Characterization of porous calcium hexaluminate ceramics produced from calcined alumina and microspheres of Vaterite (μ-CaCO3)

The evolution in the microstructure of porous calcium hexaluminate (CaAl₁₂O₁₉ or CA₆) formed in situ is significantly affected by the characteristics of precursor raw materials. In this study, microspheres of Vaterite (μ-CaCO₃) of different average sizes (5, 7 and 15?μm) were mixed with fine α-Al₂O₃ and sintered. During heating (590–753?°C), the Vaterite particles collapsed and produced spherical pores surrounded by regions of high CaO concentration. Different quantities of low-density-and-melting-point calcium aluminates (C₁₂A₇, CA and CA₂) were formed above 1100?°C. Small portions of a liquid phase were observed for samples containing thin Vaterite particles, which resulted in a denser structure comprised of CA₆. The samples prepared with coarse Vaterite significantly expanded and the asymmetric shape of the CA₆ crystals prevented densification and generated highly porous structures. Therefore, the tailoring of Vaterite particles’ characteristics enables the design of physical properties of CA₆ porous structures according to the application aimed.     

Phase and porosity changes in a calcium aluminate cement and alumina system under hydrothermal conditions

When refractory castables are dried, hydrothermal conditions may result inside the bodies if the H₂O cannot escape from the material. Under such high-pressure conditions, problems such as explosive spalling can arise. As different curing temperatures during the hydration of calcium aluminate cement (CAC)-bound castables lead to the formation of different hydrate phases, different microstructures can develop in the hardened material. This study presents the changes in porosity and in the mineralogical composition of a refractory castable model system under hydrothermal conditions depending on the curing temperature (5, 23 and 40 °C).Quantitative X-ray diffraction (QXRD) measurements show that different hydrate phases are formed during curing, while C₃AH₆ and boehmite are formed in the same quantities after hydrothermal treatment in an autoclave at ～11 bar/180 °C. Although the mineralogical composition after autoclaving is not different, the three samples differ in their microstructure. Mercury intrusion porosimetry measurements reveal that although the total porosity after autoclaving is the same, the 40 °C samples have a higher proportion of large pores. SEM images also show that the appearance of C₃AH₆ in the 40 °C autoclaved samples varies, which originates from the starting phase composition and microstructure after curing.     

Gas corrosion behavior of calcium hexaluminate materials for hydrogen metallurgy

Calcium hexaluminate (CA₆) has good chemical stability under H₂ atmosphere, which indicates its application prospect in the hydrogen metallurgy field. However, iron ores usually contain SiO₂ impurities, which can react with H₂ to form Si-containing gases such as SiO (g) and SiO₂ (g). The Si-containing gases might corrode CA₆-based lining refractories during hydrogen metallurgy. The silica and magnetite were heated in a hydrogen furnace to simulate the gaseous environment of hydrogen metallurgy in this paper. At the same time, the corrosion of CA₆ materials at elevated temperature (600 °C–1800 °C) under the hydrogen metallurgy atmosphere were studied. In addition, three SiO₂ based raw materials (silica, fused silica and waste silica brick) with different SiO₂ contents were used to investigate the effect of impurities on the their H₂ corrosion. The result showed that SiO₂ based raw materials had good chemical stability under H₂ atmosphere at the temperature lower than 1200 °C, obvious weight loss appeared at 1400 °C, the impurities including Fe₂O₃ and MgO would accelerate mass loss under H₂ atmosphere. Gases including SiO (g), SiO₂ (g) and H₂O (g) could react with CA₆ to form corundum and yoshiokaite at 1800 °C, which resulted in 3.16% weight gain after heating at 1800 °C for 8 h under the hydrogen metallurgy atmosphere. No significant mass change and mineral phase transformation appeared at the reaction temperature less than 1600 °C.     

Preparation of calcium hexaluminate porous ceramics by gel-casting method with polymethyl methacrylate as pore-forming agent

Calcium hexaluminate (CA₆) porous ceramics were prepared by gel-casting method, with α-Al₂O₃ and CaCO₃ as raw materials and polymethyl methacrylate (PMMA) microspheres as pore-forming agent. The effects of the amount of pore-forming agent PMMA microspheres on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, thermal shock stability and thermal conductivity of CA₆ porous ceramics were systematically studied. The pores of CA₆ porous ceramics are mainly formed by the burning loss of PMMA microspheres and the decomposition of organic matter. Adding an appropriate amount of PMMA microspheres as pore-forming agent has a positive effect on the thermal shock stability of CA₆ porous ceramics. When the amount of pore-forming agent is 15 wt%, the volume density of CA₆ porous ceramics is 1.33 g/cm³, the porosity is 63%, the flexural strength is 13.9 MPa, the thermal shock times can reach 9 times, and the thermal conductivity is 0.293 W/(m·K), which can meet the application in refractory, ceramics or high temperature cement industries.     

Hydration of calcium aluminates at 60°C – Development paths of C2AHx in dependence on the content of free water

The influence of water loss during the hydration of calcium aluminates on the phase development is investigated at 60°C. This is relevant for applications in which calcium aluminate cement (CAC) based formulations are exposed to quick drying during hydration. The presented results provide new insights into the well-known conversion processes occurring in CAC pastes. Using in situ XRD two different routes of the development of initially formed C₂AH₈ are determined: (a) transformation to C₃AH₆ + AH₃ in the presence of sufficient free water and (b) dehydration to C₂AH₅ at a lack of free water. Moreover, the influence of precuring of the pastes at 23°C before heating to 60°C is investigated. The increasing loss of free water with increasing precuring time resulting from both, precipitation of hydrate phases and evaporation, causes incomplete hydration of CA or CA₂ as well as dehydration of C₂AH₈ instead of conversion into C₃AH₆. Comparative investigations of sealed samples always revealed complete hydration of CA and CA₂ as well as complete conversion of C₂AH₈.     

Revisiting CA6 formation in cement-bonded alumina-spinel refractory castables

This work revisits the proposed mechanisms presented in the literature for CA₆ formation in Al₂O₃-MgO and Al₂O₃-MgAl₂O₄ castables bonded with calcium aluminate cement. New experimental tests, thermodynamic simulations and re-evaluation of the chemical composition and microstructural aspects observed for samples fired in the temperature range of 1150 °C to 1500 °C were carried out. Based on these data, a new interpretation of the CA₆ generation process, as well as the features which influence the location and morphology of this phase were proposed. CA₆ formation via solid and liquid states are suggested to take place in all evaluated compositions, where the former (solid-state) is the main reaction predicted for the silica-free refractories (AM0MS and AS0MS), whereas the liquid-state one prevails in the AM1MS and AS1MS materials. The CA₆ crystal morphology should be affected by these different reaction mechanisms. According to the experimental results, it was also discussed the role of the calcium hexaluminate features in the overall corrosion behavior of the designed refractories when they were placed in contact with molten slag at high temperatures. Such aspects have not been previously reported in published papers related to this subject.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.     

Second-phase evolution and densification behavior of AlN with CaO–Y2O3–C multicomponent additive system

AlN compacts with different CaO–Y₂O₃–C mixtures were sintered between 1100 and 1850 °C to understand the effects of the in situ formed reducing atmosphere on the densification behavior and evolution of the second-phases. AlN with Y₂O₃ densified at 1750 °C, but the addition of C changed the second-phases evolution towards Y-rich phases that delayed the densification. For AlN containing CaO, the second-phases were little influenced by the reducing atmosphere, but the addition of C increased the evaporation of the second-phase compounds during sintering, limiting the densification due to the reduction of the liquid-phase fraction and the gas trapping inside the pores. AlN with CaO–Y₂O₃ mixtures could be completely densified at 1650 °C, but the addition of C inhibited the densification below this sintering temperature because liquid-phase had poor wetting and spreading characteristics and the second-phase a high melting point (>1800 °C).     

Microstructure and mechanical properties of cold sintered porous alumina ceramics

New innovative approach to fabricate porous alumina ceramics by cold sintering process (CSP) is presented using NaCl as pore forming agent. The effects of CSP and post-annealing temperature on the microstructure and mechanical strength were investigated. Al₂O₃–NaCl composite with bulk density of 2.92 g/cm³ was compacted firstly using CSP and then a porous structure was formed using post-annealing at 1200°C–1500°C for 30 min. Brazilian test method and Vickers hardness test were used to determine the indirect tensile strength and hardness of the porous alumina, respectively. Meanwhile, the phases and the microstructure were respectively examined using X-ray diffractometer and scanning electron microscope (SEM) complemented by the 3D image analysis with X-ray tomography (XRT). SEM structural and XRT image analysis of cold sintered composite showed a dense structure with NaCl precipitated between Al₂O₃ particles. The NaCl volatization from the composite was observed during the annealing and then complete porous Al₂O₃ structure was formed. The porosity decreased from 48 vol% to 28 vol% with the annealing temperature increased from 1200 °C to 1500 °C, while hardness and mechanical strength increased from 14.3 to 115.4 HV and 18.29–132.82 MPa respectively. The BET analysis also showed a complex pore structure of micropores, mesopores and macropores with broad pore size distribution.     

Strengthening effect and mechanism of titanium extraction slag on the mechanical property of calcium aluminate cement

In heavy oil recovery, calcium aluminate cement (CAC) is in the working environment of “low-temperature hardening and ultrahigh temperature service.” However, the formation of C₃AH₆ under low temperatures results in a decrease in strength and reduce the cementing quality. In this study, titanium extraction slag (TES) was used to inhibit CAC strength deterioration. TES, characterized by a high Ti content, presents challenges in terms of utilization and poses significant ecological risks owing to its large accumulation. Cementite hydration with 0%, 20%, 30%, and 40% TES relative to CAC was examined at 30 °C for 28 d. The high C₃AH₆ content of the pure CAC increased the strength deterioration, pore size, and cementite carbonation. With 20% TES, a dilution effect was observed without strength improvement. Furthermore, 30% TES generated layered double hydroxides and converted C–S–H into C–A–S–H, thereby increasing compressive strength. By-products were generated with 40% TES, which inhibited the strength development while generating C–A–S–H to maintain the compressive strength. Therefore, TES can inhibit the strength decline of CAC, and the byproducts of the LDH structure can improve corrosion resistance.     

Evolution of elastic properties and microstructural changes versus temperature in bonding phases of alumina and alumina–magnesia refractory castables

The microstructural evolutions of high alumina refractory concretes, based on the systems CaO–Al₂O₃ and CaO–Al₂O₃–MgO, have been studied by the way of ultrasonic high temperature measurements. Since such a refractory concrete can be considered as a composite material with two constituents, a continuous matrix (so called bonding phase) and aggregates, investigations of matrices made of mixtures containing cement, reactive alumina and/or magnesia, constitute a preliminary study which is presented in this paper. The elastic behaviour of these matrices has been followed from room temperature to 1550 °C via a specific ultrasonic method. During the first thermal treatment, different changes of slope are observed in the curve E = f(T). Between 200 °C and 400 °C, dehydration mechanisms involve a microstructural reorganisation correlated with a strong decrease of the elastic properties. At high temperature, the Young's modulus evolutions are associated with the expansive formations of CA₂^b and/or in-situ spinel at 1100 °C and then CA₆ (see endnote b) at 1450 °C, which directly depend on the CaO/Al₂O₃ and MgO/Al₂O₃ ratios in the mix. The forming of bond linkage between CA₆ and in-situ spinel grains in the matrix is believed to enhance the elastic properties at high temperature.     

Single-source-precursor synthesis of soft magnetic Fe3Si- and Fe5Si3-containing SiOC ceramic nanocomposites

We present here the single-source-precursor synthesis of Fe₃Si and Fe₅Si₃-containing SiOC ceramic nanocomposites and investigation of their magnetic properties. The materials were prepared upon chemical modification of a hydroxy- and ethoxy-substituted polymethylsilsesquioxane with iron (III) acetylacetonate (Fe(acac)₃) in different amounts (5, 15, 30 and 50 wt%), followed by cross-linking at 180 °C and pyrolysis in argon at temperatures ranging from 1000 °C to 1500 °C. The polymer-to-ceramic transformation of the iron-modified polysilsesquioxane and the evolution at high temperatures of the synthesized SiFeOC-based nanocomposite were studied by means of thermogravimetric analysis (TGA) coupled with evolved gas analysis (EGA) as well as X-ray diffraction (XRD). Upon pyrolysis at 1100 °C, the non-modified polysilsesquioxane converts into an amorphous SiOC ceramic; whereas the iron-modified precursors lead to Fe₃Si/SiOC nanocomposites. Annealing of Fe₃Si/SiOC at temperatures exceeding 1300 °C induced the crystallization of Fe₅Si₃ and β-SiC. The crystallization of the different iron-containing phases at different temperatures is considered to be a consequence of the in situ generation of a Fe–C–Si alloy within the materials during pyrolysis. Depending on the Fe and Si content in the alloy, either Fe₃Si and graphitic carbon (at 1000–1200 °C) or Fe₅Si₃ and β-SiC (at T > 1300 °C) crystallize. All SiFeOC-based ceramic samples were found to exhibit soft magnetic properties. Magnetization versus applied field measurements of the samples show a saturation magnetization up to 26.0 emu/g, depending on the Fe content within the SiFeOC-based samples as well as on the crystalline iron silicide phases formed during pyrolysis.     

Effect of Zn(OH)2 on properties of corundum based castables bonded with calcium aluminate cement

In this work, the effect of Zn(OH)₂ on properties of corundum based castables bonded with calcium aluminate cement (CAC) was investigated. The phase composition and microstructure of castable matrixes containing Zn(OH)₂ after firing 800 °C, 1100 °C and 1550 °C were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectra, respectively. The results indicate that a small amount of Zn(OH)₂ can dramatically improve the medium temperature strength of castables because the generation of zinc aluminate spinel increases the ceramic bonding of castables. In addition, the addition of Zn(OH)₂ also improves the volume stability of CAC-bonded castables due to the enhanced formation of pores from Zn(OH)₂ decomposition in castables.     

Synthesis,Characterization, and Microstructure of ZrC/SiC Composite Ceramics via Liquid Precursor Conversion Method

The ceramic precursor for ZrC/SiC was prepared via solution‐based processing using polyzirconoxane, polycarbosilane, and divinylbenzene. The precursor could be transformed into ZrC/SiC ceramic powders at relative low temperature (1500°C). The cross‐linking process of precursor was studied by FT–IR. The conversion from precursor into ceramic was investigated by TGA, XRD. The ceramic compositions and microstructures were identified by element analysis, Raman spectra, SEM, and corresponding EDS. The results indicated that the ceramic samples remained amorphous below 1000°C and t–ZrO₂ initially generated at 1200°C. Further heating to 1400°C led to the formation of ZrC and SiC with the phase transformation of ZrO₂ and almost pure ZrC/SiC could be obtained upon heat‐treatment at 1500°C. During heat treatments, the ceramic sample changed from compact to porous due to carbothermal reduction. The ceramic powders with particle size of 100 nm~400 nm consisted of high crystalline degree ZrC and SiC phases, and Zr, Si, C were well distributed at the different sites in ceramic powders. The free carbon content was lowered to 1.60 wt% in final ZrC/SiC composite ceramics.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Production of calcium hexaluminate porous planar membranes with high morphological stability and low thermal conductivity

High-quality Al₂O₃ porous ceramic planar membranes suffer from severe deformation and cracking, which occur during sintering process. This study reports on solving this problem, by introducing calcium hydroxide powder in the alumina slurry. Phase-inversion tape-casting technology, applied during molding, and sintering at 1550 °C, favored an in-situ expansion reaction, which effectively suppressed deformation, and well-formed and crack-free calcium hexaluminate porous planar membranes were obtained. The produced membranes had a low thermal conductivity (0.69 W·m⁻¹ K⁻¹ at 85 °C), ascribed to the in-situ formed plate-like structure of calcium hexaluminate (CA₆) and to the high porosity. After hydrophobic modification, the membranes were applied in membrane distillation processing. High rejection rate (>99.9%) and water flux (19.8 L·m^-2 h⁻¹) were achieved at 85 °C, using a 4 wt% NaCl solution as a feed solution.     

Mechanical properties and microstructural evolution of Cansas-III SiC fibers after thermal exposure in different atmospheres

Cansas-III SiC fibers were exposed in argon, air and wet oxygen (12%H₂O+8%O₂+80%Ar) atmospheres for 1 h at 1000–1500 °C. The pristine fiber consisted of β-SiC, free carbon and SiC_xO_y phases. After exposure in air and wet oxygen, an amorphous SiO₂ layer with embedding α-cristobalite crystals formed, while stacking faults were generated in the SiC core to release the residual stress. With the increasing oxidation temperature, lots of pores formed in the oxide layer, accompanied with the thickening, cracking and spallation of oxide layer. The average tensile strength decreased with the exposure temperature increasing and the exposure atmosphere deteriorating (argon→air→wet oxygen). After exposure at 1400 °C in argon and air, the fiber strength retention rates were 84% and 70%, respectively. However, after exposure at 1300 °C in wet oxygen, the strength retention rate was only 51%, indicating the accelerating oxidation and severe strength degradation of fibers.     

Novel Synthesis Method and Characterization of Porous Calcium Hexa‐Aluminate Ceramics

Porous calcium hexa‐aluminate (CA₆) ceramics were in‐situ synthesized by heated CaCO₃ and α‐Al₂O₃ in a NaCl‐based salt at 1400°C for 3 h, and then characterized by X‐ray diffraction (XRD), scanning electron microscopy, (SEM) and mercury porosimetry. The size and morphology of the CA₆ crystals and pores were gradually changed with the increase of NaCl addition in the raw material, indicating that the molten salt not only provided a liquid environment for synthesis of CA₆, but also generated considerable pores.     

Non-shrinkage porous ceramics by In-situ hollow sphere method based on the Kirkendall effect of Al particle

The significant shrinkage of porous ceramics after sintering has produced a number of issues with their use and development. As a result, we proposed an in-situ hollow sphere method for producing non-shrinkage alumina porous ceramics. The obtained green samples were made up of Al₂O₃ and Al powders, with pores emerging inside the materials due to the Kirkendall effect of Al particles after sintering. The expansion of hollowing particles exactly offsets the shrinkage generated by sintering throughout the process. When 50 vol. % Al powder (10 µm) is added, the linear shrinkage rate of the sample after sintering at 1500 °C can reach −3.47 %, and its apparent porosity and flexural strength are 30.69 % and 44.03 MPa, respectively. According to approximate calculations, the pores formed by the oxidation of Al powder are smaller than the initial size of Al powder. This method suggests a novel approach for producing controlled shrinkage porous ceramics.     

Phase equilibrium in binary La2O3-Dy2O3 and ternary CeO2-La2O3-Dy2O3 systems

Cerium oxide doped with oxides of rare earth elements is a multifunctional material, a wide range of uses which is associated with its unique physicochemical properties. Phase diagrams of multicomponent systems are the physicochemical basis for the creation of new materials with improved characteristics.In this work, phase equilibria in ternary CeO₂–La₂O₃–Dy₂O₃ and binary La₂O₃–Dy₂O₃ systems in the whole concentration range were studied. No new phases have been identified in these systems. An isothermal section of the phase diagram of the CeO₂–La₂O₃–Dy₂O₃ system at a temperature of 1500 °С is constructed. No new phases have been detected in the system. It was found that in the studied ternary system solid solutions are formed on the basis of (F) modification of CeO₂ with structure of fluorite type, monoclinic (B), cubic (C) and hexagonal (A) modifications of Ln₂O₃.In the La₂O₃–Dy₂O₃ binary system (1500–1100 °С) three types of solid solutions are formed: based on hexagonal modification A-La₂O₃, monoclinic modification B-Dy₂O₃ and cubic modification C-Dy₂O₃ separated by two-phase fields (A+B) and (B+C), respectively. The boundaries of the regions of homogeneity of solid solutions based on A-La₂O₃ are determined by compositions containing 35–40, 20–25, 15–20 mol% Dy₂O₃ at 1500, 1250, 1100 °C, respectively. From the obtained data it follows that the solubility of Dy₂O₃ in the hexagonal modification of lanthanum oxide is 39 mol% at 1500 °C, 23 mol. % at 1250 °C and 16 mol% at 1100 °C. The limits of existence of solid solutions based on monoclinic B-modification are determined by compositions containing 30–35, 65–60 (1250 °С), 35–40, 55–60 (1100 °С) 40–45, 70–75 (1500 °C) mol% Dy₂O₃.In the studied system, with a decrease in temperature from 1500° to 1100°C, there is a decrease in the solubility of La₂O₃ in the crystal lattice of cubic solid solutions of C-type from 16 to 10 mol%.