The filling of nanometer and sub-nanometer channels/ tubes with water governs applications ranging from desalination and filtration to nanoscale energy conversion. Here, we report the most nonintuitive entropy-dominated filling of mildly hydrophilic boron nitride nanotubes (BNNTs) with diameters ranging from 0.85 to 1.69 nm. For all the BNNT sizes, water inside the BNNT is more stable than water in the bulk. The factor dictating the favorable nature of the entropy depends on the specific-BNNT-diameter-dictated structure of the water molecules. For example, for the 0.85 nm BNNT, the rotational entropic component dominates due to the presence of a significant fraction of the dangling water OH bonds that do not participate in hydrogen bonding. The fraction of such dangling OH bonds (the average HBs per molecule) decreases (increases) with an increase in the BNNT diameter, leading to a progressively reduced contribution of the rotational entropy. For the 1 nm BNNT, the translational entropic component dominates due to the enhanced in-plane motion of the water molecules caused by the single-file nature of water molecules spanning a significant radial expanse inside the BNNT. For larger BNNTs, translational entropy decreases with an increase in the BNNT diameter, although it remains the dominant factor governing the entropy-driven filling of BNNTs. This favorable translational entropy for larger BNNTs can be associated with (1) the presence of chain-like regions formed by water in 1.13 nm BNNT and (2) the presence of a high specific water volume and reduced number of HBs per molecule (as compared with bulk water) in 1.27, 1.41, 1.55, and 1.69 nm BNNTs.

A key challenge encountered by printed electronics is that the conductivity of sintered metal nanoparticle (NP) traces is always several times smaller than the bulk metal conductivity. Identifying the relative roles of the voids and the residual polymers on NP surfaces in sintered NP traces, in determining such reduced conductivity, is essential. In this paper, we employ a combination of electron microscopy imaging and detailed simulations to quantify the relative roles of such voids and residual polymers in the conductivity of sintered traces of a commercial (Novacentrix) silver nanoparticle-based ink. High resolution transmission electron microscopy (TEM) imaging revealed details of the morphology of the inks before and after being sintered at 1500C. Prior to sintering, NPs were randomly close packed into aggregates with nanometer thick polymer layers in the interstices. The 2D porosity in the aggregates prior to sintering was near 20%. After heating at 1500C, NPs sintered together into dense aggregates (nanoaggregates or NAgs) with sizes ranging from 100-500 nm and the 2D porosity decreased to near 10%. Within the NAgs, the NPs were mostly connected via sintered metal bridges, while the outer surfaces of the NAgs were coated with a nanometer thick layer of polymer. Motivated by these experimental results, we developed a computational model for calculating the effective conductivity of the ink deposit represented by a prototypical NAg consisting of NPs connected by metallic bonds and having a polymer layer on its outer surface placed in a surrounding medium. The calculations reveal that a NAg that is 35-40% covered by a nanometer thick polymeric layer has a similar conductivity compared to prior experimental measurements. The findings also demonstrate that the conductivity is less influenced by the polymer layer thickness or the absolute value of the NAg dimensions. Most importantly, we are able to infer that the reduced value of the conductivity of the sintered traces is less dependent on the void fraction and is primarily attributed to the incomplete removal of the polymeric material even after sintering.