Transition Metals and Energy Bands

Chemistry of Engineering Materials - Metals

• Metals are held together by a sea of delocalized valence electrons.
• This allows them to conduct electricity and be relatively strong without being brittle.
• Metallic solids consist entirely of metal atoms.

Alloys

• An alloy is a material containing more than one element with the characteristic properties of a metal.
• Alloying metals is vital for modifying the properties of pure metallic elements.
• Common Uses often involve alloy compositions.
  • Example: Bronze (copper and tin) and brass (copper and zinc)
  • Pure gold is too soft for jewelry but alloys of gold are harder.
• Nitinol (Nickel-Titanium): A 1:1 nickel-titanium alloy with 'shape memory'. Discovered accidentally during a 1961 meeting where a folded wire was heated by a lighter to regain its original shape.

Four Categories of Alloys

• Substitutional Alloys: Solute atoms occupy positions normally occupied by solvent atoms. Components have similar atomic radii and chemical-bonding characteristics, like silver and gold alloys.
• Interstitial Alloys: Solute atoms occupy interstitial positions in the holes between solvent atoms (e.g., carbon in steel). Solute atoms have much smaller bonding atomic radii than the solvent atoms and these strengthen the lattice structure, and make them harder but less ductile compared to substituted alloys.
• Heterogeneous Alloys: Components are not uniformly distributed e.g., pearlite in iron. Properties depend on the composition and the method of formation (rapid or slow cooling).
• Intermetallic Compounds: Compounds rather than mixtures. Have definite properties and composition (e.g., Ni3Al). These are often formed with ordered distribution of atoms and generally have better structural stability and higher melting points than constituent metals. However, they are often more brittle.

Types of Steels

• Carbon Steels: Contain trace amounts of elements besides carbon and iron. Account for around 90% of steel production. Sub-divided based on carbon percentage

  • Low Carbon Steel (Mild Steel) up to 0.3% carbon, used in cables, nails, and chains.
  • Medium Carbon Steels(0.3-0.6% carbon), tougher than mild steels and used for girders and rails.
  • High Carbon Steel (0.6-1.5% carbon), used in cutlery, tools, and springs.

• Alloy Steels: Steels that have elements other than carbon and iron, providing specific properties.

  • Examples: Stainless steel (chromium, nickel) used for its high corrosion resistance; Tool steels (tungsten/molybdenum/cobalt) with high hardness and heat resistance suitable for tools.

Types of Stainless Steel

• Contains 10%-20% chromium as an alloying element, and other components like Nickel, silicon, and manganese.
• Widely used in construction, medical equipment, piping and food processing equipment because of their high corrosion resistance and weather stability.
• Example types of stainless steel: 304 Stainless steel.

Types of Tool Steels

• Tool steels are notable for their high heat resistance qualities, making them useful for cutting and drilling equipment or environments that require such qualities.
• Contains metals such as tungsten/molybdenum/cobalt leading to durability and high hardness.

Heterogeneous Alloys

• Components are not uniformly distributed, examples include pearlite in iron.
• Properties depend on composition and formation methods (e.g., rapid or slow cooling.).

Intermetallic Compounds

• Compounds, not mixtures, with definite compositions and properties.
• Atoms are ordered rather than randomly distributed in the structure.
• Advantages: Improved structural stability, higher melting points.
• Disadvantages: Often more brittle than substitutional alloys.
  • Common types include Ni3Al and Nb3Sn (superconductor)
• Examples of compounds include AuAl2

Alloys of Gold

• Pure gold is termed 24 karat.
• Karat number decreases with decreasing gold percentage.
• Common alloys: 14 karat (58% gold), 18 karat (75% gold.)
• Alloying with other metals (e.g., silver, copper) alters color and properties such as strength and hardness.

Metallic Solids

• Held together by a sea of delocalized valence electrons.
• This allows them to conduct electricity and be relatively strong without being brittle.
• Consist entirely of metal atoms.

Metallic Properties

• Metals have a characteristic luster (shine).
• They have high thermal conductivity and electrical conductivity (e.g., silver, copper).
• They are often malleable (can be hammered into thin sheets) and ductile (can be drawn into wires).

Models of Metallic Bonding

• Electron Sea Model: Valence electrons are delocalized and move freely throughout the solid (relatively simple, good for explaining malleability and ductility)
• Molecular Orbital Model (Band Theory): Atomic orbitals combine to form molecular orbitals that extend over the entire molecule. Electrons occupy energy bands, and the behavior of these bands (how they are filled and the energy gaps between them) determines the material's properties.

Semiconductors

• Intermediate between metals and insulators.
• Have a small energy gap between the valence band and conduction band.
• Electrical conductivity increases with temperature.
• Examples: Silicon, germanium, certain compound semiconductors.
• Doping can enhance conductivity.
• Understanding doping allows for control over conductivity.
• n-type doping (adding electron donors) increases negative charge carriers
• p-type doping (adding electron acceptors) increases positive charge carriers.

Artificial Lighting (LEDs)

• LEDs are semiconductor devices.
• They convert electrical energy into light.
• The color of light emitted by an LED depends on the specific semiconductor material band gap.