rietveld method and was shown to be a partially inverse spinel with a degree of inversion near 15%. According to nitrogen adsorption measurements, the specific surface of the powders was 5.2 m²/g. The electrical conductance of a film produced from fine-particle zinc ferrite was found to be very sensitive to the hydrogen sulfide concentration in air.     

Structural, electrical and magnetic properties of copper substituted Zn–Mn ferrites

Optimum composition (x = 0.0–0.75) of copper substituted Zn_0.25Mn_0.75?xCu_xFe₂O₄ ferrites system have been synthesized by simple chemical co-precipitation method. X-ray diffraction pattern of all samples shows characteristic features of nanocrystalline cubic spinel structure of ferrites. The average grain size of the samples was measured using scanning electron micrographs. The saturation magnetization, coercivity and remanance were studied as a function of copper content in the host ferrite materials. Electrical resistivity of the samples was measured by two probe method. The semiconducting nature of the samples was noted from variation of electrical resistivity with temperature. The effect of copper substitution in ferrites was studied through the measurements of structural, electrical and magnetic properties.     

Effect of CaF2 on the electrical properties of yttria-stabilized zirconia

The effects of CaF₂ addition on the phase constitutions and electrical properties of 8 mol% yttria-stabilized zirconia were investigated. Three samples were prepared, one being doped with 10 mol% CaF₂ and sintered at 1473 K and the other two doped with 20 mol% CaF₂ and sintered at 1473 and 1673 K, respectively. Single phased fluorite-type solid solution with cubic symmetry was observed for each sample. However, F ion exists only in the sample doped with 20 mol% CaF₂ and sintered at 1273 K. Electrical measurements showed that addition of CaF₂ may enhance the ionic conductivity of the yttria-stabilized zirconia.     

Effect of in situ spinel seeding on synthesis of MgO-rich MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> composite

MgO-rich MgAl₂O₄ spinel was prepared by adopting solid-state reaction of commercially available sintered seawater magnesia and α-alumina. Starting materials were mixed in weight ratio (Al₂O₃: MgO) of 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, where the developed MgAl₂O₄ spinel crystal seed in calcined powder varied (5–50%) with respect to addition of MgO content and temperature. Around 70% spinellisation could be noticed in Al₂O₃: MgO (1:1) batch through double stage sintering. Densification and grain size of the sintered specimen was found to be greatly dependent on initial calcination temperature and content of primary spinel seed. The diametrical compressive strength of sintered specimen was ∼115 MPa for highest amount of spinel content. The hardness of MgO-rich spinel composite was varied with spinel addition.     

Advanced symmetrically graded ceramic and ceramic-metal composites

The potential of electrophoretic deposition as a shaping process of free-standing objects with a gradient in composition is investigated. The capability to produce relatively thick continuously graded ZrO₂-Al₂O₃ and WC-Co-Ti(C,N) discs was explored. After electrophoretic deposition, the dried deposits were cold isostatically pressed and pressureless sintered. In order to create a symmetric gradient profile along the thickness of the plates, a mathematical model for the EPD process was developed in order to predict the composition and slope of the gradient profile in the discs from the starting composition of the suspension, the EPD operating parameters and the powder-specific EPD characteristics. The sintered graded materials have a continuous variation in composition, microstructure and mechanical properties. In the illustrated ZrO₂-Al₂O₃ example, the ZrO₂ content increases from 0 vol% on the outer surface up to 25 vol% in the core. For the WC-Co-Ti(C,N) discs, the Ti(C,N) content decreases from 5 wt% on the outer edge to 0 wt% in the bulk of the material. In both materials, a hard outer rim and a tough core are obtained. Finally, the residual surface stresses of the densified graded ZrO₂-Al₂O₃ discs were investigated.     

Studies on structural,magnetic, and DC electrical resistivity properties of Co0.5M0.37Cu0.13Fe2O4 (M = Ni,Zn and Mg) ferrite nanoparticle systems

Ferrite nanoparticles of the composition Co_0.5M_0.37Cu_0.13Fe₂O₄ (M?=?Ni, Zn, and Mg) have been synthesized using sol-gel auto combustion method. Structural properties are investigated using powder X-ray diffraction technique and the results showed a pure spinel crystal structures of the synthesized materials with lattice parameters in the range of 8.3331–8.4793?Å. Crystallite sizes are calculated using the Scherrer’s formula and the results are found to be in the range of 62.22–82.41?nm. The surface texture and morphological nature of the samples are studied by scanning electron microscopy which depicted clear crystalline nature of the materials. Characteristic vibrational states prevailing in the samples are studied using room temperature Fourier transform infrared spectroscopy. A two-probe DC electrical resistivity measurement of the samples showed a high resistive nature. Room temperature vibrating sample magnetometer measurements revealed the magnetic properties of the samples.