Mrinal C. Saha

Multi-walled carbon nanotubes coated by multi-layer silica for improving thermal conductivity of polymer composites

Silica has been non-covalently coated on multi-walled carbon nanotubes (MWCNTs) using the sol–gel chemistry, where tetraethoxy silane (TEOS) was used to form an inorganic silica layer immediately next to surface of MWCNTs and octyl triethoxy silane was coated over the TEOS. Transmission electron microscopy (TEM) measurements show that the diameter of MWCNTs increases with increasing the number of coating layer, indicating that the silica has been coated on MWCNTs. Quantitative analysis from thermogravimetric analysis (TG) also indicates that the inorganic and organic silica has been successfully coated on MWCNTs. Further, quantitative analysis found that the amount of silica measured by TG agrees well with the increase of thickness of coated MWCNTs obtained from TEM, indicating that little or no free silica exists in the system. The thermal conductivity of epoxy/MWCNTs composite was studied and the results show that the thermal conductivity of the composite is improved by coating MWCNTs in this manner and increases with increasing the number of coatings.

Effect of carbon nanotube persistence length on heat transfer in nanocomposites: A simulation approach

Monte Carlo simulations were employed to investigate the effective thermal conductivity (Keff) of multi-walled carbon nanotube-epoxy (MWNT-epoxy) nanocomposites with and without coating the MWNTs with silica. The numerical approach was validated with experimental data and values of the Kapitza resistance for the silica-coated MWNT-epoxy composite were calculated for realistic configurations of the MWNTs. While the Kapitza resistance was found to be 40% smaller than for the case of pristine MWNTs, it was also observed that the effect of persistence length of the MWNT on Keff is as important as the effect of the Kapitza resistance.

Ultrasound Assisted Hybrid Carbon Epoxy Composites Containing Carbon Nanotubes

A simple approach has been reported toward the development of hybrid nano/microfiber composite structures with improved mechanical properties. Ultrasound assisted atomization process has been utilized for depositing carbon nanotubes (CNTs) on the surface of carbon fiber (CF) cloth using dilute solutions of CNTs in N, N-dimethylformamide (DMF). Dilute solutions with three different CNT concentrations such as 1 × 10−4 g/ml, 5 × 10−4 g/ml, and 10 × 10−4 g/ml were fed into an ultrasonic atomizer probe using a positive displacement syringe pump and sprayed directly on CF cloth rested on a hot plate inside a deposition chamber. Several layers of hybrid CF cloths containing CNTs were used to fabricate composite laminates using a vacuum assisted resin transfer molding (VARTM). Although the dispersion of CNTs in DMF was found very well for all three concentrations, the distribution of CNTs on CFs was only found homogeneous for 1 × 10−4 g/ml solution. It was found that the hybrid composite containing 0.3 wt. % CNTs loading fabricated using 1 × 10−4 g/ml solution showed about 25% improvement in flexural strength, although moderate improvement in flexure modulus was achieved for all three concentrations. The improved strength is believed to be due to homogeneous distribution of CNTs, which resulted in increased surface roughness and mechanical interlocking between fibers and matrix.

Poly dimethylsiloxane/carbon nanofiber nanocomposites: fabrication and characterization of electrical and thermal properties

ABSTRACT This article presents the fabrication and characterization of poly dimethylsiloxane/carbon nanofiber (CNF)-based nanocomposites. Although silica and carbon nanoparticles have been traditionally used to reinforce mechanical properties in PDMS matrix nanocomposites, this article focuses on understanding their impacts on electrical and thermal properties. By adjusting both the silica and CNF concentrations, 12 different nanocomposite formulations were studied, and the thermal and electrical properties of these materials were experimentally characterized. The developed nanocomposites were prepared using a solvent-assisted method providing uniform dispersion of the CNFs in the polymer matrix. Scanning electron microscopy was employed to determine the dispersion of the CNFs at different length scales. The thermal properties, such as thermal stability and thermal diffusivity, of the developed nanocomposites were studied using thermogravimetirc and laser flash techniques. Furthermore, the electrical volume conductivity of each type of nanocomposite was tested using the four-probe method to eliminate the effects of contact electrical resistance during measurement. Experimental results showed that both CNFs and silica were able to impact on the overall properties of the synthesized PDMS/CNF nanocomposites. The developed nanocomposites have the potential to be applied to the development of new load sensors in the future.

Effect of Distribution Media Length and Multiwalled Carbon Nanotubes on the Formation of Voids in VARTM Composites

The first part of this paper characterizes the effect of tooling and process parameters such as the length of distribution media used in vacuum assisted resin transfer molding (VARTM) of composite laminates. To achieve this goal, a number of 6-ply, woven carbon fiber=epoxy laminates are fabricated by using various lengths of distribution media. The spatial variations of mechanical properties of these laminates are characterized using a three-point bending fixture. It is shown that for relatively thinner laminates, extending the distribution media degrades the flexural properties by as much as 14%, possibly due to air pockets entrapped during through-the-thickness impregnation of the fibrous fabric. In the second part, a minimum distribution media length is used to investigate the mechanical property and microstructure changes due to multiwalled carbon nanotubes (MWNTs) dispersed in the composite laminates. In addition, effects of different nanotube functionalization and morphology are characterized via scanning electron microscopy and optical microscopy. To achieve adequate nanotube dispersion in the epoxy resin, both tip sonication and mechanical mixing have been used. The effect of sonication time on the dispersion of nanotubes is reported by monitoring the temporal changes in the nanotube cluster size. Even at volume fractions less than 1%, almost 10% improvements in flexural properties is observed. Extensive void formations are reported for laminates containing MWNTs, possibly preventing greater improvements in mechanical properties. [DOI: 10.1115/1.4004700]

Fostering Innovation Through Experiential Learning

Globalization has put engineering education and the profession at a challenging crossroad. The impact of rapid technological innovations on modern societies has been amplified by the globalization of the economy. The competitiveness of the U.S., which is linked to our standard of living, is dependent on our ability to produce a large number of sufficiently innovative engineers prepared to address issues related to complex systems. Hence, our focus is on the research and development of instructional activities that address the engineering competencies related to innovation. Engineering educators and practitioners have suggested that collaborative-competitive team design events promote innovation. These competitions are popular, and they attract sponsors and participants. Beyond being popular, they are believed to provide rich learning opportunities for students. The University of Oklahoma’s Formula SAE (Society of Automotive Engineers) Racing team is highly ranked in the U.S. and world. We are in the early stages of designing, implementing, and testing a four course curriculum, around the FSAE race car, that fosters meaningful learning, innovation, systems level thinking, and the attainment of career-sustaining skills as a result of authentic experiences. We plan to identify the activity features that match with the theoretical frameworks of innovation, match them to the professional competencies, translate the events from extracurricular to curricular activities, and assess their effects on student learning and development in four technical courses our curriculum. With a view to stimulating discussion, in this paper, we highlight some of the salient features of our plan and some issues that warrant further investigation.Copyright

Effect of Sizing Removal Method and POSS Coating on Flexural Properties of Carbon Fiber Epoxy Composites

Effects of sizing and surface modification on flexural properties of carbon fiber reinforced epoxy composites have been investigated. Carbon fiber was desized using three types of treatments, namely heat, acetone, and acetone-acid. In addition, these fibers were coated with three different types of Polyhedral Oligomeric Silsesquioxane (POSS) molecule. Composite panels were fabricated using the vacuum assisted resin transfer molding and samples were tested in flexure. Scanning electron microscopy analysis was performed to investigate the surface morphology and failure mechanisms. It was found that removal of sizing significantly reduced the flexural strength. About 19% and 29% reduction of flexural strength was reported for acetone treatment and heat treatment, respectively. Composites with POSS coated fibers showed improved properties, except for the heat treated fibers. Among POSS molecules investigated, the S10455 was found to be the best for improving the flexural properties of carbon fiber composites.Copyright

PDMS/CNF Nanocomposite Sensor Array for Pressure Sensing

Pressure sensing is a field that is rapidly advancing as driven by vast applications including biomedical devices, industrial automation, human-machine interfacing, and skin-like electronics. Recently significant amount of effort has been reported to develop new piezoresistive strain sensor made from polymers with carbon nanofillers. The widely used nanofillers with high electrical conductivity include carbon nanotubes, carbon nanofibers, and other carbon nanoparticles. However, most previous research focused on the improvement of material properties for sensing applications. Limited work balanced the sensor design and material innovation for real time pressure sensing. This paper presented a piezoresistance based pressure sensor array using flexible and stretchable polydimethylsiloxane (PDMS) polymer/carbon nanofiber (CNF) nanocomposite. In order to increase the sensitivity of the proposed sensor array, each sensing unit was optimized by selecting the appropriate shape and adjusting critical geometrical parameters using both finite element analysis and optimization algorithms. Considering different pressure range, two different set of sensing unit geometries were designed for the best performance in the pressure range of 50.100, and 150 psi maximum pressures. The developed sensor arrays were connected using a mesh grid approach. Preliminary sensor array experiments were tested to prove the concept. Finally, the developed pressure sensor arrays were characterized under both static and cyclic load conditions to demonstrate its sensor capability.

Improved Mechanical Property in Fiber Reinforced Plastic Composites Using Spray Coated CNTs

Carbon nanotubes have been employed as prominent nanofillers in composite matrix systems to improve the mechanical properties as well as thermal and electrical properties. A spray coating method is being developed as an effective way to deposit carbon nanotubes onto carbon fiber fabrics with good control of network formation. Compared to conventional approaches, such as chemical vapor deposition and solvent based dispersion approach, the proposed method is simple and versatile with the potential for industrial scale-up. In this paper carbon nanotubes are dispersed in solvent with optimized fabrication experimental processing procedures and sprayed on carbon fiber fabrics. Once all the solvent has been evaporated, the coated carbon fiber fabrics are examined using scanning electron microscopy system. We fabricate carbon fiber reinforced composites using an optimized wet layer approach. Both the prepared carbon nanotube coated carbon fiber fabrics and pristine carbon fiber fabrics were used with the conventional epoxy for the composite fabrication. The enhanced mechanical properties of the composites are experimentally characterized, especially focusing on the Mode-I toughness and strength. The experimental results showed that the spray coated CNTs can significantly improve the mechanical properties of composite, especially under the Mode-I fracture load conditions.Copyright

Investigation of Mechanical Properties of Nanoclay Incorporated Room Temperature Vulcanized (RTV) Silicone Foams

Room temperature vulcanized (RTV) silicone foams (SFs) have unique thermal and chemical properties due to the presence of inorganic Si-O backbones with organic methyl side groups. However, their low mechanical strength and low tear resistance are major drawbacks for many applications. We have incorporated Nanoclay as reinforcing filler to improve mechanical properties of silicone foams. A three step blending process was used to disperse Nanoclay in silicone elastomers. Initially, Nanoclay in the concentration range of 0.5%–1% by weight were mixed to silicone polymer using a mechanical mixer at 1200 rpm for 10 min followed by a tip sonication at 20% amplitude for 1 hr. Finally, a high speed mechanical mixer was used at 2000 rpm for 2 hours. Two different types of Nanoclays with different sizes were investigated. Both compression and tear properties were found to improve with addition of 0.5 wt% Nanoclay. It was found that the smaller Nanoclay particle size showed the best compressive property while the Nanoclay with larger particle size improved tear strength the greatest.Copyright