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Energy transfer in Eu3+ doped scheelites: use as thermographic phosphor

Katrien W. Meert, Vladimir A. Morozov, Artem M. Abakumov, Joke Hadermann, Dirk Poelman, and Philippe F. Smet

Open Access Open Access

Abstract

In this paper the luminescence of the scheelite-based CaGd2(1-x)Eu2x(WO4)4 solid solutions is investigated as a function of the Eu content and temperature. All phosphors show intense red luminescence due to the 5D0 – 7F2 transition in Eu3+, along with other transitions from the 5D1 and 5D0 excited states. For high Eu3+ concentrations the intensity ratio of the emission originating from the 5D1 and 5D0 levels has a non-conventional temperature dependence, which could be explained by a phonon-assisted cross-relaxation process. It is demonstrated that this intensity ratio can be used as a measure of temperature with high spatial resolution, allowing the use of these scheelites as thermographic phosphor. The main disadvantage of many thermographic phosphors, a decreasing signal for increasing temperature, is absent.

© 2014 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 a) Excitation spectrum of CaGd1.8Eu0.2(WO4)4 upon monitoring the emission at 612 nm. The excitation peak at 313 nm related to Gd is indicated by (*). b) Emission spectrum of CaGd1.8Eu0.2(WO4)4 upon excitation at 395 nm. The electronic transitions for the main excitation and emission peaks are indicated.

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Fig. 2
Fig. 2 Concentration dependence of the 5D0 - 7F2 emission intensity in CaGd2(1-x)Eu2x(WO4)4.

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Fig. 3
Fig. 3 Decay of the 5D0 emission of CaGd2(1-x)Eu2x(WO4)4 for x = 0.1,0.5 and 1 at 75K (a) and 475K (b).

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Fig. 4
Fig. 4 Temperature dependence of the emission output (λexc = 465 nm) of the low concentration (CaGd1.8Eu0.2(WO4)4) (a) and the high concentration (CaEu2(WO4)4) (b) samples. The inset shows the emission spectrum at two different temperatures. The intensities are obtained by integrating over the wavelength ranges 535 to 545 nm (5D1-7F1) and 585 to 600 nm (5D0-7F1).

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Fig. 5
Fig. 5 Decay of the 5D1 emission (λexc = 385 nm) at 75 K and 475 K for CaEu2(WO4)4. The fast decay component remains constant and equals 1.8µs. The slow decay component equals the decay constant of the 5D0 emission at the respective temperatures.

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Fig. 6
Fig. 6 Eu3+ energy level scheme illustrating the phonon-assisted cross-relaxation process.

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Fig. 7
Fig. 7 a) Ratio (R) of the integrated intensities of the 5D1 to 5D0 emission for CaEu2(WO4)4 and the corresponding Arrhenius plot (inset). b) Calculated relative sensitivity Srel as a function of temperature.

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Fig. 8
Fig. 8 Patterned resistive heater with cross-sections for the vertical and horizontal profile indicated by the black lines (left). Temperature plot of the patterned resistive heater, imaged with the use of the thermographic phosphor (middle) and with an infrared camera (right).

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Fig. 9
Fig. 9 Horizontal (a) and vertical (b) temperature profiles extracted from the temperature plots in Fig. 8.

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Tables (1)

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Table 1 Decay constant and fraction of the variable decay time component of the 5D0 emission of CaGd2(1-x)Eu2x(WO4)4

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Equations (5)

Equations on this page are rendered with MathJax. Learn more.

I ( t ) = I 1 . exp ( − t τ 1 ) + I 2 . exp ( − t τ 2 )
f 2 = I 2 τ 2 I 1 τ 1 + I 2 τ 2
( D 5 0 ) i o n 2 + ( F 7 2 ) i o n 1 + p h o n o n s ( 885 c m − 1 ) → ( D 5 1 ) i o n 2 + ( F 7 0 ) i o n 1
R= I 1 I 0 =B.exp( − ΔE kT )
S rel = 1 R dR dT = ΔE k T 2
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