Fullerton Susan K, Maranas Janna K
Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA.
J Chem Phys. 2004 Nov 1;121(17):8562-70. doi: 10.1063/1.1804156.
The mobility of vitreous boron oxide is studied by molecular dynamics simulation. A polarization model that incorporates induced dipoles arising both from charges and from other induced dipoles on atoms with nonzero polarizability is used to simulate boron oxide glass at various temperatures above the glass transition temperature. Particle mobility is investigated through the calculation of the self-intermediate scattering function and the mean-squared displacement. The calculations clearly reveal a two-step relaxation with a plateau at intermediate times for all investigated temperatures. With respect to atomic species, boron atoms are less mobile than oxygen atoms at all temperatures within the plateau region. Through analyzing particle trajectories, it is revealed that BO(3) groups move as one unit and follow each other in a stringlike manner. Three connected BO(3) groups comprise a six-membered boroxol ring, which is shown to move in a collective manner, requiring the simultaneous movement of all ring atoms. The boroxol ring is observed to be confined, or caged, during the plateau region, and jumps to a new location at longer times. This observation is linked to the concept of strong versus fragile glass formers and the potential energy landscape. In addition to the caging feature, an overshoot or dip occurs in the plateau regions of the mean-squared displacement and self-intermediate scattering functions respectively. These features are followed by a ringing pattern, previously associated with finite size effects in other strong glass formers, which persist for the duration of the plateau region. Both features are shown to be consistent with the bending of atomic "cages" from the plane of the boroxol ring, and arise due to the displacement of atoms from local minimum energy configurations.
通过分子动力学模拟研究了玻璃态氧化硼的迁移率。采用一种极化模型来模拟玻璃化转变温度以上不同温度下的氧化硼玻璃，该模型考虑了由电荷以及具有非零极化率的原子上的其他诱导偶极产生的诱导偶极。通过计算自中间散射函数和均方位移来研究粒子迁移率。计算结果清楚地表明，在所有研究温度下，中间时刻都存在一个平台期的两步弛豫。就原子种类而言，在平台期区域内的所有温度下，硼原子的迁移率都低于氧原子。通过分析粒子轨迹发现，BO(3)基团作为一个整体移动，并以串状方式相互跟随。三个相连的BO(3)基团构成一个六元硼氧环，研究表明该环以集体方式移动，需要环上所有原子同时移动。观察到硼氧环在平台期区域内受到限制或被“囚禁”，并在较长时间后跳跃到一个新位置。这一观察结果与强玻璃形成体和弱玻璃形成体的概念以及势能景观有关。除了“囚禁”特征外，均方位移和自中间散射函数的平台期区域分别出现了过冲或凹陷。这些特征之后是一种振荡模式，之前在其他强玻璃形成体中与有限尺寸效应有关，在平台期区域持续存在。研究表明，这两个特征都与硼氧环平面上原子“笼子”的弯曲一致，并且是由于原子从局部能量最低构型的位移而产生的。
