Ceramic bodies

Figure C2.11.4. A commercial spark plug witli its electrically insulating ceramic body comprised of alumina and glass (white portion).

Morse T 1979 Handbook of Organic Additives for Use in Ceramic Body Formuiation (Butte, MA Montana Energy and MHD Research and Deveiopment institute)... 

Acryhc modifiers for cement impact strength and adhesion to substrates are discussed in reference 211. Both water-soluble acryhc and acryhc emulsion polymers are used in the ceramic industry as temporary binders, deflocculants, and additive components in ceramic bodies and glazes (212) (see Ceramcs). 

Deteriora.tlon. Ceramic objects are fragile, and mechanical damages through breakage and abrasions are the most likely source of destmction. Low fired ceramics can suffer through the rehydration of the body material this process results ia a complete loss of mechanical streagth. The preseace of soluble salts ia porous ceramic bodies has the same disastrous results as ia stoae (136). 

Filtered-Particle Inspection. Solids containing extensive inteiconnected porosity, eg, sintered metallic or fired ceramic bodies formed of particles that ate typically of 0.15-mm (100-mesh) screen size, are not inspectable by normal Hquid penetrant methods. The preferred test medium consists of a suspension of dyed soHd particles, which may be contained in a Hquid vehicle dyed with a different color. Test indications can form wherever suspensions can enter cracks and other discontinuities open to the surface and be absorbed in porous material along interior crack walls. The soHd particles that form test indications ate removed by filtration along the line of the crack at the surface where they form color or fluorescent indications visible under near-ultraviolet light (1,3). 

Mixing (24,39) is used to combine the constituents of a ceramic body to produce a more chemically and physically homogeneous system for forming (see... 

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

The processes employed in manufacturing a ceramic are defined and controlled to produce a product with properties suited to a specific appHcation. Processing—microstmcture—property relationships are deterrnined by characterizing the ceramic raw materials, mixes, and the formed ceramic body intermittently during processing and after final thermal consoHdation. It is possible to modify and optimize processes to optimize properties and to identify and correct processing deficiencies when less than optimal properties are obtained. Examples of some process—microstmcture—property relations in advanced ceramics are outlined in Figure 4. 

Characterization. Ceramic bodies are characterized by density, mass, and physical dimensions. Other common techniques employed in characterizing include x-ray diffraction (XRD) and electron or petrographic microscopy to determine crystal species, stmcture, and size (100). Microscopy (qv) can be used to determine chemical constitution, crystal morphology, and pore size and morphology as well. Mercury porosknetry and gas adsorption are used to characterize pore size, pore size distribution, and surface area (100). A variety of techniques can be employed to characterize bulk chemical composition and the physical characteristics of a powder (100,101). 

There are a number of ways to obtain color in a ceramic material (1). First, certain transition-metal ions can be melted into a glass or dispersed in a ceramic body when it is made. Although suitable for bulk ceramics, this method is rarely used in coatings because adequate tinting strength and purity of color caimot be obtained this way. 

The addition of oxides to ceramic bodies and to glasses to produce color has been known since antiquity (2). The use of iron and copper oxides predates recorded history. Cobalt was introduced into Chinese porcelain about 700 AD. Chromium compounds have been used since 1800 AD. 

Using 2eohte catalysts, the NO reduction takes place inside a molecular sieve ceramic body rather than on the surface of a metallic catalyst (see Molecularsieves). This difference is reported to reduce the effect of particulates, soot, SO2/SO2 conversions, heavy metals, etc, which poison, plug, and mask metal catalysts. ZeoHtes have been in use in Europe since the mid-1980s and there are approximately 100 installations on stream. Process applications range from use of natural gas to coal as fuel. Typically, nitrogen oxide levels are reduced 80 to 90% (37). 

Masse-schlamm, m. Ceram.) body slip, -schlicker, m. Ceram.) body paste. 

Scherbe, /. potsherd, shard fragment, piece crock, pot (esp. fiowerpot) cupel scorifier (Ceram.) body. 

This property is very hard to define, as articles may be subjected to very varied forms of abrasion, and in general a given ceramic body will react quite differently to different types of abrasion. This is a question on which the manufacturer should be able to give considerable guidance. Many types of standard abrasion test have been proposed, but none has proved satisfactory and experience must continue to be the main guiding factor. 

If the temperature and supersaturation are sufficiently high in a CVD reaction, the product is primarily powder precipitated from the gas phase (see Ch. 2). Such powders have few impurities provided that the CVD precursors are carefully purified. Their small diameter and great uniformity are important factors in the production of high quality hot-pressed or sintered ceramic bodies with good mechanical and electrical properties. In addition, the sintering temperatures required for CVD powders are lower than those for conventional powders. 

Applications. CVD ceramic powders such as SiC and Si3N4 are used to produce ceramic bodies for a wide variety of applications, either experimentally or in production. These include structural applications in high temperature or corrosive environments where metals are not suitable, in such areas as reciprocating engines, gas turbines, turbochargers, bearings, machinery, and process equipment. 

Uniform microstractuie is cracial to the superior performance of advanced ceramics. In a cerantic material, atoms are held in place by strong chentical bonds that ate impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concentrate at minute imperfections in the microstmcture, initiating a crack. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack propagates throughout the material causing a catastrophic brittle failure of the ceramic body. The rehability of a ceramic component is directly related to the number and type of imperfections in its microstmcture. 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

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