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 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

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,1, aZengxin Ren,1Jie Guo,1Pingzhang Si,2and Mingze Sun3

1School of Science, Shenyang Ligong University, Shenyang 110159, China

2College of Materials Science and Engineering, China Jiliang University,

Hangzhou 310018, China

3Shenyang No 2 High School, Shenyang 110016, China

(Presented 3 November 2016; received 21 September 2016; accepted 31 October 2016;

published online 24 January 2017)

Plate-shaped La0.6Pr0.4Fe11.4Si1.6B0.2Hxbulk samples have been achieved with sin-tering 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 Fe2B 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 ∆Sm of 16.2 J/kg·K and a favorable small hysteresis of 0.6 J/kg for a field change from 0 to 5 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 refriger-ants of La(Fe, Si)13Hxhydrides © 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, com-pared with other giant MCE materails such as Gd5Ge2Si2 and, MnFeP1-xAsx , have low

mate-rial costs and consist of non-toxic constituting elements However, the Curie temperature (TC),

∼210 K, of the compound is well below the working temperature for room-temperature applica-tion.3Although substitution of Fe and/or interstitial substitution of small atoms can increase TC,4 7 hydrogenation of La(Fe, Si)13 is still the most efficient method to shift TC to room temperature while the large MCE is retained.8 A suitable structure for a typical active magnetic regenera-tor (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 1 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 La0.5Pr0.5Fe11.4Si1.6Hx thin plates with high magnetic-refrigeration performance were prepared by sintering in a high-pressure H2 atmosphere of 50 MPa to sup-press 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 LaFe11.5Si1.5

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

signifi-cantly decreases the amount of residual α-Fe.14It has been reported that addition of B improves the formation of NaZn13-type phase in the as-cast LaFe11.5Si1.5Bx alloys and is also benefi-cial for the reduction of hysteresis loss.15 In this paper, thin plates of La0.6Pr0.4Fe11.4Si1.6B0.2Hx

have been fabricated in a high-pressure H2 atmosphere and the effect of B-doping on the pre-cipitation of α-Fe, the magnetism and the MCE of the plate-shaped hydrides are systematically explored

The parent compound La0.6Pr0.4Fe11.4Si1.6B0.2was 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 oC 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 H2atmosphere of 50 MPa at 500oC for five hours The hydro-genated samples were pressed into thin plates (10 mm diameter, 0.8 mm thickness) and sintered for

24 hours at 600 oC 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 ther-mal 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 supercon-ducting 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 3 or 4 K

Figure 1(a) shows selected XRD profiles of bulk La0.6Pr0.4Fe11.4Si1.6B0.2 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 La0.6Pr0.4Fe11.4Si1.6B0.2 Besides, a small amount of α-Fe,

Fe2B and Pr2Fe14B 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 La0.5Pr0.5Fe11.4Si1.6Hx

remarkably increases from 2.8% to 9.1%.13 As shown in TableI, for the samples sintered for 24

Fig 1 (a) XRD patterns of bulk La0.6Pr0.4Fe11.4Si1.6B0.2 and its hydrides The inset shows a sintered thin plate of a hydride (b) and (c) Thermogravimetric curves of samples A and B, respectively.

TABLE I Lattice constants and phase concentrations (wt%) in bulk La0.6Pr0.4Fe11.4Si1.6B0.2 and its hydrides.

to 48 hours, the amount of α-Fe in La0.6Pr0.4Fe11.4Si1.6B0.2Hx is 3% and 4.1%, respectively and the amount of Fe2B 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 Fe2B Figures1 (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, imply-ing a hydrogen-desorption process The H content x of La0.6Pr0.4Fe11.4Si1.6B0.2Hx, as determined

by mass loss percentage of 0.26% and 0.21%, is 2.1 and 1.7 for the samples A and B, respec-tively The difference in H content may come from the more precipitation of α-Fe and Fe2B for sample B

Figures2(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 La0.6Pr0.4Fe11.4Si1.6B0.2

and its hydrides was measured in a field of 0.02 T in both warming and cooling processes (Fig.3(a))

The Curie temperature, T C , 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 T C of La0.6Pr0.4Fe11.4Si1.6B0.2

hydrides compared with La0.5Pr0.5Fe11.4Si1.6 hydrides13 can be mainly ascribed to the B addition

The M(B) curves of La0.6Pr0.4Fe11.4Si1.6B0.2 and its hydrides are shown in Figs 3(b)–(d)

Favorably, around T C 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 2 (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 3 (a) Temperature dependence of the magnetization at 0.02 T of bulk La0.6Pr0.4Fe11.4Si1.6B0.2 and samples A and B, Magnetic isotherms of (b) bulk La0.6Pr0.4Fe11.4Si1.6B0.2, (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.18As shown in Fig.3(a), the thermal hysteresis is lower than 1 K, which means that a standard process can be used for the present measurement In LaFe11.6Si1.4the M–H loops show “flaring” as the field rate is increased from 1 to 18 mT s-1resulting

in increased magnetic hysteresis.9For comparison, the same field rate of 5.5 mT s-1is employed for all the Isothermal measurement of La0.6Pr0.4Fe11.4Si1.6B0.2Hxand La0.5Pr0.5Fe11.4Si1.6Hx.13

As shown in Fig.5(a), the maximal hysteresis loss, is only 1.5 J/kg for a field change of 5 T for B-doped bulk La0.6Pr0.4Fe11.4Si1.6 The maximal hysteresis loss of La0.5Pr0.5Fe11.4Si1.6 for a field change of 1.5 T is 9 J/kg,13 which means that B-doping remarkably reduces the hysteresis loss in La(Fe, Si)13 For thin plates of La0.6Pr0.4Fe11.4Si1.6B0.2Hx, sintered for 24 and 48 hours, the maximal hysteresis loss is 0.6 and 2.7 J/kg, respectively In contrast, for La0.5Pr0.5Fe11.4Si1.6Hx

thin plates, sintered at the same conditions and for the same time, the maximal hysteresis loss is 1.1 and 4 J/kg, respectively.13 These results show that B-doping is beneficial for the reduction

of hysteresis loss in both bulk La0.6Pr0.4Fe11.4Si1.6 and its hydrides It is also worthwhile not-ing 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

fer-romagnetic impurity phases with higher T C of 1043 K for α-Fe, 1015 K for Fe2B and 569 K15 for Pr2Fe14B.21

The formation of nanoparticles in MnAs-based compounds22,23and the reduction of particle size

in La(Fe, Si)1324have been shown to be effective to reduce thermal losses in magnetic-refrigeration applications More importantly, it has been reported that, in La0.67Sr0.33MnO3 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

FIG 4 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 790×790×960 µm.

analyze the micropores in the present La0.6Pr0.4Fe11.4Si1.6B0.2Hx thin plates Three typical recon-structed 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 4 to 39 µm can be clearly observed on the slices After recon-struction, the 3-D morphology of micropores in a volume of 790×790×960 µm is shown in Fig.4(d) Different colors represent the distribution of micropores with different sizes The equivalent diameter26distribution, volume and porosity of pores from the XRM analysis are listed in TableII 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 accompa-nied by a large volume expansion of up to 1.35%,27leading to a remarkable reduction of hysteresis loss in the La0.6Pr0.4Fe11.4Si1.6B0.2Hxthin plates

The temperature dependence of ∆S m (T,B) derived by means of the Maxwell relationship is

shown in Figs5(b)–(d) The maximum value of ∆S mfor a magnetic field change of 5 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 La0.6Pr0.4Fe11.4Si1.6B0.2Hxthin 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.8The calculated RC eff for a magnetic field change of 5 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.

056419-6 Sun et al. AIP Advances 7, 056419 (2017)

FIG 5 (a) Hysteresis loss as a function of temperature for bulk La0.6Pr0.4Fe11.4Si1.6B0.2 and its hydrides Temperature

dependence of ∆Smfor (b) the bulk material, (c) sample A and (d) sample B.

RC eff at 340 K is comparable to that of Gd5Ge1.9Si2Fe0.1(about 355 J/kg at 305 K)1and is much larger than the value for Ni50Mn34In16(about 181 J/kg at 304 K).28

In conclusion, addition of B leads to formation of Fe2B and Pr2Fe14B impurity phases in the bulk

La0.6Pr0.4Fe11.4Si1.6and the phase concentrations remarkably increases during the H2high-pressure sintering process At 340 K, a high-density sintered bulk sample shows a large magnetic-entropy

change ∆Smof 16.2 J/kg·K and an effective refrigerant capacity of 311 J/kg for a field change of 5 T The result shows that La0.6Pr0.4Fe11.4Si1.6B0.2Hx 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 National Natural Science Foundation of China (No.51271179), by the Liaoning Provincial Natural Science Foundation (No 2013020105) and by the Shenyang Science and Technology Foundation (No F13-316-1-39)

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