Masoud Safdari

Hybrid carbon fiber/carbon nanotube composites for structural damping applications

Carbon nanotubes (CNTs) were grown on the surface of carbon fibers utilizing a relatively low temperature synthesis technique; graphitic structures by design (GSD). To probe the effects of the synthesis protocols on the mechanical properties, other samples with surface grown CNTs were prepared using catalytic chemical vapor deposition (CCVD). The woven graphite fabrics were thermally shielded with a thin film of SiO2 and CNTs were grown on top of this film. Raman spectroscopy and electron microscopy revealed the grown species to be multi-walled carbon nanotubes (MWCNTs). The damping performance of the hybrid CNT-carbon fiber-reinforced epoxy composite was examined using dynamic mechanical analysis (DMA). Mechanical testing confirmed that the degradations in the strength and stiffness as a result of the GSD process are far less than those encountered through using the CCVD technique and yet are negligible compared to the reference samples. The DMA results indicated that, despite the minimal degradation in the storage modulus, the loss tangent (damping) for the hybrid composites utilizing GSD-grown MWCNTs improved by 56% compared to the reference samples (based on raw carbon fibers with no surface treatment or surface grown carbon nanotubes) over the frequency range 1-60xa0Hz. These results indicated that the energy dissipation in the GSD-grown MWCNTs composite can be primarily attributed to the frictional sliding at the nanotube/epoxy interface and to a lesser extent to the stiff thermal shielding SiO2 film on the fiber/matrix interface.

Electrical conductivity of synergistically hybridized nanocomposites based on graphite nanoplatelets and carbon nanotubes

In this investigation, a recent model for assessing the electrical conductivity of nanocomposites comprising a single type of conductive nanofiller was expanded to cases with mixtures of nanofillers. The extended model considers electron tunneling as the effective mechanism for insulator-conductor transition. The model was validated with relevant experimental data based on a mono-nanofiller. Using the extended model, the effective electrical conductivity of a nanocomposite comprising both graphite nanoplatelets and carbon nanotubes was investigated. It was observed that the hybridized nanocomposites filled with a mixture of these conductive nanofillers attain, synergistically, enhanced electrical conductivities at lower volume fractions. The lower filler contents assist in preserving the intrinsic properties of the host polymer in support of several applications. It was also observed that the relative aspect ratios of the conductive fillers play significant roles on the electrical conductivity of the hybrid nanocomposite. Simulations revealed that, generally, the addition of minimal amounts of a higher aspect ratio auxiliary phase to a lower aspect ratio main phase enhances the electrical conductivity of the composite by orders of magnitude.

Effects of Carbon Nanotubes Geometrical Distribution on Electrical Percolation of Nanocomposites: A Comprehensive Approach

The main objective of this study was to study the effects of length, alignment and diameter distribution of the carbon nanotubes (CNTs) on the percolation threshold of nanocomposites using computational simulations. Furthermore, the effects of the aforementioned parameters on the efficiency of the produced networks are investigated. The best distribution for optimum connectivity and the lowest CNTs concentration for the onset of percolation is determined via analyzing the geometrical characteristics of carbon nanotubes. The critical volume fraction of CNTs for percolation was found to be 0.1% while the mean number of bonds per object was 1.3 at the best distribution condition. The results from this study are compared to available experimental data and good agreement was found.

A modified strong-contrast expansion for estimating the effective thermal conductivity of multiphase heterogeneous materials

To evaluate the effective thermal conductivity of a general anisotropic multiphase microstructure, a modified version of statistical strong-contrast expansions is formulated here. The proposed method takes into account the shape, orientation, and distribution of each phase through two-point and threepoint correlation functions. By applying a recently developed method, three-point correlation functions are approximated from the two-point correlation functions. Numerically, it is shown that for high contrast constituents, the solution of the third-order strong-contrast expansions is very sensitive to the selection of the reference medium. A technique is proposed to minimize the sensitivity of the solution. To establish the validity of the methods developed, the effective thermal conductivity of a number of isotropic and anisotropic two-phase and three-phase microstructures is evaluated and compared to their corresponding finite element (FE) simulations. Good agreement between the FE simulations and the proposed method predictions in the cases studied confirms its validity. When there are orders of magnitude disparity between the properties of the constituents, the developed method can be applied to better estimate the effective thermal conductivity of the multiphase heterogeneous materials in comparison with previous strong contrast model and other homogeneous methods. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4768467]

Incorporation of electron tunnelling phenomenon into 3D Monte Carlo simulation of electrical percolation in graphite nanoplatelet composites

The percolation threshold problem in insulating polymers filled with exfoliated conductive graphite nanoplatelets (GNPs) is re-examined in this 3D Monte Carlo simulation study. GNPs are modelled as solid discs wrapped by electrically conductive layers of certain thickness which represent half of the electron tunnelling distance. Two scenarios of impenetrable and penetrable GNPs are implemented in the simulations. The percolation thresholds for both scenarios are plotted versus the electron tunnelling distance for various GNP thicknesses. The assumption of successful dispersion and exfoliation, and the incorporation of the electron tunnelling phenomenon in the impenetrable simulations suggest that the simulated percolation thresholds are lower bounds for any experimental study. Finally, the simulation results are discussed and compared with other experimental studies.

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

Effect of Carbon Nanotube Growth Conditions on Strength and Stiffness of Carbon and Glass Fiber Polymer Composites

Carbon nanotubes (CNTs) and Carbon nanofilaments (CNFs) were grown on the surface of carbon and glass sheets from fuel rich ethylene/oxygen combustion mixtures on certain catalytic metals. Employing a well-known catalyst deposition method, incipient wetness allowed growing CNTs/CNFs on the surface of carbon and fiber glass bidirectional sheets. Two-layer fiber reinforced polymer (FRP) composite plates were fabricated using a vacuum assisted hand lay-up technique of the carbon and glass fiber sheets after CNTs/CNFs were grown using 1.0% Nickel deposits. In this paper we report on the growth process and we examine the significance of the CNTs/CNFs growth conditions including the sizing burning temperature (250 and 500 oC), the growth time period and the fiber base type (carbon or glass) on the strength and stiffness of this new multi-scale FRP composite. The ultimate tensile strength, tensile modulus (stiffness) and ultimate strain at failure was determined using ASTM D3039 test. It is shown that the sizing burning temperature has a significant effect on the strength and strain at failure of the new carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composites. Little effect on the composite stiffness was observed. Microstructural investigations of the failed specimens shed light on the fracture surface of the multi-scale CFRP and GFRP composites.

Modeling of Biologically Inspired Adhesive Pads Using Monte Carlo Analysis

Recently, the analysis and prototyping of biologically inspired adhesive pads have been the subject of growing interest. Similar to biological counterparts, these synthetic adhesives consist of rafts of tiny protruding fibers. The adhesion performance of these micro-engineered products is highly dependent on the geometrical and mechanical properties of the micro-fibers and the surface they adhere to. Small fluctuations in these parameters can drastically change their adhesion performance. In this investigation, a comprehensive mathematical model of a single micro-fiber with adhesion capability in contact with an uneven surface has been developed and the behavior of the model studied. To provide more realistic results, this analytical model could be extended to an array of micro-fibers. Thus, in a further step, using a Monte Carlo simulation, we studied an array of these micro-fibers under more realistic conditions with several degrees of uncertainty. The results deduced by this novel modeling approach are in good agreement with the experimental measurements of adhesion performance in synthetic adhesive pads available in literature.

Fuzzy Sliding Mode Control of a Ball Screw Driven Stage

Ball screw driven systems are widely used for motion control applications. In such systems, friction dominates the resulting performance and it should be compensated to achieve high precision in positioning and tracking. Hereby a friction model is introduced to describe dynamic behavior of the system and then three control strategies i.e. sliding mode control (SMC), complementary sliding mode control (CSMC) and fuzzy sliding mode control (FSMC) are applied. Performances of the controllers are compared in the sense of positioning, tracking error and control input. It is observed that the proposed FSMC can eliminate the steady state error and chattering of the control input.

Theoretical and Experimental Study of a Novel Microfabricated Fibrillar Structure

Survival of species such as geckos, spiders, flies, and crickets crucially depends on the interaction between hundreds of thousands of hairs or setae on their feet. Recently, many efforts have been made to fabricate adhesive pads inspired by natural biological systems. Fibres developed from nano molding can mimic the hairs of these species’ feet and act as a dry adhesive pad. In this study, for the nanocasting of nano fibres, several porous silicon structures with desired dimensional and morphological characteristics were made by an electrochemical etching system. The adhesive strengths of produced adhesive pads were measured about 0.07 N/cm2 in the normal direction and about 0.045 N/cm2 in the shear direction in contact with a glass surface. Besides the experimental work, a quantitative model has been developed to model van der Waals interactions in adhesive pads. The results from the theoretical model show consistency with experiments.