Agnieszka Lekawa-Raus

Piezoresistive Effect in Carbon Nanotube Fibers

The complex structure of the macroscopic assemblies of carbon nanotubes and variable intrinsic piezoresistivity of nanotubes themselves lead to highly interesting piezoresistive performance of this new type of conductive material. Here, we present an in-depth study of the piezoresistive effect in carbon nanotube fibers, i.e., yarnlike assemblies made purely of aligned carbon nanotubes, which are expected to find applications as electrical and electronic materials. The resistivity changes of carbon nanotube fibers were measured on initial loading, through the elastic/plastic transition, on cyclic loading and on stress relaxation. The various regimes of stress/strain behavior were modeled using a standard linear solid model, which was modified with an additional element in series to account for the observed creep behavior. On the basis of the experimental and modeling results, the origin of piezoresistivity is discussed. An additional effect on the resistivity was found as the fiber was held under load which led to observations of the effect of humidity and the associated water adsorption level on the resistivity. We show that the equilibrium uptake of moisture leads to the decrease in gauge factor of the fiber decrease, i.e., the reduction in the sensitivity of fiber resistivity to loading.

Soldering of Carbon Materials Using Transition Metal Rich Alloys

Joining of carbon materials via soldering has not been possible up to now due to lack of wetting of carbons by metals at standard soldering temperatures. This issue has been a severely restricting factor for many potential electrical/electronic and mechanical applications of nanostructured and conventional carbon materials. Here we demonstrate the formation of alloys that enable soldering of these structures. By addition of several percent (2.5-5%) of transition metal such as chromium or nickel to a standard lead-free soldering tin based alloy we obtained a solder that can be applied using a commercial soldering iron at typical soldering temperatures of approximately 350 °C and at ambient conditions. The use of this solder enables the formation of mechanically strong and electrically conductive joints between carbon materials and, when supported by a simple two-step technique, can successfully bond carbon structures to any metal terminal. It has been shown using optical and scanning electron microscope images as well as X-ray diffraction patterns and energy dispersive X-ray mapping that the successful formation of carbon-solder bonds is possible, first, thanks to the uniform nonreactive dispersion of transition metals in the tin-based matrix. Further, during the soldering process, these free elements diffuse into the carbon-alloy border with no formation of brazing-like carbides, which would damage the surface of the carbon materials.

Extreme Magneto-transport of Bulk Carbon Nanotubes in Sorted Electronic Concentrations and Aligned High Performance Fiber

We explored high-field (60 T) magneto-resistance (MR) with two carbon nanotube (CNT) material classes: (1) unaligned single-wall CNTs (SWCNT) films with controlled metallic SWCNT concentrations and doping degree and (2) CNT fiber with aligned, long-length microstructure. All unaligned SWCNT films showed localized hopping transport where high-field MR saturation definitively supports spin polarization instead of a more prevalent wave function shrinking mechanism. Nitric acid exposure induced an insulator to metal transition and reduced the positive MR component. Aligned CNT fiber, already on the metal side of the insulator to metal transition, had positive MR without saturation and was assigned to classical MR involving electronic mobility. Subtracting high-field fits from the aligned fiber’s MR yielded an unconfounded negative MR, which was assigned to weak localization. It is concluded that fluctuation induced tunnelling, an extrinsic transport model accounting for most of the aligned fiber’s room temperature resistance, appears to lack MR field dependence.

Graphene nano-flakes and carbon nanotube-based sensors via screen printing technology for acetone gases detection

In the world of constant facilities of human life, as well as improving the comfort of patients, there are more and more reports on non-invasive methods of testing and health related procedures. One of the most common invasive procedures performed by patients is the procedure of taking blood from the fingertip to the glucose test. It is not surprising, therefore, that the attention of researchers around the world is focused on eliminating the need for invasiveness of these tests. The tendency to facilitate and minimize interference in body coherence concerns all tests with which diabetic patients come in contact. In the light of this trend, a promising idea seems to be the possibility of non-invasive measurements of one of the conditions associated with diabetes - ketoacidosis. Such novel and non-invasive procedure is for example monitoring the amount of acetone exhaled with air by a diabetics suffering from ketoacidosis. In this work we present the sensors of acetone vapors based on titanium oxide and graphene nano-flakes or carbon nanotubes fabricated using a screen printing technology on the ceramic substrate. We have also performed test of sensitivity of fabricated sensors into the acetone gases presence in both room temperature and 150° degrees.

Carbon nanotube fibers doped with iron via Fenton reaction

In a wildly spreading research on carbon nanotube fibers as a potential material for the future, one of the most promising fields are electrical and electronic engineering. As it was mentioned repeatedly the main thing that need to be dealt with for a serious consideration of carbon nanotube structures in application as good conducting wires is a necessity of improvement their electrical conductivity values. In the many possibilities of such electrical properties improve, one of the best is chemical doping. In this work we present a oxidative doping treatment on carbon nanotube fibers via Fenton reaction. However the first assumptions on introduction hydroxide ion doping has changed after performing experiments. The reaction resulted in iron doping on carbon nanotube fibers. Such a result most probably is associated with a great reactivity of carbon nanotube with iron particles. This reactivity is being used in carbon nanotube structures production procedure, due to catalytic action of iron in the issue of carbon nanotube synthesis. This result, however differs from intentions gave us new carbon nanotube-iron composite, which seems to have a great potential for further research.