MME391 Ceramic Materials - Bursa Teknik...

Asst. Prof. Dr. Ayşe KALEMTAŞ

MME391 CeramicMaterials


Bursa Technical University, Metallurgical and Materials Engineering Department

Office Hours: Tuesday, 10:00-11:[email protected], [email protected]

mailto:[email protected]











Clay Products – Main Components

Clay

SilicaFeldspar

Traditional Ceramics


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 temperaturesrequired 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

SilicaFeldspar



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.

Feldspar

Traditional CeramicsFluxes are raw materials with high amountof alkaline oxides, mainly K2O and Na2O,whereby, in reaction with silica and alumina,promote liquid phase formation thatfacilitates the densification.

The liquid phase surrounds the solid particlesand by surface tension enablesrearrangement of particles and decreasingthe porosity.

Feldspars are used as a flux in the firing ofceramic ware. When a body is fired, thefeldspar melts at a lower temperature thanclay or silica, due to the presence of Na+ , K+or Ca2+ ions, and forms a molten glasswhich causes solid particles of clay to clingtogether: when the glass solidifies it givesstrength and hardness to the body. Themolten glass also reacts with the silica andclay particles.


Earth’s CrustEarth’s crust refers to the outher siliceous shell ofthe earth which is about 16 km thick.

Metallic ores and mineral products are producedfrom this crust. Therefore, the original source ofall metals is the earth’s crust.

Ores are naturally occuring deposits in theearth’s crust. The ores are mined and treatedby various• unit operations (mechanical processes)• unit processes (chemical metallurgical

processes)to extract metals and to convert them intothe metallic form.

ORES AND MINERALS

The mineral content of the ore must be in sufficientconcentration to make its extraction commerciallyviable.

Mine : It is a large area having an abundant quantity of

mineral deposits that can be easily and economically

extracted.


Minerals are the constituents of rocks, which make up theentire inorganic, solid portion of the earth. Rocks are usuallynot composed of a single mineral but, rather, an aggregate oftwo or more minerals. Broadly speaking, geologists dividerocks into three types:

igneous, metamorphic, and sedimentary.

Rocks: • They are a combination of minerals along with impurities.

A rock can contain either a single mineral or a number ofminerals.

• Although over 2000 minerals have been discovered, onlya few are abundantly found in most of the rocks.

ORES AND MINERALS


Metamorphic rocks have undergone structural and/or chemical transitions(metamorphism or metamorphosis) from their original form as a result of hightemperatures and pressures deep beneath the earth’s surface. These transitionsoccur in the solid state without melting and result in the formation of newminerals, such as kyanite, staurolite, sillimanite, andalusite, and some garnets.

Other minerals, such as some of the igneous minerals, may be present inmetamorphic rocks but are not necessarily the result of metamorphism. Theword “metamorphic” has a Greek origin coming from meta, meaning“change,” and morpe, meaning “shape”.

Igneous rocks form when magma cools and solidifies. Magma is a complexmolten material that originates deep within the earth.

The word igneous comes from the Latin word ignis, which means “fire”;igneous rocks then are “formed from fire.”

Magma is rich in the elements Si, O, Al, Na, K, Ca, Fe, and Mg.

Feldspars are alumino-silicate minerals found in nearly all igneous rocks.

ORES AND MINERALS


Sedimentary rocks are formed when small particles or precipitatedcrystals become cemented together. Sedimentary rocks are classified aseither clastic or chemical.

Clastic sedimentary rocks form when rock particles produced bymechanical and chemical weathering are transported by water, ice, andwind to new locations where they become cemented together.

Chemical sedimentary rocks form when highly soluble ions, such as, Na+,K+, Ca2+, Mg2+, Cl-, F-, (SO4)2-, (CO3)2-, (PO4)3- from existing rocks aredissolved in water and subsequently precipitate, forming layers inoceans and lakes, where they become cemented together.

The composition of sedimentary rocks depends on:▪ Composition of original source rocks▪ Chemical and mechanical resistance of each mineralcomponent▪ Distance traveled

ORES AND MINERALS


Resistant minerals such as quartz are common constituents ofsedimentary rocks, and some more rare minerals (e.g., garnet, rutile,zircon) have similar properties.

Feldspar is less resistant but is so common that it is a majorconstituent of many sedimentary rocks. Precipitated minerals includethe carbonates (e.g., calcite, dolomite), the sulfates (e.g., gypsum,anhydrite), chlorides, and chalcedonic silica (e.g., chert, flint).

Igneo

us

Rocks formed by

cooling and

solidification of

magma Met

am

orphic

Rocks that have

undergone

structural

and/or chemical

transitions Sed

imen

tary

Rocks formed

when smaller

particles become

cemented

ORES AND MINERALS


Average Analysis of the Earth’s Crust

It is clear that :

Oxygen Silicon

Aluminum Iron

are the 4 mostabundant elements

Since engineering metals Al, Fe,Mg and Ti are far moreabundant than the othermetals, there is never likely tobe a shortage of those metalsdue to exhaustion of the oredeposits.

The commonly known metalslike Au, Pb, Sn are in so smallquantities in the earth’s crustthat their commercial recoveryis impossible.

Most commonelements in the crust


FeldsparsFel

dsp

ars

A group of alumino-

silicates, tetrahedra form

three-dimensional

frameworks with Ca, Na

and K as the balancing

cations.

Fel

dsp

ars

The very abundant

feldspar are subdivided in

K-Na bearing alkali

feldspars and the Ca-Na

solid-solution series called

the plagioclase feldspars.


FeldsparsFeldspar Group

light silicates (K-Na-Ca, Al)

2-directionsof cleavage

(at 90 degrees)

Orthoclase

Plagioclase

K-feldspar

Ca/Na-feldspar

Most common mineral group


FeldsparsK-feldspar

... which can weather further to bauxite: AlO(OH)

... weathers to kaolinite:Al2Si2O5(OH)4

orthoclase KAlSi3O8 ...

Fel

dsp

ars

Is very

hard Fel

dsp

ars Has

multiply

colors Fel

dsp

ars Most

common

mineral

group


FeldsparsFel

dsp

ars

Feldspars are used as

a flux in the sintering

of ceramic ware.

Feldspar produces a

glassy phase during

firing increasing the

strength and

translucency of the

body.

Fel

dsp

ars Feldspars are

alumino-silicate

minerals found in

nearly all igneous

rocks.

Fel

dsp

ars The plagioclase

feldspars are solid

solutions of albite

and anorthite.

Na+ Si4+ is replaced

by Ca2+ Al3+

Fel

dsp

ars The alkali feldspars

are solid solutions

of albite and

orthoclase.

Na is replaced by K

Fel

dsp

ars

There is virtually no

solid solution

between anorhite

and orthoclase.

Fel

dsp

ars

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.


FeldsparsFel

dsp

ars

Four feldspathic

minerals are likely

to enter the

composition of

silicate ceramic

pastes. They are:

Fel

dsp

ars Orthoclase, a

mineral rich in

potassium with the

composition

K2O.Al2O3.6SiO2

Fel

dsp

ars Albite, a mineral

rich in sodium

with the

composition

Na2O.Al2O3.6SiO2

Fel

dsp

ars

Anorthite, a mineral

rich in calcium with

the composition

CaO.Al2O3.2SiO2

Fel

dsp

ars

Petalite, a mineral

rich in lithium with

the composition

Li2O.Al2O3.8SiO2

Fel

dsp

ars

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.


FeldsparsFel

dsp

ars

Feldspars rarely

occur in nature as

pure minerals. Fel

dsp

ars

Albite and anorthite

form a complete

solid solution series. Fel

dsp

ars

Even though

orthoclase and albite

form only limited

solid solutions,

deposits of

orthoclase always

contain some albite.

Fel

dsp

ars The rock nepheline

syenite is a mixture

of orthoclase, albite

and nepheline with

minor impurities.

Fel

dsp

ars

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.Fel

dsp

ars

The Republic of

Korea is the largest

producer of feldspar

in the world.


FeldsparsFel

dsp

ars

Orthoclase and

albite, which form

eutectics with

silica, respectively,

at 990 and

1050°C, are

widely used as flux.

Fel

dsp

ars

Anorthite is rather

regarded as a

substitute to chalk. Fel

dsp

ars The use of petalite,

especially owing to

its negative

expansion

coefficient, is

marginal.

Fel

dsp

ars

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.

Fel

dsp

ars

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.


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 ofpolymerized silicon-aluminum-oxygen tetrahedra in the liquid.

Despite lower melting points, the alkali feldspars produce much more viscous liquidsthan anorthite.

In the case of albite this is interpreted as evidence for a higher degree ofpolymerization 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 inceramic processing.

Albite melts at the lower temperature than orthoclase, but the addition of anorthiteincreases the melting temperature of soda feldspar while decreasing that of the potashfeldspar (down to a minimum at about 22% anorthite). Similarly a 50%: 50% mixtureof albite and orthoclase melts at a lower temperature than either end member.

Often mixtures of fluxes are employed in order to take advantage of eutecticmelting.

Lithium bearing minerals are often very effective fluxes when used in conjunction withfeldspar since such combinations form deep eutectics.


FeldsparsFel

dsp

ars The materials

treated at higher

temperatures or in

the presence of a

large quantity of

flux are generally

the least porous.

Fel

dsp

ars Whiteness is

primarily the result

of the use of raw

materials free from

iron and titanium or

containing only

small contents of

transition metals.

Fel

dsp

ars

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).

Fel

dsp

ars

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.

Fel

dsp

ars 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. In contrast,

dental porcelains must fuse at low

temperatures to be compatible with

metal substructures and therefore may

contain in excess of 80% feldspars.


FeldsparsFel

dsp

ars

Feldspars are commercially

used in the glass and

ceramics industries because

of their chemical

composition, particularly for

their alkali content. The

consumption of feldspars

within these two industries

is estimated at 85–90 % of

the market.

Fel

dsp

ars

Feldspar is also used in

fillers and abrasive

industries but not as

much as in the ceramic

and glass industries. Fel

dsp

ars

The greatest consumption of

feldspar raw materials is in

ceramic products. Ceramic

products comprise sanitary

ware, ceramic tiles,

dinnerware, ceramic glazes,

and electrical porcelain. In

ceramic industries feldspar

is a basic component in the

raw material batch for

ceramic bodies.

Fel

dsp

ars

The typical feldspar content

in floor and wall tiles is from

10–55 wt.%, from 15–30

wt.% in white ware, chemical

porcelain and hotel china,

25–35 wt.% in sanitary

ware and 30–50 wt.% in

electrical porcelain. Feldspar

is also used in ceramic glazing

in amounts ranging from

30–50 wt.%; by providing

alumina and alkali it also acts

as a flux in fine ceramics.

Fel

dsp

ars In addition, feldspar also has

various significant usages such

as for ceramic shields. The

NASA space programme has

used a ceramic thermal shield

to protect spacecraft during

the re-entry to the

atmosphere.

Fel

dsp

ars

In pottery, a high amount of

potash feldspar increases

their strength. Potash

feldspar is also involved in

the ceramic materials which

increase the efficiency of gas

turbines. A particular

application that consumes a

significant amount of feldspar

is porcelain industry, which

uses 60–80 %.


Beneficiation is the process that most minerals need to go through beforethey can be used in producing ceramics.

Physical beneficiation includes crushing and grinding of coarse rocks. Theparticle size of the raw material may affect subsequent steps in theproduction process. An example that we use is producing alumina frombauxite, a process that involves a chemical reaction.

Chemical beneficiation includes processes of separating the desired mineralfrom unwanted waste products—for example, by dissolution in a suitablesolvent followed by filtration. The Bayer process for producing alumina isalso a good example of chemical beneficiation. Bauxite contains manyimpurities. The purity of the raw materials is reflected in the composition ofthe final product. For many ceramics, careful control over purity isrequired. For these applications, the raw materials are synthesized.

Furthermore, several important ceramics do not occur naturally in mineralform and must be fabricated chemically. Synthesis of ceramic powders canhave advantages not only in purity but also in allowing the generation offine particle sized powders having a well-defined morphology.

Beneficiation


• Structural clay products : bricks, sewer pipe, roofing tile

• Triaxial bodies: Whiteware, porcelain, chinaware, sanitary ware.



Triaxial whiteware chemical composition



Fel

dsp

ars

Feldspars are the

most abundant

group of rock-

forming minerals in

the nature. They

are components of

many rock types,

including:

pegmatite, granite,

syenite (especially

nepheline syenite),

feldspathic sand

and sandstone

(arkose).

Feldsp

ars

The major commercial

value of feldspars rests

on their chemical

composition, especially

on the content of

alkalis, i.e. K2O (min.

10%) – preferred in

the production of

whiteware, or Na2O

(min. 7%) – in glass

and ceramic tile

manufacture. In the

majority of

applications the lowest

possible content of

colouring oxides, in

particular of Fe2O3

and TiO2, is required.

Feldsp

ars

For example, in the

production of ceramic

wares of high

whiteness (porcelain),

the content of Fe2O3

and TiO2 cannot

exceed 0.15% and

0.05% respectively,

while in the case of

high quality glass –

max. 0.08% Fe2O3 is

acceptable. The term

‘feldspar raw material’

or ‘feldspar’ refers to

both K and Na

feldspar-rich grades as

well as to feldspar-

quartz ones.

Feldspars


QUESTIONS

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