No abstract

666.762.2.017:620~!79oi T. I. Litovskaya, and S. P. Shmitt-FogelevichThe formation of cristobalite during the preparation of articles made from quartz ceramic leads to the emergence of cracks in the articles and to a loss of strength [i] as a result of the ~ + B cristobalite transition on cooling.As a result, the properties of the articles are degraded in use.It is therefore necessary to eliminate as far as possible the supply to the customer of products in which a critical amount of cristobalite has been formed.Sometimes the formation of cristobalite leads to cracks which emerge on the surface of the article.In that case defective articles can be revealed by visual inspection.Since the reasons for the formation of cristobalite may vary and be associated with very slight and unrecorded deviations in the production process, the output of articles in which the concentration of cristobalite goes beyond permissible limits is random in character.At the present time there are no inspection methods which make it possible to reveal~ reliably and immediately, those articles which contain cristobalite.Normally under laboratory conditions the concentration of cristobalite is determined using an x-ray method [2] but for several reasons this cannot be used for production control.According to [3,4] as well as from some theoretical considerations, the emergence of cristobalite must produce a change in the dielectric properties of a quartz ceramic. This fact may be used as the basis of a method which makes it possible to organize on-line production control of the concentration of cristobalite in the articles.The aim of the studies described in the present paper was to investigate the dependence of the dielectric properties, porosity, and density, on the phase composition of the ceramic.The studies were carried out on cylindrical specimens made by a slip-casting method. As the starting materials we used vitreous silica with a mass fraction of SiO2 > 99%. The specimens were fired in a furnace with silicon carbide heaters at a heating rate of 200~ with a 3-h dwell.It was possible to produce specimens with different concentrations of cristobalite by changing the firing temperature between I150~and 1300~ The open porosity (Pop) and apparent density (Papp) of the specimens were determined in accordance with GOST 2409-80; the concentration of cristobalite was determined using an x-ray method.Samples were prepared by quartation down to 2-5 g and milling in a chalcedony mortar to particles <0.063 nun. The measurement error in the concentration of crystalline phase depends on the amount and extent of defectiveness in the cristobalite [2].The dielectric properties of the specimens were studied in the UHF range at wavelengths of % = 3.935 and 0.884 cm.The measurements at % = 3.935 cm were carried out in an INF-2 instrument [5] using horn-type antennae with an aperture of 2.4 x 2.4 cm; the measurements at X = 0.884 cm were carried out in equipment assembled from standard units in accordm~ce with the diagram in Fig. i; horn antennae with an ...

No abstract

No abstract

No abstract