AaComposite Materials Based on Carbon

7/24/2019 AaComposite Materials Based on Carbon

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Composite materials based on Carbon Nanotubes for aerospace

applications

S. Belluccia*, C. Balasubramaniana, F. Manciab, M. Marchettic, M. Regic, F. TomboliniaaINFN-Laboratori Nazionali di Frascati, Via E. Fermi 40, 00044 Frascati, Italy

bC.S.M. Centro Sviluppo Materiali, Via di Castel Romano 100, 00128 Roma, ItalycUniversity of Rome La Sapienza, Department of Aeronautics and Astronautics Engineering

Via Eudossiana 18, 00184 Roma, Italy

ABSTRACT

Electrical and mechanical properties of composite materials based on Carbon Nanotubes are considered for aerospaceapplications. Nanostructured materials gained great importance in the past decade, owing to their wide ranging potential

applications in many areas, e.g. mechanical, structural, sensor, biomedical, electronics. Of particular interest are carbonnanotubes, which can be used as a main constituent of composite materials with exceptional mechanical and electrical

properties, very suitable for aerospace applications, also due to their light weight, mechanical strength and flexibility. We

present results obtained recently in our laboratories concerning the electrical and mechanical properties (including resilience

measurement, stress analysis, conductivity) of carbon nanotubes we synthesized by arc discharge and other techniques,embedded in a polymer matrix.

Keywords:Carbon Nanotubes, Composite materials, Mechanical properties, Aerospace structuress, Anisogrid lattices

1. INTRODUCTION

Carbon nanotubes (CNTs) can be distinguished into: single wall nanotubes (SWNTs), i.e. a cylinder with only one externalwall; multi wall nanotubes (MWNTs), made of multiple coaxial cylinders. The SWNT parameters are the diameter and the

chiral vector (n,m) or helicity, i.e. the graphite rollup direction wth respect to the cylinder axis. On the other hand, a MWNT

is made of coaxial SWNTs. The main properties of CNTs are: mechanical properties; conductivity; gas adsorbtion and

capillarity; field emission (FE); fluorescence.The mechanical resistence of CNTs is due to one of the strongest bonds in nature, i.e. the C-C sp

3 one. Their flexibility

allows one to bend CNTs repeatedly up to 90 without breaking or damaging them. The exceptional mechanical propertiesof CNTs find two different applications: the strengthening of fibers in high-performance composite materials, replacing

standard C fibers, kevlar, glass fibers; the probes for scanning tunneling microscopes.

Owing to their extreme properties, CNTs yield suitable candidates for aerospace applications, such as CNT-based electron

field emitters and composite materials for extremal conditions (aerospace environment, large temperature and pressuregradients, radiation etc.). Their high structural perfection, lightweight and chemical inertness may be ideal for spacecraft

atmosphere re-entering applications.

2. CARBON NANOTUBES LIFECYCLE: PHASES AND REQUIREMENTS

CNTs can be synthesized by different methods (see e.g. [1-4]), including arc discharge, laser vaporization, chemical vapourdeposition (CVD). The lat ter yields the most important technique for potential industrial applications (see Table 1).

One needs to implement purification techniques, as synthesized CNTs are always contaminated by u nwanted elements, suchas: other forms of Carbon; metallic particles from the catalyst; substrate granules (for CVD). The role of defects in CNTs

consists essentially in increasing the range of application of their properties (similarly to what happens for doping in

semiconductors). Defects can be induced by various methods, e.g. high energy ion bombardment, high thermal processes.

Defects are useful in many applications, as they can be used to functionalize the NTs and change their properties

considerably. Defect induced property enhancement can be observed in connection with the mechanical properties of CNTs.

Changes in stiffness are observed, i.e. the stiffness decreases with the unavoidable topological defects (intrinsically forming

in the synthesis pro cess) and increases with functionalization (on the defect si tes); defect generation and growth areobserved during the plastic deformation and fracture of NTs; composite properties can be improved with chemical bonding

between matrix and NT.

Table 1 Lifecycle of CNTs.

Third Intl. Conf. on Experimental Mechanics and Third Conf. of the Asian Committee

on Experimental Mechanics, edited by Quan, Chau, Asundi, Wong, Lim, Proc. of SPIEVol. 5852 (SPIE, Bellingham, WA, 2005) 0277-786X/05/$15 doi: 10.1117/12.621441

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3. MATERIALS AND SYNTHESIS CRITERIA

Our DC arc plasma, struck between two graphite rods technique yields high quantity of CNTs. We completed recently athermal CVD, useful for patterned substrate and large area deposition.SWNTs and MWNTs are obtained in our laboratories

under varying synthesis conditions, using different parameters e.g. the plasma current, thermal gradients. The samples arestudied with electron microscopy (Fig. 1, coll. CNR-IFN, Rome, Italy) for determining optimal conditions for maximum

yield of CNTs (in relation to amorphous material, onion-like strutures, nanoparticles and so on) [5-7].

High-yield raw material:

cathodic deposit obtained by a

arc-discharge technique, (SEM

imaging in coll. with CNR-

IFN, Roma).

SEM image of a nanotube carpet

synthesized at INFN-LNF with a

different arc-discharge setup, (SEM

imaging in coll. with CNR-IFN, Roma).

Fig. 1 SEM images of CNTs synthesized at INFN-LNF.

PLANNING AND

SYNTHESIS

CHARACTERIZATION

MANUFACTURING TECHNOLOGICAL PROCESS

NTS LIFECYCLE

FUNCTIONALIZATION

APPLICATIONS

Requirements:Choice and use of the

procedure, chemical

composition of the materialand the substrate for the

selected application

Methods :

Inert Gas Phasecondensation

Plasma Synthesis (CVD,

Arc discharge)Laser ablationLaser PyrolysisSol-gel SynthesisElectrochem. etching

Ball milling

Requirements :Define and measure the

properties of the deposited

NTsMethods :

SEMTEMRaman spectroscopyAFM

STMFTIRXRDParticle Channeling

Requirements:Post-syntesis treatment of

the NTs, for aligning the

achieved properties withthe expected ones

Methods :

Ion-Beam BombardmentUV Laser Light IrradiationChemical Purification

Use of e.m. fields

PatterningInteraction sample-substrate (AFM)

Note: Some techniquescan be applied during

deposition, for achieving

adequate properties

Requirements:Manufacturing of Prototypes of

the selected applicative driver,

to be forwarded to the industrialand the production phases

Methods :

Diverse, depending on the

specific selected application

Main Applications

Biological and space research

Micro/nanostructured films, forX-ray sources and gasdischarge tubes.

Vacuum technology, sensors,electro- medical devices

Applications in radiotherapy.

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Concerning the characterization of INFN-LNF CNTs, a morphological analysis of our samples by SEM, TEM, AFM, STM

yields ratio and dimensions of the CNTs. SEM images show that the ratio of NTs is very high (more than 70%). SWNTs

have an average diameter 1.3 nm and a length of several microns. They exist in bundles of 20 40 nm transverse size.MWNTs have a wide range of diameter (20 60 nm). Through AFM studies of our samples we evaluated, for different

types of substrate, the friction coefficient of the nanotube with the substrate surface. Contact mode AFM was used for theimaging under a constant force (coll. IMM-CSIC Madrid, Spain ). The nanotube samples were initially dispersed in

isopropyl alcohol and sonicated for a few hours. Drops of this were then suspended on three different substrates, namelyhighly oriented pyrolithic graphite (HOPG), Silicon wafer and Mica sheet (Fig. 2).

600nm

H

O

P

G

600nm

Si

600nm

m

i

c

a

Fig. 2 AFM images of CNT on (a) HOPG (b) Mica and (c) Silicon surface

The adhesion of nanotubes was found to be highest for HOPG, as compared to mica and silicon (where the nanotubes got

displaced easily by the AFM tip). Also it was observed that the nanotubes got laterally compressed on a HOPG surface (dueto Van der Waals interaction of the nanotubes with the substrate) to a ratio of 0.5. For mica and silicon there was lateral

elongation of the order of nearly 3 times , indicating that the interaction of the tube with tip was far greater than the

interaction of the tube surface with substrate. From such results we can say e.g. that mica and silicon can be used for

nanomechanics and HOPG for applications where strong bonding of nanotubes to substrates is required.

4. DISPERSION TEST OF NANOMETRIC PARTICLES IN AN EPOXY RESIN MATRIX

In the manufacturing of nanostructured composite materials [8], the primary step is the definition of a procedure necessaryto obtain an homogeneous dispersion of a nanometric particles in the polymeric matrix (epoxy resin). The mechanical

behaviour of this new kind of composite material is related to the interface between the resin and the nanoparticles(chemical characterization, adhesion problems, etc.). The materials employed in manufacturing composite samples are:

commercial epoxy resin curing agent: developed by Chemical Department of La Sapienza University Rome

nanometric graphite powder with carbon nanotubes graphite powder granulometry 20 m.

Fig. 3 (left) shows an examples of the specimens produced (dimension: 10x10x120 mm) with a different percentage of

embedded nanoparticles (0%, 10% and 20% in wt) [9-11].

Fig. 3 Specimens for mechanical tests (left); Optical analysis of composite sample (right)

The following curing process were adopted:

room temperature curing x 24 hrs furnace curing 80 C x 3 hrs.

Dynamic tests where performed obtaining the following preliminary results:1. the reduction of powder granulometry increases the impact resistance properties

2. a good surface finishing improves the mechanical properties.

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Fig. 3 (right) shows the fracture surface of a sample containing 20 % of nanoparticles. The pre-crack length is 2 mm. A

brittle behaviour of crack propagation is evidenced. With the SEM characterizations it is possible to understand the fracture -

mechanic composite behaviour. Fig. 4 shows the SEM images of the samples containing 10% or 20% of powders. In thearea A (crack initiation) and B (propagation) there is no presence of preferentialdirections of crack propagation.

Fig. 4 SEM analysis of fracture surface of composite specimen:10% powder addition (left); 20% addition (center and right)

Instead, in the sample containing 20% powder, preferential directions of crack propagations are observed (Fig. 4 center,

zones A,B, C andD). The presence of a preferential direction is due to the non-uniformity of powders dispersion in the

matrix and to the curing process. With another SEM investigation, it is possible to observe a fracture lines change directionin correspondence of cavities (or voids). In Fig. 4 right, two fracture lines (A & B) are deviated by the presence of a void

(see points C & D), and are stopped in point E. The next step is to apply this new composite material to the aerospacestructures. The anisogrid lattice (Fig. 5) is a particular structure configuration characterized by helicoidal rib (resistant to the

compression load), and circumferential rib (to ensure stability against local and global buckling).

Fig. 5 Anisogrid lattice structures

With the assigned values of the radius (R) and height ( H) of a cylindrical structure, of the applied external compression load

(W) and of the employed material, it is possible to calculate the ribs dimensions (Fig. 5) with the Vasiliev model [12],satisfying the following three requirements:

Minimum mass of the element Static resistance Local and global buckling stability.

The Vasiliev model is only a preliminary design. A FEM analysis is always requested (Fig. 6, left). A MATLAB programs

was developed to evaluate the dimensions of the anisogrid element, when a set of the structures characteristic dimensionsare given (radius R, height H and the applied load W). With the Vasilev model it is possible to calculate the variation of the

geometry of the anisogrid element, when radius Rand height H of the structure are simultaneously varied (see in Fig. 6,center, the plot 3D of the mass structure vs. RandH). This is a important application for the launchers design.

Fig. 6 FEM analysis (left); Structure Mass Mvs Radius Rand Length H (center); Dynamic analysis (right)




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It is necessary to verify that, in correspondence of structure calculated dimension (with the Vasiliev Model), the applied

load is exactly the buckling load (unitary eigenvalue). Different kinds of load and constraints distributions have been

studied. An eigenvalue 0.98 was reached, thanks to the following configuration: The load is uniformly distributed on all the nodes of the FEM model

The basement of the structure is constrained against translation All the remaining nodes of FEM model are constrained against rotation.

The 0.98 eigenvalue demonstrates that, with the above loads and constraints configuration, the buckling stability in theminimum mass (Vasiliev model) is verified. With the same configuration it is possible to satisfy the static resistance

condition. Also a dynamic analysis of the element has been performed (Fig. 6, right). This analysis is important for theintegration of a space system (for example a satellite) inside a launcher. The Vasiliev model is useful, in order to evaluate

the mass reduction of the structure, when different materials (for typical aerospace applications) are used.In particular: a) aluminium alloy Al 2024, b) composite epoxy resin carbon fibres reinforced (Hs/Ep), c) composite epoxy

resin reinforced by carbon fibres and by a dispersion of carbon nanotubes, 5% by weight. The Vasiliev Theory is applied on

a cylindrical anisogrid lattice geometry (radius R=1.5 m, height H = 4 m, applied load W = 3 MN) (Table 2).

Table 2 Structure Mass calculated for different materials

Table 2 shows how the use of CNTs, with the given particular geometric configuration (anisogrid), yields the possibility todesign a very advanced aerospace structure. The validation of the structure requires the following steps :

preliminary design numerical FEM analysis prototype design prototype manufacturing final test.After a computerised calculation and verification by FEM analysis, a flat prototype of anisogrid lattice structure was

produced. The final target is the realization of a full scale anisogrid cylindrical prototype.

After a preliminary design (Fig. 7, left, dimensions: 21x17 cm), the mould was prepared (Fig. 7, center) by traditional

mechanical tooling. For the manufacturing of the prototype the following materials are used: epoxy resin curing agent (Triaethylentetramin)

glass fibres (12000 per single filament) nanometric powder (graphite) + carbon nanotubes,with the curing process below

room temperature (21 C) curing x 24 hours furnace curing at 80 C x 3 hrs.

Fig. 7 Structure preliminary design (left); Mould (center); Flat anisogrid lattice prototype (right)

Fig. 7 (right) shows the produced prototype after some preliminary and successful mechanical tests (vibration and tension

applied loads). For a future industrialization in the anisogrid elements manufacturing, a CAD design was performed (seeFig. 8, left ), necessary to realization (with the rapid prototyping technology) of a positive simulacra (Fig. 8, right). Thenegative mould was obtained in Si resin (Fig. 9).

Fig. 8 CAD design of the anisogrid lattice flat element (left); Rapid prototyping positive mould (center);

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Fig. 9 Negative silicon mould and the relative flat anisogrid prototype manufactured

For the cylindrical geometry the same procedure is necessary (Fig. 10).

Fig. 10 Cylindrical anisogrid element manufacturing procedure

5. CONCLUSIONS

The addition of carbon nanotubes in a composite material (epoxy fibre reinforced polymers) improves the mechanicalproperties. In particular, the increase of the Young Modulus offers the possibility to further reduce the mass of the structure

in correspondence of the same mechanical properties and performance. Besides, with the innovative design of anisogrid

lattice structures (manufactured with composite materials containing carbon nanotubes), it is possible to obtain a strong

overall mass reduction of the structural component, maintaining the stability requirements.

REFERENCES

1. H. Takikama, O. Kusano, T. Sakakibara,Appl. Phys. 32, 2433 (1999).2. H. Zhang, et al. ,Physica B 325, 224 (2003).

3. H. Zeng, L. Zhu, G. Hao, R. Sheng, Carbon 36 , 259 (1998).

4. H. Lange, et al., Carbon 41, 1617 (2003).5. S. Bellucci, Nanotubes for Particle Channeling, Radiation and Electron Sources, in "Relativistic Channeling and

Coherent Phenomena", special issue of Nucl. Instr. and Meth. B, edited by S. Bellucci, V. Biryukov (Assisting Editors), (inpress): web site http://www.lnf.infn.it/conference/2004/rc2004

6. S. Bellucci, "Nanotubi di Carbonio come sorgente di elettroni", I Simposio sulle Tecnologie Avanzate: Sviluppo

delle nanotecnologie: applicazioni per la Difesa, Roma, Italy, 30/06/2004, V Reparto Tecnologico, Ministero della Difesa.

7. S. Bellucci, "Carbon nanotubes: physics and applications", Phys. Stat . Sol. (c) (to appear), Proc. XVI InternationalConference on Defects in Insulating Materials, Riga, Latvia, July 2004.

8. K. Lau, D. Hui, Composite: Part B 33, 263 (2002).9. M. Regi, et al. ,Aerotecnica Missili e Spazio , 83 , n 1.

10. M. Regi, M. Marchetti, F. Mancia, G. Allegri,Aerotecnica Missili e Spazio , 82, n 4.11. M. Regi, F. Mancia, M. Marchetti, Produzione e caratterizzazione di nanotubi in carbonio, I Simposio sulle

Tecnologie Avanzate: Sviluppo delle nanotecnologie: applicazioni per la Difesa, Roma, Italy, 30/06/2004, V RepartoTecnologico, Ministero della Difesa.

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*[email protected]; phone 39 06 9403-2888 (or -8222); fax 39 06 9403-2427 (or -2716)


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