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Abundant rare earth-based Ce2O3 was employed as the doping for improving the magnetic performance of low-cost Mn-Zn ferrites. To achieve the best doping effect, the influence of sintering temperature on the microstructure and magnetic properties was also investigated. Based on the characterization of the samples prepared at different sintering temperatures (1250–1350°C) and with various Ce2O3 contents (0–0.05 wt.%), it has been found that the magnetic properties are determined by the grain size, the distribution of the pores, and the internal stress associated with impurities. The amplitude permeability for the samples with small grains shows good frequency stability in the range of 10–100 kHz. Increasing the sintering temperature or doping a certain amount of Ce2O3 can increase the amplitude permeability and reduce the magnetic loss of the ferrite. However, excessive Ce addition or excessive sintering temperature led to increasing magnetic loss. For the sample doped with 0.01 wt.% Ce2O3, the maximum amplitude permeabilities of 4105 (20 kHz and 100 mT) and 2830 (100 kHz, 100 mT) can be obtained when sintered at 1350°C, while the minimum power loss with 5.4 W/kg (20 kHz, 100 mT) and 46 W/kg (100 kHz, 100 mT) can be achieved at a sintering temperature of 1280°C. The present results indicate that Ce doping cannot only effectively increase the amplitude permeability and decrease the magnetic loss but can also reduce the sintering temperature. The produced ferrites show great potential for the applications in induction heating systems and other fields.

Room temperature elastocaloric effect as well as the effects of strain rate and loading stress have been studied in polycrystalline Ni51Mn34In8Sn7. The increase of strain rate reduces plastic deformation and lattice invariant strain, thus improving the superelasticity. Higher strain rate and stress facilitate the stress-induced structural transition and adiabatic environment, and so the temperature change (ΔT) increases at a rate of 1.77 K (1% s−1)−1 and 1.78 K (102 MPa)−1. A large ΔT of 4.2 K is obtained by optimizing the strain rate and loading stress, which is comparable to or even larger than those of many NiMn-based alloys at room temperature. This result suggests polycrystalline Ni51Mn34In8Sn7 as a desirable material for room temperature elastocaloric refrigeration.

The effects of Cu substitution for Ni atoms on the magnetic and magnetocaloric properties have been investigated in Mn0.8Fe0.2Ni1-xCuxGe (x = 0.05–0.3) compounds. The substitution of Cu leads to the larger separation of Mn atoms, which stabilizes the hexagonal phase and thus reduces the structural transition temperature Tstr. Moreover, the larger separation of nearest-neighbor Mn atoms weakens the ferromagnetic (FM) coupling in TiNiSi-type orthorhombic phase and results in the spiral antiferromagnetic (AFM) and more antiparallel AFM state with increasing Cu content. Similarly, the Ni2In-type hexagonal phase also changes from FM state to weak-magnetic spin-glass-like state with the introduction of Cu. The x = 0.05 compound shows a conventional magnetocaloric effect (MCE) due to the magnetostructural transformation from PM hexagonal phase to FM orthorhombic phase. On the contrary, the x = 0.1 compound undergoes the antiferromagnetically magnetostructural transition from FM hexagonal phase to AFM orthorhombic phase, which leads to a high inverse MCE. This large inverse MCE could be utilized in novel magnetic refrigerator to improve the refrigeration efficiency and applied for magnetic refrigeration of natural gas liquefaction.

The magnetic and rotating magnetocaloric properties in textured Tb3NiGe2 compound were investigated. Two successive magnetic transitions, a ferrimagnetic to antiferromagnetic (AFM)-like transition at Tt = 130 K and an AFM to paramagnetic (PM)-like transition at TN = 144 K, are observed. Besides, a magnetic-field-induced antiferromagnetic-ferromagnetic metamagnetic transition occurs in the temperature range of Tt ∼ TN. Large rotating magnetic entropy changes, together with considerable adiabatic temperature changes and refrigeration capacity, are obtained in textured Tb3NiGe2 compound. Moreover, the equivalence of different calculation methods for rotating magnetocaloric effect (RMCE) is discussed mathematically. The large RMCE with excellent thermal and magnetic reversibility suggests textured Tb3NiGe2 compound as an attractive candidate for novel magnetic refrigerant.

Textured Ho2GdAl2 alloy exhibits highly anisotropic magnetic properties due to the coexistence of preferred crystallographic orientation and magnetocrystalline anisotropy (MCA). The variation in MCA energy (Ea) induced by the rotation of textured Ho2GdAl2 from a perpendicular to parallel direction under constant field results in a rotating magnetocaloric effect (MCE), which is larger than the conventional MCE. Moreover, the peaks of rotating magnetic entropy change (ΔSR) and rotating adiabatic temperature change (ΔTad,R) broaden asymmetrically towards low temperatures due to the different variation of MCA below and above the Curie temperature TC, thus leading to a table-like rotating MCE over the liquefaction temperature of natural gas.

The Gd x Ho 3−x Al 2 ( x = 0.5–2.5) compounds undergo two successive magnetic transitions: a second-order magnetic transition from paramagnetism (PM) to ferromagnetism (FM) around T C followed by a FM to ferrimagnetism (FIM)-like transition at T t . The T C increases linearly with the increase of Gd content due to the de Gennes factor of Gd being larger than that of Ho, while the T t remains nearly constant around 45 K independent of the variation of Gd content. The working temperature range is expanded by substituting Ho with Gd atoms due to the higher transition temperature of Gd 3 Al 2 and the different composition dependence of successive magnetic transitions, and thus leading to an enhanced refrigerant capacity ( RC ). For a relatively low field change of 2 T, a giant RC value of 272 J/kg with a wide working temperature range of ∼80 K is obtained for x = 1.0 compound due to the partial overlap of successive Δ S M peaks. This is probably the largest RC value so far reported in the temperature range of liquid natural gas (111 K). Consequently, the large Δ S M and giant RC suggest that Gd x Ho 3−x Al 2 compounds could be good candidates of magnetic refrigerants in a wide temperature range.

In this paper, we present a robust, low-power and wide-range MEMS thermal wind sensor, which is based on a glass reflow process. A general analytical model is first proposed for obtaining the heater’s temperature and the temperature distribution on the sensor surface. Based on this model, the effect of the sensor’s geometry and package on the sensor’s performance is investigated. These results demonstrate how the measurement range and the sensitivity of the sensor can be optimized with a change in structural geometries. Besides, it has shown that the package structure of the sensor is also important for obtaining the required performance. Then, calculated results are validated with 2-D finite element method (FEM) solver CMOSOL. It is found that although there is some deviation between theoretical and simulation results, the model can have good reference value for latter MEMS thermal wind sensor design. Finally, the sensor is characterized in a wind tunnel. At a constant heating power of 14.5 mW, measurement results show that the sensor can detect airflow speeds of up to 33 m/s, with an accuracy better than 0.5 m/s at low speeds and 5% Full-Scale at high speeds. Airflow direction can be determined in a 360° range with an accuracy better than 5°. In addition, the sensor also shows a good repeatability.

The phase relationships, magnetic transitions, and magnetocaloric effect (MCE) of MnCo1 − xCuxGe (x = 0–0.5) alloys have been investigated. The substitution of Cu for Co reduces the structural transition temperature between Ni2In-type hexagonal phase and TiNiSi-type orthorhombic phase. The Curie temperature of orthorhombic martensite (TCM) also decreases with increasing x from 0 to 0.2, which is likely due to the weakening of Co-Mn and/or Co-Co interactions by non-magnetic element Cu substitution for Co atoms. In addition, two successive first-order magnetic transitions, a ferromagnetic (FM) to antiferromagnetic (AFM)-like transition around T2 followed by an AFM to FM-like transition around T1, are observed for 0.17 ≤ x ≤ 0.27, which have not been reported in MnCoGe-based alloys so far. An inverse MCE is observed under low field changes and then a universal curve of ΔSM is successfully constructed, proving the applicability of universal curve for AFM materials with inverse MCE. Finally, the variation of magnetic states is explained in terms of the expansion of nearest-neighbor Mn-Mn distance d1. These new findings and related discussions are very helpful for future tailoring of magnetostructural coupling and metamagnetic transition in MM'X (M, M′ = transition metals, X = carbon or boron group elements) alloys. Keywords: Magnetic properties, Martensitic transformations, Magnetocaloric effect

For micro-electro-mechanical systems (MEMS) thermal wind sensor based on calorimetric principle, asymmetry effect, rarely reported in literature, exerts significant influence upon sensor’s performance. In this paper, for the first time, experiments are performed to investigate this phenomenon and a solution is proposed to optimize the performance of the sensor. The MEMS thermal wind sensor is fabricated packaged and tested in the wind tunnel. Experiments demonstrate that for the fixed wind speed, the voltage variation with direction can be reduced from over 20% to about 5% after improvement.

The effect of different atomic substitutions at Mn sites on the magnetic and magnetocaloric properties in Ni50Mn35Co2Sn13 alloy has been studied in detail. The substitution of Ni or Co for Mn atoms might lower the Mn content at Sn sites, which would reduce the d-d hybridization between Ni 3d eg states and the 3d states of excess Mn atoms at Sn sites, thus leading to the decrease of martensitic transformation temperature TM in Ni51Mn34Co2Sn13 and Ni50Mn34Co3Sn13 alloys. On the other hand, the substitution of Sn for Mn atoms in Ni50Mn34Co2Sn14 would enhance the p-d covalent hybridization between the main group element (Sn) and the transition metal element (Mn or Ni) due to the increase of Sn content, thus also reducing the TM by stabilizing the parent phase. Due to the reduction of TM, a magnetostructural martensitic transition from FM austenite to weak-magnetic martensite is realized in Ni51Mn34Co2Sn13 and Ni50Mn34Co2Sn14, resulting in a large magnetocaloric effect around room temperature. For a low field change of 3 T, the maximum ∆SM reaches as high as 30.9 J/kg K for Ni50Mn34Co2Sn14. A linear dependence of &Delta, SM upon &mu, 0H has been found in Ni50Mn34Co2Sn14, and the origin of this linear relationship has been discussed by numerical analysis of Maxwell&rsquo, s relation.

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.

The study on the field dependence of magnetocaloric effect (MCE) is considered to be of fundamental and practical importance, since it not only guides us in understanding and optimizing the MCE, but also helps us estimate the MCE for higher magnetic field which is not available in some laboratories. The magnetic field (0H) dependence of magnetic entropy change (△SM) has been studied extensively in many materials with second-order magnetic transition. However, the field dependence of MCE for first-order magnetic transition (FOMT) materials has not been sufficiently studied due to their complexity and diversity. In the present work, polycrystalline Mn0.6Fe0.4NiSi0.5Ge0.5, Ni50Mn34Co2Sn14, and LaFe11.7Si1.3 compounds with FOMT are prepared, and the magnetic and magnetocaloric properties are investigated systematically. In order to avoid a spurious △SM, the M-0H curves are measured in a loop process. The M-0H curves are corrected by taking into account the demagnetization effect, i.e. Hint=Hext-NdM. It is found that the -△SM follows a linear relationship -△SM=-△S0 +0H with the variation of magnetic field in Mn0.6Fe0.4NiSi0.5Ge0.5 compound when 0H 1 T. In addition, it is also noted that the △SM is approximately proportional to the square of 0H at low field. The origin of this linear relationship between △SM and 0H at high field and the deviation at low field are discussed by numerically analyzing the Maxwell relation. In addition to the △SM peak value, it is found that other △SM values at different temperatures also follow the linear relation at high field by performing the same numerical analysis. Moreover, it is found that the fitted △SM curve matches the experimental data very well. This result indicates that the linear relationship between △SM and 0H could be utilized to predict the △SM for higher magnetic field change when the field is lower than the saturation field. The applicability of this linear relationship is also verified in other systems with first-order magnetostructural transformation, such as Ni50Mn34Co2Sn14. However, it fails to describe the field dependence of △SM in LaFe11.7Si1.3, which exhibits a strong field dependence of transition temperature. Consequently, our study reveals that a linear dependence of △SM on 0H could occur in magnetostructural transition materials, which show the field independence of transition temperature.