FA: Structural analysis and failure assessment of optical cell windows in ammonothermal method

21. Oktober 2025

Sai Kumar Eedupalli

—

This project investigates the structural integrity and failure mechanisms of optical cell win-dows used in high-pressure, high-temperature ammonothermal reactors for gallium nitride (GaN) crystal growth. Ammonothermal synthesis operates under extreme conditions—pres-sures of 100–300 MPa and temperatures up to 600 °C—requiring robust optical viewing win-dows to enable in-situ monitoring. These windows, typically fabricated from sapphire or bo-ron carbide, are subjected to complex thermomechanical loads during mounting, pressuriza-tion, and thermal cycling, leading to various failure modes, including fracture, leakage, and coating degradation. Using a combined finite element analysis (FEA) and experimental corre-lation approach, this study simulates the stress distribution, deformation, and failure initiation within the optical window assembly. Simulations were performed in both SolidWorks Simu-lation and ANSYS Mechanical under three loading scenarios: mounting preload (30 kN force, 30 N.m torque), intermediate operation (160 MPa, 300 °C), and whole operation (300 MPa, 600 °C). The results consistently identify the inner aperture of the sapphire window as the critical stress-concentration region, with peak von Mises stresses of 118 MPa during mount-ing, 210 MPa at intermediate conditions, and 310 MPa under full operational loads. The anal-ysis reveals that mounting-induced tensile stresses, combined with pressure-induced bending and thermal expansion mismatch, drive the initiation of failure. Gold seal creep, torque relax-ation, and surface defects further exacerbate the risk of catastrophic window fracture and seal-ing failure. Based on these findings, design improvements are proposed, including geometric optimization of the window seat, use of thinner gold foils, enhanced surface polishing, and operational guidelines for step-wise pressurization. This work bridges the gap between reac-tor design and operational reliability, providing actionable insights for developing safer, more durable optical viewing cells. The outcomes support enhanced in-situ monitoring capability and operational safety in ammonothermal GaN growth systems.