The crystal morphology of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was predicted by the advanced Burton− Cabrera−Frank (BCF) model with consideration of non-centrosymmetric growth units. The present modeling showed that the advanced BCF model provides reliable results and understanding of the growth habit of RDX crystals grown from acetone and γbutyrolactone, in which the {210} and {111} faces are dominantly developed. In the present work, the kink rate, which is the net flux input and output from kink sites along the edge, is found to be a critical factor for the crystal growth of RDX, where the energy required for detaching the growth unit from a kink site determines the existence probability of a growth unit. The {210} and {111} faces were found to show slow kink rates originated from the kink sites with a low existence probability and consequently became morphologically important faces. Moreover, the effect of solvent on the crystal growth habit of RDX, the difference in morphology of RDX grown from acetone and γ-butyrolactone, was confirmed by utilizing local concentration contained in the kink rate term.

Experimentally, β-cyclotetramethylene-tetranitramine (β-HMX) crystals were found to dramatically elongate to the [100] direction when a relatively high supersaturation was imposed. A sudden growth of β-HMX to the [100] direction is closely associated with a mechanistic transition from spiral growth to two-dimensional (2D) nucleation for the (110) face. The onset supersaturation for the growth by 2D nucleation, σ 2D , was found to play a key role in the growth of β-HMX. The present simulation results based on first-principles models such as the spiral growth model and the 2D nucleation model show that the values of σ 2D on the (101) and (101̅ ) faces are smaller than those on the (020), (110), and (011) faces. This leads to the prediction of rapid growth rates for the (101) and (101̅ ) faces by 2D nucleation at low supersaturation and the appearance of a typical shape of β-HMX. On the other hand, the needle-like shape of β-HMX begins to prevail when the supersaturation exceeds the σ 2D for the (110) face because its growth mechanism is transformed from the spiral growth mechanism to the 2D nucleation mechanism which accompanies rapid growth of the (110) face. As a result, the present predictions are in remarkable agreement with the experiments. Furthermore, the kinetic Monte Carlo (KMC) simulation also shows that the σ 2D for the (110) face is lower than that for the (011) face because the (011) face provides the surface topology on which growth units are unfavorably incorporated into the lattice sites. It evidently shows that the relative positions of σ 2D bring on the advent of needle-like growth of β-HMX.

Amphiphilic compounds such as oleylamine, oleyl alcohol, and N-dodecyl-2-pyrrolidone (NDP) were found to clearly promote the nucleation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) from acetone. A statistical approach of induction time distribution of RDX showed that the nucleation rate increases significantly with addition of a small amount of amphiphilic additives, and experimental data were found to be well-represented by a compressed exponential model with an average induction time of a nonhomogeneous Poisson process. Molecular simulation also supported the fact that the molecular aggregates of RDX are easily covered by those additives, and thus interfacial energy seems to be reduced by the additives embedded onto the crystal surface.

In the cooling crystallization of 1,1-diamino-2,2dinitroethylene (DADNE), it was found that the aspect ratio of crystals decreases as the cooling rate of the solution increases. To reveal the effect of supersaturation on the growth shape of DADNE, molecular modeling was carried out by the step energy calculation and kinetic Monte Carlo (KMC) simulation. The rodlike shape of DADNE with basal {111̅ } faces was accurately predicted by the step energy calculation, resulting in remarkable agreement with the experiments. The reason behind the slowest growth rate of the {111̅ } faces was found to originate from the high energy barrier in the formation of a 2D nucleus on the crystal face. Furthermore, it was shown that the aspect ratio of DADNE decreases by the lowered free energy of 2D nucleation at high supersaturation, in which the distinctive characteristics on the anisotropic growth behavior of DADNE are blurred. The KMC simulation results also provided an understanding of the growth kinetics of growth units on each crystal face: the {111̅ } face shows a lower sticking fraction, which means that the {111̅ } face offers the surface topology where growth units are difficult to incorporate into the lattice sites. However, as the supersaturation increases, the crystal faces start to be strongly roughened, and the aspect ratio becomes reduced.

A new process of drowning-out combined with fines dissolution and cooling crystallization (the DFC process) was proposed to recover 1,3,5-trinitro-1,3,5-triazinane (RDX) from RDX/γ-butyrolactone solution with relatively large crystal size and high yield. In this process, fine crystals induced by drowning-out are dissolved into the solution by heating so that the dissolved solute can contribute to grow seed crystals by cooling. The DFC process is repeated to reach the desired yield. Estimation of the amount of antisolvent for drowning-out is made by a simple mass balance model in which the mass of fine particles produced by drowning-out equals to that dissolved. Considering dissolution of seeds at the fines dissolution stage, an empirical parameter α which is smaller than 1 was employed for the estimation of the optimum amount of antisolvent added. α = 1 indicates that effect of dissolution of seeds is negligible. In the present work, initial seeds were prepared by rapid injection of 15 g of water into a saturated RDX/γ-butyrolactone solution of 30 °C. Heating temperature for fines dissolution and cooling temperature for further crystal growth were 40 and 20 °C, respectively. In those operating conditions with α = 0.8, the average crystal size of RDX with the yield of 90% was 67.6 μm, which was a remarkably improved result when compared with that of 40.4 μm by drowning-out only.