Pedro A. P. Nascente

Chemical and structural characterization of V2O5/TiO2 catalysts

Univ Sao Paulo, Inst Fis Sao Carlos, Dept Fis & Ciencia Mat, BR-13560970 Sao Carlos, SP, Brazil

Ti-Nb thin films deposited by magnetron sputtering on stainless steel

Thin films of Ti-Nb alloys were deposited on AISI 316L stainless steel substrate by magnetron sputtering, and the structure, composition, morphology, and microstructure of the films were analyzed by means of x-ray diffraction (XRD), (scanning) transmission electron microscopy (TEM) coupled with energy-dispersive x-ray spectroscopy, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS). Thin films of four compositions were produced: Ti85Nb15 (Ti-26 wt. % Nb), Ti80Nb20 (Ti-33 wt. % Nb), Ti70Nb30 (Ti-45 wt. % Nb), and Ti60Nb40 (Ti-56 wt. % Nb). Structural characterization by XRD indicated that only the β phase was present in the thin films and that the increase in the Nb content modified the alloy film texture. These changes in the film texture, also detected by TEM analysis, were attributed to different growth modes related to the Nb content in the alloy films. The mean grain sizes measured by AFM increased with the Nb amount (from 197 to 222 nm). XPS analysis showed a predominance of ox...

Ordering of ethylidyne on clean and adsorbate covered Pd(111): Influence of the coadsorption of oxygen

The adsorption and ordering of acetylene on clean and on oxygen‐covered Pd(111) at low temperature (150–170 K) and at room temperature were studied by low‐energy electron diffraction and high‐resolution electron energy‐loss spectroscopy. At low temperature a poorly ordered overlayer of molecularly chemisorbed C2H2 was formed on both clean and O‐covered Pd(111). At room temperature C2H2 converted to ethylidyne (C2H3), which weakly ordered in a ((3)1/2×(3)1/2) R30° structure. However, preadsorption of oxygen was found to induce better ordering of C2H3 on Pd(111) by (a) possibly removing the free hydrogen atoms on the surface and (b) interacting with the hydrocarbon species via the coadsorbate induced ordering mechanism. It is proposed that the Pd(111)‐((3)1/2×(3)1/2) R30°–O+C2H3 ordered overlayer can arise from a combination of dipole–dipole interaction and charge transfer through the substrate.

Cathodic cage plasma deposition of TiN and TiO2 thin films on silicon substrates

Cathodic cage plasma deposition (CCPD) was used for growing titanium nitride (TiN) and titanium dioxide (TiO2) thin films on silicon substrates. The main advantages of the CCPD technique are the uniformity, tridimensionality, and high rate of the film deposition that occurs at higher pressures, lower temperatures, and lower treatment times than those used in conventional nitriding treatments. In this work, the influence of the temperature and gas atmosphere upon the characteristics of the deposited films was investigated. The TiN and TiO2 thin films were characterized by x-ray diffraction, scanning electron microscopy, and Raman spectroscopy to analyze their chemical, structural, and morphological characteristics, and the combination of these results indicates that the low-cost CCPD technique can be used to produce even and highly crystalline TiN and TiO2 films.

Surface characterization of Zr/Ti/Nb tri-layered films deposited by magnetron sputtering on Si(111) and stainless steel substrates

Among metallic materials, commercially pure titanium and titanium alloys are very often used as biomaterials for implants. Among these alloys, titanium-aluminum-vanadium alloy Ti-6 A-4 V is one of the most commonly used due to its excellent biocompatibility and ability to allow bone-implant integration. A new class of Ti alloys employs Zr for solid-solution hardening and Nb as β-phase stabilizer. Metals such as Ti, Nb, and Zr—known as valve metals—usually have their surfaces covered by a thin oxide film that forms spontaneously in air. This oxide film constitutes a barrier between the metal and the medium. The Ti-Nb-Zr alloys have mechanical and corrosion resistance characteristics which make them suitable for use as implants. Tri-layered films of Ti-Nb-Zr were deposited on both Si(111) and stainless steel (SS) substrates using dc magnetron sputtering equipment, under an argon atmosphere according to the following methodology: a 100 nm thick layer of Nb was deposited on the substrate, followed by a 200 nm...

Studies on plasma-nitrided iron by scanning electron microscopy, glancing angle x-ray diffraction, and x-ray photoelectron spectroscopy

The tribological and mechanical properties of several iron alloys are improved by nitriding processes. In this work the plasma process was employed because it offers various advantages as compared to other methods used in surface modifications. For example, it is a nonpollutant method and the nitriding times are reduced. Four samples were prepared by nitriding the iron substrates in a gas mixture of 80% of H2 and 20% of N2 under a pressure of 900 Pa, discharge frequency of 10 kHz and temperature of 500 °C, for 1, 2, 4, and 6 h. The samples were analyzed by optical microscopy, scanning electron microscopy (SEM), microhardness technique, glancing angle x-ray diffraction (GXRD), and x-ray photoelectron spectroscopy (XPS). SEM and GXRD results showed the presence of γ′-Fe4N and a small amount of e-Fe3N phase. XPS was employed in order to obtain the chemical-state and quantitative informations on the plasma-nitrided iron surfaces. The surface N/Fe atomic ratios obtained by XPS were close to the ideal value of 0.25 for Fe4N. The nitrogen concentration close to the surface is lower than that found in the bulk for all samples. This effect is explained by the presence of H2 in the gas mixture, which not only cleans the surface but also removes nitrogen by sputtering.The tribological and mechanical properties of several iron alloys are improved by nitriding processes. In this work the plasma process was employed because it offers various advantages as compared to other methods used in surface modifications. For example, it is a nonpollutant method and the nitriding times are reduced. Four samples were prepared by nitriding the iron substrates in a gas mixture of 80% of H2 and 20% of N2 under a pressure of 900 Pa, discharge frequency of 10 kHz and temperature of 500 °C, for 1, 2, 4, and 6 h. The samples were analyzed by optical microscopy, scanning electron microscopy (SEM), microhardness technique, glancing angle x-ray diffraction (GXRD), and x-ray photoelectron spectroscopy (XPS). SEM and GXRD results showed the presence of γ′-Fe4N and a small amount of e-Fe3N phase. XPS was employed in order to obtain the chemical-state and quantitative informations on the plasma-nitrided iron surfaces. The surface N/Fe atomic ratios obtained by XPS were close to the ideal value of ...

Effect of cooling conditions on plasma-carbonitrided iron surfaces

Plasma carbonitriding (nitrocarburizing) has been used to improve the tribological and mechanical properties of materials, especially iron-based alloys. In this work, the pulsed glow discharge technique was used for nitrocarburizing pure iron in order to investigate the effect of the cooling conditions on the characteristics of the formed layers. Three sample sets were nitrocarburizing in a gas mixture of 2 vol % CH4, 78 vol % H2, and 20 vol % N2, under a pressure of 400 Pa, discharge frequency of 9 kHz, temperature of 580 °C, during 90 min. One set was cooled under a vacuum, the second set was cooled in air, and the third set was rapidly removed from the furnace at the treatment temperature and quenched in oil. All samples were characterized by optical microscopy, scanning electron microscopy, x-ray diffraction, and Vickers microhardness. Differences in surface composition and morphology were observed, indicating the importance of the cooling rate on the formed layer in these three processes. For all of ...

Avaliação dos níveis de desgaste e de microdureza em amostras de ferro sinterizado com e sem tratamento de ferroxidação e nitretado a plasma

Nitretacao a plasma e uma tecnica de modificacao superfi-cial que objetiva melhorar propriedades mecânicas dos ma-teriais tais como a dureza, resistencia ao desgaste e resis-tencia a fadiga. Neste trabalho, este processo foi empregado na modificacao superficial de uma liga de ferro sinterizado. Foram feitos ensaios de desgaste e microdureza na liga sem tratamento e nas amostras submetidas ao tratamento de fer-roxidacao e ao tratamento a plasma realizado sob diferen-tes condicoes de tempo e temperatura. Os resultados mos-traram que tanto a resistencia ao desgaste como a microdu-reza aumentam com o tempo e a temperatura de nitretacao, sendo que a variavel que mais influenciou nas caracteristi-cas mencionadas foi a temperatura.