Environmental Engineering Reference
In-Depth Information
By applying the Legendre transformation to the
rst law of thermodynamics
(Eq.
1.1
), we can derive the de
nition of enthalpy as:
h
¼
u
l
0
HM
ð
1
:
11
Þ
Applying Eq. (
1.5
), the derivative of the speci
c enthalpy is:
h
o
h
Þ¼
o
o
dh
ð
s
H
ds
þ
dH
¼
T ds
l
0
MdH
ð
1
:
12
Þ
;
s
o
H
H
s
In most publications relating to the characterization of magnetocaloric materials,
the isothermal entropy change is presented as a function of the temperature and
the internal magnetic
eld. It is one of the most widely published properties related
to the magnetocaloric effect. The entropy change in an isothermal process can be
de
ned using Eqs. (
1.6
1.10
) and written as follows:
-
s
o
H
M
o
T
Þ¼
o
¼ l
0
o
ð
ð
1
:
13
Þ
dsT
;
H
dH
dH
T
H
For a certain increase (or decrease) in the magnetic
eld between the two states
of different magnetic
elds under isothermal conditions, the isothermal entropy
change is de
ned as follows:
Z
H
2
o
s
o
s
¼
s
2
s
1
¼
dH
ð
1
:
14
Þ
D
H
T
H
1
H
2
Z
l
0
o
M
o
D
s
¼
dH
ð
1
:
15
Þ
T
H
H
1
Z
H
2
c
T
T
dH
D
s
¼
ð
1
:
16
Þ
H
1
Another important parameter that is often used for the characterization of
magnetocaloric materials is the adiabatic temperature change. It denotes the
increase or decrease in the temperature due to the increase or decrease of the
magnetic
eld in the absence of a heat
fl
ow (adiabatic-isentropic magnetization or
demagnetization). In the adiabatic
isentropic process, the total speci
c entropy
-
does not alter (ds = 0). From Eqs. (
1.6
) and (
1.9
), it follows that:
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