Effect of pre-nitridation treatment on the formation of anatase TiO2 films by anodization

High performance-anatase TiO₂ films were successfully formed on metallic titanium by anodization in an acidic electrolyte composed of H₂SO₄, H₃PO₄ and H₂O₂ subsequent to pre-nitridation treatment. The pre-nitridation treatment was carried out by pre-annealing metallic titanium under a nitrogen atmosphere of 0.1 MPa. The anodized films showed photocatalytic activity in photooxidization of the iodide anion into the tri-iodide anion. The nitridation treatment had a significant effect not only on the formation of anatase TiO₂ films but also on the photocatalytic activity of the anodized films.     

Preparation of photocatalytic TiO2 coatings by gel-dipping with polysaccharides

Commercial tiles are being produced in vast quantities. The main properties of tiles are well established but there is an increasing interest in producing ceramics with tailored-properties and advanced functionalities. One way of adding value to commercial tiles is to deposit a photocatalytic coating to obtain ‘smart’ tiles for environmental reasons, e.g. for the (photo) degradation of organic pollutants in air or in a liquid. Here, we show the manufacture of ‘smart’ tiles by formation of TiO₂ coatings onto commercial tiles by a colloidal processing route based on the immersion of the substrate into a homogeneous aqueous ceramic suspension and its consolidation by agar thermogelation. The effect of the processing parameters (withdrawal rate, solid loading and gelling agent content) and the grain size on the photocatalytic activity of the final coated tiles is reported and discussed. Final coatings properties depend on the viscosity of the suspension, particle size, withdrawal rate, solid loading and gelling agent content, and hence, this dependence affects the photocatalytic activity of the coatings.     

Adjustable dielectric properties of Li2CuxZn1−xTi3O8 (x=0 to 1) ceramics with low sintering temperature

Single-phase dielectric ceramics Li₂Cu_xZn_1−xTi₃O₈ (x=0–1) were synthesized by the conventional solid-state ceramic route. All the solid solutions adopted Li₂MTi₃O₈ cubic spinel structure in which Li/M and Ti show 1:3 order in octahedral sites whereas Li and M are distributed randomly in tetrahedral sites with the degree of Li/M cation mixing varying from 0.5 to 0.3. The substitution of Cu for Zn effectively lowered the sintering temperatures of the ceramics from 1050 to 850 °C and significantly affected the dielectric properties. As x increased from 0 to 0.5, τ_f gradually increased while the dielectric constant (ε_r) and quality factor value (Q×f) gradually decreased, and a near-zero τ_f of 1.6 ppm/°C with ε_r of 25.2, Q×f of 32,100 GHz could be achieved for Li₂Cu_0.1Zn_0.9Ti₃O₈ ceramic sintered at 950 °C, which make it become an attractive promising candidate for LTCC application. As x increases from 0.5 to 1, the dielectric loss significantly increases with AC conductivity increasing up to 2.3×10⁻⁴ S/cm (at 1 MHz).     

Mechanochemical synthesis of Ti1−xZrxB2 and Ti1−xHfxB2 solid solutions

Solid solutions of TiB₂-ZrB₂ and TiB₂-HfB₂ were obtained under an inert atmosphere by high-energy ball-milling mixtures of Ti/Zr/B and Ti/Hf/B, respectively. Milling promoted mechanically induced self-sustaining reactions (MSR), and the ignition time was dependent on the initial composition of the mixture. The stoichiometry of Ti_1−xZr_xB₂ and Ti_1−xHf_xB₂ solid solutions was controlled by adjusting the atomic ratio of the reactants. The solid solutions were characterised by X-ray diffraction, transmission electron microscopy, electron diffraction, and energy dispersive X-ray spectroscopy. The results revealed that TiB₂-ZrB₂ possessed a nanometric microstructure and good chemical homogeneity. However, in the TiB₂-HfB₂ system, an inhomogeneous solid solution was obtained when a Ti-rich mixture was employed. The solid solutions showed good thermal stability; thus, can be used as raw materials for the development of technological materials for structural applications.     

Effect of Pt addition on the isomerization properties of MoO2 − x(OH)y deposited on TiO2

Isomerization of n-hexane and n-pentane were studied using equivalent 5 monolayers of MoO₃ deposited on TiO₂. Addition of 2.5% Pt by weight of MoO₃ on the Mo catalyst resulted in an increase in the catalytic activity of the system in favor of hydrocracking products. Surface characterization by XPS-UPS and ISS reveal that the sample surface contains Oxygen, Molybdenum, Platinum and Titanium. Apparently, the metallic properties of the deposited Pt favors the hydrocracking reactions and becomes dominant at reaction temperatures higher than 623 K. Balanced metal-acid functions in MoO_2 − x(OH)_y phase seems to be in optimized condition toward the hydroisomerization process. The contribution of Platinum addition to this catalytic reaction is not obvious. Combination of surface XPS-UPS, ISS and catalytic reactions carried out at similar experimental conditions enabled us to have better insight concerning the catalytic activities of the different chemical species present on the sample surface.     

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.     

TiO2 as a sintering additive for KNbO3 ceramics

Improved densification during the conventional sintering of KNbO₃ ceramics was achieved by using small additions of TiO₂. This improved densification can be explained on the basis of high-temperature chemical reactions in the system. X-ray diffractometry and electron microscopy were used in combination with diffusion-couple experiments in order to elucidate the chemical reactions between KNbO₃ and TiO₂. TiO₂ reacts with KNbO₃ forming KNbTiO₅, and a low concentration of Ti incorporates in the KNbO₃ structure resulting in the formation of oxygen vacancies and, consequently, in an improvement in the densification. At ∼1037 °C eutectic melting between the KNbO₃ and the KNbTiO₅ further improves the densification of the KNbO₃ ceramics.     

Effect of Ag addition on the properties of Pd–Ag/TiO2 catalysts containing different TiO2 crystalline phases

Anatase and rutile TiO₂ were used for preparation of the TiO₂ supported Pd and Pd–Ag catalysts for selective hydrogenation of acetylene. It was found that Pd/TiO₂-anatase exhibited higher acetylene conversion and ethylene selectivity than rutile TiO₂ supported ones. However, addition of Ag to Pd/TiO₂-anatase catalyst resulted in lower ethylene selectivity while that of Pd/TiO₂-rutile increased. It is suggested that Ag addition suppressed the beneficial effect of the Ti³⁺ sites presented on the anatase TiO₂ during selective acetylene hydrogenation whereas without Ti³⁺, Ag promoted ethylene selectivity by blocking sites for over-hydrogenation of ethylene to ethane.     

Mechanical properties of plasma sprayed nanostructured TiO2 coatings on mild steel

This paper discusses the effects of plasma spray parameters on the mechanical properties of nanostructured TiO₂ coatings deposited on mild steel substrates. The design of experiment method was applied to investigate the significant effects of each property and to optimize the operational spray parameters. Plasma power, powder feed rate, and stand-off distance were selected as independent variables. Agglomerated and sintered nano-TiO₂ powder was deposited on A-36 commercial mild steel. The microstructural and mechanical properties of the coatings such as porosity, microhardness, surface roughness, and wear rate were evaluated. Both plasma power and powder feed rate were found to be the main factors affecting all four responses. It was also noted that the stand-off distance was a significant factor mainly in influencing the surface roughness of the coatings. All in all, the optimized properties can be achieved by applying a plasma power of 30 KW (high level), a powder feed rate of 22 g/min (high level), and a stand-off distance of 80 mm (low level).     

Photocatalytic properties of TiO2 sol-gel modified nanocomposite films

TiO₂ nanocomposite films with different concentrations of TiO₂ MT-150A nanoparticles were immobilized on glass substrates using a dip coating process. The crystalline structure and surface chemical state of nanocomposite film properties were examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The specific surface area and morphology of TiO₂ MT-150A nanoparticles were evaluated by the BET method and Field Emission Scanning Electron Microscopy (FE-SEM). The photocatalytic activities of films were evaluated by the methyl orange decoloring rate. XPS measurements showed that the oxygen amount (%) was related to the film composition. The composite film with 10 g/L MT-150A loading yielded the highest amount of surface oxygen (26.82%) and TiO₂ rutile showed the lowest amount of surface oxygen (13.67%) in the form of surface hydroxyl groups. The remaining oxygen was identified as lattice oxygen. In addition, the nanocomposite film with 10 g/L MT-150A loading yielded the highest photocatalytic activity.     

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.     

Effect of CeO2 addition on densification and microstructure of Al2O3-YSZ composites

Alumina (Al₂O₃) and alumina-yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO₂) were prepared by spark plasma sintering (SPS) at temperatures of 1350-1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al₂O₃ based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl₁₁O₁₈ grains was observed in the ceria containing composites sintered at 1400 °C. Al₂O₃-YSZ composites without CeO₂ had higher hardness than monolithic Al₂O₃ sintered body and the hardness of Al₂O₃-YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO₂ increased from 10 to 30 vol%. The fracture toughness of Al₂O₃ increased from 2.8 MPa m^1/2 to 5.6 MPa m^1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m^1/2, was achieved with the addition of 30 vol% YSZ.     

Preparation and microstructural characterization of (Nd1−xGdx)2(Ce1−xZrx)2O7 solid solutions

(Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ (0 ≤ x ≤ 1.0) powders with an average particle size of 100 nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ powders was studied by pressureless sintering at 1600–1700 °C for 10 h in air. The relative densities of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions agree well with the Vegard's rule.     

Effects of powder size and metallic bonding layer on corrosion behaviour of plasma-sprayed Al2O3-13% TiO2 coated mild steel in fresh tropical seawater

This study focuses on the effects of powder size and Ni–Al bonding layer on the electrochemical behaviour of plasma-sprayed Al₂O₃-13% TiO₂ coating in fresh tropical seawater. The presence of the metallic bonding layer reduces the coating porosity and increases the surface roughness for both microparticle and nanoparticle coatings. The nanoparticle exhibits better corrosion rate of 1.9×10⁻⁶ mmpy compared to the microparticle coating, with a corrosion rate of 3.05×10⁻⁶ mmpy. However, the presence of the metallic bonding layer increases the corrosion rate for both micro and nanoparticle coatings. The corrosion mechanism for the coating with and without the metallic bonding layer is discussed in detail.     

Sintering mechanism and dielectric properties of LiTaO3 matrix composites with the addition of Al2O3 particles

Highly densified Al₂O₃/LiTaO₃ (ALT) ceramic composites were fabricated by hot-pressing in a nitrogen atmosphere. The addition of Al₂O₃ particles could significantly improve the densification of LiTaO₃. Sintering mechanism of the LiTaO₃ ceramic incorporated with Al₂O₃ particles is proposed. Dielectric constant of 5 vol.% Al₂O₃/LiTaO₃ (5ALT) composite ceramic was slightly increased in the range from 30 kHz to 10⁶ Hz, but the dielectric loss was lowered in the whole range from 10³ Hz to 10⁶ Hz. Piezoelectric constant (d₃₃) of the 5ALT ceramic composite is about 50% of that of LiTaO₃ single crystal.     

Photocatalytic activity of TiO2 supported on multi-walled carbon nanotubes under simulated solar irradiation

TiO₂ particles supported on multi-walled carbon nanotubes (MWCNTs) were prepared using a sol–gel method to investigate their photocatalytic activity under simulated solar irradiation for the degradation of methyl orange (MO) in aqueous solution. The prepared composites were analyzed using XRD, SEM, EDS and UV–vis absorption spectroscopy. The results of this study indicated that there was little difference in the shape and structure of MWCNTs/TiO₂ composite and pure TiO₂ particles. The composite exhibited enhanced absorption properties in the visible light range compared to pure TiO₂. The degradation of MO by MWCNTs/TiO₂ composite photocatalysts was investigated under irradiation with simulated solar light. The results of this study indicated that MWCNTs played a significant role in improving photocatalytic performance. Different amounts of MWCNTs had different effects on photodegradation efficiency, and the most efficient MO photodegradation was observed for a 2% MWCNT/TiO₂ mass ratio. Photocatalytic reaction kinetics were described using the Langmuir–Hinshelwood (L–H) model. The photocatalyst was reused for eight cycles, and it retained over 95.2% photocatalytic degradation efficiency. Possible decomposition mechanisms were also discussed. The results of this study indicated that photocatalytic reactions with TiO₂ particles supported on MWCNTs under simulated solar light irradiation are feasible and effective for degrading organic dye pollutants.     

Enhanced thermoelectric properties of carbon fiber reinforced cement composites

Thermoelectric properties of carbon fiber reinforced cement composites (CFRCs) have attracted relevant interest in recent years, due to their fascinating ability for harvesting ambient energy in urban areas and roads, and to the widespread use of cement-based materials in modern society. The enhanced effect of the thin pyrolytic carbon layer (formed at the carbon fiber/cement interface) on transport and thermoelectric properties of CFRCs has been studied. It has been demonstrated that it can enhance the electrical conduction and Seebeck coefficient of CFRCs greatly, resulting in higher power factor 2.08 µW m⁻¹ K⁻² and higher thermoelectric figure of merit 3.11×10⁻³, compared to those reported in the literature and comparable to oxide thermoelectric materials. All CFRCs with pyrolytic carbon layer, exhibit typical semiconductor behavior with activation energy of electrical conduction of 0.228-0.407 eV together with a high Seebeck coefficient. The calculation through Mott’s formula indicates the charge carrier density of CFRCs (10¹⁴–10¹⁶ cm⁻³) to be much smaller than that of typical thermoelectric materials and to increase with the carbon layer thickness. CFRCs thermal conductivity is dominated by phonon thermal conductivity, which is kept at a low level by high density of micro/nano-sized defects in the cement matrix that scatter phonons and shorten their mean free path. The appropriate carrier density and mobility induced by the amorphous structure of pyrolytic carbon is primarily responsible for the high thermoelectric figure of merit.     

Low-temperature firing and microwave dielectric properties of 16CaO–9Li2O–12Sm2O3–63TiO2 ceramics with V2O5 addition

The sintering behaviors and microwave dielectric properties of the 16CaO–9Li₂O–12Sm₂O₃–63TiO₂ (abbreviated CLST) ceramics with different amounts of V₂O₅ addition had been investigated in this paper. The sintering temperature of the CLST ceramic had been efficiently decreased by nearly 100 °C. No secondary phase was observed in the CLST ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLST ceramics with small amounts of V₂O₅ addition could be well sintered at 1200 °C for 3 h without much degradation in the microwave dielectric properties. Especially, the 0.75 wt.% V₂O₅-doped ceramics sintered at 1200 °C for 3 h have optimum microwave dielectric properties of Kr = 100.4, Q × f = 5600 GHz, and TCF = 7 ppm/°C. Obviously, V₂O₅ could be a suitable sintering aid that improves densification and microwave dielectric properties of the CLST ceramics.     

Effect of annealing on the phase transition and morphology of Ag NPs on/in TiO2 rods synthesized by a polyol method

In this study, we report the effect of annealing (250–700 ^oC) on the phase transition and morphology of silver (Ag) nanoparticles (NPs) on/in titanium dioxide (TiO₂) rods prepared using a polyol method. The annealed samples showed not only morphological change, i.e., a solid-to-liquid (melting) transition of Ag NPs due to its partial dissolution into the TiO₂ rods, but also early stage anatase crystallization and anatase–rutile transformation of TiO₂ rods under low annealing temperatures. Such findings, together with XRD and FE-SEM analyses, confirm that, upon higher annealing treatment, diffusion and coalescence leads to changes in the size and shape of the metal particles not only in the outermost regions, but also a random distribution and progressive growth of Ag clusters in the inner interface region. Here, it was shown that annealing can induce changes in morphology, as well as the chemical state and structure of Ag–TiO₂. The present polyol-synthesized Ag–TiO₂ composite also showed improved thermal stability.     

Low temperature and microwave dielectric properties of TiO2/ZBS glass composites

TiO₂ based ceramic/glass composites were prepared by a non-reactive liquid phase sintering (NLPS) using zinc borosilicate (ZBS) glass having the deformation temperature of 588 °C. The compounds of Zn₂SiO₄ and Zn₄B₆O₁₃ were formed after the sintering process, indicating that the ZBS glass was a non-reactive one in this system. For TiO₂/50 vol% ZBS glass composite, the two-stage sintering behavior was conducted as the sintering temperature increased. The former might be correlated to the NLPS process and the latter appeared to be related to the crystallization. The dielectric constant (?_r) was mainly affected by the porosity and obeyed the logarithmic mixing rule. The quality factor (Q × f₀) showed an increase and then a steep decrease after the maximum at 850 °C. TiO₂/50 vol% ZBS glass composite sintered at 900 °C demonstrated 36 in the dielectric constant (?_r) and 7500 GHz in the quality factor (Q × f₀) for an application to LTCC filters.