![]() ![]() They are also usually hard, brittle, and chemically inert. Intrinsically, ceramics usually have high melting points and are therefore generally described as refractory. In contrast, many of the properties critical to the engineering applications of materials are strongly dependent on the microstructure (e.g., mechanical strength, dielectric constant, and electrical conductivity). The intrinsic properties are determined by the structure at the atomic scale and are properties that are not susceptible to significant change by modification of the microstructure, properties such as the melting point, elastic modulus, coefficient of thermal expansion, and whether the material is brittle, magnetic, ferroelectric, or semiconducting. It is sometimes useful to distinguish between the intrinsic properties of a material and the properties that depend on the microstructure. The second type of structure is at a larger scale: the microstructure, which refers to the nature, quantity, and distribution of the structural elements or phases in the ceramic (e.g., crystals, glass, and porosity). The first type of structure is at the atomic scale: the type of bonding and the crystal structure (for a crystalline ceramic) or the amorphous structure (if it is glassy). In ceramics as well as in metals, we are concerned with two types of structure, both of which have a profound effect on properties. The difficulty and expense of growing single crystals means that, normally, crystalline ceramics (and metals) are actually polycrystalline-they are made up of a large number of small crystals, or grains, separated from one another by grain boundaries. ![]() Structurally, all materials are either crystalline or amorphous (also referred to as glassy). There are also compounds based on nitride–oxide or oxynitride systems (e.g., ′-sialons with the general formula Si6-zAlzN8-zOz, where 0 ⬍ z ⬍ ⬃4). (B4C), nitrides such as silicon nitride (Si3N4) and boron nitride (BN), borides such titanium diboride (TiB2), silicides such as molybdenum disilicide (MoSi2) and halides such as lithium fluoride (LiF). In addition, there are nonoxides including carbides such as silicon carbide (SiC) and boron carbide 1 Examples are the silicates such as kaolinite and mullite (Al6Si2O13), simple oxides such as alumina (Al2O3) and zirconia (ZrO2), complex oxides other than the silicates such as barium titanate (BaTiO3), and the superconducting material YBa2Cu3O6Ⳮ␦ (0 ␦ 1). Chemically, with the exception of carbon, ceramics are nonmetallic, inorganic compounds. There is much to be gained through continued interaction between the traditional and the advanced sectors. Although the distinction between traditional and advanced ceramics may be referred to in this book occasionally for convenience, we do not wish to overemphasize it. Advanced ceramics include ceramics for electrical, magnetic, electronic, and optical applications (sometimes referred to as functional ceramics) and ceramics for structural applications at ambient as well as at elevated temperatures (structural ceramics). ![]() Whereas traditional ceramics still represent a major part of the ceramics industry, the interest in recent years has focused on advanced ceramics, ceramics that with minor exceptions have been developed within the last 50 years or so. They are pottery, structural clay products, and clay-based refractories, with which we may also group cements and concretes and glasses. Traditional ceramics bear a close relationship to those materials that have been developed since the earliest civilizations. The use of the term advanced has, however, not received general acceptance and other forms including technical, special, fine, and engineering will also be encountered. Recent attempts have been made to divide it into two parts: traditional ceramics and advanced ceramics. 1 Ceramic Fabrication Processes An Introductory Overviewġ.1 INTRODUCTION The subject of ceramics covers a wide range of materials. ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |