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Combustion synthesis of silicate phosphors
Optical Materials 29 (2007) 1066–1070 www.elsevier.com/locate/optmat
Combustion synthesis of silicate phosphors V.B. Bhatkar a
a,*
, S.K. Omanwar a, S.V. Moharil
b
Department of Physics, Amaravati University, Amravati 444602, India b Department of Physics, Nagpur University, Nagpur 440010, India
Received 5 January 2006; received in revised form 3 April 2006; accepted 3 April 2006 Available online 3 July 2006
Abstract Silicate materials are useful for various applications. Combustion synthesis of some silicate phosphors is described. The synthesis is based on the exothermic reaction between urea and ammonium nitrate and subsequent heat transfer to the reactants. It is argued that the combustion synthesis is a simple and fast method for preparing silicate materials. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Silicates; Phosphor; Combustion synthesis; Zn2SiO4:Mn; BaSi2O5:Pb
1. Introduction Silicate materials are useful in many applications of technological importance. Zeolites, a type of microporous aluminosilicates, are widely used as molecular sieves and catalysts [1]. The lithium ceramics are promising breeder materials for fusion reactors, among which lithium orthosilicate (Li4SiO4) is strongly considered because of its good tritium solubility [2]. Mg2SiO4:Tb is used in the commercial systems for the thermoluminescence dosimetry of the ionising radiations [3]. BaSi2O5:Pb is an efficient black light phosphor which finds use in photocopying and sun-tanning lamps [4]. Zn2SiO4:Mn is one of the earliest known phosphors [5] . Zn2SiO4:Mn was also used as the green component in the first tri-colour lamp [6], as well as the first colour television. Several silicates find place in the list of cathodoluminescent materials approved by JEDEC. Phosphors P1 and P39 are the commercial lamp and CRT phosphors based on Zn2SiO4:Mn. Y2SiO5:Ce (P47) and CaSiO3:Pb, Mn (P25) are also cathodoluminescent materials of commercial importance. Y2SiO5 doped with cerium (P47) was investigated as an alternative to the standard
*
Corresponding author. E-mail address: [email protected] (V.B. Bhatkar).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.04.006
P22 (ZnS:Ag) blue phosphor [7], currently used in television sets, for low-voltage field emission displays (FEDs). Saturation measurements indicated that the high-saturation resistance of the Ce-doped silicate can yield better performance than ZnS:Ag when operated at low-voltages (1–3 kV). Manganese doped zinc silicate is a well established, green emitting phosphor for use in plasma display panels [8]. Cerium activated oxyorthosilcates Lu2SiO5:Ce (LSO:Ce [9]), Y2SiO5:Ce (YSO:Ce) and Gd2SiO5:Ce, (GSO:Ce) have proved to be excellent scintillation phosphors because of their relatively high-density, good light output, and fast decay time [9]. Bi12SiO20 (BSO) single crystals have attracted increasing attention in optoelectronics as an efficient photorefractive material and also in applications such as spatial-time light modulators, holographic information storage, two- and four-wave mixing, and several other applications in the field of nonlinear optics [10]. Using shallow traps application optical fixing through two-step gated recording in a crystal of La3Ga5SiO14 doped with praseodymium has been demonstrated [11]. Recently, Shen and Kacharu [12] introduced a new process for parallel data storage using coherent time domain optical memory and proposed Y2SiO5:Eu for practical developments. Photostimulated luminescence in silicates can be used for developing X-ray storage phosphors. Utility of Ba5SiO4Br6:Eu2+ [13] and Y2SiO5:Ce, Sm [14] for such
V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
purpose has been demonstrated. The high-conversion efficiency and short response time make it a good alternative to BaFBr:Eu as a storage phosphor in computerised radiography. The long lasting phosphorescence is observed in Ce3+ doped Ca2Al2SiO7 at room temperature. Under UV-irradiation at 365 nm it produces broadband emission with peak at 417 nm, though the intensity is weak [15]. Rare-earth-containing silicate glasses have attracted great attention as potential materials for optical and magnetooptical devices such as an upconversion laser, a hole burning memory, and an optical switch [16], because they show a high-temperature persistent spectral hole burning (PSHB)
1067
and have a large potential for a high-density frequencydomain optical data memory [17]. Most of the silicates have high-melting points. Moreover, they can appear in crystalline as well as glassy form. Synthesis of silicates is rather tricky for these reasons. Conventionally, solid state diffusion methods have been used for the synthesis of silicates. During later years novel techniques like sol–gel synthesis have been used which enable the preparation of fine, homogenous powders [18]. Recently, we have reported combustion synthesis of Zn2SiO4:Mn [19]. The synthesis is based on the exothermic reaction between urea and ammonium nitrate. In this paper we report
Table 1 Details of the starting materials used in the combustion synthesis of various silicates Sr. no.
Name of the compound
Starting materials
1
Zn2SiO4:Mn2+ Mole ratio:
ZnO 1.92
SiO2 Æ xH2O 1
MnSO4 0.08
NH4NO3 20
Urea 20
2
BaSi2O5:Pb2+ Mole ratio:
BaCO3 0.99
SiO2 Æ xH2O 2
Pb(NO3)2 0.01
NH4NO3 25
Urea 25
3
BaSi2O5:Eu2+ Mole ratio:
BaCO3 0.98
SiO2 Æ xH2O 2
Eu2O3 0.01
NH4NO3 25
Urea 25
4
BaSi2O5:Ce3+ Mole ratio:
BaCO3 0.99
SiO2 Æ xH2O 2
CeCl3 0.01
NH4NO3 25
Urea 25
5
BaMgSiO4:Eu2+ Mole ratio:
BaCO3, Mg(NO3)2 0.98, 1
SiO2 Æ xH2O 1
Eu2O3 0.01
NH4NO3 15
Urea 20
6
CaMgSiO4:Eu2+ Mole ratio:
CaCO3, Mg(NO3)2 0.98, 1
SiO2 Æ xH2O 1
Eu2O3 0.01
NH4NO3 15
Urea 20
7
BaSrSiO4:Eu2+ Mole ratio:
BaCO3, SrCO3 1, 0.98
SiO2 Æ xH2O 1
Eu2O3 0.01
NH4NO3 20
Urea 20
8
Ca2MgSi2O7:Eu2+ Mole ratio:
CaCO3, Mg(NO3)2 0.98, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
9
Ca1.5Sr0.5MgSi2O7:Eu2+ Mole ratio:
CaCO3, SrCO3, Mg(NO3)2 1.48, 0.05, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
10
Ca0.5Sr1.5MgSi2O7:Eu2+ Mole ratio:
CaCO3, SrCO3, Mg(NO3)2 0.05, 1.48, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
11
Sr2MgSi2O7:Eu2+ Mole ratio:
SrCO3, Mg(NO3)2 1.98, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
12
Sr2SiO4:Pb2+ Mole ratio:
SrCO3 1.98
SiO2 Æ xH2O 1
Pb(NO3)2 0.02
NH4NO3 20
Urea 20
13
CaMgSiO4:Pb2+ Mole ratio:
CaCO3, Mg(NO3)2 0.99, 1
SiO2 Æ xH2O 1
Pb(NO3)2 0.01
NH4NO3 15
Urea 20
14
BaSrSiO4:Pb2+ Mole ratio:
BaCO3, SrCO3 1, 0.99
SiO2 Æ xH2O 1
Pb(NO3)2 0.01
NH4NO3 20
Urea 20
15
CaMgSiO4:Ce3+ Mole ratio:
CaCO3, Mg(NO3)2 0.99, 1
SiO2 Æ xH2O 1
CeCl3 0.01
NH4NO3 15
Urea 20
16
BaSrSiO4:Ce3+ Mole ratio:
BaCO3, SrCO3 1, 0.99
SiO2 Æ xH2O 1
CeCl3 0.01
NH4NO3 20
Urea 20
1068
V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
combustion synthesis of several silicate phosphors, which demonstrate that the combustion synthesis can serve as a fast, efficient method for preparing the silicate materials. 2. Experimental Phosphors were prepared by combustion synthesis. The detailed description of the methods can be found in the original works of Patil and co-workers [20,21]. Ingredients used were metal carbonates, silicic acid (SiO2 Æ xH2O), and the dopant salts. Urea was used as a fuel and ammonium nitrate as oxidizer. Fuel to oxidizer ratio, optimised as described in our earlier work [19], was used. All constituents in stoichiometric proportions (Table 1), along with fuel and oxidizer were mixed together and small quantity of double distilled water was added. The mixture on thoroughly grinding was transferred to a pre-heated furnace at 500 °C. On rapid heating the mixture evaporates and ignites at 450 °C to yield silicates. Photoluminescence spectra were recorded on Hitachi F-4000 spectro-fluorimeter with spectral slit width of 1.5 nm. 3. Results and discussion In the system Li2O:SiO2, Li2SiO3 and Li4SiO4 are the predominant phases. Glasses of various compositions are also possible. Zinc silicate also occurs in two forms, viz. Beta and Willemite. To confirm the structure of the synthesized phosphors, powder photographs were obtained using Philips diffractometer, PW 1710. The powder photograph for the lithium silicate matched with the JCPDS file 37-1472 corresponding to Li4SiO4. Similarly the powder photograph of Zn2SiO4 matched with JCPDS file 37-1485 corresponding to Willemite phase of Zn2SiO4. As an example, data for Li4SiO4 are given in Table 2. The XRD measurements thus confirm that the desired silicates could be synthesized in a single step by the combustion method. However, the as-prepared Zn2SiO4:Mn powder was slightly yellowish. This is probably due to presence of some Mn4+. The sample was annealed at 900 °C for 1 h in the reducing atmosphere provided by burning charcoal. Fig. 1 shows PL spectra for Zn2SiO4:Mn. The emission in Zn2SiO4:Mn (curve e), which corresponds to the transition 4T1(4G) ! 6A1(6S), has a maximum situated at 523 nm with half width of 37 nm, which agrees with the literature value [22]. The corresponding excitation spectrum (curve a) shows a maximum around 243 nm corresponding to CT transition of Mn2+ [23]. Intensity of the emission was compared with the green standard CeMgAl11O19:Tb phosphor and was found to be about 90% of the commercial phosphor. Fig. 1 also shows PL spectra for BaSi2O5:Pb phosphor. Emission in BaSi2O5:Pb (curve f) is in form of a broad band around 350 nm and a shoulder at longer wavelengths. The excitation maximum is around 245 nm (curve b). This agrees with the literature data. The spectra correspond to
Table 2 XRD data for Li4SiO4 Sr. no.
2h
37-1472
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
16.721 17.2 22.19 22.583 24.03 24.13 28.174 29.247 33.799 33.9 34.13 34.802 36.961 38.2 38.788 40.97 41.11 41.61 41.74 45.28 47.853 48.755 49.3 49.43 51.244 51.57 51.74 51.79 53.325 54.015 55.005 55.563 56.21 56.365 57.04 57.205 58.375 58.527 60.005 60.105 60.577 61.983
19 15 100 85 31 38 52 19 100 32 23 82 2 38 2 11 11 3 4 1 1 13 26 19 8 11 7 5 0.1 2 1 3 3 2 3 3 12 6 6 15 35 9
1
This work 32 21 97 94 56 63 14.4 100
52 4.2 22 7 10
7.3 19 14 5.5 1.5
8
28 5.8
S0 ! 1P1 transitions. PL spectra for Eu2+ and Ce3+ activators in BaSi2O5 host are also included in Fig. 1. Eu2+ emission is in form of a broad band around 500 nm (curve g). Ce3+ emission peaks at 390 nm (curve h). Poort et al. [24] have described interesting PL of Eu2+ in silicate hosts. The excitation can be achieved by long wavelength UV bordering onto visible region and the emission is in blue or green region of the spectrum. Such phosphors can be useful for photoluminescent liquid crystal display (PLLCD) applications [25]. Combustion synthesis of these materials was attempted. The as-prepared phosphors did not show emission well above the background level, probably the activator Eu is not incorporated in divalent form. The phosphors reheated in reducing atmosphere provided by the burning charcoal at 900 °C for 1 h showed intense Eu2+ emission. Results for the silicates containing SiO4 group are presented in Fig. 2.
V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
Fig. 1. PL spectra for Zn2SiO4:Mn and BaSi2O5:Pb phosphors. (a) Zn2SiO4:Mn excitation for 524 nm emission; (b) BaSi2O5:Pb excitation for 350 nm emission; (c) BaSi2O5:Eu excitation for 350 nm emission; (d) BaSi2O5:Ce excitation for 350 nm Emission (e) Zn2SiO4:Mn Emission for 254 nm excitation; (f) BaSi2O5:Pb Emission for 254 nm excitation; (g) BaSi2O5:Eu excitation for 350 nm emission; (h) BaSi2O5:Ce excitation for 350 nm emission.
2+
Fig. 2. PL spectra for Eu doped orthosilicate phosphors. (a) BaMgSiO4:Eu excitation for 500 nm emission; (b) CaMgSiO4:Eu excitation for 524 nm emission; (c) BaSrSiO4:Eu excitation for 512 nm emission; (d) BaMgSiO4:Eu emission for 385 nm excitation; (e) CaMgSiO4:Eu emission for 385 nm excitation; (f) BaSrSiO4:Eu emission for 385 nm excitation.
BaMgSiO4:Eu (curve d) exhibits a very broad emission around 500 nm. The broad excitation band peaks at 350 nm. There is considerable excitation intensity up to 400 nm. The excitation curve falls rapidly thereafter. For BaSrSiO4:Eu, the emission is bluish green peaking at 512 nm (curve f). The excitation spectrum (curve c) is located at longer wavelengths compared to BaMgSiO4:Eu. CaMgSiO4:Eu exhibits very interesting emission spectrum (curve e) consisting of blue (446 nm) and green (524 nm) bands. Again, the excitation spectrum lies in near UV and violet–blue region of the spectrum. Efficient excitation by 385 nm, which is required for PLLCD applications, was
1069
Fig. 3. PL spectra for Eu2+ doped disilicate phosphors. (a) Ca2MgSi2O7 excitation for 447 nm emission; (b) Ca1.5Sr0.5MgSi2O7 excitation for 448 nm emission; (c) Ca0.5Sr1.5MgSi2O7 excitation for 478 nm emission; (d) Sr2MgSi2O7 excitation for 460 nm emission; (e) Ca2MgSi2O7 emission for 385 nm excitation; (f) Ca1.5Sr0.5MgSi2O7 emission for 385 nm excitation; (g) Ca0.5Sr1.5MgSi2O7 emission for 385 nm excitation; (h) Sr2MgSi2O7 emission for 385 nm excitation.
Fig. 4. PL spectra for Pb2+ doped orthosilicate phosphors. (a) Sr2SiO4:Pb excitation for 400 nm emission; (b) CaMgSiO4:Pb excitation for 360 nm emission; (c) BaSrSiO4:Pb excitation for 400 nm emission; (d) Sr2SiO4:Pb emission for 254 nm excitation; (e) CaMgSiO4:Pb emission for 254 nm excitation; (f) BaSrSiO4:Pb emission for 254 nm excitation.
observed for some Eu2+ doped disilicates also (Fig. 3). Ca2MgSi2O7 (curve e), and Ca1.5Sr0.5MgSi2O7 (curve f) exhibit blue emission around 448 nm. Emission in Sr2MgSi2O7 (curve h) peaks around 460 nm. Emission in Ca0.5Sr1.5MgSi2O7 (curve g) peaks at still longer wavelengths. Excitation spectrum of all the samples (curves a–d) show considerable intensity at 385 nm. Sr2MgSi2O7: Eu appears to be best suited as a blue phosphor for PLLCD applications. Pb2+ exhibits very efficient emission in BaSi2O5 host. Luminescence of Pb2+ (1 mol%) in some alkaline earth silicates was therefore studied. The results are presented in Fig. 4. CaMgSiO4:Pb (curve e) exhibits UV emission with
1070
V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
Acknowledgements One of us (VBB) received fellowship from University Grants Commission, New Delhi, India. The financial assistance is gratefully acknowledged. References
Fig. 5. PL spectra for Ce3+ doped orthosilicate phosphors. (a) CaMgSiO4:Ce excitation for 385 nm emission; (b) BaSrSiO4:Ce excitation for 400 nm emission; (c) CaMgSiO4:Ce emission for 330 nm excitation; (d) BaSrSiO4:Ce emission for 340 nm excitation.
a maximum around 360 nm. Emission in BaSrSiO4:Pb and Sr2SiO4:Pb (curves d and f) was at longer wavelengths. Excitation spectra (curves a–c) show reasonable overlap with 253.7 nm Hg emission. However, the emission intensities are poor. This is most probably due to thermal quenching of the luminescence. Poort et al. [24] observed such quenching for Eu2+ emission in these lattices. As seen from Figs. 2 and 3, Eu2+ excitation in silicate hosts are situated at longer wavelengths. It was thought that f–d transitions of other rare earth impurities also might show this trend. PL spectra of some Ce3+ activated orthosilicates are presented in Fig. 5. As expected, the excitation bands (curve a and b) are situated at wavelengths longer than 320 nm, whereas in most materials they appear around 265 nm [26]. The emission bands are around 385 nm for CaMgSiO4 (curve c) and 400 nm for BaSrSiO4 (curve d). 4. Conclusions Several types of silicates are successfully prepared using the combustion synthesis. The well known phosphors Zn2SiO4:Mn and BaSi2O5:Pb could be reproduced using this synthesis. Characteristic emission of activators such as Ce3+, Eu2+ and Pb2+ could be observed in the silicates prepared by this method. The combustion synthesis thus furnishes a fast and simple method for preparing silicate phosphors.
[1] H. van Bekkum, E.M. Flanigen, J.C. Jansen, Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, 1991. [2] G. Piazza, F. Scaffidi-Argentina, H. Werle, J. Nucl. Mat. 283–287 (2000) 1396. [3] T. Hashizume, Y. Kato, T. Nakajima, T. Tovya, H. Sakamato, N. Kotera, S. Iguchi, in: Proc. Symp. Adv. Rad. Dete., Vienna, IAEA, 1971, p.91. [4] R.C. Ropp, Luminescence and the Solid State, Elsevier, Amsterdam, 1991. [5] G.R. Fonda, J. Phys. Chem. 43 (1939) 561. [6] H.H. Haft, W.A. Thornton, JIES (Oct) (1972). [7] C. Stoffers, R.Y. Lee, J. Penczek, B.K. Wagner, C.J. Summers, Appl. Phys. Lett. 76 (8) (2000) 949. [8] K.S. Sohn, B. Cho, H. Chang, H.D. Park, Y.G. Choi, K.H. Kim, J. Europ. Ceramic. Soc. 20 (2000) 1043. [9] D.W. Cooke, B.L. Bennett, R.E. Muenchausen, K.J. McClellan, J.M. Roper, M.T. Whittaker, J. Appl. Phys. 86 (1999) 5308; D. Cooke, K.J. WmcClellan, B.L. Bennett, J.M. Roper, M.T. Whittaker, R.E. Muenchausen, R.C. Sze, J. Appl. Phys. 88 (2000) 7360. [10] V. Arinova, M. Veleva, D. Petrova, I.M. Kourmoulis, D.G. Papazoglou, A.G. Apostolidis, E.D. Vanidhis, N.C. Deliolanis, J. Appl. Phys. 89 (2001) 2686. [11] T. Nikolajsen, P.M. Johansen, J. Opt. A: Pure Appl. Opt. 2 (2000) 255. [12] X.A. Shen, R. Kachru, J. Alloy. Comp. 250 (1997) 435. [13] A. Meijerink, G. Blasse, L. Struye, Maters. Chem. Phys. 21 (1989) 261. [14] A. Meijerink, W.J. Scipper, G. Blasse, J. Phys. D.: Appl. Phys. 24 (1991) 997. [15] N. Kodama, T. Takahashi, M. Yamaga, Y. Tanii, J. Qiu, K. Hirao, Appl. Phys. Lett. 75 (1999) 1715. [16] K. Fujita, K. Hirao, K. Tanaka, N. Soga, H. Sasaki, J. Appl. Phys. 82 (1997) 5114. [17] Y. Mao, P. Gavrilovic, P. Singh, S.A. Bruce, W.H. Grodkiewicz, Appl. Phys. Lett. 68 (1996) 3677. [18] T.S. Ahmadi, M. Haase, H. Weller, Mat. Res. Bull. 35 (2000) 1869. [19] V.B. Bhatkar, S.K. Omanwar, S.V. Moharil, phys. Stat. Solidi (a) 191 (2002) 272. [20] S. Ekambaram, K.C. Patil, J. Alloy. Comp. 217 (1995) 104. [21] J.J. Kingsley, K. Suresh, K.C. Patil, J. Mat. Sci. 25 (1990) 1305. [22] C. Barthou, J. Benoit, P. Benalloui, A. Morell, J. Electrochem. Soc. 141 (1994) 524. [23] E. van der Kolk, P. Dorenbos, C.W.E. van Eijk, H. Bechtel, T. Justl, H. Nikol, C.R. Ronda, D.U. Wiechert, J. Lum. 87–89 (2000) 1246. [24] S.H.M. Poort, H.M. Reijnhoudt, H.O.T. van der Kuip, G. Blasse, J. Alloy. Comp. 241 (1996) 75. [25] A. Vecht, A.C. Newport, P.A. Bayley, W.A. Crossland, J. Appl. Phys. 84 (1998) 3827. [26] P. Dorenbos, Phys. Rev. B 64 (2001) 125117.
Combustion synthesis of silicate phosphors V.B. Bhatkar a
a,*
, S.K. Omanwar a, S.V. Moharil
b
Department of Physics, Amaravati University, Amravati 444602, India b Department of Physics, Nagpur University, Nagpur 440010, India
Received 5 January 2006; received in revised form 3 April 2006; accepted 3 April 2006 Available online 3 July 2006
Abstract Silicate materials are useful for various applications. Combustion synthesis of some silicate phosphors is described. The synthesis is based on the exothermic reaction between urea and ammonium nitrate and subsequent heat transfer to the reactants. It is argued that the combustion synthesis is a simple and fast method for preparing silicate materials. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Silicates; Phosphor; Combustion synthesis; Zn2SiO4:Mn; BaSi2O5:Pb
1. Introduction Silicate materials are useful in many applications of technological importance. Zeolites, a type of microporous aluminosilicates, are widely used as molecular sieves and catalysts [1]. The lithium ceramics are promising breeder materials for fusion reactors, among which lithium orthosilicate (Li4SiO4) is strongly considered because of its good tritium solubility [2]. Mg2SiO4:Tb is used in the commercial systems for the thermoluminescence dosimetry of the ionising radiations [3]. BaSi2O5:Pb is an efficient black light phosphor which finds use in photocopying and sun-tanning lamps [4]. Zn2SiO4:Mn is one of the earliest known phosphors [5] . Zn2SiO4:Mn was also used as the green component in the first tri-colour lamp [6], as well as the first colour television. Several silicates find place in the list of cathodoluminescent materials approved by JEDEC. Phosphors P1 and P39 are the commercial lamp and CRT phosphors based on Zn2SiO4:Mn. Y2SiO5:Ce (P47) and CaSiO3:Pb, Mn (P25) are also cathodoluminescent materials of commercial importance. Y2SiO5 doped with cerium (P47) was investigated as an alternative to the standard
*
Corresponding author. E-mail address: [email protected] (V.B. Bhatkar).
0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.04.006
P22 (ZnS:Ag) blue phosphor [7], currently used in television sets, for low-voltage field emission displays (FEDs). Saturation measurements indicated that the high-saturation resistance of the Ce-doped silicate can yield better performance than ZnS:Ag when operated at low-voltages (1–3 kV). Manganese doped zinc silicate is a well established, green emitting phosphor for use in plasma display panels [8]. Cerium activated oxyorthosilcates Lu2SiO5:Ce (LSO:Ce [9]), Y2SiO5:Ce (YSO:Ce) and Gd2SiO5:Ce, (GSO:Ce) have proved to be excellent scintillation phosphors because of their relatively high-density, good light output, and fast decay time [9]. Bi12SiO20 (BSO) single crystals have attracted increasing attention in optoelectronics as an efficient photorefractive material and also in applications such as spatial-time light modulators, holographic information storage, two- and four-wave mixing, and several other applications in the field of nonlinear optics [10]. Using shallow traps application optical fixing through two-step gated recording in a crystal of La3Ga5SiO14 doped with praseodymium has been demonstrated [11]. Recently, Shen and Kacharu [12] introduced a new process for parallel data storage using coherent time domain optical memory and proposed Y2SiO5:Eu for practical developments. Photostimulated luminescence in silicates can be used for developing X-ray storage phosphors. Utility of Ba5SiO4Br6:Eu2+ [13] and Y2SiO5:Ce, Sm [14] for such
V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
purpose has been demonstrated. The high-conversion efficiency and short response time make it a good alternative to BaFBr:Eu as a storage phosphor in computerised radiography. The long lasting phosphorescence is observed in Ce3+ doped Ca2Al2SiO7 at room temperature. Under UV-irradiation at 365 nm it produces broadband emission with peak at 417 nm, though the intensity is weak [15]. Rare-earth-containing silicate glasses have attracted great attention as potential materials for optical and magnetooptical devices such as an upconversion laser, a hole burning memory, and an optical switch [16], because they show a high-temperature persistent spectral hole burning (PSHB)
1067
and have a large potential for a high-density frequencydomain optical data memory [17]. Most of the silicates have high-melting points. Moreover, they can appear in crystalline as well as glassy form. Synthesis of silicates is rather tricky for these reasons. Conventionally, solid state diffusion methods have been used for the synthesis of silicates. During later years novel techniques like sol–gel synthesis have been used which enable the preparation of fine, homogenous powders [18]. Recently, we have reported combustion synthesis of Zn2SiO4:Mn [19]. The synthesis is based on the exothermic reaction between urea and ammonium nitrate. In this paper we report
Table 1 Details of the starting materials used in the combustion synthesis of various silicates Sr. no.
Name of the compound
Starting materials
1
Zn2SiO4:Mn2+ Mole ratio:
ZnO 1.92
SiO2 Æ xH2O 1
MnSO4 0.08
NH4NO3 20
Urea 20
2
BaSi2O5:Pb2+ Mole ratio:
BaCO3 0.99
SiO2 Æ xH2O 2
Pb(NO3)2 0.01
NH4NO3 25
Urea 25
3
BaSi2O5:Eu2+ Mole ratio:
BaCO3 0.98
SiO2 Æ xH2O 2
Eu2O3 0.01
NH4NO3 25
Urea 25
4
BaSi2O5:Ce3+ Mole ratio:
BaCO3 0.99
SiO2 Æ xH2O 2
CeCl3 0.01
NH4NO3 25
Urea 25
5
BaMgSiO4:Eu2+ Mole ratio:
BaCO3, Mg(NO3)2 0.98, 1
SiO2 Æ xH2O 1
Eu2O3 0.01
NH4NO3 15
Urea 20
6
CaMgSiO4:Eu2+ Mole ratio:
CaCO3, Mg(NO3)2 0.98, 1
SiO2 Æ xH2O 1
Eu2O3 0.01
NH4NO3 15
Urea 20
7
BaSrSiO4:Eu2+ Mole ratio:
BaCO3, SrCO3 1, 0.98
SiO2 Æ xH2O 1
Eu2O3 0.01
NH4NO3 20
Urea 20
8
Ca2MgSi2O7:Eu2+ Mole ratio:
CaCO3, Mg(NO3)2 0.98, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
9
Ca1.5Sr0.5MgSi2O7:Eu2+ Mole ratio:
CaCO3, SrCO3, Mg(NO3)2 1.48, 0.05, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
10
Ca0.5Sr1.5MgSi2O7:Eu2+ Mole ratio:
CaCO3, SrCO3, Mg(NO3)2 0.05, 1.48, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
11
Sr2MgSi2O7:Eu2+ Mole ratio:
SrCO3, Mg(NO3)2 1.98, 1
SiO2 Æ xH2O 2
Eu2O3 0.02
NH4NO3 30
Urea 35
12
Sr2SiO4:Pb2+ Mole ratio:
SrCO3 1.98
SiO2 Æ xH2O 1
Pb(NO3)2 0.02
NH4NO3 20
Urea 20
13
CaMgSiO4:Pb2+ Mole ratio:
CaCO3, Mg(NO3)2 0.99, 1
SiO2 Æ xH2O 1
Pb(NO3)2 0.01
NH4NO3 15
Urea 20
14
BaSrSiO4:Pb2+ Mole ratio:
BaCO3, SrCO3 1, 0.99
SiO2 Æ xH2O 1
Pb(NO3)2 0.01
NH4NO3 20
Urea 20
15
CaMgSiO4:Ce3+ Mole ratio:
CaCO3, Mg(NO3)2 0.99, 1
SiO2 Æ xH2O 1
CeCl3 0.01
NH4NO3 15
Urea 20
16
BaSrSiO4:Ce3+ Mole ratio:
BaCO3, SrCO3 1, 0.99
SiO2 Æ xH2O 1
CeCl3 0.01
NH4NO3 20
Urea 20
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V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
combustion synthesis of several silicate phosphors, which demonstrate that the combustion synthesis can serve as a fast, efficient method for preparing the silicate materials. 2. Experimental Phosphors were prepared by combustion synthesis. The detailed description of the methods can be found in the original works of Patil and co-workers [20,21]. Ingredients used were metal carbonates, silicic acid (SiO2 Æ xH2O), and the dopant salts. Urea was used as a fuel and ammonium nitrate as oxidizer. Fuel to oxidizer ratio, optimised as described in our earlier work [19], was used. All constituents in stoichiometric proportions (Table 1), along with fuel and oxidizer were mixed together and small quantity of double distilled water was added. The mixture on thoroughly grinding was transferred to a pre-heated furnace at 500 °C. On rapid heating the mixture evaporates and ignites at 450 °C to yield silicates. Photoluminescence spectra were recorded on Hitachi F-4000 spectro-fluorimeter with spectral slit width of 1.5 nm. 3. Results and discussion In the system Li2O:SiO2, Li2SiO3 and Li4SiO4 are the predominant phases. Glasses of various compositions are also possible. Zinc silicate also occurs in two forms, viz. Beta and Willemite. To confirm the structure of the synthesized phosphors, powder photographs were obtained using Philips diffractometer, PW 1710. The powder photograph for the lithium silicate matched with the JCPDS file 37-1472 corresponding to Li4SiO4. Similarly the powder photograph of Zn2SiO4 matched with JCPDS file 37-1485 corresponding to Willemite phase of Zn2SiO4. As an example, data for Li4SiO4 are given in Table 2. The XRD measurements thus confirm that the desired silicates could be synthesized in a single step by the combustion method. However, the as-prepared Zn2SiO4:Mn powder was slightly yellowish. This is probably due to presence of some Mn4+. The sample was annealed at 900 °C for 1 h in the reducing atmosphere provided by burning charcoal. Fig. 1 shows PL spectra for Zn2SiO4:Mn. The emission in Zn2SiO4:Mn (curve e), which corresponds to the transition 4T1(4G) ! 6A1(6S), has a maximum situated at 523 nm with half width of 37 nm, which agrees with the literature value [22]. The corresponding excitation spectrum (curve a) shows a maximum around 243 nm corresponding to CT transition of Mn2+ [23]. Intensity of the emission was compared with the green standard CeMgAl11O19:Tb phosphor and was found to be about 90% of the commercial phosphor. Fig. 1 also shows PL spectra for BaSi2O5:Pb phosphor. Emission in BaSi2O5:Pb (curve f) is in form of a broad band around 350 nm and a shoulder at longer wavelengths. The excitation maximum is around 245 nm (curve b). This agrees with the literature data. The spectra correspond to
Table 2 XRD data for Li4SiO4 Sr. no.
2h
37-1472
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
16.721 17.2 22.19 22.583 24.03 24.13 28.174 29.247 33.799 33.9 34.13 34.802 36.961 38.2 38.788 40.97 41.11 41.61 41.74 45.28 47.853 48.755 49.3 49.43 51.244 51.57 51.74 51.79 53.325 54.015 55.005 55.563 56.21 56.365 57.04 57.205 58.375 58.527 60.005 60.105 60.577 61.983
19 15 100 85 31 38 52 19 100 32 23 82 2 38 2 11 11 3 4 1 1 13 26 19 8 11 7 5 0.1 2 1 3 3 2 3 3 12 6 6 15 35 9
1
This work 32 21 97 94 56 63 14.4 100
52 4.2 22 7 10
7.3 19 14 5.5 1.5
8
28 5.8
S0 ! 1P1 transitions. PL spectra for Eu2+ and Ce3+ activators in BaSi2O5 host are also included in Fig. 1. Eu2+ emission is in form of a broad band around 500 nm (curve g). Ce3+ emission peaks at 390 nm (curve h). Poort et al. [24] have described interesting PL of Eu2+ in silicate hosts. The excitation can be achieved by long wavelength UV bordering onto visible region and the emission is in blue or green region of the spectrum. Such phosphors can be useful for photoluminescent liquid crystal display (PLLCD) applications [25]. Combustion synthesis of these materials was attempted. The as-prepared phosphors did not show emission well above the background level, probably the activator Eu is not incorporated in divalent form. The phosphors reheated in reducing atmosphere provided by the burning charcoal at 900 °C for 1 h showed intense Eu2+ emission. Results for the silicates containing SiO4 group are presented in Fig. 2.
V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
Fig. 1. PL spectra for Zn2SiO4:Mn and BaSi2O5:Pb phosphors. (a) Zn2SiO4:Mn excitation for 524 nm emission; (b) BaSi2O5:Pb excitation for 350 nm emission; (c) BaSi2O5:Eu excitation for 350 nm emission; (d) BaSi2O5:Ce excitation for 350 nm Emission (e) Zn2SiO4:Mn Emission for 254 nm excitation; (f) BaSi2O5:Pb Emission for 254 nm excitation; (g) BaSi2O5:Eu excitation for 350 nm emission; (h) BaSi2O5:Ce excitation for 350 nm emission.
2+
Fig. 2. PL spectra for Eu doped orthosilicate phosphors. (a) BaMgSiO4:Eu excitation for 500 nm emission; (b) CaMgSiO4:Eu excitation for 524 nm emission; (c) BaSrSiO4:Eu excitation for 512 nm emission; (d) BaMgSiO4:Eu emission for 385 nm excitation; (e) CaMgSiO4:Eu emission for 385 nm excitation; (f) BaSrSiO4:Eu emission for 385 nm excitation.
BaMgSiO4:Eu (curve d) exhibits a very broad emission around 500 nm. The broad excitation band peaks at 350 nm. There is considerable excitation intensity up to 400 nm. The excitation curve falls rapidly thereafter. For BaSrSiO4:Eu, the emission is bluish green peaking at 512 nm (curve f). The excitation spectrum (curve c) is located at longer wavelengths compared to BaMgSiO4:Eu. CaMgSiO4:Eu exhibits very interesting emission spectrum (curve e) consisting of blue (446 nm) and green (524 nm) bands. Again, the excitation spectrum lies in near UV and violet–blue region of the spectrum. Efficient excitation by 385 nm, which is required for PLLCD applications, was
1069
Fig. 3. PL spectra for Eu2+ doped disilicate phosphors. (a) Ca2MgSi2O7 excitation for 447 nm emission; (b) Ca1.5Sr0.5MgSi2O7 excitation for 448 nm emission; (c) Ca0.5Sr1.5MgSi2O7 excitation for 478 nm emission; (d) Sr2MgSi2O7 excitation for 460 nm emission; (e) Ca2MgSi2O7 emission for 385 nm excitation; (f) Ca1.5Sr0.5MgSi2O7 emission for 385 nm excitation; (g) Ca0.5Sr1.5MgSi2O7 emission for 385 nm excitation; (h) Sr2MgSi2O7 emission for 385 nm excitation.
Fig. 4. PL spectra for Pb2+ doped orthosilicate phosphors. (a) Sr2SiO4:Pb excitation for 400 nm emission; (b) CaMgSiO4:Pb excitation for 360 nm emission; (c) BaSrSiO4:Pb excitation for 400 nm emission; (d) Sr2SiO4:Pb emission for 254 nm excitation; (e) CaMgSiO4:Pb emission for 254 nm excitation; (f) BaSrSiO4:Pb emission for 254 nm excitation.
observed for some Eu2+ doped disilicates also (Fig. 3). Ca2MgSi2O7 (curve e), and Ca1.5Sr0.5MgSi2O7 (curve f) exhibit blue emission around 448 nm. Emission in Sr2MgSi2O7 (curve h) peaks around 460 nm. Emission in Ca0.5Sr1.5MgSi2O7 (curve g) peaks at still longer wavelengths. Excitation spectrum of all the samples (curves a–d) show considerable intensity at 385 nm. Sr2MgSi2O7: Eu appears to be best suited as a blue phosphor for PLLCD applications. Pb2+ exhibits very efficient emission in BaSi2O5 host. Luminescence of Pb2+ (1 mol%) in some alkaline earth silicates was therefore studied. The results are presented in Fig. 4. CaMgSiO4:Pb (curve e) exhibits UV emission with
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V.B. Bhatkar et al. / Optical Materials 29 (2007) 1066–1070
Acknowledgements One of us (VBB) received fellowship from University Grants Commission, New Delhi, India. The financial assistance is gratefully acknowledged. References
Fig. 5. PL spectra for Ce3+ doped orthosilicate phosphors. (a) CaMgSiO4:Ce excitation for 385 nm emission; (b) BaSrSiO4:Ce excitation for 400 nm emission; (c) CaMgSiO4:Ce emission for 330 nm excitation; (d) BaSrSiO4:Ce emission for 340 nm excitation.
a maximum around 360 nm. Emission in BaSrSiO4:Pb and Sr2SiO4:Pb (curves d and f) was at longer wavelengths. Excitation spectra (curves a–c) show reasonable overlap with 253.7 nm Hg emission. However, the emission intensities are poor. This is most probably due to thermal quenching of the luminescence. Poort et al. [24] observed such quenching for Eu2+ emission in these lattices. As seen from Figs. 2 and 3, Eu2+ excitation in silicate hosts are situated at longer wavelengths. It was thought that f–d transitions of other rare earth impurities also might show this trend. PL spectra of some Ce3+ activated orthosilicates are presented in Fig. 5. As expected, the excitation bands (curve a and b) are situated at wavelengths longer than 320 nm, whereas in most materials they appear around 265 nm [26]. The emission bands are around 385 nm for CaMgSiO4 (curve c) and 400 nm for BaSrSiO4 (curve d). 4. Conclusions Several types of silicates are successfully prepared using the combustion synthesis. The well known phosphors Zn2SiO4:Mn and BaSi2O5:Pb could be reproduced using this synthesis. Characteristic emission of activators such as Ce3+, Eu2+ and Pb2+ could be observed in the silicates prepared by this method. The combustion synthesis thus furnishes a fast and simple method for preparing silicate phosphors.
[1] H. van Bekkum, E.M. Flanigen, J.C. Jansen, Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, 1991. [2] G. Piazza, F. Scaffidi-Argentina, H. Werle, J. Nucl. Mat. 283–287 (2000) 1396. [3] T. Hashizume, Y. Kato, T. Nakajima, T. Tovya, H. Sakamato, N. Kotera, S. Iguchi, in: Proc. Symp. Adv. Rad. Dete., Vienna, IAEA, 1971, p.91. [4] R.C. Ropp, Luminescence and the Solid State, Elsevier, Amsterdam, 1991. [5] G.R. Fonda, J. Phys. Chem. 43 (1939) 561. [6] H.H. Haft, W.A. Thornton, JIES (Oct) (1972). [7] C. Stoffers, R.Y. Lee, J. Penczek, B.K. Wagner, C.J. Summers, Appl. Phys. Lett. 76 (8) (2000) 949. [8] K.S. Sohn, B. Cho, H. Chang, H.D. Park, Y.G. Choi, K.H. Kim, J. Europ. Ceramic. Soc. 20 (2000) 1043. [9] D.W. Cooke, B.L. Bennett, R.E. Muenchausen, K.J. McClellan, J.M. Roper, M.T. Whittaker, J. Appl. Phys. 86 (1999) 5308; D. Cooke, K.J. WmcClellan, B.L. Bennett, J.M. Roper, M.T. Whittaker, R.E. Muenchausen, R.C. Sze, J. Appl. Phys. 88 (2000) 7360. [10] V. Arinova, M. Veleva, D. Petrova, I.M. Kourmoulis, D.G. Papazoglou, A.G. Apostolidis, E.D. Vanidhis, N.C. Deliolanis, J. Appl. Phys. 89 (2001) 2686. [11] T. Nikolajsen, P.M. Johansen, J. Opt. A: Pure Appl. Opt. 2 (2000) 255. [12] X.A. Shen, R. Kachru, J. Alloy. Comp. 250 (1997) 435. [13] A. Meijerink, G. Blasse, L. Struye, Maters. Chem. Phys. 21 (1989) 261. [14] A. Meijerink, W.J. Scipper, G. Blasse, J. Phys. D.: Appl. Phys. 24 (1991) 997. [15] N. Kodama, T. Takahashi, M. Yamaga, Y. Tanii, J. Qiu, K. Hirao, Appl. Phys. Lett. 75 (1999) 1715. [16] K. Fujita, K. Hirao, K. Tanaka, N. Soga, H. Sasaki, J. Appl. Phys. 82 (1997) 5114. [17] Y. Mao, P. Gavrilovic, P. Singh, S.A. Bruce, W.H. Grodkiewicz, Appl. Phys. Lett. 68 (1996) 3677. [18] T.S. Ahmadi, M. Haase, H. Weller, Mat. Res. Bull. 35 (2000) 1869. [19] V.B. Bhatkar, S.K. Omanwar, S.V. Moharil, phys. Stat. Solidi (a) 191 (2002) 272. [20] S. Ekambaram, K.C. Patil, J. Alloy. Comp. 217 (1995) 104. [21] J.J. Kingsley, K. Suresh, K.C. Patil, J. Mat. Sci. 25 (1990) 1305. [22] C. Barthou, J. Benoit, P. Benalloui, A. Morell, J. Electrochem. Soc. 141 (1994) 524. [23] E. van der Kolk, P. Dorenbos, C.W.E. van Eijk, H. Bechtel, T. Justl, H. Nikol, C.R. Ronda, D.U. Wiechert, J. Lum. 87–89 (2000) 1246. [24] S.H.M. Poort, H.M. Reijnhoudt, H.O.T. van der Kuip, G. Blasse, J. Alloy. Comp. 241 (1996) 75. [25] A. Vecht, A.C. Newport, P.A. Bayley, W.A. Crossland, J. Appl. Phys. 84 (1998) 3827. [26] P. Dorenbos, Phys. Rev. B 64 (2001) 125117.