A Review of Mechanoluminescence in Inorganic Solids: Compounds, Mechanisms, Models and Applications

Abstract

Mechanoluminescence (ML) is the non-thermal emission of light as a response to mechanical stimuli on a solid material. While this phenomenon has been observed for a long time when breaking certain materials, it is now being extensively explored, especially since the discovery of non-destructive ML upon elastic deformation. A great number of materials have already been identified as mechanoluminescent, but novel ones with colour tunability and improved sensitivity are still urgently needed. The physical origin of the phenomenon, which mainly involves the release of trapped carriers at defects with the help of stress, still remains unclear. This in turn hinders a deeper research, either theoretically or application oriented. In this review paper, we have tabulated the known ML compounds according to their structure prototypes based on the connectivity of anion polyhedra, highlighting structural features, such as framework distortion, layered structure, elastic anisotropy and microstructures, which are very relevant to the ML process. We then review the various proposed mechanisms and corresponding mathematical models. We comment on their contribution to a clearer understanding of the ML phenomenon and on the derived guidelines for improving properties of ML phosphors. Proven and potential applications of ML in various fields, such as stress field sensing, light sources, and sensing electric (magnetic) fields, are summarized. Finally, we point out the challenges and future directions in this active and emerging field of luminescence research.

Keywords: defects; elastic deformation; mechanoluminescence; mechanoluminescence mechanism; persistent luminescence; piezoelectricity; stress distribution sensing.

Figure A1

Schematic view of a disc with diameter $2R$ and thickness l subjected to a diametrical load P.

Figure A2

The distribution of components of stress tensor ${\sigma }_x$ (a); ${\sigma }_y$ (b); and ${\tau }_{xy}$ (c) respectively and the corresponding distribution of maximum tensile stress ${\sigma }_{max}$ (d) and maximum shear stress ${\tau }_{max}$ (e). An optical image of ML in CaYAl${}_3$O${}_7$: Eu phosphor (f) (Reproduced from Ref [165]. Copyright 2008, The Electrochemical Society.) in a composite disc (diameter $2R=25$ mm, thickness $l=15$ mm) under load $P=1000$ N. Solid circles exhibit the boundary of the disc.

Figure 1

ML intensity-load relationship (a) and optical ML images (b) of SrAMgSi${}_2$O${}_7$:Eu${}^{2+}$ for A = Ca, Sr and Ba, noted as SCMSE, SMSE and SBMSE, respectively. Reproduced from Ref [11] Copyright (2009) The Japan Society of Applied Physics.

Figure 2

The strain evolution under step stress load (a) and stress evolution under step strain load (b) for elastic, viscous and viscoelastic materials. Reproduced from Ref [56], with permission from Cambridge University Press.

Figure 3

A setup for testing the ML response under compressive load (a); and a setup for testing ML for a single microparticle (b). Reproduced from Ref [57], Copyright (2012), with permission from Elsevier and Ref [58] with permission of The Royal Society of Chemistry, respectively.

Figure 4

Crystal structure of ZnS (a) along [00$\overline{1}$] and crystal structure of CaZnOS (b) as a 2 × 2 × 1 supercell (dashed line) along [001]. Unit cells are outlined by solid lines. (S${}^{2-}$: yellow, Ca${}^{2+}$: green, O${}^{2-}$: red.)

Figure 5

Structure of tridymite (a) along [001], crystal structure of CaAl${}_2$O${}_4$ unit cell (b) along [010], crystal structure of SrAl${}_2$O${}_4$ supercell ($1\times 2\times 2$) (c) along [100], and crystal structure of BaAl${}_2$O${}_4$ supercell ($1\times 2\times 1$) (d) along [001]. Unit cells are outlined by black lines, and the direction of tetrahedra was marked as U(up) and D (down). (Ca${}^{2+}$: green, Sr${}^{2+}$: deep green, Ba${}^{2+}$: heavy green, O${}^{2-}$: red.)

Figure 6

The structure of CaAl${}_2$Si${}_2$O${}_8$ (a) along [001], the structure of SrAl${}_2$Si${}_2$O${}_8$ (b) along [001], and the structure of melilite Ca${}_2$MgSi${}_2$O${}_7$ along [001] (c), and along [100] (d). (Ca${}^{2+}$: green, Sr${}^{2+}$: deep green, O${}^{2-}$: red, [SiO${}_4$]: earth, [AlO${}_4$]: brown, [MgO${}_4$]: deep cayan, [(Al, Si)O${}_4$]: orange.)

Figure 7

Crystal structure of Ruddlesden-Popper structures Sr${}_{n+1}$Sn${}_n$O${}_{3n+1}$ (a) viewed along [010], where (i) n = ∞, (ii) n = 1, and (iii) n = 2; crystal structure of calcium niobates mCaO·Nb${}_2$O${}_5$ (b) for (i) m = 1, and (ii) m = 2. Reproduced from Refs [29,183] with the permission of AIP Publishing and Copyright (2016) American Chemical Society, respectively.

Figure 8

The crystal structure (top), silicate layers along [100] (middle) and cation coordination (bottom) in CaSi${}_2$O${}_2$N${}_2$ (a); SrSi${}_2$O${}_2$N${}_2$ (b); and BaSi${}_2$O${}_2$N${}_2$ (c). Tetrahedra with vertices up and down are depicted in dark gray, and in light respectively (N${}^{3-}$: black, O${}^{2-}$: light gray). Reproduced from Ref [241], Copyright (2009), with permission from Elsevier.

Figure 9

The order parameter—internal strain ${\epsilon }_{23}$, and ML intensity data (deep green) as a function of Sr content in Ca${}_{1-x}$Sr${}_x$Al${}_2$Si${}_2$O${}_8$ series. Reproduced from Refs [170,216] with permission from Mineralogical Society of America and Copyright 2010, The Electrochemical Society, respectively.

Figure 10

Part of the cation subset along [001] (a), the nearest neighbors of the Eu dimer (b) in Na${}_x$Eu${}^{3+}$O${}_{2/3-x/3}$MoO${}_4$, and (c) the lifetime of ${}^5{D}_0$ levels as a function of relative amount of Eu${}^{3+}$ clusters at 77 K and 293 K. (In (a), Eu${}^{3+}$: dark, Na${}^{+}$: light blue, vacancies: white, Eu${}^{3+}$-Eu${}^{3+}$: yellow, different cation distribution in adjacent cells: red box.) Reproduced from Ref [283] with permission of The Royal Society of Chemistry.

Figure 11

Vacuum referred (VRBE) and host referred (HRBE) binding energy of lanthanides R (R = Eu${}^{3+}$(red), Eu${}^{2+}$(black and orange), Dy${}^{3+}$(pink), Dy${}^{2+}$(purple)) and oxygen vacancies levels (green) with respect to the valence band of SrAl${}_2$O${}_4$ host, with all R being assumed to occupy the Sr1 site. Energy unit is eV. Reproduced with permission from Ref [92] Copyright (2006) American Chemical Society and from Ref [288] Copyright (2014) American Physical Society .

Figure 12

(a) The ML intensity and applied load for two different loading rates; and (b) their comparison with calculated results (colour line) where the inset shows boundaries of m and $K_T$ for downward convexity of ML. Reproduced from Ref [308], with permission. © 2009 Optical Society of America.

Figure 13

(a) Fitting of the ML intensity with a model with loading frequency of 5 Hz (the inset shows the corresponding FFT results), and the hysteresis loops (b) at various frequencies (1–5 Hz) of SrAl${}_2$O${}_4$:Eu${}^{2+}$/Dy${}^{3+}$ composites. Reproduced from Ref [314], with permission. © 2014 Optical Society of America.

Figure 14

ML intensity (top, blue circles) for varying loads (bottom, green) without (a) and with (b) holding at maximum loading for some time in the glass composite containing SrAl${}_2$O${}_4$: Eu${}^{2+}$, Dy${}^{3+}$, and their fitting by the model (top, red lines). Reproduced from Ref [315], with the permission of AIP Publishing.

Figure 15

Snapshots of a domain boundary (a) in SrAl${}_2$O${}_4$: Eu${}^{2+}$ during nano-indentation, and the schematic diagram (b) showing the proposed mechanism. Reproduced from Ref [30], Copyright (2013), with permission from Elsevier.

Figure 16

Schematic diagram (a) depicted the ML induced by movement of dislocation, and the ML intensity (b) as a function of square of pressure of KBr and KCl (A, B indicate their elastic limit and red rectangle in the insect indicates the load history). Reproduced from Ref [330], Copyright (2010), with permission from Elsevier and Ref [66], Copyright (1982), with permission from Elsevier, respectively.

Figure 17

Digital Image of ML (a) in the vicinity of the crack tip of a compact tension specimen under a load of 61 N (rate: 80 N/s) where white arrows show the load manner, and its comparison (b) (isostress contour lines (black)) with theoretical calculations (white line). Reproduced from Ref [19], Copyright (2013), with permission from Elsevier.

Figure 18

Health monitoring at a bridge in use with full view (a) by attaching ML sheet type sensor (b) to the original concrete girder surface (c) (dotted line), and the ML integral image using the historical-log recording system (d). © 2013 IEEE. Reprinted, with permission from Ref [337].

Figure 19

The schematic illustration of the image acquisition and processing system (a) (schematic structure of ZnS composite film in iii)), which is capable of mapping 2D planar pressure i) and detecting single-point dynamic pressure ii), and the digital visualization (b) and its pressure distribution contour (c) of a handwritten “e” via ML method. Reproduced from Ref [108], with permission from John Wiley and Sons.

Figure 20

The simulation (a) of the ultrasound intensity at different distances from the source (beam transducer), and the ML intensity distribution (b) of the BaSi${}_2$O${}_2$N${}_2$:Eu sensing material. Reproduced from Ref [16], with the permission of AIP Publishing.

Figure 21

Schematic illustration of set-up (a) for wind-driven ML display, and photographs showing the emission form ML phosphors upon N${}_2$ flow (b) and images of “ML” letter in different background colours: dark (c) and yellowish (d) due to different fabrication structures. Reproduced from Ref [17] Published by The Royal Society of Chemistry.

Figure 22

The structure of the piezo-phototronic luminescence device (a) and its luminescence intensity $\lambda =$ 508 nm (b) under a square-wave voltage (blue dashed line) stimulus (Reproduced from Ref [13], Copyright (2015), with permission from Elsevier); the change of emission spectrum with increasing electric field of ZnS:Cu${}^{+}$, Al${}^{3+}$ films (c) (Ref [123], with permission from John Wiley and Sons), and of ZnS:Cu${}^{+}$ phosphor (d) (Ref [121], with the permission of AIP Publishing).

Figure 23

The time dependent luminescence profiles (a); the emission profile (b); and the normalized emission intensity (c) of ZnS:Cu${}^{+}$, Al${}^{3+}$ emitting at 472 and 503 nm under a sinusoidal magnetic field with various frequencies and the dependency on the square of the magnetic-field strength (d) with the corresponding linear fittings. Reproduced from Ref [125], with permission from John Wiley and Sons.