In this work we report on the structural, magnetic and magnetocaloric properties of polycrystalline HoNi2 and ErNi2 Laves-phase compounds. Powder X-ray diffraction studies at room temperature show ...that the compounds crystallize in the cubic C15 structure (MgCu2 type). The magnetic measurements reveal that the second-order magnetic phase transition from ferromagnetic to paramagnetic state appears at TC equal 13.5 K for HoNi2 and 6.5 K for ErNi2. Using direct measurements over a wide field range (up to 11 T), along with indirect methods, we have found low and high field regularities of the magnetocaloric effect characterized by the adiabatic temperature change, ΔTad, and the isothermal magnetic entropy change, ΔSmag. The magnetocaloric properties near the phase transition are discussed in the framework of the Landau theory for the second-order phase transitions.
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•Magnetocaloric effect of the intermetallic HoNi2 and ErNi2 Laves-phase compounds.•Direct measurements of magnetocaloric effect over a wide field range along with indirect methods.•Applicability of the Landau theory for description of the observed MCE is shown and discussed.
The magnetocaloric effect (MCE) in single crystals of MnAs compound was studied both experimentally and theoretically. Direct measurement of MCE showed that the adiabatic temperature change (ΔTad) in ...a magnetic field of 10 T was 15 K. But direct measurement of the quasi-adiabatic heat transfer (ΔQ) of the sample yielded a value of 9500 J/kg in a magnetic field of 10 T. To date, it is the highest experimentally recorded value for ΔQ by direct measurement. Phenomenological considerations that take into account the interaction of magnetic and structural order parameters explain a number of anomalies in the magnetic and magnetocaloric properties of MnAs that were observed during the experiments.
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•The MCE of MnAs single crystal by a direct method is investigated.•Adiabatic temperature change is 15 K in 10 T at temperature of 311 K.•Isothermal heat transfer is 9300 J/kg in10 T at temperature of 318 K.•Phenomenological model explain a number of anomalies in the magnetic properties.•The jumps in magnetization change decrease the MnAs magnetocaloric characteristics.
Structural, magnetic, magnetocaloric and mechanical properties of Ni49-xMn39Sb12Cox (x = 0,2,4) alloys prepared by arc-melting have been studied. It has been observed that with increase in Co ...content, the system transforms from martensite to austenite phase. Magnetic measurements show that the martensite transition shifts from 338 K to 290 K. The effective refrigerant capacity is found to increase from 23 J/kg to 119 J/kg with Co doping under an applied magnetic field of 7T. The adiabatic temperature change evaluated from zero field heat capacity was found to be −3.75 K in Co doped alloy at 7 T. Our studies show that Co doping in Ni-Mn-Sb alloy also improves mechanical properties of the alloy and thus can be considered as a competent magnetic refrigerant alloy.
•Co-doping in Ni-Mn-Sb alloy leads to enhancement in magnetocaloric properties.•The martensite transition decreases from 338 K to 290 K with Co doping in Ni-Mn-Sb alloys.•Compresseive stress enhances from 550 MPa to 1390 MPa in Co doped Ni-Mn-Sb alloys.
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•Gd70Y30−xFex (x = 0, 10 and 20) ribbon alloys were prepared by single-roller melt spinning method.•Curie temperature is 277 K, 283 K and 291 K for Gd70Y30−xFex alloys with x = 0, 10 ...and 20, respectively.•The magnetic entropy change gets a maximum of 8.8 J·kg−1 K−1 for a magnetic field change of 0–5 T.•The values of FWHM and RC under applied field 5 T are 55% and 6% larger than pure Gd respectively.
The Gd70Y30−xFex (x = 0, 10 and 20) ribbon alloys were prepared by single-roller melt spinning method. The results show that all samples are single phase and crystallize with a hexagonal structure. The magnetic entropy change at 5 T are 8.8, 8.1 and 5.7 J·kg−1·K−1 near the Curie temperature of 277 K, 283 K and 291 K for Gd70Y30−xFex alloys with x = 0, 10 and 20, respectively. Furthermore, the values of full width at half maximum and refrigeration capacity under applied field 5 T increase by 55% and 6% compared with pure Gd respectively. The obtained spontaneous magnetization and adiabatic temperature (Δ2T) are 238, 241, 197 emu/g and 4.0, 3.5, 2.4 K for samples with x = 0, 10, and 20, respectively. The Arrott plots and universal curve are employed to investigate the magnetic phase transition, showing all samples undergo a second order ferro-paramagnetic phase transition. The results indicate that Gd70Y30−xFex alloys with large magnetocaloric effect and wide temperature span are a good candidate for room temperature magnetic refrigeration.
Electrocaloric cooling is a method for flexible electronics and compact chips with a promising ability to subvert traditional vapor compression refrigeration technology. Nevertheless, most reported ...EC materials exhibit relatively low adiabatic temperature (ΔT) and low breakdown field strength (Eb) at room temperature, limiting their further applications. Here, we develop a ferroelectric nanocomposite, composed of relaxor terpolymer poly(vinylidene fluoride-trifluoroethylen-chlorofluoroethylene) (PVTC), ceramic nanofillers 0.68(BaZr0.2Ti0.8O3)-0.32(Ba0.7Ca0.3TiO3) (BCZT), boron nitride nanosheets (BNNSs), and BiFeO3 nanofibers (BFO NFs) with different morphologies and vary components ratios. The most optimal 12BBBP nanocomposite demonstrates gigantic ECE values including an isothermal entropy change (ΔS) of 94.5 J·kg−1·K−1 and a ΔT of 13.5 K under 220 MV/m electric field at 303 K, ΔS and ΔT are 3.5 times and 2.2 times larger than PVTC under 100 MV/m. It also exhibits an excellent Eb with a value of 290 MV/m. The phase-field simulation is used to simulate Eb behavior of nanocomposites with fillers of different nano-morphologies under an electrostatic field. Simultaneously, the interfacial coupling and ferroelectric domain structures are also simulated based on the time-dependent Landau-Ginzburg-Devonshire (TLGD) theory through finite element analysis (FEA) to present a better explanation for ECE performance. The experimental and simulation results simultaneously denote the ferroelectric nanocomposite possesses outstanding ECE and high Eb performance, laying foundation for the ferroelectrics in fields of flexible electronics and compact chip cooling. Coupled with the facile processability of terpolymer and lead-free nature of electroactive ceramics, this work paves a new way to develop a scalable, environmentally friendly and high-performance EC materials for next generation of refrigeration.
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•Giant ECE of ΔT = 13.5 K and ΔS = 94.5 J·kg−1·K−1 are attained in 12BBBP with different morphologies.•12BBBP nanocomposite has a large breakdown field strength (Eb=290 MV/m).•Large Eb due to presence of NFs that disperse electric field and hinder growth of breakdown phase.•Coexistence of tetragonal and rhombohedral phase, and strong interface coupling, lead to large ECE.•Excellent ECE and Eb provide effective way for design and development of subsequent ECE devices.
To foster suitable materials to be useful in the magnetic refrigeration devices, we have studied the magnetic and magnetocaloric properties of the MnFe4Si3 system, based on the Monte Carlo method. ...The obtained properties are in the harmony with reported data. The structure of this system shows a hexagonal lattice with Fe (S = 3/2) and Mn (S = 3/2). The 2ed order transition is predicted at the Curie-temperature: Tc=290K. The ΔSm: magnetic entropy change, the ΔTad: adiabatic temperature change and the magnetizations are obtained and discussed for different physical parameters. The values of (ΔSm and ΔTad) reach respectively 4.53 J/(kg.K) and 11 k at T = 289K when the applied magnetic fields (h) is of 4T. The RCP: relative cooling power reaches the value of 448 J.kg−1 at h = 4T and presents a good proof for MnFe4Si3 to be a favorite material in the magnetic refrigeration systems.
•The maximum of the magnetocaloric effect, and the critical magnetic behavior are corresponding to the same transition temperature Tc = 290 K.•The magnetic entropy change ΔSm reaches the values 1.53, 2.6, 3.68 and 4.53 J/(kg. K) at T = 289 K.•The exchange couplings (J1 = 92 K and J2 = 20 K) correspond to the stable phase (3/2, 3/2).•The possibility of MnFe4Si3 to be a very promising candidate for the magnetic refrigeration applications.
The influence of La3+-doping on the electrocaloric effect (ECE) of the 0.75(Pb1-3x/2Lax)(Mg1/3Nb2/3)O3-0.25(Pb1-3x/2Lax)TiO3 (L-PMN-PT: x/75/25 x = 0.0–4.0 mol.%) ceramics was studied. With the ...increase of La3+-doping in these ceramics, the relaxor ferroelectric properties increase, while the average grain size, coercive field Ec and Curie temperature TC decrease. As a result, a maximum adiabatic temperature change ΔT as high as 0.68 K, measured by direct measurement method, was achieved under a lower electric field (E) intensity of 5 kV/mm at 383 K for samples with x = 2.0 mol.%. Besides the high ΔT value, the La3+-doping also leads to a temperature insensitivity of ΔT. The variation of ΔT is less than 0.15 K, or 28.3%, over a wide temperature range of 303–423 K. The La3+-doped PMN-PT ceramics, with large and temperature insensitive ΔT over a broad temperature range, is very helpful for practical application. The reduction of Ec and TC can be attributed to the suppression of long-range coupling between oxygen octahedrons due to the partial replacement of Pb2+ by La3+. The temperature insensitivity of ECE is attributed to the enhancement of relaxor ferroelectric properties.
The relaxation processes of the adiabatic temperature changes (ΔTad) at the phase transitions in Ni45Mn43CoSn11, Ni50Mn36.5In13.5, and Ni50Mn35In14.25B0.75 Heusler alloys with different magnetic ...structures have been studied using a direct extraction method in magnetic fields up to 14 T. It has been found that ΔTad exhibits short relaxation times (less than 10−1 (s)) in the vicinity of the second order phase transitions at the Curie temperatures. The relaxation times of the first order martensitic transitions strongly depend on the latent heat of the transition and can be characterized by a logarithmic law.
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•The direct of MCE in the vicinity of Tc follow the application of field.•At the inverse MST the relaxation times of the ΔTad can reach 10 s or more.•The magnitude of ΔTr depends on the magnitude of the latent heat of transition.•The process of relaxation of the ΔTad can be described by logarithmic law.•Logarithmic law indicates the presence of a wide range of relaxation times.
In the present work, the simultaneous substitution of Co element for Mn element and Ni element in the MnNiGe0.75Si0.25 alloy systems resulted in Mn1−xCo2xNi1−xGe0.75Si0.25 (x = 0, 0.18, 0.19, 0.20, ...0.21, 0.22) alloys with coupled magnetic and structural transitions. A magnetic-field-induced entropy change (ΔSM) as large as −53.7 J∙kg−1 K−1 was obtained at 264 K under a 5 T magnetic field in the Mn0.8Co0.4Ni0.8Ge0.75Si0.25 alloy. In addition, the ΔSM was as large as −25.2 J∙kg−1 K−1 and −36.1 J∙kg−1 K−1 at 262K under 2 T and 3 T magnetic fields. Moreover, an adiabatic temperature change as large as 1.08 K and a reversible adiabatic temperature change as large as 0.43 K were observed under a 1.5 T permanent magnetic field in a Mn0.8Co0.4Ni0.8Ge0.75Si0.25 epoxy-bonded composite.
•The magneto-structural coupling was realized in the MnCoNiGeSi alloys.•Large low magnetic field magnetocaloric effects were observed.•Large adiabatic temperature change would be up to 5 K for 5T magnetic field.•Reversible adiabatic temperature changes were gained in the epoxy-bonded alloys.
Magnetic field–induced adiabatic temperature change, ΔTad, measures the temperature gradient that enables a magnetic refrigerant to transfer heat from reservoirs. The ΔTad of Mn5Ge3 was evaluated by ...directly recording the temperature of a Mn5Ge3 sample while subjecting it to an external field. The peak ΔTad of 0.89 ± 0.006 at 0.8 T for Mn5Ge3 at its Curie temperature was consistent with the value previously obtained from specific heat capacity measurements. In addition, the peak values of ΔTad for Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 were measured to be 0.77 ± 0.009 and 0.93 ± 0.011, respectively, at 0.8 T near their respective Curie temperatures. ΔTad measurements under cyclic application of a 0.8 T field showed that the alloys, whose magnetocaloric effect stems from the second-order magnetic phase transition, exhibited no hysteresis in ΔTad and maintained their peak ΔTad within 1.3% under cyclic field. The ΔTad of sandwiched Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3 plates was also measured to demonstrate that a combination of the alloys can be used to form a graded regenerator for the active magnetic regenerator cycle.
•ΔTad of Mn5Ge3 and Mn4.75(Co, Fe)0.25Ge3 was directly measured.•Peak ΔTad of Mn5Ge3 was measured to be 0.89 K at 0.8 T near its Curie temperature.•Peak ΔTad of 0.77 K and 0.93 K at 0.8 T for Mn4.75(Co, Fe)0.25Ge3 were measured.•Peak ΔTad was maintained within 1.3% with no hysteresis under cyclic field.•ΔTad of a composite sample was evaluated for the graded magnetic refrigerant.
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