Chng I CU TRC TINH TH CA VT LIUPh thuc vo iu kin to thnh ( nhit , p sut,) v tng tc gia cc phn t cu thnh (lc lin kt gia cc phn t, nguyn t), vt cht c th tn ti trng thi rn, lng, hoc kh (hi). Tnh cht ca vt rn (vt liu) ph thuc ch yu vo cc cch sp xp ca cc phn t cu thnh v lc lin kt gia chng. Trong chng ny cc khi nim c bn s c cp li: cu to nguyn t, cc dng lin kt v cu trc tinh th, khng tinh th (v nh hnh) ca vt rn.

1.1 CU TO V LIN KT NGUYN TTrong mc ny s trnh by mt s khi nim c bn v cu to nguyn t v cc dng lin kt gia chng, nhng yu t ng vai tr quan trng i vi cu trc v tnh cht ca vt rn v vt liu. 1.1.1 KHI NIM C BN V CU TO NGUYN T Nguyn t l mt h thng bao gm ht nhn mang in dng v cc in t (electron) mang in m chuyn ng xung quanh. Ht nhn nguyn t cu to t nhng proton v nowtron. Ht nowtron khng mang in cn ht proton mang in dng, c in tch bng in tch cu nguyn t. trng thi thng, nguyn t chung ha in v s lng proton bng s lng in t. S c c trng bng s th t nguyn t (Z) trong bng tun hon Meneleev. V khi lng ca proton v ntron ln hn rt nhiu so vi in t (khong 1830 ln) cho nn khi lng nguyn t c xc nh bng khi lng ht nhn ca n. Vi cng khi lng in t v proton, ht nhn c th cha s lng ntron khc nhau v to thnh nhng ng v ca cng mt nguyn t ha hc. Theo c hc lng t, xc sut tm thy in t trn mt qu o no xung quang ht nhn c xc nh bng bn tham s, gi l s lng t. Ni mt cch khc, trng thi nng lng ca mi in t trong nguyn t c xc nh bng bn s lng t sau y: S lng t chnh n: c cc gi tr bng 1, 2, 3, 4, , xc nh nng lng cho php ca in t. Nhng in t c cng s n hp thnh mt lp in t. K hiu cc lp in t ln lt l K, L, M, N,tng ng vi n = 1, 2, 3, 4 S lng t phng v l: xc nh cc gi tr cho php ca mmen xung lng qu o, c tr s bng 0, 1, 2,(n-1). Cc in t vi l khc nhau ca cng lp to thnh nhng phn lp tng ng, k hiu ln lt l s, p, d, f, ng vi l = 0, 1, 2 ,3 , S lng t t : xc nh kh nng nh hng cho php ca vect mmen xung lng 1, 2, l; S qu o i vi chiu ca t trng bn ngoi, c tr s bng 0,

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lng t spin

: xc nh kh nng nh hng ngc chiu nhau cu vect mmen xung

lng spin ca in t, Ngoi ra, s phn b in t theo cc mc trng thi (kh nng c mt ti mt phn lp no vi mt trng thi nng lng xc nh) cn tun theo nguyn l pauli: mi trng thi vi ba s lng t n, l, xc nh ch c th cha hai in t vi spin ngc chiu nhau. Da vo nguyn l ny, c th d on s in t cho php trn cc bc nng lng (ng vi cc lp v phn lp) khc nhau (bng 1.1) Bng 1.1 S lng in t (s trng thi nng lng) trn mt s lp v phn lp K hiu S lng S lng in t c th S lng K hiu phn lp trng thi Trn phn t chnh lp in t Trn lp in t c th lp 1 K s 1 2 2 s 1 2 2 L 8 p 3 6 s 1 2 3 M p 3 6 18 d 5 10 s 1 2 p 3 6 4 N 32 d 5 10 f 7 14 V d. nguyn t ng (Cu) vi s th th nguyn t Z nh sau: 1 K 2 L 3 M 3 N 4 , c phn phi in t

Trong s phn b in t ny (cn gi l cu hnh in t) ch r: s lng t chnh (cc s nguyn 1, 2, 3), k hiu phn lp (cc ch s, p, d), s in t thuc phn lp (s m trn k hiu phn lp) v k hiu lp in t (hng ch hoa K, L, M, N pha di). Cn lu thm rng, mt iu kin xc nh, in t c th chuyn t trng thi ny sang trng thi khc (thay i phn lp hoc lp), khi n s pht ra hoc hp th nng lng di dng cc lng t nh sang c tn s : h. h- hng s planck (h 6.627. ec.s) (1.1)

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1.1.2 CC DNG LIN KT NGUYN T TRONG CHT RN 1.1.2.1. Lin kt ng ho tr Lin kt ny to ra khi hai hoc nhiu nguyn t gp chung nhau mt s in t c tm in t lp ngoi cng (in t ho tr). V d, nguyn t ca cc nguyn t ho hc trong nhm VII B ( bng tun hon Menelv, bng 1.2) coa cu trc in t lp ngoi cng l s2p5 (by in t). c tm in t. cn kt hp hai nguyn t li bng cch gp chung hai in t lp ngoi. Do vy to ra mt lien kt ng ho tr. Lin kt ng ho tr gia hai nguyn t Clo (Cl) trong phn t cl2 c m t trn hnh 1.1a. Lin kt ng ho tr trong vt rn c thc hin nh s tp th ho in t gia mt nhm cc nguyn t ln cn. Trn hnh 1.1b nu m hnh lien kt ng ho tr ( tp th ho in t ca bn in t lp ngoi cng s2p2) trong mng tinh th Ge. Nhng c im ca lin kt ng ho tr l: - Lin kt c tnh nh hng, ngha l sc xut tn ti cc in t tham gia lin kt ln nht theo phng ni tm cc nguyn t ( hnh 1.1); - Cng lin kt ph thuc rt mnh vo c tnh lin kt gia cc in t ho tr vi ht nhn. V d, Cacbon dng th hnh kim cng c lin kt ng ho tr rt mnh v bn in t ho tr ( trong tng s su in t) lin kt cu hnh nh trc tip vi ht nhn, trong khi , Sn th hin tnh lin kt ng ho tr rt yu v bn in t ho tr ( trong tng s 50 in t) nm xa ht nhn, do c lc lin kt yu i vi ht nhn. V vy kim cng c nhit nng chy trn 3550C trong khi Sn nng chy 270C;

Hnh 1.1 Lin kt cng ho tr trong phn t kh CH4

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1.1.2.2 Lin kt ion

1.1.2.3 Lin kt kim loi 1.1.2.4 Lin kt hn hp Thc ra , lin kt ng ha tr thun ty ch c c trong trng hp lin kt ng cc (gia cc nguyn t ca cng mt nguyn t ha hc). Trong trng hp lin kt d cc (gia cc nguyn t ca cc nguyn t khc nhau). in ha tr tham gia lin kt chu hai nh hng tri ngc : B ht bi ht nhn ca mnh B ht bi ht nhn nguyn t th hai to nguyn t chung

Kh nng ca ht nhn ht in t ha tr c gi l tnh m in ca nguyn t. S khc nhau v tnh m in gia cc nguyn t trong lin kt ng ha tr lm cho m my in t chung b bin dng v to thnh ngu cc in , tin t ca lin kt ion. :Tnh ion ca lin kt s cng ln nu s khc nhau v tnh m in ca cc nguyn t cng ln. V d . Na c tnh m in bng 0,9 cn Cl bng 3,0 . Do vy lin kt gia Na va Cl trong hp cht NaCl gm khong 52% lin kt ion v 48% lin kt ng ha tr . Tt c nhng lin kt d cc u mang tnh cht hn hp gia lin kt ion v ng ha tr. 1.1.2.5 Lin kt yu ( lin kt Van der Waals) Lin kt ng ha tr cho php l gii s to thnh nhng phn t nh nc ( O) hoc polyetylen ( ) . Nhng khng cho php l gii s to thnh mt s vt rn t nhng phn t trung ha nh nc polyme Trong nhiu phn t c lin kt ng ha tr , do s khc nhau v tnh m in ca cc nguyn t , trng tm in tch dng v in tch m khng trng nhau, ngu cc in s to thnh , phn t b phn cc . Lin kt van der waals l lin kt do hiu ng ht nhau gia cc nguyn t hoc phn t b phn cc trng thi rn . Lin kt ny l loi

[4]

lin kt yu , rt r b ph v do ba ng nhit ( khi tng nhit ). V vy nhng cht rn trn c s lin kt van der waals c nhit nng chy thp.

[5]

[6]

[7]

1.2. SP XP NGUYN T TRONG VT RN1.2.1 Khng trt t hon ton, cht kh Cht kh chim ton b th tch cha n c th nn c. Cc nguyn t (phn t) trong cht kh lun lun chuyn ng do ba ng nhit s nguyn t (phn t) trn 1 n v th tch thay i. Ph thuc vo nhit v p sut. V tr tng ng gia chng lun thay i theo quy lut ngu nhin. Trung bnh mi nguyn t ( phn t) chim 1 th tch tng ng hnh cu. ng knh trung bnh 4 nm. Ly 8 nguyn t ( phn t) ca mt cht kh no lm gc ta , v ng cong xc sut tm thy 1 nguyn t ( phn t) khc. Ti khong cch d k t gc ta . Thy rng vi 0 < d < (tng ng ng knh nguyn t, phn t) xc sut bng 0. T gi tr d xc sut P(d) s tng lin tc bng mt ti gi tr d tng ng ng knh trung bnh ca khng gian hnh cu b nguyn t (phn t) chim ch . 1.2.2 Trt t l tng vt rn tinh th Trong vt rn tinh th mi nguyn t c v tr hon ton xc nh khng ch so vi nhng nguyn t gn nht m c nhng nguyn t khc bt k xa hn . Khng gian xung quanh cc nguyn t c cu to hon ton ng nht. Ni cch khc tinh th c trt t xa. Coi cc nguyn t l nhng qu cu rn ging nhau , xp xt nhau lin tip theo ba trc vung gc x,y,z trong khng gian. Ni cc tm ca qu cu nguyn t s c hnh nh ca 1 mng tinh th lp phng n gin.Hnh lp phng nh nht vi 8 nh l tm ca 8 nguyn t c gi l c s. Mi nguyn t l nh chung ca 8 c s gi l nt mng . Khong cch gn nht gia cc nt( gn bng ng knh khn gian hnh cu b nguyn t chim ch) chnh bng cnh a ca c s. ng vi gi tr nh nht ca nng lng lin kt. Hnh 1.7 L th xc sut tm thy nguyn t t khong cch d so vi nguyn t chn lm gc ta . khc vi cht kh, trong cht rn tinh th , P(d)=1 (cc pic trn th) ti nhng gi tr d hon ton xc nh l d = a, , a , tng ng khong cch gia cc nguyn t theo cnh ,ng cho mt v ng cho khng gian ca c s. rng ca cc pic trn hnh 1.7 c trng cho giao ng ca cc nguyn t quanh v tr cn bng ( v tr nt mng) hiu ng ca ba ng nhit. Do s sp xp c trt t ca cc nguyn t , ln trong vt rn tinh th xut hin c tnh quan trng:[8]

tnh d hng, tc l s khc nhau v tnh cht (c, l , ha tnh ) theo cc hng khc nhau ca tinh th. minh ha munyoung ( E) ca ng ( Cu) bng 67 GPa v 192 GPa tng ng theo cc hng cnh v ng cho khng gian ca c s . Cui cng mun lu rng s sp xp cc nguyn t trong vt rn theo mng tinh th ph thuc vo nhiu yu t nh : lin kt gia cc nguyn t s lng cc nguyn t gn nht hoc l s sp xp m mi nguyn t c th chp nhn. 1.2.3 Cht lng, vt rn vi tinh th v v nh hnh. Mt cch gn ng, th tch ca mt khi lng cht lng l i lng khng i . Ging nh trong vt rn cc nguyn t c xu th tip xc vi nhau v chim mt khng gian hnh cu kch thc khong 0.25 nm . Nn cht long khng c tnh chu nn. S khc nhau gia cht lng v vt rn th hin nh sau : Cc nguyn t lun lun chuyn ng do ba ng nhit, v vy v tr ca chng khng c xc nh. trn hnh 1.8a biu din s m t v tr cc nguyn t ca mt cht lng ti mt thi im trong khn gian hai chiu. Nhn thy rng, trong mt vng khng gian nh (c kch thc nguyn t), mt s nguyn t sp xp c trt t ( gii hn bi ng t on trn hnh 1.8a), nhng khng n nh lun lun b ph v do ba ng nhit. Nh vy cht lng c trt t gn. c im trt t gn c th hin trn ng cong xc sut P(d) ph thuc vo khong cch d : P(d) =1 ti im ng vi khong cch trung bnh gia cc nguyn t trong vng trt t gn xa hn na, gi tr xc sut P(d) lun bng 1, tc ging nh trong cht kh. Ngc vi tnh d hng trong cht rn ca vt rn, cht lng c tnh ng hng v trong cht lng s lng nguyn t, phn t trung bnh trn mt n v chiu di v lc lin kt gia chng nh nhau. Theo mt hng trong khng gian : sp xp cht (t l gia th tch do cc nguyn t chim ch trn tng th tch) ca cht lng km hn so vi vt rn (qu trnh kt tinh hoc ng rn thng km theo s gim th tch. Mt cch gn ng c th minh ha cht khi, cht lng, cht rn bng hnh nh tng ng : hi trng ha nhc trt khan gi khi ci bo ng (kh ) khi kt thc bui ha nhc (lng) v hang ng b i chun b duyt binh trn mt qung trng (rn ). Hy lm quen vi m c trng na ca cht lng l tnh linh ng ca cc nguyn t (phn t), nh gi bng st. Cht lng c st thp nn tnh chy (kh nng in y th tch cha n) cao. Thc nghim chng minh,trong chuyn bin pha lng - rn ( nhit nng chy ) st khng thay i. i vi mt s cht,trng thi lng c st cao,cc nguyn t (phn t) khng linh hot sp xp li theo trt t xa khi chuyn pha lng - rn : vt rn to thnh khng c cu trc tinh th v c gi l vt rn v nh hnh.V d,thu tinh c cu to[9]

bi cc phn t nh l cc ion

,c cu trc hnh khi 4 mt tam gic u (hnh 1.9),trong cc , cn tm khi l ion .

iu kin ch to thu tinh bnh thng,khi 4 mt [SiO4]4- (n v cu trc ca SiO2 c chung nh O2-) (hnh 1.9b) v cho cu trc v nh hnh.N gim tc ngui ca thu tinh lng,sao cho cc phn t SiO2 c thi gian sp xp theo trt t xa,s c thu tinh c cu trc tinh th (hnh 1.9c). Ging nh cht lng,vt rn v nh hnh c tnh ng hng.

Hnh 1.9 Cu trc khi 4 mt [SiO4]4- (a) v m hnh 2 chiu ca thu tinh SiO2 (b) tinh th SiO2 (c)

Cn lu rng,nu lm ngui kim loi hoc hp kim lng vi tc ln (ln hn 10 - 109 oC/s),vt rn nhn c s c cu trc v nh hnh hoc cu trc tinh th vi kch thc rt nh (khong nanomet), gi l vt rn v nh hnh hoc vi tinh th (s trnh by mc 3.1.6)4

[10]

1.2.4. QUAN H GIA CU TRC V TNH CHT T nhng trnh by cc phn trn v s c lm r thm cc chng tip theo,c th thy rng yu t quyt nh tnh cht ca vt liu l cc dng lin kt v cu trc ca n Hnh 1.10 m t cc dng lin kt gia cc nguyn t (phn t) v bng 1.3 cho nhng gi tr minh ha mt vi c trng tnh cht c hc,vt l,ha hc ca mt s vt rn. Tnh do v gin ca vt liu c th gii thch d dng thng qua dng lin kt tng ng.i vi vt liu c lin kt ng ha tr, vi gc nghing c nh gia cc nguyn t (phn t),kh nng bin dng do rt km: di tc dng ca ngoi lc,vt liu b ph hy (tch t ) khi ng sut t tr s ph v lin kt.Tng t nh vy vi vt liu c lin kt ion. Nhit nng chy rt cao v tr ha hc ln ca vt liu gm trn c s cc oxit kim loi nh MgO,Al2O3 ,SiO2 c gii thch bng nng lng lin kt ng ha tr (hoc ion) rt cao ca nhng vt liu ny so vi vt liu kim loi. Trong vt liu kim loi (lin kt kim loi), cu trc mng ca cc ion dng trong khng gian in t t do chung cho php cc ion x dch di tc dng ngoi lc: vt liu kim loi c tnh do cao.Ngoi ra,cc in t t do trong mng tinh th kim loi vi tnh linh ng cao,dc dng chuyn ng theo hng tc dng ca in trng bn ngoi v gradient nhit .Kim loi do vy c dn in v dn nhit cao hn so vi cc vt liu khc.ng ho tr (Kim cng, Si, Ge, GaAs Ceramic Graphit Polyme Van der waals CsCl Ion SiO2 Mica As Sn Fe, Ni, Co Cr Kim loi (Cu, Na, Ag

Hnh 1.10 S m t cc dng lin kt trong mt s loi vt liu thng gp

[11]

Nh trnh by mc 1.1.2.4 v hnh 1.10,trong thc t phn ln cc loi vt liu c dng lin kt hn hp (kim loi ng ha tr; kim loi ion; ng ha tr - ion;.v..v..) v vy tnh cht ca vt liu s l tng hp ca cc tnh cht c trng cho dng lin kt nu trn.c im lin kt v tnh cht va mt s vt liu nu bng 1.3. Bng 1.3. c im lin kt v tnh cht va mt s vt liu Tnh cht Cht v nng Dng M hnh lin lng lin kt lin kt kt Tc E (eV/nguyn t) ng ho tr Kim cng (7,0) Cho (hoc nhn) in t ho tr Lc ht, y tnh in Mng ca cc ion trong khng gian in t t do chung Cao Cao Cao Cao

Thp Thp

Cao Cao

Tnh do Thp Thp

ion

NaCl (3,3)

Kim loi Van der waals

Na (1,1) cu (3,5) Ti (4,8) H2 (0,01) CH4 Cl2

trung bnh Cao

trung bnh Cao

trung bnh Thp

trung bnh

Cao

Lc ht ngu Thp cc

Thp

Cao

Cao

trung bnh

kt th phn ny,xin lu rng tnh cht ca vt liu cn ph thuc rt nhiu vo cu trc ca chng.Nhng nghin cu mi nht trong tinh th hc cho thy ,bng cch to ra hp kim v nh hnh v vi tinh th c th nhn c nhng vt liu c c,l,ha tnh rt c bit,khc hn nhng tnh cht thng thng m cc vt liu ny c,vi cu trc tinh th quen thuc.

1.3 KHI NIM V MNG TINH TH nghin cu s sp xp ca cc nguyn t (ion,phn t ) trong vt rn tinh th v so snh cc kiu sp xp khc nhau, cn a ra khi nim v mng khng gian tinh th (mng tinh th).Mng tinh th l i tng nghin cu u tin v quan trng ca vt liu hc v n rt cn thit nghin cu nhiu vn khc. mc ny s trnh by nhng khi nim c bn nht v mng tinh th. 1.3.1.TNH I XNG CA TINH TH i xng l mt trong nhng tnh cht quan trng ca tinh th,n th hin hnh dng bn ngoi,cu trc bn trong cung nh cc tnh cht[12]

Tnh i xng ca tinh th c c trng bi cc yu t i xng.Mi yu t i xng tng ng vi mt thao tc i xng,tc l vi mt bin i hnh hc xc nh mt h thng im ,ng ,phn t ,... t trng lp vi chnh mnh trong khng gian Nhng yu t i xng quan trng l -Tm i xng (tm nghch o,k hiu C hoc i) : cc im a1, a2, a3 ,..gi l i xng qua tm vi b1, b2, b3 ,..tng ng nu chng trng lp nhau tng cp mt bng php nghch o qua tm C (hnh 1.11a) -Mt chiu gng (k hiu P hoc m) : a1, a2, a3 ,..c gi l i xng gng vi b1, b2, b3 ,..tng ng nu chng trng lp nhau tng cp mt bng php chiu gng qua mt phng P (hnh 1.11b).T hp ca cc im a1 c gi l i xng gng vi t hp ca b1 qua P nu tng im a1 i xng gng tng ng vi tng im b1 qua mt phng P -Trc i xng (k hiu L hoc n) :cc im a1 c th trng lp nhau bng php quay quanh trc L mt gc ,khi chng c gi l i xng nhau qua trc L.S nguyn n= gi l bc ca trc i xng. chng minh c rng ch tn ti trc

i xng vi n = 1,2,3,4,6 (khng tn ti trc i xng bc 5 v bc cao hn 6 (hnh 1.11c)a3 C a1 a2 b3 a2 b2 b1 b2 b1 a3 P b3

a1

Hnh 1.11. Cc yu t i xng : tm i xng (a) , mt i xng (b) v trc i xng (c) - Php tnh tin (k hiu t) l mt trong nhng yu t i xng quan trng ca cu trc mng tinh th,ng vi thao tc tnh tin mng tinh th theo mt hng no trong khng gian i mt s nguyn ln trn di xc nh tinh th trng vi chnh n.S trng lp y cn hiu l trng lp cc yu t hnh hc gii hn tinh th v c cc tnh

[13]

cht khc. di n v tin ( hng khng gian cho.

c gi l chu k tun hon ca mng tinh th theo

Ngoi nhng yu t i xng n,cn tn ti nhng yu t i xng phc hp gm hai hay nhiu yu t i xng hn, l: trc quay chiu gng, trc quay chiu tm, trc xon, mt nh trt.c gi quan tm c th tm hiu thm nhng ti liu chuyn su v tinh th hc. 1.3.2. C S, H TINH TH c nhng khi nim u tin v mng tinh th,hy xut pht t khi nim n gin v c s nh nu mc 1.2.2 v hnh 1.6.Thy rng, do tnh i xng ca tinh th,t mt c s,bng thao tc i xng,tnh tin theo 3 chiu trong khng gian s nhn c ton b mng tinh th. c s c xy dng trn 3 vect n v vect a = vect , , v ,b= ,c= , , tng ng 3 trc ta Ox, Oy

v Oz.Tm ca cc nguyn t (ion hoc phn t) nh l cc nt mng.Mdun ca 3 l kch thc c s,cn gi l hng s mng hay chu k ( l gc gia v , gia v tun hon (chu k tnh tin) ca mng tinh th theo ba chiu tng ng.Cc gc to bi 3 ,khi hp tng i mt k hiu l ) , gia

Thy rng trong cng mng tinh th c th chn c nhiu kiu c s khc nhau (hnh 1.12a).Tuy nhin,v c s l n v tun hon nh nht ca mng tinh th cho nn vic la chn phi tha mn nguyn tc sao cho n i din y cho tnh cht v cu trc ca ton b tinh th .Cc nguyn tc l: -Tnh i xng ca c s phi l Tnh i xng ca tinh th (v hnh dng bn ngoi v cc tnh cht); -S cnh bng nhau v s gc (gia cc cnh) bng nhau ca phi nhiu nht; -Nu c cc gc vung gia cc cnh th s gc phi nhiu nht; -C th tch nh nht hoc cc cnh bn ngn nht. c s c th tch nh nht v ch c tm nguyn t (ion,phn t) nm nh (nt mng) gi l c s n gin (hnh 1.12b). Ph thuc v tng quan gia ba vect by h tinh th khc nhau (hnh 1.4). Bng cch tnh tin, a cc phn t (nguyn t,ion hay phn t) ln tm cc mt bn,tm y hoc tm cc c s n gin,hay ni cch khc,nu khng quan tm n tnh i xng ca cc phn t ti nt mng tinh th,ch kho st v tr ca chng,thy rng[14]

,

,

;ba gc

ngi ta phn bit

ch c mi bn cch tnh tin cc phn t trong khng gian nhn c mi bn kiu mng tinh th khc nhau thuc by h tinh th ni trn. l mi bn kiu mng tinh th Bravais.Tt c cc mng tinh th ca cht rn u biu din bng mt trong mi bn kiu mng Bravais (bng 1.4).

H th

tinh Quan h Quan h Cc kiu mng Bravais gia cc gia cc n gin Tm y trc gc a # b #c # # # 90 = = 90 #

Tm Khi

Tm mt

Ba nghing Mt Nghing

a # b #c

Trc thoi Ba Phng (thoi) Su Phng

a # b #c

== = 90 == # 90 = = 90 = = 120 == = 90

a=b=c

a=b#c

Bn Phng

a=b#c

Lp Phng

a = b =c

== = 90

Bng 1.4 Cc kiu mng Bravais[15]

1.3.3. PHNG V MT TINH TH Phng php biu din tinh th bng c s nh trnh by trn c u im l tnh trc quan cao nhng khng phi lc no cng d thc hin,nht l i vi cc mng tinh th phc tp ca nhiu nguyn t (nhiu nguyn t,ion hay phn t to thnh).N cng khng thun tin khi cn xc nh v tr tng i gia cc phn t hnh hc (im,ng,mt) trong mng. Mt phng php khc cng rt thng dng trong tinh th hc l biu din cc phn t hnh hc ca mng tinh th bng cc k hiu ton hc phng php gii tch m t tinh th - s c nhc n phn ny. p dng thun li phng php biu din ny,thng chn h truc ta vi ba trc ,Oy,Oz tng ng vi 3 vect 1.3.3.1 Ch s ca nt mng V tr nt mng c k hiu bng ba s,tng ng ta ca nt mng trong h trc ta chn,t trong ngoc vung kp ([[..]]);gi tr m ca cc ta c k hiu bng du (-) trn ta tng ng .V d nt A trn hnh 1.13 c k hiu [[111]].Do tnh i xng ca mng tinh th nn ta ca mi nt mng c th suy ra bng php tnh tin cc nt trong c s vi cc bc bng s nguyn ln hng s mng Bng 1.4. By h tinh th v mi bn mng Bravais a,b,c. V d,nu ta ca mt nt trong c s l x0, y0, z0 th ta ca mt nt khc s l : x1 = x0 + n1a y1 = y0 + n2b z1 = z0 + n3c trong n1 ,n2 ,n3 - cc s nguyn Ta cn c th biu din di dng vect : = + n1 + n2 + n3 (1.29) (1.28) , , ca c s,nt O l gc ta (hnh 1.13).n v o trn ba trc l gi tr hng s mng a,b,c.

1.3.3.2. Ch s ca phng tinh th Phng tinh th l ng thng i qua cc nt trong mng tinh th. Cch nhau nhng khong cch theo quy lut xc nh v c k hiu bng ba s nguyn u, v,w

[16]

t l thun vi ta ca mt nt gn gc ta nht, nm trn phng . Ch s m c k hiu (-) trn. Trn hnh 1.13 nu mt s phng [111]. [110]. [221] Vect n v ca phng s l: =u +v +w Do tnh i xng, mun tm ch ca mt phng no . Ch cn tm ch s ca phng song song vi n. i qua gc ta . Nhng phng song song nhng c tnh cht ging nhau to thnh h phng, k hiu [uvw], Nhng phng khng song song nhng c tnh cht ging hnhau to Hnh 1.13 Ch s ng v im trong mng tinh th thnh h phng. K hiu . Cc phng trong mt h c tr s tuyt i u, v, w ging nhau, ch i chon hay. V d (hnh 1.13) h phng gm su phng : [010], [001], [100], [0 0], [00 ] v [ 00]. 1.3.3.3. Ch s Miller ca mt tinh th Mt phng tinh th l mt phng trong khng gian mng tinh th c to nn bi nhng nt mng. sp sp theo mt trt t xc nh. Ch s Miller ca mt phng tinh th c k hiu bng ba s nguyn h, k, l t l nghch vi nhng on thng, k t gc ta n giao im mt phng vi cc trc ta tng ng Ox. Oy, Oz. C th xc nh nhng ch s h, k, l ca mt mt phng tinh th theo cc bc (v d mt phng P trn hnh 1.14) nh sau : Tm giao im ca mt phng vi ba trc ta Ox, Oy, Oz;

- Xc nh di on thng t gc ta n cc giao im tng ng ni trn(1; ; 1/3 trn hnh 1.14). ly gi tr nghch o ca chng (1;2;3). - Quy ng mu s chung cc s nghch o tm c, ba s nguyn h, k .l, trn phn t s s l ch s Miller ca mt phng ang xt. Mt Phng P trn Hnh 1.14 c ch s (1.2.3) Phng trnh ca mt Phng trong khng gian l:

[17]

Nu mt phng song song vi truc ta , ch s Miller tng ng s t l vi 1/ ngha kaf n bng (v d,mt (001) l mt ca c s song song trc Ox v Oy). Gi Tr m c k hiu bng (-) trn ch s tng ng. H mt phng tinh th. K hiu(h, k, l) l nhng mt song song, c tnh cht ging nhau, v vy mun xc nh ch s ca mt mt bt k ch cn xc nh ch s ca mt phng song song vi n, nm c s cha trc . Cc ,mt phng tuy khng song song nhng c tnh chy ging nhau to mt h mt phng. Ch s Miller ca cc mt phng trong h c k hiu di dng {hkl}. Gi tr tuyt i h,k,l ca chng l nh nhau, ch i v tr cho nhau, v d {100} trong mng tinh th c c s l hnh lp phng gm : (100), (101), (001). ( 00). (0 0) v (00 ) tc l cc mt bn v y ca c s. 1.3.3.4. Ch s Miller-Bravais trong h su phng Cc ch s Miller trong h ta ba trc t ra khng thch hp i vi h tinh th su phng, v cc phng hoc mt cng h c ch s khc nhau. biu din phng v mt tinh th trong h trong h su phng, phi dng ch s Miller- Bravais, tng ng vi h ta gm bn trc : Ox, Oy, Oz v Ou (hnh 1.15), Ba trc Ox, Oy, Ou nm trn cng mt phng y ca c s, tng cp hp vi nhau mt gc 120 vung gc vi trc Oz. Gc ta O l tm ca mt y. Cch xc nh ch s Miller-Bravais hon ton ging nh trng hp ch s Miller. k hiu mt tinh th, cc ch s c vit trong ngoc n c dng (hkil). C th chng minh ng quan h: i = - (h +k) (1.32)

Trn hnh 1.15 ch s ca cc mt BCH, ABHG v AGLF tng ng l (010). (10 0) v (1 00). Nhng mt phng ny thuc cng mt h, vi tp hp cc gi tr s tuyt i ca cc ch s l nh nhau {01 10}. Nu dng ch s Miller. K hiu cc mt phng tng ng l (010), (100) v (1 0). R rang ch s Miller-Bravais th hin ng hn tnh i xng ca tinh th su phng.

Hnh 1.14. Cch xc nh ch s Miller ca mt phng P

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1.3.4. MT NGUYN T TRONG MNG TINH TH H S XP CHT 1.3.4.1. Mt xp Trong tinh th hc, vic xc nh mt xp theo mt phng trn mt mt hoc trong khng gian (trong c s) nhiu kh rt cn thit. V d, nu bit mt sp xp ca cc mt v phng trong tinh th c th xc nh kh nng bin dng do ca tinh th, di tc dng ngoi lc bin dng do s xy ra trc tin theo nhng mt v phng trnh tinh th c maatjd d xp ln nht (xem mc 4.2.1, chng 4). Mt sp xp theo phng trn mt mt hoc trong mt mng tinh th c

Hnh 1.15. Ch s Miller-Bravais trong h su phng trng cho kh nng chim ch ca nguyn t trong khng gian mng, ln lt ng xc nh theo cc biu thc : (1.33) Trong :Ml, Ms, Mu L, v, s L, S,V th. - mt xp theo phng, mt th tch (mng) - chiu di, din tch hoc th tch b chim ch bi t - tng chiu di, din tch, th tch ang xt trong tinh

[19]

Hnh 1.16. Cch xc nh mt sp xp Ml, Ms, Mv ca c s (mng tinh th)

Hnh 1.16 l v d cch tnh Ml, Ms, Mv cho mt tinh th lp phng tm mt (bng 1.4). Coi cc nt mng l nguyn t hnh cu, bn knh r, c s kch thc , kh : Mt xp theo phng [010]:

Mt xp theo phng [110]:

Mt xp theo mt (100):

Mt xp theo mt (111):

[20]

Mt th tch cuat mng lp phng tm mt

1.3.4.2. Cch sp xp nguyn t trong mng tinh th, s xp cht Nh trnh by trn, mt xp c trng kh nng xp cht (kh nng xp st, tip xc vi nhau) ca cc nguyn t. Gi s cc nguyn t l nhng qu cu rn cng kch thc c sp xp sao cho mi nguyn t tip xc vi su nguyn t xung quanh, chung s to ra mt lp nguyn t xp cht (mt xp cht) (lp A rn hnh 1.17). Mun m bo tnh xp cht (c li v mt nng lng khi to mng tinh th). Lp nguyn t tip theo ch c th xp vo lm B (cho lp B) hoc lm C tng ng lp C (hnh 1.17). nh vy s c hai kh nng nguyn t xp cht : theo trt t ABABABhoc ABCABCABC. 1.4.1. Hy quay li khi nim nt mng. cho n by gi nt mng vn c quan nim nh mt im trong khng gian mng tinh th, tng ng vi n ch mt nguyn t (hoc ion). l trng hp mng tinh th cc kim loi nguyn cht vi lin kt kiem loi. Trong mng tinh th cc hp cht gm nhiu nguyn t khc loi vi lin kt ion hoc ng ha tr, ng vi mt nt mng c th hai hoc nhiu nguyn t (ion), to thnh nt phc. Cc nt phwucs hon ton tng ng vi nhau v s lng, s loi nguyn t, nh hng v.v... Hnh 1.18 cho v d v cch sp xp nt mng trong mng tinh th ng (Cu) v nt phc trong NaCl, cristobalit-B (SiO2). 1.3.5. L HNG TRONG MNG TINH TH L hng trong mng tinh th l khng gian trng. gi hn bi hnh khi nhiu mt, mi nh khi l tm nguyn t (ion) ti nt mng tinh th.

Hnh 1.17. Cch sp xp nguyn t theo nt phc (b.c)

S ph v trt t xp gi l khuyt tt xp, Trt t sp xp s c cp li mc

[21]

Hnh 1.19. Cc loi l hng trong mng lp phng tm mt (a) V lp phng tm khi (b) Kch thng ca l hng c nh gi bng bn knh qu cu nht c th t vo l hng . Hnh dng l hng ph thuc vo cu trc ca mng tinh th V d, mng lp phng n gin c l hng lp phng, nh l nt mng c s ngoi ra, l hng hay gp c dng khi tm mt, bn mt. Trn hnh 1.19 m t hai loi l hng ny trong mng lp phng tm mt v lp phng tm khi.Bng 1.5 cho thy cc dng v s lng l hng trong mt vi mng tinh th in hnh. L hng c vai tr quan trng trong nhiu trng hp, v d trong qua trnh to thnh hp kim hoc chuyn pha (xem chng 3). Trong nhng iu kin xc nh mt s nguyn t ca nguyn t hp kim chim ch trong cc loi l hng khc nhau ca mng tinh th kim loi nn , nu chng c kch thc tng thch, kt qu s cho nhng hiu ng mnh thay i cu trc v tnh cht ca vt liu. Bng 1.5: dng v s lng l hng trong mt s mng tinh th Cu trc mng tinh th Lp phng tm mt Su phng xp cht Dng l hng Khi 4 mt Khi 8 mt Khi 4 mt[22]

S lng l hng trong c s 8 4 12

S lng l hng trn mt nguyn t 2 1 2

Lp phng tm khi Lp phng n gin

Khi 8 mt Khi 8 mt Khi 4 mt Khi lp phng

6 12 6 1

1 6 3 1

1.4 MT S CU TRC TINH TH IN HNH CA VT RN phn ny, mt s cu trc tinh th in hnh ca vt rn vi cc dng lin kt khc nhau s c cp. 1.4.1. CU TRC TINH TH CA VT RN VI LIN KT KIM LOI. Lin kt kim loi l lin kt khng nh hng (xem muc 1.1.2.3), cc nguyn t (ion, phn t) lun c xu hng sp xp st cht, vi nhiu lin kt ngn, mnh. iu ny gii thch ti sao vt rn vi lin kt kim loi (phn ln cc nguyn t kim loi trong bng tun hon Menleev) c cu trc mng vi mt xp nhng vt rn khc. (lptk), lp phng tm mt , v tnh i xng cao so vi

Ba cu trc tinh th in hnh ca lin kt kim loi l lp phng tm khi (lptm) v su phng xp cht (spxc) (hnh 1.20). 1.4.1.1. Mng tinh th lp phng tm khi( c s l hnh lp phng vi cnh bng a, v vy mng ny ch c mt hng s mng. Cc nguyn t nm nh v trung tm (hnh 1.20a) s nguyn t n ca c s c tnh nh sau: mi nguyn t nh ng thi l ca 8 c s nn thuc v mt ch c 1/8 nguyn t, nguyn t tm hon ton thuc c s. n .8 + 1 nguyn t

Trong m t tinh th. Thng quy c nguyn t nh c s trng vi gc ta l mt trong nhng nguyn t gc. nh vy ta ca cc nguyn t gc trong mng l v [ ]].

[23]

Hnh 1.20 Cch sp xp nguyn t trong c s mng A2 (a) A1 (b) v A3 (c) Mi nguyn t trong mng tm ( hoc k 8+6) T hnh 1.20a c th thy trong mng , theo phng 111 v theo mt {110} mt xp cao nht. tnh c mt xp p dng cng thc (1.33) (theo(110)) = = ( theo c s)= Trong = c bao quanh b tm nguyn t vi khong cch

v 6 nguyn t khc xa hn vi khong cch l a. Nh vy s sp xp ca mng k bng

, n- s nguyn t trong mt (110) v trong c s;

R, a- bn knh nguyn t v kch thc c s. Cho rng theo cc nguyn t xp st nhau (hnh 1.20a), ta c quan h gia r v a l: 4r = a , suy ra r = (1.41)

[24]

Kt hp (1.40) vi (1.38) v (1.39) tnh c: 1.4.1.2 Mng lp phng tm mt (A1: lptm)

[25]

1.4.1.3 Mng tinh th su phng xp cht

1.4.2 Mng tinh th ca vt rn c lin kt ng ho tr 1.4.2.1 Mng tinh th kim cng (A4) Kim cng l mt d ng tn t i (th h nh) ca cacbon vi c u h nh electron l 1s2 2s2 2p2, vy s e lp tham gia li n k t l N = 4, s s p x p s l 4 tc l mi mt nguy n t cacbon c 4 nguy n t bao quanh gn nh t.

Hnh 1.22 Mng tinh th ca kim cng a,b v lin kt trong tinh th c[26]

c s m ng kim cng (h nh 1.14a), c to thnh trn c s ca c s A1 c thm bn nguyn t bn trong vi cc ta (xem hnh 1.14b): 1/4, 1/4, 1/4 (1); 3/4, 3/4, 1/4 (2); 1/4, 3/4,3/4 (3); 3/4, 1/4, 3/4 (4). nm tm ca bn khi 1/8 c ch u nhau. Cc nguyn t cacbon u c lin kt ng ha tr vi nng lng ln nn kim cng c cng rt cao (cao nht trong thang cng). 1.4.2.2 Mng Graphit C mng lc gic lp (hnh 1.15a), trong mt lp khong cch gia cc nguyn t a = 0,246nm, lin kt ng ho tr . Khong cch gia cc lp c = 0,671nm, tng ng vi lin kt yu Van der Waals, garafit rt d b tch lp, rt mm, n c coi nh l mt trong nhng cht rn c cng thp nht.

Hnh 1.23. Cu trc mng ca grafit (a), si cacbon (b) v fullerene (c). 1.4.2.3. Cu trc si cacbon v fullerene Si cacbon v fullerene l hai dng th hnh na ca cacbon. Thnh tu mi ca cc cng trnh nghin cu trong lnh vc vt lieu trong nhng nm gn y. Si cacbon c s dng lm ct trong vt liu compozit vi nu l nhng vt liu khc (xem mc 12.4). Cu trc s cacbon c th tng tng nh nhng lp v nguyn t cacbon, sp sp theo hnh lc gic, c lien kt ng ha tr mnh (ging nh lp y c s mng graphit). Cun xung trc si (hnh 1.23b). Nng lng lin kt ng ha tr mnh trn cc lp v ca si cho pho to c nhng vt liu compozit c bn gp ba ln nhng nh hn bn ln so vi tho. Phn t cacbon C60, gi l fullerene, do hai nh bc hc ngi Anh H. Kroto v M R.Smalley tng hp c ln u tin vo nm 1985. tuy nhin gi thit v s tn taih cu trc l ca phn t cacbon c cc nh vy l thin vn xng khi nghin cu cu trc v thanh phn cc vt th v tr t nhng nm by mi ca th k ny. Cu trc phn t fullerene (C60) trnh by trn hnh 1.23c : 60 nguyn t cacbon sp xp trn mt mt cu theo nh ca 12 ng gic u v 20 lc gic u cc ng gic lien kt nhau qua lc gic. Mt phn t fuuleren c hnh dng ging nh qu bong

[27]

nhiu mi. Lin keeys gia cc nguyn t cacbon trong fullerene lun lun l ng ha tr thun ty. Cu trc i xng trn, cng cao ca nhng qu bng mini fullerene ang l ti nguyn cu hp dn ca cc nh vt liu hc khng nhng trong nghin cu c bn m c nhng lnh vc ng dingj trong k thut v cng ngh. 1.4.2.4. Cc cu trc mng tinh th lien kt ng ha tr khc Ging trng hp kim cng cu trc mng tinh th ca cc hp cht c lin kt ng ha tr mnh ph thuc vo gc gia cc lin kt, V d, mng tinh th ca SiO2c cu to bi nhng kh t din tam gic u (hnh 1.9)b trong mi mt ion Si4+ c bao quanh bng bn ion O2- . m bo trung ha in, mi inon O2- l nh chung ca hai khi t din. Ph thuc iu kin to thnh, cch xp xp cc khi t din. SiO2 c th cho tinh th thch anh vi cu trc su phng hoc -cristobalit vi cu trc lp phng (hnh 1.24). Trong iu kin ngui nhanh s nhn c thy tinh (cu trc v inh hnh) (xem mc 1.2.3, hnh 1,9b).

Hnh 1.24 Sp xp khi t din (SiO4) trong thch anh (a), cristobalit (b). Cu trc khi t din c m t trn lun lun l c s ca cc mng tinh th silicat. Tuy nhin. Ph thuc vo thnh phn ha hc v iu kin to thnh, khng phi lc no cc khi t din cng lin kt vi nhau bng mt nh chung nh tinh th SiO 2 . V d, trong tinh th forsterit, Mg2SiO4, khi t din (SiO4)4- khng lin kt bng nh ching m bng ion Mg2+ lm trung gian (hnh 1.24c) Slicat l u vo quang trng trong cng ngh ch to vt liu gm (ceramic) nh s trnh by chng 9. 1.4.3. MANH TINH TH CA VT RN C LIN KT ION Cu trc tinh th ca vt rn (hp cht) kin kt ion ph thuc vo hai yu t: t s ca s lng ion m (anion) trn s lng ion dng (cation) : t s ny c nh i vi tng hp cht, ph thuc vo s lng in t tham gia lin kt, m bo tnh trung ha v in ca h thng;

[28]

Tng quan kich thc gia anion v cation ; lin kt ion l loi khng inh hng (xem mc 1.1.4.3) v vy trong mng tinh th, cc ion lun c xu hng sp xp t xp cht v tnh i xng cao nht. Sau y nu mt s kiu mng in hnh. 1.4.3.1. Mng tinh th hp cht dng MX Xt mng tinh th ca NaCL v CsCL (hnh 1.25). Mng tinh th NaCL ( kiu B1) to nn trn c s mng lptm ca anion Cl- v ngc li. T s cc ion Cl- trn s ion Na+ lun lun bng 1. Khc vi mang NaCl, mng tinh th ca CsCl(kiu B2) c to nn trn c s mng lp phng n gin ca ion CL- , ion Cs+ nm l hng khi lp phng ( tm khi lp phng)(hnh 1.25b), t s lng ion Cl- trn ion Cs+ lun l 1 nhng trong mng NaCl. Mi ion Cs+ bao quanh bng tm ion Cl- v ngc li.

Cl Cs

a

b

c

Hnh 1,25. Mang tinh th ca hp cht Mx ; a)NaCl; b) CsCl; C)ZnS S khc nhau v cu trc mng tinh th gia NaCl v CsCl (mc d Na v CS c cng ha tr) l do s khc nhau v tng quan kch thc gia cc ion Na+ v Cs+ so vi ion Cl- . T s tng ng l dNa/dCl = 0,91; cc ion trong mang CsCl sp xp theo kiu B2 s c mt xp MV = 68% cao hn so vi sp xp theo kiu B1 ca NaCl (Mv = 53%). Mt mng tinh th tiu biu khc ca hp cht kiu MX l ca ZnS (kiu B3). To bi mng lptm ca cc ion S2- , ion Zn2+ chim v tr trong ca mng kim cng (hnh 1.25c), t s lng ion ns/nZn= 4/4= , s ion S2- bao quanh ion Zn2+ bng bn v ngc li.

[29]

1.4.3.2. Mng tinh th hp cht dng MX2 (hoc M2X)

c

Hnh 1,26 c s mng tinh th : a) CaF2; b) Cu2O; c) TiO2 v BaTiO3

Cc iu kin nX = 2nM v KMX phi lun lun c m bo trong mng tinh th hp cht ny. iu hnh c ba kiu mng sau: Mng tinh th fluorit CaF2 (kiu C1), to bi c s mng lptm ca Ca2+ tm ion F- nm tm ca tm hnh khi nh, c th tiinhs c d dng n F = 2 nc82+ = 8 v KF-Ca= 2KCa-F = 8, t s 1:2 tha mn;-

Mng tinh th cuprit Cu2O (kiu C3) : cc ion O2- to mng lptk, bn ion Cu+ chim v tr ging nh bn nguyn t bn trong c s kim cng (hnh 1.26b). no2 = 2nCu+ = 4; KCu-O = 4, t s 2:1 lun lun tha mn;-

Mng tinh th cuprit Cu2O (kiu C4), to thnh bi mng bn phng tm khi ca Ti vi = 0.45 nm ; c = 0.29 nm, Cc ion O2- chim v tr m tartreen hnh 1.26c. S ion Ti4+ trong c s l 1/8x8 + 1 =2, ca O2- l 1/2x4 + 2 = 4.Mi ion Ti4+ c su[30]

ion O2- gn nht bao quanh ( KO-Ti = 6), cn mi ion O2- c bao quanh bi 3 ion Ti4+ .T s 1:2 tha mn. 1.4.3.3. Mng tinh th hp cht dng Mm Nn Xp Mng tinh th mt s vt liu gm (ceramic) cng c th c tao thnh trn c s mng tinh th ca hai hay nhiu li cation (M , N). Bari titanat c c hai loi cation Ba 2+ v Ti4+. Hp cht c cu trc mng lp phng 120C. (hnh 1.26d). Nh vy , c th hnh dung mng tinh th hp cht ion c to thnh trn c s c s ca anion, cc cation chim mt phn hoc ton b l hng ca mng anion .Bng 1.7 l c trng cu trc ca mt s hp cht ion in hnh

Bng 1.7. Mt s hp cht ion v cu trc mng tinh th ca chng Dng hp cht Mng tinh Dng l hng th ca do cation chim anion lptm Khi 4 mt spxc Khi 4 mt lptm Khi 8 mt T l s l hng b Hp cht in hnh chim 1/2 ZnS, SiC, ZnO, CdS 1/2 ZnS, ZnO NaCl, MgO, CaO, MnS, 1 TiC 1 NiAs, FeS CsCl TiO, CdCl2 CaF2, ZnO2 Na2O, Li2O Al2O3, Fe2O3, Cr2O3 BatiO3, SrZnO3 MgAl2O4, FeAl2O4

MX

MX2 M2X M2X3 MNX3 MN2X4

spxc Khi 8 mt lp n Khi lp 1 gin phng lptm Khi 8 mt 1/2 lp n Khi lp 1/2 gin phng lptm Khi 4 mt 1 spxc Khi 4 mt 2/3 lptm lptm -

1.4.4 CU TRC CA POLYME Khc vi cu trc tinh th cc vt rn xt, trong mi n v cu thnh ( c s) to nn t mt vi nguyn t (ion, phn t), polymer c to nn t rt nhiu phn t, mi phn t li c th to bi hang triu nguyn t, ly polyetylen phn t, mi phn t li[31]

c th to bi hang triu nguyn t. ly polyetylen (C2H4)n lm v d : mi phn t ca polyetylen cu taoh bi cc lin kt ng ha tr mnh v c hng gia cacbon v hyro:

Lin kt theo (1.44) cho phn t polyetylen mch kn. Nu lin kt kp gia hai nguyn t C m sang hai pha: Lc c th s c v hn cc phn t (1.45) lin kt vi nhau, cho mt mch h dng: Mch polymer to thnh theo (1.46) gi l mch thng. Tuy nhin, khi nim thng ch l tng i bi v gc to bi gia cc lin kt ca nguyn t cacbon l 109,5 ging nh trong kiem cng (hnh 1.27a v 1.27b). Cu lu rng, khng phi lc no mch polymer cng thng, gi tr n trong cng thc phn t y t 103 n 106, v vy c nhng mch di n 10m. Lin kt gia cc mnh trong polymer thc hin bng lc van der Waals.Thng thng polymer l vt liu v nh hnh (hnh 1.27b), tuy nhin khi cc mch sp xp theo mt t xc nh s c polymer tinh th (hnh 1.27d). Qu trnhkt tinh ca polymer ph thuc vo tng iu kin c th v cu trc ca cc mch thnh phn (xem mc 11.2.4). Trong thc t, khng c c polymer hon ton trt t (tinh th). Gia cc vng trt t bao gi cng tn ti nhng vng khng trt t ( v nh hnh). l polyme bn tinh th(hnh 1.27c). vng sp xp trt t trong polymer cng cu to t cc c s (hnh 1.27d). 1.4.5 DNG TH HNH l s tn ti hai hay nhiu cu trc mng tinh th khc nhau ca cng mt nguyn t hoc mt hp cht ha hc. Qu trnh thay i cu trc mng t dng th hnh ny sang dng th hnh khc gi l chuyn bin hnh. Cc dng th hnh thng c k hiu bng cc ch ci Hy Lp nh : , , , Cacbon l mt trong nhng nguyn t c nhiu dng th hnh rt nhiu dng th hnh rt khc nhai, khng nhng v cu trc mng m cn tnh cht: b hng (v nh hnh), graphit (mng A9), kim cng (mng A4), s cacbon ( cu trc lp cun), fullere (cu trc cu C60, C100,)(xem mc 1.4.2)

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S tn ti cc dng th hnh ph thuc vo hai yu t c bn l nhit v p sut. V d, graphit c th to c iu kin bnh thng, trong khi chuyn bin th hnh graphit sang kim cng ch xy ra nhit v p sut rt cao. Cc dng th hnh ca st (Fe) cng c th c coi l mt v d in hnh, c nhiu ngha trong k thut. Cu trc mng A2 ca -Fe tn ti nhit thp hn 911C, -Fe l vt liu st t in hnh nhit di 769C. T 911C n 1392C l Fe, mng A1. Nhit 1932C l nhit chuyn bin th hnh -Fe -Fe (mng A2 ging -Fe). S khc nhau v cu trc mng tinh th (s lng. Kch thc l hng.xem mc 1.3.5, 1.4.1.1, 1.4.1.2) cho php -Fe c kh nng ho tan c nhiu hn ng k cc nguyn t ca mt s nguyn t khc nhau (v d, 2,14% C (trng lng) 1147C) so vi -Fe (tng ng o,02%C(trng lng) 727C). chnh l c s ca cc cng ngh nhieeyj luyn nh ti, ram, ha nhit luyn nhm iu chnh tnh cht ca tho theo yu cu v iu kin lm vic c th (xem mc 3.3 v .3.4)

1.5 SAI LCH MNG TINH THCc cu trc tinh th trnh by trn gi l cu trc ca tinh th l tng v khi xen k b qua dao ng nhit v cc sai hng trong trt t sp xp ca cc nguyn t (ion, phn t). nhng sai hng c gi l sai lch mng tinh th hay khuyt tt mng v ni dung phn ny mun cp n iu . Mt s tnh cht ca vt rn tinh th nh mun Young (E), h s gin n nhit, tnh cht t,ch ph thuc ch yu vo dng v lc lin kt , m hu nh khng thay i do sai lch mng. Ngc li, i vi mt s tnh cht khc nh hnh vi ca tinh th di tc dugnj ngoi lc (bin cng, bin dng do, xem chng 4), dn nhit ca cc cht cch in, bn dn, th sai lch mng ng vai tr rt quan trng. Ph thuc vo kch thc theo ba chiu trong khng gian, sai lch mng chia thnh: sai lch im, ng, mt v khi. 1.5.1.sai lch im. l loi sai lch c kch thc rt nh (c kch thc nguyn t) theo ba chiu khng gian (hnh 1.28).Mt s sai lch in hnh l nt trng,nguyn t xen k,nguyn t tp cht.

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1.5.1.1.Nt trng v nguyn t xen k. Trong tinh th,nguyn t lun tham gia dao ng nhit quanh v tr cn bng ca nt mng. nhit T xc nh,nng lng dao ng ca mi nguyn t tun theo l thuyt thng k Maxwell_boltzman nn khng ging nhau.khi mt s nguyn t no c nng lng cao,vi bin dao ng ln.Chng c kh nng bt khi nt mng, li nt mng khng c nguyn t,gi l nt trng (hnh 1.28a,b).sau khi ri khi nt mng,nguyn t c th chuyn sang v tr gia cc nt (c ch to nt trng Frenkel) (hnh 1.28a)to ra sai lch im dng nguyn t xen k.C ch th hai gi l c ch to nt trng ca Schottky,khi nguyn t ri v tr cn bng ra b mt tinh th (hinh 1.28b). S xut hin nt trng v nguyn t xen k lun lm xut hin trng ng sut hnh cu (trng ng sut ko xung quanh nt trng v nn xung quanh nguyn t xen k).Nng lng to nguyn t xen k ln hn nhiu so vi to nt trng (trong kim loi tng quan ny vo khong 7:1) vi vy nng thc t nguyn t xen k t hn nhiu so vi nt trng (bng 1.7) v chng t nh hng n tnh cht ca mng tinh th. Mt nt trng c th biu din theo cng thc: Trong n,N _S nt trng v s nt mng; Q _ nng lng to nt trng; k _hng s Boltzman;[34]

T _ Nhit tuyt i. Nh vy nng nt trng ph thuc vo nhit theo hm m,tng nhanh theo nhit v c gi tr ln nht kim loi lng. Nt trng c nh hng ln n c ch v tc khuch tn ca kim loi (hp kim) trng thi rn .V vy c ngha quan trng thc t,v d,khi ng u ha thnh phn hp kim sau khi nu luyn hoc khi ha gi dung rn qu bo ha (xem 3.3 , 3.4).Ngoi ra n cng nh hng n chuyn ng (leo)ca lch trong qu trnh bin dng do nhit cao (hin tng do nhit cao,xem mc 4.5.4). 1.5.1.2.Nguyn t tp cht. Trong thc t khng th c vt liu hoc kim loi sch tuyt i.Cc cng ngh nu luyn hin i nht trong phng th nghim cng ch cho php t c sch khong 99,999% hoc cao hn mt cht .Ph thuc vo kch thc cc nguyn t tp cht c th thay th nguyn t nn nt mng (hnh 1.28d) hoc xen k gia cc nt (hnh 1.28c) (xen k ca nguyn t tp cht khc so vi cc nguyn t t xen k mc 1.5.1.1).y cng l sai lch im v lun km theo trng ng sut hnh cu xung quanh. Nguyn t xen k d khuch tn hn so vi nguyn t thay th v khng cn nt trng lm trung gian (xem mc 3.4). Cc nguyn t tp cht,trong iu kin xc nh c th tng tc v cn tr chuyn ng ca lch,v chng c nh hng n c ch,hnh thi qu trnh bin dng do,ha bn (xem mc 4.2 v mc 1.5.2). Trong tinh th ng ha tr,nguyn t tp cht nh hng n tnh cht in.V d,trong mng Ge,bn in t ha tr to lin kt gia cc nt,nu nguyn t tp cht As (nm in t ha tr) thay th nguyn t Ge,in t ha tr th nm (tha)ca As s c t do chuyn ng di tc dng in trng ngoi.Ngc li nu nguyn t tp cht l Ga (ba in t ha tr) th s thiu mt in t cho lin kt, chnh l c s ch to cc cht bn dn (dng p hoc n) t Ge hoc hay Si. Sai lch im trong mng ion lun mang tnh phc hp.Cu trc mng ion lun phi m bo trung ha in tch,v vy mt nt trng cation lun tn ti song somg vi mt nt trng anion bn cnh, l sai lch im Schottky (hnh 1.28e)hay mt cation xen k s cnh mt nt trng cation (hnh 1.28f): sai lch im Frenkel. Nng sai lch im trong mng ion nh hng n dn in ca vt rn ion. Cui cng cn ni thm rng,trong thc t th rt t khi s dng kim loi tinh khit m thng ch to hp kim bng cch ha tan mt nguyn t khc (tp cht c ) vo kim loi nn.Nguyn t ca nguyn t hp kim s sp xp theo cch xen k hoc thay th nh trn trong mng tinh th,to dung dch rn xen k hoc thay th (xem mc 2.2).

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1.5.2. Sai lch ng. Sai lch ng,v sau s c gi l lch (dislocation) l loi sai lch c kch thc nh (c kch thc nguyn t) theo hai chiu v rt ln theo chiu th ba trong tinh th. L thuyt lch l c s l thuyt bn,b phn quan trng ca vt l cht rn ni chung v vt l kim loi ni ring.Da vo l thuyt lch ngy nay c th gii thch nhiu vn v c tnh,l tnh ca kim loi v hp kim m trc y khng gii thch c bng l thuyt c in.Hn na l thuyt lch cho php m ra trin vng mi ,to ln trong vic ch to kim loi v hp kim c nhng tnh cht c bit (v d, bn cao,tnh siu do). 1.5.2.1.Nhng khi nim chung a) Lch bin C th hnh dung bng cch chn them mt na mt phng nguyen t ABCD vo na phn trn ca tinh th l tng (hnh 1.29a). S xut hin thm na mt lm cho cc mt phng nguyn t khc nm v hai pha tr nn khng hon ton song song vi nhau na.Na tinh th trn pha ng AB s chu mt ng sut nn, na di ng sut ko (hnh 1.29 a,b). ng AB c chiu di hng vn hng s mng,gi l trc lch,l bin gii pha trong ca na mt ABCD ,v vy gi l lch bin. Nu nga mt nm pha trn,lch bin c gi l lch dng k hiu l (hnh 1.29b) trong trng hp ngc li s c lch m, k hiu du T. Cng c th hnh dung lch bin c tp bng cch trt p m t trn hnh 1.29 nh sau: Ct tinh th theo mt phng P (hnh 1.29c),p phn pha trn mt P sao cho pha phi dch i mt on bng hng s mng,pha tri vn gi nguyn v tr c.Kt qu nhn c (hnh 1.29d) cng ging m hnh biu din hnh 1.29b, mt phng p,cha trc lch gi l mt trt.

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Hnh 1.29 M hnh to lch trong mng tinh th b)Lch xon C th hnh dung bng m hnh trt p (hnh 1.30) ct tinh th l tng theo mt na mt phng ABCD (hnh 1.30a),x dch hai mp ngoi ngc chiu nhau sao cho cc nguyn t mt ngoi x dch mt on bng hng s mng theo ng CD; nh vy cc nguyn t s sp xp li quanh AB theo ng xon c (hinh 1.30b), AB goi l trc ca lch xon.

Hnh 1.30 M hnh to lch xon (b) v s sp xp cc nguyn t xung quanh ng lch xon (c)[37]

Lch xon phi c k hiu ,nu ng xon c nguyn t xung quanh trc lch theo chiu kim ng h,ngc li s c lch tri.Mt ABCD mt trt ca lch xon.Xung quanh lch xon cng tn ti trng ng sut n hi.C th c so snh v tr cc mt phng tinh th trong mng l tng,cha lch bin.v lch xon trn hnh 1.31. c)Lch hn hp Trong thc t rt nhiu trng hp mt lch c c trng ca c lch bin v lch xon, l lch hn hp.Hnh 1.32 biu din cch to lch hn hp bng trt p.ng AC l trc lch ,cc thnh phn bin (CC)v xon (AA)cng nh m hnh sp xp nguyn t xung quanh trc lch c biu din trn hnh 1.32b.

d)c trng v hnh thi ca lch C th tng tng d dng rng do trt t sp xp nguyn t b ph v,yinh th xung quanh lch b bin dng,cng gn ng lch,bin dng cng ln.vng tinh th xung quanh ng lch (trc lch) vi bn knh c hng s mng gi l li lch (tm lch).vecto Bugers ( b) l vector c trng x lch mng,xc nh phn nng lng t do gia tng khi c lch (xem mc 1.5.2.2a). C th xc nh vecto Bugers nh sau:Trn mt ngoi tinh th l tng chn nt A bt k lm im xut pht,ln lt ni vecto t A n cc nt khc,theo chiu ty to mt vng vecto khp kn xung quanh ng lch gi l vong Bugers ( hnh 1.33a). Nu v li vng Bugers ny trn mt ngoi tinh th thc cha lch (hnh 1.32b,c) n s khng khp kn,vecto BA =b l vecto cn thit khp kn vng Bugers gi l vecto Bugers. Lch bin c vecto Bugers vung gc vi ng lch,cn vecto Bugers ca lch xon song song vi ng lch,vecto b ca lch hn hp s c c hai thnh phn. Ga tr ca vecto Bugers c tnh bng :[38]

Trong a hng s mng: U,v,w _ ch s phng tinh th cha b Nu : n =1,ta c lch n v n = s nguyn :lch hon chnh n # s nguyn :lch khng hon chnh.

Mt lch,k hiu p,l tng chiu di trc lch trong mt n v th tch ca tinh th,th nguyn .Bng 1.8 cho gi tri mt lch trong mt s kim loi v tinh th ion. 1.5.2.2.Nng lng v chuyn ng ca lch a) Nng lng ca lch (F) Nng lng ca lch l phn gia tng nng lng t do ca mng do trng bin dng n hi xut hin xung quanh lch,c tnh bng : Trong a h s c gi tr t 0,5 n 1: G _ mdun trt,c trng lin kt (kh nng chng li trt) ca mng tinh th b _ gi tr ca vecto Bugers L _ chiu di lch. T ( 1.49) c nhn xt : lch vi: Khc vi sai lch im,nng lng ca lch khng ph thuc vo nhit . E t l vi chiu di lch,v vy lun tn ti lc cng T tc dng theo ng

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Nhm ko dui thng ng lch,gim nng lng t do.Hiu ng ny c ngha quan trng trong tng tc gia lch v chng ngi khi n chuyn ng di tc dng ca ng sut bn ngoi (xem mc 1.5.5,4.2.3 v 4.7). - E t l vi bnh phng vecto Bugers (b) v vy lch vi vecto nb lun c xu hng phn li thnh n lch vi vecto bn gim nng lng t do v (nb) V d :-

=

+

hoc

+

=

+

..Kt qu ca cc phn ng lch phn

ly hoc tng hp c th to ra nhng li lch trong mng tinh th. Lch c th tng tc ln nhau (ht nhau khi khc du, y nhau khi cng du) v vy chng c th hu ln nhau (khi cng vect b nhng khc du) hoc to ra nhng tng lch, bc lch, vng lch. Cui cng, lch cng c tng tc vi cc loi khuyt tt khc (Xem mc 1.5.5) nn c nh hng n tnh do, bn ca tinh th.-

b) Chuyn ng ca lch Mt khc bit gia lch v khuyt tt im l lch cn bng v nhit ng km hn. V vy n lun c xu hng chuyn ng. Gi s c mt lch bin (hnh 1.34a) trong tinh th, di tc dng ng sut ngoi t,lch s chuyn ng theo c ch trt trn mt trt P. Lc na mt phng tha (A trn hnh 1.35a.d) s ln lt i ch cho nhng na mt khc ng trc n pha trn mt P, theo c ch tip sc (hnh 1.34b,e,f) cho n khi ra n mt ngoi, to mt bc bin dng bng gi tr b (hnh 1.34c.g) Ngoi trt nh trn, lch cn c th leo khi n thay i mt phng cha n (mt trt). Chuyn ng leo c ch xy ra vi lch bin. Gi s trong tinh th ch c lch, chng tn ti c lp, khng nh hng ln nhau v khng c cc khuyt tt khc th lc tc dng ti thiu (fth) cn cho lch chuyn ng (tinh th bt u bin dng do) l : fth fp = exp (9-. ) ( 1.51)

trong : b Vect Burgers G - M un trt - H s Poisson - khong cch gia cc mt nguyn t, song song vi mt trt b - khong cch gia cc nguyn t theo phng trt

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fp - lc Peierls-Nabarro, c ngha tng t lc ma st trong chuyn ng ca lch. T (1.51) thy rng theo mt v phng xp cht nht ca tinh th, ti t gi

tr max, fp l min, lch chuyn ng d dng nht, tinh th d bin dng nht (xem mc 4.2). T nhng ni dung trnh by,c th rt ra rng tc bin dng do () ph thuc vo chuyn ng ca lch. Theo Orowan, quan h nh sau : = .b. trong : - mt lch b gi tr vect Burgers - tc chuyn ng ca lch. Nhng khi nim c bn v lch va m t l c s ca l thuyt bn, cho php gii thch nhng tnh cht nh do, bn, ph hu, dos c cp t m chng 4. 1.5.3 SAI LCH MT Sai lch mt l loi sai lch c kch thc ln theo hai chiu v nh theo chiu th ba. Trong tinh th, sai lch mt ch yu l bin gii ht, bin gii siu ht, sai lch xp, mt i tinh v mt ngoi tinh th. 1.5.3.1. Bin gii ht v siu ht a) Bin gii ht (1.52)

Bin gii ht l vng tip gip gia cc ht trong a tinh th ( xem mc 1.6). Cc nguyn t bin gii ht khng sp xp theo trt t mng v c th coi l vng cu trc v nh hnh. Sc cng b mt bin gii ht () c trng phn gia tng nng lng t do vng bin gii so vi cc vng pha trong duy tr s cn bng nguyn t b mt. Hnh 1.35 l m hnh sp xp nguyn t trn bin gii ht (hnh 1.25a.b) v nh chp t vi bin giiht trong vt liu a tinh th (hnh 1.35c)[41]

Chiu dy bin gii ht ph thuc vo d sch ca kim loi hoc vt liu, c th t gi tr hng trm ln hng s mng. Kim loi cng tinh khit, bin gii ht cng mng. Do cu trc khng tinh th, tnh cht ca bin gii ht so vi cu trc tinh th nh sau : C nhit nng chy thp hn so vi vng bn trong, v vy nhit cao bin gii ht d b chy do, gy hin tng b nng.-

Hot tnh ho hc cao hn, d b n mn ho hc. Da vo c tnh ny, dng phng php tm thc pht hin bin gii ht trn knh hin vi quang hc hoc knh hin vi in t qut.-

Bin gii ht c tng tc mnh vi cc loi sai lch khc (nguyn t tp cht, nt trng lch), v vy qu trnh khuych tn y xy ra mnh hn so vi vng bn trong. Trong chuyn pha, bin gii ht l ni d to tm mm nht.-

Bin gii ht nh hng n in tr sut v n tn x in t dn. Ngoi ra, lch rt kh vt qua bin gii ht nhit thp v vy kim loi c kch thc ht cng nh (bin gii ht cng nhiu) in tr sut v bn cng cao ( xem mc 1.5.5.3)-

b) Bin gii siu ht Siu ht (block) l nhng vng tinh th nh (kch thc trung bnh 102 104), vi cu trc tinh th kh hon chnh nh hng lch nhau mt gc nh, ngn cch bng bin gii siu ht. Bin gii siu ht thc cht l nhng tng lch, to nn do cc lch cung du, tng tc ln nhau khi chng trt trn nhng mt trt song song (xem mc 1.5.2.2b) v gi nhau v tr cn bng ( hnh 1.36). Hai siu ht ln cn nh hng lch nhau mt gc : = Trong D l khong cch trung bnh gia cc lch bin cng du trn trng lch. Bin gii siu ht thng xut hin trong qu trnh kt tinh li (xem mc 4.2.6), c tc dng cn tr lch trt v tng tc vi cc loi sai lch im khc nn cng ng vai tr trong tnh do ca vt liu. 1.5.3.2. Sai lch xp, mt i tinh Sai lch xp (SLX), (cn gi l khuyt tt xp) l s ph v tr t xp cht ca tinh th ( xem mc 1.3.4.2). Gi s mng A1 c trt t sp xp l ABCABCABC, v l do no[42]

, trt t b ph v, v d tha mt lp xp cht c SLX tha ABAC(hnh 1.37a). Nu trong trt t sp xp thiu mt lp, s to SLX thiu (v d ABCABABC , hnh 1.37b) Sai lch xp c trng bng nng lng slx , gi l nng lng SLX (phn gia tng nng lng t do c SLX). SLX lun gii hn vi vng xung quanh bng nhng lch khng hon chnh (hnh 1.37c). iu kin cn bng, rng (d0) ca SLX l: d0= Trong : G mun trt h2.h3 vect Burgers ca cc lch khng hon chnh gii hn SLX slx nng lng SLX. SLX c vai tr rt ln trong lnh vc ho bn vt liu (xem mc 4.6), v vy mt trong nhng phng hng quan trng nng cao bn lhp kim ho, lm gim slx , tng rng d0 ,tc l tng cn tr chuyn ng ca lch bng SLX. Mt i tinh thung to thnh trong chuyn pha lng - rn hoc trng thi rn (xem chng 3) cng l mt dng ca SLX. V d, tinh th mng A1 vi tr t ABCACBAC(hnh 1.38) gm hai na tinh th hai pha mt A (lp A) i xng gng vi nhau ( hnh 1.38a). Nh vy chng s nh hng lch nhau mt gc xc nh. Mt A l mt i tinh, cng c tc dng cn tr chuyn ng ca lch. Trn t chc t vi, cc mt i tinh th hin bng cc ng song song (hnh 1.38b) 1.5.3.3. Cu trc mt ngoi tinh th Mt ngoi tinh th c hnh thi tn ti khc so vi nhng mt phng tinh th bn trong. C th tng tng d dng rng i vi cc nguyn t mt ngoi (mt gii hn tinh th) khng c s sp xp nh cc nguyn t bn trong, v vy mt ngoi chng khng sp xp theo trt t quy nh ca tinh th (hnh 1.39) v to thnh vng sai lch. Sc cng b mt tinh th (nng lng b mt) c trng phn gia tng nng lng t do gi cc nguyn t b mt trng thi n nh. Do c im cu trc sai lch cho nn lp mt ngoi tinh th cng c nhng tng tc c bit vi cc loi sai lch khc. 1.5.4 SAI LCH KHI Nhng sai lch c kch thc ln theo ba chiu trong mng tinh th gi l sai lch khi. Sai lch khi v m l nhng sai hng sinh ra[43]

,

khi nu, c hp kim nh r co, tp trung tp cht, x trong vt c. T kha cnh vi m c th k nhng sai lch khi nh cc pha th hai to thnh khi tit pha t dung dch rn. y l nhng sai lch c c to ra vi mc ch nng cao bn, ci thin tnh cht theo mun. Tu thuc vo cu trc, hnh thi tn ti, kch thc (hnh 1.40) m hiu qu ho bn ca nhng pha th hai ny s khc nhau (xem mc 1.5.5.4 v mc 4.7.3). 1.5.5. VAI TR CA SAI LCH I VI TNH CHT S c mt ca sai lch mng tinh th v tng tc gia chng nh hng n tnh cht ca vt liu. Vai tr ca chng c gii thiu s b khi m t tng loi trn. Quan h gia sai lch v cc tnh cht c hc, vt l v ha hc ca vt liu s c trnh by cc chng 4, 5 v 6. gii thch tnh cht c hc (tnh do, bn, cng )cn phi da vo l thuyt bn, trong cc m hnh cu trc vi m c p dng l gii kt qu thc t. Nhng vn s c cp chng4, y, mun nhn mnh mt s v d in hnh v tng tc gia lch vi nhau v vi cc sai lch khc trong hnh vi c tnh ca vt liu. 1.5.5.1. Tng tc gia cc lch v khi nim v ho bn Xt c ch bin dng do thng dng l trt ca lch (xem mc 4.2). mc gn ng, tc bin dng do c th xc nh theo biu thc (1.52). Trong bin dng s c hai hiu ng ngc nhau xy ra : Mt lch () tng do cc ngun khc nhau hot ng nh bin gii ht, ngun Frank Read (hnh 4.9b)lm tng .-

Mt khc khi tng, tng tc gia lch vi nhau v vi cc sai lch khc tng mnh, lm gim ng k, mun duy tr = const, cn tng ng sut bn ngoi mt i lng d, chnh l hiu ng ho bn bin dng s c xt mc 4.2.3. H s ho-

bn bin dng

c trng bi nghing ng cong trn gin th ko.

C s ca tt c cc phng php ho bn vt liu tinh th l lm gim tc chuyn ng () ca lch. Trt gp cc lch trong mt trt giao nhau khc, trn mi lch hnh thnh mt bc lch. Bc cn tr chuyn ng lch hoc khi cng chuyn ng vi lch s sinh ra khuyt tt im hoc cc vng lch pha sau n, gy ho bn ( hnh 1.41)

1.6. n tinh th v a tinh th1.6.1 n tinh th n tinh th (hnh 1.20a): l mt khi cht rn c mng ng nht (cng kiu v hng s mng), c phng mng khng i trong ton b th tch. Trong thin nhin: mt[44]

s khong vt c th tn ti di dng n tinh th . Chng c b mt ngoi nhn, hnh dng xc nh, l nhng mt phng nguyn t gii hn (thng l cc mt xp cht nht). Cc n tinh th kim loi khng tn ti trong t nhin, mun c phi dng cng ngh "nui" n tinh th . c im: c tnh cht rt c th l d hng v theo cc phng mt xp cht nguyn t khc nhau. n tinh th ch c dng trong bn dn. 1.6.2 a tinh th Trong thc t hu nh ch gp cc vt liu a tinh th. a tinh th gm rt nhiu (n) tinh th nh (c m) c gi l ht tinh th, cc ht c cng cu trc v thng s mng song phng li nh hng khc nhau (mang tnh ngu nhin) v li n kt vi nhau qua vng ranh gii c gi l bin ht (hay bin gii ht) nh trnh by hnh 1.20b. T m hnh thy r: - Mi h t l mt khi tinh th hon ton ng nht, th hin tnh d hng. - Cc ht nh hng ngu nhin vi s lng rt ln nn th hin tnh ng hng - Bin ht chu nh hng ca cc ht xung quanh nn c cu trc trung gian v v vy sp xp khng trt t (x lch) nh l v nh hnh, km xt cht vi tnh cht khc vi bn thn ht. - C th quan st cu trc ht a tinh th hay cc ht nh knh hin vi quang hc (h nh 1.20c). + ht ca tinh th ht c th quan st nh tnh qua mt gy, chnh xc phi xc nh trn t chc t vi. Cp ht theo tiu chun ASTM: phn thnh 16 cp chnh nh s t 00, 0, 1, 2...., 14 theo trt t ht nh dn, trong t 1 n 8 l thng dng. Cp ht N=3,322lgZ+1, vi Z l s h t c trong 1inch2 (2,542 6,45cm2) di phng i 100 ln. Ngi ta thng xc nh cp ht bng cch so snh vi bng chun phng i (thng l x100) hoc xc nh trn t chc t vi. Bin gii siu ht

[45]

Nu nh khi a tinh th gm cc h t (kch thc hng chc - hng trm m) vi phng mng lch nhau mt gc ng k (hng chc ), n lt mi h t n cng gm nhiu th tch nh hn (kch thc c 0,1 10m) vi phng mng lch nhau mt gc rt nh ( 1-2o) gi l siu h t hay block (hnh 1.20d). Bi n gii siu h t cng b x lch nhng vi mc rt th p. 1.6.3 Textua

H nh 1.22. M hnh textua trong dy nhm sau khi ko si (vect V biu th hng ko, trc textua l [111]). Do bi n dng d o lm phng mng nh hng to nn textua. V d, khi ko si nhm (hnh 1.22), tinh th hnh tr khi c, khi ph. Cu trc a tinh th c textua vt liu c tnh d hng. ng dng cho thp bin th, tnh cht t cc i theo chiu textua, cc tiu theo phng vung gc gim tn tht.

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Chng 4 TNH CHT VT L CA VT LIU 4.1. TNH CHT IN4.1.1. S DN IN 4.1.1.1. dn in Mt trong nhng c tnh in quan trng nht ca vt liu rn l kh nng dn in ca n. Hnh 5.1 nu s b tr th nghim o dn in. Theo nh lut Ohm, cng dng in I lin h vi in p ( hiu s in th ) U theo biu thc: U= I R ,( 5.1) Trong R- in tr ca mu vt li m dng in chy qua. Gi tr ca R ph thuc vo hnh dng mu o v i vi nhiu loi vt liu, khng ph thuc vo cng dng in in tr sut biu thc: khng ph thuc v hnh hc ca mu o nhng lin h vi R qua = ( 5.2)

Trong l - khong cch gia hai im o in p; s- tit din vung gcvi hng dng in dn in l nghch o ca in tr sut: l: ( )-1 ( 5.4) (5.3)

Biu th kh nng dn inca vt liu. n v o phn : J=

Cng vi phng trnh ( 5.1 ), nh lut Ohm cn c th biu din di dng vi Trong J= I/S- mt dng in, tc l dng in i qua mt n v din tch tit din mu, cn cng in trng. E= (5.5)

Cc vt liu rn c dn in tri rng trn 27 c s , c l khng c mt tnh cht vt l no khc c c di bin i rng nh th!. Thc vy, cn c vo kh nng dn in cc vt liu rn c phn thnh 3 loi: dn in, bn dn v in mi ( cch in . Cc kim loi l nhng cht dn in tt, thng c dn in c 107 ( liu c dn in trung gian, ni chung t 10-6 n 104 ( cht bn dn. )-1 . Cc vt )-1), c gi l nhng

Dng in to thnh do chuyn ng ca cc ht mang in tch di tc dng lc ca mt in trng ngoi t vo. Cc ht mang in dng c gia tc theo hng ca[47]

in trng , cn cc ht mang in m th theo hng ngc li. Trong phn ln cc vt liu rn, dng in c ti bi cc dng in t, l s dn in bng in bng in t. Ngoi ra, trong cc vt liu ion, s chuyn ng thun ty ca cc ion cng c th to ra dng in, gi l s dn in bng ion. Di y ta ch ni s dn in bng in t, cn dn in bng ion s c nu tm tt trong mc 5.1.3 4.1.1.2. Cu trc vng nng lng trong cc vt rn Trong tt c cc cht dn in, bn dn v trong nhiu vt liu in mi, ch tn ti s dn in bng in t v dn in ny ph thuc mnh vo s lng cc in t c kh nng tham gia dn in. Tuy nhin khng phi mi in t trong nguyn t u u c gia tc khi c mt in trng. Trong mt loi vt liu cho, s in t c kh nng dn in lin quan ti s xp xp cc trng thi in t ( hay l mc nng lng in t ) v cn vi cch thc m cc in t chim lnh trong cc trng thi . S kho st c bn v cc vn ny kh phc tp, cn vn dng nguyn l ca c hc lng t, iu vt qua khi khun kh ca cun sch ny. Cch trnh by di y b qua mt s khi nm n gin v n gin ho mt s khi nim khc. Trong ring mi nguyn t tn ti cc mc nng lng gin on. Cc in t sp xp thnh cc tng ( cng vi cc s lng t chnh m= 1,2,3... ) v cc lp c ch bi cc ch s, p, d, v f (ng vi cc s lng t qu o ). Trong mi lp li c tng ng 2 +1= 1.3,5 v 7 qu o khc nhau. Trong s phn t, in t chim lnh, cc trng thi c nng lng thp nht, c hai c spin i song chim mt: qu o. Ph hp vi nguyn l loi tr. Cu hnh lng t ca mt nguyn t c lp biu th sp xp in t vo cc trng thi cho php. Ta hy ngoi suy mt s quan h quan nim cho cc vt liu rn. Mt cht c th xem nh cu to bi mt s ln, gi s l N nguyn t, c a vo sp xp vi nhau c trt t trong mt mng tinh th. nhng khong cch tng i xa, mi nguyn t l c lp vi cc nguyn t khc v s c mc nng lng trong nguyn t v cu hnh in t ging nh nguyn t ng c lp. Tuy nhin, khi cc nguyn t cng xch li gn nhau th cc in t cng b kch thch ( hay l nhiu lon) bi cc in t v cc ht nhn ca cc nguyn t ln cn. nh hng ny lm cho mi mt trng thi in t trong nguyn t ring bit b phn tch thnh mt lot cc trng thi in t nm st nhau, hnh thnh nn mt vng nng lng in t. S gin ( m rng ) t mt mc nng lng in t trong nguyn t hnh thnh mt vng nng lng trong vt rn tu thuc vo khong cch gia cc nguyn t, s gin ny bt u t cc in t ngoi cng ca nguyn t bi v chng b nhiu lon trc tin khi cc nguyn t lin kt li vi nhau ( hnh 5.2 ). Trong mi vng cc mc nng lng vn l gin on, tuy nhin khong cch gia cc mc k nhau l ht sc nh. khong cch nguyn t cn bng, s to thnh vng nng lng c th xy ra vi cc lp in t gn ht nhn nht, nh minh ho trn hnh 5.2 b. Ngoi ra gia cc[48]

vng k nhau c th tn ti nhng khe nng lng ( hay cn gi l nhng vng cm): bnh thng th cc in t khng c php chim lnh nhng mc nng lng nm trong cc khe ny. Cch thc thng thng biu din cu trc vng nng lng in t trong vt rn ch nh trn hnh 5.2a. S cc trng thi nng lng in t ( hay l s mc nng lng in t) trong mi vng s bng tng s tt c nhng trng thi do N nguyn t ng gp. Chng hn vng s s chim N trng thi, cn vng p cha 3N trng thi , mi mt trng thi nng lng in t c th cha c hai in t, chng phi c spin i song. Khi xp tt c cc cc in t vo mc nng lng s xut hn cc vng cn trng v cng c th xut hin nhng vng ch b lp y mt phn. Cc tnh cht in ca vt rn ph thuc vo cu trc vng nng lng in t ca n, c th l vo s sp xp cc vng ngoi cng v cch thc lp y chng bi cc in t . Theo quan im ny, vng cha cc in t c nng lng cao nht ( hay cc in t ho tr ) c gi l vng ho tr . Cn vng dn s l vng c nng lng cao hn k trn m trong a s cc trng hp, v c bn l b trng. nhit 0K, c th c bn kiu cu trc vng khc nhau. loi th nht ( hnh 5.3a ). Vng ho tr ch mi b lp y mt phn . Nng lng ng vi mc cao nht b chim 0K c gi l nng lng Fermi EF. y l cu trc vng in hnh cho cc kim loi, c bit l cc in t kim loi c mt ho tr s ng n l ( v d ng ). Mi nguyn t Cu c mt in t 4s . Vng 4s c kh nng cha ti 2N in t. Nh vy ch mi c mt na s mc nng lng c trong vng ho tr 4s l b lp y. loi cu trc vng th hai, cng tm thy trong cc kim loi ( hnh 5.3b. ) vng ho tr b lp y cn ph ln c vng dn: vng ny nu nh khng b ph th hon ton cn trng. Magi c cu trc vng kiu ny. Mi nguyn t Mg c hai in t ho tr 3s nhng khi to thnh vt rn th cc vng 3s v 3p ph ln nhau. Trong trng hp ny 0K, nng lng Fermi c chp nhn m mc di c N nguyn t, N trng thi b chim, c hai in t cha mt trng thi. Hai cu trc vng cui cng tng t nh nhau: mi cu trc tt c cc trng thi trong vng ho tr u b in t chim ht. Tuy nhin y khng c s dn ph vi vng cn trng; iu ny to ra mt khe nng lng xen gia. i vi cc kim loi rt tinh khit, cc in t khng th c nng lng trong khe ny. S khc nhau gia hai cu trc vng l ln ca khe nng lng. i vi vt liu cch in, khe vng tng i rng ( hnh 5.3.c ). Nng lng Fermi i vi hai cu trc vng ny nm chnh gia khe. 4.1.1.3. Tnh cht dn in lin quan vi m hnh vng v lin kt nguyn t . Ch c nhng in t no c nng lng ln hn mc Fermi th mi chu tc dng v c gia tc khi c mt in trng. y l nhng in t tham gia vo qa trnh dn[49]

in. Chng c gi l nhng in t t do. Trong cc cht bn dn v cht cch in cn tm thy mt thc th in t khc gi l l trng, c nng lng thp hn EF v cng tham gia dn in bng in t. Nh sau ny s thy dn in l mt hm s trc tip ca s in t t do v l trng, thm na s phn bit gia cc cht dn in v khng dn in ( cch in v bn dn ) nm cc ht ti in (in t t do hoc l trng ). Kim loi cho in t tr nn t do, cn phi kch thch n ln mt trong nhng trng thi nng lng cho php v cn trng trn EF . i vi cc kim loi c cu trc vng nh hnh 5.3a hoc 5.3b sn c mc nng lng trng nm st k ngay mc b chim cao nht ti EF . Do vy ch cn mt nng lng rt nh a in t ln cc trng thi trng nm di nh nu trn hnh 5.4. Ni chung nng lng do mt in trng cung cp l kch thch c mt s ln in t ln nhng trng thi dn in ny. Theo m hnh lin kt kim loi, tt c in t ho tr u c th chuyn ng t do, chng to thnh kh in t phn b ng u khp trong mng li cc li ion dng. Mc d nhng in t ny khng b lin kt nh x vi bt c mt nguyn t ring r no, chng vn phi nh mt kch thch nht nh mi c th tr thnh in t dn thc s t do. Khi ch c mt b phn b kch thch cng gy nn mt s tng i ln in t t do v hu qu c mt dn in cao. Cch in bn dn i vi cc cht cch in v cht bn dn, khng c nhng trng thi cn trng nm ngay bn trn ca vng ho tr y. Do tr thnh t do cc in t phi c nng mc vt qua khe vng nng lng, nhy vo trng thi cn trng vng dn. iu ny ch c th xy ra bng cch cp cho in t mt nng lng bng hiu nng lng gia hai trng thi xp x bng nng lng khe E g .Qu trnh kch thch ny c biu din trn hnh 5.5 . i vi nhiu vt liu, khe vng ny rng vi Electronvon. Thng nng lng kch thch l t ngun phi in nh nhit hoc nh sng. S cc in t b kch thch nhit chuyn ln vng dn ph thuc vo b rng khe v nhit . Ti mt nhit cho Eg cng ln th sc xut chuyn in t ln mt trng thi nng lng nm trong vng dn cng thp. kt qu l c t in t dn hn. Vng cng rng th dn in cng thp. Nh vy s phn bit gia cc cht bn dn v cc cht cch in l ch rng ca cc khe nng lng: i vi cc cht bn dn khe ny l hp , cn trong cc cht cch in th li kh rng. Tng nhit ca cht bn dn hoc cht cch in u lm tng nng lng nhit kch thch in t. Nh vy c nhiu c nhiu in t c nng ln vng dn, iu ny lm tng dn in.

[50]

dn in ca cc cht cch in v cc cht bn dn cng c th c nhn nhn t gc m hnh lin kt nguyn t. i vi cc vt liu cch in, lin kt gia cc nguyn t l lin kt ion hoc lin kt ng ho tr mnh. Nh vy cc in ho b gn cht hoc l phn chia vo cc nguyn t ring l. Ni cch khc, cc in t nh x cao v khng th no t do di chuyn trong khp tinh th c. Cn lin kt trong cc bn dn l ng ho tr ( hoc chim u th ng ho tr ) v tng i yu, c ngha cc in t ho tr khng lin kt mnh vi nguyn t. Hu qu l cc in t ny d b ly i bi kch thch nhit hn l cc in t ho tr trong cht cch in. 4.1.1.4. linh ng ca in t Di tc dng lc ca in trng t vo, tt c mi in t t do u thu c gia tc theo hng ngc vi hng in trng . Theo c hc lng t trong mng tinh th hon ho s khng xy ra tng tc gia in t ang gia tc v cc nguyn t. Trong iu kin , tt c mi in t t do c gia tc chng no in trng cn tc dng, do s gy ra s tng lin tc ca dng in theo thi gian. Tuy nhin ta li thy dng in t ti mt gi tr khng i ngay sau khi t in trng, nh l c mt lc ma st chng li s gia tc bi in trng ngoi. Nhng lc ma st ny l kt qu ca s tn x ca cc in t bi cc sai lch mng tinh th, bao gm cc nguyn t tp cht, l trng, cc nguyn t xen k v ngay c cc dao ng nhit ca chnh bn thn cc nguyn t. C mi mt ln tn x li lm cho in t mt ln mt ng nng v thay i hng chuyn ng ca mnh, nh minh ho trong . Tuy nhin vn c mt s chuyn ng nht nh ca in t theo hng ngc ca in trng v dng in tch ny chnh l dng in. Hin tng tn x c biu th nh l lc cn i vi s i qua ca dng in. C nhiu thng s c dng m t mc tn x, trong c tc y v linh ng ca in t. Tc y vd biu th tc trung bnh ca in t theo hng lc tc dng ca in trng t vo. N t l thun vi in trng: vd = e . ( 5.6 )

hng s t l e c gi l linh ng in t, c th nguyn m2/V.s. dn in ca a s cc loi vt liu c th biu th bng : = n | e| e. ( 5.7 ).

y n-s in t t do tc n t dn trong mt n v th tch; |e|- gi tr tuyt i ca din tch mt in t (1,6.10-9C) Nh vy d dn n t l vi s in t t do v linh ng in t.

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4.1.1.5 in tr ca kim loi Nh ni trn, a s cc kim loai l nhng cht dn in rt tt; dn in nhit phng ca nhiu kim loi ph thng nht c cho trong bng 5.1. Bng 5.1. dn in nhit phng ca cc kim loi v cc hp kim ph thng Kim loi Bc ng Vng Nhm St ng thau (70Cu 30Zn) Thp ccbon tm Thp khng g dn in (.m)-1 6,8.107 6,0.107 4,3.107 3,8.107 1,0.107 1,6.107 0,6.107 0,2.107

S d kim loi c dn in cao l v chng c mt s ln in t t do c kch thch ln cc trng thi trng nm trn mc Fermi. Nh vy n trong trong biu thc (5.7) c gi tr ln. tin trnh by, di y s dng khi nim in tr sut,nghch o ca dn in.Cc khuyt tt mng tinh th chnh l nhng tm tn x i vi in t dn trong kim loi, bi vy s khuyt tt tng ln s nng cao in tr sut (tc h thp dn in). Nng cc khuyt tt ny ph thuc vo nhit ,thnh phn v tc kt tinh ca mu kim loi.Qua thc nghim chng t rng cc c ch tn x l c lp v in tr sut ton phn ca mt kim loi l tng cc ng gp t dao ng nhit , tp cht v bin dng n hi: ( Ton phn )= t +i +d. ( 5.8 )

Trong t, , i v d ln lt l in tr sut ring ca nhit, tp cht v bin dng..Phng trnh (5.8) i khi cn c gi l quy tc Mathiessen. Hnh 5.7 nu nh hng ca cc tham s ti n tr sut ton phn. nh hng ca nhit nhit khong trn 2000C, in tr sut ca kim loi nguyn cht v tt c cc hp kim ng- niken cho trn hnh 5.7 tng tuyn tnh theo nhit . Nh vy t = 0 + aT, ( 5.9 )

[52]

Trong 0 v a- l cc hng s cho ring tng kim loi. s ph thuc ny ca thnh phn in tr sut vo nhit l do tng theo nhit ca cc dao ng nhit v cc khuyt tt khc trong mng ( v d, cc nt trng ) c xem l nhng tm tn x in t. nh hng ca tp cht Khi pha thm mt tp cht n to dung dch rn, in tr sut tp cht i lin h vi nng tp cht Ci ( tnh bng phn trm nguyn t ) nh sau : i = A Ci (1- Ci ). Trong A - Mt hng s khng ph thuc thnh phn nhng ph thuc c tp cht v kim loi nn. Hnh 5.8 cho thy nh hng ca km ln in tr nhit phng ca dung dch rn Cu Zn (pha ). Cc nguyn t Zn trong Cu tc dng nh nhng tm tn x, do s nng cao in tr sut. tnh gn ng in tr sut ca mt kim loi gm hai pha v c th s dng biu thc theo ng quy tc hn hp sau y: i = V + V (5.11)

Trong VV v - phn trm th tch v in tr sut ring phn ca cc pha tng ng. nh hng ca bin dng n hi Bin dng n hi cng lm tng in tr sut do tng s lng cc tn x in t. nh hng ca bin dng ln in tr sut c biu th trn hnh 5.7 5.1.1.6. c trng in ca cc hp kim thng mi. Tnh cht in v cc tnh cht khc ca ng khin n tr thnh vt dn kim loi c dng rng ri nht. ng OFHC c hm lng oxy v tp cht khc rt thp v dn in cao c dung cho nhiu ng dng in. Nhm c dn ch bng khong mt na ca ng, cng thng c dung lm vt dn in. Bc c dn in cao hn c nhm v ng, tuy nhin ng dng b hn ch v gi t. C khi cn phi ci thin bn c hc ca hp kim m khng lm gim ng k dn in ca n. To hp kim dung dch rn v bin dng ngui, c hai cch u c th ci thin bn nhng li gy tn hi dn in. Nh vy cn c s tho hip gia hai tnh cht m khng lm gim mnh tnh dn in. V d, cc hp kim ng berili bn v cng, nhng dn in ch gim chng nm ln so vi ng tinh khit cao.

[53]

Trong mt s ng dng khc, chng hn nh dy hoc thanh nung trong l, li cn c in tr cao, tn hao nng lng do cc in t b tn x c chuyn thnh nng lng nhit. Cc vt liu nh th cn phi c bn chng xy ho nhit cao v ng nhin c nhit nng chy cao. Nicrm (hp kim niken crm) c s dng ph bin trong cc linh kin l nung. 4.1.2. Bn dn in Cc vt liu bn dn c dn in khng cao nh cc kim loi, nhng chng li c mt s c trng in c o nn c ng dng rng ri. Tnh cht in ca cc vt liu ny ht sc nhy cm vi s c mt ca cc tp cht ngay c nng thp. Cc cht bn dn, trong tnh cht in c quy nh bi cu trc in t vn c trong vt liu tinh khit, c gi l bn dn tinh khit. Cn khi cc c trng in do cc nguyn t tp cht quyt nh th vt liu c gi l cht bn dn tp cht. 4.1.2.1. Bn dn tinh khit. Cc cht bn dn tinh khit c c trng bi cu trc vng in t nu trn hnh 53d. OK hai cht bn dn nguyn t Silic (Si) v genmani (Ge) c khe nng lng tng ng vo c 1,1 v 0,7 eV. C hai u nm trong nhm IV A ca bng tun hon cc nguyn t v u c lien kt ng ho tr. Ngoi ra, nhiu hp cht cng c bn cht bn dn tinh khit. Mt nhm vt liu nh th c hnh thnh gia cc nguyn t ca cc nhm III A v V A, v d nh: gali asenit (GaAs) v ini antimonite (InSb); chng thng c gi l cc tp cht III V. Cc hp cht ca cc nguyn t nhm II B v VI A cng biu hin tnh bn dn, trong c catmi sunfit (CdS) v km telurit (ZnTe). Khi hai nguyn t to thnh nhng hp cht ny nm v tr cch nhau cng xa trong bng tun hon th lin kt nguyn t cng tr nn ion hn v ln ca nng lng khe tng ln. Vt liu c xu hng tr nn cch in hn. Bng 5.2 cho bit khe nng lng ca mt s hp cht bn dn. Khi nim v l trng. Trong cht bn dn tinh khit c khi mi in t c kch thch ln vng dn s li mt ch thiu in t trong lin kt ng ho tr, tc l mt trng thi in t cn trng trong vng ho tr nh ch trn hnh 5.5b. C th tng tng rng, di nh hng ca in trng, v tr thiu in t ny s chuyn ch trong mng tinh th do chuyn ng ca in t ho tr khc ti, in vo mi lin kt cha (hnh 5.9) Qu trnh ny xy ra trong min ho tr nh l chuyn ng ca mt ht mang in dng gi l l trng . Mt l trng c xem nh c in tch vi ln bng in tch ca mt in t nhng ngc du (+ 1,6.10-19C). Nh vy, khi c mt in trng, cc in t v cc l trng u b tn x bi nhng ch khng hon ho trong mng tinh th. dn in tinh khit[54]

V trong cht bn dn tinh khit c hai loi ht ti in (in t t do v l trng). Cho nn biu thc (5.7) ca dn in phi c bin i bao gm c s hng ng gp ca dng l trng. Do ta c: = n e + p b (5.12)

Trong : p - s l trng trong n v th tch b- linh ng l trng i vi cc cht bn dn, ln ca b lun lun nh hn e . Trong cht bn dn tinh khit, c mi mt in t c kch thch nhy qua khe n li mt l trng trong vng ha tr, nh vy: n=p v =n e+ b) = p (e + b) (5.13) (5.14)

dn in tinh khit, linh ng in t v linh ng l trng nhit phng ca mt s vt liu bn dn c trnh by trn bng 5.2Bng 5.2. Khe nng lng, linh ng in t v l trng v dn in tinh khit nhit phng cu mt s vt liu bn dn. Vt liu Nguyn t Si Ge Hp cht III V GaP GaAs InSb Hp cht II VI CdS ZnTe Khe nng dn in, linh ng linh ng l lng eV in t, m2/V.s trng m2/V.s (.m)-1 1,11 0,67 2,25 1,42 0,17 2,40 2,26 4x10-4 2,2 10-5 2x104 0,14 0,38 0,05 0,85 7,7 0,03 0,03 0,05 0,18 0,002 0,45 0,07 0,01

4.1.2.2. Bn dn tp cht Thc ra cc cht bn dn thng mi u l bn dn tp cht. Khi c mt tp cht ngay c vi nng thp cng to ra in t hoc l d tha. Chng hn, mt nng tp cht 10-12 (tc mt nguyn t tp cht trong 1012nguyn t) cng lm cho silic tr thnh bn dn tp cht nhit phng. Bn dn tp cht kim loi n minh ho tnh bn dn tp cht hnh thnh nh th no, ta li xt bn dn nguyn t silic. Mt nguyn t silic c bn in t vng ngoi, mi mt in t ny lin kt ng ho tr vi mt in t ca bn nguyn t silic ln cn. By gi gi s mt[55]

nguyn t tp cht c ho tr nm c thay th vo, c th l nguyn t ca nhm V A bng tun hon (v d, P, As, Sb). Ch c bn trong nm in t ho tr ca nhng nguyn t tp cht ny c th tham gia vo lin kt. Hnh 5.10. M hnh lin kt in t ca bn dn tp cht loi n Mt in t tha ra ch nh theo mt cch lng lo vo vng xung quanh nguyn t tp cht bi mt lc ht tnh in yu nh minh ho trn hnh 5.10a. Nng lng lien kt ca in t ny tng i nh (vo c 0,01 eV), do d b tch khi nguyn t tp cht, khi n s tr thnh in t t do, tc in t dn (hnh 5.10b v 5.10c) Mi mt in t gn lng lo chim mt mc nng lng n nm trong khe cm v ngay di y vng dn (hnh 5.11a). kch thch in t nhy t mt trong cc trng thi (mc) tp cht ny ln mt mc trong vng dn i hi mt nng lng tng ng vi nng lng lin kt in t. V mi ln kch thch (hnh 5.11b) s cp mt in t n vo vng dn. Tp cht loi ny c gi l n (tp cht cho). Bi v mi in t n c kch thch t mt mc tp cht nn khng to ra l trng tng ng trong vng ho tr. nhit phng, nng lng nhit cng kch thch c mt s ln in t ri khi cc trng thi n. Thm vo cn xy ra mt s chuyn di vng ho tr vng dn, nh trn hnh 5.5bl; nhng mc c th b qua. Nh vy s in t trong vng dn vt xa s l trng trong vng ho tr (tc n >>p), v s hng th nht v phi ca phng trnh (5.12) ln t s hng th hai, ngha l: n e (5.15)

Vt liu loi ny c gi l bn dn tp cht loi n. Do nng ca chng, cc in t l nhng ht ti a s, cn l trng l nhng ht ti thiu s. i vi cc cht bn dn loi n, mc Fermi dch ln pha trn ti khu vc trng thi n; v tr chnh xc ca n ph thuc vo c nhit ln nng n. Bn dn tp cht loi p Mt hiu ng i lp xut hin khi pha vo silic hoc gecmani cc tp cht thay th c ho tr ba nh Al, B v Ga thuc nhm III A ca bng tun hon. Mt trong cc mi lien kt ng ho tr xung quanh mi nguyn t ny s b thiu mt in t. Ch thiu nh th c th xem nh mt l trng lin kt yu vi nguyn t tp cht. C th gii phng l trng ny khi nguyn t tp cht bng cch iu chuyn mt in t t mt mi lin kt cnh nh minh ho trn hnh 5.12. Thc cht l in t v l trng i ch ln nhau. Mt l trng chuyn ng c xem nh l trng thi kch thch v tham gia vo qu trnh dn in tng t mt in t n b kch thch, nh m t trn. Hnh 5.12. M hnh (lin kt in t) cc bn dn tp cht loi p

[56]

Cng c th dng m hnh vng nng lng din t cc kch thch tp cht to ra l trng. Mi mt nguyn t tp cht a vo khe cm mt mc nng lng nm rt st pha trn nh ca vng ho tr (hnh 5.13a). Mt l trng s c to ra trong vng ho tr khi kch thch nhit do mt in t chuyn t vng ho tr ln trng thi in t tp cht ny nh biu th trn hnh (5.13b). Vi mt chuyn di nh th ch c mt ht ti (mt l trng) c sinh ra trong cng ho tr m khng c mt in t t do no c to ra hoc mc tp cht hoc trong vng dn. Tp cht loi ny c gi l acxept (tp cht nhn, bi v n c kh nng nhn in t t vng ho tr v li mt l trng sau ny gi mc nng lng trong khe cm do loi tp cht ny dy ra l trng thi (mc) acxept. i vi bn dn tp cht loi ny, l trng c mt vi nng cao hn nhiu so vi in t (tc l p>>n), v trong trng hp vt liu c gi l cht bn dn loi p bi v tnh dn in ch yu do cc ht tch in dng m nhim. tt nhin, cc l trng l ht ti a s cn in t ch xut hin nng thiu s. iu lm cho s hng th hai trong v phi ca phng trnh (5.12) chim u th, tc l: p b (5.16)

i vi cc cht bn dn loi p, mc Fermi nm trong khe cm v gn mc acxept. Cc cht bn dn tp cht (c hai loi n v p) u c ch to t cc vt liu ban u c tinh khit ht sc cao, ni chung c tng hm lng tp vo c 10-7% nguyn t. Sau , bng cch s dng nhng k thut khc nhau, ngi ta ch nh pha them nhng nng n hc acxept c th. Quy trnh hp kim ho vt liu bn dn nh vy c gi l ping (s pha tp) Trong cc cht bn dn tp cht, s ln ht ti (hoc in t hoc l trng, tu thuc loi tp cht) c to ta nhit phng do nng lng nhit cung cp. H qu l cc cht bn dn tp cht c dn in nhit phng tng i cao. a s cc vt liu ny c s dng trong cc linh kin in t hot ng trong iu kin mi trng xung quanh. 4.1.2.3. S thay i nhit ca dn in v nng ht ti in. Hnh 5.14 trnh by th s ph thuc nhit dn in ca silic tinh khit v ca silic pha tp vi 0,0013 v 0,0052% nguyn t bo. dn in trong mu tinh khit tng mnh theo nhit . S in t v s l trng u tng ln theo nhit bi v c nhiu nng lng nhit hn kch thch cc in t chuyn t vng ho tr ln vng dn. Nh vy, gi tr ca c n v p trong biu thc dn tinh khit (5.14) u tng ln, cn linh ng in t v l trng th li gim nh khi tng nhit do s tng tn x in t v l trng bi dao ng nhit.[57]

Tuy nhin, s gim ev b rt nh so vi s tng n v p, nn hiu q ca s tng nhit vn l s tng dn in. Biu thc ton hc gn ng s ph thuc dn tinh khit vo nhit T l: ln C(5.17)

Thng s C l mt hng s, khng ph thuc nhit v Eg v k ln lt l nng lng khe v hng s Boltzmann. S ph thuc nhit ca nng ht ti ca bn dn tinh khit trn thc t cng ging nh ca dn in, tc l: ln n ln p C (5.18)

Thng s C cng l mt hng s, khng ph thuc nhit . Nh phng trnh (5.18) s c mt phng php khc biu din s ph thuc nhit ca dn in ca cc cht bn dn nh trn hnh 5.15 vi cc s liu ly t hnh 5.14. Hy ch ti on ng thng gy bi vt liu tinh khit c th nhanh chng xc nh c nng lng khe. Theo phng trnh (5.18), dc ca on thng ny l bng Eg/2k, tc l Eg c th c xc nh nh sau: Eg = - 2k = -2k (5.19)

iu ny c ch r bng th trn hnh 5.16. Mt c trng quan trng khc ca cc quan h nu trn hnh 5.14 v 5.15 l cc nhit di khong 800K (5270C) cc vt liu pha tp bo u l bn dn tp cht loi p; ngha l trn thc t tt c l trng ti in u c c t cc kch thch tp cht cc chuyn di in t t vng ho tr ln cc mc acxept bo, li cc l trng trong vng ho tr (hnh 5.13). Nng lng nhit c c cc nhit l thc hin mt s ng k nhng kch thch ny nhng li cha kch thch c nhiu in t t vng ho tr vt qua khe vng. Nh vy, dn tp cht vt xa dn ca vt liu tinh khit. V d, 400K (1270C) dn in ca silic tinh khit v ca vt liu pha 0,0013 % nguyn t bo tng ng c 10-2 v 600 (.cm)-1(hnh 5.14). S so snh ny cho thy dn in nhy vi ngay c nhng nng cc k nh ca mt s nguyn t tp cht. Hn na, dn tp cht cng nhy vi c nhit , nh ch trn hnh 5.14 cho c hai vt liu pha them bo. Bt u t khong 75K (-2000C). Trc tin dn tng theo nhit , t n mt cc i v sau li gim dn trc khi tr thnh dn tinh khit. Ni theo thut ng nng ht ti (tc l trng, hnh 5.15), lnp lc u tng tuyn tnh khi 1/T gim (tc T tng) c th xy ra s lng ln cc kch thch tp cht ngay nhng

[58]

nhit tng i thp ny bi v mc acxept nm st ngay trn nh ca vng ho tr, khi nhit tng hn na (1/T gim), nng l trng c khi li tr nn c lp vi nhit (hnh 5.15). im ny thc t tt c mi nguyn t bo u nhn c in t t vng ho tr. Tc chng bo ho theo thut ng vng bo ho trn hnh 5.16 (i vi cc tp cht n ngi ta ni chng tr nn cn kit thay cho bo ho). S l trng trong vng ny gn bng s nguyn t tp cht pha vo (tc bo). S gim dn kh tng nhit trong vng bo ho i vi hai ng cong tp cht trn hnh 5.14 c th gii thch bng s gim linh ng l trng. Trn biu thc dn tp cht (5.16), c n v p trong vng ny u c lp vi nhit v s ph thuc nhit y ch l do linh ng. Trn hnh 5.14 v 5.15 cng cn mt iu ng ch na l khong 800K (527 C), dn in ca c hai vt liu pha tp bo u tr v dn tinh khit. ch bt u c tnh bn dn tinh khit, s cc chuyn di tinh khit t vng ho tr ln vng dn tr nn ln hn s l trng do tp cht sinh ra.0

Vi nhn xt cui cng v nh hng ca hm lng acxept bo i vi tnh cht in ca silic. Trc ht, cc dn tpk cht v bo ho cng nh cc nng l trng u ln hn vt liu c hm lng bo cao hn (hnh 5.14 v 5.15). Mt kt qu nh mong i, bi v c mt nhiu nguyn t bo hn th s l trng c sinh ra t chng cng nhiu hn. Nh vy nhit bt u tnh bn dn tinh khit s nng cao khi hm lng pha tp tng ln. 4.1.2.4. Linh kin bn dn Do nhng tnh cht in c o ca mnh, cc cht bn dn c ng dng trong nhng linh kin thc hin cc chc nng in t c bit. it v transito thay th cc n in t chn khng kiu c, l hai v d quen thuc. u im ca cc linh kin bn dn l kch thc nh, tiu th cng sut thp v khng cn thi gian ch nng my. Cc con s khng l cc mch in cc k nh m mi mt mch li c to bi nhiu linh kin in t, c th t hp trn mt chip silic nh b. S pht minh ra cc linh kin in t dn n s vi mch ho, chnh l c s cho s xut hin v ln mnh cc k nhanh chng ca mt ngnh ng u cc ngnh cng nghip mi trong my nm qua. Ghp chnh lu p n Chnh lu l linh kin in t cho php dng in ch chy theo mt chiu. Trc khi xut hin chnh lu bn dn p n , ngi ta s dng n it chn khng. Ghp chnh lu p n cu to bi mt tm bn dn pha tp loi n trn mt mt v loi p trn mt kia (hnh 5.17a). Nu em cc tm vt liu loi n v p ghp li vi nhau s c mt chnh lu km, bi v s xut hin b mt gia hai phn lm cho linh kin km hiu qu. Do vy

[59]

trong tt c cc linh kin u phi s dng vt liu bn dn n tinh th trnh cc hin tng in t xut hin bin ht s ph hu hot ng ca linh kin. Trc khi t mt in p ln mu p n, ht ti chnh bn p l l trng, cn bn n l in t nh minh ho trn hnh 5.17a. Mt in p ngoi c th thit lph trn ghp p n bng hai phn cc khc nhau. Khi dung mt b pin, u dng c th ni vo bn p v u m vo bn n, gi l ni thun. S u cc ngc li (m vo p v dng vo n) c gi l ni ngc. Di tc dng ca mt in p ni thun (hnh 5.17b), cc l trng bn p v in t bn n u b ht v mt tip xc. Khi l trng v in t gp nhau gn , chng lin tc ti hp v trit tiu ln nhau, tng ng vi: in t + l trng nng lng (5.20)

Nh vy kiu ni ny, s lng ln cc ht ti i qua cht bn dn v ti ch tip xc, nh c minh chng bi mt dng in mnh v mt in tr sut thp. c trng vn ampe khi ni thun c nu trn na bn phi ca hnh 5.18. Khi ni nghch (hnh 5.17c), c l trng v in t l ht ti a s u b ko nhanh khi ch tip xc. S phn cch in tch dng v m (tc l s phn cc) li ch tip xc mt vng tng i t cc ht ti linh ng. S ti hp s khng xy ra ng k, nh vy ch tip xc by gi tr nn cch in cao. Hnh 5.18 cng minh ho c tnh vn ampe khi ghp ngc. in p u vo v dng in u ra ca qu trnh chnh lu c biu th trn hnh 5.19. y in p vo bin thin hnh sin theo thi gian (hnh 5.19a). Dng in cc i IR khi ni ngc l ht sc nh so vi dng IF khi ni thun (hnh 5.19b). S tng ng gia IF v IR v in p cc i Vo t vo c ghi ch trn hnh 5.18. Vi cc in p lp ngc cao, c khi c hang trm vn, s pht minh s lng ln cc ht ti in (in t v l trng). iu ny gy nn s tng vt ca dng in gi l hin tng m xuyn, cng c ch ra trn hnh 5.18 v c cp chi tit hn trong mc 5.1.4.5 ( bn in mi ). Transito, linh kin bn dn cc k quan trng mch vi in t ngy nay c th m nhim hai chc nng c bn sau. Th nht, chng c th hot ng nh n in t chn khng trc y, n triot, ngha l chng c th khuych i cc tn hiu in. Th hai, chng cng c dng nh cc phng tin chuyn mch trong my tnh x l v lu tr thng tin. C hai loi chnh l transito tip xc v transito hiu ng trng kim loi oxyt bn dn (vit tt l MOSFET. Theo t ting Anh metal oxide semiconductor field effect transistor). Transito tip xc

[60]

Transito tip xc t hp bi hai tip xc p n ghp ta lng vo nhau theo cu hnh n-p-n. Hnh 5.20 nu s ca mt transito tip xc p-n-p cng vi mch in ca n. Mt vng (base) loi n rt mng c t xen k vo gia vng pht (emitter) v vng gp (collector) loi p. Mch bao gm tip xc pht - (tip xc 1) c lp thun, trong khi mt in p lp ngc c t ln tip xc d - gp (tip xc 2). Hnh 5.20. S ca transito tip xc p-n-p v mch in hot ng ca n, cc c trng in p vo v ra cho thy s khuych i in p. Hnh 5.21 minh ho cc c ch vn chuyn ca ht ti in. V cc pht thuc loi p v tip xc 1 c ni thun nn c nhng s lng ln l trng i vo vng loi n, y chng l nhng ht ti thiu s v mt s s ti hp vi cc in t a s. Tuy nhin, nu nh cc ht sc hp v cc vt liu bn dn c ch to ring th a s cc l trng s trn qua vng m khng b ti hp, sau qua tip xc 2 v i ti cc gp loi p; By gi chng tr nn mt phn hp thnh ca mch pht gp. Mt s tng nh in p u vo mch pht - s gy nn mt s tng mnh ca in p trn in tr ph ti trong mch (hnh 5.20). Nh vy, mt tn hiu in p qua transito tip xc th c khuych i ln. Hiu ng ny cng c minh ho trn hnh 5.20 bi hai th in p thi gian. Hot ng ca transito n-p-n cng c l gii tng t trong thay cho l trng, in t c n qua vng v i vo cc gp. MOSFET Hnh 5.22 nu mt ct ca mt mu MOSFET gm c hai o nh bn dn loi p c to ra trong mt cht silic loi n; cc o thng vi nhau bng mt knh hp loi p. Cc u ni kim loi thch hp (u ngun v u ra) t ln cc o mt lp cch in silic ioxyt c to ra bng cch oxy ho b mt silic. Sau mt u mi kt thc (ca) t ln trn b mt ca lp cch in . Hot ng ca mt MOSFET khc vi transito tip xc ch l y ch c mt loi ht ti (hoc l in t hoc l l) hot ng m thi. dn in ca knh c bin i do s c mt ca mt in trng tng ng t ln ca s y cc ht ti (trong trng hp ny l cc l) ra khi knh, do lm gim dn in. Nh vy, mt bin i nh ca in trng ca s gy nn mt bin i tng i ln ca dng in gia cc ngun v thot. Cc qu trnh khc trong MOSFET rt ging nh m t transito tip xc. S khc nhau c bn l ch dng ca l rt nh vi dng c s (base) ca transito tip xc. Do , cc MOSFET c s dng nhng ni m ngun tn hiu cn khuych i khng th cho c mt dng in ng k. Cc cht bn dn trong my tnh.[61]

Cng vi kh nng khuch i tn hiu, cc transito cng c th hot ng nh nhng linh kin chuyn mch, mt tnh nng c s dng cho cc x l s hc v logic v cng s dng lu tr thng tin trong cc my tnh. Cc con s v hm s my tnh u c vit thnh hai s: 0 v 1 thuc h m c s 2. By gi th cc transito v it trong mch s s hot ng nh nhng chuyn mch vi hai trng thi ng v ngt. Nh vy, mt s n c th biu din bng mt tp hp cc phn t mch cha cc transito c chuyn mch thch hp. Mch vi in t. Trong my nm va qua, s chuyn di ca mch vi in t, trong hng ngn phn t v mch in t c t hp trong mt khng gian rt nh, lm mt cuc cch mng trong lnh vc in t hc. Cuc cch mng ny trc ht phc v cho ngnh hang khng v t, ni cn c nhng my tnh, nhng cu kin in t rt nh v i hi cng sut thp. Nh nhng thnh tu mi trong cng ngh ch to. c gi thnh ca mch t hp mt cch ng ngc nhin. Kt qu l m y my tnh c nhn ( PC vit tt l t ting Anh Personnal Computer) c s dng rt ph bin . ng dng cc mach t hp cng xm nhp v nhiu lnh vc ca cuc sng hin nay : tnh ton thng tin, cnh bo sn xut v iu khin cng nghip cng nh mi khu ca cng nghip in t. Nhng vi mch khng t c sn xut hng lot vi k thut ch to tinh xo, bt u t vic nui n tinh th Silic hnh tr kh ln v tinh khit cao t ct thnh nhng tm mng hnh trn. Nhiu mch vi in t ( mch t hp ). thng gi l Chip , c ch to ngay trn mt tm n. Mt chip hnh vung kch c 1/4 inch ( 6 mm ) trn mt mt cha ti hng ngn phn t nh it transito, in tr v t in. Hnh 5.23b chp ton b mt chip vi x l khc cho thy r s phc tp ca mt mach t hp nh th . Hin nay nhng chip vi 2.000 000 phn t ang c ch to vi c nhng b nh cao hn cng s chc chn c pht trin trong tng lai Nhng mch vi in t gm nhiu lp nm trn qua tm silic theo mt s chi tit chnh xc . D ng k thut quang khc ( Photolithographic ) v cc k thut vi tnh khc ( v d, khuch tn, hoc cy ion ) v to ra cc khu vc bn dn kim loi n, loi p, in tr hoc dn cao trn mt lp. Quy trnh ny c lp li t lp ny sang lp khc cho n khi ton b mch t hp c ch to xong. 4.1.3. TNH DN IN TRONG GM IN V TRONG POLYME. phn ln cc polyme v gm ion u l vt liu cch in nhit phng v do cu trc vng nng lng in t tng t nh biu din trn hnh 5.3 c: khe nng lng tng i rng, thng ln hn 2 eV. Nh vy nhit bnh thng ch c rt t in t nh c cung cp nng lng c th b kch thch nhy qua khe, iu ng vi gi tr rt nh ca dn in. Bng 5.3 cho dn in nhit phng ca mt lot vt liu nh th. ng nhin nhiu vt liu c s dng trn c s kh nng cch in ca[62]

chng nh vy, in tr sut cao li l iu mong mun. Khi nng cao nhit dn ca cc vt liu cch in c th ln hn ca cc cht bn dn. Bng 5.3. dn in tiu biu nhit phng ca mt s vt liu phi kim loi Vt liu Graphit Gm: Nhm Oxyt S Thu tinh Na Ca Mica Polyme Phenolformaldehyt Nylon6.6 Polymetylmetacrylat Polyetylen Polystyren polytetrafloetylen 4.1.3.1. Dn in trong cc vt liu ion Cc cation v anion trong vt liu ion u c in tch v do chng u c kh nng di chuyn hoc khuch tn khi c mt in trng. Nh vy mt dng in s c to thnh do chuyn ng thun tu ca nhng ion ny. ng nhin, anion v cation s chuyn di ngc hng nhau. dn tng ca mt vt liu ion bng tng ca cc ng gp ca in t ion nh sau : ton phn = in t + ion ( 5.21 ) dn in ( . m)-1 105 10-10 - 10-12 10-10 - 10-12 ER nh sng thy c c bc sng ngn nht v ngn nht

do nng lng cc tiu ca nh sng nhn thy tng ng bng: = = ( 5.66a) ( 5.66b)

Nhng kt qu ny ni ln rng ,nhng vt liu ph kim no c khe vung ln hn 3,1 eV s khng hp th mt nh sng mt nh sng nhn thy no .Nu c tinh khit cao chng s hin ra trong sut v c mu .Mt khc ,cc bn dn no c khe cng hp hn 1,8eV s hp th ton b ph nh sng nhn thy nh cc chuyn di in t t vng ha tr sang vng dn :nh vy chng u c ,Cn nhng vt liu c khe trong khong gia 1,8eV v 3,1eV th ch hp th mt phn ph nh sng nhn thy ,do ta nhn thy chng m v c mu.Mi vt liu phi kim loi tr nn c ti mt s bc sng nht nh ,ty thuc vo ln ER.V d kim cng c khe vng 5,56eV ,l c i vi bc x c bc sng ngn hn 0,22m . S hp th bc x nh sng cng c th xy ra trong cc vt rn in mi c khe vng rng do nhng chuyn ri in t khng phi t vng ha tr ln vng dn. Cc tp cht hoc khuyt tt khc c th to nn nhng mc in t trong khe vng nh l nhng mc on (cho) v acxepto (nhn) (xem hnh 5.1.2.2.) .Bc x nh sng c bc sng thch hp c th b hp th bi kt qu chuyn di in t t hoc n nhng mc ny ,nh minh ha trn hnh 5.61a .Nng lng in t b hp th bi cc kch thch in t[108]

phi c tiu tn bng mt phng thc no theo nhiu c ch khc nhau.V d, bng s ti hp trc tip in t v l trng thep phn ng (hnh 5.60b): C th xy ra nhng chuyn di in t nhiu bc nh c cc mc tp cht nm trong khe vng .Hnh 5.61b cho thy kh nng pht x hai photon bng s ging in t t mt trng thi trong vng dn xung mt mc tp cht ,ri li t nhy v vng ha tr .Hoc l mt trong cc bc chuyn di c th sn sinh mt photon (hnh 5,61c), y phn nng lng tng ng c tiu tn di dng nhit 4.4.3.4 .Truyn qua Xt trng hp nh sng i qua mt vt rn trong sut(hnh 5.62). i vi mt chm tia c cng I0 chiu vo mt nc ca mu cht c dy l v h s hp th ,cng c truyn qua ti mt sau ca mu l: IT=I0(1-R)2e-t Trong R- phn x v gi thit rng c cng mt mi trng tn ti pha ngoi c mt trc v mt sau ca mu. Nh vy thnh phn ca tia ti truyn qua vt liu trong sut ph thuc vo nhng tn hao do hp th v phn x. Hn na, cc thng s nh phn x, hp th v truyn qua u ph thuc vo bc sng nh sng v phi tha mn biu thc(5.55). Hnh 5.63 th hin cc mi quan h ny i vi thy tinh xanh. V d, vi nh sng c bc sng 0.4m th t phn truyn qua,hp th v phn x ln lt vo khong 0.90;0.05 v 0.05. Tuy nhin vi bc sng 0.55m th cc t phn ny c cc gi tr tng ng khong 0.5;0.48 v 0.02. 4.4.3.5 .Mu sc Cc vt liu trong sut hin mu l do c nhng di bc sng nh sng b hp th chn lc, mu sc cm nhn c l kt qu t hp ca nhng bc sng c truyn qua. Nu s hp th xy ra ng u vi tt c cc bc sng thy c th vt liu hin ra khng mu, v d nh cc loi thy tinh v c tinh khit cao, kim cng v saphia n tinh th tinh khit cao. Thng thng, hp th chn lc l bng kch thch in t. Chng hn nh trng hp cc cht bn dn c khe vng trng vi di nng lng photon nh sng nhn thy(1.8eV n 3.1eV). phn nh sng nhn thy c nng lng ln hn E b hp th chn lc bi cc chuyn di in t t vng ha tr ln vng dn. ng nhin, mt s bc x b hp th ny li th pht khi cc in t kch thch nhy tr v nhng trng thi nng lng thp ban u ca chng. S th pht ny xy ra khng nht thit cng tn s nh khi hp th m trong trng hp chuyn ri bc x nhiu bc(hnh 5.61b) hoc khng bc x (hnh 5.61c) th li l nh hn. Kt qu l mu sc ph thuc vo s phn b tn s ca c chm sng truyn qua v chm sng th pht.[109]

V d, cacmi sunfit(Cds) c khe vng khong 2.4V, do hp th nhng photon c nng lng ln hn 2.4eV tng ng vi phn xanh v tm ca ph nh sng nhn thy; mt s li th pht thnh nh sng vi nhng bc sng khc. Cn nh sng nhn thy khng b hp th ch gm nhng photon c nng lng t 1.8eV n 2.4eV. Cds hin mu vng da l do t hp chm truyn qua. V d, nhm oxit hoc saphia n tinh th v tinh khit cao khng c mu. Nhng rubi(saphia cha thm 0.5 n 2% crom oxit) li c mu chi. Ion Cr3+ thay th Ion Al+3 trong cu trc tinh th Al2O3 v do a vo cc mc tp cht nm trong khe vng rng ca saphia. C nhng bc sng c th s b hp th chn lc do h qu ca nhng chuyn di hoc t nhng mc tp cht ny. truyn qua nh l mt hm s ca bc sng i vi s