Anodic Electro-deposition of Carbon Films in Molten Salt
Supervisor:xu zuo song qiu shi
Carbon films have many excellent physico-chemical properties such as high hardness,wear resistance,resistivity,thermal conductivity,low friction coefficient,good chemical inertness,corrosion resistance,and optical transmittance,leading to their extensive applications in the fields of mechanical,electronic,light energy,aerospace,and biomedical science.Physical vapor deposition and chemical vaper deposition are the commonly used methods for carbon films production.Although the carbon film produced by vapor deposition methods has favorable physico-chemical properties,the residual thermal stress in the carbon film is relatively high due to the high temperature of the substrate during the deposition process,which leads to the obscission of the carbon film from the substrate.In addition,vapor deposition requires complex equipments and the deposition rate of carbon is relatively slow,resulting in high production costs.Although the process for carbon films production by electro-deposition in molten salt is not as highly developed as the vapor deposition process,the relevant equipment is simpler and the deposition rate is higher.Moreover,low-temperature molten salt media have some annealing effects on both the substrate and the carbon film,which are beneficial for decreasing the residual thermal stress in the carbon film,and further enhance the binding force between the carbon film and substrate.The molten salt electro-deposition can be classified into anodic deposition and cathodic deposition according to the electrochemical reactions during the deposition process of carbon.The carbon film obtained by cathodic reduction is mainly sp2 hybridization,whereas the carbon film produced via anodic process has relatively high percentage of sp3 hybridization.Due to the strict requirement for the anodic electro-deposition of carbon film and high degree of difficulty for the formation of successive carbon film on the metal substrate,relevant research is very limited from open literature.In the present study,anodic electro-deposition of carbon films on Ni,Cu,and Ti substrates were investigated in the molten salt using CaC2 as the carbon source.Successive carbon films with favorable adhesion was prepared rapidly by a two-step potentiostatic electro-deposition process at the anode and characterized subsequently.The anodic process of carbon films electro-deposition,the growth mechanism of carbon films,and the effects of varied conditions during the electro-deposition process on the micro-structure of the carbon film were investigated.The main results were obtained as following.(1)In the LiCl-KCl-CaCl2-CaC2 molten salt system,C22-ions can be oxidized at the anode to form carbon atoms on graphite,Ni,Cu,and Ti electrodes.Compared to graphite electrode,the oxidation potentials of C22-ions are more negative on Ni,Cu,and Ti electrodes.The electro-deposited carbon can form carbon-metal solid solutions with Ni and Ti metal substrate,which has depolarization effects on the electrode during the anodic electro-deposition process.Cu electrode gives catalytic effects on the oxidation reactions of C22-ions,making the anodic electro-deposition potential shift 0.2 V to negative side compared to graphite electrode.(2)Successive carbon film with a Ni-C solid solution transition layer can be formed rapidly on the Ni substrate by a two-step potentiostatic electro-deposition process at anode.The carbon film is consisted of closely packed round like thin slice secondary particles,which are mixed phases of graphite and amorphous carbon.Prolonging the second stage of electro-deposition,the carbon particle size was increased and the adhesion between the carbon film and the substrate was decreased gradually.(3)Carbides or solid solutions were not formed between Cu substrate and the carbon film,therefore,carbon-containing transition layer can be hardly formed between the carbon film and substrate,leading to weak binding force between the carbon film and substrate.Successive and uniform carbon film with favorable adhesion can be obtained by preparing a Ni layer on Cu substrate via chemical plating or electroplating and subsequently electro-deposition the carbon film in the molten salt.(4)The carbon film can be hardly prepared on the Ti substrate directly by anodic electro-deposition process,because the oxidation potential of C22-ions on the Ti electrode is close to that of Ti metal.However,successive and dense carbon film with favorable adhesion can be obtained by preparing a Ni layer on Ti substrate.