Synthesis of nanocrystalline TiO<Subscript>2</Subscript> by tartarate gel method

A gel was formed when a mixture of TiOCl₂ and tartaric acid was heated on a water bath. Ultrafine powders of TiO₂ in the anatase phase were formed, when the gel was decomposed at 623 K and the mole ratio of tartaric acid to titanium was 2. The anatase phase was converted into rutile phase on annealing at higher temperatures, > 773 K. When initial ratio of titanium to tartaric acid was < 2, the decomposition of gel leads to the formation of mixed phases of rutile and anatase. However, pure rutile phase was not formed by the decomposition of gel for any ratio of tartaric acid and titanium. These powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface area measurements. The average particle size obtained for anatase phase was 3 nm whereas it was 30 nm for rutile phase. Raman scattering experiments were also performed to confirm both anatase and rutile phases.     

TiO2 nanorod films grown on Si wafers by a nanodot-assisted hydrothermal growth

We report the controlled hydrothermal growth of rutile TiO₂ nanorods on Si wafers by using an anatase TiO₂ nanodot film as an assisted growth layer. The anatase nanodot film was prepared on the wafer by phase-separation-induced self-assembly and subsequent heat-treatment at 500 °C. The nanodots on the wafer were then subjected to hydrothermal treatment to induce the growth of rutile TiO₂ nanorod films. The size and dispersion density of the resulting TiO₂ nanorods could be varied by adjusting the Ti ion concentration in the growth solution. The TiO₂ nanorods were of the rutile phase and grew in the [001] direction. The growth mechanism reveals that the growth of the rutile nanorods was wholly dependent on the existence of rutile TiO₂ seeds, which could be formed by the dissolution-reprecipitation of the anatase nanodots during hydrothermal treatment or under the high-temperature conditions of the subsequent heat-treatment of the as-prepared nanodots. In controlling the rutile nanorod growth, the anatase nanodots show more efficiency than a dense anatase film. Preliminary evaluations of the rutile nanorod films have demonstrated that the wettability changed from highly hydrophobic to superhydrophilic and that the photocatalytic activity was enhanced with increasing nanorod dispersion density.     

Review of the anatase to rutile phase transformation

Titanium dioxide, TiO₂, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.     

Dielectric properties of Fe-doped TiO2 nanoparticles synthesised by sol–gel route

Fe-doped nanocrystalline samples of titanium oxide have been synthesised by sol–gel route and conventional sintering process at 450°C under atmospheric conditions. These samples are characterised by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, transmission electron microscopy (TEM) and dielectric property measurement. Samples with Fe content more than 4?mole% crystallises in rutile phase and those with less than 4?mole% crystallises in anatase phase. Nanocrystallite size has been controlled by Fe doping. Crystallite size was found to decrease with Fe concentration in the anatase phase samples whereas reverse happens in rutile phase samples of titanium oxide. There is a slight variation in numerical values of crystallite sizes measured by the two techniques: TEM and XRD peak broadening. The highest crystallite size was 86?nm in 10?mole% Fe-doped samples and the lowest 20?nm in 4?mole% Fe-doped sample. Large dispersions and anomalous values of the dielectric constant, εr were observed at low frequency in anatase phase samples. Rutile phase samples exhibit little dispersion over the measurement frequency range of 20?Hz to 10?MHz. The dielectric constant value of all the samples stabilises to a constant value at higher frequencies. This value is dependent on the final crystalline phase but independent of the crystallite size. The anomalous dielectric behaviour of anatase samples at low frequencies is assigned to the adsorbed –OH ions on the sample surface.     

Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels

Abstract

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe–(15–30) wt-%Mn alloys with additions of Al and Si have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α′ (bcc)- and ε (hcp)-martensite in the γ (fcc) matrix during plastic deformation was analysed by optical microscopy, X-ray diffraction, and scanning electron microscopy. Depending on the content of the alloying elements different phase transformations γ → ε, γ → α′ (TRIP effect), or the formation of deformation twins (TWIP effect) occurred. Additions of Al increased the stacking fault energy (γ_fcc) and suppressed the γ → ε transformation while Si decreased γ_fcc and sustained the γ → ε transformation. These steels with reduced densities of about 7.3 Mg m^?3 exhibit high tensile ductility up to 95% with true tensile strength of about 1100 MPa. The excellent plasticity induced by twinning or phase transformation up to extremely high strain rates of about <disp-formula><graphic href="splitsection2-m1.tif"/></disp-formula> results in an extraordinary shock resistance and allows for deep drawing and backward extrusion operations of parts with complex shapes.     

Solvothermal-induced phase transition and visible photocatalytic activity of nitrogen-doped titania

Nitrogen-doped titania nanoparticles consisting of pure anatase, pure rutile and bicrystallites (anatase+rutile and anatase+brookite) have been prepared in TiCl(3)-HMT (hexamethylene tetramine)-alcohol solution under solvothermal process. The effect of the solvent type and amount of HMT as pH adjuster on the phase composition of titania and its visible photocatalytic activity for degradation to MO (methyl orange) was investigated. It is found that anatase gradually transferred to rutile with increase of carbon chain using methanol, ethanol, 1-propanol and 1-butanol as solvent. The pure anatase formed at the pH value of 1-2, while bicrystalline titania (anatase+rutile and anatase+brookite) at that of 7-10 in the presence of methanol. The bicrystalline (anatase+brookite) titania have the best visible photocatalytic activity among all the samples. The -(NO) and -(NH) dopants with an N (1s) binding energy of 400 eV may have positive effects on the visible light photocatalytic activity.     

Atomistic study of the effect of B addition in the FeAl compound

First principle calculations have been carried out to study energetic of boron atom impurities in bulk and symmetric Σ5(310) tilt grain boundaries of the ordered stoechiometric B2 FeAl intermetallic. A set of configurations was considered for studying the bulk behaviour: B in tetrahedral and octahedral interstitial positions or substituting Al and Fe. For the analysis of the segregation at the grain boundary, calculations were done for B substituting Al and Fe at three different locations and for B filling empty spaces along the interface. In each case, the defect formation energies were calculated to determine the site preference and their relative stability. The results indicate that B doping is metastable in the bulk and tends to segregate along the grain boundary. The overall behaviour of the B atoms at the boundary is essentially driven by the strong Fe–B interactions.     

Study of effect of chromium on titanium dioxide phase transformation

MTiX samples with different atomic chromium percentages were synthesized by sol–gel method and calcined at 400 °C under air. The effects of Cr and temperature on titanium dioxide phase transition were studied. In situ measurement showed the presence of anatase phase for all samples at temperature < 500 °C. Without Cr content, the anatase–rutile transition takes place at 600 °C and the rutile fraction increases with increase of temperature. In the presence of Cr content, rutile phase appeared at 700 °C. Cr₂O₃ phase was shown only in the case of CrTi20 content at 800 °C which indicates that the segregation remains modest. We have also studied the anatase–rutile transition kinetics by using in situ X-ray measurements. It was found that the anatase phase stability increases as the chromium content increases. Results confirm that the transformation of anatase–rutile is of first order.     

氟对锂铝硅系统玻璃结构及析晶的影响

通过引入氟以降低锂铝硅玻璃的高温黏度,使"温度-黏度"曲线适应浮法工艺的需求,并采用激光拉曼光谱、XRD、高温旋转黏度计等测试手段研究氟对锂铝硅系统玻璃的结构及随后析晶的影响。结果表明:氟的引入使母体玻璃网络结构中的[Si(Al)O4]四面体相互之间形成的环结构向小环结构发展,[Si(Al)O4]四面体之间的相互连接程度降低,Si(Al)—O—Si(Al)键的振动减弱,从而能有效地降低母体玻璃的高温黏度。随着氟引入量从0.00mol增加到0.28mol,母体玻璃在晶化过程中析出的主晶相并未发生改变,皆为β-锂辉石固溶体,且析出的主晶相含量随氟引入量的增加而增加。此外,结合Raman光谱分析可以发现,试样中有微量的锐钛矿晶相析出。     

Formation and stability of anatase phase of phosphate incorporated and carbon doped titania nanotubes

Phase transformation analysis of the phosphate containing and carbon doped titania nanotubes, prepared by a simple anodization method, reveals complete transformation from amorphous to anatase phase in air between 360 and 400 °C. Activation energies for formation of anatase phase are evaluated and compared for the two types of titania nanotubes. A detailed analysis of the phase transformation characteristics and stability of the anatase phase is reported.     

The effect of Nb substitution on synthesis and photo-response of TiO2 thin films prepared by direct current magnetron sputtering

TiO₂ has a limited photoefficiency for solar energy harvesting and conversion due to its large bandgap. Nb substitution into the TiO₂ lattice was investigated to red-shift its photoresponse by introducing defect energy levels in the host bandgap. Ti_1−xNb_yO₂ thin films (0 ≥ y ≤ 0.52) were prepared by direct current reactive magnetron sputtering. The effect of Nb concentration on crystal phase formation and subsequent change in optical and electronic properties were determined. Films had an increasing rutile proportion as Nb concentration increased, and all films prepared for y > 0.25 were pure rutile. Nb substituted as Nb(V) or a mixture of Nb(IV)/(V) while Ti remained Ti(IV). Therefore, in these materials cation vacancies formed to electronically compensate for group V doping. Nb-doped rutile films showed no red-shift in optical response while Nb-doped anatase/rutile composites were strongly red-shifted. Photocurrent measurements were made to determine if the enhanced visible light absorption generated mobile electron-hole pairs or was due to localized excitations at color centers. At low Nb concentrations (~ 2 at.%), visible light absorption generated mobile charge carriers, while at higher concentrations ( > 9 at.%), no mobile charge carriers were created. At higher Nb concentrations, the formation of mobile charge carriers was hindered by charge recombination at cation vacancies.     

Dependence of crystalline phases in titania on catalytic properties during CO hydrogenation of Co/TiO2 catalysts

The present research showed dependence of crystalline phases in titania on the catalytic properties of Co/TiO₂ catalysts during CO hydrogenation. A comparative study of anatase TiO₂- and rultile-anatase coupled TiO₂-supported Co catalysts was conducted. It was found that the presence of rutile phase (19 mol%) in titania resulted in a significant increase in the catalytic activity during CO hydrogenation. It was proposed that the role of rutile phase was to increase the stability of the support. The impact of water vapor produced during reduction on the formation of cobalt species strongly interacted with the support was probably inhibited by the presence of rutile phase in titania leading to a decrease in the reducibility loss during reduction.     

Ge、Si元素对ZrO2和Zr(Fe,Cr)2能量与电子结构的影响

采用基于密度泛函理论的第一性原理计算方法,研究Ge、Si元素对锆合金中与腐蚀相关的ZrO_2氧化膜相和Zr(Fe,Cr)_2第二相能量与电子结构的影响。合金形成热、结合能的计算结果表明:ZrO_2四方相结构不稳定,立方相易形成且结构稳定,氧化膜晶体结构从四方相向立方相发生转变影响锆合金的耐腐蚀性能;Ge、Si元素均降低ZrO_2立方相的结构稳定性和形成能力,与Ge相比,Si易取代Zr(Fe,Cr)_2第二相中的Cr,增加锆合金Fe/Cr原子比。电子态密度和Mulliken电子占据数的计算结果表明:ZrO_2中Zr与O存在杂化共振与较强的离子键作用,Ge、Si降低ZrO_2立方相结构稳定性的原因主要在于削弱了Zr-O之间的离子键作用;ZrO_2氧化膜相和Zr(Fe,Cr)_2第二相是影响锆合金耐腐蚀性能的两个重要因素,对Si而言,形成含Si的Zr(Fe,Cr)_2第二相对锆合金耐腐蚀性能产生不利影响,改善锆合金耐腐蚀性能需要ZrO_2晶体结构改变占主导地位;对Ge而言,含Ge的Zr(Fe,Cr)_2第二相难形成,第二相对锆合金耐腐蚀性能的影响相对Si较小,减缓ZrO_2由四方相向立方相的转变倾向,是Ge改善锆合金耐腐蚀性能的重要原因。     

The selective growth of rutile thin films using radio-frequency magnetron sputtering

Using unbalanced radio-frequency (RF) magnetron sputtering crystalline rutile films were synthesised on glass substrates at (combined Ar and O₂) pressures of 0.4 Pa or less, at RF powers of 500 and 600 W with substrate to magnetron distances of 40 mm or longer. Anatase films were deposited at the greater pressure of 1.2 Pa (substrate to magnetron distance of 40 mm) or shorter substrate to magnetron distance of 20 mm (at 0.4 Pa). A mixture of anatase and rutile was formed at 0.5 Pa with all other conditions being as for those required for rutile or the power was reduced along with the substrate to magnetron distance (500 W and 20 mm). The crystallite sizes of rutile obtained were 1 - 3 nm. It is proposed that the greater the energy imparted to the substrate surface by the impinging positive species the greater the activation energy to crystalline phase formation that can be overcome. Hence the formation of rutile over anatase is favoured at greater power, longer magnetron to substrate distances and decreased pressure. Moreover, not only is it possible to control the phase of TiO₂ formed it appears to be possible to control the degree of oxygen non-stoichiometry in the rutile films formed. Smaller O₂ partial pressures, shorter substrate to magnetron distances and greater RF power are believed to produce an environment of reduced reaction of sputtered Ti species with O₂ and to result in the formation of non-stoichiometric rutile structures resulting in increased band gap energies and decreased refractive indices.     

Kinetic analysis on photocatalytic degradation of gaseous acetaldehyde,ammonia and hydrogen sulfide on nanosized porous TiO2 films

The characteristics of the UV illumination-assisted degradation of gaseous acetaldehyde, hydrogen sulfide, and ammonia on highly active nanostructured-anatase and rutile films were investigated. It was found that the anatase film showed a higher photocatalytic activity than the counterpart did, however, the magnitude of difference in the photocatalytic activity of both films decreased in the order ammonia > acetaldehyde > hydrogen sulfide. To elucidate the reasons for the observation, the adsorption characteristics and the kinetics of photocatalytic degradation of the three reactants on both films were analyzed. The adsorption analysis examined using a simple Langmuir isotherm, showed that adsorbability on both films decreased in the order ammonia > acetaldehyde > hydrogen sulfide, which can be explained in terms of the decreasing electron-donor capacity. Acetaldehyde and ammonia adsorbed more strongly and with higher coverage on anatase film (1.2 and 5.6 molecules/nm², respectively) than on rutile (0.6 and 4.7 molecules/nm², respectively). Conversely, hydrogen sulfide molecules adsorbed more strongly on rutile film (0.7 molecules/nm²) than on anatase (0.4 molecules/nm²). Exposure to UV light illumination brought about the photocatalytic oxidation of the three gases in contact with both TiO₂ films, and the decrease in concentration were measured, and their kinetics are analyzed in terms of the Langmuir–Hinshelwood kinetic model. From the kinetic analysis, it was found that the anatase film showed the photocatalytic activities that were factors of ～8 and ～5 higher than the rutile film for the degradation of gaseous ammonia and acetaldehyde, respectively. However, the activity was only a factor of ～1.5 higher for the photodegradation of hydrogen sulfide. These observations are systematically explained by the charge separation efficiency and the adsorption characteristics of each catalyst as well as by the physical and electrochemical properties of each reactant.     

Silicon lithium-ion battery anode with enhanced performance: Multiple effects of silver nanoparticles

Silicon has been regarded as one of the most promising next generation lithium-ion battery anode. However, the poor cyclic stability of the Si based anode has severely limited its practical applications, which is even worse with high mass loading density (>1?mg?cm^?2). A new concept has been developed to enhance the electrochemical performance of the Si nanoparticle anode. Silver nanoparticles are composited with the silicon nanoparticles in a facile way for the first time. It is found that the mechanical properties of the Si electrode have been significantly improved by the incorporation of the silver nanoparticles, leading to enhanced cyclic performance. With the Si/Ag mass ratio of 4:1, the reversible specific discharge capacity is retained as 1156?mA?h?g^?1 after 100 cycles at 200?mA?g^?1, which is more than three times higher than that of the bare silicon (318?mA?h?g^?1). The rate performance has been effectively improved as well due to excellent electron conductivity of the silver nanoparticles.     

Interfacial microstructures and properties of aluminum alloys/galvanized low-carbon steel under high-pressure torsion

A new composite processing technology characterized by hot-dip Zn–Al alloy process was developed to achieve a sound metallurgical bonding between Al–7 wt% Si alloy (or pure Al) castings and low-carbon steel inserts, and the variations of microstructure and property of the bonding zone were investigated under high-pressure torsion (HPT). During hot-dipping in a Zn–2.2 wt% Al alloy bath, a thick Al₅Fe₂Zn_x phase layer was formed on the steel surface and retarded the formation of Fe–Zn compound layers, resulting in the formation of a dispersed Al₃FeZn_x phase in zinc coating. During the composite casting process, complex interface reactions were observed for the Al–Fe–Si–Zn (or Al–Fe–Zn) phases formation in the interfacial bonding zone of Al–Si alloy (or Al)/galvanized steel reaction couple. In addition, the results show that the HPT process generates a number of cracks in the Al–Fe phase layers (consisting of Al₅Fe₂ and Al₃Fe phases) of the Al/aluminized steel interface. Unexpectedly, the Al/galvanized steel interface zone shows a good plastic property. Beside the Al/galvanized steel interface zone, the microhardnesses of both the interface zone and substrates increased after the HPT process.     

Fe-F/TiO_2复合微球的制备与光降解部分水解聚丙烯酰胺机制

以Span-80为调控剂,钛酸四丁酯(TBOT)为钛源,采用低温水解-回流法制备了Fe-F共掺杂TiO_2介孔复合微球(Fe-F/TiO_2)。通过XRD、SEM、FTIR、TG-DTA、BJH和UV-vis DRS测试方法对样品进行了结构性能表征;以部分水解聚丙烯酰胺(HPAM)为目标降解物,研究了Fe-F/TiO_2复合催化剂的光催化性能。结果表明,制得的Fe-F/TiO_2是由直径为10~15nm的纳米粒子堆砌而成的锐钛矿型介孔微球,其中Fe3+可以有效促进锐钛矿而抑制金红石相的生成,使其具有较高的热稳定性;比表面积、孔容积及平均孔径分别是145.11 m2/g、0.26cm3/g和6.23nm。在光降解HPAM的过程中,Fe3+和F-的协同效应可以提升材料的光催化性能,使FeF/TiO_2具有最高的催化活性。在紫外光及可见光条件下,0.1g的Fe-F/TiO_2降解100mL浓度为500mg/L的HPAM溶液120min,其COD去除率分别为81%和74%。     

On the selective growth of titania polymorphs in acidic aqueous medium

The influence of preparation conditions on the phase composition and morphology of titania was studied for the solids synthesized by hydrothermal treatment (HT) and peptizing of hydrous TiO₂ sols in acidic medium. Mutual influence of peptizing and of additive anions (SO₄²⁻, Cl⁻) on the nature of obtained polymorphs was for the first time systematically studied and coherently explained. The solids were characterized by XRD, Raman spectroscopy, and transmission electron microscopy. It was found that peptizing step preceding HT and the presence of anions play a crucial role for the selective formation of TiO₂ anatase or rutile polymorphs. Low temperature peptizing leads to acicular rutile particles, whereas HT produces highly dispersed anatase. However if the HT was preceded by peptizing step, rutile was obtained in most cases. The influence of additives strongly depends on the moment of their introduction. Sulfate and chloride species can act as phase growth controllers, or as morphology modifiers. Sulfate hindered formation of rutile and favored anatase al low temperatures, but for already formed rutile seed, sulfate acted only as a shape controller. By contrast, chloride showed a strong tendency to promote rutile growth, whatever the conditions. A qualitative model was proposed explaining the effects observed, supported by ground state DFT and semi-empirical calculations of the aqueous Ti species.     

A heterostructured SnO2–TiO2 thin film prepared by Langmuir–Blodgett technique

SnO₂–TiO₂ heterostructure films were prepared through Langmuir–Blodgett (LB) route. LB films of octadecyl amine (ODA)–titanyl oxalate multilayer deposited on Si (100) and decomposed at 600 °C showed rutile and anatase phases of ultrathin TiO₂ film. Subsequently, multilayer LB film of ODA–stannate deposited on the pre deposited TiO₂ film after decomposition at 600 °C resulted in thin SnO₂ films on the TiO₂ thin film. The phase analysis of the SnO₂–TiO₂ film showed cassiterite phase of SnO₂ as well as the rutile/anatase mixture of TiO₂ indicating a SnO₂–TiO₂ heterostructured film. Surface morphology of the pure TiO₂ film and SnO₂–TiO₂ film were analyzed by using AFM. Electrical characterization by AC impedance analysis suggested SnO₂–TiO₂ heterostructure formation. DC current voltage measurement showed increase in photocurrent indicating visible light absorption and efficient charge separation under the sunlight type radiation.