超导体MgB_2成相过程研究

Research on the phase formation process of superconductor MgB_2

根据镁和硼的基本化学性质、杂化轨道理论、前线轨道理论和粉末反应理论,分析了粉末反应中MgB2晶核的形成及生长过程。这一过程可分三步完成:(1)两种粉粒碰撞接触,做反相微幅受迫振动,产生MgB2成相区;(2)两个硼原子相遇,价轨道经sp2杂化生成B2,镁原子的两个3s价电子自旋相反成对填入B2的π轨道形成π键,生成MgB2,此即MgB2初始晶核。初始晶核有四个外露半满电子轨道,具有顺磁性,电场分布不对称;(3)初始晶核以确定的杂化轨道平面方向,分别沿a轴和c轴相互接近反应,形成晶核沿三个轴六个方向的生长,π键演化成π63键,镁离子处于硼层的六角中心,最终形成MgB2单晶晶粒。固-液界面有利较大晶粒形成。温度升高,晶粒变大。由初始晶核可能生成MgB4和MgB7等。用初始晶核顺磁性和晶粒表面局域电子,对成相过程可做进一步试验检验。

The formation and the growth process of the crystal nuclei of MgB2 were analyzed based on the basic properties of boron and magnesium, the hybrid orbital theory, the frontier orbital theory and the theory of powder reaction. The formation process could be divided into three steps. First, two kinds of powder particles contacted and vibrated with opposite phase to gen- erate the phase formation region of MgB2. Secondly, two boron atoms reacted to generate B2 with sp2 hybrid obits, two valence electrons of magnesium atom filled the π orbit of B2 with opposite spin and pairing to form π bond. As a result, the initial crystal nuclei of MgB2 was formed. The electric field distribution of the initial crystal nuclei was not asymmetric, and it had a four exposed half full hybrid orbits. Thirdly, in determined direction of the hybrid orbit plane, the initial crystal nucleus contacted and reacted constantly along the six directions of a - axis and c - axis to make the growth of the nucleus, the π bond becomed π6^3, crystal grains were formed finally. Solid - liquid interface might be more conducive to the forming larger crystal grains. Larger grains might present at higher temperature, MgB4 and MgB7 might be formed by the initial crystal nuclei. The phase formation process might be tested using the paramagnetism of the initial crystal nuclei and the spin resonance of the local electron .