Precursors for ceramics synthesis

Size: px

Start display at page:

Download "Precursors for ceramics synthesis"

Gabriel Ramsey
5 years ago
Views:

Precursors for ceramics synthesis Decrease diffusion lengths by using intimately mixing of cations. Solid precursors containing the desired cations.

Precursor: Cations are incorporated in the same (stoichiometric) solid. Thermal decomposition at relatively low temperature poorly cryst. Solid.

1 Precursors for ceramics synthesis Decrease diffusion lengths by using intimately mixing of cations. Solid precursors containing the desired cations. Coprecipitation: salts of different metals are precipitated together ( as low solubility solids). Either a solid solution or an intimate mixture of two salts. Precursor: Cations are incorporated in the same (stoichiometric) solid. Thermal decomposition at relatively low temperature poorly cryst. Solid. Either with oxidation state preservation or by changing oxidation state. 1 Coprecipitation Soluble salts of the desired cations are dissolved (usually in water) Coprecipitation by heating or by addition of a precipitating agent (forming insoluble salts) Hydroxides, carbonates, oxalates, formates, citrates (how about nitrates?) ZnFe 2 O 4 : zinc and iron oxalate are dissolved in water (?) Heated to evaporate water. precipitate as fine powder, solid solution? Heat to decompose: Fe 2 (C 2 O 4 ) 3 + Zn(C 2 O 4 ) ZnFe 2 O 4 + 4CO + 4CO 2 2

2 Carbonate precursors Formation of mixed oxides: M 1-x M x O, (M, M = Ca, Mg, Mn, Fe, Co, Zn, Cd) Calcite type solid solutions used as precursors. Ideal for formation of mixed oxides with rock salt structure due to structural similarity. 3 The low decomposition temperature may allow formation of oxides preserving a high oxidation state. Formation of CaMnO 3 by standard methods requires 1300 C, days of heating and repeated crushing/heating. Using a carbonate precursor CaMnO 3 may be prepared at 900 C for 30 minutes. May also be used for preparation of low temperature phases Note changes in oxidation state 4

3 Oxides prepared from carbonate precursors 5 6

4 Preparation method Prepare a slightly acidic solution of the cations (e.g. nitrates) with the wanted poportions. Add ammonium carbonate, (NH 4 ) 2 CO 3 to precipitate Also used with hydroxide, nitrate, and cyanide solid solutions. E.g. Ln 1-x M x (OH) 3 Coprecipitation depends on: Similar solubility of the metal salts Similar precipitation rate Formation of solid solution Filter and wash the precipitate Calcine under appropiate atmosphere for hours at C Successful preparation by calcination may depend on: Similar decomposition temperatures for the metal precursors Structural similarity and compatibility of the oxides 7 Ln 1-x M x (OH) 3, (Ln =La, Nd, M = Al, Cr, Fe, Co, Ni) La 1-x-y M x M y (OH) 3, (M = Ni, M = Co, Cu) A 1-x Pb(NO 3 ) 2 f.eks BaPbO 3 LaFe 0.5 Co 0.5 (CN) 6 5H 2 O LaFe 0.5 Co 0.5 O 3 2A x (NH 4 ) y Mo 3 S 9 + (10-y)H 2 A 2x Mo 6 S H 2 S + 2yNH 3 ((NH 4 ) 2 MoS 4 + AlCl n to prepare the precursor) 8

Redox (NH 4 ) 6 [Mo 7-x W x O 24 ] H 2 Mo-W alloys (NH 4 ) 6 [Mo 7-x W x O 24 ] O 2 Mo-W alloys Eu-Ba-Cu alloys O 2 EuBa 2 Cu 3 O 7 Organomelattic precursors are used in a range of syntheses, e.g. organoaluminium silicate for aluminosilicates.

5 Redox (NH 4 ) 6 [Mo 7-x W x O 24 ] H 2 Mo-W alloys (NH 4 ) 6 [Mo 7-x W x O 24 ] O 2 Mo-W alloys Eu-Ba-Cu alloys O 2 EuBa 2 Cu 3 O 7 Organomelattic precursors are used in a range of syntheses, e.g. organoaluminium silicate for aluminosilicates. 9 Precursor method Stoichiometric (mixed metal) compounds NiFe 2 O 4 may be prepared from: Ni 3 Fe 6 O 3 (OH)(OAc) 17 2py 2-300ºC to burn off the organic part Heating in air at 1000ºC for 2-3 days 10

6 Examples: LaCo(CN) 6 5H 2 O LaCoO 3 LaFe(CN) 6 5H 2 O LaFeO 3 Ba[TiO(C 2 O 4 ) 2 ] BaTiO 3 Li[Cr(C 2 O 4 ) 2 (H 2 O) 2 ] LiCrO 2 M 3 Fe 6 (CH 3 COO) 17 O 3 (OH) 12C 2 H 5 N MFe 2 O 4 (M = Mg, Mn, Ni, Co) Mn 1-x M x CO 3 Mn 1-x M x O (M = Mg, Ca, Co, Cd) Ca 2 Fe 2 (CO 3 ) 4 Ca 2 Fe 2 O 5 11 Precursor redox reactions (NH 4 ) 2 Cr 2 O 7 Cr 2 O 3 + N 2 + 2H 2 O Formation of MCr 2 O 4 spinel from (NH 4 ) 2 M(CrO 4 ) 2 6H 2 O (NH 4 ) 2 Mg(CrO 4 ) 2 6H 2 O MgCr 2 O 4 + N H 2 O Decomposition of nitrates: Complex, but involves reduction of N and oxidation of O 2-, e.g.: KNO 3 KNO 2 + ½ O 2 Zn(NO 3 ) 2 ZnO + 2NO 2 + ½ O 2 12

7 Formation of metastable solids Often the thermodynamically stable material is not the most interesting regarding properties and application Metastable compound are difficult to prepare using classical high temperature routes. Soft chemistry (Chimie douce, myk kjemi) methods must be used. Preparation of metastable phases: 1. Synthesis under conditions where the material is thermodynamically stable followed by quenching to ambient. 2. Preparation of thermodinamically stable compounds, and transformation of these to a metastable compound by a low temperature, soft chemistry, method. 3. Synthesis under non-equilibrium conditions; kinetic control of product formation. 13 K 2 Ti 4 O 9 ion echange in acid to: H 2 Ti 4 O 9 followed by heating (dehydroxylation, condensation): H 2 Ti 4 O 9 4TiO 2 + H 2 O 14

Chapter 17 Solubility and Complex Ion Equilibria

Chem 1046 General Chemistry by Ebbing and Gammon, 8th Edition Prof George W.J. Kenney, Jr Last Update: 27Nov2008 Chapter 17 Solubility and Complex Ion Equilibria These Notes are to SUPPLIMENT the Text,