The electric conductivity and the activation energy of ionic migration of molten salts and their mixtures
The electric conductivity and the activation energy of ionic migration of molten salts and their mixtures
复制标题
熔盐及其混合物的电导率和离子迁移活化能
DOI：
10.1098/rspa.1947.0016
复制
发表时间：
1947
期刊：
影响因子：
--
通讯作者：
E. Heymann
中科院分区：
文献类型：
--
作者：
H. Bloom;E. Heymann
许多以离子为主的熔盐的电导率(k)及其随温度的变化可以用一个简单的指数方程k = const来表示。x英汉/ RT。部分共价化合物（如ZnCl2、PbCl2）有时会偏离这一关系，因为它们的结构可能随着温度的变化而变化。离子迁移的活化能(C)总是小于粘性流动的活化能。这一事实归因于两个过程中发生的配置变化的差异。对于氯碱，C随负离子半径与正离子半径之比的增大而减小。对于含有多价离子的电解质，C大于单价离子。在一定的组成和温度范围内测量了多种电解质混合物（CdCl2- cdbr2、CdCl2- PbCl2、CdCl2- nacl、CdCl2- kcl、PbCl2- kcl）的电导率。计算了离子迁移的活化能，并在可能的情况下计算了等效电导率，并与其他研究人员在其他系统中得到的结果一起进行了讨论。在迄今所研究的体系中，电导率没有一个是用摩尔分数表示的组成的线性函数。在混合物中没有复合离子形成证据的体系中，电导率通常显示出与可加性的适度负偏差（例如CdCl2-CdBr2）。到目前为止，只有一个体系显示出正偏离可加性（CdCl2-PbCl2）。在混合物中可能存在络合离子的体系（PbCl2-KCl, CdCl2-KCl, CdCl2-NaCl）中发现了与可加性的强烈负偏差。在CdCl2-KCl和PbCl2-KCl体系中，电导率等温线在研究的所有温度下都是最小的；在这些体系中，相图表明了一种完全熔化的化合物。在相图显示不一致熔融化合物的成分附近，电导率等温线存在额外的最小值，但仅在低温下；在较高的温度下，这些极小值消失了。活化能(C)在与不稳定化合物对应的成分附近有最大值；在这种情况下，C包含了从络合离子到简单离子转变所涉及的部分能量变化。在某些情况下，随着结晶温度的接近，熔融体系的活化能(C)上升到非常高的值。这被解释为由于熔点以上的熔体中存在高度有序。进一步讨论了电导率与其温度系数、离子迁移活化能和熔盐混合物组成之间的关系。
The electric conductivity (k), and its variation with temperature, of many molten salts of predominantly ionic character can be represented by a simple exponential equation k = const. x e-C/RT. Deviations from this relation are sometimes found for partially covalent compounds (e. g. ZnCl2, PbCl2) where constitutional changes may be expected with change of temperature. The activation energy of ionic migration (C) is always smaller than the activation energy of viscous flow. This fact is attributed to the difference in the configurational changes that occur in the two processes. For alkali chloride, C decreases with increasing ratio of anion to cation radius. For electrolytes involving multivalent ions, C is greater than for uni-univalent ones. Increasing amount of covalency of the bonds involved tends to lower C. The conductivities of a number of mixtures of electrolytes (CdCl2-CdBr2, CdCl2 PbCl2, CdCl2-NaCl, CdCl2-KCl, PbCl2-KCl) were measured over a range of compositions and temperatures. The activation energies of ionic migration and, where possible, the equivalent conductivities were calculated, and the results discussed together with those obtained in other systems by various investigators. In no system so far investigated is the conductivity a linear function of the composition expressed as mole fraction. In systems which give no evidence of complex ion formation in the mixture, the conductivity usually shows moderate negative deviations from additivity (e. g. CdCl2-CdBr2). Only one system so far shows a positive deviation from additivity (CdCl2-PbCl2. Strong negative deviations from additivity are found in systems in which complex ions are likely to exist in the mixtures (PbCl2-KCl, CdCl2-KCl, CdCl2-NaCl). In the systems CdCl2-KCl and PbCl2-KCl, the conductivity isotherms have minima at all temperatures investigated; in these systems, the phase diagram indicates a congruently melting compound. Additional minima in the conductivity isotherms are found near compositions at which the phase diagram indicates incongruently melting compounds, but only at low temperatures; at higher temperatures, these minima disappear. The activation energies (C) have maximum values near compositions that correspond to unstable compounds; in this case, C contains part of the energy change involved in the transition from the complex to the simple ions. In some cases, the activation energy (C) of the molten systems rises to very high values as the crystallization temperature is approached. This is interpreted as being due to the existence of a high degree of order in the melt just above the melting-point. Further relations between conductivity, and its temperature coefficient, the activation energy of ionic migration and the constitution of the molten salt mixtures are discussed.