Particle Size Effects in YAG:CR Phosphors
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The Cr content was 0.5 mol% based on Al concentration. The mixture was stirred before being added to a bath of aqueous ammonia. As the nitrates were being added to the ammonia bath, the pH was continuously monitored and adjusted to 9-10 by adding ammonia. After all of the nitrates were added to the ammonia bath, the resulting cloudy solution was stirred for 1-3 hours to ensure that the solution had fully reacted. The precipitates were rinsed with isopropyl alcohol in a centrifuge and then either filtered in a buchner funnel or heated in a rotating evaporator to remove the excess isopropyl alcohol. The gelatinous precipitates were dried overnight at 120'C, crushed with a mortar and pestle and subsequently heat treated at temperatures ranging from 100016000C. Laser spectroscopy was employed to determine the relative light output and spectral energy distribution of the phosphors. Independent particle size measurements were performed by BET analysis, X-Ray line broadening calculations and SEM observations. X-Ray diffraction was also used to calculate lattice parameter changes as a function of annealing temperature. FTIR spectroscopy was performed in the range 500-3800 cm- 1. RESULTS AND DISCUSSION Figure 1 shows the photoluminescence (PL) spectra for the YAG:Cr powders annealed between 1000*C-1600"C for 2 hours. As shown, the light intensity increases with increasing annealing temperature. The shape of the spectral energy distribution, however, remains the same indicating that the only luminescent specie present is YAG:Cr. It has been previously demonstrated that if a slight excess of A1203 is present an A120 3:Cr peak appears [8]. Similarly, if a slight excess of Y2 0 3 is present, a YA10 3 :Cr peak will appear.
80 YAG:Cr 3:5
YAG.Cr
S60 Z uJ Z
I...
140YC
40
LU
112 _u cc 20
0 600 Figure 1
650
700 750 WAVELENGTH (nm)
800
Photoluminescence spectra of the YAG:Cr powders after annealing for 2 hours between 1200C to 1600C.
For the Cr3 ÷ ion to be activated, it must be in an octahedral site. Although Cr 3+ ions prefer octahedral coordination in these materials, because of the atomic mixing in the as-synthesized powders, it is possible that Cr ions are randomly situated in both octahedral and tetrahedral sites. A possible explanation for the increase in light output with increasing annealing temperature could be diffusion of Cr ions from tetrahedral to octahedral sites. The integrated light output as a function of reciprocal temperature is plotted in Figure 2. As shown, a linear Arrhenius 520
relationship is found with an activation energy of 19 U/mole. This suggests that a diffusional mechanism is not the basis for the increasing light output as activation energies for diffusion in YAG are at least an order of magnitude greater [14]. The FTIR data, as discussed below, also supports this conclusion.
6.00 1 0 I-
5.50 F
II--
5.00
0
I
I
I
F
4.50 [
N --I
4.00 3.50 -
Q = 19.0 ± 5 kJ/mole
-J
3.00 0.5
Figure 2
0.54
0.58 0.62 l/T (X1000)
0.66
0.7
Plot of the integrated light output as a function o
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