Effects of phase structure on up-conversion photoluminescence and dielectric performance in Er 3 þ doped (Bi 0. 5 Na 0. 5 )TiO 3 -BaTiO 3 lead- free ceramics

Effects of phase structure on up-conversion photoluminescence and dielectric performance in Er ^{3 þ} doped (Bi _{0. 5} Na _{0. 5} )TiO ₃ -BaTiO ₃ lead- free ceramics

Chao Chen

^a,*

, Xiang Xia

^a

, Laiqi Zheng

^a

, Laihui Luo

^b

, Na Tu

^a

, Zongyang Shen

^a

, Xiangping Jiang

^a,**

, Junming Liu

^c

aJiangxi Key Laboratory of Advanced Ceramic Materials, Department of Material Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen, 333001, China

bDepartment of Microelectronic Science and Engineering, Ningbo University, Ningbo, 315211, China

cLaboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China

a r t i c l e i n f o

Article history:

Received 19 March 2019 Received in revised form 8 June 2019

Accepted 11 June 2019 Available online 13 June 2019

Keywords:

Lead-free ceramic Phase structure Up-conversion Dielectric property

a b s t r a c t

The effects of phase structure on up-conversion (UC) photoluminescence (PL) and dielectric properties of lead-free ceramics of Er^3þ-dopedx(Bi_0$5Na0.5)TiO3-(1-x)BaTiO3(xNBT-BT: Er^3þ, 0.00x0.90) were investigated. X-ray diffraction and Rietveld structural refinement analyses were conducted to charac- terize the phase structure. On increasing the NBT concentration, the crystal structure changes from tetragonalP4mm toP4bm phase. The UC emission is significantly enhanced with the increment of tet- ragonality, while it begins to decrease upon the occurrence of theP4bm phase. Temperature-dependent UC emission spectra have shown that the optimal emission sample of 0.70NBT-BT: Er^3þexhibits a high color-tuning performance (from green to red). Furthermore, an outstanding thermal stability of UC green emission (65.6% of the initial intensity at 483K) was achieved. The intensity ratio of UC green emission was adopted to reveal the temperature-sensing properties ofxNBT-BT: Er^3þceramics.

©2019 Elsevier B.V. All rights reserved.

1. Introduction

The rare-earth (RE) ion-activated up-conversion (UC) photo- luminescence (PL) material has been a favorite topic for its broad applications in photonic areas, such as information processing, optical computing, andflat displays [1]. Among these RE ions, Er^3þ displays green and red UC emissions when excited by 980 nm laser beam due to its ladder-like 4f energy levels [2]. Besides the acti- vator, the performance of PL materials strongly depends on the characteristics of their host materials. In past decades,fluoride and glass have been regarded as two main kinds of host materials.

However,fluoride is toxic to human health and the environment [3]. The main drawbacks of glass are low laser-induced damage threshold, fragility, and manufacturing difficulty [4]. Hence, it is urgent to develop replacement materials. Oxide ceramic is an appropriate substitute with superior thermal stability, mechanical

strength, and chemical durability [5].

Recently, ferroelectric oxide x (Bi_0$5Na_0.5)TiO₃-(1-x)BaTiO₃ (xNBT-BT) has been extensively studied because of its significance as a potential lead-free piezoelectric material [6e9]. Most impor- tantly, these highly functional perovskite-type oxides possess high chemical and mechanical stability, as well as the lower phonon energy that makes them suitable as UC phosphor host materials [10]. Additionally, NBT-based ferroelectric materials have displayed high photorefractive sensitivity and electro-optic performance [11].

The realization of UC emission in RE-doped ferroelectric materials provides the opportunity to develop a class of ferroelectric mate- rials with luminescence properties that change in response to external stimuli including electric field, mechanical force, and temperature [12].

Most studies of thexNBT-BT solid solution have been conducted near a rhombohedral-tetragonal morphotropic phase boundary, as the enhanced piezoelectric properties were realized at the boundary when the composition is close to 0.94NBT-BT [7e9].

However, the BT-rich end of this solid solution series has received less attention. In this study, we prepared a series of 0.1mol% Er^3þ-

*Corresponding author.

**Corresponding author.

E-mail addresses:cc2762@163.com(C. Chen),jiangxp64@163.com(X. Jiang).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u rn a l h o m e p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m

https://doi.org/10.1016/j.jallcom.2019.06.147 0925-8388/©2019 Elsevier B.V. All rights reserved.

dopedxNBT-BT (xNBT-BT: Er^3þ) ceramics with the composition of 0.00x0.90, which is located in the tetragonal phase region at the BT-rich end of this solid solution. The relationship among their dielectric performance, crystal structure, and UC PL was studied in detail. Furthermore, the temperature-dependent UC emission spectra were obtained to reveal the thermal stability, chromatic, and optical temperature-sensing properties.

2. Experiment details

x (Bi_0$5Na_0.5)TiO₃-(1-x)BaTiO₃ (x¼0.00, 0.10, 0.40, 0.70, and 0.90) doped with 0.1mol% Er^3þ (xNBT-BT: Er^3þ) were fabricated through the conventional solid-state reaction. Note that the con- centration of Er^3þwas maintained at 0.1mol% to thexNBT-BT host.

The raw materials Na2CO3 (99.8%), TiO2 (99.0%), Bi2O3 (99.0%), BaCO3 (99.0%), and Er2O3 (99.99%) were weighed according to stoichiometric ratio. The weighed materials were mixed and ball- milled in polyethylene bottles with zirconium balls in alcohol for 24 h, then dried and calcined for 3 h at 1120 K. The calcined mixture was ground again, then granulated with 5 wt% polyvinyl alcohol binder. The granulated powder was then pressed into disk-shaped pellets with a diameter of 14 mm and a thickness of 1.5 mm. All green pellets were sintered at 1374 Ke1473 K for 2 h. Some samples were coated on both surfaces with silver paste and annealed at 1073 K for 10 min for dielectric electrical properties measurement.

The crystal structure of ceramic samples was characterized by X- ray diffractometer (XRD) (D8 Advanced, Bruker AXS, Karlsruhe, Germany). Rietveld refinement for structural analysis was carried out by the GSAS program [13]. An InVia micro-Raman microscope with a 514.5 nm Ar ion-exciting laser (Renishaw, UK) was used to perform Raman scattering experiments. An impedance analyzer (4294A, Agilent, USA) combined with a high-temperature stove was employed to measure the temperature-dependent dielectric constant and loss at different frequencies. The UC emission under the excitation of a 980 nm diode laser was measured with an Ocean Optics USB4000 spectrofluorometer. The sample temperature varying from 303 K to 693 K was controlled using a Linkam HFS600E-PB2 temperature-controlled stage.

3. Results and discussion

Fig. 1(a) shows the spectra of visible UC PL emission forxNBT-BT:

Er^3þ(x¼0.00, 0.10, 0.40, 0.70, and 0.90) ceramics. All samples show two typical strong green emission bands and one relatively weak

red one, peaked at 531 nm, 551 nm, and 665 nm, respectively. The PL process can be illustrated with the simplified energy level dia- gram (Fig. 1(b)). The Er^3þat the ground⁴I15/2level are pumped to the⁴I11/2level under a 980 nm laser through ground state absorp- tion (GSA) [14,15]. Some Er^3þ at the ⁴I_11/2 level can be further excited to the higher⁴F7/2level via excited state absorption (ESA1).

Because of the small energy gaps, the unstable energy level⁴F7/2

quickly relaxes nonradiatively to the²H_11/2,⁴S_3/2, and⁴F_9/2levels, producing two green emissions (²H11/2/⁴S3/2/⁴I15/2) and one red emission (⁴F9/2/⁴I15/2), respectively. Meanwhile, some ions at level⁴I_11/2decay through multiphonon relaxation to the⁴I_13/2levels and further increase the red emission through the ESA2 process (⁴I13/2þphonon/⁴F9/2). InFig. 1(a), we observe that the PL in- tensity increases with the increase of NBT(x) content. It should be noted that the PL intensity exhibits an abrupt rise atx¼0.40 and reaches a maximum value atx¼0.70. However, the PL intensity drops drastically whenx>0.70. Previous work on RE ion-activated photoluminescence material has indicated that the variation of the PL intensity strongly correlates to the crystal symmetry of their host materials [16e18].

The XRD patterns ofxNBT-BT: Er^3þhost materials are illustrated inFig. 2. The pure perovskite phase is confirmed by XRD pattern.

The splitting of the tetragonal (200)/(002) double peaks, as shown in the inset ofFig. 2, provides evidence of the tetragonal phase.

Fig. 1.(a) UC emission spectra ofxNBT-BT: Er^3þceramics (x¼0.00, 0.10, 0.40, 0.70, and 0.90) under 980 nm excitation; (b) the simplified energy level scheme for the photo- luminescence mechanism of Er^3þ.

Fig. 2.Room temperature XRD ofxNBT-BT: Er^3þceramics (x¼0.00, 0.10, 0.40, 0.70, and 0.90). Inset: Doublet {002}/{200}.

Additionally, the diffraction peaks shift slightly to the right side upon increasing the content of NBT(x), implying a slight shrinkage of unit cell volume. This might be attributed to the occupancy of A- sites by Na^þ(1.39 Å)/Bi^3þ(1.17 Å), whose average ionic radius is less than that of Ba^2þ(1.61 Å) [19]. Hence, the incorporation of NBT will possibly cause large local lattice strains that could influence the local crystal symmetry and ultimately the photoluminescence performance.

The Rietveld structural refinement analysis of XRD patterns was conducted with GSAS software to obtain more meticulous corre- lation between crystal structure and PL behavior. Results forxNBT- BT: Er^3þ(x¼0.10, 0.40, 0.70, and 0.90) samples are presented in Fig. 3. The lattice parameters for each composition are listed in Table 1. The crystal structure of ceramics withx0.70 belongs to a tetragonal phase structure withP4mm symmetry. Although the cell volume decreases from 64.543ų for x¼0.10 to 63.023ų for x¼0.70, the tetragonal distortion (c/ae1) increases and achieves a maximum of 1.795% for x¼0.70. Interestingly, this trend is consistent with the composition dependence of PL intensity when x0.70. This indicates that the incorporation of NBT could increase the tetragonal distortion and then promote the structure asym- metry of the NBT-BT host. This fact leads to the lower symmetry near the site of Er^3þ, enhancing the PL emission of the samples [16,20]. However, the PL emission begins to decrease when further increasing the NBT content (x) to 0.90. Based on the phase diagram ofxNBT-(1-x)BT solid solution performed by TEM studies [21], the P4mm andP4bm phase coexist in the 0.90NBT-0.10BT. Accordingly, a singleP4mm model could notfit the XRD patterns of 0.90NBT- 0.10BT samples. We have obtained satisfactory fitting of the diffraction peaks by using two models of P4mm and P4bm. As shown inFig. 3(d), the phase fractions of theP4mm andP4bm phase were found to be 32.61 wt% and 67.39 wt%, respectively. We note that the tetragonal distortion of theP4bm phase, defined as ( ffiffiffi

p2

c/a-1) [22], is calculated to be 0.096% and is much smaller than that of theP4mm phase (c/a-1¼1.916%). Thus, the occurrence of theP4bm phase with weaker tetragonal distortion tends to cause lower asymmetry around Er^3þ, resulting in the reduction of the PL intensity.

It is generally accepted that Raman spectroscopy can reveal the short-range lattice deformations induced by incorporating NBT into the lattice of BT. Fig. 4illustrates the room-temperature Raman spectra ofxNBT-BT: Er^3þceramics. The mode assignment was car- ried out following Pinzcuk et al. [23]. Some features of single crystals are observed in spectrum for ceramic samples with x¼0.10, including an anti-resonant dip around 180 cm¹(labeled as 1), and two transverse (TO) modes of E (sharp) and A1around 305 cm¹and 515 cm¹, respectively. The presence of both the E

(TO) mode at 305 cm¹ and the resonance dip at 180 cm¹ is common to the spectra ofxNBT-BT: Er^3þforx¼0.10, 0.40, and 0.70.

These two spectral features of tetragonal BaTiO3 are widely believed to indicate the presence of long-range-order ferroelec- tricity [24]. However, the long-range ferroelectricity disappears in thex¼0.90 sample due to the absence of these features. It should be noted that Raman bands broaden at higher NBT concentration.

This is due to the disorder on the A sites of ABO3-type perovskite ferroelectrics [25].

Fig. 5shows the temperature dependence of dielectric constant (εr) and loss (tand) for the composition ofx¼0.00 andx¼0.70,

Fig. 3.Rietveldfitted room temperature XRD plots for selected compositions ofxNBT-BT: Er^3þsamples (a)x¼0.10; (b)x¼0.40; (c)x¼0.70; (d)x¼0.90.

Table 1

Structural results forxNBT-BT: Er^3þceramics.

Symmetry Lattice parameters xNBT-BT: Er^3þceramics

x¼0.0 x¼0.1 x¼0.4 x¼0.7 x¼0.9

P4mm a¼b(Å) 3.996 3.994 3.980 3.956 3.965

c(Å) 4.042 4.045 4.048 4.027 3.889

Cell volume (ų) 64.543 64.526 64.122 63.023 61.139 Lattice distortion (%) 1.151 1.277 1.709 1.795 1.916

P4bm a¼b(Å) … … … … 5.561

c(Å) … … … … 3.936

Cell volume (ų) … … … … 121.719

Lattice distortion (%) … … … … 0.096

Mass fraction ofP4bmphase (wt

%)

0 0 0 0 67.39

Fig. 4. Room temperature Raman spectra forxNBT-BT: Er^3þ(x¼0.10, 0.40, 0.70, and 0.90) ceramics (from bottom to top).

respectively. As presented in Fig. 5(a), the dielectric anomaly of x¼0.00 is sharp and is representative of a normal ferroelectric behavior [26]. In comparison to the composition ofx¼0.00, the sharp dielectric anomaly ofx¼0.70 becomes diffuse and frequency dependent, as shown in Fig. 5(b). Additionally, the frequency dependence of the anomaly temperature could be captured in the curve oftand(T), where it rises from 443 K at 10 kHz to 448 K at 1 MHz, implying relaxor ferroelectric behavior for this composition.

Theεrversus temperature of all compositions is given inFig. 6(a).

With increasing NBT concentration, except for a crossover from a normal to a diffuse/relaxor transition, a distinct increase in the temperature of dielectric maximum (Tm) is observed. The rise inTm

can be attributed to the doping that has resulted in shortening of the TieO bonds [26]. With the incorporation of NBT, the degree of the compositional fluctuation or chemical disorder is greatly enhanced, which contributes to the diffusive phase transition. The diffuseness is adopted to describe the degree of diffusive phase transitions for samples, and the fitting function is the modified CurieeWeiss law [27,28]: 1/εr-1/εm¼(T-Tm)^g/C, wheregandTare the degree of diffuseness and absolute temperature, respectively, εmis the dielectric maximum at transition temperatureTm, andCis a constant. It is known thatg_¼1 for normal ferroelectrics andg_¼2 for typical relaxor ferroelectrics in extremity. The plots of ln (1/εr-1/

εm) versus ln (T-Tm) for xNBT-BT: Er^3þ ceramics are shown in Fig. 6(b). The dielectric curves aboveTmcan befitted well by the quadratic law, and we obtainedg_¼1.60, 1.61, and 1.76 forx¼0.40, 0.70, and 0.90, respectively. The relative high value ofgsuggests that the incorporation of NBT(x) drives the system towards a diffuse/relaxor state. On close inspection of the inset ofFig. 6(b), a distinct increase ingcan be noted when the composition increased fromx¼0.70 tox¼0.90. This observation corresponds well with the Raman data provided above, which shows the disappearance of the long-range order ferroelectricity atx¼0.90. On the other hand, the XRD results show a sudden decrease in the tetragonality from

x¼0.70 tox¼0.90, giving evidence for the occurrence of theP4bm phase. Meanwhile, the onset of theP4bm phase also has bearing on the shape of the temperature dependence of the dielectric constant (Fig. 6(a)). Forx¼0.90, theεr(T) shows two anomalies character- ized by an abrupt jump at depolarization temperature Td~400 K followed by a broad maximum atTm~550 K, whilex0.70,εr(T) exhibits only one diffuse anomaly. The nanodomains withP4bm symmetry have been reported to play an important role in this dielectric anomaly [21]. Thus, the enhancedgatx¼0.90 can be attributed to the development of theP4bm phase in the matrix of theP4mm phase.

Since the 0.70NBT-BT: Er^3þ ceramic exhibits the optimal PL emission, its thermal quenching behavior should be considered for practical application. Fig. 7(a) presents the UC emission spectra versus temperature of the 0.70NBT-BT: Er^3þsample ranging from 303 K to 693 K. InFig. 7(a), except for the emission intensity, no obvious variation is observed in the peak position and the shape of emission spectra. With increasing temperature, the UC green emission intensity decreases while the red one rises. This behavior contributes to a color-tuning performance of 0.70NBT-BT: Er^3þ ceramics. The emission colorfirst changes from green to yellow, then to orange, and ultimately to red, with increasing temperature from 303 K to 693 K, which is reflected in the corresponding chromaticity diagram of Commission Internationale de I'Eclairage (CIE) (Fig. 7(b)). The thermal color-tuning effect can be explained by the JuddeOfelt (JeO) theory [29,30], the spontaneous emission probability between the initial J manifoldj½S;LJ and½S⁰;L⁰J⁰for Er^3þ are written as:

A_ed¼ 64

p

⁴e² 3hð2Jþ1Þ

l

³

"

n n²þ22

9

#

S_ed (1)

wherelis the mean wavelength of the transition,nis the index of Fig. 5.The temperature dependence of dielectric constant (εr) and loss (tand) for the compositions of (a)x¼0.00 and (b)x¼0.70.

Fig. 6.(a) Temperature dependence of dielectric constant at 10 kHz forxNBT-BT: Er^3þceramics (x¼0.00, 0.10, 0.40, 0.70, and 0.90); (b) a plot of ln (1/εr-1/εmax) vs ln (TeTm) ofxNBT- BT: Er^3þceramics for three representative compositions (x¼0.40, 0.70, and 0.90).

refraction,hande are the Plank's constant and electron charge, respectively, and

S_ed¼ X

t¼2;4;6

U

t4f½S;LJU^ðtÞ4f S⁰;L⁰

J⁰² (2)

where the4f½S;LJU^ðtÞ4f½S⁰;L⁰J⁰²is a constant, and 3 JeO intensity parametersUt(t¼2, 4, 6) depend on single electron radial integrals and the crystal field [31]. The band of green UC emission is considered to be the hypersensitive transition of Er^3þ, depending onUt. Meanwhile, the value ofUt depends strongly on the sym- metry of the ionic sites of Er^3þ[32]. The higher symmetry of the crystal structure leads to a lower value ofUt and thus a weaker emission [33]. With increasing temperature, the crystal structure of the samples changes from tetragonal to cubic phase [26]. This leads to the higher crystal symmetry and then reduces the green emis- sion intensity. On the other hand, the probability of a cross relax- ation process (one kind of non-radiative transition) rises with increasing temperature; thus the red emission is enhanced and the red UC emission begins to be dominant. Consequently, the observed variation of the emission color could be attributed to both the change in crystal structure and probability of cross relaxation. It should be noted that the color-tuning performance of 0.70NBT-BT:

Er^3þceramics could possibly be applied in solid-state lighting [34].

Er^3þ-activated UC PL materials hold the promise for optical temperature sensing due to the suitable energy separation of two thermal coupled energy levels (²H11/2and⁴S3/2), and the distinct temperature responses of these two UC green emissions (i.e.,²H_11/

2/⁴S3/2/⁴I15/2) [35,36]. The UC green emission spectra versus temperature, ranging from 303 K to 693 K, are presented inFig. 8(a).

The normalization to the intensity of the emission peak at 551 nm was performed. The relative intensity of the green emission band centered at 531 nm (I531) gradually increases with increasing tem- perature. Due to the small energy separation between the²H11/2and

4S_3/2levels, the²H_11/2can be populated easily from⁴S_3/2through thermal agitation, which increases the ratio of I531/I551. This feature makes the 0.70NBT-BT: Er^3þceramic a promising material for con- structing optical temperature sensors according to thefluorescence intensity ratio (FIR) technique. If the self-absorption effect is ignored, the FIR of UC green emissions is given by:

FIR¼I₅₃₁ I₅₅₁¼N

2H₁₁₌₂

N

4S₃₌₂ ¼Dexp

D

E₁₂

kT

(3)

where N (⁴S_3/2) and N (²H_11/2) are the population number of⁴S_3/2 and²H_11/2levels, respectively;DE₁₂is the energy separation be- tween these two levels; k is the Boltzmann constant (8.62910⁵eV/K); andDis a constant. The FIR of green emissions versus temperature at 531 nm and 551 nm in the range of 303 Ke693 K is presented inFig. 8(b). Obviously, the value of FIR increases with the increasing of temperature, and the maximum is achieved at 693 K. Fitting the measured data to Eq.(4), the energy separation DE12 and the coefficient D are determined to be 979 cm¹and 8.2, respectively. To evaluate the sensing ability of the ceramic, the sensitivity (S) can be determined by Refs. [3,37]:

S¼d_FIR

d_T ¼FIR

D

E₁₂

kT² (4)

The inset of Fig. 8(b) presents the sensor sensitivity versus temperature. The sensitivity monotonously increases with tem- perature, and the maximal value of 0.0035 K¹is achieved at 693 K.

The optical temperature-sensing performances of RE-doped ma- terials, including oxides,fluorides, and glasses, are summarized in Table 2. Compared with other temperature sensing materials, for example NaYF4: Er/Yb, Ba5Gd8Zn4O21: Er/Yb, and Na0$5Er0$5Bi4Ti4O15: Er. [37e43], the temperature sensitivity for 0.70NBT-BT: Er^3þhas been promoted.

Fig. 9presents the decay curve of UC green emission (⁴S3/2/⁴I15/

2) for 0.70NBT-BT: Er^3þceramics. We see that the decay curve exhibits an exponential function characteristic. And the PL lifetime of⁴S_3/

2/⁴I15/2green emission is determined to be 100ms, which is slightly longer than that of the (K0$5Na0.5)NbO3-ErBiO3transparent ceramics [34]. To further study the thermal quenching behavior, the integral of UC green emission intensity for the sample is normalized to that of 303 K, as presented inFig. 10. The normalized intensity monoto- nously increases in the range from 303 K to 393 K because of the thermoluminescence effect. Similar phenomena were observed in Ca1x(LiHo)x/2Bi4Ti4O15and Sm^3þ/Zr^4þco-doping (K0$5Na0.5)NbO3

materials [44,45]. Due to the thermal quenching effect, the normal- ized intensity starts to decrease above 393 K. The intensity of green emission at 483 K still remains about ~65.6% of the original value (at 303 K), suggesting its great luminescence thermal stability.

Fig. 7.(a) Temperature-dependent UC emission spectra of 0.70NBT-BT: Er^3þceramic ranging from 303 to 693 K; (b) temperature dependence of CIE chromaticity coordinates of 0.70NBT-BT: Er^3þceramic based on the UC emission spectra.

4. Conclusions

The correlations among phase structure, UC, and dielectric behavior of lead-free xNBT-BT: Er^3þ ceramics were studied. The XRD results indicated the crystal structure transformation from the P4mm toP4bm tetragonal phase occurred when the NBT content was increased. In the P4mm phase region, the increment of tetragonal lattice distortion enhances the UC emission, while the occurrence of theP4bm phase leads to the decrement of emission.

Temperature-dependent dielectric measurements showed that an abrupt rise in diffusive factor was observed across the P4mm to P4bm phase transition, which was confirmed by the Raman spectra.

In addition, the temperature-dependent spectra of UC emissions revealed the color-tunable (from green to red) UC emissions of

0.70NBT-BT: Er^3þ ceramics. The optical temperature-sensing properties from 303 K to 693 K were studied by using the FIR technique of two green UC emissions. Furthermore, the ceramic presents a good thermal stability of UC green emission (65.6% of the initial intensity at 483K). Our results showed that 0.70NBT-BT: Er^3þ ceramics present a promising potential in optical-electrical multi- functional devices.

Acknowledgments

This work was supported by the National Natural Science Foun- dation of China (51602135, 51862016, 51562014, 51762024, and 61671224), the Foundation of Jiangxi Province (20171BAB216012, GJJ170789, GJJ170804, GJJ180718, and GJJ170794). The author (Chao Chen) wishes to acknowledge the support from the China Scholarship Council.

Fig. 8. (a) UC green emission spectra in the temperature range of 303e693 K for 0.70NBT-BT: Er^3þceramic; (b) FIR values of green emissions at 531 nm and 551 nm and the sensor sensitivity (the inset) as a function of temperature. (For interpretation of the references to color in thisfigure legend, the reader is referred to the Web version of this article.)

Table 2

Optical temperature sensing properties of rare-earth (RE) doped materials under the excitation of a 980 nm laser.

RE ions: host lattice Temperature range (K) Maximum sensitivity (K¹) References

0.70NBT-0.30BT: Er 303e693 0.0035 This work

Na_0$5Bi_0$5TiO3: Yb/Pr 100e500 0.0031 [37]

NaLuF4: Ho/Yb 390e780 0.0014 [38]

ceramic glass: Ho 303e643 0.0010 [39]

NaYF4: Er/Yb 160e300 0.0026 [40]

Na_0$5Er_0$5Bi4Ti4O15: Er 173e503 0.0017 [41]

ceramic glass: Tm/Yb 293e703 0.0028 [42]

Ba5Gd8Zn4O21: Er/Yb 298e573 0.0024 [43]

Fig. 9.The decay curve of UC green emission (⁴S3/2/⁴I15/2) for 0.70NBT-BT: Er^3þ ceramics. (For interpretation of the references to color in thisfigure legend, the reader is referred to the Web version of this article.)

Fig. 10.Temperature dependence of the integrated UC emission intensity normalized to 303 K.

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