D.N. Boccaccini

Dependence of Ultrasonic Velocity on Porosity and Pore Shape in Sintered Materials

A new approach to predict the longitudinal and transverse ultrasonic velocities in porous materials is presented. The model is based on a previously derived Youngs modulus-porosity correlation assuming spheroidal geometry of the pores. It is also assumed that the Poissons ratio of porous materials does not change significantly with porosity. The longitudinal and transverse ultrasonic velocities are given as functions of the Youngs modulus, Poissons ratio, density of the pore-free material and of the porosity and axial ratio (z/x) of the spheroidal pores. Experimental data drawn from the literature on different porous sintered materials including SiC, Al2O3, YBa2Cu3O7−x, porcelain, sintered iron, Si3N4, and sintered tungsten, were used to verify the model. A strong relationship between pore shape and the slope of the ultrasonic velocity–porosity curve was confirmed. In general, the calculated values are in fairly good agreement with the experimental data. When the actual shape (axial ratio) of the pores was known, the approach was shown to predict the experimental data better than a similar model derived by Phani. It is suggested that the present approach, coupled with the measurement of the ultrasonic velocity, may constitute a simple nondestructive technique to gain knowledge of the morphology of the porosity in sintered materials.

Volcanic ash as alternative raw materials for traditional vitrified ceramic products

Abstract Investigation on the use of volcanic ash as 100% raw materials for traditional vitrified ceramic products is reported. X-ray diffraction (XRD), thermogravimetric–thermal differential analyses (DTA–TGA) and chemical analysis were used to characterise raw samples. Fired specimens were used to evaluate their ceramic properties. Volcanic ash contains essentially classical traditional ceramic oxides, plagioclase, pyroxene and olivine as principal minerals. In the temperature range 1100–1150°C, they present dense structure, low open porosity, without isolated quartz grains. The fired materials properties were found to be in agreement with those of stoneware class BI referring to standard ISO 13006 (i.e.the water absorption values were <1·5%). The presence of a sufficiently extended glassy phase capable of embedding crystalline phases developed during sintering allows comparison of the microstructure of fired volcanic ash with that of traditional porcelain or stoneware.

Service life prediction for refractory materials

Ultrasonic pulse velocity testing and image analysis were used to predict the thermal stability of cordierite–mullite refractories. Two compositions used as substrates in fast firing of porcelain whiteware, characterized by different microstructure and crack propagation behavior, were investigated. Fracture strength and fracture toughness values were obtained from three point bending test and chevron notched specimen technique, respectively. The measurement of the ultrasonic velocity was used to assess the material degradation with increasing number of thermal-shock cycles and specimen damage was monitored using image analysis to obtain further evidence of material degradation. The correlation between thermo-mechanical properties, ultrasonic velocity, microstructure, crack-propagation behavior and thermal-shock resistance was discussed. A remarkable similarity was found between the variation of ultrasonic velocity (when measured through the length of the refractory plates) and fracture strength with number of thermal shock cycles. On the other hand, the development of surface microcracking, as monitored by image analysis, is in good agreement with the variation of KIC with the number of thermal-shock cycles. The variation of the

Thermal Shock Resistance of Cordierite-Mullite Refractory Composites

\frac{d\sigma_{\rm f}}{dE_{\rm dyn}}

Alkali-ions diffusion, mullite formation, and crystals dissolution during sintering of porcelain bodies: Microstructural approach

ratio with number of thermal-shock cycles shows the highest gradient of the investigated trends and it is proposed as a promising parameter to differentiate refractory materials regarding their different thermal shock behavior. Service life prediction models for refractory plates, from measured values of ultrasonic velocity and surface damage analysis, were proposed and validated.

Microwave thermal inertisation of asbestos containing waste and its recycling in traditional ceramics

Abstract The characterisation of thermal shock damage in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware has been investigated. Two different refractory compositions (termed REFO and CONC), characterised by different silica to alumina ratios, were studied. Thermal shock damage was induced in as received samples by water quenching tests from 1250°C. Thermal and mechanical properties were measured at room temperature by means of standard techniques and then the thermal shock resistance parameter R was calculated. The fracture toughness of selected samples was measured before and after thermal shock by the chevron notched specimen technique. The reliability of this technique for evaluation of small differences in fracture toughness after a given number of thermal shock cycles was investigated. The suitability of K Ic measurements by the chevron notched specimen technique to characterise the development of thermal shock damage in refractory materials was proved in this investigation.

Characterisation of porcelain compositions using two china clays from Cameroon

The design of composite materials leads to the development of multi-component systems where each constituent has a specific function in the material, from technological and/or application points of view. Examples of such composite systems are the cordierite-mullite refractory materials investigated in this contribution. Two different commercially available compositions were selected for evaluation of the influence of microstructure on fracture behaviour under thermal shock conditions. The materials were exposed to water-quench tests from 1250°C and subsequently the fracture toughness was evaluated using the chevron notched specimen technique. The results were compared to those obtained on as-received materials. Microstructural damage was also studied applying fractographic techniques with the aim to gain knowledge on the thermal shock damage mechanisms acting in the materials.

Determination of thermal shock resistance in refractory materials by ultrasonic pulse velocity measurement

Abstract A complete investigation on the sintering behaviour, involving ceramic transformation, of volcanic ash is reported. Sintering and softening points, vitrification and fusion of finely ground powders of volcanic ash were obtained by hot stage microscope observation. Then, a suitable thermal cycle, which matches the better microstructure and mechanical properties, has been performed. The low quartz content of the final product, the relative high density together with the particular structural complexity of the matrix consequence of the interlocking of various crystalline phases conferred to fired volcanic ash relevant ceramic characteristics. Nucleation and microcrystallisation of pyroxene together with oxidation and cation enrichment are indicated as the main sintering mechanism of fired volcanic ash. Differently from conventional vitrified ceramics, i.e. quartz and mullite in vitreous matrix, the microstructure of the fired products presents spinel, anorthite, diopside, enstatite, pyroferrite, fayalite and hedenbergite crystals embedded in the high viscous liquid phase resulting in dense and resistant materials.