Search results for "MINERALOGY"
showing 10 items of 1516 documents
Mechanism of water adsorption in the large pore form of the gallium-based MIL-53 metal-organic framework
2016
Abstract Water adsorption in the large pore ( lp_empty ) form of Ga-MIL-53 was studied by TGA, DSC and in situ XRD and FTIR at 298 K. The large pore form can be stabilized at room temperature after activation under vacuum at 553 K. The isotherm of water adsorption in this large pore form (pore dimensions: 1.67 × 1.33 nm) is very similar to that measured on the narrow pore ( np_empty ) form (pore dimensions: 1.97 × 0.76 nm). Such a similarity is rather unusual given that the pore sizes of these two phases are very different. In order to understand the origin of this effect in situ XRD and FTIR measurements were particularly helpful. It was found that the adsorption of even small amount of wa…
Influence of the crystallization process on the photoelectrochemical behaviour of anodic TiO2 films
1997
On the basis of kinetic and photoelectrochemical studies we show that the formation of amorphous or strongly disordered TiO2 films on electropolished titanium rods can occur upon anodization in 0.5 M H2SO4 solution in a range of thickness which depends on the anodization rate. This finding is confirmed both by the changes in the shape of the photocurrent vs. potential curves with the energy of the incident photons, and by the impedance behaviour of the junction. Our data indicate that TiO2 films having different degree of disorder are formed depending on the anodization rate and oxide thickness. Crystalline films are formed at very low growth rates since very low thicknesses. Amorphous or s…
Influences of Structure and Composition on the Photoelectrochemical Behaviour of Anodic Films on Zr and Zr-20at.%Ti
2008
Abstract A photoelectrochemical investigation on anodic films of different thickness grown on sputter-deposited Zr and Zr–20 at.%Ti was carried out. The estimated optical band gap and flat band potential of thick ( U F ≥ 50 V) anodic films were related to their crystalline structure and compared with those obtained for thinner ( U F ≤ 8 V/SCE) anodic oxides having undetermined crystalline structure. The E g values obtained by photocurrent spectroscopy were also compared with the experimental band gap estimated by other optical ex situ techniques and with the available theoretical estimates of the zirconia electronic structures in an attempt to reconcile the wide range of band gap data rep…
A photoelectrochemical investigation of conversion coatings on Mg substrates
2005
Abstract The structure, morphology and composition of conversion coatings grown in stannate bath on pure Mg were studied using potential–time, polarization curves, X-ray diffraction, scanning electron microscopy and photocurrent spectroscopy. The coating is manly constituted by crystalline magnesium–tin hydroxide, whose morphology and distribution depends on the conversion bath composition and temperature. The photoelectrochemical investigation allowed to estimate the band gap value of MgSn(OH) 6 and flat band potential. A sketch of the metal/passive film/electrolyte junction formed during conversion on the metal substrate is reported to account for the overall photoelectrochemical behaviou…
Photocurrent spectroscopy study of passive films on hafnium and hafnium–tungsten sputtered alloys
2003
Abstract Anodic and air-formed films on sputtered Hf and W–Hf alloys of different composition have been investigated by Rutherford back scattering, photocurrent spectroscopy (PCS) and transmission electron microscopy (TEM) techniques. In alkaline solutions the PCS data suggest the formation on Hf metal of a duplex layer with anodic hafnia covered by an external layer of composition close to HfO(OH) 2 . This last compound is also present on Hf air-formed films. In acidic solutions the initial oxy-hydroxide film disappears at high anodising potentials ( V f >10 V). In the case of W–Hf alloys films of different composition and semiconducting behaviour are formed by air exposure or by anodising…
Recent advances in photocurrent spectroscopy of passive films
2003
Abstract The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers. The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-me…
Spontaneous oxygen loading into SiO2 glass by thermal anneal
2004
The interstitial oxygen molecules (O 2 ) in SiO 2 glass were detected down to ∼10 15 cm -3 by photoluminescence of O 2 at 1272nm excited at 765nm by a continuous-wave titanium sapphire laser. It was evidenced that SiO 2 glass thermally annealed in air between 800 and 1100°C spontaneously absorbs ∼10 16 cm -3 of O 2 from the ambient atmosphere. The time-dependent concentration change of the interstitial O 2 allows the determination of both the diffusion coefficient and the solubility of the interstitial O 2 .
Excited state absorption and energy-transfer mechanisms of up-conversion luminescence in Er3+-doped oxyfluoride glass ceramics at different temperatu…
2010
Abstract Oxyfluoride silicate glass SiO2–Al2O3–Na2CO3–NaF–LaF3–ErF3 was synthesized. The glass transition and crystallization temperatures were determined by differential thermal analysis. Glass ceramics containing LaF3:Er3+ crystallites of size ∼20 nm were formed in the glass matrix after the heat treatment of the precursor glass in the vicinity of the crystallization temperature. Up-conversion luminescence, excitation spectra as well as time-resolved up-conversion luminescence of the glass and glass ceramics were studied at different temperatures. The up-conversion transients showed that at room temperature the dominant mechanism of the up-conversion luminescence in the glass ceramics is …
Oxygen-excess amorphous SiO2 with 18O-labeled interstitial oxygen molecules
2011
Abstract Exchange between oxygen molecules embedded in amorphous SiO 2 (interstitial O 2 ) and oxygen atoms in the a -SiO 2 network is found to be remarkably slow at 500 °C. Thermal loading of 18 O 2 at this temperature yields a -SiO 2 containing 18 O-labeled interstitial O 2 whose 18 O fraction is as high as ~ 90%. The 18 O fraction of interstitial O 2 in this sample is quickly decreased by thermal annealing at or above 700 °C because of the oxygen exchange accompanied by the release of 16 O from the a -SiO 2 network. This finding indicates that the oxygen exchange starts at much lower temperatures than indicated by previous works, based on monitoring of the isotopic composition of oxygen …
Photoluminescence in AlN: macro‐size and nano‐powder
2007
Photoluminescence (PL) properties in AlN are studied when size of polycrystalline grains is reduced from macro-size (AlN ceramics and macro-size powder) to nano-size (nano-powder). It was found that in all these materials within the UV- blue spectral region two PL bands are observed at ∼400 nm (caused by the oxygen-related defects) and at 480 nm (with unknown defect structure) but the ratio of their intensities depends on the grain size. Reduction of AlN grain size results in decrease of the 400 nm band and increase of the 480 nm band. It allows the following conclusions: i) reduction of AlN grain size results in decrease of oxygen (the native impurity of AlN) content in the crystalline lat…