Bilayer Polymer Metacomposites Containing Negative Permittivity Layer for New High-k Materials

Negative Capacitance in Nanocomposite Based on High-Density Polyethylene (HDPE) with Multiwalled Carbon Nanotubes (CNTs)

Negative capacitance (NC), already observed in conducting polymer-based nanocomposites, was recently reported and evidenced at low frequencies (<10 kHz) in non-conducting polymer-based nanocomposites containing conductive particles. In this contribution, we demonstrate that it is possible to produce economic high-density polyethylene (HDPE) nanocomposites exhibiting an NC effect at low frequencies via a convenient and environmentally friendly extrusion-like process by only adjusting the duration of melt-mixing. Nanocomposite materials are produced by confining a limited quantity, i.e., 4.6 wt.%, of multiwalled carbon nanotubes (CNTs) within semi-crystalline HDPE to reach the percolation threshold. With increasing melt processing time, crystallites of HDPE developing at the surface of CNTs become bigger and perturbate the connections between CNTs leading to a dramatic change in the electrical behavior of the systems. More specifically, the link between NC and current oscillations is stressed while the dependence of NC with the size of polymer crystallites is evidenced. NC tends to appear when space charge effects take place in HDPE/MWCNT interfaces, in structures with convenient crystallite sizes corresponding to 10 min of melt-mixing.

Frequency stable dielectric constant with reduced dielectric loss of one-dimensional ZnO-ZnS heterostructures

The synthesis of one-dimensional heterostructures having high dielectric constant and low dielectric loss has remained a great challenge. Until now, the dielectric performance of ZnO-ZnS heterostructures was scarcely investigated. In this work, large-scale ZnO-ZnS heterostructures were synthesized by employing the chemical vapor deposition method. High resolution transmission electron microscopy (HRTEM) confirms the formation of heterostructures. X-ray photoelectron spectroscopy (XPS) shows that S atoms fill up the oxygen vacancy (V_O) in ZnO, leading to the suppression of charge carrier's movement from ZnO to ZnS; instead there is charge transfer from ZnS to ZnO. Conductivity mismatch between adjacent ZnO and ZnS materials leads to the accumulation of free charges at the interface of the heterostructure and can be considered as a capacitor-like structure. The electrical behaviors of the potential phases of ZnO, ZnS and the ZnO-ZnS heterostructure are well interpreted by a best fitted equivalent circuit model. Each heterostructure acts as a polarization node with a specific flip-flop frequency and all such nodes form continuous transmission of polarization, which jointly increase the dielectric energy-storage performance. The orientational polarization of the polarons and Zn²⁺-V_O dipoles present at the heterostructure interface contributes to the frequency stable dielectric constant at ≥10³ Hz. Our findings provide a systematic approach to tailor the electronic transport and dielectric properties at the interface of the heterostructure. We suggest that this approach can be extended for improving the energy harvesting, transformation and storage capabilities of the nanostructures for the development of high-performance energy-storage devices.

Adjustable Graphene/Polyolefin Elastomer Epsilon-near-Zero Metamaterials at Radiofrequency Range

While epsilon-near-zero (ENZ) metamaterials have marvelously shown various application prospects, the way to construct intrinsic ENZ metamaterials and adjust their ENZ properties precisely is still uncovered. The realization of stable and broadband ENZ properties at the radiofrequency range is of great significance. Herein graphene/polyolefin elastomer (POE) intrinsic ENZ metamaterials are initially constructed via the nanohybrid process. The metamaterials possess excellent adjustable ENZ properties by adjusting the content and reduction methods of graphene. The permittivities maintain between -1 and 1 steadily with increasing graphene content, which is attributed to the moderated carrier concentration of the conductive networks in the nanohybrids. Besides, different reduction methods also have significant impacts on ENZ properties. The hydrazine hydrate reduction method increases the maximum ENZ frequency region to 126 MHz. Lorentz type resonance is reasonable for the positive-negative transition in the ENZ frequency regions. As a significant indicator of the emergence of ENZ property, the sudden peak of dielectric loss tangent is observed. This work offers novel routes to construct intrinsic ENZ metamaterials with excellent adjustability in both values of permittivity and ENZ frequency regions.

Dielectric Property and Space Charge Behavior of Polyimide/Silicon Nitride Nanocomposite Films

Polymeric materials have many applications in multiple industries. In this paper, silicon nitride nanoparticles (Si₃N₄) were incorporated into a polyimide (PI) matrix to obtain composite films via the in situ polymerization method. The Si₃N₄ nanoparticles were consistently scattered in the composites, and the thickness of PI/Si₃N₄ films was around 50 µm. The effects of nanoparticle content on the dielectric constant, loss tangent and breakdown strength were simultaneously studied. A 3 wt.% doped PI/Si₃N₄ film revealled excellent dielectric properties, a dielectric constant (ε) of 3.62, a dielectric loss tangent (tanδ) of 0.038, and a breakdown strength of 237.42 MV/m. The addition of Si₃N₄ formed an interface layer inside PI, resulting in a large amount of space charge polarization in the electric field. The space charge of materials from the microscopic point of view was analyzed. The results show that there are trapenergy levels in the composites, which can be used as a composite carrier center and transport channel, effectively improving the performance of a small amount of nanoparticles film.

Flexible silver nanowire/carbon fiber felt metacomposites with weakly negative permittivity behavior

The weakly negative permittivity behavior and its generation mechanism in flexible silver nanowires/carbon fiber felt metacomposites.