Surface control and cryogenic durability of transparent CNT coatings on dip-coated glass substrates.

Transparent carbon nanotube (CNT) coatings were deposited on boro-silicate glass substrates by dip-coating. Ultraviolet-visible (UV) spectra, surface resistance measurement, and the wettability tests were used to investigate the optical transmittance and electrical properties of these CNT coatings. The changes in electrical and optical properties of these coatings were observed to be functions of the number of dip-coating cycles. The surface resistance of the CNT coated substrates decreased dramatically as the number of dip-coatings was increased, whereas the increases in the CNT layer thickness beyond that for the first dipping cycle had little effect on the transparent-properties. Static contact angle measurements proved to be an effective means for evaluating the surface morphology of CNT coatings. The interfacial durability of the CNT coatings on a glass substrate was much better than that of ITO coatings over the temperature range from -150°C to +150°C.

Improvement of interfacial adhesion and nondestructive damage evaluation for plasma-treated PBO and Kevlar fibers/epoxy composites using micromechanical techniques and surface wettability.

Comparison of interfacial properties and microfailure mechanisms of oxygen-plasma treated poly(p-phenylene-2,6-benzobisoxazole (PBO, Zylon) and poly(p-phenylene terephthalamide) (PPTA, Kevlar) fibers/epoxy composites were investigated using a micromechanical technique and nondestructive acoustic emission (AE). The interfacial shear strength (IFSS) and work of adhesion, Wa, of PBO or Kevlar fiber/epoxy composites increased with oxygen-plasma treatment, due to induced hydrogen and covalent bondings at their interface. Plasma-treated Kevlar fiber showed the maximum critical surface tension and polar term, whereas the untreated PBO fiber showed the minimum values. The work of adhesion and the polar term were proportional to the IFSS directly for both PBO and Kevlar fibers. The microfibril fracture pattern of two plasma-treated fibers appeared obviously. Unlike in slow cooling, in rapid cooling, case kink band and kicking in PBO fiber appeared, whereas buckling in the Kevlar fiber was observed mainly due to compressive and residual stresses. Based on the propagation of microfibril failure toward the core region, the number of AE events for plasma-treated PBO and Kevlar fibers increased significantly compared to the untreated case. The results of nondestructive AE were consistent with microfailure modes.

Comparison of interfacial properties of electrodeposited single carbon fiber/epoxy composites using tensile and compressive fragmentation tests and acoustic emission.

Interfacial and microfailure properties of carbon fiber/epoxy composites were evaluated using both tensile fragmentation and compressive Broutman tests with an aid of acoustic emission (AE). A monomeric and two polymeric coupling agents were applied via the electrodeposition (ED) and the dipping applications. A monomeric and a polymeric coupling agent showed significant and comparable improvements in interfacial shear strength (IFSS) compared to the untreated case under both tensile and compressive tests. Typical microfailure modes including cone-shaped fiber break, matrix cracking, and partial interlayer failure were observed under tension, whereas the diagonal slipped failure at both ends of the fractured fiber exhibited under compression. Adsorption and shear displacement mechanisms at the interface were described in terms of electrical attraction and primary and secondary bonding forces. For both the untreated and the treated cases AE distributions were separated well in tension, whereas AE distributions were rather closely overlapped in compression. It might be because of the difference in molecular failure energies and failure mechanisms between tension and compression. The maximum AE voltage for the waveform of either carbon or large-diameter basalt fiber breakages in tension exhibited much larger than that in compression. AE could provide more likely the quantitative information on the interfacial adhesion and microfailure.

Interfacial adhesion and microfailure modes of electrodeposited carbon fiber/epoxy-PEI composites by microdroplet and surface wettability tests.

Interfacial properties and microfailure modes of electrodeposition (ED)-treated carbon fiber-reinforced polyetherimide (PEI) toughened epoxy composite were investigated using microdroplet test and the measurement of surface wettability. ED was performed to improve the interfacial shear strength (IFSS). As PEI content increased, IFSS increased due to enhanced toughness and plastic deformation of PEI. In the untreated case, IFSS increased with adding PEI content, and the IFSS of the pure PEI matrix showed the highest. On the other hand, for the ED-treated case IFSS increased with PEI content with rather low improvement rate. In the untreated case, neat epoxy resin appeared brittle microfailure mode, whereas the pure PEI matrix exhibited a more likely ductile microfailure mode. In the ED-treated case, neat epoxy exhibited a more ductile fracture than that of the untreated case. Critical surface tension and polar surface free energy of ED-treated carbon fiber was higher than those of the untreated fiber. The work of adhesion between fiber and matrix was not directly proportional to IFSS for both the untreated and ED-treated cases. The matrix toughness might contribute to IFSS more likely than the surface wettability. Interfacial properties of the epoxy-PEI composite can be affected efficiently by both the control of matrix toughness and ED treatment.

Interfacial Aspects of Electrodeposited Conductive Fibers/Epoxy Composites using Electro-Micromechanical Technique and Nondestructive Evaluation.

Interfacial adhesion and nondestructive behavior of the electrodeposited (ED) carbon fiber reinforced composites were evaluated using the electro-micromechanical technique and acoustic emission (AE). Interfacial shear strength (IFSS) of the ED carbon fiber/epoxy composites was higher than that of the untreated case. This might be expected because of the possible chemical and hydrogen bonding based on an electrically adsorbed polymeric interlayer. Logarithmic electrical resistivity of the untreated single-carbon fiber composite increased suddenly to infinity when the fiber fracture occurred, whereas that of the ED composite increased relatively broadly up to infinity. This may be due to the retarded fracture time as a result of the enhanced IFSS. In single- and 10-carbon fiber composites, the number of AE signals coming from the interlayer failure of the ED carbon fiber composite was much larger than that of the untreated composite. As the number of each first fiber fracture increased in the 10-carbon fiber composite, the electrical resistivity increased stepwise, and the slope of logarithmic electrical resistance increased. In the three-graphite filament composite with a narrow 1 time inter-filament distance, the total numbers of the filament fracture and the IFSS were smaller than those of the wider 5 times case. This might be because the interacting fracture energy caused by a filament break could affect the adjacent filaments. Copyright 2001 Academic Press.

Interfacial Properties of Two-Carbon Fiber Reinforced Polycarbonate Composites Using Two-Synthesized Graft Copolymers as Coupling Agents.

Two model coupling agents, water-dispersible (WDGP) and tetrahydrofuran (THF)-soluble graft copolymers (TSGP), were synthesized for carbon fiber/polycarbonate (PC) composites. WDGP contains a long polyacrylamide (PAAm) chain grafted on a PC backbone, whereas TSGP contains a short grafted PAAm chain. Measurements of the interfacial shear strength (IFSS) and other interfacial properties were evaluated using a fragmentation test for two-fiber composites (TFC) to provide the same loading state. Optimal conditions for the treatment was established as a function of treatment time, temperature, initial concentration, and melting procedure. The amount adsorbed on the carbon fiber was higher for TSGP then for WDGP; the maximum improvements in IFSS for WDGP and TSGP were 54% and 74%, respectively. Mechanisms of energy adsorption for WDGP and intermolecular interaction for TSGP can be considered to contribute differently to IFSS improvement. The improvement in IFSS for both coupling agents may be due to chemical and hydrogen bonding in the interface between functional groups in the carbon fiber and PAAm in the coupling agents and to interdiffusion in the interface between PC in coupling agents and matrix PC. Copyright 2000 Academic Press.