6. Chemick reakce v tuhch keramikch :

Kinetika heterogennch reakc

k = Z exp (EA/RT)

Kinetika homogennch chemickch proces (v kapaln nebo plynn fzi)

Fyzikln-geometrick model kinetiky chemickch proces v tuh fzi

Kontinuln model pemny stice na produkt Pozvoln zmna koncentrace na fzovm rozhran typick pro difuzn pemny

Diskontinuln model zmenujc se stice Skokov zmna koncentrace na fzovm rozhran, typick nap. pro nukleaci

Klasifikace chemickch proces v tuh fzi s vyuitm modelu kulov stice

N nukleace D difuze R povrchov chemick reakce E vypaovn, sublimace, rozpoutn M bezdifuzn martenzitick pemna G rstov proces zen difuz nebo povrchovou chemickou reakc

MgO

Mikrostruktura reakn vrstvy vznikajc reakc zenou difuz

( )21

'tkx =

Al2O3 + MgO Al2O3.MgO

Al2O3.MgO

Typ kinetiky reakce kulovch stic, pi kter se na povrchu stic tvo vrstva produktu pes kterou difunduj reagujc komponenty, zvis na rychlosti nejpomalejho kroku, kter se stv dcm procesem:

1. dcm procesem je chemick reakce (povrchov zen reakce) Reakn rychlost je dna rychlostn rovnic: 1 (1 )n = kt/r0 je stupe pemny, k rychlostn

konstanta, t as ,r0 velikost stic a n je d reakce (n je rovno 1/3, 1/2, a 1 pro t, dvou a

jednorozmrnou symetrii reagujcch stic)

2. dcm procesem je penos hmoty (difuzn zen reakce) Reakn rychlost je dna rychlostn rovnic: (1 (1 )1/3)2 = 2kDt/r20 je stupe pemny, k rychlostn

konstanta, t as ,r0 velikost stic a D je difuzn konstanta

= V / Voo

Procesy zen nuklean-rstovm mechanizmem

- ln (1 - ) = ktm J-M-A-J-K rovnice

ln(- ln (1 - )) = ln k + m ln t

Vyhodnocen mechanismu procesu

z asov zvislosti stupn pemny

ln(- ln (1 - ))

ln t

Vyhodnocen teplotn zvislosti rychlostn konstanty

pomoc Arrheniovy rovnice

ln k = ln Z EA/RT

ln k

1/T

Pednosti reakc v tuh fzi

Snadno se provdj Vchoz reaktanty jsou dostupn Produkty mohou bt pipraveny ve velkm mnostv

Dlouh reakn doba Nzk chemick homogenita Produkty maj velk zrna Je teba pout vysokch teplot Nedostaten kontrola sloen prdukt

Nevhody reakc v tuh fzi

Cvien Pprava keramickch materil syntzou v tuh fzi

Solid State Reactions (SSR)

Solid state reaction: the direct reaction of a mixture of starting reagents (usually powders) at high temperature (700 - 1600C)

High temperature provides the necessary energy for the reaction to occur

Contact between grains of the reagents

High temperature

Diffusion, starts at the interface

Conventional Synthetic Routes

Inorganic materials with important physical properties are prepared by SSR, like ferroelectrics (BaTiO3) and high Tc superconductors (YBa2Cu3O7-)

MgO Al2O3

Al2O3 MgO

MgAl2 O

3

Solid state reaction - Reaction rates

Solid state reactions are usually slow (from 8 hours to several days)

large amount of structural reorganization

bonds break and ions migrate through a solid

Unlike gas phase and solution reactions, the limiting factor in solid-solid reactions is usually diffusion.

The rate controlling step in a solid state reaction is the diffusion of the cations through the product layer

Reasons:

1= kxdtdx

or ( )21

'tkx =

=

=

=

',kktx thickness of the product layer

time

rate constants Rate law governing diffusion through a planar layer

A solid-state reaction will not occur until the temperature reaches at least 2/3 of the melting point of one of the reactants.

100 200

5

10

1

5

20

time (hours) x2

1

06 (c

m2 )

1500C

1400C

1300C

MgO + Al2O3 MgAl2O4

x = thickness of the product

Solid state reaction - Temperature dependance

The reaction occurs much more quickly with increasing temperature

Reagents and Equipment Reagents (synthesis of BaTiO3)

BaCO3 + TiO2 BaTiO3 (1200C)

BaO + CO2(g)

Pestle and mortar Starting reagents should be thoroughly mixed in stoichiometric amounts

Alkaline earth oxides are moisture sensitive and therefore not used as starting reactants Hydroxides, nitrates, oxalates and carbonates are often used as starting reactants instead of oxides.

BaO + TiO2 BaTiO3

Furnaces

Crucibles containing starting reagents are put into high temperature furnaces. Furnaces provide the temperature to initiate and carry on the reaction

Reaction vessels (crucibles)

Crucibles are made of refractory materials (Al2O3, ZrO2, Pt) for 2 reasons: 1. Chemically inert to the reactants 2. High melting point

WORKED PROBLEM

gCuO...gSrCO...

gin)CuOSr(moles..

.)CuOSr(RMM.)CuO(RMM

.)SrCO(RMM

832100104605457908843010460631472

3010460783286

3783286

5457963147

3

32

32

3

=

=

=

==

=

Calculate the amount of reactants needed to prepare 3g of Sr2CuO3

1. Reagents: SrCO3 and CuO (watch the oxidation state of the TM)

2. Reaction:

3. Calculate the amount of reagents:

( ) ( ) ( ) ( )gCOsCuOSrsCuOsSrCO 2323 22 ++

Reactions in controlled atmospheres

Compounds containing metals in an unstable oxidation state cannot be prepared in air, but need either a reducing or an oxidising atmosphere

Experimentally: a gas (H2/N2, O2) is passed over the reaction mixture in a tubular furnace

H2 extracts O2 via formation of H2O

1/2O2 is added to the chemical formula

Reaction in a reducing atmosphere (V5+ V2+)

Reaction in an oxidising atmosphere (Ni2+ Ni3+)

( ) ( )sVOsOV N/H 2252

( ) ( )sLaNiOsLaNiO O. 352 2

Snmek slo 1Snmek slo 2Snmek slo 3Snmek slo 4Snmek slo 5Snmek slo 6Snmek slo 7Snmek slo 8Snmek slo 9Snmek slo 10Snmek slo 11Snmek slo 12Snmek slo 13Snmek slo 14Snmek slo 15Snmek slo 16Snmek slo 17Snmek slo 18Snmek slo 19Snmek slo 20