Ultrasonic Dispersion of TiO2 Nanoparticles in Aqueous Suspension

Aggregation and dispersion behavior of nanometer and submicrometer scale TiO₂ particles in aqueous suspension were investigated using three kinds of mechanical dispersion methods: ultrasonic irradiation, milling with 5-mm-diameter balls, and milling with 50 μm beads. Polyacrylic acids with molecular weights ranging from 1200 to 30 000 g/mol were used as a dispersant, and the molecular weight for each dispersion condition was optimized. Viscosities and aggregate sizes of the submicrometer powder suspensions were not appreciably changed in the ultrasonic irradiation and 5-mm-ball milling trials. In contrast, in the trials in which nanoparticle suspension was used, ultrasonic irradiation produced better results than 5-mm-ball milling. Use of ultrasonication enabled dispersion of aggregates to primary particle sizes, which was determined based on the specific surface area of the starting TiO₂ powders, even for relatively high solid content suspensions of up to 15 vol%. Fifty-micrometer-bead milling was also able to disperse aggregates to the same sizes as the ultrasonic irradiation method, but 50-μm-bead milling can be used only in relatively low solid content suspensions. It was concluded that the ultrasonic dispersion method was a useful way to prepare concentrated and highly dispersed nanoparticle suspensions.     

Dispersion of SiC in Aqueous Media with Al2O3 and Y2O3 as Sintering Additives

It has been well accepted that polyethylene imine (PEI) is an effective dispersant for silicon carbide (SiC) in aqueous media. However, after the addition of sintering additives (Al₂O₃ and Y₂O₃), this dispersing effect is reduced significantly. In this work, a second dispersant, citric acid, was used to resolve this problem. It was found that citric acid could decrease the slurry viscosity (without sintering additives) and enhance the PEI adsorption on SiC particle surface. The optimal amount of citric acid required to achieve a minimum viscosity for 55 vol% SiC suspensions was equal to ∼0.87 wt% (at pH ∼6.8). With the aid of citric acid, well-stabilized SiC suspensions (containing sintering additives) were realized, which exhibited slight shear thinning rheologies. After tape casting and SPS sintering, dense SiC samples were obtained with a homogeneous fine-crystalline microstructure. Results showed that citric acid was an effective dispersant for improving the dispersion of SiC particles containing sintering additives.     

Colloidal Consolidation of Ceramic Nanoparticles by Pressure Filtration

The applied pressure and suspension height during consolidation of an aqueous suspension of nanometer-sized particles (24 nm hydroxyapatite, 30 nm SiC, 68 nm 8 mol% yttria-stabilized zirconia, 150 nm Al₂O₃, and 800 nm SiC) were continuously recorded using a pressure filtration apparatus. The packing density decreased when the particle size was less than 70 nm. The final packing density of 150−800 nm particles at 19 MPa was strongly influenced by the surface charge. However, surface charge does not affect the packing density of particles less than 70 nm. The ratio of the energy applied to two particles during consolidation to the interaction energy between two particles in a suspension was correlated to the packing density. The low packing density of 20−30 nm particles was improved by steric stabilization. The estimated thickness of the dispersant layer adsorbed on the particle surfaces was less than 1 nm and nearly independent of the molecular weight of the dispersants. When the applied pressure was released, the height of the consolidated cake increased because of the release of the elastic strain stored in the dispersant layer.     

Low-Temperature Atomic Layer-Deposited TiO2 Films with Low Photoactivity

Atomic layer deposition (ALD) has been successfully utilized for the conformal and uniform deposition of ultrathin titanium dioxide (TiO₂) films on high-density polyethylene (HDPE) particles. The deposition was carried out by alternating reactions of titanium tetraisopropoxide and H₂O₂ (50 wt% in H₂O) at 77°C in a fluidized bed reactor. X-ray photoelectron spectroscopy confirmed the deposition of TiO₂ and scanning transmission electron microscopy showed the conformal TiO₂ films deposited on polymer particle surfaces. The TiO₂ ALD process yielded a growth rate of 0.15 nm/cycle at 77°C. The results of inductively coupled plasma atomic emission spectroscopy suggested that there was a nucleation period, which showed the reaction mechanism of TiO₂ ALD on HDPE particles without chemical functional groups. TiO₂ ALD films deposited at such a low temperature had an amorphous structure and showed a much weaker photoactivity intensity than common pigment-grade anatase TiO₂ particles.     

Photocatalytic Activity of Monodispersed Spherical TiO2 Particles with Different Crystallization Routes

Monodispersed spherical TiO₂ particles were prepared by hydrothermal crystallization and/or calcination of spherical amorphous particles, synthesized by thermal hydrolysis of TiCl₄. The crystallized spherical particles were secondary agglomerates of primary nanocrystallites. Different crystallization routes and conditions provided the spherical TiO₂ particles with wide particle characteristics, such as the fraction of crystallization, the size and shape of the primary nanocrystallites, and the specific surface area. The photocatalytic activity showed complex dependence on the crystallization routes and conditions. The complex dependence behavior could be explained by combining the effects of the fraction of crystallization, the specific surface area, and the adsorption ability for hydroxyl ions. Especially, in the present study, the hydrothermally crystallized TiO₂ particles with large primary nanocrystallites showed the highest photocatalytic activity. The high photocatalytic activity mainly resulted from the high surface adsorption ability for hydroxyl ions, which was closely related to the well-developed (flat and faceted) morphology of primary nanocrystallite.     

Effect of TiO2 on Initial Sintering of Al2O3

Alpha alumina with additions of TiO₂ sintered more rapidly than "pure" alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO₂ in Al₂O₃. For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The "solubility limit" increased with decreasing particle size, indicating an excess surface concentration of TiO₂. The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO₂ concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient.     

Polar-Oriented Crystallization of Fresnoite (Ba2TiSi2O8) on Glass Surface Due to Ultrasonic Treatment with Suspensions

Improved polar ( c -axis) oriented growth of fresnoite (Ba₂TiSi₂O₈) was observed for surface crystallization of a glass 33.3BaO· 16.7TiO₂· 50SiO₂ (in mol%) due to ultrasonic surface treatment with an aqueous suspension of Ba₂TiSi₂O₈, and an oriented film more than 50 μm in thickness was prepared. The effect of ultrasonic treatment on the polar orientation depends on the suspending particles; of these, foreign particles of a phase other than the crystallizing Ba₂TiSi₂O₈ give poorer efficiency than the Ba₂TiSi₂O₈ particles. It is assumed that fresnoite has a tendency toward preferred polar growth on the glass, and seeding of a large number of fine particles through ultrasonic bombardment realizes the tendency. The polar-oriented growth kinetics of Ba₂TiSi₂O₈ were also discussed.     

Surface Chemistry, Dispersion Behavior, and Slip Casting of Ti3AlC2 Suspensions

The surface chemistry and dispersion properties of aqueous Ti₃AlC₂ suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti₃AlC₂ particle had complex surface hydroxyl groups, such as ≡Ti–OH,=Al–OH, and −OTi–(OH)₂, etc. The surface charging of the Ti₃AlC₂ particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti₃AlC₂ suspensions. PAA dispersant was added into the Ti₃AlC₂ suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti₃AlC₂ suspensions with 2.0 dwb% PAA had an excellent stability at pH=∼5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti₃AlC₂ materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.     

Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions

It was observed that slurries of oxide powders in oxidized polybutene fluids can be caused to change reversibly between fluid, nearly Newtonian behavior and plastic behavior by modest changes in temperature. This phenomenon was believed to result from changes in the dispersion vs association among the particles. The rheological effects of temperature, polymer oxidation, and particle size were observed for 30 vol% slurries of TiO₂, Al₂O₃, and ZrO₂ powders in polybutene fluids. Elasticity (in oscillation) and low-shear-rate viscosity (in steady shear) were observed to increase with increasing temperature for TiO₂ and Al₂O₃ particles in oxidized polybutene fluids. This behavior was attributed to the creation of interparticle structures. The attainment of this structure on heating was observed to be inhibited by increased oxidation of the polymer and increased particle size. It was concluded that the adsorption of oxidized molecules from the polymer liquid, along with the high viscosity of the bulk polymer, resulted in suspensions that were metastable against coagulation. Increased temperature resulted in lower viscosities of the liquid, allowing coagulation on a short time scale. The presence of the adsorbed polymer, however, prevented intimate contact among the particles so that the coagulated structure was easily destroyed upon subsequent cooling and shearing.     

Synthesis of Nanocrystalline TiO2 Particles by Hydrolysis of Titanyl Organic Compounds at Low Temperature

Fourier transform infrared analysis, nuclear magnetic resonance, and thermogravimetric analysis show that most of the solid product prepared from the reaction of Ti(OC₄H₉)₄ and excess (CH₃CO)₂O is a mixture of titanyl organic compounds. Nanocrystalline TiO₂ particles, which include anatase TiO₂, rutile TiO₂, and a mixture of anatase and rutile, can be obtained from hydrolysis of the titanyl organic compounds under normal pressure at 60°C. The particle size, shape, and formation process of the crystals have been studied using X-ray diffraction and transmission electron microscopy. The specific-surface-area data for a rutile TiO₂ sample and the powders obtained after calcination at different temperatures have been measured by the Brunauer–Emmett–Teller method.     

Scandium-Doped Anatase (TiO2) Nanoparticles Directly Formed by Hydrothermal Crystallization

Anatase-type TiO₂ solid solutions doped with 0–10 mol% scandium were formed by hydrothermal crystallization under weak basic conditions above 180°C for 5 h from amorphous co-precipitates that were obtained from the aqueous precursor solutions of TiOSO₄ and Sc(NO₃)₃ using aqueous ammonia. The anatase particles were spindle-like and consisted of nanosized-crystallites (23–25 nm). The lattice parameter c ₀ of anatase and the length and width of the spindle-like anatase gradually increased when the scandium content was increased. The diffuse reflectance spectra of the as-prepared TiO₂ doped with scandium showed that the onset of absorption slightly shifted to longer wavelengths with increasing scandium content. The band gap of anatase was slightly increased by making solid solutions with scandium oxide.     

Nanostructure of TiO2 Nano-Coated SiO2 Particles

We characterized SiO₂–TiO₂ nano-hybrid particles, prepared using the sol–gel method, using high-resolution transmission microscopy. A few nanometer-ordered TiO₂ anatase crystallites could be observed on the monodispersed SiO₂ nanoparticle surface. The quantum size effect of the TiO₂ anatase crystallites is attributed to the blue shift of the absorption band. The rough surface of the SiO₂–TiO₂ nano-hybrid particles was derived from the developed growth planes of the TiO₂ anatase crystallites, grown from fully hydrolyzed Ti alkoxide that did not react with acetic acid during the crystallization process at 600°C thermal annealing.     

Preparation of TiO2-Coated Al2O3 Particles by Chemical Vapor Deposition in a Rotary Reactor

A process of coating Al₂O₃ particles with TiO₂ by hydrolysis of Ti(OC₄H₉)₄ using chemical vapor deposition in a rotary reactor has been developed. The process resulted in (1) a coating film of TiO₂ which was compact and uniform with the fraction of TiO₂ being 0.1%–10.0% and (2) an amorphous TiO₂ coating at a low reaction temperature converted to anatase at a reaction temperature higher than 673 K.     

Environmentally friendly coloured materials: cellulose/titanium dioxide/inorganic pigment composite spherical microbeads prepared by viscose phase-separation method

In order to develop environmentally friendly coloured materials, cellulose composite spherical microbeads hybridised with titanium dioxide (TiO₂) particles and inorganic pigment were prepared by a phase-separation method using viscose and an aqueous solution containing sodium polyacrylate. Findings regarding the relationships between cellulose xanthate and the electronic characteristics of TiO₂ particles used in the cellulose/inorganic material composite sphering process are also reported. These findings suggest that the location of TiO₂ particles in cellulose microbeads is related to electrical repulsion between the xanthate (CSS⁻) group and TiO₂. The use of TiO₂ powder as colour pigment is limited, as its colour is white. The cellulose composite spherical microbeads covered with TiO₂ and Fe₂O₃ particles were developed by addition of iron oxide (Fe₂O₃). Their surfaces were viewed by laser microscope and using SEM images. These composite microbeads retained the photocatalytic property of TiO₂. Cellulose/TiO₂/Fe₂O₃ composite spherical microbeads with both colour function and photocatalytic properties were successfully prepared.     

Mullite Transformation Kinetics in P2O5-, TiO2-, and B2O3-Doped Aluminosilicate Gels

Mullite transformation kinetics of sol-gel-derived diphasic mullite gels doped with P₂O₅, TiO₂, and B₂O₃ were studied using quantitative X-ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P₂O₅ doping because of phase separation and formation of α-alumina and cristobalite. In TiO₂-doped samples, the mullite transformation temperature decreased with TiO₂ doping, and the transformation rate increased with decreasing TiO₂ particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid-state mass transport through lattice defects. B₂O₃ doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Photoactive and Adsorptive Niobium-Doped Anatase (TiO2) Nanoparticles: Influence of Hydrothermal Conditions on their Morphology, Structure, and Properties

Anatase-type TiO₂ nanoparticles with adsorptivity and improved photocatalytic activity for the decomposition of methylene blue (MB) in its aqueous solution, which contained up to 10 mol% niobium by forming solid solutions with niobium oxide, were directly synthesized from precursor solutions of TiOSO₄ and NbCl₅ under three hydrothermal conditions in the absence and presence of urea and aqueous ammonia at 180°C for 5 h. The influence of the hydrothermal conditions on the crystallite growth, morphology, specific surface area, adsorptivity, and photocatalytic activity of niobium-doped TiO₂ was investigated. The crystallite growth of anatase was enhanced by the presence of the niobium component. The 10 mol% niobium-doped TiO₂ that was prepared under the hydrothermal condition in the presence of urea had fine crystallites (11 nm) and high specific surface areas (135 m²/g), which showed the most enhanced photocatalytic activity and the highest adsorptivity. The hydrothermal treatment under weak basic conditions and formation of solid solutions with niobium oxide brought about a considerable increase in the adsorption of MB for the anatase-type TiO₂.     

Temperature-Induced Gelation of Concentrated Sialon Suspensions

A temperature-induced gelation method has been developed for the preparation of sialon green bodies composed of Si₃N₄, AlN, Y₂O₃, and Al₂O₃. Using a polyester/polyamine condensation polymer as dispersant, stable suspension containing sialon, precursor powders could be prepared in methyl ethyl keton (MEK)/ethanol (E) solvent mixture with solids loading as high as 60 vol%. The solvency of the dispersant in MEK/E decreased dramatically on cooling. The sedimentation tests showed that the settling behavior changed from slow accumulation at ∼20°C to fast deposition of clusters at −15°C, because of the collapse of dispersant chains adsorbed at the surface of the particles. Steady shear viscosity and oscillatory measurements performed for 60 vol% suspensions as functions of temperature and added amounts of dispersant proved that suspensions were of low viscosity and high stability at room temperature, becoming predominantly very highly viscous and elastic as temperature decreased. The gelation mechanism was mainly based on the coiling up of dissolved dispersant molecules, inducing in situ gelation and the formation of a rigid network bridging the suspended particles. The green bodies showed negligible shrinkage during gelation, a small shrinkage during drying, homogeneous microstructures, narrow pore size distributions, and high relative density. It was possible to achieve dense α-sialon ceramics through pressureless sintering at 1750°C for 2 h.     

Preparation of TiO2 Nanoparticles on Different SiO2 Supports by the Adsorption Phase Technique

The influence of supports on the preparation of TiO₂ nanoparticles by the adsorption phase technique is studied in detailed. Series temperature experiments of two types of supports (named as SiO₂ A and B) were used. Energy-dispersive analysis by X-ray indicates that the concentration of TiO₂ on both supports decreases with temperature increasing. TiO₂ quantity on SiO₂ A decreases sharply between 40° and 60°C, whereas the temperature range for SiO₂ B is between 30° and 50°C. X-ray diffraction (XRD) shows that grain size of TiO₂ particles on two SiO₂ surfaces is all below 7 nm. It is also shown by XRD that particles on SiO₂ A decrease sharply as in the quantity curve of TiO₂, but particles on SiO₂ B all change gradually and TiO₂ particles on SiO₂ B are more uniform in transmission electron spectroscopy. The similarly of both supports is considered to be the reason for the similar changes in Ti concentration, and the different characteristics of the internal/external surface lead to variant quantity and grain size, as well as characteristics of TiO₂.     

Dopants in Flame Synthesis of Titania

The effect of dopants on the characteristics of titania particles made by oxidation of TiCl₄ in a laminar diffusion flame reactor is presented. Introduction of dopant SiCl₄ inhibits the transformation of anatase to rutile, due to the formation of interstitian solid solution of SiO₂ and TiO₂. Silica decreases the sintering rate of titania and decreases the primary particle size, and, as a result, the specific surface area increases. Intruduction of SnCl₄ enhances the transformation of anatase to rutile, due to the similar crystalline structure of SnO₂ and rutile titania. However, the presence of SnO₂ and rutile titania. However, the presence of SnO₂ does not affect the primary particle size or the specific surface area of titania particles. Introduction of AICI₃ enhances the transformation of anatase to rutile, due to the formation of excess oxygen vacancies as Al₂O₃ and TiO₂ form a substitutional solid solution.