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Origin of Ultralow Thermal Conductivity in n-Type Cubic Bulk AgBiS2: Soft Ag Vibrations and Local Structural Distortion Induced by the Bi 6s(2) Lone Pair
No full textCrystalline materials with ultralow thermal conductivity are essential for thermal barrier coating and thermoelectric energy conversion. Nontoxic n-type bulk cubic AgBiS2 exhibits exceptionally low lattice thermal conductivity (kappa(lat)) of 0.68-0.48 W/m K in the temperature range of 298-820 K, which is near the theoretical minimum (kappa(min)). The low kappa(lat) is attributed to soft vibrations of predominantly Ag atoms and significant lattice anharmonicity because of local structural distortions along the 011] direction, arising because of the stereochemical activity of the 6s(2) lone pair of Bi, as suggested by pair distribution function analysis of the synchrotron X-ray scattering data. The low-temperature heat capacity of AgBiS2 shows a broad hump because of the Ag-induced low-energy Einstein modes as also suggested from phonon dispersion calculated by first-principle density functional theory. Low-energy optical phonons contributed by Ag and Bi strongly scatter heat-carrying acoustic phonons, thereby decreasing the kappa(lat) to a low value. A maximum thermoelectric figure of merit of similar to 0.7 is attained at 820 K for bulk spark plasma-sintered n-type AgBiS2
On induced colourful paths in triangle-free graphs
No full textGiven a graph G = (V, E) whose vertices have been properly coloured, we say that a path in G is colourful if no two vertices in the path have the same colour. It is a corollary of the Gallai-Roy-Vitaver Theorem that every properly coloured graph contains a colourful path on chi(G) vertices. We explore a conjecture that states that every properly coloured triangle free graph G contains an induced colourful path on chi(G) vertices and prove its correctness when the girth of G is at least chi(G). Recent work on this conjecture by Gyarfas and Sarkozy, and Scott and Seymour has shown the existence of a function f such that if chi(G) >= f (k), then an induced colourful path on k vertices is guaranteed to exist in any properly coloured triangle-free graph G. (C) 2018 Elsevier B.V. All rights reserved
CH3NH3PbBr3 quantum dots for visible wavelength photodetector applications
No full textPerovskite halide materials with their salient optoelectronic features such as high absorption coefficient, large charge carrier diffusion lengths and low carrier recombination, have revolutionised the field of photodetectors. In this paper we report the synthesis of CH3NH3PbBr3 quantum dots for visible wavelength photodetector applications. Quantum dots of size 2-4.5nm were obtained. UV-Vis and photoluminescence spectroscopy revealed an optical band gap of 2.3eV. An electrochemical band gap of 2.09eV was obtained using cyclic voltammetry. CH3NH3PbBr3 quantum dots based visible wavelength photodetector device was fabricated. The time dependent photoresponse was found to be stable over several ON-OFF cycles of the lamp. We have achieved a responsivity, external quantum efficiency and specific detectivity of 6.11AW(-1), 1380.2% and 3.81x10(12) Jones under AM 1.5 G, 1 sun illumination
Experimental investigations on mixing characteristics in the critical regime of a low-area ratio supersonic ejector
No full textEjectors have many applications in aerospace and energy conversion technologies. An ejector is a passive device that pumps a secondary fluid by energy augmentation from a primary fluid. The performance of the ejector is primarily dependent on the complex gas dynamic interactions between the primary and secondary flows in a variable area duct. The maximum mass flow rate of the ejector in the critical flow regime is limited by choking of both the primary and secondary flows. This paper focuses on experimental investigations on the mixing characteristics of the ejector having an area ratio of 2 in the critical flow regime for a range of stagnation pressure ratios varying between 5.49 and 11.12 and a primary Mach number of 1.5, 2.0, and 2.5. The gas dynamic flow field is visualized using non-invasive high-speed schlieren and Mie-scattering techniques. Mie-scattering images are used to gain insight into the mixing characteristics and estimate the non-mixed length. The optical measurements are complimented with the wall static pressure measurements. The experimental performance data are compared with two well-established analytical models. The characterization of the mixing in the critical operating regime of the ejector using optical tools is being reported here for the first time. An important outcome of the paper is the observation of a significant increment in the non-mixed length to the tune of 55% increase in the critical flow regime in comparison to the mixed flow regime. Published under license by AIP Publishing
Response of the Salinity-Stratified Bay of Bengal to Cyclone Phailin
No full textCyclone Phailin, which developed over the Bay of Bengal in October 2013, was one of the strongest tropical cyclones to make landfall in India. We study the response of the salinity-stratified north Bay of Bengal to Cyclone Phailin with the help of hourly observations from three open-ocean moorings 200 km from the cyclone track, a mooring close to the cyclone track, daily sea surface salinity (SSS) from Aquarius, and a one-dimensional model. Before the arrival of Phailin, moored observations showed a shallow layer of low-salinity water lying above a deep, warm ``barrier'' layer. As the winds strengthened, upper-ocean mixing due to enhanced vertical shear of storm-generated currents led to a rapid increase of near-surface salinity. Sea surface temperature (SST) cooled very little, however, because the prestorm subsurface ocean was warm. Aquarius SSS increased by 1.5-3 psu over an area of nearly one million square kilometers in the north Bay of Bengal. A one-dimensional model, with initial conditions and surface forcing based on moored observations, shows that cyclone winds rapidly eroded the shallow, salinity-dominated density stratification and mixed the upper ocean to 40-50-m depth, consistent with observations. Model sensitivity experiments indicate that changes in ocean mixed layer temperature in response to Cyclone Phailin are small. A nearly isothermal, salinity-stratified barrier layer in the prestorm upper ocean has two effects. First, near-surface density stratification reduces the depth of vertical mixing. Second, mixing is confined to the nearly isothermal layer, resulting in little or no SST cooling
Synthesis and evaluation of PVDF-MgTiO3 polymer-ceramic composites for low-k dielectric applications
No full textPVDF exhibits ferroelectric, pyroelectric, and dielectric properties as a function of its unique crystalline phases, namely , , , , and epsilon. Of these, the crystalline phase shows unique electrical and mechanical properties due to its highly ordered crystalline lamellae and is of industrial interest. Electrospinning is a unique technique to produce long flawless fibers for various applications. In this study, several MgTiO3-PVDF composites were prepared by the electrospinning and solution-casting methods. The objective of this was to examine the combined effect of a popular, low-loss ceramic dielectric like MgTiO3 and a high-dielectric polymer like PVDF in a new architecture. The effect of the filler dispersion on the microstructure, phase transition, and mechanical and electrical properties of the MgTiO3/PVDF composites was investigated in the frequency range of 100 Hz-100 MHz and at 10 GHz. The dynamic mechanical properties of the composites (1-100 Hz) have also been examined. These polymer composites composed of highly crystalline PVDF nanofibers and MgTiO3 appear to have promising dielectric properties distinctly different from their constituent components
Tumor Chemosensitization through Oncogene Knockdown Mediated by Unique alpha-Tocopherylated Cationic Geminis
No full textHerein, siRNA transfection efficiency of a unique set of alpha-tocopherylated gemini lipids has been established in vitro and in vivo. High efficacy of oncogene silencing achieved using the biomacromolecular assembly, formed from siRNA complexes of co-liposomes containing an alpha-tocopherylated gemini lipid, has been utilized for tumor regression via chemosensitization. Delivery studies with the gemini bearing hydroxyethyl headgroup with octamethylene spacer (TH8S) pointed to a higher siRNA transfection efficacy than its analog without hydroxyethyl group (T8S). Owing to p53 upregulation, transfected cells showed enhanced sensitivity to the chemotherapeutic agent, doxorubicin. Studies in murine model revealed significantly low levels of survivin mRNA in xenograft tumors injected with siRNA lipoplexes, leading to effective inhibition of tumor growth and an increase in sensitivity of the tumors toward doxorubicin. These findings enable us to propose the anti-survivin siRNA carrying TH8S co-liposomes as a potent member of cancer management strategies using suicide gene therapy
Modelling the Inhibition of Selenoproteins by Small Molecules Using Cysteine and Selenocysteine Derivatives
No full textSmall molecule-based electrophilic compounds such as 1-chloro-2,4-dinitrobenzene (CDNB) and 1-chloro-4-nitrobenzene (CNB) are currently being used as inhibitors of cysteine- and selenocysteine-containing proteins. CDNB has been used extensively to determine the activity of glutathione S-transferase and to deplete glutathione (GSH) in mammalian cells. Also, CDNB has been shown to irreversibly inhibit thioredoxin reductase (TrxR), a selenoenzyme that catalyses the reduction of thioredoxin (Trx). Mammalian TrxR has a C-terminal active site motif, Gly-Cys-Sec-Gly, and both the cysteine and selenocysteine residues could be the targets of the electrophilic reagents. In this paper we report on the stability of a series of cysteine and selenocysteine derivatives that can be considered as models for the selenoenzyme-inhibitor complexes. We show that these derivatives react with H2O2 to generate the corresponding selenoxides, which undergo spontaneous elimination to produce dehydroalanine. In contrast, the cysteine derivatives are stable towards such elimination reactions. We also demonstrate, for the first time, that the arylselenium species eliminated from the selenocysteine derivatives exhibit significant redox activity by catalysing the reduction of H2O2 in the presence of GSH (GPx (glutathione peroxidase)-like activity), which suggests that such redox modulatory activity of selenium compounds may have a significant effect on the cellular redox state during the inhibition of selenoproteins
Texture Evolution in Severe Plastic Deformation Processes
No full textSevere plastic deformation processes involve large grain rotations due to the action of different modes of plastic deformation and other microstructural changes which lead to characteristic texture formation. The present review deals with the evolution of texture during the most important severe plastic deformation processes, namely Equal Channel Angular Pressing (ECAP), High Pressure Torsion (HPT), Friction Stir Processing (FSP), Accumulative Roll Bonding (ARB) and Multi-Axial Forging (MAF). First three of the processes are shear based, while the latter two are plane-strain based. The textures formed during ECAP are visually different from simple shear textures due to (i) the inclination of the shear plane, (ii) additional contribution of non-shear based deformation. The relative intensities of texture components are function of deformation micro-mechanisms, amount of straining and configuration of the strain path. The texture evolved during HPT is very similar to simple shear texture, with additional consequences of microstructural changes that occur due to very large deformations. The textures formed in FSP process also resemble shear textures. On the other hand, texture evolution during ARB and MAF can be described using plane strain deformation. The present review deals with texture evolution during severe plastic deformation as a function of nature of processes and type of materials
Universal quantized thermal conductance in graphene
No full textThe universal quantization of thermal conductance provides information on a state's topological order. Recent measurements revealed that the observed value of thermal conductance of the 5/2 state is inconsistent with either Pfaftian or anti-Pfaftian model, motivating several theoretical articles. Analysis has been made complicated by the presence of counter-propagating edge channels arising from edge reconstruction, an inevitable consequence of separating the dopant layer from the GaAs quantum well and the resulting soft confining potential. Here, we measured thermal conductance in graphene with atomically sharp confining potential by using sensitive noise thermometry on hexagonal boron-nitride encapsulated graphene devices, gated by either SiO2/Si or graphite back gate. We find the quantization of thermal conductance within 5% accuracy for nu =1;4/3;2 and 6 plateaus, emphasizing the universality of flow of information. These graphene quantum Hall thermal transport measurements will allow new insight into exotic systems like even-denominator quantum Hall fractions in graphene