High Purity Alumina Powder Research Articles

Effects of mixing sequence on alumina prepared by injection molding

Two mixing sequences of binder system were used to prepare the feedstocks containing high purity alumina (A-16SG) powder. The properties of the feedstocks were analyzed by various techniques, including FT/IR, capillary rheometry, and TGA. Besides, the green density and pore size distribution of injection molded Al 2O 3 parts, and the sintered densities, the fracture strength and Weibull distribution of the densified test bars were measured. The results indicated that the sequence of binder addition is better adding polypropylene (PP) into preheated alumina powder, then stearic acid (SA), followed by paraffin wax (PW). This mixing step was found effective for the preparation of a good mixture of plastics/powder system to produce feed-stocks with a lower viscosity and better homogeneity for injection molding. The statistics of fracture strength and the results of fractography reveal that the sintered test bars made by the better feedstocks have a uniform microstructure which is free from large cracks and pore clusters. They result in a higher fracture strength (375 MPa) and Weibull modulus (up to 20.9) than those produced by the other process.

Enhanced sinterability of alumina particles by pretreating in liquid ammonia

Abstract The sinterability of high purity alumina powders was enhanced by ultrasonication in liquid ammonia. The surfaces of the resulting powders contained agglomerated regions of submicron-sized particles that were metallic in appearance. When heated to 1400 °C, these particles densified into a solid with observable grain bonding and necking.

Mechanical properties and microstructure of centrifugally compacted alumina and hot-isostatically-pressed alumina

High purity alumina powder was compacted under a high centrifugal force. Mechanical properties of the sintered body were studied by the three-point bending test at room temperature and by the compressive test at elevated temperatures. Comparison was made with hot-isostatically-pressed (HIP) alumina. The room-temperature flexural strength of the centrifugally compacted (CC) alumina was found to be 1330 MPa compared with 585 MPa of the HIP alumina. The difference in the room-temperature strength was attributed to the presence of the amorphous phase along the grain boundaries of the HIP alumina caused by the segregation of carbon and sulfur during HIP. A large ductility was observed above 1473 K in the CC alumina and above 1573 K in the HIP alumina. High-temperature ductility was lost in the HIP alumina at 1773 K where the amorphous grain-boundary phase was considered to be melted.

Formation and Control of Agglomerates in Alumina Powder

High‐purity alumina powder with submicrometer particle size was colloidally dispersed and classified in either water or ethanol to remove agglomerates from as‐received powder. After the slurries were dried, fine alumina particles cohered to form agglomerates, which were “hard” when formed in a water slurry, and “soft” in an ethanol slurry. Firing of the powder compacts with “hard” agglomerates resulted in inhomogeneous microstructures, and homogeneous microstructures were formed with “soft” agglomerates. The reasons for their formation are discussed and experimentally confirmed. In the case of water slurry, a small amount of transition alumina reacted with water at low pH. After drying, alumina gels were formed and acted as a strong binder between α‐alumina particles to form “hard” agglomerates.

Synthesis of functionally gradient aluminium titanate/alumina composites

By virtue of its very low thermal expansion coefficient and low thermal conductivity, aluminium titanate (A12TiOs) is highly suitable for applications where thermal insulation and thermal shock resistance are required. However, AlzTiO5 does present two important problems, which have restricted its wider industrial use. The first relates to the extensive microcracking, which results in low thermal expansion but also in low strength. In view of this, composite materials like AlzTiOs-mullite [1], AI2TiOs-ZrO 2 [2], AlzTiO5-TiZrO4-ZrO 2 [3] and A12TiOs-mullite-ZrO2 [4] have been developed. Thermal decomposition of the material at temperatures below 1280 °C is the second problem [5]. On the other hand, alumina is a very versatile engineering material with relatively high strength, high hardness, and excellent oxidation and chemical resistance. However, like most ceramics, it has an intrinsically poor thermal shock resistance and low fracture toughness, which restricts its use in structural applications. It follows that the marriage of A12TiO5 and alumina should lead to a layered composite that will display all of the advantages of both materials. This concept has been successfully exploited by Marple and Green [6-8] and Low and colleagues [9-11] in synthesizing, respectively, layered mullite/alumina and mullite/ZTA with improved mechanical properties such as strength and fracture toughness. The method involved encasing an alumina or ZTA host body with a mullite (3A1203"2SiO2) protective layer [6, 9]. This was done by infiltrating the host body with a silica-rich solution (e.g. ethyl-silicate), followed by firing the composite at an appropriate temperature. Due to thermal expansion mismatch between mullite and the host body, desirable macroscopic compressive stresses can be induced in the casing. The protective layer inhibits crack growth, thereby improving the effective fracture toughness and strength. In this letter, we report the preliminary results of functionally-gradient A12TiOs-alumina composites by encasing an alumina host with an A12TiO5 layer using an infiltration technique. The microstructureproperties relationship is discussed. The procedure for sample preparation was based on an infiltration method adopted by Marple and Green [6] and Low et al. [9] in their processing of layered mullite-ZTA composites. High purity alumina (A1000G, Alcoa, USA) powder was used to make the ceramic preform for infiltration. The powder was

Strain-rate effects in high-purity alumina

Ultrahigh-strength alumina specimens made from disks produced by vacuum hot pressing high-purity alumina powders were subjected to uniaxial compressive loads at a range of strain rates. It was observed that the material exhibited a failure strength far superior to commercially available alumina. The failure strength was strongly strain-rate dependent and varied from 5.5 GPa at 10-4 s-1 to 8.3 GPa at 103 s-1. Microscopic studies on the fragments of the specimens deformed under uniaxial strain revealed extensive twinning and dislocation activity. Based on the experimental results and microscopic observations, the factors and mechanism responsible for the observed high compressive strength are discussed.

Empirical Evaluation of Process Operations Affecting Sintered Density

The effect of fabrication process operations on the sintered density was empirically discussed in order to optimize operation conditions. High purity alumina powders were used as raw powders: One η/θ-phase and three a-phase alumina powders. Their crystal phase, specific surface area, particle size, degree of aggregation and shape of primary particle are characterized. Alumina with the η/θ-phase had a large specific surface area. Three a-phase alumina powders were different each other in the primary particle shape or the degree of aggregation. The alumina powders were formed by uni-axial pressing and sintered. The following process operations were changed: milling, additive, binder, spray drying, CIPing. Each green compact was sintered under the same condition. An empirical equation is established for correlating the sintered density with the compact green density under any process conditions. A diagram is proposed from the empirical equation, and the effect of process operations on the green and sintered densities was discussed on the diagram. The effects of the process operations on the green and sintered bodies depended on the primary particle properties, especially particle shape. The CIPing and addition of additives always increased the green and/or sintered densities. However, spray drying and milling were efficient in powders consisting of dendritic primary particles, but no or negative effect of spray drying and milling on the green and sintered densities was observed in powders consisting of round primary particles and weak aggregates.

Influence of Powder Characteristics on Sinterability and Wear Resistance of Alumina Ceramics

In line with ACC's stride towards new materials, the Research & Consultancy Directorate of the company has successfully developed and scaled up the process for manufacture of high and ultra high purity alumina powders at semi-commercial levels for high performance, structural and electronic applications.Among the various applications of high purity alumina powders, wear resistant parts and thermal barrier coatings are of prime importance for the domestic market. In order to optimise the powder characteristics for wear resistant applications, a study was undertaken to evaluate the influence of various parameters such as chemical purity, phase composition, particle size distribution, specific surface area (BET) of starting powders and their effect on sintering and wear properties. The paper highlights the influence of powder characteristics on sintering and wear resistance properties of the fabricated grinding media during milling.

Centrifugal compaction of submicron high purity alumina powder: S. Tashima, H. Kuroki (University of Hiroshima, Hiroshima, Japan)

Centrifugal compaction of submicron high purity alumina powder: S. Tashima, H. Kuroki (University of Hiroshima, Hiroshima, Japan)

Compaction and Sintering Behavior of a High Purity Alumina Powder Treated with Coupling Agents

Compaction and sintering behavior of a high purity alumina powder (>99.99%) treated with aluminate (A), silane (S) and titanate (T) coupling agents were investigated. The surface treatment increased the green density of pressed-bodies, the effectiveness of the coupling agents on the density increase being S (57.2%)>T(56.4%)>A(53.9%)>non-treated (51.4%) at 2wt% addition. Microstructure development after firing for 2h at 1600°C was considerably dependent on the types of metals contained in coupling agents. Alumina treated with 2wt% of aluminate (Al) exhibited uniform and fine grains similar to the non-treated system. Grain growth and densification were enhanced in the alumina treated with 2wt% of titanate (Ti) and columnar grains were formed in the alumina treated with 2wt% of silane (Si). Both bending strength and fracture toughness decreased by the addition of coupling agents. Intragranular fracture was observed in the alumina treated with 2wt% of silane although intergranular fracture was prominent in other systems.

高純度アルミナ超微粉の遠心成形

A centrifugal compaction process applying a 10-20kG centrifugal force to a slip of ceramic powder in a die has been developed. The slip was prepared by dispersing a submicron(0.22μm) high purity alumina powder in 22-28mass % of ion-exchanged water with a ball mill. The coefficient of viscosity of the slip is 80-2000mP⋅sec.The compaction process is as follows: 1) when the centrifugal force acts, the alumina particles are sedimented in the bottom part of the die cavity, leaving upper clear water in the top part of the cavity; 2) the centrifugal machine is stopped, and the upper clear water is removed; 3) the bottom of the die is replaced by a porous bottom so as to reduce the moisture in the sediment by about 2 mass % with a 10-20kG centrifugal force applied again to the sediment; 4) after dehydration, the bottom is removed and the compact is stripped from the die.This process has the following merits;1) preparing and regulating the slip is easy, because the influence of the slip's concentration and viscosity is small.2) neither a vacuum unaerating process nor unaerating agents are necessary.3) compacts with a relative green density level above 60% can be obtained in a short time.4) there is no contamination from the plaster mold.5) the time for drying and sintering compacts can be shortened.

Empirical Density Estimation of Green Body Formed by Uniaxial Press

In order to estimate green density from forming condition and powder properties, uniaxial pressing was studied. Many kinds of high purity alumina powders were used as sample. They have different crystal phases and were made by many production process. As-received powders were pressed after added with a certain amount of water, and powders which were milled, spray dried were also pressed. Sample powders were evaluated by specific surface area, particle size, bulk density, tap density, XRD and TEM observation of the primary particles. The green density was analyzed by the powder properties and the following results were obtained. (1) The experimental equation connecting the green density and the pressing pressure was reconfirmed. Two constants in the experimental equation can be correlated to the tap density, and the estimated green density by the empirical equation agreed well with the experimental one. (2) Powders having round or bulky primary particles are compacted well without powder preparation, but the densification can be hardly improved by the powder preparation, such as milling and spray drying, on the contrary, powders having dendritic primary particles are not densified, but the powder preparation is effective to improvement of densification. (3) The relationship between the green density and the pressing pressure become straight lines which pass through one point on semilogarithmic graph, if the crystal phase of powder is the same.

Surface Chemistry of Various Poly(vinyl butyral) Polymers Adsorbed onto Alumina

Organic polymeric materials, often used as ceramic processing aids to assist in the fabrication of green bodies, are thought to be of a temporary nature, nature, being easily removed by thermolysis at temperatures between 250° and 600°C. The interaction of various poly(vinyl butyral) binders with fine, high‐purity alumina powder was investigated using variable‐temperature Fourier‐transform infrared spectroscopy and thermal gravimetric analysis. It was found that there was interaction between the functionality of the polymers and the surface of the alumina powders. If these interactions are strong enough to persist to initial sintering temperatures, the polymers would not then be merely innocuous temporary processing aids, but could have an effect on the development of ceramic microstructures.

The phase shift and multiple scattering in small angle neutron scattering: Application to beam broadening from ceramics

The single-particle cross-section for small angle neutron scattering from a uniform sphere of arbitrary size is incorporated into a general theory of small angle incoherent multiple scattering from a monodisperse population of particles. The synthesis of these formalisms provides a theory of multiple scattering that is applicable over a wide range of values of the single-particle phase shift, v — encompassing the transition from diffraction ( v → 0) to refraction ( v → ∞) — and a useful tool for extracting particle size and volume fraction from SANS data dominated by multiple scattering. Single particle and multiple scattering aspects of the theory can be discussed from common elements of the formalism. The single-particle diffraction limit (Born approximation) remains applicable up to v ≈ 2, corresponding, typically, to particle radius ≈ 2 µ m, but breaks down soon thereafter, while the Porod law ( Q → ∞) is stable against both phase shift and multiple scattering. Wavelength dependence of scattering data signals multiple scattering, but not necessarily refraction. These methods have been used to characterize high purity alumina powder, and have obtained good quantitative agreement with results from laser light scattering and X-ray sedigraph microscopy.

Characterization of alumina powder using multiple small-angle neutron scattering. I. Theory

Microstructural parameters of high-purity alumina powder are determined quantitatively throughout the bulk of the material using small-angle neutron scattering techniques. A unified theoretical and experimental approach for analyzing multiple scattering data is developed to obtain values for particle size, volume fraction and surface area. It is shown how particle size and volume fraction can be measured in a practical way from SANS data totally dominated by incoherent multiple scattering (`beam broadening'). The general phase-shift dependence of single-particle scattering is incorporated into the multiple scattering formalism, and it is also shown that the diffractive limit (small phase shift) applies even for phase shifts as large as unity (particle radii of order 1 μm). The stability of the Porod law against multiple scattering and the phase-shift scale are described, a useful empirical formula for analysis of beam broadening data is exhibited, and the applicability of the formulations to polydispersed systems is discussed.

Characterization of alumina powder using multiple small-angle neutron scattering. II. Experiment

Small-angle neutron scattering techniques developed in the preceding paper [Berk & Hardman-Rhyne (1985). J. Appl. Cryst. 18, 467–472] are used to obtain microstructural parameters of high-purity alumina powder. The values of the particle size, volume fraction and surface area have been obtained and are compared to data from techniques such as laser light scattering, X-ray sedigraph and scanning electron microscopy. The particles exhibit a log–normal distribution and are spherical in shape with a mean particle size of 342 nm determined from SANS analyses of both beam broadening and Porod regions.