Transcript Giant Magnetocaloric Materials

Magnetocaloric effects in intermetallic compounds
• Introduction
- Magnetic phase transitions
- Magnetocaloric effects & Magnetic refrigeration
- Magnetic-refrigerant materials
• Experimental results & discussion
- 2nd order phase transition & MCE
- 1st order phase transition & MCE
• Conclusions
Introduction
Magnetic phase transitions
200
PM
FM
180
160
2
M (Am /kg)
140
120
100
80
60
40
20
0
Tc
0
50
100
150
200
T (K)
250
300
350
400
25
PM
AFM
15
2
M (Am /kg)
20
10
TTNN
5
0
0
50
100
150
T (K)
200
250
Magnetic field-induced transition
25
15
FM
2
M (Am /kg)
20
10
PM
5
0
0
1
2
3
0H (T)
4
5
First-order phase transition
Magnetization
 B T , P

M  
 G 
M
Entropy
 G 
S  

 T  p
Volume
 G 
V 

 P T
Second-order phase transition
  2G 
Cp  T  2 
 T  P
TC
Magneto-caloric effect & Magnetic refrigeration
T
T+ΔT
S
ΔTad
N
ΔQ
ΔQ
Isothermal
ΔSm
Absorb heat
T
S
N
T-ΔT
Adiabatic
Cooling effect
Thermodynamics
 S 
 S 
 S 
dS    dT    dB    dp
 T  B , p
 B T , p
 p T , B
 M 
S   
 dB
 T 
B
m
Large
ΔB
Large
M
T
Small
CB,p
0
B
T  M 
T   

 dB
C  T 
B
ad
0
B,p
B,p
Metal Gd sphere 3 kg
Magnetic field
Energy efficiency 20%-60%
Cooling power 200 W-600 W
C.O.P 2-9
ΔT = 4.5 K for 1.5 T ΔT = 11K
for 5 T
Superconducting magnet
Gd
Permanent magnetic field
Space: 114 x 128 x 12.7 mm3
Field strength:  2 T
Nd2Fe14B magnet
Lee et al. JAP (2002)
Magnetic refrigerant materials
B: 0--2 T
MnAs
30
- Sm (J/kgK)
25
20
La(Fe0.89Si0.11)13H1.3
Gd5Si2Ge2
Fe49Rh51
MnFeP0.45As0.55
15
10
Gd
5
0
270
280
290
300
T (K)
310
320
330
Adiabatic temperature change
B: 0--2 T
8
La(Fe0.89Si0.11)13H1.3
Gd5Si2Ge2
Gd
6
Tad (K)
Fe49Rh51
MnAs
4
2
0
270
280
290
300
T (K)
310
320
330
Ordering T:
What are important for MR?
TC = 295 K
Field change:
9
8
-SM (max)
MAX entropy change:
-Sm (J/kgK)
7
-ΔSm(max) = 8.5 J/kgK
TC
6
Relative cooling power
5
4
RCP(S) = -ΔSm(max)*δTFWHM
=552 J/kg
TFWHM
3
Cooling power
2
1
240
δTFWHM = 65 K
FWHM :
Gd
ΔB = 5 T
260
280
300
T (K)
320
340
360
T2
Q   S m (T ) dT
T1
Experimental results & discussion
Second order magnetic phase transition & MCE
9
200
8
180
Gd
160
7
6
-Sm(J/kg K)
2
M (Am /kg)
140
120
100
80
PM
FM
Tc
60
5
Gd
0-1T
0-2T
0-3T
0-4T
0-5T
4
3
2
40
1
20
0
0
50
100
150
200
T (K)
250
300
350
0
240
260
280
300
320
340
360
T(K)
Sth(max) = RLn(2J+1)=17.3 J/molK; Sth(max) = 110 J/kgK
<10%
10
TC = 298 K
Mn5Ge3
-Sm(J/kgK)
8
0-2 T
0-5 T
ΔB = 2 T
6
ΔTad = 1.7 K
4
2
0
255
270
285
300
315
330
T(K)
Hashimoto et al (1982)
First-order magnetic phase transition & MCE
Phase diagram of Gd5Ge4-xSix
400
350
Orthorhombic
PM
300
T (K)
250
Orthorhombic
PM
Monoclinic
PM
200
150
100 AFM
50
0
0
Gd5Ge4
Orthorhombic
FM
FM
FM
1
2
X
3
4
Gd5Si4
Pecharsky et al (1997)
What makes Gd5Ge4-xSix have giant MCE?
Single crystal
25
Gd5Si1.7Ge2.3
a = 7.585 Å
b = 14.800 Å
c = 7.777 Å
β = 93.290
Gd5Si1.7Ge2.3
20
2
M (Am /kg)
Monoclinic
(P1121/a)
TC=240.4±1 K
// a-axis
// b-axis
// c-axis
15
10
5
0
0.05 T
0
50
100
150
200
T (K)
250
300
350
400
B (T)
B-T phase diagram
5
Gd5Ge2.3Si1.7
4
Tc = 240.1 K
T'c = 232.7 K
3
T= 7.4 K
FM
PM
2
TC/ = 4.4 K/T
1
0
232
236
240
244
248
T (K)
252
256
260
264
Magnetization
Field-induced
magnetic phase
transition
Gd5Si1.7Ge2.3
40
5K
230 K
257.5 K
10
260 K
B//a-axis
0
0
1
2
3
0H(T)
PM
FM
Field hysteresis
1T
252.5 K
20
240 K
M (B/f.u.)
30
4
5
6
Magnetic entropy changes
50
// a-axis
40
TC = 240 K
1T
2T
3T
4T
5T
30
20
10
ΔB = 5 T
ΔS(max) = 30.5J/kgK
-Sm (J/kgK)
0
50
40
// b-axis
1T
2T
3T
4T
5T
30
20
10
δTFWHM = 18K
RCP(S) = 549 J/kgK
0
50
40
// c-axis
Effect of magnetic
anisotropy is small
1T
2T
3T
4T
5T
30
20
10
0
200
220
240
260
T (K)
280
300
320
Specific heat capacity
Gd5Si1.7Ge2.3
370
8
365
7
360
D = 237 K
5
 = 32.3 mJ/mol.K
2
cp/T (J/molK )
S (J/kgK)
Tc = 239 K
6
2
4
355
350
345
340
3
335
2
330
230
240
245
250
255
T (K)
1
0
235
at TC
0
50
100
150
T (K)
200
250
300
ΔS = 11.0 ± 0.5 J/molK
Latent heat L = 2.63 ± 0.12 kJ/mol
Tc = 239 K
cp (J/mol.K)
1750
T'c = 245.6 K
1500
Gd5Si1.7Ge2.3
1250
TC/B = 3.3 K/T
1000
= Tc•ΔSm/Cp
0T
2T
750
> 15 K
500
250
0
ΔTad
195
210
225
T (K)
240
255
270
Thermal expansion ΔL/L = (L(T)-L(T = 5 K))/L(T = 5 K)
Transition at
TC = 240.0 ±1.0 K
T’C = 236.0 ±1.0 K
Thermal hysteresis
ΔT = 4 K
ΔLa/La = 6.8x10-3 >0
ΔLb/Lb = -2.0x10-3 <0
ΔLc/Lc = -2.1x10-3 <0
Relative volume change
ΔV/V = 2.7x10-3
Clausius-Clapeyron relation
dTC/dp = 3.2 ± 0.2 K/kbar
M. Nazih et al. 2002
Transition-metal based compound: MnFeP1-xAsx
Crystal structure (0.15  x  0.65)
At transition
Fe2P-type; Hexagonal
Space group P-62m
Fe-layer
Mn-layer
Fe-layer
3g
1b/2c 3f
Δc/c > 0
Δa/a < 0
ΔV/V < 0
There is no crystallographic
symmetry change.
Magnetic moment
4 µB/f.u.
X-T phase diagram
Composition dependence of TC
H
340
PM
320
300
PM
280
O
FM
T (K)
AF
T
260
240
220
FM
200
X
180
160
0.2
0.3
0.4
0.5
X
Bacmann et al. JMMM(1994)
0.6
0.7
Magnetization
120
120
100
160-330 K
100
B=1T
80
M(Am /kg)
2
60
2
M (Am /kg)
80
MnFeP0.45As0.55
MnFeP0.45As0.55
40
20
0
270
300 K
304 K
308 K
312 K
312 K
60
40
20
285
300
315
330
345
T (K)
Thermal hysteresis 3.4 K
360
0
0
1
2
3
4
0H(T)
Field hysteresis 0.5 T
5
B – T phase diagram of MnFeP0.45As0.55
Ordering T:
6
5
TC = 306 K
T’C = 302.2 K
MnFeP0.45As0.55
FM
 = 3.8 K
B (T)
4
Thermal hysteresis:
3
3.8 K
2
PM
1
0
300
ΔTC/ΔB = 4.2 K/T
304
308
312
316
T (K)
320
324
328
First order phase transition
Specific heat capacity
Tp= 296 K
1400
MnFeP0.45As0.55
1200
1000
cp (J/kgK)
Latent heat :
Zero field
Tp = 296 K
L = 526 J/mol
800
600
Cp = 550 J/kgK
(T > 300 K)
400
cooling
200
0
240
260
280
300
320
T (K)
340
360
380
400
Magnetic entropy changes
lSMl (J/kgK)
TC = 306 K
20
18
16
14
12
10
8
6
4
2
0
ΔB = 5 T
MnFeP0.45As0.55
-ΔS(max) = 18.3 J/kgK
5T
Decreasing field
δT = 21.3 K
2T
RCP(S) = 390 J/kg
ΔTad =Tc•ΔSM/Cp
ΔTad = 10 K (ΔB=5 T)
280
290
300
310
T (K)
320
330
340
Magnetic entropy change in different compositions
Isothermal magnetic
entropy changes:
MnFeP
As
1-x
x
35
30
MnFeP1-xAsx
x=0.25
-Sm(J/kgK)
25
x=0.35
x=0.45
2T
5T
x=0.5
x=0.55
x=0.65
20
15
10
5
0
150 175 200 225 250 275 300 325 350 375
T (K)
Conclusions
1. MCE is closely related to the critical behavior of magnetic
phase transition.
Second order transition gives broad MCE peak. MCE is small.
First order transition gives sharp MCE peak. MCE can be large.
2. Gd5Si1.7Ge2.3 has a simultaneous structural and magnetic phase
transition at 239 K. This transition is a first order transition with
thermal hysteresis  7.4 K and with field hysteresis 1 T.
The MCE related with first order phase transition is quite large.
Effect of magnetic anisotropy on MCE in this material is negligible.
3. MnFeP1-xAsx (0.25<x<0.65) has a first order phase transition
with thermal hysteresis  3.4 K and field hysteresis  0.5 T.
The MCE related with this transition is also quite large.
4. Advantages of MnFeP1-xAsx as a magnetic refrigerant
1. Large MCE
2. Tunable ordering temperature( between 168 and 332 K)
3. Small hysteresis
4. Lower cost : MnFe(P,As):
Mn,Fe,P,As(99%, 150$/kg)
Gd-Si-Ge Gd:
Gd(4N): 4000 $/kg.
Fe-Rh:
Rh: 12000$/kg
Acknowledgment
This work is supervised by E. Brück, J.H.K. Buschow,
F.R. de Boer.
Collaborators: L. Zhang, W. Dagula, X.W. Li
Financially supported by the STW.
Bean-Rodbell model
V  V0
TC  T0 [1  
]
V0
Tc: Curie temperature
T0: Curie temperature (not compressible)
V : volume
V0 : volume(absent of exchange interaction)
Gibbs free energy
G = Gex + GH + Gdist + Gentr + Gpress
N: number of atoms/V0
K: compressibility
σ: relative magnetization
(J =1/2)
Volume change is due to the effect of magnetization.
G
0
V
V  V0
 NKk BT0  2 / 2  pK.
V0
Bean et al. PR(1962)
G
0

T / T0  ( / tanh1  )(1   2 / 3  pK )
  3Nk B KT0  / 2.
2
η= 0
σ
J=1/2
Set P = 0
η = 0; σ = 0
TC = T0
η < 1 corresponds to 2nd order phase transition
η > 1 corresponds to 1st order phase transition
For MnFeP0.5As0.5 η = 1.62, J = 2, T0 =250 K
R. Zach et al. JAP (1998)
1 2
Heat capacity in field
Adiabatic T change