Alumina Compacts Research Articles

SiC-particulate aluminum composite foams produced from powder compacts: foaming and compression behavior

The foaming behavior of SiC-particulate (SiCp) aluminum composite powder compacts containing titanium hydride blowing agent was investigated by heating to 750°C in a pre-heated furnace. Aluminum powder compacts were also prepared and foamed using similar compaction and foaming parameters in order to determine the effect of SiCp-addition on the foaming and compression behavior. The SiCp-addition (10 wt%) was found to increase the linear expansion of the Al powder compacts presumably by increasing the surface as well as the bulk viscosities. The compression tests conducted on Al and 10 and 20% SiCp foams further showed a more brittle compression behavior of SiCp/Al foams as compared with Al foams. The collapse stresses of Al and 10% SiCp/Al foams were also predicted using the equations developed for the open and closed cell foams. Predictions have shown that Al foam samples behaved similar to open cell foams, while 10% SiCp/Al foam collapse stress values were found between those of open and closed cell foams, biasing towards those of the open cell foams.

Preparation and Properties of Porous α‐Al2O3 Membrane Supports

Strong and permeable macro‐porous α‐Al2O3 membrane supports are made by colloidal filtration of 20 vol% dispersions of α‐Al2O3 with an average particle size of 600 nm. Intact compacts with very good surface quality were obtained at an optimum pH of 9.5 and dosage of 0.2 wt% ammonium aurintricarboxylate (Aluminon), based on dry alumina. The colloidal stability of the aluminon‐stabilized slurries is confirmed by ξ potential measurements. Slight sintering of dense‐packed α‐Al2O3 compacts was found to result in >67% packing density and a bimodal pore‐size distribution as derived from shrinkage behavior and gas adsorption studies. Non‐stationary single gas permeation measurements showed improved gas permeability, compared with α‐Al2O3 compacts prepared using powder with a smaller particle size (300 nm). The strength of the disk‐shaped alumina compacts within the porosity range of 30%–20% increased from 100 to 300 MPa with a standard deviation of 20 and 50 MPa, respectively.

Transparent Polycrystalline Alumina Ceramic with Sub-Micrometre Microstructure by Means of Electrophoretic Deposition

The optical quality attainable in coarse-grained polycrystalline alumina is severely limited by grain-boundary scattering, which is inherent to non-cubic materials. The optical properties of sub-micrometre polycrystalline alumina are of growing interest triggered by the fact that a decrease in the grain sizes of the final sintered material yields an improvement in the optical quality while the scattering mechanism changes as the grain size becomes comparable with the wavelength of light. To achieve transparent alumina ceramics with a fine-grained microstructure, however, porosity and other defects must be avoided. This necessitates the optimization of processing and sintering procedures. Electrophoretic deposition (EPD) is a colloidal process in which ceramic bodies are directly shaped from a stable suspension by application of an electric field. Electrophoretic deposition enables the formation of homogeneous, uniform green microstructures with high density, which can be sintered to transparency. It is a simple and precise technique to synthesize not only monoliths, but also composites with complex geometries [1]. Alumina green bodies were deposited from stabilized aqueous suspensions with and without doping. Green alumina compacts were evaluated based on their pore size distribution and density. Densification behaviour was characterized by dilatometric studies conducted at constant heating rate. Samples were sintered at different temperatures with subsequent post-densification by hot isostatic pressing. Transparency was evaluated by means of spectroscopic measurements. The measured in-line transmission of the samples at 645 nm was more than 50 % and that is 58 % of the value of sapphire. The influence of dopings on transparency was investigated. The mechanical properties of the samples were tested.

2757 傾斜発泡によるアルミ粉末成形体の形状変化と気孔分布(S39-2 新機能多孔質材料の創製と評価(2),S39 新機能多孔質材料の創製と評価)

2757 傾斜発泡によるアルミ粉末成形体の形状変化と気孔分布(S39-2 新機能多孔質材料の創製と評価(2),S39 新機能多孔質材料の創製と評価)

温度制御半固形モールド粉体成形法によるセラミックス微小複雑構造体の作製

New powder compaction process using a Bingham semi-solid/fluid mold was developed to fabricate micro parts. In the present work, a powder material was filled as slurry in a solid wax mold, dried and compressed by conventional pressing methods, such as isostatic pressing or die compaction. It is important to use slurry for filling because dry powder is hard to fill in the micro cavity. It is also essential to control the process temperature to treat micro parts. The wax mold was heated during compaction and became semi-solid state, which could acts as a pressurized medium for isostatic compaction. Since the compacted micro parts were very fragile, the mold's temperature was controlled to higher than its melting point during unloading, to avoid breakage of the compacts. To demonstrate effectiveness of the present process, some micro compacts of alumina are shown as examples.

Influence of Particle Size Distributions with Various Geometrical Standard Deviations on Slip-Cast Forming and Sintering Behavior in Submicron Alumina Powder Compacts

The influence of particle size distributions of submicron high-purity α-alumina powders on forming and sintering behavior in an agglomerate-free condition were investigated. Nine kinds of alumina powders with the same median diameter (d PM50 = 0.53 μm) and different geometrical standard deviations ((7=1.2-2.0) in lognormal particle size distributions were prepared by blending spherical-like commercial α-alumina powders, and were also adjusted to near ideal distributions so that σ of a mass distribution was comparable with one of the number distribution. In order to prevent the agglomerate and contamination of the impurities, forming was performed by means of the colloid process, i.e., the slip casting method using the porous alumina molds. Relative densities of green compacts and bodies sintered at up to 1300'C increased with increasing a, but densities of bodies sintered at 1500°C or more increased with decreasing a. The width of the grain size distributions of those bodies increased with increasing a. The abnormal grains over 30/μm of the grain size generated at more than 1.7 of a for alumina bodies sintered at 1700°C and those grains contained many pores. The powder with a narrow distribution in size is better to fabricate a translucent material.

Anisotropic Sintering Shrinkage of Spherical Alumina Particle Compact Aligned in High Magnetic Field

Spherical alumina particle compacts were prepared by drying alumina slurry in high magnetic field (0-10 T) and cold isostatic pressing (CIP). Anisotropy of shrinkage during sintering was examined for the alumina compacts in detail. The spherical alumina particle compact prepared in 10 T showed sintering shrinkage anisotropy. The sintering shrinkage was larger in the direction parallel to magnetic field direction (i.e., the c-axis direction of alumina crystal) than that in its perpendicular direction. On the other hand, isotropic sintering shrinkage occurred in the compacts prepared in 0 T. The experimental results indicate that the sintering shrinkage of spherical alumina particle compact depends on alumina crystal axis direction. Origin of the sintering shrinkage anisotropy for the spherical alumina particle compacts can be attributed to the particle orientation caused by high magnetic field.

Ignition dynamics and activation energies of metallic thermites: From nano- to micron-scale particulate composites

Ignition behaviors associated with nano- and micron-scale particulate composite thermites were studied experimentally and modeled theoretically. The experimental analysis utilized a CO2 laser ignition apparatus to ignite the front surface of compacted nickel (Ni) and aluminum (Al) pellets at varying heating rates. Ignition delay time and ignition temperature as a function of both Ni and Al particle size were measured using high-speed imaging and microthermocouples. The apparent activation energy was determined from this data using a Kissinger isoconversion method. This study shows that the activation energy is significantly lower for nano- compared with micron-scale particulate media (i.e., as low as 17.4 compared with 162.5kJ∕mol, respectively). Two separate Arrhenius-type mathematical models were developed that describe ignition in the nano- and the micron-composite thermites. The micron-composite model is based on a heat balance while the nanocomposite model incorporates the energy of phase transformation in the alumina shell theorized to be an initiating step in the solid-solid diffusion reaction and uniquely appreciable in nanoparticle media. These models were found to describe the ignition of the Ni∕Al alloy for a wide range of heating rates.

Liquid Phase Sintering of Alumina, I. Microstructure Evolution and Densification

The microstructure evolution and densification of alumina containing 10 vol% calcium aluminosilicate glass and 0.5 wt% magnesium oxide sintered at 1600°C were quantified by measuring the evolution of pore‐size distribution, the redistribution of liquid phase, and the fraction of closed and open pores. The densification stopped at a limiting relative density during the final stage of sintering, and the small and large pores were filled simultaneously by glass during sintering. In addition, the results indicate that the pressure build‐up of the trapped gases in pores causes a significantly negative contribution to the driving force, and consequently the observed reduction in densification during the final stage of liquid phase sintering.

Influence of Nb 2O 5 additive on the densification and microstructural evolution of fine alumina powders

The effect of Nb 2O 5 additive on the densification and microstructural evolution of fine alumina powders has been investigated. The dopant concentration added ranged from 0.5 to 2.0 mol%. It was observed that the doping of Nb 2O 5 improved the densification of alumina. Besides, the sintering temperature could be lowered about 100–150 °C by doping a small amount of Nb 2O 5 for sintering at the same period. In addition, the Nb 2O 5 dopant promoted the grain growth of alumina and second-phase AlNbO 4 particles were generated in the doped samples.

Increasing Wet Green Strength of Alumina Body During Microfabrication by Colloidal Isopressing

Colloidal Isopressing involves formulating a slurry with a weakly attractive particle network that can be pre‐consolidated to a high relative density by pressure filtration and still retain fluid‐like characteristics. The pre‐consolidated slurry is injected into an elastomeric mold and isopressed. Isopressing rapidly converts the slurry into an elastic body that can be removed from the mold without shape distortion. Not only is this process rapid, but since the water saturated compact produced by this method does not shrink during drying, it can also be converted into a green body without a long drying period. It is demonstrated that micron‐size surface features, such as 5 μm wide channels with a depth/width ratio of 2, can be rapidly produced on the surface of alumina powder compacts. The fracturing of thin vertical portions of a micro‐patterned surface during pressure release and demolding has been an obstacle to obtaining micron‐sized features with high aspect ratios. A method is shown here that enables the fabrication of such features by strengthening the saturated isopressed body. It is shown that concentration controlled gelation of a poly(vinyl alcohol)–Tyzor® Triethanolamine Tritanate (TE) additive effectively increases the strength of the elastic, isopressed body, saturated with water, while maintaining the low viscosity of the pre‐consolidated slurry, which is required for transferring the pre‐consolidated slurry into a rubber mold prior to isopressing.

Examination of morphological changes of pore channel network during the intermediate stage of sintering of undoped, MgO-doped and ZrO2-doped alumina compacts by modified statistical theory of sintering

The modified statistical theory of sintering was used to evaluate the microstructural parameters, such as the average pore radius, grain size, and total length per unit volume of pore, of undoped and doped alumina compacts during the intermediate stage of sintering. The present results further justify the validity of the modified statistical theory of sintering in characterizing the morphological changes of pore channel network. The evaluated data of the evolution of these morphological parameters provide deep insight of the understanding of the mechanism of the effects of particle size distributions, MgO and ZrO2 on breakup of pore channel network. The possible roles of MgO and ZrO2 in the intermediate stage of sintering have been discussed.

High‐Temperature Young's Modulus of Alumina During Sintering

High‐temperature Young's modulus of a partially sintered alumina ceramic has been studied dynamically during the sintering process. Comparative, room‐temperature Young's modulus data were obtained for a suite of partially sintered alumina compacts with different porosities. The dynamic Young's modulus of a 1200°C partially sintered material was observed to decrease linearly with temperature, but then above 1200°C it increased sharply as sintering and densification of the alumina became dominant. The evolution of the Young's modulus due purely to sintering exhibited an exponential relationship with porosity in excellent agreement with room‐temperature measurements of equivalent porous alumina ceramics.

Preparation, microstructure and properties of Al–Al 4C 3 system produced by mechanical alloying

Dispersion strengthened aluminium compacts have been prepared by powder metallurgy. The base microstructure is aluminium matrix strengthened with dispersed ceramic particles. The strengthening is direct by dislocation movement retardation, and indirect by deformation induced microstructure modification in the next technological steps. The method of mechanical alloying process is described. Carbon transformation to carbide Al 4C 3 is characterised for different heat treatment schedules and nine commercial carbon powders tested. The micromechanism of carbon incorporation into the metallic powder and the compacting of it are described. The influence of dispersed carbides on mechanical properties is evaluated together with the influence of deformation on microstructure and properties. Ductility anomalies up to a type of superplasticity were observed at certain tensile testing strain rates. High temperature tensile test, resistance to creep are available.

多孔質アルミニウム焼結体の特性に及ぼす原料粒径の影響

In the powder metallurgical process, porous aluminium compacts were prepared by using pure aluminium powders and resin particles as the raw materials. The resin powders classified at more than 250 μm, 110∼250 μm, and less than 110 μm mixed with pure-aluminium powder having average particle size of 20 μm, 10 μm, and 3 μm at the content of 56 vol.%. This mixtures were press-molded into column shape (d=11 mm, L=16∼17 mm). To form pores, resin particles in the green compacts were removed by heat-treatment at 380°C in Ar-gas flow, and then the compacts were sintered at 655°C for 2 hours in vacuum. Effects of particle size of raw materials on the densities and static compressive properties at room temperature for sintered compacts were mainly investigated. The relative densities of sintered compacts were within the range of 39%∼47% on the above-mentioned process conditions. Reduction in particle size of either aluminiun powder or resin particle resulted in higher density. The compressive strength increased with dcreasing the size of aluminium powder. When aluminium powder was 10 μm and 3 μm, the compressive strength increased with decreasing size of resin particle. Especially, the sintered compacts originating in resin particle less than 110 μm had much higher strength than the others regardless of size for aluminium powder.

A Novel Processing Route to Control Grain Growth in Submicrometer Alumina Compacts

A novel processing procedure for significantly suppressing grain growth in submicrometer alumina compacts has been developed and implemented with the intent of ultimately using the same processing route to control grain size in nanophase alumina compacts. In this study, partially sintered alumina pellets made from 0.5 µm starting powders are altered by the chemical infiltration of Si3N4. The control and infiltrated pellets are then heated to 1650°C for 4 h. The fully sintered pellets are approximately 97% dense. Suppressed grain growth is observed in the infiltrated pellets. The average grain size in the control pellets after densification is 4.2 and 1.2 µm in the infiltrated pellets. Depth of infiltration is measured using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Depending on the specific infiltration conditions used, the outer 15-50% of the infiltrated pellets exhibit a graded microstructure consisting of a region of abnormal grain growth and a region of suppressed grain growth. Abnormal grain growth is visible on the outer surfaces of the infiltrated pellets where a relatively high ratio of Si to N is present. Further into the pellet, after some depletion of the Si source gas has occurred, regions of suppressed grain growth are apparent. Based on these results, an infiltration profile is determined. A mathematical model is developed to describe the infiltration process and to determine optimal infiltration conditions. High-resolution electron microscopy (HREM), energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS) are used to study the infiltrated samples.

Effect of Surface Impurities on the Microstructure Development during Sintering of Alumina

Microstructural evolution during sintering of alumina powder compacts prepared by cold isostatic pressing (CIP) was monitored. For CIP, rubber molds lubricated with silicone oil were used so that a very small amount of impurity was introduced to the surface of the powder compacts. During sintering at 1600°C, grain growth in the surface region was inhibited up to sintering for 1 h, but subsequently abnormal grain growth occurred. In the inner region, however, the grains grew uniformly without abnormal grain growth. Impurities that initially drag the boundary migration but form liquid at the end are suggested to cause abnormal grain growth.

Effects of monomer composition on the mechanical and machinability properties of gel-cast alumina green compacts

In the present work, machinable green gel-cast alumina compacts were prepared by using polyethyleneglycol (400) diacrylate (P4) and polyethyleneglycol (600) dimethacrylate (P6) together with the acrylamide (AAm) comonomer. The glass transition temperatures of copolymers decreased with the increasing of P4 or P6 amount in total copolymer. The green samples obtained in an aqueous system were mechanically analyzed by means of three-point bending. Flexural strength values increased, from 2 MPa to 25 MPa, as the weight ratio of P4 or P6 in total copolymer (AAm-P4 or AAm-P6) decreased from 90% to 3.5%, respectively. The green gel-cast samples prepared by using P4 or P6 were machined easily by using a lathe, drill and milling machine without damaging the samples, which have good surface finish. The binder removal was achieved at lower temperatures than those samples prepared by using only AAm.

Microstructural evolution based on the pressure-assisted master sintering surface

Abstract Master sintering curve (MSC), in which the sintered density is a unique function of the integral of a temperature function over time, is insensitive to the heating path. The present research was undertaken to explore microstructural evolution with densification of alumina during hot pressing, and to develop the pressure-assisted master sintering surface for alumina. Densification of alumina was continuously recorded during heating at 10 °C/min at fixed pressures from 6.9 to 34.5 MPa. Significant grain growth occurred with increasing pressure. The pressure-assisted master sintering surface was successfully constructed. Using this surface, the final density can be predicted to about 1% accuracy for a fixed pressure and an arbitrary temperature–time path.

Facet-Defacet Transition of Grain Boundaries in Alumina

Grain boundaries in pure alumina powder compacts sintered at 1400°C are smoothly curved, indicating that they have atomically rough structures. When these specimens are heat-treated at temperatures between 900° and 1100°C, a small fraction of the grain boundaries develop either hill-and-valley or kinked shapes with flat segments. Some of these flat boundary segments lie on the {011[Twomacr]} plane of one of the grain pairs. These grain boundaries thus appear to become singular at these temperatures. When a corundum crystal with a basal surface is sintered in alumina powder at 1400°C, all grain boundaries formed between the corundum basal surface and small grains, as well as those between the small grains, are smoothly curved, indicating their rough structure. When heat-treated at 900°C for 3 days, about 30% of the grain boundaries between the corundum basal surface and the small grains develop kinks with flat boundary segments, and some of these flat segments lie on the basal plane of the corundum. When heat-treated again at 1400°C, all grain boundaries are curved, indicating that they become reversibly rough. These observations show that at least some of the grain boundaries in alumina undergo roughening-singular transitions at temperatures between 900° and 1100°C.