S. S. Bhattacharya

Size effect on the lattice parameters of nanocrystalline anatase

Crystallite size dependence on the lattice parameters of nanocrystalline anatase was examined by x-ray diffraction pattern analysis, x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The lattice parameter a increased while the parameter c decreased with reducing crystallite sizes. However, the cell volume increased with decreasing crystallite size indicating an overall expansion due to confinement effect. The observed shift in Eg Raman mode (at 144 cm−1) was in accordance with an empirical phonon confinement model while no substoichiometric titania could be detected by XPS. It was concluded that the lattice expansion was purely due to electrostatic relaxation as a result of size confinement.

Microstructure and mechanical properties of Sc modified Al–Cu alloy (AA2219) electron beam welds

Abstract Al alloy AA2219 is a high strength alloy belonging to the 2000 series Al alloys widely used for aerospace application. One of the drawbacks of most of the high strength Al alloys is that they suffer from poor weldability. However AA2219 is an exception due to the presence of more Cu that helps in healing cracks by providing extra eutectics. Although AA2219 has excellent weldability the strength of a welded joint is only 35–45% of the base metal. The loss of strength is due to the dissolution of the strengthening precipitates during melting. Therefore, there is a need to improve the fusion zone strength of AA2219 welds. In this study an attempt was made experimentally to use electron beam welding and modify the base metal chemistry with addition of Sc and Zr in order to improve the mechanical properties of the AA2219 weld joints. Fine equiaxed microstructure was obtained in the fusion zone of electron beam welds with improved mechanical properties. Eutectic was found to be distributed discontinuously which resulted in improved yield strength, tensile strength and ductility compared to welds made without Sc and Zr.

Synthesis, characterization and sintering of nanocrystalline titania powders produced by chemical vapour synthesis

The chemical vapour synthesis (CVS) route is a versatile process that can be used for the synthesis of nanocrystalline ceramics with very small crystallite sizes having a narrow particle size distribution. In this study, a CVS technique was used to prepare nanocrystalline titania from tetraisopropyl orthotitanate at a processing temperature of 1273 K (1000 °C). High resolution transmission electron microscopy, x-ray diffraction and nitrogen adsorption techniques were used for the characterization of the as-synthesized powders. Green bodies were produced by a combination of uniaxial and cold isostatic pressing, which were then sintered. A simple pressureless sintering route was established that led to the production of a dense titania ceramic with a uniform microstructure and an average grain size well in the nanophase regime.

Integration of perovskite PZT thin films on diamond substrate without buffer layer

Integration of lead zirconate titanate (PZT) thin film on diamond substrate offers a great deal of potential for the application of multifunctional devices under extreme conditions. However, fabrication of perovskite PZT thin films on diamond substrate without a buffer layer has not been realized to date. We report for the first time on the successful deposition of PZT thin film directly on a diamond substrate without any buffer layer using the pulsed-laser deposition technique. The perovskite phase was realized only under specific growth conditions. X-ray diffraction and Raman studies confirmed the perovskite phase. The ferroelectric behaviour of the deposited PZT thin film was confirmed using piezo response microscope phase image and ferroelectric hysteresis loop.

Multicomponent equiatomic rare earth oxides

ABSTRACT Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF2-type (Fm-3 m) structure. The addition of more than six elements resulted in the formation of a secondary phase. Annealing at 1000°C for 1 h led to the formation of a single-phase (Ia-3) even in the 7-component system. In the absence of cerium (Ce4+), secondary phases were observed irrespective of the number of cations or the extent of thermal treatment indicating that cerium cations played a crucial role in stabilizing the multicomponent REOs into a phase pure structure. GRAPHICAL ABSTRACT IMPACT STATEMENT Multicomponent equiatomic rare earth oxides pioneer a new group of materials that crystallize into a single-phase structure with the dominant role of a single element instead of entropy.

Effect of process parameters on the chemical vapour synthesis of nanocrystalline titania

In this investigation nanocrystalline titania powders were produced by the chemical vapour synthesis (CVS) route and characterized by standard techniques of XRD, HRTEM and BET. The effects of precursor/gas flow rates, hotwall temperature and system pressure on the particle size and distribution, as well as phase composition of the synthesized nanocrystalline titania powder, were studied. It was demonstrated that by suitably adjusting the process parameters during CVS it becomes possible to control the anatase crystallite size, specific surface area and the rutile content in the synthesized nanocrystalline titania powders.

A generic analysis for high-temperature power-law deformation: the case of linear In (strain rate)-In(stress) relationship

Isostructural low-stress high-temperature deformation of different classes of materials is often represented by a power law that connects the strain rate to the flow stress through a stress exponent. The temperature dependence of the rate of deformation is assumed to be exponential. In Mukherjeeet als popular approach the temperature dependence of the stress exponent is ignored by assuming a mean value for the stress exponent for the temperature range of interest and the stress is normalized with respect to the elastic constant. In the approach adopted by the engineers the stress is normalized with respect to a reference stress and it is possible to take into account the temperature dependence of the stress exponent while evaluating the activation energy for the rate-controlling process. Experimental data pertaining to 27 systems drawn from metals and alloys, superalloys, ceramics, glass ceramics, metal-matrix composites and an intermetallic, have been analysed using the latter approach to determine an activation energy for the rate-controlling process. It is demonstrated that this is an accurate description of high-temperature power-law deformation and that it involves less numbers of empirical constants than the former approach.

Spark Plasma Welding of Austenitic Stainless Steel AISI 304L to Commercially Pure Titanium

Abstract In modern fast-breeder nuclear power plants, on-site fusion welding of austenitic stainless steel AISI 304L and commercially pure titanium pipes is called for. The application imposes stringent requirements on these joints in terms of tensile strength, bend ductility, and corrosion resistance in boiling nitric acid. However, fusion welding of stainless steel to titanium directly is a near impossibility because of inevitable brittle intermetallic formation between iron and titanium and, consequently, serious weld cracking problem. All things considered, the best way to realize these joints is to make use of solid-state welded stainless steel/titanium (SS/Ti) dissimilar pipe joining inserts or adaptors. Towards this end, many solid-state welding techniques such as diffusion bonding, friction welding, and explosive welding have been tried in past, but without much success. In the current study, solid-state welding of SS rods to Ti rods was attempted in spark plasma sintering equipment. The potential of this new process, termed spark plasma welding, for dissimilar metal welding are discussed.

Growth and characterization of diamond particles, diamond films, and CNT-diamond composite films deposited simultaneously by hot filament CVD

It is important to understand the growth of CNT-diamond composite films in order to improve the inter-link between two carbon allotropes, and, in turn, their physical properties for field emission and other applications. Isolated diamond particles, continuous diamond thin films, and thin films of carbon nanotubes (CNTs) having non-uniformly distributed diamond particles (CNT-diamond composite films) were simultaneously grown on unseeded, seeded, and catalyst pre-treated substrates, respectively, using a large-area multi-wafer-scale hot filament chemical vapor deposition. Films were deposited for four different growth durations at a given deposition condition. The changes in surface morphology and growth behavior of diamond particles with growth duration were investigated ex situ using field emission scanning electron microscopy and 2D confocal Raman depth spectral imaging, respectively. A surface morphological transition from faceted microcrystalline nature to nanocrystalline nature was observed as a function of growth duration in the case of isolated diamond particles grown on both unseeded and catalyst pre-treated substrates. However, such a morphological transition was not observed on the simultaneously grown continuous diamond thin films on seeded substrates. 2D confocal Raman depth spectral imaging of diamond particles showed that the local growth of CNTs did not affect the growth behavior of neighboring diamond particles on catalyst pre-treated substrates. These observations emphasize the importance of surface chemical reactions at the growth site in deciding sp2 or sp3 carbon growth and the final grain size of the diamond films.

Structure, thermal stability, and optical properties of boron modified nanocrystalline anatase prepared by chemical vapor synthesis

Boron modified nanocrystalline anatase titania powders with boron contents varying from 0.5 to 6.2 wt % were synthesized by a chemical vapor synthesis process. High temperature x-ray diffraction studies revealed that the anatase powders containing more boron were more stable at higher temperatures. When present in small quantities (about 0.5 wt % or less), boron went into the structure of titania and lowered the direct band gap to 3.41 eV. On the other hand, higher boron contents resulted in smaller anatase crystallite sizes and partially offset the redshift. However, the excess boron formed boron oxide which probably coated the surface of the nanoparticles/crystallites resulting in improved high temperature stability of the anatase phase.