O N Srivastava
Articles written in Bulletin of Materials Science
Volume 23 Issue 6 December 2000 pp 475-481 Superconductors
In this paper we have shown that polycrystalline films corresponding to Tl-2223 phase can be grown by employing high thalliation temperatures and short thalliation times. Ultrasonically deposited precursor films corresponding to Ba2Ca2.2Cu3.3Ox(Agy) have been thalliated under high vacuum (∼ 10-5 torr) at 890°C to obtain single phase Tl-2223 films. An off-stoichiometric and unreacted pellet of composition Tl2.05Ba2Ca2Cu3Oz has been used as source of Tl. We have shown that oxygen ambient is not necessary for the growth of Tl-2223 phase. The as-thalliated films have Tc’s in the range 123 K ±0.70 K. The
Volume 24 Issue 3 June 2001 pp 257-264 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
Volume 24 Issue 5 October 2001 pp 523-528 Superconductors
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.
Volume 24 Issue 6 December 2001 pp 595-601 Superconductors
Synthesis of H𝑔(Tl)Ba2Ca2Cu3O8+𝛿 superconducting tapes have been accomplished by annealing the precursor tape, Ba2Ca2Cu3O𝑦 (fabricated by doctor blade tape casting technique) in an environment of H𝑔(Tl) vapour. Characterization of superconducting HTSC tape sample was carried out through XRD, TEM, SEM and R–T measurements. Surface morphological investigations of the as-synthesized H𝑔(Tl)Ba2Ca2Cu3O8+𝛿 HTSC tapes by scanning electron microscope have shown the occurrence of curious growth characteristics resembling spiral like features. These growth spirals encompass nearly the whole grain suggesting that spiral growth led to the formation of small crystal like grains of superconducting material H𝑔(Tl)Ba2Ca2Cu3O8+𝛿. The likely mechanism for the generation of these screw dislocations has been elucidated in terms of incoherent coalescence of growth fronts formed from H𝑔(Tl):1223 and H𝑔(Tl):1234 nuclei.
Volume 26 Issue 5 August 2003 pp 553-558 Glasses
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.
Volume 27 Issue 2 April 2004 pp 113-119 Review---Thin Films
The synthesis of high temperature superconducting films of Y:247 (𝑇c ∼ K) have been successfully achieved. The difficulty in synthesis owing to narrow range of stability of Y:247 has been taken care of through several quenching modes, e.g. quenching of the films synthesized at ∼ 850°C, in air or in liquid nitrogen. The energy dispersive analysis of X-rays (EDAX) and transmission electron microscopy (TEM) studies of the as processed film, Y2Ba4Cu7O14+𝑥, exhibit nearly correct cationic stoichiometry of 2 : 4 : 7; also narrow regions (< 50 Å) of minority Y:124 phase and stacking faults capable of working as flux pinning sites have been invariably found to be present. In addition to Y:247, Ag admixed films have also been investigated. The 𝑇c here is ∼ 70–75 K which is similar to that of the film without silver. Representative estimates of transport critical current density (𝐽c) for Y:247 films is ∼ 103 A/cm2, and with silver corresponding to Y2Ba4Cu7O14+𝑥 (Ag0.1) is found to be ∼ 104 A/cm2.
Volume 28 Issue 2 April 2005 pp 151-154 Superconductors
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.
Volume 31 Issue 3 June 2008 pp 313-318
The macroscopic coaxial carbon cylinders (dia. ∼ 0.5 cm with varying lengths, ∼ 7–10 cm) consisting of aligned carbon nanotube (CNT) stacks have been prepared by controlled spray pyrolysis method. The coaxial carbon cylinders of CNT stacks have been formed directly inside the quartz tube. Another study is done on multi-walled CNTs (MWNTs)–polymer (e.g. polyethylene oxide (PEO), polyacrylamide (PAM)) composite films. We have investigated the structural, electrical and mechanical properties of MWNTs–PEO composites. Composites with different wt% (between 0 and 50 wt% of MWNTs) have been prepared and characterized by the scanning electron microscopic technique. Enhanced electrical conductivity and mechanical strength were observed for the MWNTs–PEO composites. We have also studied the electrical property of MWNTs–PAM composite films.
Volume 31 Issue 3 June 2008 pp 319-325
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  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.
Volume 31 Issue 3 June 2008 pp 545-550
Nanocrystalline semiconducting materials are attracting much attention due to their potential applications in solar energy conversion, nonlinear optics, and heterogeneous photocatalysis. In the present investigation, we have synthesized nanostructured TiO2 photocatalysts, which have been used in the photocatalytic degradation of phenol (one of the most common water pollutants). These catalysts have been prepared through sol–gel technique using titanium tetra-isopropoxide as a raw material for synthesis. Characterization techniques such as XRD, SEM and TEM have been employed for structural/microstructural investigations. XRD results show that the as synthesized TiO2 nanopowder exhibit anatase phase, TiO2. The average sizes of the TiO2 nanopowders are ∼ 5–10 nm. The optical properties of the samples were investigated through UVvisible and fluorescence techniques. It has been observed that absorption edge corresponds to ∼ 410 nm (bandgap, ∼ 3.02 eV). The emission peak in the fluorescence spectrum at ∼ 418 nm corresponds to the bandgap energy of ∼ 2.97 eV. Concentration of phenol (initial concentration, ∼ 100 ppm) with illumination time was monitored by measuring the absorbance of pure and illuminated phenol through UV-visible spectrophotometer. Salient feature of this study relates to the fact that the present sol–gel synthesized TiO2 nanopowders have been found to be better photocatalysts for phenol degradation than the presently employed commercial TiO2 (P-25, Degussa) photocatalyst. Thus, whereas phenol concentration, with the presently synthesized TiO2 nanopowders, the concentration of phenol decreases up to ∼ 32% but for commercial TiO2 nanopowder (P-25, Degussa), it decreased only up to ∼ 25%. The improved surface area is considered as an important factor for the aforesaid decrease in phenol concentration.
Volume 34 Issue 4 July 2011 pp 607-614
In the present study, we report the synthesis of carbon nanotubes (CNTs) using a new natural precursor: castor oil. The CNTs were synthesized by spray pyrolysis of castor oil–ferrocene solution at 850°C under an Ar atmosphere. We also report the synthesis of carbon nitrogen (C–N) nanotubes using castor oil–ferrocene–ammonia precursor. The as-grown CNTs and C–N nanotubes were characterized through scanning and transmission electron microscopic techniques. Graphitic nanofibres (GNFs) were synthesized by thermal decomposition of acetylene (C2H2) gas using Ni catalyst at 600°C. As-grown GNFs reveal both planar and helical morphology. We have investigated the structural and electrical properties of multi-walled CNTs (MWNTs)–polymer (polyacrylamide (PAM)) composites. The MWNTs–PAM composites were prepared using as purified, with ball milling and functionalized MWNTs by solution cast technique and characterized through SEM. A comparative study has been made on the electrical property of these MWNTs–PAM composites with different MWNTs loadings. It is shown that the ball milling and functionalization of MWNTs improves the dispersion of MWNTs into the polymer matrix. Enhanced electrical conductivity was observed for the MWNTs–PAM composites. Graphene samples were prepared by thermal exfoliation of graphite oxide. XRD analysis confirms the formation of graphene.
Volume 42 | Issue 5