Piezoresistive Effect in Carbon Nanotube FibersACS Nano 8, 11214 (2014)
Agnieszka Lekawa-Raus , Krzysztof K. K. Koziol , Alan H. Windle

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 fibres i.e. yarn-like assemblies made purely of aligned carbon nanotubes, which are expected to find applications as electrical and electronic materials. The resistivity changes of carbon nanotube fibres were measured on initial loading, through the elastic/plastic transition, on cyclic loading and on stress relaxation. The various regimes of stress/strain behaviour were modelled using a standard linear solid model, which was modified with an additional element in series to account for the observed creep behaviour. Based on the experimental and modelling results the origin of piezoresistivity is discussed. An additional effect on the resistivity was found as the fibre 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 fibre decrease i.e. the reduction in the sensitivity of fibre resistivity to loading.

DOI: 10.1021/nn503596f
Hysteresis during field emission from chemical vapor deposition synthesized carbon nanotube fibersAppl. Phys. Lett. 105, 173107
M. Cahay, P. T. Murray, T. C. Back, S. Fairchild, J. Boeckl, J. Bulmer, K. K. K. Koziol, G. Gruen, M. Sparkes, F. Orozco, W. O’Neill

Hysteresis in the field emission (FE) data of a chemical vapor synthesized carbon nanotube fiber cathode is analyzed in the regime where self-heating effects are negligible. In both the forward and reverse applied field sweeps, various FE modes of operation are identified: including Fowler-Nordheim (FN) tunneling and space-charge limited emission from the fiber tip and FN emission from the fiber sidewall. Hysteresis in the FE data is linked to the difference in the field enhancement factors in the different FE modes of operation in the forward and reverse sweeps and related to changes in the fiber morphology.
Steam reforming on reactive carbon nanotube membranesJournal of Industrial and Engineering Chemistry 25, 222 (2015)
D. Janas, S.K. Kreft, K.K. Koziol

We developed self-heated catalytic membrane made of carbon nanotubes (CNT) and used it for steam reforming. We selected the most promising route of catalyst preparation and manufactured Ni@CNT membranes. Then, we used them to produce hydrogen from methanol, ethanol and iso-propanol whilst current was passed through to deliver thermal energy required for reactions to occur. By employing Joule heating, we reached temperatures up to about 450°C and probed catalytic activity of the system. Self-heated metal-CNT hybrid has shown promises for microscale steam reforming and other chemical transformations that require efficient and uniform heating conditions.

DOI: 10.1016/j.jiec.2014.10.038
A role of nanotube dangling pyrrole and oxygen functions in the electrochemical synthesis of polypyrrole/MWCNTs hybrid materialsApplied Surface Science 317, 794 (2014)
K. Krukiewicz, A.P. Herman, R. Turczyn, K. Szymańska, K.K.K. Koziol, S. Boncel, J.K. Zak

The effect of the functionalization of multi-walled carbon nanotubes (MWCNTs) on the process of electrochemical co-deposition of MWCNTs and polypyrrole (PPy), as well as the morphology of obtained composites have been demonstrated. As the nanotube components of the hybrids, three types of MWCNT were used, namely c-CVD derived (pristine) MWCNTs, their oxidized counterparts MWCNT-Ox and pyrrole-modified MWCNT-Py. The stability of pristine and functionalized MWCNTs (f-MWCNT) dispersions in tetrahydrofuran and water was studied together with the description of the process of formation PPy/(f-)MWCNT hybrid materials via electrochemical co-deposition. The structural and morphological properties of the hybrids were characterized by Raman spectroscopy, scanning electron microscopy and atomic force microscopy revealing substantial differences among hybrid materials in their surface morphology and the influence of MWCNT functionalization on the orientation of growing PPy chains.

Aligned carbon nanotube reinforced high performance polymer composites with low erosive wearComposites Part A: Applied Science and Manufacturing, 67, 86 (2014)
J. Chen, J.A. Trevarthen, T. Deng, M.S.A. Bradley, S.S. Rahatekar, K.K.K. Koziol

The erosive wear behaviour of epoxy composites reinforced with aligned, as-produced carbon nanotube (CNT) films was investigated. The CNT film composites were fabricated in two different configurations, where the unidirectional (0°) and bi-directional (0°/90°) aligned CNT films were exposed to the particle stream. Results have shown that the unidirectional (0°) CNT film/epoxy composite exhibit superior erosive wear resistance compared to the unidirectional (0°) carbon fibre reinforced epoxy composite. Furthermore, the bi-directional (0°/90°) CNT film/epoxy composite shows even better resistance to erosion compared to the unidirectional (0°) CNT film/epoxy composite due to additional impact energy absorption resulted from CNT networks. Scanning Electron Microscopy (SEM) provides further insight into the erosive wear mechanisms of CNT film composites at different impingement angles. This work has successfully introduced aligned as-produced CNT films fabricating epoxy composites using traditional composite manufacturing processes with low erosive wear and high electrical performance which deliver potential for engineering applications.

doi: 10.1016/j.compositesa.2014.08.009
Direct evidence of delayed electroluminescence from carbon nanotubes on the macroscaleApplied Physics Letters 104, 261107 (2014)
D. Janas, N. Czechowski, S. Mackowski, K. Koziol

Spectrally resolved and kinetic response of electroluminescence was monitored from resistively heated carbon nanotube (CNT) macroassemblies. Sensitive detection system and custom-made setup for high-speed optoelectronic measurements were employed to investigate unsorted and single chirality-enriched CNTs. By increasing the content of (7,6) or (6,5) CNTs in a sample, the E11 emission peak in the infrared region became more narrow (∼150 nm), hence approaching that of commercial emitters for this spectral range. Moreover, electroluminescence initiation in CNTs occurred very rapidly and reached its full intensity within tens of milliseconds. Interestingly, observed delay between bias voltage application and electroluminescence proved triplet-triplet annihilation in the macroscopic assembly of CNTs.

doi: 10.1063/1.4886800
Carbon nanotube fiber – silver hybrid electrical conductorsMaterials Letters 133, 186 (2014)
A. Lekawa-Raus, P. Haladyj, K. Koziol

Carbon nanotubes are very promising materials for many electrical and electronic application. Particularly, it is expected that macroscopic assemblies made purely of carbon nanotubes may form highly efficient and reliable electrical conductors which for the next generation of electrical wires. It has already been demonstrated that wire-like nanotube assemblies – carbon nanotube fibres can successfully work as electrical wires, however, their performance needs further research and development. This paper presents the possible method of enhancement of electrical conductivity and current-carrying capacity of the carbon nanotube fibres via incorporation of silver nanoparticles into the nanotube network and formation of a hybrid carbon nanotube fibre-silver conductor.

doi: 10.1016/j.matlet.2014.06.177
Experimental and theoretical studies on the mechanism for chemical oxidation of multiwalled carbon nanotubes RSC Advances 4, 28826 (2014)
B.M. Maciejewska, M. Jasiurkowska-Delaporte, A.I. Vasylenko, K.K. Koziol, S. Jurga

In this study, the oxidation of multiwalled carbon nanotubes (MWCNTs) sonicated and/or refluxed in acids (H2SO4/HNO3) was investigated using a combination of high-resolution transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and ab initio computational methods. Sonication of the sample has the strongest effect, leading to the highest concentration of defects and carboxyl groups on the walls. Substantial correlations between treatment protocols, CNT size, and the types of chemical moieties are observed. Finally, based on experimental and computational results, we suggest the mechanism of the oxidation process for attaching the functional groups on the sidewalls.

DOI: 10.1039/C4RA03881A