Procedure for studying the structure of aluminum oxide ceramics

In the Soviet Union and other countries, ceramic materials having a predetermined permittivity and a small tangent of the angle of dielectric losses in the range of the centimeter waves are used for producing the substrates of the radio engineering circuits. These materials must exhibit stable dielectric properties within a given specimen and in the specimens of different batches. Such ceramics must possess a high mechanical strength in order to ensure a high class of surface finish during their processing (machining). In order to obtain substrates having a surface roughness corresponding to the 13-14 classes, it is necessary to use a ceramic having a fine-grained structure and consisting, as far as possible, of a single phase system with minimum content of the glass phase [i]. Numerous basic technological routes are available for obtaining ceramics having the predetermined grain sizes, viz., introduction of special alloying additives that hinder grain growth; using finely dispersed raw materials; and selection of the firing regime and the atmosphere for firing [2]. The grain size of the ceramic is one of the main parameters determining most of its properties. In view of this, developing a reliable method that offers a possibility of revealing the structure of the ceramic is an important practical problem. Unfortunately, this is not a simple problem since ceramics belong to acid resistant materials and the etching methods commonly used in metallography are not applicable to them. It is particularly difficult to reveal the structure of the "polykor" type high-alumina oxide ceramics that do not virtually contain a glass phase and consist mainly (99.7-99.9%) of AI203.