Magnetic Transition Behavior And Optimization Of Magnetic Cooling Performance For La-Fe-Si Magnetic Microwires

In this thesis,several batches of one-dimensional small-sized La-Fe-Si alloy microwires have been primarily prepared by melt-extraction method.Therefore,the disadvantages of La-Fe-Si bulk alloy,like long annealing time,high hysteresis loss,poor machinability and heat transfer ability have been solved by using the characteristics of small-sized material,and the practicability in magnetic refrigeration area has also been enhanced.For preliminarily optimizing the geometric parameters of microwires,a one-dimensional passive regenerator model with parallel wire geometry based on the establishment of one-dimensional microwire physical model has been built in Matlab to calculate the viscous resistance,heat transfer capacity and axial dispersion.On this basis,the geometrical parameters of microwires were optimized and the extraction paramenters were optimized focusing on the different compositions of La-Fe-Si microwires.Besides,the microstructure and magnetic-field driven phase transition were adjusted by heat treatment process and the microwires which could meet the property requirements of parallel wire active magnetic regenerator were obtained.Finally,a model of active magnetic regenerator with parallel wire geometry was built in Matlab comprehensively in this thesis and the cooling performances of La-Fe-Si microwires which could met requirements have been optimized by this model.The study showed that the geometrical parameters were optimized to be 0.03mm-0.05mm and the extraction parameters were closely correlated to the compositions of the La-Fe-Si microwires.When the Si content of La-Fe-Si microwires was low,the master alloy of microwires were not required to be homogenized in advance.After optimization,the heating power was 21 k W,the rotation speed of wheel was 1600 r/min,and the feed rate was 50 r/min-80 r/min.The microwires were with diameters of 0.03 mm-0.04 mm and lengths of 4 cm-5 cm.When Si content of La-Fe-Si microwires was high,the master alloy needed to be homogenized before melt extraction,otherwise the high-quality microwires could not be fabricated.After optimization,the heating power was 22 k W,the rotation speed of wheel was 1700 r/min,and the feed rate was 30 r/min-50 r/min.The microwires were with diameters of 0.03 mm-0.05 mm and lengths of 10 cm-20 cm.Plenty of nanoscale La-rich andα-Fe phases were found to be distributed alternatively in as-extracted LaFe_11.7Si_1.3 microwires,and a few nanoscale La（Fe,Si）₁₃phases were also found in the microstructure.After the adjustment of heat treatment process,the annealing time of LaFe_11.7Si_1.3 was shortened to 5 min from several weeks.It was reasonable to deduct that the alternatively distributed nanoscale La-rich andα-Fe phases could provide large amounts of interfaces for peritectic reaction and the nanoscale La（Fe,Si）₁₃ phases could also provide plenty of nucleation sites for the formation of La（Fe,Si）₁₃ phases,as a result,the peritectic reaction was able to be basically finished in5 minutes at 1353K.Similarly,the annealing time of LaFe_11.2Si_1.8 microwires was shortened to 20 min at 1373 K.However,although the rapid formation of La（Fe,Si）₁₃phases could also be achieved by peritectic reaction,it needed a longer time to homogenize the compositions of La（Fe,Si）₁₃ phases due to a great amount of pre-exist La（Fe,Si）₁₃ phases in as-extracted LaFe_11.2Si_1.8 microwires.The LaFe_11.7Si_1.3 microwires annealed at 1353K for 5min exhibited a weak first-order transition character and negligible thermal hysteresis.After calculation by Landau theories,the energy barrier needed to be overcome during the first-order transition was only 1.8 k J/mol.This value was much smaller than those of La-Fe-Si bulk alloy and ribbon,and thus the first-order character of LaFe_11.7Si_1.3 microwire was sluggish.The LaFe_11.2Si_1.8 microwires annealed at 1373 K for 20 min displayed a second-order transition character.With an extension of annealing time,the microstructural evolution was basically divided into two stages,which were the formation of La（Fe,Si）₁₃ phase and the homogenization of La（Fe,Si）₁₃ phase.After annealed at 1353K for 5 min,the LaFe_11.7Si_1.3 microwires exhibited a high magnetic entropy change of 9.0 J/kg.K,a working temperature range of 13 K and a refrigeration capacity of 97.9 J/kg under 2 T.For the LaFe_11.2Si_1.8 microwires annealed at 1373K for 20 min,a magnetic entropy change of 6.2 J/kg.K,a working temperature range of 33 K and a refrigeration capacity of 154.0J/kg were obtained under 2 T.After the validation of simulation,the magnetocaloric effect of both microwires could meet the property requirements of parallel wire active magnetic regenerator.Finally,one-dimensional parallel wire active magnetic regenerator model was built by Matlab,which was used to optimize the cooling performances of both microwires.It was found that the arrangement of microwires and flow direction of working fluid had great effects on the cooling performances of regenerator.Compared with triangular array,the square array produced less viscous dissipation but weaker heat transfer ability.Furthermore,when both arrays worked in parallel flow,they were able to exhibit stronger heat transfer ability and lower viscous dissipation simultaneously and thus were predicted to generate higher cooling performance.Moreover,the cooling power and coefficient of performance of two parallel wires geometries were both affected by the aspect ratio of regenerator and working conditions like temperature span,working frequency,the hot temperature and cold temperature of regenerator.When the working frequency was 2 Hz and the temperature span was 10 K,the cooling performances of LaFe_11.7Si_1.3 and LaFe_11.2Si_1.8 parallel wire geometry were optimized.For LaFe_11.7Si_1.3 parallel wire geometry,the cooling power and coefficient of performance were optimized to be 126.35W and 4.57.For LaFe_11.2Si_1.8 parallel wire geometry,the cooling power and the coefficient of performance were optimized to be 120.88 W and 2.17.