recent development in refrigeration

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2010 International Symposium on Next-generation Air Conditioning and Refrigeration Technology, 17 – 19 February 2010, Tokyo, Japan

RECENT DEVELOPMENT OF MAGNETIC REFRIGERANT

MATERIALS NEAR ROOM TEMPERATURE

Hirofumi Wada,Professor, Department of Physics, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan

Department of Physics, Kyushu University, Koji Sadamatsu, Graduate Student, Department of Physics, Kyushu University,Hakozaki 6-10-1,

Higashi-ku, Fukuoka 812-8581, Japan Nobushi Mutsuki, Graduate Student, Department of Materials Science and Engineering,

Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Naoki Hirano, Chubu Electric Power Co.Inc.,Oodaka, Midori-ku,

Nagoya 459-8522 , Japan Shigeo Nagaya, Chubu Electric Power Co.Inc.,Oodaka, Midori-ku,

Nagoya 459-8522 , Japan

Abstract: The phase stability and magnetocaloric effect of La(FexCoySi1-x-y)13 compounds have been examined in wide concentration ranges of 0.84 x 0.90 and 0 y 0.08. It was found that the Curie temperature, TC, is decreased, while |SM| shows a maximum at an intermediate value of y, as y is increased for x=0.84 and 0.86. The substitution of Co for Si induces a broad itinerant electron metamagnetic transition, which is responsible for large |SM|. A complete map of TC and |SM| was constructed in the Fe-Co-Si ternary diagram, which suggests that the La(FexCoySi1-x-y)13 compounds

with 0.84 x 0.86 and 0.08 1-x-y 0.10 are promising candidates as magnetic refrigerant materials near room temperature.

Key Words: Magnetocaloric effect, Magnetic refrigeration 1 INTRODUCTION The magnetocaloric effect (MCE) means the isothermal magnetic entropy change (SM) or the adiabatic temperature change (Tad) by applying or removing a magnetic field on the magnetic materials. The magnetic refrigeration is a cooling technology based on the MCE. Because of high energy efficiency and environmental safety, magnetic refrigeration is expected to be an alternative cooling technology to the traditional gas refrigeration. Recently, several magnetic refrigeration systems working near room temperature have been developed. For practical application, it is strongly desired to search new magnetic materials with large MCEs near room temperature. In the last decade, several systems, such as Gd5Si2Ge2, MnAs1-xSbx, MnFeP0.45As0.55 and La(Fe1-xSix)13 have been found to exhibit giant MCEs near room temperature [1-4]. All of these compounds undergo a first-order magnetic transition (FOMT) from a ferromagnetic state to a paramagnetic state at their Curie temperatures, TC’s. This is because a ferromagnetic state is induced by a magnetic field above TC, which is accompanied by considerable entropy changes. In other words, we can make use of the latent heat at the FOMT in the entropy change. In this respect, this is analogous to conventional gas refrigeration, which utilizes the heat of vaporization of the refrigerants. However, there are some barriers to be overcome in the FOMT systems for practical application of the magnetic refrigerant materials. First, the FOMT systems show giant MCEs in the narrow temperature range. The temperature range for large MCEs is strongly dependent on a magnetic field and it is in the range of 5 – 10 K in a field change of 2 T. These values are much lower than the practical working temperature range of the refrigerator. In order to cover a wide temperature range, it is necessary to design composite materials, in which each constituent material works in a different temperature range. Second, the FOMT compounds show a large volume change at TC more or less. If a large volume change takes place frequently, the materials would break into smaller pieces. Finally, the FOMT is accompanied by thermal hysteresis, which is disadvantageous for refrigeration cycle. One method to improve refrigerant properties of the FOMT systems is to broaden a magnetic transition by alloying. Broadening the transition makes it possible to overlap a temperature range for large MCEs of

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the materials, although alloying reduces the MCE to some extent. Moreover, a broad magnetic transition results in a smooth variation of atomic volume with temperature and reduces thermal hysteresis. As an example, we report the development of magnetocaloric properties of the FOMT system, La(Fe-Si)13, by alloying. The La(Fe1-zSiz)13 compounds crystallize in the cubic NaZn13-type structure in a narrow Si concentration range of 0.11 < z 0.19 [5]. Fujita et al. have found that the compound with z = 0.12 undergoes a ferromagnetic to paramagnetic FOMT with the Curie temperature, TC, of 196 K [6]. Just above TC, the ferromagnetic state is induced by a magnetic field, which is called itinerant electron metamagnetism (IEM). Hu et al. have first observed that LaFe11.4Si1.6 (z = 0.123) exhibits a giant MCE with the magnetic entropy change, |SM|, of 19.4 J/K kg at TC = 208 K in a field change of 5 T [4]. In order to increase the magnetic ordering temperature without losing a large MCE, many studies have been reported on the influence of the fourth element substitutions in various La(Fe1-zSiz)13 based alloys [7-11]. Fujita et al. have reported that hydrogen absorption into La(Fe1-zSiz)13 increases TC up to 336 K with retaining a FOMT [7]. As a result, a giant MCE is obtained near room temperature in the La(Fe1-

zSiz)13 hydrides. However, the disadvantages of the FOMT remain in the hydrides. Since the isostructural LaCo13 compound is a ferromagnet with TC = 1260 K [5], the substitution of Co for Fe or Si is effective to raise the magnetic ordering temperature. The first report of a large MCE in the La(FexCoySi1-x-y)13 system was done by Hu et al. for LaFe11.2Co0.7Si1.1 in 2002 [8]. Subsequently, the MCE has been widely studied for different Si compositions [9-11]. To optimize the compounds for magnetic refrigerant materials near room temperature, it is necessary to study the magnetic and magnetocaloric properties of La(FexCoySi1-x-y)13. in wide composition ranges of x and y. In this paper, we report the effect of Co content on the structure and MCE of La(FexCoySi1-x-y)13 compounds with fixed x values of 0.84 x 0.90. 2 EXPERIMENTS Samples of La(Fe0.84CoySi0.16-y)13, La(Fe0.86CoySi0.14-y)13, La(Fe0.88CoySi0.12-y)13 and La(Fe0.90CoySi0.10-

y)13 with 0 y 0.08 were prepared by arc melting in an argon atmosphere. The ingots were sealed in an evacuated quartz tube and were annealed at 1050°C for two weeks. X-ray diffraction is used for phase identification. The temperature dependence of magnetization, M, was measured using a commercial SQUID magnetometer. The Curie temperature was determined from the M-T curves at a magnetic field of 0.01 T. The isothermal magnetic entropy change, SM, was estimated from the M-T curves in various magnetic fields by using the Maxwell relation,

H

HM dH

dT

dMS

0

. (1)

Although most of previous studies reported SM in 0-5 T, we mainly present SM in 0-2 T in this

study, because permanent magnets are used in real magnetic refrigeration systems at room temperature, of which maximum field is less than 2 T. 2 RESULTS AND DISCUSSION

Figure 1 shows X-ray diffraction patterns of La(Fe0.86CoySi0.14-y)13. The compound with y = 0 is almost single phase with the NaZn13-type structure, while the other compounds contain -Fe as a secondary phase. The amount of the secondary phase increases with increasing Co content. According to the previous studies [8-11], the occurrence of -Fe is inevitable in La(FexCoySi1-x-y)13 with high Co content. However, it has been reported that a small amount (10 wt%) of -Fe does not affect the MCE so much in the present system [8-11]. The amount of a secondary phase of La(Fe0.86CoySi0.14-y)13 with 0.04 y 0.07 was estimated as in the range of 415 wt% from the magnetization analysis. A further increase in the Co content causes appearance of a large amount of -Fe. Similar results were observed for the compounds with other Fe concentrations. The increase in the Fe content makes the formation of NaZn13-type compound difficult. In the case of La(Fe0.90CoySi0.10-

y)13, a large amount (30 wt%) of -Fe was detected for y = 0.04. We found that the lattice parameter of La(FexCoySi1-x-y)13 depends only on the Si content. For all the

series with different x, a decreases, as the Si content is increased except La(Fe0.86Co0.07Si0.07)13 and

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La(Fe0.84Co0.08Si0.08)13. This is because the atomic volume of Fe is nearly the same as that of Co in the NaZn13-type structure, while that of Si is much smaller. The compounds of La(Fe0.86Co0.07Si0.07)13 and La(Fe0.84Co0.08Si0.08)13 have exceptionally large a values, which are probably due to the magnetovolume effect near the magnetic ordering temperature . It is known that La(FexCoySi1-x-y)13 shows very large volume expansion below TC [8]. As described later, the Curie temperatures of La(Fe0.86Co0.07Si0.07)13 and La(Fe0.84Co0.08Si0.08)13 are 291 K and 307 K, respectively. The other compounds are in a paramagnetic state at room temperature.

The concentration dependence of the Curie temperature of La(FexCoySi1-x-y)13 was examined. In the ternary system of La(Fe1-zSiz)13, TC is decreased from 239 K to 190 K with increasing the Fe content from 0.84 to 0.90. The substitution of Co for Si of La(FexCoySi1-x-y)13 at a given x increases TC. This means that the Co substitution is effective to increase TC in this system. When TC is plotted as a function of the Fe content at the constant Si concentration, we found that TC is lowered with increasing Fe content nearly linearly with a slope of 15 K/%Fe, irrespective of Si concentration. Figure 2 shows the temperature dependence of the magnetic entropy change, |SM|, in 0-1 T and 0-2 T for La(Fe0.88CoySi0.12-y)13. The compound with y = 0 undergoes a FOMT at TC = 199 K. Sharp IEM was observed just above TC. The |SM| shows a sharp peak at around TC with a maximum value of Smax = 16.5 J/K kg in 2 T. With increasing y, the magnetic transition becomes broad and the IEM almost disappears at y=0.05. Correspondingly, Smax is decreased, as y is increased. However, the compound with y=0.05 still has large Smax of more than 10 J/K kg in 2 T. Unfortunately, the highest TC of x = 0.88 is 263 K (y=0.05), which is below room temperature. The |SM| of La(Fe0.86CoySi0.14-y)13 is depicted in Fig. 3 as a function of temperature. In contrast to x = 0.88, Smax has the highest value at y = 0.04 and is decreased with further increasing y. This is surprising, because TC increases monotonically, as y is increased. In order to study the nature of magnetic transition, we measured magnetization curves of y = 0.04 near TC and observed broad S-shaped magnetization curves. This indicates that broad IEM takes place above TC at y = 0.04, which is responsible for a large |SM|. Further increase in y broadens the magnetic transition and decreases Smax. Similar enhancement of |SM| was also observed in La(Fe0.84CoySi0.16-y)13. The present results have revealed that the substitution of a small amount of Co for Si increases the Curie temperature and at the same time induces broad IEM in La(FexCoySi1-x-y)13.

y=0

y=0.02

y=0.04

y=0.06

y=0.07

y=0.08

calculated pattern

Inte

nsity

(a.

u.)

2 (degree)

La(Fe0.86CoySi0.14-y)13

20 40 60 80

Fig. 1 X-ray diffraction patterns of La(Fe0.86CoySi0.14-y)13. Arrows indicate a diffraction peak due to the -Fe phase.

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The S-shaped magnetization curve was observed only in the limited composition ranges of x and y. In Fig. 4, we summarize the Smax and TC values of La(FexCoySi1-x-y)13 obtained in the present study plotted

|S M

| (J/

K k

g)

T (K)


y=0

y=0.02 y=0.04y=0.05

0-2T 0-1T

175 200 225 250 275 3000

5

10

15

20

Fig. 2 Temperature dependence of magnetic entropy change, |SM|, in 0-1 T and 0-2 T of

La(Fe0.88CoySi0.12-y)13.


y=0 y=0.02

y=0.04

y=0.06

y=0.07

|S M

| (J/

K k

g)

T (K)

0-2T 0-1T

175 200 225 250 275 300 3250

5

10

Fig. 3 Temperature dependence of magnetic entropy change, |SM|, in 0-1 T and 0-2 T of

La(Fe0.86CoySi0.14-y)13.

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in the Fe-Co-Si ternary diagram. The compositions of any point are represented by grid lines parallel to three edges of the triangle, as shown in the upper left triangle. The hatched portion is the area where a large amount of -Fe coexists with the NaZn13-type phase or where no NaZn13-type compound is formed. This means that at least 6~7% Si is necessary to form the NaZn13-type compound as a main phase for 0.84 x 0.90.

From these results, it is concluded that La(FexCoySi1-x-y)13 has TC near room temperature and a

relatively large |SM| value in the concentration ranges of 0.84 x 0.86 and 0.08 1-x-y 0.10 (shown by the shadow area in Fig. 4).

Compared with the La(Fe1-zSiz)13 hydrides, the present system has smaller Smax. On the other hand, the half width of |SM| peak, T, of La(FexCoySi1-x-y)13 is larger. This is because the transition becomes smooth in La(FexCoySi1-x-y)13, while a sharp FOMT is retained in the hydrides. The half width of |SM| in 0-2 T of La(Fe0.84Co0.06Si0.10)13 is about 20 K, which is almost twice as that of La(Fe0.88Si0.12)13H1.0 with a comparable Curie temperature [7]. This means that the La(FexCoySi1-x-y)13 material can work as magnetic refrigerants in a wide temperature range. Tishin and Spinichkin introduced the relative cooling power, RCP as a measure of efficiency of cooling cycle [3], which is given by,

.RCP max TS (2)

Fig. 4 Maximum value of the magnetic entropy change, Smax, (J/K kg) in 0-2 T and

TC of La(FexCoySi1-x-y)13 plotted in the Fe-Co-Si ternary diagram. The hatched portion is the area where the NaZn13-type phase is not a main phase. The compounds in the shadow area exhibit a relatively large Smax value with TC near room temperature (see text).

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Figure 5 displays the temperature dependence of |SM| of La(Fe0.86Co0.06Si0.08)13 in various magnetic field changes. From the results, RCP was evaluated, which is illustrated as a function of a maximum field in Fig. 6. It is found that RCP is a linear function of a maximum field. This behaviour is characteristics of the FOMT, because Smax is independent of a magnetic field, while the width, T, increases with increasing a maximum field in the FOMT systems. In contrast, the second-order magnetic transition systems, such as Gd, show parabolic field dependence of RCP. Although both |SM| and T are field dependent, as shown in Fig. 5, the present system shows a linear relation between RCP and a magnetic field, suggesting a first-order character of magnetic transition, though it is broadened. The values of RCP of La(FexCoySi1-x-y)13 with 0.84 x 0.86 and 0.08 1-x-y 0.10 are about 160-170 J/ kg in 0-2 T, which is comparable to that of Gd, 175 J/kg.

In conclusion, we have studied the phase stability, TC and |SM| of La(FexCoySi1-x-y)13 in a wide composition ranges of 0.84 x 0.90 and 0 y 0.08. For x = 0.84 and 0.86, TC is increased with increasing the Co content, while Smax shows a maximum at an intermediate value of y. A broad IEM transition is observed, which is responsible for the enhancement of Smax. We have constructed a complete map of TC and Smax in the Fe-Co-Si ternary diagram. The results suggest that La(FexCoySi1-x-

y)13 with 0.84 x 0.86 and 0.08 1-x-y 0.10 are quite attractive as magnetic refrigerant materials near room temperature.

This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO).

T (K)

|S M

| (J/

J kg

)

La(Fe0.86Co0.06Si0.08)13

0-1.0T0-1.2T0-1.4T0-1.6T0-1.8T0-2.0T

240 260 280 3000

2

4

6

8

10

12

Fig. 5 |SM| T curves of La(Fe0.86Co0.06Si0.08)13 in various magnetic field changes.

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4 REFERENCES [1] V. K. Pecharsky and K. A. Gschneidner, Jr., Phys. Rev. Lett. 78 4494 (1997). [2] H. Wada and Y. Tanabe, Appl. Phys. Lett. 79, 3302 (2001). [3] O. Tegus and E. Brück, K. H. J. Buschow and F. R. de Boer, Nature 415, 150 (2002). [4] F. X. Hu, B. G. Shen, J. R. Sun, Z. H. Cheng, G. H. Rao and X. X. Zhang, Appl. Phys. Lett.

78, 3675 (2001). [5] T. T. M. Palstra, J. A. Mydosh, G. J. Nieuwenhuys, A. M. van der Kraan and

K. H. J. Buschow, J. Magn. Magn. Mater. 36, 290 (1983). [6] A. Fujita, Y.Akamatsu and K. Fukamichi, J. Appl. Phys. 85, 4756 (1999). [7] A. Fujita, S. Fujieda, Y. Hasegawa and K. Fukamichi, Phys. Rev. B 67, 104416 (2003). [8] F.X. Hu, B.G. Shen, J. R. Sun, G. J. Wang and Z. H. Cheng, Appl. Phys. Lett. 80,

826 (2002). [9] F. X. Hu, X. L. Qian, J. R. Sun, G. J. Wang, X. X. Zhang , Z. H. Cheng and B.G. Shen, J. Appl. Phys. 92, 3620 (2002). [10] X. B. Liu and Z. Altounian, J. Magn. Magn. Mater. 264, 209 (2003). [11] M. Balli, D. Fruchart and D. Gignoux, J. Phys. Condens. Matter 19, 236230 (2007). [12] A.M. Tishin and Y.I. Spinichkin, The Magnetocaloric Effect and Its Applications, (IOP

Publishing, Bristol, 2003).

0

50

100

150

200

0 0.5 1 1.5 2

RC

P (

J/kg

)

H (T)

La(Fe0.86Co0.06Si0.08)13

Fig. 6 RCP of La(Fe0.86Co0.06Si0.08)13 as a function of a maximum magnetic field.

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recent development in refrigeration