Bulk Al2O3 Research Articles

Coal Slag Protection of Silicon Carbide with Chemically Vapor Deposited Mullite Coatings

The corrosion characteristics of bulk alumina, SiC, mullite, and CVD mullite coatings on SiC in contact with coal slag were investigated. Uncoated SiC corroded in the presence of coal slag, forming mixed FeSi phases and carbon. Bulk Al2O3 and slag formed a diffusional phase believed to be the spinel hercynite (Fe,Mg)O·(Al,Fe)2O3. After exposure to coal slag, a compositional difference was observed at bulk mullite/coal slag interfaces, yet this diffusional phase did not appreciably degrade the mullite samples and no cracking was observed. CVD mullite coatings offered protection to SiC in a simulated coal gasification atmosphere with corrosion protection dependent on the uniformity of the coating. Microprobe analysis of the CVD mullite coating/slag interface showed the formation of a Fe(Mg)Al.

Microstructure, Texture, and Thermal Conductivity of Single‐Layer and Multilayer Thermal Barrier Coatings of Y2O3‐Stabilized ZrO2 and Al2O3 Made by Physical Vapor Deposition

Thermal conductivity is one of the most important properties in the design of thermal barrier coatings (TBCs) for high‐temperature gas‐turbine blades. In the present study, the thermal conductivities of several TBCs were studied. Single‐layer and multilayer coatings consisting of Al2O3 and ZrO2 partially stabilized with 8 wt% Y2O3 (8YSZ) were deposited on nickel‐base superalloy substrates by electron‐beam physical vapor deposition (EB‐PVD). A columnar microstructure with (200) texture and [200] direction oriented nearly perpendicularly to the plane of the coating was observed in the single‐layer 8YSZ coating. In multilayer coatings with alternating layers of Al2O3 and 8YSZ, the tendency to develop columnar structure, as well as the intensity of (200) texture, decreased as the number of layers increased. In both the single‐layer and multilayer coatings, the 8YSZ layer consisted almost entirely of tetragonal (t) phase with a negligible amount of monoclinic (m) phase. This was in contrast to the microstructure of bulk 8YSZ (source material used for deposition), which contained a significant amount of m phase, in addition to t phase. Whereas bulk Al2O3 consisted of α‐phase, only γ‐phase was found in the Al2O3 layers of coatings. The thermal conductivities of these coatings were measured by the laser flash method. The thermal conductivity data of single‐layer 8YSZ coating compared well with the data on bulk ZrO2 stabilized by 5.3 wt% Y2O3 reported in the literature. This was due to the similarity of microstructures between the two. The thermal conductivity data of multilayer coatings were well described by a series heat‐transfer model calculation within the limits of experimental errors involved in the measurements.

Stability and Surface Acidity of Aluminum Oxide Grafted on Silica Gel Surface

ABSTRACTThe synthesis of aluminum oxide grafted on silica gel surface was carried out by the reaction of a suitable aluminum precursor with the surface hydrolysis of the oxide support. The chemical and physical properties of the attached oxide, SiO2/ Al2O3, can be quite different than those found for bulk Al2O3. The advantage of this preparation method, compared to the conventional ones (impregnation, precipitation and calcination), is that the oxide is highly dispersed on the surface (monolayer or submonolayer). We characterized the surface oxides treated at the temperature range of 423 to 1573 K employing X-ray photoelectron spectroscopy (XPS), solid state nuclear magnetic resonance spectroscopy (NMR), and diffuse reflectance spectroscopy (DRS). XPS was used to identify the oxidation states and atomic ratios. Al27 NMR detected two species for samples heated up to 1023 K, and another one above this temperature. DRS, using pyridine as a molecular probe, showed that both Lewis and Brönsted acid sites are stable up to 1023 K. We concluded that the aluminum oxide is highly dispersed on the silica gel surface and it remains stable up to 1023 K.

Oxygen Involvement in the Partial Oxidation of Methane on Supported and Unsupported V2O5

O exchange of O2 with V2O5/SiO2, V2O5/Al2O3, V2O5, SiO2, and Al2O3 was studied using steady-state isotopic transient kinetic analysis. It was found that bulk V2O5, V2O5/SiO2, V2O5/Al2O3, and Al2O3 exhibited some oxygen exchange capability, whereas SiO2 exhibited negligible oxygen exchange capability under the conditions used. Oxygen exchange reactions were also studied over supported vanadia catalysts under steady-state oxidation reaction conditions. O exchange of O2 with V2O5/SiO2 increased significantly in the presence of methane. This is attributed to a redox reaction making the surface more active for O exchange in the presence of the methane. The involvement of catalyst oxygen in the formation of the products HCHO, CO, and CO2 was demonstrated. However, an estimation of the contribution of the lattice oxygen in the formation of the products is complicated by the presence of secondary O exchange. The total amount of 16O exchanged with the feed O2 and the products indicates that the oxygen associated with silica or the vanadia-silica interface is also involved in the exchange process. In general, O exchange behavior of various product species with V2O5/Al2O3 was found to be similar to that with V2O5/SiO2. However, unlike V2O5/SiO2, the O exchange of O2 with V2O5/Al2O3 did not increase significantly in the presence of methane. This was attributed to the differences in the interactions of vanadia with silica and alumina. It is speculated that the O associated with highly dispersed tetrahedral surface vanadia is involved in a primary oxidation reaction, whereas O associated with the bulk-like vanadia and with the support is involved either in secondary oxidation reactions of HCHO or CO or in secondary O exchange of various O containing product species. Caution must be taken in making any conclusions about the source of reactive oxygen during partial oxidation on oxide catalysts based on isotopic oxygen studies due to the ease of secondary oxygen exchange.

Properties of alumina-based coatings deposited by plasma spray and detonation gun spray processes

Alumina, Al2O3 + 3 to 40 wt% TiO2, and Al2O3 + 40 wt% ZrO2 coatings were deposited by atmospheric plasma spraying (APS) and detonation gun spraying (DGS). The coatings were evaluated by optical microscopy, microhardness measurements, and X- ray diffraction. Wear resistance of the coatings was evaluated by rubber wheel sand abrasion and particle erosion test methods. Detonation gun- sprayed coatings exhibited more homogeneous microstructures and somewhat higher microhardness than corresponding plasma- sprayed coatings. Small additions of TiO2 (3 wt%) improved both the abrasion and erosion wear resistance, whereas 40 wt% TiO2 significantly decreased the erosion wear resistance of both APS and DGS coatings. Alumina + 40% ZrO2 coatings exhibited the best abrasion wear resistance of both APS and DGS coatings, but the erosion wear resistance of these coatings was lower than that of the Al2O3 and Al2O3 + 3 wt% TiO2 coatings. The best abrasion wear resistance of the coatings studied was obtained with DGS Al2O3 + 40 wt% ZrO2 and Al2O3 + 3 to 40 wt% TiCh coatings. These coatings exhibited lower wear rates than bulk Al2O3. The best erosion wear resistance was obtained with the DGS Al2O3 + 3 wt% TiO2 coating; however, it exhibited a higher wear rate than bulk Al2O3. In general, detonation gun- sprayed coatings showed significantly enhanced abrasion and erosion wear resistance than the corresponding plasma- sprayed coatings.

Solid-solution range of mullite up to 1800 °C and microstructural development of ceramics

Tetraethoxysilane (TEOS) and Al-sec-butylate (Al-O-Bu) were used for the sol-gel synthesis of mullite ceramics. The starting materials had bulk compositions corresponding to values between 72 and 78 wt% Al2O3, and 28 and 22 wt% SiO2, respectively, and were calcined at 400 °C (A-series) and 1100 °C (B-series). B-series samples, despite their higher green densities, could only be sintered to about 65–70% TD (theoretical density) at 1650 °C, whereas A-series samples achieve values of about 93–98% TD. Ceramics with relatively high amounts of glass phase from large tabular mullite crystals, which are embedded in a finer-grained mullite matrix. As soon as the bulk Al2O3 content increases, equiaxed mullite grains appear and the mean grain size becomes smaller, showing a significant difference between the nucleation and crystal growth mechanisms of mullites formed in samples with the lower and higher Al2O3-bulk compositions. Depending on the bulk composition of the samples, the temperature-controlled solid-solution of mullite ranges between about 72.7 and 74.3 wt% Al2O3 at 1600 °C and 74.1 and 75.4 wt% Al2O3 at 1800 °C, indicating that the solid-solution region bends over towards the Al2O3-side of the Al2O3-SiO2 phase diagram.

Anhydrous PT phase relations of an Aleutian high-MgO basalt: an investigation of the role of olivine-liquid reaction in the generation of arc high-alumina basalts

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H2O, which will affect the influence of bulk CaO, MgO, and Al2O3 contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than ∼12 kbar.

Thermal distribution produced by short pulse lasers in bulk Al2O3

The paper deals with an one-dimensional model of short pulse laser irradiation of the Al2O3 target and with simulation of the temperature distribution induced by it. In the model the temperature-dependent specific heat was considered. Also the latent heats of melting and evaporation were involved. The influence of energy density and absorption coefficient on the effect of overheating was examined. Experimental results on the ablation rate of the Al2O3 target by pulsed CO2 TEA laser in the evaporation regime and the threshold energy agreed well with the model.

Properties of magnetron-sputtered electrically insulating Al2O3 coatings on copper

Amorphous Al2O3 coatings were deposited on Cu substrates with metallic bond layers by different magnetron-sputtering processes. Such sputtering conditions as the type of discharge, target material, total pressure, sputtering gas composition and substrate temperature were varied to study the process effects on the structure and properties of the coatings deposited. The structure and general properties were found to be strongly dependent on the type of process and parameters. The breakdown voltages did not show any significant lowering as the temperature was increased from 20 to 400–500 ‡C. The maximum temperature without formation of cracks in the Al2O3 coating on Cu was about 700 ‡C. The d.c. electrical conductivities of the Al2O3 were similar to that of bulk Al2O3 at different temperatures. The results reveal the potential use of magnetron-sputtered Al2O3 coatings on Cu for electrical insulation and oxidation protection in different high-temperature applications, e.g. on metallic magnetohydrodynamic electrode and insulator modules.

Geochemistry and petrology of spinel lherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico: Partial melting and mantle metasomatism

Spinel Iherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico, are divided into two distinct groups according to their major element and trace element characteristics. Group Ia xenoliths are characterized by light rare earth element (LREE) depletion ((La/Lu)N = 0.10–0.77 in clinopyroxene) and linear major and compatible trace element relationships. Group Ib xenoliths are characterized by FeO and Na2O enrichment and higher (La/Lu)N ratios (0.80–4.1 in clinopyroxene) and complex major element relationships. These samples, which have a range of equilibrium temperatures of 910°–1070°C, exhibit protogranular textures and typical orthopyroxene+clinopyroxene+spinel clusters. Modal abundances and chemical compositions of the group Ia xenoliths vary from primitive (15.2% clinopyroxene, 38.5% MgO, 1824 ppm Ni) to moderately depleted (6.4–8.7% clinopyroxene, 43.8–44.1% MgO, 2192 ppm Ni). Systematic variations of major elements and compatible trace elements in the group Ia xenoliths are interpreted to result from various degrees (<25%) of partial melting and melt extraction, followed by subsolidus equilibration and annealing. The extracted melts have a range of compositions similar to picritic basalts. Abundances of moderately incompatible trace elements, Sc and Cr, in the group Ia minerals have been substantially redistributed during subsolidus equilibration. In a few of these xenoliths there appears to be vestiges of incipient metasomatism, but metasomatism has not substantially influenced the group as a whole. Group Ib xenoliths have been substantially influenced by metasomatic processes. The ∑FeO and Na2O contents of the cores of clinopyroxenes in group Ib xenoliths are higher than clinopyroxenes in group Ia samples. The higher La contents and La/Lu ratios in group Ib clinopyroxenes (compared to group Ia), together with this FeO and Na2O enrichment, suggest that equilibration of basanites with residual mantle has been a major process in the evolution of group Ib peridotites. Clinopyroxenes in the group Ib xenoliths are reversely zoned with higher Mg # (100×Mg/(Mg+∑Fe)), CaO, and TiO2 and with lower Na2O and Al2O3 in their rims than in their cores. Bulk rock Al2O3 and SiO2 contents of the group Ib xenoliths tend to be lower than those of group Ia at the same MgO contents. The Al2O3 and SiO4 depletion in bulk rocks and Al2O3 and Na2O depletion in the rims of the group Ib minerals result from reaction with an alkaline‐element‐enriched, H2O dominated fluid. The history of group Ib xenoliths appear to be dominated by an early phase of melt extraction followed by interaction with a basanitic melt to produce the FeO, Na2O, and LREE enrichment in clinopyroxenes. These peridotites then appear to have been affected by a H2O‐rich fluid that depleted Al2O3 and Na2O from the rims of clinopyroxenes and produced incipient melt veins in the xenoliths.

Selective electron-induced decomposition of CVD SiO2 overlayers on oxides

The electron-induced decomposition (EID) of SiO2 overlayers on various high surface area oxide substrates was studied using Auger electron analysis (AES). Bulk SiO2 was stable upon electron bombardment, but bulk Al2O3 decomposed readily to elemental Al when the primary electron beam energy was 2 keV and the beam current was 6–16 μA. On the other hand, SiO2 overlayers having an average thickness of about 10 Å on Al2O3 decomposed selectively under the same conditions. In contrast to Al2O3 substrates, SiO2 overlayers on TiO2, ZrO2, and MgO were quite stable to the same electron bombardment. Therefore it is concluded that Al2O3 accelerates the EID of SiO2 overlayers and the SiO2 overlayer suppresses EID of Al2O3 substrates.

Surface segregation and oxidation studies of AlBe alloys

AbstractSurface segregation and oxidation of AlBe alloys have been studied using the LEED‐AES technique. Results obtained from one polycrystalline sample and three single crystals with (001), (011) and (111) orientations show no segregation of Be to the clean Al surface. The annealing temperatures ranged from 300 °C to 600 °C for a few minutes to a few hours. Results from the polycrystalline sample suggest that in the presence of oxygen or Al2O3 on the surface, Be diffuses to the surface during annealing (at 400 °C or above) to form BeO and reduces Al2O3. The difference in the free energies of formation of the bulk BeO and Al2O3 favors this reaction. A one‐to‐one correspondence has been found between the surface coverage of BeO formed to that of Al2O3 present prior to annealing the sample. The three single crystals had much less Be in the bulk and this affects the reduction reaction by limiting the rate of Be diffusion to the surface. LEED investigation for the (111) oriented crystal showed that BeO grows as the (0001) basal plane on the Al(111) surface. The magnitude of the misfit between unrelaxed Al(111) and BeO(0001) as measured from the relative positions of superlattice beams indicate some compression or expansion in the interface region.

Role of Sodium Citrate in Aqueous Milling of Aluminum Oxide

A study was initiated to identify the mechanism(s) by which chemical additives affect aqueous milling of oxide suspensions. Properties of milled Al2O3 suspensions containing sodium citrate were correlated with ground-powder characteristics. Maximum grinding efficiency was obtained at the sodium citrate concentration which resulted in the maximum (absolute value) zeta potential and the minimum suspension viscosity. The results indicate that sodium citrate indirectly affects comminution by altering the particle arrangements (state of dispersion) in suspension. Fracture strength measurements, determined on porous bulk Al2O3 samples pretreated in distilled water and aqueous sodium citrate solutions, did not support chemomechanical mechanisms.

Extended appearance potential fine structure analysis of oxidized metal surfaces

Like EXAFS, extended appearance potential fine structure (EAPFS) probes the immediate neighborhood of a particular element and thus, unlike most other structural probes, does not require long-range order. Furthermore, the use of low-energy electron makes the technique surface sensitive. Because no dipole selection rules apply to electron excitation, explicit calculation must determine the angular momentum of the outgoing final-state electron and thus the feasibility of Fourier inversion of the data. For excitation of the O 1s core, we find the outgoing electron is overwhelmingly s-wave. Reaction of Al(100) with about 1-1/2 equivalent monolayers of oxygen disorders the surface completely (i.e., extinguishes the LEED pattern). For this surface, analysis of the O 1s edge gives an O–Al distance of 1.98±0.05 Å, the longer of the two Al–O spacings in bulk Al2O3, which indicates that the oxygen goes under the top layer of Al. For a thick oxide layer on Ni(100), diffraction effects precluded use of the O 1s edge. From Ni 2p data, for which the angular momentum selection is weaker, we can extract the bulk Ni–O distance by using plausible phase shifts.