• R S Tiwari

      Articles written in Bulletin of Materials Science

    • Role of Fe substitution and quenching rate on the formation of various quasicrystalline and related phases

      Varsha Khare R S Tiwari O N Srivastava

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      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 ∥[001]bcc I and [351]bcc II, 5-fold ∥ [113]bcc II and 3-fold ∥ [110]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.

    • Effect of cationic size in Hg(Tl/Bi)Ba2Ca2Cu3O8+𝛿 on superconducting and microstructural characteristics

      Rajiv Giri H K Singh R S Tiwari O N Srivastava

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      In this paper we have reported investigations on the effect of simultaneous substitution of Bi and Tl at the H𝑔 site in the oxygen deficient H𝑔O𝛿 layer of H𝑔Ba2Ca2Cu3O8+𝛿 cuprate superconductor. Bulk polycrystalline samples have been prepared by the two-step solid state reaction process (precursor route). It has been observed that the as grown H𝑔Bi0.2–𝑥Tl𝑥Ba2Ca2Cu3O8+𝛿 (with 𝑥 = 0.00, 0.05, 0.10, 0.15, 0.20) corresponds to the 1223 phase. It has been found that the 𝑇c varies with the average cationic size $\langle R_d \rangle$ of the dopantcations. The optimum 𝑇c of ∼ 131 K has been found for the composition H𝑔Bi0.15Tl0.05Ba2Ca2Cu3O8+𝛿. This composition leads to the average dopant cation size of ∼ 1.108 Å which is very close to the size of H𝑔2+ (∼ 1.11 Å). The microstructure for H𝑔Bi0.15Tl0.05Ba2Ca2Cu3O8+𝛿 has been found to be most dense and this phase exhibits the highest stability. The 𝐽c of the optimum material H𝑔Bi0.15Tl0.05Ba2Ca2Cu3O8+𝛿 is found to be ∼ 1.29 × 103 A/cm2 at 77 K.

    • Devitrification of rapidly quenched Al–Cu–Ti amorphous alloys

      D K Misra R S Tiwari O N Srivastava

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      X-ray diffraction, transmission electron microscopy and differential scanning calorimetry were carried out to study the transformation from amorphous to icosahedral/crystalline phases in the rapidly quenched Al50Cu45Ti5 and Al45Cu45Ti10 alloys. In the present investigation, we have studied the formation and stability of amorphous phase in Al50Cu45Ti5 and Al45Cu45Ti10 rapidly quenched alloys. The DSC curve shows a broad complex type of exothermic overlapping peaks (288–550°C) for Al50Cu45Ti5 and a well defined peak around 373°C for Al45Cu45Ti10 alloy. In the case of Al50Cu45Ti5 alloy amorphous to icosahedral phase transformation has been observed after annealing at 280°C for 73 h. Large dendritic growth of icosahedral phase along with 𝛼-Al phase has been found. Annealing of Al50Cu45Ti5 alloy at 400°C for 8 h results in formation of Al3Ti type phase. Al45Cu45Ti10 amorphous alloy is more stable in comparison to Al50Cu45Ti5 alloy and after annealing at 400°C for 8 h it also transforms to Al3Ti type phase. However, this alloy does not show amorphous to icosahedral phase transformation.

    • Effect of annealing temperature on the structural–microstructural and electrical characteristics of thallium bearing HTSC films prepared by chemical spray pyrolysis technique

      K K Verma R S Tiwari O N Srivastava

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      In order to get good quality reproducible films of Tl : HTSC system, we have studied the different annealing conditions to finally achieve the optimized annealing condition. In the present investigation, Tl–Ca–Ba–Cu–O superconducting films have been prepared on YSZ (100) and MgO (100) single crystal substrates via precursor route followed by thallination. The post deposition heat treatments of the precursor films were carried out for various annealing temperatures (870°C, 890°C) and durations (1 and 2 min). The optimized thallination procedure occurred at 870°C for 2 min into good quality films with 𝑇c (𝑅 = 0) ∼ 103 K for YSZ and 𝑇c (𝑅 = 0) ∼ 98 K for MgO substrates, respectively. Further we have correlated the structural/microstructural characteristics of the films.

    • Synthesis, characterization and application of semiconducting oxide (Cu2O and ZnO) nanostructures

      D P Singh Jai Singh P R Mishra R S Tiwari O N Srivastava

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      In the present study, we report the synthesis, characterization and application of nanostructured oxide materials. The oxide materials (Cu2O and ZnO) have been synthesized by electrolysis based oxidation and thermal oxidation methods. Cuprous oxide (Cu2O) nanostructures have been synthesized by anodic oxidation of copper through a simple electrolysis process employing plain water (with ionic conductivity, ∼ 6 𝜇S/m) as electrolyte. In this method no special electrolytes, chemicals and surfactants are needed. The method is based on anodization pursuant to the simple electrolysis of water at different voltages. Two different types of Cu2O nanostructures have been found. One type got delaminated from copper anode and was collected from the bottom of the electrochemical cell and the other was located on the copper anode itself. The nanostructures collected from the bottom of the cell are either nanothreads embodying beads of different diameters, ∼ 10–40 nm or nanowires (length, ∼ 600–1000 nm and diameter, ∼ 10–25 nm). Those present on the copper anode were nanoblocks with preponderance of nanocubes (nanocube edge, ∼ 400 nm). The copper electrode served as a sacrificial anode for the synthesis of different nanostructures. Aligned ZnO nanorod array has been successfully synthesized by simple thermal evaporation catalyst free method. Detailed structural characterizations revealed that the as synthesized aligned ZnO nanorods are single crystalline, with a hexagonal phase, and with growth along the [0001] direction. The room-temperature photoluminescence spectra showed a weak ultraviolet emission at 380 nm, a broad blue band at 435 nm and a strong orange–red emission at 630 nm. Structural/microstructural characterization of these nanomaterials have been carried out employing scanning (XL-20) and transmission electron microscopic (Philips EM, CM-12 and Technai 20G2) techniques and X-ray diffraction techniques having graphite monochromater with CuK𝛼 radiation (𝜆 = 1.54439 Å) (X’Pert PRO PAN analytical). The UV-visible absorption spectra were recorded on Model–VARIAN, Cary 100, and Bio UV-visible spectrophotometer. The photoluminescence (PL) measurement was carried out at room temperature with a He–Cd, a laser excited at 325 nm.

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