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Luminescent solar concentrators using uranyl-doped silicate glasses
Solar Energy Materials 10 (1984) 303-307 North-Holland, Amsterdam
303
LUMINESCENT SOLAR CONCENTRATORS USING U R A N Y L - D O P E D SILICATE GLASSES G. F O L C H E R , N. KELLER, J. PARIS Dbpartement de Physico-chimie, LRMCI (LA 331), CEN-Saclay, 91191 Gif sur Yoette Cedex, France
Received in revised form 6 April 1984 We describe solar concentrators containing uranyl. A fluorescence quantum yield of 0.7 is measured at low uranyl concentration (less than 1%) and decreases when the concentration increases. This variation is correlated with the lifetime of the excited state and is due to uranium-uranium interactions. These quantitative determinations can be used to optimize the thickness of the concentrator sheet.
I. Introduction Luminescent solar concentrators containing inorganic fluorescent compounds have been studied with the main objective of obtaining very stable transparent sheets. In order to attain a suitable efficiency for the concentrator, the emission quantum yield must be as high as possible and the thickness of the plate should be minimized. From these points of view, utilization of uranyl ion as the luminescent species and glass as the matrix material seems quite attractive, as emphasized by Reisfeld and Jorgensen [1]. Though very few values have been reported [2], uranyl is known to give high fluorescence quantum yields, and doped glasses generally show good stability at moderate temperatures. The absorption coefficient of uranyl is not high but leads nevertheless to a reasonable thickness for the concentrators. However, the overlap of the solar emission spectrum and uranyl absorption is not broad. This disadvantage can be largely avoided by using hybrid solar concentrators as suggested by Reisfeld et al. [1] in which a uranyl plate is associated with a plastic material containing an appropriate dye. In the case of silicon cells, uranyl has the advantage of shifting the cell absorption spectrum from an unfavorable region of the solar spectrum to a much more efficient energy. Another application of the solar concentrator is in the use of solar light to accelerate the growth of various algae which need a cold light at the fight wavelength. The lack of quantitative data on these materials prompted us to make some measurements on a cheap uranyl-doped glass. We were interested in the effect of uranium concentration on various optical properties, particularly the fluorescence yield. Taking into account the difficulties of accurate yield determination, we also measured the lifetime of the excited state and showed that an approximate value of the fluorescence yield can be deduced from these results. 0165-1633/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
304
2.
G. Folcher et al.
/
L u m i n e s c e n t solar concentrators
Preparation
We have used the most common type of glass: Na, Ca silicate. The compounds (typically SiO 2 68%, CaCO 3 27% UO 3 1%) were powdered and mixed with a small amount of water. The mixture was dried in an aluminium crucible and slowly heated in a furnace to 1250°C. This temperature was maintained for 12 h before the furnace was cooled to 600 o C. The glass was then annealed at 700 °C and allowed to cool to room temperature in 24 h. The uranium concentration can be varied from 0.1 to 5% without any effect on the glass. The pieces of glass were cut and polished to obtain samples suitable for optical measurements: small plates of 1 2 cm 2 and about 2-5 mm thick.
3. Optical properties Fig. 1 shows the absorption and emission spectra of glass at room temperature. Both are typical of UO22+ ion in a low symmetry environment. We did not detect any uranium (IV) in the glass. From 10 -2 to 5 × 10 -1 i o n / d m 3, all the samples give the same spectrum. Thus, we infer that the uranyl ion is in the same site in the matrices. The absorption is roughly proportional to the concentration of uranium used to make the glass when this concentration is lower than 2%. The extinction coefficient is (22 + 2) mol-ldm3cm-1, which is not very different from the values of 20.6 found in phosphate glass [4]. The quantum yield results are displayed in fig. 2. The quantum yield measurements were made using a comparative method in which an optical cell of the same geometric configuration as the uranyl glasses was filled with a fluoresceine solution with a previously established quantum yield Q0.
/1
....
absorption emission
/i? "4. I
300
I
i
/-00
I
i
500
I
600
nm
Fig. 1. Absorption and emission spectra.
G. Folcher et al. / Luminescent solar concentrators
305
I00%
75
50
25
[
L
L
-3
-2
-1
log ( UO 2 )
Fig. 2. Quantum yield.
The solution was then irradiated under the same conditions as those for the uranyl glasses. Thus, the uranyl glass fluorescence quantum yield is Q = QoI/Io,
where I and I 0 are the fluorescence intensities of the uranyl glass and the fluoresceine solution, respectively. Many problems arise when an accurate determination is attempted. The most important one concerns the use of fluoresceine solutions with absorbance greater than 0.5. The reduction of the quantum yield at [U] > 0.5% is only qualitatively reliable. On the other hand, at low uranium concentration the poor optical quality of the glass leads to underestimation of the measured yield. A correction can be made in this case. An appropriate evaluation of the quantum yield can be made when the uranium concentration is lower than 0.5%: 0.66 + 0.05. In order to measure the effects of the concentration on the deactivation of the excited uranyl ion, luminescent lifetime determinations were undertaken. A nitrogen laser was used either directly, hex = 337 nm, or to pump a dye (coumarin 120), hex = 420 nm. The pulses of 2 ns were focused in a cell and the emitted light was analysed at 90 ° to the excitation beam direction with a photomulplier tube equipped with a green filter centered at 520 nm. The lifetime ¢ of the excited state does not depend upon the excitation wavelength and can be measured without considering the geometry of the sample. The decay can always be fitted by a single exponential. The results are presented in fig. 3. ~']/2 is constant for dilute glasses between 0.1 and 0.5% and decreases at concentrations higher than 1%. The 380 ~s half-lifetime for dilute samples compares well with the value of 367 ~s found for phosphate glasses [4], while in aqueous solution the quenching processes shorten this time to 2 ~s [6]. In solids, the decrease in the lifetime of the excited state (also found in phosphate glass [4] in the same range of concentration), is generally explained by some kind of non-radiative energy transfer, such as dipole-dipole interaction [7]. The critical distance corresponding to the
306
G. l"blcher eta[. / Luminescent solar concentrators
T~s
z,O0
300
÷
\..
200
100
-3
-2
-1
Iog(UO 2)
Fig. 3. Excited state life time.
occurrence of the observed deactivation, evaluated from the concentration limit of 2%, is 2 × 10 - 6 c m . This value corresponds to an appreciable dipole-dipole interaction between two uranium atoms. The ratio ~ = ~'/%, where T0 is the lifetime without uranium-uranium interaction, can be considered as an estimate of the quantum yield. In this approximation, the comparison between the values of ~ at low concentration gives a rough value of the quantum yield = 0.5 at 1% and 0.3 at 5%. Taking into account the absorption coefficient of UO22+ at the wavelength employed, we can try to estimate an optimized size for the plate. For instance, if it is acceptable to lose 10% of the incident light, a 1 cm thickness of 0.5% doped glass seems to be a suitable set of parameters. In conclusion, these preliminary investigations of solar concentrators using uranyl as a mineral dye show that an inexpensive glass preparation is very easy and that quantum yields of 0.6-0.7 can be obtained with low concentration of uranium. Improvements in this yield could be obtained by varying the nature of the glass. An improvement in the absorption coefficient can also be expected, which will allow the thickness of the plates to be reduced. Acknowledgement We wish to thank the "Laboratoire de Pbotochimie Solaire, CNRS" for fluorescence measurements. References [1] R. Reisfeld, C.K. Jorgensen, Struct. Bonding Solar Energy Mater. 49 (1982) 1. [2] C.K. Jorgensen, J. Luminescence 18 (1979) 63.
G. Folcher et al. / Luminescent solar concentrators
307
[3] S.G. Schulman, Fluorescence and Phosphorescence Spectroscopy: Physical-chemical Principles and Practice (Pergamon Press, London, New York, 1977). [4] N. Lieblich-Sofer, R. Reisfeld and C.K. Jorgensen, lnorganica Chim. Acta 30 (1978) 259. [5] L.S. Forster and D. Dudley, J. Phys. Chem. 66 (1962) 838. [61 A. Cox, T. Kemp, U.J. Reed and O. Traverso, J. Chem. Soc. Faraday Trans. 76 (1980) 804. [7] M. lnokuty and F. Hirayama, J. Chem. Phys. 43 (1965) 1978.
303
LUMINESCENT SOLAR CONCENTRATORS USING U R A N Y L - D O P E D SILICATE GLASSES G. F O L C H E R , N. KELLER, J. PARIS Dbpartement de Physico-chimie, LRMCI (LA 331), CEN-Saclay, 91191 Gif sur Yoette Cedex, France
Received in revised form 6 April 1984 We describe solar concentrators containing uranyl. A fluorescence quantum yield of 0.7 is measured at low uranyl concentration (less than 1%) and decreases when the concentration increases. This variation is correlated with the lifetime of the excited state and is due to uranium-uranium interactions. These quantitative determinations can be used to optimize the thickness of the concentrator sheet.
I. Introduction Luminescent solar concentrators containing inorganic fluorescent compounds have been studied with the main objective of obtaining very stable transparent sheets. In order to attain a suitable efficiency for the concentrator, the emission quantum yield must be as high as possible and the thickness of the plate should be minimized. From these points of view, utilization of uranyl ion as the luminescent species and glass as the matrix material seems quite attractive, as emphasized by Reisfeld and Jorgensen [1]. Though very few values have been reported [2], uranyl is known to give high fluorescence quantum yields, and doped glasses generally show good stability at moderate temperatures. The absorption coefficient of uranyl is not high but leads nevertheless to a reasonable thickness for the concentrators. However, the overlap of the solar emission spectrum and uranyl absorption is not broad. This disadvantage can be largely avoided by using hybrid solar concentrators as suggested by Reisfeld et al. [1] in which a uranyl plate is associated with a plastic material containing an appropriate dye. In the case of silicon cells, uranyl has the advantage of shifting the cell absorption spectrum from an unfavorable region of the solar spectrum to a much more efficient energy. Another application of the solar concentrator is in the use of solar light to accelerate the growth of various algae which need a cold light at the fight wavelength. The lack of quantitative data on these materials prompted us to make some measurements on a cheap uranyl-doped glass. We were interested in the effect of uranium concentration on various optical properties, particularly the fluorescence yield. Taking into account the difficulties of accurate yield determination, we also measured the lifetime of the excited state and showed that an approximate value of the fluorescence yield can be deduced from these results. 0165-1633/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
304
2.
G. Folcher et al.
/
L u m i n e s c e n t solar concentrators
Preparation
We have used the most common type of glass: Na, Ca silicate. The compounds (typically SiO 2 68%, CaCO 3 27% UO 3 1%) were powdered and mixed with a small amount of water. The mixture was dried in an aluminium crucible and slowly heated in a furnace to 1250°C. This temperature was maintained for 12 h before the furnace was cooled to 600 o C. The glass was then annealed at 700 °C and allowed to cool to room temperature in 24 h. The uranium concentration can be varied from 0.1 to 5% without any effect on the glass. The pieces of glass were cut and polished to obtain samples suitable for optical measurements: small plates of 1 2 cm 2 and about 2-5 mm thick.
3. Optical properties Fig. 1 shows the absorption and emission spectra of glass at room temperature. Both are typical of UO22+ ion in a low symmetry environment. We did not detect any uranium (IV) in the glass. From 10 -2 to 5 × 10 -1 i o n / d m 3, all the samples give the same spectrum. Thus, we infer that the uranyl ion is in the same site in the matrices. The absorption is roughly proportional to the concentration of uranium used to make the glass when this concentration is lower than 2%. The extinction coefficient is (22 + 2) mol-ldm3cm-1, which is not very different from the values of 20.6 found in phosphate glass [4]. The quantum yield results are displayed in fig. 2. The quantum yield measurements were made using a comparative method in which an optical cell of the same geometric configuration as the uranyl glasses was filled with a fluoresceine solution with a previously established quantum yield Q0.
/1
....
absorption emission
/i? "4. I
300
I
i
/-00
I
i
500
I
600
nm
Fig. 1. Absorption and emission spectra.
G. Folcher et al. / Luminescent solar concentrators
305
I00%
75
50
25
[
L
L
-3
-2
-1
log ( UO 2 )
Fig. 2. Quantum yield.
The solution was then irradiated under the same conditions as those for the uranyl glasses. Thus, the uranyl glass fluorescence quantum yield is Q = QoI/Io,
where I and I 0 are the fluorescence intensities of the uranyl glass and the fluoresceine solution, respectively. Many problems arise when an accurate determination is attempted. The most important one concerns the use of fluoresceine solutions with absorbance greater than 0.5. The reduction of the quantum yield at [U] > 0.5% is only qualitatively reliable. On the other hand, at low uranium concentration the poor optical quality of the glass leads to underestimation of the measured yield. A correction can be made in this case. An appropriate evaluation of the quantum yield can be made when the uranium concentration is lower than 0.5%: 0.66 + 0.05. In order to measure the effects of the concentration on the deactivation of the excited uranyl ion, luminescent lifetime determinations were undertaken. A nitrogen laser was used either directly, hex = 337 nm, or to pump a dye (coumarin 120), hex = 420 nm. The pulses of 2 ns were focused in a cell and the emitted light was analysed at 90 ° to the excitation beam direction with a photomulplier tube equipped with a green filter centered at 520 nm. The lifetime ¢ of the excited state does not depend upon the excitation wavelength and can be measured without considering the geometry of the sample. The decay can always be fitted by a single exponential. The results are presented in fig. 3. ~']/2 is constant for dilute glasses between 0.1 and 0.5% and decreases at concentrations higher than 1%. The 380 ~s half-lifetime for dilute samples compares well with the value of 367 ~s found for phosphate glasses [4], while in aqueous solution the quenching processes shorten this time to 2 ~s [6]. In solids, the decrease in the lifetime of the excited state (also found in phosphate glass [4] in the same range of concentration), is generally explained by some kind of non-radiative energy transfer, such as dipole-dipole interaction [7]. The critical distance corresponding to the
306
G. l"blcher eta[. / Luminescent solar concentrators
T~s
z,O0
300
÷
\..
200
100
-3
-2
-1
Iog(UO 2)
Fig. 3. Excited state life time.
occurrence of the observed deactivation, evaluated from the concentration limit of 2%, is 2 × 10 - 6 c m . This value corresponds to an appreciable dipole-dipole interaction between two uranium atoms. The ratio ~ = ~'/%, where T0 is the lifetime without uranium-uranium interaction, can be considered as an estimate of the quantum yield. In this approximation, the comparison between the values of ~ at low concentration gives a rough value of the quantum yield = 0.5 at 1% and 0.3 at 5%. Taking into account the absorption coefficient of UO22+ at the wavelength employed, we can try to estimate an optimized size for the plate. For instance, if it is acceptable to lose 10% of the incident light, a 1 cm thickness of 0.5% doped glass seems to be a suitable set of parameters. In conclusion, these preliminary investigations of solar concentrators using uranyl as a mineral dye show that an inexpensive glass preparation is very easy and that quantum yields of 0.6-0.7 can be obtained with low concentration of uranium. Improvements in this yield could be obtained by varying the nature of the glass. An improvement in the absorption coefficient can also be expected, which will allow the thickness of the plates to be reduced. Acknowledgement We wish to thank the "Laboratoire de Pbotochimie Solaire, CNRS" for fluorescence measurements. References [1] R. Reisfeld, C.K. Jorgensen, Struct. Bonding Solar Energy Mater. 49 (1982) 1. [2] C.K. Jorgensen, J. Luminescence 18 (1979) 63.
G. Folcher et al. / Luminescent solar concentrators
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[3] S.G. Schulman, Fluorescence and Phosphorescence Spectroscopy: Physical-chemical Principles and Practice (Pergamon Press, London, New York, 1977). [4] N. Lieblich-Sofer, R. Reisfeld and C.K. Jorgensen, lnorganica Chim. Acta 30 (1978) 259. [5] L.S. Forster and D. Dudley, J. Phys. Chem. 66 (1962) 838. [61 A. Cox, T. Kemp, U.J. Reed and O. Traverso, J. Chem. Soc. Faraday Trans. 76 (1980) 804. [7] M. lnokuty and F. Hirayama, J. Chem. Phys. 43 (1965) 1978.