Network solids ceramics

Network solids are typically hard and rigid they have high melting and boiling points. Ceramic materials are commonly network solids. 

All metals conduct electricity on account of the mobility of the electrons that bind the atoms together. Ionic, molecular, and network solids are typically electrical insulators or semiconductors (see Sections 3.f3 and 3.14), but there are notable exceptions, such as high-temperature superconductors, which are ionic or ceramic solids (see Box 5.2), and there is currently considerable interest in the electrical conductivity ol some organic polymers (see Box 19.1). 

Silicon nitride (Si3N4), a high-temperature ceramic useful for making engine components, is a covalent network solid in which each Si atom is bonded to four N atoms and each N atom is bonded to three Si atoms. Explain why silicon nitride is more brittle than a metal like copper. 

Silicates are network solids containing Si—O—Si bridges that form the basis for many rocks, clays, and ceramics... 

Most metals are malleable, which means that they can be hammered into thin sheets, and ductile, which means that they can be drawn into wires ( FIGURE 12.10). These properties indicate that the atoms are capable of slipping past one another. Ionic and covalent-network solids do not exhibit such behavior. They are typically brittle and fracture easily. Consider, for example, the difference between dropping a ceramic plate and an aluminum cooking pan onto a concrete floor. 

Solution (a) Si3N4 is a ceramic material. We can think of it as analogous to silicon carbide, a very hard covalent-network solid (Section 11.8). It should have high -melting and boiling points and be very hard (Table 12.4). Because Si and N are both nonmetals, the bonding between them should be polar covalent. 

Silicon nitride (Si3N4) has long been considered a promising material for use in ceramic gas turbines. A hard network solid ( 9 on the Mohs scale), Si3N4 features a structure in which each nitrogen atom is surrounded by three silicon atoms, and each silicon atom is tetrahedrally surrounded by four nitrogen atoms. 

Sols arc dispersions of colloidal particles in a liquid. Colloids arc nanoscaled entities dispersed in a fluid. Gels are viscoelastic bodies that have interconnected pores of submicrometric dimensions. A gel typically consists of at Icasl two phases, a solid network that entraps a liquid phase. Sol-gel technology is the preparation of ceramic, glass, or composite materials by the preparation of a sol, gelation of the sol. and removal of the solvent. 

Before and after the works described above, contributions to the design and fabrication of similar multicomponent films or gels of cholesteric character, mainly based on HPC, EC, or their derivatives were also made [202, 219-224], Some of these [219,220,224] dealt with shear-deformed network systems preserving a unique banded structure, so that the disappearance and recovery of the optical anisotropy could be controlled thermo-reversibly. Special mention should be made of the successful preparation of two novel classes of solid materials maintaining cholesteric liquid-crystalline order. One consists of essentially pure cellulose only, and the other is a ceramic silica with an imprint of cellulosic chiral mesomorphy. 

Finally, ceramic composites belonging to the ternary Si—C—N system are of high technical relevance. Crystalline solids of composition SiQN,25 have a polymeric network structure they are made according to ... 

The earliest routes for forming ceramics from sol-gel solutions involved the precipitation of metal oxide particles from solutions. These form a true colloidal suspension a sol. Upon destabilization of this sol, aggregation takes place and a rigid network is formed a gel. A gel is intermediate between a solid and a liquid. The term sol—gel has since been used by the materials science community to describe, albeit erroneously, virtually all chemical processing of ceramics from solutions (e.g., metal oxide particle precipitation or metalorganic decomposition). This discussion focuses on the gel aspects of sol—gel synthesis and not on the sol aspects, which are treated separately in this book. 

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). 

Figure 2. (a) The surface of an alumina ceramic from which all porosity has been removed during sintering the microstructure consists of grains and the boundaries between them, (b) The sintering of silicon results in the formation of a continuous network of solid material (white) and porosity (black). The microstructural change is not accompanied by any densification. (Reprinted from R. J. Brook, ed., Concise Encyclopedia of Advanced Ceramic Materials, eopyright ( 1991, p. 4, with kind permission from Elsevier Science Ltd.)... 

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