Coatings

Abstract

Many materials emit light during the application of a mechanical energy. This phenomenon is usually referred to as mechanoluminescence (ML) or triboluminescence. The more historical term is “triboluminescence.” It stands for tribo‐induced luminescence, and this was the term used for more than a century to refer to light emission induced by any type of mechanical energy. The term “mechanoluminescence” was not proposed until 1978. The prefix “mechano” is correlated to the general mechanical way used for exciting luminescence, including concepts such as deformation, piezo, tribo, stress, cutting, grinding, rubbing, and fracto. In recent years mechanoluminescence (ML) has become the preferred nomenclature. Although the transfer of mechanical stress into light radiation is very complex, successes in experimental applications suggest possible uses of the ML phenomena in stress sensors, mechanical displays, and various smart systems. In general, ML can be divided into fractoluminescence (destructive ML) and deformation luminescence (nondestructive ML); these correspond to the luminescence induced by fracture and mechanical deformation of solid, respectively. Roughly 50% of solid materials gives fractoluminescence by fracture: the well‐known materials include sugar, molecular crystals, alkali halides, quartz, silica glass, phosphors, piezoelectric complex, metals, various minerals, and biomaterials. Recently, the fractoluminescence of rare‐earth complexes was investigated in order to build smart damage sensors capable of simple real‐time detection of the magnitude and location of structural damage within materials. Deformation luminescence can be induced by mechanical deformation without fracture, and this is of interest in nondestructive evaluation. Deformation luminescence can be further divided into plasticoluminescence and elasticoluminescence. The former is produced during plastic deformation of solids, where fracture is not required, and the later is produced during the elastic deformation of solids where neither plastic deformation nor fracture is required. Nondestructive ML due to plastic deformation has been observed in several materials such as colored alkali halides, II–VI semiconductors, and rubbers. However, ML in the elastic region has been observed only for the irradiated alkali halides, and some piezoelectric materials. So far nondestructive luminescence intensities of materials have been reported to be too weak and difficult to repeat, and this has deferred any practical application of the phenomenon. For application of ML in developing new materials, repetitive ML must occur with undiminished intensity.

Creating Recoverable Mechanoluminescence in Piezoelectric Calcium Niobates through Pr3+ Doping

“…Recoverable mechanoluminescent (ML) materials are an emerging class of mechano-optical phosphors that repeatedly respond to mechanical stimuli and produce light emissions. − As recoverable ML materials generally preserve a high structural integrity during the luminescence process, they are advantageous over conventional destructive ML materials in practical applications requiring a continuous response. Since the first demonstration of using recoverable ML phosphors in artificial skin by Xu and co-workers in 1999, , there has been an increasing focus on both fundamental research and technological application of recoverable ML materials. − It is now generally accepted that an important prerequisite for recoverable ML is the need for carrier traps to store energy. Another common factor in satisfying recoverable ML is the presence of stress-induced piezoelectric field that triggers the release of the trapped energy as light emissions, , except for a few nonpiezoelectrics showing recoverable ML due to triboelectricity. , Accordingly, previous attempts to search for recoverable ML materials are primarily based on screening of existing trap-controlled luminescent materials (i.e., persistent phosphors) displaying piezoelectricity. − ,, Given the limited number of available candidate persistent phosphors, the family of recoverable ML materials known to date only comprises about a dozen members, of which an extremely small portion shows high ML performance. , The shortage of material systems has become a major obstacle to the understanding and application of recoverable ML.…”

Color Manipulation of Intense Multiluminescence from CaZnOS:Mn2+ by Mn2+ Concentration Effect

“…In recent years, research on the mechanoluminescent (ML) phosphors, whose luminescence is especially excited by mechanical stimuli, has made dramatic advances. − With the landmark exploitation of intense, durable, and multicolor ML semiconductors ZnS:Mn 2+ /Cu + series by Jeong et al, − the applications of ML in mechanically driven excitation sources, , colorful displays, , and white-light sources, , as well as stress sensors and stress imaging, − seem to be turning into reality.…”

Innovative First Step toward Mechanoluminescent Ubiquitous Light Source for Trillion Sensors

“…For example, TiO 2 photocatalysts efficiently absorb ultraviolet (UV) light for activation, (25) and photo-responsible materials for photo-recording and switching also efficiently respond to blue and green light. (23,24) For appropriate combinations of ML light sources and photosensitive materials, we have succeeded in developing ML materials with various emission colors, (1)(2)(3)(4)6,(27)(28)(29)(30)(31)(32)(33)(34)(35) especially with UV, (27) blue, (27)(28)(29)(30)(31)(32)(33) and green (1)(2)(3)(4)34) achieved so far, as shown in Fig. 4.…”

“…1(a)], friction, and impact, even in an elastic deformation region. (1)(2)(3)(4) The ML intensity is proportional to the strain energy of the material. Thus, when dispersedly coated onto a structure, each particle acts as a sensitive mechanical sensor, while the two-dimensional (2D) emission pattern of the whole assembly reflects the dynamical strain/stress distribution [ Fig.…”