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71.
The low work-function ZrO/W(100) surface was examined with the aim of understanding the reducing mechanism of the work function. Low-energy electron diffraction (LEED) was employed to analyze the surface atomic arrangement, and X-ray photoelectron spectroscopy (XPS) was used to identify the surface chemical condition. The ZrO/W(100) surface was made as follows: (i) around three monolayers of Zr were deposited on a clean W(100) surface, (ii) the sample was heat treated in an oxygen ambience of 1.3x10−5 Pa for several tens of minutes at 1500 K, and (iii) the sample was flash heated at 2000 K in ultrahigh vacuum (UHV). During heat treatment in O2, the deposited Zr was oxidized to ZrO2, and the LEED pattern formed was p(2×1). The work function increased to 5.3 eV. Subsequent flash heating in UHV changed the p(2×1) LEED pattern into a c(4×2) pattern, and transformed ZrO2 into the so-called Zr–O complex, the oxidized level of which is between ZrO2 and metallic Zr. A drastic decrease in the work function to 2.7 eV ensued. The angular dependence of XPS showed that the Zr–O complex segregated within a few monolayers at the surface. 相似文献
72.
The peculiar behavior of active crystals is due to the presence of evolving phase mixtures the variety of which depends on the number of coexisting phases and the multiplicity of symmetry-related variants. According to Gibbs’ phase rule, the number of phases in a single-component crystal is maximal at a triple point in the p-T phase diagram. In the vicinity of this special point the number of metastable twinned microstructures will also be the highest—a desired effect for improving performance of smart materials. To illustrate the complexity of the energy landscape in the neighborhood of a triple point, and to produce a workable example for numerical simulations, in this paper we construct a generic Landau strain-energy function for a crystal with the coexisting tetragonal (t), orthorhombic (o), and monoclinic (m) phases. As a guideline, we utilize the experimental observations and crystallographic data on the t-o-m transformations of zirconia (ZrO2), a major toughening agent for ceramics. After studying the kinematics of the t-o-m phase transformations, we re-evaluate the available experimental data on zirconia polymorphs, and propose a new mechanism for the technologically important t-m transition. In particular, our proposal entails the softening of a different tetragonal modulus from the one previously considered in the literature. We derive the simplest expression for the energy function for a t-o-m crystal with a triple point as the lowest-order polynomial in the relevant strain components, exhibiting the complete set of wells associated with the t-o-m phases and their symmetry-related variants. By adding the potential of a hydrostatic loading, we study the p-T phase diagram and the energy landscape of our crystal in the vicinity of the t-o-m triple point. We show that the simplest assumptions concerning the order-parameter coupling and the temperature dependence of the Landau coefficients produce a phase diagram that is in good qualitative agreement with the experimental diagram of ZrO2. 相似文献
73.
74.
Polysiloxanes in combination with metal oxides show interesting properties as nanocomposites for optical or medical applications. The formation of covalent connections between the metal oxide and the polysiloxane is an important method to overcome phase separation between the two components, but it also can have an influence on the morphology of the final materials. In this contribution we report a method for the synthesis of hybrid materials based on polysiloxanes and various metal oxides in which both components are tightly connected to each other. Alkoxysilane modified polysiloxanes were obtained by hydrosilation reactions between vinyl triethoxysilane and poly(dimethylsiloxane-co-methylhydrosiloxane) (PDMS-co-PMHS). The thus functionalized polymers were used in a sol-gel process applying Stöber conditions and hybrid nanoparticles were obtained. Following the same pathway, different metal alkoxides (M(OR)4; M = Ti, Zr; R = ethyl, isopropyl) were coordinated to allyl acetoacetate (AAA) and the resulting complexes were applied in a hydrosilation reaction with PDMS-co-PMHS. Metal oxide hybrid nanoparticles were obtained through a sol–gel process. 相似文献
75.
76.
Fian A Haase A Stadlober B Jakopic G Matsko NB Grogger W Leising G 《Analytical and bioanalytical chemistry》2008,390(6):1455-1461
Here we report on the fabrication and characterization of ultra-thin nanocomposite layers used as gate dielectric in low-voltage
and high-performance flexible organic thin film transistors (oTFTs). Reactive sputtered zirconia layers were deposited with
low thermal exposure of the substrate and the resulting porous oxide films with high leakage currents were spin-coated with
an additional layer of poly-α-methylstyrene (PαMS). After this treatment a strong improvement of the oTFT performance could be observed; leakage currents
could be eliminated almost completely. In ellipsometric studies a higher refractive index of the ZrO2/PαMS layers compared to the “as sputtered” zirconia films could be detected without a significant enhancement of the film
thickness. Atomic force microscopy (AFM) measurements of the surface topography clearly showed a surface smoothing after the
PαMS coating. Further studies with X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) also
indicated that the polymer definitely did not form an extra layer. The polymer chains rather (self-)assemble in the nano-scaled
interspaces of the porous oxide film giving an oxide–polymer “nanocomposite” with a high oxide filling grade resulting in
high dielectric constants larger than 15. The dielectric strength of more than 1 MV cm−1 is in good accordance with the polymer-filled interspaces. 相似文献
77.
The methods for the formation of zirconia including precipitation from aqueous salts, sol–gel synthesis from zirconium alkoxides,
and the templated synthesis using surfactants are described in this review. The surface areas obtained vary widely but invariably
decrease upon prolonged calcination. Digestion of hydrous zirconia and incorporation of dopants such as lanthanum, yttrium,
or sulfate ions can increase the surface area and thermal stability. However, these methods also affect the crystal phase
of zirconia. The transformation from the metastable tetragonal to the monoclinic phase occurs during the cooling phase of
calcination. Mechanisms for the stabilization of the tetragonal phase are discussed. Zirconia with well-ordered mesopores
or in the form of hollow spheres can be prepared but lack thermal stability, unless doped with phosphates, silicates or sulfates. 相似文献
78.
A novel method for the immobilization of hemoglobin (Hb) and preparation of reagentless biosensor was proposed using a biocompatible non-toxic zirconia enhanced grafted collagen tri-helix scaffold. The formed membrane was characterized with UV-vis and FT-IR spectroscopy, scanning electron microscope and electrochemical methods. The Hb immobilized in the matrix showed excellent direct electrochemistry with an electron transfer rate constant of 6.46 s−1 and electrocatalytic activity to the reduction of hydrogen peroxide. The apparent Michaelis-Menten constant for H2O2 was 0.026 mM, showing good affinity. Based on the direct electrochemistry, a new biosensor for H2O2 ranging from 0.8 to 132 μM was constructed. Owing to the porous structure and high enzyme loading of the matrix the biosensor exhibited low limit of detection of 0.12 μM at 3σ, fast response less than 5 s and high sensitivity of 45.6 mA M−1 cm−2. The biosensor exhibited acceptable stability and reproducibility. ZrO2-grafted collagen provided a good matrix for protein immobilization and biosensing preparation. This method was useful for monitoring H2O2 in practical samples with the satisfactory results. 相似文献
79.
We have used electron spin resonance spectroscopy to study the defects induced in yttria-stabilized zirconia (YSZ) single crystals by 2.5-MeV electron irradiations. Two paramagnetic centers are produced: the first one with an axial d 111 symmetry is similar to the trigonal Zr 3+ electron center (T center) found after X-ray irradiation or thermo-chemical reduction, whereas the second one is a new oxygen hole center with an axial d 100 symmetry different from the orthorhombic O m center induced by X-ray irradiation. At a fluence around 10 18 e/cm 2 , both centers are bleached out near 600 v K, like the corresponding X-ray induced defects. At a fluence around 10 19 e/cm 2 , defects are much more stable, since complete thermal bleaching occurs near 1000 v K. Accordingly, ageing of as-irradiated samples shows that high-dose defects at more stable than the low-dose ones. 相似文献
80.
The most separations in HILIC mode are performed on silica-based supports. Nevertheless, recently published results have indicated that the metal oxides stationary phases also possess the ability to interact with hydrophilic compounds under HILIC conditions. This paper primarily describes the retention behaviour of model hydrophilic analytes (4-aminobenzene sulfonic acid, 4-aminobenzoic acid, 4-hydroxybenzoic acid, 3,4-diaminobenzoic acid, 3-aminophenol and 3-nitrophenol) on the polybutadine modified zirconia in HILIC. The results were simultaneously compared with a bare zirconia and a silica-based HILIC phase. The mobile phase strength, pH and the column temperature were systematically modified to assess their impact on the retention of model compounds. It was found that the retention of our model hydrophilic analytes on both zirconia phases was mainly governed by adsorption while on the silica-based HILIC phase partitioning was primarily involved. The ability of ligand-exchange interactions of zirconia surface with a carboxylic moiety influenced substantially the response of carboxylic acids on the elevated temperature as well as to the change of the mobile phase pH in contrast to the silica phase. However, no or negligible ligand-exchange interactions were observed for sulfanilic acid. The results of this study clearly demonstrated the ability of modified zirconia phase to retain polar acidic compounds under HILIC conditions, which might substantially enlarge the application area of the zirconia-based stationary phases. 相似文献