Class 1 - The Nature of Materials - 2025

The Nature of Materials

• Dr. Lauren Priddy

• January 15, 2025

TED Talk: Doris Kim Sung

• Doris Kim Sung is a biology student turned architect.

• Focuses on thermo-bimetals, which are smart materials that respond dynamically to temperature changes.

• The talk was recorded in May 2012.

• Available at: TED Talk Link

Atoms and Molecules

• Fundamental building blocks of materials.

Electromagnetic Forces

• Govern the interaction of atoms.

• Interactions are:

  • Weak in liquids.

  • Strong in solids.

Particles

• Single neutral particles: Atoms with no charge.

• Neutral particles composed of 2+ atoms: Molecules formed by combining atoms.

• Charged particles: Ions, which can be either positively or negatively charged.

Types of Solids

• Crystalline solids categorized based on the forces that hold their components together:

  • Ionic: Formed by ionic bonds.

  • Metallic: Consist of metal atoms bonded by metallic bonds.

  • Covalent (network): Atoms linked in vast networks by covalent bonds.

  • Molecular: Composed of discrete molecules held together by intermolecular forces.

• All solids consist of packed discrete molecules or atoms in a lattice or repeating array.

• The nature of these interactions determines the solid's properties.

Properties Variation

• The strength of interactions in solids correlates with differences in their physical properties.

Examples of Ionic Solids

• Magnesium oxide (MgO)

• Calcium fluoride (CaF2)

• Potassium sulfate (K2SO4)

• Ammonium phosphate ((NH4)3PO4)

Metallic Solids

• Composed of metals such as:

  • Iron

  • Silver

  • Copper

  • Various alloys

Covalent Solids

• Strengthened by covalent bonds.

Molecular Solids

• Made up of neutral molecules.

• Vary widely in strength due to:

  • Different molecular sizes.

  • Presence of dipole moments.

• Smaller, nonpolar molecules (e.g., H2, N2) have low melting points, while larger nonpolar and polar molecules exhibit higher melting points.

Properties of Solids

• Crystalline solids have distinct melting temperatures due to uniform atomic or molecular interactions, requiring uniform energy to break them.

Amorphous Solids

• Characteristics:

  • No definite geometrical shape.

  • Gradual softening over a range of temperatures rather than a sharp melting point.

  • Irregular surfaces when cut.

  • Isotropic properties (uniform in all directions), unlike the anisotropic properties of crystalline solids.

• Can be seen as supercooled liquids with high viscosity that prevents flow.

Common Examples of Amorphous Solids

• Rubber

• Glass

• Plastic

• Wax

• Structure is influenced by cooling rate:

  • Crystalline quartz has an ordered arrangement, while quartz glass lacks this order due to rapid cooling (4 K/min).

  • Aluminum crystallizes rapidly, whereas amorphous aluminum forms under extreme cooling rates (4 × 10^13 K/s).

Hierarchical Materials

• Nature produces nano/micro-structured hierarchical materials.

• Processing techniques can introduce hierarchical structures, influencing bulk and surface properties.

  • Example: Lotus leaf surface structure includes 3D epicuticular wax tubules leading to nanostructures.

  • A water droplet's behavior on a lotus leaf showcases these properties.

Hierarchical Structure of Spider Silk

• Noted for its unique structural characteristics.