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In Situ High Pressure Hydrogen Tribological Testing of Common Polymer Materials Used in the Hydrogen Delivery Infrastructure
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Surface melting-driven hydrogen absorption for high-pressure polyhydride synthesis.

Ryuhei Sato1,2, Lewis J Conway1,3, Di Zhang1

  • 1Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.

Proceedings of the National Academy of Sciences of the United States of America
|May 29, 2025
PubMed
Summary

High pressure facilitates the formation of calcium polyhydrides by promoting surface melting and lowering reaction energy barriers. This research offers new criteria for synthesizing high-temperature superconductors.

Keywords:
high pressuremachine learning potentialmolecular dynamicspolyhydridesurface melting

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Area of Science:

  • Materials Science
  • Condensed Matter Physics
  • Computational Chemistry

Background:

  • Synthesizing polyhydrides with high superconducting transition temperatures (Tc) is difficult due to extreme pressure and temperature requirements.
  • Understanding the fundamental mechanisms of polyhydride formation under pressure is crucial for discovering new superconducting materials.

Purpose of the Study:

  • To investigate the initial stages of polyhydride formation in calcium hydrides using advanced simulation techniques.
  • To elucidate the role of high pressure in the thermodynamics and kinetics of polyhydride synthesis.
  • To propose experimentally relevant and computationally efficient structure-search criteria for polyhydride discovery.

Main Methods:

  • Machine-learning potential molecular dynamics (MLP-MD) simulations were employed to study calcium hydride systems.
  • Thermodynamic analysis of hydrogenation and liquid polyhydride formation enthalpies under varying pressures.
  • Investigation of surface melting phenomena and intermediate liquid phase formation.

Main Results:

  • Surface melting of CaH2 in contact with high-pressure H2 was observed, proceeding via a CaH4 liquid intermediate.
  • High pressure was found to decrease the enthalpy for both hydrogenation and liquid polyhydride formation, favoring the reaction.
  • The surface melting process becomes thermodynamically more favorable than bulk polyhydride fusion under high pressure.
  • High pressure lowers the activation energy for hydrogenation, accelerating the reaction.

Conclusions:

  • High pressure plays a dual role in polyhydride synthesis: shifting equilibrium and reducing activation energy.
  • Surface melting is a key intermediate step in polyhydride formation under high pressure.
  • Proposed structure-search criteria based on melting temperature and hydrogenation enthalpy can guide experimental efforts for discovering new polyhydrides.