Glass-to-Metal Seal Composition for Thermal Cycling Reliability
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Solution Overview
Problem
The mismatch in thermal expansion coefficients between glass and metal components in solar energy vacuum tube collectors leads to cyclical thermal stresses, resulting in high mechanical breakdown rates and increased manufacturing costs due to the need for intermediary glasses and complex sealing designs.
Innovation Solution
A borosilicate glass composition with a thermal expansion coefficient closely matched to a nickel-iron-cobalt ferrous alloy (DIN 17745) is used to form a direct glass-to-metal bond, eliminating the need for intermediary glasses and reducing thermal stresses, thereby enhancing the reliability and manufacturability of the glass-to-metal joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a housekeeper seal design with thin metal tongue is used to absorb thermal stresses, then the glass-to-metal joint can accommodate thermal expansion differences, but the mechanical breakdown rate increases to more than 4% per annum
Solution Approach 1:
The patent changes the material parameters by selecting a metal alloy (nickel-iron-cobalt ferrous alloy DIN 17745) with specific thermal expansion coefficient (4.89×10−6 K−1) that closely matches the glass thermal expansion coefficient. This parameter matching eliminates the need for compensatory designs like thin metal tongues while maintaining joint reliability and mechanical strength.
2Reliability
If intermediary glasses are used to form glass-to-metal joints, then the thermal expansion mismatch is better managed, but the manufacturing cost and engineering work increase
Solution Approach 1:
The patent extracts and eliminates the intermediary glass layer from the glass-to-metal joint structure. By directly bonding glass to the specially selected metal alloy, the design removes unnecessary components while achieving better thermal expansion matching, thereby simplifying manufacturing and reducing costs.
Solution Approach 2:
The patent creates an effective composite system by directly bonding glass to a specifically engineered metal alloy (nickel-iron-cobalt ferrous alloy). This composite structure achieves optimal thermal expansion matching without requiring additional intermediary materials, simplifying the overall construction.
3Adaptability or versatility
If the thermal expansion coefficients of glass and metal are mismatched, then the glass-to-metal joint can accommodate different material properties, but cyclical thermal stresses cause joint failure
Solution Approach 1:
The patent optimizes the thermal expansion parameter by selecting a metal alloy with thermal expansion coefficient (4.89×10−6 K−1) that closely matches the glass thermal expansion coefficient. This parameter matching allows the joint to withstand temperature cycling without generating damaging thermal stresses, thereby improving reliability while maintaining material compatibility.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution provides a robust and reliable glass-to-metal seal with reduced material costs and manufacturing time, improving temperature cycling reliability and receiver efficiency by minimizing thermal stresses and eliminating the need for multiple glass grades and complex sealing components.
Implementation Method 1
The mismatch in the thermal expansion coefficients of typical glass envelope tubes and the metal at the glass-to-metal seal result in cyclical thermal stresses
Implementation Method 2
The cyclical thermal stresses, however, frequently lead to undesirably high mechanical breakdown rates of the glass-metal joint
Data Source
AI summary
A matched glass-to-metal connecting device for use in a vacuum tube collector for a solar energy collecting apparatus is made of a glass envelope and a metal sleeve directly bonded to the glass envelope. The glass envelope is made of a glass having a composition, in percent by weight on the basis of oxide content, consisting essentially of B2O3, 19; Al2O3, 8; Na2O, 2; K2O2, 3; BaO, 3; LiF, 1; and balance of SiO2 and the metal sleeve consists of metal material number 1.3981 of DIN 17745. The glass envelope has a thermal expansion coefficient that deviates from the metal part's thermal expansion coefficient by no more than 4% in the temperature range from 25° C. to 350° C.