Mehran Tehrani

Patterning the Stiffness of Elastomeric Nanocomposites by Magnetophoretic Control of Cross-linking Impeder Distribution

We report a novel method to pattern the stiffness of an elastomeric nanocomposite by selectively impeding the cross-linking reactions at desired locations while curing. This is accomplished by using a magnetic field to enforce a desired concentration distribution of colloidal magnetite nanoparticles (MNPs) in the liquid precursor of polydimethysiloxane (PDMS) elastomer. MNPs impede the cross-linking of PDMS; when they are dispersed in liquid PDMS, the cured elastomer exhibits lower stiffness in portions containing a higher nanoparticle concentration. Consequently, a desired stiffness pattern is produced by selecting the required magnetic field distribution a priori. Up to 200% variation in the reduced modulus is observed over a 2 mm length, and gradients of up to 12.6 MPa·mm−1 are obtained. This is a significant improvement over conventional nanocomposite systems where only small unidirectional variations can be achieved by varying nanoparticle concentration. The method has promising prospects in additive manufacturing; it can be integrated with existing systems thereby adding the capability to produce microscale heterogeneities in mechanical properties.

Hybrid Composites Based on Carbon Fiber/Carbon Nanofilament Reinforcement

Carbon nanofilament and nanotubes (CNTs) have shown promise for enhancing the mechanical properties of fiber-reinforced composites (FRPs) and imparting multi-functionalities to them. While direct mixing of carbon nanofilaments with the polymer matrix in FRPs has several drawbacks, a high volume of uniform nanofilaments can be directly grown on fiber surfaces prior to composite fabrication. This study demonstrates the ability to create carbon nanofilaments on the surface of carbon fibers employing a synthesis method, graphitic structures by design (GSD), in which carbon structures are grown from fuel mixtures using nickel particles as the catalyst. The synthesis technique is proven feasible to grow nanofilament structures—from ethylene mixtures at 550 °C—on commercial polyacrylonitrile (PAN)-based carbon fibers. Raman spectroscopy and electron microscopy were employed to characterize the surface-grown carbon species. For comparison purposes, a catalytic chemical vapor deposition (CCVD) technique was also utilized to grow multiwall CNTs (MWCNTs) on carbon fiber yarns. The mechanical characterization showed that composites using the GSD-grown carbon nanofilaments outperform those using the CCVD-grown CNTs in terms of stiffness and tensile strength. The results suggest that further optimization of the GSD growth time, patterning and thermal shield coating of the carbon fibers is required to fully materialize the potential benefits of the GSD technique.

3D reconstruction of carbon nanotube networks from neutron scattering experiments.

UNLABELLED Structure reconstruction from statistical descriptors, such as scattering data obtained using x-rays or neutrons, is essential in understanding various properties of nanocomposites. Scattering based reconstruction can provide a realistic model, over various length scales, that can be used for numerical simulations. In this study, 3D reconstruction of a highly loaded carbon nanotube (CNT)-conducting polymer system based on small and ultra-small angle neutron scattering (SANS and USANS, respectively) data was performed. These light-weight and flexible materials have recently shown great promise for high-performance thermoelectric energy conversion, and their further improvement requires a thorough understanding of their structure-property relationships. The first step in achieving such understanding is to generate models that contain the hierarchy of CNT networks over nano and micron scales. The studied system is a single walled carbon nanotube (SWCNT)/poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) ( PEDOT PSS). SANS and USANS patterns of the different samples containing 10, 30, and 50 wt% SWCNTs were measured. These curves were then utilized to calculate statistical two-point correlation functions of the nanostructure. These functions along with the geometrical information extracted from SANS data and scanning electron microscopy images were used to reconstruct a representative volume element (RVE) nanostructure. Generated RVEs can be used for simulations of various mechanical and physical properties. This work, therefore, introduces a framework for the reconstruction of 3D RVEs of high volume faction nanocomposites containing high aspect ratio fillers from scattering experiments.

Electromagnetic Shielding Effectiveness of a Hybrid Carbon Nanotube/Glass Fiber Reinforced Polymer Composite

A relatively low-temperature carbon nanotube (CNT) synthesis technique, graphitic structure by design (GSD), was utilized to grow CNTs over glass fibers. Composite laminates based on the hybrid CNTs–glass fibers were fabricated and examined for their electromagnetic interfering (EMI) shielding effectiveness (SE), in-plane and out-of-plane electrical conductivities and mechanical properties. Despite degrading the strength and strain-to-failure, improvements in the elastic modulus, electrical conductivities, and EMI SE of the glass fiber reinforced polymer (GFRP) composites were observed.

USING MULTISCALE CARBON FIBER / CARBON NANOTUBES COMPOSITES FOR DAMPING APPLICATIONS

A novel technique to grow carbon nanotubes (CNTs) on the surface of carbon fibers in a controlled fashion using simple lab set up is developed. Growing CNTs on the surface of carbon fibers will eliminate the problem of dispersion of CNTs in polymeric matrices. The employed synthesis technique retains the attractive feature of uniform distribution of the grown CNTs, low temperature of CNTs’ formation, i.e. 550 °C, via cheap and safe synthesis setup and catalysts. A protective thermal shield of thin ceramic layer and subsequently nickel catalytic particles are deposited on the surface of the carbon fiber yarns using magnetron sputtering. A simple tube furnace setup utilizing nitrogen, hydrogen and ethylene (C2 H4 ) were used to grow CNTs on the carbon fiber yarns. Scanning electron microscopy revealed a uniform areal growth over the carbon fibers where the catalytic particles had been sputtered. The structure of the grown multiwall carbon nanotubes was characterized with the aid of transmission electron microscopy (TEM). Dynamical mechanical analysis (DMA) was employed to measure the loss and storage moduli of the hybrid composite together with the reference raw carbon fiber composite and the composite for which only ceramic and nickel substrates had been deposited on. The DMA tests were conducted over a frequency range of 1–40 Hz. Although the storage modulus remained almost unchanged over the frequency range for all samples, the loss modulus showed a frequency dependent behavior. The hybrid composite obtained the highest loss modulus among other samples with an average increase of approximately 25% and 55% compared to composites of the raw and ceramic/nickel coated carbon fibers, respectively. This improvement occurred while the average storage modulus of the hybrid composite declined by almost 9% and 15% compared to the composites of reference and ceramic/nickel coated samples, respectively. The ultimate strength and elastic moduli of the samples were measured using standard ASTM tensile test. Results of this study show that while the addition of the ceramic layer protects the fibers from mechanical degradation it abolishes the mechanisms by which the composite dissipates energy. On the other hand, with almost no compromise in weight, the hybrid composites are good potential candidate for damping applications. Furthermore, the addition of CNTs could contribute to improving other mechanical, electrical and thermal properties of the hybrid composite.Copyright

Magnetically enhanced mechanical and creep properties of a structural epoxy

The current investigation presents the benefit influence of the magnetic processing of a structural epoxy system via moderate permanent magnetic field. During the curing process of a liquid-crystalline epoxy resin, a magnetic field of 0.5 Tesla was applied, and the mechanical properties of the cured resin were investigated using nanoindentation and nanocreep tests. Compared to samples processed without magnetic field under the same thermal conditions, the magnetically processed samples show an improvement in their modulus, hardness and creep resistance. The two-dimensional stretching of both main chains and the crosslinks of the amorphous epoxy - due to the applied magnetic field - is shown to improve the properties along both the parallel and lateral directions of the applied field.

Measurement of the thermoelectric power factor of films over the 10-400 K range

The design and development of a novel apparatus for the simultaneous measurement of electrical resistivity and Seebeck coefficient of films is reported here. Mounting stage is integrated inside a cryostat chamber enabling measurements over the 10-400 K temperature range, intended for organic thermoelectrics. Finite element method was used to analyze the thermo-mechanical response of the sample holder. The apparatus was validated against high purity nickel film, and a very good agreement was found.

IMPACT AND QUASI-STATIC MECHANICAL PROPERTIES OF A CARBON FIBER REINFORCED CARBON NANOTUBE/EPOXY

Carbon fiber reinforced plastics (CFRPs) possess superior inplane mechanical properties and are widely used in structural applications. Altering the interphase of CFRPs could alleviate the shortcomings of their out-of-plane performance. In this work, the effects of adding multi-walled carbon nanotubes (MWCNTs) to the epoxy matrix of a CFRP are investigated. Two sets of CFRPs with matrices comprising MWCNTs/epoxy and neat epoxy, respectively, were fabricated. The tensile properties of the two systems, namely the stiffness, the ultimate strength, and the strain to failure were evaluated. The results of the tension tests showed slight changes on the on-axis (along the fiber) tensile modulus and strength of the carbon fiber reinforced epoxy/MWCNT compared to composites with no MWCNTs. The addition of MWCNTs to the matrix moderately increased the strain to failure of the composite. Energy absorption capabilities for the two sets of composites under an intermediate impact velocity (100 m.s -1 ) test were measured. The energy dissipation capacity of the CFRPs incorporating MWCNTs was higher by 17% compared to the reference CFRPs.

Mechanical Characterization of a Hybrid Carbon Nanotube/Carbon Fiber Reinforced Composite

Carbon fiber reinforced polymer composites (CFRPs) are renowned for their superior in-plane mechanical properties. However, they lack sufficient out-of-plane performance. Integrating carbon nanotubes (CNTs) into structures of CFRPs can enhance their poor out-of-plane properties. The present work investigates the effect of adding CNTs, grown on carbon fibers via a relatively low temperature growth technique, on the on and off-axis tensile properties as well as on transverse high velocity impact (∼100 m.s−1) energy absorption of the corresponding CFRPs. Two sets of composite samples based on carbon fabrics with surface grown CNTs and reference fabrics were fabricated and mechanically characterized via tension and impact tests. The on-axis and off-axis tests confirmed improvements in the strength and stiffness of the hybrid samples over the reference ones. A gas gun equipped with a high-speed camera was utilized to evaluate the impact energy absorption of the composite systems subjected to transverse spherical projectiles. Due to the integration of CNTs, intermediate improvements in the tensile properties of the CFRP were achieved. However, the CFRPs’ impact energy absorption was improved significantly.Copyright

Synthesis and characterisation of nano alumina dental filler

The production of alumina nanoparticles using a low power plasma torch and creating a nanocomposite for use in dental applications were investigated. Upon fabricating the nanocomposite based on alumina nanoparticles, the mechanical properties were studied using instrumented nanoindentation and compared to three standard dental fillers. Several nanocharacterisation techniques were employed in the current study for these dental fillers and for natural dental materials; enamel and dentine. The mechanical tests carried out in this investigation include nanoindentation and nanoscratch. The newly developed nanocomposite outperformed the commercial nanocomposite in hardness and elasticity. However, the results of the mechanical tests suggest that the silver amalgam had the best mechanical properties among the four dental fillers investigated.