Nanosized zinc ferrite spinel powders of various compositions were produced in

an RF thermal plasma from oxide mixtures and from co-precipitated hydroxides. The

products were characterised for bulk chemical (ICP-AES), and phase composition

(XRD). In spite of the short residence time of the reagents in the plasma reactor

considerable extent of spinel formation was achieved. Magnetization and Curie point

measurements indicated that the spinel phases exhibited ferrimagnetic behaviour.

Ferrites synthetized in the RF plasma reactor were compared to that produced by the

conventional ceramic route

Gubicza, Jenő

Mohai, Ilona

Szépvölgyi, János

English

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SYNTHESIS OF ZINC FERRITE NANOPOWDERS IN AN RF THERMAL PLASMAI. Mohai1, J. Szépvölgyi1, J. Gubicza21Research Laboratory of Materials and Environmental Chemistry, Chemical Research Center,Hungarian Academy of Sciences, P.O.B. 17, Budapest, H-1525, Hungary2Department of General Physics, Eötvös University, P.O.B. 32, Budapest, H-1518, HungaryAbstractNanosized zinc ferrite spinel powders of various compositions were produced inan RF thermal plasma from oxide mixtures and from co-precipitated hydroxides. Theproducts were characterised for bulk chemical (ICP-AES), and phase composition(XRD). In spite of the short residence time of the reagents in the plasma reactorconsiderable extent of spinel formation was achieved. Magnetization and Curie pointmeasurements indicated that the spinel phases exhibited ferrimagnetic behaviour.Ferrites synthetized in the RF plasma reactor were compared to that produced by theconventional ceramic route.1. IntroductionThe spinel ferrites contain a close-packed cubic structure of oxygen ions in whichtetrahedral (A) an octahedral (B) interstitial sites are occupied by cations [1]. Zinc ferritebelongs to the class of normal spinels which presumably have a cation distribution of(Zn2+)A((Fe3+)2)B(O2-)4. It is paramagnetic at room temperature. However, in special synthesisconditions (e.g. co-precipitation [2], rapid quenching [3] and mechanochemical activation [4])some Fe3+ ions may occupy tetrahedral, as well as octahedral sites forming the inverse spinelstructure. The strong superexchange interaction among these sites results in an unusually highmagnetisation as compared to normal spinels [3]. A direct correlation was found between theparticle size and the saturation magnetisation: the finer the particles were, the more saturationmagnetisation could be measured [5,6]. The relatively high magnetisation of pure Fe3O4 canbe further increased by Zn substitution and the highest saturation magnetisation ever found inspinel ferrites can be measured for zinc ferrous ferrites (ZnxFe3-xO4 where x≤0.5) [7].Radiofrequency thermal plasmas offer unique advantages for the synthesis of specialceramic powders due to the high temperatures and energy densities in these systems. Inaddition, a high temperature gradient exists between the hot plasma flame and the surroundinggas phase. The resulting rapid quenching rate is favourable for producing fine particles withunstable structures in thermodynamic terms. In this paper results on the thermal plasmasynthesis of zinc ferrites and zinc ferrous ferrites are presented.2. ExperimentalPrecursors for the synthesis of ZnXFe3-XO4 (0<x≤1) were prepared by two methods:x Fe2O3 and ZnO powders of analytical grade were thoroughly mixed in an agatemortar in a molar ratio of 2:1.x Fe(NO3)3·6H2O and ZnO, with Fe:Zn = 2:1 and 6:1 molar ratio, were dissolved indeionised water. The pH of the solution was set to 8 by adding NH4OH solution.The precipitated hydroxides were washed with deionized water, filtered and dried at80ºC.The precursors were treated in a laboratory scale RF thermal plasma reactor (Linn,27 MHz, 1-7 kW) connected to an air cooled, two-stage powder collector. Argon was used asthe central plasma gas (7 l·min-1) and as the sheath gas (19 l·min-1), as well. The powder wasinjected continuously into the plasma tail flame region by carrier gas (argon or air) passedthrough a fluidised and vibrated powder-bed. Products deposited on the reactor wall(Samples R) and in the powder collector (Samples C) were collected and analysed separately.To make a comparison between ferrites prepared in the plasma reactor with those obtained bythe conventional ceramic processing the oxide mixture and the co-precipitated hydroxideswere calcined in air at 900ºC for 6h, as well.The bulk chemical composition of dissolved samples was analysed by ICP-AES(Labtest PSX75). Crystalline phases were identified by X-ray diffraction (XRD) using aPhilips Xpert diffractometer and CuKD radiation (O=0.15418 nm). The lattice parameters (a)of the crystalline phases were determined from the positions of diffraction peaks. The meangrain-size was calculated from the full widths at the half maximum (FWHM) of the mostintensive diffraction profiles according to the Scherrer’s equation. Magnetisation measure-ments were performed using a SQUID-magnetometer. The Curie point of the samples wasdetermined by measuring mass changes against the temperature (Perkin-Elmer TGS-2thermobalance) applying a constant magnetic field.3. Results and discussionConditions of sample preparation and results of XRD analysis are collected in Table 1.The X-ray diffractograms are shown in Fig. 1 (for Samples 1 and 2C), in Fig. 2 (for Samples3, 4R and 4C) and in Fig. 3 (for Samples 5, 6R, 6C, 7R, 7C). The XRD pattern of the heat-treated oxide mixture (900ºC, 6h) shows the formation of the spinel phase (Fig 1. Sample 1.).Reflections of Fe2O3 and ZnO almost completely disappeared. The calculated latticeparameter (a) is equal to that of ZnFe2O4 within the limits of the experimental error (Table 1,Sample 1). Contrary to the “conventional” synthesis, the plasma treatment of the oxidemixture resulted in the formation of magnetite (Fe3O4) with the incorporation of a smallamount of Zn only (Fig. 1. Sample 2C). The spinel composition was estimated by assumingthe increase of lattice parameter due to Zn incorporation to be proportional with the Znconcentration. The calculated composition indicated the presence of about 0.1 mol of Zn inthe spinel structure (Zn0.1Fe2.9O4). The Particular ferrite probably formed through thecondensation of oxide vapours. The small mean size of spinel grains (D = 34 nm) supportsthis assumption. During the plasma treatment of the oxide precursor the spinel formation wasnot complete. In addition to the unreacted ZnO a high amount of Fe2O3 was found in theproduct, as well. We suppose that in spite of the high specific energy (4 kWh/g oxide) thelarger, aggregated grains could not decompose and evaporate because of their short residencetime in the hot plasma region.Table 1 Methods of sample preparation and results of XRD measurements. Plasma treatedsamples were collected from the reactor (R) and the from the cyclone (C). Composition of thespinel phase was estimated from the lattice parameter (a). The mean grain size (D) wascalculated from the FWHM of XRD profiles.Sample Precursor Fe:Zna Method Spinel phase Other phasesbNo. a (Å) Composition D (nm) H/S Z/S1 Oxide 2:1 900°C, 6h 8.439 ZnFe2O4 100 0 02C Oxide 2:1 Plasma (Ar) 8.400 Zn0.1Fe2.9O4 34 0.49 0.553 Hydroxide 2:1 900°C, 6h 8.441 ZnFe2O4 119 0.05 04R Hydroxide 2:1 Plasma (Ar+air) 8.438 ZnFe2O4 49 0.05 0.064C Hydroxide 2:1 Plasma (Ar+air) 8.4388.392ZnFe2O4Fe3O450570.05 0.085 Hydroxide 6:1 900°C, 6h 8.439 ZnFe2O4 65 0.89 06R Hydroxide 6:1 Plasma (Ar) 8.414 Zn0.4Fe2.6O4 48 0.06 0.096C Hydroxide 6:1 Plasma (Ar) 8.415 Zn0.4Fe2.6O4 42 0 0.17R Hydroxide 6:1 Plasma (Ar+air) 8.424 Zn0.6Fe2.4O4 25 0.7 07C Hydroxide 6:1 Plasma (Ar+air) 8.4388.393ZnFe2O4Fe3O447620.17 0a Molar ratiob H: Fe2O3 (hematite), Z: ZnO (zincite). The most intense reflection are related to the mostintense spinel (S) reflection.25 30 35 40 452 4 (°)Intensity (a.u.) HHH ZZZSS SSSSSSSample 1Sample 2CFig. 1. X-ray diffractograms of samples preparedfrom Fe2O3+ZnO mixture by the conventionalceramic route (Sample 1) and by plasma treatment(Sample 2C). S: spinel, H: hematite (Fe2O3), Z:zincite (ZnO)Extent of spinel formation fromthe Fe:Zn=2:1 hydroxide precursor waspractically as high in the plasma treatedsamples as in the sample calcined at900°C for 6h (Fig. 2.), although thespecific energies applied in the case ofhydroxide precursors were in the rangeof 0,3-0,5 kWh/g hydroxide (which isabout one tenth as compared to thespecific energy applied in the case ofoxide precursor). Sample collectedfrom the reactor wall (Sample 4R)differed from the sample deposited inthe powder collector (Sample 4C) bothin chemical and phase compositions.Iron and zinc content of Sample 4R(Fe=46.4, Zn=26.7 w/w%) was actuallyequal to the theoretical composition ofZnFe2O4.However, in Sample 4C the iron contentincreased and the zinc content decreased(Fe=50.5% and Zn=22.9%) as comparedto Sample 4R. Phase composition ofrelevant samples supported the results ofchemical analysis: in Sample 4C thediffraction peaks of the spinel phasecould be deconvoluted to two compo-nents; ZnFe2O4 (a=8.438 Å) and Fe3O4(a:8.392 Å), respectively. Suchsignificant differences of chemical andphase compositions in the sameexperiment indicate different formationmechanism. We suppose that in case ofsamples R the solid phase reactionbetween the oxides is the dominatingprocess. This reaction can be relativelyfast in the case of hydroxide precursor).Sample C collected from the powdercollector was probably formed bycondensation of vapours.Treatment of the hydroxide-precursor (Fe:Zn = 6:1) in air at 900°Cfor 6h resulted in the formation ofZnFe2O4 phase (Table 1, Sample 5,Fig.3.).  Formation  of  Fe2O3 could  be25 30 35 40 4524(°)Intensity (a.u.)HHHSSSSSSSSSSSSZSample 3Sample 4RSample 4CFig. 2. X-ray diffractograms of samples preparedfrom hydroxide precursor (Fe:Zn=2:1) by theconventional ceramic route (Sample 3) and byplasma treatment (Samples 4R and 4C). S: spinel,H: hematite (Fe2O3), Z: zincite (ZnO)25 30 35 40 452 4 (°)Intensity (a.u.) Sample 5HHHHSSSSSSSSSSSSZ WSSSH HHSample 6CSample 7CSample 7RFig. 3. X-ray diffractograms of samples preparedfrom hydroxide precursor (Fe:Zn=6:1) by theconventional ceramic route (Sample5) and byplasma treatment (Samples 6C, 7R and 7C). S:spinel, H: hematite (Fe2O3), Z: zincite (ZnO), W:wuestite (FeO).attributed to the excess of iron. Thecomposition of zinc ferrites couldhardly be influenced using theconventional ceramic technique, unlessthe oxygen partial pressure ismaintained in a proper range. However,the thermal plasma treatment of co-precipitated hydroxides in argonatmosphere resulted in a spinel phasewith an estimated composition ofZn0.4Fe2.6O4 (Samples 6R and 6C). Partof the Fe2O3 was reduced in theseconditions and FeO (wuestite) wasformed. The product contained someZnO, as well. Spinel compositioncalculated from the cell parameter isvery close to that calculated from theFe:Zn ratio of the precursor(Zn0.3Fe2.7O4) supposing a completechemical transformation.The role of oxygen in the spinel formation could be observed in Samples 7R and C.(Fig. 3.). In Sample 7R spinel phase was identified as Zn0.6Fe2.4O4 , which has a smaller Fe:Znmolar ratio (4:1) than the precorsor powder had (6:1). In sample 7C, like in the case of 4C, thediffraction peaks of the spinel phase could be deconvoluted to two components (ZnFe2O4,a=8.438 Å and Fe3O4, a:8.393 Å, respectively). Phases formed in the presence of air (Samples4C and 7C) consisted of two separate spinel phases instead of forming a solid solutionbetween ZnFe2O4 and Fe3O4 which is practically the case for zinc ferrous ferrites. Spinelformation from the Fe:Zn = 6:1 hydroxide precursor in the presence of air was not complete:rather high amount of Fe2O3 remained unreacted, especially in Sample 7R.Te400 50098.899.099.299.4Mass(%)Fig. 4. Curie point The thermal analysis in a constantmagnetic field is a very simple method for themagnetic characterization of materials. Welldefinied Curie points could be measured inplasma synthetized ferrites (Fig. 4.). Tc1 at590°C is present both in Sample 4R and C andcan be attributed to the magnetite (Fe3O4)phase. The magnetite content of Sample 4R isprobably very low, so it can not be identifiedby XRD analysis (Table 1.). However, inThis measurement referconditions. In Sample 4Cthe sample.Magnetization Sample 2C indicated tization of 61 emu/g at 5300K (Fig. 5.). Semiquathe XRD measuremeconsideration chemicalwell) revealed that mZn0.1Fe2.9O4 spinel phasFe2O3 and ZnO phases50%. This way the emagnetization of the sabove 100 emu/g. It is msaturation magnetizationemu/g at 293K). Thisagreement with the databy Brabers [7].Sample 4RSample 4CTc1Tc2mperature (°C)600 700 800measurementsSample 4R a different Curie point can beobserved at about 530°C (Tc2). Thistemperature in not a characteristic one forferrimagnetic substances. We may suppose,that particular Curie temperature indicates thepresence  of  ferrimagnetic  zinc  ferrite  spinel.s to the formation of inverse zinc ferrite spinels in thermal plasma Tc2 can not be measured because of the lower zinc ferrite content ofmeasurements ofsaturation magne-K and 50 emu/g atntitative analysis ofnts (taking into analysis data asass ratio of thee to the unreacted was far less thanstimated saturationpinel phase can beuch higher than the of magnetite (92 result is in good published recently0 10000 20000 30000 40000 50000010203040506070300 K5 KM (emu/g)H (Oe)Fig. 5. Magnetization curves of Sample 2C4. ConclusionNanosized spinel zinc ferrite phases of various compositions can be synthesised in anRF thermal plasma reactor in spite of the very short residence time of reagents in the hotplasma region. The iron and zinc containing precursors probably first atomise then vaporisenear to the plasma flame. Spinels are forming during the condensation. This mechanism issupported by the fine (20-50 nm) particle size of powders produced. Conversion of theprecursors in the plasma reactor was not complete, especially in the case of oxide precursor.Nearly the total amount of the hydroxide transformed into spinel phase. It suggests theimportance of the grain size and the intimate mixing of precursors. Partial pressure of theoxygen can easily be adjusted in the plasma reactor. Thus, zinc ferrous ferrites can also besynthesised in these systems. Curie points of the plasma treated samples can be attributed tomagnetite (Tc=590°C) and zinc ferrite (Tc=530°C), respectively. From the results ofmagnetization measurements a saturation magnetization of the Zn0.1Fe2.9O4 spinel phase ofmore than 100 emu/g could be estimated.AcknowledgementThe authors thank Emma Pekker for TG and László F. Kiss for magnetic measurements.This work was supported by the National Scientific Research Fund (OTKA Nos.T032272 andD29339).References[1] B.D. Cullity, Introduction to Magnetic Materials, Addison-Werley, Reading, MA,1972.[2] M.R. Anantharaman, S. Jagatheesan, K.A. Malini, S. Sindhu, A. Narayanasamy, C.N.Chinnasamy, J.P. Jacobs, S. Reine, K. Seshan, R.H.H. Smits, H.H. Brongersma, J.Magn. Magn. Mat. 189 (1998) 83.[3] K. Tanaka, M. Makita, Y. Shmizugawa, K. Hirao, N. Soga, J. Phys. Chem. Solids 59(1998) 1611.[4] V. Šepelák, U. Steinike, D. Chr. Uecker, S. Wissmann, K. D. Becker, J. Solid StateChem. 135 (1998) 52.[5] T. Sato, K. Haneda, N. Seki, T. Iijima, Appl. Phys. A 50 (1990) 13.[6] T. Kamiyama, K. Haneda, T. Sato, S. Ikeda, H. Asano, Solid State Commun. 81(1992) 563.[7] V.A.M. Brabers, J. Phys. IV. France 7 (1997) C1-223.

Synthesis of zinc ferrite nanopowders in an RF thermal plasma

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