Analysis Of Deposition Mechanism Of Silicon-Alloyed Low Temperature Isotropic Pyrocarbon

Silicon-alloyed Low temperature isotropic pyrocarbon(Si-LTIC)is widely used in medical field such as artificial heart valves for its low elastic modulus,excellent flexural strength,breaking deformation greater than 2 %,good hardness,wear resistance,chemical inertness and blood compatibility.However,the Si-LTIC used for the domestic bileaflet mechanical heart valve usually appear cracks,delamination,pores and other defects,resulting in low yield and high production cost.For manufacturing the high-quality Si-LTIC coating,it's necessary to research the effects of deposition conditions on Si-LTIC microstructure and property.And which investigating the deposition process of Si-LTIC material clearly is essential for selecting and controlling deposition conditions.Therefore,problems which should be solved urgently are finding clearly the deposition process of Si-LTIC and the effects of deposition conditions on microstructure and property.The Steady-state fluidized bed chemical vapor deposition was used to prepared Si-LTIC at different temperature from 1250 to 1450 ? with different propane concentration from 25% to 60%.Microstructure of Si-LTIC was characterized by using EDS?XRD?SEM and TEM,then researched effects of deposition temperature and propane concentration on microstructure of Si-LTIC.According to the results analyzed above,analysing homogeneous gaseous reaction and heterogeneous surficial reaction of SFBCVD,then discussing deposition mechanism of Si-LTIC,finally researching effects of deposition temperature and propane concentration on the deposition model of Si-LTIC.The characters of dense gas-solid two phase flow was numerically simulated by fluent computational fluid dynamics software,then analysis effects of cone angle and inlet gas velocity on flow state.At last,the deposition mechanism of Si-LTIC prepared by SFBCVD was complemented and improved.The main contents and conclusions of this paper can be divided into the follow sections:(1)The Si-LTIC is mainly composed of carbon element and silicon element,as well as a small amount of oxygen element.The content of silicon element is between 5.7~22.1 wt%,and the content of oxygen element is very low,about 0.5 wt%.The XRD structural parameters of Si-LTIC which was prepared under different deposition condition are different.In general,with deposition temperature and propane concentration increased,the pyrocarbon's crystallite size is becoming smaller.(2)With propane concentration increased,laminar structure gradually decreases in Si-LTIC,the quantity of globular-like particles increases and the size becomes bigger.With increasing deposition temperature,the globular-like particles are refined in Si-LTIC,which causes the quantity of globular-like particles with a smaller diameter increasing,the globular-like particles replace laminar structure gradually,and the texture of the out layer around carbon blacks inside globular-like particles decreasing.The density of Si-LTIC decreases as deposition temperature and propane concentration increasing.When deposition temperature and propane concentration are too high,the average coating rate of Si-LTIC is decreased.(3)The deposition process of Si-LTIC: formation of carbon black in gas phase,carbon black growing into droplet by adsorbing small molecular chain hydrocarbon and aromatic molecular,co-deposition of many intermediate including droplet,small molecular chain hydrocarbon,aromatic molecular and ?-SiC.(4)The Si-LTIC's gaseous nucleation-surficial growth mechanism is proposed: gaseous nucleation mechanism controls the growth of globular-like particles,and surficial growth mechanism controls the formation of laminar structure.with increasing deposition temperature and propane concentration,the Si-LTIC's deposition process is transforming from surficial growth mechanism to gaseous nucleation mechanism.(5)The particulate fluidized bed owning “ring-core” structure forms in a furnace of a cone angle of 60°under experimental operating conditions in this paper,gas volume fraction of the central region in furnace is about 0.37.With increasing inlet gas velocity,gas and particle average velocity are increasing in central region,but gas volume fraction is changing little.(6)The deposition mechanism of Si-LTIC prepared by SFBCVD is complemented and improved.Deposition temperature and propane concentration are internal factors which effect the formation of Si-LTIC,and the furnace structure and inlet gas velocity are external factors which offer possibility of space and time for the formation of Si-LTIC.