BaSi2O2N2:Eu is a very intense elastico-mechanoluminescent and photoluminescent materials. The microcrystalline BaSi2O2N2:Eu&nbsp; can be prepared using solid state reaction technique in reduced atmosphere. Then the microcrystalline phosphor was mixed epoxy and the samples of desired dimension size were prepared. For the ML excitation the sample was pressed using a material testing machine.&nbsp; When a compressive load is applied on the BaSi2O2N2:Eu sample, mixed in epoxy, then initially the elastico-mechanoluminescence (EML) intensity increases linearly with time, attains a peak value and later on it decreases with time. The recovery of EML intensity of previously to compressed crystals with ultraviolet light reveals non-destructive phenomenon of EML. The EML of&nbsp; BaSi2O2N2:Eu crystals can be understood on the basis of the piezoelectrically induced electron detrapping model

Chandra, B P

Jha, P

Vhandra, V K

English

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Recent Research in Science and Technology 2012, 4(8): 64-66 ISSN: 2076-5061 Available Online: http://recent-science.com/     Luminescence induced by elastic deformation of BaSi2O2N2:Eu crystals  P. Jha1, V.K.Vhandra2 and B. P. Chandra3  1Department of Applied Physics, Raipur Institute of Technology, Mandir Hausad, Raipur, 492101(C.G.), India. 2Department of Electrical and Electronics Engineering, Chhatrapati Shivaji Institute of Technology, Shivaji Nagar, Kolihapuri,  Durg 491001 (C.G.), India. 3Deparment of Postgraduate Studies and Research in Physics, Rani Durgavati University, Jabalpur 482001(M.P.), India.  Abstract  BaSi2O2N2:Eu is a very intense elastico-mechanoluminescent and photoluminescent materials. The microcrystalline BaSi2O2N2:Eu  can be prepared using solid state reaction technique in reduced atmosphere. Then the microcrystalline phosphor was mixed epoxy and the samples of desired dimension size were prepared. For the ML excitation the sample was pressed using a material testing machine.  When a compressive load is applied on the BaSi2O2N2:Eu sample, mixed in epoxy, then initially the elastico-mechanoluminescence (EML) intensity increases linearly with time, attains a peak value and later on it decreases with time. The recovery of EML intensity of previously to compressed crystals with ultraviolet light reveals non-destructive phenomenon of EML. The EML of  BaSi2O2N2:Eu crystals can be understood on the basis of the piezoelectrically induced electron detrapping model.  Keywords: Luminescence, crystals, BaSi2O2N2:Eu  INTRODUCTION       Mechanoluminescence (ML) is a type of luminescence induced by any mechanical action on solids. The light emissions induced by elastic deformation, plastic deformation and fracture of solids are called elastico-ML, plastico-ML and fracto-ML, respectively. The ML induced by rubbing of solids or separation of two solids in contact is known as tribo-ML or triboluminescence [1]. In recent years, ML materials exhibiting intense ML during their elastic deformation and fracture have been reported to be useful for stress sensors [2, 3], fracture sensors [4, 5], safety management monitoring system [6], damage sensors [7] and in the fuse systems for army warheads [8]. ML has also been reported to be useful in online monitoring of the grinding process in milling machines [9] and in radiation dosimetry [1].      Persistent luminescent materials are phosphors which are able to emit light for a long time after excitation [9]. This remarkable ‘afterglow’ of persistent luminescence originates in energy storage in the phosphor by trapping of charge carriers in long-lived energy levels inside the band gap. This stored energy can then be released by emission of light after thermal de-trapping of the charge carriers. Many details concerning persistent luminescence, such as the origin of the traps or the transfer pathway from the traps to the luminescent centers remain unclear [10]. This paper reports the strong non-destructive elastico ML of BaSi2O2N2:Eu phosphors.   Theory        For the measurement of EML of persistent mechanoluminescent materials, the microcrystals or nanocrystals are mixed in epoxy resin and then the samples of suitable dimension are made using a mold. The EML in the sample is excited by applying a load. If Ω is the activation volume near an activator ion at which the piezoelectric constant is high, Nc is the number of crystallites in the sample, Nl is the number of activators  in a crystallite and Nt is the concentration of traps, then the total number of traps in the sample is given by, N0 = ΩNcNlNt. Considering the exponential distribution of traps in the activation volume near the activator ions in the crystallites, the number of traps Nc (Ei) of energy Ei is given by the following Boltzmann statistical formula [11-13]  )exp()( 0 ii ZEZNEN −=          …(1)  where N0 is the total number of traps in the activation volume of the crystallites and Z=1/kT  is the distribution coefficient, in which k is the Boltzmann constant and T is the absolute temperature of the crystals.      Up to the threshold energy Eth the trap-depths are comparable to kT, and therefore, the shallow traps lying between zero and Eth are thermally detrapped. Thus, the detrappable traps will be present only from Eth to E and using Eq. (1), the total number Nt of detrappable traps from Eth to E can be expressed as ∫ −=EE0tthdEZEZNN )exp( or, )]exp()[exp( ZEZENN th0t −−−=      …(2)      When an external pressure is applied, then the piezoelectric field F is produced. If α is the decrease in the trap-depth per unit electric field, then the decrease in trap-depth for the field F will be αF. Due to the decrease in trap-depth detrapping of electrons will take place and consequently the total number of filled traps will decrease. If Nd is the total number of detrapped traps after change in the trap-depth αF, then from Eq. (2), we get               Recent Research in Science and Technology 2012, 4(8): 64-66  65{ }[ ])exp()exp(0 FZFZNN thd αα −−−=                     …(3)  where Fth is the threshold piezoelectric field for the EML.     Differentiating Eq. (3), we get [ ])exp(0 FZZNdFdNd αα −=           …(4)  Equation (4) can be expressed as [ ])exp(0 tFZFZNdtdNd && αα −=        …(5)                                                  where  dtdFF /=&        As the detrapped electrons are transferred to the conduction band the rate of generation of electrons in the conduction band is given by =g [ ])exp(0 tFZFZNdtdNd && αα −=        …(6)                                                                  It is evident from Eq.(6) that, for significant value of ZαF&, the value of g will decrease with increasing deformation time of the crystals.             If τ is the lifetime of electrons in the conduction band, then for 1/τ>>α, the change in the number of electrons in the conduction band, that is, the change in the carrier density can be expressed as  ==∆ τgn [ ]ταα )exp(0 tFZFZN && −           …(7)        If σ is the capture cross-section of the electrons, vd is the drift velocity of electrons and nr is the concentration of the energy states for the excited Eu2+ ions, then the rate of generation of  the excited Eu2+ ions can be expressed as   rd nvR σ= =∆n [ ]ταασ )exp(0 tFZFZNnv rd && −  …(8)  If µ is the mobility of electrons in the crystals, then vd = µF, and Eq. (8) can be expressed as   [ ]ταασµ )exp( tFZFZNFnR 0r && −=       …(9)                                                                            It is to be noted that up to Fth there are no detrapping of charge carriers because the trap-depths up to this energy are comparable with kT and consequently traps are thermally detrapped. Thus, the detrapping takes place from the trap-depth Eth to E or for the piezoelectric field Fth to F. If η is the efficiency for the radiative decay of excited Eu2+ ions, then using Eq.(9) the EML intensity can be expressed as                                     …(10)                If B is the correlating factor between F and the piezoelectric charge Q, then F = BQ. For the piezoelectric constant d0 near the localized piezoelectric region in the crystal, applied pressure P and pressing rateP&, PBdQBF &&& 0== and PBdBQF 0== , and therefore, Eq. (10) can be expressed as                                                           In the elastic region, P is low, and therefore, PBdZ 0α <<1,and Eq.(11)can be written as [ ])(1)( 02020 PBdZPPPdBZNnI thr αατησµ −−= &         …(12)  where  Pth is the threshold pressure for the EML emission. For the crystals having low value of Z,α and d0, the product PBdZ 0α  is much less than 1,and  therefore, Eq.(12) can be written as )(2020 thr PPPdBZNnI −= &ατησµ          …(13)                  The number  Nt of trapped electrons is given by, Nt=(N0-Nd)=[N0-N0{1-exp(-ZαBd0P)}] =N0 exp(-ZαBd0P) =N0 exp(-βP), where β=ZαBd0. Thus, β=ZαBd0is the rate constant for deformation detrapping or coefficient of deformation detrapping. Substituting the value of N0, Eq. (13) can be written as                 )(N 202c thtle PPPdBZNNnI −Ω= &ατησµ     …(14)              Equation (14) can be written as                 )(N 02202c ttPdBZNNnI tle −Ω= &ατησµ      …(15)                                                                       Equation (15) can also be expressed as                             )(N 022202c ttYdBZNNnI tle −Ω= εατησµ &    …(16)                                                                         where PPt th &/0 = , that is, the time at which the EML emission starts after pressing the sample at a fixed pressing rate, Y is the Young’s modulus of elasticity and ε& is the strain rate. Equation (14) indicates that for a given pressing rate or strain rate, after the threshold pressure Pth ,the EML intensity should increase linearly with pressure P. Equation (16) indicates that for a given strain rate the EML intensity should increase linearly with the deformation time t.      Using the Eq. (14), equations of Im, total ML intensity (IT) , fast decay (Idf) and slow decay (Ids) of ML intensity,   εατµση &)( thm202ctlem PPYdB ZN NN n  I −Ω=        …(17)      +Ω=mmmectleT tPdBZnNNNnI τατησµ 212 2202          …(18) 0m202ectledf YPdBZnNNN nI εατησµ &Ω=  [ ])(exp mtt −−φ                                  …(19) )]( exp[)(exp 00s csscsd ttIttII −−= −−= χτ          … (20) EXPERIMENTAL [ ])exp()( tFZFFFZNnRI th0r && αατησµη −−==[ ])exp()( PBdZPPPdBZNnI 0th2020r αατησµ −−= &Jha et al.,   66      BaSi2O2N2:Eu powders were synthesized by (Botterman et al. 2012 [13]) using a high temperature solid state reaction. Stoichiometric amounts of starting materials BaCO3 (99.95%) and Si3N4 (α-phase, 99.5%) were weighed and thoroughly mixed in a mortar. In order to dope with Eu, appropriate amounts of BaCO3 were substituted by EuF3 (99.5%). The powders were prepared with 2 mol% of Eu. The obtained mixtures were put in zirconia crucibles and fired at 1425°C for 4 hours in a horizontal tube furnace under a flowing atmosphere of forming gas (90% N2 , 10% H2).  Fig 1. Typical mechanoluminescence (ML) behavior of stress-stimulated BaSi2O2N2:Eu during application of a force of 20 kN (64 MPa). The solid line shows the ML intensity (on a linear scale) and the dashed line shows the force profile (after Botterman et al. 2012, ref [13]).   Fig 2. Semilog plot of ML intensity I versus (t − tm).   Fig 3. Mechanoluminescence (ML) of BaSi2O2N2:Eu, stimulated by a periodically applied external force of 20 kN (64 MPa). The ML intensity on a linear scale (solid line) follows the ¼ Hz square wave profile of the external force (dashed line) (after Botterman et al. 2012, ref [13]).).  Correlation between theoretical and experimental results        Fig.1 shows typical mechanoluminescence behavior of stress-stimulated BaSi2O2N2:Eu during application of a force of 20 kN (64 MPa). Such time dependence of EML can be understood using Eq. (13 ).       Fig.2 shows the semilog plot of I versus  (t − tm). It is seen that initially the ML intensity decreases at a fast rate and then decreases at a slow rate.This results is in accord with Eq.(17). The fast decay time 2.33 ms  and slow decay time is 36 ms.       Fig.3 Mechanoluminescence (ML) of BaSi2O2N2:Eu, stimulated by a periodically applied external force of 20 kN (64 MPa).  CONCLUSIONS        The ML intensity decreases with the number of applied stress stimulations, but after irradiating the sample with UV light, the recovery of ML intensity occurs completely, indicating that the ML process is non-destructive. On the basis of the piezoelectrically induced detrapping model of elastico ML, expressions are derived for the peak ML intensity, total ML intensity, fast decay of ML intensity and slow decay of ML intensity, in which a good agreement is found in theoretical and experimental results.  REFERENCES  [1] Chandra  B.P. In Luminescence of Solids; Vij, D.R., Ed., Plenum Press: NewYork, 1998, pp. 361. [2] Xu  C.N., Watanabe  T., Akiyama  M.; Zheng  X.G. Appl. Phys. Lett., 1999, 74, 1236. [3] Chandra  B.P., Baghel  R.N., Luka A.K.,  Sanodia T.R., Kurariya  R.K., Kurariya  S.R., J. Lumin. 2009, 129, 760. [4] Chandra  B.P., Zink, J.I. Phys. Rev. B 1980, 21, 816. [5] Kim  J.S., Kwon Y.N.,  Shin  M.,  Sohn  K.S., Appl. Phys. Lett., 2007, 90, 241916-1. [6] Xu, C.N.  AIST Today, 2010, 10, 18. [7] Sage  I., Humberstone L.,  Geddes  N., Kemp  M., Bishop  S., Bourhill G., Smart Mater. Struct., 2001, 10, 332. [8] Dante  J.G., Reports by the Secretary of the Army. US Patent No. 4372211, 1983. [9] Amans J., Powder Technol.,  2004, 146, 147. [10] Van den Eeckhout K, Smet PF, Poelman D. Persistent luminescence in Eu2+-doped compounds: a review. Materials 2010, 3, 2536. [11] Gomez-Ros  J.M., Correcher  V., Garcia- Guine J., Delgado A., Rad. Prot. Dosimetry,119 (2006) 93. [12] Yazici A. N., Oztas M., Bedir M., Kayali R., Turk J. Phys. 26 (2002) 277. [13] Correcher V., Gomez-Ros  J.M., Garcia-Guinea J., Lis M., Sanchez-Munoz L., Radiation Measurements, 43 (2008) 269. [14] Botterman J., Eeckhout K. V., Baere I. D., Poelman D. and Smet P. F. Acta Materilia, 2012 (in press).     

Luminescence induced by elastic deformation of BaSi2O2N2:Eu crystals

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Luminescence induced by elastic deformation of BaSi2O2N2:Eu crystals

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