Glass‐Ceramics in the System BaO–SrO–ZnO–SiO₂ with Adjustable Coefficients of Thermal Expansion

Abstract

Glasses from the system BaO-SrO-ZnO-SiO 2 with different Ba/Sr ratios were characterized regarding crystallization behavior as well as the thermal expansion of almost fully crystallized glasses. Depending on the SrO concentration, different crystalline phases precipitate from the glasses. Those with low SrO concentrations precipitate crystals with the structure of low-temperature BaZn 2 Si 2 O 7 as one of the major phases. Higher SrO concentrations cause the formation of Ba 1 -x Sr x Zn 2 Si 2 O 7 solid solutions with the structure of high-temperature BaZn 2 Si 2 O 7 . Both, the low-as well as the high-temperature phase exhibit very different thermal expansion behaviors ranging from a very high coefficient of thermal expansion in the case of the low-temperature phase to a very low coefficient of thermal expansion in the case of the high-temperature phase. The glass-ceramics with the highest and that with the lowest coefficient of thermal expansion measured between 100°C and 800°C show a difference of 7.9 3 10 -6 K -1 , which is caused solely by a substitution of BaO with SrO. In contrast, the maximum variation in the thermal expansion of the glasses was only 1.5 3 10. The microstructure of sintered and afterward crystallized glass powders was analyzed via scanning electron microscopy and showed crack-free samples with low porosity.

Redox effects and formation of gold nanoparticles for the nucleation of low thermal expansion phases from BaO/SrO/ZnO/SiO2 glasses

“…the right side, while for polyvalent elements in which the number of the transferred electrons is only 1, the reaction enthalpy is usually much lower. That has been reported for Ag I /Ag 0 , 16 (5) formed by other redox pairs, it is, however, known that the most pronounced changes in the respective concentrations take place at temperatures T g À 100K < T < T g + 100 K. 18,19 That means that redox reactions are still possible at temperatures below T g . In this temperature range, the establishment of the equilibrium during temperature change is already affected by the kinetics, as shown by high temperature spectroscopy.…”

Variable thermal expansion of glass-ceramics containing Ba1−xSrxZn2Si2O7

“…The extremely high thermal expansion at low SrO-concentrations is due to the precipitation of LT-BaZn 2 Si 2 O 7 and the re-increase of the CTE at higher SrO-concentrations is caused by the formation of Sr 2 ZnSi 2 O 7 , with a CTE of around 9.5·10 −6  K −1   9 . The CTEs strongly depend on the temperature and hence the CTEs in the lower right part of Fig.…”

“…Glass-ceramics based on alkaline earth silicates possess high thermal expansion, which strongly depends on the composition of the base glass 9 . Typical CTE values of such materials are in the range from 8 to 12·10 −6 K −1 , according to the CTEs of the respective crystalline phases 7 , 10 .…”

“…Above the phase transition temperature, very low and temperature dependent thermal expansion can be observed 14 . Recently, it was reported that the CTEs of glass-ceramics in the base system BaO-ZnO-SiO 2 decrease drastically if SrO is added to the glass composition due to the formation of Ba 1−x Sr x Zn 2 Si 2 O 7 solid solutions with the crystal structure of the high-temperature polymorph of BaZn 2 Si 2 O 7 (HT-BaZn 2 Si 2 O 7 ) 9 , 15 . These solid solutions exhibit very low thermal expansion and hence sealing glasses with much lower CTEs can be obtained 9 , 16 .…”

Negative Thermal Expansion in Ba0.5Sr0.5Zn2SiGeO7

“…Hence, a detailed knowledge on the phase transition temperature for both the HT- as well as the LT-phase is necessary in order to control the thermal expansion behavior of materials, especially of glass-ceramics being able to precipitate the described solid solutions [8]. …”