why study structures and properties of ceramics?
some of the properties of ceramics may be explained by their structures. for example: (a) the optical transparency of inorganic glass materials is due, in part, to their noncrystallinity (b) the hydropinglasticity of clays( i.e., development of plasticity upon the addition of water) is related to interactions between water molecules and the clay structures (c) the permanent magnetic behaviors of some ceramic materials are explained by their crystal structures .
ceramic structures:
because ceramics are composed of at least two elements, and often more, their crystal structures are generally more complex than those for metals. the atomic bonding in these materials ranges from purely ionic to totally covalent many ceramics exhibit a combination of these two bonding types, the degree of ionic character being dependent on the electronegativities of the atoms. table 12.1 presents the percent ionic character for several common ceramic materials these values were determined using equation below and the electronegativities in figure 2.7.
electronegativity a measure of how willing atoms are to accept electrons (subshells with one electron low electronegativity subshells with one missing electron high electronegativity). electronegativity increases from left to right. the atomic bonding in ceramics is mixed, ionic and covalent, the degree of ionic character depends on the difference of electronegativity between the cations (+) and anions (-).
ceramic crystal structures:
ceramics that are predominantly ionic in nature have crystal structures comprised of charged ions, where positively charged (metal) ions are called cations, and negatively charged (non-metal) ions are called anions. the crystal structure for ceramics depends upon two characteristics:
1. the magnitude of electrical charge on each component ion, recognizing that the over all structure must be electrically neutral.
2. the relative size of the cation(s) and anion(s),which determines the type of interstitial site(s) for the cation(s) in an anion lattice.
stable ceramic crystal structures: anions surrounding a cation are all in contact with that cation. for a specific coordination number there is a critical or minimum cation-anion radius ratio rc/ra for which this contact can be maintained.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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