Ba(1-x)Sr(x)Zn2Si2O7--A new family of materials with negative and very high thermal expansion

Elastic mechanics solution of thermal expansion of bi-material curved beam and its application to negative thermal expansion metamaterials

Thermal stress impacts various engineering fields significantly, such as aerospace and precision instruments. This adverse effect can be greatly reduced, if not eliminated, by the application of micro-thermal expansion materials, and bi-material beams are widely utilized in the design of micro-thermal expansion structures, thereby exhibiting great application potentials. The elasticity solution of bi-material curved beam under free thermal expansion has been proposed by scholars. Based on this solution, the simplified form is proposed in this paper, and extended to the case where the rotation angles at both ends of the circular arc are constrained under thermal loads. Besides, the geometric parameters and the nonlinear problems of the thermal expansion of bi-material curved beam are analyzed. In addition, a novel type of negative thermal expansion material has been designed by applying the bi-material curved beam to the tetra chiral and anti-tetra chiral materials. The proposed material has greater negative thermal expansion effect than the traditional tetra and anti-tetra chiral materials that are with straight beams.

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.

Thermomechanical properties of zero thermal expansion materials from theory and experiments

Origin and composition dependence of the anisotropic thermomechanical properties are elucidated for Ba_1-xSr_xZn₂Si₂O₇ (BZS) solid solutions. The high-temperature phase of BZS shows negative thermal expansion (NTE) along one crystallographic axis and highly anisotropic elastic properties characterized by X-ray diffraction experiments and simulations at the density functional theory level. Ab initio molecular dynamics simulations provide accurate predictions of the anisotropic thermal expansion in excellent agreement with experimental observations. The NTE considerably decreases with increasing Sr content x. This is connected with the composition dependence of the vibrational density of states (VDOS) and the anisotropic Grüneisen parameters. The VDOS shifts to higher frequencies between 0-5 THz due to substitution of Ba with Sr. In the same frequency range, vibrational modes contributing most to the NTE are found. In addition, phonon calculations using the quasi-harmonic approximation revealed that the NTE is mainly connected with deformation of four-membered rings formed by SiO₄ and ZnO₄ tetrahedra. The thermomechanical and vibrational properties obtained in this work provide the basis for future studies facilitating the targeted design of BZS solid solutions as zero or negative thermal expansion material.

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.

Morphology, topography, and crystal rotation during surface crystallization of BaO/SrO/ZnO/SiO2 glass

The initial stages of surface crystallization of glass with the composition 8BaO·8SrO·34ZnO·50SiO₂ (mol%) are studied with respect to crystal orientation and morphology.

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.

Surface crystallization of low thermal expansion Ba0.5Sr0.5Zn2Si2O7 from an 8 BaO·8 SrO·34 ZnO·50 SiO2 glass

Thermal treatment of a glass with the composition 8 BaO·8 SrO·34 ZnO·50 SiO₂ has led to the crystallization of a Ba_1−xSr_xZn₂Si₂O₇ solid solution at the surface.

Structurally Efficient Three-dimensional Metamaterials with Controllable Thermal Expansion

The coefficient of thermal expansion (CTE) of architected materials, as opposed to that of conventional solids, can be tuned to zero by intentionally altering the geometry of their structural layout. Existing material architectures, however, achieve CTE tunability only with a sacrifice in structural efficiency, i.e. a drop in both their stiffness to mass ratio and strength to mass ratio. In this work, we elucidate how to resolve the trade-off between CTE tunability and structural efficiency and present a lightweight bi-material architecture that not only is stiffer and stronger than other 3D architected materials, but also has a highly tunable CTE. Via a combination of physical experiments on 3D fabricated prototypes and numeric simulations, we demonstrate how two distinct mechanisms of thermal expansion appearing in a tetrahedron, can be exploited in an Octet lattice to generate a large range of CTE values, including negative, zero, or positive, with no loss in structural efficiency. The novelty and simplicity of the proposed design as well as the ease in fabrication, make this bi-material architecture well-suited for a wide range of applications, including satellite antennas, space optical systems, precision instruments, thermal actuators, and MEMS.

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

The thermal expansion behavior of sintered glass-ceramics containing high concentrations of Ba1-xSrxZn2Si2O7, a phase with very low and highly anisotropic thermal expansion behavior, was investigated. The observed phase has the crystal structure of the high-temperature phase of BaZn2Si2O7, which can be stabilized by the introduction of Sr(2+) into this phase. The high anisotropy leads to microcracking within the volume of the samples, which strongly affects the dilatometric thermal expansion. However, these cracks also have an influence on the nominal thermal expansion of the as-mentioned phase, which decreases if the cracks appear. Below a grain size of approximately 80 μm, the sintered glass-ceramics have almost no cracks and show positive thermal expansion. Hence, coefficients of thermal expansion between -5.6 and 6.5 × 10(-6) K(-1) were measured. In addition to dilatometric studies, the effect of the microstructure on the thermal expansion was also measured using in situ X-ray diffraction at temperatures up to 1000 °C.

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.

Very high or close to zero thermal expansion by the variation of the Sr/Ba ratio in Ba1−xSrxZn2Si2O7 – solid solutions

The compound BaZn₂Si₂O₇ shows a highly positive coefficient of thermal expansion, while replacement of more than 10% of Ba²⁺ by Sr²⁺ results in a negative coefficient of thermal expansion.