Surface characterization of UV irradiated nanocrystalline diamond

1
Electrical conductivity phenomena in an epoxy
resin-carbon-based materials composite
www.polito.it/micronanotech www.polito.it/carbongr
oup
M. Castellino, A. Chiolerio, M. Rovere, M.I.
Shahzad, P. Jagdale A. Tagliaferro
Applied Science Technology Department -
Polytecnich of Turin , IIT Torino - Center
for Space Human Robotics10129 Turin,
Italy micaela.castellino_at_polito.it
Aim of this work A thermoset commercial epoxy
resin, used in the automotive field, has been
chosen for this study together with 16 different
kinds of Carbon based materials 13 different
commercial Carbon NanoTubes (CNTs), including
Single and Multi-Walled CNTs both as grown and
functionalized, carbon beads and powders.
Different weight (1 and 3 wt.-)
concentrations of CBMs in polymer resin were
tried to study the electrical behaviors of the
polymer Nano-Composites (NCs). Therefore the best
composite has been chosen in order to study its
conductivity behavior much more in details (from
1 to 5 wt.-).
Introduction The effective utilization of Carbon
Based Materials (CBMs) in composite applications
depends strongly on their ability to be dispersed
individually and homogeneously within a matrix.
To maximize the advantage of CBMs as effective
reinforcement for high strength polymer
composites, they should not form aggregates and
must be well dispersed to enhance the interfacial
interaction within the matrix. Our protocol for
solution processing method includes the
dispersion of CBMs in a liquid medium by vigorous
stirring and sonication, mixing the CBMs
dispersion solvents in a polymer solution and
controlled evaporation of the solvent.
N Type Diameter (nm) Length (µm) Purity (weight )
1 Multi wall 30-50 10-20 gt 95
2 Multi wall lt 8 10-30 gt 95
3 Short thin MW 9.5 1.5 gt 95
4 Single wall 1-2 5-30 gt 90
5 Single wall 1-2 0.5-2 gt 90
6 Single wall 2 several gt 70
7 -COOH Functional SW 2 several gt 70
8 -COOH Functional MW 9.5 1.5 gt 95
9 CNT powder - - -
10 Carbon balls 1000 - -
11 Carbon mix - - -
12 Multi wall annealed lt 8 10-30 gt 95
13 Graphitized Multi wall 20-30 10-30 gt99.5
14 Multi wall 18-35 gt10 97
15 Multi wall 25-45 gt10 98.5
16 Multi wall 6-10 gt10 gt90
Resin 1 wt.- of MWCNTs type 13
Thermoset epoxy resin

Samples
Electrical Properties
Finite Element Method (FEM) simulation was
performed, using the commercial code Comsol
Multiphysics, of a composite material slab
characterized by different resistivities, having
the same dimensions of real samples, with the aim
of evaluating the volume interested by the higher
fraction of current density and estimating the
penetration depth of DC currents into the sample
thickness. An example of the simulation control
volume is given in Figure 1, where the
tetrahedral mesh of Lagrangian cubic elements is
shown. In Figure 2, the current density is
distributed almost in the whole sample, with the
exception of the portions close to the
electrodes, where the effective path avoids the
sample bottom and edges. Based on these
simulations, the effective electrical path was
estimated to be 3 mm thick (same thickness of
the sample), 3 cm width (same width of the
sample) and 1 cm long (sample length reduced by
the electrode size and dead ends).
Electrical measurements were performed using the
so called Two Point Probe (TPP) method
(Schroder, 1990) with a Keithley-238 High Current
Source Measure Unit, used as high voltage source
and nano-amperometer. NCs samples showed three
different electrical behaviors noisy, linear and
non-linear responses, which depend on dispersoids
amount and characteristics.
N CBMs(wt-) Resistance (Ohm) Comments
1 1 50 109 noisy signal
1 3 240 106 not linear
2 1 240 109 noisy signal
2 3 165 109 noisy signal
3 1 120 103 not linear
3 3 90 linear
4 1 47 109 noisy signal
4 3 17 106 almost linear
5 1 500 109 noisy signal
5 3 470 106 not linear
6 1 160 109 noisy signal
6 3 3 106 almost linear
7 1 120 109 noisy signal
7 3 11 106 almost linear
8 1 20 103 not linear
8 3 80 linear
9 1 200 109 noisy signal
9 3 100 109 noisy signal
10 1 160 109 noisy signal
10 3 190 109 noisy signal
11 1 170 109 noisy signal
11 3 120 109 noisy signal
12 1 500 109 noisy signal
12 3 120 109 noisy signal
13 1 120 109 noisy signal
13 3 170 106 noisy not linear
14 1 500 109 noisy signal
14 3 27 106 almost linear
15 1 100 109 noisy signal
15 3 2 109 almost linear
16 1 60 109 noisy signal
16 3 8 106 almost linear
Resin 3 wt.- of type 9
Noisy signal
Fluctuation-induced tunnelling
Percolation theory
Resin 1 wt.- of type 3
s ? (p-pc)t
ln s ? -Ap-1/3
D . Stauffer, et al. Introduction to percolation
theory. Taylor and Francis, London, 1994
B . Kilbride, et al., JAP 92, p. 4024, 2002
Not linear
pc 0.36 v. t 1.8
A 1.44
Resin 3 wt.- of type 8

• New model in progress...
• 3D statistical resistor network taking into
  account
• tunneling effect between neighboring CNTs
• CNTs dimensions, structure and orientation.

Linear
Conclusions A detailed electrical
characterization, made making use of
sophisticated Finite Element Method (FEM)
simulations and a careful realization of a
measurement setup, allowed to collect confident
estimates for the resistances for each of the
samples above described. Several conduction
behaviors have been found from highly conductive
NCs, which showed linear Ohmic curve (i.e.
samples 3 and 8), to non-linear diode-like trend
up till completely insulating one (R gt 109 W). We
have applied physical models such as the
percolation theory and the fluctuation-mediated
tunneling theory. Parameters extracted from the
model fitting allowed us to conclude that the
lowest percolation threshold may be found for our
resin. Nevertheless a new conductivity model is
needed, which has to take into account for CNTs
dimensions and spatial distribution inside the
polymer matrix. Some of the results here reported
have been already published in A. Chiolerio et
al (2011) Electrical properties of CNT-based
Polymeric Matrix NanoComposites. In Yellampalli
Siva (ed). Carbon Nanotubes-Polymer
Nanocomposites, p. 215-230.