High-Temperature Properties of Unburned MgO-C Bricks Containing Al and Si Powders

Changes in the mechanical and thermal properties, as well as in the microstructure, of unburned MgO-C bricks containing Al and Si powders were investigated at selected temperatures. Specimens with heat treatments at 500°C shrank and exhibited higher apparent porosity than untreated specimens. The bending strength and elastic modulus at 500°C were much lower than those of untreated specimens, and the apparent porosity increased and the mechanical properties at 500°C decreased with each repeating heat treatment. It was predicted that when the volatile matter was no longer generated, microstructure shrinkage would stop and the mechanical properties become constant. The bending strength and static elastic modulus at 800°, 1000°, and 1300°C were higher than those at 500°C because of the binding effect of the reaction products (i.e., Al₄C₃, SiC, and MgAl₂O₄), although the apparent porosity was higher than at 500°C. Repeated heat treatment from room temperature (RT) to the respective temperature, however, degraded the properties to nearly the same level as at 500°C because of the increased apparent porosity and the cracks generated in magnesia particles by the reaction products. Plastic deformation appeared to occur at 1300°C just before bricks were fractured. In addition, the thermal expansion ratio decreased through repeated heating and cooling from RT to 500°, 1000°, or 1300°C, and finally decreased to a constant value, as predicted.     

Fabrication and Characterization of Dual-Channeled Zirconia Ceramic Scaffold

A novel porous ceramic with a structure containing two three-dimensional (3D) pore channels in a tetragonal zirconia polycrystals (TZP) ceramic was fabricated using a combination of a CNC-machining method and slurry coating process. A graphite scaffold with a single interconnected 3D channel as a template was prepared using CNC machining and lamination. The surfaces of the graphite scaffold were then coated uniformly with the TZP slurry, followed by heat treatment at 900°C for 3 h in air to remove the graphite material completely via thermal oxidation and at 1400°C for 3 h in air to sinter the TZP walls. This process produced a dual-channeled TZP scaffold with an additional 3D channel, which replicated the 3D graphite structure with the pre-existing channel. The fabricated scaffold showed ultra-high porosity (91%), high surface area, and high compressive strength (2.04 MPa), as well as a tightly controlled pore structure.     

Control of Phase and Pore Structure of Titania Powders Using HCl and NH4OH Catalysts

Porous titania powders were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) and were characterized at various calcination temperatures by nitrogen adsorption, X-ray diffraction, and microscopy. The effect of HCl or NH₄OH catalysts added during hydrolysis on the crystallinity and porosity of the titania powders was investigated. The HCl enhanced the phase transformations of the titania powders from amorphous to anatase as well as anatase to rutile, while NH₄OH retarded both phase transformations. Titania powders calcined at 500°C showed bimodal pore size distributions: one was intra-aggregated pores with average pore diameters of 3–6 nm and the other was interaggregated pores with average pore diameters of 35–50 nm. The average intra-aggregated pore diameter was decreased with increasing HCl concentration, while it was increased with increasing NH₄OH concentration.     

Cyclic voltammetric and FTIR studies of powdered carbon electrodes in the electrosorption of 4-chlorophenols from aqueous electrolytes

Several types of carbon materials (activated carbon, carbon black, multiwalled carbon nanotubes) differing in porosity and surface chemistry were used to prepare powdered electrodes. Activated carbon (Norit R3-ex) was demineralized and modified by oxidation with conc. HNO₃, heat treatment in NH₃ at 900 °C or heat treatment in argon at 1800 °C. Carbon black (Vulcan XC72) was flushed with an organic solvent, while the MWCNTs were functionalized to the hydroxyl and carboxyl forms. Nitrogen adsorption isotherms were used to characterize the pore structure of these materials. Their surface chemistry was assessed using thermogravimetry (TG), elemental analysis, FTIR, EDS and XPS. The ability to adsorb (isotherms) 4-chlorophenol (4-CP) in aqueous solution was determined. Cyclovoltammetric (CV) measurements of powdered carbon electrodes were carried out for blank electrolyte solution (0.1 M Na₂SO₄) and with different concentrations of 4-CP. Changes in the electric double layer capacity and other electrochemical parameters were estimated from the CV curves. The dependence of the electrochemical behavior of a powdered carbon bed on porosity and surface chemistry is analyzed and discussed. The electrochemical properties were related to chlorophenol adsorption ability and FTIR spectral analysis of the adsorption layer.     

Surface Fractal Dimension of Perovskite-Doped Alumina Membrane: Influence of Calcining Temperature

The surface fractal dimension, D , and pore size distribution of perovskite-doped alumina membrane prepared via the sol–gel method were determined from their nitrogen adsorption isotherms. The D value was calculated using the Frenkel–Halsey–Hill method. The D value increased with increasing temperature due to membrane shrinkage. The pore size distribution pattern of perovskite-doped alumina membrane showed a narrow pore size when the temperature was increased from 400° to 800°C. The isotherm type was characterized as Type IV. Smoothing and sintering effects contributed to the decreasing trend of surface fractal dimension at a high temperature. The surface fractal dimension increased when the perovskite ratio in alumina membrane was increased.     

Effect of Swelling on Thermal Conductivity and Postirradiation Densification of U3Si

The effect of swelling porosity on the thermal conductivity of irradiated U₃Si fuel was deduced from electrical conductivity measurements on samples with pore volume fractions of 0.01 to 0.27. The Maxwell-Eucken expression adequately represents the data, with β= 1.9±0.1. Postirradiation annealing of samples at 400° to 750°C produces densification. Calculations confirm that the pores in as-irradiated fuel are voids possibly containing a small amount of fission gas. Those retained after annealing are fission gas bubbles with internal pressure nearly in equilibrium with the surface tension of the surrounding solid.     

Sintering Behavior of Gehlenite. Part I: Self-Forming, Macro-/Mesoporous Gehlenite—Pore-Forming Mechanism, Microstructure, Mechanical, and Physical Properties

A novel kind of pore self-forming macro-/mesoporous gehlenite (2CaO·Al₂O₃·SiO₂) ceramic (abbreviated C₂AS) having a highest porosity of 80% corresponding to a volume expansion of 134% during sintering has been developed. The pore self-forming ability, microstructure, mechanical, and thermal physical properties of the porous ceramic are related to the sintering temperature. The gehlenite ceramic shows a very good pore self-forming ability over a very wide range of temperature from 900° to 1450°C. No vesicant is required and no hydrothermal treatment is needed, as is generally the case for other kinds of porous ceramics or glasses. The pore self-forming ability of the C₂AS porous ceramic can be attributed to the escape of the adsorbed water vapor during the sintering process, due to automatic hydration of the fine, amorphous, flakey-shaped starting C₂AS powder particles synthesized by the organic steric entrapment (PVA) method, as well as to their fine, porous microstructure. The pores of the ceramics can be either open or closed, and the average pore size ranges from 0.6 to 1.1 μm, corresponding to a porosity of 75%–80%, respectively. The porous ceramic can preserve nanometer-sized (26–50 nm) crystallites up to 1000°C. Sintered or thermally treated under different conditions, the porous ceramics exhibit relatively high flexural strengths ranging from 9.1 to 15.4 MPa, with a standard deviation of 0.3 and 4.2 MPa, respectively. Thermal properties of the porous ceramic up to 1000°C, including thermal expansion coefficient, thermal diffusivity, specific heat, and thermal conductivity, were investigated, and the stability of the porous ceramic in boiling water was also studied.     

Effect of Sodium on Crystallite Size and Surface Area of Zirconia Powders at Elevated Temperatures

Fine ZrO₂ powders were synthesized by an aqueous precipitation method using zirconyl nitrate. By adding the precursor salt to NaOH, single-phase ZrO₂ powders were formed, and the monoclinic phase did not appear upon heat treatment up to 1000°C. The samples were digested in NaOH for different amounts of time. Different levels of washing of digested samples produced surface area at 900°C for 4 h ranging from 8 to 100 m²/g. It was found that the properties of the powders at elevated temperatures were sensitive to the sodium content. The surface area decreased while the crystallite size and pore size of the samples increased with increased sodium content. Our results indicated that sodium is detrimental to the stabilization of surface area at elevated temperatures.     

Preparation of High-Silica Glasses from Colloidal Gels: III, Infrared Spectrophotometric Studies

The gel process for making silica was studied by infrared spectrophotometry of solid samples in various stages of dehydration. Spectra were recorded in the high-frequency overtone region (2500 to 8000 cm^-1) as well as in the region of fundamental absorption (200 to 4000 cm^-1), depending on the nature of the sample. Molecular water was distinguished from silanol groups and the effects of hydrogen bonding were observed. Drying and heat treatment of gels at low temperatures (<150°C) causes a loss of unbonded included water, but significant loss of hydrogen-bonded water occurs only with relatively high-temperature heat treatments (>800°C). Hydration of siloxane groups on pore surfaces and attachment of water molecules by hydrogen bonding to these surface silanol groups is reversible for heat treatments up to 1025°C. At higher temperatures (>1025°C), active sintering takes place, converting the porous structure into a homogeneous silica network. In this process, surface silanol groups become isolated within the silica network, giving rise to vibrational frequencies characteristic of internal -OH groups. Also, at the higher temperatures silanol groups react to form siloxane groups with loss of H₂O. A model is presented for formation of a gel in which hydrogen bonding initiates association of colloidal particles, followed by strengthening through interparticle solid silica precipitation.     

High Surface-Area Ceria Aerogel

Ceria (CeO₂) aerogels with high surface area and high porosity have been prepared. Ce-methoxyethoxide diluted in excess methoxyethanol was slowly hydrolyzed to yield a gel, which was then supercritically dried in CO₂. Both as-synthesized and annealed aerogels were examined by X-ray powder diffraction, infrared spectroscopy, scanning electron microscopy, and BET surface area and pore-size analyses. Thermal analysis of the as-synthesized gel showed it to contain only ∼5 wt% residual organics, which were removed by 300°C under oxygen. The unheated ceria aerogel was crystalline and exhibited a specific surface area of 349 m²/g with average pore diameter of ∼21.2 nm and 90% porosity. Heat treatment led to a reduction of porosity and pore size, as would be expected, but the extremely narrow pore-size distribution of the aerogel was retained.     

Transverse Thermal Conductivity of Thin C/SiC Composites Fabricated by Slurry Infiltration and Pyrolysis

Thin C/SiC composites were fabricated by infiltrating a woven carbon fiber fabric with a slurry of SiC powder and polymer precursor for SiC, followed by heat treatment for pyrolysis. The effects of heat treatment parameters on the crystallization of the polymer-derived SiC, the composite microstructure, and the transverse thermal properties were assessed. Whereas composites heat-treated at 1000°C were crack-free and nearly fully dense, composites that were subjected to further multiple reinfiltration and heat treatment cycles at 1600°C developed porosity and cracking. However, the transverse thermal conductivity was increased significantly by the higher-temperature heat treatment, to values higher than that of a composite with a chemical-vapor-infiltration SiC matrix and the same fiber reinforcement.     

Microstructure Refinement of Sintered Alumina by a Two-Step Sintering Technique

For a few oxide ceramics, the use of an initial precoarsening step prior to densification (referred to as two-step sintering) has been observed to produce an improvement in the microstructural homogeneity during subsequent sintering. In the present work, the effect of a precoarsening step (50 h at 800°C) on the subsequent densification and microstructural evolution of high-quality alumina (Al2O3) powder compacts during constant-heating-rate sintering (4°C/min to 1450°C) was characterized in detail. The data were compared with those for similar compacts that were sintered conventionally (without the heat treatment step) and used to explore the mechanism of microstructural improvement during two-step sintering. After the precoarsening step, the average pore size was larger, but the distribution in pore sizes was narrower, than those for similar compacts that were sintered conventionally to 800°C. In subsequent sintering, the microstructure of the precoarsened compact evolved in a more homogeneous manner and, at the same density, the amount of closed porosity was lower for the compacts that were sintered by the two-step technique, in comparison to the conventional heating schedule. Furthermore, a measurably higher final density, a smaller average grain size, and a narrower distribution in grain sizes were achieved with the two-step technique. The microstructural refinement that was produced by the two-step sintering technique is explained in terms of a reduction in the effects of differential densification and the resulting delay of the pore channel pinch-off to higher density.     

Sol-Gel Synthesis of Amorphous Calcium Phosphate and Sintering into Microporous Hydroxyapatite Bioceramics

A new route for preparing hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) bioceramic has been described. An amorphous, nanosized, and carbonate-containing calcium phosphate powder that had a Ca:P ratio of 1.67 was synthesized from calcium diethoxide and phosphoric acid in ethanol via a sol-gel method. The powder was pressed at 98 MPa into green specimens and then heated to a temperature range of 500°-1300°C. At 600°C, the powder crystallized to a carbonated hydroxyapatite and a trace of ß-tricalcium phosphate before converting to hydroxyapatite at 900°C. The thermal crystallization was associated with grain growth, shrinkage, and active surface diffusion. The activation energy of grain growth was 37 ± 2 kJ/mol. After sintering at 1100°C, the decomposition of carbonated hydroxyapatite generated a microporous ceramic with an average pore size of 0.2 µm and an open porosity of 15.5%. This microporous bioceramic can be used as a bone filler.     

Rearrangement of Porous CaO Aggregates During Calcite Decomposition in Vacuum

High-resolution SEM photographs, N₂ adsorption isotherms, Hg porosimetry, and micrometer measurements were used to characterize CaO particle shapes and pore-size distributions that result when calcite crystals are decomposed in vacuum at 686°C. The surface area of the CaO produced from large calcite crystals is constant at 116.4 m²/g independent of the extent of reaction. The volume occupied by a CaO aggregate is 98±2% that of the original calcite crystal. The ∼54% total porosity is comprised of 42% pores of ∼5 nm cross section and 12% pores of ∼10 μ m cross section. The duplex pore structure is formed by a diffusionless repacking of CaO particles that initially form with a more uniform distribution of particles and pores.     

Influence of Porosity on Deformation and Fracture of UO2

The effects of porosity on the deformation characteristics of sintered polycrystalline UO₂ were determined at 500°, 1250°, and 1600°C at a strain rate of ∼0.1 h⁻¹. At 500°C, fracture was brittle and transgranular and initiated from the large pores present after fabrication. An increase in porosity from 5 to 16 vol% caused a 79% reduction in strength. At 1250° and 1600°C, plastic deformation preceded fracture, and the effect of porosity was much more complex. At 1250°C, an increase in porosity increased the strength but lowered the ductility, and the fracture mode changed from intergranular to transgranular. In contrast, at 1600°C porosity had little effect on strength. This behavior was associated with the relative magnitudes of the stress necessary to extend a preexisting flaw and the yield stress and their influence on the brittle-to-ductile transition temperature, T_c.     

Permeable Porosity of Refractory Castables Evaluated by the Water-Expulsion Porosimetry Technique

In this work, the water-expulsion porosimetry technique was used to quantify the permeable pores of self-flow ultralow-cement refractory castables that were treated at 600°–1650°C. Results have shown that the method as proposed in previous works was not valid, because water was not removed continuously from pores, relative to increased pressure. Nevertheless, the maximum pore diameter obtained according to ASTM Method E128-89 could be successfully correlated with other castable physical properties (such as the permeability constants from the Forchheimer equation) and the apparent porosity obtained via the Archimedes technique. Although the apparent porosity decreased as the thermal treatment temperature increased, the pore morphology changed continuously, with the generation of less-tortuous and more-permeable paths for fluid flow.     

Interaction of BaCl2 and BaSO4 at Elevated Temperatures

The effect of adding BaCl₂ to BaSO₄ at elevated temperatures was studied. Differential thermal analysis and hot-stage microscopy revealed that a eutectic occurs at 865°C between BaCl₂ and 13.5 mol% BaSO₄. The microstructure in the reaction zone indicated that BaS04 recrystallizes from the eutectic melt. Thus, BaCl₂ additions can activate BaSO₄ at a temperature as low as 865°C.     

Structure of Duplex Multilayer Pb(Zr0.53Ti0.47)O3> Films Prepared by Sol-Gel Processing

Multilayer PZT films were prepared by sol-gel processing, and their structure and composition were investigated by cross-sectional TEM and EDX analyses. The films made by firing at 600°C (as-deposited films) are composed of alternating porous and dense layers, and the porosity increases by a factor of 2 through heating at 750°C. The pyrochlore phase observed in the as-deposited films turns to the perovskite phase by the heat treatment at 750°C. The Ti/Zr ratio along the film stacking direction is shown to change with the elevation of heat treatment temperature. The porous soft layers are considered to reduce the thermal stress and prevent the introduction of cracks during the firing process.     

Micro-/Macroporous Ceramics from Preceramic Precursors

Porous silicon oxycarbide (SiOC) ceramics in particular bulk and cellular structures are produced via polymer pyrolysis. By using optimal pyrolysis parameters (i.e., heating rate, maximum temperature) the addition of either solid fillers or chemically active additives is efficient in preventing the collapse of pore structure and controlling pore formation through decomposition. Fast pyrolysis can lead to crack formation and a loss of specific surface area at temperatures above 600°C, whereas slow pyrolysis is able to preserve mesopores up to 1200°C combined with high surface areas. These SiOC ceramics with bimodal pore size distribution are potential candidates for adsorption/separation processes under severe conditions.     

N2-sorptionsmessungen zur bestimmung der spezifischen oberfläche und der porenverteilung von erhitztem normalbeton

The specific surface and pore size distribution of normal concrete is influenced by temperature effects. Therefore, the specific surface area of concrete specimens has been computed from nitrogen adsorption isotherms after a preceding temperature treatment in a thermal balance. It was found that the specific surface area increases from 2,0 m²/g at 100°C to 4,5 m²/g at 400°C. The mean pore radius decreases from 53 Å to 38 Å. A temperature treatment at 665°C is connected with a further decomposition of concrete and yields a decrease of the specific surface area to 2,4 m²/g and the pore size distribution shifts to larger radii ($\text{r}\text{= 51}\text{A}\text{?}$). Due to sintering processes, at temperatures of 1000°C the porosity of the material is decisively altered and a specific surface area of 1,1 m²/g has been calculated.