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Oxidation of Molybdenum 550deg to 1700degCTo cite this article E A Gulbransen et al 1963 J Electrochem Soc 110 952

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Oxidation of Molybdenum 550 ~ to 1700~

E A Gulbransen K F Andrew and F A Brassart Physical Chemistry Department Westinghouse Electric Corporation Pittsburgh Pennsylvania

ABSTRACT

Weight change and oxygen consumption measurements were used to study the oxidation of molybdenum from 550 ~ to 1704~ for pressures of 5 to 76 Torr For temperatures of 550~176 two processes occurred simultaneously oxide scale formation and molybdenum trioxide volatili ty Above 800~ at pressures up to 76 Tor t molybdenum trioxide volatilized as fast as it formed At 900~ and 76 Torr using 12 cm 2 samples the pr imary chemical reaction gave a rate of about 10 is at molybdenumcm2sec Above this temperature for 12 cm 2 specimens the reaction was l imited by gaseous diffusion of oxygen Little change was found in the rate of oxidation to 1615~ Pressure had only a small effect on the rate of reaction for these react ion conditions However in the chemically con- trolled region pressure had an impor tant effect on the rate of oxidation To extend the tempera ture region where the p r imary chemical reaction was rate controlling samples of small area were used A sample having a total area of 012 cm 2 gave a react ion rate of 8 x 10 is atcm2sec at 1410~ For these very fast reactions appreciable tempera ture rises occurred and the actual sample tempera ture had to be estimated A log K vs 1T plot of the p r imary chemical react ion data gave an energy of act ivation of 197 kcal mole Reaction conditions where gaseous diffusion processes are rate controll ing were determined All of the earlier studies were made for these reaction conditions The activated state theory of surface reactions was applied to the p r imary chemical reac- t ion in the oxidation of molybdenum A mechanism of mobile adsorption was found to be the pr imary chemical reaction This adsorption process probably occurred on a surface already covered with a layer of adsorbed oxygen atoms since MoO3 was volatilized

M o l y b d e n u m and its al loys have m a n y use fu l h i g h - t e m p e r a t u r e mechan i ca l proper t ies However its res i s tance to ox ida t ion is poor A l though m a n y s tudies have been made the m e c h a n i s m s of ox ida- t ion have not b e e n establ ished

The reac t ion of m o l y b d e n u m w i t h oxygen is complex and involves severa l types of ox ida t ion processes The oxide volat i l izes p a r t i a l l y at 600 ~ and 76 Torr oxygen pressure and mel t s at 795~ Except for ox ida t ion be low 450~ e x p e r i m e n t a l resul t s are somewha t conflicting Resul ts appear to depend on the i n d i v i d u a l r eac t ion system spec imen size and gas flow In most studies the p r i m a r y chemica l r eac t ion has been masked b y t r an spo r t processes of o x y g e n t h rough vola t i l ized m o l y b d e n u m t r iox ide to the me ta l surface

The p re sen t work has severa l object ives (a) to d e t e r m i n e the n a t u r e of the ox ida t ion m e c h a n i s m b e t w e e n 550 ~ and 800~ where bo th oxide films are fo rmed and w h e r e oxide vo la t i l i ty occurs (b) to separa te e x p e r i m e n t a l l y the p r i m a r y chemical r e - act ion f rom the diffusion reg ion of reac t ion (c) to d e t e r m i n e the n a t u r e of the p r i m a r y chemica l r e - act ion (d) to define the t r ans i t i on b e t w e e n c he m- ical control and diffusion cont ro l of oxida t ion and (e) to d e t e r m i n e the factors affecting ox ida t ion in the diffusion cont ro l led reg ion of the react ion

Severa l r ev iews of ear l ie r w o r k have been m a d e (1-3) G u l b r a n s e n and Wysong (1) and Gor - b o u n o v a and A r s l a m b 6 k o v (4) f ound a d h e r e n t oxide films fo rmed w h e n the me ta l was oxidized be - low 400~ The da ta were fitted to the pa rabo l i c r a t e law and an e n e r g y of ac t iva t ion of about 360

k c a l m o l e was calculated Above 400~ devia t ions f rom the parabol ic r a t e l aw occurred Vola t i l i za t ion of m o l y b d e n u m t r iox ide occurred at 475~ u n d e r v a c u u m condi t ions (1)

Be t w e e n 500 ~ a nd 1000~ several s tudies have b e e n made (5 -7) S i m n a d and Sp i lne r s (5) f ound the da ta could be fitted by the parabol ic ra te l aw at 500~ and by the l i n e a r ra te l aw above 500~ Vapor iza t ion of MoO3 occurred at 650~ in 1 a rm of oxygen Catas t rophic ox ida t ion took place at 725~

Jones Mosher Speiser and S p r e t n a k (2) oxidized m o l y b d e n u m in sti l l a ir b e t w e e n 701 ~ a nd 983~ At 938~ (MOO3)3 vola t i l ized as fast as it was formed The ac tua l r a t e was less t h a n t ha t f ound at 816~ L u s t m a n (6) also found a n e a r l y cons tan t ra te of ox ida t ion above 795~ Pe te r son and Fassel (7) s tudied the ox ida t ion reac t ion as a func t ion of pressure The ra te of ox ida t ion fol lowed a n e a r l y l i nea r ra te law at all t e m p e r a t u r e s and pressures ind ica t ing a nonpro t ec t i ve oxide was formed MoO3 was the on ly oxide observed in the scale

Three s tudies have b e e n made for condi t ions above 1000~ S e m m e l (8) s tud ied the reac t ion in free flowing air b e t w e e n 982 ~ and 1371~ A l i nea r ra te l aw was found The reac t ion was insens i t ive to the flow ra te and to the reac t ion t e m p e r a t u r e Bar t l e t t and Wi l l i ams (9) s tud ied the reac t ion in air b e t w e e n 760 ~ and 1204~ us ing a flow system The ra te of ox ida t ion increased s lowly w i th t e m - p e r a t u r e and flow rate

Modiset te and Sch rye r (10) inves t iga ted the role of gaseous diffusion in the ox ida t ion of m o l y b d e n u m

952

Vol 110 No 9 O X I D A T I O N O F M O L Y B D E N U M 55~176

for the t e m p e r a t u r e r ange of 1063~176 and for flow velocit ies of 36-195 cmsec The ox ida t ion ra te inc reased s lowly w i th t empe ra tu r e flow veloci ty and diffusivi ty

Since the vo la t i l i ty of solid and l iqu id MoO3 is d i rec t ly invo lved in the ox ida t ion of m o l y b d e n u m we have r ev i ewed the l i t e r a tu re and p resen ted a s tudy of the vapor p ressure of solid MoO3 (11) in a separa te paper

Experimental Since oxygen reacts w i th m o l y b d e n u m u n d e r

ce r t a in condi t ions to fo rm both oxide scale a nd a vola t i le m o l y b d e n u m tr ioxide it is essent ia l to fol low the reac t ion by both oxygen consumpt i on and weigh t change me thods (12 13) A gold p l a t ed I n v a r b e a m ba l ance enclosed in the reac t ion sys tem was used (14) The ba lance had a per iod of less t h a n 2 sec and a sens i t iv i ty of 66 t~g0001 cm deflec- t ion at 725 cm us ing a sample weigh t of 0872g The oxygen pressure was cont ro l led by l eak ing in oxygen f rom a ca l ib ra ted v o l u m e to m a i n t a i n con- s t an t p ressure in the reac t ion system The p ressure in the aux i l i a ry v o l u m e was accura te ly r ead and the oxygen used was calculated A n 8-rai l p l a t i n u m wi re was used to suppor t the spec imen in the hot zone of the furnace

The fu rnace tubes were h i g h - p u r i t y v a c u u m - t i g h t a lumina T e m p e r a t u r e s up to 1600~ were ob ta ined by use of a special K a n t h a l - S u p e r f u rnace (12 15) Ca l ib ra t ed P t - - P t 5 1 0 Rh the rmocoup les were used to m e a s u r e the t e m p e r a t u r e ins ide the f u r - nace t ube and ad jacen t to the samples

Spec imens were m a c h i n e d f rom p u r e m o l y b d e - n u m rod and pol ished t h r o u g h 4 0 po l i sh ing paper Samples were t h e n c leaned in p e t r o l e u m e ther a nd alcohol

The s t a n d a r d spec imen was a cy l inder w i th h e m - i spher ica l ends 0316 cm in d iameter 15 cm long we igh ing about 0872g and h a v i n g a sur face a rea of about 1220 cm 2 Sma l l e r spec imens h a v i n g s u r - face areas of abou t 0610 0304 and 012 cm 2 were used to d e t e r m i n e the effect of surface area on the reac t ion rate A spectroscopic ana lys i s showed the fo l lowing impur i t i e s in pa r t s per mi l l i on Cu 10 Cr 45 Mn 5 A1 40 Fe 200 Ca 10 Ni 70 Sn 10 Mg 5 Si 50 and B 1 The e lements Ba Sr Pb Co Ag Cd V Nb and Ti were no t detected

Thermochemicai Calculations

Thermochemica l da ta have been d e t e r m i n e d for the two oxides MoO2 and MoOs (16) A r ecen t r e - v iew of the vapor p re s su re da ta has been made (11) Table I shows five reac t ions of i n t e re s t i n this work Values of the s t a n d a r d free energies of reac t ion and e q u i l i b r i u m pressures are listed

The e q u i l i b r i u m da ta show tha t bo th MoO3 a nd MoO2 are s table to direct decomposi t ion MoO3 can dissociate to MoO2 in h igh v a c u u m above its m e l t i n g point 795~ (11) MoOs is r educed at al l t e m p e r - a tures by Mo to form MoOe MoO3 has an apprec i - able vapor p ressure above 500~ (1 11) At 600~ the vapor pressure of (MoO3) is 608 x 10 -6 atm whi le at 700~ the vapor p ressure is 469 x 10 -4 (11) At the mp of 795~ the vapor p ressure is 01 a tm (11) t he bp is 1155~

Table I Thermochemical data oxides of molybdenum

1 M o ( s ) + O 2 ( g ) ~-~ M o O ~ ( s ) AF ~ k c a l m o l e 2 M o (s) + 3 2 0 ~ (g) ~- 2MoOa (s1) AF ~ k c a l r n o l e 3 2MoO3(s 1 ) ~-- 2MoO~(s 1 ) + O ~ ( g ) P o 2 a r m 4 3 M o O z ( s ) ~- 2MoOs(s 1 ) + M o (s) AF ~ k c a l 5 n M o O a ( s ) ~ - ( M o O a ) n ( g ) a t 6 0 0 ~ n = 3 24

9 5 3

Temp ~

1 2 3 4 AF ~ AF ~ Pos AF~

kcalmole kcalmole arm kcal

298 --t2745 --1597 538 X 10 -4s -55630 400 --1229 --15345 444 X 10 -24 556185 600 --1141 --14135 142 X 10 -2o -5596 800 --10555 --1296 723 X 10 -14 -55745

1000 --9715 --1181 710 X 10 -1~ -5553 1200 --8895 --1127 951 X 10 -2 -5504 1400 --8095 --10825 156 X 10 -6 -55437 1 6 0 0 - - 7 3 0 5 - - 9 9 5 5 - - - -

1 8 0 0 - - 6 5 3 5 - - - - - -

2000 --580 - - - - - -

Results The e x p e r i m e n t a l work was p l a n n e d a r o u n d

three object ives Firs t i t was essent ia l to d e t e r m i n e the n a t u r e of the reac t ion b e t w e e n 550 ~ and 1500~ Second the k ine t ics of ox ida t ion was s tud ied over a wide p ressure a nd t e m p e r a t u r e r ange to d e t e r m i n e the p r i m a r y chemica l react ion Third it was essent ia l to d e t e r m i n e the t r a n s i t i o n zone b e t w e e n chemical a nd t r a n s p o r t cont ro l led ox ida - t ion Here it was necessa ry to in t roduce the sur face area as a n e w var iab le

Oxidation p~ocesses at 600~ and 76 Tovr p~es- sure--Curves A a nd B of Fig 1 show oxygen con- s u m p t i o n a nd we igh t change m e a s u r e m e n t s Both m e a s u r e m e n t s a re i n un i t s of m i l l i g r a m s per squa re cen t imeter The oxygen c onsumpt i on da ta show a n e a r l y l i nea r ra te l a w af ter an in i t i a l per iod of fast react ion The we igh t change da ta show a slow in i - t ia l r eac t ion fo l lowed b y a per iod of inc reas ing ra te of react ion

Equa t i on [1] re la tes the oxygen used to the for- m a t i o n of solid l iquid or gaseous m o l y b d e n u m t r i - oxide

Mo(s ) -k 32 02 ~ MoO3(slg) [1]

The we igh t change g iven b y the ba lance read ings indicates the difference b e t w e e n the oxide fo rmed and m o l y b d e n u m lost as vola t i l ized oxide according to the equa t ion

16 ~ A

~ ~o~ ~o ~ ~ ~ ~

~ 0

~ E ~ -~o--=~ C

80 40 80 120 160 200 2a) ~0 ~20 360 ~00 ~ 0

Time (min)

Fig 1 Oxidation of molybdenum 600~ 76 Torr curve A 02 cosumed B weight change C molybdenum lost D 02 in oxide scale E volatility of MoO3 in vacuum

954 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y

3MoO~(s1) ~ (MoOs)s(g) [2]

F rom Eq [1] and [2] and the laws of s to ichiometry we set down the fol lowing equations

Wo = x W o + (1 - -x )Wo [3]

W B = x W o - W M o [ 4 ]

Here Wo is the weight of oxygen consumed WB the weight change of the balance WMo the weight of mo lybdenum volati l ized and x and 1--x the f rac - t ion of oxygen used to form oxide scale and volat i le oxide fol lowing Eq [1] and [2] F rom the atom weights of Mo and O in MoO3 we have

VVMo = 2 ( l - - x ) Wo [5]

Subt rac t ing [4] and [3] and subst i tu t ing [5] we have

Wo - - WB = 3 ( l - - x ) [6]

WMo = 23 ( W o - - WB) [7]

Using Eq [7] we calculate the weight of mo lyb - denum lost This is shown as curve C of Fig 1 Using Eq [4] and [7] we calculate the weight of oxygen forming oxide scale This is shown in Curve D of Fig 1 Curve E is the vo la t i l i ty curve for molybdenum t r ioxide in vacuum (11)

To re la te curve D to oxide thickness in angstroms a factor of 66500 is used (1) Thickness ma rke r s are placed on Fig 1 This evaluat ion assumes MoO3 as the oxide a surface roughness rat io of un i ty and the oxide is not porous or full of cracks

The to ta l weight of mo lybdenum reac t ing can be calcula ted f rom curve A using the s toichiometr ic

3 rat io of M o - ~ 02 of 200 while the surface recession

in angstroms can be calculated f rom curve A using the stoichiometric ra t io of 200 and the dens i ty of 102 A factor of 19600 is evaluated

F igure 1 shows severa l in teres t ing facts for the 600~ 76 Torr react ion conditions Both oxide scale format ion and oxide vola t i l i ty occur Eighty per cent of the oxygen used goes to oxide scale formation A near ly l inear ra te of oxidat ion is observed Loss of mo lybdenum occurs ve ry r ap id ly dur ing the ini t ia l per iod of reaction This ra te decreases as oxidat ion proceeds A s tudy of Fig 1 shows the inadequacy of using weight change methods alone to descr ibe the react ion in this t empera tu re range

F igure 2A and B shows photographs of the u n r e - acted and oxidized specimens The oxidized specimen shows a poor qua l i ty oxide scale was formed

Oxidat ion studies at 650 ~ and 700~ at 76 Torr oxygen pressure show similar phenomena to tha t observed at 600~ The percentage of oxygen forming oxide decreases as the t empera tu re is raised At 700~ only 30 of the oxygen used forms oxide scale At 800~ all of the oxygen used forms vola- t i le molybdenum tr ioxide At 795~ the vapor p res - sure of mo lybdenum t r iox ide is 117 Torr

Oxidat ion processes at 1000~ and 76 Tor t pres- s u r e - - A t 1000~ al l of the oxygen used forms vo la - t i le mo lybdenum tr ioxide Curves A and B of Fig 3 show the oxygen consumption and weight change

S e p t e m b e r 1963

Fig 2 Photographs of molybdenum specimens A unreacted B 600~ 76 Torr 420 min C 1200~ 76 Tort 889 min D 1600~ 76 Torr 7 min Magnification approximately 5X

T E

j j

20 v d ~

B

~80 ~

100

120 0 2 4

P f

- C

5 $ 10 12 14 Time (min

c

1P

Fig 3 Oxidation of molybdenum 1000~ (I047) 76 Torr A oxygen consumption B weight loss A - - A C calculated from A O - - O

measurements Near ly l inear rates of react ion are found The smal l decrease in slopes can be re la ted to the decrease in specimen area dur ing reaction

3 Using the s toichiometr ic ra t io of Mo-~- Oe curve

A can be used to calculate the expected weight loss of molydenum assuming all oxygen forms volat i le molybdenum tr ioxide Good agreement is found which confirms the na ture of the reaction The recession of mo lybdenum can be calcula ted using the re la t ion 1 m g c m 2 ~ 19600A

Many units are used in present ing react ion rates We prefe r the unit of atoms of Mo per cm 2 per sec The ini t ia l ra te of react ion of the 1000~ oxidat ion

Vol 110 No 9 O X I D A T I O N O F M O L Y B D E N U M 55~176 955

at 76 Torr is 108 x 10 TM at cm2sec This is a ve ry rap id react ion

For these h igh ra tes of reac t ion it is essent ia l to have a more real is t ic va lue for the surface t e m p e r a - ture The surface t e m p e r a t u r e d u r i n g reac t ion can be es t imated We assume (a) tha t r ad ia t ion is the ma jo r source of loss of heat (b) the emiss iv i ty of the sur face and wal ls is 05 and the hea t source is the sum of the hea t of fo rma t ion of MoO3(s1) (16) and the hea t of vapor i za t ion of the oxide (11) For the condi t ions of the p resen t e x p e r i m e n t at 1000~ we es t imate a sample sur face t e m p e r a t u r e of 1047~ In all of our tables and figures we list bo th the f u r - nace t e m p e r a t u r e and the ca lcula ted t e m p e r a t u r e

We conclude tha t b e t w e e n 600 ~ and 800~ both oxide sca]e f o rma t ion and oxide vo la t i l i ty occur Above 800~ on ly vola t i le m o l y b d e n u m t r iox ide is formed We wi l l n e x t p resen t the effect of t e m p e r a - t u r e on the ox ida t ion react ion

Effect o] temperatureiFigures 4 and 5 and Tab le I I show the effect of t e m p e r a t u r e on the ox ida t ion of m o l y b d e n u m at 76 Torr oxygen pressure The weigh t change in m g c m 2 is p lo t ted aga ins t t ime in minu tes F igu re 4 shows e x p e r i m e n t s for the t e m - p e r a t u r e r ange 550~176 Oxide scale fo rma t ion

-20 - - - - - - -

- - 817~ Reacted -so I

0 l] 20 30 40 50 60 Time (rain)

B

~A

0 ~ 96

Fig 4 Effect of temperature on oxidation of molybdenum 550 ~ 1000~ 76 Torr 02 A 550~ B 600~ C 650~ D 700~ E 800~ (829) F 900~ (957) G 1000~ (1047)

e7

15

25

e ~ 35

o

~ 45

55

65

7~

amp

E j

2 4 6 8 Time (rain)

Fig 5 Effect of temperature on oxidation of molybdenum 1000~176 76 Torr 02 A 1000~ (1047) B 1100~ (1135) C 1200~ (1227) D 1400~ (1418) E 1600~ (1614)

Table II Effect of temperature on initial rates of oxidation P = 76 Torr surface area = 1215 cm 2

F u r n a c e C a l c u l a t e d dndt l og t e m p ~ t e m p ~ a t o m s c m 2 s e c dndt

700 - - 1493 X 1017 1717 800 829 430X 1017 1763 900 957 1124 X 10 TM 1805

1000 1047 1080 X 10 TM 1803 1100 1135 1099 bull 10 TM 1804 1200 1227 1049 X 10 TM 1802 1400 1418 1280 X 10 TM 1811 1600 1614 1112 X 10 TM 1805

and oxide evapora t ion occur d u r i n g ox ida t ion at t e m p e r a t u r e s be t w e e n 550 ~ and 700~ Tab le II shows the in i t i a l ra tes of to ta l r eac t ion ca lcula ted on the basis of o x y g e n used in the reac t ion a nd the ca lcula ted reac t ion t empera tu res The ra tes of r e - act ion are g iven in un i t s of a toms of m o l y b d e n u m reac t ing per cm 2 per sec F igu re 4 i l lus t ra tes the t r ans i t i on in ox ida t ion p h e n o m e n a b e t w e e n oxide scale fo rma t ion and oxide evapora t ion

F i g u r e 5 shows the resUlts for the t e m p e r a t u r e r ange of 1000 ~ to 1600~ The ca lcu la ted t e m p e r a - tures are g iven in brackets The curves show a smal l decrease in ra te of reac t ion due to the change in surface area On the basis of these resul t s a lone we wou ld conclude tha t t e m p e r a t u r e has l i t t le effect on the ra t e of oxidat ion This wou ld be p red ic ted for ox ida t ion react ions w he r e the ra te of reac t ion is l imi ted by gaseous diffusion of oxygen (10) We wi l l show la te r tha t these conclus ions are i ncom- plete

F igu re 2C and D show pho tographs of the oxidized spec imens af ter r eac t ion at 1200 ~ and 1600~

Effect of pressure--Table I I I shows a s u m m a r y of the da ta us ing the 1215 cm 2 area samples The effect of p re s su re at th ree t e m p e r a t u r e s was studied The in i t i a l ra tes of reac t ion are t a b u l a t e d in mg cm2 sec and in a toms of m o l y b d e n u m reac t ing cm2sec At 800~ the effect of p ressure on the ra te of reac t ion is la rge and the ra t e fol lows the 15 pow e r of the pressure Whi le at 1600~ the effect of p ressure is smal l a nd follows the 014 power of the pressure

Classification of oxidation phenomena--A classi- f ication scheme of the p h e n o m e n a f ound d u r i n g the ox ida t ion of m o l y b d e n u m is shown in Tab le IV

Table III Effect of pressure on initial rates of oxidation surface area = 1215 cm 2

P r e s - F u r n a c e C a l c u l a t e d sure dwd~t dndt l o g t e m p ~ t e m p ~ T o r t m g c m ~ s e c a t c m J Z s e c dndt

800 829 76 00684 430 X 1017 1763 800 808 38 00180 113 X 1017 1705 800 803 19 000733 460 X 10 TM 1666 800 801 5 000327 205 X 10 TM 1631

1200 1227 76 0167 1049 X 10 TM 1802 1200 I224 38 0150 942 bull 1017 1797 1200 1219 19 0116 728 bull 1017 1786 1200 1207 5 0044 276 bull 1017 1744

1600 1614 76 0177 1112 X 10 TM 1805 1600 1612 38 0156 980 X 1017 1799 1600 1611 19 0141 885 X 1017 1795 1600 1610 5 0127 798 X 1017 1790

956 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y S e p t e m b e r 1963

Table IV Classification scheme oxidation of molybdenum

Class R e a c t i o n c o n d i t i o n s O x i d a t i o n p h e n o m e n a R a t e - c o n t r o l l i n g p r o c e s s

1 Below 450~

2 500~176

801~ to t ransi t ion t empera tu re

Above transi t ion tempera ture

Adheren t oxide films or scales form

Oxide scales form also oxide volatilizes low pressure fa- vors volat i l i ty of oxide

Liquid oxide can form vola t i l - izes as soon as oxide forms

Oxide volati l izes as fast as it forms

Wagner type diffusion of meta l or oxygen through oxide

Oxide scales not protect ive Probably chemical - type proc-

esses on meta l interface

Chemical processes on meta l in terface

Transpor t of oxygen to meta l interface Turbulence in gas phase important

F o u r t e m p e r a t u r e reg ions are proposed P r e s s u r e can change the t e m p e r a t u r e l imi t s of the s e v e r a l reg ions w i t h low pressures f a v o r i n g vo l a t i l i t y of the oxide T h r e e types of r a t e - c o n t r o l l i n g processes a re g i v e n in Tab l e IV (a) A W a g n e r t y p e of d i f fu- sion of m e t a l or o x y g e n t h r o u g h the ox ide he re an e n e r g y of ac t i va t i on of 36 kca l has been found (1 4) (b) In the i n t e r m e d i a t e t e m p e r a t u r e r ange w h e r e oxides are not p re sen t a su r face t y p e of chemica l r e - ac t ion is r a t e cont ro l l ing These processes are ad - sorpt ion chemica l react ion and desorpt ion (c) A b o v e a c e r t a i n t r ans i t ion t e m p e r a t u r e a c o m p l e x t y p e of t r a n s p o r t process is found S i m p l e diffusion of o x y g e n t h r o u g h a s t agnan t l aye r as p roposed by Modise t t e and S c h r y e r (10) is no t adequa te This t ype of r eac t ion wi l l be discussed f u r t h e r in a l a t e r section

Study of the transition between chemical control and transport control of oxidation o molybdenum--Gas f low me thods h a v e been used to s tudy the m e c h a n i s m of ox ida t ion (8 -10) U n f o r t u n a t e l y the gas flow was not v a r i e d ove r a sufficient r ange to change the m e c h a n i s m of react ion If t r a n s p o r t of o x y g e n to the sur face and reac t ion p roduc t s a w a y f r o m the sur face to a cold zone is con t ro l l ing the r a t e of oxida t ion the i m p o r t a n t fac to r is the to ta l a m o u n t of r eac t ion occur r ing pe r second The r a t e of o x i d a - t ion per un i t area dndt can be v a r i e d by chang ing the sur face a rea ove r a w i d e range

Table V Effect of sample area on initial rates of oxidation P = 7G Torr

F u r n a c e C a l c u l a t e d S a m p l e dndt log t e m p ~ t e m p ~ a r e a e r o s atcmSsec dndt

1000 1047 1215 108 X 10 is 1803 1000 1124 0604 222 X 10 is 1835 1000 1159 0304 349 bull 10 is 1854

1200 1227 1213 105 bull 10 is 1802 1200 1262 0605 246 bull 10 is 1839 1200 1296 0301 390 bull 1018 1859 1200 1410 0121 792 X 10 is 1890

1400 1418 1216 128 bull 10 is 1811 1400 1451 0605 295 bull 10 is 1847 1400 1509 0304 659 bull 1018 1882

1600 1614 1218 111 bull 10 is 1805 1600 1634 0608 273 bull 1018 1844 1600 1660 0303 494 X 1018 1869

1650 1704 0302 484 bull 10 is 1868

F i g u r e 5 and Tab le II show the r a t e of ox ida t ion to be n e a r l y i n d e p e n d e n t of t e m p e r a t u r e above 800~ us ing a s ample a rea of 12 cm 2 S a m p l e s w e r e n e x t p r e p a r e d h a v i n g areas of abou t 0605 0304 and 012 cm 2 Tab le V shows a s u m m a r y of the data T h e r a t e of ox ida t ion is n e a r l y i n v e r s e l y p ropo r t i ona l to the area ie

dn d t 9 A -~ K (pT) [8]

H e r e dnd t is the ra te of oxida t ion A is the s ample area and K(p T ) is a cons tan t d e p e n d i n g on the p re s su re and t e m p e r a t u r e of oxygen

F i g u r e 6 shows a log dnd t vs 1T plot of the da ta at 76 Tor r pressure The ca lcu la t ed su r face t e m p e r a t u r e s a r e used P a r t of the da ta fa l l a long a s t r a igh t l ine AB We i n t e r p r e t ox ida t ions a long AB as be ing u n d e r chemica l con t ro l w i t h an a c t i v a - t ion e n e r g y of 197 k c a l m o l e Ra te cons tants fa l l ing to the r i gh t of t he l ine AB we i n t e r p r e t as be ing in the r eg ion of t r a n s p o r t process control S m a l l e r va lues for dnd t at a g iven t e m p e r a t u r e are found

The r a t e da ta for the s eve ra l s ample a reas l ie on cu rves C D and E Po in t F is tha t for a 01 cm 2 sample area A r eac t i on r a t e of n e a r l y 1019 a t c m 2 sec was found S ince this po in t lies on the l ine AB w e s ta te t ha t t he r eac t ion is l im i t ed by chemica l control

T h e reac t ion r a t e of 1019 a t c m 2 s e c is t h e h ighes t r eac t ion we h a v e seen r eco rded for an ox ida t ion r e -

Temp ~ 600 700 800 900 I000 1200 1400 1600

20 -- i i i i I i

195

19 -- ~ B

185

~ 18 E

v 175 _

17 I -

14

J

A

13 12 L1 10 09 0$ 07 06 05 iTbull )

Z000 I [ -

Z

C

Z

04

Fig 6 Log dndt vs 1T oxidation of molybdenum 600 1704~ 76 Torr 02 line F-B chemical control (AHAB ~ 197 kcalmole) area to right of A-B diffusion control areas of samples C 0304 cm2 D 0604 cm~ E 1215 cm~ F 012 cm 2

Vol 110 No 9

act ion Us ing a flow s y s t e m w i t h a i r a t 1371~ S e m - m e l (8) f o u n d a r e a c t i o n r a t e of 198 x 10 is a t c m 2 sec M o d i s e t t e and S c h r y e r (10) u s i n g a flow s y s t e m a n d a 215 o x y g e n - h e l i u m m i x t u r e f o u n d a v a l u e of 109 x 10 TM a t cm2sec Bo th v a l u e s l ie c lose to l ine E of Fig 6

F i g u r e 6 also shows ev idence t h a t a m a x i m u m is r e a c h e d in t he o x i d a t i o n r e a c t i o n a t a t e m p e r a t u r e of 1400~176 A b o v e th is t e m p e r a t u r e t h e r a t e decreases This effect m a y be r e l a t e d to t he d i s - soc ia t ion of t he ( M o O s ) complex

We conc lude t h a t v a r i a t i o n of s a m p l e a r e a m a k e s poss ib le a s t u d y of t h e t r a n s i t i o n in m e c h a n i s m s of o x i d a t i o n of m o l y b d e n u m Also v e r y h igh r e a c t i o n r a t e s can be m e a s u r e d us ing s m a l l spec imens

Capabi l i ty of a reac t ion s y s t e m for m e a s u r e m e n t of f a s t r e a c t i o n s - - T h e r e su l t s of t he p r e v i o u s sec- t ion h a v e s h o w n t h a t t r a n s p o r t p rocesses l i m i t t he m e a s u r e m e n t of fas t r eac t ions in a r e a c t i o n sys tem w h e r e v o l a t i l e r e a c t i o n p r o d u c t s a r e fo rmed W e d e - fine t he c a p a b i l i t y as t he m a x i m u m o b s e r v e d to t a l r e a c t i o n r a t e in un i t s of a toms p e r second E q u a - t ion [8] g ives t he r e l a t i o n s h i p b e t w e e n c a p a b i l i t y K ( p T ) d n d t and su r f ace area F o r m o l y b d e n u m ox iu ized at 76 T o r r p r e s s u r e and 1400~ ou r s y s t e m h a d a v a l u e of K ( p T ) of 155 to 20 x 10 is at of m o l y b d e n u m r e a c t i n g p e r second L a r g e r v a l u e s w o u l d be f o u n d at h i g h e r p re s su res T e m p e r a t u r e also has an effect as can be d e t e r m i n e d f r o m the d a t a in T a b l e V I f a s a m p l e a r e a of 01 cm 2 is used a r eac t i on r a t e of a b o u t 2 x 1019 a t c m 2 s e c could be m e a s u r e d A c a p a b i l i t y of 2 x 1018 a t s ec a l lows one to m e a s u r e t he p r i m a r y c h e m i c a l r e a c t i o n ove r a t e m p e r a t u r e r a n g e of 600~176

Ca lcu l a t i ons on S e m m e l s (8) and Mod i se t t e s and S c h r y e r s (10) s y s t e m us ing a flow s y s t e m and o x y - gen at a b o u t 150 T o r r a n d 1371~ showed c a p a b i l i t y cons tan ts K ( p T ) of 72 x 1018 a n d 62 x 1018 r e - spec t ive ly M o d i s e t t e and S c h r y e r used s a m p l e s of a b o u t 63 cm 2 I t was no t pos s ib l e to e s t i m a t e t he va lue s for S e m m e l s sys tem

C o n s i d e r i n g the p r e s s u r e f ac to r in the c a p a b i l i t y n u m b e r w e conc lude t h a t t he use of gas flow b y M o d i s e t t e and S c h r y e r (10) i n c r e a s e d the s y s t e m c a p a b i l i t y b y a f ac to r of 2 to 3

Discussion S u m m a r y of k ine t i c w o r k - - I n t he p r e v i o u s sec-

t ions t he p r i m a r y c h e m i c a l r e a c t i o n of p u r e m o l y b - d e n u m was s t ud i ed us ing c y l i n d r i c a l spec imens of s e v e r a l sizes A b o v e 800~ and a t p r e s s u r e s up to 76 T o r r t he o x y g e n c o n s u m p t i o n and w e i g h t c h a n g e curves s h o w e d no ev idence of an in i t i a l p i c k u p of o x y g e n to fo rm an o x i d e film A l l of t he o x y g e n r e - ac t ed to f o r m vo la t i l e m o l y b d e n u m t r iox ide The o b - s e r v e d w e i g h t loss and o x y g e n c o n s u m p t i o n cu rves w e r e n e a r l y l i n e a r w i t h t ime F o r a shor t p e r i o d of r e a c t i o n the d a t a cou ld be f i t t ed to t he e q u a t i o n W = A t w h e r e W is t he w e i g h t loss in m g c m 2 A is a cons tan t a n d t is t he t ime F o r l onge r p e r i o d s of t ime su r f ace a r e a changes o c c u r r e d w h i c h d e c r e a s e d the r a t e of w e i g h t loss

The in i t i a l r a t e cons t an t d n d t could be fit~ed to an e x p o n e n t i a l e q u a t i o n d n d t = Ze -z~IRT A h e a t of a c t i va t i on of 197 k c a l m o l e was found w h i l e t h e

O X I D A T I O N O F M O L Y B D E N U M 55~176 957

f r e q u e n c y fac to r has t he un i t s of a t o m s of m o l y b - d e n u m r e a c t i n g p e r c m 2 p e r sec

The effect of p r e s s u r e on the o x i d a t i o n of m o l y b - d e n u m was s t u d i e d a t 800 ~ 1200 ~ a n d 1600~ A t 800~ t h e r e su l t s f o l l o w e d t h e 15 p o w e r of t h e p r e s s u r e w h i l e a t 1600~ the r e su l t s f o l l o w e d t h e 014 p o w e r of t he p r e s su re

P r o v i d i n g s m a l l s a m p l e s w e r e used t h e r e a c t i o n of m o l y b d e n u m w i t h o x y g e n cou ld be s t u d i e d in t he c h e m i c a l c o n t r o l l e d r eg ion to 1400~

M e c h a n i s m o~ r e a c t i o n - - A su r face r e a c t i o n m a y be s e p a r a t e d into a t l e a s t five d i s t i nc t processes t h e s lowes t of w h i c h d e t e r m i n e s t he r a t e of r eac t i on (a ) t r a n s p o r t of o x y g e n gas to t h e su r f ace (b ) c h e m i s o r p t i o n of t he oxyge n (c) c h e m i c a l r e a c t i o n a t t he su r face (d ) de so rp t i on (e ) t r a n s p o r t of r e - ac t ion p r o d u c t s away P rocess ( a ) a n d (e) a r e t r a n s p o r t p rocesses and if r a t e con t ro l l ing t h e t e m - p e r a t u r e d e p e n d e n c e of t h e r e a c t i o n r a t e m a y v a r y as T 12 w h e r e T is t he abso lu t e t e m p e r a t u r e C h e m - ical r e ac t i ons ( b ) ( c ) a n d (d ) u s u a l l y h a v e h igh a c t i v a t i o n ene rg i e s a n d a r e u s u a l l y d i s t i n g u i s h e d b y th is f ac to r f r o m di f fus ion processes

Predic t ions o f absolute reac t ion ra te t h e o r y - - T h i s t h e o r y a s sumes t h e f o r m a t i o n of a c o m p l e x b e t w e e n the r e a c t i n g gas and the sur face t he c h e m i s o r b e d gas and the sur face and t h e c h e m i s o r b e d r e a c t i o n p r o d u c t a n d the sur face The r a t e of a n y one of these su r f ace r eac t i ons m a y be c ons ide r e d in t e r m s of r e a c t i o n c o m p l e x e s p a s s i n g f r o m one r e g i o n of conf igu ra t ion space to ano the r A c c o r d i n g to E y r i n g and c o - w o r k e r s (17 18 ) t he n u m b e r of r e a c t i o n c o m p l e x e s c ross ing the e n e r g y b a r r i e r is g iven b y the p r o d u c t of t he n u m b e r of c o m p l e x e s in t he i n i - t i a l s t a t e a t t i m e t t h e p r o b a b i l i t y t h a t t he r e a c t i o n c o m p l e x crosses the b a r r i e r in a n y one a t t e m p t a n d the f r e q u e n c y w i t h w h i c h the c o m p l e x e s cross the e n e r g y b a r r i e r

A d s o r p t i o n - - E y r i n g a n d c o - w o r k e r s (17 18) have g iven the f o l l o w i n g exp re s s ions fo r t h e s e v e r a l t y p e s of a d s o r p t i o n processes

1 I m m o b i l e adso rp t ion a d s o r p t i o n of molecu le r a t e - d e t e r m i n i n g

~r h 4 Vl = C g C s - e -elkT [9]

~$ 8~r2I ( 2~rmkT ) 32

2 I m m o b i l e adso rp t ion d i s soc ia t ion is r a t e - c o n - t r o l l i n g p rocess

h82 Vl = Cgl2CskT e-el kT [10]

(2~rmkT) 34 ( 8~r2ikT) 12

3 Mob i l e a d s o r p t i o n

k T h vl = Cg - - e-el~T [11]

h (2~rmkT) 12

4 Mobi l e adso rp t ion no a c t i va t i on e n e r g y

P Vl = [12]

( 2~rmkT) 12

He re t he symbo l s h a v e the fo l lowing def in i t ions C~ c o n c e n t r a t i o n of mo lecu le s p e r cubic c e n t i m e t e r in t h e gas phase Cs c o n c e n t r a t i o n of a d s o r p t i o n si tes p e r squa re c e n t i m e t e r ~ s y m m e t r y n u m b e r of t he gas mo lecu le 0-$ s y m m e t r y n u m b e r of t h e a c t i -


va ted complex h P l a n c k s cons tan t I m o m e n t of i n - er t ia k Bo l t zmann s cons tan t m mass of molecule T absolu te t e m p e r a t u r e and e ene rgy of ac t iva t ion R a t e o f d e s o r p t i o n - - D e s o r p t i o n f rom an immobi l e l ayer m a y be r ega rded as i nvo lv ing an ac t iva ted s tate in which a molecule a t tached to an adsorb ing cen te r acquires the necessa ry conf igura t ion and ac- t i va t ion ene rgy to p e r m i t it to escape f rom the s u r - face In the fo l lowing ra te express ions g iven by Eyr ing and co -worke r s (17 18) both ac t iva ted com- plexes and adsorbed molecules are cons idered i m - mobile

k T V2 = C a - - e - e 2 k T [13]

h

Here ve represen t s the ra te of desorp t ion in mole - cules per square cen t ime te r per second C r e p r e - sents the concen t ra t ion of adsorbed molecules per square cen t imete r and ee is the ene rgy of ac t iva t ion C h e m i c a l r e a c t i o n - - L e t us a s sume the reac t ion i n - volves one molecule of oxygen and the act ive su r - face site S This ac t ive site is a s sumed to consist of a si te on which oxygen has b e e n p rev ious ly ad - sorbed The ac t iva ted complex consists of an adsorbed molecule which has acqu i red the appropr i a t e a m o u n t of ene rgy and the p roper configurat ion

F i r s t - O r d e r K i n e t i c s - - C o n s i d e r the case w h e n the ac t ive sites a l r eady have an oxygen a tom a t - tached to the m o l y b d e n u m atoms If the sur face is covered w i th Mo a toms hav ing one oxygen a tom adsorbed per m o l y b d e n u m atom the concen t r a t i on of sites Cs is n e a r l y cons tan t and iden t ica l w i th the n u m b e r of sites for a ba re surface U n d e r these con- d i t ions the r a t e of the reac t ion is p ropor t iona l to the concen t r a t i on of the molecules in the gas phase Cg and the reac t ion is of first order

The ra te express ion is

Yzsh 4 V = C g C s - - - - s X e - e 3 k T [14]

8~r2I ( 2~rmkT) 32 ~4

where s is the tota l n u m b e r of possible sites ad jacen t to a n y reac t ion center (r a nd ~$ are the s y m m e t r y n u m b e r s of the molecules of r eac t an t a nd ac t iva ted complex respect ively a nd e8 is the ene rgy of act i - va t i on for this type of react ion

Z e r o - O r d e r K i n e t i c s - - L e t us assume the act ive site a l r eady has an oxygen a tom a t t ached a nd tha t these sites are covered by adsorbed molecules to an apprec iab le extent The va l ue of Cs var ies w i th the p ressure of the gas If the surface is n e a r l y covered by adsorbed molecules Cs is n e a r l y cons tant and the ra te of reac t ion is n e a r l y i n d e p e n d e n t of the pressure The fo l lowing equa t i on t rea t s the reac t ion f rom the v i e w po i n t of the adsorbed molecules w i th the surface ac t iva t ion ene rgy be ing the difference in ene rgy b e t w e e n the ac t iva ted s tate and the ad - sorbed reac tants or Eo + E

k T V2 = C a ~ e - E R T [15]

h

where E is the observed ac t iva t ion energy e is the heat of adsorpt ion and so is the difference in ene rgy b e t w e e n the ac t iva ted s ta te and the in i t i a l gaseous reac tan t

C o m p a r i s o n of t h e o r y w i t h e x p e r i m e n t - - T a b l e VI shows a compar i son of the ra tes of the va r ious processes at 900~ and 76 Tor r oxygen p res su re as pred ic ted f rom the abso lu te reac t ion ra te theory wi th the e x p e r i m e n t a l l y d e t e r m i n e d ra te of r eac - t ion The ca lcula t ions were based on an exper i - m e n t a l heat of ac t iva t ion of 197 kca l mo le The fact t ha t several processes occur w i th a theore t ica l ra te s lower t h a n the e x p e r i m e n t a l va lue m e a n s the hea t of ac t iva t ion was too h igh for this pa r t i cu l a r process The compar i son was s ignif icant on ly for those processes which give r easonab le ag reemen t

The only feasible m e c h a n i s m according to Tab le VI is mobi le adsorp t ion of oxygen molecules on a m o l y b d e n u m surface a l r eady covered w i th a surface l ayer of oxygen

Table VI Correlation of predictions of absolute reaction rate theory with experimental rate of oxidation of molybdenum at 900~ 76 Torr pressure of oxygen

M e c h a n i s m

R a t e dndt A t o m s of Mo t = 0 ~ s e e

Equation Theory Experiment

Immobile adsorption adsorption of molecule rate controll ing

h 4

v = CgCs o$ 8n2I (2~mkT) s2

Immobile adsorption dissocia- k T tion rate control l ing v ~ CgI2Cs

e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12

Mobile adsorption k T h V ~ Cg e - e k T

h ( 2 ~ m k T ) l2

Mobile adsorption no activation p energy v --~ ( 2 n m k T ) 12

Desorption k T V ~ Ca e - e l k T

h

Chemical reaction first order r 8 9 4 kinetics v ----- CgCs aS 8 ~ I ( 2 ~ m k T ) 32

Chemical reaction zero order kinetics

e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM


Condensation i O~ ii = - Condensed (MOO]) s Zone- = ii ~iit

9 9

Hot Zone

Fig 7 Schematic picture of reaction system

Furnace TubeDiam ( D )

Cloud (MOO3) n Mo Sample Diam (d)

In Tab le VI we a s s u m e d t h a t a l l of t he gas was i n v o l v e d in mob i l e a d s o r p t i o n w i t h an a c t i v a t i o n e n - e r g y of 197 k c a l m o l e A c t u a l l y w e p o s t u l a t e a m o n o l a y e r of o x y g e n is p r e l i m i n a r i l y a d s o r b e d w i t h a m u c h l o w e r a c t i v a t i o n ene rgy The r e a c t i o n m e c h a n i s m is

O

Mo 9 Mo--O + 02-gt Mo 9 Mo~O

O

mobile adsorption of 02 on Mo--O monolayer

This complex undergoes chemical reaction

O

Mo 9 Mo~O -~ Mo--MoO3

O

and desorption to form gaseous molybdenum trioxide

n M o - - M o O 8 ~ n Mo + (MoO3)n

The e x p e r i m e n t a l v a l u e in T a b l e VI of 108 x 10 TM at M o c m 2 s e c shou ld be r e d u c e d b y 13 to 72 x 1017 to accoun t for o x y g e n p r e l i m i n a r i l y a d s o r b e d in the m o n o l a y e r The a g r e e m e n t of t h e o r y and e x - p e r i m e n t is w i t h i n a f ac to r of 2

Interpretation of transport phenomena--Equa- t ion [8] r e l a t e s t he o b s e r v e d r a t e o2 o x i d a t i o n to s p e c i m e n area Va lues of dndt a re a l w a y s less t h a n the v a l u e s for c h e m i c a l con t ro l (dndt)c A s c h e - ma t i c d r a w i n g of t he r e a c t i o n s y s t e m is shown in Fig 7 D u r i n g r e a c t i o n the s p e c i m e n is s u r r o u n d e d b y a zone of (MoO~)8 vapor F r o m the e q u a t i o n

3Mo + 9 2 02-gt (MoO3)3(g)

a c h a n g e in v o l u m e of 3I2 is obse rved O x y g e n gas is a c c e l e r a t e d t o w a r d the s a m p l e due to t he v o l u m e

959

c h a n g e a n d to t he r eac t ion L o c a l i z e d h e a t i n g occurs and l a r g e a m o u n t s of MoO3 a r e fo rmed W e v i sua l i ze t he r e a c t i o n zone as v e r y t u r b u l e n t As long as t he ra t ios of L 1 and Dd (see Fig 7) a r e l a rge o x y g e n diffuses to t he i n n e r t u r b u l e n t r e a c t i o n zone U n d e r r e a c t i o n cond i t ions t he t r a n s p o r t of o x y g e n to t he r e a c t i o n zone equa l s t h e t o t a l r a t e of r eac t ion The r e l a t i o n of a r e a A a n d dndt b r e a k s d o w n as dndt a p p r o a c h e s (dndt)c I n t he t u r b u l e n t r e g i o n a n d for l a r g e va lue s of L 1 a n d Dd t h e s p e c i m e n r e - acts w i t h a l l o x y g e n in t h e r e a c t i o n zone

Manuscr ip t rece ived Jan 28 1963 rev ised m a n u - script received Apr i l 1 1963 This paper has been scheduled for presenta t ion at the New York Meeting Sept 29-Oct 3 1963

Any discussion of this paper wi l l appear in a Discus- sion Sect ion to be publ i shed in the June 1964 JOURNAL

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6 B Lustman Met Prog 57 629 (1950) 7 R C Pe te rson and W M Fassel Jr Technical

Repor t VI A r m y Ordnance Contract DA-04-495 ORD-237 Sept 1 1954

8 J W Semmel Jr High Tempera tu r e Mater ia ls p 510-19 J Wi ley amp Sons Inc New York (1959)

9 E S Bar t l e t t and D N Wil l iams Trans AIME 212 280 (1958)

10 J L Modiset te and D R Schryer N A S A - T N - D - 222 March 1960

11 E A Gulbransen K F Andrew and F A Bras - sart This Journal 11 242 (1963)

12 E A Gulbransen K F Andrew and F A Bras - sart Kinet ics of Oxida t ion of Pure Tungsten 1150~176 West inghouse Research Lab Sci- entific Pape r 62-123-121-P1 Apr i l 2 1962

13 E A Gulbransen K F Andrew and F A Bras - sart This Journal l l 0 476 (1963)

14 E A Gulbransen and K F Andrew Vacuum Mi- crobalance Techniques Vol 2 p 129 P lenum Press Inc New York (1962)

15 E A Gulbransen K F A n d r e w and F A Bras - sart Vacuum Microbalance Techniques Vol III P lenum Press Inc New York 1963 To be published

16 E G King W W Weller and A U Christensen U S Dept of Int Bureau of Mines RI 5664 (1960)

17 S Glasstone K J Laidler and H Eyring Theory of Rate Processes McGraw-Hi l l Book Co New York (1941)

18 K J Laidler S Glasstone and H Eyring J Chem Phys 8 659 (1940)

Oxidation of Molybdenum 550 ~ to 1700~

E A Gulbransen K F Andrew and F A Brassart Physical Chemistry Department Westinghouse Electric Corporation Pittsburgh Pennsylvania

ABSTRACT

Weight change and oxygen consumption measurements were used to study the oxidation of molybdenum from 550 ~ to 1704~ for pressures of 5 to 76 Torr For temperatures of 550~176 two processes occurred simultaneously oxide scale formation and molybdenum trioxide volatili ty Above 800~ at pressures up to 76 Tor t molybdenum trioxide volatilized as fast as it formed At 900~ and 76 Torr using 12 cm 2 samples the pr imary chemical reaction gave a rate of about 10 is at molybdenumcm2sec Above this temperature for 12 cm 2 specimens the reaction was l imited by gaseous diffusion of oxygen Little change was found in the rate of oxidation to 1615~ Pressure had only a small effect on the rate of reaction for these react ion conditions However in the chemically con- trolled region pressure had an impor tant effect on the rate of oxidation To extend the tempera ture region where the p r imary chemical reaction was rate controlling samples of small area were used A sample having a total area of 012 cm 2 gave a react ion rate of 8 x 10 is atcm2sec at 1410~ For these very fast reactions appreciable tempera ture rises occurred and the actual sample tempera ture had to be estimated A log K vs 1T plot of the p r imary chemical react ion data gave an energy of act ivation of 197 kcal mole Reaction conditions where gaseous diffusion processes are rate controll ing were determined All of the earlier studies were made for these reaction conditions The activated state theory of surface reactions was applied to the p r imary chemical reac- t ion in the oxidation of molybdenum A mechanism of mobile adsorption was found to be the pr imary chemical reaction This adsorption process probably occurred on a surface already covered with a layer of adsorbed oxygen atoms since MoO3 was volatilized

M o l y b d e n u m and its al loys have m a n y use fu l h i g h - t e m p e r a t u r e mechan i ca l proper t ies However its res i s tance to ox ida t ion is poor A l though m a n y s tudies have been made the m e c h a n i s m s of ox ida- t ion have not b e e n establ ished

The reac t ion of m o l y b d e n u m w i t h oxygen is complex and involves severa l types of ox ida t ion processes The oxide volat i l izes p a r t i a l l y at 600 ~ and 76 Torr oxygen pressure and mel t s at 795~ Except for ox ida t ion be low 450~ e x p e r i m e n t a l resul t s are somewha t conflicting Resul ts appear to depend on the i n d i v i d u a l r eac t ion system spec imen size and gas flow In most studies the p r i m a r y chemica l r eac t ion has been masked b y t r an spo r t processes of o x y g e n t h rough vola t i l ized m o l y b d e n u m t r iox ide to the me ta l surface

The p re sen t work has severa l object ives (a) to d e t e r m i n e the n a t u r e of the ox ida t ion m e c h a n i s m b e t w e e n 550 ~ and 800~ where bo th oxide films are fo rmed and w h e r e oxide vo la t i l i ty occurs (b) to separa te e x p e r i m e n t a l l y the p r i m a r y chemical r e - act ion f rom the diffusion reg ion of reac t ion (c) to d e t e r m i n e the n a t u r e of the p r i m a r y chemica l r e - act ion (d) to define the t r ans i t i on b e t w e e n c he m- ical control and diffusion cont ro l of oxida t ion and (e) to d e t e r m i n e the factors affecting ox ida t ion in the diffusion cont ro l led reg ion of the react ion

Severa l r ev iews of ear l ie r w o r k have been m a d e (1-3) G u l b r a n s e n and Wysong (1) and Gor - b o u n o v a and A r s l a m b 6 k o v (4) f ound a d h e r e n t oxide films fo rmed w h e n the me ta l was oxidized be - low 400~ The da ta were fitted to the pa rabo l i c r a t e law and an e n e r g y of ac t iva t ion of about 360

k c a l m o l e was calculated Above 400~ devia t ions f rom the parabol ic r a t e l aw occurred Vola t i l i za t ion of m o l y b d e n u m t r iox ide occurred at 475~ u n d e r v a c u u m condi t ions (1)

Be t w e e n 500 ~ a nd 1000~ several s tudies have b e e n made (5 -7) S i m n a d and Sp i lne r s (5) f ound the da ta could be fitted by the parabol ic ra te l aw at 500~ and by the l i n e a r ra te l aw above 500~ Vapor iza t ion of MoO3 occurred at 650~ in 1 a rm of oxygen Catas t rophic ox ida t ion took place at 725~

Jones Mosher Speiser and S p r e t n a k (2) oxidized m o l y b d e n u m in sti l l a ir b e t w e e n 701 ~ a nd 983~ At 938~ (MOO3)3 vola t i l ized as fast as it was formed The ac tua l r a t e was less t h a n t ha t f ound at 816~ L u s t m a n (6) also found a n e a r l y cons tan t ra te of ox ida t ion above 795~ Pe te r son and Fassel (7) s tudied the ox ida t ion reac t ion as a func t ion of pressure The ra te of ox ida t ion fol lowed a n e a r l y l i nea r ra te law at all t e m p e r a t u r e s and pressures ind ica t ing a nonpro t ec t i ve oxide was formed MoO3 was the on ly oxide observed in the scale

Three s tudies have b e e n made for condi t ions above 1000~ S e m m e l (8) s tud ied the reac t ion in free flowing air b e t w e e n 982 ~ and 1371~ A l i nea r ra te l aw was found The reac t ion was insens i t ive to the flow ra te and to the reac t ion t e m p e r a t u r e Bar t l e t t and Wi l l i ams (9) s tud ied the reac t ion in air b e t w e e n 760 ~ and 1204~ us ing a flow system The ra te of ox ida t ion increased s lowly w i th t e m - p e r a t u r e and flow rate

Modiset te and Sch rye r (10) inves t iga ted the role of gaseous diffusion in the ox ida t ion of m o l y b d e n u m

952














9 5 3

Temp ~

1 2 3 4 AF ~ AF ~ Pos AF~


298 --t2745 --1597 538 X 10 -4s -55630 400 --1229 --15345 444 X 10 -24 556185 600 --1141 --14135 142 X 10 -2o -5596 800 --10555 --1296 723 X 10 -14 -55745

1000 --9715 --1181 710 X 10 -1~ -5553 1200 --8895 --1127 951 X 10 -2 -5504 1400 --8095 --10825 156 X 10 -6 -55437 1 6 0 0 - - 7 3 0 5 - - 9 9 5 5 - - - -

1 8 0 0 - - 6 5 3 5 - - - - - -

2000 --580 - - - - - -





Mo(s ) -k 32 02 ~ MoO3(slg) [1]


16 ~ A

~ ~o~ ~o ~ ~ ~ ~

~ 0

~ E ~ -~o--=~ C

80 40 80 120 160 200 2a) ~0 ~20 360 ~00 ~ 0

Time (min)



3MoO~(s1) ~ (MoOs)s(g) [2]


Wo = x W o + (1 - -x )Wo [3]

W B = x W o - W M o [ 4 ]


VVMo = 2 ( l - - x ) Wo [5]


Wo - - WB = 3 ( l - - x ) [6]

WMo = 23 ( W o - - WB) [7]












T E

j j

20 v d ~

B

~80 ~

100

120 0 2 4

P f

- C

5 $ 10 12 14 Time (min

c

1P











-20 - - - - - - -


0 l] 20 30 40 50 60 Time (rain)

B

~A

0 ~ 96


e7

15

25

e ~ 35

o

~ 45

55

65

7~

amp

E j

2 4 6 8 Time (rain)




700 - - 1493 X 1017 1717 800 829 430X 1017 1763 900 957 1124 X 10 TM 1805









800 829 76 00684 430 X 1017 1763 800 808 38 00180 113 X 1017 1705 800 803 19 000733 460 X 10 TM 1666 800 801 5 000327 205 X 10 TM 1631


1600 1614 76 0177 1112 X 10 TM 1805 1600 1612 38 0156 980 X 1017 1799 1600 1611 19 0141 885 X 1017 1795 1600 1610 5 0127 798 X 1017 1790




1 Below 450~

2 500~176
















1000 1047 1215 108 X 10 is 1803 1000 1124 0604 222 X 10 is 1835 1000 1159 0304 349 bull 10 is 1854



1600 1614 1218 111 bull 10 is 1805 1600 1634 0608 273 bull 1018 1844 1600 1660 0303 494 X 1018 1869

1650 1704 0302 484 bull 10 is 1868


dn d t 9 A -~ K (pT) [8]





Temp ~ 600 700 800 900 I000 1200 1400 1600

20 -- i i i i I i

195

19 -- ~ B

185

~ 18 E

v 175 _

17 I -

14

J

A

13 12 L1 10 09 0$ 07 06 05 iTbull )

Z000 I [ -

Z

C

Z

04


Vol 110 No 9


















~r h 4 Vl = C g C s - e -elkT [9]

~$ 8~r2I ( 2~rmkT ) 32



(2~rmkT) 34 ( 8~r2ikT) 12


k T h vl = Cg - - e-el~T [11]

h (2~rmkT) 12


P Vl = [12]

( 2~rmkT) 12




k T V2 = C a - - e - e 2 k T [13]

h





8~r2I ( 2~rmkT) 32 ~4



k T V2 = C a ~ e - E R T [15]

h





M e c h a n i s m




h 4



e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12


h ( 2 ~ m k T ) l2



h



e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM



9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959




































9 5 3

Temp ~

1 2 3 4 AF ~ AF ~ Pos AF~


298 --t2745 --1597 538 X 10 -4s -55630 400 --1229 --15345 444 X 10 -24 556185 600 --1141 --14135 142 X 10 -2o -5596 800 --10555 --1296 723 X 10 -14 -55745

1000 --9715 --1181 710 X 10 -1~ -5553 1200 --8895 --1127 951 X 10 -2 -5504 1400 --8095 --10825 156 X 10 -6 -55437 1 6 0 0 - - 7 3 0 5 - - 9 9 5 5 - - - -

1 8 0 0 - - 6 5 3 5 - - - - - -

2000 --580 - - - - - -





Mo(s ) -k 32 02 ~ MoO3(slg) [1]


16 ~ A

~ ~o~ ~o ~ ~ ~ ~

~ 0

~ E ~ -~o--=~ C

80 40 80 120 160 200 2a) ~0 ~20 360 ~00 ~ 0

Time (min)



3MoO~(s1) ~ (MoOs)s(g) [2]


Wo = x W o + (1 - -x )Wo [3]

W B = x W o - W M o [ 4 ]


VVMo = 2 ( l - - x ) Wo [5]


Wo - - WB = 3 ( l - - x ) [6]

WMo = 23 ( W o - - WB) [7]












T E

j j

20 v d ~

B

~80 ~

100

120 0 2 4

P f

- C

5 $ 10 12 14 Time (min

c

1P











-20 - - - - - - -


0 l] 20 30 40 50 60 Time (rain)

B

~A

0 ~ 96


e7

15

25

e ~ 35

o

~ 45

55

65

7~

amp

E j

2 4 6 8 Time (rain)




700 - - 1493 X 1017 1717 800 829 430X 1017 1763 900 957 1124 X 10 TM 1805









800 829 76 00684 430 X 1017 1763 800 808 38 00180 113 X 1017 1705 800 803 19 000733 460 X 10 TM 1666 800 801 5 000327 205 X 10 TM 1631


1600 1614 76 0177 1112 X 10 TM 1805 1600 1612 38 0156 980 X 1017 1799 1600 1611 19 0141 885 X 1017 1795 1600 1610 5 0127 798 X 1017 1790




1 Below 450~

2 500~176
















1000 1047 1215 108 X 10 is 1803 1000 1124 0604 222 X 10 is 1835 1000 1159 0304 349 bull 10 is 1854



1600 1614 1218 111 bull 10 is 1805 1600 1634 0608 273 bull 1018 1844 1600 1660 0303 494 X 1018 1869

1650 1704 0302 484 bull 10 is 1868


dn d t 9 A -~ K (pT) [8]





Temp ~ 600 700 800 900 I000 1200 1400 1600

20 -- i i i i I i

195

19 -- ~ B

185

~ 18 E

v 175 _

17 I -

14

J

A

13 12 L1 10 09 0$ 07 06 05 iTbull )

Z000 I [ -

Z

C

Z

04


Vol 110 No 9


















~r h 4 Vl = C g C s - e -elkT [9]

~$ 8~r2I ( 2~rmkT ) 32



(2~rmkT) 34 ( 8~r2ikT) 12


k T h vl = Cg - - e-el~T [11]

h (2~rmkT) 12


P Vl = [12]

( 2~rmkT) 12




k T V2 = C a - - e - e 2 k T [13]

h





8~r2I ( 2~rmkT) 32 ~4



k T V2 = C a ~ e - E R T [15]

h





M e c h a n i s m




h 4



e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12


h ( 2 ~ m k T ) l2



h



e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM



9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959
























3MoO~(s1) ~ (MoOs)s(g) [2]


Wo = x W o + (1 - -x )Wo [3]

W B = x W o - W M o [ 4 ]


VVMo = 2 ( l - - x ) Wo [5]


Wo - - WB = 3 ( l - - x ) [6]

WMo = 23 ( W o - - WB) [7]












T E

j j

20 v d ~

B

~80 ~

100

120 0 2 4

P f

- C

5 $ 10 12 14 Time (min

c

1P











-20 - - - - - - -


0 l] 20 30 40 50 60 Time (rain)

B

~A

0 ~ 96


e7

15

25

e ~ 35

o

~ 45

55

65

7~

amp

E j

2 4 6 8 Time (rain)




700 - - 1493 X 1017 1717 800 829 430X 1017 1763 900 957 1124 X 10 TM 1805









800 829 76 00684 430 X 1017 1763 800 808 38 00180 113 X 1017 1705 800 803 19 000733 460 X 10 TM 1666 800 801 5 000327 205 X 10 TM 1631


1600 1614 76 0177 1112 X 10 TM 1805 1600 1612 38 0156 980 X 1017 1799 1600 1611 19 0141 885 X 1017 1795 1600 1610 5 0127 798 X 1017 1790




1 Below 450~

2 500~176
















1000 1047 1215 108 X 10 is 1803 1000 1124 0604 222 X 10 is 1835 1000 1159 0304 349 bull 10 is 1854



1600 1614 1218 111 bull 10 is 1805 1600 1634 0608 273 bull 1018 1844 1600 1660 0303 494 X 1018 1869

1650 1704 0302 484 bull 10 is 1868


dn d t 9 A -~ K (pT) [8]





Temp ~ 600 700 800 900 I000 1200 1400 1600

20 -- i i i i I i

195

19 -- ~ B

185

~ 18 E

v 175 _

17 I -

14

J

A

13 12 L1 10 09 0$ 07 06 05 iTbull )

Z000 I [ -

Z

C

Z

04


Vol 110 No 9


















~r h 4 Vl = C g C s - e -elkT [9]

~$ 8~r2I ( 2~rmkT ) 32



(2~rmkT) 34 ( 8~r2ikT) 12


k T h vl = Cg - - e-el~T [11]

h (2~rmkT) 12


P Vl = [12]

( 2~rmkT) 12




k T V2 = C a - - e - e 2 k T [13]

h





8~r2I ( 2~rmkT) 32 ~4



k T V2 = C a ~ e - E R T [15]

h





M e c h a n i s m




h 4



e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12


h ( 2 ~ m k T ) l2



h



e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM



9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959




























-20 - - - - - - -


0 l] 20 30 40 50 60 Time (rain)

B

~A

0 ~ 96


e7

15

25

e ~ 35

o

~ 45

55

65

7~

amp

E j

2 4 6 8 Time (rain)




700 - - 1493 X 1017 1717 800 829 430X 1017 1763 900 957 1124 X 10 TM 1805









800 829 76 00684 430 X 1017 1763 800 808 38 00180 113 X 1017 1705 800 803 19 000733 460 X 10 TM 1666 800 801 5 000327 205 X 10 TM 1631


1600 1614 76 0177 1112 X 10 TM 1805 1600 1612 38 0156 980 X 1017 1799 1600 1611 19 0141 885 X 1017 1795 1600 1610 5 0127 798 X 1017 1790




1 Below 450~

2 500~176
















1000 1047 1215 108 X 10 is 1803 1000 1124 0604 222 X 10 is 1835 1000 1159 0304 349 bull 10 is 1854



1600 1614 1218 111 bull 10 is 1805 1600 1634 0608 273 bull 1018 1844 1600 1660 0303 494 X 1018 1869

1650 1704 0302 484 bull 10 is 1868


dn d t 9 A -~ K (pT) [8]





Temp ~ 600 700 800 900 I000 1200 1400 1600

20 -- i i i i I i

195

19 -- ~ B

185

~ 18 E

v 175 _

17 I -

14

J

A

13 12 L1 10 09 0$ 07 06 05 iTbull )

Z000 I [ -

Z

C

Z

04


Vol 110 No 9


















~r h 4 Vl = C g C s - e -elkT [9]

~$ 8~r2I ( 2~rmkT ) 32



(2~rmkT) 34 ( 8~r2ikT) 12


k T h vl = Cg - - e-el~T [11]

h (2~rmkT) 12


P Vl = [12]

( 2~rmkT) 12




k T V2 = C a - - e - e 2 k T [13]

h





8~r2I ( 2~rmkT) 32 ~4



k T V2 = C a ~ e - E R T [15]

h





M e c h a n i s m




h 4



e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12


h ( 2 ~ m k T ) l2



h



e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM



9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959


























1 Below 450~

2 500~176
















1000 1047 1215 108 X 10 is 1803 1000 1124 0604 222 X 10 is 1835 1000 1159 0304 349 bull 10 is 1854



1600 1614 1218 111 bull 10 is 1805 1600 1634 0608 273 bull 1018 1844 1600 1660 0303 494 X 1018 1869

1650 1704 0302 484 bull 10 is 1868


dn d t 9 A -~ K (pT) [8]





Temp ~ 600 700 800 900 I000 1200 1400 1600

20 -- i i i i I i

195

19 -- ~ B

185

~ 18 E

v 175 _

17 I -

14

J

A

13 12 L1 10 09 0$ 07 06 05 iTbull )

Z000 I [ -

Z

C

Z

04


Vol 110 No 9


















~r h 4 Vl = C g C s - e -elkT [9]

~$ 8~r2I ( 2~rmkT ) 32



(2~rmkT) 34 ( 8~r2ikT) 12


k T h vl = Cg - - e-el~T [11]

h (2~rmkT) 12


P Vl = [12]

( 2~rmkT) 12




k T V2 = C a - - e - e 2 k T [13]

h





8~r2I ( 2~rmkT) 32 ~4



k T V2 = C a ~ e - E R T [15]

h





M e c h a n i s m




h 4



e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12


h ( 2 ~ m k T ) l2



h



e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM



9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959























Vol 110 No 9


















~r h 4 Vl = C g C s - e -elkT [9]

~$ 8~r2I ( 2~rmkT ) 32



(2~rmkT) 34 ( 8~r2ikT) 12


k T h vl = Cg - - e-el~T [11]

h (2~rmkT) 12


P Vl = [12]

( 2~rmkT) 12




k T V2 = C a - - e - e 2 k T [13]

h





8~r2I ( 2~rmkT) 32 ~4



k T V2 = C a ~ e - E R T [15]

h





M e c h a n i s m




h 4



e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12


h ( 2 ~ m k T ) l2



h



e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM



9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959

























k T V2 = C a - - e - e 2 k T [13]

h





8~r2I ( 2~rmkT) 32 ~4



k T V2 = C a ~ e - E R T [15]

h





M e c h a n i s m




h 4



e--ekT 1054 bull 10 TM 108 X 10 TM

hSS

(2~zmkT) 34 (15) 12 (8~2ikT) 12


h ( 2 ~ m k T ) l2



h



e - -e kT

k T V C a - - e - e k T

h

e--ekT 954 X 1015 108 X 10 ls

36 X 1017 108 X l0 TM

138 bull 1022 108 bull 10 TM

129 X 1024 108 X 10 TM

4216 X 10 TM 108 X 1018

129 X 1024 108 X 10 TM



9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959

























9 9

Hot Zone





O


O



O


O


n M o - - M o O 8 ~ n Mo + (MoO3)n



3Mo + 9 2 02-gt (MoO3)3(g)


959























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