【作者】 王利刚；

【导师】 李国栋；

【作者基本信息】 内蒙古大学 ， 凝聚态物理， 2005， 硕士

【摘要】 采用电弧熔炼的方法在氩气保护下制备了Gd_0.6(Tb_1-xCo_x)_0.4、(Dy_1-xGd_x)(Co_0.99Si_0.01)₂和(Dy_1-xGd_x)(Co_0.98Si_0.02)₂系列合金。利用振动样品磁强计分别测量了各样品在低磁场（B=0.05T）下的M-T曲线,确定了样品的居里温度T_c。然后在居里温度附近测量了各合金在多组温度下的恒温磁化曲线,外加磁场变化为0～1.5T（或1.0T）,然后采用数值计算方法得出了各样品的磁熵变△S_M。研究结果表明,通过改变合金中的部分成分,样品的居里点有了明显提高,同时还能够保持可观的磁熵变。考虑将此系列合金制成更适合磁埃里克森循环的复合材料,此系列合金是可供选择的近室温磁制冷材料。 鉴于南京大学对类钙钛矿的研究,本论文尝试着采用溶胶-凝胶法制备了类钙钛矿化合物La_0.62Ce_0.05Ca_0.33MnO₃和La_0.67Ca_0.33MnO₃,通过X射线衍射分析了它们的晶体结构,并对其磁性能进行了研究。实验结果表明,退火温度越高,退火时间越长,则样品的粒径越小,导致居里点越低,磁熵变越小。用少量的稀土元素铈替代金属镧后,材料的居里点有所提高,磁熵变的值也增大了近一倍。 本论文针对能否用Maxwell关系计算一级相变体系磁熵变的问题进行了适当的讨论。分析表明,Maxwell关系可用于一级相变体系的磁熵变计算,并且在实际体系中,与Clausius-Clapeyron公式相比可以给出更合理的结果。 最后,本论文还在总结前人工作的基础上,针对磁相变的性质对磁热效应的影响作了较为详细地分析,并从中得出了一些结论。更多还原

【Abstract】 A series of Gd0.6(Tb1-xCox)0.4 (Dy1-xGdx)(Co0.99Si0.01)2 and (Dy1-xGdx)(Co0.98Si0.02)2 alloys have been prepared by arc-melting method. All magnetic measurements were performed using a vibrating sample magnetometer (VSM). The Curie temperature Tc was determined from the temperature dependence of magnetization measured under a field of 0.05T. The magnetization isotherms were measured on field increase in a wide temperature range covering the Curie temperature with different temperature steps. The magnetic entropy changes (△Sm) as a function of the temperature in magnetic field (1.5T or 1.0T) for these alloys have been obtained approximately from the isothermal magnetization through numerical calculation. The results from research suggested that by means of changing the content of some components, the Curie points of these samples obtained obvious improvement and the magnetic entropy changes still could maintain impressive magnitude. Considering the practicability, we can use these alloys to make complex materials that are suitable for the Ericsson cycle. So these alloys are suitable candidates as magnetic refrigerant materials near room temperature.In consideration of the study on perovskite-type oxide in Nanjing University, the nano-La0.62Ce0.05Ca0.33MnO3 and La0.67Ca0.33MnO3 were prepared by sol-gel in this paper. Their microstmcture has been analyzed by XRD, the magnetic capability has been researched. The experimental results showed that higher the thermal annealing temperature and longer the thermal annealmg time, smaller the particle size, then lower the Curie temperature and smaller the magnetic entropy changs. At the same time, we found that the magnetic entropy changs rised twice times and the Curie temprature was improved after a few Ce atoms substiting for La atoms.This paper has carried on the proper discussion about whether Maxwell’s relationship could calculate the magnetic entropy changes of the first-order phase transition. The results from analysing indicated that Maxwell’s relationship could calculate above magnetic entropy changes, and it could give more reasonable results in the actual system than Clausius-Clapeyron formula.Finally, based on formerly researches, a detailed analyse was made on the effects of the properties of magnetic-phase transition on the MCE in some typical magnetic refrigerant materials, then some conclusions were presented.更多还原