Electrical transport,optical and thermal properties of polyaniline–pumice composites

In this study, electrical conductivity, photoconductivity, absorbance and thermal properties of polyaniline (PANI) and polyaniline–pumice composites were investigated. Temperature dependent conductivity and photoconductivity measurements were carried out in the temperature range of 80–400 K. The measurements revealed that the dominant conduction mechanisms in polyaniline and 15% pumice doped composite were hopping conduction. The low activation energies calculated for 36% pumice doped composite indicated that this sample has highly defective and degenerate structure due to the high pumice content. Polyaniline and pumice doped composites showed semiconductor behavior with the exponential variation of inverse temperature dependence of electrical conductivity. Photoconductivities of the PANI and PANI–pumice composites under various illumination intensities were studied and it was found for all samples that the conductivity increased with increasing temperature and light intensity, but decreased with increasing pumice content in the structure. Absorbance spectrum has been determined in the wavelength range of 300–700 nm and it was found that the band gap values decreased as the pumice content was increased. Thermogravimetric analysis have shown for all samples that the mass loss has started above around 300 K due to the loss of moisture from the structures. As a result of this work, it was found that polyaniline and polyaniline–pumice composites had low resistivity and high band gaps and could be used as a window layer semiconductor in heterojunction solar cell applications.     

Conductive network formation in the melt of carbon nanotube/thermoplastic polyurethane composite

This research concerns the effect of conductive network formation in a polymer melt on the conductivity of multi-walled carbon nanotube/thermoplastic polyurethane composite systems. An extremely low percolation threshold of 0.13 wt.% was achieved in hot-pressed composite film samples, whereas a much higher CNT concentration (3–4 wt.%) is needed to form a conductive network in extruded composite strands. This is explained in terms of the dynamic percolation behaviour of the CNT network in the polymer melt. The temperature and CNT concentration needed for dynamic percolation to take effect were studied by the conductivity versus temperature behaviour of extruded strands, in an attempt to optimise the processing conditions.     

Composite Yarns of Multiwalled Carbon Nanotubes with Metallic Electrical Conductivity

Unique macrostructures known as spun carbon‐nanotube fibers (CNT yarns) can be manufactured from vertically aligned forests of multiwalled carbon nanotubes (MWCNTs). These yarns behave as semiconductors with room‐temperature conductivities of about 5 × 10² S cm^?1. Their potential use as, for example, microelectrodes in medical implants, wires in microelectronics, or lightweight conductors in the aviation industry has hitherto been hampered by their insufficient electrical conductivity. In this Full Paper, the synthesis of metal–CNT composite yarns, which combine the unique properties of CNT yarns and nanocrystalline metals to obtain a new class of materials with enhanced electrical conductivity, is presented. The synthesis is achieved using a new technique, self‐fuelled electrodeposition (SFED), which combines a metal reducing agent and an external circuit for transfer of electrons to the CNT surface, where the deposition of metal nanoparticles takes place. In particular, the Cu–CNT and Au–CNT composite yarns prepared by this method have metal‐like electrical conductivities (2–3 × 10⁵ S cm^?1) and are mechanically robust against stringent tape tests. However, the tensile strengths of the composite yarns are 30–50% smaller than that of the unmodified CNT yarn. The SFED technique described here can also be used as a convenient means for the deposition of metal nanoparticles on solid electrode supports, such as conducting glass or carbon black, for catalytic applications.     

Effect of thermal treatment conditions on the properties of onion-like carbon based polymer composite

Dielectric properties of onion-like carbon (OLC) and polyurethane composite prepared using different procedures were investigated in the frequency range up to 1 MHz. We show that broadband dielectric spectroscopy is powerful tool to determine technological fingerprints in the studied materials. It is demonstrated that cured samples annealed at temperature close to the melting point (450 K) exhibit substantially higher dielectric permittivity and electrical conductivity in comparison with untreated samples. With the increase of temperature of an untreated sample, its dielectric permittivity, electric conductivity and critical frequency increase, while Maxwell–Wagner mean relaxation time and aggregate sizes of OLC decreases. Annealing of the composite at temperature close to the melting temperature produce sample with more homogeneous distribution of OLC. The temperature dependence of conductivity of the homogeneous sample is mainly caused by a weak positive temperature coefficient effect.     

Electrical transport mechanisms in a-Se95M5 films (M = Ga, Sb, Bi)

The temperature dependence of direct current (dc) conductivity has been reported in thin films of a-Se₉₅M₅ (where M = Ga, Sb, Bi), in the temperature range 219-375 K, in order to identify the conduction mechanism and to observe the doping effect of different metals on amorphous selenium. It is found that the conduction in high temperature range (314-375 K) is due to thermally activated tunneling of charge carriers in the band tails of localized states; and in the low temperature range (219-314 K) conduction takes place through variable range hopping in the localized states near the Fermi level. Current-voltage (I-V) measurements at high electric fields (the field dependence of dc conductivity) have also been carried out for the samples of present system. The analysis of data shows the existence of space charge limited conduction (SCLC) in these glassy alloys. The density of localized states near the Fermi level is calculated for these alloys using dc conductivity (Mott parameters) and SCLC measurements data. The properties have been found to be highly composition dependent.     

Study of transport properties and conduction mechanism of pure and composite resorcinol formaldehyde aerogel doped with Co-ferrite

A series of resorcinol formaldehyde aerogels (RF aerogels) composite with nanoparticles of CoFe₂O₄ have been prepared by sol–gel method. Four samples of pure RF aerogels were prepared at different concentrations of Na₂CO₃ as catalyst (0.02, 0.025, 0.03, and 0.04 wt.%) and four samples of composite RF aerogels were prepared at different concentration of doped CoFe₂O₄ (0.075, 0.1, 0.125, and 0.15 wt.%; Na₂CO₃ concentration = 0.03 wt.%). DC electrical conductivity as a function of temperature was studied in the temperature range 25 °C–200 °C for all samples. AC electrical conductivity and dielectric properties were determined using RLC Bridge in the frequency range 100 Hz–1 MHz at different temperature (25–200 °C). The pore size of the samples was determined using positron annihilation lifetime spectroscopy (PALS). RF aerogels are found to exhibit a semiconducting behavior and characterized by two transition temperatures T₁ and T₂. Also σ_DC increases with increase of Co-ferrite contents. Pure RF aerogels posses a very low dielectric constant, where the lowest value of ?′ is ∼4 times as that of air. ?′ decreases with increase of frequency, and increases with increase of temperature. Large overlapping polaron (OLP) is found to be the preferred conduction mechanism in these materials. The results of PALS show that there are two types of pore size in these samples; the first ranges from 1.9 to 2.5 nm, while the second ranges from 3.2 to 5.3 nm.     

Electric field effects on CNTs/vinyl ester suspensions and the resulting electrical and thermal composite properties

In this study, electrical conductivity of a vinyl ester based composite containing low content (0.05, 0.1 and 0.3 wt.%) of double and multi-walled carbon nanotubes with and without amine functional groups (DWCNTs, MWCNTs, DWCNT-NH₂ and MWCNT-NH₂) was investigated. The composite with pristine MWCNTs was found to exhibit the highest electrical conductivity. Experiments aimed to induce an aligned conductive network with application of an alternating current (AC) electric field during cure were carried out on the resin suspensions with MWCNTs. Formation of electric anisotropy within the composite was verified. Light microscopy (LM), scanning electron (SEM) and transmission electron microscopy (TEM) were conducted to visualize dispersion state and the extent of alignment of MWCNTs within the polymer cured with and without application of the electric field. To gain a better understanding of electric field induced effects, glass transition temperature (T_g) of the composites was measured via Differential Scanning Calorimetry (DSC). It was determined that at 0.05 wt.% loading rate of MWCNTs, the composites, cured with application of the AC electric field, possessed a higher T_g than the composites cured without application of the AC electric field.     

Enhancement of thermoelectric figure of merit by doping Dy in La0.1Sr0.9TiO3 ceramic

Ceramic samples of La_0.1Sr_0.9−xDy_xTiO₃ (x = 0.01, 0.03, 0.07, 0.10) have been prepared by the solid-state reaction method. Characterization from the powder X-ray diffraction indicates that their crystal structure changes from cubic to tetragonal phase. Their electrical and thermal transport properties are measured in the temperature range of 300-1100 K. n-Type thermoelectric is obtained with large Seebeck coefficient. The figure of merit is markedly improved, due to relatively lower electrical resistivity and thermal conductivity by Dy doping effect. A much lower electrical resistivity of 0.8 mΩ cm at room temperature is obtained in La_0.1Sr_0.8Dy_0.1TiO₃, and with a relatively lower thermal conductivity of 2.5 W/m K at 1075 K. The maximum figure of merit reaches ∼0.36 at 1045 K for La_0.1Sr_0.83Dy_0.07TiO₃, which is the largest value among n-type oxide thermoelectric ceramics.     

Temperature and frequency dependent dielectric properties of lithium orthophosphate LiBaPO4

The lithium barium orthophosphate LiBaPO₄ compound has been synthesized by the classic ceramic method and characterized by X-ray diffraction (XRD) technique. The electrical conductivity and modulus characteristics of the system have been investigated in the temperature and the frequency range 681–872 K and 200 Hz–1 MHz respectively by means of impedance spectroscopy. The activation energy value of LiBaPO₄ sample is bigger than of the LiMPO₄ (M = Mn, Co, Ni, Fe) compounds. The frequency dependent conductivity of the present system shows the power law feature. Dielectric data were analyzed using complex electrical modulus M* at various temperatures. The peak positions ω_m of the above spectra shift towards higher frequencies with increase in temperature. The above spectra have been characterized in terms of Kohlrausch–Williams–Watts (KWW) relaxation function to understand relaxation behavior. The activation energy responsible for relaxation calculated from the modulus spectra is found to be almost the same as the value obtained from temperature variation of dc conductivity. The electrical modulus and its scaling behavior are also investigated and the relationship between power-law exponent n and stretched exponential exponent β is found.     

Synthesis, structure and dielectric properties of a rare earth double perovskite oxide Ba2CeTaO6

A rare earth double perovskite oxide barium cerium tantalate, Ba₂CeTaO₆ (BCT) is synthesized by solid-state reaction. The X-ray diffraction pattern of the sample at room temperature (25 °C) shows monoclinic structure, with the lattice parameters, a = 9.78 Å, b = 9.02 Å and c = 4.27 Å and β = 93.8°. A scanning electron micrograph shows the formation of grains with average size ∼ 2 μm. Impedance spectroscopy is applied to investigate the ac electrical properties of BCT in a temperature range from 303 to 673 K and in a frequency range from 100 Hz to 1 MHz. Complex-impedance-plane plots show grain contribution for BCT. The frequency-dependent electrical data are analyzed in the framework of the conductivity and modulus formalisms. The frequency-dependent conductivity spectra follow a power law. The conductivity at 110 Hz varies from 3.5 × 10⁻⁷ S m⁻¹ to 1.2 × 10⁻² S m⁻¹ with increasing temperature from 303 to 673 K, respectively. The scaling behaviour of M″ and Z″ suggest that the relaxation describes the same mechanism at various temperatures.     

Dielectric relaxation in complex perovskite oxide In(Ni1/2Zr1/2)O3

The dielectric study of indium nickel zirconate, In(Ni_1/2Zr_1/2)O₃ (INZ) synthesized by solid state reaction technique is performed in a frequency range from 500 Hz to 1 MHz and in a temperature range from 303 to 493 K. The X-ray diffraction analysis shows that the compound is monoclinic. A relaxation is observed in the entire temperature range as a gradual decrease in ?′(ω) and as a broad peak in ?″(ω) in the frequency dependent real and imaginary parts of dielectric constant, respectively. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with activation energy of 0.66 eV. The Cole-Cole model is used to study the dielectric relaxation of INZ. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependent conductivity spectra follow the universal power law.     

Low temperature amorphous growth of semiconducting Y-Ba-Cu-O oxide thin films in view of infrared bolometric detection

YBa₂Cu₃O_6 + x (YBCO) compounds are well known to exhibit superconducting properties for x > 0.5 and semiconducting properties for lower oxygen content. In this work, YBCO oxide thin films of the semiconducting phase were deposited by direct-current (DC) hollow cathode sputtering at low temperature in the 100 to 400 °C range. Structural, electrical and optical properties are investigated and discussed in relation with the envisaged bolometric detection application. Structural characterizations show that films are amorphous, with a granular structure of low roughness (3 nm rms). DC electrical measurements both reveal that films grown at 100 °C exhibit a high temperature coefficient of resistance (TCR ~ − 3% K^− 1 to − 4% K^− 1 at 300 K) and an optimized low resistivity value of 345 Ω·cm at 300 K. Consequently, this material is suitable for uncooled infrared bolometer application and can be deposited at 100 °C in a complementary metal-oxide-semiconductor compatible technological process for co-integration with readout circuitry. In addition, optical measurements performed in the 0.5 to 2.2 μm wavelength range on films grown at 100 °C highlight optical conductivity values in line with those expected for YBCO material, as well as the presence of two optical band gaps that are discussed with respect to the film nanostructure.     

Conduction and dielectric polarization in Se thin films

Se films were prepared by thermal evaporation technique in thickness range 150-8500 Å. X-ray diffraction measurements showed that Se films are in the amorphous state. The ac conductivity and dielectric properties of the amorphous Se films have been investigated in the frequency range 100-100 KHz and 100-400 K temperature range. The ac conductivity σ_ac(ω) is found to be proportional to ω^s where s<1. The temperature dependence of both ac conductivity and the parameter s is reasonably well interpreted by the correlated barrier (CBH) model. The dc conductivity at the room temperature was also studied in the same thickness range. It was concluded that the same mechanism of carrier motion might be dominant in both ac polarization and dc conduction. This carrier transport mechanism might be electronic.     

Synthesis and electrical properties of cubic NaxWO3 thin films across the metal-insulator transition

Highly oriented (1 0 0) Na_xWO₃ thin films were fabricated in the composition range 0.1 ≤ x ≤ 0.46 by pulsed laser deposition technique. The films showed transition from metallic to insulating behaviour at a critical composition between x = 0.15 and 0.2. The pseudo-cubic symmetry of Na_xWO₃ thin films across the transition region is desirable for understanding the composition controlled metal-insulator transition in the absence of any structural phase transformation. The electrical transport properties exhibited by these films across the transition regime were investigated. While the resistivity varied as T² at low temperatures in the metallic regime, a variable range hopping conduction was observed for the insulating samples. For metallic compositions, a non-linear dependence of resistivity in temperature was also observed from 300 to 7 K, whose exponent varied with the composition of the film.     

Poly(vinyl alcohol) reinforced with large-diameter carbon nanotubes via spray winding

For practical application of carbon nanotube (CNT)/polymer composites, it is critical to produce the composites at high speed and large scale. In this study, multi-walled carbon nanotubes (MWNTs) with large diameter (∼45 nm) and polyvinyl alcohol (PVA) were used to increase the processing speed of a recently developed spraying winding technique. The effect of the different winding speed and sprayed solution concentration to the performance of the composite films were investigated. The CNT/PVA composites exhibit tensile strength of up to 1 GPa, and modulus of up to 70 GPa, with a CNT weight fraction of 53%. In addition, an electrical conductivity of 747 S/cm was obtained for the CNT/PVA composites. The good mechanical and electrical properties are attributed to the uniform CNTs and PVA matrix integration and the high degree of tube alignment.     

The electronic properties of metal complexed poly(3-alkylthiophene) films

Dielectric spectroscopy has been used to extract the AC and DC electrical conductivities of P3MT complexed with the paramagnetic ion Fe²⁺, and these were compared to data for uncomplexed P3MT in the frequency range of 1 Hz-10 MHz. The polymers were nominally undoped; however, the presence of residual Fe³⁺ ions from the polymerisation reaction renders the materials p-type semiconductors. Measurements were carried out over the temperature range 123-323 K. An analysis of the frequency-dependent complex conductivity together with the DC conductivity can be used to elucidate whether free charge conduction, charge hopping or quantum tunnelling is the dominant conduction mechanism. The results were compared to the predictions of a variety of theoretical conduction models. It was found that the presence of the Fe²⁺ ions produced greater long-range order at low temperatures by complexing between the sulphur heteroatoms of four 3-methylthiophene monomer units. This provided an additional barrier hopping conduction mechanism at low temperatures (183-243 K), and the hopping occurred over a barrier height within the range 0.45-0.6 eV.     

Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.     

The unusual electrical response in polyaniline-threonine composites

Polymer-amino acid composites are a new class of polymer materials with unusual electrical property. Novel polyaniline-threonine composites combining the synthesized polyaniline and proteinogenic amino acid threonine have been prepared. The samples were characterized by Fourier-transform infrared spectra, UV-vis absorption spectra and atomic force microscopy. The unusual electrical response was found from the d.c. conductivity measurement. The carboxyl group of the amino acid threonine played an important role in the electrical conductivity. Analysis of d.c. conductivity data revealed that in the temperature range of 300 K-200 K, the d.c. conductivity was governed by Mott's 3-dimensional variable range hopping.     

Electrical conductivity and Hall mobility in p-type TlGaSe2 crystals

Systematic dark electrical conductivity and Hall mobility measurements have been carried out in the temperature range of 200-350 K on p-type TlGaSe₂ crystals. The analysis of the temperature-dependent electrical conductivity and carrier concentration reveals the extrinsic type of conduction with an acceptor impurity level located at 0.33 eV, and donor and acceptor concentrations of 9.0×10¹⁵ and 1.3×10¹⁶ cm⁻³, respectively. A hole and electron effective masses of 0.520m₀ and 0.325m₀, respectively, with a donor to acceptor compensating ratio of 0.69 are also being identified. The Hall mobility is found to be limited by the hole-phonon short-range interactions scattering with a hole-phonon coupling constant of 0.17.     

Elevated temperature tensile properties and thermal expansion of CNT/2009Al composites

1.5 vol.% and 4.5 vol.% carbon nanotubes reinforced 2009Al (CNT/2009Al) composites with homogeneously dispersed CNTs and refined matrix grains, were fabricated using powder metallurgy (PM) followed by 4-pass friction stir processing (FSP). Tensile properties of the composites between 293 and 573 K and the coefficient of thermal expansion (CTE) from 293 to 473 K were tested. It was indicated that load transfer mechanism still takes effect at temperatures elevated up to 573 K, thus the yield strength of the 1.5 vol.% CNT/2009Al composite at 423–573 K, was enhanced compared with the 2009Al matrix. However, for the 4.5 vol.% CNT/2009Al composite, the yield strength at 573 K was even lower than that for the matrix, due to the quicker softening of ultrafine-grained matrix. Compared with the 2009Al matrix, the CTEs of the composites were greatly reduced for the zero thermal expansion and high modulus of the CNTs and could be well predicted by the Schapery’s model.