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Electrocaloric (EC) and barocaloric (BC) materials undergo reversible thermal changes in response to changes in applied electric field and pressure, respectively. These materials could potentially be exploited in novel solid-state cooling systems that may replace current vapour-compression systems, which are environmentally harmful, noisy, and relatively energy inefficient. In this work, I studied EC effect and BC effects in multicaloric organic materials, which promise large caloric effects near room temperature. The dissertation is structured as follows. Chapter 1 introduces the background for conventional refrigeration and traditional caloric materials. Chapter 2 then surveys the literature on EC materials and BC materials, as well as the literature on EC prototype devices. Chapter 3 reviews the experimental and modelling methods used for this work. These include sample preparation methods, dielectric spectroscopy and ferroelectric polarisation measurements, calorimetry and infrared imaging, dilatometry and Landau models. Chapter 4 describes the study of EC effects in two dabco-based organic salts, namely [Hdabco][BF4] and [AH][ReO4], where dabco is 1,4-diazabicyclo[2.2.2]octane and AH is a variant of dabco, 1-azabicyclo[2.2.1]heptanium. Experiments and modelling demonstrates that [Hdabco][BF4] shows giant EC effects (isothermal entropy change |∆S| = 15.5 J K-1 kg-1 for |∆E| = 12 kV cm-1) that are one order-of-magnitude larger than those observed in traditional EC oxides such as BaTiO3 (|∆S|= 2.1 J K-1 kg-1 for |∆E| = 4 kV cm-1). [AH][ReO4] shows smaller EC effect of |∆S|~ 7.5 J K-1 kg-1 for |∆E| = 11.2 kV cm-1, but displays better mechanical integrity and operates closer to room temperature. It is concluded that dabco-based organic salts exhibit promising performance due to their large EC effects, non-toxicity and great tunability via chemical alterations. However, electrical leakage and breakdown remains an important challenge to be overcome for their use as EC working bodies. Chapter 5 describes the study of BC effects in the aforementioned dabco-based organic salts. BC effects in these materials have the advantage to be driven using hydrostatic pressure instead of electric field, thus avoiding issues related to electrical leakage and breakdown. Three compositions were selected for BC studies, namely [Hdabco][BF4], [Hdabco][ClO4] and [Hdabco][ReO4]. Among these, [Hdabco][ClO4] shows the largest reversible BC effects, |∆S| = 73.2 J K-1 kg-1 for |∆p| = 1200 bar, which compare well with those reported in state-of-the-art BC materials. Notably, BC effects in [Hdabco][BF4] largely outperform EC effects in the same compound, thus demonstrating that pressure is a useful driving parameter for leaky organic ferroelectrics. Chapter 6 describes BC studies in ureasil polymeric materials. These compounds show large changes in entropy when transforming from liquid to solid. By exploiting a gelation method, the liquid to solid phase transition in these compounds is transformed to a gel-to-solid phase transition, which is desirable for some caloric applications. By driving these transitions using pressure, very large BC effects of |ΔS| = 263 J K−1 kg−1 for |∆p| = 1200 bar are found, which are similar to those observed in commercial vapour-compression refrigerants, e.g. R134a. Moreover, the studied polymers have other advantages, in terms of being stretchable, non-toxic, inexpensive and have great tunability of transitions temperatures. Finally, chapter 7 summarises the main results of this work and discusses interesting avenues for future work.

There 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.

Heat pumps (HPs) are an excellent solution to supply heating and cooling for indoor space conditioning and domestic hot water (DHW) production. Conventional HPs are typically electrically driven and operate with a vapour-compression thermodynamic cycle of refrigerant fluid to transfer heat from a cold source to a warmer sink. This mature technology is cost-effective and achieves appreciable coefficients of performance (COP). The HP market demand is driven up by the urge to improve the energy efficiency of building heating systems coupled with the increase of global cooling needs for air-conditioning. Unfortunately, the refrigerants used in current conventional HPs can have large greenhouse or ozone-depletion effect. Alternative gaseous refrigerants have been identified but they present some issues regarding toxicity, flammability, explosivity, low energy efficiency or high cost. However, several non-vapour-compression HP technologies have been invented and could be promising alternatives to conventional systems, with potential for higher COP and without the aforementioned refrigerant drawbacks. Among those, the systems based on the so-called “caloric effects” of solid-state refrigerants are gaining a lot of attention. The caloric effects are large entropy and adiabatic temperature changes caused by the application or removal of an external field in certain specific solid materials. There are 4 main caloric effects: magnetocaloric, elastocaloric, electrocaloric and barocaloric. Each of them is characterized by the nature of the field and the response that induces the entropy and adiabatic temperature change: variation of the magnetic field, uniaxial mechanical stress, electrical field or hydrostatic pressure, respectively. A HP cycle can be based on these caloric effects and several heating/cooling prototypes were developed and tested over the last few decades. Although not mature technologies yet, some of these caloric systems are well suited to become new efficient and sustainable solutions for indoor space conditioning and DHW production. This paper aims to raise awareness in the building community about these innovative caloric systems. It sheds some light on the recent progress in that field and compares the performance of caloric systems with that of conventional vapour-compression HPs for building applications.Presentation of this paper at the CLIMA 2022 conference can be found here: https://youtu.be/oAjfT2_HEYo

Solid-state cooling is an emerging technology embracing cryogenic and near room temperature devices. For further developments of this machinery, prototypes and caloric materials must be optimized in order to obtain a better final result than the well-known competitors based on gas compression technology. Thus, researches on materials and prototype optimization have been intensively carried out; and the present mini-review will focus on materials, specifically on quantum materials. We comprehensively describe their ability to produce heat, such as the magneto- and barocaloric effects on graphene, as well as the enhanced caloric properties of graphene-based nanocomposite. Quantum dots and frustrated magnets are also reviewed, linked with their potential to produce heat under an external perturbation, such as magnetic field (magnetocaloric effect) or mechanical pressure (barocaloric effect). This work shows that quantum materials are special materials, with advanced and emerging features that can be explored for application in devices for solid-state cooling.

cited By 2; International audience; In the framework of solid-state cooling technologies, four kinds of caloric effects, i.e., magnetocaloric (MC), electrocaloric (EC), barocaloric (BC) and elastocaloric (eC) effects, have been compared from the viewpoint of environmental issues and application cases. This field is primarily developed with the intention of protecting the environment. Currently, alternative caloric materials (called “good” materials) with properties such as being environmentally friendly, low-cost and practicable (low stimulus field) need to be found. All current caloric effects/materials are investigated for the common objective of cooling. Due to the variety of stimuli and intrinsic properties (different caloric performances for different dimensions) of caloric materials, they can exhibit unique advantages for various application cases. A “good” caloric material for large-scale cooling application is still rare and requires more research. © 2016 Elsevier Ltd

Perovskite oxides show an amazing diversity of electronic and magnetic properties along with a myriad of structural variants and phase transitions. Large thermal changes may be driven near the ferroic phase transitions in perovskite oxides using magnetic, electric and stress fields to manipulate conjugate order parameters. The ensuing magnetocaloric, electrocaloric and mechanocaloric effects can be utilized for environment-friendly and high-efficiency solid-state cooling applications. Here we describe the details of these caloric effects in perovskite oxides both from a chronological perspective and from the viewpoint of the recent advances in multiple caloric phenomena. We highlight the role of interfaces in oxide thin films for the different caloric effects and address some of the outstanding challenges for the fundamental understanding and practical implementation of perovskite oxides in solid state refrigeration.