Research Article Enhancement of Through-Thickness Thermal Transport in Unidirectional Carbon Fiber Reinforced Plastic Laminates due to the Synergetic Role of Carbon Nanofiber Z-Threads Alexander M. Scruggs, Sebastian Kirmse , and Kuang-Ting Hsiao Department of Mechanical Engineering, University of South Alabama, Mobile, AL 36688, USA Correspondence should be addressed to Kuang-Ting Hsiao; kthsiao@southalabama.edu Received 30 July 2018; Accepted 21 October 2018; Published 3 January 2019 Academic Editor: Victor M. Castaño Copyright © 2019 Alexander M. Scruggs et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This study experimentally and analytically examined the influence of carbon nanofiber (CNF) z-threads on the through-thickness (i.e., z-direction) thermal conductivity of unidirectional carbon fiber reinforced plastics (CFRPs). It was hypothesized that a network of CNF z-threads within CFRPs would provide a thermally conductive microstructure throughout the sample thickness that would increase the through-thickness thermal conductivity. The experiments showed that the through-thickness thermal conductivity of the CNF z-threaded CFRPs (9.85 W/m-K) was approximately 7.53 times greater than that of the control CFRPs (1.31 W/m-K) and 2.73 times greater than that of the unaligned CNF-modified CFRPs (3.61 W/m-K). Accordingly, the CNF z-threads were found to play a substantial role in increasing the through-thickness thermal conductivity of CFRPs. To better understand the role of the CNF z-threads in through-thickness thermal transport, simple logical models of the CFRPs were constructed and then compared with the experimental results. Through these analyses, it was determined that CNF z- threads substantially enhance the through-thickness thermal conductivity by creating carbon fiber-CNF linkages throughout the CFRP laminate; these linkages allow the heat flow to largely bypass the resistive resin that envelops the carbon fibers. In addition, thermal infrared tests illustrated that the increased through-thickness thermal conductivity of the CNF z-threaded CFRP enabled the location and visualization of defects within the laminate, which was not possible with the control CFRP. 1. Introduction Carbon fiber reinforced plastics (CFRPs) exhibit high modulus-to-weight and strength-to-weight ratios, which makes them particularly desirable for use in industries where weight-savings is critical, such as the aerospace and automo- tive industries. However, the absence of z-direction fiber reinforcement in CFRPs results in the polymer matrix mate- rial dominating the z-direction performance of the laminate. Moreover, polymer matrices typically have very low thermal conductivity. Therefore, under good fiber-wetting condi- tions, the polymer matrix acts as an insulating barrier that surrounds individual carbon fibers. Accordingly, CFRPs tend to have relatively poor z-direction thermal conductivity (approximately 1 W/m-K) in comparison to their in-plane thermal conductivity (approximately 10 W/m-K) [1–4]. If utilized effectively, the high aspect ratio of carbon nanofibers (CNFs) enables the potential formation of a multiscale network throughout the composite structure. Fur- thermore, as high heat-treated (HHT) carbon nanofibers possess a thermal conductivity value of approximately 1950 W/m-K [5], CNFs have the potential to substantially enhance the through-thickness thermal conductivity of CFRPs. For comparison, the thermal conductivity of an AS4 carbon fiber tow is approximately 6.83 W/m-K [6], and the thermal conductivity of stainless steel is approximately 15.26 W/m-K [7]. However, the ideal thermal properties of CNFs have not been effectively translated into increases in the macroscale properties of CFRPs due to technical barriers regarding the processing and positioning of CNFs. Most studies that involve the creation of multiscale reinforcements within FRPs by using high aspect ratio carbon-based Hindawi Journal of Nanomaterials Volume 2019, Article ID 8928917, 13 pages https://doi.org/10.1155/2019/8928917