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.     

Properties and microstructural aspects of TiO2‐doped sintered Alumina‐Zirconia composite ceramics

The effect of titania content on the densification, the phase transformation, the microstructures, and mechanical properties of 50 wt% Al₂O₃‐50 wt% ZrO₂ (12 mol% CeO₂) was evaluated. Ceramic composites with different TiO₂ content (0.27, 5, 10 wt%) were prepared by pressureless sintering at low temperature (1400°C) for 2 hours in air. Dense ceramic was obtained by adding 5 wt% of TiO₂ loading to improved mechanical properties. The microstructure analysis provided lots of information about solid‐state reactivity in alumina‐zirconia‐titania ternary system. The content of TiO₂ strongly affected the phases evolution and the grain growth during sintering. Furthermore, a significant effect on mechanical properties and fracture behavior was also observed.     

Characterization of 3Y-TZP/TiO2 hybrid experimental dental ceramics

The addition of titania to zirconia dental implants has been considered a promising choice to improve its bioactivity. This study aimed to evaluate the effect of different sintering conditions on the microstructure, density, optical properties and flexural strength of a 3Y-TZP/TiO₂ dental ceramic based on zirconia with two different titania contents (7.5 mol% and 12.5 mol%). 3Y-TZP/TiO₂ ceramic powders were synthesized by coprecipitation, uniaxially pressed and sintered at six different sintering conditions. Microstructural analysis of the sintered samples was performed by scanning electron microscopy and X-ray diffraction. Optical properties were measured using a spectrophotometer. The density was determined by Archimedes principle. Flexural strength was estimated by the biaxial flexure device. The microstructure and flexural strength of the 3Y-TZP/TiO₂ dental ceramic with 7.5% and 12.5 mol% were affected by the sintering conditions. Sintering the specimens at 1460 °C for 2 h increased the grain size and significantly decreased the flexural strength of 3Y-TZP/TiO₂ dental ceramic. The interaction (titania content x sintering conditions) affected the relative density and optical properties. A relative density greater than 98% was achieved for the T7.5 groups (sintered at 1260 °C/1 h, 1300 °C/1 h and 1300 °C/2 h) and for the T12.5 groups (sintered at 1260 °C/1 h, 1260 °C/4 h, 1300 °C/1 h and 1300 °C/2 h). The highest values of L*, a* and b* were respectively 87.2 (T7.5 group sintered at 1460 °C/2hs), 4.3 (T12.5 group sintered at 1300 °C/2hs) and 15.8 (T12.5 group sintered at 1300 °C/1 h). The material developed with 12.5 mol% of titania and sintered at 1300 °C/2 h showed high densification, flexural strength of 670 MPa and has good potential to be used in dentistry.     

Low temperature hydrothermal synthesis of monodispersed flower-like titanate nanosheets

Monodispersed flower-like titanate superstructure was successfully prepared by simple hydrothermal process without any surfactant or template. N₂-sorption analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD) observation of as-synthesized product revealed the formation of flower-like titanate with diameter of about 250–450 nm and BET surface area (S_BET) of 350.7 m² g^?1. Upon thermal treatment at 500 °C, the titanate nanosheets were converted into anatase TiO₂ with moderate deformation of their structures. The as-prepared flower-like titanate showed high photocatalytic activity for H₂ evolution from water splitting reaction. Moreover, the sample heat treated at 500 °C exhibited higher photocatalytic activity than that of commercial TiO₂ anatase powder (ST-01).     

Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength

Coarse alumina powder compacts doped with various amounts of titania and copper oxide were pressurelessly sintered from 900 °C to 1600 °C. Their phase assemblages and microstructural evolution, as well as their properties, were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry/thermogravimetric (DSC/TG) analysis, and three-point bending and wetting test. The role of TiO₂ and CuO during the sintering is discussed in detail. The experimental results show that the liquid phase from the copper oxide appeared at approximately 1200 °C, so the solid-state reaction between alumina and titania took place at a lower temperature. Such solid state-reaction sintering had a strong impact on the grain growth and greatly promoted the densification of the alumina compact. In addition, the liquid phase inhibited the abnormal grain growth and microcracking. As a result, the coarse alumina powder compacts doped with 5 wt% TiO₂–CuO were fully densified and exhibited sufficient flexural strength (342±21 MPa) when sintered at a temperature of 1450 °C for 2 h.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

Formation of monolithic SrTiO3-TiO2 ceramic heterostructures by reactive hydrothermal sintering

In a one-pot approach, monolithic SrTiO₃-TiO₂ ceramic heterostructures were obtained using the reactive hydrothermal liquid phase densification (rHLPD). Structural, morphological, and photocatalytic properties of the obtained ceramics were analyzed. The relative density of the formed components reached about 80% with reaction time, temperature, and NaOH concentration variation. It was observed via Rietveld refinement that there was no XRD detectable phase other than TiO₂ and SrTiO₃ in the final structure. The monolithic SrTiO₃-TiO₂ ceramics obtained by hydrothermal reaction at 120 °C for 24 h in 1 M NaOH concentration showed a dielectric constant being around 500, and the dielectric loss was below 0.25 at frequencies higher than 10 kHz. The SrTiO₃-TiO₂ heterostructured monoliths having only 20 vol% total porosity and low specific surface area, demonstrated ∼60% efficiency (in 5 h) in degrading Methylene Blue photo-catalytically.     

Hydrothermal synthesis and formation mechanism of tetragonal barium titanate in a highly concentrated alkaline solution

Tetragonal cube-shaped barium titanate (BaTiO₃) was produced by the hydrothermal treatment of a peroxo-hydroxide precursor, a single-source amorphous barium titanate precursor, in a highly concentrated sodium hydroxide solution. Phase pure barium titanate with cube-shaped morphology and particle-sizes in the 0.2–0.5 µm range were formed at temperatures above 80 °C. Also, the cube-shaped morphology of the BaTiO₃ product was preceded by spherical- and plate-like morphologies with, respectively, a Ti-excess and Ba-excess. Coinciding with these morphological observations, changes in the reaction product were also observed. The formation of crystalline BaTiO₃ proceeded alongside secondary BaTi₂O₅ and Ba₂TiO₄ phases. These secondary phases disappeared as the reaction time was increased leaving only BaTiO₃ as the sole reaction product. Kinetic analysis of the formation of hydrothermal BaTiO₃ crystallization by the Johnson-Mehl-Avrami method showed that BaTiO₃ crystallization is a homogeneous dissolution-precipitation reaction. The mechanism is governed by nucleation and growth in the beginning of the reaction and dissolution-precipitation dominating throughout the hydrothermal reaction process.     

Thermal coarsening of individual titanate nanowires and their assemblies: Surface vs. bulk diffusion

The thermal coarsening of titanate nanowires was investigated from room temperature to 1000 °C by AFM, SEM, XRD and Raman spectroscopy. Phase transformation kinetics and the external morphology were studied in two configurations: i) as individual and suspended titanate nanowires on SiO₂ surface (denoted as 2D geometry); and ii) as large assembly in form of sintered pellets (denoted as 3D geometry). The individual titanates nanowires were coarsened far below the melting temperature (1843 °C) of TiO₂ and two temperature dependent geometrical transformation changes were identified. In the first one, the height decreases around 200 °C (~20% shrinkage) due to the dehydration of the layered titanate, followed by the titanate to anatase recrystallization until 600 °C. Interestingly, the surface area of the SiO₂ supported high temperature stabilized individual nanowires increased or remained constant, which may be due to the topotactic effects and epitaxial strain stabilization. In the second one, at sintering temperature of 600–1000 °C, an intense nanowire coarsening was observed driven by a surface diffusion mechanism on substrate and a surprising stabilization of the anatase TiO₂. In the case of a 3D assembly of the nanowires the densification with significant particle coarsening is accompanied by the phase transformation from anatase to rutile around the known phase transition temperature. These results suggest that the surface area evolution of individual titanium oxide nanowires on SiO₂ appreciably differs from the 3D assembly, and the surfaces may induce an extra stabilization effect and deviation from the classical Ostwald ripening surface diffusion model.     

Phase development of barium titanate from chemically modified-amorphous titanium (hydrous) oxide precursor

A synthesis procedure for barium titanate involving a chemically modified titanium precursor has been developed. Using a titanium isopropoxide precursor modified with acetylacetone and barium acetate, coprecipitated gels were obtained by addition to a KOH solution. Direct precipitation of cubic BaTiO₃ from such precursor suspensions was obtained under hydrothermal conditions. The pH value was found to be a critical reaction parameter such that production of phase pure BaTiO₃ required high pH (>13.0), a finding consistent with thermodynamic predictions of the Ba–Ti–H₂O stability system and prior hydrothermal syntheses. It was determined that phase-pure barium titanate can be synthesized at temperatures as low as 50 °C and that higher reaction temperatures accelerate the crystallization process. The particle size of the synthesized powder ranged from 50 to 350 nm for the synthesis conditions explored in the current work.. It was demonstrated that particle size can be controlled by proper selection of the hydrothermal synthesis conditions such as reaction concentration, temperature, and time. The chemically modified synthesis produces barium titanate more rapidly at lower reaction temperatures than previously reported for similar syntheses.     

Microstructure and mechanical properties of Al2O3–20 wt%Al2TiO5 composite prepared from alumina and titania nanopowders

In the present work, Al₂O₃–20 wt%Al₂TiO₅ composite was prepared from reaction sintering of alumina and titania nanopowders. The nano-sized raw powders were reconstituted into nanostructured particles by ball milling. Then, the nanostructured reconstituted powders were pressed and pressureless-sintered into bulk ceramics at 1300, 1400, 1500 °C for 2 h. The phase composition and microstructures of reconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope and energy-dispersive spectrometer (EDS). The microstructural analysis of the ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increasing the temperature. Also, SEM proved the existence of a proper interface between Al₂TiO₅ and Al₂O₃ grains and preferential distribution of aluminium titanate particles in the grain boundaries. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. The hardness of the samples sintered at 1300, 1400, 1500 °C were 4.8, 6.2 and 8.5 GPa, respectively.     

Photocatalytic Nb2O5-doped TiO2 nanoparticles for glazed ceramic tiles

TiO₂ nanoparticles are currently used as coating for self-cleaning building products. In order to achieve high self-cleaning efficiency for outdoor applications, it is important that titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in nano-range, so that a large enough surface area is available for enhanced catalytic performance. In this work, TiO₂ nanoparticles doped with 0–5 mol% Nb₂O₅ were synthesized by co-precipitation. Nb₂O₅ postponed the anatase to rutile transformation of TiO₂ by about 200 °C, such that after calcination at 700 °C, no rutile was detected for 5 mol% Nb₂O₅-doped TiO₂, while undoped TiO₂ presented 90 wt% of the rutile phase. A systematic decreasing on crystallite size and increasing on specific surface area of TiO₂ were observed with higher concentration of Nb₂O₅ dopant. Photocatalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under ultraviolet and daylight illumination and compared to commercial standard catalyst (P25). The results showed enhanced catalysis under UV and visible light for Nb₂O₅-doped TiO₂ as compared to pure TiO₂. In addition, 5 mol% Nb₂O₅-doped TiO₂ presented higher photocatalytic activity than P25 under visible light. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap.     

Selective Hydrogenation of Cinnamaldehyde over Cobalt Supported on Alumina, Silica and Titania

Abstract  

The liquid phase selective hydrogenation of cinnamaldehyde has been investigated on cobalt (15 wt%) impregnated on alumina, silica and various phases of titania supports. The multiple reduction stages observed in the TPR studies suggest the presence of cobalt aluminate/silicate/titanate species, and DRIFT spectra results seem to corroborate this observation. An optimum level of conversion and selectivity to cinnamyl alcohol was observed at 120 °C and 10 kg/cm² hydrogen pressure. Co/TiO₂ exhibited a greater conversion (47.4%) and selectivity to cinnamyl alcohol (58%) than Co/Al₂O₃ and Co/SiO_2, which may be attributed to the presence of TiO _x (x < 2) species on the catalyst surface and to the preferential adsorption of C=O on the catalyst surface. The stability of Co/TiO₂ was found to be better than Co/Al₂O₃ and Co/SiO₂. Between the various phases of titania (high surface area, anatase and rutile), the crystalline phases exhibited a better conversion and selectivity to cinnamyl alcohol, while the stability was found to be better for high surface area titania.     

Low-temperature densification of Mg-doped hydroxyapatite fine powders under hydrothermal hot processing conditions

Densification of calcium hydroxyapatite fine powders doped with different concentrations of Mg (2, 4 and 6 mol% Mg, MgHA) was successfully achieved for the first time in a nearly fully dense state using the hydrothermal hot pressing (HHP) technique at low temperatures. Consolidation of MgHA powders was studied under different temperatures (150–240 °C), reaction times (1–6 h), and powder particle size (20 nm–1.5 µm). X-Ray diffraction analyses indicated that the particle densification under HHP conditions proceeded without any variation in the crystalline structure and regardless of the Mg content. The results from this work showed that an increase in temperature accelerates the reaction between MgHA particles and water (solvent) mixed during the hydrothermal treatment. Particle packing associated with bulk densification was achieved through a massive dissolution-recrystallisation mechanism, which induced the formation of small particles that rapidly crystallised on the surface of the partially dissolved original MgHA particles. The optimum conditions to obtain pellets with a high apparent density of 3.0758 ± 0.001 g/cm³ and tensile strength value of 12.6 ± 0.6 MPa were 10 wt% of water at a temperature of 240 °C with a 6 h reaction time and 6 mol% of Mg (MgHA3). The use of the HHP technique coupled with the fine particle size and reactivity of the MgHA precursor powders with water allowed us to produce disks that were compacted to a nearly full dense state with a low content of open porosity of 2.0%.     

PZT-cobalt ferrite particulate composites: Densification and lead loss controlled by quite-fast sintering

During densification at 1100–1200 °C of particulate lead zirconate titanate (PZT)/cobalt ferrite (CF, 26–81  mol%) composites, side reactions do occur that are detrimental to the properties of the so-obtained material. Such reactions are promoted by initial PbO loss, the extent of which can be determined by means of XRD analysis of the densified samples taking into account the amount of ZrO₂ and the variations of the perovskite’s tetragonality. In this process, titania is produced which reacts with CF to form cobalt titanate. Microstructural characterization showed that CF grain size distribution can be mono- or bi-modal, and CF overgrowth was found to affect the coercivity of the material. In the case of the PZT:CF 74:26 composites, full densification and prevention of unwanted side reactions were achieved by designing a quite-fast sintering process. The high coercivity (789 Oe) displayed by these composites is related to the good dispersion of 250 nm euhedral CF grains in the PZT matrix and limited PZT grain growth (240 nm).     

Impact of synthesis route on the microstructure and energy-storage properties of (Bi1/2K1/2)TiO3–SrTiO3 relaxor ferroelectric ceramics: Solid-state and hydrothermal approaches

Bismuth potassium titanate (Bi_1/2K_1/2)TiO₃-based relaxor ferroelectrics are promising materials for high-energy-density ceramic capacitors. Herein, we compare the microstructure and energy-storage properties of (Bi_1/2K_1/2)_0.5Sr_0.5TiO₃ (BKST50) ceramics fabricated via two different routes: solid-state and hydrothermal reactions. A BKST50 fine powder composed of well-dispersed cubic nanoparticles was obtained via the hydrothermal reaction, whereas the conventional solid-state reaction resulted in the aggregation of primary particles. The grain size of the ceramics prepared from the hydrothermal powder could be controlled between 273 ± 24 and 936 ± 69 nm while maintaining a relative density of over 95% by simply varying the sintering temperature. On the other hand, ceramics prepared via the solid-state reaction could not be fully densified even at 1200 °C (the highest tested sintering temperature). The hydrothermally derived ceramics withstood higher electric field owing to dense and fine-grained microstructure, leading to a high recoverable energy-storage density of 2.25 J cm⁻³ at 240 kV cm⁻¹.     

Aluminum titanate based ceramics from aluminum sludge waste

This work aims at obtaining aluminum titanate-based ceramics (Al₂TiO₅: AT) composites from industrial wastes. Al-sludge waste and rutile ore were used as rich sources of alumina and titania instead of pure materials. Sludge-(0–40 wt%) rutile mixtures were mixed, formed and fired at 1350 °C for various times. Phase composition, microstructure, densification, mechanical and thermal behaviors of the obtained AT composites have been investigated. Complete conversion of the starting materials to AT with bulk density of 3.199 g/cm³, compressive strength and modulus of rupture of 326.425 MPa and 30.84 MPa, respectively and very low CTE (−0.927*10⁻⁶ K⁻¹) were achieved by firing the sludge-(30 wt%) rutile at 1350 °C for 4 h. These results suggest that the obtained AT-ceramics from Al-sludge waste-rutile ore are a promising and an ecofriendly route.     

Carbon nanotube/graphene oxide‐added CaO‐B2O3‐SiO2 glass/Al2O3 composite as substrate for chip‐type supercapacitor

A CaO‐B₂O₃‐SiO₂ (CBS) glass/40 wt% Al₂O₃ composite sintered at 900°C exhibited a dense microstructure with a low porosity of 0.21%. This composite contained Al₂O₃ and anorthite phases, but pure glass sintered at 900°C has small quantities of wollastonite and diopside phases. This composite was measured to have a high bending strength of 323 MPa and thermal conductivity of 3.75 W/(mK). The thermal conductivity increased when the composite was annealed at 850°C after sintering at 900°C, because of the increase in the amount of the anorthite phase. 0.25 wt% graphene oxide and 0.75 wt% multi‐wall carbon nanotubes were added to the CBS/40 wt% Al₂O₃ composite to further enhance the thermal conductivity and bending strength. The specimen sintered at 900°C and subsequently annealed at 850°C exhibited a large bending strength of 420 MPa and thermal conductivity of 5.51 W/(mK), indicating that it would be a highly effective substrate for a chip‐type supercapacitor.     

Sintering and microstructural study of mullite prepared from kaolinite and reactive alumina: Effect of MgO and TiO2

Mullite ceramic was prepared using kaolinite and synthesized alumina (combustion route) by solid-state interaction process. The influence of TiO₂ and MgO additives in phase formation, microstructural evolution, densification, and mechanical strengthening was evaluated in this work. TiO₂ and MgO were used as sintering additives. According to the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the raw materials, ie kaolinite, synthesized alumina, and different wt% of additives were wet mixed, dried, and uniaxially pressed followed by sintering at different temperature. 1600°C sintered samples from each batch exhibit enhanced properties. The 1 wt% TiO₂ addition shows bulk density up to 2.96 g/cm³ with a maximum strength of 156.3 MPa. The addition of MgO up to 1 wt% favored the growth of mullite by obtaining a density and strength matching with the batch containing 1 wt% TiO₂. These additives have shown a positive effect on mullite phase formation by reducing the temperature for complete mullitization by 100°C. Both additives promote sintering by liquid phase formation. However, the grain growth, compact microstructure, and larger elongated mullite crystals in MgO containing batch enhance its hardness properties.     

Effect of pressure on TiO2 crystallization kinetics using in‐situ high‐temperature synchrotron radiation diffraction

The phase transformation behavior of TiO₂ sol‐gel synthesized nanopowder heated in a sealed quartz capillary from room temperature to 800°C was studied using in‐situ synchrotron radiation diffraction (SRD). Sealing of the capillary resulted in an increase in capillary gas pressure with temperature. The pressures inside the sealed capillary were calculated using Gay‐Lussac's Law, and they reached 0.36 MPa at 800°C. The as‐synthesized material was entirely amorphous at room temperature, with crystalline anatase first appearing by 200°C (24 wt% absolute), then increasing rapidly in concentration to 89 wt% by 300°C and then increasing more slowly to 97 wt% by 800°C, with there being no indication of the anatase‐to‐rutile transformation up to 800°C. The best estimate of activation energy for the amorphous‐to‐anatase transformation from the SRD data was 10(2) kJ/mol, which is much lower than that observed when heating the material under atmospheric pressure in a laboratory XRD experiment, 38(5) kJ/mol. For the experiment under atmospheric pressure, the anatase crystallization temperature was delayed by ~200°C, first appearing after heating the sample to 400°C, after which crystalline rutile was first observed after heating to 600°C. The estimated activation energy for the anatase‐to‐rutile transformation was 120(18) kJ/mol, which agrees with estimates for titania nanofibers heated under atmospheric pressure. Thus, heating the nanopowders material under pressure promoted the amorphous‐to‐anatase transformation, but retarded the anatase‐to‐rutile transformation. This behavior is believed to occur in an oxygen‐rich environment and interstitial titanium is also expected to form when the material is heated under high gas pressure. This suggests that atmospheric oxygen appears to accelerate the amorphous‐to‐anatase transformation, whereas interstitial titanium inhibits TiO₂ structure relaxation, which is required for the anatase‐to‐rutile transformation.