CERAMIC MATERIALS I
Asst. Prof. Dr. Ayşe KALEMTAŞ
Office Hours: Thursday, 09:30-10:30 am.
[email protected], [email protected], Phone: 211 19 17 Metallurgical and Materials Engineering Department
Asst. Prof. Dr. Ayşe KALEMTAŞ
Clay products – Main Components
Clay
Silica Feldspar
Asst. Prof. Dr. Ayşe KALEMTAŞ
Clay products – Main Components
When mixed with water the crystals can easily slide over each other (like a pack of cards), and this phenomenon
gives rise to the plasticity of clays.
Provides plasticity, when mixed with water
Hardens upon drying and firing (without losing
the shape)
Adding water to clay
-- allows material to shear easily along weak
van der Waals bonds
-- enables extrusion
-- enables slip casting
Silica, SiO2, is mixed with clay to reduce shrinkage
of the ware while it is being fired, and thus
prevent cracking, and to increase the rigidity of the ware so that it will
not collapse at the high temperatures required for firing. Silica is useful for this purpose becasue
it is hard, chemically stable, has a high
melting point and can readily be obtained in a pure state in the form of
quartz.
Feldspars are used as a flux in the firing of
ceramic ware. When a body is fired, the
feldspar melts at a lower temperature than clay or
silica, due to the presence of Na+, K+ or Ca2+ ions, and forms a
molten glass which causes solid particles of
clay to cling together: when the glass solidifies
it gives strength and hardness to the body.
Clay
Silica
Feldspar
Asst. Prof. Dr. Ayşe KALEMTAŞ
SILICA (SiO2)
Silica (SiO2) is an important raw material for the production of glass,
glazes, enamels, refractories, abrasives and whiteware.
Major SiO2 sources are in the polymorphic form quartz, which is the
primary constituent of sand, sand-stone, and quartzite. Quartz is the
second most abundant mineral in Earth’s crust.
The major use (accounting for about 38% of U.S. production) is in glass
manufacture. For example, incandescent lamp bulbs are made of a soda-
lime silicate glass containing about 70 wt% SiO2.
The SiO2 content of high-quality optical glasses can be as high as
99.8 wt%.
The United States is the largest producer of industrial sand in the world.
Annual production of silica in the United States is approximately 30 Mt,
valued at around $700 million.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Quartz Minerals
Quartz
Silica tetrahedra alone can form a neutral
three-dimensional framework structure with
no need for other cations. This arrangement
forms a very stable structure.
Popular as ornamental stone and as gemstones
•Amethyst is the purple gemstone variety.
•Citrine is a yellow to orange gemstone variety that is rare in nature but is often
created by heating Amethyst.
•Milky Quartz is the cloudy white variety.
•Rock crystal is the clear variety that is also used as a gemstone.
•Rose quartz is a pink to reddish pink variety.
•Smoky quartz is the brown to gray variety.
Asst. Prof. Dr. Ayşe KALEMTAŞ
SILICA (SiO2)
Polymorphs are materials that have the same chemical
composition but different crystal structures.
Many ceramic materials show this behavior, including SiO2,
BN, BaTiO3, ZrO2 and BeO.
Transitions between the different polymorphs may occur as a
result of changes in temperature or pressure.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Phase Transitions
Reconstructive transition is a transition which involves a major reorganization of the
crystal structure and a change of local topology, during which primary bonds are
broken and reformed so that there is no immediate relationship between the crystal
structures of the parent and product phases.
Displacive transition is a transition in which a displacement of one or more kinds of
atoms or ions in a crystal structure changes the lengths and/or directions of bonds,
without severing the primary bonds.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Phase Transitions
Reconstructive
867C
Reconstructive
1470C
High
Quartz
High Tridymite
High Cristobalite
Low
Quartz
Low
Cristobalite
Middle
Tridymite
Low
Tridymite
Displacive
160C
Displacive
105C
Displacive
200 270C
Displacive
573C
The relationships between the polymorphic forms of silica
Asst. Prof. Dr. Ayşe KALEMTAŞ
Phase Transitions
The transformations between the basic structures (quartz, tridymite,
and cristobalite) are necessarily reconstructive. Therefore, they are
relatively slow and thermally activated.
Some conversion of quartz to cristobalite may occur during anneals at
high temperature, but the reverse transformation will not occur simply
because the kinetics of transformation are extremely slow in the
temperature regime in which, say, quartz is thermodynamically stable.
For whitewares the duration of firing is usually too short for
reconstructive transformations to be significant, although this may not the
case for silica refractories which are typically fired above 1400 C.
Significant conversion to tridymite and cristobalite may also occur during
service at high temperature, and the presence of a lime flux will enhance
the rate of conversion. In sharp contrast, the displacive transitions within
a polymorph cannot be suppressed and are extremely rapid.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Phase Transitions
The key consequence of interest in ceramic processing is the dimensional
changes which occur during the - to -quartz, low- to high-tridymite, and
low- to high-cristobalite transformations.
The molar volume of quartz is a smooth function of temperature except for
the phase transformation which occurs at 573 C which causes a sudden
change in volume on the order of one percent.
Interestingly, high quartz exhibits a negative thermal expansion coefficient.
Tridymite exhibits a smaller displacive volume change at 105 C, but also
exhibits negative expansion above 575 C.
In the case of cristobalite the transformation occurs at a low temperature,
about 215 C, and the volume change is roughly three percent, that is,
significantly larger than in the case of the other two polymorphs.
Asst. Prof. Dr. Ayşe KALEMTAŞ
SILICA (SiO2)
At pressures around 2 GPa, quartz transforms into coesite.
At even higher pressures, around 7.5 GPa, coesite transforms to stishovite.
The high pressure forms have been prepared experimentally and are also
found at the famous Canon Diablo Meteor site in Arizona.
Polymorph Density (g/cm3) Crystal Structure
Tridymite 2.28 Hexagonal
Cristobalite 2.33 Cubic
Quartz (Beta) 2.53 Hexagonal
Quartz (Alpha) 2.65 Rhombohedral
Cristobalite and tridymite also have high and low forms. Low tridymite is orthorhombic and
pseudohexagonal, low cristobalite is tetragonal and pseudo-cubic.
Asst. Prof. Dr. Ayşe KALEMTAŞ
SILICA (SiO2)
Influence of temperature on the expansion of the various forms of silica
Asst. Prof. Dr. Ayşe KALEMTAŞ
SILICA (SiO2)
When a ceramic is fired, the sand can react, particularly with the fluxes. This
reaction is seldom complete.
The transformation of residual quartz into cristobalite can then start from
1200 C onwards. It is favored by the rise in temperature, the use of fine
grained sand, the presence of certain impurities and a reducing atmosphere.
The form in which silica is found determines the thermal properties of silicate
ceramics. Thus, quartz and cristobalite do not have the same influence on the
expansion of the shard.
Quartz can also cause a deterioration of the mechanical properties of the
finished product owing to the abrupt variation in dimensions (ΔL/L ≅ –0.35%)
associated, at 573 C, with the reversible transformation quartz β → quartz α.
As the crystal of cristobalite formed from the flux are usually small, the
transition cristobalite β → cristobalite α, which occurs at about 220 C, often
causes less damage to the finished product. It can even contribute to the
shard/enamel fit by compressing it after cooling at room temperature.
Asst. Prof. Dr. Ayşe KALEMTAŞ
SILICA (SiO2)
For each form, at low temperatures (the α phase) we find a structure
that is a distortion of the high-temperature form (the β phase).
In each case, changing from the α to β structure involves a displacive
phase transformation; the atoms need to move only slightly relative to
one another.
However, to change from one form to another requires breaking
bonds. This process is much more difficult and is known as a
reconstructive phase transformation.
The Si–O–Si arrangement of ions does not always lie exactly on a
straight line, especially for the low temperature forms.
If the bonding were purely ionic, the line would be straight and the
O2− should lie exactly in the middle: the reason in each case is that we
want to maximize the electrostatic attractive forces and minimize the
electrostatic repulsion.
However, the Si–O bond is ∼60% covalent, so there is a strong
tendency toward directional bonding.
Asst. Prof. Dr. Ayşe KALEMTAŞ
CARBON
Elemental carbon exists in nature mainly as two allotropes,
diamond and graphite.
Graphite has a variety of uses: writing medium in pencils; electrodes; high-temperature devices (crucibles, rocket nozzles);
and strong graphite fibers.
High-quality diamonds are used in jewelry.
Most diamonds are used as abrasives, in industrial drill bits, saw blades, etc. Diamond is the hardest substance known and has a
high thermal conductivity (dissipates heat quickly).
Carbon also exists in amorphous forms, such as coke, charcoal, and carbon black.
Asst. Prof. Dr. Ayşe KALEMTAŞ
CARBON FORMS
Elemental carbon exists in nature mainly as two allotropes,
diamond and graphite.
An allotrope is a variant of a substance consisting of only one
type of atom. It is a new molecular configuration, with new
physical properties.
Substances that have allotropes includecarbon, oxygen, sulfur,
and phosphorous.
Allotropes of a given substance will often have substantial
differences between each other. For example, one allotrope of
carbon, fullerene, is many times stronger and lighter than steel.
An allotrope should not be confused with phase, which is a
change in the way molecules relate to each other, not in the way
that individual atoms bond together.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Carbon Forms - Diamond
Carbon black – amorphous – surface area ca. 1000 m2/g
Diamond:
Carbon with a cubic crystalline structure with covalent bonding between atoms
*hard – no good slip planes
*brittle – can cleave (cut) it
*large diamonds – jewelry
*small diamonds
often man made - used for cutting tools and polishing
diamond films
*hard surface coat – cutting tools, medical devices, etc.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Carbon Forms - Diamond
Carbon with a cubic crystalline structure with covalent bonding
between atoms
• This accounts for high hardness
Industrial applications: cutting tools and grinding wheels for machining hard, brittle materials, or materials that are very abrasive; also used in dressing tools to sharpen grinding
wheels that consist of other abrasives
Industrial or synthetic diamonds date back to 1950s and are fabricated by heating graphite to around 3000C (5400F) under very high pressures
Asst. Prof. Dr. Ayşe KALEMTAŞ
Carbon Forms - Graphite
Form of carbon with a high content of crystalline C in the form of layers
Bonding between atoms in the layers is covalent and therefore strong, but the parallel layers are bonded to each other by weak van der Waals forces
This structure makes graphite anisotropic; strength and other properties vary significantly with direction
• As a powder it is a lubricant, but in traditional solid form it is a refractory
• When formed into graphite fibers, it is a high strength structural material
Asst. Prof. Dr. Ayşe KALEMTAŞ
Carbon Forms - Graphite
layer structure
– weak van der Waal’s
forces between layers
– planes slide easily,
good lubricant
Asst. Prof. Dr. Ayşe KALEMTAŞ
Carbon Forms - Fullerenes and Nanotubes
Adapted from Figs. 12.18 & 12.19, Callister 7e.
Fullerenes or carbon nanotubes wrap the graphite sheet by curving into ball or tube
Buckminister fullerenes Like a soccer ball C60 - also C70 + others
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Feldspars constitute an abundant mineral group and make up an
estimated 60% of the earth’s crust. They are present in many
sedimentary deposits and are found in almost all igneous and
metamorphic rocks.
The glass industry uses most of the feldspar produced. Feldspar is a
source of Al2O3, which improves the mechanical properties of glass
such as its scratch resistance and its ability to withstand thermal
shock.
Feldspar is also used in whiteware bodies as a flux, which produces
a glassy phase during firing increasing the strength and translucency
of the body.
The Republic of Korea is the largest producer of feldspar in the
world.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Four feldspathic minerals are likely to enter the composition of silicate
ceramic
pastes. They are:
orthoclase, a mineral rich in potassium with the composition
K2O.Al2O3.6SiO2
albite, a mineral rich in sodium with the composition Na2O.Al2O3.6SiO2
anorthite, a mineral rich in calcium with the composition CaO.Al2O3.2SiO2
petalite, a mineral rich in lithium with the composition Li2O.Al2O3.8SiO2
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Orthoclase and albite, which form eutectics with silica, respectively, at 990
and 1050 C, are widely used as flux.
Anorthite is rather regarded as a substitute to chalk.
The use of petalite, especially owing to its negative expansion coefficient, is
marginal.
Potassic feldspar is particularly appreciated by ceramists because its reaction
with silica leads to the formation of a liquid whose relatively high viscosity
decreases slightly when the temperature increases. This behavior is
considered as a guarantee against the excessive deformation of the pieces
during the heat treatment.
Natural feldspars used for the preparation of ceramics are mineral mixtures.
Thus, the commercial potassium products can contain between 2.5 and 3.5%
of albite mass, whereas anorthite and a small quantity of orthoclase, between
0.5 and 3.2%, are often present in the available sodium feldspars.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Feldspars rarely occur in nature as pure minerals.
Albite and anorthite form a complete solid solution series and they
occur in nature as alloys.
Even though orthoclase and albite form only limited solid solutions,
deposits of orthoclase always contain some albite.
The rock nepheline syenite is a mixture of orthoclase, albite and
nepheline with minor impurities.
These materials are typically ground to a relatively coarse powder, on
the order of 70 to 100 m, for use in ceramic (or glass) production.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
The tendency to form a glass is strongly correlated to the viscosity of a melt.
In general, molten feldspars are rather viscous which is ascribed to the existence of
polymerized silicon-aluminum-oxygen tetrahedra in the liquid (Barth, 1969).
Despite lower melting points, the alkali feldspars produce much more viscous liquids
than anorthite.
In the case of albite this is interpreted as evidence for a higher degree of
polymerization in the melt.
In the case of orthoclase it is due to the formation of leucite, KAlSi2O6, crystals.
In all cases glasses are produced under the cooling rates normally encountered in
ceramic processing.
Albite melts at the lower temperature than orthoclase, but the addition of anorthite
increases the melting temperature of soda feldspar while decreasing that of the potash
feldspar (down to a minimum at about 22% anorthite). Similarly a 50%: 50% mixture of
albite and orthoclase melts at a lower temperature than either end member.
Often mixtures of fluxes are employed in order to take advantage of eutectic melting.
Lithium bearing minerals are often very effective fluxes when used in conjunction with
feldspar since such combinations form deep eutectics.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional Ceramics
The materials treated at higher temperatures or in the presence of a large quantity
of flux are generally the least porous.
Whiteness is primarily the result of the use of raw materials free from iron and
titanium or containing only small contents of transition metals.
Increasing the relative amount of ball clay generally improves the plasticity as well
as the green strength, but often leads to discoloration as a result of contamination by
iron-bearing accessory minerals. Therefore applications where green strength is at a
premium, and color is of less importance, employ larger amounts of ball clay.
Fine china represents the opposite end of the spectrum where aesthetics take
priority. China clays are used in these formulations, because they are nearly phase
pure and do not occur as iron bearing solid solutions (as do the smectites and illites).
The extent of glass formation affects properties such as dimensional stability and
degree of densification. In systems which require high dimensional stability such as
structural clay products (i.e., large ceramic pipes and tiles) the extent of glass
formation is kept to a minimum (Brownell, 1976). In contrast, dental porcelains must
fuse at low temperatures to be compatible with metal substructures and therefore
may contain in excess of 80% feldspars.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional Ceramics
The substitution of alumina for quartz
The substitution of alumina for quartz increases the strength of the fired ceramic
However, it increases density, decreases translucency, and reduces the effective
thermal expansion coefficient.
The density of alumina, 3.96 g/cm3, is roughly 50% larger than quartz, 2.65 g/cm3,
therefore in formulations containing fifty weight percent filler the density difference is
substantial.
There is also a decrease in translucency due to the alumina's higher index of
refraction, 1.76, which leads to a greater degree of internal light scattering.
The reduction in thermal expansion coefficient is the result of a modestly lower
thermal expansion coefficient and the absence of a phase transformation.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional ceramics
Whitewares:
• Dinnerware
• Floor and wall tile
• Electrical porcelain
• Decorative ceramics
Cement:
• Concrete roads, bridges, buildings, dams, sidewalks, bricks/blocks
Abrasives:
• Natural and synthetic abrasives
Refractories:
• Brick and monolithic products used in iron and steel, non-ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemicals industries, kiln furniture
Glasses:
• Flat glass (windows), container glass (bottles), pressed and blown glass (dinnerware), glass fibers (home insulation)
Structural clay products:
• Brick, sewer pipe, roofing tile
Asst. Prof. Dr. Ayşe KALEMTAŞ
Glasses
• Amorphous solid
– Vitreous (noncrystalline) structure
– Amorphous
– Cooled to semi-solid condition without crystallization
• Silica Glass – Optical properties
– Thermal stability
• Products
– Window glass
– Fiber optics
– Chemical containers
– Lenses
Asst. Prof. Dr. Ayşe KALEMTAŞ
Glass Ceramics
• Crystalline solid
– 0.1 to 1.0 m grains
– Use of nucleating agents
• Glass Ceramic – Efficient processing in
glassy state
– Net shape process
– Good mechanical properties versus glass
– Low porosity
– Low thermal expansion
– Higher resistance to thermal shock
• Products
– Cookware
– Heat exchangers
– Missile radomes
Asst. Prof. Dr. Ayşe KALEMTAŞ
REFRACTORIES
Firebricks for furnaces and ovens.
Brick products are used in the manufacturing plant for iron and steel, non-
ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and
chemical industries.
• Retain properties at high
temperature
– Mechanical
– Chemical
• Products
– Fire brick
– Insulating fibers
– Refractory linings
– Coatings
• Silica
• Alumina
• Magnesium Oxide etc.
Asst. Prof. Dr. Ayşe KALEMTAŞ
REFRACTORIES
• Need chemical insensitivity.
• Improve properties further by including pores – Less thermal expansion/contraction upon thermal cycling
– Resistance to thermal shock
– Increased insulation
– Lighter
• But some disadvantages:
– Worse resistance to chemical attack
– Weaker load bearing capability
Refractory Brick
Asst. Prof. Dr. Ayşe KALEMTAŞ
REFRACTORIES
Refractory SiO2 Al2O3 MgO Fe2O3 Cr2O3
Acidic
Silica 95-97
Superduty firebrick 51-53 43-44
High-alumina firebrick 10-45 50-80
Basic
Magnesite 83-93 2-7
Olivine 43 57
Neutral
Chromite 3-13 12-30 10-20 12-25 30-50
Chromite-magnesite 2-8 20-24 30-39 9-12 30-50
Compositions of typical refractories (weight percentage)
Asst. Prof. Dr. Ayşe KALEMTAŞ
Thanks for your kind
attention
THE END
Asst. Prof. Dr. Ayşe KALEMTAŞ
Any
Questions