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High Pressure Mechanochemistry

Mechanochemistry studies the effect of nonhydrostatic stresses and plastic strains on various structural changes (SCs), which include solid-solid, solid-liquid, and solid-gas chemical reactions (CRs) and phase transitions (PTs). Plastic strain-induced SCs under high pressure and plastic shear are widespread in nature, physical experiments, and modern technologies. Interpretation of a number of geophysical experiments and other phenomena is related to the analysis of various SCs under pressure and shear. In particular, one of the mechanisms of deep earthquakes is related to the instability caused by shear strain-induced PT. Shear ignition of energetic materials is subject to intensive study with the goal to assess safety issues. Mechanosynthesis (or mechanical alloying), i.e. strain-induced synthesis of various chemical compounds by ball milling, is another example. We also mention the importance of mechanochemical processes for understanding friction and wear, shear-induced metallization and oxidation.

The interaction between plasticity and structural changes under high pressure including phase transformations, chemical reactions and microstructural changes is an extremely challenging, multiscale and multidisciplinary problem requiring expertise in the field of physics, material science, mechanics and chemistry. The diamond anvil cell, has proven to be an effective tool in achieving very high pressure while providing the opportunity for in-situ measurements and the study of various structural changess. It is well known that the addition of plastic shear, through the rotation of one of the anvils, leads to findings that have both fundamental and applied importance.

Selected Results

Selected Publications

  1. Levitas V. I. and Zarechnyy O. M. Kinetics of strain-induced structural changes under high pressure.
    J. Physical Chemistry B, 2006, Vol. 110, 16035-16046.       pdf 

  2. Levitas V. I., Ma Y., Hashemi J., Holtz M. and Guven N. Strain-induced disorder, phase transformations and transformation induced plasticity in hexagonal boron nitride under compression and shear in a rotational diamond anvil cell: in-situ X-ray diffraction study and modeling. Journal of Chemical Physics, 2006, Vol. 25, 044507, pp. 1-14.                   pdf 

  3. Ma Y., Selvi E., Levitas V.I.and Hashemi J. Effect of shear strain on the α-ε phase transition of iron: a new approach in the rotational diamond  anvil cell, J. Phys.: Cond. Matt., 2006, Vol. 18, 1075-1082.  pdf
  4. Levitas, V. I., Ma, Y. Z., and Hashemi, J. Transformation-induced plasticity and cascading structural changes in hexagonal boron nitride under high pressure and shear. Appl. Phys. Lett, Vol. 86 (2005), 071912.  pdf

  5. Levitas, V. I., Hashemi, J., and Ma, Y. Strain-induced disorder and phase transformation in hexagonal boron nitride under quasi-homogeneous pressure: in-situ X-ray study in a rotational diamond anvil cell. Europhysics Letters, 2004, Vol. 68, No. 4, pp. 550-556. pdf
  6. Levitas V. I. High-Pressure Mechanochemistry: Conceptual Multiscale Theory and Interpretation of Experiments. Phys. Rev. B, 2004, 70, No. 18, 184118, pp. 1-24. pdf
  7. Levitas V. I. A microscale model for strain-induced phase transformations and chemical reactions under high pressure. Europhysics Letters, 2004, Vol. 66, No. 5, 687-693. pdf
  8. Levitas V. I. Continuum Mechanical Fundamentals of Mechanochemistry. In: High Pressure Surface Science and Engineering. Section 3. Institute of Physics, Bristol, Eds. Y. Gogotsi and V. Domnich, 2004, pp. 159-292.pdf
  9. Levitas V. I. Strain-induced nucleation at a dislocation pile-up: a nanoscale model for high pressure mechanochemistry. Phys. Letters A, 2004, Vol. 327, 180-185.pdf
  10. Levitas V.I. and Shvedov, L.K. Low Pressure Phase Transformation from Rhombohedral to Cubic BN: Experiment and Theory. Physical Review B, 2002, Vol. 65, No. 10, 104109(1-6). pdf.file

 

Crystal-amorphous and crystal-crystal  phase transformations via virtual melting

 A new mechanism of crystal-amorphous and  crystal-crystal phase transformations and internal stress relaxation via  virtual melting induced by internal stresses was revealed and justified thermodynamically and kinetically. The virtual melt represents a short-lived melt (transitional state) of the parent phase below the thermodynamic melting temperature.  Material melts when the pressure-temperature loading trajectory crosses the low temperature continuation of  nonequilibrium melting line and when crystal-crystal transformation is suppressed due to elastic energy, interface      friction and kinetic barriers. The virtual melt is stable with respect to parent crystalline phase but unstable
with respect to product (crystalline or amorphous) phase, that is why it solidifies immediately to the crystalline phase (above the glass transition temperature) or to the amorphous phase (below the glass transition temperature). 
Virtual melting removes interface friction, reduces kinetic barrier, increases atomic mobility and can reduce thermodynamic melting temperature. We combine virtual melting  and nonequilibrium phase transformation diagrams
to develop new scenarios of crystal-amorphous and  crystal-crystal phase transformations. Our theoretical predictions allowed us to resolve some known contradictions and interpret some experimental data which could not be interpreted using existing approaches. Results are applied for a new interpretation of crystal-amorphous and  crystal-crystal transformation mechanisms in ice Ih. In addition to ice, virtual melting is expected in amorphization of alpha-quartz and coesite, polymet, germanium and silicon, jadeite,  Zn_43Sb_57 and Cd_43Sb_57, BN and graphite. 
Note that amorphization and consequently virtual melting, e.g. in Si and Ge occur at more than 1000K below the thermodynamic melting temperature at the same pressure!

  1. Levitas V.I. Crystal-amorphous and crystal-crystal  phase transformations via virtual melting. Phys. Review Letters,  2005, Vol. 95, No. 7, 075701.  pdf