Nanoparticles α-Al2O3 prepared by combustion synthesis method

Nanometer α-Al2O3 powders were synthesized by the method of low-temperature combustion synthesis （LCS） with aluminum nitrate nonahydrate and urea as raw materials. The prepared powders were studied by XRD, TG-DTA, FT-IR and TEM. It is found that the average size of particles is 60 -80 nm. The optimal synthetic conditions are obtained, i.e. , the suitable fuel is urea; the molar ratio of oxidizer to fuel is 1 ： 2 and the igniting temperature is 700℃. The results show that the size of particles is governed by synthesizing temperature, the fuel and the molar ratio of oxidizer to fuel. TEM image of the particles collected shows that the crystal habits of particles have a spheric structure and particles are polycrystal.     

Wetting of (0001) α-Al2O3 single crystals by molten Al

The wetting behavior of (0 0 0 1) α-Al₂O₃ by molten Al was studied over a wide temperature range between 700 and 1500 °C. The increase in the contact angle with time at temperatures lower than 1200 °C is attributed to the surface structural reconstruction of the (0 0 0 1) α-Al₂O₃. High-temperature annealing of the substrate does not have a significant influence on the wettability.     

Oxidation of γ-TiAl Intermetallic Coated with Electrodeposited Nickel-La2O3 Composite Films

A coating of nickel withLa₂O₃ particles waselectrodeposited on the surface of -TiAl toimprove its high-temperature oxidation resistance. Theexperimental results showed that contrary to theoxidation of bare specimens, the electrodepositedNi-La₂O₃ composite-coated onesexhibited much better resistance to both isothermal andcyclic oxidation in air at 1000 and 900°C. For theoxidation of the Ni-La₂O₃ composite-coated -TiAl EPMA/EDX microanalyses revealed that after the composite film had beenthoroughly oxidized, a La₂O₃-richNiO layer close to the gas-scale interface and twoalumina-rich layers, with one beneath the NiO layer and the otheradjacent to the -TiAl matrix, were produced inthe scale. The results indicated that the preferentiallyformed La₂O₃-doped NiO layerretarded the growth of rutile and favored the formation of twoalumina-rich layers. The mechanism of the effect of theelectrodeposited Ni-La₂O₃composite on the oxidation behavior of -TiAl isdiscussed in detail.     

Isothermal Oxidation of α2-Ti3Al

The isothermal oxidation behavior of binary Ti–25 at.% Al was studiedat 1073, 1173, and 1273 K in air and oxygen with emphasis on themicrostructure of the subsurface zone underneath the external oxidescale. Thermogravimetric analysis, acoustic-emission (AE) analysis, lightmicroscopy, scanning electron microscopy (SEM), atomic force microscopy(AFM), and cross-sectional transmission electron microscopy (TEM) werecarried out. Three layers could be identified in the subsurface regionconsisting of an internal oxidation zone of -Al₂O₃,and -Ti, a ternary phase with the empirical formulaTi–21Al–15O (at.%), and an oxygen penetration zone consisting of₂-Ti₃Al(O).     

Influence of AlF3 and hydrothermal conditions on morphologies of α-Al2O3

Homogeneous α-Al2O3 platelets were synthesized by introducing AlF3 to alumina precursor.The effects of AlF3 additive on the phase transformation and morphology of the prepared α-Al2O3 platelets were investigated.The results show that a single phase of α-Al2O3 with an average particle size of 8μm can be obtained at 900℃ with 2% AlF3 additive.The transformation temperature decreasing IS attributed to introduction of Al3 vacancy and to the formation of intermediate compound of AlOF,which is considered to accelerate the mass transportation from transitional Al2O3 to α-Al2O3.AlF3 concentration and hydrothermal temperature can also affect the morphology of α-Al2O3.When hydrothermal temperature is 120℃,the morphology of α-Al2O3 transforms from irregular to flat hexangular platelet with increasing AlF3 concentration.As hydrothermal temperature increases,the morphology of α-Al2O3 with 2% AlF3 additive changes from polyhedron to hexangular platelet and then to vermicular.     

Influence of AlF3 and hydrothermal conditions on morphologies of α-Al2O3

Homogeneous α-Al2O3 platelets were synthesized by introducing AlF3 to alumina precursor.The effects of AlF3 additive on the phase transformation and morphology of the prepared α-Al2O3 platelets were investigated.The results show that a single phase of α-Al2O3 with an average particle size of 8μm can be obtained at 900℃ with 2% AlF3 additive.The transformation temperature decreasing IS attributed to introduction of Al3 vacancy and to the formation of intermediate compound of AlOF,which is considered to accelerate the mass transportation from transitional Al2O3 to α-Al2O3.AlF3 concentration and hydrothermal temperature can also affect the morphology of α-Al2O3.When hydrothermal temperature is 120℃,the morphology of α-Al2O3 transforms from irregular to flat hexangular platelet with increasing AlF3 concentration.As hydrothermal temperature increases,the morphology of α-Al2O3 with 2% AlF3 additive changes from polyhedron to hexangular platelet and then to vermicular.     

The formation of α-Al2O3 scales at 1500°C

The growth of Al₂O₃ scales on -NiAl was studied at 1500°C. Oxidation rates, diffusion mechanisms, and microstructures were examined in order to achieve a complete understanding of the scale development. Variation of the Al content within the phase field had little effect on the oxidation behavior. Ionimplanted yttrium (2×10¹⁶/cm²) was observed to provide a short-term improvement in scale adhesion but little long-term effect. When doped with Y or Zr, the first 1 m of -Al₂O₃ was observed to grow mainly by an inward oxygen growth mechanism. At longer times when the implant was ineffective, microstructural observations indicate a mixed-growth mode.     

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al₂O₃–ZrO₂–Y₂O₃ composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al₂O₃ particles and Al₂O₃–Y₂O₃ sol. Their microstructure showed that YSZ particles were covered with spatial-network Al₂O₃–Y₂O₃ blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al₂O₃–Y₂O₃ or ZrO₂–Y₂O₃ coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al₂O₃–Y₂O₃, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.     

18O/SIMS characterization of the growth mechanism of doped and undoped α-Al2O3

Sequential oxidation experiments at 1200°C and 1500°C using¹⁶O and >95%¹⁸O-enriched environments were conducted on undoped and Y- and Zr-doped -NiAl and FeCrAl alloys. After oxidation, samples were analyzed by SIMS sputter depth profiling. At 1200°C, a clear pattern was established where the undoped -Al₂O₃ was found to grow by the simultaneous transport of both Al and O. Zr-doped -Al₂O₃ was found to grow mainly by the inward transport of oxygen. The profiles in all cases indicate O diffusion primarily by shortcircuit pathways. Results at 1500°C (only on -NiAl) indicated a similar behavior but were less conclusive. Y and Zr were found to segregate to the oxide grain boundaries at 1200°C and 1500°C. The segregation of these dopants is believed to impede the cation transport in the -Al₂O₃ scale and thereby change the oxidation mechanism.     

The Morphology of Thermally Grown α-Al2O3 Scales on Fe-Cr-Al Alloys

The morphology and growth kinetics of protective-Al₂O₃ scales formed duringhigh-temperature oxidation of the Fe-Cr-Al andFe-Cr-Al-Y alloys were studied. Scanning electronmicroscopy observations were focused on the oxidewrinkling phenomenonand formation of interfacial cavities in the course ofoxidation of the yttrium-free alloy. In this paper,emphasis is also placed on the effects of surface preparation and yttrium additions on the scaleand scalealloy interface structures. Several features ofalumina morphology, which have not received sufficientattention previously, are discussed in the context of existing models; some criticalissues are outlined.     

Preparation and formation mechanism of Al2O3 nanoparticles by reverse microemulsion

Al2O3 nanoparticles were prepared by polyethylene glycol octylphenyl ether（Triton X-100）/n-butyl alcohol/cyclohexane/ water W/O reverse microemulsion. The proper calcination temperature was determined at 1 150 ℃ by thermal analysis of the precursor products. The structures and morphologies of Al2O3 nanoparticles were characterized by X-ray diffraction, transmission electron microscopy and UV-Vis spectra. The influences of mole ratio of water to surfactant on the morphologies and the sizes of the Al2O3 nanoparticles were studied. With the increase of surfactant content, the particles size becomes larger. The agglomeration of nanoparticles was solved successfully. And the formation mechanisms of Al2O3 nanoparticles in the reverse microemulsion were also discussed.     

Preparation, Characterization and Photocatalytic Property of Cubic α-Fe2O3 Nanoparticles

设计了一种简单的水热合成法合成晶型可控的立方结构的α-Fe2O3,通过扫描电子显微镜,透射电子显微镜,和X射线衍射对所制备产物的结构和形态特征进行了分析。制备的立方结构的α-Fe2O3的尺寸范围在130~150 nm之间。研究结果表明,α-Fe2O3纳米材料作为一种有效的光催化剂,在过氧化氢存在的条件下可以用来光催化降解废水中的有机物（如罗丹明B）,这为设计和开发高效率可见光催化剂在去除工业废水中有机染料的应用提供了新的探索和基础。     

Preparation and Characterization of LiTaO3 Films Derived by an Improved Sol-Gel Process

In this paper, an improved sol-gel method was suggested to obtain high-concentration LiTaO3 precursor solution for simplified experimental conditions and thicker films, by mixing lithium acetate and tantalum ethoxide in a 1, 2-Propylene glycol solution. Compared to traditional methods, the process was done without weak acidic solution and absolute dry experimental condition. Results of a comparative study of LiTaO3 thin films derived by the improved sol-gel process and a traditional process using 2-methoxy ethanol as solvent were presented. Nano-crystalline LiTaO3 films with rhombohedral structures were formed in both methods after annealing at 650 for 5 min. The thickness of each LiTaO3 layer coated onto the substrate increased from 25 nm to 110 nm when 2-methoxy ethanol was replaced by 1, 2-Propylene glycol. LiTaO3 films with a stronger preferential orientation were obtained in 1, 2-Propylene glycol due to its higher boiling point and slower volatilization rate. On the other hand, the diffraction peak intensity of LiTaO3 thin films prepared using 1, 2-Propylene glycol was weaker than that of the films prepared using 2-methoxy ethanol due to decreased times of annealing.     

Oxidation of Al2O3-dispersion chromizing coating by pack-cementation at 800℃

Preparation and oxidation of an Al2O3-dispersed chromizing coating were investigated by chromizing an aselectrodeposited Ni-Al2O3 nanocomposite film using a conventional pack-cementation method at a greatly decreased temperature (800℃).For comparison,chromizing was also performed with the same condition on an as-deposited Ni film without Al2O3 nanoparticles.Oxidation at 900℃ indicates that,compared with the Al2O3-free chromizing coating,the Al2O3-dispersed chromizing coating exhibits a increased oxidation resistance,due to the formation of purer and denser chromia scale.The effect of Al2O3 on the coating formation and the coating oxidation behavior was discussed in details.     

Preparation of Y2O3：Eu^3＋ phosphor by molten salt assisted method

A kind of fine and quasi-spherical Y2O3：Eu^3＋ phosphor was prepared by firing a preparative precursor at 1 200 ℃ for 2 h with the molten salts of Na2CO3, S and NaCl. The precursor was obtained by homogeneous precipitation of yttrium and europium with oxalic acid when using EDTA, citric acid or starch as complexant. The structure and morphology of the phosphors were characterized by XRD and SEM, respectively. The influence of complexing environment, firing temperature and molten salts on formation of the phosphor Y2O3：Eu^3＋ was discussed. The result show that the prepared Y2O3：Eu^3＋ phosphor is of quasi-spherical structure with size of 2-3 μm. Its luminescent intensity is 30% higher than that of the same phosphor prepared by the same procedure but without molten salts, and is 5% higher than that of commercial Y2O3：Eu^3＋ red phosphor.     

Establishment and Breakdown of α-Al2O3 Scales on Ni-Al Alloys at High Temperatures

Abstract

The isothermal oxidation behaviour of Ni-7% Al and Ni-12·5% Al in 1atm oxygen at 800, 1000 and 1200°c has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis and scanning electron microscopy. The ease of formation of an initially complete, protective, external α-Al₂O₃ scale is greater the higher the alloy aluminium content and the higher the temperature. Once developed, this external α-Al₂O₃ scale tends to fail mechanically in localised regions, the subsequent oxidation behaviour depending upon the composition of the exposed alloy. Under conditions favouring breakaway, Ni-7% Al yields rapid oxidation rates as NiO-rich nodules are formed on the exposed alloy substrate, although the rate declines later as a healing α-Al₂O₃ layer develops at their bases. If the α-Al₂O₃ fails on Ni-12·5% Al, the aluminium content of the revealed alloy is still high enough to ensure rapidproduction of a new external α-Al₂O₃ layer.     

微米γ-Al2O3高压相变制备亚微米α-Al2O3

采用微米γ-Al₂O₃为原料,利用国产铰链式六面顶压机,在5 GPa压强下,经不同的温度和保温时间,实现了由γ-Al₂O₃向α-Al₂O₃的相转变。X射线衍射分析结果表明,得到的粉体和块体材料中均仅含α-Al₂O₃单相;扫描电镜结果显示合成的α-Al₂O₃最小晶粒尺寸为80 nm。在5 GPa压强、800 ℃条件下合成的样品平均晶粒尺寸为340 nm,实现了用微米γ-Al₂O₃高压相变制备亚微米α-Al₂O₃。与常压烧结方法相比,采用高温高压烧结,可在600 ℃实现γ-Al₂O₃向α-Al₂O₃的转变,显著降低α-Al₂O₃材料的合成温度。      

The Effect of Yttrium on the Growth Process and Microstructure of α-Al2O3 on FeCrAl

The oxidation behavior of undoped and Y-dopedhigh-purity FeCrAl at 1200°C was investigated. Thescale-growth processes were studied using two-stage¹⁶O/¹⁸O experiments andhigh-resolution imaging SIMS. SEM, TEM, and scanning transmission electronmicroscopy-electron-dispersive X-ray spectrometry(STEM-EDS) studies were performed in order to determinethe microstructure of the oxide and the chemistry of the metal-oxide interface. An equiaxed-Al₂O₃ scale was found togrow by simultaneous outward Al and inward oxygendiffusion on the undoped FeCrAl. In contrast, a columnar-Al₂O₃ grain structure on the Y-doped FeCrAl was produced by inwardoxygen diffusion; outward Al diffusion was suppressed.Y was shown to segregate to-Al₂O₃ grain boundaries andto the metal-oxide interface.