No abstract

We report on a rapid sintering protocol, which optimizes the preparation of 0‐29 mol% Gd‐doped ceria ceramics with density ≥98% of the theoretical crystal lattice value. The starting material is a nanometer grain‐sized powder prepared by carbonate co‐precipitation and calcined with minimal agglomeration and loss of surface area. Slow (5°C/min) heating of the green‐body from 500°C to the optimum temperature of rapid sintering (T1opt, dwell time <1 minute) followed by 20°C/min cooling to 1150°C with 6 minutes dwell time, produces maximum pellet density. T1opt increases from 1300 to ~1500°C with increase in Gd‐content, while the average grain size in the maximally dense pellets, as determined by scanning electron microscopy, ranges between 600 nm and ~1 μm. For each doping level, the logarithm of the average grain size decreases linearly with 1/T1. By avoiding extended exposure to sintering temperatures, this protocol is expected to minimize undesirable Gd segregation.

Incorporation of carbon nanotubes (CNTs) into liquid crystalline phases of cellulose nanocrystals (CNCs) may be used for preparation of hybrids with novel optical, electrical, and mechanical properties. Here, we investigated the effect of nanoparticle diameter, geometry, aspect ratio, and flexibility on the exclusion of dispersed carbon nanostructures (CNs) from the chiral nematic phase (N*) of the CNCs. Although the CNs are nicely dispersed in isotropic suspensions of CNCs, we observe that fullerenes, carbon black, and CNTs are depleted from the N* phase. This observation is surprising as theoretical predictions and previous observations of nanoparticles indicate that nanometric inclusions would be incorporated within the N* phase. Cryogenic transmission electron microscopy imaging reveals that the dispersed CNs induce misorientation of the CNCs, irrespective of their geometry and size. Rheological measurements suggest that about 10% of the CNCs are affected by the CNs. The multiparticle nature of the interaction may be the origin of the nonsize selective exclusion of the CNs. Re-entrant behavior is observed at high CNC concentrations (about 13 wt %), where a (nematic) gel-like phase kinetically traps the CNs. These phases exhibit non-Newtonian flow behavior and birefringence, offering a pathway for preparation of nonisotropic CNCs−CNT composites and thin films via liquid processing.

Hybrids comprising cellulose nanocrystals (CNCs) and percolated networks of single-walled carbon nanotubes (SWNTs) may serve for the casting of hybrid materials with improved optical, mechanical, electrical, and thermal properties. However, CNC-dispersed SWNTs are depleted from the chiral nematic (N*) phase and enrich the isotropic phase. Herein, we report that SWNTs dispersed by non-ionic surfactant or triblock copolymers are incorporated within the surfactant-mediated CNC mesophases. Small-angle X-ray measurements indicate that the nanostructure of the hybrid phases is only slightly modified by the presence of the surfactants, and the chiral nature of the N* phase is preserved. Cryo-TEM and Raman spectroscopy show that SWNTs networks with typical mesh size from hundreds of nanometers to microns are distributed equally between the two phases. We suggest that the adsorption of the surfactants or polymers mediates the interfacial interaction between the CNCs and SWNTs, enhancing the formation of co-existing meso-structures in the hybrid phases.

Swelling of non-fully hydrolyzed polyvinyl-alcohol in ethanol–water mixtures induces stability in dispersions of multi-wall carbon nanotubes.