Stimulation von ZnS-Phosphoren mit langwelligem Infrarot

Stimulation experiments with ZnS phosphors, using IR wavelengths from 2 to 15μ, were performed at the temperature of liquid helium. For this purpose a cryostat was constructed which allowed to keep the samples and the screening device at the temperature of liquid helium. Moreover glow curves after different decay times at 6 °K were taken. After excitation of ZnS phosphors a strong release of carriers from relatively deep traps is taking place although the phosphor is kept at the temperature of liquid helium. This emptying of traps is accompanied by a luminous recombination of the released electrons with the activator levels producing an intense afterglow which can be observed over a long time. This phenomenon cannot be explained by thermal release of trapped electrons into the conduction band, followed by recombination with the activator levels because of the depth of the emptied traps. The rate of thermal carrier release was calculated to be about 10^?29 sec^?1 for 0,05 eV deep traps, but the observed rate was of the order of 10^?4 sec^?1. An emptying of traps by IR-stimulation can be excluded because the phosphor was surrounded by a concentric screening device kept at 4.2 °K. The effect can be explained by luminous tunneling of trapped electrons to the activator levels. An estimate of the tunneling rate gives a value agreeing with the experimental results. If such a “tunnel afterglow” does exist a spectral shift to longer wavelengths compared to the usual fluorescence and phosphorescence is to be expected. This could be actually observed. Additional experiments included IR-stimulated emission after various decay times, glow curves taken after such stimulation, and studies of the influence of temperature. The results rule out the possibility that the carriers were released from the traps by IR light. Apparently, IR radiation affects the potential barrier between the traps and the activators so that the rate of recombination by tunneling increases.     

Long lasting phosphorescent properties of Ti doped ZrO2

A novel long-lasting afterglow phosphor ZrO₂:Ti is prepared by the conventional solid-stated method and their luminescent properties are investigated. A bluish white strong and broad emissive band, which is attributed to originate from the recombination of electrons trapped by F⁺ centers and the holes created in Valence band, is observed under 254 nm UV irradiation. The identical color long afterglow, which lasts about 1 h, is found in the ZrO₂:Ti phosphor after removing the 254 nm UV light. The mechanism of the long lasting phosphorescence (LLP) has been discussed based on the thermoluminescence (TL) results. The replacement of Zr by Ti produces more anion vacancies, resulting in the enhanced photoluminescence (PL) and LLP of ZrO₂:Ti sample. These results indicate that ZrO₂:Ti phosphor has potential promising applications.     

Crystal-Size Dependence of Replenishment Effect in BaFBr:Eu

Excitation spectra of photo-stimulated luminescence (PSL) in BaFBr:Eu and BaFBr:Eu ? Na crystals were studied. Replenishment of PSL at liquid helium temperature was investigated in crystals of various sizes including a commercially available X-ray storage phosphor plate (Image Plate). Although a weak replenishment effect was observed in the fine powder crystals, no replenishment effects were observed in Image Plate, while it showed strong PSL at liquid helium temperature. Results indicate that there are two types of PSL centers, one consists of spatially correlated electrons and holes and the other consists of electrons and holes trapped at isolated centers. Lack of the replenishment effect in Image Plate indicates that trapped electrons are stabilized by the surface and need thermal activation to recombine with holes which are possibly trapped at doped Eu^2?.     

Mechanism of long-lasting phosphorescence of Eu 2+ in melilite

UV excitation for several Eu 2+ -doped melilite crystals, Eu 2+ :Ca 2 Al 2 SiO 7 (CASM), Eu 2+ :CaSrAl 2 SiO 7 (CSASM), and Eu 2+ :Sr 2 Al 2 SiO 7 (SASM) produces long-lasting phosphorescence (1-10 2 v s) from Eu 2+ ions besides the intrinsic Eu 2+ luminescence. The distribution of the radiative decay rates is due to the recombination of distant pairs of trapped electrons and holes in the crystals. The intensities of the phosphorescence for these crystals were measured as functions of temperature and time. The most intense phosphorescence was obtained from the Eu 2+ : SASM crystal. The decay curves measured for Eu 2+ :SASM below 400 v K fit t m n ( n h 1). This fact shows that the recombination of the distant pairs occurs through tunneling below 400 v K. The temperature dependence of the intensities integrated in a time domain obeys the Arrhenius's equation with two thermal activation energies including radiative and non-radiative processes. These results suggest that holes and electrons recombine radiatively at Eu 2+ sites in the SASM crystal through thermal hopping and tunneling. On the other hand, Eu 2+ :CASM and Eu 2+ :CSASM show different behavior on the decay curves at low temperatures, satisfying t m n ( n >1). This discrepancy may be removed by several electron and/or hole centers with different trapping energy levels.     

Luminescence properties of a new Mn-activated red long-afterglow phosphor

A new phosphor, CaZnGe₂O₆:Mn²⁺, which emits red long-lasting phosphorescence centered at 648 nm upon UV light excitation, is prepared by the conventional high-temperature solid-state method and its luminescent properties are systematically investigated in this paper. XRD, photoluminescence, thermoluminescence spectra and afterglow decay curve are used to characterize the synthesized phosphor. This phosphor is well crystallized by calcination at 1150 °C for 3 h and possesses excellent performance. The color coordinate values of this phosphor are x=0.64, y=0.26 under 250 nm UV light excitation. Under 250-nm UV light irradiation, this phosphor shows obvious long-lasting phosphorescence that can be seen with the naked eye in the dark clearly after the irradiation source has been removed for more than 3 h. The possible mechanism of this red-light-emitting long-afterglow phosphor is also investigated based on the experiment results.     

The probabilities of triplet-singlet transitions in aromatic hydrocarbons and ketones

Calculations have been performed on the probabilities of triplet-singlet transitions in benzene and acetone in connection with the decay times of the phosphorescence. The transition probability was determined from the extent to which excited singlet states are mixed with the triplet state. The assumption that in benzene the phosphorescent state has ³ B_2u properties—so that the ¹ B_1u singlet state is mixed with it-—yields the best agreement with the experimental data. In the case of acetone the phosphorescence was considered as a triplet-singlet transition in which only the electrons of the CO group are involved. In both cases the agreement with experiment is as satisfactory as in other calculations on transition probabilities, the figures for the decay times being 190 and 7 seconds for benzene and 7·8 and 0·6 milliseconds for acetone. The ratio of the calculated decay times is in good agreement with the experimental result (2·4×10⁴ and 1·2×1O⁴).     

掺稀土离子的电子俘获材料

可利用不等价离子取代法或高能射线辐照法制备电子俘获材料,如长余辉发光材料znO-SiO₂-B₂O₃-MnO、CaS:Eu²⁺,Tm³⁺、CaGa₂S₄:Eu²⁺,Ho³⁺和固体剂量材料Zn_0.96(BO₂)₂:0.04Tb,Sr_0.93Dy_0.07B₄O₇等.利用俘获在缺陷中的电子-空穴对的复合,并把能量传递给Eu²⁺或Mn²⁺等激活离子而产生长余辉发光或热释光.它们将在弱光照明与显示,信息存储和辐射治疗与探测、监控等方面获得应用.     

Morphology and photoluminescence of BaAl12O19:Mn2+ green phosphor prepared by flux method

This paper reports that the green phosphor BaAl11.9O19:0.1Mn2+ is prepared by a flux assisted solid state reaction method.The effect of flux systems on the crystal structure,morphology and luminescent properties of the phosphor are studied in detail.The samples are characterized by the application of x-ray diffraction patterns,scanning electron microscopy patterns,luminescent spectra and decay curves.The results show that a pure phase BaAl12O19 can be achieved at the firing temperature above 1300℃ by adding the proper flux system,the firing temperature is reduced at least 200℃ in comparison with the conventional solid state reaction method.Maximum photoluminescence emission intensity is observed at 517 nm for(AlF3+Li2CO3) flux system under vacuum ultraviolet region(147 nm) excitation.The photoluminescence emission intensity and the decay time of these phosphor is found to be more superior to that of the corresponding sample prepared by the conventional solid state reaction method implying the suitability of this route for the preparation of display device worthy phosphor materials.     

Gas-phase thermometry in a high-pressure cell using BaMgAl<Subscript>10</Subscript>O<Subscript>17</Subscript>:Eu as a thermographic phosphor

We demonstrate the feasibility of laser-induced phosphorescence thermography for gas-phase temperature field measurement in a high-pressure cell. BaMgAl₁₀O₁₇:Eu (BAM) was used as a thermographic phosphor; it shows a blue-shifted laser-induced emission spectrum with increasing temperature. Local temperature was determined from the intensity ratio of two disjunctive emission bands. A new seeding device was developed to suspend the solid thermographic phosphor particles in a gas environment. The particle suspension time was modeled and validated by experiments. The influence of multiple scattering and other aspects of quantitative measurement were examined. The technique is currently capable of measuring up to 650 K, limited by signal intensity. The 2D temperature distributions were measured with a precision better than 60 K at 650 K. Multiple scattering limits the spatial resolution to only about 10 mm along the line of sight.     

Effects of dopant concentrations and firing temperatures on decay kinetics of manganese doped willemite nanopowders

Nanocrystallline willemite, Zn_2−xMn_xSiO₄ (0.5≤x≤5 mol%), doped with variable concentration of divalent manganese ions, phosphor powders were prepared using the simple wet-chemical sol-gel method combined with furnace firing at 800, 900, and 1000 °C. X-ray diffraction (XRD) and high resolution X-ray photoelectron (HR-XPS) scans confirm the presence of willemite phase of Zn₂SiO₄. Laser-induced phosphorescence decay measurements of Zn_2−xMn_xSiO₄ nanophosphors were investigated using high peak power pulsed UV nitrogen laser (λ=337.1 nm). The decay curves show non-single exponential behavior with long term decay rate. Various parameters describing the strength of optical transitions in atoms and molecules such as, Einstein's A and B coefficients, ‘f’, integrated cross-section, and transition dipole moment values have been calculated. The long term decay rate of optical transition parameters was found to be somewhat temperature and concentration dependent.     

Size-dependent photoluminescence properties under vacuum ultraviolet excitation of Zn2SiO4:Mn nanophosphors

The Zn₂SiO₄:Mn²⁺ nanophosphors with different particle sizes were synthesized via the hydrothermal method by adjusting the concentrations of surfactant and the hydrothermal temperature. The behavior of the photoluminescence as a function of phosphor particle sizes under vacuum ultraviolet excitation was investigated. Higher critical quenching concentration with decreasing particle size of the Zn₂SiO₄:Mn²⁺ nanophosphors was observed. This is ascribed to the hindrance of energy transfer between luminescence centers under vacuum ultraviolet excitation. The prolonged decay time in smaller samples provides further evidence that the energy transfer confinement has an effect on the photoluminescence properties.     

气体传热对多层绝热性能影响的试验研究

文中通过建立的能进行夹层气体置换的稳态量热器试验系统,试验分析了夹层气体传热对多层绝热材料有效热导率的影响,重点对置换气体种类、气体压强、材料层数及冷热边界温度对多层材料的影响进行试验研究。试验表明在10—60层/cm层密度范围,真空度低于100Pa时,Kn数属于自由分子状态区域和中间压强区域,此时材料的有效热导率随残留气体热适应系数的增大而减小,并随着真空度的降低而增大,当残留气体为空气时,为保证多层材料的绝热性能,应尽量维持真空度不低于10-2Pa。同时,分析表明为有效降低低真空下稀薄气体传热对多层绝热性能的影响,可以采用综合热适应系数较低的气体置换夹层中的空气,以减少低真空多层绝热材料的有效热导率,改善绝热性能。     

Annihilation kinetics for triplet excitations in organic glasses

The results of investigations of delayed luminescence decay are presented for disordered phenanthrene in the microsecond, millisecond, and second time ranges. At liquid nitrogen temperature, the highly non-exponential decay of both the phosphorescence and the annihilation delayed fluorescence is observed. This character of decay is caused by the relaxation of the electron-excitation energy in the system of energy-disordered centers when a quantity kT is much less than the width of the distribution for the excited-state energy. At the same time, the analysis of the time dependence for the triplet-triplet-annihilation rate exhibits two time intervals in which different kinetics is observed for the triplet excitations annihilation. In the microsecond range, the classical relationship between the phosphorescence and the delayed fluorescence takes place for the system under investigation; i.e., the triplet-triplet-annihilation rate is constant. At large times, the reaction-rate time dependence is described by the power law characteristic of inhomogeneous and low-dimensional systems. When the temperature increases, a transition to the classical behavior is observed throughout the entire time interval.     

石榴石型Cd3Al2Ge3O12:Tb化合物的发光

对具有石榴石型的镉铝锗酸盐Cd3Al2Ge3O12(CAGG)磷光体的合成、光致发光、阴极射线发光及磷光衰减特性进行了研究.254nm紫外光停止激发后,CAGG:Tb呈现强而长的Tb3+离子的特征的黄绿光辐射.基质和磷光体的磷光衰减都符合B=At-α关系式.对这种磷光现象进行了讨论.     

Recombination luminescences resulting from the photoionization of N,N, N′, N′-tetramethyl-p-phenylenediamine in the low temperature matrices. II

The delayed luminescences and thermal glow curves from ultraviolet-irradiated rigid solutions of N, N, N′, N′-tetramethyl-p-phenylenediamine (TMPD) were investigated. The luminescence which comprises the flourescence and phosphorescence of TMPD was affected by the composition of the matrix and an added electron trap, that is, benzene, diphenyl, carbon dioxide, oxygen, etc. It was found that the glow curves for systems containing these electron-trapping reagents show two components, which are concluded to be due, respectively, to electrons trapped by the solvent and these reagents. The electrons trapped by the solvent are released at the lower temperature. These results are indicative of the capabilities of these added molecules to capture electrons in the rigid organic matrices. It has also been found that the glow curve for a concentrated solution of TMPD shows two maxima, suggesting that TMPD itself acts as an electron trap. Oxygen quenches the delayed luminescence and thermoluminescence strongly. The intensity ratios between fluorescence and phosphorescence were measured and correlated with the glow curves.     

Effect of electron cooling on the electron lifetime on liquid 4He

Electron trapped in image potential induced surface states on liquid ⁴He can escape into the gas by thermal ionization. We have calculated the rate of ionization on the assumption that the electrons are strongly coupled to each other and weakly coupled to the liquid and gas. The first electrons to leave the surface extract their ionization energy from the kinetic energy of the remaining electrons. This adiabatic cooling effect rapidly lowers the electron temperature to a small fraction of the temperature of the surrounding gas and liquid.     

Determination of surface normal temperature gradients using thermographic phosphors and filtered Rayleigh scattering

Wall temperature as well as the temperature distribution within or close-by the boundary layer of an electrically heated axisymmetric jet impinging on a flat plate were monitored to deduce wall-normal temperature gradients. The radial surface temperature profile of the plate was determined by coating it with thermographic phosphors (TPs), materials whose phosphorescence decay time is dependent on their temperature. The TP was excited electronically by a frequency-tripled Nd:YAG laser (355 nm) and the temporal decay of the phosphorescence intensity was measured zero-dimensionally by a photomultiplier tube. In this case the 659-nm emission line of Mg₃F₂GeO₄:Mn was monitored. The non-intrusive measurement of gas temperatures near the surface was performed two-dimensionally by filtered Rayleigh scattering (FRS). A tunable frequency-tripled single-longitudinal-mode alexandrite laser beam at 254 nm was formed into a light sheet pointing parallel to the surface. The scattered light was imaged through a very narrow linewidth atomic mercury filter onto an intensified charged coupled device (ICCD). The elastic stray light from surfaces was strongly suppressed, whereas Doppler-broadened light was detected. Thermographic phosphors proved to be reliable for the measurement of surface temperatures. Dependent on the specific experimental conditions, problems appeared with signals interfering with the FRS radiation close-by the surface. Results and challenges of this approach are discussed. PACS 07.20.Dt; 32.50.+d; 44.20.+b; 42.65.Es; 33.20.Fb     

Actual transient photocurrents from a one-donor (1D) photoconductor

Transient responses to be expected from a single-level model, after a light is switched on and then off, are analysed through computer simulations of the complete differential equations of the model. A strong dissymmetry arises between growth and decay transients for free electrons and free holes. Transients for trapped electrons display decreasing or increasing modes. Time constants for growth for free and trapped electrons and free holes vary over many decades of time as functions of both temperature, T, and light intensity, G. A renewed insight is cast into the actual internal processes of charge exchanges between free and localized levels, and a direct quantitative study is made of photoconductivity gain. Gain is shown to display either extremely large values at temperatures far below a so-called temperature T ₀, or a strong loss of sensitivity for temperatures ranging in an extended vicinity of T ₀, including T?>?T ₀.     

Kinetics of the Decay of Prolonged Luminescence of Disordered Chrysene under the Conditions of Nonequilibrium Transfer of Energy

We present the results obtained in investigation of the decay of the prolonged luminescence of disordered chrysene in a microsecond range. At the temperature of liquid nitrogen, a highly nonexponential decay of both phosphorescence and annihilation of delayed fluorescence is observed. The observed character of decay is due to relaxation of the energy of electronic excitation in a system of energydisordered centers, when the value of kT is much smaller than the width of the excitedstate energy distribution. At the same time, in the system investigated there is a classical relationship between phosphorescence and delayed fluorescence, i.e., the rate coefficient for the reaction of triplettriplet annihilation is a constant.     

Decay pathway and high-temperature luminescence of Eu in Ca2Gd8Si6O26

The temperature-dependent luminescence of Eu:Ca₂Gd₈Si₆O₂₆ and its decay pathways are investigated in order to assess the utility of the material as a thermometric phosphor. Non-radiative decays are found to compete with radiative processes even at room temperature. A decay pathway involving decay through charge-transfer states is proposed based on the decay profiles of emissions from ⁵D₁ and ⁵D₀ levels and on the temperature sensitivity of the spectra as observed after excitation by several wavelengths. The implications of this on solid-state lighting are also discussed.