This article was downloaded by: [Rutgers University]On: 07 July 2014, At: 20:39Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

Synthesis and Reactivity in Inorganic, Metal-Organic,and Nano-Metal ChemistryPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lsrt20

Synthesis of Single-Phase Barium HexaferriteNanopowder via a Novel EDTA Precursor-Based Routeand its DC Resistivity and Magnetic PropertyAmit B. Rajput a , Subhenjit Hazra a , Gerard F. Fernando b & Narendra N. Ghosh aa Department of Chemistry , BITS-Pilani, K. K. Birla Goa Campus , Zuarinagar, Goa, Indiab School of Metallurgy and Materials , University of Birmingham , Birmingham, UnitedKingdomPublished online: 07 Nov 2011.

To cite this article: Amit B. Rajput , Subhenjit Hazra , Gerard F. Fernando & Narendra N. Ghosh (2011) Synthesisof Single-Phase Barium Hexaferrite Nanopowder via a Novel EDTA Precursor-Based Route and its DC Resistivity andMagnetic Property, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:9, 1114-1121, DOI:10.1080/15533174.2011.591355

To link to this article: http://dx.doi.org/10.1080/15533174.2011.591355

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. Taylor and Francis shallnot be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:1114–1121, 2011Copyright C© Taylor & Francis Group, LLCISSN: 1553-3174 print / 1553-3182 onlineDOI: 10.1080/15533174.2011.591355

Synthesis of Single-Phase Barium Hexaferrite Nanopowdervia a Novel EDTA Precursor-Based Route and its DCResistivity and Magnetic Property

Amit B. Rajput,1 Subhenjit Hazra,1 Gerard F. Fernando,2 and Narendra N. Ghosh1

1Department of Chemistry, BITS-Pilani, K. K. Birla Goa Campus, Zuarinagar, Goa, India2School of Metallurgy and Materials, University of Birmingham, Birmingham, United Kingdom

An ethylenediamine tetraacetic acid (EDTA) precursor-basedchemical method has been reported for preparation of single-phase barium hexaferrite nanopowder. Synthesized powders werecharacterized by thermogravimetric analysis, differential scan-ning calorimetric analysis, x-ray diffraction, particle size analy-sis, transmission electron microscopy, and scanning electron mi-croscopy. DC electrical resistivity and magnetic properties of syn-thesized BaFe12O19 were measured by using a two-probe methodand a vibrating sample magnetometer, respectively. This EDTAprecursor-based method has the capability of producing nanos-tructured pure single-phase BaFe12O19 powder at a comparativelylower calcination temperature and offers the potential of a simpler,more cost-effective route than other reported methods.

Keywords chemical synthesis, magnetic materials, magnetic proper-ties, x-ray diffraction

INTRODUCTIONBarium hexaferrite (BaFe12O19) is one of the important

members of the family of hard ferrites because of its use invarious applications, including the fabrication of permanentmagnets, high-density magnetic and magneto-optic recordingmedia, magnetic fluids, and microwave devices.[1–5] Due to itslarge magnetocrystalline anisotropy, high Curie temperature,relatively large magnetization, excellent chemical stability, and

Received 21 March 2011; accepted 22 April 2011.N. N. Ghosh gratefully acknowledges financial support from

DRDO, New Delhi India (ERIP/ER/0904500/M/01/1204). N. N. Ghoshand G. F. Fernendo acknowledge the financial support provided by theRoyal Academy of Engineering, UK, Research Exchanges with Chinaand India award. The authors also express thanks to Dr. Rahul Mohan,NCAOR, Goa, India, for recording the SEM micrographs of the sam-ples and to Mr. M. K. Patra and Dr. S. R. Vadera, Defence Lab Jodhpur,for recording VSM measurements.

Address correspondence to Narendra N. Ghosh, Department ofChemistry, BITS-Pilani, K. K. Birla Goa Campus, Zuarinagar, Goa403726, India. E-mail: naren70@yahoo.com

corrosion resistivity, research associated with barium hexaferritecontinues to attracted significant attention.[6–8]

Traditionally, barium hexaferrite is prepared by a ceramicmethod using BaCO3 and iron oxide as starting materialsand sintering temperatures above 1200C for up to 12 h.[9–11]

However, to achieve single-phase barium hexaferrite, high-temperature (above 1200C) sintering with prolonged heat-ing time (up to 12 h) and occasional ball milling are re-quired. Due to the high sintering temperature, the productionof nanoparticles is not possible by this method. Furthermore,the intermediate milling process can yield a non-homogeneousmixture on a microscopic scale and induces lattice strains inthe materials.[12] The introduction of impurities due to the ball-milling process is also a possibility. In recent years, a rangeof wet chemical methods has been developed to overcome thealready-mentioned issues associated with the high-temperaturepreparation of barium hexaferrite nanopowders. These includesynthesis routes based on sol–gel,[13–17] co-precipitation,[18–21]

precursors,[21,22] hydrothermal,[23] reverse micelle,[24] autocom-bustion,[16,17] microemulsion,[25] under applied DC electricfield,[26] and mechano-combustion.[27] A citrate-based sol–gelmethod has been employed extensively to prepare barium hex-aferrite.[13–17] In this method, the precursor was prepared byreacting a mixture of Ba2+ and Fe3+ with citric acid. The driedprecursor powder thus formed was then heat treated at 450Cfor 24 h, followed by calcination at 800C for 6 h.[16] Jun-liang et al. have modified the citrate-based method by addingethylene glycol, ethylenediamine tetraacetic acid, and ammo-nium nitrate in a mixture of barium nitrate, ferric nitrate, andcitric acid. Ammonia solution was added to the reaction mix-ture to adjust the pH to 6.5. A freeze-drying procedure wasused to remove moisture, and the resulting yellowish dry pow-der was irradiated in a microwave oven, which was operatedat 450 W. Barium hexaferrite powder thus formed was com-posed of many tiny crystallites, with particle size ranging from50 to 100 nm.[14] Moghaddam et al. have prepared barium hex-aferrite using a co-precipitation technique where an aqueoussolution of iron and barium chloride was co-precipitated bythe addition of NaOH. The dried precipitate was milled in a

1114

Dow

nloa

ded

by [

Rut

gers

Uni

vers

ity]

at 2

0:39

07

July

201

4

EDTA-PRECURSOR ROUTE FOR BaFe12O19 NANOPOWDER 1115

high-energy planetary mill for 5–40 h and calcined at 1100C.It has been reported that chloride ion can persist until 800Cand this can delay the formation of barium hexaferrite phase.However, BaFe2O4 and BaFeO3-x were also detected as impu-rities, along with the final barium hexaferrite phase.[20] In thehydrothermal method, maghemite and barium hydroxide weremixed in water and then NaOH was added to this suspension.The mixture was placed in an autoclave and heated at 180 to280C for 1–5 h.[23] Xu et al. have used a reverse microemul-sion technique to prepare BaFe12O19. Here, cetyl trimethyl am-monium chloride, n-hexanol, and cyclohexanol were used toprepare the reverse micelle, and that was mixed with aqueoussolution of barium nitrate and ferric nitrate. (NH4)2CO3 wasadded to the reaction mixture to initiate precipitation. After ag-ing the reaction mixture for 12 h, the precursor formed waswashed with dry ethanol and water to remove the surfactantsand organic residues. The precursor was then calcined at 950Cfor 6 h.[24]

The wet-chemical methods, which have been reported toprepare BaFe12O19 nanoparticles, also are associated with somedisadvantages. For example, in most of the cases strong reagents(such as NaOH), organic solvents, or organic reagents are used.This is particularly true for sol–gel, co-precipitation, hydrother-mal, and microemulsion methods. Therefore, these synthesisroutes are relatively complex and expensive. Moreover, cal-cination or sintering above 900C for several hours is gener-ally required to form barium hexaferrite. In the case of thehydrothermal-based method, the requirement for an autoclavelimits scaled-up production. However, the most critical prob-lem associated with most of these methods is the formation ofsome undesired impurities (e.g., BaFe2O4, Fe2O3, etc.) alongwith the final barium hexaferrite phase.[9–11,13–16,21–23,26,27] Theimportance of synthesizing a single-phase material should notbe underestimated, as the electrical and magnetic properties offerrites are highly sensitive to the presence of impurities. There-fore, there is a need to develop a simple and cost-effective chem-ical method for preparation of single-phase barium hexaferritenanopowder and study its electrical and magnetic property.

In this article, we report the synthesis of single-phase bar-ium hexaferrite nanopowder using an aqueous solution-basedethylenediamine tetraacetic acid (EDTA) precursor method.The synthesized BaFe12O19 was characterized using room-temperature wide-angle powder x-ray diffraction (XRD), ther-mogravimetric analysis (TGA) and differential scanning calori-metric (DSC) analysis, particle size analysis, high-resolutiontransmission electron microscopy (HRTEM), and scanning elec-tron microscopy (SEM). We have systematically investigatedthe variation in DC electrical resistivity with the change of mea-surement temperature and the microstructure of the synthesizedbarium hexaferrite at different sintering condition. To the bestof our knowledge, a comprehensive investigation on the DC re-sistivity behavior of the nanosized barium hexaferrite particlescorrelating with the change of microstructures, evolved duringdifferent sintering temperature, has not yet reported. A vibrating

sample magnetometer (VSM) was used to measure the room-temperature magnetic properties of the synthesized barium hex-aferrite nanopowders. The Curie temperature was measured byusing the “gravity method” described by Soohoo.[28]

EXPERIMENTAL

Chemical Synthesis of Barium HexaferriteBaCO3 (99.9%), Fe(NO3)3·9H2O (99.9%), EDTA (99.9%),

NH4NO3 (99.9%), and nitric acid were purchased from Merck,India, and used without further purification. Ba(NO3)2 was pre-pared by dissolving BaCO3 in aqueous nitric acid. The stepsinvolved in the synthesis of barium hexaferrite are illustrated inScheme 1.

With reference to Scheme 1, stoichiometric amounts of bar-ium nitrate and ferric nitrate were dissolved in distilled wateraccording to a molar ratio of 1:12. An aqueous solution of EDTAwas prepared by dissolving EDTA in hot water with dropwiseaddition of NH4OH. After complete dissolution of EDTA, thesolution was boiled to remove the excess NH3. The pH of thesolution was found to be ∼6. To prepare precursors from var-ious total metal ions:EDTA ratios, aqueous solutions of metalnitrates and EDTA were mixed in various molar ratios (rang-ing from 1:1 to 1:5). The mixtures were then stirred for 1 hat room temperature using a magnetic stirrer. Black precursorswere formed when the mixtures were evaporated to dryness on

Ba(NO3)2 Fe(NO3)3

Aqueous solution of Ba2+ and Fe3+

Reaction Mixture

Precursor Powder

Calcination

BaFe12O19

+ H2O

+ Aqueous solution of EDTA

Drying at~1100C

+

SCH. 1. Preparation route for barium hexaferrite nanoparticles by the EDTAprecursor method.

Dow

nloa

ded

by [

Rut

gers

Uni

vers

ity]

at 2

0:39

07

July

201

4

1116 A. B. RAJPUT ET AL.

a hot-plate at ∼110C. Three to four drops of 10% NH4NO3

aqueous solution was added to the dried precursor powder andthen the precursor powders were calcined in air for 4 h at differ-ent temperatures ranging from 450 to 850C to obtain bariumhexaferrite nanopowder. NH4NO3 was added to facilitate theoxidation of carbonaceous mass of the precursor.

Sample CharacterizationRoom temperature x-ray diffraction spectra of the precursors

and the calcined powders was recorded by using a powder x-raydiffractometer (Mini Flex II, Rigaku, Japan) with Cu Kα (λ =0.15405 nm) radiation. Thermogravimetric analysis (TGA) anddifferential scanning calorimetric (DSC) analysis were carriedout on the precursors using a DTG-60 and a DSC-60 (Shimadzu,Japan), respectively. Thermal analyses experiments were per-formed in air at a heating rate of 10C min−1 from 40 to 900C.The particle size distribution for the calcined powders was de-termined by dynamic light scattering technique (DLS) using aparticle size analyzer (Delsa Nano S, Beckman Coulter, USA),and also using HRTEM (JEOL JEMS FEG-TEM-2100, Japan).For DLS study, sample was prepared by dispersing ∼1 mg ofpowder in an aqueous dispersing medium containing 10 mLof double distilled water and 2–3 drops of aqueous solutionof Tween 20 (10% v/v). The dispersion was sonicated for 10min. Three milliliters of the dispersion was placed in a quartzcuvette and DLS measurement was performed at 30C. Pelletswere prepared using BaFe12O19 nanopowders, obtained by cal-cining the precursor at 850C. These pellets were sintered at1100, 1200, and 1350C for 6 h in an air atmosphere and theresultant morphology was studied using SEM (JSM-6360LV,JEOL, Japan). Variation of DC resistivity with changing mea-surement temperature of the as-synthesized nanopowders andthe sintered pellets was measured by using a two-probe methodemploying a Keithley electrometer (6517A, USA). For this pur-pose, both surfaces of the pellets were manually coated withsilver paste. Room-temperature magnetization with respect tothe external magnetic field was measured for the synthesizedpowder (calcined at 850C) by using a vibrating sample magne-tometer (EV5, ADE Technology, USA). Curie temperature wasmeasured by using a gravity method.[28]

RESULTS AND DISCUSSION

Thermal AnalysisThe thermal decomposition behavior of the precursor was

studied using TGA. The TGA thermogram (Figure 1) revealedthat a total weight loss of ∼88% occurred in two steps when theprecursor powder was heated from 40 to 900C in air. Initially∼8% weight loss occurred between 40 and 180C due to the lossof moisture from the sample; ∼80% weight loss was observed inthe range of 200 to 450C. This may have been due to the thermo-oxidative decomposition of precursor and evolution of CO2 andNOX gases. This decomposition was also reflected in the DSCthermogram, as shown in Figure 1, where an exothermic peak

FIG. 1. TGA-DSC thermograms of barium hexaferrite precursor.

was observed at 435C. Heating the sample beyond 450C didnot result in any further weight loss in TGA. Thus it can beassumed that decomposition of carbonaceous content of theprecursor occurred in between 300 and 450C.

X-Ray DiffractionIn order to determine the optimum condition (total metal ion:

EDTA ratio and calcination temperature) to prepare single-phase

FIG. 2. XRD spectra of the barium hexaferrite powders obtained by usingprecursors with metal ion:EDTA molar ratio of 1:5 and calcined at specifiedtemperatures. Impurity peak of hematite marked with asterisk [JCPDS 80-2377].The absence of any impurity phase is clearly observed at 850C.

Dow

nloa

ded

by [

Rut

gers

Uni

vers

ity]

at 2

0:39

07

July

201

4

EDTA-PRECURSOR ROUTE FOR BaFe12O19 NANOPOWDER 1117

TABLE 1Crystallite size of BaFe12O19 at different calcination

temperatures

Calcination temperature (C) Crystallite size (nm)

650 32.9750 33.0850 34.9

nanosized barium hexaferrite, room temperature powder XRDanalysis of the calcined powders was performed. It was ob-served that, precursor powders were amorphous in nature andBaFe12O19 phase started to form when calcination tempera-ture was 650C (Figure 2). For all the calcined powders thediffraction peaks corresponding to the (110), (008), (107), (114),(203), (205), (206), (217), (2011), (220), and (2014) diffrac-tion planes of barium hexaferrite were present [JCPDS 84-0757] in the x-ray diffractogram. The increase in peak inten-sities with increasing calcination temperature occurred due tothe increase in crystallinity and crystallite size during the cal-cination process. The crystallite size of the calcined powderswas calculated using x-ray peak broadening of the diffractionpeaks (107) using Scherrer’s formula[29] and are summarized inTable 1.

It is important to note that peaks corresponding to α-Fe2O3

(hematite, as an impurity phase) were detected for the calcinedpowders obtained from the precursors prepared with total metalions and EDTA molar ratio ranging from 1:1 to 1:4 (Figure 3)

FIG. 3. Room-temperature XRD spectra of the calcined powders preparedfrom different precursors (total metal ion:EDTA ratio ranging from 1:1 to 1:5)by calcining at 850C. Impurity peak of hematite marked with asterisk [JCPDS80-2377]. The absence of any impurity phase is clearly observed for the 1:5ratio.

and when the calcination temperatures were lower than 850C(Figure 2). When the total metal ion:EDTA ratio was 1:5, for-mation of the pure single-phase barium hexaferrite occurred ata calcination temperature of 850C (Figure 3).

The XRD diffraction pattern of the calcined powder indi-cated the presence of a hexagonal crystal structure of the syn-thesized BaFe12O19 with space group P63/mmc (194). The val-ues of lattice parameters, a = b and c, and lattice volume (V)were found to be 5.89 Å, 23.203 Å, and 697.12 Å3, respec-tively. These values agreed well with the reported values inliterature.[19]

Particle Size and TEM AnalysisThe intensity-weighted particle size distribution of the as-

synthesized BaFe12O19 nanopowder (obtained by calcining theprecursor at 850C), obtained from DLS study at 30C (Fig-ure 4a), exhibited a single particle size distribution with thepeak average at 9.6 ± 3.9 nm and cumulant mean diameter14.8 nm with a polydispersity index of 0.123. As the pow-der used for DLS study was obtained by calcination at higher

FIG. 4. (a) Particle size distribution and (b) HRTEM micrograph of synthe-sized barium hexaferrite nanopowder.

Dow

nloa

ded

by [

Rut

gers

Uni

vers

ity]

at 2

0:39

07

July

201

4

1118 A. B. RAJPUT ET AL.

FIG. 5. SEM micrographs of barium hexaferrite pellets: (a) unsintered, and pellets sintered at (b) 1100C, (c) 1200C, and (d) 1350C.

temperature, the polydispersity index was found to have a highvalue. Figure 4b illustrated an HRTEM micrograph of the cal-cined powder where the average particle size was estimated tobe ∼20 nm.

SEM AnalysisFour pellets of the synthesized barium hexaferrite were

prepared to investigate the microstructure. To prepare pellets,BaFe12O19 nanopowder was used, obtained by the calcining theprecursor, with total metal ions:EDTA molar ratio 1:5, at a calci-nation temperature of 850C. One pellet was kept unsintered tocharacterize the microstructure of the as-synthesized nanopow-der. The other three pellets were sintered at 1100, 1200, and1350C for 6 h without using any additional binder. The densityof the pellets was measured via the of mass/volume method.The density was increased with increasing sintering tempera-ture. A density of 4.83 g/cm3 was achieved after sintering at1350C for 6 h, whereas the theoretical density of BaFe12O19

is 5.30 g/cm3.[30] Thus, ∼91.13% of the theoretical density wasachieved without the use of sintering aids.

From SEM micrographs of unsintered and sintered pellets(Figure 5), it was observed that (i) in the case of unsinteredsample, nanoparticles were loosely agglomerated and formedinter-granular pores (Figure 5a); and (ii) sintering at 1100Cresulted in formation of bigger particles (micrometer sized) butwith distinct morphologies (Figure 5b). The nanocrystals thatwere formed at 850C grew into larger rod-shaped particlesand acted as “nutrients” for the formation of bigger crystals[31];(iii) formation of several-micrometer-long rods, with variouslengths, was observed for the sample that was sintered at 1200C

(Figure 5c). These rods were formed due to the fusion of hexag-onal particles; (iv) however, the destruction of the clear grainstructure was observed at 1350C sintering temperature alongwith the formation of larger particles with irregular size andshape (Figure 5d).

40 60 80 100 120 140 160 180 2000

5

10

15

20

25

1150C

(d)

(c)

(b)

(a)

Temperature (°C)

Res

isti

vity

(x10

7 ) cm

Ω

0

2

4

6

8

10

12

Resistivity(x10

3) Ωcm

FIG. 6. Variation of DC resistivity with temperature of barium hexaferrite for(a) unsintered pellets, and pellets sintered at (b) 1100C, (c) 1200C, and (d)1350C. Left y-axis shows the scale for (a) and right y-axis shows the scale for(b), (c), and (d).

Dow

nloa

ded

by [

Rut

gers

Uni

vers

ity]

at 2

0:39

07

July

201

4

EDTA-PRECURSOR ROUTE FOR BaFe12O19 NANOPOWDER 1119

-15 -10 -5 0 5 10 15

-60

-40

-20

0

20

40

60

Ms

(em

u/g

)

Hc (kOe)

FIG. 7. Room-temperature hysteresis loop for barium hexaferrite nanopowder.

DC ResistivityThe DC resistivity of barium hexaferrite was measured using

the same pellets that were used to investigate microstructurevia SEM, because of understanding the relationship betweenmicrostructure and the electrical property of BaFe12O19. Figure6 shows the variation in the DC resistivity with the change ofmeasurement temperature for the unsintered and sintered pel-lets. It was observed that there was a clear difference in theresistivity between the unsintered and sintered samples. Theunsintered sample exhibited the typical negative temperature

coefficient of resistance (NTCR) behavior of ferrites only be-yond 115C, whereas the sintered samples exhibited the typ-ical NTCR behavior starting from room temperature (27C).The room-temperature resistivity (ρRT) of the unsintered sam-ple was found to be 9.3 × 106 -cm and this increased up to22.26 × 107 -cm at 115C. The resistivity was then decreasedwith increasing temperature and reached a value of 3.3 × 106

-cm at 200C. The increase in resistivity of the unsinteredsample from room temperature to 115C might be attributed tothe presence of open porosity and loose agglomeration in thenanopowders (as evident from SEM micrograph in Figure 5a)and entrapped moisture inside the pores of the as-synthesizednanopowders.[32,33] Increasing the measurement temperature upto 115C is likely to have caused the evaporation of moisturefrom the sample and therefore maximum resistivity was attained.Humidity recorded in the laboratory was ∼90% at 27C. Thevalue of ρRT was found to be 12.87 × 103, 4.34 × 103, and 1.13× 103 -cm for the pellets sintered at 1100, 1200, and 1350Crespectively (Figure 6, b, c, and d). The room-temperature re-sistivity value decreased with increasing sintering temperature.This was quite obvious, because of the formation of lager grainat higher sintering temperature (Figure 5), and larger grainswould imply a lesser resistance to electron flow. All the sin-tered samples exhibited NTCR behavior starting from roomtemperature and reached a value of 0.46 × 103, 0.22 × 103,and 0.35 × 103 -cm at 200C for the sample sintered at 1100,1200, and 1350C, respectively. As the sintering caused graingrowth and reduction in the porosity (Figure 5, b, c, and d), theconsequences of adsorbed moisture in these samples were notobserved.

TABLE 2Comparison of Ms and Hc of barium hexaferrite particles prepared by different synthesis methods

Synthesis method Ms (emu/g) Hc (Oe) Reference

Aerosol-derived precursor 51.9–56.5 2000–2600 [13]Microwave-assisted sol–gel autocombustion 64.1 1000 [14]Sol–gel combustion ∼ 60 A-m2/kg — [15]Autocombustion 43–55 2587–5000 [16]Sol–gel autocombustion 58.57–60.13 A-m2/kg 415 -433 kA/m [17]Co-precipitation 2.3–57 — [18]Co-precipitation 60.175 860 [19]Acetate precursor method 52 2600 [21]Sugar-nitrate precursor process 42.34 1624.9 [22]Hydrothermal 40 A-m2/kg 0.096 × 106 A/m [23]Reverse microemulsion 54.5–64.3 3357.2–5483.3 [24]Microemulsion 61.2 5397 [25]Mechano-combustion 67.1 493.5 kA/m [27]Molten salt 65.8 5251 [39]Solid state 34.44–59.38 2159–3598 [40]Pyrolysis of aerosol 5.7–75.5 1325–5470 [41]EDTA precursor route 56.5 4914 This work

Dow

nloa

ded

by [

Rut

gers

Uni

vers

ity]

at 2

0:39

07

July

201

4

1120 A. B. RAJPUT ET AL.

Magnetic MeasurementThe room-temperature magnetization curve for the synthe-

sized BaFe12O19 nanopowder was obtained by using VSM withan applied field of 15 kOe as shown in Figure 7. The mag-netic property measurement yielded coercivity (Hc) of 4914 Oeand a saturation magnetization (Ms) of 56.5 emu/g. The coer-civity and saturation magnetization values are comparable withthe values reported by other authors using different BaFe12O19

preparation techniques. The variations in the magnetic proper-ties of BaFe12O19, obtained by different synthesis routes, aresummarized in Table 2. In the present case Ms (56.5 emu/g)and Hc (4914 Oe) values of the synthesized barium hexaferritenanopowder were found to be lower than the theoretical val-ues of Ms (72 emu/g) and Hc (6700 Oe) for the single crystalof BaFe12O19.[24,25] Several reasons, such as size effects, spin-canting phenomenon, particle size effect, and others, have beenproposed by Junliang et al. to explain the lowering of saturationmagnetization values of nanoparticles.[14] The critical domainsize of BaFe12O19, as estimated by several researchers, is ∼1µm.[34–36] In the current case, the smaller size of the synthesizedBaFe12O19 nanoparticle (average particle size ∼20 nm) plays acritical role for the observed lower values of Ms. However, theHc value of the synthesized BaFe12O19 was found to be com-paratively higher than that of most of the reported values (Table2). The Curie temperatures (Tc) of the as-synthesized bariumhexaferrite and sintered samples were found to vary from 462 to495C and the values increased with increasing sintering tem-perature. These Tc values are slightly higher than that reportedas the ASM international value of Tc (450C) for BaFe12O19

[37] and the value reported by Ram et al.[38]

CONCLUSIONSA simple EDTA precursor-based chemical method has been

successfully developed for the preparation of single-phase bar-ium hexaferrite nanopowder. TGA–DSC, XRD, and HRTEManalysis of the synthesized precursor and calcined powdersconfirmed that oxidative decomposition of precursor leads tothe formation of single-phase barium hexaferrite. The chemicalprocess involves the homogeneous mixing of an aqueous solu-tion of Fe(NO3)3, Ba(NO3)2, and EDTA. The chelating agentEDTA plays a critical role in the formation of single-phaseBaFe12O19 nanopowder. It not only prevents the segregationor intermittent precipitation of metal ions from solution duringevaporation but also helps the formation of a fluffy, voluminous,porous carbon-rich precursor. During the decomposition of theprecursor, nascent metal oxides form, which are basically smallatomic clusters with proper chemical homogeneity and imbed-ded into the precursor. Final calcination of precursor leads tothe formation of single-phase barium hexaferrite nanoparticles(∼ 20 nm average diameters). The coercivity (Hc) of the syn-thesized BaFe12O19 nanopowders was 4914 Oe along with asaturation magnetization (Ms) of 56.5 emu/g. The Curie tem-perature (Tc) of these barium hexaferrite nanopowders (462C)

was found to be a higher value than that reported by previousresearchers.[37,38]

The synthetic method reported here offers the followingadvantages: (i) This method is simple and requires only themixing of water-soluble starting materials followed by evap-oration and calcination, (ii) water is the reaction medium in-stead of an organic solvent; and (iii) the starting materialsare relatively cheap and are not moisture-sensitive. Hence,complex experimental setups or multistage synthetic proce-dures are not required, (iv) the synthesis is carried out un-der atmospheric conditions, (v) corrosive reagent such asNaOH is not required, (vi) the calcination temperature re-quired for this method is comparable to[10,13,16,18,19,39] or lowerthan[9,11,15,17,20–22,24,25,27,40,41] that of other reported synthesisroutes for the production of single-phase barium hexaferrite, and(vii) pure single-phase barium hexaferrite is formed, whereasin other reported methods impurity phases remain present withthe final products.[9–11,13–16,21–23,26,27] Overall, the novelty of thisEDTA precursor-based method lies in its simplicity and its capa-bility of producing pure single-phase BaFe12O19 nanopowdersat a comparatively lower calcination temperature. This EDTAprecursor-based method has proved to be a convenient methodfor preparation of ferrite nanopowders such as various compo-sitions of Ni(1-x)ZnxFe2O4,[42] α-Fe2O3.[43,44]

REFERENCES1. Fujiwara, T. Barium ferrite media for perpendicular recording. IEEE. T.

Magn. 1985, 21, 1480–1485.2. Harris, V.G.; Geiler, A.; Chen, Y.; Yoon, S.D.; Wu, M.; Yang, A. Recent

advances in processing and applications of microwave ferrites. J. Magn.Magn. Mater. 2009, 321, 2035–2047.

3. Bregar, V.B. Advantages of ferromagnetic nanoparticle composites in mi-crowave absorbers. IEEE. T. Magn. 2004, 40, 1679–1684.

4. Lim, K.M.; Kim, M.C.; Lee, K.A.; Park, C.G. Electromagnetic wave ab-sorption properties of amorphous alloy-ferrite-epoxy composites in quasi-microwave band. IEEE. T. Magn. 2003, 39, 1836–1841.

5. Castro, S.; Gayoso, M.; Rodrıguez, C.; Mira, J.; Rivas, J.; Paz, S.; Greneche,J.M. Synthesis and characterization of small BaFe12O19 particles. J. Magn.Magn. Mater. 1995, 140–144, 2097–2098.

6. Paul, K.B. Magnetic and structural properties of Ba M-type ferrite-composite powders. Physica B 2007, 388, 337–343.

7. Yu, H.; Liu, P. Effects of pH and calcination temperatures on the formationof citrate-derived hexagonal barium ferrite particles. J. Alloys Compounds2006, 416, 222–227.

8. Wang, S.; Ng, W.K.; Ding, J. Preparation and characterization of Aldoped longitudinal barium ferrite thin film media. Scripta Mater. 2000,42, 861–865.

9. Hessien, M.M.; Radwan, M.; Rashad, M.M. Enhancement of magneticproperties for the barium hexaferrite prepared through ceramic route. J.Anal. Appl. Pyrol. 2007, 78, 282–287.

10. Benito, G.; Morales, M.P.; Requena, J.; Raposo, V.; Vazquez, M.; Moya, J.S.Barium hexaferrite monodispersed nanoparticles prepared by the ceramicmethod. J. Magn. Magn. Mater. 2001, 234, 65–72.

11. Sharma, P.; Rocha, R.A.; de Medeiros, S.N.; Paesano, A., Jr. Structural andmagneticstudies on bariumhexaferritesprepared by mechanicalalloying andconventional route. J. Alloys Compounds 2007, 443, 37–42.

12. Kojima, H. Ferromagnetic Materials: A Handbook on the Properties ofMagnetically Ordered Substances, ed. E.P. Wohlfarth; North-Holland: Am-sterdam, vol. 3, ch. 5, 1982.

Dow

nloa

ded

by [

Rut

gers

Uni

vers

ity]

at 2

0:39

07

July

201

4

EDTA-PRECURSOR ROUTE FOR BaFe12O19 NANOPOWDER 1121

13. Yu, H.; Lin, H. Preparation and thermal behavior of aerosol-derivedBaFe12O19 nanoparticles. J. Magn. Magn. Mater. 2004, 283, 190–198.

14. Junliang, L.; Yanwei, Z.; Cuijing, G.; Wei, Z.; Xiaowei, Y. One-step syn-thesis of barium hexaferrite nano-powders via microwave-assisted sol-gelauto-combustion. J. Eur. Ceram. Soc. 2010, 30, 993–997.

15. Ataie, A.; Mali, A. Characteristics of barium hexaferrite nanocrystallinepowders prepared by a sol-gel combustion method using inorganic agent.J. Electroceram. 2008, 21, 357–360.

16. Bahadur, D.; Rajakumar, S.; Kumar, A. Influence of fuel ratios on autocombustion synthesis of barium ferrite nano particles. J. Chem. Sci. 2006,118, 15–21.

17. Xu, G.; Ma, H.; Zhong, M.; Zhou, J.; Yue, Y.; He, Z. Influence of pH oncharacteristics of BaFe12O19 powder prepared by sol–gel auto-combustion.J. Magn. Magn. Mater. 2006, 301, 383–388.

18. Lisjak, D.; Drofenik, M. The mechanism of the low-temperature formationof barium hexaferrite. J. Eur. Ceram. Soc. 2007, 27, 4515–4520.

19. Shepherd, P.; Mallick, K.K.; Green, R.J. Magnetic and structural propertiesof M-type barium hexaferrite prepared by co-precipitation. J. Magn. Magn.Mater. 2007, 311, 683–692.

20. Moghaddam, K.S.; Ataie, A. Role of intermediatemilling in the processingof nano-size particles of bariumhexaferrite via co-precipitationmethod. J.Alloys Compounds 2006, 426, 415–419.

21. Ataie, A.; Heshmati-Manesh, S.; Kazempour, H. Synthesis of barium hexa-ferrite by the co-precipitation method using acetate precursor. J. Mater. Sci.2002, 37, 2125–2128.

22. Tang, X.; Zhao, B.Y.; Hu, K.A. Preparation of M-Ba-ferrite fine powdersby sugar-nitrates process. J. Mater. Sci. 2006, 41, 3867–3871.

23. Drofenik, M.; Kristl, M.; Znidarsic, A.; Hanmel, D.; Lisjak, D. Hydrother-mal synthesis of Ba-hexaferrite nanoparticles. J. Am. Ceram. Soc. 2007, 90,2057–2061.

24. Xu, P.; Han, X.; Wang, M. Synthesis and magnetic properties of BaFe12O19

hexaferrite nanoparticles by a reverse microemulsion technique. J. Phys.Chem. C 2007, 111, 5866–5870.

25. Pillai, V.; Kumar, P.; Multani, M.S.; Shah, D.O. Structure and magneticproperties of nanoparticles of barium ferrite synthesized using microemul-sion processing. Colloid. Surfaces A 1993, 80, 69–75.

26. Kuznetsov, M.V.; Morozov, Y.G.; Busurin, S.M.; Chernega, M.L.; Parkin,I.P. Phase composition and magnetism of combustion products in Ba-Fe-Ocompounds synthesized under applied DC electric field. J. Magn. Magn.Mater. 2007, 309, 202–206.

27. Ataie, A.; Zojaji, S.E. Synthesis of bariumhexaferrite nano-particles viamechano-combustion route. J. Alloys Compounds 2007, 431, 331–336.

28. Soohoo, R.F. Theory and Application of Ferrites; Prentice Hall: EnglewoodCliffs, NJ, 1960.

29. Klug, H.P.; Alexander, L.E. X-ray Diffraction Procedures for Polycrys-talline and Amorphous Materials; Wiley: New York, 1967.

30. Mallick, K.K.; Shepherd, P.; Green, R.J. Dielectric properties of M-typebarium hexaferrite prepared by co-precipitation. J. Eur. Ceram. Soc. 2007,27, 2045–2052.

31. Sharma, R.; Agarwala, R.C.; Agarwala, V. Development of radar absorb-ing nano crystals by microwave irradiation. Mater. Lett. 2008, 62, 2233–2236.

32. Cantalini, C.; Faccio, M.; Ferri, G.; Pelino, M. The influece of water vapouron carbon monoxide sensitivity of α-Fe2O3 microporous ceramic sensors.Sensor Actuat. 1994, 18–19, 437–442.

33. Chauhan, P.; Annapoorni, S.; Trikha, S.K. Humidity-sensing properties ofnanocrystalline hematite thin films prepared by sol-gel processing. ThinSolid Films 1999, 346, 266–268.

34. Haneda, K.; Morrish, A.H. Magnetic properties of BaFe12O19 small parti-cles. IEEE. Trans. Magn. 1989, 25, 2597–2601.

35. Kittle, C. Physical theory of ferromagnetic domains. Rev. Mod. Phys. 1949,21, 541–583.

36. Goto, K.; Ito, M.; Sakurai, T. Studies on magnetic domains of small particlesof barium ferrite by colloid-SEM method. Jpn. J. Appl. Phys. 1980, 19,1339–1346.

37. ASM. ASM Handbook, Properties and Selection: Nonferrous Alloys andSpecial-Purpose Materials; ASM International: Materials Park, OH, Vol.2, 1990.

38. Ram, S.; Krishnan, H.; Rai, K.N.; Narayan, K.A. Magnetic and electricalproperties of Bi2O3 modified BaFe12O19 hexagonal ferrite. Jpn. J. Appl.Phys. 1989, 28, 604–608.

39. Yu, J.; Tang, S.; Zhai, L.; Shi, Y.; Dua, Y. Synthesis and magnetic prop-erties of single-crystalline BaFe12O19 nanoparticles. Physica B 2009, 404,4253–4256.

40. Sozeri, H. Simple recipe to synthesize single-domain BaFe12O19 with highsaturation magnetization. J. Magn. Magn. Mater. 2009, 321, 2717–2722.

41. Cabanas, M.V.; Gonzalez-Calbet, J.M.; Vallet-Regi, M. Synthesis of bariumhexaferrite by pyrolysis of aerosol. J. Mater. Res. 1994, 9, 712–716.

42. Sarangi, P.P.; Naik, B.D.; Vadera, S.R.; Patra, M.K.; Prakash, C.; Ghosh,N.N. Development of simple chemical method for synthesis of single phaseNi-Zn ferrite nanopowders. Mater. Technol. 2009, 24, 97–99.

43. Sarangi, P.P.; Naik, B.; Ghosh, N.N. Synthesis of single-phase α-Fe2O3

nanopowders by using a novel low temperature chemical synthesis route.J. Am. Ceram. Soc. 2008, 91, 4145–4147.

44. Sarangi, P.P.; Vadera, S.R.; Patra, M.K.; Prakash C.; Ghosh, N.N. DC electri-cal resistivity and magnetic property of single-phase α- Fe2O3 nanopowdersynthesized by a simple chemical method. J. Am. Ceram. Soc. 2009, 92,2425–2428.D

ownl

oade

d by

[R

utge

rs U

nive

rsity

] at

20:

39 0

7 Ju

ly 2

014