Phase Separation in Lix_xFePO4_4 Induced by Correlation Effects

We report on a significant failure of LDA and GGA to reproduce the phase stability and thermodynamics of mixed-valence Li$_x$FePO$_4$ compounds. Experimentally, Li$_x$FePO$_4$ compositions ($0 \leq x \leq 1$) are known to be unstable and phase separate into Li FePO$_4$ and FePO$_4$. However, first-principles calculations with LDA/GGA yield energetically favorable intermediate compounds an d hence no phase separation. This qualitative failure of LDA/GGA seems to have its origin in the LDA/GGA self-interaction which de localizes charge over the mixed-valence Fe ions, and is corrected by explicitly considering correlation effects in this material. This is demonstrated with LDA+U calculations which correctly predict phase separation in Li$_x$FePO$_4$ for $U-J \gtrsim 3.5$eV. T he origin of the destabilization of intermediate compounds is identified as electron localization and charge ordering at different iron sites. Introduction of correlation also yields more accurate electrochemical reaction energies between FePO$_4$/Li$_x$FePO$_ 4$ and Li/Li$^+$ electrodes.Comment: 12 pages, 5 figures, Phys. Rev. B 201101R, 200

Experimental performance of compact UWB antenna for breast cancer screening

This paper presents experimental performances of the algorithm used for detection of malignant tissue in breast. The algorithm uses ultra-short pulse radar (USPR) technique along with confocal microwave imaging (CMI) algorithm based on synthetic aperture radar (SAR) to detect tumor cells or tissue. A compact Vivaldi antenna having ultra-wideband (UWB) performance with bandwidth 3 GHz to 8 GHz is used as transceiver and an impulse with no distortion is used as an excitation signal in our experiment. We used various synthetic breast phantom and metallic or high-k dielectric balls as tumor. The skin subtraction method using Teflon cover seems to be promising for this experiment

Development of a Limb prosthesis by reverse mechanotransduction

Recent developments in the field of limb prosthesis have focused on the use of body signals of the user to generate the desired motion in the prosthesis. Unlike earlier designs, this approach is more effective and less stressful for the amputee. The signals that have been used up till now are EMG signals, EEG signals and neural signals. Another possible source of body signal is the pH value of the neuromuscular junction, which depends upon the ion movements across the muscle tissue. Hence, it is safe to assume that changes in the pH can accurately mimic the intended changes in the amputated limb muscles, and therefore can be used to turn the user’s desired motion into actual motion of the limb prosthesis. In the current model, this is achieved through the means of a pH-to-voltage converter that converts the pH value into voltage that is in turn used to drive the motor. The direction of movement is controlled by a microcontroller-based circuit. Further improvements can be made upon the model presented in this thesis, if the pH values could be more accurately read and employed to determine the direction of the movement of the finger too. Also, attempts can be made to apply the same working principle on more complex models of hand prosthesis, thus producing more applicable results

Surface modification of 316L stainless steel by Sol-Gel method

Medical implants made from 316L stainless steel have been used widely and successfully for various types of dental and orthopaedic implants. It is believed that oxide films covering implant surfaces are of crucial importance for biocompatibility and successful osseo-integration. A uniform TiO2 coating on 316L stainless steel coupons has been prepareds using sols-gel methodology followed by hydrothermals post-treatmentsk. Sol-gel process is one of the best wet chemical routes to deposit the coating with convenient production of required thin films and controlled distribution of pore-size, processed at relatively lower temperature than any other conventional coating methods. Different coating thicknesses have been achieved by coating TiO2 sol-gel on samples having different surface finish. The structure and morphology of thef coatingsr weref analyzedh using optical microscope and X-rayf diffractionh technique. When the samples are observed under optical microscope, the results indicate that TiO2 sol-gel coating on 316l stainless steel is uniform and dense; we can see a crack-free surface. The XRD patterns show different strong peaks of TiO2 coating on the substrate. Bioactivity study of the coated samples is done by dipping them in SBF solution. We observe firmly attached hydroxy apatite layer on the TiO2 coated substrate and XRD patterns show the presence of calcium phosphate which increases the osseo-integration. The surface free energy have been calculated for the steel samples having different surface finish and also the contact angle has been measured. Ground samples show the best characteristics with enhanced roughness

Size-dependent spinodal and miscibility gaps for intercalation in nano-particles

Using a recently-proposed mathematical model for intercalation dynamics in phase-separating materials [Singh, Ceder, Bazant, Electrochimica Acta 53, 7599 (2008)], we show that the spinodal and miscibility gaps generally shrink as the host particle size decreases to the nano-scale. Our work is motivated by recent experiments on the high-rate Li-ion battery material LiFePO4; this serves as the basis for our examples, but our analysis and conclusions apply to any intercalation material. We describe two general mechanisms for the suppression of phase separation in nano-particles: (i) a classical bulk effect, predicted by the Cahn-Hilliard equation, in which the diffuse phase boundary becomes confined by the particle geometry; and (ii) a novel surface effect, predicted by chemical-potential-dependent reaction kinetics, in which insertion/extraction reactions stabilize composition gradients near surfaces in equilibrium with the local environment. Composition-dependent surface energy and (especially) elastic strain can contribute to these effects but are not required to predict decreased spinodal and miscibility gaps at the nano-scale