Properties of Ceramics

Ceramics offer stiffness, hardness, resistance to wear, and resistance to corrosion (particularly by oxygen and water), even at high temperature. They are less dense than most metals, which makes them desirable metal substitutes when weight is a factor. Most are good electrical insulators at ordinary temperatures, a property that is exploited in electronics and power transmission. Ceramics retain their strength well at high temperatures. Several important structural metals soften 

One important property of ceramics is their porosity. Porous ceramics have small openings into which fluids (typically air or water) can infiltrate. Fully dense ceramics have no channels of this sort. Two ceramic pieces can have the same chemical composition but quite different densities if the first is porous and the second is not. 

FIGURE 22.7 Microstructures of aluminosilicate ceramics, viewed by the different colors of light emitted after bombardment by electrons. 

The raw materials for traditional ceramics are natural clays that come from the earth as powders or thick pastes and become plastic enough after adjustment of their water content to be formed freehand or on a potter s wheel. Special ceramics (both oxide and nonoxide) require chemically pure raw materials that are produced synthetically. Close control of the purity of the starting materials for these ceramics is essential to produce finished pieces with the desired properties. In addition to being formed by hand or in open molds, ceramic pieces are shaped by the squeezing (compacting) of the dry or semidry powders in a strong, closed mold of the desired shape, at either ordinary or elevated temperatures (hot pressing). 

Brittleness and ductiUty are not intrinsic properties of materials. The evaluation of the degree of brittleness or ductility of a material is associated to the relationship existing between the characteristic speed of the processes (kinetic) and the duration of observation, and depends on the temperature. 

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Table 11. Physical Properties of Ceramic and Cemented-Carbide Cutting Tools ...

Properties and Mature of Bonding. The metaUic carbides are interesting materials that combine the physical properties of ceramics (qv) with the electronic nature of metals. Thus they are hard and strong, but at the same time good conductors of heat and electricity. 

As before, the data presented here are approximate, intended for the first phase of design. When the choice has narrowed sufficiently, it is important to consult more exhaustive data compilations (see Further Reading) and then to obtain detailed specifications from the supplier of the material you intend to use. Finally, if the component is a critical one, you should conduct your own tests. The properties of ceramics are more variable than those of metals the same material, from two different suppliers, could differ in toughness and strength by a factor of two. 

In this chapter we examine the mechanical properties of ceramics and, particularly, what is meant by their "strength". 

R. W. Davidge, Mechanical Properties of Ceramics, Cambridge University Press, 1979. 

While mechanical properties of ceramic materials are usually quite adequate for the duties which they have to perform, it is essential to realise the limitations of the material, and to design and install any articles made from it in such a way as to minimise any weakness. Table 18.12 gives typical values for the mechanical properties of the different materials which are available. 

Recently, interesting composite materials incorporating polymeric materials into the sol-gel glasses have been reported by Wilkes and his co-workers [9]. These materials are named ceramers . The properties of ceramers strongly depend on the reaction conditions, i.e., acidity, water content, reaction temperature, the amount of organic polymer, the molecular weight of polymer, solvent, and so on. 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

Crockery preferably is made from ceramic materials, although it is brittle and can break rather easily. Properties of ceramics, such as resistance to absorb flavours and low heat conductivity, however, are superior compared to metals and plastics. Therefore, we defined a task to design crockery with improved mechanical strength. 

The expert tried to optimise the design of unbreakable crockery. First he wanted to find reasons for using ceramic as the main material in terms of desired properties. From this first step, he concluded that ceramic had some advantages over metals or composites. The expert made a sharp distinction between intrinsic and extrinsic properties. The choice of type of ceramics was not relevant becanse the desired properties are extrinsically determined. Thns relevant properties cannot be much influenced by the difference in bonding strength due to the different types of ions of the material. Consequently, the properties of ceramic crockery are not mnch inflnenced by the actual choice of ceramic material. Because of this, the expert did not inclnde the ionic stmcture in his reasoning. When he was asked why he did not nse this snb-microscopic level, he explained it was not necessary becanse this [the desired properly] is not imdeigoing influences at atomic level at all . 

With the following example, we illustrate how in a sequence of activities the students intuitive notions about the influence of particle size and the sintering temperature of the clay on the properties of ceramic materials have productively been used (Klaassen Lijnse, 1996 Mortimer Scott, 2003 Duit Treagust, 2003 ... 

Common ancient ceramic materials often found in archaeological excavations, such as fired brick and pottery, were made mostly from a mixture of a secondary clay and fillers. The nature, composition, and properties of clay have been already discussed the nature of the fillers, the changes undergone by the clay as well as by the fillers during their conversion to ceramics, and the unique properties of ceramic materials, are reviewed in the following pages. Attention is drawn also to studies that provide information on the composition and characteristics of ancient ceramic materials. 

Table 5.4 Intrinsic semiconducting properties of ceramic oxides...

Zhou X, Ma J, Deng F, Meng G, and Liu X. Preparation and properties of ceramic interconnecting materials, I.a07Ca0 Cr() n doped with GDC for IT-SOFCs. J. Power Sources 2006 162 279-285. 

As we have mentioned, the piezoelectric properties of ceramics are generated by a poling process. Apparently, if a strong electric field in a direction other than the poling direction is applied, the piezoelectric property is altered or lost. The safe value of an ac field to avoid causing depoling, Ej, can be found in the product specifications. 

The creep properties of ceramics are of particular importance, especially at high temperatures. In general, the principles of Section 5.2.2 still apply, but the ceramic... 

Most of the important magnetic ceramics are of the ferrimagnetic class. However, some ceramics do exhibit other types of magnetic behavior. These ceramic materials will be described first, followed by a more thorough description of an important class of ferrimagnetic ceramics called ferrites. Finally, a topic related to the magnetic properties of ceramic superconductors will be introduced. 

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