Surface characterization of UV irradiated nanocrystalline diamond

Surface characterization of UV irradiated nanocrystalline diamond

Electrical conductivity phenomena in
an epoxy resin-carbon-based
materials composite

www.polito.it/micronanotech
www.polito.it/carbongroup

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 Robotics
10129 Turin, Italy
[email protected]

Aim of this work

Introduction

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 MultiWalled 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.-%).

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.

epoxy

Samples

Thermoset
resin

+

N

Type

Diameter
(nm)

1
2
3
4
5
6
7

Multi wall
Multi wall
Short thin MW
Single wall
Single wall
Single wall
-COOH Functional
SW
-COOH Functional
MW
CNT powder
Carbon balls
Carbon mix
Multi wall annealed
Graphitized Multi
wall
Multi wall
Multi wall
Multi wall

8
9
10
11
12
13
14
15
16

30-50
<8 9.5 1-2 1-2 2 2 Lengt h (m)m) 10-20 10-30 1.5 5-30 0.5-2 several several Purity (weight %) > 95
> 95
> 95
> 90
> 90
> 70
> 70

9.5

1.5

> 95

1000
<8 20-30 10-30 10-30 > 95
>99.5

18-35
25-45
6-10

>10
>10
>10

97
98.5
>90

Resin + 1 wt.-% of MWCNTs type
13

Electrical Properties
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.
20

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).

Resin + 3 wt.-% of type 9

Voltage (V)

10

0

-10

Noisy signal

-20
-2,0x10

20

-10

0,0

-10

-1,0x10

Current (A)

-10

1,0x10

2,0x10

2- Current density distribution in
a composite volume

-10

Fluctuation-induced tunnelling

Percolation theory

Resin + 1 wt.-% of type 3

(p-pt theory. Taylor and Francis,
D . Stauffer, et al. Introduction to percolation
p
)
c
London, 1994

10

Voltage (V)

1- Mesh distribution on the
control
volume of the FEM simulation

ln -Ap-1/3
B . Kilbride, et al., JAP 92, p. 4024, 2002

0

-1

10

-1

0

-3

10

-2

-20

Not linear
-1,8x10

-4

-1,2x10

-4

-5

-6,0x10

0,0

Current (A)

-5

-4

6,0x10

1,2x10

-4

1,8x10

-7

10

-9

10

Exp
Fit

-11

10

pc = 0.36 v.%
t = 1.8

-13

10
10

-15

10

Resin + 3 wt.-% of type 8

0,0

0,3

0,6

Volume (%)

0,9

1,2

log (Conductivity)

-10

Conductivity (S/cm)

-5

10

-3
-4
-5

Exp
Fit

-6
-7

A = 1.44

-8
3,5

4,0

4,5

5,0

5,5

6,0

6,5

7,0

(Volume fraction)^-1/3

Voltage (V)

5

New model in progress...

0

3D statistical resistor network taking into account:

-5

tunneling effect between neighboring CNTs;
Linear

-10
-1

-1,0x10

-2

-5,0x10

0,0

Current (A)

-2

5,0x10

CNTs dimensions, structure and orientation.

-1

1,0x10

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 > 109 ).
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.

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