High-pressure mechanochemistry: Conceptual multiscale theory and interpretation of experiments

Abstract

Fifteen mechanochemical phenomena observed under compression and plastic shear of materials in a rotational diamond anvil cell (RDAC) are systematized. They are related to strain-induced structural changes (SCs) under high pressure, including phase transformations (PTs) and chemical reactions. A simple, three-scale continuum thermodynamic theory and closed-form solutions are developed which explain these phenomena. At the nanoscale, a model for strain-induced nucleation at the tip of a dislocation pile-up is suggested and studied. At the microscale, a simple strain-controlled kinetic equation for the strain-induced SCs is thermodynamically derived. A macroscale model for plastic flow and strain-induced SCs in RDAC is developed. These models explain why and how the superposition of plastic shear on high pressure leads to (a) a significant (by a factor of 3-5) reduction of the SC pressure, (b) reduction (up to zero) of pressure hysteresis, (c) the appearance of new phases, especially strong phases, which were not obtained without shear, (d) strain-controlled (rather than time-controlled) kinetics, or (e) the acceleration of kinetics without changes in the PT pressure. Also, an explanation was obtained as to why a nonreacting matrix with a yield stress higher (lower) than that for reagents significantly accelerates (slows down) the reactions. Some methods of characterization and controlling the SCs are suggested and the unique potential of plastic straining to produce high-strength metastable phases is predicted.

Hallmarks of mechanochemistry: from nanoparticles to technology

“…99 Second, it does not relate, on a fundamental basis, the chemical reactivity to the severity of the mechanical deformation. 100 Third, it does not connect the overall kinetics with the characteristic features of the mechanical processing method. 101 Regarding the first issue, it must be noted that any novel achievement in such a direction would be significant progress in fundamental knowledge in the field.…”

Modeling and simulation of strain-induced phase transformations under compression in a diamond anvil cell

“…While most of our results below may be equally applicable to both PTs and chemical reactions, 7,8 we will focus on PTs here. It was recently recognized that strain-induced PTs differ fundamentally from the similar pressure-induced PTs.…”

“…It was recently recognized that strain-induced PTs differ fundamentally from the similar pressure-induced PTs. 7,8,24 Pressure-and stress-induced PTs start predominantly at pre-existing defects ͑various dislocation configurations and internal boundaries͒ at stress levels below the yield strength. It is necessary to increase pressure to drive the PTs further.…”

“…[15][16][17][18][19][20][21][22][23] It was found that the superposition of plastic shear under constant compressive load, due to the rotation of an anvil, leads to findings that have both fundamental and applied importance. 7,8,[15][16][17][18][19][20] They are enumerated below and reviewed in detail in Refs. 7 and 8.…”

“…All of these processes can be considered under the umbrella of high-pressure mechanochemistry. [6][7][8] One can also mention mechanochemically enhanced PTs under nanoindentation, 9 plastic-strain-enhanced reactions in rocks, 10 and mechanically induced change in reaction pathways in polymers. 11 Pioneering studies on the unique mechanochemical behavior of materials under high pressure and large plastic strains have been initiated using rotational Bridgman anvils.…”