Optimization of magnetocaloric properties of arc-melted and spark plasma-sintered LaFe11.6Si1.4
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LaFe11.6Si1.4 alloy has been synthesized in polycrystalline form using both arc melting and spark plasma sintering (SPS). The phase formation, hysteresis loss and magnetocaloric properties of the LaFe11.6Si1.4 alloys synthesized using the two different techniques are compared. The annealing time required to obtain the 1:13 phase is significantly reduced from 14 days (using the arc melting technique) to 30 min (using the SPS technique). The magnetic entropy change (ΔSM) for the arc-melted LaFe11.6Si1.4 compound, obtained for a field change of 5 − 0T (decreasing field), was estimated to be 19.6 J kg−1 K−1. The effective RCP at 5T of the arc-melted LaFe11.6Si1.4 compound was determined to be 360 J kg−1 which corresponds to about 88 % of that observed in Gd. A significant reduction in the hysteretic losses in the SPS LaFe11.6Si1.4 compound was observed. The ΔSM, obtained for a field change of 5 − 0T (decreasing field), for the SPS LaFe11.6Si1.4 compound decreases to 7.4 J kg−1 K−1. The TC also shifts from 186 (arc-melted) to 230 K (SPS) and shifts the order of phase transition from first to second order, respectively. The MCE of the SPS LaFe11.6Si1.4 compound spreads over a larger temperature range with the RCP value at 5T reaching 288 J kg−1 corresponding to about 70 % of that observed in Gd. At low fields, the effective RCP values of the arc-melted and spark plasma-sintered LaFe11.6Si1.4 compounds are comparable, thereby clearly demonstrating the potential of SPS LaFe11.6Si1.4 compounds in low-field magnetic refrigeration applications.
Conventional refrigeration technologies based on the gas-compression/expansion cooling mechanisms may gradually be replaced by the environmentally friendly and more efficient magnetic refrigeration in the near future [1, 2, 3, 4, 5]. Magnetic refrigeration is based on the magnetocaloric effect (MCE) which results from the coupling of a system of magnetic moments with an external magnetic field resulting in the cooling or heating of a system. Recently, a new class of magnetic materials has emerged, called giant magnetocaloric effect (GMCE) materials, which undergo a phase transition from one form of magnetic order to another with an associated “giant” change in entropy. The La(FexSi1−x)13 material system belongs to this GMCE group of materials. It also has an added advantage of consisting of low-cost elements. The only drawback of this material system, however, is the problem of low productivity in manufacturing La(FexSi1−x)13-type materials. The NaZn13 phase is rarely directly generated in ingots prepared by conventional methods such as arc melting due to an incomplete peritectic reaction which results in the mixed phases of α-Fe + La(Fe,Si)13(τ1a) + LaFeSi(τ4) . Accordingly, a long annealing time (typically 14–30 days) is necessary to form the NaZn13 structure. Therefore, it is imperative to find a quicker and cheaper way of synthesizing these alloys, promoting the affordability of the La(FexSi1−x)13-based magnetic refrigerators.
This work seeks to explore the synthesis of the La(FexSi1−x)13-type materials using the spark plasma sintering (SPS) technique. In the spark plasma sintering technique, the powders are heated by the joule effect and the spark which is generated in the spaces between the powder particles activates the surface of these particles generating a self-heating approach which leads to significantly shorter sintering times required and results in fine grain structures . Compared to arc melting, the SPS technique occurs at relatively low average temperatures and is completed in shorter periods of time resulting in a tight control over grain growth and microstructure . Due to the above-mentioned reasons, we expect the La(FexSi1−x)13 alloys prepared by SPS to require significantly shorter heat treatment times for obtaining the cubic NaZn13 crystal structure-type phase. Shorter heat treatment times will significantly reduce the cost of synthesizing the La(FexSi1−x)13 alloys which in turn have a positive impact on the affordability of La(FexSi1−x)13-based magnetic refrigerators. Also, we expect the resulting microstructure in SPS La(FexSi1−x)13 to influence the magnetic hysteresis in this material system, similar to what was observed by Idza et al.  for the polycrystalline nickel–zinc ferrite Ni0.3Zn0.7Fe2O4 . If indeed the magnetic hysteresis is significantly reduced in the SPS La(FexSi1−x)13, this will consequently improve its magnetocaloric properties.
In this work, the effect of a solid-state reaction using the spark plasma sintering (SPS) technique to form the NaZn13 structure is investigated and compared with the conventional arc melting method. The SPS synthesis of LaFe11.6Si1.4 has been investigated for a range of sintering conditions such as temperature, pressure, and holding time. We then performed a heat treatment study to optimize the 1:13 phase yield in the SPS LaFe11.6Si1.4. Previously, LaFe11.6Si1.4 has been synthesized using the SPS technique but to date, there has been one report on the magnetocaloric properties of SPS synthesized La(FexSi1−x)13 alloys [10, 11]. The results reported here will make a significant contribution in demonstrating the potential of SPS La(FexSi1−x)13 alloys for magnetic refrigeration applications making use of permanent magnets.
2 Experimental procedure
Spark plasma sintering and annealing conditions for spark plasma-sintered LaFe11.6Si1.4 alloys
Particle size of starting powders (μm)
Applied pressure (MPa)
Holding time (min)
Holding temp (K)
Annealing temp (K)
Annealing time (h)
The resulting LaFe11.6Si1.4 pellets were ground, polished, and wrapped in tantalum foil, annealed at 1373–1523 K for 30 min to 72 h in an evacuated quartz tube and then quenched in water in order to obtain the NaZn13-type main phase. The structure was determined using room-temperature powder X-ray diffraction (XRD) with Cu Kα radiation. The microstructures of the spark plasma-sintered and arc-melted LaFe11.6Si1.4 alloys were investigated using an FEI Inspect F (FEI, Netherlands) scanning electron microscope (SEM). The magnetization measurements were taken using the vibration sample magnetometer option of a Quantum Design 6T MPMS SQUID VSM system in the temperature range of 150–300 K at applied fields of up to 5T.
3 Results and discussion
MCE properties of benchmark magnetocaloric materials
Order of transition
ASM (5T) (J/kg K)
RCP (5T) (J/kg)
SPS LaFe11.4Si1.6 
Arc-melted Gd 
Arc-melted LaFe11.4Si1.6 
Arc-melted La0.8Gd0.2Fe11.4Si1.6B0.3 
SPS LaFe11.6Si1.4 (this work)
Arc-melted LaFe11.6Si1.4 (this work)
Spark plasma sintering of the LaFe11.6Si1.4 alloys has been investigated in this work. Using this method, the heat treatment times required to yield or obtain the 1:13 phase was drastically reduced from 14 days to 30 min making SPS a quicker and cheaper way of synthesizing these alloys, consequently presenting advantages in terms of energy consumption. Reducing the cost of synthesizing these alloys will have a positive impact on the affordability of LaFe11.6Si1.4 alloy-based magnetic refrigerators.
The main conclusions of this work are as follows: (1) The MCE of SPS LaFe11.6Si1.4 alloys is lower than the MCE reported for benchmark magnetocalorics such as Gd and Gd5Si2Ge2 at 5T but competitive at 2T, as shown in Table 2. (2) The SPS LaFe11.6Si1.4 alloys require a short heat treatment time of 30 min, thus making its synthesis cost effective.
Also a broader working temperature range of the MCE is obtained in the SPS LaFe11.6Si1.4 alloy as compared to the arc-melted LaFe11.6Si1.4 alloy. In the SPS LaFe11.6Si1.4 alloy, the order of transition is shifted to second order eliminating both thermal and magnetic hysteresis. In this work, we have also shown that in the field range, 0–2T, the relative cooling power of the SPS LaFe11.6Si1.4 alloy is competitive with the arc-melted LaFe11.6Si1.4 alloy. We are currently exploring various methods (such as varying particle size of starting powders and heat treatment investigations) to significantly increase the amount of 1:13 phase in our SPS LaFe11.6Si1.4 alloys, and if these efforts prove successful, they will be reported in due course.
This research was funded from the Engineering and Physical Sciences Research Council (EPSRC) (Grant Number L017563). Professor Damian Hampshire and Dr Mark Raine (Physics Department, Durham University) are gratefully acknowledged for help with the magnetic measurements.
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