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PFG Alumni: Anson Ma

Anson Ma

The Rheology, Microstructure and Film Processing of Carbon Nanotube Suspensions

Introduction

Anson's project was concerned with the rheology, microstructure, and film processing of different types of Carbon Nanotube (CNT) suspensions. CNTs, typically with a diameter of less than 100 nanometers and a length of several-hundred microns, belong to a relatively new class of fibrous nano-material.

Because of their high mechanical strength, low density, high thermal and electrical conductivity, CNTs are being extensively researched and evaluated as polymer fillers. The key challenge is how to process this material that is in the form of powder, into macroscopic forms that are of practical use and with controlled properties.

Project Objectives

  • To advance our understanding in the rheology of CNT suspensions.

  • To investigate how microstructure evolves under different flow conditions

  • To produce CNT thin films and characterize their internal nanostructure

Carbon nano tubes

Carbon nano tubes

SEM images of Multi-walled CNTs produced by the Department of Materials Science and Metallurgy at the University of Cambridge

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Rheology 

The figure below shows the typical shear-thinning characteristic observed for CNT suspensions in a simple shear flow. Such behaviour was simulated using an aggregation/orientation model jointly developed with Prof. F. Chinesta of ENSAM Paris.

 The rheology of CNT suspensions
 The variation of the apparent viscosity of carbon nano tube suspensions of different weight percent with shear rate.

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Microstructure

Figures below show the quiescent optical texture of multi-walled CNTs suspended in epoxy at different concentration levels.

 Morphology of CNTs in epoxy, 0.025% Morphology of CNTs in epoxy, 0.05% 
 Morphology of CNTs in epoxy, 0.1%  Morphology of CNTs in epoxy, 0.5%
(a) 0.025% CNT (b) 0.05% CNT (c) 0.1% CNT (d) 0.5% CNT

As a simple shear flow is applied to a CNT suspension (0.1%), its optical texture changes accordingly as shown below.

 CNT suspension sheared at 0-1s  CNT suspension sheared at 0.5s-1
 CNT suspension sheared at 10s-1 CNT suspension sheared at 100s-1 
0.1% CNT suspension sheared at (a) 0 s-1 (b) 0.5 s-1 (c) 10 s-1 (d) 100 s-1 (with shear time in excess of 300s)

With appropriate flow conditions and suspending medium, a highly heterogeneous mesostructure can be formed. This mesostructure is also referred as the Mesoscopic Helical Filament (MHF). Interestingly, the formation of MHFs is analogous to the way in which cotton is spun into fibres.

 MHF formation, unsheared sample

Diffuse isotropic
aggregates

MHF formation, sample sheared for 180s 

MHF formation, sample sheared for 600s

Helical Mesoscopic
Filaments (MHFs)

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Film processing

CNT films were produced using the dip coating method. Their nanostructure was characterised by an Atomic Force Microscope (AFM) and selective etching.

 CNT dip coating process at different speeds The figure on the left shows CNT films dip coated and withdrawn at different rates in the dip coating process. 

 

CNT AFM image of unetched sample

AFM image of CNT film
without etching

CNT AFM image of etched sample

AFM image of CNT film with
selective etching to expose the
nanostructure of underlying CNTs

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