Mechanoluminescence Intensity Research Articles

Trap Assisted Dynamic Mechanoluminescence Toward Self-Referencing and Visualized Strain Sensing.

Strain sensors utilizing mechanoluminescent (ML) materials have garnered significant attention and application due to their advantages, such as self-powering, non-contact operation, and real-time response. However, ML-based strain sensing techniques typically rely on the establishing of a mathematical relationship between ML intensity and mechanical parameters. The absolute ML intensity is vulnerable to environmental factors, which can result in measurement errors. Herein, an color-resolved visualized dynamic ML and self-referencing strain sensing is investigated in Ca9Al(PO4)7: Tb3+, Mn2+. By analyzing the ML performance under various mechanical stimulations and adjustable strain parameters, a relationship between strain and the ML intensity ratio of Tb3+/Mn2+ is aimed to bed established. This will enable the development of a self-referencing and visualized strain sensing technology. Through a comparison of luminescence characteristics under continuous mechanical stimulation (stretching) and continuous X-ray irradiation, it is discovered that the ratiometric dynamic ML is primarily driven by the dynamic filling and continuous release of carriers form traps, which compensates for the ML of Mn2+. Leveraging the self-referencing and color-resolved (from green to red) visualized ML characteristics, an application scenario for monitoring human joint movement is developed. This approach offers new insights into the use of dynamic ML materials in strain sensing and human-machine interaction.

Near-Infrared Mechanoluminescence from Cr3+-Doped Spinel Nanoparticles for Potential Oral Diseases Detection.

Mechanoluminescence (ML) phosphors have found various promising utilizations such as in non-destructive stress sensing, anti-counterfeiting, and bio stress imaging. However, the reported NIR MLs have predominantly been limited to bulky particle size and weak ML intensity, hindering the further practical applications. For this regard, a nano-sized ZnGa2O4: Cr3+ NIR ML phosphor is synthesized by hydrothermal method. By improving the synthesis method and regulating the chemical composition, the NIR ML (600-1000nm) intensity of such nano-materials has been further enhanced about four times. The reasons for the ML performance difference between micro-/nano- sized phosphors also have been preliminarily analyzed. Additionally, this work probes into the ML mechanism deeply in traps' aspect from band structure and defect formation energy, which can supply significant references for a new approach to develop efficient NIR ML nanoparticles. Finally, due to excellent tissue penetration capability, nano-sized ZnGa2O4:Cr3+ NIR ML phosphor shows great potential applications in biomedical fields such as for the detection of clinical oral diseases.

Persistent Photoluminescence and Mechanoluminescence of a Highly Sensitive Pressure and Temperature Gauge in Combination with a 3D-Printable Optical Coding Platform.

Distinct types of luminescence that are activated by various stimuli in a single material offer exciting developmental opportunities for functional materials. A versatile sensing platform that exhibits photoluminescence (PL), persistent luminescence (PersL), and mechanoluminescence (ML) is introduced, which enables the sensitive detection of temperature, pressure, and force/stress. The developed Sr2MgSi2O7:Eu2+/Dy3+ material exhibits a linear relationship between ML intensity and force and can be used as an ML stress sensor. Additionally, the bandwidth of the PL emission band and the PL lifetime of this material are remarkably sensitive to temperature, with values of ≈0.05nmK-1 and 1.29%/K, respectively. This study demonstrates PersL pressure sensing for the first time, using long-lasting (seconds) lifetime as a manometric parameter. The developed material functions as an exceptionally sensitive triple-mode visual pressure sensor; specifically, it exhibits: i) a sensitivity of ≈-297.4cmGPa-1 (8.11nmGPa-1) in bandshift mode, ii) a sensitivity of ≈272.7cm-1/GPa (14.8nmGPa-1) in bandwidth mode, and iii) a sensitivity of 42%GPa-1 in PL-lifetime mode, which is the highest value reported to date. Notably, anti-counterfeiting, night-vision safety-sign, 8-bit optical-coding, and QR-code applications that exhibit intense PersL are demonstrated by 3D-printing the studied material in combination with a polymer.

Mechanoluminescent/Electric Dual-Mode Sensors Enabled by Trace Carbon Nanotubes.

Mechanoluminescence (ML)-based sensors are emerging as promising wearable devices, attracting attention for their self-powered visualization of mechanical stimuli. However, challenges such as weak brightness, high activation threshold, and intermittent signal output have hindered their development. Here, a mechanoluminescent/electric dual-mode strain sensor is presented that offers enhanced ML sensing and reliable electrical sensing simultaneously. The strain sensor is fabricated via an optimized dip-coating method, featuring a sandwich structure with a single-walled carbon nanotube (SWNT) interlayer and two polydimethylsiloxane (PDMS)/ZnS:Cu luminescence layers. The integral mechanical reinforcement framework provided by the SWNT interlayer improves the ML intensity of the SWNT/PDMS/ZnS:Cu composite film. Compared to conventional nanoparticle fillers, the ML intensity is enhanced nearly tenfold with a trace amount of SWNT (only 0.01wt.%). In addition, the excellent electrical conductivity of SWNT forms a conductive network, ensuring continuous and stable electrical sensing. These strain sensors enable comprehensive and precise monitoring of human behavior through both electrical (relative resistance change) and optical (ML intensity) methods, paving the way for the development of advanced visual sensing and smart wearable electronics in the future.

Suppressed emission intensity of mechanoluminescence film on steel

The emission intensity from mechanoluminescence (ML) film (SrAl2O4:Eu2+) on a steel was lower than that on an aluminum alloy. The phenomenon was confirmed by utilizing detachable ML films where two films with identical properties were adhered to testpieces of dissimilar materials. The ML intensity from the one adhered on an Al-Mg-Si alloy testpiece was six times higher than that from the other on a ferric stainless steel testpiece, under the identical tensile testing conditions. Furthermore, the stainless steel testpiece, of which only single side was adhered with an aluminum foil, was prepared, the ML tests were held on both sides with the same detachable ML films. As the result, the intensity from the aluminum side showed more than twice than that from the other side without aluminum. Therefore, it is newly found suppressed ML performance on steel sheet can be avoided with the aluminum insert.

Shear Stiffening‐Based Mechanoluminescent Device for Impact‐Thermal Coupling Protection and Impact Visualization

AbstractIntelligent impact‐protection wearable devices often require intricate circuitry to operate, which hinders the timely display of impact‐related injuries. Consequently, it is imperative to develop intelligent protective materials that are self‐sufficient and capable of visualization. In this work, the impact protection material shear‐stiffening gel (SSG) is combined with the mechanoluminescent (ML) material ZnS:Cu/PDMS@SiO2 to create ML‐SSG. This material embodies various protective features, including impact protection, force visualization, flame resistance, and long‐distance passive control, making it ideal for intelligent wearable devices. In light of the significant shear stiffening effect of SSG, ML‐SSG effectively dissipates up to 80% of the impact energy and exhibits excellent impact resistance. Concurrently, ML‐SSG is also capable of visualizing impact injuries, displaying and warning in real‐time via mechanoluminescence, and assessing the impact force based on the intensity of mechanoluminescence. The incorporation of SiO2 and ZnS:Cu has resulted in ZnS:Cu/PDMS@SiO2 with remarkable flame‐retardant property. This innovative material significantly improves the performance of ML‐SSG in complex environments. In addition, ML‐SSG realizes human–computer interaction through neural network based on mechanoluminescence display characteristics. This research significantly expands the potential applications of multifunctional protective materials in various complicated environments, thereby promoting the development of wearable protective devices.

Deeply trapped electrons induced long-time sustainable mechanoluminescence of X-ray irradiated La2Ti2O7:Pr3+ for detection of stress burst

X-ray irradiated La2Ti2O7:Pr3+ was found to show long-time sustainable mechanoluminescence (ML), which can realize the high sensitivity and real time detection of stress burst for more than 72 h. Under the excitation of X-ray, La2Ti2O7:Pr3+ shows red ML emissions located at 610, 624 and 638 nm, which are similar to the afterglow emission spectrum and can be ascribed to1D2-3H4, 3P0-3H4 and 3P0-3H2 of Pr3+. And the linear increases of compressive load can induce the linear increases of ML intensity of the sample, indicating that the X-ray induced ML emission can be utilized to accurately detect the stress applied on the object. Furthermore, X-ray irradiated La2Ti2O7:Pr3+ possesses excellent stability and the repeatability of ML emission and the ML signals can keep stable even 50 load cycles after 1 h decay. The thermoluminescence results suggest that abundant electrons are trapped in shallow and deep answered for stable ML emission. More importantly, the deeply trapped electrons induced ML emissions from X-ray irradiated La2Ti2O7:Pr3+ can be utilized to detect the stress bursts for a long time. When the sample is recharged by 5 min X-ray irradiation once, within 24 h decay, the 1500 N burst among many 1000 N cycles can be clearly detected, and the signal to noise ratio (S/N) reaches 1.32 and 1.26 after 12 and 24 h decay, respectively. And even after 72 h decay, the S/N of 2500 N still arrives at 1.23 and that of 4000 N reaches 1.97, indicative of long-time high sensitivity detection of abnormal stress. All these results suggest that the X-ray-irradiated La1.97Ti2O7:0.03Pr3+/resin sample can realize the high sensitivity and real time detection of stress burst for more than 72 h, which thus possesses great potential application in early warning of the emergency disasters such as bridge fracture, tunnel collapse and so on.

Mechanoluminescence of ZnS: Cu@Al2O3 enabled optical fiber microlens passive tactile sensor for hardness recognition.

We propose an optical fiber microlens (OFM) passive tactile sensor (OFMPTS) for hardness recognition. The sensor features a core-shell structure, with an OFM as the core and an elastic mechanoluminescence (ML) matrix shell, which is composed of ZnS: Cu@Al2O3 doped with 10 nm SiO2 particles and polydimethylsiloxane (PDMS). Utilizing the Hertz model, the ML intensity of the sensor is correlated to the elastic modulus of the sample, which enables precise hardness detection. The microlens fiber structure significantly enhances photon collection efficiency, thereby allowing for effective coupling of the ML signal. In press mode, OFMPTS differentiates between five PDMS hardness levels. It can also operate in scan or tap mode, identifying hidden foreign bodies and tissue masses through response curve analysis. The sensor, which requires no external light source, expands the capabilities of optical hardness measurement.

Intense and sensitive green mechanoluminescence by Tb3+ doping in Y3GaO6

In this study, an efficient mechanoluminescent material, namely Y3GaO6:Tb3+, with green emission was prepared by high-temperature solid-phase synthesis. X-ray diffraction (XRD) shows that the samples belong to the orthorhombic crystal system with the space group Cmc21. The luminescence intensity of the materials is related to the content of the luminescent centers, and both photoluminescence (PL) and mechanoluminescence (ML) show that the optimum doping level of Tb3+ is 0.07. In both tensile and frictional applications, the ML intensity is linearly correlated with the magnitude of applied stress. Analysis of the thermoluminescence (TL) spectra indicates that the ML of the material arises from carriers released from the trap under stress stimulation, thus revealing its mechanism of ML property. Finally, the study demonstrates the mechanoluminescence pattern of Y3GaO6:Tb3+.

Achieving Tunable Mechanoluminescence in CaZnOS:Tb3+, Sm3+ for Multicolor Stress Sensing.

Mechanoluminescent (ML) materials can exhibit visible-to-near-infrared mechanoluminescence when responding to the fracture or deformation of a solid under mechanical stimulation. Transforming mechanical energy into light demonstrates promising applications in terms of visual mechanical sensing. In this work, we synthesized the phosphor CaZnOS:Tb3+, Sm3+, which exhibited intense and tunable multicolor mechanoluminescence without pre-irradiation. Intense green ML materials were obtained by doping Tb3+ with different concentrations. Tunable multicolor mechanoluminescence (such as green, yellow-green, and orange-red) could be realized by combining green emission (about 542 nm), attributed to Tb3+, and red emission (about 600 nm) generated from the Sm3+ in the CaZnOS substrate. The tunable multicolor ML materials CaZnOS:Tb3+, Sm3+ exhibited intense luminance and recoverable mechanoluminescence when responding to mechanical stimulation. Benefiting from the excellent ML performance and multicolor tunability in CaZnOS:Tb3+, Sm3+, we mixed the phosphor with PDMS and a curing agent to explore its practical application. An application for visual mechanical sensing was designed for handwriting identification. By taking a time-lapsed shot while writing, we easily obtained images of the writer's handwriting. The images of the ML intensity were acquired by using specific software to transform the shooting data. We could easily distinguish people's handwriting through analyzing the different ML performances.

Innovative 3D printing of mechanoluminescent composites: Vat photopolymerization meets machine learning

Mechanoluminescent (ML) materials, which refers to a class of material that emits light upon being subjected to external mechanical stimuli, have been drawing attention due to their unique multifunctionality and potential applications in next-generation structural health monitoring. However, the practical use of ML materials has been limited by the lack of a universally acceptable framework for producing high-intensity ML composites and the challenges in fabricating complex 3D shapes. As a breakthrough, this paper presents a novel approach for producing SrAl2O4:Eu2+, Dy3+ particle-based ML composites using Vat photopolymerization (VP)–based 3D printing, and its process optimization via machine learning-based algorithm. In this study, multi-objective Bayesian optimization (MBO) with Gaussian process regression (GPR) as surrogate model is adopted to fine-tune three critical process parameters in VP process: ML particle content, layer thickness, and cure ratio, aiming for both strong ML properties and reduced printing time. GPR models the complex process input-output relationship, utilizing data collected from experiments. The Pareto-optimal solutions identified by MBO not only enabled the rapid production of high-performance ML specimens but also provided empirical insights into how process parameters affect the end product’s ML property and overall printing time. Furthermore, a micromechanical method for analyzing ML particle volume fraction dependence of the ML intensity is presented to analyze the result. Finally, the practical applicability of the proposed framework was validated by testing ML-based stress sensors and mechanical components using the optimized VP process.

Thermally recovered mechanoluminescence in Ca6BaP4O17:Eu2.

Stable reproducibility of mechanoluminescence (ML) is of vital importance for trap-controlled ML materials. Photo/electric excitation is usually needed for ML recovery of trap-controlled materials. In this work, it is demonstrated that thermal treatment can be applied to achieve recovery of ML, which is ascribed to the unique trap level configuration. The Ca6BaP4O17:Eu2+ performing robust trap-controlled ML has been proposed, and the corresponding repetitive ML can be realized by thermal treatment. TL spectra reveal that the thermally induced reproducible ML benefits from the dual defect level electronic structure of Ca6BaP4O17:Eu2+. The ML intensity is dependent on the electrons in shallow traps, and the electron transfer from deep traps to shallow traps induced by thermal treatment leads to repetitive ML.

Quantifying the interfacial triboelectricity in inorganic-organic composite mechanoluminescent materials

Mechanoluminescence (ML) sensing technologies open up new opportunities for intelligent sensors, self-powered displays and wearable devices. However, the emission efficiency of ML materials reported so far still fails to meet the growing application requirements due to the insufficiently understood mechano-to-photon conversion mechanism. Herein, we propose to quantify the ability of different phases to gain or lose electrons under friction (defined as triboelectric series), and reveal that the inorganic-organic interfacial triboelectricity is a key factor in determining the ML in inorganic-organic composites. A positive correlation between the difference in triboelectric series and the ML intensity is established in a series of composites, and a 20-fold increase in ML intensity is finally obtained by selecting an appropriate inorganic-organic combination. The interfacial triboelectricity-regulated ML is further demonstrated in multi-interface systems that include an inorganic phosphor-organic matrix and organic matrix-force applicator interfaces, and again confirmed by self-oxidization and reduction of emission centers under continuous mechanical stimulus. This work not only gives direct experimental evidences for the underlying mechanism of ML, but also provides guidelines for rationally designing high-efficiency ML materials.

Realizing Near Infrared Mechanoluminescence Switch in LAGO:Cr Based on Oxygen Vacancy.

Mechanoluminescence (ML) materials are featured with the characteristic of "force to light" in response to external stimuli, which have made great progress in artificial intelligence and optical sensing. However, how to effectively enable ML in the material is a daunting challenge. Here, a Lu3Al2Ga3O12:Cr3+ (LAGO: Cr3+) near infrared (NIR) ML material peaked at 706nm is reported, which successfully realizes the key to unlock ML by the lattice-engineering strategy Ga3+ substitution for Al3+ to "grow" oxygen vacancy (Ov) defects. Combined with thermoluminescence measurements, the observed ML is due to the formation of defect levels and the ML intensity is proportional to it. It is confirmed by X-ray photoelectron spectroscopyand electron paramagnetic resonance that such a process is dominated by Ov, which plays a crucial role in turning on ML in this compound. In addition, potential ML emissions from 4T2 and 2E level transitions are discussed from both experimental and theoretical aspects. This study reveals the mechanism of the change in ML behavior after cation substitution, and it may have important implications for the practical application of Ov defect-regulated turn-on of ML.

Theoretical understanding of defects-driven mechanoluminescence for Pr3+-doped NaNbO3/LiNbO3 heterojunctions

Mechanoluminescence (ML), which involves the emission of light under mechanical stimuli, shows great potential in various applications such as sensing, imaging, and energy harvesting. Current research suggests that the luminescence mechanism of ML is typically connected to specific defects present within the material. In this study, we focus on the investigation of ML defects in Pr3+-doped NaNbO3/LiNbO3 heterojunctions, employing a combination of experimental and theoretical approaches. Through experimental analysis, we confirmed the presence of the heterojunction and its influence on ML intensity, and the optimal doping ratio for the heterojunction in ML was established. Furthermore, we examined the influence of varying Pr3+ doping concentrations on ML behavior and a proof-of-concept was demonstrated using the X-rays charged heterostructural phosphor as a stress sensor for biological applications. The position and concentration of internal defects in the ML material were scrutinized through thermoluminescence tests employing the variable heating rate method and positron annihilation. Complementing the experimental findings, theoretical simulations were conducted to elucidate the underlying mechanisms responsible for the observed ML defects. Density functional theory calculations were employed to investigate the energy levels, charge transfer processes, and lattice distortions within the heterojunctions under mechanical stress. Theoretical predictions were compared and validated against the experimental results. The integration of experimental and theoretical approaches provides a comprehensive understanding of the ML behavior of Pr3+-doped NaNbO3/LiNbO3 heterojunctions. The insights gained from this research contribute to the development of novel ML materials and pave the way for their applications in next-generation sensing and energy conversion devices.

Impact detection method based on the ratio of mechanoluminescence intensity

This paper proposes an impact detection method based on the ratio of mechanoluminescence (ML) intensity, as an improvement over traditional methods that rely on absolute ML intensity. The study utilized two types of ML materials, ZnS:Cu2+ and ZnS:Mn2+, which were physically mixed and embedded them in polydimethylsiloxane (PDMS) to fabricate a flexible film sensor. A drop ball experimental setup was constructed, and the ML spectrum of the film sensor under the impact of a small ball was collected through a spectrometer. This established the relationship between impact velocity and the ratio of ML intensities. Experiments proved that the distance of spectral collection has a minimal effect on the measurement results of this method. Compared to traditional methods based on absolute ML intensity, this method has stronger resistance to environmental interference and better applicability. This new impact detection method based on the ratio of ML intensities has broad application prospects in future engineering applications, safety assessments, and material testing.

Realizing Red Mechanoluminescence of ZnS: Mn2+ Through Ferromagnetic Coupling

AbstractSelf‐recoverable mechanoluminescence (ML) is becoming a novel technology widely used in the fields of sensing, display, and artificial intelligence. The dominant ML material, ZnS: Mn2+, is reported to solely present a yellow emission color, which limits the applications of self‐recoverable ML materials to a large extent. Herein, an effective strategy to extend the ML emission range of ZnS: Mn2+ by the ferromagnetic coupling of Mn2+ ions are reported. Under the thermal carbon‐reduction atmosphere (TCRA), the emission ranges of ML, photoluminescence (PL), and persistent luminescence spectra of ZnS: Mn2+ phosphors are all successfully broadened from yellow to red. Furthermore, as for the PL and ML intensities of ZnS: Mn2+, they are intensified to 1.76 and 3.23 folds larger under the TCRA treatment than those in pure nitrogen, respectively. Various spectra and magnetic test results reveal that the red emission bands of ZnS: Mn2+ @TCRA phosphors originate from the ferromagnetic coupling of Mn2+ ions. This study is the first to realize strong red emission and tunable multicolor luminescence in the conventional ZnS‐based phosphors, which introduces opportunities for discovering the multiband emissions of Mn2+ ion in other compounds. Brightly multicolored ZnS: xMn2+ @TCRA elastic films have been fabricated to demonstrate their anti‐counterfeiting and security applications.

Stimulus response of mechanoluminescence of SrAl2O4:Eu,Dy mixed with epoxy resin using uniaxial load

This paper reports the mechanoluminescence (ML) stimuli by a uniaxial load on rectangular-shaped molded SrAl2O4:Eu,Dy phosphor mixed with epoxy resin. The Sr0.97Al2O4: Eu0.01, Dy0.02 phosphor has been prepared through a solid-state reaction technique in the reducing atmosphere created through activated charcoal. The XRD pattern of this phosphor confirms its monoclinic structure. When a load is applied on the rectangular-shaped (RS) molded ML sample, the stimuli ML intensity initially increases with time, attains a peak value, and then decreases with time. It has also been found that the peak ML intensity of the phosphor is linearly proportional to the increasing applied loads. A minute change in tm of the phosphor sample with increasing loads has been recorded. The ML intensity increases with the thickness of the RS molded phosphor samples. The peak of photoluminescence (PL) spectrum of the prepared phosphor excited with 345nm wavelength of light is found at 518 nm.

Luminescence properties of a green-emitting mechanoluminescent phosphor CaSrGa4O8:xTb3+ without pre-excitation.

In this study, both mechanoluminescence (ML) and long persistent luminescence (LPL) characteristics were first observed in CaSrGa4O8 doped with Tb3+ ions, which confirmed that CaSrGa4O8 is a high-quality host for luminescent material research. Notably, the samples show stronger mechanoluminescent intensity with increasing Tb3+ doping. Additionally, the introduction of Tb3+ led to a shift of the thermoluminescence peak towards higher temperatures and a substantial increase in its intensity, suggesting that Tb3+ doping enhances the overall trap concentration and introduces deeper trap energy levels. Presumably, the free carriers in the system recombine upon mechanical stimulation, releasing energy that is transferred to Tb3+ ions. Investigations into the intrinsic structure, matrix effects, and trap evolution of the material confirmed that deep and shallow traps are responsible for the observed ML and LPL phenomena, respectively. The elucidation of the unique luminescent properties of the material provides us with some guidance for the development of new multi-functional luminescent materials.

Diagnosis of Mechanoluminescent Crack Based on Double Deep Learning in Al 7075

The phenomenon of mechanoluminescence (ML) refers to the emission of light induced by mechanical stimulation applied to mechano-optical materials for example SrAl<sub>2</sub>O<sub>3</sub>:Eu,Dy (SAO). Numerous technologies on the basis of ML have been presented to visualize the stress or strain in various structures for the applications including structural health monitoring. As a result, extensive attention has been devoted to the design, synthesis, characteristics, optimizations, and applications of ML materials. However, challenges still remain in the standardization of ML measurement and evaluation, thereby commercially viable ML applications are currently unavailable. To overcome these difficulties, present study proposes ML measurement and evaluation techniques employing the ML fracture mechanics, finite element method, and dual deep learnings. For the effective normalization of visualized ML images under fixed initial ML intensity condition, continuous UV irradiation above the critical ML power density has been subjected to tensile and compact tension (CT) specimens. Therefore, Plastic Stress Intensity Factor (SIF) as well as crack tip stress field have been extracted successfully from normalized ML images based on ML fracture mechanics. To complement and verify the ML analysis, numerical FEM simulation and analytical ASTM calculation have been also provided. Finally, a double deep learning consists of Generative Adversarial Networks (GAN) and Convolutional Neural Networks (CNN) has been trained and tested for the standard evaluation of in-situ ML images.