Effect of B doping on the structure and magnetocaloric properties of plate shaped La0 6Pr0 4Fe11 4Si1 6Hx sintered in high pressure H2 atmosphere Effect of B doping on the structure and magnetocaloric[.]
Effect of B-doping on the structure and magnetocaloric properties of plate-shaped La0.6Pr0.4Fe11.4Si1.6Hx sintered in high-pressure H2 atmosphere Naikun Sun, Zengxin Ren, Jie Guo, Pingzhang Si, and Mingze Sun Citation: AIP Advances 7, 056419 (2017); doi: 10.1063/1.4974979 View online: http://dx.doi.org/10.1063/1.4974979 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics AIP ADVANCES 7, 056419 (2017) Effect of B-doping on the structure and magnetocaloric properties of plate-shaped La0.6 Pr0.4 Fe11.4 Si1.6 Hx sintered in high-pressure H2 atmosphere Naikun Sun,1,a Zengxin Ren,1 Jie Guo,1 Pingzhang Si,2 and Mingze Sun3 School of Science, Shenyang Ligong University, Shenyang 110159, China of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China Shenyang No High School, Shenyang 110016, China College (Presented November 2016; received 21 September 2016; accepted 31 October 2016; published online 24 January 2017) Plate-shaped La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx bulk samples have been achieved with sintering in a high-purity H2 atmosphere at 50 MPa The effect of B-doping on the structure, magnetism and magnetocaloric properties of the plate-shaped hydrides has been systematically explored The results show that B-doping unfavorably leads to a remarkable increase of Fe2 B during the sintering process and has not helped much in the 1:13 phase stabilization and/or in the magnetocaloric properties At 340 K, a high-density sintered thin plate shows a large magnetic-entropy change ∆S m of 16.2 J/kg·K and a favorable small hysteresis of 0.6 J/kg for a field change from to T High-resolution X-ray microtomography analysis shows that micropores exist in the thin plates causing a porosity of 0.26% and leading to a remarkable reduction of the hysteresis This work opens an effective route for synthesizing thin magnetic refrigerants of La(Fe, Si)13 Hx hydrides © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4974979] The ternary La(Fe, Si)13 compounds show a large magnetocaloric effect (MCE) and, compared with other giant MCE materails such as Gd5 Ge2 Si2 and, MnFeP1-x Asx , have low material costs and consist of non-toxic constituting elements However, the Curie temperature (T C ), ∼210 K, of the compound is well below the working temperature for room-temperature application.3 Although substitution of Fe and/or interstitial substitution of small atoms can increase T C ,4–7 hydrogenation of La(Fe, Si)13 is still the most efficient method to shift T C to room temperature while the large MCE is retained.8 A suitable structure for a typical active magnetic regenerator (AMR) can be designed by aligning thin plates of the refrigerants parallel to each other Lovell et al demonstrated the importance of sample shape in terms of engineering the hysteretic behavior for giant MCE material beyond the obvious role of demagnetization.9 It is a challenge for La(Fe, Si)13 to be made into thin-plate form with thickness well below mm,10 because microcracks form upon cutting the material.11 Thin plates of La(Fe,Si)13 refrigerants have been successfully fabricated through a thermally induced decomposition and recombination process.12 However, it is well expected that upon hydrogenation, the thin plates can be destroyed by hydrogen embrittlement In our previous work,13 La0.5 Pr0.5 Fe11.4 Si1.6 Hx thin plates with high magnetic-refrigeration performance were prepared by sintering in a high-pressure H2 atmosphere of 50 MPa to suppress the desorption of hydrogen It has been found that, upon increasing the sintering time, a large amount of α-Fe precipitates from the main phase, leading to a remarkable reduction of the magnetic-entropy change ∆Sm The influence of B addition on secondary phase α-Fe in LaFe11.5 Si1.5 a naikunsun@163.com 2158-3226/2017/7(5)/056419/7 7, 056419-1 © Author(s) 2017 056419-2 Sun et al AIP Advances 7, 056419 (2017) melt-spun ribbons has been investigated by Xie et al and the result shows that B-doping significantly decreases the amount of residual α-Fe.14 It has been reported that addition of B improves the formation of NaZn13 -type phase in the as-cast LaFe11.5 Si1.5 Bx alloys and is also beneficial for the reduction of hysteresis loss.15 In this paper, thin plates of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx have been fabricated in a high-pressure H2 atmosphere and the effect of B-doping on the precipitation of α-Fe, the magnetism and the MCE of the plate-shaped hydrides are systematically explored The parent compound La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 was synthesized in an induction furnace from the starting materials La, Pr and Fe with commercial purity of 99%, and Si and B with purity of 99.99% and the obtained ingots were annealed at 1200 o C for 12 hours in a high-purity Ar atmosphere The samples were then ground to particles of 100–150 mm and hydrogenated in a self-made furnace in a high-purity H2 atmosphere of 50 MPa at 500 o C for five hours The hydrogenated samples were pressed into thin plates (10 mm diameter, 0.8 mm thickness) and sintered for 24 hours at 600 o C in a high-purity H2 atmosphere of 50 MPa (named sample A) Then, part of the as-sintered sample was pulverized, pressed into pellets again and sintered for another 24 hours (named sample B) X-ray diffraction (XRD) was performed using Cu − Kα radiation in a Rigaku d/Max-γA diffractometer and JADE 5.0 software was employed to refine the cell parameters.16 The quantitative analysis of different phases was studied by K-value method and the percentage error is 5%, which was described in detail in Ref 17 The H contents were determined by thermal gravimetric (TG) analysis using a Netzsch STA 449C instrument The microstructures were examined by means of a Hitachi-3400N scanning electron microscope (SEM) The micropores have been analyzed using high-resolution X-ray microtomography (XRM) with Versa XRM-500 and an acceleration voltage of 140 kV The magnetic properties were measured by means of a superconducting quantum interference device (SQUID) magnetometer and the magnetization isotherms were measured using a standard process18 with a field sweep rate of 5.5 mT s-1 and temperature span of or K Figure 1(a) shows selected XRD profiles of bulk La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 and of samples A and B The inset shows sintered thin plates The bulk crystallizes well in the cubic NaZn13 -type structure, indicating H insertion into La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Besides, a small amount of α-Fe, Fe2 B and Pr2 Fe14 B exists as impurity phases The lattice parameter has increased from 11.492 Å for bulk to 11.681 and 11.697 Å for samples A and B, respectively In our previous experiments, with sintering time increasing from 24 to 48 hours, the amount of α-Fe in La0.5 Pr0.5 Fe11.4 Si1.6 Hx remarkably increases from 2.8% to 9.1%.13 As shown in Table I, for the samples sintered for 24 Fig (a) XRD patterns of bulk La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 and its hydrides The inset shows a sintered thin plate of a hydride (b) and (c) Thermogravimetric curves of samples A and B, respectively 056419-3 Sun et al AIP Advances 7, 056419 (2017) TABLE I Lattice constants and phase concentrations (wt%) in bulk La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 and its hydrides Bulk Sample A Sample B A (Å) α-Fe Fe2 B Pr2 Fe14 B 1:13 phase 11.492(5) 11.681(7) 11.697(8) 1.6 4.1 1.1 2.8 4.9 1.7 3.2 95.6 91 88 to 48 hours, the amount of α-Fe in La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx is 3% and 4.1%, respectively and the amount of Fe2 B almost doubled These results indicate that B-doping is beneficial to suppress the precipitation of α-Fe from the 1:13 main phase during the process of sintering, but unfavorably leads to a remarkable increase of Fe2 B Figures (b) and (c) show the TG curves of samples A and B From about 450 K, the TG curve exhibits an abrupt weight loss until about 650 K, implying a hydrogen-desorption process The H content x of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx , as determined by mass loss percentage of 0.26% and 0.21%, is 2.1 and 1.7 for the samples A and B, respectively The difference in H content may come from the more precipitation of α-Fe and Fe2 B for sample B Figures 2(a), (b) show the SEM micrographs of the samples A and B, respectively The surface evenness of sample B is much better than of sample A and no microcracks are observed at the surface of the thin plates A small amount of La-rich phase (white) and α-Fe (black) can be clearly seen.19,20 Figures 2(c) and (d) show the fracture morphology of sample A It can be clearly seen that the thickness of the thin plate is from 0.7 to 0.8 mm and a small amount of α-Fe (black) is distributed in the main phase (gray) The temperature dependence of the magnetization of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 and its hydrides was measured in a field of 0.02 T in both warming and cooling processes (Fig 3(a)) The Curie temperature, TC , defined as the minimum of dM/dT vs T curves, is found to be 200, 340 and 335 K for the bulk, A and B samples, respectively The higher TC of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 hydrides compared with La0.5 Pr0.5 Fe11.4 Si1.6 hydrides13 can be mainly ascribed to the B addition The M(B) curves of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 and its hydrides are shown in Figs 3(b)–(d) Favorably, around TC the warming and cooling magnetization curves for the three samples almost overlap, indicating a small hysteresis loss (defined as the enclosed area in the field cycle) Isothermal Fig (a) and (b) SEM images of samples A and B, respectively (c) and (d) Fracture morphology of sample A 056419-4 Sun et al AIP Advances 7, 056419 (2017) Fig (a) Temperature dependence of the magnetization at 0.02 T of bulk La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 and samples A and B, Magnetic isotherms of (b) bulk La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 , (c) sample A and (d) sample B around TC measurement process shows a remarkable influence on MCE When thermal hysteresis is larger than the step width of the magnetization isotherms, a loop process under equilibrium conditions is needed to ensure the validity of the measurement result.18 As shown in Fig 3(a), the thermal hysteresis is lower than K, which means that a standard process can be used for the present measurement In LaFe11.6 Si1.4 the M–H loops show “flaring” as the field rate is increased from to 18 mT s-1 resulting in increased magnetic hysteresis.9 For comparison, the same field rate of 5.5 mT s-1 is employed for all the Isothermal measurement of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx and La0.5 Pr0.5 Fe11.4 Si1.6 Hx 13 As shown in Fig 5(a), the maximal hysteresis loss, is only 1.5 J/kg for a field change of T for B-doped bulk La0.6 Pr0.4 Fe11.4 Si1.6 The maximal hysteresis loss of La0.5 Pr0.5 Fe11.4 Si1.6 for a field change of 1.5 T is J/kg,13 which means that B-doping remarkably reduces the hysteresis loss in La(Fe, Si)13 For thin plates of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx , sintered for 24 and 48 hours, the maximal hysteresis loss is 0.6 and 2.7 J/kg, respectively In contrast, for La0.5 Pr0.5 Fe11.4 Si1.6 Hx thin plates, sintered at the same conditions and for the same time, the maximal hysteresis loss is 1.1 and J/kg, respectively.13 These results show that B-doping is beneficial for the reduction of hysteresis loss in both bulk La0.6 Pr0.4 Fe11.4 Si1.6 and its hydrides It is also worthwhile noting that the isothermal magnetization measurements show a rather large magnetization value in the paramagnetic phase of samples A and B, which can be ascribed to the great increase of ferromagnetic impurity phases with higher TC of 1043 K for α-Fe, 1015 K for Fe2 B and 569 K 15 for Pr2 Fe14 B.21 The formation of nanoparticles in MnAs-based compounds22,23 and the reduction of particle size in La(Fe, Si)13 24 have been shown to be effective to reduce thermal losses in magnetic-refrigeration applications More importantly, it has been reported that, in La0.67 Sr0.33 MnO3 films, the epitaxial strain plays a significant role in determining the peak position of isothermal magnetic-entropy change with improved cooling capacity.25 In order to explore the mechanism of the hysteresis reduction, high-resolution XRM, which allows in situ microtomography observations have been employed to 056419-5 Sun et al AIP Advances 7, 056419 (2017) FIG Reconstructed 2-D slices of sample A (a) The transverse section, (b) and (c) two vertical longitudinal sections, and (d) 3-D morphology of micropores in a volume of 790ì790ì960 àm analyze the micropores in the present La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx thin plates Three typical reconstructed 2-D slices, representing the transverse and two vertical longitudinal sections of the bar sample, are shown in Figs 4(a), (b) and (c) respectively The black regions represent the pore phase and micropore ranges from to 39 µm can be clearly observed on the slices After reconstruction, the 3-D morphology of micropores in a volume of 790ì790ì960 àm is shown in Fig 4(d) Different colors represent the distribution of micropores with different sizes The equivalent diameter 26 distribution, volume and porosity of pores from the XRM analysis are listed in Table II The porosity in the whole analyzed volume is 0.26% It can well be expected that the micropores with a large size distribution relieve the internal strains during the magneto-structural transition accompanied by a large volume expansion of up to 1.35%,27 leading to a remarkable reduction of hysteresis loss in the La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx thin plates The temperature dependence of ∆Sm (T,B) derived by means of the Maxwell relationship is shown in Figs 5(b)–(d) The maximum value of ∆Sm for a magnetic field change of T (or 2T) is 18.1 J/kg·K (or 13.1 J/kg·K) at about 204 K for the bulk parent alloy, 16.2 J/kg·K (or 9.7 J/kg·K) at about 340 K for sample A and 11.9 J/kg·K (7.6 J/kg·K) at about 335 K for sample B Compared to sample B, sintered for 48 hours, sample A, which has been sintered for 24 hours, exhibits a larger magnetic-entropy change and smaller hysteresis loss This result means that 24-hour sintering is the optimal sintering time for the fabrication of La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx thin plates From the aspect of technological application, the effective refrigerant capacity (RC eff ) is considered to be another important factor for assessing magnetic refrigerant materials.8 The calculated RC eff for a magnetic field change of T is 311 J/kg for sample A and 248 J/kg for sample B For sample A, the value of TABLE II Equivalent diameter distribution (P1 -P5 —1-5, 5-10, 10-20, 20-50 and 50-100 µm, respectively), volume (VP —volume of pores, VT —total volume) and porosity (VP /VT ) of pores from XRM analysis P1 P2 P3 P4 P5 VP (µm3 ) VT (µm3 ) Porosity 895 795 285 40 1570000 604000000 0.26% 056419-6 Sun et al AIP Advances 7, 056419 (2017) FIG (a) Hysteresis loss as a function of temperature for bulk La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 and its hydrides Temperature dependence of ∆Sm for (b) the bulk material, (c) sample A and (d) sample B RC eff at 340 K is comparable to that of Gd5 Ge1.9 Si2 Fe0.1 (about 355 J/kg at 305 K)1 and is much larger than the value for Ni50 Mn34 In16 (about 181 J/kg at 304 K).28 In conclusion, addition of B leads to formation of Fe2 B and Pr2 Fe14 B impurity phases in the bulk La0.6 Pr0.4 Fe11.4 Si1.6 and the phase concentrations remarkably increases during the H2 high-pressure sintering process At 340 K, a high-density sintered bulk sample shows a large magnetic-entropy change ∆Sm of 16.2 J/kg·K and an effective refrigerant capacity of 311 J/kg for a field change of T The result shows that La0.6 Pr0.4 Fe11.4 Si1.6 B0.2 Hx compounds fabricated by high-pressure sintering are promising candidates for high-performance thin magnetic refrigerants for AMR ACKNOWLEDGMENTS This work has been supported by the 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Effect of B- doping on the structure and magnetocaloric properties of plate- shaped La0. 6 Pr0.4 Fe11.4 Si1.6 Hx sintered in high- pressure H2 atmosphere Naikun Sun,1,a Zengxin Ren,1 Jie Guo,1 Pingzhang... achieved with sintering in a high- purity H2 atmosphere at 50 MPa The effect of B- doping on the structure, magnetism and magnetocaloric properties of the plate- shaped hydrides has been systematically