Computational and experimental studies of strain sensitive carbon nanotube films

• Main Author: Bu, Lei
• Other Authors: TU Chemnitz, Fakultät für Elektrotechnik und Informationstechnik/Institut für Mikrosystem- und Halbleitertechnik
• Format: Doctoral Thesis
• Language: English
• Published: Universitätsbibliothek Chemnitz 2014
• Subjects: Kohlenstoff-Nanoröhren; k-Faktor; Ultraschallszeit; Widerstandsnetzwerk; carbon nanotubes; strain sensing; resistance; gauge factor; sonication time; percolation theory; resistor network; polymer composite; exclude area; reproducibility; ddc:620; Dehnungsmessung; Widerstand; Perkolationstheorie; Reproduzierbarkeit
• Online Access: http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-156473; http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-156473; http://www.qucosa.de/fileadmin/data/qucosa/documents/15647/Bu_Lei_Dissertation_2014.pdf; http://www.qucosa.de/fileadmin/data/qucosa/documents/15647/signatur.txt.asc

The excellent electrical and mechanical properties of carbon nanotubes (CNTs) provide interesting opportunities to realize new types of strain gauges. However, there are still challenges for the further development of CNT film strain gauges, for instance the lack of design rules, the homogeneity, stability and reproducibility of CNT films. This thesis aims to address these issues from two sides: simulation and experiment. Monte Carlo simulations show that both the sheet resistance and gauge factor of CNT films are determined essentially by the two-dimensional exclude area of CNTs. It was shown, for the first time, that the variation of the CNT film gauge factor follows the percolation scaling law. The sheet resistance and gauge factor both have a power-law divergence when approaching the percolation threshold. The standard deviation of film resistances, however, also increases correspondingly. These findings of simulations provide a general guide to the tailoring of material property of CNT films in strain sensing applications: a compromise should be made between the reproducibility, conductivity and sensitivity of CNT films depending on application purposes. From the experimental side, the processing parameters for the preparation of CNT dispersions were first investigated and optimized. The reproducibility of the film resistance is significantly improved by selecting a suitable sonication time. In strain measurements it was found that for most CNT films the film resistance responses nonlinearly to the applied strain. The dependence of the film resistance on the strain can be roughly divided into two regions with nearly linear behavior respectively. The gauge factor varies with the quality of CNTs and the depositing method. A gauge factor up to 8 was achieved in the high strain region. The nonlinear response behavior was found in simulations when the CNT waviness is properly taken into account. To achieve a high gauge factor and simultaneously retain the high conductivity and reproducibility, good-quality MWCNTs were integrated in polyethylene oxide (PEO). A high gauge factor up to 10 was achieved for the composite film with CNT weight fraction of 2.5%. The resistance and gauge factor can be tuned by changing the MWCNT weight fraction with respect to PEO. A careful comparison of simulation and experiment results show that a good qualitative agreement can be achieved between them in many respects.