Magnetocaloric Systems Research Articles

Multi-phase transition behavior over a wide temperature range in magnetocaloric (Mn, Fe, Ni)2(P, Si) alloys

This study investigates the magnetocaloric property of MnFe0.95-xNixP0.6Si0.4 (x = 0.02, 0.03, 0.04) alloys over a wide temperature range and their related phase transitions. These P-rich composition alloys, with a P/Si ratio of 1.5, exhibit multi-phase transition behavior across a broad temperature range. In-situ 2D X-ray diffraction reveals overlapping diffraction peaks of the Fe2P-type hexagonal structure throughout the ferromagnetic-paramagnetic transition, rather than a single, drastic transition. This observation suggests the formation of a multi-phase structure within the alloys. Consequently, these alloys exhibit high refrigeration capacity (RC) values ranging from 168.6 to 218.1 J kg−1 under an applied magnetic field of 2 T. Notably, these RC values are superior to those observed in other P/Si ratio compositions without multi-phase transitions or even Gd, a representative magnetocaloric material. This improved performance demonstrates the promising potential of these multi-phase transition alloys as candidates for magnetocaloric refrigeration systems.

A DFT+U and Monte Carlo study of Gd2Zr2O7 pyrochlore

Pyrochlore oxides, specifically Gd2Zr2O7, have been a subject of interest due to their unique structural, physical, and magnetic properties. These properties make pyrochlore oxides potential candidates for various technological applications, including magnetocaloric refrigeration. Magnetocaloric refrigeration is a promising alternative to conventional refrigeration methods, as it is more energy-efficient and environmentally friendly. By studying the magnetocaloric effect of pyrochlore oxides like Gd2Zr2O7, researchers can gain a deeper understanding of the underlying mechanisms and potential applications of these materials in magnetocaloric refrigeration systems. By employing advanced computational methods such as Monte Carlo simulations and first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) a technique is employed to examine the magnetic structure, researchers can investigate the magnetocaloric effect of pyrochlore oxides with high accuracy and precision. This information has the potential to aid in the advancement of more effective magnetocaloric systems materials and ultimately lead to the design and optimization of advanced magnetocaloric refrigeration systems.

Impact of dimensionality on the magnetocaloric effect in two-dimensional magnets

Magnetocaloric materials, which exploit reversible temperature changes induced by magnetic field variations, are promising for advancing energy-efficient cooling technologies. The potential integration of two-dimensional materials into magnetocaloric systems represents an emerging opportunity to enhance the magnetocaloric cooling efficiency. In this study, we use atomistic spin dynamics simulations based on first-principles parameters to systematically evaluate how magnetocaloric properties transition from three-dimensional (3D) to two-dimensional (2D) ferromagnetic materials. We find that 2D features such as reduced Curie temperature, sharper magnetic transition, and higher magnetic susceptibility are beneficial for magnetocaloric applications, while the relatively higher lattice heat capacity in 2D can compromise achievable adiabatic temperature changes. We further propose GdSi2 as a promising 2D magnetocaloric material. Our calculation predicts that GdSi2 exhibits an isothermal entropy change ΔSM of 22.5 J kg−1 K−1 and an adiabatic temperature change ΔTad of 6.2 K, near the hydrogen liquefaction temperature (TC≈25 K). Our analysis offers valuable theoretical insights into the magnetocaloric effect in 2D ferromagnets and demonstrates that 2D ferromagnets hold promise for cooling and thermal management applications in compact and miniaturized nanodevices.

Predicting the dynamic behavior of a magnetocaloric cooling prototype via artificial neural networks

Although magnetocaloric cooling is considered a promising long-term alternative to vapor compression, recent prototype developments have not yet made this technology commercially competitive, primarily due to its high energy consumption and lack of cost-effective, long-term mechanically-chemically stable materials. To address the first issue and understand how the efficiency of magnetocaloric systems can be improved, dynamic models can offer valuable insights into their transient operation. This work focuses on the development of an artificial neural network with experimental data to model the dynamic operation of a magnetic refrigeration system. Through a design of experiments approach, we propose excitation signals for the identification experiment, involving five manipulated variables and one selected disturbance as inputs, with the output temperature of the cold manifold and power consumption as the target parameters. We chose a nonlinear autoregressive artificial neural network with exogenous inputs to model the transient operation of the system. The temperature model achieved R2 values of 0.995 and 0.955 for the 1-step and 90-step ahead predictions, respectively. Similarly, the power consumption model achieved R2 values of 0.988 and 0.949 for the 1-step and 90-step ahead predictions, respectively. These performance metrics were evaluated on the test sets that were not used for training the models, highlighting the robustness and accuracy of the models in both short-term and long-term predictions.

Research on microstructure evolution and surface quality of WEDM for magnetic refrigerant rare-earth gadolinium

Rare-earth gadolinium (Gd) is preferable for manufacturing regenerators of the core components of room-temperature magnetic refrigeration owing to its unique magnetocaloric and mechanical properties. However, the surface quality of the regenerator plays a crucial role in the heat transfer effect and service life of magnetocaloric systems during wire electrical discharge machining (WEDM) when fabricating rare-earth Gd array microstructure regenerators. In this study, different process parameters were used to conduct a process experiment of the WEDM of rare-earth Gd. First, the evolution of the surface microstructure and its causes were analyzed using a single-factor experiment, while a corrosion test was conducted on the samples. The analysis showed that the pulse-on time and open voltage considerably affected the surface quality of the processed samples, while the samples with better surfaces exhibited good corrosion resistance. Additionally, a Taguchi experiment was designed, and a regression analysis used to establish regression models between the process parameters (pulse-on time, pulse-off time, peak current, open voltage, and water pressure) and both surface roughness (SR) and material removal rate (MRR). The results showed that the average prediction errors of SR and MRR were only 5.34% and 5.48%, respectively.

Exploring magnetocaloric and heat capacity behavior in Fe doped Mn5Ge3 alloy

Magnetocaloric properties of hexagonally structured Mn5−xFexGe3 (x=0.15, 0.3, and 0.5) alloys have been investigated using DC magnetization and heat capacity measurements. The maxima of entropy change, −ΔSmmax∼5.04(5.57) J/kg K, along with an adiabatic temperature change of ΔTadmax∼5.05(7.25) K was observed for x=0.15(0.5) at an applied magnetic field H=5 T. With the scaling analysis of −ΔSm, the rescaled curves collapse onto a single universal curve anticipated by the mean-field theory, revealing a second-order type of magnetic transition. Furthermore, −ΔSmmax follows a power law of Hn with n=0.597(3), 0.591(3), and 0.586(3) for Mn5−xFexGe3 (x=0.15, 0.3, and 0.5) alloys, respectively. The refrigerant capacity (RC) is increased from 400 J/kg (for x=0.15) to 420 J/kg (for x=0.5) with Fe doping in Mn5Ge3. Moreover, the coefficient of refrigerant performance (CRP) enhances with Fe doping from 0.06 (for x=0.15) to 0.1 (for x=0.5). Thus, high RC and reasonable CRP values for earth-abundant Mn-based Mn–Fe–Ge alloys promise the potential to replace the high-cost rare-earth (Gd) and heavy metal-based metallic magnetocaloric systems for use in environment-friendly magnetic refrigeration technology.

A conceptual design of a thermal switch capacitor in a magnetocaloric device: experimental characterization of properties and simulations of operating characteristics

The quest for better performance from magnetocaloric devices has led to the development of thermal control devices, such as thermal switches, thermal diodes, and thermal capacitors. These devices are capable of controlling the intensity and direction of the heat flowing between the magnetocaloric material and the heat source or heat sink, and therefore have the potential to simultaneously improve the power density and energy efficiency of magnetocaloric systems. We have developed a new type of thermal control device, i.e., a silicon mechanical thermal switch capacitor (TSC). In this paper we first review recently developed thermal switches based on micro-electromechanical systems and present the operation and structure of our new TSC. Then, the results of the parametric experimental study on the thermal contact resistance, as one of the most important parameters affecting the thermal performance of the device, are presented. These experimental data were later used in a numerical model for a magnetocaloric device with a thermal switch-capacitor. The results of the study show that for a single embodiment, a maximum cooling power density of 970 W m−2 (510 W kgmcm −1) could be achieved for a zero-temperature span and an operating frequency of 5 Hz. However, a larger temperature span could be achieved by cascading multiple magnetocaloric elements with TSCs. We have shown that the compact TSC can be used in caloric devices, even with small temperature variations, and can be used in a variety of practical applications requiring thermal regulation.

Effect of Nd doping on the crystallographic, magnetic, and magnetocaloric properties of NdxGd3−xCoNi

The crystal structure, magnetic and magnetocaloric properties, and the critical behavior of representative compounds in the pseudo-ternary NdxGd3−xCoNi series have been investigated (x = 0.15, 0.5, 1.0, and 1.5). All these phases are isotypic with the parent compound Gd3CoNi, crystallizing with the monoclinic Dy3Ni2-type (mS20, C2/m, No. 12). All samples present a paramagnetic to ferromagnetic (PM-FM) second order phase transition with decreasing Curie temperature as the Nd concentration is increased (TC = 171, 150, 120, and 96 K, respectively) and, at lower temperatures, there is a spin reorientation, which leads to a complex magnetic ground state. The critical exponents (β, γ, and δ) have been retrieved for the PM-FM transitions. On the one hand, in x = 0.15, 0.5, and 1.5 the value of γ ≈ 1 indicates that the magnetic interactions are long-range order while the values of β point to a certain deviation from the 3D-Heisenberg universality class; on the other hand, NdGd2CoNi has a particular critical behavior, as β is close to the mean field model while γ is close to the uniaxial 3D-Ising one. Concerning the magnetocaloric properties, the magnetic entropy change and refrigerant capacity present competitive values, interesting for cryogenic applications. Finally, the thermal diffusivity values of these compounds are extremely good for practical magnetocaloric refrigeration systems, as they are in the range 1.5–3 mm2/s.

ElastoCaloric (eC) Cooling: recent studies and preliminary results on materials and devices

Refrigeration is a process aimed at lowering the temperature of an environment compared to the outside temperature. It has always played a fundamental role in modern industry: food, for the preservation of food; in the air conditioning of buildings, to make them habitable and even in the field of medicine and biology, for the preservation of samples and particular therapies. Today refrigeration systems, are responsible for about 17% of the world's electricity consumption. Therefore, the scientific community is researching solid-state refrigeration, which is 50% more efficient than vapour compression systems. Solid-state refrigeration is based on some materials' caloric effect, which can release or absorb latent heat when subjected to an external field (magnetic, electric, pressure or strain) release. Elastocaloric Refrigeration is one of the most minor explored frontiers, unlike magnetocaloric and electro-caloric systems. Some preliminary studies confirm that the eC systems have the advantage of being able to decrease the intensity of the external field (with a relative increase of the COP). In this work, a review on Ec systems is carried out, with particular attention to the description of the properties of the materials used, the various types of stress application. The results have been presented in terms of temperature, entropy, latent heat and COP.

The Critical Behaviour and Magnetism of MnCoGe0.97Al0.03 Compounds

The critical behaviour associated with the field-induced martensitic transformation heavily relies on the vacancy and transition of the magnetic phase in MnCoGe based-compounds. Due to this revelation, an intensive investigation was brought forth to study the substitution of Ge (atomic radius = 1.23 Å) by Al (atomic radius = 1.43 Å) in MnCoGe0.97Al0.03 alloy compound. The room-temperature X-ray diffraction indicated that the reflections were identified with the orthorhombic structure (TiNiSi-type, space group Pnma) and minor hexagonal structure (Ni2In-type, space group P63/mmc). The substitution of Al in the supersession of Ge transmuted the crystal structure from TiNiSi-type to Ni2In-type structure. The MnCoGe0.97Al0.03 compound’s magnetism was driven by interactions that are long in range, as indicated by the study of the critical behaviour in the proximity of TC. The magnetic measurement and neutron diffraction revealed that the structural transition took place with the decrease in temperature. The results from neutron diffraction signify that the transformation of the magnetic field-induced martensitic has a crucial function in producing the immense effect of magnetocaloric systems such as these. This outcome serves a critical function for investigations in the future.

Effect of Composition on the Phase Structure and Magnetic Properties of Ball-Milled LaFe11.71-xMnxSi1.29H1.6 Magnetocaloric Powders

Magnetocaloric alloys are an important class of materials that enable non-vapor compression cycles. One promising candidate for magnetocaloric systems is LaFeMnSi, thanks to a combination of factors including low-cost constituents and a useful curie temperature, although control of the constituents’ phase distribution can be challenging. In this paper, the effects of composition and high energy ball milling on the particle morphology and phase stability of LaFe11.71-xMnxSi1.29H1.6 magnetocaloric powders were investigated. The powders were characterized with optical microscopy, dynamic light scattering, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). It was found that the powders retained most of their original magnetocaloric phase during milling, although milling reduced the degree of crystallinity in the powder. Furthermore, some oxide phases (<1 weight percent) were present in the as-received and milled powders, which indicates that no significant contamination of the powders occurred during milling. Finally, the results indicated that the Curie temperature drops as Fe content decreases (Mn content increases). In all of the powders, milling led to an increase in the Curie temperature of ~3–6 °C.

Tailoring the magnetocaloric, magnetic and thermal properties of Dy6(Fe,Mn)X2 intermetallics (X[dbnd]Sb, Te, Bi)

The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe2P-type Dy6(Fe,Mn)X2 (XSb, Bi, Te) intermetallics (space group P6¯2m, N 189, hP9) have been experimentally studied. They present a paramagnetic to ferromagnetic transition (in the range 129–370 K), followed, as temperature decreases, by a spin-reorientation one (from 52 to 170 K) and a ground magnetic state at 2 K with antiferromagnetic components. This state turns into a ferromagnetic state when a magnetic field is applied. The critical exponents β,γ,δ related to the PM-FM transition point to long range order interactions but in most compounds their values severely deviate from the Mean Field class, presenting an unconventional critical behavior, probably due to magnetocrystalline anisotropies. This magnetic complexity has the consequence that in every intermetallic three MCE effects arise: Two direct magnetocaloric effects (DMCE) with a table-like effect in between (from 40 K to more than 400 K), with moderate values of the magnetic entropy maxima (up to 6.9 J/kgK for μ0ΔH = 5 T, with the tableau in-between being around 4 J/kgK, for Dy6FeSb2 and Dy6FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy6FeSb2 and Dy6FeSbTe) close to other rare earth based high entropy alloys described in literature. The seven compounds present a relevant third MCE, inverse, below 25 K, with a value as high as 17.8 J/kgK (μ0ΔH = 5 T) for Dy6FeSbTe. The maximum of the magnetic entropy change at the Curie temperature has been shown to scale with the critical exponents found and universal curves have been built. Finally, the thermal diffusivities in the range of the DMCE have been measured, with the result that they present good values (between 1 and 3 mm2/s) to be used in real magnetocaloric refrigeration systems.

Intense ferromagnetic fluctuations preceding magnetoelastic first-order transitions in giant magnetocaloric LaFe13−xSix

First-order magnetic transitions are of both fundamental and technological interest given that a number of emergent phases and functionalities are thereby created. Of particular interest are giant magnetocaloric effects, which are attributed to first-order magnetic transitions and have attracted broad attention for solid-state refrigeration applications. While the conventional wisdom is that atomic lattices play an important role in first-order magnetic transitions, a coherent microscopic description of the lattice and spin degrees of freedom is still lacking. Here, we present a comparative neutron scattering study on the lattice and spin dynamics in intermetallic $\mathrm{La}{\mathrm{Fe}}_{11.6}{\mathrm{Si}}_{1.4}$ and $\mathrm{La}{\mathrm{Fe}}_{11.2}{\mathrm{Si}}_{1.8}$, which represent one of the most classical giant magnetocaloric systems and undergo first-order and second-order magnetic transitions, respectively. While their spin-phonon coupling effects are quite similar, $\mathrm{La}{\mathrm{Fe}}_{11.6}{\mathrm{Si}}_{1.4}$ exhibits a much stronger magnetic diffuse scattering in the paramagnetic state preceding its first-order magnetic transition, corresponding to intense ferromagnetic fluctuations. These dynamic insights suggest that the magnetic degree of freedom dominates this magnetoelastic transition and ferromagnetic fluctuations might be universally relevant for this kind of compounds.

Electrocaloric cooling over high device temperature span

Summary Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems.

Predicting the performance of magnetocaloric systems using machine learning regressors

Since refrigeration, air-conditioning and heat pump systems account to 25–30% of all energy consumed in the world, there is a considerable potential to mitigate the Global Warming by increasing the efficiency of the related appliances. Magnetocaloric systems, i.e. refrigerators and heat pumps, are promising solutions due to their large theoretical Coefficient Of Performance (COP). However, there is still a long way to make such systems marketable. One barrier is the cost of the magnet and magnetocaloric materials, which can be overcome by decreasing the materials quantity, e.g. by optimizing the geometry with efficient dimensioning procedures. In this work, we have developed a machine learning method to predict the three most significant performance values of magnetocaloric heat pumps: temperature span, heating power and COP. We used 4 different regressors: ordinary least squares, ridge, lasso and K-Nearest Neighbors (KNN). By using a dataset generated by numerical calculations, we have arrived at minimum average relative errors of the temperature span, heating power and COP of 23%, 29% and 31%, respectively. While the lasso regressor is more appropriate when using small datasets, the ordinary least squares regressor shows the best performance when using more samples. The best order of polynomials range between 3, for the heating power, to 5, for the COP. The worse performance in predicting the three performance values occurs when using the KNN regressor. Furthermore, the application of regressors to the dataset is more adequate to evaluate the temperature span rather than energetic performance values.

Modeling of hydrogen liquefaction using magnetocaloric cycles with permanent magnets

Abstract Hydrogen (H2) is promising alternative to replace fossil fuels, but its transport and storage has been challenging. As H2 fuel cell vehicles are gaining traction, the infrastructure for storing large amounts of liquid H2 is needed. However, liquid H2 would suffer from boil-off loss, and traditional vapor compression refrigeration systems would not be able to economically recover the lost H2 due to the low efficiencies at cryogenic temperature. Magnetocaloric (MC) refrigeration systems could possess much higher coefficient of performance (COP) at cryogenic temperature compared to the vapor compression ones. Previous work on cryogenic MC systems, however, have only focused on large scale applications which use superconducting magnets to provide a large magnetic field but are prohibitively expensive to operate for small scale applications, such as that of a H2 refilling station. In this work, we modeled the performance of a MC refrigeration cycle using 1-Tesla permanent magnets for H2 liquefaction, with the objective of cooling H2 from 80 K (using liquid nitrogen as the heat sink) to 20 K (boiling point of hydrogen). We evaluated main performance metrics including the total work input to the refrigeration system, COP, total MCM mass in the system, and total volume of the permanent magnets, etc. Our modeling results indicate that such a permanent magnet-based MC cooling system is feasible for small-scale H2 liquefaction, with projected COP values significantly higher than those of vapor compression systems. This work provides design guidelines for future experimental efforts on permanent magnet MC cooling systems for cryogenic cooling.

Broad Multi-Parameter Dimensioning of Magnetocaloric Systems Using Statistical Learning Classifiers

Prototyping innovative energy devices is a complex multivariable dimensioning problem. For the case of magnetocaloric systems, one aims to obtain an optimized balance between energy conversion performance, useful power generated and power consumed. In these devices, modeling is entering a mature phase, but dimensioning is still time consuming. We have developed a technique that dimensions any type of magnetocaloric system by training statistical learning classifiers that are used to simulate the computation of a very large number of systems with different combinations of parameters to be dimensioned. We used this method in the dimensioning of a magnetocaloric heat pump aiming at optimizing the temperature span, heating power, and coefficient of performance, obtaining an f-score of 95%. The respective classifier was used to mimic over 940 thousand computed systems. The gain in computation time was 300 times that of computing numerically the system for each combination of parameters.

Modeling and computing magnetocaloric systems using the Python framework heatrapy

Among all ferroic-based thermotechnologies, magnetocaloric refrigeration has become one of the most reported alternatives to vapor-compression systems. Hence, the modeling, and respective computation, of magnetocaloric systems has become of paramount importance in designing new devices. The need to optimize various adjustable model parameters makes overall computational costs a real-life limitation to these computational explorations. Recently, the heatrapy Python framework was made available, which aims at simulating caloric effects and thermal devices. In this work, two simple models are implemented in the heatrapy framework and are described and validated: one fully solid state magnetocaloric system, and one hydraulic active magnetic regenerative system. Both models show considerably reduced computational costs.

Low computational cost semi-analytical magnetostatic model for magnetocaloric refrigeration systems

The analysis of the active magnetic refrigeration (AMR) cycle for different waveforms of both the magnetic field and the velocity of the heat transfer fluid is an essential challenge in designing and implementing heating and cooling systems based on the magnetocaloric effect. One of the most important issue is the correct modelling of the magnetic and thermal behavior of the active magnetocaloric materials (MCM) in order to estimate precisely cooling capacity of the magnetocaloric system. As the multiphysics coupling implies successive calls for both the thermal and the magnetic modelling subroutines, the execution time of these subroutines has to be as short as possible. For this purpose, a new magnetostatic model based on reluctance network has been performed to calculate the internal magnetic field and the internal magnetic flux density of the active magnetocaloric material (gadolinium, Gd) inside the air gap of the magnetic circuit. Compared to a 3D Finite Element Model (FEM), our magnetostatic semi-analytical model leads to a sharp drop of the computation time, while offering a similar precision for all magnetic quantities in the whole magnetocaloric system.

Plastically deformed Gd-X (X = Y, In, Zr, Ga, B) solid solutions for magnetocaloric regenerator of parallel plate geometry

Despite significant progress in the study of materials undergoing first-order magnetic phase transitions accompanied by the so-called giant magnetocaloric effect (MCE), Gd metal still remains the most widely used material in prototypes of magnetic refrigerators due to its significant MCE, good machinability and reasonable mechanical and chemical stabilities. Alloying of Gd enables fine-tuning the Curie temperature of Gd-based solid solutions (all show second-order phase transitions), for graded magnetocaloric materials. Commonly, Gd packed spheres are used as a magnetocaloric working substance in the active magnetic regenerator (AMR) cycle. In this work, we show that the optimized stacking parallel-plate geometry of AMR bed made of Gd is more effective for application at frequencies 1–10 Hz then the packed spheres. We also give a short review on magnetocaloric properties of cold-rolled Gd-X (X = Y, In, Zr, Ga, B) solid solutions. These materials can be produced in the form of thin (∼100 μm) foils/plates to ensure rapid heat exchange between to the heat transfer fluid. Although the magnetocaloric effect decreases in the as-rolled foils, it can be recovered by thermal treatment of the final stacked-plates regenerators. Gd-Y, Gd-In and Gd-Zr solid solutions have magnetocaloric properties, comparable to the MCE of pure Gd in a wide temperature working span up to 37 K, 36 K, and 16 K respectively, which makes them suitable magnetocaloric material systems for testing the fundamental heat exchangers geometries at ambient temperature and in frequencies of 1–10 Hz.