A facile method to fabricate monolithic alumina–silica aerogels with high surface areas and good mechanical properties

In this study, monolithic alumina–silica aerogels with high surface areas and good mechanical properties were synthesized via a facile sol–gel method without solvent exchange. Furthermore, surface areas, microstructures (up to 1300 °C), and mechanical properties of the prepared alumina–silica aerogels were investigated. The sintering and phase transformations of metastable alumina nanoparticles are suppressed owing to the uniformly distributed Si in the alumina–silica aerogels; therefore, the alumina–silica aerogels can maintain much higher specific surface areas after being calcined at 800 °C (575.5 m²/g), 1000 °C (443.2 m²/g), and 1200 °C (120.6 m²/g) compared to pristine alumina aerogels. In addition, the prepared high surface area alumina–silica aerogels show considerably higher strengths than those obtained in previous works. The compressive stress (3 % strain) and elastic modulus of the alumina–silica aerogels reached 1.78 and 65.6 MPa, respectively. The reported alumina–silica aerogels in this study can be good candidates as high-temperature thermal insulators and catalysts.     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

Effect of Alumina Reactivity on the Densification and Properties of Al2O3–Cr2O3 Refractories

Alumina‐chrome (Al₂O₃–Cr₂O₃) refractories with Al₂O₃:Cr₂O₃ molar ratio 1:1 were synthesized in the temperature range of 1400–1700°C by conventional solid–oxide reaction route. The effect of different aluminas (viz., hydrated and calcined) on the densification, microstructure, and properties of Al₂O₃–Cr₂O₃ refractories was investigated without changing the Cr₂O₃ source. The starting materials were analyzed to determine the chemical composition, mineralogy, density, surface area, and particle size. Sintered materials were characterized in terms of densification, phase assemblage, and mechanical strength at room temperature and at higher temperatures. Microstructural evolution at different sintering temperature was correlated with sintering characteristics. It can be concluded that the Al₂O₃–Cr₂O₃ refractories prepared with hydrated alumina as Al₂O₃ source show better densification and hot mechanical strength than corresponding calcined variety.     

Fabrication and characterization of the monolithic hydrophobic alumina aerogels

The monolithic hydrophobic mesoporous alumina aerogels were successfully synthesized by acid–base sol–gel polymerization of aluminium chloride hexahydrate (AlCl₃·6H₂O) in deionized water/alcohol solution (v/v = 3:7). To minimize shrinkage during drying, alumina hydrogels were aged in tetraethylorthosilicate (TEOS)/acetonitrile solution, and modified using trimethylchlorosilane (TMCS)/acetonitrile solution. Properties of the final product were examined by contact angle measurement, FTIR, FESEM, TEM and BET analyses. Surface modification was confirmed by FTIR spectroscopy. It was found that hydrophobic property of the alumina aerogels was affected by the contents of TMCS. When the molar ratio of TMCS to AlCl₃·6H₂O is 0.35, hydrophobic alumina aerogels shows lower bulk density (0.453 g/cm³) and higher surface area (495 m²/g) than those of unmodified alumina aerogels (0.933 g/cm³, 413 m²/g).     

Thermal and electrical properties of epoxy composites at high alumina loadings and various temperatures

Epoxy microcomposites with high loading micro alumina (Al₂O₃, 100–400 phr) were prepared by casting method and their thermal and electrical properties were studied at temperatures from 25 to 150 °C. The electric resistance device and the dielectric electrode device were designed to measure the electrical properties of the composites. Thermogravimetric analysis (TGA) and scanning electron microscopic proves the homodispersion of Al₂O₃ microparticles in epoxy. TGA indicates that the temperature of 5 % weight loss of epoxy/Al₂O₃ (100 phr) composite is 366 °C, 34 °C higher than that of pure epoxy. Differential scanning calorimetry shows that the glass transition temperature of epoxy/Al₂O₃ composite (400 phr) increases to 114.7 °C, 9.2 °C higher than that of pure epoxy. Thermal conductivity test demonstrated that with increasing Al₂O₃ content at 25 °C, thermal conductivity of epoxy/Al₂O₃ composites increased to 1.382 W/(m K) which is 5.62 times that of pure epoxy. Electrical tests demonstrate that by increasing of Al₂O₃ content and temperature, the electric resistance and dielectric properties of the composites show great dependencies on them. Resistivities of all the specimens decreased with the increasing of temperature owing to the increasing molecular mobility in the higher temperature. Resistivity of pure epoxy at 25 °C is about 9.56 × 10¹⁶ Ω cm, about one order of magnitude higher than that of pure epoxy at 125 °C and two orders of magnitude higher than that of pure epoxy at 150 °C. These results can give some advice to design formulations for practical applications in power apparatus.     

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%.     

Synthesis of Spinel from Caustic Magnesite and Alumina Dust

The sintering of a mixture of a caustic dust and an alumina dust collected from electric filters taken in MgO/Al₂O₃ ratios of 0.1, 0.28, 0.5, and 0.75 at 1650°C is studied. Materials with superior physicomechanical properties are obtained: open porosity, 1.2 – 8.4%; density, more than 3.5 g/cm³, and compressive strength, 160 – 410 MPa. High-density pellets (free of additions) are prepared at a MgO/Al₂O₃ ratio of 0.75, with compressive strength as high as 160 MPa.     

Effect of Pluronic F-127 debinding temperature on the sintering and densification of alumina by direct ink writing

Hydrogel-based alumina (Al₂O₃) inks were prepared using Pluronic F-127 with 65 wt% of solid loading (Al₂O₃). The Al₂O₃ inks were deposited, and the freestanding samples were studied using TGA/DTA. Significant weight loss was observed between 180 and 360°C. A two-stage hydrogel debinding process of Al₂O₃ samples was carried out at 180 and 360°C with holding times of 30, 60, 90, 120, and 150 min. The Al₂O₃ samples were then sintered at 1600°C. X-ray diffraction was used for the phase analysis of the alumina inks, and a scanning electron microscope was used microstructural analysis. Based on the TGA/DTA analysis, a two-stage debinding process was adopted. Significant effect of hydrogel debinding temperature was observed on the sintering and densification behavior of alumina. It was observed that the porosities in the alumina samples were increasing when the debinding time was increased from 30 to 150 min, with the debinding temperature at 180 and 360°C. Moreover, the nature of the porosities was changing from closed porosities to interconnected porosities.     

Novel Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> aerogel/porous zirconia composite with ultra-low thermal conductivity

Highly porous zirconia fibers networks with a quasi-layered microstructure were successfully fabricated using vacuum squeeze moulding. The effects of inorganic binder content on the microstructure, room-temperature thermal and mechanical properties of fibrous porous zirconia ceramics were systematically investigated. Al₂O₃–SiO₂ aerogel was impregnated into fibrous porous ceramics, and the microstructures, thermal and mechanical properties of Al₂O₃–SiO₂ aerogel/porous zirconia composites were also studied. Results show that the Al₂O₃–SiO₂ aerogel/porous zirconia composites exhibited higher compressive strength (i.e., 1.22 MPa in the z direction) and lower thermal conductivity [i.e., 0.049 W/(m/K)]. This method provides an efficient way to prepare high-temperature thermal insulation materials.     

Effect of glass additives on the strength and toughness of polycrystalline alumina

The effect of stress at grain boundaries on the mechanical properties of alumina ceramics was investigated. Residual stresses at grain-boundaries resulted from a mismatch in thermal expansion coefficient (TEC) between the alumina matrix and the glass-phase segregated at grain-boundaries. The BaO–Al₂O₃–SiO₂ (BAS) system and the Li₂O–Al₂O₃–SiO₂ (LAS) system glasses were chosen to have a higher and a lower TEC than that of alumina, respectively, resulting in microscopic tensile and compressive stresses at grain-boundaries for Al₂O₃/BAS and Al₂O₃/LAS composites, respectively. The experimental results showed that the Al₂O₃/BAS composite fractured intergranularly with a fracture toughness higher than that of monolithic alumina. On the other hand, the Al₂O₃/LAS composite experienced transgranular fracture and high bending strength despite its low toughness. Both composites could be sintered to full density at 1500°C for 2 h due to the presence of a liquid phase. It was concluded that strengthening and toughening of alumina ceramics could be tailored by designing their grain-boundary microstresses.     

Influence of Ageing, Silver Loading and Type of Reducing Agent on the Lean NO x Reduction Over Ag–Al2O3 Catalysts

The influence of ageing temperature, silver loading and type of reducing agent on the lean NO _x reduction over silver–alumina catalysts was investigated with n-octane and bio-diesel (NExBTL) as reducing agent. The catalysts (2 and 6 wt% Ag–Al₂O₃) were prepared with a sol–gel method including freeze drying and the evaluation of NO _x reduction and aging were performed using a synthetic gas-flow reactor. The results indicate a relatively high NO _x reduction for both reducing agents. The hydrothermally treated 6 wt% Ag–Al₂O₃ sample displays a maximum NO _x reduction of 78 % at 350 °C for n-octane as reductant and the corresponding value for NExBTL is 60 %. Furthermore, the catalysts show high durability and an increase in activity for NO _x reduction after ageing at temperatures up to 650 °C, with n-octane as reducing agent.     

Catalysis of NiO–Al2O3 aerogels for the CO2-reforming of CHF4

Uniform and monolithic NiO–Al₂O₃ aerogels were prepared from cyclic nickel glycoxide, (CH₂O)₂Ni, and boehmite sol, AlOOH, and the catalyst performance of the aerogels for the CO₂-reforming of methane was investigated. The NiO–Al₂O₃ aerogels showed higher activity than impregnation NiO/Al₂O₃ catalysts, while the aerogels exhibited much less activity for coking than the impregnation catalysts. Less deactivation was also observed on the aerogel catalysts than on the impregnation catalysts in the continuous-flow reaction. The Ni was uniformly incorporated throughout alumina where both the metal and the support exist in the aerogel form, i.e., Ni–O–Al bond was considered to be formed in the aerogels. As a result, fine Ni particles appeared after H₂ reduction throughout the alumina support with high dispersion, which brought about not only higher activity but also much less activity for coking on the aerogels. Retardation of catalyst deactivation was ascribed to the suppression of both coking and sintering of Ni particles on the aerogels. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enhanced thermal shock response of Al2O3–graphite composites through a layered architectural design

The use of ceramics such as alumina in moving components often requires the addition of low friction materials such as graphite. A new strategy for improving toughness, strength, and thermal-shock resistance of Al₂O₃–graphite self-lubricating composites was proposed in this study. Alumina layers embedded between Al₂O₃–graphite layers were fabricated and tested after thermal shock conditions ranging between 500 °C and 800 °C maximum temperature. Retained strength and apparent fracture toughness after the tests were compared to room temperature values. Results show that compressive residual stresses generated in the outer Al₂O₃–graphite layers during cooling down from sintering improve the failure resistance of the materials. The introduction of heat-resistant particles (Al₂O₃ particles) into graphite layers combined with a layered architecture can greatly decrease the oxidation degradation of the materials below 500 °C. In addition, the retained strength and toughness in the layered architectures after thermal shock between 550 °C and 800 °C remains constant, thus indicating that the new-developed Al₂O₃/Al₂O₃–graphite laminated composites may be reliable candidates for self–lubricating applications also for elevated temperatures.     

Optimization of Dielectric Constant of Polycarbonate/Polystyrene Modified Blend by Ceramic Metal Oxide

We modified a polycarbonate (PC)/polystyrene (PS) blend by loading alumina (Al₂O₃) into the blend. X-ray diffraction (XRD) shows a decrease in crystallinity. The optical microscopy and atomic force microscopy (AFM) confirms the homogeneous dispersion of alumina. The differential scanning calorimetry (DSC) data shows improved results in glass transition (T_g) and melting temperature (T_m) of blend systems. The electrical properties of polymer blends modified by Al₂O₃ were studied as a function of frequency (50 Hz–35 MHz) at 323 K. The alumina showed a significant effect in resulting in a high dielectric constant with low dielectric loss and dissipation factor.     

Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part II: Effect of Al2O3

The hydration and microstructural evolution of three alkali activated slags (AAS) with Al₂O₃ contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O. The formation of C(A)–S–H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al₂O₃ content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(A)-S-H and led to the formation of strätlingite. Increasing Al₂O₃ content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al₂O₃ content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.     

Arsenate and arsenite adsorbents composed of nano-sized cerium oxide deposited on activated alumina

Cerium oxide composited activated alumina was prepared and used as arsenate and arsenite adsorbents by oxidation of cerium chloride in H₂O₂ solution. The preparation conditions affected arsenic adsorption capabilities in the Al₂O₃–CeO₂. Efficient adsorption of arsenic was achieved when CeO₂ was deposited on 6.0 g of powdered activated alumina at 0.01 M Ce³⁺ concentration and H₂O₂/Ce = 0.5. The arsenic adsorption was particularly enhanced in the presence of the nano-size CeO₂ on the Al₂O₃ support, obeying the Langmuir adsorption isotherm model with maximum adsorption capacities of 13.6 and 10.5 mg/g, respectively, for arsenate and arsenite.     

On the Production of Hydrogen from Bio-Oil: A Representative Study Using Propylene Glycol

In a bio-refinery focused on fast pyrolysis, hydrogen (H₂) producible from reforming of the aqueous fraction of bio-oil with steam can be utilized for upgrading pyrolytic lignin into fuels by hydrotreatment. In this work, propylene glycol (PG) was chosen as a typical compound symbolizing higher polyols in the bio-oil aqueous fraction. Catalytic processing of PG into H₂ at low temperature (T = 500°C) was investigated using several commercial catalysts such as Ni/Al₂O₃, Ru/Al₂O₃, Ru/C, Pt/C, and Pd/C in a laboratory-scale fixed-bed reactor. The efficiencies of the catalysts were presented as selectivity to CO, CO₂, CH₄ and H₂, and PG conversion into gaseous phase. Wide ranges of temperature (300–500°C), W/F_O (18.6–92.9 g h/mol), and S/C ratio (5.6–12.7 mol/mol) were examined using Ni/Al₂O₃. At T = 500°C, H₂ selectivity (73.7%) and PG conversion (66.2%) were maximized using ratios of catalyst mass to molar flow rate of PG (W/F_O) = 18.6 g h/mol and steam to carbon (S/C) = 12.7 (10 wt% PG solution). It was found that Ni/Al₂O₃ demonstrates stable operation for at least 6 h of time-on-stream. Finally, a plausible reaction pathway for PG reforming was proposed.     

Influence of Alumina Particles on Thermal Behavior of High Density Polyethylene (HDPE)

This study evaluated the effects of alumina (Al₂O₃) particles on thermal properties of High Density Polyethylene (HDPE). HDPE and HDPE/5, 10 & 15 wt% Al₂O₃ composites were prepared by compression molding. Differential scanning calorimetery (DSC) was used to analyze the thermal and crystallization behavior of the samples. The results indicated that the alumina particles affected the crystallization behavior of HDPE matrix, significantly. However, the DSC results showed that alumina content did not influence the melting temperature of HDPE in this composite. The results also showed that the incorporation of alumina particles caused the decrease of specific heat capacity coefficient and entropy.     

Nickel Oxide Based Supported Catalysts for the In-situ Reactions of Methanation and Desulfurization in the Removal of Sour Gases from Simulated Natural Gas

Supported nickel oxide based catalysts were prepared by wetness impregnation method for the in-situ reactions of H₂S desulfurization and CO₂ methanation from ambient temperature up to 300 °C. Fe/Co/Ni (10:30:60)–Al₂O₃ and Pr/Co/Ni (5:35:60)–Al₂O₃ catalysts were revealed as the most potential catalysts, which yielded 2.9% and 6.1% of CH₄ at reaction temperature of 300 °C, respectively. From XPS, Ni₂O₃ and Fe₃O₄ were suggested as the surface active components on the Fe/Co/Ni (10:30:60)–Al₂O₃ catalyst, while Ni₂O₃ and Co₃O₄ on the Pr/Co/Ni (5:35:60)–Al₂O₃ catalyst.     

Effect of Sodium on Microstructures and Thermoelastic Properties of Calcium Aluminate Cement–Bonded Refractories

The effect of sodium on refractory phase formation in a model Calcium Aluminate Cement–bonded refractory was investigated from 700°C to 1500°C. Sodium reacts with α‐alumina to form sodium β‐alumina (β‐Al₂O₃) via the intermediate NaAlO₂. Formation of β‐Al₂O₃ disrupts the reaction path of calcia with alumina, delaying crystallization of calcium hexaluminate, CaO·6Al₂O₃, from 1350°C to 1500°C. β‐Al₂O₃ is also shown to reduce Young's modulus and delay sintering. The presence of NaAlO₂ and β‐Al₂O₃ result in an increase in internal friction. Increased linear expansion of up to 47% is observed when 1 wt% Na is added. The expansion is shown to scale with the amount of dopant with only 0.3 wt% Na leading to an additional 31% linear expansion. On cooling, the presence of β‐Al₂O₃ can be demonstrated by a peak in internal friction between 1200°C and 1000°C which could be caused by Na⁺ ion hopping along the spinel‐like planes.