keramikos / ceramics

8/10/2019 keramikos / ceramics

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Ceramics Basic Science

Introduction

The word ceramics in derived f rom the Greek Keramikos

which means earthen. A ceramic is therefore an ear thy mater ial ,

usually of a silicate nature and may be defined as a combination of one

or more metals with a nonmetall ic elements usually o!y"en #Gilman,

$%&'(.

History

)eramics are the ear li es t "rou* of inor"anic mater ia ls to be

structurally modified by man and his early history in *rinci*ally traced

throu"h these materials.

Gla+ed *or ce la in t he o ri "i n o f " la+i n" i n an i nt er es ti n"

advance. Gla+ed *orcelain is the only restorat ive material from

which bacterial *la-ue can be easily removed.

The ear l ies t "la+in" techni-ue was a umerian invent ion made

famous about /0001) as "y*tian 1lue 3aience.

uro*ean develo*ment of *orcelain came in $8 t h century.

$'88 4) )hemant *ublished a book on artificial teeth.


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The f i rs t ceramic ar t i f icial teeth where launched by an 5 tal ian

dentist in $808 Guie**an"elo 3an+i.

These where brittle

The dental *rofession did not real ly master the art of ceramics

until the end of $% t h century.

4r. )harles 6. 7and f i led the f irs t *atient in $88% for *orcelain

arket corwn.

The old trick of baskin" *orcelain .) was re*orted in $%08.

5n $%29 with 4r. Albert 7e Gro:s book on ce ramics in dentistry

*orcelain became firmly established.

$%&2 t he use o f "ol d a ll oys f or *or ce la in b in"i n" became

universal.

5n $%&; or advances were made in firin" dental ceramics by the

use of vacuum firin" techni-ues.

STRUCTURE

The lar"er o!y"en atoms serve as a matri! with smaller metal

atoms tucked in to the s*aces between the o!y"en atoms.


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Atomic bonds in ceramic crystal have both a covalent and ionic

charac ter. The s tron"er bonds are r es*onsible for a "rea ter

s tabil i ty of ceramics and im*art very useful *ro*ert ies such as

hardness hi"h molecules of elas t ici ty, resis tance to heat and

chemical attack.

At the same t ime the nature of the bondin" creates di f f icul t ies

for the dental ceramist since all ceramic materials are brittle.

THE NATURE OF GLASSES

4ental *orcelains are "lassy materials and in order to understand

their formulation knowled"e of "lass formation in essential.

Glasses may be re"arded as su*er cooled l i-uids or as non

crystalline solids.

This d is tin"uishes them f rom other sol ids and the ir a tomic

structu re a nd resultant * ro* erties d e*e nd not o nly o n

com*osition but also on thermal history.

5m*ure so lu tions are selec ted in so lid and molten s ta te . To

modify *hysical *ro*erties l ike viscosity, melt in" tem*erature,

chemi ca l dur ab il it y, t he rmal e!*ansi on and r es is tance t o

devitrification.

Glass formation


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= 2 ion is the *rinci*al anion *resent in al l "lasses which form

very s table bonds wi th smal l mult ivalent cation l ike i lica, 1oron,

Germanium, ?hos*horus "ivin" rise to structural units such as the i= /

tetrahedral which form a random network in "lass. Thus these ions are

termed "lass formers.

$%;2, #@achoriasen( andam network theory of "lass structures

a. 5nteratomic forces in "lasses and crystals are similar .

b. Atoms in "lass oscillate about definite e-uilibrium *osition.

c . 5n " las s # l ike crys ta ls ( a toms mus t be l inked in the form of ;

dimensional network but because they do not "ive a shar* Bray

differentiation s*ectra they could not be *eriodic.

d . The unit s of s t ructure in " lasses #i=/ tetrahedra( and crystals

are *ractical ly identical but in crystals these structural units are

built u* to "ive a re"ular lattice and in "lasses there is sufficient

dis tor t ion of bond an"les to *ermit the s t ructural uni ts to be

arran"ed in a random network.

3or dental *ur*ose the two "lass formin" o!ides used are i, 1.

They form the *rimary network around which dental "lass can be built.

Alumina under cer ta in c ir cums tances may be re"arded as a " lass

formin" o!ide.

ental !orcelain


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4enta l *or ce la ins use t he bas ic i = net work a s t he " la ss

f or mi n" ma tr i! b ut a dd it io na l * ro *e rti es s uc h a s l ow fu sin "

tem*eratures, hi"h viscosity, and resistance to diversification are built

in by the addition of other o!ides to the "lass formin" lattice and i= / .

These o!ides "enerally consist of K, Ca, )a, Al, 1oric o!ides.

Flu"es

K, Ca, )a o!ides are used as "lass modifies and act as f lu!es by

interru*tin" the inte"rity of i= / network.

The *ur*ose of a f lu!es is to lower the softenin" tem*erature of

a "lass by reducin" the amount of crossl inkin" between = 2 and

"lass formin" elements #e.".( silicon

#.".( oda #Ca 2=( is introduced into a s i l icate melt to *roduce

odium silicate "lasses. 6ere structural chan"es occur.

5ns tead of br id"in" o!y"en ion which formed the l ink between

the 2i= / , there are now two nonbrid"in" o!y"en one of which

has been contributed by the Ca 2=.

6ence a "a* is *roduced in the i= / network and the CaD ions are

accommodated in the interstices or holes in the random network.

5ncreased CaD

ion added i=i bonds are broken.

=E i ratio is very im*ortant

5t will affect the viscosity and thermal e!*ansion of "lass.


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#e.".( 5n dental *orcelain used for bondin" to metal, i t is a usual

*ractice to increase the soda content to raise the thermal

e!*ansion of *orcelain near to that of Au al loys . 7i , K o!ides

also have a function


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e" ula r o r a lu min ou s * or ce la in a re mo re su sc e* ti bl e to

devitrification due to their hi"her silicaE alkali ratio.

Glass modifiers

Addition will

4ecrease softenin" *oint

4ecrease viscosity

Addi tion result s in the ;d imens ional s il ica l ne twork which

contains may l inear chains of i= / which are able to move eas i ly at

lower tem*erature than are the atoms locked into the ;dimensional

structure of i= / .

5ncreased ease of movement

Greater fluidity

4ecreased softenin" tem*erature


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? orce la in with less firin " te m* era tu re decreased chemical

*ro*erties.

%ater

Cot added intentionally

5m*ortant "lass modifier

6edonism ions can re*lace the


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wher e a t r a t i o s o f $2H 1 2= ; the *hysical *ro*er ties of "lass

chan"es -uite markedly.

5nit ial addit ion of 1 2=; 1=/ te trahedra formed "ivin" low

e!*ansion and "ood chemical resistance.

Above $2H no 1= / tetrahedra formed but less s table form 1= ;

takes over. 1oric o!ide is therefore used in -uanti ties below the

initial $2H value.

A twin lat t ice is formed with i= / in which 1= / tetrahedra still

acts as a flu! by interru*tin" i= / network but at the same time a

"ross weakenin" of "lass i s not *roduced by format ion of less

stable 1= ; trian"les.

Intermediate o"ides

Addition of "lass modifiers or f lu!es to the basic "lass formin"

network i=/ in dental *orcelain wi ll decrease the sof tenin"

tem*erature and v iscosity.

4enta l *or ce la ins r e-ui re a h i"h o f r es is tance t o s lum* o r

*yr o*last ic flow hence we need to *roduce "lasses with hi"h

viscosity and low fir in" tem*erature. This can be done by usin"

intermedia te o!ides which a lthou"h not usually ca*able of

formin" a "lass can take *art in the "lass network.

The hardness and viscosity of "lass can be increased by the use

of intermediate o!ide such as Al2=; .


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5 t canno t be consi de red a s a t rue " la ss f or mer because t he

dimensions of the ion e!cludes the *oss ibi l i ty of Al 2=; bein"

formed and =E Al ratios *recludes formation of Al= / tetrahedra.

5n crys ta l s the Al ion can be / or & coordinated wi th o!y"en

"ivin" rise to Al=/ and Al= & .

The tetrahedral "rou* can re*lace i= / in silicate lattices.

ach Al has D;

ach i ion has char"e of D/ hence an addi t ional uni t *os i t ive

char"e must be *resent to o btain electro neutrality.

hen metallic o!ides such as Ca 2= are *resent, one alkali metal

ion *er Al= / tetrahedra would sat isfy this re-uirement and the

a lka li metal ions could be accommodated in the inter st ices

between tetrahedral "rou*s. 5n this way each Al D ; ion re*laces a

i/ D ion network and Ca D ion is taken in to * reserve neutrality.

Thi s i s found in many a luminosi li ca tes l ike 3elds*ar and

+eolites.

C$emical com!osition of dental !orcelain

The stabil i ty of "lass is hi"hly de*endent on the i= lat t ice and

the covalent bonds must not be reduced too much, other wise

*roblem of hyd rolytic stability and devitrification may arise.

Avera"e dental *o rcelain &0H i= 2

1alancin" o!ides, f lu!es are carefully controlled to *rovide the

re-uired *ro*er ti es such as r es is tance to *yro*las ti c f low,


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hardness, hydrolyt ic s tabil ity, low mel tin" t em*era ture and

resistance to devitrification.

Frittin&

The term frit is used to describe the final "lass *roduct.

aw mat er ia l *owder # =2 , )= ;( a re m i! ed in a r ef ra ct or y

crucible and heated to a tem*erature wi l l above thei r ul t imate

maturin" tem*erature.

=!ides melt to form a molten "lass.

Gasses esca*e, the melt is then -uenched in water.

The red hot "lass s tr ikes the cold water immediately breaks u*

into fra"ments and is termed on frit.

?rocess of blendin" melt in", -uenchin" the "lass com*onents is

called frittin".

FELS'ATHIC 'ORCELAIN

3elds*ar is the *recursor of common clay

Catural felds*ars are mi!tures of albite


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?otash felds*ar has hi"h viscos i ty and this wi l l decrease only

relatively slowly with increased tem*erature.

hen felds*ar i s mel ted at about $290 0) to $;00 0) the alkal i

uni te w it h a lumi na and s il ica t o f or m Ca o r K a lumi ni um

silicates.

A "lassy *hase i s formed wi th a f ree crys tal l ine s i l ica *hase

3elds*ar has been used to make dental *orce la in for decades

#hi"h fusin"(

Catural felds*ar 89H, Iuart+ $9HJ Then demand for low

fusion tem*erature came into *icture

6ence t he f el ds *a r was modif ied by t he add it ion o f " la ss

modif iers f lu!es intermediate o!ides . )hemical analys is of natural

felds*ar shows that the elements necessary for "lass makin" are *resent

in this mineral.

II( Felds!ar in dental !orcelain

)ontain *otash D alumina D silica

sed in *re*aration of nearly al l dental *orcelain desi"ned for

me ta l c era mic c ro wn a nd m an y o th er d en ta l " la ss es a nd

ceramics.


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hen felds*ar i s mi!ed wi th var ious metal o!ides and f i red to

hi"h tem*erature i t can form a "lass *hase that is able to soften

and flow sli"htly at *orcelain firin" tem*erature.

The softenin" of "lass *hase durin" f ir in" al lows the *orcelain

*owder *articles to coalesce to"ether by sinterin" 5t is a term that

refers to the fus in" of *ar t icle to"ether at hi"h tem*erature wi thout

com*lete meltin".

Role in leucite formation

3elds*ar has a tendency to form the crystal l ine mineral leucite

when it is melted.

7euci te i s *otassium aluminium s i licate mineral wi th a lar"e

coefficient of thermal e!*ansion #20 to 29 ! $0 &L 0)(

3elds*ar when heated to tem*eratures of $$90 to $9;0 under"oes

incon"ruent meltin" to "ive crystals of leucite in li-uid *hase

5ncon"ruent melt in" is a *rocess by which one material melts to

form a li-uid *lus a different crystalline material

Colorin& and o!acifyin& com!onents

1asic 4ental *orcelain material wi ll vary in color de*endin"

u*on whether it is a sin"le *hase or multi*hase "lass #mi!ture(

5f *orcelain sin"le *hase "lass then all o!ides constituents are

com*letely taken into solution. The resultant *roduct should be

as trans*arent as "ood window "lass.


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5n case of fused felds*athic *orcelain some manufacturer may

limit the de"ree of fusion and *yrochemical reaction in the frit to

the e!tent that a *ro*ort ion of felds*ar remains undissolved in

"lass . These "rains remain discretely in the "lass matr i! and

because of difference in 5 the final "lass frit may a**ear

o*alescent or assume a "ray b lue t rans lucency s imilar to

internal enamel.

Colorin& !i&ments

)ol or ed " la ss es a re * roduced by f ri tt in" h i"h t em*era tu re

resistant *i"ments "enerally metall ic o!ides into the basic "lass

used in *orcelain manufacturer.

Glass i s t hen a h i"hl y col or s at ur at ed and " round t o a f ine

*owder.

Colour !i&ments used are)

?ink)hromium tin or )hromiumalumin

This el iminates the "reenish hue of "lass and adds a warm tone

to the *orcelain.

Mellow5ndium or *raesodynium ,this "ives a ivory shade

1lue)obalt salts ,*roduces enamel shades

Green )hromic o!ide , not "enerally *referred

Grey iron o!ide or * latinum "rey, "ive an effect of translucency.


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O'ACIF*ING AGENTS

These are added to *roduce a more life like tooth affect

An o*acifyin" a"ent "enerally consists of a metal o!ide "round

to a very f ine *ar ticle s i+e less than 9m to *revent a s*eckled

a**earance in the *orcelain

) om mo n o !id es u se d a re c er iu m o !id e, ti ta ni um o !id e,

+irconium o!ide.

STAINS AN COLOUR + OIFIERS

They are su**lied in a kit.

A stain is more concentrated than a colou r modifier

tain used as surface colourent and to *rovide enamel check

lines, decalcification s*ots.

)olour modifier used for "in"ival effects.

Fluorescence

The usual *rocedure for *roducin" fluorescence is by addition of

the uranium salt sodium diurinate. This sal t *roduces a s tron"

"reenish yellow effect and when small amount of cerium is added

a bluish white fluorescence can be achieved.

The use of uranium has been banned.

Cew nonradioactive *orcelains are available containin" rare

earth o!ides.


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Ad#anta&es

$ . Good aes thet ic s

2. 5ncreased com*ress ive s t ren"th

;. Good chemical durabi li ty

/ . 1 iocom*at ib le

eficiencies

$. 4ecreased tens ile s t ren"th

2. 1ri tt leness

Ne!$eline synite

This material has been tried as a re*lacement for felds*ar #$%//(

Ce*hiline is the essential mineral of this syenite.

This never became *o*ular for makin" dental*ocelain because of

its hi"h *yro*lasticity


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$stedE99$$2.

; . 6ondr um =E A r evi ew o f t he s tr en"t h * ro*e rt ie s o f den ta l

ceramics. ?rosthet 4entJ $%%2E&'E89%8&9.

/ . < c7 ea n , = do nt 4 E vo lu ti on o f d en ta l c era mic s in t he

twentieth century. ?rosthet 4entJ 200$E89E&$&&.

keramikos / ceramics

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