Les effets caloriques représentent la capacité d’un matériau à voir son entropie varier sous l’effet d’une sollicitation externe et peuvent être utilisés pour des systèmes de refroidissement à l'état solide en remplacement (ou complément) des dispositifs traditionnels à base de fluides frigorifiques. Dans cette thèse, nous avons cherché à étudier l'effet élastocalorique du caoutchouc naturel. Après une présentation des différents matériaux caloriques et du caoutchouc naturel, le chapitre 2 détaille la caractérisation élastocalorique du caoutchouc naturel, et les résultats sont interprétés à partir de la notion de cristallisation induite par la déformation. Le changement de température adiabatique élastocalorique et la variation d’entropie associée atteignent 9K et 50kJ.m-3.K-1, ce qui est très important comparé aux autres matériaux caloriques. L’effet élastocalorique étant maximum pour une déformation voisine de 4,5, une pré-déformation peut être appliquée pour éviter la zone moindre activité élastocalorique. La mesure directe de l’effet élastocalorique est ensuite comparée à une méthode indirecte déduite du facteur de Clapeyron, et les divergences sont discutées. Dans le chapitre 3, la contrainte et la température élastocalorique sont simulées par un modèle de Flory modifié sur la base de la cristallisation. Il est possible de prédire le comportement contrainte-déformation à différentes températures, ainsi que les variations de température élastocaloriques à température ambiante. Dans le chapitre 4, les effets de la fatigue sur l’effet élastocalorique du caoutchouc naturel sont ensuite étudiés. La résistance à la fatigue pour de grandes amplitudes de déformation est très faible (< 800 cycles). Trois régimes de déformation intermédiaire sont ensuite testés : 0-3, 2-5, et 4-7, et permet d’établir que le régime 2-5 est le plus performant (jusqu’à 100 000 cycles). Dans le dernier chapitre, un modèle de système régénératif de refroidissement à base de matériaux caloriques est développé afin d’établir des lignes directrices pour le choix des matériaux élastocaloriques / In this thesis, we aimed to study the eC effect of natural rubber (NR) and to prove its potential to act as an eC material primarily. The method for improving the eC effect efficiency and fatigue life of NR were also proposed. The eC effect of NR is characterized directly, and interpretation based on the theory of strain-induced crystallization/crystallite (SIC) is proposed. The eC adiabatic temperature change and isothermal entropy change of NR can be up to 9 K and 50 kJ.m-3.K-1 (56 J.kg-1.K-1), which are larger than most of caloric materials. Two coefficients, eC strain coefficient and eC stress coefficient , are defined for evaluating the eC performance at different strains, where is the specific entropy, is the engineering strain, is the temperature and is the stretching stress. It’s found that both coefficients are maximum for a strain around 4.5, indicating that the highest eC performance occurred at middle strain, which is attributed to the occurrence of SIC. To improve the eC performance, it is proposed to apply a pre-strain, so that the low strain regime where eC performance is low can be skipped. Moreover, the large needed deformation can be reduced by the pre-strain and thus the possibility of a compact cooling system designed based on NR is improved. The fatigue property of eC effect of NR is then investigated. The fatigue life at large deformation strain amplitudes (strain of 1-6) is about 800 cycles for the tested NR, which is too short to be used for a cooling system. Decreasing strain amplitude is necessary to extend fatigue life up to requirement of a cooling device. For the same small strain amplitude of 3, the fatigue property is compared at amorphous strain regime (strain of 0-3), onset strain of melting (strain of 2-5) and high strain of SIC (strain of 4-7). It’s found that a larger eC temperature change and a better fatigue property can be obtained at two SIC strain regimes (strain of 2-5 and 4-7) than amorphous strain regime. Especially, the fatigue property at the onset strain of melting (strain of 2-5) is better than that at high strain of SIC (strain of 4-7). A high-cycle fatigue was applied at the strain of 2-5 (most promising strain regime) up to 1.7×105 cycles. It was observed that there is no crack of the sample, as well as a degradation degree of 12% of the eC temperature change. Furthermore, the eC stress coefficient (4.4 K/MPa) at onset strain of melting is larger than that at high strain of SIC (1.6 K/MPa). As a result, the middle strain regime (onset strain regime of melting) can get a higher eC performance, larger temperature change, and better fatigue life, which should be chosen for eC cooling system.

Metal oxides crystallized in perovskite structure are generally modified in two different ways. According to the general structural formula ABO3, the two ways are A-site modification and B-site modification. The primary significance of perovskite metal oxides rests on their importance in electronic devices. A particular class of perovskites, namely Lead Zirconate or modified Lead Zirconate has received a special attention because of their unique antiferroelectricity and various applications in devices. Among the other modifications, A-site modification of PbZrO3 by La is rare and not much explored. Chapter 1 describes various applications of antiferroelectric thin films along with the synthesis and characterization of pure and La modified PbZrO3, which are relevant to the work presented in this thesis. Sol-gel processing and spin coating technique to deposit solid oxide thin films are well known for their low cost of deposition as well as for their ability to achieve better stoichiometric chemical composition. Common crack formation problem of sol-gel grown films can be prevented by ‘drying control chemical adhesive’ like polyvinylpyrrolidone (PVP). Heat treatment of sol-gel derived thin films is generally determined by TGA and DTA. Crystalline phase of deposited solid thin films is determined by XRD whereas effect of modification can be ascertained by XRD peak assignment and relative crystalline peak shifting. Sol-gel grown film thickness is measured by common cross sectional SEM whereas AFM can detail the surface morphology. Chapter 2 summarizes the deposition and characterization of pure and La modified PbZrO3 thin films. Any nonmetal, which is insulator, is dielectric material and show dielectric dispersion in a frequency domain of low field alternative current. Among the most common feature of dielectric dispersion, Maxwell – Wagner type dispersion is well known. Similar kind of dielectric dispersion, named Maxwell – Wagner like dispersion, can be observed while the equivalent circuit consists of parallel G – C along with a series R. Universal power law of ac conductivity is the deciding factor to distinguish the nature of dispersion. Structural phase transition can be determined by dielectric response and it is widely known as dielectric phase transition. Effect of La modification on dielectric phase transition of PbZrO3 thin films depends on stabilization or destabilization of antiferroelectricity. Maximum dielectric constants of pure and modified PbZrO3 thin films depend on the crystallographic orientations of the growth. Chapter 3 presents dielectric properties of pure and La modified PbZrO3 thin films and these properties are correlated to the stabilization or destabilization of antiferroelectricity, relative integrated intensity of (202)O film orientation and trapped electron charge due to oxygen vacancies. Charge storage property of a capacitor is determined by the polarization of the capacitor on application of electric field whereas field dependent integrated area of polarization on withdrawal of electric field determines the recoverable capacitive energy storage. Among the three kinds of capacitors like linear or paraelectric, ferroelectric and antiferroelectric capacitors, antiferroelectric capacitor is known to be best for their ability to store huge amount of recoverable energy. The recoverable energy in antiferroelectrics can be increased by increasing squareness of the P – E hysteresis loop, applicable electric field, polarization or by the all possible combinations of them. Chapter 4 describes the correlation of relative integrated intensity of (202)O [RI(202)O] with critical applied electric field of P – E saturation to provide enhanced squareness of the hysteresis loops. This chapter also describes the variation of charge and recoverable energy storage properties with respect to RI(202)O. Like magnetocaloric effect, electrocaloric effect is capable to alter the temperature of a system by adiabatic polarization or depolarization. From the Maxwell’s relation of thermodynamics, assuming, (∂p ) = (∂s )electrocaloric effect can be calculated from temperature dependent polarization value of a paraelectric, ferroelectric or an antiferroelectric. Chapter 5 presents the electrocaloric effect of pure and La modified PbZrO3 thin films. Summary of present study and discussion have been delineated in Chapter 6 along with the future work which can give more insight into the understanding of antiferroelectric PbZrO3 thin films with respect to Pb and Zr site modification and with respect to different electrodes. (For formulas pl see the pdf file of the thesis)

Abstract In this thesis, dielectric and thermal behaviours due to the application of an electric field were studied in relaxor ferroelectric (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) and (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) systems of great technological importance. Special attention was given to the behaviour of the electric-field-induced phase transitions and electrocaloric effect, which are closely related to the existing and potential applications. Reactive sintering or columbite methods were used to fabricate polycrystalline PMN-PT ceramics with various compositions (x=0−0.3). In addition, commercial PMN-PT single crystals with composition close to the morphotropic phase boundary region were used. A studied PZN-PT crystal composition was grown by solution gradient cooling technique. Materials were mainly studied by means of dielectric and direct temperature measurements. The electrocaloric effect observed in a ceramic PMN-PT system was found to show distinct maximum values close to the thermal depolarization temperatures with low electric fields. The temperature range and magnitude of the electrocaloric effect was significantly expanded to high temperatures with increasing electric fields due to the contribution of polar nanoregions. The maximum electrocaloric temperature change was in the range of 0.77−1.55 °C under an electric field of 50 kV/cm. In addition, temperature change measurements on depoled PMN-0.13PT ceramics demonstrated that the electrocaloric effect is accompanied with an irreversible part below its depolarization temperature due to hysteresis loss and a possible phase transition type response related to the evolution of the macroscopic polarization. An electric field application to the &lt;001&gt; and &lt;011&gt; directions in PMN-PT crystals was found to cause distinct anomalies in the dielectric and temperature change responses. These anomalies were attributed to the complex polarization rotation routes and different phase stability regions in the electric-field-temperature phase diagrams of PMN-PT. Furthermore, measurements on PMN-PT crystals provided the first direct indications of a temporarily reversed electrocaloric effect with an increasing electric field. In addition, the measured electrocaloric trends in PZN-PT crystal were reproduced by a simple lattice model and mean-field approximation around the transition temperature. This demonstrated that the electrocaloric effect is driven mainly by the dipolar entropy lowering. / Tiivistelmä Tässä työssä tutkittiin dielektristen ominaisuuksien ja lämpötilan käyttäytymistä teknologisesti merkittävissä (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) ja (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) ferrosähköisissä relaksorimateriaaleissa sähkökentän vaikutuksen alaisena. Tutkimuksen erityishuomion kohteena olivat sähköisesti indusoidut faasimuutokset sekä sähkökalorinen ilmiö, jotka liittyvät läheisesti nykyisiin sekä tulevaisuuden sovellutuksiin. Monikiteisiä PMN-PT keraamikoostumuksia (x=0−0,3) valmistettiin sekä reaktiivisella sintrauksella että kolumbiittimenetelmällä. Lisäksi tutkimuksessa käytettiin kaupallisia PMN-PT erilliskiteitä, joiden koostumus on lähellä morfotrooppista faasirajaa. Työssä käytetty PZN-PT erilliskide kasvatettiin jäähdyttämällä korkean lämpötilan liuoksesta. Materiaaleja tutkittiin pääosin lämpötilan ja dielektristen ominaisuuksien mittauksilla. Kun PMN-PT keraamisysteemiin kohdistettiin alhainen sähkökenttä, sähkökalorisen ilmiön selkeä maksimiarvo havaittiin lähellä materiaalin termistä depolarisaatiolämpötilaa. Suuremmilla sähkökentän arvoilla sähkökalorinen ilmiö voimistui ja sen lämpötila-alue laajeni korkeampiin lämpötiloihin polaaristen nanoalueiden kytkeytymisen vuoksi. Sähkökalorisen lämpötilamuutoksen maksimi vaihteli välillä 0,77−1,55 °C sähkökentän arvolla 50 kV/cm. Lisäksi lämpötilamittaukset depoolatulle PMN-0,13PT koostumukselle osoittivat, että sähkökalorisen ilmiön ohella materiaalissa esiintyy makroskooppisen polarisaation muodostumiseen liittyvä palautumaton lämpöenergia depolarisaatiolämpötilaa pienemmissä lämpötiloissa hystereesihäviön ja mahdollisen faasimuutoksen vaikutuksesta. PMN-PT erilliskiteiden dielektrisyys- ja lämpötilavasteessa havaittiin selkeitä muutoksia sähkökentän vaikuttaessa &lt;001&gt; ja &lt;011&gt; kidesuuntiin. Nämä muutokset ovat selitettävissä PMN-PT:n polarisaation kompleksisten rotaatiosuuntien ja erityyppisten sähkökenttä-lämpötila -faasidiagrammien stabiilisuusalueiden avulla. PMN-PT kiteiden mittauksissa havaittiin myös ensimmäinen suora osoitus väliaikaisesti käänteisestä sähkökalorisesta ilmiöstä sähkökentän kasvaessa. Lisäksi mitatut PZN-PT erilliskiteen sähkökaloriset ominaisuudet transitiolämpötilan läheisyydessä pystyttiin pääpiirteittäin mallintamaan käyttämällä yksinkertaista hilamallia ja keskimääräisen kentän approksimaatiota. Mallinnuksen mukaan sähkökalorinen ilmiö aiheutuu pääasiassa sähköisesti indusoidusta dipolientropian alenemisesta.

Les matériaux caloriques à l'état solide, qui subissent un changement de température adiabatique ou un changement d'entropie isothermal lorsque certains stimuli externes (champ électrique, champ magnétique, contrainte ou pression mécanique) est appliquée ou retirée, sont prometteurs pour la réfrigération à l'état solide, comme alternative aux dispositifs de refroidissement conventionnels inventé il y a cent ans qui utilisent des gaz dangereux. Compte tenu des améliorations des systèmes de réfrigération à compression de vapeur approchant très vite de leur limite d'efficacité théorique, en plus des préoccupations environnementales accrues, il y a eu récemment une recrudescence de la recherche mondiale pour de nouvelles solutions de réfrigération plus économiques et respectueuses de l'environnement. Les caloriques les plus importants sont les matériaux "ferroiquement" ordonnés (ferroélectriques, ferroélastiques et ferromagnétique / antiferromagnétique) qui présentent souvent des effets caloriques géants près de leurs transitions ferroïques. Dans cette thèse, nous présentons nos résultats théoriques et expérimentaux sur l'effet électrocalorique, élastocalorique, barocalorique et magnétocalorique dans différents matériaux ferroïques. Nos résultats montrent que tous ces effets caloriques peuvent donner des solutions de réfrigération prometteuses avec un faible impact environnemental. Nous abordons les ferroélectriques qui apparaissent comme matériaux idéaux permettant à la fois des réponses électrocaloriques, élastocaloriques et barocaloriques géantes près de la température ambiante. Pour la première fois, nous mettons en évidence un effet électrocalorique négatif dans des films minces antiferroélectriques et nous proposons un nouveau mécanisme pour comprendre la réponse calorique dans antiferroiques en général incluant antiferroélectrique et antiferromagentique. Par ailleurs, pour la première fois en utilisant une caméra infra-rouge, nous effectuons la mesure résolue spatialement sur l'effet électrocalorique dans des condensateurs multicouches, l'un des systèmes les plus étudiés considérés comme le prototype électrocalorique le plus prometteur. Nos résultats fournissent la première preuve expérimentale directe sur le flux de chaleur électrocalorique à la fois temporellement et spatialement dans un dispositif électrocalorique spécifique. En outre, pour la première fois, nous concevons un cycle de réfrigération multicalorique combinant effet électrocalorique avec des effets élastocaloriques / magnétocaloriques via des matériaux ferroélectriques. Nous avons réalisé ce cycle mutlicalorique pour résoudre un problème réel et de longue date, à savoir une grande hystérésis magnétique qui a empêché l'utilisation pourtant prometteuse de FeRh découvert il y a près de 26 ans en tant que matériau magnétocalorique. Nous espérons que cette thèse fournira non seulement des connaissances utiles pour comprendre fondamentalement l'effet calorique à l'état solide dans les matériaux ferroïques et ce qui est véritablement mesuré, mais pourra aussi servir de guide pratique pour exploiter et développer les ferrocalorics vers la conception de dispositifs appropriés. / Solid-state caloric materials, which undergo an adiabatic temperature change or isothermal entropy change when some external stimulus (electric field, magnetic field, stress and pressure) is applied or withdrawn, are promising for solid-state refrigeration, as an alternative to hazardous gases used in conventional cooling devices invented a hundred years ago. Given that the highly refined vapor-compression refrigeration systems asymptotically approach their theoretical efficiency limit in addition to the concern on environment, there has been a recent upsurge in worldwide search for new refrigeration solution which is economical and environmentally friendly. The most prominent calorics are ferroically ordered materials (ferroelectric, ferroelastic and ferromagnetic/antiferromagentic) that often exhibit giant caloric effects near their ferroic transitions. In this thesis, we present our theoretical and experimental results on electrocaloric effect, elastocaloric effect, barocaloric effect and magnetocaloric effect in different ferroic materials. Our findings show that all these caloric effects may appear promising with low environmental impact. We address ferroelectrics emerging as ideal materials which permit both giant elastocaloric, electrocaloric and barocaloric responses near room temperature. For the first time, we find a large negative electrocaloric effect in antiferroelectric thin films and we propose a new mechanism to understand the caloric response in antiferroics including antiferroelectric and antiferromagentic. In addition, for the first time using Infra-red camera we carry out spatially-resolved measurement on electrocaloric effect in multilayer capacitors, one of the most studied systems which are regarded as the most promising electrocaloric prototype. Our findings provide the first direct experimental evidence on the electrocaloric heat flux both temporally and spatially in a specific electrocaloric device. Moreover, for the first time, we design a multicaloric refrigeration cycle combining electrocaloric effect with elastocaloric/magentocaloric effects bridged by ferroelectric materials. We realized such mutlicaloric cycle to solve a real and longstanding problem, i.e., a large hysteresis that impeded reversibility in an otherwise promising magnetocaloric material FeRh discovered almost 26 years ago. We hope that this thesis will not only provide a useful background to fundamentally understand the solid-state caloric effect in ferroics and what we are really measuring, but also may act as a practical guide to exploit and develop ferrocalorics towards design of suitable devices.

Le domaine du refroidissement est en constante expansion, le système actuel est basé sur la compression/décompression des fluides. Face aux problèmes environnementaux et économiques que ce système présente (natures des fluides frigorigènes et leurs recyclages, nuisances sonores et vibratoires, réglementations contraignantes), de nouvelles solutions techniques alternatives émergent. Ainsi ce travail de thèse porte sur de nouveaux systèmes de refroidissement basés sur les effets électrocalorique et magnétocalorique, respectivement présents dans des films minces de polymère fluoré et dans des composites à matrice polymère et à charges magnétocaloriques. A travers des caractérisations physico-chimiques, électriques, électrocaloriques et magnétocaloriques ces travaux se proposent d’identifier l’origine de l’effet électrocalorique dans des films minces de terpolymère P(VDF-TrFE-CTFE) ferroélectrique relaxeur, mais également d’étudier l’influence de la dispersion des particules magnétocaloriques La(Fe,Si)H dans une matrice polymère de poly(propylène) sur le phénomène magnétocalorique. De plus, dans le cadre de cette thèse, un appareil de mesure directe de l’effet électrocalorique a été développé avec le Dr. Basso de l’INRIM de Turin. La comparaison avec la méthode de mesure indirecte permet d’aborder ce phénomène d’un point de vue thermodynamique afin de faire le point sur la validité des hypothèses thermodynamiques utilisées dans le cas d’un polymère ferroélectrique relaxeur. / The cooling sector is in constant expansion, the current system is based on the compression/decompression of fluids. In front of environmental and economic problems of this system (nature of frigorigen fluids and their recycling, noise and vibration issues, restrictive regulations), new alternative technological solutions emerge. Thus this thesis provides new cooling systems based on the magnetocaloric and electrocaloric effects respectively present in thin films of fluoropolymer and composites with polymer matrix and magnetocaloric loads. Through physicochemical, electrical, electrocaloric and magnetocaloric characterizations, this work intends to identify the origin of electrocaloric effect in thin terpolymer films P(VDF-TrFE-CTFE) which is a ferroelectric relaxor, but also to study the influence of the magnetocaloric particles La(Fe,Si)H dispersion in a polymer matrix of poly(propylene) on the magnetocaloric phenomenon. In addition, as part of this thesis, a direct measurement device of the electrocaloric effect was developed with Dr. Basso from the INRIM of Turin. The comparison with the indirect measurement method comes up with this phenomenon from a thermodynamic point of view to take stock of the validity of thermodynamic assumptions used in the case of a ferroelectric polymer relaxor.

The temperature and/or entropy of electrically polarisable materials can be altered by changing electric field E. Research into this electrocaloric (EC) effect has focussed on increasing the size of the EC effects, with the long-term aim of building a cooler with an EC material at its heart. Materials and experimental methods are briefly reviewed. A ‘resetting’ indirect route to isothermal entropy change ∆S for hysteretic first-order transitions is described. An indirect route to adiabatic temperature change ∆T, without the need for field-resolved heat capacity data, is also described. Three temperature controllers were built: a cryogenic probe for 77-420 K with ∼5 mK resolution, a high-temperature stage with vacuum enclosure for 295-700 K with ∼15 mK resolution, and a low-temperature stage for 120-400 K with electrical access via micropositioners. Automation enables dense datasets to be compiled. Single crystals of inorganic salts (NH4)2SO4 , KNO3 and NaNO2 were obtained. Applying 380 kV cm−1 across (NH4)2SO4 , it was found that |∆S| ∼ 20 J K−1 kg−1 and |∆T | ∼ 4 K, using the indirect method near the Curie temperature TC = 223 K. Without the ‘resetting’ indirect method, |∆S| ∼ 45 J K−1 kg−1 would have been spuriously found. Preliminary indirect measurements on KNO3 and NaNO2 give |∆S| ∼ 75 J K−1 kg−1 for ∆E ∼ 31 kV cm−1 near TC = 400 K and |∆S| ∼ 14 J K−1 kg−1 for ∆E ∼ 15 kV cm−1 near TC = 435 K, respectively. A cation-ordered PbSc0.5Ta0.5O3 ceramic showing a nominally first-order transition at 295 K was obtained. The Clausius-Clapeyron phase diagram is revealed via indirect measurements where |∆S| ∼ 3.25 J K−1 kg−1 and |∆T | ∼ 2 K, and direct measurements where |∆T | ∼ 2 K. Clamped samples show broadening of the field-induced transition. Epitaxial, ∼64 nm-thick SrTiO3 films were grown by pulsed laser deposition on NdGaO3 (001) substrates with a La0.67Sr0.33MnO3 bottom electrode. The indirect method gives |∆S| ∼ 8 J K−1 kg−1 and |∆T | ∼ 3.5 K near 180 K with |∆E| = 780 kV cm−1. Finite element modelling (FEM) was used to optimise the geometry of multilayered capacitors (MLCs) for EC cooling. Intrinsic cooling powers of 25.9 kW kg−1 are predicted for an optimised MLC based on PVDF-TrFE with Ag electrodes.