ABSTRACT
Graphite nanoplatelets (GNPs)-the segregated ultra-high molecular weight polyethylene (UHMWPE)-based composites with hybrid filler-decorated with Fe3O4 were developed. Using X-ray diffraction and scanning electron microscopy, it was shown that the decorated component has the shape of separate granules, or their clusters were distributed evenly over the GNPs surface. The individual Fe3O4 nanoparticles are predominantly rounded, with diameters of approximately 20-60 nm. The use of GNPs/Fe3O4 as a filler leads to significant decreases in the percolation limit φc, 0.97 vol% vs. 0.56 vol% for GNPs/UHMWPE- and (GNPs/Fe3O4)/UHMWPE segregated composite material (SCM), respectively. Modification of the GNP surface with Fe3O4 leads to an essential improvement in the electromagnetic interference shielding due to enhanced microwave absorption in the 26-37 GHz frequency range in its turn by abundant surface functional groups and lattice defects of GNPs/Fe3O4 nanoparticles.
ABSTRACT
The microwave properties of epoxy composites filled with 30 wt.% of BaFe12-xGaxO19 (0.1 ≤ x ≤ 1.2) and with 1 wt.% of multi-walled carbon nanotubes (CNTs) were investigated in the frequency range 36-55 GHz. A sufficient increase in the microwave shielding efficiency was found for ternary 1 wt.%CNT/30 wt.% BaFe12-xGaxO19/epoxy composites compared with binary 1% CNT/epoxy and 30 wt.% BaFe12-xGaxO19/epoxy due to the complementary contributions of dielectric and magnetic losses. Thus, the addition of only 1 wt.% of CNTs along with 30 wt.% of barium hexaferrite into epoxy resin increased the frequency range where electromagnetic radiation is intensely attenuated. A correlation between the cation Ga3+ concentration in the BaFe12-xGaxO19 filler and amplitude-frequency characteristics of the natural ferromagnetic resonance (NFMR) in 1 wt.%CNT/30 wt.% BaFe12-xGaxO19/epoxy composites was determined. Higher values of the resonance frequency fres (51.8-52.4 GHz) and weaker dependence of fres on the Ga3+ concentration were observed compared with pressed polycrystalline BaFe12-xGaxO19 (fres = 49.6-50.4 GHz). An increase in the NFMR amplitude on the applied magnetic field for both random and aligned 1 wt.% CNT/30 wt.% BaFe12-xGaxO19/epoxy composites was found. The frequency of NFMR was approximately constant in the range of the applied magnetic field, H = 0-5 kOe, for the random 1 wt.% CNT/30 wt.% BaFe12-xGaxO19/epoxy composite, and it slightly increased for the aligned 1 wt.% CNT/30 wt.% BaFe12-xGaxO19/epoxy composite.
ABSTRACT
Polyethylene (PE) based composites with segregated carbon nanotubes (CNTs) network was successfully prepared by hot compressing of a mechanical mixture of PE and CNT powders. Through comparison with a composite comprising randomly distributed carbon nanotubes of the same concentration, we prove that namely the segregated CNT network is responsible for the excellent electrical properties, i.e. 10-1 S/m at 0.5-1% and 10 S/m at 6-12% of CNT. The investigation of the complex impedance in the frequency range 1 kHz-2 MHz shows that the sign of real part of the dielectric permittivity changes from positive to negative in electrically percolated composites indicating metal-like behavior of CNT segregated network. The obtained negative permittivity and AC conductivity behavior versus frequency for high CNT content (3%-12%) are described by the Drude model. At the same time, in contrast to reflective metals, high electromagnetic shielding efficiency of fabricated PE composites in the frequency range 40-60 GHz, i.e. close to 100% at 1 mm thick sample, was due to absorption coursed by multiple reflection on every PE-CNT segregated network interface followed by electromagnetic radiation absorbed in each isolated PE granule surrounded by conductive CNT shells.
ABSTRACT
The paper describes preparation features of functional composites based on ferrites, such as "Ba(Fe1-xGax)12O19/epoxy," and the results of studying their systems; namely, the correlation between structure, magnetic properties and electromagnetic absorption characteristics. We demonstrated the strong mutual influence of the chemical compositions of magnetic fillers (Ba(Fe1-xGax)12O19 0.01 < x < 0.1 solid solutions), and the main magnetic (coercivity, magnetization, anisotropy field and the first anisotropy constant) and microwave (resonant frequency and amplitude) characteristics of functional composites with 30 wt.% of hexaferrite. The paper presents a correlation between the chemical compositions of composites and amplitude-frequency characteristics. Increase of Ga-content from x = 0 to 0.1 in Ba(Fe1-xGax)12O19/epoxy composites leads to increase of the resonant frequency from 51 to 54 GHz and absorption amplitude from -1.5 to -10.5 dB/mm. The ability to control the electromagnetic properties in these types of composites opens great prospects for their practical applications due to high absorption efficiency, and lower cost in comparison with pure ceramics oxides.
ABSTRACT
The article reports about electric field-induced alignment of the carbon nanoparticles embedded in epoxy matrix. Optical microscopy was performed to consider the effect of the electric field magnitude and configuration, filler morphology, and aspect ratio on alignment process. Characteristic time of aligned network formation was compared with modeling predictions. Carbon nanotube and graphite nanoplatelet rotation time was estimated using an analytical model based on effective medium approach. Different depolarization factor was applied according to the geometries of the particle and electric field.Solid nanocomposites were fabricated by using AC electric field. We have investigated concentration dependence of electrical conductivity of graphite nanoplatelets/epoxy composites using two-probe technique. It was established that the electrical properties of composites with random and aligned filler distribution are differ by conductivity value at certain filler content and distinguish by a form of concentration dependence of conductivity for fillers with different morphology. These differences were explained in terms of the dynamic percolation and formation of various conductive networks: chained in case of graphite nanoplatelets and crossed framework in case of carbon nanotubes filler.
ABSTRACT
This work presents the results of computer modeling and experimental measurements of microwave transmission properties for one-dimensional periodic multi-layered photonic structures (PCs), composed of epoxy layers and composite layers filled with nanocarbon particles-multi-walled carbon nanotubes and graphite nanoplatelets. The results show that the characteristics of observed photonic band gaps in transmission spectra of PC can be controlled by varying the parameters of layers, namely, the complex permittivity and the layer thickness. It was found that the insertion of the defects (for instance, magnetic layer) into photonic structure can change the EMR transmission spectrum. The comparative analysis of EMR transmission spectra for investigated photonic structures has showed good agreement between the experimental and simulated data. It was found that EMR absorption in composite layers of photonic structures shifts the transmission spectra to the smaller values of EMR transmission index and reduces the sharpness of photonic band gaps. Thus, by changing the parameters of composite layers in photonic structure, we can obtain the tunable photonic band gaps, necessary for technological applications in devices, capable of storing, guiding, and filtering microwaves.
