Experimental investigations on B2O3-Al2O3-SiO2 glass-ceramics with different K2O/Na2O ratios for sealing to Kovar alloy

This work studied the effects of the K2O/Na2O ratio on the viscosity of B2O3-Al2O3-SiO2 parent glass and the microstructure, physical, and chemical properties of the resulting glass-ceramics after heat treatment. Additionally, the effect of sealing temperatures on the sealing strength of glass-ceramics-to-Kovar was studied. The results show that the viscosity of parent glass increases with the increase in the R (R=n(K2O)/[n(Na2O)+n(K2O)]) value, and the viscous activation energy of BAS parent glass is about 262.65–293.85 KJ/mol. After heat treatment, the parent glass precipitates the main crystalline phase of Al0.52Zr0.48O1.74 (PDF#53–0294) and ZrO2 (PDF#37–1484). As the R value gradually increases, the thermal expansion coefficient of glass-ceramics initially decreases before increasing within the range of 44.74×10−7 to 52.29×10−7 ℃−1. The density follows a similar trend, fluctuating between 2.40 and 2.45 g/cm3. Meanwhile, the resistivity of the glass-ceramics steadily increases from 1×1012.07 to 1×1014.82 Ω·cm. Moreover, the resistivity of GcN-3 (R=1/2) sample remains excellent at high temperatures, maintaining 3.94×109 Ω·cm at 600 ℃. Subsequently，the BAS glass-ceramics sample GcN-3 (R=1/2) was sealed with Kovar alloy at temperatures range from 930 to 970℃. With the increasing of sealing temperature, the sealing strength increased from 0.51 to 2.10 MPa. In the process of sealing, with the increase of temperature, the iron in the Kovar alloy and the silicon in the glass-ceramics diffused to produce effective sealing and formed different sealing layers. The sealing area was observed to consist of four distinct regions: the matrix alloy, the alloy iron-deficiency area, the iron-rich glass-ceramics and the matrix glass-ceramics.