Stress Anisotropy Severely Affects Zinc Phosphate Network Formation

Abstract

Using density-functional theory based simulations, we study how initially disconnected zinc phosphate molecules respond to different externally imposed deformations. Hybridization changes are observed in all cases, in which the coordination of zinc atoms changes irreversibly from tetrahedral to seesaw and square pyramidal, whereby the system stiffens substantially. The point at which stiff networks are formed does not only depend on the hydrostatic pressure. Stress anisotropy generally reduces the required hydrostatic network formation pressure. Moreover, networks obtained under isotropic deformations turn out stiffer, elastically more isotropic, and lower in energy after decompression than those produced under anisotropic stresses. We also find that the observed stress-memory effects are encoded to a significant degree in the arrangement of atoms in the second neighbor shell of the zinc atoms. These findings refine previously formulated conjectures of pressure-assisted cross-linking in zinc phosphate-based anti-wear films. Graphical Abstract

Mechanochemical Ionization – Differentiating Pressure-, Shear-, and Temperature-Induced Reactions in a Model Phosphate

“…The more anisotropic the deformation, the more linear becomes the final Lewis structure. The spatial separation of cations and anions increases with the degree of anisotropy, which is consistent with the lower energy of reaction for anisotropic deformation reported previously 24 . For mode (a) and (b) only one net or charging proton transfer occurs, while mode (c) leads to two proton transfers.…”

“…Even those hydrogen atoms became immobile after the systems were relaxed to the next available, zero-stress energy minimum, and then re-equilibrated at 450 K. During the initial compression, hydrogen atoms started to become mobile, even those not formally participating in the reaction, when the deformation exceeded roughly 50% of the critical deformation, at which enthalpy suddenly decreased. Given that hydrostatic pressures p and shear stresses τ (to be precise, the second deviatoric stress invariants), at which the transitions are expected to occur quasi-instantaneously are approximately 24 (p, τ) = (a) (4.0,1.2) , (b) (3.1,2.6) and (c) (1.2, 2.1), each time in units of GPa, the onset of ion mobility could already occur at a hydrostatic pressures being just a little above 0.6 GPa. These numbers might become even smaller in experimental systems, depending on the time scale of asperity collision.…”

“…Given our results, the occasionally made assumption that mechanochemistry results predominantly from local heating, as advocated in the hot-spot model 26 , is certainly false for our system of interest, and, as we believe, for most mechanochemical systems that we are aware of. In fact, we expect solids that are mechanically produced to be softest in the direction of largest compressive stress after the stress is released 24 , while temperature-induced reactions of originally isotropic system should not have preferred stiff directions after cooling, unless anisotropic single crystals happened to be produced. In addition, we are somewhat skeptical of typical flash-temperature estimates as they tend to assign the dissipated energy to a small interfacial zone, although important dissipative processes contributing substantially to friction, such as viscoelastic or plastic deformation, also take place far away from it.…”

“…Density-functional theory 50,51 simulations are done using the same CP2K open-source package 52 and similar methodology as our precedent paper 24 , including the Perdew-Burke-Ernzerhof exchange-correlation functional 53 with Grimme empirical corrections to the dispersion interactions 54 and Goedecker-Teter-Hutter pseudopotentials 55,56 with the double-ζ Gaussian basis sets 57 . Energy cutoffs were 400 Ry in molecular dynamics simulations and 600 Ry in static calculations.…”

“…We have recently studied the response of a bulk system containing triphosphoric acid and zinc phosphate molecules to externally imposed deformations 24 . All compression/decompression cycles lead to exothermic chemical changes, however, the reaction energy, i.e., the energy difference between the fully relaxed initial and final structures, generally turned out largest in magnitude under isotropic deformation and smallest when the deformation was most anisotropic.…”