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.这一过程可分为三步完成：1.两种粉粒碰撞接触后,做反相微幅受迫振动,这种振动产生晶核的形成及生长区;2.两个硼原子相遇,价轨道经sp2杂化后形成B2,镁原子的两个3s价电子填入B2的π轨道形成π键,生成MgB2,此即MgB2初始晶核;3.初始晶核以确定的杂化轨道平面方向,分别沿c轴和a轴相互接近反应,形成晶核沿a和c三个轴六个方向的生长,B2的π键演化成共轭大π36键,镁处于硼层的六角中心,最终形成MgB2单晶晶粒.固-液界面处更有利于较大晶粒的形成.合成温度较高时晶粒较大,由初始晶核可能会形成MgB4和MgB7等.
The formation of the crystal nuclei of MgB2 and the process of growth of that are analyzed based on the basic properties of boron and magnesium,the hybrid orbital theory,the frontier molecular orbital theory and the theory of powder reaction,which can be divided into three steps.First,two kinds of powder particle contact together and vibrate with opposite phase to generate the region where the crystal nucleus form and grow.Second,two boron atoms react to generate B2 with sp2 hybrid obits,two valence electrons of magnesium atom fill the π orbit of B2 to form π bond,as a result,the initial crystal nuclei of MgB2 has been formed.Third,in determined direction of the hybrid orbit plane,the initial crystal nucleus contact and react constantly each other along a-axis and c-axis respectively to make the growth of the nucleus along these three axes,crystal grains are formed finally.Solid-liquid interface may be more conducive to the formation of larger crystal grains.Larger grains may present and MgB4 and MgB7 may be formed by the initial crystal nuclei,at higher temperature.
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