Department of Metallurgical & Materials Engineering

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IIT Jodhpur

Guanidinium lead iodide (GAPbI3) has been synthesized experimentally, but stability remains an issue, which can be modulated by the insertion of bromine (Br) into the system. We have performed a systematic theoretical investigation to see how bromination can tune the stability of GAPbI3. The optical properties were also determined, and we have found formation enthalpy-based stability in the perovskite systems, which are active in the visible and IR region even after bromine insertion and additionally more active in the IR range with the transition from GAPbI3 to GAPbBr3. The spin orbit coupling effect is considered throughout the band structure calculations. The ensemble of the primary and secondary gaps in the half and fully brominated hybrid perovskites leads to the phenomenon of a multipeak response in the optical spectra, which can be subsequently attributed as multivalley optical response behaviour. This multivalley optical behaviour enables the brominated guanidinium-based hybrid perovskites to exhibit broad light harvesting abilities, and this can be perceived as an idea for natural multi-junction solar cells. © 2017 The Royal Society of Chemistry.

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Journal of Materials Chemistry A

Bromination-induced stability enhancement with a multivalley optical response signature in guanidinium [C (NH 2) 3]+-based hybrid perovskite solar cells

We report a versatile route for the preparation of metal phosphides using PH3 plasma for supercapacitor applications. The high reactivity of plasma allows rapid and low temperature conversion of hydroxides into monometallic, bimetallic, or even more complex nanostructured phosphides. These same phosphides are much more difficult to synthesize by conventional methods. Further, we present a general strategy for significantly enhancing the electrochemical performance of monometallic phosphides by substituting extrinsic metal atoms. Using NiCoP as a demonstration, we show that the Co substitution into Ni2P not only effectively alters the electronic structure and improves the intrinsic reactivity and electrical conductivity, but also stabilizes Ni species when used as supercapacitor electrode materials. As a result, the NiCoP nanosheet electrodes achieve high electrochemical activity and good stability in 1 M KOH electrolyte. More importantly, our assembled NiCoP nanoplates//graphene films asymmetric supercapacitor devices can deliver a high energy density of 32.9 Wh kg−1 at a power density of 1301 W kg−1, along with outstanding cycling performance (83% capacity retention after 5000 cycles at 20 A g−1). This activity outperforms most of the NiCo-based materials and renders the NiCoP nanoplates a promising candidate for capacitive storage devices. © 2017 Elsevier Ltd

Nano Energy

Low temperature synthesis of ternary metal phosphides using plasma for asymmetric supercapacitors

Chip formation ability and chip characteristics during machining in as-cast as well as friction stir processed hypereutectic Al-Si alloy are investigated. The effect of microstructural refinement by friction stir processing on chip size, shape and morphology are characterized using optical microscopy and scanning electron microscopy techniques. It was observed that with the increase in number of FSP passes morphology of chips transformed from an irregular brittle powder to regular curly ductile chips with the corresponding decrease in the surface roughness values, thus enhancing the machinability. The change of chip behavior in cast and that after FSP was primarily due to the reduction in diameter and uniform redistribution of primary silicon. © 2016 The Society of Manufacturing Engineers.

Journal of Manufacturing Processes

Enhancing the machinability of hypereutectic Al-30Si alloy by friction stir processing

Annealing behavior of Al-Mg-Sc-Zr alloy processed by equal-channel angular pressing (ECAP) at 325°C to various strains and undergone subsequent cold rolling to ∼80% reduction was investigated. ECAP to strain, e ∼3 formed partially recrystallized structure with ultrafine-grain (UFG) mantle zone along the boundaries of initial grains of ∼25 μm size in as-cast and homogenized condition, whilst ECAP to e ∼8 and 10 resulted in nearly 1 μm grain size, with their volume fractions being 0.7 and 0.9, respectively. These structures after ECAP exhibited sufficient stability, within normal grain growth up to ∼5 μm grain size only during annealing for 1 h in the temperature range of 350-520°C. However, subsequent to ECAP cold rolling and further annealing at 425°C and above revealed a distinct effect of prior ECAP. While the materials undergone ECAP to e ∼3 and e ∼8 and having lower UFG fractions revealed quite similar behavior, the material undergone ECAP to e ∼10 (and nearly complete recrystallization at ECAP) exhibited abnormal grain growth. The sources of microstructure evolution at different stages of processing are analyzed along with their effects on hardness at ambient temperature. © 2016 Elsevier B.V.

Journal of Alloys and Compounds

Microstructural evolution in Al-Mg-Sc-Zr alloy during severe plastic deformation and annealing

Here we demonstrate a method to tune a ferroelectric imprint, which is stable in time, based on the coupling between the non-switchable polarization of ZnO and switchable polarization of PbZrxTi(1-x)O3. SrRuO3/PbZrxTi(1-x)O3/ZnO/SrRuO3 heterostructures were grown with different ZnO thicknesses. It is shown that the coercive voltages and ferroelectric imprint vary linearly with the thickness of ZnO. It is also demonstrated that the ferroelectric imprint remains stable with electric field cycling and electric field stress assisted aging. © 2016 Author(s).

Fulltext

APL Materials

Tunable and stable in time ferroelectric imprint through polarization coupling

Synthesis route can have a significant influence on the process of phase-choice and -evolution in High Entropy Alloys (HEAs). With the objective of instituting awareness, this communication on the foundation of phase diagrams, adopted using the CALPHAD approach, attempts at describing and deciphering the cause for such gradation. © 2016 Elsevier Ltd. All rights reserved.

Intermetallics

Phase-evolution in high entropy alloys: Role of synthesis route

The Hf-40 wt pct Ti (Hf-Ti) alloy was developed for neutron poison application in the spent nuclear fuel reprocessing plant. The furnace-cooled Hf-Ti sample exhibited the microstructure comprising equiaxed-α, lamellar-α, and feathery-α. The water-quenched Hf-Ti sample confirmed the presence of lath and internally twinned martensite. In comparison to the furnace-cooled sample, low corrosion current density and passivation current density values obtained for the water-quenched Hf-Ti in 6 M HNO3 at 298 K (25 °C) indicated better passivation ability. The martensitic structure exhibited high hardness (660 HV) and negligible corrosion rate in 6 M nitric acid at 298 K (25 °C). X-ray photoelectron spectroscopic (XPS) analysis confirmed that passivation behavior of this alloy was due to the protective passive film composed of TiO2 and HfO2. © 2016, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Microstructure and Corrosion Behavior of Hf-40 Wt Pct Ti Alloy in Nitric Acid Medium for Reprocessing Applications

The well known localized surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) supported on a dielectric substrate depends on the particle shape, size and type of dielectric material. The particle size and shape mainly vary with the method of preparation and the parameters involved there in. In this report, we show preparation of AuNPs supported on quartz substrate by direct current sputtering followed by thermal annealing at an optimized temperature of 400 °C. The samples were characterized using optical absorption spectra, scanning electron microscopy (SEM) and the energy dispersive x-ray spectrum. The LSPR position could be tuned by varying annealing temperature. The LSPR was found to be blue shifted up to 10 nm with annealing temperature varying from 400 °C to 800 °C. The change in LSPR was ascribed to the morphology of AuNPs over quartz. © 2016 Author(s).

AIP Conference Proceedings

Temperature dependent localized surface plasmon resonance properties of supported gold nanoparticles

We solve the transport equations of the electrons and phonons to understand the thermoelectric behaviour of the technologically important half-Heusler alloys MNiSn (M: Ti, Zr, Hf). Doping is simulated within the rigid band approximation. We clarify the origin of the electron dominated thermoelectric response and determine the carrier concentrations with maximal figures of merit. The phonon mean free path is studied to calculate the grain size below which grain refinement methods can enforce ballistic heat conduction to enhance the figure of merit. © The Owner Societies 2016.

Physical Chemistry Chemical Physics

Electron dominated thermoelectric response in MNiSn (M: Ti, Zr, Hf) half-Heusler alloys

We present the first report in which the thermoelectric properties of two-dimensional MXenes are calculated by considering both the electron and the phonon transport. Specifically, we solve the transport equations of the electrons and phonons for three MXenes, M2CO2, where M = Ti, Zr, or Hf, in order to evaluate the effect of the metal M on the thermoelectric performance. The lattice contribution to the thermal conductivity, obtained from the phonon lifetimes, is found to be lowest in Ti2CO2 and highest in Hf2CO2 in the temperature range from 300 to 700 K. The highest figure of merit is predicted for Ti2CO2. The heavy mass of the electrons due to flat conduction bands results in a larger thermopower in the case of n-doping in these compounds. © 2016 American Chemical Society.