SLIDE 3 CERAMIC PROPERTIES-new.pdf

8/10/2019 SLIDE 3 CERAMIC PROPERTIES-new.pdf

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IMPERFECTIONS & APPLICATION

OF CERAMIC


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Point defect Point defects are localized disruptions in an other ise perfect ato!ic or

ionic arran"e!ents in a cr#stal structure$ !a# %e introduced %# !o e!ent of the ato!s or ions hen the# "ain

ener"# %# heatin"' durin" processin" of the !aterial or %# introductionof other ato!s$

The distinction %et een an i!purit# and a dopant (

I!purities ) ele!ent*co!pounds that are present fro! ra !aterials orprocessin" +e$"$ Si sin"le cr#stals "ro n in ,uartz cruci%les containo-#"en as an i!purit#.

/opants ) ele!ents*co!pounds that are deli%eratel# added' in 0no nconcentrations' at speci1c locations in the !icrostructure' ith anintended %ene1cial effect on properties or processin" +e$"$ P & 2 areadded to Si cr#stals to i!pro e or alter the electrical properties of puresilicon

the effect of i!purities is deleterious' hereas the e3ect of dopants onthe properties of !aterials is useful$


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4 t#pe of point defects( 5. i!perfection) defecti e structure( ato! !a# re!o e' add'

or displacedeter!ined %# T' P & co!position 4. i!purit#' can occur in t o a#s(

i. an interstitial i!purit# ) ato! occupies aninterstitial site

ii. an i!purit# ato! replaces an ato! in theperfect lattice


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I!perfections in cera!ics cr#stalline defect ) a lattice irre"ularit# ha in" one or !ore

of its di!ensions on the order of an ato!ic dia!eter The si!plest of the point defects is a vacancy, or vacant

lattice site A acanc# is produced hen an ato! or an ion is !issin"

fro! its nor!al site in the cr#stal structure


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Interstitial defect An interstitial defect is for!ed hen an e-tra ato! or ion is

inserted into the cr#stal structure at a nor!all#unoccupied position

Interstitial ato!s or ions' althou"h !uch s!aller than the

ato!s or ions located at the lattice points' are still lar"erthan the interstitial sites that the# occup#$ Conse,uentl#' the surroundin" cr#stal re"ion is co!pressed

and distorted$ Interstitial ato!s such as h#dro"en are often present as

i!purities6 hereas car%on ato!s are intentionall# addedto iron to produce steel$

7nli0e acancies' once introduced' the nu!%er of interstitialato!s or ions in the structure re!ains nearl# constant'e en hen the te!perature is chan"ed$


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Su%stitutional /efects introduced hen one ato! or ion is replaced %# a different t#pe of

ato! or ion occup# the nor!al lattice sites$ !a# either %e lar"er than the nor!al ato!s or ions' in hich case the

surroundin" interato!ic spacin"s are reduced' or s!aller causin" thesurroundin" ato!s to ha e lar"er interato!ic spacin"s$

alter the interato!ic distances in the surroundin" cr#stal$ introduced either as an i!purit# or as a deli%erate addition once introduced' the defects is relati el# te!p$ independent E-a!ples dopants such a +P. or %oron +2. into Si$ 8hether ato!s or ions "o into interstitial or su%stitutional sites

depends upon the size and alence of "uest ato!s or ions co!pared tothe size and alence of host ions$

The size of the a aila%le sites also pla#s a role in this$


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Ato!ic defects in ol in" host ato!sfor each ion ) acancies & interstitial are possi%lesince cera!ic !aterials contain ions of at least t o 0inds +9 & :.'defects for each ion t#pe !a# occurNaCl ( Na & Cl ) each interstitials & acancieshi"hl# i!pro%a%le that there ould %e apprecia%le concentrationsof anion +Cl. interstitials$

The anion is relati el# lar"e' and to 1t into a s!all interstitialposition' su%stantial strains on the surroundin" ions !ust %eintroduced$


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2ecause the ato!s e-ist as char"ed ions' hen defectstructures are considered' conditions of electroneutralit#!ust %e !aintained$

Electroneutrality ) the state hen there are e,ualnu!%ers of positi e and ne"ati e char"es fro! the ions$

) conse,uentl#' defects in cera!ics do not occur alone in A; !aterials' defect is a cation acanc#


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Defect of a cationvacancy and a cationinterstitial pair ) aFrenkel defect

formed by a cation leaving its normal position vacant andmoving into an interstitial site.

There is no change in charge because the cation maintainsthe same positive charge as an interstitial.


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The ratio of cations to anions is not altered %# thefor!ation of either a Fren0el or a Schott0# defectIf no other defects are present' the !aterial is said to %estoichio!etric$Stoichiometry ) as a state for ionic co!pounds

herein there is the e-act ratio of cations to anionspredicted %# the che!ical for!ula$For e-a!ple' NaCl is stoichio!etric if the ratio of Na9ions to Cl= ions is e-actl# 5(5$A cera!ic co!pound is nonstoichiometric if there is an#de iation fro! this e-act ratio


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Nonstoichio!etr# !a# occur for so!e cera!ic !aterials in hich t oalence +or ionic. states e-ist for one of the ion t#pes$Iron o-ide +FeO. can %e present in %oth Fe49 and Fe>9 states6 dependson te!perature and the a!%ient o-#"en pressure$

The for!ation of an Fe>9 ion disrupts the electroneutralit# of the cr#stal%# introducin" an e-cess 95 char"e' hich !ust %e offset %# so!e t#peof defect$

This !a# %e acco!plished %# the for!ation of one Fe49 acanc# +orthe re!o al of t o positi e char"es. for e er# t o Fe>9 ions that arefor!ed

The cr#stal is no lon"er stoichio!etric %ecause there is one !ore O ionthan Fe ion6 ho e er' the cr#stal re!ains electricall# neutral$


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I!purities in cera!ics T#pe( solid solutions of %oth su%stitutional and

interstitial For an interstitial' the ionic radius of the i!purit#

!ust %e relati el# s!all in co!parison to theanion$

A su%stitutional i!purit# ill su%stitute for thehost ion +c*a. to hich it is !ost si!ilar in anelectrical sense(

if the i!purit# ato! nor!all# for!s a cation in acera!ic !aterial' it !ost pro%a%l# ill su%stitutefor a host cation$

For e-a!ple' in NaCl' i!purit# Ca49 and O4=ions ould !ost li0el# su%stitute for Na9 and Cl=ions' respecti el#$


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To achie e an# apprecia%le solid solu%ilit# ofsu%stitutin" i!purit# ato!s' the ionic size and char"e!ust %e er# nearl# the sa!e as those of one of thehost ionsFor an i!purit# ion ha in" a char"e different fro! thehost ion for hich it su%stitutes' the cr#stal !ustco!pensate for this difference in char"e so thatelectroneutralit# is !aintained ith the solid$One a# this is acco!plished is %# the for!ation

acancies or interstitials of %oth ion t#pes


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APPLICATIONS


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?lasses The "lasses are a fa!iliar "roup

of cera!ics6 e$"$ containers'lenses' indo and 1%er"lass

Non cr#stalline silicates ith

other o-ide +e$"$ CaO' Na4O'@4O & Al4O>. in uence the"lass properties$

A t#pical soda


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?lass Cera!ics ?lass can %e transfor!ed to cr#stalline %# hi"h T heat

treat!ent ) cr#stallisation Product( "lass:cera!ics +fine:"rained pol#cr#stalline. Process in ol es nucleation & "ro th sta"e

A nucleation a"ent +fre,uentl# TiO4. is addded to pro!otecr#stallization6

Co!!onl# used as o en are' ta%le are' o en indo etc


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Cla# Products er# popular products +a%undant' ine-pensi e' eas# to %e

for!ed. Contain nonplastic in"redient hich affect the chan"e that

ta0e place durin" the dr#in" and 1rin" processes ) thecharacteristics of the 1nished piece

t o %road classi1cations( the structural cla# products andthe hite ares$

Structural cla# products include %ric0s' tiles' and se erpipes applications in hich structural inte"rit# isi!portant$

hite are cera!ics %eco!e hite after thehi"h:te!perature 1rin" )"roup are porcelain' potter#'ta%le are' china' and plu!%in" 1-tures' sanitar# are


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Refractories Properties( the capacit# to ithstand hi"h te!peraturesithout !eltin" or deco!posin"' endure at hi"h T' capacit#

to re!ain inert in se ere en iro!ent' pro ide ther!alinsulation

Co!!on product ) %ric0s Application( !etal refinin"' "lass !anufacturin"'

!etallur"ical heat treat!ent' po er "eneration the perfor!ance of a refractor# cera!ic' to a lar"e de"ree'

depends on co!position$ there are se eral classi1cations(


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For !an# co!!ercial !aterials' the ra in"redients consistof %oth lar"e and 1ne particles' hich !a# ha e differentco!positions$

7pon 1rin"' the 1ne particles nor!all# are in ol ed in thefor!ation of a %ondin" phase' hich is responsi%le for theincreased stren"th of the %ric0

The ser ice te!perature is nor!all# %elo that at hichthe refractor# piece as 1red$

the opti!u! porosit# depends on the conditions of ser ice$


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Porosit# is one !icrostructural aria%le that !ust %econtrolled to produce a suita%le refractor# %ric0$ Stren"th'load:%earin" capacit#' and resistance to attac0 %#

corrosi e !aterials all increase ith porosit# reduction$

At the sa!e ti!e' ther!al insulation characteristics andresistance to ther!al shoc0 are di!inished$


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A%rasi es used to ear' "rind' or cut

a a# other !aterial' hichnecessaril# is softer$

Properties( hardness* ear

resistance' tou"h /ia!ond6 silicon car%ide'tun"sten car%ide'alu!iniu! o-ide' silicasand' dia!onds

used in se eral for!s( 5.%onded to "rindin" heels'4. as coated a%rasi es'and >. as loose "rains$


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In the 1rst case' the a%rasi e particles are %onded to aheel %# !eans of a "lass# cera!ic or an or"anic resin

Coated a%rasi es are those in hich an a%rasi e po der iscoated on so!e t#pe of paper or cloth !aterial6 sandpaper

is pro%a%l# the !ost fa!iliar e-a!ple$ 8ood' !etals'cera!ics' and plastics are all fre,uentl# "round andpolished usin" this for! of a%rasi e$

?rindin"' lappin"' and polishin" heels often e!plo# loosea%rasi e "rains that are deli ered in so!e t#pe of oil: or

ater:%ased ehicle$


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Ce!ents ce!ent' plaster cement of paris' and

li!e Characteristic( for! paste hen

!i-ed ith ater su%se,uentl# set &hardens

act as a %ondin" phase thatche!icall# %inds particulatea""re"ates into a sin"le cohesi estructure at Tr

the role of the ce!ent is si!ilar tothat of the "lass# %ondin" phase thatfor!s hen cla# products and so!erefractor# %ric0s are 1red

The properties of portland ce!ent'includin" settin" ti!e and 1nalstren"th' to a lar"e de"ree depend onits co!position$

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