Magnetocaloric effect and thermal transport management in lanthanum manganites
File(s)
Author(s)
Turcaud, Jeremy
Type
Thesis
Abstract
This thesis investigates two challenges associated with the use of manganites for magnetocaloric
applications. The first challenge is associated with methods to engineer the
thermal conductivity, K. The second challenge is to understand the limits of the entropy
change achievable in magnetocaloric manganites.
Thermal management has been achieved via different microstructuring routes and their
influence on thermal transport properties such as K, resistivity and thermopower, have
been studied. A factor of two increase in K is demonstrated by using density and grain
size optimization, while three-fold and six-fold increases are seen by employing the introduction
of a second highly conductive phase via: (1) silver impregnation and silver particle
coating and (2) copper electroplating, respectively.
Understanding the magnetocaloric effect (MCE) characteristics in manganites has been
achieved by bringing together magnetisation, magneto-structural, magneto-Seebeck, and
neutron diffraction independent measurements. We first show that the temperature T* up
to which a spontaneous magnetisation is observed in the inverse magnetic susceptibility
of La0.7Ca0.3MnO3 and La0.7Ba0.3MnO3 above Tc, is related to the transition temperature
of the low temperature (high-magnetic field and high-magnetisation) magnetic phase. In
the most widely studied La(1-x)CaxMnO3 (x = 0.2, 0.25, 0.3), we then conclude that unlike
between the degree of static Jahn-Teller distortion and the interval [T*-Tc]/Tc where we
show that there exists a close relationship, there is no apparent correlation between the
magnitude of the MCE and [T*-Tc]/Tc . We then unravel the competing strength of the various
degrees of freedom and show that the inhibition of a large magnetocaloric response
is due to the strong correlations that underpin the collosal magnetoresistance effect: both
clustering of magnetic Mn atoms due to polaron formation and the insulator to metal transition.
Finally we discuss prospects to improve material properties for application in light of
these findings.
applications. The first challenge is associated with methods to engineer the
thermal conductivity, K. The second challenge is to understand the limits of the entropy
change achievable in magnetocaloric manganites.
Thermal management has been achieved via different microstructuring routes and their
influence on thermal transport properties such as K, resistivity and thermopower, have
been studied. A factor of two increase in K is demonstrated by using density and grain
size optimization, while three-fold and six-fold increases are seen by employing the introduction
of a second highly conductive phase via: (1) silver impregnation and silver particle
coating and (2) copper electroplating, respectively.
Understanding the magnetocaloric effect (MCE) characteristics in manganites has been
achieved by bringing together magnetisation, magneto-structural, magneto-Seebeck, and
neutron diffraction independent measurements. We first show that the temperature T* up
to which a spontaneous magnetisation is observed in the inverse magnetic susceptibility
of La0.7Ca0.3MnO3 and La0.7Ba0.3MnO3 above Tc, is related to the transition temperature
of the low temperature (high-magnetic field and high-magnetisation) magnetic phase. In
the most widely studied La(1-x)CaxMnO3 (x = 0.2, 0.25, 0.3), we then conclude that unlike
between the degree of static Jahn-Teller distortion and the interval [T*-Tc]/Tc where we
show that there exists a close relationship, there is no apparent correlation between the
magnitude of the MCE and [T*-Tc]/Tc . We then unravel the competing strength of the various
degrees of freedom and show that the inhibition of a large magnetocaloric response
is due to the strong correlations that underpin the collosal magnetoresistance effect: both
clustering of magnetic Mn atoms due to polaron formation and the insulator to metal transition.
Finally we discuss prospects to improve material properties for application in light of
these findings.
Version
Open Access
Date Issued
2014-07
Date Awarded
2014-10
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Cohen, Lesley
Sandeman, Karl
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
EP/G060940/1
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)