Transient behaviour of the mechanoluminescence induced by impulsive deformation of fluorescent and phosphorescent crystals

When a crystal is fractured impulsively by the impact of a moving piston, then initially the mechanoluminescence (ML) intensity increases quadratically with time, attains a peak value and later on it decreases with time. Considering that the solid state ML and gas discharge ML are excited due to the charging and subsequent production of electric field near the tip of moving cracks, expressions are derived for the transient ML intensity I, time t_m and intensity I_m corresponding to the peak of ML intensity versus time curve, respectively, the total ML intensity I_T, and for fast and slow decays of the ML intensity. It is shown that the decay time for the fast decrease of the ML intensity after t_m, is related to the decay time of the strain rate of crystals, and the decay time of slow decay of ML, only observed in phosphorescent crystals, is equal to the decay time of phosphorescence. The value of t_m decreases with the increasing impact velocity, I_m increases with the increasing impact velocity, and I_T initially increases and then it tends to attain a saturation value for higher values of the impact velocity. The values of t_m, I_m and I_T increase linearly with the thickness, area of cross-section and volume of the crystals, respectively. So far as the rise, attainment of ML peak, and fast decay of ML are concerned, there is no any significant difference in the time-evolution of solid state ML, gas discharge ML, and the ML emission consisting of both the solid state ML and gas discharge ML. From the time-dependence of ML, the values of the time-constant for decrease of the surface area created by the movement of a single crack, the time-constant for the decrease of strain rate of crystals, and the decay time of phosphorescence of crystals can be determined. A good agreement is found between the theoretical and experimental results. The importance of fracto ML induced by impulsive deformation of crystals is discussed.     

Mechanoluminescence by impulsive deformation of γ-irradiated Er-doped CaF2 crystals

An impulsive technique has been used for mechanoluminescence (ML) measurements in γ-irradiated Er doped CaF₂ crystals. When the ML is excited impulsively by the impact of moving piston on to γ-irradiated CaF₂:Er crystals, two peaks are observed in ML intensity with time and it is seen that the peak intensities of first and second peaks (I_m1 and I_m2) increase with increasing impact velocity. However the time corresponding to first and second peaks (t_m1 and t_m2) shifts towards shorter time values with increasing impact velocity. It is also seen that the total ML intensity I_Total initially increases with the impact velocity and then it attains a saturation value for higher values of the impact velocity. We have presented a theoretical explanation for the observed results.     

Strong mechanoluminescence induced by elastic deformation of rare-earth-doped strontium aluminate phosphors

When rare-earth-doped strontium aluminate phosphor mixed in an epoxy resin, is deformed elastically by applying a uniaxial pressure, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value I_m at a particular time t_m, and later on it decreases with time. After t_m, initially, the ML intensity decreases exponentially at a fast rate and then it decreases exponentially at a slow rate. The ML appears after a threshold pressure and then, initially at low pressure, the peak intensity I_m of ML increases linearly with the magnitude of applied pressure, and for high pressure, I_m increases exponentially with the magnitude of applied pressure. The value of I_m increases linearly with the density of filled hole traps. The ML emission also takes place during the release of applied pressure. There should be a significant effect of temperature on the ML intensity of rare-earth-doped strontium aluminate phosphors. The ML intensity of rare-earth-doped strontium aluminates decreases with successive number of the applications of pressure and the diminished ML intensity can be recovered with the exposure of the samples to UV-radiation. The ML spectra of rare-earth-doped strontium aluminate phosphors are similar to their photoluminescence spectra. As only the piezoelectric-phase of the strontium aluminate phosphors exhibit ML during their elastic deformation, the ML emission can be attributed to the piezoelectrification of the crystals. Considering that the piezoelectric field causes decrease in the trap-depth of the hole traps and, therefore, the holes transferred from traps to the valence band recombine with (Eu¹⁺)^* ions, whereby the Eu²⁺ ions are excited, expressions are derived for different parameters of ML, which are able to satisfactorily explain the experimental results. It is shown that the lifetimes of holes in the shallow traps in stressed and unstressed materials, and the threshold pressure P_t for the ML emission, and other parameters of the ML, can be determined from the ML measurements. Finally, the criteria for tailoring strong elasico-mechanoluminescent materials are explored.     

Mechanoluminescence Study of Europium Doped CaZrO<Subscript>3</Subscript> Phosphor

Behaviour displayed by mechanoluminescence (ML) in CaZrO₃:Eu³⁺ doped phosphors with variable concentration of europium ions are described. When the ML is excited impulsively by the impact of a load on the phosphors the ML intensity increases with time, attains a maximum value and then it decreases. In the ML intensity versus time curve, the peak increases and shifts towards shorter time values with increasing impact velocities. Sample was synthesized by combustion synthesis method with variable concentration of Eu³⁺ ions (0.1, 0.2, 0.5, 1, 1.5 mol%) and characterized by X-ray diffraction technique. The total ML intensity I_T is defined as the area below the ML intensity versus time curve. Initially I_T increases with impact velocity V₀ of the load and then it attains a saturation value for higher values of impact velocities which follow the relation I_T = I_T ⁰ exp.(?V_c/V₀) where I_T ⁰ and V_c are constants. Total ML intensity increases linearly with the mass of the phosphors for higher impact velocities. The ML intensity I_m, corresponding to the peak of ML intensity versus time curve increases linearly with the impact velocities. The time t_m, is found to be linearly related to 1000/V₀. The mechanoluminescence induced by impulsive excitation in europium doped CaZrO₃ phosphors plays a significance role in the understanding of biological sensors and display device application.     

Mechanoluminescence response to the plastic flow of coloured alkali halide crystals

The present paper reports the luminescence induced by plastic deformation of coloured alkali halide crystals using pressure steps. When pressure is applied onto a γ-irradiated alkali halide crystal, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value and later on it decreases with time. The ML of diminished intensity also appears during the release of applied pressure. The intensity I_m corresponding to the peak of ML intensity versus time curve and the total ML intensity I_T increase with increase in value of the applied pressure. The time t_m corresponding to the ML peak slightly decreases with the applied pressure. After t_m, initially the ML intensity decreases at a fast rate and later on it decreases at a slow rate. The decay time of the fast decrease in the ML intensity is equal to the pinning time of dislocations and the decay time for the slow decrease of ML intensity is equal to the diffusion time of holes towards the F-centres. The ML intensity increases with the density of F-centres and it is optimum for a particular temperature of the crystals. The ML spectra of coloured alkali halide crystals are similar to the thermoluminescence and afterglow spectra. The peak ML intensity and the total ML intensity increase drastically with the applied pressure following power law, whereby the pressure dependence of the ML intensity is related to the work-hardening exponent of the crystals. The ML also appears during the release of the applied pressure because of the movement of dislocation segments and movements of dislocation lines blocked under pressed condition. On the basis of the model based on the mechanical interaction between dislocation and F-centres, expressions are derived for the ML intensity, which are able to explain different characteristics of the ML. From the measurements of the plastico ML induced by the application of loads on γ-irradiated alkali halide crystals, the pinning time of dislocations, diffusion time of holes towards F-centres, the energy gap E_a between the bottom of acceptor dislocation band and the energy level of interacting F-centres, and work-hardening exponent of the crystals can be determined. As in the elastic region the strain increases linearly with stress, the ML intensity also increases linearly with stress, however, as in the plastic region, the strain increases drastically with stress and follows power law, the ML intensity also increases drastically with stress and follows power law. Thus, the ML is intimately related to the plastic flow of alkali halide crystals.     

Luminescence induced by elastic deformation of ZnS:Mn nanoparticles

When the thin film of ZnS:Mn nanoparticles deposited on a glass substrate is elastically deformed by applying a load, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value I_m at a particular time t_m, and later on it decreases with time. The rise and decay characteristics of the ML produced during release of the load are also similar to those produced during the application of load. Similar rise, occurrence of peak and then decrease in ML intensity are also found, when the film is deformed impulsively by dropping a steel ball of small mass from a low height; however, in this case, the time durations for the occurrence of ML and decay time of ML are very short. In the cases of loading and impulsive deformation ,after t_m, initially the ML intensity decreases at a fast rate and then at a slow rate, in which the decay time of fast decrease is equal to the time-constant for rise of pressure and the decay time for slow decrease is equal to the relaxation time of the surface charges. In the case of loading, the peak intensity I_m and the total intensity I_T of ML increase quadratically with the magnitude of applied pressure; however, in the case of impulsive deformation, both the I_m and I_T increase linearly with the height through which the ball is dropped on to the sample. In the case of deformation of the samples at a fixed strain rate, I_m should increase linearly with the applied pressure. The elastico ML in ZnS:Mn nanoparticles can be understood on the basis of the piezoelectrically-induced electron detrapping model, in which the local piezoelectric field near the Mn²⁺ centres reduces the trap-depth, and therefore, the detrapping of filled electron traps takes place, and subsequently the energy released non-radiatively during the electron-hole recombination excites the Mn²⁺ centres and de-excitation gives rise to the ML. The equal number of photons emitted during the application of pressure, release of pressure, and during the successive applications of pressure, indicates that the detrapped electron-traps get filled during the relaxation of the surface charges induced by the application and release of pressure because the charge carriers move to reduce the surface charges. On the basis of the piezoelectrically-induced electron detrapping model, expressions are derived for different characteristics of the ML of ZnS:Mn nanoparticles and a good agreement is found between the theoretical and experimental results. The expressions explored for the dependence of ML intensity on several parameters may be useful in tailoring the suitable nanomaterials capable of exhibiting ML during their elastic deformation. The values of the relaxation time of surface charges, time-constant for the rise of pressure, and the threshold pressure can be determined from the measurement of the time-dependence of ML. It seems that the trapping and detrapping of charge carriers in materials can be studied using ML.     

Mechanoluminescence and thermoluminescence of Mn doped ZnS nanocrystals

This paper reports the synthesis of ZnS:Mn nanocrystals by the chemical route in which mercaptoethanol was used as the capping agent. The particle size of such nanocrystals was measured using XRD and TEM patterns and was found to be in between 3and 5 nm. It was found that the peak position of TL glow curve and the TL intensity of ZnS:Mn nanoparticles increases as the particle size is decreased. The isothermal decay technique is used to determine the trap-depth. The stability of the charge carriers in the traps increase with the decrease in size of the nanoparticles. The higher stability may be attributed to the higher surface/volume ratio and also to the increase in the trap-depth with decreasing particle size. When a ZnS:Mn nanocrystal is deformed the peak intensity I_m increases linearly with the increasing height of the load. After I_m, initially the ML intensity decreases at a fast rate, and later on it decreases at a slow rate. The ML in ZnS:Mn nanocrystals can be understood on the basis of the piezoelectrically induced electron detrapping model.     

Mechanoluminescence and lyoluminescence characterization of Li2BaP2O7:Eu phosphor

In this work mechanoluminescence and lyoluminescence properties of Li₂BaP₂O₇: Eu phosphor are reported. Phosphor was synthesized through high temperature solid state diffusion method. Analysis of phosphor was made through various characterization techniques such as mechanoluminescence (ML), lyoluminescence (LL), x-ray powder diffraction (XRD), scanning electron microscope (SEM) and photoluminescence (PL). It was observed that ML intensity showed good enhancement with variation in time, concentration of dopant Eu, mass of piston and impact velocity. Lyoluminescence intensity was also found to increase with change in time and mass of the sample. Variation in gamma doses imparted to Li₂BaP₂O₇: Eu phosphor was observed to affect both the ML and LL intensities' respectively. Both the ML and LL intensity attain a maximum value I_m at a particular time t_m but afterwards, it decreases and finally disappears. Morphology of Li₂BaP₂O₇: Eu luminescent material was also studied using scanning electron microscope technique. The average particle size in Eu doped lithium barium diphosphate phosphor was around 2 μm.     

Development of mechanoluminescence technique for impact studies

A new technique called, mechanoluminescence technique, is developed for measuring the parameters of impact. This technique is based on the phenomenon of mechanoluminescence (ML), in which light emission takes place during any mechanical action on solids. When a small solid ball makes an impact on the mechanoluminescent thin film coated on a solid, then initially the elastico ML (EML) intensity increases with time, attains a maximum value I_m at a particular time t_m, and later on it decreases with time. The contact time T_c of ball, can be determined from the relation T_c=2t_c, where t_c is the time at which the EML emission due to compression of the sample becomes negligible. The area from where the EML emission occurs can be taken as the contact area A_c. The maximum compression h is given by h=A_c/(πr), where r is the radius of the impacting ball, and thus, h can be determined from the known values of A_c and r. The maximum force at contact is given by F_m=(2mU₀)/T_c, where m is the mass of the impacting ball and U₀ is the velocity of the ball at impact. The maximum impact stress σ_m can be obtained from the relation, σ_m=F_m/A_c=(2mU₀)/(T_cA_c). Thus, ML provides a real-time technique for determining the impact parameters such as T_c, A_c, h, F_m and σ_m. Using the ML technique, the impact parameters of the SrAl₂O₄:Eu film and ZnS:Mn coating are determined. The ML technique can be used to determine the impact parameters in the elastic region and plastic region as well as fracture. ML can also be used to determine the impact parameters for the collision between solid and liquid, if the mechanoluminescent material is coated on the surface of the solid. The measurement of fracto ML in microsecond and nanosecond range may provide a tool for studying the fragmentations in solids by the impact. Using the fast camera the contact area and the depth of compression can be determined for different intervals of time.     

Pulse-induced mechanoluminescence of ultraviolet-irradiated SrAl2O4:Eu,Dy phosphors

The SrAl₂O₄:Eu,Dy phosphors prepared by solid state reaction technique in a reduced atmosphere of 95% Ar+5% H₂ exhibit very intense mechanoluminescence (ML) which can be seen in daylight with naked eye. When the phosphors are deformed by the impact of a low-power electric hammer, initially the ML intensity increases with time, attains a maximum value and then decreases with time. After the threshold pressure, the peak of ML intensity I_m and the total ML intensity I_T increase with the increasing value of the impact pressure. For the ML excited by the pressure pulse of short duration, two decay times of ML are observed; however, for the ML excited by the pressure pulse of long duration, only one decay time is observed. The ML intensity decreases with successive applications of pressure on SrAl₂O₄:Eu,Dy phosphors. For the low applied pressure in the range below the limit of elasticity recovery of ML intensity takes place when the sample is exposed to ultraviolet (UV) light. This fact indicates that the vacant traps produced during the application of pressure pulses get filled during the exposure of the sample to UV light. The ML in the elastic region of SrAl₂O₄:Eu,Dy phosphors can be understood on the basis of the piezoelectrically induced detrapping model. The non-irradiated SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors exhibit ML during the fracture of the compact mass of phosphors whose ML intensity is less when compared to that of the UV-irradiated compact masses. The ML induced by pressure pulses may be useful for determining the magnitude and rise time of unknown pressure pulses and to determine the lifetime of charge carriers in shallow traps.     

Mechanoluminescence and thermoluminescence of BaFCl:Sm 2+ and BaFBr:Sm 2+ crystals

The alkaline-earth fluorohalide crystals MFX, where M=Ca, Sr, Ba, Pb and X=Cl, Br, I, form an important class of materials crystallizing in the PbFCl-type tetragonal structure which is also called the matlockite structure. These compounds have long been of interest because of the various defect species which can be detected by spin resonance and associated techniques. The crystals were prepared by slow cooling of the melt of a stoichiometric mixture of BaF ₂ and the corresponding chloride or bromide under 0.2 bar of ultrapure argon (5N5), often slightly fluorinated. We have studied the mechanoluminescence (ML) of BaFBr:Sm ²⁺ and BaFCl:Sm ²⁺ crystals. It is seen that after the impact of a moving piston, initially the ML intensity increases with time, attains a maximum value and then it decreases with time up to a particular minimum value, and then it increases again, attaining a peak value and finally disappears. The first peak lies in the deformation region and the second peak lies in the post-deformation region. The ML intensity of the BaFCl:Sm ²⁺ crystal is much higher than the ML intensity of the BaFBr:Sm ²⁺ crystal. For different impact velocities, the ML intensity increases with velocity; and the total ML intensity attains a saturation value for higher impact velocities. The total ML intensity increases with the increase in the applied load. It is suggested that the moving dislocation produced during deformation of crystals captures holes from hole-trapped centers (like H centers), and the subsequent radiative recombination of the dislocation holes with electron gives rise to ML. Thermoluminescence (TL) of BaFBr:Sm ²⁺ and BaFCl:Sm ²⁺ crystals was studied after exposure to ultraviolet rays with the help of a TLD reader. The peak of TL for the BaFBr:Sm ²⁺ crystal is found at ～247°C and for BaFCl:Sm ²⁺ crystals at 283°C. The TL intensity initially increases with increase in the UV radiation and then it attains saturation for higher values of UV exposure. The absorption spectrum was recorded with the help of a UV–visible spectrophotometer (Shimadzu). The band found at 275 nm was attributed to H centers.     

Modeling of transient electroluminescence overshoot in bilayer organic light-emitting diodes using rate equations

When a voltage pulse is applied under forward biased condition to a spin-coated bilayer organic light-emitting diode (OLED), then initially the electroluminescence (EL) intensity appearing after a delay time, increases with time and later on it attains a saturation value. At the end of the voltage pulse, the EL intensity decreases with time, attains a minimum intensity and then it again increases with time, attains a peak value and later on it decreases with time. For the OLEDs, in which the lifetime of trapped carriers is less than the decay time of the EL occurring prior to the onset of overshoot, the EL overshoot begins just after the end of voltage pulse. The overshoot in spin-coated bilayer OLEDs is caused by the presence of an interfacial layer of finite thickness between hole and electron transporting layers in which both transport molecules coexist, whereby the interfacial energy barrier impedes both hole and electron passage. When a voltage pulse is applied to a bilayer OLED, positive and negative space charges are established at the opposite faces of the interfacial layer. Subsequently, the charge recombination occurs with the incoming flux of injected carriers of opposite polarity. When the voltage is turned off, the interfacial charges recombine under the action of their mutual electric field. Thus, after switching off the external voltage the electrons stored in the interface next to the anode cell compartment experience an electric field directed from cathode to anode, and therefore, the electrons move towards the cathode, that is, towards the positive space charge, whereby electron–hole recombination gives rise to luminescence. The EL prior to onset of overshoot is caused by the movement of electrons in the electron transporting states, however, the EL in the overshoot region is caused by the movement of detrapped electrons. On the basis of the rate equations for the detrapping and recombination of charge carriers accumulated at the interface expressions are derived for the transient EL intensity I, time t_m and intensity I_m corresponding to the peak of EL overshoot, total EL intensity I_t and decay of the intensity of EL overshoot. In fact, the decay prior to the onset of EL overshoot is the decay of number of electrons moving in the electron transporting states. The ratio I_m/I_s decreases with increasing value of the applied pulse voltage because I_m increases linearly with the amplitude of applied voltage pulse and I_s increases nonlinearly and rapidly with the increasing amplitude of applied voltage pulse. The lifetime τ_t of electrons at the interface decreases with increasing temperature whereby the dependence of τ_t on temperature follows Arrhenius plot. This fact indicates that the detrapping involves thermally-assisted tunneling of electrons. Using the EL overshoot in bilayer OLEDs, the lifetime of the charge carriers at the interface, recombination time of charge carriers, decay time of the EL prior to onset of overshoot, and the time delay between the voltage pulse and onset time of the EL overshoot can be determined. The intense EL overshoot of nanosecond or shorter time duration may be useful in digital communication, and moreover, the EL overshoot gives important information about the processes involving injection, transport and recombination of charge carriers. The criteria for appearance of EL overshoot in bilayer OLEDs are explored. A good agreement is found between the theoretical and experimental results.     

Mechanoluminescence induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates

Mechanoluminescence (ML) emission from coloured alkali halide crystals takes place during their elastic and plastic deformation. The ML emission during the elastic deformation occurs due to the mechanical interaction between dislocation segments and F-centres, and the ML emission during the plastic deformation takes place due to the mechanical interaction between the moving dislocations and F-centres. In the elastic region, the ML intensity increases linearly with the strain or deformation time, and in this case, the saturation region could not be observed because of the beginning of the plastic deformation before the start of the saturation in the ML intensity. In the plastic region, initially the ML intensity also increases linearly with the strain or deformation time, and later on, it attains a saturation value for large deformation. When the deformation is stopped, initially the ML intensity decreases at a fast rate; later on, it decreases at a slow rate. The decay time for the fast decrease of the ML intensity gives the relaxation time of dislocation segments or pinning time of the dislocations, and the decay time of the slow decrease of the ML intensity gives the diffusion time of holes in the crystals. The saturation value of the ML intensity increases linearly with the strain rate and also with the density of F-centres in the crystals. Initially, the saturation value of the ML intensity increases with increasing temperature, and for higher temperatures the ML intensity decreases with increasing temperature. Therefore, the ML intensity is optimum for a particular temperature of the crystals. From the ML measurements, the relaxation time of dislocation segments, pinning time of dislocations, diffusion time of holes and the energy gap between the bottom of the acceptor dislocation band and interacting F-centre level can be determined. Expressions derived for the ML induced by elastic and plastic deformation of coloured alkali halide crystals at fixed strain rates indicates that the ML intensity depends on the strain, strain rate, density of colour centres, size of crystals, temperature, luminescence efficiency, etc. A good agreement is found between the theoretical and experimental results.     

Persistent mechanoluminescence induced by elastic deformation of ZrO2:Ti phosphors

ZrO₂:Ti phosphors show such a strong mechanoluminescence (ML) that it can be seen in day light with naked eye. When a pellet of ZrO₂:Ti phosphor mixed in epoxy resin is deformed in the elastic region at a fixed strain rate using a testing machine, ML intensity increases linearly with time, and when the deformation is stopped, ML intensity decreases exponentially with time. For a given strain rate, ML intensity increases linearly with pressure, and for a given pressure, ML intensity increases linearly with the strain rate. The total ML intensity, in the deformation region, increases quadratically with pressure; however, the total ML intensity in the post-deformation region increases linearly with pressure. ML intensity decreases with successive number of pressings, whereby the reduced ML intensity can be recovered by UV-irradiation of the sample. ML intensity increases linearly with density of filled electron traps and it is optimum for a particular concentration of Ti in ZrO₂. ML intensity should change with increasing temperature of the phosphors. Although ZrO₂ is non-piezoelectric as a whole, it seems that the local structures near the Ti ions in ZrO₂ crystals are in the piezoelectric phase. The elastico ML in ZrO₂ phosphors can be understood on the basis of the localized piezoelectrification-induced detrapping model. According to this model, the localized piezoelectric field near Ti ions causes detrapping of electrons and subsequently the detrapped electrons moving in the conduction band are captured by the energy state of excited Ti⁴⁺ ions, whereby excited Ti⁴⁺ ions are produced and consequently the decay of excited Ti⁴⁺ ions gives rise to the light emission. The expressions derived on the basis of this model are able to explain satisfactorily the characteristics of ML. The relaxation time of localized piezoelectric charges and the threshold pressure for the ML emission can be determined from ML measurements. The long decay of elastico ML indicates the possibility of exploring persistent elastico ML, which may be useful for the fabrication of dim light sources capable of operating without any external power.     

Tetrahedral manganese (II) complexes—intense and unique type of mechanoluminophores

Intense and unique type of mechanoluminescence (ML) is found in tetrahedral manganese (II) complexes. During the excitation of ML by the impact of a piston onto the crystal, the ML intensity initially increases with time, attains a maximum value and then decreases. After retardation of the piston, the decay rate of ML is faster during crystal deformation; however, its value decreases after cessation of the deformation and becomes equal to the decay rate of phosphorescence. The ML disappears below the melting point. Since the crystals of tetrahedral manganese (II) complexes are centrosymmetric, the local non-centrosymmetric sites near the defects are attributed to be responsible for the mechanoluminescence excitation.     

Possibility of elastico-mechanoluminescence dosimetry using alkali halides and other crystals

The elastico-mechanoluminescence (EML) intensity of X or γ-irradiated alkali halide crystals can be used in radiation dosimetry. The EML intensity of X or γ-irradiated alkali halide crystals increases linearly with the strain of the crystals, and when the crosshead of the testing machine deforming an X or γ-irradiated crystal is stopped, then the EML intensity decreases with time. The semilog plot of the EML intensity versus (t − t_c) (where t_c is the time where the crosshead of the testing machine is stopped) indicates that, in the post-deformation region, the EML intensity initially decreases exponentially at a fast rate and later on it decreases exponentially at a slow rate. The EML intensity increases linearly with the density of the F-centres in the crystals. This fact indicates that elastico-ML can suitably be used for the radiation dosimetry. The EML spectra of X or γ-irradiated alkali halide crystals are similar to their thermoluminescence spectra. Based on the detrapping of electrons during the mechanical interaction between the dislocation segments and F-centres, an expression is derived, which indicates that the EML intensity should increase linearly with the density of F-centres in the crystals. The expression derived for the decay of EML indicates that the decay time for the fast decrease of EML should gives the pinning time of dislocation segments (lifetime of interacting F-centres), and the decay time for the slow decrease of EML intensity should gives the lifetime of electrons in the shallow traps. As the elastic deformation is non-destructive phenomenon and the EML intensity depends on the radiation dosage given to the alkali halide crystals, similar to the thermoluminescence and photo-stimulated luminescence, the EML of alkali halide crystals and other crystals may be used for the radiation dosimetry. In EML dosimetry, the same crystal can be used number of times because the elastic deformation does not cause permanent deformation in the crystals, and moreover, comparatively the devices needed for the EML measurements are of low cost and very simple. In recent years, a large number of elastico mechanoluminescent materials have been investigated, and the study of their suitability for the radiation dosimetry may be interesting.     

Unconventional photoluminescence effect of SrTiO3 thin film under UV-illumination

We find that PL intensity I(t) of SrTiO₃ thin film measured under UHV condition increases with UV-laser illumination over long time scale of ∼ 2 h. The intensity increase takes place at lower sample temperature as well, 200, 100 K, and 20 K. When O₂ and N₂ gas are introduced into the sample chamber the PL intensity decreases with the UV-illumination time, opposite to the UHV-case. We consider a quantitative thermal energy flow model of the laser-power and heat absorption by the sample, but find that temperature change of the sample is not large enough to account for the time dependent I(t). We propose photo-catalysis effect on STO surface as possible scenario of the PL intensity change.     

混响强度及其衰减规律与脉宽的关系

本文由短脉宽下混响强度的普遍表达式I_r(t,τ)=Kτt^-me^-βt出发,导出对应的可用于长脉宽的混响强度精确表达式,并给出了忽略脉宽尺度范围的指数衰减损失后相应的近似表达式。当指数衰减系数β足够小和脉宽大于有效脉宽τ_m=a_mt(a_m随m增大而减小,m是大于2的实数)时,近程混响强度趋向饱和且随时间t的(m-1)次幂衰减。实验结果与理论符合较好。     

The self-similarity of wall-bounded temporally accelerating turbulent flows

From the study of viscous flow it is known that certain time-dependent laminar problems, such as the impulsively started flat plate and the diffusion of a vortex sheet, possess self-similar solutions. Previous studies of turbulent channel and pipe flows accelerating between two steady states have shown that the flow field evolves in three distinct stages. Furthermore, recent direct numerical simulations have shown that the perturbation velocity, i.e. the surplus velocity from the initial value, in an impulsively accelerating turbulent channel and pipe flow also possesses a self-similar distribution during the initial stage. In here, these results are developed analytically and it is shown that accelerating flows in which the centreline velocity develops as U^∧_c(t) = U₀(t/t₀)^m will possess a self-similar velocity distribution during the initial stage. The displacement thickness of the perturbation velocity is shown to be dependent only on the type of acceleration, and not on the initial Reynolds number, the acceleration rate or the change in Reynolds number. The derived formulas are verified with good agreement against measurements performed in a linearly accelerating turbulent pipe flow and with data from channel flow simulations.