Definition of Ceramics

To illustrate, consider the following examples Magnesia, or MgO, is a ceramic since it is a solid compound of a metal bonded to the nonmetal O2. Silica is also a ceramic since it combines an NMES and a nonmetal. Similarly, Tie and ZrB2 are ceramics since they combine metals (Ti,Zr) and the NMES (C,B). SiC is a ceramic because it combines two NMESs. Also note ceramics are not limited to binary compounds BaTi03, YBa2Cu303, and Ti3SiC2 are all perfectly respectable class members. 

It follows that the oxides, nitrides, borides, carbides, and silicides (not to be confused with silicates) of all metals and NMESs are ceramics which, needless to say, leads to a vast number of compounds. This number becomes even more daunting when it is appreciated that the silicates are also, by definition, ceramics. Because of the abundance of oxygen and silicon in nature, silicates are ubiquitous rocks, dust, clay, mud, mountains, sand — in short, the vast majority of the earth s crust — are composed of silicate-based minerals. When it is also appreciated that even cement, bricks, and concrete are essentially silicates, the case could be made that we live in a ceramic world. 

In addition to their ubiquitousness, silicates were singled out above for another reason, namely, as the distinguishing chemistry between traditional and modern ceramics. Before that distinction is made clear, however, it is important to explore how atoms are arranged in three dimensions. 

The difference between the two is best illustrated schematically, as shown in Fig. 1.1. From the figure it is obvious that a solid possesses long-range order when the atoms repeat with a periodicity that is much greater 

A note on nomenclature The addition of the letter a to the end of an element name implies that one is referring to the oxide of that element. For example, while silicon refers to the element, silica is Si02 or the oxide of silicon. Similarly, alumina is the oxide of aluminum or AliO magnesium magnesia etc. 

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In 1925 493 university graduates were already employed in the ceramic industry. By then the American Ceramic Society had been founded and it consisted of 7 departments art, enamel, glass, fireproof materials, whiteware, terra cotta and heavy clay. Unfortunately World War II caused a decrease in the number of students of ceramics. In order to postpone the conscription of ceramic engineers an attempt was made to have ceramic studies and ceramic engineering pronounced essential for the war, to no avail however. The fact that there was no good definition of ceramics was partly to blame for this failure. 

A literature search will not result in an unequivocal definition of ceramic matrix composites or CMCs. The following definition which was presented at a symposium in Eindhoven, The Netherlands on 11 October 1994 gives us something to go by ... 

Ceramics are inorganic, nonmetallic materials. The structure of ceramics may be crystalline or partly crystalline (having intergranular amorphous phases). The definition of ceramic is often restricted to inorganic crystalline materials. 

The term ceramics refers to a broad range of materials including not only polycrystalline materials, but also powdered materials, thin films and single crystals, and glassy inorganic materials. In this section, we will use a slightly narrower definition of ceramics as polycrystalline sintered materials. 

The definition of ceramics has proved perennially difficult. Scientific or technological definitions have attempted to be accurate and exhaustive, whether based on the type of material or whether operational definitions based on the processes used to make ceramics. 

As we all know, ceramic materials are inorganic, non-metallic, solid, and inert materials. Things are made from them by the action of heat and subsequent cooling, which may be crystalline or partly crystalline. The definition of ceramic is often restricted to inorganic crystalline materials, as opposed to the noncrystalline glasses, which involve several steps of the ceramic process, and their mechanical properties behave similarly to ceramic materials. 

We define sol-gel rather broadly as the preparation of ceramic materials by preparation of a sol, gelation of the sol, and removal of the solvent The sol may be produced fi om inorganic or organic precursors (e.g., nitrates or alk-oxides) and may consist of dense oxide particles or polymeric clusters. We expand the definition of ceramics to include oi nically modified materials, often called ORMOSILs or CERAMERs [48]. 

In the Dictionary of Ceramic Science and Engineering [OBA 84], we find a rather restrictive definition of ceramics Any inorganic and non-metallic product prepared by treatment at temperatures higher than 540°C (1,000°F) or used under conditions implying these temperatures, which includes metallic oxides and borides, carbides, nitrides and mixtures of these compounds . 

This document provides definitions of the terms most commonly used in relation to sol-gel processing and ceramization. It embraces all categories of materials and their processing. The definitions result from the efforts of a working party drawn from the membership of the lUPAC Polymer and Inorganic Chemistry Divisions. 

Thermistor basedflow-through calorimetric sensors. Enzyme thermistors make the most widely developed type of heat measurement-based sensors. The thermistors are normally used as temperature transducers in these devices. Thermistors are resistors with a very high negative temperature coefficient of resistance. They are ceramic semiconductors made by sintering mixtures of metal (manganese, nickel, cobalt, copper, iron) oxides. Like the two previous groups, thermistor sensors do not comply strictly with the definition of "sensor" as they do not consist of transducers surrounded by an immobilized enzyme rather, they use a thermistor at the end of a small... 

Ion-Conducting Ceramics and Glasses. In addition to the conduction of charge via electrons, charge can be conducted via ions. Ions are present in most crystalline ceramic materials such as oxides and halides. This process is termed ionic conduction and may occur either in conjunction with or separately from electronic conduction. As a result, we must expand our definition of conductivity to include both types of conduction ... 

Calcium carbonate, mother-of-pearl, magnesite, dolomite and other inorganic materials which nature uses to provide an organism with solidity, have been categorized under the name bioceramic materials. Later some synthetic materials were added to this group. This means that the definition of a ceramic material, as given at the beginning of this book, must somewhat be adjusted as the heat treatment of a ceramic material obviously does not apply in the animal world. 

Figure 9.1 Definitions of the various species used as starting materials and condensation products during pyrolytic conversion to ceramics.

Initially, packed beds were also used. They, however, were no success, and at present monoliths are applied exclusively. This should not be misunderstood. Monolith means literally a single stone. However, metal-based analogues are also included in the definition of monolith. In fact, for catalytic converters in cars, in addition to ceramics, metal-based monoliths have been and still are used. A major advantage of metal was the thin wall thickness that could be achieved. Later, industry succeeded in manufacturing ceramic structures of comparable wall thickness. In view of their higher resistance against corrosion, ceramic monoliths are now more generally applied than metal ones. 

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