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Register a new userPrintable, castable, nanocrystalline cellulose-epoxy composites exhibiting hierarchical nacre-like toughening
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Due to their exceptional mechanical and chemical properties and their natural abundance, cellulose nanocrystals (CNCs) are promising building blocks of sustainable polymer composites. However, the rapid gelation of CNC dispersions has generally limited CNC-based composites to low CNC fractions, in which polymer remains the dominant phase. Here we report on the formulation and processing of crosslinked CNC-epoxy composites with a CNC fraction exceeding 50 wt.%. The microstructure comprises sub-micrometer aggregates of CNCs crosslinked to polymer, which are analogous to the lamellar structure of nacre and promotes toughening mechanisms associated with bulk ductile behavior, despite the brittle behavior of the aggregates at the nanoscale. At 63 wt.% CNCs, the composites exhibit a hardness of 0.66 GPa and a fracture toughness of 5.2 MPa.m$^{1/2}$. The hardness of this all-organic material is comparable to aluminum alloys, and the fracture toughness at the centimeter scale is comparable to that of wood cell wall. We show that CNC-epoxy composite objects can be shaped from the gel precursors by direct-write printing and by casting, while the cured composites can be machined into complex 3D shapes. The formulation, processing route, and the insights on toughening mechanisms gained from our multiscale approach can be applied broadly to highly loaded nanocomposites.
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Carbon Nanotubes (CNTs)-polymer composites are promising candidates for a myriad of applications. Ad-hoc CNTs-polymer composite fabrication techniques inherently pose roadblock to optimized processing resulting in microstructural defects i.e., void formation, poor interfacial adhesion, wettability, and agglomeration of CNTs inside the polymer matrix. Although improvement in the microstructures can be achieved via additional processing steps such as-mechanical methods and/or chemical functionalization, the resulting composites are somewhat limited in structural and functional performances. Here, we demonstrate that 3D printing technique like-direct ink writing offers improved processing of CNTs-polymer composites. The shear-induced flow of an engineered nanocomposite ink through the micronozzle offers some benefits including reducing the number of voids within the epoxy, improving CNTs dispersion and adhesion with epoxy, and partially aligns the CNTs. Such microstructural changes result in superior mechanical performance and heat transfer in the composites compared to their mold-casted counterparts. This work demonstrates the advantages of 3D printing over traditional fabrication methods, beyond the ability to rapidly fabricate complex architectures, to achieve improved processing dynamics for fabricating CNT-polymer nanocomposites with better structural and functional properties.
Radio, millimetre and sub-millimetre astronomy experiments as well as remote sensing applications often require castable absorbers with well known electromagnetic properties to design and realize calibration targets. In this context, we fabricated and characterized two samples using different ratios of two easily commercially available materials: epoxy (Stycast 2850FT) and magnetite ($mathrm{Fe_{3}O_{4}}$) powder. We performed transmission and reflection measurements from 7 GHz up to 170 GHz with a VNA equipped with a series of standard horn antennas. Using an empirical model we analysed the data to extract complex permittivity and permeability from transmission data; then we used reflection data to validate the results. In this paper we present the sample fabrication procedure, analysis method, parameter extraction pipeline, and results for two samples with different epoxy-powder mass ratios.
Daytime radiative cooling has attracted considerable attention recently due to its tremendous potential for passively exploiting the coldness of deep-sky as clean and renewable energy. Many advanced materials with novel photonic micro-nanostructures have already been developed to enable highly efficient daytime radiative coolers, among which the flexible hierarchical porous coatings (HPCs) are a more distinguished category. However, it is still hard to precisely control the size distribution of the randomized pores within the HPCs, usually resulting in a deficient solar reflection at the near-infrared optical regime under diverse fabrication conditions of the coatings. We report here a three-phase (i.e., air pore-phase, microsphere-phase and polymer-phase) self-assembled hybrid porous composite coating which dramatically increases the average solar reflectance and yields a remarkable temperature drop of ~10 degC and 30 degC compared to the ambient circumstance and black paint, respectively, according to the rooftop measurements. Mie theory and Monte Carlo simulations reveal the origin of the low reflectivity of as-prepared two-phase porous HPCs, and the optical cooling improvement of the three-phase porous composite coatings is attributed to the newly generated interfaces possessing the high scattering efficiency between the hierarchical pores and silica microspheres hybridized with appropriate mass fractions. As a result, the hybrid porous composite approach enhances the whole performance of the coatings, which provides a promising alternative to the flexible daytime radiative cooler.
Modelling of single cellulose fibres is usually performed by assuming homogenous properties, such as strength and Young s modulus, for the whole fibre. Additionally, the inhomogeneity in size and swelling behaviour along the fibre is often disregarded. For better numerical models, a more detailed characterization of the fibre is required. Herein, we report a method based on atomic force microscopy to map these properties along the fibre. A fibre was mechanically characterized by static colloidal probe AFM measurements along the fibre axis. Thus, the contact stress and strain at each loading point can be extracted. Stress strain curves can be obtained along the fibre. Additionally, mechanical properties such as adhesion or dissipation can be mapped. The inhomogeneous swelling behaviour was recorded via confocal laser scanning microscopy along the fibre. Scanning electron microscopy measurements revealed the local macroscopic fibril orientation and provided an overview of the fibre topology. By combining these data, regions along the fibre with higher adhesion, dissipation, bending ability and strain or differences in the contact stress when increasing the relative humidity could be identified. This combined approach allows for one to obtain a detailed picture of the mechanical properties of single fibres.
Metal-Nb$_{2}$O$_{5-x}$-metal memdiodes exhibiting rectification, hysteresis, and capacitance are demonstrated for applications in neuromorphic circuitry. These devices do not require any post-fabrication treatments such as filament creation by electroforming that would impede circuit scalability. Instead these devices operate due to Poole-Frenkel defect controlled transport where the high defect density is inherent to the Nb$_{2}$O$_{5-x}$ deposition rather than post-fabrication treatments. Temperature dependent measurements reveal that the dominant trap energy is 0.22 eV suggesting it results from the oxygen deficiencies in the amorphous Nb$_{2}$O$_{5-x}$. Rectification occurs due to a transition from thermionic emission to tunneling current and is present even in thick devices (> 100 nm) due to charge trapping which controls the tunneling distance. The turn-on voltage is linearly proportional to the Schottky barrier height and, in contrast to traditional metal-insulator-metal diodes, is logarithmically proportional to the device thickness. Hysteresis in the I-V curve occurs due to the current limited filling of traps.
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