High-purity Al2O3 Research Articles

Néhány nagy tisztaságú Al2O3 műszaki kerámia hajlítószilárdsági vizsgálata

Néhány nagy tisztaságú Al2O3 műszaki kerámia hajlítószilárdsági vizsgálata

A quick method for the determination of the Al/Si weight ratio in alumino-silicates using an Am-Be neutron source

An irradiation procedure with fast and thermal neutrons from a 5 Ci Am-Be isotopic neutron source irradiation facility in combination with a 3''3' NaI(Tl) detector system has been used to determine Al/Si weight ratios in alumino-silicates. Samples were irradiated with and without Cd cover for 10 minutes and counted for 10 minutes after a waiting time of 1 minute. The peak area analysis of the 1779 keV gamma-ray line of 28Al product radionuclide produced via 27Al(n,γ)28Al and 28Si(n,p)28Al reactions in combination with the neutron flux parameter at the irradiation site and nuclear data were used to determine Al/Si ratios. Due to discrepancy in literature data, Am-Be neutron source spectrum averaged cross sections of (n,p) reactions on 27Al, 28Si and 56Fe were determined by the activation technique using 115In(n,n')115mIn as the fast neutron flux monitor reaction. The method was tested using mixtures of high-purity Al2O3 and SiO2 with known weight ratios of Al/Si and validated by a certified reference material BCS-CRM 348 (Ball Clay). Results are presented for bentonite, kaolin, bauxite, feldspar and ball clay samples from Nigeria. The method is non-destructive, rapid and suitable for use in-situ for large-scale exploration works and industrial process control.

The effects of solid phase additives on sintering properties of alumina bioceramic

In order to reduce the sintering temperature and improve the preparing conditions of alumina bioceramics, the Mg−Zr−Y composite solid phase additives, were added into high purity Al2O3 micro-powder by chemical coprecipitation method. The powder was shaped under 200MPa cold isostatic pressure, and then the biscuits were sintered at 1600°C under normal pressure. The sintered alumina materials were tested and the sintering mechanism was discussed. The results show that physical properties of the material were improved comparatively. The Mg−Zr−Y composite solid additives could promote the sintering of alumina bioceramics and the mechanism is solid phase sintering.

Preparation of Ceramic Surfaces for Microscopic Observation by Penning Discharge Microetching.

In this study, a new microetching technique for the observation of ceramic surfaces by Penning discharge microsputtering is developed and applied to ceramic surfaces. Each ceramic surface was prepared by mechanical polishing and subsequent etching and the resultant surfaces were examined under both an optical and a scanning electron microscope. The results in the case of using with a gold target revealed high-definition images of the surface structure of the Si3N4, ZrO2 and Al2O3-ZrO2-Al6Si2O13 ceramics after surface etching using this process. However, in the case of using a gold target, high-purity Al2O3 ceramics and Al2O3-ZrO2 complex ceramics, the resultant surface preparation was unsuccessful. However, when using a titanium target, the procedure in both cases, was successful. This difference could be attributed to the difference in the effect of oxygen on both metals.

Analysis of Trace Amounts of Emitted Gases during Fracture of High-Purity Alumina under Ultrahigh Vacuum. Effects of Sintering Atmosphere and Sintering Time on Gas Emission Behavior.

The effects of sintering atmosphere and sintering time on gas emission behavior during fracture of high-purity Al2O3 were investigated under ultrahigh vacuum using a mass spectrometer to examine the behavior of gases in the material. The gas in the sintering atmosphere was predominantly emitted at the moment of fracture for all specimens. The total amount of gas emitted from a specimen sintered in O2 was less than that of a specimen sintered in air or Ar, showing that the mobility of O2 in the material was much higher than that of N2 or Ar. During prolonged sintering, the density of specimens decreased, while the amount of emitted gas increased. Entrapped gases in the sintered body were concluded to bring about the density decrease during grain growth.

Improvement of creep resistance in polycrystalline Al2O3 by Lu-doping

High-temperature creep resistance in high-purity Al2O3 (nominal purity of 99.99%) and Al2O3 doped with 0.05 mol% Lu2O3 was examined by uniaxial compression testing in air at a constant load at temperatures between 1150 and 1350°C and the applied stress range of 10∼200 MPa. The creep resistance was found to highly improve in this temperature range by doping of Lu2O3 into polycrystalline Al2O3 even at the level of 0.05 mol%. The stress exponent was about 2 in the two materials, but the activation energy for creep was different; 410 kJ/mol for undoped Al2O3 and 780 kJ/mol for Lu2O3-doped Al2O3. Lutetium ions were found to segregate in Al2O3 grain boundaries without forming amorphous phase or second-phase particles. The improved creep resistance in polycrystalline Al2O3 due to Lu2O3 doping was attributed to the segregation of Lu ions in Al2O3 grain boundaries probably due to the suppression of grain boundary diffusion. The change of chemical bonding state in grain boundaries with the segregation of Lu ions is supposed by HREM–EELS analysis.

Two kinds of roles of MgO in the densification and grain growth of alumina under various atmospheres: Sensitive and insensitive roles to the experimental procedures

Two kinds of high-purity Al2O3 powder were studied with respect to the effects of not only MgO doping but also atmosphere on both densification and grain growth. Controversial results for MgO doping are explained in terms of two roles that MgO can play during these processes: sensitive and insensitive to the experimental procedures. Atmospheres, whether dry or wet, had little influence on densification or grain growth in the early stage. After closed pores appreciably formed, however, both N2 and Ar atmospheres quickly reduced the densification rate.

Grain boundary bonding state and fracture energy in small amount of oxide-doped fine-grained Al2O3

Grain boundary structure and chemical bonding state were characterized in high-purity Al2O3, 0.1 wt% MgO, 0.1 wt% Y2O3 or 0.1 wt% ZrO2-doped Al2O3. High resolution electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS) revealed that all samples examined have single phase structure, and that doped cations segregate along grain boundaries. Electron energy loss spectroscopy (EELS) spectra taken from grain boundaries in doped Al2O3 shows slight chemical shift in comparison with those from grain interior. This result suggests that the chemical bonding state in grain boundaries changes by the segregated ions. The change in chemical bonding state seems to affect the grain boundary fracture energy of Al2O3.

Improvement of high-temperature creep resistance in fine-grained Al2O3 by Zr4+ segregation in grain boundaries

The high-temperature creep resistance in Al2 O3 is improved greatly by ZrO doping. Zirconium ions are found to be segregated in Al O grain boundaries. The activation energies for creep in high-purity Al2 O3 and ZrO-doped Al O are - estimated to be 430 and 650 kJ mol1, respectively. The grain boundary diffusivity of Al ions is expected to be reduced by the segregation of Zr4+ in the grain boundaries.

Anomalous stochastic behavior of partial discharge on aluminum oxide surfaces

The stochastic properties of pulsating partial discharge (PD) generated by applying a low-frequency sinusoidal alternating voltage to a point electrode touching an aluminum oxide (Al2O3) surface in air have been investigated. The time dependence of such statistical characteristics as mean numbers of positive and negative PD pulses per half cycle and the amplitude and phase distributions of individual positive and negative PD pulses selected according to their order of occurrence in a cycle were extracted from records of the amplitudes and phases of all PD events that occurred while the voltage was applied for times up to 40 min. The discharge characteristics exhibit a dramatic sensitivity to the impurity content of Al2O3. In the case of high-purity (99.9%) Al2O3, the positive-PD pulses cease within 30 s after application of the voltage from which time the negative-PD pulses persist indefinitely in a relatively stationary pattern. The cessation of positive PD was not observed for Al2O3 samples of lower purity (96% or lower). A modified version of a previously developed Monte Carlo simulator of ac-generated PD that includes effects of transport and decay of surface charge between PD events was used to gain insight into the conditions that could give rise to the observed long-term behavior of PD for high purity Al2O3.

Preparation and mechanical properties of high-purity Al2O3 fibre/Al2O3 matrix composite

Al2O3 matrix composites with unidirectionally oriented high-purity Al2O3 fibre with and without carbon coating, were fabricated by the filament-winding method, followed by hot-pressing at 1573–1773 K. The composite with non-coated Al2O3 fibre exhibited a bending strength (594 MPa) comparable to that of monolithic Al2O3 (589 MPa). While the composite with a carbon-coated fibre had lower strength (477 MPa), it showed improved fracture toughness (6.5 MPa m1/2) compared to the composite with an uncoated fibre (4.5 MPa m1/2) and monolithic Al2O3 (5.5 MPa m1/2). This toughness enhancement was explained based on the increased crack extension resistance caused by the fibre pull-out observed by SEM at the notch tip.

Interfacial morphologies between alumina and silver-copper-titanium alloy

Joints of high-purity Al2O3 were made with a Ag57Cu38Ti5 alloy by vacuum brazing at 800 °C and 900 °C for 30 min. The microstructures at the interface between the ceramic and the alloy were examined in cross-section by scanning electron microscope (SEM) and on different planes parallel to the interface layer-by-layer by optical microscope (OM), respectively. The chemical composition of the reaction product was also analysed by energy dispersive X-ray (EDX). A thin reaction layer about 1.0 μm thick was formed on the Al2O3 surface at a temperature of 800 °C. This layer had three kinds of morphologies and they were identified as Ag, Cu2Ti4O and AlTi, respectively. According to SEM and OM results, there were two distinct layers 4.0 μm thick interfacial reaction layer for sample heated at 900 °C, one layer in the vicinity of the ceramic consisting mainly of Ti2O and TiO and the other layer near the alloy was CuTi2. A transition layer structure composed of Al2O3/Ti2O + TiO/Ti2O + TiO + CuTi2/CuTi2/Ag–Cu was formed at the interface. The Al concentration at the interface was relatively high and confirmed that the reduction of Al2O3 by Ti occurred, which was consistent with the result of a thermodynamic analysis.

Microwave Sintering of Pure and Doped Nanocrystalline Alumina Compacts

AbstractA single-mode cavity microwave furnace, operating in the TE103 mode at 2.45 GHz is being used to investigate sintering of pure and doped nanocrystalline alumina. The purpose of these experiments is to determine the effect of additives on the sintering process in the nanocrystalline regime. Using the sol-gel method, high purity Al2O3 nanocrystalline powders were synthesized. These powders were calcined at 700°C and then CIP'ed to 414 MPa, producing 0.4 in. diameter, 0.25 in. high cylindrical compacts. The compacts were heated in the microwave furnace to temperatures between 1100°C to approximately 1800°C and were then brought back to room temperature using a triangular heating profile of about 30 minutes duration. A two-color IR pyrometer was used to monitor the surface temperature of the workpiece. The additives tested in this work lowered the temperature needed for densification but this effect was offset by increased grain growth. Initial grain growth from < 5 nm to ∼ 50 nm was closely correlated with the γ to α-alumina phase transition.

Improvement of Fracture Toughness of Al<sub>2</sub>O<sub>3</sub> Composites by Micro-Dispersion of Flaky Refractory Metals Mo, Ta and Nb

New simultaneous toughening and strengthening method for Al2O3 composites has been investigated. The study on flake-forming process of refractory metal powders such as Mo, Ta and Nb was carried out using high purity Al2O3 (AKP-30) and easy-sintering Al2O3 (AES-11) matrices applying the wet ball milling. Three-point flexural strength and fracture toughness by means of SEVNB method of Al2O3/Mo, Al2O3/Ta and Al2O3/Nb composites fabricated by hot pressing were evaluated. The micro-dispersion of flaky particles of Mo, Ta and Nb in Al2O3 matrices contributes remarkably to increase of the flexural strength and fracture toughness. The mechanism of improvement in strength and toughness can be induced from SEM observation of the fracture surface and TEM observation of the microstructure. These properties are closely related to the fracture manner of flaky particles and the distribution of nano particles of refractory metals.

Oxidation Kinetics of Chemically Vapor‐Deposited Silicon Carbide in Wet Oxygen

The oxidation kinetics of chemically vapor‐deposited SiC in dry oxygen and wet oxygen (PH2O= 0.1 atm) at temperatures between 1200° and 1400°C were monitored using thermogravimetric analysis. It was found that in a clean environment, 10% water vapor enhanced the oxidation kinetics of SiC only very slightly compared to rates found in dry oxygen. Oxidation kinetics were examined in terms of the Deal and Grove model for oxidation of silicon. It was found that in an environment containing even small amounts of impurities, such as high‐purity Al2O3 reaction tubes containing 200 ppm Na, water vapor enhanced the transport of these impurities to the oxidation sample. Oxidation rates increased under these conditions presumably because of the formation of less protective sodium alumino‐silicate scales.

Wear of Media Balls during the Ball Milling of Si<sub>3</sub>N<sub>4</sub> Powder

Wear of six kinds of media balls during the ball milling of Si3N4 raw powder with additives was tested. The materials of the ball were α′/β′-SiAlON, β-Si3N4, high purity Al2O3, Al2O3 (HD-11), SiC and tetragonal ZrO2. The material of the least wear was α′/β′-SiAlON. The Si3N4 powder milled with SiC ball resulted in the mixture of Si3N4 with 13vol% of SiC. The powder consolidated to almost full density by the conventional gas pressure sintering.

Densification and Pore Elimination of Ultra High Purity Al2O3.

This investigation aimed at the characterization of monosized ulta high purity Al2O3, observing the microstructural evolution with the measurement of pore elimination during sintering. The small particle size of 0.39μm Al2O3 powder with a most frequent pore size of 0.12μm in the green stage gave almost full density Al2O3 at the low sintering temperature of 1400°C. Brook's diagram predicted the pore-grain boundary separation, however, no separation was observed. The experimental result of pore elimination agreed with Kingery and Francois' study of pore growth/separation by the parameter of equilibrium dihedral angle. The reduction of the coordination number of grains surrounding a pore is also of importance to obtain high density Al2O3.

Densification Behavior of High-Purity Al<sub>2</sub>O<sub>3</sub> and Al<sub>2</sub>O<sub>3</sub>-15vol% ZrO<sub>2</sub>

The densification and grain growth of high purity Al2O3 and Al2O3-15vol% ZrO2 were examined in the final stage of sintering. Both densification and grain growth took place much faster in Al2O3 than in Al2O3-15vol% ZrO2. The densification rate of Al2O3 was explained by the rate equation proposed by Ashby et al., which was slightly modified by taking into account the actual distribution of pores. On the other hand, the densification rate of Al2O3-15vol% ZrO2 was much sluggish than that estimated from the modified rate equation. The grain-boundary diffusion of constituent ions might be retarded by the zirconia addition.

Microstructure of Alumina Brazed with a Silver‐Copper‐Titanium Alloy

Joints of high‐purity Al2O3 were made with a 56Ag‐36Cu‐6Sn‐2Ti (wt%) experimental alloy by vacuum brazing at 900°C for 20 min. The microstructure at the ceramic‐metal interfaces was examined in cross‐sectional specimens of the joints, and phases formed by reaction between the braze filler metal and the Al2O3 were analyzed for composition and crystal structure in a transmission electron microscope. A 0.1‐ to 0.2‐μm‐thick layer formed directly on the Al2O3 surface. This layer was Ti‐rich, had the face‐centered‐cubic crystal structure with a lattice parameter of 0.423 nm, and was identified as oxygen‐deficient γ‐TiO. The second reaction layer was ∼3 μm thick, and it separated the TiO layer from the metallic phases of the braze filler metal. The structure of this layer, as determined by electron diffraction, was diamond cubic of the space group Fd3m, with a lattice parameter of 1.137 nm. The combination of microanalysis and diffraction data confirmed that this phase was Ti3(Cu0.76Al0.18Sn0.06)3O and indicated that it can be classified as an n phase.

Influence of surface parameters and doping on the sensitivity and on the response times of tin oxide resistive sensors

A systematic analysis of the correlations between preparation conditions and physical properties has been carried out on tin oxide films, deposited onto high purity Al2O3 substrates using a variant of the chemical vapour deposition (CVD) technique, with the aim of optimizing the gas-sensing properties of these devices.It has been demonstrated that the nature of the precursor used for tin oxide preparation is of importance for the effective gas performance of the sensor; tin oxide from organometallic precursors is the most suitable active layer for H2 detection and also results in a very fast response time. Sb doping is a very effective way of extending the sensitivity to the largest hydrogen partial pressures.The experimental determination of the surface trap level relative to the chemisorbed oxygen has been performed using a novel fitting procedure of the conductivity transients.