Wolfgang M. Sigmund

Low temperature synthesis of ultrafine Pb(Zr, Ti)O 3 powder by sol-gel combustion

Lead zirconate titanate powders are derived from a novel aqueous-based citrate-nitrate/oxynitrate sol-gel combustion process. Aqueous solutions of metal nitrates or oxynitrates are transformed into gels with citric acid under heating. The received gels undergo a self-propagating combustion reaction on heating to 180{degree}C and subsequently yield voluminous ashes. These ashes form single phase perovskite Pb(Zr{sub 0.53}Ti{sub 0.47})O{sub 3} powder with a specific surface area of 8m{sup 2}/g upon calcination at 550{degree}C. The ashes show a homogeneous distribution of lead, zirconium, and titanium ions which guarantees short diffusion paths in solid state formation of PZT perovskite. The redox behavior of the gels was studied with the help of DTA experiments. Powders are characterized in terms of XRD, SEM, and EDX analysis. {copyright} {ital 1997 Materials Research Society.}

Zinc oxide nanowires on carbon nanotubes

Multiwalled nanotubes (MWNTs) were heat-treated with zinc (Zn) at various temperatures. Hereby, zinc oxide (ZnO) was produced on the outermost shells of MWNTs in the forms of ultrathin films, quantum dots, or nanowires/nanorods. ZnO nanowires were grown on the surface of MWNTs without the presence of a catalyst, suggesting the vapor–liquid–solid mechanism is not applicable for the current growth process. The average diameter of nanowires on MWNTs prepared at 600 °C was about 30 nm and the length was up to 1.5 μm, yielding aspect ratios of 7 to 50. ZnO nanorods grown on MWNTs at 800 °C had diameters from 80 to 150 nm with lengths from 0.5 to 1 μm (aspect ratios of 6 to 10).

Control of growth orientation for carbon nanotubes

Laterally aligned carbon nanotubes were synthesized on substrates over iron nanoparticles using chemical vapor deposition. In addition, aligned carbon nanotubes grown vertically and with tilt angle to the substrates were produced, which means that it is possible to grow aligned carbon nanotubes at any angle relative to the substrate. The growth direction of the carbon nanotubes was controlled by a magnetic field that is applied in the process of adhering catalyst particles on silicon oxide substrates from dispersion. The ferromagnetic property of the iron nanoparticles fixes them in a defined orientation under magnetic field, which results in aligned growth of the carbon nanotubes. These results indicate that carbon nanotubes preferentially grow from certain facets of the catalyst particles, suggesting a crucial clue in investigating the growth mechanism of carbon nanotubes. The laterally aligned carbon nanotubes could make it possible to integrate them in nanoelectronic devices, such as a channel for fie...

Novel synthesis of cerium oxide nanoparticles for free radical scavenging

AIMS The aim of this article is to present a novel synthetic route to form CeO(2) nanoparticles that protects against the detrimental influence of oxidative stress in mammalian cells. METHODS The noncytotoxic surfactant lecithin was used to synthesize CeO(2) nanoparticles and the products were colloidally stabilized in a biocompatible tri-sodium citrate buffer. These nanoparticles were delivered into murine insulinoma betaTC-tet cells, and intracellular free radical concentrations responding to exposure to hydroquinone were measured in a variety of extracellular CeO(2) concentrations. RESULTS Well-dispersed, highly crystallized CeO(2) nanoparticles of 3.7 nm in size were achieved that are chemically and colloidally stable in Dulbeccos modified Eagles medium for extended periods of time. Treating betaTC-tet cells with these nanoparticles alleviated detrimental intracellular free radical levels down to the primary level. CONCLUSION CeO(2) nanoparticles synthesized from this route are demonstrated to be effective free radical scavengers within betaTC-tet cells. Furthermore, it is shown that CeO(2) nanoparticles provide an effective means to improve cellular survival in settings wherein cell loss due to oxidative stress limits native function.

Coaxially electrospun PVDF-Teflon AF and Teflon AF-PVDF core-sheath nanofiber mats with superhydrophobic properties.

This work reports the coaxial electrospinning of poly(vinylidene fluoride) (PVDF)-Teflon amorphous fluoropolymer (AF) and Teflon AF-PVDF core-sheath nanofiber mats yielding superhydrophobic properties. The coaxial electrospinning configuration allows for the electrospinning of Teflon AF, a nonelectrospinnable polymer, with the help of an electrospinnable PVDF polymer. PVDF-Teflon AF and Teflon AF-PVDF core-sheath fibers have been found to a have mean fiber diameter ranging from 400 nm to less than 100 nm. TEM micrographs exhibit a typical core-sheath fiber structure for these fibers, where the sheath fiber coats the core fiber almost thoroughly. Water contact angle measurements by sessile drop method on these core-sheath nanofiber mats exhibited superhydrophobic characteristics with contact angles close to or higher than 150 degrees. Surprisingly, PVDF-Teflon AF and Teflon AF-PVDF nanofiber mat surface properties were dominated by the fiber dimensions and less influenced by the type of sheath polymer. This suggests that highly fluorinated polymer Teflon AF does not advance the hydrophobicity beyond what surface physics and slightly fluorinated polymer PVDF can achieve. It is concluded that PVDF-Teflon AF and Teflon AF-PVDF core-sheath electrospun nanofiber mats may be used in lithium (Li)-air batteries.

Formation of anatase TiO2 nanoparticles on carbon nanotubes

Anatase TiO2 nanoparticles with a size range of 2 to 10 nm have been formed on carbon nanotubes by the controlled hydrolysis and condensation of titanium bis-ammonium lactato dihydroxide in water and electrosterically dispersed carbon nanotubes.

Synthesis of YAG phase by a citrate-nitrate combustion technique

Abstract A white precipitate is yielded on heating a solution of Y(NO3)3 (equivalent to 8 wt.% Y2O3), Al(NO3)3 (equivalent to 7 wt.% Al2O3 and molar ratio of 1:1.9378 for Y2O3 to Al2O3) and citric acid in isopropanol with a citrate–nitrate molar ratio of 0.098 to 60°C. The dried precipitate is then combusted at 200°C and produces a solid product (ash). X-ray powder diffraction patterns of the ash and its calcined forms show that the ash is amorphous and remains amorphous up to 600°C. The ash starts crystallizing to form a YAG phase at 800°C and completely transforms into YAG below 900°C. A certain amount of YAM phase co-exists with the YAG phase between 850°C and 900°C. Finally at 900°C, only the YAG phase exists. This combustion technique for synthesis of YAG phase can be used for incorporation of sintering additives in non-oxide ceramics such as Si3N4.

NANOSTRUCTURED YTTRIA POWDERS VIA GEL COMBUSTION

Nanostructured yttria powders were prepared by a gel combustion technique. The technique involves exothermic decomposition of an aqueous citrate-nitrate gel. The decomposition is based on a thermally induced anionic redox reaction. A variety of yttria powders with different agglomerate structures can be made by altering the citrate-nitrate ratio γ. The gel with γ = 0.098 in situ yields nanostructured yttria powder at 258 °C that is porous and agglomerated with an average of 25 nm primary particles. Its specific surface area is 55 m 2 /g. The decomposition of the gels was investigated by simultaneous thermogravimetry analysis (TGA) and differential thermal analysis (DTA) experiments. The produced ashes and calcined powders are characterized by x-ray diffraction (XRD), ir spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer, Emmett, and Teller (BET) analysis.

Defect chemistry and defect engineering of TiO2-based semiconductors for solar energy conversion

This tutorial review considers defect chemistry of TiO2 and its solid solutions as well as defect-related properties associated with solar-to-chemical energy conversion, such as Fermi level, bandgap, charge transport and surface active sites. Defect disorder is discussed in terms of defect reactions and the related charge compensation. Defect equilibria are used in derivation of defect diagrams showing the effect of oxygen activity and temperature on the concentration of both ionic and electronic defects. These defect diagrams may be used for imposition of desired semiconducting properties that are needed to maximize the performance of TiO2-based photoelectrodes for the generation of solar hydrogen fuel using photo electrochemical cells (PECs) and photocatalysts for water purification. The performance of the TiO2-based semiconductors is considered in terms of the key performance-related properties (KPPs) that are defect related. It is shown that defect engineering may be applied for optimization of the KPPs in order to achieve optimum performance.

AFM study of repulsive van der Waals forces between Teflon AF thin film and silica or alumina

Abstract An atomic force microscope (AFM) was used to measure the interactions between a flat Teflon AF™ surface and an α-alumina or amorphous silica sphere in cyclohexane as a function of distance of separation. Repulsive van der Waals (vdW) forces were predicted for the interactions between Teflon AF™ and α-alumina or amorphous silica according to the negative Hamaker constants, which were calculated from the dielectric response functions of materials using the Lifshitz theory. The measured forces agree well with the theoretically calculated forces, which include the retardation contribution. The retardation effects are apparent at large distance of separation (>4–5 nm) where the measured forces are weaker than the non-retarded prediction.