Journal of Magnetism and Magnetic Materials 346 (2013) 175–177

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Journal of Magnetism and Magnetic Materials

0304-88http://d

n CorrE-m

(J.-H. Hs

journal homepage: www.elsevier.com/locate/jmmm

Letter to the Editor

Structural and magnetic properties of γ-Fe2O3 nanostructuredcompacts processed by spark plasma sintering

P. Saravanan a,b,n, Jen-Hwa Hsu a,n, D. Sivaprahasam c, S.V. Kamat b

a Department of Physics, National Taiwan University, Taipei 106, Taiwanb Defence Metallurgical Research Laboratory, Hyderabad 500058, Indiac International Advanced Research Centre for Powder Metallurgy and New Materials, Chennai 600113, India

a r t i c l e i n f o

Article history:Received 4 June 2013Available online 18 July 2013

Keywords:Spark plasma sinteringIron oxideMagnetic nanoparticlesNanomaterials

53/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jmmm.2013.07.023

esponding authors. Tel.: +886 233 665 162; faail addresses: psdrdo@gmail.com (P. Saravanau).

a b s t r a c t

Gram quantities of γ-Fe2O3 nanopowders having mean particle size of 2074 nm were synthesized usinga hydrothermal method and then consolidated into dense nanostructured compacts by spark plasmasintering (SPS) at relatively low temperatures: 300–350 1C. The cubic spinel structure of the as-synthesized γ-Fe2O3 nanoparticles (NPs) did not get altered by the SPS process; nevertheless, a moderateincrease in their grain sizes was evident in the SPSed compacts (80–125 nm). The physical propertiessuch as density (ρ), coercivity (Hc) and magnetization (Ms) values of γ-Fe2O3 NPs were affected by the SPStemperature. Significantly, higher values of ρ (4.45 g/cm3), Hc (274 Oe) and Ms (67.2 emu/g) wereachieved for the bulk compact SPSed at 350 1C. This work highlights the merits of sintering γ-Fe2O3

NPs by SPS –as a new method of compaction with useful magnetic properties; which cannot be realizedwith the conventional sintering techniques.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Among various magnetic metal oxides, the cubic spinel structuredmaghemite (γ-Fe2O3) is technologically important and is widely usedfor the production of permanent magnetic materials. Besides, atnanoscale, it has tremendous potential in electronics, catalysis andbiomedical engineering [1,2]. Unfortunately, pure maghemite trans-forms into hematite (α-Fe2O3) at a rather low temperature: �380 1C.For this reason, the processing of γ-Fe2O3 magnetic nanocompositeshas often been carried out at very low temperatures using watersoluble polymers [3,4]. The potential of γ-Fe2O3 NPs for hightemperature applications can be harnessed by utilizing thermoplasticpolymers (polyphenylene sulfide and polyether imide) as dispersingmatrices. In this regard, newer compaction methods with a capabilityof sintering γ-Fe2O3 NPs below their phase transformation tempera-ture need to be explored. At this juncture, it should be mentionedthat while the synthesis of γ-Fe2O3 powders has been the subject ofmany investigations aimed at obtaining fine nanoparticles withcontrolled sizes and shapes; almost no studies have been devotedto their consolidation by sintering. The applicability of the SPStechnique in sintering various magnetic oxides, viz. CoFe2O4 [5],Ni–Zn–Fe2O4 [6] and Ba2Fe12O19 [7] has already been demonstratedat low sintering temperatures: 500–700 1C. In light of the above, we

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x: +886 233 665 892.n), jhhsu@phys.ntu.edu.tw

exploit the advantages of the SPS technique, such as lower sinteringtemperature and time, to consolidate γ-Fe2O3 NPs below their phasetransformation temperature.

2. Experimental

Fe3O4 NPs were initially synthesized using a method describedelsewhere [8], by alkalizing an aqueous mixture of iron (II) and iron(III) chlorides and subjecting them to a hydrothermal treatment at250 1C for 12 h. A black precipitate consisting of anionic Fe3O4 NPswere isolated by centrifugation and acidified with nitric acid. Theparticles were then oxidized to form γ-Fe2O3 using ferric nitrate;centrifuged and dispersed in water after several washings withacetone. The nanopowders were finally dried and subjected to SPSusing Dr. Sinter 1050. In the SPS technique, 5 g of γ-Fe2O3 was placedin a tungsten carbide die (10 mm diameter) and the sample was(i) first heated from room temperature (RT) to 200 1C at 100 1C/min,(ii) then heated from 200 1C to the sintering temperature (Ts) at 50 1C/min, (iii) held at Ts for 5 min and (iv) finally cooled to RT at 100 1C/min.The whole sintering process was performed under an applied uniaxialpressure of 120MPa and in a high vacuum atmosphere (�6 Pa). Thebulk compacts obtained after SPS were polished with SiC paper andcut for further processing and characterization. The Archimedesmethod was employed to estimate the bulk sample density. Structuraland morphological characterizations were carried out using scanningelectron microscope (SEM) with energy dispersive X-ray spectroscopy(EDS) analysis, transmission electron microscope (TEM) and X-ray

P. Saravanan et al. / Journal of Magnetism and Magnetic Materials 346 (2013) 175–177176

diffractometer (XRD); while the magnetic properties were evaluated atRT using a vibrating sample magnetometer (VSM) up to a maximumfield of 20 kOe.

3. Results and discussion

Fig. 1 shows the temperature profile and cross-head displace-ment during SPS for the sample sintered at 350 1C under apressure of 120 MPa. The cross-head displacement is a measureof density change that occurs during the SPS process and thus thedensification of γ-Fe2O3 powders during SPS can be divided intotwo stages (marked as I and II in Fig. 1). In the early stage i.e.during the continuous heating stage (Tso250 1C), there is anobvious shrinkage of the powder compact. On the other hand,when Ts exceeds 250 1C, the cross-head displacement continues toincrease gradually and tends to attain steady state at 350 1C. Thedependence of compacted density on Ts is given in Table 1. Thetrue density of bulk γ-Fe2O3 is 5.24 g/cm3 [9] and this has beenchosen as the reference (100%) for calculation of relative density inour study. The density of the SPSed material increased from 3.93 g/cm3 (75%) for the specimen sintered at 300 1C to 4.45 g/cm3 (85%)for the specimen sintered at 350 1C. It should be noted thatTs4700 1C was employed during SPS for consolidating ferriteNPs to attain bulk densities up to 98% [7]. In this study, sinceour intention was to sinter γ-Fe2O3 NPs at or below 350 1C; theobtained density values are significantly lower. Nevertheless, thesevalues are quite reasonable in comparison to that obtained byconventional sintering (CS), wherein Ts of 41000 1C with aduration of several hours was employed [10]. In the SPS method,a high-energy pulsed current momentarily generates extremelyhigh and localized temperatures between the powder particlesthrough a ‘Joule-heating mechanism’. The bonding of powderparticles is thus quite rapid and hence, higher densification canbe achieved even at significantly lower SPS temperatures.

The XRD pattern of as-synthesized γ-Fe2O3 NPs confirmed its phasepurity (Fig. 2a) and all the diffraction peaks were indexed to the cubicspinel structure of maghemite with space group (P4132), in line with

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Fig. 1. Temperature–cross-head displacement–sintering profile during SPS ofγ-Fe2O3 NPs.

Table 1Grain size (D), lattice constant (d), density (ρ), saturation magnetization (Ms),coercivity (Hc), measured on γ-Fe2O3 NPs and SPSed bulk compacts.

γ-Fe2O3 NPs D (nm) d (Å) ρ (g/cm3) Ms (emu/g) Hc (Oe)

As-synthesized 2074.0 8.349670.0007 – 46.9 62SPSed at 300 1C 8776.2 8.349970.0006 3.93 57.0 191SPSed at 325 1C 10279.5 8.350470.0005 4.21 65.4 243SPSed at 350 1C 110715.0 8.350870.0005 4.45 67.2 274

the results of Horner et al. [8]. The lattice constant (a) for γ-Fe2O3 NPcalculated from the powder diffraction data was 8.349670.0007 Å,which is in good agreement with that of bulk γ-Fe2O3 (a¼8.3505 Å)[11]. The XRD pattern showed a considerable line-broadening—indi-cating the nanocrystalline nature of γ-Fe2O3. A mean particle size of18 nm was estimated using the Scherer′s equation and this isconsistent with the TEM analysis (Fig. 2b) which shows that theparticles are nearly spherical and monodispersed with average size of2074 nm. The selected area electron diffraction (SAED) patternobtained for the γ-Fe2O3 NPs (Fig. 2c), shows that they are polycrystal-line in nature. The reflections of (2 2 0), (3 1 1), (4 4 0), (4 2 2) and(5 1 1) which correspond to the cubic spinel structure of γ-Fe2O3 areclearly seen in the SAED pattern. The XRD patterns of all the SPSedcompacts exhibited diffraction peaks quite similar to that of thestarting material. A typical XRD pattern obtained for the sample SPSedat 350 1C is shown in Fig. 2d, which can be compared with thatobtained for the as-synthesized NPs. The diffraction peaks of SPSedsample can be indexed to the cubic spinel structure of γ-Fe2O3 and thecalculated lattice constant values also showed a close proximity to theas-synthesized γ-Fe2O3 NPs (Table 1). This confirms that no significantphase transformation of γ-Fe2O3 has occurred during SPS. This is thenoteworthy aspect of SPS, as the phase transformation of γ-α-Fe2O3 isusually observed during calcination between 300 and 400 1C [10].Fig. 2e shows a representative cross-sectional microstructure of the γ-Fe2O3 bulk compact SPSed at 350 1C. The micrograph reveals sphericalgrains with mean grain size of about 110 nm which suggests thatsome, albeit limited, grain growth has occurred during SPS. The darkerareas are cavities caused by removal of particles during mechanicalpolishing as well as pores resulting from the sintering process [12].EDS elemental analysis revealed that the SPSed compact contains onlyFe and O, with no other impurities (Fig. 2f).

Fig. 3 shows the typical hysteresis loops obtained at RT for the as-synthesized γ-Fe2O3 NPs and their corresponding SPSed counterparts.Clearly, both coercivity enhancement and increase in magnetizationare evident in the hysteresis curves due to the SPS treatment. Themagnetic properties of γ-Fe2O3 bulk compacts obtained at different Tsare given in Table 1. The as-synthesized γ-Fe2O3 NPs showed asaturation magnetization (Ms) of 46.9 emu/g and coercivity (Hc) of61.5 Oe. Though, these values were significantly lower than that of thebulk γ-Fe2O3 (73–74 emu/g and 250–350 Oe) [13]; they tend toincrease with increase of Ts. Maximum values of Ms (67.2 emu/g)and Hc (274 Oe) were obtained for the sample SPSed at 350 1C and thisenhancement can be attributed to the combined effect of crystal-lization and inter-grain exchange coupling occurring at higher Ts.

At this stage, it is essential to highlight the advantages of SPS overthe CS process for the compaction of γ-Fe2O3 NPs. Both extensive graingrowth and occurrence of α-Fe2O3 were reported for the CSed γ-Fe2O3

[10]—mainly attributed to the slow heating and longer sinteringduration. Similarly, increased pressure during CS can promotepressure-induced phase transformation of γ-α-Fe2O3 rapidly andhence, compaction at elevated pressure is highly detrimental to thecubic-spinel structure of γ-Fe2O3 [14]. It is therefore not trivial toconsolidate γ-Fe2O3 NPs by conventional heat-treatment processes. Incontrast, SPS has high thermal efficiency and utilizes lower pressureand the whole thermal transmission process is completed in aninstant by uniform self-heating and surface activation of powderparticles due to the effect of spark plasma generated momentarily[15]. This method thus makes it possible to consolidate γ-Fe2O3 NPswith good phase homogeneity, minimal grain growth and highdensification.

4. Conclusions

Consolidation of γ-Fe2O3 NPs was successfully accomplished bySPS at relatively low temperatures: 300–350 1C. The effect of Ts on

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6.00 9.00 12.00 15.00 keV

30 40 50 60 70 80

Fig. 2. XRD, TEM micrograph and SAED pattern of as-synthesized γ-Fe2O3 NPs: (a), (b) and (c). XRD, SEM micrograph and EDS of SPSed sample at 350 1C: (d), (e) and (f),respectively.

-9000 -6000 -3000 0 3000 6000 9000-75

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Fig. 3. RT hysteresis loops of γ-Fe2O3 NPs and their SPSed bulk compacts. Insetshows a close view of coercivity changes with respect to SPS temperature.

P. Saravanan et al. / Journal of Magnetism and Magnetic Materials 346 (2013) 175–177 177

the structural and magnetic properties of γ-Fe2O3 bulk compactswas investigated and compared with the as-synthesized powder.The cubic spinel structure of as-synthesized γ-Fe2O3 NPs wasretained even after the SPS process. The average grain size ofγ-Fe2O3 NPs (2074 nm) increased after SPS; however, their sizeswere still in the nanometer range (110 nm). The density andmagnetic parameters such as Ms and Hc of SPSed compact werefound to increase with increase in Ts. This study has demonstratedthe potential of SPS as a new compaction method for γ-Fe2O3 NPs.

Acknowledgments

The financial support for the present study was extended byDefence Research and Development Organization (DRDO), Govern-ment of India. The keen interest shown by the Director, DMRL inthis work is gratefully acknowledged. One of the authors PS thanks

National Science Council of Taiwan for providing postdoctoralfellowship under Contract no. NSC-99–2112-M-002–020-MY3.

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