Factors Influencing the Reaction of Solids What Are the Consequences of High Reaction Temperatures? Techniques, concepts, factors different from nal solid state ynthetic preparations ally carried out at high molecular solids, liquids, solutions, gases temperature. This has the following disad e Reaction mechanism alt can be difficult to incorporate ions that readily form e Reaction conditions volatile species (i.e. Ag) eIt is not possible to access low temperature, metastable e Surface area Defect concentration, type e High (cation)oxidation states are often unstable at high Nucleation diffusion rates temperature, due to the thermodynamics of the following Surface reactivity, structure, free energy Dn-(8)+O2() e Due to the preseed by e the contribution to the free energy become increasingl Nucleation and Diffusion Concepts in Methods for Increasing Solid State Solid state reactions Reaction rates Nucleation, requires structural similarity of reactants and products, less reorganization energy, faster nucleation of aHot pressing densification of particles product phase within reactants mposite precursor Mgo, Al, O. MgAl,o, as example: mGo and MgAl,o: rocksalt and spinel, similaracp 0z e Coated particle mixed component reagents, Al,O: hcp O core/shell precursors Spinel nuclei, matching of structure at Mgo interface a Oxide arrangement essentially continuous across Aimed to increase interfacial reaction area a nd decrease interface thickness x ottom line: structural similarity of reactants and products promotes nucleation and growth of one phase within another Factors Influencing Direct solid State reaction on diffusion rates e -AG but extremely slow at RT aCharge, mass and temperature e Reaction complete in several days at interstitial versus substitutional diffusion MgO ALo, e Heterogeneous nucleation on existing Mgo aDepends on number and types of defects in reactant AlDa crystal surfaces e Interfacial growth rates 3:1 aPoint, line, planar defects, grain boundaries eNhanced ionie diffusion with defects and grain MgO/MgAl-O4 Reactant/Product Interface MgAl,O/AL,O3 Product/Reactant Interface MgAl,O,SpinelProduct]Layer5 Factors Influencing the Reaction of Solids Techniques, concepts, factors different from conventional synthesis and characterization of molecular solids, liquids, solutions, gases Reaction mechanism Reaction conditions Surface area Defect concentration, type Nucleation, diffusion rates Surface reactivity, structure, free energy Structural considerations What Are the Consequences of High Reaction Temperatures? To speed the rate of diffusion, conventional solid state synthetic preparations are usually carried out at high temperature. This has the following disadvantages: It can be difficult to incorporate ions that readily form volatile species (i.e. Ag+ ) It is not possible to access low temperature, metastable (kinetically stabilized) products. High (cation) oxidation states are often unstable at high temperature, due to the thermodynamics of the following reaction: 2MOn (s) ® 2MOn-1 (s) + O2 (g) Due to the presence of a gaseous product (O2 ), the products are favored by entropy, and the entropy contribution to the free energy become increasingly important as the temperature increases. Methods for Increasing Solid State Reaction Rates Decreasing particle size Hot pressing densification of particles Atomic mixing in composite precursor compounds Coated particle mixed component reagents, core/shell precursors Nanocrystalline precursors Aimed to increase interfacial reaction area A and decrease interface thickness x Nucleation and Diffusion Concepts in Solid State Reactions Nucleation, requires structural similarity of reactants and products, less reorganization energy, faster nucleation of product phase within reactants MgO, Al2O3 , MgAl2O4 as example: MgO and MgAl2O4 : rocksalt and spinel, similarccp O2- Al2O3 : hcp O2- Spinel nuclei, matching of structure at MgO interface Oxide arrangement essentially continuous across MgO/MgAl2O4 interface Bottom line: structural similarity of reactants and products promotes nucleation and growth of one phase within another Factors Influencing Cation Diffusion Rates Charge, mass and temperature Interstitial versus substitutional diffusion Depends on number and types of defects in reactant and product phases Point, line, planar defects, grain boundaries Enhanced ionic diffusion with defects and grain boundaries Direct Solid State Reaction –DG°f , but extremely slow at RT Reaction complete in several days at 1500°C Heterogeneous nucleation on existing MgO , Al2O3 crystal surfaces Interfacial growth rates 3:1 Overall reaction: MgO + Al2O3 ® MgAl2O4 MgO/MgAl2O4 Reactant/Product Interface MgAl2O4 /Al2O3 Product/Reactant Interface MgAl2O4 Spinel Product Layer MgO MgO Al2O3 Al2O3 Mg2+ Al3+ x/4 3x/4