Key Engineering Materials Vols. 512-515

Abstract: 2024Al and SiC/2024Al were used to encapsulate the AD95 Al₂O₃ ceramics by Vacuum-high Pressure Infiltration Method. Their anti-bullet properties are not good with Efm of 5.17 and 4.65 respectively, and is far lower than that of the ceramic without encapsulation. The latter one can generate higher hydrostatic pressure between the bullet and ceramic during penetration process. However, in encapsulated targets, the ductile materials in front of the ceramic can decelerate the bullet, so the pressure between bullet and ceramic is not great enough to break the bullet. Propagated microcracks along with the bonding interface were observed in plates consisting of encapsulated Al₂O₃. These microcraks lead to the interfacial debonding between Al₂O₃ and encapsulating materials. Consequently, the anti-bullet property of the ceramic encapsulated by metal or composite is depressed. Further more, the different interface bonding conditions and fracture modes cause the discrepancy of anti-bullet property in two kinds of targets with encapsulation structure.

Abstract: To improve the thermal conductivity of alumina ceramics, different inorganic materials such as AlN, BN, Si3N4 and SiC which own high thermal conductivity were integrated with 96% alumina ceramic. The effect of addition of AlN, BN, Si3N4 and SiC on water absorption, flexural strength and thermal conductivity of the as-prepared alumina-based composite ceramics were investigated. Field emission scanning electron microscopy (FESEM) and X-ray diffractometer (XRD) were employed to characterize the microstructure and phase compositions of the composites. The results showed that sintering temperature of 96% alumina ceramic matrixes were improved in varying degree due to adding AlN, BN, SiC and Si3N4. A small quantity of Al6Si2O13 crystals were formed in the case of adding Si3N4 and SiC, which is due to the oxidation of Si3N4 and SiC into SiO2 in the air and the subsequent reaction with alumina. At appropriate firing temperature, the flexural strength of the composites with the addition of AlN is the best and higher than corundum matrixes. Meanwhile, the thermal conductivity of the composites with AlN has 110% improvement. It is obvious that AlN with high thermal conductivity is suitable for improving the thermal conductivity of corundum ceramics.

Abstract: Alumina (Al₂O₃) and Al₂O₃-based ceramics are candidate wear-resistant materials for many applications. In the past decades, the friction and wear behaviors of Al₂O₃ and Al₂O₃-based ceramics in dry sliding and lubricated conditions have been extensively investigated. In dry sliding, surface modification of Al₂O₃ and Al₂O₃-based ceramics at tribo-interface can be observed. In other words, the original surfaces of Al₂O₃ and Al₂O₃-based ceramics can be modified by friction and friction-induced material interaction (fracture, mechanical mixing, tribo-sintering) at the tribo-interface, and hence give birth to a tribo-layer. The formation and evolution of the tribo-layer on the worn surfaces of Al₂O₃ and Al₂O₃-based ceramics are the keys to understand the friction and wear behaviors of Al₂O₃ and Al₂O₃-based ceramics. This is conducted by scanning electron microscopy (SEM) observation and classification of the morphology and composition of the worn surfaces of Al₂O₃ and Al₂O₃-based ceramics at different sliding stages. The typical characteristics of the tribo-layers on the worn surfaces of Al₂O₃ and Al₂O₃-based ceramics in sliding against Si₃N₄ in a reciprocating motion are smooth surface with many interlinked cracks in morphology, mechanical mixing in chemical composition. The condition allowing the formation of the tribo-layer is briefly discussed.

Abstract: Al₂O₃ materials were used in a very wide range due to its good mechanical properties and relative low manufacture cost. The corrosion resistance of Al₂O₃ materials in acid, alkaline and sea water solutions gained more and more attention because many application situations are severe and the working life is shorten due to the corrosion. In this paper, the corrosion behavior of Al₂O₃ based materials in acid and alkaline solutions was studied and effect of corrosion time on the corrosion behavior of Al₂O₃ materials was investigated. The microstructure of as prepared Al₂O₃ materials was characterized by SEM. Possible corrosion process and mechanism was discussed in details. The results reveal that the as prepared Al₂O₃ materials show better corrosion resistance in alkaline solution than in acid solutions. In both acid and alkaline conditions, the corrosion mainly occurs in the grain boundary. Mass loss increased with increasing corrosion time, while the corrosion rate was decreased.

Abstract: Corrosion resistance of solid state (SSS) and liquid phase sintered (LPS) alumina under hydrothermal conditions (subcritical water and sodium chloride solution at temperatures up to 290 °C, and pressures up to 7 MPa) was evaluated. The influence of sintering additives as well as the conditions of the test (temperature, corrosion medium) on dissolution of ceramics was studied. For evaluation of the corrosion mechanisms special attention was paid to determination of the eluate chemistry combined with the chemical and phase analysis of corroded surfaces. Corrosion of LPS alumina in both media was controlled by dissolution of calcium aluminosilicate grain boundary glass. Corrosion rates of SSS ceramics were several times lower and were largely controlled by dissolution of highly resistant alumina matrix. Corrosion of the ceramics was moderately faster in sodium chloride solution. The data are supplemented by calculations of phase equilibria in the corrosion solutions.

Abstract: The study of this paper is La and Ba(La⁄Ba=1:1) doped modifying alumina. And we focus on the need for influence on content(La+Ba) and calcination temperature on the S.A of modified alumina. Factors considered were the S.A, it’s attenuation and phase in calcined etc. Screening out the sample of better comprehensive performance. At the same time the less content(La+Ba) can reduce the manufacturing cost of the catalyst.

Abstract: In this paper, carbon nanotubes (CNT)/Fe powders were synthesized by a simple catalytic pyrolysis routine and the Fe-CNT/Al2O3 bulk composites were fabricated by hot-pressing sintering. The CNT/Fe powders were characterized by X-ray diffraction (XRD) and TGA-DSC. The dielectric properties of the Fe-CNT/Al2O3 bulk composites were measured by using Agilent E4991A impedance analyzer from 100MHz to 1GHz. Both real part and imaginary part of the permittivity increased as added CNT into the composites. The imaginary part of permittivity (ε'') ranged from 137 to 1300 with the frequency, which indicated a good dielectric loss properties.

Abstract: The traditional method of preparation alumina insulation material includes the addition of pore-forming agent, direct foaming, foam impregnation and gel-casting. In this experiment, α-alumina as raw material, silica fume as an additive, Combination freeze-drying method, add pore-forming agent and direct foaming successfully prepared low-density, high strength, low thermal conductivity of alumina insulation material. Change the particle size of pore-forming agent can be get different properties of the sample. The SEM photograph was clearly observed that the hole wall dense uniform, α-alumina particles sufficient contact, no significant ice sublimation hole left. There are also the reasons of the sample with higher value of bending strength and compressive strength. This can make a control of porosity, as well as pore size, pore shape and pores space topology of alumina insulation material.

Abstract: As automotive catalyst support, alumina thermal stability is a importent problem of catalyst. This paper focus on the need for influence on calcination temperature to the pore structure and phase of modified alumina. Screen better modified alumina raw materials by characterization and analysis of the S.A, T.P.V and A.P.D.

Abstract: A Coprecipitation Method Was Applied to Synthesize Al₂O₃/GdAlO₃ Compound Powder, Using Ammonia as the Precipitator. Gadolinium Oxide and Aluminium Nitrate Were Used as the Raw Materials with the Eutectic Ratio( 77 mol% Al ³⁺ – 23 mol% Gd ³⁺ ). the Precursor Was Calcined at Different Temperatures from 1200 to 1600 °C. the Phase Identifications at Different Temperatures Were Characterized by X-ray Diffractometry (XRD). the Growth Morphology of Particles Were Investigated Using Field Emission Electro Microscopy (FE-SEM). the Results Reveal that GdAlO₃ Crystallized Earlier than α-Al₂O₃. the Diffraction Peaks of α-Al₂O₃ Phase Were Observed after Calcination at 1300°C for 1 H. Metastable Phase Gd₃Al₅O₁₂ Underwent Complete Decomposition at 1600°C for 1 H. Gadolinium Aluminate and α-Al₂O₃ Showed Different Growth Mechanism during the Calcination Process. the Average Grain Size of the Calcined Powder Increased from ~40 to ~900 Nm as the Calcination Temperature Increased from 1200 to 1600 °C.