Zhiyong Liang

Functionalized Carbon‐Nanotube Sheet/Bismaleimide Nanocomposites: Mechanical and Electrical Performance Beyond Carbon‐Fiber Composites

Since their discovery in 1991, carbon nanotubes (CNTs) have been considered as the next-generation reinforcement materials to potentially replace conventional carbon fibers for producing super-high-performance lightweight composites. Herein, it is reported that sheets of millimeter-long multi-walled CNTs with stretch alignment and epoxidation functionalization reinforce bismaleimide resin, which results in composites with an unprecedentedly high tensile strength of 3081 MPa and modulus of 350 GPa, well exceeding those of state-of-the-art unidirectional carbon-fiber-reinforced composites. The results also provide important experimental evidence of the impact of functionalization and the effect of alignment reported previously on the mechanical performance and electrical conductivity of the nanocomposites.

Statistical characterization of single-wall carbon nanotube length distribution

This paper describes an effective method for quantifying the length and length distribution of large populations of single-wall carbon nanotubes using atomic force microscopy and SIMAGIS software. The results of the measurements were modelled with the Weibull distribution, resulting in a statistically confirmed fit. The fitted Weibull distribution was used to predict the length effect factor and elastic modulus as functions of nanotube properties in composite materials. The prediction shows that the length factor for the elastic modulus tends to increase with enhanced loading but decrease with rising rope diameter. The statistical characterization presented indicates a pathway for the future theoretical modelling and related experimental investigation of carbon nanotube application.

Controlled nanostructure and high loading of single-walled carbon nanotubes reinforced polycarbonate composite

This paper presents an effective technique to fabricate thermoplastic nanocomposites with high loading of well-dispersed single-walled carbon nanotubes (SWNTs). SWNT membranes were made from a multi-step dispersion and filtration method, and then impregnated with polycarbonate solution to make thermoplastic nanocomposites. High loading of nanotubes was achieved by controlling the viscosity of polycarbonate solution. SEM and AFM characterization results revealed the controlled nanostructure in the resultant nanocomposites. Dynamic mechanical property tests indicated that the storage modulus of the resulting nanocomposites at 20 wt% nanotubes loading was improved by a factor of 3.4 compared with neat polycarbonate material. These results suggest the developed approach is an effective way to fabricate thermoplastic nanocomposites with good dispersion and high SWNT loading.

EXPERIMENTAL DESIGN AND OPTIMIZATION OF DISPERSION PROCESS FOR SINGLE-WALLED CARBON NANOTUBE BUCKY PAPER

This paper presents a new processing method to produce nanotube reinforced composites by using single-walled nanotube (SWNT) bucky paper as reinforcement. The SWNT bucky paper is an entangled mat of SWNTs, which is a highly porous mesh structure. To successfully produce SWNT bucky paper reinforced composites, it is essentially important to prepare a high-quality bucky paper. In this study, the quality of the bucky paper related to the dispersion of nanotubes within the bucky paper, which was quantitatively-characterized with rope size distribution and pore size distribution. An experimental study was conducted to investigate the effects of various process parameters on the dispersion of nanotubes using design of experiments (DOEs) approach. Based on the experimental analysis, the dispersion process of nanotubes within the bucky paper was optimized by selecting appropriate process parameters.

Precise cutting of single-walled carbon nanotubes

We precisely cut single-walled carbon nanotubes (SWNTs) to create short nanotubes with controlled length and open ends using an ultra-microtome and magnetically aligned SWNT membranes. At −60 °C, multiple layers of SWNT membranes were stacked and frozen together, and then cut at lengths of 50 and 200 nm, respectively. Transmission electron microscopy (TEM) and Raman characterizations clearly indicated that nanotubes were mechanically chopped to create open ends without notable sidewall damage. Atomic force microscopy (AFM) characterization and statistical analysis showed that the length distributions of the cut SWNTs can be controlled. Short SWNTs are very promising for applications in biomolecular transportation, field emission, field effect transistor and nanocomposites.

In situ measurement and monitoring of whole-field permeability profile of fiber preform for liquid composite molding processes

Abstract For liquid composite molding (LCM) processes, such as resin transfer molding (RTM), the quality of final parts is heavily dependent on the uniformity of the fiber preform. However, the conventional permeability measurement method, which uses liquid (oil or resin) as its working fluid, only measures the average preform permeability in an off-line mode. This method cannot be used to create an in situ permeability profile because of fiber pollution. Further, the conventional method cannot be used to reveal preforms local permeability variations. This paper introduces a new permeability characterization method that uses gas flow to detect and measure preform permeability variations in a closed mold assembly before resin injection. This method is based upon two research findings: (1) resin permeability is highly correlated with air permeability for the same fiber preform with well-controlled gas flow, and (2) the whole-field air permeability profile of a preform can be obtained through measuring the pressure field of gas flow. In this study, first the validity of the gas-assisted, in situ permeability measurement technique was established. Then the technique was demonstrated as effective by qualitatively detecting non-uniformities and permeability variations in fiber performs. Finally, a two-dimensional flow model, based on the finite difference scheme, was developed to quantitatively estimate the whole-field preform permeability profile using predetermined pressure distribution. The efficacy of the new method was illustrated through experimental results.

Electrical conductivity modeling and experimental study of densely packed SWCNT networks

Single-walled carbon nanotube (SWCNT) networks have become a subject of interest due to their ability to support structural, thermal and electrical loadings, but to date their application has been hindered due, in large part, to the inability to model macroscopic responses in an industrial product with any reasonable confidence. This paper seeks to address the relationship between macroscale electrical conductivity and the nanostructure of a dense network composed of SWCNTs and presents a uniquely formulated physics-based computational model for electrical conductivity predictions. The proposed model incorporates physics-based stochastic parameters for the individual nanotubes to construct the nanostructure such as: an experimentally obtained orientation distribution function, experimentally derived length and diameter distributions, and assumed distributions of chirality and registry of individual CNTs. Case studies are presented to investigate the relationship between macroscale conductivity and nanostructured variations in the bulk stochastic length, diameter and orientation distributions. Simulation results correspond nicely with those available in the literature for case studies of conductivity versus length and conductivity versus diameter. In addition, predictions for the increasing anisotropy of the bulk conductivity as a function of the tube orientation distribution are in reasonable agreement with our experimental results. Examples are presented to demonstrate the importance of incorporating various stochastic characteristics in bulk conductivity predictions. Finally, a design consideration for industrial applications is discussed based on localized network power emission considerations and may lend insight to the design engineer to better predict network failure under high current loading applications.

The fabrication of single-walled carbon nanotube/polyelectrolyte multilayer composites by layer-by-layer assembly and magnetic field assisted alignment

Single-walled carbon nanotube (SWNT)/polymer composites are widely studied because of their potential for high mechanical performance and multifunctional applications. In order to realize highly ordered multilayer nanostructures, we combined the layer-by-layer (LBL) assembly method with magnetic force-induced alignment to fabricate SWNT/poly(ethylamine) (PEI) multilayer composites. The SWNTs were functionalized with the anionic surfactant sodium dodecylbenzenesulfonate (NaDDBS) to realize negative charge at pH>7, while the PEI is positively charged at pH<7. The LBL method is based on the electrostatic absorption between the charged SWNTs and PEI resin to form multilayer composites on a solid substrate polydimethylsiloxane. Since the fabricated thickness of each SWNT-NaDDBS/PEI bilayer is uniform ( approximately 150 nm), the multilayer film thickness can be strictly controlled via the number of deposition cycles. A high magnetic field (8.5 Tesla) was used to align the SWNTs during the LBL process. The resultant LBL composite samples demonstrated high SWNT loading of approximately 50 wt% and uniform distribution of SWNTs in the multilayer structures, which was verified using a quartz crystal microbalance. Good alignment was also realized and observed through using high magnetic fields to align the nanotubes during the LBL deposition process. The results indicate that the LBL/magnetic alignment approach has potential for fabricating nanotube composites with highly ordered nanostructures for multifunctional materials and device applications.

Resin infusion between double flexible tooling: prototype development

The Resin Infusion between Double Flexible Tooling (RIDFT) technique is a novel two-stage process, which incorporates resin infusion and wetting with vacuum forming. The flow front of the infused resin is two-dimensional and avoids flow complexities prevalent in the three-dimensional flow seen in other liquid composite molding techniques. It employs a one-sided mold, which provides obvious cost benefits when compared with resin transfer molding. On-going prototype development of the RIDFT process has yielded positive results. Composite laminates with good surface quality, micro structural characteristics, and mechanical properties have been repeatedly produced with cost savings of 24% when compared with SCRIMP. This paper describes the RIDFT process, outlining its merits and presenting its challenges, whilst identifying potential benefits to industry. Current work being undertaken include the refining of production parameters, the construction of a larger prototype to examine the full extent of its suitability for the manufacture of large composite components and the incorporation of the UV curing technique to reduce the cycle time in the manufacture of large structures.

Resin Infusion between Double Flexible tooling: Evaluation of process parameters

Manufacturers of lean weight vehicular structures are discovering the advantages of utilizing the high strength to weight ratios found in composite materials. However, traditional closed mold techniques require a long cycle time and relatively expensive tooling. This paper reports on a new technique called Resin Infusion between Double Flexible Tooling (RIDFT). In this innovative process, the resin infusion and resin-fiber wetting are finished between two flexible tools in a two dimensional flat shape and then the entire wetted reinforcement and flexible tooling is formed into a specified part shape by using a vacuum. This process avoids complex resin flow and mold filling issues and saves mold preparation and fiber stacking time. Using a RIDFT prototype, several components were produced. A larger industrial RIDFT machine was constructed to establish its viability for large component manufacture. An evaluation of production parameters, including infusion pressure, resin temperature, number of fiber layers and hardener content, ascertained their effect on mechanical properties and infusion times for the manufactured parts. The interaction between the hardener and fiber layers were found to directly affect tensile strength. The interaction between hardener and infusion pressure was also shown to affect tensile strength.