Ratiometric temperature sensing based on non-thermal coupling levels in BaZrO₃: Yb³⁺/Er³⁺ ceramics

1. Introduction

The optical temperature sensor based on upconversion (UC) emissions of rare earth (RE) ions doped materials have drawn great attention [1–8]. Compared with conventional temperature monitoring technique, this kind of temperature sensing technique which is achieved by fluorescence intensity ration of UC emission have many advantages, especially as noncontact, no effect the temperature field. Nowadays, the UC emission has been observed and studied in many RE doped materials. Among the RE ions, Er³⁺ doped luminescent materials are extensively explored for UC phosphors because of the ability of efficiently emit photons from visible to near infrared regions. Er³⁺ ion absorbed photons are easily populated in near-infrared (NIR) region due to the long lifetimes of the excited states and many radiative transitions between the electronic energy levels of Er³⁺ ions. Most of the previous thermometric material studies focus on the thermally coupling level-pairs (TCL) of Er³⁺ ions, ²H_11/2, ⁴S_3/2 levels and ⁴F_9/2(1), ⁴F_9/2(2) level [9–13], such as Wang et.al found that the thermal quenching ratio and temperature sensitivity from thermally coupled energy levels of Er³⁺ doped materials were dependent on the pump power [14, 15], while the non-thermal coupling levels are neglected to the influence of the temperature sensor dependent on pump power.

Lanthanide ions doped fluorides are very popular due to abundant energy levels for UC luminescence and a high luminescent quenching concentration. However, they are easily oxidized at high temperature. In recent years, ferroelectrics doped with RE has been a subject of research interest due to their excellent multifunctional properties, especially well chemical stability at high temperature [16–21]. Meanwhile, RE-doped lead-free ferroelectrics were found to be exhibited strong UC emissions, and the UC luminescent intensities were significantly dependent on temperature. BaZrO₃ is favorable doped divalent or trivalent RE ions due to typical cubic perovskite structure and it possesses some singular properties, including high thermal, chemical stability, wide band gap and so on [22–24]. During the past few years, many researchers have studied the synthesis, structural and photoluminescence properties of BaZrO₃ doped with rare-earth (Eu³⁺, Tb³⁺, Tm³⁺,Yb³⁺) [25–28]. The UC emission of BaZrO₃: Er³⁺ also have been reported about these studies [29–32]. However, limited attention has been paid to the temperature sensing characteristics of Er³⁺/Yb³⁺ co-doped BaZrO₃. Moreover, ceramic materials have good qualities, such as high temperature resistance, corrosion resistance, high strength and great hardness, easy to prepare and shape. To the best of our knowledge, no previous studies have reported the application of Er³⁺/Yb³⁺ co-doped BaZrO₃ ceramic for the temperature sensing characteristics.

In this work, BaZrO₃ doped with Er³⁺, Yb³⁺ powders were synthesized through the sol-gel method. Er³⁺/Yb³⁺ co-doped BaZrO₃ ceramics were prepared by dry pressing method. The influences of Er³⁺ ion concentration to luminescence properties are discussed. Through a 980 nm laser excitation, UC green, red and NIR luminescence had been acquired. For implement accurate high temperature measurement, an appropriate temperature measurement method was used. Three UC fluorescence emissions from BaZrO₃: Yb³⁺/Er³⁺ were investigated. The fluorescence intensity ratio (FIR) of non-thermal coupling level ⁴S_3/2,⁴I_9/2→⁴I_15/2, and ⁴F_9/2, ⁴I_9/2→⁴I_15/2 were studied as a function of pump power and temperature around the range of 300~500 K. The sensor sensitivity and accuracy were evaluated for non-thermal coupling level ⁴S_3/2,⁴I_9/2→⁴I_15/2, and ⁴F_9/2, ⁴I_9/2→⁴I_15/2. BaZrO₃: Er³⁺/Yb³⁺ demonstrated that great potential for use as high temperature sensors with high sensitivity and accuracy.

2. Experiment

2.1 Synthesis preparation of BaZrO₃ doped with Er³⁺, Yb³⁺

BaZrO₃ doped with Er³⁺, Yb³⁺ powders were synthesized through the sol-gel method. For instance BaEr_0.1Yb_0.15Zr_0.75O_3, 1.3067 g barium nitrate (Ba(NO₃)₂, Aladdin Reagent Inc., 99.5%), 1.6100 g zirconium nitrate pentahydrate (Zr(NO₃)₂·5H₂O, Aladdin Reagent Inc., 99.5%), 0.2217 g erbium nitrate pentahydrate (Er(NO₃)₃·5H₂O, Aladdin Reagent Inc., 99.9%) and 0.3368 g ytterbium nitrate pentahydrate (Yb(NO₃)₃·5H₂O, Aladdin Reagent Inc., 99.9%) were dissolved in deionized water and heated at 90 °C in a glass beaker. After the powers were completely dissolved, 1.9214 g CA (Kemiou Chemical Reagent Co., AR) and 2.1918 g EDTA (Kemiou Chemical Reagent Co., AR) were added. The pH of the resulting solution was adjusted to about 7 using ammonium hydroxide. Water was evaporated and the resultant gels were combusted to remove organic compounds at 250 °C for 2 h, which then was ignited to form fine powder. The product of the combustion reaction was calcined at 1100 °C for 4 h in air to obtain the final powders. The dried powders were compacted into disk samples under 200 MPa. All ceramics were sintered at 1600 °C for 6 h in air.

2.2 Characterization

X-ray diffraction (XRD) was employed to characterize the structures of the samples by an X-ray diffractometer (X’Pert PRO, PANalatical) using Cu Kα radiation (λ = 1.5405 Å, 40 kV, 40 mA). The scanning angle 2θ is from 10° to 90° with a step size of 0.02° at room temperature. The sample was excited by using 980 nm continuous wave laser source and the UC emission spectra were obtained by spectrometer (Ocean optics QE Pro).

3. Results and discussion

The XRD patterns of BaZrO₃: samples with various Er³⁺ ion concentrations are shown in Fig. 1. All the peak positions are in accord with the standard cubic phase of BaZrO₃ (JCPDS, No. 06-0399). It means that no other phases exist in the product and the Er³⁺, Yb³⁺ ions are completely dissolved into the host lattice of BaZrO₃. Figure 1(b) provides clear information about the crystal structure of the cubic BaEr_xYb_0.15Zr_0.85-xO₃ perovskite structure, in which the larger cation Ba located at its center, the smaller cation Zr located at the corners and the anions O located at the center of the edges of the cubic. The doped Er and Yb ions occupy the Zr cation sites. The materials crystallize in the cubic symmetry in the space group of Pm-3m.

 

Fig. 1 (a). The XRD patterns of the BaZrO₃-xEr, 15mol%Yb ceramic doped with different Er³⁺ ion concentrations; (b). Schematic illustration of the cubic BaEr_xYb_0.15Zr0_.85-xO₃ perovskite structure.

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Under an excitation wavelength of 980 nm, UC emission spectra of the BaZrO₃-xEr, 15mol%Yb³⁺ ceramic with different Er³⁺ concentrations at room temperature are presented in Fig. 2(a). The pumping power is set as 90 mW. The UC emission exhibits very bright red which can be easily observed by the naked eye. Compared to other Er³⁺ doped photoluminescence materials, it is found that the UC emission contains four parts: an intense red emission band at 640~700 nm corresponds to ⁴F_9/2→⁴I_15/2 transitions, as shown in Fig. 2(b); ⁴S_3/2→⁴I_15/2 transition achieved a relatively weak green emission band at 546 nm; a weak green emission band at 526 nm is ascribed to ²H_11/2→⁴I_15/2 transitions; and a weak NIR emission band at 800~880 nm is the ⁴I_9/2→⁴I_15/2 transitions of Er³⁺ ions . The green, red and NIR UC luminescence intensities increase as the value of x increases, but the green reaches its maximum at x = 0.05, while the red and NIR reaches its maximum at x = 0.07, then the UC emission intensity decreases with further increasing the concentration of Er³⁺ doping. The phenomenon may be ascribed to concentration quenching effect.

 

Fig. 2 (a) UC emission spectra of the BaZrO₃-xEr, 15mol%Yb ceramic doped with different Er³⁺ concentrations under the 980 nm excitation. (b) Energy-level diagrams and proposed UC energy transfer pathways in the Yb³⁺-Er³⁺

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To investigate the influence of laser power for emission intensity, the UC emission spectra of the BaZrO₃: 15mol%Yb³⁺/5mol%Er³⁺ ceramic were examined as a function of pump power. Under 980 nm excitation, Yb³⁺ absorb the pump photon and undergo the ²F_7/2→²F_5/2 transition. The excited Yb³⁺ ions donate as-absorbed energy to adjacent Er³⁺ ions resonantly, promoting Er³⁺ ions to generate the ⁴I_15/2→⁴I_11/2, ⁴I_11/2→⁴I_9/2, ⁴I_13/2→⁴F_9/2, ⁴I_11/2→⁴F_7/2 upward transitions, and then nonradiative relaxations to the ²H_11/2, ⁴S_3/2,⁴F_9/2 and ⁴I_9/2, giving the green, red and NIR emission. Figure 3 shows log–log plots of up-conversion intensity and pumping power for green, red, and NIR emissions. In general, for an unsaturated UC process, the UC emission intensity depends on the excitation power and obey to the relationship I∝Pⁿ, where I is the UC intensity, P is the pump laser power and n is the number of laser photons involved to populate the upper emitting levels and can be determined by the slope value. The values of slope were 1.85, 1.51, 2.47, 2.27 and 0.45 for UC emission peaks at 526, 546, 658, 679 and 875nm, respectively. These results indicate that the two green and two red emissions come from two photon UC process and NIR emission comes from one photon UC process. These results were consistent with previously reported data for Er³⁺ doped Sr_0.69La_0.31F_2.31 [15].

 

Fig. 3 Log–log plots of intensity and pumping power for 526 nm, 546 nm, 658 nm, 679 nm, and 875 nm emissions at room temperature

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In order to investigate the influence of temperature and pump power for photoluminescence properties, the UC emission spectra of 15mol% Yb³⁺/ 5mol% Er³⁺ co-doped BaZrO₃ ceramic were measured as a function of temperature at the range from 300 K to 500 K. The heat effect caused by the excitation power should be excluded in our experimental process. Figure 4 shows UC emission spectra under different pump power at temperature of 350 K. From the spectra, the power density of 115mW at the emission band of ⁴F_2/9→⁴I_15/2 transitions is slightly higher than the others. However, there are no obvious difference at 75 mW and 98 mW. It is indicated that the heating effect can be neglected when the power is not larger than 98mW. The pumping power of 980 nm laser is set as 90 mW and 65 mW, respectively, as shown in Fig. 5. It can clearly observed that the red and green emission noticeably decrease with the increasing temperature, but there is hardly change for the NIR emission in 875 nm at 90 mW excitation power in Fig. 5(a), while under 65 mW excitation power, the red, green and NIR emission obviously decrease with the increasing temperature, as shown in Fig. 5(b). Based on this finding, the non-thermal coupling levels red-NIR FIR (I_C/I_E and I_D/I_E) and green-NIR FIR (I_A/I_E and I_B/I_E) are more suitable for use as a function of temperature around the high temperature relevant range than the TCL green-green and red-red. Due to the similar intensity of I_C/I_E and I_D/I_E as well as I_A/I_E and I_B/I_E, the non-thermal coupling levels of I_C/I_E and I_B/I_E were considered as temperature sensing.

 

Fig. 4 UC spectra of the BaZrO₃: 15mol%Yb³⁺/5mol%Er³⁺ ceramic under different pump power at 350 K.

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Fig. 5 UC emission spectra of the BaZrO₃: 5 mol% Er³⁺, 15 mol% Yb³⁺ ceramic at various temperatures (peak A: 526 nm, peak B: 546nm, peak C: 658 nm, peak D: 679nm, peak E: 875 nm) (a) 90 mW excitation power. (b) 65 mW excitation power.

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The FIR as a function of temperatures in the range of 300~500 K were calculated and plotted in Fig. 6 with different pump power. The experimental data were well linear fitted. Wang et.al proposed a linear equation to establish the relation between fluorescence intensity ratios and temperature [15]. FIR of their non-thermally coupling levels can be expressed as:

 

Fig. 6 The fluorescence intensity ratio as a function of temperature in the range of 300~500 K under 980 nm excitation. (a) 90 mW excitation power. (b) 65 mW excitation power.

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Considering practical applications of the Yb³⁺/Er³⁺ co-doped BaZrO₃ ceramic, it is necessary to understand the optical temperature sensing behavior. For ratiometric luminescent thermometry, the relative sensitivity S_r can be expressed as [2]:

Figure 7 shows the relative sensitivity S_r at temperature range 300 K to 500 K. The S_r increase with the increasing temperature, which S_r of C vs E is always greater than B vs E. The maximum value of the relative sensitivity of the prepared material has been achieved ∼1.39% K⁻¹ and ~1.22% K⁻¹ in non-thermal coupling levels I_C/I_E and I_B/I_E at 500 K with 90 mW excitation power, respectively, as shown in Fig. 7(a). Under 65 mW excitation power, the maximum value of relative sensitivity is 0.64% K⁻¹ and 0.46% K⁻¹ for I_C/I_E and I_B/I_E at 500 K, respectively, as shown in Fig. 7(b). The result demonstrated I_C/I_E emission is more suitable for temperature sensing than the I_B/I_E at temperature range from 300 K to 500 K and high pump power is more sensitive than low. The sensor sensitivity for different samples are compared and listed in Table 1. The result illustrated that the Yb³⁺/Er³⁺ co-doped BaZrO₃ ceramic is promising candidate to be used as an efficient optical temperature sensor.

 

Fig. 7 Relative sensitivity for BaZrO₃: 15mol%Yb³⁺/5mol%Er³⁺ at (a) pump power 90 mW, (b) 65 mW at temperatures from 300 to 500 K.

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Table 1. comparative data table for sensor sensitivity for different samples

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The accuracy of temperature measurement is critical for temperature sensing in practical applications. The measurement error (ΔT) is a significant factor for evaluating temperature sensor. From the Eq. (1), the relation between ΔT and ΔFIR is described as:

4. Conclusion

In summary, BaZrO₃ doped with Er³⁺, Yb³⁺ ceramics were fabricated to investigate temperature sensing properties at range from 300~500 K. The sample possesses many advantages as an optical temperature sensor, such as excellent red color luminescence intensity, higher sensitivity, and accuracy for wide range temperature detection. The temperature relative sensitivity of our sample reaches 1.39% K⁻¹ for non-thermal coupling levels ⁴F_9/2, ⁴I_9/2→⁴I_15/2 at 500K under 90 mW excitation power. It mean that the BaZrO₃: Er³⁺, Yb³⁺ can achieve higher sensitivity and accuracy, as well as it could easy be used as a temperature sensor.