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.     

Error analysis of the rapid lifetime determination method for double-exponential decays and new windowing schemes

The rapid lifetime determination method (RLD) is a mathematical technique for extremely rapid evaluations of lifetimes in exponential decays. It has been applied in luminescence microscopy and single-molecule lifetime evaluation. To date, the primary application has been in single-exponential evaluations. We present extensions of the method to double exponentials. Using Monte Carlo simulations, we assess the performance of both the double-exponential decay with known lifetimes and the double-exponential decay with unknown preexponential factors and lifetimes. Precision is evaluated as a function of the noise level (Poisson statistics), the ratios of the lifetimes, the ratios of their preexponential factors, and the fitting window. Optimum measurement conditions are determined. RLD is shown to work well over a wide range of practical experimental conditions. If the lifetimes are known, the preexponential factors can be determined with good precision even at low total counts (10(4)). With unknown preexponential factors and lifetimes, precisions decrease but are still acceptable. A new gating scheme (overlapped gating) is shown to offer improved precision for the case of a single-exponential decay. Theoretical predictions are tested against actual experimental data from a laser-based lifetime instrument.     

Measurements of hydroxyl concentrations and lifetimes in laminar flames using picosecond time-resolved laser-induced fluorescence

Picosecond time-resolved laser-induced fluorescence (PITLIF) can potentially be used to obtain measurements of minor species concentrations in rapidly fluctuating flames. Previous studies demonstrated this potential for atomic sodium by monitoring the temporal fluorescence signal with both an equivalent-time and a real-time sampling method. In this developmental study, PITLIF is used to determine hydroxyl concentrations in laminar CH(4)-O(2)-N(2) flames by the measurement of both the integrated fluorescence signal and the fluorescence lifetime. The quenching environment can be monitored with real-time sampling, and thus the necessary quenching rate coefficient is obtained in 348 us, which is fast enough for use in many turbulent flows. Fluorescence lifetimes of OH are also measured at different equivalence ratios in laminar flames by the use of the equivalent-time sampling technique. These results compare favorably with predicted lifetimes based on relevant quenching cross sections and calculated species concentrations.     

Dynamic windowing algorithm for the fast and accurate determination of luminescence lifetimes

An algorithm for the accurate calculation of luminescence lifetimes in near-real-time is described. The dynamic rapid lifetime determination (DRLD) method uses a window-summing technique and dynamically selects the appropriate window width for each lifetime decay such that a large range of lifetimes can be accurately calculated. The selection of window width is based on an optimal range of window-sum ratios. The algorithm was compared to alternative approaches for rapid lifetime determination as well as nonlinear least-squares (NLLS) fitting in both simulated and real experimental conditions. A palladium porphyrin was used as a model luminophore to quantitatively evaluate the algorithm in a dynamic situation, where oxygen concentration was modulated to induce a change in lifetime. Unlike other window-summing techniques, the new algorithm calculates lifetimes that are not significantly different than the slower, traditional NLLS. In addition, the computation time required to calculate the lifetime is 4 orders of magnitude less than NLLS and 2 orders less than other iterative methods. This advance will improve the accuracy of real-time measurements that must be made on samples that are expected to exhibit widely varying lifetimes, such as sensors and biosensors.     

Effect of Temperature and X-Ray Illumination on the Oxygen Ordering in La2CuO4.1 Superconductor

Structural properties of La₂CuO_4.1 single crystal are studied by high-resolution synchrotron X-ray diffraction as a function of temperature and of X-ray fluence. Superstructures with a periodicity 2, along the c axis, due to a 3D long-range oxygen ordering, have been observed. The temperature-dependent study has allowed us to distinguish two order–disorder phase transitions, at 350 and 375 K for two different ordered phases, respectively. After rapid quenching from 380 to 100 K we were able to induce disorder-to-order-like transition because of a 2D rearrangement of the excess oxygen atoms in the temperature range of 130–180 K. The oxygen ordering could also be produced by X-ray beam illumination; a clear signature of X-ray-photo-induced phase transition has been found by placing the sample under high X-ray flux at 300 and 220 K.     

High-temperature remote thermometry using laser-inducedfluorescence decay lifetime measurements ofY2O3:Eu and YAG:Tb thermographic phosphors

An Nd:YAG-laser-based remote sensing system for determining the surface temperature of substrates coated with thermographic phosphors has been developed. The fluorescence decay lifetime versus temperature measurements obtained for Y₂O₃:Eu (4.52% Ec) and YAG:Tb (5% Tb) over the temperature range 27°C-1095°C are presented. Both phosphors can be used for continuous temperature measurements over the range of approximately 550°C-1000°C. Stepwise regression analysis is used to calculate empirical relationships expressing temperature as a function of the measured fluorescence lifetimes for both phosphors beyond the onset temperature quenching point. The accuracy of the temperature measurements was estimated to be ±5%. The intensities of the 611-nm Y₂O ₃:Eu and the 544-nm YAG:Tb emission lines as a function of temperature are compared     

Effect of Temperature and X-Ray Illumination on the Oxygen Ordering in La2CuO4.1 Superconductor

Structural properties of La₂CuO_4.1 single crystal are studied by high-resolution synchrotron X-ray diffraction as a function of temperature and of X-ray fluence. Superstructures with a periodicity 2, along the c axis, due to a 3D long-range oxygen ordering, have been observed. The temperature-dependent study has allowed us to distinguish two order–disorder phase transitions, at 350 and 375 K for two different ordered phases, respectively. After rapid quenching from 380 to 100 K we were able to induce disorder-to-order-like transition because of a 2D rearrangement of the excess oxygen atoms in the temperature range of 130–180 K. The oxygen ordering could also be produced by X-ray beam illumination; a clear signature of X-ray-photo-induced phase transition has been found by placing the sample under high X-ray flux at 300 and 220 K.     

Dual fluorescence sensor for trace oxygen and temperature with unmatched range and sensitivity

An optical dual sensor for oxygen and temperature is presented that is highly oxygen sensitive and covers a broad temperature range. Dual sensing is based on luminescence lifetime measurements. The novel sensor contains two luminescent compounds incorporated into polymer films. The temperature-sensitive dye (ruthenium tris-1,10-phenanthroline) has a highly temperature-dependent luminescence and is incorporated in poly(acrylonitrile) to avoid cross-sensitivity to oxygen. Fullerene C70 was used as the oxygen-sensitive probe owing to its strong thermally activated delayed fluorescence at elevated temperatures that is extremely oxygen sensitive. The cross-sensitivity of C70 to temperature is accounted for by means of the temperature sensor. C70 is incorporated into a highly oxygen-permeable polymer, either ethyl cellulose or organosilica. The two luminescent probes have different emission spectra and decay times, and their emissions can be discriminated using both parameters. Spatially resolved sensing is achieved by means of fluorescence lifetime imaging. The response times of the sensor to oxygen are short. The dual sensor exhibits a temperature operation range between at least 0 and 120 degrees C, and detection limits for oxygen in the ppbv range, operating for oxygen concentrations up to at least 50 ppmv. These ranges outperform all dual oxygen and temperature sensors reported so far. The dual sensor presented in this study is especially appropriate for measurements under extreme conditions such as high temperatures and ultralow oxygen levels. This dual sensor is a key step forward in a number of scientifically or commercially important applications including food packaging, for monitoring of hyperthermophilic microorganisms, in space technology, and safety and security applications in terms of detection of oxygen leaks.     

Time-resolved luminescence from annealed quartz

The influence of annealing on the lifetime and luminescence intensity has been studied in quartz annealed at 873, 1073, 1173, and 1273 K and in unannealed quartz. Luminescence was stimulated by pulsed green (525 nm) light. Luminescence intensity increased as a function of temperature from 293 K to a peak at 373 K, and decreased thereafter to 473 K, the maximum temperature in the investigations. Luminescence lifetimes from both unannealed quartz and samples annealed at 873 K for up to 5 min were constant at about 40 micros below 373 K. Lifetimes decreased strictly monotonically with temperature in samples annealed above 873 K. Values of the activation energy for thermal assistance accounting for the increase in luminescence intensity, and of thermal quenching describing the decrease of luminescence intensity and luminescence lifetimes are presented.     

Open air calibration with temperature compensation of a luminescence quenching-based oxygen sensor for portable instrumentation

This report addresses the task of calibrating an optical sensor for oxygen determination. Detailed analyses of the functional dependences from our measurement system results have been carried out with the additional aim of temperature compensation. As a result, an empirical calibration function has been successfully derived for the luminescent quenching-based oxygen sensor included in a self-designed portable instrument. This function also compensates for the temperature influence on the quenching luminescence process in the range from 0 to 45 degrees C. Moreover, the calibration procedure is extremely simple because only a single standard is needed. In fact, the oxygen measurement system can be calibrated with exposure to an open air atmosphere, and therefore, neither laboratory standards nor trained personnel are required. The method has been applied to a set of 11 units of the mentioned sensor (up to 24% oxygen concentration) giving an overall deviation between our calibrated system results and the laboratory standards of 0.3% oxygen concentration (calculated with 95% confidence level). The proposed calibration function has shown itself to be applicable for different sensing film thicknesses and luminophore concentrations using the same fittings parameter. Additionally, this function has been successfully applied to other oxygen dyes. Good agreement has also been found when the performance of the instrument was compared to a commercially available portable instrument based on an electrochemical sensor. We believe that this work could be an interesting finding for spreading the use of optical sensors for atmospheric oxygen determination in commercial measurement equipment for different purposes in confined working atmospheres, such as mines, undergrounds, warehouses, vehicles, and ships.     

Lifetime-based pH sensor system based on a polymer-supported ruthenium(II) complex

A new luminescence lifetime-based pH sensor system is described. The system is based on [Ru(Ph2phen)2DCbpy]2+ (DCbpy = 4,4'-dicarboxy-2,2'-bipyridine) immobilized in a mixed domain network copolymer utilizing hydrophobic regions in a hydrophilic, water-swellable, poly(ethylene oxide) matrix. The metal complex binds irreversibly to the hydrophobic domains leaving the pH-sensing COOHs projecting into an aqueous-rich poly(ethylene oxide) region. The complex shows a strong pH dependence of its lifetime (3-4-fold) and provides a usable pH range of about 3-5. The long (approximately 1 micros) excited-state lifetime and visible absorption of the sensor simplifies measurements. A model for the combined pH and oxygen-quenching sensitivity of the complex is provided; this allows use of the pH system over a wide range of oxygen concentrations. The combined polymer sensor is easy to prepare and requires no covalent chemistry. Further, the polymers enhance the luminescence of the complex and minimize interference from oxygen quenching.     

ON LIFE TIME PREDICTIONS WITH THE STRAIN RANGE PARTITIONING METHOD

Abstract— Computer optimization techniques were applied to the strain range partitioning method for lifetime prediction under high temperature low cycle fatigue loading. The experimental data were evaluated using a simultaneous parameter fit determination of the baseline coefficients. This approach gave good predictions whereas a more conventional method did not work. The significance of the linear damage rule and the interaction damage rule is discussed on the basis of a generalised rule containing both as special cases. This formalism was applied to a ferritic steel and a cast nickelbase-super-alloy. In both cases a very good agreement between calculated and measured lifetimes was found.     

Effect of Annealing Temperature on Magnetic Aftereffect and Positron Lifetime in Fe73.5Cu1Nb3Si13.5B9 Alloy

1. IntroductionFe-based nanocrystalline materials, obtained byappropriately annealed amorphous ribbons above thecrystallization temperature, consist of a--FeSt finegrains and some residual amorphous phase. Thismicrostructure leads to their excellent soft magneticproperties, such as high saturation magnetization andhigh permeabilityll'2]. The quite effective methodo f studying the structure on an atomic scale is themeasurement of the MAE[3-5]. On the other hand,positron annihilation character…     

Thermal degradation of pearlitic steels: influence on mechanical properties including fatigue behaviour

With the aim to predict the durability of railway wheels, thermomechanical damage was studied for two steels with different alloying levels of silicon and manganese in the temperature range of 500–725°C. Softening caused by cementite spheroidisation in pearlite leads to changes in the mechanical behaviour and an accompanying decrease in fatigue lifetimes. It was found that higher contents of Si and Mn lead to better resistance to softening of both virgin and plastically deformed material. Correspondingly, the high Si-Mn alloyed steel loses much less in fatigue lifetime than the lower alloyed steel.     

Optical properties of pure Ytterbium Alluminium perovskites

We present the first report about growth and optical properties of pure Ytterbium Alluminium perovskites single crystals (YbAlO₃). The sample crystal structure was studied and assigned by means of Raman spectroscopy while the photoluminescence measurements from Yb³⁺ charge-transfer state show a broad ultraviolet emission bands with nanosecond lifetimes at room temperature. Yb emissions are also studied as a function of temperature revealing an abrupt quenching in the 180-240 K range. The fast time decay and the high material density suggest Ytterbium Alluminium perovskites crystal as a good candidate for the development of fast scintillators for high energy physics applications.     

Phosphorescent cyclometalated complexes for efficient blue organic light-emitting diodes

Phosphorescent emitters are extremely important for efficient organic light-emitting diodes (OLEDs), which attract significant attention. Phosphorescent emitters, which have a high phosphorescence quantum yield at room temperature, typically contain a heavy metal such as iridium and have been reported to emit blue, green and red light. In particular, the blue cyclometalated complexes with high efficiency and high stability are being developed. In this review, we focus on blue cyclometalated complexes. Recent progress of computational analysis necessary to design a cyclometalated complex is introduced. The prediction of the radiative transition is indispensable to get an emissive cyclometalated complex. We summarize four methods to control phosphorescence peak of the cyclometalated complex: (i) substituent effect on ligands, (ii) effects of ancillary ligands on heteroleptic complexes, (iii) design of the ligand skeleton, and (iv) selection of the central metal. It is considered that novel ligand skeletons would be important to achieve both a high efficiency and long lifetime in the blue OLEDs. Moreover, the combination of an emitter and a host is important as well as the emitter itself. According to the dependences on the combination of an emitter and a host, the control of exciton density of the triplet is necessary to achieve both a high efficiency and a long lifetime, because the annihilations of the triplet state cause exciton quenching and material deterioration.     

Temperature-dependent luminescence of gallium-substituted YAG:Ce

The temperature-dependent lifetime of trivalent cerium was determined in (Y_1−x Ce _x )₃Al_2.5Ga_2.5O₁₂ (where x = 0.1 and 0.2) over a temperature range of 20−120 °C. In both samples, the quenching temperature is significantly lower compared to (Y_1−x Ce _x )₃Al₅O₁₂. The difference in quenching temperatures is explained by evaluating the changes in the lattice, which occur as a result of substituting the Al³⁺ for Ga³⁺. The information presented in this report is useful for future design of phosphors for use as non-contact temperature sensors.     

Luminescent Mononuclear and Dinuclear Iridium(III) Cyclometalated Complexes Immobilized in a Polymeric Matrix as Solid-State Oxygen Sensors

Oxygen quenching of the luminescence of mononuclear and dinuclear Ir(III) cyclometalated complexes immobilized in the pPEGMA matrixes has been studied. Linear Stern-Volmer plots, even when experiments at different emission wavelengths have been performed, were evidenced. Despite the different luminescence lifetimes of the chromophores in the absence of quencher, similar Stern-Volmer slopes have been calculated. This behavior was tentatively interpreted by taking into account the size and charge of the chromophores. Increased sizes and lower charges seem to enhance the sensitivity of the systems. Such findings could be of interest for the design of new solid-state luminescent oxygen sensors with improved performance.     

Two-Dimensional Visualization of Fluorescence Lifetimes by use of a Picosecond Laser and a Streak Camera

Two-dimensional distributions of the effective lifetime of the fluorescence emission induced by short-pulsed laser radiation are obtained from two-dimensional images recorded with a streak camera and a charge-coupled device by means of a separation algorithm method (SAM). In theory, the best response with respect to noise is obtained for lifetimes corresponding to a range of pixels of 5-50 in the CCD, that is, 5-50 ps at the fastest streak speed. In experiments the SAM is compared with pure time-resolved measurements, and it is used for two-dimensional lifetime evaluation. The laser-pulse duration is 25 ps, and the lower limit of the lifetime resolution as used in the experiments is estimated to be 200-250 ps. The results demonstrate the possibility of performing pattern recognition independently of the relative distribution of emission intensity between regions of different fluorescence lifetimes. The technique is demonstrated for static objects but can in principle be extended to nonstationary objects if two detectors are used.     

Fluorescence lifetime standards for time and frequency domain fluorescence spectroscopy

A series of fluorophores with single-exponential fluorescence decays in liquid solution at 20 degrees C were measured independently by nine laboratories using single-photon timing and multifrequency phase and modulation fluorometry instruments with lasers as excitation source. The dyes that can serve as fluorescence lifetime standards for time-domain and frequency-domain measurements are all commercially available, are photostable under the conditions of the measurements, and are soluble in solvents of spectroscopic quality (methanol, cyclohexane, water). These lifetime standards are anthracene, 9-cyanoanthracene, 9,10-diphenylanthracene, N-methylcarbazole, coumarin 153, erythrosin B, N-acetyl-l-tryptophanamide, 1,4-bis(5-phenyloxazol-2-yl)benzene, 2,5-diphenyloxazole, rhodamine B, rubrene, N-(3-sulfopropyl)acridinium, and 1,4-diphenylbenzene. At 20 degrees C, the fluorescence lifetimes vary from 89 ps to 31.2 ns, depending on fluorescent dye and solvent, which is a useful range for modern pico- and nanosecond time-domain or mega- to gigahertz frequency-domain instrumentation. The decay times are independent of the excitation and emission wavelengths. Dependent on the structure of the dye and the solvent, the excitation wavelengths used range from 284 to 575 nm, the emission from 330 to 630 nm. These lifetime standards may be used to either calibrate or test the resolution of time- and frequency-domain instrumentation or as reference compounds to eliminate the color effect in photomultiplier tubes. Statistical analyses by means of two-sample charts indicate that there is no laboratory bias in the lifetime determinations. Moreover, statistical tests show that there is an excellent correlation between the lifetimes estimated by the time-domain and frequency-domain fluorometries. Comprehensive tables compiling the results for 20 (fluorescence lifetime standard/solvent) combinations are given.