Si-C-N三元系中新型亚稳相的第一性原理研究

发布时间：2018-10-30 19:05

【摘要】：由Si、C和N元素所组成的二元化合物及三元化合物具有优秀的机械、化学和热性能,在实际工业生产中有广泛的应用。关于这类材料的实验与理论研究从未间断过,然而具有明确化学计量比和结构的Si-C-N化合物仍然非常少。本论文采用CALYPSO结构预测程序对Si CN和Si3N4进行了结构的预测,采用CASTEP第一性原理计算模块创建结构,并对预测和创建的结构进行稳定性判定,研究了亚稳相的能带结构,态密度,弹性模量以及理论维氏硬度。采用CASTEP计算模块,通过对N原子在3C-SiC结构中取代C原子、取代Si原子以及占据第一类或第二类间隙位置的结构的计算,研究了N原子在3C-SiC结构中不同占位对结构稳定性的影响,及N原子的含量对SixCyNz的电子性质、弹性模量和维氏硬度的影响。结果显示仅当N原子取代C原子并且C原子被取代的比例为1/4和1/8时的结构是稳定的,由于与C原子相比,N原子的多一个外层电子,因此C原子被N原子取代后的结构具有电子导电的特性,而随着N原子比例的增加,硬度逐渐降低。采用CALYPSO程序和CASTEP模块相结合的方法,预测了Si:C:N=1:1:1的SiCN的晶体结构,并与Kawamura所提出的c-SiCN结构进行了对比研究。发现与c-SiCN结构相比之下,本论文所预测的三个结构,不论是常压相t-SiCN,还是高压相o-SiCN和h-SiCN,在能量上都更具有优势。t-SiCN是一种具有窄带隙的超硬材料,o-SiCN和h-SiCN是具有空穴导电性的硬材料。采用CALYPSO程序和CASTEP模块相结合的方法,预测了Si3N4的晶体结构,发现了两种能量介于稳定β相和高压γ相之间的结构t-Si3N4和m-Si3N4,以及一种相对于γ相的相变压力点为198 GPa的更高压的相o-Si3N4。三种结构都是具有宽带隙的半导体,硬度与α、β和γ相的硬度相当,并且随着压力的增加而变高。随着压力的增加,在常压下都是脆性材料的三种结构的Si3N4,先后变成韧性材料。随着常压相向高压相的转变过程中,对于它们在零压下的弛豫结构中,每个原子的配位键数的逐渐增多,并且键长逐渐增长。采用CASTEP计算模块以常压相中相类似的Si-N层为基础层,通过一系列的对称操作,构建了多种类β-Si3N4相的氮化硅结构,并对它们的形成焓、稳定性、各种性质进行了计算。由于α相中的原子层堆垛顺序ABCD(CD层为AB层的镜面反射层)与传统意义上的堆垛顺序有冲突,因此本论文对类β-Si3N4结构中的原子层的堆垛顺序进行了重新的定义。通过计算发现当结构满足相邻两Si-N层中仅有一层为镜面反射结构,并且相邻两Si-N层的中心N原子不位于同一c轴方向时的能量较低,但是这两个条件仅是能量较低的必要非充分条件。在众多的类β-Si3N4结构中,发现了一个潜在的δ相结构,其为α相的一半的结构,即α相中从上下两个方向看去的两个β相的相邻两Si-N层所组成的结构。
[Abstract]:Binary compounds and ternary compounds composed of Si,C and N have excellent mechanical, chemical and thermal properties and are widely used in industrial production. The experimental and theoretical studies on this kind of materials have never stopped, but the Si-C-N compounds with definite stoichiometric ratio and structure are still very few. In this paper, the structure of Si CN and Si3N4 is predicted by CALYPSO structure prediction program, and the structure of Si CN and Si3N4 is established by CASTEP first-principle calculation module. The stability of the predicted and created structure is determined, and the energy band structure and density of states of metastable phase are studied. Elastic modulus and theoretical Vickers hardness. In this paper, the CASTEP calculation module is used to calculate the structure in which N atoms replace C atoms, Si atoms and the gap positions of the first or second type in the 3C-SiC structure. The effects of different positions of N atoms on the structural stability of 3C-SiC and the effects of N atom content on the electronic properties, elastic modulus and Vickers hardness of SixCyNz were studied. The results show that the structure is stable only when the N atom replaces the C atom and the ratio of the C atom to the C atom is 1 / 4 and 1 / 8, because the N atom has one more outer electron than the C atom. Therefore, the structure of C atom replaced by N atom has the property of electron conduction, and the hardness decreases gradually with the increase of N atom ratio. The crystal structure of Si:C:N=1:1:1 SiCN is predicted by combining CALYPSO program with CASTEP module, and compared with the c-SiCN structure proposed by Kawamura. It is found that compared with the c-SiCN structure, the three structures predicted in this paper, whether at atmospheric pressure phase t-SiCN-high-pressure phase or high-pressure phase o-SiCN and h-SiCN-have more advantages in energy. T-SiCN is a kind of superhard material with narrow band gap. O-SiCN and h-SiCN are hard materials with hole conductivity. The crystal structure of Si3N4 was predicted by combining CALYPSO program with CASTEP module. Two kinds of structure t-Si3N4 and m-Si3N4 with energy between stable 尾 phase and high pressure 纬 phase were found. And a more high-pressure phase o-Si _ 3N _ 4 with a phase transition pressure of 198 GPa relative to 纬 phase. The three kinds of structures are semiconductor with wide band gap. The hardness is equivalent to that of 伪, 尾 and 纬 phase, and increases with the increase of pressure. With the increase of pressure, the Si3N4, with three structures of brittle materials under normal pressure has been transformed into ductile materials. With the transition from atmospheric phase to high pressure phase, the number of coordination bonds per atom increases and the bond length increases gradually for their relaxation structure at zero pressure. The CASTEP calculation module is used to construct the silicon nitride structure of 尾-Si3N4 phase through a series of symmetrical operations, and the formation enthalpy, stability and various properties of these layers are calculated by using the Si-N layer similar to the normal pressure phase as the basic layer. Because the order of ABCD (CD stacking in 伪 phase is the specular reflection layer of AB layer) and the traditional stacking order is in conflict, this paper redefines the stacking order of atomic layer in 尾-Si3N4 structure. It is found that when only one of the adjacent Si-N layers is a mirror reflection structure and the central N atom of the adjacent two Si-N layers is not located in the same c-axis direction, the energy of the N atom is lower. But these two conditions are only necessary and insufficient conditions for lower energy. Among the many 尾-Si3N4 like structures, a potential 未 phase structure is found, which is the structure of the two adjacent Si-N layers of the 伪 phase, which is seen from the upper and lower directions.
【学位授予单位】：燕山大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ174.1

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