Effect of sintering temperature and Ho2O3 on the properties of TiO2-based varistors

In this study, a series of TiO₂-based ceramics doped with different contents of Ho₂O₃ in the range of 0–0.6?mol% are prepared by means of a conventional solid-state reaction method. Phase composition, microstructure and element distribution are studied by use of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) separately. The influence of sintering temperature and Ho₂O₃ on the properties of samples is explored. The results show that the breakdown voltage decreases continuously while both the nonlinear coefficient and the relative dielectric constant ascend firstly and then descend with the sintering temperature increasing. Meanwhile, the relative dielectric constant and nonlinear coefficient of samples firstly ascend and then descend with the increasing of Ho₂O₃. Although the minimum breakdown voltage (3.3?V/mm) is obtained when sample is sintered at 1450?°C, the sample doped with 0.45?mol% Ho₂O₃ sintered at 1400?°C exhibits high comprehensive electrical properties, with breakdown voltage of 6?V/mm, the nonlinear coefficient of 5.5 and the relative dielectric constant of 1.88?×?10⁵.     

Densification behavior of mullite-Al2TiO5 composites by reaction sintering of natural andalusite and TiO2

In this work, mullite-Al₂TiO₅ composites were fabricated by natural andalusite with TiO₂ as an additive. The densification characteristic, phase composition and mullitization process of andalusite with TiO₂ addition was investigated by the Archimedes’ principle, dilatometry, X-ray diffraction and scanning electron microscopy (SEM-EDS) techniques. The results showed that the incorporation of TiO₂ not only enhanced the thermal stability of in-situ Al₂TiO₅ in the silica liquid yielded from the mullitization of andalusite, but also accelerated andalusite decomposition and retarded mullite formation, which facilitated the sintering and densification of mullite-Al₂TiO₅ composites.     

Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process

The mechanisms of aqueous wollastonite carbonation as a possible carbon dioxide sequestration process were investigated experimentally by systematic variation of the reaction temperature, CO₂ pressure, particle size, reaction time, liquid to solid ratio and agitation power. The carbonation reaction was observed to occur via the aqueous phase in two steps: (1) Ca leaching from the CaSiO₃ matrix and (2) CaCO₃ nucleation and growth. Leaching is hindered by a Ca-depleted silicate rim resulting from incongruent Ca-dissolution. Two temperature regimes were identified in the overall carbonation process. At temperatures below an optimum reaction temperature, the overall reaction rate is probably limited by the leaching rate of Ca. At higher temperatures, nucleation and growth of calcium carbonate are probably limiting the conversion, due to a reduced (bi)carbonate activity. The mechanisms for the aqueous carbonation of wollastonite were shown to be similar to those reported previously for an industrial residue and a Mg-silicate. The carbonation of wollastonite proceeds rapidly relative to Mg-silicates, with a maximum conversion in 15 min of 70% at , 20 bar CO₂ partial pressure and particle size of . The obtained insight in the reaction mechanisms enables the energetic and economic assessment of CO₂ sequestration by wollastonite carbonation, which forms an essential next step in its further development.     

Effect of TiO2 on sinterability and physical properties of pressureless sintered Ti3AlC2 ceramics

TiO₂ was selected as effective sintering aid for pressureless sintering of Ti₃AlC₂ ceramics in this study. The addition of only 5?wt% TiO₂ largely promotes the densification and nearly dense Ti₃AlC₂ ceramic was obtained by pressureless sintering at 1500?°C. Significant strengthening and toughening effects were observed with the addition of TiO₂. High Vickers hardness, flexural strength and fracture toughness of 3.22?GPa, 298?MPa and 6.2?MPa?m^?1/2, respectively, were achieved in specimen pressureless sintered with 10?wt% TiO₂. Additionally, the addition of 5?wt% TiO₂ had no deleterious effect on the excellent oxidation resistance of Ti₃AlC₂ ceramic under 1200?°C water vapor atmosphere, while addition of 10?wt% TiO₂ accelerates the oxidation rate by two orders of degree.     

Kinetic modeling of tungsten silicide chemical vapor deposition from WF6 and Si2H6: Determination of the reaction scheme and the gas-phase reaction rates

Kinetic studies were carried out on tungsten silicide (WSi_x) chemical vapor deposition from WF₆/Si₂H₆ in a hot-wall type tubular reactor, focusing on the axial distributions of step coverage and composition ratio. The growth-rate profile within the tubular reactor showed exponential decay, which suggests first-order reaction kinetics. The silicon content of the film increased downstream in the reactor, but the step coverage quality was independent of axial position. The reactive sticking probabilities obtained from the step coverage profile were about 0.33. Two reaction models were investigated to explain these experimental results. The first model is a parallel deposition model, in which W-containing species and Si-containing species are depositing separately. The other model is a consecutive reaction model, in which W-species and Si-species react in the gas phase to form intermediate species containing W and Si. Considering these plausible kinetic mechanisms, consecutive reactions in the gas phase to form W and Si containing species were found to be the controlling factor in the WF₆/Si₂H₆ process. These gas-phase homogeneous reaction-rate constants of the WF₆/Si₂H₆ process were obtained from axial distribution of film composition ratio based on this kinetic model, and the activation energy is about 25 kJ/mol.     

Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

Industrial fluidised bed direct reduction kinetics of hematite ore fines in H2 rich gases at elevated pressure

Reduction kinetics of iron ore fines were investigated under fluidised bed conditions with hydrogen-rich gas mixtures in the temperature range from 400 to 700 °C at a pressure of 10 bar. Kinetic parameters were determined for the reactions hematite to magnetite, magnetite to wuestite and wuestite to iron from experimental data. To be able to investigate the influence of both temperature and reducing gas composition on the reaction rate, a laboratory scale pressurised fluidised bed reactor was used which is able to apply process conditions close to those in industrial plants. Appropriate to heterogeneous reactions, a rate expression considering the specific surface area was derived. The time dependent function for the transient evolution of specific surface area was generated from measured mercury porosimetry data. Kinetic parameters such as frequency factor, energy of activation and reaction orders were determined for the consecutive reactions from hematite to magnetite, magnetite to wuestite and wuestite to iron by numerical integration and parameter estimation from measured rate data. A comparison to the evaluation of kinetic parameters without considering specific surface area confirm the results of this work.     

Low temperature preparation of nanocrystalline SrTiO3 and BaTiO3 from alkaline earth nitrates and TiO2 nanocrystals

Using low-melting-point Sr(NO₃)₂ (melting point: 570 °C) and high-reactive-activity TiO₂ nanocrystals (Degussa P25 TiO₂) as the raw materials, phase-pure SrTiO₃ nanocrystals were prepared at 600 °C. X-ray diffraction and X-ray photoelectron spectroscopy revealed that the as-prepared products were cubic phase SrTiO₃ with a surface composition of Sr_0.97TiO_2.92. Transmission electron microscopy image showed that the as-prepared SrTiO₃ comprised nanocrystals with the size of about 24-44 nm. UV-vis absorption spectrum of the as-prepared SrTiO₃ nanocrystals displayed a wide absorption peak centered at around 365 nm (3.4 eV), together with a tail at the lower energy side. This kind of low temperature and cost-effective method can also be extended to prepare BaTiO₃ nanocrystals, simply by substituting Ba(NO₃)₂ for Sr(NO₃)₂.     

Characterization of doped TiO2 nanoparticle dispersions

Nanomaterial suspensions with different dopant types and compositions were investigated to examine their effects on agglomeration through the measurement of hydrodynamic diameter (HD), surface charge, and isoelectric point (IEP). Four different types of nanoparticles, all synthesized by a flame aerosol reactor, were considered for the analysis. The nanoparticles considered were pristine TiO₂, Cu–TiO₂, V–TiO₂, and Pt–TiO₂ with dopant concentrations ranging from 1 to 6 wt%. Measurements were conducted over a broad range of pH (3–11) and ionic strengths (0.001–0.1 M) to understand the roles of pH and ionic strength (IS) on dispersion characteristics. Calculations were made using the classical DLVO theory to explain the agglomeration behavior. The results indicate that dopant addition can change surface charge, hydrodynamic diameter, and shift the IEP to higher or lower pH than pristine TiO₂, depending on the type of dopant and composition. Vanadium and platinum doping shifted the IEP to lower pH values, whereas copper doping shifted it to higher pH values. For each of the nanoparticles considered, pH and IS were found to have significant effects on the surface charge and HD, which were also verified by calculation from DLVO theory.     

Effects of sintering temperature and KBT content on microstructure and electrical properties of (Bi.5Na.5)TiO3-BaTiO3-(Bi.5K.5)TiO3 Pb-free ceramics

A route exploring the morphotropic phase boundaries (MPB) region in (Bi_.5Na_.5)TiO₃-BaTiO₃-(Bi_.5K_.5)TiO₃ ternary system has been designed based on the phase diagram. X-ray diffraction (XRD) has been performed to determine the phases of the prepared samples. The dielectric, ferroelectric, and piezoelectric properties of [(1-x) 0.9363(Bi_.5Na_.5)TiO₃–0.0637BaTiO₃]-x(Bi_.5K_.5)TiO₃ (BNKBT100x) ternary lead-free piezoelectric ceramics are investigated as the functions of x and sintering temperature. When x was varied from 0 to 0.11, the BNKBT100x ceramics show single perovskite structure sintered at 1130–1210?°C. These ceramics show large dielectric permittivity, small dielectric loss, and diffused phase transition behavior. Well-defined ferroelectric polarization-electric field (P-E) hysteresis loop and relative large piezoelectric and electromechanical coefficients are also found in these ceramics. When increasing x, the electrical performances first increase, then decrease. The same rule is found when varying the sintering temperature. The optimized composition and sintering temperature are finally obtained.     

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.     

Correlation of compaction pressure, green density, pore size distribution and sintering temperature of a nano-crystalline 2Y-TZP-Al2O3 composite

Highly sinter-active nano crystalline composite powder of 2 mol% yttria doped tetragonal zirconia polycrystal (2Y-TZP) with 2 wt% alumina was synthesized by co-precipitation method. Crystallization temperature of the amorphous precursor powder, measured from simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) techniques was found to be ∼470 °C. The powder was calcined at different temperatures in the range of 700-1000 °C. XRD patterns of the calcined powders revealed the presence of a single tetragonal phase. Particle size of the calcined powder measured by different techniques (X-ray line broadening, BET surface area and laser scattering technique) indicated an increase in the average particle size with calcination temperature. The study of compaction behaviour revealed the presence of soft agglomerates in the calcined powder. Pore size distribution of the green compacts obtained from a mercury porosimeter was found to be monomodal above a critical pressure. The onset temperature of sintering was found to increase with calcination temperature. Powders calcined at 800 °C and 900 °C had shown better sinterability at 1200 °C owing to the presence of finer pores with a narrow size distribution in the green compacts. Sintering behaviour of the powder calcined at 700 °C was found to be marginally poorer in comparison to the other samples, whereas the powder calcined at 800 °C had demonstrated best densification behaviour, especially when compacted at 300 MPa.     

Nanocrystalline TiO2 on single walled carbon nanotube arrays: Towards the assembly of organized C/TiO2 nanosystems

Arrays of single walled carbon nanotube bundles organized following different architectures have been coated by a homogeneous deposit of nanocrystalline titania. The nanotubes were grown treating nanosized C powders with atomic H in a purpose-designed chemical vapor deposition (CVD) reactor, the subsequent TiO₂ deposition was performed at 400 °C using the metal-organic CVD (MOCVD) technique and titanium tetraisopropoxide Ti(OⁱPr)₄ as a precursor. X-ray diffraction and Raman spectroscopy evidence the anatase structure of the TiO₂ coatings, formed by grains with an average size of about 55 nm. The structural and compositional characteristics of the TiO₂ deposits are not sensitive to the organization of the nanotube arrays, which maintain their pristine architectures. The adopted synthetic procedure opens a new route for the immobilization of anatase-type TiO₂ nanocrystallites onto geometrically varied structures and for the integration of composite nanotube/TiO₂ systems in effective devices.     

Effect of particle diameter on the permeability of polypropylene/SiO2 nanocomposites

In this contribution the permeation behaviour of polypropylene composites containing silica nanospheres of different diameters is reported. The composite materials were produced by melt compounding polypropylene with silica nanospheres, with diameters ranging from 12 to 150 nm, produced using standard sol-gel methods. The permeability of O₂, N₂ and H₂O through the materials was tested. The results indicate that the addition of nanospheres with diameters of ≥30 nm results in an increase in the O₂ and N₂ permeability and is directly proportional to the nanosphere diameter. In the case of the water vapour transmission rate, it was found that there is also an increase, but the effect is inversely related with the nanospheres diameter. Incorporation of spheres with a diameter of 12 nm produces a decrease in the permeability of polypropylene composites in all instances, due to its formation of aggregates in composites.     

AFM investigation of solid product layers of MgSO4 generated on MgO surfaces for the reaction of MgO with SO2 and O2

The nucleation, growth and arrangement of MgSO₄ product layers during the reaction of single-crystal MgO with SO₂ and O₂ were investigated via atomic force microscopy (AFM). The morphology of the single-crystal MgO surface after reacting with SO₂ and O₂ consisted of three-dimensional cone-shaped MgSO₄ product islands. Most of the islands appeared at locations with terraces, steps and kinks. With increasing reaction time, small product islands grew to large islands, finally coalescing into continuous islands. The nucleation, growth and arrangement of the solid products on the reactant surface are controlled by thermodynamics and kinetics. The morphology of the MgSO₄ product islands is governed by the competition between the intrinsic chemical reaction rate and the product molecular diffusion rate, which are both significantly affected by the reaction temperature. As the reaction temperature increases, the mean size of the MgSO₄ islands increases while the island density decreases. The nucleation, growth and arrangement of the solid product layers and the product layer morphology on the solid reactant surface directly affect the mechanism of the gas–solid reaction.     

Effect of nano TiO2 particles on microhardness and microstructural behavior of AA7068 metal matrix composites

Present work deals with the fabrication of AA7068 composites reinforced with different percentages of Nano TiO₂ (3, 6 and 9?wt%) using powder metallurgy technique along with as sintered AA7068 material. The pressure applied for compacting the composites was 318?MPa and sintered at 560?°C in a sintering furnace for one hour. Vickers micro hardness test have been conducted for finding the microhardness. Optical Microscopy was carried out to analyze the microstructural behavior. The worn surface was investigated by the Field Emission Scanning Electron Microscope (FESEM) after the wear test. The microhardness was found to increase by increasing the weight percentage of TiO₂ particles. It was observed to be a maximum of 68 VHN by adding 9% Nano TiO₂ particles. Abrasion, oxidation and delamination were the dominant wear mechanisms present in the composites. Energy Dispersive Spectroscopy analysis was done to confirm the presence of the TiO₂ particles.     

Vapor phase oxidation of dimethyl sulfide with ozone over V2O5/TiO2 catalyst

The removal of volatile and odorous emissions from pulp and paper industrial processes usually generates secondary pollution which is treated further by scrubbing, adsorption, and catalytic incineration. Studies using a flow reactor packed with 10% vanadia/titania (V₂O₅/TiO₂) catalyst showed complete conversion of dimethyl sulfide (DMS) in the presence of ozone. The molar yields of partial oxidation products were only 10–20%. Small amounts of partial oxidation products, such as and dimethyl sulfone (DMSO₂), dimethyl disulfide (DMDS), and dimethyl sulfoxide (DMSO), were also formed. The results of the oxidation of DMS using ozone only, ozone plus catalyst, and oxygen plus catalyst suggest that the combined use of O₃ with catalyst is essential for the complete destruction of DMS to CO₂ and SO₂. A Box-Behnken design was used to determine the factors that have a significant effect on the conversion and selectivity of the products. It was concluded that product selectivity is strongly influenced by temperature, gas hourly space velocity (GHSV), and ozone concentration. The catalysts were characterized using XRD, surface area measurements, and SEM techniques. Time-on-stream studies carried out in a 500 ppmv gas stream held at 150 °C for 6 h, using 2 g of the catalyst, an ozone-to-DMS molar ratio of 0.9, and a GHSV of 37,000 h⁻¹, yielded 99.9% conversion of DMS. A plausible reaction mechanism has been proposed for the oxidation of DMS based on reaction product distribution and possible intermediates formed.     

Dopant-free, polymorphic design of TiO2 nanocrystals by flame aerosol synthesis

A dopant-free aerosol synthesis of highly crystalline TiO₂ nanoparticles (20–35 nm) with tunable polymorphic content is demonstrated by rapid flame spray pyrolysis. By controlling precisely the total ambient oxygen partial pressure of the combustion in a quartz tube enclosure, anatase content as high as 96 wt% (4 wt% rutile) was obtained at high oxic flame conditions, while rutile content as high as 94 wt% (6 wt% anatase) was obtained under anoxic flames. The polymorphic variability lies within a narrow range of combustion equivalence ratios, that is, 1.0<Φ<1.5. Unlike any other flame aerosol syntheses, the anatase and rutile crystallite sizes were similar within each sample. Under highly oxic flame conditions (Φ<1.0), twinnings between anatase {0 1 1} planes could be observed, inferring oriented attachment taking place. Such mechanism could not, however, be seen under anoxic flame (Φ>1.0) possibly due to physical hindrance by surface carbonaceous content (typically <2 wt%). The carbon content can be easily removed by short calcination without significantly affecting the surface areas and crystallite properties of the original TiO₂ nanocrystals, preserving hence its pristine state.     

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.