فهرست مطالب

  • Volume:8 Issue: 4, 2011
  • تاریخ انتشار: 1390/08/24
  • تعداد عناوین: 7
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  • M. S. Kaiser, A. S. W. Kurny Page 1
    Microstructure and properties of the Al-6Si-0.3Mg alloys containing scandium (0.2 to 0.6wt %) were investigated. The microstructure was observed by optical microscopy, the hardness was determined by Vickers tester and phase transformation was investigated by differential scanning calorimetry (DSC) technique. The results showed that scandium can refine dendrites, enhance hardness in the aged alloys and suppress softening effect during prolonged ageing treatment.
    Keywords: Al, Si, Mg alloys, age hardening, grain, refinement, precipitates, recrystallization
  • A. Allahverdi, E. Najafi Kani, M. Fazlinejhad Page 9
    The linear expansion, early-age compressive strength and setting times of the binary mixtures of gypsum and Portland cement clinkers of relatively low C3A-contents were investigated. For this reason, type 1, 2, and 5 of Portland cement-clinkers were selected and a number of binary mixtures were designed. At relatively lower percentages of gypsum (about 5%), the early strength behavior is improved. Results obtained for compressive strength of mixtures with 5% gypsum confirm the possibility of achieving 28- and 90-day compressive strengths up to values higher than 100 MPa and 130 MPa, respectively. At relatively higher percentages of gypsum (more than 25%), excessive expansion caused by ettringite formation results in the formation of micro-cracks effectively weakening the strength behavior. The work suggests that type S expansive cements could be produced from Portland cement clinkers of relatively low C3Acontents.
    Keywords: Portland cement, Gypsum, Linear expansion, Compressive strength
  • S. Janitabar Darzi, A. R. Mahjoub, A. R. Nilchi, S. Rasouli Garmarodi Page 20
    TiO2/SiO2 nanocomposite with molar ratio 1:1 was synthesized by a free calcination sol-gel method using titanium tetra chloride and tetraethylorthosilicate as raw materials. In the composite, TiO2 nanocrystals are highly dispersed in the amorphous SiO2 matrix and the mater showed size quantization effect arising from the presence of extremely small titanium oxide species having a low coordination number. Thermal phase transformation studies of the as-prepared composite were carried out by means of X-ray diffraction (XRD) patterns and thermogravimetry–differential scanning calorimetry (TG–DSC) analyses. The studies showed existence of anatase phase in all the tested temperatures. When temperature exceeds 400°C, brookite phase was formed beside anatase phase. At 950°C amorphous silica matrix was transformed to crystobalite and brookite phase disappeared. Finally, small peaks of rutile phase were detectable at 1100°C.
    Keywords: composite, mesoporous, TiO2, SiO2, crystobalite, anatase, phase transformation
  • A. Mohassel, A. H. Kokabi, P. Davami Page 27
    The wide-gap aluminothermic rail welds with root opening of 50-70 mm were produced using plain carbon steel rail and non-alloy aluminothermic charge. Mechanical properties and micro-structure of the weld metal and HAZ as well as the impact energy and the fracture toughness of the welds were investigated. The yield and tensile strength of wide-gap welds were about 98% and 95% of the base metal, respectively. Both minimum and maximum hardnesses of the joint were seen in HAZ which were related to the grain coarsening and normalizing, respectively. The mean value of wide-gap weld fracture toughness is more than narrow-gap weld. Moreover, trans-granular cleavage indicated the brittle fracture mode of the weld metal.
    Keywords: Wide, gap weld, Aluminothermic weld, Fracture toughness, Microstructure
  • F. Kashaninia, H. Sarpoolaky, A. R. Bagheri, R. Naghizadeh, M. Zamanipour Page 34
    There have been lots of studies to control the poor hydration resistance of dolomite refractories; one of the most effective solutions has been the addition of magnesia to doloma. Using a co-clinker of magnesia-doloma as a starting material would provide more homogeneity in the properties of the product and has been published recently.On the other hand, addition of iron oxide to doloma has been found to increase the hydration resistance. In this paper,the effect of iron oxide addition on hydration phase analysis and microstructure of two different magnesia- doloma samples, one with CaO content of 25 wt% and the other one with that of 35 wt% has been investigated. Ten samples were prepared by pressing followed by firing at 1750 ºC for 3hrs. Results showed that the hydration resistance of the samples improved by decreasing the CaO content, because CaO is much more prone to hydration comparing to MgO.Besides, iron oxide addition lead to the formation of iron-containing phases which increased the hydration resistance of the samples both by capsulating the CaO and MgO grains and by promoting the liquid phase sintering.
    Keywords: Magnesia, doloma, Iron oxide, CaO content, Hydration resistance
  • M. Rezvani Page 41
    The effect of Y2O3, CeO2, P2O5, ZrO2 and TiO2 in single, double and triple form on crystallization mechanism of Li2OAl2O3- SiO2(LAS) glass-ceramic system was investigated. The nucleation and crystallization peak temperatures of optimized samples in each group were determined by Ray & Day method. The crystalline phase was determined by the X-ray diffractometery. The micro-structure of the samples was studied by SEM techniqe. Crystallization activation energy, E, and kinetic constants(n, m) were determined by differential thermal analysis (DTA) through Marotta and Augis-Bennet methods. According to the results, the Avrami constants(m, n) derived from the Marotta and Augis- Bennett, glasses containing both ZrO2 and TiO2 nuclei were showed bulk crystallization. The crystallization mechanism of specimens containing ZrO2, TiO2 and CeO2 in the triple nuclei series represent two-dimensional bulk crystallization. By comparison of Avrami constants and activation energy of crystallization of optimized samples with other results they gave much lower value of E(255.5 kJ/mol) and higher value of n in 4.38.The lattice constants of the main phase(-eucryptite solid solution)in samples were determined according to the XRD results.
    Keywords: LAS glass ceramics, Eucryptite, h, quartz, Kinetic parameters, Crystallization mechanism
  • M. M. Kashani Motlagh, A. A. Youzbashi, Z. Amiri Rigi Page 50
    A natural bentonite obtained from Khorasan, Iran, was submitted to acid activation with sulphuric acid. Sample aliquots (5gr) were leached with 100 ml H2SO4 solutions of various concentrations (2–7N) at 80±2 oC for 2 hours. X–ray diffraction, chemical analysis, infrared spectroscopy and specific surface area measurements were performed in order to evaluate important structural modifications occurring as a result of acid attack. Octahedral sheet was affected by acid activation resulting into the dissolution of cations (Mg2+, Fe2+, Al3+) and consequent decomposition of montmorillonite structure. Bentonite samples were then tested in order to verify their capacity to bleach colza–soybean oil, and their performances were compared to that of a commercial bleaching clay. The bleaching ability of the natural clay was poor when compared with that of the industrial adsorbent. Acid activation of the bentonite sample with 7N sulphuric acid yielded an adsorbent material which was highly efficient in the bleaching of the oil functioned better than the commercial clay product under the same conditions.
    Keywords: Acid activation, Bentonite, Oil bleaching