Mossbauer study of low temperature oxidation in natural magnetite

Scripta

METALLURGICA

Vol. 17, pp. 45-48, 1983 Printed in the U.S.A.

MOSSBAUER

STUDY OF LOW T E M P E R A T U R E IN N A T U R A L

Pergamon Press Ltd. All rights reserved

OXIDATION

}~GNETITE

A.Gediko~lu D e p a r t m e n t of Physics Science Facultv of A n k a r a U n i v e r s i t y Be~evler, (Received (Revised

Ankara,Turkey

J a n u a r y 29, 1982) N o v e m b e r 5, 1982)

Introduction

M a g n e t i t e is one of the o l d e s t f e r r o m a g n e t i c materials known (7). The Fe ions in m a g n e t i t e are located in the t e t r a h e d r a l and o c t a h e d r a l sites in face c e n t e r e d cubic (fcc) latuices created bv o x y g e n atoms. A l t h o u q h ferric ions are l o c a t e d in the o c t a h e d r a l and also in t e t r a h e d r a l sites, ferrous ion3 can only be located in o c t a h e d r a l sites. There is a rapid exchanc~e of e l e c t r o n s b e t w e e n ferric and ferrous ions at the t e m p e r a t u r e s above the Verwev t r a n s i t i o n t e m p e r a t u r e w h i c h has a value cf 120°K. ~ h e r e f o r e Fe may be c o n s i d e r e d as Fe 2.5~ at the o c t a h e d r a l sites. There is a m e t a l - d l e l e c t r i c t r a n s i t i o n b e [ c w the V ~ r w e v temperature. As a rusult, the M ~ s s b a u e r spectra of m a g n e t i t e c o n s i s t s of two six-line spectrums above Verwey t e m p e r a t u r e (1,2,8). The chemical

formula

of m a g n e t i t e

is given

(Fe3#)tetra (Fe2+ F e ~ 2 x

as follows

Vx)octa

(5),

04

l--j~ where, V is the numbec of vacanci6s in the o c t a h e d r a l sites and this indicates that the m a q n e t i t e is an u n s a t u r a t e d oxide. H a n e d a and M o r r i s h (3) reported that m a g n e t i t e p a r t i c a l s with nm sizes can be o x i d i z e d c o m p l e t e l y if they are t r e a t e d at 120°C for 1/2 h in the open air. But the size of the particles is a very i m p o r t a n t factor in the o x i d i z i n g procedure. S e l w o o d (4) observed the c o m p l e t e o x i d i z i n g of ~m size m a g n e t i t e p a r t i c l e s at the temperature of 220°C. Haneda and M o r r i s h (3) also r e p o r t e d the r e l a t i o n b e t w e e n o x i d a t i o n t e m p e r a t u r e and time with the size of the particles. Since oxidation is d e p e n d e n t on the spetific surface, m a g n e t i t e - m a g h e m i t e t r a n s f o r m a tion is r e l a t e d to the p a r t i c l e size. Hence, t r a n s f o r m a t i o n is faster and p o s s i b l e at r e l a t i v e l y lower t e m p e r a t u r e s for the p a r t i c l e s with small sizes. C r y s t a l l o g r a p h i c a l l y m a g n e t i t e and m a g h e m i t e have i s o m o r p h o u s structures, The only d i f f e r e n c e b e t w e e n them is that m a g n e t i t e has both ferric and ferrous ions but m a g h e m i t e has only ferric. Thus m a g h e m i t e gives a M ~ s s b a u e r spectra with only one six-line spectrum.

te,

It was thought w o r t h w h i l e to examine the o x i d a t i o n since it has a wide a p p l i c a t i o n in industry.

kinetics

45 0036-9748/83/010045-04503.00/0 Copyright (c) 1983 P e r g a m o n Press Ltd.

of m a g n e t i -

46

OXIDATION

IN M A G N E T I T E

Vol.

17, No.

1

Results and D i s c u s s i o n A M 6 s s b a u e r s p e c t r o m e t e r w o r k i n g in c o n s t a n t a c c e l e r a t i o n m o d e was u s e d during the study and the v e l o c i t y scale was c a l i b r a t e d w i t h a s t a n d a r d Fe m e t a l spectrum. The M ~ s s b a u e r source was a Co57 d i f f u s e d in palladium. N a t u r a l m a g n e t i t e o b t a i n e d f r o m the e a s t e r n part of T u r k e y was u s e d as the absorber. The p a r t i c l e sizes of the p o w d e r e d and 51.21% Fe c o n t a i n i n g samples were m e a s u r e d m i c r o s c o p i c c a l l y f r o m 0.5 to 4 ~m. The M 6 s s b a u e r s p e c t r u m of this sample m e a s u r e d at room t e m p e r a t u r e is shown in Fig.l. S p e c t r u m of the same sample h e l d in the o p e n air for 28 m o n t h s is also given in Fig.2. Both spectra are the s u p e r p o s i t i o n of two s u b - s p e c t r a since magnetite has two d i f f e r e n t crystal s y m m e t r y forms and i o n i z a t i o n states. The relative w e i g h t s of these s u b - s p e c t r a are 1/2 for the pure magnetite. This ratio is also equal to the ratio of f e r r i c ions in t e t r a h e d r a l sites to the f e r r o u s and f e r r i c ions in o c t a h e d r a l sites. The scatter in 1/2 ratio should be a t t r i b u t e d to the o x i d a t i o n effect, b e c a u s e f e r r o u s ions w i t h o c t a h e d r a l s y m m e t r y t r a n s f o r m into f e r r i c and the amount of f e r r i c ions i n c r e a s e s w i t h the oxidation. Thus the i n t e n s i t y of lowest line, i.e. -8 m m / s e c e x t e r n a l h y p e r f i n e line, becomes increased. The p a r a m e t e r s given in Table 1 were c a l c u l a t e d f r o m the r e l a t i v e ratios of sub-spectra. It may be o b s e r v e d in Figures 1 and 2 that m a g n e t i t e becomes o x i d i z e d at r o o m t e m p e r a t u r e and some f r a c t i o n of it is t r a n s f o r m e d to maghemite. The 8.5% v o l u m e ratio of m a g h e m i t e in the o r i g i n a l sample was i n c r e a s e d to 19% in the sample held for 28 m o n t h s at room temperature. This also shows the slow rate of oxidation. The o r i g i n a l sample was h e a t e d at 100°C and 150°C for 40 m i n u t e s in the o p e n air to o b s e r v e the d e p e n d e n c e of o x i d a t i o n on temperature. The M 6 s s b a u e r spectra m e a s u r e d on these samples are shown in Figures 3 and 4. The m a g h e m i t e p e r c e n t a g e s w e r e f o u n d f r o m the s p e c t r a as 29.5% and 32.5% relatively. The small d i f f e r e n c e in the m a g h e m i t e p e r c e n t a g e s indicates that the o x i d a t i o n becomes m o r e d i f f i c u l t in this case. This means, o x i d a t i o n takes place only on the s p e c i m e n surface and t e m p e r a t u r e has a r e l a t i v e l y large effect on the p r o g r e s s of the oxidation. This result agrees w i t h that of Meisel (9) w h i c h showed the e x i s t e n c e of a 0.1 ~m layer caused by the effect of v a p o u r in the o p e n air. The author also n o t e d the start of o x i d a t i o n f r o m the surface. But it should be taken into c o n s i d e r a t i o n that the t e m p e r a t u r e and time may also change the d i f f u s i o n d e p t h of o x y g e n and t h e r e f o r e the v a l u e s given by Meisel. The chemical f o r m u l a of o x i d i z e d m a g n e t i t e can be given as, z.Fe304 ÷ y.Fe203

-

Fe3_xO 4 "......'.

•

L

i

i

i

l

i

l

"

~

I

l

FIG. 1 M 6 s s b a u e r s p e c t r u m of m a g n e t i t e

FIG.2 M ~ s s b a u e r s p e c t r u m of m a g n e t i t e afte~ 28 months.

Vol.

17,

No.

1

OXIDATION

IN M A G N E T I T E

47

CO,,T~ ,,IOX;

:':

'""~": /7" )/~\. j~~~"...." ,,t~'" i~ ;' "~"

~

sg",~ :~ p ;- F '~ ,~

I j

I

'q

.i

I

FIG.3 M ~ s s b a u e r s p e c t r u m of m a g n e t i t e h e a t e d at 100°C in the open air for 40 minutes.

FIG.4 M 6 s s b a u e r s p e c t r u m of m a g n e t i t e h e a t e d at 150oc in the open air for 40 m i n u t e s

w h e r e x = 3 - 4(4z ÷ 2y)/(4z + 3y) , z is the ratio of m a g n e t i t e in the s p e c t r u m and y is the m a g h e m i t e ratio, z ÷ y should be i, and these v a l u e s are g i v e n in Table i. The x p a r a m e t e r shows the t r a n s f o r m a t i o n ratio of m a g n e t i t e and its v a l u e as 0.33 i n d i c a t e s the c o m p l e t e d t r a n s f o r m a t i o n . The v a l u e s g i v e n in Table 1 d e p e n d on the r e l a t i v e ratios of the s u b - s p e c t r a and the m e a n e x p e r i m e n t a l scatter on these v a l u e s is about 2%, t h e r e f o r e the same scatter arises in all v a l u e s presented. The internal m a g n e t i c f i e l d v a l u e s of t e t r a h e d r a l and o c t a h e d r a l s y m m e t r y forms were d e t e r m i n e d as H 1 = (40.6 ~ 0.4) 106 A / m and H 2 = (38.6 ~ 0.4) 106 A / m r e l a t i v e l y by u s i n g the m a g n e t i c dipole m o m e n t v a l u e s as ~ e = - 0 . 1 5 3 ~n and ~ , = 0.0903 ~n (i0) The i s o m e r i c a l shifts were also calculate~ r e l a t i v e to Fe metal and f o u n d for both f o r m s as ~i = 0 . 3 7 ~ 0.10 m m / s e c and ~2 = 0.77 ; 0.i0 mm/sec. It was o b s e r v e d d u r i n g the e x p e r i m e n t s that the internal m a g n e t i c f i e l d and i s o m e r i c a l shift v a l u e s are c o n s t a n t and not d e p e n d on the o x i d a t i o n effect. The p r e s e n t results are in good a g r e e m e n t with the l i t e r a t u r e (5,6). The p a r t i c l e s of the sample were c o n s i d e r e d as spherical and o x i d a t i o n depths were calculated. E x i s t e n c e of two spheres w i t h the radius of r I and r? and same o r i g i n was a s s u m e d for the two s t r u c t u r e s and r I- r 2 was taken as t h e - t h i c k n e s s of m a g h e m i t e . If the average p a r t i c l e size is taken as 2 ~ m the m a x i m u m o x i d a tion d e p t h is c a l c u l a t e d as 0.240 ~ m w h i c h agrees with the v a l u e g i v e n by Meisel (9). face

The r2/r I v a l u e s effect.

The results sample

given

obtained

in Table

f r o m hhe

sub-spectrum octa

1 indicates

that

Table 1 room t e m p e r a t u r e

in% F e 2 e / F e 3÷

tetra

the o x i d a t i o n

M~ssbauer

is a sur-

spectra

magnetite

maghemite

z(%)

v(%)

x

r2/rl

original

61

39

0.48

91.5

8.5

0.022

0.97

e x p o s e d to air for 28 m o n t h s

54

46

0.45

81.0

19.0

0.050

0.93

h e a t e d in air at 100°C for 40 min.

47

53

0.41

70.5

29.5

0.080

0.89

h e a t e d in air at 150°C for 40 min.

45

55

0.40

67.5

32.5

0.088

0.88

48

OXIDATION IN MAGNETITE

Vol.

17, No. 1

As a result it can be stated that, the magnetite-maghemite transformation is a surface effect and oxidation should be taken into consideration when magnetite is used as a thin film or powder. Oxidation starts even at the room temperature and humidity may play an important role in this progress. Another important result is the saturation of oxidakion and its dependence on particle size and oxidation temperature. Although there is a saturation state, if the temperature is raised sufficiently high the diffussion of oxygen becomes easier and magnetite transforms into maghemite completely. This study reveals that the M~ssbauer spectroscopy technique is also a useful method to investigate the oxidation effect. Acknowledgement The author thanks to the Turkish Iron-Steel Corporation for providing the samples and T.N.Durlu for helpful discussions. References i. 2. 3. 4. 5. 6. 7. 8. 9. i0.

Yu F. Krupyanskii and I.P.Suzdalev, J.de Physique C6,407(1974) S.Morup, H.Topsoe and J.Lipka, J.de Physique C6,287(1976) K.Haneda and A.H.Morrish, J.de Physique CI,321(1977) P.W.Selwood, Interscience New York, 306(1956) G.W.Simmons and H.Leidheiser Jr., in "Application of M~ssbauer Spectroscopy" ed.by R.L.Cohen, Acad.Press,p.92(1976) N.N.Greenwood and T.C.Gibb, "M~ssbauer Spectroscopy", Chapman and Hall Ltd., p.251 (1971) S.Iida, K.Mizushima, J.Mada, S.Umemura, J.Yoshida and K.Nakao, J.de Physique, CI,73 (1977) H.Topsoe, J.A.Dumesic and M.Boudart, J.de Physique C6,411(1974) W.Meisel, J.de Physique Ci,63 (1980) H.Wegener,"Der MSssbauer Effekt",p.95, B.I.HochschultaschenbHcher 2/2a, Bibliographisches Institut,Mannheim 1966.