Mechanoluminescence, thermoluminescence and photoluminescence studies on Al2O3:Tb phosphors

Mechanoluminescence, thermoluminescence and photoluminescence studies on Al2O3:Tb phosphors

Journal of Luminescence 132 (2012) 210–214 Contents lists available at SciVerse ScienceDirect Journal of Luminescence journal homepage: www.elsevier...

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Journal of Luminescence 132 (2012) 210–214

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence journal homepage:

Mechanoluminescence, thermoluminescence and photoluminescence studies on Al2O3:Tb phosphors R.K. Rai a, A.K. Upadhyay b, R.S. Kher c, S.J. Dhoble d,n a

Department of Physics, Government Engineering College, Ujjain 456010, India Department of Physics, O.P. Jindal Institute of Technology, Raigarh 496001, India c Department of Physics, Government E.R.R.P.G. Science College, Bilaspur 495006, India d Department of Physics, R.T.M. Nagpur University, Nagpur 440033, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 April 2011 Received in revised form 18 July 2011 Accepted 3 August 2011 Available online 22 August 2011

Terbium activated Al2O3 phosphors were synthesized by combustion technique using hydrazine as a reductive non-carbonaceous fuel. X-ray diffraction (XRD) patterns of the samples were recorded to confirm the formation of the sample. Scanning electron microscope (SEM) images were taken to study the surface morphology of the sample. The photoluminescence (PL), thermoluminescence (TL) and mechanoluminescence (ML) properties of the g-ray irradiated samples were studied. ML was excited impulsively by dropping a piston on the sample. In ML glow curves one peak with a shoulder was observed. ML intensity increases with activator concentration. Optimum ML was observed for the sample having 0.5 mol% of Tb ions. In the TL glow curve two distinct peaks, one around 222 1C and another around 280 1C, were observed for the samples having 0.5 mol% of activator concentration. In the PL spectra the 5D4-7F5 line at 544 nm in the green region is observed, which is the strongest in Al2O3 system. It is suggested that de-trapping of trapped charge carriers followed by recombination is responsible for ML and TL in this system. & 2011 Elsevier B.V. All rights reserved.

Keywords: Aluminum oxide Mechanoluminescence Thermoluminescence Photoluminescence X-ray diffraction

1. Introduction Phosphors containing rare earth ions have received increasing attention in recent years due to their technological importance [1]. The various aspects of luminescence and complex process involved in the origin of light emission offer challenging problems to the research workers in this field [2]. Mechanoluminescence (ML) is an interesting luminescence phenomenon, which is caused by mechanical stimulus such as grinding, cutting, collision, striking and friction [3]. In the recent past, intense ML materials have been prepared, whose ML emission can be seen in daylight with naked eye and such materials were finding important applications in novel self-diagnosis systems, optical stress sensors, stress imaging devices, wireless fracture sensor systems and in damage sensors. The use of ML paint enabled to accurately detect the crack tip even on a micro-scale not by detecting the crack opening displacement but by identifying the light emission that accompanies the crack tip stress field. The ML technique is much simpler, cheaper and more suited for the detection of fast crack than the conventional techniques such as Laser photo-elasticity, Laser


Corresponding author. E-mail address: [email protected] (S.J. Dhoble).

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.08.003

ultrasound, Raman spectroscopy, electrical resistance techniques, etc., which involves complicated instrumentation [4]. It is known that host materials and intentionally added impurity play an important role in the luminescence process; usually fluorides are attractive host materials as the efficiency of up-conversion is quite high due to their low phonon energies [5]. Compared to fluorides, oxides have attractive properties such as high chemical stability and ease of synthesis [6]. It is also well known that the quality of luminescent material is largely influenced by the synthesis technique. In recent years the combustion method has displayed unique advantages of lower synthesis temperature, shorter synthesis time and controlled size of the particles [7]. Aluminum oxide is a material of technological importance, because it offers a large transparent window for UV to near infrared, low permeability to alkali impurity, excellent mechanical properties and good chemical stability; therefore, it is a good candidate to be used as a host material of the rare earth ions [8]. Aluminum oxide has a number of advantages as a thermoluminescence phosphor. It is abundantly available, chemically very inert and since it is a ceramic material, high temperature stable glow peaks can be expected, which may find application in surveying radiation levels in heated environments [9]. The survey of literature shows that no systematic investigations have been made on the luminescence (specially ML) of g-ray

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irradiated Al2O3:Tb phosphors. Hence in the present investigation Al2O3:Tb phosphors were prepared by combustion synthesis technique and its mechanoluminescence (ML), thermoluminescence (TL) and photoluminescence (PL) properties were studied.

2. Experimental The Al2O3:Tb (0.1, 0.2, 0.5 and 1.0 mol%) samples were prepared by the combustion technique as reported by Mackittrick et al. [10] using hydrazine as a fuel. The starting materials taken were Al(NO3)3  9H2O, Tb(NO3)3  6H2O compounds of ultra high purity (99.9%). All of them obtained from Alfa Aesar Lancaster, USA. Hydrazine was used as a reductive non-carbonaceous fuel that prevents carbon contamination. A flow chart for the preparation of terbium doped Al2O3 phosphor is shown in Fig. 1. The crystalline structure and particle morphology of the resulting samples were investigated by X-ray diffraction analysis (XRD model D8 Advance ˚ Data have been Bruker AXS) using Cu Ka radiation (l ¼1.5418 A). collected by step scanning 2y from 201 to 701and 9.6 s swept time at each step at room temperature. In order to study the surface morphology of phosphor, scanning electron microscopy (model LEO 0435 VP) has been carried out. Gamma irradiation was carried out using a 60Co source having exposure rate 0.930 kGy/h. ML was excited by dropping a piston (mass—0.7 kg) on to the sample from various heights. The impact velocity of the piston was calculated pffiffiffiffiffiffiffiffi using the formula: v ¼ 2gh. The ML was monitored using a photomultiplier tube (RCA-931A) connected to a digital storage

Aluminum Nitrate, Terbium Nitrate and Hydrazine In de-ionized distilled

Stirring for 1 hour at room temperature

Mixture of metal nitrate and Hydrazine solution

Combustion at 290 C in Muffle furnace

Al O :Tb Powder

XRD and SEM and PL of Al O :Tb Powder

ML and TL of γ irradiated Al O :Tb sample Fig. 1. Flow chart for the preparation of Al2O3:Tb phosphor.


oscilloscope (Scientific SM-340). Schematic diagram of the experimental set-up used for deforming the sample and measuring the ML is shown in Fig. 2. PL was recorded using fluorescence spectrophotometer (Shimadzu RF-5301 XPC) and emission was recorded using a spectral slit width of 1.5 nm. The TL glow curve was recorded using TL reader (Nucleonix TL 1009I) by heating the sample with heating rate 12 1C/s.

3. Results and discussion Fig. 3 shows the XRD pattern of Al2O3:Tb phosphor. The small amount of doped rare earth ions (Tb3 þ ) has virtually no effect on the phase structures. The observed XRD pattern was found to match well with the standard JCPDs [11] data of the compound Al2O3. Fig. 4 shows the XRD reference pattern of the standard Al2O3 compound (JCPDs file no. 00-001-1243). Fig. 5 shows the surface morphologies of the powder sample. The microstructure of the sample reflects the inherent nature of the combustion process. When a gas is escaping under high pressure during combustion process, pores are formed with the formation of small particles near the pores. The non-uniform and irregular shapes of the particles as shown can be attributed to the non-uniform distribution of temperature and mass flow in the combustion flame. Fig. 6 shows the time dependence of ML intensity of g-ray irradiated Al2O3:Tb phosphors. A distinct peak along with the shoulder was observed. It is also observed that the ML intensity of Al2O3:Tb3 þ is approximately 103 times less than the sugar sample (non-irradiated) excited by dropping the same load from the same height. The ML intensity increases with the increase in the concentration of Tb ions, attains an optimum value for a 0.5 mol% then decreases with further increase in the concentration of Tb3 þ . The decay time for the first peak is 0.84 ms and for the second peak is 1.5 ms. 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. 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 center, either by traveling in the conduction band (or valence band) or by electron (or holes) tunneling. As for ML materials, in particular, the recombination process is facilitated by the assistance of dislocation in the crystal [12–16]. In the present investigation the probability of involvement of dislocation is very low because of the particle size of the crystal; probably, piezo-electrification during the impact is responsible for the de-trapping of the trapped charge carriers [17]. The occurrence of the shoulder (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. The release of trapped charge carriers from shallow traps may take place later on due to thermal vibration of lattices and therefore a delayed ML (second peak) may be produced, which may lie in the post deformation region of the phosphor [18]. Fig. 7 shows the gamma dose dependence of ML intensity. It was observed that ML intensity increases with the increase in gamma dose because more charge carriers are trapped with the increase in gamma dose. When the load is applied on to the sample impulsively, de-trapping of charge carriers followed by recombination with the luminescence centers causes emission of light. Fig. 8 shows TL glow curves of Al2O3:Tb3 þ phosphors. Two distinct peaks were observed in TL glow curve around 222 1C and 280 1C. It is observed that TL peak intensity initially increases


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1 4 5

11 8



9 10


1. Stand

5. Guiding Cylinder

9. Wooden Block

2. Pulley

6. Aluminum foil

10. Photomultiplier tube

3. Metallic wire

7. Sample

11. Digital storage oscilloscope

4. Load

8. Transparent Lucite plate

12. Iron base mounted on table

Fig. 2. Schematic diagram of experimental set-up used for deforming the sample and measuring the ML.


500 400 300 200 100 0 20






2 Theta Deg. Fig. 3. XRD pattern of Al2O3:Tb (0.1 mol%) powders.

Fig. 5. SEM photograph of Al2O3:Tb (0.1 mol%) phosphor.

2 ML Intensity (arb. units.)

Intensity (arb. units)


0.1 mol %


0.2 mol % 0.5 mol % 1.0 mol %

1 0.5 0 0

Fig. 4. XRD reference pattern of Al2O3 (JCPDs file No. 00-001-1243).


2 Time (ms)


Fig. 6. Time dependence on ML of g irradiated Al2O3:Tb phosphor.


R.K. Rai et al. / Journal of Luminescence 132 (2012) 210–214


0.456 kGy 0.930 kGy 1.86 kGy

1 0.5 0 1


2 Time (ms)




emi 0.1 mol %

600 500

emi 0.2 mol % emi 1.0 mol %

400 300 200


0 200


0.1 mol% 0.2 mol %


1.0 mol %

25000 20000 15000 10000 5000 0 70


170 220 Temperature (°C)



Fig. 8. TL glow curve of Al2O3:Tb phosphor (g-ray irradiation 0.930 kGy/h).

50000 0.456 kGy



350 400 450 Wavelength (nm)




Fig. 10. (a) Excitation and (b) emission spectra of Al2O3:Tb phosphor having different concentrations of Tb3 þ .

0.5 mol %


emi 0.5 mol %



Fig. 7. Dependence of ML intensity of Al2O3:Tb (0.5 mol%) phosphor on g-dose.

TL Intensity (arb. units)

ext 0.1 mol %



PL Intensity (arb. units)

ML Intensity (arb. units)


TL Intensiy (arb.units)


0.930 kGy 1.86 kGy


get un-trapped successively at different temperatures, depending on their thermal stability. The excited impurity ions by decaying to its ground state give characteristic emission of RE3 þ . The increase in the TL/ML intensity with g-dose attributed to the increase of active luminescent centers with g-ray irradiation and subsequent emission of TL/ML is due to re-oxidation of RE2 þ into RE3 þ during heating/deformation. Thus the intensity increases in the initial stage. The dosage saturation can be explained on the assumption that only limited number of RE ions are available for charge reduction with g-ray irradiation [19]. Fig. 10 shows the PL excitation (wavelength 240 nm) and emission spectrum of the Al2O3:Tb phosphors. The 4f–4f emissions from the 5D4-7Fj (j ¼6, 5, 4, 3) states of Tb3 þ are found at 490 nm, 544 nm, 588 nm and 590 nm, respectively. There are two additional emissions shown in the figure at 424 nm and 441 nm corresponding to the Tb3 þ transition from the 5D3-7F5 state and 5 D3-7F4 state [20]. The 5D4-7F5 line at 544 nm is the strongest in nearly all host crystals [21]. The reason is that this transition has the largest probability for both the electric dipole and magnetic dipole induced transition.

4. Conclusion

20000 10000 0 80


180 230 Temperature (°C)


Fig. 9. Dependence of TL intensity of Al2O3:Tb (0.5 mol %) phosphor on g-dose.

with Tb3 þ concentration, attains an optimum value for 0.5 mol% of Tb3 þ then further decreases with the increase in activator concentration. Fig. 9 shows the dependence of TL intensity on g-dose of Al2O3:Tb3 þ (0.5 mol%) sample. Initially TL intensity increases with the g-dose, attains optimum value at 0.930 kGy and then decreases with a further increase in g-dose. Under exposure to g-ray, electron hole pairs are created. Some of the released electrons are captured by the impurity RE3 þ ions that convert to RE2 þ . The hole is captured in the host related centers. Warming of the irradiated samples causes these holes to

The polycrystalline Al2O3:Tb (0.1, 0.2, 0.5 and 1.0 mol%) phosphors were successfully synthesized by the combustion technique at an initiating temperature of  300 1C and its luminescence properties were investigated. This new synthesis condition allows rare earth ions to incorporate easily into the Al2O3 lattice, in spite of the large size difference between rare earth ions and the aluminum cations in the Al2O3 structures. ML/TL intensity initially increases with Tb3 þ concentration, approaches to an optimum value for 0.5 mol% of Tb and decreases with a further increase in Tb3 þ concentration. Al2O3:Tb3 þ shows the characteristic emission of Tb3 þ ions and the principal emission was observed in the green region, which is suitable for commonly used photomultiplier tubes.

Acknowledgement One of the authors (RKR) is thankful to Dr. S. A. Khan, Principal, Government College Seepat, Bilaspur (C.G.), for their guidance and valuable suggestion regarding the synthesis of the sample and is also thankful to the Head of the Department Institute Instrumentation


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