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Studies on the formation of zinc ferrite
J. inorg, nucl. Chem., 1974, Vol. 36, pp. 569-573. Pergamon Press. Printed in Great Britain.
STUDIES ON THE FORMATION OF ZINC FERRITE K. R. KRISHNAMURTHY, J. GOPALAKRISHNAN, G. ARAVAMUDAN and M. V. C. SASTRI Department of Chemistry, Indian Institute of Technology, Madras 600036, India (Received 1 June 1973)
Abstract--The reactions of zinc oxide with x-Fe203 and with ?-Fe203 leading to the formation of zinc ferrite (ZnFe204) have been studied at different partial pressures of oxygen in the temperature range 500800°C. The ferrite spinel is formed more readily when ~-Fe203 is used as the reactant. The DTA and X-ray diffraction results indicate that the 2;-Fe203 phase is stabilized in presence of ZnO. The lowering of partial pressure of oxygen in the ambient atmosphere favours the formation of zinc ferrite, indicating a cyclic redox process featuring FeS+, Fez+ and oxygen at the phase boundaries. At very low oxygen pressures, conversion to zinc ferrite is not facilitated, apparently due to the formation of an intermediate species,F%_ xZn=04, which, under these conditions, is not easily converted to ZnFe20,.
INTRODUCTION
Table 1. X-ray diffraction data for ~.Fe20 3
THE VOmVa~rIONof ferrite spinels, A F e 2 0 , (where A is a divalent metal ion, Mg 2÷, Mn2 +, Ni 2÷, Zn 2+, etc.) from their constituent oxides AO and Fe20~ has been the subject of a number of investigations in view of their technical importance[l]. The reaction between oxides AO and Fe2Os is known to be influenced by a variety of factors, such as, particle size[2], efficiency of mixing[3], presence of lattice defects and associated nonstoichiometry[4] in the reactants, and changes in the ambient pressure of oxygen in the reaction atmosphere[5]. An understanding of the factors that control the formation of ferrites is desirable in the context of their technical production. Previous studies of ferrite formation have invariably employed ct-Fe20 3 as the starting material[l, 6-8]. In the present work, the formation of ZnFe204 is studied in detail employing ?-Fe2Oa as the reactant. The kinetics of the reaction between ?-Fe203 and ZnO under atmospheric and at low oxygen partial pressures in the temperature range 500-800"C is studied with a view to establishing the conditions for the production of Z n F e 2 0 , and to understanding the mechanism of ferrite formation in general. For comparative purposes, the reaction between x-Fe203 and ZnO at the same temperature range under atmospheric pressure is also studied. EXPERIMENTAL
Observed values
569
i
dtA)
I
3"680 2'947 2"695 2"510 2'081 1'836 1.694 1"605 1'485
13 28 33 100 20 15 20 23 30
3'720 2.944 2'708 2'510 2"082 1"818 1.699 1'602 1"472
25 90 20 100 90 80 40 90 90
* ASTM Card No. 13-458. of hydrated Fe203 at 500"C. ZnO was obtained by the decomposition of zinc oxalate, ZnC204.2H20, at 400"C in air.
•
7
0.6
o
0.2
g u.
?-Fe203 (cubic form) was prepared by thermal decomposition of ferrous oxalate, FeC204.2H20, around 250"C in a stream of air and characterized by X-ray diffraction ('fable 1). • -Fe203 (hexagonal form) was obtained by the dehydration
Reported values*
d(A)
0
2
4
6
Reoction
time,
8
I0
12
hr
Fig. 1. Isothermal kinetic curves forthe formation of ZnFe20, from ~<-Fe20~ and ZnO.
570
K. R. KRISHNAMURTHY,J. GOPALAKRISHNAN,G. ARAVAMUDANand M. V. C. SASTRI Table 2. X-ray diffraction data for ?-Fe20 3 heated in air at 500°C for 2 hr in the presence and in the absence of ZnO* d spacings observed in presence of Z n O 3-68 (13) 2-95 (21) 2.806 (60) 2.691 (33) 2.595 (50) 2.510 (100) 2.468 (100) 2.108 (11) 1.906 (17) 1.840 (13) 1-694 (11) 1.623 (37) 1.605 (7) 1.487 (20) 1.474 (37)
d spacings reported d spacings observed for ~'-Fe203, Z n O in the absence d spacings reported and ZnFe204 of ZnO for a-Fe203 3.721 (60)* 2.944 (90r 2.816 (71)b 2.708 (20)° 2.602 (56)b 2.510 (100)* 2.476 (100)b 2-102 (12)c 1.911 (29)b 1.818 (80)" 1.699 (40)" 1.626 (40)b 1.602 (90)" 1.477 (35)b 1-472 (90)*
3"680 (20) 2.695 (100) 2.510 (80) 2"205 (35) 1'836 (35) 1'694 (35) 1'480 (25)
3'66 (25) 2"69 (100) 2.51 (50) 2-201 (30) 1.838 (40) 1'690 (60) 1"484 (35)
* 1 : 1 molar mixture of 7-Fe203 and ZnO. a, b and c refer to lines corresponding to 7-Fe20 3 , ZnO and Z n F e 2 0 , respectively.
Kinetic studies
Table 3. Rate constants for the formation of ZnFe204 in air
Intimately mixed equimolar quantities of ZnO and • -Fe203 or y-Fe203 were reacted in air at selected temperatures in the range 500-800"C (controlled to within +5"C) for periods of 2, 4, 6, 8, 10 and 12 hr. After the reaction, the contents were analysed for unreacted ZnO by extraction with 2 N HCI in cold and titration with EDTA. The fraction of reaction completed (x) was calculated by material balance and plotted against time to get isothermal kinetic curves (Fig. 1): The kinetic data were treated on the basis of Jander's equation to get rate constants and Arrhenius activation parameters (Tables 3 and 4). In addition, the reaction
Reaction temperature (*C)
Rate constant (hr- 1) ~-Fe20 3 + ZnO
500 550 600 650 700 750
1"218 x 10 -4 1'207 4"519 7-784 1'456
x x x x
10 -3 10 -3 10 -3 10 -2
Table 4. Kinetic parameters for the formation of Z n F e 2 0 4 in air
System
Frequency factor, A (hr- 1)
AH* (kcal/mole)
AG # (kcal/mole)
AS * (cal. deg- x)
7-Fe2Oa + Z n O ~,-Fe2Oa + Z n O
1.00 x 104 7"3 x 104
25.1 31'1
76'4 83'0
-58"5 -54.5
Table 5. Influence of oxygen partial pressure on the formation of zinc ferrite at 600"C Reaction time (hr) 2 4 6 8 10 12
Fraction of reaction completed, x At atmospheric , At 350 _+ 2 torr At ~ 1 tort pressure air pressure air pressure 0-293 0-340 0-429 0-473 0"519 0'553
0-615 0"665 0-624 0-714 0"748
0.500 0"652 0'653 0.543
Rate constant (hr- 1) ~-Fe20 3 + Z n O 8"195 2"299 4"580 1"258 2"388
x x x x x
10-* 10 -s, 10 -3 10 -2 10 -2
Formation of zinc ferrite
571
Table 6. M6ssbauer data for ?-Fe203 + ZnO system* Isomer shift (6) Quadrupole splitting (A) in (mm/sec) in (mm/sec)
Samples 1. Pure ZnFe20, 2. 1 : 1 molar mixture of ?-Fe20 3 + ZnO heated at 600"C for 1 hr at 1 mm Hg air pressure 3. 1 : 1 molar mixture of ~,-Fe203 + ZnO heated at 600"C for 2 hr at 350 mm of Hg air pressure
0"347
0-371
0.359
0.345
0.351
0'384
* The data were obtained at room temperature and the isomer shifts are with respect to Fe-foil at room temperature.
between7-Fe20 3and ZnO was studied at 350Torr and 1Torr of air pressure and 600~C (Table 5). The DTA curves for ?-Fe203 and ?-Fe20 3 and ZnO mixture (1 : 1) heated in air were recorded with a Netzsch apparatus (Netzsch Ger~itebau GmbH, Selb, West Germany) employing Pt crucibles and Pt-Pt/Rh thermocouples. A heating rate of 10*C/rain was used. X-ray diffractograms of the starting materials, reaction intermediates and final products were recorded with Mn-filtered CoK= radiation. M6ssbauer spectral data for ?-Fe203-ZnO samples treated differently (Table 6) were kindly supplied by Professor N. N. Greenwood, University of Leeds, U.K.
-I'O I
-2.0 -
O
A
~3.0
RESULTS AND DISCUSSION P h a s e transformation in 7-Fe20 3
-4.e
v-Fe2Oa is a metastable form of iron oxide having a cubic, spinel-type, structure. The distribution of Fe a+ ions among the octahedral and tetrahedral sites of the oxide lattice is denoted as [9] [Fes3 + ]t[Fel33¢ + 1-12~]0Oa2 • Such a formulation indicates the presence of 2{ vacant octahedral cation sites per unit cell that are normally occupied in an ideal spinel lattice. On heating ?-Fe20 a is known to undergo transformation to the stable antiferromagnetic ~t-variety. The temperature of transformation is however, not well-defined and it varies as widely as 450-6000C with individual samples, depend-
300
400
I
1
~.oo
I
Ho
liT
*K
1.2 o
~-3 o
x 103
Fig. 2. A~henius plots for the formation of ZnFe204 from,
(a) ?-Fe203 + ZnO, and (b) =-Fe203 + ZnO. hag on particle size, degree of crystallinity etc.[10]. In our studies it is found that ?-Fe203, obtained by the decomposition of FeC204 . 2 H 2 0 in air, transforms to • -Fe203 at 425-450°C as revealed by an exothermic peak in the D T A curve. However, in the DTA curve of the 1:1 molar mixture of ZnO and T-Fe2Oa, the exothermic peak at 450°C is significantly absent (Fig. 3).
500
Ternpero'~ure,
600
700
*C
Fig. 3. DTA curves for, (a) 7-Fe203, and (b) y-Fe203 + ZnO (1 i l mixture).
80C
572
K. R. KRISHNAMURTHY,J. GOPALAKRISHNAN,G. ARAVAMUDANand M. V. C. SASTRI
Instead, it has only broad exothermic dents attributable to ZnFezOa formation in the high temperature side (500-700"C). In order to substantiate the DTA results an equimolar mixture of ~-Fe203 and ZnO heated for about 2 hr at 50tYC in air was examined by XRD (Table 2). The diffraction lines correspond mainly to the original ~/-Fe2Oz and ZnO excepting for a faint line due to ZnFe204. It is to be noted that when ~-Fe203 alone (in the absence of ZnO) is heated under similar conditions, complete transformation to u-form takes place within the same time (Table 2). The stabilization of ~-FezOz by other metal ions like Na +, A13+ and Cr 3+ has already been reported in literature[Ill. From the foregoing, it is evident that ~-Fe203 reacts directly with ZnO to form the ferrite without transforming to u-Fe2Oz.
in Table 5. It is seen that ferrite formation is in general accelerated at moderately low oxygen pressures, although, at very low oxygen partial pressures, the overall conversion to ZnFe20, decreases. These findings may be explained by assuming that the reaction occurs through the following steps[11]: (i) Initial phase boundary reaction between Fe203 and ZnO resulting in the formation of two interfaces. (ii) Reduction of,Fe 3+ at the Fe203/ZnFe2Og interface 3Fe203 --, 2F%O4 + ½02. (iii) Reaction between FeaO4 and ZnO to form a solid solution followed by diffusion of cations through the product layer xZnO + 2F%O4 --*2FezZnx/20(4 +x/2)(- Fez -.,ZnxO,,). (iv) Oxidation of Fe 2+ to Fe 3+ giving zinc ferrite
ZnFe20, formation in air
2FezZnx/2Ot4+ ~/2)+ ~)2 ~ xZnFe204 + (3 - x)Fe203.
The kinetic data for the reactions of ZnO with u-Fe2Oz and with ?-Fe203 summarized in Tables 3 and 4, show that there is a fast initial reaction followed by a fairly slow one. Presumably, the initial surface reaction results in the formation of a ZnFe20# layer that creates two phase boundaries, ZnO/ZnFe204 and ZnFe2Od Fe203 and further reaction occurs by ionic diffusion through the product layer. The kinetic data for the formation of zinc ferrite can be satisfactorily analysed on the basis of Jander's equation
The above scheme would show that the reaction involves counter diffusion of Fe 2+ and Zn 2+ across the product layer with an effective oxygen transport through the gas phase. A similar mechanism has been proposed by other authors for the formations of MgFezO,[Ib] and NiFe204[11]. Lowering the oxygen pressure would favour the reduction of Fe z+ to Fe 2+ (step 2) thereby increasing the Fe z+ concentration at the Fe2Oa/ZnFezO, interface which in turn would increase the diffusion flux of Fe 2+ ions across the product layer. Hence the observed acceleration of ZnFe20# formation at moderately low oxygen pressures (~ 70 Torr). However, at very low oxygen pressures (~0.2 Torr) step 4, which involves oxidation of Fe z + to Fe z +, may be retarded, thus accounting for the net decrease in the conversion to zinc ferrite. Evidence for the formation of a ferrous-containing intermediate, ZnxFez-xO4, has been obtained from a study of the sample prepared by heating ?-Fe2Oa and ZnO at ~ 1 Torr air pressure and 600°C for 1 hr. The sample has a Fe 2+ content of 3.3-3.5 per cent which corresponds to the value required for a solid solution ZnxFez_ xO# with x ",, 0.85. The electrical conductivity of this sample at 300*K is around 10 -6 fl-1 crn-1 whereas the conductivity of either pure ZnFe20# or unreacted mixture of y-Fe2Oz and ZnO is much lower (~10 -s f1-1 cm-1). The M6ssbauer spectral parameters (isomer shift and quadrupole splitting) for the samples of ~-FezOz and ZnO treated differently are given in Table 6. All the samples have a quadrupole split pattern with no magnetic hyperfine splitting. The parameters for the sample heated at 1 torr pressure compare favourably with those for the phase Zn~Fea_~O4 with x = 0.8 reported by Dobson et al. [12].
[1 - (1
-
x)1/3] 2 =
Kt
where K is the rate constant and x is the fraction of reaction completed at time t. The average rate constants and the Arrhenius activation parameters are given in Tables 3 and 4 respectively. The fact that the rate data fit Jander's equation shows that the reaction follows a mechanism involving diffusion of ions after the initial reaction. The values of activation enthalpy, AH*, and free energy, AG *, obtained in this work (,,, 30 kcal/mole and ~ 80 kcal/mole) for the reaction between ZnO and u-Fe2Oz compare favourably with those reported by Cholet al.[7] and Duncan and Stewart[6] for the same reaction. The enthalpy of activation for the reaction with 7-Fe2Oz is found to be 25 kcal/mole which is about 6 kcal/mole lower than the value obtained for u-FezOz reaction, showing thereby that ~,-Fe2Oa is more reactive than the u-form towards zinc ferrite formation. The greater reactivity of y-FezOz may be traced to its spineltype structure. The cubic close packed lattice of oxide ions found in the product spinel is already available in ),-Fe203, whereas u-Fe2Oz having a corundum structure, has first to undergo a lattice rearrangement from h.c.p, to c.c.p, of oxide ions prior to reaction with ZnO. Reaction between ?-Fe20 a and ZnO at low oxygen partial pressures
The results obtained for the reactions under air pressures of 760, 350 _ 2 and 1 Torr at 600°C are presented
Marker studies
To determine the relative magnitudes of ionic diffusion, marker studies have been carried out in air. Pellets of ~,-Fe203 and ZnO were pressed together with
Formation of zinc ferrite a thin gold wire in between and the composite pellet heated at 800"C for 15 hr. On cleaving the gold wire was found close to the ZnO/ZnFe20+ interface. The reacted pellet gave way at the same interface while the other interface was held intact. These results suggest that, although the reaction involves a counter-diffusion of Zn 2+ and Fe 2 +, the diffusion of Zn 2+ is faster while that of Fe 2÷ is rate-controlling. Acknowledgements--This work forms part of a research project sponsored by the National Bureau of Standards, Washington, D.C., under contract No. NBS(G)-133. One of the authors (K.R.K.) is thankful to C.S.I.R. (India) for the award of a Junior Research Fellowship. REFERENCES
1. (a) H. Schmalzried, Z. Phys. Chem. 33, 111 (1962); (b) P. Reijnen, Reactivity of Solids, Fifth International Symposium (Edited by G.-M. Schwab), p. 562. Elsevier, Amsterdam (1965); (c) C. Kooy, ibid. p. 21. (d) G. C. Kuczynskii, Ferrites, Proc. Int. Conf., 1970 (Edited by Y. Hoskino, S. Lida and M. Sugimoto), p, 87. Univ. Tokyo Press, Japan (1971).
J.I.N.C., Vol. 36, No. 3 - - G
573
2. J. P. Auradon, F. Damay and G. Chol, IEEE Trans. Magnetics 5 (3), 276 (1969). 3. G. Chol and J. P. Aubaile, Ferrites, Proc. Int. Conf., 1970 (Edited by Y. Hoshino, S. Iida and M. Sugimoto), p. 243. Univ. Tokyo Press, Japan (1971). 4. Z. G. Szabo, I. Batta and F. Solymosi, Reactivity of Solids, Fourth International Symposium (Edited by J. H. De Boer), p. 409. Elsevier, Amsterdam (1960). 5. E. S. Savranskaya, Yu. D. Tret'yokov and V. I. Fadeeva, Porosh. Met. 11, 65 (1971); Chem. Abstr. 76, 28206q. 6. J. F. Duncan and D. J. Stewart, Trans. Faraday Soc. 63, 1031 (1967). 7. G. Chol, Y. Gros, J. C. Pebay-Peyroula and J. F. Barbe, Mater. Res. Bull. 2, 753 (1967). 8. H. Parker, C. J. Rigden and C. J. Tinsley, Trans. Faraday Soc. 65, 219 (1969). 9. N. N. Greenwood, Ionic Crystals, Lattice Defects, and Nonstoichiometry, p. 104. Butterworths, London (1970). 10. R. C. Mackenzie, Differential Thermal Analysis, Vol. I, p. 274. Academic Press, New York (1970). 11. D. Elwell, R. Parker and C. J. Tinsley, Solid St. Commun. 4, 67 (1966); Czech. J. Phys. B17, 382 (1967). 12. D. C. Dobson, J. W. Linnett and M. M. Rahman, J. Phys. Chem. Solids, 31, 2727 (1970).
STUDIES ON THE FORMATION OF ZINC FERRITE K. R. KRISHNAMURTHY, J. GOPALAKRISHNAN, G. ARAVAMUDAN and M. V. C. SASTRI Department of Chemistry, Indian Institute of Technology, Madras 600036, India (Received 1 June 1973)
Abstract--The reactions of zinc oxide with x-Fe203 and with ?-Fe203 leading to the formation of zinc ferrite (ZnFe204) have been studied at different partial pressures of oxygen in the temperature range 500800°C. The ferrite spinel is formed more readily when ~-Fe203 is used as the reactant. The DTA and X-ray diffraction results indicate that the 2;-Fe203 phase is stabilized in presence of ZnO. The lowering of partial pressure of oxygen in the ambient atmosphere favours the formation of zinc ferrite, indicating a cyclic redox process featuring FeS+, Fez+ and oxygen at the phase boundaries. At very low oxygen pressures, conversion to zinc ferrite is not facilitated, apparently due to the formation of an intermediate species,F%_ xZn=04, which, under these conditions, is not easily converted to ZnFe20,.
INTRODUCTION
Table 1. X-ray diffraction data for ~.Fe20 3
THE VOmVa~rIONof ferrite spinels, A F e 2 0 , (where A is a divalent metal ion, Mg 2÷, Mn2 +, Ni 2÷, Zn 2+, etc.) from their constituent oxides AO and Fe20~ has been the subject of a number of investigations in view of their technical importance[l]. The reaction between oxides AO and Fe2Os is known to be influenced by a variety of factors, such as, particle size[2], efficiency of mixing[3], presence of lattice defects and associated nonstoichiometry[4] in the reactants, and changes in the ambient pressure of oxygen in the reaction atmosphere[5]. An understanding of the factors that control the formation of ferrites is desirable in the context of their technical production. Previous studies of ferrite formation have invariably employed ct-Fe20 3 as the starting material[l, 6-8]. In the present work, the formation of ZnFe204 is studied in detail employing ?-Fe2Oa as the reactant. The kinetics of the reaction between ?-Fe203 and ZnO under atmospheric and at low oxygen partial pressures in the temperature range 500-800"C is studied with a view to establishing the conditions for the production of Z n F e 2 0 , and to understanding the mechanism of ferrite formation in general. For comparative purposes, the reaction between x-Fe203 and ZnO at the same temperature range under atmospheric pressure is also studied. EXPERIMENTAL
Observed values
569
i
dtA)
I
3"680 2'947 2"695 2"510 2'081 1'836 1.694 1"605 1'485
13 28 33 100 20 15 20 23 30
3'720 2.944 2'708 2'510 2"082 1"818 1.699 1'602 1"472
25 90 20 100 90 80 40 90 90
* ASTM Card No. 13-458. of hydrated Fe203 at 500"C. ZnO was obtained by the decomposition of zinc oxalate, ZnC204.2H20, at 400"C in air.
•
7
0.6
o
0.2
g u.
?-Fe203 (cubic form) was prepared by thermal decomposition of ferrous oxalate, FeC204.2H20, around 250"C in a stream of air and characterized by X-ray diffraction ('fable 1). • -Fe203 (hexagonal form) was obtained by the dehydration
Reported values*
d(A)
0
2
4
6
Reoction
time,
8
I0
12
hr
Fig. 1. Isothermal kinetic curves forthe formation of ZnFe20, from ~<-Fe20~ and ZnO.
570
K. R. KRISHNAMURTHY,J. GOPALAKRISHNAN,G. ARAVAMUDANand M. V. C. SASTRI Table 2. X-ray diffraction data for ?-Fe20 3 heated in air at 500°C for 2 hr in the presence and in the absence of ZnO* d spacings observed in presence of Z n O 3-68 (13) 2-95 (21) 2.806 (60) 2.691 (33) 2.595 (50) 2.510 (100) 2.468 (100) 2.108 (11) 1.906 (17) 1.840 (13) 1-694 (11) 1.623 (37) 1.605 (7) 1.487 (20) 1.474 (37)
d spacings reported d spacings observed for ~'-Fe203, Z n O in the absence d spacings reported and ZnFe204 of ZnO for a-Fe203 3.721 (60)* 2.944 (90r 2.816 (71)b 2.708 (20)° 2.602 (56)b 2.510 (100)* 2.476 (100)b 2-102 (12)c 1.911 (29)b 1.818 (80)" 1.699 (40)" 1.626 (40)b 1.602 (90)" 1.477 (35)b 1-472 (90)*
3"680 (20) 2.695 (100) 2.510 (80) 2"205 (35) 1'836 (35) 1'694 (35) 1'480 (25)
3'66 (25) 2"69 (100) 2.51 (50) 2-201 (30) 1.838 (40) 1'690 (60) 1"484 (35)
* 1 : 1 molar mixture of 7-Fe203 and ZnO. a, b and c refer to lines corresponding to 7-Fe20 3 , ZnO and Z n F e 2 0 , respectively.
Kinetic studies
Table 3. Rate constants for the formation of ZnFe204 in air
Intimately mixed equimolar quantities of ZnO and • -Fe203 or y-Fe203 were reacted in air at selected temperatures in the range 500-800"C (controlled to within +5"C) for periods of 2, 4, 6, 8, 10 and 12 hr. After the reaction, the contents were analysed for unreacted ZnO by extraction with 2 N HCI in cold and titration with EDTA. The fraction of reaction completed (x) was calculated by material balance and plotted against time to get isothermal kinetic curves (Fig. 1): The kinetic data were treated on the basis of Jander's equation to get rate constants and Arrhenius activation parameters (Tables 3 and 4). In addition, the reaction
Reaction temperature (*C)
Rate constant (hr- 1) ~-Fe20 3 + ZnO
500 550 600 650 700 750
1"218 x 10 -4 1'207 4"519 7-784 1'456
x x x x
10 -3 10 -3 10 -3 10 -2
Table 4. Kinetic parameters for the formation of Z n F e 2 0 4 in air
System
Frequency factor, A (hr- 1)
AH* (kcal/mole)
AG # (kcal/mole)
AS * (cal. deg- x)
7-Fe2Oa + Z n O ~,-Fe2Oa + Z n O
1.00 x 104 7"3 x 104
25.1 31'1
76'4 83'0
-58"5 -54.5
Table 5. Influence of oxygen partial pressure on the formation of zinc ferrite at 600"C Reaction time (hr) 2 4 6 8 10 12
Fraction of reaction completed, x At atmospheric , At 350 _+ 2 torr At ~ 1 tort pressure air pressure air pressure 0-293 0-340 0-429 0-473 0"519 0'553
0-615 0"665 0-624 0-714 0"748
0.500 0"652 0'653 0.543
Rate constant (hr- 1) ~-Fe20 3 + Z n O 8"195 2"299 4"580 1"258 2"388
x x x x x
10-* 10 -s, 10 -3 10 -2 10 -2
Formation of zinc ferrite
571
Table 6. M6ssbauer data for ?-Fe203 + ZnO system* Isomer shift (6) Quadrupole splitting (A) in (mm/sec) in (mm/sec)
Samples 1. Pure ZnFe20, 2. 1 : 1 molar mixture of ?-Fe20 3 + ZnO heated at 600"C for 1 hr at 1 mm Hg air pressure 3. 1 : 1 molar mixture of ~,-Fe203 + ZnO heated at 600"C for 2 hr at 350 mm of Hg air pressure
0"347
0-371
0.359
0.345
0.351
0'384
* The data were obtained at room temperature and the isomer shifts are with respect to Fe-foil at room temperature.
between7-Fe20 3and ZnO was studied at 350Torr and 1Torr of air pressure and 600~C (Table 5). The DTA curves for ?-Fe203 and ?-Fe20 3 and ZnO mixture (1 : 1) heated in air were recorded with a Netzsch apparatus (Netzsch Ger~itebau GmbH, Selb, West Germany) employing Pt crucibles and Pt-Pt/Rh thermocouples. A heating rate of 10*C/rain was used. X-ray diffractograms of the starting materials, reaction intermediates and final products were recorded with Mn-filtered CoK= radiation. M6ssbauer spectral data for ?-Fe203-ZnO samples treated differently (Table 6) were kindly supplied by Professor N. N. Greenwood, University of Leeds, U.K.
-I'O I
-2.0 -
O
A
~3.0
RESULTS AND DISCUSSION P h a s e transformation in 7-Fe20 3
-4.e
v-Fe2Oa is a metastable form of iron oxide having a cubic, spinel-type, structure. The distribution of Fe a+ ions among the octahedral and tetrahedral sites of the oxide lattice is denoted as [9] [Fes3 + ]t[Fel33¢ + 1-12~]0Oa2 • Such a formulation indicates the presence of 2{ vacant octahedral cation sites per unit cell that are normally occupied in an ideal spinel lattice. On heating ?-Fe20 a is known to undergo transformation to the stable antiferromagnetic ~t-variety. The temperature of transformation is however, not well-defined and it varies as widely as 450-6000C with individual samples, depend-
300
400
I
1
~.oo
I
Ho
liT
*K
1.2 o
~-3 o
x 103
Fig. 2. A~henius plots for the formation of ZnFe204 from,
(a) ?-Fe203 + ZnO, and (b) =-Fe203 + ZnO. hag on particle size, degree of crystallinity etc.[10]. In our studies it is found that ?-Fe203, obtained by the decomposition of FeC204 . 2 H 2 0 in air, transforms to • -Fe203 at 425-450°C as revealed by an exothermic peak in the D T A curve. However, in the DTA curve of the 1:1 molar mixture of ZnO and T-Fe2Oa, the exothermic peak at 450°C is significantly absent (Fig. 3).
500
Ternpero'~ure,
600
700
*C
Fig. 3. DTA curves for, (a) 7-Fe203, and (b) y-Fe203 + ZnO (1 i l mixture).
80C
572
K. R. KRISHNAMURTHY,J. GOPALAKRISHNAN,G. ARAVAMUDANand M. V. C. SASTRI
Instead, it has only broad exothermic dents attributable to ZnFezOa formation in the high temperature side (500-700"C). In order to substantiate the DTA results an equimolar mixture of ~-Fe203 and ZnO heated for about 2 hr at 50tYC in air was examined by XRD (Table 2). The diffraction lines correspond mainly to the original ~/-Fe2Oz and ZnO excepting for a faint line due to ZnFe204. It is to be noted that when ~-Fe203 alone (in the absence of ZnO) is heated under similar conditions, complete transformation to u-form takes place within the same time (Table 2). The stabilization of ~-FezOz by other metal ions like Na +, A13+ and Cr 3+ has already been reported in literature[Ill. From the foregoing, it is evident that ~-Fe203 reacts directly with ZnO to form the ferrite without transforming to u-Fe2Oz.
in Table 5. It is seen that ferrite formation is in general accelerated at moderately low oxygen pressures, although, at very low oxygen partial pressures, the overall conversion to ZnFe20, decreases. These findings may be explained by assuming that the reaction occurs through the following steps[11]: (i) Initial phase boundary reaction between Fe203 and ZnO resulting in the formation of two interfaces. (ii) Reduction of,Fe 3+ at the Fe203/ZnFe2Og interface 3Fe203 --, 2F%O4 + ½02. (iii) Reaction between FeaO4 and ZnO to form a solid solution followed by diffusion of cations through the product layer xZnO + 2F%O4 --*2FezZnx/20(4 +x/2)(- Fez -.,ZnxO,,). (iv) Oxidation of Fe 2+ to Fe 3+ giving zinc ferrite
ZnFe20, formation in air
2FezZnx/2Ot4+ ~/2)+ ~)2 ~ xZnFe204 + (3 - x)Fe203.
The kinetic data for the reactions of ZnO with u-Fe2Oz and with ?-Fe203 summarized in Tables 3 and 4, show that there is a fast initial reaction followed by a fairly slow one. Presumably, the initial surface reaction results in the formation of a ZnFe20# layer that creates two phase boundaries, ZnO/ZnFe204 and ZnFe2Od Fe203 and further reaction occurs by ionic diffusion through the product layer. The kinetic data for the formation of zinc ferrite can be satisfactorily analysed on the basis of Jander's equation
The above scheme would show that the reaction involves counter diffusion of Fe 2+ and Zn 2+ across the product layer with an effective oxygen transport through the gas phase. A similar mechanism has been proposed by other authors for the formations of MgFezO,[Ib] and NiFe204[11]. Lowering the oxygen pressure would favour the reduction of Fe z+ to Fe 2+ (step 2) thereby increasing the Fe z+ concentration at the Fe2Oa/ZnFezO, interface which in turn would increase the diffusion flux of Fe 2+ ions across the product layer. Hence the observed acceleration of ZnFe20# formation at moderately low oxygen pressures (~ 70 Torr). However, at very low oxygen pressures (~0.2 Torr) step 4, which involves oxidation of Fe z + to Fe z +, may be retarded, thus accounting for the net decrease in the conversion to zinc ferrite. Evidence for the formation of a ferrous-containing intermediate, ZnxFez-xO4, has been obtained from a study of the sample prepared by heating ?-Fe2Oa and ZnO at ~ 1 Torr air pressure and 600°C for 1 hr. The sample has a Fe 2+ content of 3.3-3.5 per cent which corresponds to the value required for a solid solution ZnxFez_ xO# with x ",, 0.85. The electrical conductivity of this sample at 300*K is around 10 -6 fl-1 crn-1 whereas the conductivity of either pure ZnFe20# or unreacted mixture of y-Fe2Oz and ZnO is much lower (~10 -s f1-1 cm-1). The M6ssbauer spectral parameters (isomer shift and quadrupole splitting) for the samples of ~-FezOz and ZnO treated differently are given in Table 6. All the samples have a quadrupole split pattern with no magnetic hyperfine splitting. The parameters for the sample heated at 1 torr pressure compare favourably with those for the phase Zn~Fea_~O4 with x = 0.8 reported by Dobson et al. [12].
[1 - (1
-
x)1/3] 2 =
Kt
where K is the rate constant and x is the fraction of reaction completed at time t. The average rate constants and the Arrhenius activation parameters are given in Tables 3 and 4 respectively. The fact that the rate data fit Jander's equation shows that the reaction follows a mechanism involving diffusion of ions after the initial reaction. The values of activation enthalpy, AH*, and free energy, AG *, obtained in this work (,,, 30 kcal/mole and ~ 80 kcal/mole) for the reaction between ZnO and u-Fe2Oz compare favourably with those reported by Cholet al.[7] and Duncan and Stewart[6] for the same reaction. The enthalpy of activation for the reaction with 7-Fe2Oz is found to be 25 kcal/mole which is about 6 kcal/mole lower than the value obtained for u-FezOz reaction, showing thereby that ~,-Fe2Oa is more reactive than the u-form towards zinc ferrite formation. The greater reactivity of y-FezOz may be traced to its spineltype structure. The cubic close packed lattice of oxide ions found in the product spinel is already available in ),-Fe203, whereas u-Fe2Oz having a corundum structure, has first to undergo a lattice rearrangement from h.c.p, to c.c.p, of oxide ions prior to reaction with ZnO. Reaction between ?-Fe20 a and ZnO at low oxygen partial pressures
The results obtained for the reactions under air pressures of 760, 350 _ 2 and 1 Torr at 600°C are presented
Marker studies
To determine the relative magnitudes of ionic diffusion, marker studies have been carried out in air. Pellets of ~,-Fe203 and ZnO were pressed together with
Formation of zinc ferrite a thin gold wire in between and the composite pellet heated at 800"C for 15 hr. On cleaving the gold wire was found close to the ZnO/ZnFe20+ interface. The reacted pellet gave way at the same interface while the other interface was held intact. These results suggest that, although the reaction involves a counter-diffusion of Zn 2+ and Fe 2 +, the diffusion of Zn 2+ is faster while that of Fe 2÷ is rate-controlling. Acknowledgements--This work forms part of a research project sponsored by the National Bureau of Standards, Washington, D.C., under contract No. NBS(G)-133. One of the authors (K.R.K.) is thankful to C.S.I.R. (India) for the award of a Junior Research Fellowship. REFERENCES
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