Influence of surface modification on the dispersion of nanoscale α‐Al2O3 particles in a thermoplastic polyurethane matrix

In this study, two types of nanoscale α‐Al₂O₃ particles were used for preparation of α‐Al₂O₃/thermoplastic polyurethane (TPU) composites. These α‐Al₂O₃ particles were either coated or uncoated with stearic acid. For the uncoated α‐Al₂O₃/TPU composite, the results of field‐emission scanning electron microscopy (FE‐SEM) and energy dispersive X‐ray spectrometry indicate that uncoated α‐Al₂O₃ particles are significantly aggregated together. This aggregation is due to the poor compatibility between the inorganic filler (α‐Al₂O₃) and the organic matrix (TPU). The size of clusters is in the range from 5 to 20 μm. For the coated α‐Al₂O₃/TPU composite, FE‐SEM results indicate that most coated α‐Al₂O₃ particles are well dispersed in the TPU matrix. This phenomenon results from the effect of surface modifier (i.e., stearic acid) on α‐Al₂O₃ particles. Stearic acid can act as a compatibilizer to bridge the boundary between the TPU matrix and the α‐Al₂O₃ particle. Stearic acid is not only a suitable surface modifier for the nanoscale α‐Al₂O₃ particle, but also a good dispersant for the dispersion of nanoscale α‐Al₂O₃ particles in the TPU matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Fabrication and Characterization of Near‐Net‐Shape In Situ Reaction‐Bonded Porous Cordierite/SiC Ceramics

Porous cordierite/SiC ceramics were fabricated by in situ reaction bonding using α‐SiC, α‐Al₂O_3, and MgO powders as the starting materials. During sintering, part SiC is oxidized to SiO₂ and then the latter reacts with Al₂O₃ and MgO to form cordierite. As a result, porous cordierite/SiC ceramics were obtained, and the ceramics are strengthened by the residual SiC. Due to the large volume expansion introduced by the oxidation of SiC, the ceramics exhibit small sintering‐induced dimension variations. In addition, a fine‐grained microstructure and good thermal and mechanical properties were obtained for the porous cordierite/SiC ceramics.     

Selective Corrosion Preparation and Sintering of Disperse α‐Al2O3 Nanoparticles

Disperse fine equiaxed α‐Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe₂O₃ as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe₂O₃ acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al₂O₃ nanoparticles, resulting in disperse fine equiaxed α‐Al₂O₃ nanoparticles. These α‐Al₂O₃ nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al₂O₃ ceramics. The two‐step sintered bodies are nanocrystalline α‐Al₂O₃ with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al₂O₃ ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al₂O₃ nanoparticles have an excellent sintering activity.     

Preparation of Fe3Si‐Al2O3 Nanocomposite Powders by Mechanochemical Reaction of Fe3O4‐Si‐Al Powder Mixtures

Phase evolution and morphology of Fe₃O₄‐Si‐Al powder mixtures during ball milling from 30 min to 20 h were investigated. A 3‐h critical milling was necessary for the occurrence of mechanically activated combustion reaction. The reaction results in the formation of Fe (Si), Fe₃Si, and α‐Al₂O₃. During ball milling from 3 to 20 h, Fe (Si) and Fe₃Si were combined into disordered Fe₃Si intermetallic and Fe₃Si‐Al₂O₃ composite powder was formed. The presence of in situ formed alumina leads to a decrease in crystallite and particle sizes. The Fe₃Si‐Al₂O₃ particles after milling for 20 h had a crystalline size of 10~12 nm.     

α‐Al2O3 improves the properties of gel polyacrylonitrile nanocomposite electrolytes used as electrolyte materials in rechargeable lithium batteries

In this investigation, a series of gel polyacrylonitrile (PAN)/α‐Al₂O₃ nanocomposite electrolyte materials that incorporate various fractions of PAN, α‐Al₂O₃ inorganic powders, propylene carbonate and ethylene carbonate as cosolvents, and LiClO₄ were prepared. X‐ray diffraction revealed that the gel nanocomposite electrolyte materials contained amorphous PAN in which was uniformly dispersed α‐Al₂O₃. The gel PAN/α‐Al₂O₃ nanocomposite electrolytes had lower glass‐transition temperatures (as determined by dynamic mechanical analysis) and higher conductivity than a similar electrolyte prepared in the absence of α‐Al₂O₃. The conductivity of the PAN/α‐Al₂O₃ nanocomposite films was inversely proportional to the size of the α‐Al₂O₃ particles and directly proportional to (I) the amount of α‐Al₂O₃ (up to 7 wt %), (II) the F value [LiClO₄/CH₂CH(CN) ratio], and (III) the amount of plasticizer (propylene carbonate/ethylene carbonate = 1 : 1). Cyclic voltammetry revealed that adding α‐Al₂O₃ significantly increased the electrochemical stability of the composite electrolyte system. A rechargeable lithium battery prepared using this gel nanocomposite electrolyte system exhibited good cyclability and a stable capacity. The coulombic efficiency for the recharge/discharge process was approximately 75%, even after 100 cycles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Sintering of α-Al2O3—Amorphous silica compacts

Sintering of α-Al₂O₃—fused SiO₂ compacts at temperatures of 1350–1500 °C is affected by metastable and stable phase reactions. Shrinkage maxima in the range 10 to 30 mole % Al₂O₃ are the result of cristobalite in the fused SiO₂ and its reaction with α-Al₂O₃ to form the eutectic of the metastable phase equilibrium diagram for SiO₂ (cristobalite)—α-Al₂O₃ without mullite. The retardation of densification of α-Al₂O₃ compacts with additions of SiO₂ up to about 50 mole % at temperatures about 1400°C and above is associated with the appearance of mullite.     

Tribological properties of ethylene–propylene–diene rubber + polypropylene + thermal‐shock‐resistant ceramic composites

This work is part of a program on composites used in thermoelectric devices. Tribological properties of dynamic vulcanizate blends of polypropylene and ethylene‐propylene‐diene rubber filled with 5 wt% of microscale powder have been studied. The microscale thermal‐shock‐resistant ceramic filler contains α‐Al₂O₃, mullite (3Al₂O₃ · 2SiO₂ or 2Al₂O₃SiO₂), β‐spodumene glass‐ceramic and aluminium titanate. We found that our ceramic particles are abrasive; they cause strong abrasion of softer steel ball surfaces during dry sliding friction. To overcome the difficulty of particle dispersion and adhesion, the filler was modified through grafting using three types of organic molecules. Dry sliding friction was measured using four types of counter‐surfaces: tungsten carbide, Si₃N₂, 302 steel and 440 steel. Thermoplastic vulcanizate filled with neat ceramic powder shows the lowest friction compared to composites containing the same but surface‐treated powder. We introduce a ‘bump’ model to explain the tribological responses of our composites. ‘Naked’ or untreated ceramic particles protrude from the polymer surface and cause a decrease of the contact area compared to neat polymer. The ball partner surface has only a small contact area with the bumps. As contact surface area decreases, so does friction and the amount of heat generated during sliding friction testing. Chemical coupling of the ceramic to the matrix smoothens the bumps and increases the contact surface, giving a parallel increase in friction. Copyright © 2012 Society of Chemical Industry     

Phase transitions and glass transition in a hyperquenched silica‐alumina glass

We investigate phase transitions, glass transition, and dynamic behavior in the hyperquenched 69SiO₂–31Al₂O₃ (mol%) glass (SA glass). Upon reheating, the SA glass exhibits a series of thermal responses. Subsequent to the sub‐T_g enthalpy release, the glass undergoes a large jump in isobaric heat capacity (ΔC_p) during glass transition, implying the fragile nature of the SA glass. The mullite starts to form before the end of glass transition, indicating that the SA glass is extremely unstable against crystallization. After the mullite formation, the remaining glass phase exhibits an increased T_g and a suppressed ΔC_p. The formation of cristobalite at 1553 K indicates the dominance of silica in the remaining glass matrix. The cristobalite gradually re‐melts as the isothermal heat‐treatment temperature is raised from 1823 to 1853 K, which is well below the melting point of cristobalite, while the amount of the mullite remains unchanged.     

The Effect of La2O3 Addition on Sol–Gel Derived Mullitization

Monophasic gel with stoichiometric 3Al₂O₃·2SiO₂ composition and gels with 0.99, 1.96, and 2.91 mol% La₂O₃ added were sol–gel derived. The crystallization path, structure evolution, microstructure, and morphology of calcined premullite powders and sintered ceramic bodies have been investigated as a function of La₂O₃ content and sintering temperature. In addition to mullite, spinel phase at about 980°C, and α‐alumina at above 1000°C were determined; however, neither La₂O₃ nor La‐related compounds had crystallized. The La₂O₃ predominately incorporated into the glassy phase, enhanced with La₂O₃ level, which affected both mullite structure and composition, as confirmed by electron microscopy, Rietveld structure refinement, determination of unit cell parameters, electron microscopy, and achieved density of the sintered bodies. Increased thermal treatment changes the alumina/silica ratio in mullite (towards 3:2 below 1200°C, and toward 2:1 above), and decreases the mullite/amorphous ratio. Sintered dense ceramic bodies revealed a positive densification effect and increased sinterability as a result of the lanthanum‐induced increase in glassy phase.     

Influence of sintering additives on densification kinetics and high‐temperature strength in γ‐Y2Si2O7 ceramics

Pressureless sintering of pure γ‐Y₂Si₂O₇ powders that had been synthesized by a solid‐liquid reaction method using Y₂O₃ and SiO₂ powders with Li₂O, MgO, and Al₂O₃ additives was reported. The sintering kinetics of γ‐Y₂Si₂O₇ powders was analyzed to track details of densification evolution. Apparent activation energies of the densification of γ‐Y₂Si₂O₇ powders were reported for the first time, which was 57.1, 96.6, and 100.2 kJ/mol for the powders with Li₂O, MgO, and Al₂O₃ additives, respectively, indicating that Li₂O could promote the densification behavior effectively. The flexural strengths as a function of temperature for the γ‐Y₂Si₂O₇ ceramics with different additives were also investigated. The degradation of high‐temperature flexural strength was mainly ascribed to the softening of grain‐boundary glassy phase. γ‐Y₂Si₂O₇ specimens fabricated using the powders with MgO or Al₂O₃ additives exhibited better high‐temperature mechanical properties.     

High‐Pressure Behavior of Mullite: An X‐Ray Diffraction Investigation

Using synchrotron X‐ray diffraction and diamond anvil cells we performed in situ high‐pressure studies of mullite‐type phases of general formula Al_4+2xSi_2?2xO_10?x and differing in the amount of oxygen vacancies: 2:1‐mullite (x = 0.4), 3:2‐mullite (x = 0.25), and sillimanite (x = 0). The structural stability of 2:1‐mullite, 3:2‐mullite, and sillimanite was investigated up to 40.8, 27.3, and 44.6 GPa, respectively, in quasi‐hydrostatic conditions, at ambient temperature. This is the first report of a static high‐pressure investigation of Al₂O₃–SiO₂ mullites. It was found that oxygen vacancies play a significant role in the compression mechanisms of the mullites by decreasing the mechanical stability of the phases with the number of vacancies. Elevated pressure leads to an irreversible amorphization above ~20 GPa for 2:1‐mullite and above 22 GPa for 3:2‐mullite. In sillimanite, only a partial amorphization is observed above 30 GPa. Based on Rietveld structural refinements of high‐pressure X‐ray diffraction patterns, the pressure‐driven evolution of unit cell parameters is presented. The experimental bulk moduli obtained are as follows: K₀ = 162(7) GPa with K₀′ = 2.2(6) for 2:1‐mullite, K₀ = 173(7) GPa with K₀′ = 2.3(2) for 3:2‐mullite, K₀ = 167(7) GPa with K₀′ = 2.1(4) for sillimanite.     

High performance of la‐promoted Fe2O3/α‐Al2O3 oxygen carrier for chemical looping combustion

Iron oxide supported oxygen carrier (OC) is regarded to a promising candidate for chemical looping combustion (CLC). However, phase separation between Fe₂O₃ and supports often occurs resulted from the severe sintering of supports during calcination, which leads to the sintering and breakage of Fe₂O₃ thus the decrease of redox reactivity. In this article, La‐promoted Fe₂O₃/α‐Al₂O₃ were used as OCs for CLC of CH₄ and for the first time found that the OC with the addition of 18 wt % La exhibited outstanding reactivity and redox stability during 50 cycles of CLC of CH₄. Such a superior performance originated from the formation of LaAl₁₂O₁₉ hexaaluminate (La‐HA) phase with not only small particle size but also excellent thermal stability at CLC conditions, which worked as a binder to prevent the phase separation thereby the sintering and breakage of active species α‐Fe₂O₃ were avoided during reaction. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2827–2838, 2017     

Preparation and Performance Study of Mullite/Al2O3 Composite Ceramics for Solar Thermal Transmission Pipeline

For lowering sintering temperature of mullite/Al₂O₃ composite ceramics for solar thermal transmission pipeline, kaolin, potassium feldspar, quartz, and γ‐Al₂O₃ were used as raw materials to in situ synthesize the composite ceramics with pressureless sintering method. Densification, mechanical properties, thermal expansion coefficient, thermal shock resistance, phase composition, and microstructure were investigated. The experiment results demonstrated that the introduction of potassium feldspar and quartz decreased the lowest sintering temperatures greatly to 1300°C. The optimum sample A3 sintered at 1340°C obtained the best performances. The water absorption, apparent porosity, bulk density, bending strength, and thermal expansion coefficient of A3 were 0.04%, 0.12%, 2.71 g/cm³, 94.82 MPa, and 5.83 × 10^?6/°C, respectively. After 30 thermal shock cycles (wind cooling from 1100°C to room temperature), no cracks were observed on the surfaces of the sample, and the bending strength increased by ?7.96%. XRD analysis indicated that the main phases of samples before and after 30 thermal shock cycles were consistently mullite, corundum, and α‐cristobalite, while the content of mullite increased after thermal shock. SEM micrographs illustrated that the mullite grains growth and micro‐cracks appeared after thermal shock endowed the composite ceramics with excellent thermal shock resistance.     

Examination of the Dye‐Fixing Ability of Porous χ‐Alumina Flake Powders

Porous χ‐Al₂O₃ is a new material for fabricating coated inkjet printing paper. Paper coated with χ‐Al₂O₃ powders has excellent water fastness. This study investigated the dye‐fixing ability of χ‐Al₂O₃ powders through measurements of adsorption isotherms. The variations in zeta potential, pore volume, pore size distribution, and specific surface area of χ‐Al₂O₃ powders during the dye‐fixing process were also examined. The results show that χ‐Al₂O₃ powders have a strong affinity to dye colorants. The adsorption isotherm is classified as HA‐type and the adsorption data fit the Langmuir model. In addition to the positive‐charged surface of χ‐Al₂O₃ provides cationic sites for fixing dye colorants via electrostatic interaction, the porous structure on the surface of χ‐Al₂O₃ particles plays an indispensable role in trapping the colorant.     

Sintering of α-Al2O3/quartz, and α-Al2O3/cristobalite related to mullite formation

Cristobalite and quartz react differently in mixtures with α-Al₂O₃ at 1415°C. With cristobalite, an eutectic liquid forms in accordance with the metastable phase equilibrium diagram for α-Al₂O₃-SiO₂ (cristobalite) in the absence of mullite. With quartz, a liquid first forms on the surfaces of quartz because of the occurence of an intermediate liquid phase on transformation of quartz to cristobalite. These liquids act as precursors to the formation of mullite by reacting with α-Al₂O₃. Mullite was detected earlier in the cristobalite-containing mixtures under similar firing conditions because the growth of mullite becomes significant with the formation of the eutectic liquid at the α-Al₂O₃-cristobalite interface since it is already saturated with Al₂O₃. The kinetics of sintering are affected by the rates of the step reactions.     

Mechanical and thermal conductive properties of fiber‐reinforced silica‐alumina aerogels

We report the formation of Al₂O₃‐SiO₂ fiber‐reinforced Al₂O₃‐SiO₂ aerogels with the content of fibers in the range from 40 wt% to 55 wt% by sol‐gel reaction, followed by supercritical drying. The structure and physical properties of fiber‐reinforced Al₂O₃‐SiO₂ aerogels are studied. We find that the fiber‐reinforced Al₂O₃‐SiO₂ aerogels can be resistant to the temperature of 1200°C. The integration of fibers significantly improves the mechanical properties of Al₂O₃‐SiO₂ aerogels. We find that the bending strength of fiber‐reinforced Al₂O₃‐SiO₂ aerogels increases 0.431 MPa to 0.755 MPa and the elastic modulus increases from 0.679 MPa to 1.153 MPa, when the content of fibers increases from 40 wt% to 50 wt%. The thermal conductivity of the fiber‐reinforced Al₂O₃‐SiO₂ aerogels is in the range from 0.0403 W/mK to 0.0545 W/mK, depending on the content of fibers.     

Phase Equilibrium Studies of “Cu2O”–SiO2–Al2O3 System in Equilibrium with Metallic Copper

Phase equilibria of the “Cu₂O”–Al₂O₃–SiO₂ system have been experimentally investigated at metallic copper saturation. High‐temperature equilibration, rapid quenching, and electron probe X‐ray microanalysis (EPMA) techniques have been used. Containerless equilibration technique has been developed to enable the phase equilibrium of this chemically reactive system to be investigated. The microstructures and compositions of all phases present in the quenched sample were measured accurately using EPMA. The isothermals between 1150°C and 1300°C have been determined in the “Cu₂O”–Al₂O₃–SiO₂ system at metallic copper saturation. The following primary phase fields were identified in the system: SiO₂ (tridymite), Cu₂O (cuprite), Cu₂O^.Al₂O₃ (delafossite), Al₂O₃ (corundum), and 3Al₂O₃·2SiO₂ (mullite). The implications of the phase diagram on making of the copper aluminosilicate glass have been demonstrated.     

Integrated experimental phase equilibria study and thermodynamic modelling of the binary ZnO–Al2O3, ZnO–SiO2, Al2O3–SiO2 and ternary ZnO–Al2O3–SiO2 systems

Phase equilibria of the ZnO–SiO₂, Al₂O₃–SiO₂ and ZnO–Al₂O₃–SiO₂ systems at liquidus were characterized at 1340–1740 °C in air. The ZnO–Al₂O₃ subsolidus phase equilibria were derived from the experiments with the SiO₂- and CaO + SiO₂-containing slags. High-temperature equilibration on silica or platinum substrates, followed by quenching and direct measurement of Zn, Al, Si and Ca concentrations in the phases with the electron probe X-ray microanalysis (EPMA) was used to accurately characterize the system. Special attention was given to zincite phase that was shown to consist of two separate ranges of compositions: round-shaped low-Al zincite (<2 mol.% AlO_1.5) and platy high-Al zincite (4–11 mol.% AlO_1.5). A technique was developed for more accurate measurement of the ZnO solubility in the low-ZnO phases (corundum, mullite, tridymite and cristobalite) surrounded by the ZnO-containing slag, using l-line for Zn instead of K-line, avoiding the interference of secondary X-ray fluorescence. Solubility of ZnO was found to be below 0.03 mol.% in corundum and cristobalite, and below 0.3 mol.% in mullite. Present experimental data were used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in this system using FactSage computer package. The modified quasichemical model with two sublattices (Zn²⁺, Al³⁺, Si⁴⁺) (O^2?) was used for the liquid slag phase; the compound energy formalism was used for the spinel (Zn²⁺,Al³⁺)[Zn²⁺,Al³⁺,Va]₂O^2-₄ and mullite Al³⁺₂(Al³⁺,Si⁴⁺) (O^2?,Va)₅ phases; the Bragg-Williams formalism was used for the zincite (ZnO, Al₂O₃); other solid phases (tridymite and cristobalite SiO₂, corundum Al₂O₃, and willemite Zn₂SiO₄) were described as stoichiometric. Present study is a part of the research program on the characterization of the multicomponent Pb–Zn–Cu–Fe–Ca–Si–O–S–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni system.     

Nano‐engineered nickel catalysts supported on 4‐channel α‐Al2O3 hollow fibers for dry reforming of methane

A nickel (Ni) nanoparticle catalyst, supported on 4‐channel α‐Al₂O₃ hollow fibers, was synthesized by atomic layer deposition (ALD). Highly dispersed Ni nanoparticles were successfully deposited on the outside surfaces and the inside porous structures of hollow fibers. The catalyst was employed to catalyze the dry reforming of methane (DRM) reaction and showed a methane reforming rate of 2040 Lh^?1gNi^?1 at 800°C. NiAl₂O₄ spinel was formed when Ni nanoparticles were deposited on alpha‐alumina substrates by ALD, which enhanced the Ni‐support interaction. Different cycles (two, five, and ten) of Al₂O₃ ALD films were applied on the Ni/hollow fiber catalysts to further improve the interaction between the Ni nanoparticles and the hollow fiber support. Both the catalyst activity and stability were improved with the deposition of Al₂O₃ ALD films. Among the Al₂O₃ ALD coated catalysts, the catalyst with five cycles of Al₂O₃ ALD showed the best performance. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2625–2631, 2018