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WHY CERAMICS AND WHICH ONES

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الكلية كلية هندسة المواد     القسم قسم هندسة المعادن     المرحلة 2
أستاذ المادة علي هوبي حليم الخزرجي       20/12/2015 04:09:44
The first decision to be made in selecting material candidates for an application is to determine which types of materials to consider. This commonly entails both fabrication and cost issues discussed below, especially where ready availability is desired or required, and significant development is not realistic. However, a basic question for many needs, especially longer term ones, is, What material candidates have the best intrinsic property potential to meet the requirements true for ceramics and ceramic composites, since there is such a diversity of materials and properties, with much of their potential partially or substantially demonstrated, but often untapped. This potential arises from both the extremes diversity of ceramic materials. Perspective on diversity can be obtained by remembering that solid materials can be divided into nominally single-phase materials that are polymeric (mainly plastics or rubbers), metallic, or ceramic, or into two- or multiphase composites of constituents from any one of the three basic single-phase materials, or combinations of two or three of the single-phase materials. Ceramics, or more specifically monolithic ceramics, are thus defined as nominally single-phase bodies that are not composites nor metals or polymers. While this includes a few elemental materials such as sulfur, or much more importantly, the various forms of carbon, the great bulk and diversity of ceramics are chemical compounds of atoms of one or more metallic elements with one or more metalloid or nonmetallic elements.
The more developed ceramics are mostly compounds of two types of atoms, that is, binary compounds, which are typically classified by the nonmetallic or metalloid anion element they contain—for example, compounds of metals with nonmetals, such as oxides and non-oxides, the latter including borides, carbides, halides, nitrides, silicides, and sulfides.. However, there is a variety of known ternary ceramic compounds formed with a third atom constituent. Those that contain either two metallic and one metalloid or nonmetallic types of atom continue to be classified as carbides, oxides, etc., as for binary ceramics. However those containing one type of metallic atom with two types of atoms of either metalloid or nonmetallic designation or a combination of one of each, are named by their latter atoms, e.g., as carbonatites and oxysilicides, for compounds containing carbon and nitrogen or oxygen ceramic compounds consisting of four or more atomic constituents that are generally much less known. Such higher-order compounds offer opportunity for extending ceramic technology via more diverse properties.
The diversity of ceramics and their properties is significantly extended by the fact that the properties of a given ceramic compound can be varied, often substantially,
by changing microstructure via differences in fabrication/processing, which is extensively discussed elsewhere [11,12]. The diversity is also significantly extended by addition of one or more other ceramic compounds that form a solid solution with the b as ceramic compound. The limitations of such solid solution extension of properties are the limits of solubility due either to precipitation or reaction, or both. However, these limits on solubility also provide more specialized ways of extending the range of ceramic properties via ceramic composites, i.e., ceramic bodies consisting of two or more ceramic phases that have limited or no mutual solubility and a considerable range of chemical compatibility.
More extensively, ceramic composites are made by consolidating mixtures of composite phases, which are classified by the character of the additional, usually second, phase, that is, particulate, whisker, platelet, or fiber composites, which are addressed in this book, generally in decreasing extent in the order listed. The resultant diversity of ceramic properties from all of the ceramic compounds, their solid solutions, and composites is illustrated in part by a much-abbreviated listing of some properties of the more refractory members of the more common and more extensively developed binary ceramic materials in Table 1.1. Note that other binary systems have refractory compounds, for example, sulfides and phosphides with melting points of 2000 to 2500-2700°C, and many systems with compounds having melting points of 1500-2200°C or above. Also, note that, while ternary and higher-order compounds typically have lower melting temperatures than the more refractory binary compounds, this is not always true.
The property diversity of ceramics is further shown by the following observations addressing the six categories of functional properties: (1) thermal chemical, (2) mechanical, (3) thermal conduction, (4) electrical, (5) magnetic, and (6) electromagnetic. Thus, there are a number of ceramics that have among the highest potential operating temperatures, approaching their melting points at and above those of their only other competitors, the refractory metals (Table 1.1), and have the highest energies for ablation, especially in the absence of melting. Further, the diversity of ceramic compositions provides candidates for sulfides for sulfur environments, as well as the formation or application of at least partially protective coatings that are chemically compatible with the ceramic substrate.
Considering mechanical performance, many ceramics have high stiffness and high melting points, reflecting the strong atomic bonding. While stiffness generally decreases with increasing temperature, as for other materials, it is typically an important attribute of many ceramics across the temperature spectrum.
High bond strengths of many refractory ceramics also correlates with their high hardness s, which tends to correlate some with armor performance and especially with much wear and erosion resistance, as well as with compressive strengths that can also be of importance at high temperatures, but are typically more important at modest temperatures [2, 11, and 12]. Tensile strengths, though being particularly sensitive to microstructural and thus to fabrication process parameters, also correlate in part with elastic moduli, and can be quite substantial over a broad range of temperatures. Also, note that some ternary compounds (such as mulita and perhaps higher-order compounds) can have much higher creep resistance than their more refractory binary constituents. At the extreme of mechanical precision, many ceramics offer the highest degrees of precision elastic stability, i.e., dimensional stability under mechanical and thermal loading, which is typically most pronounced and important at modest temperatures [12].
Different ceramics have among the lowest intrinsic thermal conductivities and others the highest ranges of thermal conductivities, with even more extremes shown for electrical conductivity or resistivity. This includes both highest-temperature ternary and higher-oxide superconductors of extensive interest for about the past 10 years. Some ceramics also have the highest resistance to dielectric breakdown, hence the ability to be good insulators even under very high electrical fields, as well as other important electrical properties [2, 11, and 12]. These include high-temperature semiconductors for a variety of applications and ionic conductors for diverse applications, such as advanced fuel cells and batteries, as well as sensors. Of particular importance for many technological applications, which like many other properties, are most often of particular importance at or near room and moderate temperatures. This is commonly also the case for their important magnetic and electromagnetic properties, but elevated temperature Performance of such functions can also be important. While a single property may drive applications, unique combinations of properties are commonly an important factor. Thus, for example, good magnetic properties in nonconductive, i.e., dielectric, ceramics is an important factor in their magnetic applications, while application of the transparency of dielectric ceramics to ultraviolet (UV), visible, infrared (IR), microwave, and other electromagnetic waves is often made due in part to the temperature capabilities of many ceramics. These and other applications are also often partly driven by the substantial hardness s of many ceramics as reflected in their resistance to wear, erosion, and ballistic impact, for example, for transparent armor windows.


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