一级相变磁制冷材料的基础问题探究

由于一级相变磁制冷材料发生磁相变时有晶胞体积的突变,相变过程中有相变潜热存在,其磁化过程中有许多磁学问题有待于进一步探究.本文以LaFe13-xSix合金为研究对象,在现有对磁一级相变基础问题的分析基础上,对一级相变材料中系统熵变、等温熵变、绝热温变、热滞、磁滞、铁磁与顺磁态两相共存的温度区间和磁场区间、制冷能力的计算等磁学基础问题进行了较为细致的探究.分析表明,在忽略完全铁磁态和顺磁态对磁热效应的贡献时,Maxwell方程和Clausius-Clapeyron方程计算熵变的值具有等效性.等温磁化过程中升温和降温曲线包围的面积SABCE(磁滞的大小),实际上是升温过程和降温过程中磁场做的净功,等于相变潜热之差.磁滞和热滞的大小与磁化过程数据测量的时间有关,测量时间越长则滞后越小,当相变是平衡相变则滞后为零.另外,对温度和磁场诱导磁相变过程进行了分析,提出了一级相变磁制冷材料制冷能力的不同计算模型.本文对一级相变磁制冷材料的磁学基础问题研究有一定的参考价值.

Basic problem in the first-order phase transition magnetic refrigeration material

Due to the cell volume mutations and the phase transition latent heat existing during phase transition of the first-order phase transition magnetic refrigeration material, many basic problems need to further explore in the magnetization process. In this paper, taking LaFe_13-xSi_xalloys as the research object, we discuss in detail some problems, such as a phase-change, entropy change, isothermal entropy change, adiabatic temperature change, thermal and magnetic hysteresis, the temperature range and magnetic field range in which the ferromagnetic and paramagnetic state coexist, and magnetic refrigeration capacity calculation, The analysis shows that the magnetic entropies calculated by Maxwell equation and Clausius-Clapeyron equation are equivalent when neglecting the contributions of ferromagnetic and paramagnetic state to magnetocaloric effect. The area surrounded by the curve in heating of isothermal magnetization process and curve in cooling of isothermal magnetization process (hysteresis size) is actually the net work done by magnetic field during the heating process and cooling process. The values of magnetic and thermal hysteresis are related to the measurement time: the longer the measurement time, the smaller the hysteresis is. When the transformation is of the equilibrium phase, the hysteresis should be equal to zero. In addition, the temperature and magnetic field induced magnetic transition processes are discussed, and different calculation models of the first-order phase transition material for magnetic refrigeration refrigeration capacity are proposed.
Keywords： first-order phase transition magnetic entropy hysteresis two-phase coexistence