Design principles of ceramic-metal sealing structure

(1) Matching principle of linear expansion coefficient

    The linear expansion coefficients of metal parts and ceramic parts in ceramic metal sealing should be consistent or close. The difference should be within the range of 7% to 10% in the entire area from room temperature to welding temperature, such as 4J34 and 95% Al2O3 porcelain.

(2) The principle of low elastic modulus and low yield limit

    In non-matched sealing, due to the large difference in thermal expansion, metal materials with low elastic modulus and low yield limit should be selected as parts, such as oxygen-free copper.

(3) The principle of thermal conductivity close to

    When choosing the mating material, in addition to the matching of thermal expansion, the thermal conductivity of the two is relatively close, which is beneficial to reduce the thermal stress of the sealing member.

(4) Principle of compressive stress

    Since the tensile strength of ceramics is about 1/10 of the compressive strength, when designing the sealing parts, the ceramic parts should be subjected to compressive stress as much as possible. For example, for high-strength, high-linear expansion coefficient stainless steel, an outer envelope structure should be adopted.

(5) Principle of reducing stress

    Under the premise of ensuring sufficient strength of the sealing member, the metal thickness on the sealing surface should be as thin as possible to relieve part of the stress. For example, the thickness of the metal on the sealing surface is usually 0.5 to 1.0 mm. In addition, a tubular thin tube is better than a solid needle.

(6) The principle of avoiding stress concentration

    Stress concentration in the sealing parts is very harmful, often causing poor reliability and even catastrophic consequences. For example, the output window sealing parts should be round windows as much as possible, and rectangular or square windows should be avoided.

(7) Transitional sealing principle

    During the sealing process, especially the needle sealing structure, transitional sealing should be adopted as much as possible to reduce stress. For example, when Mo and Kovar needles φ≥lmm, solid needles should not be used for direct sealing, but transitional sealing should be used.

(8) Principle of knife edge sealing

    Knife-edge sealing has been widely adopted recently. The main reasons are: simple parts processing, lower cost; convenient parts matching and clamping, easy for large-scale production; small sealing stress, which is beneficial to improving product quality and reliability. When the structural conditions permit, knife-edge sealing should be used as much as possible. For example, the sealing of a large-diameter porcelain ring and a stainless steel cylinder.

(9) Flexible structure principle

    In addition to the above-mentioned reduction in the thickness of the metal parts to reduce the stress, the same goal can be achieved by adopting a flexible structure. Therefore, the distance between the seals should be extended as much as possible in the design, and the amount of solder should be reduced. The entity cannot be formed during welding. Or "dead lump".

(10) Principle of solder optimization

    When selecting solder, use materials that have good plasticity and do not form brittle compounds with the base material during welding. In terms of shape, wire-like materials should be used. According to reports, under the same conditions, the wire-like solder is 20%~35% higher than the sheet-like sealing strength, and the solder with lower soldering temperature should be used as much as possible.

(Huiputech)