A MÖSSBAUER STUDY OF MOLYBDENUM OXIDATION KINETICS

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A MÖSSBAUER STUDY OF MOLYBDENUM OXIDATION KINETICS

A. Zhetbaev, Sh-Sh. Ibragimov, A. Shokanov

To cite this version:

A. Zhetbaev, Sh-Sh. Ibragimov, A. Shokanov. A MÖSSBAUER STUDY OF MOLYBDE- NUM OXIDATION KINETICS. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-387-C1-388.

�10.1051/jphyscol:19801148�. �jpa-00219633�

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JOURNAL DE PHYSIQUE Colloque C1, supplgment au n ^O1 , Tome 41, janvier 1980, page C1-387

A M6SSBAUER STUDY OF YOLYBDENLIM OXIDATION KINETICS

A.K. Zhetbaev, Sh-Sh. Ibragimov and A.K. Shokanov

I n s t i t u t e of NucZear Physics, Kazakh Academy of Sciences, Alma-Ata, USSR.

The MBssbauer e f f e c t of 5 7 ~ e nuclei was successfully applied f o r investigation of many metal oxides /l/ and f o r determina-

tion of t h e i r parameters. However, t h i s method has not been used up t o now f o r

studying molybdenum oxides.

It i s lmown /2/ t h a t molybdenum direct- ly combines with owgen through comparat- ively low heating, oxides of d i f f e r e n t valences being produced depending on the r e l a t i v e content of owgen and on temperature.

When being heated in air, molybdenum is f i r s t coated with an oxide film repre- senting the tetravalent oxide Moo2. Bi-,

tri-, and pentavalent molybdenum oxides are also hown. A t a high temperature of oxidation molybdenum i s capable t o dis- p l w its maximam valence and to form Moo3 t h a t begins intensively sublimating al- ready a t 4 0 0 ~ ~ . This condition reduces the heat-resistance of molybdenum, so it i s not always possible t o use it a s a refrac- tory element. The temperature of the melt- ing and sublimation of the oxide depends on the valence s t a t e of molybdenum. Owing to t h i s , investigation of the k i n e t i c s of the formation of molybdenum oxides depend-

ing on the air-annealing temperature and on t h e i r stslucture i s of special signific- ance. To t h i s aim, the l15ssbauer e f f e c t can successfully be used.

Samples were prepared from a 99.5% molybdenum f o i l 20fi i n thickness. The ex- periments were carried out using an emis- sion method. The sample with 5 7 ~ o isotope on i t s surface was subjected t o diffusion annealing f o r two hours a t 1 0 0 0 ~ ~ in by- drogen current i n a qua~tz-tube furnace.

Samples of %odoped Moo3 were prepared using the same method, but the annealing was carried out i n a i r . The monitoring absorber was 57~e-enriched potassium f er-

rocyanide. A l l the measurements w e r e car- r i e d out a t room temperature, using a con- s t a n t acceleration spectrometer. The MBss- bauer spectrum of the metallic sample re- presented a single l i n e f =0.&.03 d s in width and with

6

=-0.2350.03 mm/s isomer s h e t r e l a t i v e t o 5 7 ~ o i n C r . The ef-

f e c t value was equal t o 23%.

lChe metallic l o sample was subjected to isochronous annealing in a i r i n a wide range of temperatures. The Figwe below.

i l l u s t r a t e s t h a t the absorption spectrum shape and the e f f e c t value greatly vary depending on the annealing temperature.

After the annealing a t 1 9 0 ~ ~ a. weak ad- d i t i o n a l l i n e appears with

d

-2.41 .mm/s isomer s h i f t r e l a t i v e t o %o in l o , which i s indicative of the oxide film growth. The blackening of the sanple surface on 1 9 0 ~ ~ annealing indicates the formation of Moo2 with a s t m c t u r e of t h e r u t i l e type. The l i n e with the -2.41 mm/s s h i f t apparently appertains t o a doublet the other component of which is in the range of the main peak,

A computer analysis has shown t h a t the value of the quadrypole s p l i t t i n g of t h i s doublet i s ~E=1.68 muds, the iaomer s U r e l a t i v e t o 57Co i n C r equals -1.56 d s . These parameters indicate t h a t the iron

in Moo2 i s bivalent.

On attaining the annealing temperature of 350°c, the ~ 6 s s b a u e r spectrum of the sample begins t o change greatly.The spectrum centroid gradually s h i f t s t o the side of negative v e l o c i t i e s , the main ljne widens, and a new line appears nearby it.

A t the 5 0 0 ~ ~ annealing temperature the metallic molybdenum l i n e completely d i s - appears . A t this temperature the compacted sample turns i n t o a powder, which is indL- cative of i t s complete oxidation. The Mtfssbauer spectrvm of the powder is of a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801148

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Cl-388 JOURNAL DE PHYSIQUE

complicated shape. The l i n e with the s h i f t On a t t a i n i n g the annealing temperature of -2.41 mm/s i s s t i l l p r e s e n t i n i t , b u t

gradually disappears while the annealing temperature increases.

Fig.

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Mt5ssbauer s p e c t r a of m e t a l l i c molybdenum before annealing (a) and a f t e r

annealing i n the a i r a t d i f f e r e n t t e m c e o atures: (b) a t l 9 0 ~ ~ , ( c ) a t .500°c, and ( d ) a t 7 0 0 ~ ~ (Moo3)

about 700°c, the sample spectrum becomes i d e n t i c a l t o the MOO ^'spectrum. A computer a n a l y s i s of the l a t t e r permits t o decom- 3

pose it i n t o two doublets with parameters 4=-1.35 mm/s, a4=1.20 mm/s, and

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^-0.37

mm/s, aE2 =00.5 5 s . This means t h a t the i r o n i n t h i s sample i s present i n two d i f - f e r e n t s t a t e s : I?e3+ and ~ e ~ + . The r a t i o of these s t a t e s is approximately 1 : 3.

A computer a n a l y s i s of the spectrum produced on 5 0 0 ' ~ annealing permits t o de- compose it i n t o t h r e e doublets, two of which correspond t o MOO and t h e t h i r d

-

t o 1JoO2. 3

It should be noted t h a t a s the anneal- i n g temperature i n c r e a s e s , the ac t i v i t y of the sample g r e a t l y decreases.

A conclusion can be drawn t h a t molybdenum i s coated with an oxide f i l m a t about 1 9 0 ~ ~ i n the air, which hinders a f u r t h e r oxidation of molybdenum up t o about 350'~. A t a temperature over 350'~

the oxidation -mocess recommences. A t 5 0 0 ' ~ the oxidation proceeds e s p e c i a l l y quickly as a t t h i s tem_oerature IlrIo03 sub- limates i n t e n s i v e l y exposing the metal surface

.

m e n being oxidized i n the a i r , molybdenum shows valences 4 and 6 , o t h e r valence s t a t e s do not make an appreciable contribution.

References

/1/ Nicholson,W.J,, Burns,G., Phys.Rev.

129 (1963) 2490.

/2/ Perelman,l?.I., Zvorykin,A.Ya., Kolyb- denum and Tungsten ,"Naukatt &I. ( 1968)