Kwan Han Yoon

Smart Materials and Structures Based on Carbon Nanotube Composites

Since the first discovery of carbon nanotubes (CNTs) in 1991, CNTs have generated enormous research activities in many areas of science and engineering due to their combined exceptional mechanical, thermal and electronic properties. These properties make nanotubes ideal, not only for a wide range of applications but also as a test-bed for fundamental scientific studies (Baughman et al., 2002). They can be described as a graphite sheet rolled up into a nanoscale tube. Two structural forms of CNTs exist: single-walled (SWCNTs) and multi-walled (MWCNTs) nanotubes. CNT lengths can be as short as a few hundred nanometers or as long as several micrometers. SWCNT have diameters between 1 and 10 nm and normally capped ends. In contrast, MWCNT diameters range from 5 to a few hundred nanometers because their structure consists of many concentric cylinders held together by van der Waals forces. CNTs are synthesized in a variety of ways, such as arc discharge, laser ablation, high pressure carbon monoxide (HiPCO), and chemical vapor deposition (CVD) (Dresselhaus, 1997). CNTs exhibit excellent mechanical, electrical, thermal and magnetic properties. The exact magnitudes of these properties depend on the diameter and chirality of the nanotubes and whether their structure is singleor multi-walled. Fig. 1 shows a segment of a single graphene plane that can be transformed into a carbon nanotube by rolling up into a cylinder. To describe this structure, a chiral vector is defined as OA = na1 + ma2, where a1 and a2 are unit vectors for the hexagonal lattice of the graphene sheet, n and m are integers, along with a chiral angle θ, which is the angle of the chiral vector with respect to the x direction. Using this (n, m) scheme, the three types of nanotubes are characterized. If n = m, the nanotubes are called ‘‘armchair”. If m = 0, the nanotubes are called ‘‘zigzag”. Otherwise, they are called ‘‘chiral”. The chirality of nanotubes has significant impact on their transport properties, particularly the electronic properties. For a given (n, m) nanotube, if (2n + m) is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor. Each MWCNT contains a multi-layer of graphene, and each layer can have different chiralities, so the prediction of its physical properties is more complicated than that of SWCNT (Jin & Yuan, 2003).

Design, manufacturing, and characterization of high-performance lightweight bipolar plates based on carbon nanotube-exfoliated graphite nanoplatelet hybrid nanocomposites

We report a study on manufacturing and characterization of a platform material for high-performance lightweight bipolar plates for fuel cells based on nanocomposites consisting of carbon nanotubes (CNTs) and exfoliated graphite nanoplatelets (xGnPs). The experiments were designed and performed in three steps. In the preexperimental stage, xGnP-epoxy composite samples were prepared at various xGnP weight percentages to determine the maximum processable nanofiller concentration. The main part of the experiment employed the statistics-based design of experiments (DOE) methodology to identify improved processing conditions and CNT: xGnP ratio for minimized electrical resistivity. In the postexperimental stage, optimized combinations of material and processing parameters were investigated. With the aid of a reactive diluent, 20wt.% was determined to the be maximum processable carbon nanomaterial content in the epoxy. The DOE analyses revealed that the CNT: xGnP ratio is the most dominant factor that governs the electrical properties, and its implications in relation to CNT-xGnP interactions and microstructure are elucidated. In addition, samples fabricated near the optimized condition revealed that there exists an optimal CNT: xGnP ratio at which the electrical performance can be maximized. The electrical and mechanical properties of optimal samples suggest that CNT-xGnP hybrid nanocomposites can serve as an alternative material platform for affordable, lightweight bipolar plates.

Damping properties and transmission loss of polyurethane. II. PU layer and copolymer effect

Polyurethane (PU) layer and copolymer consisted of the different molecular weights (1000 and 2000 g/mol) of poly(propylene glycol) (PPG) were prepared. The damping and mechanical properties of these materials were compared with PU1000 made by PPG having the molecular weight of 1000 g/mol. The optimum composition of PU2000 used for PU layer and copolymer was diphenylmethane diioscynate (MDI)/propylene glycol (PPG)/butanediol (BD) (1/0.3/0.7) based on the damping and mechanical properties. The damping peak of PU copolymer was higher than those of PU layer and PU1000 in low temperature range (−30–10 °C). For application in noise reduction, the transmission loss of the mechanical vibration through solid structure was measured. PU layer and copolymer were used as a damping layer. The transmission loss of PU copolymer was more effective than those of PU layer and PU1000 in the experimental frequency range.

Thermal and mechanical properties of modified CaCO3 filled poly (ethylene terephthalate) nanocomposites

Poly(ethylene terephthalate) (PET)/CaCO3 and PET/modified-CaCO3 (m-CaCO3) nanocomposites were prepared by melt blending. The morphology indicated that m-CaCO3 produced by reacting sodium oxalate and calcium chloride, was well dispersed in PET matrix and showed good interfacial interaction with PET compared to CaCO3. No significant differences in the thermal properties such as, glass transition, melting and degradation temperatures, of the nanocomposites were observed. The thermal shrinkage of PET at 120 °C was 10.8 %, while those of PET/CaCO3 and PET/m-CaCO3 nanocomposites were 2.9–5.2 % and 1.2–2.8 %, respectively depending on filler content. The tensile strength of PET/CaCO3 nanocomposite decreased with CaCO3 loading, whereas that of PET/m-CaCO3 nanocomposites at 0.5 wt% loading showed a 17 % improvement as compared to neat PET. The storage modulus at 120 °C increased from 1660 MPa for PET to 2350 MPa for PET/CaCO3 nanocomposite at 3 wt% loading, and 3230 MPa for PET/m-CaCO3 nanocomposite at 1 wt% loading.