Introduction

Mechanoluminescence (ML) is a phenomenon of light emission resulting from mechanical action on a solid. Depending on the nature of the mechanical stimulus, ML can be divided into three categories: elastico-ML, plastico-ML and fracto-ML [1]. Elastico-ML is light emission induced by elastic deformation. So, the process is non-destructive and reproducible. With those advantages, elastico-ML has shown potential applications for stress sensing, lighting, imaging and energy conversion [2,3,4,5,6,7,8].

In recent years, much effort has been expended on elastico-ML. However, only a few materials are found to exhibit intensive elastico-ML, which are suitable for the potential applications [9, 10]. Among them, SrAl2O4:(Eu,Dy) is one of the most promising elastico-ML materials and has been extensively studied since the first report on the elastico-ML property of SrAl2O4:Eu in 1999 [11]. The ML of SrAl2O4:(Eu,Dy) phosphor can be observed in day light with naked eyes and is relatively easier to be detected by characterization apparatus. In the elastic deformation region, the ML intensity of SrAl2O4:(Eu,Dy) shows a good linear relationship with the deformation rate, impact energy and torsion [12,13,14,15]. Based on that, the phosphor has been successfully used to measure the stress and visualize the stress distribution in solids [16,17,18,19,20].

Even so, the practical application of SrAl2O4:(Eu,Dy) as stress-sensing devices still remains challenges. SrAl2O4:(Eu,Dy) phosphor is widely known as a persistent luminescence material that can emit light for several tens of hours after the end of excitation. Generally, the persistent luminescence of SrAl2O4:(Eu,Dy) is related to the charge carriers caught by the so-called traps located during excitation. After the excitation ceases, the trapped charge carriers are gradually released from the traps, return to the activators and finally produce luminescence [21]. Similarly, the ML of SrAl2O4:(Eu,Dy) is also believed to attribute to the trapped charge carriers which can be released by mechanical stimulus and then recombine with the luminescence center, resulting in photoemission [16]. Therefore, it is easy to deduce that the concentration of the trapped charge carriers is one of the factors affecting the intensity of elastico-ML of SrAl2O4:(Eu,Dy) [16]. However, the concentration of the trapped charge carriers deceases as the time elapses after the excitation, due to the thermal release of the trapped charge carriers. Accordingly, the intensity of ML is also related to the afterglow decay time elapsed until the onset of mechanical stimulus [22, 23]. This fact makes difficulty in qualitative stress measurement and hampers the application of SrAl2O4:(Eu,Dy) as a stress-sensing material in the real world.

In this paper, we found an effective way to solve the problem that the ML of SrAl2O4:(Eu,Dy) was affected by the afterglow decay time. Steady-state and enhanced ML of SrAl2O4:(Eu,Dy) was achieved by the assistance of continuous UV irradiation.

Experimental

In this study, a commercial SrAl2O4:(Eu,Dy) phosphor (Shenzhen Yaodexing Technology Ltd., KYD-7) was used as the elastico-ML material. The SrAl2O4:(Eu,Dy) powder mixed thoroughly with an optical epoxy resin (Truer MC003) in a weight ratio of 1:1. The composite was coated onto polycarbonate substrates by the doctor blade method [24] and then cured in the oven at 65 °C for 2 h to obtain the ML-sensing film with a thickness of 350 μm.

The photoluminescence of the ML-sensing film was characterized by a Shimadzu RF-5301PC fluorescence spectrometer. The ML of SrAl2O4:(Eu,Dy) was measured by a homemade setup as shown in Fig. 1. The measurement of ML was carried out in an optical enclosure to shield against ambient light. The ML film sample was placed on a plate that inclined 45° to the horizontal line. The impact force applied on the sample was realized by dropping a stainless steel ball (mass, 13.6 g; diameter, 15 mm) from a fixed height through a PVC pipe onto the ML film. The ML intensity of the film was measured using an amplified Si photodiode detector (Thorlabs PDA100A). The output voltage of the Si photodiode detector was collected via a PC-based data acquisition card (Art Technology USB 2813).

Figure 1
figure 1

Schematic diagram of the homemade setup for measurement of impact-induced ML

Full size image

Results and discussion

Figure 2a shows the excitation and emission spectra of SrAl2O4:(Eu,Dy) ML film. The excitation spectrum contains a broad band with peaks at about 365 nm and 420 nm. Under 365 nm excitation, a broad emission centered at 513 nm was observed. The emission band can be ascribed to 4f65d → 4f7 electronic transition of Eu2+ ions in SrAl2O4:(Eu,Dy) phosphor. Besides, the photoluminescent property of the epoxy resin film without SrAl2O4:(Eu,Dy) powder was also investigated. No emission was detected by the fluorescence spectrometer. Figure 2b, c shows the afterglow decay curves of SrAl2O4:(Eu,Dy) ML film measured by the fluorescence spectrometer and the amplified photodiode, respectively. The persistent luminescence process usually contains a rapid decay process and a slow decay process, which can be described by a double exponential equation. The equation is shown as follows [25]

Figure 2
figure 2

a Excitation and emission spectra of the SrAl2O4:(Eu,Dy) film. Afterglow decay curves of the SrAl2O4:(Eu,Dy) film measured by b the fluorescence spectrometer and c the amplified photodiode

Full size image
$$ I = I_{0} + I_{1} \exp ( - \frac{t}{{\tau_{1} }}) + I_{2} \exp ( - \frac{t}{{\tau_{2} }}) $$
(1)

where I represents the phosphorescent intensity; I 0, I 1 and I 2 are three constants; t represents the time; τ 1 and τ 2 are decay constants which decide the decay rate for the rapid and the slow exponential decay components, respectively. By fitting the decay curves using Eq. (1), the values of the decay constants τ 1 and τ 2 are 2.01 and 41.89 s for the ML film measured by the fluorescence spectrometer, and 2.08 and 45.77 s for the ML film measured by the amplified photodiode. The result obtained by the amplified photodiode is in good agreement with that obtained by the fluorescence spectrometer, which means that the detector can well reflect the actual persistent luminescence process.

Figure 3 presents the impact-induced ML intensity of SrAl2O4:(Eu,Dy) film after different afterglow decay time. In this experiment, the ML films were pre-irradiated by 365 nm UV LED (voltage = 3.2 V; current = 0.10 A) for 5 min before the ML measurement. The impacts were applied on the ML films after various decay times (30, 60, 90, 120, 150 s). The typical ML curves are shown in the inset of Fig. 3. The baselines of the curves result from the persistent luminescence decay of the SrAl2O4:(Eu,Dy) film after the excitation ceases. The peaks in the curves can be attributed to the ML induced by the impact of the steel ball. It is obvious that the ML intensity exponentially decreases as the decay time increases, which means that the ML intensity strongly depends on the afterglow decay time of SrAl2O4:(Eu,Dy) phosphors. So, it is important to consider the decay time as a crucial factor for stress evaluation.

Figure 3
figure 3

Dependence of impact-induced ML intensity of the SrAl2O4:(Eu,Dy) film on afterglow decay time. The inset is the typical ML curves of the SrAl2O4:(Eu,Dy) film after different afterglow decay time

Full size image

Under controlled conditions, it is feasible to keep the decay time at a certain fixed value to study the relationship between the ML intensity and the impact-induced stress. Figure 4 shows the ML intensity variations with the impact energy, i.e., the steel ball dropped from the different height. In each test, the afterglow decay time before applying the impact on the ML films was fixed at 30 s. At each dropping height, ten tests were carried out to obtain an average ML intensity for the same sample. The ML intensity increases linearly with the impact energy, which is consistent with the results reported in other literatures [26, 27]. This implies that the SrAl2O4:(Eu,Dy)-based ML films are suitable for sensing the impact stress quantitatively. However, in the real world, the occurrence of an impact is unpredictable. As a result, the decay time of the ML film should be treated as a variant. A much more complicated model should be established to predict the stress from the ML intensity along with the decay time [22].

Figure 4
figure 4

Variations of ML intensity of the SrAl2O4:(Eu,Dy) film with impact energy. The error is estimated using the standard deviation of the average ML intensity from ten separate drops for each dropping height

Full size image

Here, a practical method is proposed to obtain stable ML irrespective of the decay time. Under continuous UV irradiation, the ML intensity of the SrAl2O4:(Eu,Dy) film can keep consistent when the film is subject to the same successive impacts. To show the effect of UV irradiation on the impact-induced ML of the SrAl2O4:(Eu,Dy) film, five stainless steel balls successively dropped from a height of 100 cm onto the SrAl2O4:(Eu,Dy) film at an interval of 30 s, and the SrAl2O4:(Eu,Dy) film was irradiated by 365 nm UV LED (voltage = 3.2 V; current = 0.02 A) during the whole process. As shown in Fig. 5, the intensities of the ML induced by five successive impacts are almost at the same level. The baseline of the luminescence curve can be attributed to the photoluminescence of the SrAl2O4:(Eu,Dy) film excited by 365 nm light source. It is also observed that valleys in the curve appear after the occurrence of ML, due to the SrAl2O4:(Eu,Dy) film shaded by the steel ball from the UV light during the impacts. For comparison, the impact-induced ML of the SrAl2O4:(Eu,Dy) film after different afterglow decay time is also presented in Fig. 5. In these tests, the SrAl2O4:(Eu,Dy) films were pre-irradiated for 5 min by 365 nm UV LED (voltage = 3.2 V; current = 0.02 A) and decayed for a certain time before the impact. It is clear that the ML intensity of the SrAl2O4:(Eu,Dy) film is largely enhanced by the assistance of continuous UV irradiation, compared to the film just with pre-irradiation.

Figure 5
figure 5

ML curve of the SrAl2O4:(Eu,Dy) film under continuous UV irradiation and ML curves of the pre-irradiated SrAl2O4:(Eu,Dy) film after different afterglow decay time

Full size image

The underlying differences between the ML processes with and without continuous UV irradiation are depicted in Fig. 6. Persistent luminescence and ML of the SrAl2O4:(Eu,Dy) films are both related to migration of detrapping electrons from traps to Eu2+ luminescence center and transition from the 5d level of Eu2+ ions to the 4f ground level. However, the way of the electron detrapping in persistent luminescence process is distinct from that in ML process. The release of electrons from the traps in persistent luminescence is assisted by thermal activation or tunneling [28,29,30] while that in ML involves the piezoelectric field effect induced by the impact [31]. Even though, traps filled with electrons are prerequisite for the occurrence of persistent luminescence or ML. So, the SrAl2O4:(Eu,Dy) films are needed to be pre-irradiated by UV light to charge the empty traps before measurement of persistent luminescence or ML. After the UV light is off, the SrAl2O4:(Eu,Dy) films start to exhibit persistent luminescence which consumes the electrons in the traps. As the persistent luminescence decays, the number of electrons in the traps decreases. In the ML theoretical study by Chandra [16], the ML intensity is proportional to the total number of detrappable electrons caught by traps. Therefore, the ML intensity greatly depends on the afterglow decay time. Specifically, the ML intensity decreases as the decay time increases, as shown in the inset of Fig. 3.

Figure 6
figure 6

Mechanism of ML process a without and b with UV irradiation

Full size image

Under continuous UV irradiation, four processes, excitation, emission, trapping and detrapping, are involved in the SrAl2O4:(Eu,Dy) films. At the very beginning of UV irradiation, electrons are excited from the ground state to the excited state of Eu2+ ions. A part of them sequentially return back to the ground state, and the other part get trapped. As soon as the electrons get trapped, they can get detrapped and cause afterglow emission. In this period, the trapping of the electrons occurs at a higher rate than the detrapping, so there is a fast buildup of the number of filled traps and an associated increase in the afterglow emission. As a result, the luminescent emission, consisting of photoluminescence emission and afterglow emission, increases as well. After a short period of time (several seconds in our experiment), the intensity of the luminescent emission gets saturated and a steady-state luminescence will be established [32]. Meanwhile, the trapping and detrapping rate of electron will reach equilibrium, which suggests that the number of the trapped electrons is maintained constant. Thus, the ML intensity can be kept at a stable level.

Figure 7 shows the impact-induced ML variations with the impact energy for the SrAl2O4:(Eu,Dy) film under continuous 365 nm irradiation. In this experiment, ten tests were carried out to obtain an average ML intensity for the same sample at each dropping height. The ML intensity shows a good linear relationship with the impact energy, which means that this method has a great potential for quantitative estimation of the impact stress.

Figure 7
figure 7

Variations of UV-assisted ML intensity of the SrAl2O4:(Eu,Dy) film with impact energy. The error is estimated using the standard deviation of the average ML intensity from ten separate drops for each dropping height

Full size image

Conclusion

The impact-induced ML of SrAl2O4:(Eu,Dy) has been investigated in this paper. The ML intensity of SrAl2O4:(Eu,Dy) strongly depends on the afterglow decay time after ceasing the excitation source. Only when the decay time before the occurrence of the impact is kept constant, the ML intensity is proportional to the impact energy. Aiming to achieve a steady-state ML that is not influenced by the decay time, a method is proposed where SrAl2O4:(Eu,Dy) is continuously irradiated by UV light source during the measurement of ML. Due to the establishment of the equilibrium between trapping and detrapping of electrons in the traps, the number of the trapped electrons is maintained constant. Therefore, stable ML signals can be obtained under continuous UV irradiation. In addition, the ML intensity still linearly correlates with the impact energy, which suggests that this method is beneficial for the application of SrAl2O4:(Eu,Dy) ML material in impact sensors.