Permeability of Refractory Castables at High Temperatures

Temperature is an important factor affectng the permeation of fluids in refractory castables. Because of experimental difficulties, however, permeability parameters are usually obtained at room temperature and then extrapolated to the temperature of interest, with no concern regarding porous structure modification (thermal expansion, etc.). In this work, an apparatus has been developed to evaluate the air-flow permeability of refractory castables at temperatures up to 800°C. The objective is to investigate modifications in the permeability constants obtained by Forchheimer's equation to provide a more realistic relationship with castable processing and molten-metal corrosion parameters.     

Dynamic Permeability Behavior during Drying of Refractory Castables Based on Calcium-Free Alumina Binders

Considerable efforts have been made to understand the dewatering behavior of refractory castables. Modeling of this process must be based on realistic heat and mass transfer data; hence, consistent values of castable properties are required, particularly permeability values, since these are the most important parameters that govern the process. Permeability values, however, are usually obtained at room temperature and therefore may not reflect the remarkable microstructural changes that take place during water removal. The problem has become more critical for castable compositions based on calcium-free binders, in which the dehydration process is still unclear and explosive spalling is more likely to occur. This study has investigated the behavior of the dynamic permeability of high-alumina calcium-free refractory castables subjected to a drying process up to 700°C. Samples were previously treated at temperatures ranging from 110° to 1650°C to provide information on the castables' reversible and irreversible microstructural changes. The results revealed fluctuations and a dramatic decrease in the permeability level near 200°C, which may help to explain the occurrence of explosive spalling in this class of castables.     

Assessment of Mass Loss and Permeability Changes during the Dewatering Process of Refractory Castables Containing Polypropylene Fibers

This work describes the use of a new permeametry technique to evaluate microstructural changes that take place in castables with high alumina-fiber content during the dewatering process. It is shown that the mass loss due to cement dehydration and the permeability increase caused by fiber burnout can be independently assessed, based on the fluid dynamic conditions established for the experiment. Control of the pressure drop through the sample was the key procedure used to analyze the phenomenon of interest.     

Permeability of High-Alumina Refractory Castables Based on Various Hydraulic Binders

This work investigated the changes in the permeability of high-alumina self-flowing refractory castables based on calcium aluminate cement (CAC) and hydratable alumina binder (HAB) pretreated between 110° and 1650°C. Permeability constants k ₁ and k ₂ were fitted from Forchheimer's equation based on airflow tests conducted at room temperature. The results indicated that dehydration was the main contributing factor for increased permeability in bodies pretreated up to 600°C and that sintering effects prevailed between 900° and 1650°C. Castables based on HAB were less permeable than those based on CAC, a behavior ascribed to the type of hydrates formed and to the particle-packing features of the matrix and the matrix–aggregate interfaces.     

Effective Medium Theory of the Porosity Dependence of Bulk Moduli

Effective medium theory is used to obtain a closed- form expression for the porosity dependence of the bulk moduli of ceramics. The theory is illustrated with an application to data for high-purity alumina that spans a volume fraction of porosity ranging from 0.5% to 90%.     

In Situ Coagulation of High-Alumina Zero-Cement Refractory Castables

Coagulation methods originally developed for colloidal processing were investigated in this paper as alternative approaches to consolidate high-alumina refractory castables free of hydraulic binders (zero-cement). Three in situ reactions based on the direct coagulation casting (DCC) technique were evaluated to promote castable coagulation: (1) the autocatalytic hydrolysis of gluconic acid lactone, (2) the gradual dissolution of hydroxyaluminum diacetate particles in water, and (3) the enzyme-catalyzed hydrolysis of urea. The coagulating behavior of castables and matrix-representative suspensions was investigated with the help of zeta potential analysis, pH measurements, castable free-flow evaluation, and oscillatory rheological tests. The enzyme-catalyzed hydrolysis of urea seemed to be the most appropriate mechanism to promote the coagulation of initially self-flow zero-cement refractory compositions.     

Positronium Annihilation Study of Porosity in Silica Xerogels Prepared by a Sol–Gel Process

Nano- and mesopores of bulk silica xerogels were studied by means of positron annihilation lifetime measurement as well as nitrogen physisorption. Effects of an alkaline catalyst concentration in the sol–gel process on the pore sizes and specific surface area were investigated. The pore size was estimated from the ortho -positronium lifetime. Remarkable structural changes were found around a relative alkaline concentration of 10⁻⁴ (a molar ratio of sodium hydroxide to tetramethyl ortho -silicate). Above this particular catalyst concentration, the nanopore radius decreased from 0.45 to 0.35 nm, whereas the mesopore radius increased from 1 to 8 nm. The positron annihilation lifetime technique was effective in the evaluation of the nano- and mesoscopic structural changes of silica xerogel.     

Porous Silica Matrix Obtained from Pyrex Glass by Hydrothermal Treatment: Characterization and Nature of the Porosity

A novel porous silica matrix has been prepared from Pyrex glass, using hydrothermal treatment under saturated-steam condition. This process makes it possible to obtain, in one step, a silica support formed of a homogeneously distributed and interconnected macropore microstructure. The new matrix contains silanol groups that can be used in reactions of surface modification to provide a hybrid material and a selective macrofiltration membrane, and also it can improve chemical inertness. The porous matrix is noncrystalline as obtained and, after thermal treatment at temperatures higher than 950°C, exhibits an X-ray pattern characteristic of α-cristobalite and low volume contraction. The present samples were characterized by scanning electron microscopy, mercury intrusion porosimetry, nitrogen adsorption-desorption isotherms, infrared spectroscopy, X-ray powder diffractometry, atomic absorption, and high-resolution solid-state nuclear magnetic resonance. The results present a new way of producing a macroporous silica matrix.     

Influence of Air Compressibility on the Permeability Evaluation of Refractory Castables

Air compressibility is important in attaining an accurate quantification of the permeability parameters for refractory castables. Nevertheless, this effect is often neglected in the permeability equations that are found in ceramics literature. The consequences of omitting the compressibility effect in two of the main permeability equations are highlighted and discussed. The permeability to air flow of high-Al₂O₃ ultra-low-cement refractory castables has been quantified by Darcy's law according to U.S. ASTM C577–96 and European PRE-16th permeability methods, which propose distinct approaches for air compressibility. Forchheimer's equation, which establishes a more realistic dependence between fluid pressure and fluid velocity, also has been used to obtain the permeability constants k ₁ and k ₂. Results show that equations proposed by the ASTM and the PRE methods may yield very distinct values of Darcian permeability k ₁ for the same sample. Forchheimer's equation for compressible fluids provides the best fitting with experimental data.     

Porosity effect on microstructure,mechanical, and fluid dynamic properties of Ti2AlC by direct foaming and gel‐casting

In this study, Ti₂AlC foams were fabricated by direct foaming and gel‐casting using agarose as gelling agent. Slurry viscosity, determined by the agarose content (at a fixed solids loading), as well as surfactant concentration and foaming time were the key parameters employed for controlling the foaming yield, and hence the foam porosity after sintering process. Fabricated foams having total porosity in the 62.5‐84.4 vol% range were systematically characterized to determine their pore size and morphology. The effect of the foam porosity on the room‐temperature compression strength and elastic modulus was also determined. Depending on the amount of porosity, the compression strength and Young's modulus were found to be in the range of 9‐91 MPa and 7‐52 GPa, respectively. Permeability to air flow at temperatures up to 700°C was investigated. Darcian (k₁) and non‐Darcian (k₂) permeability coefficients displayed values in the range 0.30‐93.44 × 10^?11 m² and 0.39‐345.54 × 10^?7 m, respectively. The amount of porosity is therefore a very useful microstructural parameter for tuning the mechanical and fluid dynamic properties of Ti₂AlC foams.     

Drying Stages during the Heating of High-Alumina, Ultra-Low-Cement Refractory Castables

The purpose of this work was to investigate the drying kinetics of high-alumina, ultra-low-cement refractory castables under continuous heating conditions. Three main drying stages were identified during the castable heat-up and were related to the phase change of free water and to the decomposition of hydrated products present in the body. A clear correlation was found between the actual heating profile inside the castable and the dewatering stages under various heating schedules. Thermal analysis was used to assess the drying temperature that represents the highest risk of steam pressure buildup and, thus, of explosive spalling.     

Effects of Matrix Porosity on the Mechanical Properties of a Porous-Matrix, All-Oxide Ceramic Composite

The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregnation and pyrolysis of a ceramic precursor solution. Mechanical tests are performed to assess the role of the matrix in both matrix-dominated and fiber-dominated loading configurations. The results demonstrate a loss in damage tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are obtained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a consequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double-notched shear specimen are broadly consistent with the limited values obtained by the short beam shear method, although the former exhibit large variability. In addition, effects of precursor segregation during drying on through-thickness gradients in matrix properties and their role in composite performance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabling damage tolerance in porous-matrix composites. Deficiencies in the understanding of the mechanisms that enable damage tolerance in this class of composites are discussed.     

Depth-Resolved Porosity Investigation of EB-PVD Thermal Barrier Coatings Using High-Energy X-rays

Demands for designing prime reliant, energy-efficient, and high-performance thermal barrier coatings (TBCs) in gas turbines have led to a growing interest toward comprehensive microstructural characterization. Here we investigate the novel use of high-energy X-rays for small-angle X-ray scattering (SAXS), together with wide-angle scattering and radiography, for the depth-resolved characterization of TBCs grown by electron beam physical vapor deposition (EB-PVD). The coating microstructure is found to consist of columns perpendicular to the substrate, extending through the thickness, with a [001] growth texture and significant intercolumnar porosity. In addition, overshadowing effects during deposition together with gas entrapment give rise to nanoscale intracolumnar porosity consisting of featherlike and globular pores. Radiography showed an increase in the total porosity, from 15% near the substrate to 25% near the coating surface, which is ascribed to an increase in the intercolumnar spacing at the top of the coating. By contrast, the small-angle scattering studies, which are sensitive to fine features, showed the pore internal surface area to be greatest near the substrate.     

Effect of Porosity on the Electrical Properties of Polycrystalline Sodium Niobate: I, Electrical Conductivity

The electrical behavior of NaNbO₃ ceramic samples with different relative densities was investigated by ac impedance spectroscopy in a range of 13 MHz to 10⁻³ Hz between 400° and 800°C in dry air. Measurements were performed during heating and cooling cycles. The Nyquist impedance diagrams of dense sodium niobate exhibit only one semicircle representing the grain contribution with depression angles as small as 1°, indicating a high homogeneity of the specific electrical properties. In the case of porous samples, the data reveal an additional low-frequency semicircle related to microstructure. For all studied samples, the Arrhenius conductivity plots show a change in the activation energy around 640°C, attributed to the tetragonal-cubic phase transition. The electrical conductivity of porous samples appears to be higher than that of dense ones.     

Microstructure and Mechanical Properties of Silicon Nitride Ceramics with Controlled Porosity

Porous silicon nitride ceramic with a porosity from 0–0.3 was fabricated by partial hot-pressing of a powder mixture of α-Si₃N₄ and 5 wt% Yb₂O₃ as sintering additive. Irrespective of the porosity, the samples exhibited almost the same microstructural features including grain size, grain aspect ratio, and pore size. Porosity dependences of Young's modulus, flexural strength, and fracture toughness ( K _{I C} ) were investigated. All these properties decreased with increasing porosity. However, because of the fibrous microstructure, the decreases of flexural strength and fracture toughness were moderate compared with the much greater decrease of Young's modulus. Thus, the strain tolerance (fracture strength/Young's modulus) increased with increasing porosity. The critical energy release rate also increased slightly with an increasing volume fraction of porosity to 0.166 and remained at the same level with that of the dense sample when the porosity was 0.233. They decreased as porosity increased further.     

Vaporization Processes and Pressure Buildup during Dewatering of Dense Refractory Castables

This work describes an experimental investigation on the dewatering kinetics of high-alumina refractory bodies under several heating conditions. The drying stages involving the removal of unbound water have been correlated with the two consecutive thermal processes by which liquid water transforms into vapor during heating: evaporation and ebullition. Thermogravimetric data have been used as the basis for a discussion of the parameters that affect the performance of both vaporization processes and guide the design of suitable heating schedules aimed at minimizing the drying time and the risk of harmful pore pressurization.     

Effect of Porosity on the Electrical Properties of Polycrystalline Sodium Niobate: II, Dielectric Behavior

The dielectric behavior of dense and porous NaNbO₃ ceramic samples, synthesized by a suitable chemical route, was investigated by impedance spectroscopy between room temperature and 800°C in dry air. The dielectric behavior and thermal stability of the samples were evaluated as a function of several thermal cycles. The dielectric constant was calculated from the relaxation frequency, and from an alternative approach based on the variation of the opposite of the imaginary part of impedance as a function of reciprocal angular frequency. The values obtained using both relations were in a good agreement. After the porosity was corrected, the porous and dense samples presented the same dielectric constant. All samples evaluated displayed a broad dielectric anomaly between 300°–400°C. Neither the orthorhombic-tetragonal- nor the tetragonal-cubic-phase transitions were detected by dielectric measurements. The Curie-Weiss law was found to be valid above the transition temperature, whereas the corresponding phase transition presented a diffuse nature. The origin of the related thermal hysteresis is discussed herein.     

Fluid Dynamics and Thermal Aspects of the Dewatering of High-Alumina Refractory Castables: Removal of Physically Absorbed Water

This article reports on an experimental investigation of the dewatering process of cement-free high-alumina refractory castables. Simultaneous fluid dynamic, thermal, and mass loss effects were investigated during the removal of physically absorbed water at temperatures of 25° to 700°C. The release of steam was decisively affected by the castable's permeability level and the heating rate applied. The analysis of fluid dynamics revealed that at 1°C/min, the main bulk of physical water was released as steam under saturated conditions at 100°C. However, at 5°C/min, steam was trapped within the pores, and water loss was chaotically released and shifted to higher temperatures. Thermal analysis showed that the endothermic boiling of water may result in a critical thermal shock in the castable's structure. Both steam entrapment and thermal shock were more severe with the reduction in the castable's permeability level.