CERAMIC MATERIALS Imetalurji.mu.edu.tr/.../Sayfa/A_Kalemtas_Ceramics_Materials_09_10_2013.pdfPhase...

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

mailto:[email protected]







Clay products – Main Components

Clay

Silica Feldspar


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


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.


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.

http://mineral.galleries.com/minerals/silicate/quartz/quartz.htm

http://mineral.galleries.com/minerals/gemstone/amethyst/amethyst.htm

http://mineral.galleries.com/minerals/gemstone/citrine/citrine.htm

http://mineral.galleries.com/minerals/silicate/milky_qu/milky_qu.htm

http://mineral.galleries.com/minerals/gemstone/rock_cry/rock_cry.htm

http://mineral.galleries.com/minerals/gemstone/rose_qua/rose_qua.htm

http://mineral.galleries.com/minerals/gemstone/smoky_qu/smoky_qu.htm


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.


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.


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


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.


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.


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.


SILICA (SiO2)

Influence of temperature on the expansion of the various forms of silica


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.


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.


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.


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.


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.


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


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


Carbon Forms - Graphite

layer structure

– weak van der Waal’s

forces between layers

– planes slide easily,

good lubricant


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


Feldspars


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.


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


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.


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.


Feldspars


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.


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.


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.


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


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

http://www.amazon.com/exec/obidos/tg/detail/-/B000093URB/103-7692725-2738240?v=glance&s=electronics&me=ATVPDKIKX0DER&vi=pictures&img=14


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


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.


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


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)


Thanks for your kind

attention

THE END


Any

Questions

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