Sidananda Sarma

Synthesis of polymer–PbS nanocomposite by solar irradiation-induced thermolysis process and its photovoltaic applications

In this study, we report the development of a new route for the synthesis of polymer–PbS nanocomposite thin film by a novel solar irradiation-induced reaction which is completely free from any complexing agents and toxic chemicals. The as synthesised polymer–PbS thin films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis absorption spectroscopy. The optical studies showed a direct allowed band gap of PbS lying in the range 1.78–2.2 eV. The XRD pattern shows the cubic structure of PbS with a lattice parameter of 5.927 Å. The SEM micrograph of the as-synthesised PbS thin films shows clusters in a relatively loose compact structure in the poly(vinyl alcohol) matrix and separated by voids, which are clearly observed in the surface. However, after annealing, most of the voids disappear in the PbS surface and it is likely that the clusters coalesce giving rise to an homogeneous and compact film. The PbS thin film after annealing is almost pinhole free and as such suitable for application in solar cell. A proto-type thin film solar cell of CdS/PbS was fabricated (1 × 1 cm2) on glass substrates using this deposition technique for PbS. The CdS layer was deposited by heat-induced thermolysis technique. The current–voltage (I–V) characteristics of the cells were measured with a high-impedance (∼1014) ECIL electrometer amplifier (model EA812). The efficiency of the solar cell was found as 2.54%.

Magnetic Properties of Fe/Mn Substituted Co‐Ni‐Ga Alloys

Co‐Ni‐Ga ferromagnetic shape memory alloys exhibit high ductility and magneto crystalline anisotropy. But these alloys have lower saturation magnetization (Msat) than Ni‐Mn‐Ga alloys. Partial substitution of Co48Ni22Ga30 alloy with Mn/Fe has been attempted to improve its magnetic properties. The alloys were prepared by arc melting high purity elements followed by homogenization at 1443 K and ice water quenching. Curie temperatures and the saturation magnetization of the alloy increase with increase in Mn/Fe content. But, the effective magnetic anisotropy constant Keff changes its value slightly with the substitution of Mn/Fe.

Influence of Annealing Temperature on the Properties of Co-Ni-Ga Ferromagnetic Shape Memory Alloy

Polycrystalline ingot of Co47Ni23Ga30 alloy was prepared by arc melting constituent elemental powders under argon atmosphere. The alloy ingot was then vacuum sealed in a fused silica ampoule, homogenized at 1230 °C for 24 hours and quenched in liquid nitrogen. X-Ray diffraction patterns of the as-quenched samples revealed single-phase tetragonal structure. The quenched alloy was then separately annealed at 900 °C, 1000 °C and 1150 °C for 6 hours and subsequently quenched in ice water. The alloys annealed at 1150 °C and 1230 °C exhibited a singlephase martensite structure (β′-phase) at room temperature, whereas, presence of a face centred cubic (γ) phase along with the martensite phase was observed in alloy pieces annealed at 900 °C and 1000 0C, respectively. The martensite-austenite structural phase change in this alloy was observed using a Differential Scanning Calorimeter. It was found that the martensite-austenite and austenitemartensite transition temperatures (As, Af, Ms and Mf) shifted to higher temperatures when the annealing temperature was increased. The Curie temperature shifted towards lower temperatures as the percentage of γ-phase increased in the alloy. The saturation magnetization did not show any appreciable change when the annealing temperature was changed. Presence of the additional γ- phase in the alloy annealed below 1150 °C was confirmed by Optical Microscopy and Scanning Electron Microscopy analysis. The influence of the annealing temperature on the properties of this ferromagnetic shape memory alloy composition is discussed in the paper.

Development of Co-Ni-Ga Ferromagnetic Shape Memory Alloys with Enhanced Properties

Polycrystalline ingots of Co70-xNixGa30 (22 ≤ x ≤ 25) alloys were prepared by a sequence of arc melting high purity Co, Ni and Ga in argon atmosphere, followed by homogenization at 1150°C under a pressure of 10-3 Pa, and quenching in ice water. Structural characterisation of the quenched alloys was carried out to verify the presence of the martensite phase at room temperature. The martensite start (Ms), martensite finish (Mf), austenite start (As) and austenite finish (Af) temperatures for the alloys were determined using a differential scanning calorimeter. The ferromagnetic to paramagnetic phase transition temperature (TC) of the alloys was determined using an indigenously developed ac susceptometer. All the alloys are FSMAs with Ms, Af and TC above room temperature. The composition dependence of the properties of these alloys could be understood on the basis of the e/a (electrons to atom) ratio and the Co/Ni ratio. Presence of γ-phase precipitates along with the β-phase in these alloys enhances the ductility as well as influences the physical properties of these alloys.

Structural Characterization of Co70-xNixGa30 Ferromagnetic Shape Memory Alloys

Polycrystalline ingots of Co70–xNixGa30 (20 ≤ x ≤ 26) ferromagnetic shape memory alloy (FSMA) were prepared by arc melting elemental powders followed by homogenization at 1230 °C for 24 hrs and quenching in liquid nitrogen. Room temperature X-Ray diffraction (XRD) patterns of as-quenched samples exhibited single-phase tetragonal structure for alloy compositions with x = 21 to 26, and a two-phase structure (cubic A2-phase along with weak tetragonal phase) for the alloy with x = 20. Rietveld refinement was performed on the X-ray diffraction patterns to obtain the refined structural parameters. Differential Scanning Calorimeter (DSC) curves recorded from 30 °C to 250 °C revealed martensite-austenite and austenite-martensite transformations in all alloys except the alloy with composition x = 20. Low temperature ac magnetic susceptibility measurements confirmed the existence of martensitic transformations in the alloy with x = 20. The structural transformation temperatures show a linear variation with e/a ratio. All the alloys were ferromagnetic at room temperature. Curie temperature was determined using a high temperature ac magnetic susceptibility measurement set-up.

Co-Ni-Ga Alloys with Room Temperature Ferromagnetic Martensite Phase

Co-Ni-Ga and Co-Ni-Al alloys are expected to be good ferromagnetic shape memory alloys (FSMAs) owing to their higher ductility resulting from the presence of γ-phase precipitates and higher stability in preparation since they do not contain the highly volatile element Mn. Co-Ni- Ga alloys have a wide range of martensitic transformation and Curie temperatures. In order to explore the possibility of obtaining Co-Ni-Ga alloys with room temperature ferromagnetic martensitic phase, two series of compositions, viz., Co70-xNixGa30 (20 ≤ x ≤ 26) and CoxNi25Ga75-x (43 ≤ x ≤ 50) were taken up for investigation. Polycrystalline ingots of these alloys were prepared by arc melting followed by homogenization and quenching at ice water. Analysis of room temperature X-ray diffraction patterns revealed that most of these alloys had a single-phase (tetragonal) structure typical of a martensitic phase, while some of the alloys exhibited a two-phase (cubic and tetragonal) structure due to the presence of both martensite and austenite phases. All alloys having single martensite phase at room temperature showed martensitic transformation at elevated temperature as well as a linear change of the characteristic martensitic transformation temperatures (As, Af, Ms and Mf) with the number of valence electron to atom ratio (e/a). As, Af, Ms and Mf showed distinctive variations when aged in the martensite phase and austenite phase. All the alloys were ferromagnetic at room temperature and the Curie temperature was determined by high temperature ac magnetic susceptibility and magnetization measurements. The typical twin lamellar structure of martensite phase was observed by optical microscope and the development of the cubic γ-phase along with the parent β′-phase was investigated for different ageing temperatures and annealing temperatures. These studies provide useful information about the potential of these alloys for actuator applications.