Effect of Nb doping on the phase transition and optical properties of sol-gel TiO2 thin films

Various content Nb-doped TiO₂ thin films were prepared by sol-gel process. XRD analysis shows that the existence of crystalline TiO₂ in anatase and rutile form depends on the Nb content in the examined samples. It is observed that Nb promotes the anatase to rutile phase transition but has a depression effect on the anatase grain growth. It is found that incorporation of about 4 at.% of Nb completely transforms anatase TiO₂ to the rutile form at a calcination temperature as high as 900 °C. The mechanism is proposed. Optical analyses show that the films have an average of 60% transmission in visible region. The energy gap values using Tauc's formula have also been estimated. The band gap of rutile Ti_1−xNb_xO₂ solid solutions increases with increasing x.     

氢化TiO2纳米棒的制备及其对Q235碳钢的光生阴极保护特性

随着海洋开发的逐步推进,海洋工程中的金属防腐蚀问题显得愈加重要。目前,可利用半导体的光电效应实现对金属的光生阴极保护,为改善常用的TiO₂光电极材料的弱光吸收和低转换效率问题,文中利用水热法在FTO导电玻璃表面构建一维有序TiO₂纳米棒阵列,并通过氢化处理提高TiO₂对太阳光的吸收和光电流密度。考察了氢化TiO₂纳米棒阵列在海水环境下对Q235碳钢的光生阴极保护特性,结果表明氢化TiO₂纳米棒的光电流密度达到了2.12 mA/cm²,且稳定性良好;当Q235碳钢耦连于模拟太阳光照下的氢化TiO₂纳米棒电极时,其界面反应电阻变小,电极电位较原先的腐蚀电位降低约349 mV,说明氢化TiO₂纳米棒阵列能够对碳钢产生良好的光生阴极保护效应,且该效应在无光条件下能保持至少7 h。     

Photoresponse and donor concentration of plasma-sprayed TiO<Subscript>2</Subscript> and TiO<Subscript>2</Subscript>-ZnO electrodes

The photoelectrochemical characteristics of plasma-sprayed porous TiO₂, TiO₂-5%ZnO, and TiO₂-10%ZnO electrodes in 0.1 N NaOH solution were studied through a three-electrode cell system. The microstructure, morphology, and composition of the electrodes were analyzed using an electron probe surface roughness analyzer (ERA-8800FE), scanning electron microscopy, and x-ray diffraction. The results indicate that the sprayed electrodes have a porous microstructure, which is affected by the plasma spray parameters and composition of the powders. The TiO₂-ZnO electrodes consist of anatase TiO₂, rutile TiO₂, and Zn₂Ti₃O₈ phase. The photoresponse characteristics of the plasma-sprayed electrodes are comparable to those of single-crystal TiO₂, but the breakdown voltage is close to 0.5 V (versus that of a saturated calomel electrode). The short-circuit photocurrent density (J _SC) increases with a decrease of donor concentration, which was calculated according to the Gartner-Butler model. For the lowest donor concentration of a TiO₂-5%ZnO electrode sprayed under an arc current of 600 A, the short-circuit J _SC is approximately 0.4 mA/cm² higher than that of the TiO₂ electrodes under 30 mW/cm² xenon light irradiation. The J _SC increases linearly with light intensity. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Effect of Nb on oxidation behavior of high Nb containing TiAl alloys

The isothermal oxidation behavior of Ti-45Al-8Nb and Ti-52Al-8Nb alloys at 900 °C in air was investigated. The early oxidation behaviors were studied by using XPS and AES. And the microstructure and the composition of the oxidation scale were studied by using XRD and SEM. The results show that the oxidation behavior of TiAl alloy is significantly improved by Nb addition. Nb substitutes for Ti in TiO₂ as a cation with valence 5, and thus to suppress TiO₂ growth. The (Ti,Nb)O₂-rich layer is a dense and chemically uniform which is more protective than the TiO₂ layer. Nb addition also lowers the critical Al content to form an external alumina. Nb₂Al phase is formed in the metallic matrix at the oxide–metal interface on the high Nb containing TiAl alloys.     

Effect of Nb on the high temperature oxidation of Ti–(0–50 at.%)Al

The oxidation behavior of Ti–Nb, Ti₃Al–Nb and TiAl–Nb (Nb: 0–30 at.%) has been investigated at 1173 K in air. When Nb is in solid solution with TiO₂, the addition of Nb can improve the oxidation resistance of the alloys by impeding mass transfer in TiO₂. However, Nb decreases the oxidation resistance when the amount of Nb is too high and forms TiNb₂O₇ or AlNbO₄ phases in the scale.     

TiO2/膨润土复合光催化材料的常温制备及性能研究

采用改进的溶胶-凝胶法,在以水为主要溶剂的反应体系中,控制钛酸四丁酯充分水解、缓慢聚合,在 常压、低温(70℃)的温和条件下制备出稳定的TiO₂纳米晶溶胶,并利用TiO₂纳米晶溶胶在膨润土表面负载,获得TiO₂/膨润土复合光催化材料。采用 X 射线衍射、扫描电镜、比表面积测定等研究手段对样品的结构形貌进行了表征,并考察了其光催化活性。结果表明：较高的水用量有利于TiO₂晶体形成,当去离子水:钛酸四丁酯摩尔比大于167:1时,在溶胶体系中出现了锐钛矿型TiO₂纳米晶体;TiO₂纳米晶主要负载于膨润土表面,并未嵌入到膨润土层间结构,但相对于单一膨润土,TiO₂负载显著提高了材料比表面积;当去离子水:钛酸四丁酯摩尔比=192:1时,在紫外光照射下,复合光催化材料表现出最高的光催化活性,对亚甲基蓝的降解率达到93.8%。     

Oxidation behavior of gamma alloys designed for high temperature applications

The oxidation behavior of gamma alloys was investigated in air and scale spallation was observed. The gamma alloys were superior in oxidation behavior to binary TiAl alloys in both isothermal and cyclic exposure. The mass gain of the alloy with high Nb content was smaller than low Nb alloys at 760 and 815 °C, the difference between them decreased at 870 °C. The improved oxidation resistance of the alloys is attributable to Nb addition resulting from reduced TiO₂ growth through the so-called doping effect. Not only the alloys with low Nb but also the high Nb alloys suffered from scale spallation, especially after longer and/or higher temperature exposure. The density of the scale formed on the gamma alloys was found to be about one half of bulk oxide (Al₂O₃, TiO₂) and was comparable to that formed on binary TiAl alloys. The coefficient of the thermal expansion (CTE) of the gamma alloys was larger than that of the oxides and the alloying addition reduced the CTE value of the alloys. Although the Nb addition suppressed the oxide growth and reduced the CTE difference between alloy and oxides, the prevention of the scale spallation could not be attained by single addition of Nb.     

Mo对NbCr2/ Nb合金组织及性能影响

采用机械合金化+热压工艺制备了NbCr_2/Nb-XMo (X=0,2.5,5.0,7.5,10, at%)合金,研究了合金元素Mo对NbCr_2/Nb合金组织及性能的影响。结果表明:合金元素Mo主要存在于Nb基体中,对合金的物相不产生明显影响,合金仍由Nb固溶体和NbCr_2组成;Mo的添加使得NbCr_2/Nb的相界面处应力增加,导致NbCr_2颗粒中的层错/孪晶的密度增加,并促进了Nb基体中位错的运动,从而使得NbCr_2/Nb合金在保持高强度的同时,具有良好的塑性和韧性。     

Modification of TiO2 nanorods by Bi2MoO6 nanoparticles for high performance visible-light photocatalysis

In this work, TiO₂ nanorods were prepared by a hydrothermal process and then Bi₂MoO₆ nanoparticles were deposited onto the TiO₂ nanorods by a solvothermal process. The nanostructured Bi₂MoO₆/TiO₂ composites were extensively characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectroscopy. The photocatalytic activity of the Bi₂MoO₆/TiO₂ composites was evaluated by degradation of methylene blue. The Bi₂MoO₆/TiO₂ composites exhibit higher catalytic activity than pure Bi₂MoO₆ and TiO₂ for degradation of methylene blue under visible light irradiation (λ > 420 nm). Further investigation revealed that the ratio of Bi₂MoO₆ to TiO₂ in the composites greatly influenced their photocatalytic activity. The experimental results indicated that the composite with Bi₂MoO₆:TiO₂ = 1:3 exhibited the highest photocatalytic activity. The enhancement mechanism of the composite catalysts was also discussed.     

Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.     

Properties of TiO2 film prepared from titanium tetrachloride

The properties of TiO₂ film prepared by titanium tetrachloride were investigated with respect to annealing temperatures in terms of phase change, crystallite size, and band gap energy. The TiO₂ film dried at room temperature exhibited an amorphous phase, while films calcined above 281 and at 990°C displayed anatase TiO₂ and a mixture of anatase and rutile, respectively. The TiO₂ film was transformed to an anatase phase through three stages during the annealing processes: (1) removal of water, (2) decomposition of a peroxo group, and (3) amorphous-anatase phase transformation. It was also found that the bandgap energy of TiO₂ film was changed with increasing annealing temperature. This is attributed to the quantum size effect in the range of 475–675°C and to the formation of rutile phase having lower band gap energy than anatase in the range of 675–990°C.     

Nanoscale Studies of the Early Stages of Oxidation of a TiAl-Base Alloy

The strategy to perform nanoscale studies of the initial stages of oxidation of TiAl involved first gaining some information on the electronic structure of pure TiO₂ surfaces and then on TiAl surfaces before and after oxidation both in low- and high-oxygen potentials. Both materials were studied in atomically-cleaned states generated by repeated sputtering and heating. It was found that the oxygen vacancies created additional defect states in the band gap of stoichiometric TiO₂. The results obtained on TiO₂ were used as fingerprints to study the oxide nucleation. The results on the initial stages of oxidation of TiAl confirm the nucleation of Ti₂O₃ islands of nanometer size and monolayer height in a low-oxygen-pressure environment, whilst a TiO₂ layer developed in an atmospheric environment. The ledges on atomically-cleaned surfaces usually acted as nucleation sites.     

First principles study on the phase stability and mechanical properties of MoSi2 alloyed with Al,Mg and Ge

The phase stability, mechanical properties and electronic structure of C11_b and C40 MoSi₂ with alloying elements Al, Mg and Ge were systematically investigated using first principles methods. The calculated lattice constants and elastic constants of C11_b and C40 MoSi₂ are in good agreement with the previous results. It is found that there is a phase transition from C11_b to C40 when the concentrations of Al and Mg reach ∼7 at.% and ∼6 at.%, respectively. Based on the elastic constants, the anisotropy, ductility, hardness and melting temperatures are presented for MoSi₂ with alloying elements. For C11_b, the ductility will be enhanced by increasing the concentrations of Al or Mg. Simultaneously, hardness will be reduced by the increasing of Al or Mg. Ge have a reverse effects. For C40, the ductility is reduced weakly by Al or Mg. In addition, the effects of substitution of Mo by Nb with Si substitution of Si by Al, Mg and Ge are also investigated. Nb and Mg codoping can improve the ductility of MoSi₂. Finally, the density of states is used to analysis the effects of alloying elements on the mechanical properties, and the results are in consistent with the predictions based on elastic constants.     

Synthesis and characterization of titanium doped sodium beta″-alumina

High dense Na-β″-Al₂O₃ electrolyte materials have been synthesized by solid state reaction with boehmite, magnesia, sodium carbonate and titania as the starting materials. The effects of TiO₂ doping percentage on the properties and microstructures of the prepared samples were investigated by X-ray diffraction (XRD), scan electron microscope (SEM), three point bending and ionic conductivity tests. The results indicated that both the relative densities and the phase purities of the samples could effectively improved after doping with TiO₂. The proper doping amount of TiO₂ was to form the transient liquid phase during the sintering process, which would reduce the steric effect and accelerate the diffusivity. Moreover, the mechanical performance of the obtained sample increased to the value above 280 MPa as the doping amount of TiO₂ was larger than 1 wt%. As to the electric properties, if the doping amount was less than 1 wt%, the grain boundary resistivity reduced as the density increased, so the ionic conductivity of the Na-β″-Al₂O₃ was enhanced obviously. However, when the doping amount was above 1 wt%, the ionic conductivity was deteriorated because of the increased resistivity caused by the broadening grain size.     

Theoretical study on the electronic and optical properties of (N, Fe)-codoped anatase TiO2 photocatalyst

Electronic and optical properties of pure, N-doped, Fe-doped and (N, Fe)-codoped anatase TiO₂ were evaluated, respectively, by using the density functional theory. The results indicate that the elemental doping narrows the band gap of TiO₂ and realize its visible-light response activity; and incorporation of Fe into N-doped TiO₂ further increases the photocatalytic activity under visible-light irradiation compared with that of the N-doped TiO₂.     

Electrochemical behaviors of TiO2−x films synthesized by plasma-based ion implantation and deposition in fibrinogen containing PBS solution

TiO_2−x films were synthesized on carbon by plasma-based ion implantation and deposition (PBII-D). Electrochemical behaviors of the prepared films were investigated in phosphate buffer solution (PBS) and fibrinogen containing PBS solution (PBS(Fn)), to probe charge transfer phenomena between TiO_2−x film and fibrinogen. Electrochemical impedance spectroscopy (EIS) as well as simulated values of equivalent circuit units including reaction resistance and electric double layer has been obtained, indicating different charge transfer rate occurred across the interfaces. The shape of Mott-Schottky spectroscopy around the rest-open potential indicates that TiO_2−x films are typical n-type semiconductor. Donor density results calculated by Mott-Schottky theory show that TiO_2−x films exhibit higher donor density in PBS(Fn) than in PBS, indicating charge transfer from fibrinogen to TiO_2−x films, and the space charge layers bend lower.     

Capacitance performance enhancement of TiO<Subscript>2</Subscript> doped with Ni and graphite

Nano-amorphous TiO₂ was prepared by a sol-gel method. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the composite electrode material (TiO₂-NiO-C) is made of powder with a grain size of 36.2 nm. Doping of nickel and graphite can increase the electrical conductivity and the specific surface area of nano-amorphous TiO₂. The electrochemical properties of TiO₂-NiO-C, such as self-discharge, leakage current, and cycle life, were studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge test. With a charge-discharge current density of 500 mA/g, the specific capacity of the TiO₂-NiO-C composite material reaches 12.88 mAh/g. Also, the expense of capacity is only 3.88% after 500 cycles. The electrochemical capacitor with the electrode material of TiO₂-NiO-C shows excellent capacity and cycling performance.     

The corrosion of Nb-modified Ti3Al in a combustion gas with and without Na2SO4−NaCl deposits at 600–800°C

The corrosion behavior of a Nb-modified Ti₃Al intermetallic compound containing 11 at.% Nb in a simulated combustion gas with and without deposits of a Na₂SO₄–NaCl mixture was examined at 600–800°C for times up to four days. In the absence of salt deposits the corrosion rates were rather low and increased only slightly with temperature, producing very thin scales of mixed oxides of Ti, Al, and Nb without sulfides. The presence of the salt deposits produced higher weight gains during an initial stage of one to two days at 600 and 700°C, after which the reaction stopped. A more important and longlasting effect was observed instead at 800°C, when the kinetics of hot corrosion became nearly linear. The scales formed by hot corrosion were complex mixtures of various corrosion products at all temperatures and showed a porous outer region containing a mixture of unreacted salts with oxides (mainly TiO₂), an intermediate region of a mixture of variable composition of oxides of the three metals, and a TiO₂-rich layer beneath it. At 800°C the scales tended to form a thin, discontinuous Al₂O₃-rich layer in the middle and contained an additional innermost region presenting a large concentration of sulfur, very likely as Nb and Ti sulfides. The high rate of hot corrosion at 800°C is attributed to the appearance of sulfides in the inner region of the scale and to a more efficient scale fluxing.     

First-principles investigation of phase stability,elastic and thermodynamic properties in L12 Co3(Al,Mo,Nb) phase

First-principles calculations have been performed to investigate the phase stability, elastic, and thermodynamic properties of Co₃(Al,Mo,Nb) with the L1₂ structure. Calculated elastic constants show that Co₃(Al,Mo,Nb) is mechanically stable and possesses intrinsic ductility. It is found that the shear and Young's moduli of Co₃(Al,Mo,Nb) are smaller than those of Co₃(Al,W). Calculated density of states indicate the existence of covalent-like bonding in Co₃(Al,Mo,Nb). Temperature-dependent thermodynamic properties of Co₃(Al,Mo,Nb) can be described satisfactorily using the Debye-Grüneisen approach, including entropy, enthalpy, heat capacity and linear thermal expansion coefficient, showing their significant temperature dependences. Furthermore the obtained data can be employed in the modeling of thermodynamic and mechanical properties of Co-based alloys to enable the design of high temperature alloys.     

Photo-catalytic effect enhanced by the chemisorption of phenylethyl-mercaptan-assembled monolayers on Au-clusters/TiO2-anatase thin film

A simple process is employed to increase the efficiency of TiO₂ photo-catalytic activity, for which the recombination probability of electron–hole pairs is relaxed. Au is selectively deposited on a high-transparency TiO₂-anatase thin film on a glass substrate, and then phenylethyl mercaptan (PEM) is chemisorbed onto the selectively covered Au-clusters/TiO₂-anatase thin film. The enhancement of the photo-catalytic activity on the PEM/Au-clusters/TiO₂-anatase thin film is evaluated via the induced degradation of methylene blue. The results demonstrate that the Au coverage ratio on TiO₂-anatase thin film and the photo-catalytic activity of the chemisorbed PEM/Au-clusters/TiO₂-anatase are related. The photo-catalytic contribution of PEM/Au-clusters/TiO₂-anatase differs from that of Au on a TiO₂-anatase thin film. An optimized photo-catalytic system, a composite of PEM/3.8% Au-clusters/TiO₂-anatase thin film, is proposed. The efficiency of the PEM/3.8% Au-clusters/TiO₂-anatase thin film is 52.1% higher than that of the as-deposited TiO₂-anatase thin film.