Deformation Behavior of Polycrystalline Aluminum Oxide

The steady-state creep behavior of polycrystalline aluminum oxide from 97 to 100% dense has been examined at temperatures from 1600° to 1800°C over a range of stresses from 100 to 2000 psi under four-point transverse bending. Grain size is shown to have an important effect on the deformation behavior. Stress-strain rate dependence for the fine-grained aluminum oxide (3 to 13μ) is of viscous behavior (°_ε∼σ). The strain rate is also inversely proportional to the square of the grain size (°_ε∼ 1/(GS)²). The creep behavior can be described by the Nabarro-Herring diffusional flow model although the diffusion coefficient does not agree with that for oxygen diffusion in sapphire. When the grain size becomes coarse, the Nabarro-Herring model no longer applies. Coarse-grained aluminum oxide (50 to 100μl) behaves plastically when deformed (°_ε∼σ). The strain rates observed on coarsegrained aluminum oxide are higher than one would predict from the deformation behavior of fine-grained aluminum oxide. This plastic flow probably occurs through some dislocation movement or glide mechanism.     

Modeling of Grain-Boundary Segregation Behavior in Aluminum Oxide

It is believed that the segregation of oversized dopant ions to grain boundaries in Al₂O₃ hinders grain-boundary diffusion, thereby reducing the tensile creep rate in this system by ∼2–3 orders of magnitude. In order to explain this improvement in creep behavior, it is helpful to characterize both the effective cation and interstitial volumes at grain boundaries, because the relative openness of some boundary structures suggests a great accommodation of oversized ions. In this study, the boundary volume is determined by a spatially local Voronoi construction, which highlights cation (Al³⁺) substitutional sites as well as large interstitial voids. In particular, we examine the spatial distribution of free volume near grain boundaries and, in addition, the dependence of the driving force for segregation on misfit strain in doped Al₂O₃. We interpret our results in light of recent evidence that selective codoping can provide a more efficient means of filling available space near boundaries, thereby further enhancing creep resistance.     

Effects of Milling Liquid on the Reaction-Bonded Aluminum Oxide Process

The reaction-bonded aluminum oxide process begins with aluminum, Al₂O₃, and usually ZrO₂ powders that have been attrition-milled in an organic liquid. The attrition-milled powder is then compacted and heat-treated in air to produce polycrystalline, Al₂O₃-based ceramics. Safety considerations have made it desirable for the milling liquid to be changed from acetone to a less-flammable solvent. In this paper, mineral spirits, ethanol, and mineral spirits that contains 2 wt% stearic acid are presented as viable alternatives to acetone. The effects of changing the milling liquid on the reaction process and the properties of the final fired ceramic are investigated.     

Refractive Index of Liquid Aluminum Oxide at 0.6328 μm

The real part, n , of the refractive index of pendant drops of laser-heated liquid Al₂O₃ was measured by laser ellipsometry at 0.6328 μm. At temperatures from 2327 to 2600 K, n for the liquid was 1.744 ± 0.016. No significant differences were observed in the results obtained on ruby specimens and in oxygen or argon atmosphres.     

Dispersion-Strengthened Aluminum Oxide

Finely divided molybdenum particles were dispersed in aluminum oxide to serve as inclusions for the inhibition of grain growth and secondary recrystallization during sintering. Conventional powder metallurgy techniques were used to fabricate compacts of aluminum oxide containing up to 16 vol% molybdenum. Three methods for developing a uniform dispersion of molybdenum particles in aluminum oxide are compared on the basis of the resulting microstructures. Specimens sintered in vacuum had densities approaching 98% of theoretical density with an average grain size of 2μ or less with no evidence of secondary recrystallization. The grain refinement is reflected in transverse rupture strength data with average strengths exceeding 80 kpsi and maximum strengths exceeding 100 kpsi.     

Effect of Yttrium and Lanthanum on the Tensile Creep Behavior of Aluminum Oxide

The tensile creep behavior of two rare-earth dopant systems, lanthanum- and yttrium-doped alumina, are compared and contrasted in order to better understand the role of oversized, isovalent cation dopants in determining creep behavior. It was found that, despite some microstructural differences, these systems displayed qualitatively a similar improvement in creep resistance, supporting the hypothesis that creep is strongly influenced by segregation. Differences in primary creep behavior and activation energy for steady-state creep were, however, observed for these systems. Given these results, it is expected that creep behavior can be further optimized by adjusting the dopant level and by controlling the microstructure.     

Tensile Creep Behavior of a Vitreous-Bonded Aluminum Oxide under Static and Cyclic Loading

Creep deformation and rupture behavior of a vitreousbonded aluminum oxide was investigated under uniaxial static and cyclic tensile loadings at 1000°, 1100°, and 1175°C. The material was more creep resistant, i.e., having lower creep strain rates, under cyclic loading compared to that under static loading. For the same maximum applied stress, the ratio of steady-state creep rate under static loading to that under cyclic loading at 1100°C was approximately 100. However, the value of this ratio decreased to about 10 when the testing temperature was raised to 1175°C or lowered to 1000°C. Under static loading the material had more propensity to develop creep damage in the form of micro- and macrocracks, leading to early failure, whereas under cyclic loading the creep damage was more uniformly distributed in the form of cavities confined to the multigrain junctions. Viscous bridging by the grain boundary second phase may be the primary contributor to the lower creep deformation rate and improved lifetime under cyclic loading.     

Synthesis of Aluminum Oxide Platelets

Aqueous solutions of boehmite and hydrofluoric acid (HF) were used to prepare homogeneous mixtures of alumina and aluminum fluoride. Calcination at temperatures as low as 1000°C resulted in the formation of well-defined hexagonal-shaped α-alumina platelets. Containment of the aluminum fluoride by covering the calcination crucible promoted crystal growth presumably by a reaction of continuous evaporation–condensation of aluminum fluoride. Hexagonal-shaped platelet α-alumina was observed with average diameters ranging from 7 to 33 μm. Large platelets with a narrow size distribution and average diameter of over 25 μm were prepared by controlling the initial concentration of HF and the calcination time, temperature, and atmosphere.     

Hot Pressing of Aluminum Oxide

The hot-pressing characteristics of two aluminum oxides were studied in order to acquire a better understanding of the densification process and to develop a method of predicting the effects of experimental conditions other than those actually tested. A rapid hot-pressing technique was developed which greatly reduced the time element. The rate equation proposed by Murray, Livey, and Williams for hot-pressing ceramics was found to hold for the two aluminas, thus making it possible to predict the effects of changes in hotpressing conditions. The equation used was     . Viscosity, η, was calculated as a function of temperature using this equation and the experimental hot-pressing results. Values varied from 1.2 × 10¹² poises at 1300°C. to 4.6 × 10¹⁰ at 1600°C.     

Low-Temperature Sintering of Aluminum Oxide

A fine-sized (∼0.1 μm), agglomerate-free Al₂O₃ dispersion was used to prepare homogeneous green bodies with ∼69% relative density and ∼10-nm median pore radius. Samples could be sintered at 1150°C to a relative density >99.5% and an average grain size of 0.25 μm.     

Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.     

Investigation of Hot Tensile Behavior of Silicon Carbide and Magnesium Oxide Reinforced Aluminum Matrix Composites

Silicon - The aim of the current study is to investigate the mechanical behaviour of aluminium alloy and reinforced with Nano-particle composites such as magnesium oxide and silicon carbide via...     

Aluminum Oxide-Titanium Oxide Solid Solution

An oxide of titanium, apparently Ti₂O₃, showed solid solubility in Al₂O₃ of 1.0, 1.8, and 2.5 mole % at 1400°, 1600°, and 1700°C, respectively, when heated in hydrogen with a dew point of about -50°C. At the same temperatures in air, however, no indication of solid solubility (of TiO₂, presumably) was found by X-ray diffraction. It is concluded, therefore, that titania effects densification and grain growth of alumina by grain-boundary action rather than by a defect mechanism in the corundum lattice resulting from the substitutional solution of Ti⁴⁺ ions.     

Undercooled Melt Formation and Shaping of Alumina–Yttrium Aluminum Garnet Eutectic Ceramics

Undercooled melt was produced in the alumina-rich portion of the Al₂O₃–Y₂O₃ system, heating the metastable eutectic structure to above the metastable eutectic temperature but below the equilibrium eutectic temperature, followed immediately by solidification along the equilibrium path in the undercooled melt. Coupling the melting and solidification enabled a nearly adiabatic transformation from the metastable eutectic structure to the equilibrium eutectic structure. The eutectic structure produced through the solidification was uniform and fine throughout the castings. A fin-type casting with a thickness of 300 μm was successfully produced. This paper proposes a novel casting process using the undercooled melt formation.     

Optimization of Synthesis of Aluminum Hydroxides and Highly-Disperse Aluminum Oxide

The results of producing highly-disperse cubic aluminum oxide by the chemical precipitation method from aluminum hydroxide solutions with subsequent dehydration are described.     

Feedback-Controlled Firing of Reaction-Bonded Aluminum Oxide

The reaction-bonded aluminum oxide (RBAO) process utilizes the oxidation of intensely milled aluminum/alumina powder compacts that are heat treated in air to make alumina-based ceramics. RBAO samples are typically oxidized in a furnace which is heated at 1^°C/min to 1100^°C. Heat-treating samples with a characteristic dimension >1 mm, without adjusting the furnace temperature program, usually results in a cracked ceramic. Cracking is caused by the excessive thermal and chemical stresses that result from steep temperature gradients (>30^°C/mm) and compositional gradients (>5000 mol·(m³·mm)⁻¹), which develop under the deleterious ignition and shrinking core reaction regimes. While adjustments to the furnace temperature program based on continuum models have had some success, the use of feedback-controlled firing is investigated as a means to avoid the furnace temperature program design step and to decrease the firing time. Feedback-controlled firing is shown to improve yields and significantly reduce the time required to completely oxidize the aluminum. For example, a 16 g sample with a characteristic dimension of 7.56 mm, which previously took >100 h to oxidize completely, was successfully oxidized crack free in 18.3 h using feedback control. Using the typical heat-treatment cycle, a 1 mm sample was fired in 18 h. With feedback-controlled firing, the same sized sample was fired in only 5 h.     

纳米氧化铝刹车油特性

纳米氧化铝刹车油(Brake nanofluid)由本校自行设计的电浆放电纳米制造系统生产,电浆电弧放电所产生的高温将铝钯材瞬间汽化产生纳米颗粒氧化铝,以压力差将汽化金属吸入冷却液(Dot3),而纳米氧化铝会均匀融入冷却液而得纳米氧化铝刹车油.对所生产出的纳米悬浮氧化铝颗粒大小进行检测分析,经由透射电子显微镜(TEM),X光衍射仪来鉴定所生产纳米悬浮液外观以及组成成分,并对刹车油共沸点及粘度特性作具体探讨;生产的纳米氧化铝粒粒径约为30 nm,平均粒径也可达到50 nm左右,而且纳米氧化铝流体颗粒外形呈现圆形度,分布均匀,能提供刹车系统较低起始摩擦力,同时提升刹车油共沸点及粘度性,能增加刹车安全性.     

氧化铝中钯的测定

试验了氧化铝为载体的含钯催化剂中钯的AAS测定条件,方法具有良好的精密度,分析结果准确。