Elastico-mechanoluminescence Intensity Research Articles

Largely enhanced elastico-mechanoluminescence of CaZnOS: Mn2+ by co-doping with Nd3+ ions

In this paper, a large enhancement of red elastico-mechanoluminescence (EML) intensity has been observed in piezoelectric semiconducting CaZnOS: Mn2+ phosphors via the co-doping of Nd3+ ions. The optimal EML intensity of CaZnOS: 1%Mn2+, 1.2%Nd3+ was about ten times than that of CaZnOS: 1%Mn2+. In addition, the relationship between the EML intensity and the applied load was almost linear and the phosphor exhibited recoverable performance under ultraviolet (UV) (254 nm) excitation, which is a potential candidate for pressure sensing sensor applications. The thermoluminescence (TL) results indicated the co-doping of Nd3+ ions not only increased the concentration of traps but also introduced two new types traps. Further investigations revealed that the shallow trap contributed the most to the long afterglow and two deep traps were mainly responsible for the EML performance. Based on the piezoelectricity-induced detrapping model, all the results suggested the enhanced EML intensity of CaZnOS: Mn2+ can attribute to the increased trap quantity and regulated trap concentration generated by the co-doping of Nd3+ ions.

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 − tc) (where tc 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.

Piezoelectrically-induced trap-depth reduction model of elastico-mechanoluminescent materials

Considering the detrapping of charge carriers due to reduction in trap-depth caused by piezoelectric field produced by applied pressure, an expression is derived for the detrapping rate of electrons. Then, an expression is obtained for the rate of generation of excited ions produced during capture of detrapped electrons by Eu3+ ions in persistent luminescent materials or by the energy released during electron-hole recombination in ZnS:Mn crystals. Finally, an expression is explored for the elastico-mechanoluminescence (EML) intensity, which is able to explain satisfactorily the characteristics of EML for the application of static pressure as well as for impact pressure. The total number of detrapped electrons and the total EML intensity are found to increase linearly with the electrostatic energy of the crystals in piezoelectric field. It is shown that the EML intensity should increase with the EML efficiency, number of crystallites (volume of sample), concentration of local piezoelectric regions in crystallites, piezoelectric constant of local piezoelectric regions, average length of the local piezoelectric regions, total number of electron traps, pressing rate, and applied pressure, and it should be higher for the materials having low value of threshold pressure and low value of trap-depth in unstressed condition. On the basis of the piezoelectrically-induced trap-depth reduction model of EML reported in the present investigation novel intense elastico mechanoluminescent materials having repetitive EML with undiminished intensity for successive loadings can be tailored which may find applications in sensing, imaging, lighting, colored displays, and other mechano-optical devices.

Elastico-mechanoluminescent enhancement with Gd^3+ codoping in diphase (Ba,Ca)TiO_3:Pr^3+

Elastico-mechanoluminescence (EML) in diphase (Ba,Ca)TiO3:Pr3+ with 60 mol% Ca content was greatly enhanced by Gd3+ codoping. The optimal EML intensity of (Ba,Ca)TiO3:Pr3+,Gd3+ is higher by 235% than that of (Ba,Ca)TiO3:Pr3+. The decreases of both photoluminescent intensity and reflectivity induced by Gd3+ codoping suggest the introduction of new trap centers. The thermoluminescent (ThL) measurement has been performed to investigate the effect of codoping on the trap depth and concentration. The consistent dependency correlations of EML intensity and ThL integral intensity on the Gd3+ concentration illuminate that the improved EML originates from the increased concentration of traps with suitable depth. A possible EML mechanism for (Ba,Ca)TiO3:Pr3+,Gd3+ is proposed on the basis of these experimental observations.

Elastico-Mechanoluminescence of Thermoluminescent Crystals

Elastico-mechanoluminescence (EML) is a type of luminescence induced by elastic deformation of solids. The present paper reports the elastic-ML of thermoluminescent crystals such as X-or γ-irradiated alkali halide crystals, ZnS:Mn, and ultraviolet irradiated persistent luminescent crystals. Generally, all the elastico-mechanoluminescent crystals are thermoluminescent, but all the thermoluminescent crystals are not the mechanoluminescent. The elastico-mechanoluminescence spectra of crystals are similar to their thermoluminescence spectra. Both the elastico-mechanoluminescence and thermoluminescence arise due to the de-trapping of charge carriers. As elastico-ML of persistent luminescent crystals depends on both the density of filled traps and piezoelectric field, the intense thermoluminescent crystals may not be the intense mechanoluminescent crystals. When a sample of X-or γ-irradiated alkali halide crystal, UV-irradiated persistent luminescent microcrystals mixed in epoxy resin, or a film of ZnS:Mn nanoparticles is deformed in the elastic region by the pressure rising at fixed pressing rate for a particular time, or by a pressure of triangular form, or by a pressure pulse, then after a threshold pressure, initially the EML intensity increases with time, attains a maximum value and later on it decreases with time. In the first case, the fast decay time of EML is related to the time-constant for stopping the moving crosshead of the testing machine; in the second case, generally the fast decay does not appear; and in the third case, the fast decay time is equal to the rise time of the pressure pulse. However, in all the cases, the slow decay time is related to the lifetime of re-trapped charge carriers in the shallow traps lying in the region where the piezoelectric field is negligible. When the sample is deformed by the pressure rising at fixed pressing rate for a particular time, or pressure of triangular form, then the ML appears after a threshold pressure and the transient EML intensity increases linearly with the applied pressure; however, the total EML intensity increases quadratically with the applied pressure. The EML intensity of persistent luminescent crystals decreases with increasing number of pressings. However, when these crystals are exposed to UV light, then the recovery of EML intensity takes place. The mechanical interaction between the bending segment of dislocations and filled electron traps is able to explain the elastico-ML of X-or γ-irradiated alkali halide crystals. However, the piezoelectrically-induced de-trapping model is suitable for explaining the ML of persistent luminescent crystals and ZnS:Mn. The investigation of elastico-ML may be helpful in understanding the thermoluminescence and the investigation of thermoluminescence may be helpful in understanding elastico-ML. Furthermore, similar to the thermoluminescence, the mechanoluminescence may also find application in radiation dosimetry. Expressions are derived for the elastico-ML of thermoluminescent crystals, in which a good agreement is found between the experimental and theoretical results. Finally, the application of the elasticoML of thermoluminescent crystals in light sources, displays, imaging devices, sensing devices, radiation dosimetry and in non-destructive testing of materials are discussed.Contents of Paper

Elastico-mechanoluminescence in CaZr(PO_4)_2:Eu^2+ with multiple trap levels

We report on a novel elastico-mechanoluminescence (EML) phosphor of CaZr(PO4)2:Eu2+ for simultaneous luminescent sensing and imaging to mechanical load by the light-emitting of Eu2+ ions. The EML properties of CaZr(PO4)2:Eu2+ show an intense luminance (above 15 mcd m(-2)), a low load threshold (below 5 N), a broad measurement range for the dynamic load (up to 2000 N), and an accurate linear relationship of EML intensity against the applied load. The excellent EML characteristics are considered to originate from the piezoelectric crystal structure and the multiple trap levels with appropriate depths. An EML mechanism based on the electrons as the main charge carriers is proposed.

Mechanoluminescence of Nanoparticles

Certain nanoparticles exhibit intense light emission during their elastic deformation, called the phenomenon of elastico mechanoluminescence (EML).When ZnS:Mn and SrAl2O4:Eu nanoparticles are deformed in the elastic region by the stress initially increasing with time and then attaining a fixed value or by the stress initially increasing and then decreasing with time or by the impact stress, then after a threshold pressure, initially the EML intensity increases with time, attains a maximum value and later on it decreases with time. For the fixed pressure or slowly decreasing pressure, the slow decay time of EML is equal to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals.In these cases, sometimes, fast decay of EML also appears and it is related to the machine -constant, that is, the decay time of strain- rate after the power to the cross-head of the machine is switched off. For the fast-deformation caused by impact stress, the fast decay time of EML is controlled by the rise time of impact stress, and the slow decay of EML gives the lifetime of retrapped electrons in the shallow traps. For ZnS:Mn and SrAl2O4:Eu nanoparticles, the total EML intensity increases quadratically with the applied pressure. The diminished EML intensity of SrAl2O4:Eu nanoparticles caused by number of pressings can be recovered by exposing the sample to UV-radiation. The tribo ML of ZnS:Mn nanoparticles increases with increasing value of the load placed on the rotating rod inducing tribo ML. The EML spectra of ZnS:Mn and SrAl2O4:Eu nanoparticles are similar to their photoluminescence and electrolumi- nescence spectra. The piezoelectrically-induced detrapping model is found to be suitable for the EML of nanoparticles and the expressions derived on the basis of this model are able to explain successfully the characteristics of the EML of nanoparticles. The EML of nanoparticles has been found useful in stress sensor and an EML-based safety-management monitoring system has been developed. The EML of nanoparticles can be used for the visualizations of stress distribution in solids, stress field near crack-tip, quasidynamic crack-propagation in solids and internal defect in a pipe. The EML intensity of nanoparticles is so intense that it is able to drive a solar cell system and it can be used as a light-source for specific applications. Furthermore, several parameters of nanoparticles can be determined from their EML measurements.

Dynamics of the mechanoluminescence induced by elastic deformation of persistent luminescent crystals

When a composite of suitable dimension formed by mixing the microcrystalline or nanocrystalline persistent luminescent materials in epoxy resin is deformed at a fixed pressing rate, then the elastico mechanoluminescence (EML) emission takes place after a threshold pressure, in which the EML intensity increases linearly with the applied pressure. When the applied pressure is kept constant or decreased linearly, then the EML intensity decreases with time, in which depending on the prevailing condition, the EML intensity initially decreases at a fast rate and then at a slow rate or sometimes it decreases exponentially having only one decay time. When a small ball is dropped from a low height onto the film of a persistent luminescent material, then initially the EML intensity increases with time, attains a peak value and then it decreases initially at a fast rate and later on at a slow rate. In this case, both the peak EML intensity and the total EML intensity increase linearly with the height through which the ball is dropped onto the film. Considering the piezoelectrically induced detrapping model based on successive detrapping of exponentially distributed traps a theoretical approach is made to the dynamics of light emission induced by elastic deformation of persistent luminescent crystals and thin films. It is shown that the EML intensity depends on several parameters such as pressure, pressing rate or strain rate, temperature, density of filled electron traps, piezoelectric constant near defect centers, etc. Both, in the case of slow deformation and impact stress, the fast decay time is related to the time-constant for the decrease of pressing rate of the samples and the slow decay time of EML is related to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. Both, the EML produced during the release of pressure and the EML produced during the successive applications of pressure take place due to the detrapping of retrapped electrons in the vacant electron traps near activator ions, in which retrapping is caused by the thermally released electrons from the filled shallow traps lying in the normal piezoelectric region of the crystals, which get filled during the detrapping of stable traps at the time of increase of pressure. On the basis of the proposed model, the dependence of EML intensity on different parameters, dynamics of EML and physical concepts of the threshold pressure, characteristic piezoelectric field for detrapping, coefficient of deformation detrapping, nonlinear increase of the EML intensity of some crystals at high pressure and higher EML intensity in the crystals having higher coefficient of deformation detrapping can be satisfactorily understood. A good agreement is found between the theoretical and experimental results. It is shown that the present study may be helpful in tailoring the intense persistent elastico mechanoluminescent materials having long lasting time.

Real-time mechanoluminescence sensing of the amplitude and duration of impact stress

The present paper reports the real-time sensing of the amplitude and duration of impact stress using mechanoluminescence (ML) of the films such as ZnS:Mn and SrAl 2O 4:Eu. After the impact of a small ball from a low height onto the film, initially the elastico mechanoluminescence (EML) intensity increases with time, attains a peak value and then it decreases with time, initially at a fast rate and later on at a slow rate. The fast decay time of the EML intensity is related to the rate constant for the rise of impact stress and the slow decay time of EML is equal to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals, which get filled during the detrapping of thermally stable traps at the time of the increase of pressure. Both the peaks of EML intensity and total EML intensity increase linearly with the height through which the ball is dropped onto the films. The EML spectra are similar to the corresponding photoluminescence and electroluminescence spectra. On the basis of the localized piezoelectrically induced electron detrapping model, expressions are derived for different parameters of the impact stress-induced EML of the films, whereby a good agreement is found between the experimental and theoretical results. As the EML intensity depends on the impact stress, the impact stress can be sensed by measuring the EML intensity. Furthermore, the duration of stress is related to the time t m corresponding to the peak of the EML intensity versus time curve; hence, the pulse duration of the impact stress can be monitored by measuring the value of time t m .

Suitable materials for elastico mechanoluminescence-based stress sensors

Whereas the elastico mechanoluminescence (EML) of certain crystals increases linearly with the stress, nonlinearity occurs in the EML intensity versus stress plot of several crystals. The EML of crystals can be understood on the basis of piezoelectrically-induced detrapping model, whereby the localized piezoelectric field causes detrapping of electrons or holes and subsequently the capture of electrons in the excited states of activator ions, recombination of electrons in hole captured centres, recombination of holes in electron-captured centres or simply the electron–hole recombination gives rise to the light emission. Considering the piezoelectrically-induced detrapping model of EML expression is derived for the stress dependence of the EML intensity. It is shown that the crystals having uniform distribution of traps show linear relationship between the EML intensity and stress and the crystals having exponential distribution of traps show nonlinear relationship between the EML intensity and stress. The crystals having linear dependence of EML intensity on stress are suitable for the fabrication of EML-based stress sensors. The values of coefficient of deformation detrapping, relaxation time of the crosshead of the machine used to deform the samples and lifetime of the charge carriers in the shallow traps lying in the normal piezoelectric region of the crystals can be determined from the EML measurements. The values of the coefficient of deformation detrapping are 0.310, 0.018 and 0.021 MPa −1 for SrMgAl 6O 11:Eu, Sr 2MgSi 2O 7:Eu and SrCaMgSi 2O 7:Eu crystals, respectively. The coefficient of deformation detrapping is low for SrAl 2O 4:Eu, SrAl 2O 4:Eu, Dy, SrBaMgSi 2O 7:Eu and ZnS:Mn crystals and such crystals are suitable for EML-based stress sensors. A good agreement is found between the theoretical and experimental results.

Mechanoluminescence glow curves of rare-earth doped strontium aluminate phosphors

The present paper reports the mechanoluminescence (ML) glow curves of rare-earth doped strontium aluminate phosphors. When Sr3Al2O6:Eu, Dy phosphor mixed in epoxy resin is compressed at a fixed pressing rate or fixed strain rate, its elastico ML (EML) intensity increases linearly with deformation time or pressure and attains a maximum value Im at the time tm, at which the deformation is stopped. Under the pressed condition, the decay time for fast decrease of EML after tm, gives the time-constant for stopping the cross-head of the testing machine used to deform the sample, and decay time for slow decrease of EML gives the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. When SrAl2O4:Eu phosphor mixed in resin is compressed at a fixed rate, then the EML intensity increases linearly with pressure and when the pressure is decreased at a fixed rate, then the EML intensity decreases exponentially with time, in which the decay time of EML is equal to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. The EML intensity of SrAl2O4:Eu film excited by the impact stress, initially increases with time, attains a peak value and later on it decreases exponentially with time, in which the fast decay of EML intensity gives the decay time of impact stress and the decay time of the slow decrease of the EML intensity gives the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. The piezoelectrically-induced detrapping model is found to be suitable for the EML of rare-earth doped strontium aluminate phosphors. Expressions derived on the basis of the piezoelectrically-induced detrapping model are able to explain satisfactorily the characteristics of the EML of the phosphors. It is shown that several parameters of the phosphors can be determined from the ML glow curves.