Francis Maury

Characterization of chromium nitride and carbonitride coatings deposited at low temperature by organometallic chemical vapour deposition

Abstract Chromium nitride and carbonitride hard coatings were deposited by organometallic chemical vapour deposition in the temperature range 350–550 °C. Bis(benzene) chromium and NH 3 or N 2 H 4 were used as chromium and nitrogen vapour sources respectively. The chemical and structural characterization of these coatings is presented. Amorphous films were obtained below 450 °C. Above this temperature, a different phase, namely Cr 7 C 3 , Cr 2 (N,C) or CrN could be prepared depending on the gas phase composition. Electron probe microanalysis and X-ray photoelectron spectroscopy gave evidence for a slight contamination of the films with free carbon. The experimental results are in good agreement with a thermodynamic calculation which has been fruitful in giving information about the feasibility of growing chromium nitride phases under these chemical vapour deposition conditions. The microhardness of the coating in each case was comparable with that of similar material deposited by other techniques.

Iridium coatings grown by metal–organic chemical vapor deposition in a hot-wall CVD reactor

Abstract Deposition of uniform coatings on relatively large size and/or complex shaped pieces require generally isothermal rather than cold-wall chemical vapor deposition (CVD) reactor. After a review of the state-of-the-art of Ir CVD processes aiming the selection of the starting materials, Ir thin films were deposited on W substrates by thermal decomposition of Ir(COD)(MeCp) either in presence of H2 or O2. The growth was carried out in a horizontal hot-wall metal–organic chemical vapor deposition reactor under reduced pressure and low temperature (573–673 K). Using this CVD reactor the process is more difficult to control using H2 rather than O2 as co-reagent. The purity, the microstructure, the growth rate and the thickness uniformity depend on the deposition conditions. Oxygen avoids carbon incorporation in the layers and enhances significantly the growth rate. However, co-deposition of Ir and IrO2 was observed using a high excess of O2. Polycrystalline, compact, untextured and pure Ir coatings were deposited with a satisfactory thickness uniformity over a length of approximately 15 cm and with a typical thickness of 1–2 μm. These coatings have attractive properties to be used as oxidation barriers at high temperature. Optimal deposition conditions were found using the trends predicted by a kinetic model simulating the growth rate along the CVD reactor. A good thickness uniformity along the reactor requires a very short residence time of the reactive species. As a result, the conversion rate is low leading to a poor efficiency of the process.

Chemical vapor deposition of SnO2 coatings on Ti plates for the preparation of electrocatalytic anodes

Abstract SnO2 coatings have been deposited by metal-organic chemical vapor deposition (MOCVD) on Ti plates using SnEt4 and O2 as reactive gas mixture. The thickness, morphology and microstructure of these coatings are controlled by the growth conditions. The deposition process has been used for the preparation of Ti/IrOx/SnO2 electrodes that were tested for the anodic oxidation of organic pollutants in industrial wastewater. These anodes exhibit a high overpotential for oxygen evolution and a good efficiency for the elimination of total organic carbon (TOC) from wastewater. Electrochemical impedance measurements were used to characterize the activity of the interface SnO2/aqueous media as a function of time during the oxidation process. The influence of the grain size, thickness and surface pre-treatment of the Ti substrate on the electrocatalytic properties is discussed.

Elucidating the crystal-chemistry of Jbel Rhassoul stevensite (Morocco) by advanced analytical techniques

Abstract The composition of Rhassoul clay is controversial regarding the nature of the puremineral clay fraction which is claimed to be stevensite rather than saponite. In this study, the raw and mineral fractions were characterized using various techniques including Fourier transform infrared spectroscopy and magic angle spinning nuclear magnetic resonance (MAS NMR). The isolated fine clay mineral fraction contained a larger amount of Al (>1 wt.%) than that reported for other stevensite occurrences. The 27Al MAS NMR technique confirmed that the mineral is stevensite in which the Al is equally split between the tetrahedral and octahedral coordination sites. The 29Si NMR spectrum showed a single unresolved resonance indicating little or no short-range ordering of silicon. The chemical composition of the stevensite from Jbel Rhassoul was determined to be ((Na0.25K0.20)(Mg5.04Al0.37Fe0.20⃞0.21)5.61(Si7.76Al0.24)8O20(OH)4). This formula differs from previous compositions described from this locality and shows it to be an Al-bearing lacustrine clay mineral.

Structural characterization of chromium carbide coatings deposited at low temperature by low pressure chemical vapour decomposition using dicumene chromium

Abstract The growth of chromium carbide coatings by pyrolysis of dicumene chromium in a hot-wall low pressure chemical vapour deposition reactor has been investigated between 300 and 550 °C. Amorphous chromium carbide films were obtained in the low temperature range 300 - 500 °C whereas a textured crystalline Cr 7 C 3 phase was grown above 500 °C. The total carbon content was independent of the deposition temperature and amounted to a carbon excess of about 30% compared with the Cr 7 C 3 stoichiometry. Electron spectroscopy for chemical analysis of both amorphous and crystalline coatings confirms the presence of this carbon excess since about 30% - 40% of free carbon was found. A heterogeneous structural model composed of the Cr 7 C 3 phase and free carbon is proposed both for crystalline and amorphous coatings.

A miniaturised silicon based enzymatic biosensor: towards a generic structure and technology for multi-analytes assays

Abstract A miniaturised and generic biosensor has been developed using microsystem technologies for l- and d- lactate, l -malate, l -alanine and ethanol assays. The biosensor is based on the association between an enzymatic solution including diaphorase and dehydrogenases enzymes confined in the microstructure by a membrane and two gold microelectrodes weakly polarised. The pre-treatment of the gold electrode surface is a critical parameter, the sensor sensitivity can be significantly enhanced by processing chemical cleaning steps with sulphochromic solution, H2SO4+H2O2 solution or oxygen plasma. X-ray photoelectron spectroscopy (XPS) analyses put in evidence that the enhanced sensitivity of electrodes can be in part attributed to the removal of an organic contamination. The assays of l -lactate, ethanol and alanine are also presented and compared to d -lactate assay.

Evaluation of tetra-alkylchromium precursors for organometallic chemical vapour deposition. I: Films grown using Cr[CH2C(CH3)3]4

Abstract Chromium carbide thin films have been deposited by organometallic chemical vapour deposition using the σ-alkylchromium complex Cr[CH 2 C(CH 3 ) 3 ] 4 . The growth was achieved in a hot wall, low pressure chemical vapour deposition reactor in the temperature range 250–350 °C under an inert or hydrogen atmosphere. As a result of the high reactivity of this precursor towards oxygen, the films were partially oxidized. Oxygen incorporation decreases on increasing the deposition temperature. Characterization of their heterogenous structure, mainly by electron diffraction and X-ray photoelectron spectroscopy, gives evidence, in spite of a poor crystallinity, for a mixture of cubic chromium carbide CrC 1− x phase, amorphous chromium metal, chromium oxides (mainly Cr 2 O 3 ) and free carbon, whose proportions depend on the deposition conditions.

INFLUENCE OF ORGANOCHROMIUM PRECURSOR CHEMISTRY ON THE MICROSTRUCTURE OF MOCVD CHROMIUM CARBIDE COATINGS

Abstract Chromium carbide coatings were prepared in the same hot-wall low-pressure chemical vapour deposition reactor using Cr(CO)6, Cr(C6H6)2, Cr(C6H5C3H7)2 and Cr(C5H5)2, chosen as representative organochromium precursors. The structural characterization of the films is described and the ability of the organochromium compounds to deposit a particular phase is discussed. A cubic CrC1−x phase is obtained using the carbonyl derivative, and the main component of the coatings prepared with bis(arene) chromium is the Cr7C3 phase. Free carbon contamination of the films is found regardless of the precursor.

Electrical Switching in Semiconductor-Metal Self-Assembled VO2 Disordered Metamaterial Coatings.

As a strongly correlated metal oxide, VO2 inspires several highly technological applications. The challenging reliable wafer-scale synthesis of high quality polycrystalline VO2 coatings is demonstrated on 4” Si taking advantage of the oxidative sintering of chemically vapor deposited VO2 films. This approach results in films with a semiconductor-metal transition (SMT) quality approaching that of the epitaxial counterpart. SMT occurs with an abrupt electrical resistivity change exceeding three orders of magnitude with a narrow hysteresis width. Spatially resolved infrared and Raman analyses evidence the self-assembly of VO2 disordered metamaterial, compresing monoclinic (M1 and M2) and rutile (R) domains, at the transition temperature region. The M2 mediation of the M1-R transition is spatially confined and related to the localized strain-stabilization of the M2 phase. The presence of the M2 phase is supposed to play a role as a minor semiconducting phase far above the SMT temperature. In terms of application, we show that the VO2 disordered self-assembly of M and R phases is highly stable and can be thermally triggered with high precision using short heating or cooling pulses with adjusted strengths. Such a control enables an accurate and tunable thermal control of the electrical switching.

Physicochemical Study of Photocatalytic Activity of TiO2Supported Palygorskite Clay Mineral

This study deals with the influence of physicochemical parameters, namely, the photocatalyst loading, dye concentration, and pH of polluted solutions, on the degradation efficiency of Orange G (OG) solutions containing TiO2 nanoparticles supported on palygorskite clay mineral (TiO2-Pal). The TiO2 photocatalyst attached to natural palygorskite fibers was elaborated by colloidal solgel route. It exhibits the anatase structure that is themost photoactive crystallographic form.The highest performances of supported photocatalyst on OG degradation were found using an optimumamount of TiO2-Pal around 0.8 g⋅L−1, which corresponds properly to ca. 0.4 g⋅L−1 of TiO2.This amount is interestingly lower than the 2.5 g⋅L−1 generally reported when using pure unsupported TiO2 powder.The photodegradation rate increases by decreasing OG initial concentration, and it was found significantly higher when the OG solution is either acidic (pH < 4) or basic (pH ≈ 11). For OG concentrations in the range 5 × 10 −6–5 × 10 −4 M, the kinetic law of the OG degradation in presence of TiO2-Pal is similar to that reported for unsupported TiO2 nanopowder. It follows a Langmuir-Hinshelwood model with a first-order reaction and an apparent rate constant of about 2.9 × 10 −2 min−1.