Presentation on “A Study of Carbon Nanotubes as

Smart Reinforcements for Glass/Glass Ceramic Matrix Composites”

PREPARED BY:-RAHUL DUBEY

26 OCT 2016 COMPOSITE MATERIALS MM 5001 ODD SEM(2016-17)

INTRODUCTION Carbon Nanotubes (CNT) have amazing physical and

mechanical properties. Only atomic scale defects attributed to their nano size. Extensively small size provides very high surface area

for matrix-reinforcement interaction. Being used in polymeric, metallic and ceramic matrix

composites. Not as popular in amorphous glasses and partially

crystalline glass ceramics silicate based composites. The enhancement of fracture toughness along with other

mechanical, functional and technological properties of these composites with brittle matrices are of interest.

CARBON NANOTUBESLong, thin cylinders of carbon.Discovered in 1991 by Sumio Iijima. They can be thought of as a sheet of graphite (a

hexagonal lattice of carbon) rolled into a cylinder. Graphene layers in CNTs are rolled up to form

seamless hollow tubes. An individual atomic carbon layer forms a single

walled carbon nanotube.Multiple layers rolled up concentrically constitute

a multi-walled carbon nanotube.

CARBON NANOTUBES

A structure containing pentagons or half fullerenes closes the tube ends.

The interlayer spacing of multi-walled CNTs is around 3.35 Ǻ.

The diameter of the innermost tube can be as small as 0.4 nm and up to a large fraction of the outer diameter, which possibly can reach more than 100 nm.

These intriguing structures have sparked much excitement in recent years and a large amount of research has been dedicated to their understanding.

PRODUCTION OF CNTS Electric Arc Discharge(EAD)

Laser Ablation or Evaporation(LAE)

Chemical Vapour Deposition(CVD)

ELECTRIC ARC DISCHARGE20-40V VOLTAGE

AND 50-100A CURRENT

PASSED THROUGH GRAPHITE

ELECTRODES OF 6-12 mm DIA

ELECTRODE DISTANCE 1-4 mm

PLACED IN INERT ATM

CARBON VAPOURISES FROM ANODE

MWCNTs DEPOSIT ON THE CATHODE AS

SOFT DARK BLACK FIBROUS MATERIAL

LASER ABLATION OR EVAPORATION

Introduced in 1995

Heating done using a high power laser

Inert gas flow drives the vapourized carbon atoms on

a copper collector

Transition Metal Catalysts used to give

SWCNTs

LAE

CHEMICAL VAPOUR DEPOSITION OR CATALYTIC GROWTH PROCESSSOURCE OF

CARBON (HC OR CO)

HEATED INSIDE A QUARTZ TUBE AT

500-1100°C

PRESENCE OF A CATALYST AND

INERT ATM OF He OR Ar

HC DECOMPOSES INTO H AND C

CARBON ATOMS REARRANGE

INTO HEXAGONAL NETWORKS

CNTs GROW ON THE METAL

CATALYST

KEY PROPERTIES• By measurement of the amplitude of their

thermal vibrations in a transmission electron microscope, the moduli value of MWCNTs and SWCNTs were found to be 1.8 TPa and 1.25 Tpa respectively.

• Axial Stiffness of highly crystalline CNTs = 1.1 TPa

• CNTs produced by CVD have lower stiffness (large number of structural defects)

Tensile Strength

Modulus of Elasticity

•Theoretical Tensile Strength = 75-135 GPa •Tensile loadings of SWCNTs lead to the failure by brittle fracture mode.•MWCNTs fail in tension via a sword-in-sheath mechanism; bending failure of multi walled CNTs is due to stepwise or brittle fractures.•Presence of defects is responsible for the low strength values observed experimentally. The defects provide ideal sites for failure.

•Metallic single walled CNTs = 105-106 S/m•Semiconductor single walled CNTs = 10 S/m•In-plane conductivity of graphite = 2.5×106 S/m (for comparison)•Multi walled CNTs = 20-2×107 S/m•The presence of defects affect the measured electrical conductivity of CNTs significantly.

Electrical Conductivity

• Theoretically predicted value for individual single walled CNTs = 6600W/mK

• The experimental thermal conductivities of individual multi walled CNTs are very high, e.g. 3000 W/mK

Thermal Conductivity

CRACK BRIDGING MECHANISM

APPLICATIONS OF CNTsSupercapacitorsTransparent electrodes for organic light emitting diodesLithium ion batteriesNanowiresField-effect transistorsMolecular switches, sensors and filtersElectrochemical electrodes and catalyst supports Immobilization of biomacromoleculesOrganic reaction catalysisNanoscale reinforcing elements in composites

GLASS AND GLASS-CERAMIC MATRIX COMPOSITES

Glasses and glass-ceramics are favourite matrix materials for aerospace, automotive, electronics, biomedical and other specialized field applications because of the flexibility in terms of composition, properties and processing.

It is possible to manufacture glasses with a wide range of thermal expansion coefficients (TECs) which match the TECs of the reinforcement.

High composite densities can be achieved by promoting viscous flow during sintering by controlling the viscosity of glasses.

Manufacturing Process: CNT Reinforced Glass/glass-ceramic matrix composites

Manufacturing Requirements:The availability of high quality CNTs with intrinsically

good mechanical properties.A uniform dispersion of CNTs in the matrixThe development of a suitable interfacial bonding.The prevention of oxidation of CNTs during high

temperature sintering, and The consolidation of composites to near theoretical

densities

STAGES IN MANUFACTURING

Composite powder preparation using suitable dispersion processes such as conventional powder mixing, sol-gel techniques, colloidal mixing processes and in-situ CNT synthesis in the matrix material.

Composite Densification by an Appropriate Sintering Technique.

The densification techniques are:• Hot-Press Sintering (HPS)• Spark Plasma Sintering (SPS)• Pressureless Sintering (PLS)• Laser Treatment• High Pressure Techniques

Mechanical Properties1. HardnessThe following tendencies have been observed during

experiments:• Dense matrices and good interfacial bonding increases

hardness. • There is no proof of a strong interfacial bonding. • Simple mechanical bonds exist.• Inhomogeneous dispersion of CNTs in matrices and poor

densities of sintered composites cause decrement in the hardness value.

• If there is a combination of the above two, a compromise is generally observed in the hardness value.

• Sharp increases at very low filler contents are most likely related to changes in matrix morphology or crystallinity.

 

2. Elastic Modulus• Considering the high stiffness of CNTs, greater than that of

glasses/glass-ceramics, an increase in elastic modulus of the composites is expected.

• For three-dimensional randomly aligned short fibre composites, a modified rule of mixtures called Krenchel’s rule incorporating an orientation efficiency factor (η0) and length efficiency parameter (ηL) is often used:

EC = ηO ηL EF VF + EM (1-VF) in which EC, EF and EM are the elastic moduli of the composite,

fibres and matrix respectively and VF is the fibre volume fraction.

• The value of ηo for three dimensional random orientation of fibres is 0.2 and the value of ηL depends upon the fibre length, diameter, inter-fibre spacing and shear modulus of the matrix.

3. Fracture Strength• Significant increase in the fracture strength has been observed at low to moderate CNT

loadings using three and four point flexural and compression strength tests.• The main parameters in this case are microstructure and the degree of agglomeration.

4. Fracture Toughness• Glass based systems are brittle. • The aim has been to introduce additional toughening mechanisms through the

incorporation of CNTs. • Vickers Indentation Fracture Toughness Technique (VIF) has been employed. • The composites developed have higher fracture toughness. • High CNT loadings lead to a drop again due to inhomogeneous CNT dispersion. • Improvements in inorganic matrix CNT composite toughness are usually attributed to

conventional fibre mechanisms, such as crack deflection, CNT bridging and CNT pull-out.

• Scaling considerations highlight the lower absolute performance expected for nanofibres compared to microfibers, if only these conventional mechanisms operate.

• Thus, the possibility of new toughening mechanisms such as shear banding of hollow nanostructures or pull-out of flexible single walled CNTs over convoluted contour lengths can’t be ruled out.

FUNCTIONAL PROPERTIESElectrical Conductivity

• The electrical conductivity of glasses and glass ceramics increases by a huge margin .

• At very low CNT contents, electrical conductivity is not increased significantly.

• The formation of a conducting percolating network of CNTs rapidly increases electrical conductivity by several orders of magnitude following the scaling law according to percolation theory:

σ C = σCNT (Ф-ФC)t

where σ C and σ CNT are the conductivities of composites and CNTs respectively, Ф is the volume fraction of CNTs in composites, ФC is the critical volume fraction (percolation threshold) and the exponent t is the dimensionality of the system.

Thermal Conductivity

• The thermal diffusivity (α) and specific heat capacity (CP) of are measured by laser flash technique and differential scanning calorimetry, respectively.

• Thermal conductivity (KC) is calculated by computing the two measured values along the sintered density (ρ) of composites using the following equation:

KC = αCP ρ• The increment in thermal conductivity due to CNT reinforcement in glass/glass

ceramic matrix is not very high due to the high interfacial density associated with nanoscale particles.

• Many reasons may limit the enhancement, including the presence of structural defects in CNTs, non-uniform dispersion of CNTs producing agglomerates, insufficient densification of composites leading to porosity, preferential alignment of CNTs producing anisotropic composite properties and the presence of new phases at the CNT-glass interfaces.

• The thermal resistance due to large interfacial surface area between CNTs and glass matrix is the most likely reason for the reduced increment of thermal conductivity of composites.

APPLICATIONS • Have low density, improved fracture toughness, enhanced electrical

and thermal conductivity, resistance to oxidation at moderate to high temperatures and higher hardness and stiffness than polymers and some metals.

• Space structural applications.• As heat sink materials to dissipate heat from electronic

components. • Applications requiring a sudden temperature change due to their

resistance to thermal shock and cycling, for example in the handling of low-melting metals and glasses.

• Thermal barrier coatings. • Low friction resistance applications demanding anti-friction

materials such as in pump manufacturing and components for the automotive industry.

• Components such as bearings, seals, brake and gear systems • Applications requiring controlled (nano)porosity such as in filters,

catalyst supports or sorbents.

CONCLUSIONS AND SCOPE• There is much about carbon nanotubes that is still unknown. • Challenges include cheaper mass-production.• There is no doubt that carbon nanotube will play a significant

role in a wide range of commercial applications in future.• Although the effect of randomly dispersed CNTs on the

hardness and stiffness of glass/glass-ceramic matrices is still not clearly understood, moderate improvements in fracture strength and toughness and thermal conductivity have been conclusively observed

• The significant increase in electrical conductivity is particularly promising.

• More research needs to be done regarding the environmental and health impacts of producing large quantities .

• Incorporating highly aligned CNTs of high aspect ratio in one or more well-specified directions remains to be achieved.

• A useful increase in functional properties, moderate increase in mechanical characteristics and improved technological properties suggest the potential in numerous industrial applications.

• The possibility of tailoring the functional properties and the simplicity of production (in comparison with continuous fibre reinforced composites) justifies continued R&D efforts in this field.

• Combinations of CNT fillers with conventional fibre reinforcements may prove particularly beneficial in the near term, as has begun to be demonstrated in polymer matrix systems.

REFERENCES

• Ceramic Nanocomposites, Woodhead Publishing Series In Composites Science And Engineering, Edited by Rajat Banerjee and Indranil Manna

• http://www.answers.com/topic/timeline-of-carbon-nanotubes?cat=technology

• http://www.pa.msu.edu/cmp/csc/nttimeline.html

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