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.     

Hydrothermal synthesis of carbonate-free submicron-sized barium titanate from an amorphous precursor: Synthesis and characterization

In this paper, the amorphous barium titanate precursor was prepared by the peroxo-hydroxide method and post-treated by various drying procedures, such as: room temperature drying, room temperature vacuum drying and vacuum drying at 50 °C. The objective in the latter two treatments was to increase the Ti-O-Ba bonds of the precursor. The post-treated precursors were compared with the untreated (i.e., ‘wet’) precursor. Also, a barium titanate precursor was prepared by an alkoxide route. Afterwards, the precursors were hydrothermally treated at 200 °C in a 10 M NaOH solution. Vacuum drying of the precursor seemingly promoted the formation of Ti-O-Ti bonds in the hydrothermal end-product. The low Ba:Ti ratio (0.66) of the alkoxide-route prepared precursor lead to a multi-phase hydrothermal product with BaTiO₃ as the main phase. In contrast, phase pure BaTiO₃, i.e. without BaCO₃ contamination, was obtained for the precursor which was dried at room temperature. Cube-shaped and highly crystalline BaTiO₃ particles were observed by electron microscopy for the hydrothermally treated peroxo-hydroxide-route prepared precursor.     

Low-temperature synthesis of superfine barium strontium titanate powder by the citrate method

Ba_0.6Sr_0.4TiO₃ powder was synthesized by a citrate method. The phase development was examined with respect to calcining temperature and heating rate during the calcining process. The results reveal a crucial role of the heating rate to the formation of a pure perovskite phase at low calcining temperatures. It was found that keeping relatively low heating rates ≤0.7 °C/min during the calcining process after 300 °C was favorable to a sufficient decomposition of (Ba,Sr)₂Ti₂O₅·CO₃ intermediate phase at low temperatures and consequently led to the formation of a pure perovskite phase at 550 °C. Ba_0.6Sr_0.4TiO₃ powder calcined at the temperature under the heating rate of 0.7 °C/min showed a superfine and uniform particle morphology and high sintering reactivity. As a result, the ceramic specimens prepared from the powder attained reasonable relative densities (94–95%) at sintering temperatures of 1250–1270 °C.     

Hydrothermal synthesis of hierarchical hydroxyapatite: Preparation, growth mechanism and drug release property

Hydroxyapatite (HAP) hierarchical microspheres were synthesized by a facile hydrothermal method using calcium nitrate and ammonium dihydrogen phosphate through controlling complexing agents. The influences of two kinds of complexing agents (potassium sodium tartrate tetrahydrate and trisodium citrate) and reaction time on the morphology of HAP crystals have been investigated. These results indicate that complexing agents have a great influence on the morphology of HAP. When potassium sodium tartrate tetrahydrate was used as complexing agent, HAP flowers were composed of the network of nanosheet building blocks. Well-crystallized HAP dandelions with nanorods radiating from the center can be obtained by the introduction of trisodium citrate. Broader XRD diffraction peaks imply a nanometer scale size. Based on XRD and SEM results, the formation mechanism of HAP crystals has been discussed. The hierarchically structured HAP microspheres were explored as drug carriers. The results indicate that HAP flowers and dandelions showed a favorable sustained release property for ibuprofen; thus, they are very promising for application in drug delivery.     

Antimony doping effect on barium titanate structure and electrical properties

Nanopowders of pure and antimony doped barium titanate (BaTiO₃-BT) were synthesized by polymeric precursors method based on Pechini process. Obtained powders were pressed and sintered at 1300 °C for 8 h. XRD analysis showed the formation of cubic crystal structure in all nanopowders and tetragonal in BT ceramics. The influence of antimony concentration on structure change, grain size reduction and microstructure development was analyzed. Dielectric behavior of pure and antimony doped ceramics was studied as a function of temperature and frequency. The significant dielectric properties modification as a consequence of doping with different antimony concentration was noticed. The electrical resistivity measurements pointed out that antimony concentration influenced also on materials change from insulator to semiconductor.     

Hydrothermal synthesis of NaNbO3 with low NaOH concentration

NaNbO₃ fine powders were prepared by reacting niobium pentoxide with low NaOH concentration solution under hydrothermal conditions at 160 °C. The reaction ruptured the corner-sharing of NbO₆ octahedra in the reactant Nb₂O₅, yielding various niobates, and the structure and composition of the niobates depended on the [OH⁻] and reaction time. The fine Nb₂O₅ powder first aggregated to large particles and then turned to metastable intermediates with multifarious morphology. The reaction was fast for the situation of [OH⁻] = 2 M. The [OH⁻] determined the structure of final products, and three types of NaNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries were obtained, respectively, depending on the [OH⁻]. The low [OH⁻] was propitious to yield orthorhombic NaNbO₃. The present work demonstrated that higher [OH⁻] was not favored to synthesize NaNbO₃ powders and the conversion speed in this reaction was not in proportion to the [OH⁻].     

Highly efficient green synthesis of highly pure microporous nanosilica from silicomanganese slag

Highly pure nanosilica was synthesized through a facile hydrometallurgy-based method from silicomanganese slag as a low cost silica source. The synthesis route included short-term nitric acid dissolution at room temperature, gelation, washing, drying, and calcination steps. The experimental dissolution conditions resulted in a dissolution efficiency of 98%. The crystalline structure, chemical composition, chemical bonding, microstructure and elemental analyses, particle size distribution, and surface area of the extracted silica were then studied by appropriate characterization techniques. The characterization findings indicated that the amorphous silica particles had a microporous nature with an average particle size of ~24 nm, exhibiting a high purity of more than 99% and a high specific surface area of ~474 m² g^?1. The overall results indicated that the proposed synthesis route is a promising feasible alternative method to produce highly pure microporous nanosilica from a low cost secondary resource. The proposed method can treat 98% of the slag and uses less chemicals than conventional methods and is therefore a greener nanosilica production process. The current process also competes with the traditional process and other recently introduced processes in terms of process economy and the quality of the produced product.     

Growth mechanism and photocatalytic activity of chrysanthemum-like anatase TiO2 nanostructures

Chrysanthemum-like hierarchical anatase TiO₂ nanostructures self-assembled by nanorods have been successfully fabricated by a simple solvothermal route without using template materials or structure-directing additives. The products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectrometer system (Raman), UV–vis absorption spectroscopy (UV–vis) and N₂ adsorption–desorption measurement. The results indicate that synthesized chrysanthemum-like hierarchical anatase TiO₂ nanostructures have a spherical shape with an average diameter of 1.5 μm and they are composed of nanorods with a width of about 30 nm and a length of about 300 nm. The pore distribution of the sample exhibits two kinds of pores. Such mesoporous structure of the sample might be extremely useful in photocatalysis because they possess efficient transport pathways to the interior and supplies higher specific area for more pollutant molecules to be absorbed. In addition, the synthesized TiO₂ nanostructures show enhanced photocatalytic activity compared with commercial P25 for the degradation of RhB under UV light irradiation, which can be attributed to their special hierarchical structure and high light-harvesting capacity.     

Hydrothermal synthesis and mechanism of triangular prism-like monocrystalline CeO2 nanotubes via a facile template-free hydrothermal route

Monocrystalline CeO₂ tablet-like nanostructures and triangular prism-like nanotubes were synthesized by thermal conversion of cerium carbonate hydroxide (CeOHCO₃) precursors prepared by a simple template-free hydrothermal method using Ce(NO₃)₃·6H₂O as cerium source, CO(NH₂)₂ as both precipitator and carbon source and polyvinylpyrrolidone (PVP) as surfactant. X-ray diffractometer (XRD) images inferred that the as-synthesized Ce(CO₃)(OH) has a hexagonal structure, and the CeO₂ obtained by calcining the Ce(CO₃)(OH) at 500 °C for 5 h has a cubic fluorite structure. Scanning electron microscope (SEM) was employed to reveal the transformation from tablet-like to triangular prism-like structures, and then to triangular prism-like nanotubes with the increase of temperature from 120 up to 200 °C. Monocrystalline structure was revealed by high resolution transmission electron microscope (HRTEM) and select area electron diffraction (SAED) patterns. The thermal decomposition process of the as-synthesized Ce(CO₃)(OH) was investigated by thermo-gravimetric differential thermal analysis (TG–DTA) apparatus, and the possible formation mechanism of CeO₂ has been discussed. The spectral properties were characterized by Fourier transform infrared spectrum (FT-IR), Raman scattering, Photoluminescence (PL) spectra and UV–vis spectroscopy. There is a red-shifting in the band gap of the material compared to bulk one, which is mainly attributed to the influences of the Ce³⁺ ions, oxygen vacancies and the change of morphology.     

One-step hydrothermal synthesis of yttria-stabilized tetragonal zirconia polycrystalline nanopowders for blue-colored zirconia-cobalt aluminate spinel composite ceramics

A novel approach for the preparation of blue-color giving zirconia nanopowders by doping of 3?mol% Y₂O₃ through simple one-step hydrothermal process is proposed. A blue-color giving yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powders were prepared by urea-based solution, zirconium acetate, CoCl₂·6H₂O and AlCl₃ precursors in hydrothermal vessel at 24?h, 150?°C and 20 bars. Based on the results, the synthesized blue-color giving Y-TZP nanopowders have entirely tetragonal structure as mono phase with 3.8?±?0.2?nm average grain size and (Y, Co, Al)_xZrO₂; x?≤?0.03?at. with chemical composition. Thermal treatment was also applied to synthesized Y-TZP powders at 1200?°C and 1450?°C to observe the color evolution. Only sharp blue was obtained in Y-TZP powders resulting the development of zirconia-cobalt aluminate spinel (ZrO₂-CoAl₂O₄) composite ceramic structure for both temperatures after heat-treatment. Herein, not only formation of CoAl₂O₄ but also incorporation of cobalt (Co) and aluminum (Al) into the Y-TZP grains plays a critical role on evolving of blue color. This synthesized Y-TZP nanopowders can be a good candidate for one-step production of blue-color sintered ZrO₂-CoAl₂O₄ spinel composite ceramics in numerous ceramic applications due to their superior structural and functional properties.     

Structural and Raman spectroscopic studies of poled lead-free piezoelectric sodium bismuth titanate ceramics

Lead-free piezoelectric sodium bismuth titanate (NBT) ceramics are synthesized by a solid state reaction method. Poled NBT ceramics possess rhombohedral R3c structure. Vibrational phonon frequencies associated to Bi–O, Na–O, TiO₆ octahedra and oxygen vibrations are observed. Experimentally unreported lowest frequency region peak centered at 111 cm⁻¹ is assigned to E(TO1) mode related to Bi–O vibrations. Appearance of lowest frequency mode may be due to well-polished surfaces with reduced Rayleigh scattering background and well-defined ordering of Bi and Na ions in the lattice.     

Diethylene glycol mediated synthesis of Gd2O3:Eu3+ nanophosphor and its Judd–Ofelt analysis

Gd_1.9Eu_0.1O₃ nanophosphor with enhanced photoemission is successfully synthesized by a controlled combustion method of metal–citrate complex in diethylene glycol medium. It is found that the phase formation and spectral properties including quantum efficiency are highly sensitive to sintering temperature and the role of citric acid both as a chelating agent and as a fuel for combustion was evident in the study. The formation of cubic Gd₂O₃:Eu nanocrystalline phosphor was confirmed by X-ray diffraction, Raman and Infrared measurements. Transmission electron microscopic images together with the SAED patterns indicate the formation of spherical Gd₂O₃ nanocrystalline powder with well defined planes separated by 0.312 nm of cubic Gd₂O₃. The high degrees of homogeneity observed in the electron micrographs are attributed to the glycol medium used. The intensities of transitions between different J levels depend on the symmetry of the local environment of Eu³⁺ activators and are described using the Judd–Ofelt analysis. The spectral properties of the samples prepared under optimum conditions have shown large asymmetric ratio, indicating high degree of distortion in local environment around Eu³⁺ ions and a maximum quantum efficiency of 78% was obtained. The life-time measurements indicated an increase from 0.83 to 1.28 ms with increase in sintering temperature from 700 to 1000 °C. The chromaticity and correlated colour temperature of the emission were evaluated based on the 1931 CIE chromaticity diagram. The absorptions observed in the excitation spectra show the suitability of the nanophosphor obtained in this study for getting excited in UV, NUV and visible regions for a variety of device applications.     

Photoluminescence emission in zirconium-doped calcium copper titanate powders

Intense photoluminescence (PL) emission was observed in Zr-doped calcium copper titanate powders. The compounds were synthesized by a soft chemical method and heat treated at temperatures between 300 and 850 °C. The decomposition of the precursors was examined by X-ray diffraction, Fourier transform infrared, Fourier transform Raman, and ultraviolet–visible spectroscopies; as well as PL analysis. Here, we discuss the role of the structural ordering, which facilitates the self-trapping of electrons and charge transfer, and review the mechanism that triggers the PL. The most intense PL emission was obtained for the sample with 5% Zr calcined at 750 °C, which is neither highly disordered nor completely ordered at ~520 nm.     

Soft chemistry routes for the preparation of Ag-CoFe2O4 nanocomposites

Silver-cobalt ferrite nanocomposites (Ag-CoFe₂O₄) were synthesized through wet ferritization process and self-propagating combustion method. The structure, morphology, surface chemistry and magnetic properties of the nanocomposites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM). X-ray diffraction patterns confirmed the formation of CoFe₂O₄ and Ag nanoparticles with cubic symmetry. The average crystallite size of CoFe₂O₄ by wet ferritization ranged between 60 Å and 87 Å; for those obtained by self-propagating combustion was in the range 232–290 Å. SEM micrographs revealed different morphological features of nanocomposites. Ag-CoFe₂O₄ obtained by wet ferritization exhibited typical superparamagnetic behaviour. The antimicrobial and anti-biofilm properties of all silver-cobalt ferrites were evaluated. The results revealed that the Ag-CoFe₂O₄ nanocomposites exhibited good microbicidal and anti-biofilm features.     

Detonation synthesis of nanoscale silicon carbide from elemental silicon

Direct reaction of precursors with the products of detonation remains an underexplored area in the ever-growing body of detonation synthesis literature. This study demonstrated the synthesis of silicon carbide during detonation by reaction of elemental silicon with carbon products formed from detonation of RDX/TNT mixtures. Continuum scale simulation of the detonation showed that energy transfer by the detonation wave was completed within 2–9 μs depending on location of measurement within the detonating explosive charge. The simulated environment in the detonation product flow beyond the Chapman-Jouguet condition where pressure approaches 27 GPa and temperatures reach 3300 K was thermodynamically suitable for cubic silicon carbide formation. Carbon and added elemental silicon in the detonation products remained chemically reactive up to 500 ns after the detonation wave passage, which indicated that the carbon-containing products of detonation could participate in silicon carbide synthesis provided sufficient carbon-silicon interaction. Controlled detonation of an RDX/TNT charge loaded with 3.2 wt% elemental silicon conducted in argon environment lead to formation of ～3.1 wt% β-SiC in the condensed detonation products. Other condensed detonation products included primarily amorphous silica and carbon in addition to residual silicon. These results show that the energized detonation products of conventional high explosives can be used as precursors in detonation synthesis of ceramic nanomaterials.     

Solution combustion synthesis of bioceramic calcium phosphates by single and mixed fuels—A comparative study

Calcium phosphate based bioceramics have been synthesized by a modified combustion synthetic route using both citric acid and succinic acid separately and in mixture as fuels and nitrate and nitric acid as oxidants. Calcium nitrate and diammonium hydrogen phosphate were used as calcium and phosphate sources. The effects of citric acid to succinic acid ratio on the phase formation have been investigated. The precursors and the calcined products have been characterized by powder X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy. Succinic acid has been used as a fuel for the first time to synthesize hydroxyapatite.     

Effect of vibro-milling time on phase formation and particle size of barium titanate nanopowders

Barium titanate (BT) nanopowder was synthesized by a solid state reaction via a rapid vibro-milling technique. The effect of milling time on phase formation and particle size of BT powder was investigated. Powder samples were characterized using XRD (X-ray diffraction) and SEM techniques. It was found that the resulting BT powders have a range of particle size depending on milling times. Production of a single-phase BT nanopowder can be successfully achieved by employing a combination of 30 h milling time and calcination conditions of 1200 °C for 2 h.     

Molten salt synthesis of titanate pyrochlore waste-forms

Actinide chlorides, such as those arising from pyrochemical reprocessing operations can be problematic to immobilise, as the high chloride content often makes their solubilities in melts very low, and even in small quantities can seriously affect the properties of the waste-form. Rather than attempt to immobilise the chlorine, one potential approach is to utilise the chloride salt as a reaction medium from which the An(III) cations can be extracted and immobilised. To this end, lanthanide titanate pyrochlores have been prepared by molten salt synthesis in CaCl₂:MgCl₂, CaCl₂:NaCl and MgCl₂:NaCl eutectics. Single-phase pyrochlore is found to be formed at temperatures as low as 650 °C in the CaCl₂:NaCl system, whereas in the CaCl₂:MgCl₂ and MgCl₂:NaCl eutectics reaction with Mg produces a magnesium titanate secondary phase. Compositions of Yb₂Ti₂O₇ doped with Sm³⁺ as an actinide surrogate have been synthesised, and cold-pressing and sintering at 1500 °C yields fully dense pellets.     

Synthesis of β-TCP and CPP containing biphasic calcium phosphates by a robust technique

Biphasic calcium phosphate (BCP) compositions consisting of β-tricalcium phosphate (β-TCP) and calcium pyrophosphate (CPP) are potential biodegradable ceramics for bone regeneration. The present work demonstrates the formation of such dense ceramics by first preparing the precursors of nano-sized, amorphous, and equiaxed calcium pyrophosphate particles, and then sintering the precursors at 900 °C to transform them into desired BCP. However, if the complex of calcium tripolyphosphate was used, only CPP could be generated. It was also observed that the incorporation of Mg²⁺ had several effects on the resulting products including: (1) promoting the generation of meso-porous precipitates; (2) favoring the formation of β-TCP instead of CPP; (3) reducing the grain size and increasing the density of the sintered ceramics, and (4) enhancing the negative electric charge of the BCP surface. Thus, the as-prepared BCP ceramics can serve as potential bone substitute materials in orthopedic applications.     

Efficient synthesis of silver-reduced graphene oxide composites with prolonged antibacterial effects

In this work, we successfully synthesized the silver-reduced graphene oxide (Ag-rGO) composites based on the in situ growth of Ag nanoparticles (NPs) on the surfaces of the GO sheets through a one-pot solvothermal method without using any surfactants or modifiers. The as-synthesized Ag-rGO composites have been characterized by X-ray diffraction, X-ray photoelectron spectrometry and electron microscopy. Results showed that high-density Ag NPs and dots with uniform size distribution were grown on the surfaces of the GO sheets, accompanied by the reduction of GO to rGO. By simply changing the concentration of Ag⁺ ions in the solvothermal reaction system, the size and distribution of the Ag nanocrystals could be controlled. The antibacterial properties of the Ag-rGO composites were investigated by combined techniques of inhibition zone method, plate colony-counting method and bacterial growth curve. The Ag-rGO composites have been found to exhibit excellent antibacterial properties with long-term effects, which could effectively inhibit the growth of bacteria of Vibrio natriegens and Bacillus sp. 1NLA3E. The strongly-coupled interaction between the Ag nanocrystals and the rGO supports as well as the presence of the Ag dots contributed equally to the prominent long-term antibacterial performance of the Ag-rGO composites. The present Ag-rGO composites may find important environmental applications.