Post on 26-Feb-2018
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Atomic Difusion
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A thought experiment
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Typo on HW 05
______ 23. The process described in question 22is known as:
A. Grain growth B. Carburizing C. Sintering . i!!usion bonding
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Beore heating
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Ater short time inurnace
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Ater additional time inurnace
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Ater very long time inurnace
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Difusion:
Difusionis the movement ofatoms within a material
usually to reduce aconcentration gradient, or inresponse to an applied force
or voltage
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el difusion: atoms move aboutwithin a material, exchanging latticesites
Alloy difusion: in an alloy, atomsof the various components willdiuse until a homogeneous
distribution is achieved
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What is difusion!
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The important role o difusion: Eliminate concentration dierences
Heat treat metals
Manufacture ceramics, transistors andsolar cells
Electrical conductivity in ceramics
Alter surface chemistry
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"ossi#le $echanisms oDifusion
acancy
!nterstitial
!nterstitialcy "ing
#ote: acancy and interstitial aremost important$
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Difusion mechanisms
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Difusion mechanisms%contd&'
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(irst )a* o tudent"erormance:
%uccess & Ability x Motivatio
where %uccess is graduation'
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(ic+s (irst )a* o Atomic
Difusion:
(ic)s (irst *aw+rate of atommovement
& -. /dc0dx1
& 2ux /rate of net 2ow across aninterface1
. & diusion coe3cientdc0dx & concentration gradient
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Difusion ,oe-cient:. & .
oe-40"5
.o& diusion constant
4 & activation energy
" & gas constant
5 & absolute temp
. has strong temperature dependence'
T D
500C 10-14
600C 10-13
1000C 10-9
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The .ole o Temperature:
5he higher the temperature, thegreater the rate of atomic diusion
.iusion becomes signi6cant whenthe temperature reaches orexceeds 7$8 times the meltingpoint /on an absolute temperaturescale'1
5 9 7$8 5mp/o1
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Activation /nergy 1:
4 is lower for:
*ow M;
Metals /vs$ ceramics1 %maller atoms
More open structures
!nterstitial vs$ vacancy
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Activation /nergy
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Difusion ,oe-cients
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Types o Difusion:
rain boundary diusion
?$ olume diusion
Examples: Values of Q
Th in W Ag in Ag
Surface 66,400 8900
Grain Boundary 90,000 22,750
Volume 120,000 45,700
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/fect o time 2 (ic+s
econd )a*:(or the same result:
.
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Applications o Difusion
arburiBing a gear High arbon & wear resistant
*ow arbon & fracture resistant
>rain growth
%intering of ;0M or ceramic Csed to fuse together particles
.iusion bondingoins material w0o welding
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,ar#uri3ing
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,ar#uri3ing
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4rain 4ro*th
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intering
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Difusion Bonding