Low Pressure Sensing Characteristics of Mechanoluminescence Film with 3D Microstructure
Authors: Fujio Yuki , Kurihara Kazuma , Nakajima Tomohiko , Kikunaga Kazuya , Xu Chao-Nan , Terasaki Nao 
Published: December 2024

Mechanoluminescence (ML), which is light emitting repeatedly induced by external mechanical stimuli, has received widespread attention worldwide because it has been applied to various applications [1-3]. Among different applications, pressure sensors using a composite of ML material and organic resin have superior advantages in wireless detection using light-emitting and multi-scalable applications compared with electric devices. For this interest worldwide, we have applied an ML phenomenon into the pressure sensing technique using an ML composite sheet composed of europium doped strontium aluminate (SrAl2O4:Eu) ML material and epoxy resin. As a result of evaluating the load response of the flat ML composite sheets formed on rigid acrylic plates, it was found that the load distribution on the sole due to human weight bearing can be visualized two-dimensionally with relatively high spatial resolution. However, we also found that such a simple configuration, like a flat composite film on a rigid substance, limits detection sensitivity to several hundred newtons. Here, we conceived the improvement technique in the low-pressure response of the ML composite film. It is well-known that the ML intensity is proportional to the equivalent strain and its rate. When the same load is applied to different structures, the ML intensity of a configuration with the augmentation of stress or with the stress concentration region will be higher. Thus, it could be expected that we can control the pressure sensitivity of the ML sensor by designing a surface structure of the ML composite sheet. In this study, therefore, we challenged improving low-pressure sensing characteristics with a focus on the surface geometry of the ML composite film.

In our pursuit of enhancing low-pressure sensing characteristics, we embarked on an innovative approach the use of micro-sized pillar array films. These films were prepared using a thermal imprinting method with cyclo olefin polymer (COP) or laminated poly methyl methacrylate/polycarbonate (PMMA/PC) films. The micro-pillar array pattern was transferred onto these polymer films, and an ML ink consisting of a mixture of SrAl2O4:Eu ML material and epoxy resin was directly sprayed on the surface of those polymer films with micro-pillar array patterns. Before forming the ML layer on the COP film, we introduced a SiO2 interlayer using a photo-assisted chemical solution deposition (PCSD) process [4] to enhance the adhesive strength between the ML layer and COP film. The SrAl2O4:Eu ML material was synthesized by a high-temperature solid-state reaction method [5]. We then evaluated the pressure-sensing characteristics under compression testing in the load range of 0 – 90 N using the prepared ML films with different configurations.

Considering the amplified mechanism of the ML intensity for the micro-pillar array film, we can predict that the ML intensity is primarily affected by the configuration of the micro-pillar arrays, such as height, width (diameter), and pillar density, as well as the elastic modulus of the base polymer. Thus, we investigated the effect of such a configuration and mechanical factors of the micro-pillar array on the pressure-sensing characteristics. As a result, we found that high height and low elastic modulus micro-pillar array exhibited the highest ML intensity and responded even to small loads of 1 N or less. This finding suggests the potential of micro-array films for high-sensitivity pressure sensing applications. To verify the amplified mechanism of the ML intensity for the micro-pillar array film, we observed cross-sectional ML images under dynamic compression tests. The cross-sectional ML distribution image showed that the micro-pillars made of soft resin (COP) were more easily deformed and exhibited higher ML intensity than micro-pillars made of hard resin (PMMA/PC). Furthermore, since bending deformation is observed as the height of the micro-pillar increases, it is concluded that soft and tall micro-pillars produce high ML intensity by more considerable deformation and bending mode.

References

[1] C.N. Xu, T. Watanabe, M. Akiyama, X.G. Zheng, Appl. Phys. Lett., 74 (1999) 2414-2416.

[2] J.C. Zhang, X. Wang, G. Marriott, C.N. Xu, Prog. Mater. Sci., 103 (2019) 678-742.

[3] N. Terasaki, “Interfacial phenomena in adhesion and adhesive bonding”, chapter 4 (2024) 209-290.

[4] T. Nakajima, S. Hanawa, T. Tsuchiya, J. Mater. Chem. C, 7 (2019) 5193.

[5] Y. Fujio, C.N. Xu, N. Terasaki, Sensors, 22 (2022) 5476.