Nucleation studies in supersaturated aqueous solutions of KH2PO4 doped with KBr and K2Cr2O7

Induction periods were measured for various supersaturated aqueous solutions of potassium dihydrogen orthophosphate doped separately with potassium bromide and potassium dichromate by the direct vision method. Various critical nucleation parameters were calculated based on the classical theory for homogeneous crystal nucleation and the results are reported and discussed. The critical nucleation parameters increased with increase in doping concentration for both the dopants considered.     

Nucleation studies in supersaturated aqueous solutions of (NH4)H2PO4 doped with (NH4)2C2O4·H2O

Induction periods were measured for various supersaturated aqueous solutions of ammonium dihydrogen orthophosphate doped with ammounium oxalate monohydrate by the direct vision method. Various critical nucleation parameters were calculated based on classical theory for homogeneous crystal nucleation and the results reported and discussed. The critical nucleation parameters increased with increase in doping concentration.     

Spectroscopic properties of B2O3–PbO–Bi2O3–GeO2 glass doped with Sm3+ and gold nanoparticles

Heavy metal oxide B₂O₃–PbO–Bi₂O₃–GeO₂ transparent glass doped with Sm³⁺ was synthesized and implanted with Au⁺ using energy of 300 keV and fluence of 1 × 10¹⁶ cm⁻². The annealing of the implanted glass at moderate temperature below the glass transition temperature induced the nucleation of gold nanoparticles, confirmed by the characteristic absorption band in the visible range and by transmission electron microscopy. Using Miés and Doylés theories for the surface plasmon resonance, the average size of the gold nanoparticles was about 4.6 nm, similar to the values observed by transmission electron microscopy. It was also observed the crystallization of a thin layer of the glass at the implanted surface after annealing, detected by X-ray diffraction and scanning electron microscope. Visible and near-infrared emission of Sm³⁺ was enhanced after annealing of the glass implanted with gold. Judd–Ofelt parameters and radiative parameters were calculated for the glass doped with Sm³⁺ with and without gold nanoparticles.     

Epitaxial growth of Ag on Ag-doped NaCl

NaCl crystals doped with AgSO₄ have been grown in the laboratory and pure silver films of different thicknesses have been grown on the cleavages of doped and undoped crystals in a vacuum of the order of 10^-5 torr at room temperature by thermal evaporation. The films have been examined in an electron microscope and the nucleation and growth processes have been studied and compared. It is observed that: (i) preferential nucleation occurs on doped cleavages whereas uniform nucleation occurs on the undoped cleavages; (ii) coalescence of nuclei takes place earlier in films grown on doped substrates than on undoped ones; and (iii) films on doped substrates grow epitaxially with (100) orientation. The implications are discussed.     

Nucleation studies and surface SHG analysis of l-arginine phosphate monohydrate (LAP) family crystals

The metastable zone width and the nucleation parameters, such as interfacial tension, radius of the critical nucleus and critical free energy change have been estimated for pure l-arginine phosphate (LAP), potassium thiocynate (KSCN) mixed LAP (LAP:KSCN) and sodium sulfite (Na₂SO₃) mixed LAP LAP:Na₂SO₃ single crystals. Pure and additive mixed LAP single crystals are grown by slow cooling technique. The surface second harmonic generation (SHG) analysis is done on (100) face of the grown crystals. The SHG intensity on (100) face of the crystals are measured.     

Low-temperature sintering and electrical properties of Co-doped ZnO varistors

ZnO–Bi₂O₃–B₂O₃-based varistors doped with each kind of cobalt oxides were prepared by conventional ceramic processing. The effects of CoO, Co₂O₃ and Co₃O₄ on the microstructure and the electrical characteristics of varistor samples sintered at 880 °C were investigated separately. Analysis of microstructure indicated the cobalt cations were distributed both in grain regions and grain boundary regions and no crystalline phases containing cobalt were detected in XRD patterns for the samples with various cobalt oxides. All these cobalt oxides could effectively enhance the varistor performance by effectively increasing the nonlinear coefficient and lowing the leakage current, while the breakdown voltage fields increased slightly. Capacitance–voltage characteristics showed the potential barriers of varistor samples increased with the addition of each cobalt oxide. It was found that the addition of same amount of cobalt cations in various cobalt oxides had a different effect on the varistor samples. Best electrical properties were obtained for the varistor sample containing Co₃O₄, in which the nonlinearity coefficient is 28.5, the leakage current density is 3.4 μA and the breakdown voltage field is as low as 260 V/mm.     

Effects of nonstoichiometry and doping on the curie temperature and defect structure of lithium niobate

LiNbO₃ crystals doped with Gd, Zn, Cu, and Er to various levels were grown by the Czochralski technique. The structural parameters, Curie temperature, and density of the LiNbO₃<Gd> crystals were determined as a function of composition. With increasing Gd content, thea parameter of the hexagonal cell increases, c remains constant within the experimental error, density increases, and T_c gradually decreases. The defect structure and properties of LiNbO₃ doped with Gd, Zn, Cu, Er, Mg, and Ta are contrasted with those of nominally undoped LiNbO₃ crystals differing in Li/Nb ratio. The results suggest that the variation of the Curie temperature with composition is determined primarily by the position that the dopant cations occupy in the structure of lithium niobate.     

Superconducting Performance of Ex-situ SiC Doped MgB2 Monofilamentary Tapes

The superconducting performance of the ex-situ SiC doped MgB₂ monofilamentary tapes are reported. Polycrystalline powders of MgB₂ doped with 5 and 10 wt% SiC were synthesized by a conventional solid-state reaction route and characterized for their superconducting performances. It was found that the superconducting parameters viz., upper critical field (H _c2), irreversibility field (H _irr) and critical current density (J _c) were improved significantly with SiC addition. It was also found that relatively lower synthesis temperature resulted in further improved superconducting parameters in comparison to higher synthesis temperature. Thus, synthesized powders are used for the fabrication of ex-situ powder-in-tube (PIT) monofilamentary tapes. The superconducting performance in terms of critical current density (J _c), being determined from both magnetization (J _cm) and transport (J _ct) measurements, was improved significantly. In particular, the SiC doped MgB₂ tapes (fabricated using 700 °C heat treated bulk powder) exhibited the transport J _ct of nearly 10⁴ A/cm² under applied fields of as high as 7 Tesla. Further, it was found that the J _ct anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB₂ tapes was responsible for decreased anisotropy and improved J _c(H) performance.     

Synthesis,characterization and optical properties of bismuth germanate doped with trivalent europium

Bismuth germanate single crystals doped with trivalent europium were grown by the Czochralski method. Procedures and results of synthesis experiments are reported. Microprobe analysis reveals the segregation of doped rare-earth elements. Spectroscopic studies of Eu³⁺ ions suggests C_3v point symmetry for the cationic site. Oscillator strengths and τ_λ parameters for Eu³⁺ were calculated.     

Pore Structure and Transport Properties in Bulk YBa2Cu3O7?δ Doped with Sb2O3

In this work we have studied the pore structure and electrical transport properties of superconducting YBa₂Cu₃O_7?y polycrystalline samples doped by the addition of different Sb₂O₃ concentrations, i.e. resulting in (YBa₂Cu₃O_7?y )_1?x (Sb₂O₃) _x . The samples were prepared through the solid-state reaction method. Rietveld analyses of X-ray diffraction data were used to investigate how the lattice parameters are modified by doping. Specific superficial area measurements identified the principal characteristics of the pore structure of the samples and how these properties change with doping. The superconducting properties were studied by using zero field cooling magnetization and transport critical current measurements. The critical temperature of the samples does not depend on the doping level, but their transport critical current density strongly decreases as the Sb₂O₃ concentration is increased. Our experimental results suggest that for the samples studied here there is not a direct correlation between the modification by doping of both, the pore structure and the transport critical current density.     

Effect of Molecular Rotation on Vapor Phase Nucleation: Fullerenes C60 and C70

Abstract

The effect of rotational free energy of fullerenes C₆₀ and C₇₀ on homogeneous nucleation of crystals from vapor phase have been studied. Classical nucleation parameters have been estimated for the case of nucleation and crystal growth of C₆₀ and C₇₀ by Physical Vapor Transport (PVT) method under different supercooling in the range of 20–150 K, keeping the source temperatures constant. The result shows that the growth of large size single crystals is more feasible in the lower supercooling range. The effect of change in interfacial tension on the critical free energy of formation of the nuclei is also studied.     

The role of TiO2 and composition in the devitrification of near-stoichiometric cordierite

Devitrification behavior and thermal expansion of glasses and glass-ceramics, doped with TiO₂, near the stoichiometric cordierite composition were investigated. The activation energy for growth of surface nucleated crystals was shown to be approximately 433 kJ/mol. and was independent of TiO₂ content in the glass. Volume nucleation was achieved by the addition of approximately 8 wt% TiO₂, but the mechanism of volume nucleation was different depending upon cordierite composition. Regions in the MgO-Al₂O₃-SiO₂ phase diagram near the stochiometric cordierite compound were investigated using thermal analysis, X-ray powder diffraction and dilatometry. Glass in glass phase separation was postulated for MgO-lean glasses, whereas precipitation of mullite and Al₂TiO₅ preceded devitrification in other compositions. In each case, the formation of a silica-rich glass is believed to initiate the devitrification. Coefficients of thermal expansion for the glass-ceramics increased with increasing TiO₂ content resulting from increasing levels of uncrystallized glass and the formation of mullite and rutile during crystallization.     

Effect of impurities on the strengthening of CaF2 single crystals

The critical resolved shear stress (CRSS) of pure and aliovalently doped (with Y⁺³ and Na⁺) CaF₂ single crystals have been measured in compression along [1 1 1] axis between room temperature and 873 K. Strain rate cycling and/or stress relaxation experiments were performed to obtain the necessary parameters for the determination of rate-controling mechanism for plastic flow. The measured temperature and concentration dependences of the activation parameters suggest that the elastic interaction of dislocations with impurity cation-lattice anion defect pairs determine the CRSS as in alkali halide crystals. The CRSS for CaF₂ single crystals doped with trivalent Y⁺³ were found an order of magnitude larger than those doped with the monovalent Na⁺. Based on the valence of dopant cations and the magnitude of hardening produced, solution hardening in CaF₂ could be divided into two groups: rapid and gradual hardening.     

Microstructure characteristics for anorthite composite glass with nucleating agents of TiO2 under non-isothermal crystallization

The anorthite-based composite glass doped with TiO₂ and B₂O₃ was prepared by quenching of molten droplets. Phase development and crystals microstructure of glass were investigated under non-isothermal conditions. A glass transition temperature of 770°C and an exothermal peak around 870°C in the DTA trace was associated with anorthite crystallization (CaAl₂Si₂O₈). For glass specimens under nucleation and crystallization heat-treatment, the final predominant phase was identified as anorthite. Anorthite crystals show preferential nucleation at specific sites with rutile TiO₂ crystals precipitated from the glassy matrix and anorthite crystallization is governed by heterogeneous volume nucleation. The introduced TiO₂ plays the role of nucleating agents to reduce the crystallization temperature lower than 900°C for anorthite-based glass-ceramics. Chemical compositions could be related to the crystal microstructures on different characteristic regions. It was observed that the sintering aid of B₂O₃ neither reacted with nor dissolved in the anorthite or rutile TiO₂ crystals, and remained a glassy phase in the matrix. Occurrence of acicular precipitations was attributed to the orientation growth of TiO₂ crystals. Anorthite crystals were observed to grow with the forms of feathery-spherical particles, having a tendency to coalescence into a huge domain.     

Applications of Judd–Ofelt theory to praseodymium and samarium ions in phosphate glass

The Judd–Ofelt (J–O) theory has had a remarkable success in the characterization of radiative transitions in lanthanide doped solids. The purpose of this paper is to review the applications of the J–O theory to the Pr³⁺ and Sm³⁺ ions and to use this system as an occasion to appraise its validity and to clarify its limits. In this paper we dwell at length on the absorption and luminescence measurements of two glass samples doped with Pr³⁺ and Sm³⁺, because they are basic for the J–O treatment. The results obtained for the J–O theory application to phosphate glasses doped with Pr³⁺ and Sm³⁺ present two undesirable outcomes: (1) a positive value of parameter Ω₂ and (2) large uncertainties with which the three Judd–Ofelt parameters were obtained. The validity of the J–O theory for intensity analysis was also tested for Sm³⁺ doped in phosphate glass. The resulting Ω₂ was much lower than Ω₄. The first set parameters were obtained using all the levels for which f_exp. was available. The second set parameter values were evaluated without the ⁶F_1/2 and ⁶H_15/2 levels. The Ω_2,4,6 values given in these two sets clearly suggest that particular care should be taken while evaluating the Judd–Ofelt parameters as well as when these parameters are compared to other systems due to their strong dependence on the nature of levels.     

The role of bulk nucleation in the formation of crystalline cordierite coatings produced by air plasma spraying

Near stoichiometric cordierite (2MgO–2Al₂O₃–5SiO₂) glasses doped with 6.3, 8.3 and 10.1 weight percent (wt.%) TiO₂ were plasma sprayed onto SiO₂-based refractory concrete substrates. Substrate temperatures were controlled such that the maximum surface temperature exceeded the crystallization temperature during plasma spraying. Only the cordierite powders containing 8.3 and 10.1 wt.% TiO₂ produced crystalline coatings. For compositions with less than 8 wt.% TiO₂ the crystallization is believed to be controlled by surface nucleation at imperfections. Surface nucleation became more difficult with increasing preheat temperature and is related to improved wetting between coating splats. Bulk nucleation-controlled crystallization was observed at compositions above 8 wt.% TiO₂ and the presence of Al₂TiO₅ was detected in these compositions.     

E.s.r. during crystallization in Gd3+ doped lithium-aluminium-silicate glasses

Measurements of the electron spin resonance spectrum have been made in a Gd³⁺ doped lithium-aluminium-silicate glass at various stages during the crystallization of the glass. The crystallization was induced by heat-treatment at various temperatures and took place either from the surface or from nucleation centres produced by the presence of TiO₂ in the glass.The changes in the resonance spectra have been interpreted in terms of the stability of the gadolinium sites. In the presence of titanium, it is suggested that complexes are formed in the glass which involve both titanium and gadolinium ions.Supporting measurements show that the presence of these complexes is also detectable from the more conventional structural and thermal characterization of the glasses.     

Applications of Judd–Ofelt theory to praseodymium and samarium ions in phosphate glass

The Judd–Ofelt (J–O) theory has had a remarkable success in the characterization of radiative transitions in lanthanide doped solids. The purpose of this paper is to review the applications of the J–O theory to the Pr³⁺ and Sm³⁺ ions and to use this system as an occasion to appraise its validity and to clarify its limits. In this paper we dwell at length on the absorption and luminescence measurements of two glass samples doped with Pr³⁺ and Sm³⁺, because they are basic for the J–O treatment. The results obtained for the J–O theory application to phosphate glasses doped with Pr³⁺ and Sm³⁺ present two undesirable outcomes: (1) a positive value of parameter Ω₂ and (2) large uncertainties with which the three Judd–Ofelt parameters were obtained. The validity of the J–O theory for intensity analysis was also tested for Sm³⁺ doped in phosphate glass. The resulting Ω₂ was much lower than Ω₄. The first set parameters were obtained using all the levels for which f_exp. was available. The second set parameter values were evaluated without the ⁶F_1/2 and ⁶H_15/2 levels. The Ω_2,4,6 values given in these two sets clearly suggest that particular care should be taken while evaluating the Judd–Ofelt parameters as well as when these parameters are compared to other systems due to their strong dependence on the nature of levels.     

Yellow and warm white light emitting Zn doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> for near UV excitable phosphor converted WLED

Optical properties of Zn doped Y₂O₃ microsheets prepared by sol–gel combustion method have been investigated and their application in phosphor converted white LED has been examined. The formation of single phase, well crystalline cubic Y₂O₃ is confirmed from powder XRD results. Effective substitution of Zn in Y₂O₃ crystal lattice is inferred from shifting of diffraction peaks. SEM images have showed that undoped as well as Zn doped Y₂O₃ formed as microsheets. Doping of Zn enhanced the growth of the sheets and its length increased from 1.5 to 19 µm. Development of structural disorder in Y₂O₃ crystal structure after Zn doping and confirmation of the conserved cubic structure of Zn doped Y₂O₃ without any secondary phase have been revealed from micro-Raman spectra. The optical band gap of Y₂O₃ has been altered after Zn doping and it is found to be decreased from 5.6 to 5.22 eV as increasing Zn concentration. Both undoped and Zn doped Y₂O₃ showed a broad visible emission from blue to green region due to various defects and impurities present in it. Broad PL excitation spectrum inferred the possibility to attain the visible emission under the excitation of light with wide range of wavelength from near UV to blue region. Excitation of pure Y₂O₃ under near UV (375 nm) LED chip lead to the emission of yellow light whereas Zn doped Y₂O₃ emitted warm white light with color coordinate of (0.42, 0.35), colour rendering index of 77.6 and correlated color temperature (CCT) of 2840 K. Hence, Zn doped Y₂O₃ discussed in the present work can be a better replacement for various rare earth doped phosphors in the application of phosphor converted WLED (pc-WLED).