Nearly constant magnetic entropy change involving the enhancement of refrigerant capacity in (La0.6Ba0.2Sr0.2MnO3)1−x/(Co2O3)x composite
Introduction
The search for magnetic materials with enhanced magnetocaloric effect (MCE) for their utilization in magnetic cooling for ׳green׳ refrigeration is a very active field of research due to the fact that it has higher energy efficiency and less impact on environment than traditional method based on gaseous compression [1], [2]. Magnetic refrigeration technology is of special interest because of its great social effect and economical benefit [3]. This technology yields a much higher cooling efficiency (about 20–30%) than vapor-compression technology [4].
The MCE is the tendency of magnetic materials to heat up when placed in a magnetic field and cool down when removed from the field. The study of materials exhibiting MCE is an intriguing area of research from the standpoint of both application and basic understanding due to their potential possibilities for use as working bodies in thermodynamic cycles of modern and effective alternative solid-state refrigeration technologies [5], [6], [7]. The materials showing the large MCE are preferred [8], [9]. Many materials have been investigated in recent years such as rare-earth based compounds [10], Heusler alloys [11], pnictides [12], manganites [13], [14], [15], [16] and some of these materials show substantial MCE around room temperature. Manganites have great promise as magnetocaloric materials due to their combined magnetic and structural transitions. These materials also present a higher resistivity, which is favorable for reducing eddy current heating [17]. Physical properties in these systems come from the interplay between the charge, orbital, spin, and lattice degrees of freedom. These factors can be modified by calcination temperature [18], pressure [19], impurities [20], [21] and particle size [22], [23].
At present, there are very intense research activities to enhance the colossal magnetoresistance effect present in manganites by making composites of these materials with secondary phases [24], [25], [26]. Additionally, these materials are very convenient for the preparation routes, and their Curie temperature can be adjusted under the various doping conditions. Moreover, their extraordinary chemical stability and tunable phase transition are also added advantages to choose manganites as magnetic refrigerant. Therefore, the new trends have been focusing on studying the MCE of manganite composite [27], [28], [29]. Besides, developing new materials with interesting MCE, improving and optimizing the magnetocaloric properties of the existing materials are a very active field of research [30], [31]. For practical application, in addition to the large magnetic entropy change (ΔS), the magnetic refrigeration compound needs a large refrigerant capacity (RC) [32], [33]. The key issue for a magnetic refrigeration material is increasing RC with keeping a large ΔS. An enhancement of RC has been observed in materials having magnetic multi-phases. Such observation is due to the broadened width of the δTFWHM as compared to those in single phase, though the former materials exhibited smaller ΔS value [34], [35].
The Ericsson cycle has been considered as the optimal process for magnetic cooling at room temperature [36]. In this process, a nearly constant high magnitude of ΔS over a wide temperature span (so called table-like) is required. The efficiency and feasibility of the optimal refrigeration cycle maximize both RC and δTFWHM, which is perfectly implemented in the composite materials. Moreover, the presence of two transitions broadens the ∆S curves which improved the refrigerant capacity [37], [38]. The RC is an important figure of merit for evaluating magnetocaloric properties. It allows an easy comparison of different magnetic materials. This parameter quantifies the measure of how much heat is transferred between the hot and cold sides in an ideal thermodynamic cycle [39]. Therefore, from a practical point of view, it is important to enhance several MCE properties of materials in order to broaden the entropy changes and maximize both δTFWHM and RC. This fact is not easy to be accomplished by a single material. To prevail over this limitation, the composite of the second phase compound and manganite could be an effective route to improve the magnetic refrigeration as well as the RC properties. Recently, Anwar et al. studied 10% cobalt oxide as a secondary phase to modify the properties of the La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 samples, and suggested that the cobalt oxide addition could effectively enhance their magnetocaloric properties [40]. This work suggests that searching and designing a new composite, based on La0.6Ba0.2Sr0.2MnO3 and cobalt oxide Co2O3, with successive transitions is a solution to improve magnetic cooling performance. The occurrence of maximum entropy change near room temperature makes it of much practical importance and it can be used for magnetic refrigeration.
Section snippets
Materials and methods
La0.6Ba0.2Sr0.2MnO3 (LBSMO) sample was synthesized by the conventional solid-state reaction method at high temperatures in air. Synthesis method is reported elsewhere [41]. The obtained LBSMO powders with single-phase perovskite structure were completely mixed with a commercial Co2O3 powder according to the nominal ratio of (LBSMO)1−x/(Co2O3)x with x=0, 0.05 and 0.1. The resulting (LBSMO)1−x/(Co2O3)x powders were pelletized and sintered for 13 h. The high sintering temperature was chosen to
Results and discussion
Fig. 1 depicts the Rietveld refinement of X-ray diffraction patterns of LBSMO sample recorded at room temperature. The sample exhibits sharp reflections, every detectable peak can be indexed via the space group of R–3c, and the structure of the sample is rhombohedral perovskite with (La, Ba and Sr) located at 6a (0, 0, 1/4), Mn at 6b (0, 0, 0), O at 18e (x, 0, 1/4) and there is no detectable impurity phase. For LBSMO, the lattice parameters are found to be a=b=5.496(1) Å and c=13.471(7) Å and the
Conclusion
It has been shown that the strong enhancement of RC can be obtained by combining La0.6Ba0.2Sr0.2MnO3 and Co2O3. The two closely spaced magnetic transitions (TC1 and TC2) of (LBSMO)0.95/(Co2O3)0.05 lead to a nearly flat magnetic entropy change in a wide temperature range which is favorable to an Ericsson-type refrigerator. ΔS (T) composite with 5% of Co2O3 is a promising candidate for near room temperature magnetic refrigeration applications because of the large refrigerant capacity and the wide
Acknowledgment
This study was supported by the Tunisian Ministry of Higher Education and Scientific Research and the Neel Institute.
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