Show simple item record

dc.contributor.authorKim, Ji-Yeob
dc.date.accessioned2022-04-20T12:38:23Z
dc.date.available2022-04-20T12:38:23Z
dc.date.submitted2021-08-01
dc.identifier.urihttps://www.repository.cam.ac.uk/handle/1810/336256
dc.description.abstractThere has been much effort to develop next generation refrigeration systems that do not use harmful fluids. Magnetocaloric, electrocaloric and mechanocaloric materials have been studied independently as potential replacement of these fluids refrigerants but each has its own limitations. More recently, multicaloric materials that respond to multiple fields have been proposed in order to overcome individual limitations. Here, I studied three different multicaloric materials. The first study focused on extrinsic magnetocaloric effects in La0.7Ca0.3MnO3 (LCMO) driven using strain. LCMO’s intrinsic caloric effect near its ferromagnetic transition is small (-1.2 J kg−1 K−1 T−1 ) [1]. Moya et al. have discovered that LCMO thin films grown on BaTiO3 (BTO) substrates can produce extrinsic magnetocaloric effects as large as -9 J kg−1 K−1 T−1, driven by a structural phase transition in the substrate, and strain-mediated feedback [2]. However, producing high-quality thin films can be challenging, and thin films do not provide large thermal mass for cooling. Here, LCMO || BTO core-shell particles with different compositions were investigated to see whether they present similar behaviour to the film-substrate heterostructures, while simultaneously providing larger thermal mass. The second study focused on surface magnetocaloric effect in near stoichiomet- ric Ni-Mn-Ga (NMG) thin films. NMG is interesting as its ferromagnetic transition is accompanied by a structural transition, producing large magnetocaloric effects that can be driven using magnetic field and/or stress. In this study, the surface magnetisation was studied on 200-nm-thick NMG thin films grown on MgO sub- strate. Previous studies have reported that magnetisation is suppressed at the surface in NMG thin films, therefore compromising their magnetocaloric perfor- mance. Here, we observed that suppression of the magnetisation can be avoided by optimising growth conditions. Furthermore, using synchrotron techniques, we confirmed the presence of the magnetostructural phase transition at the surface, thus demonstrating that the surface is magnetocalorically active. The third study focused on barocaloric effects in fluorinated ferroelectric poly- mers of polyvinylidene fluoride – co – trifluoroethylene (PVDF-TrFE) with different compositions. P(VDF-TrFE) polymers have been previously investigated for elec- trocalorics effects driven by electric fields; here their barocaloric performance on applying and removing pressure is studied. Other ferroelectric materials have been studied for their barocaloric effects, but their performance was limited due to brittleness, low transition temperature, or small entropy changes. PVDF-TrFE is interesting as it is ductile, inexpensive and a good electrocaloric material. Here, we achieved the maximum reversible entropy change (|∆S| = 221.86 J kg−1 K−1) and temperature change (|∆T | = 29.05 K) in PVDF-TrFE 20 and PVDF-TrFE 25, respec- tively, with a pressure change of 2.7 kbar. These are much larger than the values obtained from the electrocaloric effect. [1] C. M. Xiong, J. R. Sun, Y. F. Chen, B. G. Shen, J. Du, and Y. X. Li, “Relation between magnetic entropy and resistivity in La 0.67Ca0.33MnO3,” IEEE Trans. Magn., vol. 41, no. 1 I, pp. 122–124, 2005. [2] X. Moya, L. E. Hueso, F. Maccherozzi, A. I. Tovstolytkin, D. I. Podyalovskii, C. Ducati, L. C. Phillips, M. Ghidini, O. Hovorka, A. Berger, M. E. Vickers, E. Defay, S. S. Dhesi, and N. D. Mathur, “Giant and reversible extrinsic magne- tocaloric effects in La0.7Ca0.3MnO3 films due to strain,” Nat. Mater., vol. 12, no. 1, pp. 52–58, 2012.
dc.rightsAll Rights Reserved
dc.rights.urihttps://www.rioxx.net/licenses/all-rights-reserved/
dc.subjectFerroic materials
dc.subjectCaloric effects
dc.subjectStrain-mediated
dc.subjectSurface magnetism
dc.subjectFerroelectric polymer
dc.subjectPerovskite
dc.subjectHeusler alloy
dc.titleCaloric effects in ferroic materials
dc.typeThesis
dc.type.qualificationlevelDoctoral
dc.type.qualificationnameDoctor of Philosophy (PhD)
dc.publisher.institutionUniversity of Cambridge
dc.date.updated2022-03-10T06:19:00Z
dc.identifier.doi10.17863/CAM.83676
rioxxterms.licenseref.urihttps://www.rioxx.net/licenses/all-rights-reserved/
rioxxterms.typeThesis
dc.publisher.collegeNewnham
cam.supervisorMoya, Xavier
cam.depositDate2022-03-10
pubs.licence-identifierapollo-deposit-licence-2-1
pubs.licence-display-nameApollo Repository Deposit Licence Agreement
rioxxterms.freetoread.startdate2023-04-20


Files in this item

Thumbnail

This item appears in the following Collection(s)

Show simple item record