Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H− yield

International audienceHighly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56MHz plasma reactor. Relative yields of surface-produced H− ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (∼3-5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing H− production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions

Study of the interaction between a Zirconium thin film and an EN C100 steel substrate: temperature effect

Zirconium thin films were grown on high carbon steel substrates EN C100 (1 %wt. of carbon) by RF magnetron sputtering. In order to study the reactivity of the film/substrate system as a function of the temperature, one hour vacuum annealing was carried out for different temperatures between 600°C and 1100°C. The films were then analyzed by X-ray diffraction, scanning electron microscopy, glow discharge optical emission spectroscopy (GDOES) and nanoindentation. The obtained results showed a progressive transformation of zirconium film to zirconium carbide. Carbon atoms diffusion from substrate toward the film induced this transformation. The sample annealed at 900°C exhibited the best mechanicals properties (H =17 GPa and E = 220 GPa). Samples treated at higher temperature were affected by oxidation and high microporosity. Even if the conversion is uncomplete, annealing significantly promotes adhesion of the film on the substrate

Electrochemical Hydrogen Insertion in Substoichiometric Titanium Carbide TiC0.6: Influence of Carbon Vacancy Ordering

International audienceSubstoichiometric titanium carbide of formula TiC0.6 with different degrees of carbon vacancy ordering has been synthesized first by reactive sintering at high temperature (2100 °C) and then by annealing at low temperature (730 °C) for different durations. The effect of annealing on the structure of the carbide and its capacity to electrochemically insert hydrogen has been investigated. The XRD study reveals two phase transitions in the carbide during annealing. First, annealing of disordered TiC0.6 (space group Fm3̅m) for 40 h leads to a trigonal superstructure of space group R3̅m thanks to ordering of carbon vacancies. After the longest annealing time of 120 h, the structure of the carbide becomes cubic anew, but with a space group of Fd3̅m. The electrochemical hydrogen insertion in these different types of TiC0.6, as studied by cyclic voltammetry, strongly depends on the crystalline structure of the carbide. The maximum storage capacity is obtained for 40 h, corresponding to the R3̅m ordered phase. Because the R3̅m structure consists of alternately empty and full (111) carbon atomic planes, we show that the hydrogen insertion and diffusion in the material is rendered possible by the presence of vacant (111) planes. This behavior is very similar to that of a cation-ordered manganese-nickel spinel vis-à-vis lithium ion insertion

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HYDROGEN INSERTION IN SUBSTOICHIOMETRIC TITANIUM CARBIDE TiCx THIN FILM

International audienceThere is nowadays a growing need for the development of cheap and effective energy storage and conversion materials. That is why more and more recent studies do not focus on original chemical compositions, but on the tailoring of structures and shapes of already existing materials. Titanium carbide is ceramic mainly known for its refractarity (melting point ca. 3067 °C), its metallic conductivity (σ ≈ 106 Ω-1 m-1) and its high hardness (HV = 28 GPa) making him is one of the most widely applied hard coating materials. TiC presents a large range of composition from TiC0.55 to TiC0.98. In two recent electrochemical studies [1-2], it has been shown that hydrogen can be inserted in the substoichiometric titanium carbide TiC0.6, but not in TiC0.9, this latter being closer to the stoichiometry. Moreover, in contrast to carbon or nanoporous gold, TiC seems to represent a more viable, stable, cathode for aprotic Li-O2 cells [3].Objective of this work is to investigate the electrochemical properties of TiC in the form thin films that would be suitable as an electrode. Substoichiometric titanium carbide thin films TiCx were synthesized by reactive magnetron sputtering using titanium target and a variable flow of gaseous benzene. Poisoning of the target was studied by optical emission spectroscopy and the monitoring of the cathodic voltage, pressure and deposition rate. The samples present a dense microstructure composed with columnar grains of 30 nm width. X-Ray Diffraction reveals that the films present a (111) preferred orientation. In complement to XRD, X-Ray Photoelectron Spectroscopy measurements suggest that for a low carbon content, a Ti-rich metallic phase is present while for a high carbon content, nanocrystallites of TiCx could be embedded by an amorphous carbon matrix. The compositions TiC and TiC0.6 were approximately obtained for 30 and 22 sccm respectively. The electrochemical measurements were carried out at room temperature, in a standard three-electrode cell. The solution was a 1 M H2SO4 aqueous solution deaerated by pure Ar bubbling. TiCx thin films were mounted on a 2000 rpm rotating disk as a working electrode. Results show that hydrogen insertion is possible in the thin film TiC but not in TiC0.6. This behaviour is totally different from bulk TiCx synthesized by classic sintering.[1] Gringoz, A., Glandut, N., Valette, S.Electrochemical hydrogen storage in TiC0.6, not in TiC0.9(2009) Electrochemistry Communications, 11 (10), pp. 2044-2047.[2] Nguyen, J., Glandut, N., Jaoul, C., Lefort, P.Electrochemical hydrogen insertion in substoichiometric titanium carbide TiC0.6: Influence of carbon vacancy ordering(2013) Langmuir, 29 (38), pp. 12036-12042. [3] Ottakam Thotiyl, M.M., Freunberger, S.A., Peng, Z., Chen, Y., Liu, Z., Bruce, P.G.A stable cathode for the aprotic Li-O2 battery(2013) Nature Materials, 12 (11), pp. 1050-105

Stockage électrochimique d'hydrogène dans le carbure de titane

Ce travail concerne l'étude de la faisabilité de l'insertion électrochimique de l'hydrogène dans le carbure de titane sous-stoechiométrique de formule TiCx obtenu par des procédés de frittage réactifs conventionnels (naturel et sous charge) et sous forme de couches minces par pulvérisation cathodique magnétron. L'insertion électrochimique de l'hydrogène dans ce matériau dépend grandement de plusieurs facteurs : le procédé d'élaboration, la structure cristalline et la stoechiométrie du carbure. Le frittage sous charge des carbures TiCx avec x inférieur ou égal a 0,70 mènent à l'obtention d'une structure cristalline ordonnée où les plans (111) de carbone sont partiellement vides permettant l'insertion de l'hydrogène dans le matériau. A l'inverse, les carbures préparés par frittage naturel à haute température (2100C) ne permettent pas l'insertion de l'hydrogène quelle que soit la stoechiométrie du carbure car les lacunes de carbone sont désordonnées dans leur structure cristalline. Néanmoins, il est possible d'ordonner ces lacunes de carbones par des recuits thermiques à basse température (730C) et d'identifier de nouveau les plans (111) de carbone partiellement vides comme responsable de l'insertion de l'hydrogène dans le carbure de titane dont le coefficient de diffusion a ete estimé a 1,2 X 10-13 cm2.s-1 dans TiC0,60. La réaction électrochimique d'oxydation du carbure de titane a aussi été étudiée, où il est démontré que le carbure de titane s'oxyde en TiO2 avec un rejet de CO2.This work deals with the feasibility of the electrochemical hydrogen insertion into the substoichiometric titanium carbides TiCx (0.5 <= x <= 1) obtained by conventional reactive sintering (natural and hot pressing), and under the form of thin films, as obtained by magnetron reactive sputtering. The electrochemical hydrogen insertion in this material strongly depends on several parameters : (i) the elaboration process ; (ii) the crystalline structure ; and (iii) the stoichiometry of the carbide. The carbides TiCx obtained by hot pressing with x lower or equal to 0.70 present an ordered crystalline structure where the (111) carbon plans are partially empty, allowing the hydrogen insertion into the material. On the contrary, the carbides prepared by reactive sintering at high temperature (2100C) do not allow the hydrogen insertion whatever the carbide stoichiometry, because of the disorder of the carbon vacancies inside the crystalline structure. Nevertheless, it is possible to order these carbon vacancies by annealing at low temperature (730C), this treatment rendering again the carbon plans (111) partially empty, and so, allowing the hydrogen to penetrate inside the titanium carbide with a diffusion coeffcient estimated at 1.2 X 10-13 cm2.s-1 in TiC0.60. The electrochemical reaction of oxidation of the titanium carbide was also studied, and it is demonstrated that TiC oxidizes into TiO2 accompanied by a CO2 release.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Hydrogen insertion in substoichiometric titanium carbide

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Hydrogen insertion in titanium carbide based thin films (nc-TiC x /a-C:H) - comparison with bulk TiC x

International audienceNanocomposites composed of titanium carbide nanosized grains embedded in an amorphous hydrogenated carbon matrix (nc-TiCx/a-C:H) are prepared by hybrid Magnetron Sputtering - PECVD process using a titanium metal target and gaseous C6H6. By controlling the benzene flow rate, thin films with different carbon content are obtained. The structures of nc-TiCx/a-C:H materials are analyzed by X-ray diffraction, X-ray photoelectron and Raman spectroscopic methods. The electrochemical hydrogen insertion, as studied by cyclic voltammetry, strongly depends on the carbon content in the thin films. The correlation between the hydrogen insertion ability and the structure of materials are discussed. Furthermore, we show that the hydrogen insertion in these thin films reaches values much more significant than in bulk substoichiometric titanium carbide obtained by reactive sintering