Tadahiko Watanabe

Direct view of stress distribution in solid by mechanoluminescence

Visualization of stress distribution has been realized by a nondestructive mechanoluminescence (ML) from SrAl2O4:Eu, which can emit three magnitudes higher visible light than that of well-known ML substance of quartz. A simulation result confirms that such a ML image successfully reflects the stress distribution. A kinetic model for ML of SrAl2O4:Eu is proposed.

Artificial skin to sense mechanical stress by visible light emission

The idea and successful practice of a stress sensor to sense mechanical stress by an artificial skin, i.e., self-diagnosis thin film, has been realized, through the fabrication of a high-luminescence thin piezoelectric film which can reproducibly emit strong visible light upon stressing. The strongest luminescent film consists of nanosized crystallites of ZnS doped with 1.5 at. % Mn, in which Mn acts as the emitting center. The intensity of the emitted luminescence responds to stress applied directly onto the film or to the underlying material reversibly and reproducibly, so it can be used as an artificial skin to sense mechanical stress.

Dynamic visualization of stress distribution by mechanoluminescence image

We report the realization of the dynamic image of stress distribution by developing a remarkably strong mechanoluminescence (ML) material of Sr0.975Al2O3.985:Eu0.01, which can emit four orders of magnitude larger intensity than that of the reported strong ML material of quartz crystal. This ML material can be mixed in the target composite or coated on the surface to sense stress by emitting visible light. This method is applicable to the dynamic visualization of stress distribution in a solid not only in the atmosphere but also in an aqueous environment.

Preparation and characteristics of highly triboluminescent ZnS film

Triboluminescence (TL) is the emission of light induced by the application of mechanical energy. The triboluminescent intensities of various inorganic bulk materials and films were investigated with a photon counting system. It was found that ZnS doped with 5 at% Mn exhibited the strongest TL intensity among the materials investigated. Increase in the crystallinity of ZnS also greatly enhanced TL intensity. Optimizing preparation conditions produced a strong triboluminescent film, which gave an intense visible light emission when friction was applied to it.

INTENSE VISIBLE LIGHT EMISSION FROM SR3AL2O6:EU, DY

The triboluminescence intensity from stress-activated Sr3Al2O6:Eu,Dy (SAO-ED) was so strong that we could see it with the naked eye in the atmosphere. The luminescence integrated intensity was about five hundred times as high as that of crystal sugar. We think that the light emission is due to the movement of dislocations and the 4f7–4f65d transition in the doped Eu2+ ions from the analysis of the emitted light. Furthermore, we have discovered the new phenomenon that the luminescence intensity of the SAO-ED is recovered by the irradiation of ultraviolet light.

Enhancement of adhesion and triboluminescence of ZnS:Mn films by annealing technique

Abstract Highly triboluminescent and strongly adherent ZnS films doped with Mn (ZnS:Mn) were fabricated on glass substrates by ion-plating and a post-plating heat treatment in a vacuum-sealed quartz glass tube. A crystalline ZnS:Mn film was prepared from a high-crystallinity ZnS:Mn pellet by ion-plating. The crystallinity and the friction-induced photon-emitting capability (triboluminescence) of the films were greatly enhanced by a post annealing in vacuum-sealed quartz glass tube. Of greater interest, we found that this annealing treatment strengthened the bond to the glass substrate together with enhancing the endurance to friction, which greatly enhanced its practicability to act as a stress/friction sensor for various applications.

Recovery phenomenon of mechanoluminescence from Ca2Al2SiO7:Ce by irradiation with ultraviolet light

We have investigated the mechanoluminescence (ML) from Ca2Al2SiO7:Ce. The ML is clearly visible to the naked eye in the atmosphere. The luminescence integrated intensity is about 400 times as high as that of crystal sugar. The ML center has been identified as the Ce3+ ion from spectra of the ML and also from the photoluminescence studies of Ca2Al2SiO7:Ce. The ML intensity decreases on repetitive application of stress but is completely recovered by irradiation with ultraviolet light. It is suggested that the ML mechanism arises from the movement of dislocations and recombination between electrodes and holes released from these traps which are associated with the Ce3+ centers.

Electrical power generation characteristics of PZT piezoelectric ceramics

The electrical power generation characteristics of Mn-doped PZT ceramics responding to slow mechanical stress as well as to impact stress have been investigated. Although both the slow and impact stresses induce a reversible electrical response, the generation properties are distinctly different. Slow stress releases two output current peaks with opposite directions, responding to the increasing and decreasing part of the stress, respectively. However, impact stress produces a nearly one-directional signal. The output charge and energy by slow stress are found to be two orders of magnitude higher than that produced by impact stress. This work shows that the energy conversion efficiency of piezoelectric ceramics strongly depends on the method of stress application.

Humidity sensors using manganese oxides

Abstract Unique humidity sensing properties of electrolytic manganese dioxide (EMD) and its calcination products (Mn2O3 and Mn3O4) were investigated. These oxides gave resistive type elements which reduced d.c. resistance on exposure to humidity at room temperature. The humidity sensing behavior was strongly dependent on the oxides, reflecting the differences in microstructure. The use of a mixture of these oxides (hybrid system) was found to be effective for optimizing humidity sensing performances not only in d.c. mode but also in a.c. mode. The elements attached with a pair of Pt and Cu electrodes, on the other hand, showed response to humidity in EMF mode in the high relative humidity (RH) range above 60%. This suggests that proton conduction in the oxides increases with increasing RH, becoming comparable to or larger than electronic conduction at high RH. The EMF response was discussed on an electrochemical cell model constructed on a mixed conducting diaphragm.

Statistical approach for optimizing sputtering conditions of highly oriented aluminum nitride thin films

Design of experiments and the analysis of variance (ANOVA) are very efficient methods to optimize the sputtering conditions for highly c-axis oriented aluminum nitride (AlN) thin films. The effects of the five sputtering control factors; substrate temperature, nitrogen concentration, rf power, sputtering pressure and sputtering time, were simultaneously investigated by only 16 experiments. It could be proved statistically at the 5% level that the substrate temperature, the rf power and the sputtering pressure are significant control factors for the crystal orientation of the films. Especially, the rf power is the most important control factor. On the other hand, the effects of the nitrogen concentration and the sputtering time are not statistically significant. Our results were ascertained at the 95% confidence level. The full width at half-maximum (FWHM) of the X-ray rocking curves of the film deposited under the optimized sputtering conditions was 2.7° (σ=1.6°). The orientation of the film was the highest of the films deposited on glass substrates reported to our knowledge.