Thermodynamic properties of aluminum titanate-mullite composite ceramics containing heat stabilizer

Using Al₂O₃ and TiO₂ as raw materials, adding MgO as heat stabilizer and mullite as enhancer, aluminum titanate-mullite multiphase ceramics were successfully prepared by solid phase synthesis. The effects of MgO and mullite were systematically studied on the phase composition, microstructure, thermal stability, sintering properties, and mechanical properties of aluminum titanate ceramics. The results showed that the introduction of Mg²⁺ can partially replace Al³⁺ to form Mg_xAl_2(1-x)Ti_(1+x)O₅ solid solution, improved the thermal stability of aluminum titanate ceramics, and promoted the formation and growth of grains, which reduced the sintering temperature. The crack deflections caused by mullite particles improved the mechanical properties. The filling effect of mullite particles and the formation of silica in mullite raw materials were conducive to ceramic densification. The statistics of Mg4M10 sample were as follows: the porosity was only 2.9%, the flexural strength was as high as 64.15 MPa, and the thermal expansion coefficient was 1.35 × 10⁻⁶ K⁻¹ (RT-700°C), encouraging the application of ceramics with high thermal mechanical properties.     

A TiO2 modified whisker mullite hollow fiber ceramic membrane for high-efficiency oil/water emulsions separation

Design and preparation of membranes with ultrahigh separation performance and antifouling property for oil-in-water (O/W) emulsions remains challenging. In this study, a high flux mullite/TiO₂ ceramic composite membrane was prepared via multi-precipitation of TiO₂ on a whisker mullite hollow fiber support synthesized by combining phase inversion and high-temperature sintering techniques. The results showed that the generated whisker mullite structure improved the permeation flux, and the micro-nano structured TiO₂ functional layer endowed the membrane surface with superhydrophility and stability. The retention of the optimal composite membrane (M20T13) that was soaked in the titanium solution 20 times for 13 min each time for the O/W emulsions like n-hexane, toluene and engine oil maintained over 98 %, and the flux after 6 h filtration was 668.34 L·m⁻²·h⁻¹, 487.25 L·m⁻²·h⁻¹ and 258.66 L·m⁻²·h⁻¹, respectively, much higher than that of the optimal substrate (F3A1, mass ratio of fly ash: Al₂O₃ = 3:1). Moreover, the flux recovery rate of M20T13 was much higher than that of F3A1 after chemical backwashing. This work manifests great potential in O/W treatment fields.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

Preparation and characterization of transparent indium- doped TiO2 films deposited by MOCVD

Titanium dioxide (TiO₂) films doped with different indium (In) concentrations have been prepared on SrTiO₃ (STO) substrates by high vacuum metalorganic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) analyses revealed the TiO₂ films doped with low In concentrations to be [001] oriented anatase phase and the films with high In concentrations to present polycrystalline structures. The 1.8% In-doped TiO₂ film exhibited the best electrical conductivity properties with the lowest resistivity of 8.68×10⁻² Ω cm, a Hall mobility of 10.9 cm² V⁻¹ s⁻¹ and a carrier concentration of 6.5×10¹⁸ cm⁻³. The films showed excellent transparency with average transmittances of over 85% in the visible range.     

Formation of TiO2 nano fibers on a micro-channeled Al2O3-ZrO2/TiO2 porous composite membrane for photocatalytic filtration

In this study, needle-shape TiO₂ fibers were successfully fabricated inside a micro-channeled Al₂O₃-ZrO₂ composite porous membrane system using sol-gel method. The micro-channeled Al₂O₃-ZrO₂ composite was fabricated using the fibrous monolithic (FM) process. Pure anatase phase TiO₂ was crystallized from the as-coated amorphous phase during calcination at 510 °C. The TiO₂ fibers grew on the surface frame of the micro-channeled Al₂O₃-ZrO₂ composite membrane and fully covered the inside of the micro-channeled pores. The specific surface area of the TiO₂ coated membrane system was dramatically increased by over 100 fold compared to that of the non-coated system. The photocatalytic activity of the membrane was also assessed and was shown to very effectively convert organic materials. Thus, this novel membrane holds promise for use as an advanced filtration system.     

Catalytic Activities of Al2O3-promoted NiSO4/TiO2 for Acid Catalysis

A series of catalysts, NiSO₄/Al₂O₃–TiO₂, for acid catalysis was prepared by the impregnation method, where support, Al₂O₃–TiO₂ was prepared by the coprecipitation method using a mixed aqueous solution of titanium tetrachloride and aluminum nitrate solution followed by adding an aqueous ammonia solution. The addition of nickel sulfate (or Al₂O₃) to TiO₂ shifted the phase transition of TiO₂ from amorphous to anatase to higher temperature because of the interaction between nickel sulfate (or Al₂O₃) and TiO₂. 15-NiSO₄/5-Al₂O₃–TiO₂ containing 15 wt% NiSO₄ and 5 mol% Al₂O₃, and calcined at 400°C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The charge transfer from Ti atoms to the neighboring Al atoms strengthens the Al–O bond between Al and the surface sulfate species. The addition of Al₂O₃ up to 5 mol% enhanced the acidity, thermal property, and catalytic activities of NiSO₄/Al₂O₃–TiO₂ gradually due to the interaction between Al₂O₃ and TiO₂ and consequent formation of Al–O–Ti bond.     

Low thermal expansion behavior and thermal durability of ZrTiO4–Al2TiO5–Fe2O3 ceramics between 750 and 1400 °C

The thermal-shock-resistant materials in the system Al₂TiO₅–ZrTiO₄ (ZAT) were synthesized by oxide process. The range of ZAT compositions investigated had showed very low thermal expansions of 0.3∼1.3×10⁻⁶/K compared to 8.29×10⁻⁶/K of pure ZrTiO₄ and 0.68×10⁻⁶/K of polycrystalline Al₂TiO₅, respectively, compared with the theoretical thermal expansion coefficient for a single crystal of Al₂TiO₅, 9.70×10⁻⁶/K. The composites also had high thermal durability between 750 and 1400° for 100 h. The low thermal expansion and high thermal durability are apparently due to a combination of microcracking caused by the large thermal expansion anisotropy of the crystal axes of the Al₂TiO₅ phase and the limitation of grain growth Al₂TiO₅ by the ZrTiO₄. The microstructural degradation of the composites is presented here analyzed by scanning electron microscopy, X-ray diffraction, and dilatometry.     

Study on the formation process of Al2O3-TiO2 composite powders

Fine particles of anatase were suspended in solutions of ammonium alum with Al₂O₃/TiO₂ molar ratios from 0.1:1 to 7:1. By spray drying the suspensions and calcining the spray-dried powders, Al₂O₃-TiO₂ composite particles were obtained. The results show that after the spray drying, coatings of ammomium alum are formed on the surface of the anatase particles, leading to composite precursor powders (CCPs) with larger particle sizes. Upon calcining the CCPs, ammomium alum pyrolyzes to amorphous Al₂O₃ and anatase transforms into rutile. Both are mainly responsible for the observed particle size reductions as well as the densification of each composite particle. The in-situ formed α-Al₂O₃ and rutile may have higher reactivities, forming aluminum titanate at 1150 °C, about 130 °C lower than the theoretical temperature for the formation of Al₂TiO₅ by solid reaction. The reaction between α-Al₂O₃ and rutile starts from the interface between the anatase and the alum coating and mainly takes place in the single particles formed by spray drying. The molar ratio of Al₂O₃ to TiO₂ influences the final crystalline phases in the composite powders, but not stoichiometrically.     

TiO2 nanofibers doped with rare earth elements and their photocatalytic activity

In the present study rare earth doped (Ln³⁺–TiO₂, Ln = La, Ce and Nd) TiO₂ nanofibers were prepared by the sol–gel electrospinning method and characterized by XRD, SEM, EDX, TEM, and UV-DRS. The photocatalytic activity of the samples was evaluated by Rhodamine 6G (R6G) dye degradation under UV light irradiation. XRD analysis showed that all the synthesized pure and doped titania nanofibers contain pure anatase phase at 500 °C but at 700 °C it shows both anatase and rutile phase. XRD result also shows that Ln³⁺-doped titania probably inhibits the phase transformation. The diameter of nanofibers for all samples ranges from 200 to 700 nm. It was also observed that the presence of rare-earth oxides in the host TiO₂ could decrease the band gap and accelerate the separation of photogenerated electron–hole pairs, which eventually led to higher photocatalytic activity. To sum up, our study demonstrates that Ln³⁺-doped TiO₂ samples exhibit higher photocatalytic activity than pure TiO₂ whereas Nd³⁺-doped TiO₂ catalyst showed the highest photocatalytic activity among the rare earth doped samples.     

Surface modification of macroporous Al2O3 tubes with carbon-doped TiO2 intermediate layer and preparation of highly permeable palladium composite membranes for hydrogen separation

ABSTRACT

To prepare H₂-permeable palladium composite membranes, a novel carbon-doped microporous TiO₂ intermediate layer was introduced to modify the surface of macroporous Al₂O₃ substrates. The Pd/TiO₂–C/Al₂O₃ membrane was prepared via electroless plating, and thereafter, carbon residue in the intermediate layer was removed by calcination in air, yielding a Pd/TiO₂/Al₂O₃ membrane. Experimental results indicate that the carbon residue shrinks the pore size of the intermediate layer and facilitates a decrease of membrane defects. Additionally, carbon removal induces a higher effective membrane area at the permeate side, which enhances hydrogen permeability. Furthermore, the apparent activation energy (E_A) and stability of the as-prepared Pd/TiO₂/Al₂O₃ membrane were investigated.     

Characterisation by EPR spectroscopy

The fresh and used EUROCAT II V₂O₅–WO₃/TiO₂ catalysts have been studied by electron paramagnetic resonance. For both samples, V⁴⁺ in clusterlike arrangement and two sites corresponding to VO²⁺ ions in octahedral symmetry axially distorted were found. It has been shown that the TiO₂ phase transformation (anatase → rutile) occurs at lower temperature (1073 K) for the EUROCAT II catalysts than the WO₃/TiO₂ support. Impurities of Fe³⁺ ions were detected both on the catalysts and on the WO₃/TiO₂ support. Q-band measurements have allowed a difference in the environment of the Fe³⁺ ions caused by the SCR process to be observed. Finally, traces of Ti³⁺ ions in the anatase phase have been detected.     

Colored films produced by electron beam deposition from nanometric TiO2 and Al2O3 pigment powders obtained by modified polymeric precursor method

Synthesis and the characterization of TiO₂:5%Co (green), TiO₃:5%Fe (brown-reddish), TiO₂:2%Cr (brown), Al₂O₃:5%Co (blue), Al₂O₃:5%Fe (brown-reddish) and Al₂O₃:2%Cr (light green) nanometric pigment powders using polymeric precursor (modified Pechini's method) is reported. Colored thick films were deposited on amorphous quartz substrates by electron beam physical vapor deposition (EB-PVD) using pellets of the pigment powders as target. The evaporation process was carried out in vacuum of 4 × 10⁻⁶ Torr and the amorphous quartz substrates were kept at 350 °C during deposition. The TiO₂-based pigment powders presented crystalline anatase phase and the Al₂O₃-based pigment powders showed corundum phase, investigated by X-ray diffraction (XRD). The average particle size of the pigment powders was about 20 nm, measured by scanning electron microscopy with field emission gun (SEM-FEG). Diffuse reflectance spectra and colorimetric coordinates L¹, a¹, b¹ using the CIE-L¹a¹b¹ method are shown for the pigment powders, in the 350–750 nm range. The colored thick films were characterized by transmittance (UV–Vis) and atomic force microscopy (AFM). The average film roughness was ∼5.5 nm and the average grain size obtained in the films was around 75 nm. Films with thickness from 400 nm to 690 nm were obtained, measured by talystep profiler. Transmission spectra envelop method has been used to obtain refractive index and thickness of the Al₂O₃ colored thick films.     

钯复合膜的制备及其在氢气氮气分离中的应用

Thin palladium composite membranes were prepared by modified electroless plating method on a-alumina supports and a dense Pd/α-Al2O3 composite membrane with high hydrogen flux, good selectivity for hydrogen was obtained. It was tested in a single gas permeation system for hydrogen permeance and hydrogen selectivity over mtrogen. The hydrogen permeance of the corresponding membrane was ashigh as 2.45×10^-6mol·m^-2·s^-1.Pa^-1 and H2/N2 selectivityover700 at 623K and a pressure difference of 0.1MPa. The-main resistance of the composite membrane to H2 permeation lies in the aluminum ceramic support rather than the thin Pd layer.     

Photocatalytic performance of Al and Nb-doped TiO2 nanoparticles prepared by microwave-assisted hydrothermal method

Microwave-assisted hydrothermal synthesis of aluminum (Al)-doped TiO₂ (ALT) and niobium (Nb)-doped TiO₂ (NBT) nanoparticles were carried out. Investigations were performed to examine the crystallinity, vibrational modes, surface morphology, and composition using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), respectively. The XRD study indicated the higher crystallinity of the ALT particles compared to the Al and Nb-doped TiO₂ (ALNBT) particles, and only the presence of the anatase phase was observed in all samples. As a result of doping, the Raman mode at ∼147 cm⁻¹ is found to be shifted toward a higher wavenumber for both samples. SEM analysis showed the spherical morphology of ALT and NBT nanoparticles. The elemental composition peaks of Al, titanium, Nb, and oxygen were noticed by EDS measurements. Furthermore, both prepared nanoparticles were used as photocatalytic materials. The Nb and Al-doped samples showed an improvement in the photocatalysis response in relation to the pure TiO₂ sample, in which Al-doped sample was able to decolorize 100% of rhodamine B in 75 min of analysis.     

Alkaline earth metal doped nanoporous TiO2 for enhanced photocatalytic mineralisation of bisphenol-A

Magnesium and barium doped TiO₂ nanoparticles were synthesised by sol-gel technique. The materials were characterised by XRD, BET, UV-Vis, FT-IR, SEM and TEM techniques. The pure TiO₂ nanoparticles gave both anatase and rutile phases together, but the presence of either Mg²⁺ or Ba²⁺ metal ion in TiO₂ gave only anatase phase. The framework substitution of Mg²⁺ in TiO₂ was established by XRD and FT-IR techniques. However, Ba²⁺ was retained only on the surface of TiO₂ as BaCO₃ and the absence of framework substitution of Ba²⁺ in TiO₂ was evident from XRD and FT-IR analysis. The band gap values of Mg²⁺ and Ba²⁺ doped nano TiO₂ were higher than the parent nano TiO₂. The photocatalytic activity in the degradation of bisphenol-A was found to be higher for metal doped nano TiO₂ than both pure nano TiO₂ and commercial TiO₂ (Degussa P-25).     

Boron-doped rutile TiO2/ anatase TiO2/ ZrTiO4 ternary heterojunction photocatalyst with optimized phase interface and band structure

To improve the photocatalytic performance of TiO₂-based heterostructures, Z-scheme/Ⅱ-type rutile TiO₂ (R)/anatase TiO₂ (A)/ZrTiO₄ ternary heterojunction photocatalyst was designed and prepared via a facile one-step calcining strategy. Phase interface and band structure of the materials were controlled and optimized by regulating R–TiO₂/A–TiO₂ mass ratio in the TiO₂ (A, R)/ZrTiO₄ structures using boron doping. The highest photocatalytic performance and excellent catalytic stability of Rhodamine B removal was observed from the heterojunction with a low R–TiO₂/A–TiO₂ mass ratio of 0.066, even after five testing cycles, accompanying with low photoluminescence intensity and electrochemical impedance, high photocurrent and charge carrier density (5.12 × 10²² cm⁻³), and a positive shift of valence band position (from +2.06 to + 2.16 eV). The increased photodegradation behaviour was due to the remarkably enhanced separation efficiency and improved redox ability of the photo-induced charge carriers as a result of the high content of oxygen vacancies and the formed anatase TiO₂/rutile TiO₂ Z-scheme heterojunction.     

Effect of Mg-and Fe-ions on formation mechanism of aluminium titanate

The presence of Mg²⁺- and Fe³⁺-ions has an effect on the formation of Al₂TiO₅. Crystalline phases produced under the influence of the heat treatment have been identified in a heated X-ray diffraction chamber in the temperature range of 20–1500 °C. In the presence of Mg²⁺- and Fe³⁺-ions transitional phases are formed in the temperature range of 1000–1350 °C during Al₂TiO₅ formation. The XRD technique was used to identify the crystalline phases formed. On addition of MgO, chemical composition of the transitional phase formed is Mg_0.3Al_1.4Ti_1.3O₅, whereas on addition of Fe₂O₃ we have calculated a Powder Diffraction File card data for the transitional phase. Determination of the lattice parameters of the Al₂TiO₅ ceramics produced enabled verification of incorporation of Mg²⁺- or Fe³⁺-ions into the crystal lattice of Al₂TiO₅, i.e. the formation of Mg²⁺- and Fe³⁺-containing solid solutions.     

Fabrication of Al2O3/AlN composite ceramics with enhanced performance via a heterogeneous precipitation coating process

To improve the thermal and mechanical properties of Al₂O₃/AlN composite ceramics, a novel heterogeneous precipitation coating (HPC) approach was introduced into the fabrication of Al₂O₃/AlN ceramics. For this approach, Al₂O₃ and AlN powders were coated with a layer of amorphous Y₂O₃, with the coated Al₂O₃ and AlN powders found to favor the formation of an interconnected YAG second phase along the grain boundaries. The interconnected YAG phase was designed to act as a diffusion barrier layer to minimize the detrimental interdiffusion between Al₂O₃ and AlN particles. Compared with samples prepared by a conventional ball-milling method, the HPC Al₂O₃/AlN composites exhibited less AlON formation, a higher relative density, a smaller grain size and a more homogeneous microstructure. The thermal conductivity, bending strength, fracture toughness and Weibull modulus of the HPC Al₂O₃/AlN composite ceramics were found to reach 34.21 ± 0.34 W m⁻¹ K⁻¹, 475.61 ± 21.56 MPa, 5.53 ± 0.29 MPa m^1/2 and 25.61, respectively, which are much higher than those for the Al₂O₃ and Al₂O₃/AlN samples prepared by the conventional ball-milling method. These results suggest that HPC is a more effective technique for preparing Al₂O₃/AlN composites with enhanced thermal and mechanical properties, and is probably applicable to other composite material systems as well.     

Significantly improved photoluminescence of the green-emitting β-sialon:Eu2+ phosphor via surface coating of TiO2

The β-sialon:Eu²⁺ phosphor particles were successfully coated by TiO₂ nanoparticles via the sol-gel method. The TiO₂-coated β-sialon:Eu²⁺ phosphor had a significantly improved photoluminescence (PL) performance under the 365 nm excitation, due to the localized surface plasmon resonance (LSPR) at the interface between the TiO₂ coating layer and phosphor surface. The emission intensity of the TiO₂-coated β-sialon:Eu²⁺ prepared with the titanium (IV) tetrabutoxide (Ti(OC₄H₉)₄, TTBO):H₂O = 1:0.5 volume ratio was dramatically increased by ~24%. When the preparation temperature was 500°C, it was responsible for superior PL intensity by considering the important domination factors of higher anatase content and spherical particle shape of the TiO₂ coating layer to the LSPR effect. The coating around the phosphor surface by the TiO₂ nanoparticles would be an effective technique to improve the PL efficiency of phosphor for the application in the white light-emitting diodes (LEDs), by utilizing the LSPR effect of the semiconductor coating layer, instead of conventional metal plasmonic materials.     

Structural and optical investigations of Eu3+-doped TiO2 nanopowders

This paper presents an evaluation of photoluminescent and structural properties of Eu³⁺-doped TiO₂ materials in powder form prepared by sol–gel process. The increase of heat-treatment temperature of synthesis results in the TiO₂ phase transition, obtaining anatase, rutile and sometimes brookite, examined by Raman and XRD techniques. Crystallite size as well as microstrains in the crystal structure were evaluated as a function of Eu³⁺ and heat-treatment temperature. It was found that when Eu³⁺ is introduced in anatase phase an intense color emission is observed under excitation at 394 and 463 nm. The rutile phase presents inversion center symmetry, and apparently the Eu³⁺ tends to occupy this site, which decreases the intensity of the emission assigned to the ⁵D₀→⁷F₂ transition. The materials obtained at 700 °C showed most intensity of emission in the red region, verified by the ratio between ⁵D₀→⁷F₂/⁵D₀→⁷F₁ transitions of Eu³⁺. The results showed that the materials are interesting absorber in the ultraviolet and red region and can be used for improvement of energy conversion in solar cells devices.