Direct Synthesis of a Spinel Example: MgAL2O4 .Structural considerations .Mass transport necessary due to structural AgO+Al2O3→MgAl2O4 differences of reactants and product tMgO: ap 02- Mg+in O, sites rEaction only occurs at contact points between MgAL2O4∞pO2 Mg+ in 1/8 T, sties grains of Mgo and AlO3 山3inl/2o, sites sGet nucleation near contact point and then growth B+ in 2/30, sites of product PBond breaking and formation sGrowth requires diffusion of Mg /Al through the +Topotaxy at MgO/spinel interface (cep for both) WVery slow +Epitaxy at Al,O / spinel interface (hep to ccp) Kirkendall Effect Because of the different into the other. the front In 1947, Smigelkas and Kirkendall reported etween the two metals is the movement of the interface between a diffusion couple, i. e copper and zinc in brass as the result of the different diffusion rates of The diffusion of zinc into the these two species at an elevated temperature copper is faster, and so the This phenomenon, now called the Kirkendall the zinc(gray) and copper Effect, was the first experimental proof that atomic diffusion occurs through vacancy the right. As the zinc ions xchange and not by the direct interchange of opper, they leave vacancies that can fuse into pores Kirkendall Effect Another Example of the Kirkendall Effect determining sten usually is the rate a Mg, Al srO+TiO2→ a Reaction slows as MgAlO, layer grows Longer distance for cations to diffuse 0*. Sr& in a Spinel growth faster on one side to maintain SrTiO acp Sr//0=(4: %) Ti*+ in 140, sites a 3Mg= diffuse to right, balances 2Al+ to left MgO/MgAlO, Reactant/Product Interface A3-3Mg2+4MgO→ MaLo4 SrO/SrTiO, Reactant/Product Interface MgALOJALO, Product/Reactant Interface BMg-2A++ 4ALO3-3MgALOA SrTiO/TiO, Product/Reactant Interface Mgo Fe Oa- MgFe,O4. colored spinel interface, can 3sr0+3TiO2→3 SrTio3 easily monitor growth rate6 Direct Synthesis of a Spinel Structural considerations Mass transport necessary due to structural differences of reactants and products MgO: ccp O2- Mg2+ in Oh sites MgAl2O4 ccp O2- Mg2+ in 1/8 Td sties Al3+ in 1/2 Oh sites Al2O3 hcp O2- Al3+ in 2/3 Oh sites Bond breaking and formation Topotaxy at MgO/spinel interface (ccp for both) Epitaxy at Al2O3 /spinel interface (hcp to ccp ) Example: MgAl2O4 Reaction only occurs at contact points between grains of MgO and Al2O3 Get nucleation near contact point and then growth of product Growth requires diffusion of Mg2+/Al3+ through the product Very slow MgO+ Al2O3 ® MgAl2O4 Kirkendall Effect In 1947, Smigelkas and Kirkendall reported the movement of the interface between a diffusion couple, i.e., copper and zinc in brass, as the result of the different diffusion rates of these two species at an elevated temperature. This phenomenon, now called the Kirkendall Effect, was the first experimental proof that atomic diffusion occurs through vacancy exchange and not by the direct interchange of atoms. Because of the different diffusion rates of each metal into the other, the front between the two metals is observed to move. The diffusion of zinc into the copper is faster, and so the brass alloy boundary between the zinc (gray) and copper (brown) appears to move to the right. As the zinc ions diffuse into the copper, they leave vacancies that can fuse into pores. Kirkendall Effect Mg2+, Al3+ diffusion usually is the rate determining step Reaction slows as MgAl 2O4 layer grows Longer distance for cations to diffuse Spinel growth faster on one side to maintain charge-balance 3Mg2+ diffuse to right, balances 2Al 3+ to left MgO MgO Al2O3 A l2O 3 M g2 + A l3 + x/4 MgO/MgAl 3x/4 2O4 Reactant/Product Interface 2Al3+ –3Mg2+ + 4MgO ® MgAl2O4 MgAl2O4 /Al2O3 Product/Reactant Interface 3Mg2+ –2Al3+ + 4Al2O3 ® 3MgAl2O4 4MgO + 4Al2O3 ® 4MgAl2O4 MgO + Fe2O3 ® MgFe2O4 , colored spinel interface, can easily monitor growth rate Another Example of the Kirkendall Effect SrO + TiO2 ® SrTiO3 RockSalt Rutile Perovskite SrO: ccp O2- , Sr2+ in all Oh sites TiO2 : hcp O2- , Ti4+ in 1/2 Oh sites SrTiO3 : ccp Sr2+/O2- (¼:¾ ), Ti4+ in ¼Oh sites SrO SrO TiO 2 TiO 2 Sr2 + Ti 4+ x/3 2x/3 SrO/SrTiO3 Reactant/Product Interface Ti4+ –2Sr2+ + 3SrO ® SrTiO3 SrTiO3 /TiO2Product/Reactant Interface 2Sr2+ –Ti4+ + 3TiO2 ® 2SrTiO3 3SrO + 3TiO2 ® 3SrTiO3