Cold Spraying of TiO<Subscript>2</Subscript> Photocatalyst Coating With Nitrogen Process Gas

Titanium dioxide (TiO₂) is a promising material for photocatalyst coatings. However, it is difficult to fabricate a TiO₂ coating with anatase phase by conventional thermal spray processes due to a thermal transformation to rutile phase. In this paper, anatase TiO₂ coatings were fabricated by the cold spray process. To understand the influence of process gas conditions on the fabrication of the coatings, the gas nature (helium or nitrogen) and the gas temperature are investigated. It was possible to fabricate TiO₂ coatings with an anatase phase in all spraying conditions. The process gas used is not an important factor to fabricate TiO₂ coatings. The thickness of the coatings increased with the process gas temperature increasing. It indicates that the deposition efficiency of the sprayed particles can be enhanced by controlling the spray conditions. The photocatalytic activity of the coatings is similar or better than the feedstock powder due to the formation of a large reaction area. Concludingly, cold spraying is an ideal process for the fabrication of a TiO₂ photocatalyst coating.     

Dominant microstructural feature over photocatalytic activity of high velocity oxy-fuel sprayed TiO2 coating

TiO₂ photocatalytic coatings were deposited through high velocity oxy-fuel spray using anatase powder and rutile powder as feedstock. The as-sprayed TiO₂ coating was composed of anatase phase and rutile phase. The anatase content in the coating was significantly influenced by fuel gas flow and melting condition of spray powder. A high anatase content of 35% was achieved for the coating deposited using rutile powder. The anatase content in the coating deposited using anatase powder reached 55-65%. The as-sprayed TiO₂ coating was photocatalytically reactive for degradation of acetaldehyde in air. The photocatalytic activity was influenced by spray conditions. The surface morphology and phase structure of coatings deposited at different spray conditions were investigated to clarify the relationship between the coating microstructure and activity. It is found that the photocatalytic activity is significantly influenced by anatase content and surface area.     

Synthesis of B4C-Nano TiB2 Composite Powder by Sol-Gel Method

Production of (B₄C-nano TiB₂) composite powder by chemical method was evaluated in this study. Starting materials were boron carbide, carbon, and titanium (IV) iso propoxide (TTIP). Water was used as a hydrolyser agent. TTIP was hydrolyzed with water and, consequently, amorphous Ti(OH)₄ was formed. Heat-treatment of Ti(OH)₄ at 100 and 850 °C led to the production of TiO₂ and TiB₂ phases, respectively. The effect of heat-treatment time and temperature on the phase transformation and size of the produced nano powder were investigated. The produced nano powder was characterized by XRD, SEM, and DTA. It was found that heat-treatment time and temperature have significant effects on the amount and size of the produced TiB₂ powder. The data also reveal that the minimum temperature for TiB₂ formation is 650 °C.     

Synthesis of TiO<Subscript>2</Subscript> Nanoparticles from Ilmenite Through the Mechanism of Vapor-Phase Reaction Process by Thermal Plasma Technology

Synthesis of nanoparticles of TiO₂ was carried out by non-transferred arc thermal plasma reactor using ilmenite as the precursor material. The powder ilmenite was vaporized at high temperature in plasma flame and converted to a gaseous state of ions in the metastable phase. On cooling, chamber condensation process takes place on recombination of ions for the formation of nanoparticles. The top-to-bottom approach induces the disintegration of complex ilmenite phases into simpler compounds of iron oxide and titanium dioxide phases. The vapor-phase reaction mechanism was carried out in thermal plasma zone for the synthesis of nanoparticles from ilmenite compound in a plasma reactor. The easy separation of iron particles from TiO₂ was taken place in the plasma chamber with deposition of light TiO₂ particles at the top of the cooling chamber and iron particles at the bottom. The dissociation and combination process of mechanism and synthesis are studied briefly in this article. The product TiO₂ nanoparticle shows the purity with a major phase of rutile content. TiO₂ nanoparticles produced in vapor-phase reaction process shows more photo-induced capacity.     

Application of statistical methodology for the evaluation of mechanically activated phase transformation in nanocrystalline TiO2

Response surface methodology was employed to study the effect of three factors, namely as milling time, milling speed and ball to powder weight ratio, on the mechanical activation of polymorphic phase transformation in nanocrystalline TiO₂ powder and identify the probable interactions between these factors. The response was the rutile percentage after annealing the samples. Based on the statistical analysis, ball to powder weight ratio was found as the most effective factor and just one statistically significant interaction was found between milling speed and ball to powder ratio. It was also shown that increasing the milling time has no significant effect on the phase transformation since the required activation energy for the phase transformation is unattainable under these conditions. The rutile percentage was calculated from X-ray diffraction patterns of the samples via Rietveld refinement method. Raman spectroscopy was employed to verify the phase composition analysis based on X-ray diffraction results.     

N-doped TiO2/ZnO composite powder and its photocatalytic performance for degradation of methyl orange

The N-doped TiO₂/ZnO composite powder with a molar ratio of Ti to Zn of 3/1 was prepared via sol–gel process and then ammonia treated with NH₃ mass fractions of 0%, 7%, and 28% for 24 h at room temperature followed by thermal calcinations in air for 2 h at various temperatures of 500, 600, and 700 °C. The as-prepared composite powder was characterized in detail through thermo-gravimetric analyzer, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The results showed that the phase transformation of anatase to rutile has been successfully retarded via the ammonia treating process, leading to the presence of anatase phase in the composite. The particle crystallization of the composite powder was significantly promoted with the increase of the calcining temperature. The photocatalysis evaluation through MO degradation revealed an enhanced photocatalytic activity for the composite powder that might be related to the good crystallization, the presence of anatase phase, and the particle size reduction of the powder.     

Preparation of low-oxygen Ti powder from TiO2 through combining self-propagating high temperature synthesis and electrodeoxidation

An effective method was reported to prepare low-oxygen Ti powder, which included two experimental steps: the fast conversion of TiO₂ to TiO_x<1 powder by self-propagating high-temperature synthesis (SHS) process and the generation of low-oxygen Ti powder by electrodeoxidizing TiO_x<1 powder at the cathode in molten CaCl₂. The key intermediate steps were analyzed by XRD, SEM and electrochemical testing techniques. The results demonstrated that TiO_x<1 powder (TiO_0.325 and TiO_0.97) was generated after acid leaching MgO in SHS products with TiO₂/Mg molar ratio of 1:2, and the TiO_x<1 powder with 16.3 wt.% oxygen could be transformed into pure titanium powder with 0.121 wt.% oxygen by electrodeoxidation at a constant potential of -3.3 V for 10 h. The electrodeoxidation of TiO_x<1 powder in CaCl₂ molten salt follows the step-by-step deoxidation mode, and the lattice of TiO_x<1 powder after electrodeoxidation shrinks.     

Phase formation of nano-TiO<Subscript>2</Subscript> particles during flame spraying with liquid feedstock

The nanostructured TiO₂ photocatalytic coatings were synthesized through flame spraying with liquid feedstock under different conditions. The nanostructured TiO₂ deposit of substantial anatase phase was annealed at different temperatures. X-ray diffraction analysis showed that significant transformation from anatase to rutile occurred at a temperature above 600 °C. However, thermal analysis suggested that the phase transformation from anatase to rutile started at a temperature from 400 to 500°C. It was found that the grain size of rutile phase was larger than that of anatase. The deposits annealed at temperatures lower than 450°C were photocatalytically active. However, the deposit annealed at 500°C, which contained 95% anatase crystalline, became photocatalytically inactive. Based on the experimental findings, a model is proposed to explain the phase transformation of the nano-TiO₂ particles and the phase formation in flame-spraying of nanostructured TiO₂ deposit with liquid feedstock. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Photocatalytic activity and kinetics for acid yellow degradation over surface composites of TiO2-coated activated carbon under different photocatalytic conditions

TiO₂-coated activated carbon surface (TAs) composites were prepared by a sol–gel method with supercritical pretreatment. The photocatalytic degradation of acid yellow (AY) was investigated under UV radiation to estimate activity of catalysts and determine the kinetics. And the effects of parameters including the initial concentration of AY, light intensity and TiO₂ content in catalysts were examined. The results indicate that TAs has a higher efficiency in decomposition of AY than P25, pure TiO₂ particles as well as the mixture of TiO₂ powder and active carbon. The photocatalytic degradation rate is found to follow the pseudo-first order kinetics with respect to the AY concentration. The new kinetic model fairly resembles the classic Langmuir-Hinshelwood equation, and the rate constant is proportional to the square root of the light intensity in a wide range. However, its absorption performance depends on the surface areas of catalysts. The model fits quite well with the experimental data and elucidates phenomena about the effects of the TiO₂ content in TAs on the degradation rate.     

Electromagnetic wave absorbing properties of Fe<Subscript>73</Subscript>Si<Subscript>16</Subscript>B<Subscript>7</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> base P/M sheets mixed with dielectric fine particles

Fe-based nanocrystalline powder sheets with dielectric TiO₂ powder additives were investigated to improve the characteristics of electromagnetic (EM) wave absorption. The amorphous ribbons of Fe₇₃Si₁₆B₇Nb₃Cu₁ (at.%) alloys were prepared by a planar flow casting (PFC) process, and the ribbons were pulverized using an attrition mill. Fe-based flake powder crystallized at 550°C for 1h was mixed with a nano-sized and a micro-sized TiO₂ powder. The powder mixtures were then tape-cast with binders to become EM wave-absorbing sheets. The absorbing properties of the fabricated sheet sample, such as complex permittivity and permeability, were measured by a network analyzer. The properties of EM wave absorption improved with the increase of TiO₂ powder in the mixture. The mixture with micro-sized TiO₂ powder was a little more effective in causing power loss of EM waves than the mixture with nano-sized TiO₂ powder.     

Properties of TiO2 film prepared from titanium tetrachloride

The properties of TiO₂ film prepared by titanium tetrachloride were investigated with respect to annealing temperatures in terms of phase change, crystallite size, and band gap energy. The TiO₂ film dried at room temperature exhibited an amorphous phase, while films calcined above 281 and at 990°C displayed anatase TiO₂ and a mixture of anatase and rutile, respectively. The TiO₂ film was transformed to an anatase phase through three stages during the annealing processes: (1) removal of water, (2) decomposition of a peroxo group, and (3) amorphous-anatase phase transformation. It was also found that the bandgap energy of TiO₂ film was changed with increasing annealing temperature. This is attributed to the quantum size effect in the range of 475–675°C and to the formation of rutile phase having lower band gap energy than anatase in the range of 675–990°C.     

Effects of heat treatment scheme on the photocatalytic activity of TiO2 nanotube powders derived by a facile electrochemical process

TiO₂ nanotubes (NTs) in powder form were synthesized by a facile electrochemical process in a perchlorate-containing electrolyte. Transmission electron microscopy results indicate that the TiO₂ NT-powder is in an amorphous structure and has outer diameter of 20 nm and tube-wall thickness of 5 nm. X-ray diffraction analysis reveals that phase composition of the annealed TiO₂ NT-powders is related to the heat treatment scheme. Methyl orange was employed as a representative dye pollutant to evaluate the ultraviolet photocatalytic activity of the TiO₂ NT-powders. It was found that different post heat treatment schemes affect greatly the photocatalytic activities of the TiO₂ NT-powders, which should be ascribed to the changes in phase structural and morphological properties of the TiO₂ NT-powders. These results indicate that there should be a balance between crystal phase and nanotubular configuration for achieving the best photocatalytic activity of the TiO₂ nanotube powders.     

The effect of Cu and Zn on the phase stability of L12 Al3Hf intermetallic compound synthesized by mechanical alloying

Elemental powders of Al and Hf were mechanically alloyed to produce L1₂ Al₃Hf powder with a nanocrystalline structure. The effect of the addition of ternary elements on the mechanical alloying behavior and the thermal stability of L1₂ phase was investigated. The start and finish temperatures of the phase transformation from L1₂ to D0₂₃ varied as a function of time and temperature in binary and ternary alloys. L1₂ Al₃Hf alloy was readily prepared by mechanical alloying for 6 h. It is thought that the homogeneous distribution of Al and Hf and following formation of the L1₂ phase are related with the attainment of the ultra-fine grain size of the powder. However, L1₂ phase was formed in the (Al+12.5 at.% M)₃Hf (M=Cu, Zn) alloys after 10 hours milling. The delayed formation of L1₂ phase in the ternary alloys was related to the retardation of the microstructural evolution or grain size refinement. The start temperature of the L1₂ to D0₂₃ phase transformation in the binary Al₃Hf alloy was only about 650 K, but it was increased to about 970 and 1170 K with the addition of 10 at.% Cu and 12.5 at.% Zn, respectively. Their grain size was less than 20 nm after 20 min heat treatment at each temperature. The start temperature of the phase transformation was not significantly affected by annealing time but the finish temperature of the transformation was decreased significantly with increasing annealing time. It is worth noting that the results in this study will be presented as useful data for the various consolidation processes of L1₂ nanocrystalline Al₃Hf powder.     

Direct synthesis and characterization of high temperature stable anatase TiO2 nanospheres from peroxo-titanium complex

Phase pure anatase nanospherical TiO₂ has been obtained by sol dissolution method using TiCl₃ as a precursor. The effect of annealing temperature on the properties of TiO₂ has been studied and discussed. The structural, morphological and optical properties of the synthesized powder were determined using XRD, SEM, AFM, TEM, UV-Visible and flourimetry. The anatase phase of synthesized TiO₂ using this method has a higher thermal stability up to 800 °C after which it is transformed into rutile phase. The crystallite size increases with the annealing temperature. The shift in absorption towards the visible region is observed for the anatase phase. The material showed fluorescence properties emitting light of 410 nm. The study also evaluated the photodegradation of a congo red as model dye. The TiO₂ powder has shown good catalytic properties due to its high surface area. The proposed method is simple, fast, and efficient and reproducible for the synthesis of homogeneous nanospheres of TiO₂, which will be applicable in possible optoelectronic devices.     

Influence of Feedstock Powder Modification by Heat Treatments on the Properties of APS-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-40% TiO<Subscript>2</Subscript> Coatings

The formation and decomposition of aluminum titanate (Al₂TiO₅, tialite) in feedstock powders and coatings of the binary Al₂O₃-TiO₂ system are so far poorly understood. A commercial fused and crushed Al₂O₃-40%TiO₂ powder was selected as the feedstock for the experimental series presented in this paper, as the composition is close to that of Al₂TiO₅. Part of that powder was heat-treated in air at 1150 and 1500 °C in order to modify the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including EDS of metallographically prepared cross sections. Only a maximum content of about 45 wt.% Al₂TiO₅ was possible to obtain with the heat treatment at 1500 °C due to inhomogeneous distribution of Al and Ti in the original powder. Coatings were prepared by plasma spraying using a TriplexPro-210 (Oerlikon Metco) with Ar-H₂ and Ar-He plasma gas mixtures at plasma power levels of 41 and 48 kW. Coatings were studied by XRD, SEM including EDS linescans of metallographically prepared cross sections, and microhardness HV1. With the exception of the powder heat-treated at 1500 °C an Al₂TiO₅-Ti₃O₅ (tialite–anosovite) solid solution Al_2?xTi_1+xO₅ instead of Al₂TiO₅ existed in the initial powder and the coatings.     

Effect of titanium addition on mechanical properties of Mo-Si-B alloys

In this study, we investigated the effect of titanium addition on microstructure and mechanical properties in Mo-Si-B alloys. The Mo-Ti-Si-B alloy (Mo-3.9Ti-3Si-1B, wt%), which has α-Mo, Mo₃Si, Mo₅SiB₂ and TiO₂ phases_, was fabricated by a powder metallurgy (PM) method. The starting materials were pulverized by using a high-energy ball milling and the resultant powder was subjected to a reduction process followed by cold isostatic pressing (CIP) compaction and pressureless sintering. In the microstructure, intermetallic compound phases were uniformly distributed in the α-Mo matrix. Some titanium atoms solved into the α-Mo matrix and the others formed a TiO₂ phase caused by reaction with oxygen at the grain boundary. Fracture toughness of the Mo-Ti-Si-B sintered body was recorded as 10.42 MPa·m^1/2, which is lower than that of the Mo-Si-B sintered body without addition of titanium. In the Mo-Ti-Si-B sintered body, the fracture mode is similar to the Mo-Si-B sintered body where intergranular fracture through the Mo grain boundary and transgranular fracture cross the intermetallic compound phase. The decrease of fracture toughness is due to the relatively large TiO₂ at the grain boundary, promoting intergranular fracture.     

Polyethyleneimine surfactant effect on the formation of nano-sized BaTiO3 powder via a solid state reaction

Barium titanate powder is widely used in multilayer ceramic capacitors (MLCC) because of its’ superior ferroelectric and dielectric properties. Presently, barium titanate powder is mass-produced using a conventional solid state reaction. However, the conflicting literature reports clearly show that the solid state reaction mechanism is not well understood. Synthesizing nano-sized barium titanate powders with less agglomeration using a solid state reaction method at a low calcination temperature has become a hot research topic. This study examines the effect of polyethyleneimine (PEI) surfactant addition on BaTiO₃ formation in a solid sate reaction. XRD, DTA and TEM are used in this investigation. The results indicate that PEI addition can promote BaCO₃ and TiO₂ mixing homogeneity, which enhances the interfacial reaction between BaCO₃ and TiO₂. BaTiO₃ formation can be obtained through the interfacial reaction between BaCO₃ and TiO₂ at the intimate contact between the reactants and Ti⁺⁴ ion diffusion along the surfaces or grain boundaries of BaTiO₃ powders, thereby obtaining a single phase BaTiO₃ powder when calcining at low temperatures for a long period.     

Control of microwave dielectric properties in 0.42ZnNb<Subscript>2</Subscript>O<Subscript>6</Subscript>−0.58TiO<Subscript>2</Subscript> ceramics by the quantitative analysis of phase transition

Phase constitutions of ZnNb₂O₆−TiO₂ mixture ceramics were significantly changed according to the sintering temperature. Phase transition procedures and their effect on the microwave dielectric properties of 0.42ZnNb₂O₆−0.58TiO₂ were investigated using X-ray powder diffraction and a network analyzer. The fractions of the phases composing the mixture were calculated by measuring integral intensities of each reflection. The structural transitions in 0.42ZnNb₂O₆−0.58TiO₂ were interpreted as the association of two distinct steps: the columbite and rutile to ixiolite transition present at lower temperatures (900–950°C) and the ixiolite to rutile transition at higher temperatures (1150–1300°C). These transitions caused considerable variation of microwave dielectric properties. Importantly, τ_f was modified to around 0 ppm/°C in two sintering conditions (at 925°C for 2 hr and at 1300°C for 2 hr), by the control of phase constitution.     

Alumina-base plasma-sprayed materials—Part II: Phase transformations in aluminas

Aluminum oxide is widely used for plasma spraying. Alumina deposits consist of a number of metastable crystallographic modifications, which at elevated temperatures, transform to the stable α phase. It was shown that additions of various oxides changed the phase composition and shift phase transformation temperatures. This paper addresses the variation of phase compositions and temperatures of the phase changes for plasma-sprayed alumina deposits manufactured with alumina-base materials containing O₂O₃ and TiO₂. This study combines the results obtained from energy dispersive analyzer of X-rays (EDAX) and scanning electron microscopy (SEM) chemical analysis, differential thermal analysis (DTA), and X-ray powder diffrac-tion (XRD) quantitative phase analysis of as-sprayed and annealed samples of alumina deposits and shows how the two additives change the phase composition and the α-phase formation temperature. This transformation temperature varied by nearly 200 °C. The metastable alumina sequences were also influenced by the chemical composition; for example, the content of 6 alumina varied between 0 and 55 wt %. Part I of this article was printed in the September issue of JTST Vol 6 (No. 3), September 1997, p 320–326     

Early-Stage Oxidation Behavior of Pt-Modified γ′-Ni3Al-Based Alloys with and without Hf Addition

The early-stage oxidation behavior in air of Pt-modified γ′-Ni₃Al-based alloys of composition (in at.%) Ni–22Al–30Pt with and without 0.5Hf was investigated in terms of oxidation kinetics, scale evolution and Al₂O₃ phase transformation. Oxidation exposures included heating to and short-term holds at 1,150 °C. Hafnium addition did not appear to affect microstructural evolution and growth rate of the oxide scales during heating to 1,150 °C; however, it was found that Hf delayed the metastable-to-α-Al₂O₃ phase transformation, thus allowing continued fast growth of oxide scale. After the transient oxidation stage of up to about 10 min (including heating time), Ni-rich metallic particles precipitated in the lower part of the metastable Al₂O₃ layer, due to a decrease in the oxygen potential resulting from scale evolution. The present results indicated that the period of oxide phase transformation was followed by the establishment of steady-state oxidation kinetics. However, the steady-state kinetics were different for the two alloy systems. Specifically, after complete phase transformation to α-Al₂O₃, rapid growth of oxide grains occurred on the Hf-free alloy; whereas, the oxide grain size remained small for the Hf-containing alloy. Such a difference of transformation and subsequent grain-growth behavior greatly affected oxide thickening kinetics.