RESUMO
We developed a ReaxFF reactive force field for NASICON-type Li1+xAlxTi2-x(PO4)3 (LATP) materials, which is a promising solid-electrolyte that may enable all-solid-state lithium-ion batteries. The force field parameters were optimized based on density functional theory (DFT) data, including equations of state and the heats of formation of ternary metal oxides and metal phosphate crystal phases (e.g., LixTiO2, Al2TiO5, LiAlO2, AlPO4, Li3PO4 and LiTi2(PO4)3 (LTP)), and the energy barriers for Li diffusion in TiO2 and LTP via vacancies and interstitial sites. Using ReaxFF, the structural and the energetic features of LATP were described properly across various compositions - Li occupies more preferentially the interstitial site next to Al than next to Ti. Also, as observed in experimental data, the lattice parameters decrease when Ti is partly substituted by Al because of the smaller size of the Al cation. Using this force field, the diffusion mechanism and the ionic conductivity of Li in LTP and LATP were investigated at T = 300-1100 K. Low ionic conductivity (5.9 × 10-5 S cm-1 at 300 K) was obtained in LTP as previously reported. In LATP at x = 0.2, the ionic conductivity was slightly improved (8.4 × 10-5 S cm-1), but it is still below the experimental value, which is on the order of 10-4 to 10-3 S cm-1 at x = 0.3-0.5. At higher x (higher Al composition), LATP has a configurational diversity due to the Al substitution and the concomitant insertion of Li. By performing a hybrid MC/MD simulation for LATP at x = 0.5, a thermodynamically stable LATP configuration was obtained. The ionic conductivity of this LATP configuration was calculated to be 7.4 × 10-4 S cm-1 at 300 K, which is one order of magnitude higher than the ionic conductivity for LTP and LATP at x = 0.2. This value is in good agreement with our experimental value (2.5 × 10-4 S cm-1 at 300 K) and the literature values. The composition-dependent ionic conductivity of LATP was successfully demonstrated using the ReaxFF reactive force field, verifying the applicability of the LATP force field for the understanding of Li diffusion and the design of highly Li ion conductive solid electrolytes. Furthermore, our results also demonstrate the feasibility of the MC/MD method in modeling LATP configuration, and provide compelling evidence for the solid solution sensitivity on ionic conductivity.
RESUMO
To avoid unexpected environmental mechanical failure, there is a strong need to fully understand the details of the oxidation process and intrinsic mechanical properties of reactive metallic iron (Fe) nanowires (NWs) under various aqueous reactive environmental conditions. Herein, we employed ReaxFF reactive molecular dynamics (MD) simulations to elucidate the oxidation of Fe NWs exposed to molecular water (H2O) and hydrogen peroxide (H2O2) environment, and the influence of the oxide shell layer on the tensile mechanical deformation properties of Fe NWs. Our structural analysis shows that oxidation of Fe NWs occurs with the formation of different iron oxide and hydroxide phases in the aqueous molecular H2O and H2O2 oxidizing environments. We observe that the resulting microstructure due to pre-oxide shell layer formation reduces the mechanical stress via increasing the initial defect sites in the vicinity of the oxide region to facilitate the onset of plastic deformation during tensile loading. Specifically, the oxide layer of Fe NWs formed in the H2O2 environment has a relatively significant effect on the deterioration of the mechanical properties of Fe NWs. The weakening of the yield stress and Young modulus of H2O2 oxidized Fe NWs indicates the important role of local oxide microstructures on mechanical deformation properties of individual Fe NWs. Notably, deformation twinning is found as the primary mechanical plastic deformation mechanism of all Fe NWs, but it is initially observed at low strain and stress level for the oxidized Fe NWs.
RESUMO
We report reactive dynamics (RD) studies on: the decomposition of bulk hydrazine (N(2)H(4)); the decomposition of bulk monomethyl-hydrazine (CH(3)N(2)H(3)), hereafter referred to simply as methyl-hydrazine; the decomposition of hydrazine in the presence of hydrogen peroxide (H(2)O(2)); and decomposition hydrazine on catalytic surfaces Pt[100] and Pt[111] under various conditions. These studies use the ReaxFF reactive force field to describe the multitude of chemical reactions in these systems for a variety of reaction conditions in order to show that this approach leads to realistic decomposition mechanisms and rates. In particular, we determined how the decomposition of hydrazine is affected by temperature, pressure, and heating rate. We analyzed chemical reaction mechanism of the decomposition of hydrazine at the studied conditions and found that at lower temperatures the initial product from hydrazine decomposition is NH(3), whereas at higher temperatures H(2) and N(2) are the dominant early products. Prominent intermediates observed during these decompositions include N(2)H(3), N(2)H(2,) and NH(2), in agreement with quantum mechanical studies (7.3 ps at 3000 K). As the heating rate is decreased, the onset for hydrazine decomposition shifts to lower temperatures. Using a constant heating rate, we found that higher pressure (increased density) favors formation of NH(3) over N(2) and H(2). In studies of the catalytic decomposition of hydrazine on surfaces Pt[100] and Pt[111], we found that the presence of a Pt-catalyst reduces the initial decomposition temperature of hydrazine by about 50%. We found that the Pt[100]-surface is 20 times more active for hydrazine decomposition than the Pt[111]-surface, in qualitative agreement with experiments. These studies indicate how ReaxFF RD can be useful in understanding the chemical processes involved in bulk and catalytic decomposition and in oxidation of reactive species under various reaction conditions.
