Due to the huge potential applications of large single crystal metal foils in crystal epitaxy, catalysis, electronics and thermal engineering, the preparation of large single crystal metal foils with different crystal plane indices has always been one of the key research directions in materials science. In the single crystal metal foil, the high-index crystal plane is more complete in principle and can obtain a richer variety of surface structures and properties, but it is very difficult to control and prepare a single-crystal foil with a high-index crystal plane because it is compared to a low index The crystal plane and the high-index crystal plane are at a disadvantage in thermodynamics and kinetics. At present, the existing preparation method is to cut or deposit from a large single crystal metal ingot on a non-metal single crystal, and its cost is very high. At this stage, there is an urgent need to develop a special preparation method that can efficiently produce large single crystal metal foils with various high-index crystal planes.
A few days ago, researchers from Peking University, Southern University of Science and Technology and other units have jointly developed a pre-oxidation treatment technology. Polycrystalline copper foil undergoes pre-oxidation-annealing to form a large single crystal foil with high index crystal plane. The world's largest single crystal copper foil (300 × 200mm, close to a4 paper size). Related papers were published in Nature on May 27 with the title "Seeded growth of large single-crystal copper foils with high-index facets".
Paper link:
https://www.nature.com/articles/s41586-020-2298-5
In this study, polycrystalline copper foil (thickness 25μm, purity 99.8%) was first oxidized at 150-650 ° C for 1-4h (in air) using a vapor deposition furnace (CVD), then reduced and annealed at 1020 ° C for 3-10h ( 800cm3 / min Ar + 50cm3 / minH2), cooled to room temperature in the environment of Ar and H2, and finally obtained a large single-crystal copper foil. Optical imaging was performed after heating at 200 ° C for 230 min. Due to different oxidation barriers, crystal planes with different indices showed different colors, and a uniform single-crystal copper region could be observed.
The study found that the Cu (111) plane is no longer the only thermodynamically favorable surface, but the high-index surface (such as Cu (112)) can be synthesized. The researchers repeated this process many times, and prepared more than 30 large-index single crystals with large index planes (250-390mm long and 210mm wide). Local oxidation experiments verified the important role of pre-oxidation in the production of high-index crystal planes. The pre-oxidized part was converted to Cu (235) and more than 30 types of crystal planes were observed. The unoxidized part was only Cu (111) ).
The study found that the strain energy and surface energy in the metal foil are competitive driving forces for abnormal grain growth during annealing. When not pre-oxidized, the copper foil is annealed for a long time before the abnormal grain growth, and most of the strain energy is released. At this time, the grain growth is mainly driven by the surface energy. Therefore, annealing results in the Cu (111) single crystal having the lowest surface energy. When the copper foil is pre-oxidized, the upper and lower surfaces will be covered with a layer of CuxO grains with different orientations to form a Cu-CuxO interface. The interface energy is weakly dependent on the orientation of the Cu grains themselves. Therefore, seeds with different indices (Cu (hkl)) have a certain probability of abnormal growth. As the annealing progresses, Cu (hkl) seeds with size advantages spread to the entire copper foil through abnormal grain growth.
In order to controllably produce the high-index single-crystal copper foils required in this study, this study proposes a high-index crystal face copied by the crystal face transfer method, and a small piece is cut from the large single-crystal copper foil obtained by pre-oxidation annealing and placed in another As a "seeded" on a piece of polycrystalline copper foil, at a higher annealing temperature, the polycrystalline copper in the "seed" position will assimilate from the top until it spreads to the entire piece of copper foil, forming a large single-crystal copper Foil. This method can also be applied to the preparation of bulk single crystal copper.
In summary, this study prepared the largest single-crystal copper foil with high index facets. The abnormal grain growth technique proposed in this study has also been successfully applied to single-crystal nickel. The research is expected to make further progress in the basic exploration and technical application of large high-index single crystal metal foils, and may be applied in selective catalysis, low-impedance electrical conduction, and heat dissipation.