Christian Müller

Flexible carbon nanotube films for high performance strain sensors.

Compared with traditional conductive fillers, carbon nanotubes (CNTs) have unique advantages, i.e., excellent mechanical properties, high electrical conductivity and thermal stability. Nanocomposites as piezoresistive films provide an interesting approach for the realization of large area strain sensors with high sensitivity and low manufacturing costs. A polymer-based nanocomposite with carbon nanomaterials as conductive filler can be deposited on a flexible substrate of choice and this leads to mechanically flexible layers. Such sensors allow the strain measurement for both integral measurement on a certain surface and local measurement at a certain position depending on the sensor geometry. Strain sensors based on carbon nanostructures can overcome several limitations of conventional strain sensors, e.g., sensitivity, adjustable measurement range and integral measurement on big surfaces. The novel technology allows realizing strain sensors which can be easily integrated even as buried layers in material systems. In this review paper, we discuss the dependence of strain sensitivity on different experimental parameters such as composition of the carbon nanomaterial/polymer layer, type of polymer, fabrication process and processing parameters. The insights about the relationship between film parameters and electromechanical properties can be used to improve the design and fabrication of CNT strain sensors.

Electromechanical Behavior of Chemically Reduced Graphene Oxide and Multi-walled Carbon Nanotube Hybrid Material

In this paper, we propose strain-sensitive thin films based on chemically reduced graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs) without adding any further surfactants. In spite of the insulating properties of the thin-film-based GO due to the presence functional groups such as hydroxyl, epoxy, and carbonyl groups in its atomic structure, a significant enhancement of the film conductivity was reached by chemical reduction with hydro-iodic acid. By optimizing the MWCNT content, a significant improvement of electrical and mechanical thin film sensitivity is realized. The optical properties and the morphology of the prepared thin films were studied using ultraviolet-visible spectroscopy (UV-Vis) and scanning electron microscope (SEM). The UV-Vis spectra showed the ability to tune the band gap of the GO by changing the MWCNT content, whereas the SEM indicated that the MWCNTs were well dissolved and coated by the GO. Investigations of the piezoresistive properties of the hybrid nanocomposite material under mechanical load show a linear trend between the electrical resistance and the applied strain. A relatively high gauge factor of 8.5 is reached compared to the commercial metallic strain gauges. The self-assembled hybrid films exhibit outstanding properties in electric conductivity, mechanical strength, and strain sensitivity, which provide a high potential for use in strain-sensing applications.

Evaluation of the piezoresistive behavior of multifunctional nanocomposites thin films

The piezoresistive properties of thin films based on carbon nanotubes dispersed in conductive polymer were investigated. Conductive multiwalled carbon nanotubes / Poly (3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) films were prepared and deposited on flexible polymer substrates (Kapton HN, 250 μm) by drop casting and the aqueous dispersions with different amounts of MWCNTs in PEDOT:PSS were prepared at different sonication times. The influences of the MWCNT load on the film properties such as electrical conductivity; strain sensitivity and hysteresis were investigated. Negative resistance coefficient was observed when the films are subjected to a force from 10 N to 110 N. Furthermore, it was observed that at low amounts of MWCNTs dispersed in PEDOT:PSS with sonication time of 15 min have better strain sensitivity then pure PEDOT:PSS and other prepared thin film composites. Moreover, the strain sensitivity was found to be strongly dependent on the MWCNT concentration. In general, MWCNT/PEDOT:PSS films have potential as a highly linear strain sensitive material.

Surface electronic structure of monolayer Sb on InP(110)

Abstract The electronic band structure of an ordered 1 × 1 overlayer of Sb on InP(110) has been investigated by angle resolved photoelectron spectroscopy using synchrotron radiation. Experimental energy dispersion of the features in the spectra has been mapped along high symmetry directions in the surface Brillouin zone. A number of criteria is employed to distinguish surface-related features and these are compared with a tight-binding, total energy minimisation calculation based on a zig-zag chain model of Sb on the InP(110) surface. Two bands near the valence band maximum are found to be in reasonable agreement with predicted states with bonding character associated with the adsorption on anion and cation sites. The origin of these and the remaining bands is discussed with reference to measurements which probe the symmetry of the states with respect to the mirror plane and the surface normal.

Transformation of epitaxial NiMnGa/InGaAs nanomembranes grown on GaAs substrates into freestanding microtubes

We report the fabrication of Ni2.7Mn0.9Ga0.4/InGaAs bilayers on GaAs (001)/InGaAs substrates by molecular beam epitaxy. To form freestanding microtubes the bilayers have been released from the substrate by strain engineering. Microtubes with up to three windings have been successfully realized by tailoring the size and strain of the bilayer. The structure and magnetic properties of both, the initial films and the rolled-up microtubes, are investigated by electron microscopy, X-ray techniques and magnetization measurements. A tetragonal lattice with c/a = 2.03 (film) and c/a = 2.01 (tube) is identified for the Ni2.7Mn0.9Ga0.4 alloy. Furthermore, a significant influence of the cylindrical geometry and strain relaxation induced by roll-up on the magnetic properties of the tube is found.

Single-wall carbon nanotubes based near-infrared sensors on flexible substrate

We demonstrate near-infrared sensors based on single wall carbon nanotubes. In order to fabricate near-infrared sensors, single wall carbon nanotubes were dispersed in the aqueous phase. Thin films of single wall carbon nanotubes dispersions have been deposited on flexible polymer substrates. The physical and optical characteristics of both the single wall carbon nanotubes dispersions and the deposited films were studied using a variety of methods. These single wall carbon nanotubes based near-infrared sensors shows excellent sensitivity up to 7.2% at 1.5 mW/mm2 laser light illumination and can be used, for instance, for a variety of scientific and industrial applications.

Characterization of the dielectric properties of multiwalled carbon nanotubes (MWCNTs) /PEDOT:PSS nanocomposites

Poly (3,4-ethylenedioxythiophene) poly (4-styrene-sulfonate) (PEDOT:PSS) polymer shows remarkable electrical properties which can be enhanced by using multi-walled carbon nanotubes (MWCNTs) as fillers. The MWCNTs/PEDOT:PSS nanocomposites containing 0.025, 0.05, 0.075 and 0.1 wt. % of MWCNTs were prepared and deposited on a flexible thin substrate (Kapton HN) by using the drop casting technique. The dielectric characterization of nanocomposite was conducted by using the electrochemical impedance spectroscopy over a wide range of frequency from 40 Hz to 110 MHz. It was observed that the real part of complex impedance decreases due to generation of more conductive paths in nanocomposite with increasing MWCNTs content and it shows the existence of both frequency dependent and frequency independent regions. The imaginary part of impedance indicates the presence of dipolar relaxation which is considered as a peak in the corresponding frequency. Furthermore, the polarization and the conduction mechanism of PEDOT:PSS nanocomposites of different MWCNTs content has been analyzed by designing an equivalent circuit based on the impedance data of the nanocomposites.

Tuning the fabrication parameters of multi-walled carbon nanotubes-epoxy based flexible strain sensitive composites

Over the past two decades, carbon nanotubes (CNTs) have attracted a great deal of interest owing to their superior electrical properties and they are being used in many engineering applications including highly sensitive strain sensors. However, realization of well-dispersed CNTs within the polymer matrix is challenging due to the strong tendency of carbon nanotubes to form bundles. Therefore, each processing parameters should be well characterized. In this work, the optimization of the processing parameters for synthesizing viable dispersions of multi-walled carbon nanotubes (MWCNTs)/epoxy based films was performed. Dispersions ranging from 0.3 wt.% to 1 wt.% MWCNTs were synthesized and deposited on flexible substrate by the stencil printing technique at different deposition speeds up to 90 mm/s. The scanning electron microscopy (SEM) images confirmed that MWCNTs are homogeneously dispersed within the epoxy matrix. Furthermore, the piezoresistive properties of composites for optimized deposition speed were also analyzed by electrochemical impedance spectroscopy (EIS). It was observed that, sensors at low concentrations shows higher sensitivity (13.6 at 0.3 wt.%). A non-linear piezoresistivity was observed at low concentrations due to dominant effect of tunneling and in contrast with that, at high concentrations sensor shows higher linearity.

Investigation of physical aging of carbon nanotube/PEDOT:PSS nanocomposites by electrochemical impedance spectroscopy

In this work, thin films based on multi-walled carbon nanotubes (MWCNT)- poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS) were prepared by solution mixing method. The dispersions were deposited on a flexible thin polyimide Kapton-HN 500 substrate by drop casting technique. The physical aging effect on the thin films as a function of MWCNT concentration ranging from 0.025 wt. % to 0.1 wt. % were investigated at room temperature by electrochemical impedance spectroscopy (EIS) over a wide range of frequency from 40 Hz to 110 MHz. It was found that the MWCNT concentration has a considerable influence not only on the conductivity but also on the aging rate of the nanocomposite films. It was also observed that the influence of aging on the electrical properties of the nanocomposites decreases with increasing amount of MWCNT concentration, due to the electron restriction mobility in the polymer chains at the vicinity of PEDOT:PSS/MWCNT interfaces. While the relative resistance change in the pure PEDOT:PSS polymer is 21.2 %, this change is found to be 6.8 % at 0.1 wt. % of MWCNT. Moreover, the aging effect on the MWCNT/PEDOT:PSS nanocomposites was considered within an equivalent complex R-C circuit model based on the obtained impedance data. This model was used to extract the electrical fitting parameters of the nanocomposites at different MWCNT concentrations.

The piezoresistive performance investigation of multifunctional genuine nanocomposites thin films

In this work, piezoresistive properties of thin films based on carbon nanomaterials dispersed in intrinsically conductive polymer were investigated. The high conductive multiwalled carbon nanotubes / graphene oxide and Poly (3,4-ethylenedioxythiophene) polymerized with poly (4-styrenesulfonate) (PEDOT:PSS) thin films were prepared and deposited on flexible polymer substrates (Kapton HN500, 125 μm) by solution drop casting. The influences of mixing ratio of MWCNT:GO to PEDOT:PSS and of the chemical reduction on the electromechanical thin film properties such as electrical conductivity and strain sensitivity were investigated. For the non-treated thin films, a negative resistance coefficient was observed for the applied strain force with low strain sensitivity around -1.9. This behavior was independent on the mixing ratio between the MWCNT:GO to PEDOT:PSS. However, and for the chemically reduced thin films, two strain regions were distinguished with sensitivity up to 92. In general, MWCNT:GO/ PEDOT:PSS films have potential as a high conductive, high strain sensitive material for advanced structural health monitoring and aerospace applications.