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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.
According to the basic chemical properties of magnesium and boron, hybrid orbital theory, frontier orbital theory and powder reaction theory, the formation and growth of MgB2 nuclei in the powder reaction are analyzed. This process can be divided into three steps: After the collision of the powder particles, do the opposite micro-forced vibration, which generates nucleation and growth of vibration; 2. The two boron atoms meet, the valence orbit sp2 hybrid formed B2, two magnesium atoms 3s valence electrons fill B2 π orbit π bond formed to generate MgB2, that MgB2 initial nuclei; 3. The initial nucleus to determine the direction of the hybrid orbital plane, along the c-axis and a-axis, respectively, approaching each other to form a crystal The nuclei grow in six directions along a and c axes, and the π bond of B2 evolves into a large conjugate π36 bond, and the magnesium is in the hexagonal center of the boron layer, finally forming MgB2 single crystal grains.The solid-liquid interface is more favorable Larger grains are formed. Larger grains are formed at higher temperatures and MgB4 and MgB7 may form from the initial nuclei.