Study of magnesium ferrite nano particles with excess iron content

T.P. Sumangala a,n, C. Mahender a, B.N. Sahu b, N. Venkataramani a, Shiva Prasad b

a Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, Indiab Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

a r t i c l e i n f o

Keywords:MgFe2þδO4

FEG-TEMMagnetismGas sensing

a b s t r a c t

Stoichiometric and non stoichiometric magnesium ferrite (MgFe2þδO4, δ¼0.0, 0.1) were synthesized bythe sol gel combustion method resulting in nanocrystalline powders with size ranging from 10 to100 nm. These powders were calcined at various temperatures (300–800 1C). One part of the calcinedpowder was quenched in liquid nitrogen and the other part furnace cooled. α-Fe2O3 was observed in allcalcined samples by XRD and this was also reflected in the magnetization data. Electrical response ofMgFe2.1O4þδ spinel phase to 75 ppm ethanol was found to be greater than that for a stoichiometricmagnesium ferrite.

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1. Introduction

Spinel ferrites are metal oxides with a general formulaMFe2O4. Their magnetic properties have been extensively studiedbecause of their applications [1]. Recently, there have also beenstudies on the gas sensing behavior of ferrites [2] as the electricalresistance of ferrites can change in response to their gaseousenvironment. Various ferrites including Magnesium ferrite, Zincferrite and Nickel ferrite has been studied for sensing ethanol [2–5]. Other than ferrites, hematite is also useful for ethanol sensing[6]. It is also known that the defects in a system may improve gassensing performance [7]. Recently the effect of non stoichio-metric zinc ferrite with deficient and excess Fe on the ethanolsensing behavior of zinc ferrite was demonstrated [5]. It wasshown that excess Fe in spinel helps in improving responsetowards ethanol.

It is not clearly known, if there is any relationship between themagnetic and gas sensing properties of ferrites. Magnetic proper-ties in magnesium spinel ferrite are largely governed by the siteoccupancies of ions and are quite sensitive to the thermal treat-ment given to them [8]. Keeping this in mind and to check if therecould be any link between the magnetic properties and the gassensing properties, we decided to study non stoichiometric mag-nesium ferrite containing excess iron. In this paper, we reporta preliminary study of magnetic and gas sensing properties ofmagnesium ferrite with 5% excess iron.

2. Experimental details

Stoichiometric and non stoichiometric MgFe2O4 were preparedfrom Mg (NO3)2 �6H2O and Fe (NO3)3 �9H2O by sol gel combustionsynthesis. The nitrate salts were taken in molar ratio 2:1 and 2.1:1for stoichiometric and non stoichiometric samples respectivelyand were dissolved in distilled water. Citric acid was added to thissolution to have 1:1 ratio with metal salts and the resultantsolution kept over a hot plate at 80 1C with constant stirring. Afterthe complete dissolution of added salts, ammonia solution (25%)was added until the pH reached 7. The resultant mixture washeated with constant stirring for about 15 h, at the end of which, ittransformed to a gel. This gel was further heated without stirringand at an optimum viscosity of the gel, the gel self ignited. Thecombustion continued till all of the gel had been transformed to afluffy powder. This powder was then milled in a ball mill withyttria stabilized zirconium balls for 24 h. The resultant powderwas then heated in air in the temperature range 300–800 1C. Apart of the powder was quenched in liquid nitrogen and the otherpart was allowed to cool slowly inside a furnace. These powderswere then crushed and characterized.

The following scheme was used to name the samples. All the nonstoichiometric samples were named with extension ‘_5’. Furnacecooled samples were indicated as ‘FC’ and quenched ones as ‘QN’.Thus non stoichiometric sample furnace cooled from 800 1C wasnamed as MgF800FC_5% and stoichiometric sample as MgF800FC.

3. Characterization

X-ray diffraction experiments were carried out using a Panaly-tical X-Ray diffractometer with a Cu target (Kα radiation,

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n Corresponding author. Tel.: þ91 2225764610.E-mail address: sumampoornima@gmail.com (T.P. Sumangala).

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λ¼1.54060 Å). The powder samples were scanned in the 2θ range20–801 with a step size 0.021. Field Emission Gun TransmissionElectron Microscopy (FEG-TEM) images were taken using JEOLJEM-2100F microscope. M–H measurements were carried out at 5 Kon a Quantum design Physical property Measurement System(PPMS) having a Vibrating Sample magnetometer (VSM) attachment.

Gas sensing measurements were performed using a homemadeset up. Two set of samples were prepared for gas sensing. In thefirst case, powder samples were pressed using a uniaxial pressureof 1.44 MPa. These are termed as green pellets in the presentpaper. Electrical contacts were made in these pellets using silverpaint. In the second case, powder was mixed with 10% binder(poly vinyl alcohol) and then pressed using a uniaxial pressure of1.44 MPa. The second set of pellets was sintered at 800 1C follow-ing a binder burnout cycle at 500 1C for 12 h. These pellets aretermed as sintered pellets here. These pellets were metalizedusing a thin film of silver deposited using DC sputtering. For thegas sensitivity test, 75ppm ethanol and air (0% humidity) wereused. The test gases were streamed alternatively into the chambercontaining the test sample attached to a heater. Resistancemeasurements were performed using a Keithely 220 Nanovolt-meter and a Keithley 181 current source connected to computerusing Labview software.

4. Results and discussions

4.1. X-ray diffraction (XRD)

Fig. 1a and b show the XRD pattern of furnace cooled andquenched Fe excess MgFe2O4 powder samples. The observed peakswere matched with JCPDS card no 73-1720 and 33-0664 forMgFe2O4 and α-Fe2O3, respectively.

From Fig. 1a and b, it can be seen that the samples have formedthe magnesium ferrite phase. In addition to that a phase ofα-Fe2O3 is also seen for all samples. It is also seen that the relativeintensity of (1 0 4) peak of α-Fe2O3 (highest intensity peak ofα-Fe2O3) is not the same in all spectra. The intensity of (1 0 4) peakof α-Fe2O3 initially increases with calcination temperature, with amaximum observed for MgF600FC_5 and MgF500QN_5 and thendecreases at higher temperatures. This indicates that some of theexcess Fe added to the system is precipitating as α-Fe2O3 phase forall samples. Similar observation has been reported by otherworkers [9].

Table 1 gives the amount of α-Fe2O3 in all samples obtainedfrom Rietveld refinement. It can be seen from Table 1 that, thisamount is not same in all the samples. The amount is greater than5 mol%, for samples calcined and furnace cooled or quenched from500 and 600 1C. For sample MgF800FC_5, the amount of α-Fe2O3

phase is only 0.2%.The ability of spinel structure to accommodateexcess Fe ions has been already shown and the quantity of excessFe ions that can be accommodated increases as a function ofcalcination temperature [10,11].

4.2. FEG-TEM

The FEG-TEM images of MgF600FC_5 furnace cooled sample isshown in Fig. 2 along with the selected area diffraction (SAED)pattern in inset. MgF600FC sample was chosen as it is having thelargest amount of α-Fe2O3 phase (8.1%) from the Rietveld refine-ment. It can be seen from Fig. 2 that MgF600FC_5 sample containscluster of both large particles and small particles. The SAED onsmaller particles (shown in the inset), shows that these particlesare in α-Fe2O3 phase.

4.3. Magnetic measurements

Magnetic measurements were performed on all the furnacecooled and quenched samples at 5 K. Due to the non saturation ofmagnetization in these samples, value of magnetization wascalculated using the law of approach to saturation magnetization[12] and the value at H-1 is being reported. The change in thevalue of magnetization is plotted as a function of calcinationtemperature in Fig. 3a and b. It can be seen that the values ofmagnetization is not increasing monotonically both for furnacecooled and quenched samples. For samples calcined from 500 to800 1C, the value of magnetization for quenched samples is higherthan the corresponding furnace cooled ones. The conversion of

Fig. 1. XRD of (a) Furnace cooled and (b) Quenched samples. n represents (1 0 4)peak of α-Fe2O3.

Table 1Mol% of α-Fe2O3 and χ2 from Rietveld.

Calcinationtemperature (1C)

Furnace cooled Quenched

Mol% of α-Fe2O3 χ2 Mol% of α-Fe2O3 χ2

300 2.9 1.73 1.9 1.42400 4.5 1.75 4.4 1.64500 6 1.55 7.3 1.46600 8.1 1.42 5.7 1.49700 4.5 1.47 5.1 1.54800 0.2 1.69 2.8 1.58

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α-Fe2O3 to γ-Fe2O3 may be ruled out as one of the reasons for thishigh value of magnetization for quenched samples since the latteris not stable at that temperature (500–800 1C) [13]. The otherpossible reason is the conversion of Fe3þ ion to Fe2þ ion. This wasinvestigated by Xray photoelectron spectroscopy and no definitivefeature of Fe2þ could be seen in the obtained spectra. The

improvement in magnetization may thus be attributed to thechanged cation distribution. By Néel's model, this increase inmagnetization may be attributed to the fact that the additionalFe ions added will prefer octahedral site to tetrahedral at highertemperature and hence lead to higher magnetization in quenchedsamples. But when the samples are furnace cooled, the Fe ionsgo back to the tetrahedral sites and causing a reduction inmagnetization. For stoichiometric MgF800FC sample, the magne-tization obtained was 43 emu/g.

4.4. Gas sensing

We used our samples to test their response towards ethanol. Ourpreliminary experiments showed that samples calcined at 800 1Cshowed better response than other samples. Hence samples,MgF800FC_5 and MgF800QN_5 and MgF800FC were used for gassensing measurement. Stoichiometric sample was also measured inorder to compare the effect of excess Fe ion addition towards theresponse to a test gas. Resistance of the sample decreased whenethanol was inserted in the measurement chamber. When theresistance value reached saturation, ethanol was purged out usingair. It was observed that the resistance increased in the presence ofair. Response of the sensor is defined as the change in resistance inthe presence of test gas to the resistance in air. The time required bythe sample in the presence of test gas to decrease the resistance to90% of its saturation value is the response time and the time toreach 90% of its initial value is the recovery time. Fig. 4 shows thechange in resistance of MgF800QN_5 sintered pellet with ethanol.Table 2 gives the response of furnace cooled and quenched Feexcess sample and furnace cooled stoichiometric sample to 75ppmethanol measured at 200 1C. It can be seen from Table 2 that there isa significant improvement in the response towards ethanol for thefurnace cooled Fe excess sample compared to its stoichiometriccounterpart. The response of Fe excess furnace cooled sample wasgreater than the corresponding quenched sample.

Sintered pellets are preferred over green pellets for gas sensingapplication due to their mechanical strength which is required. Itcould also be seen from Table 2 that the response of MgF800FC_5sample does not change significantly with the thermal treatmenton the pellet. On the other hand it could be seen that for the othertwo samples, the response has decreased.

The response of samples towards ethanol increases in the orderMgF800FCoMgF800QN_5oMgF800FC_5. To compare the mag-netization of these samples the magnetization values was cor-rected for the quantity of α-Fe2O3 phase. Magnetization values forthese samples increases in the order MgF800FC_5 (39.6 emu/g)oMgF800FC (43 emu/g)oMgF800QN_5 (57.2 emu/g). This showsthat though excess Fe has contributed to an improvement in

Fig. 2. FEG-TEM image of MgF600FC_5.

Fig. 3. Magnetization vs. calcination temperature of (a) furnace cooled and(b) quenched series measured at 5 K.

Fig. 4. Gas sensing response of MgF800FC_5 sample.

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response towards ethanol, it is not the case for the magnetizationof the sample. This shows that though magnetization may be usedto have an idea on the probable cation distribution, the exact siteoccupancy which will improve the gas sensing response could notbe correlated from the magnetization values.

5. Conclusion

Fe excess containing magnesium ferrite (MgFe2þδO4, δ¼0.1)nanocrystalline powders with grain size in the range (10–100 nm)were synthesized by the sol gel combustion method. The resultantsample on heat treatment showed the precipitation of α-Fe2O3

phase. The quantity of α-Fe2O3 phase initially increased and thendecreased with calcination temperature. The variation in magne-tization as a function of calcination temperature may be due to thechange in site occupancy and the precipitation of a second phase.The addition of excess Fe into the MgFe2O4 system is seen toimprove the gas sensing behavior of the sample towards ethanolover the stoichiometric composition.

Acknowledgments

The authors thank PPMS central facility at IIT Bombay and SAIF,IIT Bombay for FEG-TEM facility measurements. One of the

authors, Sumangala T.P. thanks Crompton Greaves Ltd. for theresearch fellowship.

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Table 2Gas sensing response of various samples.

Sample Green pellet Sintered pellet

Response (%) Response time (min) Recovery (h) Response (%) Response time (min) Recovery (h)

MgF800FC_5 88 5 16 84 9 22MgF800QN_5 75 3 5 54 10 10MgF800FC 71 1 16 64 32 15

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