M S Ramachandra Rao

We investigated the formation of extended defects in CaFeO2.5, predominantly appearing as antiphase boundaries (APBs), as a function of the synthesis method and temperature. While CaFeO2.5 is known to adopt an ordered oxygen defect structure showing long range order of the (FeO 4)∞ chains in its bulk form, interestingly, we demonstrated that the length of these (FeO4)∞ chains can be considerably scaled down to few nanometers by adopting a modified sol-gel method (low temperature synthesis) while the grain size of the resulting nano-phase CaFeO2.5 is around 50 nm. We discuss the synthesis dependent modulation of the length of APBs, characterized by X-ray diffraction and high resolution TEM, to be at the origin of an amplified switching dynamics of the (FeO4)∞ chains. This can accordingly explain the reduction of the onset temperature for oxygen diffusion to set in from 450 °C for bulk-CaFeO2.5 to 320 °C for nano-CaFeO2.5, as determined by 18O/16O oxygen isotope exchange reactions. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effect of doping of transition metal ions (Fe and Co) on transport properties of ZnO has been studied in both bulk and thin films. The solubility limit of these ions have been found to be higher in thin films compared to bulk. Optical measurements reveal the presence of Fe in both 2+ and 3+ state. Co is believed to be in 2+ states. Electrical resistivity measurements show that while for bulk Fe doped ZnO samples there is a decrease in ρ (T) compared to undoped ZnO, it increases for bulk Co doped ZnO samples. However, thin film samples of both types of doped compounds show a decrease in ρ (T) compared to undoped ZnO. This difference in bulk and thin film behaviour has been explained on the basis of experimental results. © 2007 Materials Research Society.

Electronic phase separation in Er3+-doped La0.8Sr0.2MnO3 was investigated. Er3+ doping showed an increase in Curie/peak resistivity temperature (TC/TP) above a certain nominal dopant concentration. It was found that with increase in TP the overall resistivity of the compounds also decreases.

We report the effect of cobalt ferrite (CoFe O 2 4) nanoparticles on dielectric and ferroelectric properties of poly(vinylidene fluoride)(PVDF). Large enhancements in dielectric constant and remanent polarization upon addition of 5 wt.% CoFe O 2 4 nanoparticles to PVDF have been observed. Nearly 17% increase of dielectric constant and 31% increase in remanent polarization was observed in PVDF-CoFe O 2 4 nanocomposite films as compared to pristine PVDF films prepared under identical conditions. These enhancements were accompanied by a decrease in crystallinity and an increase in electroactive β-phase content as evidenced by x-ray diffraction and fourier transform infrared spectroscopy, respectively. A 5% increase in the amount of β-phase content is also observed in nanocomposite films. These enhancements in dielectric constant and remanent polarization are attributed to the influence of CoFe O 2 4 nanoparticles on nucleation, morphology, dipole aligning and dipole pinning behavior of PVDF.

This work investigates the high-pressure structure of freestanding superconducting (Tc=4.3 K) boron-doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30 GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients, and the grain boundary components undergo an irreversible phase change at 14 GPa. We show that the blueshift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of Tc in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of B0=510±28 GPa. The comparative analysis of high-pressure data clarified that the sp2 carbons in the grain boundaries transform into hexagonal diamond.

Nanocrystalline CrN coatings, widely required for surface engineering application covering wear and corrosion resistance, need to be investigated for atomic scale morphology, surface roughness, local stiffness, phase uniformity, and homogeneity. Evolution of these properties as a function of thickness need to be studied. In this paper, we have attempted to address these issues through use of a multimode scanning probe microscope (SPM) equipped to carry out Atomic Force Microscopy (AFM) and Atomic Force Acoustic microscopy (AFAM) of Chromium nitride films (100-500 nm thick) on Si prepared under high vacuum by pulsed Laser Ablation using Nd-YAG Q-switched laser. Prior to SPM analysis, the coatings were annealed in N 2 atmosphere at 700°C for 30 minutes for improving crystallanity and coating substrate adhesion. The GIXRD patterns of these annealed specimens showed formation of nanocrystalline CrN. Also signature of amorphous phases was seen. The grain size was estimated to be less than 30 nm. Contact mode AFM imaging revealed a roughness value less than 50 nm. Local stiffness values were calculated from AFM force-distance curves. Imaging of frictional force and surface flaws are being investigated by Frictional Force Microscopy (FFM), resonance spectroscopy, and AFAM, respectively. The contrast in AFAM images is seen due to variation in surface elasticity in reference and CrN samples. Stiffness constant and elastic modulus were calculated for both the samples and compared. Copyright © 2007 American Scientific Publishers All rights reserved.

Realization of diverse functionalities by tuning magnetic interactions in rare-earth perovskite oxide thin films opens up exciting technological prospects. Strain-induced tuning of magnetic interactions in rare-earth cobaltates and nickelates is of central importance due to their versatility in electronic transport properties. Here we report the spin reorientation induced switching of anisotropic magnetoresistance (AMR) and its tunability with strain in epitaxial LaCo0.5Ni0.5O3-δ thin films across the ferromagnetic transition. Moreover, with strain tuning, we observe a twofold to fourfold symmetry crossover in AMR across the magnetic transition temperature. The magnetization measurements reveal a ferromagnetic transition around 50 K. At temperatures below this transition, there is a subtle change in the magnetization dynamics, which reduces the ferromagnetic long-range ordering in the system. X-ray absorption and x-ray magnetic circular dichroism spectroscopy measurements at the Co and Ni L edges reveal a Co spin state transition below 50 K, leading to the AMR switching and also the presence of Ni2+ and Co4+ ions evidencing the charge transfer from Ni to Co ions. Our work demonstrates the tunability of magnetic interactions mediated electronic transport in cobaltate-nickelate thin films, which is relevant in understanding Ni-Co interactions in oxides for their technological applications such as in AMR sensors.

Dielectric materials with permittivity higher than that of SiO2 are becoming increasingly important in ULSI technology as they allow the physical thickness of the gate dielectric to be higher while maintaining the same gate capacitance as in a conventional MOS process. This reduces the gate leakage current, which has become a serious problem in modern MOSFETs with a gate oxide thickness less than 4 nm. In this paper we present the characteristics of MTOS capacitors with a Ta2O5/SiO2 stacked dielectric layer and compare the results with MOS capacitors having an equivalent dielectric thickness. It is shown that the leakage current is considerably smaller and the breakdown voltage significantly higher in the MTOS devices.

This work is focused on the development of a dual-layered diamond-coated tungsten carbide tool for machining titanium Ti-6Al-4V alloy. A hot-filament chemical vapor deposition technique was used to synthesize diamond films on tungsten carbide tools. A boron-doped diamond interlayer was added to a microcrystalline diamond layer in an attempt to improve the interface adhesion strength. The dual-layered diamond-coated tool was employed in machining at cutting speeds in the range of 70 to 150 m min-1 with a lower feed and a lower depth of cut of 0.5 mm rev-1 and 0.5 mm, respectively, to operate in the transition from adhesion- to diffusion-tool-wear and thereby arrive at suitable conditions for enhancing tool life. The proposed tool was then compared, on the basis of performance under real-time cutting conditions, with commercially available microcrystalline diamond, nanocrystalline diamond, titanium nitride and uncoated tungsten carbide tools. The life and surface finish of the proposed dual-layered tool and uncoated tungsten carbide were also investigated in interrupted cutting such as milling. The results of this study show a significant improvement in tool life and finish of Ti-6Al-4V parts machined with the dual-layered diamond-coated tool when compared with its uncoated counterpart. These results pave the way for the use of a low-cost tool, with respect to, polycrystalline diamond for enhancing both tool life and machining productivity in critical sectors fabricating parts out of titanium Ti-6Al-4V alloy. The application of this coating technology can also be extended to the machining of non-ferrous alloys owing to its better adhesion strength.