Magnetocaloric Refrigeration Research Articles

Metamagnetic phase transition induced large magnetocaloric effect in a Dy0.5Ho0.5MnO3 single crystal.

Magnetic refrigeration based on the magnetocaloric effect is gaining interest in orthogonal or hexagonal rare-earth manganite. However, a more comprehensive understanding of the underlying mechanism is still required. We grew a high-quality single crystal of Dy0.5Ho0.5MnO3 using the optical floating zone method, since the parent crystals DyMnO3 and HoMnO3 have orthogonal and hexagonal structures, respectively. The magnetic and magnetocaloric properties and refrigeration mechanisms are thoroughly investigated. Doping modifies the magnetism according to the results obtained from the investigation of magnetic and dielectric properties and heat capacity. The spin reorientation transition shifts towards low temperature in comparison to HoMnO3. Near the Néel temperature of rare-earth sublattices (5 K), the highest changes in negative magnetic entropy under 0-70 kOe are 18 J kg-1 K-1 and 13 J kg-1 K-1 along the a- and c-axes, respectively. The low-temperature metamagnetic phase transition caused by the alterations in the magnetic symmetry of Ho3+ contributes to an increased magnetocaloric effect in comparison to the parent crystals, rendering it a promising choice for magnetic refrigeration applications. Dy0.5Ho0.5MnO3 exhibits a clear magnetocrystalline anisotropy with enhanced refrigeration capacity and negative magnetic entropy change along the a-axis. The adiabatic temperature change of Dy0.5Ho0.5MnO3 is 8.5 K, larger than that of HoMnO3, rendering it a promising choice for low-temperature magnetic refrigeration applications.

Optimisation of a packed particle magnetocaloric refrigerator: A combined experimental and theoretical study

Magnetocaloric refrigeration has strong potential towards achieving high efficiency hydrogen liquefaction. Optimising parameters such as particle size and operating cycles can have a significant impact on liquefaction performance. This work reports on a room temperature magnetocaloric refrigeration prototype designed with helium as the heat transfer fluid and packed particle Gd0.8875Ce0.1025Si0.84Cr0.19 as the refrigerant. The temperature spans and cooling power with three different particle sizes at two different operating cycles were measured. A maximum temperature span of 16.7 K and a maximum cooling power of 8.6 W kg−1 were obtained. Reduction of performance in the faster cycle was observed in larger particles due to insufficient thermal diffusion time and its implications for cryogenic temperatures are discussed. A numerical model was developed based on the experimentally observed variation of the temperature span and cooling power with particle size and cycle duration. This model was then used to study the competing effects of viscous dissipation and thermal diffusion for different particle sizes and cycle durations.

A low-frequency ferrohydrodynamic pump for a magneto-caloric refrigerator

Ferrohydrodynamic or magnetic pumping enables the design of a magnetocaloric refrigerator with no moving parts. Existing magnetic pumps utilize travelling wave magnetic fields with frequencies in the range of 100 to 1000 Hz. Such high frequencies when utilized in the proposed refrigerator could cause heating which is detrimental to its performance. Hence, a magnetic pump that works with low magnetic field frequencies (¡ 1 Hz) is designed and its performance is experimentally characterized and compared against an one-dimensional model. The design of the magnetic pump consists of a rising and falling pipe, circumscribed by an electromagnetic coil. On application of a magnetic field, due to the inward acting force on either end of the pipes, the ferrofluid progresses in the rising pipe and reaches the falling pipe. On removal of the magnetic field, the portion of the fluid in the falling pipe falls down due to gravity, thereby achieving a net pumping action. Thus on continuously cycling the magnetic field, an intermittent motion of the ferrofluid is obtained. The maximum cross-sectional area and time-averaged mass flow rate of the proposed design is 1.8 g s-1 cm-2 at 0.74 Hz and 35.7 mT. This mass flow rate is comparable to pump designs that work on travelling wave magnetic fields, whose operational frequency is three orders of magnitude higher.

Magnetic ground state of holmium nitride

Rare-earth mononitrides such as HoN exhibit a wide range of useful properties leading to potential applications as magnetic semiconductors, spintronic half-metals, or magnetocaloric refrigerants in hydrogen liquefaction systems. First-principle calculations of electronic structures and related properties of such materials should correctly reproduce their magnetic moment. First, we identify the unusually high number of unoccupied electronic states which guarantees that the energy minimum identified is the global one. Second, we develop a method which allows the experimentally relevant magnetisation to constitute an energy minimum, emphasising the favourable distribution of the spins in an exceptionally large simulation cell. Third, we examine the dependence of selected HoN characteristics on the cell size and on the magnetisation. The results provide a theoretical insight into the spin structure of rare-earth nitrides and allow one to use the correct methodology of similar calculations of properties of strongly correlated materials.

Thermodynamic comparison of magnetocaloric and vapor compression wine coolers

This paper presents a comparison of the thermodynamic performances of magnetocaloric and vapor compression cooling technologies when operating the same wine cooler cabinet. The vapor compression system used is a commercially available 31-bottle domestic wine cooler, while the magnetocaloric cooling device is a state-of-the-art (TRL-5) prototype connected to the retrofitted wine cooler cabinet. The test procedures involved characterizing the unsteady and steady-state behavior of both technologies. Detailed instrumentation of both systems allowed for a unique low-level performance comparison, considering first- and second-law-based parameters, which has not been explored in the literature. Overall, the wine cooler was found to be more efficient when operated by the vapor compression system. For an ambient temperature of 25 °C and a cabinet temperature of approximately 12 °C, the coefficient of performance (COP) and second-law efficiency of the wine cooler were 0.7 and 3.6% respectively when using the vapor compression system, whereas they were 0.49 and 2.0% respectively when using the magnetocaloric refrigerator system. However, despite the contrast between the internally ideal coefficient of performance of the magnetocaloric wine cooler and the vapor compression wine cooler, significant but realistic design improvements were suggested. These suggestions were evaluated and quantified using a system-level magnetocaloric refrigerator model. The numerical results indicate that with changes in the current pumping system and magnetocaloric materials, an improved version of the magnetocaloric wine cooler could surpass the COP of its original version by 88% and the vapor compression system by 31%.

Integration of Multifunctional Epitaxial (Magnetic) Shape Memory Films in Silicon Microtechnology

AbstractMagnetic shape memory alloys exhibit various multifunctional properties, which range from high stroke actuation and magnetocaloric refrigeration to thermomagnetic energy harvesting. Most of these applications benefit from miniaturization and a single crystalline state. Epitaxial film growth is so far only possible on some oxidic substrates, but they are expensive and incompatible with standard microsystem technologies. Here, epitaxial growth of Ni–Mn–based Heusler alloys with single crystal‐like properties on silicon substrates is demonstrated by using a SrTiO3 buffer. It is shown that this allows using standard microfabrication technologies to prepare partly freestanding patterns. This approach is versatile, as its applicability for the NiTi shape memory alloy is demonstrated and spintronic and thermoelectric Heusler alloys are discussed. This paves the way for integrating additional multifunctional effects into state‐of‐the‐art microelectronic and micromechanical technology, which is based on silicon.

Two magnetic entropy peaks during one-step martensitic transformation in a Ni42Co8Mn37.5In12.5 alloy

Magnetic shape memory alloys with multiple entropy change (ΔSM ) peaks are promising to extend the working temperature window (ΔT eff) in active magnetic refrigeration. They usually have multi-step martensitic transformations, since the multiple variations of structure and magnetic states lead to the one-to-one corresponding ΔSM peaks. In this work, we report an interesting Ni42Co8Mn37.5In12.5 magnetocaloric alloy, which exhibits only one-step martensitic transformation but generates two considerable ΔSM peaks with the value of 7.2 J K−1 kg−1 and 9.9 J K−1 kg−1 under 6 T respectively. This phenomenon is attributed to the restriction internal stresses stemming from the lattice mismatch between martensite and austenite, the random local stresses produced by defects, the interfacial hindrances of wide phase boundaries, and the inhibition of dispersed precipitates, which divide the one-step transformation into two discrete stages. Such multiple ΔSM peaks are beneficial for expanding the working temperature window of magnetocaloric refrigerators.

Design and comparison of electro-permanent magnetic field sources for magnetocaloric heat pumps

We present the results of a comprehensive study of the design, numerical analyses and experimental validation of various electro-permanent magnetic structures that can be used in magnetocaloric refrigeration. Most magnetic field sources currently used in conceptual magnetic cooling devices require moving parts, which produce noise, vibrations and require maintenance. Furthermore, the operating frequency of such magnetic structures is relatively low and limited by aspects of the mechanical construction. Unlike any magnetic structure currently used in a magnetocaloric refrigeration device, the presented structure consists of a hybrid assembly of an electromagnet and a permanent magnet. We report on how different configurations of permanent magnets and the windings of electromagnets, including the air gaps, affect the energy efficiency, the input power and the total mass of the magnetic field source. Some guidelines for future research and the development of electro-permanent magnetic field sources for magnetocaloric refrigeration and heat pumps are presented, supported by information on how such structures can enable higher operating frequencies that could increase the compactness of future magnetocaloric devices.

Magnetocaloric and electrocaloric heat engine and refrigeration cycles in the open Fermi-Hubbard optical dimer with attractive interactions

Quantum heat engines and refrigerators can be realized in a Fermi-Hubbard optical dimer through immersion in a mean-field Fermi gas bath using the magnetocaloric and electrocaloric effects. It is demonstrated that in the presence of onsite and heat bath-induced attractive interactions, singlet-to-triplet transitions are achieved with a finite external magnetic field. Similarly, triplet-to-singlet transitions occur in the presence of an applied finite electric field. Results show that to lift the degeneracies, considerable electric potential magnitudes are needed. These magnitudes are greater than the magnetic potential strengths where the degeneracies occur. Furthermore, in the attractive regime, combinations of direct and inverse caloric effects can be combined to operate quantum heat engines and refrigerators, like what happens in the repulsive regime.

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.

Transition metal (dis)order in single crystal multicomponent rare earth perovskites

The utilization of configurational entropy in materials design has accelerated the tailoring of oxides for a variety of technological applications. So called “high entropy” oxides (HEO) have more recently been investigated for their use in magnetocaloric refrigeration due to tunability of the spin glass phase. To advance our understanding of these magnetic interactions, we have grown single crystals ( 5 mm diameter, 3 cm length) of the multicomponent perovskite SmCo1/4Cr1/4Fe1/4Mn1/4O3 using the laser diode floating zone technique. This material deviates from its parent SmMO3 (MCo, Cr, Fe, Mn) magnetic behaviors by having two broad transitions with a large bifurcation in the zero-field cooled vs field cooled measurements. Measurements of magnetization as a function of field show spin canting of the transition metal ions, with exchange bias emerging as a function of temperature. Heat capacity measurements are utilized alongside the non-magnetic rare earth analog LaCo1/4Cr1/4Fe1/4Mn1/4O3 to understand the low temperature superexchange interactions. Such comparison supports the two magnetic sublattices operating at different energy scales. Overall single crystal growth of such HEO should be utilized due to the ease of growth using the floating zone technique and insight it gives into the magnetic characterization.

Landau theory-based relaxational modeling of first-order magnetic transition dynamics in magnetocaloric materials

The magnetocaloric effect is often largest within the neighborhood of a first-order phase transition. This effect can be utilized in magnetocaloric refrigeration, which completely eliminates the need for the greenhouse gases utilized in conventional refrigeration. However, such transitions present unique dynamical effects and are accompanied by hysteresis, which can be detrimental for such refrigeration applications. In this work, a Landau theory-based relaxational model is used to study the magnetic hysteresis and dynamics of the first-order magnetic transition of LaFe13−x Si x . Fitting the experimental magnetization data as a function of applied magnetic field under different field sweep rates with this model provided the Landau parameters (A, B, and C) and the kinetic coefficient of the studied material. We demonstrate the tendency of the magnetic hysteresis to increase with the magnetic field sweep rate, underlining the importance of studying and minimizing the magnetic hysteresis in magnetic refrigerants at practical field sweep rates. While evaluating the temperature dependence of the time required for a complete transition to occur, a nonmonotonic behavior and a sharp peak were found for temperatures near the transition temperature. Such peaks occur at the same temperature as the peak of the magnetic entropy change for low fields, whereas for higher fields the two peaks decouple. This information is critical for technological applications (such as refrigerators/heat pumps) as it provides guidelines for the optimization of the magnetic field amplitude in order to reduce the transition timescale and consequently maximize the machine operational frequency and amount of heat that is pumped in/out per second.

Evaluation of magnetocaloric behavior and refrigeration performance of Gd3Co10Al87 nanostructured thin film alloy with multiple Curie temperatures

Evaluation of magnetocaloric behavior and refrigeration performance of Gd3Co10Al87 nanostructured thin film alloy with multiple Curie temperatures

Current perspective in magnetocaloric materials research

Magnetocaloric refrigeration has remained a promising alternative to conventional refrigeration for the last few decades. The delay in reaching the market is significantly based on materials’ related issues, such as hysteresis/reversibility, mechanical stability, or formability. This perspective paper shows the current trends in magnetocaloric materials research, highlighting the families of alloys and compounds that are gaining attention in the recent years. It also includes an overview of novel approaches that can be used to analyze these properties that could improve the applicability of magnetocaloric materials.

Correlating the properties of near-room-temperature first- and second-order magnetocaloric materials

Several magnetocaloric materials have been proposed since the discovery of the Giant Magnetocaloric Effect. Although some have great potential as magnetocaloric refrigerants or working materials in thermomagnetic motors/generators, only a few have been tested experimentally or had their properties incorporated into validated mathematical models. While experiments are limited by material costs and specialized equipment to determine magnetic field-dependent properties such as specific heat capacity and magnetization, the development of correlation methods must ensure data quality and resolution over a wide range of conditions to reduce interpolations errors. Aiming to keep the number of baseline experimental data points at a minimum, we propose a fitting procedure to correlate thermomagnetic quantities (i.e., isofield specific heat capacity, magnetization and isothermal entropy change) that is accurate at intermediate (i.e., not directly measured) temperatures and applied magnetic fields. The method has been applied to different first-order materials (La(Fe,Mn,Si)13Hy and MnFePxAs1−x) and second-order materials (Gd and Gd100−xYx) with a good reproducibility of the isofield specific heat capacity, entropy–temperature diagram, adiabatic temperature change and magnetization behavior around room temperature at applied fields between 0 and 2 T.

Efficient modulation of the magnetocaloric refrigerator capacity

Magnetocaloric energy conversion devices (e.g., room air conditioners and household refrigerators) have the potential to significantly reduce the emissions associated with refrigerant leakage into the atmosphere but still have lower efficiencies compared to mature vapor compression systems. The efficiency of a magnetocaloric cooling device derives not only from its design characteristics (e.g., solid refrigerant, hydraulic system, and magnet system) and its operating temperature span but also from its modulating capability. Owing to the lack of experimental data regarding this topic, the advantage of modulating the cooling capacity (i.e., the part-load performance) of an active magnetic regenerator prototype is demonstrated experimentally for the first time. The capacity modulation is carried out by means of regulating both the cycle frequency of the device and the volumetric flow rate of the heat transfer fluid. At a 14 K temperature span and a 1.4 Hz frequency, the magnetocaloric refrigerator prototype using 3.8 kg of gadolinium provided a maximum cooling capacity of 452 W with an appreciable coefficient of performance of 3.2, which corresponds to a second-law efficiency of 15.5 %. At part-load operating conditions, the device can produce a cooling capacity of 245 W with an increased second-law efficiency of 29.7 %, or a coefficient of performance of 6.2, making it more competitive with traditional vapor compression systems. In future studies, the experimental data obtained may be implemented in a dynamic building energy model to quantify the energy-saving benefits of part-load operation by estimating the overall system efficiency during a typical cooling season.

Contrasting response on magnetocaloric effect and refrigeration capacity due to Ni or Mn substitution by Fe in Ni-Mn-In-Co-Fe alloys

We set the composition of two Ni-Mn-In-Co-Fe alloys as (Ni46.8Fe1.2)Co2Mn36.5In13.5 and Ni48Co2(Mn34.5Fe2)In13.5 in order to display the martensitic transformation near to room temperature. Under a magnetic field change of 5 T the polycrystalline low Co content alloys displayed a large magnetocaloric effect ΔSMag of 15.5 and 20.0 Jkg-1K-1; and giant refrigeration capacity RCFWHM up to 422 and 292 Jkg-1 for (Ni46.8Fe1.2)Co2Mn36.5In13.5 and Ni48Co2(Mn34.5Fe2)In13.5, respectively. The XRD patterns showed cubic L21 structure with space group 225 in both alloys and via Rietveld refinement negligible differences were found in lattice parameter of 0.598(7) and 0.598(9) nm for (Ni46.8Fe1.2)Co2Mn36.5In13.5 and Ni48Co2(Mn34.5Fe2)In13.5, respectively. Nevertheless, the magnetocaloric response in the alloys exhibited remarkable dissimilarities. Whilst the applied magnetic field shifted the martensitic transformation − 3.4 KT-1 in the Ni48Co2(Mn34.5Fe2)In13.5 alloy this sensibility increased up to − 6.8 KT-1 in the (Ni46.8Fe1.2)Co2Mn36.5In13.5 alloy; consequently the span in the Full Width Half Maximum of the ΔSMag(T) curve was 17 and 32 K, respectively.

What is the speed limit of martensitic transformations?

ABSTRACT Structural martensitic transformations enable various applications, which range from high stroke actuation and sensing to energy efficient magnetocaloric refrigeration and thermomagnetic energy harvesting. All these emerging applications benefit from a fast transformation, but up to now their speed limit has not been explored. Here, we demonstrate that a thermoelastic martensite to austenite transformation can be completed within 10 ns. We heat epitaxial Ni-Mn-Ga films with a nanosecond laser pulse and use synchrotron diffraction to probe the influence of initial temperature and overheating on transformation rate and ratio. We demonstrate that an increase in thermal energy drives this transformation faster. Though the observed speed limit of 2.5 × 1027 (Js)1 per unit cell leaves plenty of room for further acceleration of applications, our analysis reveals that the practical limit will be the energy required for switching. Thus, martensitic transformations obey similar speed limits as in microelectronics, as expressed by the Margolus – Levitin theorem.

Improving efficiency of transcritical CO2 cycles through a magnetic refrigeration subcooling system

Subcooling methods for transcritical CO2 plants are being studied in order to improve the behaviour of these systems in hot climates, where basic configurations are not competitive enough. To achieve important improvements in the transcritical CO2 performance, it is necessary to perform the subcooling with a refrigeration cycle working with a Coefficient of Performance higher than that of the CO2 system without subcooling. Magnetic refrigeration devices can achieve high Coefficient of Performance values when the temperature difference between the hot sink and cold source is small, and therefore they meet the requirements to be applied as a CO2 subcooling method. This work presents the coupling of two refrigeration technologies: vapour compression and magnetocaloric refrigeration, which has not yet been presented in the literature. The magnetic refrigeration system is, based on the experimental results of the existing prototype, analysed semi-empirically and further evaluated, as a subcooling method for a transcritical CO2 cycle in a wide range of ambient conditions. Gas-cooler pressure, subcooling degree and operating parameters of the magnetic refrigerator were optimized for each condition to obtain the maximum Coefficient of Performance. We show that subcooling with the existing prototype of magnetic refrigeration system can enhance the overall Coefficient of Performance of transcritical CO2 cycle by up to 9%.

Machine learning predicted magnetic entropy change using chemical descriptors across a large compositional landscape

Magnetocaloric refrigeration has drawn considerable attention in the last few decades as it can positively disrupt the current cooling technology. Most research efforts focus on developing new magnetic materials in the laboratory by trial and error. Here we report a materials dataset developed using past experimental work comprising several important magnetocaloric material classes such as La(Fe,Si/Al)13, heusler alloys, manganites, Gd5(Si,Ge)4 family, rare-earth and metallic glasses as well as Laves phase compounds with their reported magnetic entropy changes, -ΔSM(T,H). Notable linear and non-linear machine learning models are implemented to predict the -ΔSM(T,H) of materials. Our analyses indicate that the Random Forest model outperforms the others with R2 of 0.82. We then use this model to screen a large magnetic materials database with nearly 40,000 compounds to identify potential new magnetocaloric materials operating near room temperature. MnGa2Sb2, CrGa2Sb2, SbSCl0.1I0.9, Sm3Te4, LaRhSn, SbSI, Tl0.58Rb0.42Fe1.72Se2, Cs0.86Fe1.66Se2, La2.1MnGe2.2 are some of the newly predicted compounds that could yield large magnetocaloric cooling performance.