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Effect of grain size on TL yield of polycrystalline BaS: Bi phosphors
Solid State Communications, Vol. 30, pp. 3 15—317. Pergamon Press Ltd. 1979. Printed in Great Britain. EFFECT OF GRAIN SIZE ON IL YIELD OF POLYCRYSTALUNE BaS: Bi PHOSPHORS R.P. Rao and D.R. Rao* Materials Science Centre, Indian Institute ofTechnology, Kharagpur 721302, India (Received 30 November 1978 by A.R. Verina) Effect of grain size on thermoluminescence (TL) of BaS: Bi (0.36% by wt.) phosphors was studied by X-irradiation at RT (30°C).It is observed that the the intensity of phosphorescence and IL output decrease with the decrease of the grain size of the powders, which has been attributed to the loss of incident excitation energy due to scattering.
IT IS KNOWN that luminescence efficiency of phosphors depending upon the temperature and type of excitation can be improved by taking proper care of the energy losses in the material. Apart from various factors like surface damage, non-radiative recombination centres, energy storage properties of the material and ambient atmospheric conditions during the process of excitation, the loss of excitation energy in the case of powdered samples is mainly accounted for due to scattering of incident radiation by the particles of the phosphors. Thus, while considering the industrial applications of polycrystalline phosphors in fluorescent lamps, screens, luminescent paints, etc., the effect of grain size on the light output has to be specially studied and estimated. Several reports are available in the literature [1—3] on sulphide phosphors which indicated, in general, a decrease of brightness and luminescence yield with the decrease of grain size; the diminution in the fluorescence intensity was attributed to the loss of excitation energy due to the various factors as mentioned above. Information on a similar study of the effect of grain size on thermoluminescence (TL) of phosphors is very meagre. A study of this nature helps a general understanding of the trapping phenomenon which enables one to estimate how much the material, depending upon the grain size and process of excitation retains the excitation energy. In this direction the energy storage properties of BaS : Bi phosphors are studied by X-irradiation at RT and the results on the TL output obtained for different grain sizes of the powders are presented in this paper. Phosphors were prepared by reducing pure BaSO 4 and the details of material preparation were described in our earlier papers [4, 5] The material after firing around 1000°C is cooled to RT and was ground in a
pulverizer. The powder obtained in this way was sieved using sieves of different mesh numbers and the grain sizes indicated in our results are thus the average values. In all the cases, X-irradiation (30 kV, 10 mA, 10 mm) was done at RT by packing the powders in a brass well and the TL glow curves were recorded at a constant rate of heating (28 ±1°Cmint). The experimental details of the setup to record the TL glow curves was given elsewhere [6]. Results on the long period afterglow decay and TL of BaS : Bi phosphors containing different concentrations of Bi have been reported in our recent paper [5]. In Fig. I results on TL glow curves of a typical sample of BaS : Bi (0.36% by wt.) where all the three TL glow peaks are well defined, of different grain sizes are presented. It is observed that the TL output decreases with the decrease of grain size and the corresponding effect of grain size on the glow peak at 200°Cwhich was related with Bi impurity [51is also shown as an inset in Fig. 1. The effect of irradiation dose, X-ray and heat cycles have also been studied for samples of different grain sizes and containing different concentrations of Bi. All the samples, independent of grain size, exhibited the same number of TL glow maxima showing similar behaviour as with the increase of irradiation-time, the TL output increases and attains a saturation value in about 30 mm of irradiation at RT. The TL glow curves of one such sample obtained for different times of X-ray exposure are shown in Fig. 2. The growth curve of the glow peak at 200°Cshown as an inset in this Fig. 2 clearly indicates the saturation
.
behaviour after 30 mm of irradiation. The results of this investigation suggest that the storage of excitation energy becomes less as the grain size of the powders decreases gradually and thereby the TL yield also diminishes. It may be mentioned here that the material used in this study was pulverised once and the particles were separated using sieves of different mesh numbers. Further, each sample was studied four to
___________
*
Presently at Department of Materials Science, University of Southern California, Los Angeles, CA 90007, U.S.A. 315
316
IL YIELD OF POLYCRYSTALUNE BaS Bi PHOSPHORS
Vol. 30, No.6
/~—~\ /Z N\
I4
I — 45 MICRONS 2—75 “ 3106 “ 4—212&300”
2 12 U) I.7
~ 10
100 200 300 AVERAGE GRAIN SIZE (MICRoNS) I
I 2
I 3
I 4
I I 5 6 7 TIME (MINUTES)
I
B
I 9
I 10
II
12
Fig. 1. IL glow curves of X-irradiated BaS: Bi (0.36% by wt.) phosphors of different grain sizes. 1-2 14
12
MINUTES
2— 5 3- 10 4-20 5-30 6—60 7-120
-
“ “ “ “ “ “
U)
I—
~I0-
7 3
65
2
—B-
4
—I
IU) 7
~
6_lI.
I—
30 I
2
3
4
60 TIME
90 5
120
6 7 TIME (MINUTES)
B
9
10
II
12
Fig. 2. IL glow curves of BaS : Bi (0.36% by wt.) powder (75 ,.zm grain size) irradiated by X-rays (30 kV, 10 mA) for different times at RI.
Vol. 30, No.6
IL YIELD OF POLYCRYSTALUNE BaS: Bi PHOSPHORS
317
five times to check reproducibility. The powders treated effects, may be attributed to scattering as one of the in this way are observed to exhibit the same number of main sources of the loss of excitation energy. IL peaks and the area under the glow curve in each case remained almost the same. Our results on the Acknowledgements One of the authors (RPR) is intensity and decay of phosphorescence obtained for indebted to CSIR India for the award of a fellowship in this scheme. different grain sizes of BaS: Bi phosphors [5] also indicated similar features of IL as mentioned above. REFERENCES With the decrease of the grain size an increase of loss in the available excitation energy occurs and thus less I. V.L. Levshin & B.D. Ryzhikov, Opt. Spectrosc. volume of the material gets affected during irradiation. 10, 259 (1961). This may be understood considering the phenomenon 2. Gy. Gergely, J. Phys. Chem. Solids 24, 681 (1963). 3. S. Kuboniwa, I. Hoshina, I. Narahara & of scattering by which a major fraction of the incident M. Kanamaru, J. Electrochem. Soc. 120, 1734 radiation is lost due to scattering by smaller grains thus (1973). making available less excitation energy to the bulk of the 4. R.P. Rao, D.R. Rao & H.D. Banerjee, Mater. Res. material. So in a general way it may be concluded that BulL 13,491(1978). the decrease in the intensity of emission (fluorescence, 5. R.P. Rao & D.R. Rao (to be published). H.D. Banerjee & D.R. Rao,J. Mater. Sc!. 11,2333 phosphorescence and IL) of BaS: Bi phosphors with the 6. (1976). decrease of grain size, apart from the other surface —
IT IS KNOWN that luminescence efficiency of phosphors depending upon the temperature and type of excitation can be improved by taking proper care of the energy losses in the material. Apart from various factors like surface damage, non-radiative recombination centres, energy storage properties of the material and ambient atmospheric conditions during the process of excitation, the loss of excitation energy in the case of powdered samples is mainly accounted for due to scattering of incident radiation by the particles of the phosphors. Thus, while considering the industrial applications of polycrystalline phosphors in fluorescent lamps, screens, luminescent paints, etc., the effect of grain size on the light output has to be specially studied and estimated. Several reports are available in the literature [1—3] on sulphide phosphors which indicated, in general, a decrease of brightness and luminescence yield with the decrease of grain size; the diminution in the fluorescence intensity was attributed to the loss of excitation energy due to the various factors as mentioned above. Information on a similar study of the effect of grain size on thermoluminescence (TL) of phosphors is very meagre. A study of this nature helps a general understanding of the trapping phenomenon which enables one to estimate how much the material, depending upon the grain size and process of excitation retains the excitation energy. In this direction the energy storage properties of BaS : Bi phosphors are studied by X-irradiation at RT and the results on the TL output obtained for different grain sizes of the powders are presented in this paper. Phosphors were prepared by reducing pure BaSO 4 and the details of material preparation were described in our earlier papers [4, 5] The material after firing around 1000°C is cooled to RT and was ground in a
pulverizer. The powder obtained in this way was sieved using sieves of different mesh numbers and the grain sizes indicated in our results are thus the average values. In all the cases, X-irradiation (30 kV, 10 mA, 10 mm) was done at RT by packing the powders in a brass well and the TL glow curves were recorded at a constant rate of heating (28 ±1°Cmint). The experimental details of the setup to record the TL glow curves was given elsewhere [6]. Results on the long period afterglow decay and TL of BaS : Bi phosphors containing different concentrations of Bi have been reported in our recent paper [5]. In Fig. I results on TL glow curves of a typical sample of BaS : Bi (0.36% by wt.) where all the three TL glow peaks are well defined, of different grain sizes are presented. It is observed that the TL output decreases with the decrease of grain size and the corresponding effect of grain size on the glow peak at 200°Cwhich was related with Bi impurity [51is also shown as an inset in Fig. 1. The effect of irradiation dose, X-ray and heat cycles have also been studied for samples of different grain sizes and containing different concentrations of Bi. All the samples, independent of grain size, exhibited the same number of TL glow maxima showing similar behaviour as with the increase of irradiation-time, the TL output increases and attains a saturation value in about 30 mm of irradiation at RT. The TL glow curves of one such sample obtained for different times of X-ray exposure are shown in Fig. 2. The growth curve of the glow peak at 200°Cshown as an inset in this Fig. 2 clearly indicates the saturation
.
behaviour after 30 mm of irradiation. The results of this investigation suggest that the storage of excitation energy becomes less as the grain size of the powders decreases gradually and thereby the TL yield also diminishes. It may be mentioned here that the material used in this study was pulverised once and the particles were separated using sieves of different mesh numbers. Further, each sample was studied four to
___________
*
Presently at Department of Materials Science, University of Southern California, Los Angeles, CA 90007, U.S.A. 315
316
IL YIELD OF POLYCRYSTALUNE BaS Bi PHOSPHORS
Vol. 30, No.6
/~—~\ /Z N\
I4
I — 45 MICRONS 2—75 “ 3106 “ 4—212&300”
2 12 U) I.7
~ 10
100 200 300 AVERAGE GRAIN SIZE (MICRoNS) I
I 2
I 3
I 4
I I 5 6 7 TIME (MINUTES)
I
B
I 9
I 10
II
12
Fig. 1. IL glow curves of X-irradiated BaS: Bi (0.36% by wt.) phosphors of different grain sizes. 1-2 14
12
MINUTES
2— 5 3- 10 4-20 5-30 6—60 7-120
-
“ “ “ “ “ “
U)
I—
~I0-
7 3
65
2
—B-
4
—I
IU) 7
~
6_lI.
I—
30 I
2
3
4
60 TIME
90 5
120
6 7 TIME (MINUTES)
B
9
10
II
12
Fig. 2. IL glow curves of BaS : Bi (0.36% by wt.) powder (75 ,.zm grain size) irradiated by X-rays (30 kV, 10 mA) for different times at RI.
Vol. 30, No.6
IL YIELD OF POLYCRYSTALUNE BaS: Bi PHOSPHORS
317
five times to check reproducibility. The powders treated effects, may be attributed to scattering as one of the in this way are observed to exhibit the same number of main sources of the loss of excitation energy. IL peaks and the area under the glow curve in each case remained almost the same. Our results on the Acknowledgements One of the authors (RPR) is intensity and decay of phosphorescence obtained for indebted to CSIR India for the award of a fellowship in this scheme. different grain sizes of BaS: Bi phosphors [5] also indicated similar features of IL as mentioned above. REFERENCES With the decrease of the grain size an increase of loss in the available excitation energy occurs and thus less I. V.L. Levshin & B.D. Ryzhikov, Opt. Spectrosc. volume of the material gets affected during irradiation. 10, 259 (1961). This may be understood considering the phenomenon 2. Gy. Gergely, J. Phys. Chem. Solids 24, 681 (1963). 3. S. Kuboniwa, I. Hoshina, I. Narahara & of scattering by which a major fraction of the incident M. Kanamaru, J. Electrochem. Soc. 120, 1734 radiation is lost due to scattering by smaller grains thus (1973). making available less excitation energy to the bulk of the 4. R.P. Rao, D.R. Rao & H.D. Banerjee, Mater. Res. material. So in a general way it may be concluded that BulL 13,491(1978). the decrease in the intensity of emission (fluorescence, 5. R.P. Rao & D.R. Rao (to be published). H.D. Banerjee & D.R. Rao,J. Mater. Sc!. 11,2333 phosphorescence and IL) of BaS: Bi phosphors with the 6. (1976). decrease of grain size, apart from the other surface —