High Purity Alumina Powder Research Articles

The Preparation and Properties of Alumina Ceramics through a Two-Step Pressureless Sintering Process

In this paper, a high-dense alumina ceramics were prepared through the two-step pressureless sintering process with high-purity alumina powder as raw materials and high-purity MgO as sintering aids. The effects of the sintering temperature in the first-step (T1) and the soaking time (t) in the second sintering step (T2) on the density, microstructure and mechanical properties of the alumina ceramics were studied. The results indicated that the relative density increased with the increase of T1 temperature whereas it increased and then decreased with the increase of MgO content. Higher T1 temperature and extended soaking time caused larger grain size, which accompanied with the Ostwald ripening of the grain and led to non-uniformity of grain size distribution. The addition of MgO was beneficial to the decrease in grain size due to pinning effect of the second phase. For samples with shorter soaking time, sintering with higher T1 temperature led to better mechanical properties because of its high density. However, for the long soaking time, all samples after sintering at different T1 temperature were fully-densified, so the grain size become to the dominant factor of strength, thus samples with lower T1 temperature exhibited better mechanical properties due to the refinement grain. Excessive addition of MgO resulted in defects, by which the strength increased firstly and then decreased slightly with the increased MgO content. For the samples with 2.5wt.% MgO, the optimum condition for the two-step pressureless sintering was T1=1450°C and T2=1400°C for 20h, and the obtained sample achieved the relative density of 96% and the strength of 507±32MPa.

Development ceramic composites based on Al2O3, SiO2 and IG-017 additive

Based on high purity alumina and quartz powders and IG-017 bio-original additives the authors have developed new ceramic composite materials for different industrial purposes. The main goal was to fine a material and morphological structures of high performance ceramic composites as frames for development complex materials for extreme consumptions in the future. For this the mixed powders of Al2O3, SiO2 and IG-017 bio-original additive were uniaxially pressed at different compaction pressures into disc shapes and were sintered in electric kiln under air (1) and nitrogrn (2) atmosphere. The grain size distributions of the raw materials were determined by laser granulometry. There thermo-physical properties were also determined by derivatography.The prepared and sintered specimens were tested on geometrical sizes, microstructure and morphology by scanning electron microscopy, porosity and water absorption. In this work the authors present the results of their research and investigation.

Determination of trace elements in high purity alumina powder by helium enhanced direct current glow discharge mass spectrometry

Trace impurities in high purity alumina powder were determined by fast flow direct current glow discharge mass spectrometry (GD-MS). The non-conductive samples were prepared with high purity graphite powder and used as a sample binder and as a secondary cathode. To improve the sensitivity of the GD-MS analysis, helium was introduced as an additional glow discharge gas to argon plasma. The quantification results of the GD-MS measurement were calculated by external calibration with matrix matched certified reference materials. The GD-MS results for the determination of Na, Mg, Si, Ca, Ti, V, Cr, Fe, Cu, Zn and Ga in the alumina samples agreed well with the certified values of a reference material and the results of chemical analysis using wet sample digestion with inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). The GD-MS analysis is a rapid analysis technique to determine trace elements in non-conductive alumina to below mg·kg−1 levels.

Qualification Methods of Al2O3 Injection Molding Raw Materials

For producing ceramic arc tube parts (plugs), there are used two different major components for producing injection molding raw material (feedstock): high purity alumina powder as the main component, and an organic paraffin wax as a binder material. It is expressly important to know the material, physical and chemical properties of these components, since mainly these have effect on the homogenity of feedstock, and therefore on the quality of end product. In this research, both of the main components and the moldable raw material was investigated by visual, physical, and thermal methods. As most important and main statement, the researchers found that the dynamic viscosity of raw material depends more on the applied temperature, than on the deformation speed gradient.Applied analitycal methods were laser granulometry, sieve analysis, differential thermal analysis and rheology analysis.

Quality Control Methods of Al<sub>2</sub>O<sub>3</sub> Based Ceramic Injection Molding Raw Materials

In the illuminant industry, for producing arc tube parts for high intensity discharge lamps the applied method is the ceramic injection molding. The ceramic arc tube parts are made of high purity alumina powder. By producing ceramic parts, one of the most critical step is to optimizing the injection molding process, [1] but first of all we need to know the properties of injection molding raw material, because later the molding process will be optimized for this material, to decrease the amount of cracked ceramics.For producing ceramic arc tube parts (plugs), there are used two different major components for producing injection molding raw material (feedstock): high purity alumina powder as the main component, and an organic paraffin wax as a binder material. It is expressly important to know the material, physical and chemical properties of these components, since mainly these have affect on the homogenity of feedstock, and therefore on the quality of end product. [3]In this research, both of the main components and the moldable raw material was investigated by visual, physical, chemical and thermal methods. As most important and main statement, the researchers found that the dynamic viscosity of the injection molding raw material depends on the used mixer equipment and the applied deformation velocity.Applied analitycal methods were laser granulometry, differential thermal analysis, and rheological analysis.

Phase-field simulation of platelike grain growth during sintering of alumina

Abstract The toughness of alumina can be improved by utilizing the in situ formation of platelike anisotropic grains during sintering, that is, abnormal grain growth (AGG). Computer simulations of AGG may be effective to realize the conditions for obtaining the desired self-composite microstructure. In the first part of this study, sintering experiments of high-purity alumina powders were conducted to confirm the effects of powder size distribution as well as the amounts of additives. In the second part, a phase-field method for simulating the platelike grain growth was proposed. The large platelike grains were reproduced when the critical driving force of coarsening was set up. The incubation time of AGG was also observed in the case of the narrow size distribution. Although the morphology of the platelike grains did not exactly agree with the experimental observations, a possibility of the present method as a computational tool for simulating platelike AGG was verified.

Advances in High-Purity Alumina and Resource Distribution

High-purity alumina has excellent performance and wide application in high-tech industries and areas of modern materials, so industrialized, low-cost, non-polluting and sustainable development preparation process has become one of the priorities of present high-purity alumina powder research. The use of high purity alumina and its major domestic craft preparation methods are described in the paper.

Compressive mechanical properties of porous alumina powder compacts

Abstract In this study, compressive mechanical properties, relative density, porosity, grain size, and grain boundary area of porous alumina ceramics, were examined at room temperature to clarify the influence of microstructures. High purity alumina powder of median size 150 nm was dispersed in an aqueous solution at pH 3.0 and consolidated by pressure filtration to achieve a uniform microstructure. The grain boundary development was analyzed using an initial sintering model and the decreased Brunauer–Emmett–Teller specific surface area during sintering. When the alumina compacts were sintered at 500–1400 °C, the relative density (60.2–62.9%) was almost constant up to 1000 °C, while the specific surface area decreased above 800 °C. The Young's modulus and compressive strength of porous alumina compacts increased exponentially with sintering temperature. The abovementioned mechanical properties were well explained by the increased grain boundary area during sintering. Another factor that contributes to the observed increase in the mechanical strength of porous alumina compacts is the increase in packing density, leading to an increase in the total grain boundary area. An effective compressive strength (830 MPa) for dense alumina compacts of 0% porosity was estimated from the relationship between measured compressive strength and grain boundary area, which was 1/4–1/3 times that of the compressive strength reported for dense alumina materials.

Transparent sub-micrometre alumina from lanthanum oxide doped common grade alumina powder

Common grade high purity alumina (99.99%) powder doped with lanthanum oxide was slip cast, dried and pressureless sintered to sub-micrometre grain sized alumina at 1380–1410 °C. Samples that attained >96% theoretical density (TD) were subsequently hot isostatically pressed to fully dense alumina having ≥99.95% TD. Sintered sub-micrometre alumina samples, ground to 2 mm thickness and polished to 0.5 μm surface finish, displayed >82% transmission in the 3–5 μm wavelength range. Samples of 0.5 mm thickness also exhibited 40–60% transmission in the visible wavelength region of 400–900 nm. Lanthanum oxide addition within 0.10 wt% was shown to be favourable for improved transparency in the visible region. The alumina samples also exhibited high Vickers hardness values of 2144+45 kg/mm 2 at 5 kg indentation load.

Measurable properties of Al2O3 ceramic injection molding raw materials

Powder injection molding (PIM), which encompasses metal injection molding (MIM) and ceramic injection molding (CIM), is a process which enables large scale production of complex-shaped components for use in a diverse range of industries. Ceramic injection molding (CIM) is a technology for manufacturing complex, precision, netshape components from ceramic powder. In the illuminant industry, for producing arc tube parts for high intensity discharge lamps the applied method is the ceramic injection molding. The ceramic arc tube parts are made of high purity alumina powder. By producing ceramic parts, one of the most critical steps is to optimize the injection molding process, to determine and control the influential machine parameters, which have an effect on the quality of the end product. Nevertheless, the properties of injection molding raw material is needed to be known before any optimization of the injection molding process, because later the molding process is optimized for this material, to decrease the amount of cracked ceramics. For producing ceramic arc tube parts (plugs), there are two different major components used for producing injection molding raw material (feedstock): high purity alumina powder as the main component, and an organic paraffin wax as a binder material. It is expressly important to know the physical and dimensional properties of the alumina powder, since mainly these have affect on the homogeneity and viscosity of feedstock, and therefore on the quality of the end product. In the present research, both the main components and the moldable raw material were investigated by visual, physical, chemical and thermal methods. It was found – most importantly – that the viscosity of the raw material linearly decreases with the increasing grinding time of the alumina powder. Applied analytical methods in terms of laser granulometry, tap density analysis, differential thermo-analysis and rheology analysis were used.

Cylindrical EBG antenna for short range gigabit wireless communications at millimetre-wave bands

An omnidirectional millimetre-wave antenna based on a cylindrical electromagnetic bandgap (EBG) resonator is presented. The cylindrical woodpile is fabricated using the ceramic extrusion freeforming technique with high purity alumina powder. The designed antenna can achieve a narrow beam with 6.5° measured half power beam-width in the elevation plane without using an array configuration. The overall antenna gain is approximately 5 dBi at 94.2 GHz, suitable for future short range gigabit wireless communications.

On the Sintering Activation Energy of α-Al<sub>2</sub>O<sub>3</sub>

The sintering activation energy of high-purity alumina powders with different particle sizes was evaluated under non-isothermal condition. It was found that, during sintering, the activation energy for the lower temperature stage is higher than that for higher temperature stage. The value of the activation energies for the powder compact with larger particle size was higher than that for the powder compact with smaller particle size. If the selected temperature interval for calculation was narrow enough, the evaluated activation energy values varied with the increasing temperature continuously.

Directive millimetre-wave antenna based on freeformed woodpile EBG structure

A high directivity millimetre-wave antenna based on woodpile electro-magnetic bandgap (EBG) material at W-band is presented. The woodpile EBG structure is fabricated using the ceramic extrusion freeforming technique with high purity alumina powder. The designed antenna can achieve a narrow beam without using an array. The measured half power beam widths are 14deg and 10deg for E-plane and H-plane, respectively. Measurements results are good compared with simulations

Normal and Abnormal Grain Growth in Ceramics - NbC-Co and Alumina

Normal and abnormal grain growth has been observed in 70NbC-30Co with varying B concentrations at 1450°C and in alumina with varying impurity and additive concentrations at 1600°C -1650°C as typical systems with and without liquid matrix. The grain growth behavior depends on the roughening of the interfaces as indicated by the grain and grain boundary shapes. When 4% B is added to 70NbC-30Co, the NbC grains in Co-rich liquid matrix are spherical and undergo diffusion controlled normal growth, because the grain-liquid interface is rough. As the B concentration is decreased to 3, 2, 1, and 0%, the NbC grains become more cubic and the tendency for abnormal grain growth increases because of the step growth mechanism of the flat singular surface segments. When compacts of high purity alumina powder are sintered at 1650°C, the grain boundaries are smoothly curved, indicating their atomically rough structures. With increasing impurity content—in particular SiO2—in the alumina powder, abnormal grain growth becomes more pronounced with increasing number of flat grain boundaries. These singular grain boundaries are expected to move by a step mechanism and thus cause the abnormal grain growth. These results show that the interface roughening and hence the grain growth mode changes gradually with the additive or impurity concentrations. Therefore, the abnormal grain growth cannot be sharply distinguished from the normal grain growth as has been previously suggested in general and for alumina in particular.

On the evaluation of the activation energy of sintering

High-purity alumina powders with different particle morphology were prepared for the study of the evaluation of the activation energy of sintering. In this investigation, the evaluation of the activation energy is based on nonisothermal and isothermal sintering, in which the evaluated values were higher for the former than the latter. The abnormally higher value of the activation energy based on the nonisothermal sintering for the classified powder compact may be resulted from the low heating rate, which is attributed to the fact that the relative contribution of D l and D b to densification is different. Pore morphology also plays a role in evaluating the activation energy of sintering.

Superplastic behavior of zirconia-reinforced alumina nanocomposites from powder alcoxide mixtures

Here we report the synthesis, microstructure and superplastic creep behavior of alumina-based composites reinforced with 5–20 wt% ZrO 2, prepared from high-purity alumina powders and zirconium alcoxide precursor mixtures. The composites reached 100% true strain at 1350 °C, deformed by stationary creep in the range of strain rates ~10 −7 to 10 −4 s −1. The microstructure remained stable during creep; no evidence of grain growth, grain boundary opening or cracking were observed in deformed samples. We conclude from the analysis of creep parameters that creep takes place mainly by interface-reaction-accommodated grain boundary sliding, and that the mobility of alumina grain boundaries controls the creep rate. The observed behavior is explained by the role of alumina–zirconia interphases, stronger and with a faster relaxation kinetics than alumina interfaces, to achieve an effective grain boundary pinning. Zirconia particles (~0.2–0.4 μm) are homogeneously placed at alumina grain boundaries (~1–2 μm).

Alumina nanocomposites from powder–alkoxide mixtures

This paper deals with a colloidal processing route that uses alkoxides to obtain alumina nanocomposites. These alkoxides, which include zirconium IV-propoxide, tetraethyl orthosilicate (TEOS), yttrium methoxyethoxide (ABCR, Germany) are used both separately and in different mixtures as coating for alumina particles in non-aqueous media. As a consequence the grain boundaries of the high-purity alumina powder are modified by segregation of the secondary phases or by the formation of well-distributed zirconia, YAG and mullite nanoparticles. The presence of these oxides at the grain boundaries (GBs) drastically affects the evolution of the microstructure, and the properties of the resulting materials. Thus high dense composites with microstructures formed by an alumina matrix and a very homogeneous distribution of secondary nanophases can be obtained.

Pressure filtration of dispersed and flocculated alumina slurries

The pressure-filtration behaviors of dispersed and flocculated alumina slurries at high percent solids were investigated with the objective of determining the effect of surfactant type and concentration on slurry properties. A high-purity alumina powder was used as a model solid. Dispersion of the alumina was controlled by pH and by surfactants that are known to act as electric and steric stabilizers. The effects of both ionic and nonionic surfactants were investigated. Surface charge measurements of the slurries were carried out by an electrokinetic sonic amplitude (ESA) method to better understand the mechanisms. The effect of surfactant type and concentration on the kinetics of pressure filtration and cake properties are discussed.

Rheology of concentrated alumina suspension to improve the milling output in production of high purity alumina powder

In production of high purity alumina, milling is an important processing step during which particle size has to be reduced to sub-micron size using minimum energy input and avoiding contamination from the milling system. In this investigation, to increase the efficiency of the milling process, the effect of two anionic dispersants were studied in concentrated alumina slurry. The optimized slurry was granulated using freeze drying and the properties of granules and sinters were compared with that using spray drying.

高速遠心成形法による高純度アルミナの焼結中における気孔組織の変化

To elucidate the causes of the superior mechanical properties of the alumina developed by the high-speed centrifugal compaction process, its microstructure development during sintering is investigated. A high purity alumina powder with a purity of 99.99% and a mean particle size of 0.22μm is dispersed in 25 mass% of ion-exchanged water, compacted applying acentrifugal force of 10, 000-20, 000g for 3ks, and sintered at 1423-1773 K for 5.4ks in air.Homogeneous sintered compacts with a relative density of 99% or more and free from abnormal grain growth are obtained without using any additives in a wide range of sintering conditions. On tens of fracture surfaces observed, no sign of inhomogeneity such as large grain, large pore, or inclusion is observed. Instead, each specimen contains a little intergranular porosity. At low sintering temperatures, pores remain at the fbur-grain corners which. are thermodynamically the most stable sites. As the sintering temperature increases, the pores change their locations gradually from four-grain corners to three-grain edges, then to two-grain boundaries.The strongest specimen, the one sintered at 1503K for 5.4ks, shows homogeneous equiaxed grains with equiaxed and simple-shaped pores located at four-grain comers.