Che5700 陶瓷粉末處理
Firing (Sintering)
•To develop desired microstructure, hence
desired product properties; to turn green body
into final products
• Usually last step of process, may sometime has
an additional “post-sintering finish”
•Three stages: (a) organic burnout, elimination
of gaseous products from decomposition and
oxidation; (b) sintering; and (c) cooling, may
include thermal and chemical annealing;
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Purpose of sintering: develop necessary
strength (through consolidation of particles; or
densification)  particles join together via solid
state diffusion and/or reaction to form
compound (sometimes with the help of small
amount of liquid phase)  elimination of pores
 shrinkage of product
 Sometimes, the end product can be very
“porous” (e.g. membrane, filter, etc)
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Equipments
furnaces, tunnel kilns (picture taken from
Google search)
 many different designs: provide enough
temperature, time and atmosphere for sintering
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Presintering Processes
 Thermolysis: organic burnout; important step
before sintering
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Thermolysis Behaviour
May be influenced by:
binder concentration,
product size, product
placement configuration,
heating rate, furnace
atmosphere;
 Time required:
probably by diffusion
length of vapor phase,
may take hours, or days
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Other Considerations
 Endothermic or exothermic reactions may
alter the internal temperature and rate of
reactions
Gas permeation rate slow in compact from
fine particles; amount of gaseous product;
 ex: PVA, initial endothermic, later exothermic
reactions
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Thermal Analysis
TGA, DTG, DTA, DSC (differential scanning
calorimetry), TMA (thermal mechanical analysis)
 Many variations
of TMA: measure
volume changes
(e.g. creep
behaviour), etc.
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Quite
different
behavior
between
different
cases
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More Examples
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Effect of Atmosphere
Plasticizer (low MW) helpful to generate
interconnecting pores for later decomposition of
binders
 for binders (high MW): degradation & oxidation
 Ash from
burnout:
may not be
overlooked
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Decomposition of Inorganic Compounds
Dehydration reactions (water of crystallization) :
clay 450 – 700oC; talc 900-1000oC
 Carbonates: usually 700 – 920oC
 Sulfates: higher temperature is often required
 some organic matter may produce “carbon”,
cause black color in final product
 C + deficiency in oxygen  CO, may have
effect on oxidation state of final product, e.g.
Mn2O3 & MnO or Fe3O4 & Fe2O3 & FeO
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Solid State Sintering
Pure sintering, sintering with reactions and
dissolving, sintering in the presence of glass
particles, etc.
 Density
change to
indicate the
degree of
sintering
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Driving force of sintering: Gt = Gv + Gb +
Gs (t: total; v: volume; b: boundary; s: surface
of grain)
 Simply put: elimination of pores, elimination of
free surface area associated with these pores
 Densification rate & final degree of
densification: hot press > sintering with fine
particle > sintering with coarse particle
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surface diffusion: produce surface smoothing,
particle joining, pore rounding (no shrinkage)
Viscous flow & plastic deformation: major effect
on volume change
 Pores: source of vacancy; grain boundary:
vacancy sink
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 Homogeneous vs inhomogeneous systems
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 Ball mill to reduce aggregate size helps
sintering
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Grains with more than 6 sides,
have concave boundary
 log-normal grain size
distribution; uniformity is key
Arrows
indicate direction
of movement
 Large
becomes larger;
to minimize
grain boundary
 grains with
less than 6 sides,
have convex
boundary. Tend
to shrink
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Regular grain growth vs exaggerated (or
discontinuous) grain growth
 average grain size d = 1.56L with L = read
from mean intercept length between boundaries
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Dopant
accumulate in
the boundary,
reduce surface
tension
 grain growth
inhibitor:
segregated at
grain boundary,
help to get high
density product
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Many models proposed for grain growth, or pore
mobility: e.g. Mp = K Ds/(T rp^4) [Ds suface
diffusion coeff.; rp pore radius]  small pores
moves fast
 vacancy diffuse from small pores to large pores
 an example of Ostwald ripening
 densification faster: if vacancy diffusivity >
surface diffusivity (grain coarsening); [fast heating
rate is useful]
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 Rapid heating rate :
densification with small
grain size;
 Surface diffusion
dominate at low
temperature  favors
grain coarsening
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The main results are:
(i) both n-TiO2 and ZrO2 undergo densification at
temperature much lower than they do in more
conventional sized powders;  due to small size
(ii) they densify without significant grain growth until the
density reaches ,- 90% bulk density and the porosity
becomes closed;
(iii) at densities above 90%, grain growth can be rapid;
abnormal grain growth, however, was never
observed;
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 source:
Nanostructured
Materials, 1 (1992)
173-178
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(iv) grain growth can be controlled by pressure-assisted
sintering or by Y doping in n-TiO2, although complete
densification appears to require some grain growth
(v) Vickers hardness in dense nanophase ceramics are as
high as in single crystal TiO2, but it decreases with
increasing grain size on annealing.
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 Impurity doping: effective to limit grain growth
 Vickers hardness ~ d^-1/2
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Atmosphere important: gas trapped in closed
pore will limit its shrinkage unless gas is soluble in
the ceramics
 SO2, Cl2 may come from impurity in raw
particles;
 oxygen may be important in determining the
stoichiometry of product (e.g. ZnO, PbO. Ferrites
etc.); lattice vacancy & oxidation state
 in nitrogen atmosphere: reaction bonded Si3N4
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 Compact pressed at high
pressure  narrow & smaller
pore size  higher density
In a uniform
compact: uniform
interstices smaller
than grain size
shrink at a fast rate;
 coarse micropore: usually grow
 very large
macro-pore: remain
the same during
sintering
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Sintering in the presence of small
amount of wetting liquid
Usually a glassy
phase wetting
grain;
 better
densification at
lower temperature
This glassy phase is also beneficial to
adherence to substrate or glaze
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 Less than 1%
glassy phase is
sufficient if
distributed
uniformly;
 High diffusivity in the liquid phase increase
mass transport and shrinkage
 Liquid penetrate between grains, inhibiting
exaggerated grain growth
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Sintering of whiteware bodies (vitrification)
Densification occurs simultaneously with
reaction and dissolving of raw materials producing
new glassy and crystalline phases
 Fine clay coat quartz and feldspar (KAlSi3O8 –
NaAlSi3O8 – CaAl2Si2O8)
 quartz dissolving slowly into feldspar liquid
above 1250C, producing mullite; whole process
very dependent on particle size & impurities
 high green density & control of heating
program are important
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Around 560C, dehydration from clay;
Shrinkage at 950C to formation of mullite
(3Al2O3 2 SiO2) from metakolin
 1160C change in vitrification
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Sintering of glaze and glassy thick film
Glaze commonly < 1 mm in thickness
Spreading of the vitreous phase: gravity &
surface tension & viscosity of the glaze (both
dependent on temperature & composition)
 lead frit is often added; yet lead oxide is
volatile (air pollution problem)
 Bubbles diffuse out of glaze
 Some glaze penetrate & react with substrate
body; develop strong bonding; pigment may settle
in glaze
 one-fire, two-fire processes
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 One example of
manganese zinc ferrite:
presintering, sintering,
annealing, cooling;
different atmosphere;
 Sintering with high
oxygen pressure  to
prevent loss of ZnO;
during cooling,
reduced O2 pressure
to get ferrous iron for
single phase product;
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Summary
Ceramics are produced in a variety of size,
shape, composition, & production rate;
 General procedure often contains presintering,
sintering, annealing and cooling stages; heating
rate, time & atmosphere can be changed
 elimination of pores is desired; closed pores
will limit final density;
 grain growth also occur during sintering 
final microstructure  final property
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