Suitable materials for elastico mechanoluminescence-based stress sensors
Highlights
► Explores suitable materials for elastico mechanoluminescence-based stress sensors. ► Piezoelectrically-induced detrapping is responsible for the EML emission. ► Materials having uniform distribution of traps are suitable for EML-based sensors. ► The coefficient of deformation detrapping can be determined from the EML measurements.
Introduction
The phenomenon of mechanoluminescence (ML) has generated extensive research interest over the years because of its potential applications for fracture detection, impact detection, damage detection and visualization of stress distribution in solids. Mechanoluminescence is the emission of light from solid materials when they are deformed elastically or plastically or fractured. The light emissions induced by elastic deformation, plastic deformation and fracture of solids are called elastico ML (EML), plastico ML (PML) and fracto ML (FML), respectively [1], [2]. Whereas nearly 50% of all organic and inorganic solids show ML during their fracture, only a few solids show ML during their elastic and plastic deformation [1], [2].
Although the phenomenon of ML is known for a long time, in the past no practical application could be made because of the low ML intensity of the materials and lack of reproducibility. In the recent past, systematic material research has resulted in producing a variety of materials that emit an intensive and repeatable ML during their elastic deformation without any destruction. The examples of elastico mechanoluminescent materials are: x or γ-irradiated alkali halide crystals, ZnS:Mn, SrAl2O4:Eu, SrAl2O4:Ce, SrAl2O4:Ce,Ho, SrMgAl6O11:Eu, SrCaMgSi2O7:Eu, SrBaMgSi2O7:Eu, Sr2MgSi2O7:Eu, Ca2MgSi2O7:Eu,Dy, CaYAl3O7:Eu, (Ba, Ca)TiO3:Pr3+, ZnGa2O4:Mn, MgGa2O4:Mn, BaAl2Si2O8:rare earth element, Ca2Al2SiO7:Ce, ZrO2:Ti and ZnS:Mn, Te [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. The rare earth element can be Eu. The elastico ML has also been observed in the nanoparticles of ZnS:Mn [18], SrAl2O4:Eu [27], [28] and ZnMnTe [24]. A few polymers [29], [30] and rubbers [31], [32] have also been reported to be elastico mechanoluminescent. Certain materials such as SrAl2O4:Eu, SrMgAl6O11:Eu, Ca2Al2SiO7:Ce and ZrO2:Ti, show such an intense EML that it can be seen in day light with naked eye. The EML materials have potential for their use in stress sensors [17], [18], [33], impact sensor [22]; damage sensor [34] and visualizations of stress distribution in solids [17], stress field near the tip of crack [20], quasidynamic crack-propagation in solids [21] and internal defects of a pipe [35]. As the EML materials can be used for the stress imaging of the orthopedic devices, they have potential for analyzing the suitable design of the artificial organs such as legs [36]. An EML-based safety-management monitoring system has been developed [37] that includes EML sensors that emit light upon mechanical stimulation, image sensors nodes that monitor the distribution of EML intensity of the EML sensors, wireless photo-detector nodes, a database for diagnosing the stress anomaly in the structure on the basis of the EML intensity, and a network system that coordinates the operation of these component. The appearance of a crack that occurs in the concrete of bridges and buildings can be visualized by using this system, when the EML sensors of the system are placed on the structure surfaces. Furthermore, one can predict the generation and propagation of the crack by diagnosing the stress anomaly in the structure. Such EML-based monitoring system is expected to be useful for the inspection and maintenance of various structures including those made of concrete.
For EML-based stress sensors a linear relation between the EML intensity and stress is needed. In several intense elastico mechanoluminescent materials nonlinearly occurs in the dependence of EML intensity on stress. The present paper reports the studies made on the nonlinearity in the stress dependence of the EML of crystals and makes a comparison between the theoretical and experimental results. The causes for nonlinearity in the EML intensity versus stress plot are explored and it is shown that the present study may be helpful in tailoring the suitable EML materials for the stress sensors. It is shown that, from the nonlinearity in the EML intensity versus stress plot the coefficient of deformation detrapping can be determined.
Section snippets
Mechanisms of the EML of crystals
To date the rare earth doped aluminate and silicate crystals and ZnS:Mn crystals have been reported to have the potential for stress sensors, therefore, the mechanisms of the EML of these crystals will be discussed.
EML of rare earth doped aluminate silicate crystals
The EML appears only in the crystals doped with suitable activators. This result shows that local regions near the activators are responsible for the EML emissions. As the mechanical energy is not able to cause EML by direct conversion of mechanical energy into light energy, it seems that the EML has electrical origin. In this connection, the local piezoelectric field near the defects seems to be responsible for the EML emissions [13].
If there is exponential distribution of traps near the
Experimental support to the proposed theory
Fig. 2 shows the stress dependence of EML intensity of SrMgAl6O11:Eu phosphors [10]. It is seen that initially the EML intensity increases linearly and then the nonlinearity starts after the stress of 0.835 MPa.
Fig. 3 shows the stress dependence of the EML of Sr2MgSi2O7:Eu (SMSE),. SrCaMgSiO7:Eu (SCMSE) and SrBaMgSi2O7:Eu(SBMSE) phosphors [25]. It is seen that whereas the EML intensity of SBMSE phosphors increases linearily with stress, the nonlinearity occurs in the EML intensity versus stress
Conclusions
The important conclusions drawn from the present investigation are as given below:
- (i)
Whereas the elastico mechanoluminescence (EML) of certain crystals increases linearly with the stress, nonlinearity occurs in the EML intensity versus stress plot of several crystals.
- (ii)
The EML of crystals can be understood on the basis of piezoelectrically-induced detrapping model, whereby the localized piezoelectric field causes detrapping of electrons or holes and subsequently the capture of electrons in the
Acknowledgment
The authors are thankful to Prof. Chittranjan Kar of the Department of English and Linguistics, Pt Ravishankar Shukla University, Raipur(C.G.),India for his help.
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