Mechanical and dielectric properties of Ni/Al2O3 composites

Ni/Al2O3 composites were prepared by hot pressing approach. The relationship between their microstructure, mechanical, dielectric and magnetic properties with Ni particle content was studied. By increasing the amount of metal in the composite, the relative density and the bending strength decrease gradually. The possible reason is that non-wetting between Ni and alumina in the preparation results in weak adhesion of the Ni/A; interface. For the composites, the maximum fracture toughness is 6.4 MPa. m^1/2, which is about 25% higher than that of pure alumina ceramic. The increase in toughness of the Ni/Al2O3 composites is due to the deformation of nickel particles. The complex dielectric constant measurements indicate that the real part and the imaginary part increase greatly with the Ni content in the frequency range of 8.2-12.4 GHz. The real part and the imaginary part of complex permeability of the composites also increase with increasing Ni content.     

Mechanical properties of polytypoidally joined Si3N4–Al2O3

Crack-free joining of alumina and silicon nitride has been achieved by a unique approach introducing sialon polytypoids as a functionally graded materials (FGM) bonding layer. The polytypoid compositions are identified in the phase diagram of the Si₃N₄–Al₂O₃ system. The thermal stresses of this FGM junction were analyzed using a finite element analysis program taking into account both coefficient of thermal expansion and modulus variations. From this analysis, the result showed a dramatic decrease in radial, axial and hoop stresses as the FGM changes from three layers to 20 graded layers. Scaling was considered, showing that the graded transition layer should constitute about 75% or more of the total sample thickness to reach a minimal residual stress. Oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were minimally or not deflected at the sialon layers, implying strong interfaces. Finally, flexural testing was conducted at room temperature and at high temperature. The average strength at room temperature was found to be 581 MPa and the average strength at high temperature (1200°C) was found to be 262 MPa. Scanning electron microscope observation of fracture surfaces at a different loading rates indicated that the strength loss at higher temperatures was consistent with a softening of glassy materials present at grain junctions.     

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基于分形理论,建立了Al2O3凝聚态夹杂物分形碰撞凝聚长大新数学模型,并利用Fluent软件将此新数学模型应用于某钢铁公司冶炼铝镇静钢的中间包中,从夹杂物的分布特征角度比较了新数学模型与传统等体积球状数学模型的差异。结果表明:与传统等体积球状数学模型相比,Al2O3凝聚态夹杂物分形碰撞凝聚长大新数学模型的计算结果数值较大,且夹杂物粒子分布特征复杂;新数学模型显示出了夹杂物粒子容易在出水口和入水口富集的特征,这与实际生产中Al2O3夹杂物容易堵塞水口现象一致;实际生产中很难找到球形Al2O3夹杂物,大多是簇群状形貌夹杂物,分形维数在1.78~1.85,表明Al2O3夹杂物分形碰撞凝聚长大新数学模型具有一定的现实意义。     

Solidification and remelting of Al through Al2O3 fibrous preform under centrifugal force

The solidification and remelting of molten aluminum through a porous preform under centrifugal force field were modeled numerically. The results show that the transient solidification and remelting phenomena appear on the infiltration fi-ont and can be divided into two distinct regions： the remelting region and solid-liquid congruent melting region. The decrease of porosity always results in the increase of moving velocity difference between the infiltration front and the remelting fi＇ont, which leads to the increase of the solid-liquid congruent region extent. But for the decrease of the rotational fi＇equency, the difference of moving velocity between infiltration front and remelting fi＇ont decreases, which leads to the decrease of regional extent. The infiltration fi＇ont moving velocity is mainly influenced by the centrifugal infiltration pressure, whereas the remelting fi＇ont moving velocity is mainly influenced by the material thermodynamics. The transient solidification and remelting phenomena are the intercoupling results between the centrifugal infiltration dynamics and the material thermodynamics.     

Al2O3·SiO2/Zn-RE复合材料的界面及凝固组织

在扫描电镜上观察和研究了Al2 O3 ·SiO2 /Zn RE复合材料的凝固组织及其界面。结果表明 ,在复合材料中 ,Al2 O3 ·SiO2 纤维与基体间存在致密界面层 ,适当的化学反应可改善纤维与基体的结合 ;在凝固过程中 ,Al2 O3 ·SiO2 纤维可作为锌合金共晶体非自发形核的衬底 ,纤维 /基体界面上的硅在共晶体的共生生长过程中起了领先相作用 ,导致复合材料的共晶转变由铝硅共晶转变和锌铝共晶转变两者组成     

Aging behavior of Al2O3 short fiber reinforced Al-Cu alloy composites

Al2O3 short fiber reinforced AI-Cu composites containing 1%, 3%, 5% and 7% Cu were fabricated by a squeeze casting technique. The as-cast Al2O3/Al-Cu composites were solution treated at 535 ℃ and then aged at 170, 190 and 210 ℃, respectively. Age hardening behavior of the Al2O3/Al-Cu composites was analyzed by measuring the hardness of the samples at different aging temperatures and aging time. Microstructures of the composites were observed by transmission electron microscope（TEM）. The results indicate that the hardness of the Al2O3/Al-Cu composites containing 7% Cu is much higher than that containing 1%-5% Cu because of the large amount of CuAl2 precipitant in the Al2O3/Al-Cu composite. With the increase of Cu content from 1% to 7%, the time needed for the appearance of peak hardness shortened, indicating that the addition of Cu can accelerate the kinetic of CuAl2 precipitation in the Al2O3/Al-Cu composites. The Al2O3/Al-Cu composite containing 7% Cu shows the highest increment of hardness by aging treatment. Therefore, in order to get a higher peak hardness, the Al2O3/Al-Cu composites need more Cu addition as compared with the un-reinforced Al-Cu alloys.     

20CaO·13Al2O3·3MgO·3SiO2的合成与氧化铝的浸出性能

使用分析纯试剂配料,在1 500 ℃、保温1 h的条件下得到了四元化合物20CaO·13Al2O3·3MgO·3SiO2 (C20A13M3S3),研究了其氧化铝浸出性能,并通过XRD和SEM等分析了其在碳酸钠溶液中的作用机理.结果表明:C20A13M3S3具有一定的氧化铝浸出能力,其浸出率随着浸出时间的延长而提高,并在浸出2 h后达到最大值68.87%,低于同条件下12CaO·7Al2O3的氧化铝浸出率(92.78%);C20A13M3S3和Na2CO3反应的主要产物为NaAl(OH)4和CaCO3,并含有少量的Ca2SiO4和Mg(OH)2;生成的Ca2SiO4具有较高的活性,浸出2 h后,其分解率可达到19.35%.     

3Al2O3·2SiO2/A1-3.5Cu复合材料界面研究

用挤压铸造方法制备硅酸铝短纤维增强Al-3.5Cu复合材料;用XRD、SEM和分析透射电镜(ATEM)对该复合材料的微观组织、增强纤维的微观结构、复合材料界面及界面反应产物的结构特征、化学组成进行研究.结果表明,增强纤维为莫来石结构,晶型为正交晶系;该复合材料界面存在断续分布的CuAl2O4尖晶石结构的界面反应产物,界面附近存在高密度位错结构.     

The Microstructure and Mechanical Behavior Evaluation of Ce-TZP／Al2O3 Nanocomposites

Ce-TZP/Al2O3 nanocomposites were prepared from mixture of self-made Ce-TZP nanometer powder and commercial Al2O3 powder by hot-pressing method. Influences of nanometer ZrO2 particles on the mechanical properties and microstructure of Ce-TZP/Al2O3 ceramics were investigated. Meanwhile, t→m transformation toughening mechanism was investigated by x-ray diffractometry (XRD) method, and deflection of samples under applied stress were recorded too. The results show that when the percentage of ZrO2 was 20%, the mechanical properties and microstructures of materials were preferential. Additionally, TEM observation show that dislocation structures form both in the Al2O3 grain and on the Al2O3 grain boundary.     

SiO2与液态Al反应生成Al2O3/Al复合材料的研究

采用SiO2粉与液态Al反应制备Al2O3／Al复合材料，讨论了SiO2的加入量、反应温度、反应时间对反应速度的影响，分析了不同反应温度和保温时间下生成复合材料的微观结构。试验证明：当SiO2含量较低时，SiO2与Al发生完全反应，形成均匀Al2O3／Al复合层；当ω(SiO2)达一定量时，反应速度反而降低；保温时间为6～8h，反应速度最快，之后变慢。     

A Comparative Study of High Temperature Corrosion of Al2O3, SiO2 and Al2O3–SiO2 Forming Alloys in a Na2SO4–NaCl Atmosphere

The present paper is designed to provide a summary of our study on the high temperature corrosion of Al₂O₃, SiO₂ and Al₂O₃–SiO₂ forming alloys in the gas phase and liquid phase of Na₂SO₄–NaCl system by comparing their corrosion resistance at 1000 °C. The obtained results show that the alumina-forming alloy experiences severe internal corrosion in the gas phase compared to in the liquid phase due to oxide cracking. This results in an increase in the inward diffusion and/or penetration of constituents of the salts and oxygen to form internal Al-oxide and Cr-sulfides. In the liquid phase, however, the formation of yttrium sulfide beneath a continuous double oxides layer of Al₅Y₃O₁₂ and Al₂O₃ may be related to the high affinity of yttrium for sulfur. On the other hand, it is apparent from the cross-sectional observations that a SiO₂ and Al₂O₃–SiO₂ forming alloys form a continuous and dense oxides layer, and demonstrate a high resistance against internal oxidation and corrosion in both corrosive environments.     

Hydrophilic property of SiO2-TiO2 overlayer films and TiO2／SiO2 mixing films

The photo-induced hydrophilicity of SiO2 overlayer on TiO2 films prepared by sol-gel method was investigated by means of soak angle measurement, XPS, UV-VIS and FTIR spectra. The results show that, compared with the TiO2 film without SiO2 overlayer, when the TiO2 film is thoroughly covered by SiO2 overlayer, the hydrophilicity and the sustained effect are enhanced. It is found that the significant growth of the OH- group occurs in the surface of SiO2 overlayer. The different mechanism of enhanced hydrophilicity between SiO2 overlayer on TiO2 films and TiO2/SiO2 mixing films was analyzed. The result suggests that the photo-generated electrons created in the interface between TiO2 and SiO2 tend to reduce the Ti(Ⅳ) cation to the Ti( Ⅲ ) state, and the photogenerated holes transmit through the SiO2 layer to uppermost surface efficiently. Once the holes go up to the surface， they tend to make the surface hydrophilic. The stable hydrophilicity of SiO2 overlayer which adsorbs more stable OH groups, enhances the sustained effect, i.e. the super-hydrophilic state can be maintained for a long time in dark place.     

Mechanical Properties and the Microstructure of Al2O3/Al/Al2O3 Joints with the Surface Modification of Alumina by a Thin Layer of Ti + Nb

Al₂O₃/Al/Al₂O₃ joints were formed by liquid-state bonding of alumina substrates covered with a thin Ti + Nb coating of 900 nm thickness with the use of an Al interlayer of 30 μm at 973 K under a vacuum of 0.2 mPa for 5 min. The bond strength of the joints was examined by a four-point bending test at 295, 373, and 473 K. Optical, scanning, and transmission electron microscopies were applied for detailed characterization of the interface structure and failure characteristics of fractured joint surfaces. The analysis of the results has shown that (i) bonding occurred due to the formation of a reactive interface on the metal side of the joint in the presence of Al₃Nb(Ti) precipitates and (ii) modification of Al₂O₃ by a thin layer of Ti + Nb increases the hardness at the interface and makes it possible to achieve reliable joints working at elevated temperatures.     

Al2O3对等离子喷涂Cr2O3/TiO2/Al2O3/SiO2复合陶瓷涂层性能影响研究

目的研究Al_2O_3添加量对Cr_2O_3/TiO_2/Al_2O_3/SiO_2四元复合陶瓷涂层性能的影响。方法采用等离子喷涂技术在油气管道X80管线钢基体表面制备出具有不同Al_2O_3含量的四元复合陶瓷涂层。另外,为探究基体温度对涂层性能的影响,所有涂层均在等离子喷枪预热及室温的两种基体上制备。所制涂层的气孔率、硬度、结合力及电化学腐蚀性能分别采用煮沸称重法、维氏硬度计、划痕仪、电化学工作站进行检测,并用X射线衍射仪(XRD)、扫描电镜(SEM)分析不同Al_2O_3含量涂层的物相组成和形貌特征,研究Al_2O_3含量对涂层各性能的影响。结果随着Al_2O_3含量的增加,Cr_2O_3/TiO_2/Al_2O_3/SiO_2四元复合陶瓷涂层的气孔率呈现先降低后增加的趋势,相对应的四元复合陶瓷涂层的结合力、维氏硬度则先增加后降低。当Al_2O_3质量分数为60%时,四元复合陶瓷涂层的性能最优,气孔率为3.6%,硬度为824.6HV,结合力为53.8N。电化学腐蚀测试表明,Al_2O_3能增强涂层的耐腐蚀性能,Al_2O_3质量分数为60%时,涂层自腐蚀电位最高,为-0.28 V。另外,在基体预热和不预热条件下,所制涂层性能随Al_2O_3含量的变化一致,但是基体预热比不预热更有利于涂层性能的提高。结论 Al_2O_3的添加不仅能够有效降低涂层Cr含量,还能显著提升四元复合陶瓷涂层的各项性能,特别是耐腐蚀性。此外,等离子喷涂前对基体进行预热,有利于涂层性能提高。     

Phase reaction and sintering behavior of a Al2O3–20wt%AlN–5wt%Y2O3 system

Two major problems exist in the processing of AlN. The first is the difficulty in achieving full densification even at relatively high sintering temperatures. The second is the formation of the spinel phase, AlON. Pure AlN sintered at temperatures up to 2000°C have produced no more than 90–93% densification in the former case, while AlN rich ternary systems (AlN–Al₂O₃-sintering agent) have resulted in the detrimental formation of AlON well before full densification can occur. This paper reports on the phase reaction and sintering behavior of a ternary Al₂O₃–AlN–Y₂O₃ system near the critical temperature range of 1600–1700°C, in a carbo-thermal reduction furnace in a fully nitrogen environment. Full densification (>98%) for AlN without the formation of AlON was achieved by sintering an initial Al₂O₃ rich ternary system (Al₂O₃–20wt%AlN–5wt% Y₂O₃) at a relatively low temperature of 1680°C. Formation of the AlON was delayed until 1700°C, at which a stoichiometric γ-AlON (Al₃O₃N) with spinel type structure was obtained. Thermal conductivity values for a sintered substrate obtained with low oxygen content within the AlN matrix reached 125 W m⁻¹ K⁻¹.     

泡沫镍负载TiO2和TiO2/3Al2O3·2SiO2薄膜的光催化性能

以泡沫镍为载体,3Al2O3·2SiO2作为过渡中间层,用溶胶-凝胶法在泡沫镍上负载锐钛矿相的TiO2薄膜,制成泡沫金属基的TiO2和TiO2/3Al2O2·2SiO2光催化材料,利用TG-DSC、XRD和FE-SEM等测试手段对其进行表征,用乙醛气体的光催化降解测试其光催化活性.研究表明泡沫镍负载的TiO2和TiO2/3Al2O3·2SiO2薄膜均具有良好的光催化活性,其中,由于负载的3Al2O3·2SiO2过渡中间层增大了载体的比表面积,使负载光催化剂的活性位大大增加,因此,TiO2/3Al2O3·2SiO2薄膜的光催化活性较单一的TiO2薄膜有非常显著的提高.     

Mechanical and wear properties of Mo5Si3–Mo3Si–Al2O3 composites

Mo₅Si₃ and Mo₅Si₃–Mo₃Si–Al₂O₃ composite were synthesized use MoO₃, Mo, Si and Al as raw materials by mechanically induced self propagating reaction and then consolidated by hot-pressing. The microstructure of the materials was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS). The effects of the Al₂O₃ on the mechanical and tribological properties of Mo₅Si₃–Mo₃Si–Al₂O₃ composite have been studied. It was found that benefits associated with the addition of the Al₂O₃ to Mo₅Si₃ and Mo₃Si include finer microstructure, higher strength, higher fracture toughness and higher hardness. The dry sliding wear properties of the composite were investigated using against GCr15 bearing steel in ball-on-disk system at room temperature. The results indicated that the friction coefficients and specific wear rates of Mo₅Si₃–Mo₃Si–Al₂O₃ composite were significantly reduced by the addition of Al₂O₃, and its specific wear rates decreased by an order of magnitude compare with the monophase Mo₅Si₃. The friction coefficients of test materials decrease with an increasing load. The dominant wear mechanism of the composites was interpreted by several different wear models involving plastic deformation, adhesion, brittle fracture and reaction to form a tribo-oxidation layer.     

Effects of Mg content on aging behavior of sub-micron Al2O3p/Al-Cu-Mg composites

30%Al2O3p/Al-Cu-2.0Mg composite and Al2O3p/Al-Cu-2.5Mg composite with 0.3 μm-Al2O3 particles were fabricated. Age-hardening behaviors of two composites and the related matrix alloys were studied by means of Brinell-hardness measurement, DSC and TEM. The results show that the hardness of the composite is improved obviously because of the addition of sub-micron Al2O3 particles. But the hardness increment of Al2O3p/Al composite after aging is lower than that of the related matrix alloy. Moreover, the formation of GP region is suppressed by the addition of sub-micron Al2O3 particles, which broadens the exothermic peak of S＇ phase. The increment of Mg content has a different influence on accelerating the aging processes of aluminum alloys and the composites, and the hardness also increases.     

Preparation of Al2O3–ZrO2–SiO2 ceramic composites by high-gravity combustion synthesis

By a furnace-free technique of high-gravity combustion synthesis, Al₂O₃–ZrO₂–SiO₂ ceramic composites were prepared via melt solidification instead of conventional powder sintering. The solidification kinetics and microstructure evolution of the ceramic composites in high-gravity combustion synthesis were discussed. The phase assemblage of the ceramic composites depended on the chemical composition, where both (Al₂O₃ + ZrO₂) and (mullite + ZrO₂) composites were obtained. The ceramic composites consisted of ultrafine eutectics and sometimes also large primary crystals. In the (mullite + ZrO₂) composites, two different morphologies and orientations were observed for the primary mullite crystals, and the volume fraction of mullite increased with increasing SiO₂ content. The ceramic composites exhibited a hardness of 11.2–14.8 GPa, depending on the chemical composition and phase assemblage.     

The influence of BaO additives on the reaction of Al2O3–SiO2 ceramics with molten Al and Al–Si alloys

BaO considerably affects the composition and the microstructure of the reaction zone formed between BaO-doped aluminosilicate ceramics and molten aluminium alloys under vacuum. The reduced Ba and Si form AlBaSi precipitates, found adhered to the interface and dispersed in the metal Al-matrix, whose formation apparently controls the reaction kinetics.