Glass sealing requirements for glass

    The good sealing of glass and other materials and the performance of the sealing products depend to a large extent on the performance of the sealing glass. Therefore, before sealing, understand the performance of the sealing glass used and choose the appropriate glass Sealing with other materials is extremely important. For sealing with other materials, the performance requirements of glass are: it does not deform under heated working conditions, maintains a rigid solid state, is firmly combined with other materials, and the joint can maintain good air-tightness, and the sealed product must have a certain amount of heat. Performance, electrical properties and related physical and chemical properties. Therefore, the selected glass should have good thermal shock resistance, electrical insulation properties, mechanical properties and chemical stability.

    (1) Thermal shock resistance of glass

    At room temperature, glass is a hard and brittle material. Under the action of external force, glass cannot be plastically deformed like metal, and it is prone to brittle fracture. When there is a sudden change in temperature, the glass body is destroyed because it cannot withstand the thermal shock. However, the thermal shock damage of glass is divided into two situations: when subjected to rapid heating, the surface of the glass body generates compressive stress and the interior is subjected to tensile stress; when subjected to rapid cooling, the situation is the opposite, that is, tensile stress is generated on the surface and compressive stress is exerted on the interior. Due to the effect of instantaneous thermal stress, the glass breaks first from the place where the stress is concentrated, that is, the junction between the glass and other materials or where there are surface defects. However, glass is "pressure-resistant and not tensile", that is, its tensile strength is only about 1/10 of the compressive strength. Therefore, when the thermal shock temperature difference is the same, the rapid heating or the rapid cooling rate is the same, the glass changes from a hot state to a cold state. The thermal shock conditions from cold to hot are much harsher, that is to say, the destructiveness of glass subjected to rapid cooling is much greater than that of rapid heat. It must be pointed out that whether it is rapid heat or rapid cold, if it is under the action of local temperature, thermal shock will be more destructive.

    The most important factor affecting the thermal shock is the thermal expansion coefficient a of the glass. The larger the a, the worse the thermal shock resistance. Therefore, when choosing the glass sealed with other materials, a material with a lower a should be used to make the sealing body have better thermal shock resistance. In addition, the thermal shock resistance of glass is also related to the geometry and size of the sealing member. When the glass is sealed with other materials, if the surface area of the glass is larger and the thickness is thicker, the thermal shock resistance of the sealing member will be worse.

    Except for special circumstances, the glass used for sealing with other materials is divided into two categories: one is the so-called hard (quality) glass, and its a is approximately (32~50)×10^-7/℃; the other is It is soft (quality) glass, and its a is slightly higher than 88×10^-7/℃. Since soft glass has a larger a than hard glass, the thermal shock resistance of soft glass is generally worse than that of hard glass.

    (2) Thermal expansion coefficient of glass

    Choose the glass that is sealed with other materials, and the a of the two must be as close as possible to make the stress as small as possible after the sealing. If the difference of the thermal expansion coefficient △a exceeds ±10%, a large internal stress will be generated at the sealing interface. Once the stress exceeds its strength limit, the sealing part will be destroyed. At this time, it will be at the junction of the sealing glass. Longitudinal linear cracks appear. In order to obtain a crack-free seal, it is generally required to be below the strain point of the glass, and the thermal expansion curves of the two are basically close.

    (3) Electrical insulation properties of glass

    As a sealing glass, it is generally required to have good electrical insulation properties, otherwise it cannot be widely used in the electronic field. At room temperature, the resistivity of quartz glass is as high as 10¹⁶Ω·cm or more, and the resistivity of ordinary glass is not lower than 10¹³Ω·cm. However, the resistivity of glass drops sharply as the temperature rises. In electronic glass, a corresponding parameter called T point has long been specified, which is the temperature when the volume resistivity is 10⁸Ω·cm to indicate the insulation performance of the glass. The higher the T point, the better the electrical insulation performance of the glass.

    When the temperature exceeds 1000°C, the glass almost becomes a conductor due to the linear decrease in resistivity. Because glass is a typical ion conductive substance, its conductive mechanism is obviously caused by the movement of alkali metal ions under the action of field strength. Therefore, for sealing parts that require high insulation performance, the monovalent metal oxide introduced into the glass composition should be reduced accordingly.

    In addition, when moisture or other impurities are adsorbed on the surface of the glass, the surface resistance will also decrease significantly. If alkali metal ions are precipitated on the surface, the resistance will drop sharply. For example, whenever the rainy season comes, many electronic components or other sealing products without a "protective layer" tend to have lower impedance, which is due to the increase in electrical conductance by capturing hydroxyl ions (OH-) from the moist air. Therefore, reducing the alkali metal oxide content in the glass composition and strengthening the surface treatment of the glass are all helpful to improve the electrical performance of the glass seal.

    (4) Water resistance of glass

    Most glasses have good resistance to water, acids, alkalis, gases and chemical reagents. As the electronic glass for sealing, the requirements are higher, otherwise it will limit its use in various harsh environments. The water resistance of glass is similar to thermal shock resistance, which mainly depends on the chemical composition of the glass, especially alkali metal oxides. The more alkali metal oxides in the glass composition, the worse the water resistance; on the contrary, the less alkali metal oxides, the better the water resistance. Secondly, increasing the content of Al2O3, ZrO2 or ZnO in the glass composition is conducive to the improvement of water resistance. Furthermore, heat treatment or surface treatment is also an effective way to improve the water resistance of glass.

    (5) Softening point of glass

    When selecting sealing glass, in addition to the above-mentioned main properties, it is also hoped that the softening temperature of the glass should not be too high. Because too high will lead to an increase in the sealing temperature on the one hand, and on the other hand, it will not be conducive to the fluidity during sealing. If the fluidity is poor, the glass body will not be able to cover the entire sealing space, and the wetting will be insufficient, and the sealing will be insufficient. Low intensity, which is also an important cause of chronic leakage.

    (6) Requirements for glass quality

    In the clarification process of the glass used for sealing in the later stage of melting, bubbles must be fully eliminated, and there should be no stones and obvious streaks. If the tensile stress formed by the stripes in the glass is added to the stress formed by the sealing of other materials, this tensile stress can cause the sealing member to burst. Of course, the faint stripes are not so dangerous. However, bubbles and pores are more harmful than streaks. Their existence is often the reason why glass cannot be used. Especially in the process of manufacturing tubular seals, when glass processing is performed on a lathe, one end of the pores will be affected. If no measures are taken, air bubbles will often form due to the expansion of air when processing the other end of the glass tube. If bubbles are formed in these sealing parts, they may break on the surface where the glass is in contact with other materials and form cracks in the sealing parts.

(Huiputech)