预测材料结晶能力的理论模型

【作者】 彭坤；

【导师】 宁西京；

【作者基本信息】 复旦大学 ， 原子与分子物理， 2011， 硕士

【摘要】 单晶材料与其相同化学组分的多晶材料相比,具有明显优越的力、热、光、电、磁等特性,制备体积更大、结构更完美的单晶材料是长期以来人们努力的目标。事实上,不同材料形成单晶体的能力有很大差异。例如,石英在一般条件下便可以形成线径为数十厘米的单晶体,而在各种极端条件(包括采用2000℃高温和109pa高压)下所能制备出的金刚石晶体也只有毫米量级线径。因此在新材料的设计中不仅需要考察特定原子分子构型所对应的物理化学特性,还需要预测其形成这种特定原子分子构型(单晶体)的能力。然而,当前已有的理论都难以对材料的这种结晶能力做出准确预测。前不久,我们建立了一个凝结势模型用以预测材料形成单显体的能力,研究表明单元体材料(Ni、Al、Cu、Ar和Mg)的结晶能力随凝结势的增大而单调增强。考虑到大量材料是由多元以上的化学组分构成,本文将凝结势模型推广于二元体系,由此涉及如何定义二元组分材料凝结势等一系列基础问题。我们首先在普遍分析的基础上建立了二元体系的凝结势模型,然后结合大量分子动力学模拟具体研究了(0-6wt.%)Al原子掺杂对于Ni单晶材料结晶能力及(6,17,30wt.%)Au元素的掺杂对Cu单晶结晶能力的影响。分子动力学模拟表明,6%Al元素的掺杂会大大地减弱Ni单晶的结晶能力,而30% Au元素掺杂对Cu单晶的结晶过程几乎没有影响。实验中人们发现添加少于6wt.%的Al元素(一般5.6 wt.%左右)会大大提高镍基单晶高温合金的蠕变寿命,反之则影响合金的综合性能,与我们的分子动力学结果相符。在此基础上我们确定了二元体系凝结势的定义式,并将其应用于二元材料Ni3Al和C3N4的结晶能力的预测,得到Ni3Al单晶形成能力较强而β-C3N4单晶难以合成的结论,均与实验事实吻合,表明凝结势模型可广泛应用于预测体材料的结晶能力。更多还原

【Abstract】 Materials of single crystal structures that exhibit a lot of superior properties have been widely used in many fields. Because of these excellent properties, continuous efforts have been made to prepare single crystals with larger volume and better structure. In fact, the ability to form different structures of single crystals may considerably differ from one another. For example, single crystal quartz with size of several 10 cm can be easily produced in common experiments, while it is difficult to get diamond grains of only several millimeters even under extreme conditions. So it is urgent in materials design to develop a theory to accurately predict the crystallizing ability.Recently, we proposed a condensing potential (CP) model to evaluate the ability of materials to form single crystals, and demonstrated that the ability of some single-component crystals (Ni, Al, Cu, Ar and Mg) increases monotonically with increasing CP. Since the prediction can be easily performed via common ab initio methods, the theory is promising to predict the ability of multi-component materials to form single crystals, which are more common in practice. In the present work, we extended the CP model to two-component systems, and specifically investigated the Al doping (0-6 wt.%) influence on the crystallizing ability of Ni-based crystal and the Au doping (6 wt.%,17 wt.%,30 wt.%) influence on the crystallizing ability of Cu-based crystal. Extensive molecular dynamic (MD) simulations were performed to examine the crystal growth and showed that the A1 doping of only 6 wt.% will considerably decrease the crystallizing ability of the pure crystal but even 30% Au does not. As expected, the ability of the two-component materials to form single crystals also increased monotonically with increasing CP. Then we employed the extended CP model to investigate the crystallizing abilities of Ni3Al crystal and C3N4 crystal. Our predictions were well in agreement with the experimental data, exhibiting a prospect of CP model in predicting the ability of multi-component materials to form single crystals.更多还原