Nanoparticle Enlarged Interfacial Effect on Phase Transition of 1-Octadecanol/Silica Composites

Three-Dimensional Hollow Reduced Graphene Oxide Tube Assembly for Highly Thermally Conductive Phase Change Composites and Efficient Solar-Thermal Energy Conversion

Due to their extraordinary mechanical strength and electrical and thermal conductivities, graphene fibers and their derivatives have been widely utilized in various functional applications. In this work, we report the synthesis of a three-dimensional (3D) hollow reduced graphene oxide tube assembly (HrGOTA) using the same wet spinning method as graphene fibers. The HrGOTA has high thermal conductivity and displays the unique capability of encapsulating phase change materials for effective solar-thermal energy conversion. The HrGOTA comprises layers of moisture-fused hollow reduced graphene oxide tubes (HrGOTs), whose individual thermal conductivity is up to 578 W m^-1 K^-1. By impregnating 1-octadecanol into HrGOTs, a 1-octadecanol-filled HrGOT phase change composite (PCC) with a latent heat of 262.5 J g^-1 is obtained. This high latent heat results from the interfacial interaction between 1-octadecanol and the reduced graphene oxide tube, as evidenced by the shifts in XRD patterns of 1-octadecanol-filled and 1-octadecanol/multiwalled carbon nanotube-filled HrGOTA samples. In addition, 1 wt % multiwalled carbon nanotubes are added to the PCC to enhance visible light absorption. Because of their high thermal conductivity and visible light absorption rates, these new PCCs display high solar-thermal energy conversion and storage efficiencies of up to 81.7%, commensurate with state-of-the-art carbon-based PCCs but with significantly lower carbon weight percentages.

Composite Films of HDPE with SiO2 and ZrO2 Nanoparticles: The Structure and Interfacial Effects

Herein, we investigated the influence of two types of nanoparticle fillers, i.e., amorphous SiO₂ and crystalline ZrO₂, on the structural properties of their nanocomposites with high-density polyethylene (HDPE). The composite films were prepared by melt-blending with a filler content that varied from 1% to 20% v/v. The composites were characterized by small- and wide-angle x-ray scattering (SAXS and WAXS), small-angle neutron scattering (SANS), Raman spectroscopy, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). For both fillers, the nanoaggregates were evenly distributed in the polymer matrix and their initial state in the powders determined their surface roughness and fractal character. In the case of the nano-ZrO₂ filler, the lamellar thickness and crystallinity degree remain unchanged over a broad range of filler concentrations. SANS and SEM investigation showed poor interfacial adhesion and the presence of voids in the interfacial region. Temperature-programmed SANS investigations showed that at elevated temperatures, these voids become filled due to the flipping motions of polymer chains. The effect was accompanied by a partial aggregation of the filler. For nano-SiO₂ filler, the lamellar thickness and the degree of crystallinity increased with increasing the filler loading. SAXS measurements show that the ordering of the lamellae is disrupted even at a filler content of only a few percent. SEM images confirmed good interfacial adhesion and integrity of the SiO₂/HDPE composite. This markedly different impact of both fillers on the composite structure is discussed in terms of nanoparticle surface properties and their affinity to the HDPE matrix.

Alkylated Meso-Macroporous Metal-Organic Framework Hollow Tubes as Nanocontainers of Octadecane for Energy Storage and Thermal Regulation

Alkylated meso-macroporous Cr-MIL-101 hollow tubes (ACHT) are synthesized through the coordination of Cr³⁺ with alkylated bridged ligands. The alkylated ligands work as both bridging ligands and structure-directing agents, which presents a new strategy for synthesizing meso-macroporous metal-organic frameworks (MOFs). This is the first example of 1D meso-macroporous MOF hollow tubes with large pore diameter (5-136 nm). Furthermore, the octadecane@ACHT form-stable phase change materials (FSPCMs) are prepared using ACHT as nanocontainers for octadecane. The nanocavity of ACHT possesses enough storage space for octadecane, and the large number of alkyl chains on the ACHT framework induces octadecane molecules to crystallize from α- to β-crystals. Considering the comfortable phase transition temperature (27.7 °C) and the relatively large thermal storage capacity (187 J g^-1 ), the octadecane@ACHT FSPCMs can be used in interior wall of building to improve the indoor thermal comfort or in the field of thermal-regulated textiles and clothing.

Thermodynamic and kinetic anisotropies in octane thin films

Confinement breaks the translational symmetry of materials, making all thermodynamic and kinetic quantities functions of position. Such symmetry breaking can be used to obtain configurations that are not otherwise accessible in the bulk. Here, we use computer simulations to explore the effect of substrate-liquid interactions on thermodynamic and kinetic anisotropies induced by a solid substrate. We consider n-octane nano-films that are in contact with substrates with varying degrees of attraction, parameterized by an interaction parameter ϵS. Complete freezing of octane nano-films is observed at low temperatures, irrespective of ϵS, while at intermediate temperatures, a frozen monolayer emerges at solid-liquid and vapor-liquid interfaces. By carefully inspecting the profiles of translational and orientational relaxation times, we confirm that the translational and orientational degrees of freedom are decoupled at these frozen monolayers. At sufficiently high temperatures, however, free interfaces and solid-liquid interfaces close to loose (low-ϵS) substrates undergo "pre-freezing," characterized by mild peaks in several thermodynamic quantities. Two distinct dynamic regimes are observed at solid-liquid interfaces. The dynamics is accelerated in the vicinity of loose substrates, while sticky (high-ϵS) substrates decelerate dynamics, sometimes by as much as two orders of magnitude. These two distinct dynamical regimes have been previously reported by Haji-Akbari and Debenedetti [J. Chem. Phys. 141, 024506 (2014)] for a model atomic glass-forming liquid. We also confirm the existence of two correlations-proposed in the above-mentioned work-in solid-liquid subsurface regions of octane thin films, i.e., a correlation between atomic density and normal stress, and between atomic translational relaxation time and lateral stress. Finally, we inspect the ability of different regions of an octane film to explore the potential energy landscape by performing inherent structure calculations, and observe no noticeable difference between the free surface and the bulk in efficiently exploring the potential energy landscape. This is unlike the films of model atomic glass formers that tend to sample their respective landscape more efficiently at free surfaces. We discuss the implications of this finding to the ability of octane-and other n-alkanes-to form ultrastable glasses.