Formation of nanocrystalline manganese oxide in flames: oxide phase governed by classical nucleation and size-dependent equilibria
Formation of nanocrystalline manganese oxide in flames: oxide phase governed by classical nucleation and size-dependent equilibria
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DOI：
10.1039/d0ce00734j
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发表时间：
2020-09-07
期刊：
影响因子：
3.1
通讯作者：
Camacho, Joaquin
中科院分区：
文献类型：
--
作者：
Dasappa, Shruthi;Camacho, Joaquin
在补充的实验和模型研究中，研究了晶态氧化锰纳米颗粒的成核和生长。在火焰辅助化学气相沉积过程中，气体到颗粒的转化发生在预混滞留火焰驱动高温合成的过程中。采用伪一维法和轴对称二维法计算了滞留火焰的结构，以评估火焰沉积设计中采用更快的基于相似性的计算的准确性。对于目前研究的窄宽高比滞流，伪一维计算结果与两种计算方法的计算结果吻合较好。根据火焰条件的不同，观察到具有II、II-III、III或IV氧化态的氧化锰纳米颗粒。这些观察结果可以用纳米氧化锰和周围的气相氧之间的尺寸相关平衡来解释。在颗粒温度-氧气-时间历史过程中评估了局部平衡，以深入了解火焰中氧化物的形成。对火焰中形成凝聚MnO的饱和度比的分析表明，成核可能受到热力学势垒的限制。测量的颗粒尺寸小于有限凝聚生长过程的预期颗粒尺寸，支持这种成核机制。首次报道了在贫氧火焰中合成纳米晶MnO的方法。当颗粒接近沉积表面时，预计MnO2是热稳定的相，但在许多被检查的火焰中也产生了其他亚稳态氧化物相。事实上，只有在最小的颗粒尺寸条件下才能观察到MnO2，这可能表明较高的冷却速度将相平衡限制在较小质量的颗粒上。
Particle nucleation and growth of crystalline manganese oxide nanoparticles was examined in a complementary experimental and modelling study. Gas-to-particle conversion occurred in a flame-assisted chemical vapor deposition process whereby a premixed stagnation flame drove the high-temperature synthesis. The structure of the stagnation flame was computed using pseudo one-dimensional and axisymmetric two-dimensional methods to assess the accuracy of using a faster similarity-based calculation for flame-deposition design. The pseudo one-dimensional computation performs reasonably well for the narrow aspect ratio stagnation flow currently studied as evidenced by reasonable agreement between the measured flame position and both computational methods. Manganese oxide nanoparticles having II, II-III, III or IV oxidation states were observed depending on the flame conditions. These observations may be explained by size-dependent equilibria between nano-scale manganese oxide and surrounding gas-phase oxygen. Local equilibrium was assessed during the particle temperature-oxygen-time history to gain insight into oxide formation in the flame. Analysis of the saturation ratio for formation of condensed MnO in the flame indicates that nucleation may be limited by a thermodynamic barrier. This nucleation mechanism is supported by measured particle sizes smaller than what would be expected from a coagulation limited growth process. Nanocrystalline MnO, reported here for the first time by flame synthesis, was obtained in oxygen lean flames. MnO2 is the phase predicted to be thermally stable as the particles approach the deposition surface, yet other metastable oxide phases were produced in many of the flames examined. In fact, MnO2 was only observed in the smallest particle size conditions which may indicate that high cooling rates limit phase equilibrium to less massive particles.