Forschungsbericht 2011
Development and Electrical Characterization of Aligned CNT/Polymer Nanocomposites (APNCs)
This project aims on the manufacturing and characterization of densely packed, axial aligned carbon nanotubes (CNTs) structures embedded in polymeric matrix. So called aligned polymer nanocomposites (APNCs) can exhibit orders of magnitudes higher electrical conductivity and improved thermal conductivity compared to CNT/polymer composites based dispersed CNTs and are proposed for future electronic and sensor applications. Basic idea is to fully exploit the high axial CNT properties by maximized degree of CNT orientation and maximized volume loading inside a composite. Due to high requirements on the CNT component, like orientation, low waviness and low entanglement and extreme high length, no established manufacturing processes exists. Many questions on the dependency of processing, specific CNTs properties and possible changed polymer interaction are still open and not well characterized yet. Morphological aspects have to be correlated to macroscopic electrical properties of these composites to penetrate theoretical aspects of the predominating electron transport processes.
Introduction: CNTs and polymer nanocomposites (PNCs) based on dispersedly embedded CNTs Due to the need of light weight structures with integrated functionality huge affords have been made to gain from the extraordinary intrinsic properties of carbon nanotubes (CNTs). Their large aspect ratio (millimeter length and several nanometers in diameter) leads to high surface areas which can interact with surrounding matrix. Seamless wrapped up graphene layers with strictly ordered arrangement of carbon atoms lead, due to the specific orbital configuration, to high binding energies (high stiffness and strength and thermal/chemical stability) and interesting electron transportation properties. Since their (re)discovery in 1991 theoretical and experimental research has lead to a basic knowledge of the CNT properties and advantage in CNT synthesis. Functionalization and doping intend to influence the molecular structure of CNTs in order to influence interface properties or electrical conduction. Electrical properties of CNTs can reach from semiconducting over metallic to ballistic electron transport. Besides the use of single CNTs in electronic components, like transistors, and also some intended medical use as drug delivery system, CNTs are prospected to be broadly used in polymers. First, due to the low density, high surface area, stiffness, strength and crack bridging abilities neat polymers and also fiber reinforced composites can be significantly improved. The natural disadvantage of most polymer systems, the lack of electrical conductivity, can be solved even with extreme low CNT addition. Simultaneously new functionality like mechanical-electrical sensing abilities for load monitoring and failure forecast and can be added to structural elements. These nanocomposites will have a high technological impact on light weight structures e.g. in transportation systems or wind turbines. In order to manufacture these kinds of low volume fraction nanocomposites dispersion steps are widely used. For best performance it is necessary to break up initial agglomerated and entangled CNTs (primary agglomerates of commercial CNTs) and separate them homogeneously. Advanced processing routes make use of the high shear forces in three roll milling. In re-agglomeration processes formation of electrically conductive networks and pathways of CNTs take place, which define the resulting composite properties. The analysis of network formation, morphology and their dependency on components and manufacturing steps involved is extreme complex and a broad field of investigation. Advantage of this manufacturing process is its high scalability for mass production, and already industry relevant availability of standard CNTs. Key issues of this manufacturing route are related to the technological limitations due to a high viscosity increase at higher CNT loadings >10-15 vol% and very restricted influence on the CNT-orientation. But both, higher volume fraction and the direct yield of the pronounced axial CNT properties (so highly aligned) CNTs, are key elements to increase electrical conductivity of CNT/polymer nanocomposites for use as high performance electronic conductors.
Aligned polymer nanocomposites (APNCs) based on vertically aligned CNT-forests (VACNTs) Recent advantages on CNT synthesis via several available chemical vapour deposition (CVD) processes, opens ability to synthesize arrays of aligned CNTs with superior low waviness, low structural defects and outstanding length in the millimetre range. Not yet commercial available these `vertical aligned CNTs¿ (VACNTs) or simple `CNT forests¿ raise growing attention of scientists to gain more of the intrinsic CNT properties in a CNT/polymer composite. Thus, in order to exploit the remarkable high electrical conductivity of CNTs in the axial direction, basic idea of APNCs can be described as follows: 1) In contrast to the described dispersed use of CNTs, the manufacturing of APNCs relies on high quality arrays of highly oriented CNTs. 2) In order to manufacture polymer based composites with maximized CNT volume fraction and maximized CNT orientation, mechanical compaction of the CNT forests is necessary. At this point it is crucial to ensure that the degree of CNT orientation is not negatively altered -or even better- the flexible (not structure related) waviness of CNTs is decreased. 3) A capillary force driven impregnation process with a resin/hardener system should lead to lowest possible - but complete wetting of the compacted and aligned CNTs structures.
APNCs can overcome the specific conductivity restrictions of just dispersed CNTs and are predestinated for high performance anisotropic conducting layers and sensor elements in electronically devices. Concerning the high proposed filler loadings, it is necessary to reduce inter-CNT distances into the scale of polymeric chains; the high requirement of extreme good CNT alignment should be obvious. Thus only a few groups are able to provide such high quality CNT forests for APNC production. The new electrical and mechanical anisotropic properties of APNCs are still not well known and need detailed investigations. Until now, no qualified standard method for APNC production has been evolved and established. Basic problems like low reproducibility and the ability to overcome dimensional limitation have to be overcome in order to provide further scientific investigations. Due to the experimental critical processing, technological and achievable electrical conductivity boundaries are not explored so far. This project covers a full range of aspects, including: work on increased length and improved structure of the aligned CNT-forests, work on the investigation and introduction on the several specific manufacturing steps and the characterization of electrical properties of resulting composites. In order to establish improved and new methods for APNC manufacturing, the analytical correlation between morphological aspects and resulting composite properties has to be fulfilled. Therefore methods for morphological component characterization (CNTs: TEM, SEM, TGA, Raman spectroscopy, ¿) and the resulting composites (APNCs: TEM, SEM, Raman spectroscopy, WAXS, SAXS,¿) are correlated with results of impedance spectroscopy and 4-point measurements after van der Pauw. Wissenschaftliche Kontakte und Kooperationen
Publikationen [1] J. Sumfleth, M. H. G. Wichmann, K. Prehn, S. Wedekind, K. Schulte, Compos. Sci. Technol. 70 (2010) 173 [2] J. Sandler, M. S. P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A. H. Windle, Polymer 40 (1999) 5967 [3] W. Bauhofer, J. Z. Kovacs, Compos. Sci. Technol. 69 (2009) 1486 [4] K Prehn, A. Warburg, T. Schilling, M. Bron, K. Schulte K, Compos. Sci. Technol. 69 (2009) 1570 [5] A. de la Vega, J. Z. Kovacs, W. Bauhofer, K. Schulte, Compos. Sci. Technol. 69 (2009) 1540 This project aims on the manufacturing and characterization of densely packed, axial aligned carbon nanotubes (CNTs) structures embedded in polymeric matrix. So called aligned polymer nanocomposites (APNCs) can exhibit orders of magnitudes higher electrical conductivity and improved thermal conductivity compared to CNT/polymer composites based dispersed CNTs and are proposed for future electronic and sensor applications. Basic idea is to fully exploit the high axial CNT properties by maximized degree of CNT orientation and maximized volume loading inside a composite. Due to high requirements on the CNT component, like orientation, low waviness and low entanglement and extreme high length, no established manufacturing processes exists. Many questions on the dependency of processing, specific CNTs properties and possible changed polymer interaction are still open and not well characterized yet. Morphological aspects have to be correlated to macroscopic electrical properties of these composites to penetrate theoretical aspects of the predominating electron transport processes.
Introduction: CNTs and polymer nanocomposites (PNCs) based on dispersedly embedded CNTs Due to the need of light weight structures with integrated functionality huge affords have been made to gain from the extraordinary intrinsic properties of carbon nanotubes (CNTs). Their large aspect ratio (millimeter length and several nanometers in diameter) leads to high surface areas which can interact with surrounding matrix. Seamless wrapped up graphene layers with strictly ordered arrangement of carbon atoms lead, due to the specific orbital configuration, to high binding energies (high stiffness and strength and thermal/chemical stability) and interesting electron transportation properties. Since their (re)discovery in 1991 theoretical and experimental research has lead to a basic knowledge of the CNT properties and advantage in CNT synthesis. Functionalization and doping intend to influence the molecular structure of CNTs in order to influence interface properties or electrical conduction. Electrical properties of CNTs can reach from semiconducting over metallic to ballistic electron transport. Besides the use of single CNTs in electronic components, like transistors, and also some intended medical use as drug delivery system, CNTs are prospected to be broadly used in polymers. First, due to the low density, high surface area, stiffness, strength and crack bridging abilities neat polymers and also fiber reinforced composites can be significantly improved. The natural disadvantage of most polymer systems, the lack of electrical conductivity, can be solved even with extreme low CNT addition. Simultaneously new functionality like mechanical-electrical sensing abilities for load monitoring and failure forecast and can be added to structural elements. These nanocomposites will have a high technological impact on light weight structures e.g. in transportation systems or wind turbines. In order to manufacture these kinds of low volume fraction nanocomposites dispersion steps are widely used. For best performance it is necessary to break up initial agglomerated and entangled CNTs (primary agglomerates of commercial CNTs) and separate them homogeneously. Advanced processing routes make use of the high shear forces in three roll milling. In re-agglomeration processes formation of electrically conductive networks and pathways of CNTs take place, which define the resulting composite properties. The analysis of network formation, morphology and their dependency on components and manufacturing steps involved is extreme complex and a broad field of investigation. Advantage of this manufacturing process is its high scalability for mass production, and already industry relevant availability of standard CNTs. Key issues of this manufacturing route are related to the technological limitations due to a high viscosity increase at higher CNT loadings >10-15 vol% and very restricted influence on the CNT-orientation. But both, higher volume fraction and the direct yield of the pronounced axial CNT properties (so highly aligned) CNTs, are key elements to increase electrical conductivity of CNT/polymer nanocomposites for use as high performance electronic conductors.
Aligned polymer nanocomposites (APNCs) based on vertically aligned CNT-forests (VACNTs) Recent advantages on CNT synthesis via several available chemical vapour deposition (CVD) processes, opens ability to synthesize arrays of aligned CNTs with superior low waviness, low structural defects and outstanding length in the millimetre range. Not yet commercial available these `vertical aligned CNTs¿ (VACNTs) or simple `CNT forests¿ raise growing attention of scientists to gain more of the intrinsic CNT properties in a CNT/polymer composite. Thus, in order to exploit the remarkable high electrical conductivity of CNTs in the axial direction, basic idea of APNCs can be described as follows: 1) In contrast to the described dispersed use of CNTs, the manufacturing of APNCs relies on high quality arrays of highly oriented CNTs. 2) In order to manufacture polymer based composites with maximized CNT volume fraction and maximized CNT orientation, mechanical compaction of the CNT forests is necessary. At this point it is crucial to ensure that the degree of CNT orientation is not negatively altered -or even better- the flexible (not structure related) waviness of CNTs is decreased. 3) A capillary force driven impregnation process with a resin/hardener system should lead to lowest possible - but complete wetting of the compacted and aligned CNTs structures.
APNCs can overcome the specific conductivity restrictions of just dispersed CNTs and are predestinated for high performance anisotropic conducting layers and sensor elements in electronically devices. Concerning the high proposed filler loadings, it is necessary to reduce inter-CNT distances into the scale of polymeric chains; the high requirement of extreme good CNT alignment should be obvious. Thus only a few groups are able to provide such high quality CNT forests for APNC production. The new electrical and mechanical anisotropic properties of APNCs are still not well known and need detailed investigations. Until now, no qualified standard method for APNC production has been evolved and established. Basic problems like low reproducibility and the ability to overcome dimensional limitation have to be overcome in order to provide further scientific investigations. Due to the experimental critical processing, technological and achievable electrical conductivity boundaries are not explored so far. This project covers a full range of aspects, including: work on increased length and improved structure of the aligned CNT-forests, work on the investigation and introduction on the several specific manufacturing steps and the characterization of electrical properties of resulting composites. In order to establish improved and new methods for APNC manufacturing, the analytical correlation between morphological aspects and resulting composite properties has to be fulfilled. Therefore methods for morphological component characterization (CNTs: TEM, SEM, TGA, Raman spectroscopy, ¿) and the resulting composites (APNCs: TEM, SEM, Raman spectroscopy, WAXS, SAXS,¿) are correlated with results of impedance spectroscopy and 4-point measurements after van der Pauw. Wissenschaftliche Kontakte und Kooperationen
Publikationen [1] J. Sumfleth, M. H. G. Wichmann, K. Prehn, S. Wedekind, K. Schulte, Compos. Sci. Technol. 70 (2010) 173 [2] J. Sandler, M. S. P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A. H. Windle, Polymer 40 (1999) 5967 [3] W. Bauhofer, J. Z. Kovacs, Compos. Sci. Technol. 69 (2009) 1486 [4] K Prehn, A. Warburg, T. Schilling, M. Bron, K. Schulte K, Compos. Sci. Technol. 69 (2009) 1570 [5] A. de la Vega, J. Z. Kovacs, W. Bauhofer, K. Schulte, Compos. Sci. Technol. 69 (2009) 1540 Stichworte
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