Slide 1 Ceramic Structures-new

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STRUCTURES OF CERAMICS REFF:  Materials Science & Engineering; An Introduction Callister, W. , !r, "##$, !o%n Wile & Sons  Funda'ental o( Cera'ics, )arsou', M. W., "##*, Mc+ra-ill Engineering Materials "; An Introduction to Microstructures, /rocessing and esign, As%0, M. F and !ones, . R. , 1234, /erga'on /ress Introduction CERAMICS: +ree5  5era'i5 era'i5os 6 0urn 0urn stu((  stu((  %e at 8&9/ a77licatio a 77lications ns (olloed 0 solid co'7ounds (or'ed 0 %eat cooling 7ro7erties are ac%ieed t%roug% %ig%-T 7rocess 8(iring desira0le 7ro7erties • %as lost its 7lasticit & no longer ca7a0le to re%drat re%drate e at least " ele'ents; 1 is a non-'etal, t%e ot%er 'a 0e 8a 'etal8s or 8anot%er non alence • • • • • • • T%e alence o( an ato' is t%e nu'0er o( electrons in an ato' t%at 7artici7ate in 0onding or c%e'ical reactions Usuall, t%e alence is t%e nu'0er o( electrons in t%e outer s and 7 energ leels. T%e nu'0er o( coalent 0onds is deter'ined 0 t%e nu'0er o( alence electrons For N alence electrons, an ato' can 0ond it% ot%er ato's 3 < N For e=a'7le, N6 $ (or c%lorine, and 3 - N6 1, %ic% 'eans t%at one Cl ato' can 0ond to onl one ot%er ato'. T%e alence o( an ato' is related to t%e a0ilit o( t%e ato' to enter into c%e'ical co'0ination it% ot%er ele'ents E=a'7les o( t%e alence are: Mg: 1s" "s" "74 *s" alence 6 " electronegatiit • • • • • • • • • • It is 7ossi0le to %ae interato'ic 0onds t%at are 7artiall ionic and 7artiall coalent t%e degree o( eit%er 0ond t7e de7ends on t%e relatie 7ositions o( t%e constituent ato's in t%e 7eriodic ta0le or t%e di((erence in t%eir electronegatiities. Electronegatiit 8e greed t%e tendenc o( an ato' to gain an electron; t%e 7oer o( ato' to attract electrons to itsel(  Ato's it% al'ost co'7letel >lled outer energ leels?suc% as c%lorine?are strongl electronegatie and readil acce7t electrons. oeer, ato' it% nearl e'7t outer leels?suc% as sodiu'?readil gie u7 e ectrons and ae o e ectronegatiit. T%e ider t%e se7aration 80ot% %ori@ontall and erticall (ro' t%e loer le(t to t%e u77er-rig%t-%and corner 8i.e.,t%e greater t%e di((erence in electronegatiit  in general t%e 'ore ionic t%e 0ond. Conersel, t%e closer t%e ato's are toget%er 8i.e., t%e s'aller t%e di((erence in electronegatiit, t%e greater t%e degree o( coalenc. I( t%e electronegatiit di((erence 0eteen t%e' 8∆=is large 8indicating 1 ele'ent is greedier t%an ot%er, e attracted to t%e 'ore electronegatie ele'ent ion attract eac% ot%er In general ∆=  1.$ ionic ∆= B 1.$ coalent IONIC BONDING • • • • W%en one ato' 'a donate its alence electrons to a di((erent ato', >lling t%e outer energ s%ell o( t%e second ato' )ot% ato's no %ae >lled9e'7tied outer energ leels, 0ut 0ot% %ae acuired an electrical c%arge and 0e%ae as ions. T%e ato' t%at contri0utes t%e electrons is le(t it% a net 7ositie c%arge and is called a cation, %ile t%e ato' t%at acce7ts t%e electrons acuires a net . T%e o77ositel c%arged ions are t%en attracted to one anot%er and 7roduce t%e ionic 0ond. • • • • • • Occured 0 trans(er electron; (or' 0eteen er actie 'etallic & non 'etallic ele'ents Ato's o( a 'etallic ele'ent easil gie u7 t%eir alence electrons to t%e non'etallic ato's to (or' AD ionic 0onding, A loses e easil, D acce7ts e it%out too 'uc% ener in ut It (ollos t%at (or ionic 'aterials to 0e sta0le, all 7ositie ions 'ust %ae as nearest neig%0ors negatiel c%arged ions in a t%ree di'ensional sc%e'e, and ice ersa T%e 7redo'inant 0onding in cera'ic 'aterials is ionic. COVALENT BONDING • • • • • • • t%e s%aring o( coalent 0onding electrons 0eteen adacent ato's. To ato's ill eac% contri0ute at least one electron to t%e 0ond, and t%e s%ared electrons 'a 0e considered to 0elong to 0ot% ato's. (or's %en ato's %ae t%e sa'e electronegatiit  energies o( 0onding electrons o( A & D are co'7ara0le I( t%e electron energ o( t%e ato's is di((erent  trans(er energ 8ionic 0onding Eac% instance o( s%aring re7resents one coalent 0ond e.g: silicon ato', %as a alence o( (our, o0tains eig%t electrons in its outer energ s%ell 0 s%aring its electrons it% (our surrounding silicon ato's  eac% silicon ato' is 0onded to  neig%0oring ato's 0  coalent 0onds Man non'etallic ele'ental 'olecules " Cl" F" etc as ell as 'olecules containing dissi'ilar ato's, suc% as C,"O, GO*, and F, are coalentl C%aracteristics o( ions %ic% a((ect crstal structure: 1. 'agnitude o( electrical c%arged o( eac% ions Crstal electricall neutral 8H c%arges 'ust 0e 0alanced 0 an eual nu'0er o( 8< c%e'ical (or'ula indicates ratio o( H to < E= CaF"  calciu' ions 8H" & (luoride 8- • • • • • ". relatie si@e o( H and < ion Inole si@e9ionic radii 8rc & ra Metalic ele'ents gie u7 electrons %en ioni@ed  cations are s'aller t%an an ons rc ra B Eac% cation 7re(ers as 'an neig%0our anions, anions also desire a 'a=i'u' nu'0er o( cation. • • • • Sta0le structures reuire t%at cations and anions are in touc%J Coordination nu'0er • t%e nu'0er o( ato's touc%ing a 7articular ato', or t%e nu'0er o( nearest neig%0ors (or t%at 7articular ato'. • nu'0er o( anions neig%0ors (or a cation related to rc9ra • T%is is one indication o( %o tig%tl and e((isientl ato's are 7ac5ed toget%er. • • For ionic solids, t%e coordination nu'0er o( cations is de>ned as t%e nu'0er o( nearest anions. T%e coordination nu'0er o( anions is t%e nu'0er o( nearest cations. • Ta0le: Coordination nu'0ers and geo'etries (or arious rc9ra • red  cation • whte   anion • • Co''on coor ination nu'0ers (or cera'ic: , 4 and 3 rc9ra1  coordinate no. 1" T%e si@e o( an ion de7end seeral (actors, e.g: 1. coordination nu'0er Ionic radius increase as t%e nu'0er o( o77osite c%arge neig%0or ions increases ionic radii (or 8coord no. B4B3 • • • • • ". c%arge on an ion Re'oing e (ro' ato'9ion, t%e re'aining alence electrons 0eco'e 'ore Ionic si@e increases %en electrons are added to an ato' or ion Radii (or Fe: Fe"H: Fe*H 6 #.1": #.#$$: #.#42 Ato' arrange'ent • 1 unit cell: t%e s'allest grou7 o( ato's (or' a re7etitie 7attern in descri0ing crstal structure  re7resent crstal stucture T7es o( ato'ic or ionic arrange'ents: 1.Go Order T%ese 'aterials rando'l >llu7 %ateer s7ace is aaila0le to t%e'. In 'onoato'ic gases, suc% as argon 8Ar ato's or ions %ae no orderl arrange'ent. ". S%ort-Range Order 8SRO A 'aterial dis7las s%ort-range order 8SRO i( t%e s7ecial arrange'ent o( t%e K  A'or7%ous9glass9non crstalline 'aterial; e.g. glass *. Long-Range Order 8LRO t%e s7ecial ato'ic arrange'ent e=tends re7eat 7eriodicit 0ond lengt% oer 'uc% larger 1## n' u7 to (e c' T%e ato's or ions in t%ese 'aterials (or' a regular re7etitie, gridli5e 7attern, in t%ree di'ension  crstalline 'aterials; e.g. cera'ics • SRO-non crstalline solid • lack a systematic and regular arrangement • arrange'ent o( ato's oer relatiel large ato'ic distances. • • • also called amorphous or supercooled liuids! inas'uc% as t%eir ato'ic structure rese'0les t%at o( a liuid. W%et%er a crstalline or a'or7%ous solid (or's de7ends on t%e ease it% %ic% a rando' ato'ic structure in t%e liuid can trans(or' to an ordered state during An a'or7%ous condition 'a 0e illustrated 0 co'7arison o( t%e crstalline and noncrstalline structures o( t%e cera'ic co'7ound silicon dio=ide 8SiO", %ic% 'a e=ist in 0ot% states. Crstal structure • • • • • 0ased on t%e unit cell geo'etr onl. Wit%in t%is (ra'eor5, an x, y, z coordinate system is established with its origin at one o( t%e unit cell corners; eac% o( t%e x, y, and z axes coincides with one o( t%e t%ree 7arallele7i7ed edges t%at e=tend (ro' t%is corner. T%e unit cell geo'etr is co'7letel de>ned in ter's o( si= 7ara'eters: t%e t%ree edge lengt%s a, b, and c, and the three interaxial angles a, ß, and γ . These se"en crystal sste's are cu0ic, tetragonal, %e=agonal, ort%or%o'0ic, r%o'0o%edral 8also called trigonal, 'onoclinic, and triclinic T%e cu0ic sste', (or %ic% a = b = c and a = ß = γ = 90 , %as t%e greatest degree o( s''etr. Least s''etr is dis7laed 0 t%e triclinic sste', since a ≠ b ≠ c and a ≠ß≠γ. Single crstal • • %en t%e 7eriodic and re7eated arrange'ent o( ato's is 7er(ect or e=tends t%roug%out t%e entiret o( t%e s7eci'en it%out interru7tion, t%e result is a single crstal. All unit cells interloc5 in t%e sa'e a and %ae t%e sa'e single crstal orientation. /olcrstalline 'aterial • • A 7olcrstalline 'aterial is co'7rised o( 'an crstals it% aring orientations in s7ace. T%ese crstals in a 7olcrstalline 'aterial are 5non as grains. T%e 0orders 0eteen tin crstals, %ere t%e crstals are in 'isalign'ent and are 5non as grain 0oundaries. • • #tages in the solidi$cation o% a polycrystalline: Initiall, s'all crstals or nuclei (or' at arious 7ositions. T%ese %ae rando' crstallogra7%ic orientations. T%e s'all grains gro 0 t%e successie addition (ro' t%e surrounding liuid o( ato's to t%e structure o( eac%. T%e e=tre'ities o( adacent grains i'7inge on one anot%er as t%e solidi>cation 7rocess a77roac%es co'7letion. T7e o( crstal structure • AD: structure o( GaCl, CsCl, NnS • A'D7 • A')nD7 AD-t7e crstal structures • eual nu'0er o( A 8cation & D 8anion • Re(erred as AD • * structures: roc5 salt, CsCl and NnS • Ionic & or coalent 0onding • Ionic  MgO; " e o( A trans(erred to D, result in Mg"H & O"- • Coalent NnS; s%aring ele5tron Roc5 salt 8GaCl structure • • • • • • T%e 'ost co''on AD crstal structure Electrostatic attraction 0eteen GaH & Cl- %old t%e crstal toget%er Ma=. electrostatic interaction H -, H neig%0ours 8ice ersa Coordination nu'0er (or 0ot% H & - is 4 8octa%edral 1 unit cell  generated (ro' FCC o( anion it% 1 cation in cu0ic center & 1 at centered o( eac% o( 1" cu0e edge GaCl, MgO, MnS, LiF and FeO Cesiu' cloride 8CsCl stucture • • • • Coordination nu'0er (or 0ot% ions is 3 8cu0ic T%e anions are at eac% o( t%e corners o( a cu0e ng e cat on s at t e cu0e center T%is structure is 7ossi0le %en t%e anion and t%e cation %ae t%e sa'e alence Ninc )lende 8NnS structure • • Coordinate nu'0er (or 0ot% ions is  8tetra%edral all corner and (ace 7ositions o( t%e cu0ic cell are occu7ied 0 S ato's • • • • tetra%edral 7ositions Eac% Nn ato' 0onded to  S ato's, ice ersa Most o(ten t%e ato'ic 0onding is %ig%l coalent NnS, NnTe, and SiC A'D7 < T7e crstal structures • • • • C%arges o( H & - are not t%e sa'e, '  7 E=a'7le: AD"  CaF" Ca ion at t%e centers o( cu0e F ion in t%e corner 1 unit cell consists o( 3 cu0es A')nD7 < T7e crstal structure • " t7es o( cation, A & ) • C%e'ical (or'ula A=)nD7 • E=. )aTiO* corners o( t%e cu0e, single TiH is at t%e centre, O"- ions is at t%e centre o( 4 (aces silica • • • • • • T%e 'ost si'7le: silicon dio=ide9silica /ure silica  no 'etal ions, eer o=gen 0eco'es a 0ridge 0eteen " silicon ato's Eer corner o=gen ato' is s%ared 0 adacent tetra%edra T%e 'aterialKs electricall neutral, all ato's %ae sta0le electronic structures Ratio Si to O 1:" 8indicated 0 c%e'ical (or'ula I( tetra%edras are arranged in a regular & order  crstalline SILICATE CERAMIC • • • • • Silicates are co'7osed 7ri'aril o( silicon and o=gen, a0undant ele'ents in eart%Ks crus%; soil, roc5, cla Eac% silicon ato' 0ond strongl to  o=gen ato' )asic unit in all silicates tetra%edron 8o=gen are situated at t%e corners, o=gen is at t%e centre T%e 'ain t7es o( silicate cera'ics: 1 alu'osilicates 85aolin- or cla-0ased cera'ic suc% as 7orcelain and 0ric5s; sste' P"O-Al"O*-SiO" and " 'agnesiu' silicates 8talc-0ased tec%nical cera'ics; sste' MgO-Al"O*-SiO". Silicate cera'ics are conentionall su0diided into coarse or (ine and, according to ater a0sor7tion, into dense 8B " Q (or (ine and B 4 Q (or coarse or 7orous cera'ics 8 "Q and  4 Q, res7ectiel. Silica +lasses • • • • • • A.5.a (used silica9itreous silica As a result, glasses are 'ec%anicall rigid li5e solids, et %ae t%e disordered arrange'ent o( 'olecules li5e liuids noncrstalline solid or glass, %ig% rando'ness )asic unit tetra%edron 8sa'e as t%e crstalline /ure silica (or's glass it% %ig% so(tening T 81"## C +reat strengt% and sta0ilit, lo t%er'al e=7ansion 0ut %ard to or5 it% 0ecause %ig% in iscosit