Magnetic properties of mechanochemically synthesized γ-Fe2O3 nanoparticles

The synthesis of mono-dispersed γ-Fe₂O₃ nanoparticles by mechanochemical processing was demonstrated for the first time, via the solid-state exchange reaction Fe₂(SO₄)₃ + 3Na₂CO₃ → Fe₂(CO₃)₃ + 3Na₂SO₄ → Fe₂O₃ + 3Na₂SO₄ + 3CO₂(g) and subsequent heat treatment at 673 K. The nanoparticles had a volume-weighted mean diameter of 6 nm and a narrow size distribution with the standard deviation of 3 nm. The particles showed a superparamagnetic nature with the superparamagnetic blocking temperature of 56.6 K. The anisotropy constant was 6.0 × 10⁶ erg/cm³, two orders of magnitude larger than the magnetocrystalline anisotropy constant of bulk γ-Fe₂O₃. The detailed analysis of the magnetic properties indicated that the γ-Fe₂O₃ nanoparticles had a core-shell structure, consisting of a ferrimagnetic core of ∼4 nm in diameter having a collinear spin configuration and a magnetically disordered shell of ∼1.2 nm in thickness.     

Synthesis and characterization of nanosized urchin-like α-Fe2O3 and Fe3O4: Microwave electromagnetic and absorbing properties

Urchin-like α-Fe₂O₃ and Fe₃O₄ nanostructures were prepared from the precursor urchin-like α-FeOOH under reducing atmosphere. The dependence of reduction temperature on their morphology, microstructure, and microwave electromagnetic and absorbing characteristics were systematically studied. It is found that the reduction temperature plays an important role in the microstructure and electromagnetic characteristics of the resulting products. In present study, the urchin-like α-Fe₂O₃ with dual absorption peaks can be formed at the relatively low temperature (e.g. 300 °C). Urchin-like Fe₃O₄ can be obtained just at 350-400 °C, which presents excellent microwave absorption property, with the minimum reflection loss of −29.96 dB and below −20 dB in 3.76-8.15 GHz corresponding to 3-4 mm thickness. The excellent microwave-absorption properties are a consequence of a proper electromagnetic matching and enhanced absorbing abilities resulting from the urchin-like shape and inverse spinel-type crystal structure.     

Thermal stability of γ-Al2O3 coatings for challenging cutting operations

Crystalline PVD Al₂O₃ coatings offer great potential for their use in cutting operations. They promise high hot hardness and high oxidation resistance at elevated temperatures. Alumina exists in different crystallographic phases. α-Al₂O₃ appears to be the only thermodynamically stable phase at all common temperatures and pressures. Today there are many efforts to generate α-Al₂O₃ by means of physical vapour deposition. In this regard one problem is the high deposition temperature, which does not allow the deposition on temperature-sensitive materials.Another promising candidate is γ-Al₂O₃ which is more fine-grained than α-Al₂O₃ and can be deposited at lower temperatures. At high temperatures γ-Al₂O₃ might be transformed into α-Al₂O₃, which could limit the application temperature. But until now it is not clearly proved, up to which temperatures γ-Al₂O₃ thin films are stable and which mechanisms influence the stability. In the present work different (Ti,Al)N/γ-Al₂O₃ coatings are deposited on cemented carbides by means of Magnetron Sputter Ion Plating (MSIP). The (Ti,Al)N bond coat was employed to improve adhesion of γ-Al₂O₃ on the substrate. It could be shown that the γ-phase is stable in vacuum up to 1200 °C. In the atmosphere the formation of α-Al₂O₃ begins at 900 °C and it is influenced by the choice of transition zone between the (Ti,Al)N interlayer and γ-Al₂O₃. The results show that the thermal stability of the γ-phase and therefore the application temperature of the coating can be enhanced by the choice of interlayer.     

In-situ formed γ-Al2O3 nanocrystals repaired and toughened Al2O3 coating prepared by plasma spraying

During process of plasma spraying of alumina with the stable α phase in the as-received powder, metastable γ phase tends to form in the as-deposited coating. In this paper, plasma-sprayed Al₂O₃ coating was investigated using transmission electron microscopy. In situ formation of γ-Al₂O₃ nanocrystals was observed in the pre-existing cracks of the as-sprayed Al₂O₃ coating, which contributes to self-repairing and self-toughening of the Al₂O₃ coating. Except for the predominant intergranular fracture, the pre-formed γ-Al₂O₃ nanocrystals would effectively obstruct the crack development in the coating and then the coating fracture toughness is enhanced.     

Amorphous-like nanocrystalline γ-Al2O3 films prepared by MOCVD

Alumina (Al₂O₃) films were prepared by metalorganic chemical vapor deposition using aluminum tri-acetylacetonate as a precursor. The effects of deposition conditions on film phase, microstructure, and deposition rate were investigated. γ-Al₂O₃ films were obtained at substrate temperatures ranging between 1173 and 1373 K and total chamber pressures ranging between 400 and 1000 Pa, whereas α-Al₂O₃ films incorporating a small amount of the γ phase were obtained at 1373 K and 800 Pa. Al₂O₃ films prepared at 1173 K showed a halo in X-ray diffraction patterns, consistent with amorphous structures. However, TEM observations suggested that these films consisted of a nanocrystalline γ-Al₂O₃ phase containing trace amounts of carbon.     

Surface Carbonization of Mo-La2O3 Cathode

The carbonized structures of Mo-La₂O₃ cathode specimens have been investigated by means of FESEM and XRD, respectively. The substructure of carbonized layer in the Mo-La₂O₃ cathode has been found for the first time. The results showed that the carbonized layer with uniform Mo₂C was helpful to emission, while the demixing carbonized layer with a compact MoC outside layer was harmful to emission. The uniform Mo₂C layer consists of coarse particles with lots of grain boundary crevices as well as holes arranging perpendicular to the wire axle and up to surface, which was beneficial to the migration of activated rare-earth in activation and operating.     

Oxidation behavior of γ-TiAl based alloy with Al2O3-Y2O3 composite coatings prepared by electrophoretic deposition

Electrophoretic deposition (EPD), a versatile and cost-effective process, was employed to prepare Al₂O₃-Y₂O₃ composite coatings on a γ-TiAl based alloy. SEM observations showed that the composite coatings were compact and consisted of uniform nano-particles. Cyclic oxidation at 1000 °C indicated that the γ-TiAl alloy exhibited a cyclic spallation-oxidation behavior under cyclic oxidation while the Al₂O₃-Y₂O₃ composite coatings improved the oxidation and scale spallation resistance of γ-TiAl alloy significantly due to the suppression of outward diffusion of Ti in the γ-TiAl substrate and the promotion of selective oxidation of Al in the γ-TiAl alloy induced by the composite coating.     

Al2O3/Au/Al2O3层状阻氚薄膜

采用磁控溅射法在316L不锈钢基体上分别沉积单层Al2O3膜、单层Au膜以及Al2O3/Au/Al2O3层状薄膜。采用气相渗透法在500℃,氘分压为0.06 MPa条件下测试了薄膜的阻氘性能。结果表明,3种薄膜氘渗透后,薄膜的形貌良好,无开裂、无剥落的现象,氘渗透率减低因子均比316L不锈钢基材增大一个数量级以上,阻氘效能按单层Al2O3膜、单层Au膜以及Al2O3/Au/Al2O3层状薄膜依次递升。Al2O3/Au/Al2O3层状薄膜的优异阻氘效能可归因于,延性的Au夹层使层状薄膜的力学性能得到显著提高;Al2O3层能阻止Au与基体间互扩散,使Au能充分发挥阻氘效能。本研究表明,由贵金属与陶瓷阻氚材料构成的层状薄膜是发展阻氚涂层的新途径。     

Plasma spraying of Al2O3–WO3 composite coatings

Al₂O₃–WO₃ oxide composites are of considerable interest as solid acid catalysts in the petrochemical industry. Typically they consist of a monolayer of WO₃ dispersed over a high surface area alumina support. The aim of this work was to form a single layer Al₂O₃–WO₃ composite coating by plasma spraying. Ammonium metatungstate was dispersed through porous alumina particles and transformed to WO₃ by heat treatment. WO₃ doped powders were plasma sprayed using “low” and “high” enthalpy plasma systems. Plasma spraying led to dramatic changes in the WO₃ oxide to form metallic tungsten, a metastable Al₂O₃–W solid solution and the mixed oxide Al₂(WO₄)₃. The mechanisms accounting for these transformations are discussed.     

Sol-gel formation of γ-Fe2O3/SiO2 nanocomposites

Fe₂O₃–SiO₂ nanocomposites with Fe/Si molar concentrations ranging between 0.25 and 0.57 were prepared by the sol-gel route. Tetraethoxysilane and iron(III) nitrate solution in ethanol were mixed and the hydrolysis reaction was promoted by the hydration water of the salt. The sols were gelated in air and then treated at 400°C for 4 h. The effect of the gelation process on the formation of the final nanocomposites has been investigated by studing different factors, such as temperature and surface of evaporation/volume ratio of the sol. Nanoparticles of γ and/or α iron oxide phase of different size were obtained in the silica matrix depending on the gelation conditions and on the later heat treatments. γ-Fe₂O₃–SiO₂ nanocomposites were obtained only when organic material was present into the pores of the gel silica matrix supporting their formation through a magnetite phase. At temperatures >400°C, the γ-phase begins to transform to the α-phase.     

Dry Sliding Wear Behaviour of Al2O3-Cr2O3 Coatings with Different Contents of Cr2O3EI北大核心CSCD

Cr₂O₃对 Al₂O₃-Cr₂O₃复合涂层与高硬度陶瓷接触时的摩擦磨损行为及磨损机制的影响尚未揭示。采用大气等离子喷涂的方法制备 Cr₂O₃含量不同的 Al₂O₃-Cr₂O₃复合涂层以研究 Cr₂O₃的影响机制。试验结果表明：Cr₂O₃明显减少了涂层的微观孔隙;复合涂层中 α-Al₂O₃ / γ-Al₂O₃的相对含量比明显高于 Al₂O₃ 层中的 37%;Al₂O₃-40%Cr₂O₃涂层的硬度与 Al₂O₃涂层相比提高了 48%,断裂韧性是 Al₂O₃涂层的 2 倍多;当载荷为 5 N、10 N 和 15 N 时,Al₂O₃-40%Cr₂O₃复合涂层的摩擦因数最低,磨损率依次降低 60%、85% 和 79%。但是当载荷为 20 N 时,Al₂O₃-20%Cr₂O₃复合涂层的摩擦因数最低,磨损率降低了 50%。微观脆性断裂是涂层的主要磨损机制。复合涂层耐滑动磨损性能与 Cr₂O₃含量及磨损条件是密切相关的。微观结构、硬度、断裂韧性、导热系数等是影响 Al₂O₃-Cr₂O₃ 复合涂层耐磨损性能的重要因素。研究结果可为高耐磨性 Al₂O₃基涂层的设计和应用提供指导。     

Lu2O3/TiO2/Nb2O5共稳定ZrO2热障涂层材料研究

采用Lu2O3/TiO2/Nb2O5作为ZrO2热障涂层材料的稳定剂,研究Lu2O3/TiO2/Nb2O5共稳定ZrO2(15%LTNSZ)热障涂层材料在900℃、40 mol%V2O5+60 mol%Na2SO4熔盐中的热腐蚀行为。结果表明:8%YSZ热障涂层材料热腐蚀2 h后,t相ZrO2全部相变为m相ZrO2,热腐蚀产物为尺寸较大的YVO4晶体;15%LTNSZ热障涂层材料热腐蚀20 h后,m相ZrO2的比例仅为5.5%,热腐蚀产物为尺寸较小的LuVO4晶体。与Y2O3稳定剂相比,Lu2O3/TiO2/Nb2O5稳定剂使ZrO2在40 mol%V2O5+60 mol%Na2SO4熔盐中的抗热腐蚀性能提高1个量级以上。     

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。     

B2O3含量对SiO2–B2O3–Al2O3–Na2O系陶瓷结合剂结构与性能的影响

制备不同B₂O₃含量的SiO₂–B₂O₃–Al₂O₃–Na₂O系玻璃试样和陶瓷结合剂试样,利用电子多功能实验机、扫描电镜、显微硬度仪、平面流淌法、热膨胀系数测试仪等分别测试不同玻璃试样的密度和显微硬度,陶瓷结合剂试样的抗折强度、微观形貌和热膨胀系数等,并用X射线衍射仪、傅里叶变换红外光谱仪对陶瓷结合剂的结构和成分变化进行分析。结果表明:将B₂O₃引入陶瓷结合剂中可有效降低其烧结温度,提高其热稳定性并调节其热膨胀系数等。在陶瓷结合剂中加入摩尔分数为15%的B₂O₃时,其样条抗折强度最高为78.11 MPa,密度和硬度最高分别为2.45 g/cm3和856 MPa,且该陶瓷结合剂的热膨胀系数与金刚石最匹配。X射线衍射分析结果表明陶瓷结合剂是典型的玻璃相结构,且对磨料有良好的包覆效果。      

孔道Al2O3/SiO2对NaAlH4-Tm2O3体系储氢性能的影响

以机械球磨法制备具有可逆吸放氢性能的NaAlH4-Tm2O3储氢材料体系。利用相同制备方法进一步研究两种不同孔道材料(大孔Al2O3与介孔SiO2)对NaAlH4-Tm2O3体系储氢性能的影响,测试样品的循环吸放氢性能,并对样品吸放氢前后的结构进行表征。结果表明:大孔Al2O3材料的添加并不能明显改善NaAlH4-Tm2O3体系的放氢速率和放氢量,而介孔SiO2的加入使NaAlH4-Tm2O3体系在150℃条件下5 h内的首次放氢量(质量分数)达到4.61%,高于NaAlH4-Tm2O3体系的4.27%,增加了约8.0%。此外,添加介孔SiO2的NaAlH4-Tm2O3体系放氢速率也有所提高。     

α-Fe2O3 nanoplates: PEG-600 assisted hydrothermal synthesis and formation mechanism

Quasi-hexagonal α-Fe₂O₃ nanoplates with lateral sizes of 40-60 nm and thickness of ca. 10 nm were fabricated by a facile poly(ethylene glycol 600) (PEG-600) assisted hydrothermal technique in combination with calcination method. The final α-Fe₂O₃ nanoplates inherited perfectly the morphology of the preliminarily hydrothermal products with phases of dominant α-Fe₂O₃ and minor α-FeOOH. The platelets could be tailored from nano- to meso- and to micro-scale via adjusting PEG-600 quantities. An adsorption-extension-attachment model was proposed to explain the formation and growth mechanism of the platelets. The as-obtained α-Fe₂O₃ nanoplates exhibited a specific Langmuir surface area of 59 m²/g and a maximum N₂ adsorption of 137.3 cm³/g at 1 atm. UV-vis measurement showed a strong absorption in a wide range from UV to visible light with a blue-shifting band gap of 2.33 eV due to the nanosize effect.     

ZnFe2O4基材料在NaF-AlF3-Al2O3熔盐中的腐蚀

采用锌铁尖晶石材料作为铝电解惰性阳极,考察了这种阳极在 熔盐中的腐蚀 行为,阳极电流密度为0～2.5A/cm2．实验结果表明,锌铁尖晶石材料在阳极极化条件下的NaF-AlF3-Al2O3熔盐中具有很好的耐腐蚀性能．在低电流密度下,阳极材料的腐蚀速度随电流密度的增大而增大,最高的腐蚀速度出现于0.5～0.75A/cm2．此后,腐蚀速度随电流密度的增大而降低．实验证明,高阳极电流密度(＞1.5A/cm2)、熔盐电解质中保持高Al2O3含量和低NaF/AlF3摩尔比,对降低阳极材料的腐蚀速度有利,这也将是惰性阳极应用的重要条件．     

退火对0.3%Y2O3/0.3%La2O3/0.3%Al2O3/Cu复合材料组织和性能的影响（英文）

采用氧/氮气氛喷射沉积技术结合内氧化工艺制备了0.3%Y2O3/0.3%La2O3/0.3%Al2O3/Cu复合材料,并对其在不同温度下进行退火处理。研究了退火对复合材料显微组织、断口形貌、硬度、强度、导电性和电弧侵蚀表面特征的影响。结果表明,随着退火温度的增高,复合材料的晶粒明显长大,其电导率逐渐增加,而硬度和抗拉强度下降。当退火温度达到1000℃时,基体开始出现退火孪晶。复合材料的拉伸断口形貌表现为大而深的韧窝,是典型的微孔缩聚型断裂,韧窝里的质点为金属氧化物颗粒。其电弧侵蚀表面呈现出大量的浆糊状凝结物和气泡。     

一种可内生Al2O3扩散障的δ-Ni2Al3-Cr2O3/ Ni-Cr2O3涂层体系

通过对Ni-Cr₂O₃复合镀层620 ℃部分渗铝制备了δ-Ni₂Al₃-Cr₂O₃/Ni-Cr₂O₃涂层体系。Cr₂O₃颗粒在渗铝的过程中和Al反应生成更为稳定的Al₂O₃。1000 ℃恒温氧化20 h后发现,铝化物涂层和复合镀层内掺杂的Cr₂O₃颗粒完全转化为Al₂O₃,并在铝化物涂层/Ni镀层界面自发形成了一层Al₂O₃富集层,该富集层起扩散障作用,阻碍铝化物涂层因互扩散所致的退化。