Amorphous covalent ceramics

The processes occurring during cross linking and ceramization as well as the structure of the preceramic network and the amorphous covalent ceramic depend strongly on the chemical structure of the synthesized precursor molecule. The influence of the type of the polymer molecule, of the catalysts, steric effects of the side groups, bond stren s, as well as radical stabilities are crucial factors. 

Beyond all doubt, the state of the amorphous covalent ceramics forms a specific and, by knowing the preparation route, a more or less exactly definable state which occurs as an intermediate during the transformation of the silicon organic polymer into the final crystalline ceramics. This state appears to be stable at room temperature for a long time. And, of course, materials in this state are distinguished by specific properties different from the state of crystalline ceramics. 

Hitherto we have discussed the formation of amorphous covalent ceramics only on the basis of polymer derived materials. In Sects. 2.2 and 4.2.2.2, thin amorphous, hydrogen stabilized SiC layers (a-SiC H) are also considered which are formed, first of all, by gas phase processes (CVD, PVD). They represent another type of amorphous covalent ceramics. And though it is not expected that properties of such layers agree completely with those of the polymer derived ACC, the basic ideas of stability and transformability of the ACC state discussed above should be transferable to this type of amorphous covalent ceramics, too. 

Polymer pyrolysis to form advanced ceramics allows the production of highly covalent refractory components (fibers, films, membranes, foams, joints, monolithic bodies, ceramic matrix composites) that are difficult to fabricate via the traditional powder processing route [1-4]. Yajima was the first to demonstrate the feasibility of producing high-strength SiC-based fibers from pyrolysis of polycarbosilane [5]. In this process, a thermoplastic pre-ceramic polymer is first shaped into the desired form, cross-linked into a pre-ceramic network and finally converted into a ceramic material by a pyrolysis process in a controlled atmosphere (Fig. 1). A common feature of the polymer route is the formation of intermediates called amorphous covalent ceramics (ACC) [6]. These are formed after removal of the organic components and before crystallization that occurs at higher temperatures. 

Polymer pyrolysis appears to be a very promising processing route to advanced covalent ceramics. This process allows the formation of new amorphous covalent ceramic phases in the general system Si-M-C-N-O (with M = B, Ti, Al, Zr...) which show exceptional oxidation and creep resistance at high temperature up to... 

Ceramics are inorganic, nonmetallic, solid materials. They can be crystalline or noncr5 talline. Noncrystalline ceramics include glass and a few other materials with amorphous structures. Ceramics can possess a covalent-network structure, ionic bonding, or some combination of the two. (Section 11.8, Table 11.6) They are normally hard and brittle and are stable to very high temperatures. Ceramic materials include familiar objects such as pottery, china, cement, roof tiles, refractory bricks used in furnaces, and the insulators in spark plugs. 

Presently it is unclear what particular mechanism is responsible for the shrinkages observed during the annealing of latser powders. The results are very encouraging if they do in fact represent the possibility of sintering 3 4 perhaps other covalent ceramics) in an amorphous state. 

Figures 9.1-9.3 illustrate these interconnected relationships.13 Figure 9.1 defines some of the terms used in this chapter. Small molecules are species with molecular weights below about 1,000. They are volatile at temperatures below say 200 100 °C. Clusters are oligomers derived from covalently linked small molecules. They have a lower volatility than small molecules and, if large enough, can be shaped by melting or by solvent evaporation methods. Linear polymers can be simple chain structures or may consist of rings linked together. In either case they are usually non-volatile and easily fabricated. Cross-linked systems can be produced from polymers or from clusters. The final ceramic may be amorphous or crystalline.

Ceramics tend to possess ionic and covalent bonding, and Si02, for example, can exist in either form—the crystalline form occurs when Si02 melt is slowly cooled from above the mp (1723°C), while the amorphous form is obtained if rapid cooling is applied. The type of bonding formed affects the properties (e.g. amorphous ceramics are poorer conductors of heat due to lack of an ordered lattice). 

As previously stated, ceramics are characterized either by the ionic or covalent bonding of their constituents and, consequently, with some exceptions, they exhibit brittle behavior. Also note that the field of ceramics covers a broad range of structures, from completely crystalline to amorphous (mostly glassy structures). Therefore, the main deformation at ambient temperatures is elastic (tending to brittleness) only at elevated temperatures may one speak about plastic deformation, since most ceramics show ductility. Clearly, the temperature level is a... 

The word ceramics is derived from the Greek keramos, meaning solid materials obtained from the firing of clays. According to a broader modern definition, ceramics are either crystalline or amorphous solid materials involving only ionic, covalent, or iono-covalent chemical bonds between metallic and nonmetallic elements. Well-known examples are silica and silicates, alumina, magnesia, calcia, titania, and zirconia. Despite the fact that, historically, oxides and silicates have been of prominent importance among ceramic materials, modern ceramics also include borides, carbides, silicides, nitrides, phosphides, and sulfides. 

Ceramics are inorganic solids, usually oxides, which contain ionic and covalent bonds. The material, formed by sintering at high temperatures, ranges from amorphous glass-like material to highly crystalline solids, from insulators to conductors or semiconductors. They include earthenware, which is fired at 1,100-1,300 K and a porosity of about 8% fine china or bone china, fired at 1,400-1,500 K with a porosity of less than 1% stoneware, fired at over 1,500 K with a porosity of about 1% before glazing and porcelain which is fired at over 1,600 K and has a much finer microstructure than either stoneware or bone china. 

Ceramics and glasses are generally multicomponent sohds that are chemically bonded by ionic or covalent bonding such that there are no free electrons. Therefore, the electrical conductivity and the thermal conductivity are low and the material is brittle. If there is crystallinity the material is called a ceramic and if there is no crystallinity (i.e. the material is amorphous) the material is called a glass. Ceramics and glasses are characterized by low ductility and low fracture toughness. Some elemental materials, such as boron, carbon, and silicon, can be formed as amorphous materials, so the definitions must be taken with some exceptions. 

Polymer derived ceramics have been known for the last four decades and are prepared via solid-state thermolysis of preceramic polymers. They exhibit a unique combination of remarkable properties due to their covalent bonding and amorphous nature. Thus, silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) based ternary PDCs have been shown to possess outstanding high-temperature properties such as stability with respect to crystallization and decomposition, oxidation and corrosion resistance as well as excellent thermomechanical properties (e.g., near zero steady state creep resistance up to temperatures far beyond 1000 °C). Their properties are directly influenced by the chemistry and the architecture of the preceramic precursors, thus there is an enormous potential in tuning the microstructure and properties of the PDCs by using tailored polymers. Furthermore, suitable chemical modification of the preceramic precursors leads to quaternary and multinary ceramics, as it has been shown for instance for silicon boron carbonitride ceramics in the last 25 years, which in some cases exhibit improved properties as compared to those of the ternary materials. 

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