Melt-extracted Fabrication And Their Magnetocaloric Effects (MCE) Of GdAlCo Alloy Fibers

Magnetic refrigeration technique based on magnetocaloric effect（MCE）shows high-efficiency,environmental friendly and good reliability,which is a potential method instead of traditional gas compression and expansion.The Gd-based amorphous alloys with characteristics of metallic glass and large magnetic moment of Gd elements show excellent MCE properties at transition area,which considered as ideal materials using in cooling system.The fibers show high heat exchange efficiency due to their large specific surface area compared with bulk metallic glass（BMG）,and the near one-dimensional shape enables them to be geometrically adaptable to a diversity of cooling systems.Thus,we have adopted melt-extracted method for preparing rapid quenched Gd-based micro fibers and explored the impacts of component changes on MCE.Also,we have studied the enhanced MCE due to the interactions and the MCE anisotropy behaviors,then the biphases and fiber composite method have been used for improving the MCE properties.The above researches are very important for the fibers applied in magnetic refrigeration.The quality of fabricated Gd Al Co fibers show high relationship to the melt feeding speed.The fibers with high roundness cross section but Rayleigh wave defect on the surface were obtained with feeding speed of 10 μm/s,which due to the large surface tension and perturbation sensibility of too little melt extracted by the cupper wheel.On the contrary,the obtained fiber with feeding speed of 40 μm/s will show groove defect on the surface due to the solidification o f too much melt extracted by the wheel with low surface tension before roundness completly.The optimized melt-extracted fabrication process for Gd AlCo amorphous fiber are the heating power of 4 k W,wheel rotate speed of 1700 r/min（line speed of 30 m/s）a nd feeding speed of 20 μm/s.The microstructure analysis of the fibers showed that there are some nano crystallines distributed inhomogeneously on the amorphous matrix,which resulting in the inhomogeneous innerstress during fabrication process.Notably,the Gd Al Co fibers show high tensile stress and fracture reliability,which satisfied the mechanical requirements of the magnetic cooling systems.The Gd Al Co fibers exhibit good MCE poperties and achieve a ferromagnetic to paramagnetic transition process near Curie temperature as shown by the thermomagnetic results and universal curve analysis of magnetic entropy changes.The fibers show superior MCE performances than their components in BMG,and which working temperatures are lower than room temperature.The maximum magnetic entropy changes（-ΔSM max）of 10 J/kg·K at magnetic change（μ0ΔH）of 5 T for the Gd Al Co fibers and excellent cooling efficiency was obtained due to the broad phase transition region.The MCE performances almost kept unchanged except the Curie temperature（TC）when the element contents changed slightly.The critical exponents and magnetic states of these Gd Al Co fiber are deviated from mean field theory.which resulting from the nanocrytals distributed inhomogeneously on the amorphous matrix.The Gd₅₃Al₂₄Co₂₀Zr₃ alloy in multi-fibers form exhibits superior MCE performances such as-ΔSM max of 6.94 J/kg·K at μ0ΔH=3 T compared with same component in forms of single fiber and BMG.The calcuated critical exponents and magnetic states of the three forms are all close to the mean-field theory,indicating the amorphous structures with negligible nanocrystals.The multi-fibers display higer magnetic entropy changes owing to the high magnetization at ferrimagnetic state,which caused by the demagnetizing effect among the fibers.Obviously,the near one-dimensional shape of the fiber resulted in the magnetocaloric anisotropy for both single and multi fibers.The fibers have low saturation fields when the applied field along the direction of easy magnetization and shows almost the same saturation magnetizations at all directions.However,the multi fibers show higher saturation fields than single fiber at both directions due to the demagnetizing effect.The cooling efficiency of the fiber was improved by structure of nanocrystal/amorphous biphases,which owing to the interactions and transit ions superposition.The nanocrystal/amorphous biphases were also obtained by thermal treatment which also released the inner stress thus improved the MCE performances.The composites show greatly enchaned cool efficiency due to the transitions superposition,which fabricated by the Gd-based fibers with close MCE but different TC temperatures.The calculations and experimental results explored that enhanced cool efficiency only obtained when the TC intervals larger than 10 K and increased with the increase of TC intervals.Moreover,the composites with three components would obtain broader table-like MCE performance compare with composites with two components.These composites accord with the requirement of property stability in magnetic cooling systems when working in varying temperatures.