Measurements of the high temperature dielectric properties of ceramic materials at microwave frequencies have been made using two different experimental techniques.Data has been collected at frequencies from O.2GHz to 4.0GHz and for sample temperatures up to 1200°C. Detailed cross checking of the high temperature dielectric data obtained by the two techniques has been carried out with the help of other laboratories worldwide. An investigation of the applicability of dielectric mixture equations to practical measurement techniques is reported. The most reliabl~ estimates of permittivity were given by the Landau-Lifshitz, Looyenga equation or by a cube root extrapolation technique.Permittivity data obtained for a series of yttria stabilised zirconia samples, three differently processed silicon nitride samples and ten related glass compositions are presented. Analysis of the frequency and temperature dependence of both components of complex permittivity has been undertaken· in an attempt to identify the physical origins of the dielectric loss mechanisms. For the yttria doped zirconia samples results indicate two distinct loss mechanisms dominant over different temperature ranges. Below approximately 950K a hopping model involving short range motion of oxygen vacancies around fixed dopant ions is proposed. Above 950K thermally activated quantum mechanical tunneling of electrons is suggested as the dominant mechanism. A single loss mechanism for the entire temperature range involving the lattice loss of the silicon nitride network itself is indicated from the measurements of the hot pressed and pressureless sintered silicon nitride samples. For the reaction bonded silicon nitride samples there is evidence of a second loss mechanism due to additional ion impurities above 1410K. The measurements on the oxide glass systems add support to the belief that + 1 charged metal ions will dominate the dielectric properties of glass systems when present. The loss process has an increasing activation energy with increasing temperature which is seen to be consistent with ionic motion within the previously proposed random potential energy model. Differences in the complex permittivity with composition are attributed to variation in ionic size and metal ion-oxygen ion bond strength.

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Low cement castable (LCC) refractory linings, over recent years, have superseded refractory bricks in many high temperature applications. There are concerns however, pertaining to their dry-out. In particular the catastrophic destruction of LCC shapes or linings as a result of phenomenate rmede xplosives palling. This thesis describes the study of both the on-site treatments of LCCs and the physicochemical changes that occur during heating. Two types of explosive spalling event are classified,b asedo n on-siteo bservationsa ndp ost-mortems tudy. A model is presented that describes the physicochemical changes in a LCC system as it is heated from ambient to 450°C. From this model a zone of high vulnerability to explosion has been identified between 230-280°C. The dehydration of the hydrated alumina phase gibbsite (AH3), within this zone, is identified as a possible cause of explosive spalling. An investigation of polypropylene fibres, which are added to LCCs as anti-explosion additives, found that the fibres block porosity between1 60-240°CI.t is suggested that this additive system is not optimised and that polymer fibre blends may be more advantageous. The diffusion characteristics of a LCC system cured at 5 and 20°C is presented. It was found that at the lower curing temperature the diffusion rate was lower. This supports the recommendation for the use of high curing temperature to facilitate the drying of LCCs and reduce the risk of explosive spalling. A list of guidelines that could be used to establish benchmark standards for the development of best practice in industry has been compiled from the results of these studies.

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