磁制冷工质的熵特性分析

Entropy Characteristics of Magnetic Refrigerants

磁制冷的基本原理是应用磁工质的磁热效应。外加磁场作用下的磁工质总熵组成包括磁熵S_M、品格熵S_L和电子熵S_E,其中只打磁熵变化可用于制冷,而晶格熵和电子嫡却是磁制冷的重要热负荷,影响磁制冷效率。因此提高磁制冷效率的一种关键途径是选用具有大的磁熵变,而晶格熵和电子熵都较小的磁制冷工质,这就需对磁制冷工质进行正确的熵分析。本文分别由量子力学分子场近似理论、德拜理论及电子能级理论推导出计算磁工质磁熵、晶格嫡、电子熵的数学模型,运用上述模型研究了高温磁制冷典型工质钆的总熵、磁熵、晶格熵和电子熵等随温度和磁场强度的变化关系,讨论了熵的各种形态变化影响因素及其对磁制冷的性能影响,从而得出了磁制冷工质正确选择的一般原则,为研制实用磁制冷机打下了基础。

Magnetic refrigeration is based on the magnetocaloric effect of magnetic working substances and it's expected to become one of most practical methods for CFC substitute due to its high efficiency and doing no harm to the ozonosphere. The total entropy of magnetic refrigerants under the action of external magnetic field contains magmetic entropy .lattice entropy and electron entropy. Of these entropies .only magnetic entropy can be used for refrigeration, while the lattice entropy and electron entropy .however .play the roles of thermal load in magnetic refrigeration processes .resulting in a decrease of refrigerating efficiency, so the essential means for increasing the magnetic refrigerating efficiency is to choose suitable magnetic refrigerants,which must possess high magnetic entropy.low lattice entropy and electron entropy. In this paper, mathematical models for predicting magnetic entropy .lattice entropy and electron entropy are carried out from the molecular field approximation,the Debye model and the electron energy level theory respectively of quantum mechanics, using the above models the temperature and magnetic field dependences of the calculated magnetic entropy. lattice entropy, electron entropy and the total entropy of gadolinum.which is a typical magnetic refrigerant near room temperature,are obtained. The influences of kinds of entropy on the performance of magnetic refrigeration are discussed,and finally,some general rules for selecting magnetic refrigerants are drawn to help the development of highly economic magnetic refrigerators.