Materials Science Forum Vols. 775-776

Abstract: The magnetic properties of MnZn ferrites are highly influenced by the partial oxygen pressure during sintering. In this work MnZn ferrites with different chemical compositions were pressed in a toroidal form and sintered at 1290 °C for 6 h at different oxygen pressures. X ray diffraction using K-edge radiation of Fe, Zn and Mn were performed in order to identify the atomic preference in the MnZn ferrite spinel structure, by means of Rietveld analysis. Magnetic losses were measured at 25 °C and 100 °C (200mT, 100 kHz). The high temperature loss decreased with the increase of oxygen content during sintering whereas at room temperature this effect was opposite. The losses behavior as function of frequency was analyzed according the loss separation model.

Abstract: This study proposes to evaluate the influence of the variation of sintering temperature on microstructural characteristics and magnetic ferrite Ni_0,5Zn_0,5Fe₂O₄ sintered by microwave energy. The samples were sintered at 900, 1000, 1100 and 1200°C for exposure time of 10 minutes, with rate 50°C/minutes and characterized by density and porosity, X-ray diffraction, scanning electron microscopy and magnetic measurements. The results indicate that the values of density and apparent porosity were 4.2, 4.5, 4.4 and 4.5 g/cm³ and 3.4, 2.1, 2.2 and 2.4% for the sintering temperatures of 900, 1000, 1100 and 1200°C respectively. The formation of the ferrite phase Ni_0,5Zn_0,5Fe₂O₄ been identified for all conditions of sintering, with grain sizes of 52, 62, 71 and 58nm and saturation magnetization values of 63, 68, 69 and 27 emu/g to temperatures sintering 900, 1000, 1100 and 1200°C respectively.

Abstract: This work involved a study of the reproducibility of the process of combustion synthesis to produce Ni-Zn ferrites. The structural, morphological and magnetic characteristics of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetometry using an alternating gradient magnetometer (AGM). The XRD diffractograms of the samples indicated that they are monophasic, crystalline, with crystallite sizes ranging from 21 to 38 nm, and have a homogeneous morphology consisting of agglomerates of spherical particles. The samples behaved as soft magnetic materials, with magnetization levels ranging from 37 to 47 emug^-1. The combustion synthesis was found to be efficient in producing Ni-Zn nanoferrites, yielding reproducible results.

Abstract: Ni_0.5Zn_0.5Fe₂O₄, Mn_0.5Zn_0.5Fe₂O₄ and Ni_0.2Cu_0.3Zn_0.5Fe₂O₄ nanoferrites were synthesized, characterized and evaluated in terms of their performance as catalysts in the methyl esterification reaction of soybean oil. The nanoferrites were synthesized by combustion and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The XRD patterns show the presence of inverse spinel B(AB)₂O₄ phase. The EDX results confirmed the stoichiometry of the nanoferrite systems, whose morphology consisted of large block-like agglomerates with a brittle aspect and a wide agglomerate size distribution. The results indicate that the Ni_0.5Zn_0.5Fe₂O₄ nanoferrite was the most active catalyst in the esterification reaction, with conversion rates ranging from 40 to 91%.

Abstract: It is well known that iron has a magnetocrystalline anisotropy and, therefore, the crystallographic texture has great influence on its magnetic properties. In most applications of non-oriented grain electrical steels, it is desirable that the magnetic properties are isotropic. In this work, modern quantitative texture analysis methods are used to characterize the crystallographic textures of many types of non-oriented grain electrical steels and their relation with the magnetic properties. The magnetocrystalline anisotropy coefficient is the parameter of texture analysis that is directly related to the magnetic properties. This paper analyzes the correlation between the magnetic properties of electrical steels with 3 wt.% to 5 wt.% silicon and their magnetocrystalline anisotropy coefficients.

Abstract: The Stoner-Wohlfarth model can be used for predicting hysteresis curves of either isotropic or anisotropic nanocrystalline Sm₂Co₁₇ type magnets. For isotropic magnets, with Mr/Ms=0.5, the Stoner-Wohlfarth model predicts coercive force of 48% of the anisotropy field. Here, Mr is remanence and Ms is magnetization of saturation. Two texture distributions were compared: Pearson VII and cosⁿ. The calculations indicate that increasing the alignment degree of the grains, the coercivity increases. However, this increase of the coercive force is small. It is found that a very well aligned magnet, with Mr/Ms ratio of 0.96, presents coercivity only ~20% higher than that of the isotropic magnet.

Abstract: In the first part of this article, the SW-CLC (Stoner-Wohlfarth with CLC modification) model is discussed for nanocrystalline magnets, as melt-spun exchange coupled NdFeB. In the second part, the effect of grain size of coercivity for large grain size, above the single domain particle size, is addressed. The Kondorsky law observed for large grain size only can be due to domain wall displacement phenomena, where there is nucleus expansion. There are two main situations, one for nanocrystalline grain size, where the SW-CLC model is obeyed, and other for large grain size, where the coercive field decreases proportionally to the square root of the grain size.

Abstract: Thermoelectric materials are able to generate an electric potential when subjected to a temperature gradient. The efficiency of this conversion is based on the value of the figure of merit (ZT) of the material. One way to increase the value of ZT is producing nanostructured ceramics, because it should interfere in the thermal conductivity of the material. SrTiO₃ is also a thermoelectric oxide material, due it n-type semiconductor nature. In this study, ceramic powders were obtained by solid-state reaction, mixing stoichiometric amounts of titanium oxide (TiO₂) and strontium carbonate (SrCO₃) in a polypropylene jar with zirconium balls for three hours. This mixture was calcined at 1000°C for 2 hours. These samples were pressed into pellets with 12mm in diameter and were sintered under different conditions: by a conventional method in an resistive oven at 1400o C for 3 hours and in a microwave oven at temperatures of 1220°C 1350°C 1375°C 1400°C and 1450°C for thirty minutes. The powders were subjected to structural and ceramics were subjected to microstructural characterizations.

Abstract: The search of variations in the methodology for obtaining nanoferrites has attracted the interest of researchers in search of better results with regard to the structure and morphology of these materials. This study evaluates the effect of microwave power (50 and 70 W) in the structural and morphological characteristics of NiZn ferrite, using aniline as a fuel for combustion reaction. The aluminas were characterized by X-ray diffraction and scanning electron microscopy. The results showed that only the variation in microwave power is sufficient to change the structure of nanoferrites. The sample synthesized in power of 50 W was presented monophasic, illustrating the ferrite phase with crystallite size of 50.04 nm; and for 70 W, it was appeared, besides the ferrite phase, hematite and zinc oxide with a crystallite size of 17.07 nm. The morphology did not change significantly, the nanoferrites showed particles with similar geometry.

Abstract: Membranes can be defined as a barrier that separates two phases and is widely applied in industrial separation processes. The development of membrane separation processes (PSM) has the major advantage of phase change without separation of components, besides selectivity and simplicity of operation. The addition of mineral fillers in polymers is intended for the cost reduction and increase in rigidity of the materials. When these loads have sizes of nanometric dimensions, this system is called nanocomposites, which exhibit improved properties compared to the pure polymers or conventional composites. Microporous membranes were obtained from polyamide 6.6 nanocomposites of varying amounts of regional montmorillonite clay through the immersion-precipitation process. In general, the membranes showed a filter skin, which contains very small pores and a porous layer, with variations in microstructure, detected by scanning electron microscopy (SEM).