Thermal Treatment Of Crystals Research Articles

Effect of ultraviolet exposure and thermal treatment on NaF micro-crystallization mechanism in photo-thermo-refractive glasses

Photo-thermo-refractive (PTR) glass is a promising recording medium for volume-holographic devices. The recording processes involve a series of steps including ultraviolet (UV) exposure, nucleation, and crystallization thermal treatment. Sodium fluoride (NaF) microcrystals precipitate after crystallization, and modulate the refractive index locally of the glass matrix. In this study, to simulate the volume Bragg grating recording processes and study the effect of photothermal-induced processes on the crystallization kinetics in PTR glass, Si-Al-Na-Zn-K PTR glasses with and without Br− ions were prepared using the melt-quenching method. The UV exposure duration, thermal treatment temperature and crystallization time were regulated during subsequent treatment. During these processes, Br-free samples were used for comparison to demonstrate the potential roles of Br−ion in the nucleation and crystallization. The Ag nanoparticle (NP) nuclei surrounded by AgBr (AgF) microcrystals were characterized using UV–visible absorption spectroscopy and transmission electron microscopy, and the changes in the precipitated NaF were characterized using X-ray diffraction, Raman spectroscopy as well as XPS. The mechanism of NaF micro-crystallization is disclosed based on above experimental results. It is found the formation of Ag nanoparticle surrounded by AgBr (AgF) microcrystals inhibits the crystallization kinetics of NaF microcrystals in PTR glass.

On the Structure of Luminescenting Centers in Lu2SiO5:Се Crystals

The absorption and luminescence spectra of cerium-activated lutetium oxyorthosilicate crystals Lu2SiO5:Се subjected to irradiation by gamma rays and neutrons and thermal treatment under reductive conditions have been studied. It has been demonstrated that the radiative and thermal treatment of crystals leads to the removal of oxygen from the oxygen heptahedrons surrounding cerium ions with the conversion of Ce1 centers into Ce2.

Synthesis of waste‐derived glass‐ceramics from MSWI fly ash and EAF dust: Kinetics of nucleation and crystallization

The kinetics of crystal nucleation and growth critically affect the conversion of waste into high‐quality glass‐ceramics. This study investigates the effects of prenucleation treatment on the kinetic behavior of a waste‐derived glass‐ceramicsynthesized from a mixture of municipal solid waste incinerator fly ash and electric arc furnace dust. The crystallization kinetic parameters of annealed and prenucleated glass samples were determined by differential thermal analysis under nonisothermal conditions. Prenucleation experimental results revealed a temperature and time of maximum nucleation of 700°C and 30 min, respectively. Crystallographic and microstructural analyses revealed that the main crystalline phases were melilite and augite, with an equiaxed grain morphology, and were embedded in the glassy matrix. The activation energies (Ec) for the crystallization of the annealed and prenucleated glass samples, determined using modified Kissinger and Ozawa equations, were in the ranges of 367.4–395.2 and 199.8–214.6 kJ/mol, respectively. The Avrami constant (n) was 1.8 for the annealed glass and 1.5 for the prenucleated glass. These results confirm a significant difference between the Ec values of the annealed glass and prenucleated glass, suggesting that mixed ash‐based glass is suitable for use in the two‐stage crystallization thermal treatment adopted in this study. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 480–488, 2013

Influence of impurities and thermal treatment on spectroscopic properties and laser performance of thulium-doped yttrium vanadate crystals

Influence of impurities and thermal treatment on spectroscopic properties and laser performance of thulium-doped yttrium vanadate crystals was examined. YVO4 crystals nominally pure, single doped with Tm3+ and co-doped with Tm3+ and Ca2+ were grown by the Czochralski method and then thermally treated at 1150 °C for several hours in a reducing atmosphere (vacuum) or oxidizing atmosphere (air). Samples of crystals were investigated by nuclear magnetic resonance spectroscopy (NMR) and by optical spectroscopy methods. Laser performance of samples was examined upon laser diode pumping. For pure YVO4 and for YVO4 containing 0.5 at. % of Tm a single-site NMR spectrum of 51V nuclei was observed with central line widths of 2.5 and 3.2 kHz (FWHM), respectively. For samples containing 5 at. % of Tm the NMR spectrum was a superposition of multi-site spectra indicating at least three kinds of vanadium sites with axial symmetry. Optical absorption spectra did not contain bands that could be ascribed to V4+ ions in tetragonal sites. Level of matrix absorption in the visible region and its increase with decreasing wavelength from about 600 to 370 nm was found to be substantially dependent on conditions of thermal treatment. Thermal treatment of crystals and additional doping with Ca did not influence the 3 F 4 lifetime of thulium and laser performance of crystals, however. Infrared absorption spectra revealed OH- contamination in all samples. It has been concluded that the quenching of the 3 F 4 emission in samples containing 5 at. % of thulium is related to migration-accelerated energy transfer to hydroxyl ions acting as energy sinks.

Engineering of the magnetic properties of Finemet based nanocrystalline glass-coated microwires

The magnetic properties of Finemet nanocrystallized ribbons have been extensively studied and well understood. However, results obtained on microwires of such compositions are ambiguous considering the form of the hysteresis loop after the crystallization thermal treatment. In this paper, we show results on the static characterization of annealed Finemet based microwires (Fe73.2Si15.8B6.9Cu1.2Nb2.9) of different geometries and illustrate how the magnetic anisotropy of Finemet based nanocrystalline microwires depends on the annealing temperature and the geometric characteristics of the microwires.

Influence of the crystallization thermal treatment on the structural and electrical properties of PZT thin films

Different methods have been proposed in the literature to deposit PZT thin films. Among these, chemical based processes have been revealed as a promising preparation route due to their low cost processing and the facility for controlling the stoichiometry of complex systems. The objective of this work was to investigate the influence of the time and temperature of crystallization on the physical properties of the Pb(Zr0.53Ti0.47)O3 thin films prepared by a hybrid method. The structural and microstructural dependence on the crystallization conditions of the PZT films were investigated by X-ray diffraction and atomic force microscopy. Dielectric and ferroelectric properties were also characterized and their behaviors were related to the structural evolution of films.

Influence of compensation level on EL2 concentration in semi‐insulating gallium arsenide

The concentration of the intrinsic mid-gap donor EL2 in semi-insulating GaAs is known to be dependent on melt composition during crystal growth and on post-growth thermal treatment of crystals. A detailed analysis of data emerged from monitoring the production process of state-of-the-art GaAs substrates revealed discrepancies to the compensation model. By means of a combination of methods for quantitative point defect analysis we can show that EL2 concentration depends also on compensation ratio or position of Fermi level, respectively.

Recycling of zinc-hydrometallurgy wastes in glass and glass ceramic materials

The aim of Brite-Euram project CT94 -1018 has been to recycle jarosite (JW), an iron rich hazardous waste resulting from the hydrometallurgy of zinc ores, to obtain glass and glass-ceramic materials. Granite scraps and mud (GW), generated by the caving, cutting and sawing of the blocks, and glass cullet have been utilized as additives to define the batch composition. The average compositional ranges are 40–70% for JW, 20–40% for GW and 10–40% for glass-cullet. Several compositions were prepared and tailored for the final product destination; melted at the 1400–1450°C temperature range. Glass-ceramic materials have been obtained from the glassy-phase by means of a controlled nucleation and crystallization thermal treatment, yielding a 40–55% crystalline volume fraction. The properties of the glass and glass-ceramics have been tailored to favor the commercial exploitation of the products in the building industry, in the form of paving tiles wall covering panels, glass fibers for insulation and coloring pigments for ceramics. A 1 ton/day vitrification pilot plant, able to work in a continuous or discontinuous way, has been realized and tested using different waste streams.

Growth of bulk YBa2Cu3O7- delta single crystals and their properties

The effects of the charge composition, temperature-time conditions, value and direction of temperature gradients in a melt on the crystallization of bulk YBa2Cu3O7- delta single crystals have been studied. The morphology, chemical composition, structural, magnetic and resistive properties of these crystals have been characterized. The width of a metastable zone of YBCO is found to be about 0.8 K. At overcooling less than 0.8 K a bulk crystal of polyhedral type grows on a seed. At overcooling greater than 0.8 K a spontaneous crystallization takes place and crystals in the form of thin plates are produced. Orientation figures are defined for as-grown YBCO faces by a chemical etching method. The distribution of the main impurity (Al) is shown to be homogeneous in the volume of the growing crystal (the homogeneity level is 3%), its concentration varying from 0.05 to 0.11 at/FU in different runs. The absence of the block structure in (001) planes of the YBCO single crystal is established using X-ray topography. The contrast unevenness on the (100) topogram indicates that a certain amount of the block structure occurs on (100) planes, comparable with the resolution of the method of 0.02 degrees . The lattice parameters and the M(T) and rho (T) dependences are measured before and after the thermal treatment of crystals.