No abstract

Meso-scale structure in polymeric foams determines the mechanical properties of the material. Density variations, even more than variations in the anisotropic void structure, can greatly vary the compressive and tensile response of the material. With their diverse use as both a structural material and space filler, polyurethane (PU) foams are widely studied. In this manuscript, quantitative measures of the density and anisotropic structure are provided by using micro X-ray computed tomography (microCT) to better understand the results of mechanical testing. MicroCT illustrates the variation in the density, cell morphology, size, shape, and orientation in different regions in blown foam due to the velocity profile near the casting surface. "Interrupted" in situ imaging of the material during compression of these sub-regions indicates the pathways of the structural response to the mechanical load and the changes in cell morphology as a result. It is found that molded PU foam has a 6 mm thick "skin" of higher density and highly eccentric morphological structure that leads to wide variations in mechanical performance depending upon sampling location. This comparison is necessary to understand the mechanical performance of the anisotropic structure.

As an analogue of the mineral pollucite (CsAlSi2O6), CsTiSi2O6.5 is a potential host phase for radioactive Cs. However, as 137Cs and 135Cs transmute to 137Ba and 135Ba, respectively, through the beta decay, it is essential to study the structure and stability of this phase upon Cs → Ba substitution. In this work, two series of Ba/Ti‐substituted samples, CsxBa(1−x)/2TiSi2O6.5 and CsxBa1−xTiSi2O7−0.5x, (x = 0.9 and 0.7), were synthesized by high‐temperature crystallization from their respective precursors. Synchrotron X‐ray diffraction and Rietveld analysis reveal that while CsxBa(1−x)/2TiSi2O6.5 samples are phase‐pure, CsxBa1−xTiSi2O7−0.5x samples contain Cs3x/(2+x)Ba(1−x)/(2+x)TiSi2O6.5 pollucites (i.e., also two‐Cs‐to‐one‐Ba substitution) and a secondary phase, fresnoite (Ba2TiSi2O8). Thus, the CsxBa1−xTiSi2O7−0.5x series is energetically less favorable than CsxBa(1−x)/2TiSi2O6.5. To study the stability systematics of CsxBa(1−x)/2TiSi2O6.5 pollucites, high‐temperature calorimetric experiments were performed at 973 K with or without the lead borate solvent. Enthalpies of formation from the constituent oxides (and elements) have thus been derived. The results show that with increasing Ba/(Cs + Ba) ratio, the thermodynamic stability of these phases decreases with respect to their component oxides. Hence, from the energetic viewpoint, continued Cs → Ba transmutation tends to destabilize the parent silicotitanate pollucite structure. However, the Ba‐substituted pollucite co‐forms with fresnoite (which incorporates the excess Ba), thereby providing viable ceramic waste forms for all the Ba decay products.

Silica aerogels are ultra low-density, high surface area materials that are extremely good thermal insulators and have numerous technical applications. However, their mechanical properties are not ideal, as they are brittle and prone to shattering. Conversely, single-walled carbon nanotubes (SWNTs) and graphene-based materials, such as graphene oxide, have extremely high tensile strength and possess novel electronic properties. By introducing SWNTs or graphene-based materials into aerogel matrices, it is possible to produce composites with the desirable properties of both constituents. We have successfully dispersed SWNTs and graphene-based materials into silica gels. Subsequent supercritical drying results in monolithic low-density composites having improved mechanical properties. These nanocomposite aerogels have great potential for use in a wide range of applications.

No abstract