Formation, microstructure and magnetic properties of nanocrystalline MgFe2O4

Materials Chemistry and Physics 132 (2012) 782–787

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Formation, microstructure and magnetic properties of nanocrystalline MgFe2 O4 V.M. Khot, A.B. Salunkhe, M.R. Phadatare, S.H. Pawar ∗ Center for Interdisciplinary Research, D.Y. Patil University, Kolhapur 416006, Maharashtra, India

a r t i c l e

i n f o

Article history: Received 1 July 2011 Received in revised form 21 November 2011 Accepted 6 December 2011 Keywords: Nanostructures Thermodynamic properties Electron microscopy Magnetic properties

a b s t r a c t Nanocrystalline powder of MgFe2 O4 was successfully synthesized by a cost effective novel combustion route. Nitrates of the constituent elements and glycine were respectively used as an oxidizer and fuel to drive the reaction. The effect of glycine to nitrate molar ratio (G N−1 ) on the structure and formation of MgFe2 O4 was studied in view of thermodynamic considerations like adiabatic flame temperature and gas evolved during the combustion. The as prepared powder was characterized by X-Ray Diffraction (XRD), Fourier Transform Infra Red (FTIR) spectroscopy and Scanning Electron Microscopy (SEM) for formation and microstructure analysis at various G N−1 ratios. XRD results revealed that the crystallinity of MgFe2 O4 is insensitive to G N−1 variations and fuel lean combustion also lead to appropriate MgFe2 O4 phase formation. Thermo Gravimetric-Differential Thermal Analysis (TG-DTA) for the precursor gel demonstrated the occurrence of rapid chemical reaction between glycine and nitrates at around 194 ◦ C corresponding to ignition of precursors at this temperature. Transmission electron microscopy image for as prepared stoichiometric sample shows formation of nanoparticles of sizes from 28 nm to 50 nm. SEM images of MgFe2 O4 nanoparticles at G N−1 ratio show remarkable change in microstructure regarding porosity and grain size. Room temperature magnetic measurements for stoichiometric sample show the magnetization (Ms ) and remanence (Mr ) of about 31.56 emu g−1 and 9.60 emu g−1 at ±10 kOe respectively. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Spinel ferrites with general formula MFe2 O4 (M = Co, Mg, Mn, Ni, etc.) have been extensively used in various technological and biomedical applications in the past decades. It covers wide range of applications including humidity sensor, switching circuits, contrast agent in magnetic resonance imaging, tissue repair, immunoassays, detoxification of biological fluids, targeted drug delivery and magnetic fluid hyperthermia [1–3]. The effective applicability of these nanoparticles is driven by their physical and chemical property which is highly sensitive to their shape and size. The shape and size of these nanoparticles can be effectively controlled by the synthesis route employed [4,5]. Among the various ferrites, MgFe2 O4 is an interesting magnetic material where magnetic couplings purely originate from the magnetic moment of Fe cations and may be relatively weaker due to non magnetic Mg2+ metal ions. Magnetic anisotropy in MgFe2 O4 could be lower than that of other spinel ferrites in which all the metal cations have large magnetic moments. In recent years, various physical and chemical techniques such as co-precipitation [6], sol–gel [7], ball milling [8] and combustion [9] have been successfully employed for the synthesis of MgFe2 O4 nanoparticles.

∗ Corresponding author. Tel.: +91 231 2601202/235; fax: +91 231 2601595. E-mail address: pawar s [email protected] (S.H. Pawar). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.12.012

Although chemical co-precipitation method is suitable for mass production of magnetic nanoferrites, it does require careful adjustment of the pH value of the solution for particles formation. On the other hand, combustion synthesis (glycine nitrate process) offer many distinct advantages for synthesizing magnesium ferrites as it produces high-surface-area with less reaction time, compositionally homogeneous powder, usually with low levels of residual carbon. These advantages are mainly due to the nature of the fueloxidant combustion reaction, which is rapid, self-sustaining and exothermic in nature. The surface area, size-distribution and agglomeration of the particles in final product depend on the adiabatic flame temperature which in turn related to nature of fuel as well as fuel to oxidant ratio (F/O). Adiabatic flame temperature helps in crystallization and formation of desired phase of compound. However, high adiabatic flame temperature adversely affects the particle characteristics like increase in crystallite size and increased agglomerates. Very little work is done to figure out the effect of F/O ratio on characteristics of product in terms of thermodynamic considerations including adiabatic flame temperature and heat absorbed by the products, i.e. heat of reaction [10–12]. The fuel used in reaction should be able to maintain homogeneity among constituents and also undergo combustion with oxidizer at low ignition temperature. In the present work, glycine (NH2 CH2 COOH) was used as a fuel because it turns out to be a cost effective alternative to urea and citric acid. It has a relatively negative heat of combustion

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(−3.24 kcal g−1 ) as compared to urea (−2.98 kcal g−1 ) or citric acid (−2.76 kcal g−1 ) [13]. The effect of glycine to nitrate ratio on powder characteristics of MgFe2 O4 was reported in terms of thermodynamic considerations. The product obtained was characterized by XRD, SEM, TEM and Vibrating Sample Magnetometer (VSM). Adiabatic flame temperatures were calculated theoretically for the combustion reactions for different G N−1 ratios and systematically discussed. 2. Materials and method Analytical grade ferric nitrate nonahydrate Fe(NO3 )3 ·9H2 O, magnesium nitrate hexa hydrate Mg (NO3 )2 ·6H2 O were used as oxidants and glycine as a fuel to accomplish the combustion reaction. All the reagents used were of high degree purity. The oxidation valences of metal nitrates were balanced by reducing valences of the fuel so that equivalence ratio was unity and energy released was maximum [14]. In a typical procedure the stoichiometric amount of reactants were hand mixed in a beaker. The mixture was turned into the slurry due to hygroscopic nature of nitrates. The beaker was then kept on hot plate preheated to 300 ◦ C. During combustion, the spark was occurred at one corner which spread over the mass resulting brown fluffy product that gets transformed into powder by slightest touch. As the amount of glycine (fuel) plays an important role in combustion synthesis; in present case, the glycine to nitrate ratio (G N−1 ) was varied as 0.48, 0.74, 1.48, 2.22 and 2.56. This respectively makes three combustion systems: fuel lean, fuel efficient and fuel rich wherein G N−1 = 1.48 represents stoichiometric ratio for combustion. Thermal properties of precursor gel were recorded by the Trans-analytical instrument (SDT 2960) operated in temperature 35–1000 ◦ C with heating rate of 10 ◦ C in flowing air ambiance to investigate the decomposition behavior of nitrate and fuel mixture. The samples were indexed as V1, V2, V3, V4 and V5 for G N−1 ratios 0.48, 0.74, 1.48, 2.22 and 2.56 respectively. All the samples were characterized by Philips PW-3710 automated X-ray diffractometer equipped with crystal monochromator employing Cr-K␣ radiation of wavelength 2.28970 A˚ for structural and phase identification. The crystallite size of the as-synthesized product was estimated from the full-width at half-maximum (FWHM) of the strongest diffraction peak using the Scherrer formula [15], D=

0.9 ˇcos

(1)

where D is the crystallite size,  is the wavelength of Cr-K␣ radiation, ˇ is FWHM and  is the diffraction angle of the strongest characteristic peak. The particle shape, size and morphology were investigated by SEM (JEOL JSM 6360) and TEM (Philips CM 200 ˚ FTIR specmodel, operating voltage 20–200 kV, resolution 2.4 A). trum was recorded with the help of PerkinElmer spectrometer, (Model No. 783, USA) in the range of 400–2000 cm−1 to confirm the formation of spinel phase and purity of the samples. Magnetization and coercivity for all the samples was measured by VSM at room temperature [Lake Shore 7307]. 3. Result and discussion 3.1. Thermal analysis The simultaneous TG-DTA curves for stoichiometric precursor gel were recorded in temperature range of room temperature to 1000 ◦ C in air ambiance (Fig. 1). TG curve shows a weight loss of about 30% below 194 ◦ C which is due to complete evaporation of water and organic contents in the precursor gel. The sudden weight loss of 56% was observed between temperature range of 194–200 ◦ C which is attributed to rapid chemical reaction between

Fig. 1. TG-DTA curve for stoichiometric precursors gel.

metal nitrates and glycine. This maximum weight loss occurs in narrow temperature range which corresponds to decomposition step. The overall weight loss of 86% for sample was observed which is in good agreement with theoretically predicted weight loss of (84%). This amounts to the overall reaction yield of about 14%. The sharp exothermic peak observed in DTA curve around 194 ◦ C was attributed to ignition of precursors at this temperature. The acceleration of reaction rate and lowering of ignition temperature results in combustion reaction between metal nitrates and glycine. 3.2. Thermodynamic analysis According to the principle of propellant chemistry, when stoichiometric amount of fuel is mixed with metal nitrates then product of reaction consists of environment friendly gases like H2 O, N2 and CO2 . The combustion reaction between metal nitrates and glycine can be expressed as: Mg(NO3 )2 6H2 O + 2Fe(NO3 )3 9H2 O + 4.44˚CH2 NH2 COOH + (9.99˚ − 10)O2 → MgFe2 O4 + 8.88˚CO2 ↑ + (4 + 2.22˚)N2 ↑ +(24 + 11.1˚)H2 O ↑

(2)

where ˚ is molar ratio between glycine and nitrates. Here ˚ = 4.44/3 = 1.48 is the stoichiometric ratio which implies that product can be formed directly from the reaction without consuming external oxygen. Thus different values of ˚ in above equation represent the different G N−1 ratios. The theoretical calculations based on thermodynamic consideration such as enthalpy of reaction and flame temperature helps in estimation of exact ignition condition and initiation of combustion reaction. Enthalpy of reaction depends on the heat of formation of products and reactants. The following equation is used to calculate the enthalpy of reaction: H ◦ =



nHf ◦



products

−



nHf ◦



(3) reactants

where n is the number of moles, Hf ◦ is heat of formation and H◦ is enthalpy of reaction. The thermodynamic data for various reactants and products involved in combustion is available in literature [11–13,16] and listed in Table 1. The enthalpy of reaction as a function of ˚ can be calculated by using thermodynamic data (Table 1) and Eq. (2) as follows: H ◦ = 464.23 + ˚(−1122.72) (25 ◦ C, kcal)

(4)

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Table 1 Thermodynamic data required for calculation of adiabatic flame temperature [11–13,15]. Compound

Heat of formationa Hf ◦ (kcal mol−1 )

Heat capacitiesa cp (cal mol−1 K)

Fe(NO3 )3 ·9H2 O (s) Mg(NO3 )2 ·6H2 O (s) CH2 NH2 COOH (s) H2 O (g) CO2 (g) N2 (g) O2 (g) MgFe2 O4 (s)

−785.2 −624.48 −79.71 −57.79 −94.05 0 0 −343.69

– – – 7.2 + 0.0036 T 10.34 + 0.00274 T 6.5 + 0.001 T 5.92 + 0.00367 T 34.16

a

All values considered at ambient temperature T = 25 ◦ C.

The heat absorbed by product during combustion reaction can be theoretically approximated as:



Q = −H ◦ =

Tad



 ncp

298

products

dT.

(5a)

Eq. (5a) can be modified to calculate adiabatic flame temperature (Tad ) as follows, Tad = T +

Q Cp

(5b)

where Q is the heat absorbed by products under adiabatic condition, T is the reference temperature (T = 298 K) and Cp is the heat capacity of the products at constant pressure. Using data from Table 1, Eqs. (4), (5a) and (5b); the adiabatic flame temperatures and heat absorbed by products for various G N−1 ratios were calculated and tabulated in Table 2. As expected, the values of theoretically calculated Tad and heat absorbed by product are increase with increase in amount of glycine. The reaction temperature and Tad increases with increase in G N−1 ratio. However beyond the optimum value of temperature, the decrease in reaction temperature with further increase in G N−1 ratio is observed attributed to the amount of gases released during reaction which may dissipates heat. This gives rise to lower values of reaction temperature than that of theoretically calculated Tad .

Fig. 2. XRD patterns of samples V1, V2, V3, V4 and V5 using different G N−1 ratio: 0.48, 0.74, 1.48, 2.22 and 2.96.

in this case are nanocrystallites with sizes ranging from 28 nm to 46 nm and can be confirmed by TEM image. The variation of crystallite size (D) and lattice parameter (L) with different G N−1 ratio are summarized in Table 3. From Table 3 it is seen that the crystallite size decreases with increasing G N−1 ratio, attains minimum

3.3. Structural analysis Fig. 2 exhibits, XRD patterns of MgFe2 O4 powder prepared by combustion method at various G N−1 ratios. The diffraction peaks corresponding to planes (2 2 0), (1 1 1), (3 1 1), (4 0 0), (4 2 2) and (5 1 1) were well matched with JCPDS card no. 36-0398 which confirms the formation of pure MgFe2 O4 phase with space group fd3m. Fig. 3a shows the TEM image recorded for the stoichiometric sample V3 which revealed particle size of MgFe2 O4 powder in the range of 28–46 nm. The presence of ring pattern in selected area electron diffraction (SAED) pattern assures the formation of polycrystalline MgFe2 O4 (Fig. 3b). The crystallite size was calculated from highest intensity peak (3 1 1) observed in the XRD patterns for the samples prepared by various G N−1 ratios. It was estimated that all the samples obtained Table 2 Variation of adiabatic flame temperature (Tad ), heat absorbed (Q) by product and number of moles of gases evolved during combustion at different G N−1 ratio. Sample

G N−1

Q (kcal mol−1 )

Tad (K)

Number of moles of gases evolved

V1 V2 V3 V4 V5

0.48 0.74 1.48 2.22 2.96

−94.18 97.13 658.49 1219.85 1781.21

11 565 1711.34 2442.15 2948.89

35.32 39.1 50.2 61.3 72.4

Fig. 3. (a) TEM image for stoichiometric sample (V3) MgFe2 O4 . (b) Selective area electron diffraction pattern for stoichiometric sample (V3).

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Table 3 Effect of variation in G N−1 ratio on crystallite size (D), lattice parameter (L), magnetization (Ms ), coercivity (Hc ) and remenance (Mr ) of MgFe2 O4 . Sample ID

G N−1 ratio

D(3 1 1) (nm)

L(3 1 1) (nm)

Ms (emu g−1 ) at 10 kOe

V1 V2 V3 V4 V5

0.48 0.74 1.48 2.22 2.96

37 33 28 31 32

0.8386 0.8386 0.8430 0.8386 0.8394

19.33 27.29 31.56 21.14 17.25

value at sample V3 and then slightly increases. It is also interesting to note that the intensity of Bragg peak is also minimum for stoichiometric sample corresponding to smaller crystallite size of sample compared to other variations. The results indicate a small dependence of crystallite size on the synthesis conditions such as adiabatic flame temperature, number of mole of gases escaped during combustion and enthalpy of reaction. The large amount of gases produced during the combustion may carry heat from system and thereby hindering the growth of particles. The properties observed for stoichoimetric condition are due to dominant effect of number of gas molecules escaped over adiabatic flame temperature. The dependence in values of crystallite size and lattice parameter with G N−1 ratios is probably due to competition between adiabatic flame temperature and number of gases evolved during synthesis [10].

3.4. FT-IR analysis The formation of the spinel phase in the nanocrystalline MgFe2 O4 samples is supported by FT-IR spectra. The FT-IR spectra for variable G N−1 ratios were recorded in the range 400–2000 cm−1 (Fig. 4). The absorption bands appeared at 450 cm−1 and 561 cm−1 corresponding to stretching vibration of metal–oxygen bonds at tetrahedral and octahedral sites respectively are observed in case of spinel ferrites. The band observed at 1638 cm−1 is due to N O stretching and the intensity of band is found to increase with increase in the G N−1 ratio. The shift in values of absorption bands from lower wavenumber (559 cm−1 ) to higher wavenumber (567 cm−1 ) with increasing G N−1 ratio can be attributed to shifting of Fe3+ and Mg2+ ions towards oxygen ion on occupation of tetrahedral and octahedral sites, which decreases the Fe3+ –O2− and Mg2+ –O2− distances [16]. On the basis of this data it can be

± ± ± ± ±

0.28 0.40 0.47 0.31 0.25

Hc (Oe)

Mr (emu g−1 )

119.07 93.26 182 110.68 103.25

4.03 4.02 9.60 3.69 3.10

suggested that the MgFe2 O4 spinel phase is formed with cation distribution, (Fe1−x 3+ Mgx 2+ )A [Mg1−x 2+ Fe1+x 3+ ]B O4 2− where x is inversion parameter while A and B are tetrahedral and octahedral places in spinel structure respectively. The inversion parameter is equal to 0 for inverse spinel and is 1 when structure is normal spinel. The observed values illustrates that the frequency band appeared at ∼560 cm−1 and ∼450 cm−1 are responsible for the formation of spinel MgFe2 O4 [17]. The remarkable increase in the intensity and area of investigated bands with increase in G N−1 ratio suggests the enhancement in MgFe2 O4 production. 3.5. Microstructural analysis The SEM images of MgFe2 O4 nanoparticles with different G N−1 ratio are shown in Fig. 5. Obtained images show remarkable change in the microstructure regarding porosity, grain size of samples. This shows the dependence of microstructure on different reactant composition [16]. From Fig. 5a and b it can be concluded that lower G N−1 ratio favors frothy and small holes within structure, which may be due to escaping large number of gases during the combustion. While for intermediate G N−1 ratio, small spherulitic porous structure dominates. For higher G N−1 ratio, (Fig. 5d and e) foamy agglomerated particles with a wide distribution and larger voids in their structure are observed. It can be seen from figure that all samples exhibit larger grains in the range of 200–500 nm and having a network with voids and pores. The porosity in all cases is found to be entirely intergrannular. The formation of pores is attributed to the release of large amount of gases during combustion process. The formation of multigrain agglomerates observed in all samples consists of very fine crystallites as they show strong tendency to form agglomerates [18]. The appearance of spongy structure with increasing G N−1 ratio was attested a better crystallinity of spinel phase. The minimum amount of fuel used in the case of the fuellean results in a small enthalpy and hence the local temperature of the particles remains low, which may prevent the formation of a dense structure. Associated gas evolution results in highly porous structure, i.e., as the amount of gas increases agglomerates are more likely to break up and more porosity will be observed as in case of higher G N−1 ratio. 3.6. Magnetic properties

Fig. 4. FT-IR spectra of V1, V2, V3, V4 and V5 samples at different G N−1 ratios: 0.48, 0.74, 1.48, 2.22 and 2.96 respectively.

The specific magnetization curves of the investigated samples, obtained from room temperature VSM measurements are shown in Fig. 6. These curves are typical for a soft magnetic material and indicate hysteresis loops [9]. From these measurements magnetization (Ms ), remenance (Mr ) and coercivity (Hc ) are derived and listed in Table 3. It can be seen that the magnetization of sample V5 is much smaller as compared to other samples. This is attributed to its poor crystallization and relatively small grain size at fuel rich combustion. Sample V1 has an average grain size of about 37 nm and magnetization about19 emu g−1 at room temperature. The magnetization of stoichiometric sample V3 is about 31.56 emu g−1 which is close to that of bulk MgFe2 O4 material

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Fig. 5. SEM images of (a) V1, (b) V2, (c) V3, (d) V4 and (e) V5 using different G N−1 ratios: 0.48, 0.74, 1.48, 2.22 and 2.96 respectively.

(approximately 30 emu g−1 ) [19]. It should also be noted that the Mr value (9.60 emu g−1 ) is also maximum for stoichiometric MgFe2 O4 . From Table 3 it is clear that the magnetization, remanance and coercivity increases with increase in G N−1 ratio attains maximum at stoichiometric condition then decrease further for higher glycine amount. The change in the magnetic properties may be attributed

to the low crystalline anisotropy, which arises from crystal imperfection and the high degree of aggregation. G N−1 ratio strongly influences, the maximum reaction temperature Tm : when ˚ (thus G N−1 ) increases, Tm first increases until (˚ = 1) stoichiometric condition is reached and then decreased in fuel-rich conditions because large amounts of gases (CO2 , H2 O, N2 ) are released which dissipates

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of gases evolved. From FT-IR analysis, the spinel structure, purity and formation of MgFe2 O4 was confirmed. Transmission electron microscopy image shows the formation of MgFe2 O4 nanocrystals with average particle size of about ∼40 nm which is in good agreement with the particle size calculated from XRD analysis. The microstructural analysis of products with different G N−1 ratio shows very pronounced effect on microstructure which is attributed to the effect of adiabatic flame temperature and number of moles of gases evolved during combustion. The magnetization for sample increases with increase in G N−1 ratio attains maximum value at stoichiometric condition and then decreases with further increase in G N−1 ratio. Thus glycine–nitrate process can be explored to obtain high quality pure and homogeneous MgFe2 O4 without subsequent heating treatment. Acknowledgments

Fig. 6. M–H measurements at room temperature for V1, V2, V3, V4 and V5 at different G N−1 ratios: 0.48, 0.74, 1.48, 2.22 and 2.96 respectively.

Authors are grateful to Board of Research in Nuclear Science and Department of Science and Technology India for their financial support. References

the heat of the process [19]. As a consequence, for the highest G N−1 values, the released energy was not sufficient to burn all the organic matter which in turn affects the magnetic properties of MgFe2 O4 . Also it has been proved that combustion method is able to induce the redistribution of cations along A and B sites. The variations in magnetic properties can also be attributed to the change in the distribution of Mg2+ and Fe3+ ions at A and B site of the spinel structure with increase in G N−1 ratio and surface structure disorder [3,15].

[1] [2] [3] [4] [5] [6] [7] [8] [9]

4. Conclusion The nanocrystalline MgFe2 O4 powder with average particle size of around 40 nm was successfully prepared by glycine nitrate synthesis with different G N−1 ratios. Thermodynamic considerations show that calculated values of heat absorbed by product, number of moles of gases evolved and adiabatic flame temperature increase with increase in G N−1 ratio. XRD result reveals that amount of glycine has no significant effect on formation of single phase MgFe2 O4 powder and fuel lean condition also leads to proper MgFe2 O4 phase formation. Slight variation in crystallite size and lattice parameter with different G N−1 ratios may be attributed to competition between adiabatic flame temperature and number

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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