7 Solid State Synthesis Flashcards
4 types of SS syn
- ceramic
- mechanochemical
- combustion
- low temperature
What is Ceramic method?
- advantages/disadvantage
- rxn solid components at high T, solid percussors mixed and heated
- adv: readily available percussors, Inexpensive powder production
- dis: time consuming, power intensive
Steps in ceramic
Grind/mix, press into pellet, in boat/furnace, heat, anneal, cool
why slow heating in ceramic method
avoid vigorous rxn
types of boats for cermaic
Quarts, Alumina
What is annealing
- extended process at high T, ~50% rxn temp
- passivates defects
Why grind reagents
Increase SA
Why press into pellet
decrease path lenth
In __________ atoms/ions _______ through lattice to passivate defects
annealing, migrate
rule of thumb what T rxns occur
2/3 MP of one component , provides high mobility w/o melting material
Rxn rate _______ as rxn progresses, equation?
decreases
rate= kx^-1 x=[prod]
Factors that affect kinetics of SS rxns
1) contact area b/w surface
2) rates of diffusion ion through various phases
3) rate nucleation of product phases
Where does the new compound form? defects?
In middle,
more defects as progresses
Experimental considerations (4)
1) use salt to decompose high T
2) rxns start at structural defects (extended defects: grain bound, dislocations ect)
3) Need inert boat (silica, alumina, graphite)
4) precursors volatile/air sen use ampule
how to increase SA (5)
grind, use ball mill, pellets, periodic cooling/grinding
when possible (usually not) do rxn in melt
Disadvantages of ceramic (5)
- make undesirable phases
- inhomogeneous distribution of dopants
- can’t do in-situ monitoring
- difficult to separate prod mixtures
- reactants need to have sim decomposition T
Molecular mixing: Coprecipitation
- challenges
- salts 2 metals ppt from solution to allow for molecular mixing
- need sim solubilities/ppt rates
Molecular Mixing: Precursor
- challenges
- uses precursors that incorporate the atoms/ions wanted in prod with correct stoich ratios
- need to be able to remove impurities, not everything has a precursor
Affecting Kinetics: Nucleation
- crystal structure prod diff then react/ lattice parameters largely diff maybe no nuc
Oriented rxns
Epitactic: structure sim interface b/w prod and reac
Topotactic: structure sim bulk b/w prod and reac
Mechanical methods
- reactive surfaces form when reagents place with hardon steal/ WC (tungston carbide)
- lots of defects , anneal but be carful not to make new pdt
- high diffusion even at RT, some localize heating but does not affect
most defects of all methods
mechanical
rank path lenths
mechanical> co-ppt > single precursor
Combustion
- Fast, started using: electric arc, laser pulse, spark, chemical reaction
- less E, short rxn time, low operating/processing cost, vigorous, make not at eq pdt
- very exo
2 types combustion: self propagation vs simultaneous combustion
self propagation (SHS): rxn progress through waves
simultaneous combustion (SCM): throughout solid once ignition T reached
Low T rxns - Intercalation rxn:
- layered structures with weak intermolecular interactions
intercalation vs insertion rxn
intercalation: layered material
insertion: 3D units
low T: Insertion rxn
- have large lattice
- cations inserted
Low T: Ion exchange
exchange weakly bond with stronger
Solid from gas phase
- non volatile reacts with gas to make stuff
-exo if sink hotter source
-endo is sink cooler source
light bulbs
W (halogen( better, has lower vapour P then C (incandescent) and bigger MP
- halogen in inter gas to reduc eveaperation
- transort to gas phase and undergo reations in gas phase (CVT)
Chemical vapour deposition (cvd)
make films
- don’t want gas phase rxn
- 1+ precusor in vapor phase deposide on heated substrate chamber
-
General Aspects – CVD
- Reagent transport in gas phase
- Diffusion of precursors to the substrate
- Adsorption of precursors onto growth surface
- Diffusion to growth sites
- Surface chemical reactions leading to film
deposition - Desorption of byproducts
- Transport of byproducts
Precursor characteristics
Volatile liquid.
– High thermal stability (evaporation/
transport).
– Clean decomposition make stable by products readily removed from the reaction
zone.
– High purity, readily available, low cost.
– Non-toxic and non-pyrophoric
Percussor for CVD
multiple and single source
