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 rate
6 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
Formation of Hollow Nanocrystals Through the nanoscale kirkendall effect As show, reaction of high quality single"crystal 海。 stals of companying transpo A ed by the injection of sulfur in provide a fast transport path for outward diffusion of cobalt atoms that can then spread on the inner shell surface · 20 nm Evolution of Cose hollow nanocrystals with time by injection of a e ol utom of oe hollow oanmeryatal e r time mD espe hle to a stream aperyista aolution at is k cbo beren t bottom isht oa.10 molar ratio was 1:1 lodal solution at 455 K (A to D) TEM images of the after flow of O /Ar for (A)O min, (B)30 min, (C)80 min, and D)210 min Inset: HRTEM of a Coo hollow nanocrystal Choosing the Optimum Temperature Overcoming the Diffusion Barrier (a)calcined at 500C aks of Li-CO3+ Zroz Need intimate mixture of reactants Can be obtained in several ways overy gmall particle size reactants peaks of LizZ-Oa (monoclinic) ofind molecular precursor that has the needed elements (d) calcined at1400°C in the correct ratio: eg. BalTio(C,)I for BaTiO peaks of LizZrO3+ZrOz out without demixing the components Optimum temperature. carbonates for Brownmillerite (CaFe,Oy XRD Patterns of the Li, ZrO3 crystallize from gels prepared using sol-gel chemistry Calcined at Various Temperatures
7 As show, reaction of high quality single-crystal cobalt nanocrystals with oxygen, sulfur, or selenium at relatively low temperatures produces hollow polycrystalline nanocrystals of cobalt oxide, sulfide, or selenide, respectively. As the reaction proceeds in time, more cobalt atoms diffuse out to the shell, and the accompanying transport of vacancies leads to growth and merging of the initial voids. This results in the formation of bridges of material between the core and the shell that persist until the core is completely consumed. These bridges provide a fast transport path for outward diffusion of cobalt atoms that can then spread on the inner shell surface. Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect (A) TEM image of cobalt nanocrystals. (B ) TEM image of the cobalt sulfide phase synthesized by the injection of sulfur in o-dichlorobenzene (5 ml) into cobaltnanocrystal solution with a Co/S molar ratio of 9:12. (C ) HRTEM images of Co3S4 (left) and Co9S8 (right). (D) TEM image of the cobalt sulfide phase synthesized with a Co:S molar ratio of 9:8. Evolution of CoO hollow nanocrystals over time in response to a stream of O2 /Ar mixture (1: 4in volume ratio, 120 ml/min) being blown through a cobalt colloidal solution at 455 K. (A to D) TEM images of the solutions after flow of O2 /Ar for (A) 0 min, (B) 30 min, (C) 80 min, and (D) 210 min. Inset: HRTEM of a CoO hollow nanocrystal. Evolution of CoSe hollow nanocrystals with time by injection of a suspension of selenium in o-dichlorobenzene into a cobalt nanocrystal solution at 455 K, from top-left to bottom right: 0 s, 10 s, 20 s, 1min, 2 min, and 30 min. The Co/Se molar ratio was 1:1. XRD Patterns of the Li2ZrO3 Calcined at Various Temperatures (a)Calcined at 500°C peaks of Li2CO3 + ZrO2 (b) Calcinedat 700°C peaks of Li2ZrO3 (monoclinic) (c) Calcinedat 850°C-1200 °C peaks of Li2ZrO3 (monoclinic) (d) Calcined at 1400°C peaks of Li2ZrO3 + ZrO2 Optimum temperature: 850-1200oC ( a) ( b) (c) ( d) Choosing the Optimum Temperature Overcoming the Diffusion Barrier Need intimate mixture of reactants Can be obtained in several ways: very small particle size reactants find molecular precursor that has the needed elements in the correct ratio：eg. Ba[TiO(C2O4 )2 ] for BaTiO3 Make a solution of needed metals and dry the solution out without demixing the components co-precipitate reactants in a solid solution salt：e.g. carbonates for Brownmillerite (Ca2Fe2O5 ) crystallize from gels prepared using sol-gel chemistry
Solid state precursors Synthesis of Magnetic Garnets a Crystalline, phase-pure material Magnetic garnets(tunable magnetic materials) aqueous m Great for spinels, e.g., chromite Y(NO) 3+Gd(NO)+FeCl+NaOH-Y Gda FesO, a Chromite Spinel Precursor NH2Mg(CO少26H2O1150°C Firing at 900"C, 18-24 hrs, pellets, regrinding, repelletizing peated firings, removes RFeO, perovskite impurity CNH2NCrO2·6HO somorphous replacement of Y* for Gd*on dodecahedral MnCr o MnCr O2“5CH2N 1100°C ites, solid solution, similar rare earth ionic radii CoCr O, CoCr,. 5C,HN 1200°C Cucr.O (NH)Cu(CrO去2·2NH 750°C loda, 2Fe+ occupied in On sites, 3Fe+Ta sites, 3RE+ ZnCrO (NH),Zn(CrO ), 2NH, 1400C ight for rmula units in a unit cell of 160 atoms. cubic lattice unit, cell parameter follows Gda- FesO1g)=Px3(Y3 O1g)+P(3-x/3(Gd, FesO12) Synthesis of Magnetic Garnets Coprecipitation Technique Coprecipitation applicable to nitrates, acetates, aAny property(P)of a solid-solution member is the atom oxalates. alkoxide and so forth weighted average of the end-members ole magnetic properties by tuning the x value Requires: try garnet Y Gda-yFesO12 . Y GdrxFe, O creates a tunable magnetic garnet that is strongly temperature and composition dependent asimilar precipitation rates applications in permanent magnets, magnetic recording "no supersaturation media, magnetic bubble memories and so forth, similar Useful for spinels and the like concepts apply to magnetic spinels Disadvantage: difficult to prepare high purity, Combinatorial Materials Chemistry- Self Propagating High Temperature Robots Can Do Solid State Synthesis! Combinatorial materials chemistry, the wave of the Synthesis (SHS) Used for parallel synthesi of compounds, rapid eXtreme exothermicity of a reaction can be used to ts, clever analytical provide high temperatures needed for diffusion plied to high Te superconductors, inorganic phosphors, tHermite Fe.0.+Al>ALO. +Fe has been used to ake a number of useful materials including fractory ceramic parts that can be pressed and machined to final size
8 Solid State Precursors Crystalline, phase-pure material Decomposes on heating Great for spinels, e.g., chromite spinels Chromite Spinel Precursor Ignition(°C) MgCr2O4 (NH4 )2Mg(CrO4 )2•6H2O 1150°C NiCr2O4 (NH4 )2Ni(CrO4 )2•6H2O 1100°C MnCr2O4 MnCr2O7•5C5H5N 1100°C CoCr2O4 CoCr2O7•5C5H5N 1200°C CuCr2O4 (NH4 )2Cu(CrO4 )2•2NH3 750°C ZnCr2O4 (NH4 )2Zn(CrO 4 )2•2NH3 1400°C Magnetic garnets(tunable magnetic materials) aqueous precursor technique: Y(NO3 ) 3+Gd(NO3 ) 3+FeCl3+NaOH®YxGd3-xFe5O12 Firing at 900°C, 18-24 hrs., pellets, regrinding, repelletizing, repeated firings, removes RFeO3 perovskite impurity Isomorphous replacement of Y3+ for Gd3+ on dodecahedral sites, solid solution, similar rare earth ionic radii: 0<x<3, 2Fe3+ occupied in Oh sites, 3Fe3+ Td sites, 3RE3+ dodecahedral sites eight formula units in a unit cell, total of 160 atoms, cubic lattice unit, cell parameter follows Vegard’s law behavior: P(YxGd3-xFe5O12)=Px/3(Y3Fe5O12)+P(3-x)/3(Gd3Fe5O12) Synthesis of Magnetic Garnets Any property (P) of a solid-solution member is the atom fraction weighted average of the end-members Tunable magnetic properties by tuning the x value in the binary garnet YxGd3-xFe5O12 YxGd3-xFe5O12 creates a tunable magnetic garnet that is strongly temperature and composition dependent, applications in permanent magnets, magnetic recording media, magnetic bubble memories and so forth, similar concepts apply to magnetic spinels Synthesis of Magnetic Garnets Coprecipitation applicable to nitrates, acetates, oxalates, alkoxides, and so forth Requires: ßsimilar salt solubilities ßsimilar precipitation rates ßno supersaturation ßUseful for spinels and the like Disadvantage: difficult to prepare high purity, accurate stoichiometric phases Coprecipitation Technique Combinatorial Materials Chemistry¾¾ Robots Can Do Solid State Synthesis! Combinatorial materials chemistry, the wave of the future, beware traditional solid state chemists! Used for parallel synthesis of series of compounds, rapid screening by parallel measurements, clever analytical techniques, massive amounts of data . Applied to high Tc superconductors, inorganic phosphors, giant magnetoresistant (GMR) mixed valency perovskites, high dielectric constant rutile type oxides, mixed metal catalysts and electrocatalysts, and even hydrothermal synthesis of zeolites. Self Propagating High Temperature Synthesis (SHS) Extreme exothermicity of a reaction can be used to provide high temperatures needed for diffusion Thermite Fe2O3 + Al ® Al2O3 +Fe has been used to make a number of useful materials including refractory ceramic parts that can be pressed and machined to final size
Solid state metathesis read Solid state metathesis reactions can en two salts lIc MoCl(s)+5/2Na S(s)-,MoS, (s)+5NaCI(s)+1/2S( are chosen, a highly exot hermic reaction MoCI, +5/2 Nas- Mos, nAcL+ 1/2s The enthalpy of this reaction is DH=213 kcal/mol started (by grinding spark. ete )the heat generated d by the reaction itself leads to a rapid increase in temperature The maximum temperature attained is-1700K. The reaction is completed in 48 The percent yield is typically 80%6 of theoretical. Washing with CH, OH re Reaction reaches 1050 C and is over in 300 ms B with H-O removes Nacl Washing with chloroform removes remaining s SSM(Solid State Metathesis) olid State Metathesis Reactions to Ill-V Group AB+CD→AC+BD Metal, B:halogen C: alkali metals, D: S, P, As, Sb) En. MC1+2LiS-MS, +4LiCl (M=Ti, Zr, Hf and V)(>400PC) CldANHn SigN, +12HCI MCI+Li Fe OoMFe O+2LiCl(400-500PC) N→+ZN+ LicH1/6N Snl4+2Ii2E→SnEa+4l(E=O,S,Se(500°O PbCl2+Ii2E→PbE+2 LiCI(E=0,Se,Te)(50°C LnCl,+Li N-LnN+3LiCl (Ln=Y, La, Pr, Nd, Sm, Eu, Gd, Tb MCL(8+(x/)Li3N(S)MN(s)+xLiCI+(x-3)/6N,(g) MCI(s)+xNaN(s),MN(8)+xNaCl(s)+(3x-1)/2N(g) Disadvantages of SSM Sintering Overview he product is impure, often contains the impurities such as including is a process specifically for powdered materials, Since the actual reaction temperature is far higher than the uct licl compressed powder is heated to a temperature close to but not with the product I is used on laboratory at particles may bond by solid state bonding, but not melt. research, and ca sed for large scale production. GThe method to All+NayAs--AlAs +Nal+Al +As (1)1000oC/8h (2)wash As CGaNor inn can not be obtained in the similar system of GaCl,or InCl, and Li,N under similar condition, which prod corresponding metals Ga or In and release nitrogen gas. a Surface area reduction e Powder process
9 Solid State Metathesis Reactions Can Be Very Exothermic MoCl5 (s)+5/2Na2S(s)®MoS2 (s)+5NaCl(s)+1/2S(s) Reaction reaches 1050ºC and is over in 300 ms Solid State Metathesis Reactions ß A metathesis reaction between two salts merely involves an exchange of anions, although in the context we will use there can also be a redox component. If the appropriate starting materials are chosen, a highly exothermic reaction can be devised. ß MoCl 5 +5/2 Na2 S → MoS2 + 5NaCl + 1/2S ß The enthalpy of this reaction is DHrxn = -213 kcal/mol ß Due to the highly exothermic nature of this reaction, once it is started (by grinding, spark, etc.) the heat generated by the reaction itself leads to a rapid increase in temperature ß The maximum temperature attained is ~ 1700 K. ß The reaction is completed in < 4 s. ß The percent yield is typically 80% of theoretical. ß Washing with CH3OH removes remaining MoCl 5 ß Washing with H2O removes NaCl ß Washing with chloroform removes remaining S SSM (Solid State Metathesis) AB + CD ® AC + BD (A:metal, B:halogen; C: alkali metals, D: S, P, As, Sb) Examples： MCl4+2Li2S®MS2+4LiCl (M=Ti, Zr, Hf and V) (>400°C) 3SiCl4+4NH3®Si3N4+12HCl MCl2+Li2Fe2O4®MFe 2O4+2LiCl (400-500°C) ZrCl4+4/3Li3N®ZrN+4LiCl+1/6N2 SnI4+2Li2E®SnE2+4LiI (E=O, S, Se)(500°C) PbCl2+Li2E®PbE+2LiCl (E=O, Se, Te) (500°C) PbCl2+Li2O®Pb2O2Cl+LiCl+[Pb] (500°C) LnCl3+Li3N®LnN+3LiCl (Ln=Y,La,Pr,Nd,Sm,Eu,Gd,Tb) MClx (s)+(x/3)Li3N(s)®MN(s)+xLiCl+(x-3)/6N2 (g) MClx (s)+xNaN3 (s)®MN(s)+xNaCl(s)+(3x-1)/2N2 (g) Solid State Metathesis Reactions to III-V Group Reactants Condition Products Reactants Condition Products AlI3+Na3P Bomb ignition Amorphous GaI3+Na3S b Bomb ignition GaAs+Sb 990°C/42h AlP <600°C/12h GaAs +Sb(trace) AlI3+Na3As Bomb ignition Amorphous GaF3+Na3Sb Bomb ignition GaAs+Sb 220°C/12h AlAs+Al+As InI3 +Na3P Bomb ignition InP+In+InI 2+ trace P 550°C/17h AlSb+trace As <600°C/12h, InP+In+InI2 AlI3+Na3Sb Bomb ignition AlSb+Al+Sb >600°C/12h InP 550°C/12h AlSb+ trace Al+Sb InF3+Na3P Bomb ignition InP+In+trace impurities >600°C/18h AlSb+ Al+Sb InI3 +Na3As Bomb ignition InAs+In+InI2 GaF3+Na3P Bomb ignition GaP+P(red)+ Ga(trace) <600°C/12h InAs+In+InI2 GaCl3+Na3P Bomb ignition GaP+P(red)+ Ga(trace) >600°C/12h InAs+In GaI3 +Na 3P Bomb ignition GaP+P(red)+ Ga(trace) InF3+Na3As Bomb ignition InAs+In+trace impurities >220°C/8h GaP +Ga(trace) InI3 +Na3Sb Bomb ignition InAs+In+Sb GaI3 +Na 3As Bomb ignition GaAs +Ga(trace) 550 °C/12h InAs+Sb <570°C/12h GaAs +As(trace) 950°C/8h GaAs The product is impure, often contains the impurities such as metals. Since the actual reaction temperature is far higher than the melting point of LiCl, the by-product LiCl is sintered together with the product. Up to now, SSM is used on laboratory research, and can not used for large scale production. The method to remove the impurities: GaN or InN can not be obtained in the similar system of GaCl3 or InCl3 and Li3N under similar condition, which produce corresponding metals Ga or In and release nitrogen gas. Disadvantages of SSM AlAs C/8h (2)wash o (1)1000 AlAs NaI Al As ignite As 3 Na 3 AlI ¾¾¾¾¾¾¾¾¾® ¾¾¾¾¾¾® + ¾¾¾ ¾® + + + Sintering Overview Sintering is a process specifically for powdered materials, including metals, ceramics, and plastics. In this process, the compressed powder is heated to a temperature close to but not at melting, in a controlled-atmosphere furnace. This is done so that particles may bond by solid state bonding, but not melt. Surface area reduction Powder process
tering Sintering Sintering: powder pressing firing below melting T e mechanism 888 A neck forms between two islands and thickens as atoms ransported into the eThe driving force for neck growth is to reduce the total surface energy of An animation showing urfaces have a greater activity than metal or ceramic powder is toms situated in the concave neck; an effective concentration gradient between these mass transport into the neck. Density Ceramics +1 Structure and Properties Cracks and pores e3 New ceramics <4 Glass 1o Important Elements x Introduction Introduction Classification of ceramic products ●Anin mpound consisting of a metal ay products and one or more nonmetals refractory ceramics ■ Cement More abundant and widely used materials Clay product, glass, cement, concrete and Whiteware (1) Traditional Ceramics ● High hardness electrical and thermal (2) New Ceramics )Glasses melting temperatures (4) Glass-cer ● But too brittle n Nuclear fuels
10 Sintering Sintering: powder pressing + firing below melting T An animation showing the changes in pore shape when metal or ceramic powder is sintered. Sintering is a coalescence mechanism involving islands in contact. A neck forms between two islands and thickens as atoms are transported into the region. The driving force for neck growth is to reduce the total surface energy of the system. Since atoms on the convex island surfaces have a greater activity than atoms situated in the concave neck; an effective concentration gradient between these regions develops. ® mass transport into the neck. Sintering Cracks and Pores Density Increases Grain Growth Sintering Ceramics 1 Structure and Properties 2 Traditional Ceramics 3 New Ceramics 4 Glass 5 Important Elements Introduction An inorganic compound consisting of a metal and one or more nonmetals. More abundant and widely used materials ¾ Clay product, glass, cement, concrete and modern ceramics High hardness, good electrical and thermal insulation, chemical stability and high melting temperatures. But too brittle Introduction Classification of Ceramic Products Clay products Refractory ceramics Cement Whiteware Glass products Glass fibers Abrasives Cutting tools Ceramics insulators Magnetic ceramics Nuclear fuels Bioceramics (1) Traditional Ceramics (2) New Ceramics (3) Glasses (4) Glass-ceramics