High Permittivity Composites Research Articles

Dielectric Properties and 3D-Printing Feasibility of UV-Curable Resin/Micron Ceramic Filler Composites

To prepare high-permittivity composite materials for dielectrically functional gradient materials (d-FGMs) by the stereolithographic 3D-printing technique, three ceramic powders (i.e., alumina, barium titanate, and strontium titanate) are selected as functional fillers for a UV-curable resin matrix. The viscosity and UV curing depth of the uncured slurry are tested for feasibility of 3D printing. Comprehensive electrical properties, including volume resistivity, permittivity, dielectric loss, and breakdown strength of the cured composites are measured. The effects of the filler types, morphologies, particle sizes, and volume fractions on the UV curing characteristics of the slurry and dielectric properties of cured composites are systematically analyzed. The experimental results show that spherical fillers with large particle sizes, smooth surfaces, and high permittivity are conducive for reducing the slurry viscosity, increasing curing depth and adjusting the composite’s permittivity over a wide range. We believe that the proposed strategy for material system establishment can improve the 3D printability of high-permittivity composites and promote other applications of the d-FGMs by the stereolithography technique.

Flexible multi-walled carbon nanotubes/polydimethylsiloxane membranous composites toward high-permittivity performance

As reported, flexible composites with high-permittivity performance have played a significance role in thin-film capacitors, stretchable electronic devices, etc. However, the flexibility of traditional high-permittivity composites with metal particles or conductive polymers was damaged near percolation threshold. In this work, an in situ polymerization process was used to prepare the flexible percolating multi-walled carbon nanotubes/polydimethylsiloxane (MWCNTs/PDMS) membranes. Meanwhile, the high-permittivity performance w as observed when the MWCNTs content was near but not exceeded the percolation threshold, attributing to the Maxwell-Wagner-Sillars effect. As MWCNTs content increased, the conduction mechanism of the MWCNTs/PDMS composites was changed from the hopping conduction to metal-like conduction, the conductivity values were enhanced as well, and thus, the main dielectric loss resulted from the ohmic dissipation. Furthermore, the tensile strength and elongation results demonstrated that the membranes were flexible and had good mechanical properties. This work not only revealed the mechanism of high permittivity in percolating composites but also promoted the applications of flexible high-permittivity membranes in capacitors, electromagnetic interference shielding, microwave absorption, and so on.

The Impact of 3D Printing Process Parameters on the Dielectric Properties of High Permittivity Composites

Fused filament fabrication (FFF) is a well-known and greatly accessible additive manufacturing technology, that has found great use in the prototyping and manufacture of radiofrequency componentry, by using a range of composite thermoplastic materials that possess superior properties, when compared to standard materials for 3D printing. However, due to their nature and synthesis, they are often a great challenge to print successfully which in turn affects their microwave properties. Hence, determining the optimum printing strategy and settings is important to advance this area. The manufacturing study presented in this paper shows the impact of the main process parameters: printing speed, hatch spacing, layer height and material infill, during 3D printing on the relative permittivity (εr), and loss tangent (tanδ) of the resultant additively manufactured test samples. A combination of process parameters arising from this study, allowed successful 3D printing of test samples, that marked a relative permittivity of 9.06 ± 0.09 and dielectric loss of 0.032 ± 0.003.

Low loss and high permittivity composites based on poly(vinylidene fluoride-chlorotrifluoroethylene) and lead lanthanum zirconate titanate

Lanthanum doped lead zirconate titanate stannate antiferroelectric ceramic (Pb0.97La0.02)(Zr0.63Sn0.3Ti0.07)O3 (PLZST) is proposed as an inorganic filler. P(VDF-CTFE)/PLZST composites with various compositions are fabricated via the solution cast process. The dielectric permittivity of the composites is significantly increased and the inherent loss is reduced in a wide range of frequencies at the same time. At 100Hz, the 40% PLZST loaded composite shows a dielectric permittivity of 28 (3 times more than that of the neat polymer), and a low loss of 0.045 (near half of that in the neat polymer). The low loss of the composite is most likely due to the slower increasing rate of the conductivity by comparison to the dielectric permittivity.

Design of grid-type microwave absorbers with high-permittivity composites of Ag-coated Ni-Zn ferrite particles

We investigate the microwave absorbing characteristics in grid-shaped rubber composite sheets containing Ag-coated Ni-Zn ferrite particles. The improvements in the microwave absorbance are obtained through the insertion of a periodic air cavity in the high-permittivity composites. In the bulk specimens containing conductive and magnetic Ag-coated ferrite microspheres, the impedance matching is not satisfied due to the high dielectric permittivity of the composite. The insertion of an air cavity in those absorbers reduces the permittivity and permeability, and thereby leading to impedance matching at a higher frequency. In the grid-type absorber with an optimum air cavity volume rate, the reflection loss can be decreased to as low as −30 dB at 10.5 GHz with a small layer thickness of 2 mm. The proposed grid-type microwave absorber has advantages of a small matching thickness and a considerable reduction in weight in comparison with conventional ferrite composite absorbers.

Soft dielectrics for capacitive sensing in robot skins: Performance of different elastomer types

In a capacitive tactile sensor the dielectric layer plays an important role: both its electrical and mechanical properties affect the capacitance variation and the sensor response in terms of sensitivity, since the deformation ability of the dielectric layer allows for the detection of small pressures, and spatial resolution, since the dielectric layer acts as a low-pass spatial filter decreasing the spatial resolution. In this work we comparatively evaluate the effect of different elastomers on the performance of a capacitive sensor designed to cover large areas of a robot body. In particular, we compare the sensitivity, the spatial resolution and the durability of the sensor using foams, bulk elastomers and high permittivity composites.

Electric stress grading on bushings of combined instrument transformers using high permittivity polymeric composites

Bushings are used in high voltage equipment to bring the high voltage conductors through a grounded structure. Due to the high electric field around the area next to the grounded structure it is required to keep the intensity of the field as low as possible in order to avoid surface discharges. In this paper, it is discussed how high permittivity composites prepared with doped and functionalized barium titanate powder can be used to control the electric stress on the surface of a polymeric bushing which is used in a combined instrument transformer with two central conductors, one at high voltage and the other at ground potential. First, it is described how the properties of the filler were improved and then it is shown how the prepared polymer/ceramic composite can modify the voltage profile along the surface of a prepared sample. By using three dimensional finite element simulations the performance of the composite bushing is compared with the stress grading obtained in a ceramic bushing with capacitive grading. According to the results, by using composite materials of increased permittivity, the electric stress on the surface of the bushing can be reduced to acceptable values without need of conductive foils.

Graphene: Powder, Flakes, Ribbons, and Sheets

Graphene's unique physical and electrical properties (high tensile strength, Young's modulus, electron mobility, and thermal conductivity) have led to its nickname of "super carbon." Graphene research involves the study of several different physical forms of the material: powders, flakes, ribbons, and sheets and others not yet named or imagined. Within those forms, graphene can include a single layer, two layers, or ≤10 sheets of sp² carbon atoms. The chemistry and applications available with graphene depend on both the physical form of the graphene and the number of layers in the material. Therefore the available permutations of graphene are numerous, and we will discuss a subset of this work, covering some of our research on the synthesis and use of many of the different physical and layered forms of graphene. Initially, we worked with commercially available graphite, with which we extended diazonium chemistry developed to functionalize single-walled carbon nanotubes to produce graphitic materials. These structures were soluble in common organic solvents and were better dispersed in composites. We developed an improved synthesis of graphene oxide (GO) and explored how the workup protocol for the synthesis of GO can change the electronic structure and chemical functionality of the GO product. We also developed a method to remove graphene layers one-by-one from flakes. These powders and sheets of GO can serve as fluid loss prevention additives in drilling fluids for the oil industry. Graphene nanoribbons (GNRs) combine small width with long length, producing valuable electronic and physical properties. We developed two complementary syntheses of GNRs from multiwalled carbon nanotubes: one simple oxidative method that produces GNRs with some defects and one reductive method that produces GNRs that are less defective and more electrically conductive. These GNRs can be used in low-loss, high permittivity composites, as conductive reinforcement coatings on Kevlar fibers and in the fabrication of large area transparent electrodes. Using solid carbon sources such as polymers, food, insects, and waste, we can grow monolayer and bilayer graphene directly on metal catalysts, and carbon-sources containing nitrogen can produce nitrogen-doped graphene. The resulting graphene can be transferred to other surfaces, such as metal grids, for potential use in transparent touch screens for applications in personal electronics and large area photovoltaic devices. Because the transfer of graphene from one surface to another can lead to defects, low yields, and higher costs, we have developed methods for growing graphene directly on the substrates of interest. We can also produce patterned graphene to make GNRs or graphane/graphene superlattices within a single sheet. These superlattices could have multiple functions for use in sensors and other devices. This Account only touches upon this burgeoning area of materials chemistry, and the field will continue to expand as researchers imagine new forms and applications of graphene.

Low-Loss, High-Permittivity Composites Made from Graphene Nanoribbons

A new composite material was prepared by incorporation of graphene nanoribbons into a dielectric host matrix. The composite possesses remarkably low loss at reasonably high permittivity values. By varying the content of the conductive filler, one can tune the loss and permittivity to desirable values over a wide range. The obtained data exemplifies how nanoscopic changes in the structure of conductive filler can affect macroscopic properties of composite material.