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Chapter 4
Synthesis of LSMO
Important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. - Sir William Bragg
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 64
‘Nano… Big Events happen in small worlds…’ Richard Feynman
4.1. Introduction
The main aspect for the successful use of nanoparticles (NPs) in biomedical field is to
make them monodispersive and biocompatible. This problem is solved at earlier stage of the
time of particles preparation. The suitable method which produces narrow size and
biocompatible nanoparticles is highly desirable. As described in Chapter 1, the solution
combustion synthesis is employed for the synthesis of LSMO nanoparticles. The combustion
methodology of LSMO nanoparticles still requires refinement, including the ability to control
many aspects, including suitable fuel choice, colloidal monodispersity, nanoparticle size
tuning, cohesive particle sizing and anisotropy. In this chapter, the development of an
optimized combustion method to LSMO nanoparticles is presented. Notably the particles
made from this study were later shown to be the best ever particles to date for magnetic
hyperthermia.
4.2. Combustion technique
Combustion synthesis (CS) is becoming one of the most popular methods for the
preparation of a variety of materials, ranging from non-oxides, such as borides, nitrides,
carbides, etc., to simple and complex oxides [1]. Combustion synthesis processes are
characterized by high- temperatures, short reaction times and fast heating rates. These
characteristic features make CS an attractive method for the manufacture of technologically
useful materials at lower costs compared to conventional ceramic processes [2]. Some other
advantages of CS are
(i) Use of relatively simple equipment
(ii) Formation of high-purity products
(iii) Stabilization of metastable phases
(iv) Formation of virtually any size ( micro to nano) and shape (spherical to Hexagonal)
products
(v) Uniform distribution of dopants takes place throught the host material due to the atomic
mixing of the reactants in the initial solution.
(vi) This process not only give ups nanosize oxide materials but also allows uniform
(homogeneous) doping of trace amounts of rare-earth impurity ions in a single step.
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 65
Depending upon the nature of reactants: elements or compounds (solid, liquid or gas);
type and the exothermicity (adiabatic temperature, T), CS is described as: self-propagating
high temperature synthesis (SHS); low-temperature combustion synthesis (LCS), solution
combustion synthesis (SCS), gel-combustion, sol–gel combustion, emulsion combustion,
volume combustion (thermal explosion), etc.
The solution combustion synthesis (SC) method of preparing oxide materials is a
fairly recent development compared to SSC or SHS techniques described above. Today, SCS
is being used all over the world to prepare oxide materials for a variety of applications.
During the short span (15 years) of SCS synthesis history, hundreds of papers on SCS of
oxides have been published.
An aqueous solution of a redox system constituted by the nitrate ions of the metal
precursor, acting as oxidizer, and a fuel like glycine, urea, citric acid or many others, is
heated up to reasonable temperatures and, leading dehydration, the strongly exothermic
redox reaction occurs, which is generally self-sustaining and provides the energy for the
formation of the oxide. All these fuels serve two purposes: (a) they are the source of C and H,
which on combustion form CO2 and H2O and liberate heat; (b) these fuels form complexes
with the metal ions facilitating homogeneous mixing of the cations in solution.
Oxide materials produced with this method include several ferrites and spinels, tin
oxide and antimony tin oxide (ATO), ceria, ferroelectric materials, iron oxide, zinc oxide,
protonic conductors and various solid solutions.
To understand the highly exothermic nature of this reaction, concepts used in
propellant chemistry were employed [3]. A solid propellant contains an oxidizer like
ammonium per chlorate and a fuel like carboxy terminated polybutadiene together with
aluminum powder and some additives. The specific impulse (Isp) of a propellant, which is a
measure of energy released during combustion, is specified by the ratio of thrust produced
per pound of the propellant. It is expressed as
MolecularWt. of gaseous products
TcIsp k (4.1)
The highest heat Tc (chamber temperature in the rocket motor) is produced when the
equivalence ratio (φe = oxidizer/fuel ratio) is unity.
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 66
φe =
(Coefficient of oxidizing elements in
specific formula) × (Valency)
(Coefficient of reducing elements in( 1)
specific formula) × (Valency)
(4.2)
A mixture is said to be stoichiometric when φe = 1, fuel lean when φe >1, and fuel
rich when φe < 1. Stoichiometric mixtures produce maximum energy. The oxidizer/fuel
molar ratio (O/F) required for a stoichiometric mixture (φe = 1) is determined by summing
the total oxidizing and reducing valencies in the oxidizer compounds and dividing it by the
sum of the total oxidizing and reducing valencies in the fuel compounds. In this sort of
calculation oxygen is the only oxidizing element; metal cations, hydrogen and carbon are
reducing elements and nitrogen is neutral. Reducing elements have negative valencies and
oxidizing elements have positive valencies. In solution combustion calculations, the valency
of the oxidizing and reducing elements are considered similar to the oxidation number
concept familiar to chemists.
4.3. Experimental
In the present study the solution combustion technique by using glycine and polyvinyl
alcohol (PVA) has been employed for the synthesis of LSMO NPs. Short process duration
and the formation of various gases during combustion inhibit particle size growth and favor
synthesis of nano-size powders with high specific surface area. To achieve this, choice of
organic fuel having lower decomposition temperature with evolution of gases (CO2, H2O) is
important. This helps for generating sufficient local heating to system fuel for the completion
of combustion synthesis and at the same time creates porosity within the mixture and also
prevents particle agglomeration.
The perovskite type La0.7Sr0.3MnO3 (LSMO) NPs have been prepared with the
solution combustion method by using two different fuels such as glycine and polyvinyl
alcohol. The representation of the process is graphically shown in the figure 4.1. The nitrates
are used as starting materials which decompose at lower temperature (400-500 ºC) and also
act as oxidants in reaction. The stoichiometric amounts of the nitrate precursors La
(NO3)3.6H2O, Sr (NO3)2 and Mn (NO3)2.4H2O of analytical grade were dissolved in double
distilled water to form the solution of 0.1 M. The equimolar solution of glycine and PVA
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 67
were prepared in double distilled water. The mixture of oxidants and fuels kept onto a
magnetic stirrer for 0.5 h at 100 ºC to get uniform mixing and evaporation of water to form
gel of precursors and then the gel was kept onto a heating coil (≈ 300 ºC) for the burning
process. During the heating process the ignition takes place, followed by the combustion of
the reactants mixture with the appearance of a high speed propagating flame. The high
temperature reached within the raw material mixture led to the formation of dried fluffy foam
of La0.7Sr0.3MnO3. Assigning the +4, +1, +3, +2 and +2 valencies to the C, H, La3+, Sr2+, and
Mn2+, reducing elements, respectively, the -2 valency to O2- oxidizer and considering
nitrogen with the valence 0, 4 then the φe is calculated according to the equation 4.2.
Figure 4.1. Flow chart of the preparation of La0.7Sr0.3MnO3 (LSMO) nanoparticles by
solution combustion technique using glycine and PVA fuel.
The combustion reaction mostly influenced by oxidant to fuel equivalent ratio
denoted by Φe (O/F). The oxygen content of oxidizer can be completely reacted to oxidize or
consume fuel and no more heat exchange is required for the reaction time. The value of Φe is
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 68
calculated by taking ratio of the total oxidizing and reducing valencies. When Φe becomes
unity, it serves maximum heat release at the time of combustion. This kind of combustion is
called stoichiometric combustion and is expected to yield desired phase.
Table 4.1. Different properties of fuels used for solution combustion synthesis
Table 4.2. Thermodynamic data of reactants and products involved in combustion reaction.
Enthalpy ΔfJ.mol-1)
Gibbs free energy
Δ GfJ.mol-1)
Entropy SJ.deg-1.mol-1)
Specific heat CpJ.deg-1.mol-1)
Mn(No3)2 -576.26 -451.0 218.0 -121.0
La(No3)3 -1254.4 ----- ----- ------
Sr(No3)2 -978.22 -780.0 194.56 149.87
N2 0 0 191.609 29.124
H2O -241.826 -228.61 188.835 33.60
Co2 -393.51 394.41 213.785 37.13
Glycine -528.5 -368.6 103.5 99.2
PVA -119.4 ----- ------ 74.82
According to propellant chemistry for getting the same oxidation and reduction
valencies in the solution, 2.29 mole % solution of PVA and 2.54 mole % solution of glycine
is required. Glycine is inexpensive and produced combustion heat (−3.24 Kcal/g) which is
more negative as compared to urea (−2.98 Kcal/g) and citric acid (−2.76 Kcal/g). Hence,
glycine is used as a fuel for synthesis of La0.7Ca0.3MnO3 powder. On the other side, polymer
such as PVA has not been exploited as a fuel in combustion technique. However, very few
authors have reported combustion technique with PVA for preparation of ZnO
nanoparticles [4]. Heat source induces polymer bond fission which results into increment in
the temperature of polymeric material. The volatile fraction of the resulting polymer
Properties Organic fuel components Glycine PVA
Structural formula H2N -CH2- COOH (CH2CHOH)n
Molecular weight (g/mol) 75.0 44.05 (monomer)
Heat of combustion (KJ/g) 13.0 25.056
Decomposition temperature(ºC) 262 228
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 69
fragments diffuses into the air and creates a combustible gaseous mixture (also called fuel).
This gaseous mixture ignites when the auto-ignition temperature is reached and liberates heat
which induces formation of fine nanoparticles. The prepared LSMO powder by combustion
technique is annealed at 800 ºC for 5h and used for further characterization. The plausible
chemical reactions for glycine and PVA fuel assuming complete combustion can be
represented by Equations (4.3) and (4.4), respectively.
0.7La3+ (NO3)33-
+ 0.3Sr2+ (NO3)23-
+ Mn2+ (NO3)22-
+ 2.54 (NH2-CH2-COOH) + 1.33O2
La0.7Sr0.3MnO3 + 6.35H2O + 3.62N2 + 5.08CO2 (4.3)
0.7La3+ (NO3)33-
+ 0.3Sr2+ (NO3)23-
+ Mn2+ (NO3)22-
+ 2.29 (-C2H4O) + 1.5O2
La0.7Sr0.3MnO3+4.58H2O + 2.35N2 + 4.58CO2 (4.4)
4.4.Results and discussions
4.4.1. Thermogravimetric Analysis
The thermo-gravimetric analysis (TGA) of as synthesized LSMO powder was carried
out for phase confirmation of LSMO from room temperature to 1000 ºC in air atmosphere.
The TGA graphs of as synthesized LSMO are shown in the figure 4.2 (a) and (b). The TG
curve of both the samples shows the initial weight loss due to removal of atmospheric water
content. The second weight loss is attributed to the decomposition of unburnt carbon content
which starts from 200 ºC and ends at 550 ºC for both samples. Above the 600 ºC there is
no weight loss in the material for both samples, which implies that the material goes into the
desired stable phase.
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 70
Figure 4.2. Thermogravimetric analysis of La0.7Sr0.3MnO3 synthesized by combustion
technique (a) by using glycine and (b) by using PVA.
LSMO prepared with glycine fuel shows 13.64 % weight loss due to carbon residue,
while LSMO prepared with PVA fuel shows 17.04%. The higher decomposition temperature
of glycine burns the maximum carbon content during the combustion which implies lower
carbon content in the powder observed from TG analysis. From the figure 4.2 we conclude
that the stable phase formation for LSMO occurs above the 600 ºC. For the further
characterization the LSMO prepared by combustion technique with glycine and PVA fuels is
heated at 800 ºC for 5h.
4.4.2. X-Ray diffraction study
Figure 4.3 shows the X-ray diffraction patterns of the LSMO samples prepared by
using glycine and PVA as fuels. All the reflection peaks are indexed with JCPDS card
(reference code: 00-051-0409) and showing pseudo-cubic perovskite structure (spacegroup
R-3c). The calculated lattice parameters in both samples are a = 5.4907 Å, and c = 13.324 Å.
The study also shows that the glycine and PVA fuels provide sufficient reaction temperature
and help into the formation nuclei of LSMO. Further, the post annealing temperature (800
ºC for 5 h) removes all the impurities and develop pure phase LSMO.
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 71
Figure 4.3. XRD patterns of LSMO prepared by solution combustion technique with
glycine and PVA as fuels and annealed at 800 ºC for 5 h.
The Gaussian fit of the most intense peak (110) was used to calculate the full width at
half maxima for determination of crystallite size (D) by equation D = 0.9λ/βcosθ, where λ =
1.5405Å wavelength of incident X-ray, θ is the corresponding Bragg’s diffraction angle and
β is full width at half maxima of the (110) peak. The average crystallite size obtained by
above equation is about 25 and 20 nm for LSMO prepared by glycine and PVA,
respectively. From the XRD patterns of both samples it is seen that the reflection peaks are
quite broad, suggesting their nanocrystallinity.
4.4.3. FT-IR analysis
The FTIR spectra of LSMO powder annealed at 800 oC for 5h and are shown in the
figure 4.4. The characteristic band around 600 cm-1 observed in both samples corresponding
to Mn-O. This indicates both samples strongly contain the metal-oxygen bonds which
involves the internal motion of a change in Mn-O-Mn bond length. The stretching mode is
associated with the change of Mn-O-Mn bond length while the bending mode involves the
change of Mn-O-Mn bond angle. The peaks at 1630 and 3440 cm-1 are due to surface-
adsorbed water on the particle of LSMO.
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 72
Figure 4.4. FTIR spectra of LSMO prepared by using glycine and PVA.
4.4.4. Morphological and elemental analysis
Figure 4.5. FE-SEM images of LSMO samples prepared by (a) glycine and (b) PVA.
The surface morphologies of the samples were analyzed by FE-SEM and
corresponding images for two different samples are shown in the figure 4.5 (a and b). From
the images one can observe that most of the grains are spherical in shape with unvarying
distribution. It is further observed that the grain size in sample prepared by glycine is
distributed from 50-60 nm, whereas sample prepared by PVA is in the range of 40-50 nm.
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 73
Figure 4.6. EDAX spectra of LSMO nanoparticles prepared by (a) glycine and (b) PVA.
The EDAX spectra were used for quantitative elemental analysis and composition of
the LSMO prepared by using glycine and PVA fuels (Figure 4.6(a) and (b)). Spectra indicate
that both samples are consistent with their elemental signals and stoichiometry as expected.
The corresponding peaks are due to the La, Sr, Mn and O elements, whereas not any
additional impurity peak is observed and it implies that the prepared samples are pure in
nature. The detailed analysis of both samples shows the atomic weight ratio of (La, Sr): Mn ≈
1.0 and suggests the obtained LSMO samples are stoichiometric. The observed atomic
percentage from EDAX is presented in the table 4.3.
Table 4.3. Elemental compositions of La, Sr, Mn and O atoms evaluated by using
EDAX Technique.
Element LSMO by Glycine Mass% At%
LSMO by PVA Mass% At%
O 3.13 20.74 4.23 20.27
Mn 30.98 39.33 29.24 40.85
Sr 7.28 6.57 6.53 5.72
La 58.61 33.36 60.00 33.15 Total 100.00 100.00 100.00 100.00
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 74
4.4.5. TEM and DLS studies
Figure 4.7. TEM images of LSMO nanoparticles prepared by solution combustion
synthesis (a) glycine (b) PVA. (Inset: there corresponding SAED pattern).
Figure 4.8. DLS histogram of LSMO nanoparticles prepared by solution
combustion synthesis (a) glycine and (b) PVA.
The TEM micrographs of LSMO nanoparticles prepared by solution combustion
synthesis by using glycine and PVA fuel are shown in figure 4.7 (a) and (b), respectively.
The average particle size of LSMO by using glycine as a fuel is of the order of 30 nm while
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 75
by using PVA fuel is about 20 nm. The smaller particle size of LSMO synthesized by PVA
fuel is attributed to lower decomposition temperature of PVA compared to glycine which
controls the combustion flame temperature. At the same time, it creates the porosity in the
powder to prevent agglomeration. In both samples the particle size distribution is almost
homogeneous. The particle diameters are slightly larger than the observed crystal sizes
obtained from XRD, due to the presence of noncrystalline surface layers as well as high
temperature calcinations (800 ºC) which causes the grain growth and it results into increasing
the particle size that is not determined by XRD. The corresponding selected area electron
diffraction (SAED) patterns (inset in the figure 4.7 (a) and (b)) show bright rings, indicating
the polycrystalline nature of the LSMO MNPs.
The hydrodynamic diameter of nanoparticles was finally determined by DLS (Figure
4.8 (a, b)). The main peaks were centered on 20-25 nm for sample prepared with PVA and
30-40 nm for sample prepared by glycine, which are consistent with TEM results. Moreover,
one should observe the absence of large aggregates in the sample prepared by PVA.
4.4.6. Magnetization study
Figure 4.9. (a) M-T measurements of LSMO nanoparticles prepared with glycine and
PVA as fuels, respectively at 500 Oe. (b) dM/dt curves obtained from
M-T measurement of both samples from which Curie temperature is
determined.
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 76
Figure 4.9 (a) shows the variation of magnetization M as a function of temperature
(T) of LSMO prepared with glycine and PVA in the range 5 to 375 K in an external magnetic
field of 500 Oe recorded in zero-field-cooled (ZFC) and field-cooled (FC). From both the
curves it is clearly observed the superimposition of the ZFC and FC curves take place at a
certain temperature (TSEP) which is different for two samples. TSEP for LSMO prepared with
glycine (30 nm) is 230 K while for LSMO prepared with PVA (20 nm) is 175 K, which
indicates that the TSEP is a function of particle size. The superimposition of ZFC and FC
curves is one of the characteristic features of superparamagnetic system [5] and from the two
graphs it has been observed that the LSMO prepared with glycine and PVA is
superparamagnetic in nature. The superparamagnetism is induced in the system when the
system comes from maultidomain to single and uniformly magnetized domains. Then, the
overall system is in a state of uniform magnetization and its phase transition occurs from
ferromagnetic to superparamagnetic and system behaves like a small permanent magnet.
The size of the single-domain particle depends on the material and its anisotropy
energy constant. Transformation from multidomain behavior (ferromagnetic) to single
domain (superparamagnetic) occurs at a certain radius of particle called critical radius rc, the
mathematical expression of the same is presented in chapter 2 (equation 2.3) The typical
values for rc is about 15 nm for Fe, 35 nm for Co, 30 nm for γ-Fe2O3 [5] and about 40 nm for
LSMO, [6] thus La0.7Sr0.3MnO3 nanoparticles prepared by solution combustion method with
glycine and PVA fuels are below the critical size (i.e. single domain).
The important parameter, involved in our measurements, is the blocking temperature
(TB). The temperature at which the magnetic anisotropy energy of a nanoparticles system is
overcome by thermal energy and the whole system becomes superparamagnetic above TB or
magnetization of the particle is blocked below TB [7]. The experimental blocking temperature
observed for two different samples are given in Table 4.4. Rostamnejadi et al. used the AC
magnetic susceptibility measurements for the estimation of blocking temperature on the
principle that the below TB, the magnetization direction of nanoparticles can follow the
direction of the applied field and the total magnetization increases with decreasing
temperature. However, we estimated the blocking temperature based on earlier reports of the
magnetic measurements for ferrite nanoparticles [8, 9]. The theoretical blocking temperature
is estimated from the equation TB = KV/25kB where K is modified magnetic anisotropy
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 77
constant and for LSMO nanoparticles and it is 2.25×104 erg/cm3 [10], V is a particle volume
calculated from TEM images for LSMO NPs. Calculated values are given in the table 4.4.
The TC is calculated by taking the differentiation of ZFC curve. The observed nature is
shown in the figure 4.9 (b). The observed value is about 350 K for both samples, which is
less than the reported value for the same composition (~ 370 K).
Figure 4.10. M-H curves of LSMO prepared by glycine and PVA (a) at 300 K (b) at 5 K.
To understand the physics behind the superparamagnetic behavior of La0.7Sr0.3MnO3
system we carried out the M versus H measurements as a function of applied field and
temperature. Figure 4.10 (a) and (b) shows the M-H curves of La0.7Sr0.3MnO3 nanoparticles
at 300 and 5 K prepared by glycine and PVA, respectively. Magnetization value of sample
prepared using glycine at the particular magnetic field is more than that using PVA due to the
particle size effect. Former sample has the larger particle size as compared to the latter one.
The similar observation is reported by Dyakonov et al. [10]. The magnetization increases
with decreasing temperature from 300 to 5 K due to overcome of the magnetic anisotropic
energy over the thermal effect. The hysteresis parameter (coercivity) of both samples is
almost zero at 300 K, which reveals the superparamagnetic nature of La0.7Sr0.3MnO3.
The M-H curve at 300 K for sample prepared using glycine reports the magnetization
value MS ~ 54.91emu/g, which is slightly higher than the value 46.8 emu/g reported by
Daengsakul et al.[11] for the same composition. We observed MS value of 34.91emu/g for
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 78
sample prepared using PVA. This implies that the effect of fuel choice strongly influences
not only on morphology, but also magnetic properties of the La0.7Sr0.3MnO3.
Table 4.4. Magnetization parameters of LSMO observed from SQUID measurement.
4.5. Summary
The structural, magnetic and dispersion properties of the La0.7Sr0.3MnO3 nanoparticles
(20~30nm) prepared with novel solution combustion method have been studied in great
detail. The properties of the LSMO have been affected by the fuel used for combustion. The
glycine and PVA both produce pure-phase LSMO with almost identical particle size
distribution. FC and ZFC measurements strongly support the superparamagnetic behaviour of
the LSMO and the TC determined is smaller than bulk value of the same composition. The
coercivity observed is almost zero for both samples at room temperature, which is a
characteristic feature of superparamagnetism. It is observed that the magnetization influenced
by the fuel used in the combustion method. LSMO prepared by PVA shows lower
aggregation and well dispersion stability. So, we conclude from FE-SEM, TEM and DLS,
study that the LSMO prepared by using PVA as a fuel is excellent candidate for the
biomedical application especially hyperthermia. However, magnetization is lower as
compared to LSMO prepared by glycine, but the observed magnetization is enough for the
successful application in hyperthermia treatment. Hence, in further study, surface
functionalization is done only on LSMO prepared by PVA and results are presented in the
next chapters.
Magnetization (emu/g)
5 K 300 K
Coercivity (Oe)
5K 300 K
Remanent magnetization
(emu/g)
5 K 300 K
Blocking temperature
(K)
Curie temperature
(K)
LSMO by
Glycine 84.43 54.91 76 7 16 1 ~75 350
LSMO by PVA
71.22 34.90 70 1 10 0 ~30 350
Chapter- IV Synthesis of LSMO
Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 79
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