SiC-Based Materials

Researchers have been developing SiC-CMCs in order to obtain an oxidation-resistant, tough thermostructural material. In general, a SiC-based material easily forms a protective oxide layer on its surface at high temperatures... 

In the case of PSCSs when x > 0.1, most of the products were found soluble and were converted into the corresponding PCS at 450 °C, under atmospheric pressure. These spinnable PCS were transformed into SiC-based materials with ceramic yields very close to those of Yajima ( 60 %). PSCSs of formula (MeRSi),.j-(HR SiCH2SiR H)j also were prepared in order to appreciate the influence of R and R on the carbon content of the ceramic. Otherwise, multinuclear solid state NMR studies indicated that the network was first built around silicon atoms (formation of SiC4) then, at higher temperature, aroimd carbon atoms (formation of CSi4). 

SiC-based materials have also shown very good corrosion resistance in HI. Both sintered and chemical vapor deposition (CVD) SiC have very low corrosion rates when tested in HI at elevated temperatures. In addition, Si-infiltrated C-based materials (Si-SiC) also have good potential. This method of manufacturing may become an attractive option in the future, as it promises an extremely low-cost alternative to manufacture SiC-based corrosion-resistant materials and reduce the potential joining problems. Effort is continuing to resolve the manufacturing techniques and improve the inherent mechanical properties of these SiC-based materials. 

Since both HI and H3PO4 are reducing in nature, it is possible to use materials that are common to both in the iodine separation reaction (see table 4.3). The material of construction used to fabricate the I2 separation reactor must be able to resist the combination of HI and H3PO4. Preliminary test results show that the corrosion behavior of the various materials tested in the HI,-H3P04 acid mixture is similar to that in HI, at high temperatures, with Ta and Nb alloys and SiC-based materials the most promising construction materials candidates. 

II. POLYMETHYLSILANE AND PCS PROMISING PRECURSORS FOR SIC-BASED MATERIALS VIA THE POLYMER BLEND TECHNIQUE... 

Kiln furniture must be made of materials resistant to considerable temperature cycling in particular, during fast firing the changes in temperature are quite rapid (heating rate up to 30 — 40 ""C min ). A low thermal expansion coefficient and/or high thermal conductivity are therefore required. These requirements are best met by cordierite-mullite materials up to 1300 ""C and SiC-based materials for higher temperatures. 

Magnani,G. and Beaulardi,L.. Properties of liquid phase pressureless sintered SiC-based materials... 

High creep resistance coupled with low density as well as good oxidation resistance of pure SiC and most SiC-based materials compared with highly advanced metallic alloys predestine this ceramic for devices operating at high temperatures, up to 1500 °C, in the fields of ... 

Balanced research of oxide and non-oxide materials on their specific life-limiting characteristics appears to be necessary because neither class of materials can satisfy design and service life requirements for all of the anticipated applications. For example, SiC-based materials have the high thermal conductivities and low thermal expansion coefficients essential for some components, particularly in high performance turbines for which oxides are inadequate. Conversely, in some corrosive environments (e.g., hot gas filters in coal-fired power systems), oxides provide necessary corrosion resistance. 

Because most CMC applications are dominated by thermal loads, the material temperature depends not only on the component environment (e.g., gas temperature in a turbine engine), but also on material properties and thermal boundary conditions. In a typical environment, material temperatures decrease as their thermal conductivity increases. Therefore, in common thermal environments and boundary conditions, SiC-based materials have lower material temperatures than oxides because of the higher thermal conductivity of SiC. 

Thus, fundamental questions remain about what controls the creep rates of SiC-based materials. What is the role of carbon, of solid solution dopants Do the stacking faults in P-SiC grains play a role Is the a-SiC microstructure intrinsically more creep resistant All of these questions remain to be answered. 

Compilations of corrosion data on SiC-based materials have been given in some hand- and textbooks [2,35,52,53], many papers on it are included in the proceedings of specific meetings [3,5,54]. 

In reality the attack already starts at the melting point of Na2S04. This is attributed to internal controlling factors. Many SiC-based materials contain free carbon, which promotes the dissociation of Na2S04 and makes the silicate melt more basic. The dramatic attack of Na2S04 on SiC with free carbon has been confirmed experimentally [72]. 

In the latter case SiC and/or additions are reacted to an intermediate liquid which not only provides densification at reduced temperatures but since it is consumed in the reaction yields a SiC-based material without glass at the grain boundaries. 

V. Kevorkiijan, A. Bizjak, J. Vizintin, F. Thevenot, G. Interdonato, and C. Reimondi, B4C-SiC based material for wear applications, in 4th Euro Ceramics Vol. 4, Basic Science, A. Bellosi (Ed.), Faenza Editrice, 1995, pp. 209-216. 

Jerzy Lis, Y, Miyamoto, Roman Pampuch et al., Ti3SiC (sic ) based materials prepared by HIP-SHS techniques. Materials Letters 22,163 (1995). 

Nanoporous SiC-based materials are difficult to obtain due to the high formation temperature. Disordered nanoporous silicon carbide ceramics can be fabricated by a solid-gas reaction of ordered mesoporous carbon replica with silicon vapor [82], and a chemical vapor infiltration of dimethyldichlorosilane inside mesoporous silica following by the removal of silica [83]. Recently, highly ordered mesoporous SiC materials with uniform pore sizes and ultralarge surface areas have been synthesized by fully impregnating polycarbosilane... 

G. Magnani, and L. Beaulardi, Properties of Liquid Phase Pressureless Sintered SiC-Based Materials Obtained without Powder Bed, J.Aus.Ceram.Soc., 41(1), 31-6 (2005). 

The present studies are mainly devoted to silicon carbide (SiC)-based materials in the form of isolated clusters, nanopartides, or several architectures, which exhibit various original features. The interests in SiC is motivated by the large offered possibihties from structural aspects as well as physical responses such as electronic, optics, photovoltaic or dielectric properties. Additionally, beyond good thermal stability and mechanical hardness, the SiC is versatile from structural aspect (more than 170 polytypes), electronic behavior (a variable energy gap from 2.4 to 3.3 eV) as well as photorefractive properties (Vonsovici et al. 2000). As matter of fact, the nanocrystalline size modulates all the intrinsic parameters involved in the parent bulk materials. When nanopartides are associated with suitable matrixes, promising new potentialities... 

---