SiO2 Trace Research Articles

Synthesis and characterization of WS2/SiO2 microfibers

Tungsten disulfide polycrystalline microfibers were successfully synthesized by a process involving electrospinning, calcination, and sulfidation steps. We used an aqueous solution of silicotungstic acid (H4SiW12O40) and polyvinyl alcohol as precursors for the synthesis of composite fibers by the needle-less electrospinning technique. The obtained green composite fibers (av. diam. 460 nm) were converted by calcination in air to tungsten oxide WO3 fibers with traces of SiO2 and a smaller diameter (av. diam. 335 nm). The heat treatment of the WO3 fibers under flowing H2/H2S/N2 stream led to conversion to tungsten disulfide WS2 with retention of the fibrous morphology (av. diam. 196 nm). Characterization of the intermediate and final fibers was performed by the XRD, SEM, TEM, HAADF STEM EDS, elemental analyses ICP-OES, and IR spectroscopy methods.

Effect of non-metallic inclusions (NMI) on crack formation in forged steel

Effect of non-metallic inclusions on crack formation in forged steel components is presented in this work. Ultrasonic phased array system was used to locate and identify the defects in the forged steel samples. X-Ray Computed Tomography (XCT), Scanning Electron Microscopy (SEM), optical metallographic and image analysis, Optical Emission Spectrometry (OES) techniques were used for defect analysis and material characterization. Few subsurface cracks were found in the rejected sample of quench tempered steel acquired after the forging process. Defects were clustered near the core of the specimen. Ultrasonic phased array transducer system and XCT confirmed the presence of approximately 100 µm wide and 10 mm long cracks. Optical and SEM images suggested that the cracks were discontinuous and revealed a predominant presence of MnS and Al2O3 inclusions along with traces of SiO2 and SiC inclusions. Further, alumina inclusions were found to be clustered near the core of specimen, which is the crack zone. Cracks could have been initiated due to quench stress and augmented by the clustered alumina inclusions and propagated through the grain boundaries. Strain analysis using EBSD was performed in a sample near the crack to study the distortion of crystal lattice around the alumina inclusions. Considerable strain distribution could be observed in the matrix containing alumina inclusion, but strain seems to be relieved near the crack. It was presumed that alumina inclusions in the matrix acted as a stress concentration factor and led to the crack initiation mechanism.

Synthesis and Characterization of Cobalt Ferrite Nanoparticles

Cobalt ferrite can be synthesized using several techniques. However, to have wide applications in electronic, medical fields etc., the synthesis technique should be simple and inexpensive. The synthesis technique used to prepare nanoparticles should not consume lot of time and energy. Also it should yield narrow particle size distribution and homogeneity in the prepared material. It was observed that surface modification such as with silica coating on the cobalt ferrite will have significant effect on the structural and magnetic properties. It is also observed that, silica coated nanoparticles could be used in biomedical applications (Hong et al., 2013). In this work we have chosen sol-gel method to synthesize pure cobalt ferrite (CoFe2O4) and silica coated (CoFe2O4 / SiO2) nanoparticles. To observe the effect of silicate coating on the structural and magnetic properties of CoFe2O4 we have carried out the present study. CoFe2O4 nanoparticles were synthesized with SiO2 coating and in pure form by sol-gel method. The obtained particle sizes were 24 and 26 nm in both the cases. The X-ray diffraction patterns showed the formation of CoFe2O4 spinel structure without any traces of SiO2 in the prepared samples. The Infrared spectra showed the bands corresponding to tetrahedral and octahedral sites as feature of typical spinel ferrites and also band due to SiO2. The particle size and morphology of CoFe2O4 / SiO2 was found to be uniform but in the case of pure CoFe2O4 somewhat agglomerated which is accounted for magnetization of ferrites. The magnetization value for CoFe2O4 / SiO2 showed a drastic decrease when compared to pure CoFe2O4 due to presence of non-magnetic coating layer.

Cobalt-Based Alloys for High Temperature Applications

The isothermal oxidation behavior and thermal stability of a cobalt base alloy was investigated up to a period of 312 hr in air from 1000 to 1200°C. A comparison of oxidation behavior of this alloy with a conventional nickel-base superalloy (Inconel 713C) has been conducted in detail. This experimental alloy oxidizes by forming layers of Al2O3, Cr2O3, TiO2, CoO and traces of SiO2 with WO2 oxides on the surface of the specimen in contact with air. Scanning electron microscopy (SEM) was used to study the microstructure, morphology and compositions of oxides formed after the exposure. Thermal stability of the alloy after extended periods of exposures to air at 1000, 1100 and 1200°C was studied using transmission electron microscopy (TEM).

Processing of the Fine Mullite Powder from Metal-Alkoxides and Its Sintering

The controlled hydrolysis of metal-alkoxides yielded an amorphous precursor powder with the composition of 3Al2O3⋅2SiO 2. The SiO2/Al2O3 ratio of the resultant fine powder determined by X-ray fluorescence analysis was 3:2 and mullite was identified above 1200°C. However, the traces of SiO2 and α-Al2O3 were detected above 1400°C, probably due to the preferential hydrolysis of Al(OPri)3. A sintered body with a relative density of about 95% was obtained by firing at 1600°C for 2h. The Vickers hardness, Young's modulus and stress intensity factor of the sintered body was about 11GPa, 230GPa and 3MPa·m1/2, respectively.