New advances in long-life friction/force-induced luminescent materials

Friction/force luminescence refers to a luminous behavior produced by materials under the stimulation of tribology, mechanics and so on. Due to its unique tribological/mechanical-optical response characteristics, tribological/force-induced luminescence provides new ideas and new ways to realize tribological/mechanical sensing and its visualization. Most of the tribological/force-induced luminescent materials found so far only exhibit dynamic tribological and mechanically stimulated transient emission behavior, which greatly limits its application in the visualization and imaging of tribology/mechanics.

The development of long-life friction/force-induced luminescent materials is an effective way to address these problems, after researchers obtained tribological, long-life friction/force luminescence phenomena in specific material systems through trap engineering design. However, this type of long-life friction/force-induced luminescent material must undergo pre-irradiation before use, and pre-store energy inside its structure, which not only increases the difficulty of operation in practical application, but also makes it difficult to achieve the cyclic stable use of this type of material.

Researcher Wang Zhaofeng of the National Key Experimental Friction Physics and Sensing Research Group of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, has long been committed to the study of friction/force luminescence.

Previously, the team developed a variety of long-life friction/force-induced luminescent materials through trap engineering (Materials Horizons, 2019, 6, 2003-2008; Nano Energy, 2021, 83, 105860; Advanced Optical Materials, 2021, 9, 2100137), and aiming at the phenomenon of non-piezoelectric and non-trap friction/force-induced luminescence, an interfacial frictional electric field induced electron bombardment luminescence model is proposed (Nano Energy, 2022, 96, 107075).

Under the guidance of the above research work, the team recently developed a self-energizing, long-life friction/force-induced luminescence material – Sr3Al2O5Cl2:Dy3+/PDMS (SAOCD/PDMS), which can produce bright long-life friction/force-induced luminescence without pre-irradiation under the stimulation of tribology and mechanics (Figure 1), effectively avoiding the pre-irradiation needs of previous materials in use, and greatly improving the application convenience of long-life friction/force-induced materials.

Figure 1 Self-energizing, long-life friction/force-induced luminescence performance of SAOCD/PDMS

In order to reveal the principle of self-energization and long-life friction/force-induced lightluminescence of SAOCD/PDMS, the researchers further analyzed the thermal emission, cathode ray luminescence, triboelectric properties and matrix effects of luminescence of the material. The results show that the physical process of electron bombardment induced by the interfacial frictional electric field is the key to the self-charging and long-life friction/force luminescence of SAOCD/PDMS. Under mechanical stimulation, the interfacial triboelectric effect between the SAOCD particles and the PDMS chain causes electrons to migrate from SAOCD to PDMS. Later, under the action of the interfacial frictional electric field, the electron acceleration on the PDMS bombarded SAOCD, causing the electrons inside SAOCD to transition from the valence band to the guide band. Part of the electrons that transition to the conduction band are transferred to the characteristic energy level of Dy3+, resulting in self-activating friction/force-induced luminescence, and the other part of the electrons are trapped by traps (mainly shallow traps) in the structure, which is a process of self-charging. After mechanical stimulation, these self-charging electrons will spontaneously transfer to the characteristic energy levels of the conduction band and Dy3+ at room temperature, resulting in a unique self-energizing, long-life friction/force luminescence phenomenon (Figure 2).

Figure 2 Self-energizing, long-life friction/force-induced luminescence mechanism of SAOCD/PDMS

The discovery of self-energizing and long-life friction/force-induced luminescence overcomes the need for pre-irradiation of traditional long-life friction/force-induced luminescence materials, and has important application value in continuous mechanical display and imaging. At the same time, the researchers have further developed a storage and visual reading technology of tribological and mechanical information by using the unique self-charging physical processes in SAOCD/PDMS materials. As shown in Figure 3, under mechanical stimulation, tribological and mechanical information will be stored inside the material in the form of trap capture carriers, and then, under thermal stimulation, the stored tribological and mechanical information will be read in a visual form, and the stored and read tribological and mechanical information mainly include tribological/mechanical strength, occurrence time and its spatial distribution.

Fig. 3 Tribological, mechanical information storage and visual reading based on SAOCD/PDMS self-energizing process

The results were recently published in Advance under the title “Self-charging persistent mechanoluminescence with mechanics storage and visualization activities.” In the journal Science (。 Bai Yongqing, a doctoral student at the University of Chinese Academy of Sciences, is the first author of the paper, and Researcher Wang Zhaofeng is the corresponding author. Professor Liang Yongmin of Lanzhou University and Dengfeng Peng, a researcher at Shenzhen University, provided important guidance for this work.

The above work has been supported by the Gansu Provincial Outstanding Youth Fund, the Strategic Pioneering Science and Technology Special Project of the Chinese Academy of Sciences (Category B), and the “13th Five-Year Plan” Key Cultivation Project of Lanzhou Institute of Chemicals. (Source: Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences)

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