Kedar Singh

Thermal properties of pineapple leaf fiber reinforced composites

Simultaneous measurment of effective thermal conductivity (λ) and effective thermal diffusivity (κ) of pineapple leaf fiber reinforced phenolformaldehyde (PF) composites have been studied by transient plane source (TPS) technique. The samples of different weight percentage typically (15, 20, 30, 40 and 50%) have been taken. It is found that of effective thermal conductivity and effective thermal diffusivity of the composites decrease, as compared with pure PF as the fraction of fiber loading increases. Using Y. Agari, model thermal conductivity of pure fiber is evaluated and compared with the thermal conductivity of fiber determined by extrapolated experimental value of composite. Also, we have compared the results of thermal conductivity of composites with two models (Rayleigh–Maxwell and Meredith–Tobias model). Good agreement between theoretical and experimental result has been found.

Quantum Phase Transition from Superparamagnetic to Quantum Superparamagnetic State in Ultrasmall Cd1–xCr(II)xSe Quantum Dots?

Despite a long history of success in formation of transition-metal-doped quantum dots (QDs), the origin of magnetism in diluted magnetic semiconductors (DMSs) is yet a controversial issue. Cr(II)-doped II-VI DMSs are half-metallic, resulting in high-temperature ferromagnetism. The magnetic properties reflect a strong p-d exchange interaction between the spin-up Cr(II) t(2g) level and the Se 4p. In this study, ultrasmall (~3.1 nm) Cr(II)-doped CdSe DMSQDs are shown to exhibit room-temperature ferromagnetism, as expected from theoretical arguments. Surprisingly, a low-temperature phase transition is observed at 20 K that is believed to reflect the onset of long-range ordering of the single-domain DMSQD.

Simultaneous Measurements of Thermal Conductivity and Diffusivity of Se85–xTe15Sbx (x = 2, 4, 6, 8, and 10) Chalcogenide Glasses at Room Temperature

Measurements of thermal conductivity and thermal diffusivity of twin pellets of Se80Te20-xInx (x = 2, 4, 6 and 10) glasses, prepared under a load of 5 tons were carried out at room temperature using transient plane source (TPS) technique. The measured values of both thermal conductivity and diffusivity were used to determine the specific heat per unit volume of the said materials in the composition range of investigation. Results indicated that both the values of thermal conductivity and thermal diffusivity increased with the addition of indium at the cost of tellurium whereas the specific heat remained almost constant. This compositional dependence behaviour of the thermal conductivity and diffusivity has been explained in terms of the iono-covalent type of bond which In makes with Se as it is incorporated in the Se-Te glass.

Investigations on structural, optical and second harmonic generation in solvothermally synthesized pure and Cr-doped ZnO nanoparticles

At the forefront of the current scientific revolution of nanoscience nanocrystals (NCs), crystalline particles grown in liquid media, stand out over other classes of inorganic nanomaterials due to the high degree of control with which their crystal structure, size, shape, and surface functionalities can be engineered in the synthesis stage and to the versatility with which they can be processed and implemented into a large spectrum of devices and processes. Doped semiconductor nanostructures can yield both high luminescence efficiencies and lifetime shortening at the same time. In the present manuscript pure and Cr-doped ZnO nanoparticles were successfully synthesized from the solution phase chemistry and investigated with respect to their structural and optical properties. The resulting powder consisting of nanocrystalline particles were characterized by X-ray diffraction (XRD), UV-Visible spectroscopy, photoluminescence spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX) techniques. A UV emission peak was observed from the exciton transition at 380 nm in the room temperature photo luminescent (PL) spectra. The blue emission band was assigned to the Zn interstitial and vacancy level transition. Even though Cr ions are known to act as an efficient non-radiative loss centre for near band gap emission (NBE), a pronounced NBE is obtained at room temperature even for a nominal Cr concentration of 8 at. %. XRD data analysis shows that the chromium dopant atoms are incorporated into the wurtzite host lattice. The grain size decreases with increasing dopant concentration. The lattice constants extracted by the Rietveld method from XRD data vary slightly with doping concentration.

Evidence of a ZnCr2Se4 spinel inclusion at the core of a Cr-doped ZnSe quantum dot.

Herein we report doping of ZnSe by Cr ions leads to formation of small ZnCr2Se4 spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr2Se4 spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II−VI semiconductor lattice with a half-filled t2g d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie−Weiss (C−W) temperature of θ = −125 K, and a nearest-neighbor exchange coupling constant of JNN = −12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported. ■ INTRODUCTION The ability to incorporate paramagnetic ions into a semiconductor quantum dot (QD) lattice is of great interest to the solid-state materials community due to the ability to couple s− p−d orbital exchange-induced magnetic properties with the size-dependent QD energy levels. The excitement of incorporating a magnetic ion into an otherwise nonmagnetic host lattice reflects the potential of such materials for quantum computing, spintronics, magneto-resistive, and magneto-optical applications. While chemists are intrigued by the ability to control the location and concentration of the dopant ions as a crystallite grows, physicists are interested in the novel spin properties that arise and the influence of QD surfaces. For years it was considered nearly impossible to consistently dope a II−VI lyothermally grown QD due to selfannealing effects. Over the past several years the realization that a single source cluster precursor can be used to generate a predoped nucleus has led to the opportunity to efficiently incorporate a range of dopant ions into the whole II−VI series. In II−VI semiconductors, incorporation of a dopant metal ion with the same charge (i.e., Mn(II), Cr(II), and Co(II)) leads to simple substitutional incorporation of the ion of interest up to its solubility limit. Incorporation of an ion that has a different charge, such Cu(I) or Eu(III), results in formation of ion vacancies in the QD in order to reach charge neutrality. One magnetic dopant ion that has received little attention in the QD community is chromium. In Cr-doped II−VI semiconductors, the incorporation of Cr(I), Cr(II), and Cr(III) have been reported by solid-state synthesis. Of the three oxidation states, the Cr(III) oxidation state is preferred due to its one-half-filled t2g level and has been shown to be naturally introduced into octahedral (Oh) sites formed by ion vacancies that charge compensate the lattice in the otherwise tetrahedral coordination environment observed in II−VI semiconductors (ZnS, ZnSe, CdS, CdSe). Synthesis of the Cr(III)-doped II−VI semiconductor occurs via a peritectoid decomposition reaction leading to formation of a AB2X4 spinel structure by the group II metal occupying the tetrahedral (Td) site (A site) and the Cr(III) ion occupying the Oh site (B site). 28 The spinel phase is a singularity in the phase Received: November 1, 2011 Published: March 1, 2012 ArticleHerein we report doping of ZnSe by Cr ions leads to formation of small ZnCr(2)Se(4) spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr(2)Se(4) spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II-VI semiconductor lattice with a half-filled t(2g) d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie-Weiss (C-W) temperature of θ = -125 K, and a nearest-neighbor exchange coupling constant of J(NN) = -12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported.

Calorimetric studies of glass transition phenomenon in glassy Se80?xTe20Snx alloys

Glass transition kinetics has been studied in a glassy Se80?xTe20Snx (0 ?x ?10) system. Differential scanning calorimetry (DSC)?runs were taken at four different heating rates (5, 10, 15 and 20?K?min?1) for this purpose. The heating rate dependence of the glass transition temperature (Tg) has been studied using different non-isothermal methods. The activation energy of glass transition (Eg) has been evaluated using Kissingers relation and the theory of Moynihan. The composition dependence of Tg and Eg has also been discussed.

Thermal properties of neutron irradiated Se80Te10In10 glass

Abstract The thermal transport properties of Se 80 Te 10 In 10 chalcogenide glass irradiated by slow neutrons for different time exposures were studied using the transient plane source method. The effective thermal conductivity ( λ ) and effective thermal diffusivity ( χ ) increase with exposure time of the material and reaches to a saturation value. Constant values of effective thermal conductivity and effective thermal diffusivity beyond a certain time exposure could be explained on the basis of decrease of concentration of Se–Te polymeric chains and Se–Te rings.

Electro-optical, UV absorbance, and UV photoluminescence analysis of Se95In5 chalcogenide glass microparticle doped ferroelectric liquid crystal

The dandelion like Se95In5 chalcogenide glass microparticle (CGMPs) doped ferroelectric liquid crystal (FLC) has been investigated. The electro-optical parameters of the pure and doped FLC were carried out as a function of applied voltage. The experimental response time and polarization curves for the Se95In5 CGMPs doped FLC have also been theoretically fitted. The presence of Se95In5 CGMPs affects the molecular dynamics of the FLC molecules, which was proved by the Fourier transformed infrared spectroscopy. The UV absorbance of the pure FLC material has been enhanced in the presence of CGMPs, which is analogous to the coupling between phonons of CGMPs and radiation field. The photoluminescence (PL) of the pure FLC has also been enhanced and blue shifted with the addition of Se95In5 CGMPs. The enhanced PL is attributed to the constructive interaction between low energy phonons of the CGMPs and incident photons of the monochromatic light in the LC medium. The blue shifting of PL emission is due to the enha...

Effect of indium additive on glass-forming ability and thermal stability of Se–Zn–Te chalcogenide glasses

We report the measurements on Se93− x Zn2Te5In x (0 ≤ x ≤ 10) chalcogenide glasses using differential scanning calorimetry. The thermal stability and glass forming ability of these glasses are studied by different criteria. The composition dependence of the various kinetic parameters of crystallization is discussed.

Simultaneous measurements of thermal conductivity and diffusivity of Se80Te20-xInx (x = 2, 4, 6 and 10) chalcogenide glasses at room temperature

Measurements of thermal conductivity and thermal diffusivity of twin pellets of Se80Te20-xInx (x = 2, 4, 6 and 10) glasses, prepared under a load of 5 tons were carried out at room temperature using transient plane source (TPS) technique. The measured values of both thermal conductivity and diffusivity were used to determine the specific heat per unit volume of the said materials in the composition range of investigation. Results indicated that both the values of thermal conductivity and thermal diffusivity increased with the addition of indium at the cost of tellurium whereas the specific heat remained almost constant. This compositional dependence behaviour of the thermal conductivity and diffusivity has been explained in terms of the iono-covalent type of bond which In makes with Se as it is incorporated in the Se-Te glass.