Ceramics discussions

The microwave ceramics discussed so far are prepared by sintering pellets of pressed power. Usually, the sintering temperatures are very high (typically around 1200 to 1500°C)... 

This chapter provides an overview of recent advances in our understanding of the mechanics and micromechanisms of creep-fatigue crack growth in discontinuously reinforced ceramics. (Discussions of fatigue in continuously reinforced ceramics can be found in Chapter 5 of this volume.) The chapter is arranged in the following sequence. Section 7.2 begins with a description of the... 

The oxide ceramics discussed so far in this chapter all consisted of single chemical compounds, except for minor additives. A natural idea for new ceramics is to make materials that contain two (or more) oxides in equal or nearly equal molar amounts. Thus, if BaCOs and TiOi are mixed and heated to high temperature, they react to give the ceramic barium titanate ... 

Effect of Anisotropy in Grain Shape and Conductivity. In the ceramics discussed so far, the shape and conductivity of the grains were isotropic. This, is not, however, a rule for ceramics. Two examples are given of materials whose conduction or grain structure are anisotropic. 

As with a number of the other ceramics discussed, ferrites are most often prepared by traditional processing methods using oxide and carbonate precursors of the various cations. Doping and firing atmosphere control are also frequently used, depending on the particular application, to improve specific material properties, such as resistivity. ... 

The comparison of piezoelectric properties of several of the major piezoelectric ceramics discussed above is given in Table 13.3. hi this table, PZT 4 is hard PZT (PZT doped with acceptor ions, such as K or Na at the A site, or Fe % Al % or Mn at the B site), PZT 5H is soft PZT (PZT doped with donor ions, such as La " at the A site, or Nb or Sb at the B site), LF4T is (K Na Li, ) (Nbj j Ta, jSbj f )Oy and PVDF is piezoelectric polymer synthesized using copolymerization of vini-lydene difluoride with trifluoroethylene (TrFE). 

One counter-example is the transformation toughening of ceramics discussed in section 7.5.4. 

The term brittleness has always loomed large in discussions of ceramics because it suggests serious problems which may be holding back the use of the engineering ceramics discussed in this text. Because of those... 

Although the traditional ceramics discussed previously account for the bulk of production, the development of new and what are termed advanced ceramics has begun and will continue to establish a prominent niche in advanced technologies. In particular, electrical, magnetic, and optical properties and property combinations unique to ceramics have been exploited in a host of new products some of these are discussed in Chapters 18, 20, and 21. Advanced ceramics include materials used in microelectromechanical systems as well as the nanocarbons (fullerenes, carbon nanotubes, and graphene). These are discussed next. 

Although this discussion has been in temis of molecules in solution, the same principles apply to other cases, such as precipitates in an alloy or composites of ceramic particles dispersed in a polymer. The density, p(r), is... 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

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). 

Unlike conventional ceramic materials, glass-ceramics are fully densifted with zero porosity. They generally are at least 50% crystalline by volume and often are greater than 90% crystalline Other types of glass-based materials that possess low amounts of crystallinity, such as opals and mby glasses, are classified as glasses and are discussed elsewhere (see Glass). 

Given the three key stmctural variables discussed under design, glass-ceramics can be engineered to provide a broad range of physical properties. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Some of the many ceramic materials and precursors discussed in the following sections are Hsted in Table 1. 

The observed dielectric constant M and the dielectric loss factor k = k tan S are defined by the charge displacement characteristics of the ceramic ie, the movement of charged species within the material in response to the appHed electric field. Discussion of polarization mechanisms is available (1). 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

The effective interfacial area depends on a number of factors, as discussed in a review by Charpentier [C/j m. Eng.J., 11, 161 (1976)]. Among these factors are (1) the shape and size of packing, (2) the packing material (for example, plastic generally gives smaller interfacial areas than either metal or ceramic), (3) the liquid mass velocity, and (4), for smaU-diameter towers, the column diameter. 

Another growing field is that of nonmetallic heat exchanger designs which typically are of the shell and tube or coiled-tubing type. The graphite units were previously discussed but numerous other materi- s are available. The materials include Teflon, PVDF, glass, ceramic, and others as the need arises. 

Most ceramics have enormous yield stresses. In a tensile test, at room temperature, ceramics almost all fracture long before they yield this is because their fracture toughness, which we will discuss later, is very low. Because of this, you cannot measure the yield strength of a ceramic by using a tensile test. Instead, you have to use a test which somehow suppresses fracture a compression test, for instance. The best and easiest is the hardness test the data shown here are obtained from hardness tests, which we shall discuss in a moment. 

Low-grade ceramics - stone, and certain refractories - are simply mined and shaped. We are concerned here not with these, but with the production and shaping of high-performance engineering ceramics, clay products and glasses. Cement and concrete are discussed separately in Chapter 20. We start with engineering ceramics. 

We have tried to present the material in an uncomplicated way, and to make the examples entertaining, while establishing basic physical concepts and their application to materials processing. We found that the best way to do this was to identify a small set of "generic" materials of each class (of metals, of ceramics, etc.) which broadly typified the class, and to base the development on these they provide the pegs on which the discussion and examples are hung. But the lecturer who wishes to draw other materials into the discussion should not find this difficult. 

XPS has been used in almost every area in which the properties of surfaces are important. The most prominent areas can be deduced from conferences on surface analysis, especially from ECASIA, which is held every two years. These areas are adhesion, biomaterials, catalysis, ceramics and glasses, corrosion, environmental problems, magnetic materials, metals, micro- and optoelectronics, nanomaterials, polymers and composite materials, superconductors, thin films and coatings, and tribology and wear. The contributions to these conferences are also representative of actual surface-analytical problems and studies [2.33 a,b]. A few examples from the areas mentioned above are given below more comprehensive discussions of the applications of XPS are given elsewhere [1.1,1.3-1.9, 2.34—2.39]. 

The transport of charged ions in alkali halides and, later on, in (insulating) ceramics is a distinct parepisteme, because electric fields play a key role. This large field is discussed in Schmalzried s 1995 book, already mentioned, and also in a review by one of the pioneers (Nowick 1984). This kind of study in turn led on to the developments of superionic conductors, in which ions and not electrons carry substantial currents (touched on again in Chapter 11, Section 11.3.1.1). 

A book edited by Levinson (1981) treated grain-boundary phenomena in electroceramics in depth, including the band theory required to explain the effects. It includes a splendid overview of such phenomena in general by W.D. Kingery, whom we have already met in Chapter I, as well as an overview of varistor developments by the originator, Matsuoka. The book marks a major shift in concern by the community of ceramic researchers, away from topics like porcelain (which is discussed in Chapter 9) Kingery played a major role in bringing this about. 

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