Struktur Logam

8/9/2019 Struktur Logam

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METALS AND ALLOYS

LOGAM dan PADUAN

Prof. Dr. Timbangen Sembiring, M. Sc.


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Struktur Logam

Metallic Structures

2

http://upload.wikimedia.org/wikipedia/en/8/8e/Close-packed_spheres.jpg


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Struktur

• Sifat-2 kebanyakan bahan/materi

berkaitan langsung dengan struktur

kristallnya.

• Perbedaan sifat secara signifikan

terdapat antara bahan kristal dan non

kristal yang memiliki komposisi sama.

3


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• !on dense" random packing

• #ense" ordered packing

Struktur yang rapat dan penyusunan teratur $ordered packed%

cenderung memiliki energi rendah/kecil.

&nergi dan Penyusunan/Packing

Energy

r

typical neighbor bond length

typical neighbor bond energy

Energy

r

typical neighbor bond length

typical neighbor bond energy


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'ristal Logam'ristal Logam

• Logam umumnya bahan padatan $highmelting points%.

• (da ikatan kuat antara atom-atom logam.• )katan pada logam dan paduan $alloy%

berbeda berdasarkan komposisinya* intipositif terdapat pada lautan elektron%

+

+

+

+

+

+++

+ –

–

–

–

– – –

–

–

– – –

– – – –

– –


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+

• atom tersusun secara periodik dan

dalam 3 #

,ahan kristalin.

-logam

-beberapa keramik

-beberapa polimer

• atom tersusun bukan periodik

,ahan non kristallin

-struktur kompleks-pendinginan tiba2/

rapid cooling

kristallin Si2

!on kristallin Si2 (morf 0 !oncrystalline

Materi dan Penyusunan/Packing

Si Oksigen

• 1ontoh*

• teradi pada*


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Struktur Kristal Logam

•,gm kita menyusun atom-atom logam untukmeminimalkan ruang kosong $dalam 2-

dimensi%4

vs.


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5

• 1enderung tersusun rapat $densely packed%.• (lasan tersusun rapat*

- )katan logam $Metallic bonding% tidak berarah

- 6arak atom tetangga terdekat $nearest neighboratom% cenderung mengecil untuk memperkecil

energi ikat.

- 'abut elektron $electron cloud% menutupi satu

sama lainnya.

• Merupakan struktur kristal paling sederhana.

Struktur Kristal Logam


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7

• 6arang ditemukan disebabkan kerapatan penyusuan

$packing density% sangat kecil $hanya Po memiliki struktur S1%• 1lose-packed directions merupakan sisi kubus.

• 1oordination 8 0 +

$8 nearest neighbors%

Simple 1ubic Structure $S1%


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9:

• (P; untuk struktur kubik sederhana 0 :.olume of unit cell

?diasumsikan bola padat

close-packed directions

a

R 0:.<

a

contains 5 @ 9/5 09 atom/unit cell


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99

• 1oordination 8 0 5

• (tom bersentuhan satu sama lainnya sepanang diagonal

kubus.• Semua atom identik

,ody 1entered 1ubic Structure $,11%

1ontoh*

1r" A" ;e $α%" Bantalum" Molybdenum

2 atoms/unit cell* 9 center C 5 corners @ 9/5


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92

(tomic Packing ;actor* ,11

a

(P; 0

3π $ 3a/%32

atoms

unit cell atom

=olume

a3

unit cell

=olume

length 0 R 0

1lose-packed directions*

3 a

• (P; untk struktur body-centered cubic 0 :.+5

aR

a2

a3


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93


• (tom-atom bersentuhan satu sama lainnya sepanangdiagonal sisi bidang.

;ace 1entered 1ubic Structure $;11%

1ontoh* (l" 1u" (u" Pb" !i" Pt" (g

atoms/unit cell* + face @ 9/2 C 5 corners @ 9/5


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9<

• (P; untuk struktur face-centered cubic 0 :.

(tomic Packing ;actor* ;11

(P; 0

3π $ 2a/%3

atoms

unit cell atom=olume

a3

unit cell

=olume

1lose-packed directions*

length 0 R 0 2 a

Dnit cell contains* + @ 9/2 C 5 @ 9/5

0 atoms/unit cella

2 a


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9+

Ee@agonal 1lose-Packed Structure

$E1P %


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9


• (,(,... Stacking SeFuence

• (P; 0 :.

• 3# Proection • 2# Proection

Ee@agonal 1lose-Packed Structure

$E1P%

+ atoms/unit cell

e@* 1d" Mg" Bi" Gn

• c /a 0 9.+33

c

a

( sites

, sites

( sites ,ottom layer

Middle layer

Bop layer


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(,(,... Stacking SeFuence


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!"ay Diffraction

97


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Xray-Tube

Detector

Sample

Bragg´s Equation

d = λ2 sin!

d " distance bet#een t$e same atomic planesλ " monochromatic #a%elengt$! " angle o& di&&racto-

meter

Xray-Tube

Detector

'etal Target

()u or )o*

s a m p l e

X-Ray Diffractometer


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Kera#atan Teoritis,

dimana n 0 umlah atom/sel satuan

A 0 berat atom

V C 0 >olume sel satuan 0 a3 untuk kubik

N ( 0 bilangan (=ogadro

0 +.:22 @ 9:23 atoms/mol

'erapatan/#ensity 0 ρ 0

V C

N (

n Aρ 0

1ellDnitof >olumeBotal

1ellDnitin (tomsof Mass


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23

• 1ontoh* 1r $,11%

A $berat atom% 0


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2

;aktor-2 yng menentukan struktur kristal9. Dkuran relatif )on H ;ormasi strukur yang stabil*

--umlah maksimum muatan ion tetangga yang berlainan.- -

- -C

unstable

- -

- -C

stable

- -

- -C

stable

2. 'enetralan Muatan * --!et charge in ceramic

should be Iero.

--Jeflected in chemicalformula*

1a;2*1a2C

cation

;-

;-

anionsC

(mKp

!ilai m" p untuk mencapaikenetralan muatan


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+enis ,atan

Types o& Bonding

,onic

Bonding

.an Der /aals

Bonding

'etallic

Bonding

)o%alent

Bonding

0ydrogen

Bonding

$ig% Melting

Point

$ar& an& 'rittle

(on con&ucting

soli&

(a)l, )s)l, *nS

Lo Melting Points

Soft an& 'rittle

(on!)on&ucting

(e, r, Kr an& e

-ariable

Melting

Point

-ariable

$ar&ness

)on&ucting

e, )u, g

-ery $ig%

Melting

Point

-ery $ar&

/sually not

)on&ucting

Diamon&, 0ra#%ite

Lo Melting

PointsSoft an& 'rittle

/sually(on!)on&ucting

1ce,

organic soli&s


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2+

• ,onding*

-- 1an be ionic and/or co=alent in character. -- ionic character increases ith difference in

electronegati=ity of atoms.

• #egree of ionic character may be large or small*

(tomic ,onding in 1eramics

Si1* small1a;2* large


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2

• 1oordination 8 increases ith

1oordination 8 and )onic Jadii

2

r cationr anion

1oord

8

N :.9


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)om#utation of Minimum )ation!nion"a&ius "atio

• #etermine minimumr

cation /r

anion for an octahedral site $1.!. 0 +%

a = 2r anion

2r anion + 2r cation = 2 2r anion

r anion + r cation = 2r anion r cation = $ 2 −9%r anion

ar r 222 cationanion =+

9.:92

anion

cation=−=

r

r

a Measure the radii $blue

and yello spheres%

Substitute for 2a3 in theabo=e eFuation


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3:

• n the basis of ionic radii" hat crystal structure ould you

predict for ;e4

• (nser*


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39

"ock Salt Structure

Same concepts can be applied to ionic solids in general.

&@ample* !a1l $rock salt% structure

r !a 0 :.9:2 nm

r !a/r 1l 0 :.


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MgO an& eO

2- r 0 :.9: nm

Mg2C r Mg 0 :.:2 nm

r Mg/r 0 :.


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)rystal Structures

737.:959.:

9:.:

1l

1s==

−

+

r

r

1esium 1hloride structure*

∴ Since :.32 N :.737 N 9.:"

cubic sites preferredSo each 1sC has 5 neighbor 1l-

(KHBype 1rystal Structures include !a1l" 1s1l" and Iinc blende


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3

4 )rystal Structures

• 1alcium ;luorite $1a;2%

• 1ations in cubic sites

• D2" Bh2" Gr2" 1e2

• (ntifluorite structure H

positions of cations and

anions re=ersed

;luorite structure


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3<

'5 )rystal Structures

•Pero=skite structure

&@* comple@ o@ide

,aBi3


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3+

• (toms may assemble into crystalline or amorphous structures.

• Ae can predict the density of a material" pro=ided e kno the

atomic eight" atomic radius" and crystal geometry $e.g." ;11"

,11" E1P%.

SDMM(JO

• 1ommon metallic crystal structures are ;11" ,11 and E1P.

1oordination number and atomic packing factor are the same

for both ;11 and E1P crystal structures.

• 1eramic crystal structures are based on*

-- maintaining charge neutrality

-- cation-anion radii ratios.

• )nteratomic bonding in ceramics is ionic and/or co=alent.

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