High-k polymer nanocomposites have received increased research interest by virtue of integrating high dielectric constant nanofiller with high breakdown strength, flexibility, and ease of processing ...of a matrix. With outstanding anisotropy, high-aspect-ratio nanofillers have proved to be much more efficient enhancers of the dielectric properties of nanocomposites when compared with traditional zero-dimensional (0D) fillers, leading to many dielectric and energy storage applications. This review summarizes the latest research on one-dimensional (1D) and quasi-1D fillers based high-k polymer nanocomposites with the focus on the superiority of 1D or quasi-1D high-k fillers in enhancing the dielectric properties and energy storage capability of polymer nanocomposites. Dielectric anisotropy, which plays a critical role in determining the dielectric properties and energy storage capability of polymer nanocomposites, was highlighted and the experimental methodologies for achieving anisotropic dielectric polymer nanocomposites were reviewed. The fundamental electrical parameters, such as dielectric constant, dielectric nonlinearity, dielectric loss and electrical conduction, and breakdown strength of dielectric polymer composites, are also discussed. Given the recent progress, guidelines for the future development of high-k polymer nanocomposites with dielectric and energy storage applications were proposed.
Metal–organic frameworks (MOFs) are hybrid porous crystalline networks with tunable chemical and structural properties. However, their excellent potential is limited in practical applications by ...their hard‐to‐shape powder form, making it challenging to assemble MOFs into macroscopic composites with mechanical integrity. While a binder matrix enables hybrid materials, such materials have a limited MOF content and thus limited functionality. To overcome this challenge, nanoMOFs are combined with tailored same‐charge high‐aspect‐ratio cellulose nanofibrils (CNFs) to manufacture robust, wet‐stable, and multifunctional MOF‐based aerogels with 90 wt% nanoMOF loading. The porous aerogel architectures show excellent potential for practical applications such as efficient water purification, CO2 and CH4 gas adsorption and separation, and fire‐safe insulation. Moreover, a one‐step carbonization process enables these aerogels as effective structural energy‐storage electrodes. This work exhibits the unique ability of high‐aspect‐ratio CNFs to bind large amounts of nanoMOFs in structured materials with outstanding mechanical integrity—a quality that is preserved even after carbonization. The demonstrated process is simple and fully discloses the intrinsic potential of the nanoMOFs, resulting in synergetic properties not found in the components alone, thus paving the way for MOFs in macroscopic multifunctional composites.
A novel sustainable material strategy is reported to fabricate wet‐stable, mechanically robust, and elastic multifunctional nanoMOF‐CNF aerogels with the highest reported nanoMOF loading of 90 wt%, enabled by high‐aspect‐ratio cationic CNFs. The carbonized and non‐carbonized aerogels are effectively demonstrated for water purification, CO2 and CH4 gas storage and separation, flame retardancy, and energy storage.
High‐stress field generated by electroplating of lithium (Li) in pre‐existing defects is the main reason for mechanical failure of solid‐state electrolyte because it drives crack propagation in ...electrolyte, followed by Li filament growth inside and even internal short‐circuit if the filament reaches another electrode. To understand the role of interfacial defects on mechanical failure of solid‐state electrolyte, an electro–chemo–mechanical model is built to visualize distribution of stress, relative damage, and crack formation during electrochemical plating of Li in defects. Geometry of interfacial defect is found as dominating factor for concentration of local stress field while semi‐sphere defect delivers less accumulation of damage at initial stage and the longest failure time for disintegration of electrolyte. Aspect ratio, as a key geometric parameter of defect, is investigated to reveal its impact on failure of electrolyte. Pyramidic defect with low aspect ratio of 0.2–0.5 shows branched region of damage near interface, probably causing surface pulverization of solid‐state electrolyte, whereas high aspect ratio over 3.0 will trigger accumulation of damage in bulk electrolyte. The correction between interfacial defect and electro–chemo–mechanical failure of solid‐state electrolyte is expected to provide insightful guidelines for interface design in high‐power‐density solid‐state Li metal batteries.
Electro–chemo–mechanical failure of solid‐state electrolyte induced by interfacial defects is revealed by multiphysics simulation, through visualization of stress distribution, relative damage, and crack formation. It is found that the geometry of defect and aspect ratio are the dominating factors for the failure process of solid‐state electrolyte. This work shows promising guidelines for suppressing failure of solid‐state electrolyte.
Abstract
Researches on flexible thermoelectric materials usually focus on conducting polymers and conducting polymer-based composites; however, it is a great challenge to obtain high thermoelectric ...properties comparable to inorganic counterparts. Here, we report an n-type Ag
2
Se film on flexible nylon membrane with an ultrahigh power factor ~987.4 ± 104.1 μWm
−1
K
−2
at 300 K and an excellent flexibility (93% of the original electrical conductivity retention after 1000 bending cycles around a 8-mm diameter rod). The flexibility is attributed to a synergetic effect of the nylon membrane and the Ag
2
Se film intertwined with numerous high-aspect-ratio Ag
2
Se grains. A thermoelectric prototype composed of 4-leg of the Ag
2
Se film generates a voltage and a maximum power of 18 mV and 460 nW, respectively, at a temperature difference of 30 K. This work opens opportunities of searching for high performance thermoelectric film for flexible thermoelectric devices.
•N-rich carbon nanotubes with long aspect ratio are constructed by a simple template method.•The nitrogen content in the carbon nanotubes is up to 15.7%.•Excellent cycle durability and rate ...capability are obtained in sodium/lithium ion batteries.•Quantitative kinetic analysis reveals the dominated capacitive contribution.
An effective design of carbon materials with appropriate nanoarchitecture and optimized physicochemical property is critically demanded for superb lithium/sodium storage capacities. N-rich (up to 15.7%) hollow carbon nanotubes (NCNT) with long aspect ratio are well developed via a template method by exploring polypyrrole as the high nitrogen-containing carbon precursor. Owing to the hollow structure with large cavity and long aspect ratio, the NCNT exhibits Li+/Na+ storage capability with favorable volume buffer and rapid ion and electron transfer at high rate, resulting in long-term cycling and high-rate property. Meanwhile, the high N content creates abundant active sites and extrinsic defects, facilitating the improved specific capacity and rate performance. Impressively, our NCNT-600 electrode displays a favorable reversible capacity of 132 mAhg−1 after 5000 cycles at 4000 mA g−1 for SIBs and 170 mAhg−1 after 2000 cycles at the same current for LIBs. Further quantitative kinetic analysis reveals the dominated capacitive contribution of Li+/Na+ storage in NCNT, which is attributed to the porous hollow nanotubes and N-rich carbon with volume strain mitigation and enhanced electronic/ionic transfer capability. Given the cost-effectiveness and material sustainability, our work will shed light on the further design of other carbon-based materials for advanced energy storage devices.
A tunable response frequency is highly desirable for practical applications of microwave absorption materials but remains a great challenge. Here, hollow lightweight polydopamine@α-MnO2 microspindles ...were facilely synthesized with the tunable absorption frequency governed by the aspect ratio. The size of the hard template is a key factor to achieve the unique shape; the polymer layer with uniform thickness plays an important role in obtaining spindles with homogeneous size. With the aspect ratio increasing, the maximum reflection loss, as well as the absorption bandwidth (<−10 dB), increases and then decreases; meanwhile, the microwave absorption band shifts to the low frequency. The optimized aspect ratio of the cavity about the hollow polydopamine@α-MnO2 microspindles is ∼2.8. With 3 mm thickness at 9.7 GHz, the strongest reflection reaches −21.8 dB, and the width of the absorbing band (<−10 dB) is as wide as 3.3 GHz. Via electron holography, it is confirmed that strong charge accumulates around the interface between the polydopamine and α-MnO2 layers, which mainly contributes to the dielectric polarization absorption. This study proposes a reliable strategy to tune the absorption frequency via different aspect ratio polymer@α-MnO2 microspindles.
Integrating synthetic low‐dimensional nanomaterials such as metal–organic framework (MOF) nanosheets with a sustainable biopolymer is a promising strategy to endow composites with attractive ...structural and functional properties for expanded applications. Herein, aggregation‐induced‐emission luminogen (AIEgen)‐based MOF bulk crystals are successfully exfoliated into ultrathin 2D nanosheets. Seaweed cellulose nanofibrils (CNFs) are assembled with low amounts (0.3 to 4.0 wt%) of the 2D nanosheets to generate luminescent composites. The 2D nanosheets are adsorbed onto the CNFs in dilute water suspensions owing to the flexibility of the MOF nanosheets and the high aspect ratio of the CNFs. Transparent films are prepared by solution casting from a water suspension of the CNF‐MOF assembly. The fluorescence emission of the composite films is enhanced because of the favored affinity between MOF nanosheets and CNFs. Remarkably, these films demonstrate excellent UV‐shielding capacity and high optical transmittance at the visible wavelength range. The composite films also show reversible changes in fluorescence emission intensity in response to ambient humidity. The tensile strength and modulus of the composite films are also enhanced owing to the increased adhesion between CNFs through the adsorbed MOF nanosheets. This work provides a novel pathway to fabricate luminescent CNFs‐based composites with tunable optical properties for functional materials.
Seaweed cellulose nanofibrils (CNFs)‐based composites, with strong fluorescence emission sensitive to relative humidity, are fabricated by integrating 4.0 wt% aggregation‐induced‐emission luminogen (AIEgen)‐based 2D metal–organic framework (MOF) nanosheets in a water suspension. The composite films also show efficient UV‐shielding capacity without compromise of the optical transmittance and excellent mechanical properties, owing to the metal–carboxylate coordination between the 2D MOF nanosheets and the CNFs.
•Flow boiling experiments in the high-aspect-ratio interconnected microchannels.•Earlier onset of nucleate boiling and higher CHF are obtained.•Regular liquid film redevelopment and annular flow are ...observed alternately.•Highly stable flow boiling is achieved from the boiling incipience to near the CHF point.
High aspect ratio microchannels have the advantage of dissipating more heat per base area. However, flow boiling instability of bubble expansion towards upstream and downstream is more easily triggered as soon as boiling starts due to the narrower channel width of high aspect ratio microchannel. To suppress flow boiling instability, in this study, the flow boiling heat transfer performance is experimentally investigated in the high-aspect-ratio (aspect ratio of 2.5) interconnected microchannels (IM) with micro connection slots. Two slots of 20 μm and 40 μm in width, denoted as IM20 and IM40, respectively, are considered. Experiments are conducted at mass fluxes of 446-963 kg/m2•s and effective heat fluxes of 36.2-427.9 W/cm2, using deionized water as the working fluid. The outlet of the channel is maintained at atmospheric pressure with a saturation temperature of 100 °C. Results show that obviously earlier onset of nucleate boiling (ONB) in IM is demonstrated, compared to that in plain-wall microchannel (PM). Meantime, the nucleation bubble is easily captured in IM, whereas almost no bubbly flow is observed in PM. More importantly, significantly stable flow boiling from the incipience of boiling to near the critical heat flux (CHF) point is found in IM. It is observed that regular liquid film redevelopment and annular flow appear alternately among parallel microchannels in IM. By contrast, due to flow maldistribution of PM, various irregular flow patterns appear simultaneously among parallel microchannels. These micro connection slots promote nucleate boiling and fluid mixing of neighboring channels. Eventually, remarkable heat transfer enhancement is achieved in IM without compromising the two-phase pressure drop. The enhanced heat transfer coefficient and critical heat flux of IM are respectively around 25.7%-86.9% and 13.9%-59.1% over the entire range of tested cases. In addition, owing to enhanced fluid mixing and more liquid maintaining in the larger slot, IM40 shows more heat dissipation than IM20. This study demonstrates that the microchannel heat sink with the interconnected configuration with tens of micro-slots can be an effective approach for suppressing flow boiling instability and enhancing flow boiling heat transfer performance.
A reconfigurable, droplet‐directing surface is developed based on high‐aspect‐ratio shape‐memory polymer (SMP) pillars. The water droplet on the original or recovered SMP pillars can slide off the ...surface at a finite angle of inclination while being fully pinned on the deformed pillar array. This wettability contrast allows directed water shredding from the straight pillars to the deformed ones.
This study investigated the feasibility of reducing fiber content in ultra-high-performance fiber-reinforced cement composites (UHP-FRCC). For this, three different types of steel fibers were ...considered, and three different aspect ratios were applied for the case of straight fibers. To quantitatively evaluate the cost effectiveness of reducing the fiber content of UHP-FRCC, cost analysis was also performed. Test results indicated that at low fiber volume fractions (Vf≤1.0%), the twisted fibers provided the highest flexural strength, but they exhibited similar strength and poorer toughness than the straight fibers at a Vf equal to or higher than 1.5%. Smaller flexural strength and toughness were observed in the specimens with hooked fibers than those with straight ones at a Vf equal to or higher than 1.0%. In the case of straight fibers, the one with the highest aspect ratio was more effective in improving the flexural performance than those with lower aspect ratios. The medium-length straight fibers were most efficient at improving the flexural performance of UHP-FRCC at a Vf equal to or higher than 1.5%. The total production costs of commercially available UHP-FRCC could be reduced by as much as 32–35% by replacing short straight fibers with medium-length or long straight fibers.
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