Rosenburg, Felix (2018)
The state of carbon and the piezoresistive effect in silicon oxycarbide ceramics.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
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Item Type: | Ph.D. Thesis | ||||
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Type of entry: | Primary publication | ||||
Title: | The state of carbon and the piezoresistive effect in silicon oxycarbide ceramics | ||||
Language: | English | ||||
Referees: | Riedel, Prof. Dr. Ralf ; Werthschützky, Prof. Dr. Roland | ||||
Date: | 1 June 2018 | ||||
Place of Publication: | Darmstadt | ||||
Date of oral examination: | 8 March 2018 | ||||
Abstract: | The present work reports on the morphology of carbon, the electrical properties and the piezoresistve effect in polymer-derived silicon oxycarbides (SiOC/C nanocomposites) within carbon concentrations of 1 to 45 vol.%. The nanocomposites have been prepared by pyrolysis of poly-organosilicon precursors or preceramic polymers and a subsequent densification step using spark plasma sintering (1000 < T < 1800 °C; P = 50 MPa, Argon). The obtained samples are characterized by the means of spectroscopic (Raman, TGA-FTIR, XRD, XPS) and electrical (dc conductivity, impedance, Hall effect) investigations. The SiOC/C composites consist of a glassy matrix (silica, SiOxC4-x), silicon carbide and segregated carbon and is simplified as a two phase system (glass/carbon) for the description of the electrical and piezoresistive properties. The state of carbon within SiOC/C depends on the carbon content of the precursor and the thermal treatment. According to UV Raman the microstructure of carbon changes from a disordered (amorphous) to nano-crystalline state within 1000 < T < 1800 °C. The progressive ordering or graphitization, respectively, is illustrated by the increasing lateral crystal size (7.5 < La < 20 nm) and the corresponding decrease of the density of defects as derived from the intensity ratio AD/AG of the Raman D- and G-band. Vacancies have been identified as main type of defects. The electrical and piezoresitive properties of SiOC/C mainly depend on the state of carbon with the exception of samples with very low carbon content (C < 1 vol.%). For samples treated at T = 1600 °C the resistivity decreases by several orders of magnitude at a critical carbon concentration of about 6 vol.%. For samples treated at 1400 and 1100 °C the percolation threshold shifts from 8 to 20 vol.% of carbon because of the lower degree of graphitization. Beyond the percolation threshold weakly activated transport (Ea < 0.1 eV) occurs in conjugated sp² bonds within a continuous 3D network. The charge carrier mobility (μ ≈ 3 cm²/Vs) and density (N = 1018 – 1020 cm-³) of carriers are comparable to those of glassy carbon. A change from band-like transport to conduction in localized states at the percolation threshold is indicated by an increase in Ea ≈ 0.3 eV and presumably arises from electron/hole confinement when the localization length approaches a lateral crystal size of La ≲ 10 nm (La decreases from about 8 to 4 nm within 1 to 10 vol.% carbon). SiOC/C changes its resistivity with strain and, accordingly, the piezoresistive effect also relies on the carbon content and its degree of graphitization. Within 1 to 6 vol.% the gauge factor (GF) increases up to the percolation threshold (GF ≈ 45) and appears to arise from charge carrier tunneling. The extremely high GF values (> 1000) at the percolation threshold suggest either divergence of the GF vs. carbon concentration function or local strain concentration. Above the percolation threshold GF is related to the density of defects within the continuous carbon network. A strain-induced increase of the density of states near EF is identified as origin of the piezoresistive effect, a change in mobility is unable to explain the experimental decrease of the resistance with applied strain. The obtain results on the piezoresistivity favor the development of high-temperature strain sensors able to detect static and dynamic excitation. As a proof of concept a demonstrator to sense the natural frequency of a planetary gear has been developed. |
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URN: | urn:nbn:de:tuda-tuprints-75175 | ||||
Classification DDC: | 500 Science and mathematics > 500 Science 500 Science and mathematics > 540 Chemistry 600 Technology, medicine, applied sciences > 620 Engineering and machine engineering |
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Divisions: | 11 Department of Materials and Earth Sciences > Material Science > Dispersive Solids | ||||
Date Deposited: | 10 Jul 2018 13:29 | ||||
Last Modified: | 09 Jul 2020 02:08 | ||||
URI: | https://tuprints.ulb.tu-darmstadt.de/id/eprint/7517 | ||||
PPN: | 433466758 | ||||
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