Structural and magnetic characterization of La0.7Sr0.3MnO3 nanoparticles obtained by the citrate-gel combustion method: Effect of fuel to oxidizer ratio

Author’s Accepted Manuscript Structural and Magnetic Characterization of La0.7Sr0.3MnO3 Nanoparticles Obtained by the Citrate-Gel Combustion Method: Effect of Fuel to Oxidizer Ratio C.O. Ehi-Eromosele, B.I. Ita, E.E.J. Iweala, K.O. Ogunniran, J.A. Adekoya, F.E. Ehi-Eromosele www.elsevier.com

PII: DOI: Reference:

S0272-8842(15)01684-3 http://dx.doi.org/10.1016/j.ceramint.2015.08.158 CERI11255

To appear in: Ceramics International Received date: 29 April 2015 Revised date: 28 July 2015 Accepted date: 28 August 2015 Cite this article as: C.O. Ehi-Eromosele, B.I. Ita, E.E.J. Iweala, K.O. Ogunniran, J.A. Adekoya and F.E. Ehi-Eromosele, Structural and Magnetic Characterization of La0.7Sr0.3MnO3 Nanoparticles Obtained by the Citrate-Gel Combustion Method: Effect of Fuel to Oxidizer Ratio, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.08.158 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structural and Magnetic Characterization of La0.7Sr0.3MnO3 Nanoparticles Obtained by the Citrate-Gel Combustion Method: Effect of Fuel to Oxidizer Ratio Ehi-Eromosele, C.O.1*, Ita, B.I.1&2, Iweala, E.E.J.3, Ogunniran, K.O.1, Adekoya, J.A.1, EhiEromosele, F.E.4 1. Department of Chemistry, Covenant University, PMB 1023, Ota, Nigeria. 2. Department of Pure and Applied Chemistry, University of Calabar, Calabar, Nigeria. 3. Department of Biological Sciences, Covenant University, PMB 1023, Ota, Nigeria. 4. Department of Mechanical Engineering, University of Benin, Benin City, Nigeria.

*Corresponding Author E-mail and Phone: [email protected] +234-8039576084 Abstract The effect of fuel to oxidizer ratio on the processing of nano-crystalline La0.7Sr0.3MnO3 by the solution combustion technique is reported. The results show that the structural, morphological and magnetic properties of La0.7Sr0.3MnO3 could be controlled by using different combinations of citric acid fuel and metal nitrates ratio (C/N). Thermodynamic considerations of the combustion processes show that the exothermicity and the amount of gases released increases with increase in C/N. The post-annealed powders were characterized by Thermo GravimetricDifferential Thermal analysis (TG-DTA), X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-ray (EDAX) analysis and Vibrating Scanning Magnetometer (VSM) measurements. Only the fuel rich composition produced pure perovskite phase without any secondary phase. All samples had comparable crystallite sizes (≤ 1

37 nm). FESEM images of La0.7Sr0.3MnO3 showed that the C/N ratio had pronounced effect on the microstructure regarding shape and porosity. Room temperature magnetization measurements revealed unusually low saturation magnetization but near super-paramagnetic behaviour in all the samples. Keywords: La0.7Sr0.3MnO3, perovskite manganite, Combustion method, fuel to oxidizer ratio, citric acid fuel, magnetism. 1.0 Introduction Perovskite manganites have attracted much research interests during the past decade due to their colossal magnetoresistance (CMR) and potential applications in spintronics and solid oxide fuel cells [1-10]. These materials possess interesting physical properties of concurrent ferromagnetism and metallic conductivity in the intermediate composition [11]. Recently, perovskites like lanthanum strontium manganites (LSMO) have found interesting biomedical applications. LSMO nanoparticles have been studied for self controlled hyperthermia for cancer treatment [12-14]. The increasing attention on LSMO magnetic nanoparticles (MNPs) is due to their tunable Curie temperature (Tc), biocompatibility and superparamagnetic nature [14]. The La1−xSrxMnO3 compounds show wide range of ferromagnetic–paramagnetic transition temperature, Tc, from 283 to 370 K whereas Fe3O4 (ferrites) has Tc of ~823 K (550 °C) which is much greater than the therapeutic hyperthermia temperature [15]. Among CMR materials, La0.7Sr0.3MnO3 is a typical composition which is of interest in this context due to its high Tc of ~380 K and a large magnetic moment at room temperature [16, 17] making it to be a very promising material in room temperature applications. 2

The dependence of the physical and chemical properties of nanocrystalline materials on the shape and size of nanoparticles and the thermal history of sample preparation as well have always motivated the study of new synthetic routes to produce these materials. Various methods of synthesis such as solid state reaction [11, 18, 19], co-precipitation [19-21], sol-gel technique [19] and combustion methods [17, 19, 22-26] have been used for the synthesis of LSMO nanoparticles. The wet chemical methods of synthesis usually require careful control of pH of the solution, temperature and concentration like parameters for formation of particles; and in most cases, products obtained from them usually contain matrix components at the surfaces of particles which affect the structural and magnetic behavior of the material [25]. On the other hand, the solid state reaction route requires repeated grinding and very high sintering temperature for phase formation resulting in increase in particle size of the material. To overcome some of these challenges, low temperature combustion synthesis have been exploited. Combustion reaction is a vigorous exothermic redox reaction between a suitable fuel which also acts as a complexing agent and an oxidizer (i.e., corresponding metal nitrates). Combustion synthesis has the advantage of high temperatures, fast heating rates and short reaction times thus inhibiting particle size growth and promoting the formation of homogeneous, crystalline nanopowders. Nature of the fuel and fuel to oxidizer ratio play an important role in combustion synthesis since they can influence the morphology, phase and particulate properties of the final product [27]. Several fuels have been used in the combustion synthesis of LSMO, like oxalyl-dihydrazide [24], polyvinyl alcohol (PVA) [25, 26], urea [27, 29], glycine [25, 27, 29], etc. The fuels differ in their reducing power, the combustion temperature and the amount of gases they generate, which affects the characteristics of the reaction product [30]. Also, fuels having lower decomposition 3

temperature with evolution of larger amount of gases (CO2 and H2O) are important. These help to generate sufficient local heating for the completion of combustion reaction during the synthesis and at the same time creates materials of high porosity, good crystallinity and also prevents particle agglomeration [31, 32]. Thorat et al. [25] investigated the importance of synthesis technique and fuel choice (glycine and PVA) on the combustion synthesis of La0.7Sr0.3MnO3, and showed that PVA was a more efficient fuel for synthesis of superparamagnetic La0.7Sr0.3MnO3 nanoparticles for biomedical applications. Citric acid has a lower decomposition temperature (175oC) as compared to glycine (262oC) and PVA (228oC). Citric acid is a polyhydroxy carboxylic acid with three carboxylic acid groups and one hydroxyl group and hence could be a very good complexing agent, in addition to acting as a fuel for combustion [33]. Moreover, citrate-nitrate process is a much simpler, cheaper and safer process than glycine-nitrate process and has recently been established as a novel technique to prepare ceramic oxides with diverse properties [34]. Although coated La0.7Sr0.3MnO3 nanoparticles have been synthesized using the citrate-gel combustion route [17, 22, 23, 35], there is hardly any information available on the effect of reactant composition on the properties of the final product. Hence, the objective of this work is to study the effect of using citric acid fuel in solution combustion synthesis (SCS) on the phase formation, structural, morphological and magnetic properties of La0.7Sr0.3MnO3 nanoparticles. As the thermochemistry of SCS and the particle size are controlled by the fuel to oxidizer ratio, the product characteristics have also been studied in this context.

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2.0 Experimental 2.1 Materials Analytical grade La(NO3)3.6H2O (99.9% purity from Alfar Aesar, USA), Sr(NO3)2 (99 % purity) and Mn(NO3)2.6H2O (99.99% purity) obtained from Sigma Aldrich were used as the oxidants while citric acid monohydrate (99.5 % purity) obtained from Merck Ltd., Mumbai, India was used as the fuel (propellant) for the combustion process.

2.2 Experimental Method It is well known that the properties of synthesized powders obtained by combustion reaction are mostly influenced by the fuel to oxidant equivalent ratio. The value of this ratio is calculated by taking the ratio of the total oxidizing (metal nitrates) and reducing (citric acid) valencies. In the present work, citric acid to nitrate (C/N) molar ratio has been varied as 0.33, 0.66 and 0.99 to obtain fuel lean, stoichiometry and fuel rich conditions, respectively. For fuel stoichiometric composition sample (C/N = 0.66), 3.03 g La(NO3)3.6H2O, 0.64 g Sr(NO3)2, 2.51 g Mn(NO3)2.4H2O and 2.74 g citric acid monohydrate were dissolved in 20 ml of distilled water and the solutions were heated to 80oC to form a viscuous gel of precursors under magnetic stirring. After that, the gel was transferred to a hot plate pre-heated to 300oC. Finally, after a short moment, the solution precursors boiled, swelled, evolved a large amount of gases and ignited, followed by the yielding of puffy black products. The auto combusted powder was annealed at 800oC for 5 hours in air and used for further characterization. Similarly, for fuel lean/oxidizer rich sample (C/N = 0.33) and fuel rich/oxidizer lean sample (C/N = 0.99), same

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procedures were followed except that 1.37 g and 4.11 g of citric acid monohydrate were used, respectively. 2.3 Characterisation Methods The precursor gel was characterized by TG-DTA by means of STA 409 PC Luxx simultaneous DSC-TG-DTA instrument from NETZSCH-Geratebau Germany at a temperature range of 301000oC in air atmosphere with a heating rate of 10oC/min. The X-ray diffractograms of the autocombustion and annealed powders were recorded using an X-ray diffractometer (D8 Advance, Brucker, Germany), equipped with a Cu Kα radiation source (λ = 1.5406 A˚) and the crystallite size (D) is calculated from X-ray line broadening of the (110) diffraction peak using the wellknown Scherrer relation. D

0.9  Cos

(1)

where β is the full width at half maxima of the strongest intensity diffraction peak (311), λ is the wavelength of the radiation and θ is the angle of the strongest characteristic peak. The surface morphology was examined with a field emission-scanning electron microscope (FE-SEM) using FEI NOVA NANO SEM 600. The magnetic characterizations were carried out with a Vibrating Scanning Magnetometer (Lake Shore cryotronics-7400 series) under the applied field of ±20,000 G at room temperature.

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3.0 Results and Discussion 3.1 Combustion Reaction and Thermodynamic Analysis The theoretical stoichiometric equations for the formation of La0.7Sr0.3MnO3 nanoparticles can be written as follows:

0.7La(NO3)3.6H2O + 0.3Sr(NO3)2 + Mn(NO3)2.4H2O + 1.31øC6H8O7 + (9.99ø − 10)O2 → La0.7Sr0.3MnO3 + 7.86øCO2↑ + (8.2 + 5.24ø)H2O↑ + 2.35N2↑

(2)

where ø is the multiplication factor in order to get fuel lean (< 1), and fuel rich condition (> 1). In the reaction above (Eq. 2), ø = 1 represents C/N = 1.31/2 = 0.66 and it is the stoichiometric condition (i.e., the ratio at which oxygen content of oxidizer can be reacted to consume fuel entirely and no heat exchange is required for the complete reaction). The fuel lean/oxidizer rich condition is when the quantity of oxygen in the combustible mixture is in excess of that required for the complete combustion of the fuel and so a portion of the oxygen does not participate in the reaction. While the fuel rich/oxidizer lean condition is when the quantity of oxygen in the combustible mixture is lower than that required for complete combustion of the fuel and so, a portion of the atmospheric oxygen would be needed for completion of the reaction [36].

The precursor solution of fuel stoichiometric and fuel lean samples were colourless and the fuel rich sample was light yellow. In all samples, after heating the precursor solutions at 80oC, the resulting gels turned yellow and after combustion process turned black powder which even became darker after annealing at 800oC for 5 hrs. The combustion type for fuel stoichiometric 7

and fuel rich samples were flamy combustion while fuel lean was smouldering combustion with lots of yellow fumes given off. In fuel lean sample, there are more oxidants than reductants, i.e., the system is set as an over-oxidising state hence, there is not enough fuel to completely combust the excess oxidants. This might also account for the lesser combusted powder volume for fuel lean as compared with fuel rich and fuel stoichiometric samples. The combustion process with citric acid did not result in much foaming as seen with glycine and urea.

For the different fuel to oxidizer combinations, there will be a different mechanism of the combustion reaction [28]. The theoretical calculations based on thermodynamic consideration such as enthalpy of reaction and flame temperature helps in estimation of exact ignition condition and to predict the exothermicity of the different fuel to oxidizer combinations used. Enthalpy of reaction depends on the heat of formation of products and reactants [37]. The following equation is used to calculate the enthalpy of reaction:

     n f 

products

   n f 

(3)

reac tants

where n is the number of moles,  f is heat of formation and   is enthalpy of reaction. The thermodynamic data [38] for the various reactants and products involved in the combustion reaction are given in Table 1. The enthalpy of formation for La0.7Sr0.3MnO3 was calculated from the components’ binary oxides according to the following reaction [39]:

0.35La2O3(s) + 0.3SrO(s) + 0.5Mn2O3(s) + 0.075O2(g) → La0.7Sr0.3MnO3(s)

8

(4)

The enthalpy of reaction for the formation of La0.7Sr0.3MnO3 using the fuel rich, stoichiometric and fuel lean compositions can be calculated by using the thermodynamic data (Table 1) and Eqs. 2 and 3. The enthalpy of reaction for the formation of La0.7Sr0.3MnO3 using the fuel rich, stoichiometric and fuel lean compositions were −1217.73 kcal mol-1, −938.39 kcal mol-1 and −659.05 kcal mol-1, respectively. The results show that the fuel rich composition had the highest exothermicity while the fuel lean composition had the lowest. The values for enthalpy of reaction for the formation of La0.7Sr0.3MnO3 are consistent with the conditions of the combustion process. The amount of gases released during the combustion process was also seen to decrease with decrease in citric acid to nitrate ratio. Exothermic chemical reaction was observed with all the fuel compositions resulting in the release of heat to raise the temperature of the products. The results imply that the enthalpy of combustion is significant for all fuel compositions and the citric acid to nitrate ratio plays a role in the characteristics of the final product.

3.2 Thermal Analysis The thermal behavior of the precursor gels of the various fuel compositions were investigated by TG-DTA measurements. The simultaneous TG-DTA curves of the precursor gels for fuel rich, stoichiometric and fuel lean samples are shown in Fig. 1. TG curves of all samples show a weight loss of about 40% below 200oC which is due to complete evaporation of water and organic content in the precursor gel. The sudden weight loss (about 26% for fuel rich, 30% for stoichiometric and 20% for fuel lean) were observed at about 190oC and it is attributed to the rapid chemical reaction between the metal nitrates and citric acid. In the stoichiometric sample (Fig. 1b), no weight loss is observed after the combustion reaction (at about 190 oC) giving a 9

product free of residual reactants and carbonaceous product. This is understandable by recalling that the stoichiometric sample was prepared by calculating the ratio at which oxygen content of oxidizer can be reacted to consume fuel entirely and no heat exchange is required for the complete reaction. It is not so with the fuel rich sample (Fig. 1a) which was calculated to have an excess of fuel, hence the thermogram shows a decomposition of the excess fuel (about 7%) after the combustion reaction had taken place at about 190oC. Also, the fuel lean sample (Fig. 1c) which was calculated to have a minimum amount of fuel (i.e., more oxidants than reductants) recorded the least weight loss (20%) during the combustion reaction as a result of the insufficient fuel to burn the metal nitrates hence, the thermogram shows a decomposition of the metal nitrates after the combustion reaction had taken place at about 190oC. The DTA curves complements the weight loss regime reported in the TG. The endothermic peaks at about 130oC were attributed to complete evaporation of water and organic content in the precursor gel while the sharp exothermic peaks observed in the DTA curves (Fig. 1a-c) around 190oC were attributed to the ignition of nitrate-citrate precursors dried gel at this temperature. Only fuel rich sample show a second exothermic peak at about 300oC corresponding to the decomposition of the excess fuel (about 7%) after the combustion reaction. These results show that the citric acid to nitrate ratio influences the thermal effects supporting the results obtained from the thermodynamic considerations of the combustion process. 3.3 Structural and Phase Analysis Fig. 2a-c shows the X-ray diffraction patterns of the fuel rich, stoichiometric and fuel lean La0.7Sr0.3MnO3 annealed (done at 800ºC for 5 hrs) samples, respectively. The obtained peaks are well matched to the rhombohedral perovskite structure (R-3c (167) space group) with standard 10

JCPDS card no. 00-051-0409. The matched lattice parameters in all the samples are a = b = 5.4905 Å, and c = 13.3077 Å. The diffractogram of the fuel rich sample (Fig. 2a) exhibited pure perovskite phase and better intense peaks than the other two which indicated that the sample had higher crystallinity. The stoichiometric and fuel lean sample had two secondary phases indexed to SrMnO3 (JCPDS 241221) showing that the annealing (done at 800oC for 5 hrs) was not enough to crystallize the perovskite phase. The presence of the excess fuel in the fuel rich sample that recorded the highest adiabatic flame temperature (maximum combustion temperature) might have aided the crystallization of a pure perovskite phase. The results show that the excess fuel provides sufficient reaction temperature and help in the formation of the nuclei of La0.7Sr0.3MnO3 with further annealing resulting in the removal of all impurities and development of a pure LSMO phase. The average crystallite size calculated from the Scherrer’s equation (Eq. 1) for peak (110) obtained for fuel rich, stoichiometric and fuel lean samples are 36 nm, 35 nm and 37nm, respectively. The XRD patterns of all the samples show that the reflection peaks are quite broad, indicating their nanocrystallinity. 3.4 Morphological and Chemical Composition Analysis The surface morphologies of the samples annealed at 800oC for 5 hrs were analysed by FE-SEM and the images of fuel rich, stoichiometric and fuel lean LSMO powders corresponding to different C/N ratio are shown in Fig. 3a-c, respectively. Obtained images show remarkable changes in the morphology regarding porosity and shape of the samples. The fuel rich sample (Fig. 3a) shows that most of the grains are spherical in shape with fairly uniform distribution, the stoichiometric sample show a pseudo-spherical shape while the fuel lean sample shows different shapes. These differences in the shapes might be due to the crystallinity of the LSMO phase (see 11

XRD results) which decreased with decrease in fuel to oxidizer ratio. The formation of multigrain agglomerates observed in all samples consists of very fine crystallites as they show tendency to form agglomerates. Also, all the samples showed structures having voids with the fuel lean sample having the least voids. The appearance of voids might be due to escaping large number of gases during combustion which is minimum in fuel lean. EDAX analysis was used to investigate the quantitative chemical composition for the constituent elements of La0.7Sr0.3MnO3 synthesised using different citric acid to nitrate ratio. The EDAX spectra for fuel rich, stoichiometric and fuel lean samples are in Figs. 4a-c, respectively. The corresponding peaks are due to La, Sr, Mn, and O elements with no additional impurity peak indicating the purity of the prepared samples. An analysis of the compositions of the samples shows a slight difference in the amounts of the elements. However, the EDAX spectra indicate that all the samples are consistent with their elemental signals and stoichiometry close to the nominal composition. The quantitative analyses of all samples gave an atomic ratio of (La, Sr) : Mn ≈ 1 (Table 3). It can also be seen that this atomic ratio decreased with increase in the citric acid to nitrate ratio. This might be due to the loss of elements with increased exothermicity as the citric acid to nitrate ratio increased. Also, the oxygen over-stoichiometry of all samples increased with increase in the citric acid to nitrate ratio with the fuel rich sample recording the highest and the fuel lean the lowest. 3.5 Magnetic Study The specific magnetization curves of the La0.7Sr0.3MnO3 samples obtained from room temperature VSM measurements are shown in Fig. 5. The magnetic properties of the La0.7Sr0.3MnO3 powders are given in Table 4. The ferromagnetism seen in LSMO comes from 12

the hole-doping of the parent perovskite manganite (LaMnO3) by the replacement of La with Sr. The substitution of lanthanum with a divalent metal ion (Sr2+) at the A-site creates the mixed valencies of the manganese ions (Mn3+ − Mn4+) at the B-site and significantly increases the electrical conductivity and magnetization of the compound due to double exchange interactions between pairs of Mn3+ and Mn4+ ions through an oxygen atom [40]. Fig. 5a-c shows curves that are typical of a soft magnetic material and indicate hysteresis loops. From these measurements, saturation magnetisation, remanence magnetisation, coercivity and loop squareness ratio were derived and listed in Table 4. All samples recorded unusually low saturation magnetization (9-14 emu/g) compared with the reported values [17, 22, 25]; even though the applied magnetic field at 20, 000 G was obviously insufficient to saturate all the magnetic moments in the sample. It is a well known fact that the ferromagnetic order in LSMO is dominated by the double exchange interaction between Mn3+ and Mn4+ ions. If the Mn4+/Mn3+ ratio decreases, it will cause a reduced ferromagnetic moment [41]. It has also been suggested that there is an enhancement of magnetic moment of annealed LSMO due to oxygen incorporation [42]. It is possible that the oxygen over-stoichiometry of the samples seen from EDAX analysis might be due to oxygen from extraneous sources like the sample holder and not from oxygen incorporated in the samples. The remanence magnetization and coercivity were very low for all samples suggesting the near superparamagnetic nature of the samples. Transformation from multidomain behaviour (ferromagnetic) to a single domain (superparamagnetic) occurs at certain radius of the particle called the critical radius. The typical values for the critical radius for LSMO is about 40 nm [25], thus all the La0.7Sr0.3MnO3 magnetic nanoparticles are below the critical size (i.e. single domain). It can also be seen from the hysteresis curve that fuel to oxidizer ratio influenced not only the 13

morphology but also the magnetic properties of La0.7Sr0.3MnO3 as different C/N ratio gave different values of the magnetic properties.

4.0 Conclusion The effect of C/N molar ratio on the phase stability, microstructure, size and magnetic properties of La0.7Sr0.3MnO3 nanoparticles was demonstrated using the citrate-gel auto-combustion route. Like glycine and PVA, the use of citric acid as a fuel was found to produce nanocrystalline purephase LSMO although a secondary phase appeared in both stoichiometric and fuel lean samples. All samples had comparable crystallite sizes signaling that the C/N molar ratio had no clear effect on it but was seen to affect the microstructure of the samples. A thermodynamic consideration of the combustion process shows that when C/N molar ratio increases, the amount of gas produced and adiabatic flame temperature also increased. FESEM images of the fuel rich sample showed that most of the grains are spherical in shape with fairly uniform distribution, the stoichiometric sample showed a pseudo-spherical shape while the fuel lean sample showed different shapes. EDAX analysis confirms the purity of the prepared samples and an analysis of the compositions of the samples shows a slight difference in the amounts of the constituent elements which varied with the exothermicity of the combustion process. The room temperature magnetic measurements revealed unusually low saturation magnetization but near superparamagnetic behaviour in all the samples.

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Acknowledgements This work would not have been possible without the visiting research grant given to Mr. EhiEromosele C.O. by the International Centre for Materials Science, Jawarharlal Nehru Centre for Advanced Scientific Research, Bangalore, India. The corresponding author would also want to thank Dr. Padaikathan N.P. of the Department of Mechanical Engineering, IISc, India for helping with the TG-DTA measurements.

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Table 1: Standard thermodynamic data for the reactants and products of the combustion reaction [38]

La(NO3)3.6H2O (s)

Heat of formation,  f (kcal/mol) −299.81

Mn(NO3)2.4H2O (s)

−137.74

Sr(NO3)2 (s)

−233.80

C6H8O7 (s)

−368.98

H2O (g)

−57.79

CO2 (g)

−94.05

N2 (g)

0

O2 (g)

0

La0.7Sr0.3MnO3 (s)

−323.57a

Compound

All values considered at standard temperature T= 298 K. a Value from Laberty et al. [39]

21

Table 2: Effect of citric acid to nitrate (fuel to oxidizer ratio) on adiabatic flame temperature (Tad), heat absorbed (Q) by product and number of moles of gases evolved during combustion Sample

G/N

  , (kcal mol-1)

Amount of gases produced during combustion (moles)

Fuel rich

0.99

-1217.73

30.2

Stoichiometric

0.66

-938.39

23.65

Fuel lean

0.33

-659.05

17.1

Table 3: % concentration of the constituent elements of La0.7Sr0.3MnO3 system by EDAX Sample

La

Sr

Mn

O

Fuel rich

13.80

5.86

19.80

60.54

Stoichiometric

13.92

5.94

19.93

60.21

Fuel lean

13.94

5.96

19.94

60.16

Table 4: Magnetic Properties of La0.7Sr0.3MnO3 Powders Sample

Saturation Magnetisation, Ms (emu/g)

stoichiometric Fuel rich Fuel lean

14 11 9

Remanence Magnetisation, Mr (emu/g) 3.0 4.20 1.17

22

Coercivity, Hc (Gauss)

29 34 29

Loop squareness ratio, Mr/ Ms 0.21 0.38 0.13

23

(128)

(208)

(214)

(122)

(024)

(x) - SrMnO3

(202)

(012)

Intensity (a.u.)

(110)

Fig. 1: TG-DTA curves of the La0.7Sr0.3MnO3 precursor gels samples: (a) fuel rich (b) stoichiometric (c) fuel lean

(x)

(x)

(a)

(b)

(c) 20

30

40

50

60

70

80

2 Theta (degrees)

Fig. 2: X-ray diffraction patterns of La0.7Sr0.3MnO3 annealed powders: (a) fuel rich sample (b) stoichiometric sample and (c) fuel lean sample

24

Fig. 3: FE-SEM images of La0.7Sr0.3MnO3 samples annealed at 800ºC for 5 hrs: (a) fuel rich (b) stoichiometric (c) fuel lean

25

Fig. 4: EDAX spectra of La0.7Sr0.3MnO3: (a) fuel rich (b) stoichiometric (c) fuel lean

26

Magnetisation (emu/g)

20

(a) (b) (c)

10

0 -20000

-10000

0

10000

20000

Field(G) -10

-20

Fig. 5: Magnetic hysteresis curves of La0.7Sr0.3MnO3 powders measured at room temperature for (a) the stoichiometric sample (b) the fuel rich sample (c) the fuel lean sample

27