Morito Akiyama

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.

Enhancement of Piezoelectric Response in Scandium Aluminum Nitride Alloy Thin Films Prepared by Dual Reactive Cosputtering

Adv. Mater. 2009, 21, 593–596 2009 WILEY-VCH Verlag Gm The industrial demand for higher-temperature piezoelectric sensors is drastically increasing, for the control of automobile, aircraft, and turbine engines and the monitoring of furnace and reactor systems, because environmental problems, such as carbon dioxide (CO2) and nitrogen oxide (NOx) reduction, are becoming more globally serious. The sensors are also desirable for health monitoring coal-fired electric-generation plants and nuclear plants. It is generally known that piezoelectric materials with a higher Curie temperature possess a lower piezoelectric coefficient. Furthermore, the results of a study (Fig. 1) of the relationship between maximum use temperature and piezoelectric coefficient d33 shows that the piezoelectric materials with a higher maximum use temperature possess a lower piezoelectric coefficient d33. [3–9] For example, the Curie temperature and piezoelectric coefficient d33 of lead zirconium titanate (PZT), which is widely used in many electronic devices, are 250 8C and 410 pCN , respectively. The maximum use temperature and d33 of aluminum nitride (AlN), which is a typical hightemperature piezoelectric material, are 1150 8C and 5.5 pCN . It is difficult to achieve a good balance between high maximum use temperature and large piezoelectricity in a material, and no effective piezoelectric materials with these characteristics have yet been found. In this communication, we report a hightemperature piezoelectric material that exhibits a good balance between high maximum use temperature and large piezoelectricity. This was achieved by the combination of the discovery of a phase transition in scandium aluminum nitride (ScxAl1 xN) alloy thin films and the use of dual co-sputtering, which leads to nonequilibrium alloy thin films. Sc0.43Al0.57N alloys exhibit a large piezoelectric coefficient d33 of 27.6 pCN , which is at least 500% larger than AlN. The large piezoelectric coefficient d33 is the highest piezoelectric response among the tetrahedrally bonded semiconductors, despite the fact that the crystal structure of scandium nitride (ScN) is rock-salt (nonpolar). Moreover, the large piezoelectricity is not changed by annealing at 500 8C for 56 h under vacuum. This work demonstrates the new route to design of this high-temperature piezoelectric material. ScN has a rock-salt structure (nonpolar). However, Takeuchi reported the existence of a (meta)stable wurtzite structure in ScN, and the possible fabrication of Sc-IIIA-N nitrides by firstprinciples calculations. Farrer et al. predicted that the wurtzite structure is unstable in ScN, and that the hexagonal structure is (meta)stable in ScN, unlike the wurtzite structure. The piezoelectric responses of hexagonal ScxGa1 xN and ScxIn1 xN alloys can be enhanced by an isostructural phase transition (from wurtzite to layered hexagonal). However, the piezoelectric responses and Curie temperatures of the nitride alloys have not yet been confirmed by experiments. AlN, GaN, and InN are IIIA nitrides and have a wurtzite structure (polar). In particular, the thermal stability and piezoelectricity of AlN are the highest among the IIIA nitrides. AlN is a piezoelectric material compatible with the Complementary metal–oxide– semiconductor (CMOS) manufacturing process, and is a promising material for integrated sensors/actuators on silicon substrates. Wurtzite and rocksalt structures have rather different lattice forms and unit sizes. The formation of

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.

Influence of Eu, Dy co-doped strontium aluminate composition on mechanoluminescence intensity

Abstract We have investigated the mechanoluminescence (ML) of Eu,Dy co-doped strontium aluminates. The SrAl 2 O 4 and Sr 3 Al 2 O 6 indicate high ML intensity, which is clearly visible to the naked eye in the air. SrAl 2 O 4 :Eu,Dy shows the highest ML intensity and has many filled traps, high quantum efficiency and moderate stiffness. On the other hand, Sr 3 Al 2 O 6 :Eu,Dy does not have many filled traps and high relative quantum efficiency, but Sr 3 Al 2 O 6 :Eu,Dy shows relatively high ML intensity. Sr 3 Al 2 O 6 :Eu,Dy indicates low stiffness. SrAl 4 O 7 :Eu,Dy has many filled traps and high relative quantum efficiency, but SrAl 4 O 7 :Eu,Dy shows very small ml intensity. SrAl 4 O 7 :Eu,Dy indicates high stiffness. This suggests that the stiffness is an important factor for the ML intensity of the Eu,Dy co-doped strontium aluminate system.

Preparation and properties of nanostructured TiO2 electrode by a polymer organic-medium screen-printing technique

An organic-medium screen-printing technique was developed for making porous TiO2 electrodes. The TiO2 pastes were prepared by mixing only 100% polyalkylene glycol and commercial nanocrystalline TiO2 powders. The obtained paste is highly printable and hard to evaporate during printing. The TiO2 electrodes have a very porous structure with large cavities. The dye-sensitized solar cell based on these meso-macroporous TiO2 electrodes exhibits high overall conversion efficiency of 4.3–5.8%, which is comparable to those of prepared by water or terpineol medium.

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.

Flexible piezoelectric pressure sensors using oriented aluminum nitride thin films prepared on polyethylene terephthalate films

We have investigated the high sensitive piezoelectric response of c-axis oriented aluminum nitride (AlN) thin films prepared on polyethylene terephthalate (PET) films. The AlN films were deposited using a radio frequency magnetron sputtering method at temperatures close to room temperature. The c axes of the AlN films were perpendicularly oriented to the PET film surfaces. The sensor consisting of the AlN and PET films is flexible like PET films and the electrical charge is linearly proportional to the stress within a wide range from 0to8.5MPa. The sensor can respond to the frequencies from 0.3 to over 100Hz and measures a clear human pulse wave form by holding the sensor between thumb and middle finger. The resolution of the pulse wave form is comparable to a sphygmomanometer at stress levels of 10kPa. We think that the origin of the high performance of the sensor is the deflection effect, the thin thickness and high elastic modulus of the AlN layer, and the thin thickness and low elastic modulus of the ...