Interface-engineered calcium phosphate-hydroxyapatite heterostructure with enhanced full-spectrum mechanoluminescence for bridge stress distribution mapping

Mechanoluminescent (ML) materials, with their self-powered and real-time response characteristics, have demonstrated significant potential in structural health monitoring. However, limitations such as narrow spectral width and insufficient quantitative analysis capability still restrict their practical applications. This work presents an Eu-doped Ca 3 (PO 4 ) 2 /Ca 5 (PO 4 ) 3 OH (CPO/CPOH) heterostructured ML material. By synergistically optimizing interfacial electron behavior and phase composition, this heterostructure design increases ML intensity by 230% compared to single-phase materials. Notably, an interface-induced Eu 3+ →Eu 2+ self-reduction phenomenon is first observed . Introducing a reducing atmosphere , ML intensity is further amplified by 10.7 times, and full-spectrum ML emission with a full width at half maximum (FWHM) of 170 nm is successfully realized. Density functional theory (DFT) calculations confirm that the oxygen-bridge-connected interface structure exhibits thermodynamic stability, and directional electron transfer from CPO to CPOH reduces the system band gap (2.73 eV), providing an efficient pathway for carrier migration. Combined with thermoluminescence analysis, cycling stability, and interfacial electric field correlation, the ML mechanism is clarified as an interfacial electric field-driven carrier detrapping process. Furthermore, by integrating digital image correlation (DIC) technology with Lagrange transformation, this material successfully visualizes and enables quasi-quantitative analysis of stress concentration regions (0.05 - 1.16 MPa) in bridge structures, validating compressive performance differences among T-beam bridges, cylindrical bridges, and arch bridges. This study not only provides a novel heterostructure ML system and regulating spectral properties but also establishes a reliable visual quasi - quantitative analysis platform for real-time health monitoring of large-scale structures. This study overcomes the limitations of conventional mechanoluminescent (ML) materials by designing a novel heterostructure that achieves simultaneous 230% ML intensity enhancement and full-spectrum emission (170 nm FWHM), enabling quantitative stress mapping in structural diagnostics. We comprehensively investigated the Eu-doped Ca 3 (PO 4 ) 2 /Ca 5 (PO 4 ) 3 OH heterostructure system and summarize the key advances: 1.Novel Heterostructure-Driven Self-Reduction Mechanism: We pioneer a heterostructure-induced Eu 3+ →Eu 2+ self-reduction strategy, regulated by phase composition (90% CPOH at 1300°C), which boosts ML intensity by 230% and achieves full-spectrum emission (170 nm FWHM). This approach further amplifies Eu 2+ emission 20.3-fold under reducing atmospheres, resolving spectral narrowness in existing ML systems. 2.Quantitative Stress-to-Light Transduction: Integration of full-spectrum ML with digital image correlation (DIC) enables real-time visualization and Lagrangian transformation-based quantification of stress concentrations (0.09-1.16 MPa) in bridge structures. This eliminates qualitative limitations in ML-based structural health monitoring. 3.Electro-Optical Mechanism Validation: Rigorous experimental and computational studies confirm electrostatic-driven carrier detrapping as the ML mechanism, evidenced by trap depths (0.73/0.81 eV), electric field correlation (R² > 0.98), and >80-cycle stability-validated via DFT-calculated interfacial charge transfer (bandgap: 2.73 eV).