Formation of TiC particle during carbothermal reduction of TiO2

Formation of TiC particle during carbothermal reduction of titanium dioxide (TiO₂) was investigated. The mixture with TiO₂ and carbon resin was reacted at 1500 °C for 0–45 min under flowing Argon atmosphere. The powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The partially reduced TiO₂ particles were conglomerated in the initial stage of the reduction and the size of this conglomerate ranged from 500 to 1000 nm. After the complete reaction between Ti as a reduction product and C, the large conglomerates separated to homogeneous and fine TiC particles with a size of 80 nm.     

Thermochemical modeling and experimental studies on the formation of TiB2 through carbothermic synthesis from TiO2 and B2O3 or B4C

In the present study, a combined modeling and experimental approach were chosen to compare the carbothermal reaction mechanisms of TiO₂ with B₂O₃ or B₄C and to obtain a high yield of TiB₂ as a final product. A thermochemical modeling software, FactSage, was used to estimate the possible products and their quantities by means of Gibbs Energy Minimization approach. In the experimental step, different reaction temperature (1400–1700 °C) and time (0–60 min) combinations on TiB₂ synthesis were investigated at fixed initial molar ratios of TiO₂:B₂O₃:C (1:1:5) and TiO₂:B₄C:C (1:0.5:1.5). The experimental products were characterized by using X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and BET analysis methods. It was found that calculated modeling results obtained through FactSage agree well with the experimental results. The Magneli phases formed as intermediate products only when B₂O₃ was used as starting material. Full conversion to TiB₂ was obtained at 60 min and 1600 °C for both initial sources of boron.     

In-situ fabrication of Al2O3–SiC nanocomposites using B2O3 as sintering aid

Al₂O₃–SiC nanocomposites with 5 and 10 vol% SiC have been in-situ fabricated by sol-gel method followed by carbothermal reduction of alumina–silica gel using B₂O₃ as sintering aid. Green bodies were formed by cold isostatic pressing of calcined gel, which was prepared by an aqueous sol-containing aluminum chloride, TEOS, sucrose and boric acid. Pressureless sintering was carried out in Ar–12%H₂ atmosphere at 1700 °C. Addition of B₂O₃ (1 or 3 wt%) was an effective densification aid in the Al₂O₃–5 vol% SiC composites, while the densification of Al₂O₃–10 vol% SiC composites was not affected by adding B₂O₃. The composite material containing 5 vol% SiC doped with 3 wt% B₂O₃ reached 98.7% of full density. Nano-sized β-SiC particles were formed in-situ by means of a reaction between mullite and carbon at 1600 °C. Scanning electron microscopy revealed that the spherical in-situ synthesized SiC nanoparticles were well distributed through the composite and located predominantly to the interior of alumina matrix grains.     

Synthesis of sub-micro sized high purity zirconium diboride powder through carbothermal and borothermal reduction method

Sub-micro sized zirconium diboride (ZrB₂) powders were successfully prepared via the boro/carbothermal reduction method using zirconium oxide and boron carbide as the primary raw materials. The prepared mixtures were thermally reacted at 1250 °C for 1 h. The optimized composition range containing the lowest oxide and carbide impurity, which was 0.14% of oxygen and 0.3% of carbon contents, was determined using crystallographic and elemental analysis. The particle size was reduced from 5 µm to 245 nm by the addition of B₄C as a reductant within a composition range that maintained the highest purity. The morphology changed from faceted to angular hexagonal bar-like with a simultaneous growth in particle size. Changes in the particle structure were a result of the existing liquid B₂O₃ phase during the reaction. The 245-nm particles contained 12.1% oxygen content and 16.2% oxygen content for the 5-μm particle in the circumstance in which limited oxides could be produced.     

Enhanced production of benzyl alcohol in the gas phase continuous hydrogenation of benzaldehyde over Au/Al2O3

Exclusive hydrogenation of benzaldehyde to benzyl alcohol in gas phase continuous operation (393–413 K, 1 atm) was achieved over Au/Al₂O₃, Au/TiO₂ and Au/ZrO₂. Synthesis of Au/Al₂O₃ by deposition–precipitation generated a narrower distribution (2–8 nm) of smaller (mean = 4.3 nm) Au particles relative to impregnation (1–21 nm, mean = 7.9 nm) with increased H₂ uptake under reaction conditions and higher benzaldehyde turnover. Switching reactant carrier from ethanol to water resulted in a significant enhancement of selective hydrogenation rate over Au/Al₂O₃ with 100% benzyl alcohol yield, attributed to increased available reactive hydrogen. This response extends to reaction over Au/TiO₂ and Au/ZrO₂.     

Microstructure of TiO2 porous ceramics by freeze casting of nanoparticle suspensions

During freeze casting of TiO₂ porous ceramics, the porous architecture is strongly influenced by TiO₂ particle size, solids loading, and cooling temperature. This work investigates the influences of particle size, freezing substrate, and cooling temperature on the TiO₂ green bodies prepared by freeze casting. The results show that the lamellar channel width with 100 nm particles is larger than that of 25 nm particles, yet the ceramic wall thickness is noticeably decreased. The lamellar structure is more ordered when using a copper sheet than glass as its freezing substrate. A finer microstructure results when frozen at − 50 ℃ than − 30 ℃. Such porous materials have application potentials in a wide range of areas such as photocatalysis, solar cells, and pollutant removal and should be further studied.     

Protective properties of Al2O3 + TiO2 coating produced by the electrostatic spray deposition method

Mechanical resistance of Al₂O₃ + TiO₂ nanocomposite ceramic coating deposited by electrostatic spray deposition method onto X10CrAlSi18 steel to thermal and slurry tests was investigated. The coating was produced from colloidal suspension of TiO₂ nanoparticles dispersed in 3 wt% solution of Al₂(NO₃)₃, as Al₂O₃ precursor, in ethanol. TiO₂ nanoparticles of two sizes, 15 nm and 32 nm, were used in the experiments. After deposition, coatings were annealed at various temperatures, 300, 1000 and 1200 °C, and next exposed to cyclic thermal and slurry tests. Regardless of annealing temperature and the size of TiO₂ nanoparticles, the outer layer of all coatings was porous. The first five thermal cycles caused a rapid increase of aluminum content of the surface layer to 30–37 wt%, but further increase in the number of thermal cycles did not affect the aluminum content. The oxidation rate of coating-substrate system was lower during the thermal tests than during annealing. The oxidation rate was also lower for smaller TiO₂ particles (15 nm) forming the coating than for the larger ones (32 nm). The protective properties of Al₂O₃ + TiO₂ coating against intense oxidation of substrate were lost at 1200 °C. Slurry tests showed that coatings annealed at 1000 °C had the best slurry resistance, but thermal tests had weakened this slurry resistance, mainly due to decreasing adhesion of the coating.     

Effects of heat-treatment temperature and starting composition on morphology of boron carbide particles synthesized by carbothermal reduction

Carbothermal reduction using B₂O₃ and carbon black was applied for synthesis of B₄C powder and the effects of heat-treatment temperature and starting composition of raw mixture on morphology of B₄C particles were investigated. Morphology of B₄C particles synthesized at 1450 °C was mainly spherical shapes. The B₄C powder synthesized at 1550 °C was large and changed in morphology from polyhedral to skeletal shape, and particle size of B₄C increased with an increase in the amount of B₂O₃ in the starting mixtures. The B₄C powder synthesized beyond 1650 °C consisted from dendrite-like particles aggregated by small primary particles. Morphology of the primary B₄C particles synthesized at 1750 °C changed from polyhedral to rounded shape with increasing the amount of B₂O₃ in the starting mixtures. It is clarified that heat-treatment temperature and the starting compositions of raw mixtures mainly affected B₄C nuclei number along with primary particle size and morphology of primary B₄C particles, respectively.     

Piezo-spectroscopic characterization of alumina-aluminium titanate laminates

A multilayered alumina-aluminium titanate composite was prepared by a colloidal route from aqueous suspensions. The structure of the laminate was symmetric and constituted of two external Al₂O₃ layers (width ≅ 1750 μm), one central Al₂O₃ layer (width ≅ 1200 μm) and two intermediate thin (width ≅ 315–330 μm) Al₂O₃–Al₂TiO₅ layers.Additional monolithic materials with the same compositions as those of the layers were fabricated as reference materials. Young's modulus of the monoliths was determined by three point bending. Dilatometry determinations were performed on green specimens, following the same heating and cooling schedules as those used for sintering the laminate, in order to determine the actual dimensional changes on cooling after sintering. The dimensional changes of the sintered specimens on heating and on cooling were also determined. Microscopic distributions of residual stresses were evaluated by fluorescence piezo-spectroscopy, and they revealed the existence of weak tensile and compressive hydrostatic stresses in the aluminium titanate and alumina layers, respectively. The level and sign of these stresses was in good agreement with those predicted based on analysis of the Young's modulus and the dimensional variations during cooling after sintering of the monoliths with the same compositions as those of the layers. Dimensional variations during cooling after sintering were different from those for sintered materials, which presented hysteresis between heating and cooling. In spite of the presence of compressive residual stresses in the external layers of the laminate, strength values of notched samples of the laminated specimens were lower than those for monoliths of the same composition as the external layers.     

The effect of TiO2 particle size on the characteristics of Au–Pd/TiO2 catalysts

The nanocrystalline TiO₂ materials with average crystallite sizes of 9 and 15 nm were synthesized by the solvothermal method and employed as the supports for preparation of bimetallic Au/Pd/TiO₂ catalysts. The average size of Au–Pd alloy particles increased slightly from sub-nano (< 1 nm) to 2–3 nm with increasing TiO₂ crystallite size from 9 to 15 nm. The catalyst performances were evaluated in the liquid-phase selective hydrogenation of 1-heptyne under mild reaction conditions (H₂ 1 bar, 30 °C). The exertion of electronic modification of Pd by Au–Pd alloy formation depended on the TiO₂ crystallite size in which it was more pronounced for Au/Pd on the larger TiO₂ (15 nm) than on the smaller one (9 nm), resulting in higher hydrogenation activity and lower selectivity to 1-heptene on the former catalyst.     

Photooxidation of iodide ion on some semiconductor and non-semiconductor surfaces

In 80% aqueous ethanol, TiO₂ (anatase), ZrO₂, ZnO, V₂O₅, Fe₂O₃ and Al₂O₃ photocatalyze the oxidation of iodide ion but CdO and CdS do not; the wavelength of illumination is 365 nm. However, Fe₂O₃ fails to bring in a sustainable photocatalysis in 60% aqueous ethanol. The photooxidation of iodide ion on TiO₂, ZrO₂, ZnO, V₂O₅ and Al₂O₃ in 60% aqueous ethanol was studied as a function of [I⁻], amount of catalyst suspended, airflow rate, light intensity and solvent composition. The metal oxides examined show sustainable photocatalytic activity. Iodine formation is larger with illumination at 254 nm than at 365 nm. The mechanisms of photocatalysis on semiconductor and non-semiconductor surfaces have been discussed. Photocatalytic generation of iodine has been analyzed using a kinetic model. The photocatalytic efficiencies are of the order V₂O₅ > TiO₂ > ZrO₂ > ZnO > Al₂O₃ and V₂O₅ > TiO₂ > ZrO₂ > ZnO=Fe₂O₃ > Al₂O₃ in 60% and 80% aqueous ethanol.     

Mechanism and kinetics of combustion-carbothermal synthesis of AlN nanopowders

In this work, the carbothermal reduction-nitridation process of low-temperature combustion synthesis (LCS) (Al₂O₃+C) precursor was investigated in detail. Compared with conventional precursor, the LCS precursor possesses many advantages such as amorphous structure, nanosized particles, homogeneous mixing at molecular level. The experimental results indicate that the methods for preparing precursor exert great influence on phase transformation of Al₂O₃, onset temperature of nitridation and reaction activity. During the calcination, the phase transformation of Al₂O₃ is hindered by a large amount of surrounding C particles rendering Al₂O₃ maintains high reactivity. Accordingly, the nitridation reaction initiates at 1300 °C and completes at 1500 °C for 2 h. Furthermore, the reaction mechanism was also discussed on the basis of experiments. More significantly, it is established that the activation energy of carbothermal reduction-nitridation reaction using LCS precursor is Eα=336 KJ/mol.     

Ignition and chemical mechanisms of volume combustion synthesis of titanium diboride

Reaction ignition and chemical mechanisms in volume combustion synthesis of TiB₂ via TiO₂–B₂O₃–Mg precursors were studied using in-situ differential thermal analysis, X-ray diffraction, scanning electron microscopy and thermochemical modeling. Mg–TiO₂ samples ignited at 607 °C through a sudden single step solid-solid reaction while Mg–B₂O₃ samples ignited at 810 °C after melting of magnesium. X-ray diffraction analysis revealed that reduction of TiO₂ occurs in multiple steps and forms intermediate compounds. Results showed that heat released from the first reaction between TiO₂ and Mg ignites the reactions between Mg, Ti and B₂O₃ resulting in the formation of TiB₂. Samples with larger TiO₂ particle size or a higher sample surface to volume ratio showed a two-step reaction behavior and the released heat in the first solid state reaction was insufficient for the propagation of the reaction throughout the sample. In addition, Mg₃B₂O₆ undesired by-product was formed as a result of this two-step reaction.     

Colored films produced by electron beam deposition from nanometric TiO2 and Al2O3 pigment powders obtained by modified polymeric precursor method

Synthesis and the characterization of TiO₂:5%Co (green), TiO₃:5%Fe (brown-reddish), TiO₂:2%Cr (brown), Al₂O₃:5%Co (blue), Al₂O₃:5%Fe (brown-reddish) and Al₂O₃:2%Cr (light green) nanometric pigment powders using polymeric precursor (modified Pechini's method) is reported. Colored thick films were deposited on amorphous quartz substrates by electron beam physical vapor deposition (EB-PVD) using pellets of the pigment powders as target. The evaporation process was carried out in vacuum of 4 × 10⁻⁶ Torr and the amorphous quartz substrates were kept at 350 °C during deposition. The TiO₂-based pigment powders presented crystalline anatase phase and the Al₂O₃-based pigment powders showed corundum phase, investigated by X-ray diffraction (XRD). The average particle size of the pigment powders was about 20 nm, measured by scanning electron microscopy with field emission gun (SEM-FEG). Diffuse reflectance spectra and colorimetric coordinates L¹, a¹, b¹ using the CIE-L¹a¹b¹ method are shown for the pigment powders, in the 350–750 nm range. The colored thick films were characterized by transmittance (UV–Vis) and atomic force microscopy (AFM). The average film roughness was ∼5.5 nm and the average grain size obtained in the films was around 75 nm. Films with thickness from 400 nm to 690 nm were obtained, measured by talystep profiler. Transmission spectra envelop method has been used to obtain refractive index and thickness of the Al₂O₃ colored thick films.     

Highly transparent AlON ceramics sintered from powder synthesized by carbothermal reduction nitridation

Aluminum oxynitride (AlON) powders were synthesized by the carbothermal reduction and nitridation process using commercial γ-Al₂O₃ and carbon black powders as starting materials. And AlON transparent ceramics were fabricated by pressureless sintering under nitrogen atmosphere. The effects of ball milling time on morphology and particle size distribution of the AlON powders, as well as the microstructure and optical property of AlON transparent ceramics were investigated. It is found that single-phase AlON powder was obtained by calcining the γ-Al₂O₃/C mixture at 1550 °C for 1 h and a following heat treatment at 1750 °C for 2 h. The AlON powder ball milled for 24 h showed smaller particles and narrower particle size distribution compared with the 12 h one, which was benefit for the improvement of optical property of AlON transparent ceramics. With the sintering aids of 0.25 wt% MgO and 0.04 wt% Y₂O₃, highly transparent AlON ceramics with in-line transmittance above 80% from visible to infrared range were obtained through pressureless sintering at 1850 °C for 6 h.     

Continuous flame aerosol synthesis of carbon-coated nano-LiFePO4 for Li-ion batteries

Core–shell, nano-sized LiFePO₄-carbon particles were made in one step by scalable flame aerosol technology at 7 g/h. Core LiFePO₄ particles were made in an enclosed flame spray pyrolysis (FSP) unit and were coated in-situ downstream by auto thermal carbonization (pyrolysis) of swirl-fed C₂H₂ in an O₂-controlled atmosphere. The formation of acetylene carbon black (ACB) shell was investigated as a function of the process fuel-oxidant equivalence ratio (EQR). The core–shell morphology was obtained at slightly fuel-rich conditions (1.0<EQR<1.07) whereas segregated ACB and LiFePO₄ particles were formed at fuel-lean conditions (0.8<EQR<1). Post-annealing of core–shell particles in reducing environment (5 vol% H₂ in argon) at 700 °C for up to 4 h established phase pure, monocrystalline LiFePO₄ with a crystal size of 65 nm and 30 wt% ACB content. Uncoated LiFePO₄ or segregated LiFePO₄–ACB grew to 250 nm at these conditions. Annealing at 800 °C induced carbothermal reduction of LiFePO₄ to Fe₂P by ACB shell consumption that resulted in cavities between carbon shell and core LiFePO₄ and even slight LiFePO₄ crystal growth but better electrochemical performance. The present carbon-coated LiFePO₄ showed superior cycle stability and higher rate capability than the benchmark, commercially available LiFePO₄.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Preparation of microfiltration membrane supports using coarse alumina grains coated by nano TiO2 as raw materials

To increase the mixing uniformity of coarse alumina grains with a small amount of nano TiO₂ particles, TiO₂ particles were prepared on the surface of coarse Al₂O₃ grains by in-situ hydrolysis of TiCl₄. The coated coarse Al₂O₃ powder was used to prepare microfiltration membranes supports. The effects of TiO₂ content and sintering temperatures on the bending strength, porosity and pore size distribution of the obtained supports were studied. The results show that the melted nano TiO₂ grains locate mainly at the neck of Al₂O₃ grains, which increases the bending strength of the support by increases the neck area. However, the bending strength is weakened if the TiO₂ content is excessive. No aggregated nano TiO₂ grainsare found. The resulting supports sintered at 1650 °C for 2 h yields a bending strength of 55.4 MPa, a porosity of 38% with a mean pore size of 8.0 μm.     

Effect of BaCu(B2O5) on the sintering and microwave dielectric properties of Ca0.4Li0.3Sm0.05Nd0.25TiO3 ceramics

The sintering behaviors and microwave dielectric properties of the Ca_0.4Li_0.3Sm_0.05Nd_0.25TiO₃ (abbreviated CLSNT) ceramics with different amounts of BaCu(B₂O₅) addition were investigated in this paper. Adding BaCu(B₂O₅) to CLSNT lowered its sintering temperature from 1300 °C to 925 °C. No secondary phase was observed in the CLSNT ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLSNT ceramics with small amounts of BaCu(B₂O₅) addition could be well sintered at 925 °C without much degradation in the microwave dielectric properties. Especially, the 1.75 wt.% BaCu(B₂O₅)-doped CLSNT ceramic sample sintered at 925 °C for 3 h had optimum microwave dielectric properties of ε_r = 93.5 ± 3.2, Q × f = 6486 ± 434 GHz, and τ_f = 5 ± 1.5 ppm/°C (at 3–4 GHz), enabling it a promising candidate material for LTCC applications. Obviously, BaCu(B₂O₅) could be a suitable sintering aid to facilitate the densification and microwave dielectric properties of the CLSNT ceramics.