纳米碳颗粒/碳化硅陶瓷基复合材料的氧化行为

以微米硅(Si)和纳米碳黑(Cp)粉体为主要原料,采用经机械化学法合成的碳化硅(SiC)和15%和25%的纳米碳颗粒与碳化硅(Cp-SiC)的复合粉体,并经无压烧结得到了Cp/SiC陶瓷基复合材料,分析了在不同温度条件下Cp/SiC烧结体的氧化行为。结果表明:当温度小于700℃时,Cp/SiC复合陶瓷在空气中的氧化受C—O2反应控制,致使其为均匀氧化;700℃时,氧化后的复合材料显气孔率最大,弯曲强度达极小值;大于700℃,氧化过程受O2的气相扩散控制,呈非均匀氧化;700~900℃之间,O2通过微裂纹的扩散控制着Cp/SiC的氧化过程;900~1 100℃之间,O2通过SiC缺陷的扩散控制着Cp/SiC的氧化过程,并在1 000℃时的最初的2 h内,复合材料弯曲强度增大,且达到了极大值。同时表明,纳米碳含量是影响复合材料强度及氧化行为的关键因素,添加纳米碳质量分数为15%的Cp/SiC复合陶瓷可以作为一种抗氧化性能优良的玻璃夹具材料。     

Oxidation of Chemically-Vapor-Deposited Silicon Carbide in Carbon Dioxide

Chemically-vapor-deposited silicon carbide (CVD SiC) was oxidized in carbon dioxide (CO₂) at temperatures of 1200–1400°C for times between 96 and 500 h at several gas flow rates. Oxidation weight gains were monitored by thermogravimetric analysis (TGA) and were found to be very small and independent of temperature. Possible rate-limiting kinetic mechanisms are discussed. Passive oxidation of SiC by CO₂ is negligible compared to the rates measured for other oxidants that are also found in combustion environments, oxygen and water vapor.     

Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was measured in an atmosphere of dry oxygen between 900° and 1600°C. The rate was studied by using a thermogravimetric apparatus and was found to be diffusion controlled. The products of oxidation were amorphous silica and cristobalite, depending on the temperature. The effect of surface area was determined, and a correlation between the various sizes was made with the aid of an equation derived on the assumptions that (1) the reaction was diffusion controlled, (2) the particles were essentially spherical, and (3) the interfacial area was constantly changing. The presence of water vapor in the gaseous atmosphere was found to be extremely critical.     

Oxidation Resistance of Multiwalled Carbon Nanotubes Coated with Silicon Carbide

Multiwalled carbon nanotubes (MWCNTs) were coated with a SiC layer using SiO vapor. The growth mechanism of SiC and the oxidation resistance of the SiC-coated MWCNTs were studied. The growth of the SiC layer was controlled by adjusting the partial pressure of CO₂ using carbon felt placed in a crucible. The nanometer-sized SiC particles were deposited onto the tubes by the reaction between SiO( g ) and CO( g ). On the other hand, the thin surface of the MWCNTs was converted to the SiC layer when the carbon felt was not used. The oxidation durability of MWCNTs was improved by the SiC coating. MWCNTs were oxidized completely in air at 650°C for 60 min. However, about 90 mass% of the SiC-coated MWCNTs remained after the same oxidation test.     

Effect of Carbon and Boron on the High-Temperature Oxidation of Silicon Carbide

Silicon carbide has good oxidation resistance, due to the formation of a protective silica layer. Although amorphous silica is an excellent oxygen barrier, it is very sensitive to impurity elements, which affect its viscosity, oxygen diffusivity, and crystallization kinetics. This paper compares the oxidation rates of CVD SiC, sintered α-SiC, and CVD SiC- coated graphite in 1 atm oxygen at 1500^deg;C to determine the effects of small additions of boron and carbon. The formation of bubbles in the silica scale formed on sintered α-SiC in oxygen between 1230° and 1550^deg;C is also discussed.     

Oxidation Behavior of Silicon Carbide

The oxidation of purified green silicon carbide in controlled atmospheres was studied by weight-gain measurement and by observation of the surface reaction products, including optical measurement of the thickness of the oxide surface film. The rate of oxidation was much greater for silicon carbide in contact with fluid silicate glasses than for silicon carbide alone. In a vacuum of 0.1 mm., oxidation proceeded with loss of weight, because of the formation of volatile SiO₂, and at a greater rate than at atmospheric pressure. It is postulated that the rate-controlling process in the normal oxidation of silicon carbide is the formation of solid SiO₂ on the surface.     

Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Oxidation of Submicroscopic Fibrous Silicon Carbide

The oxidation rate of silicon carbide fibers of submicroscopic dimensions in static air was investigated by a gravimetric technique at 800°, 900°, and 1000°C. The fibers can be held near 800°C for several hours without significant oxidation, but they rapidly oxidize at 1000°C. A theoretical model for diffusion-controlled oxidation of the fibers, taking into account a changing reaction interfacial area, was obeyed to more than 60% conversion of the silicon carbide to silica. For the diffusion-controlled oxidation an enthalpy of activation of 55.8 or 39.8 kcal/mole was calculated depending on whether an amorphous silica sheath was initially present.     

High-Resolution Electron Microscopy of the Silicon Carbide/Aluminum Carbide Interface

The interface of single-crystal SiC and Al brazed at 1273 K is investigated by high-resolution electron microscopy. The orientation relationship of SiC to the Al₄C₃ reaction layer that forms between the SiC and the Al can be expressed as (0001)_SiC∥(0001)_Al4C₃ and [11∼00]_SiC∥[11∼00]_Al4C₃. Furthermore, a very thin (two tetrahedral layers thick) transition phase and misfit dislocations are observed between the SiC and Al₄C₃ lattices. The structure of the transition phase is discussed based on the high-resolution electron microscopy, the stacking of the (Al,Si)C₄ tetrahedral layers, and the charge balance. The same reaction product, with the same orientation relationship, is observed at the interface of a polycrystalline SiC and Al brazed joint.     

Carbon Inclusions in Sintered Silicon Carbide

The carbon additions in the pressureless sintering of SiC are commonly used for the removal of SiO₂ layers on the starting powders. In practice, it is common to add more C than is necessary for stoichiometric removal to ensure a complete deoxidation. As a result, inclusions of excess free C are a general feature of the microstructure of sintered SiC. This phenomenon was studied by high-resolution Auger electron spectroscopy on ultra-high-vacuum-exposed fracture surfaces as well as by high-resolution transmission electron microscopy of B- and C-doped materials.     

Composite Ceramic Based on Silicon Carbide

Refractories and Industrial Ceramics - An effective method for preparing dense granular ceramic from SiC with a residual porosity of less than 5.0% is the use of additives of oxide system eutectic...     

Stress-Oxidation Behavior of a Carbon/Silicon Carbide Composite in a High-Temperature Combustion Environment

A carbon/silicon carbide composite with a silicon carbide coating was prepared by chemical vapor infiltration. Stressed oxidation testing was performed on the composites in a self-built high-temperature combustion environment. The gas in this environment contained oxygen, steam, carbon dioxide, and some nitrogen. Test conditions were controlled at temperatures of 1300°, 1500°, and 1800°C, and the stress was sustained at 40, 80, 120, 160, and 200 MPa. The effect of combustion environment and applied load on stress-oxidation behavior was discussed by analyzing the residual strength and weight loss. The morphology of the fracture surface of the tested specimens was observed by scanning electron microscopy. The high-temperature combustion environment and the high sustained stress above 80 MPa enhanced the material failure and led to strength reduction by determining crack openings and thus oxidation of fibers. However, sustained stress below 80 MPa resulted in no strength degradation after exposure for 10 min at 1500°C.     

Active-to-Passive Transition in the Oxidation of Silicon Carbide and Silicon Nitride in Air

The active-to-passive transition in the oxidation of SiC and Si₃N₄ was determined in a flowing air environment as a function of temperature and total pressure. The experimentally observed transition temperatures ranged from a low of 1347°C to a high of 1543°C for partial pressures of oxygen of 2.5 and 123.2 Pa, respectively. The SiC and Si₃N₄ samples had approximately the same transition point for a given pressure. In general, the higher the flow rate, the higher the transition temperature for a given pressure. The transitions for SiC measured in this study agree with previous data for the transition of SiC measured in pure oxygen at reduced pressures and in oxygen inert gas mixtures.     

Process and Mechanical Properties of in Situ Silicon Carbide-Nanowire-Reinforced Chemical Vapor Infiltrated Silicon Carbide/Silicon Carbide Composite

A SiC nanowire/Tyranno-SA fiber-reinforced SiC/SiC composite was fabricated via simple in situ growth of SiC nanowires directly in the fibrous preform before CVI matrix densification; the purpose of the SiC nanowires was to markedly improve strength and toughness. The nanowires consisted of single-crystal β-phase SiC with a uniform ∼5 nm carbon shell; the nanowires had diameters of several tens to one hundred nanometers. The volume fraction of the nanowires in the fabricated composite was ∼5%. However, the composite did not show significant increase in strength and toughness, likely because of strong bonding between the nanowires and the matrix caused by the very thin carbon coating on the nanowires. Little debonding and pullout of SiC nanowires from the matrix were observed at the fracture surfaces of the composite.     

Oxidation of Silicon and Silicon Carbide in Ozone-Containing Atmospheres at 973 K

The oxidation behavior of a silicon wafer, chemically vapor-deposited SiC, and single-crystal SiC was investigated in an oxygen—2%–7% ozone gas mixture at 973 K. The thickness of the oxide film that formed during oxidation was measured by ellipsometry. The oxidation rates in the ozone-containing atmosphere were much higher than those in a pure oxygen atmosphere. The parabolic oxidation kinetics were observed for both silicon and SiC. The parabolic rate constants varied linearly with the ozone-gas partial pressure. Inward diffusion of atomic oxygen formed by the dissociation of ozone gas through the SiO₂ film apparently was the rate-controlling process.     

碳化硅和碳纤维对甲基乙烯基硅橡胶导热复合材料性能的影响

研究碳化硅和碳纤维对甲基乙烯基硅橡胶导热复合材料性能的影响。结果表明:碳化硅和碳纤维能够均匀分散于硅橡胶基体中,不同粒径的碳化硅复配能够进一步提高复合材料的导热性能;在碳化硅填充硅橡胶中加入碳纤维不仅能够在基体内部形成串联的导热网链,进一步提高复合材料的导热性能,还能提高复合材料的物理性能。     

Push-Out Tests on a New Silicon Carbide/Reaction-Bonded Silicon Carbide Ceramic Matrix Composite

Fiber push-out tests have been performed on a ceramic matrix composite consisting of Carborundum sintered SiC fibers, with a BN coating, embedded in a reaction-bonded SiC matrix. Analysis of the push-out data, utilizing the most complete theory presently available, shows that one of the fiber/coating/matrix interfaces has a low fracture energy (one-tenth that of the fiber) and a moderate sliding resistance τ∼ 8 MPa. The debonded sliding interface shows some continuous but minor abrasion, which appears to increase the sliding resistance, but overall the system exhibits very clean smooth sliding. The tensile response of a full-scale composite is then modeled, using data obtained here and known fiber strengths, to demonstrate the good composite behavior predicted for this material.     

渗硅碳化硅材料的高温氧化

研究了全碳粉反应渗硅碳化硅(PCRBSC)材料,在1300℃静态空气中的高温氧化行为.研究结果表明:PCRBSC材料的氧化过程遵循直线-抛物线规律,其结构对高温氧化有很大的影响,特别是游离硅fsi和游离碳fc的含量对氧化影响更大,fsi含量高的PCRBSC材料单位面积氧化增重(Δm/s)明显,fc含量高的PCRBSC材料氧化后表现为先减重后增重,氧化层断口经扫描电镜观察有明显的气孔存在.     

In Situ Polysilane-Derived Silicon Carbide Particulates Dispersed in Silicon Nitride Composite

A dense (97% of theoretical density) Si₃N₄—SiC composite containing 10 wt%β-SiC was prepared by introducing a SiC phase by the pyrolysis of a polymeric SiC precursor. The composite material was produced by mixing an alkyl/aryl-substituted polysilane with Si₃N₄ powder and, by subsequently forming green compacts, pyrolyzing the polymeric species, and finally sintering the sample. Synthesis and characterization of the polymeric compound was described. Its transformation reactions to SiC and the characterization of the ceramic residue were also studied. High ceramic yields were obtained by curing the as-synthesized polysilane at 500°C in an Ar atmosphere. The heat treatment had no effect on the good solubility of the polymeric precursor in organic solvents. This was important for processes such as infiltration, sealing, and coating and for the mixing of the polymer with powders for the preparation of homogeneous composite ceramics. The dense microstructure of the pyrolyzed and sintered Si₃N₄ powder–polysilane mixture exhibited reduced grain growth of the Si₃N₄ particles and a very homogeneous distribution of the in situ-formed β-SiC phase.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.