Environmental Engineering Reference
In-Depth Information
Even though the shape-memory and superelastic effects are known for more than
50 years and already applied in various areas, the EsCE was not seriously proposed
for application in a cooling (or heat pumping) device until recently.
In general, the EsCE is analogous to the MCE and the ECE in detail presented in
the previous chapters. Here, instead of a magnetic
eld change,
a change of the mechanical stress is required, which causes strain changes in the
materials (instead of magnetization and polarization changes). Therefore,
eld or an electrical
the
Maxwell relation can be written in the following form:
T
¼
o
e
o
o
s
o
r
1
q
ð
10
:
20
Þ
T
r
where T, s,
are the temperature, entropy, stress, strain and density,
respectively. Furthermore, the isothermal entropy change and the adiabatic tem-
perature change of the EsCE loaded from the initial (
˃
,
ε
and
ˁ
˃
1
) to the
nal stress (
˃
2
) can
be written as follows:
1
q
o
e
o
T
1
q
o
e
o
T
r
2
ds
¼
d
r
!
D
s
ist
¼
d
ð
10
:
21
Þ
r
r
r
r
1
T
c
r
1
q
T
c
r
o
e
o
dT
¼
ds
!
dT
¼
d
r
!
D
T
ad
T
r
ð
10
:
22
Þ
1
q
T
c
r
o
e
o
r
2
¼
d
r
T
r
r
1
Figure
10.25
shows a schematic example of the
rst-order elastocaloric phase
transition at
two different cases, both isothermally. Figure
10.25
a shows a
stress
strain dependence diagram for different applied stresses during heating and
cooling, while Fig.
10.25
b shows a strain
-
temperature dependence diagram at
different temperatures during loading and unloading. The great majority of so far
known elastocaloric materials undergo a
-
rst-order phase transition, which is
associated with hysteresis behaviour by different mechanical behaviours of the
material during loading and unloading or heating and cooling, as shown in
Fig.
10.25
. This further results in different EsCE properties (Eqs.
10.20
and
10.21
)
for forward and reverse transformations. The differences are the irreversibility
losses associated with the hysteresis behaviour of the transformation, which rep-
resented an additional heat source of the transformation [
98
].
In addition to the Maxwell relation, the EsCE is often described and charac-
terized with the Clausius
Clapeyron relation, written as follows:
-
¼q
o
s
o
e
or
crit
o
ð
10
:
23
Þ
T
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