Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Content on Electrical Breakdown Properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

Al₂O₃/Cu composites were prepared by external addition of Al₂O₃, and the effect of Al₂O₃ content on microstructure, density, hardness, electrical conductivity and vacuum electrical breakdown properties was studied. The results show that with increasing Al₂O₃ addition, the density of Al₂O₃/Cu composite significantly decreases, the hardness sharply increases and then slowly decreases, but the electrical conductivity invariably decreases. The vacuum breakdown test shows that with increasing Al₂O₃ addition, the breakdown strength first sharply increases and then decreases when the Al₂O₃ content exceeds 1.2 wt.%; the chopping current always exhibits a decreasing trend and the arc life first increases and then decreases. According to the morphology of arc erosion and analysis, the arc erosion resistance increases and then decreases sharply. In the range of experiments, the optimal arc erosion resistance of Al₂O₃/Cu composite can be obtained with the addition of 1.2 wt.% Al₂O₃.     

Structure and electric property comparison between Ge nanoclusters embedded in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript>

Metal-insulator-semiconductor (MIS) structures containing Ge nanocrystals embedded in both Al₂O₃ and ZrO₂/Al₂O₃ are fabricated by an ultra-high vacuum electron-beam evaporation method. Secondary ion mass spectroscopy (SIMS) results indicate that Ge embedded in Al₂O₃ diffuses towards the surface of the Al₂O₃ layer after annealing at 800°C in N₂ ambient for 30 min. Ge embedded in ZrO₂/Al₂O₃ is stable, thus inducing less leakage current. Capacitance voltage studies indicate that annealing can effectively passivate the negatively charged trapping centers. Memory effect of the Ge nanoclusters is verified by hysteresis in the C-V curves in the Al₂O₃/Ge+Al₂O₃/Al₂O₃ and ZrO₂/Ge+Al₂O₃/Al₂O₃ samples. This article is based on a presentation in “The 7th Korea-China Workshop On Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24–27, 2003.     

Theoretical investigation of the CO oxidation on Al<Subscript>12</Subscript>Zr Cluster

Equilibrium geometries of Al₁₂Zr cluster were systematically studied on the basis of density functional theory with generalized gradient approximation. To gain insights into high catalytic activity we use the CO oxidation as a benchmark probe. In Al–Zr bimetallic clusters, Zr site is the catalytically active centre, the adsorption of CO and O₂ on the same site respectively (single-site mechanism), a Langmuir-Hinshelwood (LH) mechanism is proposed, which proceed via two steps, CO + O₂ → CO₂ + O and CO + O → CO₂. Two CO oxidation mechanisms of two CO₂ molecules as product have been simulated. For the later mechanism, the key step is the O–O bond scission in the OCOO* intermediate, which is significantly accelerated due to the attack of the neighboring CO molecule. The calculated barriers for the later reactions are lower compared with the former reaction. Detailed reaction paths corresponding to this case are calculated. Our study suggests that the CO oxidation catalyzed by Al₁₂Zr cluster is likely to occur at the room temperature.     

Studies on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> high K gate dielectrics applied in a fully depleted SOI MOSFET

Al₂O₃/ZrO₂/Al₂O₃ gate stacks were prepared on ultrathin SOI (Silicon on insulator) substrates by ultrahigh vacuum electron beam evaporation and post-annealed in N₂ at 450°C for 30 min. Three clear nanolaminate layered structure of Al₂O₃(2.1 nm)/ZrO₂(3.5 nm)/Al₂O₃(2.3 nm) was observed with a high-resolution cross-sectional transmission electron microscope (HR-XTEM). High frequency capacitance voltage (C-V) characteristics of a fully depleted (FD) SOI MOS capacitor at 1 and 5 MHz were studied. The minority carriers determine the high frequency C-V properties, which is opposite to the case of bulk MOS capacitors. The series resistance of the SOI substrate is found to be the determinant factor of the high frequency characteristics of FD SOI MOS capacitors. This article is based on a presentation in “The 7th Korea-China Workshop on Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24≈27, 2003.     

Effect of Microstructure on the Thermal Conductivity of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-YSZ Coatings

The microstructures of three atmospheric plasma-sprayed (APS) Al₂O₃-ZrO₂ coatings were investigated using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The differences in the microstructures of the three Al₂O₃-ZrO₂ coatings, including their phase compositions, cracks, pores, grain sizes, and solid solutions, were analyzed in detail. A close relationship was observed between the thermal conductivities of the coatings and the microstructures, and the Al₂O₃-YSZ coatings with more spherical pores, fewer vertical cracks, and finer grains exhibited the lowest thermal conductivity of 0.91 W/m·K. Compared with YSZ coatings, Al₂O₃-YSZ coatings can exhibit lower thermal conductivity, which may be attributed to the formation of an amorphous phase, smaller grains, and Al₂O₃-YSZ solid solution.     

Formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al Composites by Directed Melt Oxidation of Al-Si-Zn Alloy

Observations are presented on the initiation and growth of Al₂O₃/Al composites by the directed melt oxidation of Al-Si alloys containing metallic Zn or using external dopant ZnO. Thermal gravimetric analysis, optical microscopy, and x-ray diffraction analysis were employed to characterize the progress of oxidation and the nature of oxidation products. Both Zn and ZnO dopants were able to initiate the directed melt oxidation of Al-Si alloys without any Mg being present. Al₂O₃/Al composites were produced when the alloying Zn concentration exceeding 3 wt.%. The incubation period of the oxidation process for Al-Si-Zn alloys was shortened markedly and the amount of composite products increased with the increasing of Zn content in the alloy. In addition, doping with ZnO powder resulted in dense composite formation. A macroscopically planar surface and a fine microstructure promote oxidation growth in Al₂O₃/Al composites. Doping with ZnO powder offers a significant advantage over using metallic Zn for the directed melt oxidation of Al-Si alloy.     

New ternary compound La<Subscript>8</Subscript>Al<Subscript>13</Subscript>Sn<Subscript>3</Subscript>

A new ternary compound La₈Al₁₃Sn₃ was synthesized and studied by means of X-ray powder diffraction technique. The compound La₈Al₁₃Sn₃ crystallizes in a hexagonal AlB₂-type structure with space group P6/mmm (No. 191) and the lattice parameters a = 0.44919(1) nm and c = 0.43835(1) nm. The Smith and Snyder figure of merit for index, F _N , is F ₃₀ = 197.3(30). The Rietveld refinement for the crystal structure of the compound was carried out successfully. The liability R-factors of Rietveld refinement are R _p = 0.114 and R _wp = 0.148. The compound La₈Al₁₃Sn₃ decomposes into LaAl₂ with cubic structure and space group Fd $ \bar 3 $ \bar 3 m (227), LaAl₃ with hexagonal structure and space group P6₃/mmc (194), and Sn at 720°C.     

The Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Powder Morphology on the Properties of Cu-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composites Designed for Functionally Graded Materials (FGM)

In order to meet the requirements of an increased efficiency applying to modern devices and in more general terms science and technology, it is necessary to develop new materials. Combining various types of materials (such as metals and ceramics) and developing composite materials seem to be suitable solutions. One of the most interesting materials includes Cu-Al₂O₃ composite and gradient materials (FGMs). Due to their potential properties, copper-alumina composites could be used in aerospace industry as rocket thrusters and components in aircraft engines. The main challenge posed by copper matrix composites reinforced by aluminum oxide particles is obtaining the uniform structure with no residual porosity (existing within the area of the ceramic phase). In the present paper, Cu-Al₂O₃ composites (also in a gradient form) with 1, 3, and 5 vol.% of aluminum oxide were fabricated by the hot pressing and spark plasma sintering methods. Two forms of aluminum oxide (αAl₂O₃ powder and electrocorundum) were used as a reinforcement. Microstructural investigations revealed that near fully dense materials with low porosity and a clear interface between the metal matrix and ceramics were obtained in the case of the SPS method. In this paper, the properties (mechanical, thermal, and tribological) of composite materials were also collected and compared. Technological tests were preceded by finite element method analyses of thermal stresses generated in the gradient structure, and additionally, the role of porosity in the formation process of composite properties was modeled. Based on the said modeling, technological conditions for obtaining FGMs were proposed.     

Dual-Layer Oxidation-Protective Plasma-Sprayed SiC-ZrB<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Carbon Nanotube Coating on Graphite

Graphite is used in high-temperature gas-cooled reactors because of its outstanding irradiation performance and corrosion resistance. To restrict its high-temperature (>873 K) oxidation, atmospheric-plasma-sprayed SiC-ZrB₂-Al₂O₃-carbon nanotube (CNT) dual-layer coating was deposited on graphite substrate in this work. The effect of each layer was isolated by processing each component of the coating via spark plasma sintering followed by isothermal kinetic studies. Based on isothermal analysis and the presence of high residual thermal stress in the oxide scale, degradation appeared to be more severe in composites reinforced with CNTs. To avoid the complexity of analysis of composites, the high-temperature activation energy for oxidation was calculated for the single-phase materials only, yielding values of 11.8, 20.5, 43.5, and 4.5 kJ/mol for graphite, SiC, ZrB₂, and CNT, respectively, with increased thermal stability for ZrB₂ and SiC. These results were then used to evaluate the oxidation rate for the composites analytically. This study has broad implications for wider use of dual-layer (SiC-ZrB₂/Al₂O₃) coatings for protecting graphite crucibles even at temperatures above 1073 K.     

Fabrication and Properties of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> Composite Coatings

Al₂O₃ /xZrO₂ (where x = 0, 3, 13, and 20 wt.%) composite coatings were deposited onto mild steel substrates by atmospheric plasma spraying of mixed α-Al₂O₃ and nano-sized monoclinic-ZrO₂ powders. Microstructural investigation showed that the coatings comprised well-separated Al₂O₃ and ZrO₂ lamellae, pores, and partially molten particles. The coating comprised mainly of metastable γ-Al₂O₃ and tetragonal-ZrO₂ with trace of original α-Al₂O₃ and monoclinic-ZrO₂ phases. The effect of ZrO₂ addition on the properties of coatings were investigated in terms of microhardness, fracture toughness, and wear behavior. It was found that ZrO₂ improved the fracture toughness, reduced friction coefficient, and wear rate of the coatings.     

Microstructure and properties of Al<Subscript>3</Subscript>Zr/2024Al in situ composites after forging

To research the influencing rule of severe plastic deformation on Al matrix in situ composites, 10 wt% Al₃Zr/2024Al in situ particle-reinforced composite was prepared by direct melt reaction (DMR), and then, the composite was hot-forged by one direction with 90 % plastic deformation. Then, the microstructure of the forging state composite was observed, and the change law of mechanical properties and friction performances after and before plastic deforming was compared. The results indicate that the Al₃Zr-reinforced particle rotates and breaks into smaller size of 10–20 µm with the matrix flow; in addition, these smaller particles distribute more orderly in orientation and dispersedly. The 2024 matrix grain changes from as-cast tissue to fibrous tissue with length of 100 µm along the forging direction. Some mechanical properties of composites hot-forged by one direction with 90 % plastic deformation are improved much obviously. For example, the tensile strength is improved by 39.75 %, reaching 225 MPa, and the hardness is improved by 35.84 %, reaching HBW 76.83. Besides, the wear rate reduces, wear depth and area decline, and the wear-resisting property is improved after hot forging.     

Effect of Milling Time on Electrical Breakdown Behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

In order to clarify the relationship between the microstructure and the arc erosion behavior of metal-matrix composite, Al₂O₃/Cu composites with different distributions of Al₂O₃ particles were prepared by high energy ball milling and powder metallurgy. The effect of milling time on microstructure, properties, and arc erosion behavior of Al₂O₃/Cu composite was investigated. The results show that the distribution of Al₂O₃ particles improves significantly with increase of milling time, but Al₂O₃ particles will be aggregated if milling time is too long. The optimal milling time is 24 h in the range of experiments. A uniform distribution of Al₂O₃ particles in copper matrix can improve the hardness, electrical conductivity, average breakdown strength, chopping level, and arc life. With improvement in the distribution of Al₂O₃ particles, the erosion area becomes larger, and the erosion pits become shallower and are dispersed more uniformly.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nano-Particles on Corrosion Performance of Plasma Electrolytic Oxidation Coatings Formed on 6061 Aluminum Alloy

Corrosion resistance improvement of plasma electrolyte oxidation coatings on 6061 aluminum alloy in silicate electrolyte containing Al₂O₃ nano-particles was studied, with particular emphasis on the microstructure, coating growth, and corrosion behavior in 3.5 wt.% NaCl solution. The microstructure of coatings, their thickness, and phase composition were characterized using scanning electron microscopy and x-ray diffraction. All characterization data showed that the maximum coating thickness and lowest amount of porosity were obtained in a low concentration of KOH, a high concentration of Na₂SiO₃, and moderate concentration of Al₂O₃ nano-particles in the electrolyte. This combination describes the optimum plasma electrolytic oxidation electrolyte, which has the best conductivity and oxidizing state, as well as the highest incorporation of electrolyte components in the coating growth process. On the other hand, incorporation and co-deposition of Al₂O₃ nano-particles were more pronounced than SiO₃ ^2? ions in some level of molar concentration, which is due to the higher impact of electron discharge force on the adsorption of Al₂O₃ nano-particles. The electrochemical results showed that the best protective behavior was obtained in the sample having a coat with the lowest porosity and highest thickness.     

Phase Relations in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> System in the Solid State. The Crystal Structure of AlVO<Subscript>4</Subscript>

Phase relations in the ternary oxide system Al₂O₃-V₂O₅-MoO₃ in the solid state in air have been investigated by using the x-ray diffraction (XRD) and differential thermal analysis/thermogravimetric (DTA/TG) methods. It was confirmed that in the subsolidus area of the Al₂O₃-V₂O₅-MoO₃ system, there exist seven phases, that is Al₂O₃, V₂O_5(s.s.), MoO₃, AlVO₄, Al₂(MoO₄)₃, AlVMoO₇, and V₉Mo₆O₄₀. Seven fields, in which particular phases coexist at equilibrium, were isolated. The crystal structure of AlVO₄ has been refined from x-ray powder diffraction data. Its space group is triclinic, , Z = 6, with a = 0.65323(1) nm, b = 0.77498(2) nm, c = 0.91233(3) nm, α = 96.175(2)°, β = 107.234(3)°, γ = 101.404(3)°, V = 0.42555 nm³. The crystal structure of the compound is isotypic with FeVO₄. Infrared (IR) spectra of AlVO₄ and FeVO₄ are compared.     

Drying of liquidized products of pyrolysis on composite sorbent CaCl<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Statics,dynamics, and modeling

Our investigation on the equilibrium of water sorption by composite sorbent CaCl₂/Al₂O₃ in a model system based on benzene and its use for the dynamic drying of liquidized products of pyrolysis is reported. To describe the dynamics of drying, a mathematical model has been developed that has been used to estimate the operating parameters of an industrial drying plant for liquidized products of pyrolysis.     

Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particle Size on Microstructure,Mechanical Properties and Abrasive Wear Behavior of Flame-Sprayed and Remelted NiCrBSi Coatings

The influence of micrometric alumina (low surface area-to-volume ratio) and nanometric alumina (high surface area-to-volume ratio) on microstructure, hardness and abrasive wear of a NiCrBSi hardfacing alloy coating applied to an AISI 304 substrate using flame spraying (FS) combined with surface flame melting (SFM) is studied. Remelting after spraying improved the mechanical and tribological properties of the coatings. Microstructural characterization using XRD, SEM and EDS indicated that alumina additions produced similar phases (NiSi, Ni₃B, CrC and Ni₃₁Si₁₂) regardless of the alumina size, but the phases differed in morphology, size distribution and relative proportions from one coating to another. The addition of 12 wt.% nanometric Al₂O₃ increased the phases concentration more than five- to sixfold and reduced the hard phases size about four-to threefold compared with NiCrBSi + 12 wt.% micrometric Al₂O₃. Nanoalumina led to reduced mass loss during abrasive wear compared to micrometric alumina and greater improvement in hardness.     

Effect of Surface Roughness and Structure Features on Tribological Properties of Electrodeposited Nanocrystalline Ni and Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

Metal matrix composite coatings obtained by electrodeposition are one of the ways of improving the surfaces of materials to enhance their durability and properties required in different applications. This paper presents an analysis of the surface topography, microstructure and properties (residual stresses, microhardness, wear resistance) of Ni/Al₂O₃ nanocomposite coatings electrodeposited on steel substrates from modified Watt’s-type baths containing various concentrations of Al₂O₃ nanoparticles and a saccharin additive. The residual stresses measured in the Ni/Al₂O₃ coatings decreased with an increasing amount of the co-deposited ceramics. It was established that the addition of Al₂O₃ powder significantly improved the coatings’ microhardness. The wear mechanism changed from adhesive-abrasive to abrasive with a rising amount of Al₂O₃ particles and coating microhardness. Nanocomposite coatings also exhibited a lower coefficient of friction than that of a pure Ni-electrodeposited coating. The friction was found to depend on the surface roughness, and the smoother surfaces gave lower friction coefficients.     

Phase Equilibria in the System Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-CoO and Gibbs Energy of Formation of Ca<Subscript>3</Subscript>CoAl<Subscript>4</Subscript>O<Subscript>10</Subscript>

An isothermal section of the system Al₂O₃-CaO-CoO at 1500 K has been established by equilibrating 22 samples of different compositions at high temperature and phase identification by optical and scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy after quenching to room temperature. Only one quaternary oxide, Ca₃CoAl₄O₁₀, was identified inside the ternary triangle. Based on the phase relations, a solid-state electrochemical cell was designed to measure the Gibbs energy of formation of Ca₃CoAl₄O₁₀ in the temperature range from 1150 to 1500 K. Calcia-stabilized zirconia was used as the solid electrolyte and a mixture of Co + CoO as the reference electrode. The cell can be represented as: From the emf of the cell, the standard Gibbs energy change for the Ca₃CoAl₄O₁₀ formation reaction, CoO + 3/5CaAl₂O₄ + 1/5Ca₁₂Al₁₄O₃₃ → Ca₃CoAl₄O₁₀, is obtained as a function of temperature: /J mol⁻¹ (±50) = −2673 + 0.289 (T/K). The standard Gibbs energy of formation of Ca₃CoAl₄O₁₀ from its component binary oxides, Al₂O₃, CaO, and CoO is derived as a function of temperature. The standard entropy and enthalpy of formation of Ca₃CoAl₄O₁₀ at 298.15 K are evaluated. Chemical potential diagrams for the system Al₂O₃-CaO-CoO at 1500 K are presented based on the results of this study and auxiliary information from the literature.     

Oxidation behavior of Mo-Si-B-(X) alloys: Macro- and microalloying (X= Cr,Zr, La<Subscript>2</Subscript>O<Subscript>3</Subscript>)

High melting Mo-Si-B based alloys exhibit good mechanical properties and oxidation resistance at very high temperatures. The alloy composition Mo-9Si-8B (at.%) shows excellent oxidation behavior between 900°C–1,300°C as a consequence of protective silica scale formation. Below 900°C, a protective oxide layer does not form as a consequence of simultaneous and competitive Mo- and Si-oxide formation. Macro- and microalloying strategies as well as pre-treatment prior to oxidation in air were examined in order to determine their impact on the oxidation performance. Alloying with Cr is a suitable means for protective Cr-oxide scale formation at intermediate temperatures; adding small amounts of reactive elements was found to have a strong impact on the silica scale forming ability. Furthermore, the oxidation behavior can be controlled and enhanced by selective oxidation in oxygen-deficient atmospheres prior to exposure to air.