Introduction of Solid State Scopes of Solid State Synthesis Synthesis Method .Solids: crystals, fibers, films, foams, ceramics, powders eDirect reaction .Crystallization: solution, melt, glass, sol-gel pRecursor method Soft Chemistry for"novel" metastable phases apor Phase Transport (VPT) a, electrochemical .Thin films: chemical, electrochemical, physical methods .Single crystal growth: vapor, liquid, solid phase elon exchange Introduction of solid State Synthesis Methods Solid State Reaction Processing e Steps in Conventional Solid State Synthesis Diffusion mechanisms and fick's law Optimum Conditions for Conventional Solid State Synthesis 1. Solid State Reaction Processing r Self Propagating High Temperat 2.Ceramics--From Solid State Reactions Synthesis (SHS) 3. Important Methods for Solid State Materials Solid State Reaction Processing Steps in Conventional Solid State Synthesis Typical Solid State Reaction Processing 1. Select appropriate starting material powders to maximize surface area a Raw materials with additives c)Well defined compositions e Forming(Green bodv) ng (Products 3. Mix starting materials together b) Aaam motar and pe stle orga nie solvent optional 4. Pelletize te contact of re b) Minimizes contact with the crucible c)Organic binder may be used to help keep pellet together
1 Introduction of Solid State Synthesis Methods Scopes of Solid State Synthesis Solids: crystals, fibers, films, foams, ceramics, powders, nanoparticles, morphology Direct reaction Crystallization: solution, melt, glass, sol-gel Precursor method Solvothermal: high P, T Soft Chemistry for “novel”metastable phases Injection, intercalation: chemical, electrochemical Vapor Phase Transport (VPT) Thin films: chemical, electrochemical, physical methods Combustion synthesis Single crystal growth: vapor, liquid, solid phase Ion-exchange Introduction of Solid State Synthesis Methods 1. Solid State Reaction Processing 2. Ceramics ¾¾ From Solid State Reactions 3. Important Methods for Solid State Materials Solid State Reaction Processing Steps in Conventional Solid State Synthesis Diffusion Mechanisms and Fick’s law Kirkendall Effect Optimum Conditions for Conventional Solid State Synthesis Self Propagating High Temperature Synthesis (SHS) Sintering Solid State Reaction Processing Typical Solid State Reaction Processing Raw materials with additives Mixing/Grinding Forming (Green body) Sintering (Sintered body) Machining (Products) 1 2 3 4 5 6 7 Steps in Conventional Solid State Synthesis 1. Select appropriate starting materials a) Fine grain powders to maximize surface area b) Reactive starting reagents are better than inert c) Well defined compositions 2. Weigh out starting materials 3. Mix starting materials together a) Agate mortar and pestle (organic solvent optional) b) Ball Mill (Especially for large preps > 20g) 4. Pelletize a) Enhances intimate contact of reactants b) Minimizes contact with the crucible c) Organic binder may be used to help keep pellet together
Reactivity, strength, cost, ductility all important Ca) Ceramic refractories (crucibles and boats ta)Factors influencing choice of temperature include Tamman's rule and potential for volatilization Zb recio z Metals (ru cibles colts and tubes) Tamman's Rule: Extensive reaction will not occur until the -Pt 1770 C 10 ml crucibles $500 or more of the reactants Au 1063 C 10 ml crucibles $340 ab)Initial heating cyele to lower temperature can help to prevent spillage and volatilization 2450C 10 ml crucibles $930 Ce)Atmosphere is also critical Steel: 1400 C (under inert gas 1Pt-1600C PtOT -Oxides(Oxidizing Conditions)-Air, O Low Temps Mo:~2000°C oXides(Reducing Conditions)-H/Ar, CO/COz High T Ta:~2500°C .Nitrides -NH. or Inert(N. Ar, etc. Sulfides-HS Pyrex: borosilicate glass(76%SiO,, 16% B,O Bao.. sEaled tube reactions, Vacuum furnaces 7. Grind product and analyze (x-ray powder diffraction) Quartz Pure SiO2, Tmax 1 100C 8. If reaction incomplete return to step 4 and repeat. Possible reaction paths between Sulfurization method Two Solid grains BaCo, +(1-xNiO+xCoo->BaNi,- Co,S, A B gas phase diffusion interface diffusion urface diffusion Solid solid reaction Atom movement in materials odel for a classical solid-solid reacti (below melting point ! Diffusion; is required for the heat treatment of metals, he solidification Planar interface between two crystals manufacture of transistors and solar cells nd the electrical conductivity of many ceramic materials MgO+ Al O3, MgAl,O, (SpineD Stability of Atoms Atoms possess thermal energy can move from orman reeds a)anormal lattice d another normal lattice(sel Diffusion) O pALO Mgo AL,O3 )) a normal lattice a vacancy(Vacancy Diffusion) (Interstitial Di (d) one side of bor e other side of boundary
2 5. Select sample container Reactivity, strength, cost, ductility all important a) Ceramic refractories (crucibles and boats) -Al2O3 1950°C 10 ml crucibles $30 -ZrO2 /Y2O3 2000°C 10 ml crucibles $94 b) Precious Metals (crucibles, boats and tubes) -Pt 1770°C 10 ml crucibles $500 -Au 1063°C 10 ml crucibles $340 -Ag 960°C 10 ml crucibles $43 -Ir 2450°C 10 ml crucibles $930 -Steel: ~ 1400°C (under inert gas) -Pt: ~ 1600°C (PtO) -Mo: ~ 2000°C -Ta: ~ 2500°C c) Glass Tubes Pyrex : borosilicate glass (76% SiO2 , 16% B2O3 , BaO ...) Tmax. ~ 400°C Quartz Pure SiO2 , Tmax. ~ 1100°C 6. Heat a)Factors influencing choice of temperature include Tamman’s rule and potential for volatilization Tamman’s Rule: Extensive reaction will not occur until the temperature reaches at least 2/3 of the melting point of one or more of the reactants. b)Initial heating cycle to lower temperature can help to prevent spillage and volatilization c)Atmosphere is also critical Oxides(Oxidizing Conditions) –Air, O2 , Low Temps Oxides(Reducing Conditions) –H2 /Ar, CO/CO2 , High T Nitrides –NH3 or Inert (N2 , Ar, etc.) Sulfides – H2S Sealed tube reactions, Vacuum furnaces 7. Grind product and analyze (x-ray powder diffraction) 8. If reaction incomplete return to step 4 and repeat. Sulfurization Method 1 x x 2 CS / Ar BaCO3 +(1- x)NiO+ xCoO¾¾2 ¾®BaNi - Co S Possible Reaction Paths Between Two Solid Grains A and B A B gas phase diffusion volume diffusion interface diffusion surface diffusion Model for a classical solid-solid reaction (below melting point !): Planar interface between two crystals MgO + Al2O3 ® MgAl2O4 (Spinel) Phase 1: formation of seeds Phase 2: growth of seeds MgO Al2O3 Al MgO 2O3 Solid Solid Reaction Atom Movement in Materials Diffusion: is required for the heat treatment of metals, the manufacture of ceramics, the solidification of materials, the manufacture of transistors and solar cells, and the electrical conductivity of many ceramic materials. Stability of Atoms Atoms possess thermal energy can move from (a) a normal lattice Ë another normal lattice (SelfDiffusion) (b) a normal lattice Ë a vacancy (Vacancy Diffusion) (c) a interstitial site Ë another interstitial site ( Interstitial Diffusion) (d) one side of boundary Ë the other side of boundary
Diffusion mechanisms Activation Energy for Diffusion Diffusion of unlike atoms Vacancy Diffusion Interstitial 舒游 88。 000 3 000000 Figure: Diffusion mechanisms in :33 substitutional atom diffusion and b) interstitial diffusion 8888888 Aspects of Solid-Solid Reactions Rate of Diffusion( Fick's First Law COnventional solid state synthesis techniques involve Ficks first law ting mixtures of two or more solids to form a solid phase roduct. Unlike gas phase and solution reactions. the here D=Doexpeey Ficks la J: the flux (atoms/ems) dFlux =-d deldx D: diffusion coefficient (emy/s) a per unit ti Ac Ax: the concentration gradient (atoms/em .cm) OD is the diffusion coefficient (em/sec). this is the average istance the molecule would travel in the direetion of flow hrough unit thickness in unit time .D is independent of concentration only at low The flux during diffusion is centration de/dx is the concentration gradient across the boundary of through a plane of unit tme Table: Diffusion data Fick's First Law selected materiala for Types of Diffusion Volume diffusion 2. Grain Boundary Diffusion where D=Do exp(R 3. Surface Diffusion Table: The effect of the type of diffusion for thorium in ngsten and for self-diffusion in silver 3:8 0格p(-890k7 00ep-12007 099 npt-4s30eRn
3 Diffusion Mechanisms Diffusion of unlike atoms Vacancy Diffusion & Interstitial Diffusion Figure: Diffusion mechanisms in materials: (a)vacancy or substitutional atom diffusion and (b) interstitial diffusion Activation Energy for Diffusion Aspects of Solid-Solid Reactions Conventional solid state synthesis techniques involve heating mixtures of two or more solids to form a solid phase product. Unlike gas phase and solution reactions, the limiting factor in solid-solid reactions is usually diffusion. Fick’s law : Flux = -D dc/dx Flux is mass moving through unit area per unit time D is the diffusion coefficient (cm2 /sec), this is the average distance the molecule would travel in the direction of flow through unit thickness in unit time D is independent of concentration only at low concentrations dc/dx is the concentration gradient across the boundary of interest Rate of Diffusion ( Fick’s First Law ) Fick’s First Law ) RT Q where D D exp( x c J D 0 - = D D = - J : the flux (atoms/cm2 ·s) D : diffusion coefficient (cm2 /s) Dc/Dx: the concentration gradient (atoms/cm3 · cm) The flux during diffusion is defined as the number of atoms passing through a plane of unit area per unit time Fick’s First Law Table: Diffusion data for selected materials ) RT Q where D D exp( x c J D 0 - = D D = - Types of Diffusion 1. Volume Diffusion 2. Grain Boundary Diffusion 3. Surface Diffusion Table: The effect of the type of diffusion for thorium in tungsten and for self-diffusion in silver
To obtain good rates of reaction, you typically 1)The area of contact between reacting solids need the diffusion coefficient to be larger than he contact between reactants we want to use starting reagents with large surface area. Consider the . The diffusion coefficient increases wit numbers for a temperature, rapidly as you approach the melting point. This concept is leads to lumber of Crystallites=1 Tamman's Rule: Extensive reaction will not occur til the temperature reaches at least 2/3 of the umber of Crvst Iting point of one or more of the reactants. urface Area=6x10= number of Crystallites=10 Surface Area= 6x106 cm2 Pelletize to encourage intimate contact between crystallites! 2) The rate of diffusion Consider for Example: the tWo ways to increase the rate of diffusion are to Synthesis of Sr, CrTaO Encrease introduce defects by starting with reagents that 1) Possible starting reagents decompose prior to or during reaction, such as PSr Metal- Hard to handle, prone to oxidation carbonates or nitrates sSrO· Picks up CO2& water,mp=2430°C MSI(NO ,-mp=570C, may pick up some water 3)The rate of nucleation of the product phase PSrCO,-decomposes to SrO at 1370.C dWe can maximize the rate of reactants with crvstal structur nucleat Ta metal-mp=2996°C lilar to that of the pa2O3-mp=1800°C roduct (topotactic and epitactic reactions). MCr Metal- Hard to handle, prone to oxidation rO3-mp=2435°C 2) we To make 5.04 g of Sr, CrTaO, (FW=504.2 g/mol: 0.01 moD Ta2O3mp=2070K→引3mp=1380K(107°O complete the Cr2O3mp=2710K→mp=1807K(1534°O CO,+Ta, O +Cr, 0 2Sr CrTao+cOA Although you may get a complete reaction by heating to 1150C, in practice there will still be a fair amount of ou need unreacted CrO TaO322095g(0.005mo) Cr30.7600g(O preliminary fire at 1400C should be used to prevent the a 500-1600C. The initial heating cycle should be slow, or 3)Grind in a mortar and pestle for 5-15 minutes, then CO, from violently decomposing and spilling out of press a pellet crucible 4) Applying Tammans rule to each of the reagents 5)If the sample is pelletized, the reaction with an alumina crucible should be rather small For the highest purity rCO3→SrO1370°(1643K products, a platinum crucible should be used SrO mp=2700K→引3mp=1800K(1527O 6)All of the elements are in stable highly oxidized states the product, so that heating in air should be appropriate
4 To obtain good rates of reaction, you typically need the diffusion coefficient to be larger than ~ 10-12 cm2/s. The diffusion coefficient increases with temperature, rapidly as you approach the melting point. This concept is leads to Tamman’s Rule: Extensive reaction will not occur until the temperature reaches at least 2/3 of the melting point of one or more of the reactants. Rates of Reaction are controlled by three factors: 1) The area of contact between reacting solids To maximize the contact between reactants we want to use starting reagents with large surface area. Consider the numbers for a 1 cm3 volume of a reactant Edge Length = 1 cm number of Crystallites = 1 Surface Area = 6 cm2 Edge Length = 10 mm number of Crystallites = 109 Surface Area = 6x103 cm2 Edge Length = 100Å number of Crystallites = 1018 Surface Area = 6x106 cm2 Pelletize to encourage intimate contact between crystallites! 2) The rate of diffusion Two ways to increase the rate of diffusion are to Increase temperature Introduce defects by starting with reagents that decompose prior to or during reaction, such as carbonates or nitrates. 3) The rate of nucleation of the product phase We can maximize the rate of nucleation by using reactants with crystal structures similar to that of the product (topotactic and epitactic reactions). Consider for Example: the Synthesis of Sr2CrTaO6 1) Possible starting reagents Sr Metal – Hard to handle, prone to oxidation SrO - Picks up CO2 & water, mp = 2430°C Sr(NO3 ) 2 – mp = 570°C, may pick up some water SrCO3 –decomposes to SrO at 1370°C Ta Metal –mp = 2996°C Ta2O5 –mp = 1800°C Cr Metal – Hard to handle, prone to oxidation Cr2O3 – mp = 2435°C Cr(NO3 ) 3•nH2O –mp = 60°C, composition inexact 2) Weigh out starting reagents To make 5.04 g of Sr2CrTaO6 (FW = 504.2 g/mol; 0.01 mol) to complete the reaction: 4SrCO3+Ta2O5+Cr2O3®2Sr2CrTaO6+4CO2 you need: SrCO3 2.9526 g (0.02 mol) Ta2O5 2.2095 g (0.005 mol) Cr2O3 0.7600 g (0.005 mol) 3) Grind in a mortar and pestle for 5-15 minutes, then press a pellet 4) Applying Tamman’s rule to each of the reagents: SrCO3 ® SrO 1370°C (1643 K) SrO mp = 2700 K ® 2 / 3 mp = 1800K (1527°C) Ta2O5 mp = 2070 K ® 2 / 3 mp = 1380K (1107°C) Cr2O3 mp = 2710 K ® 2 / 3 mp = 1807K (1534°C) Although you may get a complete reaction by heating to 1150°C, in practice there will still be a fair amount of unreacted Cr2O3 . Therefore, to obtain a complete reaction it is best to heat to 1500-1600°C. The initial heating cycle should be slow, or a preliminary fire at 1400°C should be used to prevent the SrCO3 from violently decomposing and spilling out of the crucible. 5) If the sample is pelletized, the reaction with analumina crucible should be rather small. For the highest purity products, a platinum crucible should be used. 6) All of the elements are in stable highly oxidized states in the product, so that heating in air should be appropriate
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]Layer
5 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