Materials Chemistry and Physics 82 (2003) 991–996

Influence of oxygen pressure on combustion synthesis of ZnFe2O4

Yao Li a,∗, Jiupeng Zhaob, Jiuxing Jianga, Xiaodong Heaa Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, PR China

b Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China

Received 17 April 2003; received in revised form 22 August 2003; accepted 3 September 2003

Abstract

The influences of oxygen pressure on the phase composition, microstructural evolution of the combustion products during combustionsynthesis of ZnFe2O4 through iron, iron oxide and zinc oxide powders are discussed. Single spinel-phase zinc ferrite can be obtained whenthe oxygen pressure is 1.0 MPa and non-stoichiometric oxides, Fe1−xO and Fe1−zO are formed at high or low oxygen pressure. Moreover,an analysis of the dependence of the degree of conversion to ferrite,η, on oxygen pressure has also been made. For a given porosity andcombustion temperature, the degree of conversion increases with the oxygen pressure increasing and for any given porosity and degree ofconversion, higher oxygen pressure is required at higher temperature.© 2003 Elsevier B.V. All rights reserved.

Keywords: Zinc ferrite; Combustion synthesis; Oxygen pressure; Degree of conversion

1. Introduction

Zinc ferrite of the stoichiometric composition ZnFe2O4,possesses a normal spinel structure. It is a 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 thek value, the higher the molarratio of Fe/Fe2O3 in the reactants should be. Combustionsynthesis of ferrites belongs to the solid–gas combustionreaction, and during the process of the combustion, agaseous reactant, oxygen, is involved and plays an importantrole. Although preparation and characterization of ferritesthrough combustion synthesis method have been reportedby 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 oxygen pressure on the combustion temperatureand combustion wave velocity. In addition, the dependencies

∗ Corresponding author. Tel./fax:+86-451-86412513.E-mail address: liyao@hit.edu.cn (Y. Li).

of the phase composition, microstructural evolution of thecombustion products and their degree of conversion to ferriteon oxygen pressure were discussed in detail.

2. Experimental procedure

The raw materials used in the experiments were iron, ironoxide and zinc oxide, with an average particle size of 25,0.8 and 0.5�m, respectively. 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 thenwere dried in air. The mixture of powders were pressedinto 80 mm high columns (diameter 20 mm) with differentporosity, then they were put into a quartz container. A spiraltungsten wire was used to initiate the reactants by an elec-tric current which was in contact with the powder. The ex-periments were carried out in an air-tight combustion vesselwith separate inputs and outputs for supply and pumping outof gases, respectively. The pressure in the vessel was mea-sured by a vacuum gauge and a pressure gauge. The oxygenpressure was given in the range from 0.1 to 1.5 MPa. Thestructure of the combustion reactor is identical with that ina previous work[6].

The major characteristics of the combustion synthesis pro-cess (combustion temperature,Tc and combustion wave ve-locity, Uc) were measured with Pt/Rh thermocouples pressed

0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2003.09.005

992 Y. Li et al. / Materials Chemistry and Physics 82 (2003) 991–996

into the mixture and registered. The propagation depth ofthe combustion front was determined by the ratio of pene-tration depth of the combustion front,l, to the length of thereactant,L. Phase transformation of the as-synthesized prod-ucts and the particle size were inspected by X-ray diffrac-tion (XRD) (Siemens 5000). Morphology of the sampleswere characterized by scanning electron microscopy (SEM)(JSM 5410). The Mössbauer spectra of the samples wererecorded at room temperature using a constant accelerationMössbauer spectrometer (Oxford MS-500, UK).

3. Results and discussion

3.1. Combustion temperature and combustionwave velocity

The oxygen gas determines the processing parametersTcand Uc. Fig. 1 shows the values ofTc and Uc under dif-ferent oxygen pressures. With the increase of oxygen pres-sure,Tc andUc increase obviously. Moreover, the increment

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

φ =0.5k=0.5

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

1

2

3

4

U c,m

m/s

PO2,MPa

φ =0.5k=0.5

Fig. 1. Combustion temperature and combustion wave velocity versus oxygen pressure (k is the exothermic coefficient,φ the initial porosity of the sample).

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

of combustion temperature at high oxygen pressures werelarger than that of at low oxygen pressures. The possible rea-son is that the oxygen permeability and proximity betweenparticles are improved sharply with the increase of oxygenpressure in low pressure region, accelerating the oxidationreaction of iron and increasing the quantity of the heat re-leased. Consequently, the combustion temperature increasesintensively, increasing driving energy for propagating thecombustion 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.

Fig. 2 shows the propagation depth of the combustionfront versus the oxygen pressure. Under low oxygen pres-sures, the propagation depth of the combustion front in-creases asymptotically with the oxygen pressure. At lowpressure of the gas, the oxygen is not enough to sustain theoxidation of the iron and the pressure difference betweenthe ambient medium and the powder pores is small whichleads to the low penetration velocity of the oxygen into theinner, and the combustion ceases before reaches to the end.

Y. Li et al. / Materials Chemistry and Physics 82 (2003) 991–996 993

ZnFe2O4

ZnO

Fe2O3

Fe1-xO

5

4

3

2

1

20 25 30 35 40 45 50 55 60

2 (deg) q

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.

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. The crystallitesize calculated from X-ray line broadening using Scherrer’sequation is in the range 0.5–0.8�m at 1.0 MPa and in therange 2–3�m at 1.5 MPa, indicating the growth of crystal-lite size with the oxygen pressure increase.

In addition, two diffraction lines, existing at 2θ =42.2◦ and 61.1◦, respectively, are attributable to a non-

Fig. 4. Mössbauer spectra of the samples synthesized under different oxygen pressures.

stoichiometric composition, Fe1−xO (x < 1), deduced fromthe JCPDS card. However, these two diffraction lines dis-appear when the oxygen pressure is at 1.0 MPa. When theoxygen pressure is at 1.5 MPa, the corresponding diffrac-tion lines split apart into a doublet which belongs to anothernon-stoichiometric phase, Fe1−zO (z < 1), cited from theJCPDS card.

Mössbauer spectra of the samples measured at room tem-perature are shown inFig. 4. In (a), the doublet is related toZnFe2O4 and the sextets is related to�-Fe2O3. In (b), thespectrum is built from three components: a sextet and twodoublets. One of the doublet is also related to ZnFe2O4, theother is Fe1−xO or Fe1−zO. While the sextets is close to�-Fe2O3 according to Ref.[7]. Compared with the�-Fe2O3crystallized completely, the hyperfine fields decrease andthe value of the quadrupole splitting broadens which indi-cate that the�-Fe2O3 crystallized incompletely under thegiven condition and the asymmetry of the iron nucleus. Thepresence of�-Fe2O3 proves that the combustion products

994 Y. Li et al. / Materials Chemistry and Physics 82 (2003) 991–996

are formed under a high cooling rate due to the high com-bustion temperature under high oxygen pressure during thecombustion 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 process iscarried out under low oxygen pressure (below 1 MPa), theferritization degree of the combustion products will be lowdue to the lowTc andUc. When the oxygen pressure is high(above 1.5 MPa), the crystal lattice might not have enoughtime to search for the more stable positions due to the hightemperature and high combustion wave velocity. Therefore,both a low and a high oxygen pressure would lead to anon-equilibrium condition under which some intermediatephases of iron oxide with a non-stoichiometric compositionand crystal defects were formed. Similar results were ob-tained by Murin et al.[8] during the oxidation of iron.

Dependence of the lattice parametera0 of ZnFe2O4 onthe oxygen pressure is shown inFig. 5. The lattice parametera0 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 degree of crystallinity of thecombustion products obtained under 1.0 MPa is higher thanthat formed under 1.5 MPa, as shown inFig. 6. It clearlyshows spherical-shaped particles for lower oxygen pressure(1.0 MPa) and with a tendency towards an anomalous shapefor higher oxygen pressure(1.5 MPa). The particle grainsize increases substantially with the oxygen pressure dueto the faster kinetics of the crystal growth under higher

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 the lattice parameter of zinc ferrite with oxygen pressure.

oxygen pressure. When oxygen pressure is above 1.5 MPa,the phase that has melted can be observed in the samplesand the combustion product is very hard to be crushed intopowders due to self-sintering.

3.4. The relationship between oxygen pressure and degreeof conversion

During the combustion process, there exist three kineticpotential barriers, i.e., the penetration of oxygen from theenvironment into the combustion zone; the oxidation of iron;the infiltration of oxygen through the melting product layer.If the combustion reaction is carried out under the low oxy-gen pressure, the ferritization degree of the combustion prod-ucts will be low. This is caused by the shortage of oxygenand the lack of permeability. The degree of conversion toferrite for the powders depends on the oxygen pressure andthe porosity of the sample. Ideally, the amount of oxygengas occupying the total volume of pores is stoichiometricallyequal to that for the total conversion and is independent onthe permeation of gas. We use the Clapeyron–Mendeleevequation to calculate their relationship, i.e.

PO2(φV) = nO2RTc (2)

wherePO2 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 andTc 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 can also be calculated as

VFe = nFeMFe

ρ(4)

Y. Li et al. / Materials Chemistry and Physics 82 (2003) 991–996 995

Fig. 6. SEM photomicrograph of the samples obtained under different oxygen pressures.

whereMFe is the atomic weight of iron,nFe the number ofmoles of iron in the reactants.

Suppose the combustion reaction is determined by thequantity of oxygen that penetrated into the pores, we definethe parameterη as the degree of conversion to ferrite, and

η = nO2

snFe(5)

wheres is the ratio of the mass of oxygen to the mass ofiron. CombiningEqs. (2)–(5), η can be given as follows:

η = 2(2 + k)

3k

MFe

ρRTc

φ

1 − φPO2 (6)

the above equation described the dependence of the degreeof conversion on the oxygen pressure, porosityφ and com-bustion temperature. For a given porosity and combustiontemperature,Eq. (6)simplifies to

η = k1PO2 (7)

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.

wherek1 is a constant, and

k1 = 2(2 + k)

3k

MFe

ρRTc

φ

1 − φ(8)

It can be seen that the degree of conversion increases withthe increase in oxygen pressure. Oxygen pressure corre-sponding to varying degrees of conversion to zinc ferritewas calculated by means ofEq. (6) for three porosity val-ues and two temperatures. The results are shown inFig. 7.The porosity range was selected to reflect upper and lowerlimits which occur in practice. The upper limit (φ = 0.65)represents a value typical of uncompacted powders whilethe lower limit (φ = 0.35) is typical of highly compactedpowders.Fig. 7(a) shows that in order to obtain completeconversion (η = 1), oxygen pressure should be raised from0.28 MPa forφ = 0.65 to 0.98 MPa forφ = 0.35. For agiven oxygen pressure, larger and more numerous pores ofthe reactant favor higher degrees of conversion and lowerdependence on the permeation of oxygen gas through the

996 Y. Li et al. / Materials Chemistry and Physics 82 (2003) 991–996

compact. For any given porosity and degree of conversion,higher pressure is required at higher temperature.

For a complete conversion and a given porosity,Eq. (6)becomes

PO2 = 3k

2(2 + k)

ρR

MFe

1 − φ

φTc (9)

the above equation means that at a certain combustiontemperature, there is a critical oxygen pressure valuePc.When the practical oxygen pressure is in excess of thecritical value Pc, a complete conversion to ferrite can beobtained; while the practical oxygen pressure is lower thanthe critical pressure, the degree of conversion is dependenton the permeation of oxygen gas, and the reactant willnot be converted to ferrite completely due to the lack ofoxygen.

4. Conclusions

1. With the increase of oxygen pressure,Tc andUc increaseobviously. Under low oxygen pressures, the propagationdepth of the combustion front increases asymptoticallywith the oxygen pressure.

2. The results of XRD, Mössbauer spectra and SEM showthat single spinel-phase zinc ferrite with nearly spherical-shaped particles can be obtained when the oxygen pres-sure is 1.0 MPa and a non-stoichiometric oxides, Fe1−xOor Fe1−zO is formed under a very low or high oxygenpressure.

3. Analysis of the dependence of the degree of conversionto ferrite, η, on the oxygen pressure shows that for agiven porosity and combustion temperature, the degree ofconversion increases with the oxygen pressure increasingand for any given porosity and degree of conversion,higher pressure is required at higher temperature.

Acknowledgements

The first author is grateful to the Hei Longjiang ProvinceNatural Science Foundation of China (02E-08) and the Mul-tidiscipline Scientific Research Foundation of Harbin Insti-tute of Technology (grant No. MD 2002.03) that supportedthis research.

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