Polyisoprene-carbon black nanocomposites as tensile strain and pressure sensor materials
Introduction
Regardless to a wide technological use of polymer composites fundamental and applied studies of the materials [1], [2], [3], [4], [5] are still of acute interest. Electrically conductive polymer composite (ECPC) is obtained when particles of good conductors (carbon black, graphite powder, carbon fibre, micro-particles of metals) are implanted into an insulating polymer matrix. Most often such polymer composites are used in electric heating elements and resistors as the so-called thermodynamically inactive materials. Recent efforts [1], [2], [3], [4], [5] have been focused on active ECPC the conductivity of which would strongly depend on external thermodynamic parameters—pressure, temperature, and other. Such materials might become the basis for a new generation of cheap large-size sensors. In case of micro-size particles an irreversible dependence of electrical resistance at stretch or pressure has been observed [1], [2].
New interesting properties are expected if the composite contains dispersed nano-size conducting particles [3], [4], [5]. In earlier studies of composites containing conductive carbon black nano-size particles in a polyisoprene matrix [5] we have observed a reversible change of electric conductivity by many orders at stretch. In other words, a giant and reversible tenso-resistive effect is observed in the electrically conductive polymer nanocomposite (ECPNC). To develop a nano-structure model of the extremely strong tenso-resistive effect the carbon black conductive network of the insulating matrix was mapped by a conductive-tip atomic force microscope.
Experience with ferroelectrics suggests that maximum sensitivity of a thermodynamic active material to any impacts is observed at phase transitions [6]. Thus, the maximum sensitivity of ECPNC materials to thermodynamic forces may be expected at the percolation threshold of electric conductivity.
The present study of the electrical resistance of ECPNC at tensile strain and pressure near the percolation threshold mainly concerns obtaining the best composition for sensor applications.
Section snippets
Experimental
The nano-composites for studies were made of polyisoprene and highly structured nano-size conductive carbon black (NCB). For measurements of electrical resistivity versus pressure plates of 12 mm in diameter were cut of 1 mm thick cm sheets vulcanized at high pressure. Samples of the size cm were cut to study stretch deformation and dependence of the electrical resistance of ECPNC on the stretching force. Copper foil electrodes were glued on both sides at the ends and each pair of
Sensing of tensile strain
Electrical resistance R of the nanocomposites was examined with regard to the force of uniaxial stretch and the absolute mechanical deformation Δl in the direction of the acting force (Fig. 1). No change of the resistance is seen in the sample of eight mass fractions of NCB (Fig. 3) even at 100% deformation (Δl=50 mm) of the composite. Similarly, there is a negligible dependence of R on Δl at 20 mass parts of NCB. The pattern is essentially different near the percolation transition. The best
Theory and fitting the experimental results on sensing of tensile strain
A continuous insulator–conductor transition is observed in two-component systems at gradual increase of the number of randomly dispersed conductor particles in an insulator matrix. Most often such transitions (called percolation transitions) are described by the model of statistical percolation [7]. The volume concentration of conductor particles VC at which the transition proceeds is called the percolation threshold or the critical point. According to the statistical model, conductor
Conclusions
Of all the composites examined, the best results were obtained on samples with 10 mass fractions of carbon nano-particles, which apparently belonged to the region of percolation phase transition. Electrical resistance of the samples changed by more than four orders upon a 40% stretch and more than three orders upon a 0.30 MPa pressure. Resistance practically returns to its previous value after the samples are relaxed.
Nanocomposites exhibit a weak semiconductor-like thermal dependence of
Acknowledgements
The study has been supported by the Latvian Council of Science. The authors are thankful to Dr. Donats Erts and Boris Polyakov from the Institute of Chemical Physics of the University of Latvia for the AFM measurements. The BFG Stock Company is acknowledged for preparing the samples.
Maris Knite is a 1978 graduate of the Faculty of Physics and Mathematics of the University of Latvia and has been awarded the PhD degree by the same faculty in 1989. In 1999, he was awarded the Dr. habil. phys. degree by the Institute of Solid State Physics of the University of Latvia. In 1991–1992, he was a visiting scientist at the Institute of Experimental Physics at the University of Vienna, Austria. Since 1999, he is a full time professor at Solid State Physics Division of the Institute of
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Cited by (0)
Maris Knite is a 1978 graduate of the Faculty of Physics and Mathematics of the University of Latvia and has been awarded the PhD degree by the same faculty in 1989. In 1999, he was awarded the Dr. habil. phys. degree by the Institute of Solid State Physics of the University of Latvia. In 1991–1992, he was a visiting scientist at the Institute of Experimental Physics at the University of Vienna, Austria. Since 1999, he is a full time professor at Solid State Physics Division of the Institute of Technical Physics of the Riga Technical University. His field of scientific interest includes: phase transitions and optical, thermo-optical, and electro-optical properties in ferroelectrics, ferroelastics and silicides; laser-induced phase transitions in ferroelectric and silicide thin films; formation and investigation of electrically conductive nanocomposites for sensor application.
Valdis Teteris is a 1962 graduate of the Faculty of Electronics and Telecommunications of the Riga Technical University. In 1965–1968, he was postgraduate at the Moscow Energetics Institute. Since 1992, he has been a head of the Laboratory of Materials for Electric and Electronic Engineering of the Riga Technical University. His field of scientific interest includes materials for electrical and radio engineering, design of electronic equipment and devices.
Aleksandra Kiploka is a 1967 graduate of the Faculty of Physics and Mathematics of the University of Latvia. In 1975–1978, she was postgraduate at the Riga Technical University where investigated heat resistive properties of polycrystalline β-SiC films. Since 1994, she has been an assistant at the Solid State Physics Division of the Institute of Technical Physics of the Riga Technical University. Her field of scientific interest includes electrically conductive nanocomposites for sensor application.
Jevgenijs Kaupuzs is a 1984 graduate from the Faculty of Physics and Mathematics of the University of Latvia, and has been awarded the PhD degree by the Institute of Solid State Physics of University of Latvia in 1995. Since 1997, he has been a scientist-researcher at the Institute of Mathematics and Computer Science of the University of Latvia. His field of scientific interest includes: ferroelectrics; phase transitions and critical phenomena; charge transfer in semiconductors.