Dielectrics made of Al2O3 and SiO2 for high temperatures

Abstract

Dielectrics produced from corundum with commercial alumina and from quartz ceramics by hot pressing and by the conventional method with the use ofF-9 silicone resin are described. The effect of technological factors on the properties of the specimens is studied.In choosing dielectrics for parts of radio devices operating at a high temperature under conditions of one-sided heating, when surface fusion is possible, it is important to take into account the change in the dielectric properties of the materials in heating, namely, the permittivity s and the loss parameter tan 8. In a search for materials that would provide an admissible loss of power in devices operating at superhigh frequencies we conducted experiments with corundum refractories and quartz ceramics. Both kinds of material are known to be dielectrics with high quality parameters [I, 2]. Below we present results of an investigation of various technological factors on the properties of the articles.The initial materials for alumina dielectrics were electrofused corundum and commercial alumina that contained 99 and 99.3% AI203, respectively. The composition of the corundum differed in the content of fi'actions Nos. 63, 32, and 3. The commercial alumina was fired at a temperature ranging from 1450 to 1750°C, crushed to a grain size of less than 28 -40 gm, andmixed with electrocorundum in an optimum ratio (with respect to the final strength) of about 45 to 55 vol.%. It should be noted that the strength of the specimens depended on the degree of crushing of the alumina (Fig. 1). It seems that with excessive dispersity fine alumina particles positione.d between grains of electrocorundum form more microcracks.The specimens were pressed at a pressure of 5 0 -120 MPa and fired at different temperatures. An increase in the pressure was accompanied by an increase in ¢~c to over 100 MPa, and the open porosity Pop decreased from 27 to 15 -17%. In firing the specimens we established that their strength depended substantially on the rate of their cooling. For specimens cooled at a rate of 30 -35°C/h ¢~c was 60 -70MPa, and for specimens cooled at 130-135°C/h the strength was 110-120 MPa. This is explainable by forma-I St. Petersburg Institute of Refractudes, St. Petersburg, Russia.tion of a structure with finer grains and slower crystal growth in rapid cooling; this phenomenon has been observed in other works too [3].The data presented in Fig. 2 show that the alumina for the binder and the final articles should be fLred at higher temperatures, because their strengzh is an important factor promoting stability in a rapidly moving gas flow. The thermal stability of articles fired at 1750°C is quite high (over 60% of the initial strength was preserved after three heat cycles of 2000-20°C). The finished articles had the following properties: apparent density Pap = 3.1 g/cm 3, Pop--21 -23%, t~ c = 120 MPa at 20°C, c b = 83 MPa at 20°C and 40 MPa at IO00°C, o~ = 15 and 5 MPa, respectively.The dielectric properties of the specimens were determined by two methods, namely, a vol...