Crystallizing silicate glass seals

Sealing glasses are advantageous materials for joining of metals, ceramics or other glasses. For many applications, the sealed composite has to withstand high temperatures. Above the softening temperature of the glass, simple sealing glasses can usually not be used and crystallizing sealing glasses are required. The most important properties of the crystallized sealing glass are good adherence to the material to be sealed and a good match of their coefficients of thermal expansion (CTE). The chemical composition of a glass seal can be varied in a wide range and hence also its physical properties. For the joining process, glass powder is given in between the materials to be joined. During subsequent heating, sintering of the glass by viscous flow and finally nucleation and crystal growth takes place. The supplied temperature/time schedule should be designed in such a manner that densification occurs before a notable quantity of crystal phase is formed. The formed crystalline phases and their CTEs are decisive for the CTE of the crystallized seal, because a mismatch in the CTEs results in stresses during cooling. For non-cubic crystalline phases, also the anisotropy of the CTEs is of high importance, because it leads to internal stresses within the crystallized seal. A high degree of anisotropy should hence be avoided. The chemical composition of the crystallizing glass seal has to be tailored in such a manner that the type and quantity of the respective crystallized phases as well as the composition of the residual glass phase result in the aimed CTE as well as in good adherence to the materials to be joined. During application, both CTE and adherence should not change notably with time. Hence, during application at high temperature, further crystallization should not take place and type and volume concentration of the crystalline phase should remain constant.