Impedance spectroscopy was conducted on colloidal ITO thin films that had been subjected to alternating oxygen and argon plasma treatments, followed by air annealing from 150 to 750 °C. An equivalent circuit consisting of an RC element nested within another RC element, featuring a negative resistance and a negative capacitance, fitted the data well. These results are interpreted as being due to surface plasmons that are a function of the presence of nanoporous ITO-rich regions surrounded by isolated ITO nanoparticles coated with an amorphous polymer that intertwines with the ITO-rich regions as a function of annealing treatment.

The electrical conductivity of insulating polymer matrix composites undergoes radical increase at a certain concentration of conductive filler, which is known as the percolation threshold. Polymer matrix conductive nanocomposites were fabricated by compression molding the mechanically mixed poly (methyl methacrylate) (PMMA) and antimony tin oxide (ATO) nanoparticles, as has been done with other polymer composites before. The electrical conductivity of PMMA/ATO nanocomposites increased by several orders of magnitude at a small concentration of ATO (∼ 0.27 vol %). The continuous 3D network like distribution of ATO nanoparticles contributed to this percolation at subcritical filler concentrations. The effects of processing parameters on these unique microstructures and electrical properties were investigated. The tetrakaidecahedron-like microstructure was observed by scanning electron microscopy (SEM) and was found to be affected by the molding pressure, temperature and amount of nanoparticles. The viscoelastic flow of matrix under the optimum processing conditions allowed the shape transformation of PMMA into space filling polyhedra and an ordered distribution of ATO nanoparticles along the sharp edges of the PMMA. Parametric finite element analysis was performed to model this unique microstructure-driven percolation. The 2D simplified model was generated in AC/DC frequency domain mode in COMSOL Multiphysics® to solve the effects of ordered distribution of conductive nanoparticles on the electrical properties of the composite. There was excellent agreement between experimental and simulated values of electrical conductivity and percolation concentration. This model can be used to predict percolation threshold and electrical properties for any types of composite systems containing insulating matrix and conductive fillers that can form this unique microstructure.

Dispersions containing 1 mg/mL of several carbon nanomaterials were used to deposit films containing 1 to 20 layers. The electrical properties of the composite films were characterized via impedance spectroscopy along two directions: in-plane on the film topmost surface and also through the thickness. It was found that carbon black nanoparticles never achieved full in-plane interconnection while the multiwalled carbon nanotube (MWNT) and single-walled carbon nanotube were already percolated at one layer. Graphite flakes showed a complete percolation curve that allowed its resistance to change by 6–7 orders of magnitude. The differences in the microstructure, electrical response, and thermal decomposition behavior of these carbon nanomaterial–paper substrate films were explained by detailed equivalent circuit analysis of the impedance spectra. Interpretation was supplemented by scanning electron microscopy images and thermal analysis via Differential Scanning Calorimetry/Thermogravimetric Analysis (DSC/TGA). Thru-plane electrical properties were for the most part similar, although only films with short MWNT showed a clearly infiltrated network structure.

Four-layer multiwalled carbon nanotube (MWNT) thin films were deposited via dropcasting (1 mg/mL MWNTs and 10 mg/mL SDBS) onto filter papers that vary in pore size (1, 5, 25, and 40 µm) to determine the effect of the underlying substrate structure on the in-plane properties of the films. The films (<100 nm thick) were dried using vacuum filtration, and drying in a 65 °C heater with and without a ceramic heating board. DC resistance of the films ranged from 6 × 103 to 9.3 × 109 Ω. Impedance spectroscopy analysis revealed a low and a high frequency inductive response and two parallel R–C circuits for the more conducting thin films. High resistance films were fit by a single RC circuit with a constant-phase element. The differences in the in-plane electrical responses of the different MWNT films can be explained by the degree of carbon nanotube surface coverage, obtained as a result of using different pore size filter papers. The drying method utilized also affected the CNT network formation and its resultant electrical properties.

Concentration- and layer-dependent percolation thresholds can be determined for carbon nanotube (CNT) films deposited from aqueous dispersions on paper substrates at both the surface of the deposited film (in-plane) and through the thickness of the paper (thru-plane) using impedance spectroscopy. By analyzing the impedance spectra as a function of the number of layers (solution concentration is constant) or the solution concentration (number of layers is constant), the electrical properties and percolation thresholds for CNT-paper composites can be determined. In-plane measurements show that percolation occurs at 4 layers when 1 mg/mL solution concentration is used. In the thru-plane direction, the films are already percolated at 1 mg/mL concentration, which is confirmed by varying the concentration of the solution used to deposit 1 layer films. A second percolation event happens between 8 and 12 layers due to an increased number of interconnections of CNTs within the paper substrate. The lowest sheet resistance achieved was 100 Ω/□.

Recently, there has been much interest in the creation of 3D networks of nanowires. One possible way to do this is to encase the nanowires inside transparent polymer matrices since there is also a demand for obtaining conducting transparent composites. If the filler of the composite is made from a strongly conducting material, the degree of connectivity of the networked nanowires can be tested by measuring its conductivity. Though much work has been done with ITO (Tin-doped indium oxide), little has been done with the chemically similar, but cheaper, ATO (Antimony-doped tin oxide). In this study, ATO nanoparticles were added into a polystyrene matrix and simultaneously pressed and heated so that a 3D network of the nanoparticles would form. The effecti veness of the conducting pseudo-nanowire networks was measured as the concentration of ATO in polystyrene was varied. Another variable utilized was the temperature at which the samples were pressed. The optical transmittance of the composites was also measured in order to quantify their transparency. It was found that, once the nanowire networks had percolated at a concentration of about 1.25 PHR, the conductivity and, consequently, the coherence of the networks increased at a decreasing rate as the concentration was increased. The effect of the pressing temperature was complex and required many additional sets of specimens to understand. Samples pressed at the highest temperature had the least coherent networks, as the polystyrene became too fluid and disrupted the ATO networks while at lower temperatures the opposite occurred. The optical transmittance dropped sharply as the concentration of ATO reached and surpassed 1.0 PHR. Nanowire networks were, indeed, formed through this process using these materials, but use as a conducting transparent composite in the visible range is unlikely as the percolation threshold occurs at a concentration greater than that of the optical transmittance drop, creating a trade-off between conductivity and transparency. The resistivity did drop as much as six orders of magnitude and may be useful for other applications.

Commercially available antimony tin oxide (ATO) nanoparticles were dispersed in water using tetramethylammonium hydroxide (TMAH) as a dispersing agent and deposited onto glass substrates by spin coating. Films of one to five layers were made. These thin films were characterized using impedance spectroscopy and ultraviolet-visible spectroscopy to obtain their sheet resistances and optical transmittance, respectively. The films displayed sheet resistances around 105-106 kΩ/☐ and optical transmittance in the near infrared to near ultraviolet range above 95%. Films were then made using a higher concentration ATO solution and found to achieve sheet resistances on the order of 102 kΩ/☐ but had decreased transmittance as low as 65% at some wavelengths. Impedance measurements, along with optical micrographs, were taken at different locations on the films. These experiments demonstrated that films of more than one layer showed greater uniformity. Additional sets of films were also produced with varying substrate preparation and dispersion deposition parameters. Aside from dispersion concentration, high humidity during film measurement was found to be the most crucial parameter for achieving low sheet resistances.

The percolation threshold in a ceramic composite depends on the processing conditions used to fabricate them along with the size and shape of the filler. In this study, borosilicate glass microspheres were used as the matrix material and nanosized antimony tin oxide (ATO) particles were used as the filler. The microsphere/ATO composites were fabricated by hot pressing around the glass transition temperature in order to control the viscosity. The pressure and temperature applied allowed the ATO to be confined to the spaces between certain glass particles, forming percolating networks at low volume fractions of the ATO. The electrical properties were examined using ac impedance spectroscopy. The impedance, electric modulus, and tan δ were studied which allowed for valuable insights in structure-property-processing relationships in these materials, along with determination of the percolation behavior in these composites. This analysis on samples right before percolation indicated that there was a highly resistive component affecting long range conductivity which is likely due to porosity at the triple points while the dielectric response is affected by the clusters of ATO nanoparticles. Based on this, the percolation of ATO should reduce down to lower concentrations if the processing conditions are improved to reduce this porosity and further segregate the ATO.

Composite rods consisting of Alumina (Al2O3) and Silicon Carbide whiskers (SiCw) are used to fabricate microwave cooking racks because they effectively act as a microwave intensification system that allows cooking at much faster rates than conventional microwave ovens. The percolation behavior, electrical conductivity and dielectric properties of these materials have been reported previously. However, it has been observed that the electrical response of the extruded bars is a function of the rod length and that long rods show substantially different behavior than thinner disks cut from them. A percolation model has been proposed that describes the effect of the alignment of the semiconducting SiC whiskers and the quality of the interfaces present in the composite rods: SiC-SiC and SiC-Al2O3-SiC for example. This study was undertaken with the goal of testing out whether the response of the individual sections could be used to generate the response of the full length rods and to assess the importance of the homogeneous distribution of the SiC fillers on the resultant impedance response.

Spheroidal colloidal indium tin oxide (ITO) nanoparticles, about 6 nm in diameter, were synthesized in-house and films were fabricated from them on glass substrates by spin coating. These films had high electrical resistivity due to the presence of organic capping ligands around each nanoparticle. Although high temperature annealing has been shown to reduce film resistivity by over eight orders of magnitude, lower temperature processing is desirable for applications like flexible electronics. Colloidal ITO films were subjected to a series of alternating RIE treatments in oxygen (5 minutes duration per cycle) and in argon (1 minute duration per cycle); and parameters such as gas pressure, RIE power and number of cycles were varied. These RIE treatments were found to reduce the film resistivity significantly. Among the parameters studied, gas pressure during RIE was found to be the most important parameter that determined the effectiveness of the treatment. Residual carbon content variation characterization done by XPS depth profiles also indicated similar trends.

Polystyrene (PS), polymethyl-methacrylate (PMMA) and multi-walled carbon nanotubes (MWNT) were used to fabricate conductive nanocomposites using various mixing methods, followed by compression molding to analyze their electrical properties. The main objective of this research project was to evaluate how using different mixing techniques alter the composite microstructure and hence the properties of the resultant composite material. Three fabrication techniques were selected to be investigated: mechanical mixing, melt-mixing, and solution mixing. The concentration of the fillers was kept constant at 2 wt% MWNT to simplify comparisons. After mixing, the composite mixtures were compression molded at the same temperature of 180°C. It was found that each mixing method yielded uniquely different AC conductivity profiles which can be attributed to how the fabrication method used affected the arrangement of the CNTs in the composite structure. This newfound control of the electrical properties of the composite materials could definitely be useful to researchers because one can choose the proper fabrication technique based on what properties are desired.

The use of super acids such as chlorosulfonic acid (CSA) has proven to be extremely effective at exfoliating different forms of graphite in high concentrations without covalently functionalizing the surface of the graphene. Once quenched, the acid solutions can then be vacuum filtered through acid resistant polypropylene filter paper with an average pore size of 0.2 μm to collect the exfoliated carbon into a free standing retentate film. These films can then be easily washed, removed, and redispersed into solution by sonicating the films in a surfactant solution. Films were deposited onto various substrates using a range of spin coating parameters. This study has found that exfoliated CNTs provide the best conductivity out of the four types of chemically exfoliated carbon structures studied. CNTs have also proven to be the easiest type of exfoliated carbon to disperse and are able to stay in solution with less than 1%wt surfactant. The findings have shown that the electrical conductivity of the spin coated films actually increases with RPM and is inversely proportional to the film thickness. It is possible to achieve electrical conductivities as high as 10,507 ± 3728.64 [S/m] while still maintaining the transparency of the thin films. The initial spin coating step is more efficient at low ramp rates around 100 rpm/s and results in very smooth films. High spin speeds of 1800 rpm during the casting stage are found to play a large role in improving the conductivity of the films. Lastly, drying the samples on a hot plate for 5 min. on high has significantly improved the films electrical properties and virtually eliminated the need for tedious and expensive plasma cleaning treatments.

Fabrication of transparent, conductive phase-segregated ITO/PC composites was accomplished by compression molding of dry mixed constituents. Mixtures of 50nm indium tin oxide particles in a polycarbonate matrix of compositions ranging from phr = 1.0 to 0.001 were measured by optical and electrical methods. Measured transmittance ranged from 5% to 68% at 400nm, 15% to 80% at 700nm, and averaged 10% to 75% across the visible spectrum. Resistance was measured by impedance spectroscopy. Resistivities of the different composites spanned 10 orders of magnitude. Electrical percolation was found to occur near phr = 0.15.

Indium Tin Oxide (ITO) films were deposited by RF sputtering onto glass and quartz substrates with no external heating. An ITO target containing 10 wt% SnO2 was used for the deposition in a Kurt Lesker PVD75 system, in an atmosphere of 50% O2 + 50% Ar. Post-deposition heat treatments were done on these coatings at 150°C, 300°C and 450°C in an atmosphere of commercial air or argon. The effects of these heat treatments on the microstructure and the properties of the films were evaluated using atomic force microscopy, resistivity measurements, and UV-visible absorption spectroscopy. The heat treatments were observed to significantly affect the properties such as transmittance in the visible region, optical band gap and the electrical resistivity of the films. The main differences are caused by the differences in thermal expansion coefficient of the substrates as compared to the sputtered ITO films.

Waspaloy specimens aged at 800°C from 0.5h to 88.5h were evaluated via small angle neutron scattering (SANS), ultra small angle X-ray scattering (USAXS), electrical resistivity, and SEM. The average γ' precipitate size and volume fraction, obtained from modeling the small angle scattering data, was used to calculate a figure of merit of electron scattering. This figure of merit is designed to correlate the electron scattering ability of the material to the precipitate microstructure. The USAXS data shows a secondary precipitate population at smaller diameters that is absent from the SANS data, since the SANS measurements were not obtained at high enough values of Q. It is believed that this secondary population makes the USAXS-derived figure of merit more sensitive to the actual measured resistivity response than the SANS-derived values; however, the SANS derived primary precipitate sizes are believed to more accurate due to a larger sample volume.

The results of SPM-based localized electrical examination of precipitation hardened Waspaloy microstructures using Electrostatic Force Microscopy (EFM) and Current-Atomic Force Microscopy (I-AFM) are reported herein. The measurements were conducted on two differently etched specimens with the same initial microstructure. Selective etching by preferentially removing the γ′ or the γ phase resulted in a non-uniform surface topography leaving the less reactive phase standing in relief relative to the depressed phase. The presence of a non-uniform surface topography affected the measured EFM response by causing an inhomogeneous surface voltage distribution. A non-linear tip-surface interaction could have further complicated the measured EFM response by making it non-localized. The EFM phase obeyed a supplementary behavior upon reversing the polarity of the DC bias. Using I-AFM, the tip current was found to be the highest at γ-γ′ interphase boundaries, which was attributed to the relaxation of the lattice atoms in the relief zone formed upon etching.

Nickel-base superalloys are an important class of metallic ‘nanocomposite’ structural materials known for their good strength retention abilities at high homologous temperatures for long service times. Literature on electrical resistivity studies of age-hardening superalloys is limited. The current work is focused on developing microstructure-electrical resistivity correlations in controlled Waspaloy microstructures. The microstructures are ‘controlled’ as the size distribution of g¢ precipitates is varied systematically. The microstructures are produced upon aging the initial homogenized alloy at nominal temperatures of 700°C, 800°C and 875°C for times up to 100 hrs. Resistivity measurements did not reveal a g¢ nucleation regime for the sampled aging intervals. The primary microstructural evolution mechanism contributing to the observed changes in resistivity was g¢ coarsening. Interestingly, the microstructures resulting from progressive aging at 700°C showed a slow transformation of etch-pits from perfect polygonal shapes to more irregular shapes.

Poly(acrylonitrile-co-butadiene-co-styrene) (ABS) is a thermoplastic polymer that is used in numerous structural applications as a result of its excellent mechanical properties. For those applications where good electrical conductivity is also desired, carbon black is often used as the filler of choice. Most reports in the literature indicate that at least 8 wt% carbon black filler is needed in order to achieve percolation. Our group recently reported that by manual mixing of ABS pellets and carbon black to create a segregated microstructure, percolation was achieved at an unprecedented low filler fraction of less than 0.01 wt% carbon black, a value which is comparable to or even better than that obtained using single wall carbon nanotubes as the filler. While the ABS/CB composites had excellent electrical performance, with a conductivity as high as 10-1 S/m, their mechanical strength was compromised.

In this paper we report on new experiments designed to maintain high electrical conductivity while improving on the mechanical behavior of percolating ABS/CB nanocomposites. The experiments were aimed at controlling the processing parameters such as temperature, pressure and time during hot pressing of the mechanically mixed precursor materials. Using data obtained at the various temperature-pressure combinations used, it will be shown that similar volume percentages of carbon black and carbon nanotubes can be used to obtain equivalent conductivities, suitable for EMI shielding, while still maintaining good mechanical properties.

Aluminum nitride (AlN), a wide band gap semiconductor (Eg = 6.2eV), has potential applications in microelectronics due to its excellent insulating properties and compatibility with silicon [1,2]. More recently, the use of AlN thin films in high electron mobility transistors, light emitting diodes and UV sources is explored by altering the band gap of the material [3]. The present work describes the combinatorial synthesis of (Al,Ti)N thin films via pulsed laser deposition (PLD) technique to obtain desirable compositional spreads and corresponding variations in the electrical properties. Films of AlN, TiN and (Al,Ti)N were deposited on 6H-SiC (0001) substrates held at a temperature of 680°C. The surface quality of the films examined using an AFM revealed island growth of SiO2 and other growth patterns possibly related to substrate defects.

X-ray diffraction studies indicated that the growth of AlN and TiN films occurred with corresponding habit planes of (0002) and (111) parallel to the substrate surface. Compositional investigations conducted using energy dispersive spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS) showed systematic changes in the Al and Ti composition across the thickness of the compositional spread film. Cross-sectional analysis of (Al,Ti)N films conducted in a high-resolution transmission electron microscope revealed that the films were multi-layered. Several orders of magnitude decrease in the measured resistivity across a 15 mm length (Al,Ti)N film was noted corresponding to a systematic increase in the Ti content. Further optimization of deposition conditions is essential for producing thicker films.