Effect of partial substitution of alumina–chromium slag for Al2O3 on microstructures and properties of Al2O3–SiC–C trough castables

Alumina–chromium slag (ACS), a cheap and abundant refractory raw material comprising aluminum–chromium oxides and β-Al₂O₃, is a byproduct of ferrochrome smelting. For this reason, we investigated the relationships between composition and mechanical properties, abrasion resistance, oxidation resistance, and resistance to iron slag erosion for Al₂O₃–SiC–C trough castables in which ACS was substituted for alumina. Due to the presence of β-Al₂O₃ in ACS, the aluminum-chromium slag reacted with SiO₂ to form a low-melting phase of albite and promoted the formation of mullite, which filled the pores at high temperatures and reduced the porosity, thereby promoting densification and strengthening of the sample. The cold mechanical properties of the sample and the normal temperature wear resistance were enhanced, but the high-temperature mechanical properties and the resistance to iron slag corrosion of the sample were impaired. According to the results of the anti-oxidation experiment, the presence of β-Al₂O₃ in the ACS reduced the porosity and made the sample more dense, which remarkably improved oxidation resistance of the sample. For industrial production requirements, ACS substitution should not exceed 48?wt% due to of thermomechanical properties and anti-slag corrosion performance in Al₂O₃–SiC–C trough castables.     

The corrosion resistance of microsilica-containing Al2O3–MgO and Al2O3–spinel castables

Microsilica addition in Al₂O₃–MgO and Al₂O₃–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA₆ was formed in the matrix, mainly for the Al₂O₃–spinel composition, which also favored the castable dissolution into the molten slag.     

Effect of different corundum sources on microstructure and properties of Al2O3–Cr2O3 refractories

Al₂O₃–Cr₂O₃ refractories are completely substitutional solid solutions and exhibit better corrosion and abrasion resistance. To enable the comprehensive utilization of it, the microstructure and properties of Al₂O₃–Cr₂O₃ samples with different corundum sources were investigated in this study. The starting sources of corundum sources included sintered tabular corundum, fused white corundum, or brown corundum with minor impurities of β-Al₂O₃ and TiO₂. The results of mechanical test showed that the introduction of white corundum deteriorates the physical structure, while brown corundum acts in an opposite manner. The optimum bonding strength of the Al₂O₃–Cr₂O₃ brick was reached by combining white and brown corundum, whereby rapid neck growth occurred via surface diffusion during solid-phase sintering.     

Effect of raw material composition on microstructures and properties of microporous MgO–Mg(Al,Fe)2O4 refractory aggregates

Microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates were prepared by the in-situ decomposition synthesis method using the magnesite, Al(OH)₃ and Fe₂O₃ as raw materials. The effect of raw material composition (theoretical Mg(Al, Fe)₂O₄ contents were 0–55 wt %) on their microstructure and strengths was investigated. When the theoretical Mg(Al, Fe)₂O₄ contents were relatively low (0–5.5 wt %), the number of neck connections between the particles in the microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates was small. As the theoretical Mg(Al, Fe)₂O₄ contents increased to be 11–22 wt %, the number of neck connections increased and the compressive strengths were enhanced. When the theoretical Mg(Al, Fe)₂O₄ contents increased to be excessive (33–55 wt %), the inter-particle pore size further increased due to the increase of volume expansion caused by the formation of more spinel, resulting in a decrease of compressive strength. Overall, when the theoretical Mg(Al, Fe)₂O₄ contents were 11–22 wt %, the microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates showed the excellent performances with the median pore sizes of 17.37–25.46 μm, the apparent porosities of 23.4–28.1%, the bulk densities of 2.57–2.79 g/cm³ as well as the compressive strengths of 41.2–75.8 MPa.     

Effect of MgO/Al2O3 ratio on the crystallization behaviour of Li2O–MgO–Al2O3–SiO2 glass-ceramic and its wettability on Si3N4 ceramic

The composition governs the crystallization ability, the type and content of crystal phases of glass-ceramics. Glass-ceramic joining materials have generated more research interest in recent years. Here, we prepared a novel Li₂O–MgO–Al₂O₃–SiO₂ glass-ceramic for the application of joining Si₃N₄ ceramics. We investigated the influence of the MgO/Al₂O₃ composition ratio on microstructure and crystallization behaviour. The crystallization kinetics demonstrated that the glasses had excellent crystallization ability and high crystallinity. β-LiAlSi₂O₆ and Mg₂SiO₄ were precipitated from the glass-ceramics, and the increase of MgO concentration was conducive to the precipitation of Mg₂SiO₄. Among the glass-ceramic samples, the thermal expansion coefficient of LMAS2 glass-ceramic was 3.1 × 10^?6/°C, which was very close to that of Si₃N₄ ceramics. The wetting test showed that the final contact angle of the glass droplet on the Si₃N₄ ceramic surface was 32° and the interface was well bonded.     

Al2O3-based binders for corrosion resistance optimization of Al2O3–MgAl2O4 and Al2O3–MgO refractory castables

This work addresses the main aspects related to the use of alternative binders [hydratable (HA) or colloidal alumina (ColAlu)] in castables containing different spinel sources (pre-formed or in situ generated), in order to point out: (i) the features that control the corrosion behavior of these materials, and (ii) the key factors to better select a refractory composition. Thermodynamic calculations, corrosion cup-test and SEM analyses were carried out in order to evaluate the slag attack of the designed refractory compositions. According to the attained results, the alumina-based binders (HA or ColAlu) induced a more effective sintering process due to their high specific surface area, improving the physical properties and the binding level of the generated microstructure. The spinel grain size also played an important role in the corrosion behavior of these refractories, as the finer the particles, the greater their dissolution was into the molten liquid, leading to further precipitation of spinel in the solid–liquid interface as a continuous and thick layer. Among the evaluated compositions and considering the presence of silica fume, the most suitable formulation with optimized corrosion resistance was the one with in situ spinel generation and HA as a binder.     

The influence of Al2O3 nanoparticles addition on the microstructure and properties of bauxite self–flowing low-cement castables

In this research, the impact of Al₂O₃ nanoparticles addition on microstructure, mechanical, and physical properties of bauxite self–flowing low-cement castables were investigated. Also, the optimum amount of Al₂O₃ nanoparticles is determined. For this propose, up to 3 wt% Al₂O₃ nanoparticles were added to the bauxite castable compositions. The physical and mechanical properties of castable compositions such as bulk density (BD), apparent porosity (AP), self-flow values (SFV), and cold crushing strength (CCS) were examined. Also, the X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDX) techniques were used for detection the ceramic phase's formation and microstructural analysis of the castables compositions, respectively. Results show that addition of Al₂O₃ nanoparticles up to 1 wt% improved the properties of bauxite self–flowing low-cement castables. As well as, the use of Al₂O₃ nanoparticles led to the formation of the platy and needle crystalline phases such as hibonite (CaO·6Al₂O₃), calcium dialuminate (CaO·2Al₂O₃), and mullite (3Al₂O₃·2SiO₂), between the grain boundaries of the bauxite particles. Also, Al₂O₃ nanoparticles addition led to aforementioned phase formation occur at the lower temperatures.     

Thermal shock resistance of Cr2O3–Al2O3–ZrO2 bricks with the microstructure of Cr2O3 aggregates coated by ZrO2

Zirconia-coated Cr₂O₃ aggregates synthesized by mixing ZrO₂ powders and Cr₂O₃ aggregates with a phenolic resin binder followed by thermosetting treatment were employed as modified Cr₂O₃ aggregates to obtain Cr₂O₃–Al₂O₃–ZrO₂ bricks (high-chromia bricks). The elastic modulus (E) and cold modulus of rupture (CMOR) of these high-chromia bricks before and after thermal shock cycles were systematically investigated, and the residual ratios of CMOR and E were calculated. The thermal shock resistance of the high-chromia bricks was significantly improved by the factor of modification of Cr₂O₃ aggregates. The mechanism of the improved thermal shock resistance of these high-chromia bricks was studied via microstructure analysis, and the crack propagation behavior was analyzed by scanning electron microscopy (SEM). The fracture work (γ_WOF), thermal shock damage factor (R′′′′), and thermal stress crack stability parameter (R_st) were measured and calculated using the wedge splitting test (WST). The results indicate that the porous ZrO₂ coating layer wrapped the Cr₂O₃ aggregates, forming modified Cr₂O₃ aggregates that can increase crack deflection, free path of crack propagation, and fracture work, thus improving the thermal shock resistance of high-chromia bricks. The thermal shock resistance of the fabricated high-chromia bricks was highly correlated with the thickness of the ZrO₂ coating layer surrounding the Cr₂O₃ aggregates. The variation trend of R_st is well consistent with the experimental results, which is suitable to evaluate the thermal shock resistance of high-chromia bricks.     

Effect of surface properties of MgO on its dissolution and spinel growth in CaO–SiO2–Al2O3 slag

The dissolution behavior of MgO in CaO–SiO₂–Al₂O₃ ternary slag at the interface of single-crystal, dense poly-crystal, and porous poly-crystal MgO was investigated to evaluate the effect of the surface properties of the MgO. The experimental results revealed that a detached spinel layer formed at the MgO interface due to the change in thermodynamic condition of the slag, which was independent of the surface properties. On the other hand, it was also confirmed that the growth rate and morphology of the detached spinel layer strongly depended on the surface properties, such as porosity and curvature of MgO. During the formation of the spinel layer at the interface during MgO dissolution, a kinetic approach adopting parabolic relation theory was employed to determine the correlation between the surface properties and the spinel growth mechanism.     

Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.     

Simulation and experimental investigation of preferred β-Sialon growth and its effects on thermo-mechanical properties of Al2O3–C refractories

β-Sialon bonded Al₂O₃–C refractories possess high strength and superior thermal shock performance. In this study, the growth of preferred β-Sialon (Si₃Al₃O₃N₅) and its effects on thermo-mechanical properties of Al₂O₃–C refractories were investigated via simulations and experiments. The results indicate that the additive Fe₂O₃ contributed to the formation of β-Sialon and helped its column structure become plate-like. Transmission electron microscopy confirmed that the (101) crystal plane was a growth plane of plate-like β-Sialon. The growth mechanism of β-Sialon was suggested by density functional theory; calculation results revealed that the key step for the formation and growth of β-Sialon was the adsorption of the gaseous molecule Al₂O on the Si₃N₄ (101) crystal plane. It was found that the existence of Fe atoms could significantly reduce the adsorption energy. Additionally, Al₂O₃–C refractories containing plate-like β-Sialon possessed a high cold modulus of rupture and crushing strength, which increased by 40% and 15%, respectively, compared with the specimens containing column β-Sialon. It was also found that the formation of plate-like β-Sialon resulted in significantly better thermal shock resistance for the Al₂O₃–C refractory specimens, and the residual strength loss ratio of the sintered specimens was only 4% after five thermal shock cycles.     

Mechanism of in-situ formation of spinel and its effect on the mechanical properties of Al2O3–C refractories

This work looked at the in-situ formation mechanism of magnesia alumina spinel in Al₂O₃–C refractories with magnesia addition at different firing temperatures. A comprehensive study on the mechanical properties of Al₂O₃–C refractories was performed in comparison to traditional analogs. The magnesia alumina spinel was in-situ formed at the firing temperature of 1150 °C in Al₂O₃–C refractories. With the increase of the firing temperature, the Al₂O₃ phase was gradually dissolved in spinel phase to form aluminum rich spinel phase, resulting in a decrease in its lattice constant due to the defects formation. The formed spinel phase was homogenously distributed and bonded well with corundum, improving the interfacial bond, load transferring capacity and crack propagation resistance. The formation of spinel phase also enhanced the sintering of the alumina matrix owing to the solid solution of alumina in the spinel. Therefore, the mechanical properties such as cold modulus of rupture and hot modulus of rupture in Al₂O₃–C refractories achieved a substantial enhancement compared with traditional refractories.     

Preparation and study of the mechanical and optical properties of infrared transparent Y2O3–MgO composite ceramics

In this study, fine Y₂O₃–MgO composite nanopowders were synthesized via the sol–gel method. Dense Y₂O₃–MgO composite ceramics were fabricated by pre-sintering the green body in air at different temperatures for 1 h and then subjecting the sintered bodies to hot isostatic pressing at 1300°C for 1 h. The effects of pre-sintering temperature on the microstructural, mechanical, and optical properties of the resulting ceramics were studied. The average grain size of the ceramics was increased, whereas their hardness and fracture toughness were decreased with increasing pre-sintering temperature. A maximum fracture toughness of 1.42 MPa·m^1/2 and Vickers hardness of 10.4 GPa were obtained. The average flexural strength of the ceramics was 411 MPa at room temperature and reached 361 MPa at 600°C. A transmittance of 84% in the 3–5 µm region was obtained when the composite ceramics were sintered at 1400°C. Moreover, a transmittance of 76% in the 3–5 µm region was obtained at 500°C.     

Phase transformation and microstructure of MgO–Al2O3–SiO2 system glass–ceramics under different heat treatment conditions

Abstract

Effects of heat treatment conditions on phase transformation, microstructure and thermal expansion coefficient (TEC) in MgO–Al₂O₃–SiO₂ system glass–ceramics were investigated by means of differential thermal analysis, X-ray diffraction and scanning electron microscopy. The magnesium aluminium titanate (MAT) precipitated firstly at 850°C and β-quartz solutions (β-QSS) formed at 950°C. Further increasing temperature to 1000°C, MAT disappeared and β-QSS became master phase, following little amount of α-cordierite, MgTi₂O₅, rutile and sapphirine. When glass was treated at 1050°C, β-QSS content decreased and α-cordierite became master phase. As temperature reached higher than 1100°C, β-QSS and sapphirine disappeared, and α-cordierite became master phase accompany with rutile and MgTi₂O₅ as secondary phase. The microstructure transformed gradually from particle shape crystallites to slat shape network with the increase in heat treatment temperature. By controlling heat treatment condition, an ideal glass–ceramics with proper TEC for matching sealing to 4J29 alloy has been obtained.     

Effect of carbon black on corrosion resistance of Al2O3–SiC–C castables to SiO2–MgO slag

Metallurgical solid waste recycling is the shape of things to come in green development of Chinese iron and steel industry. Utilization of ironworks slag for producing mineral wool at high temperature is an important approach. However, refractory lining is seriously corroded by the SiO₂–MgO based slag at 1600 °C during the production process. Different production steps need different atmospheres, the changeable service atmospheres (air and reducing atmosphere) put forward high requirements for slag resistance. The Al₂O₃–SiC–C castables containing carbon black are usually used in iron runner, which faces high-temperature service condition of 1450 °C–1500 °C. Nevertheless, the function of carbon black in the Al₂O₃–SiC–C castables at 1600 °C is till essentially unknown. In the current study, the carbon black was introduced to tabular alumina based Al₂O₃–SiC–C castables to improve corrosion resistance to SiO₂–MgO based slag at 1600 °C. The result showed that 0.4 wt% carbon black was suitable for the castables, which the slag resistance of castables was significantly improved. The carbon black had contributed to block slag by wettability resistance. By comparison with the castables without carbon black, the corrosion index and penetration index had been reduced by 20.2% and 28.0%, respectively, under air atmosphere. And there were little corrosion or penetration under reducing atmosphere for castables with 0.4 wt% carbon black. For the mechanical properties, the Al₂O₃–SiC–C castables with 0.4 wt% carbon black could serve production process although the carbon black impaired the physical properties.     

Effect of TiO2 on crystallization and mechanical properties of MgO–Al2O3–SiO2 glasses containing P2O5

MgO–Al₂O₃–SiO₂ glass-ceramics have been widely used in military, industrial, and construction applications. The nucleating agent is one of the most important factors in the production of glass-ceramics as it can control the crystallization temperature or the grain size. In this study, we investigated the effect of replacing P₂O₅ with different amounts of TiO₂ on the crystallization, structure, and mechanical properties of an MgO–Al₂O₃–SiO₂ system. The crystallization and microstructure were investigated by differential scanning calorimetry, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The mechanical properties were investigated by measuring the Vickers hardness, Young's modulus, and fracture toughness. The results showed that adding TiO₂ favored the precipitation of fine grains and significantly increased the Vickers hardness, Young's modulus, and fracture toughness of the glasses. Introducing an appropriate amount of TiO₂ can make a glass structure more compact, promote crystallization, and improve the mechanical properties of MgO–Al₂O₃–SiO₂ glass-ceramics.     

Effect of bimodal microstructure on high-temperature properties of nano-reinforced Al2O3–MgO–C refractories

The beneficial effects of adding nanostructured expandable graphite (EG) hybridized yttrium aluminium garnet (EG\YAG) powder as a composite reinforcement in improving the oxidation resistance, hot-strength, and microstructure development in Al₂O₃–MgO–C refractories were studied. The refractory components reinforced with EG\YAG exhibited more than 60% of oxidation resistance enhancement and as high as 200% increase in hot-strength performance over the standard refractories, formulated without EG\YAG. Correlating the damage parameter (D_E) calculations based on ultrasonic measurements with residual strength data (R_c, R_b) showed that there was a progressive increase in R_c and R_b values with consistent reduction in the oxidative damage of EG\YAG reinforced refractories. Analysis indicated that these beneficial features were majorly ascribed to the in-situ development of bimodal microstructure with EG\YAG sintered framework throughout the refractory interior in these new class of reinforced systems. Additionally, the mechanism of toughening and implications of these results to materials design are discussed.     

Preparation of AlN–Al2O3 composites exhibiting antibacterial and water purification properties

This study aimed to develop new antibacterial and water purification materials without heavy metal contamination. Herein, AlN–Al₂O₃ composites were prepared by changing the content of AlN in raw material. The results showed that AlN, Al₂O₃, aluminum oxynitride, and yttrium aluminum garnet phases were generated by adjusting the AlN: Al₂O₃ ratio. The difference in the ability of AlN and aluminum oxynitride to release substances such as ammonium ions and aluminum hydroxide when reacting with water resulted in remarkably different pH values of the sample immersion solution, which led to an increase in the material antibacterial efficiency with the addition of AlN. Similar results were also obtained with zinc ion absorption. Therefore, AlN–Al₂O₃ composite ceramics can potentially be used as novel antibacterial and water purification materials without heavy metal contamination through the release of ammonium ions for conferring antibacterial effects and of aluminum hydroxide for absorbing heavy metals and suspended impurities.     

Preparation and properties of porous SiC–Al2O3 ceramics using coal ash

In this paper, we first reported that porous SiC–Al₂O₃ ceramics were prepared from solid waste coal ash, activated carbon, and commercial SiC powder by a carbothermal reduction reaction (CRR) method under Ar atmosphere. The effects of addition amounts of SiC (0, 10, 15, and 20 wt%) on the postsintering properties of as-prepared porous SiC–Al₂O₃ ceramics, such as phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal shock resistance, and thermal diffusivity have been investigated. It was found that the final products are β-SiC and α-Al₂O₃. Meanwhile, the SEM shows the pores distribute uniformly and the body gradually contacts closely in the porous SiC–Al₂O₃ ceramics. The properties of as-prepared porous SiC–Al₂O₃ ceramics were found to be remarkably improved by adding proper amounts of SiC (10, 15, and 20 wt%). However, further increasing the amount of SiC leads to a decrease in thermal shock resistance and mechanical properties. Porous SiC–Al₂O₃ ceramics doped with 10 wt% SiC and sintered at 1600°C for 5 hours with the median pore diameter of 4.24 μm, room-temperature compressive strength of 21.70 MPa, apparent porosity of 48%, and thermal diffusivity of 0.0194 cm²/s were successfully obtained.     

Raman and X-ray photoelectron spectroscopy study on the influence of La2O3 on the melt structure of SiO2–CaO–Al2O3–MgO

In order to gain more insights into the influence of rare earth elements on the melt structure of SiO₂–CaO–Al₂O₃–MgO glass ceramics, Raman and X-ray photoelectron spectroscopy techniques were used to study the influence of La₂O₃ on the Si–O/Al–O tetrahedron structure within SiO₂–CaO–Al₂O₃–MgO–quenched glass samples in this study. Results showed that some Raman peak shapes at low frequencies (200–840 cm^?1) changed significantly after the addition of La₂O₃, compared to the high frequency (840–1200 cm^?1) region that corresponds to the [SiO₄] structure, suggesting that the depolymerization of the low-frequency T–O–T (T=Si or Al) structure was more prevalent with La³⁺ addition. Besides, the depolymerization extent of the Si–O/Al–O tetrahedral network varied when the melt composition altered. Most notably, depolymerization is the most significant at a low CaO/SiO₂ ratio (0.25) and a high Al₂O₃ content (8%). Meanwhile, La³⁺ can promote the transformation of Si–O–Si and Al–O–Al bonds to the Si–O–Al ones, thereby forming a complex ionic cluster network interwoven with Si–O and Al–O tetrahedrons.