Papers by Keyword: Magnetocaloric Effect

Abstract: Mn_1.25Fe_0.65-xSn_xP_0.50Si_0.50 (0, 0.02, 0.04, 0.05, 0.06, 0.08, 0.09, 0.10, 0.20) series compounds were prepared by mechanical alloying and solid-phase sintering, and their mechanical and magnetic properties were studied. The XRD measurement results show that all the compounds crystalize in Fe₂P hexagonal structures, with a space group of P-62m. With the increase in Sn content, the compressive strength is significantly improved, the Curie temperature of the compound gradually decreases, and the nature of magnetic transition is tuned from a weak to strong first-order one, which is confirmed by the increase of thermal hysteresis of the compounds. The maximum magnetic entropy change of the compound increases from 9.3 J/kg·K at x = 0 to 17.2 J/kg·K at x = 0.04 under a magnetic field change of 0 - 3 T.

Abstract: Recent theoretical and experimental studies suggested that P can enter the structure of La(Fe,Si)₁₃ alloys and lead to a significant enhancement of the isothermal entropy change, one of the two main quantities characterizing the magnetocaloric effect. Here, we report a systematic study of P for Si substitutions in La(Fe,Si)₁₃ alloys. Eight LaFe_11.5Si_1.5-xP_x polycrystalline bulk samples with 0 ≤ x ≤ 0.2 were prepared by arc-melting followed by heat treatment. Powder x-ray diffraction and SEM/EDX analyses show that the α-Fe secondary phase progressively increases with the increase in P substitutions and that a La-rich LaP secondary phase appears. We therefore found that P does not actually enter the main La(Fe,Si)₁₃ phase. Magnetization and DSC measurements confirm this interpretation as the Curie temperatures of La(Fe,Si,P)₁₃ alloys are nearly insensitive to P for Si substitutions and the latent heat of the first-order ferromagnetic transition decreases with the increase in nominal P substitutions. Our work put into questions former reports of the literature on P addition in La(Fe,Si)₁₃ and highlights the particularly complex synthesis of these alloys.

Abstract: This paper study the effect of structural behaviour and electrical properties in cubic NaZn13-type La (FeSi)₁₃ compounds annealed at different high temperatures from 1000oC to 1200oC. The Raman spectroscopy measurement revealed a different level of coexistence of La (FeSi)₁₃-type phase and α-Fe in 1000oC, 1100oC and 1200oC annealed samples. The different level coexistence of the phases suggested that the samples show a significant phase change with different annealing temperature methods. This finding has also supported by impedance analyzer measurement, where the pattern shows that the 1000oC and 1100oC samples have similar behaviour waveform pattern compared to the 1200oC, which has different behaviour. Besides, the impedance pattern clearly shows that the propagation of the signal for the 1000oC and 1100oC before it arrives in the conductor state (R=0.00) has high resistive values compared to 1200oC annealed samples. Further investigation on the electrical properties was done with the conductivity and tan delta measurements. The results show that 1200oC annealed sample have a high value of conductivity (S=18μ S/m) compares with 1100oC (S= 0.1μ S/m) and 1000oC (S=4.89n S/m). The tan delta measurements found that samples annealed at 1200oC temperature has low value of resistivity (tan ɗ = 0.117) compares with 1100oC (tan ɗ = 0.335) and 1000oC (tan ɗ = 0.482) respectively.

Abstract: In this work, the effect of different annealing treatments on the phase transition, structural behaviour and thermal analysis of MnCoGe alloys has been analysed. The changes in the transition temperatures have been studied by Simultaneous Thermal Analysis (STA). The results show that the structural transition temperature (T_str) depends on the annealing treatment of the samples preparation. However, under the same heat treatment no significant change is observed on the curie temperature (T_c). The thermal analysis reveals endothermic peak which corresponds to the structural phase transition of the compounds. The microstructural evolution has been monitored using in-situ X-ray diffraction which made known this compound produce three type of structures which are hexagonal, orthorhombic and mix structure (hexagonal and orthorhombic).

Abstract: Since the discovery of magnetocaloric effect (MCE), numbers of method in producing magnetocaloric materials has been studied. Among those methods, ball milling has been shown as a very versatile technique with several advantages compared to other preparation methods. In this work, the effect of ball milling preparation technique on the phase structural behaviour and electrical properties of MnCoGe alloys has been analysed. The changes in the structural behaviour have been studied by X-ray powder diffraction (XRD) and Raman Spectroscopy. The results suggest that the samples show significant structural changes with different method of ball milling running time. This finding has also been supported by electrical properties where the measurement found that the frequency also plays important role in the structure changes. The absolute impedance value,|Z| (Ω) suggest that structure start to change at initial frequency structure of hexagonal at point 3.22 Ω and 44.1 MHz region. The permittivity and dielectric loss (tan delta) graph that corresponds to a frequency (Hz) up to 100 kHz shown that the 2-hours milling time MnCoGe compound has the lowest permittivity value which make it had lower energy and required more frequency to react.

Abstract: Natural gas is rapidly gaining in geopolitical importance. Gas has grown from a marginal fuel in regionally disconnected markets to an energy source that is transported across great distances for consumption in many different economic sectors. Natural gas is the fuel of choice for consumers seeking for relatively low environmental impacts. As a result, the world’s gas consumption is projected to more than double over the next three decades, rising from 23 – 28 % of the total primary energy demand by 2030 and surpassing coal as the world's number two energy source and potentially overtaking oil's share in many large industrialized economies. This paper is devoted to a short review of materials used in the novel approach to natural gas liquefaction – magnetic cooling process.

Abstract: The multicomponent Tb_0.2Dy_0.8-xGd_xCo₂ and Tb_0.2Dy_0.8-xGd_xCo_0.9Al_0.1 alloys (x≤0.5) were studied in a large temperature range (80 – 350 K) and fields up to 1.8 T. Temperature dependencies of lattices parameters, surface topology features, Curie temperature and magnetocaloric effect near it, of these polycrystalline cubic Laves phase alloys have been obtained and analyzed. The effect of Gd and Al substitution within the rare earth and cobalt sublattices on the structural and magnetocaloric properties of Tb_0.2Dy_0.8Co₂ has been discussed.

Abstract: 0.85La_0.7Sr_0.3MnO₃/0.15GeO₂ composite material and pure La_0.7Sr_0.3MnO₃ were investigated by X-ray diffraction, scanning electron microscopy, magnetometry and magnetic resonance methods. It was observed that both samples demonstrate the ferromagnetic properties, while the absolute value of the magnetization, the magnetic entropy change and the magnetic ordering temperature decrease in composite in comparison with pure La_0.7Sr_0.3MnO₃. The magnetic resonance spectra of investigated (1-x)La_0.7Sr_0.3MnO₃/xGeO₂ (x=0, 0.15) can be attributed to the superposition of magnetic resonance spectra from magnetically anisotropic particles with different orientations.

Abstract: This paper reports the effect of Cu doping in first order phase transition material Mn_1.28Fe_0.67P_0.48Si_0.52 on its phase structure, magnetocaloric effect and mechanical properties. The results of XRD, SEM and EDS analysis show that the Mn_1.28Fe_0.67P_0.48Si_0.52 in this composite forms Fe₂P hexagonal structure and the space group is P-62m;Most of Cu exists as a simple substance, and a small amount of Cu and Mn form a solid solution. When the mass ratio of Cu reaches 10:4, the (Mn,Fe)₃Si phase appears in Mn_1.28Fe_0.67P_0.48Si_0.52. The magnetic measurement results show that the saturation magnetization of Mn_1.28Fe_0.67P_0.48Si_0.52 after Cu doping has no obvious change, the Curie temperature decreases, and the thermal hysteresis increases. The maximum magnetic entropy change becomes smaller as the Cu content increases. Under a 1.5 T external magnetic field, the maximum magnetic entropy ΔS_m of the composite decreases rapidly from 11 J/kg·K at x = 0 to 4 J/kg·K at x = 5,the half width of the magnetic entropy change gradually increases. The Vickers hardness of the composite is reduced, the compressive strength has been greatly improved, and the mechanical properties have been significantly enhanced after Cu doping.

Abstract: A unidireon heating, followed with a ferromagnetic- paramagnetic transition in austenite for the produced unidirectional crystal. Under a magnetic field change of 3 T, the total effective refrigeration capacity was strikingly enhanced up to 125 J/kg, nearly 64% higher than that of polycrystalline master alloy ctional crystal of Heusler Ni₅₀Mn₃₇Sn₁₃ material was produced using a modified high-pressure optical zone-melting furnace. A structural transformation between weak-magnetization martensite and ferromagnetic austenite occurred first up(76 J/kg). The modified high-pressure optical zone-melting technique demonstrated high potentials for the fabrication of super-performance Heusler Ni-Mn-based magnetocaloric materials.