Unit 3: Nanomaterials and Composites

I. Orientation: The Nanoscale Regime

Nanomaterials are materials with at least one external dimension in the range 1–100 nm (1 nm = 10⁻⁹ m), where properties diverge sharply from the bulk. The field traces to Richard Feynman's 1959 lecture "There's Plenty of Room at the Bottom" and the term "nanotechnology" coined by Norio Taniguchi (1974).

Defining features that later sections rely on:

• Surface-to-volume ratio: rises steeply as size falls; for a sphere it scales as 3/r, so a large fraction of atoms sit at the surface and become chemically reactive.
• Quantum confinement: when a dimension approaches the electron de Broglie wavelength, energy levels become discrete and the band gap widens.
• Property tunability: colour, melting point, conductivity and strength all shift with size and shape, not just composition.
• Dominant characteristic length: phenomena governed by mean free path, exciton radius or grain size become size-sensitive at the nanoscale.

II. Classification by Dimensionality

Nanomaterials are grouped by how many dimensions escape the nanoscale (remain "bulk-like").

A. 0D — all three dimensions confined

• Definition: every dimension lies in 1–100 nm; electrons confined in all directions.
• Examples: quantum dots (CdSe, ~2–10 nm), nanoparticles, gold colloids.
• Behaviour: full quantum confinement; discrete, atom-like energy levels.

B. 1D — one dimension unconfined

• Definition: two dimensions nanoscale, one extended (length).
• Examples: nanowires, nanorods, nanotubes.
• Behaviour: electron transport free along the axis, confined across it.

C. 2D — two dimensions unconfined

• Definition: one dimension (thickness) nanoscale.
• Examples: graphene, nanosheets, thin films.
• Behaviour: carriers move in a plane; confinement along thickness only.

D. 3D — bulk nanostructured solids

• Definition: no dimension confined, but built from nanoscale units.
• Examples: nanocrystalline powders, nanocomposites, dispersions of 0D/1D/2D fillers.

III. Carbon-Based Nanomaterials

Carbon's sp², sp³ hybridisation lets it form distinct nanostructures of differing dimensionality.

A. Fullerenes (buckyballs)

• Structure: closed cage of carbon; C₆₀ has 60 atoms in 20 hexagons + 12 pentagons, ~0.7 nm diameter (0D).
• Discovery: Kroto, Curl and Smalley, 1985 (Nobel 1996).
• Bonding: sp²-hybridised carbons with some strain-induced sp³ character on the curved surface.
• Uses: electron acceptors in organic solar cells, lubricants, drug carriers.

B. Carbon nanotubes

• Structure: rolled graphene cylinders (1D); diameter ~1 nm, length up to micrometres.
• Types: single-walled (SWCNT) and multi-walled (MWCNT); chirality vector (n,m) sets metallic vs semiconducting character.
• Properties: tensile strength ~ up to ~63 GPa, current density far above copper.

C. Graphene

• Structure: single sheet of sp²-carbon in a hexagonal honeycomb lattice (2D), one atom thick.
• Isolation: Geim and Novoselov, 2004 (Nobel 2010), by mechanical exfoliation.
• Electronics: electrons behave as massless Dirac fermions; carrier mobility ~200,000 cm²/V·s.

IV. Properties and Size-Dependent Effects

Reducing size to the nanoscale rewrites the electronic, optical, mechanical and thermal response.

A. Electronic properties

• Band-gap widening: quantum confinement raises the gap as size falls, per particle-in-a-box scaling E ∝ 1/L².
• Ballistic transport: in CNTs and graphene, electrons travel with negligible scattering over the mean free path.

B. Optical properties

• Surface plasmon resonance: gold nanoparticles appear red at ~20 nm and shift with size due to collective electron oscillation.
• Tunable emission: quantum dots fluoresce across the spectrum; smaller dots emit blue, larger emit red.

C. Mechanical properties

• Enhanced strength: fewer defects per nano-volume gives near-theoretical strength (CNTs).
• Hardness (Hall–Petch): yield strength rises as grain size drops:

σ_y = σ_0 + k / √d
σ_y = yield strength, σ_0 = friction stress,
k = strengthening coefficient, d = grain diameter

D. Thermal properties

• Melting-point depression: gold melts near 1064 °C in bulk but well below ~300 °C for very small nanoparticles, as surface atoms are loosely bound.
• High thermal conductivity: graphene reaches ~3000–5000 W/m·K along the plane.

E. Size-dependent effects (summary of the driver)

• Surface dominance: at ~5 nm a large fraction of atoms are surface atoms, driving reactivity and lower melting.
• Confinement threshold: effects switch on once a dimension nears the exciton Bohr radius.

V. Methods of Nanomaterial Synthesis

Two philosophies build nanostructures from opposite directions.

A. Top-down approach

• Principle: carve nanostructures from bulk by removing material.
• Methods: ball milling, lithography, laser ablation, etching.
• Limits: surface defects, poor size uniformity, hard below ~10 nm.

B. Bottom-up approach

• Principle: assemble nanostructures atom-by-atom or molecule-by-molecule.
• Methods: sol–gel, chemical vapour deposition (CVD, used for CNT/graphene), self-assembly, chemical reduction.
• Advantages: finer control, fewer defects, uniform sizes.

Contrast:

1. Top-down: bulk → nano; subtractive; scalable but coarse.
2. Bottom-up: atoms → nano; additive; precise but slower.

VI. Applications of Nanomaterials

Nanoscale properties translate into function across sectors.

A. Catalysis

• High surface area: more active sites per gram; platinum nanoparticles cut catalyst loading in fuel cells.
• Selectivity: shape-controlled particles favour specific reaction facets.

B. Electronics and telecommunication

• Miniaturisation: CNT and graphene transistors push beyond silicon limits.
• Optoelectronics: quantum dots in QLED displays; nanophotonic components in optical fibre systems.

C. Biomedical and pharmaceutical fields

• Targeted drug delivery: functionalised nanoparticles carry drugs to tumour sites, reducing systemic dose.
• Imaging and diagnostics: superparamagnetic iron-oxide nanoparticles as MRI contrast agents; gold nanoparticles in biosensors.

D. Energy-related technologies

• Batteries: nano-silicon and nanostructured electrodes shorten Li⁺ diffusion paths, raising capacity.
• Supercapacitors: graphene and CNT electrodes give huge surface area for fast charge storage.
• Solar cells: quantum dots and fullerene acceptors widen light absorption and improve charge separation.

VII. Fundamental Concepts of Composite Materials

A composite combines two or more chemically distinct phases to yield properties neither achieves alone; the phases remain physically separable.

A. Role of matrix and reinforcement

• Matrix: continuous phase that binds, protects and transfers load to the reinforcement.
  • Types: polymer (epoxy), metal (aluminium), ceramic (SiC).
• Reinforcement: dispersed phase that carries most of the load and gives stiffness or strength.
  • Load transfer: stress passes through the matrix–reinforcement interface, so bonding quality controls performance.

B. Composition, structure and characteristic properties

• Composition: defined by volume fraction; the rule of mixtures estimates stiffness:

E_c = E_f V_f + E_m V_m
E_c = composite modulus, E_f/E_m = fibre/matrix modulus,
V_f/V_m = fibre/matrix volume fractions (V_f + V_m = 1)

• Structure: reinforcement geometry (particle, fibre, laminate) and orientation set anisotropy.
• Characteristic properties: high strength-to-weight ratio, corrosion resistance, tailorable stiffness, good fatigue resistance.

VIII. Classification of Composites by Reinforcement

Reinforcement geometry defines the composite family.

A. Particulate composites

• Definition: matrix reinforced with roughly equiaxed particles.
• Behaviour: near-isotropic; particles restrict matrix deformation.
• Examples: concrete (aggregate in cement), cemented carbide (WC in cobalt).

B. Fibre-reinforced composites

• Definition: thin, high-strength fibres in a matrix; load carried mainly by fibres.
• Orientation: continuous aligned fibres give strong anisotropy; short/random fibres are more isotropic.
• Examples: glass-fibre-reinforced plastic (GFRP), carbon-fibre-reinforced polymer (CFRP).

Contrast:

1. Continuous fibres: high strength along the fibre axis, weak transverse.
2. Discontinuous fibres: lower peak strength but easier to mould and more uniform.

C. Structural composites

• Definition: composites whose properties depend on geometry of layering, not just constituents.
• Laminates: stacked plies at varied angles for balanced in-plane strength.
• Sandwich panels: stiff skins over a light core (honeycomb/foam) for high bending stiffness at low weight.

IX. Engineering Applications of Composites

Composites are chosen where weight, stiffness and corrosion resistance matter together.

• Aerospace: CFRP wings and fuselage panels cut weight and fuel burn.
• Automotive: GFRP body panels and leaf springs reduce mass and rust.
• Civil infrastructure: fibre-reinforced polymer rebar and concrete resist corrosion in bridges.
• Sports and marine: carbon composite frames, hulls and blades combine stiffness with light weight.
• Wind energy: long glass/carbon composite turbine blades exploit high strength-to-weight ratio.
• Electronics: printed circuit boards use glass-fibre/epoxy (FR-4) laminates for dimensional stability.