Robert Olejnik

Thermoelectric properties of carbon nanotube and nanofiber based ethylene-octene copolymer composites for thermoelectric devices

Polymer composites have been created from multiwalled carbon nanotubes or carbon nanofibers and ethylene-octene copolymer. The composites have thermoelectric properties and exhibit thermoelectric effect, that is, the conversion of temperature differences into electricity. The thermoelectric efficiency of created composites with nanotube or nanofiber concentration of 30wt% evaluated by a thermoelectric power at room temperature is 13.3 µV/K and 14.2 µV/K, respectively. The flexible thermoelectric device (thermopile) was constructed with three different composite legs to produce electric current and the output voltage was measured in the range of temperature difference from -15 to 25°C.

Effect of compressive strain on electric resistance of multi-wall carbon nanotube networks

The network of entangled multi-wall carbon nanotubes is shown as a conductor whose resistance is sensitive to compressive strain, both in the course of strain growth and when loading/unloading cycles are imposed. If the compression is applied, the resistance decrease is up to 25% at the maximum applied deformation. The experimental data are analysed using the Weibull distribution model and a contact network model to get an estimate of the contact resistance between carbon nanotubes and the formation of contacts in the course of compression.

An electrically conductive and organic solvent vapors detecting composite composed of an entangled network of carbon nanotubes embedded in polystyrene

A composite composed of electrically conductive entangled carbon nanotubes embedded in a polystyrene base has been prepared by the innovative procedure, when the nonwoven polystyrene filter membrane is enmeshed with carbon nanotubes. Both constituents are then interlocked by compression molding. The mechanical and electrical resistance testing show that the polymer increases nanotube network mechanical integrity, tensile strength, and the reversibility of electrical resistance in deformation cycles. Another obvious effect of the supporting polymer is the reduction of resistance temperature dependence of composite and the reproducibility of methanol vapor sensing.

Electromechanical properties of carbon nanotube networks under compression

The network of entangled multiwall carbon nanotubes and the composite consisting of a polystyrene filter-supported nanotube are introduced as conductors whose conductivity is sensitive to compressive stress both in the course of monotonic stress growth and when loading/unloading cycles are imposed. The testing has shown as much as a 100% network conductivity increase at the maximum applied stress. It indicates the favorable properties of the multiwall carbon nanotube network for its use as a stress-electric signal transducer. To model the conductivity–stress dependence, it is hypothesized that compression increases local contact forces between the nanotubes, which in turn leads to a decrease in the contact resistance between them. The lack of detailed knowledge of the mechanism as well as an unclear shift from individual contacts to the whole network conductance behavior is circumvented with a statistical approach. In this respect, the conductivity/compression data were fitted well using the Weibull distribution for the description of the nanotube contact resistance distribution.

Electromechanical Sensors Based on Carbon Nanotube Networks and Their Polymer Composites

A network of entangled multiwall carbon nanotubes and the composite consisting of filter-supported multiwall carbon nanotube network are conductors whose conductivity is sensitive to compressive stress both in the course of monotonic stress growth and when loading/unloading cycles are imposed. The testing has shown as much as 100% network conductivity increase at the maximum applied stress. The entangled carbon nanotube networks are prepared by vacuum filtration method and peeled off from the filter. The carbon nanotubes are used in pristine condition or chemically functionalized. The filter-supported entangled networks are prepared by the nanotube dispersion filtration through a non-woven flexible polystyrene filter. The nanotubes infiltrate partly into the filter surface pores and link the accumulated filtrate layer with the filtering mat. The filter-support increases nanotube network mechanical integrity, the composite tensile ultimate strength and affects favorably the composite electrical resistance. Other obvious effect of the supporting polymer is reduction of the resistance temperature dependence. Moreover, the conductivity of carbon nanotube networks manifests also organic vapor dependence. The dependence is reversible, reproducible, selective as well as influenced by nanotube oxidation.

Pre-Strain Stimulation of Electro-Mechanical Sensitivity of Carbon Nanotube Network/Polyurethane Composites

The change of electrical resistance of a highly extensible composite sensors consisting of a network of entangled multi-wall carbon nanotubes (CNTs) and thermoplastic polyurethane elastomer in the course of elongation was stimulated by initial tensile deformation. Though the initial deformation irreversibly changes the arrangement of CNT network, subsequent cyclic elongation and corresponding resistance change is stable. The resistance sensitivity, quantified by a gauge factor, which defines the sensitivity of strain sensor as the relative resistance change divided by the applied strain, increases nearly five times from the value of about five for not initially elongated composites. This is a substantial increase, which ranks the composites among materials with the highest electromechanical sensitivity. The observed sensitivity increase is discussed on basis of the cracking of a nanotube network with extension when the number of contacts between nanotubes decreases and thus the network has fewer interconnections that can carry an electric current.

Characterization of Carbon Nanotube Based Polymer Composites Through Rheology

Poly(methyl methacrylate)/Multi‐wall carbon nanotubes (MWCNT) nanocomposites were prepared by solution method. Here the dispersions of MWCNT in PMMA solutions were sonicated for appropriate time followed by dispersions coagulation. Both types of CNT materials were used such as pure MWCNT and the same MWCNT after their adequate surface treatment. The aim of treatment was to covalently attach organic material onto surface of CNT to process their better dispersion in polymeric matrix leading to more effective CNT reinforcement effect. The state of CNT dispersion was characterized thought rheology measurements with help of parameters like elasticity and viscosity of the melt. Also the effect of sonication onto pure PMMA matrix was determined.

Tuning the Molecular Sensitivity of Conductive Polymer Resistive Sensors by Chemical Functionalization

Polyaniline thin films are chemically functionalized by nucleophilic addition of thiols beareing different functional groups: carboxylic, amine, -dodecyl and sulfonic. The modification is tested using FTIR and XPS spectroscopy. Then, the films are used as resistive sensors for different volatile organic compounds in a static measuring system. The sensitivity of conducting polymers to alcohols and heptane is strongly affected by the chemical functionalization of the materials. Polyaniline show an increasing signal when the chain length of different alcohols is increased. The incorporation of hydrophilic groups on the PANI chains seems to maintain the trend. On the other hand, the polymer modified with a long alkyl chain (PANI-DOT) show negative response for methanol and the signal increases up to propanol to decrease for longer chains. PANI-DOT show the largest signal for heptane of all polymers tested. Therefore, the functional group attached to the conductive polymer chain can be used to tune the molecular sensitivity of the resistive sensor

Antenna of silver nanoparticles mounted on a flexible polymer substrate constructed using inkjet print technology

This article describes the construction of an antenna that operates at frequencies of 1.07, 1.5, and 2.49 GHz and that is fabricated on a flexible polymer substrate using inkjet printing technology. In particular, this article is focused on the preparation and characterization of an antenna starting from the ink formulation for printing a homogeneous, electrically conductive layer using silver nanoparticles. The diameter of the prepared silver nanoparticles ranges from 50 to 200 nm. The inkjet printing technology on flexible polymer substrates offers a wide range of applications where there are high demands for flexibility. In combination with the polymer substrate, inkjet printing enables the production of more complex shapes and curves for antennas that are widely applicable not only in wearable electronic devices but also in plastic cases for portable communication devices.

Improved Selectivity of Oxidized Multiwall Carbon Nanotube Network for Detection of Ethanol Vapor

Two kinds of Multiwall carbon nanotubes (MWCNT) networks “Buckypaper” were made by the vacuum filtration method of MWCNT aqueous suspension. The first one was prepared from pure CNT and the second from its oxidized form by acidic KMnO4 as oxidizing agent. The CNT oxidation increase content of oxygen bonded to the surface of CNT decreasing their hydrophobic character. The sensitivity of MWCNT networks to two kind of organic solvent vapors (ethanol and hepane) has been investigated by resistance measurements. The solvents had different polarities given by Hansen solubility parameters and nearly the same volume fractions of saturated vapors at the condition of experiment. CNT oxidation significantly increases the sensitivity of CNT resistive sensor to vapors of ethanol and decrease response to heptane vapors. The present paper demonstrates the effective way how to add proper selectivity for organic vapor detection.