Uttandaraman Sundararaj

Highly electrically conductive and high performance EMI shielding nanowire/polymer nanocomposites by miscible mixing and precipitation

Metal nanowire/polymer nanocomposites are advanced materials for electrically conductive applications. Metal nanowires have high surface area, high aspect ratios, and high electrical conductivity, which are critical for the synthesis of conductive polymer nanocomposites using extremely low amounts of conductive filler. In this work, lightweight, thin, and highly conductive copper nanowire/polystyrene nanocomposites were prepared using a novel method of nanocomposite preparation termed miscible solvent mixing and precipitation (MSMP). Suspensions of high aspect ratio copper nanowires were mixed with polystyrene solutions to produce polymer nanocomposites with segregated nanowire networks resembling cell-like structures. Highly electrically conductive networks of nanowires were obtained beyond a percolation threshold of ϕc = 0.67 vol% and percolated nanocomposites showed electrical conductivities up to 104 S m−1, which exceeds the conductivity range generally reported for carbon nanofiller-based nanocomposites. The significant potential of these nanocomposites for electrical applications like electromagnetic interference (EMI) shielding was further demonstrated. Metal nanowire/polymer nanocomposites sheets of 0.21 mm in thickness exhibited EMI SE of more than 20 dB for copper nanowire concentrations of only 1.3 vol%.

Sheet formation in immiscible polymer blends : model experiments on initial blend morphology

Abstract Model experiments were performed to determine the controlling parameters in the evolution of the phase morphology of immiscible polymer blends from pellets to micrometre-sized particles. It has been established that during melting in both twin-screw extruders and batch mixers, the dispersed phase is stretched into sheets. These sheets develop into cylinders and the cylinders ultimately break into spherical droplets via Rayleigh-type instabilities. Here, we show that micrometre thick sheets can be created from millimetre-sized pellets by shearing in the parallel discs geometry. Extensional flow is not required to generate the sheets. Pellets are seen to break up in three ways: (1) by stretching into cylinders with drops streaming off the end; (2) by extending sheets that form fingers at the edges; and (3) by stretching into thin sheets that break up by forming holes. A map of the different regions of breakup is given using the Deborah number and the ratio of the first normal stress difference of the matrix to the restoring stress of the pellet (drop). The drop-restoring stress is the sum of the surface stress resulting from interfacial tension and the first normal stress difference of the drop. The masterplot explains why sheets can be easily formed from large drops and gives a window where the sheets are stable and do not form holes.

X-band EMI shielding mechanisms and shielding effectiveness of high structure carbon black/polypropylene composites

The electromagnetic interference (EMI) shielding effectiveness (SE) and EMI shielding mechanisms of high structure carbon black (HS-CB)/polypropylene (PP) composites in the X-band frequency range were studied. Composite plates with three different thicknesses and five different electrical conductivities were studied. The reflection loss and absorption loss of the composites were quantified based on the electromagnetic radiation power balance. The results showed that for HS-CB/PP composites, absorption loss contribution to the overall attenuation is more than the contribution of the reflection loss. The ability of the theoretical model to predict the EMI shielding by reflection and absorption was found to be a function of the shielding plate thickness and conductivity.

Boron/Nitrogen Co-Doped Helically Unzipped Multiwalled Carbon Nanotubes as Efficient Electrocatalyst for Oxygen Reduction

Bamboo structured nitrogen doped multiwalled carbon nanotubes have been helically unzipped, and nitrogen doped graphene oxide nanoribbons (CNx-GONRs) with a multifaceted microstructure have been obtained. CNx-GONRs have then been codoped with nitrogen and boron by simultaneous thermal annealing in ammonia and boron oxide atmospheres, respectively. The effects of the codoping time and temperature on the concentration of the dopants and their functional groups have been extensively investigated. X-ray photoelectron spectroscopy results indicate that pyridinic and BC3 are the main nitrogen and boron functional groups, respectively, in the codoped samples. The oxygen reduction reaction (ORR) properties of the samples have been measured in an alkaline electrolyte and compared with the state-of-the-art Pt/C (20%) electrocatalyst. The results show that the nitrogen/boron codoped graphene nanoribbons with helically unzipped structures (CNx/CBx-GNRs) can compete with the Pt/C (20%) electrocatalyst in all of the key ORR properties: onset potential, exchange current density, four electron pathway selectivity, kinetic current density, and stability. The development of such graphene nanoribbon-based electrocatalyst could be a harbinger of precious metal-free carbon-based nanomaterials for ORR applications.

Segregated Hybrid Poly(methyl methacrylate)/Graphene/Magnetite Nanocomposites for Electromagnetic Interference Shielding

Nanocomposites of poly(methyl methacrylate)/reduced graphene oxide (PMMA/rGO) without and with decorated magnetite nanoparticles with a segregated structure were prepared using emulsifier-free emulsion polymerization. Various characterization techniques were employed to validate the presence of the nanofillers and the formation of the segregated structure within the nanocomposites. The percolation threshold of the nanocomposites was found to be 0.3 vol %, while a maximum electrical conductivity of 91.2 S·m-1 and electromagnetic interference shielding effectiveness (EMI SE) of 63.2 dB (2.9 mm thickness) were achieved for the PMMA/rGO nanocomposites at a loading of 2.6 vol % rGO. It was also observed that decorating rGO with magnetite nanoparticles (hybrid nanocomposites) led to a tremendous increase in EMI SE. For instance, 1.1 vol % PMMA/rGO nanocomposites indicated an EMI SE of 20.7 dB, while adding 0.5 vol % magnetite nanoparticles enhanced EMI SE to 29.3 dB. The excellent electrical properties obtained for these nanocomposites were ascribed to both superiorities of the segregated conductive structure and magnetic properties of the magnetite nanoparticles.

Tunneling Conductivity and Piezoresistivity of Composites Containing Randomly Dispersed Conductive Nano-Platelets

In this study, a three-dimensional continuum percolation model was developed based on a Monte Carlo simulation approach to investigate the percolation behavior of an electrically insulating matrix reinforced with conductive nano-platelet fillers. The conductivity behavior of composites rendered conductive by randomly dispersed conductive platelets was modeled by developing a three-dimensional finite element resistor network. Parameters related to the percolation threshold and a power-low describing the conductivity behavior were determined. The piezoresistivity behavior of conductive composites was studied employing a reoriented resistor network emulating a conductive composite subjected to mechanical strain. The effects of the governing parameters, i.e., electron tunneling distance, conductive particle aspect ratio and size effects on conductivity behavior were examined.

Helical and Dendritic Unzipping of Carbon Nanotubes: A Route to Nitrogen-Doped Graphene Nanoribbons

Bamboo structured nitrogen-doped multiwalled carbon nanotubes (CN(x)-MWCNTs) have been successfully unzipped by a chemical oxidation route, resulting in nitrogen-doped graphene nanoribbons (CN(x)-GNRs) with a multifaceted microstructure. The oxidation of CN(x)-MWCNTs was carried out using potassium permanganate in the presence of trifluoroacetic acid or phosphoric acid. On the basis of the high resolution transmission electron microscopy studies, the bamboo compartments were unzipped via helical or dendritic mechanisms, which are different from the longitudinal unzipping of open channel MWCNTs. The product graphene oxide nanoribbons were simultaneously reduced and doped with nitrogen by thermal annealing in an ammonia atmosphere. The effects of the annealing temperature, time, and atmosphere on the doping level and types of the nitrogen functional groups have been investigated. X-ray photoelectron spectroscopy results indicate that a wide range of doping levels can be achieved (4-9 at %) simply by changing the annealing conditions. Pyridinic and pyrrolic nitrogen functional groups were the dominant species that were attached to the edges of the CN(x)-GNRs. The GNRs, with a faceted structure and pyridinic and pyrrolic groups on their edges, have abundant nitrogen sites. These active sites could play a vital role in enhancing the electrocatalytic performance of GNRs.

Effects of Nitrogen Doping on X-band Dielectric Properties of Carbon Nanotube/Polymer Nanocomposites.

Nitrogen-doped and undoped carbon nanotubes (CNTs) were synthesized by selective passing of source and carrier gases (ethane, ammonia, hydrogen, and argon) over an alumina-supported iron catalyst in a quartz tubular reactor at 650 °C. Synthesized CNTs were mixed with polyvinylidene fluoride with an Alberta polymer asymmetric minimixer (APAM) mixer at 240 °C and 235 rpm, and the resulting nanocomposites were compression molded. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and thermogravimetric analysis (TGA) techniques revealed that introducing nitrogen into the crystalline structure of CNTs resulted in higher crystalline defects. Dielectric measurements showed that nitrogen doping significantly increased dielectric permittivity for a known dielectric loss. This was ascribed to the role of the crystalline defects and nitrogen atoms, which acted as polarizing centers, blocked the nomadic charges, polarized them, and prevented them from moving along CNTs. The obtained results introduce nitrogen doping as a regulative tool to control the dielectric properties of CNT/polymer nanocomposites.

Outstanding electromagnetic interference shielding of silver nanowires: comparison with carbon nanotubes

Silver nanowires (AgNWs) were synthesized by AC electrodeposition of Ag into porous aluminum oxide templates. AgNWs were embedded into polystyrene via a solution processing technique to create a nanocomposite. For comparison, carbon nanotube (CNT)/polystyrene nanocomposites were identically generated. TEM and XRD analyses confirmed the synthesis of AgNWs with an average diameter and length of 25 nm and 3.2 μm, respectively. TEM images also revealed that at the molding temperature (240 °C) AgNWs transformed into a chain of nanospheres. At low filler loadings, the AgNW/polystyrene nanocomposites presented inferior electrical properties compared to the CNT/polystyrene nanocomposites. This was attributed to a lower aspect ratio, fragmentation phenomenon and poorer conductive network for AgNWs. However, at high filler loadings, the electrical properties of the AgNW/polystyrene nanocomposites significantly increased. It seems that at high filler loadings, the conductive network was well-established for both types of nanocomposites and thus, the higher innate conductivity of AgNWs played a dominant role in presenting superior electrical properties.

Impact of foaming on the broadband dielectric properties of multi-walled carbon nanotube/polystyrene composites

This study investigated the impact of foaming on the broadband dielectric properties of multi-walled carbon nanotube/polystyrene (MWCNT/PS) nanocomposites. Different carbon nanotube concentrations were prepared by blending of a 20 wt.% MWCNT/PS masterbatch and pure PS using a twin-screw extruder. A chemical blowing agent was used to foam the nanocomposites in a micro injection molding machine. Compression molding was applied to fabricate unfoamed nanocomposites for comparison purposes. Comparing the dielectric properties of unfoamed and foamed nanocomposites showed that foaming increased the percolation threshold, reduced DC and AC conductivities, widened the insulator–conductor transition window, and reduced the dissipation factor of the MWCNT/PS composites. These were attributed to deteriorated conductive network and inferior dispersion and distribution of MWCNTs coming from the presence of foam cells in the nanocomposites. The obtained results propose foaming as a promising technique to improve the dielectric properties of MWCNT/polymer composites.