Phase Transition Mechanism of Graphite to Nano-Polycrystalline Diamond Resolved by Molecular Dynamics Simulation
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摘要: 以往研究发现，纳米多晶金刚石的硬度可超越单晶金刚石，因此利用石墨制备纳米多晶金刚石获得了广泛研究。然而，由石墨碳源制备的金刚石同时具有均匀精细结构和层状结构，其形成机理尚未获得很好的理解。为此，针对不同层间距的石墨，进行了一系列分子动力学模拟。研究发现，不同受压情况下石墨存在不同的相变行为，即在马氏体转变下获得片层状金刚石，而在扩散型相变下获得无特定取向的精细纳米金刚石。在静水压条件下，或者[002]方向上的压力大于横向压力且石墨层滑移不受限时，石墨将转化为层状结构立方金刚石；当[002]方向上的压力大于横向压力，但石墨层横向滑移受阻时，石墨转化为多晶六方和立方金刚石混合物；当最大压力方向在[210]或[010]方向时，石墨转化为无特定取向的均匀多晶金刚石。通过原子运动的微观尺度分析，揭示了由石墨制备的纳米多晶金刚石同时具有均匀精细结构和层状结构的机理，有望为大规模合成超硬纳米多晶金刚石提供理论支持。Abstract: Previous studies have found that the nano-polycrystalline diamond (NPD) is harder than single crystal diamond, consequently NPD prepared from graphite has been widely studied. Previous experiments revealed that the NPD originated from graphite contains both homogeneous fine structure and lamellar structure, while the mechanism has not been fully understood. In this work, molecular dynamics simulation was carried out, in which graphite models with different interlayer spacings were built up and compressed. The results showed that the graphite under different compression conditions exhibit different phase transition behaviors, namely, lamellar diamond is obtained under martensite transformation, and fine nanodiamonds without a specific orientation are obtained under diffusive transformation. Under hydrostatic pressure, or, if the slip of the graphite layer is not limited and [002] is the maximum pressure direction, the graphite converts into lamellar cubic diamond; if the maximum pressure is in [210] or [010] direction, the phase transition product is the polycrystalline diamond; if the maximum pressure is in [002] direction, the slip of graphite layers is hindered, the product is a mixture of polycrystalline hexagonal and cubic diamond. The microscopic analysis of atomic motion reveals the formation mechanism of NPD transformed from graphite with homogeneous fine structure and lamellar structure, which is expected to provide insights for large-scale synthesis of superhard NPD.
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Key words:
- nano-polycrystalline /
- diamond /
- graphite /
- phase transition /
- molecular dynamics /
- high pressure and high temperature
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图 1 石墨在18 GPa、2500 ℃下合成多晶金刚石的透射电镜图像[11]：(a) 层状金刚石结构，(b) 均匀无特定取向金刚石结构，(c) 混合型金刚石结构（A区域为均匀精细结构，B区域为层状结构）
Figure 1. Transmission electron microscope images of polycrystalline diamond synthesized directly from graphite at 18 GPa and 2500 ℃[11]: (a) lamellar structure, (b) homogeneous fine structure, (c) mix-type structure (area A: homogeneous fine structure, area B: lamellar structure.)
图 3 3类受压情况下的典型应力-应变曲线：(a) d=0.315 nm（类型Ⅰ，[210]为最大压力方向）；(b) d=0.260 nm，相变分两阶段进行（第1阶段中，[002]为最大压力方向，属于类型Ⅱ；第2阶段中，[210]为最大压力方向，属于类型Ⅰ）；(c) d=0.228 nm（第1次相变为石墨-金刚石相变，属于类型Ⅲ，[002]方向为最大压力方向；第2次相变为HD-CD相变）
Figure 3. Typical stress-strain curves for three types of compression conditions: (a) d=0.315 nm (Type Ⅰ with [210] as the maximum pressure direction); (b) d=0.260 nm (Phase transition process is split into two stages. The first stage belongs to type Ⅱ with [002] being the maximum pressure direction, while the second stage belongs to type Ⅰ with [210] as the maximum pressure direction.); (c) d=0.228 nm (The first transition is the graphite to diamond transition, belonging to type Ⅲ with [002] being the maximum pressure direction. The second phase transition is the HD-CD phase transition.)
图 4 具有不同层间距的石墨在沿x轴（[210]方向）压缩下的部分模拟结果（橙色原子属于HD相，蓝色原子属于CD相，灰色原子属于石墨或非晶相。层间距较大时，获得晶粒较大的CD多晶，且层间距越大，晶粒越小；层间距较小时，获得晶粒较小的金刚石多晶（HD和CD相混合），且层间距越小，晶粒越小）
Figure 4. Partial simulation results of graphite with varying interlayer spacing under x-axis compression ([210] direction) (Orange atoms belong to hexagonal diamond phase, blue atoms belong to cubic diamond phase, and gray atoms belong to graphite or amorphous phase. Graphite with large interlayer spacing obtains CD polycrystals with larger grains, and the grains become smaller with increasing interlayer spacing. Graphite with small interlayer spacing obtains diamond polycrystals with smaller grains (mixing of HD and CD phase), and the grains become smaller with decreasing interlayer spacing.)
图 5 3类受压条件及静水压下石墨的结构演化：(a) d=0.315 nm（类型Ⅰ），石墨层在压力作用下发生屈曲，随后多个位点相变为CD；(b) d=0.260 nm，第1阶段（类型Ⅱ）石墨发生马氏体相变，生成层状结构CD，第2阶段剩余石墨层倾斜并进一步相变为CD（类型Ⅰ）；(c) d=0.228 nm（类型Ⅲ），多个位点同时产生取向不一的HD核；(d) 静水压模拟结果
Figure 5. Structural evolution of graphite under three types of compression condition and hydrostatic pressure: (a) d=0.315 nm, type Ⅰ, the graphite layers buckle under pressure, followed by multiple-site transition to CD; (b) d=0.260 nm, in the first stage (type Ⅱ), some graphite layers transform into lamellar CD, and in the second stage (type Ⅰ), the remaining graphite layers tilt and further transform into CD; (c) d=0.228 nm, type Ⅲ, HD nuclei with different orientations emerge simultaneously in multiple sites of graphite; (d) hydrostatic pressure simulation result
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