Porous Alumina Matrix Research Articles

Effect of Yttrium Aluminum Garnet Additions on Alumina‐Fiber‐Reinforced Porous‐Alumina‐Matrix Composites

The effects of incorporating yttrium aluminum garnet (YAG) into a porous alumina matrix reinforced with Nextel 610 alumina fibers were investigated. Composites with various amounts of YAG added to the matrix were prepared to determine its effect on retained tensile strengths after heating to 1100° and 1200°C. Strengths of YAG‐containing composites were slightly lower than those of an all‐alumina‐matrix composite after heating for 5 h to 1100°C. However, after heating for 5 or 100 h at 1200°C, all the YAG‐containing composites displayed greater strengths and greater strains to failure than the all‐alumina composite. At the higher temperature, the presence of YAG is believed to inhibit the densification of the matrix, which helps to maintain higher levels of porosity and weaker interparticle bonding that allows for crack‐energy dissipation within the matrix. A reduction in grain growth of the fibers by the presence of segregated Y was also observed, which may also contribute to higher fiber strength, thereby increasing the retained strengths of the YAG‐containing composites.

Transient and alternating current conductivity of nanocrystalline porous alumina thin films on silicon, with embedded silicon nanocrystals

Structural characterization and surface topography of porous alumina thin films on silicon with embedded silicon nanocrystals were performed using scanning and transmission electron microscopy. The nature of porous alumina thin films is nanocrystalline with a high density of uniformly distributed silicon nanocrystals. The pores were randomly distributed with an average size of 35 nm. ac impedance spectroscopy measurements were performed at room temperature, from 0.05 up to 3.0 V in the range of 1–105 Hz for both porous alumina thin films with and without embedded silicon nanocrystals. Transient current measurements were also performed from 0.5 up to 50.0 V in the time interval 1–100 s both in forward and reverse bias conditions. The electrical conduction is dominated by the porous alumina matrix and there is no evidence of participation of the contacts to the electrical properties of the thin films. ac conductivity results follow the dielectric universal response through the whole frequency range of investigation. The real part of the specific electrical conductivity σ′ is voltage independent, in the samples studied, implying the presence of a conduction mechanism. The analysis of the experimental data reveals that the conductivity is governed by two different conduction mechanisms regardless of bias conditions, forward or reverse. In the low applied voltage region the conduction is due to thermally excited electrons, hopping from one state to another. This conduction mechanism is ohmic. For higher voltages the electrical conduction is space charge limited.

Influence of porous anodic alumina matrix upon europium luminescence from sol–gel-derived films

Photoluminescence (PL) and PL excitation (PLE) spectra of Eu-doped TiO 2, SnO 2, and In 2O 3 xerogel films fabricated on porous anodic alumina (PAA) and flat substrates of monocrystalline silicon are analyzed. PAA matrix changes significantly the excitation conditions of Eu in TiO 2 xerogel film, resulting in broad intense band at 280 nm in PLE spectra that is not presented for the same xerogel film fabricated on Si substrate. Moreover, PAA changes the PL behavior of xerogel subjected to thermal treatment resulting in non-monotonous dependence PL intensity on annealing temperature. It is revealed that increase of annealing temperature from 200 to 500 °C leads to decrease of intensity of predominant spectral band at 611 nm, whereas further temperature increase up to 900 °C increases the PL intensity over the value obtained at initial annealing temperature.

Formation of complex anodic films on porous alumina matrices

The kinetics of growth of complex anodic alumina films was investigated. These films were formed by filling porous oxide films (matrices) having deep pores. The porous films (matrices) were obtained voltastatically in (COOH)2 aqueous solution under various voltages. The filling was done by reanodization in an electrolyte solution not dissolving the film. Data about the kinetics of reanodization depending on the porosity of the matrices were obtained. On the other hand, the slopes of the kinetic curves during re-anodization were calculated by two equations expressing the dependence of these slopes on the ionic current density. A discrepancy was ascertained between the values of the calculated slopes and those experimentally found. For this discrepancy a possible explanation is proposed, related to the temperature increase in the film, because of that the real current density significantly increases during re-anodization.

Fabrication of Alumina-YAG Composites by Immersion of Liquid into Porous Alumina Matrix

Fabrication of Alumina-YAG Composites by Immersion of Liquid into Porous Alumina Matrix

Optical properties of nanostructured thin films containing noble metal clusters: AuN, (Au0.5Ag0.5)N and AgN

The optical properties of nanocomposite thin films of gold, silver and bimetallic silver-gold clusters embedded in a porous alumina matrix have been investigated in the size range 2–6.7 nm. The metallic particles are produced by laser vaporization of either an Au0.5Ag0.5 alloy or a pure metal target whereas the dielectric matrix is evaporated by an electron gun. Samples involving a low metal concentration have been characterized by several complementary techniques in order to determine their composition, morphology and cluster size distribution. The mixed particles have the same stoichiometry as the target rod. Optical absorption spectra exhibit a surface plasmon resonance whose position is shifting with cluster mean size, giving evidence of finite size effects. Theoretical calculations in the framework of Time-Dependent-Local-Density-Approximation (TDLDA), taking into account an inner skin of ineffective screening and the porosity of the matrix, are consistent with observed size evolutions of the Mie frequency in each type of sample.

An Efficient Template Synthesis of Aligned Boron Carbide Nanofibers Using a Single-Source Molecular Precursor

The synthesis of aligned, monodispersed boron carbide nanofibers has been achieved by the 1025 °C pyrolysis of the compound 6,6‘-(CH2)6(B10H13)2 absorbed into a porous alumina matrix, followed by dissolution of the alumina with aqueous HF. SEM studies show that the nanofibers are ∼45 μm in length with diameters of ∼250 nm. XRD and TEM studies demonstrated that fibers pyrolyzed at 1000 °C (3 h) are amorphous, but those heated at 1025 °C (3 h) showed onset of boron carbide crystallization.

Bismuth nanowire arrays: Synthesis and galvanomagnetic properties

This paper reports galvanomagnetic properties of arrays of single-crystal bismuth nanowires, with diameters of 7 to 200 nm, embedded in an amorphous porous anodic alumina matrix. A sample preparation technique is described that makes it possible to obtain nanowires with diameters below 10 nm. The wires are single crystals, with their long axes oriented in the bisectrix/trigonal plane, about 19\ifmmode^\circ\else\textdegree\fi{} from the bisectrix axis. The temperature dependence $(1.4\mathrm{K}l~Tl~300\mathrm{K})$ of the electrical resistance, longitudinal magnetoresistance $(0\mathrm{T}l~Bl~5\mathrm{T}$ with $1.4l~Tl~75\mathrm{K},$ and $0Tl~Bl~1\mathrm{T}$ with $80l~Tl~300\mathrm{K})$ and transverse magnetoresistance $(0\mathrm{T}l~Bl~5\mathrm{T}$ with $1.4l~Tl~75\mathrm{K})$ of the nanowires are given. The results extend previous work to wires of narrower diameter, and confirm the existence of the semimetal-semiconductor phase transition seen in the magnetoresistance. The data are discussed qualitatively in terms of the interplay between the electron cyclotron radii, electron scattering on the wire walls, size-induced energy level quantization, and the transfer of carriers between the different carrier pockets of the Fermi surface. Nanowires of Bi are theoretically predicted to have a much higher thermoelectric figure of merit than bulk Bi.

Oxidation Kinetics of Nickel Particles: Comparison Between Free Particles and Particles in an Oxide Matrix

The degradation of selective solar absorbers through oxidation has been studied. We compare the oxidation kinetics of nickel particles of various sizes. Both free particles and particles embedded in an oxide matrix were studied. The oxidation kinetics of polycrystalline nickel nanorods was determined by IR spectroscopy in the temperature range 300–500°C. The particles were oxidized when situated in the porous alumina matrix of an electrochemically deposited solar absorber coating. The oxidation kinetics was compared to that of free nanometer-particles at the same temperature and to micron-sized polycrystalline nickel particles, which were studied by thermogravimetry in a wider temperature range. It was found that the rate constant was markedly lower for the particles in the matrix. Implications for the durability of selectively solar absorbing coatings are discussed.

Quadrupolar vibrational mode of silver clusters from plasmon-assisted Raman scattering

Absorption and low-frequency Raman-scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (\ensuremath{\approx}420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, the maximum of which depends on the cluster diameter D at the maximum of the cluster-size distribution in the sample according to the approximate law ${\ensuremath{\omega}}_{\mathrm{vib}}\ensuremath{\propto}{D}^{\ensuremath{-}1}.$ The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters, via the plasmon-phonon interaction. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is shown to reflect the ellipsoidal shape distribution of part of the embedded clusters.

Size dependence of the energy relaxation in silver nanoparticles embedded in dielectric matrices

The electron dynamics is studied in silver nanoparticles of different sizes embedded either in a silicate glass or in a porous alumina matrix, using time-resolved optical spectroscopy. The femtosecond pump–probe measurements, made under various conditions of excitation, reveal that the energy relaxation is faster as the particle size decreases. For particles with identical size and excitation conditions, the relaxation time is found to be faster for clusters in the alumina matrix. The influence of the surface acoustic modes and of the heat conductivity of the matrix on the energy relaxation is discussed.

Plasmon-phonon coupling and resonant raman scattering of silver clusters

Absorption and low-frequency Raman scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (≈420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 cm-1, the maximum of which depends on the mean cluster diameter 〈D〉 in the sample according to the approximate law ωvib∝〈D〉-1. The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is thought to reflect the ellipsoidal shape distribution of part of the embedded clusters.

A New Granular Composite with High Selectivity for Cesium Ion Prepared from Phosphomolybdic Acid Hydrate and Inorganic Porous Material

Ammonium molybdophosphate (AMP) was incorporated in the porous alumina matrix (JA) by successive impregnation with H3Mo12O40P and NH4NO3. The loading percentage of AMP crystals on JA increased with the repetition times of impregnation. The distribution coefficients of Cs+ on AMP-loaded granular composite (JAAMP) was above 104 cm3/g in the presence of 1 mol/dm3 while those of other nuclides were less than 10, indicating high selectivity of JAAMP to Cs+. The separation factor of 137Cs to other nuclides 85Sr,60Co, 152Eu,241Am,22Na) (αCs/Sr;Co;Eu;Am;Na =K d,Cs/K d,Sr;Co;Eu;Am;Na) was estimated to be above 103. The breakthrough curve for Cs+ through the column packed with JAAMP exhibited a symmetrical S-shaped profile, and this exchanger proved to be effective for the selective isolation of radiocesium from a simulated high-level liquid waste.

Preparation and emission spectra of Eu(III) in nanostructured γ-alumina

The preparation of fibrous and spheroidal nanonstructured composites of γ-alumina matrices impregnated with Eu2O3 together with their physico-chemical morphology are described. The emission properties of Eu3+ ion were investigated in both nanocomposites. A more intense emission in the spheroidal nanocomposite than in the fibrous has been found. In addition to the Eu3+ emission two broad emission bands centered at 770 and 500 nm were observed. These latter are inherently associated with porous alumina matrices.