Mixture Of Al2O3 Research Articles

Pressureless sintering of Al2O3/Ni nanocomposites produced by heterogeneous precipitation method with varying nickel contents

Al2O3/Ni nanocomposites with varying Ni contents (1, 3, 5vol.%) were produced by pressureless sintering of Al2O3 and nano Ni powder mixture, prepared by a heterogeneous precipitation method. Better densification was achieved by increasing the sintering temperature and decreasing the Ni content. The highest density was obtained from the samples with 1vol.% Ni content sintered at 1550°C. X-ray diffraction analysis was used in order to verify the Ni particle formation in the sintered specimens. SEM analyses were conducted on the polished and thermally etched specimens and Al2O3 grain and Ni metal particle sizes were measured. It was observed that Ni particles were dispersed homogeneously in the alumina matrix and they were commonly located at the grain boundaries or at the triple junctions. Vickers hardness of the specimens was measured to investigate the hardness-material relationship which was strongly dependent on microstructure which is affected by the sintering temperature and Ni content.

Effect of Sandblasting Process on the Oxidation Behavior of HVOF MCrAlY Coatings

Surface characteristics, such as topography and roughness (R a), are important in affecting the isothermal oxidation behavior of MCrAlY coatings. In this study, the effect of sandblasting on the oxidation behavior of HVOF thermal-sprayed MCrAlY coatings was investigated. Oxidation tests were conducted under isothermal conditions at 1050 °C for different periods. The R a of the bondcoat was approximately 12 μm. Before and after oxidation, the thermally-grown scale composition and morphology were analyzed using scanning electron microscope (SEM/EDX) and X-ray diffraction. The results showed that the oxide scale that formed on the as-sprayed coatings was a mixture of Al2O3 and spinel, while the oxide formed on the sandblasted coating was composed mainly of Al2O3. In addition, with increasing R a of the bondcoat, the oxidation rate progressively decreased and the amount of Al2O3 increased compared with the as-sprayed coating. Scanning electron microscope analysis showed that the thickness of the scale layer on the sandblasted CoNiCrAlY coating at 1050 °C was much thinner than that on the as-sprayed one. Isothermal oxidation kinetics of HVOF CoNiCrAlY coatings with different surface modifications at 1050 °C. This figure indicates the effect of the surface process on the oxidation kinetics of the CoNiCrAlY coating at the temperature of 1050 °C. As it can be observed, in both samples, oxidation rate is initially high and gradually lowers with the passage of time. The reason for this reduction in the longer times is the formation of a uniform and dense oxide layer at the beginning of the process within which the rate of oxygen penetration reduces and gets harder with lasting oxidation time. The type of oxidation kinetics of these coatings is parabolic. According to the results obtained, the oxidation rate constant (Kp) for the coating in the as-sprayed and sandblasted states equals 1.75 × 10−13 and 0.63 × 10−13 mg/cm2, respectively. The reduction in the oxidation rate signifies the effect of the surface morphology on the oxidation behavior of these coatings.

Interactions in Ternary Mixtures of MnO2, Al2O3, and Natural Organic Matter (NOM) and the Impact on MnO2 Oxidative Reactivity.

Our previous work reported that Al2O3 inhibited the oxidative reactivity of MnO2 through heteroaggregation between oxide particles and surface complexation of the dissolved Al ions with MnO2 (S. Taujale and H. Zhang, "Impact of interactions between metal oxides to oxidative reactivity of manganese dioxide" Environ. Sci. Technol. 2012, 46, 2764-2771). The aim of the current work was to investigate interactions in ternary mixtures of MnO2, Al2O3, and NOM and how the interactions affect MnO2 oxidative reactivity. For the effect of Al ions, we examined ternary mixtures of MnO2, Al ions, and NOM. Our results indicated that an increase in the amount of humic acids (HAs) increasingly inhibited Al adsorption by forming soluble Al-HA complexes. As a consequence, there was less inhibition on MnO2 reactivity than by the sum of two binary mixtures (MnO2+Al ions and MnO2+HA). Alginate or pyromellitic acid (PA)-two model NOM compounds-did not affect Al adsorption, but Al ions increased alginate/PA adsorption by MnO2. The latter effect led to more inhibition on MnO2 reactivity than the sum of the two binary mixtures. In ternary mixtures of MnO2, Al2O3, and NOM, NOM inhibited dissolution of Al2O3. Zeta potential measurements, sedimentation experiments, TEM images, and modified DLVO calculations all indicated that HAs of up to 4 mg-C/L increased heteroaggregation between Al2O3 and MnO2, whereas higher amounts of HAs completely inhibited heteroaggregation. The effect of alginate is similar to that of HAs, although not as significant, while PA had negligible effects on heteroaggregation. Different from the effects of Al ions and NOMs on MnO2 reactivity, the MnO2 reactivity in ternary mixtures of Al2O3, MnO2, and NOM was mostly enhanced. This suggests MnO2 reactivity was mainly affected through heteroaggregation in the ternary mixtures because of the limited availability of Al ions.

The formation of feldspar strontian (SrAl2Si2O8) via ceramic route: Reaction mechanism, kinetics and thermodynamics of the process

The reaction mechanism, the equilibrium composition, the temperature range of stability of formed intermediates as well as the kinetics and thermodynamics of activated state during the formation of monoclinic strontium-aluminum-silicate feldspar stroncian (SrAl2Si2O8) via the ceramic route from the mixture of SrCO3, Al2O3 and SiO2 is described in this work. Strontian does not appear up to the temperature of 1150°C and is the only stable phase at the temperature ≥1600°C. Three independent reactions lead to two parallel reaction pathways, i.e. the formation of strontian from single or binary oxides (1) and with Sr-gehlenite as the intermediate (2). Since the reaction rate constants ratio is higher than one (k1/k2>1), the first reaction route is favored according to the Wegscheider principle. The kinetics of chemical reaction of 1.5 order corresponding to the kinetic function F2/3 ((1−α)−1/2−1) was determined as the rate determining the mechanism of formation of strontian. The integral and differential methods show that the process requires average apparent activation energy of 229.3kJmol−1. The determined average value of frequency factor is 2.1×105s−1.

Fine grained Al2O3–ZrO2 (Y2O3) ceramics by controlled crystallization of amorphous phase

It is classic to prepare nanoceramics by crystallization of amorphous solids; however, this method was seldom reported in the classic system of Al2O3–ZrO2. In present work, fine grained Al2O3–ZrO2 (Y2O3) ceramics were obtained through controlled crystallization of dense amorphous bulks by heat treatment. Phase composition, microstructure and mechanical property were investigated during heat treatment. Crystallization and densification were almost completed after heat treatment at 1400°C for 2h with relative density of 98.5%, and as-prepared sample exhibited homogenous mixture of Al2O3 and ZrO2 grains at about 440nm. The fine grained ceramic exhibited high hardness of 15.5GPa and fracture toughness of 5.2MPam1/2. This work may provide new guidance for preparing AZY nanoceramics through crystallization of amorphous phase in both experiment and theory.

Activation of consolidation processes of alumina ceramics

The methods for activating sintering ceramics based on Al2O3 by mechanical activation in the planetary mill, by adding in the mixture of nanopowders (NP) Al, Al2O3, and submicron powder TiO2, and by applying the technology of spark plasma sintering (SPS) are developed. It has been shown that adding the nanopowder up to 20 wt. % Al2O3 in a coarse powder α-Al2O3 activates the sintering process resulting in increased density and hardness of the sintered alumina ceramics. Substantial effect of increasing density of alumina ceramics due to adding the submicron powder TiO2 in the compound of initial powder mixtures has been established.

Catalytic pyrolysis of waste plastic into liquid fuel

Process of pyrolysis is a thermochemical process conducted at high temperatures and usually in presence of catalysts. Different type of catalysts, natural and synthetic, can be used for conversion of organic wastes into valuable fuels. The aim of this work is conversion of waste polyolefin mixture and production of liquid fuel using mixture of Al2O3 and SiO2 as a catalyst. Waste mixture was pyrolyzed at temperature range 400-550oC and obtained products were liquid fuel, gas and minor solid residue. Under the optimized reaction conditions, the condensed liquid fraction is much larger than the gaseous fraction. Different amounts of catalyst and polyolefin mixture as a feedstock were used. According to the obtained results, the retention time and the percent of SiO2 in the catalyst mixture have predominant effect on the amount of liquid product. Decreasing the quantity of SiO2 in the catalyst mixture increased the yield of liquid product. The physical properties of obtained liquid products were characterized and according to the measured values, liquid fuel belongs to light fraction of diesel fuel.

The development of effective CaO-based CO2 sorbents via a sacrificial templating technique.

A carbon-based sacrificial templating approach was employed to realize single-pot synthesis of cyclically stable CaO-based CO2 sorbents. The sacrificial carbonaceous template was formed through resorcinol-formaldehyde polymerization reaction. The resultant sorbents following the thermal decomposition of the carbonaceous template featured an inverse opal-like macrostructure composed of a highly porous nanostructured backbone. In addition to pure CaO, sorbents supported with Al2O3, MgO, Y2O3, and ZrO2 were synthesized. SEM and XRD were utilized to characterize the morphology and the chemical composition of the synthetic CO2 sorbents, respectively. The cyclic CO2 uptake performance of the synthetic sorbents was assessed by TGA and compared to limestone. All of the synthetic sorbents exhibited an improved CO2 uptake performance when compared to limestone. The performance enhancement became more pronounced in the case of supported sorbents. The sorbent with the best CO2 uptake performance was supported by a mixture of Al2O3 and Y2O3, and exhibited a CO2 uptake of 0.61 g CO2/g CaO after 10 cycles of calcination and carbonation under practically relevant operating temperatures, which exceeded the CO2 uptake of limestone by more than 350%.

Microstructure and mechanical behaviour characterisation of Al-Al<SUB align="right">2O<SUB align="right">3 MMC processed by DIMOX and Al-Al<SUB align="right">2O<SUB align="right">3/MnO<SUB align="right">2 MMC processed via stir casting route

In the present study two types of composite were developed - Al-Al2O3 composite by direct metal oxidation (DIMOX) and by reinforcements using both ex situ and in situ approaches together by a dispersing powder mixture of Al2O3 and MnO2 in a ratio of 1:1, through stir casting route in aluminium matrix. Al-Al2O3 composites were synthesised successfully under ambient conditions and it was found that the particles increased as the holding time and the number of stirrings was increased resulting in improved mechanical properties. Generation of Al2O3 particles in the matrix was confirmed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) examination. Hence, oxidation, which is known as a major problem in composite preparation by liquid metallurgy route, can be used for preparation of composites and for improving the properties. In Al-Al2O3/MnO2 composites the mechanical properties as well as particle distribution improved as Al2O3 + MnO2 were introduced and also with their increasing weight percent.

Production and characterization of A5083–Al2O3–TiO2 hybrid surface nanocomposite by friction stir processing

In this study, the production and characterization of A5083–Al2O3–TiO2 hybrid surface nanocomposite by friction stir processing have been investigated. The effect of different ratios of nanosized Al2O3 and TiO2 particles on microstructural and mechanical properties was investigated. A threaded cylindrical hardened steel tool was used with the rotation speed of 500 r/min and travel speed of 56 mm/min and a tilt angle of 3°. Microhardness of base metal and treated surfaces as well as tensile strength was evaluated. The samples were characterized by means of optical and scanning electron microscopy. The results showed that the maximum tensile strength and hardness value were achieved for a mixture of Al2O3 and TiO2 in the ratio of 25–75, respectively. The microhardness and tensile strength were respectively increased by 50% and 182% while ductility was reduced by 60%.

Frictional properties of Ti3AlC2 ceramic against different counterparts in deionized water and artificial seawater

In this study, the tribological behavior of Ti3AlC2 in both deionized water and artificial seawater was investigated. The tribological mechanisms of different tribopairs in the two aqueous environments were discussed in detail. When sliding against AISI 316L stainless steel, Al2O3 and Si3N4 in the two environments, Ti3AlC2 displayed poor tribological properties, and the mainly wear mechanisms were mechanical wear by counterparts. The tribo-chemical products mainly consisted of TiO2 and Al2O3 in both the liquids. However, the Ti3AlC2/SiC tribopair could provide excellent lubrication in both deionized water and artificial seawater. A smooth tribofilm was formed on the Ti3AlC2 worn surface, where the tribofilm was the mixture of TiO2, Al2O3 and SiOx. The tribofilm was believed to be responsible for the excellent lubricating effect. In light of these results, the Ti3AlC2/SiC tribopair held promise to serve in marine tribological applications.

Study of atomic and molecular emission spectra of Sr by laser induced breakdown spectroscopy (LIBS).

Laser Induced Breakdown Spectroscopy (LIBS) is an ideal analytical technique for in situ analysis of elemental composition. We have performed a comparative study of the quantitative and qualitative analysis of atomic and molecular emission from LIBS spectra. In our experiments, a mixture of SrCl2 and Al2O3 in powder form was used as a sample. The atomic emission from Sr and molecular emission from SrCl and SrO observed in LIBS spectra were analyzed. The optimum laser energies, gate delays, and gate widths for selected atomic lines and molecular bands were determined from spectra recorded at various experimental parameters. These optimum experimental conditions were used to collect calibration data, and the calibration curves were used to predict the Sr concentration. Limits of detection (LODs) for selected atomic and molecular emission spectra were determined.

Phase relations in the SiC–Al2O3–Y2O3 system

Phase relations in the SiC–Al2O3–Y2O3 system were determined by phase analyses of the solid-state reacted samples from powder mixtures of SiC, Al2O3 and Y2O3. The phase diagram at 1700°C of the ternary system was presented. SiC was compatible with each of the three yttrium aluminates of Y4Al2O9 (YAM), YAlO3 (YAP) and Y3Al5O12 (YAG). Stability of the YAP phase and possible effect of the SiO2 impurities on the phase relations in the system were discussed.

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO2/Y2O3 ceramics from mixture of Al2O3, SiO2, ZrO2 and Y2O3 by gradually adding of α–β Si3N4 nanopowder from 1 to 5wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400°C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si3N4 additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33g/cm3, while for traditionally sintered samples, the value is 2.55g/cm3, and the compressive strength for SPS grows with Si3N4 additives, reaching 600N/mm2. In the case of traditional sintering, it decreases to approximately 100N/mm2. The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO2 grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si3N4 additive, are filled with mullite crystalline formations.

Synthesis and luminescence spectra of two europium(II) aluminates, EuAl2O4 and EuAl12O19

Two europium(II) aluminates, EuAl2O4 and EuAl12O19, were prepared by a solid-state reaction between europium(III) oxide (Eu2O3) and alumina (Al2O3) powders under a mixed H2 and N2 gas flow. In the preparation of EuAl2O4 from a stoichiometric mixture of Eu2O3 and Al2O3, subtle differences in the products, such as whether the Eu4Al14O25 phase co-occurred or not, were observed between δ- and α-Al2O3 powders, which had an effect on the luminescence spectra. Europium(II) aluminates were formed above 1100°C. A single phase of EuAl2O4 obtained above 1350°C was confirmed by powder X-ray diffraction, infrared spectroscopy, and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The 27Al MAS NMR spectrum of EuAl2O4 is reported for the first time. The green emission at 520nm became more intense with increasing calcination temperatures up to 1300°C but diminished at higher temperatures. The EuAl12O19 phosphor was also prepared from a stoichiometric mixture of Eu2O3 and Al2O3 at 1200°C and its luminescence spectrum was compared with that of SrAl12O19:Eu2+.

Kinetics of oxygen uncoupling of a copper based oxygen carrier

Here, an oxygen carrier consisting of 60wt% CuO supported on a mixture of Al2O3 and CaO (23wt% and 17wt% respectively) was synthesised by wet-mixing powdered CuO, Al(OH)3 and Ca(OH)2, followed by calcination at 1000°C. Its suitability for chemical looping with oxygen uncoupling (CLOU) was investigated. After 25 repeated redox cycles in either a thermogravimetric analyser (TGA) or a laboratory-scale fluidised bed, (with 5vol% H2 in N2 as the fuel, and air as the oxidant) no significant change in either the oxygen uncoupling capacity or the overall oxygen availability of the carrier was found. In the TGA, it was found that the rate of oxygen release from the material was controlled by intrinsic chemical kinetics and external transfer of mass from the surface of the particles to the bulk gas. By modelling the various resistances, values of the rate constant for the decomposition were obtained. The activation energy of the reaction was found to be 59.7kJ/mol (with a standard error of 5.6kJ/mol) and the corresponding pre-exponential factor was 632m3/mol/s. The local rate of conversion within a particle was assumed to occur either (i) by homogeneous chemical reaction, or (ii) in uniform, non-porous grains, each reacting as a kinetically-controlled shrinking core. Upon cross validation against a batch fluidised bed experiment, the homogeneous reaction model was found to be more plausible. By accurately accounting for the various artefacts (e.g. mass transfer resistances) present in both TGA and fluidised bed experiments, it was possible to extract a consistent set of kinetic parameters which reproduced the rates of oxygen release in both experiments.

Synthesis of calcium aluminate nanoflakes and nanochains from woody biomass

Studies on the fabrication of nanoscaled calcium aluminate materials and their growth mechanisms still present key challenges. In this work, we report the catalyst-free chemical vapor deposition of calcium aluminate nanostructures including nanoflakes and nanochains during the carbonization of woody biomass at 1500°C in H2. The mixture of Al and Al2O3 is prepared as a starting material with henequén fibers for the synthesis. Hexagonal CaAl12O19 nanoflakes and monoclinic CaAl2O4 nanochains are found as main products after the synthesis. The CaAl12O19 nanoflakes have thickness of approximately 100nm. The CaAl2O4 nanochains exhibit a regular periodic structure that is composed of nanowires and nanobeads of 100nm and 300nm in diameter, respectively. The nanochains oriented along the (100) plane grow to lengths of dozens of microns. The surface of nanochains is covered by an amorphous layer with a thickness of approximately 5nm. Cathodoluminescence measurement reveals that both CaAl12O19 nanoflakes and CaAl2O4 nanochains have the potential in optical applications.

Fabrication and Characterization of Hydroxyapatite/Mullite and Tricalcium Phosphate/Al<sub>2</sub>O<sub>3</sub> Composites Containing 30 wt% of Bioactive Components

Mullite-matrix and Al₂O₃-matrix composites were fabricated with 30 wt% hydroxyapatite (HA) and tricalcium phosphate (TCP), respectively, as additives to give bioactivity. A diphasic gel process was employed to lower the densification temperature of the mullite matrix to 1320℃. A polymer complexation process was used to synthesize a TCP powder that was fully densified at 1250℃, for application to the matrix. For the HA/mullite composite, HA decomposed during sintering by reactions with the matrix components of Al₂O₃ and SiO₂, resulting in a mixture of Al₂O₃, TCP, and other minor phases with a low densification of less than 88% of the theoretical density (TD). In contrast, the TCP/Al₂O₃ composite was highly densified by sintering at 1350oC to 96%TD with no reaction between the components. Different from the TCP monolith, the TCP/Al₂O₃ composite also showed a fine microstructure and intergranular fracture, both of which characteristics are advantageous for strength and fracture toughness.

Gelcasting and reactive sintering of sheet-like YAG transparent ceramics

A transparent yttrium alumina garnet (YAG) ceramic was successfully prepared by gelcasting a mixture of Al2O3 and Y2O3 powders aided by a reactive sintering process. A water-soluble epoxy resin gelling system was employed to fabricate large, sheet-like ceramic pieces. A stable slurry with 75 wt% solid loading and low viscosity was prepared. After casting, drying, and pre-sintering, vacuum sintering was performed at 1720 °C for 6 h. Homogeneous microstructures of both pre-sintered and sintered bodies were obtained. The resulting YAG ceramic (60 × 50 × 0.5 mm3) with an average grain size of 10 μm had an in-line transmittance of 83.0% and 80.7% at the wavelength of 1064 nm and 400 nm, respectively.

Effect of temperature on the reaction sintering of mechanically activated ZrSiO4–Al2O3 mixture

In the present study, the effect of temperature on the dissociation of ZrSiO4 and formation of mullite during the reaction sintering of mechanically activated ZrSiO4–Al2O3 powder mixture was investigated. In this regard, a mixture of ZrSiO4 and Al2O3 with the stoichiometric composition was mechanically activated for different times up to 60h and then reaction sintered at different temperatures. Structural changes during the mechanical activation and reaction sintering were determined by X-ray diffraction (XRD). The results showed that by 20h of mechanical activation, the starting temperature for mullite formation during the reaction sintering decreased from 1550°C to 1200°C. The dissociation of ZrSiO4 and formation of mullite increased by increasing both the mechanical activation time and the reaction sintering temperature. Complete formation of mullite–zirconia composite was observed after 60h of mechanical activation and subsequent reaction sintering at 1400°C for 2h, which is about 200–250°C lower than the temperature commonly used in the reaction sintering of ZrSiO4–Al2O3 mixture.