Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Hydrodesulphurization performance of NiW/TiO2-Al2O3 catalyst for ultra clean diesel

TiO₂-Al₂O₃ binary oxide supports were obtained by sol–gel methods from Tetra-n-butyl-titanate and pseudoboehmite/aluminium chloride resources. The typical physico-chemical properties of NiW/TiO₂-Al₂O₃ catalysts with different TiO₂ loadings and their supports were characterized by means of BET, XRD and UV–vis DRS, etc. The BET results indicated that the specific surface areas of NiW/TiO₂-Al₂O₃ catalysts were as higher as that over pure γ-Al₂O₃ support, and the pore diameters were also large. The XRD and UV–vis DRS analyzing results showed that the Ti-containing supported catalysts existed as anatase TiO₂ species and the incorporation of TiO₂ could adjust the interaction between support and active metal, and impelled the higher reducibility of tungsten. The hydrodesulphurization (HDS) performance of the series catalysts were evaluated with diesel feedstock in a micro-reactor unit, and the HDS results showed that NiW/TiO₂-Al₂O₃ catalysts exhibited higher activities of ultra deep hydrodesulphurization of diesel oil than that of NiW/Al₂O₃ catalyst. The optimal TiO₂ content of NiW/TiO₂-Al₂O₃ catalysts was about 15 m%, and the corresponding HDS efficiency could reach to 100%. The sulphur contents of diesel products over NiW/TiO₂-Al₂O₃ (from pseudoboehmite/AlCl₃) catalysts with suitable TiO₂ content could be less than 15 ppmw, which met the sulphur regulation of Euro IV specification of ultra clean diesel fuel.     

Formation of the Ti4O7 phase through interaction between coated carbon and TiO2

Fine particles ofphotocatalytic anatase TiO₂ prepared through hydrolysis of titanium tetraisopropoxide were coated by carbon. A reduced phase, Ti₄O₇, was formed through interaction between TiO₂ and the coating carbon. EXAFS analysis on this Ti₄O₇ phase showed an intermediate Ti-Ti distance between those in anatase and rutile, which agreed with the structure composed of two-dimensional slabs of Ti-O octahedra separated by a shear plane. This carbon-coated Ti₄O₇ was confirmed to have photocatalytic activity, even though a little lower than anatase, examining the decomposition of methylene blue in water under LTV irradiation.     

The preparation of Cu-coated Al2O3 composite powders by electroless plating

Cu-coated Al₂O₃ composite powders were synthesized by using the electroless plating method. The influence of the components proportion and the pH value of the plating solution on the Cu layer were analyzed with XRD and SEM. The results showed that the proportion of the plating solution components plays an important role for synthesizing the Al₂O₃/Cu composite powders. The content of copper in the composite powders could be effectively controlled by adjusting the content of copper sulfate and formaldehyde in the plating solution. Furthermore, the pretreatment of the Al₂O₃ powders is also a key factor to form a uniform Cu layer coating Al₂O₃ particles. The optimum technical parameters for producing Al₂O₃/Cu composite powders with uniform Cu coat were obtained.     

Al2O3-coated SnO2/TiO2 composite electrode for the dye-sensitized solar cell

A novel Al₂O₃-coated SnO₂/TiO₂ composite electrode has been applied to the dye-sensitized solar cell. In such an electrode, two kinds of energy barriers (SnO₂/TiO₂ and TiO₂/Al₂O₃) were designed to suppress the recombination processes of the photo-generated electrons and holes. After the SnO₂ was modified by colloid TiO₂, the photoelectric conversion efficiency of the SnO₂/TiO₂ composite cell increased to 2.08% by a factor of 2.8 comparing with that of the SnO₂ cell. The Al₂O₃ layer on the SnO₂/TiO₂ composite electrode further suppressed the generation of the dark current, resulting in 37% improvement in device performance comparing with the SnO₂/TiO₂ cell.     

The reduction behavior of Mo/TiO2-Al2O3 catalyst

The effect of TiO₂ modified Al₂O₃ surface on the reducibility of MoO₃ has been studied by TPR and XPS. The results show that Mo⁶⁺ in Mo/TiO₂-Al₂O₃ can be reduced to much lower valency, especially at low Mo loading. The influence of the calcination temperature on the reduction of Mo⁶⁺ on Al₂O₃ and TiO₂-Al₂O₃ carriers is different. The data reveals that the reducibility of Mo⁶⁺ on Al₂O₃ slightly decreased, while that on TiO₂-Al₂O₃ increased when the calcination temperature was raised. It is suggested that the stronger tetrahedral site of the Al₂O₃ surface was first occupied by TiO₂ and main octahedral Mo⁶⁺ in polymeric species-; and a small crystalline MoO₃ formed on TiO₂-Al₂O₃, whereas the formation of tetrahedral Mo⁶⁺ species and Al₂(MoO₄)₃ phase was inhibited.     

Size effects on χ- to α-Al2O3 phase transformation

Nano-scaled χ-Al₂O₃ powders with d₅₀ mean particle sizes from 17 to 314 nm were prepared to investigate the size effect on their phase transformation. Structural properties and crystallization behavior as a function of thermal treatments of various-sized χ-Al₂O₃ particles were examined by DTA, XRD and TEM characterizations. It was confirmed that the decrease of particle size allows for stable α-Al₂O₃ formation at relatively low temperature. Furthermore, the phase transformation route of χ-Al₂O₃ to α-Al₂O₃ was also modified due to the decrease of particle size. A critical size of χ-Al₂O₃ that determines the phase transformation behavior was found to be around 40 nm. For particles larger than 40 nm, a transition phase of κ-Al₂O₃ is formed before obtaining final α-Al₂O₃ phase. Nevertheless, for those smaller than the critical size, starting χ-Al₂O₃ particles have to grow to 40 nm and then directly transform to α-Al₂O₃ bypassing κ-Al₂O₃ at a temperature as low as 1050 °C.     

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.     

Agglomeration of α-Al2O3 powders prepared by gel combustion

Ultrafine α-Al₂O₃ powders were prepared by a gel combustion method and the agglomeration characteristic of the resultant powders was studied. A variety of fine crystallite α-Al₂O₃ powders with different agglomeration structures could be obtained by altering the citrate-to-nitrate ratio γ and calcining the precursors at 1050 °C for 2 h. All the powders were of nearly equivalent crystallite size (60–80 nm) except for the P1 powder (113 nm) from the gel with γ = 0.033. The primary crystallites of the obtained α-Al₂O₃ powders were formed into large secondary particles with different degree of agglomeration. Except for the powder P1, the mean particle sizes from specific surface area and particle size distribution measurement increase with increasing citrate-to-nitrate ratio in the fuel-lean condition and decrease in the fuel-rich condition. Densities of alumina ceramics from powders P4 and P5 sintered at different temperatures were relatively low due to the wide particle size distribution.     

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.     

Preparation of ceramic nanospheres by CO2 laser vaporization (LAVA)

In order to prepare ceramic nanoparticles by CO₂ laser vaporization (LAVA) coarse ceramic powders (e.g. ZrO₂, Al₂O₃, and TiO₂) were used as raw materials. The raw powder was vaporized into a flowing process gas under normal pressure. Expanding into the gas, the vapor instantly cools down. Gas-phase condensation leads to the formation of nanoscale particles with a composition that corresponds to that of the raw powder. LAVA nanoparticles are chemically pure, spherical, crystalline, exhibit a narrow size distribution, and form merely soft agglomerates. The co-laser vaporization of mixtures of ceramic raw powders allows for the preparation of nanoparticles of multi-phase (e.g. Fe₂O₃-SiO₂) or single-phase (e.g. CaTiO₃) mixed-oxides and dispersion ceramics (e.g. ZrO₂-Al₂O₃). In order to modify the surface of the nanoparticles they can be coated in-process with an organic additive. Thus, the LAVA method allows for the targeted development of a wide range of ceramic nanopowders with tailored properties.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Detonation spraying of TiO2-2.5 vol.% Ag powders in a reducing atmosphere

In the present work, rutile powders containing additions of metallic silver (2.5 vol.%) were detonation sprayed in a reducing atmosphere formed by gaseous detonation products of the C₂H₂ + 1.05O₂ mixture. The initial volume of the C₂H₂ + 1.05O₂ mixture - explosive charge - used for a detonation pulse was computer-controlled as the fraction of the barrel volume filled with the mixture. Using a previously developed model of the detonation process, the particle temperatures and velocities were calculated to explain the observed phase and microstructure development in the coatings. With increasing explosive charge, the temperature of the sprayed particles increased and rutile was partially reduced to oxygen-deficient TiO_2−x and then to Ti₃O₅. When the melting temperature of rutile was not reached, the coatings were porous; semi-molten particles formed denser coatings obtained with higher spraying efficiency. Silver inclusions in the titanium oxide matrix experienced melting and substantial overheating, but remained well preserved in the coatings.     

Effects of the starting materials and mechanochemical activation on the properties of solid-state reacted Li4Ti5O12 for lithium ion batteries

Li₄Ti₅O₁₂ was synthesized by a solid-state reaction between Li₂CO₃ and TiO₂ for applications in lithium ion batteries. The effects of the TiO₂ phase and mechanochemical activation on the Li₄Ti₅O₁₂ particles as well as the corresponding electrochemical properties were investigated. Rutile TiO₂ was more desirable in acquiring high purity Li₄Ti₅O₁₂ than anatase due to the anatase to rutile phase transformation, which was found to be more rigid in the solid-state reaction than the intact rutile phase. Mechanochemical activation of the starting materials was effective in decreasing the reaction temperature and particle size as well as increasing the Li₄Ti₅O₁₂ content. The specific capacity depended significantly on the Li₄Ti₅O₁₂ content, whereas the rate capability improved with decreasing particle size due to the enhanced contact area and reduced diffusion path. Overall, a 200 nm-sized Li₄Ti₅O₁₂ powder with a specific capacity of 165 mAh/g could be synthesized by optimizing the milling method and starting materials.     

Investigation of surface modification of rutile TiO2 nanoparticles with SiO2/Al2O3 on the properties of polyacrylic composite coating

Surface modification and characterization of TiO₂ nanoparticles as an additive in a polyacrylic clear coating were investigated. For the improvement of nanoparticles dispersion and the decreasing of photocatalytic activity, the surface of nanoparticles was modified with binary SiO₂/Al₂O₃. The surface treatment of TiO₂ nanoparticles was characterized with FTIR. Microstructural analysis was done by AFM. The size, particle size distribution and zeta potential of TiO₂ nanoparticles in water dispersion was measured by DLS method. For the evaluation of particle size and the stability of nanoparticles in water dispersions with higher solid content the electroacoustic spectroscopy was made. To determine the applicability and evaluate the transmittance of the nano-TiO₂ composite coatings UV–VIS spectroscopy in the wavelength range of 200–800 nm was employed. The results showed that surface treatment of TiO₂ nanoparticles with SiO₂/Al₂O₃ improves nanoparticles dispersion and UV protection of the clear polyacrylic composite coating.     

In situ synthesis,mechanical and cyclic oxidation properties of Ti3AlC2/Al2O3 composites

ABSTRACT

Ti₃AlC₂/Al₂O₃ composite materials were successfully fabricated from TiO₂/TiC/Ti/Al powders by the in situ reactive hot pressed technique. The microstructure, mechanical and oxidation properties of the composites were investigated in the paper. Vickers hardness increased with the Al₂O₃ content. The relative density of Ti₃AlC₂/Al₂O₃ composites exhibits a declining tendency with Al₂O₃ content especially exceeds 10 vol.?%. The Ti₃AlC₂/Al₂O₃ composites show excellent electrical conductivity. The flexural strength and fracture toughness of Ti₃AlC₂/10 vol. % Al₂O₃ are 461 ± 20?MPa and 6.2?±?0.2?MPa m^1/2, respectively. The cyclic oxidation behaviour of resistance of Ti₃AlC₂/10 vol. % Al₂O₃ composites at 800–1000°C generally obeys a parabolic law. The oxide scale of sample consists of a mass of α-Al₂O₃ and TiO₂, forming a dense and adhesive protect layer. The result indicates that the Al₂O₃ can greatly improve the oxidation resistance of Ti₃AlC₂.     

Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO₂, CeO₂ and CeO₂-doped TiO₂ have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N₂-physisorption, Transmission Electron Microscopy (TEM) and UV-Vis spectrophotometry. The TiO₂ and CeO₂-doped TiO₂ nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10-13 nm for Ce doping concentrations of 5-50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO₂. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce^3+/4+ in the TiO₂ matrix, which generated an n-type impurity band.     

In situ growth of TiC whiskers in Al2O3 matrix for ceramic machine tools

TiC whiskers have been synthesized via a carbothermal reduction technique in an α-Al₂O₃ matrix within a temperature range of 1380–1580 °C in an argon atmosphere. The raw materials consist of TiO₂, carbon, nickel and NaCl. Various mixing procedures and reaction temperatures were used. The yield of the whiskers is mainly dependent on the mixing procedures and the morphology of the synthesized composite powders is mainly dependent on the synthesis temperatures. The majority of the synthesized whiskers display an ideal aspect ratio of 10–30 with a diameter of 1–3 μm. No significant influence on the growth of the TiC whiskers by the present of the Al₂O₃ matrix powder can be noted.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.