Types of Solids. Chapter 12. Solids and Modern Materials

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1 Chpter 12 Solids nd 1 Types of Solids Four generl types of solids. Metllic solids shre network of highly deloclized electrons. Ionic solids re sets of ctions nd nions mutully ttrcted to one nother. 2

2 Bonding in Solids Covlent-network solids re joined y n extensive network of covlent onds. Moleculr solids re discrete molecules tht re linked to one nother only y vn der Wls forces. 3 Other Solids Polymers: long chins of toms held together y covlent onds. The chins re often held to one nother y weker intermoleculr forces. Nnomterils: solids whose crystls hve dimensions on the order of 1-100nm. 4

3 Crystlline vs. Amorphous solids In crystlline solids toms re rrnged in very regulr pttern. Amorphous solids re chrcterized y distinct lck of order in the rrngement of toms. 5 Crystl Lttices One cn deduce the pttern in crystlline solid y thinking of the sustnce s lttice of repeting shpes formed y the toms in the crystl. 6

4 Crystl Lttices (2 dimensions) The individul shpes of the lttice, then, form "tiles," or unit cells, tht must fill the entire spce of the sustnce. lttice point lttice vectors 7 Crystl Lttices(3-dimensions) There re seven sic threedimensionl lttices: Cuic Tetrgonl Orthorhomic Rhomohedrl Hexgonl Monoclinic Triclinic 8

5 Three Types of Cuic Lttice Primitive lttice vs. Centered lttice Within ech mjor lttice type, dditionl types re generted y plcing lttice points in the center of the unit cell or on the fces of the unit cell. 9 Filling the Unit Cell Once one plces toms within unit cell, the structure of the compound cn e seen y onding the toms to one nother cross unit cells. 10

The metl nuclei re seen to exist in se of deloclized vlence electrons.

6 Metllic Solids Metllic solids or metls consist solely of metl toms. Metllic onding results from delocliztion of vlence electrons throughout the solid. The metl nuclei re seen to exist in se of deloclized vlence electrons. Importnt physicl properties of pure metls: mllele: cn e hmmered into thin sheets ductile: cn e pulled into wires good electricl nd therml conductivity 11 Metllic Structure The structures of mny metls conform to one of the cuic unit cells. 12

7 Cuic Structures One cn determine how mny toms re within ech unit cell which lttice points the toms occupy. 13 r s 2r = s 4r = s 2 s=(r 8)=(r2 2) 4r = s(3) 1/2 S= 4r 3 14

Volume of Unit Cells Primitive: 3 3 Volume = 2r = 8r Bcc: Volume = 4r 3 3 Fcc: Volume = r 8 3 15 Close Pcking Hexgonl close pcking (hcp): The

Cuic close pcking (ccp): The third lyer toms do not sit directly ove the spheres in either of the first two lyers.

8 Volume of Unit Cells Primitive: 3 3 Volume = 2r = 8r Bcc: Volume = 4r 3 3 Fcc: Volume = r Close Pcking Hexgonl close pcking (hcp): The third lyer toms re in the depressions tht lie directly over the first lyer. Cuic close pcking (ccp): The third lyer toms do not sit directly ove the spheres in either of the first two lyers. In oth hcp nd ccp ech sphere hs 12 equidistnt nerest neighors: 6 in the sme lyer, 3 from the lyer ove, nd 3 from the lyer elow. Ech sphere hs coordintion numer of 12. CN: the numer of toms immeditely surrounding given tom in the crystl structure. 16

9 Cuic close-pcking --c c.c.p vs h.c.p Hexgonl close-pcking -- c c c c c c c c c c c c c c c c c c c c ccp hcp 17 Close Pcking The toms in crystl pck s close together s they cn sed on the respective sizes of the toms. 18

etween the spheres. Pcking efficiency is the frction of spce in crystl tht is ctully occupied y toms.

Solution Volume of toms = Prctice Exercise Determine the pcking efficiency y clculting the frction of

10 Smple Exercise 12.1 Clculting Pcking Efficiency It is not possile to pck spheres together without leving some void spces etween the spheres. Pcking efficiency is the frction of spce in crystl tht is ctully occupied y toms. Determine the pcking efficiency of fcecentered cuic (fcc) metl. Solution Volume of toms = Prctice Exercise Determine the pcking efficiency y clculting the frction of spce occupied y toms in ody-centered cuic metl. Answer: 0.68 or 68% 19 Alloys comintions of two or more elements, the mjority of which re metls. Addition of second (or third) element chnge in the properties of the mixture 20

Alloys sustitutionl lloys: metl tom replced y second element interstitil lloys: second element(nonmetls) fills spce in the lttice of metl toms. 21 second element in Sustitutionl lloys.

(nonmetl) significntly smller rdius much hrder, stronger nd less ductile thn the pure metl (incresed onding etween nonmetl nd metl). An exmple is steel (contins up to 3% cron). mild steels (<0.

11 Alloys sustitutionl lloys: metl tom replced y second element interstitil lloys: second element(nonmetls) fills spce in the lttice of metl toms. 21 second element in Sustitutionl lloys. similr tomic rdii. similr onding chrcteristics. second element in Interstitil lloys. (nonmetl) significntly smller rdius much hrder, stronger nd less ductile thn the pure metl (incresed onding etween nonmetl nd metl). An exmple is steel (contins up to 3% cron). mild steels (<0.2% cron; useful for chins, nils, etc) medium steels ( % cron; useful for girders, rils, etc.) high-cron steels ( % cron; used in cutlery, tools, springs) Other elements my lso e dded to mke lloy steels. Addition of V nd Cr increses the strength of the steel nd improves its resistnce to stress nd corrosion. The most importnt iron lloy is stinless steel. It contins C, Cr (from ferrochrome, FeCr 2 ), nd Ni. 22

12 Heterogeneous lloys: The components re not dispersed uniformly (e.g., perlite steel hs two phses: lmost pure Fe nd cementite, Fe 3 C). Intermetllic compounds: homogeneous lloys with definite properties nd compositions. Exmples include: Ni 3 Al ( mjor component of jet ircrft engines). Cr 3 Pt (used to cot rzor ldes (to increse hrdness nd ility to mintin shrp edge), Co 5 Sm (used in permnent mgnets in lightweight hedsets). LN 5 (used s the node in nickel-metl hydride tteries). 23 Metllic Bonding In elementl smples of nonmetls nd metlloids, toms generlly ond to ech other covlently. Metls, however, hve lck of vlence electrons; insted, they form lrge groups of toms tht shre electrons mong them. 24

25 Prolems with the electronse model As the numer of electrons increses, the strength of onding should increse, nd the

13 Electron-Se Model Metl: s group of ctions suspended in se of electrons. The electricl nd therml conductivity, ductility, nd mlleility of metls is explined y this model. 25 Prolems with the electronse model As the numer of electrons increses, the strength of onding should increse, nd the melting point should increse. However, group 6B metls (t the center of the trnsition metls) hve the highest melting points in their respective periods. cf) p, strength, H fusion W Mo Cr 26

14 A Moleculr-Oritl Approch -nd structure- As the numer of toms in chin increses, the energy gp etween moleculr oritls (MOs) essentilly disppers, nd continuous nds of energy sttes result. Bnds 27 Bnd Structure of Nickel 28

15 In ionic solids, the lttice comprises lterntely chrged ions. Ionic solids hve very high melting nd oiling points re quintessentil crystls. Ionic Solids 29 30

16 Structures of Ionic Solids The different-sized ions in n ionic compound minimize the distnce etween oppositely chrged ions while keeping likechrged ions wy from ech other

17 33 Moleculr Solids The physicl properties of moleculr solids re governed y vn der Wls forces. soft. gses or liquids t RT Moleculr solids show poor therml nd electricl conductivity. sucrose. 34

Covlent-Network Solids Covlent-network solids consist of toms held together, in lrge networks or chins, with covlent onds. much higher melting points nd much hrder thn moleculr solids.

18 Covlent-Network Solids Covlent-network solids consist of toms held together, in lrge networks or chins, with covlent onds. much higher melting points nd much hrder thn moleculr solids. Due to the strong covlent onds tht connect the toms. Ex) dimond, grphite, qurtz (SiO 2 ), nd silicon cride (SiC). 35 Covlent-Network nd Moleculr Solids Dimonds re n exmple of covlent-network solid, in which toms re covlently onded to ech other. They tend to e hrd nd hve high melting points. 36

Covlent-Network nd Moleculr Solids Grphite is n exmple of moleculr solid, in which toms re held together with vn der Wls forces. They tend to e softer nd hve lower melting points.

19 Covlent-Network nd Moleculr Solids Grphite is n exmple of moleculr solid, in which toms re held together with vn der Wls forces. They tend to e softer nd hve lower melting points. 37 Semiconductors Elementl semiconductors (mde of only one type of tom) Elementl semiconductors include silicon, germnium, nd gry tin. These elements dopt the crystl structure of dimond. In this structure, four toms in tetrhedrl coordintion geometry surround ech tom. There re 4 vlence electrons per tom thus, ech hyrid oritl contins single electron. The result is tht semiconductors re conductive ut less so thn metls due to the presence of the nd gp. Compound semiconductors (mde up of two or more elements) Compound semiconductors include GAs, InP, nd CdTe. These lso mintin the verge vlence electron count s elementl semiconductors (4 per tom) 38

20 Semiconductors gp etween the occupied MOs (vlence nd) nd the unoccupied ones (conduction nd) : 0.08 to 3.05 ev (7 to 300 kj/mol) 39 Semiconductors E g : C Si Ge Sn (decrese)///p( metl) Due to the decrese in overlp 40

Doping By introducing very smll mounts of impurities tht hve more (n-type) or fewer (p-type) vlence electrons, one cn increse the conductivity of semiconductor. 41 Smple Exercise 12.

21 Doping By introducing very smll mounts of impurities tht hve more (n-type) or fewer (p-type) vlence electrons, one cn increse the conductivity of semiconductor. 41 Smple Exercise 12.3 Qulittive Comprison of Semiconductor Bnd Gps Will GP hve lrger or smller nd gp thn ZnS? Will it hve lrger or smller nd gp thn GN? Solution Eg(ZnS)>Eg(GP) : The electronegtivity difference to e lrger for ZnS, which should result in ZnS hving lrger nd gp thn GP. Eg(GN)>Eg(GP) : P nd N. Nitrogen is locted ove phosphorus in group 5A. Bsed on incresed oritl overlp,we would expect to GN hve lrger nd gp thn GP. Check Externl references show tht the nd gp of GP is 2.26 ev, ZnS is 3.6 ev, nd GN is 3.4 ev. Prctice Exercise Will ZnSe hve lrger or smller nd gp thn ZnS? Answer: Becuse zinc is common to oth compounds nd selenium is elow sulfur in the periodic tle, the nd gp of ZnSe will e smller thn ZnS. 42

silicon, would yield n n-type semiconductor: G, As, or C?

Prctice Exercise Suggest n element tht could e used to dope silicon to yield p-type mteril.

22 Smple Exercise 12.4 Identifying Types of Semiconductors Which of the following elements, if doped into silicon, would yield n n-type semiconductor: G, As, or C? Solution As, if doped into silicon, would yield n n-type semiconductor. Prctice Exercise Suggest n element tht could e used to dope silicon to yield p-type mteril. Answer: Becuse Si is in group 4A, we need to pick n element in group 3A. Boron nd luminum re oth good choices oth re in group 3A. In the semiconductor industry oron nd luminum re commonly used dopnts for silicon

23 Polymers Polymers re molecules of high moleculr mss mde y sequentilly onding repeting units clled monomers. 45 Plstics Plstics re mterils tht cn e formed into vrious shpes, usully with het nd pressure. Thermoplstic mterils cn e reshped. Recycling of polypropylene Thermosetting plstic mterils re shped y n irreversile process. not redily reshped. Elstomers re mterils tht exhiit elstic or ruery ehvior. If moderte mount of deforming force is dded, the elstomer will return to its originl shpe. 46

24 Some Common Polymers 47 Addition Polymers Addition polymers re mde y coupling the monomers y converting onds within ech monomer to onds etween monomers. 48

25 Condenstion Polymers Condenstion polymers re mde y joining two suunits through rection in which smller molecule (often wter) is lso formed s y-product. These re lso clled copolymers. 49 Synthesis of Nylon Nylon is one exmple of condenstion polymer. 50

51 Properties of Polymers Stretching the polymer chins s they form cn

26 Properties of Polymers - Mw distriution - Interctions etween chins of polymer lend elements of order to the structure of polymers. 51 Properties of Polymers Stretching the polymer chins s they form cn increse the mount of order, leding to degree of crystllinity of the polymer. LDPE vs. HDPE 52

Plsticizers We cn modify the polymeric properties y the ddition of sustnces with lower moleculr mss. Plsticizers re molecules tht interfere with interctions etween polymer chins.

27 Plsticizers We cn modify the polymeric properties y the ddition of sustnces with lower moleculr mss. Plsticizers re molecules tht interfere with interctions etween polymer chins. These mke polymers more plile. 53 Cross-Linking Chemiclly onding chins of polymers to ech other cn stiffen nd strengthen the sustnce. Nturlly occurring ruer is too soft nd plile for mny pplictions. 54

28 Cross-Linking 55 In vulcniztion, chins re cross-linked y short chins of sulfur toms, mking the ruer stronger nd less susceptile to degrdtion Nnomterils nm Semiconductors on the Nnoscle Semiconductor prticles with dimeters in the 1 to 10 nm rnge re clled quntum dots Semiconductor nd gps chnge sustntilly with size in the 1-10 nm rnge By tuning the nd gp, ll colors of the rinow cn e otined from one mteril. 56

wvelengths of light, depending on the size of

.. Quntum well, quntum wire 57 photoluminiscence

quite different properties thn lrger smples of

29 Nnoprticles Different size prticles of semiconductor (like Cd 3 P 2 ) cn emit different wvelengths of light, depending on the size of the energy gp etween nds... Quntum well, quntum wire 57 photoluminiscence Metls on the Nnoscle Finely divided metls cn hve quite different properties thn lrger smples of metls. men free pth of n electron in metl t room temperture is on the nm scle. 58

Buckyll or uckminsterfullerene my e prepred y electriclly evporting grphite in helium tmosphere.

30 Fullerenes In 1985 molecules composed of 60 cron toms, C 60 molecules, were first descried. C 60 molecules re mong clss of molecules of cron toms known s fullerenes. Buckyll or uckminsterfullerene my e prepred y electriclly evporting grphite in helium tmosphere. Becuse fullerenes re composed of individul molecules, they dissolve in vrious orgnic solvents while dimond nd grphite do not. 59 Cron Nnotues Cron nnotues cn e mde with metllic or semiconducting properties without doping. 60

Grphene In 2004 sheets of cron toms with honeycom structure were isolted nd identified. Grphene hs interesting properties: It is very strong nd hs high therml conductivity.

31 Grphene In 2004 sheets of cron toms with honeycom structure were isolted nd identified. Grphene hs interesting properties: It is very strong nd hs high therml conductivity. Its electronic structure is like tht of semiconductor with n energy gp of zero. It cn sustin very high electricl current densities. 61 Prolems 10, 26, 32, 46, 58, 64, 81,

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