Sebastian Geier

Carbon nanotube array actuators

Experimental investigations of highly vertically aligned carbon nanotubes (CNTs), also known as CNT-arrays, are the main focus of this paper. The free strain as result of an active material behavior is analyzed via a novel experimental setup. Previous test experiences of papers made of randomly oriented CNTs, also called Bucky-papers, reveal comparably low free strain. The anisotropy of aligned CNTs promises better performance. Via synthesis techniques like chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD), highly aligned arrays of multi-walled carbon nanotubes (MWCNTs) are synthesized. Two different types of CNT-arrays are analyzed, morphologically first, and optically tested for their active characteristics afterwards. One type of the analyzed arrays features tube lengths of 750–2000 �m with a large variety of diameters between 20 and 50 nm and a wave-like CNT-shape. The second type features a maximum, almost uniform, length of 12 �m and a constant diameter of 50 nm. Different CNT-lengths and array types are tested due to their active behavior. As result of the presented tests, it is reported that the quality of orientation is the most decisive property for excellent active behavior. Due to their alignment, CNT-arrays feature the opportunity to clarify the actuation mechanism of architectures made of CNTs.

Experimental and finite element analyses of multifunctional skins for morphing wing applications

As a consequence of operational efficiency because of rising energy costs, future transport systems need to be mission-adaptive. Especially in aircraft design the limits of lightweight construction, reduced aerodynamic drag and optimized propulsion are pushed further and further. The first two aspects can be addressed by using a morphing leading edge. Great economic advantages can be expected as a result of gapless surfaces which feature longer areas of laminar flow. Instead of focusing on the kinematics, which are already published in a great number of varieties, this paper emphasizes as major challenge, the qualification of a multi-material layup which meets the compromise of needed stiffness, flexibility and essential functions to match the flight worthiness requirements, such as erosion shielding, impact safety, lighting protection and de-icing. It is the aim to develop an gapless leading edge device and to prepare the path for higher technology readiness levels resulting in an airborne application. During several national and European projects the DLR developed a gapless smart droop nose concept, which functionality was successfully demonstrated using a two-dimensional 5 m in span prototype in low speed (up to 50 m/s) wind tunnel tests. The basic structure is made of commercially available and certified glass-fiber reinforced plastics (GFRP, Hexcel Hexply 913). This paper presents 4-point bending tests to characterize the composite with its integrated functions. The integrity and aging/fatigue issues of different material combinations are analyzed by experiments. It can be demonstrated that only by adding functional layers the mentioned requirements such as erosion-shielding or de-icing can be satisfied. The total thickness of the composite skin increases by more than 100 % when required functions are integrated as additional layers. This fact has a tremendous impact on the maximum strain of the outer surface if it features a complete monolithic build-up. Based on experimental results a numerical model can be set up for further structural optimizaton of the multi-functional laminate.

Li1.4Al0.4Ti1.6(PO4)3 Used as Solid Electrolyte for Structural Supercapacitors

Structural supercapacitors are very interesting multifunctional devices combining the properties of an electrical energy storage device and a structural component simultaneously. These types of supercapacitors are mostly equipped with solid state electrolytes, instead of traditional liquid electrolytes, avoiding leakage and safety problems and supporting the mechanical performance of the composite materials. In the present study, the Lithium-ion based solid ceramic electrolyte Li1.4Al0.4Ti1.6(PO4)3 was successfully synthesized by sol-gel method. Its electrical properties were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Results show that Li1.4Al0.4Ti1.6(PO4)3 possesses a conductivity of 2.94×10−4 S/cm at room temperature and a specific capacity of 55.57 μF/g. The as-prepared samples were embedded into fiber composite material using the aviation approved resin RTM6 with an injection process making the composite structure flexible. Subsequently, the specific capacity and conductivity were tested getting values of 53.44μF/g and 2.00×10−4 S/cm respectively. The reason for electrical properties loss was investigated by computerized tomography (CT) and EIS tests and the results provide reference for the future research.Copyright

Morphology- and ion size-induced actuation of carbon nanotube architectures

ABSTRACT Future adaptive applications require lightweight and stiff materials with high active strain but low energy consumption. A suitable combination of these properties is offered by carbon nanotube-based actuators. Papers made of carbon nanotubes (CNTs) are charged within an electrolyte, which results in an electrical field forming a double-layer of ions at their surfaces and a deflection of the papers can be detected. Until now, there is no generally accepted theory for the actuation mechanism. This study focuses on the actuation mechanism of CNT papers, which represent architectures of randomly oriented CNTs. The samples are tested electrochemically in an in-plane set-up to detect the free strain. The elastic modulus of the CNT papers is analyzed in a tensile test facility. The influence of various ion sizes of water-based electrolytes is investigated. During the tests, four parameters that have a significant influence on the mechanical performance of CNT papers were identified: the test conditions, the electrical charging, the microstructure and the ion size. All of these influencing factors point to the mechanically weak inter-tube linking at which the actuation seems to take place. Quadratic voltage-strain correlation suggests a combination of electrostatic and volumetric effects as the possible reason for CNT paper actuation. Abbreviations: CNT: carbon nanotubes; CV: cyclic voltammetry; CVD: chemical vapor deposition; HiPCO: high pressure carbon monoxide; IL: ionic liquid; MWCNT: multi-walled carbon nanotube; MW: multi-walled; NHE: normal hydrogen electrode; PDMS: polydimethylsiloxane; PMMA: polymethylmethacrylate; PPy: polypyrrole; PVDF: polyvinylidenefluoride; SCE: saturated calomel electrode; SWCNT: single-walled carbon nanotube; SW: single-walled; 1M: one molar concentration

Experimental Investigations of Actuators Based on Carbon Nanotube Architectures

Commercially successful actuators typically meet a mechanical profile which combines high flexibility and stiffness. Current smart materials used as electromechanical actuators suffer from low or unstable mechanical properties. This is the reason why these actuators are additionally fixed on structures. This kind of actuators represents an additional weight when they are switched off. A new class of carbon nanotube actuators shows promising electromechanical properties combining low density, high Young’s modulus and comparatively high free strains up to 1\(\%\). Paper-like architectures made of carbon nanotubes are tested in capacitor mode—two electrodes are immersed into an electrolyte. As a result an in-plane deflection of the electrodes can be detected. The actuation-mechanism is still subject of controversy. Different experiments indicate different physical effects. A comparison of the results reveals a possible dependency on the specimen-composition. Actuated tensile tests are carried out addressing the dependencies between specimen composition and possible physical effects. Two architectures are tested and compared: papers made of randomly oriented single-walled carbon nanotubes and multi-walled carbon nanotube-arrays, which feature single, continuous carbon nanotubes in one dimension of the specimen. The tests are conducted in dry, wet and wet/actuated condition to determine further effects of swelling and mechanical weakening. Different actuation potentials and electrolytes are tested. The mechanical performance of the carbon nanotube paper strongly depends on the conditions, which is demonstrated by a significant reduction of the Young’s modulus. Additionally, electrical charging seems to start an irreversible mechanical degradation. A general statement for CNT-arrays cannot be easily given because of the variation in the results. If the best results are considered to be the ideal results, no condition dependency can be detected. According to the experimental set-up, the sample composition and the testing method a quantum-mechanical effect might be most likely the reason for the array-actuation.

CARBON NANOTUBE STRAIN MEASUREMENTS VIA TENSILE TESTING

Discovering and characterizing new smart materials is an urgent need to close the performance-gaps of standard active materials. Still there is a need for a material of high modulus, low density and large strain. Carbon materials catch scientific attention since a while but one sort among these is of special interest, carbon nanotubes (CNTs). Beside excellent material properties another interesting feature was first mentioned 1999 - the active behavior of paper-like mats made of CNTs. The CNT-papers are electrical activated using a double-layer interaction of ions provided by an electrolyte and the charged high surface area of the paper formed by carbon tubes. Until now the mechanism behind the strain generation of CNT-based architectures is unknown but the it is of high interest. The origin of the mechanism reveals the active potential of carbon tubes to be or not to be a resilient smart material in order to use the strong carbon bonds instead of weak van der Waals force as linking between the tubes. This paper presents further investigations about the mechanical composition of CNT-papers as well as vertical aligned CNT-arrays using an actuated tensile test set-up. For better comparison the experiments are conducted in dry, wet and wet/charged conditions. Especially in the case of the CNT-arrays it is essential to preload the specimens because their curly CNT-structure superimposes the vertical orientation of the arrays. While the CNT-paper is tested in an one molar sodium chloride solution, the hydrophilic character of CNT-arrays requires an ionic liquid (IL). It is found that the mechanical properties of CNT-papers drop significantly by wetting and further more by charging what indicates an electro-static dominated effect. In contrast the CNT-arrays show similar results independent of their test-conditions and an active, reversible behavior of tube-elongation by charging. These results indicate strongly a quantum-mechanical effect of the single tubes.

Clarifying the Carbon Nanotube Actuation Mechanism: An Experimental Approach

Experimental investigations of different architectures made of pure, as produced carbon nanotubes (CNTs) are the main focus of this presented article. Different types of experimental set-ups are used to analyze the free strain of the CNT-based architectures. According to their build-up different experimental set-ups like actuated tensile tests, in-plane and out-of-plane strain measurements are realized to investigate the actuation mechanism and possible dependencies. The first analyzed architecture can be characterized as a 2D paper of randomly oriented, entangled single walled CNTs, also called Bucky-paper. In contrast the second investigated architecture consists of highly oriented, vertically aligned multi walled CNTs grown on a substrate of glassy carbon. The results are evaluated according to findings of various other material quality tests in order to find a significant statement for their possible actuation mechanisms.

Fundamental investigations of carbon nanotubes working as actuators

Excellent properties like low density, high mechanical stiffness as well as an outstanding thermal and electrical conductivity make researchers focusing on carbon nanotubes (CNTs) since years. Beside that it is found that structures made of CNTs can be actuated when they are set up like a capacitor. Usually two dimensional (2D) CNT-papers with randomly oriented CNTs, called Bucky-papers, are used. They are charged and divided by an electrically insulating but ionic conductible electrolyte. Experiments demonstrate low voltages for actuation (±1V). Although the mechanism of CNT-actuation is still an open issue theoretical studies suggest a charge and ion induced lengthening of the C-bonds, which predict theoretical strains up to 1%. These characteristics make CNTs a potential candidate for lightweight and powerful actuators of future adaptive aerospace applications. The presented work gives an overview of possible CNT-actuator configurations. Comprehensive analysis tools for 2D mats of randomly oriented CNTs have been developed to guarantee a consistent data base for the comparison of different CNT-configurations. It is focused on the electro-mechanical properties with respect to the processing and configuration of CNT-actuators. For a more efficient use of the mechanical advantages of the CNT-geometry a new aligning manufacturing approach is presented, to get highly oriented 2D CNT-papers. Their properties are compared with randomly oriented CNT-papers. Finally a new test set-up will be introduced, which enables deflection measurements directly on the top of vertically aligned CNTs (CNT-arrays). The buildup and necessary prework are shown, as well as results of the first experiments. The method of measuring along the axis of aligned CNTs qualifies this set-up to get a deeper understanding about the actuation mechanism of CNTs. Vertically aligned CNTs promise to be a more efficient actuator configuration because of their high stiffness in direction of actuation.

The preparation of a composite structure for a first large scale ground test of a smart and gapless wing leading edge

At the Institute of Composite Structures and Adaptive Systems (FA, Prof. Wiedemann) of the DLR the structure of a flexible and gapless wing leading edge has been developed for testing in large scale structure-system ground tests. The absence of gaps in a flexible wing leading edge allows for a significant noise reduction and provides an additional key technology for realizing wings with a fully natural laminar flow. In the years 2009 and 2010 the work in the project SmartLED within the 4th German Aviation Research Program (LuFo) was focused on the preparation and realization of the first ground test of the in the project developed overall system. The overall smart droop nose concept arose from the cooperation of Airbus and EADS, whereas the DLR Institute FA dealt with the structural design, the test of the material systems, the simulation of the overall system, and the development of manufacturing technologies for the composite structures to be employed in the planned tests. The detailed presentation of this work forms the content of this paper which has been made possible through the application of the process chain for composite structures established at the Institute FA of the DLR.

Investigations of the key mechanism of carbon-nanotube actuators and their dependencies

Future adaptable applications require electro-mechanical actuators with a high weight-related energy. Among modern multi-functional materials carbon nanotubes (CNTs) have some special characteristics which give them the potential to solve this demand. On the one hand raw CNTs have excellent mechanical properties like their low density (1330kg/m3) and very high estimated stiffness of about 1TPa. On the other hand CNTs have the ability under presence of ions, wired like a capacitor and activated by a charge injection to perform a dimensionchange (length of C-C bondings). Calculations and experiments present achievable active strains of 1% at low voltage of ±1V what qualifies CNT-based materials for leightweight powerful actuators. In this paper the former work done with actuators using CNT-containing mats and Nafion as solid electrolyte is evaluated by analyzing the two main-components in more detail. On the one hand the CNT-based modelmaterial SWCNT-mats called Bucky-paper (BP) and on the other hand ion donating electrolytes in liquid-phase like a NaCl-solution and its solid equivalent Nafion as thin-foils are tested. Additional methods of fabrication, preparation and characterization of the CNT-powder and the manufactured BPs containing randomly oriented single-walled carbon nanotubes (SWCNTs) are presented which provide a deeper system-understanding. Both materials (BPs and Nafion-foils) are intensively investigated in different deflection-test-rigs due to their structural assembly. This paper presents a method for electro-mechanical measurements of BPs in an in-plain test set-up which avoids sensing secondary effects like thermal expansion or mass-transport and confirm that BP-deflection should only be a capacity-driven effect. Nafion as solid electrolyte will be tested in an out-of-plane facility to measure its possible actuation within the lamellar-direction. With this approach the dependencies of each component and their individual characters on the deflection can be estimated. The active response can be referred to the internal structure of both components as well as of the whole structural assembly. The results give a certain direction to a BP-optimization referring to active strain, density, structural integrity and conductibility. In addition to these facts the active character of BPs using CNTs of different suppliers and Nafion is analyzed. These investigations are of particular importance for detection of global dependencies and using both materials in a hybrid-assembly like solid actuators which are needed for structural applications.