Lonsdaleite,also known as hexagonal diamond,is an allotrope of carbon with a hexagonal crystal structure,which was discovered in the nanostructure of the Canyon Diablo meteorite.Theoretical calculations have shown tha...Lonsdaleite,also known as hexagonal diamond,is an allotrope of carbon with a hexagonal crystal structure,which was discovered in the nanostructure of the Canyon Diablo meteorite.Theoretical calculations have shown that this structure gives it exceptional physical properties that exceed those of cubic diamond,making it highly promising for groundbreaking applications in superhard cutting tools,wide-bandgap semiconductor devices,and materials for extreme environments.As a result,the controllable synthesis of hexagonal diamond has emerged as a cutting-edge research focus in materials science.This review briefly outlines the progress in this area,with a focus on the mechanisms governing its key synthesis conditions,its intrinsic physical properties,and its potential applications in various fields.展开更多
Diamond has the strongest three-dimensional network structure and its cubic configuration is extremely stable under high pressure,thus limiting the experimental synthesis of diamond polymorphs.Hexagonal diamond,a typi...Diamond has the strongest three-dimensional network structure and its cubic configuration is extremely stable under high pressure,thus limiting the experimental synthesis of diamond polymorphs.Hexagonal diamond,a typical polymorph of diamond,has attracted considerable attention in recent decades,yet synthesizing pure and large-sized hexagonal diamond remains technically challenging,preventing an accurate understanding of its properties and formation mechanism.Here,we report the direct synthesis of millimeter-sized,nearly pure hexagonal diamond from graphite under high-pressure and high-temperature conditions using our developed high-pressure technique in a multi-anvil press.The synthesized hexagonal diamond is highly oriented polycrystalline,exhibiting an ultrahard hardness(165±4 GPa)on(100)planes,which is∼50%harder than single-crystal cubic diamond.Structural characterizations and molecular dynamics simulations indicate that hexagonal diamond is formed through a martensitic transformation process whereby hexagonal graphite is transformed into hexagonal diamond by sliding and then direct bonding between graphite sheets.Furthermore,we show that the transformations from graphite to cubic or hexagonal diamonds are strongly temperature-pressure dependent.With this understanding,we further synthesized cubic/hexagonal diamond composites with unusual heterostructures at a lower pressure.This work not only established a fundamental framework for high-pressure phase transformations in graphite but also provided insight into the structural evolution of two-dimensional materials at high pressures and a potent strategy for exploring their new high-pressure phases.展开更多
Hexagonal diamond(HD),also named lonsdaleite,is one of diamond polytypes with an AB stacking sequence different from ABC of cubic diamond[1].Based on X-ray diffraction data,natural HD was first claimed to exist in the...Hexagonal diamond(HD),also named lonsdaleite,is one of diamond polytypes with an AB stacking sequence different from ABC of cubic diamond[1].Based on X-ray diffraction data,natural HD was first claimed to exist in the Canyon Diablo meteorite[2,3],arising from the transformation of graphite under the high pressure and high temperature induced by meteorite impacts.On the one hand,HD has been questioned as a distinct material.展开更多
Motion estimation is an important issue in H.264 video coding systems because it occupies a large amount of encoding time.In this paper,a novel search algorithm which utilizes an adaptive hexagon and small diamond sea...Motion estimation is an important issue in H.264 video coding systems because it occupies a large amount of encoding time.In this paper,a novel search algorithm which utilizes an adaptive hexagon and small diamond search (AHSDS) is proposed to enhance search speed.The search pattern is chosen according to the motion strength of the current block.When the block is in active motion,the hexagon search provides an efficient search means;when the block is inactive,the small diamond search is adopted.Simulation results showed that our approach can speed up the search process with little effect on distortion performance compared with other adaptive approaches.展开更多
基金the National Natural Science Foundation of China(12274170 and 52225203)。
文摘Lonsdaleite,also known as hexagonal diamond,is an allotrope of carbon with a hexagonal crystal structure,which was discovered in the nanostructure of the Canyon Diablo meteorite.Theoretical calculations have shown that this structure gives it exceptional physical properties that exceed those of cubic diamond,making it highly promising for groundbreaking applications in superhard cutting tools,wide-bandgap semiconductor devices,and materials for extreme environments.As a result,the controllable synthesis of hexagonal diamond has emerged as a cutting-edge research focus in materials science.This review briefly outlines the progress in this area,with a focus on the mechanisms governing its key synthesis conditions,its intrinsic physical properties,and its potential applications in various fields.
基金supported by the National Key R&D Program of China(2018YFA0305900)the National Natural Science Foundation of China(U23A20561,12274383,21703004,and 52172240)+1 种基金Fundamental Research Funds for the Central Universities,Sun Yatsen University(23qnpy04)Open Project of State Key Laboratory of Superhard Materials,Jilin University(202403).
文摘Diamond has the strongest three-dimensional network structure and its cubic configuration is extremely stable under high pressure,thus limiting the experimental synthesis of diamond polymorphs.Hexagonal diamond,a typical polymorph of diamond,has attracted considerable attention in recent decades,yet synthesizing pure and large-sized hexagonal diamond remains technically challenging,preventing an accurate understanding of its properties and formation mechanism.Here,we report the direct synthesis of millimeter-sized,nearly pure hexagonal diamond from graphite under high-pressure and high-temperature conditions using our developed high-pressure technique in a multi-anvil press.The synthesized hexagonal diamond is highly oriented polycrystalline,exhibiting an ultrahard hardness(165±4 GPa)on(100)planes,which is∼50%harder than single-crystal cubic diamond.Structural characterizations and molecular dynamics simulations indicate that hexagonal diamond is formed through a martensitic transformation process whereby hexagonal graphite is transformed into hexagonal diamond by sliding and then direct bonding between graphite sheets.Furthermore,we show that the transformations from graphite to cubic or hexagonal diamonds are strongly temperature-pressure dependent.With this understanding,we further synthesized cubic/hexagonal diamond composites with unusual heterostructures at a lower pressure.This work not only established a fundamental framework for high-pressure phase transformations in graphite but also provided insight into the structural evolution of two-dimensional materials at high pressures and a potent strategy for exploring their new high-pressure phases.
文摘Hexagonal diamond(HD),also named lonsdaleite,is one of diamond polytypes with an AB stacking sequence different from ABC of cubic diamond[1].Based on X-ray diffraction data,natural HD was first claimed to exist in the Canyon Diablo meteorite[2,3],arising from the transformation of graphite under the high pressure and high temperature induced by meteorite impacts.On the one hand,HD has been questioned as a distinct material.
基金Project (Nos.60505017 and 60534070) supported by the National Natural Science Foundation of China
文摘Motion estimation is an important issue in H.264 video coding systems because it occupies a large amount of encoding time.In this paper,a novel search algorithm which utilizes an adaptive hexagon and small diamond search (AHSDS) is proposed to enhance search speed.The search pattern is chosen according to the motion strength of the current block.When the block is in active motion,the hexagon search provides an efficient search means;when the block is inactive,the small diamond search is adopted.Simulation results showed that our approach can speed up the search process with little effect on distortion performance compared with other adaptive approaches.
