Synthesis of cobalt-doped barium cerate-zirconate and its evaluation for hydrogen production and electrochemical characterization

Cobalt-doped barium cerate-zirconate was synthesized using an oxalate co-precipitation route. The material was characterized using X-ray diffraction, transmission and scanning transmission electron microscopy coupled with X-ray energy dispersive spectroscopy. Results indicated that homogeneous cubic phase material was obtained at very high heat-treatment temperatures. Catalytic activity of the material toward CH₃OH partial-oxidation was tested at different temperatures and O₂:CH₃OH ratios. High hydrogen yields were obtained indicating that the material was a suitable catalyst for hydrogen generation. Impedance spectroscopy tests were conducted at different conditions to understand conduction processes occurring in the material. Results suggested mixed protonic–electronic conductivity in the presence of hydrogen. Thus, material is a potential candidate for the bi-functional role of electro-ceramic catalyst for simultaneous hydrogen generation and purification.     

Glass forming ability and stability: Ternary Cu bearing Ti,Zr, Hf alloys

Four Cu bearing alloys of nominal composition Zr₂₅Ti₂₅Cu₅₀, Zr₃₄Ti₁₆Cu₅₀, Zr₂₅Hf₂₅Cu₅₀ and Ti₂₅Hf₂₅Cu₅₀ have been rapidly solidified in order to produce ribbons. All the alloys become amorphous after melt-spinning. In the Zr₃₄Ti₁₆Cu₅₀ alloy localized precipitation of cF24 Cu₅Zr phase can be observed in the amorphous matrix. The alloys show a tendency of phase separation at the initial stages of crystallization. The difference in crystallization behavior of these alloys with Ni bearing ternary alloys can be explained by atomic size, binary heat of mixing and Mendeleev number. It has been observed that both Laves and Anti-Laves phase forming compositions are suitable for glass formation. The structures of the phases, precipitated during rapid solidification and crystallization can be viewed in terms of Bernal deltahedra and Frank–Kasper polyhedra.     

Alloying behaviour,thermal stability and phase evolution in quinary AlCoCrFeNi high entropy alloy

An equiatomic quinary AlCoCrFeNi high entropy alloy (HEA) has been synthesized by mechanical alloying. Milled powder after 30?h shows good chemical homogeneity and refined morphology with a mean particle size of ～4?μm. Solid solution phase with BCC crystal structure (a?=?2.89?±?0.02?Å) has been confirmed from XRD and transmission electron microscopy in the as-synthesized high entropy alloy. The milled alloy powder is not thermally stable. Differential scanning calorimetric (DSC) thermogram of 30?h milled powder exhibits the presence of a small peak at ～600?°C (873?K) with a thermal shift near the peak. This thermal shift indicates the diffusive type of phase transformation in this alloy while heating. The analysis of the in-situ heating X-ray diffraction patterns at various temperatures extends support to the diffusive nature of the phase transformation. Upon heat treatment, the disordered BCC solid solution phase partially transforms to Ni₃Al prototype L1₂ phase which precipitates at a lower temperature (350?°C (623?K)) as observed by in-situ XRD experiments. However, at high temperature annealing (575–800?°C (848–1073?K)) the evolution of a partially ordered BCC phase (B2) with lattice parameter (a?=?2.87?±?0.02?Å), and L1₂ phase (a?=?3.58?±?0.05?Å), along with tetragonal σ phase (a?=?8.8?Å and c?=?4.53?Å) are observed. Similar types of phases have also been identified after annealing and microwave sintering at 800?°C (1073?K) & 900?°C (1173?K) respectively. The transformation of ordered BCC phases along with two intermetallics such as L1₂ phase and σ phase suggests that the evolution of the high entropy phase in the milled condition leads to a combination of high entropy and medium entropy phases in the annealed condition.     

Homogeneous and polymorphic transformations to ordered intermetallics in nanostructured Au–Cu multilayer thin films

Journal of Materials Science - Atomic arrangements in the nanostructured grains and interfaces of thermally evaporated Au/Cu multilayer thin films on polycrystalline Si substrate have been explored...     

Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices

We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5-0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50-60% were also obtained.     

Single-step growth dynamics of core–shell GaN on Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> freestanding nanoprotruded microbelts

Freestanding wurtzite GaN nanoprotruded microbelts with Ga₂O₃ core, with typical thickness 1–10 μm, and length of few millimeters are synthesized by thermal annealing of Ga metal and subsequent reaction with ammonia at a low flow rate. They are of distinctive rectangular shape with a typical width of 10–100 μm. Thickness of the belt is about 1/10th of the width and length up to a few millimeters. The GaN, Ga₂O₃ layers and the GaN–Ga₂O₃ interface are characterized by high-resolution transmission electron microscopy after focused ion beam sectioning of the belt. Initially, Ga₂O₃ nucleates after reaction with the O₂ available in the environment, and subsequent reaction with NH₃ results in the formation of core–shell structure in the catalyst-free vapor–solid growth process. Having a low-symmetry phase, Ga₂O₃ can grow only in certain preferred directions thus controlling the final morphology of the belt. Nanoscale protrusions ~50–100 nm found on the surface of the belts could be an ideal system for building functional devices.     

Bulk metallic glasses: A new class of engineering materials

Bulk glass-forming alloys have emerged over the past fifteen years with attractive properties and technological promise. A number of alloy systems based on lanthanum, magnesium, zirconium, palladium, iron, cobalt and nickel have been discovered. Glass-forming ability depends on various factors like enthalpy of mixing, atomic size and multicomponent alloying. A number of processes is available to synthesise bulk metallic glasses. The crystallisation behaviour and mechanical properties of these alloys pose interesting scientific questions. Upon crystallisation many of these glasses transform to bulk nanocrystals and nanoquasicrystals. A detailed study of the structure and the crystallisation behaviour of glasses has enabled the elucidation of the possible atomic configuration in liquid alloys. Their crystallisation behaviour can be exploited to synthesise novel nanocomposite microstructures and their mechanical properties can be enhanced. A broad overview of the present status of the science and technology of bulk metallic glasses and their potential technological uses is presented.     

Microstructure and interfacial chemistry of pure and La‐doped BiFeO3 thin films

We report on the microstructure and interfacial chemistry of thin films of pure and La‐doped multiferroic bismuth ferrite (Bi_1‐xLa_xFeO₃ or BLFO), synthesized on Indium Tin Oxide‐coated glass substrates by solution‐deposition technique and studied using scanning transmission electron microscopy. Our results show that undoped and La‐doped thin films are polycrystalline with distorted rhombohedral structure without any presence of any line or planar defect in the films. In addition, the films with La doping did not show any structural change and maintain the equilibrium structure. Cross section compositional analysis using X‐ray energy dispersive spectrometry did not reveal either any interdiffusion of chemical species or formation of reaction product at the film‐substrate interface. However, a closer examination of the microstructure of the films shows tiny pores along with the presence of ～2–3 nm thin amorphous layers, which may have significant influence on the functional properties of such films. Microsc. Res. Tech. 76:1304–1309, 2013. © 2013 Wiley Periodicals, Inc.     

Thermodynamic Basis of Non-equilibrium Phase Transformations of bcc β-Phase in Ti–Mo System

This study reports the experimental identification of transformation products of high temperature bcc β phase in Ti–xMo (x = 1, 7, 15, 25 wt%) alloys and aims to understand the transformations by thermodynamic modelling. The high temperature bcc β phase had undergone martensitic transformation in Ti–1Mo and Ti–7Mo alloys, resulting in acicular martensitic structure within large β grains. X-ray diffraction (XRD) analysis confirmed the martensite to be hcp (α′) and orthorhombic (α″) in Ti–1Mo and Ti–7Mo alloys respectively. Combined analysis of XRD and transmission electron microscopy (TEM) suggested the formation of fine plates of α″, omega (ω) and bcc β phases in Ti-15 and Ti-25 Mo alloys. Calculation of enthalpy of formation supported the stability of solid solution phase over the amorphous phase in the entire concentration range of Mo.     

Synthesis and Structural Characterization of V–4Ti–4Cr Alloy

V–(4–10)Ti–(4–5)Cr alloys are the potential candidate materials for the high performance structural applications in fusion power systems due to their favorable mechanical and physical properties. In the present study, the alloy design has been attempted through model based approach for pre mentioned composition range. Thermodynamic calculations have been carried out through Miedema model for this alloy system. The enthalpy difference—composition plot for the ternary V–Ti–Cr system indicate the stability of the solid solution phase in this composition range, which is also in agreement with the atomic size variation which is well within the 15 % size variation limit. Based on these plots, an alloy with a composition of V–4Ti–4Cr is considered as the reference composition for our study. The alloys have been prepared by melting the desired compositions of highly pure metals in a water cooled vacuum arc melting unit. For structural information, X-ray diffraction experiment has been carried out. The XRD pattern does not contain any signature for the secondary phase formation. The structure is bcc structure with small variation in lattice parameter from that of pure vanadium. Transmission electron microscopy characterization has also been carried out, which confirms the absence of fine secondary phases and the crystal structure as b.c.c.