Search Results - (Author, Cooperation:V. P. Dravid)

2015-06-27

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2015-11-28

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2012-09-22

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2014-04-18

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2018-06-02

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

2018-02-10

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Materials Science

1089-7550

AIP Digital Archive

Physics

The distribution of Cd, In, and Sn cations on the spinel lattice was investigated across the solid solution, Cd1+xIn2−2xSnxO4. Convergent beam electron diffraction was used to establish the presence of the spinel crystallography throughout the solid solution. Atom location by channeling enhanced microanalysis was employed to determine the distributions of cations on the octahedral and tetrahedral lattice sites. CdIn2O4 was observed to be a normal-type spinel. As x was increased, the cation distribution became more random as Cd and Sn filled the octahedral sublattice. These observations may correlate with previously observed changes in optical gap and conductivity across the solution. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

InAs0.3Sb0.7 layers with mirrorlike morphology have been grown on GaAs substrates by low-pressure metalorganic chemical vapor deposition. A room-temperature electron Hall mobility of 2×104 cm2/V s has been obtained for a 2-μm-thick layer. Low-temperature resistivity of the layers depended on TMIn flow rate and layer thickness. Hall mobility decreased monotonically with decreasing temperature below 300 K. A 77 K conductivity profile has shown an anomalous increase in the sample conductivity with decreasing thickness except in the near vicinity of the heterointerface. In order to interpret the experimental data, the effects of different scattering mechanisms on carrier mobility have been calculated, and the influences of the lattice mismatch and surface conduction on the Hall measurements have been investigated by applying a three-layer Hall-effect model. Experimental and theoretical results suggest that the combined effects of the dislocations generated by the large lattice mismatch and strong surface inversion may lead to deceptive Hall measurements by reflecting typical n-type behavior for a p-type sample, and the measured carrier concentration may considerably be affected by the surface conduction up to near room temperature. A quantitative analysis of dislocation scattering has shown significant degradation in electron mobility for dislocation densities above 107 cm−2. The effects of dislocation scattering on hole mobility have been found to be less severe. It has also been observed that there is a critical epilayer thickness (∼1 μm) below which the surface electron mobility is limited by dislocation scattering.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A new method to prepare bicrystals with well defined planar interfaces in YBa2Cu3Ox (Y123) has been developed. The bicrystal misorientation is controlled by dual seeding using Nd1+xBa2−xCu3Oy single crystals. The grain boundary plane orientation is influenced by the positioning of the seeds and by control of the temperature gradient. The macro-, meso-, and microscopic planarity of the grain boundaries has been established by optical and electron microscopy. In addition, a difference in critical current density between the low (≤10°) and the high (≥20°) misorientation angle regime of nearly two orders of magnitude has been established in a series of [001]-tilt grain boundaries. Thus, this type of grain boundary may allow a less ambiguous interpretation of the relationship between microstructure and transport properties than is possible from bicrystal thin film boundaries by eliminating the potential variations in properties associated with a varying grain boundary plane. © 1996 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The fabrication of nonvolatile ferroelectric random access memories based on SrBi2Ta2O9 (SBT) or other ferroelectric capacitors require exposure of these capacitors to processing gases mixtures including hydrogen. This results in a strong degradation of the capacitor electrical properties, mainly due to the interaction of hydrogen with the ferroelectric layer. Using surface analysis methods, we previously determined that the hydrogen-induced degradation of SBT capacitors might be partially due to the degradation of the near surface region of the SBT layer. It was also demonstrated that oxygen annealing after the hydrogen exposure results in the recovery of the degraded SBT surface layer and the electrical properties of the capacitor. We have now performed detailed cross sectional transmission electron microscopy studies of virgin, hydrogen, and oxygen annealed SBT/Pt/TiO2/SiO2/Si heterostructures. These studies combined microstructural imaging with a nanoscale compositional analysis of the SBT layer as a function of depth from the free surface of the film and across grain boundaries. High resolution energy dispersive x-ray spectroscopy revealed that hydrogen annealed SBT layers have a deficiency of Bi up to a depth of ∼30 nm underneath the free surface, and that there is accumulation of Bi at the grain boundaries of pristine, hydrogen, and oxygen-recovery annealed SBT layers. This suggests that the diffusion of Bi is largely controlled by grain boundaries and they play a critical role in controlling the electrical properties of SBT capacitors. © 2000 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have developed a novel method to synthesize "clean'' carbon nanotubes with relatively higher yield using a hydrogen arc discharge. The quality and yield of the tubes depend sensitively on the gas pressure in the arc discharge. We have observed sharp, open-ended nanotubes with clear lattice fringes at the edges and empty interiors. The existence of these frozen-open-ended tubes in the buckybundles provides evidence for an open-ended growth model for nanotubes. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Phase-pure epitaxial Tl2Ba2Ca2Cu3O10 thin films have been grown on single crystal (110) LaAlO3 substrates using an improved metal-organic chemical vapor deposition process. First, Ba-Ca-Cu-Ox precursor films are grown on LaAlO3 (110) substrates using Ba(hfa)2⋅tet, Ca(hfa)2⋅tet, and Cu(hfa)2 (hfa=hexafluoroacetylacetonate; tet=tetraglyme) as volatile metalorganic precursors. Thallium is then incorporated into the films during a post-anneal in the presence of a Tl2O3, BaO, CaO, CuO powder mixture at 820 °C for 12 h in a flowing 10% O2/Ar atmosphere. The films have a transport-measured Tc=115 K and Jc=1.5×105 A/cm2 (80 K), while magnetic hysteresis measurements yield Jc=6×105 A/cm2 (77 K). Preliminary surface resistance measurements give Rs=0.35 mΩ at 5 K, 10 GHz.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report on a novel technique for synthesizing large quantities of single-shell carbon nanotubes in a dc arc with a composite anode of graphite and copper. Based on systematic studies involving a varying diameter and distribution of copper rods inside the graphite anode, we have correlated the presence of copper to the formation of monolayer buckytubes. We speculate that copper atoms in the discharge disrupt the orderly formation of multilayer buckytubes observed in earlier experiments.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Buckybundles (bundles of buckytubes), approximately 50 μm in diameter and 1 cm in length, have been observed in a deposited carbon rod on the cathode electrode of a dc arc. Scanning electron microscopy images clearly show an evenly spaced array of parallel buckybundles. High resolution electron microscopy reveals that each buckybundle consists of closely packed buckytubes with their axes parallel to the bundle axis. Within each bundle it is estimated that there are about 107 buckytubes with diameters in the range (20–300 A(ring)). We have measured the deposited rod growth rate as a function of the He gas pressure and have evaluated the influence of the graphite feed rod diameter on the yield of buckybundles. The magnetic susceptibilities parallel and perpendicular to the bundle axes were measured. The results show that the bundles have anisotropic diamagnetic properties.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The use of a stable glow discharge for the synthesis of carbon nanotubes is reported. Bundles of buckytubes are synthesized with remarkably large diameters (up to 200 μm). The bundles are evenly spaced, parallel, and occupy the entire central region of the deposited rod. High resolution electron microscopy (HREM) images of the deposited rod produced by the glow discharge revealed higher yield and improved quality buckytubes as compared to those produced by an arc discharge. The behavior of the two deposition modes (glow and arc) has been compared and their effects on the formation of buckytubes are discussed. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Phase-pure epitaxial thin films of the YBa2Cu3O7−δ lattice-matched, low dielectric loss perovskite insulator NdGaO3 have been grown on (110) LaAlO3 substrates by a metalorganic chemical vapor deposition (MOCVD)/post-annealing process. Amorphous Nd-Ga-O films are first prepared by MOCVD using the volatile metalorganic β-diketonate precursors Nd(dpm)3 and Ga(dpm)3 (dpm=dipivaloylmethanate). Subsequent postannealing affords phase pure, highly oriented [(001) and/or (110) orientations perpendicular to the substrate surface] and epitaxial NdGaO3 films as assessed by x-ray diffraction θ-2θ, ω, and φ scans. Cross-sectional high-resolution electron microscopic images show that the epitaxial growth occurs with atomically abrupt film-substrate interfaces and with coexisting NdGaO3 (001) and (110) orientation domains.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A carbon nitride phase with primarily sp3-bonded carbon is stabilized in CNx/ZrN superlattice coatings. Rutherford backscattering spectrometry measurements give a N/C atomic ratio ∼1.3 for the CNx layers. High-resolution transmission electron microscopy and Z-contrast imaging from the superlattice show a well-defined layer structure. Raman and near-edge x-ray absorption fine structure spectroscopy results reveal the formation of sp3-bonded carbon atoms in short-period CNx/ZrN superlattices. © 2000 American Institute of Physics.

Electronic Resource

1551-2916

Blackwell Publishing Journal Backfiles 1879-2005

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

This study investigates thermal mismatch stresses in the lamellar microstructure of the solid solution directionally solidified eutectic (DSE) oxide Co1−xNixO/ZrO2(CaO). X-ray and neutron diffraction measurements were performed on isolated eutectic domains to measure the residual strain and stress tensors. Maximum principal residual stresses on the order of 1 GPa were recorded, with the Co1−xNixO and the ZrO2(CaO) phases maintaining states of tensile and compressive stress, respectively. The stress tensors for these materials are compared with measurements for similar DSE oxide systems and suggest that solid solution DSEs might be used to tailor the residual stress states in DSE composites.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Infinite-layer samples with the nominal chemical formula (Sr,_JCCaJ〉,CuO2 (j〉 = 0.9, 1.0 and 1.1; jt = 0.3) were prepared using pre-synthesized precursors of low-pressure forms of (Sr,_,Cax)CuO2 with additions of CuO and (Sr,_xCa,)2CuO3, followed by a high-pressure treatment. The synthesis ...

Electronic Resource

1573-4870

SrTiO3 ; grain boundary ; embedded cluster method ; DV-Xα method

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract We have performed atomic-level first principle electronic structure calculations on doped grain boundaries (GB) in SrTiO3. This was motivated by the electron holography experiments, which were able to quantify the electrostatic potential and the associated space charge distribution across the Mn-doped GB in this material. The embedded cluster Discrete Variational (DV)-Xα method was used to determine the charge and the densities of states for several idealized models of a single crystal and symmetrical tilt grain boundaries in SrTiO3. Special attention was given to the role of Mn+2 and Mn+3 acceptors substituting for Ti+4 resulting in charge segregation across the grain boundaries, which was shown in the electron holography experiments. We have found that Mn replacing Ti prefers to have valence charge around +2 and this picture agrees with the experimental observation of negative grain boundary charges in the GB core.

Electronic Resource