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Raman Spectroscopy Study of Phonon Liquid Electron Crystal in Copper Deficient Superionic Thermoelectric Cu2- xTe
No full textSuperionic Cu2-xTe (CT) is an interesting and emerging p-type thermoelectric (TE) material due to the existence of various polymorphic phases and crystal structures, which undergo several structural phase transitions. On the basis of the stoichiometry of the CT compounds, the structure parameters, the carrier concentration (np), and the thermal conductivity (κ) can be modulated for optimum TE performance. Further, the understanding of the fundamental properties and their impact on TE parameters is not well understood because of their complex structures. We have investigated the vibrational properties of CT compounds such as Cu1.25Te (CT1.25), Cu1.6Te (CT1.6), and Cu2Te (CT2) using temperature dependent Raman studies in the temperature range of 300-773 K. Several structural phases are probed through remarkably distinct spectra for the CT compounds. The temperature transitions are complex such as (i) eutectic melting into CuTe and Te for both CT1.6 (above �593 K) and CT1.25 (above �613 K) and (ii) the structural transition from trigonal to orthorhombic and cubic phase for CT2 (above �553 K), which are strongly manifested in the Raman study. Further, the role of np in the Raman spectra has also been investigated. The intensity of the Raman modes (>100 cm-1) showed strong np dependence due to strong plasmon-phonon coupling. The analysis of full width at half-maximum (fwhm) of Raman peaks and qualitative estimation of phonon lifetime (�i) showed that CT2 has the minimum lattice thermal conductivity
Effect of Fe alloying on the thermoelectric performance of Cu2Te
No full textCopper telluride belongs to the �Phonon Liquid Electron Crystal� (PLEC) class of materials owing to its mobile Cu ions which migrate easily inside the crystal lattice. The mobile copper ions scatter phonons and lead to low thermal conductivity making it a suitable choice as a thermoelectric material. However, the formation of copper vacancies results in high hole carrier concentration (�1021 cm�3) leading to poor Seebeck coefficient and degrades its overall thermoelectric efficiency. In this work, we aim at improving the thermoelectric transport properties of Cu2Te by alloying with Fe. The alloys of Cu2-xFexTe (where x varies from 0 to 0.06) were synthesized via solid-state synthesis followed by compaction in an induction hot press system. The formation of the trigonal phase (space group P3m1) of Cu2Te and the solubility of Fe � x = 0.05 in Cu2-xFexTe is confirmed by means of X-ray Diffraction and backscattered electron micrographs. The decrease in electrical conductivity and the Hall carrier concentration and increase in the Seebeck coefficient with increasing Fe content signifies the compensation of holes by the electrons contributed from the trivalent Fe. The specific heat capacities of the Fe substituted samples were higher than the Dulong Petit limit of 3NkB, which implies that �liquid-like� behaviour of the Cu ions has been suppressed. This is because the Fe atoms act as barriers to the Cu ion diffusion. Due to the maximum power factor (�0.47 mW m�1 K�2) and minimum thermal conductivity (�2.19 W m�1 K�1), the maximum figure of merit of �0.16 at 750 K was obtained for the sample Fe0.03. It is hypothesized that the substitution of Fe not only helped to control the high carrier concentration but also restricted the cationic diffusion in Cu2Te. Hence, the present study points to a possibility of synergistically improving the thermoelectric properties as well as suppressing the ionic diffusivity in PLEC materials
Evolution of local flame displacement speeds in turbulence
No full textIn this study, we assess the veracity of models for density-weighted local flame displacement speed of turbulent premixed flames. It will be shown that a combination of two models, one for the weakly stretched laminar flame state and another derived for a configuration where a curved laminar flame interacts with itself to annihilate, can describe most local realizations of a turbulent premixed flame. To that end, we have performed direct numerical simulations of a reactive mixture of hydrogen-air at atmospheric pressure using a detailed chemical reaction mechanism and analysed the dataset with recently developed flame particle tracking techniques. Forward tracking a large number of flame particles from the generating locations of the corresponding flame surfaces (given by backward tracking) to the corresponding annihilating locations, creates a manifold of local states that can represent nearly all possible states realizable for the turbulent premixed flame under consideration. With all the states of the flame accessible over time, we first assess the applicability of the two-parameter Markstein length based flame speed model. It is found that the model prediction is reasonably accurate for a significant part of the flame particles' lifetime, for turbulent premixed flames with Karlovitz number O(10) However, during the final stage of annihilation of the flame particles in the negatively curved trailing regions, the local structure of the flame no longer resembles a standard premixed flame, even qualitatively. A new interaction model for the flame displacement speed, during these final stages of annihilation of the flame elements, has been derived
Surface modification influenced properties of silicon nanowires grown by Ag assisted chemical etching with ECR hydrogen plasma treatment
No full textSilicon nanowires (SiNWs) are fabricated by Ag assisted chemical etching and are treated with hydrogen plasma created by electron cyclotron resonance (ECR) plasma system at 600 watts microwave power for various time durations (0�30 min). The hydrogen plasma exposure on the surface of the SiNWs reduced the surface roughness and increased the crystalline nature. SEM analysis revealed that the diameter of the SiNWs decreased on plasma exposure. The electrical conduction measurements suggested that the hydrogen plasma exposure for 5 min on the SiNW surface enhanced the electrical conductivity when compared to as fabricated SiNW surface. The hydrophobic nature of fabricated SiNWs was transformed to hydrophilic at plasma exposure for lower time duration. On plasma exposure of NWs for 30 min the sample turned hydrophobic. Study of different properties of the SiNWs before and after plasma treatment revealed that there is pronounced effect of plasma on the nature of SiNWs
Shuffle-nanodomain regulated strain glass transition in Ti-24Nb-4Zr-8Sn alloy
No full textThe unprecedented properties of multi-functional metastable β-Ti alloys, including superelasticity over a wide temperature range, ultra-low modulus, and Invar and Elinvar anomalies, have attracted a great deal of attention. Persistent research efforts have been made towards the understanding of the origins of these unique properties. In this article we report a novel shuffle-nanodomain regulated strain glass transition in a metastable β-Ti alloy, Ti-24Nb-4Zr-8Sn (wt., Ti2448), which could be the dominant transformation pathway that offers these unique properties. Using the ex-situ aberration-corrected scanning transmission electron microscopy and in-situ cooling transmission electron microscopy, we find that randomly distributed 011�01¯1�β O� phase (orthorhombic, shuffle only) nanodomains embedded in the β phase (BCC) matrix at room temperature transform to α� phase (orthorhombic) with a continuous increase in the amount of 21¯1�1¯1¯1�β shear upon cooling or loading. Crystallographic analysis shows that the shuffle of the O� phase will restrain the twelve possible shears that transform a BCC lattice to α� martensite to only two. Thus, the randomly distributed O� nanodomains prevent the formation of long-range-ordered, self-accommodating transformation-strain domain patterns seen in normal martensitic transformations and suppress completely the sharp first-order, auto-catalytic and avalanche-like martensitic transformation into a high-order-like (continuous) strain glass transition. Such a continuous β � O� � α� strain glass transition has been confirmed by dynamic mechanical analysis, resistivity and differential scanning calorimetric measurement. This unique transition pathway allows us to offer new insights into the unique properties found in this alloy
On the dynamics of sprays in complex gas turbine swirl injectors
No full textAbstract: Coupling of spray with the coherent structures of highly turbulent flow has been a long-standing problem, especially in the context of liquid fuel delivery systems in gas turbine combustors. In this work, we analyze the evolution of the hydrodynamic topology and consequent spray-flow interactions in a dual swirl injector assembly. We have shown (using time-resolved particle image velocimetry) that the geometry of the swirl cup (exit flare angle and mixing length), as well as the flow orientation (counter vs. co) in the primary and secondary swirlers, ascertain the hydrodynamic transitions in the resultant flow field. Width of the recirculation zone (r/Ro) is identified as the key length-scale used to ascertain the global characteristics of the flow field. For a given flare angle, reduction in length scale (r/Ro) is witnessed with orientation switch from counter-rotation to co-rotation configuration. Proper orthogonal decomposition (POD) is implemented over instantaneous flow field data to extract energetic spatial flow structures and temporal modes. POD revealed the existence of distinct frequency bands depending on the relative dominance of the primary or secondary swirler flow fields. Dynamic mode decomposition (DMD) also has been carried out to delineate the evolution of dominant frequency values with respect to the experimental variables
DEKTV and YVG motifs in the Lsm domain are important for the activity of Scd6, a conserved translation repressor protein
No full textScd6 is a conserved RGG-motif protein which represses translation by binding eIF4G through its RGG-motif. Lsm and FDF are two other conserved domains present in the protein, however the role of both these domains is unclear. We provide evidence in this report that the Lsm domain is important for the role of Scd6 in translation. Mutant of Scd6 lacking the Lsm domain does not cause overexpression growth defect in a manner comparable to the wild type. Similar results were observed with two distinct point mutants of Scd6 wherein putative RNA-binding motifs DxEKxTV and YVG were mutated. Upon overexpression, the three mutants were defective in inducing formation of P-bodies and stress granules which are conserved sites of translation repression. Importantly localization to granules in response to glucose deprivation and sodium azide stress was defective for Lsm domain mutants indicating that the inability to localize to granules could be a reason for their defective role in translation. Deletion of scd6 impairs Lsm1 foci formation upon glucose deprivation stress which could not be rescued by complementation with Lsm-domain deletion mutant of Scd6 when compared to the full-length protein. Put together, our results highlight the role of Lsm domain and its specific motifs in Scd6 activity and provide crucial insight into its function. © 2020 Elsevier B.V
Search for electroweak production of a vector-like T quark using fully hadronic final states
No full textA search is performed for electroweak production of a vector-like top quark partner T of charge 2/3 in association with a top or bottom quark, using proton-proton collision data at s = 13 TeV collected by the CMS experiment at the LHC in 2016. The data sample corresponds to an integrated luminosity of 35.9 fb�1. The search targets T quarks over a wide range of masses and fractional widths, decaying to a top quark and either a Higgs boson or a Z boson in fully hadronic final states. The search is performed using two experimentally distinct signatures that depend on whether or not each quark from the decays of the top quark, Higgs boson, or Z boson produces an individual resolved jet. Jet substructure, b tagging, and kinematic variables are used to identify the top quark and boson jets, and also to suppress the standard model backgrounds. The data are found to be consistent with the expected backgrounds. Upper limits at 95 confidence level are set on the cross sections for T quark-mediated production of tHQq, tZQq, and their sum, where Q is the associated top or bottom heavy quark and q is another associated quark. The limits are given for each search signature for various T quark widths up to 30 of the T quark mass, and are between 2 pb and 20 fb for T quark masses in the range 0.6�2.6 TeV. These results are significantly more sensitive than prior searches for electroweak single production of T � tH and represent the first constraints on T � tZ using hadronic decays of the Z boson with this production mode
Effect of Mn addition on magnetoelectric coupling behavior of BiFeO3-Pb/BaTiO3 multiferroics
No full textHybrid multiferroic materials exhibiting morphotropic phase boundary (MPB) with enhanced ferroelectric and ferromagnetic properties has shown great potential for future technologies. In this paper, we report structural, ferroelectric, piezoelectric, magnetic and magnetoelectric characteristics of 0.7BiFeO3-0.3Pb0.5Ba0.5TiO3 (BFPTBT-Pure) and 0.7BiFeO3-0.3Pb0.5Ba0.5TiO3 + Mn0.5 (BFPTBT-Mn5) ceramic compositions synthesized via conventional solid state reaction route. The crystallinity of the compositions exhibits polymorphs of rhombohedral (R3c) and tetragonal (P4mm) symmetries forming morphotropic phase boundary (MPB). Highly dense SEM micrographs were observed with an average grain size 0.57 μm and 0.62μm for BFPTBT-Pure and BFPTBT-Mn5, respectively. Mn doped ceramic sample. Improved ferroelectric behavior has been observed with Mn doping in the composition as the value of remnant polarization increases from 2.46 μC cm-2 to 7.63 μC cm-2 recorded at an applied frequency of 50 Hz. The piezoelectric coefficients for BFPTBT-Pure and BFPTBT-Mn5 were found to be 36pC/N and 57pC/N respectively. M-H hysteresis loops depicted that remnant magnetization increases with Mn addition in the sample. The Curie transition temperature (T c) was observed to be 447 °C and 467 °C for BFPTBT-Pure and BFPTBT-Mn5 ceramics, respectively. The magnetoelectric coupling was confirmed through the observation of magnetic field induced relative change in dielectric constant (Magnetocapacitance: MC). MC was found to be 9.49 and 11.81 for BFPTBT-Pure and BFPTBT-Mn5, respectively
Origin of visible and near IR upconversion in Yb3+-Tm3+-Er3+ doped BaMgF4 phosphor through energy transfer and cross-relaxation processes
No full textNear infrared and green, red emission through upconversion and energy transfer processes in BaMgF4 doped with Yb3+-Tm3+ and Yb3+-Tm3+-Er3+ excited at 980 nm were investigated. The BaMgF4:Yb3+,Tm3+ phosphor showed a dominating UC emission at 800 nm. The origin of this 800 nm emission peak was explored through different possible cross-relaxation processes. The optimized composition of Yb3+ and Tm3+ was further co-doped with Er3+, these compositions also showed dominating 800 nm emission for lower concentrations of Er3+ which was gradually suppressed at higher Er3+ concentration. The enhancement of the visible emission of Er3+ in the BaMgF4:Yb3+,Tm3+,xEr3+ doped phosphor suggests the efficient energy transfer from Yb3+ to Tm3+ and Tm3+ to Er3+. By tuning the concentrations of the dopants, a near white light emission under infrared excitation was also achieved. © 2019 Elsevier B.V