Novel composite architectures based on low dimensional C/BN with enhanced mechanical and thermal responses for sports applications
Date of Issue2018-12-31
School of Materials Science and Engineering
Articular cartilage injury of the knee/ankle is one of the most common sports injuries due to the complex physiological loadings they encounter during intense activities. There are two strategies to address this: 1) to avoid, that is to protect the healthy from such injury by advancing the sports accessories (e.g., footwear); 2) to cure, which means to treat the patients with appropriate therapy (e.g., total cartilage replacement). However, as the most promising midsole materials, vertically aligned carbon nanotube (VACNT) arrays and three-dimensional reduced graphene oxide (3D rGO) aerogels still exhibit limited compressibility and energy dissipation, while the conventional artificial cartilages (poly (vinyl alcohol) (PVA) hydrogels) still possess insufficient mechanical strength, toughness and heat transfer. These unsatisfactory characteristics of the candidate materials have severely restricted their further applications. To date, boron nitride nanotubes (BNNT) and nanosheets (BNNS) with superior mechanical and thermal characteristics, have demonstrated significant reinforcing effects in the physicochemical properties of carbon based architectures due to their highly coherent atomic configuration and lattice constants with graphene. In light of this, it is expected that the integration of BN nanomaterials is able to enhance the overall performances of the abovementioned candidate materials, while which has not been investigated yet. In this thesis, coaxial C@BNNT arrays with significantly improved compressive strength, shape recovery, fatigue resistance, energy dissipation and heat transfer have been firstly fabricated by encapsulating outer BNNT onto the CNT arrays. These enhancements in mechanical and thermal properties of the C@BNNT arrays endow their outstanding energy return and dissipation characteristics. Inspired by this, unique rGO/BN aerogels with ultralow density, high compressibility and excellent recoverability have been further constructed with cell walls of assembled rGO and BNNS, showing high potential as multifunctional compressible mattress. Despite these progresses, the inadequate shape recoverability and viscoelasticity of the C@BNNT arrays under cyclic compression as well as their limited dimensions (hundreds µm in length) still cannot meet the practical application requirements. To address these issues, commercially available CNT arrays with length of ~4 mm were chosen and further encapsulated with outer graphene layers. Notably, the resulting CNT@Gr arrays exhibit superior compressibility (~80% recovery after 1000 cycles at a 60% strain), strength, and even outstanding strain- and frequency-dependent viscoelasticity that is constant over an exceptionally broad temperature range (−100−500 °C) in air, attributing to the more intense synergistic effect between graphene and CNT. These CNT@Gr arrays with excellent characteristics will inspire a wide range of promising mechanical support and damping applications. On the other hand, to enable the PVA hydrogels with outstanding compressive responses to fulfill the requirements for artificial cartilages, the wafer-scale VACNTs were incorporated into the PVA hydrogels to fabricate composite VACNT/PVA hydrogels. Interestingly, such hydrogels perform outstanding mechanical responses upon both static and dynamic compressions due to the reinforcements of uniformly distributed VACNTs and large CNT-PVA interfaces. Furthermore, considering the better biocompatibility of BNNS than that of the CNT, highly hydrophilic BNNS were synthesized and subsequently introduced into the PVA to fabricate BNNS/PVA hydrogels. As expected, the resulting hydrogels exhibit not only remarkably improved mechanical properties but also excellent heat transfer, which can be attributed to the homogeneously distributed BNNS with excellent physical properties and the hydrogen bonding interactions between the BNNS and PVA chains. Therefore, these biocompatible BNNS/PVA hydrogels are promising in addressing the mechanical failure and locally overheating issues as cartilage substitutes and may also have broad utility for other biomedical applications.