NaCl–H2O-assisted preparation of SrTiO3 nanoparticles by solid state reaction at low temperature

An environmentally friendly NaCl–H₂O system was developed to synthesize monodisperse strontium titanate (SrTiO₃) nanoparticles from commercially available raw materials (SrCO₃ and rutile) by solid state reactions. The formation rate of SrTiO₃ was accelerated by the addition of NaCl and water vapor. Single phase SrTiO₃ was obtained by calcination at 700 °C for 2 h in water vapor (H₂O flow rate of 2.0 mL/min) by the addition of 50 wt% NaCl, although 900 °C and 750 °C for 2 h were required to complete the reaction by calcinations in air and air by the addition of 50 wt% NaCl, respectively. The results demonstrate that both NaCl and H₂O played vital roles to accelerate the formation of SrTiO₃ nanoparticles at relatively low temperature. On the basis of experiments and analysis, a rational growth mechanism has been proposed and discussed.     

Molten salt synthesis of SrTiO3 nanocrystals using nanocrystalline TiO2 as a precursor

A molten salt method was proposed to synthesize SrTiO₃ nanocrystals in the eutectic NaCl–KCl at 700 °C for 6 h, by using the homemade TiO₂ nanocrystals and commercial Sr(NO₃)₂ powder as raw materials. Besides, a control experiment with the commercial TiO₂ submicron-sized crystallites as a precursor was also conducted. The structure and composition of the obtained products were characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results from XRD and XPS revealed the preparation of pure cubic phase SrTiO₃ powders. The TEM observation demonstrated that the nanocrystalline TiO₂ precursor played an important role in the current molten salt synthesis of SrTiO₃ nanocrystals, which were likely formed by the template formation mechanism.     

Photocatalytic activities of N-doped nano-titanias and titanium nitride

TiO₂ doped with various loadings of nitrogen was prepared by nitridation of a nano-TiO₂ powder in an ammonia/argon atmosphere at a range of temperatures from 400 to 1100 °C. The nano-TiO₂ starting powder was produced in a continuous hydrothermal flow synthesis (CHFS) process involving reaction between a flow of supercritical water and an aqueous solution of a titanium salt. The structures of the resulting nanocatalysts were investigated using powder X-ray diffraction (XRD) and Raman spectroscopy. Products ranging from N-doped anatase TiO₂ to phase-pure titanium nitride (TiN) were obtained depending on post-synthesis heat-treatment temperature. The results suggest that TiN started forming when the TiO₂ was heat-treated at 800 °C, and that pure phase TiN was obtained at 1000 °C after 5 h nitridation. The amounts and nature of the Ti, O and N at the surface were determined by X-ray photoelectron spectroscopy (XPS). A shift of the band-gap to lower energy and increasing absorption in the visible light region, were observed by increasing the heat-treatment temperature from 400 to 700 °C.     

Hydrothermal synthesis of 3D TiO2 nanostructures using nitric acid: Characterization and evolution mechanism

Various morphologies of TiO₂ nanostructures were synthesized by HNO₃ assisted hydrothermal treatment with respect to the acid molarity (1 M, 3 M, and 8 M), temperature (110, 140, and 180 °C), and time (1, 3, and 6 h). An additional sample was synthesized inside the protonated titanate nanoribbon coated vessel with the acid molarity of 8M at 140 °C for 3 h. The crystal structure and morphology of the nanostructures synthesized were investigated using X-Ray diffractometer, scanning electron microscope, and transmission electron microscope. The results revealed that lower acid concentrations, longer synthesis durations and higher temperatures favored anatase phase formation. Meanwhile, a phase pure 3D lotus structure rutile TiO₂ could be obtained by hydrothermal synthesis at 8M HNO₃ concentration at 140 °C for 3 h using protonated H-titanate nanoribbons. A probable mechanism for the evolution of 3D rutile lotus structure was highlighted.     

Effect of microstructural evolution on wettability and tribological behavior of TiO2 nanotubular arrays coated on Ti–6Al–4V

Self-organized TiO₂ nanotubular arrays were fabricated by electrochemical anodization of Ti–6Al–4V plates in an NH₄F/H₃PO₄ electrolyte. The effect of microstructural evolutions on the wettability and tribological behavior of the TiO₂ nanotubes was investigated. Based on the XRD profiles of the fabricated material, the characteristic TiO₂ peaks were not recognized after anodization; however, highly crystalline TiO₂ (anatase and rutile) was formed due to crystallization during annealing at 500 °C for 1.5 h. The nanotube arrays were converted entirely to rutile at 700 °C. From a microstructure point of view, a highly ordered nanotube structure was achieved when the specimen was annealed at 500 °C, with a length of 0.72 μm and a pore diameter of 72 nm. Further increasing the annealing temperature to 700 °C resulted in the complete collapse of the tubular structure. The results indicate that the improved wettability of the anodized specimens was due to the combination of the effects of both the surface oxide layer and the increased surface roughness achieved after anodization. Moreover, the wear resistance and wettability of the sample annealed 500 °C were improved due to the high hardness (435 HV) and low coefficient of friction (0.133–168) of the highly crystalline structure of the TiO₂ nanotubes.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

The effect of the hydrothermal synthesis variables on barium titanate powders

In this work, the influence of (a) Ba excess in the starting hydrothermal mixture with TiO₂, (b) hydrothermal reaction temperature, and (c) washing cycles on the hydrothermal synthesis of barium titanate (BaTiO₃) were investigated to assess their relative contributions to the final characteristics of the sintered oxide. BaTiO₃ cake was prepared by hydrothermal synthesis at 150 °C and 180 °C using BaOH₂·8H₂O and TiO₂·xH₂O as starting hydrothermal mixture with an excess of Barium (+1 Ba mol% and +2 Ba mol%). The obtained BaTiO₃ cake was washed several times from 0 to 14 (W_n<15) using simple de-ionized water and then sintered at 1120 °C for 3 h. All considered hydrothermal syntheses variables strongly contribute to the final characteristics of the sintered BaTiO₃ powders in terms of Ba²⁺/Ti⁴⁺ molar ratio, crystalline structure and mean particle size. In particular, it is clear from these experiments that the removal of the unfavorable barium salts from BaTiO₃ cake by long washing cycles before final calcination is a critical step in the hydrothermal synthesis of BaTiO₃.     

Synergy effect in the photocatalytic degradation of methylene blue on a suspended mixture of TiO2 and N-containing carbons

N-containing carbon materials were obtained from waste plum stones submitted to pyrolysis under Ar flow at 700 °C or to activation under steam at 800 °C and enriched with nitrogen by heating in a NH₃/air mixture at 270 °C or in NO at 300 °C. In situ mixtures of TiO₂ and carbons were prepared by the slurry method and methylene blue photodegradation was chosen as a model reaction to verify the influence of N-containing carbons on the photocatalytic activity of TiO₂ under artificial visible light irradiation. From the kinetics of methylene blue degradation an important synergy effect between both solids was detected with a remarkable increase up to a factor of 5.3 higher in the photocatalytic activity on TiO₂–C than that on TiO₂ alone. A mechanism for the photoassisting role of N-containing carbons upon the photoactivity of TiO₂ under visible light is discussed.     

Preparation of Li2TiO3 by hydrothermal synthesis and its structure evolution under high energy Ar+ irradiation

Ultrafine lithium titanate (Li₂TiO₃) powder was synthesized by hydrothermal method. The phase formation and transition condition among α, β, and γ-Li₂TiO₃ were discussed. XRD and ICP-AES showed the single α-phase was formed at 180 °C with 2 h hydrothermal reaction, and it transited into β-phase at 400 °C. SEM observation and EDS analysis confirmed the dissolution of TiO₂ and the formation of α-Li₂TiO₃ proceeded simultaneously with preferable growth direction of (-133) lattice. During the phase transition, the powder maintained the small crystallite, which facilitated the fabrication of Li₂TiO₃ bulk with small grain size. After the Ar⁺ irradiation, the surface region to the depth of 3 μm of Li₂TiO₃ ceramic was affected, where the decrease of crystallization and disturbance of short-range order were confirmed by GIXRD and Raman spectroscopy. In spite of the structure change at the surface area, the ceramic bulk maintained the same.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

The effects of deposition temperature on structure and dielectric properties of Ba0.6Sr0.4TiO3 thin films produced by pulsed laser deposition

The effects of deposition temperature on orientation, surface morphology and dielectric properties of the thin films for Ba_0.6Sr_0.4TiO₃ thin films deposited on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition were investigated. X-ray diffraction patterns revealed a (2 1 0) preferred orientation for all the films. With rising substrate temperature from 650 °C to 700 °C, the crystallinity and crystal grain size of the films increase, the relative dielectric constant increases, but the dielectric losses have not obvious difference. The film deposited at 350 °C and annealed at 700 °C has strongly improved roughness and dielectric permittivity compared with the film only deposited directly at 700 °C. Three distinct relaxation processes within tan(δ) were found for the Ba_xSr_1?xTiO₃ film: a broadened process of the film relaxation, an intermediate peak which originates from Maxwell–Wagner–Sillars polarization, and an extremely slow process ascribed to leak current. The complex dielectric permittivity and loss can be fitted by an improved Cole–Cole model corresponding to a stretched relaxation function.     

Evaluating the photocatalytic activity of Pt/TNT film catalyst

TiO₂ nanotubes (TNT) were prepared by a hydrothermal method from the commercially available TiO₂-P25. Five types of TNT were produced at different temperatures (120 °C, 130 °C, and 150 °C) and by using different reaction times (12 h, 24 h, and 30 h). The photocatalytic reactor that was used is a film catalytic reactor, in which the height of the catalyst is 1.0 mm. The BET and FESEM analysis results showed that TNT130-24 (130 °C, 24 h) and TNT150-12 (150 °C, 12 h) possessed well-formed tubular structures with a high specific surface area (282.9–316.7 m² g⁻¹) and large pore volumes (0.62–0.70 cm³ g⁻¹). However, TNT120-30 (120 °C, 30 h) presented the best photocatalytic activity upon CO removal due to the synergistic effect of TiO₂ nanotubes and TiO₂ particles. After the TNT catalysts were modified with Pt particles, the removal efficiency was in the order of Pt/TNT120-30>Pt/TNT130-24>Pt/P25. Pt/TNT120-30 showed 99% removal efficiency in a continuous photoreactor with a high space velocity of 1.79×10⁴ h⁻¹. The results of the TEM and DRS analyses confirmed that the Pt particles enhanced the photocatalytic reaction, which was attributed to the well-dispersed nature of the 1 nm nanoscaled Pt particles on the surfaces of the TNT catalysts, and narrowed the band gap from 3.22 eV to 3.01 eV.     

Preparation of cerium titanate brannerite by solution combustion,and phase transformation during heat treatment

An aqueous solution route was employed to prepare cerium titanate oxide brannerite. Thermal analysis, X-ray diffraction, Raman spectroscopy, transmission and scanning electron spectroscopy, were used to investigate the brannerite structure formation and bulk properties. Mixed metal oxides (TiO₂, CeO₂ and brannerite CeTi₂O₆) were formed upon calcination at 800 °C for 12 h. The amount of brannerite phase decreased to form the constituent oxides with increasing calcination temperature and only pure TiO₂ and CeO₂ were present after 1200 °C calcination. The brannerite CeTi₂O₆ phase reformed at 1300 °C, and its relative amount was increased with dwell time. After 48 h calcination at 1300 °C, brannerite with only minor metal oxide impurities (<1%) was observed. The sample melted at 1400 °C which led to the collapse of brannerite back to its constituent oxides. Further, the phase formation was influenced by pelletization of the powders, which may be explained by a molar volume increase during brannerite formation. Attempts to confirm this hypothesis using Pu brannerite were inconclusive.     

Temperature stable and high-Q microwave dielectric ceramics in the Li2Mg3−xCaxTiO6 system (x=0.00–0.18)

Microwave dielectric properties of Li₂Mg_3−xCa_xTiO₆ (x=0–0.18) ceramics were studied using a conventional solid-state route to find temperature stable and high Q microwave ceramics. As the calcination temperature was 500 °C, the Li₂TiO₃ phase with monoclinic rock salt structure in C2/c space group started to form. When the samples were calcined from 600 °C to 900 °C, the XRD patterns exhibited a remarkable chemical reaction between the MgO and Li₂TiO₃ phases, which eventually formed the Li₂Mg₃TiO₆ phase. The results indicated the Li₂Mg₃TiO₆and CaTiO₃ co-existed with each other and formed a stable composite system when the calcium content was added. The SEM photographs indicated that the pores caused by the Li evaporation could be effectively reduced due to the appearance of CaTiO₃. As x was increased from 0 to 0.18, the relative density was significantly improved due to the elimination of pores. As the Ca content increased, the dielectric constant (εr) increased from 14.8 to 20.6; the quality factor (Q×f) decreased from 148,713 GHz to 79,845 GHz, and the temperature coefficient of resonant frequency (τf) significantly increased from −42.4 to +10.8 ppm/°C due to the increased amount of CaTiO₃. Therefore, at x=0.12, the LMCxT ceramics sintered at 1280 °C for 6 h displayed excellent comprehensive properties of εr=17.8,Q×f=102,246 GHz and τf=−0.7 ppm/°C.     

Dielectric and magnetic properties of BiFeO3 ceramics prepared by hydrothermal synthesis

Single-phase BiFeO₃ powders were prepared at a temperature of 200 °C by a hydrothermal synthesis. BiFeO₃ ceramics were prepared with the powders by a conventional ceramic process. The BiFeO₃ ceramics with no impurity phase were prepared at the sintering temperature of 650–800 °C. The dense microstructure was observed in the BiFeO₃ ceramics sintered at a temperature of 700 °C and higher. BiFeO₃ ceramics show linear M–H curves in low H, which are antiferromagnetic behaviors. The dielectric dispersion was observed at the frequency range of 10 kHz to 1 MHz in the BiFeO₃ ceramic sintered at 700 °C or lower. The dielectric constant and loss of the BiFeO₃ ceramics sintered at 750 °C or higher were about 85 and 0.4 at 100 kHz, respectively.     

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.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.     

Microstructure and photocatalytic activity of electrospun carbon nanofibers decorated by TiO2 nanoparticles from hydrothermal reaction/blended spinning

Recently, carbon nanofibers@TiO₂ (CNFs@TiO₂) composites as photocatalysts for dye degradation have attracted intense attention. However, only few contributions had been made to investigate systematically the differences between the various preparation approaches and the influence of thermal treatment on the photocatalytic activity. In this work, the electrospun CNFs@TiO₂ composites which were prepared by hydrothermal reaction and blended spinning, respectively, have been fabricated via stabilization in air at 280 °C and then carbonization in N₂ at heat treatment temperature between 500 and 1100 °C. The composites which were prepared by hydrothermal reaction and blended spinning showed the outstanding photocatalytic activity at 900 °C and 1100 °C, respectively. And the photocatalytic activity of composites prepared by hydrothermal reaction was higher than that prepared by blended spinning, but reversibility of the composites showed a reverse trend. These results indicated that the effect of heat treatment temperature on the photocatalytic activity depended on the synergistic effect among the adsorptive property of CNFs, TiO₂ loading amount and anatase phase content in composites. Hence, combining the merits of hydrothermal reaction and blended spinning, a novel method for preparing CNFs@TiO₂ composites with high TiO₂ loading amount and strong interfacial interaction could be envisioned.     

Densification behavior and properties of alumina–chrome ceramics: Effect of TiO2

Highly dense alumina–chrome bodies with low porosity are usually used as corrosion and thermal resistant refractories. Alumina–chrome refractory with molar ratio 1:1 was developed using chemical grade hydrated alumina and chromium (III) oxide by conventional sintering route. Batch materials were attrition milled, isostatically pressed and sintered in the temperature range from 1000 °C to 1700 °C with 2 h soaking at peak temperature. Phase development of the sintered materials with temperature was studied by X-ray diffraction. Sintering temperature, sintering condition and addition of sintering aid (TiO₂) have immense effect on the densification of the alumina–chrome refractory. Highly dense alumina–chrome refractory with almost nil apparent porosity was developed at 1500 °C in reducing atmosphere. Flexural strength of the sintered materials at room temperature and at 1200 °C was also measured. 1 wt% TiO₂ gives the optimum result with respect to densification and flexural strength.     

Effects of SrTiO3 additives on the structure and microwave dielectric properties of Ba4.2Sm9.2Ti18O54 ceramics

The effects of SrTiO₃ additives on the microstructure and the dielectric properties of Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST) materials have been investigated. The microstructure of BST doped with SrTiO₃ was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA). The XRD, SEM and EPMA results showed that small amounts of SrTiO₃ (less than 4 wt.%) could be accommodated in BST while a secondary metatitanate phase appeared in the samples with 8 wt.% SrTiO₃ added, the addition of SrTiO₃ increased the lower solubility limit (x values) of Ba_6?3xSm_8+2xTi₁₈O₅₄. The relative permittivities and the TC_f values showed an approximately linear increase with increasing additions of SrTiO₃. It was observed that near zero TC_f values could be achieved with the addition of SrTiO₃ to BST ceramics. With 4 wt.% SrTiO₃ addition and sintered at 1360 °C for 2 h, BST ceramics exhibited excellent dielectric properties: ?_r = 81.2, Q_f = 8470 GHz and TC_f = ?1.5 ppm/°C.