Solid State Synthesis •  Solid forms: crystals, powders, fibers, films, foams,

ceramics, nanoparticles, morphology •  Direct reaction •  Crystallization: solution, melt, glass, sol-gel •  Precursor method •  Solvothermal: high T, P •  Soft Chemistry: “novel” metastable phases •  Intercalation: injection (chemical, electrochemical),

ion-exchange •  Vapor Phase Transport (VPT) •  Combustion synthesis •  Thin films: chemical, electrochemical, physical

•  West, Ch. 4; Smart & Moore, Ch. 3

Factors Influencing the Reaction of Solids

Ø  Structural considerations Ø  Defect concentration, type

Ø  Reaction conditions

Ø  Reaction “mechanism”

Ø  Nucleation, diffusion rates

Ø  Surface area

Ø  Surface reactivity, structure, free energy

Ø  Concepts, techniques different from conventional synthesis and characterization of molecular solids, liquids, solutions, gases

Direct Solid State Reaction

•  –ΔG°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

•  Linear dependence of x on t2

•  Overall rxn: MgO + Al2O3 → MgAl2O4

MgAl2O4 Spinel Product Layer

MgO / MgAl2O4 Reactant / Product Interface

MgAl2O4 / Al2O3 Product / Reactant Interface

MgO

MgO

Al2O3

Al2O3

Mg2+

Al3+

x/4

3x/4

Section 4.2

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 breakage and formation •  Topotaxy at MgO/spinel interface (ccp for both) •  Epitaxy at Al2O3/spinel interface (hcp to ccp) •  Substitutional or interstitial hopping of Mg2+, Al3+ across

growing spinel interface •  High T process

Kirkendall Effect •  Mg2+, Al3+ diffusion usually rate determining step •  Reaction slows as MgAl2O4 layer grows •  Longer distance for cations to diffuse •  Spinel growth faster on one side due to charge-balance •  3Mg2+ diffuse to right, balances 2Al3+ to left

•  MgO / MgAl2O4 Reactant / Product Interface: 4MgO – 3Mg2+ + 2Al3+ → MgAl2O4

•  MgAl2O4 / Al2O3 Product / Reactant Interface: 4Al2O3 – 2Al3+ + 3Mg2+ → 3MgAl2O4

4MgO + 4Al2O3 → 4MgAl2O4

•  MgO + Fe2O3 → MgFe2O4, colored spinel interface, can easily monitor growth rate

rhs growth rate lhs growth rate = 3/1

MgO

MgO

Al2O3

Al2O3

Mg2+

Al3+

x/4

3x/4

The Sol-Gel Method •  Soluble metal source •  e.g. metal halide, tetraorthosilicate (TEOS), titanium

isopropoxide [Ti(OiPr)4], etc.

•  Covalent liquid, add alcohol and water

•  Acid or base hydrolysis: Cl– + H+ + ROM(OR)3 → HOM(OR)3 + RCl

HO– + M(OR)4 → (HO)M(OR)3 + OR–

•  Condensation polymerization:

(RO)3M-OH + HO-M(OR)3 → (RO)3M-O-M(OR)3 + H2O

•  Careful control of water, pH, temperature, time, gel calcination

•  Determines viscosity, structure of metal oxide product

•  e.g. MgAl2O4 spinel from gel, heat only to 250°C, but expensive metal alkoxide reagents, e.g. Mg(OCH3)2 and aluminum tri-sec-butoxide

Section 4.3.1

Materials via the Sol-Gel Method

•  Silica glass from TEOS (SiO2 melt is too viscous, even at 2000°C) •  Alumina fibers (insulating material) from Al(OsBu)3 •  ITO coating: indium tin oxide, semiconducting, transparent

•  Reflects IR, used to coat building windows •  Substrate dip-coated into Sn & In alkoxides, polymerized in situ •  Crack-free and adherent if thin

•  YSZ: yttria-stabilized zirconia, from Y(OPr)3 & Zr(OPr)4

•  Zeolites •  Abrasives: disperse AlOOH in acid to yield sol; mix with other metals,

polymerize to a gel; dry; mill; burn → sharp metal oxide particles

Solid State Precursors •  Crystalline, phase-pure material •  Decomposes on Δ

•  Limited choice

•  Great for spinels, e.g. chromite spinels, tune the magnetic property via the CFSE of the metal ions in the Td and Oh sites:

Chromite Spinel Precusor 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(CrO4)2•2NH3 1400°C

Section 4.3.4

Hydrothermal Synthesis •  Aqueous solvent, high T and P •  (Teflon-lined) sealed autoclave (“bomb”) •  Pressure introduced externally, or internal autogenous pressure via degree of filling (dashed lines)

•  AB saturated steam curve (compressed liquid above, liquid and vapor below)

•  Supercritical fluid above B

•  Heat gel to form zeolite •  Piezoelectric SiO2 quartz

using seeds, 1M NaOH mineralizer, temperature gradient (400°C nutrient, 360°C seed)

•  600 tons / year

Section 4.3.5

B(g) A

Vapor Phase Transport (VPT) •  Growth of single crystals for purification,

passivation or new compounds •  A: reactant(s); B: vapor phase transporting agent •  Temperature gradient furnace, ΔT > 50°C •  A & B react at T1

•  A(s) + B(g) ↔ AB(g) small K •  AB volatile, unstable, decomposes at T2

•  Deposits crystals of purified A •  Section 4.4.1

AB(g) A A

T1 T2

The VPT Equilibrium •  Equilibrium with small K necessary •  Driving force for diffusion: concentration gradient of AB(g)

•  A(s) + B(g) ↔ AB(g) K depends on T

•  K = exp(–ΔG°/RT)

Formation of AB Exothermic •  AB(g) forms at cold end •  Decomposes to A(s), B(g) at hot

end •  ΔG° negative ⇒ K larger for

lower T •  Equilibrium shifts to right @

lower T, left @ higher T •  ⇒ T1 < T2

Formation of AB Endothermic •  AB(g) forms at hot end •  Decomposes to A(s), B(g) at cold

end •  ΔG° positive ⇒ K larger for

higher T •  Equilibrium shifts to right @

higher T, left @ lower T •  ⇒ T1 > T2

Pt(s) + O2(g) ↔ PtO2(g)

•  Endothermic •  T ≥ 1200°C:

PtO2(g) forms at hot end Pt crystallizes at cold end

•  T1 > T2

•  Observed in furnaces with Pt heating elements •  Pt crystals form on furnace walls

O2(g) Pt PtO2(g) Pt Pt

T1 T2

van Arkel Method

•  Purification of metals from their carbides, nitrides, oxides

•  Ti/Hf/Th, V/Nb/Ta, Cr, Fe, Cu

Cr(s) + I2(g) ↔ CrI2(g)

•  Exothermic

•  CrI2(g) forms at cold end

•  Cr(s) crystallizes at hot end

•  T1 < T2 I2(g) Cr CrI2(g) Cr Cr

T1 T2

Synthesis with VPT •  Coupling VPT with subsequent reaction •  Can make binary, ternary, quaternary compounds

A(s) + B(g) AB(g)

AB(g) + C(s) AC(s) + B(g)

A(s) + C(s) AC(s)

T1

T2

T1

T2

T1

T2

AB(g) A AC

T1 T2

B(g) A C

VPT Synthesis of Metal Sulfides

•  Passivation (unwanted):

2Al(s) + 3S(s) Al2S3(s) – coated Al(l)

•  Large crystals in presence of I2(g):

Al2S3(s) + 3I2(g) 2AlI3(g) + 3/2S2(g)

•  Same for ZnS •  Gases react much faster than solids due to mobility •  Can occur between crystals of an isothermal solid

state reaction

800°C

700°C

800°C

Double Transport •  Endothermic in one direction, exothermic in the other •  Separation of W and WO2

•  H2O and I2 transporting agents

•  Endothermic: WO2(s) + I2(g) WO2I2(g)

•  Exothermic W(s) + 2H2O(g) + 3I2(g) WO2I2(g) + 4HI(g)

1000°C

800°C

1000°C

800°C

I2(g) W/WO2 WO2I2(g) W/WO2 WO2

1000ºC 800ºC

H2O(g) & I2(g) W/WO2 WO2I2(g) & HI(g) W W/WO2

1000ºC 800ºC

Double VPT Synthesis

•  Direct reaction very slow at high T:

SnO2(s) + 2CaO(s) Ca2SnO4(s)

•  Fast with CO transport agent:

SnO2(s) + CO(g) SnO(g) + CO2(g) SnO(g) + CO2(g) + 2CaO(s) Ca2SnO4(s) + CO(g)

•  NiCr2O4, Nb5Si3, ZnWO4: Section 4.4.1, p.216

Combustion Synthesis •  Controlled explosion •  Highly exothermic, high T maintained, self propagating

once started •  Reaction complete in seconds or minutes

Fe2O3 + 2Al 2Fe + Al2O3

•  Al “fuel”, iron oxide “oxidant” •  T = 3000°C •  Cut or weld metal •  Extraction of metal •  Section 4.2.3

High Pressure Synthesis •  Static pressure up to several hundred kbar •  Ambient or high T

•  Shock wave for higher P, T

•  Sample placed in center of opposed anvils or tetrahedral anvil

•  e.g. SiO2 → stishovite polymorph above 100 kbar

•  Rutile structure: Si in Oh sites

•  graphite (3 C.N.) → diamond (4 C.N.) at 130 kbar, 3000°C

•  Others: Table 4.5

•  Unusual oxidation states

•  e.g. Cr: 3+ Oh, 6+ Td; Cr4+ in perovskite PbCrO3, CaCrO3, …

•  Section 4.5

Crystal Growth

Czochralski Method (Section 4.6.1) •  Growth from melt of desired composition

•  Seed in contact with melt surface, rotated slowly out of melt

•  Melt held just above MP •  Cylindrical single crystals

•  Si, Ge, GaAs, …

•  Inert atmosphere & high P to prevent loss of As, P, etc.

•  Inert oxide on top of melt (B2O3) to prevent loss of more volatile As component

•  Section 4.6 •  Growth from vapor, liquid or solid •  First two give larger crystals •  Property measurement •  Device fabrication

•  Melt inside temperature gradient furnace

•  Stationary

•  Furnace gradually cooled

•  Crystallizes at cool end (< Tm)

Stockbarger Method

Bridgman Method

•  Melt passed through temperature gradient

•  Crystallizes at T < Tm

Section 4.6.2

•  Similar to Stockbarger Method

•  Boat pulled through furnace

•  Melt only in one section, initially in contact with seed

•  Oriented nucleation onto seed

Zone Melting

•  Impurities more concentrated in liquid phase than solid phase

•  Impurities swept out by moving molten zone

•  Purification of W, Si, Ge, …

Zone-Refining Technique

Section 4.6.3

Verneuil Flame Fusion Method

•  First used 1904, high melting oxides

•  Artificial gemstones of ruby, sapphire

•  Fine powder passed thru O2 / H2 flame

•  Melts, drops to seed or growing crystal, e.g. CaO

•  All above methods lead to rapid growth rate of large crystals

Section 4.6.5