富硼体系中立方氮化硼晶体的生长

本文在hBN-Li3N体系中添加不同含量的单质硼(B),研究了cBN晶体在富B条件下的生长特性.结果表明,B进入晶体的位置具有明显的区域选择性.B以占据N(111)面内N空位的方式进入晶体,并与原有的N原子一起形成一个B原子和N原子的(111)面.随着B进入量的增加,越来越多的B原子取代N空位,B和N原子的混合面与邻近的B(111)面叠加,在晶体的中心部位形成颜色较深的三角形阴影并逐步扩展,最后,使晶体完全变成黑色.由于B占据N空位造成原来N的(111)面上有大量硼原子存在,使得晶体沿＜111＞方向生长困难而有利于沿＜100＞方向生长, 从而形成了八面体或类球形晶体.同时,由于加入的B与部分Li3N发生化学反应也可生成cBN,因此,体系中cBN晶体的形成受两种机制控制:一种为溶解析出过程,另一种为化学反应过程.cBN的产率随着B添加量的增加而降低的实验结果表明,溶解析出过程占主导地位.     

Magnetic properties of (Nd,Pr)–Fe–B nanocomposite over quenched and annealed ribbons

The base alloys of nominal composition (Nd_0.75Pr_0.25)_yFe_balanceB_x (y=10−9.2 and x=6−19.2) were chosen to study the influence of RE/B ratio, smaller than stochiometric composition on magnetic properties of over quenched and annealed ribbons. From X-ray diffraction analysis of these ribbons, the α-Fe and Fe₃B phases were observed along with (Nd,Pr)₂Fe₁₄B major phase. The average grain size was calculated using these patterns as: 35 nm for α-Fe, 45 nm for (Nd,Pr)₂Fe₁₄B and 22 nm for Fe₃B particles. TEM analysis also supported the nano distribution of the above phases. These X-ray graphs support the idea of exchange coupling between hard and soft phases responsible for the observed magnetic properties. In these ribbons the saturation magnetization J_s and remnant magnetization J_r increases from 1.19 T to 1.66 T and from 0.65 T to 0.91 T, respectively as RE/B ratio increases. The increase in J_s and J_r may be attributed to the presence of exchange coupling between these phases. The corresponding coercivity _jH_c decreases from 673.33 to 271.33 k Am⁻¹. The maximum energy product (BH)_max initially increases from 72.42 kJ m⁻³to 109.85 kJ m⁻³ up to RE/B≈1 and then decreases to 58.5 kJ m⁻³, depending on the shape of second quadrant B–H loop. The coercivity mechanism observed from initial hysteresis curve was considered to be nucleation of domain wall.