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Calcium silicates are abundant, but sparingly soluble, feedstocks of interest for making low-carbon alternative cements. Under hydrothermal and alkaline conditions, they can form crystalline calcium silicate hydrate (CCSH) products, which are abundant in Roman concrete, or they can form carbonates when CO2 is present. To understand when co-precipitation of CCSH and carbonate phases is possible, we studied the hydrothermal carbonation of a model calcium silicate, pseudowollastonite (-CaSiO3), at 150ºC and high pH as a function of CO2 source (CO2(g) or Na2CO3) and different concentrations of sodium, alumina, and silica. Our experiments produced a range of CCSH phases including tobermorite – 13Å, rhodesite, and pectolite, as early as one day after the start of our experiments. About 10.7% hydrated product was observed after 7 days of curing in 2 M NaOH solution. We also observed the formation of CaCO3 as both aragonite and calcite when carbon was introduced to our experimental system. The carbon source impacted the ratio of CaCO3 to CCSH phases in the reaction products. Availability of Na2CO3 produced a balance between CaCO3 and CCSH phases whereas CO2(g) produced more CaCO3 at about 36.4% by mass at the highest. Higher concentrations of Na+ increased precipitation of both CaCO3 and/or CCSH phases. The presence of excess silica, in the form of dissolved borosilicate glass from our reaction vessels under alkaline reaction conditions, also enhanced the formation of CCSH phases formed in some experiments. Supplemental Al2O3, a common constituent in many silicate feedstocks, also enhanced CCSH formation, likely by forming aluminum substituted phases under the conditions tested here. These chemical insights can be enabling in designing formulation and curing guidelines for novel cementitious materials. 

Abstract Temperature limitations in nickel‐base superalloys have resulted in the emergence of SiC‐based ceramic matrix composites as a viable replacement for gas turbine components in aviation applications. Higher operating temperatures allow for reduced fuel consumption but present a materials design challenge related to environmental degradation. Rare‐earth disilicates (RE 2 Si 2 O 7 ) have been identified as coatings that can function as environmental barriers and minimize hot component degradation. In this work, single‐ and multiple‐component rare‐earth disilicate powders were synthesized via a sol‐gel method with compositions selected to exist in the monoclinic C 2/ m phase ( β phase). Phase stability in multiple cation compositions was shown to follow a rule of mixtures and the C 2/ m phase could be realized for compositions that contained up to 25% dysprosium, which typically only exists in a triclinic, P , phase. All compositions exhibited phase stability from room temperature to 1200°C as assessed by X‐ray diffraction. The thermal expansion tensors for each composition were determined from high‐temperature synchrotron X‐ray diffraction and accompanying Rietveld refinements. It was observed that ytterbium‐containing compositions had larger changes in the α 31 shear component with increasing temperature that led to a rotation of the principal axes. Principal axes rotation of up to 47° were observed for ytterbium disilicate. The results suggest that microstructure design and crystallographic texture may be essential future avenues of investigation to ensure thermo‐mechanical robustness of rare‐earth disilicate environmental barrier coatings. 

Abstract Materials with tunable thermal properties enable on-demand control of temperature and heat flow, which is an integral component in the development of solid-state refrigeration, energy scavenging, and thermal circuits. Although gap-based and liquid-based thermal switches that work on the basis of mechanical movements have been an effective approach to control the flow of heat in the devices, their complex mechanisms impose considerable costs in latency, expense, and power consumption. As a consequence, materials that have multiple solid-state phases with distinct thermal properties are appealing for thermal management due to their simplicity, fast switching, and compactness. Thus, an ideal thermal switch should operate near or above room temperature, have a simple trigger mechanism, and offer a quick and large on/off switching ratio. In this study, we experimentally demonstrate that manipulating phonon scattering rates can switch the thermal conductivity of antiferroelectric PbZrO 3 bidirectionally by −10% and +25% upon applying electrical and thermal excitation, respectively. Our approach takes advantage of two separate phase transformations in PbZrO 3 that alter the phonon scattering rate in different manners. In this study, we demonstrate that PbZrO 3 can serve as a fast (<1 second), repeatable, simple trigger, and reliable thermal switch with a net switching ratio of nearly 38% from ~1.20 to ~1.65 W m −1 K −1 .