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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