Applicability of Dy-doped yttrium aluminum garnet (YAG:Dy) in phosphor thermometry at different oxygen concentrations

Author’s Accepted Manuscript Applicability of Dy-doped yttrium aluminum garnet (YAG:Dy) in phosphor thermometry at different oxygen concentrations Naohiro Ishiwada, Kazuki Tsuchiya, Takeshi Yokomori www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)31364-4 https://doi.org/10.1016/j.jlumin.2018.12.016 LUMIN16145

To appear in: Journal of Luminescence Received date: 29 July 2018 Revised date: 1 December 2018 Accepted date: 6 December 2018 Cite this article as: Naohiro Ishiwada, Kazuki Tsuchiya and Takeshi Yokomori, Applicability of Dy-doped yttrium aluminum garnet (YAG:Dy) in phosphor thermometry at different oxygen concentrations, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.12.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Applicability of Dy-doped yttrium aluminum garnet (YAG:Dy) in phosphor thermometry at different oxygen concentrations

Naohiro Ishiwada*, Kazuki Tsuchiya, Takeshi Yokomori

Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi Kohoku-ku, Yokohama-shi, Kanagawa, Japan

*

Corresponding author. Phone & Fax: +81-45-566-1640, E-mail: naohiro.ishiwada@

keio.jp

Abstract

Phosphor thermometry is a method used to measure temperature based on the temperature-dependent phosphorescence of phosphors and elucidate heat transfer phenomena, such as high-temperature gas flow. Although various rare earth-doped thermographic phosphors are in use, the effect of oxygen concentration on their phosphorescence has not been sufficiently explored. We explore herein the applicability of Dy-doped yttrium aluminum garnet (YAG:Dy), a well-known rare earth-doped phosphor with temperature sensitivity above 1000 K, in 1

phosphor thermometry at different oxygen concentrations. A third-harmonic Nd:YAG laser excited the sample. Phosphorescence was measured using a photomultiplier tube for lifetime detection. A spectrometer was used to detect the intensity ratio between two emission lines. The chamber was filled with a nitrogen–oxygen mixture with a controlled concentration. The phosphorescence intensity ratio depended on temperature over a wide temperature range and varied with the oxygen concentration, especially above 1000 K. The YAG:Dy lifetimes could be detected over the entire temperature range and remained constant up to 1000 K. In addition, the lifetimes decreased with the increasing oxygen concentration, especially above 1000 K, confirming the oxygen quenching effect. Consequently, YAG:Dy is confirmed to be sensitive to oxygen concentration for determining the intensity ratio and lifetime, especially above 1000 K.

Abbreviations: PMT, photomultiplier tube; YAG, yttrium aluminum garnet; XRD, X-ray powder diffraction; and SEM, scanning electron microscopy

Keywords: Phosphor thermometry, Intensity ratio method, Lifetime method, Oxygen quenching

2

1. Introduction

The emission properties of phosphors are sensitive to temperature; hence, phosphor thermometry can make remote temperature sensing a low-cost, simple, and precise technique. Accordingly, phosphor thermometry has received considerable attention as a versatile alternative to conventional temperature measurement techniques involving the use of thermocouples and pyrometers [1–5].

In phosphor thermometry, the temperature dependence of phosphorescence can be utilized via a lifetime method or intensity ratio method. The lifetime method involves the detection of phosphorescence decay times. It provides a high degree of measurement precision. Numerous thermographic phosphors are available with different ranges that can be applied to various temperature ranges [1,6]. The drawback of this method is that the decay time is quite short; therefore, highly time-resolved detection devices are desirable. In contrast, the intensity ratio method is based on the ratio between the intensities of two emission lines [7,8]. While two-dimensional maps are more easily detected through two different bandpass filters, the temperature resolution is inferior to that of the lifetime method [9].

Temperature sensing using thermographic phosphors has recently been applied in unsteady environments, such as internal combustion engines and gas turbines [9]; thus, 3

thermographic phosphors could be exposed to domains with varying oxygen concentrations. Some rare earth-doped phosphors are known to be sensitive to oxygen concentration [8]. The frequency of collision between excited-state phosphors and oxygen molecules increases with the increasing oxygen concentration. Finally, the excitation energy of phosphors is transferred to oxygen molecules, and no phosphorescence emission occurs. This is termed as oxygen quenching [10].

Oxygen quenching is dependent on the partial pressure of oxygen; therefore, as the intensity and lifetime of luminescence vary with the oxygen partial pressure of the surrounding gases, the pressure distribution can be measured by using the luminescence characteristics. These phosphors are commonly known as pressure-sensitive paint (PSP). The most widely used PSP belongs to organometallic material groups, such as PtTFPP and PdTFPP, because they emit a strong luminescence, and have a long lifetime [11–14]. In contrast, as explored by many researchers, organometallic sensors are not suitable for use in higher-temperature fields.

For application in harsh environments, such as combustion, some researchers have focused on ceramic phosphors that survive high-temperature environments and studied the sensitivity of phosphorescence to oxygen concentration. For instance, the lifetimes of Y2O3:Eu and YAG:Eu are known to be highly sensitive to oxygen partial pressure [10], while phosphors,

4

such as Al2O3:Cr, YAB:Cr, and Mg3F2GeO4:Mn, are reported to have no significant sensitivity [15]. The sensitivity of phosphor to oxygen concentration depends on the crystal structure and the luminescence center material; however, the correlation between oxygen quenching and these properties of the phosphor is still unclear [8, 10, 15]. Thus, the effects of temperature and oxygen concentration on the phosphorescence of various phosphors must be studied.

This study explores the thermal and oxygen quenching properties of Dy-doped Y3Al5O12 (YAG). Dy-doped phosphors are the most widely used thermographic phosphors because they show phosphorescence even above 1000 K. YAG was selected as the host because of its high melting point (2213 K) and high mechanical strength [16]. Indeed, many researchers have reported that YAG:Dy phosphor can be used at high temperatures [17–19].

Feist et al. observed that the lifetime of YAG:Dy depended on oxygen partial pressure, implying an uncertainty in temperature measurements [20]. In contrast, Yu et al. measured the intensity ratios of YAG:Dy in air and in pure oxygen and reported that the difference in the emission–temperature relation for the two cases was smaller than the experimental precision [21]. Despite some previous reports, the applicability of YAG:Dy phosphor to the intensity ratio method and lifetime method at varying oxygen concentrations is still unclear. In addition, the sensitivity of phosphor to oxygen is often problematic for thermometry in environments with

5

temporally and spatially varying oxygen concentrations, such as combustion environments; therefore, the response rate of oxygen quenching and the reduction in intensity caused by oxygen quenching are also discussed herein.

2. Experimental methods

2.1 Preparation of Dy-doped YAG phosphor

Dy-doped YAG phosphor was prepared by the sol–gel method [22]. Metal nitrate hydrates Y(NO3)3·6H2O and Al(NO3)3·9H2O were used as the host lattice, while Dy(NO3)3·6H2O was used as the dopant (Wako Pure Chemical Industries, Ltd). The hydrates were dissolved in distilled water to prepare 1.0 mol/L precursor solutions with a Y:Al:Dy ratio of 2.97:5:0.03. Subsequently, citric acid was dissolved in the abovementioned solution, and the resulting mixture was stirred at 338 K for 2 h and 358 K for 3 h. The solutions were concentrated by slow evaporation at 383 K, yielding a white gel, which was then preheated at 673 K for 2 h in air and finally sintered at 1673 K for 6 h in air.

2.2. Analysis of temperature-dependent photoluminescence

6

Power Controller Beam splitter

Temperature Controller

Nd:YAG laser (355 nm) Short pass filter Motorized flip mirror

Data Logger

Thermocouple

Silica Pole

Silica Tube

Inflow N2,O2 4L/min

Tube Furnace MFC

YAG:Dy Phosphor

Outflow

PC

Laser power meter Spectrometer Long pass filter

ND filter

Dichroic Mirror

Collimate Lens

CH1 Photomultiplier

CH2 Photodetector

N2

Oscilloscope

O2

Fig. 1. Experimental setup for analyzing the effects of temperature and oxygen concentration on phosphorescence.

Fig. 1 shows the experimental setup for analyzing the effects of temperature and oxygen concentration on phosphorescence. The phosphors were placed in a quartz pipe chamber (34 mm × 570 mm), which could be heated by a tubular electric furnace (Asahi: ARF-40K). The phosphor temperature was monitored by a K-type thermocouple and controlled by a proportional-integral-derivative (PID) temperature control unit (Shimaden: PAC16 and SR91). The nitrogen–oxygen gas mixture was injected into the chamber at a total flow rate of 4 L/min. The flow rate of each gas was controlled by mass flow controllers. The temperature variation during each measurement was ±1 K. Phosphorescence was measured at temperature intervals of 200 K from room temperature to 1273 K while heating up the furnace. 7

The phosphors were excited at a wavelength of 355 nm obtained from a third-harmonic-generation Nd:YAG laser (Quantel: Q-Smart 850) operated at a pulse energy of 20 mJ. A motorized flip mirror (Thorlabs: MFF101/M) deflected the laser intensity to a laser power meter (Gentec-EO: TPM-300) to record the fluctuations in the laser intensity during experiments. These fluctuations were less than 5%. A dichroic mirror transmitted all light above 390 nm (Semrock: FF390-Di01-25x36) and reflected the excitation light from the lamp to the prepared phosphor. Simultaneously, this mirror transmitted only the emitted light from the sample back to the detector. In addition, a short-pass filter that reflected all light shorter than 425 nm (Edmund optics: OD2 #64-600) and a long-pass filter with a cut-on point of approximately 400 nm (Thorlabs: FEL0400) were installed to remove the scattered laser light. Neutral density (ND) filters (Thorlabs: NDUV10A) were used to adjust the laser beam to the desired intensity of a photomultiplier tube (PMT, Hamamatsu, H10721-210) and a spectrometer (Ocean Optics: USB2000+).

2.3. Data processing of temperature-dependent photoluminescence

Phosphorescence was measured by a spectrometer to detect the intensity ratio of the two emission lines. Data were collected with an integration time of 500 ms, and the average number of data collection times was 120. The emission intensity at 1273 K was also recorded with an

8

integration time of 500 ms. The average number of data collection times was 10 to confirm the response time for oxygen. The intensity ratios were calculated as follows:

Intensity ratio (－) 

I A,T I B ,T

,

(1)

I A,T0 I B ,T0

where IA is the intensity of the chosen counter range at 450–475 nm; IB is the intensity of the main range at 476–500 nm; T is the temperature of the phosphor sample; and T0 is the room temperature.

The lifetime signal was monitored using the PMT coupled to an oscilloscope (Tektronix, TDS 754D), which recorded the experimental decay with a resolution of 1,000,000 points in each flash. The intensity of laser pulses was monitored with a fast photodetector (Thorlabs: DET10A/M) to note the timing of pulse generation from the laser. The temporal response of the phosphor follows a double-exponential procedure because the phosphorescence of Dy3+ occurs mainly because of the 4F9/2-to-6H15/2 and 4I15/2-to-6H15/2 transitions [23].  t   t I (t )   1 exp      2 exp    1   2

  , 

(2)

where I(t) is the emission intensity at time t; α1 and α2 are the pre-exponential factors; and τ1 and τ2 are the emission time constants. The lifetime decay evaluations were analyzed using a nonlinear least-square fitting in MATLAB (MathWorks). The average decay time, τ, was calculated as follows based on the above equation [23]: 9

1 12   2 22 ,  1 1   2 2

(3)

3. Results and discussion

3.1 Morphology of YAG:Dy phosphor

The structure of the prepared phosphor was confirmed and characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) (Figs. 2 and 3, respectively). The XRD patterns well matched with the ICDD 70-7794 data corresponding to YAG [24]. In addition, the pure phase formation of YAG was confirmed because the XRD peaks corresponding to YAP, YAM, or YAH were not detected. The SEM image revealed that the particles were spherical, with sizes ranging from 1.5 to 15.0 µm.

10

Intensity [a.u.]

Y3Al5O12 (ICDD #70-7794)

10

20

30

40

50

2Theta [degree]

Fig. 2 XRD spectrum of phosphor.

30 μm

5 μm

Fig. 3 SEM image of the prepared phosphor.

11

60

70

80

3.2 Effect of oxygen quenching on the emission intensity

298 K 873 K 1073 K 1173 K 1273 K

Normalized intensity [a.u.]

1.4 1.2

N2=100%

O2=100%

1.0 0.8 0.6 0.4 0.2 0.0 -100

0

100

200

300

400

Time [s] Fig. 4 Effect of oxygen quenching on the emission intensity history of YAG:Dy.

Fig. 4 shows the effect of oxygen quenching on the emission of YAG:Dy in the range 476–500 nm. The gas composition was changed from 100% nitrogen to 100% oxygen at zero time. The horizontal axis indicates the elapsed time from the start of the measurement, while the vertical axis depicts the emission intensity normalized with the intensity at the start of the measurement. The emission intensity of YAG:Dy decreased when the gas changed from nitrogen to oxygen at temperatures above 1073 K. Additionally, beyond 1273 K, the emission 12

intensity dropped to approximately 40%, and the response time for oxygen was 5 to 10 s. In our experiment, the volume capacity of the chamber was 0.52 L, and the continuous flow rate was 4 L/min; hence, the complete replacement of nitrogen with oxygen took approximately 7.8 s. Therefore, we assumed that the response time for oxygen depends on the replacement time, and oxygen quenching occurred with a fast response after the gas is changed to oxygen.

The theoretical implications associated with the abovementioned results are next discussed. Oxygen quenching refers to the process by which the excitation energy of phosphor is quenched by collision with oxygen molecules. When the excited phosphor comes into contact with oxygen (quencher), its excited electron returns to the ground state. Therefore, this phosphor cannot emit light, and the energy is dissipated. The collision frequency increases with the increasing temperature, which consequently increases the probability of deactivation. The efficiency of energy transfer to oxygen molecules also increases as the photoluminescence excitation energy is increased. Hence, oxygen quenching caused by collisions is likely to occur at a high temperature.

This phenomenon can be explained by considering the YAG properties. The sites, on which the activators sit in a YAG crystal, have a low symmetry, which leads to high-intensity emissions [6]. In contrast, the oxygen sensitivity is closely related to the presence of oxygen

13

vacancies in the host lattice. The oxygen vacancy could shift the charge-transfer state (CTS) bands and reduce the non-radiative decay rate. The oxygen vacancy affects the non-radiative decay rate because oxygen vacancies can act as electron trap sites for the extra electrons generated by excitation. These trap sites could prevent the extra electrons from migrating and combining non-radiatively with trapped holes [25, 26]. Consequently, the oxygen vacancy in the lattice is reduced by increasing the ambient oxygen concentration; hence, the non-radiative decay rate becomes stronger with the increasing oxygen concentration. In addition, the oxygen quenching effect becomes stronger at a high-temperature range because the probability of transition through CTS interactions is enhanced by temperature.

3.3 Effect of oxygen concentration on emission spectra with increasing temperature

14

Intensity [a.u.]

F9/2 → H15/2

4

473 K, O2=0%, N2=100% 473 K, O2=10%, N2=90% 473 K, O2=20%, N2=80% 473 K, O2=100%, N2=0%

1.0 0.8 0.6 0.4 0.2

4

I15/2 → H15/2 6

0.0

4

1273K, O2=0%, N2=100% 1273K, O2=10%, N2=90% 1273K, O2=20%, N2=80% 1273K, O2=100%, N2=0%

1.0

Intensity [a.u.]

6

0.8

4

0.6

F9/2 → H15/2 6

I15/2 → H15/2 6

0.4 0.2 0.0

420

440

460

480

500

520

Wavelength [nm]

Fig. 5 Effect of oxygen concentration on the emission spectra of YAG:Dy with increasing temperature.

Fig. 5 shows the effect of the oxygen concentration on the emission spectra of YAG:Dy with the increasing temperature. The emission spectra were normalized to the peak intensity at N2 = 100% and O2 = 0%. The spectra showed two strong bands centered at 470–500 nm and 458 nm. The emission between 470 and 500 nm originated from the 4F9/2 → 6H15/2 transition, while that at 458 nm was caused by the 4I15/2 → 6H15/2 transition [27, 28]. The emissions corresponding to these two bands showed different temperature dependences; hence, the integrated intensity ratio from 450 to 475 nm as the counter range and from 476 to 500 nm as the main range is appropriate for phosphor thermometry [29]. The emission intensity

15

nonlinearly decreased with the increasing oxygen concentration, while the sensitivity of phosphor to oxygen quenching became stronger with the increasing temperature because the variations in the oxygen concentration and temperature influenced the frequency of collision with the excited luminescence centers nearby. Oxygen quenching caused by collision is unlikely to occur at low temperatures because of the low probability of transfer of the photoluminescence excitation energy to oxygen molecules; hence, the emission intensity hardly changes for the oxygen concentration in the low-temperature range. By contrast, oxygen quenching caused by collision is more probable in the high-temperature range; that is, oxygen quenching by collision easily occurs when the oxygen concentration increases at high temperatures.

Figs. 6(a) and (b) show the temperature dependence and the effect of the oxygen concentration on the intensity ratios of YAG:Dy, respectively. Fig. 6(a) presents the Arrhenius plot of the natural logarithm of the I458nm/I484nm intensity ratio and the inverse of temperature (1/T). The intensity ratio method is based on the intensity ratio between the emissions of two energy states. The emission at 484 nm from the 4F9/2 → 6H15/2 transition and that at 458 nm from the 4I15/2 → 6H15/2 transition were employed herein. The relative population of photons follows a Boltzmann distribution, and is dependent on the temperature and the energy difference between the states; thus, the experiment data were fitted with the following equation [30]:

16

ln(

 458nm E )  ln( B)  ,  484nm kbT

(4)

where ΔE is the energy difference between the 4F9/2 and 4I15/2 states; kb is the Boltzmann constant; and B is the pre-exponential factor. The agreement between the experiment data and the theoretical equation is confirmed in this figure. ΔEave is 1161 cm−1, which is determined from the slope in the experimental data points using linear regression. The obtained energy difference between the 4F9/2 and 4I15/2 states corresponds to the theoretical value of 1000 cm−1 [31].

In the abovementioned discussion, the intensity ratios were calculated using the intensity values at the peak points. However, bandpass filters are usually used in the practical application process, which means that the intensity integral of a certain band is used for the intensity ratio calculation [29]. Therefore, in this study, the stronger peak range at 476–500 nm and another range at 450–475 nm were selected to obtain the calibration curves of the intensity ratios (Fig. 6(b)). As seen in this figure, the intensity ratios drastically decreased with an increase from room temperature to 1273 K; thus, an effective temperature resolution was observed in the intensity ratio method. However, the oxygen quenching effect was confirmed from 900 K onward. At temperatures exceeding 1000 K, the intensity ratios showed agreement within the error margins of approximately 15 K between the oxygen concentrations of 0% and 10% and had an error potential of approximately 50 K between the oxygen concentrations of 17

0% and 100%. Consequently, based on the intensity ratio method, YAG:Dy was suitable for use in phosphor thermometry at temperatures below 900 K because the effect of oxygen quenching was negligible. The results showed a high-temperature resolution from room temperature to approximately 900 K.

The temperature uncertainty of the intensity ratio was evaluated from the abovementioned results. One of the uncertainties is signal detection (i.e., spectrometer). The optical resolution of the spectrometer was set to approximately 0.34 nm, which influenced the data processing to integrate the intensity. Its pixel well depth was 62,500 electrons; hence, the luminescence intensity was divided by the resolution of the pixel well depth. These resolutions in the signal detection influenced the accuracy of the intensity ratio calculation. Thus, the variation in the intensity ratio was calculated, and the coefficient of variation for the intensity ratio was 0.03%, which corresponded to the systematic temperature uncertainty of ±0.83 K.

18

(a) Ln (Intensity ratio) = Ln (I458nm/I484nm)

0

O2=0%,N2=100% O2=10%,N2=90% O2=20%,N2=80% O2=100%,N2=0%

-1

-2

-3

-4

△Eave= 1161cm-1 -5 0.0005

0.0010

0.0015

0.0020

(b)

0.0025

0.0030

0.0035

-1

1/T [K ]

Intensity ratio (I450-475nm/I476-500nm) [-]

14

O2=0%, N2=100% O2=10%, N2=90% O2=20%, N2=80% O2=100%, N2=0%

12 10 8 6 4 2 0 300

400

500

600

700

800

900

1000 1100 1200 1300

Temperature [K]

Fig. 6 Temperature dependence and effect of the oxygen concentration on the intensity ratios of YAG:Dy. (a) Arrhenius plot of the natural logarithm of the I458nm/I484nm intensity ratio and the inverse of temperature (1/T). (b) Intensity integral of the peak range at 476–500 nm and 450– 475 nm for the intensity ratio calculation.

19

The following expressions for the relative sensitivities of the intensity ratio measurement are introduced from Equation (4) to evaluate the material performance in terms of the intensity ratio:

S (T ) 

E E E  B  exp( )  LIR(T ) , 2 kbT kbT kbT 2

S R (T ) 

(5)

E 100% , kbT 2

(6)

where LIR stands for the luminescence intensity ratio; S is the temperature resolution of the luminescence intensity ratio; and SR is the relative sensitivity defined as the ratio of sensitivity. Note that the sensitivities are functions of temperature; therefore, it is important to report which temperature they were calculated.

Table 1 shows the comparison of the temperature resolutions of the promising materials, which were estimated from the experimental data of the intensity ratio. From this table, some materials had more sensitivity than YAG:Dy3+. However, these materials were more likely to suffer from black body radiation or possess a weak intensity; thus, YAG:Dy3+ is among the good candidates for phosphor thermometry using the intensity ratio method.

Table 1 Comparison of the temperature resolutions of expected materials in terms of the intensity ratio. Material

ΔE (cm−1)

Relative sensitivity (%/K)

Reference

TiO2:Eu3+

－

2.43 (at 523.15 K)

[32]

ZnGa2O4:Cr3+

1744

2.79 (at 300 K)

[33]

Bi2Ga4O9:Cr3+

390

0.62 (at 300 K)

[34]

20

MgTiO3:Mn4+

－

0.6 (at 148 K)

[35]

GdVO4@SiO2:Tm3+,Yb3+

－

0.94(at 323 K)

[36]

YAG:Dy3+

1161.1

1.86 (at 300 K)

This work

3.4 Effect of oxygen concentration on photoluminescence lifetime with increasing temperature

The lifetime of the YAG:Dy phosphor as a function of the oxygen concentration was investigated. Fig. 7 illustrates the lifetimes of YAG:Dy at various oxygen concentrations at 473 K and 1273 K. The oxygen quenching effect was observed at 1273 K, whereas the lifetime decays were insensitive to the oxygen concentration in the low-temperature range.

21

Ln (Normalized intensity) [-]

1.00 0.37 0.14 0.05

Ln (Normalized Intensity) [-]

0.02 1.00

473K, O2=0%, N2=100%, 473K, O2=10%, N2=90% 473K, O2=20%, N2=80% 473K, O2=100%, N2=0%

0.37

0.14

0.05

0.02 0.0

1273K, O2=0%, N2=100% 1273K, O2=10%, N2=90% 1273K, O2=20%, N2=80% 1273K, O2=100%, N2=0% 5.0x10

-4

1.0x10

-3

1.5x10

-3

2.0x10

-3

Lifetime [s]

Fig. 7 Phosphorescence lifetimes in the oxygen/nitrogen mixture as a function of the oxygen concentration.

Fig. 8 shows the phosphorescence lifetimes in the gas mixture of oxygen and nitrogen as a function of temperature. The lifetime of YAG:Dy did not change until 900 K; thus, the lifetimes of YAG:Dy were difficult to measure in the low-temperature range. In contrast, the lifetime of YAG:Dy varied with the temperature beyond 1000 K. The high-temperature resolution was confirmed using the lifetime method. However, the lifetimes of YAG:Dy were affected by the oxygen concentration at temperatures exceeding 1000 K. Beyond 1100 K, its lifetimes had an

22

error potential of approximately 90 K in the temperature reading when strong variations exist in the oxygen concentration of the surrounding gas phase.

The lifetime mechanism responsible for oxygen quenching was considered. The lifetime consisted of the inverse of the sum of the radiative and non-radiative decay rates. The lifetime was largely controlled by the non-radiative decay rates, whereas the radiative decay rates were relatively constant [10]. According to Fig. 8, the lifetime became shorter with the increasing oxygen concentration, suggesting that the non-radiative decay rate became stronger because of the oxygen quenching effect. Moreover, the oxygen quenching effect became stronger at a high-temperature range; thus, temperature influenced the oxygen quenching sensitivity. This phenomenon was caused by the non-radiative decay by the CTS being thermally promoted in the high-temperature range [37].

23

Lifetime [s]

1.0x10

-3

9.0x10

-4

8.0x10

-4

7.0x10

-4

6.0x10

-4

5.0x10

-4

4.0x10

-4

3.0x10

-4

O2=0%, N2=100% O2=10%, N2=90% O2=20%, N2=80% O2=100%, N2=0%

400

600

800

1000

1200

1400

Temperature [K]

Fig. 8 Phosphorescence lifetimes in the oxygen/nitrogen mixture as a function of temperature.

The relationship between intensity and lifetime when the oxygen quenching occurred was also considered. The transient luminescence intensity, I(t), after the end of excitation at t = 0 is described by a mono-exponential approach.  t I (t )  I 0 exp    ,  

(7)

where I0 denotes the intensity at t = 0, and τ is the lifetime defined by I (t) = I0/e. We calculated the time-integrated luminescence intensity from the abovementioned equation.  t I 0 exp   dt A  ,     Aref  ref   t  0 I 0 exp    ref dt  





0

(8)

where A is the time-integrated luminescence intensity, and subscript ref represents the condition 24

N2 = 100%. The decrease ratio of the oxygen quenching effect on the time-integrated intensity and time constant was evaluated and listed in Table 2. The decrease ratio of the time-integrated luminescence intensity was almost the same as that of the lifetimes; thus, the relation between the luminescence intensity and the time constant is convincing. According to Fig. 4, the intensity strongly decreased by approximately 60% at 1273 K when the atmosphere gas was changed to 100% O2 because the experimental data were normalized at the start point of N2 = 100% and O2 = 0%.

Table 2: Comparison of the oxygen quenching effects on the time-integrated intensity and the time constant. N2 = 100%, O2 = 0%

N2 = 90%, O2 = 10%

N2 = 80%, O2 = 20%

N2 = 0%, O2 = 100%

(1 − τ/τref) * 100 (%)

0%

11.9%

13.7%

16.3%

(1 − A/Aref) * 100 (%)

0%

13.8%

16.3%

18.9%

4. Conclusion

This study investigated the thermal and oxygen quenching properties of YAG:Dy to determine the feasibility of using this phosphor in temperature domains with variations in oxygen concentration.

Consequently, the time-resolved intensity dropped by nearly 60% when transitioning from pure nitrogen to pure oxygen at high temperatures, and oxygen quenching occurred with a fast response. In addition, the intensity ratios of YAG:Dy were sensitive to temperature, but 25

insensitive to oxygen concentration from room temperature to approximately 900 K; hence, the intensity ratio of YAG:Dy is a good parameter for phosphor thermometry with a varying oxygen concentration. However, above 1000 K, the intensity ratios had an error potential of approximately 15–50 K with the oxygen concentration. These results suggest that YAG:Dy is suitable for use based on the intensity ratio method at temperatures lower than 900 K because the temperature measurements are feasible, regardless of the oxygen concentration. The lifetime of YAG:Dy was strongly affected by oxygen quenching at temperatures above 1000 K and nonlinearly varied with the oxygen concentration. Therefore, beyond 1100 K, the lifetime of YAG:Dy had an error potential of approximately 90 K in the domain, where the oxygen concentration varied.

Hence, the results demonstrated that the intensity ratios and the lifetimes of YAG:Dy depend on the oxygen concentration, especially above 1000 K. Therefore, when YAG:Dy phosphor is employed as a thermographic phosphor, the oxygen concentration is an important parameter for achieving high-precision measurements.

Acknowledgments

26

This work was supported by the Council for Science, Technology and Innovation (CSTI), the Cross-ministerial Strategic Innovation Promotion Program (SIP), and the “Innovative Combustion Technology” program.

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