Effect of Al2O3 particle size on vacuum breakdown behavior of Al2O3/Cu composite

In order to clarify the effect of Al₂O₃ particle size on the arc erosion behavior of the ceramic-reinforced Al₂O₃/Cu composite, Al₂O₃/Cu composites with different sizes of Al₂O₃ particles were prepared by powder metallurgy, the effect of Al₂O₃ particle size on the characteristics of arc motion was studied, and the mechanism of arc erosion of Al₂O₃/Cu composites was discussed as well. The results show that with decrease in the size of Al₂O₃ particles, the erosion area increases significantly and the erosion pits become shallower. The vacuum breakdown is preferred to appear in the area between Al₂O₃ particle and the copper matrix. Based on the experimental results and theoretical analysis, a particle partition arc model is proposed.     

Effect of Al2O3 particle size on vacuum breakdown behavior of Al2O3/Cu composite

In order to clarify the effect of Al₂O₃ particle size on the arc erosion behavior of the ceramic-reinforced Al₂O₃/Cu composite, Al₂O₃/Cu composites with different sizes of Al₂O₃ particles were prepared by powder metallurgy, the effect of Al₂O₃ particle size on the characteristics of arc motion was studied, and the mechanism of arc erosion of Al₂O₃/Cu composites was discussed as well. The results show that with decrease in the size of Al₂O₃ particles, the erosion area increases significantly and the erosion pits become shallower. The vacuum breakdown is preferred to appear in the area between Al₂O₃ particle and the copper matrix. Based on the experimental results and theoretical analysis, a particle partition arc model is proposed.     

Spray processing and wear characteristics of Al-Cu-Al2O3-Pb based composites

The spray deposition process has been employed in synthesis of Al-4.5Cu-10Al₂O₃ and Al-4.5Cu-10Al₂O₃-10Pb based composites. The microstructure and wear characteristics of composites were investigated. The rapid solidification inherent in spray deposition processing resulted in a uniform dispersion of Al₂O₃ and Pb particles co-existing in the matrix of the- primary α-phase. The grain size of the Al-4.5Cu-10Al₂O₃-Pb composite was observed to be higher than that of the Al-4.5Cu-10Al₂O₃ composite in various sections of the spray deposit. The wear rate of composite materials decreased with addition of Pb phase. This behavior is discussed in the light of the microstructural modification induced by spray deposition and the morphology of debris particles on the wear track surfaces. The wear characteristics of the composites are compared with that of the liquid immiscible Al-4.5Cu-10Pb alloy.     

Reaction between ∼0.7- to 1-μm α-Al2O3 particles and SiO2 during high-temperature sintering

Al₂O₃:Cr³⁺ and Al₂O₃:Ti³⁺ particles with an average size of ∼1 and 0.7 μm, respectively, have been prepared through mechanical grinding of bulk crystals. Using purpose-designed accessories made from chromium and titanium, we were able to prevent Fe contamination. We studied the reaction between α-Al₂O₃ and SiO₂ particles during high-temperature sintering. The results demonstrate that, in this system at temperatures above 1300°C, corundum particles dissolve in silica to form mullite. The reaction temperature and rate depend on the particle size composition of the Al₂O₃. The heating rate is shown to influence the dissolution rate of corundum particles. Increasing the heating rate from 7 to 15°C/min shifts the dissolution range of alumina particles from 1500 to 1630°C.     

Effects of TiO2/Al ratio on microstructures and mechanical properties of TiAl base composites

Effects of TiO₂/Al ratio on the microstructures and mechanical properties of in situ Al₂O₃/TiAl based composites were investigated. The results indicate that the as-sintered products consist of grains of nearly lamellar α₂ + γ structure with a dispersion of randomly oriented Al₂O₃ particles. A 43.9Ti-38.6Al-17.5TiO₂-nNb₂O₅ system was compared to 57.46Ti-36.78Al-5.76TiO₂-nNb₂O₅ system. The lamellar spacing of the products increases and the α₂ phase volume decreases with decreasing TiO₂/Al ratio. For each system, as the volume of α₂ phase increases, the average lamellar spacing decreases. Strength increases with an increasing TiO₂/Al ratio due to the amount of α₂ phase. Al₂O₃ phase increases with increasing TiO₂/Al ratio. Toughness increases with decreasing TiO₂/Al ratio. When the Nb₂O₅ content is smaller than 6 wt.%, the lamellar spacing plays an important role in toughness than the Al₂O₃ content. When the Nb₂O₅ content is larger than 6 wt.%, the Al₂O₃ content exhibits significantly increases the values of toughness than lamellar spacing.     

Origins of hindrance in densification of Ag/Al2O3 composites

The densification and microstructure development were studied for Ag-matrix composites containing dispersed Al₂O₃ particles to examine the effect of inclusions on the densification of composites. The incorporation of Al₂O₃ particles into Ag matrix hindered densification. Microstructure observation revealed that pores larger than those in the matrix formed around Al₂O₃ particles during sintering. The pore morphology was dependent on the number density of Al₂O₃ particles. When the number density was low, pores remained around an Al₂O₃ particle and coalesced to a large circumferential void at high temperatures. When the number density was intermediate, clusters made of a few Al₂O₃ particles formed and pores within and around clusters remained up to high temperatures. When the number density was high, the distance between clusters became small and the clusters were connected, forming continuous pores (pore channels). The three-dimensional connectivity of Ag was decreased, and the shrinkage between Ag particles resulted in thickening of pore channels. The presence of these large pores was the origin of the hindrance in densification.     

The fatigue crack growth behaviour of 2524‐T3 aluminium alloy in an Al2O3 particle environment

The effect of the Al₂O₃ dust environment on the crack propagation behaviour of 2524‐T3 Al alloy was investigated. The results show that the Al₂O₃ dust environment reduces the fatigue crack growth rate (FCGR) of alloy especially at low ΔK. Many Al₂O₃ particles are deposited and stuck in the crack during fatigue loading which promotes crack closure, while this effect is gradually weakened with the increase of ΔK. The deposited Al₂O₃ particles induce the disorderly arranged slip bands (SBs) ahead of the crack tip which deflects the crack path making it more tortuous in the Al₂O₃ dust.     

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Two types of Al₂O₃/Mo composites were fabricated by hot-pressing a mixture of - or -Al₂O₃ powder and a fine molybdenum powder. For Al₂O₃/5 vol% Mo composite using -Al₂O₃ as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al₂O₃ grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m^1/2). In the system using -Al₂O₃ as a starting powder, nanometre sized molybdenum particles were dispersed within the Al₂O₃ grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al₂O₃/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al₂O₃ grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m^1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.     

Evaluation of microstructure and properties of AZ31/Al2O3 composites prepared by solid-phase synthesis

Solid-phase synthesis was used to prepare AZ31/Al₂O₃ composites and the effect of Al₂O₃ particles on its microstructure and mechanical properties was investigated. The composites were characterised by optical microscopy, scanning electron microscopy, transmission electron microscopy, vickers hardness tester and electron universal strength tester. The results show that the grains of the composites are significantly refined and the mechanical properties increase obviously. While the content of Al₂O₃ particles is 2?wt-%, the hardness, ultimate tensile strength and elongation to failure reach the maximum of 85HV, 308?MPa, 6.83%, respectively. The addition of Al₂O₃ particles promotes the dynamic recrystallisation.     

Thermal stability and mechanical properties of yttria-doped tetragonal zirconia polycrystals with dispersed alumina and silicon carbide particles

Yttria-doped tetragonal zirconia polycrystals in which were dispersed various amounts of Al₂O₃ and SiC particles were sintered at 1500° C for 3 h, and the mechanical properties and the thermal stability of the sintered bodies were evaluated. Dispersion of Al₂O₃ caused no significant effect on sinterability, and increased the hardness and elasticity of the composites. Dispersion of SiC particles decreased the relative density and the grain size of composites. Elasticity and hardness increased by dispersing less than 10 vol% SiC, but decreased above 10 vol% SiC due to the decrease of relative density. Dispersion of both Al₂O₃ and SiC particles slightly increased the fracture toughness of ZrO₂-3 mol% Y₂O₃ ceramics but significantly decreased that of ZrO₂-2 mol% Y₂O₃ ceramics. The rate of the tetragonal-to-monoclinic phase transformation decreased by dispersing both Al₂O₃ and SiC particles. The transformation depth increased rapidly and then slowly with increasing the annealing time. The rate of increase in the transformation depth greatly decreased by dispersing Al₂O₃ particles.     

The shock-wave synthesis of ultradisperse chromium-doped alumina powder

An ultradisperse alumina (Al₂O₃) powder doped with Cr³⁺ ions was synthesized for the first time by shock-wave loading of a mixture of aluminum powder and ammonium chromate. The synthesized powder is composed primarily of the chromium-doped α-and δ-Al₂O₃ particles. The luminescence spectra of the α phase are identical to those of a macroscopic ruby crystal sample, which indicates that chromium atoms are incorporated into the alumina lattice. The Cr³⁺-doped δ-Al₂O₃ modification represents a new optical material.     

Strengthening of porous alumina bodies using carboxylate-alumoxane nanoparticles

Aqueous solutions of acetate-functionalized alumina nanoparticles (A-alumoxane), with an average particle size of 28 nm, have been used as alumina precursors for the infiltration of porous -alumina bodies in order to produce composite structures with homo-interfaces between substrate and infiltrate. Alternatively, if metal doped-methoxy(ethoxyethoxy)acetic acid-functionalized alumina nanoparticles (M-doped MEEA-alumoxane; M = Ca, Er, La, Ti, and Y), with an average particle size of 67 nm, are used in combination with A-alumoxane, a hetero-interface is formed between substrate and infiltrate. Samples were characterized by SEM, BJH, hardness and bend strength measurements. The bulk hardness of the -alumina substrates increases with sintering temperature, but this increase is significantly smaller than the effect of infiltration. The composite hardness generally increases with decreased average pore size although the exceptions to this trend suggest that the identity of the infiltrate is of equal or greater importance. Overall the hetero-interfaces show higher strength than the homo-interface; the latter showing only slightly better performance than high temperature sintering. For the samples fired at 1000°C, the MgAl₂O₄/Al₂O₃ and CaAl₁₂O₁₉/Al₂O₃ combinations appear to provide the greatest enhancement, with both the LaAl₁₁O₁₈/Al₂O₃ and Y₃Al₅O₁₂/Al₂O₃ hetero-interface samples show marked increase in hardness between 1000 and 1400°C. The elastic modulus and bend strength of the -alumina substrate increases significantly for the Er₆Al₁₀O₂₄/Al₂O₃ and LaAl₁₁O₁₈/Al₂O₃ infiltrates. The identity of the hetero-interface has a significant effect on the bulk properties of the composite.     

The SciCryo Project and Cryogenic Scintillation of Al2O3 for Dark Matter

We discuss cryogenic scintillation of Al₂O₃. Room-temperature measurements with α particles are first carried out to study effect of Ti concentration on response. Measurements under X-rays between room temperature and 10 K confirm a doubling of light output. The integration of a scintillation-phonon detector into an ionization-phonon dark matter search is underway, and the quenching factor for neutrons has been verified.      

Combustion synthesis of ferromagnetic Al2O3-based cermets in thermal explosion mode

The ferromagnetic Al₂O₃-based cermets with different ratios of Co and Co–50Ni alloys were successfully prepared by combustion synthesis in thermal explosion (TE) mode. The reaction process, microstructure, and magnetic property of cermets were investigated. The relative density of cermets can be over 95% via uniaxial loading at the time of ignition when the cermets are hot and ductile. In Al₂O₃–Co cermets, β-Co and α-Co co-exist at room temperature with average size of less than 10 μm and disperse homogeneously in the matrix, while in Al₂O₃–(Co–50Ni) cermets, the network-like Co–50Ni alloy can infiltrate into the boundary gaps of Al₂O₃ particles. The ferromagnetic Co and Co–50Ni alloys are responsible of the magnetic properties of Al₂O₃-based cermets. The saturation magnetization strongly depends on the magnetic characteristics and ratios of ferromagnetic phases. Al₂O₃–(Co–50Ni) cermets have soft magnetic properties with high magnetic susceptibility and low coercive force.     

High-pressure high-temperature transitions in nanocrystallineγ Al2O3,γ Fe2O3 and TiO2

We report first observation of new polymorphs of Al₂O₃ and Fe₂O₃ in specimens of xerogelγ Al₂O₃ andγ Fe₂O₃ quenched from high pressures and temperatures. At about 5 GPa and 1400°C, xerogel gamma alumina (XGA) transformed into a polymorphic mixture of phasesα Al₂O₃, B Al₂O₃ and C Al₂O₃, while XGA containing 1 wt% Cr₂O₃ transformed into a mixture of phasesαAl₂O₃, H Al₂O₃ and k′ Al₂O₃. The phases B Al₂O₃, C Al₂O₃ and H Al₂O₃ have the monoclinic-, cubic- and hexagonal-rare earth sequioxide (Ln₂O₃) type structure, respectively. At 5·2 GPa and 1450°C, XGA yielded a mixture ofα Al₂O₃ and hexagonalμ Al₂O₃. At STP, the phaseμ Al₂O₃ was found to transform to another hexagonal phaseλAl₂O₃ over a 10 week period. At 5·2 GPa and 900°C,γ Fe₂O₃ showed transition to a new phase H Fe₂O₃ which probably has an 8 layer close packed structure. In nanocrystalline TiO₂, only the anatase to rutile transition was found. The results are discussed using the free energy vs temperature diagram for xerogel and nanocrystalline materials.     

Preparation of mullite-based iron magnetic nanocomposite powders by reduction of solid solution

In this article, the preparation of mullite-based iron magnetic nanocomposite powders by hydrogen reduction of Fe-doped mullite solid solution with a nominal composition of Al_5.4Fe_0.6Si₂O₁₃ is reported. The formation process of Al_5.4Fe_0.6Si₂O₁₃ solid solution was analyzed using X-ray diffraction analysis (XRD), Fourier Transform Infrared Spectrum (FT-IR), thermogravimetric, and differential thermal analysis (TG-DTA). It is found that doping with Fe³⁺ cation affects the crystallization temperature of mullite. During the hydrogen reduction process, more than 89% Fe³⁺ cation in solid solution were transformed into α-Fe phase when reduction temperature reached 1200 °C. Microstructure characterization of nanocomposite powders reduced at 1300 °C reveals that there are two types of α-Fe particles in mullite matrix. Fe nanoparticles with a size of approximately 10 nm were precipitated within the mullite grains, while Fe particles larger than hundreds of nanometers were located at the surfaces of the mullite grains. The measurement of the magnetic properties of nanocomposite powders indicates that large particles and nanoparticles of α-iron have the ferromagnetic and superparamagnetic behavior at room temperature, respectively.     

Alumina powder assisted carbon nanotubes reinforced Mg matrix composites

Mg matrix composites reinforced by carbon nanotubes (CNTs)-Al₂O₃ mixture, which was synthesized by in situ growing CNTs over Al₂O₃ particles through chemical vapor deposition (CVD) using Ni catalyst, were fabricated by means of powder metallurgy process, followed by hot-extrusion. By controlling synthesis conditions, the as-grown CNTs over Al₂O₃ particles possessed high degree of graphitization, ideal morphology, higher purity and homogeneous dispersion. Due to the ‘vehicle’ carrying effect of micrometer-level A₂O₃, CNTs were easy to be homogeneously dispersed in Mg matrix under moderate ball milling. Meanwhile, Al₂O₃ particles as catalyst carriers, together with CNTs, play the roles of synergistic reinforcements in Mg matrix. Consequently, the Mg matrix composites reinforced by CNTs-Al₂O₃ mixture exhibited remarkable mechanical properties.     

The structure of Al2O3-Cr2O3 powders condensed from a plasma

The structure of Al₂O₃-Cr₂O₃ powders prepared by plasma oxidation of the mixed halides has been examined by X-ray diffraction, electron microscopy and electron spin resonance. The powders consisted predominantly of faceted spherical particles of a well crystallized solid solution of Cr₂O₃ in θ-Al₂O₃, with diameters of the order of 0.1 μm. Some larger particles of α-Cr₂O₃ were present in powders containing 17.8 and 24 wt% Cr₂O₃. The maximum solid solubility of Cr₂O₃ in θ-Al₂O₃ observed was 18 wt%. It is suggested that nucleation of crystallization of liquid Al₂O₃-Cr₂O₃ droplets occurs as a structure based on cubic close packing of oxygen ions and that the presence of chromium results in ordering to the θ-Al₂O₃ form rather than the δ-Al₂O₃ form usually observed in alumina powders prepared by plasma methods.     

Fluidization behavior of nano-particles by adding coarse particles

Fluidization behaviors of SiO₂ and TiO₂ nano-particles by adding coarse particles of FCC, ordinary Al₂O₃ and heat-resistant Al₂O₃, has been investigated experimentally. The effect of size and inventory of coarse components on the fluidization behavior of nano-particles has been studied. The results reveal that the fluidization quality of the mixture systems can be improved with increasing the amount of the coarse particles. For SiO₂ nano-particles, the optimum of FCC, ordinary Al₂O₃ and heat-resistant Al₂O₃ are 30%, 40% and 50%, respectively. Similarly, the optimal amount of FCC, ordinary Al₂O₃ and heat-resistant Al₂O₃ is about 30% for TiO₂ nano-particles. Fluidization quality is almost the best for all these three coarse additives of 65–80 μm. The analysis performed from the standpoint of the Richardson–Zaki (R–Z) equation reveals that the apparent terminal velocity derived through scaling experimental data to R–Z equation affects the expansion index of the mixture systems.