Optical temperature sensing properties of Yb³⁺–Er³⁺ co-doped NaLnTiO₄ (Ln = Gd, Y) up-conversion phosphors

Abstract

Yb³⁺–Er³⁺ ion co-doped NaLnTiO₄ (Ln = Y, Gd) up-conversion (UC) phosphors were successfully synthesized by a sol–gel method. The phase purity and the structure of the samples were characterized by powder X-ray diffraction (XRD), and the optimal compositions were also determined according to their UC emission intensities. The samples emit orange light and their UC spectra were recorded with excitation by a laser diode with a 980 nm wavelength. The UC luminescence intensity could be enhanced greatly after introducing sensitizer Yb³⁺ ions, and the energy transfer (ET) from Yb³⁺ to Er³⁺ plays a vital role. The UC mechanism and processes responsible for the emissions were investigated and found to involve two-photon absorption. The lifetime of green emission in Er³⁺ singly doped and Yb³⁺–Er³⁺ co-doped samples were measured to prove the existence of ET. The temperature dependence of the fluorescence intensity ratios (FIR) for the two green UC emission bands peaked at 530 and 550 nm was studied in the range of 300–510 K under excitation by a 980 diode laser with about 4 W cm⁻² power density, and the maximum sensitivity was approximately 0.0045 K⁻¹ at 510 K for NaYTiO₄ and 480 K for NaGdTiO₄. This indicates that Yb³⁺–Er³⁺ ion co-doped NaLnTiO₄ (Ln = Y, Gd) phosphors are potential candidates for optical temperature sensors.

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1. Introduction

Temperature is a fundamental and significant physical parameter, which could be accurately measured by many methods. Conventional temperature measurements are based on the principle of liquid and metal expansion, which is realized by heat flow to an invasive probe. However, these contact methods could not be used in many cases, such as the electrical transformer in power stations, oil refineries and intracellular temperature, which promote the development of non-contract thermometry technique. Recently, the non-contract temperature sensing based on rare earth (RE) ions activated luminescent material has been given more attention.^1–7 In particular, the fluorescence intensity ratio (FIR) technique is regarded as the most promising, which involves the measurement of photoluminescence intensities from two thermally coupled energy levels (TCLs) of RE ion. FIR technique could offer excellent measurement accuracy because it is independent of spectrum losses and fluctuations in the excitation intensity. Now, optical temperature sensors based on RE activated up-conversion (UC) phosphor have been extensively investigated.^8–12 Among these RE ions, Er³⁺ is the most popular activator because its two thermally coupled levels (TCLs) ²H_11/2 and ⁴S_3/2 and characteristic green UC emission from TCLs. However, only low luminescence efficiency is obtained in Er³⁺ solely doped phosphor under the 980 nm laser excitation due to the low absorption. The Er³⁺ and Yb³⁺ are usually used in couples, which could improve the luminescence efficiency of UC phosphor remarkably due to the high and broad absorption (ranging from 850–1050 nm) of Yb³⁺ and efficiently transfer its energy to Er³⁺.^13–17

The layered perovskite titanates complex oxides ALnTiO₄ (A = Li, Na, K; Ln = rare earth) have been proven to serve as excellent phosphor hosts due to their typical two-dimensional structures and good chemical and physical stabilities.^18–20 They usually show higher critical concentration than those of other conventional inorganic phosphors because the energy transfer is restricted to quasi-two-dimensional sub-lattice in present compounds.²¹ Recently, a series of Eu³⁺ doped red emitting phosphors ALnTiO₄ (A = Na, K; Ln = La, Gd, Y):Eu³⁺ have been prepared by sol–gel method in our group, results indicate that they show high luminescence efficiencies and could be used as red components in white light-emitting diodes (w-LEDs) and field emission displays (FEDs).^22,23 However, very little research on UC phosphors using ALnTiO₄ (A = Li, Na, K; Ln = rare earth) as hosts has been carried out.²⁴

In this paper, UC phosphors based on Yb³⁺ and Er³⁺ doped NaLnTiO₄ (Ln = Y, Gd) were prepared by sol–gel method and their UC luminescence properties were investigated. The UC emission mechanisms were evaluated, and the energy transfer processes between Yb³⁺ and Er³⁺ were proved through the measurements of lifetime. Additionally, their thermometry behaviors have also been illustrated by FIR technique.

2. Experimental

2.1 Synthesis of samples

The Yb³⁺ sensitized Er³⁺ doped NaLnTiO₄ (Ln = Y, Gd) phosphors with composition NaLn_1−x−yYb_xEr_yTiO₄ were prepared using a modified sol–gel method. In present systems, Yb³⁺ and Er³⁺ ions are expected to occupy the sites of Ln³⁺ due to their identical valence and similar ionic radius. Here, the starting materials include analytical reagent (AR) Na₂CO₃, rare earth oxides Y₂O₃, Gd₂O₃, Yb₂O₃, Er₂O₃ with high purity (99.99%, Shanghai Yuelong Nonferrous Metals Ltd.) and analytical reagent tetrabutyl titanate Ti(OC₄H₉)₄, which are used as for the sources of Na, Y (or Gd), Yb, Er and Ti, respectively. In addition, anhydrous ethanol and acetic acid played the role of solvent and hydrolysis inhibitor in this experiment. According to the chemical formula of the UC phosphors NaLn_1−x−yYb_xEr_yTiO₄: (a) x = 0.1, y = 0.01, 0.03, 0.05, 0.07, 0.09, and (b) y = 0.05, x = 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, a stoichiometric amount of oxides Ln₂O₃ (Ln = Y, Gd, Yb and Er) and Na₂CO₃ (excess 30% as flux) were first dissolved in dilute HNO₃(AR) under continuous stirring, and the excess HNO₃ was evaporated at high temperature. Then, the required amount of deionized water was added to get transparent solution A after fully stirring. The calculated volume of Ti(OC₄H₉)₄, ethanol and acetic acid were mixed to form the transparent solution B. Subsequently, solution B was transferred to solution A drop by drop under constant stirring to get the final transparent solution. The final transparent solution was kept at 70 °C for 24 h until an ivory white resin formed. The resin was further dried at 120 °C for 24 h to get a dried gel. The dried gel was ground and preheated in a furnace at 500 °C for 5 h, and then sintered at required temperature at 900 °C for 2 h to obtain final samples.

2.2 Characterization

The crystal structures were identified by powder X-ray diffractions (XRD) using a Rigaku-Dmax 3C powder diffractometer (Rigaku Corp, Tokyo, Japan) with Cu-Ka radiation (λ = 1.5406 Å). UC luminescence spectra of the phosphors at room temperature (RT) were recorded using a Hitachi F-4600 spectrophotometer (Hitachi high technologies corporation, Tokyo, Japan) with an external power-controllable 980 nm semiconductor laser (Hi-Tech Optoelectronics Company, Beijing, China) as excitation source. The lifetimes for ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ at 547 nm at RT were carried out using an 980 nm optical parametric oscillator (OPO) pulsed laser as excitation source, and the signals were detected by a Tektronix digital oscilloscope (TDS 3052). In order to investigate the temperature dependence of the UC emission, the sample was placed in a temperature-controlled copper cylinder, and its temperature was increased from RT to 240 °C. The UC spectra of sample at various temperatures were obtained using a Jobin-Yvon HRD-1 double monochromator equipped with a Hamamatsu R928 Photomultiplier under the excitation of a 980 nm diode laser with 164 mW and the excitation power density was about 4W cm⁻². The signal was analyzed by an EG&G 7265 DSP Lock-in amplifier and stored into computer memories.

3. Results and discussion

3.1 Crystal structure and phase composition

The structures and compositions of the obtained products were characterized by XRD. Fig. 1 shows the XRD patterns of NaY_1−x−yYb_xEr_yTiO₄ and NaGd_0.81Yb_0.14Er_0.05TiO₄ phosphors. XRD patterns of NaY_0.90−yYb_0.10Er_yTiO₄ and NaY_0.95−xYb_xEr_0.05TiO₄ samples as a function of Er³⁺ concentrations y and Yb³⁺ concentrations x are shown in Fig. 1a, it is observed that all diffraction peaks of samples can readily be identified as the characteristic peaks of pure orthorhombic phase NaYTiO₄ with JCPDS standard card no. 50-0022. Results indicate that no noticeable diffraction peaks from any impurity phases are found within the whole range of our experiments, which illuminates that the parameters to synthesis of the present samples are fit. In addition, it is found that the positions of the strongest diffraction peak (113) gradually shift to high angle, which due to the smaller radius of Yb³⁺ (0.86 Å) or Er³⁺ (0.88 Å) ions than that of Y³⁺ (0.89 Å).²⁵ As a representative, XRD pattern of NaGd_0.81Yb_0.14Er_0.05TiO₄ phosphor is shown in Fig. 1b with the standard NaGdTiO₄ (JCPDS 86-0830) profile, all of their diffraction peaks are matched well. Above results prove that Yb³⁺ or Er³⁺ ions occupy the sites of Y³⁺ and do not alter the host structure significantly.

Fig. 1 XRD patterns of NaLnTiO₄ (Ln = Y, Gd) phosphors: (a) NaY_1−xYb_xEr_yTiO₄ phosphors and (b) NaGd_0.81Yb_0.14Er_0.05TiO₄ phosphors annealed at 900 °C for 2 h in air.

3.2 Effects of do-pant contents on up-conversion emission

It is well known that the UC emission intensity greatly depends on the concentrations of the doped ions, thus we investigated the effect of dopant contents on NaY_1−x−yYb_xEr_yTiO₄: xYb³⁺, yEr³⁺ phosphors with the continue wave (CW) excitation at 980 nm. Room temperature UC spectra of samples as function of Er³⁺ and Yb³⁺ contents are displayed in Fig. 2a and b, respectively. The obtained emission spectra of Er³⁺/Yb³⁺ codoped NaYTiO₄ UC phosphors consist of two green emission (530 and 550 nm) bands and a red emission (668 nm) band, which are assigned to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 the characteristic inner shell transitions of Er³⁺ from the excited levels to the ground state of 4f electronic configuration, respectively. The normalized integrated intensities for green (from 510 to 570 nm) and red (from 640 to 690 nm) UC emissions were also calculated and presented in the insets of Fig. 2a and b with variable Er³⁺ or Yb³⁺ concentrations for the two series of samples, respectively. From the inset of Fig. 2a, it is found that the UC luminescence intensities for both the green and red emissions increase first with increasing Er³⁺ contents and then decrease with the continual increase of Er³⁺ concentration after reaching the maximum at y = 0.05 due to the concentration quenching, which is caused by non-radiative energy transfer and the cross relaxation between Er³⁺.²⁶ Fig. 2b shows the UC spectra of samples NaY_1−x−0.05Yb_xEr_0.05TiO₄: xYb³⁺, 0.05Er³⁺ with invariable Er³⁺ concentration y = 0.05 and variable Yb³⁺ concentrations ranging from 0 to 0.20, which shows similar trend as Fig. 2a. Seen from Fig. 2b, it is observed that the UC emission of sample NaY_0.95Er_0.05TiO₄ without Yb³⁺ is too weak to be checked due the weak absorption of Er³⁺ for 980 nm laser, and increases remarkably with the growth of Yb³⁺ dosage due to the efficient energy transfer from Yb³⁺ to Er³⁺ for the larger absorption cross section of Yb³⁺ at 980 nm and excellent energy levels match between Yb³⁺ and Er³⁺.²⁷ Results indicate that the optimal concentration of Yb³⁺ is x = 0.14, and the UC emission intensity descends gradually as the concentration of Yb³⁺ beyond 0.14. The main reasons for this may come from the energy back transfer (EBT) Er³⁺ (⁴S_3/2) + Yb³⁺ (²F_7/2) → Er³⁺ (⁴I_13/2) + Yb³⁺ (²F_5/2).^28,29 The higher Yb³⁺concentration, the higher ratio of EBT from Er³⁺ to Yb³⁺ to ET from Yb³⁺ to Er³⁺.²⁴

Fig. 2 UC luminescence spectra of NaY_1−x−yYb_xEr_yTiO₄: xYb³⁺, yEr³⁺ (a) x = 0.10; y = 0.01, 0.03, 0.05, 0.07, 0.09 and (b) y = 0.05; x = 0, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20 under excitation at 980 nm.

3.3 Energy scheme and mechanism of UC emission

In order to better understand the possible UC processes and the mechanisms of the green and red emission in Yb³⁺/Er³⁺ co-doped NaLnTiO₄ (Ln = Y, Gd) phosphors, the energy level diagram for Er³⁺ and Yb³⁺, feasible excitation pathways, emission process and UC mechanisms are demonstrated in Fig. 3. The possible up-conversion populating processes of the Er³⁺ and Yb³⁺ co-doped samples under the excitation of 980 nm are shown in Fig. 3a. As drawn in the schematic, the energy level ²F_5/2 of Yb³⁺ is close to that of ⁴I_11/2 of Er³⁺, thus the energy transfer from Yb³⁺ to Er³⁺ is high efficient. Under the excitation of 980 nm LD, Yb³⁺ first absorbs an infrared photon from the ground state transition ²F_7/2 to the excited state ²F_5/2. The excitation wavelength (980 nm) is in resonance with the transition of ²F_7/2 → ²F_5/2 of Yb³⁺ and thus easy to be absorbed by this ion. For the green emission, the peaks at 530 and 550 nm come from the excited states ²H_11/2/⁴S_3/2 to ground state ⁴I_15/2 transitions of Er³⁺, and the ⁴S_3/2 level could be populated by non-radiative (NR) relaxation from the upper ²H_11/2 level populated through NR process from ⁴F_7/2 level of Er³⁺. The population ⁴F_7/2 level may follow three process: Er³⁺ (⁴I_11/2) + a photon (980 nm) → Er³⁺ (⁴F_7/2) (excited state absorption: ESA), Er³⁺ (⁴I_11/2) + Yb³⁺ (²F_5/2) → Er³⁺ (⁴F_7/2) + Yb³⁺ (²F_7/2) (energy transfer, ET),and Er³⁺ (⁴I_11/2) + Er³⁺ (⁴I_11/2) → Er³⁺ (⁴F_7/2) + Er³⁺ (⁴I_15/2) (cross relaxation: CR). Here, the intermediary level ⁴I_11/2 of Er³⁺ play an vital role in these processes, and population process of this energy includes the energy transfer (ET) process Er³⁺ (⁴I_15/2) + Yb³⁺ (²F_5/2) → Er³⁺ (⁴I_11/2) + Yb³⁺ (²F_7/2) and the ground state absorption (GSA) process from ⁴I_15/2 to ⁴I_11/2. However, the probability for the GSA of Er³⁺ is low due to the narrow absorption cross section, which illuminates that the ET process dominated the UC process.³⁰ For the red emission assigned to ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, the population of ⁴F_9/2 could involve following these processes: Er³⁺(⁴I_13/2) + a photon (980 nm) → Er³⁺(⁴F_9/2) (ESA), Yb³⁺(²F_5/2) + Er³⁺(⁴I_13/2) → Yb³⁺(²F_7/2) + Er³⁺(⁴F_9/2) (ET), and the NR from ⁴S_3/2 to ⁴F_9/2. The long-lived intermediary ⁴I_13/2 level is populated by NR relaxation from ⁴I_11/2 level.

Fig. 3 (a) Energy level diagram of the Yb³⁺, Er³⁺ ions and the proposed UC mechanisms in Er³⁺–Yb³⁺ co-doped NaYTiO₄ under 980 nm excitation; (b) and (c) dependence of UC luminescence intensity of NaY_0.81Yb_.0.14Er_0.05TiO₄ and NaGd_0.81Yb_0.14Er_0.05TiO₄ upon the pumping power, respectively.

For a multi-photon UC process, the UC luminescence intensity (I, integrated intensity) depends on the nth the pump laser powder (P), i.e., I ∝ Pⁿ, here n denotes the number of absorbed infrared (IR) photons to produce an UC emission photon³¹ and it is important for the determination of UC mechanism. It is known that which could be assigned to the slope of the straight line that obtained through the plot of ln I versus ln P. Fig. 3b and c shows the experimental data and fitting curves reflecting the relationship between the integrated UC intensities and pump power for the samples NaY_0.81Yb_0.14Er_0.05TiO₄ and NaGd_0.81Yb_0.14Er_0.05TiO₄ with optimal composition, respectively. As shown in Fig. 3b and c, it could find that the slopes of the both curves for the green peak are 2.05 and 1.99 for Ln = Y and Gd, and the slopes are 1.87 and 1.83 for the red emission, which are very close to 2.0 within the margin of error. Results indicate that the green ⁴S_3/2 → ⁴I_15/2 and red ⁴F_9/2 → ⁴I_15/2 transition in Yb³⁺/Er³⁺ co-doped NaLnTiO₄ (Ln = Y, Gd) are two-photon absorption process.

3.4 Lifetime measurements

According to above mentioned possible UC processes, the enhanced UC intensities with the addition of Yb³⁺ are attributed to the efficient energy transfer from Yb³⁺ to Er³⁺, and ET plays a dominant role in this process. In order to prove the existance of the ET between Yb³⁺ and Er³⁺, the emission decay curves of the green (⁴S_3/2 → ⁴I_15/2, 550 nm) for the samples NaY_0.95−xYb_xEr_0.05TiO₄: 0.05Er³⁺, xYb³⁺ (x = 0, 0.08.) with and without sensitizer Yb³⁺ were shown in Fig. 4 after 980 nm pulsed laser excitation, in which the decay profiles of the ⁴S_3/2 → ⁴I_15/2 (green) transitions are normalized. The lifetime could be calculated according to eqn (1):³²where I(t) is fluorescence intensity at time t. The decay curve can be well fitted with quadratic index equation, and the average lifetime can obtained through the following equation:³³

Fig. 4 The temporal evolution of the green ⁴S_3/2 → ⁴I_15/2 emissions in NaY_0.95−xYb_xEr_0.05TiO₄: 0.05Er³⁺, xYb³⁺ (x = 0, 0.08).

The corresponding lifetimes were listed in Fig. 4, it could be found that the lifetime for Yb³⁺–Er³⁺ co-doped NaY_0.87Yb_0.08Er_0.05TiO₄ is larger (84 μs) than that (36 μs) of Er³⁺ solely doped sample NaY_0.95Er_0.05TiO₄, which confirms the presence of ET from Yb³⁺ to Er³⁺ in present system.

3.5 Temperature sensing properties

As already mentioned, UC bands centered at 530 and 550 nm arise from ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions. The energy gap between the levels ²H_11/2 and ⁴S_3/2 of Er³⁺ is around 750 cm⁻¹, the state of ²H_11/2 may also be populated from ⁴S_3/2 by thermal excitation and the UC emission intensity ratio of emission band at 530 to 550 could change with the variable temperature, therefore it is could be used as optically temperature sensor for the present UC phosphors. In order to investigate the temperature sensing properties of synthesized phosphors, the green UC emission spectra for samples NaLnTiO₄: 0.05Er³⁺, 0.14Yb³⁺ (Ln = Y, Gd) at different temperature (from 300 to 510 K) were recorded and displayed in Fig. 5a and b, in which the spectra are normalized to the most intense emission peak at about 551 nm. In present cases, the excitation power density was estimated to be around 4W cm⁻², which is low enough to neglect the heating effect from the pumping source. It is found that no remarkable shift in emission wavelength for two samples while their fluorescence intensity ratio (FIR) of UC emission from ²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2 (550 nm) increases with the rise of temperature. The relative population of the thermally coupled energy levels follows the Boltzmann distribution and the FIR of two emissions can be written as follows:^34,35where I_H and I_S are intensities (the integrated areas below the emission curves) for the upper (²H_11/2 → ⁴I_15/2) and lower (⁴S_3/2 → ⁴I_15/2) thermally coupled levels transitions, respectively. N, g, σ, ω are the number of ions, the degeneracy, the emission cross-section, the angular frequency of fluorescence transitions from the ²H_11/2 and ⁴S_3/2 levels to the ⁴I_15/2 level, respectively. K is the Boltzmann constant, and ΔE is the energy gap between the ²H_11/2 and ⁴S_3/2 levels, T is absolute temperature and C is proportionality constant.

Fig. 5 Temperature dependence of the green UC luminescence spectra of NaYTiO₄: 0.14Yb³⁺, 0.05Er³⁺ (a) and NaGdTiO₄: 0.14Yb³⁺, 0.05Er³⁺ (b) under 980 nm excitation (the spectra are normalized to the most intense emission peak at about 551 nm).

Fig. 6a and b present the behaviors of FIR (I_H/I_S) for samples NaYTiO₄: 0.14Yb³⁺, 0.05Er³⁺ and NaGdTiO₄: 0.14Yb³⁺, 0.05Er³⁺ as a function of temperature, in which the fitting results analysized using eqn (3) are also included. It is found that the monolog of experimental FIR data are fitted as a straight line on the inverse of temperature from Fig. 6a. The slopes of two lines are about 1079 and 1061, and the energy gap ΔE could be calculated to about 783 and 765 cm⁻¹ in NaYTiO₄ and NaGdTiO₄, which is close to previous results in the range of 750–800 cm⁻¹.^36,37 The dependences of FIR on temperature are showed in Fig. 6b, the values of the coefficient C is about 8.55 and 8.85 by the best fit curves.

Fig. 6 Temperature depentent FIR (a), (b) and relative sensitivities (c) for NaYTiO₄:Er³⁺/Yb³⁺ (left column) and NaGdTiO₄:Er³⁺/Yb³⁺ (right column) phosphors.

The sensor sensitivity is an important reference parameter, which is defined as the rate of the FIR (R) changes with temperature and expressed using the following formula:³⁸

Fig. 6c shows the sensitivity (S) as a function of absolute temperature. With the increase of temperature, the sensitivity of Er³⁺–Yb³⁺ co-doped NaYTiO₄ reached the maximum about 0.0045 K⁻¹ at 510 K, but the sensitivity for Er³⁺–Yb³⁺ co-doped NaGdTiO₄ keep a constant after reach at maximum 0.0045 K⁻¹ at temperature of 480 K. It indicates that Yb³⁺–Er³⁺ ions co-doped NaLnTiO₄ (Ln = Y, Gd) phosphors could be used as potential candidates for optical temperature sensors based on the FIR technique.

4. Conclusions

The Yb³⁺–Er³⁺ ions co-doped NaLnTiO₄ (Ln = Y, Gd) up-conversion (UC) phosphors were prepared by sol–gel method, and the optimal preparation parameters for NaLnTiO₄ (Ln = Y, Gd) were also determined as 0.05Er³⁺, 0.14Yb³⁺ at 900 °C. Under 980 nm excitation, samples emit orange light and their UC spectra are composed of prominent green emission centered at 530, 550 and red emission peaked at 668 nm originating from ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The lifetimes of green emission centered at 550 nm, possible UC processes and mechanism investigations indicate that emission are two-photon processes and the efficient energy transfer from Yb³⁺ to Er³⁺ plays an important role in UC process. The dependences of FIR for the samples NaLnTiO₄: 0.05Er³⁺, 0.14Yb³⁺ (Ln = Y, Gd) with optimal compositions on temperature were measured in the range of 300–510 K, and the sensitivities of samples reach the same maximum 0.0045 K⁻¹ for NaYTiO₄ Er³⁺–Yb³⁺ at 510 K and NaGdTiO₄:Er³⁺–Yb³⁺ at 480 K, respectively. The results illuminate that the present UC phosphors could be used as promising candidates for optical temperature sensors.

Acknowledgements

This work was supported by the high-level talent project of Northwest University, National Natural Science Foundation of China (no. 11274251,11274299, 11374291), Ph.D. Programs Foundation of Ministry of Education of China (20136101110017), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (excellent), Natural Science Foundation of Shaanxi Province (no. 2014JM1004) and Foundation of Key Laboratory of Photoelectric Technology in Shaanxi Province (12JS094).

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