Introduction
An understanding of phase equilibria in ceramic systems is central to the utilization and development of materials in refractories, glass, and other high temperature technologies. Phase equilibria address significant questions related to the flexibility and constrains, dictated by forces of nature, on the evolution of phase assemblages in ceramic. Phase boundaries also assist in the evaluation of the service stability of a ceramic materials, both in the long and short time frames. Thus, knowledge of the stability of a ceramic or glass component in high temperature or in high pressure environments can often be obtained from an appropriate stable or metastable phase diagram. In the processing and manufacture of ceramic products, the reactions which occur are understood more clearly if the phase relations under equilibrium conditions are known. The chemical and physical properties of ceramic products are related to the number, composition, and distribution of the phases present. Temperature, pressure, and concentration are the principal variables which determine the kinds and amounts of the phases present under equilibrium conditions. To ceramists, who must understand the effects of these variables on both the processing and the properties of the finished product, the phase equilibrium relations (usually presented in the form of phase diagrams) provide the necessary fundamental information. The study of phase relations is based on the assumption that the system under consideration is at equilibrium. In the development of reliable information on phase relations, this condition must be satisfied. In a practical sense, however, as in the manufacture or service of a ceramic product, circumstances may not permit a condition of equilibrium to be established. In many cases, it is known that the system is driving toward or approaching equilibrium, and knowledge of the direction in which the reaction is progressing or the direction by which it deviates from equilibrium can be of great value. In some instances involving ceramic processing, the approach to equilibrium actually may be quite close. The progress of a ceramic system toward its stable equilibrium state can often be halted for kinetic reasons, resulting in a phase assembly which can persist metastably for an extended period. The arrest of the equilibrium phases, either inadvertently or by deliberate processing, has given rise to some useful new materials in recent years. Thus, a study of stable and metastable phase equilibrium relations is particularly relevant to ceramic and glass compositions. While most phase equilibrium diagrams have been and continue to be determined by experimental laboratory techniques, there is a growing trend toward calculation of multicomponent equilibria from thermodynamic data. The validity of many classic ceramic phase equilibrium diagrams, while not basically in doubt, continues to be questioned and revised because the experimental techniques and interpretation of data can vary from one study to another. Nevertheless, the student needs to be aware of both experimental and theoretical methods of determining phase diagram. The principles of thermodynamics are at the core of much important phase equilibrium information. It is thus appropriate for us to begin with a brief review of the definitions and principles of thermodynamics that pertain to phase relations. (Bergeron and Risbud, 1984).
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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