Geopolymer Adhesives for DUV LED Packaging: Synthesis and Bonding Mechanism

In the post-pandemic era, awareness of disinfection and sterilization had heightened, driving an increased demand for ultraviolet (DUV) light-emitting diodes (LEDs). Geopolymers effectively addressed the reliability issues of existing organic materials under DUV radiation. This study focused on investigating the bonding mechanism of geopolymer adhesive with DUV LEDs packaging alumina substrate. The bonding strengths of geopolymers with varying Si/Al and Na/Al molar ratios to alumina were tested through orthogonal experiments, identifying a sample with a maximum strength of 47.0 ± 9.0 MPa. Fourier transform infrared spectroscopy bands at 1026 and 869 cm⁻¹confirm geopolymerization in this highest-strength sample, which has a Si/Al molar ratio of 2.2 and a Na/Al molar ratio of 1. Scanning electron microscopy and nuclear magnetic resonance analyses show that this sample possesses a denser microstructure and the highest Q⁴(3Al) peak area fraction, indicating a more homogeneous geopolymerization reaction compared to other samples. Transmission electron microscopy clarified the bonding mechanism between the geopolymer and alumina. Fast Fourier transform showed the overlap of alumina’s hexagonal crystal structure with the amorphous geopolymer gel. Energy-dispersive X-ray spectroscopy revealed significant overlaps of Al and Si elements at the bonding interface. Together, these techniques provided strong evidence of chemical bonding at the interface. This study enriches the theory of heterogeneous material interconnections in fully inorganic DUV LED packaging, with the potential to further enhance packaging reliability.