Function Of Alumina Content Research Articles

Effect of Al2O3 addition on the thermal expansion of sodium alkaline‐earth silicate glasses: A molecular dynamics study

AbstractCoefficient of thermal expansion (CTE) is an important property to consider when utilizing oxide glasses in thermal treatment processes to avoid thermal damage at the interfaces of the glasses with heterogeneous materials. It is thus important to know the effect of additives on CTE for designing glasses. The use of alumina efficiently improves chemical and mechanical durability of oxide glasses while maintaining the functionality and productivity; however, alumina‐doping often induces nonlinear variation of CTE. In this work, we therefore tried to investigate the relationship between CTE and the microstructure of sodium alkaline‐earth aluminosilicate glasses using classical molecular dynamics (MD) simulations. To accurately model the glasses, we extended a force‐matching potential by optimizing the parameter sets for Ca–O, Mg–O, and Na–O pair interactions using Bayesian optimization. The MD simulations reproduced the nonlinear variation of CTE as a function of alumina content, and detailed structural analyses identified inhomogeneous expansion in the glasses. It was found that the nonuniform CTE change at the nanoscale was related to the formation of an alumina‐rich region, in which more fivefold‐coordinated aluminum exist, when alumina content exceeded Na2O content. Accordingly, the microstructural change by alumina‐doping was identified as the origin of the nonlinear variation in the CTE of the glasses.

Boson peak, heterogeneity and intermediate-range order in binary SiO2-Al2O3 glasses

In binary aluminosilicate liquids and glasses, heterogeneity on intermediate length scale is a crucial factor for optical fiber performance, determining the lower limit of optical attenuation and Rayleigh scattering, but also clustering and precipitation of optically active dopants, for example, in the fabrication of high-power laser gain media. Here, we consider the low-frequency vibrational modes of such materials for assessing structural heterogeneity on molecular scale. We determine the vibrational density of states VDoS g(ω) using low-temperature heat capacity data. From correlation with low-frequency Raman spectroscopy, we obtain the Raman coupling coefficient. Both experiments allow for the extraction of the average dynamic correlation length as a function of alumina content. We find that this value decreases from about 3.9 nm to 3.3 nm when mildly increasing the alumina content from zero (vitreous silica) to 7 mol%. At the same time, the average inter-particle distance increases slightly due to the presence of oxygen tricluster species. In accordance with Loewensteinian dynamics, this proves that mild alumina doping increases structural homogeneity on molecular scale.

Extreme behavior of Li-ion conductivity in the Li2O–Al2O3–P2O5 glass system

Aluminophosphate glasses with compositions 50Li2O–xAl2O3–(50−x)P2O5 have been synthesized and analyzed in the x=0–10 range. The influence of alumina content on the glass transition temperature (Tg), crystallization temperature, thermal stability, density and ionic conductivity has been investigated. When Al2O3 substitutes for P2O5, Tg increases in all glass compositions. Crystallization stability has been found to be a function of alumina content. Ionic conductivity has been studied by impedance spectroscopy. Ionic conductivity and density exhibit a non-linear and non-additive change with a maximum at 6.25mol% of Al2O3. IR spectroscopy study has indicated the phosphate network depolymerization. NMR technique revealed dramatic structural changes in the glass at x>6.25. These changes lead to extreme behavior of molar volume and, as a consequence, to a maximum in ionic conductivity.

Thermodynamics of Indium Dissolution Behavior in FeO-Bearing Metallurgical Slags

Indium solubility in the FeO-SiO2-Al2O3-5CaO-MgOsat slag system was measured at 1573 K (1300 °C) to confirm the thermodynamic dissolution behavior of indium at \( {\text{p}}_{{\text{O}_{2} }} = 10^{ - 11} \) atm. The indium solubility in FeO-bearing slags increased with increasing oxygen partial pressure and decreased with increasing basicity which is in proportion to the activity of FeO. The dissolution of indium in FeO-bearing slags was confirmed to progress according to the following reaction: $$ \text{In} (\text{s} ) + \frac{1}{4}\text{O}_{2} (\text{g} ) = (\text{In}^{ + } ) + \frac{1}{2}(\text{O}^{2 - } ) $$ The enthalpy change for the dissolution of indium in FeO-bearing slag was about −181 kJ/mol, indicating that indium dissolution is exothermic. The indium solubility in the FeO-SiO2-Al2O3-5CaO-MgOsat slag system was minimized as a function of alumina content at a given FeO/SiO2 ratio, which can be explained by the amphoteric behavior of Al2O3 in the slag system. To improve indium recovery by lowering indium loss to the slag phase during the pyro-recycling of In-containing materials using FeO-bearing metallurgical slags, a lower oxygen potential and lower silica content are highly favorable.

Production of high strength Al–Al 2O 3 composite by accumulative roll bonding

Recently accumulative roll bonding has been used as a novel method to produce particle reinforced metal matrix composites. In this study, aluminum matrix composite reinforced by submicron particulate alumina was successfully produced and the effects of number of ARB cycles and the amount of alumina content on the microstructure and mechanical properties of composites were investigated. According to the results of tensile tests, it is shown that the yield and tensile strengths of the composite are increased with the number of ARB cycles. Scanning electron microscopy (SEM) reveals that particles have a random and uniform distribution in the matrix by the ARB cycles and a strong mechanical bonding takes place at the interface of particle-matrix. It is also found that the tensile strength of the composite, as a function of alumina content, has a maximum value at 2 vol.%, which is 5.1 times higher than that of the annealed aluminum.

The effect of alumina addition on the electrical conductivity of Gd-doped ceria

Acceptor doped-ceria is a possible electrolyte material for the IT-SOFC (intermediate temperature solid oxide fuel cell) due to its high oxygen-ion conductivity. However, its use has been limited by its mechanical weakness and the appearance of electronic conductivity in reducing condition. In this study, alumina was selected as an additive in the doped-ceria to see if it increases the oxygen-ion conductivity and mechanical strength. Effects of alumina addition in doped ceria were studied as a function of alumina content and acceptor (Gd) content. The electrical conductivity of (Ce 1-x Gd x O 2-δ ) 1-j + (Al 2 O 3 ) y (x = 0-0.35, y = 0-0.10) was measured by using impedance spectroscopy. The grain conductivity of Ce 0,8 Gd 0,2 O 2-δ (GDC20) with 5 mol% alumina increased ∼3 times from that of GDC20 at 300°C. The grain conductivity was even ∼2 times higher than that of Ce 0,9 Gd 0,1 Ο 2-δ (GDC 10) at 300°C. The electrical conductivity of GDC20 without alumina addition, measured at 500°C in air, rapidly decreased after exposure to reducing condition (Po 2 ∼10 -22 atm) at 800°C. However, the decrease was much slower in GDC20 with alumina addition, indicating the improved mechanical strength. Among the examined compositions, (Ce 0,75 Gd 0,25 Ο 2-δ ) 0.95 + (Al 2 O 3 ) 0.05 (GDC25A5) showed the highest conductivity at most temperatures.

Ionic Conductivity of Nano-Structured Yttria Stabilized Cubic Zirconia as a Function of Alumina Content

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Aluminum co-doping of soda-lime silicate glasses: Effect on optical and spectroscopic properties

One of the limits of using silicate glasses as a host for Er 3+ is the emission bandwidth, which is smaller than in the case of other oxide glasses, such as phosphates and tellurites. The bandwidth can be increased by adding aluminum to the composition. Here we present an experimental investigation of the effect of increasing aluminum content on optical and spectroscopic properties of this glass. Glass samples were prepared by melting at 1550 °C reagent grade SiO 2, Al 2O 3, Na 2CO 3, K 2CO 3, CaCO 3, (NH 4) 3PO 4 and Er 2O 3; Al 2O 3 concentration ranged from 0.99 to 20 mol%. Both heating and cooling rates were equal to 0.17 °C s −1. Measurements of the fluorescence bandwidth were then made and as a function of alumina content. Judd–Ofelt analysis was performed and calculated radiative lifetimes were compared with experimental ones. Planar waveguides were also fabricated by using ion-exchange technology, and some optical properties measured by the prism coupling technique. These waveguides had losses of the order of 1 dB/cm at 635 nm.

Flexure Strength, Fracture Toughness, and Slow Crack Growth of YSZ/Alumina Composites at High Temperatures

Flexure strength and fracture toughness of zirconia–alumina composites, fabricated by hot pressing 10 mol% yttria‐stabilized zirconia (10‐YSZ) reinforced with 0–30 mol% alumina particulates or platelets, were determined as a function of alumina content at 1000°C in air. Both strength and fracture toughness of the two composite systems increased with increasing alumina content. For a given alumina content, flexure strength of the particulate composites was greater than that of the platelet composites at higher alumina contents (≥20 mol%); whereas, fracture toughness of the platelet composites was greater than that of the particulate counterparts, regardless of the alumina content. The susceptibility to slow crack growth (SCG), determined at 1000°C via constant stress‐rate testing, was greatest for 30 mol% particulate composite with SCG parameter n=5–8 and was least for 30 mol% platelet composite with n=33. Elastic modulus of both composite systems decreased below 400°C and then remained almost unchanged up to 1000°C, forming a unique transition around 400°C, irrespective of alumina content.

Preparation of a complete series of single phase homogeneous sol-gels of Al2O3 and MgO for basic catalysts

Homogeneous single phase sol-gels of magnesia and alumina were prepared using magnesium and aluminum alkoxides as precursors. The gels were formed through a hydrolysis reactions followed by condensation. These materials behave as basic catalysts and, therefore, are superb anionic interchangers. The solids obtained were characterized by infrared spectroscopy, thermogravimetric analysis and anionic interchange tests. No evidence for the presence of the individual binary oxides was found. An important effect on the dehydroxylation of the surface and on the anionic exchange was found as a function of alumina content in the magnesia matrix. The best sample was the one containing 50% MgO-50% Al 2 O 3 .

Sintering behaviour of alumina-modified silica aerogels

A main problem for the applicability of silica aerogels as thermal insulation materials at temperatures above 600°C is viscous sintering, which may lead to a significant density increase with time. Modification of silica aerogels via integration of alumina yields aerogels with increased stability against sintering. The onset of sintering of alumina silica aerogels occurs at about 900°C compared with about 600°C for pure silica aerogels. The sintering process was investigated as a function of alumina content, density and particle size. Alumina-modified silica aerogels were studied by dilatometry, viscosimetry and thermogravimetry during non-isothermal and isothermal heat treatment.

Isopropylamine adsorption for the characterization of acid sites in silica-alumina catalysts

The adsorption and reaction of isopropylamine has been studied using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) on a series of silica-alumina catalysts as a function of alumina content, and the results have been correlated with cracking activities for n-hexane. The TPD-TGA curves on the amorphous silica-aluminas exhibit similar features to that observed on high-silica zeolites in that a well-defined feature is observed which desorbs as propene and ammonia between 575 and 650 K. This desorption feature appears to be due to strong Bønsted sites which are not present on either pure silica or alumina. The concentration of these sites increases in a regular manner with Al content and correlates well with the n-hexane cracking activity. This suggests that the TPD-TGA measurement of isopropylamine is a useful probe of the catalytically active sites on amorphous silica-aluminas.

Acid-base properties of silica-aluminas: use of 1-butanol dehydration as a test reaction

1-Butanol dehydration was performed as a test reaction for acid-base characterization on a series of silica-aluminas covering the whole composition range (0-100% Al2O3). Alumina-rich samples (80-100% Al2O3) dehydrate 1-butanol to 1-butene and dibutylether. Silica-aluminas with 5-70% Al2O3 dehydrate 1-butanol to 1-butene which directly isomerizes to cis- and trans-2-butenes. The acid-base properties were also investigated by ammonia and carbon dioxide TPD and FT-IR spectroscopy of adsorbed pyridine, in parallel with another test reaction, 4-methyl-2-pentanol dehydration. The acidity-basicity profiles are discussed as a function of alumina content (nature, strength and amount of sites). A very good correlation between the results of test reactions and acid-base characterization methods is observed. Our study confirms previous results obtained on another series of silica-aluminas, but, in addition, the usefulness of the rapid and simple TPD method and 1-butanol dehydration as a test reaction is revealed.

STUDIES ON THE SINTERING OF MoSi2–Al2O3CERMETS

AbstractThe porosity of hydrogen-sintered bodies composed of molybdenum disilicide and alumina has been studied as a function of alumina content and grain size. It is found that oxidation-resistant bodies can be made if the grain size of the constituents is ∼ 10 μ or less, although the bodies are fairly weak.In addition, samples were sintered in dissociated ammonia and compared with those sintered in hydrogen. It became apparent that samples sintered in dissociated ammonia underwent a particularly severe form of attack when exposed to air at high temperatures. This is attributed to the formation, during sintering, of Si3N4, a phase which decomposes at high temperature in air when enclosed in the cermet bodies.