Possible Chemical Bond Formation between a Carbon Nanotube and Alumina Matrix - A Quantum Mechanics Investigation

To improve the structural properties of engineering ceramics, carbon nanotubes have been used as a reinforcement phase to produce stronger ceramic matrix composites. This paper investigates the possible chemical bond formation between a carbon nanotube and alumina with the aid of quantum mechanics analysis. The cases with and without functionalizing the nanotubes were examined. The nanotubes were modeled by nanotube segments with hydrogen atoms added to the dangling bonds of the perimeter carbons. The cleaved ceramic (0001) surface was represented by an alumina molecule with the oxygen atoms on either end terminated with hydrogen. Methoxy radicals were used to functionalize the CNTs. The study predicts that covalent bonding between Al atoms on a cleaved single crystal alumina surface and C atoms on a nanotube are energetically favorable. Introduction Ceramic materials are brittle, hard and strong in compression, but weak in shearing and tension. Engineering ceramics can be classified into oxides (Alumina, Zirconia), non-oxides (carbides, borides, silicides) and composites (combinations of oxides and non-oxides). Alumina based ceramics are by far the largest range of advanced ceramics and offer a combination of good mechanical and electrical properties leading to a wide range of applications in medicine, electrical and electronics industries. Currently, the brittleness of ceramics impedes their use as structural materials. However, this can be improved by an increase in fracture toughness or by a reduction in critical flaw size. It has been reported that the fracture toughness can be increased by incorporating nanoparticles into ceramics. Carbon nanotubes (CNTs) have exceptional mechanical properties. By combining CNTs and ceramics, if one can impart the attractive properties of both CNTs and ceramics to the resulting composites, the ceramic matrix of the composites can be toughened. However, a challenge in fabricating CNT-ceramic composites is to achieve appropriate CNT-matrix interfacial properties. For this, strong bonds should be formed at the interface which can lead to good stress transfer capability [1]. It has been reported that the interfacial bonding properties of CNT-ceramic composites can vary significantly with the processing conditions. The studies on nanocomposites have illustrated significant challenges in processing and improving properties. Poyato et al. [2] used a combination of acid treatment, aqueous colloidal processing, and spark-plasma sintering (SPS) to fabricate high-density Al2O3/single-wall carbon nanotube (SWCNT) composites with welldistributed SWCNTs and other carbon nanostructures at Al2O3 grain boundaries. Although a significant achievement has been reached in dispersing CNTs in the matrix, there are some debates about the toughening of ceramic/SWNTs composites. Some recent reviews [3-4] on CNT-ceramics composites have discussed the investigations of a variety of methods which have been used to produce ceramic and metal matrix nanotube composites. The conclusions on the improvement of mechanical properties, however, are diverse. Key Engineering Materials Vol. 443 (2010) pp 723-728 Online available since 2010/Jun/02 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.443.723 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 129.94.206.15-29/03/11,03:04:49) This work aims to examine the possibility of chemical bonds at the CNT-ceramics interface between a SWCNT and alumina when SWCNTs are used as reinforcements to toughen ceramics. In our previous work, we had discussed and demonstrated the methods of promoting covalent bonds [5-7] between Polyethylene (PE) and CNTs, a theoretical rationale [8] for chemical bonding at the interface and their important role in the composite reinforcement [9]. In the present work we will use a similar approach to examine the possibility of forming chemical bonds between CNT and single crystal alumina with and without using free radical initiators. Methodology The nature of the atomic termination layers on cleaving an alumina single crystal is important to the understanding of the reaction with CNTs. Alumina single crystal may be cleaved parallel to the (0001) plane at two different locations leading to either a surface terminated with an O layer and a surface terminated with two Al layers or two equivalent surfaces terminated with an Al layer. Work by Guo et al. [10] had shown that for the (0001) surface, an Al layer terminated surface had the lowest cleavage energy. Thus in this work, we will investigate the possible chemical bond formation between an alumina molecule of which the oxygen atoms on its either end are replaced by OH to represent a segment of Al terminated surface in ceramics, and a segment of a CNT (with and without free radical initiators) using Density Functional theory (DFT). The model of a (5,0) zigzag nanotube segment used for this study contains 60 carbon atoms (length 11.36 Å). Hydrogen atoms were added to the dangling bonds of the perimeter carbons. The following reactions were studied by fully optimizing the geometry of the corresponding reactants and products: (i) reaction of nanotube model C60H10 with alumina model (Al2O3H2) (ii) reaction of a free radical functionalized nanotube model, C60H10-OCH3 with alumina model (Al2O3H2). All calculations were performed using DFT with a hybrid functional B3LYP [11-14] and a 321G basis set [15]. The atomic spin densities and charge densities were analyzed by the Mulliken method [16]. For open-shell molecular radicals, the unrestricted formalism was used. The present level of calculation, DFT(UB3LYP)/3-21G, is known to produce reasonable results for bond lengths, bond angles and bond energies for a wide range of molecules [17]. The computations were carried out on a supercomputer using the ab initio quantum chemistry package, Gaussian 09 [18]. Table 1 Stabilization energies of various reactions Reactant A Reactant B Product A-B Difference in energy for the reaction A + B A-B (kJ/mol) C60H10 Al2O3H2 (triplet) Singlet with 6-membered ring -395.07 Singlet with 7-membered ring -431.05 Singlet with 8-memebred ring -389.51 Triplet with 7-membered ring -427.62 C60H10 OCH3 Doublet -168.0 [C60H10-OCH3] Al2O3H2 (triplet) Doublet with 7-membered ring -386.34 Results and Discussion Reaction of C T directly with Alumina. Al terminated surfaces could form when alumina single crystal cleaves parallel to the (0001) plane. This would leave the Al atoms on the surface with a free electron. The hydrogenated alumina model that has a free electron on each Al would represent the surface terminated with an Al layer. When the hydrogenated alumina in its triplet state reacts with CNT, the product could be a singlet or a triplet. Depending on the positions of the nanotube C 724 Advances in Materials Processing IX