Research On Catalytic Synthesis Of Nanodiamonds Under Atmospheric Pressure Based On CDC Principle

Nanodiamond（ND）has the combined excellent performances arising from diamond and nano-material,and has been widely used in many fields such as friction polishing,composite materials,and biomedicine.However,it is worth noting that the implementation of common synthesis methods are usually required explosive reaction or high-energy conditions for the reaction,and the synthesis conditions are very harsh.So the technical strategy for realizing the ND synthesis under moderate conditions is very attractive and still needs further investigation.Based on the principle of carbide-derived carbon（CDC）prepared by halide-etching,the synthesis of ND was successfully catalyzed at atmospheric pressure by chlorine-etching after catalyst was embedded into carbide precursor.In this paper,the influence of different catalyst introduction methods on the microstructure of CDC and the synthesis of ND was discussed.The effects of different carbide precursors and catalysts on the microstructure of CDCs and the synthesis of ND were studied.The formation mechanism of catalytic synthesis of ND based on CDC principle was preliminatively explored through the type and size distribution of NDs in different regions of CDC.The main research works and results are as follows:（1）VC and Fe were used as the precursor and catalyst.Fe was introduced into the surface and interior of VC particles by simple mixing and high stress embedding respectively.The influence of different introduction methods on the structures of CDC was discussed.When the catalyst Fe is introduced by mechanical mixing,Fe adheres to the surface of VC particles.In this case,more graphite structures can be observed in the prepared CDC compared with that without catalyst.However,when the catalyst Fe was introduced by high stress embedding,Fe was enclosed inside the VC particles,the graphite structures in CDC were greatly reduced.At the same time,many ND particles with different sizes and types were also observed.（2）Four carbides,VC,TiC,NbC andβ-Si C,were used as precursors respectively,and Fe was introduced into the precursor particles by high stress embedding mode.The correlation between the types of the precursors and the types and sizes of ND synthesized by catalytic synthesis was studied.Based on the technical route of this paper,NDs have been catalytically synthesized by using a variety of carbides as precursors.And the type of the synthesized NDs is very diverse,including 3C,4H,8H,15R,21R and so on.Further study has revealed that the chemical bond and theρ_c value（the number of carbon atoms contained per unit volume）of the carbide may have an important influence on the catalytic synthesis of ND.When catalytic etchingβ-Si C,which is composed of covalent bond,or NbC with a lowρ_c,no or almost no ND structure is obtained.When catalytic etching VC of the highρ_c,most of the synthesized ND are H-type diamond with larger size.Using TiC with a lowerρ_c as the precursor,ND synthesized are mainly C-type and R-type diamonds,and the ND size obtained is smaller.This finding may lay a foundation to develop the strategy for tailoring the size and type of NDs to better satisfy the designated applications.（3）The transition metal elements Fe,Co and Ni were used as catalysts.The three elemental metals were introduced into TiC by high stress embedding respectively.The influence of different catalysts on the catalytic synthesis of ND was discussed.Using Fe,Co and Ni as catalysts,various types of ND structures have been successfully synthesized by chlorine-etching TiC.Further comparison shows that the catalyst type has an important effect on the type and size of NDs.When Fe and Ni are used as catalysts,the synthesized NDs are mostly 3C-type and R-type diamonds,and only a small amount of H-type diamonds can be obtained.While using Co as a catalyst,the proportion of H-type diamonds and small-sized ND structures has increased significantly.（4）Based on the variation of the number,type and size of ND structures in different regions of CDC particles,the synthesis mechanism of ND was preliminarily discussed.In addition,the feasibility of the introduction of H₂ to increase ND synthesis yield was preliminarily studied.The number,type and size of ND structures in CDC particles are all related to the distance to the catalyst introduced.The synthesized ND structure mainly appears in the edge area of the CDC particles closer to the catalyst,and the farther away from the catalyst,the weaker the influence of the catalyst and stress,and the smaller the number of NDs.In addition,the closer to the catalyst,the higher the content of the 3C diamond with the lowest ground state energy level and the large-sized ND structures;the further away from the catalyst,the H-type and R-type diamonds with higher ground state energy level and the ND structure of small size increased.The use of Cl₂+H₂ mixed atmosphere etching can increase the yield of catalytically synthesized ND by about 50%,which proves that the introduction of H₂ is an effective technical path to achieve ND macro-preparation.