Study on Surface Segregation of Impurity Oxides in Wustite at High Temperatures.

在这个始于1990年的项目中，我们使用自己研制的特殊热台，研究了在973K到1223K的高温下，浮氏体中杂质氧化物的表面偏析。在这里，我们描述每一项结果。(1)杂质氧化物的高温表面偏析：去年有报道称CaO在热平衡状态下偏析在浮氏体表面。今年，我们研究了MgO、Al_2O_3和SiO_2的表面偏析。没有证据表明这三种杂质氧化物会分离。此外，从MgO的表面摩尔分数与体积分数的比较中证实了MgO在足够量的MgO中不会分离。在本项目中，我们最后将对浮氏体表面的Na_2O和K_2O进行分析。(2)表面偏析驱动力的确定：我们推导出表面偏析是由于堆积在杂质金属离子周围的应变能取代主金属离子而松弛造成的。发现与宿主离子不同的价态对分离是无效的。(3)动力学：在高于923K左右的温度下，相对较快地达到了平衡表面浓度，这与McLean方程的计算一致。(4)与浮氏体还原速率的相关性：得出CaO加速浮氏体还原的作用，导致浮氏体表面偏析的减少。

In this project started at 1990, we have been studying on the surface segregation of the oxides contained as impurity in wustite at high temperatures from 973K to 1223K, using a special hot-stage made by ourselves. Here, we describe the each item of results.(1) High temperature surface segregation of impurity oxides : It was reported in the last year that CaO segregates on the surface of wustite in thermally equilibrated state. This year, we studied the surface segregation of MgO, Al_2O_3 and SiO_2. None of the three impurity oxides was evidensed to segregate at all. Furthermor, it is confirmed that MgO never segregates from the comparison of surface molar fraction to bulk one for MgO contained in sufficient amount. We are going to make analysis of Na_2O and K_2O at wustite surface finally in this course of project.(2) Determination of the driving force surface segregation : We deduced the surface segregation to take place due to the relaxation of the strain energy piled up around the impurity metal ion substituting host metal ion. The valency different from that of host ion has been found to be ineffective to the segregation.(3) Kinetics : Relatively fast achieving of equilibrium surface concentration was evidenced at the temperatures higher than about 923K, which is in accordance with the calculation by the McLean equation.(4) Correlation with the reduction rate of wustite : The effect of CaO accelerating the reduction of wustite was concluded to bring about dute to its surface segregation.