Enhancement of bonding network for silica sol bonded SiC castables by reactive micropowder

Silica sol bonded castables have obvious advantages over low cement or hydratable alumina bonded castables in drying performance and sintering properties for SiC castables. However, they are not widely used due to their weak strength at low temperature. The efficiency of bonding network for silica sol bonded SiC castable in the presence of different reactive micropowder such as SiO₂ micropowder and α-Al₂O₃ micropowder was evaluated through oscillatory tests, sintered properties and microstructural analysis. Results show that the polymerization reaction between SiO₂ micropowders enhanced the siloxane network and reinforced the bonding strength, furthermore, the addition of α-Al₂O₃ micropowder contributed to accelerating the formation of the siloxane network and hardening of the silica sol at lower temperatures and shorter time. Silica sol performed well as a binder agent for SiC castables with an addition content of 3 wt% SiO₂ micropowder and 2 wt% α-Al₂O₃ micropowder, which showed high strength and good workability at room temperature. And Silica sol bonded SiC castable with the above micropowder contents possessed the best mechanical behavior after heat treatment due to combined binding of SiC whiskers and mullite.     

Combined effects of SiO2 ratio and purity on physical properties and microstructure of in situ alumina-mullite ceramic

The in situ formation of mullite (Al₆Si₄O₁₃) is a complex process based on solid-state reactions strongly affected by the characteristics of Al₂O₃ and SiO₂ sources. This study investigated the combined effects of variable SiO₂/Al₂O₃ ratios and the presence of low-melting-point impurities on the physical properties and microstructure of in situ alumina-mullite ceramics. Two grades of synthetic amorphous silica, known as microsilica, of similar physical properties and significant differences in the content of alkali-based impurities (0.8 and 3.6 wt%), were combined with thin calcined alumina particles in different proportions (from silica-free up to stoichiometric mullite), pressed and sintered. The samples were tested for total porosity, flexural strength, and pore size distribution, and their crystalline phases and microstructures formed were investigated. Small amounts of both microsilica grades (up to 8.9 wt%) hindered the densification of the alumina-mullite matrix and favored grain growth events, increasing porosity and reducing strength. For samples containing higher microsilica loads (16.4–28.2 wt%), the impurities content significantly affected the amount of liquid phases formed. Such impurities altered the ratio and shape of the pores and the total amount of mullite after sintering. Therefore, different microstructures and levels of flexural strength and total porosity were observed.     

Enhancement of catalytic activity of alumina by copper addition for selective reduction of nitrogen monoxide by ethene in oxidizing atmosphere

The catalytic activity of alumina for the title reaction has been found to be greatly improved by the loading of copper. The addition of copper resulted in lowering the active temperature region, the higher maximum activity, and the enhancement of the reaction rate. The maximum effect was observed at 0.3 wt% of the loading amount of copper. A similar enhancement was also confirmed on SiO₂-Al₂O₃.     

Fabrication and microstructural characterization of the novel optical ceramic consisting of α-Al2O3@amorphous alumina nanocomposite core/shell structure

In this study, α-Al₂O₃@amorphous alumina nanocomposite core-shell structure was synthesized from AlCl₃ and the commercial α-Al₂O₃ nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was transformed to the amorphous and γ-Al₂O₃ phases after the calcination process and the shell material was completely converted to γ-Al₂O₃ at 1000?°C. However, for the α-Al₂O₃@amorphous alumina core-shell nanoparticles were completely converted to α-Al₂O₃ at 1000?°C. It can be concluded that α-Al₂O₃ core particles, as the seed crystalline, help to transforming of γ-Al₂O₃ phase as the shell material directly without forming transitional phases to α-Al₂O₃. The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al₂O₃@amorphous alumina core-shell nanocomposite. The body sintered has a final density of ～99.8% and the in-line transmittance value is ～80% within the IR range.     

Fabrication of mullite-bonded porous silicon carbide ceramics by in situ reaction bonding

An in situ reaction bonding technique was developed to fabricate mullite-bonded porous silicon carbide (SiC) ceramics in air from SiC and α-Al₂O₃, using graphite as the pore-former. Graphite is burned out to produce pores and the surface of SiC is oxidized to SiO₂ at high temperature. With further increasing the temperature, the amorphous SiO₂ converts into cristobalite and reacts with α-Al₂O₃ to form mullite (3Al₂O₃·2SiO₂). SiC particles are bonded by the mullite and oxidation-derived SiO₂ to obtain porous SiC ceramics. The reaction bonding behavior, open porosity, pore size distribution and mechanical strength of porous SiC ceramics were investigated as a function of the sintering temperature, forming pressure and graphite content. In addition, the phase composition and microstructure were also studied.     

Recycling of waste lubricant oil into chemical feedstock or fuel oil over supported iron oxide catalysts

The recycling of waste lubricant oil from automobile industry was found to be best alternative to incineration. Silica (SiO₂), alumina (Al₂O₃), silica-alumina (SiO₂-Al₂O₃) supported iron oxide (10 wt% Fe) catalysts were prepared by wet impregnation method and used for the desulphurisation of waste lubricant oil into fuel oil. The extent of sulphur removal increases in the sequence of Fe/SiO₂-Al₂O₃<Fe/Al₂O₃<Fe/SiO₂ and this might be due to the presence of smaller crystalline size (7.4 nm) of Fe₂O₃ in Fe/SiO₂ catalyst. X-ray diffraction results suggest the presence of iron sulphide in the used catalyst. Gas chromatography with thermal conductivity detector analysis confirms the presence of H₂S in gaseous products. In addition, Fe/SiO₂ catalyst facilitated the formation of lower hydrocarbons by cracking higher hydrocarbons (≈C₄₀) present in waste lubricant oil.     

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.     

Effects of different additives on properties of magnesium aluminate Spinel–Periclase castable

Bauxite- and alumina-based spinels were employed as refractory aggregates, and sintered magnesia fine powder, calcium aluminate cement, microsilica, and activated α-Al₂O₃ were utilized as matrices. The effects of alumina powder, analytically pure zinc oxide, and analytically pure zirconia on the properties of magnesium aluminate spinel–periclase castables were studied. The results demonstrated that the addition of the three additives promoted the sintering of magnesium aluminate spinel–periclase castables. Simultaneously, the three additives significantly improved the high-temperature properties of the samples. The thermal shock resistance of the alumina powder sample increased by 200%, that of the pristine zinc oxide sample by 75%, and that of the zirconia sample by 125%. The additives effectively improved the thermal shock resistance of the magnesium aluminate spinel–periclase castable. In addition, the slag resistance depths of the samples with alumina powder and zirconia were 41% lower than that of the sample without additives, which significantly improved the slag resistance of the magnesium aluminate spinel–periclase castable.     

Effect of boehmite sol on the performance of alumina microfiltration membranes

Precursor film method was used to prepare highly permeable ceramic microfiltration membranes in this work. The performance of alumina microfiltration membranes was improved by adding boehmite sol in the membrane precursor film. With increasing the boehmite sol content, the effective average pore size of the membrane was continuously decreased and the separation efficiency of the membrane was increased. These improvements were due to that the boehmite sol not only helped the dispersion of the α-Al₂O₃ powder in the membrane forming slurry, but also formed γ-Al₂O₃ in the gaps among the existed α-Al₂O₃ particles which was beneficial for membrane sintering and pore size decreasing. For the membrane prepared with 45 wt% boehmite sol, the effective average filtration pore diameter was 178 nm and the water permeance of the membrane reached 1691 Lm⁻²h⁻¹bar⁻¹, which was much better than the values reported before. Moreover, the reusability of the membrane was confirmed using a recycling test.     

Effect of grain boundary state and grain size on the microstructure and mechanical properties of alumina obtained by SPS: A case of the amorphous layer on particle surface

The work was aimed at the investigation of kinetics of Spark Plasma Sintering (SPS) of the α-Al₂O₃ particles with amorphous surface layers and investigation of the effect of the amorphous layers on the grain growth and on the mechanical properties of alumina. The objects of investigations comprised:(i) submicron α-Al₂O₃ powder, (ii) submicron α-Al₂O₃ powder with the amorphous layers on the particles' surfaces, and (iii) the fine-grained α-Al₂O₃ powder. The submicron powders (i) and (ii) were used to analyze the effect of the amorphous layers on the sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle sizes on the shrinkage kinetics. The effect of the temperature regime and of the rate (V_h) on the shrinkage kinetics of the submicron and fine alumina powders has been studied. The shrinkage curves were analyzed using the Young–Cutler and Coble models. The sintering kinetics was shown to be determined by the intensity of grain boundary diffusion for the submicron powders and by simultaneous lattice diffusion and grain boundary one for the fine powders. The amorphous layers on the surfaces of the submicron α-Al₂O₃ particles were found to affect the grain boundary migration rate and the Coble equation parameters at the final stages of SPS. The abnormal characteristics of the alumina ceramics sintered from the submicron powder with the amorphous layers on the particles’ surfaces were suggested to originate from the increased concentration of the defects and of the excess free volume at the grain boundaries formed during crystallization of the amorphous layers.     

Fabrication of lightweight alumina with nanoscale intracrystalline pores

In this study, lightweight alumina containing nanoscale intracrystalline pores was fabricated by introducing zirconia sol and alumina sol into α-Al₂O₃ micropowder. The effects of zirconia sol on the sintering behavior of lightweight alumina were also investigated. With the introduction of zirconia sol, the grain growth and phase transformation of nano-alumina are delayed. Therefore, the structure of the nanoscale pores between nanoparticles can be stabilized during heat treatment, and numerous nanoscale intracrystalline pores with diameters in the range of 50-300 nm are trapped in the alumina grains. Because of the existence of these nanoscale intracrystalline pores, the thermal conductivity of lightweight alumina decreases significantly. Lightweight alumina with the addition of 0.75 wt% zirconia sol exhibits 31% lower thermal conductivity compared to alumina without zirconia sol.     

Nano-bonded refractory castables

Calcium aluminate cement (CAC) contents higher than 3 wt% in refractory castables can have some drawbacks in the various processing steps (mainly drying) and also in their refractoriness when in contact with SiO₂. The use of colloidal silica as an alternative binder has been studied by many researchers in recent years and recently reports have also explored the use of colloidal alumina for the same purpose. This article reviews the recent developments in nano-bonded refractory castables focusing on the use of colloidal silica or alumina. In the first part of the paper, a comparison of different binding systems for refractory castables is shown. The benefits of replacing CAC or hydratable alumina by colloidal binders are discussed. In the second part, the advantages of colloidal silica/alumina as a refractory binder are highlighted. Meanwhile, the characterization techniques and functional mechanisms of these binders are presented in order to understand the behavior of these systems. Finally, in the last section, the challenges for suitable use of colloidal binders are discussed and the future direction of nano-structured refractory castables is outlined.     

Sintering of α-Al2O3-seeded nanocrystalline γ-Al2O3 powders

This paper demonstrates that seeding nanocrystalline transition alumina powders is a viable option for producing high quality, alumina-based ceramics. By using α-Al₂O₃ concentrations of ⩾1.25 wt.% α-Al₂O₃ seed particles (equivalent to 5 ×10¹⁴ seeds/cm³ of γ-Al₂O₃) the sintering temperature is reduced from 1600°C for unseeded γ-Al₂O₃ to 1300–1400°C in dry pressed powders. The scale of the sintered microstructure is related to N_v^−1/3 and thus a 100-nm grain size is obtained. It is apparent that seeding is necessary for producing dense, alumina-based ceramics from nanocrystalline transition alumina powders.     

Synthesis and two-step sintering behavior of solution derived corundum abrasives with plate-like grains

Corundum abrasives with plate-like grains were fabricated by a two-step sintering technique using the solution-based process with the addition of the ternary compound additive Na₃AlF₆-CaO-SiO₂. The two-step sintering method showed obvious advantages over conventional sintering methods in promoting sample densification, suppressing grain growth, and homogenizing the microstructure of the corundum abrasives. The sample doped with 2.5?wt% Na₃AlF₆ and 4?wt% CaO + SiO₂ in the molar ratio of 1:1 possessed a relative density of 99.3%, average grain size of 0.54?µm, and single-particle compressive strength of 49?N. The introduction of seeds reduced the temperature of θ- to α-Al₂O₃ phase transformation. The relationship between the microstructure and the mechanical properties of the abrasives was also discussed.     

Modelling and optimizing the liquid phase sintering of alumina/CaO–SiO2–Al2O3 ceramics using response surface methodology

Alumina is an attractive material for engineering applications due to its unique properties. In this study, CaO–SiO₂–Al₂O₃ eutectic phase was used as an additive phase and liquid phase sintering of the alumina/CaO–SiO₂–Al₂O₃ samples were investigated. The liquid phase sintering was modelled and optimized by Response Surface Methodology (RSM) using Central Composite Design (CCD) to achieve maximum fracture strength and density as responses. Sintering temperature, alumina particles size distribution (PSD), lubricant and eutectic phase content were selected as independent variables. Two cubic models were developed in terms of these variables to describe the responses. The validity and accuracy of the models were checked using Analysis of Variance (ANOVA).Phase identification of the synthesized eutectic phase was evaluated by XRD and fracture strength of the sintered samples was determined by Ring-on-Ring test method. SEM was used to study the fracture surface of the samples. The obtained models for predicting fracture strength and density of the sintered samples showed high conformity with the experimental results. Sintering temperature and alumina PSD were found as the most effective parameters. Therefore, optimized condition based on the defined constraints was obtained for sintering temperature of 1533 °C, alumina PSD of 25%, and lubricant and eutectic phase content of 1.5 wt% and 7.5 wt%, respectively. Results showed that after ball milling of the eutectic phase, the fracture strength of the optimized ceramic sample was improved and it reached to maximum values at smaller amounts of the additive phase.     

M- S-H formation in MgO-SiO2slurries via wet milling for magnesia based castables

It is believed that the formation of hydration phase, MgO-SiO₂-H₂O (M-S-H), contributes to good workability and reliable comprehensive properties for magnesia based castables. In order to stimulate the formation of M-S-H in magnesia based castables and understand the minimum introduction of microslica amount, wet milling process was used to promote the dissolution of MgO and SiO₂ in this work. The slurry containing different content of microsilica with wet milling technology and the castables with/without wet milling slurry were prepared. The effects of microsilica content on the formation of hydration phases were analyzed by XRD, FT-IR and TG/DSC and the properties of magnesia based castables were evaluated by explosion resistance, CMOR, HMOR and so on. The results showed that the formation of M-S-H was accelerated because of the dissolution of Mg²⁺ and HSiO₃^? in wet milling process. Higher amount of M-S-H led to a tight bonding in the early stage, and a denser structure after firing at high temperature due to the limited formation of brucite and in-situ formation of evenly distributed forsterite phase. In addition, much higher HMOR were obtained when less microsilica was added, attributing to the suppressed formation of low-melting-point liquid. Therefore, 2–3 wt% microsilica addition was recommended in this process.     

Facile one-step precursor-to-aerogel synthesis of silica-doped alumina aerogels with high specific surface area at elevated temperatures

Silica-doped alumina aerogels offer the potential alternative to the applications as thermal insulators, catalysis, or catalytic support at elevated temperatures. However, the production process of silica-doped alumina aerogels was complicated and time-consuming. We developed a one-step precursor-to-aerogel method of silica-doped alumina aerogels with high specific surface area and thermal stability. Compared to conventional methods, the developed method reduced time and solvent waste of alumina-based aerogels production. Here, we investigated the alumina aerogels doped with silica to stabilize γ-phase at higher temperatures. XRD, FTIR, TEM, TG-DSC, and BET analysis results showed that silica stabilized the γ-Al₂O₃ at 1200 °C. The stabilization mechanism analysis showed that silica addition could significantly hinder the contact among alumina particles and the formation of necks in the sintering process, thereby retarding the transition of γ–θ phase and maintaining the high specific surface area at elevated temperatures. Silica and alumina particles formed mullite at 1200 °C, which could suppress α-phase transformation. In addition, silica-doped alumina aerogels exhibited the high specific surface area of 311 m²/g at 1000 °C and 146 m²/g at 1200 °C when the silica content was in the range of 10.6–13.1 wt%.     

Effect of MgO micropowder on sintering properties and microstructures of microporous corundum aggregates

Microporous corundum aggregates were fabricated based on superplasticity, with α-Al₂O₃ micropowder as the main raw material, by adding nano-sized alumina sol and MgO micropowder. The relationship between the superplasticity and the in-situ stress during sintering, the corresponding sintering properties, and the microstructures of the fabricated microporous corundum aggregates were investigated by means of X-ray diffraction (XRD), mercury porosimetry, and scanning electron microscopy (SEM). The experimental results show that the relationship between the superplasticity and the in-situ stress is the main factor that influences the sintering behavior of the microporous corundum aggregates. Various amounts of MgO micropowder were added to the α-Al₂O₃ micropowder for a fixed content of nano-sized alumina sol. With increasing MgO micropowder, which results in a greater in-situ stress, the total and closed porosity increased initially and decreased afterwards whereas the apparent porosity and bulk density decreased first and then increased. Microporous corundum aggregates with high closed porosity, low apparent porosity and small pore size were obtained with the addition of 1 wt% MgO micropowder. Therefore, the relationship between the superplasticity and the in-situ stress should be controlled in order to fabricate microporous refractory aggregates which have high closed porosity and small pore size.     

Stability of Pt/γ-Al2O3 Catalysts in Model Biomass Solutions

The stability of a Pt/??-Al₂O₃ catalyst in liquid water and aqueous solutions of 5?wt% glycerol or sorbitol at 225?°C is examined using a variety of physicochemical methods. It is demonstrated that the presence of glycerol and sorbitol significantly reduces the hydration of ??-Al₂O₃ to form boehmite as compared to treatment in pure water. The stability against hydration increases with increasing carbon chain length. Treatment with polyol solutions also results in reduced agglomeration of supported metal particles. The prevention of boehmite formation and agglomeration of metal particles are attributed to the formation of carbonaceous species on the surface. In addition to these effects, the deposits block a considerable portion of active metal surface area. IR spectroscopic analysis indicates that dehydration reactions play an important role in the formation of the carbonaceous deposits. The present results illustrate that water and dissolved biomass compounds can strongly affect the stability of heterogeneous catalysts under reaction conditions.     

Sintering and microstructural study of mullite prepared from kaolinite and reactive alumina: Effect of MgO and TiO2

Mullite ceramic was prepared using kaolinite and synthesized alumina (combustion route) by solid-state interaction process. The influence of TiO₂ and MgO additives in phase formation, microstructural evolution, densification, and mechanical strengthening was evaluated in this work. TiO₂ and MgO were used as sintering additives. According to the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the raw materials, ie kaolinite, synthesized alumina, and different wt% of additives were wet mixed, dried, and uniaxially pressed followed by sintering at different temperature. 1600°C sintered samples from each batch exhibit enhanced properties. The 1 wt% TiO₂ addition shows bulk density up to 2.96 g/cm³ with a maximum strength of 156.3 MPa. The addition of MgO up to 1 wt% favored the growth of mullite by obtaining a density and strength matching with the batch containing 1 wt% TiO₂. These additives have shown a positive effect on mullite phase formation by reducing the temperature for complete mullitization by 100°C. Both additives promote sintering by liquid phase formation. However, the grain growth, compact microstructure, and larger elongated mullite crystals in MgO containing batch enhance its hardness properties.