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 ?? 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
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
2xx1
Ar/CS
3 SCoBaNixCoONiO)x1(BaCO
2
???? →?+?+
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 Al2O3Al2O3MgO
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 èanother normal lattice (Self-
Diffusion)
(b) a normal lattice èa vacancy (Vacancy Diffusion)
(c) a interstitial site èanother interstitial site
( Interstitial Diffusion)
(d) one side of boundary èthe other side of boundary
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
)RTQexp(DD where xcDJ 0 ?=???=
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
)RTQexp(DD where
xcDJ
0
?=
?
??= Types of Diffusion1. 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
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 = 100?
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.
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) → 2MO n-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
–?G°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 + Al2 O3 → MgAl2O4
MgO/MgAl2O4 Reactant/Product Interface
MgAl2 O4/Al2 O3 Product/Reactant Interface
MgAl2 O4 Spinel Product Layer
MgO
MgO
Al2O3
Al2O3
Mg2+
Al3+
x/4
3x/4
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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 breaking and formation
Topotaxy at MgO/spinel interface (ccp for both)
Epitaxy at Al2O3/spinel interface (hcp to ccp )
Example: MgAl2O4
Reaction only occurs at contact points between
grains of MgO and Al2O3
Get nucleation near contact point and then growth
of product
Growth requires diffusion of Mg2+/Al3+ through the
product
Very slow
4232 OMgAlOAlMgO →+
Kirkendall Effect
In 1947, Smigelkas and Kirkendall reported
the movement of the interface between a
diffusion couple, i.e., copper and zinc in brass,
as the result of the different diffusion rates of
these two species at an elevated temperature.
This phenomenon, now called the Kirkendall
Effect, was the first experimental proof that
atomic diffusion occurs through vacancy
exchange and not by the direct interchange of
atoms.
Because of the different
diffusion rates of each metal
into the other, the front
between the two metals is
observed to move.
The diffusion of zinc into the
copper is faster, and so the
brass alloy boundary between
the zinc (gray) and copper
(brown) appears to move to
the right. As the zinc ions
diffuse into the
copper, they leave vacancies
that can fuse into pores.
Kirkendall Effect
Mg2+, Al3+ diffusion usually is the rate
determining step
Reaction slows as MgAl2O4 layer grows
Longer distance for cations to diffuse
Spinel growth faster on one side to maintain
charge-balance
3Mg2+ diffuse to right, balances 2Al3+ to left
MgO
MgO
Al2O3
Al2O 3
Mg2+
Al3+
x/4
3x/4MgO/MgAl2O4 Reactant/Product Interface
2Al3+ –3Mg2+ + 4MgO → MgAl2O4
MgAl2O4/Al2O3 Product/Reactant Interface
3Mg2+ –2Al3+ + 4Al2O3 → 3MgAl2O4
4MgO + 4Al2O3 → 4MgAl2O4
MgO + Fe2O3 → MgFe2O4, colored spinel interface, can
easily monitor growth rate
Another Example of the Kirkendall Effect
SrO + TiO2 → SrTiO3
RockSalt Rutile Perovskite
SrO: ccp O2-, Sr2+ in all Oh sites
TiO2: hcp O2-, Ti4+ in 1/2 Oh sites
SrTiO3: ccp Sr2+/O2- (?:? ), Ti4+ in ?Oh sites
SrO
SrO
TiO 2
TiO 2
Sr2+
Ti 4+
x/3
2 x/3
SrO/SrTiO3 Reactant/Product Interface
Ti4+ –2Sr2+ + 3SrO → SrTiO3
SrTiO3/TiO2 Product/Reactant Interface
2Sr2+ –Ti4+ + 3TiO2 → 2SrTiO3
3SrO + 3TiO2 → 3SrTiO3
7
As show, reaction of high quality single-crystal
cobalt nanocrystals with oxygen, sulfur, or
selenium at relatively low temperatures
produces hollow polycrystalline nanocrystals of
cobalt oxide, sulfide, or selenide, respectively.
As the reaction proceeds in time, more cobalt
atoms diffuse out to the shell, and the
accompanying transport of vacancies leads to
growth and merging of the initial voids. This
results in the formation of bridges of material
between the core and the shell that persist until
the core is completely consumed. These bridges
provide a fast transport path for outward
diffusion of cobalt atoms that can then spread on
the inner shell surface.
Formation of Hollow Nanocrystals Through
the Nanoscale Kirkendall Effect
(A) TEM image of cobalt nanocrystals. (B) TEM image of the cobalt
sulfide phase synthesized by the injection of sulfur in
o-dichlorobenzene (5 ml) into cobaltnanocrystal solution with a Co/S
molar ratio of 9:12. (C) HRTEM images of Co3S4 (left) and Co9S8 (right).
(D) TEM image of the cobalt sulfide phase synthesized with a Co:S
molar ratio of 9:8.
Evolution of CoO hollow nanocrystals over time in response to a stream
of O2/Ar mixture (1: 4in volume ratio, 120 ml/min) being blown through
a cobalt colloidal solution at 455 K. (A to D) TEM images of the
solutions after flow of O2/Ar for (A) 0 min, (B) 30 min, (C) 80 min, and
(D) 210 min. Inset: HRTEM of a CoO hollow nanocrystal.
Evolution of CoSe hollow nanocrystals with time by injection of a
suspension of selenium in o-dichlorobenzene into a cobalt
nanocrystal solution at 455 K, from top-left to bottom right: 0 s, 10
s, 20 s, 1min, 2 min, and 30 min. The Co/Se molar ratio was 1:1.
XRD Patterns of the Li2ZrO3
Calcined at Various Temperatures
(a)Calcined at 500°C
peaks of Li2CO3 + ZrO2
(b) Calcinedat 700°C
peaks of Li2ZrO3 (monoclinic)
(c) Calcinedat 850°C-1200 °C
peaks of Li2ZrO3 (monoclinic)
(d) Calcined at 1400°C
peaks of Li2ZrO3 + ZrO2
Optimum temperature:
850?1200oC
( a)
( b)
(c)
( d)
Choosing the Optimum Temperature Overcoming the Diffusion Barrier
Need intimate mixture of reactants
Can be obtained in several ways:
very small particle size reactants
find molecular precursor that has the needed elements
in the correct ratio： eg. Ba[TiO(C2O4)2] for BaTiO3
Make a solution of needed metals and dry the solution
out without demixing the components
co-precipitate reactants in a solid solution salt： e.g.
carbonates for Brownmillerite (Ca 2Fe2O5)
crystallize from gels prepared using sol-gel chemistry
8
Solid State Precursors
Crystalline, phase-pure material
Decomposes on heating
Great for spinels, e.g., chromite spinels
Chromite Spinel Precursor 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(CrO 4)2?2NH3 1400°C
Magnetic garnets(tunable magnetic materials) aqueous
precursor technique:
Y(NO3)3+Gd(NO3)3+FeCl3+NaOH→YxGd3-xFe5O12
Firing at 900°C, 18-24 hrs., pellets, regrinding, repelletizing,
repeated firings, removes RFeO3 perovskite impurity
Isomorphous replacement of Y3+ for Gd3+ on dodecahedral
sites, solid solution, similar rare earth ionic radii:
0<x<3, 2Fe3+ occupied in Oh sites, 3Fe3+ Td sites, 3RE3+
dodecahedral sites
eight formula units in a unit cell, total of 160 atoms, cubic
lattice unit, cell parameter follows Vegard’s law behavior:
P(YxGd3-xFe5O12)=Px/3(Y3Fe5O12)+P(3-x)/3(Gd3Fe5O12)
Synthesis of Magnetic Garnets
Any property (P) of a solid-solution member is the atom
fraction weighted average of the end-members
Tunable magnetic properties by tuning the x value in
the binary garnet YxGd3-xFe5O12
YxGd3-xFe5O12 creates a tunable magnetic garnet that is
strongly temperature and composition dependent,
applications in permanent magnets, magnetic recording
media, magnetic bubble memories and so forth, similar
concepts apply to magnetic spinels
Synthesis of Magnetic Garnets
Coprecipitation applicable to nitrates, acetates,
oxalates, alkoxides, and so forth
Requires:
§similar salt solubilities
§similar precipitation rates
§no supersaturation
§Useful for spinels and the like
Disadvantage: difficult to prepare high purity,
accurate stoichiometric phases
Coprecipitation Technique
Combinatorial Materials Chemistry??
Robots Can Do Solid State Synthesis!
Combinatorial materials chemistry, the wave of the
future, beware traditional solid state chemists!
Used for parallel synthesis of series of compounds, rapid
screening by parallel measurements, clever analytical
techniques, massive amounts of data .
Applied to high Tc superconductors, inorganic phosphors,
giant magnetoresistant (GMR) mixed valency perovskites,
high dielectric constant rutile type oxides, mixed metal
catalysts and electrocatalysts, and even hydrothermal
synthesis of zeolites.
Self Propagating High Temperature
Synthesis (SHS)
Extreme exothermicity of a reaction can be used to
provide high temperatures needed for diffusion
Thermite Fe2O3 + Al → Al2O3 +Fe has been used to
make a number of useful materials including
refractory ceramic parts that can be pressed and
machined to final size
9
Solid State Metathesis Reactions Can
Be Very Exothermic
MoCl5(s)+5/2Na2S(s)→MoS2(s)+5NaCl(s)+1/2S(s)
Reaction reaches 1050oC and is over in 300 ms
Solid State Metathesis Reactions
§ A metathesis reaction between two salts merely involves an
exchange of anions, although in the context we will use there can
also be a redox component. If the appropriate starting materials
are chosen, a highly exothermic reaction can be devised.
§ MoCl5 +5/2 Na2S → MoS2 + 5NaCl + 1/2S
§ The enthalpy of this reaction is DHrxn = -213 kcal/mol
§ Due to the highly exothermic nature of this reaction, once it is
started (by grinding, spark, etc.) the heat generated by the
reaction itself leads to a rapid increase in temperature
§ The maximum temperature attained is ~ 1700 K.
§ The reaction is completed in < 4 s.
§ The percent yield is typically 80% of theoretical.
§ Washing with CH3OH removes remaining MoCl5
§ Washing with H2O removes NaCl
§ Washing with chloroform removes remaining S
SSM (Solid State Metathesis)
AB + CD → AC + BD
(A:metal, B:halogen;
C: alkali metals, D: S, P, As, Sb)
Examples：
MCl4+2Li2S→MS2+4LiCl (M=Ti, Zr, Hf and V) (>400°C)
3SiCl4+4NH3→Si3N4+12HCl
MCl2+Li2Fe2O4→MFe2O4+2LiCl (400-500°C)
ZrCl4+4/3Li3N→ZrN+4LiCl+1/6N2
SnI4+2Li2E→SnE2+4LiI (E=O, S, Se)(500°C)
PbCl2+Li2E→PbE+2LiCl (E=O, Se, Te) (500°C)
PbCl2+Li2O→Pb2O2Cl+LiCl+[Pb] (500°C)
LnCl3+Li3N→LnN+3LiCl (Ln=Y,La,Pr,Nd,Sm,Eu,Gd,Tb)
MClx(s)+(x/3)Li3N(s)→MN(s)+xLiCl+(x-3)/6N2(g)
MClx(s)+xNaN3(s)→MN(s)+xNaCl(s)+(3x-1)/2N2(g)
Solid State Metathesis Reactions to III?V Group
Reactants Condition Products Reactants Condition Products
AlI3+Na3P Bomb ignition Amorphous GaI3+Na3Sb Bomb ignition GaAs+Sb
990 C/42h AlP <600 C/12h GaAs
+Sb(trace)
AlI3+Na3As Bomb ignition Amorphous GaF3+Na3Sb Bomb ignition GaAs+Sb
220 C/12h AlAs+Al+As InI3 +Na3P Bomb ignition InP+In+InI 2+ trace
P
550 C/17h AlSb+trace As <600 C/12h, InP+In+InI2
AlI3+Na3Sb Bomb ignition AlSb+Al+Sb >600 C/12h InP
550 C/12h AlSb+
trace Al+Sb
InF 3+Na3P Bomb ignition InP+In+trace
impurities
>600 C/18h AlSb+ Al+Sb InI3 +Na3As Bomb ignition InAs+In+InI2
GaF3+Na3P Bomb ignition GaP+P(red)+
Ga(trace)
<600 C/12h InAs+In+InI2
GaCl3+Na3P Bomb ignition GaP+P(red)+
Ga(trace)
>600 C/12h InAs+In
GaI3 +Na 3P Bomb ignition GaP+P(red)+
Ga(trace)
InF 3+Na3As Bomb ignition InAs+In+trace
impurities
>220 C/8h GaP
+Ga(trace)
InI3 +Na3Sb Bomb ignition InAs+In+Sb
GaI3 +Na 3As Bomb ignition GaAs
+Ga(trace)
550 C/12h InAs+Sb
<570 C/12h GaAs
+As(trace)
950 C/8h GaAs
The product is impure, often contains the impurities such as
metals.
Since the actual reaction temperature is far higher than the
melting point of LiCl, the by-product LiCl is sintered together
with the product. Up to now, SSM is used on laboratory
research, and can not used for large scale production.
The method to remove the impurities:
GaNor InN can not be obtained in the similar system of
GaCl3 or InCl3 and Li3N under similar condition, which produce
corresponding metals Ga or In and release nitrogen gas.
Disadvantages of SSM
AlAswash)2(h8/Co1000)1(
AsAlNaIAlAsigniteAs3Na3AlI
????? →????????? →?
???? →? ++++
Sintering Overview
Sintering is a process specifically for powdered materials,
including metals, ceramics, and plastics. In this process, the
compressed powder is heated to a temperature close to but not
at melting, in a controlled-atmosphere furnace. This is done so
that particles may bond by solid state bonding, but not melt.
Surface area reduction
Powder process
10
Sintering
Sintering: powder pressing + firing below melting T
An animation showing the
changes in pore shape when
metal or ceramic powder is
sintered.
Sintering is a coalescence mechanism
involving islands in contact.
A neck forms between two islands
and thickens as atoms are transported
into the region.
The driving force for neck growth is
to reduce the total surface energy of
the system.
Since atoms on the convex island
surfaces have a greater activity than
atoms situated in the concave neck; an
effective concentration gradient
between these regions develops. →
mass transport into the neck.
Sintering
Cracks and Pores
Density Increases
SinteringGrain Growth
Ceramics 1 Structure and Properties
2 Traditional Ceramics
3 New Ceramics
4 Glass
5 Important Elements
Introduction
An inorganic compound consisting of a metal
and one or more nonmetals.
More abundant and widely used materials ?
Clay product, glass, cement, concrete and
modern ceramics
High hardness, good electrical and thermal
insulation, chemical stability and high
melting temperatures.
But too brittle
Introduction
Classification of Ceramic Products
Clay products
Refractory ceramics
Cement
Whiteware
Glass products
Glass fibers
Abrasives
Cutting tools
Ceramics insulators
Magnetic ceramics
Nuclear fuels
Bioceramics
(1) Traditional Ceramics
(2) New Ceramics
(3) Glasses
(4) Glass-ceramics
11
The compressive strength is typically ten times the
tensile strength.
Transparency to light of some ceramics ? optical
applications (windows, photographic cameras, telescopes,
etc)
Good thermal insulation ? use in ovens, the exterior
tiles of the Shuttle orbiter, etc.
Good electrical isolation ? ceramics are used to
support conductors in electrical and electronic
applications.
Good chemical inertness ? applications in reactive
environments.
Applications of Ceramics 1. Structure and Properties
Structure: A crystalline material but more complex or
an amorphous material
Mechanical Properties:
Slip does not occurs in ceramics: thus, brittle.
contains imperfection such as vacancies, interstitials,
displaced atom and microcracks.
Microcracks make ceramics weaker in tension.
Strengthening methods: uniform, smaller grains,
minimizing porosity, inducing residual stress,
reinforcement, heat treatment
Crystalline ceramics: Slip dislocation motion is
difficult since ions of like charge have to be brought
into close proximity → large barrier for dislocation
motion. Ceramics with covalent bonding slip is also
not easy: covalent bond strong ? ceramics brittle
Non-crystalline ceramic: there is no regular
crystalline structure → no dislocations. Materials
deform by viscous flow, i.e. by breaking and reforming
atomic bonds, allowing ions/atoms to slide past each
other (like in a liquid).
Viscosity is measure of glassy material’s resistance
to deformation.
Plastic Deformation in Ceramics 2 Traditional Ceramics
Fired clay, cement and natural abrasives based
on silicates, silica and mineral oxides.
Being able to fire the powder and water
mixture.
Raw materials: kaolinite, silica (quartz is one
form), bauxite (a pure form is corundum),
silicon carbide
Types
Pottery and Tableware
Brick and tile
Refractories
Abrasives
3 New Ceramics
Synthetic - Oxides, carbides, nitrides, borides and
oxynitride (SiAlON)
Oxides
Alumina (Aluminum oxide) (Al2O3)
Produced synthetically from bauxite
Good hot hardness, Low thermal conductivity
corrosion resistance
Abrasive, electrical insulator, bioceramics, cutting
tool, spark plug etc.
Zirconia (Zirconium oxide)
Silica (SiO2)
New Ceramics
Carbides
SiC, WC, TiC, TaC, Cr3C2.
SiC –traditional ceramics, used as abrasives.
WC –Carburizing tungsten powders from
wolframite and scheelite
TiC –Carburizing rutile or ilmenite
TaC - Carburizing tantalum powders or tantalum
pentoxide
Chromium carbides - Carburizing chromium oxides
12
New Ceramics
Nitrides
Silicon Nitrides (Si3N4)
Oxidizes at 1200°C and decomposes at 1900°C.
resistance to thermal shock and creep
Corrosion resistance to molten nonferrous metals
Gas turbine, rocket engines and melting crucibles
Boron Nitride (BN)
Form: Hexagonal or cubic (CBN)
Extremely high hardness
Cutting tools
Titanium Nitride (TiN)
Electrical conductive material
High hardness, low friction with ferrous materials, wear
resistance
4 Glass
Glass ?? an inorganic, nonmetallic compound that cools
to a rigid condition without crystallizing. (Amorphous glassy
ceramics dated back 4000 years)
Principal ingredient: Silica(SiO2) found in mineral quartz
from sandstone and sand
Melted and cooled to form vitreous silica
resistance to thermal shock
Commercial glass products contain 50-75% silica.
Compositions:
Act as flux during heating
Increase fluidity in a molten state
Retard devitrification
Reduce chemical attack
Add colors
Alter the index of refraction (for lenses)
Silicate Glasses ? noncrystalline silicates (SiO2) containing
other oxides (CaO, NaO2, K2O, Al2O3) Containers, windows,
lenses, fiberglass, etc.
Silicate Glasses
Example:
Container/window glasses
contain
~ 30 wt% oxides (CaO, Na 2O)
whose cations are incorporated
within SiO4 network: network
modifiers.
Quartz sand + soda ash or
limestone
Solidification is gradual, through a viscous stage
(viscosity is increasing with decreasing T), without a
clear melting temperature
Specific volume (1/density) does not have abrupt
transition at fixed temperature but shows a change in
slope at the glass-transition temperature
Properties of Glasses
Important temperatures(viscosity) in glass:
Melting point: viscosity = 100P, below this viscosity
(above this T) glass is liquid
Working point: viscosity = 104P, glass is easily deformed
Softening point: viscosity = 4×107P, maximum T at
which a glass piece maintains shape for a long time
Annealing point: viscosity = 1013P, relax internal
stresses (diffusion)
Strain point: viscosity = 3 × 1014 P, above this viscosity,
fracture occurs before plastic deformation
Glass forming operations occur between softening and
working points!
Properties of Glasses Glass Formation at the Liquid-glass Transition Temperature
Below Tg, glass is a rigid brittle material
Above Tg, glass behaves as a viscous liquid with behavior
characterized by continuous deformation (at a rate inversely related
to viscosity) rather than a fixed elastic strain in response to stress
Tg:
glass transition
temperature
Tm:
Melting temperature
(point) corresponds to
a change in volume at
fixed temperature
13
Heat Treatment of Glasses
Annealing: elevate temperature to remove thermal
stresses resulting from inhomogeneous temperatures during
cooling (similar to annealing of metals)
Tempering: heating glass above glass transition
temperature but below softening point; then quench in an
air jet or oil bath.
The interior: cools later than outside, tries to contract
while in a plastic state after exterior has already become
rigid. Causes residual compressive stresses on surface and
tensile stresses inside.
In fracture: crack has to overcome residual compressive
stress, making tempered glass less susceptible to fracture.
Used in automobile windshields, glass doors, eyeglass
lenses, etc.
Glass Product
Window glass
Containers
Light Bulb Glass
Laboratory Glassware
Glass fibers
Optical glasses
Glass-Ceramics
By heat treating glass into a polycrystalline
structure (90-98%)
Grain size usually ranges from 0.1 to 1.0mm, which
makes stronger.
Opaque (gray or white) in color
After heating and forming into a desired geometry,
it is cooled and reheated to crystallize at a high
density of nucleation sites.
Efficient processing, close dimensional control and
excellent properties
Used in cooking ware and heat exchangers
5.Other Important Elements
Replace ceramics in some applications.
Silicon
A semimetallic element abundant on earth.
The same structure as diamond but lower in
hardness
Brittle, lightweight and chemically inactive at room
temperature
Used to make glass and clay, alloying ingredient,
semiconductors
Boron
Scarce on Earth, Lightweight
Electrically insulator at low temp. and conductor at
high temp.
Used in Cutting tool (CBN) , fibers
Application of Fine Ceramics
Ceramic rotors under
commercial
production materials:
Sintered silicon
nitride
Important Methods for Solid State Materials
1. Soft Chemistry
Intercalation and De-intercalation
Dehydration
Ion Exchange
Sol-Gel Process
2. Molten Salt Fluxes
3. High-Pressure Synthesis
4. CVT (Chemical Vapor Transport)
CVD (Chemical Vapor Deposition)
5. Single Crystal Growth
14
Soft Chemistry
Approach : Soft Chemistry reactions are carried out under
moderate conditions (typically T < 500°C). Soft Chemistry
reactions are topotactic, meaning that structural elements of
the reactants are preserved in the product, but the
composition changes.
Advantages : Soft Chemistry Methods are very useful for
the following applications:
Modifying the electronic structure of solids (doping)
Design of new metastable compounds (structural motif can
be selected by choice of precursor, may have unusual
properties)
Preparing reactive and/or high surface area materials used
in heterogeneous catalysis, batteries and sensors
Disadvantages : First of all, one must find the appropriate
precursor in order to carry out Soft Chemistry. Secondly,
metastable products are often unstable in applications where
high temperatures are used or single crystals are needed
Intercalation
Involves inserting ions into an existing structure,
this leads to a reduction (cations inserted) or an
oxidation (anions inserted) of the host.
Typically carried out on layered materials (strong
covalent bonding within layers, weak van der Waals
type bonding between layers, i.e. graphite, clays,
dicalchogenides, etc.).
Performed via electrochemistry or via chemical
reagents as in the n-butyl Li technique.
Examples :
TiS2 + nBu-Li → LiTiS2
De-intercalation
The reverse of intercalation, also performed using
either electrochemical methods or with reactive
chemical species
Examples :
NiMo3S4→ Mo3S4
In2Mo6S6 + 6HCl (g) → Mo6S6 + 2InCl3 (g) + 3H2 (g)
This approach can often lead to new phases
(polymorphs) of previously known compounds
CuTi2S4 → cubic TiS2
KCrSe2 → layered CrSe2
Li2FeS2 → FeS2
Dehydration
By removing water and/or hydroxide groups from
a compound, you can often perform redox
chemistry and maintain a structural framework
not accessible using conventional synthesis
approaches
Examples :
Ti4O7(OH)2*nH2O→ TiO2 (B) (500°C)
2KTi4O8(OH)*nH2O→ K2Ti8O17 (500°C)
Ion Exchange
§ Exchange charge compensating, ionically
bonded cations (easiest for monovalent cations)
§ Examples :
LiNbWO6 + H3O+ → HNbWO6 + Li+
Cubic-KSbO3 + Na+ → Cubic-NaSbO3 + K+
Molten Salt Fluxes
Solubilize reactants→Enhance diffusion → Reduce
reaction temperature
Synthesis in a solvent is the common approach to
synthesis of organic and organometallic compounds.
This approach is not extensively used in solid state
syntheses, because many inorganic solids are not
soluble in water or organic solvents. However, molten
salts turn out to be good solvents for many ionic-
covalent extended solids.
Often slow cooling of the melt is done to grow crystals,
however if the flux is water soluble and the product is
not, then powders can also be made in this way and
separated from the excess flux by washing with water.
15
Molten Salt Fluxes
Synthesis needs to be carried out at a temperature
where the flux is a liquid. Purity problems can arise,
due to incorporation of the molten salt ions in
product. This can be overcome either by using a salt
containing cations and/or anions which are also
present in the desired product (i.e. synthesis of
Sr2AlTaO6 in a SrCl2 flux) , or by using salts where
the ions are of a much different size than the ions in
the desired product (i.e. synthesis of PbZrO3 in a
B2O3 flux).
§ Example 1
4SrCO3 + Al2O3 + Ta2O5 → Sr2AlTaO6 (SrCl2 flux, 900°C)
§ Powder sample, wash away SrCl2 with weakly acidic H2O
§ Direct synthesis requires T > 1400°C and Sr2Ta2O7
impurities persist even at 1600°C
§ Example 2
La2O3 + CuO + KOH→ La2-xKxCuO4 (KOH flux, 380°C)
§ Volatility of potassium plagues direct reaction
§ Example 3
K2Tex + Cu → K2Cu5Te5 (K2Tex Flux, 350°C)
§ Example of reactive A2Qx (A = alkali metal, Q = S, Se, Te)
flux. In this approach the flux acts not only as a solvent but
also as a reactant. A large number of new compounds have
been made in the past decade using this approach.
Precursor Routes
Approach : Decrease diffusion distances through
intimate mixing of cations.
Advantages : Lower reaction temperatures, possibly
stabilize metastable phases, eliminate intermediate
impurity phases, produce products with small
crystallites/high surface area.
Disadvantages : Reagents are more difficult to work
with, can be hard to control exact stoichiometry in
certain cases, sometimes it is not possible to find
compatible reagents (for example ions such as Ta5+ and
Nb5+ immediately hydrolyze and precipitate in aqueous
solution).
Precursor Routes
Methods : With the exception of using mixed
cation reactants, all precursor routes involve the
following steps:
Mixing the starting reagents together in
solution.
Removal of the solvent, leaving behind an
amorphous or nanocrystaline mixture of cations
and one or more of the following anions: acetate,
citrate, hydroxide, oxalate, alkoxide, etc.
Heat the resulting gel or powder to induce
reaction to the desired product.
Mixed Cation Synthesis of Na2ZrTeO6
I was attempting to make Na2ZrTeO6 from Na 2CO3, ZrO2
and TeO2, using a conventional heat and beat approach. At
~700 ?750°C, I began to form my desired product, but there
was also a considerable amount of ZrO 2 still present,
together with some Na2TeO4.
Increasing the annealing temperature (850 ? 950°C) did
lead to an increase in the Na 2ZrTeO6 concentration, but
before all of the ZrO2 would react I began to volatilize a
tellurium species.
To circumvent this problem I tried pre-reacting the Na 2CO3
and ZrO2 to form Na2ZrO3 at ~1000°C. I then reacted
Na2ZrO3 with TeO2 at 750°C to form single phase
Na2ZrTeO6.
Na2CO3 + ZrO2→Na2ZrO3
Na2ZrO3 (s) + TeO2 (s) → Na2ZrTeO6 (s)
Coprecipitation Synthesis of ZnFe2O4
§ Mix the oxalates of zinc and iron together in water in a
1:1 ratio. Heat to evaporate off the water, as the amount
of H2O decreases a mixed Zn/Fe acetate (probably
hydrated) precipitates out.
§ Fe2((COO)2)3 + Zn(COO)2 → Fe2Zn((COO)2)5?xH2O
§ After most of the water is gone, filter off the precipitate
and calcine it (1000°C).
§ Fe2Zn((COO)2)5 → ZnFe2O4 + 4CO + 4CO2
§ This method is easy and effective when it works. It is not
suitable when
1. Reactants of comparable water solubility cannot be found.
2. The precipitation rates of the reactants is markedly
different.
§ These limitations make this route unpractical for many
combinations of ions. Furthermore, accurate
stoichiometric ratios may not always be maintained.
16
Sol-Gel Synthesis of Metastable ScMnO3
Begin by dissolving Sc2O3 and MnCO3 separately, in heated
aqueous solutions of formic acid to form the formate salts:
Sc2O3 + 6HCOOH → 2Sc(HCOO)3 + 3H2O
MnCO3 + 2HCOOH + 2H2O → Mn(COOH)2? 2H2O + H2CO3
Addition of Sc(HCOO)3 and Mn(COOH)2?2H2O to melted
citric acid monohydrate results in the formation of a (Sc,Mn)
citrate polymer.
Heat to 180°C → Removal of excess water and organics
Heat to 450°C → Formation of an amorphous oxide product
Heat to 690°C → Formation of crystalline ScMnO3
Direct reaction of the formates at 700°C simply gives the a
mixture of the binary oxides:
2Sc(HCOO) 3 + 2Mn(COOH)2?2H 2O →Sc2O3 + Mn2O3 + 5CO2 + 2H 2O + H 2
Alkoxide-Hydroxide Synthesis of Sr2AlTaO6
Reflux a mixture of Ta(OC2H5)5 and Al(OC2H5)3 overnight in
a solution of ethanol ? This results in the formation of
polymeric (Ta,Al) ethoxide species.
Add a stoichiometric quantity of Sr(OH)2?8H2O in acetone,
mix well and reflux overnight. The hydroxide ions and water
of hydration are sufficient to trigger a slow precipitation.
Filter off the solution and heat at 120°C to drive off
remaining solvent.
Heat to 1200 ? 1400°C to form highly crystalline Sr2AlTaO6
or heat to 800-1000°C to form high surface area Sr2AlTaO6
Direct reaction of the oxides also results in formation of
Sr2AlTaO6, but minor Sr/Ta/O impurity phases are always
present.
The alkoxides are often hygroscopic and air sensitive,
consequently it can be difficult to weigh out accurate
quantities. Furthermore, they are rather expensive.
High Pressure Synthesis
§ Approach : By increasing the pressure it allows you to
explore regions of the phase diagram not accessible at
atmospheric pressure.
§ Advantages : Often leads to formation of compounds which
cannot be formed using any other technique. The presence
of high non-metal partial pressures (i.e. high O2 partial
pressure) can be used to stabilize cations in unusually high
oxidation states.
§ Disadvantages : High pressure synthesis equipment tends
to be large and expensive. Product volume is often times so
small that characterization becomes difficult, and practical
application can be impractical.
§Methods : The various high pressure techniques vary
primarily in design of the pressure transmitting device,
which in turn leads to variations in the accessible pressure
and temperature range, as well as the sample volume.
§Quenching : A compound that is stable at high pressure
may transform back to its ambient pressure phase upon
release of the pressure. In order to prevent this it is
important to lower the temperature back to room
temperature before releasing the pressure back to ambient
pressure. This so called "quenching" of the high pressure
phase is most likely to be successful in cases where
considerable structural rearrangement (breaking and
making bonds) is involved in the transformation between
the low pressure and high pressure phase.
Piston Cylinder Press
Can achieve 50 kbar and 1800K
Sample is placed in container (Pt, Au ..) and the
container is embedded in a pyrophyllite block
?pyrophyllite acts as a pressure transmitting
medium
Squeeze sample by forcing WC (tungsten carbide)
piston into WC (tungsten carbide) cylinder
Multianvil Press
17
Belt Design
Can achieve 150 kbar,
2300K
?Relatively large sample
volume
?Sample in Au/Pr container
or for BN or MgO
The phase diagram
for diamond and
graphite (from J.
Geophys . Res. 1980,
85, B12, 6930.)
The stable condition
of diamond:
T>2,000°C,
P>50,000atm
Experiments using:
?Diamond Anvil Cell?
Multi-anvil press?
Piston cylinder ?
Laser Heating ?
Synchrotron Radiation ?
Shock
But core pressures and
temperatures remain
challenging.
Synthetic Diamond
The production of diamonds Large synthetic diamonds
Why Use High Pressures?
High pressure allows the preparation of new compositions,
new structures, unusual oxidation states
PbSnO3 does not form as a perovskite at ambient pressure,
but will at high pressure
CaFeO3 can only be prepared at high pressures. At
ambient pressures, Brownmillerite (CaFeO2.5) forms
Superconducting oxygen excess La2CuO4+δ can be
prepared at high oxygen pressure
La2Pd2O7 can be prepared at high oxygen pressure.
Normally only get Pd2+ oxides
Oxygen Pressure can be generated in-situ by
decomposition of say KClO3 (or KMnO4)
However, beware! KCl may be incorporated into the
product
Dry High Pressure Methods
of Solid State Synthesis
Pressures up to Gigabars accessible, at high temperatures,
and with in ? situ observations by diffraction, spectroscopy to
probe chemical reactions, structural transformations,
crystallization, amorphization, phase transitions and so forth
Methods of obtaining high pressures:
Anvils, diamond tetrahedral and octahedral pressure
transmission
Shock waves
Explosions
Pressure techniques useful for synthesis of unusual
structures, metastable yet stable when pressure released
Often high pressure phases have a higher density, higher
coordination number
18
Examples of High Pressure
Polymorphism for Some Simple Solids
Solid Normal structure
and coordination
number
Typical
transformation
conditions P
(kbar)
Typical
transformation
conditions T
( C)
High pressure
structure and
coordination
number
C Graphite 3 130 3000 Diamond 4
CdS Wurtzite 4:4 30 20 Rock salt 6:6
KCl Rock salt 6:6 20 20 CsCl 8:8
SiO2 Quartz 4:2 120 1200 Rutile 6:3
Li2 MoO4 Phenacite 4:4:3 10 400 Spinel 6:4:4
NaAlO2 Wurtzite 4:4:4 40 400 Rock salt 6:6:6
Hydrothermal Synthesis
Reaction takes place in superheated water, in a
closed reaction vessel called a hydrothermal bomb
(150 < T < 500°C; 100 < P < 3000 kbar).
Seed crystals and a temperature gradient can be
used for growing crystals
Particularly common approach to synthesis of
zeolites
Example :
6CaO + 6SiO2 → Ca6Si6O17(OH)2 (150-350°C)
The Use of High Pressure Stabilizes
Products With the Following Attributes:
(a)Dense packing of ions (Higher
cation coordination numbers)
(b) Higher cation oxidation states
(c) Higher symmetry
(A) Dense Packing of Ions (Higher
Cation Coordination Numbers)
For example: when prepared under ambient
conditions, the compounds Na2MTeO6 (M =
Ti4+, Sn4+) crystallize with the ilmenite
structure. Upon treatment at high
temperature (1000°C) and pressure (40?70
kbar) in a multi-anvil device, both of these
compounds transform to the perovskite
structure, which is one of the most efficiently
packed ternary oxide structures known.
(B) Higher Cation Oxidation States
For example: at ambient pressure the
reaction between calcium oxide and iron oxide
leads to the formation of CaFeO2.5, with the
Brownmillerite structure and iron in the +3
oxidation state. But upon treatment with
high oxygen pressures in a belt or piston-
cylinder device, the perovskite CaFeO3, with
iron in the +4 oxidation state is stabilized.
(C) Higher Symmetry
§ Since the volume of a polyhedron increases
upon distortion, high pressures favor
symmetric coordination environments.
§ As an example: PbSnO3, with the perovskite
structure, can only be made using high
pressure. Ambient pressure synthesis leads to
a mixture of PbO (with a pronounced Pb2+
lone pair effect) and SnO2.
19
A polycrystalline sample, A, and a transporting species,
B, are sealed together inside a tube.
Upon heating the transporting species reacts with the
sample to produce a gaseous species AB.
When AB reaches the other end of the tube, which is
held at a different temperature, it decomposes and re-
deposits A.
If formation of AB is endothermic, crystals are grown
in the cold end of the tube.
A (powder) + B (g)→ AB (g) (hot end)
AB (g) → A (single crystal) + B (g) (cold end)
(Pt, Au, Nb, Ta, W) are used.
Chemical Vapor Transport § If formation of AB is exothermic, crystals are
grown in the hot end of the tube.
A (powder) + B (g) → AB (g) (cold end)
AB (g) → A (single crystal) + B (g) (hot end)
§ Typical transporting agents include:
§ I2, Br2, Cl2, HCl, NH4Cl, H2, H2O, TeCl4, AlCl3,
CO, S2
§ Temperature gradient is typically created and
controlled using a two-zone furnace.
§ Tubes are usually SiO2, unless reactive, in which
case metal tubes
A solid is dissolved in the gas phase at one place (T=T1) by
reaction with a transporting agent (e.g. I2). At another
place (T=T2) the solid is condensed again.
Whether T1 < T2 or T1 > T2 depends on the thermochemical
balance of the reaction !
Transport can proceed from higher to lower or from lower
to higher temperature
T 1 T 2
trace of a transporting agent (e.g. I2)
Main application: purification and crystallization of solids
Chemical Transport Reaction
Selected Materials Processing Technologies
? Transport phenomena in materials processing often
involve phase transformation or conversion from liquid to
solid, solid to solid, or gas to solid. Transport phenomena
significantly affect the way these transformations or
conversions take place during materials processing and
therefore the quality of the resultant products.
Examples of Vapor Phase Transport
2-MnP2 two layer stacking variant of MnP2
Hot end of tube at ~ 800K, cool end 80 K lower, MnP2 formed at
cooler end of tube
Both components are transportable
CaSn2O4
Reaction accelerated because SnO is volatile
NiCr2O4
Reaction accelerated by the formation of volatile CrO3
Nb5Si3
Reaction does not occur in absence of H2, volatile SiO is formed
2
tubesealed,I%at1 MnPP2Mn 2 ?????? →?+
42
HorCOtracesC900
2 OCaSnSnOCaO2
2
o ??????? →?+
42
OtracesC1100
32 ONiCrOCrNiO
2
o ????? →?+
NbO6SiNbSiO3Nb11 32Htrace,C10002 2o +????? →?+
If the reaction product is transportable get faster
reaction than if only one reactant is transportable
Also can easily get single crystals
Very slow reaction in absence of I2 due to formation on
Al2S3 layer on surface of Al
With I2 get rapid reaction due to formation of volatile
S2(g) and AlI3(g)
Large crystals are formed
Examples of Vapor Phase Transport
32
tubeSiOsealedItraces SAlSAl 22 ??????? →?+
20
Chemical Vapor Deposition (CVD)
CVD is the process of chemically reacting a volatile
compound of a material to be deposited, with other gases, to
produce a nonvolatile solid that deposits atomistically on a
substrate.
Typically, the reaction is initiated by heating the
substrate. Other mechanisms of supplying the activation
energy necessary to initiate reactions include: laser CVD,
photo CVD, and plasma enhanced CVD.
For Metals, Semiconductors, Compound Films & Coatings
CVD Reaction types
Pyrolysis
Reduction
Oxidation
Compound Formation
Disproportionation
Reversible Transfer
1 Pyrolysis
thermal decomposition of gases on hot substrate
SiH4(g)→Si(s)+2H2(g) (650°C)
Ni(CO)4(g)→Ni(s)+4CO(g) (180°C)
Hydrides, Carbonyl, Organometallic compounds
2 Reduction
- Hydrogen as reducing agent
- Halide, Carbonyl halide, Oxyhalide, Oxygen-containing
compounds
- SiCl4(g)+2H2(g)→Si(s)+4HCl(g) (1200°C) : Si Epitaxy
- WF6(g) + 3H2(g) → W(s) + 6HF(g) (300°C)
- MoF6(g)+ 3H2(g) → Mo(s) + 6HF(g) (300°C)
- WF6(g)+Si(s)→W(s)+SiF4(g)(selectively fill contact hole)
3 Oxidation
- SiH4(g) + O2(g) → SiO2(s) + 2H2(g) (450°C)
- 4PH3(g) + 5O2(g) → 2P2O5(s) + 6H2(g) (450°C)
7% of P in SiO2 → “planarization”(glass film)
- SiCl4(g)+2H2(g)+O2(g)→ SiO2(g)+4HCl(g) (1500°C) optical
fiber for communications purposes
4 Compound Formation
Carbide, nitride, boride, .. films of coatings (hard, wear-
resistant)
SiCl4(g)+ CH4(g)→ SiC(s)+ 4HCl(g) (1400°C)
TiCl4(g)+ CH4(g)→ TiC(s)+ 4HCl(g) (1000°C)
BF3(g) + NH3(g) → BN(s) + 3HF(g) (110°C)
3SiCl2H2+4NH3(g)→Si3N4(s)+6H2(g)+6HCl(g) (750°C)
Precursor gases should be sufficiently volatile and reactive
in the gas phase
5 Disproportionation
Disproportionation reactions are possible when
metals can form volatile compounds having different
valence states depending on the temperature
300°C
2GeI2(g) Ge(s) + GeI4(g)
600°C
Ge, Al, B, Ga, In, Si, Ti, Zr, Be, Cr → halides
lower-valent state (stable at high T), metal transport
→ single crystal
In systems where provision is made for mass
transport between hot and cold ends
6 Reversible Transfer
In the reaction equilibrium at source and deposition
regions maintained at different temperatures within a
single reactor
GaAs epitaxial films by “chloride process”
750°C
As4(g)+As2(g)+6GaCl(g)+3H2(g) 6GaAs(s)+6HCl(g)
850°C
“Chloride VPE”(Vapor Phase Epitaxy)
In the hydride process, AsH3 and HCl ? “Hydride VPE”
Generally in CVD
? a A(g) + bB(g) → cC(s)+ dD(g)
? Reversible ; thermodynamics is applicable to reactions
Chemical Vapor Deposition： reactant gases enter the
reactor from the outside of the system
Chemical vapor transport reactions： solid or liquid
sources are contained within closed or open reactors ,
need carrier gases to transport source materials
But, type of chemical reaction is same
21
Reaction feasibility
where are chemical reactions going?
chemical thermodynamics at equilibrium : feasibility
How fast are they getting there?
chemical kinetics; growth rates, speed of reaction
For chemical reaction aA + bB ? cC
DG = DG + RTlnK,
If the system is in equilibrium, DG = 0 and DG = RTlnK
For many practical cases, DG @ DG since the ai differ little from the
standard-state activities, which are taken to be unity.
DG = DG + RTlnK fi DG from Ellingham diagram
Standard free energy change DG @ 0 for large critical sited nuclei.
(1 atm. & T)
If DG << 0, polycrystal formation is promoted.
b
B
a
A
c
C
aa
aK =
Thermodynamics of CVD
For optoelectronic devices by hydride VPE
Binary : GaAs , InP, GaP, InAs
Ternary : (Ga, In)As, Ga(As, P)
Quaternary : (Ga, In, As, P)
Gas-phase reactions
2AsH3? As2 + 3H2
2PH3 ? P2 + 3H2
2HCl + 2In ? 2InCl + H2
2HCl + 2Ga ? 2GaCl + H2
Deposition reactions at InP substrate
2GaCl + As2 + H2 ? 2GaAs + 2HCl
2GaCl + P2 + H2 ? 2GaP + 2HCl
2InCl + P2 + H2 ? 2InP + 2HCl
2InCl + As2 + H2 ? 2InAs + 2HCl
Typical Parameters
§ Pressure → 0.1 torr –1 atm
Substrate Temp. → 100°C ? 1500°C
Deposition Rate → 60?/min –300,000?/min
§ The use of organometallic precursors as gas phase
species (MOCVD) can result in significant
reduction of the substrate temperature.
WCl6 (g) + 3H2 (g) → W (s)+ 6HCl (g)
SiH4 (g) + O2 (g) → SiO2 (s) + 2H2 (g)
6TiCl4 (g) + 8NH3 (g) → 6TiN (s) + 24HCl (g) + N2 (g)
Examples
Single Crystal Growth
How do single crystals differ from polycrystalline samples?
Single crystal specimens maintain translationalsymmetry
over macroscopic distances (crystal dimensions are typically
0.1 mm –10 cm).
Why would one go to the effort of growing a single crystal?
Structure determination and intrinsic property
measurements are preferably, sometimes exclusively,
carried out on single crystals.
For certain applications, most notably those which rely on
optical and/or electronic properties (laser crystals,
semiconductors, etc.), single crystals are necessary.
What factors control the size and purity of single crystals?
Nucleation and Growth. If nucleation rates are slow and
growth is rapid large crystals will result. On the other hand
if nucleation is rapid, relative to growth, small crystals or
even polycrystalline samples will result.
Journals Related to Crystal Growth
《 J. Crystal Growth》
《 Appl. Phys. Lett》
《 J. Luminescence》
《 IEEE J. Quantum Electronics》
《 Physics Letters》
《 Applied Optics》
22
Crystal Growth Theory
Initially form a nucleus
–this is highly dependent upon the presence of
impurities and vessel surfaces
Growth of nucleus
–growth depends upon the presence of low
energy surface sites such as steps
–The faces with low surface energy grow
more slowly and are therefore larger
–presence of additives in solution may alter
relative rates for different faces
Crystal Growth — Practice
Many techniques are available
–choice of method depends upon the material
§Grow from solution
§Grow from molten solid
–this can include solvents as diverse as water and
molten PbO!
§Grow from vapor phase
§Grow from solid phase
Most widely used melt growth methods are
Czochralski, Bridgeman, and floating-zone
processes.
What Can Be Done to Increase the
Growth Rates?
§In order to attain the rapid growth rates needed to
grow macroscopic crystals, diffusion coefficients must
be large, hence crystal growth typically occurs via
formation of a solid from another state of matter :
(a) Liquid (Melt) → Solid (Freezing)
(b) Gas (Vapor) → Solid (Condensation)
(c) Solution → Solid (Precipitation)
§At this point it should be noted that defect
concentrations tend to increase as the growth rate
increases, consequently the highest quality crystals
need to be grown slowly.
What Can Be Done to Limit the
Number of Nucleation Sites?
§ Several techniques are used separately or
in combination to induce nucleation of the
solid phase at a slow and controlled rate :
(a) Slow Cooling of Melts
(b) Temperature Gradients
(c) Introduction of Seed Crystals
Growth From Solution
The most common method
Based on precipitation from a saturated solution
Saturation can be achieved in many way
–cool solution
–evaporate solvent off
–add things to solvent to reduce solubility
Solutions
Saturated Solution:
A solution containing the maximum amount of
dissolved solute at a given temperature.
Unsaturated Solution:
A solution containing less than the maximum
amount of dissolved solute at a given temperature.
Supersaturated Solution:
A solution containing more than the maximum
amount of dissolved solute at a given temperature.
A non-equilibrium situation.
23
Zone 1 - Metastable zone.
The solution may not nucleate for
a long time, but this zone will
sustain growth. It is frequently
necessary to add a seed crystal.
Zone 2 - Nucleation zone.
Product crystals nucleate and
grow.
Zone 3 - Precipitation zone.
Product does not nucleate but
precipitate out of solution.
Solutions Nucleationphenomenon whereby a “nucleus”, such as a dust
particle, a tiny seed crystal starts a
crystallization process.
Common difficulties:
1.If supersaturation is too high, too many nuclei
form, hence an overabundance of tiny crystals.
2. In supersaturated solutions that don’t experience
spontaneous nucleation, crystal growth often only
occurs in the presence of added nuclei or “seeds”.
Growth From the Vapor Phase
Not very common
Used for some sublimable compounds
Used where a material can be transported by the
addition of an appropriate reagent
ZnS(s) + I2(g) → ZnI2(g) + 1/8S8(g) (exothermic)
Growth From the Melt
Very important, but not possible for most materials
Material must be stable above melting point
Phase diagram must allow direct crystallization
Slow Cooling of the Melt
With congruently melting materials (those which maintain
the same composition on melting) one simply melts a mixture
of the desired composition then cools slowly (typically 2 ?
10°C/hr) through the melting point.
More difficult with incongruently melting materials,
knowledge of the phase diagram is needed.
Often times the phase diagram is not known, consequently
there is no guarantee that crystals will have the intended
stoichiometry.
Molten salt fluxes are often used to facilitate crystal growth
in systems where melting points are very high and/or
incongruent melting occurs.
Crystals grown in this way are often rather small, thus this
method is frequently used in research, but usually not
appropriate for applications where large crystals are needed.
Czochralski Process
§Start with a single crystal
seed of known orientation.
§Somewhere along its length
will be the interface of the
melt to air. In other words,
melt half of it.
§Slowly pull up into air.
§Crucible material will
reintroduce some impurities
at the surface of boule.
24
a rotating seed crystal is raised slowly from a
melt with equal composition
Czochralski process: Si
One step in the production of
semiconductor devices involves the
growth of a large (10 or more inches in
diameter!) single crystal of silicon by the
Czochralski process. In this process, a
solid seed crystal is rotated and slowly
extracted from a pool of molten Si.
A pure silicon seed crystal is now placed into the molten
sand bath.
This crystal will be pulled out slowly as it is rotated.
The result is a pure silicon tube that is called an ingot
Commercial LiNbO3 Crystals Gadolinium Gallium Garnet
Used as substrate for growth
of devices
Czochralski Crystal Growth Technique
§This technique originates from
pioneering work by Czochralski
in 1917 who pulled single
crystals of metals. Since crystal
pulling was first developed as a
technique for growing single
crystals, it has been used to grow
germanium and silicon and
extended to grow a wide range of
compound semiconductors,
oxides, metals, and halides. It is
the dominant technique for the
commercial production of most of
these materials.
Czochralski Method
Single crystal growth from the melt precursor(s)
Crystal seed of material to be grown placed in contact with
surface of melt
Temperature of melt held just above melting point, highest
viscosity, lowest vapor pressure
Seed gradually pulled out of the melt (not with your hands
of course, special crystal pulling equipment is used)
Melt solidifies on surface of seed
Melt and seed usually rotated counterclockwise with
respect to each other to maintain constant temperature and
to facilitate uniformity of the melt during crystal growth,
produces higher quality crystals, less defects
Inert atmosphere, often under pressure around growing
crystal and melt to prevent any materials loss
Czochralski Method
A seed crystal is attached to a rod, which is rotated slowly.
The seed crystal is dipped into a melt held at a temperature
slightly above the melting point.
A temperature gradient is set up by cooling the rod and
slowly withdrawing it from the melt (the surrounding
atmosphere is cooler than the melt)
Decreasing the speed with which the crystal is pulled from
the melt, increases the quality of the crystals (fewer defects)
but decreases the growth rate.
The advantage of the Czochralski method is that large
single crystals can be grown, thus it used extensively in the
semiconductor industry.
In general this method is not suitable for incongruently
melting compounds, and of course the need for a seed crystal
of the same composition limits its use as tool for exploratory
synthetic research.
25
Growing GaAs Crystal Requires a
Modification of the Czochralski Method
Layer of molten inert oxide like B2O3 spread on to of
the molten feed material to prevent preferential
volatilization of the both volatile component of Ga and
As, this is critical for maintaining precise stoichiometry.
For example Ga1+xAs and GaAs1+x which are
respectively rich in Ga and As, become p-doped and n-
doped .
The Czochralski crystal pulling technique has proven
invaluable for growing many large single crystals in the
form of a rod, which can subsequently be cut and
polished for various applications, some important
examples: Si, Ge, GaAs, LiNbO3 , SrTiO3 , NdCa(NbO3)2
Solid
In
Solid
P Molten InP
Liquid Encapsulate, B2O3
Heat Heat
In (l) + P (g) InP (s)
B2O3
Liquid Encapsulated Czochralski Technique
Bridgman-Stockbarger-Process
(Moving Temperature Gradient) Vertical Bridgman Crystal Growth Process
The furnace consists of three zones:
(1) the upper zone (temperature>Tmp;
melting point of the crystal),
(2) the lower zone (temperature<Tmp)
(3) an adiabatic zone (a baffle)
between the two.
The ampoule is raised into the upper
zone until only the lower portion of
the single crystal seed remain
unmelted in the lower zone. After the
temperature stabilizes, the ampoule is
lowered slowly into the lower zone to
initiate crystal growth from the seed.
Stockbarger and Bridgman Methods
Stockbarger method is based on a crystal growing from the
melt, involves the relative displacement of melt and a
temperature gradient furnace, fixed gradient and a moving
melt/crystal
Bridgman method is again based on crystal growth from a
melt, but now a temperature gradient furnace is gradually
cooled and crystallization begins at the cooler end, fixed
crystal and changing temperature gradient
Both methods are founded on the controlled solidification
of a stoichiometric melt of the material to be crystallized
Enables oriented solidification
Melt passes through a temperature gradient
Crystallization occurs at the cooler end
Both methods benefit from seed crystals and controlled
atmospheres
Bridgman-Stockbarger Crystal
Growth Technique
The Bridgman
technique is a
directional solidification
process. The ampoule
contains the melt which
moves through the axial
temperature gradient in
a furnace. Single
crystals can be grown
using either seeded or
unseeded ampoules.
26
Bridgman Technique
Geometry
Elimination Rule
Verneuil Method
Used for growing large
crystals of high melting
point solids
For example:
Ruby from Cr3+/Al2O3
powder
Sapphire from Cr26+/Al2O3
powder
Verneuil Fusion Flame Method
1904 first recorded use of the method
Useful for growing crystals of extremely high
melting metal oxides
Starting material fine powder
Passed through O2/H2 flame or plasma torch (ouch
they are hot!)
Melting of the powder occurs in the flame
Molten droplets fall onto the surface of a seed or
growing crystal
Flux Growth
Oxides like Bi2O3 and PbO have low melting
points and may be used as solvents
–PbTiO3 can be crystallized from PbO/PbF2
mixtures
–need to pick a flux that is compatible with the
desired product
Alkali and alkaline earth metal hydroxides
and halides are also frequently used as fluxes
Top?Seeded Solution Growth Technique
Top-seeded solution growth
technique is one of the "Flux growth"
techniques which is the most
commonly used growth method from
high temperature solutions. The
components of the desired materials
are dissolved in a solvent. Crystals
can be grown below the melting
temperature which prevents
decomposition before melting or a
phase transition below the melting
point.
Crystals grown using this
technique:
Pure and doped BaTiO3
Beta BaB2O4
Growing From Wet KOH/NaOH
Many superconducting copper
oxides have been grown by this
method
Synthesis of EuBa2Cu3O7-δ
take stoichiometric amounts of
CuO, Eu2O3 and Ba(OH)2?8H2O
and dissolve in molten
KOH/NaOH at 450°C
gives clear blue solution
solution held at 450°C under
flowing dry air
as water is lost the product
crystallizes out
27
Molten Metal Fluxes
Molten metals can sometimes be used as solvents.
However, metal should not form stable compounds with
reactants
Heat materials in sealed ampoule. Using a Cu solvent
avoids Mn loss due to heating at high temperatures
1:2:100 ratio of Ru, P and Sn sealed evacuated quartz
tube. Heated to 1200°C and then slow cooled. Crystals
recovered from Sn by washing with HCl
2
C1200,Culiquid MnSiSi2Mn o????? →?+
2
Snliquid RuPPRu ??? →?+
Phase Diagram Can Give Problems
Crystal growth of an
incongruently melting
phase can be very difficult
Can not just go straight
from a stoichiometric melt
? may need to use flux or
other method
A4B can be grown
directly from
stoichiometric melt AB3
can not
Zone Melting
Sweep a molten zone through the crucible in such a way
that the melt crystallize onto a seed
Method used for purifying existing crystals as
impurities tend to stay with the liquid
Method related to the Stockbarger technique
Thermal profile furnace employed
Zone Melting Crystal Growth and
Purification of Solids
Material contained in a boat (must be inert to the melt)
Only a small region of the charge is melted at any one time
Initially part of the melt is in contact with the seed
Boat containing sample pulled at a controlled velocity
through the thermal profile furnace
Zone of material melted, hence the name of the method
Oriented solidification of crystal occurs on the seed
Partitioning of impurities occurs between melt and the
crystal
This is the basis of the zone refining methods for purifying
solids
Impurities concentrate in liquid more than the solid phase,
swept out of crystal by moving the liquid zone
Used for purifying materials like W, Si, Ge to ppb level of
impurities, often required for device applications
When a small slice of the sample is molten and
moved continuously along the sample,
impurities normally dissolve preferably in the
melt. (!! icebergs in salt water don′t contain any
salt !!)
segregation coefficient k:
k = Csolid/Cliquid
(c: concentration of an impurity)
Zone melting was first used as a purification
technique. It, however, can also be used for
crystal growth.
only impurities with k < 1 can be removed by
zone melting !!
Floating Zone Melting
A phase diagram for studying dopant segregation
28
Floating Zone Growth of Silicon
Trace amount of impurities in inter-metallic compounds
have a considerable influence on the physical properties of
the material. Zone melting is a powerful technique for
purification, as it passes the material through a thermal
gradient melting only a small part of the batch. Impurities
usually concentrate in the liquid rather than in the solid
phase.
Move polycrystalline ingot into hot zone a seed formation
of single crystal
Zone melting crystal growth:
(a)horizontal
(b)floating zone
The heater, which surrounds a small portion of the
boat, produces a short molten zone in the sample.
Before crystal growth, the entire boat is filled with the
feed . To initiate the process the heater is positioned
near the seed so that a portion of the seed can be
melted. After the temperature in the sample has
stabilized, the heater is moved at a constant velocity
away from the seed to cause the crystal to grow from
upon the unmelted portion of the seed.
Zone melting crystal growth can also be
conducted without a crucible, as in the
floating-zone process.
The molten zone is sustained by the surface
tension of the melt, and also by
electromagnetic levitation if an induction
heater is used.
The advantage of the floating-zone process
is that it is free from contamination by the
crucible material, which is particularly
significant when growing single crystals
from high-melting-point or reactive
materials.
The disadvantage, however, is that the
molten zone has a tendency to collapse
under gravity.
Hydrothermal
Chemistry
When this altered water
(~350°C) comes in contact with
cold seawater (~2°C) many
reactions take place
Fe2+ + H2S →FeS solid
2Ca2+ + SO4-→CaSO4 solid
Other reduced metal sulfides
form, all insoluble at ambient
temperatures
Hydrothermal Growth of Quartz Crystals
Large quartz crystals are needed as oscillators for
timing applications
Large quartz crystals are grown from basic aqueous
solution at high P/T due to improved solubility of SiO2
Hydrothermally
grown quartz
29
Hydrothermal Growth of Quartz Crystals
Water medium
Nutrients region: 400°C
Seed region: 370°C
Pressure 1.7 Kbar
Mineralizer 1M NaOH
Uses of single crystal quartz:
Radar, sonar, piezoelectric transducers,
monochromators
Annual global production hundreds of tons of quartz
crystals, amazing
Chemical transport in supercritical
aqueous solution (H2O: Tk= 374°C, Pk=
217.7 atm)
Autoclave for the growth of SiO2 single
crystals (→ quartz)
1500 bar, T- gradient 400 →370°C
1: nutrient (powder)
2: seed crystal
3: mechanical fixing of crystal
4: product crystal
Hydrothermal Synthesis
Hydrothermal Reactor Designs
Depending on design may be useable to 10 kbar
Applicability and Value of
Hydrothermal Synthesis
Hydrothermal techniques can be used to
synthesize a wide variety of materials
–zeolites and aluminophosphates
–optical materials like KTP (KTiOPO4)
–BaTiO3 (widely used ferroelectric)
Synthesis can be carried out at low
temperature(relative to direct reaction of solids)
High quality samples can be made
Hydrothermal Synthesis of Crystals
Basic methodology
Water medium
High temperature growth, above normal boiling
point
Water acts as a pressure transmitting agent
Water functions as solublizing phase
Often a mineralizing agent is added to assist with
the transport of reactants and crystal growth
Speeds up chemical reactions between solids
Useful technique for the synthesis and crystal
growth of phases that are unstable in a high
temperature preparation in the absence of water
Crystal growth hydrothermally involves:
Temperature gradient reactor
Dissolution of reactants at one end
Transport with help of mineralizer to seed at the other
end
Crystallization at the other end.
Note that because some materials have negative
solubility coefficients, crystals can actually grow at the
hotter end in a temperature gradient hyrdothermal
reactor, counterintuitive but true, good example is α-
AlPO4 known as Berlinite, important for its high
piezoelectric coefficient (yes larger than alpha -quartz
with which it is isoelectronic) and use as a high
frequency oscillator
Hydrothermal Synthesis of Crystals
30
§ Hydrothermal crystal growth is also suitable
for growing single crystals of:
§ Ruby: Cr3+/Al2O3
§ Corundum: α-Al2O3
§ Sapphire: Cr26+/Al2O3
§ Emerald: Cr3+/Be3Al2Si6O18
§ Berlinite: α-AlPO4
§ Metals: Au, Ag, Pt, Co, Ni, Tl, As
Role of the Mineralizer
Consider the growth of quartz crystals
Control of crystal growth rate, through
choice of mineralizer, temperature and
pressure
Solubility of quartz in water is important
SiO2 + 2H2O → Si(OH)4
Solubility about 0.3 wt% even at supercritical temperatures
>374°C
A mineralizer is a complexing agent (not too stable) for the
reactants/precursors that need to be solublized (not too much)
and transported to the growing crystal
Some mineralizing reactions:
NaOH mineralizer, dissolving reaction, 1.3-2.0 KBar
3SiO2 + 6OH- → Si3O96- + 3H2O
Na2CO3 mineralizer, dissolving reaction, 0.7-1.3 KBar
SiO2 + 2OH- → SiO32- + H2O
CO32- + H2O → HCO3- + OH-
NaOHcreates growth rates about 2x greater than with
Na2CO3 because of different concentrations of hydroxide
mineralizer
Examples of Hydrothermal Crystal
Growth and Mineralizers
Berlinite α-AlPO4
Powdered AlPO4 cool end of reactor, negative solubility
coefficient!!!
H3PO4/H2O mineralizer
AlPO4 seed crystal at hot end
Emeralds Cr3+:Be3Al2Si6O18
SiO2 powder at hot end 600°C
NH4Cl or HCl/H2O mineralizer, 0.7-1.4 Kbar, cool central
region for seed, 500°C
Al2O3/BeO/Cr3+ dopant powder mixture at other hot end
600oC
6SiO2 + Al2O3 + 3BeO →Be3Al2Si6O18
Beryl contains Si6O1812- six rings
§ Metal crystals (amazing, what would you use these
for?)
§ Metal powder at cool end 480°C
§ Mineralizer 10M HI/I2
§ Metal seed at cool end 500°C
§ Dissolving reaction that also transports Au to the
seed crystal:
§ Au + 3/2I2 + I- → AuI4-
§ Metal crystals grown this way include Au, Ag, Pt, Co,
Ni, Tl, As at 480-500°C
Critical Point
§Hydrothermal synthesis necessitates knowledge of what is
going on in an autoclave under different degrees of filling and
temperature
§Pressure, volume, temperature tables of dense fluids like
water are well documented
§The critical point is the point at which the liquid-vapor line
ends.
§Physical differences between liquids and gases disappear at
the critical point and we speak of a single "fluid" state.
–Above this point (at higher temperatures or pressures)
condensation will not take place.
–eg. CH4, Tc = 190 K, CH4 cannot be liquefied at room
temperature, liquid can only be obtained below 190 K.
31
The Supercritical Region
T
P
Critical
pointTriple point
solid
gas
liquid
?Distinction between liquid and gas disappears
?Liquid and gas phase cannot be identified ? no meniscus
Critical Temperatures and Pressures
§ The super critical region is usually a region
of high pressure and temperature
Tc/K TP/atm
Carbon Dioxide 304.2 72.8
Water 647.3 217.6
Ammonia 405.6 111.3
Ethylene 282.4 49.7
Benzene 288 48
Properties of Supercritical Fluids
They are like high density gases and low density
liquids (called fluids to indicate either form)
As a high density gas they can penetrate into
solids eg: concrete, and undergo reactions
internally
As a low density liquid they can dissolve other
compounds, eg. CO2 can act as a solvent for organic
compounds. This property can be used to clean up
circuit boards, oil contaminated systems or be a
host for organic synthesis.
Stockbarger Method
Move the crucible containing a seed and the melt
through a temperature gradient so that the melt
crystallizes onto the seed crystal
Stockbarger method is
based on a crystal growing
from the melt, involves the
relative displacement of
melt and a temperature
gradient furnace, fixed
gradient and a moving
melt/crystal
Bridgman Method
Bridgman method is again based on crystal growth
from a melt, but now a temperature gradient furnace
is gradually cooled and crystallization begins at the
cooler end, fixed crystal and changing temperature
gradient
