Persistent Luminescence of Eu2☎ and Na☎ Doped Alkaline Earth Aluminates

The luminescence and afterglow properties of the Eu^2? and Na^? doped alkaline earth aluminales, stoichiometric and non-stoichiometric (M_x Al₂O₄:Eu^2?, Na^?; M = Ca or Sr, x = 0.97, 1.00 or 1.03, X_Na, = 0.02), were studied. Broad band luminescence and afterglow of the Eu^2? ion were observed in the blue (λ_max = 440nm) and green (λ_max = 520nm) region for the calcium and strontium aluminates, respectively. Both Na^? co-doping and strontium excess quenched the afterglow efficiently. The results supported the mechanism of the persistent luminescence where the cation vacancies act as traps. The results for the calcium aluminates were ambiguous, probably due to the slightly larger ionic radius of the Na^? with respect to that of Ca^2?. The sodium ions may not fit into the calcium sites and thus form (an) independent compound(s).     

Luminescence Properties of Dual Valence Eu Doped Nano-crystalline BaF2 Embedded Glass-ceramics and Observation of Eu2+ → Eu3+ Energy Transfer

Europium doped glass-ceramics containing BaF₂ nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF₂ nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm⁻¹ has been demonstrated from the emission spectra of Eu³⁺ ions showing the transitions from upper excitation states ⁵D_J (J = 1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu³⁺) ions to divalent (Eu²⁺) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu²⁺ to Eu³⁺ ion by radiative trapping or re-absorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu³⁺ to Eu²⁺ under normal air atmospheric condition has been observed in a BaF₂ containing oxyfluoride glass-ceramics system.     

Spectroscopic and Structural Properties of Ca1-xSrxAl2O4:Eu2☎, RE3☎ Persistent Luminescence Materials

The preparation, structure and luminescence of the Ca_1-xSr_xAl₂O₄:Eu^2?, RE^3? system were studied. Monoclinic CaAl₂O₄ was the major phase when the strontium content x was from 0 to 0.6, but hexagonal SrAl₂O₄ was obtained when x was 0.8 and monoclinic SrAl₂O₄ when x was 1. Only slight Ca/Sr cation solid solubility was observed. The strontium ions dissolved better into the CaAl₂O₄ phase than vice versa. Two luminescence bands were observed for mixed compositions, peaking at 440 and 530 nm, corresponding to those of the monoclinic CaAl₂O₄:Eu^2? and hexagonal SrAl₂O₄:Eu^2? ones. The persistent luminescence was enhanced by the Ca/Sr replacement. This observation supports the mechanisms where the lattice defects act as traps.     

Luminescence of Eu2+–vc− dipoles and their associates in CsI:Eu crystals

Spectral-kinetics properties of photo-scintillation excited with single light pulses of a nitrogen laser (λ=337.1 nm, t_1/2=5 ns, Q=1 mJ) have been studied in CsI:Eu crystals at temperature within 80–300 K. It is found that the exponential decay of 463 nm emission band has a time constant which grows from 0.85 μs at 78 K to 1.6 μs at 380 K. Such an anomalous temperature behavior of 463 nm emission decay kinetics is discussed in terms of the crystal thermal expansion. It has been proposed that 463 nm emission is caused by a cluster center consisting of three dipoles Eu²⁺v_c^? bounded with each other in a hexagon. Owing to the exchange resonance in the cluster, the energy passes from an excited dipole to a non-excited one and the distance between them gets longer due to thermal expansion of the crystal.     

The Oxidation Behavior of Vanadium Alloys in Air at Elevated Temperature

Vanadium alloy is one of the most important candidate structural materials for a fusion reactor. Its main advantages over other candidates are its low activity feature and feasible properties at high temperature. However, vanadium is easily oxidized at high temperature to form a non-protective surface film of V2O5. Oxygen concentration would thus get high and the properties get worse in not only the loss of the ductility but also the enhanced hydrogen embrittlement by the oxidation.     

High-Duty-Cycle Operation of GaAs/AlGaAs Quantum Cascade Laser above Liquid Nitrogen Temperature

We present a detailed study of λ- 9.75 μm GaAs/AlGaAs quantum cascade lasers. For a coated 2-rmm-long and 40-μm-wide laser, an optical power of 85 μ W is observed at 95% duty cycle at 80K. At a moderate driving pulse （1 kHz and 1% duty cycle）,the device presents a peak power more than 20mW even at 120K. At 80K, the fitted result of threshold current densities shows evidence of potential cw operation.     

Interacting Fermi Liquid in Two Dimensions¶at Finite Temperature.¶Part I: Convergent Attributions

Using the method of a continuous renormalization group around the Fermi surface, we prove that a two-dimensional interacting system of Fermions at low temperature T is a Fermi liquid in the domain , where K is some numerical constant. According to [S1], this means that it is analytic in the coupling constant λ, and that the first and second derivatives of the self energy obey uniform bounds in that range. This is also a step in the program of rigorous (non-perturbative) study of the BCS phase transition for many Fermion systems; it proves in particular that in dimension two the transition temperature (if any) must be non-perturbative in the coupling constant. The proof is organized into two parts: the present paper deals with the convergent contributions, and a companion paper (Part II) deals with the renormalization of dangerous two point subgraphs and achieves the proof. Received: 27 July 1999 / Accepted: 31 May 2000     

Behaviour of F and Fz Centres Under Thermal Stimulation in KCl:Eu2☎ Irradiated with Ionizing and UV Radiation

The optical absorption (OA) of F and F_z, centres in beta-and UV-irradiated KCl: Eu^2? have been studied. The spectra measured at room temperature and during the subsequent heatings up to 600 K, were analyzed by a deconvolution procedure. The OA bands of the F and F_z centres are well described respectively by asymmetric and symmetric gaussiane with parameters that are linearly dependent on the temperature. The thermal stability of the F_z centres correlates with the low-temperature thermostimulated luminescence (TL) peaks. The stability of the F centres is determined at least by two processes. Part of the F centres is destroyed together with the F_z centres. The remaining ones are destroyed during further heating up to 470 K, correlating with the TL peak at 425 K (at heating rate of 0.1 K/s).     

The charge states’ conversion of the activator ions in CaF2:Eu nanophosphors

According to stationary X-ray-excited luminescence spectra and thermally stimulated luminescence spectra of CaF₂:Eu nanophosphors, it was found that Eu³⁺?→?Eu²⁺ conversion can occur during thermal annealing of fine-grained (d?=?25?nm) nanoparticles in the 200–800°C range, which is accompanied by an increase in their size within 40–189?nm. An important role of the exciton mechanism of Eu²⁺ luminescence excitation was revealed according to the temperature dependence of X-ray-excited luminescence spectra of CaF₂:Eu nanoparticles of 114?nm size. The maximum of the X-ray-excited luminescence light output of CaF₂:Eu nanophosphors in the Eu²⁺ ions’ emission band was traced out at 400–500°C annealing temperature and at the size of nanoparticles of 114–180?nm. The subsequent growth of the annealing temperatures, particularly in the 800–1000°C range, causes the reduction of X-ray-excited luminescence light output because of the increment of lattice defects’ concentration due to a sharp increase in the size of nanoparticles and their agglomeration.     

Electrical Conductivity of Synthetic Quartz Crystals at High Temperature and Pressure from Complex Impedance Measurements

An electrical conductivity measurement system under high-pressure conditions with a multi-anvil high-pressure apparatus by an ac complex impedance method was set up.With this system,we have successfully measured the electrical conductivity of synthetic quartz under pressure up to approximately 1.0GPa in the temperature range 661-987K.The values of electrical conductivity decrease with the increasing pressure and increase with the increasing temperature.The activation enthalpies for the α-quartz crystals are 1.10-1.28eV.The electrical conductivity of α-quartz is ionic,with Na ions moving in channels parallel to the c-axis being the predominant current carrier.     

Explanation of Discrete Electric Relaxation Peak of CeO2: Gd3☎

Low-temperature loss peaks of Gd^3? doped CeO₂ have been investigated with defect calculation of computer simulation. Two types of possible origin of dielectric loss, low symmetry configuration around dopant and defect clusters with net dipole moment, have been modeled. The simulation has suggested that the dipole poling of defect cluster normally occurring at high temperature is the cause of the observed low-temperature loss peaks of CeO₂: Gd^3?.     

Structural,Magnetic and Optical Properties of Rh2☎ Centres in NaCl Lattices: An Explanation

A method for exploring Jahn—Teller systems in the framework of Density Functional Theory is described in the present work. Using such a method the properties of the Rh^2? centre in NaCl with remote charge compensation are studied. The calculations support the existence of a strong Jahn—Teller effect and account reasonably for the main features of EPR and optical spectra. The onset of charge transfer excitations is calculated to be at about 3.7 eV a fact which concurs with available experimental data. The elongated geometry is found to be about 200cm^?1 more stable than the compressed one in agreement with experiments.     

The Temperature Distribution in Longitudinally Pumped Ti:sapphire Lasers

l.IntroductionItiswe1lknownthatthetemperaturcofaso1id-statelasermediawillgethighwhenitispumPedbyflash-lamporlaser,x`hichwi1l1eadtohigheroscillationthersholdandlowerefficiency.Duetotheinhomogeneouspumpandtheinhomogeneouscool,thetem-Peraturedistributioninthemediabecomesinhomogeneoussothattherefractiveindexofthemediachangesinhomogeneouslyandtheheatlenseffectwillbegenerated,whichwillaffectontheosci11ationthreshold,efficiency,andotherlaserparameters.Itisessentia]andimportanttoknowthetemPeraturedis…     

A Study of Electrical Characteristics of Crystals of Homogeneously Doped LiNbO3:Zn,Mg in the Temperature Range of 450–900 K

Technical Physics - In a series of initially polydomain crystals of LiNbO3:Zn,Mg obtained by the homogeneous doping of the crystals in the concentration ranges of around 1 ± 0.02 mol % MgO and...     

Temperature dependence of persistent luminescence in β-irradiated SrAl2O4:Eu, Dy phosphor

SrAl₂O₄:Eu²⁺, Dy³⁺ is a phosphor characterized by a long persistent luminescence (PLUM) when excited with UV-vis light and ionizing radiation exhibiting intensity variation in the 10-320 K temperature range and maximum intensity around 320 K. In this work, we study the PLUM behavior of SrAl₂O₄:Eu²⁺, Dy³⁺ as a function of temperature from room temperature to 670 K in samples exposed to β irradiation. The room-temperature irradiation followed by PLUM readout revealed an integrated PLUM maximum at 323 K decreasing later. In contrast, irradiation and PLUM readout at temperatures above room temperatures produced integrated PLUM intensities maxima around 425 and 625 K. Successive cycles of preheating followed by irradiation and PLUM readout produced an increasing of the PLUM intensity as a function of cycle number. The observed phenomenon was ascribed to trapped electrons at the multiple trapping states related to the 425 and 625 K defects levels and electron transfer from one trap to another (electron hopping). Eventually, there is a return to the 5d level of Eu³⁺ cations with the characteristic PLUM emission by thermal energy supplied at room temperature (lattice vibrations) or by a preheating-irradiation-readout cycle. This property may allow keeping up the PLUM properties of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors through background radiation self exposure and adequate heating processes.     

The Antiferromagnetic Correlations in the Half—Filled Double—Exchange Model at Finite Temperature

We extend a previous result of ours [G.S.Tian,Phys.Rev.B63 (2001) 224413] on the antiferromagnetic spin correlations in the half-filled Hubbard model at finite temperature to the double-exchange model.To overcome the mathematical difficulty cused by the S=3/2 localized spin freedom in this model,we apply both Zener‘s argument and the finite-temperature spin-reflection-positivity method to show rigorously that,at any temperature T,the spin correlations in the half-filled double-exchange model are predominantly antiferromagnetic.This conclusion is completely consistent with the experimental observations and the previous theoretical results by approximate methods.     

Measurements of rotational energy transfer and quenching in OHA 2 σ +,υ′ = 0 at elevated temperature

Total rotational energy transfer rates have been measured at 1330 K for specific rotational levels in the OHA ^{2 +}, = 0 state in collisions with H₂O. Rotational levels ranging fromN = 0 to 15 were studied. Measurements were performed in the post-flame region of a stoichiometric H₂/O₂/He flame operating at 25 mbar. Quenching rates following excitation of individual upper rotational states were also measured. The RET and quenching rates both exhibit monotonic decreases with increasing rotational quantum number.     

Quantum yield of Eu2+ emission in (Ca1−xSrx)S:Eu light emitting diode converter at 20–420 K

A series of red-emitting light converters Ca_1?xSr_xS:Eu²⁺, with tunable composition-dependent emission maxima were synthesized and characterized concerning their photoluminescent (PL) properties. X-ray diffraction patterns, photoluminescence spectra, luminosities and quantum yields were compared for phosphors with strontium concentrations varying from 0 to 100%. The maxima wavelength of emission shifts from 663 down to 619 nm, originating from the dependence of Eu²⁺ 5d state energy on the surrounding crystal field. Upon increasing the temperature from 20 to 420 K, a broadening of emission spectra along with thermal quenching of emission intensity and quantum yield occurs. Satisfying PL properties and their thermal stability demonstrate that the phosphors could be used as light converters in light emitting diodes (LEDs).