Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Thermodynamic evaluation of SiC oxidation in Al2O3–MgAl2O4–SiC–C refractory castables

The literature suggests that MgAl₂O₄ can accelerate SiC oxidation in Al₂O₃–MgAl₂O₄–SiC–C refractory castables. Thus, in this work thermodynamic calculations have been carried out using FactSage^® software in order to explore, search for and understand the role of MgAl₂O₄ on the SiC oxidation. According to the thermodynamic predictions, at 1500 °C and under a reducing atmosphere, there is no evidence that spinel might directly affect SiC oxidation. The increase of SiC content in an Al₂O₃–SiC–C (AL) castable composition was mainly related to the reaction between mullite and carbon. On the other hand, the SiC generation in the Al₂O₃–MgAl₂O₄–SiC–C (SP) composition was a result of the reaction involving liquid SiO₂ and carbon from the refractory. Therefore, the lower SiC content in the SP castable resulted from the refractory's phase transformations. It was also suggested that the samples thermally treated 15 times at 1500 °C did not reach the equilibrium condition, which explains the differences between experimental and thermodynamic results.     

Poly(N-isopropylacrylamide) assisted microwave synthesis of magnesium aluminate spinel oxide for production of highly transparent films by hot compression

Magnesium aluminate spinel oxides have been prepared via poly(N-isopropylacrylamide) assisted microwave technique. The prepared MgAl₂O₄ powders showed a crystalline cubic structure with spinel phase after calcination at 600 °C only. The poly(N-isopropylacrylamide) amount showed a high effect on the crystallite size and the densification behavior of MgAl₂O₄. The increase of the amount of poly(N-isopropylacrylamide) reduced the sintering temperature of MgAl₂O₄ from 1400 °C to 1050 °C. The hot-pressed of MgAl₂O₄ powders in the presence of 3 wt% of poly(N-isopropylacrylamide) exhibited a full density at sintering temperature 1100 °C in 15 min only. The sintered films showed high transparency (81 ± 2%) in the wavelength range 500–1000 nm.     

Influence of post-HIP temperature on microstructural and optical properties of pure MgAl2O4 spinel: From opaque to transparent ceramics

Transparent MgAl₂O₄ spinel ceramics were processed from sub-micrometric commercial powder by applying a two-step procedure: pressureless sintering under vacuum followed by hot isostatic pressing. To limit grain growth and to avoid secondary reactions or impurities, no additives or sintering aids were added to the powder. First, pressureless sintering at 1500 °C during 2 h under vacuum led to opaque samples due to a high level of porosity. To improve the optical quality of the MgAl₂O₄ ceramics and the in-line transmission in the visible range, a post-treatment by hot isostatic pressing was applied. Highly transparent ceramics were obtained after a post-treatment at 1800 °C for 10 h with an in-line transmission of 81% at 400 nm and 86% from 950 to 3000 nm for a thickness of 2 mm (98.8% of the theoretical transmission).     

Strengthening of Al2O3-C slide gate plate refractories with microcrystalline graphite

The novel carbon sources (including nano-carbon black, carbon nanotubes and graphene oxide nanosheets, etc.) have been extensively researched in low carbon Al₂O₃-C refractory systems. In the present work, ultrafine microcrystalline graphite (UMCG) and nickel-loaded ultrafine microcrystalline graphite (NMCG) were added into low carbon Al₂O₃-C slide gate plate refractories to partially replace graphite flake (GF), respectively. The mechanical properties, phase compositions and microstructures were investigated by three-point bending test, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. Also, the reaction mechanisms of in-situ formed ceramic phases were discussed by thermodynamic analysis. The results indicate that the existence of UMCG powders can facilitate the in-situ formation of intertwined ceramic whiskers, leading to increased densification and mechanical properties of low carbon Al₂O₃-C slide gate plate. Moreover, multi-walled carbon nanotubes and ceramic phases intensively interlock with each other in the Al₂O₃-C refractories containing NMCG powders, which results in their better mechanical properties; the cold modulus of rupture are 36.03±0.12 MPa and 32.14±0.17 MPa for the specimens after coking at 1200 °C and 1350 °C, respectively. This work puts forward a practical application for the microcrystalline graphite as a candidate carbon source in Al₂O₃-C slide gate plate refractories.     

Phase formation in Al2O3-C refractories with Al addition

Depending on the recipe and the firing conditions, several non-oxides can be formed in Al₂O₃-C refractories. In this paper, the effect of the purity of the recipe components on the phase formation in Al₂O₃-C refractories with Al addition was investigated. Two test series were sintered from 800 °C to 1600 °C under air embedded in coke breeze. One test series was with brown fused alumina, and the other was with tabular alumina. At temperatures of up to 1200 °C the phase formation was the same for both recipes. For temperatures greater than 1400 °C, the impurities of brown fused alumina enhanced the formation of a polytype, while Al₄O₄C and Al₂₈O₂₁C₆N₆ were formed in the other series. The findings explain the occurrence of several non-oxides in disequilibrium at the chosen temperatures. The occurrence of Al₄C₃ was of particular interest due to its low hydration resistance. It was formed at 1200 °C.     

Effects of LiF on the microstructure and optical properties of hot-pressed MgAl2O4 ceramics

Transparent magnesium aluminate spinel (MgAl₂O₄) ceramics were fabricated by hot-pressing of the MgO and α-Al₂O₃ powder mixture using LiF as a sintering aid. Effects of the LiF additive on densification, microstructure and optical properties of MgAl₂O₄ ceramics were systematically investigated. It has been found that the addition of LiF can effectively remove the porosity and increase the optical transparency of MgAl₂O₄ ceramics. For the spinel ceramics HP-ed at 1550 °C for 3 h with 1 wt% LiF addition, the average grain size is about 36 µm and the in-line transmittance exceeds 60% at the wavelength of 800 nm.     

Development of mullite/zirconia composites from a mixture of aluminum dross and zircon

In order to obtain mullite/zirconia composites, mixtures of aluminum dross and zircon were sintered. Aluminum dross was collected and purified by a milling, sieving and washing process. Stoichiometric mixtures of aluminum dross and zircon were sintered at several temperatures (1400, 1450 and 1500 °C) for several periods of time (2, 4 and 6 h). After the purifying treatment the dross contained mainly Al₂O₃, AlN, MgAl₂O₄, SiO₂ and metallic Al. A homogeneous mullite matrix with small zirconia particles was obtained by sintering the aluminum dross–zircon samples at 1500 °C for 6 h.     

The effect of BaO and Al2O3 addition on the crystallization behaviour of cordierite glass ceramics in the presence of V2O5 nucleant

The effect of V₂O₅ nucleant on crystallization of stoichiometric cordierite glass ceramics in the presence of various amounts of BaO and Al₂O₃ additives were investigated by DTA, XRD and SEM. It was shown that 3 wt.% V₂O₅ and 1.5 wt.% BaO were the optimum amounts of the additives effective in inducing both surface and bulk crystallization in the above glass ceramics.This resulted in ～90 wt.% cordierite after a 3 h heat treatment at 1020 °C. The specimens possessing 4–5 wt.% Al₂O₃ in excess of the stoichiometric cordierite composition, developed mullite along with cordierite in the temperature range of 1045–1055 °C, whereas in the specimen containing 6 wt.% excess Al₂O₃, mullite was detected as the sole crystallization product.     

Hierarchical MgAl2O4 supported Pt-Sn as a highly thermostable catalyst for propane dehydrogenation

Highly thermal stability is an essential property for propane dehydrogenation (PDH) catalysts that need to be regenerated frequently in oxidative atmosphere. A kind of hierarchical MgAl₂O₄ with flower-like morphology is prepared by alcohothermal method and used to support Pt and Pt-Sn. The materials are characterized by ICP-AES, SEM, TEM, XRD, Py-IR, and N₂ physi-sorption. Hierarchical MgAl₂O₄ supported Pt shows great thermal stability under oxidizing atmosphere at 800 °C. Supported Pt-Sn catalyst for PDH shows high stability of performance during 10 cycles of dehydrogenation-regeneration runs. The high stability could originate from the existence of relatively abundant MgAl₂O₄(111) facets on MgAl₂O₄.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

In situ hot elastic modulus evolution of MgO–C refractories containing Al,Si or Al–Mg antioxidants

Metals and alloys (such as Al, Si, Al–Mg and Al–Si) are commonly added to MgO–C refractory bricks as antioxidants due to their effectiveness to prevent carbon oxidation (in the 600–1400 °C range) and their low cost. These additives act at different temperatures and react with refractory components and gases in the environment, inducing significant changes in the resultant microstructure and affecting the overall thermo-mechanical performance of these products. This work addresses the evaluation of physical properties, cold and hot mechanical resistance, as well as in situ hot elastic modulus (E) measurements in the temperature range of 30–1400 °C for MgO–C bricks containing antioxidants (Al, Si or Al–Mg alloy) in a reducing atmosphere. Cured and fired samples of the designed formulations were evaluated throughout 1 or 2 heating-cooling cycles. Despite the improved mechanical behavior (higher cold crushing strength and hot modulus of rupture) of the antioxidant-containing formulations, compared to the additive-free MgO–C sample, the interaction of the selected additives with the refractory components and CO_(g) led to a generation of phases (i.e., Al₄C₃, Al₂O₃, SiC, SiO₂, MgAl₂O₄) that could not be well accommodated in the microstructure. Consequently, the in situ E drop was observed during cooling (mainly below 600 °C) of the antioxidant-containing sample due to crack and flaw formations. Si and Al–Mg were the most promising antioxidants, whereas the Al-containing composition showed the highest E damage level after two heating/cooling cycles up to 1400 °C for cured samples. Based on the elastic modulus profiles with the temperature, the results also indicated the best working conditions for these ceramic materials.     

Dissolution behavior of a novel Al2O3-SiC-SiO2-C composite refractory in blast furnace slag

Al₂O₃-SiC-SiO₂-C composite refractories are interesting potential blast furnace hearth lining materials that feature several advantageous properties. In this study, the corrosion resistance of a novel Al₂O₃-SiC-SiO₂-C composite refractory to blast furnace slag was investigated by adopting a rotating immersion method (25 r/min) at 1450–1550 °C and comparing it against a conventional corundum-based refractory at 1550 °C as a benchmark. The results showed that the apparent activation energy of Al₂O₃-SiC-SiO₂-C composite refractory over the dissolution process in the slag is 150.4 kJ/mol. Dissolution of the Al₂O₃ and 3Al₂O₃·2SiO₂ phases appeared to be the main cause of Al₂O₃-SiC-SiO₂-C composite refractory corrosion. High-melting-point compounds in the slag layer formed a protective layer which mitigated the corrosion. The novel Al₂O₃-SiC-SiO₂-C composite refractory is better suited to blast furnace hearth lining than the conventional corundum-based refractory, because the carbon phase and SiC phase in the material are not readily wetted by the blast furnace slag and therefore are more resistant to slag penetration. Higher melting point phases also may crystallize on the hot face of the hearth lining due to the high thermal conductivity of the Al₂O₃-SiC-SiO₂-C composite refractory, promoting a more stable protective layer.     

Synthesis and characterization of a MgO-MgAl2O4-ZrO2 composite with a continuous network microstructure

Using analytically pure MgO, analytically pure Al₂O₃ and analytically pure ZrO₂ as raw materials, Mg_4.68Al_2.64Zr_1.68O₁₂ was prepared at 1993 K for 10 h, and then, a MgO-MgAl₂O₄-ZrO₂ composite with a continuous network was successfully obtained by controlling the cooling rate based on the in-situ decomposition reaction of Mg_4.68Al_2.64Zr_1.68O₁₂ at temperatures below 1887 K. The three phases of MgO, MgAl₂O₄ and ZrO₂ are highly dispersed in this continuous network microstructure, with ZrO₂ intertwined by MgO and MgAl₂O₄ and micropores with a size of less than 2 µm. Furthermore, the synthesis mechanism of Mg_4.68Al_2.64Zr_1.68O₁₂ is given as follows: first, MgAl₂O₄ is synthesized using the reaction: MgO(s)+Al₂O₃(s)=MgAl₂O₄(s) at temperatures below 1894 K; and then, Mg_4.68Al_2.64Zr_1.68O₁₂ is further prepared through MgO and ZrO₂ diffusion and dissolution into MgAl₂O₄ at temperatures above 1894 K, for example, at 1923 K or 1993 K in this work.     

Synthesizing carbon nanotubes and carbon nanofibers over supported-nickel oxide catalysts via catalytic decomposition of methane

Supported-NiO catalysts were tested in the synthesis of carbon nanotubes and carbon nanofibers by catalytic decomposition of methane at 550 °C and 700 °C. Catalytic activity was characterized by the conversion levels of methane and the amount of carbons accumulated on the catalysts. Selectivity of carbon nanotubes and carbon nanofiber formation were determined using transmission electron microscopy (TEM). The catalytic performance of the supported-NiO catalysts and the types of filamentous carbons produced were discussed based on the X-ray diffraction (XRD) results and the TEM images of the used catalysts. The experimental results show that the catalytic performance of supported-NiO catalysts decreased in the order of NiO/SiO₂ > NiO/HZSM-5 > NiO/CeO₂ > NiO/Al₂O₃ at both reaction temperatures. The structures of the carbons formed by decomposition of methane were dependent on the types of catalyst supports used and the reaction temperatures conducted. It was found that Al₂O₃ was crucial to the dispersion of smaller NiO crystallites, which gave rise to the formation of multi-walled carbon nanotubes at the reaction temperature of 550 °C and a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes at 700 °C. Other than NiO/Al₂O₃ catalyst, all the tested supported-NiO catalysts formed carbon nanofibers at 550 °C and multi-walled carbon nanotubes at 700 °C except for NiO/HZSM-5 catalyst, which grew carbon nanofibers at both 550 °C and 700 °C.     

Chemical oxidation of residual carbon from ZnAl2O4 powders prepared by combustion synthesis

The removal of carbon residue from ZnAl₂O₄ nanopowders by annealing at 500–800 °C leads to a decrease of specific surface area from 228.1 m²/g to 47.6 m²/g. At the same time, the average crystallite size increased from 5.1 nm to 14.9 nm. In order to overcome these drawbacks, a new solution for removing the carbon residue has been suggested: chemical oxidation using hydrogen peroxide. In terms of carbon removal, a H₂O₂ treatment for 8 h at 107 °C proved to be equivalent to a heat treatment of 1 h at 600 °C. The benefits of chemical oxidation over thermal oxidation were obvious. The specific surface area was much larger (188.1 m²/g) in the case of the powder treated with H₂O₂. The average crystallite size (5.8 nm) of ZnAl₂O₄ powder treated with H₂O₂ was smaller than the crystallite size (8.2 nm) of the ZnAl₂O₄ powder annealed at 600 °C.     

Experimental phase relations between MgO-saturated magnesium-aluminate spinel (MgAl2O4) and CuOx-rich liquid

Experimental phase equilibrium data for the Cu-O-Al₂O₃-MgO system is required to improve the performance of MgAl₂O₄-containing refractories and slagging in non-ferrous smelting. In this work, the phase relations of MgAl₂O₄ in the Cu-O-Al₂O₃-MgO system were studied experimentally in air within a temperature range of 1100–1400 °C using the equilibration and quenching method. The chemical compositions of the phases in the quenched samples were determined using electron probe microanalysis (EPMA). Less than 1 wt% of Al₂O₃ or MgO were found in the oxide liquid phase, whereas the solid MgAl₂O₄ and MgO phases contained up to 23 wt% and 30 wt% of ‘Cu₂O’, respectively. Discrepancies between these results and the corresponding calculated values generated by the MTDATA 6.0 software and Mtox database Version 8.2 ranged from 3 wt% to 19 wt%. The results of this work indicate that the MgAl₂O₄ spinel is chemically stable in the presence of a CuO_x-rich liquid under the conditions studied.     

Optical and mechanical properties of amorphous bulk and eutectic ceramics in the HfO2–Al2O3–Y2O3 system

Bulk glasses containing HfO₂ nano-crystallites of 20–50 nm were prepared by hot-pressing of HfO₂–Al₂O₃–Y₂O₃ glass microspheres at 915 °C for 10 min. By annealing at temperatures below 1200 °C, the bulk glasses were converted into transparent glass-ceramics with HfO₂ nano-crystallites of 100–200 nm, which showed the maximum transmittance of ～70% in the infrared region. An increase of annealing temperature (>1300 °C) resulted in opaque YAG/HfO₂/Al₂O₃ eutectic ceramics. The eutectic ceramics contained fine Al₂O₃ crystallites and showed a high hardness of 19.8 GPa. The fracture toughness of the eutectic ceramics increased with increasing annealing temperature, and reached the maximum of 4.0 MPa m^1/2.     

Phase-transformation and grain-growth kinetics in yttria-stabilized tetragonal zirconia polycrystal doped with a small amount of alumina

Microstructure development during sintering in 3 mol% Y₂O₃-stabilized tetragonal zirconia polycrystal doped with a small amount of Al₂O₃ was investigated in the isothermal sintering conditions of 1300–1500 °C. At the low sintering temperature at 1300 °C, although the density was relatively high, the grain-growth rate was much slow. In the specimen sintered at 1300 °C for 50 h, Y³⁺ and Al³⁺ ions segregated along grain boundaries within the widths of about 10 and 6 nm, respectively. In grain interiors, the cubic-phase regions were formed by not only a grain-boundary segregation-induced phase-transformation mechanism but also by spinodal decomposition. The grain-growth behavior was kinetically analyzed using the grain-size data in 1300–1500 °C, which indicated that the grain-growth rate was enhanced by Al₂O₃-doping. These phase-transformation and grain-growth behaviors are reasonably explained by the diffusion-enhanced effect of Al₂O₃-doping.     

Pressureless Sintering of Al2O3/Ni Nanocomposites

In the present study, an Al₂O₃/Ni nanocomposite containing 5 vol% Ni is prepared by pressureless sintering at 1400°C for 2 h. Most nickel inclusions, around 70% in the sintered nanocomposite, locate at the intergranular sites, the triple junctions and Al₂O₃/Al₂O₃ grain boundaries. The average size of the nickel inclusions at the triple junctions, grain boundaries and intragranular locations is 145, 131 and 73 nm, respectively. The average size of all nickel inclusions is 118 nm. The presence of nickel inclusions can prohibit the grain growth of matrix grains. The size of Al₂O₃ grains in the sintered nanocomposite is only 490 nm. The strength of the nanocomposite is thus high for the refined microstructure. The matrix Al₂O₃ grains and Ni inclusions at triple junctions underwent considerable coarsening during a post-annealing treatment at 1300°C for 2 h. The strength of the annealed composites is thus reduced significantly after annealing.