Epitaxial growth of tungsten nanoparticles on alumina and spinel surfaces

Isolated tungsten nanoparticles (α-W and β-W phase) were synthesized and epitaxially grown on alumina and spinel particle surfaces with an average tungsten size of ≤20?nm for a low tungsten content (of ≤1.5?vol%). Using tungsten (VI) ethoxide alcoholic solutions, tungsten trioxide hydrated precursors were attached to a ceramic grains surface as a nanoparticle coating. High-resolution transmission electron microscopy (HRTEM) micrographs showed epitaxial interfaces between alumina, spinel and metallic tungsten. This epitaxial growth is assumed to be due to the effect of water vapour on the sublimation of ortho-tungstic acid during the reduction process in a hydrogen atmosphere. The planes involved in the epitaxy were found to be [Formula: see text] and [Formula: see text].     

Alumina-coated hollow glass spheres/alumina composites

Coating of alumina onto the surface of hollow glass spheres was accomplished by controlled heterogeneous precipitation from aqueous solutions. The processing conditions were optimized to yield thin and uniform precursor coatings. After calcination, converting the precursor to alumina, the alumina-coated hollow glass spheres formed free-flowing powders that were used to produce glass/alumina composites with up to 35 vol% of controlled and well dispersed closed porosity. The dielectric constants and the flexural strengths of such porous composites were determined as a function of porosity. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of hollow spherical magnetite nanoparticles

We have developed and tested procedures for the synthesis of hollow spherical magnetite nanoparticles and analyzed the general mechanisms of their formation in relation to their structure and morphology. The phase composition, elemental composition, and morphology of the synthesized particles have been examined by X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy, and their magnetic properties have been studied by an induction method using a vibrating sample magnetometer. The results demonstrate that the proposed procedures enable the preparation of hollow porous spherical magnetite particles 40 nm to 100 μm in diameter, with a wall thickness ≃d/10 and a narrow size distribution.     

Assembled CuO hollow spheres from nanoparticles

CuO microspheres with hollow interiors were synthesized by a simple method using urea as an important reagent to generate soft-template. X-ray diffraction pattern, Infrared spectrum, electronic diffraction, field emission scanning electron microscopy images, transmission electron microscopy images and high-resolution transmission electron microscopy investigated the phase structures and morphologies of CuO microspheres. The hollow spheres may be formed through the assembling of nanoparticles with the assistance of gas bubbles and a self-generated template and aggregation process was proposed for the formation mechanism of the CuO spheres.     

Synthesis and properties of hollow silica nanoparticles

Hollow nanoparticles of silicon dioxide (SiO₂) have been obtained using Cu/SiO₂ core-shell nanoparticles as precursors. An original technique based on heating the precursor nanoparticles to T = 400°C followed by a nanochemical reaction of copper oxide separation from hollow silica particles has been proposed and implemented for the first time. The obtained hollow SiO₂ nanoparticles have been studied by transmission electron microscopy. Mechanisms involved in the formation of hollow silica nanoparticles are discussed.     

Size-controlled synthesis of alumina nanoparticles from aluminum alkoxides

Aluminum oxide nanoparticles were prepared by the hydrolysis of aluminum oxide alkoxides followed by calcinations, in the presence of surface stabilizing agents, such as Na(AOT) molecules. The size of alumina precursors (bohemite) was 20-30 nm, yielding aluminum oxide particles with an average size of 80 nm after calcinations at 1200 °C. The shape of the α-alumina nanoparticles was mainly spherical and the high temperature inhibited the formation of the hexagonal crystals. The introduction of Na(AOT) during the appropriate processing step, had the effect of controlling the size of the particles, the degree of aggregation and the particles shapes.     

Strengthening of porous alumina bodies using carboxylate-alumoxane nanoparticles

Aqueous solutions of acetate-functionalized alumina nanoparticles (A-alumoxane), with an average particle size of 28 nm, have been used as alumina precursors for the infiltration of porous -alumina bodies in order to produce composite structures with homo-interfaces between substrate and infiltrate. Alternatively, if metal doped-methoxy(ethoxyethoxy)acetic acid-functionalized alumina nanoparticles (M-doped MEEA-alumoxane; M = Ca, Er, La, Ti, and Y), with an average particle size of 67 nm, are used in combination with A-alumoxane, a hetero-interface is formed between substrate and infiltrate. Samples were characterized by SEM, BJH, hardness and bend strength measurements. The bulk hardness of the -alumina substrates increases with sintering temperature, but this increase is significantly smaller than the effect of infiltration. The composite hardness generally increases with decreased average pore size although the exceptions to this trend suggest that the identity of the infiltrate is of equal or greater importance. Overall the hetero-interfaces show higher strength than the homo-interface; the latter showing only slightly better performance than high temperature sintering. For the samples fired at 1000°C, the MgAl₂O₄/Al₂O₃ and CaAl₁₂O₁₉/Al₂O₃ combinations appear to provide the greatest enhancement, with both the LaAl₁₁O₁₈/Al₂O₃ and Y₃Al₅O₁₂/Al₂O₃ hetero-interface samples show marked increase in hardness between 1000 and 1400°C. The elastic modulus and bend strength of the -alumina substrate increases significantly for the Er₆Al₁₀O₂₄/Al₂O₃ and LaAl₁₁O₁₈/Al₂O₃ infiltrates. The identity of the hetero-interface has a significant effect on the bulk properties of the composite.     

Fabrication of hollow alumina microspheres via core/shell structure of polymethylmethacrylate/alumina prepared by mechanofusion

Core/shell microspheres of polymethylmethacrylate coated with Al₂O₃ or a mixture of Al₂O₃ and SiO₂ nanoparticles have been successfully prepared by a mechanofusion system. The microspheres were then subjected to removal of PMMA by firing in air, followed by sintering at 1600°C for 3 h. The shell structure was destroyed after the sintering when the shell layer consisted of only Al₂O₃. The addition of SiO₂ was found to be effective for maintaining the original shell structure, while the shell size expanded slightly and a hole was formed in each hollow microsphere during the removal process of PMMA.     

Synthesis of hollow polyaniline nanoparticles with reactive template

γ-Fe₂O₃ nanoparticles were used as the reactive templates for the fabrication of the hollow polyaniline nanoparticles via the chemical oxidative polymerization of aniline in the presence of hydrochloric acid. The particle size of the doped hollow polyaniline nanoparticles in the emeraldine form was determined by the reactive template. It was found that the reacting temperature played an important role in the formation of the hollow nanoparticles because of the low redox potential of Fe(III) ion. However, the kind of the protonic acids used had no influence on the fabrication of the hollow nanostructure. The formation mechanism of the hollow polyaniline nanoparticles was also proposed.     

Adsorption of avermectin on porous hollow silica nanoparticles by supercritical technology

Porous hollow silica nanoparticles with O.D. of approximately 100 nm and a wall thickness of approximately 10 nm were prepared by using inorganic CaCO3 templates. The produced PHSN were employed as a novel carrier to study the adsorption of avermectin in supercritical carbon dioxide by applying different adsorption pressure, adsorption temperature, adsorption time and volume of cosolvent. The results indicated that while increasing adsorption pressure and time always showed a positive effect on the avermectin adsorption until adsorption saturation is reached, both the adsorption temperature and volume of cosolvent require an optimal value for achieving the maximum adsorption. It was found that the optimal adsorption could be obtained at an adsorption pressure of 15 MPa and an adsorption temperature of 313 K for 90 minutes with 5 ml cosolvent. In addition, the desorption behavior of avermectin from the avermectin-loaded PHSN samples showed a sustained style: approximately 60% of avermectin was released in the first 20 minutes, while the other 40% followed a typical sustained desorption pattern and was dissolved out slowly for a time period of 3000 minutes, which is different from the quick and complete desorption from solid SiO2 carriers.     

氧化铝多孔支撑体的研究

用挤出成型的方法制备高性能陶瓷膜用Ａｌ２Ｏ３〉９９％支撑体，制备中受初闰度、烧成温度、粘结等因素的影响，因此可以调节一个或几个因素来控制支撑体的性能。用硬脂酸铝为造孔剂，调节支撑体的孔径和气孔率时发现造孔剂质量分数〈５％和〉５％在坯体中的分布不同，成孔的作用也不一样，造孔剂用量〈５％时，对成孔作用不大，〉５％时成孔明显，但造孔剂的添加量不宜超过１５％。     

氮掺杂空心TiO2纳米粒子的制备与研究

采用一种简便的方法合成了氮掺杂空心TiO:纳米粒子.以聚苯乙烯(PS)纳米粒子为模板、氨水为催化剂、三乙醉胺为抑制剂,采用溶胶-凝胶方法合成了PS/TiOZ(核/壳)纳米复合粒子.考察了氨水、三乙醉胺和钛酸正丁酯用量对TiO:壳层表面形态和厚度的影响.氨水用量3.OOg时TiO2壳层表面较光滑,氨水用量6.OOg时其表...     

Solvothermal synthesis of magnetite hollow submicrospheres and mesoporous nanoparticles

Magnetite hollow submicrospheres and mesoporous nanoparticles have been synthesized by a solvothermal approach with assistant of hexamethylenetetramine (HMT) and poly(vinyl pyrrolidone) (PVP). Experimental results showed that the morphology of magnetite was transformed from hollow to solid submicrospheres with increasing amount of HMT. Moreover, without addition of PVP or appropriate addition of external water, mesoporous nanoparticles were obtained. A probable gaseous bubble template mechanism was proposed for the formation of magnetite hollow and mesoporous nanostructures based on experimental observations. Magnetic measurement results revealed that all of the samples were ferromagnetic at room temperature.     

Synthesis of hollow spheres with mesoporous silica nanoparticles shell

Hollow spheres with mesoporous silica nanoparticles shell were synthesized with the use of cetyltrimethylammonium bromide (CTAB) and polystyrene (PS) hollow spheres as dual templates. The key to this study is that the uneven surface of the template provides nucleation sites for mesoporous nanoparticles, resulting in the formation of hollow spheres with mesoporous silica nanoparticles shell. The final products with hierarchical mesopores can be obtained through a simple one-step approach.     

Facile synthesis of porous hollow iron oxide nanoparticles supported on carbon nanotubes

Porous hollow iron oxide nanoparticles (PHNPs) supported on carbon nanotubes (CNTs) were facilely synthesized by etching Fe@Fe_xO_y/CNT with dilute nitric acid aqueous solution at ambient temperature without the assistance of any surfactants and ligands. The mean diameter of hollow iron oxide nanoparticles was about 17 nm, with a wall thickness of about 4 nm. The formation mechanism of PHNPs is discussed based on the characterization results from TEM, XRD and H₂-TPR. The combination of nanoscale Kirkendall effect and selective acid etching is proposed to be responsible for the formation of CNT supported PHNPs, through a transformation from core/void/shell structures to hollow nanoparticles.     

Versatile gradients of chemistry, bound ligands and nanoparticles on alumina nanopore arrays

Nanoporous alumina (PA) arrays produced by self-ordering growth, using electrochemical anodization, have been extensively explored for potential applications based upon the unique thermal, mechanical and structural properties, and high surface-to-volume ratio of these materials. However, the potential applications and functionality of these materials may be further extended by molecular-level engineering of the surface of the pore rims. In this paper we present a method for the generation of chemical gradients on the surface of PA arrays based upon plasma co-polymerization of two monomers. We further extend these chemical gradients, which are also gradients of surface charge, to those of bound ligands and number density gradients of nanoparticles. The latter represent a highly exotic new class of materials, comprising aligned PA, capped by gold nanoparticles around the rim of the pores. Gradients of chemistry, ligands and nanoparticles generated by our method retain the porous structure of the substrate, which is important in applications that take advantage of the inherent properties of these materials. This method can be readily extended to other porous materials.     

Sintering and grain growth of ultrapure alumina

The kinetics of densification and grain growth of ultrapure alumina (> 99.999%) were measured for clean sintering conditions in a pure-sapphire tube, and compared with kinetics measured during normal sintering conditions in an alumina crucible of 99.8% purity. For the clean condition, the microstructure of sintered alumina remained homogeneous and only normal grain growth was observed up to 1900°C for 5 H. However, under the normal sintering condition, both normal and abnormal grain growth were observed depending on the sintering temperature and time. Thus, abnormal grain growth in alumina could be effectively suppressed without introducing sintering aids (such as MgO) by using an ultrapure powder and by preventing the introduction of any impurities throughout the sintering process. This result strongly suggests that abnormal grain in commercially pure alumina ( 99.99%) is not an intrinsic property of alumina but an extrinsic property controlled by minor constituents that can be present in the original powder or introduced during powder processing and subsequent sintering.     

Formation of hollow ZnO through low-temperature oxidation of Zn nanoparticles

The hollow phenomenon with the oxidation of Zn nanoparticles was studied by transmission electron microscopy. Hollow ZnO was formed by oxidizing Zn nanoparticles with the diameter less than 20 nm at 423 K for 3.6 ks. Nano-hole in the center of ZnO can be considered as a vacancy cluster, which is formed as the result of the generation and aggregation of vacancies due to faster outward diffusion of Zn across ZnO in air at the initial oxidation stage.