Entwicklung und Charakterisierung von C-Nanoröhrchen-verstärkten keramischen SiCN-Fasern
Final Report Abstract
In the first part of this project, MWCNTs were used to reinforce SiCN ceramic fibres. The superior mechanical properties of these nanofillers can be transferred to the SiCN precursor and subsequently to the SiCN ceramic fibres. But this transference will depend on the interaction between the MWCNT and the matrix as well as its dispersibility. With the aid of a dispersant, Disperbyk 2070, composite MWCNTs/SiCN ceramic fibres were successfully prepared, but the understanding of the interaction of the system MWCNT/Dispersant/SiCN precursor was still not clear. To understand this interaction, the MWCNTs (CNT-MW F) used in the first part of the project, were compared to other 3 new types of MWCNTs (XX, CNT MW Dispersion and Baytubes C150HP), considered more purified than the CNT-MW F in terms of catalyst. The purification of CNTs is related to the existence of catalyst particles, which are used for the production of the CNTs. In this case, the use of purified MWCNTs is a better way to understand the interaction between CNTs and the matrix. After different mixture tests, the only purified MWCNT, which could be successfully added to the SiCN precursor for posterior processing of composite fibres was the type Baytube C150 HP. TEM, TGA and XPS measurements were performed for the characterization of both MWCNTs, CNT-MW F and Baytube C150HP. Transmission electron micrographs show how the MWCNTs CNT-MW F have catalyst particles attached in the tubes, while the other types are purified in terms of catalyst. In the TGA measurements in air, the purified MWCNTs (Baytubes C150HP) have a mass change of almost 100% and its higher thermal stability in comparison to the unpurified MWCNTs (CNT-MW F) is observed with the shift of the curve to a higher temperature. Different from what it would be expected, the oxygen content in the outer walls of the Baytubes is similar to the CNT-MW F and it is assumed that the homogeneity of Baytubes in the ABSE polycarbosilazane does not come from the functionalisation of their surface, but from the removal of the catalyst particles. Before mixing with the precursor, the MWCNTs were dissolved in a solvent, THF or toluene, milled for 24hs to have its length reduced to about 10µm. The solution of MWCNTs was then mixed with the ABSE precursor and dispersed in ultrasonic bath. The final mixture was distilled and a MWCNT/ABSE precursor solid spinning mass was obtained. Once this composite spinning mass contain a large number of MWCNT agglomerates, the addition of a dispersant was essential for the good dispersibility of the CNTs in the SiCN precursor. The dispersant Disperbyk 2070 eliminates the MWCNT agglomerates for amounts of CNTs up to 1wt%, but also interferes in the interaction between the MWCNT and the matrix, as reported in the first part of the project. Therefore, 10 other new different dispersants, including a solution of dispersant with already dispersed Baytubes C150HP from the BYK company were investigated. The stability of these dispersants was tested in the system MWCNTs/Dispersant/ABSE precursor. This includes the improvement of the dispersibility of MWCNTs in the ABSE, the rheological measurements of the composite spinning mass and the chemical stability of the composite spinning mass during the spinning of green fibres. Almost all dispersants reacted with the ABSE precursor, contributing to the cross-linking of the material. As a consequence of the material cross-linking, a rubbery material is formed, impeding it to be spun into fibres. Concerning the rheology of composite spinning masses, increasing the amount of MWCNTs changes the material from viscoelastic to elastic one, as the loss and storage modulus rapidly increase at low frequencies. Amounts higher than 1.5wt% of unpurified (CNT MW F) or purified (Baytubes C150HP) MWCNTs make the composite mass not spinnable. Increasing the amount of a dispersant, in this case Disperbyk 2070, there is almost no change in the rheological properties. The composite material has viscoelastic behaviour even with 3wt% of dispersant. During the melt-spinning of the composite spinning mass into green fibres, it is clear that even with the addition of >3wt% of dispersant, it is not possible to obtain homogeneous composite green fibres with more than 1wt% MWCNTs. Composite green fibres with 0.5wt% and 1.0wt% CNTs having a diameter of about 50µm were produced. Once the dispersibility of CNTs in the matrix is the main problem that arises for the production of composite fibres, thinner ceramic fibres with about 25µm were also prepared, but its reproducibility was very complicated. The curing of green fibres was performed by applying an electron beam dose of 600 kGy and energy of 10 MeV. As stated in the first part of the project, the curing of green fibres with electron beam dose is more sensitive to the molecular weight of the ABSE precursor than to the quantity of MWCNT added in the precursor (up to 5wt% CNT). This parameter is enough to cure a composite material with an ABSE precursor having a low molecular weight of about 4000g/mol and MWCNT content not higher than 2wt%. By using a continuous tube furnace, the cured fibres were pyrolysed in nitrogen atmosphere at 1100°C. Pyrolysis of the ceramic fibres at temperature of 1300°C could not be performed, due to the fact that a new furnace for this application was still in construction. The addition of MWCNTs in the composite fibres increased the tensile strength, which reaches a maximum of 1.8 GPa, but it is difficult to achieve a reproducibility of this tensile strength. This difficulty has again a relation with the dispersibility of CNTs as the deviation in the physical structure of the singular CNTs (length, diameter, number of tubes...) In relation to the microstructure of the composite fibres, intensive SEM analyses were performed. SEM micrographs enable the localization of CNTs agglomerates, pores and the analysis of the fibre surface, but no special information about the MWCNT/dispersant/precursor interaction or about the formation of gaps between MWCNTs and ceramic matrix was found with this technique. Oxidation resistance tests were also performed and there is no burning out of CNTs if they are protected from the oxidized environment. Even the agglomerates of CNTS, which are on the surface of the ceramic fibres, did not burn. The chemical resistance of the composite MWCNT/SiCN ceramic fibres against acid (HNO3) is good, since there is no optical change in its structure nor loss of mass. However, there is a loss of mass of about 15wt%, which is more a consequence of decomposition of Si-N and Si-O bonds of the ceramic, than the MWCNTs. The production of composite MWCNTs/SiCN ceramic fibres with thick diameters and a reasonable tensile strength is possible. But the commercial availability of MWCNTs with a reasonable deviation in terms of its physical structure (length, diameter, number of tubes…) and their complex dispersibility in the matrix makes the reproducibility of composite MWCNTs/SiCN ceramic fibres is very complicated and disadvantageous for fibre processing.
Publications
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"Synthesis of SiCN-precursors for fibres and matrices". Advances in Science and Technology 50 (2006) 24-30
G. Motz
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“Cross-linking via Electron Beam Treatment of a Tailored Polysilazane (ABSE) for Processing of Ceramic SiCN-Fibers”. Soft Materials 4 [2-4] (2007) 165-174
S. Kokott, G. Motz
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“Modification of the ABSE polycarbosilazane with Multi-Walled Carbon Nanotubes for the creation of spinable masses”. Mat. –wiss. u. Werkstofftech. 38 (2007) 894-900
S. Kokott, G. Motz
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“Rheology and processability of Multi-Walled Carbon Nanotubes – ABSE polycarbosilazane composites”. J. Europ. Ceram. Soc. 28 (2008) 1015-1021
S. Kokott, L. Heymann and G. Motz
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“Physicochemical Interactions between MWCNTs and the Ceramic Matrix in SiCN Fibers”. High Temperature Ceramic Materials and Composites, Page 35, 2010
O. Flores, D. Koch, W. Krenkel and G. Motz
