The Research And Application Exploration Of New Type Inorganic Mechanoluminescence Materials

Mechanoluminescence（ML）is a classic luminescence phenomenon induced by mechanical stimuli.During the last decade,ML has shown great potential applications in stress sensors,self-powered display/illumination,imaging,anti-counterfeiting and other fields,breaking the limitations of the existing material application process.However,most ML materials doped with rare earth ions(Eu²⁺,Tb³⁺,Nd³⁺)or transition metal ions(Mn²⁺,Cu⁺)emitting colourful lights show the drawbacks of weak luminescence intensity,low sensitivity,high cost,limited emission colors,and high cost,constricting the application of existing ML materials.In addition,it is still challenging to determine the pressure in real time through pressure-induced color changes,and the reported ML materials are all monochromatic non-adjustable ML bands which is difficult to distinguish the dynamic change process of stress and pressure with naked eye,and also limits the application of ML materials.Therefore,the development of new ML materials with super-brightness ML and adjustable luminescence colors under different stress levels which can be widely used in stress-induced display,real-time pressure visual monitoring and other fields is indeed urgent.Bi element,as a green and environmental friendly main group metal has outstanding advantages such as variable valence,adjustable emission,and low cost.It was selected as the specific activator in the oxysulfide MZnOS（M=Sr,Ca）matrix which is a new type of ML material with low threshold,color adjustability,as well as ultra-high efficiency.In addition,the oxysulfide-based ML materials can be used to realize the development and application of new color controllable stress luminescent materials by energy transfer process through co-doping with rare earth ion Sm³⁺or transition metal ion Mn²⁺.The obtained main conclusions are as follows:（1）A novel ML material CaZnOS:x%Bi³⁺was prepared prepared via a high temperature solid-state reaction method,whose ML emission color can be adjusted largely from blue（465 nm）to cyan（490 nm）.More importantly,its ML intensity is about 50 times higher than that of the reported commercial ML material Zn S:Cu⁺.In addition,CaZnOS:Bi³⁺has a low stress threshold（<100 N）and high strain sensitivity.According to the fitting results of thermoluminescence spectra,the trap depth of ML generated inside is 0.68-1.20 e V.In addition,the synthetic ultrafine ML phosphors applied fluorescent inks can print specific patterns on paper or flexible films.Due to its excellent ML performance such as ultra-high sensitivity,low threshold and adjustable color,CaZnOS:Bi³⁺exhibits great potential applications in anti-counterfeiting technology and visual stress distribution.（2）A novel stress-emitting Sr ZnOS:x%Bi³⁺phosphor was synthesized via solid-state reaction method at high temperature.The Rietveld refinement shows that the Sr²⁺site is occupied by Bi³⁺ions.The optical band gap of the matrix increases and then decreases with the concentration of Bi³⁺ion.Under the excitation of 297 nm ultraviolet light or 5000 N,the cyan light at 480 nm and the red light at 613 nm are emitted from the energy level transition of Bi³⁺ion ³P_1,0→¹S₀ and the defect emission in the matrix.As the concentration of doped Bi³⁺ion increases,the color of PL changes from blue to dark blue,and finally to red;the emission color of ML gradually changes from pink to red;thermoluminescence（Th L）emission color changes from orange to red;The afterglow after UV excitation changes from cyan to red.Therefore,the regulation of PL,ML,afterglow,and Th L luminescence colors of Sr ZnOS:Bi³⁺can be achieved.As a new type of ML material,Sr ZnOS:Bi³⁺has great potential applications in multi-mode anti-counterfeiting,mechanical light conversion,stress sensing and stress imaging.（3）A novel CaZnOS:Bi³⁺,x%Sm³⁺phosphor was prepared by the high-temperature solid-phase reaction method,and the occupancy,photoluminescence and stress luminescence characteristics and energy transfer mechanism of the activator Bi³⁺and Sm³⁺ions were studied in detail.Rietveld refinement shows that the Ca²⁺position is co-occupied by Bi³⁺and Sm³⁺ions.Under the excitation of 366 nm,CaZnOS:Bi³⁺,Sm³⁺phosphors exhibited at 490 nm the cyan emission band derived from the ³P_1,0→¹S₀energy level transition of Bi³⁺,and the wavelength range of 550-750 nm.The four red emission bands are derived from the ⁴G_5/2→⁶H_J（J=5/2,7/2,9/2,11/2）energy level transition of Sm³⁺.As the concentration of Sm³⁺increases,the emission color of PL changes from cyan and white to orange-red,and ML changes from cyan to red,and the decay time of Bi³⁺decreases sharply due to the energy transfer between Bi³⁺and Sm³⁺.In addition,compared with a single Bi ion doped sample,the co-doped phosphor has better thermal quenching performance.（4）CaZnOS:Bi³⁺,x%Mn²⁺ML materials were prepared by high temperature solid-state reaction method.Under the pressure from 0 to 5000 N,the ML emission color can be adjusted from red→orange→white→cyan.Under the excitation of 375 nm,with the increase doping concentration of Mn²⁺ion,the PL and ML luminescence colors realize the regulation within the range of cyan-white-orange-red.The co-doping Mn²⁺ion significantly reduces the afterglow of Bi³⁺ion and largely improves the contrast of real-time stress monitoring.It is worth noted that through in-situ high-pressure XRD patterns and Raman spectroscopies,CaZnOS has a reversible phase transition above 20GP.The coorelation between the crystal structure,traps and ML performance is further discussed in detail,and the mechanism of ML is proposed.As a new ML material,CaZnOS:Bi³⁺,Mn²⁺realizing multi-color operation,has great potential applications in the fields of new mechanical stress-induced display,ultrasonic monitoring and particularly advanced multi-mode anti-counterfeiting technology.