Study of the network formation of carbon nanotubes in epoxy matrices for electrical conductivity improvement [Elektronische Ressource] / von Josef-Zoltan Lott

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Informations

• Publié par: technische_universitat_hamburg-harburg
• Publié le: 01 janvier 2009
• Nombre de lectures: 123
• Langue: English
• Poids de l'ouvrage: 15 Mo

Extrait

Study of the network formation of carbon
nanotubes in epoxy matrices for electrical
conductivity improvement



Vom Promotionsausschuss der
Technischen Universität Hamburg-Harburg
zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften (Dr. rer. nat.)
genehmigte Dissertation





von
Dipl.-Phys. Josef-Zoltan Lott (geb. Kovacs)
aus
Temeschburg, Rumänien





2009


























st
1 member of committee: Prof. Dr. rer. nat. Wolfgang Bauhofer
nd2 member of committee: Prof. Dr.-Ing. Karl Schulte

th
Date of examination: August 10 , 2009
ii










For Christina
iii



iv Abstract
This thesis analyses the network formation of CNT in epoxy matrices using
scanning electron microscopic (SEM), electrical, Raman spectroscopic and
rheological techniques. Investigations are focused on the improvement of the
electrical properties of the CNT/epoxy composites.

A non-destructive method called voltage (or charge) contrast SEM was
developed for determining the real CNT shapes and distributions in a composite
over several length scales. This knowledge is crucial for the interpretation of all
upcoming experiments.

Conductivity measurements revealed two percolation thresholds, the lower one
attributed to a kinetic and the higher one to a statistic network formation process.
The kinetic percolation threshold was found to be sensitive to temperature and
the shear forces present in the liquid composite. CNT with higher aspect ratios
were found to have lower statistic and kinetic percolation thresholds, meaning
that the influence of the aspect ratio on the kinetic percolation threshold is
dominating the influence of shearing. Processing of the CNT/epoxy suspension
with a calender was found to be disadvantageous for both, the percolation
threshold and the maximum achievable composite conductivity.

Raman spectra were utilised to determine the temperature, orientation and
stress state of CNT in epoxy resins. The waviness of some CNT types was
shown to restrict determining the CNT orientation. Stresses induced by the
thermal expansion coefficient of the matrix and their relief at the glass transition
temperature could be monitored accurately by Raman spectroscopy.
Simultaneous conductivity measurements revealed that the thermal stresses
were not sufficiently high to affect the integrity of the established CNT network.
v The CNT network formation in an epoxy liquid due to shear forces was studied
under controlled conditions inside a rheometer. Rheological, electrical and
optical parameters could be monitored and analysed simultaneously. Shearing
with low shear rates was found to produce agglomerates while shearing with
high shear rates destroyed them, both being reversible processes. The
formation of electrically conductive networks was different for calendered and
non-calendered CNT/epoxy suspensions. The calendered samples needed a
pre-shearing step at high shear rates and a gradual lowering of the shear rate in
order to establish a network.

In conclusion, CNT are ideal fillers for polymer composites used in antistatic
and electromagnetic shielding applications. They yield high conductivities at low
filler concentrations without the need to be perfectly dispersed in the matrix.
Their agglomeration can be controlled most effectively by adjusting the
suspension temperature.
vi Acknowledgments
Many people contributed to my doctoral thesis over the last five years.
Especially, I would like to thank …

… my advisor, Prof. Wolfgang Bauhofer, for giving me the opportunity to
work on this exciting topic, as well as for his guidance, support and
motivation.

… Prof. Karl Schulte for offering me the support of his group and for co-
reviewing the thesis.

… Prof. Manfred Eich for providing me with essential measurement
equipment.

… Prof. Jan Luiken ter Haseborg for chairing the examination committee.

… former and present members of the research groups of Prof. Bauhofer
and Prof. Eich: Dr. Robert Schliewe, Roman Kubrin, Dr. Altan Yildirim, Dr.
Alexander Petrov, Jan-Hendrik Wülbern, Jan Hampe, Gabriele Birjukov,
Christine Kunstmann, Stefan Schön, Michael Seiler, Dr. Michael
Hossfeld, Iris Bucher, Carola Micheelsen, Dr. Matthias Schwarz, Dr.
Markus Schmidt; those in the research group of Prof. Schulte: Alejandra
de la Vega, Dr. Bodo Fiedler, Dr. Florian Gojny, Dr. Malte Wichmann,
Sam Buschhorn, Dr. Kirsten Prehn, Dr. Luis Prado, Dr. Leif-Ole Meyer,
Dr. Eduard Ilinich, Jan Sumfleth, Florian Gehrig, Lars Böger, Hella Wilde,
Ingrid Hoffmann, Dr. Georg Broza, Dr. Hans Wittich; as well as other
colleagues virtually affiliated with our group: Martin Sussiek and Dr.
Mathias Nolte for their kind assistance and inspiring conversations.
vii
… all former students who worked on term, diploma or master theses:
Roman Mandjarov, Thomas Blisnjuk, Bala Velagala, Balaji Ponnam, Jan
Roman Pauls, Kjer Andresen, Sabine Bechtle, Claudia Pardo Garcia,
Heinrich Löwe, Daniel Manuello, Jens Rein, Hendrick Oncken, Florian
Lindstaedt, Dicky Tirta Djaja, Momchil Binev and Christian Schilling for
their valuable contributions.

… Prof. Hans-Joachim Fitting and Michael Schossig for fruitful discussions
on scanning electron microscopy.

… Cord Heineking and all members of the scientific workshop group for
processing all manufacturing orders very fast.

… Dr. Yoshinobu Shimamura for a lovely stay in Japan.

… my teachers Heinz Kalheber and Wolfgang Radkovsky for persuading
me to study physics.

… my wife Christina for her love, unfailing support and encouragement.



Hamburg, August 2009 Josef-Zoltan Lott


viii Table of Contents
1 Introduction................................................................................................1

2 Materials .....................................................................................................5
2.1 Epoxy resin systems.............................................................................5
2.2 Carbon Nanotubes ...............................................................................7
2.2.1 Nanocyl CNT .................................................................................9
2.2.2 ACVD-aligned-grown CNT ............................................................9
2.2.3 CCVD-aligned-grown CNT ..........................................................10
2.2.4 Elicarb SWCNT ...........................................................................11

3 Scanning electron microscopy analyses...............................................13
3.1 Principles of image generation ...........................................................14
3.2 Experimental procedure......................................................................16
3.3 Visualization of filler particles within a polymer matrix ........................17
3.4 Dispersion quality analyses by means of voltage contrast..................20
3.5 The influence of SEM parameters on the voltage contrast .................24
3.6 SEM analyses of poorly conductive composites.................................29
3.7 High magnification imaging of individual CNT ....................................30
3.8 Imaging of electric field induced CNT networks..................................31
3.9 Summary and conclusion ...................................................................34

4 Electrical conductivity analyses.............................................................35
4.1 Literature review .................................................................................36
4.1.1 Percolation thresholds – kinetic and statistical ............................36
4.1.2 Maximum conductivity .................................................................46
4.1.3 Percolation theory........................................................................51
4.1.4 Summary and conclusion ............................................................55
ix 4.2 Experimental procedure......................................................................57
4.2.1 Composite processing.................................................................57
4.2.1.1 Nanocyl CNT composites.....................................................57
4.2.1.2 ACVD-aligned-grown CNT composites ................................58
4.2.1.3 CCVD-aligned-grown CNT composites ................................59
4.2.2 Conductivity measurement ..........................................................59
4.2.3 Electric field alignment.................................................................59
4.3 Kinetic and statistical percolation thresholds ......................................60
4.4 Charge transport through the CNT network........................................65
4.5