1. Introduction

Mechanoluminescent (ML) materials, a specific type of solid phosphors, can convert the local mechanical energy into light emission with the application of any mechanical stimulus [1,2]. As a category of ML materials, elastico-mechanoluminescent (EML) materials present an accurate linearity of ML intensity against load in the elastic deformation range, in addition to the mechano-optical conversion [3,4]. The potential of EML materials has been recognized as stress probes to monitor the stress distribution in artificial skin and bone, engineering structures, and living body in view of the advantages such as non-destruction, reproducibility, real-time, and reliability [5–11]. Thus, research for EML materials has continuously gained popularity. More than twenty kinds of inorganic EML materials with different emission spectra from ultraviolet to infrared light have been successfully developed since ZnS:Mn²⁺ and SrAl₂O₄:Eu²⁺ were firstly reported in 1999 [5,12]. Nevertheless, the number of intense EML materials is quite limited. Among them, SrAl₂O₄:Eu²⁺ [12] and ZnS:Mn/Mn,Te/Cu/Cu,Mn [5,13,14] have relatively more intense EML. The brightness can be seen even in day light with naked eyes. Unfortunately, the aluminates and sulfides are chemically unstable, especially very sensitive to moisture, which greatly limits the scope of applications. Accordingly, the water-resistant EML materials, such as silicates SrCaMgSi₂O₇:Eu²⁺ [15] and Ca₂MgSi₂O₇:Eu²⁺ [16], aluminosilicates Ca₂Al₂SiO₇:Ce³⁺ [17] and CaAl₂Si₂O₈:Eu²⁺ [18], have been synthesized, but their EML intensities are much weaker than that of SrAl₂O₄:Eu²⁺. Stable and intense EML materials are thus urgently needed.

Recently, an intense red EML has been reported in diphase (Ba,Ca)TiO₃:Pr³⁺, which is considered one of the promising candidates for the EML applications. The most intense EML brightness achieved at the 60 mol% Ca content is above 15 mcd/m², roughly 5000 times the light perception of dark-adapted eye (0.32 mcd/m²) [19,20]. More importantly, the host of this EML material has excellent chemical stability and superior water-resistant behavior [21]. Therefore, to further enhance the EML intensity of diphase (Ba,Ca)TiO₃:Pr³⁺ has very important value to realize the practical application in outdoor and day-light environments.

Considering that the EML performance is closely related to the trap properties, especially the depth and concentration of traps, we have so far done a series of rare earth ions codoping researches to modify the trap properties. Different rare earth ions (including Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) codoped diphase (Ba,Ca)TiO₃:Pr³⁺,RE (Ca 60 mol%) were synthesized to search for suitable rare earth ions favoring the enhancement of EML intensity [22]. The results have proved that Gd³⁺ has the most positive effect. Therefore, in the present work, the EML performance of diphase (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ with different Gd³⁺ codoping concentrations was systematically investigated. The results indicate that the optimal EML intensity of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ is higher by 235% than that of (Ba,Ca)TiO₃:Pr³⁺. The improved EML mechanism is discussed in terms of the results of photoluminescence (PL), reflectivity and thermoluminescence (ThL).

2. Experimental

Gd³⁺ codoped diphase (Ba,Ca)TiO₃:Pr³⁺ with the formula (Ba_0.4Ca_0.6)_0.998-xPr_0.002Gd_xTiO₃ (x = 0, 0.0005, 0.001, 0.002, 0.004, 0.006, and 0.008; hereafter denoted as (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ for simplicity) were synthesized in air by the solid-state reaction method. Raw materials of BaCO₃, CaCO₃, TiO₂, Pr₆O₁₁, and Gd₂O₃ (≥ 99.9%) were mixed thoroughly and prefired at 900 °C for 4 h, then remixed and part of powders were pressed into pellets. The powders and pellets were subsequently sintered at 1400 °C for 4 h. To evaluate the EML property, the synthesized powder products were ground and screened through a 20 μm sieve, and then mixed in a transparent epoxy resin at a weight ratio of 1:9 (0.5 g: 4.5 g) to form the plastic disks 25 mm in diameter and 15 mm thick.

The structural characterization was examined by X-ray diffraction (XRD, D8 Advance, Bruker AXS Gmbh) using Cu-Kα radiation. The morphology of the synthesized products was characterized by scanning electron microscopy (SEM, JSM-5510, JEOL). PL excitation (PLE) and PL were recorded with a fluorescence spectrometer (F-4600, Hitachi). The diffuse reflectance spectrum was measured using a UV/vis/NIR spectrophotometer (UV-2450, Shimadzu). ThL curve was obtained on a ThL-meter (FJ427A1, Beijing Nuclear Instrument Factory). In order to actually compare the PL, reflectivity and ThL curves of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ with those of (Ba,Ca)TiO₃:Pr³⁺, the screened (Ba,Ca)TiO₃:Pr³⁺ and (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ powders with the same weight (0.5 g) were prepared. Compressive stress was applied on the composite disk with a universal testing machine (WES-50, Jinan New Century Testing Machine Manufacture Co.). The EML intensity was measured with a computer-driven photon-counting system that consists of a photomultiplier tube (R2949, Zolix Instruments Co.) and a photoncounter (DCS103, Zolix Instruments Co.). The EML spectra were recorded with a photon multi-channel analyzer system (QE65000, Ocean Optics). The EML images were recorded by a Canon 7D camera with a 50 mm f/1.4 lens. Prior to the EML and ThL measurements, all the samples with the same weight were irradiated at 254 nm for 1 min with a conventional ultraviolet (UV) lamp to charge the excited electrons in the trap levels, and then the measurements were executed after the delay of 1 min to prevent thermal fading at room temperature. All measurements except ThL were performed at room temperature. All measurements were executed twice at least, and the experimental results have good repeatability. The experimental error is less than 5%.

3. Results and discussion

3.1 Structure and morphology

Our previous reports have indicated that (Ba,Ca)TiO₃:Pr³⁺ with 25~90 mol% Ca content has a diphase coexistence of the tetragonal piezoelectric phase Ba_0.77Ca_0.23TiO₃:Pr³⁺ (Ba-rich phase) and the orthorhombic phosphor phase Ba_0.1Ca_0.9TiO₃:Pr³⁺ (Ca-rich phase) [19,20]. EML in diphase (Ba,Ca)TiO₃:Pr³⁺ comes from the interaction of sandwich architectures composed of piezoelectric phase and phosphor phase. Figure 1 exhibits the powder XRD patterns of Gd³⁺ codoped (Ba,Ca)TiO₃:Pr³⁺ with 60 mol% Ca content. It is evident that all the (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ samples consist of diphase. According to the contents of raw materials, the stoichiometries of tetragonal piezoelectric phase Ba_0.77Ca_0.23TiO₃:Pr³⁺ and orthorhombic phosphor phase Ba_0.1Ca_0.9TiO₃:Pr³⁺ have been calculated to be 44.8 mol% and 55.2 mol%, respectively. In comparison with (Ba,Ca)TiO₃:Pr³⁺, the co-doping not only does not produce any impure phase, but also has no remarkable influence on the location and intensity of the diffraction peaks. Furthermore, it is considered that Pr³⁺ (CN = 9, r = 1.179 Å) and Gd³⁺ (CN = 9, r = 1.107 Å) are incorporated into Ca²⁺ (CN = 12, r = 1.34 Å) or Ba²⁺ (CN = 12, r = 1.61 Å) sites according to the acceptable ion radius percentage difference (< 30%) between the doped and substituted ions [23,24]. Since there is no available data for Pr³⁺ and Gd³⁺ with CN = 12, we take the data of CN = 9 as a reasonable approximation in this work.

 

Fig. 1 XRD patterns of (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0, 0.0005, 0.001, 0.002, 0.004, 0.006, and 0.008).

Download Full Size | PDF

The backscattered SEM (BSEM) images have been measured to reveal the microstructure of diphase (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺, as shown in Fig. 2.All the diphase samples exhibit quite dense microstructures and obviously two different composites, i.e. Ba-rich piezoelectric phase (light grains) and Ca-rich phosphor phase (dark grains), forming self-assembled sandwich architectures of piezoelectric/phosphor/piezoelectric [19,20]. As the Gd³⁺ concentration increases, the content and grain size of Ba-rich and Ca-rich phases have no significant changes. These results indicate that low concentration of Gd³⁺ codoping (x = 0.0005~0.008) has no detectable effect on the crystallization behavior of the samples.

 

Fig. 2 Backscattered SEM images of diphase (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ ceramic pellets: (a) x = 0, (b) x = 0.001, (c) x = 0.004, and (d) x = 0.008, showing coexistence of Ba-rich phase (light grains) and Ca-rich phase (dark grains).

Download Full Size | PDF

3.2 Photoluminescent and optical properties

Figure 3 shows the PLE (λ_em = 613 nm) and PL (λ_ex = 343 nm) spectra of (Ba,Ca)TiO₃:Pr³⁺ and (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺. The Gd³⁺ codoping has no effect on the locations of PLE and PL peaks, as well as the shape of spectra. However, it causes the distinct decrease of PLE and PL intensities. The dependence of PL intensity on the Gd³⁺ concentration is plotted in Fig. 4.

 

Fig. 3 (a) Photoluminescent excitation (PLE, λ_em = 613 nm) and (b) photoluminescent (PL, λ_ex = 343 nm) spectra of (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0, 0.0005, 0.001, 0.002, 0.004, 0.006, and 0.008).

Download Full Size | PDF

 

Fig. 4 Dependence of relative photoluminescent (PL) intensity on the Gd³⁺ concentration of (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0, 0.0005, 0.001, 0.002, 0.004, 0.006, and 0.008). The inset shows the PL picture of sample with x = 0.004 (λ_ex = 254 nm).

Download Full Size | PDF

Figure 3(a) illuminates that the excitation spectra are composed of four bands. A band centered at ~256 nm is ascribed to the lowest field component of the 4f5d state of Pr³⁺. B band centered at ~343 nm is assigned to the valence-to-conduction band transition [O(2p)-Ti(3d)]. C band centered at ~400 nm is attributed to a low-lying Pr-to-metal (Pr³⁺-Ti⁴⁺) intervalence charge transfer state (CTS), by which photo-electrons are radiationlessly de-excited from the ³P₀ state to the ¹D₂ state of Pr³⁺. D band composed of 456, 476, and 494 nm weak peaks originates from ³H₄ to ³P_J (J = 0, 1, 2) transitions of Pr³⁺ [25,26]. It is apparently observed that the Gd³⁺ codoping decreases the absorption of 4f5d state of Pr³⁺ (A band) and host (B band). Furthermore, there is no Gd³⁺ characteristic peak, indicating that no energy transfer from Gd³⁺ to Pr³⁺ takes place [27]. Figure 3(b) presents that the main emission peaks of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ all lie in 613 nm, which are ascribed to the ¹D₂-³H₄ transition of Pr³⁺ [25,26]. The inset of Fig. 4 displays the PL picture of sample with x = 0.004 under the 254 nm UV irradiation. It should be noted that none of Gd³⁺ or intrinsic defect related emission has been observed. It is well known that the energy absorbed by phosphors will be released mainly by three ways, i.e. light emission, energy transfer, and nonradiative transition. Therefore, the above results suggest that Gd³⁺ codoping could introduce trap centers to capture the excited charge carriers, delaying the recombination emission in Pr³⁺ and suppressing the PL intensity, or create nonradiative recombination centers to compete with Pr³⁺, quenching PL, rather than form new luminescent centers or act as sensitizers.

Figure 5 presents that the diffuse reflectance spectra of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺. There is no significant change of the absorption edge by Gd³⁺ codoping. However, it is clear that the absorption of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ is stronger than that of (Ba,Ca)TiO₃:Pr³⁺ in the range of 400~700 nm. The variation of absorption with Gd³⁺ concentration is consistent with the trend of PL intensity [Fig. 4], i.e. the sample with weaker PL intensity at 613 nm corresponding to the stronger absorption in the range of 400~700 nm. It also confirms the existence of carrier traps and/or nonradiative recombination centers.

 

Fig. 5 Diffuse reflectance spectra for (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0, 0.0005, 0.001, 0.002, 0.004, 0.006, and 0.008).

Download Full Size | PDF

3.3 Elastico-mechanoluminescent properties of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺

Figure 6(a) shows a comparison of typical EML responses for (Ba,Ca)TiO₃:Pr³⁺ codoped different concentration of Gd³⁺. An increase in the compressive load up to 900 N can induce a corresponding increase in EML intensity for all samples. The relation between EML intensity and compressive load is almost linear, which is extremely useful for the sense of the stress intensity and distribution. More importantly, the EML intensity of (Ba,Ca)TiO₃:Pr³⁺ was greatly improved by Gd³⁺ codoping in the concentration range of 0.0005~0.008. Figure 6(b) presents the Gd³⁺ concentration dependence of EML peak intensity. The largest EML intensity is obtained in the sample with x = 0.004, higher by 235% than that of (Ba,Ca)TiO₃:Pr³⁺.

 

Fig. 6 (a) Elastico-mechanoluminescent (EML) behaviors and (b) comparison among EML peak intensities of (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0, 0.0005, 0.001, 0.002, 0.004, 0.006, and 0.008).

Download Full Size | PDF

Figure 7 shows the EML spectrum of (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0.004). All the EML spectra of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ prepared in this work have the same peak location and shape. The EML spectra are in good agreement with the PL spectra, suggesting that the EML emission originates from the same luminescent center as PL, i.e. the electron transition in Pr³⁺ from the excited state ¹D₂ to the ground state ³H₄. The inset of Fig. 7 displays the typical EML image of the (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ powder/epoxy composite disk under the compressive load. An intense red-light EML emission can be clearly observed by the naked eye.

 

Fig. 7 Elastico-mechanoluminescent (EML) spectrum of (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0.004). The inset shows the EML image.

Download Full Size | PDF

3.4 Thermoluminescent properties of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺

Our previous reports have indicated that diphase (Ba,Ca)TiO₃:Pr³⁺ belongs to defect-controlled type of EML materials. The EML mechanism has been explained by a piezoelectrically induced trapped carrier de-trapping model, in which electron carriers trapped at the trap levels are released under the piezoelectric field induced by mechanical stimulus, and then recombine with the luminescent centers, resulting in photon emission [19–21]. Thus, the trap levels play an important role in the EML process.

In order to investigate the EML mechanism in (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺, the ThL curves of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ were measured from 30 °C to 200 °C at the heating rate of 1 °C/s. The ThL curves are presented in Fig. 8(a).There is only one peak centered at about 60 °C for each ThL curve, suggesting the existence of only one kind of traps. Obviously, the Gd³⁺ codoping does not have the distinct effect on the profile and the location of the ThL peak, but dramatically enhances the ThL intensity in comparison with those of (Ba,Ca)TiO₃:Pr³⁺. It is well known that the trap depth is proportional to corresponding ThL peak temperature, while the trap concentration is proportional to the area under the ThL curve. These results indicate that the Gd³⁺ codoping has not changed the type and depth of traps, but increased the concentration of traps. Figure 8(b) shows the Gd³⁺ concentration dependence of ThL integral intensity. The largest ThL integral intensity is obtained in the sample with x = 0.004, higher by 290% than that of (Ba,Ca)TiO₃:Pr³⁺. Interestingly, the variation of the ThL integral intensity with Gd³⁺ concentration [Fig. 8(b)] agrees well with that of the EML intensity [Fig. 6(b)]. Therefore, it is concluded that the increase of trap concentration induced by Gd³⁺ co-doping leads to the improved EML performance of (Ba,Ca)TiO₃:Pr³⁺.

 

Fig. 8 (a) Thermoluminescent (ThL) curves and (b) relative ThL integral intensities of (Ba,Ca)TiO₃:Pr³⁺,xGd³⁺ (x = 0, 0.0005, 0.001, 0.002, 0.004, 0.006, and 0.008).

Download Full Size | PDF

3.5 Elastico-mechanoluminescent mechanism of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺

Because of the low reduction potential, part of Pr³⁺ can be oxidized to Pr⁴⁺ when sintered in air, while Ti⁴⁺ would get the electron released by Pr³⁺ and be reduced to Ti³⁺, i.e. Pr³⁺-e→Pr⁴⁺ and Ti⁴⁺ + e→Ti³⁺, or Pr³⁺ + Ti⁴⁺→Pr⁴⁺ + Ti³⁺. Thus, several kinds of defects were formed during the synthesis process of (Ba,Ca)TiO₃:Pr³⁺, including calcium and/or barium vacancies ([V_Ca/Ba]'') to compensate [Pr_Ca/Ba]^o, Pr⁴⁺ ([Pr_Ca/Ba]^oo) which tends to form by oxidization of Pr³⁺ after thermal treatment in air, and negatively charged centers like Ti³⁺ ([Ti_Ti]') and/or interstitial oxygen [O_i]'' correlated with the presence of Pr⁴⁺ [28–30]. However, except [Pr_Ca/Ba]^o as the electron trapping center which participates in the EML process, the other defects which are not anticipated from the nominal chemical formulae act as nonradiative recombination centers [20,25].

In the present case of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺, trivalent Gd³⁺ is stable even in oxidation condition. When Gd³⁺ is codoped into (Ba,Ca)TiO₃:Pr³⁺, more point defects arising from the Gd³⁺ impurity ([Gd_Ca/Ba]^o) and the Ca or Ba vacancy ([V_Ca/Ba]'') can be created in the host lattice due to charge compensation, besides those above-mentioned defects in (Ba,Ca)TiO₃:Pr³⁺. Thus, it confirms the above-mentioned explanation on the significant decrease of PL in (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ [Figs. 3 and 4]. On the other hand, the introduced defects provide more carrier traps to improve the EML performance. Considering the same Pr³⁺ concentration in (Ba,Ca)TiO₃:Pr³⁺ and (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺, the increased trap concentration originates from the formation of [Gd_Ca/Ba]^o.

According to the results previously reported and obtained in this work [19–21,25,26], the EML mechanism of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ is explained based on electrons as the main charge carriers as follows. Figure 9 shows the schematic diagram of EML process. After the Gd³⁺ codoping, more electron trapping centers [Gd_Ca/Ba]^o are formed in the host. These traps have the similar depth with [Pr_Ca/Ba]^o. When (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ is irradiated with UV light (254 nm), the electrons are firstly excited from the ³H₄ ground level of Pr³⁺ to the conduction band of Ca-rich phosphor phase. Pr³⁺ stays now as Pr³⁺-h⁺ ionic complex. Then the electrons are trapped by the electron traps ([Pr_Ca/Ba]^o) and [Gd_Ca/Ba]^o. Under the application of stress, the local piezoelectric field induced by Ba-rich piezoelectric phase acts on the phosphor phase and leads to the de-trapping of trapped electrons to the conduction band. By comparison with the case before Gd³⁺ codoping, more trapped electrons are released from the traps of [Pr_Ca/Ba]^o and [Gd_Ca/Ba]^o to the conduction band. Subsequently, these electrons radiationlessly de-excite from CTS via the ³P₀ state to the ¹D₂ level of Pr³⁺-h⁺. Finally, the relaxation of the electrons back to the ground level of Pr³⁺ results in more intense EML.

 

Fig. 9 Proposed elastico-mechanoluminescent mechanism model for (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺, showing the possible trapping and de-trapping processes of electrons. CB: conduction band, VB: valence band, and CTS: charge transfer state.

Download Full Size | PDF

It should be pointed out that Pr³⁺ has stronger natural tendency to be oxidized to Pr⁴⁺ than to be reduced to Pr²⁺, especially sintered in air. In this work, therefore, electrons are the main charge carriers in the EML mechanism.

4. Conclusions

Different concentrations of Gd³⁺ codoped diphase (Ba,Ca)TiO₃:Pr³⁺ were synthesized by the solid-state reaction method. The crystal structure, morphology, PLE and PL properties, diffuse reflectance spectra, EML performance, and ThL curves were systematically investigated. The EML intensity has been greatly enhanced by Gd³⁺ codoping. The optimal EML intensity of (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ is higher by 235% than that of (Ba,Ca)TiO₃:Pr³⁺. The PL, reflectivity and ThL results indicate that more carrier traps have been introduced by Gd³⁺ codoping. The codoping has not changed the type and depth of traps, but increased the concentration of traps. The consistent correlations between the dependences of EML intensity and ThL integral intensity on the Gd³⁺ concentration illuminate that the improved EML originates from the introduced electron traps [Gd_Ca/Ba]^o which have similar depth with ([Pr_Ca/Ba]^o). Finally, an EML mechanism for (Ba,Ca)TiO₃:Pr³⁺,Gd³⁺ is proposed on the basis of these experimental observations.