Uses of α-Fe2O3 and fly ash as solid adsorbents

Solid adsorbents have shown great promise for control of particulate and non-particulate matter and as gas sensing devices in recent times. In the present study, adsorption of environmental toxic pollutant such as lead ions on solid adsorbents viz. α-Fe₂O₃ and fly ash, are reported. Considerable adsorption was observed on fly ash when compared to α-Fe₂O₃ surface. These studies are characterized by employing solid state and solution studies.     

Some thermodynamic aspects of the high-temperature transition in doped V2O3

Measurements of the entropy change are reported for the high-temperature metal-insulator (MI) transitions in the (V_1–xCr_x)₂O₃ and (V_1–xAl_x)₂O₃ systems. It is emphasized that the entropy of the I phase exceeds that of the M phase. Evidence is presented to show that the M and I phases coexist over a narrow temperature range. The transformation is attended by enormous hysteresis effects; these indicate that the lattice plays an important role in the transition. The probable role of Cr³⁺ and Al³⁺ as a dopant in the V₂O₃ lattice is briefly discussed. A phase diagram for the dilute V₂O₃-Al₂O₃ alloy system is presented.     

Apparent activation energy for densification of <Emphasis Type="Italic">α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powder at constant heating-rate sintering

The apparent activation energy for densification is a characteristic quantity that elucidates the fundamental diffusion mechanisms during the sintering process. Based on the Arrhenius theory, the activation energy for densification of α-Al₂O₃ at constant heating-rates sintering has been estimated. Sintering of α-Al₂O₃ powder has been executed by the way of a push rod type dilatometer. It is shown that the apparent activation energy does not have a single value but depends directly on the relative density. The apparent activation energy corresponding to lower relative density was higher than that corresponding to higher relative density. In addition, the value of the evaluated activation energy is different at the same density level when the Arrhenius plot involves different heating rates.     

Application of fracture mechanics tofailure analysis and to residual life assessment of components operating at high temperatures

Abstract

MCrAlY overlay coatings have been successfully used as a means of improving the oxidation performance of gas turbine blades operating at elevated temperatures. However, depletion of aluminium can limit the ability of such coatings to form a protective oxide layer should spallation of the original α-Al₂O₃ oxide layer occur under thermal cycling conditions. It is the objective of the current research to evaluate the potential of NiAl₃ as a reservoir phase for a NiCrAlY overlay coating on a IN738LC superalloy substrate at 1,100°C in air. The morphologies and microstructures of the conventional NiCrAlY and NiAl3-modified NiCrAlY overlay coatings in the as-sprayed and oxidised conditions were characterised using SEM, EDX and XRD techniques.     

Processing and properties of zirconia-toughened alumina ceramics

Al₂O₃:ZrO₂ ceramics have been prepared from physically mixed pure oxide powders. The results indicate that careful processing of the starting powders and a two-stage sintering process can avoid expensive processing methods like hot pressing/hot isostatic pressing used for achieving high densification. The mechanical properties were measured and the resultant microstructure studied to explain the toughening behaviour of this material.     

Effect of Ni-modified α-Al2O3 prepared by sol–gel and solvothermal methods on the characteristics and catalytic properties of Pd/α-Al2O3 catalysts

In the present study, Ni-modified α-Al₂O₃ with Ni/Al ratios of 0.3 and 0.5 were prepared by sol–gel and solvothermal method and then were impregnated with 0.3 wt.% Pd. Due to different crystallization mechanism of the two preparation methods used, addition of nickel during preparation of α-Al₂O₃ resulted in various species such as NiAl₂O₄, mixed phases between NiAl₂O₄ and α-Al₂O₃, and mixed phases between NiAl₂O₄ and NiO. As revealed by NH₃-temperature programmed desorption, formation of NiAl₂O₄ drastically reduced acidity of alumina, hence lower amounts of coke deposited during acetylene hydrogenation was found for the Ni-modified α-Al₂O₃ supported catalysts. For any given method, ethylene selectivity was improved in the order of Pd/Ni–Al₂O₃-0.5 > Pd/Ni–Al₂O₃-0.3 > Pd/Ni–Al₂O₃-0  Pd/α–Al₂O₃-commercial. When comparing the samples prepared by different techniques, the sol–gel-made samples showed better performances than the solvothermal-derived ones.     

非均相沉淀法制备纳米α-Al2O3/金属复合粉体

利用非均相沉淀法制备了平均粒径小于50 nm的α-Al₂O₃/W(Ni)复合粉体,并研究对比了纳米钨和镍对α-Al₂O₃相转变温度的影响。结果表明:纳米钨和镍的存在均可降低过渡型氧化铝向α型氧化铝转变的相变温度,但降低程度有所不同,含钨氧化铝于1000℃完成相变,而含镍氧化铝于1150℃完成相变;并且纳米第二相体积比越大,对相变温度的影响也越大。     

Non-protective alumina growth in sulfur-doped NiAl(Zr)

Abstract

The 1,200°C oxidation behavior of NiAl was examined at various levels of sulfur and zirconium dopants to test the possibility of a critical S/Zr ratio required for adhesion. Cyclic furnace testing for 200 1-h cycles and interrupted testing for 500 h were used as screening tests. Pure NiAl and NiAl(Zr) with 0.14 at. % Zr were chosen as model base compositions; they exhibited normal, slow-growing scales (3 mg/cm²) with excellent adhesion for the Zr-doped alloys. NiAl with about 120 ppma S exhibited a substantial weight loss (?20 mg/cm²) in cyclic tests and a very large weight gain (+60 mg/cm²) in interrupted tests. The major surface phase remained as α-Al₂O₃. Sulfur doping the NiAl(Zr) alloy caused massive weight gains of 80–100 mg/cm², swelling, cracking, and nearly complete conversion into NiAl₂O₄ and α-Al₂O₃. The initial objective of determining critical S/Zr ratios for adhesion was therefore unattainable. Initiation of the catastrophic attack was examined after a 10 h exposure, revealing a few sites of broad, raised, and cracked ridges. In cross-section, the ridges appeared as nodular intrusions, with a complex, fractal, oxide–metal interface. They were primarily alumina (with occasional entrapped islands of NiAl₂O₄ or pure Ni metal). They possessed a unique microstructure consisting of 0.3 μm lamellae, separated by 0.1 μm open channels. This allowed for rapid growth controlled by gaseous diffusion. The microstructure is discussed in terms of SO₂ evolution and a sulfur-driven depassivation process.     

γ-l2O3 纳米粉对氧化铝、碳化硅陶瓷纤维烧结特性的影响

研究了纳米γ-l₂O₃ 添加剂对氧化铝、碳化硅陶瓷纤维烧结特性和显微结构的影响。实验结果表明, 在纤维挤压成形过程中, 纳米γ-l₂O₃ 粉填充到微米氧化铝粉的孔隙之中, 提高了纤维的素坯密度, 当添加剂含量为40% 时, 纤维的素坯相对密度达到最大值。纳米γ-l₂O₃ 可以促进氧化铝和碳化硅纤维的烧结, 同时降低纤维的烧结温度, 提高纤维的密度。     

Vickers Indentation Fracture (VIF) modeling to analyze multi-cracking toughness of titania, alumina and zirconia plasma sprayed coatings

For massive brittle materials, the fracture toughness in mode I, K_IC, can be determined using various reliable techniques. Besides, Vickers Indentation Fracture (VIF) technique has been developed to locally determine fracture toughness. However, since the indentation test generates a complex three-dimensional crack system around the indent, fracture toughness, K_C, is calculated instead of K_IC. Consequently some authors rightly reject the VIF technique to determine standard fracture toughness by arguing that the literature counts numerous VIF crack equations thus revealing discrepancies of this technique. Nevertheless in some cases (e.g. brittle ceramic coatings) inclusive material techniques are not applicable since presence of the substrate and/or multi-crack network can modify the crack propagation into the coating.In this work, we employed VIF technique to study multi-cracking behavior of titania, alumina and zirconia ceramic oxide coatings obtained by plasma spraying. To calculate VIF toughness, we propose (i) to select two crack equations for radial-median and Palmqvist cracking modes respectively, (ii) to adjust the crack equation of Miranzo and Moya for intermediate cracking mode, (iii) to develop a mathematical approach to determine the cracking mode, (iv) to take into account the multi-crack network by defining an equivalent four-crack system and (v) to propose a universal crack equation applicable independently of the cracking mode.     

A new combustion route to γ-Fe2O3 synthesis

A new combustion route for the synthesis of γ-Fe ₂ O ₃ is reported by employing purified a-Fe ₂ O ₃ as a precursor in the present investigation. This synthesis which is similar to a self propagation combustion reaction, involves fewer steps, a shorter overall processing time, is a low energy reaction without the need of any explosives, and also the reaction is completed in a single step yielding magnetic iron oxide i.e. γ-Fe ₂ O ₃ .The as synthesized γ-Fe ₂ O ₃ is characterized employing thermal, XRD, SEM, magnetic hysteresis, and density measurements. The effect of ball-milling on magnetic properties is also presented.     

ZrO2在β"─Al2O3复合陶瓷中的作用

本文研究了四方和立方ZrO₂在与Na-β"-Al₂O₃的复合陶瓷中作为第二相所起的各种作用。结果说明ZrO₂对复合陶瓷具有细化晶粒、改善显微结构、提高强度和断裂韧性的作用,与此同时也在一定程度上降低离子电导率。在提高力学性能方面,四方ZrO₂除了具有与立方ZrO₂相同的弥散颗粒的作用外,相变所起的作用也是很显着的。ZrO₂-Na-β"-Al₂O₃复合陶瓷是一种有应用价值的固体电解质材料。     

Interatomic potential,phonon spectrum and molecular-dynamics simulation up to 1300K and 10 GPa in YBa2Cu3O7\t-\gd for δ=0 to 1for δ=0 to 1

An empirical interatomic potential for YBa₂Cu₃O_7−δ is determined for different oxygen contents (δ=0 to 1), consisting of Coulomb and short-range interactions. The calculated structure and phonon spectrum, and the results of molecular-dynamics computer simulation on the orthorhombic-to-tetragonal phase transition are in fair agreement with reported experiments.     

Epitaxial growth and relaxation of γ-Al2O3 on silicon

0.5-10 nm-thick single crystal γ-Al₂O₃ films was epitaxially grown, at high temperature, on Si(001) and Si(111) substrates using electron-beam evaporation techniques. Reflection High Energy Electron Diffraction studies showed that the Al₂O₃ films grow pseudomorphically on Si (100) up to thickness of 2 nm. For higher thicknesses, a cubic to hexagonal surface phase transition occurs. Epitaxial growth and relaxation were also observed for Si(111). The film surfaces are smooth and the oxide-Si interfaces are atomically abrupt without interfacial layers.     

Synthesis and characterization of γ-Fe2O3 — a magnetic tape material

Acicular FeC₂O₄ · 2H₂O was precipitated from glycerol and starch media. Thermal decomposition of this oxalate in dry and moist nitrogen yielded primarily FeO and Fe₃O₄ respectively. Characterization was attempted through DTA, TG, x-ray diffraction, TEM and magnetization studies. It was found that the oxalate can be completely decomposed to Fe₃O₄ in moist nitrogen (P_{H 2}O ∼ 35 torr) at 775 K and then oxidised by dry air to acicular γ-Fe₂O₃ at 575 K. The resulting material has saturation magnetization (∼ 70 emu/g), coercive field (∼ 300 Oe) and squareness ratio (∼ 0·60–0·65), which values art comparable with those of the commercial samples.     

Observation of broadband infrared luminescence in a novel Bi-doped P2O5-B2O3-Al2O3 glass

A novel Bi-doped P₂O₅-B₂O₃-Al₂O₃ glass was investigated, and strong broadband NIR (near infrared) luminescence was observed when the sample was excited by 445 nm, 532 nm, 808 nm and 980 nm lasers, respectively. The max FWHM with 312 nm, the lifetime with 580 μs and the σ_emτ product with 5.3 × 10^− 24 cm² s were obtained which indicates that this glass is a promising material for broadband optical amplifier and tunable laser. The effect of the introduction of B₂O₃ on the glass structure and Bi-ions illuminant mechanism were discussed and analyzed. It is suggested that the introduction of B₂O₃ makes the glass structure closer, and the broadband NIR emission derives from Bi⁰:²D_3/2 → ⁴S_3/2 and Bi⁺:³P₁ → ³P₀ transitions.     

Microstructural characterization in diffusion bonded TiC-Al2O3/Cr18-Ni8 joint with Ti interlayer

Ceramic matrix composite, TiC-Al₂O₂, and stainless steel, Cr18-Ni8, were joined at 1400 K by solid state diffusion bonding, making use of a Ti foil acting as thermal stress relief interlayer. The microstructure of the joint was thus formed. The diffusion bonded TiC-Al₂O₃/Cr18-Ni8 joint was investigated by a variety of characterization techniques such as scanning electron microscope (SEM) with energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD). The results indicate that Ti foil is fully fused to react with elements from substrates and Ti₃Al, TiC and α-Ti are formed in the diffusion bonded TiC-Al₂O₃/Cr18-Ni8 joint. The interfacial shear strength is up to 99 MPa and the shear fracture occurs close to the ceramic matrix composite due to the application of Ti foil acting as thermal stress relief interlayer.     

Y2O3表面改性Al2O3P增强6061Al复合材料组织与性能

采用液相包裹法对Al₂O₃微粉进行稀土Y₂O₃表面改性,用挤压铸造法制备表面经稀土Y₂O₃改性的Al₂O_3P/6061Al复合材料,并对复合材料的显微组织及拉伸性能进行分析和研究。结果表明:表面经稀土Y₂O₃改性的Al₂O₃微粉能均匀的分布于基体中,界面润湿性得以改善,复合材料组织更加均匀。TEM观察表明:改性粉体在制备复合材料前后表面存在颗粒状包裹层。对其表面进行EDAX分析,结果显示含有Y,Al和O元素。粉体XRD图谱中有Y₂O₃衍射峰的存在。拉伸性能测试表明:改性粉体对Al合金增强效果明显增加,抗拉强度提高29.8％,屈服强度提高38.4％,延伸率提高10.3％。对拉伸断口进行SEM分析,改性后复合材料断口韧窝更加均匀、丰满,材料表现出良好的塑性。     

Effects of TiO2 doping on microstructure and properties of directed laser deposition alumina/aluminum titanate composites

ABSTRACT

Al₂O₃-based composite ceramics have excellent high temperature performance and are ideal materials for preparing hot end components. However, poor fracture toughness and thermal shock resistance limit its applications. Based on the excellent low thermal expansion characteristics and thermal shock resistance of Al₂TiO₅ ceramic, different composition ratios of Al₂O₃/Al₂TiO₅ composite ceramics were prepared by directed laser deposition (DLD) technology. Effects of TiO₂ doping amount on microstructure and properties of the composite ceramics were investigated. Results show that α-Al₂O₃ phase is discretely distributed in the continuous aluminum titanate matrix when TiO₂ doping amount between 2 and 30?mol%. With the increase of TiO₂ doping amount, content of Al₂O₃ gradually decreases and its morphology changes from cellular to dendritic. When TiO₂ doping amount reaches 43.9?mol%, the microstructure transforms into fine Al₂TiO₅/Al₆Ti₂O₁₃ eutectic structure. Property test results show that Al₂O₃/Al₂TiO₅ composite ceramics have good comprehensive mechanical properties when TiO₂ doping amount between 2 and 6?mol%.