Papers by Keyword: Magnetic Refrigeration

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: A comprehension about magnetic refrigeration which develops strong materials as the refrigerant will be explained and develop in this project. Magnetic refrigeration comes from magnetocaloric effect (MCE) which is magnetic material known as refrigerant that react with a magnetic and demagnetic field. Refrigerant plays the roll in control the performance of magnetic refrigerator. One of interesting refrigerant is MnCoGe compound which found very favourable in produce high value of MCE. From this point of view, the process of producing of MnCoGe using ball milling treatment and heat treatment had been studied. Variable of temperature in heat treatment process and following by cooling down to room temperature have been implemented in order to produce good structure of MnCoGe compound. Furthermore, from structure properties measurement found this compound produce three type of structures which are hexagonal, orthorhombic and mix structure (hexagonal and orthorhombic). Based on the different temperature of heat treatment, it indicate for 1500^o C and 1200^o C treatment are dominant to the hexagonal structure type, then for 1000^o C dominant to the orthorhombic structure and lastly for 1100^o C dominant to mix structure. Electric properties measurement found the frequency play the role in change the structure as well as in 1000^o C compound was found below 5M Hz frequency show the hexagonal structure and when increasing above 5M Hz the pattern change to orthorhombic structure respectively. Moreover, for the permittivity measurement it gives the information about conductivity and tan delta value. Systematically at 1000^o C compound found this material is more to conductor behavior compare others compound. Therefore, when the temperature is increased to 1100^o C, 1200^o C and 1500^o C then the conductivity value decreasing and its resistivity value become increasing.

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: Magnetic refrigeration near room temperature is considered as an environmentally benign alternative for the current compressor-based cooling technology. Two materials that are based on ferromagnetic transition metal compounds are considered as the most promising candidates to be used in real world applications. Here we discuss the main features of these materials.

Abstract: The zircon polymorph of GdCrO₄ has a large magnetocaloric effect over a wide temperature range, with |ΔS_T| > 20 J/kg·K from 6 K to 34 K, for a magnetic field of 9 T. This unusual behaviour is very interesting on magnetic refrigeration applications, for liquefying H₂ or natural gas. The mean-field approach explains that it is due to the weaker Gd-Cr magnetic exchange relative to the Cr-Cr one, while the Gd-Gd exchange is negligible. This possibility has not been sufficiently studied and opens an interesting strategy to design more efficient materials for magnetic refrigeration.

Abstract: The single-phase Mn_1.2Co_0.8Si_0.2P_0.8 compound was fabricated by the spark plasma sintering (SPS) technology followed by vacuum annealing. The microstructure, Néel temperature (T_N) and “inverse” magnetocaloric effect of this compound were investigated. The results show that the structure of Mn_1.2Co_0.8Si_0.2P_0.8 compound prepared by SPS is a single phase with precise stoichiometric proportion. Increasing the magnetic field from 0.05 T to 1 T, the T_N of the material reduces gradually from 110 k to 45 k, and a splitting of T_N appears. The splitting of the antiferro-to-ferromagnetic transition is an intrinsic feature rather than the secondary phase. Though the maxima entropy changes is about 0.6 Jkg^-1K^-1 at B=5T, the Mn_1.2Co_0.8Si_0.2P_0.8 phase synthesized by SPS is more favorable, more overall magnetic moment. In addition to the magnetic refrigeration applications, this compound may be used in thermomagnetic generators.

Abstract: In this work we report the results of experimental investigation of the magnetocaloric effect in Gd₉₀Ga₁₀ cold rolled ribbons. A moderate entropy change ΔS = 3.5 J/(kg·K) and magnetocaloric effect ΔT = 3.4K was observed for the as-cast materials in an external magnetic field of 2T which is less by 35% in comparison with the pure gadolinium metal. It was found that a significant (up to 70%) depression of magnetization and magnetocaloric properties developed in the course of plastic deformation can be completely restored by means of a high temperature heat treatment.

Abstract: The magnetocaloric effect (MCE) in Ni_1.81Mn_1.64In_0.55 Heusler alloy has been measured by the direct method. The field dependences of the magnetization were obtained. The phase transition temperatures were determined. The maximal adiabatic temperature change ΔT_ad near the Curie temperature is 1.8 K under the magnetic field change ∆H = 18 kOe. The inverse MCE (∆T_ad = -3.72 K) in the same field change takes place near the temperature of martensitic transformation.