Review
Magnetocaloric and barocaloric effects of metal complexes for solid state cooling: Review, trends and perspectives

https://doi.org/10.1016/j.ccr.2020.213357Get rights and content

Highlights

  • Metal complexes can be used for solid state cooling at different temperature ranges.

  • Applications at cryogenic temperatures (MCE), or close to room temperature (BCE).

  • We comprehensively review the literature of metal complexes for solid state cooling.

  • We also provide perspectives and viewpoints for this interdisciplinary field.

Abstract

Solid state refrigeration is a viable alternative for the conventional gas-compression technology due to the environmental friendliness of its materials, energy efficiency, and low noise. Research in this field is focused on the development of advanced prototypes and smart materials. This Review focuses on a special family of quantum materials: metal complexes. We introduce the fundamentals of caloric effects and magnetism of these complexes, discussing their applications at different ranges of temperature, based on different physical mechanisms. At cryogenic temperatures (close to temperature of liquid He), some metal complexes present a huge value of magnetic entropy change, ranging from c.a. 10 J/kgK to c.a. 70 J/kgK (for 7 T of magnetic field change). These values make some metal complexes appealing as cryogenic coolant materials. We also present a comprehensive collection of results from the literature, organized on a chart as a function of time, for different classes of metal complexes; those with 3d-3d magnetic interactions, 3d-4f coupling, and 4f-4f interactions. We observed that those materials that achieved the maximum value of entropy change, i.e., the spin-only value, follow an exponential scaling law with time. This result helps to predict a new class of metal complexes and further outcomes for the field. On the other hand, for a small amount of applied pressure, these materials produce large barocaloric effect around the spin crossover transition (this transition occurs in a wide range of temperature, even close to room temperature). Thus, we introduce the SCO mechanism and comprehensively review this topic, along with the recent theoretical models and experimental results. The recent results of barocaloric effect are considered enormously significant (56 J/kgK for 0.9 kbar of pressure change, close to room temperature), even in comparison with traditional metallic barocaloric materials. We also provide perspectives for this subject, with discussions about new mechanisms for the models (as the Jahn-Teller distortion and orbital contribution).

Introduction

Household cooling devices represent a major environmental issue, as 17% of the electric energy produced worldwide is used to run these machines [1]. In addition, these devices work based on old-fashioned gas compression, which contributes to ozone depletion. Thus, upgrading this technology is necessary, and one possible solution is to use solid state refrigeration based on multicaloric materials [2]. These smart materials can increase/decrease their base temperature or exchange heat with a thermal reservoir when subjected to an external excitation, such as a magnetic and/or electric field or even mechanical compression. Based on this phenomenon, a number of prototypes and materials have been optimized over the years [3], [4], including Heusler alloys, manganites, pure 3d-based metals, and other families.

Metal complexes belong to the long list of materials explored for caloric applications. These materials, also called coordination complexes, are those with a metallic center surrounded by organic or inorganic ligands, thus constituting a three-dimensional framework. These materials are quite versatile; while some present a spin-crossover (SCO) transition with an associated huge lattice entropy change for small values of applied mechanical pressure and large values of barocaloric effect (BCE), others have achieved maximum entropy change corresponding to the spin-only value at cryogenic temperature (4 K), making the material appealing as a cryogenic magnetocaloric coolant.

Thus, the aim of this work is to produce an updated review for the field, addressing the trends and perspectives. For this purpose, we describe the achievements of community studying: (i) the magnetocaloric effect at cryogenic temperatures and (ii) the barocaloric effect at higher temperatures, around the SCO transition.

The present work delivers first a short introduction to the fundamentals of the caloric effect; followed by a brief survey on the magnetism of metal complexes. Section 4 discusses the cooling at cryogenic temperatures. It starts with an introduction to the physical-chemical requirements in order to obtain an optimized material with enhanced MCE at low temperatures. The section also compiles the achievements described in the literature and ends with the trends and perspectives for the subject. Section 5 discusses the cooling at higher temperatures, around the SCO transition. First, a brief survey on the SCO mechanism is presented and then the achievements from the literature. In order to conclude the section, trends and perspectives for the subject are discussed. Finally, an overall outlook is presented, as well as general concluding remarks for the field.

Section snippets

Fundamentals of caloric effects

The specific heat of an insulator material, as the metal complexes, is given, in principle, by two main terms:ctot(T,B)=cmag(T,B)+clat(T),where cmag(T,B) represents the temperature- and field-dependent magnetic contribution, and clat(T) is the temperature-dependent lattice term. The lattice specific heat is given by the Debye (and/or Einstein) contribution:clatD(T)=9RTTD30TD/Tx4ex(ex-1)2dxT/TD1125π4TTD3,while the magnetic contribution is given by the Schottky anomaly:cmag(T,B)=T2T2TlnZ(T,B)

Magnetism of metal complexes

The history of molecular magnetism is very recent, in comparison to the knowledge of magnetism of metals. The community considers the starting year for this field as 1952, after advances in the experimental techniques for measurement of very low magnetic signals from samples with a small amounts of magnetic centers. In 1952, B. Bleaney and K. Bowers [6] observed the magnetic interaction between two cooper ions in a copper acetate. Since then, several researchers have developed relevant

Physical-chemical requirements

Many metal complexes present a remarkable magnetocaloric effect at low temperatures, down to the liquid Helium region; making them attractive cryogenic coolant materials. Overall, these materials present magnetocaloric potentials at cryogenic temperatures comparable to the commercial coolant Gd3Ga5O12 (GGG) [16]; and are even more promising than other known materials, like intermetallics [17], [18], [19], [20] and paramagnetic salts [21], [22]. In order to be a viable candidate for application

Spin crossover mechanism

Spin crossover (SCO) is an effect of pure quantum nature. It changes the effective spin (and orbital) momentum of the ion due to a strong magneto-structural correlation. This change of the spin (and orbital) state, due to a strong structural change, opens a large avenue for applications, including: sensors, actuators and transducers [83]. However, before reviewing the literature of this theme, a discussion of the physical mechanisms behind the SCO shall be presented.

Most of the transition

Outlook and concluding remarks

Metal complexes present versatile physical and chemical properties, such as low toxicity, mechanical flexibility, synthesis at low temperatures, compatibility with polymers systems, solubility, among others, in addition to the huge and tunable caloric potentials. There are two branches of metal complexes for application in caloric devices: those with high (spin-only) magnetocaloric effect (MCE) at cryogenic temperatures and other materials presenting barocaloric effect (BCE) at higher

Declaration of Competing Interest

The author declare that he has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The author thanks the Brazilian agencies CNPq, FAPERJ and PROPPI-UFF for financial support. He belongs to the INCT of Refrigeração e Termofísica, funding by CNPq by Grant No. 404023/2019-3. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) – Finance Code 001. The author also thanks Renan Alves, who helped improving Fig. 4, Fig. 5, Fig. 7.

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