Precipitation of Magnesium Aluminum Spinel from Alumina-Matrix Solid Solution: I, Fundamental Concept and Precipitation Behavior

Al₂O₃ ceramics with magnesium aluminum spinel dispersion particulates were produced by a precipitation treatment of Al₂O₃-matrix solid solution, (Al_1—2x,Ti _x ,Mg _x )₂O₃. We successfully constructed the precipitation process for synthesizing the nanocomposite, using the solubility dependence of titanium and magnesium in Al₂O₃ on the valence of titanium. Changing of the valence of titanium from 4+ to 3+ was accomplished by controlling the heating atmosphere, and, thereby, magnesium precipitation was promoted. The precipitation behavior was characterized using X-ray diffractometry, and the microstructure was observed using transmission electron microscopy. We confirmed that magnesium aluminum spinel nanosized particulate were precipitated in the Al₂O₃ grain.     

Investigation of Multiply Twins in Mn2.02Co0.98O4 Ceramic by Means of Transmission Electron Microscopy

Mn_2.02Co_0.98O₄ ceramic with tetragonal spinel structure was prepared by solid‐state coordination reaction method. The phase and microstructure were studied using X‐ray diffractometer (XRD) and transmission electron microscope (TEM), respectively. The results show that the Mn_2.02Co_0.98O₄ ceramic exhibits tetragonal spinel structure owing to imperfect quenching. Several kinds of defects were formed in the sample due to the phase transition from cubic spinel at high temperature to tetragonal spinel at room temperature. Two types of {112} twin wall were found in the quenched Mn_2.02Co_0.98O₄ ceramic. The presence and the characteristics of twins were investigated. The twins arranged in a cyclic triangle form and the twin boundaries were parallel to (112) plane or plane. The possible mechanism for twinning arrangement in the triangle form was proposed according to some crystal geometry model analysis.     

Dislocation-Induced Softening of MgO·2.9Al2O3 Single Crystals

Magnesium aluminate spinel (MgO·2.9Al₂O₃) single crystals were deformed in compression along the [001] direction at temperatures between 1100° and 1500°C. Dislocations introduced by plastic deformation were observed by transmission electron microscopy. These dislocations lead to plasticity at a temperature where undeformed spinel has no ductility. Further, room-temperature Vickers hardness decreases after plastic deformation at 1500°C.     

超声波-均匀沉淀法制备纳米氧化铁

以硝酸铁和尿素为原料，利用超声波与均匀沉淀相结合的方法，研制出了长轴10nm、短轴5nm的α-Fe2O3粒子。并对样品进行了XRD、TGA、DTA、TEM等表征分析。     

Charge distribution in nano‐scale grains of magnesium aluminate spinel

Charge distribution in magnesium aluminate spinel (MAS) results in the formation of a space‐charge region that plays a critical role in assigning functional properties. Significant theoretical advances explaining this phenomenon have been accomplished, even though quantitative experimental support from nano‐scale granular MAS is only indirect. In this work, the electrostatic potential distribution in nano‐scale grains of nonstoichiometric MAS (MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃) was measured by off‐axis electron holography (OAEH) and compared to the distribution of cations and defects in this material as measured by electron energy‐loss spectroscopy (EELS). In this manner, we studied the roles of composition, grain size, and applied electric field (EF) on the formation of a space‐charge region. We quantitatively demonstrated that regardless of grain size, the vicinity of MgO·0.95Al₂O₃ grain boundaries presented an excess of Mg⁺² cations, whereas the vicinity of MgO·1.07Al₂O₃ grain boundaries included an excess of Al⁺³ cations. The degree of structural disorder (ie, the inversion parameter, i) indicated that as‐synthesized MAS were significantly disordered (i between 0.37 and 0.41), with values decreasing toward equilibrium ordering values following annealing (i between 0.27 and 0.31). The application of an external ~150 V/cm EF during annealing further enhanced lattice ordering (i between 0.16 and 0.19). Such variations in the distribution of cations and defects should determine the space‐charged potential (SCP). However, using these measurements to calculate the SCP was not possible due to the wide range of values reported for formation energies of defects (0.82‐8.78 eV). Consequently, we correlated local ionic ordering with electrostatic potential in nonstoichiometric MAS. The magnitudes of the SCP in both MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃ decreased following annealing from ?3.4 ± 0.3 V and 2.0 ± 0.2 V to ?2.0 ± 0.2 V and 1.6 ± 0.1 V, respectively.     

Full reoxidation of CuMn2O4 spinel catalyst triggered by epitaxial Mn3O4 surface nanocrystals

Transmission electron microscopy was performed on a model system of a novel CuMn₂O₄ spinel catalyst in order to study the effects of reduction by carbon monoxide and subsequent reoxidation by oxygen both processed at 600°C. A phase transition from CuMn₂O₄ spinel into separated MnO and Cu‐rich phases, which are CuO or Cu depending on the reduction period, was observed in the reduced samples. Furthermore, a surface coverage of the MnO grains by epitaxially grown, nanocrystalline Mn₃O₄ was found in all reduced samples. These Mn₃O₄ nanocrystals are assumed to act as seed crystals in the subsequent reoxidation step completely reversing the phase transitions and reconstituting single‐phase CuMn₂O₄ spinel. In addition, superlattice reflections due to cation ordering occurred as a typical feature in the as‐prepared spinel. In the reoxidized spinel, the superlattice reflections were absent, hence indicating a disordered cation distribution.     

锌冶炼渣浸出模拟液铁沉降试验研究

实验研究了不同反应条件的铁沉降效率及锌、镉损失率。结果表明,氧气做氧化剂pH为3.0~3.5时,反应7 h后,铁沉降效率不高,仅达到34%,锌、镉损失率随反应时间的增加而增加。铁沉降效率随pH及温度的升高而增大,SEM和XRD分析表明,pH为3.0时的沉铁渣中主要成分是针铁矿。     

Structural and Thermodynamic Variation in Nickel Aluminate Spinel

New structural and calorimetric data for samples of NiAl₂O₄ quenched from 600° to 1560°C are presented The spinel remains stoichiometric for all heat treatments. Based on the refinement of X-ray powder patterns, it is shown that the degree of disorder, defined as the mole fraction of tetrahedral sites occupied by Al³⁺, changes from x = 0.82 at 600°C to 0.78 at 1560°C. Simultaneously, the lattice parameter and enthalpy vary in a complex manner with quench temperature. The largest lattice parameter (0.80500 ± 0.00004 nm) and most exothermic enthalpy of annealing (heat released when sample is equilibrated at 780°C; -10.1 kJ/mol) occur for the sample quenched from 1100°C. A linear correlation exists between the heat of annealing and the lattice parameter. The results have been interpreted as a superposition of at least two effects: (1) the disordering of Ni²⁺ and Al³⁺ ions between octahedral (16d) and tetrahedral (8a) sites and (2) a second process, which may be a small amount of the disordering of ions into the usually empty (16c) sites.     

铁氧化菌对含砷溶液中砷沉淀和臭葱石晶体形成的影响

在常压、pH=1.5、铁砷摩尔比1.5、不同温度、臭葱石晶种存在的条件下,考察了嗜酸氧化亚铁硫杆菌(Acidthiobacillus ferrooxidans,At.f)、中度嗜热西伯利亚硫杆菌(Sulfobacillus sibiricu,S.s)和嗜酸热古菌(Metallosphaera Ar,M.A)菌种对溶液中砷沉淀及臭葱石晶体形成的影响.结果表明,在70℃,M.A菌种及臭葱石晶种存在的条件下,溶液中砷去除率达87%,且能形成结晶效果好、粒径大的长斜形臭葱石晶体.     

种分分解法制备EVA阻燃剂用亚微米氢氧化铝

以硝酸铝和铝酸钠溶液为主要原料,加入适量添加剂,采用中和法制备氢氧化铝晶种,并将该晶种加入铝酸钠溶液中进行种分分解,制备出亚微米氢氧化铝粉体。在添加剂为乙酸、中和反应温度为60℃、搅拌速度为400 r/min、种分分解过程分解首温为60℃、分解末温为55℃、分解时间48 h、晶种添加量(晶种的质量分数,下同)为2%的条件下,制备出了粒度中位径(D_(50))在1.80μm左右,比表面积<5.0 m~2/g的亚微米氢氧化铝粉体,该粉体结晶完整,形貌为块状。该氢氧化铝粉体和目前EVA阻燃剂所用国外氢氧化铝产品相比,在粒度分布、比表面积、吸油率和形貌成型度等方面都差异不大,可以替代国外同类产品。     

Epitactic Nucleation of Spinel in Aluminosilicate Gels and Its Effect on Mullite Crystallization

Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the ∼1000°C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at ∼1000°C in gels that otherwise crystallize directly to mullite at ∼1000°C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at ∼1000°C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at ∼1000°C, the release of alumina from the spinel-type crystal structure becomes the rate-controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nucleation frequency.     

Imaging and Diffraction Study of Continuous α-Fe2O3 Films on (0001)Al2O3

Continuous α-Fe₂O₃ films grown on bulk (0001)Al₂O₂ substrates by low-pressure chemical vapor deposition have been studied by transmission electron microscopy and the observations compared to those obtained from discontinuous films at an earlier stage of the growth process. Plan-view specimens revealed significant thermal stress in the continuous films, while cross-sectional specimens showed that cracking occurs in thicker films. The free surface of the film and the film/substrate interface appeared sharp and flat, apart from growth ledges and steps. Weak-beam imaging revealed a hexagonal misfit dislocation network consisting of perfect edge dislocations. Fine structure in the selected-area diffraction patterns which corroborates these observations is also discussed. The misfit network of partial dislocations previously observed in the discontinuous films was not observed for the continuous films, indicating an effect of film thickness, growth rate, or surface preparation on the Fe₂O₃/(0001)Al₂O₃ interface structure.     

Structural evolution of GeMn/Ge superlattices grown by molecular beam epitaxy under different growth conditions

GeMn/Ge epitaxial ''superlattices'' grown by molecular beam epitaxy with different growth conditions have been systematically investigated by transmission electron microscopy. It is revealed that periodic arrays of GeMn nanodots can be formed on Ge and GaAs substrates at low temperature (approximately 70°C) due to the matched lattice constants of Ge (5.656 Å) and GaAs (5.653 Å), while a periodic Ge/GeMn superlattice grown on Si showed disordered GeMn nanodots with a large amount of stacking faults, which can be explained by the fact that Ge and Si have a large lattice mismatch. Moreover, by varying growth conditions, the GeMn/Ge superlattices can be manipulated from having disordered GeMn nanodots to ordered coherent nanodots and then to ordered nanocolumns.PACS: 75.50.Pp; 61.72.-y; 66.30.Pa; 68.37.L.     

Iron particles in carbon nanotubes

Nanometer-size iron-rich particles in carbon nanotubes have been studied by transmission electron microscopy with and without in situ and ex situ heating. Several remarkable results were found; a high temperature phase (γ-Fe) of iron stable at low temperatures and preferential presence of iron and iron carbide in carbon nanotubes. Based upon these experimental results, thermodynamics of the Fe-C phase diagram and its kinetics were used to explain the non-uniform distribution of iron and iron carbide, which also yielded a deeper insight into the formation of carbon nanotubes. Some of the results also allowed describing the role of the graphitic structure in retaining the high temperature phase (γ-Fe) of iron at low temperatures. Furthermore, methods have been demonstrated with which γ-Fe can be produced in carbon nanotubes intentionally or in a large quantity. Selected area electron diffraction patterns of iron inside nanotubes demonstrated the crystallographic relationship of the iron to the nanotube axis along with phase changes of the iron. This paper summarizes the findings and draws further conclusions on the particle shape inside multiwalled carbon nanotubes.     

Thermal Expansion Characteristics of Alumina–Magnesia and Alumina–Spinel Castables in the Temperature Range 800°–1650°C

The thermal expansion of Al₂O₃–MgO castables containing 5.5 wt% MgO and 1.36 wt% CaO and Al₂O₃–spinel castables containing 20 wt% spinel having 95 wt% Al₂O₃ and 1.7 wt% CaO was measured in the temperature range of 800–1650°C by dilatometry. A sharp increase in expansion from around 1425° to 1525°C, followed by a sharp decrease with further increasing temperature, is characteristic of Al₂O₃–MgO castables. The sharp increase in expansion is believed to be caused by the bond linkage between the CA₆ and spinel grains in the bonding matrix, while the sharp decrease is apparently related to liquid-phase sintering. The sharp increase and decrease in expansion were not observed in Al₂O₃–spinel castables because of the much lower MgO (around 1 wt% MgO) and impurity contents. The magnitude of thermal expansion of calcium aluminate bonded castables containing self-forming or preforming spinels or both is dictated by the MgO content of the castables.     

Biological Properties of Iron Oxide Nanoparticles for Cellular and Molecular Magnetic Resonance Imaging

Superparamagnetic iron-oxide particles (SPIO) are used in different ways as contrast agents for magnetic resonance imaging (MRI): Particles with high nonspecific uptake are required for unspecific labeling of phagocytic cells whereas those that target specific molecules need to have very low unspecific cellular uptake. We compared iron-oxide particles with different core materials (magnetite, maghemite), different coatings (none, dextran, carboxydextran, polystyrene) and different hydrodynamic diameters (20–850 nm) for internalization kinetics, release of internalized particles, toxicity, localization of particles and ability to generate contrast in MRI. Particle uptake was investigated with U118 glioma cells und human umbilical vein endothelial cells (HUVEC), which exhibit different phagocytic properties. In both cell types, the contrast agents Resovist, B102, non-coated Fe₃O₄ particles and microspheres were better internalized than dextran-coated Nanomag particles. SPIO uptake into the cells increased with particle/iron concentrations. Maximum intracellular accumulation of iron particles was observed between 24 h to 36 h of exposure. Most particles were retained in the cells for at least two weeks, were deeply internalized, and only few remained adsorbed at the cell surface. Internalized particles clustered in the cytosol of the cells. Furthermore, all particles showed a low toxicity. By MRI, monolayers consisting of 5000 Resovist-labeled cells could easily be visualized. Thus, for unspecific cell labeling, Resovist and microspheres show the highest potential, whereas Nanomag particles are promising contrast agents for target-specific labeling.     

Transmission electron microscopy of milkfat globules in the low-density lipid fraction of swiss cheese whey

A low-density lipid fraction (LDLF) was recovered from Swiss cheese whey (SCW) residual lipids by high-speed centrifugation. The present study was conducted to determine the microstructural properties of the extremely small-sized milkfat globules (MFG) in LDLF by: (i) freeze-fracture transmission electron microscopy (FF-TEM) and (ii) thin-section TEM (TS-TEM). FF-TEM results revealed that MFG in LDLF were ≤1 μm in size and were embedded in what appeared to be a smooth, protein-like matrix. The MFG in FF-TEM specimens exhibited either planarly cleaved fractions with smooth cores or peripherally cleaved fractions with surface laminations. TS-TEM results revealed that the MFG in LDLF were dispersed in an aggregated nonlipid matrix.     

High-Strain-Rate Superplasticity in Y2O3-Stabilized Tetragonal ZrO2 Dispersed with 30 vol% MgAl2O4 Spinel

High-strain-rate superplasticity is attained in a 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP) dispersed with 30 vol% MgAl₂O₄ spinel: tensile elongation at 1823 K reached >300% at strain rates of 1.7 × 10⁻²– 3.3 × 10⁻¹ s⁻¹. The flow behavior and the microstructure of this material indicate that the MgAl₂O₄ dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO₂ matrix and from stress relaxation due to the dispersed MgAl₂O₄ grains.     

Effect of annealing on grain growth and Y segregation behavior in tetragonal ZrO2 thin film

In order to fabricate tetragonal yttria stabilized zirconia samples with large grain size, 3 mol% Y₂O₃ doped zirconia thin films were grown on (0001) α-Al₂O₃ substrate by pulsed laser deposition (PLD) followed by subsequent high temperature annealing. The thin film samples were annealed at 1200°C, 1250°C, 1300°C, and 1350°C in order to obtain larger grain size without Y segregation. The microstructure and chemical composition of these annealed films were analyzed using atomic force microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The as-grown thin film was found to be composed of [111]-oriented grains of ∼100 nm connected with small-angle tilt boundaries. Based on analysis of annealed thin films, it was revealed that grain growth of tetragonal zirconia occurred anisotropically. Cross section scanning transmission electron microscopy observations revealed that such grain growth behavior is affected by the step-terrace structures of the sapphire substrate. Energy-dispersive X-ray spectroscopy showed that Y was found to distribute almost uniformly below 1300°C but to segregate at the grain boundaries at 1350°C. As a conclusion, the 1300°C-annealed sample shows the largest grain size with homogeneous Y distributions.     

Polymer-grafted cellulose fibers. II. Polymer localization and induced alterations in fiber morphology

In methyl acrylate- or acrylonitrile-grafted northern softwood Kraft or southern softwood Kraft pulp fibers, polymer grafts are present almost homogeneously throughout the fiber wall, although some surface enhancement is observed. On unhydrolyzed fiber surfaces, acrylonitrile grafts protrude in clumps of tangled polymer tufts whereas methyl acrylate grafts form a more uniform, sometimes knobby surface coating. Grafting followed by hydrolysis causes extensive lengthwise splits in the S1 layer, which pulls away from the S2 layer. In the hydrolyzed solvent-dried fiber, the internal grafted polymer/cellulose domains create a unique filamentous and lamellar periodic substructure in the S2 wall. Coexisting with this substructure are numerous microvoids and occasional large pores. We believe that the enhanced absorbency of these fibers can be attributed to the S1 layer disruption, the presence of osmotically active polymer (sodium polyacrylate) incorporated extensively within the S2 wall, and the presence of a drastically altered, more accessible S2 wall substructure. Analytical electron microscopy is shown to be a useful technique for investigating polymer grafts in cellulose fibers. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1471–1485, 1997