Alternative Caloric Energy Conversions - Magnetocaloric Energy Conversion: From Theory to Applications - page 437

Fig. 10.25 a Strain - temperature dependence diagram for different applied stresses during heating
and cooling, b Stress - strain dependence diagram at different temperatures during loading and
unloading

where ð

or crit = o

T ¼ C M and C A Þ is the slope of the transformation line in the

stress temperature phase diagram, as schematically noted in Fig. 10.24 . These

transformation lines are constructed based on the critical stresses (or temperatures)
required to start (and

-

—
see Fig. 10.25 . Equation ( 10.23 ) can be further used to estimate the isothermal
entropy change and the adiabatic temperature change, as shown in Eqs. ( 10.21 ) and
( 10.22 ) for the Maxwell relation.
As explained in, e.g., Moya et al. [ 99 ], the Clausius

nish) the transformation at a particular temperature (stress)

Clapeyron relation is only

-

valid for the rst-order transitions and does not account for the other caloric effects

that arise in each of the interconvertible phases. However, these effects are normally
much smaller than caloric effects during transitions and are often neglected and
under this assumption this relation is equivalent to the Maxwell relation.

10.3.2 Elastocaloric Materials

In general, all superelastic materials can also be considered as elastocaloric mate-
rials. Of course, the transformation temperatures (in particular the austenitic

nish

temperature — A ) should be above the refrigeration (heat sink) temperature, but at

the same time not too high in order to decrease the required stress to start the
transformation. However, as explained in Moya et al. [ 99 ], the

rst EsCE was
detected in Indian rubber, which is a shape-memory polymer, already in early
nineteenth Century. That makes the EsCE the oldest-known ferroic caloric effect.
Some 50 years later, Joule reported on the EsCE in some metals and dry woods,
where he observed small temperature changes under the applied stress caused by
the reversible elastic heat. However, the rst analysis of the EsCE and the related

adiabatic temperature changes on the nowadays known superelastic (shape-