A phenylenevinylene copolymer with perylene bisimde units as organic sensitizer for dye-sensitized solar cells

An alternating phenylenevinylene copolymer P with perylene bisimide units has been used as organic sensitizer to fabricate dye-sensitized solar cells (DSSCs) based on porous and TiCl₄ modified TiO₂ photoelectrodes. As a consequence of the compact layer formed by TiCl₄ treatment to the porous TiO₂ thin film layer, an efficient electron network was formed. Dark current measurements and electrochemical impedance spectra (EIS) suggested that modified photoelectrode significantly reduced the recombination rate of electrons with redox couple in the electrolyte due to the reduced bare FTO surface and longer electron lifetime as compared to the porous TiO₂ photoelectrode. The power conversion efficiency of DSSCs utilizing this copolymer as sensitizer is about 2.60% and 3.98% with porous and modified TiO₂ photoelectrodes, respectively.     

Electrochemical stability of anodic titanium oxide films grown at potentials higher than 3 V in a simulated physiological solution

The cathodic behaviour of oxides formed on titanium electrodes in physiological solutions at potentials between 3 and 5 V (vs. SCE) was studied by cyclic voltammetry. In case of anodic polarization at potentials higher than 3 V (vs. SCE), a cathodic peak at ∼0.4 V (vs. SCE) appears in the cathodic scan, which could be due to the reduction of unstable peroxides. The results show that this peak depends on the anodic potential and the oxidation time. This behaviour supposedly is due to the formation of unstable titanium peroxides like TiO₃ during anodization. Based on repetitive oxidation-reduction processes can be concluded that the created amount of TiO₃ inside of the TiO₂ surface layer seems to be constant.     

In situ composite coating of titania-hydroxyapatite on commercially pure titanium by microwave processing

Microwave (MW) processing has been studied as an alternative method of hydroxyapatite (HA) based composite coatings on commercially pure titanium (CPTi) to enhance the bioactivity for orthopaedic and dental implant applications. The coating was formed by processing CPTi metal packed in HA and at 800 W microwave power for 22 min. The composition of the coating was found to be TiO₂ (rutile) as major phase along with HA as minor phase. The MW absorption of non-stoichiometric TiO₂ layer, which was grown during the initial hybrid heating, resulted in sintering of apatite particles interfacing them. The non-stoichiometric nature of TiO₂ was evident from the observed mid-gap bands in ultraviolet-visible diffusive reflectance (UV-VIS-DR) spectrum. The lamellar α structure of the substrate suggests that the processing temperature was above β transus of CPTi (1155 K). The oxygen stabilized α phase whose thickness increased with microwave processing time, was likely to be the reason for the increase in Young's Modulus and hardness of the substrate. The coating induced apatite precipitation in bioactivity test. The osteoblast cell adhesion test demonstrated cell spreading which is considered favourable for cell proliferation and differentiation. Thus, in situ composite coating of titania and HA on CPTi was obtained by a simple one-step process.     

Oxidation of Ti2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti₂AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti₂AlC forms a continuous layer of α-Al₂O₃ below a layer of TiO₂ at temperatures as low as 700 °C. Oxidation of the Ti₂AlC coatings is more complex, and also involves the phases Ti₃AlC₂, TiC, and Ti_xAl_y, formed during the spraying process. α-Al₂O₃ is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.     

Cyclic oxidation of Ti3AlC2 at 1000–1300 °C in air

The cyclic-oxidation behavior of Ti₃AlC₂ was investigated at 1000–1300 °C in air for up 40 cycles. It was revealed that Ti₃AlC₂ had excellent resistance to thermal cycling. The cyclic oxidation of Ti₃AlC₂ basically obeyed a parabolic law. In all cases, the scales were dense, resistant to spalling and highly stratified. The inner continuous α-Al₂O₃ layer was well adhesive, while the outermost layer changed from rutile TiO₂ at temperatures below 1100 °C to Al₂TiO₅ at 1200 and 1300 °C, respectively. At 1300 °C, a mechanical-keying structure of inner Al₂O₃ to the Ti₃AlC₂ substrate formed, which improved the resistance to scale-spallation.     

Surface characterization and corrosion behavior of 70/30 Cu-Ni alloy in pristine and sulfide-containing simulated seawater

The corrosion behavior of the 70/30 Cu-Ni alloy in stagnant, aerated pristine and sulfide-containing simulated seawater as a function of exposure time was investigated with polarization curve measurement and electrochemical impedance spectroscopy (EIS). It was demonstrated that the compact protective oxide film formed on the 70/30 Cu-Ni alloy resulted in the decrease of corrosion rate in aerated pristine seawater; while the corrosion rate of 70/30 Cu-Ni alloy in aerated sulfide-containing seawater increased dramatically due to the catalysis of the sulfide ions or sulfide scale for both the cathodic and anodic reactions. The impedance spectra and the corresponding equivalent circuits confirmed that a duplex layer of a surface film was formed on the 70/30 Cu-Ni alloy in aerated pristine seawater after a period of time and that the inner layer was responsible for the good resistance of the alloy; while only a porous and non-protective corrosion product layer formed on the 70/30 Cu-Ni alloy in aerated sulfide-containing seawater, which made small values of charge transfer resistance (R_ct) to last for a abnormally long time by interfering with the growth of the protective oxide film. The composition of the surface film on the alloy in pristine and sulfide-containing seawater for different exposure times were investigated thoroughly by XPS. It was found that the duplex corrosion product layer formed on the alloy in pristine seawater was composed of an inner Cu₂O and an outer CuO layer. The porous and non-protective corrosion product layer formed on the alloy in aerated sulfide-containing seawater was a mixture of CuCl, Cu₂S, NiS, Cu₂O and NiO with trace amounts of CuO and Ni(OH)₂ and that the most significant component was Cu₂S. In addition, SEM was used to analyze the topography of the 70/30 Cu-Ni alloy in both solutions after different exposure times.     

Oxidation inhibition of γ-TiAl alloy at 900 °C by inorganic silicate composite coatings

Inorganic silicate composite coatings on γ-TiAl were fabricated by air spraying. The oxidation behaviour of the alloy was investigated at 900 °C. The results indicated that rapid oxidation occurred in the γ-TiAl, and multilayered non-protective TiO₂ and Al₂O₃ scales formed. For coated γ-TiAl alloy, the oxidation was markedly inhibited; a thin Al₂O₃ layer was detected, which improved the oxidation resistance of the alloy. The low oxygen partial pressure at the interface of the coatings and the alloy promotes the preferentially oxidation of Al in the γ-TiAl substrate, and the outward diffusion of Ti to form TiO₂ was retarded.     

Detrimental effect of particle sol-gel coating on the corrosion behavior of A380-SiC composite

In this study, the corrosion susceptibility of aluminum matrix composites reinforced with artificially oxidized SiO₂ and sol-gel Fe/TiO₂ coated silicon carbide particles (SiC_p) has been investigated. Corrosion behavior of the composites, fabricated by the liquid metal infiltration technique, was established in chloride containing alkaline environments by cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS) techniques. It has been found that, sol-gel coating of SiC particles with Fe/TiO₂ has a detrimental effect on the corrosion characteristics of A380-SiC metal matrix composite.     

Role of chlorine in the oxidation resistance of Ti-45at.% Al-1.6at.%Mn intermetallic compound

In order to investigate the role of chlorine in the oxidation resistance of Ti-45at.%Al-1.6at.%Mn intermetallic compounds, specimens with and without chlorine were prepared by a reactive sintering and a remelting process, respectively, and then isothermally oxidized at temperatures of 800 to 1000°C. The oxidation resistance of reactive-sintered Ti-45at.%Al-1.6at.%Mn containing 0.02 wt.%Cl was superior to that of remelted specimens due to the formation of a protective Al₂O₃, layer. From the results of an EPMA line scan of a cross-section of the oxide layer, carbon and chlorine peaks were detected on the TiO₂ layer of the reactive-sintered specimens suggesting that TiO₂ can decompose to TiCl₄ through the reaction with Cl₂ and C (or CO). As a result of the chlorination of TiO₂, the growth of TiO₂ could be constrained and a protective A1₂O₃ scale was easily formed on the reactive-sintered alloy.     

Microstructure and apatite-forming ability of the MAO-treated porous titanium

Porous titanium was treated by micro-arc oxidation (MAO) in the aqueous electrolytes containing 0.1 and 0.2 M NaOH. The microstructure (including morphology, phase component, element composition and chemical species) and in vitro apatite-forming ability of the oxidized films formed on the inner-pore walls of porous titanium were investigated. It is found that continuous thin films with pore sizes of 20-60 nm are formed in both electrolytes. The films consist of an amorphous TiO₂ outmost layer, a coexisted intermediate layer of amorphous TiO₂ and rutile, and a Ti₂O₃ bottom layer, and tightly bond to the titanium substrate without any cracks. In vitro bioactivity assessment shows that both MAO films possess high apatite-forming abilities. It is also found that, compared with the film formed in the 0.1 M NaOH-containing electrolyte, the film formed in the 0.2 M NaOH-containing electrolyte has a higher roughness and more nanopores which help shorten apatite induction time. It is expected the MAO-formed bioactive porous titanium will not only be beneficial to bone ingrowth into the porous structure, but also be beneficial to achieve a tough chemical bonding at the bone/implant interface.     

Improvement in corrosion resistance of CrN coated stainless steel by conformal TiO2 deposition

A conformal titanium dioxide (TiO₂) layer was deposited onto chromium nitride (CrN) coated stainless steel by atomic layer deposition technique, and the electrochemical corrosion test on the CrN single-layer and TiO₂/CrN double-layer coated sample was carried out. The equilibrium corrosion potential of the double-layer coated sample shifted positively compare to that of the single-layer coated one. Moreover, the corrosion current density decreased significantly with the TiO₂ deposition, revealing that better corrosion resistance was obtained after the deposition of the TiO₂ layer. The improvement in corrosion resistance after the TiO₂ deposition was attributed to the blocking of the through-thickness cracks or pinholes in the CrN layer.     

Corrosion resistance of TiO2 films grown on stainless steel by atomic layer deposition

Titanium dioxide (TiO₂) films have been deposited onto stainless steel substrates using atomic layer deposition (ALD) technique. Composition analysis shows that the films shield the substrates entirely. The TiO₂ films are amorphous in structure as characterized by X-ray diffraction. The electrochemical measurements show that the equilibrium corrosion potential positively shifts from − 0.96 eV for bare stainless steel to − 0.63 eV for TiO₂ coated stainless steel, and the corrosion current density decreases from 7.0 × 10^− 7 A/cm² to 6.3 × 10^− 8 A/cm². The corrosion resistance obtained by fitting the impedance spectra also reveals that the TiO₂ films provide good protection for stainless steel against corrosion in sodium chloride solution. The above results indicate that TiO₂ films deposited by ALD are effective in protecting stainless steel from corrosion.     

Film properties and stability influence on impedance distribution during the dissolution process of low-carbon steel exposed to modified alkaline sour environment

In this work, we characterized the anodic dissolution and the hydrogen transport within carbon steel (SAE 1018) samples immersed in alkaline sour solutions (CN⁻, polysulfide-base inhibitor and H₂S(aq)). The evolution of interfacial and transport processes could be quantified by Electrochemical Impedance Spectroscopy (EIS) and hydrogen permeation measurements. EIS experimental data were analyzed and fitted by using Transmission Line Model (TLM); this latter helped to propose the mechanisms through the porous layer of the corrosion products formed. The area influencing the dissolution and the mass transfer process was quantified by the pores number, pores thickness and the interfacial passive electrical elements describing the mechanisms in different regions within the pores of the corrosion product layer. The TLM was used to analyze the active-mass transport processes occurred at different spatial positions of the porous layer, such as the mass transfer at the wall and the active-mass transfer at the base of the cylindrical pore of the non-stoichiometric Fe_xS_y.     

Corrosion behaviour of 16%Cr-4%Al and 16%Cr ODS ferritic steels under different metallurgical conditions in a supercritical water environment

The corrosion behaviour of 16%Cr and 16%Cr-4%Al ODS ferritic steels in different heat treatment conditions has been investigated in a supercritical water environment. The exposed coupons were analyzed using scanning electron microscopy (SEM), electron probe micro analysis (EPMA), Auger and X-ray diffraction analysis (XRD). It was found that the formation of oxides depends on the chemical composition and not on the metallurgical condition. The Al-free alloys formed a monolayer oxide film composed of (Cr, Fe)₂O₃. The Al-containing alloys formed an oxide film composed of an outer layer of hematite and magnetite and an inner layer of Al₂O₃. The oxidation mechanisms are discussed.     

The role of TiO2 in the air-stable hybrid organic–inorganic light-emitting diodes

The role of the TiO₂ layer in hybrid organic–inorganic light-emitting diodes (HOILEDs) was discussed. A TiO₂ layer was fabricated using a process that formed an “oxygen-rich” and “porous” TiO₂ layer in an HOILED. The importance of the surface state on the TiO₂ and the hole injection scheme in the HOILED was confirmed. All the data support the idea that HOILEDs are strongly hole-dominated LEDs, and that the TiO₂ in HOILEDs plays a supporting role. The TiO₂ seems to act as a hole-blocking layer.     

Oxidation behaviour of Ti–Al–C films composed mainly of a Ti2AlC phase

This paper addresses the oxidation behaviour of Ti–Al–C films composed mainly of a Ti₂AlC phase. The films exhibited rather low oxidation rates at 600 and 700 °C, with an oxygen-rich zone or a thin oxide layer appearing on the film surfaces. Much faster oxidation rates were observed at 800 and 900 °C. The Ti₂AlC phase was quickly consumed by oxidation. From the film surface to the inner zone, TiO₂-rich layer, Al₂O₃-rich layer, and TiO₂ + Al₂O₃ mixed layer was observed, respectively. The oxidation mechanism of the Ti–Al–C film is discussed based on the experimental results.     

Oxidation of single crystal PWA 1483 at 950 °C in flowing air

The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10⁻⁶ mg²/(cm⁴ × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO₂ and NiTiO₃ but mostly Cr in Cr₂O₃ and (Ni/Co)Cr₂O₄ besides NiTaO₄. This outer scale is connected to a discontinuous layer of Al₂O₃ and an area of γ′-depletion within the base material.     

XPS analysis of the passive film formed on austenitic stainless steel coated with conductive polymer

Improvement of the passivation behavior of Type 304 austenitic stainless steel (SS) by coating with conductive polymers (CPs), like polyaniline (PANI) and poly(o-phenylenediamine) (PoPD), followed by exposure in an acid solution has been demonstrated. The passive films formed on SSs (after peeling off the polymer layer) are compared with those formed during anodic polarization under the same exposure condition. The passive films beneath the CPs are thicker and less hydrated than those formed on uncoated stainless steel. The polymer layer enhances the enrichment of chromium and nickel in the entire passive oxide, forming a more protective film than that formed during anodic polarization. The elemental distribution within the passive film is different in the two modes of passivation. The type of the polymer influences on the composition of the passive film. The best passivation is obtained by PoPD, with the passive film resulting in significant resistance of the SS to pitting corrosion in the 3% NaCl solution. The oxide film of this steel is characterized, in its inner and outer layers, by the highest ratio of Cr(OH)₃/Cr₂O₃ and the lowest content of iron species.     

Hot corrosion behaviour of Nbss/Nb5Si3 in situ composites in the mixture of Na2SO4 and NaCl melts

Hot corrosion behaviour of Nb–16Si–24Ti–6Cr–6Al–2Hf (at.%) in the mixture of Na₂SO₄ and NaCl melts at 900 °C was studied. The results show that the corrosion kinetics of the alloy fit parabolic law. The oxides consist of a loose and porous outer layer and an internal oxidation zone. Outer oxides are mainly composed of TiO₂, TiNb₂O₇, Nb₂O₅, CrNbO₄ and SiO₂ while the internal oxidation zone is composed of TiO₂. Hot corrosion mechanism of the alloy in the presence of Na₂SO₄ and NaCl deposits is discussed.     

Effect of Fe on the high temperature oxidation of TiAl alloys

An alloy of 51.23Ti−48.73Al−0.4Fe (at.%) was oxidized at 800, 900 and 1000°C in air to determine the effect of Fe on oxidation. The scales formed consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner mixed (TiO₂ Al₂O₃) layer, typical of conventional TiAl alloys. A small amount of dissolved Fe ions was weakly segregated in the outer TiO₂ layer and also in the inner (TiO₂−Al₂O₃) mixed layer. Ti₂AIN and TiN were detected in the scale in some instances. A thin, oxygen-affected Ti₃Al sublayer formed at the oxide-substrate interface. The overall oxidation kinetics and the scale morphology were not affected by Fe-addition.