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Magnetic behavior of SmCo5-hydrogen system
Journal of Magnetism and Magnetic Materials 35 (1983) 114-116 North-Holland Pubhshing Company
114
MAGNETIC BEHAVIOR OF S m C o s - H Y D R O G E N S Y S T E M Masuhiro YAMAGUCHI,
Tokio OHTA
Faculty of Engineering, Yokohama National University, Hodogaya, Yokohama 240, Japan
and Y a s u a k i O S U M I Governmental industrial Research Institute, Osaka, Midorigaoka, Ikeda, Osaka 563, Japan
The magnetization of the hydrides of activated SmCo5 powder has been determined as a function of hydrogen pressure (less than 6 MPa) and composition, and temperature under the direct influence of hydrogen. The hydride phase transformation and the effect of hydrogen on the magnetic moment have been studied.
1. Introduction The SmCo 5 compound and some related alloys are useful permanent magnet materials. Zijlstra and Westendorp [ 1] have shown that SmCo 5 can absorb hydrogen up to the concentration of 2.5 H / S m C o 5 at room temperature and that the hydrogen absorption results in the decrease in the coercivity and the magnetization. This discovery has led to the studies of hydride formations and hydriding kinetics in the SmC%-H system [2-4]. The dissociation hydrogen pressure of the SmCo 5 hydride is about 0.4 MPa at room temperature. For less stable hydrides, magnetic measurements must be done under the direct influence of hydrogen. Larsen and Livesay [5] have developed a magnetic torque method for thin SmCo s film deposited onto glass in order to monitor hydriding kinetics. Recently, a magnetometer has been developed which can determine magnetic behavior and hydride phase transformations of less stable hydrides in in situ conditions [6]. The first reaction of bulk SmCo 5 with hydrogen is not fast usually. Rapid hydriding and dehydriding reactions take place in "activated powder" of SmCo 5, which can react readily with hydrogen. In this work, the magnetization of activated powder of SmCo 5 will be determined as a function of hydrogen pressure and composition, and temperature by the above in situ method. The hydride phase transformation in this system will also be investigated by the magnetic method.
2. Experimental The SmCo 5 compound was given to us by the Santoku Metal Industry Company. It was prepared by induction melting under argon from Sm (99.9%) and Co (99.99%). The powder X-ray diffraction patterns showed the formation of the CaCu5-type structure and the absence of extraneous phases. 0304-8853/83/0000-0000/$03.00
Magnetization measurements were made simultaneously with the determination of hydrogen composition under hydrogen atmosphere in the temperature range between 4.2 and 400 K. The details of this method has been reported elsewhere [6]. The activated powder of SmCo 5 was prepared as follows: the sample, crushed in a mortar under Ar atmosphere, was introduced in the sample holder of the magnetometer loosely. Then, it was outgassed at 150°C under a vacuum of 0.1 Pa for 1 h, exposed to high purity hydrogen at 4 MPa, and finally cooled to room temperature. Hydrogen was released at 150°C again. The hydriding and dehydriding cycle was repeated four times before magnetic measurements. By the activation process, the sample was pulverized to fine powder (particle size of several tens of microns) with large area of surface. The hydrogen used has a purity of "seven nines".
3. Results and discussion Fig. 1 shows the hydrogen pressure versus (a) saturation-magnetization (pressure-magnetization isotherm) and (b) hydrogen composition (pressure-composition isotherm) in the absorption and the desorption processes for SmC%H x at 25°C. Three regions can be distinguished in the system: (1) the a phase (0 < x < 0.3); (2) the two-phase (0.3 < x < 2.5); and (3) the fl phase (2.5 _
© 1983 N o r t h - H o l l a n d
M. Yamaguchi et al. / Magnetic behaviour of SmCo 5 -hydrogen system
I
I
I
I
I
I:
,0~
115
I
I
E
Ca)o~
a.
o.1
8( m 001 100
90
8O
70
6O" MAGNETIZATION,M (emu,~j)
I
I
1
2
COMPOSITION,x
~
__
II 3
Fig. 1. Hydrogen pressure versus (a) saturation magnetization; and (b) hydrogen composition in the absorption (open circles) and the desorption (closed circles) processes for the SmCosH~ system at 25°C. o
determined to be 94, 92, 70, and 66 e m u / g for x = 0, 0.3, 2.5, and 3.0, respectively. Fig. 3 shows the temperature dependence of magnetization in a field of 10.3 kOe for the parent compound ( x = 0 ) from 4.2 to 400 K, the quenched fl phase ( x = 3.0 + 0.1) from 4.2 K to room temperature, and the sample at a constant hydrogen pressure of 2.28 MPa between 77 and 400 K (magnetization-temperature isobar) for the S m C % H x system. Here, the quenched fl phase was prepared by the quenching method: after the sample was hydrogenated to the composition of x = 3.0 at 25°C, it was cooled quickly by liquid N 2. The magnetization-temperature isobar at 2.28 MPa shows that the system transforms from the fl to the a phases at the temperature around 340 K in the heating run and returns from the a to the fl phases at the temperature around 320 K in the cooling run. It is noted that the magnetization of the quenched fl phase agrees with that of the low temperature part of the isobar. The magnetization of SmCo 5H x at 0 K is determined to be 98 and 76 e m u / g for x = 0 and 3.0 + 0.1, respectively. Assuming that before and after hydrogen absorption the Sm moment is that of its free trivalent ion and parallel to the Co moment, the magnetic moment of the Co atom is calculated to be 1.42 and 1.07#a for x = 0 and 3.0 + 0.1, respectively. However, the possibility of a
1 2 HYDROGEN COMPOSITION, x
Fig. 2. Hydrogen composition dependence of saturation magnetization in the absorption (open circles) and the desorption (dosed circles) processes for the SmCosH x system at 25°C.
z"
8o
o
77
=2.28,IdP~a I Y
P
8C--
I
[
I
zoo
Fig. 3. Temperature dependence of magnetization in a field of 10.3 kOe for the parent compound (x = 0), the quenched /~ phase (x = 3.0+0.1), and the sample at a constant hydrogen pressure of 2.28 MPa for the SmCosH x system. The open and closed symbols indicate the data taken in the heating and cooling runs, respectively.
Table 1 Magnetic and thermodynamic constants for the SmCosH x system Phase
Metal a fl
x
0 0.3+0.1 3.0 + 0.1
~
TEMPERATURE, T (K)
Magnetization (emu/g)
Magnetic moment at 0 K (#a)
25°C
0K
Per f.u.
Per Co
94 92 66
98
7.81
1.42
76
6.06
i .07
Change in enthalpy, AH (kJ/mol H2)
Change in entropy, AS ( J / K mol H2)
-30+2
-114+8
116
M. Yamaguchi et al. / Magnetic behaviour of SmCo 5 -hydrogen system
complex magnetic structure cannot be excluded in the hydride. The changing rate of the Co moment per absorbed hydrogen is - ( 0 . 5 8 + 0 . 0 2 ) # a / H , which nearly equals to that for the YC%--,YCosH2. 8 case ( - 0.54#B/8 ) [7]. The dissociation pressure for the fl ~ a transformation is obtained to be 0.50 MPa at 25°C from the pressure-magnetization isotherm (fig. 1) and 2.28 MPa at 340 K from the magnetization-temperature isobar (fig. 3). The dissociation pressure P in a two-phase region is related to temperature T; this can be expressed approximately in a limited temperature range by In P / P o ffi - A S / R
+ AH/RT
(P0 = 1 atm),
(1)
where A H and A S are, respectively, the changes in enthalpy and entropy, and R is the gas constant. Applying eq. (1) to this system gives the thermodynamic quantities of A H = ( - 3 0 + 2 ) L l / m o l H 2 and A S = ( - 114 + 8) J / K mole H 2 In conclusion, for the activated powder of the SmC%-hydrogen system, the magnetization decreases
by 30 and 22% at room temperature and 0 K by the maximum hydrogen absorption, respectively, and the magnetic behavior gives the thermodynamic quantities of hydride phase transformation. References
[1] H. Zijlstra and F.F. Westendorp, Solid State Commun. 7 (1969) 857. [2] F.A. Kuijpers and H.H. van Mal, J. Less Common Met. 23 (1971) 395. [3] J.S. Raichlen and R.H. Doremus, J. Appl. Phys. 42 (1971) 3166. [4] J.W. Larsen and B.R. Livesay, J. Less Common Met. 73 (1980) 79. [5] J.W. Larsen and B.R. Livesay, J. Appl. Phys. 50 (1979) 7687. [6] M. Yamaguchi, T. Katamune and T. Ohta, J. Appl. Phys. 53 (1982) 2788. [7] M. Yamaguchi, T. Ohta and T. Katayama, J. Magn. Magn. Mat. 31-34 (1983) 221.
114
MAGNETIC BEHAVIOR OF S m C o s - H Y D R O G E N S Y S T E M Masuhiro YAMAGUCHI,
Tokio OHTA
Faculty of Engineering, Yokohama National University, Hodogaya, Yokohama 240, Japan
and Y a s u a k i O S U M I Governmental industrial Research Institute, Osaka, Midorigaoka, Ikeda, Osaka 563, Japan
The magnetization of the hydrides of activated SmCo5 powder has been determined as a function of hydrogen pressure (less than 6 MPa) and composition, and temperature under the direct influence of hydrogen. The hydride phase transformation and the effect of hydrogen on the magnetic moment have been studied.
1. Introduction The SmCo 5 compound and some related alloys are useful permanent magnet materials. Zijlstra and Westendorp [ 1] have shown that SmCo 5 can absorb hydrogen up to the concentration of 2.5 H / S m C o 5 at room temperature and that the hydrogen absorption results in the decrease in the coercivity and the magnetization. This discovery has led to the studies of hydride formations and hydriding kinetics in the SmC%-H system [2-4]. The dissociation hydrogen pressure of the SmCo 5 hydride is about 0.4 MPa at room temperature. For less stable hydrides, magnetic measurements must be done under the direct influence of hydrogen. Larsen and Livesay [5] have developed a magnetic torque method for thin SmCo s film deposited onto glass in order to monitor hydriding kinetics. Recently, a magnetometer has been developed which can determine magnetic behavior and hydride phase transformations of less stable hydrides in in situ conditions [6]. The first reaction of bulk SmCo 5 with hydrogen is not fast usually. Rapid hydriding and dehydriding reactions take place in "activated powder" of SmCo 5, which can react readily with hydrogen. In this work, the magnetization of activated powder of SmCo 5 will be determined as a function of hydrogen pressure and composition, and temperature by the above in situ method. The hydride phase transformation in this system will also be investigated by the magnetic method.
2. Experimental The SmCo 5 compound was given to us by the Santoku Metal Industry Company. It was prepared by induction melting under argon from Sm (99.9%) and Co (99.99%). The powder X-ray diffraction patterns showed the formation of the CaCu5-type structure and the absence of extraneous phases. 0304-8853/83/0000-0000/$03.00
Magnetization measurements were made simultaneously with the determination of hydrogen composition under hydrogen atmosphere in the temperature range between 4.2 and 400 K. The details of this method has been reported elsewhere [6]. The activated powder of SmCo 5 was prepared as follows: the sample, crushed in a mortar under Ar atmosphere, was introduced in the sample holder of the magnetometer loosely. Then, it was outgassed at 150°C under a vacuum of 0.1 Pa for 1 h, exposed to high purity hydrogen at 4 MPa, and finally cooled to room temperature. Hydrogen was released at 150°C again. The hydriding and dehydriding cycle was repeated four times before magnetic measurements. By the activation process, the sample was pulverized to fine powder (particle size of several tens of microns) with large area of surface. The hydrogen used has a purity of "seven nines".
3. Results and discussion Fig. 1 shows the hydrogen pressure versus (a) saturation-magnetization (pressure-magnetization isotherm) and (b) hydrogen composition (pressure-composition isotherm) in the absorption and the desorption processes for SmC%H x at 25°C. Three regions can be distinguished in the system: (1) the a phase (0 < x < 0.3); (2) the two-phase (0.3 < x < 2.5); and (3) the fl phase (2.5 _
© 1983 N o r t h - H o l l a n d
M. Yamaguchi et al. / Magnetic behaviour of SmCo 5 -hydrogen system
I
I
I
I
I
I:
,0~
115
I
I
E
Ca)o~
a.
o.1
8( m 001 100
90
8O
70
6O" MAGNETIZATION,M (emu,~j)
I
I
1
2
COMPOSITION,x
~
__
II 3
Fig. 1. Hydrogen pressure versus (a) saturation magnetization; and (b) hydrogen composition in the absorption (open circles) and the desorption (closed circles) processes for the SmCosH~ system at 25°C. o
determined to be 94, 92, 70, and 66 e m u / g for x = 0, 0.3, 2.5, and 3.0, respectively. Fig. 3 shows the temperature dependence of magnetization in a field of 10.3 kOe for the parent compound ( x = 0 ) from 4.2 to 400 K, the quenched fl phase ( x = 3.0 + 0.1) from 4.2 K to room temperature, and the sample at a constant hydrogen pressure of 2.28 MPa between 77 and 400 K (magnetization-temperature isobar) for the S m C % H x system. Here, the quenched fl phase was prepared by the quenching method: after the sample was hydrogenated to the composition of x = 3.0 at 25°C, it was cooled quickly by liquid N 2. The magnetization-temperature isobar at 2.28 MPa shows that the system transforms from the fl to the a phases at the temperature around 340 K in the heating run and returns from the a to the fl phases at the temperature around 320 K in the cooling run. It is noted that the magnetization of the quenched fl phase agrees with that of the low temperature part of the isobar. The magnetization of SmCo 5H x at 0 K is determined to be 98 and 76 e m u / g for x = 0 and 3.0 + 0.1, respectively. Assuming that before and after hydrogen absorption the Sm moment is that of its free trivalent ion and parallel to the Co moment, the magnetic moment of the Co atom is calculated to be 1.42 and 1.07#a for x = 0 and 3.0 + 0.1, respectively. However, the possibility of a
1 2 HYDROGEN COMPOSITION, x
Fig. 2. Hydrogen composition dependence of saturation magnetization in the absorption (open circles) and the desorption (dosed circles) processes for the SmCosH x system at 25°C.
z"
8o
o
77
=2.28,IdP~a I Y
P
8C--
I
[
I
zoo
Fig. 3. Temperature dependence of magnetization in a field of 10.3 kOe for the parent compound (x = 0), the quenched /~ phase (x = 3.0+0.1), and the sample at a constant hydrogen pressure of 2.28 MPa for the SmCosH x system. The open and closed symbols indicate the data taken in the heating and cooling runs, respectively.
Table 1 Magnetic and thermodynamic constants for the SmCosH x system Phase
Metal a fl
x
0 0.3+0.1 3.0 + 0.1
~
TEMPERATURE, T (K)
Magnetization (emu/g)
Magnetic moment at 0 K (#a)
25°C
0K
Per f.u.
Per Co
94 92 66
98
7.81
1.42
76
6.06
i .07
Change in enthalpy, AH (kJ/mol H2)
Change in entropy, AS ( J / K mol H2)
-30+2
-114+8
116
M. Yamaguchi et al. / Magnetic behaviour of SmCo 5 -hydrogen system
complex magnetic structure cannot be excluded in the hydride. The changing rate of the Co moment per absorbed hydrogen is - ( 0 . 5 8 + 0 . 0 2 ) # a / H , which nearly equals to that for the YC%--,YCosH2. 8 case ( - 0.54#B/8 ) [7]. The dissociation pressure for the fl ~ a transformation is obtained to be 0.50 MPa at 25°C from the pressure-magnetization isotherm (fig. 1) and 2.28 MPa at 340 K from the magnetization-temperature isobar (fig. 3). The dissociation pressure P in a two-phase region is related to temperature T; this can be expressed approximately in a limited temperature range by In P / P o ffi - A S / R
+ AH/RT
(P0 = 1 atm),
(1)
where A H and A S are, respectively, the changes in enthalpy and entropy, and R is the gas constant. Applying eq. (1) to this system gives the thermodynamic quantities of A H = ( - 3 0 + 2 ) L l / m o l H 2 and A S = ( - 114 + 8) J / K mole H 2 In conclusion, for the activated powder of the SmC%-hydrogen system, the magnetization decreases
by 30 and 22% at room temperature and 0 K by the maximum hydrogen absorption, respectively, and the magnetic behavior gives the thermodynamic quantities of hydride phase transformation. References
[1] H. Zijlstra and F.F. Westendorp, Solid State Commun. 7 (1969) 857. [2] F.A. Kuijpers and H.H. van Mal, J. Less Common Met. 23 (1971) 395. [3] J.S. Raichlen and R.H. Doremus, J. Appl. Phys. 42 (1971) 3166. [4] J.W. Larsen and B.R. Livesay, J. Less Common Met. 73 (1980) 79. [5] J.W. Larsen and B.R. Livesay, J. Appl. Phys. 50 (1979) 7687. [6] M. Yamaguchi, T. Katamune and T. Ohta, J. Appl. Phys. 53 (1982) 2788. [7] M. Yamaguchi, T. Ohta and T. Katayama, J. Magn. Magn. Mat. 31-34 (1983) 221.