Optimized Sealing Process and Real-Time Monitoring of Glass-to-Metal Seal Structures
Summary
Key procedures to optimize the sealing process and achieve real-time monitoring of the metal-to-glass seal (MTGS) structure are described in detail. The embedded fiber Bragg grating (FBG) sensor is designed to achieve online monitoring of temperature and high-level residual stress in the MTGS with simultaneous environmental pressure monitoring.
Abstract
Residual stress is an essential factor to keeping the hermeticity and robustness of a glass-to-metal seal structure. The purpose of this report is to demonstrate a novel protocol to characterize and measure residual stress in a glass-to-metal seal structure without destroying the insulation and hermeticity of sealing materials. In this research, a femto-laser inscribed fiber Bragg grating sensor is used. The glass-to-metal seal structure that is measured consists of a metal shell, sealing glass, and Kovar conductor. To make the measurements worthwhile, the specific heat treatment of metal-to-glass seal (MTGS) structure is explored to obtain the model with best hermeticity. Then, the FBG sensor is embedded into the path of sealing glass and becomes well-fused with the glass as the temperature cools to RT. The Bragg wavelength of FBG shifts with the residual stress generated in sealing the glass. To calculate the residual stress, the relationship between Bragg wavelength shift and strain is applied, and the finite element method is also used to make the results reliable. The online monitoring experiments of residual stress in sealing glass are carried out at different loads, such as high temperature and high pressure, to broaden functions of this protocol in harsh environments.
Introduction
Metal-to-glass sealing is a sophisticated technology that combines interdisciplinary knowledge (i.e., mechanics, materials, and electrical engineering) and is widely applied in aerospace1, nuclear energy2, and biomedical applications3. It has unique advantages such as higher temperature and pressure endurance compared with organic material sealing structures. According to the difference of coefficient of thermal expansion (CTE), MTGS can be divided into two types: matched seal and mismatched seal4. As for the matched seal, the CTE of metal (αmetal) and sealing glass (αglass) are nearly the same to reduce the thermal stress in sealing materials. However, to keep good hermeticity and mechanical robustness of the seal structure in harsh environments (i.e., high temperature and high pressure), the mismatched seal displays better performance than the matched seal. Due to the difference between αmetal and αglass, the residual stress generates in sealing glass after the annealing process of MTGS structure. If the residual stress is too large (even exceeding the threshold value), the sealing glass displays small defects, such as cracks. If the residual stress is too small, the sealing glass loses its hermeticity. As a result, the value of residual stress is an important measurement.
Analysis of residual stress in MTGS structures has aroused research interests of many groups around the world. The numerical model of axial and radial stress was built based on thin shell theory5. The finite element method was applied to obtain the global stress distribution of an MTGS structure after the annealing process, which was consistent with experimental results6,7. However, because of limitations involving small size and electromagnetic interference, many advanced sensors are not suitable for these circumstances. The indentation crack length method was reported to measure the residual stress in the sealing material of MTG; however, this method was destructive and could not achieve real-time online monitoring of stress changes in glass.
Fiber Bragg grating (FBG) sensors are small in size (~100 µm) and resistant to electromagnetic interference and harsh environments8. In addition, the components of the fiber are similar to those of sealing glass (SiO2), so FBG sensors have no effects on the hermeticity and insulation of the sealing material. FBG sensors have been applied to the residual stress measurement in composite structures9,10,11, and results showed that it displayed good measuring precision and signal response. Simultaneous temperature and stress measurements may be achieved by fiber Bragg grating arrays on one optical fiber12,13.
A novel protocol based on an FBG sensor is demonstrated in this study. The appropriate preparation for the special MTGS structure has been explored by adjusting the maximum heat temperature to ensure the good hermeticity of the MTGS structure. The FBG sensor is embedded in the prepared path of sealing glass to fuse the FBG and glass together after the heat treatment. Then, the residual stress can be obtained by the Bragg wavelength shift of the FBG. The MTGS structure with the FBG sensor is placed under high temperature and high pressure environments to achieve online monitoring of residual stress under changing loads. In this study, the detailed steps to produce an MTS structure with a FBG sensor are outlined. The results show the feasibility of this novel protocol and establish the foundation for the failure diagnosis of an MTGS structure.
Protocol
Representative Results
Discussion
The critical steps for the stress measuring of sealing material of MTGS structure at high temperature and high pressure include 1) manufacturing of the MTGS models with the FBG sensor, of which the grating region is located at the middle of sealing glass; 2) heating of the whole model using a standard heat treatment process, and after the model cools to RT, the FBG sensor will becomes well-fused with MTGS model, and the residual stress can be measured by Bragg wavelength shift; 3) placing of the complete model into the f…
Divulgazioni
The authors have nothing to disclose.
Acknowledgements
This work has been supported by the National S&T Major Project of China (ZX069).
Materials
| ABAQUS | Dassault SIMULA | ABAQUS6.14-5 | The software to carry out numerical simulation. |
| Fiber Bragg grating sensors | Femto Fiber Tec | FFT.FBG.S.00.02 Single | apodized FBG |
| Fusion splicer | Furukawa Information Technologies and Telecommunications | S123M12 | FITEL's line of fusion splicers provides an excellent solution for both field and factory splicing applications。 |
| Glass powder | Shenzhen Sialom Advanced Materials Co.,Ltd | LC-1 | A kind of low melting-point glass powder (380℃). |
| Graphite mold | Machining workshop of Tsinghua University | Graphite | The mold to locate each part of the metal-to-glass structure. |
| Heating furnace | Tianjin Zhonghuan Electric Furnace Technology Co., Ltd | SK-G08123-L | vertical tubular furnace |
| Kovar conductor | Shenzhen Thaistone Technology Co., Ltd | 4J29 | A common material used for the electrical penetration in the metal-to-glass seal structure |
| Optical interrogator | Wuhan Gaussian Optics CO.,LTD | OPM-T400 | FBG spectrum analysis modules |
| Pro/Engineer | Parametric Technology Corporation | PROE5.0 | The software to establish the 3D geometry. |
| Steel shell | Beijing Xiongchuan Technology Co., Ltd | 316 stainless steel | A kind of austenitic stainless steel |
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