Materials Science and Engineering B106 (2004) 196–201

Influence of oxygen pressure on combustionsynthesis of zinc ferrite powders

Yao Li a,∗, Jiupeng Zhaob, Xiaodong Hea

a Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, PR Chinab Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China

Received 10 January 2003; accepted 10 September 2003

Abstract

The influence of the oxygen pressure on the phase composition, microstructural evolution of the combustion products during the combustionsynthesis of zinc ferrite through iron, iron oxide, and zinc oxide powders was discussed. The results show that with the increase of oxygenpressure, combustion temperature and combustion wave velocity increase. Single spinel-phase zinc ferrite can be obtained when the oxygenpressure is 1.0 MPa and a non-stoichiometric composition, Fe1−xO or Fe1−zO is formed when the oxygen pressure is below 1.0 MPa or above1.5 MPa. Moreover, an analysis of the dependence of the degree of conversion to ferrite,η, on the oxygen pressure has also been made. For agiven porosity and combustion temperature, the degree of conversion increases with the oxygen pressure increasing and for any given porosityand degree of conversion, higher pressure is required at higher temperature.© 2003 Published by Elsevier B.V.

Keywords: Zinc ferrite; Oxygen pressure; Degree of conversion

1. Introduction

Zinc ferrite, the stoichiometric composition is ZnFe2O4and possesses a normal spinel structure, is commercially im-portant material because of its excellent electrical and mag-netic properties[1,2]. Recently, researches into combustionsynthesis of ferrites have been given much attention due tothe high productivity, low consumption of energy, and sim-plicity of the process. Synthesis of ZnFe2O4 ferrite proceedsaccording to the following equation:

ZnO+ (1 − k)Fe2O3 + 2kFe+ 1.5kO2 = ZnFe2O4 (1)

wherek is the coefficient which controls the exothermicityof the mixture. The larger the value ofk, the higher themolar ratio Fe/Fe2O3 in the reactants should be. Combustionsynthesis of ferrites belongs to the solid–gas combustionreaction, and in the process of the combustion, the gaseousreactant, oxygen, is involved in the process and plays animportant role. Although preparation and characterizationof ferrites through combustion synthesis have been reported

∗ Corresponding author. Tel.:+86-451-86412513;fax: +86-451-86412513.

E-mail address: liyao@hit.edu.cn (Y. Li).

by many researchers[3–5], there is very little work foundin the literature about the effect of the oxygen pressure onthe combustion process.

The purpose of the paper is to report the results about theinfluence of the oxygen pressure on the combustion temper-ature, the combustion wave velocity. In addition, the depen-dence of the phase composition, microstructural evolution,and the degree of conversion to ferrite on oxygen pressureof the samples was discussed in details.

2. Experimental procedure

The raw materials used in the experiments were iron(25�m diameter), iron oxide (0.8�m diameter), and zincoxide (0.5�m diameter). The purity of the raw materialsis more than 99%. The starting materials were weighed ac-cording to the required stoichiometric proportion, and weremixed in ethanol followed by ball milling for 8 h and thendried in air. The mixture of powders were pressed into 80 mmhigh columns (diameter 20 mm) with different porosity,φ,then were put into a quartz container. A spiral tungstenwire was used to initiate the reactants by an electric cur-rent which was in contact with the powder. The experiments

0921-5107/$ – see front matter © 2003 Published by Elsevier B.V.doi:10.1016/j.mseb.2003.09.022

Y. Li et al. / Materials Science and Engineering B106 (2004) 196–201 197

were carried out in an air-tight combustion vessel with sepa-rate inputs and outputs for supply and pumping out of gases,respectively. The pressure in the vessel was measured by avacuum gauge and a pressure gauge. The oxygen pressurewas given in the range from 0.1 to 1.5 MPa. The structure ofthe combustion chamber is identical with that in a previouswork [6].

The major parameters of the combustion synthesis process(combustion temperature,Tc and combustion wave veloc-ity, Uc) were measured with Pt/Rh thermocouples pressedinto the mixture and registered. Phase transformation ofthe combustion-synthesized products were inspected by anX-ray diffraction (XRD). Morphology of the samples werecharacterized by scanning electron microscopy (SEM). TheMössbauer spectra of ZnFe2O4 was recorded at room tem-perature using a constant acceleration Mössbauer spectrom-eter (Oxford MS-500, UK).

3. Results and discussion

3.1. Combustion temperature and combustionwave velocity

The oxygen gas determines the processing parametersTcandUc. Fig. 1 shows the values ofTc andUc under differ-ent oxygen pressures. With the increase of oxygen pressure,Tc andUc increase visibly. Moreover, combustion tempera-ture increases sharply in low-oxygen pressure region, whileit increases slowly in high-oxygen pressure region. The pos-sible reason is that the oxygen permeability and proximitybetween particles are improved with increasing the oxygenpressure, accelerating the oxidation reaction of iron, andincreasing the quantity of the heat dispersed. As a conse-quence, the combustion temperature increases, increasing ofdriving energy for propagating the combustion wave.

It has been found experimentally that the combustion ofthe samples ceases, not reaching the other end when theoxygen pressure is below 0.3 MPa.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.61550

1600

1650

1700

Com

bust

ion

tem

pera

ture

,K

PO2,MPa

k=0.5

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

1

2

3

4

Uc,

mm

/s

PO2,MPa

k=0.5¦Õ=0.5

¦Õ=0.5

Fig. 1. Combustion temperature, combustion wave velocity vs. oxygen pressure.

Fig. 2 shows the propagation depth of the combustionfront (the ratio of penetration depth of the combustion front,l, to the length of the reactant,L) versus the oxygen pres-sure. Under low-oxygen pressures, the propagation depth ofthe combustion front increases asymptotically with the oxy-gen pressure. At low pressure of the gas, the oxygen is notenough to sustain the oxidation of the ion and the pressuredifference between the ambient medium and the powderpores is small which leads to the low penetration velocity ofthe oxygen into the inner, and the combustion ceases beforeit reaches to the end.

3.2. Phase transformation

The influence of oxygen pressure on the crystalline phaseswas studied.Fig. 3shows the XRD patterns of the combus-tion products under different oxygen pressures. At 0.3 MPa,besides the main lines of the ferrite matrix, additional linesbelonging to the ZnO and�-Fe2O3 phases are seen in theXRD patterns. Their intensity is highly dependent on theoxygen pressure. As the oxygen pressure increases from 0.3to 1.5 MPa, the line intensity of�-Fe2O3 and ZnO decreaseconsiderably. At 1.0 MPa, the diffraction peaks of ZnO andFe2O3 disappear, and spinel peaks of ferrites can be clearlyobserved in the X-ray spectra of the products. With an in-crease in the oxygen pressure, the increase in sharpness ofXRD lines indicates the growth of crystallite size.

In addition, two diffraction lines, the intensity of whichis fairly weak and exist at 2θ = 42.2 and 61.1◦, respec-tively, are attributed to a non-stoichiometric composition,Fe1−xO (x < 1), deduced from the JCPDS card. However,the diffraction lines disappear when the oxygen pressure isat 1.0 MPa. When the oxygen pressure is at 1.5 MPa, the cor-responding diffraction lines split apart into a doublet whichbelongs to another non-stoichiometric phase, Fe1−zO (z <

1), cited from the JCPDS card.Mössbauer spectra of the samples measured at room tem-

perature are showed inFig. 4. In Fig. 4a, the doublet is re-lated to ZnFe2O4 and the sextets is related to�-Fe2O3. In

198 Y. Li et al. / Materials Science and Engineering B106 (2004) 196–201

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

1.0

l/L, %

PO2,MPa

k=0.5 ¦Õ=0.5

Fig. 2. Dependence of the propagation depth of the combustion front on the oxygen pressure.

5 4 3 2 1

2θ (deg)

ZnFe2O4 ZnO Fe2O3 Fe1-xO

605550454035302520

Fig. 3. XRD patterns of the products synthesized under different oxygen pressures (k = 0.5, φ = 0.5). (1) PO2 = 0.3 MPa; (2) PO2 = 0.5 MPa; (3)PO2 = 0.8 MPa; (4)PO2 = 1.0 MPa and (5)PO2 = 1.5 MPa.

-10 -8 -6 -4 0 2 4 6 8 1080

85

90

95

100

-10 -8 -6 -4 -2 0 2 4 6 8 10

90

92

94

96

98

100

V/mm s-1V/mm s-1

-2

(a) (b)

Fig. 4. Mössbauer spectra of the samples synthesized under different oxygen pressure. (a) PO2 = 1 MPa and (b) PO2 = 1.5 MPa.

Y. Li et al. / Materials Science and Engineering B106 (2004) 196–201 199

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.844

0.846

0.848

0.850

0.852

Latti

ce p

aram

eter

, nm

oxygen pressure,MPa

Fig. 5. Variation of lattice parameter of zinc ferrite with oxygen pressure.

Fig. 4b, the spectrum is built from three components: a sex-tet and two doublets. One of the doublet is also related toZnFe2O4, the other is Fe1−xO or Fe1−zO. While the sextetsis close to �-Fe2O3 according to the reference [7]. Com-pared with the �-Fe2O3 crystallized completely, the hyper-fine fields decrease and the value of the quadruple splittingbroadens which indicate that the �-Fe2O3 crystallized in-completely under the given condition and the asymmetry ofthe iron nucleus. The presence of �-Fe2O3 probes that thecombustion products are formed under a high cooling ratedue to the high combustion temperature under high-oxygenpressure during the combustion reaction.

From the results of XRD and Mössbauer spectra analy-sis, it can be concluded that the non-stoichiometric com-position, Fe1−xO or Fe1−zO is formed only under a verylow- or high-oxygen pressure. The reasonable interpretationmight be given as follows: when the combustion processis carried out under low-oxygen pressure (below 1 MPa),the ferritization degree of the combustion products will below due to the low Tc and Uc. When the oxygen pressureis high (above 1.5 MPa), the crystal lattice might not haveenough time to search for the more stable positions due tothe high temperature and high combustion wave velocity.Therefore, both a low and a high-oxygen pressure wouldlead to a non-equilibrium condition under which some in-termediate phases of iron oxide with a non-stoichiometriccomposition and crystal defects were formed. Similar resultswere obtained by Murin et al. during the oxidation of iron[8].Dependence of the lattice parameter a0 of ZnFe2O4 onthe oxygen pressure is shown in Fig. 5. The lattice parame-ter a0 decreases with an increase in the oxygen pressure toapproach the lattice parameter of ZnFe2O4 cited from theJCPDS card (a0 = 0.8441 nm) which is indicated by thebroken line in the figure.

3.3. Microstructure of the combustion products

The final products are very loose and could be easilymilled into powders. The crystalline degree of the combus-tion products obtained under 1.0 MPa is more perfect thanthat formed under 1.5 MPa as shown in Fig. 6. Fig. 6 clearlyshows spherical-shaped particles for low-oxygen pressure(1.0 MPa) and with a tendency towards an anomalous shapefor higher oxygen pressure (1.5 MPa). The particles grainsize increases substantially with the oxygen pressure due tothe faster kinetics of the crystal growth under higher oxy-gen pressure. When oxygen pressure is above 1.5 MPa, thephase that has melted can be observed in the samples and thecombustion product is very hard to be crushed into powdersdue to self-sintering.

3.4. The relationship between oxygen pressure and degreeof conversion

During the combustion process, oxygen exists three ki-netic potential barriers, i.e., the penetration of oxygen fromthe environment into the combustion zone; the oxidation ofiron; the infiltration of oxygen through the melting productlayer. If the combustion reaction is carried out under thelow-oxygen pressure, the ferritization degree of the com-bustion products will be low. This is caused by the short-age of oxygen and the lack of permeability. The degree ofconversion to ferrite for the powders depends on the oxy-gen pressure and the porosity of the sample. Ideally, theamount of oxygen gas occupying the total volume of poresis stoichiometrically equal to that for the total conversionand is independent on the permeation of gas. We use theClapeyron–Mendeleev equation to calculate their relation-ship, i.e.

200 Y. Li et al. / Materials Science and Engineering B106 (2004) 196–201

Fig. 6. SEM photomicrograph of the samples obtained under different oxygen pressures. (a) PO2 = 1.0 MPa and (b)PO2 = 1.5 MPa.

PO2(φV) = nO2 RT (2)

where PO2 is the oxygen pressure, φ the initial porosity ofthe sample, V the volume of the reactant, nO2 the number ofmoles of O2, ρ the density of iron, R the gas constant andT is the combustion temperature.

According to Eq. (1), the molar volume of iron in thereactant,VFe, can be given as follows:

VFe = 2k

2 + kV (3)

VFe also can be calculated as

VFe = nFeMFe

ρ(4)

where MFe is the atomic weight of iron and nFe is the numberof moles of iron in the reactants.

We define the parameter η as the degree of conversion toferrite. Combining Eqs. (2)–(4), η can be given as follows:

η = 2(2 + k)

3k

MFe

ρRTc

φ

1 − φPO2 (5)

0 1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

C,%

P,0.1MPa

100 1 2 3 4 5 6 7 8 90

20

40

60

80

100

C,%

P,0.1MPa

Fig. 7. Dependence of the degree of conversion to ferrite on oxygen pressure (k = 0.5). (a) Tc = 1500 K and (b) Tc = 1700 K.

Eq. (5) described the dependence of the degree of conver-sion on the oxygen pressure, porosity φ and combustion tem-perature. For a given porosity and combustion temperature,Eq. (5) simplifies to

η = k1PO2

where k1 is a constant, and

k1 = 2(2 + k)

3k

MFe

ρRTc

φ

1 − φ

It can be seen that the degree of conversion increaseswith the oxygen pressure increasing. Oxygen pressure cor-responding to varying degrees of conversion to zinc ferritewere calculated by means of Eq. (5) for three porosity valuesand two temperatures. The results are shown in Fig. 7. Theporosity range was selected to reflect practical upper andlower limits. The upper limit (φ = 0.65) represents a valuetypical of uncompacted powders while the lower limit (φ =0.35) is typical of highly compacted powders. Fig. 7a showsthat in order to obtain complete conversion (η = 1), oxygen

Y. Li et al. / Materials Science and Engineering B106 (2004) 196–201 201

pressure is raised from 0.28 MPa for φ = 0.65 to 0.98 MPafor φ = 0.35. And for a given oxygen pressure, larger andmore numerous pores of the reactant favor higher degreesof conversion and lower dependence on the permeation ofoxygen gas through the compact. For any given porosity anddegree of conversion, higher pressure is required at highertemperature.

For a complete conversion and a given porosity, Eq. (5)becomes

PO2 = 3k

2(2 + k)

ρR

MFe

1 − φ

φTc (6)

Eq. (6) means that at a certain combustion temperature, thereis a critical oxygen pressure value Pc. When the practicaloxygen pressure is in excess of the critical value Pc, a com-plete conversion to ferrite can be obtained; while the prac-tical oxygen pressure is lower than the critical pressure, thedegree of conversion is dependent on the permeation of oxy-gen gas, and the reactant will not be convert to ferrite com-pletely due to the lack of oxygen.

4. Conclusions

(1) With the increase of oxygen pressure, Tc and Uc increaseobviously. Under low-oxygen pressures, the propagationdepth of the combustion front increases asymptoticallywith the oxygen pressure.

(2) The results of XRD, Mössbauer spectra and SEMshow that single spinel-phase zinc ferrite with nearlyspherical-shaped particles can be obtained when theoxygen pressure is 1.0 MPa and a non-stoichiometriccomposition, Fe1−xO or Fe1−zO is formed under a verylow- or high-oxygen pressure.

(3) Analysis of the dependence of the degree of conver-sion to ferrite, η, on the oxygen pressure shows thatfor a given porosity and combustion temperature, thedegree of conversion increases with the oxygen pres-sure increasing and for any given porosity and degreeof conversion, higher pressure is required at highertemperature. Only when the practical oxygen pressureis in excess of the critical pressure value, Pc, a completeconversion to ferrite can be obtained.

Acknowledgements

The first author is grateful to the Hei Longjiang ProvinceNatural Science Foundation of China (02E-08) and ChinaPostdoctoral Science Foundation (LRB00047) that sup-ported this research.

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