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HIGH TEMPERATURE OXIDATION

OF TUNGSTEN AND MOLYBDENUM

NN

.

.MM

.

.GG

hh

oo

nn

iiee

mm

TThhee UUnniivveerrssiittyy ooffCCaalliiffoorrnniiaa aatt LLooss AAnnggeelleess ((UUCCLLAA))

LLooss AAnnggeelleess,, CCAA.. 9900009955--11559977,, UUSSAA

APEX STUDY GROUP MEETING

UCLA

November 2-3, 1998

Research Sponsored by

US Department of Energy (DOE/OFE)

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PRESENTATION OUTLINE

(1) IMPACT OF OXIDATION ON DESIGN WITH REFRACTORY ALLOYS.

(2) MODELING OF HIGH-TEMPERATURE OXIDATION.

2.1. Thermodynamics of Reactions

2.2. Non-equili brium Analysis

(3) RESULTS FOR TUNGSTEN.

(4)RESULTS FOR MOLYBDENUM.

(5)CONCLUSIONS.

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DESIGN IMPACT

@ At Normal temperatures and pressures, the chemical reaction of a gas

with the solid generally results in condensed products.

@ At high temperatures and low pressures, the formation of volatile

products is thermodynamically favored over the growth of the condensedphase.

@ The upper temperature limit for design with refractory metals with a

helium coolant will be influenced by the formation of volatile oxides.

@ The present investigation is concerned with W/He and Mo/He designs.

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MODELING HIGH-TEMPERATURE OXIDATION

APPROACH:

@ Quasi-equilibrium Treatment of Heterogeneous Reactions for the

systems: O-W and O-Mo.

@ Rate Limiting Step is the Trapping (adsorption) of Oxygen atoms until

equilibrium.

@ Assume that the helium pressure is Psys, and its temperature T*. The

oxygen partial pressure is given by:

P appm PO sys2 106

' =

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THERMODYNAMICS OF CHEMICAL REACTIONS

@ The rate at which oxygen molecules collide with unit area of the wall

surface is given by:

Z P M RTO O O2 2 22= @ For Tungsten, the following thermodynamic reactions take place:

x W s y O g W O gx y( ) ( ) ( )+ 1

22

@ For Molybdenum, similar reactions occur. In addition, for T~1500-

2500 K:

Mo s O g Mo O g Mo O g( ) ( ( ) ( ) ( ) ( )+ + 1

212 2 33- )

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FORMATION ENTHALPIES & ENTROPIES OF OXIDES

Tungsten Molybdenum

Species Hf298.15 (kcal/g.mole) Species Hf298.15(kcal/g.mole)

Sf298.15(cal/g.mole.K)

O(g) 59.559 O(g) 6611..33 1166

W O(g) 101.6 Mo O(g) 9955 2255..55

WO2 (g) 18.3 MoO2 (g) 1111..00 99..00

W O3 (g) -70.0 Mo O3 (g) --8800 --1155..55

W2 O6 (g) -278.2 Mo2 O6 (g) --227700 --7722

W3O8 (g) -408.7 Mo3O8 (g) --440000 --111188

W3O

9(g) -483.6 Mo

3O

9(g) --446633 --113322

W4O12 (g) -670.2 Mo4O12 (g) --664400 --119900

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QUASI-EQUILIBRIUM TREATMENT OF BATTY &

STICKNEY

*

@ Collisions of O2 with the surface can lead to either: (1) adsorption and

equilibration, or (2) reflection.

@ Define the equilibration probability as:

i i iZ= /@@ FFoorr ooxxyyggeenn mmoolleeccuulleess aatt aa tteemmppeerraattuurree TT

**,, ddiiffffeerreenntt ffrroomm tthhee wwaallll

tteemmppeerraattuurree TT,, tthhee eeqquuiilliibbrraatteedd ooxxyyggeenn fflluuxx iiss::

O O OZ2 2 2' ' '=

J.C. Batty and R.E. Stickney, Jour n. Chem. Physics, Volume 51, No. 10, (1969) p 4475

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QUASI-EQUILIBRIUM TREATMENT OF BATTY &

STICKNEY

*

(Cont.)

@ Let i W O g x y ( ), and the Gibbs Energy as: G T T T S T i i i( ) ( ) ( )=

@ Now, we have the following system:

K PP

G T RT

P G RT

P P P P

ii

O

y i

O O

O O O i

N

= =

=

= + +

( )exp( ( ) / )

exp( / )

/

'

2

2

2 2

2

1

i = 1,2,..., N

PO

@ PO2 is obtained from O2. Requires equilibration probability

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EQUILIBRATION PROBABILITY IS DETERMINED

EXPERIMENTALLY

For W OT2

1034982 7607 104

=

exp[ ..

] , T (oK)

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Experimental Data on W OxidationZ

O2'=1.2x10

17cm

-2s

-1

Temperatur e (K)

1000 1500 2000 2500 3000 3500

Evapora

tion

Ra

te(c

m-2s

-1)

10

12

1013

1014

1015

1016

W2O6

WO

WO3

WO2

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Experimental Data on W Oxidation

ZO2

'=1.2x1017

cm-2

s-1

Temperature (K)

1000 1500 2000 2500 3000 3500

To

talEvapora

tion

Ra

te(cm

-2s

-1)

1012

1013

1014

1015

1016

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Experimental Data on W Oxidation

ZO2

'=1.2x1017

cm-2

s-1

Temperature (K)

1000 1500 2000 2500 3000 3500

Evapora

tion

Ra

te(micron

/yr)

0

1000

2000

3000

4000

5000

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Calculated W Oxidation Rates

Experiment at ZO2

'=1.2x1017

cm-2

s-1

Temperature (K)

1000 1500 2000 2500 3000 3500

Ca

lcua

lte

dEvapora

tion

Ra

te(mm

/yr

)

0

5

10

15

20

2550 atm helium, 0.1ppm O2

Experimental

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EXPERIMENTAL RESULTS FOR MOLYBDENUM

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CONCLUSIONS

(1) HIGH-TEMPERATURE OXIDATION SETS SEVERE LIMITS ONHELIUM COOLING OF REFRACTORY ALLOYS.

(2) FOR TUNGSTEN AND MOLYBDENUM OPERATING AT 50 ATM.HELIUM COOLANT, AT 0.1 PPM OXYGEN, THE UPPER

TEMPERATURE IS ESTIMATED AT 1200- 1300 OC.

(3) IF HIGHER TEMPERATURES ARE BENIFICIAL TO DESIGNOBJECTIVES, THE FOLLOWING IS TO BE IMPLEMENTED:

@ Reduce the oxygen impurity concentration to the ppp range.

@ use oxidation-resistant coatings in the high-temperature zones.