Non Crystalline Ceramics

NON CRYSTALLINE CERAMICS

These are usually regarded super, cooled liquids. Their molecules are not arranged in regular geometric shapes. e.g. amorphous or fused SiO₂ has each Si bonded to four O and each O is bonded to two Jairo (S)

This type of ceramics is used for mirrors, optical lenses, reinforcement fibres for GRP and optical fibres for data transmission. anoites llama

Silicates and Silica

Silicates are composed of silicon and oxygen, which are abundantly available in the earth's crust. For example, rocks, soils and clay come under the classification of silicates.

A unit cell of silicate is a tetrahedron on which each atom of silicon is bounded to four atoms of oxygen as shown in fig 4.18. over of abnuo 02

Oxygen atoms are located on the edges of a tetrahedron structure and silicon atoms are located at the centre.

This basic unit of silicate is treated as negatively charged. There is a covalent bond between Si and O, i.e. Si-O. 

Silica

Silica (SiO₂) is the simple form of silicate. This is a three-dimensional network of tetrahedron where every corner oxygen atom is shared by adjacent tetrahedral. This material becomes electrically neutral but al but electronically stable.

Under this arrangement, the ratio of Si to O becomes 1:2 as given by chemical formula, SiO₂.

There are three polymorphic forms of silica: (1) quartz, (2) cristobalite and (3) tridymite. Silica is used in the manufacture of different varieties of glasses.

Structure of glasses

Generally, solids have three-dimensional periodic structures as shown in figure 4.19. This is a crystalline structure.

The materials, which do not have three-dimensional structures, but random structure as shown in figure 4.20 are said to be amorphous or glassy.

Many metal alloys, oxide compounds and non-oxide compounds form glassy structure. Fused silica or vitreous silica has high degree of atomic randomness. Similarly, oxide as B₂O₃ and GeO₂ may also form glassy structure.

The glasses that are used for containers and windows are silica glasses in which oxides such as CaO and Na₂O are added.

The thickness of solid layer depends on the length of time in mold. This process is continued until the entire mold cavity becomes solid. This process is known as slip casting and the various stages are shown in Fig. 4.21.

Advantages:

The main advantage of slip casting is the ability to form intricate shapes at relatively low cost.

The complex ceramic shapes which are produced using slip casting include turbine engine rotors, automobile wings, etc.