Formation Of Α-alumina Research Articles

Low-Temperature Formation of α-Alumina by Doping of an Alumina-Sol

In our present study, the phase transition behavior of γ-alumina doped by an alumina-sol was explored using X-ray powder diffraction and transmission electron microscopy. It was found that the addition of an alumina-sol into γ-alumina enabled the onset of the γ- to α-alumina transformation at temperatures as low as 600°C. This phenomenon was designated as the “sol-effect.” Although the onset temperature of the phase transformation was very low, the transformation was completed at 1100°C which is the same as that observed with the seeding method. Further investigation revealed that the transformation was composed of two steps. One occurs over the temperature region from 600 to 950°C where a slow increase in the content of α-alumina is observed. The other is the region from 950 to 1100°C where a rapid increase of α-alumina occurs due to “self-seeding.” The “sol-effect” produces powders with very fine α-alumina particle sizes and little agglomeration.

Mullite Transformation Kinetics in P2O5‐, TiO2‐, and B2O3‐Doped Aluminosilicate Gels

Mullite transformation kinetics of sol‐gel‐derived diphasic mullite gels doped with P2O5, TiO2, and B2O3 were studied using quantitative X‐ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P2O5 doping because of phase separation and formation of α‐alumina and cristobalite. In TiO2‐doped samples, the mullite transformation temperature decreased with TiO2 doping, and the transformation rate increased with decreasing TiO2 particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid‐state mass transport through lattice defects. B2O3 doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.

Low temperature sintering of α-alumina with the aid of abrasive powder in wet grinding

Wet grinding of aluminum hydroxide in a tumbling mill was conducted to investigate the effect of abrasive powder worn from the ball media on the preparation of low temperature sinterable α-alumina powder. The abrasive powder worn from high purity alumina balls reduces the transformation temperature to α-alumina during the calcination process. The transformation temperature decreases, from about 1300°C for the unground sample to about 1000°C for the ground product, with increasing amounts of abrasive powder. Only 0.2 mass% of contaminant causes a reduction in temperature by about 200°C in comparison with the unground one. The α-alumina powder calcined after the grinding is very fine. Therefore, low temperature sintering of such α-alumina calcined powder is easily achieved, forming a dense sintered body. The alumina abrasive powder is considered to play a significant role as a seed effect in the formation of α-alumina.

水酸化アルミニウム粉砕時に生成する湿式ボールミル摩耗粉を利用したアルミナセラミックスの低温焼結

The wet grinding of aluminum hydroxide in a tumbling mill was conducted to investigate the effect of abrasive powder from the ball media on the preparation of low temperature sinterable α-alumina powder. The abrasive powder worn from high purity alumina balls reduced the transformation temperature to a-alumina during the calcination process from about 1300°C for the unground sample to about 1000°C for the ground product, with increasing as the amount of the abrasive powder. Only 0.2mass% of contaminant caused a reduction in temperature of about 200°C in comparison with the unground one. The α-alumina powder calcined after grinding is very fine, so the low temperature sintering of such α-alumina calcined powder is easily achieved, forming a dense sintered body. The alumina abrasive powder is considered to play a singnificant role as a seed effect in the formation of α-alumina.

Characterization of Abrasion Powder Worn from Alumina Balls by Wet Milling and Its Phase Transformation during Heating

Abrasion test of high and low purity alumina balls is conducted in different types of mills under wet condition. The abrasion powder produced by abrasion of high purity alumina balls gave extremely large value of specific surface area over 30m2/g. The powder includes α-Al(OH)3 of gibbsite type, while α-Al(OH)3 is not included in an abrasion powder worn from the low purity alumina one. A considerable amount of amorphous aluminum hydroxide is contained in both abrasion powders. The amount of aluminum hydroxide in the powder increased as the milling becomes moderate and as the balls' size is small. The aluminum hydroxide in the abrasion powder is transformed into a single phase of α-alumina without forming κ- and θ-alumina phases at a relatively low temperature of about 900°C. This is completely different from the phase transformation of an ordinary powder of α-Al(OH)3. The formation of α-alumina from abrasion powder at such low temperature is attributed to a seed-effect of fine α-alumina powder mixed uniformly in the abrasion powder. An abrasion powder worn in alcohol also enables us to sinter at comparatively low temperature.

Formation of α-Al<sub>2</sub>O<sub>3</sub> from Al(OH)<sub>3</sub> in the Presence of Seed Particles and Its Sinterability

Effect of wet grinding of commercial aluminum hydroxide on the transformation to α-alumina during heating was investigated using vibration ball mill. The grinding with high purity alumina balls reduced the transformation temperature to α-alumina by 300°C. Such significant reduction in the transformation temperature was not observed in the samples obtained by the grindings with zirconia and low purity alumina balls. The formation of α-alumina from ground aluminum hydroxide at such a low temperature was possibly attributed to a seed-effect of fine α-alumina powder worn from the balls and its uniform mixing with the mother powder during grinding. The α-alumina powder made from ground aluminum hydroxide consisted of very fine particles, which enabled us to sinter at a relatively low temperature. The sintered bodies had fine-grained structure without abnormal grain growth.

Thin-film substrates for the study of solid-state transformations

A popular route to obtain α-alumina (α-Al2O3) ceramics is by the use of sol-gel chemistry in which boehmite (γ-AlOOH) is dispersed in water to form a colloidal solution which is later dried and heat treated. Heat treatment of the boehmite produces a series of metastable transitional aluminas prior to the formation of α-alumina at approximately 1200°C. The transformation sequence has associated with it a large volume decrease with significant pore formationand entrapment which results in a poorly densified, vermicular α-alumina product. The use of nucleating agents or seeds such as α-alumina or hematite (α-Fe2O3) is known to lower the transformation barrier and eliminate the vermicular microstructure. The enhanced transformation has been attributed to a solid-state homo- or heteroepitactic nucleation mechanism. Direct evidence of such a mechanism, obtained using transmission electron microscopy (TEM), has been reported. In that study, however, only the edges of large (20 μm diameter) hematite particles distributed in a partially transformed transition alumina matrix were observed. The hematite crystals in that case could not be imaged in cross-section so the extent of the transformation could not be precisely determined.

Preparation of alumina by sol-gel process, its structures and properties

Alumina monolithic gel has been prepared by ammonia method from anhydrous aluminium chloride and n-butanol using formamide as the solvent. The gel has been made by hydrochloric acid catalysis and using large quantity of water for hydrolysis (molar ratio of water to aluminium-n-butoxide, R>99). Transparent alumina xerogel of size 37.5×20×5 mm approximately has been prepared. The alumina gel has been dried and sintered at 400°C, 1000°C and 1200°C respectively and the powders formed thereby have been examined by XRD, FTIR spectroscopy, SEM and particle size analysis. These studies have confirmed the formation of α-alumina at 1200°C having particle size as low as 0.2 μm or less along with agglomerates. The density of the powder has increased gradually, where as its particle size has decreased, with the increase of the sintering temperature.

Phase evolution in Al2O3 fibre prepared from an oxychloride precursor

The formation of polycrystalline α-alumina during the heat treatment of aluminium oxychloride fibres occurs via transitions through various intermediate crystalline alumina phases. The different phases show a strict pseudomorphism, i.e. the external shape of the crystal is retained and there is an orientational relationship between the crystal axes, before and after the phase transition. Consequently, the phase sequences and microstructure are interdependent and controlled by the heat-treatment conditions, which determine the final properties of the fibres. The effects of water content in the fibre formulation, and sintering temperature and heating rate on the phase development from amorphous oxychloride to crystalline α-alumina fibres, were investigated. The transition path for the phase evolution during heat treatment to 1200°C (when the formation of α-alumina is completed) was established and correlated with fibre microstructure. In fibres with higher water content, the transition temperature for γ- and δ-alumina phase formation was higher. Only for a very high heating rate was θ-alumina observed as an intermediate phase during phase evolution. The axial porosity appeared to be eliminated more rapidly in the fibres at temperatures below 1200°C. Consequently, residual porosity, comprising closed pores, in the final stage of sintering, was mainly observed radially oriented toward, and on, the fibre surface. A high diametral shrinkage was measured and plotted as a function of the sintering temperatures used for fibre heat-treatment.

Properties and mechanism of formation of α-alumina (Corundum) film by anodic oxidation of aluminium in bisulphate melts

Aluminium can be anodically oxidized in the low temperature melts (100–250°C) of bisulphates or their mixed systems with the formation of α-alumina (Corundum) film. For example, in KHSO 4-NaHSO 4 (2 : 1 mol) mixture at 180°C, at 1 A/dm 2 by applying 0–160 V for 30 min, a white and mat oxide film is produced on aluminium, which is rather porous (density 2·96) but very hard and is not attacked by any chemicals such as HF, H 2SO 4, H 3PO 4, NaOH etc. of any concentration. Coating ratio is 1·76–1·81, nearly equal to the theoretical value (1·89). The film is very stable and can neither be sealed by steaming nor dyed by water-soluble dyes, but the paint or lacquer adhesion is good due to its porosity. Corrosion potentials in NKCl and KNO 3 were compared with those of the oxalic acid films. X-ray diffraction patterns indicated distinctly the formation of an α-alumina with a very slight amount of γ-alumina (400) and (440). The reaction mechanism was considered as the decomposition of sulphuric acid, M 2SO 4.H 2SO 4 → M 2SO 4 + SO 3 + H 2 + O. This was confirmed experimentally and thermodynamically. Electrolysis in conc. sulphuric acid gave a similar film but it consisted mainly of γ-alumina of less crystalline structure. The mechanism of direct formation of α-alumina is not yet established but the local heating over the anode surface, absence of water and the existence of a slight amount of γ-alumina (400) and (440) are possibly its factors.