Open Access
Add to BibSonomyAbstract
The fluorescence intensity ratio (FIR) method is a non-contact temperature (T) measurement technique based on thermally coupled levels of rare earth ions in a doped host. Green fluorescence originating from 2H11/2 and 4S3/2 states of Er3+ doped K0.5Na0.5NbO3 (KNN) ceramic are studied in the temperature range of 300 K to 720 K. The fluorescence intensities change dramatically around phase transition points where the crystal symmetry changes, inducing deviation of the FIR from Boltzmann’s law. The temperature determined by the FIR method deviates from thermocouple measurements by 7 K at the orthorhombic to tetragonal phase transition (TO-T) point and 13 K at the Curie point (TC). This finding gives guidance for developing fluorescent T sensors with ferroelectrics and may also provide a fluorescent method to detect phase transitions in ferroelectric materials.
© 2016 Optical Society of America
1. Introduction
Due to their unique electron structure and their resulting optical, electric, and magnetic properties, rare earth (RE) ions have been central in the development of new materials [1–3]. Ferroelectrics are materials, which have good mechanical-electric coupling, have been widely used in medical and military applications [4–6]. Recently, researchers have doped RE ions into ferroelectrics to create new multi-functional materials [7,8]. There are two exciting developments in this field: using the fluorescence ratio method (FIR) for temperature (T) measurement [9–11] and using fluorescence variation to monitor temperature or stress-induced phase transitions [12–15].
To the best of our knowledge, researchers developing temperature sensors with ferroelectrics tend to ignore the influence of phase transitions and make relatively coarse detection in a wide temperature range. They depend on the fact that usually the widely discussed thermally coupled levels such as 2H11/2 and 4S3/2 states of Er3+ ions still follow Boltzmann’s law in this situation [11]. Meanwhile, researchers investigating the fluorescence near phase transitions mainly focused on the single peak intensity variation with temperature. In this situation, they seldom discuss the different peaks’ variation and FIR around phase transition points [13]. Up to now, the study of the influence of phase transitions on FIR has been overlooked. However, it is necessary to find out whether the FIR follows Boltzmann’s law or deviates from it around phase transition points.
As one of the most studied upconversion (UC) fluorescence RE ions, Er3+ has been extensively used for FIR thermometry [16–18]. The 2H11/2 and 4S3/2 states are the most frequently discussed thermally coupled levels (TCLs) in various kinds of hosts, including ferroelectrics [11,19]. K0.5Na0.5NbO3 (KNN) ceramics are lead-free ferroelectrics, which have drawn considerable attention as they are environmentally friendly and have other intriguing properties such as high polarization and excellent temperature stability [20,21]. KNN goes from an orthorhombic phase to a tetragonal phase around 480 K and then enters a paraelectric cubic phase at around 680 K. In this work, we chose Er3+ as the photoluminescence center and doped it into a KNN ceramic material. We investigated the single peak intensity and the FIR of two fluorescence peaks originating from the 2H11/2 and 4S3/2 states to ground states through the temperature range from 300 K to 720 K. Two phase transitions occur in this process. We measured the dielectric temperature spectrum to find tetragonal phase transition (TO-T) and Currie point (TC). To verify that doping does not influence the ferroelectric properties, we checked the hysteresis loop of the doped KNN.
2. Methods
The 1 mol% Er3+-doped KNN ceramic was prepared using the high temperature solid-state reaction method. Stoichiometric K2CO3, Na2CO3, Nb2O5, and Er2O3 were accurately weighted and used as raw materials. Then, they were mixed and ground in a motor for 3 h until uniform. The resulting mixture was pressed to form disks and sintered at 880 °C for 4 h. Then, it was pulverized and mixed with 5% PVA. After prilling, particles of the correct size were selected and the pre-burning ceramic powder was pressed into a 13-mm diameter disk. The dumping temperature was set at 550 °C for 2 h, and the material was then sintered at 1150 °C for 4 h. The resulting samples were prepared in two ways: either coated with a silver electrode for electric testing or polished for fluorescence measurement.
The crystal structure of the ceramic was identified by X-ray diffraction (XRD) with an automated Rigaku D/max 2400 X-ray diffractometer using CuKa radiation. The hysteresis behavior was studied at room temperature using a Precision Premier II Ferroelectric Tester. The dielectric constant of the ceramic was measured using an automated measuring system with an Agilent E4980A impedance analyzer. The PL spectra were measured at temperatures from 300 K to 720 K with a Zolix Omni-λ 300 spectrometer under excitation from a 980 nm diode laser. The temperature was controlled by a homemade furnace. The PL measurement are done with the sample of diameter 13 mm, and thickness polished to less than 0.05 mm. The heating rate is 1 K per minute. The excitation photon flux density is 1 W/cm−1 which is small enough to avoid laser-induced thermal effect. At a given temperature the sample is heated for 10 minutes before acquiring the PL measurement. The time is totally sufficient to guarantee steady-state conditions and thermal equilibrium conditions. The signal to noise ratio has an average value of 3500 so the slight intensity change is trustable.
3. Results and discussions
The XRD pattern of the prepared sample is shown in Fig. 1. The diffraction peak is easily identified as associated with the orthorhombic phase KNbO3 structure (PDF#71-2171). The slight doping of Er3+ ions does not change the crystal structure. Figure 2 shows the hysteresis loop as a function of the electric field. The remnant polarization (Pr) is 14 µC/cm2, and the maximum polarization (Pmax) is less than 20 µC/cm2. The doping does not significantly change the coercive field (Ec), which is constant at 11 kV/cm. Based on the above results, we concluded that the Er3+ doped KNN ceramic was well prepared and could be used for subsequent tests. Figure 3 shows the variation of dielectric constant with temperature. The data demonstrate that the sample goes from the orthorhombic to tetragonal phase, and the tetragonal to cubic phase at TO-T (478 K) and TC (678 K), respectively.
We recorded the fluorescence between 510 nm and 580 nm across the temperature range of 300 K to 720 K. Fluorescence spectra at 420 K, 570 K, and 720 K are depicted in Fig. 4(a). The three spectra are recorded at different phase structures of orthorhombic, tetragonal, and cubic phases. The FIR of the three spectra is smaller than, equal to, and larger than 1, respectively. We can recognize two main fluorescence bands peaked at 525 nm and 555 nm, which originate from the 2H11/2 and 4S3/2 states to ground states, respectively. Both of the transitions are Stark split into several sub-peaks due to the host lattice field. The peak intensities are integrated area values with a line width of nearly 17 nm as shown in Fig. 4(a). The energy level diagram and the UC mechanism are shown in Fig. 4(b). The 980-nm diode laser excitation pumps the Er3+ ions in the KNN ceramic from the ground state to the first excited state 4I11/2 (GSA). Energy transfer (ET) with another Er3+ ion transitions to the 4F7/2 excited state. The transition to the 4F7/2 state can also occur by absorption of another laser photon through excited state absorption (ESA). Soon the Er3+ ions in the 4F7/2 state drop down to the 2H11/2 and 4S3/2 states through a non-radiative relaxation process. The excited ions then emit green fluorescence through radiative transitions of the 2H11/2 and 4S3/2 states to ground states.
Fig. 4 (a). Fluorescence spectrum originated from 2H11/2 and 4S3/2 states to ground state in Er3+ doped BCT ceramic excited with a 980 nm diode laser at 373 K; (b) Upconversion mechanism of 2H11/2 and 4S3/2 states of the sample excited with a 980 nm diode laser.
Before discussing the FIR of the green fluorescence in phase transition points, we take an overall look at the temperature range of 300 K to 720 K. Fig. 5 demonstrates the total variation trend of the FIR of the 525-nm and 555-nm peaks. So as to work as a temperature sensor, the experimental data must be fitted to Boltzmann’s law as follows:
where I525nm and I555nm are the peak intensities of the 525 nm and 555 nm fluorescence bands, respectively, A is a constant, B is the offset factor, ΔE is the energy difference between the TCLs, kB is the Boltzmann constant, and T is the absolute temperature. From Fig. 5, we know the experimental data can be fitted well by Boltzmann’s law. A and B are 12.3 and 0.33, respectively. The experimental fitted ΔE is 1100 cm−1, which corresponds to a theoretical value of 1030 cm−1. So far, the 2H11/2 and 4S3/2 states in Er3+ doped KNN ceramic seem suited for fluorescent thermometry by using the FIR method. The relative sensitivity Sr is defined ashere we achieve a relative big Sr of 0.4% at 450 K, and the average Sr value considering all temperature range is about 0.3%.Fig. 5 FIR variation of 525 nm and 555 nm peak fluorescence in wide temperature range of 300 K to 750 K.
We then perform a detailed study around TO-T. Fig. 6(a) plots the FIR of the 525 nm and 555 nm peaks. However, the result deviates from the Boltzmann distribution around TO-T. Fig. 6(b) shows the peak intensity variation of the 2H11/2 and 4S3/2 states to ground state transitions. The continuous decrease in intensity is mainly ascribed to thermal quenching with rising temperature. As we can see, the peak intensity changes markedly in the phase transition process. Although variations in the 525 nm peak intensity are difficult to observe, there is a clear bridge-shaped fluorescence variation around TO-T for the 555 nm peak intensity. This could only be induced by changes in the crystal symmetry. The influence of the phase transition on the 525 nm and 555 nm peaks is the major cause of the FIR deviation from Boltzmann’s law.
Fig. 6 (a)FIR variation of 525 nm and 555 nm peak fluorescence around TO-T; (b) Intensity variation of 525 nm and 555 nm peak fluorescence around TO-T.
The fluorescence intensity and FIR behave around TC as they do around TO-T. As shown in Fig. 7(a), the FIR of the green peaks again deviates from the Boltzmann distribution. As shown in Fig. 7(b), the peak intensities at 525 nm and 555 nm increase regularly until TC. This increase is the result of energy transfer enhancement between Er3+ ions. Although thermal quenching intensifies with rising temperature, the ET process is also enhanced, thus the intensity increases [22]. When the phase structure goes through tetragonal to cubic the fluorescence intensity decreases. The crystal is more symmetric after this transition, and the Er3+ fluorescence tends to decrease in a more symmetric environment. After the host ceramic completely enters the paraelectric cubic phase, the thermal quenching process outcompetes energy transfer, thus the intensity again decreases.
Fig. 7 (a). FIR variation of 525 nm and 555 nm peak fluorescence around TC; (b) Intensity of 525 nm and 555 nm peak fluorescence around TC.
Table 1 shows the deviation of the FIR-derived temperature value from Boltzmann’s law at TO-T, TC, and at 580 K, which is away from TO-T and TC. Using FIR methods, the derived temperature value can deviate from its real value by 7 K at TO-T and 13 K at TC. We attribute the larger deviation around TC to the more dramatic crystal change around TC. The small deviation of 0.2 K far from the phase transitions, at 580 K, can be ascribed to measurement uncertainty. Deviations at TO-T and TC are far larger than the measurement uncertainties. The experiment is repeated for many cycles, and the results are based on the average experimental data, thus the deviation could not be an experimental error. From Fig. 6, Fig. 7, and Table 1, we conclude that the peak intensity is sensitive to the phase structure of the KNN host and will change irregularly around TO-T and TC. The phase transitions affect different fluorescence peaks differently, and the thermally coupled excited energy levels are also affected, thus the FIR deviates from the Boltzmann distribution. Temperature measurement using the FIR method is affected in Er3+-doped KNN ceramic at TO-T and TC; experimental work in other ferroelectrics with other RE ions is necessary to reach a more comprehensive conclusion.
Table 1. Deviant temperature value from Boltzmann law around TO-T and TC.
4. Conclusion
In summary, the traditional TCL of 2H11/2 and 4S3/2 states of Er3+ ions that is often used for FIR thermometry exhibits deviations from the Boltzmann distribution in a ferroelectric KNN host. This fluorescent temperature measurement method is accurate over a wide temperature range; however, as phase transitions influence the separate peaks in distinct ways, the FIR deviates from the Boltzmann distribution around TO-T and TC. Although this may make the FIR temperature measurement unsuitable for ferroelectrics at all temperatures, it may offer promise as a non-contact method to detect phase transitions in these materials.
Funding
National Key Basic Research Program (973 Program, Grant No. 2013CB632900); Natural Science Foundation of China (61505045); China Postdoctoral Science Foundation funded project (2014M561342); Fundamental Research Funds for the Central Universities and PIRS of HIT (B201415).
References and links
1. I. D. Hughes, M. Däne, A. Ernst, W. Hergert, M. Lüders, J. Poulter, J. B. Staunton, A. Svane, Z. Szotek, and W. M. Temmerman, “Lanthanide contraction and magnetism in the heavy rare earth elements,” Nature 446(7136), 650–653 (2007). [CrossRef] [PubMed]
2. A. Volpi, A. Di Lieto, and M. Tonelli, “Novel approach for solid state cryocoolers,” Opt. Express 23(7), 8216–8226 (2015). [CrossRef] [PubMed]
3. A. Rosenflanz, M. Frey, B. Endres, T. Anderson, E. Richards, and C. Schardt, “Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides,” Nature 430(7001), 761–764 (2004). [CrossRef] [PubMed]
4. J. F. Scott, “Applications of Modern Ferroelectrics,” Science 315(5814), 954–959 (2007). [CrossRef] [PubMed]
5. C. M. Krowne, S. W. Kirchoefer, W. Chang, J. M. Pond, and L. M. B. Alldredge, “Examination of the possibility of negative capacitance using ferroelectric materials in solid state electronic devices,” Nano Lett. 11(3), 988–992 (2011). [CrossRef] [PubMed]
6. D. Niermann, C. P. Grams, P. Becker, L. Bohatý, H. Schenck, and J. Hemberger, “Critical slowing down near the multiferroic phase transition in MnWO4.,” Phys. Rev. Lett. 114(3), 037204 (2015). [CrossRef] [PubMed]
7. J. Hao, Y. Zhang, and X. Wei, “Electric-induced enhancement and modulation of upconversion photoluminescence in epitaxial BaTiO3:Yb/Er thin films,” Angew. Chem. Int. Ed. Engl. 50(30), 6876–6880 (2011). [CrossRef] [PubMed]
8. X. Wang, C. Xu, H. Yamada, K. Nishikubo, and X. Zheng, “Electro-mechano-optical conversions in Pr3+-doped BaTiO3–CaTiO3 ceramics,” Adv. Mater. 17(10), 1254–1258 (2005). [CrossRef]
9. Z. Liang, F. Qin, Y. Zheng, Z. Zhang, and W. Cao, “Noncontact thermometry based on downconversion luminescence from Eu3+ doped LiNbO3 single crystal,” Sensor. Actuat. A-Phys. 238, 215–219 (2016).
10. P. Du, L. Luo, W. Li, Q. Yue, and H. Chen, “Optical temperature sensor based on upconversion emission in Er-doped ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ceramic,” Appl. Phys. Lett. 104(15), 152902 (2014). [CrossRef]
11. P. Du, L. Luo, W. Li, and Q. Yue, “Upconversion emission in Er-doped and Er/Yb-codoped ferroelectric Na0.5Bi0.5TiO3 and its temperature sensing application,” Appl. Phys. Lett. 116(1), 014102 (2014).
12. P. Zhang, M. Shen, L. Fang, F. Zheng, X. Wu, J. Shen, and H. Chen, “Pr3+ photoluminescence in ferroelectric (Ba0.77Ca0.23)TiO3 ceramics: Sensitive to polarization and phase transitions,” Appl. Phys. Lett. 92(22), 222908 (2008). [CrossRef]
13. J. Wu, W. Mao, Z. Wu, and Y. Jia, “The photoluminescence indicating the Curie transition of Er3+-doped (Ba0.97Ca0.03)(Sn0.06Ti0.94)O3 ferroelectric ceramic,” Mater. Lett. 166, 75–77 (2016). [CrossRef]
14. D. K. Khatua, A. Kalaskar, and R. Ranjan, “Tuning photoluminescence response by electric field in electrically soft ferroelectrics,” Phys. Rev. Lett. 116(11), 117601 (2016). [CrossRef] [PubMed]
15. J. Wang, L. Luo, Y. Huang, W. Li, and F. Wang, “Strong correlation of the electrical properties, up-conversion photoluminescence, and phase structure in Er3+/Yb3+ co-doped (1−x)K0.5Na0.5NbO3-xLiNbO3 ceramic,” Appl. Phys. Lett. 107(19), 192901 (2015). [CrossRef]
16. L. Li, L. Zheng, W. Xu, Z. Liang, Y. Zhou, Z. Zhang, and W. Cao, “Optical thermometry based on the red upconversion fluorescence of Er3+ in CaWO4:Yb3+/Er3+ polycrystalline powder,” Opt. Lett. 41(7), 1458–1461 (2016). [CrossRef] [PubMed]
17. M. Ding, Y. Mizuno, and K. Nakamura, “Discriminative strain and temperature measurement using Brillouin scattering and fluorescence in erbium-doped optical fiber,” Opt. Express 22(20), 24706–24712 (2014). [CrossRef] [PubMed]
18. K. Zheng, W. Song, G. He, Z. Yuan, and W. Qin, “Five-photon UV upconversion emissions of Er3+ for temperature sensing,” Opt. Express 23(6), 7653–7658 (2015). [CrossRef] [PubMed]
19. X. D. Wang, O. S. Wolfbeis, and R. J. Meier, “Luminescent probes and sensors for temperature,” Chem. Soc. Rev. 42(19), 7834–7869 (2013). [CrossRef] [PubMed]
20. J. Koruza, B. Rožič, G. Cordoyiannis, B. Malič, and Z. Kutnjak, “Large electrocaloric effect in lead-free K0.5Na0.5NbO3-SrTiO3 ceramics,” Appl. Phys. Lett. 106(20), 202905 (2015). [CrossRef]
21. M. Senna, J. Pavlic, T. Rojac, B. Malic, and M. Kosec, “Preparation of phase pure K0.5Na0.5NbO3 fine powders by a solid state reaction at 625 °C from a precursor comprising Nb2O5 and K, Na acetates,” J. Am. Ceram. Soc. 97(2), 413–419 (2014). [CrossRef]
22. R. Reisfeld and Y. Eckstein, “Dependence of spontaneous emission and nonradiative relaxtions of Tm3+ and Er3+ on glass host and temperature,” J. Chem. Phys. 63(9), 4001 (1975). [CrossRef]
