P. Prieto

Magnetoelectric coefficient in BiFeO3 compounds

In this work, we report magnetoelectric coefficient measurements in BiFeO3 (BFO) compounds using dynamic lock-in technique to evidence magnetoelectric coupling behavior. The response of the magnetoelectric coefficient shows rapid increase from low frequencies (0.1kHz) to around 5kHz, reaching a maximum, and then decreasing monotonously to 100kHz. For a constant frequency at 7kHz, the maximum magnetoelectric coefficient was close to 7mV∕cmOe, obtained at a bias magnetic field of H=120Oe. Thus, we demonstrated the feasibility of this technique for characterizing multiferroic materials.

Tungsten carbide/diamond-like carbon multilayer coatings on steel for tribological applications

Abstract Tungsten carbide/diamond like (W–C/DLC) multilayers have been investigated as low friction coatings on high-speed steel substrates. The coatings are composed of a W–C multilayer base and an upper lubricious DLC layer and they are obtained by reactive r.f. magnetron sputtering from a single target comprising of two equal halves; one-half carbon and the other half tungsten. The whole coating structure was obtained in situ, without any interruption of the sputtering process. Transmission electron microscopy (TEM) and SIMS were used to assess the multilayer structure and XPS, XRD, and Raman spectroscopy was used to analyze its composition. The tribological properties of the coatings in sliding wear were investigated by means of scratch test and ball-on-disc test measurements. It was found that the multilayer W–C base improves the adhesion of the upper DLC layer to steel substrates while maintaining its low friction coefficient.

Characterization of diamond-like carbon (DLC) thin films prepared by r.f. magnetron sputtering

Diamond-Like Carbon (DLC) thin films were deposited on stainless steel and silicon substrates by a r.f. (13.56 MHz) magnetron sputtering technique. A carbon target (99,99%) and a gas mixture of Ar/CH 4 were used. During the deposition process the plasma discharge was monitored by Optical Emission Spectroscopy (OES) in order to analyze the state of the chemical species present in the plasma. The films were characterized by Raman Spectroscopy and by Reflection Absorption and Transmission Infrared Spectroscopy. The morphology of the deposited layers was analyzed by Scanning Electron Microscopy (SEM). The Raman intensity of the diamond and graphite peaks (I D/IG) depends on the percentage of CH4 in the gas mixture. The relationship between the lines H α, and Hβ, intensities are a measure of the relative change of the plasma electronic temperature that, for the experiment conditions, does not depend significantly on the concentration of CH 4 in the mixture and its value was of the order of 1 eV. Optical Emission Spectroscopy shows that, besides of the atomic hydrogen peak (H α, Hβ, Hγ), emission spectra are dominated by neutral CH species and the most intense peak in the spectra correspond to CH (A 2 Δ → X 2 Π at 431,5 nm) which is supposed to be the precursor species in the diamond-like films, beside in the Transmission Infrared Spectroscopy analysis were observed the sp 3 CH2 symmetric and asymmetric at 2870 and 2960 cm -1 peaks. These peaks have been observed in diamond deposited at very high CH4 concentrations and also in diamond-like carbon (DLC) films.

Optical emission spectroscopy study of r.f. magnetron sputtering discharge used for multilayers thin film deposition

Abstract An investigation by Optical Emission Spectroscopy (OES) was performed on RF (13.56 MHz) magnetron sputtering discharge used for Tungsten Carbide/Diamond-Like Carbon (W-C/DLC) multilayer thin film deposition. The multilayer deposition is obtained from a single tungsten (99.99%) and carbon (99.99%) binary compound target by gradual variation of methane (CH4) concentration in the argon/methane (Arue5f8CH4) gas mixture. The optical emission spectra supply information about the chemical species present in the plasma, and the densities of these species can be correlated with the structure and composition of the deposited layers. The electron temperature dependence on gas pressure and methane concentration in Arue5f8CH4 gas mixture by using some Ar, W and H emission line intensities, are studied in the center of the discharge. A simple model for the excitation process of the Ar, W and H atoms allows us to calculate the density of sputtered atoms in the plasma. At high methane concentrations for deposition of DLC layers, OES shows that besides the atomic hydrogen peaks Hα (656.3 nm) and Hβ (486.1 nm) of Balmer series, emission spectra are dominated by neutral CH species and the most intense peak in the spectra corresponds to CH-band head at 431.5 nm, which is supposed to be the precursor species in the diamond-like films. Owing to the transport regime of the W and C sputtered species across the discharge, the existence of correlation between the target composition, tungsten density in the plasma, multilayer thin film composition, and microstructure determined by XPS, XRD, IR and Raman spectroscopy are studied.

Mechanical and tribological properties of tungsten carbide sputtered coatings

Tungsten carbide (WC) hard coatings have been obtained on steel substrates by an r.f. magnetron sputtering process. Two-layer coatings have been deposited in order to improve the adhesion on steel. The lower layer was tungsten metal and the upper WC layer was obtained by reactive sputtering of the tungsten target in an Ar and methane gas mixture. AES and SIMS confirmed that the WC layer composition depends on the reactive sputtering gas composition. Film microhardness was measured by microindentation, and the coating adhesion by microscratch. Measurement results showed that high hardness coatings can be prepared at a relatively low temperature and that good adhesion on steel is achieved with the two-layer coating. Nanowear measurements showed a noticeable dependence of this applied functional property on the coating compositions.

Formation of highly textured BiSrCaCuO superconducting thick films by a rapid melting process

Abstract We have prepared highly textured Biue5f8Srue5f8Caue5f8Cuue5f8O thick films from mixtures of the oxides by a rapid melting process. Sample powders on (100)-MgO-substrates were melted within seconds and kept between 910 °C and 1050 °C for typically several minutes. The samples were then directly cooled to room temperature. The crystallization of Bi2Sr2CaCu2Ox and the formation of a (001) - texture was observed in situ with a high-temperature X-ray diffractometer. Resistivity measurements showed the film to exhibit a Tc of 70 K. Subsequent annealing steps in air between 850 °C and 885 °C raised Tc to 85 K. The critical current density as inferred from magnetization measurements is at least 103 A/cm2. Morphology and composition of the films were investigated by scanning electron microscopy and electron diffraction.

Improvement of Electrochemical Surface Properties in Steel Substrates Using a Nanostructured CrN/AlN Multilayer Coating

Improvement of corrosion properties on AISI D3 steel surfaces coated with [CrN/AlN]n multilayered system deposited for various periods (Λ) via magnetron sputtering has been studied in this work exhaustively. For practical effects compared were the latter properties with CrN and AlN single layers deposited with the same conditions as the multilayered systems. The coatings were characterized in terms of crystal phase; chemical composition, micro-structural, and electrochemical properties by x-ray diffractometry, energy dispersive x-ray, Fourier transforming infrared spectroscopy, atomic force microscopy, scanning electron microscopy, Tafel polarization curves, and electrochemical impedance spectroscopy. Corrosion evolution was observed via optical microscopy. Results from x-ray diffractometry analysis revealed that the crystal structure of [CrN/AlN]n multilayered coatings has an NaCl-type lattice structure and hexagonal structure (wurtzite-type) for CrN and AlN, respectively, i.e., it was made non-isostructural multilayered. The best behavior was obtained by the multilayered period: Λxa0=xa060xa0nm (50 bilayers), showing the maximum corrosion resistance (polarization resistance of 1.18xa0KΩ, and corrosion rate of 1.02xa0mpy). Those results indicated an improvement of anticorrosive properties, compared to the CrN/AlN multilayer system with 1 bilayer at 98 and 80%, respectively. Furthermore, the corrosion resistance of steel AISI D3 is improved beyond 90%. These improvement effects in multilayered coatings could be attributed to the number of interfaces that act as obstacles for the inward and outward diffusions of ion species, generating an increment in the energy or potential required for translating the corrosive ions across the coating/substrate interface. Moreover, the interface systems affect the means free path on the ions toward the metallic substrate, due to the decreasing of the defects presented in the multilayered coatings.

Gas Temperature Determination of an AC Arc Discharge at Atmospheric Pressure in Air Using a Mach–Zehnder Interferometer

To determine the gas temperature of an alternating current (ac) (50 kHz) arc discharge at atmospheric pressure in air, a Mach–Zehnder type interferometer was built, utilizing an He-Ne

FTIR studies of tungsten carbide in bulk material and thin film samples

(lambda=6328 hboxAA)

Plasma characterization of Pulsed-Laser ablation process used for fullerene-like CNx thin film deposition

laser source with vertical polarization and a power of 5 mW. Upon introducing the arc discharge into one of the interferometer arms, a displacement of interference fringes takes place with respect to its position without the discharge. Keeping in mind that displacement of the fringes is related to the difference of the optical path with the variation of the refraction index and with temperature change in the discharge zone; the latter was determined from the displacements of the interference fringes. At the center of the discharge channel, a temperature near 3000 K was calculated, diminishing gradually to room temperature toward the discharge borders. This temperature value at the center of the channel agrees with results previously reported in similar discharges. This is an alternate method for the diagnosis of plasma parameters used in the production and treatment of materials.