Two-Step Process in Homogeneous Nucleation of Alumina in Supersaturated Vapor

Abstract

Multiple pathways in crystal nucleation are now known to be more common than previously predicted; it is, therefore, crucial to understand the early stages of crystallization. Even in relatively simple vapor-phase homogeneous nucleation, the process has significant potential diversity. Here, we experimentally show crystalline Al2O3 nanoparticles forming via precisely two steps in the nucleation process from supersaturated vapor with a moderate cooling rate. In situ FT-IR measurement of nucleation allowed us to observe the formation of Al2O3 nanoparticles. Liquid-like particles first nucleated from the vapor before crystallizing. The crystalline phase was preserved by quenching without further transformation into the most stable α-Al2O3 polymorph. The precipitated phase changed from δ-Al2O3 for pure Al2O3 to γ-Al2O3 or θ-Al2O3 by adding Sb or Cr, respectively. We demonstrate that a two-step process occurs in homogeneous nucleation of refractory materials from supersaturated vapor, which may facilitate polymorphic control in industry and improve understanding of cosmic dust formation in space.

Facile synthesis of high‐melting point spherical TiC and TiN powders at low temperature

“…For the TiCl 2 –N 2 –H 2 system, there are mainly two reaction paths to form TiN in the TiCl 2 –N 2 –H 2 system (Figures and ), that is, TiCl 2 (g) → TiN(s) path (TiCl 2 (g) + H 2 (g) + N 2 (g) → TiN(s) + HCl(g)) and Ti(s) → TiN(s) path (TiCl 2 (g) → Ti(s) + TiCl 4 (g), Ti(s) + N 2 (g) → TiN(s)). All the reactions show much negative Gibbs free energy variations (∆ G , Figure ) along with big positive equilibrium constant K ( K > 10 3 , Figure A) from 750°C to 900°C, indicating that the homogeneous nucleation is feasible in the TiCl 2 –N 2 –H 2 system, because the big K values would easily lead to generate a high degree of supersaturation ( S ) for homogeneous nucleation in CVD system . For the conventional TiCl 4 –N 2 –H 2 system, it shows small ∆ G and K ( K < 10 0.6 , Figure ), implying the low supersaturation S .…”

“…All the reactions show much negative Gibbs free energy variations (∆G, Figure 8) along with big positive equilibrium constant K (K > 10 3 , Figure 8A) from 750°C to 900°C, indicating that the homogeneous nucleation is feasible in the TiCl 2 -N 2 -H 2 system, because the big K values would easily lead to generate a high degree of supersaturation (S) for homogeneous nucleation in CVD system. 32,33,37 For the conventional TiCl 4 -N 2 -H 2 system, it shows small ∆G and K (K < 10 0.6 , Figure 9), implying the (4) low supersaturation S. Hence, products obtained in the conventional TiCl 4 -N 2 -H 2 system were TiN film rather than TiN powders even at 1500°C, 38 because the heterogeneous nucleation barrier is lower than the homogeneous nucleation barrier. Theoretically, to obtain spherical powders in gas, small TiN particles have to fuse into one particle in a very short time (τ f ), which is related to the ratio of the synthesis temperature (T) to the melting point of the synthesized powder (T m ).…”

“…Currently, quantitative research about the homogeneous nucleation from supersaturated vapor is a great challenge, [32][33][34] especially for high melting point materials. 33 Herein, we try to understand the spherical TiN nucleation and growth based on the classical nucleation theory (CNT). Essentially, the formation of nuclei is a dynamic process.…”

Analyzing multistep homogeneous nucleation in vapor-to-solid transitions using molecular dynamics simulations

“…The additional nucleation process for solid-solid transitions is necessary to form a stable crystalline phase. Recent experimental studies [3][4][5] support multiple processes of nucleation for various substances. This is a common phenomenon in the first stage of condensation from vapor to solid.…”

“…The phase transitions start through nucleation, where unstable equilibrium clusters of a new phase called critical nuclei form first and then grow continuously. Although crystallization is expected below the triple point temperature, it has often been observed, in nature and experiments, that nuclei formed from vapor are supercooled liquid droplets [1][2][3][4][5]. This is an example of Ostwald's step rule [1], where a metastable phase appears first before a stable phase.…”