Study On The Preparation And Properties Of Mo₂C By MPCVD Technique

Transition metal carbides are a large material system,which have attracted much attention in the photoelectron,catalytic,electrochemical and superconductivity due to the incorporation of carbon atoms in the metal lattice.Early studies in 1967 showed that the superconducting transition temperatures T_c of NbC,ZrC,HfC,TaC,?-WC,?-Mo₂C and?-W₂C were lower than 10 K.Subsequent studies in 1971 on the superconductivity of Mo₂C showed that the superconducting transition temperatures T_c of?-Mo₂C,?-Mo₂C,?-MoC_1-x,and?-W₂C were 6-7.3 K,5.1-7.2 K,7.4-8.9 K and2.95-3.05 K,respectively.The great differences in superconducting properties are related to the preparation methods and growth parameters.The main preparation methods to prepare Mo₂C include temperature-programmed method,thermal decomposition method,liquid-phase reaction method,and microwave preparation method,which have certain deficiencies in some aspects.To provide a new preparation method for preparing high-quality Mo₂C crystals is very important for the study of transition metal carbides.In this study,microwave plasma chemical vapor deposition?MPCVD?was used to prepare Mo₂C crystals.It provided new methods and approaches for the preparation of transition metal carbides and nitrides,and promotes their application in superconducting properties.The main works were listed as follows:1.In the MPCVD equipment,high-purity molybdenum foil?³0?m,purity:99.999%?was used as growth substrate,methane was used as a carbon source,and hydrogen was used as a reducing agent and carrier gas to grow Mo₂C crystals.By adjusting and optimizing process parameters,the effects of different growth temperatures and growth times on the growth of Mo₂C crystals were investigated.The results showed that:?1?Different growth temperatures had certain influences on the phase composition of the products.At the range of 600-1200°C,when the temperature of the substrate was under 800°C,it was hard for the growth of Mo₂C crystal,and the graphitization on the substrate surface was serious.When the temperature rises,the substrate surface products were Mo₂C crystal,and there was no other heterogeneous phase;and when the temperature was further increased,the substrate surface tended to grow diamond and MoC phase.In the current experimental system,the optimum growth temperature range of Mo₂C crystal is about 950-1100°C.?2?Different growth time had great influences on the phase composition of the product.Since the growth of Mo₂C was carried out on the surface of the molybdenum substrate,it made the ratio of molybdenum and carbon in the system changeable,so the product composition also had some differences.With a short growth time,the pure phase Mo₂C crystals were grown.When the growth time was prolonged,the surface of the substrate tended to grow MoC phase.2.The superconducting and photocatalytic properties of the prepared Mo₂C crystals were researched as following:?1?Superconducting properties of pure phase Mo₂C crystal were tested.The results showed that the superconducting transition temperature T_c was about 4.0 K.This result was consistent with the reported T_c of single crystal Mo₂C,indicating that the high quality of as-prepared Mo₂C by MPCVD.?2?The superconducting properties of the samples under different growth conditions were tested.The results showed that the single-phase Mo₂C crystals showed normal state to superconducting state transition trend.But for the composite phase samples,the superconducting transition turned to be normal state-state 1-state 2-superconducting state.Through the analysis of the composite phase of the sample surface,the transition trend was related to the existence of high-temperature superconducting phase MoC and low-temperature superconducting phase Mo₂C.The superconductivity of“Zero-resistance effect”in the superconducting state was tested,further confirming the superconductivity of Mo₂C.?3?The photocatalytic performance of the samples under different growth conditions was tested.The results showed that Mo₂C had a certain ability to degrade Rhodamine B.After 60 min degradation,the degradation efficiency reaches 94.2%;the composite phase samples also had a certain photocatalytic performance,but its degradation performance was slightly lower than that of the pure phase of Mo₂C.