Functional Inorganic Materials

• Short range structures:

  • Crystalline e.g., SiO2 quartz oscillator
  • Amorphous e.g., SiO2 optic fibre
  • Polymeric e.g., silicone
  • Composite
  • Results in broad interference features in XRD

  
• Long range structures:

  • Powders e.g., pigments for incorporation into polymer composites
  • Single crystals e.g., quartz oscillators
  • Solid objects e.g., solid oxide fuel membranes
  • Fibres e.g., for structural composites
  • Thin films e.g., transparent conductors
  • Porous particles or objects e.g., catalytic converter supports
  • Results in Bragg reflections
• Solid oxide fuel cell electrolyte:

  • Made up of porous anode with hydrogen oxidation electrocatalyst and porous cathode with oxygen reduction electrocatalyst
  • Electrolyte is an oxide ion conducting ceramic membrane
  • Must have low electronic conductivity
  • Must be non-porous to hydrogen (close to fully dense)

  
• A typical oxide-ion conductor is a fluorite-type oxide ion conductor - high oxide vacancy concentrations facilitate oxide ion hopping between states. Yttrium ions stabilise cubic structure - stabilised zirconias used in oxygen sensors and solid oxide fuel cells.

  
• Various processes used to form powders into a shape that can be set a high temperature:

  • Mainly organic additives to improve mixing, lubricate particle flow and bind particles prior to firing
  • Careful drying results in crack-free, non-distorted films, fibres or objects that are turned purely ceramic on firing

  
• Sintering:

  • Diffusion is the random movement of ions at high temperature
  • Particles densify, driven by lattice enthalpy and reducing surface curvature
  • Ostwald ripening
  • Material collects at necks between particles, joining them together
  • Grain boundaries may segregate dopants or otherwise have a different composition to particles

  
• Sintering depends on the size and charge of ions involved, e.g., rule of thumb for oxides:

  • With alkali metals 500 C
  • Most TMs and larger alkaline earths 1000 C
  • Smaller alkaline earths and highly charged ions 1500 C
  • Lower temperatures for fluorides, higher for nitrides and carbides
• External routes in sintering are provided by lower energy mechanisms in volatile and low-melting components with higher mobility:

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

  
• The Born-Mayer equation is used to determine lattice enthalpy.

  
• New phase formation in solid state synthesis is driven by product lattice enthalpy.
• Reactivity in solid state synthesis is improved by:

  • Small initial particle size
  • Increased contact between particles
  • High temperature
• Synthesis under pressure obtains denser structures, and anion content can exchange O for S (H2S), N (NH3) or F (F2). Consider whether reactions need a particular gas environment.
• Metastability:

  • Metastable solids are kinetically stable and require different synthesis strategies
  • Synthesis under conditions where thermodynamically stable followed by quenching
  • Synthesis under non-equilibrium conditions (products are kinetically stable)

  
• Perovskite:

  • Large A cation in a cuboctahedral site and smaller B cation in octahedral coordination environment
  • Small tetragonal distortion leads to splitting of diffraction peaks at ambient temperature

  
• Polarisation of a BaTiO3 lattice can occur by:

  • Ti shift along c direction causes a dipole in each unit cell
  • Fully disordered in cubic (high T) phase
  • Ordered within domains in the tetragonal phase
  • This is paraelectric under normal conditions

  
• Application of an electric field is measured in a capacitor.

  
• Paraelectric: dipoles are disordered, can be an intrinsic property of the material or an effect caused by thermal motion above ordering temperature.
• Ferroelectric: dipoles are aligned parallel leading to a net polarisation and structure changes.
• Antiferroelectric: dipoles are aligned antiparallel leading to no net polarisation (but often changes to structure).
• Piezoelectric: dipoles facilitate coupling between atom positions and the electric field so changes to applied field or applied stress result in changes to the other.
• Positive temperature coefficient of resistance (PTCR effect): observed around the ferroelectric to paraelectric transition - used in thermistors.
• Heywang-Junker model of PTCR:

  • Dopant segregate mainly to the amorphous grain boundaries
  • Electric field from the polarised grain materials causes higher concentration of charge barriers in some regions of the grain boundaries, increasing conductivity
  • Polarisation ceases above the phase transition, so conductivity falls as the mechanism ceases
• Sol: a stable suspension of colloidal solid particles or polymer molecules in a liquid.
• Gel: a porous, 3D continuous solid network surrounding and supporting a continuous liquid phase.
• Hydrolysis and condensation in sol-gel synthesis are both nucleophilic substitutions. Alkoxides are used more often than Si-O as with Si-OR properties can be modified. The reactive groups in condensation are Si-OH so Si-OR must first hydrolyse.

  
• Gelation is typically irreversible.
• Base catalysis: stabilises negative transition states so reactions are preferred on most reacted sites.

  
• Acid catalysis: high electron density stabilises positive transition state. Hydrolysis and condensation are maximised where many Si-OR groups remain.

  
• Colloidal suspension: a heterogeneous mixture of particles with diameters in the range of 2 - 500 nm. Unlike suspensions, the particles in a colloid do not separate into two phases on standing.
• Acid speciation:

  • Monomers
  • Dimers and short chains, with lots of hydrolysed groups

  
• Base speciation:

  • Solid particles, often spherical
  • Hydrolysed groups quickly go on to condense

  
• Gelation: when the body becomes immobile. Hydrolysis and condensation continue after (aging).
• Drying (xerogel): causes pressure due to water surface tension and this increases as pore size gets smaller - careful drying needed to avoid fragmentation.
• Supercritical drying (aerogel): avoids surface tension by exchanging water for ethanol then flushing with CO2 in the supercritical state.
• The effect of heat treatment on sol-gel:

  • As-formed gels are most frequently amorphous
  • Silica-based materials are often heated to remove residual groups from the gel structure
  • Some other materials may be crystallised by heating in addition to removing terminal groups e.g., TiO2
  • Any heating process may involve morphology changes but are especially prominent if sintering to crystallise
• Acid sols:

  • Used for fibre growth and coating
  • As solvent evaporates condensable groups collide more frequently
  • Reactions need to occur rapidly enough to stabilise the shape otherwise the liquid phase will run off
• Aerogels are formed under supercritical drying where it maintains volume with the removal of liquid. This prevents exposure to surface tension and no shrinkage due to arrest of hydrolysis/condensation reactions.
• Sol-gel materials will have some porosity unless fully removed by heat treatment.
• Gelation of a solution provides opportunities to allow gels to form around other structures that can be removed.

  
• Most metal alkoxides hydrolyse easily:

  • Lower metal electronegativity = stronger lewis acidity
  • Most metals have several stable coordination numbers - easier expansion of coordination sphere and ligand elimination may not be required