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Lanthanides in Luminescent Thermometry Chapter 281 399 range of temperatures from a few degrees K to more than 1000 K, so they are especially suitable for high-temperature applications. Nevertheless, they are mainly proposed for in vivo applications (Hao et al., 2013; Zhang and Liu, 2013; Zhou et al., 2015a) because their IR excitation light has a large penetra- tion depth into living tissue, >2 mm, in comparison with visible or UV light (del Rosal et al., 2015; Dong et al., 2011). It is also less damaging to cells, and it can be detected without interference from tissue autofluorescence (Tian et al., 2014). The thermometric performance of UCNPs depends on the composition, the size, and the crystallinity of the host, and on the type and concentration of activators and sensitizers. UCNPs hosts should have low lattice phonon ener- gies in order to minimize nonradiative loss (Bettinelli et al., 2015; Liu, 2015; Wang and Liu, 2009; Zhou et al., 2015b). That is the case of heavy metal halides. Among them, fluorides are the most chemically stable so that they are ideal for this purpose. NaYF4 has often been the choice as an upconverting host, but it has recently been found that NaLuF4 may comparatively increase the upconversion light intensity by 10-fold (Yang et al., 2012). For instance, NaLuF4:Yb/Ho (10/0.5%) and NaLuF4:Yb/Gd/Tm (20/20/0.7%) under 980 nm excitation and at 500 K have Sr1⁄40.83 and 0.267%K1, respectively, values higher than that of 0.263%K1 for NaYF4:Pr (0.8%) at the same tem- perature (Zhou et al., 2014c). Moreover, the sensitivity of UCNPs with oxide ceramic matrices is comparable to that of fluorides. For instance, Sr1⁄40.41, 0.57, and 0.68% K1 at 500 K have been reported for YNbO4:Yb/Er (2/0.3%), La2O2S:Nd (1%), and Y4Al2O9:Dy (0.3%), respectively (Zhou et al., 2014c). A comparison of the thermal sensitivities of oxide ceramic hosts (eg, Gd2O3, BaTiO3, Al2O3, YNbO4, LiNbO3, CaWO4, La2(WO4)3, SrWO4, CaMoO4, and SrMoO4) with the Er3+/Yb3+ activator/sensitizer pair can be found in Huang et al. (2015a). We note, however, that as the authors used Sa instead of Sr the comparison is useless; Sa critically depends on the constant B in Eq. (25) (that when experimentally determined depends on the setup used). Although they are mostly known for their use in IR thermometry, oxide ceramics can also be useful as luminescent thermometers under UV excitation. An example is that of scheelite-based CaGd2(1x)Eu2x(WO4)4 solid solutions (Meert et al., 2014). The intensity ratio between the 5D1!7F1 (535–545 nm) and the 5D0 ! 7F1 (585–600 nm) transitions has a temperature dependence in the range 300–500 K, with Sm ranging from 1.4% K1 (300 K) to 0.47% K1 (475 K). Actually, this compound has been used in the form of powders mixed with silicones to map the temperature of a resis- tive heater with the help of a photographic camera. A disadvantage of this sys- tem is the low relative intensity of the 5D1 ! 7F2 emission, as compared to the 5D0 ! 7F2 one, that makes acquisition times very long. Upconverting materials have also been proposed for low temperature ther- mometry. For instance, for Y2O3:Yb3+/Ho3+ submicron porous powders under 978 nm excitation Lojpur et al. reported maximum absolute sensitivities atPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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