Combustion synthesis and luminescence characteristic of rare earth activated LiCaBO3

JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012, P. 1005

Combustion synthesis and luminescence characteristic of rare earth activated LiCaBO3 N.S. Bajaj, S.K. Omanwar (Department of Physics, SGB Amravati University Amravati-444602 (M.S.), India) Received 23 December 2011; revised 14 February 2012

Abstract: Lithium calcium borate (LiCaBO3) polycrystalline thermoluminescence (TL) phosphor doped with rare earth (Tb3+ and Dy3+) elements was synthesized by novel solution combustion synthesis. The reaction produced very stable crystalline LiCaBO3:D (D= Tb3+ and Dy3+) phosphors. These rare earth doped phosphors material showed maximum TL sensitivity with favorable glow curve shape. TL glow curve of X-ray irradiated that LiCaBO3:Tb3+ and LiCaBO3:Dy3+ samples showed two major well-separated glow peaks. The TL sensitivity of these phosphors to X-ray radiation was comparable with that of TLD-100 (Harshaw). Photoluminescence spectra of LiCaBO3:Tb3+ and LiCaBO3:Dy3+ showed the characteristic Tb3+ and Dy3+ peaks respectively. TL response to X-ray radiation dose was linear up to 25 Gy. Keywords: luminescence; LiCaBO3; combustion synthesis; rare earths; kinetics

Looking at our previous experience[1–6] of the synthesis of luminescent materials specifically for silicates, oxides, borates and aluminates, the combustion method is found to be very useful. In addition to this the method provides uniform and narrow distribution of the particle size of the product. Moreover the method is simple, easy and time saving. Thus we have attempted solution combustion method for the synthesis of lithium borates. Borate compounds are known for their wide band gap. They are the best hosts for various activators. From the literature it is found that borate compounds find several interesting applications, e.g. barium borate is a non-linear optical (NLO) material used for laser harmonic generation[7] and for forming of green component of tricolor TV phosphor[8]. In borate compounds boron atom is coordinated by oxygen atoms to form a variety of atomic groups that affect the physical properties in general and optical properties in particular[9]. Because of the low Z and high emission in UV region LiCaBO3 could be the potential candidates as luminescent materials. However, earlier workers used highly sophisticated method for the synthesis of these compounds[9–13]. Here is the attempt of adopting solution combustion method for the preparation of these new low Z alkaline-earth metal borate compounds. Also earlier worker used gamma rays as the irradiation source; here for the first time we used X-ray source for this material.

purity starting materials, Li2CO3 (G.R.), Ca(NO3)2˜4H2O (A.R.), H3BO3 (A.R.), CO(NH2)2 (G.R.), NH4NO3 (G.R.) were mixed thoroughly in agate mortar for about 5 min with drop by drop addition of double distilled water, so that the paste was formed. A stock solution of stoichiometric amount of dopant prepared by dissolving oxides of terbium into concentrated HNO3 in ratio of 1 g in 100 ml of double distilled water was then mixed in paste. The prepared paste was kept in heating menthol till we got a clear and completely dissolved solution. As prepared solution was then transferred into the pre-heated furnace (550 °C) after warming it for 5 min. The self heat generating redox reaction was completed and the fine powder of LiCaBO3:Tb was finally obtained. This raw powder was sintered for minimum 1 h at 700 ºC and quenched to room temperature on an aluminum plate. Exactly the same procedure was followed for the synthesis of the phosphors with other dopant. During the combustion synthesis proper molar ratios of fuel and oxidizers depending on the moles of precursors are very necessary for evacuation of unwanted various molecules so that fine powders of required product could be finally obtained. Therefore the reaction is to be exactly balanced. Such balanced chemical reactions of each of the phosphors prepared are listed systematically in following Table 1.

1 Experimental

2.1 Conformation of material

During this synthesis, the stoichiometric amounts of high

2 Results and discussion

Fig. 1 represents the XRD pattern for the product synthe-

Foundation item: Project supported by University Grant Commission, New Delhi (UGC/F.1 /37-332/2009(SR)) Corresponding author: N.S. Bajaj (E-mail: [email protected]; Tel.: +7588793056) DOI: 10.1016/S1002-0721(12)60169-0

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JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012

Table 1 Balanced chemical reactions during the combustion synthesis S.N.

Product

1

LiCaBO3:Dy

Corresponding reaction with balanced molar ratios of precursors 0.5Li2CO3+0.998Ca(NO3)2˜4H2O+H3BO3+ 5CO(NH2)2+4NH4NO3+0.002Dy2O3+4O2 LiCaBO3:Dy+(5.5CO2+4NH3+4NO2+17.5H2O+6N2 )

2

LiCaBO3:Tb

0.5Li2CO3+0.998Ca(NO3)2˜4H2O+H3BO3+5CO(NH2)2+ 4NH4NO3+0.002Tb4O7+4O2LiCaBO3:Tb+(5.5CO2+ 4NH3+4NO2+17.5H2O+6N2)

Fig. 1 XRD pattern for LiCaBO3 prepared by solution combustion synthesis

sized by us, which is in good agreement with the literatures[9–13]. Important lines in this pattern for the 2 values, i.e., 28.90, 34.06, 40.82, 41.82, 29.44 and 59.82 in order of decreasing intensity were in exactly matching with our data, however the order of decreasing intensity is slightly changed. In our results we found that the 2 values for decreasing intensity are 28.90, 29.44, 34.06, 41.82, 40.82 and 59.82. This agreement indicates that LiCaBO3 has been successfully prepared by using the combustion method, which we have attempted for the first time to this new material.

traps of the sample. It is known that it is better to have high TL sensitivity and the peak temperature should locate around 200 ºC for the dosimeter because in this temperature range the TL intensity of the dosimeter does not have obvious fading and the affect ion of the black radiation can also be neglected[14]. Therefore, we will center our attention on the dosimetric properties of LiCaBO3:D in the following part. 2.3 Kinetic study The TL glow curves for LiCaBO3:Tb3+ and LiCaBO3:Dy3+ phosphors were deconvoluted by using the PeakFit software as shown in Figs. (3, 4) respectively. We employed the peak shape method to analyze the activation energy of both the LiCaBO3:Tb3+ and LiCaBO3:Dy3+ phosphors using Eq. (1)[14]. E=c(kTm2/)–b(2kTm) (1) where  stands for , , or . The values of , , and  are respectively determined by low-temperature half-width (= Tm–T1), high-temperature half-width (=T2–Tm) and full width (=T2–T1). For first-order kinetics, the values of the c and b depending on , , or [14] and k is Boltzmann constant. To analyze the frequency factor s of the glow curve values the activation energy obtained from Eq. (1) and heating rate () at which the glow curves are recorded were used in Eq. (2). The summary of calculated result is represented in Table 2. E/kTm2=sexp[–E/kTm] (2)

2.2 TL glow curves TL curves of LiCaBO3 doped with Tb3+ and Dy3+ phosphors exposed to a 5 Gy X-ray irradiation are shown in Fig. 2 Compared with commercial phosphor TLD-100 purchased from harshaw, it is observed that the phosphors are well sensitive to X-ray and exhibits TL intensity approximately one third of TLD-100. It is obvious that the two glow curves are very different in shape, peak position and TL intensity. Curve for LiCaBO3:Tb3+ and LiCaBO3:Dy3+ has the main TL peak respectively at about 136 and 124 ºC with one weak shoulder at about 188 and 219 ºC. It is known that the TL curve is closely related with the traps of the material. Therefore, the difference of the TL curves in Fig. 2 shows that the different rare earth dopants have substantial effect on the

Fig. 3 Deconvoluted curves from experimental curve for LiCaBO3: Tb3+

Fig. 4 Deconvoluted curves from experimental curve for LiCaBO3: Dy3+ Table 2 Kinetic parameters for LiCaBO3:Tb3+ and LiCaBO3:Dy3+ Phosphor

Peak 3+

LiCaBO3:Tb

Fig. 2 TL glow curve for LiCaBO3:D (D=Tb3+, Dy3+) compared with commercial phosphor TLD-100

LiCaBO3:Dy3+

Order of kinetics

E/eV

s/s–1

Tm/ºC 6

Main peak

First

0.148 2.13×10

136

Shoulder peak

First

0.096 4.72×106

188

Main peak

First

0.101 2.06×105

124

Shoulder peak

First

0.096 1.18×105

219

N.S. Bajaj et al., Combustion synthesis and luminescence characteristic of rare earth activated LiCaBO3

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2.4 Photoluminescence study 2.4.1 LiCaBO3:Tb3+ The emission spectrum of LiCaBO3:Tb3+ consists of a series of sharp lines peaking at 486, 543, 585 and 627 nm corresponding to the 5D4 to 7Fj (J=3,4,5,6) transitions within the 4f8 configurations Tb3+ (as shown in Fig. 5). Among the emission lines from the 5D4 state the dominant emission is observed at 543 nm, corresponding to the 5D4 7 F5 transition. The 5D47F5 emission line is the strongest in nearly all host crystals when the Tb3+ concentration is a few mole percentage or higher because this transition has the largest probability for both electric-dipole and magnetic-dipole induced transitions. The intensity of 486 nm line, which corresponds to 5D47F6 transition of Tb3+ ions is comparable and about one half the intensity of 5D47F5 line. The intensity of the emission lines 585 and 627 nm is weak and corresponds to 5D47F4 and 5D47F3 transition of Tb3+ ions. 2.4.2 LiCaBO3:Dy3+ The excitation and emission spectra of LiCaBO3:Dy3+ phosphors are demonstrated in Fig. 6. Under 359 nm excitation, the emission spectrum of LiCaBO3:Dy3+ phosphor shows three bands at 476, 486–89 and 577 nm, which originate from the transitions of 4F9/26H15/2, 4F9/2 6 H13/2 and 4F9/26H11/2 of Dy3+, respectively. The excitation spectrum for 577 nm emissions has several excitation bands at 309, 332, 340, 359, 372 and 396 nm which come from the transitions of 6H15/24D7/2, 6P7/2, 6M21/2, 4G11/2, 4I15/2 and 6F9/2 of Dy3+, respectively. 2.5 Dose response The TL material is said to be good when its response to absorbed dose is linear over the wide range. To study the linearity five samples were irradiated simultaneously for each level of dose. Each data point corresponds to the mean of the five readings. The linearity is observed in the range from 5 to 25 Gy as shown in Fig. 7. The relationship be-

Fig. 5 Photoluminescence excitation and emission spectra for LiCaBO3:Tb3+

Fig. 6 Photoluminescence excitation and emission spectra for LiCaBO3:Dy3+

Fig. 7 Dose response of LiCaBO3:Tb3+ and LiCaBO3:Dy3+ phosphors

tween the TL response of high temperature peak and the absorbed dose for LiCaBO3:Tb3+ and LiCaBO3:Dy3+ phosphor is shown in Fig. 7 and it is found to be linear.

3 Conclusions We synthesized LiCaBO3:D (D=Tb and Dy) phosphors by one step, low cost and low temperature solution combustion synthesis. TL characteristics and some dosimetric properties of Tb3+ and Dy3+ activated LiCaBO3 phosphor were investigated in detail. The trap parameters, i.e., activation energy and frequency factor of the TL glow curve of the sample were calculated by peak shape method. The photo luminescence emission showed the characteristic transition of Tb3+ and Dy3+ at 543 and 577 nm respectively. The TL and dosimetric characteristics implied the potential of LiCaBO3: Tb3+ and Dy3+ phosphor as X-ray TL materials in the personal protection dosimetry field and radiation dosimetry. Acknowledgement: One of the authors NSB is thankful to UGC New Delhi for Financial support under the Grant given in the Major Research project.

References: [1] Bhatkar V B, Omanwar S K, Moharil S V. Combustion synthesis of the Zn2SiO4:Mn phosphor. Phys. Stat. Sol. (A), 2002, 191: 272. [2] Dhakare D S, Omanwar S K, Moharil S V, Dhopte S M, Muthal P L, Kondawar V K. Combustion synthesis of borate phosphors. Optical Materials, 2007, 29(12): 1731. [3] Bhatkar V B, Omanwar S K, Moharil S V. Combustion synthesis of silicate phosphors. Optical Materials, 2007, 29(8): 1066. [4] Sonekar R P, Omanwar S K, Moharil S V, Dhopte S M, Muthal P L, Kondawar V K. Combustion synthesis of narrow UVB emitting rare earth borate phosphor. Optical Materials, 2007, 30(4): 622. [5] Sonekar R P, Omanwar S K, Moharil S V. Combustion synthesis and photoluminescence of Eu2+ doped BaB8O13. Indian Journal of Pure and Applied Physics, 2009, 47: 441. [6] Nagpure P A, Bajaj N S, Sonekar R P, Omanwar S K. Synthesis and luminescence studies of rare earth activated lanthanum pantaborate. Indian Journal of Pure and Applied Physics, 2011, 49: 799. [7] Chen C T, Wu Y C, Li R K. The development of new NLO crystals in the borate series. Journal of Crystal Growth, 1990,

1008 99(1-4): 790. [8] Lu Chung-Hsin, Godbole S V, Natarajan V. Luminescence characteristics of strontium borate phosphate phosphors. Materials Chemistry and Physics, 2005, 93: 73. [9] Wu L, Chen X L, Li H, He M, Dai L, Li X Z, Xu Y P. Structure determination of a new compound LiCaBO3. J. Solid State Chem., 2004, 177: 1111. [10] Wu L, Chen X L, Tu Q Y, He M, Zhang Y, Xu Y P. Phase relations in the system Li2O-CaO-B2O3. J. Alloys Compd., 2003, 358: 23. [11] Jiang L H, Zhang Y L, Li C Y, Pang R, Hao J Q, Su Q. Ther-

JOURNAL OF RARE EARTHS, Vol. 30, No. 10, Oct. 2012 moluminescence characteristics of rare-earth-doped LiCaBO3 phosphor. J. Lumin., 2008, 128: 1904. [12] Li P L, Wang Z J, Yang Z P, Guo Q L, Li X. Luminescent characteristics of LiCaBo3:M (M=Eu3+, Sm3+, Tb3+, Ce3+, Dy3+) phosphor for white LED. J. Lumin., 2010, 130: 0222. [13] Anishia S R, Jose M T, Annalakshmi O, Ponnusamy V, Ramasamy V. Dosimetric properties of rare earth doped LiCaBO3 thermo luminescence phosphors. J. Lumin., 2010, 130: 1834. [14] Mckeever S W S. Thermoluminescence of Solids. Cambridge: Cambridge University Press, 1998. 88.