Trong cac bai trude, ta da ndi ddng dien trong kim loai la ddng cac electron chuyen ddng cd huang. Nhung nhu tai ca cac vat lieu khac, kim loai cung do cac nguyen td lien kei vdi nhau tao nen. Nguyen tu lai gom hat nhan tich dien duong va cac electron mang dien am quay xung quanh.
Vay cac electron trong kim loai cd dac diem gi va nd chi phdi tfnh chai cua ddng dien trong kim loai ra sao ? Dd chinh la noi dung cua bai hoc nay.
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: Gf € k er-
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^ -^ f
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Hinh 13.1
Mang tinh the tao bdi cac ion duong sdp xep CO trat tu va cac electron tu do chuyen dong hPn loan trong mang.
1 - BAN CHAT C U A DONG DIEN TRONG KIM LOAI Ban chat ciia ddng dien trong kim loai duge neu ro trong mdt li thuyet tdng qudt ggi la thuyet electron vi tfnh ddn dien cua kim loai. Thuyet dd cd ndi dung nhu sau :
1. Trong kim loai, cac nguyen tu bi mat electron hod tri trd thdnh edc ion duong (Hinh 13.1). Cdc ion ducng lien ket vdi nhau mdt cdch trat tu tao nen mang tinh the kim loai. Chuyen ddng nhiet cua cdc ion (dao ddng cua ion quanh vi tri can bdng) ed the pha huy trat tu nay. Nhiet do eang cao, dao ddng nhiet cang manh, mqng dnh tlie cdng trd nen mdt trat tu.
2 Cdc electron hoa tri tdch khdi nguyen tir, trd thdnh edc electron tu do vdi mat do n khdng ddi {n = hang sd). Chiing chuyen ddng hdn loan tao thanh khi electron tu do choan toan bd the tfch eua khdi kim loai va khdng sinh ra ddng dien nao.
3. Dien trudng £" do ngudn dien ngoai sinh ra, day khf electron trdi nguge chieu dien trudng, tao ra ddng dien.
4. Su mat trat tu ciia mang tinh the cdn trd chuyen ddng cua electron tu do, la nguyen nhan gay ra dien trd ctia kim loai. Cdc loai mat trat tu thudng gap la chuydn ddng nhiet (dao ddng nhiet) eiia cae ion trong mang tinh the, su meo mang tinh thd do bien dang eo hge vd edc nguyen tti la Idn trong kim loai. Dien trd ciia kim loai rat nhay cdm vdi cae yeu to tren.
Thuyet electron vd tfnh ddn dien cua kim loai cho tha'y hat tdi dien trong kim loai Id electron tu do. Mat dp ciia cluing rdt cao nen kim loai ddn dien rat tdt (Bdng 13.1). Nhieu tfnh chat khac eua ddng dien trong kim loai ciing cd thd suy ra tii thuyet nay.
Vay, ddng dien trong kim loai Id ddng chuyen ddi cd hudng cda ede electron tu do dudi tac dung ciia dien trudng.
Bang 13.1 Chat
Bac Bach kim Dong
Po l ^ '
1,62.10"
10,6.10- 1,69.10-
a (K-1)
4,1.10-3 3,9.10-3 4,3.10-3 Nhom
Sdt Silic Vonfam
2,75.10-8 9,68.10-3 0,25.10"
5,25.10-3
4,4.10-3 6,5.10-3 -70.10-3 4,5.10-3 .
i
Tren bang nay cd silic khong phai kim loai. Dien tra sua't cua chiing Idn hon dien trd suat ciia kim loai vai bac.
II - SU" P H U T H U O C CUA D I E N T R d SUAT CUA KIM LOAI THEO NHIET D O
Khi nhiet do tang, ehuyen ddng nhiet eua cac ion trong mang tinh thd tang, lam cho dien trd cua kim loai tang. Thi nghiem chiing td dien trd suat p eua kim loai tang theo nhiet do gan dung theo ham bae nhat:
p = p^[l + a{t-t^)] (13.1) trong dd p^ la dien trd suat d rQ°C (thudng lay la 20''C) ; a la he sd nhiet dien trd, don vi do la K"'.
Gid tri cua p^ vd a eiia mdt so kim loai dugc ghi tren Bdng 13.1.
Tuy nhien, neu lam thf nghiem mdt cdch chfnh xde, ta tha'y he sd nhiet dien trd eiia mdi kim loai khdng nhiing phu thude vao nhiet do, ma vao ed do saeh vd che do gia cdng vat lieu dd. Ngay nay, he sd nhiet dien trd ciia bach kim (platin) da dugc nghien eiiu rat cdn than, vi ngudi ta thudng diing day bach kim de lam nhiet ke diing trong cdng nghiep. H I
400 800 1200 T(K) Su bien thien dien trd suat cua dong theo nhiet do
R l Vi sao ngudi ta chpn day bach kim de lam nhiet ke dien trd dung trong cdng nghiep ?
Ill - DIEN TRd CUA KIM LOAI 6 NHIET DO THAP VA H l i N TUONG SiEU DAN
Bang 13.2
Nhiet do toi han cua mot so chat sieu dan Ten vat lieu
• Nhom ( Thuy ngan i Chi
Thiec Kem NbgSn 1 NbgAl NbgGe j DyBa2Cu307 ' HgBa2Ca2Cu30g
1
7"c(K)
1,19 4,15 7,19 3,72 0,85 18 18,7 23 92,5 134
EP Vi sao ddng dien chay trong cuon day sieu dan khong cd ngudn dien lai cd the duy tri lau dai ? Cd the dCing ddng dien ay lam cho dong co chay mai dugc khdng ?
Khi nhiet do cang giam, mang tinh the eang bdt mat trat tu nen su edn trd ciia nd den chuyen ddng eua electron edng ft, dien trd suat ciia kim loai gidm lien tue. Den gdn 0 K, di6n trd ciia cdc kim loai saeh deu rat be. Do thi bieu didn dien trd suat ciia ddng theo nhiet do dugc ve tren Hinh 13.2.
Mdt so kim loai nhu Hg, Pb..., mdt sd hgp kim nhu NbjSn, NbjGe..., va ca mdt sd gdm dxit kim loai nhu DyBa2Cu307 khi nhiet do thap hon mot nhiet do tdi han T^ thi dien trd sudt dot ngdt gidm xudng bang 0. Nhiet do tdi han cua mdt sd chat dugc ghi tren Bang 13.2. Nhieu tfnh cha't khac nhu tii tfnh, nhiet dung ciing thay ddi dot ngdt d nhiet do nay. Ta ndi rdng cae vat lieu a'y da chuyen sang trang thdi sieu ddn. Ngay nay cac cudn day sieu ddn duge diing de tao ra cdc tii trudng rdt manh ma cae nam cham dien thudng khdng thd tao ra dugc. Cho ddng dien chay qua cdc cudn day kim loai sieu ddn rdi bd ngudn dien di, ddng dien vdn tiep tue chay trong nhieu nam ma khdng yeu di. Trong tuong lai, ngudi ta du kie'n ed the diing day sieu ddn de tdi dien, vd tdn hao nang lugng tren dudng day khdng cdn nira. BE
;)
Ti T,>T^
Hmh 13.3
Electron khuech tan tu dau nong qua dau lanh lam dau nong tich dien duong.
IV - HIEN TUONG NHIET DIEN
Thuyet electron vd tfnh ddn dien eiia kim loai cdn cho thdy, neu sgi day kim loai cd mdt ddu ndng va mdt dau lanh, thi chuyen ddng nhiet cua electron se lam cho mdt phan electron tu do d ddu ndng ddn vd ddu lanh. Ddu ndng se tfch dien duong, ddu lanh tfch dien am (Hinh 13.3).
Giira ddu ndng va ddu lanh cd mdt hieu dien the nao da'y. Neu lay hai day kim loai khac loai nhau va han hai ddu vdi nhau, mdt mdi han giii d nhiet do cao, mdt mdi hdn d nhiet do thap, thi hieu dien the giOa ddu ndng vd ddu lanh cua timg day khdng gidng nhau, khien trong maeh cd mdt sua't dien ddng ^ (Hinh 13.4). f ggi Id sudt dien ddng nhiet dien, vd bd hai ddy ddn hdn hai ddu vdo nhau gpi la cap nhiet dien.
Thf nghiem ehiing td rdng "f = aj (7, - Tj), trong dd Tj - ^2 la hieu nhiet do d ddu ndng vd dau lanh ; (Zp la he sd nhiet dien ddng, phu thude vdo bdn chat cua hai loai vat lieu diing lam cap nhiet dien, den vi do la V.K"'. Suat dien ddng nhiet dien tuy nhd nhung rat dn dinh theo thdi gian va didu kien thf nghiem, nen cap nhiet dien dugc dimg pho bien dedo nhiet do. Ba loai cap nhiet dien thudng ddng la :
Cap platin - platin pha rddi cd o^ = 6,5 |J.V/K Cap crdmen - alumen cd o^ = 41 |lV/K Cap ddng - constantan cd (Zp = 40 |J.V/K.
Day (Jong Von ke
Nuoc da Ngon nen chay Hmh 13.4 Cap nhiet dien dong - constantan :
- Nhiet dp ngpn nen chay 7^:
- Nhiet dp nude da dang tan Tg = 273 K (tue 0°C).
Hat t a i d i e n t r o n g k i m loai la e l e c t r o n t U d o . M a t d o cua c h u n g rait cao nen k i m loai d a n d i e n t o t .
D o n g d i e n t r o n g k i m loai la d o n g c h u y e n d d i co hUdng cua cac electron t U d o dU6i tac d u n g cua d i e n t r U d n g .
Chuyen d o n g n h i e t cua m a n g t i n h t h e can t r d c h u y e n d o n g cua hat tai d i e n lam cho d i e n t r d cua k i m loai p h u t h u o c n h i e t d o . D e n g a n 0 K, d i e n t r d cua k i m loai rat n h d . Vat lieu sieu d a n cd d i e n t r d d o t n g o t g i a m d e n b a n g 0 k h i n h i e t d o T < T^.
Cap n h i e t d i e n la hai day k i m loai khac b a n chait, hai d a u han vao n h a u . Khi n h i e t d o hai m o i han T.^, T^ khac n h a u , t r o n g mach cd suait d i e n d o n g n h i e t d i e n ^ = : 0^(7, - T^), Oj la he so n h i e t d i e n d o n g .