Second-order Phase Transition Materials Research Articles

Magnetocaloric properties of a wire composite of La (Fe,Mn,Si)13-H alloy powders in a Gd cylindrical tube

The enhancement of magnetocaloric effect (MCE) and the extension of effective magnetic refrigeration temperature region near room temperature are critical requirements for highly efficient and environment friendly cooling technologies. The large isothermal entropy change of the first-order phase transition material and broader temperature range of the second-order phase transition material are combined into the composite form of La(Fe,Mn,Si)13-H and Gd with their transition temperatures close to room temperature. In this work, wire composites were fabricated using the La (Fe,Mn,Si)13-H powders filled inside the Gd tube. The magnetocaloric properties of the wire composite are convoluted from individual constituents. The dual transitions temperature were observed at 287 K and 292 K. The isothermal entropy change response of the wire composite can be obtained by using the simulation with the combined weight of the Gd tube and La (Fe,Mn,Si)13-H powders. The maximum isothermal entropy change (−ΔSiso) at μoH= 5 T calculated from isothermal magnetization measurements are 8.7 for Gd tube, 16.9 for La(Fe,Mn,Si)13-H powders, and 10.2 J/kg•K for a wire composite. The magnetocaloric properties including the order of phase transition of the wire composite are close to second-order type due to the higher weight fraction of Gd tube.

Magnetic properties and cryogenic magnetocaloric effect in monoclinic RE8.66(BO3)2(B2O5)O8 (RE = Er, Tm) compounds

Researchers in cryogenic magnetic refrigeration have never stopped pursuing magnetic refrigerants with lower ordering temperatures and larger magnetocaloric effects. Rare earth borates have been found to be potential cryogenic magnetocaloric materials due to their low phase transition temperatures and considerable magnetic entropy changes. Here, two rare earth borates RE8.66(BO3)2(B2O5)O8 (RE = Er, Tm) were successfully fabricated, and the crystallographic and magnetic parameters as well as cryogenic magnetocaloric effects were systematically evaluated. The compounds are deemed to crystalize in a monoclinic structure belonging to space group C2/m. They are both second-order phase transition materials without any long-range order above 2 K. These monoclinic rare earth borates, especially for Er8.66(BO3)2(B2O5)O8, show more considerable low-field magnetocaloric effects than rare earth orthoborates. For ΔH = 1 and 2 T, the values of −ΔSMmax are 5.1 and 11.2 J/(kg K) for Er8.66(BO3)2(B2O5)O8, 3.0 and 5.3 J/(kg K) for Tm8.66(BO3)2(B2O5)O8, respectively. Therefore, they are regarded as potential candidates for cryogenic magnetic refrigeration.

Predicting the magnetic measurements of first- and second-order phase transition magnetocaloric materials with artificial neural networks

Due to the large innovative applications involving magnetocaloric materials, such as magnetic cooling devices and thermomagnetic engines, the number of studies on their characterization have been increasing over the past two decades. Although indispensable, magnetic measurements are considerably time-consuming, which sometimes hinder depth studies on the subject. We propose a method that uses artificial neural networks to predict magnetic measurements of magnetocaloric materials with first- and second-order phase transitions, aiming to reduce the required number of measured field dependent magnetization curves M(H) in standard studies. We validate the method for magnetic measurements of the first- and second-order phase transition materials Gd5Ge4 and Gd5Si2.4Ge1.6, respectively. The application of the proposed method results in very high accuracies, with r2>0.98, when comparing the prediction to measured M(H) curves. The entropy change vs temperature curves upon the application of 5 T magnetic fields are also predicted.

Impact of hysteresis on caloric cooling performance

• 1D regenerator model with hysteresis is used to assess caloric cooler's performance. • Modeling of six hypothetical materials with different caloric properties. • Effect of hysteresis is greater on coefficient of performance than on cooling power. • Materials with low isothermal entropy change are very sensitive to hysteresis. • Materials with low specific heat can tolerate high hysteresis values. Caloric cooling relies on reversible temperature changes in solids driven by an externally applied field, such as a magnetic field, electric field, uniaxial stress or hydrostatic pressure. Materials exhibiting such a solid-state caloric effect may provide the basis for an alternative to conventional vapor compression technologies. First-order phase transition materials are promising caloric materials, as they yield large reported adiabatic temperature changes compared to second-order phase transition materials, but exhibit hysteresis behavior that leads to possible degradation in the cooling performance. This work quantifies numerically the impact of hysteresis on the performance of a cooling cycle using different modeled caloric materials and a regenerator with a fixed geometry. A previously developed 1D active regenerator model has been used with an additional hysteresis term to predict how modeled materials with a range of realistic hysteresis values affect the cooling performance. The performance is quantified in terms of cooling power, coefficient of performance (COP), and second-law efficiency for a range of operating conditions. The model shows that hysteresis reduces efficiency, with COP falling by up to 50% as the hysteresis entropy generation ( q hys ) increases from 0.5% to 1%. At higher working frequencies, the cooling performance decreases further due to increased internal heating of the material. Regenerator beds using materials with lower specific heat and higher isothermal entropy change are less affected by hysteresis. Low specific heat materials show positive COP and cooling power up to 2% of q hys whereas high specific heat materials cannot tolerate more than 0.04% of q hys .

Mechanism of broadened phase transition temperature range in LaFeCoSiC compounds prepared upon high-pressure annealing

The cobalt-carbon co-doped NaZn13-type compound LaFe10.95Co0.65Si1.4C0.15 (LFCSC) is one of the most promising candidates for room-temperature working substance in magnetic refrigerator due to its many excellent properties such as large reversible entropy, low cost, and short annealing time. However, owing to the narrow temperature regions of magnetic phase transition in LFCSC, the operation-temperature window for magnetic refrigeration is limited, which restricts its actual application to some extends. In this paper, it is shown that the application of high-pressure to LFCSC during annealing can tailor atomic environment and magnetic transition, which leads to a strongly expanded phase transition temperature range in LFCSC. This broadening behavior can be well understood by importing the magnetoelastic interaction of localized magnetic moments into a microscopic model. The refrigeration performance of the high-pressure annealed sample with wide phase transition temperature range is enhanced according to the relative cooling power (RCP). On the contrary, temperature averaged entropy change (TEC) exhibits a weakened value in the high-pressure annealed sample, which suggests that the magnetic cooling performance could not be effectively improved by simply expanding the phase transition temperature range in the second-order phase transition materials. However, high-pressure annealing would be helpful to the magnetic refrigeration performance for the first-order phase transition materials.

Investigation into electrocaloric effect of different types of ferroelectric materials by Landau-Devonshire theory

The electrocaloric effects in various types of materials, including inorganic perovskites, organic perovskites, organic polymers, molecular ferroelectrics and two-dimensional ferroelectric materials, possess great potential in realizing solid-state cooling devices due to the advantages of low-cost, high-efficiency and environmental friendly. Different ferroelectric materials have distinct characteristics in terms of phase transition and electrocaloric response. The mechanism for enhancing the electrocaloric effect currently remains elusive. Here, typical inorganic perovskite BaTiO<sub>3</sub>, PbTiO<sub>3</sub> and BiFeO<sub>3</sub>, organic perovskite [MDABCO](NH<sub>4</sub>)I<sub>3</sub>, organic polymer P(VDF-TrFE), molecular ferroelectric ImClO<sub>4</sub> and two-dimensional ferroelectric CuInP<sub>2</sub>S<sub>6</sub> are selected to analyze the origins of their electrocaloric effects based on the Landau-Devonshire theory. The temperature-dependent pyroelectric coefficients and electrocaloric performances of different ferroelectric materials indicate that the first-order phase transition material MDABCO and the second-order phase transition material ImClO<sub>4</sub> have excellent performances for electrocaloric refrigeration. The predicted results also strongly suggest that near the phase transition point of the ferroelectric material, the variation rate of free energy barrier height with temperature contributes to the polarizability change with temperature, resulting in enhanced electrocaloric effect. This present work provides a theoretical basis and a new insight into the further development of ferroelectric materials with high electrocaloric response.

Magnetic properties of FeNi alloys for high-temperature thermomagnetic power generation

In this paper, the magnetic properties and heat transfer performance of Fe99.3-xNixMn0.4Si0.3 (x=33, 35, 37, 39, 41, 43) alloys near the Curie temperature (TC) were investigated. The results show that Curie temperature TC for the Fe99.3-xNixMn0.4Si0.3 alloys increases almost linearly from 450 K to 647 K with the Ni content increasing from x = 33 to x = 43. The maximum change rate of the magnetic induction (ΔB/ΔT) near the Curie temperature first increases and then decreases with the increasing Ni content. The maximum ΔB/ΔT value for Fe99.3-xNixMn0.4Si0.3 (x≥37) alloys is higher than that of the second-order phase transition materials (0.0202 T/K). Therefore, the Fe100-xNixMn0.4Si0.3 alloys are suitable for thermomagnetic power generation above 473 K. The energy conversion process of a platy sample (0.2×0.2×0.001 m3) for the alloy with x=37 in the static thermomagnetic power generation was simulated, and the maximum electromotive force of 1.47 V was generated in the simulation.

A hybrid thermal diode based on phase transition materials

Ideally, a thermal rectifier is a device where heat current preferentially flows in one direction, just as the electrical diode works for electrical currents. Herein, we introduce a design of a thermal diode based on the combination of first- and second-order phase transition materials. Validation is realized by a proof-of-concept device consisting of a hybrid composite of neopentylglycol (NPG) and gadolinium (Gd) as the first- and second-order phase transition materials, respectively. Device manipulates the heat via a combined effect of molecular transformations in NPG as well as deactivation of magnons in Gd at the transition and Curie temperatures, respectively. Thermal measurements of the hybrid thermal diode demonstrate a thermal rectification factor of 1.45, which is higher than the value obtained for the reference device based only on NPG. We interpret such enhancement in the thermal rectification factor due to the presence of an asymmetry temperature jump along the heat transfer axis of the device as a consequence of the impact of both phase transitions. Results are corroborated via finite element analysis of the diode by using ANSYS. Hence, such combination of effects has been proved as successful strategy to develop enhanced-performance thermal rectifiers.

Magnetic and magnetocaloric properties in second-order phase transition La1−xKxMnO3 and their composites

In this work, we present a detailed study on the magnetic properties and the magnetocaloric effect (MCE) of La1−xKxMnO3 compounds with x=0.05–0.2. Our results pointed out that the Curie temperature (TC) could be controlled easily from 213 to 306K by increasing K-doping concentration (x) from 0.05 to 0.2. In the paramagnetic region, the inverse of the susceptibility can be analyzed by using the Curie-Weiss law, χ(T)=C/(T−θ). The results have proved an existence of ferromagnetic clusters at temperatures above TC. Based on Banerjee's criteria, we also pointed out that the samples are the second-order phase transition materials. Their magnetic entropy change was calculated by using the Maxwell relation and a phenomenological model. Interestingly, the samples with x=0.1–0.2 exhibit a large MCE in a range of 282–306K, which are suitable for room-temperature magnetic refrigeration applications. The composites obtained from single phase samples (x=0.1–0.2) exhibit the high relative cooling power values in a wide temperature range. From the viewpoint of the refrigerant capacity, the composites formed out of La1−xKxMnO3 will become more useful for magnetic refrigeration applications around room-temperature.

The Effect of Fe Doping on the Magnetic and Magnetocaloric Properties of Mn5−xFexGe3

The magnetic and magnetocaloric properties of a series of minutely doped Mn5-xFexGe3 compounds that exhibit the D88-type hexagonal crystal structure at room temperature have been investigated. For all Fe concentrations, the alloys are ferromagnetic and undergo a second-order ferromagnetic-to-paramagnetic transition near room temperature. Although the small Fe doping had little effect on the ferromagnetic transition temperatures of the system, changes in the saturation magnetization and magnetic anisotropy were observed. For x≤0.15, all compounds exhibit nearly the same magnetic entropy change of ~7 J/kg K, for a field change of 50 kOe. However, the magnitude of the refrigerant capacities increased with Fe doping, with values up to 108.5 J/kg and 312 J/kg being observed for field changes of 20 kOe and 50 kOe, respectively. As second-order phase transition materials, the Mn5-xFexGe3 compounds are not subject to the various drawbacks associated with first-order phase transition materials yet exhibit favorable magnetocaloric effects.

Applicability of scaling behavior and power laws in the analysis of the magnetocaloric effect in second-order phase transition materials

In recent years, universal scaling has gained renewed attention in the study of magnetocaloric materials. It has been applied to a wide variety of pure elements and compounds, ranging from rare earth-based materials to transition metal alloys, from bulk crystalline samples to nanoparticles. It is therefore necessary to quantify the limits within which the scaling laws would remain applicable for magnetocaloric research. For this purpose, a threefold approach has been followed: a) the magnetocaloric responses of a set of materials with Curie temperatures ranging from 46 to 336 K have been modeled with a mean-field Brillouin model, b) experimental data for Gd has been analyzed, and c) a 3D-Ising model ---which is beyond the mean-field approximation--- has been studied. In this way we can demonstrate that the conclusions extracted in this work are model-independent. It is found that universal scaling remains applicable up to applied fields which provide a magnetic energy to the system up to 8\% of the thermal energy at the Curie temperature. In this range, the predicted deviations from scaling laws remain below the experimental error margin of carefully performed experiments. Therefore, for materials whose Curie temperature is close to room temperature, scaling laws at the Curie temperature would be applicable for the magnetic field range available at conventional magnetism laboratories ($\sim 10$ T), well above the fields which are usually available for magnetocaloric devices.

Study of multi-layer active magnetic regenerators using magnetocaloric materials with first and second order phase transition

Magnetocaloric materials (MCM) with a first order phase transition (FOPT) usually exhibit a large, although sharp, isothermal entropy change near their Curie temperature, compared to materials with a second order phase transition (SOPT). Experimental results of applying FOPT materials in recent magnetocaloric refrigerators (MCR) demonstrated the great potential for these materials, but a thorough study on the impact of the moderate adiabatic temperature change and strong temperature dependence of the magnetocaloric effect (MCE) is lacking. Besides, comparing active magnetic regenerators (AMR) using FOPT and SOPT materials is also of fundamental interest. We present modeling results of multi-layer AMRs using FOPT and SOPT materials based on a 1D numerical model. First the impact of isothermal entropy change, adiabatic temperature change and shape factor describing the temperature dependence of the MCE are quantified and analyzed by using artificially built magnetocaloric properties. Then, based on measured magnetocaloric properties of La(Fe,Mn,Si)13Hy and Gd, an investigation on how to layer typical FOPT and SOPT materials with different temperature spans is carried out. Moreover, the sensitivity of variation in Curie temperature distribution for both groups of AMRs is investigated. Finally, a concept of mixing FOPT and SOPT materials is studied for improving the stability of layered AMRs with existing materials.

Direct measurements of the magnetic entropy change.

An experimental device that can accurately measure the magnetic entropy change, Δs, as a function of temperature, T, and magnetic field, H, is presented. The magnetic field source is in this case a set of counter-rotating concentric Halbach-type magnets, which produce a highly homogeneous applied field with constant orientation. The field may be varied from 0 to 1.5 T in a continuous way. The temperature stability of the system is controlled to within ±10 mK and the standard range for the current setup is from 230 K to 330 K. The device is under high vacuum and we show that thermal losses to the ambient are negligible in terms of the calorimetric determination of the magnetic entropy change, while the losses cannot be ignored when correcting for the actual sample temperature. We apply the device to two different types of samples; one is commercial grade Gd, i.e., a pure second-order phase transition material, while the other is Gd5Si2Ge2, a first order magnetic phase transition material. We demonstrate the device's ability to fully capture the thermal hysteresis of the latter sample by following appropriate thermal resetting scheme and magnetic resetting scheme.

Field Dependence of Magnetocaloric Properties in La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5

In this paper, the field dependence of magnetocaloric properties of La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5 with second-order phase transition material is studied using a phenomenological model. The model parameters were determined from the magnetization data adjustment and were used to give better fits to magnetic transition and to calculate the magnetocaloric thermodynamic quantities. The entropy curves have been observed to behave as an asymmetrical broadening of ΔS M peak with increasing magnetic field. For larger fields, the peak shifts to higher temperatures, while the overall shape of the curve broadens over a wide temperature range. The values of maximum magnetic entropy change, full width at half maximum, and relative cooling power, at several magnetic field variations, were calculated. The maximum magnetic entropy changes of 3.957(5) and 14.197(4) J kg −1 K −1 and the relative cooling power (RCP) values of 95.420(3) and 392.729(2) J kg −1 are obtained for 1 and 5 T, respectively. The theoretical calculations are compared with the available experimental data. The critical exponents associated with ferromagnetic transition have been determined from magnetocaloric effect (MCE) methods. By using the field dependence of ΔS M max≈a(μ 0 H) n and the distance (T peak−T c)≈b(μ 0 H)1/Δ, we have investigated the critical behavior of La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5. From the analysis of the relationship between the local exponent n and the gap exponent Δ, we have calculated other exponents: β, γ, and δ. The large MCE, relatively high RCP, high magnetization, and low cost jointly make the present compound a promising candidate for magnetic refrigerant near room temperature.

Construction of consistent magnetocaloric materials data for modelling magnetic refrigerators

The simulation of magnetocaloric refrigerator behaviour needs a good description of the material properties. Magnetocaloric material data are often given as a function of few values of the external magnetic field applied on a sample, but active material reacts to its internal field. The simulating of the magnetocaloric effect using experimental data can then create some artefacts. In this paper we present an alternative way to obtain material data expressed as a function of the internal magnetic field. This characterization is built on the knowledge of the zero magnetic field heat capacity of second-order phase transition materials such as gadolinium, as well as on the direct measurements of magnetocaloric data. An inverse approach is performed to calculate the data as a function of the internal magnetic field with an improved level of detail. The obtained data allow a better modelling of the magnetocaloric effect in an active magnetic regenerator.

Effect of praseodymium and cobalt substitution on magnetic properties and structures in La(Fe 1- xSi x) 13 compounds

Magnetic properties and structures in La 1- z Pr z (Fe 0.895- x Co x Si 0.105) 13 ( x=0.07, 0.08; z=0, 0.2, 0.4) compounds were investigated. When Pr and Co substituted for La and Fe, the Curie temperature of the compounds was adjusted to around room temperature. The magnetic phase transition was driven from first-order to second-order due to Co substitution. As a second-order phase transition material, the MCE of La 0.6Pr 0.4(Fe 0.825Co 0.07Si 0.105) 13, whose relative cooling power was 175 J/kg under a field change of 2 T, was larger than some representative first-order phase transition materials. Suitable Curie temperature and large values of RCP made it an attractive potential candidate for room temperature magnetic refrigeration application.