pp 257-264 June 2001 Quasicrystals
We have investigated Fe substituted versions of the quasicrystalline (qc) alloy corresponding to Al65Cu20(Cr, Fe)15 with special reference to the possible occurrence of various quasicrystalline and related phases. Based on the explorations of various compositions it has been found that alloy compositions Al65Cu20Cr12Fe3 and Al65Cu20Cr9Fe6 exhibit interesting structural phases and features at different quenching rates. At higher quenching rates (wheel speed ∼ 25 m/sec) all the alloys exhibit icosahedral phase. For Al65Cu20Cr12Fe3 alloy, however, both the icosahedral and even the decagonal phases get formed at higher quenching rates. At higher quenching rate, alloy having Fe 3 at% exhibits two bcc phases, bccI (𝑎 = 8.9 Å) and bccII (𝑎 = 15.45 Å). The orientation relationships between icosahedral and crystalline phases are: Mirror plane ∥bcc I and bcc II, 5-fold ∥ bcc II and 3-fold ∥ bcc II. At lower quenching rate, the alloy having Fe 6 at% exhibits orthorhombic phase (𝑎 = 23.6 Å, 𝑏 = 12.4 Å, 𝑐 = 20.1 Å). Some prominent orientation relationships of the orthorhombic phase with decagonal phase have also been reported. At lower quenching rate (∼ 10 m/sec), the alloy (Al65Cu22Cr9Fe6) shows the presence of diffuse scattering of intensities along quasiperiodic direction of the decagonal phase. For making the occurrence of the sheets of intensities intelligible, a model based on the rotation and shift of icosahedra has been put forward.
pp 265-271 June 2001 Inorganic–Organic Hybrid Materials
Tetravalent metal acid (TMA) salt zirconium tungstate (ZW) has been synthesized, followed by its derivatization using para-chlorophenol (𝑝CP). The resulting compound is abbreviated as ZW𝑝CP. ZW𝑝CP has been characterized for elemental analysis, spectral analysis (FTIR), X-ray analysis and thermal analysis (TGA). Its chemical stability has been assessed in various mineral acids, bases and organic solvents. Ion exchange capacity (IEC) has been determined and distribution behaviour towards several metal ions in different electrolyte solutions with varying concentrations has been studied and a few binary separations achieved.
pp 273-279 June 2001 Chemical Vapour Deposition
A CVD system for the production of continuous SiC fibre was set up. The process of SiC coating on 19 𝜇m diameter tungsten substrate was studied. Methyl trichloro silane (CH3SiCl3) and hydrogen reactants were used. Effect of substrate temperature (1300–1500°C) and concentration of reactants on the formation of SiC coating were studied. SiC coatings of negligible thickness were formed at very low flow rates of hydrogen (5 × 10–5 m3/min) and CH3SiCl3 (1.0 × 10–4 m3/min of Ar). Uneven coatings and brittle fibres were formed at very high concentrations of CH3SiCl3 (6 × 10–4 m3/min of Ar). The flow rates of CH3SiCl3 and hydrogen were adjusted to get SiC fibre with smooth surface. The structure and morphology of SiC fibres were evaluated.
pp 281-283 June 2001 Amorphous Alloys
The oxidation behaviour of Zr-based bulk amorphous alloy Zr65Cu17.5Ni10Al7.5 has been studied in air environment at various temperatures in the temperature range 591–684 K using a thermogravimetric analyser (TGA). The oxidation kinetics of the alloy in the amorphous phase obeys the parabolic rate law for oxidation in the temperature range 591–664 K. The values of the activation energy and pre-factor as calculated from the Arrhenius temperature dependence of the rate constants have been found to be 1.80 eV and 2.12 × 109 g cm–2.sec–1/2, respectively.
pp 285-289 June 2001 Nanomaterials
We investigated the optical properties of silicon clusters and Si nanocrystallites using photoluminescence (PL) and Raman scattering technique. Broad luminescence band in the red region was observed from Si-doped SiO2 thin films deposited by co-sputtering of Si and SiO2 on 𝑝-type Si (100) substrates, annealed in Ar and O2 atmosphere. Nanocrystalline Si particles fabricated by pulsed plasma processing technique showed infrared luminescence from as grown film at room temperature. Raman spectra from these films consisted of broad band superimposed on a sharp line near 516 cm–1 whose intensity, frequency, and width depend on the particle sizes arising from the phonon confinement in the nanocrystalline silicon. We also performed PL, Raman and resonantly excited PL measurements on porous silicon film to compare the optical properties of Si nanostructures grown by different techniques. An extensive computer simulation using empirical pseudopotential method was carried out for 5–18 atoms Si clusters and the calculated gap energies were close to our PL data.
pp 291-296 June 2001 Optical Properties of Materials
The optical energy gaps of WS2 single crystal were determined from the analysis of the absorption spectrum near the fundamental absorption edge at room temperature using light parallel to 𝑐-axis incident normally on the basal plane. On the basis of two- and three-dimensional models it was found that both direct and indirect band transitions took place in WS2 and the indirect transition was of the allowed type. The optical energy gaps corresponding to both transitions were determined and the phonon energies associated with the indirect transitions estimated. The implications of the results have been discussed.
pp 297-300 June 2001 Optical Properties of Materials
The thermally evaporated stoichiometric CdI2 films show good 𝑐-axis alignment normal to substrate plane for film thickness up to 200 nm. The optical absorption data indicate an allowed direct interband transition across a gap of 3.6 eV in confirmation with earlier band structure calculations. However, part of the absorption data near band edge can be fitted to an indirect band gap of 3 eV. The dependence of band gap on film thickness (> 200 nm) can be explained qualitatively in terms of decreasing grain boundary barrier height with grain size.
pp 301-304 June 2001 Alloys and Steels
25%Al-Zn alloy coating performs better than hot dip galvanized coating and 55%Al-Zn-Si coating with regard to general seawater corrosion protection. This study deals with the interfacial intermetallic layer's growth, which affects considerably the corrosion resistance and mechanical properties of 25%Al-Zn alloy coatings, by means of three-factor quadratic regressive orthogonal experiments. The regression equation shows that the intermetallic layer thickness decreases rapidly with increasing content of Si added to the Zn-Al alloy bath, increases with rise in the bath temperature and prolonging dip time. The most effective factor that determined the thickness of intermetallic layer was the amount of Si added to Zn-Al alloy bath, while the effect of bath temperature and dip time on the thickness of intermetallic layer were not very obvious.
pp 305-312 June 2001 Alloys and Steels
A two-dimensional diffusion based model is developed to describe transformation of austenite into ferrite and pearlite under continuous cooling conditions. The nucleation of ferrite is assumed to occur over grain boundaries and the nucleation of pearlite is assumed to be taking place all over the grain and at growing ferrite–austenite interfaces, when the composition and temperature conditions are favourable. A cellular automaton algorithm, with transformation rules based on this model is used for the growth of ferrite and pearlite. Model predicted results for continuous cooling transformations are verified by comparing the model predicted microstructure features with the experimental measurements for two sets of plain carbon steels of different composition and austenite grain size. Using the model, it is possible to generate results like undercooling to start ferrite and pearlite transformations, which are difficult to obtain experimentally.
pp 313-316 June 2001 Cement and Concrete Materials
Addition of bitumen emulsion to the matrix has been found to improve strength and soundness of the product while decreasing the initial setting periods. Thus, bitumen emulsion as an admixture in magnesia cement is a moisture proofing and strengthening material.
pp 317-322 June 2001 Cement and Concrete Materials
Compositional and structural information were obtained from an ancient 1600-year old Indian iron using microprobe techniques (EDS, 𝜇XRD and 𝜇PIXE). Several different local locations in the iron matrix and in the entrapped slag inclusions were analyzed. The P content of the metallic iron matrix was very heterogeneous. Lower P contents were observed in the regions near slag inclusions. This was correlated to the dephosphorization capacity of the slag. The crystallized phases identified in the slag inclusions were wüstite and fayalite. The compositions of the slag inclusions were relatively homogeneous.
pp 323-330 June 2001 Chemical Beneficiation
Chemically beneficiated high silica/alumina iron ore rejects (27–76% Fe2O3) were used to synthesize iron oxides of purity 96–98% with SiO2/Al2O3 ratio reduced to 0.03. The major impurities on chemical beneficiations were Al, Si, and Mn in the range 2–3%. A 99.73% purity Fe2O3 was also prepared by solvent extraction method using methyl isobutyl ketone (MIBK) from the acid extracts of the ore rejects. The magnesium ferrite, MgFe2O4, prepared from these synthetic iron oxides showed high resistivity of ∼ 108 ohm cm. All ferrites showed saturation magnetization, 4𝜋𝑀s, in the narrow range of 900–1200 Gauss and the Curie temperature, 𝑇c, of all these fell within a small limit of 670 ± 30 K. All ferrites had low dielectric constants (𝜀'), 12–15, and low dielectric loss, tan 𝛿, which decreased with the increase in frequency indicating a normal dielectric dispersion found in ferrites. The presence of insignificant amount of polarizable Fe2+ ions can be attributed to their high resistances and low dielectric constants. Impurities inherent in the samples had no marked influence on the electrical properties of the ferrites prepared from the iron ore rejects, suggesting the possibility of formation of ferrite of constant composition, MgFe2O4, of low magnetic and dielectric losses at lower temperatures of 1000°C by ceramic technique.
pp 331-338 June 2001 Chemical Beneficiation
Iron oxyhydroxides and hydroxides were synthesized from chemically beneficiated high SiO2/Al2O3 low-grade iron ore (57.49% Fe2O3) rejects and heated to get iron oxides of 96–99.73% purity. The infrared band positions, isothermal weight loss and thermogravimetric and chemical analysis established the chemical formulas of iron-oxyhydroxides as 𝛾-FeOOH.0.3H2O; 𝛼-FeOOH.0.2H2O and amorphous FeOOH. The thermal products of all these were 𝛼-Fe2O3 excepting that of 𝛾-FeOOH.0.3H2O which gave mainly 𝛾-Fe2O3 and some admixture of 𝛼-Fe2O3. The hydrazinated iron hydroxides and oxyhydroxides, on the other hand, decomposed autocatalytically to mainly 𝛾-Fe2O3. Hydrazine method modifies the thermal decomposition path of the hydroxides. The saturation magnetization, 𝐽s, values were found to be in the range 60–71 emu g–1 which are close to the reported values for 𝛾-Fe2O3. Mechanism of the 𝛾-Fe2O3 formation by hydrazine method is discussed.
pp 339-344 June 2001 Supramolecular Materials
Multifunctional ligand 5-cyano-6-(-4-pyridyl)-2-thiouracil (L) was prepared and allowed to react with trans [Co(en)2Cl2]+Cl– resulting into [Co(en)2LCl]2+.2Cl– which upon further reaction with equimolar ratio of ligand [L] gave the complex [Co(en)2L2]3+.3Cl–. These metal complexes were then separately reacted with thymine and adenine bases. Complexes thus prepared after characterization by their elemental analysis, FAB mass and spectral (IR, 1HNMR, UV-visible) data were studied for their powder X-ray diffraction and optical microscopic characteristics.
Volume 42 | Issue 5
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