Gas phase aluminizing of TiAl intermetallics

The aim of this study was the microstructural characterization of aluminide diffusion coatings deposited on Ti48Al2Cr2Nb and Ti46Al7Nb alloys. The coatings' deposition process was prepared using out of pack method. The X-ray diffraction phase analysis from the surface of coatings indicated that TiAl₂ phase dominates in the coating. The thickness of aluminide coating was about 10 μm.     

Preparation and characterization of Ca0.18Na0.32Bi0.50TiO3 ferroelectric thin films by metalorganic solution deposition

Ca_0.18Na_0.32Bi_0.50TiO₃ (CNBT) ferroelectric thin films were prepared by metalorganic solution deposition on silicon substrate and annealed at different temperatures. The morphology and structure of the films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The crystal structure of Ca-doped Na_0.50Bi_0.50TiO₃ films shows no obvious lattice distortion compared with that of un-doped one. The optimal heat treatment process for CNBT films were determined to be high-temperature drying at 400 °C for no less than 15 min followed by annealing at 600 °C for 5 min, which leads to the formation of compact films with uniform grains of 30–50 nm. Ferroelectric property measurement shows that the remanent polarization of CNBT films is 18 times higher than that of un-doped Na_0.50Bi_0.50TiO₃ (NBT) thin films.     

Titanium-supported Ce-, Zr-containing oxide coatings modified by platinum or nickel and copper oxides and their catalytic activity in CO oxidation

A combination of plasma electrolytic oxidation (PEO) and impregnation techniques followed by annealing in air has been used to obtain composites Pt/nZrO₂ + pTiO₂/Ti, Pt/nZrO₂ + pTiO₂ + zCeO_x/Ti, NiO + CuO/nZrO₂ + pTiO₂/Ti, NiO + CuO/nZrO₂ + pTiO₂ + zCeO_x/Ti with different zirconium and titanium contents and ZrO₂/TiO₂ phase ratio. The composites have been investigated by means of XRD, XPS and SEM/XSA methods. According to the XPS data, the platinum content on the coating surface is ~ 0.4 at.%, whereas the XSA measurements have shown that the nickel and copper contents in coatings attain 16 and 8 at.%, respectively, depending on the initial oxide coatings composition. Nickel and copper oxides form either extended islets or solid layers (“crusts”) on the coating surface. Both the composites promoted with platinum and those with the “crust” built from nickel and copper oxides are active in CO oxidation at the temperatures above 200 °C and 300 °C, respectively.     

Application of a novel microwave plasma treatment for the sintering of nickel oxide coatings for use in dye-sensitized solar cells

In this study the use of microwave plasma sintering of nickel oxide (NiO_x) particles for use as p-type photoelectrode coatings in dye-sensitized solar cells (DSSCs) is investigated. NiO_x was chosen as the photocathode for this application due to its stability, wide band gap and p-type nature. For high light conversion efficiency DSSCs require a mesoporous structure exhibiting a high surface area. This can be achieved by sintering particles of NiO_x onto a conductive substrate. In this study the use of both 2.45 GHz microwave plasma and conventional furnace sintering were compared for the sintering of the NiO_x particles. Coatings 1 to 2.5 μm thick were obtained from the sintered particles (mean particle size of 50 nm) on 3 mm thick fluorine-doped tin oxide (FTO) coated glass substrates. Both the furnace and microwave plasma sintering treatments were carried out at ~ 450 °C over a 5 min period. Dye sensitization was carried out using Erythrosin B and the UV–vis absorption spectra of the NiO_x coatings were compared. A 44% increase in the level of dye adsorption was obtained for the microwave plasma sintered samples as compared to that obtained through furnace treatments. While the photovoltaic performance of the DSSC fabricated using the microwave plasma treated NiO_x coatings exhibited a tenfold increase in the conversion efficiency in comparison to the furnace treated samples. This enhanced performance was associated with the difference in the mesoporous structure of the sintered NiO_x coatings.     

Elaboration and characterization of nanocrystalline TiO2 thin films prepared by sol–gel dip-coating

Nanocrystalline TiO₂ thin films were deposited on a ITO coated glass substrate by sol–gel dip coating technique, the layers undergo a heat treatment at temperatures varying from 300 to 450 °C. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, Atomic Force Microscopy (AFM), visible, (Fourier-Transform) infrared and ultraviolet spectroscopy, Fluorescence and spectroscopic ellipsometry. The results indicate that an anatase phase structure TiO₂ thin film with nanocrystallite size of about 15 nm can be obtained at the heat treatment temperature of 350 °C or above, that is to say, at the heat treatment temperature below 300 °C, the thin films grow in amorphous phase; while the heat treatment temperature is increased up to 400 °C or above, the thin film develops a crystalline phase corresponding to the titanium oxide anatase phase. We have accurately determined the layer thickness, refractive index and extinction coefficient of the TiO₂ thin films by the ellipsometric analysis. The optical gap decreases from 3.9 to 3.5 eV when the annealing temperature increases. Photocatalytic activity of the TiO₂ films was studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 350 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Plasma electrolytic oxidation of hafnium

This paper presents the results of the investigation of plasma electrolytic oxidation (PEO) of hafnium. Atoms ionized during the PEO micro-discharging were identified using optical emission spectroscopy. The spectral line shape analysis of the hydrogen Balmer line H_β indicated the presence of two types of micro-discharges characterized by electron number densities of around 2.5·10²¹ m^− 3 and 1.3·10²² m^− 3. Scanning electron microscopy and X-ray diffraction were employed to investigate surface morphology and phase composition of the PEO coatings obtained. The coatings were crystalline and composed of monoclinic HfO₂. Diffuse reflectance spectroscopy has shown that HfO₂ coatings have a broad absorption band in the range from 200 nm to 400 nm. Optical band gap of HfO₂ coatings was around 5.4 eV, as estimated from absorption spectra. Photoluminescence measurements show that HfO₂ coatings have broad emission band in the visible region, with a maximum at around 480 nm. The highest photoluminescence was obtained for the excitation wavelength of 270 nm. Intensity of photoluminescence increased with PEO time and is related to an increase of oxygen vacancy defects in HfO₂ coatings formed during the process.     

Ferromagnetism of Mn-doped TiO2 nanorods synthesized by hydrothermal method

Mn-doped TiO₂ nanorods which showed room temperature ferromagnetic (RTFM) behavior have been synthesized by a hydrothermal method. Analysis of X-ray diffraction and Raman spectral data reveal the formation of anatase phase without any impurity phase. From the hysteresis loop measurements, it is possible to estimate the two parameters, magnetization (M_S) and coercivity (H_C) which are 33 memu/g and 89 Oe respectively, for 5 at% Mn-doped TiO₂. The two parameters (M_S, H_C) are strongly dependent on the Mn doping concentration. The origin of the RTFM is understood in terms of the concentration of oxygen vacancies and/or defects which is created by Mn doping.     

Doping amount and temperature dependence of superplastic flow in tetragonal ZrO2 polycrystal doped with TiO2 and/or GeO2

The doping amount and temperature dependence of superplastic flow in a TiO₂- and/or GeO₂-doped tetragonal ZrO₂ polycrystal (TZP) were investigated in the doping range of 0.2–8 mol.% and in the temperature range of 1200–1550 °C. While the tensile ductility in the TZP is significantly improved by the co-doping of TiO₂ and GeO₂, there is an optimum combination of doping amount and temperature for enhancing the tensile ductility. The present study also shows that the flow stress decreases with an increase in the doping amount, but this decrease levels off with a 2–3 mol.% addition of GeO₂ or (TiO₂–GeO₂). The data for the flow behavior and thermal groove experiment indicated that TiO₂ and/or GeO₂ doping enhances the grain boundary diffusion of zirconium cations and reduces the grain boundary energy, respectively. These effects of grain boundary segregation can be regarded as the cause of the improved high-temperature ductility of (TiO₂–GeO₂)-doped TZP.     

The photoelectrochemical properties of TiO2 electrodes modified by quantum sized PbS and thiols

Photoelectrochemical electrodes have been prepared by chemical deposit of quantum sized PbS particles (Q-PbS) on TiO₂ nanocrystalline films and followed by the modification of such electrodes with thiols. The photoelectrochemical properties have been studied. The results show that Q-PbS modified TiO₂ electrodes generate photocurrent as high as 542.5 μA/cm² under irradiation of a 400 nm light, but it undergoes photodegradation. The modification with thiols leads to an improvement in terms of increased photostability, although the photocurrent decreases due to the resistance brought about by the carbon chains of thiols.     

Microstructures and mechanical properties of conventionally solidified Al63Cu25Fe12 alloy

In this study, the microstructures and mechanical properties of conventionally solidified Al₆₃Cu₂₅Fe₁₂ alloy after different heat-treatments were investigated. The microstructures of the as-cast and subsequently heat-treated samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). The XRD results showed the presence of quasicrystalline icosahedral phase (i-phase) together with crystalline phases corresponding to β-AlFe(Cu) solid solution phase (β-phase) and τ-AlCu(Fe) solid solution phase (τ-phase). The SEM investigations clearly showed the formation of i-phase with pentagonal dodecahedra structure. However, the i-phase together with β-phase was also observed in the heat-treated samples and the peak intensity of the β-phase decreased with increasing heat-treatment temperature. From the DTA curves, the melting point of i-phase was determined as 890 °C for this alloy composition. Mechanical properties of the as-cast and subsequently heat-treated samples were measured by a Vickers indenter. Results showed that the microhardness (HV) and the elastic modulus (E) of the as-cast sample were around 598 kg fmm^?2 (5.86 GPa) and 104 GPa, respectively. In addition, the characteristic of material plasticity (δ_H) value was calculated to be 0.54.     

Architecture,optical absorption,and photocurrent response of oxygen-deficient mixed-phase titania nanostructures

The optical absorption and photoelectric properties of oxygen-deficient titania (TiO₂) nanostructures consisting of anatase nanotubes and rutile film layer were investigated. The nanostructures were prepared by electrochemical anodization followed by long-time annealing at four temperatures – 450, 550, 650 and 750 °C. Various characterization techniques, including X-ray photoelectron spectroscopy depth profiling, revealed that elemental stable zones (structural regions in which the concentrations of O and Ti are stable) formed within the TiO₂ nanostructures at high annealing temperatures (650 and 750 °C) have O/Ti atomic ratios significantly less than 2. A direct relationship between oxygen vacancy concentration and annealing temperature was established on the basis of this finding. Measurement of the optical absorption spectra of the TiO₂ nanostructures revealed a blue-shift in the absorption edge along with a notable increase in the long-wavelength absorption due to the presence of oxygen vacancies. This observation is in agreement with the first-principles calculations of the absorption coefficients of anatase Ti_nO_2n?1 and Ti_nO_2n?2 structures, in which the oxygen vacancy concentration can be adjusted by varying the supercell size. The contrary photocurrent responses of the TiO₂ nanostructures under ultraviolet and visible light were measured. A strong photocurrent response under filtered visible light (λ > 500 nm) was found for the TiO₂ nanostructures annealed at 650 and 750 °C, which suggests that the dominant positive effect of oxygen vacancies exceeds the adverse impact of other features associated with thickening of the rutile film layer at high annealing temperatures, such as a reduction in the specific surface area and an increased charge recombination rate.     

Porous hierarchical TiO2 nanostructures: Processing and microstructure relationships

A dual porous hierarchical coating of TiO₂ nanotubes (～50 nm diameter) on the nanoscale and large (～1 to 20 μm) pores on the micro-scale can be fabricated on the surface of Ti by anodic oxidation. This unique coating may have potential applications as bioactive coatings for Ti bone implants. This paper details several important aspects of the coating microstructure. TiO₂ coatings were fabricated by anodic oxidation in 1 M H₂SO₄ + 0.1 M NaF solution. Microstructure characterization was carried out using scanning electron microscopy. We also report on the observation of precipitates which form as both a continuous surface layer and of a conical geometry. The mechanism for nanotube formation, precipitate layer formation, and microscopic pitting was discussed. The effect of processing variables (i.e. time, temperature, pH) on the TiO₂ microstructure was studied. Anodization time was found to affect nanotube length and also pit size and density. Lowering the electrolyte pH decreased the nanotube length and microscopic pit density. Increasing electrolyte temperature decreased nanotube length and increased pit/pore and precipitate density. Microscopic pitting, in the nanotube coating was found to occur above grain boundaries in the Ti substrate and above Ti grains with (0 0 0 1) orientation.     

x(Mg0.7Zn0.3)0.95Co0.05TiO3-(1−x)(La0.5Na0.5)TiO3 ceramic at microwave frequency with a near zero temperature coefficient of resonant frequency

Dielectric properties of x(Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃-(1?x)(La_0.5Na_0.5)TiO₃ ceramic were investigated at microwave frequencies. A nearly 0 ppm/°C temperature coefficient of resonant frequency was realized at x = 0.9. A two-phase system was confirmed by XRD analysis. A dielectric material applicable to microwave devices with a Q × f of 20,000–87,000 GHz and a dielectric constant of 21.27–26.2 was obtained at 1100 °C after 4 h of sintering. The microwave dielectric material 0.9(Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃-0.1(La_0.5Na_0.5)TiO₃ sintered at 1150 °C for 4 h has a dielectric constant of 24.56, a Q × f of 68,000 GHz, and a τ_f value of 0 ppm/°C. It is proposed as a candidate dielectric for GPS patch antennas.     

Structural and mechanical properties of corundum and cubic (AlxCr1−x)2+yO3−y coatings grown by reactive cathodic arc evaporation in as-deposited and annealed states

Coatings of (Al_xCr_1?x)_2+yO_3?y with 0.51 ? x ? 0.84 and 0.1 ? y ? 0.5 were deposited on hard cemented carbide substrates in an industrial cathodic arc evaporation system from powder-metallurgy-prepared Cr/Al targets in pure O₂ and O₂ + N₂ atmospheres. The substrate temperature and bias in all the deposition runs were 575 °C and ?120 V, respectively. The composition of the coatings measured by energy dispersive X-ray spectroscopy and elastic recoil detection analysis differed from that of the facing targets by up to 11%. Microstructure analyses performed by symmetrical X-ray diffraction and transmission electron microscopy showed that corundum, cubic or mixed-phase coatings formed, depending on the Cr/Al ratio of the coatings and O₂ flow per active target during deposition. The corundum phase was promoted by high Cr content and high O₂ flow per target, while the cubic phase was observed mostly for high Al content and low O₂ flow per active target. In-situ annealing of the cubic coatings resulted in phase transformation from cubic to corundum, completed in the temperature range of 900–1100 °C, while corundum coatings retained their structure in the same range of annealing temperatures. Nanoindentation hardness of the coatings with Cr/Al ratio <0.4 was 26–28 GPa, regardless of the structure. Increasing the Cr content of the coatings resulted in increased hardness of 28–30 GPa for corundum coatings. Wear resistance testing in a turning operation showed that coatings of Al–Cr–O have improved resistance to crater wear at the cost of flank wear compared with TiAlN coatings.     

The interdependence of structural and electrical properties in TiO2/TiO/Ti periodic multilayers

Multilayered structures with 14–50 nm periods composed of titanium and two different titanium oxides, TiO and TiO₂, were accurately produced by DC magnetron sputtering using the reactive gas pulsing process. The structure and composition of these periodic TiO₂/TiO/Ti stacks were investigated by X-ray diffraction and transmission electronic microscopy techniques. Two crystalline phases, hexagonal close packed Ti and face centred cubic TiO, were identified in the metallic-rich sub-layers, whereas the oxygen-rich ones comprised a mixture of amorphous TiO₂ and rutile phase. DC electrical resistivity ρ measured for temperatures ranging from 300 to 500 K exhibited a metallic-like behaviour (ρ_473K = 1.05 × 10^?5 to 1.45 × 10^?6 Ω m) with a temperature coefficient of resistance ranging from 1.20 × 10^?3 K^?1 for the highest period (Λ = 50.0 nm) down to negative values close to ?4.97 × 10^?4 K^?1 for the smallest one (Λ = 14.0 nm). A relationship between the dimensions of periodic layers and their collective electrical resistivity is proposed where the resistivity does not solely depend on the total thickness of the film, but also depends on the chemical composition and thickness of each sub-layer. Charge carrier mobility and concentration measured by the Hall effect were both influenced by the dimension of TiO₂/TiO/Ti periods and the density of ionized scattering centres connected to the titanium concentration in the metallic sub-layers.     

Self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

Self-assembled polyaniline (PANI) nanotubes, accompanied with nanoribbons, were synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous medium, in the presence of colloidal titanium dioxide (TiO₂) nanoparticles of 4.5 nm size, without added acid. The morphology, structure, and physicochemical properties of the PANI/TiO₂ nanocomposites, prepared at various initial aniline/TiO₂ mole ratios, were studied by scanning (SEM) and transmission (TEM) electron microscopies, FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies, elemental analysis, X-ray powder diffraction (XRPD), conductivity measurements, and thermogravimetric analysis (TGA). The electrical conductivity of PANI/TiO₂ nanocomposites increases in the range 3.8 × 10^?4 to 1.1 × 10^?3 S cm^?1 by increasing aniline/TiO₂ mole ratio from 1 to 10. The morphology of PANI/TiO₂ nanocomposites significantly depends on the initial aniline/TiO₂ mole ratio. In the morphology of the nanocomposite synthesized using aniline/TiO₂ mole ratio 10, nanotubes accompanied with nanosheets prevail. The nanocomposite synthesized at aniline/TiO₂ mole ratio 5 consists of the network of nanotubes (an outer diameter 30–40 nm, an inner diameter 4–7 nm) and nanorods (diameter 50–90 nm), accompanied with nanoribbons (a thickness, width, and length in the range of 50–70 nm, 160–350 nm, and ～1–3 μm, respectively). The PANI/TiO₂ nanocomposite synthesized at aniline/TiO₂ mole ratio 2 contains polyhedral submicrometre particles accompanied with nanotubes, while the nanocomposite prepared at aniline/TiO₂ mole ratio 1 consists of agglomerated nanofibers, submicrometre and nanoparticles. The presence of emeraldine salt form of PANI, linear and branched PANI chains, and phenazine units in PANI/TiO₂ nanocomposites was proved by FTIR and Raman spectroscopies. The improved thermal stability of PANI matrix in all PANI/TiO₂ nanocomposites was observed.     

Characterization and dielectric behavior of V2O5-doped 0.9Mg0.95Co0.05TiO3–0.1Ca0.6La0.8/3TiO3 ceramic system at microwave frequency

The microwave dielectric properties and the microstructures of Mg_0.95Co_0.05TiO₃–Ca_0.6La_0.8/3TiO₃ ceramics, prepared by a mixed oxide route, have been investigated. With small amount of V₂O₅ additions, the sintering temperatures of 0.9Mg_0.95Co_0.05TiO₃–0.1Ca_0.6La_0.8/3TiO₃ ceramics can be lowered to 1250 °C. The microwave dielectric properties are found strongly correlated with the sintering temperature as well as the amount of V₂O₅ additions. The Q × f value of 0.9Mg_0.95Co_0.05TiO₃–0.1Ca_0.6La_0.8/3TiO₃ increased with increasing temperature to 1250 °C and decreased thereafter. The decrease in Q × f value was coincident with the abnormal grain growth. A maximum Q × f value of 58,000 (GHz) associated with a dielectric constant (?_r) of 21.7 and a temperature coefficient (τ_f) of ?10 ppm/°C, was achieved for 0.25 wt.% V₂O₅-doped samples at 1250 °C. Moreover, a cross-coupled compact hairpin filter with designed center frequency of 2.0 GHz is designed and fabricated using the proposed dielectric ceramic to study its performance. It also showed a substantial reduction in both insertion loss and size in comparison with other dielectrics FR4 and alumina.     

Friction and wear mechanisms in MoS2/Sb2O3/Au nanocomposite coatings

Fundamental phenomena governing the tribological mechanisms in sputter deposited amorphous MoS₂/Sb₂O₃/Au nanocomposite coatings are reported. In dry environments the nanocomposite has the same low friction coefficient as pure MoS₂ (～0.007). However, unlike pure MoS₂ coatings, which wear through in air (50% relative humidity), the composite coatings showed minimal wear, with wear factors of ～1.2–1.4 × 10^?7 mm³ Nm^?1 in both dry nitrogen and air. The coatings exhibited non-Amontonian friction behavior, with the friction coefficient decreasing with increasing Hertzian contact stress. Cross-sectional transmission electron microscopy of wear surfaces revealed that frictional contact resulted in an amorphous to crystalline transformation in MoS₂ with 2H-basal (0 0 0 2) planes aligned parallel to the direction of sliding. In air the wear surface and subsurface regions exhibited islands of Au. The mating transfer films were also comprised of (0 0 0 2)-oriented basal planes of MoS₂, resulting in predominantly self-mated “basal on basal” interfacial sliding and, thus, low friction and wear.     

Thin nanocrystalline TiO2–SnO2 sprayed films: Influence of the dopant concentration,substrate and thermal treatment on the phase composition and crystallites sizes

Thin nanocrystalline TiO₂–SnO₂ films (0–50 mol% SnO₂) are coated on quartz and stainless steel substrates by spray pyrolysis method. The synthesized films are investigated by XRD, Raman spectroscopy and XPS.The diffraction peaks of anatase phase fade while the peaks of rutile phase appear in the X-ray profiles with increasing of the treatment temperature and the content of SnO₂ in the sprayed films. It is found that SiO₂ coming from the quartz substrate stabilizes the anatase phase up to 700 °C. A more pronounced crystallization of rutile is registered with the films deposited on stainless steel substrate, which probably is caused by combined effect of SnO₂ doping and penetration of iron and chromium from the substrate inside the films.Dopant concentration (SnO₂) influences the size of the crystallites of the titania films deposited on quartz substrates The size of crystallites in the titania films decreases from 45 to 25 nm with increasing of SnO₂ amount.The SnO₂ amount does not affects substantially the size of crystallites (about 23 nm) for the films deposited on stainless steel.     

Microstructures and cyclic oxidation behaviour of Pt-free and low-Pt NiAl coatings on the Ni-base superalloy Rene-80

Aluminizing of bare and 3 μm-Pt-electroplated specimens has been utilised to prepare NiAl and low platinum (Ni,Pt)Al coatings. Cyclic oxidation of the coatings was investigated by exposing samples to 1 h cyclic oxidation at 1100 °C. The modified coating exhibited an external layer of NiAl-25 vol.% PtAl₂ above a three-zone structure. This structure endured over the whole testing time, while the NiAl coating failed after 77 cycles. The (Ni,Pt)Al coating did not reduce the scale growth rate, but it improved scale adhesion. In addition, Pt limited the outward diffusion of Ti from substrate and hence prohibited formation of undesirable TiO₂.