Synthesis of crystalline γ-Al2O3 with high purity

Crystalline γ-AlO(OH) was synthesized by the precipitation of sodium aluminate and oxalic acids in aqueous solution. And then γ-AlO(OH) was successfully transferred to γ-Al₂O₃ after subsequent high temperature heat treatment. The effects of reaction conditions on formation of γ-AlO(OH) and γ-Al₂O₃ were further investigated in detail. The XRD analysis shows that the complete formation of crystalline γ-Al₂O₃ is at pH 8–9, reaction temperature of 93–96 °C and calcination temperature of higher than 400 °C. The product of γ-Al₂O₃ contains impurity, including iron, calcium and silicon ion with a low content of about 0.01% and has large specific surface area and high pore volume of 269.9 m²/g and 0.57 mL/g, which can be applied in catalysts and catalyst supports.     

Effect of Y203 on microstructure and oxidation of γ-Ni＋γ′-Ni3Al coatings transformed from electrodeposited Ni-Al films at 1 000℃

The electrodeposited Y2O3-dispersed γ-Ni＋γ-Ni3Al coatings on Ni substrates were developed by the conversion of electrodeposited Ni-Al-Y2O3 films with dispersed AI microparticles in Ni matrix into Ni3Al by vacuum annealing at 800 ℃ for 3 h. For comparison, Y2O3-free γ-Ni＋γ＇-Ni3Al coatings with a similar AI content were also prepared by vacuum annealing the electrodeposited microparticle-dispersed composite coatings of Ni-AI under the same condition. SEM and TEM characterizations show that the electrodeposited Y2O3-dispersed γ＋γ＇ coatings exhibit finer grains, a more homogeneous distribution of γ＇, and a narrowed γ＇ phase spacing compared with the electrodeposited Y2O3-free γ-γ＇ coatings. The oxidation at 1 000 ~C shows that the addition of Y2O3 significantly improves the oxidation resistance of the electrodeposited γ＋γ＇coatings. The effect of Y2O3 particles on the microstructure and oxidation behavior of the electrodeposited γ＋y＇ coatings was discussed in detail.     

Partial dislocation configurations in a low-angle boundary in α-Al2O3

The dislocation structures of a low-angle tilt grain boundary in alumina bicrystal were investigated by transmission electron microscopy. The grain boundary was found to consist of two regions: an area with pairs of partial dislocations and an area with groups of odd numbered partial dislocations (multiple-partial-structure). Eight kinds of multiple-partial-structures were found in the fabricated grain boundary. The Burgers vectors of each partial dislocation in the grain boundary can be distinguished by dark-field imaging, and thus the arrangement of partial dislocations in the multiple-partial-structures are determined. It is concluded that a slight twist component of the boundary is the origin of the characteristic multiple-partial-structures.     

Microstructure and properties of CVD γ-Al2O3 coatings

This work deals with the microstructures and wear properties of chemical vapour deposited γ-Al₂O₃. The γ-Al₂O₃ coatings were deposited at 800 °C on TiN and Ti(C,N) pre-coated cemented carbide substrates. The microstructures developed in the γ-Al₂O₃ coatings and the influence of the nucleation surface on the growth of γ-Al₂O₃ were characterised using transmission electron microscopy, electron energy-loss spectroscopy and X-ray diffraction. The γ-Al₂O₃ coatings were fine-grained with a high density of {1 1 1} growth twins and contained some residual sulphur. γ-Al₂O₃ was found to grow epitaxially on the investigated substrates. The mechanical properties were evaluated in metal cutting and were compared with those of κ-Al₂O₃ coated tools. As compared with the κ-Al₂O₃ coatings, the γ-Al₂O₃ coatings exhibited slightly worse adhesion and tendency for edge chipping. However, the γ-Al₂O₃ coatings showed better crater wear resistance on the rake face than κ-Al₂O₃ coatings.     

Influence of an aluminized intermediate layer on the adhesion of a γ-Al2O3 washcoat on FeCrAl

To improve the combination performance of a γ-Al₂O₃ washcoat on a FeCrAl support in auto-catalysts, an aluminized intermediate layer was deposited on the surface of metallic foils by the powder embedded aluminizing technique. A heat treatment after aluminizing and an aging test after washcoating were carried out. The microstructure, phase structure, chemical composition and inter-coat adhesion of the as-coated and heat-treated aluminized layers were studied by means of scanning electron microscopy, X-ray diffraction (XRD), energy dispersive X-ray spectrum (EDX) and ultrasonic vibration tests. The surface of the as-coated aluminized layer was inhomogeneous and a great number of nanowires were formed. Aluminum infiltrated into the support to form Al₅(Fe,Cr)₂ and/or other intermetallic compounds in the coat-support interface. After the heat treatment, aluminum segregated out and oxidized into homogeneous -Al₂O₃ whiskers on the outer surface. The aluminized layer transformed into -Fe(Cr,Al) diffusion zone after the aging test. After the aluminizing and heat treatments, no cracks were found on the successive alumina washcoat and the weight loss was only 4.0 wt.% under ultrasonic vibration for 30 min. This suggested an enhanced interface adhesion between the γ-Al₂O₃ washcoat and the FeCrAl support.     

Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings

The influence of nucleation on the microstructure and properties of CVD Al₂O₃ was investigated. The experimental α-Al₂O₃ layers were deposited (a) without nucleation control and (b) with nucleation steps resulting in pronounced , and growth textures. The experimental layers were characterised using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Wear properties of the textured coatings were evaluated in turning. The chemistry of the nucleation surface appeared to be an important factor in pre-determining the phase content and growth textures of the Al₂O₃ layers. Optimised nucleation resulted in substantially improved wear properties and these kinds of α-Al₂O₃ layers were typically composed of relatively small, defect-free grains exhibiting no porosity. The textured α-Al₂O₃ layer showed the best wear resistance.     

Growth of β-Ga2O3 nanocolumns crossing perpendicularly each other on MgO (1 0 0) surface

β-Ga₂O₃ nanocolumns straightened and crossed perpendicularly each other were deposited on MgO (1 0 0) substrate by vapor phase transport method. Growth of the nanocolumns was examined at steps of 1000, 1050, and 1200 °C in elevation of source-boat temperature. We have drawn out the substrate from deposition-tube at each source-boat temperatures of 1000, 1050, and 1200 °C. Scanning electron microscopy of the sample with source-boat temperature of 1200 °C demonstrated that the straightened and elongated nanocolumns are crossing perpendicularly each other. Typical lengths of the nanocolumns were in the range of several hundreds nanometers below 1050 °C, and those of 1200 °C were in the range of ten to fifteen hundreds nanometers. Diameters of the nanocolumns stayed in the range of few hundreds nanometers, notwithstanding variation of the source temperature. X-ray diffraction and transmission electron microscopy with energy dispersive X-ray spectroscopy revealed that the nanocolumns are monoclinic β-Ga₂O₃ crystal, and the (4 0 0) plane of β-Ga₂O₃ nanocolumns is parallel to the (1 0 0) plane of MgO substrate.     

Subsolidus phase relations in the ZnO–MoO3–B2O3, ZnO–MoO3–WO3 and ZnO–WO3–B2O3 ternary systems

The subsolidus phase relations in the ZnO–MoO₃–B₂O₃, ZnO–MoO₃–WO₃ and ZnO–WO₃–B₂O₃ ternary systems have been investigated by the means of X-ray powder diffraction (XRD). There is no ternary compound in all the systems. There are five binary compounds and five tie lines in the ZnO–MoO₃–B₂O₃ system. This system can be divided into six 3-phase regions. There are three binary compounds and three tie lines in the ZnO–MoO₃–WO₃ system. This system can be divided into four 3-phase regions. There are four binary compounds and four tie lines in the ZnO–WO₃–B₂O₃ system. This system can be divided into five 3-phase regions. The possible component regions for ZnO single crystal flux growth were discussed. The phase diagram of Zn₃B₂O₆–ZnWO₄ pseudo-binary system has been constructed, and the result reveals this system is eutectic system. The eutectic temperature is 1007 °C and eutectic point component is 70 mol% Zn₃B₂O₆.     

Thermochemistry of GaBO3 and phase equilibria in the Ga2O3–B2O3 system

Employing a Tian-Calvet-type calorimeter operating in the scanning mode at temperatures from 1120 to 1220 K, the enthalpy change, Δ_dH, associated with the decomposition of GaBO₃ (=1/2β-Ga₂O₃+1/2B₂O₃(liq.)) and the corresponding decomposition temperature, T_d, were determined: Δ_dH=30.34±0.6 kJ/mol, T_d=1190±5 K. Using the transposed-temperature-drop method the thermal enthalpy, H(T)−H(295 K), of GaBO₃ was measured as a function of temperature, T, in the region from 760 to 1610 K; the results obtained are

[H(T)−H(295 K)]/(J/mol)=104.8·(T/K)−31 300 (760 K<T<1190 K),

[H(T)−H(295 K)]/(J/mol)=138.8·(T/K)−41 480 (1190 K<T<1590 K).

On the basis of the experimental results, the enthalpy and entropy of formation, Δ_fH and Δ_fS, respectively, of GaBO₃ from the component oxides were derived:

Δ_fH=−30.34 kJ/mol,Δ_fS=−25.50 J/(K·mol) at 1190 K,

Δ_fH=−10.55 kJ/mol,Δ_fS=−5.48 J/(K·mol) at 298 K.

The enthalpy versus temperature curve shows, apart from a step associated with the decomposition of GaBO₃, a further step at 1593 K which is attributed to a monotectic equilibrium.     

Preparation and properties of sintered molybdenum doped with La2O3/MoSi2

Sintered Mo with the addition of La₂O₃/MoSi₂ was prepared via the process of solid–solid doping + powder metallurgy. X-ray diffraction experiment, hardness test, three-point bending test and high-temperature tensile test were carried out to characterize the samples. The XRD pattern of a typical sample shows that the sintered Mo was mainly composed of Mo, La₂O₃ and Mo₅Si₃. Mo₅Si₃ was probably formed through the reaction between MoSi₂ and the Mo matrix. Densities and fracture toughnesses of both doped Mo and pure Mo were measured and contrasted. Sintered Mo with the addition of 0.2 wt% La₂O₃/MoSi₂ has the highest toughness, while more addition of La₂O₃/MoSi₂ has smaller effect on improving toughness or even embrittles Mo. The results of three-point bending test and high-temperature tensile test show that the bending strength and high-temperature tensile strength of doped Mo are both higher than those of pure Mo. The formation of Mo₅Si₃ improves the high-temperature strength. The La₂O₃/Mo₅Si₃ dispersed in the Mo matrix refined the grains, and thus strengthened the Mo matrix by dispersion strengthening and grain refinement.     

Oxygen partial pressure dependence of memory effect of sputtered nc-Al/α-Al2O3 thin films

Nanocrystalline aluminum embedded in amorphous dielectric alumina matrix thin films (nc-Al/α-Al₂O₃) was synthesized via reactive magnetron sputtering. The nc-Al/α-Al₂O₃ films at different oxygen partial pressures were sputtered on p-type Si substrates from a pure Al target in the mixed ambient of Ar and O₂. Both deposition rate and aluminum concentration increase as the oxygen partial pressure decreases. X-ray photoelectron spectroscopy and high-resolution transmission electron microscope studies give the confirmation of nanocrystalline Al embedded in amorphous Al₂O₃ matrix. This nanocomposite thin film exhibits memory effect as a result of charge trapping. The flat band voltage value depends on the Al nanocrystal concentration which is related to oxygen partial pressure.     

Microstructural changes in particles detected during the transformation from β-FeOOH to α-Fe2O3 in dense aqueous suspensions

The hydrothermal transformation from β-FeOOH to α-Fe₂O₃ in dense aqueous suspensions, obtained by partial neutralization of concentrated FeCl₃ solution with concentrated NaOH solution, was investigated. Mössbauer spectroscopy was used for the phase analysis of samples, as well as the complementary techniques XRD and FT-IR. The size and morphology of the particles were inspected with FE-SEM. At the beginning of the crystallization process very fine β-FeOOH particles were formed, which transformed to α-Fe₂O₃ (end-product) with a prolonged time of heating. A small amount or traces of α-FeOOH as an intermediate phase were detected by FT-IR and FE-SEM. Gradual formation of α-Fe₂O₃ double spheres with ring was monitored. Double spheres with ring were formed by the aggregation mechanism. The orientation effect of α-Fe₂O₃ subunits in double spheres with ring was observed. α-Fe₂O₃ double spheres with ring also showed two narrow particle size distributions.     

The effect of BaZrO3 addition on melt-processed YBa2Cu3O7–x superconductors

The effects of BaZrO₃ addition on the thermal behavior and microstructure of YBCO superconductors have been investigated. The differential thermal analysis indicates that the peritectic decomposition temperature of YBCO precursor powder is not change when BaZrO₃ is doped. The solidification temperature of YBCO changes with increasing of BaZrO₃ addition, thus the window of solidification temperature is affected. The results of scanning electron microscopy show that the BaZrO₃ particles are accumulated along the growth boundary, which result in the constitutional segregation of YBCO. The pushing–trapping theory is used to explain the phenomenon of segregation in the YBCO matrix.     

Microstructural investigation of heat-treated CVD κ-Al2O3 multilayer coatings

In this work, the κ-Al₂O₃ to -Al₂O₃ phase transformation in CVD multilayer coatings has been investigated. Coatings with different degrees of transformation have been prepared by isothermal heat-treatments of as-deposited κ-Al₂O₃ multilayer coatings. These coatings have then been examined by means of transmission electron microscopy of cross-section specimens. The aim of this work was to examine the microstructural evolution during the κ-Al₂O₃ to -Al₂O₃ phase transformation and to describe how the transformation growth is affected by the κ-Al₂O₃ microstructure. The microstructure of the non-transformed κ-Al₂O₃ was not affected by the heat-treatments. During the phase transformation, a linkage of voids took place at the -Al₂O₃/TiC interfaces, resulting in large separated voids and continuous cracks. A large number of small faceted voids appeared within the -Al₂O₃ grains. It was found that during the transformation c_κ → c, i.e. the close packed planes are preserved, but rotated slightly around the c-axis. Three modes for the κ-Al₂O₃ to -Al₂O₃ phase transformation were found: (i) a columnar transformation; (ii) a lateral transformation; and (iii) a mixed columnar-lateral transformation. The predominance of any of the transformation modes is attributed to the nucleation/growth ratio. A high ratio yields a columnar transformation, while a low ratio yields a lateral transformation. The nucleation sites in all transformation modes are suggested to be at voids at the κ-Al₂O₃/TiC interfaces.     

Growth and microstructural stability of epitaxial Al films on (0001) α-Al2O3 substrates

Al films were grown epitaxially on single-crystal α-Al₂O₃ substrates by magnetron sputtering and molecular beam epitaxy, respectively. The microstructure and thermal stability of these films were analysed in detail using X-ray diffraction methods and electron microscopy techniques. The films consist of two twin-related growth variants, related by a 180° rotation around the <111> film normal resulting in a {111} Al || (0001) α-Al₂O₃, and ± < > Al || < > α-Al₂O₃ orientation relationship. The Al variants are separated by Σ3 { } Al twin boundaries possessing a rigid body translation of the {111} Al planes across the boundary plane in order to reduce their energy. Motion of the twin boundaries was observed by annealing plan-view samples in situ in a transmission electron microscope. The twin boundaries advance in jerky motion at velocities of several μm/s at temperatures of ˜400 °C, resulting in grain coarsening. In all cases, heat treatments resulted in increased area fraction of one twin variant, which finally will result in single-crystal films upon further annealing.     

Preparation and properties of multifunctional Fe@C@Y2O3:Eu nanocomposites

Multifunctional Fe@C@Y₂O₃:Eu³⁺ nanocomposites were prepared by the solvo thermal method, and their structure, magnetic and luminescent properties were characterized by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and scanning electron microscope (SEM). Results show that the nanocomposites are spherical with a mean diameter of 700 nm and there are high special saturation magnetization (47.4 emu/g) and strong red emission under UV-light. Even dispersed in water solution, the nanocomposites also exhibit a strong red emission under ultraviolet light radiation, and it could be manipulated using an external magnet. Thus it looks promising for application in biomedicine field, especially in drug targeting and fluorescence label. And we also discussed the effect of the electron transfer process between the Fe magnetic core and Y₂O₃:Eu³⁺ shell.     

Processing and properties of ZrO2(3Y2O3)–Al2O3 nanocomposites

Nanocomposites of yttria-stabilized zirconia (YSZ), containing 20 and 40 wt% alumina, were prepared by a two-step process: (1) fine-particle aggregates of the constituent phases were melted and homogenized in a high enthalpy plasma, prior to rapid quenching in water to obtain metastable starting powders, and (2) the metastable powders were consolidated by hot isostatic pressing (HIP), under conditions designed to ensure the formation of nanocomposites by controlling the metastable-to-stable phase transformation during sintering. In both cases, the resulting nanocomposites had completely uniform structures, comprising 27 and 50 vol% of -Al₂O₃ in a tetragonal YSZ matrix phase. Measurements of hardness and indentation toughness were correlated with observed structures.     

Melt processed YBa2Cu3O7−x superconductors

The superconducting YBa₂Cu₃O_7−x samples were prepared by an Arc-Cast-Annealing (ACA) and Arc-Quench-Powder-Growth (AQPG) processes as modifications of QMG and MPMG techniques. Pe'lets of YBa₇Cu₃O_7−x were quenched by arc-casting in a water cooled copper mould and then the solidified rods were annealed at different temperatures and times to store the superconductivity. Annealed at an appropriate temperature the cast rods showed rising superconducting properties with increasing the annealing time. Some of the rods after solidification were crushed to give powder which was compacted and then subjected to a melt growth process. As a result of this processing, large grained textured YBCO superconductors with dispersed 211 inclusions in the superconducting grains were produced. The microstructure and physical properties of these ACA and AQPG samples were investigated when subject to various temperature cycles. It was found that the volume fraction and size distribution of the second phase inclusions were dependent upon the maximum temperature during the melt growth process. The critical current density (J_c) for ACA and AQPG samples was estimated from magnetization loops using Bean's critical state model. It was found that the value of J_c of AQPG sample was much higher than that of ACA sample.     

载体中ZrO2/Al2O3比对Pt/ZrO2-Al2O3催化剂性能的影响

相比汽油车而言,柴油车具有高效、低油耗的优势已得到广泛应用。本实验以ZrO₂作为改性剂,探究了ZrO₂与Al₂O₃的质量比对催化剂的影响。研究结果表明：随着ZrO₂的加入,Pt粒子先减小后增大;Pt粒子与载体的交互作用先增大后减小。活性实验数据分析表明,ZrO₂的最佳添加量为40 wt%,CO和C₃H₆完全氧化温度分别降低20 _oC 、25 _oC。贵金属在催化剂的分散度以及贵金属与载体的相互作用随着ZrO₂与Al₂O₃质量比的变化而变化。Pt粒子越小,其与载体的交互作用越强,这表明催化剂性能越强。     

High temperature oxidation resistance of FeCrAl alloys covered with ceramic SiO2–Al2O3 coatings deposited by sol–gel method

One-, three- and five-layer SiO₂–Al₂O₃ coatings were deposited on a FeCrAl alloy basis by the sol–gel method. Sols in which the molar ratio of tetraethoxysilan to aluminium tri-sec-butoxide was 1:1 and 1:3 were used.As the samples were being soaked at T = 1200 °C for t = 700 h the mass of the samples increased. Thermal shock (T = 1200 °C, 10,000 cycles) causes greater degradation of the surface than soaking at a constant temperature. The XPS and EDS results show that the composition of the top layer of the coatings changes during high temperature oxidation and thermal shock. The outward movement of aluminium cations results in surface enrichment with aluminium, particularly for the single-layer coatings. The measured energies of bonds Si 2p and Al 2p in the multilayer coatings indicate that a structure of aluminosilicates with a composition between that of mullite and that of sillimanite forms during sintering.