Open Access
Add to BibSonomy
Abstract
High sensing sensitivity and accuracy is valuable for optical temperature sensors in practical applications. Er3+/Yb3+ co-doped BaZrO3 ceramics were prepared by the dry pressing method. The upconversion luminescence of Er3+/Yb3+ co-doped BaZrO3 ceramics was studied at a temperature range from 300 to 500 K under different pump power. The ratio of non-thermal coupling levels of 4S3/2, 4I9/2→4I15/2 (I546/I875), and 4F9/2, 4I9/2→4I15/2 (I658/I875) show a linear relationship as the temperature increased. The maximum relative sensitivity was estimated to be 1.39% K−1 at 500 K under 90 mW pump power. The result indicated that the BaZrO3:Er3+/Yb3+ ceramic is a promising candidate for high temperature sensing applications.
© 2017 Optical Society of America
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, 2H11/2, 4S3/2 levels and 4F9/2(1), 4F9/2(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 4S3/2,4I9/2→4I15/2, and 4F9/2, 4I9/2→4I15/2 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 4S3/2,4I9/2→4I15/2, and 4F9/2, 4I9/2→4I15/2. 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 BaEr0.1Yb0.15Zr0.75O3, 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 BaErxYb0.15Zr0.85-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 BaErxYb0.15Zr0.85-xO3 perovskite structure.
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 4F9/2→4I15/2 transitions, as shown in Fig. 2(b); 4S3/2→4I15/2 transition achieved a relatively weak green emission band at 546 nm; a weak green emission band at 526 nm is ascribed to 2H11/2→4I15/2 transitions; and a weak NIR emission band at 800~880 nm is the 4I9/2→4I15/2 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+
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 2F7/2→2F5/2 transition. The excited Yb3+ ions donate as-absorbed energy to adjacent Er3+ ions resonantly, promoting Er3+ ions to generate the 4I15/2→4I11/2, 4I11/2→4I9/2, 4I13/2→4F9/2, 4I11/2→4F7/2 upward transitions, and then nonradiative relaxations to the 2H11/2, 4S3/2,4F9/2 and 4I9/2, 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 Sr0.69La0.31F2.31 [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
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 4F2/9→4I15/2 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 (IC/IE and ID/IE) and green-NIR FIR (IA/IE and IB/IE) 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 IC/IE and ID/IE as well as IA/IE and IB/IE, the non-thermal coupling levels of IC/IE and IB/IE were considered as temperature sensing.
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.
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:
where IU/L are luminescent intensity from upper and lower, a and b are constants, T is the absolute temperature. The ratio IC/IE increases more obviously with temperature than the ratio IB/IE. It is indicated that the phenomenon is a stronger difference between the temperature dependencies of peaks C and E compared to the difference between peaks B and E.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.
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 Sr can be expressed as [2]:
Figure 7 shows the relative sensitivity Sr at temperature range 300 K to 500 K. The Sr increase with the increasing temperature, which Sr 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 IC/IE and IB/IE 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 IC/IE and IB/IE at 500 K, respectively, as shown in Fig. 7(b). The result demonstrated IC/IE emission is more suitable for temperature sensing than the IB/IE 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.
Table 1. comparative data table for sensor sensitivity for different samples
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:
According to Eq. (3), the parameter ΔT is related to the value of at signal division circuitry with the same precision, that is, the larger the value of a, the smaller the ΔT. A much smaller error can be expected at C vs E than B vs E. Therefore, non-thermal coupling levels 4F9/2, 4I9/2→4I15/2 is best suited for temperature sensing in our sample.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 4F9/2, 4I9/2→4I15/2 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.
Funding
National Key Laboratory of Tunable Laser Technology, National Natural Science Foundation of China (NSFC) (No. 61078006 and No. 61275066); National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No.2012BAF14B11).
References and links
1. A. Pandey and V. K. Rai, “Improved luminescence and temperature sensing performance of Ho3+-Yb3+-Zn2+:Y2O3 phosphor,” Dalton Trans. 42(30), 11005–11011 (2013).
2. Y. Cui, R. Song, J. Yu, M. Liu, Z. Wang, C. Wu, Y. Yang, Z. Wang, B. Chen, and G. Qian, “Dual-emitting MOF⊃Dye composite for ratiometric temperature sensing,” Adv. Mater. 27(8), 1420–1425 (2015).
3. D. Chen, Z. Wan, Y. Zhou, X. Zhou, Y. Yu, J. Zhong, M. Ding, and Z. Ji, “Dual-phase glass ceramic: structure, dual-modal luminescence, and temperature sensing behaviors,” ACS Appl. Mater. Interfaces 7(34), 19484–19493 (2015).
4. X. Wang, Q. Liu, Y. Bu, C.-S. Liu, T. Liu, and X. Yan, “Optical temperature sensing of rare-earth ion doped phosphors,” RSC Advances 5(105), 86219–86236 (2015).
5. D. Das, S. L. Shinde, and K. K. Nanda, “Temperature-Dependent Photoluminescence of g-C3N4: implication for temperature sensing,” ACS Appl. Mater. Interfaces 8(3), 2181–2186 (2016).
6. Z. Cao, X. Wei, L. Zhao, Y. Chen, and M. Yin, “Investigation of SrB4O7:Sm2+ as a multimode temperature sensor with high sensitivity,” ACS Appl. Mater. Interfaces 8(50), 34546–34551 (2016).
7. T. Wei, Z. Dong, C. Zhao, Y. Ma, T. Zhang, Y. Xie, Q. Zhou, and Z. Li, “Up-conversion luminescence and temperature sensing properties in Er-doped ferroelectric Sr2Bi4Ti5O18,” Ceram. Int. 42(4), 5537–5545 (2016).
8. R. Cao, X. Ceng, J. Huang, X. Xia, S. Guo, and J. Fu, “A double-perovskite Sr2ZnWO6: Mn4+ deep red phosphor: Synthesis and luminescence properties,” Ceram. Int. 42(15), 16817–16821 (2016).
9. F. Vetrone, R. Naccache, A. Zamarrón, A. Juarranz de la Fuente, F. Sanz-Rodríguez, L. Martinez Maestro, E. Martín Rodriguez, D. Jaque, J. García Solé, and J. A. Capobianco, “Temperature sensing using fluorescent nanothermometers,” ACS Nano 4(6), 3254–3258 (2010).
10. D. Chen, M. Xu, and P. Huang, “Core@ shell upconverting nanoarchitectures for luminescent sensing of temperature,” Sens. Actuators B Chem. 231, 576–583 (2016).
11. S. P. Tiwari, M. K. Mahata, K. Kumar, and V. K. Rai, “Enhanced temperature sensing response of upconversion luminescence in ZnO-CaTiO3: Er3+/Yb3+ nano-composite phosphor,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 150, 623–630 (2015).
12. B. S. Cao, Y. Y. He, Z. Q. Feng, Y. S. Li, and B. Dong, “Optical temperature sensing behavior of enhanced green upconversion emissions from Er–Mo:Yb2Ti2O7 nanophosphor,” Sens. Actuators B Chem. 159(1), 8–11 (2011).
13. A. Pandey, V. K. Rai, V. Kumar, V. Kumar, and H. C. Swart, “Upconversion based temperature sensing ability of Er3+–Yb3+codoped SrWO4: An optical heating phosphor,” Sens. Actuators B Chem. 209, 352–358 (2015).
14. X. Wang, Y. Wang, J. Marques-Hueso, and X. Yan, “Improving optical temperature sensing performance of Er3+ Doped Y2O3 microtubes via co-doping and controlling excitation power,” Sci. Rep. 7(1), 758 (2017).
15. X. Wang, Q. Liu, P. Cai, J. Wang, L. Qin, T. Vu, and H. J. Seo, “Excitation powder dependent optical temperature behavior of Er3+ doped transparent Sr0.69La0.31F2.31 glass ceramics,” Opt. Express 24(16), 17792–17804 (2016).
16. X. Wang, Y. Bu, X. Yan, P. Cai, J. Wang, L. Qin, T. Vu, and H. J. Seo, “Detecting the origin of luminescence in Er3+-doped hexagonal Na1.5Gd1.5F6 phosphors,” Opt. Lett. 41(22), 5314–5317 (2016).
17. P. Du, L. Luo, W. Li, Q. Yue, and H. Chen, “Optical temperature sensor based on upconversion emission in Er-doped ferroelectric 0.5 Ba (Zr0. 2Ti0. 8) O3-0.5 (Ba0. 7Ca0. 3) TiO3 ceramic,” Appl. Phys. Lett. 104(15), 152902 (2014).
18. H. Zou, X. Wang, Y. Hu, X. Zhu, Y. Sui, and Z. Song, “Optical temperature sensor through upconversion emission from the Er3+ Doped SrBi8Ti7O27 ferroelectrics,” J. Electron. Mater. 45(6), 2745–2749 (2016).
19. D. Junli, D. Peng, X. Jiadan, X. Chaoxiang, and L. Laihui, “Piezoelectric and upconversion emission properties of Er3+-doped 0.5 Ba (Zr0.2Ti0.8) O3− 0.5 (Ba0.7Ca0.3)TiO3 ceramic,” J. Rare Earths 33(4), 391–396 (2015).
20. Y. Zhao, Y. Ge, X. Zhang, Y. Zhao, H. Zhou, J. Li, and H. Jin, “Comprehensive investigation of Er2O3 doped (Li, K, Na) NbO3 ceramics rendering potential application in novel multifunctional devices,” J. Alloys Compd. 683, 171–177 (2016).
21. P. Du, L. Luo, and J. S. Yu, “Low-temperature thermometry based on upconversion emission of Ho/Yb-codoped Ba0.77Ca0.23TiO3 ceramics,” J. Alloys Compd. 632, 73–77 (2015).
22. L. R. Macario, M. L. Moreira, J. Andres, and E. Longo, “An efficient microwave-assisted hydrothermal synthesis of BaZrO3 microcrystals: growth mechanism and photoluminescence emissions,” CrystEngComm 12(11), 3612–3619 (2010).
23. I. Grinberg and A. M. Rappe, “Silver solid solution piezoelectrics,” Appl. Phys. Lett. 85(10), 1760–1762 (2004).
24. C. Shi, M. Yoshino, and M. Morinaga, “First-principles study of protonic conduction in In-doped AZrO3 (A=Ca, Sr, Ba),” Solid State Ion. 176(11-12), 1091–1096 (2005).
25. X. Liu and X. Wang, “Preparation and luminescence properties of BaZrO3:Eu phosphor powders,” Opt. Mater. 30(4), 626–629 (2007).
26. R. Borja-Urby, L. A. Diaz-Torres, P. Salas, M. Vega-Gonzalez, and C. Angeles-Chavez, “Blue and red emission in wide band gap BaZrO3:Yb3+,Tm3+,” Mater. Sci. Eng. B 174(1-3), 169–173 (2010).
27. B. Marí, K. C. Singh, M. Sahal, S. P. Khatkar, V. B. Taxak, and M. Kumar, “Preparation and luminescence properties of Tb3+ doped ZrO2 and BaZrO3 phosphors,” J. Lumin. 130(11), 2128–2132 (2010).
28. J. Oliva, E. D. Rosa, L. A. Diaz-Torres, P. Salas, and C. Ángeles-Chavez, “Annealing effect on the luminescence properties of BaZrO3:Yb3+ microcrystals,” J. Appl. Phys. 104(2), 023505 (2008).
29. V. Singh, V. K. Rai, K. Al-Shamery, M. Haase, and S. H. Kim, “NIR to visible frequency upconversion in Er3+ and Yb3+ co-doped BaZrO3 phosphor,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 108, 141–145 (2013).
30. L. A. Diaz-Torres, P. Salas, J. S. Perez-Huerta, C. Angeles-Chavez, and E. De la Rosa, “A new blue, green and red upconversion emission nanophosphor: BaZrO3:Er,Yb,” J. Nanosci. Nanotechnol. 8(12), 6425–6430 (2008).
31. L. Guo, C. Zhong, X. Wang, and L. Li, “Synthesis and photoluminescence properties of Er3+ doped BaZrO3 nanotube arrays,” J. Alloys Compd. 530, 22–25 (2012).
32. L. A. Diaz-Torres, P. Salas, J. Oliva, E. D. Rosa, C. Angeles-Chavez, and V. M. Castaño, “NaOH–controlled upconversion of nanocrystalline BaZrO3:Er,Yb phosphor,” Int. J. Nanotechnol. 10(12), 1055–1063 (2013).
33. D. Chen, W. Xu, Y. Zhou, and Y. Chen, “Lanthanide doped BaTiO3SrTiO3 solid-solution phosphors: Structure, optical spectroscopy and upconverted temperature sensing behavior,” J. Alloys Compd. 676, 215–223 (2016).
34. S. Zhou, G. Jiang, X. Li, S. Jiang, X. Wei, Y. Chen, M. Yin, and C. Duan, “Strategy for thermometry via Tm3+-doped NaYF4 core-shell nanoparticles,” Opt. Lett. 39(23), 6687–6690 (2014).
35. A. F. Pereira, K. U. Kumar, W. F. Silva, W. Q. Santos, D. Jaque, and C. Jacinto, “Yb3+/Tm3+ co-doped NaNbO3 nanocrystals as three-photon-excited luminescent nanothermometers,” Sens. Actuators B Chem. 213, 65–71 (2015).
