Time resolved luminescence of quartz from Nigeria

Characteristics of luminescence lifetimes and luminescence intensity obtained from time resolved spectra of quartz from Nigeria are presented. The luminescence was pulse-stimulated at 11 μs width at 470 nm. Samples used consisted of unannealed quartz as well as samples annealed at 500 °C, 600 °C, 700 °C, 800 °C and 900 °C. The luminescence lifetimes and luminescence intensities were studied as a function of annealing temperature, irradiation dose and measurement temperature. It was found that there is a decrease, although non-monotonic, in the luminescence lifetime with change in annealing temperature from 20 to 900 °C. In addition, lifetimes extracted from time resolved spectra of unannealed samples as well as ones annealed at 500 °C and 600 °C are initially independent of irradiation dose but do later decrease with further irradiation. Regarding the luminescence intensity it was observed that in general, the intensity passes through a peak as the measurement temperature is increased from 20 to 200 °C with slight differences in the detailed pattern dependent on preheating. Activation energies for thermal quenching and thermal assistance evaluated from temperature-dependent changes of luminescence lifetime and luminescence intensity are given. The results are discussed in terms of a model consisting of three luminescence centres with probability of hole trapping during irradiation being highest for the luminescence centre associated with the least lifetime.     

Influence of structural defects on the thermal conductivity of polycrystalline and single-crystal PbTe

We have measured the thermal conductivity of unannealed and annealed (800 K, 120 h) polycrystalline and single-crystal PbTe samples at temperatures from 80 to 303 K, evaluated the electronic and lattice components of their thermal conductivity, and determined the thermal resistivity due to structural defects, whose concentration in the unannealed single-crystal samples reaches ∼10¹⁷ cm⁻³. The results demonstrate that the thermal resistivity of the unannealed polycrystalline and single-crystal samples is 9.4 and 1.7 cm K/W, respectively. Annealing eliminates the defects, thereby increasing the lattice thermal conductivity of the material.     

Effect of thallium doping on the thermal conductivity of PbTe single crystals

The thermal conductivity of thallium-doped PbTe single crystals has been measured in the temperature range 80–310 K, and the electronic and lattice (χ_ph) components of their thermal conductivity have been evaluated. The results indicate that thallium doping markedly reduces the lattice thermal conductivity of PbTe single crystals, by up to ～29%. Heat treatment at ～473 K reduces the density of structural defects in the crystals and increases their χ_ph. Thallium doping has a significant effect on χ_ph starting at 0.05 at % before annealing and starting at 0.01 at % after annealing. We have calculated the density of structural defects in the unannealed samples and the doping-induced thermal resistance Δ W of the unannealed and annealed samples. ΔW has been shown to increase with thallium content.     

Influence of the annealing temperature on violet emission of ZnO films obtained by oxidation of Zn film on quartz glass

The photoluminescence (PL) emission properties of ZnO films obtained on quartz glass substrate by the oxidation of Zn films with the oxygen pressure of 50Pa at temperature of 773 K~973 K were studied. The strong single violet emission centering on 424 nm (or 2.90 eV) without any accompanying deep-level emission and UV emission was observed in the PL spectra of the ZnO films at room temperature. The intensity of violet emission increased with increasing annealing temperature in the range of 773 K~873 K and decreased with increasing annealing temperature in the range of 873 K~973 K. These violet emission bands are attributed to the electron transition from interstitial zinc (Zn_i) level (2.91 eV) to the valence band.     

Optical detection of phase transitions in potassium niobate

Abstract

Luminescence data from potassium niobate crystals are reported which show intensity and/or wavelength variations on heating, or cooling, through the phase transition temperatures. The luminescence signals can clearly identify the structural changes between the various crystalline phases. The transition temperatures differ between heating and cooling. They were recorded near 247 and 491 K (heating) or 220 and 478 K (cooling). The hysteresis indicates the presence of metastable material, (e.g. a supercooled structure). The data resolve the previously cited differences in transition temperatures for KNbO₃ from different laboratories. The luminescence signals show further details in the variations of spectra, intensity and transition discontinuities of the luminescence which are related to material quality between samples, even from a single supplier. The luminescence data underline the sensitivity of these crystals to structural damage from electron, X-ray or thermal treatments and offer the opportunity to assess crystalline quality prior to device fabrication.     

Study of the damage produced in 6H-SiC by He irradiation

Lattice damage and evolution in 6H-SiC under He⁺ ion irradiation have been investigated by the combination of Rutherford backscattering in channeling geometry (RBS/C), Raman spectroscopy, UV–visible spectroscopy and transmission electron microscopy (TEM). 6H-SiC wafers were irradiated with He ions at a fluence of 3 × 10¹⁶ He⁺cm⁻² at 600 K. Post-irradiation, the samples were annealed in vacuum at different temperatures from 873 K to 1473 K for isochronal annealing (30 min). Thermally annealed He irradiated 6H-SiC exhibited an increase in damage or reverse annealing behavior in the damage peak region. The reverse annealing effect was found due to the nucleation and growth of He bubbles. This finding was consistent with the TEM observation. The thermal annealing brought some recovery of lattice defects and therefore the intensities of Raman peaks increased and the absorption coefficient decreased with increasing annealing temperature. The intensity of Raman peak at 789 cm⁻¹ as a function of annealing temperature was fitted in terms of a thermally activated process which yielded activation energy of 0.172 ± 0.003 eV.     

Photoconductive and ellipsometric studies of spray pyrolysed CdS thin films

The influence of annealing temperature on photoconductivity of spray pyrolysed CdS films has been investigated. The annealing of films at 373K resulted in maximum photoconductivity. The photoconductivity at all wavelengths was found to decrease with increase of annealing temperature and became a minimum at 473K. For films annealed at or above 523K, the photoconductivity for wavelengths ⩾560 nm decreased considerably. The results are discussed with the help of ellipsometric data, visible and IR spectra of these samples. Also the variation in photoconductivity of these samples were studied during heating to analyse the effect of trapping.     

Study of the damage produced in 6H-SiC by He irradiation

Lattice damage and evolution in 6H-SiC under He⁺ ion irradiation have been investigated by the combination of Rutherford backscattering in channeling geometry (RBS/C), Raman spectroscopy, UV–visible spectroscopy and transmission electron microscopy (TEM). 6H-SiC wafers were irradiated with He ions at a fluence of 3 × 10¹⁶ He⁺cm⁻² at 600 K. Post-irradiation, the samples were annealed in vacuum at different temperatures from 873 K to 1473 K for isochronal annealing (30 min). Thermally annealed He irradiated 6H-SiC exhibited an increase in damage or reverse annealing behavior in the damage peak region. The reverse annealing effect was found due to the nucleation and growth of He bubbles. This finding was consistent with the TEM observation. The thermal annealing brought some recovery of lattice defects and therefore the intensities of Raman peaks increased and the absorption coefficient decreased with increasing annealing temperature. The intensity of Raman peak at 789 cm⁻¹ as a function of annealing temperature was fitted in terms of a thermally activated process which yielded activation energy of 0.172 ± 0.003 eV.     

Strength and toughness tradeoffs for an ultrafine-grain size ferrite/cementite steel produced by warm-rolling and annealing

For an ultrafine grain ferrite/cementite (UGF/C) steel, the Charpy impact energy was measured at temperatures from 373 K to 4.2 K, and tensile tests were carried out at temperatures between 323 K and 77 K. For the steel with annealed microstructure, the ductile-to-brittle transition appearance temperature (DBTT) was lower than the Charpy transition temperature (CTT). With increasing annealing time at 873 K, the DBTT and the CTT increased, and the DBTT approached the CTT. The DBTT decreased with decreasing effective grain size. The effective grain size correlated to the grain size of the larger grain size peak in the distribution of grains with {1 0 0} planes. The annealed microstructures had higher yield strength for equivalent toughness (including upper shelf energy, DBTT and CTT) compared to the conventional ferrite/pearlite steel.     

The electrical and thermal conductivities of dilute copper-chromium alloys at low temperatures

Electrical and thermal conductivity measurements are reported on dilute copper-chromium alloys containing 32 and 50 at. ppm chromium, in their unannealed state and also after annealing at 530–550°C for 16 and 66 h in each case, under fore-vacuum. From the electrical resistivity measurements evidence of a possible cluster formation is obtained in the annealed sample. The electronic Lorenz number of the alloys increases with decrease of temperature in the temperature range 4.2–1.6 K. Lower temperature measurements might show a maximum around the Kondo temperature (～1 K) as predicted by the available theories on dilute magnetic alloys.     

Annealing effect on the characteristics of La0.67Sr0.33MnO3 polycrystalline thin films produced by the sol–gel dip-coating process

The effect of annealing temperature on selected characteristics of polycrystalline La_0.67Sr_0.33MnO₃ films, which have been produced on quartz substrates, was investigated. X-Ray powder diffraction patterns showed that the phase formation started at 873 K and all the films had perovskite structure. By increasing the annealing temperature, the lattice parameters were decreased. Scanning electron microscope indicated that the film thicknesses were approximately 3 μm and the average grain size of the samples varied between 30–100, 50–110, 70–120, and 100–150 nm for films annealed at 873, 973, 1,073, and 1,173 K, respectively. All the films showed a paramagnetic–ferromagnetic (T_C) and metal–insulator (T_IM) phase transition. The T_C indicated a small variation [from 131 K (S4) to 124 K (S1)] as a function of annealing temperature, whereas the T_IM went down from 212 K (S4) to 110 K (S1), a strong decrease of 102 K. A colossal magneto resistance with magneto resistance ratios of 130, 139, 156, and 163% were observed near T_C and at 6 T magnetic field.     

Effect of γ-radiation on optical properties of Eu2+ ions in the KCl: Eu2+ system

Emission spectra and luminescence time decay of Eu²⁺ ions in KCl crystals annealed at 873 K and quenched to room temperature before irradiation were measured at 80 K after -irradiation which was performed at 295 and 200 K. The results are consistent with radiation-induced aggregation of the original isolated europium dipoles.     

Thermal expansion behavior and microstructure in bulk nanocrystalline selenium by thermomechanical analysis

Thermal expansion behavior of bulk nanocrystalline (NC) Se samples with a grain size range of 16–46 nm was studied by thermomechanical analysis (TMA) in the temperature range 290–373 K. Bulk NC Se samples were prepared by isothermally crystallizing the as-quenched bulk amorphous solid at 373–478 K. The glass transition and crystallization of the remaining amorphous Se in the partially crystallized samples were studied by TMA, and compared with the results of differential scanning calorimetry (DSC). The glass transition temperature, as determined from the thermal expansion behavior, was 308 K, 11 K lower than the value by DSC analysis. A structural densification phenomenon was observed in a grain growth process of an as-crystallized NC Se sample by TMA. It was found that the linear thermal expansion coefficient of the bulk NC Se sample increased with a reduction of grain size, from which the deduced thermal expansion coefficient of the interface decreased with the refinement of the grain size.     

Microstructure and Vickers hardness of Co/Cu multilayers fabricated by electrodeposition

In order to investigate thermal stability of Co/Cu multilayers fabricated by electrodeposition, Vickers hardness tests and microstructure observations were conducted on both as-deposited and annealed Co/Cu multilayers having a layer thickness of 100 nm. The multilayers were annealed at temperatures ranging from 473 to 1,273 K for 1 h. It is confirmed that even after the annealing at 1,023 K, the multilayer maintained the high hardness (Hv231) which was comparative to that of the as-deposited Co/Cu multilayer. When the annealing temperature was higher than 1,073 K, the hardness decreased rapidly with increasing temperature. Scanning electron microscopy (SEM) observation revealed that the multilayered structures were still maintained without any layer damages after the annealing at the temperatures less than 873 K. At the cross sections of the Co/Cu multilayers annealed at T > 923 K, several copper layers were fragmented. The layered structure finally disappeared by the annealing at 1,273 K. The rapid decrease in the hardness at T > 1,073 K is simply understood from the annihilation of the Co/Cu interfaces.     

Temperature sensing with fluorescence intensity ratio technique in epoxy-based nanocomposite filled with Er-doped 7YSZ

Luminescent nanocomposite of epoxy filled with Er³⁺-doped yttria-stabilized zirconia (7YSZ) is prepared with their luminescence spectra measured in the temperature range 123–423 K. Fluorescence intensity ratio (FIR) of the two Er³⁺ emissions is also obtained in the same temperature range. Er–7YSZ/epoxy nanocomposites exhibited higher sensitivity of 0.18%/K as compared with the bare Er–7YSZ particles. Luminescence thermometry is demonstrated by using the nanocomposites as temperature sensitive paint (TSP) with a resolution of 1 K. The advantage of FIR technique combined with the excellent thermal stability of epoxy matrix makes the Er–7YSZ/epoxy nanocomposites viable as temperature sensitive paint for aerodynamic applications.     

Synthesis of nitrogen-rich B–C–N materials from melamine and boron trichloride

Nitrogen-rich B–C–N materials have been prepared by the reaction between melamine and boron trichloride at different temperatures. The composition of the materials was dependent on the synthesis and annealing temperatures: C6N10.8-11H9.4B1.5-1.7 (for products synthesized and annealed at 673 K), C6N9.3-9.4H3.8-3.9B2.2-2.5 (for those synthesized at 673 K and annealed at 873 K) and C6N9.2H3.6B1.2-1.3 (for those synthesized and annealed at 873 K). Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance showed that the s-triazine rings from the melamine molecules were preserved in materials synthesized and annealed at 673 and 873 K. The sample obtained at 873 K had a graphite-like structure as suggested by X-ray and electron diffraction studies. The s-triazine rings were decomposed in the materials synthesized or annealed at 1223 K and the main product obtained was turbostratic boron nitride. © 1998 Chapman & Hall     

Influence of annealing temperature and oxygen atmosphere on the optical and photoluminescence properties of BaTiO3 amorphous thin films prepared by sol–gel method

Homogeneous and transparent BaTiO₃ thin films were prepared by sol–gel dip coating method. The prepared BaTiO₃ thin films were annealed in air and O₂ atmosphere at different temperatures. The annealed BaTiO₃ thin films were amorphous in nature. Scanning electron microscopy (SEM) revealed the nucleation and particle growth on the films. Energy-dispersive X-ray (EDX) analysis data revealed the adsorption of oxygen atoms in the BaTiO₃ film. The direct energy band gap was found to vary (3.84–3.58 eV) as functions of annealing atmosphere and temperature. Photoluminescence (PL) revealed intense emission peaks at 393 and 675 nm. Quenching of PL intensity was observed in films annealed at high temperature and in O₂ atmosphere. This is due to reduction in the oxygen vacancy by the adsorption of oxygen in the film. Luminescence spectra also have been related to the results obtained by SEM and EDX analysis. The change in luminescence intensity of BaTiO₃ thin films makes it suitable for optoelectronic temperature sensor applications.     

Search for common characteristics in the glow curves of quartz of various origins

The thermoluminescence (TL) glow curves of quartz of various origins were measured under two different conditions, (1) unannealed samples and (2) samples annealed at 500 degrees C and 900 degrees C. The different glow curves obtained were analysed using first order kinetics and glow curve deconvolution (GCD) analysis. The comparison of the glow curves obtained was mainly concentrated in studying the sensitivities of the glow peaks as a function of the annealing temperature, and in obtaining the kinetic parameters of the glow peak at '110 degrees C'. Furthermore, in four samples the detailed comparison was extended to the trapping parameters of all existing glow peaks. It was found that despite their different origin and the different shapes of the glow curves, there are several basic characteristics that are common to all samples studied.     

Effects of lead on annealing properties of cold-drawn copper wire

Cold-drawn copper wires were made by cold-drawing after hot-rolling tough pitch copper rods which had been pre-heated at 673–973 K. The wires were annealed at 293–673 K to study their annealing properties. The effects of pre-heating the hot-rolled tough pitch copper rods, including lead, on the recrystallization behaviour of the cold-drawn copper wires were investigated by tensile tests, electrical resistivity measurements and structural observations. The half-softening temperature of annealed cold-drawn copper wires decreased when the hot-rolled copper rods were pre-heated at 873 K before cold-drawing. This behaviour was attributed to separation of lead dissolved in the copper matrix which is known to lower the half-softening temperature.     

Effect of ion doping with donor and acceptor impurities on intensity and lifetime of photoluminescence from SiO2 films with silicon quantum dots

Doping with donor and acceptor impurities is an effective way to control light emission originated from quantum-size effect in Si nanocrystals. Combined measurements of photoluminescence intensity and kinetics give valuable information on mechanisms of the doping influence. Phosphorus, boron, and nitrogen were introduced by ion implantation into Si+ -implanted thermal SiO2 films either before or after synthesis of Si nanocrystals performed at Si excess of about 10 at.% and annealing temperatures of 1000 and 1100 degrees C. After the implantation of the impurity ions the samples were finally annealed at 1000 degrees C. It is found that, independently of ion kind, the ion irradiation (the first stage of the doping process) completely quenches the photoluminescence related to Si nanocrystals (peak at around 750 nm) and modifies visible luminescence of oxygen-deficient centers in the oxide matrix. The doping with phosphorus increases significantly intensity of the 750 nm photoluminescence excited by a pulse 337 nm laser for the annealing temperature of 1000 degrees C, while introduction of boron and nitrogen atoms reduces this emission for all the regimes used. In general, the effective lifetimes (ranging from 4 to 40 micros) of the 750 nm photoluminescence correlate with the photoluminescence intensity. Several factors such as radiation damage, influence of impurities on the nanocrystals formation, carrier-impurity interaction are discussed. The photoluminescence decay is dominated by the non-radiative processes due to formation or passivation of dangling bonds, whereas the intensity of photoluminescence (for excitation pulses much shorter than the photoluminescence decay) is mainly determined by the radiative lifetime. The influence of phosphorus doping on radiative recombination in Si quantum dots is analyzed theoretically.