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The investigation of physical properties such as the electrical conductivity of vinyl ester/carbon nanotube/polymer systems with different ratios of carbon nanotubes has been a remarkable phenomenon recently. For this purpose, both neat materials without carbon nanotubes and single-walled carbon nanotube (SWCNT)-loaded vinyl ester resin-based composite materials were manufactured. To establish a relationship between the surface properties of the material and the conductivity mechanism in the material, the surface microstructure and conductivity behaviors of the materials are described comparatively using the fractal analysis method and impedance spectroscopy techniques. The surface topography of the samples was determined from microphotographs obtained by scanning electron microscopy (SEM). The SEM images showed that independent and different-sized clusters aggregated on the surfaces and internal structures of the materials. Surface coverage ratios and fractal dimension values were calculated by applying the fractal characterization method to the specimens. Surface coverage ratios and fractal dimensions in SWCNT-loaded nanocomposites were analyzed considering effects such as component dispersion and particle density compared to pure polymer. It was found that the fractal behavior of the nanotube-filled composite was one of the effective components in the formation of the conductive mechanism. Nyquist diagrams of materials obtained using the impedance results also confirmed the fractal analysis data. It was noticed from the curvature degree of the Nyquist plots that as SWCNT was added to the vinyl ester matrix, the efficiency of the interchain network of the nanotube-loaded material was significantly increased. [ABSTRACT FROM AUTHOR]

In this paper, the internal structure and dielectric properties of unsaturated polyester resin-based neat and single-walled carbon nanotube reinforced composites were comprehensively evaluated with the fractal analysis using the Fast Fourier Transform (FFT). The greyscale images, bitmap (BMP) images and 3D tomographic images were obtained by converting the scanning electron microscope images of the materials. It was observed that the distributions of components in the resin for both materials are irregular and their surfaces exhibit anisotropic behaviors. The surface coating rate (SCR) and fractal dimensionality (FD) of the materials were also calculated using the power spectrum. It has been observed that the fractal dimensionality of the composites can be changed by the doping process and the fractalization of the nanotube doped sample increases compared to the pure material due to nanotube agglomeration, spatial distribution and the orientation. The increase in fractalization as a result of this agglomeration and orientation in carbon nanotubes explains the high dielectric constant values observed at low frequencies by increasing the number and size of carbon nanotubes clusters that act as micro capacitors in certain regions of the matrix. It has been reported that the calculations for the surface coverage ratios for both samples also support these results. [ABSTRACT FROM AUTHOR]

This study presents impedance characteristics of single walled carbon nanotube/vinylester (SWCNT/VE) glass fiber reinforced polymer (GFRP) composites. The impedance measurements were carried out as a function of the frequency over range of 10-2 and 107 Hz at various temperatures between 300 K and 420 K. Bode and Nyquist plots of real and imaginary parts of complex impedance (Z*) were obtained and Cole–Cole approach was used to interpret the impedance characteristics. The results indicated that the bulk resistance of the material decreases significantly as the temperature increases. The frequency-dependent AC conductivities were calculated using the complex impedance data and dimensions of specimen. It has been observed that the alternating current values are compatible with the Jonscher’s power law. The behavior of dielectric constant and loss factor at the various temperatures were analyzed as a function of applied frequency. While the sample exhibited high dielectric permittivity in the low frequency region with the Maxwell-Wagner-Sillars (MWS) effect, it was observed that the permittivity decreased as a result of the dipoles' inability to rotate themselves in the field direction at high frequencies. No dielectric relaxation peak was observed in the loss spectra in our limits. From the results, it can be said that the contribution to the dielectric relaxation is due to the interface polarization and DC conductivity. Electric modulus formalism was also used to describe the conductivity and dielectric relaxation processes of SWCNT/VE binary composite. It was found that the obtained peak maximums shifted to higher frequencies as the temperature increased. It is concluded that the frequency regime below the peak maximum defines the range of mobile charge carriers, and in the regime above the maximum, the charge carriers are limited to short distance potential wells. [ABSTRACT FROM AUTHOR]

This study aims to establish a relationship between the dielectric characteristics and fractal parameters of single walled carbon nanotube reinforced vinylester based nanocomposites manufactured by sheet molding compound process. The complex dielectric permittivity was analyzed and interpreted using electric modulus formalism. The topography of both unfilled and single-walled carbon nanotube reinforced nanocomposites has been investigated using scanning electron microscopy (SEM) to provide correlation between experimental dielectric data and fractal theory. The self-similar microstructure was observed in micro photos for both materials. The superficial particle and cluster coverage ratio were calculated with the application of the scaling theory. Additionally, critical exponent and the fractal dimensions that determine the environment-area relationship have been determined by Slit Island Method (SIM) which is used to process 2-dimensional digital microscopic images. Fractal dimensions estimated with SIM were compared with the fractal dimensions computed obtained by box counting method for reliable results. It has been revealed that the surface of the composite consists of amorphous clusters with different sizes independent of each other and the structure is self-affine. In addition, the superficial area and cluster size have been shown to have a significant effect on the fractal dimension. As a result, the relationship between the dielectric permittivity of the material, determined by the electrical module formalism, and the morphological surface formation defined by the fractal analysis method and the SIM method, were determined and the results were compared with similar studies in the literature in this article. [ABSTRACT FROM AUTHOR]

The electrical properties of composite materials over a wide frequency range are of great interest, not only for experimental applications, but also for theoretical studies such as fractal analysis. This study presents comparative analysis of alternating current (ac) conductivity and fractal structure characteristics in standard and single walled carbon nanotube (SWCNT) reinforced polymer composites based unsaturated polyester resin (UPR). The electrical characteristics of polymer matrices at 320 K have been analyzed as a function of frequency by impedance analysis method. It was found that the conductivity of the nanotube doped material in the dc conductivity region, which is the low frequency region, is independent of the frequency and takes a constant value. It was proved that conductivity obeys Jonscher's power law toward the high frequency region. The standard sample showed an insulating behavior that exhibits continuous increase with increasing frequency. The images of the samples were obtained by scanning electron microscope (SEM) to reveal the relationship between the conductivity of the materials and their fractal properties. All samples were converted to binary format for calculations. Cellular particle density for each sample was determined according to scaling theory. Then, the surface coverage ratio, fractal dimensions, cluster densities, average cluster sizes and critical interface exponent values of the samples were calculated and compared with different samples in the literature. It was determined that the coverage ratio and fractal dimension increased when carbon nanotubes were added. In addition, it was observed that the interface critical exponent decreased when the carbon nanotube was doped. [ABSTRACT FROM AUTHOR]

This paper investigates effect of different industrial resins including vinyl ester and unsaturated polyester produced by sheet moulding compound (SMC) method on conductivity and dielectric properties of single walled carbon nanotubes (SWCNT) modified glass fiber reinforced polymer composites (GFRP), comparatively. Alternating current (AC) conductivities, complex dielectric permittivity, dissipation factor (DF), electric modulus of unsaturated polyester and vinyl ester resin-based composites was examined by impedance analyzer depending on frequency range from 10-2 Hz to 107 Hz at room temperature. Cole–Cole approach was used to describe the impedance characteristics of tested samples. In order to clearly understand the effect of resin type, the glass fiber and carbon nanotube filling ratios in the produced materials were kept constant. AC conductivity results showed that VE composite exhibited better performance than UP composite in all frequency. The theoretical models including Power Law and Linear Regression (LR) algorithm also confirmed the experimental results about the conductivity of the composites. A comparison of the dielectric characteristics between SWCNT/VE and SWCNT/UP binary composites indicated that VE based composite exhibited superior dielectric properties compared to the UP based sample. In addition, scanning electron microscopy (SEM) images have been taken to observe distribution and organization of fillers in polymer composites. [ABSTRACT FROM AUTHOR]

This study presents mechanical and electrical characterization of glass fiber reinforced polymer (GFRP) filled with low specific single-walled carbon nanotubes (SWCNTs) and BYK 8003 organic compounds. For this aim, DC conductivities of materials were calculated using high speed I–V measurements. The dielectric characteristics including dielectric constant, volume resistivity, loss tangent and dielectric loss factor were examined depending on frequency ranging from 50 Hz to 100 kHz. Also, tensile and flexural tests were fixed to investigate the effect of filling process on the mechanical behavior of GRFP. The surface morphology of the composite materials was analyzed with SEM and AFM measurements. [ABSTRACT FROM AUTHOR]

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The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm2/V s along with 8.44×1012 cm-2 electron density, which resulted in a two-dimensional sheet resistance of 435 Ω/□ for the Al0.2Ga0.8N/AlN/GaN heterostructure. The sample designed with an Al0.88In0.12N barrier exhibited very high sheet electron density of 4.23×1013 cm-2 with a corresponding electron mobility of 812 cm2/V s at room temperature. A record two-dimensional sheet resistance of 182 Ω/□ was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al0.88In0.12N/AlN/GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al0.88In0.12N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs. [ABSTRACT FROM AUTHOR]