Bates, Matthew (2015) Theoretical investigation of solid state cooling using spin models. PhD thesis, University of Warwick.

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Abstract

A mean field spin model coupled to a thermal model of a solid is used as a description of the electrocaloric effect in relaxor ferroelectrics at both first and second order transitions. This theoretical model is also used in efforts to find the tricritical point of transitions to maximise the adiabatic temperature change due to the electrocaloric effect while minimising the hysteresis losses in a cooling cycle. The electrocaloric effect is the adiabatic temperature change exhibited by a material under the sequential application then removal of electric fields. Relaxor ferroelectrics are materials with two interaction scales, local interactions in polarised nano regions (PNRs) and longer range interactions between PNRs. Electric dipoles are modelled as spins and their alignment due to an external field is used to examine polarisationelectric field loops. The results are compared to experimental data to determine values of parameters that may be used in simulations to model the electrocaloric effect. I reproduced simulations of the electrocaloric effect in the ferroelectric material lead zinc niobate-lead titanate [Pb(Zn1=3Nb2=3)O3-PbTiO3] by coupling the dipolar entropy of the spins to the entropy of a thermal lattice one can examine the temperature increase that must occur as the dipolar entropy decreases in an external field. The results of this simulation agree with experiment. Novel work is carried out in a prediction of the strength of the electrocaloric effect in polyvinylidene fluoride trifloroethylene, P(VDF-TrFE), using a simulation where the material parameters are determined from the comparison of simulated polarisation-electric field hysteresis loops with experimental results. The simulations suggest that P(VDF-TrFE) is a material worth investigating experimentally because even though it has a weak electrocaloric strength, under large fields the potential adiabatic temperature change at room temperature has promise for replacing conventional refrigerants.

Due to the simplicity of this coupled model and the physical similarities between the electrocaloric effect and the magnetocaloric effect (an analogous effect observed in certain magnetic materials where an adiabatic temperature change is seen under the application and removal of a magnetic field) an investigation is also carried out on the magnetocaloric material iron rhodium (FeRh). In simulations I varied global and local stoichiometry to affect the ordering of spins and thus the entropy and strength of the magnetocaloric effect. I compared the results of our simulations with experiment and examine the variation in peak isothermal entropy change and adiabatic temperature change under variation in stoichiometry as well as the effects on the full width at half maximum. The results show that a simple model can give a qualitative representation of the effect.

Work of a different nature is presented at the end of the thesis due to it being a short and complete topic which interested me during my PhD. Details are given on how Catalan numbers may be used to determine the depth of leaves in binary tree graphs and path length between them using both diagrammatic notation and the methodology of generating functions. An analytic solution is drawn from a combinatoric problem that initially appeared to only be soluble by brute force numerics.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Solid state physics, Pyroelectricity
Official Date:	September 2015
Dates:	Date Event September 2015 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Staunton, Julie
Extent:	x, 115 leaves : charts
Language:	eng

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