Client – Brookhaven National Labs
Application requirements:
- Sapphire insulator (tube) brazed to Reactor Grade (RRR) Niobium weld adapters. The Niobium and Sapphire components are specific grades for this critical application.
- Assembly is used as a thermal break in a beam-line application. The assembly must withstand high temperature bake-outs, and cryogenic temperatures for operation. The parts will be frequently cycled through this significant thermal differential.
Critical Issues:
- The thermal extremes for this application involve significant mechanical stress due to thermal expansion of the components.
- Precise dimensional tolerances are required for the braze joint to be achieved without fracturing the sapphire insulator. The weld adapters must provide suitable mechanical strength for welding into the final assembly configuration.
Design Basis:
MPF considered similar ceramic break designs which typically involved brazing Al2O3 and Kovar components. The Reactor Grade Niobium is difficult to fabricate into complex shapes, so this limited the mechanical design of the weld adapters. Ultimately, MPF adopted a specific metallization-alloy combination and specialized fixtures in order to yield a successful braze.
Results:
MPF delivered the Sapphire-Niobium assemblies which were tested extensively for vacuum leaks, thermal cycling and mechanical stress. Vacuum and thermal performance was excellent. It was noted that the mechanical stresses of the parts were acceptable, but improvements would be beneficial in the service life of the assemblies. The design was revised based on mechanical modeling to reduce mechanical stresses at the braze interfaces.
