Red-emitting piezoluminescence (elasticoluminescence) is achieved by doping rare earth Pr into the well-known piezoelectric matrix, LiNbO . By precisely tuning the Li/Nb ratio in nonstoichiometric Li NbO :Pr , a material that exhibits an unusually high piezoluminescence intensity, which far exceeds that of any well-known piezoelectric material, is produced. Li NbO :Pr shows excellent strain sensitivity at the lowest strain level, with no threshold for stress sensing. These multipiezo properties of sensitive piezoluminescence in a piezoelectric matrix are ideal for microstress sensing, damage diagnosis, electro-mechano-optical energy conversion, and multifunctional control in optoelectronics.

This paper describes a study on the application of smart mechanoluminescence (ML) sensor to structures for the visualization of stress distribution. Smart thin film sensor of ML material SrAl2O4:Eu/polyme is coated on the surface of structure. The sensor will emit visual light when the structure is subjected to deformation. The light intensity is proportional to the strain energy density and the equivalent strain of the stressed material based on the energy viewpoint and experimental facts. The distribution of visual light intensity is recorded using CCD camera or ordinary camera. Then the stress distribution can be evaluated from the distribution of visual light intensity based on solid mechanics and the relationship between light intensity and equivalent strain. Compression test of a disk with coated SrAl2O4:Eu/epoxy film sensor is conducted to compare the visual light distribution with the stress distribution obtained from finite element analysis. Furthermore, tensile test of a rectangular specimen with a coated SrAl2O4:Eu/polymer film sensor is performed to demonstrate the relationship between visual light intensity and strain. Experimental and numerical results show that the smart sensor of ML material SrAl2O4:Eu/polymer is an effective strain sensor to directly visualize the stress distribution in real time.

Piezoelectric LiNbO 3 :Pr 3+ has recently attracted attention as a so-called multi-piezo material, exhibiting simultaneously piezoluminescence and piezoelectricity. In order to investigate and improve the performance of piezoluminescence and piezoelectricity in the newly found multi-piezo material LiNbO 3 :Pr 3+ , a systematic study has been carried out on both the crystal structural stability and the correlated multi-piezo performance through structure control between LiNbO 3 and NaNbO 3 . Full set samples of Li 1¹x Na x NbO 3 :Pr 3+ (x = 01) with various Li/Na ratios were synthesized, and the crystal structure, piezoluminescence performance, piezoelectric property d 33 , and their correlations were systematically investigated. We found that four crystal phases, LiNbO 3 -R3c, NaNbO 3 -R3c, NaNbO 3 -P2 1 ma, NaNbO 3 -Pbma, can exist stably with co-existence in Li 1¹x Na x NbO 3 :Pr 3+ by controlling the Li/Na ratio. A strong correlation between piezoluminescence and piezoelectric properties was verified. Furthermore, the highest piezoluminescence intensity and piezoelectric constant were realized near the phase boundary of NaNbO 3 -R3c and NaNbO 3 -P2 1 ma, demonstrating the crystal structure control as a promising technology to engineer multi-piezo materials.