Soldering of Carbon Materials Using Transition Metal Rich AlloysACS Nano, 2015, 9 (8), pp 8099–8107
M. Burda, A. Lekawa-Raus, A. Gruszczyk and K. Koziol

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.

DOI: 10.1021/acsnano.5b02176
A computational study of the quantum transport properties of a Cu-CNT compositePhys. Chem. Chem. Phys. 17, 18273 (2015)
M. Ghorbani-Asl, P.D. Bristowe, K. Koziol

The quantum transport properties of a Cu–CNT composite are studied using a non-equilibrium Green's function approach combined with the self-consistent-charge density-functional tight-binding method. The results show that the electrical conductance of the composite depends strongly on CNT density and alignment but more weakly on chirality. Alignment with the applied bias is preferred and the conductance of the composite increases as its mass density increases.

Low temperature electrical transport in modified carbon nanotube fibers Scripta Materialia 106, 34 (2015)
Agnieszka Lekawa-Raus, Kamil Walczak, Gregory Kozlowski, Simon C. Hopkins, Mariusz Wozniak, Bartek Glowacki, Krzysztof Koziol

Carbon nanotube fibres are a new class of materials highly promising for many electrical/electronic applications. The range of applications could be extended through the modification of their electrical transport by inclusions of foreign materials. However, the changes into the electrical transport are often difficult to assess. Here, we propose that the analysis of resistance-temperature dependencies of modified fibres supported by a newly developed theoretical model may aid the research in this area and accelerate real life applications of the fibres.

Poly(3,4-ethylenedioxythiophene) growth on the surface of horizontally aligned MWCNT electrodeApplied Surface Science 335, 130 (2015)
K. Krukiewicz , J.S. Bulmer, D. Janas, , K.K.K. Koziol, J.K. Zak,

The process of conjugated polymer deposition on the surface of horizontally aligned multi-walled carbon nanotube, HA-CNT, electrode is described. Poly(3,4-ethylenedioxythiophene), PEDOT, was grown electrochemically under variable conditions in both aqueous and non-aqueous solutions of selected electrolytes. It is shown that the mechanism of nucleation highly depends on the reaction environment. The presence of a surfactant or non-aqueous medium favors the process of progressive nucleation in which the polymer growth is uniform and a homogeneous film of PEDOT is formed. It is demonstrated that the conditions make it is possible to cover the outer walls of individual HA-CNTs instead of forming thick polymer layer. The application of overpotential is proven to be a necessary condition to generate radical cations and cause relatively fast growth of polymer layer.

Influence of atmospheric water vapor on electrical performance of carbon nanotube fibresCarbon 87, 18 (2015)
A. Lekawa-Raus, L. Kurzepa, G. Kozlowski, S.C. Hopkins, M. Wozniak, D. Lukawski, B.A. Glowacki, K.K. Koziol

Carbon nanotube assemblies are expected to find application in many areas of technology. Therefore, it is of paramount importance to understand and predict their performance in different ambient conditions. Here, we explore the influence of air exposure on the electrical conduction in carbon nanotube fibres and films produced via floating catalyst chemical vapor deposition. We recognize that on top of the previously well-explored oxygen doping effect these macroscopic materials are also significantly affected by humidity. The adsorption of water vapor causes an increase in the weight of the assemblies, increase in electrical conductivity at room temperature or changes in the resistance-temperature dependence at low temperatures. It is suggested that the water vapor is mainly adsorbed by the standard clustering mechanisms observed in other carbon materials, but the mechanisms responsible for the improvement in electrical performance are much more debatable. We present a strong indication that the carbon nanotubes are neither n-doped nor p-doped by water molecules and provide further discussion on the potential role of water in the electrical transport of carbon nanotube assemblies.

Resistance-temperature dependence in carbon nanotube fibresCarbon 84, 118 (2015)
A. Lekawa-Raus, K. Walczak, G. Kozlowski, M. Wozniak, S. C. Hopkins, K. K. Koziol

The electrical transport of a carbon nanotube assembly is determined by its morphology and composition. These vary with the assembly production processes and post-process treatments applied. Here, we present the study of the electrical – structural dependence of wire like assemblies of carbon nanotubes i.e. carbon nanotube fibres produced via floating catalyst chemical vapour deposition processes. We propose that the analysis of resistance – temperature characteristics of the fibres provides vast amount of information for the assessment of the quality of the fibres and thus the efficacy of fibre production and post-production processes. To aid qualitative and quantitative analysis of the experimental results we propose a new universal model which allows the fitting of experimental data in the full range of temperatures and a straightforward comparison of the recorded characteristics.

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.