- Email: [email protected]
Interaction of (MnZn)O solid solutions with Fe2O3 as intermediate stage of formation of MnZn ferrites
Ceramics International 14 (1988) 87 91
Interaction of ( M n Z n ) O Solid Solutions w i t h Fe2Oa.as Intermediate Stage of ormat=on of M n-Zn Ferrites V. V. P a n ' k o v Institute of Physics o1"Solids and Semiconductors, Byel. SSR Academy of Sciences, Minsk, USSR (Received 2 October 1986; accepted 30 March 1987) Abstract: The mechanism of the solid state reaction of (Mn, Zn)O solid solutions
with Fe_,O3 has been investigatedin the temperature range 1190-1320 C, using the diffusioncouple method. The electron probe microanalysismethod has been used in determining the distribution of the element concentrations over the reaction layer. The dependence of the reaction mechanism on the Mn-to-Zn ratio in Mn Zn ferrites is considered.
1 INTRODUCTION
The mechanism of reaction (1) has been studied in this work by the method of diffusion couples. This method allows us to represent by models the real interaction between the particles of the starting c o m p o u n d s of the mixture when heated. The analysis of the reaction zones formed in this case makes it possible to visualize the regularities of the solid phase reaction.
The present investigation deals with a study on the mechanism of interaction of solid solutions of zinc and manganese oxides, designated here as Z n l _ x M n x O i +r, with iron oxide Znl _xMn.,O1 +~. + F%O3 = Znl _xMnxFe20,~ + ~, (1)
2 METHODS
When a polycrystalline Mn Zn ferrite is prepared from mixtures of M n C O 3, Fe20 3 and ZnO there takes place an interaction of three kinds of powder particles. This implies the existence of intermediate reactions which ultimately yield a M n - Z n ferrite. One of these reactions is that of reaction (1). The point is, zinc oxide possesses limited solubility in manganese oxides and can dissolve these oxides itself. Besides, in the Z n O - M n O binary system there is a Z n M n 2 0 4 c o m p o u n d apart from the limited solid solutions. Zinc manganite as an intermediate c o m p o u n d has been found in preparing a M n - Z n ferrite by the cerarnic method. 1 The interaction between zinc manganite and iron oxide as Z n M n 2 0 , , + Fe20 3 ---*(Zn, M n ) F e 2 0 4
The diffusion couples were composed of plates (10x5x5mm) of the Zn I xMnxOl+ >, solid solutions and of plates (10 x 5 x 5mm) of iron oxide obtained by the hot pressing method at 1200'>C. The Z n l _ x M n x O l + ;, solid solutions with x = 0 , 0"15, 0-30, 0-50, 0'70, 0'85 and 1-0 were prepared by annealing the corresponding mixtures of MnO and ZnO powders at 1300'C for 24h. It is established that in the investigated temperature range tile Znl _xMn~O1 +;, solid solutions with x ~> 0.7 is a single phase zinc manganite, and with x < 0"7, a mixture of two phases: zinc manganite and zinc oxide. The Znl_.~MnxOl+r-Fe203 diffusion couples with differing x values were simultaneously placed in
(2)
is a particular case of reaction (1). 87
Ceramics International 0272-8842/88/$0350 ,:(', 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
88
V. V. P a n ' k o v
a furnace preheated to a given temperature. The annealing was carried out at 1190, 1227, 1250, 1290, 1320 C in air. The time of exposure at each temperature varied from 24 to 20013. To identify the initial interface a thin platinum line was precipitated on the crystals. The contact between the crystals constituting the diffusion couple was kept constant by a special laboratory-made device out of AI203 tubes. 2 After annealing, the specimens were quenched in air. The diffusion couples were cut normally to the surface of the interface and polished. The reaction zone was investigated by electron probe X-ray spectrum and microstructure analyses. While carrying out the electron probe X-ray analysis a Mno.ssZno.45Fe204 single crystal was used as a standard for manganese and zinc and iron oxide was employed as a standard for Fe, The curves of c o n c e n t r a t i o n distribution of elements in the diffusion zone, and the velocity constants of the layer growth, were determined by the technique described by Pan'kov and Bashkirov. 2
3 RESULTS A N D D I S C U S S I O N It has been found that the reaction zone for all the Zn~_.,.MnxO ~+;.-Fe203 diffusion couples is composed of the layer which has a spinel-like structure• This layer crystallizes on both sides of the initial interface. In this case, a larger fraction of the layer has been formed on the Fe203 particles. An increase in manganese content for Zn,_~MnxO~+. ,, brings about: first, an increase in the length of the layer, and second, an increase in that part formed on the (Zn, Mn)Ox+ 7 particles when compared with that formed on the Fe203 particles (Fig. 1). It is known that the availability and accumulation of pores in different areas of the reaction zone
provide information about the mechanism of the solid state reaction. At the spinel/Fe203 boundary there is an accumulation of pores elongated towards the diffusion flux. The layer with the spinel structure formed on the (Zn, Mn)O,+ 7 particle has high density and is characterized by almost total absence of pores. At the spinel/(Zn, Mn)Ol+7 boundary there is a narrow area with a high concentration of pores. For several compositions with high manganese content this area consists of a large amount of fine pores elongated towards the diffusion flux. For compositions enriched in zinc at the spinel/ (Zn, Mn)Ol +7 boundary there develops a crack. Figure 2 gives the distribution of iron, manganese and zinc along the reaction zone. Similar curves are obtained at other annealing temperatures. These curves, as well as the X-ray analysis data, show the diffusion layer to be the (Zn, Mn, Fe)304 solid solution with a spinel structure for all tile (Zn, Mn)O, + 7 solid solutions. The manganese, zinc and iron concentrations therein vary from the concentration of these elements in (Zn, Mn, Fe)30, , to their concentrations in ZnxMn~.Fe:Fe3_x_~._=O4 which contains some dissolved magnetite. It is stated that for a larger fraction of the (Zn, Mn, Fe)304 layer the ratio of Zn concentration to Mn concentration is higher than that for the initial (Zn, Mn)O, +7" Therefore, the M n - Z n spinel forrned is enriched in Zn. It is noted that in the spinel layer at the spinel/(Zn, Mn)O~+ 7 boundary the concentration of Mn changes more markedly than that of Zn. The largest length of the spinel layer is in the M n O - F e 2 Q diffusion couple, i.e. corresponding to the formation of Mn-ferrite (the smallest)--to Znferrite formation. For the (Zn, Mn)O~ +7-Fe203 diffusion couples, the increase of the Mn concentration in
{:.~.~ .,aa ~ .... ..
( Zn
:" ~....,-~,:-,-.,+..
t~L~;.
: ~.
i
'
Mn
.'
I [
~:.,7 ..,:...~-::.~.:~,."':, ; : : : •-::. "
~"
°~.'"i,-'
7 " '."
);'.3 ' r. '~'-" ,
.
:
:-
,.:
" b
:"-'~"Y'-';';t "" " ¢111~ -
"" '~" K c
~0x-~
B'g
.
. . . . ,".-" ~...., .' ,.. ;,":, : ~',.. .. : ".-.-.:j.. .,~:.. " ° .,,... :',-:,':-
Fe
:.."
..
:,.3 '"*
,.
-.,:
..v
"
Mn
.~s
~;i~. ~ 0.2
1 :~
IKa'~
~
:. " , ' : , - c > .
.,
\',-,
,.;
. ,~,
.el
~
"
"J' '
J" "~ " ~ a
L
K"~::~'
Y I" ¢ ' ,~ * 1
-." :-. '
Fig. I.
Fe)304
Typical photomicrograph of diffusion couple Z n o . s M n o . s O - F e 2 0 3 a n n e a l e d at 1320°C for 6 h.
O.t
< ISoo
I
I
iooo
5oo
r
o Distanee~
soo
X(pm)
Fig. 2. Distribution of Mn, Zn and Fe concentrations in the reaction zone for the Zno.sMno.50-F%O 3 diffusion couple annealed at t250°C for t20h.
Interaction of (MnZn)O solid solutions with Fe20 3 in ferrite formation 12g0°c ×
<~
89
Table 2. Activation energy, Q, pf g r o w t h in the spinel structure layer for the Znl_xMnxO-Fe203 systems
2s0°c
12
Systems
MnO-Fe203 0.70 MnO. 0.30ZnO-Fe203 0.50 MnO .0-50 ZnO-Fe203 0.25 MnO. 0.75 ZnO-Fe=O 3 ZnO-Fe203
4
~2
20
0
4'o
180 166 145 117 98
8'0
do time ,
Fig. 3.
Q (kcal/mole)
t
(Ix)
Reaction layer thickness vs. time at various temperatures.
(Zn, Mn)O1 + ~extends the depth of Zn diffusion. The kinetics of the (Zn, Mn, Fe)304 layer growth obey the parabolic law (Fig. 3). The values of the growth rate constants are given for various compositions and annealing temperatures in Table 1. The activation energy of the spinel layer growth increases with the growing M n content in the (Zn, Mn)O~ +~. solid solution (Table 2). The parabolic law of the spinel layer growth gives evidence of the fact that in the case of this type of solid state reaction the diffusion process through the layer of the product formed will govern the formation of the spinel. It is obvious that the vacancies diffusion mechanism takes place in the spinels. In the first approximation the activation energy of the spinel layer growth is most closely connected with that of the diffusion process. In the case of solid state reactions it is necessary to consider the effective activation energy. The tentative estimation of activation energy makes some sense when the processes of one type are compared for different systems as is the case for the Mn-, Mn-Zn-ferrite and
Zn-ferrite formation. U n d e r the assumption of vacancy diffusion mechanism one can easily show why the rate constant of the Mn-ferrite formation, and, as a result, the diffusion mobility of Mn ions is larger than the corresponding values for Zn ferrite. Since Mn 2 ÷ diffuse most likely in this temperature range and these ions are both in tetra- and octahedral sites as distinct from Zn 2 ÷ which prefer only tetrahedral sites, the process of the Mn-ferrite formation proceeds more rapidly than that of the Zn ferrite. In this case more vacancies are present in the crystal lattice of the spinel. However, each diffusion j u m p of Mn 2+ ions is more difficult when compared to that of Zn 2 + ions, since the ionic radius of Mn z + (r = 8"0/~) is larger than that of Zn 2+ (r = 7"4,~). This difference results in larger values of activation energy for the Mn-ferrite formation process. Here two cases can be identified in terms of the mechanism of ferrite formation. Firstly, for compositions with high zinc content. In this case the (Zn, Mn)O1 + ~.solid solution consists of a mixture of two phases: zinc manganite and zinc oxide ones. Zinc oxide in interacting with (Zn, Mn)O~ + ~ volatilizes and is transferred to the iron oxide particle. From Fig. 4 it is seen that the pores are formed in
Table 1. G r o w t h rate constants of spinel structure layer at different annealing temperatures for the Zn 1_xMnxO-Fe=O 3 systems Temperature (°C)
Reaction rate constants (k. 108 cm=/s) 0.3ZnO.0.7 MnO-Fe203
1 190 1 227 1 250 1 290 1 320
1.7 3-2 7.0 15.0 63.0
0.5ZnO.0.5 0.75Zn0.0.25 MnO-Fe=O a MnO-Fe=03 1.2 2'3 5.0 22.1 61 "1
1.1 2.0 4.5 15.0 55.0
Fig. 4. Photomicrograph of the 0"5ZnO-0"5MnO plate annealed in contact with the Fe203 plate at 1250°C for 72 h.
V. 1/. Pan'kor
90
place of the zinc oxide phase after interaction with F % O 3. The microstructure of pores in the spinel layer replicates very closely that of the ZnO phase in the (Mn, Zn)O 1 + r matrix at the spinel/(Mn, Zn)O 1+ ,. boundary. On reaching the surface of the FexO 3 particle Zn 2 + form the phase of Zn spinel as:
Where e represents electron. The enrichment of the M n - Z n spinel in Zn occurs at the expense of this reaction. In addition to the Zn and diffusion of electrons under the action of the concentration gradient, there takes place the interdiffusion through the initial interface between the cations of Mn and Zn and those of Fe. At the spinel/Fe203 boundary two reactions can be observed. One of them is associated with the volatilization of oxygen at this boundary
as has been reported, 3 is followed by the 11.7% volume increase and occurs at the expense of the reaction where the reduction of Fe 3+ to Fe 2+ is carried by the extra electrons. Zinc ferrite is formed in the ex-Fe203 zone due to the one-side diffusion of Zn x +, electrons and 0-4% oxygen volume increase. The 2-9% volume increase for the 0"5ZnO. 0-5ZnO FexO 3 system is indicative of a mixed-type mechanism which is realized in this case as well as the two above-mentioned mechanisms of Mn-ferrite formation. The quantity of the pores elongated towards the diffusion flux in the reaction layer is not as large in the case of the M n - Z n spinel formation as that for the N i - Z n spinel. From these considerations one can say that the fraction of the mechanism connected with the Fe x+ diffusion is not as large. At the spinel/(Zn, Mn)O~ + r boundary the crystallization of the spinel phases proceeds as follows:
M n " + , Z n 2+ + F e 2 0 3 - +
(Zn, Mn)304 + Fe3 +(Fe2 +)--+
(1-x)Zn 2+ +Fe203+2(1-x)e+3-4x/602 + = 3 - x / 3 Z n 3 1 1 -.,o:3-x F~2 c2x/3xFe204
(Zn, Mn, Fe)304 + 02 + Fe 2 +
(Zn, Mn, Fe)aO 4 + Z n 2+, Mn "+
The other one proceeds without the extraction of oxygen
The concentration profiles (Fig. 2) reveal that the composition of the spinel phase along the reaction zone changes. In the spinel phase formed at the b o u n d a r y with Fe203 the Fe content is larger than that in the spinel of the stoichiometric M e F e 2 0 4 composition which does not contain Fe 2+. The increase in the atomic fraction of Fe can be accounted for by the manganite formation. With increasing Mn content in (Zn, Mn)O1 +., the quantity of the dissolved magnetite in the spinel decreases (Table 3).
Mn" +, Z n 2 + + Fe203 ---,(Zn, Mn, Fe)304 + Fe 3 + The pores accumulate at the spinel/Fe203 boundary in the spinel layer. High porosity at this interface has been observed for the nickel -3 and nickel-zinc ferrites. 2 However, the quantity of the pores elongated towards the diffusion flux in the reaction layer is much less in the case of the M n - Z n spinel formation than that in the case of the N i - Z n spinel. It is established 3 that the porosity formed is connected with the oxygen volatilization by the reaction: 3Fe203 = 2Fe30 4 + 10 2
Table 3. C o n c e n t r a t i o n s of M n , Z n a n d F e i n s p i n e l phase at t h e spinel/Fe=O 3 b o u n d a r y and initial interface for diffusion couples annealed at 1250'C for 120h
Fe304 + Me 2+ = Fe 2+ + M e F e 2 0 4 As for the Mn-ferrite formation by this mechanism, we observed almost no change of volume AV = + 0"6%. A 2"9% volume increase in the exF e 2 0 3 zone was experimentally found in the 0"5MnO-0"5ZnO-Fe203 system. The volume increase during the spinel formation in Fe203 was detected by the shift in platinum marker planes placed perpendicularly to the diffusion axis. The above-mentioned procedure enables continuous marking of the entire diffusion zone to be made. It has been established that the volume increase is proportional to the amount of spinel formed on the Fe203 side. The Mn-ferrite formation in the ex-Fe203 zone,
Initial interface
Spinel/F%03 boundary
Zno.3M no.70-Fe203
CMn ~o
CFe
0-122 0'060 0"247
0-041 0"033 0-355
Zno.sMno.sO-Fe20a CMo Cz° CFe
0.074 0-121 0"234
0.01 2 0-055 0"362
Zno.TsMno.250-F%Oa CMn Czo CFe
0"058 0.120 0"251
0"002 0.059 0"370
Interaction o f (MnZn)O solid solutions with Fe 20 3 in ferrite formation
The second case of the reaction mechanism (1) refers to compositions with high manganese content. Here the (Zn Mn)O~ + r particles are single phase zinc manganite (Zn Mn)304. As in the previous case the manganese-zinc spinel with higher zinc content is formed. It is evident that along with the opposite diffusion of ions of zinc, manganese and iron there is additional zinc transfer to the Fe204 particle. This can be due to the manganite decomposition into oxides: Z n M n 2 0 4 ~ ZnO + Mn203 It is the volatilized zinc oxide that is the additional source of zinc. In this case the manganese oxide can also decompose at the spinel/(Zn Mn)Ol +7 boundary to release oxygen and to form manganese cations and electrons. The spinel phase will further be formed by the mechanism previously described. 3 This mechanism implies the excess of cation and anion vacancies at the spinel/(Zn, Mn)O~ + ~ boundary. As the spinel phase formed by this type of
91
mechanism has high zinc content, jt appears that not the whole quantity of the manganese oxide decomposes. The fraction that has not been decomposed forms the solid solution with manganite and enhances the manganese content in it. ZnMn204 + Mn203 --,(Zn, Mn)304 This assumption has been validated by the experiment (Fig. 2) while comparing the concentration curves of Zn and Mn in the reaction zone of the (Zn Mn)Ol +.e particle. In other respects the mechanism of M n - Z n ferrite formation enriched in Mn is similar to that of M n - Z n ferrites enriched in Zn.
REFERENCES I. CHIBA, A. & KIMURA, O., Study on formation of Mn-Zn ferrites, J. Jap. Sot'. Powder and Powder Met., 31 (1984), 75. 2. PAN'KOV, V. V. & BASHKIROV, L. A., Mechanism of Ni-Zn ferrite formation, J. Solid State Chem., 39(3) (1981), 298. 3. EVENO, P. V., Thesis, Orsay, 1974.
Interaction of ( M n Z n ) O Solid Solutions w i t h Fe2Oa.as Intermediate Stage of ormat=on of M n-Zn Ferrites V. V. P a n ' k o v Institute of Physics o1"Solids and Semiconductors, Byel. SSR Academy of Sciences, Minsk, USSR (Received 2 October 1986; accepted 30 March 1987) Abstract: The mechanism of the solid state reaction of (Mn, Zn)O solid solutions
with Fe_,O3 has been investigatedin the temperature range 1190-1320 C, using the diffusioncouple method. The electron probe microanalysismethod has been used in determining the distribution of the element concentrations over the reaction layer. The dependence of the reaction mechanism on the Mn-to-Zn ratio in Mn Zn ferrites is considered.
1 INTRODUCTION
The mechanism of reaction (1) has been studied in this work by the method of diffusion couples. This method allows us to represent by models the real interaction between the particles of the starting c o m p o u n d s of the mixture when heated. The analysis of the reaction zones formed in this case makes it possible to visualize the regularities of the solid phase reaction.
The present investigation deals with a study on the mechanism of interaction of solid solutions of zinc and manganese oxides, designated here as Z n l _ x M n x O i +r, with iron oxide Znl _xMn.,O1 +~. + F%O3 = Znl _xMnxFe20,~ + ~, (1)
2 METHODS
When a polycrystalline Mn Zn ferrite is prepared from mixtures of M n C O 3, Fe20 3 and ZnO there takes place an interaction of three kinds of powder particles. This implies the existence of intermediate reactions which ultimately yield a M n - Z n ferrite. One of these reactions is that of reaction (1). The point is, zinc oxide possesses limited solubility in manganese oxides and can dissolve these oxides itself. Besides, in the Z n O - M n O binary system there is a Z n M n 2 0 4 c o m p o u n d apart from the limited solid solutions. Zinc manganite as an intermediate c o m p o u n d has been found in preparing a M n - Z n ferrite by the cerarnic method. 1 The interaction between zinc manganite and iron oxide as Z n M n 2 0 , , + Fe20 3 ---*(Zn, M n ) F e 2 0 4
The diffusion couples were composed of plates (10x5x5mm) of the Zn I xMnxOl+ >, solid solutions and of plates (10 x 5 x 5mm) of iron oxide obtained by the hot pressing method at 1200'>C. The Z n l _ x M n x O l + ;, solid solutions with x = 0 , 0"15, 0-30, 0-50, 0'70, 0'85 and 1-0 were prepared by annealing the corresponding mixtures of MnO and ZnO powders at 1300'C for 24h. It is established that in the investigated temperature range tile Znl _xMn~O1 +;, solid solutions with x ~> 0.7 is a single phase zinc manganite, and with x < 0"7, a mixture of two phases: zinc manganite and zinc oxide. The Znl_.~MnxOl+r-Fe203 diffusion couples with differing x values were simultaneously placed in
(2)
is a particular case of reaction (1). 87
Ceramics International 0272-8842/88/$0350 ,:(', 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
88
V. V. P a n ' k o v
a furnace preheated to a given temperature. The annealing was carried out at 1190, 1227, 1250, 1290, 1320 C in air. The time of exposure at each temperature varied from 24 to 20013. To identify the initial interface a thin platinum line was precipitated on the crystals. The contact between the crystals constituting the diffusion couple was kept constant by a special laboratory-made device out of AI203 tubes. 2 After annealing, the specimens were quenched in air. The diffusion couples were cut normally to the surface of the interface and polished. The reaction zone was investigated by electron probe X-ray spectrum and microstructure analyses. While carrying out the electron probe X-ray analysis a Mno.ssZno.45Fe204 single crystal was used as a standard for manganese and zinc and iron oxide was employed as a standard for Fe, The curves of c o n c e n t r a t i o n distribution of elements in the diffusion zone, and the velocity constants of the layer growth, were determined by the technique described by Pan'kov and Bashkirov. 2
3 RESULTS A N D D I S C U S S I O N It has been found that the reaction zone for all the Zn~_.,.MnxO ~+;.-Fe203 diffusion couples is composed of the layer which has a spinel-like structure• This layer crystallizes on both sides of the initial interface. In this case, a larger fraction of the layer has been formed on the Fe203 particles. An increase in manganese content for Zn,_~MnxO~+. ,, brings about: first, an increase in the length of the layer, and second, an increase in that part formed on the (Zn, Mn)Ox+ 7 particles when compared with that formed on the Fe203 particles (Fig. 1). It is known that the availability and accumulation of pores in different areas of the reaction zone
provide information about the mechanism of the solid state reaction. At the spinel/Fe203 boundary there is an accumulation of pores elongated towards the diffusion flux. The layer with the spinel structure formed on the (Zn, Mn)O,+ 7 particle has high density and is characterized by almost total absence of pores. At the spinel/(Zn, Mn)Ol+7 boundary there is a narrow area with a high concentration of pores. For several compositions with high manganese content this area consists of a large amount of fine pores elongated towards the diffusion flux. For compositions enriched in zinc at the spinel/ (Zn, Mn)Ol +7 boundary there develops a crack. Figure 2 gives the distribution of iron, manganese and zinc along the reaction zone. Similar curves are obtained at other annealing temperatures. These curves, as well as the X-ray analysis data, show the diffusion layer to be the (Zn, Mn, Fe)304 solid solution with a spinel structure for all tile (Zn, Mn)O, + 7 solid solutions. The manganese, zinc and iron concentrations therein vary from the concentration of these elements in (Zn, Mn, Fe)30, , to their concentrations in ZnxMn~.Fe:Fe3_x_~._=O4 which contains some dissolved magnetite. It is stated that for a larger fraction of the (Zn, Mn, Fe)304 layer the ratio of Zn concentration to Mn concentration is higher than that for the initial (Zn, Mn)O, +7" Therefore, the M n - Z n spinel forrned is enriched in Zn. It is noted that in the spinel layer at the spinel/(Zn, Mn)O~+ 7 boundary the concentration of Mn changes more markedly than that of Zn. The largest length of the spinel layer is in the M n O - F e 2 Q diffusion couple, i.e. corresponding to the formation of Mn-ferrite (the smallest)--to Znferrite formation. For the (Zn, Mn)O~ +7-Fe203 diffusion couples, the increase of the Mn concentration in
{:.~.~ .,aa ~ .... ..
( Zn
:" ~....,-~,:-,-.,+..
t~L~;.
: ~.
i
'
Mn
.'
I [
~:.,7 ..,:...~-::.~.:~,."':, ; : : : •-::. "
~"
°~.'"i,-'
7 " '."
);'.3 ' r. '~'-" ,
.
:
:-
,.:
" b
:"-'~"Y'-';';t "" " ¢111~ -
"" '~" K c
~0x-~
B'g
.
. . . . ,".-" ~...., .' ,.. ;,":, : ~',.. .. : ".-.-.:j.. .,~:.. " ° .,,... :',-:,':-
Fe
:.."
..
:,.3 '"*
,.
-.,:
..v
"
Mn
.~s
~;i~. ~ 0.2
1 :~
IKa'~
~
:. " , ' : , - c > .
.,
\',-,
,.;
. ,~,
.el
~
"
"J' '
J" "~ " ~ a
L
K"~::~'
Y I" ¢ ' ,~ * 1
-." :-. '
Fig. I.
Fe)304
Typical photomicrograph of diffusion couple Z n o . s M n o . s O - F e 2 0 3 a n n e a l e d at 1320°C for 6 h.
O.t
< ISoo
I
I
iooo
5oo
r
o Distanee~
soo
X(pm)
Fig. 2. Distribution of Mn, Zn and Fe concentrations in the reaction zone for the Zno.sMno.50-F%O 3 diffusion couple annealed at t250°C for t20h.
Interaction of (MnZn)O solid solutions with Fe20 3 in ferrite formation 12g0°c ×
<~
89
Table 2. Activation energy, Q, pf g r o w t h in the spinel structure layer for the Znl_xMnxO-Fe203 systems
2s0°c
12
Systems
MnO-Fe203 0.70 MnO. 0.30ZnO-Fe203 0.50 MnO .0-50 ZnO-Fe203 0.25 MnO. 0.75 ZnO-Fe=O 3 ZnO-Fe203
4
~2
20
0
4'o
180 166 145 117 98
8'0
do time ,
Fig. 3.
Q (kcal/mole)
t
(Ix)
Reaction layer thickness vs. time at various temperatures.
(Zn, Mn)O1 + ~extends the depth of Zn diffusion. The kinetics of the (Zn, Mn, Fe)304 layer growth obey the parabolic law (Fig. 3). The values of the growth rate constants are given for various compositions and annealing temperatures in Table 1. The activation energy of the spinel layer growth increases with the growing M n content in the (Zn, Mn)O~ +~. solid solution (Table 2). The parabolic law of the spinel layer growth gives evidence of the fact that in the case of this type of solid state reaction the diffusion process through the layer of the product formed will govern the formation of the spinel. It is obvious that the vacancies diffusion mechanism takes place in the spinels. In the first approximation the activation energy of the spinel layer growth is most closely connected with that of the diffusion process. In the case of solid state reactions it is necessary to consider the effective activation energy. The tentative estimation of activation energy makes some sense when the processes of one type are compared for different systems as is the case for the Mn-, Mn-Zn-ferrite and
Zn-ferrite formation. U n d e r the assumption of vacancy diffusion mechanism one can easily show why the rate constant of the Mn-ferrite formation, and, as a result, the diffusion mobility of Mn ions is larger than the corresponding values for Zn ferrite. Since Mn 2 ÷ diffuse most likely in this temperature range and these ions are both in tetra- and octahedral sites as distinct from Zn 2 ÷ which prefer only tetrahedral sites, the process of the Mn-ferrite formation proceeds more rapidly than that of the Zn ferrite. In this case more vacancies are present in the crystal lattice of the spinel. However, each diffusion j u m p of Mn 2+ ions is more difficult when compared to that of Zn 2 + ions, since the ionic radius of Mn z + (r = 8"0/~) is larger than that of Zn 2+ (r = 7"4,~). This difference results in larger values of activation energy for the Mn-ferrite formation process. Here two cases can be identified in terms of the mechanism of ferrite formation. Firstly, for compositions with high zinc content. In this case the (Zn, Mn)O1 + ~.solid solution consists of a mixture of two phases: zinc manganite and zinc oxide ones. Zinc oxide in interacting with (Zn, Mn)O~ + ~ volatilizes and is transferred to the iron oxide particle. From Fig. 4 it is seen that the pores are formed in
Table 1. G r o w t h rate constants of spinel structure layer at different annealing temperatures for the Zn 1_xMnxO-Fe=O 3 systems Temperature (°C)
Reaction rate constants (k. 108 cm=/s) 0.3ZnO.0.7 MnO-Fe203
1 190 1 227 1 250 1 290 1 320
1.7 3-2 7.0 15.0 63.0
0.5ZnO.0.5 0.75Zn0.0.25 MnO-Fe=O a MnO-Fe=03 1.2 2'3 5.0 22.1 61 "1
1.1 2.0 4.5 15.0 55.0
Fig. 4. Photomicrograph of the 0"5ZnO-0"5MnO plate annealed in contact with the Fe203 plate at 1250°C for 72 h.
V. 1/. Pan'kor
90
place of the zinc oxide phase after interaction with F % O 3. The microstructure of pores in the spinel layer replicates very closely that of the ZnO phase in the (Mn, Zn)O 1 + r matrix at the spinel/(Mn, Zn)O 1+ ,. boundary. On reaching the surface of the FexO 3 particle Zn 2 + form the phase of Zn spinel as:
Where e represents electron. The enrichment of the M n - Z n spinel in Zn occurs at the expense of this reaction. In addition to the Zn and diffusion of electrons under the action of the concentration gradient, there takes place the interdiffusion through the initial interface between the cations of Mn and Zn and those of Fe. At the spinel/Fe203 boundary two reactions can be observed. One of them is associated with the volatilization of oxygen at this boundary
as has been reported, 3 is followed by the 11.7% volume increase and occurs at the expense of the reaction where the reduction of Fe 3+ to Fe 2+ is carried by the extra electrons. Zinc ferrite is formed in the ex-Fe203 zone due to the one-side diffusion of Zn x +, electrons and 0-4% oxygen volume increase. The 2-9% volume increase for the 0"5ZnO. 0-5ZnO FexO 3 system is indicative of a mixed-type mechanism which is realized in this case as well as the two above-mentioned mechanisms of Mn-ferrite formation. The quantity of the pores elongated towards the diffusion flux in the reaction layer is not as large in the case of the M n - Z n spinel formation as that for the N i - Z n spinel. From these considerations one can say that the fraction of the mechanism connected with the Fe x+ diffusion is not as large. At the spinel/(Zn, Mn)O~ + r boundary the crystallization of the spinel phases proceeds as follows:
M n " + , Z n 2+ + F e 2 0 3 - +
(Zn, Mn)304 + Fe3 +(Fe2 +)--+
(1-x)Zn 2+ +Fe203+2(1-x)e+3-4x/602 + = 3 - x / 3 Z n 3 1 1 -.,o:3-x F~2 c2x/3xFe204
(Zn, Mn, Fe)304 + 02 + Fe 2 +
(Zn, Mn, Fe)aO 4 + Z n 2+, Mn "+
The other one proceeds without the extraction of oxygen
The concentration profiles (Fig. 2) reveal that the composition of the spinel phase along the reaction zone changes. In the spinel phase formed at the b o u n d a r y with Fe203 the Fe content is larger than that in the spinel of the stoichiometric M e F e 2 0 4 composition which does not contain Fe 2+. The increase in the atomic fraction of Fe can be accounted for by the manganite formation. With increasing Mn content in (Zn, Mn)O1 +., the quantity of the dissolved magnetite in the spinel decreases (Table 3).
Mn" +, Z n 2 + + Fe203 ---,(Zn, Mn, Fe)304 + Fe 3 + The pores accumulate at the spinel/Fe203 boundary in the spinel layer. High porosity at this interface has been observed for the nickel -3 and nickel-zinc ferrites. 2 However, the quantity of the pores elongated towards the diffusion flux in the reaction layer is much less in the case of the M n - Z n spinel formation than that in the case of the N i - Z n spinel. It is established 3 that the porosity formed is connected with the oxygen volatilization by the reaction: 3Fe203 = 2Fe30 4 + 10 2
Table 3. C o n c e n t r a t i o n s of M n , Z n a n d F e i n s p i n e l phase at t h e spinel/Fe=O 3 b o u n d a r y and initial interface for diffusion couples annealed at 1250'C for 120h
Fe304 + Me 2+ = Fe 2+ + M e F e 2 0 4 As for the Mn-ferrite formation by this mechanism, we observed almost no change of volume AV = + 0"6%. A 2"9% volume increase in the exF e 2 0 3 zone was experimentally found in the 0"5MnO-0"5ZnO-Fe203 system. The volume increase during the spinel formation in Fe203 was detected by the shift in platinum marker planes placed perpendicularly to the diffusion axis. The above-mentioned procedure enables continuous marking of the entire diffusion zone to be made. It has been established that the volume increase is proportional to the amount of spinel formed on the Fe203 side. The Mn-ferrite formation in the ex-Fe203 zone,
Initial interface
Spinel/F%03 boundary
Zno.3M no.70-Fe203
CMn ~o
CFe
0-122 0'060 0"247
0-041 0"033 0-355
Zno.sMno.sO-Fe20a CMo Cz° CFe
0.074 0-121 0"234
0.01 2 0-055 0"362
Zno.TsMno.250-F%Oa CMn Czo CFe
0"058 0.120 0"251
0"002 0.059 0"370
Interaction o f (MnZn)O solid solutions with Fe 20 3 in ferrite formation
The second case of the reaction mechanism (1) refers to compositions with high manganese content. Here the (Zn Mn)O~ + r particles are single phase zinc manganite (Zn Mn)304. As in the previous case the manganese-zinc spinel with higher zinc content is formed. It is evident that along with the opposite diffusion of ions of zinc, manganese and iron there is additional zinc transfer to the Fe204 particle. This can be due to the manganite decomposition into oxides: Z n M n 2 0 4 ~ ZnO + Mn203 It is the volatilized zinc oxide that is the additional source of zinc. In this case the manganese oxide can also decompose at the spinel/(Zn Mn)Ol +7 boundary to release oxygen and to form manganese cations and electrons. The spinel phase will further be formed by the mechanism previously described. 3 This mechanism implies the excess of cation and anion vacancies at the spinel/(Zn, Mn)O~ + ~ boundary. As the spinel phase formed by this type of
91
mechanism has high zinc content, jt appears that not the whole quantity of the manganese oxide decomposes. The fraction that has not been decomposed forms the solid solution with manganite and enhances the manganese content in it. ZnMn204 + Mn203 --,(Zn, Mn)304 This assumption has been validated by the experiment (Fig. 2) while comparing the concentration curves of Zn and Mn in the reaction zone of the (Zn Mn)Ol +.e particle. In other respects the mechanism of M n - Z n ferrite formation enriched in Mn is similar to that of M n - Z n ferrites enriched in Zn.
REFERENCES I. CHIBA, A. & KIMURA, O., Study on formation of Mn-Zn ferrites, J. Jap. Sot'. Powder and Powder Met., 31 (1984), 75. 2. PAN'KOV, V. V. & BASHKIROV, L. A., Mechanism of Ni-Zn ferrite formation, J. Solid State Chem., 39(3) (1981), 298. 3. EVENO, P. V., Thesis, Orsay, 1974.