Lecture 4

• Andrew Hector: 'What’s the difference between crystalline and amorphous?'
• Inter-particle bonding

  In some cases need to achieve densities close to that of the crystals themselves
• Stabilised zirconias used in oxygen sensors and solid oxide fuel cells
• Inter-particle bonding


  Provides electronic and magnetic interactions, ionic conduction pathways between particles
• Yttrium ions also stabilise cubic structure (ZrO2 is monoclinic)
• Short range structure

  1. Crystalline: SiO2 quartz oscillator

  2. Amorphous: SiO2 optic fibre

  3. Polymeric: silicone

  4. Composite (at whatever length scale)
• For a single material, most often crystalline particles are joined at their edges
• Complex compositions

  • ternary or higher oxides: YBa2Cu3O7-δ superconductor
• High oxide vacancy concentrations facilitate oxide ion hopping between sites
• Inter-particle bonding

  Provides mechanical strength
• Simple compounds

  • binary oxides: TiO2 as a photocatalyst
• Fluorite-type oxide ion conductor (Y3+)x(Zr4+)1-xO2-2x
• Elements

  • graphite battery anodes
• Variety of combinations of structure and composition


  • Composition
  • Crystal structure
  • Microstructure
• Solid oxide fuel cell electrolyte

  1. H2 + O2- ⇌ H2O + 2e-

  2. ½O2 + 2e- ⇌ O2-

  3. Electrolyte is an oxide ion conducting ceramic membrane, to separate H2 + ½O2 ⇌ H2O to two half-reactions

  4. Must have very low electronic conductivity

  5. Must be non-porous to hydrogen, so close to fully dense
• Longer range structure

  1. Powders: pigments for incorporation into polymer composites

  2. Single crystals: quartz oscillators

  3. Solid objects: solid oxide fuel membranes

  4. Fibres: for structural composites

  5. Thin films: transparent conductors

  6. Porous particles or objects: catalytic converter supports
• Temperature needed for sintering depends on the size and charge of ions involved
• Synthesis below diffusion temperature involves conversion of pre-formed structures through topotactic transformation
• Quenched metastable materials undergo negative thermal expansion and are stable within specific temperature ranges
• Many complex solids are obtained by reacting simpler ones together at high temperature
• Reactivity in solid state synthesis is improved by small initial particle size, increased contact between particles, and high temperature
• Rate of solid state synthesis is inversely proportional to distance, hence small particles are preferred
• Controlling anion content and cation oxidation states in reactions may require specific gas environments
• External routes in sintering for lower energy mechanisms

  • Evaporation-condensation
  • Dissolution-precipitation
  • Melt and flow
• Sintering

  1. Diffusion of ions at high temperature

  2. Particles densify driven by lattice enthalpy and reducing surface curvature

  3. Particles grow, larger particles grow at the expense of smaller ones (Ostwald ripening)

  4. Material collects at necks between particles, joining them together
• Born-Mayer equation


  • Lattice enthalpy higher with smaller and more highly charged ions
  • Efficient packing can be achieved with two different cation sizes plus an anion
• High temperature solid state reactions usually yield thermodynamically stable phases, while metastable solids require different synthesis strategies
• Mainly organic additives used for producing ceramic bodies

  • Improve mixing
  • Lubricate particle flow
  • Bind particles together prior to firing
• Producing ceramic bodies involves various processes to form powders into a shape that can be set at high temperature
• Direct solid state reactions require differential conditions for different substituents
• Perovskite structure

  Large A cation (e.g. Sr) in cuboctahedral sites and smaller B cation (e.g. Ti) in octahedral coordination environment
• Measured polarisation in sintered BaTiO3 shows hysteresis due to a residual polarisation as the field strength is swept
• Subject: Inorganic Materials Chemistry
• Ferroelectric behaviour of BaTiO3

  1. Paraelectric under normal conditions

  2. Application of an electric field can cause domains that oppose the field to grow and hence an overall dipole

  3. Ordered domain size increases with field and can reach 0.1 µm
• Email: [e-mail]
• BaTiO3 is commonly used in multilayer ceramic capacitors with multiple plates closely spaced to minimise d
• Author: Andrew Hector
• Polarisation of BaTiO3 lattice


  1. Ti shift along c direction causes a dipole in each unit cell

  2. Fully disordered in cubic (high T) phase

  3. Ordered within domains in the tetragonal phase
• Heywang-Jonker model of PTCR: Dopants segregate mainly to the amorphous grain boundaries, electric field from the polarised grain material causes higher concentration of charge carriers in some regions of the grain boundaries, increasing conductivity, polarisation ceases above the phase transition, so conductivity falls as the mechanism ceases
• Publication date

  30/2035