Cluster-plus-glue-atom Model And Chemical Formulas Of Chromium-doped Diamond-like Carbon Possessing High Corrosion Resistance And Electrical Conductivity

Common bipolar plate materials?metal,graphite,and composite bipolar plates?cannot balance high strength,electrical conductivity,corrosion resistance,gas barrier,and low cost.Therefore,it is of great significance to uncover the mysterious veil of the bipolar plate surface treatment meterials for the development of proton exchange membrane fuel cells?PEMFC?.According to previous studies,doped amorphous carbon coatings have the greatest potential to become bipolar surface treatment materials.However,it is difficult to balance the high corrosion resistance and electrical conductivity of Chromium-doped diamond-like carbon?Cr-DLC?coatings.Researchershavenotbeenabletoestablishthe composition-structure-properties relationships and models of Cr-DLC coatings,nor to quantitatively answer the optimal addition of Cr.In this work,we first established stable amorphous structure model of Cr-DLC coatings based on the cluster-plus-glue-atom model,and derived its optimal composition formulas quantitatively.Secondly,the synergetic electrical and corrosion behavior is discussed within the framework of sp² bond content and of the percolation theory.Finally,the eutectic points of Cr-C system were successfully interpretated using the cluster-plus-glue-atom model.This work not only verifies the feasibility of the cluster-plus-glue-atom model in composition design of Cr-DLC coatings with high corrosion resistance and electrical conductivity,but also lays a theoretical foundation for the composition design of Cr-C based materials.Details are as follows:1)Established a stable amorphous structure model of Cr-DLC.Doped Cr atoms will not only form a local structure in the amorphous carbon network,but also have a catalytic effect on the formation of sp² hybrid bonds.Moreover,the increase in the content of sp² hybrid bond contributes to the conductivity of Cr-DLC coatings,so there must be a quantitative correlation between the composition-structure-properties of Cr-DLC coatings.The present work addresses the amorphous structural feature of Cr-DLC by introducing the cluster-plus-glue-atom model.In this model,a good glass former is composed of a structural unit covering a characteristic nearest-neighbor cluster plus a few next-shell glue atoms.Based on previous research on the atomic structure of Cr-DLC coatings,the prevailing cluster is[Cr-C₄],i.e.,Cr-centered and C-shelled.2)Derived the best compositional formulas of Cr-DLC coatings possessing high corrosion resistance and electrical conductivity.Based on the cluster-plus-glue-atom model,the cluster can be combined with several glue atoms to form cluster formulas,thus the quantitative relationship between the stable structure and composition of Cr-DLC coatings can be established.Here,the cluster?[Cr-C₄]?is matched with appropriate glue atoms to satisfy the electron orbital saturation principle?similar to the octet rule in chemistry?.Two composition formulas are thus proposed,[Cr-C₄]CrC₃?22.22 at.%Cr?and[Cr-C₄]Cr₃C₂?40 at.%Cr?that should show good amorphous structural stabilities.3)Revealed the corrosion resistance and conduction mechanism of Cr-DLC coatings.The stable structural cluster formulas of Cr-DLC coatings must correspond to its high chemical stability and electrical conductivity.Based on this,we first experimentally verified that Cr-DLC coatings corresponding to cluster formulas?[Cr-C₄]CrC₃ and[Cr-C₄]Cr₃C₂?have outstanding corrosion resistance and electrical conductivity.Furthermore,their superior electrical conductivity behavior is justified within the framework of percolation theory and sp²bonding contents.?ie,the percolation conduction path formed in the amorphous carbon network?This work verifies the feasibility of the cluster-plus-glue-atom model in composition design of Cr-DLC coatings with high corrosion resistance and electrical conductivity.4)Interpretated eutectic points of Cr-C system based on the cluster-plus-glue-atom model.According to our previous work,binary eutectic liquids satisfy the dual-cluster short-range-order structural model,i.e.,a eutectic liquid is composed of two stable liquid subunits respectively issued from the two eutectic phases and each one is formulated the same as ideal metallic glasses[cluster]?glue atom?_{1 or 3},where the nearest-neighbor cluster is derived from a devitrification phase.Based on the Friedel oscillation and atomic dense packing theories,the principal clusters are derived:rhombidodecahedron CN₁₄[Cr-Cr₁₄]from Cr,capped trigonal prism CN₉[C-Cr₉]from Cr₂₃C₆ and Cr₇C₃,and[C-Cr₈]from Cr₃C₂ under the criteria of the cluster distribution following spherical periodicity order and of the cluster isolation.Thus the selected principal clusters are matched with appropriate glue atoms to construct the dual cluster formulas for the Cr-C eutectics Cr₈₆C₁₄ and Cr_67.4C_32.6.i.e.,[Cr-Cr₁₄+C-Cr₉]CrC₃?Cr_86.2C_13.8 and[C-Cr₉+C-Cr₈]C₆?Cr_68.0C_32.0,respectively.