Thermal Expansion of Sintered Glass Ceramics in the System BaO-SrO-ZnO-SiO2 and Its Dependence on Particle Size

Solid solutions based on BaZn2Si2O7 with thermal expansions from negative to highly positive – a review

Replacing Ba2+ by Sr2+ stabilizes the high temperature phase and leads to zero or negative thermal expansion. Replacing Zn2+ by Mg2+ or Co2+, Ni2+, Mn2+, Cu2+ shifts the phase transition to higher temperatures and leads to high thermal expansion.

Phase stability determination of negative thermal expansion silicates by theory and experiment

Materials that exhibit zero thermal expansion have numerous applications, ranging from everyday ceramic hobs to telescope mirrors to devices in optics and micromechanics. These materials include glass ceramics containing crystal phases with negative thermal expansion in at least one crystallographic direction, such as Ba_1-xSr_xZn_2-2yMg_2ySi₂O₇ solid solutions. However, the volume increase associated with the martensitic phase transformation in these crystals often hinders their use as zero thermal expansion materials at operating temperatures near the transition temperature T_t. Here, an approach to rapidly predict T_t of such materials as a function of chemical composition based on a combination of density functional theory simulations and experiments has been developed and applied to Ba_1-xSr_xZn_2-2yMg_2ySi₂O₇. Its central element is the modeling of free energy as a function of temperature and chemical composition using a composition-dependent Debye model augmented by an empirical correction, which incorporates the effects of anharmonic lattice vibrations. This approach provides T_t predictions with an estimated uncertainty of about ±100 K, which is similar to the accuracy of computationally much more demanding simulations of polymorphous phase transitions. In addition, this approach allows computationally efficient determination of the chemical compositions at which the Ba_1-xSr_xZn_2-2yMg_2ySi₂O₇ phase with the desired thermal properties will be formed during synthesis, facilitating the targeted design of zero thermal expansion materials.

Core–shell structures with metallic silver as nucleation agent of low expansion phases in BaO/SrO/ZnO/SiO2 glasses

The role of silver as a nucleating agent in BaO/SrO/ZnO/SiO₂ glasses is studied with a range of microstructure-characterization techniques, such as scanning transmission electron microscopy, ultraviolet-visible spectroscopy, and X-ray diffraction.

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

Glasses in the system BaO/SrO/ZnO/SiO₂ containing 0.01 and 0.1 mol% gold were used to study the formation of gold nanoparticles with the aim to use them as nucleation agents. In order to promote gold clustering, the glasses were additionally doped with 0.5 mol% Sb₂O₃. Depending on the heat treatment schedule, Au particle sizes were in the range from 6 to above 50 nm. In contrast to many other gold ruby glass systems, the clustering is completely prevented by the absence of antimony; then the glasses remain colorless. Surprisingly, at higher temperatures, a re-dissolution of gold clusters was also observed, which now allows the formulation of a more comprehensive model concerning the redox and clustering behavior. This growth model is completed by the fact that a high gold concentration enables the stabilization of much smaller Au clusters. Mie theory with the aid of quantum confined size-dependent dielectric functions was successfully used to describe the optical behavior of the gold nanoparticles also for sizes below 10 nm. These results were confirmed using high resolution scanning transmission electron microscopy, including energy dispersive X-ray spectroscopy. It could also be shown that small gold particles up to a size of 50 nm are not effective as nucleating agents.

Glasses in the system BaO/SrO/ZnO/SiO₂ containing 0.01 and 0.1 mol% gold were used to study the formation of gold nanoparticles with the aim to use them as nucleation agents.

WO3 as a nucleating agent for BaO/SrO/ZnO/SiO2 glasses – experiments and simulations

In glasses from the BaO–SrO–ZnO–SiO₂ system, the addition of up to 4 mol% WO₃ leads to volume crystallization.

Variable thermal expansion of glass-ceramics containing Ba1-xSrxZn2Si2O7

Up to now, the thermal expansion behavior of multiphase glass-ceramics cannot be predicted reliably because of the nescience about the formation of the type and concentration of crystalline phases. In the system BaO-SrO-ZnO-SiO₂, recently a new phase based on Ba_1−xSr_xZn₂Si₂O₇ solid solutions was found, which exhibits unexpected low and highly anisotropic thermal expansion, which can be used for an adjustment of the thermal expansion properties. In the case of sealing materials for high-temperature reactors, the formation of this phase should be avoided. Hence, in this manuscript the concentration thresholds in which these solid solutions precipitate from glasses were determined. The phase analysis was correlated with the thermal expansion behavior of the glass-ceramics. Depending on the Ba/Sr-ratio of the glasses and the considered temperature range, the coefficients of thermal expansion of the glass-ceramics vary between 19.4·10⁻⁶ K⁻¹ and 4.8·10⁻⁶ K⁻¹. The concentration thresholds in which the as mentioned phases form via crystallization of glasses differ strongly from the literature values obtained via conventional ceramic mixed oxide route.

Negative Thermal Expansion in Ba0.5Sr0.5Zn₂SiGeO₇

Solid solutions with the composition Ba_0.5Sr_0.5Zn₂Si_2-xGe_xO₇ and BaZn₂Si_2-xGe_xO₇ were prepared with different values of x using a conventional mixed oxide route. Both compounds exhibit very different thermal expansion, which is due to the different crystal structures. Ba_0.5Sr_0.5Zn₂Si_2-xGe_xO₇ solid solutions exhibit the structure of high-temperature BaZn₂Si₂O₇ and show negative thermal expansion, which was proven via high-temperature X-ray diffraction. Up to around x = 1, the crystal structure remains the same. Above this value, the low-temperature phase becomes stable. The Sr-free solid solutions have the crystal structure of low-temperature BaZn₂Si₂O₇ and show also a limited solubility of Ge. These Sr-free compositions show transitions of low- to high-temperature phases, which are shifted to higher temperatures with increasing Ge-concentration.