Materials Science Forum Vols. 514-516

Abstract: The surface properties of carbon film electrode sensors covered by coating with Nafion polymer film, before and after electrochemical analysis of sub-micromolar traces of lead and cadmium ions, were investigated. These protective polymer films are applied to the electrode surfaces to prevent the irreversible adsorption of chemical species, such as proteins and surfactants present in natural media, which leads to a decrease in response. Electrochemical impedance spectroscopy and voltammetric techniques were used for characterisation. The effect of non-ionic surfactant molecules in solution on the behaviour of the polymer-coated electrode was also investigated. It is shown that permanent changes to the structure and morphology of the Nafion film and of the carbon-Nafion interface occur after deposition of the trace metal ions, an effect which is lessened in the presence of surfactant.

Abstract: The effect of NiCoCrAlYx-ZrO2 (1-x) interlayer on the residual stresses in the ZrO2/NiCoCrAlY thermal barrier coating due to thermal cycling was analyzed using the finite element method. Modeling results showed that the magnitude and distribution of the residual stresses in the coating were strongly influenced by the material combination characterized by the value of x and the thickness of interlayer. The maximum tensile stresses at the different regions (i.e. coating/substrate interface, coating surface, et al.) as functions of the material combination and the thickness of the interlayer were obtained. The physical meaning of the above results was also discussed.

Abstract: As TiAl based alloys begin to approach maturity, the development of successful and cost effective joining methods will be required. The growing industrial interest in these materials, particularly in aerospace and automotive industry, led to an interesting challenge - how to joint parts and components in order to produce integrated and resistant structures. Diffusion bonding of materials produces components with thinner interfaces than other joining techniques do. The absence of abrupt microstructure discontinuity and the small deformation induced maximize joint strength. This work focuses on the joining of TiAl using a thin multilayer obtained by alternating nanometric layers of titanium and aluminium. The Ti/Al layers were deposited onto the γ-TiAl samples by DC magnetron sputtering. The interfaces of these diffusion bonded joints depend on processing and deposition conditions. In this work we describe the influence of bilayer thickness (period) and on microstructure and chemical composition of the joining interfaces.

Abstract: Recently, it was demonstrated that copper thin films show good adsorption characteristics for organic polar and non-polar compounds. Also, these films when used in small cavities can favor preconcentration of these organic compounds. It is also known that copper oxide can provide catalysis of organic compounds. Therefore, the aim of this work is the study of copper thin film catalysis when used in small cavities. Copper thin films, 25 nm thick, were deposited on silicon and/or rough silicon. These films do not show oxide on the surface when analyzed by Rutherford backscattering. Also, Raman analysis of these films showed only silicon bands, due to the substrate, however infrared spectroscopy shows oxide bands for films exposed to organic compound aqueous solutions. Cavities with copper films deposited inside were tested with a continuous flow of n-hexane, acetone or 2-propanol admitted in the system. The effluent was analyzed by Quartz Crystal Microbalance. It was shown that n-hexane or acetone can be trapped. The system also shows good reproducibility. Tests of catalysis were carried out using Raman spectroscopy and heating the films up to 300°C during 3 minutes after exposure to n-hexane, 2- propanol and acetophenone – pure or saturated aqueous solution. After the exposure, Raman spectra present intense bands only for 2-propanol, indicating that adsorption easily occurs. However, after heating with all solutions it was not found only silicon bands. Raman microscopy after heating also showed copper oxide cluster formation and, eventually, graphite formation. Although the heating provides oxide copper formation, this reaction does not produce a high amount of residues, which means that catalysis is possible in this condition. Thus, a simple device using copper thin films can be useful as sample pretreatment on microTAS development.

Abstract: The heat treatment of γ-TiAl alloy (Ti-47Al-2Cr-2Nb (at.%)) diffusion brazed joints was investigated. Joining was performed using a Ti/Ni/Ti clad-laminated braze alloy foil at 1050 and 1150°C with a dwell time of 10 minutes. The joints were subsequently heat treated at 1250 and 1350°C for 240 and 30 minutes, respectively. The microstructure and the chemical composition of the interfaces were analysed by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. Microhardness tests performed across the interface were used to roughly predict the mechanical behaviour of the as-diffusion brazed and of the heat treated joints. Diffusion brazing produced interfaces with two distinct layers essentially composed of α2-Ti3Al and of TiNiAl; γ-TiAl was also detected for joining at 1150°C. After heat treating, the as-diffusion brazed microstructure of the interface was completely replaced by a mixture essentially composed of γ-TiAl and α2–Ti3Al single phase grains and of (α2 + γ) lamellar grains. Microhardness tests showed that the hardness of the as-diffusion brazed interfaces, which ranges from 567 to 844 HV (15 gf), is significantly higher than that of the titanium aluminide alloy (272 HV). All post-joining heat treatments lowered substantially the hardness of the interface, as the hardness of the main phases detected at the interfacial zone after heat treating the joints is comprised between 296 and 414 HV.

Abstract: It is known that anthraquinone derivatives act as aqueous sulphide oxidation catalysts, so the redox chemistry of the compound anthraquinone 2,7-disulphonate (AQ27DS) stimulated our interest, as reported here. AQ27DS was reduced in aqueous solution at pH 9.0 to give a deep red coloured air-sensitive solution. Cyclic voltammetry and exhaustive electrolysis indicated that the anthraquinone was reversibly reduced in a two electron, one proton process at a variety of electrode surfaces. From limiting current results at a rotating disc electrode, the diffusion coefficient of AQ27DS was calculated to be 3.37 x 10-10 m2 s-1. Spectroscopic results confirmed that AQ27DSH- was the major reduced species, but also indicated that the di-anion (AQ27DS2-) and radical species AQ27DS• were also present. ESR spectroscopy showed that the radical was formed via a comproportionation reaction between the di-anion and the AQ27DS starting material. The peak separation from voltammetry enabled the comproportionation constant (Kc) to be estimated, and it was found to be in the range of 0.4 to 4.

Abstract: Three types of bottom electrodes were deposited by RF magnetron sputtering on SiO2/Si substrates: LaNiO3 (LNO), Pt/Ti and LNO/Pt/Ti. The effect of different deposition and processing conditions for the LNO films on the ferroelectric properties of sputtered Pb(Zr0.52Ti0.48)O3 (PZT) capacitors was investigated. The LNO films were either deposited at room temperature and heattreated in O2 flow in the furnace at 500 or 600°C or made in situ in the range of 200-500°C. Other deposition parameters under study were the pressure, the RF power and the Ar:O2 ratio. The resistivity of the LNO films was measured and on some of the films with the lower values, amorphous PZT was deposited and then crystallized in the furnace. X-ray diffraction results show that the PZT films deposited over Pt/Ti had a preferential (100) orientation, while those deposited over LNO made in situ are strongly (100) oriented and the ones deposited over amorphous LNO do not exhibit any preferential orientation. The remanent polarization of the capacitors was around 28μC/cm2 when amorphous LNO or Pt/Ti electrodes were used and around 20μC/cm2 with LNO made in situ. Leakage currents were improved when LNO electrodes made in situ was used; a good ferroelectric fatigue performance of the capacitors when subjected to 1010 switching cycles was also observed

Abstract: The bottom electrode crystallization (BEC) method was applied to the crystallization of Pb(Zr,Ti)O3 (PZT) thin films deposited by RF magnetron sputtering on Pt/Ti/SiO2/Si substrates. Using a proportional-integral-differential controller, the current flowing in the Pt/Ti films provided accurately controlled Joule heating for the crystallization of the PZT films. The temperature uniformity of the heat treatments was investigated by measuring the ferroelectric properties of PZT. Platinum and tungsten wires were alternatively used as electrical contacts. Scanning electron microscopy (SEM) images were used to inspect the electrical contact regions between the platinum films and different contact wires. The PZT films showed higher remanent polarizations and lower leakage currents near the electrical contacts when Pt wires were used; the ferroelectric properties were more uniform on the PZT films heat-treated with W contact wires. The BEC method can successfully replace the more conventional means for thin film crystallization, having the advantage of being a very precise, low cost and low power consumption technique.

Abstract: Thin films of PbZr0,52Ti0,48O3 (PZT) for applications in piezoelectric actuators were deposited by the pulsed laser deposition technique (PLD) over Pt/Ti/SiO2/Si substrates. The effect of different electrode and PZT deposition and processing conditions on the ferroelectric and piezoelectric properties of the devices was investigated. X-Ray diffraction results showed that the deposition temperature for the electrodes had a strong influence on the PZT orientation; the increase in the electrode deposition temperature changes the PZT orientation from random or (111) to (001) depending also on PZT deposition pressure. From scanning electron microscope (SEM) pictures one could also observe that the deposition pressure affects the porosity of the PZT films, which increases with the pressure above 1×10-1 mbar for films deposited at room temperature. The measurement of the ferroelectric hysteresis curves confirmed that the structural changes induced by different processing parameters affected the ferroelectric properties of the material. The best ferroelectric properties including fatigue endurance were obtained for electrodes made at high temperature and for PZT deposited at 2×10-2 mbar and heat treated at 675°C for 30 minutes in an oxygen atmosphere. The piezoelectric coefficient d33, measured using a Michelson interferometer, had values in the range between 20 and 60 pm/V, and showed a strong dependence on the thickness of the PZT films.

Abstract: ZnO is a wide band gap semi-conductor that has attracted tremendous interest for its potential applications in optoelectronic, solar cell, gas detection … In this work, aluminium doped zinc oxide (ZnO:Al) films were deposited by RF magnetron sputtering on glass substrates with different RF power densities of 1.2, 2.5, 3.7 and 4.9 W/cm2. We notice that the films grown at 1.2 W/cm2 were very thin and their physical properties were not precisely determined. The electrical properties of ZnO films were investigated using the impedance spectroscopy technique in the frequency range from 5 Hz to 13 MHz. The impedance data, represented by Nyquist diagrams showed that the resistivity of the films changed during the first three months after deposition. The deposited films show good optical transmittance (over 80 %) in the visible and near infrared spectra. The band gap is around 3 eV and decreases with the increasing of the RF power density (from 3.35 to 3.05 eV). The results of this study suggest that the variation of the RF power density used for deposition allow the control of the electrical and optical properties of the films