Fracto- mechanoluminescence and thermoluminescence properties of orange-red emitting Eu3+ doped Ca2Al2SiO7 phosphors

Fracto- mechanoluminescence and thermoluminescence properties of orange-red emitting Eu3+ doped Ca2Al2SiO7 phosphors

Journal of Luminescence 183 (2017) 89–96 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate...

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Journal of Luminescence 183 (2017) 89–96

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Fracto- mechanoluminescence and thermoluminescence properties of orange-red emitting Eu3 þ doped Ca2Al2SiO7 phosphors Geetanjali Tiwari a,n, Nameeta Brahme a,n, Ravi Sharma b, D.P. Bisen a, Sanjay K. Sao a, Ayush Khare c a

School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, C.G., 492010 India Department of Physics, Govt. Arts and Commerce Girls College, Devendra Nagar, Raipur, C.G., India c Department of Physics, National Institute of Technology, Raipur - 492 010 India b

art ic l e i nf o

a b s t r a c t

Article history: Received 27 April 2016 Received in revised form 18 October 2016 Accepted 8 November 2016 Available online 14 November 2016

The suitability of nano-structured Ca2Al2SiO7:Eu3 þ phosphors for thermoluminescence and mechanoluminescence dosimeter were investigated. Europium doped di-calcium di-aluminum silicate phosphor was synthesised by the combustion assisted method and annealed at 1100 °C for 4 h in reducing and oxidizing environments. The prepared Ca2Al2SiO7:Eu3 þ phosphor was characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX), photoluminescence (PL) and decay characteristics. The phase structure of sintered phosphor has akermanite type which belongs to the tetragonal crystallography; this structure is a member of the melilite group and forms a layered compound. The chemical composition of the sintered Ca2Al2SiO7:Eu3 þ phosphor was confirmed by EDX spectra. Mechanoluminescence (ML) and thermoluminescence (TL) studies revealed that the ML and TL intensity increases with activator concentration. Optimum ML was observed for the sample having 2 mol% of Eu ions. The TL intensity of Ca2Al2SiO7:Eu3 þ was recorded for different exposure times of γ -irradiation and it was observed that TL intensity is maximum for γ dose of 1770 Gy. The PL spectra indicated that Ca2Al2SiO7:Eu3 þ could be excited effectively by near ultraviolet (NUV) light and exhibited bright orange-red emission with excellent colour stability. CIE colour coordinates of the prepared Ca2Al2SiO7:Eu3 þ phosphor was found suitable as orange-red light emitting phosphor with a CIE value of (x¼ 0.6142, y ¼ 0.3849) and correlated colour temperature (CCT) is 1250 K. Therefore, it is considered to be a new promising orange-red emitting phosphor for white light emitting diode (LED) application. & 2016 Elsevier B.V. All rights reserved.

Keywords: Ca2Al2SiO7 phosphors Mechanoluminescence Thermoluminescence

1. Introduction Phosphors are widely used for emissive displays. However, all currently used phosphors still need considerable improvement, such as higher efficiency, lower current saturation and better chromaticity [1]. Oxide phosphors (including silicates phosphors) are more chemically stable than sulfide phosphors under high Coulomb loading. Metal silicates have been widely reported as promising host materials for rare earth and transition metal ions with excellent luminescence properties in the green, blue and red spectral regions [2]. From the manufacturing point of view calcium silicate phosphor would be ideal, because both calcium and silica are abundant and relatively inexpensive [3]. Silicates are also efficient luminescent materials, mainly because of their rigid and n

Corresponding authors. E-mail addresses: [email protected] (G. Tiwari), [email protected] (N. Brahme). http://dx.doi.org/10.1016/j.jlumin.2016.11.012 0022-2313/& 2016 Elsevier B.V. All rights reserved.

stable crystal structures [4,5]. Despite of their excellent host matrix properties, the silicates are not completely free of lattice defects, which causes a drastical decrease in the luminescent efficiency or cause undesired properties, e.g. afterglow, as is the case with the Ce3 þ doped rare earth oxyorthosilicates [6,7]. A systematic study of the persistent luminescence properties, especially the effect of all the silicates is, however, lacking. Long-lasting phosphorescence has also been reported in Ca2Al2SiO7:Eu2 þ , Ca2Al2SiO7:Ce3 þ and Ca2Al2SiO7:Ce3 þ , Mn2 þ [8–10]. However, to our knowledge, there have been no attempts to investigate Ca2Al2SiO7 based phosphors as a potential component used in semiconductor based light-emitting diodes (LED) for solid state lighting. The white LED can be generated by several methods [11– 15]. The most commonly used method is to combine the red /green /blue tricolours phosphors with a GaN/InGaN chip. The presently reported red phosphors for near UV InGaN based LEDs are mainly Y2O2S:Eu3 þ [16], Na5La (MoO4)4:Eu3 þ and Na5 (MoO4)4:Eu3 þ [17]. But the red phosphor shows lower efficiency

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compared with the blue and green ones [18]. Therefore, it is urgent to search for new orange-red phosphors that can be excited efficiently under the near UV range around 400 nm with intense emission. Mechanoluminescence (ML) also known as triboluminescence or fractoluminescence, is the light emission induced as a result of a mechanical action on a solid [19]. About 36% of all inorganic and 19% of all organic compounds exhibit ML. The ML phenomenon is known since 400 years, but, the interest in ML compounds has purely been academic in nature and no practical applications have been established due to the weak ML intensity of the phosphors. The ML intensity depends on the technique used for deformation. ML materials with a high intensity have now been developed, showing promising applications of this phenomenon in advanced stress sensing techniques, including no contact, full-field sensing and self-diagnosis structures [20–24]. The ML of γ -irradiated alkali halides has been the most extensively studied in the past two decades and the correlation between ML and TL has been established. It is suggested that the recombination of electrons with free radicals (anion radicals produced by gamma irradiation) released from the traps during thermal or mechanical excitation is responsible for luminescence in this system. We have also studied the ML and TL of Ca2Al2SiO7:Ce3 þ , Ca2Al2SiO7:Tb3 þ and Ca2Al2SiO7:Ce3 þ Tb3 þ phosphors [26–28]. The present paper reports the ML and TL in gamma-irradiated Ca2Al2SiO7:Eu3 þ phosphors. The Eu3 þ ion showed good luminescence performance in Ca2Al2SiO7 host material and through the combustion assisted method high quality powders with good crystallinity and homogeneous composition was obtained at a relatively low temperature. In this study, we describe the structural and luminescent properties of Eu3 þ doped calcium aluminium silicate phosphors.

2. Experimental 2.1. Sample preparation Eu3 þ doped Ca2Al2SiO7 phosphors were prepared by the combustion assisted method without any flux material. The starting materials taken were calcium nitrate Ca(NO3)2, aluminum nitrate Al(NO3)3.9H2O, silicate SiO2, Europium nitrate Eu(NO3)3.6H2O (0.5, 1, 2, 3, 4 mol%) and urea (NH2CONH2) as a fuel. Stoichiometric composition of each nitrate and urea were mixed together and crushed in mortar and pestle for 2 hour to form a thick paste then transferred to crucible and introduced into muffle furnace maintained at 600°C. Initially the mixture boils and the spontaneous ignition occurs and the foamy product was found that can easily be milled to obtain the precursor powder. The precursor powders were ground and annealed at 1100 °C for 4 h in reducing and oxidizing environments to formation of rare earth doped Ca2Al2SiO7 phosphor.

[Model: QUANTA-200F-FEI]. Gamma irradiation was carried out using a 60Co source with 0.930 kGy/h exposure rate. ML was recorded by dropping a piston (mass 400gm) onto the sample from various heights. The ML was monitored using a photomultiplier tube (RCA-931A) connected to a digital storage oscilloscope (ScientificSM-340). PL was recorded using fluorescence spectrophotometer (ShimadzuRF-5301XPC) and emission was recorded using a spectral slit width of 1.5 nm. The TL glow curve was recorded using TL reader (NucleonixTL1009I) by heating the sample with heating rate 5 °C/s. TL emission spectra were recorded by using interference filters of different wavelengths. All measurements were carried out at the room temperature.

3. Results and discussion 3.1. XRD Analysis The XRD patterns of Ca2Al2SiO7:Eu3 þ phases with different doping contents are shown in Fig. 1. All profiles were found to be in good agreement with JCPDS-card (No. 35-0755), which indicates that no impurity phase exists. Ca2Al2SiO7 (a gehlenite structure) has a tetragonal unit cell with lattice parameters a¼ b¼ 7.686 Å, c¼ 5.068 Å of space group. P421m (no 113). In the structure of Ca2Al2SiO7, the cat ions are localized at three types of sites (an eightfold coordinated site) called Thomson cube (TC) occupied by Ca2 þ , a regular tetrahedral site (T1) fully occupied by Al3 þ ions, and a very distorted tetrahedral site (T2), where Si4 þ ions and Al3 þ ions are statistically distributed [29,30]. Therefore, based on the effective ionic radii ‘r’ of cat ions with different coordination number (CN) reported by Shannon, we propose that Eu3 þ ions are expected to and, in fact, occupy the Ca2 þ sites preferably, because the ionic radius of Eu3 þ (r¼ 1.066 Å when CN ¼8), are close to that of Ca2 þ (r ¼1.12 Å when CN¼8). Since both four- coordinated Al3 þ (r ¼0.39 Å) and Si4 þ (r¼ 0.26 Å) sites are relatively small for Eu3 þ to occupy, we thereby conclude that Eu3 þ tends to prefer the Ca2 þ sites due to size consideration. The average crystallite size was calculated from the XRD pattern using Debye Scherrer relation D ¼kλ/βcosθ, where D is the crystallite size for the (hkl) plane, k is dimensionless shape factor, with a value close to unity, λ is the wavelength of the incident X- ray radiation Cu Kα (0.154 nm), β is the full width at half maximum (FWHM) in radiation, and θ is the corresponding Bragg's angle of diffraction.The average crystallite size of

2.2. Measurements and characterization The crystalline structure and particle morphology of the prepared samples were investigated by X-ray diffraction analysis (XRD model D8 Advance Bruker AXS) using Cu Kα radiation (λ ¼0.154 nm). Data have been collected by step scanning 2θ from 20° to 90°. The phase structure of the sample was verified with the help of Joint Committee of Powder Diffraction Standard Data (JCPDS) file (JCPDS: 35-0755) and average crystallite size was calculated using Debye Scherrer formula (D ¼kλ/βcosθ). The particle size of the prepared phosphor was determined by TEM using a [Tecnai G2S -TWIN-FEI]. The morphology of the phosphor was characterized by scanning electron microscope (SEM) with EDX

Fig. 1. XRD patterns of Ca2-xAl2SiO7:Eux (x¼ 0.005, 0.01, 0.02, 0.03, 0.04) phosphors.

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91

Fig. 4. EDX image of Ca2Al2SiO7:Eu3 þ (2 mol%). 3þ

Fig. 2. TEM image of Ca2Al2SiO7:Eu

(2 mol%).

Ca2Al2SiO7:Eu3 þ phosphor was 50.9 nm. 3.2. Transmission electron microscopy (TEM) Fig. 2 shows the transmission electron microscopy images of Ca2Al2SiO7:Eu3 þ which consist of aggregated particles having size smaller than 50 nm. Variation in particle size might also be due to agglomeration of very fine particles due to heat treatment at high temperature. TEM images show that the particle has tetragonal structure and particle size ranges in between 20 and 50 nm. So we conclude that, transmission electron microscopy results are in good agreement with the XRD results. 3.3. Scanning electron microscopy (SEM) Fig. 3 shows the SEM micrograph of the Ca2Al2SiO7:Eu3 þ . The microstructure of the sample reflects the inherent nature of the combustion process. When a gas is escaping under high pressure during the combustion process, pores are formed with the formation of small particles near the pores. The non-uniform and irregular shapes of the particles could be attributed to the nonuniform distribution of temperature and mass flow in the combustion flame. 3.4. Energy dispersive X-ray spectroscopy (EDX) Fig. 4 shows the Energy dispersive X-ray spectroscopy (EDX) images of Ca2Al2SiO7:Eu3 þ . The chemical composition of the powder sample has been measured using EDX spectra. EDX is a standard procedure for identifying and quantifying elemental composition of the sample area as a few nanometers. The existence of europium (Eu) is clear in their corresponding EDX spectra. There appeared no other emission apart from that of

calcium (Ca), aluminum (Al), silicon (Si) and oxygen (O) and Eu. In EDX spectra, the presence of Ca, Al, Si, O and Eu, intense peak are present which preliminarily indicates the formation of Ca2Al2SiO7:Eu3 þ phosphor. 3.5. Photoluminescence The photoluminescence properties of Ca2Al2SiO7:Eu3 þ host were investigated by the excitation and emission spectra at room temperature. The excitation spectra of Ca2Al2SiO7:Eu3 þ (2 mol%) in the range of 200–600 nm excited at 619 nm emission is shown in Fig. 5 (a). The broad band extending from 240–340 nm is associated with charge transfer (CT) due to transition from 2p orbital of O2 ions to the 4 f orbital of Eu3 þ ions, whereas, the sharp lines correspond to direct excitation of f–f shell transitions of Eu3 þ ions 7F0-5F2 (279 nm), 7F0-5H6 (319 nm), 7F0-5H3 (328 nm), 7F0-5D4 (363 nm), 5G4 (383 nm), 5L6 (394 nm), 5D3 (419 nm), 5D2 (466 nm), 5 D1 (534 nm) and 5D0 (589 nm), respectively. The prepared Ca2Al2SiO7:Eu3 þ phosphor could be excited by near UV (NUV) at about 394 nm effectively. So, it can match well with UV and NUVLED, showing a great potential for practical applications (Zhang et al. [15]). As seen in Fig. 5(a), the intensity of the transition at 394 nm is the highest in the spectra. Fig. 5(b) shows the emission spectrum of Ca2Al2SiO7:Eu3 þ (0.5, 1, 2, 3, 4 mol %) in the wavelength range of 500–750 nm under 394 nm excitation. There are five main sharp emission peaks at near 580, 589, 619, 658 and 703 nm, amongst which the intensity of the peak at 619 nm is highest. These emissions are caused by the f–f forbidden transitions of Eu3 þ with 4f6 electron configuration, corresponding to 5D0-7F0 (579 nm), 7F1 (589 nm, 602 nm), 7F2 (615 nm, 619 nm) and 7F3 (651 nm, 658 nm), 7F4 (693 nm, 703 nm), respectively. The orange emission at about 589 nm belongs to the magnetic dipole 5D0-7F1 transition of Eu3 þ , and the transition hardly varies with the crystal field strength. The red emission at 619 nm ascribes to the electric dipole 5D0-7F2 transition of Eu3 þ , which is very sensitive to the local environment around the Eu3 þ , and depends on the symmetry of the crystal field. The two strongest peaks found that the 589 and 619 nm emissions indicate that there are two Ca2 þ sites in the Ca2Al2SiO7 lattice. One site, Ca (I), has inversion symmetry and the other site Ca (II), has non inversion symmetry. The Eu3 þ ions occupy these two sites of Ca (I) and Ca (II) when doped in Ca2Al2SiO7 phosphor. As the prepared phosphor of Ca2Al2SiO7:Eu3 þ show the dominated orange emission peak located at 589 nm, it could be concluded that Eu3 þ ions mainly occupy the inversion symmetric centre in host lattice. Fig. 5(c) shows the schematic energy level diagram of Eu3 þ ions in the Ca2Al2SiO7 host depicting different emissions bands. 3.6. CIE chromaticity coordinate

Fig. 3. SEM image of Ca2Al2SiO7:Eu3 þ (2 mol%).

In general, colour of any phosphor material is represented by

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Fig. 6. CIE Chromaticity diagram of Ca2Al2SiO7:Eu3 þ (2 mol%)phosphor.

colours of Eu3 þ doped Ca2Al2SiO7 phosphor are placed in the orange-red (x ¼0.6142, y¼0.3849) corners. The chromatic co-ordinates of the luminescence of this phosphor are measured and reached to orange-red luminescence (CIE 1931)

3.7. Correlated colour temperature (CCT) In order to investigate the prepared phosphors for suitability as a practical white light source correlated colour temperature (CCT) of the samples have been calculated. The correlated colour temperature (CCT) is a specification of the colour appearance of the light emitted by a light source, relating its colour to the colour of light with respect to a reference light source when heated up to a specific temperature, in degrees Kelvin (K). The CCT rating for a lamp or a source is a general warmth or coolness measure of its appearance. However, opposite to the temperature scale, lamps with a CCT rating below 3200 K are usually considered ‘‘warm’’ sources, while those with a CCT above 4000 K are usually considered ‘‘cool’’ in appearance [31–33]. McCamy has proposed the analytical equation to calculate the CCT which is given by

CCT=449 n3 + 3525 n2 + 6823.3 n + 5520.33

Fig. 5. (a) Excitation spectra of Ca2Al2SiO7:Eu3 þ (2 mol%) phosphor (λem ¼ 619 nm). (b) Emission spectra of Ca2Al2SiO7:Eu3 þ (0.5, 1, 2, 3, 4 mol%) phosphors (λex¼ 394 nm). (c) Schematic energy level diagram of the Ca2Al2SiO7:Eu3 þ phosphor.

means of colour coordinates. The luminescence colour of the samples excited under 394 nm has been characterized by the CIE (Commission International de I'Eclairage) 1931 chromaticity diagram. The emission spectrum of the Ca2Al2SiO7:Eu3 þ phosphor was converted to the CIE 1931 chromaticity using the photo-luminescent data and the interactive CIE software (CIE coordinate calculator) diagram as shown in Fig. 6. Every natural colour can be identified by (x, y) coordinates that are disposed inside the ‘chromatic shoe’ representing the saturated colours. Luminescence

Intensity (A. U.)

Where n¼(x  xe)/(y  ye) is the inverse slope line and (xe¼0.6142, ye¼ 0.3849) is the epicenter. It can be seen that the value of CCT from 1250 K, which is well under the acceptable range and can be considered ‘‘warm’’ in appearance.

18000 16000 14000 12000 10000 8000 6000 4000 2000 0

0

1000

2000 Time (s)

3000

4000

Fig. 7. Decay Curve of Ca2Al2SiO7:Eu3 þ (2 mol%) phosphor.

TL Intensity (A. U.)

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1800 1600 1400 1200 1000 800 600 400 200 0

93

I------ 0.5mol% II----- 1mol% III---- 2mol% IV----- 3mol% V------ 4mol%

III II IV I V

0

50

100

150

200

250

300

Temperature (°C)

Total TL Intensity (A. U.)

(a) 50000 45000 40000 35000 30000 25000 20000 15000 10000 5000 0 0

1

2

3

Concentration (mol%)

4

5

(b) 3þ

Fig. 8. (a) TL glow curve for different Eu concentration with 1770 Gy γ dose. 8(b) Eu3 þ concentration dependence of the total TL peak intensity.

3.8. Decay characteristics The decay curve of the phosphor was measured and it is exhibited in Fig. 7. The initial intensity of Ca2Al2SiO7:Eu3 þ is much stronger, which is due to the higher emission efficiency of Ca2Al2SiO7:Eu3 þ . The rate of decay of intensity is given by the formula [34].

I = I1 exp ( −t /τ1)+I2 exp ( −t /τ2 )

(1)

Where I represents the phosphorescent intensity. I1 and I2 are constants of intensity, τ1 and τ2 are average lifetime of an excited electron, deciding the rates for the rapid and the slow exponential decay components, respectively. The values of decay parameters are shown in table 1. (Table 1). 3.9. Thermoluminescence (TL) Thermoluminescence (TL) is an active field of research due to its application in radiation dosimetry and archaeological dating. TL is the light that a solid sample emits when it is heated after irradiation, by X-rays, γ-rays and UV light. TL glow curves of Eu3 þ doped Ca2Al2SiO7 phosphors were optimized for different doping concentration and different exposure time of γ radiation. 5 °C/sec heating rate was fixed for all the measurement. The TL measurement was carried out between room temperature to 300 °C. Fig. 8(a) shows the TL glow curves of Ca2Al2SiO7:Eu3 þ with different Eu3 þ concentrations (x ¼0.5, 1, 2, 3, 4 mol %) with γ dose for 1770 Gy. It was observed that total TL intensity increases with increasing doping concentrations of Eu3 þ and attained maximum

Fig. 9. (a) TL glow curves of Ca2Al2SiO7:Eu3 þ phosphors for different γ dose. (b) Graph between total TL intensity of Ca2Al2SiO7:Eu3 þ Phosphors and gamma irradiation dose.

Table 1 Decay parameters. Phosphor

τ1(min)

τ2(min)

Ca2Al2SiO7:Eu3 þ

5 min

36

for 2 mol% concentrations. Further increment in doping concentration, decreases the total TL intensity. It is well known that the luminescence intensity of the phosphor is strongly influenced by the activator concentration. An increase in the activator concentrations increases the energy transfer by the ions, causing increase in the TL intensity [15,28,35,36]. There has been an optimum value of activator concentration, as shown in Fig. 8(a) and (b). On further increase in activator ions the distance between the activators get reduced, the interaction of the ions increases resulting in a decrease in the luminescence intensity [15]. Fig. 9(a) shows the TL glow curve of Ca2Al2SiO7:Eu3 þ phosphors for different (295 Gy to 1770 Gy) γ exposure time. Fig. 9 (b) shows the total TL intensity of the Ca2Al2SiO7:Eu3 þ phosphor measured at different γ exposure time. It was observed that the total TL intensity increases linearly with increase in γ exposure time. The maximum TL intensity was observed for 1770 Gy γ-irradiation time. It may be due to the increase in charge carrier

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Table 2 TL parameters of Ca2Al2SiO7:Eu3 þ . γ dose (Gy)

Heating Rate

Tm (°C)

τ(°C)

δ(°C)

ω(°C)

m ¼δ/ω

Activation energy E(eV)

Frequency factor (S-1)

1770

5 °C/s

150

25.70

22.05

47.75

0.46

0.83

2.6*1011

Fig. 10. TL fading curve for Ca2Al2SiO7:Eu3 þ (2 mol%)phosphor.

occupancy of traps may with increasing γ dose. The thermal activation energy E for the sample with 1770 Gy γ- radiation time (associated with the trap depth) was calculated from the glow peak parameters using the following equation [17].

E=2kTm ( 1.26Tm/ω − 1)

(2)

where ω ¼ τ þ δ is the total half width intensity, τ is the half width at the low temperature side of the peak (τ ¼ Tm  T1), δ is the half width towards the fall-off side of the glow peak (δ ¼T2  Tm), where T1 and T2 are the half‐intensity temperatures on the low and high temperature side of the peak, respectively. Tm is the peak temperature at the maximum. The μ ¼ δ/ω, is a shape factor which differentiates between first and second order TL glow peak. The frequency factor was calculated by the formula s ¼[2β (1.26 Tm /ω  1)/(e 2Tm)] exp (2.52Tm/ω) where β is the (constant) heating rate E¼activation energy, Tm ¼maximum temperature. The TL parameters i.e. activation energy (E) and frequency factor (s) for the prominent glow peaks of prepared phosphor are shown in Table 2. 3.10. Fading Fading is an important factor in many applications such as archaeological, geological dating and environmental dosimetry. The TL fading curve for Ca2Al2SiO7:Eu3 þ phosphor irradiated with 1770 Gy of γ-dose were measured between 0–60 days in the multiple of 5days delay. As depicted in Fig. 10 Ca2Al2SiO7:Eu3 þ possesses a higher concentration of trapped carriers, which are liberated rapidly after excitation, leading to stronger initial intensity and longer duration of the afterglow. The initial TL intensity of Ca2Al2SiO7:Eu3 þ is high, but decreases rapidly from 0 to 20 days, indicating a liberation of a large number of trapped carriers after the removal of the excitation. After 20 days delay time, the rate of decrease in TL intensity gets slower, as the probability of liberating trapped carriers is reduced. Still the TL intensity of Ca2Al2SiO7:Eu3 þ is strong, which is high enough to be used for dosimetric purpose. The TL intensity got stable around 35 days and up to 60 days no further remarkable decrease in the intensity was seen. For dosimetric applications two basic properties are essential, firstly, linear behaviour with the dose and secondly less fading with time. Our sample showed a remarkable fall of intensity

Fig. 11. TL emission spectra of Ca2Al2SiO7:Eu3 þ phosphor for γ dose of 1770 Gy.

initially but it also showed linear behaviour with γ-dose and less fading in the intensity after 15 days time fading with enough TL intensity, hence it could be used for dosimetric purpose. 3.10.1. TL emission spectra Fig. 11 shows the TL emission spectrum of Ca2Al2SiO7:Eu3 þ (2 mol %) in the wavelength range of 400 to 700 nm. There are four main sharp emission peaks at near 540, 580, 619 and 680 nm amongst which the peak intensity at 619 nm is the highest. It could be concluded that the emissions are caused by the f–f forbidden transitions of Eu3 þ with 4f6 electron configuration, corresponding to 5D0-7F0 (589 nm), 7F2 (619 nm) respectively. The orange emission at about 589 nm belongs to the magnetic dipole 5 D0-7F1 transition of Eu3 þ and the red emission at 619 nm ascribes to the electric dipole 5D0-7F2 transition of Eu3 þ . 3.11. Mechanoluminescence (ML) Mechanoluminescence (ML) is an interesting luminescence phenomenon whereby light emission in solids is caused by grinding, rubbing, cutting, cleaving, shaking, scratching, and compressing or by crushing the solids. ML is a defect related phenomenon, associated with a trap involved process, in which electrons (or holes) dwell in the trap for some time and then recombine with the luminescence centre, either by travelling in the conduction band (or valence band) or by electron (or holes) tunnelling [24]. Since the mechanical energy cannot be supplied directly to the trapped charge carrier, deformation induced intermediate process is responsible for the de-trapping of the charge carriers [20]. Two peaks have been found in ML intensity vs time curve [12(a)]. The first peak is because of the transient response of the pressure exerted on the sample [25]. The occurrence of a second peak, which occurs in the post deformation region, may be due to the captures of carriers by the shallow traps lying away from the newly created surfaces where the electric field near the surface is not so effective [37]. The release of trapped charge carriers from shallow traps may take place later due to thermal vibration of lattices and therefore a delayed ML (second peak) may be produced [38]. In the present investigation piezoelectrification during the impact is responsible for the detrapping of the trapped

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95

(a)

Peak ML Intensity (A. U.)

60 50 40 30 20 10 0

0

300

600

900

1200 1500 1800

Gamma Dose (Gy) Fig. 12. (a) ML intensity verses time curve for different Eu3 þ concentration for γ dose of 1770 Gy. (b) Variation of peak ML intensity by Eu3 þ concentration variation.

charge carriers [28]. Fig. 12(b) shows that the ML intensity initially increases with the increase in the concentration of Eu ions attains an optimum value for 2 mol% then decreases with further increase in the concentration of Eu ions. Fig. 13 shows ML intensity versus time curve for different γirradiation time by the impact of load (400 g), dropped with same height of 50 cm. It is clearly visible that the the ML intensity linearly increases with an increase in γ dose up to 1770 Gy. As the γ dose increases, carrier (electron and hole) concentration in trap level increases, which results in the increase in the ML intensity. After 1770 Gy γ- irradiation no any remarkable change in the ML intensity was recorded. Fig. 14(a) shows the characteristics curve between ML intensity versus time for Ca2Al2SiO7:Eu3 þ (2 mol %) phosphor at different heights. The experiment was carried out for a moving piston of fixed mass (400 g) dropped with different heights 20, 30, 40, 50 cm. It is evident that the ML intensity increases linearly with the increase of falling height of moving piston in the range of 20– 50 cm. Fig. 14(b) shows the curve between ML peak intensity versus impact velocity of Ca2Al2SiO7:Eu3 þ (2 mol %) phosphor. The ML intensity increases linearly with the increase in falling height of the moving piston in the range of 20–50 cm [39]. Since v¼√(2gh), where h is the height of moving piston and ML intensity is proportional to the impact height, the ML intensity is proportional to the square of the impact velocity. As the velocity of the piston increases with height, the impact velocity will be more and hence more fractures are created in the sample which results in creation of new surface. When the moving piston hits the Ca2Al2SiO7:Eu3 þ phosphor, it produces piezoelectric field in the sintered phosphor

(b) Fig. 13. (a) ML intensity versus time curve for γ- irradiated phosphor. (b) Dependence of ML peak intensity on γ–dose.

as they are non-centro symmetric [40]. The piezoelectric field near certain defect centres may be high due to the change in the local structure [41]. The piezoelectric field reduces the trap depth of the carriers. The decrease in the trap depth causes transfer of electrons from electron traps to the conduction band. Subsequently, the moving electrons in the conduction band are captured in the excited state, located at the bottom of the conduction band, whereby excited ions are produced [42]. The subsequent recombination of electrons with the holes in centres gives rise to the light emission [43]. When the surface of an object was coated with the ML materials, the stress distribution in the object beneath the layer could be reflected by the ML brightness and could be observed. Based on the above analysis this phosphor can also be used as sensors to detect the stress of an object.

4. Conclusions We have investigated the Mechanoluminescence and Thermoluminescence phenomena in the Ca2Al2SiO7:Eu3 þ phosphor prepared by combustion assisted method, which is observed first to our knowledge. The X-ray diffraction pattern indicated that the crystal structure was tetragonal. The X-ray diffraction intensity was maximum for (2 1 1) plane having 2θ ¼ 31.4°. The highest TL / ML intensity was observed for the sample having 2 mol % concentration of Eu. The EDX spectra confirm the present elements in Ca2Al2SiO7:Eu3 þ phosphor. The excitation spectra indicate that the phosphor can be effectively excited by near ultraviolet (NUV) light, making it attractive as conversion phosphor for LED applications. The Ca2Al2SiO7:Eu3 þ phosphor exhibits bright orange-red

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[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

(a)

50 cm

Peak ML Intensity (A. U.)

60 50

[21] [22] [23] [24]

40 cm

40

30 cm

30

[19] [20]

20 cm

[25] [26]

20

[27]

10

[28]

0

10

20

30

40

50

Impact Height (cm)

(b) Fig. 14. (a) Change in ML intensity with impact height for 1770 Gy of γ-dose. (b) ML peak intensity versus impact height for 1770 Gy of γ- dose.

emission excited by 394 nm. CIE chromaticity diagram confirms Ca2Al2SiO7:Eu3 þ phosphor exhibits efficient orange-red emission and excellent colour stability, indicating that it has favourable properties for application as near ultraviolet LED conversion phosphor. The TL intensity increases with increase in γ exposure time indicating the increase in concentration of traps with γ dose. The ML intensity linearly increases with increasing impact height and with the increasing γ dose. The γ-irradiated ML and TL study of the Ca2Al2SiO7:Eu3 þ phosphor shows dependence of ML and TL intensity upon the γ dose given to the phosphor. It shows linear response up to 1770 Gy with γ-irradiation, and the TL fading curve for Ca2Al2SiO7:Eu3 þ phosphor irradiated with 1770 Gy of γ-dose were measured up to 60 days delay times, which suggests their possible applications in ML and TL dosimetry.

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