Hyper-elastic Pressure Sensors
Temperature Dependence of Piezoresistivity of Polyisoprene – Nanostructured
Carbon Composite
Juris Zavickis, Maris Knite, Artis Linarts and Raimonds Orlovs
Institute of Technical Physics, Riga Technical University, Azenes Street 14/24-322, Riga, Latvia
Keywords: Temperature Dependence, Piezoresistivity, Polyisoprene, Carbon Black, Composite.
Abstract: Our previous efforts revealed polyisoprene-nanostructured carbon composite as prospective sensitive
material for elaboration of entirely hyper-elastic piezoresistive pressure sensor element. In this article we
investigate the temperature dependence on initial electrical resistivity as well as piezoresistive properties of
such material and self-elaborated hyper-elastic pressure sensing element. Certain conclusions about the
effect of temperature on electroconductive structure and piezoresistivity are made.
1 INTRODUCTION
Pressure sensing in various ambient conditions may
play important role in modern industrial
applications. Recent attempts show polyisoprene –
nanostructured carbon composite (PNCBC) as a
prospective piezoresistive material for elaboration of
hyperelastic pressure sensors (Zavickis, 2011).
These sensors can be made in various sizes and
shapes so they can be easily tailored to specific
applications. If functional multi layer approach is
used, hyper-elastic pressure sensor can be easily
realised (Knite, 2009; Knite, 2008). These sensors
are proved to be functional for pressures up to 1
MPa and can be successfully used in different
industrial and engineering applications, like civil
security, industrial monitoring, robotic skin (Huang,
2011), medical (Chang, 2012), traffic surveillance
and many more. Additionally to that PNCBCs show
multifunctional sensing properties (Knite, 2007).
Hence polymer matrix composites are mostly linked
with comparably large coefficients of thermal
expansion and their mechanical properties typically
have strong temperature dependence, it’s
anticipated, that operational temperature will have
noticeable effect on PNCBCs electrical and
piezoresistive properties. All forementioned
branches may cover significantly large interval of
operating temperatures, therefore it’s very important
to know the permissible operational temperature
range of the PNCBC sensor element as well the
dependence of output of sensors electrical parameter
on operational temperature.
In our work we determine the temperature
dependence of single PNCBC element, as well we
elaborate layered hyper-elastic pressure sensor
element (LHPS), consisting only of PNCBC structural
parts. In the end we measure the time dependence of
piezoresistivity of ready-made LHPS element.
2 SAMPLES AND
EXPERIMENTAL
Previously known composition was used to
elaborate piezoresistive composite (Knite, 2004):
Natural polyisoprene caoutchouc was mixed with
necessary curing ingredients (sulphur, zinc oxide,
stearic acid and N – Cyclohexyl – 2 - Benzothiazole
Sulfenamide) and various concentrations of high
structure carbon black (HSCB) (Degussa Printex
Xe2 with average primary particle diameter 30nm,
DBP absorption 380ml/100g, specific surface area of
950m
2
/g) in Baltic Rubber factory (BRF) using roll
mixing. The concentration of the fillers was
expressed in mass parts per hundred rubber (p.h.r.).
The raw composition was vulcanized between two
parallel brass foil inserts under pressure in hot steel
mould using Rondol thermostated press for 15
minutes under 30 atmospheres of pressure at 150°C
to obtain finished polyisopene – nanostructured
carbon black composite (PNCBC). The brass foil
494
Zavickis J., Knite M., Linarts A. and Orlovs R..
Hyper-elastic Pressure Sensors - Temperature Dependence of Piezoresistivity of Polyisoprene – Nanostructured Carbon Composite.
DOI: 10.5220/0004038204940498
In Proceedings of the 9th International Conference on Informatics in Control, Automation and Robotics (ICINCO-2012), pages 494-498
ISBN: 978-989-8565-21-1
Copyright
c
2012 SCITEPRESS (Science and Technology Publications, Lda.)
inserts were used to provide good electrical
connection to the samples. We acquired cylindrical
shape samples with 18 mm in diameter and 1 mm in
height. The vulcanization conditions were
determined at BRF using Monsanto 100 dynamic
rheometer. Before any measurements were made the
samples were shelf aged at room temperature for at
least 24 hours. To determine electrical properties of
each PNCBC sample the electrical resistivity was
measured using a Keithley 6487
Picoammeter/Voltage source. Temperature
dependent resistivity measurements were conducted
using temperature control system Linkam THMSE
600 coupled with data acquisition unit Agilent
34970A. Mechano-electrical properties of the
samples were determined using Zwick/Roell Z2.5
universal material testing machine coupled with
Agilent 34970A data acquisition/switch unit.
Samples whose initial electrical resistivity was
higher than 10
8
Ω were not tested for piezoresistivity
due to technical limitation of measuring equipment.
Based on these results certain raw rubber
compositions were chosen to elaborate a LHPS. The
piezoresistive response of LHPS under 1 atmosphere
of pressure in different ambient temperatures was
determined using Zwick/Roell Z2.5 materials testing
machine additionally equipped with temperature
chamber and coupled with Agilent 34970A data
acquisition/switch unit. Test temperatures have been
chosen according to ASTM D1349-99 standard
“Standard practice for rubber-standard temperatures
for testing”.
2.1 Selection of Materials for LHPS.
Figure 1: The specific electrical resistivity as a function of
the HSCB fraction.
To select the most suitable PNCBC
compositions for LHPS elements a series of tests
was performed. The electrical resistivity of PNCBC
samples was measured at first. Figure 1 displays the
electrical percolation threshold of PNCBC
depending on the HSCB concentration.
As it is seen from Figure 1 the percolation
transition appears to be in the region from 5 to 9
p.h.r. of HSCB. According to the concept of
piezoresistivity, the composites are most sensitive to
external force if the concentration of conductive
filler is maintained within the range of percolation
(Balberg, 2002). The piezoresistive properties of
PNCBC samples with 7 to 9 p.h.r of HSCB under
external pressure for up to 1 MPa are shown in
Figure 2.
Figure 2: Piezoresistive behaviour of PNCBC with
different concentrations of electroconductive filler for up
to 1 MPa of uniaxial pressure.
Initial resistivity of PNCBC with lower HSCB
fractions was too high to successfully perform this
test. The speed of loading and unloading was kept
constant at 10 kPa/s. The maximal piezoresistive
response was observed for PNCBC samples
containing 8 p.h.r. of HSCB.
2.2 Elaboration of LHPS
LHPS prototype was elaborated consisting of outer
shell of non-conductive natural rubber and multiple
functional layers of PNCBC with two HSCB
concentrations. The pressure sensitive layer was
made from PNCBC with 8 p.h.r. of HSCB since the
piezoresistivity of this composition was found to be
highest – relative change of electrical resistivity
observed under operational pressure of 1 MPa was
more than 100 %. PNCBC with 10 p.h.r. of HSCB
was used to produce hyper-elastic electrode layers
on both sides of sensitive layer. All three layers were
connected in series and incorporated into non-
conductive natural rubber shell without CB filler.
Hyper-elasticPressureSensors-TemperatureDependenceofPiezoresistivityofPolyisoprene-NanostructuredCarbon
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495
Two small wires with end soldered brass foil
extensions were added to electrode layers to connect
the LHPS to measuring equipment. All structural
elements (layers) were semi-vulcanized under
pressure of 3 MPa at 140 °C for 7 minutes just
enough so they could maintain their shape. During
final vulcanization all elements were assembled
together in necessary configuration and cured under
pressure of 3 MPa at 150 °C for 11 minutes.
Figure 3: The schematic cross-cut of LHPS, consisting of:
1 – non conductive outer shell, 2 – piezoresistive PNCBC,
3 – electroconductive PNCBC, 4 – brass foil electrode, 5 –
wires.
Figure 3 shows schematic construction of LHPS
consisting of 3 active layers with wires incorporated
into protective rubber shell. Figure 4 shows the
picture of functional prototype of LHPS.
Figure 4: The image of ready-made LHPS element.
3 TEMPERTURE DEPENDENCE
OF THE ELECTRICAL
RESISTIVITY AND
PIEZORESISTIVE EFFECT
As it can been seen in Figure 1, the PNCBC exhibits
sharp and distinct percolation transition in the range
of 5 to 9 p.h.r. of HSCB. According to theory (Knite,
2004), the composition belonging to the middle of
percolation should exhibit most extensive
piezoresistivity. The determination of piezoresistive
effect was done for PNCBC samples with 7, 8 and 9
p.h.r of HSCB and samples with 8 p.h.r of
conductive filler appeared to be the most sensitive to
applied external pressure (Figure 2). Based on these
results, the piezoresistive layer for
LFHPS was
elaborated with 8 p.h.r. of HSCB.
First of all the temperature dependence of
specific electrical resistivity was determined for
PNCBC sample containing 8 p.h.r. of HSCB. Figure
5 shows positive thermal coefficient of resistivity
(PTC) that can be explained with large difference in
thermal expansion coefficients of polymer matrix
and carbon nanostructure filler (Balberg, 2004).
Therefore the thermal expansion of the elastomer
matrix leads to the partial reorganization of
electroconductive network increasing the overall
electrical resistivity of the composite. The large
coefficient of thermal expansion causes the sufficient
broadening of tunnel junctions existing in the
composite (Knite, 2004) with subsequent exponential
rise of resistivity versus temperature.
Figure 5: The temperature dependence of specific
electrical resistivity of PNCBC with 8 p.h.r. of HSCB at
elevated temperatures.
The piezoresistivity of LHPS under 0,1 MPa of
pressure was determined in various ambient
temperatures (23, 40, 55, 70 and 85 °C) which were
chosen according to standard ASTM D1349-99.
LHPS was tested for mechano-electrical properties
after it was kept and stabilized in each regime of
elevated temperature for at least 1 hour.
Temperature inside the chamber was monitored
using thermocouple.
Figure 6 shows gradual increase of piezoresistive
sensitivity with the increase of operating
ICINCO2012-9thInternationalConferenceonInformaticsinControl,AutomationandRobotics
496
temperature. This can be explained with decrease of
elastic modulus, as the sensor becomes softer thus
increasing the cross-deformation under similar
operational pressures and providing greater
structural mobility of particles of electro-conductive
filler at elevated temperatures.
Figure 6: Comparison of piezoresistive behaviour of LHPS
in various ambient temperatures for 1
st
loading cycle.
Figure 7: The relative change of electrical resistivity of
LHPS under cyclic operational pressure up to 100 kPa at
23 °C.
Figure 8: The relative change of electrical resistivity of LHPS
under cyclic operational pressure up to 100 kPa at 40 °C.
Figure 9: The relative change of electrical resistivity of
LHPS under cyclic operational pressure up to 100 kPa at
55 °C.
Figure 10: The relative change of electrical resistivity of
LHPS under cyclic operational pressure up to 100 kPa at
70 °C.
Figure 11: The relative change of electrical resistivity of
LHPS under cyclic operational pressure up to 100 kPa at
85 °C.
Hyper-elasticPressureSensors-TemperatureDependenceofPiezoresistivityofPolyisoprene-NanostructuredCarbon
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497
In the same time from Figures 7-11 it can be
seen, that the increase of operating temperature has
positive effect on repeatability of the piezoresistive
effect during cyclic loading. The noticeable decrease
of the initial electrical resistivity at the start of each
loading cycle is observed only for operating
temperatures of 55 °C and below as the initial
resistivity for repeated cycles shifts lower. If the
operating temperature was at least 70 °C or higher,
only negligible drift of initial resistivity was
observed. Thus it can be seen, that LHPS performs
considerably better at elevated temperatures which
can be attributed to better mobility of filler particles
allowing the more rapid mechanical modification
and sequencial recombination of electroconductive
network of HSCB particles to occur.
4 CONCLUSIONS
The observed positive thermal coefficient of
resistivity (PTC) for PNCBC with 8 p.h.r. of HSCB
could be explained with large difference in thermal
expansion coefficients for both: polymer matrix and
HSCB filler causing the sufficient broadening of
tunnel junctions with subsequent exponential rise of
electrical resistivity versus temperature.
Behaviour of LHPS at evaluated operating
temperatures has been successfully investigated.
It has been found out that the sensitivity of
LHPS enhances gradually with the increase of
operating temperature. The increase of the operating
temperature improves the repeatability of the
piezoresistive cycles due to greater mobility of
particles of electroconductive filler and better partial
destruction/recombination of electroconductive
structure in the PNCBC.
ACKNOWLEDGEMENTS
The research was supported by Ministry of
Education and Science of the Republic of Latvia.
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