Study Of Magnetocaloric Properties Of Gd₆₀Co_40-xFe_x?x=0,5,10?Amorphous-Nanocrystalline Composites

Amorphous alloys have characteristics such as adjustable magnetic transition temperature,low eddy current loss,low hysteresis and heat lag,corrosion resistance,high strength,high thermal stability and easy processing due to their unique long-range disordered structure.In addition,as one of the candidate materials for the magnetic refrigerant,the magnetocaloric effect of the amorphous alloy has a wide temperature range and a high relative cooling capacity.The Curie temperature of Gd-Co binary amorphous system can be adjusted to near room temperature with the increase of Co content,but the magnetic entropy variation decreases.The composite material with the platform type magnetocaloric effect can widen the working temperature range of the material without substantially reducing the magnetic entropy change value,and is more in line with the performance requirements of the Ericsson thermal cycle for the working medium.This kind of composite can be prepared by bonding or sintering method,but the heat transfer between adjacent particles during refrigeration decreases due to the temperature difference,which can be avoided by endogenous preparation of amorphous-nanocrystalline composites.In this paper,Gd₆₀Co_40-xFe_x?x=0,5,10?amorphous-nanocrystalline composites were prepared by controlling melt quenching rate and annealing heat treatment.The phase composition,microstructures,thermal properties and Magnetocaloric Properties of the composites were investigated by X-ray diffraction,high resolution transmission electron microscopy,differential scanning calorimeter and comprehensive physical testing system.The influence mechanism of the micro-composition of the composites on the magnetocaloric properties was studied.The main results are as follows:A series of amorphous nanocrystalline composite alloys with nominal composition of Gd₆₀Co_40-xFe_x?x=0,5,10?were obtained by single roll melt spinning method by adjusting cooling rate.Among them,Gd₆₀Co₃₀Fe₁₀ alloy can be synthesized at different cooling rates?50 m/s,12.5 m/s and 6 m/s on the surface of copper roll?.Amorphous,Gd-rich amorphous+?Fe,Co?₅Gd nanocrystals and Gd-rich amorphous+?Fe,Co?₅Gd nanocrystals+hcp-Gd nanocrystals can be prepared respectively,the size of the precipitated nanocrystals is¹0-30 nm.In addition,after heat treatment with different annealing processes,Gd-rich amorphous+?Fe,Co?₅Gd nanocrystals+Gd₁₂?Co,Fe?₇ nanocrystals+hcp-Gd nanocrystals,and Gd-rich amorphous+?Fe,Co?₅Gd nanocrystals+Gd₁₂?Co,Fe?₇ nanocrystals+Gd₃?Co,Fe?nanocrystals can be obtained.After annealing,the grains are dispersed in the amorphous matrix,and the average size is²0-30 nm.Under external magnetic fields of 10 Oe,50 Oe and 200 Oe,the magnetic transition temperatures of Gd₆₀Co₃₀Fe₁₀ alloy prepared at 6 m/s rotational speed were determined to be 244 K,237 K and 217 K respectively by inflection point method.With the increase of magnetic field,the temperature of magnetic transition shifts to low temperature,which is mainly caused by the local composition inhomogeneity?clusters,short program?in the alloy.Under the condition of 0-2 T magnetic field,the maximum peak value of magnetic entropy change is 2.18 Jkg^-1K^-1,corresponding temperature is about 217 K,with a larger half-peak width temperature of 140 K,better relative refrigeration capacity of 305.2 Jkg^-1,higher than the relative refrigeration capacity of Gd₅₅Co₃₅Mn₁₀?600 K/20 min?amorphous nanocrystalline alloy and Gd₅₅Co₄₅ amorphous alloy with similar Curie temperature,showing better magnetocaloric properties.However,the maximum magnetic entropy of annealed Gd₆₀Co₃₀Fe₁₀?6m/s?–553 K alloy sample is 2.55 Jkg^-1K^-1 near 165 K.In addition,the magnetic entropy curves show a plateau shape in the working temperature range around 245 K and 275 K.That is to say,the whole curve presents a step shape,but due to the content ratio between phases,the main magnetocaloric effect of the curve is located at¹65 K.