Effect of chemical vapor infiltration treatment on the wave-absorbing performance of carbon fiber/cement composites

Short carbon fibers were treated at high temperatures around 1100℃ through chemical vapor infiltration technology. A thinner layer ofpyrocarbon was deposited on the fiber surface. The dispersion of carbon fibers in a cement matrix and the mechanical properties of carbon fiber/cement composites were investigated by scanning electron microscopy （SEM） and other tests. The reflec- tivity of electromagnetic waves by the composites was measured in the frequency range of 8.0-18 GHz for different carbon fiber contents of 0.2wt%, 0.4wt%, 0.6wt%, and 1.0wt%. The results show that the reflectivity tends to increase with the increase of fiber content above 0.4wt%. The minimum reflectivity is -19.3 dB and the composites exhibit wave-absorbing performances. After pyrocarbon is deposited on the fiber, all the refiectivity data are far greater. They are all above -10 dB and display mainly wave-reflecting performances.

Effects of carrier gas on densification of porous carbon-carbon composites during chemical vapor infiltration

The densification rate of C/C composites fabricated by directional flow thermal gradient chemical vapor infiltration process from C 3H 6, C 3H 6 N 2 and C 3H 6 H 2 was investigated respectively. The mechanism on the role of carrier gas in chemical vapor infiltration was also discussed. The results shows that whether or not adding N 2 as carrier gas has little influences on the densification behavior of C/C composites with the controlled temperature, the partial pressure of hydrocarbon and the effective residence time of the gas phase remain constant. When the controlled temperature is not less than 1 173 K,using N 2 or H 2 as carrier gas makes pronounced differences in densifying of C/C composites. The average bulk density of C/C composites from C 3H 6 H 2 is eight to ten percent higher than that from C 3H 6 N 2. However, when the controlled temperature is not higher than 1 123 K,the densification rate of C/C composites from C 3H 6 H 2 is much lower than that from C 3H 6 N 2, which implies that effects of carrier gas on densification of C/C composites are closely related to the type of carrier gas and infiltration temperature. At higher temperature, using H 2 as carrier gas is favorable to the densification of C/C composites, while at lower temperature, hydrogen, acting as reactive gas, can inhibit the formation of pyrolytic carbon.

A Numerical Study of Densification Behavior of Silicon Carbide Matrix Composites in Isothermal Chemical Vapor Infiltration

We studied the characteristics of two-scale pore structure of preform in the deposition process and the mass transfer of reactant gas in dual-scale pores, and observed the physiochemical phenomenon associated with the reaction. Thereby, we established mathematical models on two scales, respectively, preform and reactor. These models were used for the numerical simulation of the process of ceramic matrix composites densified by isothermal chemical vapor infiltration(ICVI). The models were used to carry out a systematic study on the influence of process conditions and the preform structure on the densification behaviors. The most important findings of our study are that the processing time could be reduced by about 50% without compromising the quality of the material, if the processing temperature is 950-1 000 ℃ for the first 70 hours and then raised to 1 100 ℃.

Ablation properties of C/C composites with various needled preforms prepared by isothermal chemical vapor infiltration

The ablation properties of C/C composites with four different needled preforms prepared by isothermal chemical vapor infiltration （ICVI）, which are super-thin mat lay-up, 0°/90° weftless fabric lay-up, 0°/45° weftless fabric lay-up and 0°/45° twill fabric lay-up, were quantitatively evaluated by performing the ablation tests with an engine torch. And their ablation discrepancies were analyzed according to the surface characteristic, porosity and thermal diffusivity. The results show that the 0°/45° weftless composite has a fiat eroded surface with no obvious macroscopic pits. Its thickness and mass erosion rates are decreased by about 46.8% and 34.8%, 25.0% and 27.5%, and 17.5% and 19.4% compared with those of the mat, the 0°/90° weftless and the 0°/45° twill composites, respectively. The ablation properties are mainly controlled by the thermo-chemical effect （oxidation）, and a little by the thermo-mechanical effect （mechanical denudation）. The needling fiber bundles play an important role in accelerating the ablation process and resulting in the heterogeneous ablation.

Design,preparation,and structure of particle preforms for Si_3N_(4(P))/Si_3N_4 radome composites prepared using chemical vapor infiltration process

A particle preform was designed and prepared by conglomerating and cold-pressed process, which was condensed by chemical vapor infiltration （CVI） process to fabricate silicon nitride particles reinforced silicon nitride composites. The conglomerations are of almost sphericity after conglomerated. There are large pores among the conglomerations and small pores within themselves in the preform according to the design and the test of pore size distribution. The pore size of the preform is characterized by a double-peak distribution. The pore size distribution is influenced by conglomeration size. Large pores among the conglomerations still exist after infiltrated Si3N4 matrix. The conglomerations, however, are very compact. The CVI Si3N4 looks like cauliflowershaped structure. 2008 University of Science and Technology Beijing. All rights reserved.

FINITE DIFFERENCE MODEL FOR SIMULATION OF DENSIFYING CARBON-CARBON COMPOSITES BY CHEMICAL VAPOR INFILTRATIONP ROCESSES

A finite difference (FD) model is proposed to simulate the chemical vapor infiltration (CVI) processes for fabrication of carbon carbon composites. With iterative operation of many discrete values, the FD based model can be used to describe and analyze the real, continuous densification processing quantitatively. Many densification characteristics of carbon carbon composites can be predicated by modeling. The prediction results can be compared with experiment value directly, which shows its good ability for practical application. Special verification experiments are designed with Iso thermal CVI processing and slender cylindroid unidirectional C/C composites are prepared to verify the accuracy of the model. The modeling curve of density versus infiltration time is in good agreement with experiment values. According to modeling analysis, the effects of infiltration temperature and fiber volume fraction on densification are also discussed preliminarily. The conclusion obtained also accords with experiment or results in other literature, further approving the accuracy of the FD based model.

Densification mechanism of chemical vapor infiltration technology for carbon/carbon composites

Carbon/carbon composites were fabricated using pressure-gradient chemical vapor infiltration(CVI) technology with propane (C3H6) as the carbon precursor gas and nitrogen (N2) as the carrier gas. The chemical process of deposition of pyrolytic carbon was deduced by analyzing the component of molecules in gas phase and observing the microstructure of deposition carbon. The results show that the process of deposition starts from the breakdown of C—C single bond of propene (C3H6), and forms two kinds of active groups in the heterogeneous gas phase reaction. Afterwards, these active groups form many stable bigger molecules and deposit on carbon fiber surface. At the same time, hydrogen atoms of the bigger molecules absorbed on carbon fiber surface are eliminated and the solid pyrolytic carbon matrix is formed in the heterogeneous reaction process.

Rapid chemical vapor infiltration of C/C composites

With liquid petrol gas(LPG)as carbon source,carbon felt as porous perform and hydrogen as diluent,C/C composites were fast fabricated by using a multi-physics field chemical vapor infiltration(MFCVI)process in a self-made furnace.A set of orthogonal experiments were carried out to optimize parameters in terms of indices of density and graphitization degree.The results show the optimal indices can be achieved under the conditions of temperature 650℃,LPGconcentration 80%,gas flux 60 mL/s, total pressure 20 kPa,infiltration time 15 h.The verification experiment proves the effectiveness of the orthogonal experiments. Under the optimal conditions,the graphitization degree of 75%and bulk density of 1.69 g/cm are achieved with a uniform density distribution.At the same time,a new structure is obtained.

Microstructure of carbon fiber preform and distribution of pyrolytic carbon by chemical vapor infiltration

The carbon/carbon composites were made by chemical vapor infiltration(CVI) with needled felt preform. The distribution of the pyrolytic carbon in the carbon fib er preform was studied by polarized light microscope(PLM) and scanning electroni c microscope(SEM). The experimental results indicate that the amount of pyrolyti c carbon deposited on the surface of chopped carbon fiber is more than that on t he surface of long carbon fiber. The reason is the different porosity between th e layer of chopped carbon fiber and long carbon fiber. The carbon precursor gas which passes through the part of chopped carbon fibers decomposes and deposits o n the surface of chopped carbon fiber. The pyrolytic carbon on the surface of lo ng carbon fibers is produced by the carbon precursor gas diffusing from the chop ped fiber and the Z-d fiber. Uniform pore distribution and porosity in preform are necessary for producing C/C composites with high properties.

Oxidation mechanisms of C-SiC-TaC-C composites prepared by chemical vapor infiltration

To improve the oxidation-resistance properties,SiC and TaC species were introduced in C/C composites by chemical vapor infiltration(CVI) methods. The oxidation-resistance properties of C-SiC-TaC-C composites were studied by X-Ray diffractometry(XRD),JEOL-6360LV scanning electronic microscopy(SEM) and AdventurerTM electronic balance with precision of 0.1 mg. The results show that,1) the oxidation rate of the composites increases continuously with time at all experimental temperatures;2) The oxidation rate increases with temperature within 700-1 100 ℃,slowly in 700-800 ℃,acutely in 800-1 100 ℃;it reaches a maximum value at 1 100 ℃,then decreases within 1 100-1 400 ℃;3) The relationship curve of oxidation rate with temperature can be divided into three regions. The oxidation rate is controlled by reactivity in region Ⅰ,the mixed effects of reactivity and gas diffusion in region Ⅱ,gas phase diffusion in region Ⅲ;4) The composites exhibit a higher oxidation onset temperature in low temperature region and a lower oxidation rate at high temperature due to the oxidation of TaC to(Ta,O) and the formation of the dense SiO2-Ta2O5 oxide layer respectively. With the addition of SiC/TaC species,the oxidation-resistant properties of C/C composites can be improved effectively.

Modeling of gas phase diffusion transport during chemical vapor infiltration process

In order to improve the uniformity of both the concentration of gaseous reagent and the deposition of matrix within micro pores during the chemical vapor infiltration (CVI) process, a calculation modeling of gas phase diffusion transport within micro pores was established. Taken CH 3SiCl 3 as precursor for depositing SiC as example, the diffusion coefficient, decomposing reaction rate, concentration within the reactor, and concentration distributing profiling of MTS within micro pore were accounted, respectively. The results indicate that, increasing the ratio of diffusion coefficient to decomposition rate constant of precursor MTS is propitious to decrease the densification gradient of parts, and decreasing the aspect ratio (L/D) of micro pore is favorable to make the concentration uniform within pores.

GROWTH CHARACTERSAND MODEL OF PYROLYTIC CARBON IN CHEMICAL VAPOR INFILTRATION PROCESS

Chemical Vapor Infiltration (CVI) processes are the essential techniques for fabrication of high performance carbon carbon composites. Based on the polarized light and scanning electron analysis, the authors study the micro morphology and texture characteristics of pyrolytic carbon deposited in CVI process, as well as the growth behavior of pyrolytic carbon. The research shows that Rough Laminar (RL) texture has the hierarchical and self similar structural features, which reflects the stage growth and self similar behavior during the growth course of pyrolytic carbon. According to the two growth features, a laminated growth model of pyrolytic carbon is proposed with the concept of Cone Growth Units (CGU). The laminated growth model can provide a fine description for the growth course of RL pyrolytic carbon. The model indicates that formation, developing and combination of local high order structures (such as CGU structures) are the essential factors for the growth of RL texture. Smooth Laminar (SL) texture and ISO carbon come into being with long range orderliness and isotropy structure respectively, which no local high orderliness intermediate involves in.

Comparative modeling of gas transport in isothermal chemical vapor infiltration process of C/SiC composites

Two comparative models taking into account of momentum, energy and mass transport coupled with chemical reaction kinetics were proposed to simulate gas transport in isothermal CVI reactor for fabrication of C/SiC composites. Convection in preform was neglected in one model where momentum transport in preform is neglected and mass transport in preform is dominated by diffusion. Whereas convection in preform was taken into account in the other model where momentum transport in preform is represented by BRINKMAN equations and mass transport in preform includes both diffusion and convection. The integrated models were solved by finite element method. The calculation results show that convection in preform have negligible effect on both velocity distribution and concentration distribution. The difference between MTS molarities in preform of the two models is less than 5×10-5, which indicates that ignorance of convection in preform is reasonable and acceptable for numerical simulation of ICVI process of C/SiC composites.

The Infiltration Process and Texture Transition of 2D C/C Composites

2D needle-punched fiber felt was infiltrated by a kind of rapid isothermal chemical vapor infiltration technique. The infiltration process and texture transition of the infiltrated C/C composites were investigated. The porosity and the variations of the cumulative pore volume were determined by mercury porosimetry. The texture of matrix carbon was studied under a polarized light microscope. The results show that the relative mass gain of the sample increases directly as the infiltration time at the initial stage until 20 h, and subsequently the increasing rate of the relative mass gain decreases gradually with the prolonging of infiltration time. Three layers of pyrocarbon were formed around fibers. Low-textured pyrocarbon was obtained at the initial stage. With the densification going on, high-textured pyrocarbon was formed on the surface of low-textured pyrocarbon. Then, low-textured pyrocarbon was produced again during the final stage of densification. The texture transition is ascribed to the variation of the ratio of cumulative inner surface area to volume of pores and the gas partial pressure in pores.

Processing and properties of 2D SiC/SiC composites by precursor infiltration and pyrolysis

Two-dimensional plain-weave silicon carbide fiber fabric reinforced silicon carbide(2D-SiC/SiC)composites were molded by stacking method and densified through precursor infiltration and pyrolysis(PIP)process.SiC coating was deposited as the fiber/matrix interphase layer by chemical vapor deposition(CVD)technique.Fiber/matrix debonding and relatively long fiber pullouts were observed on the fracture surfaces.Additionally,the flexural strength and elastic modulus of the composites with and without fiber/matrix interphase layer were investigated using three-point bending test and single-edge notched beam test.The results show that the fiber fraction and the porosity of 2D-SiC/SiC composites with and without coating are 27.2%(volume fraction)and 11.1%,and 40.7%(volume fraction)and 7.5%,respectively.And the flexural strength and elastic modulus of 2D-SiC/SiC composites with and without coating are 363.3 MPa and 127.8 GPa,and 180.2 MPa and 97.2 GPa,respectively.With a proper thickness,the coating can effectively adjust the fiber/matrix interface,thus causing a dramatic increase in the mechanical properties of the composites.

Study on Soil Infiltration Capability and Its Impact Factors of Different Land-use Types in Purple Soil Region

Soil infiltration capability is the hot spot topic of soil erosion studies and soil physical and chemical properties have great influence on it. A new infiltration method point- source infiltration method was used to precisely evaluate the infiltration capability in different purple soil land- use types. And correlation analysis on soil physical and chemical properties and soil infiltration capability of different land- use types was performed. Results showed that:( i) there is a large difference among soil physical and chemical properties in different land- use types,soil water content,non- capillary porosity,capillary porosity,content of > 0. 25 mm aggregates and organic matter content in the top soil are greater than those in the subsoil;( ii) soil infiltration capability showed differences among different land- use types. Land use showed great effects,in general,the order of decrease on initial infiltration rate and average infiltration rate was: woodland slope > slope farmland >grassland,the order of decrease on steady infiltration rate was: slope farmland > woodland > grassland and the time reaching stable state was:slope farmland > woodland > grassland;( iii) correlation analysis showed that there was a significantly positive correlation between initial infiltration rate and wet sieve MWD value and structural damage rate,and it had a significantly negative correlation with capillary porosity;( iv)steady infiltration rate and non- capillary porosity showed the significantly positive correlation,and it had a significantly negative correlation with the soil bulk density;( v) the average infiltration rate and non- capillary porosity and structural damage rate showed a positive correlation and the correlation coefficient was large and there was a negative correlation between average infiltration rate and soil bulk density and capillary porosity,and the absolute value of correlation coefficient was relatively large. The results of this study can provide the theoretical basis for soil infiltration study in purple soil area.

Numerical simulation of isothermal chemical vapor infiltration process in fabrication of carbon-carbon composites by finite element method

The chemical vapor infiltration process in fabrication of carbon-carbon composites is highly inefficient and requires long processing time. These limitations add considerably to the cost of fabrication and restrict the application of this material. Efforts have been made to study the CVI process in fabrication of carbon-carbon composites by computer simulation and predict the process parameters, density, porosity, etc. According to the characteristics of CVI process, the basis principle of FEM and mass transport, the finite element model has been established. Incremental finite element equations and the elemental stiffness matrices have been derived for the first time. The finite element program developed by the authors has been used to simulate the ICVI process in fabrication of carbon-carbon composites. Computer color display of simulated results can express the densification and distributions of density and porosity in preform clearly. The influence of process parameters on the densification of preform has been analyzed. The numerically simulated and experimental results give a good agreement.

Mechanical and dielectric properties of porous and wave-transparent Si3N4–Si3N4 composite ceramics fabricated by 3D printing combined with chemical vapor infiltration

Porous Si3N4–Si3N4 composite ceramics were fabricated by 3D printing combined with low-pressure chemical vapor infiltration(CVI).This technique could effectively improve the designability of porous Si3N4 ceramics and optimize the mechanical and dielectric properties.The effects of process parameters including the deposition time and heat treatment on the microstructure and properties of porous Si3N4–Si3N4 composite ceramics were studied.The study highlights following:When CVI processing time was increased from 0 to 12 h,the porosity decreased from68.65%to 26.07%and the density increased from 0.99 to 2.02 g/cm3.At the same time,the dielectric constant gradually increased from 1.72 to 3.60;however,the dielectric loss always remained less than0.01,indicating the excellent electromagnetic(EM)wave-transparent performance of porous Si3N4–Si3N4 composite ceramics.The maximum flexural strength of 47±2 MPa was achieved when the deposition time attained 6 h.After heat treatment,the porosity increased from 26.07%to 36.02%and the dielectric constant got a slight increase from 3.60 to 3.70 with the dielectric loss still maintaining lower than 0.01.It has been demonstrated that the porous Si3N4–Si3N4 composite ceramics are a promising structural and EM wave-transparent material suitable for high temperature service.

Chemical vapor infiltration of pyrocarbon from methane pyrolysis: kinetic modeling with texture formation

A complete mechanism of methane pyrolysis is proposed for chemical vapor infiltration of pyrocarbon with different textures, which contains a detailed homogeneous mechanism for gas reactions and a lumped heterogeneous mechanism for pyrocarbon deposition. This model is easily applied to simulate gas compositions and pyrocarbon deposition in a vertical hot-wall flow reactor in the temperature range of 1,323–1,398 K without any adjusting parameters and presents better results than previous mechanisms. Results have shown that the consumption of methane and the production of hydrogen are well enhanced due to pyrocarbon deposition. Pyrocarbon deposition prevents the continuously increasing of acetylene composition and leads to the reduction in the mole fraction of benzene at long residence times in the gas phase. The carbon growth with active sites on the surface is the controlling mechanism of pyrocarbon deposition. C1 species is the precursor of pyrocarbon deposition at 1,323 K,and the primary source over the whole temperature range. As temperature increases, gas phase becomes more mature and depositions from acetylene, benzene and polyaromatic hydrocarbons become more prevalent. A general pyrocarbon formation mechanism is derived with the specific precursors and illustrates that the maturation of gas compositions is beneficial to forming planar structures with hexagonal rings or pentagon-heptagon pairs, namely, high textured pyrocarbon. The results are in well agreement with experiments.

Deposition kinetics and mechanism of pyrocarbon for electromagnetic-coupling chemical vapor infiltration process

Although the electromagnetic-coupling chemical vapor infiltration(E-CVI)has been proven of a highefficiency technique for producing carbon fiber reinforced pyrocarbon(Py C)matrix(C/C)composites,a deep understanding of the deposition kinetics and mechanism of Py C matrix is still lack.In this work,a deposition model with uniform electric field but gradient magnetic field was set up by using unidirectional carbon fiber bundles as the substrates to investigate the deposition kinetics and mechanism.Meanwhile,the polarizability,and the chemical adsorption and dehydrogenation barriers of hydrocarbon were simulated based on the density functional theory(DFT)and the Climb-image nudged elastic band method,respectively.The E-CVI process exhibited extremely high Py C deposition rates of 8.7,11.5,16.5 and 22.7 nm/s at 700,750,800 and 850℃,respectively,together with a significantly low apparent activation energy of 57.9 k J/mol within the first 5 min.The Py C deposited at different temperatures with different time shows a smooth laminar structure with low coherent length and graphitization degree.The theoretical calculation and simulation results indicated that the electrons existing on the carbon fibers and the accelerated motion of radicals with preferred orientation forced by the derived magnetic field have reduced the energy barrier for the deposition process,thereby resulting in low apparent activation energy and high Py C deposition rate.The results of this work may shed a light on how to better direct the preparation of C/C composites by E-CVI process with high quality and efficiency.