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A comparative study of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 compound synthesized by solid-state and sol-gel process
Accepted Manuscript A comparative study of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 compound synthesized by solid-state and sol-gel process M. Ben Rejeb, C. Ben osman, Y. Regaieg, A. Marzouki-Ajmi, C. Ben osman, W. Cheikhrouhou-Koubaa, S. Ammar, A. Cheikhrouhou, T. Mhiri PII:
S0925-8388(16)33646-5
DOI:
10.1016/j.jallcom.2016.11.166
Reference:
JALCOM 39663
To appear in:
Journal of Alloys and Compounds
Received Date: 6 April 2016 Revised Date:
22 October 2016
Accepted Date: 10 November 2016
Please cite this article as: M. Ben Rejeb, C. Ben osman, Y. Regaieg, A. Marzouki-Ajmi, C. Ben osman, W. Cheikhrouhou-Koubaa, S. Ammar, A. Cheikhrouhou, T. Mhiri, A comparative study of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 compound synthesized by solid-state and sol-gel process, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.166. 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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT
A comparative study of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 compound synthesized by Solid-state and Sol-gel process M. Ben Rejeb1, C. Ben osman2, Y. Regaieg1,2, A. Marzouki-Ajmi1, C. Ben osman2,W.
1
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Cheikhrouhou-Koubaa1,3, S. Ammar2, A. Cheikhrouhou1, T. Mhiri4
Laboratoire de Physique des Matériaux, Université de Sfax, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia 2 3
ITODYS, Université Paris Diderot, CNRS UMR 7086, 15 rue Jean Antoine de Baïf, 75205 Paris, France
Center de Recherche en Numérique de Sfax (CNRS), Technopole de Sfax, Cité El Ons, Route de Tunis, Km 9,
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Sfax, BP 275, Sakiet Ezzit, 3021 Sfax, Tunisie 4
Laboratoire de l’Etat Solide, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia
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Abstract
In this paper, we investigated the impact of the elaborating method on the structural, magnetic and magnetocaloric properties of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 powder sample, synthesized by both methods: solid state (SS) and sol gel (SG) process. The two compounds were firstly analyzed by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) to
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determine the temperature transformation into the perovskite structure. The Rietveld refinement of the X-ray powder diffraction show that both samples are single phase and crystallize in the orthorhombic structure with Pbnm space group. The surface morphology of the samples was carried out using scanning electron microscopy (SEM). Magnetization
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measurements versus temperature in a magnetic applied field of 0.05T indicate that both samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. Curie temperature TC is found to be 296 and 260K for SS and SG samples, respectively. The
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Max maximum of the magnetic entropy change, ∆S M , is lower in the SG sample than in the SS
sample, but the thermal variation of -∆SM is broader, resulting in a higher relative cooling power (RCP).
Keywords: A. oxide Materials, B. Chemical synthesis, C. magnetocaloric, D. X-ray diffraction . *Corresponding Author: Mouna BEN REJEB Laboratoire de Physique des Matériaux, Université de Sfax, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia Tel/Fax: +216 74 676 607 e-mail: [email protected]
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1 Introduction Magnetic refrigeration (MR) is becoming a promising technology to replace the conventional expansion-compression technique due to its high energy efficiency with no harmful gazes. This (MR) is based on the magnetocaloric effect (MCE). This latter was first discovered by Warburg in 1881 [1], Debye in 1926 [2] and Giauque in 1927 [3] and is defined as the
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isothermal magnetic entropy change or the adiabatic temperature change arising from the application / removal of a magnetic field to (from) a system with a magnetic degree of freedom. MCE is intrinsic to all magnetic materials and arises from the coupling of the magnetic sub-lattice with the magnetic field, which alters the magnetic part of the total
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entropy, due to a corresponding change of the magnetic field.
Nowadays, gadolinium (Gd) [4-6] and its alloys [7, 8] have been considered as the most active magnetic refrigerants near room temperature [9], since their Curie temperature is very
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close to 300K. Unfortunately, their widespread use is somehow commercially limited because of their high cost. Compared to Gd based compounds, manganites have attracted more attention as alternative candidates for magnetic refrigeration in the vicinity of room temperature [10]. In addition manganites, have many advantages: low cost, high chemical stability, low eddy current heating, high resistivity and tunable TC. In fact, recently large
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values of MCE are observed in perovskite manganese oxides [11-14]. The research is very interesting in the system La1-xCaxMnO3, which is characterized by rather large values of MCE and adjustable temperatures of phase transitions [9, 15-17]. Several methods of synthesis route have been cited in literature to elaborate manganites
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perovskite such as solid-state, the sol-gel, the ball-milling and the floating zone methods [1819]. The solid-state route is a conventional ceramic method that needs higher sintering
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temperatures and longer annealing times to obtain homogenous composition and desired structure [20]. Sol-gel is a chemical method developed by Pechini [21] and used recently to elaborate manganite with homogenous powder. Magnetic, structural properties and MCE depend on the dopant concentration [22-23], the nature of dopant [24-25] and the impact of sintering temperature [26-27]. In this context, Y. Pan et al. [28] studied the effect of elaborating methods on microstructural, electrical and magnetic properties of La0.85K0.15MnO3 compound and deduced that such properties depend strongly
on
the
synthesis
method.
S.
Mahjoub
et
al.
[29]
synthesized
the
Pr0.6Ca0.1Sr0.3Mn0.975Fe0.025O3 manganites by two different methods: the solid state process and the sol gel route, they found that structural, magnetic, magnetocaloric and critical
ACCEPTED MANUSCRIPT properties depends on the synthesis techniques. Riahi et al [30] have recently studied the impact of elaboration’s method on magnetic properties of La0.78Dy0.02Ca0.2MnO3 compound prepared by solid-state,sol-gel and ball milling , they found that regardless the elaborating technique, the orthorhombic structure with the ‘Pnma’ space group was stable and they deduced that the process of elaboration presents an intensive impact on the magnetic behavior
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and the order of transition. Similarly, F. Ayadi et al. [31] have studied the magnetic properties of La0.7Ca0.2Ba0.1MnO3 ceramics, elaborated by a combined sol–gel and spark plasma sintering (SPS) route, and the produced manganite present a large MCE with a relative cooling power of about 270 J/kg for a magnetic field change of 5 T.
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It was found that the structures and properties of these materials were highly dependent on synthesis process of its precursor powders, so the improvement of preparation method of these powders is paid much attention by researchers [10, 18].
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This paper is a continuation of our lab systematic investigations on the effect of synthesis route on structural, magnetic properties and MCE, in this context we present a comparative study of La0.65Ca0.2(Na0.5K0.5)0.15Mn03 the structural, magnetic and magneto-caloric properties of powder sample elaborated by both solid state (SS) and sol gel (SG) methods in the same grinding and annealing time.
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2 Experimental details
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 powder sample were synthesized using both solid state (SS) and sol gel (SG) methods. Solid State method
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The solid state (SS) method consists in mixing La2O3, CaCO3, Na2CO3, K2CO3 and MnO2 up to high purity (Sigma Aldrich 99.9%) in the desired proportions. The starting materials were
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mixed in an agate mortar and the resulting powder was firstly analyzed by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) using a TGA92 SETARAM analyzer in the 30-1000°C temperature range (5°C min-1) to determine the temperature transformation into the perovskite structure. After that, the powder was heated in air up to 600C° during 24 hours and then at 900°C for the same duration. The powders were then pressed into pellets (of about 1mm thickness) and sintered at 1000°C in air for 24h with intermediate regrinding and repelling. The equation of the total reaction can be written as follows: 0.325La2O3
+
0.2CaCO3
+
0.0375K2CO3
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 + 0.275CO2 + 0.125O2
+
0.0375Na2CO3
+
MnO2
→
ACCEPTED MANUSCRIPT Sol-Gel method The sol gel (SG) method consists in dissolving the precursors with high purity (La2CO3, CaCO3, K2CO3, Na2CO3 and MnO2) (Sigma Aldrich 99.9%) in a concentrated nitric acid solution (HNO3) to transform them into nitrates. The step made under at 50°C until reaching total dissolution. After that, one adds the monohydrate citric acid and the ethylene glycol to
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complete the homogeneity and the transparency of the solution. This solution was slowly evaporated to 130C° until the formation of a transparent gel. In a later step, we raise the temperature until 350C° with a speed of climb of 10C°/minute, with the purpose of having the propagation of a combustion which transforms the freezing into fine powder. The resulting
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powder was firstly analyzed by TGA and DTA. After that, the powder undergoes the same time of annealing and grinding for the sample elaborated by SS.
The morphology of the samples was observed by scanning electron microscope (SEM) using
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a Supra40 ZEISS FEG-SEM microscope operating at 10 kV. Phase purity, homogeneity and cell dimensions were determined by powder X-ray diffraction using a Empyrean PANALYTICAL diffractometer equipped with a copper source (λ λ = 1.5418Å) and a 3D PiXCEL detector. Structural analysis was carried out using the standard Rietveld technique [32] using FULLPROF software [33]. Finally, magnetization measurements versus
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temperature in the range 200-350K in a magnetic applied field of 0.05T and versus magnetic applied field up to 5T at several temperatures were carried out using a Vibrating Sample Magnetometer .The magnetic entropy change, -∆SM, was deduced from M(H) isotherms using the classical thermodynamics based on Maxwell relations [34].
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3 Results and discussion
3.1 Characterization by thermo-gravimetric analysis and differential thermal analysis
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Fig.1. shows the simultaneous TGA-DTA curves obtained during the decomposition of SS and SG compounds in the 30-1000°C temperature range. The SS compound undergoes a total mass loss of 6.5% of the initial value which is in agreement with the departure of 0.275 CO2 and 0.125 O2 (calculated weight loss, 7.25%). This decomposition process appears to be divided into 2 steps: the first mass loss observed between 30 and 450°C is recorded and attributed to the departure of 0.275 CO2, (observed weight loss, 5% and calculated weight loss, 5.45%). This transformation is accompanied by a weak exothermic peak observed on the DTA curve at 200°C. The observed second mass loss (1.5%) begins above 450°C. This process is most likely a waste of oxygen (calculated weight
ACCEPTED MANUSCRIPT loss, 1.8%). After gradually lost weight the decomposition ends at about 1000°C. At this temperature, the perovskite phase was formed. The SG compound undergoes a total mass loss of 28% of the initial value. The recorded curve appears to be divided into three parts. The first endothermic decomposition starts at 30°C and ends at 100°C, which can be attributed to the dehydration of the chemical process. The second
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decomposition observed between 100 and 600°C is characterized by a strong DTA exothermic peak, a mass loss is recorded and attributed to the departure of CO2, the combustion of the last organic residues and a waste of oxygen resulting in the final product of
phase can occur at about 600°C)[37]. 3.2 Structural and micro structural properties
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the perovskite [35-38]. Finally, any weight loss is observed at above 600°C (the perovskite
X-ray diffraction (XRD) patterns at room temperature of La0.65Ca0.2(Na0.5K0.5)0.15Mn03
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powder samples have been indexed in the orthorhombic system with Pbnm space group. The structural parameters are refined by Rietveld’s profile-fitting method. The both samples are single phase without any evidence of neither crystalline nor amorphous impurity. As an example of the good quality of the fits, the room temperature Rietveld plot for both samples is given in Fig. 2. The inferred structural parameters and χ2 fit factor values are listed in Table 1.
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As displayed in table 1, the values of cell parameters, the bonds and have been varied slightly between the SS sample and the SG sample. Regardless the process route of elaboration, the orthorhombic structure with the ‘Pbnm’ space group was stable. The average crystallite size was estimated from the XRD data using the Scherer relation [39]:
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DXRD = Kλ/βcosθ (1)
Where the constant K = 0.9 is the crystallite shape factor, λ is the X-ray wavelength (λ λ=
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0.15418 nm), β is the fullwidth at halfmaximum of intensity vs. 2θ and θ is the Bragg diffraction angle of the most intense peak (200). Using the Scherer formula,the value of crystallite size is found to be 47.79 nm for the SS compound, which is larger compared to the value observed in th SG compound (39.93 nm). The SEM images of the SS and SG samples are given in Fig. 3. They show, in both cases, various sizes of grains with inter-grain porosity. The average grain size ranges in the micrometer range for the SS sample while it is in the sub-micrometer range for the SG sample. This phenomena is explained by the temperature formation of the manganite phase, for both methods, according to the simultaneous TGA/TDA curves [37, 40-41] 3.3 Magnetic and magneto-caloric properties
ACCEPTED MANUSCRIPT Magnetization measurements versus temperature in the range 200-350K under a magnetic applied field of 0.05T showed that both samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature (Fig.4). The Curie temperature (TC) values, correspond to the temperature where dM/dT is minimum, are 296K and 260K for the SS and SG samples respectively. This discrepancy is due probably to local variation of strain near grain
found that when the crystallite size decrease the TC decrease.
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boundaries [42-44] and it may be explained by the evolution of crystallites size. In fact, it is
As shown in Fig.5 the ZFC (Zero Field cooled) and FC (Field cooled) curves for SG sample. The both curves coincide above TC and slightly split below TC, this difference can be
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explained by the fact that during (ZFC) procedure, the spins are randomly aligned in a polycrystalline sample, whereas during (FC) procedure, the spins are aligned along the applied field. Moreover, the M(T) curve observed in the SG indicates the absence of magnetic
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impurity in the sample in low temperature.
In order to confirm the ferromagnetic behavior of our compounds at low temperature, we carried out magnetization measurements versus magnetic applied field up to 5T at several temperatures
in
the
range
200-350K.
Fig.6
shows
the
isotherms
M(H)
for
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 prepared by both methods. At T > TC, the magnetization M
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varies linearly with the applied field, while at T < TC, it increases rapidly for µ0H less than 0.5T and tends to saturate for higher field values, which confirms the ferromagnetic behavior of our compounds at low temperatures. Using these data, we plotted the Arrott curves, which represent the variation of M2 versus µ0H/M at each temperature (Fig.7). They are usually used
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to determine the order of the paramagnetic-ferromagnetic transition, considering the sign of the origin plot slope [45, 46]. In the present case all the M2 versus µ0H/M curves for our
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samples elaborated by both methods exhibit a positive slope indicating that the transition between the ferromagnetic and paramagnetic phase is of the second order [47]. These results must be underlined because they traduce the fact that the grain size reduction (from the micrometer to the submicrometer range) does not affect the order of the transition [48]. Furthermore, the TC values deduced from these plots are very close to that determined from the M(T) curves. From magnetization isotherms we calculated the total magnetic entropy change ∆S M of both synthesized samples as a function of temperature and magnetic field. According to the thermodynamic theory based on Maxwell relations, ∆S M formula:
can be evaluated through the
ACCEPTED MANUSCRIPT ∆S M (T , H ) = S M (T , H ) − S M (T ,0) =
H max
∫ 0
∂M dH ∂T H
(2)
where Hmax is the maximum value of the external applied field. In practice, this relation can be approximated as
i
M i − M i +1 ∆H i Ti +1 − Ti
(3)
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∆S M = ∑
where Mi and Mi+1 are the experimental values of magnetization measured at temperatures Ti and Ti+1 respectively, under magnetic applied field Hi [34]. Fig. 8 shows ∆S M as a function temperature
under
different
magnetic
applied
field
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of
variations
for
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 compound prepared by both methods. ∆S M Increases with an
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increasing magnetic applied field change. The maximum values of the magnetic entropy change ∆ S MMax reached, for a field change of 5T, 3.8 and 3.5 J/kgK for SS and SG samples, respectively.
The magnetic phase transition can also be analyzed quantitatively according to the spin fluctuation model based on Landau theory, Amaral et al. [49, 50] have suggested a successful
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model with a contribution from magnetoelastic and electron interaction in manganites. The Gibbs free energy is expressed as:
G(M,T) = G0 + ( )A(T)M2 + ( )B(T)M4 + ( )C(T)M6 - µ0HM
(4)
The coefficients A(T), B(T), and C(T) are temperature dependent parameters usually known
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as Landau coefficients. By assuming equilibrium condition of Gibb’s free energy (
= 0) at
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TC, the magnetic equation of state is obtained as: µ0
= A(T) + B(T)M2 + C(T)M4
(5)
The values of Landau’s coefficients are determined by fitting the magnetization traces to Eq.(5). The variations of A(T) and B(T) values are shown in the Fig. 9. As expected A(T) is positive with a minimum near TC and the value of B is positive at TC confirming the second order magnetic transition. As shown in this figure, the value of Curie temperature TC =296K and 260K for the SS and SG samples respectively derived from the magnetic measurements is exactly that obtained from the A(T) behavior. These results were confirmed by an analysis of Arrott’s plots close to TC. The corresponding magnetic entropy is obtained from differentiation of the magnetic part of the free energy with respect to temperature
ACCEPTED MANUSCRIPT SM(T, H)= (
)H = - ( )A’(T)M2 - ( )B’(T)M4 - ( )C’(T)M6
(6)
A’(T), B’(T), and C’(T) are the temperature derivatives of the expansion coefficients. The same result is obtained using the equation of state and the integration of Maxwell relations. Nevertheless, in order to compare with the magnetic entropy change (∆S (T, H)) obtained for experimental measurements, we should also calculate the temperature dependence of the entropy change ∆S (T, H) is:
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magnetic entropy without magnetic field S(T, H=0). Therefore, the theoretical magnetic
∆S (T, H) = -( )A’(T) (M02-M2) –( )B’(T) (M04-M4) – ( )C’(T) (M06-M6) (7)
Here, the value of M0 can be obtained by extrapolating the magnetization at H=0. As shown
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in Fig. 10, the calculated magnetic entropy change by using Eq. (7) and the experimental data for La0.65Ca0.2(Na0.5K0.5)0.15Mn03 samples elaborated by both methods for a field change of
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5T.
A clear correspondence is found between the experimental magnetic entropy change (-∆SM) and the estimated one using Landau theory. Analysis clearly demonstrates that magnetoelastic coupling, electron coupling and the electron interaction contributions is necessary for the observed magnetocaloric data.
The change of magnetic entropy can also be calculated from the field dependence of the
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specific heat by the following integration:
(, ) (,)
ΔSM(T, µ 0H) =
(8)
From relation (8), one can calculate the specific heat changes induced by the external
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magnetic field from zero to µ0Hmax as:
ΔCP(T, µ 0H) = CP(T, µ 0H) - CP(T, 0) =
(, )
(9)
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Using relation (9), ΔCP as a function of temperature for La0.65Ca0.2(Na0.5K0.5)0.15Mn03 sample prepared by both SS and SG methods at different magnetic fields is displayed in Fig. 11. It is clearly seen that the specific heat (ΔCP) changes sharply from the negative to the positive at the Curie temperature. Since dM/dT < 0, ΔSM < 0 results, and consequently the total entropy decreases upon magnetization. Furthermore, ΔCP < 0 for T < TC and ΔCP > 0 for T > TC [5152]. The sum of the two parts is the magnetic contribution to the total specific heat, which affects the cooling or heating power of the magnetic refrigerator [53]. Specific heat presents the advantage of delivering values necessary for further refrigerator design, should the material in question be selected.
ACCEPTED MANUSCRIPT In evaluating the magnetocaloric effect, it is important to determine the Relative Cooling Power RCP from the following relation:
RCP = −∆S MMax (T , ∆H ) * δTFWHM where, δTFWHM is the full-width at half-maximum of ∆S M as a function of temperature where –∆SM is the maximum value of the magnetic entropy and δTFWHM is the width at half
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peak height for ∆S M (T ) (( δTFWHM ) is the temperature difference between the two reference points T1 and T2 that have been selected as those corresponding to ∆SM (T1) = ∆SM (T2) = ½
∆S M (T ) ) [54].
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The RCP values are 229.4 and 237.8 J/kg at 5T for SS and SG samples respectively. These values correspond to about 56 and 58 % of that observed in pure Gd (410 J/kg) [55],
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respectively.
4. Conclusion
We successfully synthesized the La0.65Ca0.2(Na0.5K0.5)0.15Mn03 ceramic by both the solid state reaction and sol gel method and investigated their structures, magnetic and magnetocaloric properties. Both samples are single phase with an orthorhombic structure. According to the thermal properties, we can note that the manganite phase was formed at 1000°C for the SS
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sample, meanwhile it was occurring at 600°C for the SG one. The SG sample had smaller grain size and more disorder surface in comparison with SS sample. The Curie temperature shifted towards lower temperature from the SS sample to SG sample. The magnetic entropy change of the SG sample is found to be smaller than that of the SS sample, while its relative
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cooling power, RCP, appears to be higher. It reaches for a field change of 5T, 237.8 J/kg in the SG sample while it 229.4 J/kg in SS sample. Our samples may be suitable for applications
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in the magnetic refrigeration near room temperature.
Acknowledgement
This work has been supported by the Tunisian Ministry of Higher Education and Scientific Research.
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[25] A. Selmi, R. M’nassri, W. Cheikhrouhou-Koubaa, N. Chniba Boudjada, A. Cheikhrouhou, Effects of partial Mn-substitution on magnetic and magnetocaloric properties in Pr0.7Ca0.3Mn0.95X0.05O3 (Cr, Ni, Co and Fe) manganites, J. Alloys Comp. 619 (2015) 627. [26] G. Venkataiah, D.C. Krishna, M. Vithal, S.S. Rao, S.V. Bhat, V. Prasad, S.V. Subramanyam, P. Venugopal Reddy, Effect of sintering temperature on electrical transport properties of La0.67Ca0.33MnO3, Physica B 357 (2005) 370. [27] I. Matos, S. Sério, M.E. Lopes, M.R. Nunes, M.E. Melo Jorge, Effect of the sintering temperature on the properties of nanocrystalline Ca1−xSmxMnO3 (0 ≤ x ≤ 0.4) powders, J. Alloys Comp. 509 (2011) 9617.
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ACCEPTED MANUSCRIPT [28] K.Y. Pan, S.A. Halim, K.P. Lim, W.M.W.Y. Daud, S.K. Chen, M. Navasery, Microstructure, electrical and magnetic properties of polycrystalline La0.85K0.15MnO3 manganites prepared by different synthesis routes, J. Mater. Sci.: Mater. Electron. 24 (2013) 1869. [29] S. Mahjoub, M. Baazaoui, E.K. Hlil, M. Oumezzine, Effect of synthesis techniques on
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structural, magnetocaloric and critical behavior of Pr0.6Ca0.1Sr0.3Mn0.975Fe0.025O3 manganites, Ceram. Int. 41 (2015) 12407.
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properties of La0.78Dy0.02Ca0.2MnO3 manganite: A comparison between sol-gel, high-energy ball-milling and solid state process J. Alloys Comp 688 (2016) 1028-1038
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Lecoq, S. Nowak, S. Ammar, L. Sicard, Preparation of nanostructured La0.7Ca0.3-xBaxMnO3 ceramics by a combined sol–gel and spark plasma sintering route and resulting magnetocaloric properties, J. Magn. Magn. Mater. 381 (2015) 215 [32] H.M. Rietveld, A Profile Refinement Method for Nuclear and Magnetic Structures, J. Appl. Cryst. 2 (1965) 65-71.
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[33] T. Roisnel, J. Rodriguez-Carvajal, Computer Program FULLPROF, LLB-LCSIM, May (2003).
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[35] P. Durán, J. Tartaj, F. Capel, C. Moure, Formation, sintering and thermal expansion behaviour of Sr- and Mg-doped LaCrO3 as SOFC interconnector prepared by the ethylene glycol polymerized complex solution synthesis method, J. Eur. Ceram. Soc. 24 (2004) 2619.
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[36] S. Bilger, G. Blab, R. Förthman, Sol-gel synthesis of lanthanum chromite powder, J. Eur. Ceram. Soc. 17 (1997) 1027. [37] M. Ben Rejeb, W. Cheikhrouhou-Koubaa, M. Koubaa, A. Cheikhrouhou, S. Nowak, L. Sicard, S. Ammar-Merah, Annealing Effect on the Structural, Magnetic and Magnetocaloric Properties of La0.65Ca0.2K0.15MnO3 Synthesized by Sol Gel Method, J. Supercond. Nov. Magn. 28 (2015) 1379. [38] J.D. Carter, H.U. Anderson, M.G. Shumsky, Structure and phase transformation of lanthanum chromate, J. Mater. Chem. 31 (1996) 551. [39] A. Taylor, X-ray Metallography, Wiley, New York, 1961.
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ACCEPTED MANUSCRIPT [40] V.S. Reddy Channu, Rudolf Holze, Edwin H. Walker, Synthesis and Characterization of La0.8Sr0.2MnO3-δ Nanostructures for Oxide Fuel Cells, New .Journal of Glass and Ceramics, 3 (2013) 29-33. [41] Z. Juan, L. Lirong, W. Gui, Synthesis and magnétocaloric properties of La0.85K0.15MnO3 nanoparticles, Adv. Powder Technology22 (2011) 68-71.
Size-induced
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of
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and
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properties
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dimensionality on the magnetic and magnetocaloric response of La0.7Ca0.3MnO3, Appl. Phys. Lett. 102 (2013) 062414.
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L. Sicard, S. Ammar-Merah, Magnetocaloric properties of La0.67Ca0.33MnO3 produced by reactive spark plasma sintering and by conventional ceramic route, J. Mater. Res. Express 1 (2014) 046105.
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second-order phase transition manganites, J. Non-Cryst. Solids 354 (2008) 5301. [46] M. Foldcaki, R. Chahine, B.R. Gopal, T.K. Bose, Investigation of the magnetic roperties of the Gd1−xErx alloy system in the x < 0.62 composition range, J. Magn. Magn. Mater. 150 (1995) 421.
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[47] V. Karpasyuk. E. Beznisko, A. Abdullina, Generalized integro-differential equation of magnetization reversal dynamics in polycrystals and its application to processes in manganite-
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based CMR materials, J. Magn. Magn. Mater. 272 (2004) 750. [48] A. Abdallah-Ben Ammar, W. Cheikhrouhou-Koubaa, M. Koubaa, S. Nowak, H. Lecoq, L. Sicard, S. Ammar, A. Cheikhrouhou, Effect of sodium substitution on the physical properties of sol-gel made La0.65Ca0.35MnO3 ceramics, J. Mater. Chem. Phys. 148 (2014) 751. [49] V.S. Amaral, J.S. Amaral, Magnetoelastic coupling influence on the magnetocaloric effect in ferromagnetic materials, J. Magn. Magn. Mater. 272 (2004) 2104-2105. [50] J.S. Amaral, M.S. Reis, V.S. Amaral, T.M. Mendonça, J.P. Araujo. M.A. Sa, P.B. Tavares, J.M. Vieira, Magnetocaloric effect in Er- and Eu-substituted ferromagnetic La-Sr manganites, J. Magn. Magn. Mater.290 (2005) 686. [51] H. Yang, Y.H. Zhu, T. Xian, J.L. Jiang, Synthesis and magnetocaloric properties of La0.7Ca0.3MnO3nanoparticles with different sizes, J. Alloys Compd. 555 (2013) 150. 13
ACCEPTED MANUSCRIPT [52] R. M’nassri, A. Cheikhrouhou, Magnetocaloric effect in different impurity doped La0.67Ca0.33MnO3 composite, J. Supercond. Nov. Magn. 27 (2014) 421. [53] X.X. Zhang, G.H. Wen, F.W. Wang, W.H. Wang, C.H. Yu, Magnetic entropy change in Fe-based compound LaFe10.6Si2.4, Appl. Phys. Lett. 77 (2000) 3072. [54] K.A. Gschneidner, V.K. Pecharsky, Magnetocaloric Materials, Annu. Rev. Mater. Sci. 30
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(2000) 387 [55]V.K. Pecharsky, K.A. Gschneidner, A.O. Tsokol, Recent developments in magneto-
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caloric materials, Rep. Prog. Phys. 68 (2005) 1479.
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ACCEPTED MANUSCRIPT Table 1: Refined structural parameters of the SS and SG samples at room temperature. SG
Space Group
Pbnm
Pbnm
a (Å)
5.444(5)
5.441(4)
b (Å)
5.474(8)
5.473(9)
c (Å)
7.682(5)
7.674(0)
V (Å )
228.99
228.57
Mn-O1 (Å)
1.959(8)
1.958(6)
Mn-O2 (Å)
2.037(1)
2.022(1)
Mn-O2 (Å)
1.851(0)
1.856(6)
˂Mn-O˃ (Å)
1.949(3)
1.945(7)
Mn-O1-Mn (°)
157.04(4)
Mn-O2-Mn (°)
166.29(9)
˂Mn-O-Mn˃ (°)
163.21(4)
RB (%) RF (%) 2
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164.56(4)
3.16
2.24
6.30
2.68
2.3
1.7
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χ
156.75(1)
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SS
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 TGA and DTA curves for SS and SG samples. Fig. 2 XRD patterns and Rietveld refinements of the SS and SG samples. Fig. 3 SEM images of the SS and SG samples. Fig. 4 Temperature dependence of the magnetization at 0.05T for the SS and SG samples.
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Fig. 5 Temperature dependence of the magnetization at 0.05T (FC-ZFC) for the SG sample. Fig. 6 Variation of the magnetization as a function of applied magnetic field at different temperatures for SS and SG samples. Fig. 7 M² versus µ0H/M isotherms for the SS and SG samples. applied field changes for the SS and SG samples. Fig. 9The variations of A(T) and B(T) values 10
The
calculated
magnetic
entropy
and
the
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Fig. 8 Magnetic entropy change -∆SM as a function of temperature at several magnetic
experimental
data
for
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 samples elaborated by both methods. Fig. 11 ∆CP as a function of temperature for La0.65Ca0.2(Na0.5K0.5)0.15Mn03 sample prepared
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Figure 1
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30
20 15 10 5
225
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275
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300
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M (emu/g)
25
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200K
70
∆Τ=10Κ
280K
50 ∆Τ=5Κ
40 320K
30
∆Τ=10Κ
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10 0 1
2
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5
6
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200 K
4000 ∆Τ = 10 Κ
3000 2000 1000
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350 K
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0.10
0.15
0.20
0.25
0.30
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3000
2000
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Samples have been elaborated by both methods. The perovskite phase was formed at 1000°C for the SS and at 600°C for the SG. The TC values are 296 K and 260 K for the SS and SG samples, respectively.
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• • •
S0925-8388(16)33646-5
DOI:
10.1016/j.jallcom.2016.11.166
Reference:
JALCOM 39663
To appear in:
Journal of Alloys and Compounds
Received Date: 6 April 2016 Revised Date:
22 October 2016
Accepted Date: 10 November 2016
Please cite this article as: M. Ben Rejeb, C. Ben osman, Y. Regaieg, A. Marzouki-Ajmi, C. Ben osman, W. Cheikhrouhou-Koubaa, S. Ammar, A. Cheikhrouhou, T. Mhiri, A comparative study of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 compound synthesized by solid-state and sol-gel process, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.166. 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 proof before it is published in its final 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.
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A comparative study of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 compound synthesized by Solid-state and Sol-gel process M. Ben Rejeb1, C. Ben osman2, Y. Regaieg1,2, A. Marzouki-Ajmi1, C. Ben osman2,W.
1
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Cheikhrouhou-Koubaa1,3, S. Ammar2, A. Cheikhrouhou1, T. Mhiri4
Laboratoire de Physique des Matériaux, Université de Sfax, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia 2 3
ITODYS, Université Paris Diderot, CNRS UMR 7086, 15 rue Jean Antoine de Baïf, 75205 Paris, France
Center de Recherche en Numérique de Sfax (CNRS), Technopole de Sfax, Cité El Ons, Route de Tunis, Km 9,
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Sfax, BP 275, Sakiet Ezzit, 3021 Sfax, Tunisie 4
Laboratoire de l’Etat Solide, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia
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Abstract
In this paper, we investigated the impact of the elaborating method on the structural, magnetic and magnetocaloric properties of La0.65Ca0.2(Na0.5K0.5)0.15MnO3 powder sample, synthesized by both methods: solid state (SS) and sol gel (SG) process. The two compounds were firstly analyzed by thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) to
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determine the temperature transformation into the perovskite structure. The Rietveld refinement of the X-ray powder diffraction show that both samples are single phase and crystallize in the orthorhombic structure with Pbnm space group. The surface morphology of the samples was carried out using scanning electron microscopy (SEM). Magnetization
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measurements versus temperature in a magnetic applied field of 0.05T indicate that both samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. Curie temperature TC is found to be 296 and 260K for SS and SG samples, respectively. The
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Max maximum of the magnetic entropy change, ∆S M , is lower in the SG sample than in the SS
sample, but the thermal variation of -∆SM is broader, resulting in a higher relative cooling power (RCP).
Keywords: A. oxide Materials, B. Chemical synthesis, C. magnetocaloric, D. X-ray diffraction . *Corresponding Author: Mouna BEN REJEB Laboratoire de Physique des Matériaux, Université de Sfax, Faculté des Sciences de Sfax, B.P. 1171, 3000 Sfax, Tunisia Tel/Fax: +216 74 676 607 e-mail: [email protected]
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1 Introduction Magnetic refrigeration (MR) is becoming a promising technology to replace the conventional expansion-compression technique due to its high energy efficiency with no harmful gazes. This (MR) is based on the magnetocaloric effect (MCE). This latter was first discovered by Warburg in 1881 [1], Debye in 1926 [2] and Giauque in 1927 [3] and is defined as the
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isothermal magnetic entropy change or the adiabatic temperature change arising from the application / removal of a magnetic field to (from) a system with a magnetic degree of freedom. MCE is intrinsic to all magnetic materials and arises from the coupling of the magnetic sub-lattice with the magnetic field, which alters the magnetic part of the total
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entropy, due to a corresponding change of the magnetic field.
Nowadays, gadolinium (Gd) [4-6] and its alloys [7, 8] have been considered as the most active magnetic refrigerants near room temperature [9], since their Curie temperature is very
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close to 300K. Unfortunately, their widespread use is somehow commercially limited because of their high cost. Compared to Gd based compounds, manganites have attracted more attention as alternative candidates for magnetic refrigeration in the vicinity of room temperature [10]. In addition manganites, have many advantages: low cost, high chemical stability, low eddy current heating, high resistivity and tunable TC. In fact, recently large
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values of MCE are observed in perovskite manganese oxides [11-14]. The research is very interesting in the system La1-xCaxMnO3, which is characterized by rather large values of MCE and adjustable temperatures of phase transitions [9, 15-17]. Several methods of synthesis route have been cited in literature to elaborate manganites
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perovskite such as solid-state, the sol-gel, the ball-milling and the floating zone methods [1819]. The solid-state route is a conventional ceramic method that needs higher sintering
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temperatures and longer annealing times to obtain homogenous composition and desired structure [20]. Sol-gel is a chemical method developed by Pechini [21] and used recently to elaborate manganite with homogenous powder. Magnetic, structural properties and MCE depend on the dopant concentration [22-23], the nature of dopant [24-25] and the impact of sintering temperature [26-27]. In this context, Y. Pan et al. [28] studied the effect of elaborating methods on microstructural, electrical and magnetic properties of La0.85K0.15MnO3 compound and deduced that such properties depend strongly
on
the
synthesis
method.
S.
Mahjoub
et
al.
[29]
synthesized
the
Pr0.6Ca0.1Sr0.3Mn0.975Fe0.025O3 manganites by two different methods: the solid state process and the sol gel route, they found that structural, magnetic, magnetocaloric and critical
ACCEPTED MANUSCRIPT properties depends on the synthesis techniques. Riahi et al [30] have recently studied the impact of elaboration’s method on magnetic properties of La0.78Dy0.02Ca0.2MnO3 compound prepared by solid-state,sol-gel and ball milling , they found that regardless the elaborating technique, the orthorhombic structure with the ‘Pnma’ space group was stable and they deduced that the process of elaboration presents an intensive impact on the magnetic behavior
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and the order of transition. Similarly, F. Ayadi et al. [31] have studied the magnetic properties of La0.7Ca0.2Ba0.1MnO3 ceramics, elaborated by a combined sol–gel and spark plasma sintering (SPS) route, and the produced manganite present a large MCE with a relative cooling power of about 270 J/kg for a magnetic field change of 5 T.
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It was found that the structures and properties of these materials were highly dependent on synthesis process of its precursor powders, so the improvement of preparation method of these powders is paid much attention by researchers [10, 18].
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This paper is a continuation of our lab systematic investigations on the effect of synthesis route on structural, magnetic properties and MCE, in this context we present a comparative study of La0.65Ca0.2(Na0.5K0.5)0.15Mn03 the structural, magnetic and magneto-caloric properties of powder sample elaborated by both solid state (SS) and sol gel (SG) methods in the same grinding and annealing time.
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2 Experimental details
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 powder sample were synthesized using both solid state (SS) and sol gel (SG) methods. Solid State method
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The solid state (SS) method consists in mixing La2O3, CaCO3, Na2CO3, K2CO3 and MnO2 up to high purity (Sigma Aldrich 99.9%) in the desired proportions. The starting materials were
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mixed in an agate mortar and the resulting powder was firstly analyzed by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) using a TGA92 SETARAM analyzer in the 30-1000°C temperature range (5°C min-1) to determine the temperature transformation into the perovskite structure. After that, the powder was heated in air up to 600C° during 24 hours and then at 900°C for the same duration. The powders were then pressed into pellets (of about 1mm thickness) and sintered at 1000°C in air for 24h with intermediate regrinding and repelling. The equation of the total reaction can be written as follows: 0.325La2O3
+
0.2CaCO3
+
0.0375K2CO3
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 + 0.275CO2 + 0.125O2
+
0.0375Na2CO3
+
MnO2
→
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complete the homogeneity and the transparency of the solution. This solution was slowly evaporated to 130C° until the formation of a transparent gel. In a later step, we raise the temperature until 350C° with a speed of climb of 10C°/minute, with the purpose of having the propagation of a combustion which transforms the freezing into fine powder. The resulting
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powder was firstly analyzed by TGA and DTA. After that, the powder undergoes the same time of annealing and grinding for the sample elaborated by SS.
The morphology of the samples was observed by scanning electron microscope (SEM) using
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a Supra40 ZEISS FEG-SEM microscope operating at 10 kV. Phase purity, homogeneity and cell dimensions were determined by powder X-ray diffraction using a Empyrean PANALYTICAL diffractometer equipped with a copper source (λ λ = 1.5418Å) and a 3D PiXCEL detector. Structural analysis was carried out using the standard Rietveld technique [32] using FULLPROF software [33]. Finally, magnetization measurements versus
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temperature in the range 200-350K in a magnetic applied field of 0.05T and versus magnetic applied field up to 5T at several temperatures were carried out using a Vibrating Sample Magnetometer .The magnetic entropy change, -∆SM, was deduced from M(H) isotherms using the classical thermodynamics based on Maxwell relations [34].
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3 Results and discussion
3.1 Characterization by thermo-gravimetric analysis and differential thermal analysis
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Fig.1. shows the simultaneous TGA-DTA curves obtained during the decomposition of SS and SG compounds in the 30-1000°C temperature range. The SS compound undergoes a total mass loss of 6.5% of the initial value which is in agreement with the departure of 0.275 CO2 and 0.125 O2 (calculated weight loss, 7.25%). This decomposition process appears to be divided into 2 steps: the first mass loss observed between 30 and 450°C is recorded and attributed to the departure of 0.275 CO2, (observed weight loss, 5% and calculated weight loss, 5.45%). This transformation is accompanied by a weak exothermic peak observed on the DTA curve at 200°C. The observed second mass loss (1.5%) begins above 450°C. This process is most likely a waste of oxygen (calculated weight
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decomposition observed between 100 and 600°C is characterized by a strong DTA exothermic peak, a mass loss is recorded and attributed to the departure of CO2, the combustion of the last organic residues and a waste of oxygen resulting in the final product of
phase can occur at about 600°C)[37]. 3.2 Structural and micro structural properties
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the perovskite [35-38]. Finally, any weight loss is observed at above 600°C (the perovskite
X-ray diffraction (XRD) patterns at room temperature of La0.65Ca0.2(Na0.5K0.5)0.15Mn03
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powder samples have been indexed in the orthorhombic system with Pbnm space group. The structural parameters are refined by Rietveld’s profile-fitting method. The both samples are single phase without any evidence of neither crystalline nor amorphous impurity. As an example of the good quality of the fits, the room temperature Rietveld plot for both samples is given in Fig. 2. The inferred structural parameters and χ2 fit factor values are listed in Table 1.
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As displayed in table 1, the values of cell parameters, the bonds
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DXRD = Kλ/βcosθ (1)
Where the constant K = 0.9 is the crystallite shape factor, λ is the X-ray wavelength (λ λ=
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0.15418 nm), β is the fullwidth at halfmaximum of intensity vs. 2θ and θ is the Bragg diffraction angle of the most intense peak (200). Using the Scherer formula,the value of crystallite size is found to be 47.79 nm for the SS compound, which is larger compared to the value observed in th SG compound (39.93 nm). The SEM images of the SS and SG samples are given in Fig. 3. They show, in both cases, various sizes of grains with inter-grain porosity. The average grain size ranges in the micrometer range for the SS sample while it is in the sub-micrometer range for the SG sample. This phenomena is explained by the temperature formation of the manganite phase, for both methods, according to the simultaneous TGA/TDA curves [37, 40-41] 3.3 Magnetic and magneto-caloric properties
ACCEPTED MANUSCRIPT Magnetization measurements versus temperature in the range 200-350K under a magnetic applied field of 0.05T showed that both samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature (Fig.4). The Curie temperature (TC) values, correspond to the temperature where dM/dT is minimum, are 296K and 260K for the SS and SG samples respectively. This discrepancy is due probably to local variation of strain near grain
found that when the crystallite size decrease the TC decrease.
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boundaries [42-44] and it may be explained by the evolution of crystallites size. In fact, it is
As shown in Fig.5 the ZFC (Zero Field cooled) and FC (Field cooled) curves for SG sample. The both curves coincide above TC and slightly split below TC, this difference can be
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explained by the fact that during (ZFC) procedure, the spins are randomly aligned in a polycrystalline sample, whereas during (FC) procedure, the spins are aligned along the applied field. Moreover, the M(T) curve observed in the SG indicates the absence of magnetic
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impurity in the sample in low temperature.
In order to confirm the ferromagnetic behavior of our compounds at low temperature, we carried out magnetization measurements versus magnetic applied field up to 5T at several temperatures
in
the
range
200-350K.
Fig.6
shows
the
isotherms
M(H)
for
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 prepared by both methods. At T > TC, the magnetization M
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varies linearly with the applied field, while at T < TC, it increases rapidly for µ0H less than 0.5T and tends to saturate for higher field values, which confirms the ferromagnetic behavior of our compounds at low temperatures. Using these data, we plotted the Arrott curves, which represent the variation of M2 versus µ0H/M at each temperature (Fig.7). They are usually used
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to determine the order of the paramagnetic-ferromagnetic transition, considering the sign of the origin plot slope [45, 46]. In the present case all the M2 versus µ0H/M curves for our
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samples elaborated by both methods exhibit a positive slope indicating that the transition between the ferromagnetic and paramagnetic phase is of the second order [47]. These results must be underlined because they traduce the fact that the grain size reduction (from the micrometer to the submicrometer range) does not affect the order of the transition [48]. Furthermore, the TC values deduced from these plots are very close to that determined from the M(T) curves. From magnetization isotherms we calculated the total magnetic entropy change ∆S M of both synthesized samples as a function of temperature and magnetic field. According to the thermodynamic theory based on Maxwell relations, ∆S M formula:
can be evaluated through the
ACCEPTED MANUSCRIPT ∆S M (T , H ) = S M (T , H ) − S M (T ,0) =
H max
∫ 0
∂M dH ∂T H
(2)
where Hmax is the maximum value of the external applied field. In practice, this relation can be approximated as
i
M i − M i +1 ∆H i Ti +1 − Ti
(3)
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∆S M = ∑
where Mi and Mi+1 are the experimental values of magnetization measured at temperatures Ti and Ti+1 respectively, under magnetic applied field Hi [34]. Fig. 8 shows ∆S M as a function temperature
under
different
magnetic
applied
field
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of
variations
for
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 compound prepared by both methods. ∆S M Increases with an
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increasing magnetic applied field change. The maximum values of the magnetic entropy change ∆ S MMax reached, for a field change of 5T, 3.8 and 3.5 J/kgK for SS and SG samples, respectively.
The magnetic phase transition can also be analyzed quantitatively according to the spin fluctuation model based on Landau theory, Amaral et al. [49, 50] have suggested a successful
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model with a contribution from magnetoelastic and electron interaction in manganites. The Gibbs free energy is expressed as:
G(M,T) = G0 + ( )A(T)M2 + ( )B(T)M4 + ( )C(T)M6 - µ0HM
(4)
The coefficients A(T), B(T), and C(T) are temperature dependent parameters usually known
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as Landau coefficients. By assuming equilibrium condition of Gibb’s free energy (
= 0) at
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TC, the magnetic equation of state is obtained as: µ0
= A(T) + B(T)M2 + C(T)M4
(5)
The values of Landau’s coefficients are determined by fitting the magnetization traces to Eq.(5). The variations of A(T) and B(T) values are shown in the Fig. 9. As expected A(T) is positive with a minimum near TC and the value of B is positive at TC confirming the second order magnetic transition. As shown in this figure, the value of Curie temperature TC =296K and 260K for the SS and SG samples respectively derived from the magnetic measurements is exactly that obtained from the A(T) behavior. These results were confirmed by an analysis of Arrott’s plots close to TC. The corresponding magnetic entropy is obtained from differentiation of the magnetic part of the free energy with respect to temperature
ACCEPTED MANUSCRIPT SM(T, H)= (
)H = - ( )A’(T)M2 - ( )B’(T)M4 - ( )C’(T)M6
(6)
A’(T), B’(T), and C’(T) are the temperature derivatives of the expansion coefficients. The same result is obtained using the equation of state and the integration of Maxwell relations. Nevertheless, in order to compare with the magnetic entropy change (∆S (T, H)) obtained for experimental measurements, we should also calculate the temperature dependence of the entropy change ∆S (T, H) is:
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magnetic entropy without magnetic field S(T, H=0). Therefore, the theoretical magnetic
∆S (T, H) = -( )A’(T) (M02-M2) –( )B’(T) (M04-M4) – ( )C’(T) (M06-M6) (7)
Here, the value of M0 can be obtained by extrapolating the magnetization at H=0. As shown
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in Fig. 10, the calculated magnetic entropy change by using Eq. (7) and the experimental data for La0.65Ca0.2(Na0.5K0.5)0.15Mn03 samples elaborated by both methods for a field change of
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5T.
A clear correspondence is found between the experimental magnetic entropy change (-∆SM) and the estimated one using Landau theory. Analysis clearly demonstrates that magnetoelastic coupling, electron coupling and the electron interaction contributions is necessary for the observed magnetocaloric data.
The change of magnetic entropy can also be calculated from the field dependence of the
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specific heat by the following integration:
(, ) (,)
ΔSM(T, µ 0H) =
(8)
From relation (8), one can calculate the specific heat changes induced by the external
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magnetic field from zero to µ0Hmax as:
ΔCP(T, µ 0H) = CP(T, µ 0H) - CP(T, 0) =
(, )
(9)
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Using relation (9), ΔCP as a function of temperature for La0.65Ca0.2(Na0.5K0.5)0.15Mn03 sample prepared by both SS and SG methods at different magnetic fields is displayed in Fig. 11. It is clearly seen that the specific heat (ΔCP) changes sharply from the negative to the positive at the Curie temperature. Since dM/dT < 0, ΔSM < 0 results, and consequently the total entropy decreases upon magnetization. Furthermore, ΔCP < 0 for T < TC and ΔCP > 0 for T > TC [5152]. The sum of the two parts is the magnetic contribution to the total specific heat, which affects the cooling or heating power of the magnetic refrigerator [53]. Specific heat presents the advantage of delivering values necessary for further refrigerator design, should the material in question be selected.
ACCEPTED MANUSCRIPT In evaluating the magnetocaloric effect, it is important to determine the Relative Cooling Power RCP from the following relation:
RCP = −∆S MMax (T , ∆H ) * δTFWHM where, δTFWHM is the full-width at half-maximum of ∆S M as a function of temperature where –∆SM is the maximum value of the magnetic entropy and δTFWHM is the width at half
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peak height for ∆S M (T ) (( δTFWHM ) is the temperature difference between the two reference points T1 and T2 that have been selected as those corresponding to ∆SM (T1) = ∆SM (T2) = ½
∆S M (T ) ) [54].
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The RCP values are 229.4 and 237.8 J/kg at 5T for SS and SG samples respectively. These values correspond to about 56 and 58 % of that observed in pure Gd (410 J/kg) [55],
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respectively.
4. Conclusion
We successfully synthesized the La0.65Ca0.2(Na0.5K0.5)0.15Mn03 ceramic by both the solid state reaction and sol gel method and investigated their structures, magnetic and magnetocaloric properties. Both samples are single phase with an orthorhombic structure. According to the thermal properties, we can note that the manganite phase was formed at 1000°C for the SS
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sample, meanwhile it was occurring at 600°C for the SG one. The SG sample had smaller grain size and more disorder surface in comparison with SS sample. The Curie temperature shifted towards lower temperature from the SS sample to SG sample. The magnetic entropy change of the SG sample is found to be smaller than that of the SS sample, while its relative
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cooling power, RCP, appears to be higher. It reaches for a field change of 5T, 237.8 J/kg in the SG sample while it 229.4 J/kg in SS sample. Our samples may be suitable for applications
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in the magnetic refrigeration near room temperature.
Acknowledgement
This work has been supported by the Tunisian Ministry of Higher Education and Scientific Research.
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structural, magnetocaloric and critical behavior of Pr0.6Ca0.1Sr0.3Mn0.975Fe0.025O3 manganites, Ceram. Int. 41 (2015) 12407.
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properties of La0.78Dy0.02Ca0.2MnO3 manganite: A comparison between sol-gel, high-energy ball-milling and solid state process J. Alloys Comp 688 (2016) 1028-1038
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Lecoq, S. Nowak, S. Ammar, L. Sicard, Preparation of nanostructured La0.7Ca0.3-xBaxMnO3 ceramics by a combined sol–gel and spark plasma sintering route and resulting magnetocaloric properties, J. Magn. Magn. Mater. 381 (2015) 215 [32] H.M. Rietveld, A Profile Refinement Method for Nuclear and Magnetic Structures, J. Appl. Cryst. 2 (1965) 65-71.
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magnetic
and
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(2000) 387 [55]V.K. Pecharsky, K.A. Gschneidner, A.O. Tsokol, Recent developments in magneto-
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caloric materials, Rep. Prog. Phys. 68 (2005) 1479.
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ACCEPTED MANUSCRIPT Table 1: Refined structural parameters of the SS and SG samples at room temperature. SG
Space Group
Pbnm
Pbnm
a (Å)
5.444(5)
5.441(4)
b (Å)
5.474(8)
5.473(9)
c (Å)
7.682(5)
7.674(0)
V (Å )
228.99
228.57
Mn-O1 (Å)
1.959(8)
1.958(6)
Mn-O2 (Å)
2.037(1)
2.022(1)
Mn-O2 (Å)
1.851(0)
1.856(6)
˂Mn-O˃ (Å)
1.949(3)
1.945(7)
Mn-O1-Mn (°)
157.04(4)
Mn-O2-Mn (°)
166.29(9)
˂Mn-O-Mn˃ (°)
163.21(4)
RB (%) RF (%) 2
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164.56(4)
3.16
2.24
6.30
2.68
2.3
1.7
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χ
156.75(1)
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SS
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 TGA and DTA curves for SS and SG samples. Fig. 2 XRD patterns and Rietveld refinements of the SS and SG samples. Fig. 3 SEM images of the SS and SG samples. Fig. 4 Temperature dependence of the magnetization at 0.05T for the SS and SG samples.
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Fig. 5 Temperature dependence of the magnetization at 0.05T (FC-ZFC) for the SG sample. Fig. 6 Variation of the magnetization as a function of applied magnetic field at different temperatures for SS and SG samples. Fig. 7 M² versus µ0H/M isotherms for the SS and SG samples. applied field changes for the SS and SG samples. Fig. 9The variations of A(T) and B(T) values 10
The
calculated
magnetic
entropy
and
the
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Fig.
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Fig. 8 Magnetic entropy change -∆SM as a function of temperature at several magnetic
experimental
data
for
La0.65Ca0.2(Na0.5K0.5)0.15Mn03 samples elaborated by both methods. Fig. 11 ∆CP as a function of temperature for La0.65Ca0.2(Na0.5K0.5)0.15Mn03 sample prepared
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Figure 1
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Figure 2
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Figure 3
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ACCEPTED MANUSCRIPT 35 SS SG
30
20 15 10 5
225
250
275
T (K)
325
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Figure 4
300
Figure 5
20
350
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M (emu/g)
25
ACCEPTED MANUSCRIPT 80
(SS)
200K
70
∆Τ=10Κ
280K
50 ∆Τ=5Κ
40 320K
30
∆Τ=10Κ
20
350K
10 0 1
2
3
4
5
6
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0
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80
(SG) 70
200K
∆Τ=10Κ
60 50
220K
∆Τ=5Κ
40
280K
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M (emu/g)
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M (emu/g)
60
30 20 10 0
1
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∆Τ=10Κ
330K
3
T (K) Figure 6
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4
5
6
ACCEPTED MANUSCRIPT 6000 (SS)
200 K
4000 ∆Τ = 10 Κ
3000 2000 1000
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M2 (emu/g)2
5000
350 K
0 0.00
0.05
0.10
0.15
0.20
0.25
0.30
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µ0H/M (T/(emu/g))
5000
200 K
∆Τ = 10 Κ
3000
2000
1000
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M2 (emu/g)2
4000
(SG)
0.05
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0 0.00
0.10
0.15
330 K
0.20
µ0H/M (T/(emu/g))
Figure 7
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0.30
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Samples have been elaborated by both methods. The perovskite phase was formed at 1000°C for the SS and at 600°C for the SG. The TC values are 296 K and 260 K for the SS and SG samples, respectively.
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• • •