Poster giving an introduction to magnetic refrigeration at room temperature. Presented at the 2008 Danish Physical Society meeting in Nyborg, Denmark.

1. Magnetic refrigeration Cool
Mag
at room temperature ridge
F
Britt Rosendahl Hansen, Luise Theil Kuhn
Risø National Laboratory for Sustainable Energy, Technical University of Denmark, DTU
How does it work? The challenge The magnetocaloric effect II
S S We need better magnetocaloric materials! 1st order versus 2nd order magnetic phase transition:
All the known materials have one or more problems, see
the Examples.
The temperature change, D T, obtained depends on the
N magnetisation and heat capacity, see below. We cannot Magnetization
N
know these properties a priori .
The physicist
He applies a magnetic field adiabatically
OUr hero is holding a magnetic material This orders the magnetic moments..
Thus, progress is slow as it is done by trial and error.
S
S Entropy preserved (adiabatic)
Entropy
S = Smagnetic + Slattice + Selectron change
Why pursue this?
Smagnetic decreases Þ
Slattice increases
Vapour-compression Nd0.5Sr0.5MnO3
Magnetic refrigeration [6]
N
N
1st order: sharp transition Þ large D
S
Mature
Ÿ
Larger efficiency
Ÿ
Why? Our physicist now cools the sample..
..and the material heats up ...but small range (peak width)
Reliable
Ÿ
Environmentally friendly
Ÿ
...and hysteresis
Low-cost
Ÿ
Ÿ noise
Less
S
S Entropy preserved (adiabatic)
S = Smagnetic + Slattice + Selectron
The magnetocaloric effect I
Smagnetic increases Þ
Slattice decreases
Wanted!
N
N The magnetic entropy change is given by:
Why?
..which reduces the entropy of ..and when the field is removed adiabatically H2
æT , H ) ö
M(
¶ Dead or alive
the material... the sample cools further
SM (T ) =
ò ¶H , P dH
D- ç ÷
T
H1 è ø
S S
where M is the magnetization. From this and the heat
capacity, C, we find the adiabatic temperature change:
H2
Materials with the following properties:
æ ¶ )ö
T M (T , H
ò ,H) ´ ÷
ç
T (T ) = dH
D- ç ÷
C(T T
¶H , P
H1 è ø l phase transition around room temperature
Magnetic
(broad or tunable)
N N We see that DT is largest, when the increase in
Cheers
Heat is transferred and the object(s) are cooled
Thermal contact with object(s) in need of cooling is established magnetization for a given temperature and applied field
l capacity at phase transition temperature
Small heat
is largest .
(c) 2008 Britt Rosendahl Hansen
l
Magnetically soft: easy to magnetize
Magnetization
l
No thermal or magnetic hysteresis
This happens around
the magnetic phase
Our group at Risø DTU l
Cheap and resistant to corrosion by water
transition temperature.
D
Materials research
T
X-ray diffraction
Do we have the right material? How pure is it?
Temperature (K)
Characterization
Dbe determined from isothermal magnetization
SM can
curves:
Examples
Idea/guess Magnetization (A/m)
Production
Gadolinium
Magnetometry T1
Does the material display hysteresis (thermal or magnetic)?
From initialcurves we can calculate dM/dT T2
Benchmark material
SM
D
Hexagonal close-packed
Metal
FM-PM transition
2nd order
Applied field (T)
H1 H2
Pro:
Large DT
[5]
Broad transition
Calorimetry Data analysis
Con:
Provides us with the heat capacity To obtain the entropy and temperature changes Expensive
Fe49Rh51 Oxidizes
Hexagonal close-packed
Metal
AFM-FM transition [4]
st
1 order
All the characterization [1]
data in this box are on
Pro:
samples of La(Fe,Co,Si)13 La0.67Ca0.33-xSrxMnO3
Very large D
T
Tc tunable
Perovskite
Ceramic
Con:
Modeling Prototype design FM-PM transition
Rh is prohibitively expensive st nd
1 /2 order
Thermal hysteresis
0.65T
p
Mechanism: double exchange
1.25T
q
1.70T
n
Pro:
[2]
Tc tunable by doping
Cheap
Easy to fabricate
References Does not corrode
Con:
[1] Hernando et al., Synthesis and properties of
Large entropy change, but
Test machine mechanically alloyed and nanocrystalline materials, 235
also large heat capacity
(1997) 675
T too small
[2] Annaorazov et al., Cryogenics, 32 (10) (1992) 867 Þ D
[3] Dinesen, Ph.D.-thesis, 2004
Conf
ident [4] Pecharsky et al., Phys. Rev. Lett., 78 (1997) 4494
ial [3]
[5] Dan’kov et al., Phys. Rev. B, 57 (6) (1998) 3478
[6] Sande et al., Appl. Phys. Lett. 79 (13) (2001) 2040
New design