Effect of Amorphous Si₃N₄ Nanoparticles on the Infrared Transmittance of α-Al₂O₃

Introduction and Objectives: This study aims to enhance the optical properties of polycrystalline alumina by controlling grain growth and minimizing structural defects. To achieve this, the influence of amorphous silicon nitride (Si₃N₄) nanoparticles as a reinforcing agent, in conjunction with MgO and La₂O₃ as sintering aids, on the infrared transmittance of PA was investigated. Furthermore, the role of the dispersing agent in improving slurry homogeneity and particle distribution was evaluated.Materials and Methods: The composite materials were synthesized via a chemical precipitation process, wherein alumina powder was mixed with Si3N4 nanoparticles, magnesium nitrate, lanthanum nitrate, and a dispersing agent in an aqueous solution.Ultrasonic waves were employed to enhance particle dispersion, and the slurry pH was adjusted to 10 to stabilize the suspension.Subsequent to powder preparation, spark plasma sintering was utilized to achieve densification and control grain growth. The microstructural and optical characteristics of the samples were then analyzed using X-ray diffraction, field emission scanning electron microscopy and infrared spectroscopy.Results: The findings indicated that the incorporation of Si3N4 nanoparticles (0.1 wt%) and a dispersing agent (2 wt%) led to an enhancement in infrared transmittance, with a maximum achieved of 85% within the 5–6 µm wavelength range. This observation was corroborated by microscopic analysis, which confirmed a reduction in grain size and an improvement in microstructural uniformity. Furthermore, X-ray diffraction analysis substantiated the preservation of the crystalline structure of alumina across all samples.Conclusion: The optimization of the sintering process, in conjunction with the incorporation of silicon nitride nanoparticles, facilitates the fabrication of transparent alumina, which exhibits augmented optical properties. These materials hold considerable promise for applications in infrared-sensitive systems, including missile guidance and optical sensor technologies.