Optimized Sealing Process and Real-Time Monitoring of Glass-to-Metal Seal Structures

This protocol can be used to perform the real-time simultaneous temperature and stress monitoring of the metal-to-glass structures for the first time. The main advantage of this technique is that the fiber Bragg grating sensor can be well-fused with the metal-to-glass structure without destroying the insulation or hermeticity of the metal-to-glass structure. Demonstrating the procedure with Zhichun Fan will be Kangjia Hu, a master student from INET.

To process granulated glass powder, pour approximately 1.1 grams of the powder into a glass cylinder mold and place the mold onto the press machine. To compact the glass into a glass cylinder, switch on the press machine and place the resulting cylinder into a heating furnace to be centered. Remove the centered glass cylinder from the heating furnace and use a graphite gasket to manufacture the glass cylinder, the steel shell, and the Kovar conductor.

For residual stress measurement, first fuse the head of an optical fiber with an FC connector by the fusion splicers and match the FC connector with an interrogator to demodulate the wavelength and the FBG spectrum. Insert the FBG through a path in the sealing glass of the manufactured MTGS model with the grating of the FBG positioned precisely within the glass. Then, fix another FBG near the sealing glass to monitor the temperature only.

Connect the interrogator to a computer and use a heat treatment claw to place the centered glass cylinder, steel shell, FBGs, and Kovar conductor graphite gasket onto the quartz septum in the heating furnace. Raise the temperature to 450 degrees Celsius in five-degree Celsius per minute increments before dropping the temperature back to room temperature in 0.5 degrees Celsius per minute increments. Then record the realtime Bragg wavelength data in the computer software.

To monitor the stress and temperature, place one FBG into a centered glass cylinder and place a second FBG outside of the glass to monitor the temperature only. Place the MTGS model with the optical fiber into the furnace as demonstrated and use the standard heat treatment to process the MTGS model with an embedded FBG sensor, then impose 100, 200, 300, and 400 degree Celsius temperatures onto the model holding each temperature for 100 minutes. In this representative experiment, the standard heat treatment to produce the MTGS models with high pressure endurance was explored, demonstrating that the models can satisfy analyses under harsh environmental conditions.

The FBG can be well-fused with the MTGS structure and the residual strain in the sealing glass will be reflected by a Bragg wavelength shift after the heat treatment. The realtime stress changes in the sealing glass from 100 to 400 degrees Celsius are monitored precisely by the FBG sensor, and the decrease of residual stress in the sealing glass can be reflected instantaneously. Treat the cladding-stripped sensor with care and make sure that the position of the FBG is correct within the glass or the stress will not be measured precisely.

This technique can be used to measure residual stress in solar energy received tube directly and accurately, an accomplishment that has not been achieved in the previous studies. The method can be applied to achieve on-line distributed strain and the temperature in monitoring of sealing structure and to detect the failure at the first time.