Dielectric properties of Lanxide™ Al2O3/Al composites

The dielectric properties of unreinforced Lanxide Al₂O₃/Al composites have been investigated over a wide range of temperatures and frequencies. These composites were formed by the directed oxidation of suitably doped aluminium-based alloy melts, with no filler or reinforcing material in the reaction path. As-grown composite materials were good electrical conductors in all directions owing to the presence of an interconnected metallic constituent. As the metallic phases were partially removed (in favour of porosity) by continuing the oxidation reaction to completion, the composites remained electrically conducting parallel to, and became insulating transverse to, the original growth direction of the composite. This anisotropy apparently was caused by different connectivity of the metal phase between the two directions. Thermal treatments at 1600°C in argon resulted in volatilization of the residual metal in the composite, thus further increasing the porosity. As the metal content was decreased, the composites changed from conducting to insulating along the growth direction. When the metallic phase was removed completely, the porous alumina ceramic maintained anisotropic dielectric properties, due to c-axis alignment of the alumina (corundum) phase along the growth direction. The dielectric constants were 8.0 and 6.4, respectively, parallel and perpendicular to the c-axis aligned directions of the porous alumina ceramic. A dielectric relaxation phenomenon was observed in some samples of both as-grown and thermally treated material, and was attributed to an unidentified impurity effect.     

Study on the effects of Cr2O3 on the reduction behavior of γ-Fe2O3

XRD and TG reduction analysis show that -Fe₂O₃ Fe-Cr catalysts, which contains 0.0 to 14.0 wt %. Cr₂O₃ and prepared by coprecipitating method, consist of crystalline -Fe₂O₃ and non-crystalline Fe₂O₃. Between 150–450 °C, three reduction stages are observed in the catalyst. The first stage is non-crystalline Fe₂O₃ reduced to non-crystalline Fe₃O₄, the second is crystalline -Fe₂O₃ to crystalline Fe₃O₄ and the third is non-crystalline Fe₂O₃ reduced to non-crystalline FeO. About 5 wt %. Cr₂O₃ can enter the lattices of -Fe₂O₃ to form solid solution. With the increasing of Cr₂O₃ content, the relative abundance of non-crystalline Fe₂O₃ and the amount of soluble Cr₂O₃ in non-crystalline increases, while the crystalline size of -Fe₂O₃ decreases.     

Influence of TiC particle size on the load-independent hardness of Al2O3–TiC composites

Seven samples of Al₂O₃–30 wt.% TiC composites were prepared by hot-pressing the Al₂O₃ powder mixed with TiC particles of different particle sizes. Knoop and Vickers hardness measurements were conducted on these samples, respectively, in the indentation load range from 1.47 to 35.77 N. The load-independent hardness numbers were then determined by analyzing the relationship between the measured indentation size and the applied indentation load. It was found that the load-independent hardness number increases with the increasing TiC particle size, and this experimental phenomenon may be attributed to the effect of the residual internal stress resulting from the mismatch between the thermal expansion of Al₂O₃ matrix and that of the TiC particles.     

Effect of the starting Al2O3 and of the method of preparation on the characteristics of Li-stabilized β″-Al2O3 ceramics

The influence of the morphology and the size of the particles of various types of starting Al₂ O₃ material on the synthesis and characteristics of Li-stabilized'-Al₂O₃ ceramics have been investigated. The use of highly dispersed oxides makes it possible to attain higher densities in the fired ceramic bodies due to their higher reactivity. In the case of oxides obtained from ammonium alum, the degree of dispersion and the reactivity may be increased by raising the amount of-Al₂O₃ up to a certain limit. Alumina prepared from Al₂ (OH)₅ NO₃ by slurry solution spray-drying also gives satisfactory results despite its lower degree of dispersion. This is connected with the morphology of the particles. In the case of synthesized materials containing an insufficient amount ofAl₂O₃-NaAlO₂ eutectic, high densities may also be achieved by applying a two-step firing schedule at temperatures above the melting point of the eutectic.     

Effect of Al2O3 and Fe3Al on the phase formation and mechanical properties of β-sialon

The paper evaluated the mechanical properties of β-sialon composites prepared by hot-pressing sintering at 1600 °C in N₂ atmosphere using α-Si₃N₄, Al₂O₃, Y₂O₃ and Fe₃Al as raw materials. The influence of Al₂O₃ and Fe₃Al content on flexure strength, fracture toughness, hardness, and relative density was investigated. And phase formation and morphology of the composites were characterized by X-ray diffraction and electron microscopy. The experimental results indicate that the raw material Fe₃Al reacts with α-Si₃N₄ to form silicides at elevated temperature, and supplies more liquid phase to assist densification. Besides, the variation of flexure strength, fracture toughness and hardness is mainly consistent, and also in good agreement with the relative density measurements. The values all increase firstly, and then decrease when the Al₂O₃ content increases. Scanning electron microscopy illustrates that the metal particles act to inhibit the crack propagation.     

Fabrication and mechanical properties of woven Al2O3 fibre – ZrO2 matrix minicomposite reinforced Al2O3 matrix composites by slurry infiltration – sintering process

Abstract

A plain woven Al₂O₃ fibre - ZrO₂ minicomposite reinforced Al₂O₃ matrix composite ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃) has been fabricated by a simple, pressureless, multiple slurry infiltration - sintering process. The fabrication process consisted of two major steps: fabrication of the woven Al₂O₃ fibre - ZrO₂ minicomposite ((Al₂O₃)_f/ZrO₂)_mc) and fabrication of the composite ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃. The woven fibre form of ((Al₂O₃)_f/ZrO₂) _mc was made by dipping a plain woven Al₂O₃ fabric into colloidal ZrO₂ solution. Then Al₂O₃ powder dispersed slurry was infiltrated into the open spaces of the woven fabric ((Al₂O₃) /ZrO₂)_mc. The infiltrated minicomposites were stacked, pressed, and sintered in ambient air to form a plate of ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃. The optimum sintering temperature of ((Al₂O₃)_f/ZrO₂)_mc/Al₂O₃ was investigated via tensile strength and matrix density. Tensile behaviour and fracture resistance of the composite fabricated under the optimum processing condition were evaluated. The tensile stress - strain behaviour of the optimised composite showed non-catastrophic fracture behaviour with bundle bridging and extensive fibre bundle pullouts. The maximum tensile stress revealed that almost full strength expected from the bare bundle strength was exploited by the proposed process.     

Fabrication and mechanical properties of α-Al2O3/β-Al2O3/Al/Si composites by liquid displacement reaction

Al₂O₃/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties were investigated. Aluminum reacted with the mullite and SiO₂-glass constituting the mullite refractories and changed them into -Al₂O₃ and Si. Simultaneously, -Al₂O₃ was formed by the reactions among -Al₂O₃ and sodium and potassium oxides in the glassy phase. Also, Al penetrated into the -Al₂O₃/-Al₂O₃/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into -Al₂O₃/-Al₂O₃/Al/Si composites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The content of -Al₂O₃ in the composite was lowest at the lowest Na₂O and K₂O contents in the refractories. Silicon in the composite had its highest content at the highest SiO₂ content. The composite fabricated from SiO₂/Al₂O₃ (in mol) (SAR)=1.85 consisted of 2–5 m Al₂O₃ grains embedded in metal, but that from SAR=1.05 showed a complicated microstructure with small and large grains. The bending strength of the composites fabricated from the refractories of SAR=1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respectively. Also, the corresponding fracture toughness values were 5.2, 6.1, and 5.5 MPam , respectively.     

Synthesis of H2Ti2O3·H2O nanotubes and their effects on the flame retardancy of bamboo fiber/high-density polyethylene composites

H₂Ti₂O₃·H₂O nanotubes (TNTs) were prepared through hydrothermal synthesis and dispersed in bamboo fiber/HDPE (BH) composites to improve the flame retardancy of the composites. TEM observation showed that TiO₂ particles were transformed into TNTs through hydrothermal treatment at 120 °C for 12 h in 8 M NaOH solution. Then, a cone calorimeter and a limiting oxygen index chamber were used to evaluate the effects of the TNTs on the flame retardancy of the BH composites. Results demonstrated that TNTs definitely improved the flame retardancy of BH composites by absorbing decomposition products from combustion due to its large specific area and tubular structure. Additionally, the TNTs reduced the free volume in the microzone, strengthened the molecular chain rigidity, and then contributed to the thermostability and flame retardancy of the BH composites.     

Effect of the addition of t-ZrO2 on the material properties of β-TCP/PCL composites

Using poly-methyl methacrylate as a pore-forming agent, porous β-tricalcium phosphate (β-TCP)/t-ZrO₂ composites were fabricated depending on the volume percentages (vol.%) of t-ZrO₂ powder. In the porous sintered bodies, hybrid pores, about 20 and 200 μm in diameter, were homogenously dispersed in the β-TCP/t-ZrO₂ matrix and showed good interconnection. On the other hand, β-TCP-(t-ZrO₂)/polycaprolactone (PCL) composites were fabricated by the melt infiltration process using porous β-TCP/t-ZrO₂ bodies. The relative density of the β-TCP-(t-ZrO₂)/PCL composites increased as the vol.% of t-ZrO₂ increased and its maximum value was about 98.6%. However, the hardness, bending strength and elastic modulus of β-TCP-(t-ZrO₂)/PCL composites decreased due to the low densification of porous β-TCP/t-ZrO₂ bodies as the volume percentages of t-ZrO₂ content increased. The values (using 20 vol.% of t-ZrO₂) were 11.4 Hv, 25.5 MPa and 17.2 GPa, respectively.     

Fabrication of Al2O3 continuous fibre reinforced Al–Cu alloys by axial infiltration process

Abstract

This paper describes the fabrication of Al₂O₃ continuous fibre reinforced Al-Cu alloys by an axial infiltration process which is expected to be used in the production of stick, bar, or platelike composites. A discussion on the infiltrating process gave equations for the critical infiltration pressure and the size of composite defects. Microscopic observations and microprobe analyses on Al-4.43Cu, Al-6.48Cu, Al-10.11Cu, and Al-4.45Cu-1.54Mg (wt-%) matrix composites identified the solidification process of matrix alloys in the presence of Al₂O₃ fibres. The approximate relationships between microstructure, interspace size, and the matrix composition are described schematically. Microsegregation of Cu and Mg in the composites are also analysed quantitatively.     

Sintering behaviour of slip-cast Al2O3 – Y-TZP composites

The sintering behaviour of alumina–Y-TZP composites prepared by slip-casting technique were studied. Slip-cast samples containing varying amounts of Y-TZP ranging up to 90 vol% were prepared and evaluated. Sintering studies were carried out at 1450°C to 1600°C. Sintered samples were characterised where appropriate to determine phases present, grain sizes, bulk density and mechanical properties. Good correlation was obtained between the calculated prepared powder density and experimental results. The sintered bulk density of the composites was observed to increase with increasing Y-TZP content and sintering temperature up to 1550°C. Maximum hardness values (>14 GPa) were obtained for all samples containing <60 vol% Y-TZP and when sintered at 1550°C. It has been found that the additions of up to 50 vol% Y-TZP was effective in suppressing Al₂O₃ grain growth.     

Stress development and relaxation in Al2O3 during early stage oxidation of β-NiAl

Abstract

Using a glancing synchrotron X-ray beam (Advanced Photon Source, Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from thermally grown oxides on NiAl samples were recorded during oxidation at 1000 or 1100°C in air. The diffraction patterns were analyzed to determine strain and phase changes in the oxide scale as it developed and evolved. Strain was obtained from measurements of the elliptical distortion of the Debye-Scherrer rings, where data from several rings of a single phase were used. Results were obtained from α-Al₂O₃ as well as from the transition alumina, in this case θ-Al₂O₃, which formed during the early stage. Compressive stress was found in the first-formed transition alumina, but the initial stress in α-Al₂O₃ was tensile, with a magnitude high enough to cause Al₂O₃ fracture. New α-Al₂O₃ patches nucleated at the scale/alloy interface and spread laterally and upward. This transformation not only puts the alpha alumina in tension, but can also cause the transition alumina to be in tension. After a complete α-Al₂O₃ layer formed at the interface, the strain level in α-Al₂O₃ became compressive, reaching a steady state level around –75 MPa at 1100°C. To study a specimen’s response to stress perturbation, samples with different thickness, after several hours of oxidation at 1100°C, were quickly cooled to 950°C to impose a compressive thermal stress in the scale. The rate of stress relaxation was the same for 1 and 3.5 mm thick samples, having a strain rate of ～1×10^?8/s. This behavior indicates that oxide creep is the major stress relaxation mechanism.     

Influence of processing defects on the measured properties of Cu–Al2O3 composites: A forensic investigation

The measured mechanical and physical properties of copper–alumina (Cu–Al₂O₃) composites are influenced by the defects induced during the manufacturing of the test samples. In this paper we shall demonstrate how the fundamental knowledge of the influence of the amount and type of micro-cracks on the mechanical and thermal properties of copper–alumina composites can be used as a forensic tool in identifying the defects induced during the manufacturing process of the test samples. This process involves several steps; the preparation of ingots from commercial powders by hot iso-static pressing (HIP), followed by their machining, hot extrusion, heat treatment, and if necessary further machining to make specimens of desired shape. Each step in this manufacturing process introduces some defects, so that the mechanical and physical properties measured using these samples, such as the Young modulus, the thermal conductivity and the coefficient of thermal expansion can be very different from the expected values, despite the fact that the density of the composite measured on the same samples is very close to its theoretical value. As a result open porosity is not the likely cause and it is reasonable to suspect that closed pores (e.g. micro-cracks) in the copper matrix and at the interfaces of the alumina inclusions formed during the multi-stage manufacture of test samples are responsible for the differences in the measured and expected mechanical and thermal properties.     

Modeling of thermally induced damage in the processing of Cr–Al2O3 composites

Thermal stresses induced during the cooling of Cr–Al₂O₃ (MMC) processed by sintering are modeled numerically using the FEA. The composite microstructure is modeled as (i) random distribution of ceramic particles (voxels) in the metal matrix, and (ii) using micro-CT scans of the real microstructure transformed into a FE mesh. Numerical simulations of the thermal residual stresses are compared with the test data measured by X-ray diffraction. A simple numerical model is then proposed to predict the overall elastic properties of the composite with account of the porosity and damage induced by the thermal stresses. Comparison of the model predictions with the measured data for Young’s modulus is presented.     

The effect of Mg add on morphology and mechanical properties of Al–xMg/10Al2O3 nanocomposite produced by mechanical alloying

Effect of Mg content on microstructure and mechanical properties of Al–xMg/10 wt.%Al₂O₃ (x = 0, 5, 10 and 15 wt.%) powder mixtures during milling was investigated. The results show that for the binary Al–Mg matrix, the predominant phase was an Al–Mg solid solution. With the increment of Mg to 15 wt.% the crystallite sizes of 20 h milled powders diminish from 44 to 26 nm and lattice strains increased from 0.22% to 0.32% caused by Mg atomic penetration into the substitution sites of the Al lattice. With up to 15 wt.% Mg (for 20 h milled composites) microhardness increases from 120 to 230 HV caused by the increment of the Mg concentration and dislocation density as well as the decrease of the crystallite size.