Multi-mode mechanoluminescence of fluoride glass ceramics from rigid to flexible media toward multi-scene mechanical sensors

Abstract

The smart mechanical sensing technology based on mechanoluminescence (ML) has potential applications in the fields of wearable mechanical sensors and remote detection of human health due to its characteristics of non-contact, visualization, and remote signal transmission. Herein, a new strategy for multi-mode ML in a rigid and flexible medium by embedding fluoride nanocrystals (CaF₂: Tb³⁺) in amorphous media was proposed and the intrinsic physical mechanism of energy conversion was clarified. For the rigid transparent CaF₂: Tb³⁺ glass ceramics (GCs), without being fabricated with any special medium, recoverable trap-controlled ML can be generated under friction after X-ray pre-irradiation. Furthermore, the ML composite device fabricated using CaF₂: Tb³⁺ GC powder and flexible stretchable polydimethylsiloxane (PDMS) can achieve self-recovery ML under multimode mechanical stimulation due to the induction of inorganic-organic interface triboelectrification. Finally, rigid accurate stress detection at non-stressed point positions is designed based on the remote transmission of a ML signal owing to the unique optical waveguide effect of GCs, and a flexible mechanical-sensing optical skin for real-time monitoring of human health status is realized by using the CaF₂: Tb³⁺ GC powder/PDMS composite device. This work opens up a new avenue for the realization of multi-scene stress detection based on a rigid/flexible ML material.

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1 Introduction

Mechanoluminescence (ML) is a mechanical–optical energy conversion process that occurs under mechanical stimulation, such as compression, stretching, friction, scratching, and impacts, forming a link between luminescence and mechanics,^1–11 which has broad applications in visualization,¹² non-contact mechanical sensing,^13–21 structural health monitoring,²² artificial intelligence skin,^23–25 and wearable devices.^26,27 Currently, various ML materials have been developed based on the structure composition of the material, mechanical stimulation mode, delocalization/recombination path of the electron–hole pair, fabricating medium of composite devices, and energy conversion modes.^28–31 The macroscopic mechanical–optical energy conversion, which is the essential physical mechanism of ML, is an indirect conversion that comprises two or more microscopic energy conversion processes, such as mechanical-electrical and electrical–optical energy conversion. In the ML generation process, an intermediate physical field, such as piezoelectricity/triboelectricity, serves as a bridge for the conversion of mechanical energy to optical energy.⁶ In addition to the structure and performance of the ML material, the intermediate physical field is often closely related to the mechanical stimulation mode and the type of fabricated medium used in ML devices.

Currently, most ML materials can only generate luminescence using ML phosphors and organic elastomers, such as epoxy resin (ER) and polydimethylsiloxane (PDMS), to form composite devices. These organic elastomers can act as a stress conduction medium, transferring the macroscopic mechanical force received by the ML composite device to the luminescent particles and producing an interface interaction with the luminescent particles when subjected to macroscopic mechanical force or deformation, both of which can determine ML generation. For example, piezoelectric crystal-based luminescent particles can generate piezoelectric ML under stress,^1,3 while triboelectric ML can be generated when the triboelectric potential is induced at the interface between the inorganic luminescent particles and organic elastomer medium.^32–42

When CaF₂: Tb³⁺ glass ceramics (GCs) were subjected to macroscopic mechanical stimulation as a typical transparent and rigid medium,^43–48 generating obvious elastic deformation related to the direction of the stress application was challenging, except for a small amount of elastic deformation in the specific direction of large area glass. Producing ML in such rigid CaF₂: Tb³⁺ GCs is challenging using the existing ML mechanisms. In our previous study, we found that trap-controlled ML was achieved in BaLaF₃: Tb³⁺ GCs after pre-irradiation with a high-energy femtosecond laser, and remote transmission of the ML signal was achieved because of the unique optical waveguide effect of CaF₂: Tb³⁺ GCs, leading to the study of rigid medium ML.^49,50 However, the difference in mechanical properties of the rigid and flexible organic media leads to different application prospects for ML. Thus, studying the same material in rigid and flexible media will further promote the popularization and application of ML in CaF₂: Tb³⁺ GCs.

This work investigates the ML performance of CaF₂: Tb³⁺ GCs in their original rigid and flexible device fabricated with PDMS (Fig. 1a). The rigid CaF₂: Tb³⁺ GCs produce recoverable ML under friction after X-ray pre-irradiation. Furthermore, the CaF₂: Tb³⁺ GCs/PDMS composite device can produce recoverable ML under mechanical stimulation. This study provides a detailed examination of ML characteristics and physical mechanisms in different media, proposes the application of CaF₂: Tb³⁺ GCs in various scenarios, and offers theoretical support for the selection of ML materials for different applications.

Fig. 1 (a) Schematic diagram of multimode ML based on CaF₂: Tb³⁺ GCs in different media. (b) XRD patterns of PG and CaF₂: Tb³⁺ GCs. (c) Crystal structure of CaF₂. (d) DTA curve of CaF₂: Tb³⁺ GCs.

2 Experimental section

2.1 Materials and synthesis

A series of glass samples were prepared by using conventional melting-quenching techniques with specific components. The raw materials B₂O₃ (99.99%), SiO₂ (99.99%), ZnO (99.9%), and Na₂CO₃ (99.99%) were ground into a powder by using an agate mortar. All chemical reagents were of analytical grade purity and were purchased from Aladdin Chemical Reagents Ltd. The evenly mixed raw material was then placed in an alumina crucible and melted in air at 1200 °C for 15 min. Furthermore, the melt was poured into a 450 °C preheated copper plate and pressed with another copper plate to form precursor glass (PG). The PG sample was cooled to room temperature (RT) and annealed in a muffle furnace to obtain the CaF₂: Tb³⁺ GCs. Finally, the obtained products were cut and polished for further characterization.

2.2 Preparation of the application device

The flexible film was fabricated by dispersing the CaF₂: Tb³⁺ GC powder in polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning). The ML powder (CaF₂: Tb³⁺ GCs, CaF₂: Eu³⁺ GCs, CaF₂: Eu²⁺ GCs) was mixed well with PDMS (1.5 : 2.5) and then uniformly placed on a mold. The mold with the composite was then placed in an oven and heated at 80 °C for 60 min. Then a flexible ML film was fabricated for the measurement (CaF₂: Tb³⁺ GCs/PDMS, CaF₂: Eu³⁺ GCs/PDMS, CaF₂: Eu²⁺ GCs/PDMS).

2.3 Characterization

An X-ray diffraction (XRD) instrument was operated in the range of 10°–80° at 2θ (Bruk BRUKER D2 PHASER, Germany). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded with a Hitachi F-7000 fluorescence spectrophotometer with a 150 W Xe lamp as the excitation source. The microstructures of the GCs were analyzed using transmission electron microscopy (TEM) and high-resolution field TEM with a JEM-F200 at 200 kV. Sample morphology was investigated by using a scanning electron microscope (SEM) TESCAN VEGA 3 SEM (Tescan China, Ltd). Energy-dispersive spectrum (EDS) and element mapping were obtained by using energy dispersive spectrometer (Ametek Materials Analysis Division). The differential thermal analysis (DTA) curve was recorded with a heating rate of 15 °C min⁻¹ (STA7300, Japan Corporation, Hitachi High-tech Science). The transmission spectrum in the wavelength range of 200–800 nm was recorded by using a Hitachi U-4100 spectrophotometer (Hitachi, Tokyo, Japan). The thermoluminescence (TL) curves were measured by using an FJ-427A pyrometer (Nuclear Instrument Factory, Beijing, China). The electron paramagnetic resonance (EPR) spectrum of the samples was measured by using a Bruker X-band A300-6\1 paramagnetic resonance spectrometer at a frequency of 9.2 GHz at RT. The ML intensity was characterized by using an MS-T3001 friction and wear testing machine, a multi-mode force-luminescence detection system (Qingdao Qingke Longnuo Environmental Technology Co., Ltd QKLN-ML-2), and the use of an Ocean Insight QE Pro fiber optic spectrometer (FLAME-S-XP1-ES). The distance between the friction interface and the electrostatic measurement probe was fixed at 10 cm, and the wavelength range of the test was from 300–1000 nm. The triboelectric properties were detected through a CSM friction testing machine (Tribometer 3, Switzerland) equipped with an electrostatic measuring probe (SK050, KEYENCE (Japan) Co., Ltd). The distance from the friction interface to the electrostatic measuring probe is fixed at 10 cm, and the CSM friction machine adopts a rotating module with a rotation radius of 3 mm and a speed of 60 rpm. The cathodoluminescence (CL) performance was tested on a modified Mp-Micro-S instrument attached to a SEM. All photographs were captured by using a digital camera (Nikon D7100) at RT.

3 Results and discussion

This work focused on investigating the physical mechanism, energy transfer process, and corresponding luminescence behavior during ML generation in different media with various mechanical stimulations. Fig. 1a depicts the ML phenomenon in different media and the flexible ML composite device fabrication processes. Transparent CaF₂: Tb³⁺ GCs after X-ray pre-irradiation generated recoverable trap-controlled green emitting ML under friction, regardless of the bright and dark environments. By grinding CaF₂: Tb³⁺ GCs (i.e., turning glass into powder) and mixing them with PDMS (CaF₂: Tb³⁺ GCs/PDMS), we designed a self-recovery ML composite device resulting from the triboelectric potential induction of the inorganic-organic mechanical interface. Bright green emitting ML can be observed in the CaF₂: Tb³⁺ GCs/PDMS composite device under different mechanical stimuli, such as compression, friction, stretching, tearing, and folding.

3.1 Phase and structure characterization

As a proof-of-concept experiment, we chose CaF₂: Tb³⁺ GCs as the research material in this work. Fig. 1b and S1 (ESI)† show the X-ray diffraction (XRD) patterns of CaF₂: Tb³⁺ GCs before and after the thermal treatment and the CaF₂: x% Tb³⁺ (x = 0.1, 0.3, 0.5, 0.7, and 0.9) samples quenched at 530 °C for 6 h, respectively. These figures demonstrate that heat treatment gradually crystallizes the original amorphous precursor glass (PG) sample, and the precipitated microcrystalline peaks belonging to the (111), (220), and (311) crystal surfaces of the CaF₂ phase are consistent with the standard card (JCPDS NO. 75-0097), indicating that the precipitated microcrystalline phase is CaF₂ and the introduction of Tb³⁺ does not form any impurity phase during the doping process. The transmission electron microscopy (TEM) image of CaF₂: Tb³⁺ GCs in Fig. S2a (ESI)† shows that the microcrystal particles are evenly distributed in the glass matrix. In addition, Fig. S2b (ESI)† shows that the (111) surface average surface spacing of CaF₂: Tb³⁺ GC crystals is 0.3164 nm, and the corresponding fast Fourier transform image verifies the successful precipitation of CaF₂: Tb³⁺ nanocrystals from the amorphous matrix (Fig. S2c ESI†). The scanning electron microscope (SEM) image and energy-dispersive spectrum (EDS) mapping of the GCs (Fig. S3 ESI†) clearly show that the elements are uniformly distributed throughout the sample. The crystal structure diagram of CaF₂ in Fig. 1c shows that F occupies eight sites of the hexahedron and Ca is surrounded by eight F to form a center cube, occupying the Ca sites after doping with Tb. The CaF₂ crystal is a cubic crystal system, with space group Fm3̄m belonging to the cubic system and a centrosymmetric structure. Previous studies revealed that a crystal with this structure is nonpiezoelectric. Fig. 1d presents the differential thermal analysis (DTA) curve of the sample at a heating rate of 15 °C min⁻¹, which was used to assess the glass characteristics of the annealed samples. The sample exhibits a weak heat absorption tendency at 539 °C, indicating the amorphicity of the sample. The thermal stability (Tc − Tg) is around 80 °C, which is a low value, indicating that the crystallization of the system starts below the crystallization temperature. Thus, according to the peak crystallization temperature of 539 °C, we selected 530 °C as the optimal annealing temperature. Further analysis determined that the glass transition temperature (Tg) is 459 °C, the starting crystallization temperature (Tx) is 525 °C, the peak crystallization temperature (Tc) is 539 °C, and the final crystallization temperature (Tp) is 589 °C.

3.2 ML performance characterization

CaF₂: Tb³⁺ GCs were obtained using the melting and quenching method as well as through heat treatment from a rigid transparent medium. The transmittance of all the CaF₂: Tb³⁺ GCs samples was more than 80% (Fig. S4a ESI†), and the corresponding transmittance and PL characteristics are presented in the ESI† (Fig. S4 ESI†). Because the CaF₂: Tb³⁺ GCs serve as a rigid medium, the physical mechanism of their ML generation is different from that of composite ML devices fabricated in a flexible medium with inorganic phosphors. Thus, the CaF₂: Tb³⁺ GCs were irradiated with X-rays to charge for ML performance characterization. Fig. 2a shows the ML spectrum of the CaF₂: Tb³⁺ GC sample surface under friction pair. The ML spectrum is similar to the PL spectrum originating from Tb³⁺ ions, indicating that only a luminescence center is present.⁴⁹ To quantitatively assess the relation between ML intensity and applied loading, Fig. 2b shows the changing relation between ML intensity and applied loading and the linear fitting data, and the insets show ML photos corresponding to various applied loadings. As shown in the photographs, the ML intensity of CaF₂: Tb³⁺ GCs gradually increases with increasing load, and the corresponding ML spectra are presented in Fig. S5a (ESI).† The linear fitting factor is 0.99132 and the friction-induced mechanical-optical response sensitivity is 1.94786. Additionally, a linear relation (Fig. S5c ESI†) was observed between the ML intensity of the CaF₂: Tb³⁺ GCs and the friction speed (Fig. S5b ESI†) under a constant friction load. Fig. 2c shows the ML spectrum before and after the X-ray pre-irradiation of CaF₂: Tb³⁺ GCs. Furthermore, ML is not generated unless the CaF₂: Tb³⁺ GCs are irradiated with X-ray, indicating that X-ray pre-irradiation plays a crucial role in the ML phenomenon of the CaF₂: Tb³⁺ GCs. High-energy ray irradiation of glass materials can cause some point defects within the glass matrix owing to photoionization. Fig. 2d shows the electron paramagnetic resonance (EPR) spectra of the CaF₂: Tb³⁺ GCs before and after X-ray irradiation. The EPR signal (g = 1.9965) was observed only after X-ray irradiation, which is attributed to the oxygen-vacancy defects in the glass,^48,55 indicating that the pre-irradiation with X-ray can generate oxygen vacancies within the CaF₂: Tb³⁺ GC matrix. The TL curves before and after X-ray pre-irradiation are shown in Fig. 2e. A series of continuously distributed traps, trap 1 (350 K), trap 2 (403 K), and trap 3 (522 K), appear after X-ray pre-irradiation which can be fitted using Gaussian fitting. The trap depth E can be estimated from the TL curves using an approximate equation as follows:

E = T/500 (K) (1)

where E is the thermo-active energy of trap depths (eV), which is the energy gap between the electron trap and the conduction band, and T is the TL peak (K).^51–53 The depths of the three traps are 0.7, 0.806, and 1.044 eV, respectively. This result reveals that the oxygen vacancy trap in the CaF₂: Tb³⁺ GCs can effectively capture and store the charge carriers. The TL and ML spectra (Fig. S6a and b ESI†) of the CaF₂: x% Tb³⁺ GCs (x = 0.1, 0.3, 0.5, 0.7, and 0.9) indicate that the TL intensity is quenched at x = 0.7, which is consistent with the PL quenching concentration. Fig. S6c and d (ESI)† show the filling time-dependent TL curves and ML spectra of the CaF₂: Tb³⁺ GCs, respectively. X-Ray irradiation at 40 kV and 30 mA can achieve the upper trap filling limit within 15 min, leading to the maximum TL and ML intensity. Furthermore, the TL after X-ray pre-irradiation follows the same tendency as the ML. This sequence of evidence demonstrates that X-ray pre-irradiation introduced and filled the trap in the glass and the fundamental mechanism of this trap-controlled ML generation is to release the carrier in the trap by mechanical stimulation.

Fig. 2 (a) ML spectrum of the CaF₂: Tb³⁺ GCs (the illustrations show an ML visualization of a handwriting trajectory through long exposure and extracted dimensional color maps from the corresponding photos). (b) The relationship between ML intensity of CaF₂: Tb³⁺ GCs and applied load (insets show the corresponding ML photographs). (c) ML spectra of CaF₂: Tb³⁺ GCs before and after X-ray pre-irradiation. (d) EPR spectra before and after X-ray pre-irradiation of CaF₂: Tb³⁺ GCs. (e) TL curves and Gaussian fitted result of CaF₂: Tb³⁺ GCs before and after X-ray pre-irradiation. (f) Cycling stability of ML/TL in CaF₂: Tb³⁺ GCs.

Theoretically, macroscopic mechanical stimulation cannot directly release the carrier for generating ML. Therefore, during ML generation, the macroscopic mechanical energy is first converted into another form of energy that can effectively release the carrier to the strap. Moreover, friction heat is generated when any two media come in contact with one another and undergo friction. Concurrently, the trap-controlled carrier is considerably sensitive to thermal stimulation. Thus, we speculate that the microscopic ML mechanism in the CaF₂: Tb³⁺ GCs is the frictional heat-induced TL. Fig. S7a and b (ESI)† depict the TL curves of the pre-irradiated CaF₂: Tb³⁺ GCs at RT at different times. It can be observed that with the increase in placement time, the TL intensity gradually reduces because of the effect of RT thermal excitation. After 14 days of placement, the shallow trap retained about 40% of its initial intensity, while the deep trap retained more than 70% of its initial intensity. The variable temperature TL curves (Fig. S7c ESI†) and ML spectra (Fig. S7d ESI†) show that the ML and TL intensity decreases with the increase in temperature, indicating that the ML intensity is directly related to the concentration of charge carriers. Fig. S7e (ESI)† depicts the temperature-dependent TL photographs of the CaF₂: Tb³⁺ GCs after X-ray pre-irradiation for 15 min. It can be seen that the TL of the CaF₂: Tb³⁺ GCs gradually increases with the increasing temperature and slowly decreases above 500 K. This indicates that after X-ray pre-irradiation, thermal stimulation at RT and high temperature can effectively stimulate carrier release in the traps to produce TL. The temperature dependence of the ML is consistent with TL, confirming our speculation about the ML mechanism in the CaF₂: Tb³⁺ GCs. To verify the repeatability of ML in CaF₂: Tb³⁺ GCs under cyclic loading, 40 experiments were performed and the energy was released through repeated charging and heat removal. The sample retained above 95% of the initial intensity (Fig. 2f), confirming that the ML performance of the prepared CaF₂: Tb³⁺ GCs was recoverable. To further demonstrate the ML environmental stability of CaF₂: Tb³⁺ GCs, we tested their ML in water, air, and alcohol every 2 hours for a duration of 24 hours (Fig. S8a–c ESI†). It was observed that the ML stability of CaF₂: Tb³⁺ GC samples was considerably retained. Furthermore, we tested the ML cycling stability of CaF₂: Tb³⁺ GCs and found that the same trends were exhibited over 10 cycles in a 60 s cycle, indicating that the ML performance was stable and repeatable (Fig. S8d ESI†).

Several studies have found that most ML materials do not exhibit ML characteristics alone but the ML phenomenon can be observed after composite encapsulation with an organic flexible medium.⁵⁴ Herein, we combined the CaF₂: Tb³⁺ GCs after grinding with various organic matrices to further investigate the effect of the medium on ML performance. As shown in Fig. 3a, no ML was observed during mechanical stimulation (grinding and compression) in the CaF₂: Tb³⁺ GC powder. Furthermore, the composite devices were fabricated with an ER elastomer (CaF₂: Tb³⁺ GCs/ER). When CaF₂: Tb³⁺ GC powder is combined with a flexible matrix, such as polyurethane (PU), silicone (SG), or PDMS (CaF₂: Tb³⁺ GCs/PU, CaF₂: Tb³⁺ GCs/SG, and CaF₂: Tb³⁺ GCs/PDMS), the composite devices can produce large deformations (>50%) at relatively low tensile stress (<5 MPa). As shown in Fig. 3a and S9 (ESI),† under the same strain action, CaF₂: Tb³⁺ GCs/PU exhibited no ML, CaF₂: Tb³⁺ GCs/SG exhibited weak ML, and CaF₂: Tb³⁺ GCs/PDMS exhibited strong ML. To further explain the variability of ML properties of CaF₂: Tb³⁺ GCs in different media, the surface potential of CaF₂: Tb³⁺ GCs, ER, PU, SG, and PDMS after rubbing with CaF₂: Tb³⁺ GCs for 1 min is obtained, as shown in Fig. 3b. It was observed that when CaF₂: Tb³⁺ GCs-CaF₂: Tb³⁺ GCs, ER, and PU as well as CaF₂: Tb³⁺ GCs were rubbed, positive potentials with smaller absolute values were generated. When CaF₂: Tb³⁺ GCs/SG and CaF₂: Tb³⁺ GCs/PDMS are rubbed, the medium obtains electrons with a negative potential, which appears as a negative potential with a large absolute value. Thus, we believe that the absence of ML after CaF₂: Tb³⁺ GCs/ER and CaF₂: Tb³⁺ GCs/PU is because of insufficient triboelectric potential generated by ER and PU friction to reach the minimum threshold for ML generation. However, owing to the large triboelectric potential generated during the contact separation process of the inorganic–organic mechanical interface in CaF₂: Tb³⁺ GCs/SG and CaF₂: Tb³⁺ GCs/PDMS, ML can directly stimulate ML particles. Additionally, the charge decay curve in Fig. 3c well explains the different ML intensities between CaF₂: Tb³⁺ GCs/SG and CaF₂: Tb³⁺ GCs/PDMS. While the luminescence behavior of the ML process is similar to that of CL in the electron–hole pair delocalization/recombination path, the CL spectrum of CaF₂: Tb³⁺ GCs/PDMS, as given in Fig. 3d, agrees with the ML spectrum.³⁸ The results suggest that the ML mechanism of CaF₂: Tb³⁺ GCs/SG and CaF₂: Tb³⁺ GCs/PDMS is a direct excitation-emission process induced by the interface triboelectric field. Furthermore, the proposed ML excitation pathway of electron bombardment under a triboelectric field is also deemed reasonable. Hence, we believe that the ML mechanism of CaF₂: Tb³⁺ GCs/SG and CaF₂: Tb³⁺ GCs/PDMS conforms to the interface friction-induced electron bombardment model of direct excitation-emission. Furthermore, the separation of ML particles from the polymer chain weakens the interfacial binding of the composite elastomer, causing a decrease in the interface triboelectric field and the interface friction intensity. Fig. 3e shows the stress–strain curves of the CaF₂: Tb³⁺ GCs/PDMS composite elastomer tested on a tensile test machine, with an elastic modulus and tensile intensity of 2.07712 and 2.08876 MPa, respectively. The ML intensity and stress variables exhibit a linear relation. The inset is an ML photograph of CaF₂: Tb³⁺ GCs/PDMS during the tensile mechanical action of different shape variables. The intensity of ML gradually increases with the increase in strain. Moreover, to demonstrate the universality of this ML mechanism in a flexible medium, we also studied the ML characteristics of CaF₂: Tb³⁺ GCs/PDMS, CaF₂: Eu³⁺ GCs/PDMS, and CaF₂: Eu²⁺ GCs/PDMS. The results reveal that the tri-color (red: CaF₂: Eu³⁺ GCs/PDMS, green: CaF₂: Tb³⁺ GCs/PDMS, and blue: CaF₂: Eu²⁺ GCs/PDMS) ML of a single matrix is achieved in the material, displaying the ML spectrum of CaF₂: Eu³⁺ GCs/PDMS, CaF₂: Tb³⁺ GCs/PDMS, and CaF₂: Eu²⁺ GCs/PDMS samples (Fig. 3f), as well as the corresponding ML visualization of handwritten traces extracted from dimension-color maps via long exposure (Fig. 3g). The photos exhibit tri-color ML (Fig. 3g(ii)), while the dimensional color diagram (Fig. 3g(i)) indicates that it can exhibit ML intensity distribution.

Fig. 3 (a) ML performance of CaF₂: Tb³⁺ GC powder in various media: powder only and with ER, PU, SG, and PDMS. (b) The surface triboelectric potential of polymer matrices when rubbed with CaF₂: Tb³⁺ GC powder for 1 min under different loads. (c) The decay rate of triboelectric potential on polymer surfaces. (d) ML/CL spectra of CaF₂: Tb³⁺ GCs/PDMS elastomers. (e) Stress–strain curves and relationship of ML intensity versus strain. (f) ML spectra (from top to bottom) of CaF₂: Eu³⁺ GCs/PDMS, CaF₂: Tb³⁺ GCs/PDMS, and CaF₂: Eu²⁺ GCs/PDMS. (g) ML photos and dimension color maps of CaF₂: Eu³⁺ GCs/PDMS, CaF₂: Tb³⁺ GCs/PDMS, and CaF₂: Eu²⁺ GCs/PDMS.

3.3 ML mechanism analysis

The mechanisms of ML generation in CaF₂: Tb³⁺ GCs differ in rigid and flexible media, which is primarily attributed to the variation in mechanical properties of the medium, mechanical stimulation mode, and the interaction between ML particles and the medium. First, as shown in Fig. 4a, the ML of a rigid CaF₂: Tb³⁺ GC medium is primarily caused by TL induced via friction heat generation, and the microscopic energy conversion process is primarily the gradual conversion of mechanical energy to thermal energy and then to optical energy. The amorphous glass network structural matrix contains numerous bridging oxygen bonds, and a large number of oxygen-vacancy traps are very easily formed under X-ray pre-irradiation, which will become an effective trap center. X-Ray irradiation effectively promotes the filling of the carriers in the trap. When the object (such as the friction pair) comes into contact with the ML CaF₂: Tb³⁺ GCs and produces relative movement, the mechanical energy generated by the friction force is converted into Joule heat to produce TL. During this, the macroscopic phenomenon presented in this instantaneous process is the ML under the action of friction. The increase in ML intensity with friction load and friction rate is attributed to the fact that the work done by the friction force increases with the load and rate, producing more Joule heat. The reproducible pre-irradiation and release of carriers in traps is the fundamental cause of recoverable trap-controlled ML.

Fig. 4 (a) Mechanism diagram of the ML in CaF₂: Tb³⁺ GCs. (b) Mechanism diagram of the ML in the CaF₂: Tb³⁺ GCs/PDMS flexible device.

The self-recovery ML generated by CaF₂: Tb³⁺ GCs/PDMS can be attributed to the induction of electron bombardment because of friction potential at the inorganic–organic mechanical interface, as shown in Fig. 4b. When subjected to multimode mechanical stimulation (such as stretching and friction), an interfacial triboelectric field between the CaF₂: Tb³⁺ GC particles and the SG or PDMS polymer chains is generated due to interfacial friction. This electric field interaction can cause excitation from the valence band to the conduction band (CB), followed by ML emission after the electrons in the CB are transferred to the Tb³⁺ excited state level and recombine with the holes in the ground state. The generation of ML in a flexible medium is primarily derived from the energy conversion of mechanical energy to electric energy and then to optical energy.

3.4 Potential application of ML devices

Based on the advantages of two different media, we have designed two different types of applications for CaF₂: Tb³⁺ GC glass fibers and plate glass. First, based on the glass optical waveguide effect, as shown in Fig. 5a, we observed a change in the intensity of the waveguide with distance and force in glass fibers and plate glass. This effect can be used to monitor stress points and stress values at the edges of glass in structures such as railways, bridges, windows, and aircraft, and for structural health monitoring. We can transmit the monitoring results to a computer via a Wi-Fi signal to achieve remote stress detection as shown in Fig. 5a. Secondly, we have also prepared CaF₂: Tb³⁺ GCs/PDMS elastomers for optical skin. The elastomer can detect mechanical data from various joints of athletes, monitor and analyze their exercise habits, and provide visual and mechanical feedback, thereby providing data support for analyzing the physical condition of athletes as shown in Fig. 5b.

Fig. 5 (a) Diagram of potential application of CaF₂: Tb³⁺ GCs. (b) Diagram of potential application of CaF₂: Tb³⁺ GCs/PDMS.

4 Conclusion

In summary, the CaF₂: Tb³⁺ GCs are synthesized using the melt-quenching method. X-Ray pre-irradiation is employed to form numerous hypoxic centers in the amorphous structure of CaF₂: Tb³⁺ GCs, serving as effective trap centers. These trap centers facilitated the transfer of carriers to the emitting center, resulting in the production of bright green ML in a rigid transparent mass. The atomic contact between CaF₂ NCs and the amorphous structure played a crucial role in promoting efficient energy transfer between the trap and the emitting center. The application of force induced the carriers in the trap to transfer to the emitting center, leading to ML generation. Furthermore, the ground CaF₂: Tb³⁺ GC powder mixed with PDMS produced flexible films that exhibited bright green ML due to direct excitation of the center through contact between the powder particles and colloids. This study achieved the first realization of rigid/flexible multi-scenario ML using the same material through trap-controlled and triboelectrification for direct excitation of two different ML mechanisms. Additionally, tri-color ML is accomplished by incorporating Eu²⁺, Tb³⁺, and Eu³⁺, enabling the potential use of full-spectrum stress displays. By considering the disparity between rigid and flexible media, precise detection devices for determining the stress-free point position and real-time monitoring of athletes' exercise states are designed separately. This comprehensive and in-depth study encompasses the mechanisms and applications of ML, providing valuable guidance for future research on ML materials in the field of rigid/flexible multi-scene.

Consent

I declare that I am the patient whose information is included in the manuscript entitled “Multi-mode mechanoluminescence of fluoride glass ceramics from rigid to flexible media toward multi-scene mechanical sensors”. I understand that the manuscript will be published and that my personal and medical information may be disclosed, and I hereby give my consent for the publication of this information. I have been informed of the purpose of the manuscript and the type of information that will be included. I am aware that my identification will be kept confidential and that my personal information will only be used for research purposes. I understand that my participation in this research is voluntary, and I am free to withdraw my consent at any time. I have been provided with an opportunity to ask any questions regarding my participation, and all my questions have been answered. I further understand that my withdrawal of consent will not affect the quality or outcome of the research. I understand that my consent is necessary for publication, and I consent to the publication of my personal and medical information in the manuscript.

Author contributions

Yingdan Song: conceptualization, investigation, validation, visualization, writing – original draft, supervision. Jianqiang Xiao: conceptualization, investigation, validation, visualization, supervision. Lei Zhao: conceptualization, supervision, writing – review & editing, funding acquisition. Zhichao Liu: writing – review & editing. Yami Ling: investigation, supervision, validation. Yingjuan Yan: investigation. Yixuan Xu: investigation. Alexey·Nikolaevich Yakovlev: investigation. Tingting Hu: investigation. Tatiana Grigorievna Cherkasova: investigation. Qiang Xu: funding acquisition. Canjun Wang: methodology. Xuhui Xu: conceptualization, supervision, writing – review & editing, funding acquisition. All authors contributed to data analysis, discussions, and manuscript preparation. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the International Science and Technology Cooperation and Exchanges Project of Shaanxi (Project No. 2021KW-39), the Scientific Research Program Funded by the Shanxi Provincial Education Department (Project No. 22JC002), the Key Science and Technology Program of Shaanxi Province (Project No. 2023-YBGY-370), the Scientific Research Program Funded by the Shaanxi Provincial Education Department (Grant No. 22JK0245), and the Yunnan Major Scientific and Technological Projects (Grant No. 202202AG050004 and 202202AG050016).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta06257k

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