RESEARCH INTERESTS

Experimental

Blast tolerant materials, radiation damage mitigation, thermal barrier coatings, high energy radiation detection, drug delivery using nanotubes, dental fillers based on nanocomposites, synthesis and characterization of lanthanides nanoparticles for gamma ray scintillators, synthesis of nanoparticles and WS2 nanofibers via plasma torch, synthesis of graphitic structures by design, processing of nanotube/polymer composites, magnetic annealing of engineering materials, residual stress development in super alloys, durability of polymeric composites, nanoindentation of thin films, nano impact and nano fatigue testing of thin films, thermomechanical analysis of polymers, light and electron microscopy and mechanical testing.

Computational

Molecular dynamics simulations of nanoscale phenomena, statistical continuum mechanics of heterogeneous media, impact simulation, constitutive modeling of viscoplasticity, numerical optimization, neural networks applications in materials science, parameters identification of dynamic systems, and Kalman filtering.

Specific Research Activities
Magnetically Enhanced Mechanical and Creep Properties of a Structural Epoxy

This 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.

Nanocreep test of epoxy samples. A maximum load of 1 mN was held constant for 30 min during the test.

Novel Growth of Multiscale Carbon Nanofilaments on Carbon and Glass Fibers

Carbon nanofilaments (CNFs) were grown on the surface of microscale carbon-fibers and glass fibers at low temperature using palladium as a catalyst to create multiscale fiber reinforcing structures with potential applications in structural composites. Employing a relatively new method, in which carbon structures are grown from fuel rich combustion mixtures on certain catalytic metals, multiscale filament structures were grown from ethylene/oxygen mixtures at 550 °C on commercial pitch carbon fibers and fiberglass. The filaments grew in different size distributions and distinct morphologies (whiskers and spirals) depending on the base fiber.

Submicron fibers (ca. 200 nm) and spirals (ca. 50 nm) were dominating the grown species over the fiberglass substrate.  Relatively short, densely spaced nanofilaments (ca. 10 nm), and a slightly less dense layer of larger (ca. 300 nm diameter) faster growing fibers were found to exist together to create a unique multiscale carbon structures over the pitch carbon fiber substrate. Transmission electron microscopy indicated poor crystallinity for the nanoscale carbon filaments   grown on both pitch carbon fibers and fiberglass.

SEM micrographs of (a-c) carbon filaments growth on fiberglass after 120 minutes deposition, and (d-e) carbon filaments growth on pitch based carbon fibers 120 minutes. In both cases the growth of filament was at 550 °C with fuel rich environment.

Tribological reliability of MEMS multilayered thin films

Tribological behaviors of polysilicon and layered metals/polysilicon microstructures are needed to design reliable microelectromechanical systems (MEMS). Nanomechanical characterization of bulk materials of polysilicon and thin films of polysilicon/chromium/gold was carried out in this study. Scratch resistance of these materials were measured using a Nano Test system. The deformation mechanisms of these materials during nanoscratching were found to be strongly dependent on the loading conditions. In the case of constant load, the scratch depth is proportional to the applied load.  Nanoscratch tests with linearly ramping loads suggests that two deformation regimes can be defined; an elasto-plastic regime at low loads, and a fully plastic regime at high loads.  Abrupt change of the scratch depth for (Au/Cr/P- Si) tri film revealed the existence of unwanted porosities in the deposited metal films.

AFM images of a nanoscratch on polysilicon surface using a constant normal scratch load of 10 mN. Z-axis magnified 25 times.

Multi-Objective Optimization Approach for Design of Blast Resistant Composite Laminates Using Carbon Nanotubes

A reliable process for the design of blast resistance composite laminates is needed. We consider here the use of Carbon nanotubes (CNT) to enhance the mechanical properties of composite interface layers. The use of CNT not only enhances the strength of the interface but significantly alters stress propagation in composite laminates. A simplified wave propagation simulation method is developed and the optimal CNT content in the interface layer is determined using means of multi-objective optimization. The optimization process targets reducing the stress to strength ratio in all the composite laminate layers. Two optimization methods are implemented to identify the optimal CNT content. A case study demonstrating the design of 5 layer composite laminate subjected to a blast event is used to demonstrate the concept. It is shown that the addition of 2% and 4% CNT by weight to the epoxy interface results in significant enhancement of the composite ability to resist blast

Synthesis and Characterization 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 was 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 nanocharacterization 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, nanoscratch and nanoimpulse. The newly developed nanocomposite outperformed both the glass ionomer and commercial nanoepoxy. However, the comprehensive mechanical tests suggest that the silver amalgam had the best mechanical properties among the four dental fillers investigated

TEM images of alumina particles                                            Alumina composite load-displacement curves  at load of  10 mN.

Investigating of the Nanomechanical and Tribological Properties of Dental Materials

This study utilizes nanoscale characterization techniques nanoindentation and nanoscratch for testing both the human enamel and dentine together with three biocompatible dental filling materials; epoxy nanocomposite, glass ionomer, and silver amalgam.

Nanoindentation tests were performed to obtain accurate hardness and reduced modulus values for the enamel, dentin and three different fillers. We utilized nano-scratch tests to obtain critical load in scratch test and resistance to sliding wear.

Testing showed the silver amalgam filling has a higher modulus of elasticity, hardness and wear resistance compared to the nanocomposite.  The relatively nondistructive mechanical characterization techniques utilized might assist in better understanding the mechanical behavior of the dental fillers and thus facilitate the design of robust fillers with excellent mechanical properties.

 AFM scans for series of 2, 6 and 10 mN indentations on the surface of the different dental materials.

Role of Polyethylene Glycol in Specific Receptor Targeting of Carbon Nanotubes to Cancer cells

Selective targeting of cancer cells is desirable to avoid exposure of noncancerous cells to cytotoxic agents, and is achieved by targeting distinct receptors on cancer cells’ surface. This study demonstrated that dispersion of single wall carbon nanotubes (SWNTs) by ultrasonication with phospholipid-polyethylene glycol (PL-PEG) fragments it, thus interfering with its ability to block non-specific uptake by cells. However, unfragmented PL-PEG promoted specific cellular uptake of targeted SWNTs to specific receptors expressed by cancer cells.

(C-F) Confocal microscopy images of SKOV-3 cells that express FRα deprived of folic acid for 24 hrs followed by incubation for 24 hrs with (C) no treatment of cells, (D) SWNT-Fluorescein without PEG, (E) SWNT-PL-PEG2000-Fluor, and (F) SWNT-PL-PEG2000-folate. Blue represents DAPI staining for nucleus. Green is fluorescein on SWNTs. Red is a membrane stain.

Polymer-Mediated Alignment Of Carbon Nanotubes Under High Magnetic Fields

We present studies of the alignment of single wall carbon nanotubes in a bulk composite, where an epoxy polymeric matrix is processed inside a high magnetic field to mediate the reorientation process of the carbon nanotubes. The alignment process was characterized by environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM) and wide-angle x-ray diffraction (WAXD).

Based on the current investigation, it is evident that magnetic alignment is a phenomenon closely related to the self-organizing process of the polymeric system. The microstructures of the polymer samples processed inside 15 and 25 Tesla fields were shown to have local orientation along the field’s directions. The WAXD show a high degree of alignment when the azimuthal scan is parallel to the field direction. The alignment was more pronounced as a function of magnetic field strength. The field-assisted reorientation of the epoxy samples induced the alignment of the carbon nanotubes at the nanoscale level, while the carbon nanotubes ropes did not align globally due to their length.

ESEM images for the morphology of the fracture surface of SWCNN-Epoxy composite. The fracture surface processed at 0 Tesla magnetic field was captured at two magnifications: (a) 50mm and (b) 10mm. The fracture surface processed at 25 Tesla magnetic field. was also captured at two magnifications: (c) 50mm and (d) 10mm. The arrow is the direction of the magnetic field. The darkest spots are clusters of the carbon nanotubes intercalated along the polymer fibrils

AFM microscopy for the SWCNT-epoxy composite cured inside a 25 Tesla magnetic field. The curing time temperature cycle was 2 hours at 25 ^oC, followed by 2 hours at 60^o C. The image on the right (b) represents the enhanced detail of the encircled area in the image on the left (a).

Enhancement of Thermal and Electrical Properties of Carbon Nano-Tube Polymer Composites by Magnetic Field Processing

We show that the thermal and electrical properties of single wall carbon nanotube (CNT)-polymer composites are significantly enhanced by magnetic alignment during processing. Raman spectroscopy is used to detect changes in nanotube and epoxy molecular vibrations as a result of composite interactions and magnetic field processing. The electrical transport properties of the composites are mainly governed by the hopping conduction with localization lengths comparable to bundle diameters. The bundling of nanotubes during the composite processing seems to be an important factor for electrical and thermal transport properties. These factors are important for further enhancement of  the performance of CNT-composites towards the theoretical limit of an isolated CNT system.   

Temperature dependence of resistivity of magnetically processed CNT-Epoxy composites. The current was applied along the thickest direction of a slab or disk shaped sample

Thermal conductivity of CNT-Epoxy composites and control epoxy.

Dynamics Simulation of Magnetic Field Induced Orientation of Nanotube-Polymer Composite

Molecular dynamics simulations are carried out to study the reorientation of single wall carbon nanotubes in a polyethylene matrix under the influence of a 25 Tesla magnetic field. The simulations are based on a variant of velocity Verlet algorithm, which relaxes the Larmor time-step restriction while preserving second-order accuracy. Simulations reveal that the unfolding and reorganization of the polyethylene chain facilitates the reorientation of the single wall carbon nanotubes closer to the direction of the applied magnetic field. Also, they bring out the difference between the behavior of the carbon nanotubes of zigzag chirality  and that of armchair chirality.

Trajectory of the armchair CNT-polyethylene (under 25 Tesla magnetic field along the z-axis) at different instants between 0 and 10 ps.

Potential energy evolution for the armchair CNT-polyethylene and zigzag CNT- polyethylene composites during 10 ps.

Mechanical and Microscopy Analysis of Fiber Carbon-Polymer Matrix Composites