Process-structure-property of multifunctional composites
Multifunctional composites including polymer matrix composites (PMC), metal matrix composites (MMC) and ceramic matrix composites (CMC) remain to be one of my major research interests. I am especially interested in the interfacial structures, reactive-diffusion phenomena and phase formation that usually need to be resolved by electron microscopy and/or in-situ testing. Currently, there are three on-going projects in this area: CNT or CNT yarn reinforced PMC, steel encapsulated MMC and reaction bonded SiC-Si and B4C-SiC-Si CMC.
CNTs are well known for outstanding mechanical and other physical or chemical properties. While significant progresses have been made in cooperating CNTs in polymers to form PMCs, multiple challenges remain to be resolved for practical applications. One effort has been to spin continuous CNT-yarns from pre-fabricated CNT forest. Highly aligned CNT-yarn promises to be an ideal structural component for the aerospace industry due to the light weight, remarkable mechanical properties, and other unique physical and chemical attributes of the CNTs. However, contrary to expectations, the mechanical properties of the CNT yarns fabricated by various methods are not only orders of magnitude below the corresponding properties of individual CNTs, but often compare poorly to those of commercial materials such as carbon fiber and Kevlar. The problem originates from a combination of insufficient interactions either from the van der Waals bonding, or from twist to give efficient load transfer between CNTs in the yarn. Thus as the yarn is strained, individual CNTs break prematurely at a relatively low tensile load.
We developed a method for the introduction of chemical cross-links on multiwall carbon nanotubes (MWCNTs) in CNT-forest or yarn involving exposing a CNT sample, together with a candidate coating material, to 172 nm photons from a vacuum-ultraviolet (VUV) lamp. Polystyrene (PS) or polymethyl methacrylate (PMMA) was directly bonded to the side wall of MWCNTs during the irradiation exposure process. Through a dynamic mechanism of CNT wall activation, polymer gas phase generation and deposition, this method simultaneously achieved the surface activation of carbon nanotubes (CNTs) in forests or yarns, de-polymerization of candidate polymers, and uniform deposition and re-polymerization. Coatings of PS and PMMA were deposited on the CNT walls in the CNT-yarn and CNT-forest after a 30 min irradiation in a N2 environment with no axillary heating. This achievement paves the way for continued research of this CNT functionalization method for high performance polymer matrix composites with reinforcements made from CNTs or CNT-yarns.
With the support of ARL and industrial collaborators, steel encapsulated MMCs are fabricated during which compressive residual stress is introduced in the MMC core due to the disparity of coefficients of thermal expansion (CTE). A variety of characterizations and tests are being conducted aiming to compare the performance of several material combinations consisting of aluminum or magnesium matrices reinforced with Al2O3, SiC, or B4C and surrounded by A36 steel, 304, or Nitronic 50 stainless steels. The effects of varying reinforcement percentage, surrounding steel thickness, and CTE mismatch between each material combination are being investigated.
Funded by the II-VI Foundation, the third on-going composite project investigates the reaction bonded SiC-Si and B4C-SiC-Si ceramic matrix composites (CMC) to establish the knowledge of phase transformation, interface and reaction zones, interphases, substructures and defects in the SiC-Si and B4C-SiC-Si CMC systems as well as an understanding of micromechanics and phase evolution by in-situ microscopic characterization and testing. Furthermore, an exploration of thermal and thermoelectric properties is to shed light on the possibility for potential material modifications and multifunctional applications. There are three interrelated research thrusts for this project specified as follows: a. Comprehensive characterization of microstructures including phase components and elemental compositions, reaction zones (rims) and grain boundaries, fine structures and defects; b. Micromechanical, electrical, and thermoelectric testing, and structure-property correlations; c. In-situ TEM and SEM observations of structural evolution and dynamic testing.
Fei Deng, Nopporn Rujisamphan, Chang Liu, Yoshinari Maezono, Stephen C. Hawkins, S. Ismat Shah and Chaoying Ni, Grafting polymer coatings onto the surfaces of carbon nanotube forests and fibers via a photon irradiation process, Applied Physics Letters, 100 (21), 2012, 213109.
Sean Fudger, Eric Klier, Prashant Karandikar, Brandon McWilliams, Chaoying Ni, Mechanical Properties of Steel Encapsulated Metal Matrix Composites, Advanced Composites for Aerospace, Marine, and Land Applications II, 2015, 121-136.
Lingyu Li, Christopher Y. Li, Chaoying Ni, Lixia Rong, Benjamin Hsiao, Structure and crystallization behavior of Nylon 66/multi-walled carbon nanotube nanocomposites at low carbon nanotube contents, Polymer, 2007, 48 (12), 3452-3460.
Lingyu Li, Christopher Y. Li, Chaoying Ni, Polymer crystallization-driven, periodic patterning on carbon nanotubes, JACS, 2006, 128(5), 1692-1699.
Funding and support:
II-VI Foundation; ARL