超轻材料

3486 www.advmat.de www.MaterialsViews.com wileyonlin C O M M U N IC A TI O N Matthias Mecklenburg , Arnim Schuchardt , Yogendra Rainer Adelung *, Andriy Lotnyk , Lorenz Kienle , and Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with O Performance The utilization of carbon nanomaterial’s fascinating properties, like carbon nanotubes’ (CNTs’) tensile strength of 63 GPa [ 1 ] or graphenes lowest electrical resistance [ 2 ] is typically limited by the lack of advanced structural design. CNTs directly assem- bled as macroscopic rope deliver ≈ 3 GPa. [ 3 ] Only if individual tubes get interconnected higher values can be achieved. [ 4 ] For graphene the underlying substrate is often limiting the conduc- tivity. [ 2 ] Although 3D interconnected graphene networks were recent and li not se Furthe from t synthe nectio the sy presen < 200 lattice Despi scopic Rec tures ductiv applic super high p high m and unl 200–400 ited lifet connecti Aerog robust t rial prop M. Me Istitute Hambu Denick A. Schu Prof. R Institu Functio University of Kiel Kaiserstr E-mail: ra Dr. A. Lo Institute Synthesis Universit Kaiserstr [+] Prese Permose DOI: 10 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimelibrary.com tion as an electrode material. A super lightweight variant of Aerographite is shown in Figure 1 e–h (SI, Figure S16). Here the walls of the tubes introduce another hierarchy level; they are formed from a hollow framework of ribbons which consists of a transition state between amorphous carbon (with a very high amount of sp 2 -hybridized bonds) and completely sp 2 -hybridized vitreous carbon, as confi rmed by the electrical conductance and the electron energy loss spectroscopy (EELS) spectra. [ 18 , 19 ] This hollow framework structure (network with sub network) mate- rial possesses the super low density of 180 μ g cm − 3 which is . 2, D-24143 Kiel, Germany @tf.uni-kiel.de tnyk , [+] Prof. L. Kienle for Materials Science and Real Structure y of Kiel . 2, D-24143 Kiel, Germany nt Address: Leibniz Institute of Surface Modifi cation (IOM) rstr. 15, D-04318, Leipzig, Germany .1002/adma.201200491 Adv. Mater. 2012, 24, 3486–3490 ly synthesized, they are only hypothetically free standing ghtweight (2 mg cm − 3 ), without a second phase they are lf-supporting and only stable as composite materials. [ 5 ] rmore they tend to collapse even as single tubes. [ 6 ] Thus - he engineering point of view - real life applications desire sis methods which enable tailoring of materials intercon- ns and morphological design, like recently presented by nthesis of nickel microlattices [ 7 ] (0.9 mg cm − 3 ). Here we t a novel cellular material called Aerographite (density µg cm − 3 ), more than 4 times lighter than the Ni micro- s, which were up to now the most lightweight materials. te its low density it can be fabricated in various macro- shapes in the order of several cubic centimeters. ently published results about graphene network struc- indicate the importance of highly porous, electrical con- e, and mechanical stable foam like structures [ 5 , 8 ] for the ations as electrode material for Li ion car batteries or capacitors. For those applications low specifi c weight, a enetrability, and accessibility of surfaces and especially which is freely ad over the Figure 1 cally opa structur adsorbin Raman s magnifi c reveals t intercon that pro than the a simila carbon n meter. O exhibit o mation of vanis S3) that dependi be shape Figure S effect of pure sp bons [ 15 , 1 mechan electrica cklenburg , Prof. K. Schulte of Polymers and Composites rg University of Technology estr. 15, D- 21073 Hamburg, Germany chardt , Dr. Y. K. Mishra , S. Kaps , . Adelung te for Materials Science nal Nanomaterials Kumar Mishra , Sören Kaps , Karl Schulte utstanding Mechanical echanical robustness are necessary. Cyclic loading oading with Li ions easily leads to volume changes of %. [ 9 ] Corresponding stresses are a major point of lim- ime performance, [ 9 – 11 ] thus introduction of direct inter- ons are advantageous. raphite is designed to be most lightweight but extremely o bear strong deformations. These challenging mate- erties are achived by Aerographite’s hirachical design realized by a novel single-step cvd sythesis based on justable ZnO networks. Figure 1 gives a small overview variety of the Aerographite family. The photograph of a shows that the material is completely black and opti- que despite its extreme low density. The open porous e of Aerographite is orders of magnitudes more light g than, e.g., carbon nanotube sponges as confi rmed in pectroscopy experiments. Figure 1 b–d show in different ations the distinct levels of the material. A closer look hat this variant of Aerographite consists of a seamless nected network of closed shell micro tubes, a structure vides ultra-low densities of ≈ 2 mg cm − 3 already lower lightest reported aerogels. [ 5 , 8 , 12 , 13 ] The walls possess r nanoscopic thickness ( ≈ 15 nm) like thin multiwall anotubes (MWCNTs) but have a microscale tube dia- ther than MWCNTs, the relatively large graphite planes rders of magnitude less curvature promoting the for- of manifold tube junctions over the sintering bridges hed template (Supporting Information (SI), Figure S2, are exceptions for CNT materials. [ 14 ] Furthermore, ng on the growth conditions, the graphite surface can d in a wrinkled manner (surface in Figure 1 d and SI, 4) to give a higher mechanical stability. The stabilizing wrinkled, e.g., non-hexagonal ring confi gurations, of 2 -carbon structures is also known from glass-like car- 6 ] (often called ‘vitreous carbons’), which have superior ical properties and chemical resistance [ 17 ] but are still lly conductive as it would be required for an utiliza- www.advmat.de www.MaterialsViews.com C O M one or other l sized, which the ho cies, se graphi face-ar damag or carb The and su plate. T in a br and su structu some o (SI, Fig The of vari remov by a h to me (SI, Fi achieve as we S16 ribbons, S13-S15 fi lled) are highly con- trollable by the CVD parameters respectively Figure thesis. closed-s archical l) Other variants Aerogra surface Adv. Mat 1 . Overview of different Aerographite morphologies by controlled derivations a) Photograph of macroscopic Aerographite. b–d) 3D interconnected struc hell graphitic Aerographite in different magnifi cations and TEM inset of wall. e– hollow framework confi guration of Aerographite in different magnifi cations. i– © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim der of magnitude lighter than all known aerogels and ightweight materials. Many more variants can be synthe- e.g. the low aspect ratio variant of interconnected spheres is depicted in Figure 1 i (SI, Figure S5). Furthermore, llow tubes can be fi lled with several kinds of carbon spe- e Figure 1 j-k (SI, Figures S13-S15). Due to the internal tic structure of these variants a much higher specifi c sur- ea is given but with the disadvantage that fl exibility and e tolerance are reduced to a level of conventional silica [ 13 ] on aerogels. [ 8 , 12 ] achieved structures of Aerographite adopt the network rface structure of the individual tailorable ZnO [ 20 ] tem- hus it is possible to tune the morphology of Aerographite oad manner with respect to its aspect ratios, diameters, rface structures (SI, Figure S5). To give a glimpse of the ral variety of the template material, Figure 2 a–d show f the templates, including the corrugated pipe structures ures S7b, S11b). developed CVD process allows a single-step synthesis ous carbon species with a simultaneous and complete al of the whole template material. This is introduced ydrogen gas fl ow, which reduces ZnO (SI, Figure S6) tallic Zn, which is precipitated to the exhaust system gures S7-S10). It is important to point out that the d carbon structures and degree of crystalline order ll as the morphology (SI, Figures S2-S4 graphitic, ments a structur measur structur rarely reported and challen impressive low density of framework structure (Fi exhibits outstanding mec and tensile strength and th Aerographite reacts with a unidirectional tensile and ible plastic deformation As a demonstration of th even possible to regain th Aerographite bulk body 100 micrometers, by subs shape, indicating an extre leading to marginal differ stress. These features a mechanical tensile loads w disentangling of CNTs or shows a sponge like behav the modulus increased to see SI, Figure S20. A te duces a pronounced self-s nism behind this could b the hollow amorphous ca load direction. The hollow ρ = 0.180 mg cm − 3 exhib ultimate tensile strength ( of Aerographite. i) Aerographite network in low aspect bubble-like confi guration. j-k) phite with nano porous graphite fi lling. l) Hollow corrugated pipe design of Aerographite by detailed adoption of template shape. er. 2012, 24, 3486–3490 3487wileyonlinelibrary.com nd Van der Waals forces. In fact this al defi ciency is the reason why tensile ements of CNT-assembled graphitic es and also silica aerogels are just ging to measure. [ 22 ] Just owing to the < 200 μ g cm − 3 the hierarchical hollow gure 1 e–g), fortifi ed by sp 3 -bonds, hanical robustness, specifi c stiffness, erefore superior merit indices. [ 23 ] The viscoelastic deformation behavior on compression stresses with a revers- part, see Figure 3 a (SI, Figure S21). is mechanical robust behavior, it is e full shape of an initial 3 mm high after compression down to a few equent expansion back to its original me low Poisson ratio (SI, Video S1), ences between true and engineering llow the application of remarkable ithout losing structural integrity, e.g., small fl akes of graphite. The material ior under compression, for 95% strain 160 kPa in a 8.5 mg cm − 3 sample, nsile and compression cycling intro- tiffening (inset Figure 3 c). A mecha- e a reorganization and alignment of rbon ribbon structure parallel to the framework variant with a density of its a smooth fracture behavior at an UTS) of 1 kPa what is superior to all M U N IC A TIO N by the design of the ZnO templates (SI, Figure S6). Microscopy of an evolutionary state (SI, Figures S7-10) and the results of TEM (SI, Figures S4, S12), EFTEM (SI, Figures S10,S12), XRD, Raman spectros- copy, and TGA measurements at different Aerographite types are discussed in the sup- porting information where one can also fi nd suggested growth model (SI, Figure S1). For mechanical testing distinct examples of the Aerographite family were created and loaded under cyclic compression and tensile stress. Tests with different geometries showed that cylindrical test specimens provided the lowest experimental error (see Experimental Section). SI, Figure S19 shows a viewgraph of the Young’s modulus in compression versus the density. This graph indicates the change in the Young’s modulus in compression for different densities and the corresponding structure types. It must be pointed out that Aerographite can carry signifi cant tensile loads as its network structure exhibits tube junctions which originate from the ZnO tem- plate. This robustness in tensile loading is an exception for foam like carbon structures which are usually kept together by entangle- of syn- ture of h) Hier- 3488 www.advmat.de www.MaterialsViews.com wileyonl C O M M U N IC A TI O N the amount of the stored electrolyte, the ion mobility and the volume of the individual report relatio higher deliver more, obtain ical Ae tion is fi rst pl resp. 2 S18). W of Aer are st low de could shield stand First e double confi rm An Ae the rec is evap of the Figure O tem- plates w tural va interme plate re pattern Aerogra Aerogra 2 . Growth process and analysis of Aerographite. a) Photograph of utilized Zn inelibrary.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Aerogra conduct pression samples inset) t porosity epoxy s turing ( We have here introdu lightweight, electrically c fl exible graphite based m lished synthesis for other graphene, the here used plate for the synthesis o has been proved that thi able substrate/template m e.g., CNTs and graphene. of the Aerographite fam network of microstructu ness. Variants come as fi or as a super lightweigh struts from amorphous c tuned from graphitic to advantage of remarkable lightweight material rea specifi c moduli observed parameters like, e.g., pore bridges keeps the opportu mechanical performance like conductivity, fl exibilit structural integrity, high o high temperature stability ed graphene or CNT based macroscopic bulk samples. In n to its density Aerographite bears an at least 35 times force as compared to the best silica based aerogel, which s an UTS of 16 kPa at density of 100 mg cm − 3 . [ 13 ] Further- a Young’s modulus of 15 kPa (slope in Figure 3 c) can be ed. This modulus at such a low density brings a hypothet- rographite free bending bar (min. defl ection if cross sec- variable by E/ ρ 2 or if the height is variable by E/ ρ 3 ) to the ace in every density selection map (0.463 × 106 m 5 kg − 1 s − 2 .572 × 106 m 8 kg − 2 s − 2 ), see Figure 3 b (SI, Figures S17, hile a straight forward free standing panel application ographite is more of academic interest, these numbers ill very important in technological applications where nsities lead to low acceleration induced forces. These be microelectromechanical systems (MEMS), electrical ing or conductive electrode applications that should with- high accelerations, e.g., caused by vibrations or impacts. xperiments with Aerographite electrodes in an electric -layer capacitor (EDLC, also known as supercapacitor) ed the mechanical robustness in a real life application. rographite electrode loaded with electrolyte even endures rystallization of the salt, which occurs when the solvent orated during drying. This procedure induced cracking activated carbon reference electrode. In order to optimize ith a volume of 1.77 cm 3 . b-d) SEM micrographs showing two examples for the struc- riety of ZnO crystal templates, here with a corrugated surface. e) Photograph of an diate state of a sample on its way from ZnO to Aerographite. f-h) SEM revealing tem- moval and formation of carbon layers (Figures S7-S10). i-k) TEM micrograph, SAED , and EELS spectrum revealing graphitic state and sp 2 -hybridization of closed shell phite. l-n) TEM micrograph, SAED pattern, and EELS spectrum of hollow framework phite revealing amorphous carbon with a high content of sp 2 -hybridization. [ 19 ] phite family exhibit a much higher ivity of 37 S m − 1 at a mechanical com- of 24%. The cm-scaled Aerographite are super hydrophobic (Figure 3 b herefore their properties and high enable an excellent wetting with ystems for nanocomposite manufac- SI, Figure S22). ced the novel synthesis of an ultra- onductive, mechanical robust, and aterial. In contrast to already estab- carbon nanostructures like CNTs or CVD process employs ZnO as tem- f bulk samples on the cm 3 -scale. It s inorganic semiconductor is a suit- aterial for sp 2 hybridized carbons, [ 25 , 26 ] The common structural motive ily is the completely interconnected res with a nanoscopic wall thick- lled and unfi lled, corrugated walls, t example of a hollow framework of arbon. The atomic structure can be glass-like pyrolytic carbon, with the mechanical properties. This most ches the highest merit indices for until now. Further optimization of size and volume density of sintering nity for future improvements of the of Aerographite. Further properties y and compressibility without losing ptical adsorption and X-ray opacity, a and chemical resistance, the bearing Adv. Mater. 2012, 24, 3486–3490 electrode, an electrolyte loaded Aerographite electrode can be compressed till it has the optimal balance between these parameters. From CNT based electrodes [ 24 ] it is known that increasing electrode length decreases the capacity. CNT electrodes with 600 μ m length show a power density of ≈ 2.3 Wh kg − 1 while 4000 μ m long Aerographite electrodes still show 1.25 Wh kg − 1 which is a promising starting point for the optimization. Besides the structural properties, the application as electrode material requires a good electrical conductivity. Referring to this, Aerographite reaches a conductivity of 0.2 S m − 1 at a den- sity of 180 μ g cm − 3 . By increasing the density under compres- sion in air, e.g., to reach optimal free volume to surface ratios, the conductivity increases to 0.8 S m − 1 (0.2 mg cm − 3 ), see inset of Figure 3 a. The increase for this light weight material is mainly linear over a wide defor- mation range of at least ε ≈ 30% which gives chance for easy electro-mechanical sensing applications. However samples of the heavy (50 mg cm − 3 ) closed shell modifi cations of www.advmat.de www.MaterialsViews.com C O M mm; d = 110 mm): At a constant temperature profi le of 200 ° C in injection zone and 760 ° C in main of bici to o for sup from of deg to s sion stru Exp T disc poly 1 to rate typi esta in. [ 2 0.8 10 m C grap tem Figu test max con den drop Aero effe mod Adv. re 3 . Mechanical and electrical properties of Aerographite. a) Tensile and comp of the 180 μ g cm − 3 Aerographite variant (Figure 1e, SI, Figure S16), under tensile l imum force and displacement was 31.1 mN and 0.4 mm. The inset shows the e ductivity against compression. b) Material selection map, plotting the Young’s mo sity 3 against the specifi c volume, parameters obtained from. [ 7 , 8 , 21 ] The inset shows let on the surface of an Aerographite sample proving the superhydrophobicity graphite. c) Tensile test of Aerographite (180 μ g cm − 3 ) until fracture. Inset: self-st © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Online Library or from the author. Acknowledgements We gratefully acknowledge par Z1, R. A. DFG Heisenberg pro grant and M. M. the Landesex Joachim Herz Stiftung. tensile and compressive loads, and the super hydropho- ty making it a remarkable multifunctional material. Next thers, potential applications might be electrode materials the increasing demand of batteries and high surface area ercapacitor materials. Proper designed carbon materials the family of Aerographites could avoid typical problems electrode materials like low mechanical cycling stability, enerating electrical contacts, or non-optimized electrolyte urface ratio which might be tuned in by simple compres- due to the negligible Poisson’s ratio of these sponge-like ctures. erimental Section emplates preparation : The ZnO templates were produced in a recently overed method. [ 27 ] Zn powder (grain size 1–5 μ m) was mixed with vinyl butyral powder (Kuraray Europe GmbH) in a mass ratio of 2. This mixture was heated in a muffl e furnace with a heating of 60 ° C min − 1 to 900 ° C. After 30 min a loose powder of cally ZnO tetrapods was obtained. Further synthesis methods and blished recipes for the fabrication of ZnO structures can be found 0 ] Compressed powder (densities ranging from 0.15 g cm − 3 to g m − 3 ) was reheated for 3 to 4 h at 1200 ° C in ceramic rings (h = m, d = 15 mm) to form junctions. VD-Synthesis : Basic Aerographite confi guration (hollow, with closed hitic shells) can be gained by