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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]

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]

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]