No abstract

Laser flash analysis (LFA), an unsteady-state technique originally developed for measuring the thermal diffusivity of homogenous materials, was used to estimate the thermal conductivity of carbon fibers consolidated in an epoxy matrix to form axially aligned unidirectional composites. Experimental studies were conducted for P-25 and K-1100 mesophase pitch-based carbon fibers whose conductivity values bracket almost two orders of magnitude ($10 and 1000 W/mÁK). Experimentally determined fiber thermal conductivity values were generally consistent with those cited in the literature after appropriate corrections were applied to account for the extremely high conductivity (low thermal resistance) of highly graphitic fibers, relative to the graphite coating. Finite element analysis was used to simulate heat flow patterns that may occur in a uniaxial fiber-polymer composite due to the large differences in the thermal conductivities of carbon fibers and polymer matrices. Simulations reveal that fiber thermal conductivity is accurately determined from composite response for high volume fraction of fibers (!0.6) regardless of fiber conductivity, or for lower conductivity fibers (10-100 W/mÁK) regardless of volume fractions. However, for composites containing high thermal conductivity fibers (100-1000 W/mÁK) at low volume fractions ( 0.2), fiber thermal conductivity may not be accurately determined due to transverse heat flow within the graphite layers that channel heat through the highly conductive fiber. Thus, under certain conditions, heat flow paths deviate from the one-dimensional heat flow assumption inherent to laser flash analysis and rule-of-mixtures.

No abstract

2016-12-23T18:49:38

Carbon nanotubes (CNTs) are one of the best candidates for utilizing in the future nanoelectronic devices. However, the semiconductivity property of CNTs varies as diameter and chirality number change. Several nanoelectronic applications require semiconductivity to be of a certain value with little variability. Therefore, it is necessary to reliably produce uniform CNTs with unique diameter and chirality. However, this still remains one of the challenging problems in the large scale production and application of CNTs. In this paper, the effect of fabrication temperature change on CNTs diameter and chirality are experimentally and theoretically studied. Utilizing chemical vapor deposition (CVD) fabrication process and by conducting experimental investigation, it is observed that a CNT possesses a larger diameter at its base compared to the section far away from deposited nanoparticles. Moreover, using MD simulation technique, it is observed that the energy of the CNTs molecular structure will increase by applying higher fabrication temperature. Usually this energy increase is greater in the thicker CNTs. However, the energy increase percentage is found to be affected by the chirality of the CNT. Among CNTs of the same diameter, the armchair conformation has the highest percent increase, followed by the chiral CNTs, and the zigzag nanotube has the lower percent increase. The obtained results can be utilized in a controllable CNTs diameter and chirality design process.