Papers by Keyword: Spinel

Abstract: Metallization techniques based on electroless coating are used to coat SiC particles reinforced to make Al-Si/SiC metal matrix composites. Nitric Acid (HNO₃), aluminum powder and different percentage of magnesium addition were used in solutions as electroless coating medium to coat the surface of SiC particles. Five different percentage of Mg was used from 0.1 to 0.5 wt% in electrolyte solution. Metal oxide coating was characterised by scanning electron microscope (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) as well as Transmission Electron Microscope (TEM). It is obtained that metal oxide layer formed on the SiC surface was MgAl₂O₄ or spinel which was analysed by XRD and confirmed by EDS. Spinel (MgAl₂O₄) layer was found at all composition of Mg, and such layer improved wettability between SiC and Al-Si. It also is found that the solution with 0.2%wt Mg content generated more homogeneous metal oxide layer on SiC particles therefore the solution with 0.2wt% Mg was selected as electroless coating medium to coat SiC particles reinforced to produce Al-Si/SiC metal matrix composites by stir casting route. The phases present in Al-Si/SiC composites was spinel (MgAl₂O₄), Si eutectic and Mg₂Si which was analysed by XRD.

Abstract: Spinel LiMn₂O₄ were prepared by a molten salt combustion method at 600°C. The phase composition of the products was investigated by X-ray diffraction (XRD), and the electrochemical performance was tested by a coin-type self battery. XRD results presented that the purity and the crystallinity of the products increased with extending calcining time. Single phase LiMn₂O₄ could be prepared at 600°C for 6 and 12h. The ratio of lithium/manganese also affected the phase compositions of the products. The purity and crystallinity of the product with the molar ratio of Li/Mn=1.1/2 were higher than these of the product with the molar ratio of Li/Mn=1.05/2. The performance tests indicated that the electrochemical performance of the product prepared at 600°C for 6h is the best. The initial discharge capacity was 112.8mAh/g, and after 20 cycles, the capacity retention is 81.8%. The product with Li/Mn=1.1/2 had better electrochemical performance than the product with Li/Mn=1.05/2.

Abstract: Zeolite NaKL was in-situ crystallized under static condition with kaolin microspheres calcined at 980 °C as silicon and aluminium sources. Influences of hydrothermal synthesis conditions such as the mole ratio of n(K₂O)/n(Na₂O), n(OH^-)/n(SiO₂) and crystallizaion time on crystallization of zeolite L were studied in detail. The crystal phase and morphology of the as-prepared zeolite samples were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The results showed that the as-synthesized samples owned characteristic peaks of zeolite L with high crystallinity and were covered uniformly by regular crystal of zeolite L on its kaolin microsphere surface , which maintained complete ball shape as before. The optimum conditions to get zeolite NaKL with high crystallinity by this method were found to be the mole ratio of n(K₂O)/n(Na₂O) =7:3 and n(OH^-)/n(SiO₂) =0.50 in the reaction mixture, an aging of the reactants at room temperature for 30 h and crystallization temperature 120 °C for 24 h.

Abstract: Spinel-perovskite multiferroics of NiFe₂O₄/BiFeO₃ nanoparticles were prepared by modified Pechini method. The structure and morphology of the composites were examined by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the composites consisted of spinel NiFe₂O₄ and perovskite BiFeO₃ after annealed at 700°C for 2h, and the particle size ranges from 40 to 100nm. VSM and ME results indicated that the nanocomposites exhibited both tuning magnetic properties and a ME effect. The ME effect of the nanocomposites strongly depended on the magnetic bias and magnetic field frequency.

Abstract: Pure and highly crystalline spinel LiMn2O4 has been successfully prepared by a modified solution combustion synthesis (MSC) method at 400oC for 5h, while strong Mn2O3 impurity is present in the product prepared by conventional solution combustion synthesis (CSC) method on the same conditions. The particle size of LiMn2O4 prepared by MSC method is about 200 nm with a uniformly distribution. Electrochemical tests indicate that the LiMn2O4 prepared by MSC method exhibits a higher capacity, better cycle life and better rate capability than that of prepared by CSC method. It is proved that some disadvantages (such as low purity and bad crystallinity) of CSC method at low temperature can be improved efficiently by MSC method.

Abstract: In terms of this work formulations of carbon bonded castables based on new binder approaches and nanoadditions will be demonstrated. The new binder system allows the manufacturing of water based magnesia carbon castables with the same properties and chemistry of pressed magnesia carbon bricks. This binder can be also applied in oxide castables offering them high refractoriness and workability during processing. According to the workability nano-additions improve significantly the spreading diameter of carbon castables and as a result their flowability.

Abstract: In this research, solution-based combustion synthesis is applied to prepare the spinel CoAl2O4 pigment from precursor solution of Al(NO3)3 .9H2O, Co(NO3)2 .6H2O and glycine. Effect of pH values (2.5, 7, 10.5), molar ratio of fuel to metal nitrates in the precursor solutions (1.5, 2) and subsequent calcination temperature (800, 1000, 1200 °C) on the powder characteristics are described. Gel formation, morphologies, specific surface area and colour of the powder are characterized using DTA/TG, XRD, TEM, BET and UV-Vis. The results indicate that the crystalline spinel CoAl2O4 is formed at all different Gl/(metal nitrates) molar ratios, pH and temperatures and higher temperature promote the increase of the crystallite size. According to TEM figures most of the particles calcined at 800 and 1000 °C has sizes less than 50 and 100 nm, respectively. Corresponding to results of BET experiment, specific surface area has its maximum values at pH 7 and decreases with increasing of temperature. Finally, colorability test indicates the complete stability of the synthesized powder in the glass matrix.

Abstract: In contrast with metals, the resistivity of ceramics decreases with increasing temperatures. This phenomenon was first discovered in 1833 by Faraday and remained a mere scientific curiosity until 1930, when Samuel Ruben proposed the fabrication of a pyrometer device, which explored the negative temperature coefficient (NTC) of resistance exhibit by Cu2O. Eight decades later, NTC ceramic thermistors constitute an important business segment for most electroceramic manufacturers. Here, we present a review of the most significant scientific and technological advances, which lead to the enormous commercial success of NTC thermistors. This review concludes with an outlook into future possible applications of NTC ceramics, providing that some current technological shortcomings (such as ageing) are resolved.

Abstract: Magnetic susceptibility measurements, magnetization and neutron diffraction results at low temperature for cobalt and manganese oxide spinel ceramics are presented. The Curie temperature varies similarly with the sample composition in ceramics and powders. The experimental molar Curie constant variation is explained by the presence of Co2+, CoIII, Mn3+ and Mn4+, and possibly Co3+ in the octahedral sites for the cobalt rich phases. The magnetic moments of the cations in tetrahedral and octahedral sites are not collinear and the global magnetization is oriented in a third direction.

Abstract: The densification mechanism during the park-plasma-sintering (SPS) processing was examined in high purity MgAl2O4 spinel. As the density ρt increases, that is, as the effective stress σeff decreases, stress exponent n evaluated from σeff dependence of densification rate varies from n ≥ 4 in the low ρt region, n ≈ 2 in the intermediate ρt region to n ≈ 1 in the high ρt region. TEM observation shows that significant stacking faults caused by partial dislocations are observed in the low ρt region, but limited in the high ρt region. The ρt dependent densification behavior and microstructure suggest that the predominant densification mechanism during the SPS processing changes with ρt from plastic flow by partial dislocation motion for the low ρt region (n ≥ 4) to diffusion-related creep for the high ρt region (n ≈ 1).