Haseeb Ullah Khan Jatoi Department of Chemical Engineering UET Lahore

Greek word Keramikos which means “Burnt Stuff” indicating that desired properties of these materials are normally achieved through a high temperature treatment. Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. For example, some of the common ceramic materials include aluminum oxide (or alumina,Al 2 O 3 ), silicon dioxide (or silica, SiO 2 ), silicon carbide (SiC), silicon nitride (Si 3 N 4 ). The traditional ceramics are composed of clay minerals such as porcelain, cement, and glass.

PROPERTIES Ceramic materials are relatively stiff and strong—and comparable to those of the metals. Very hard. Extremely brittle and are highly susceptible to fracture. Insulator of heat and electricity and are more resistant to high temperatures and harsh environments than metals and polymers.

Typical Ceramic Materials

GLASSES A familiar group of ceramics; containers, lenses, and fiberglass are typical applications. They are non-crystalline silicates containing other oxides, notably CaO, Na 2 O, K 2 O, and Al 2 O 3, which influence its properties. A typical soda–lime glass consists of approximately 74 wt% SiO 2, the balance being mainly Na 2 O (soda) and CaO (lime). They may be fabricated with ease.

The viscosity of glasses varies with temperature according to Arrhenius Type Relationship μ = A exp (B/T) A and B are constants, T is temperature, μ is viscosity. Creep rate equation is d ε /dt = B σ n n = 10 for glasses.

Most inorganic glasses can be made to transform from a non-crystalline state to crystalline state by the proper high- temperature heat treatment. This process is called crystallization. The product is a fine-grained polycrystalline material which is called a glass–ceramic. The most common uses for these materials are as ovenware, tableware, oven windows, and cooking range tops primarily because of their strength and excellent resistance to thermal shock.

CLAY PRODUCTS One of the most widely used ceramic raw materials is clay. Inexpensive ingredient, found naturally in great abundance and ease with which clay products may be formed; when mixed in the proper proportions, clay and water form a plastic mass that is very amenable to shaping. The formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength.

Most of the clay-based products fall within two broad classifications: Structural clay products include building bricks, tiles, and sewer pipes. White ware ceramics become white after the high-temperature. e.g. porcelain, pottery, tableware, china, and plumbing fixtures.

A refractory material is one that retains its strength at high temperatures. They are important for their capacity to withstand high temperatures without melting or decomposing, and the capacity to remain unreactive and inert when exposed to severe environments. Able to provide thermal insulation Typical applications include furnace linings for metal refining, furnaces, kiln and reactor. Glass manufacturing, metallurgical heat treatment, and power generation.

Performance of a refractory ceramic, to a large degree depends on its composition. Porosity is one micro structural variable that must be controlled to produce a suitable refractory brick. Strength, load-bearing capacity, and resistance to attack by corrosive materials all increase with porosity reduction.

Fireclay Refractories The primary ingredients for the fireclay refractories are high-purity fireclays, alumina and silica mixtures usually containing between 25 and 45 wt% alumina. Fireclay bricks are used principally in furnace construction, to confine hot atmospheres, and to thermally insulate structural members from excessive temperatures. Highest temperature it can withstand is 1587 ˚C

Acid or Silica Refractories The prime ingredient for silica refractories is silica, sometimes termed acid refractories. These materials, well known for their high- temperature load-bearing capacity, are commonly used in the roofs of steel- and glass- making furnaces; for these applications, temperatures as high as 1650˚C may be realized. Basic raw material is Ganister (sand stone) and Quartzite (mineral rock)

Basic Refractories The refractories that are rich in magnesia (MgO), are termed basic; they may also contain calcium, chromium, and iron compounds. Find extensive use in some steel-making open hearth furnaces. temperatures as high as ˚C may be realized. Basic raw material is Dolomite {carbonate mineral Ca Mg(Co 3 ) 2 }

One chief concern in the application of ceramic materials is the method of fabrication. FABRICATION AND PROCESSING OF GLASSES AND GLASS–CERAMICS Glassy, or non-crystalline, materials do not solidify in the same sense as do those that are crystalline. Upon cooling, a glass becomes more and more viscous in a continuous manner with decreasing temperature; there is no definite temperature at which the liquid transforms to a solid as with crystalline materials. One of the distinctions between crystalline and non- crystalline materials lies in the dependence of specific volume on temperature.

For crystalline materials, there is a discontinuous decrease in volume at the melting temperature Tm However, for glassy materials, volume decreases continuously with temperature reduction; a slight decrease in slope of the curve occurs at what is called the glass transition temperature, or fictive temperature Tg, Below this temperature, the material is considered to be a glass; above, it is first a super cooled liquid, and finally a liquid. Glass Transition Temperature. It is a temperature at which the viscosity is and viscous flow ceases.

Logarithm of viscosity versus temperature

Melting Point It is the temperature at which the viscosity is 10 Pa-s (100 P); the glass is fluid enough to be considered a liquid. Working Point It is the temperature at which the viscosity is 10 3 Pa-s ( 10 4 P); the glass is easily deformed at this viscosity. Softening Point It is the temperature at which the viscosity is 4*10 6 Pa-s (4*10 7 P), is the maximum temperature at which a glass piece may be handled without causing significant dimensional alterations.

Annealing Point It is the temperature at which the viscosity is Pa-s (10 13 P); at this temperature, atomic diffusion is sufficiently rapid that any residual stresses may be removed within about 15 min. Strain Point The strain point corresponds to the temperature at which the viscosity becomes 3 *10 13 Pa-s ( 3 * P); for temperatures below the strain point, fracture will occur before the onset of plastic deformation. The glass transition temperature will be above the strain point.

Glass is produced by heating the raw materials to an elevated temperature above which melting occurs. It is essential that the glass product be homogeneous and pore free. Homogeneity is achieved by complete melting and mixing of the raw ingredients. Porosity results from small gas bubbles that are produced; these must be absorbed into the melt or otherwise eliminated. Four different forming methods are used to fabricate glass products: pressing, blowing, drawing, and fiber forming

Heat Treating Glasses Annealing When a ceramic material is cooled from an elevated temperature, internal stresses, called thermal stresses, may be introduced as a result of the difference in cooling rate and thermal contraction between the surface and interior regions. These thermal stresses are important in brittle ceramics, especially glasses, since they may weaken the material or, in extreme cases, lead to fracture, which is termed thermal shock. Normally, attempts are made to avoid thermal stresses, which may be accomplished by cooling the piece at a sufficiently slow rate.

Once such stresses have been introduced, however, elimination, or at least a reduction in their magnitude, is possible by an annealing heat treatment in which the glassware is heated to the annealing point, then slowly cooled to room temperature. Glass Tempering The strength of a glass piece may be enhanced by intentionally inducing compressive residual surface stresses. This can be accomplished by a heat treatment procedure called thermal tempering. Tempered glass is used for applications in which high strength is important; these include large doors and eyeglass lenses. Used as a safety glasses

Glass is heated to the temperature of more than 600˚C. The glass then undergoes a high-pressure cooling procedure called "quenching." During this process, which lasts just seconds about 3 to 10 seconds, high-pressure air blasts the surface of the glass. Quenching cools the outer surfaces of the glass much more quickly than the center. As the center of the glass cools, it tries to pull back from the outer surfaces. As a result, the center remains in tension, and the outer surfaces go into compression, which gives tempered glass its strength.