Hydrothermal-assisted Sol-gel Process Research Articles

Rapid and low-temperature preparation of zircon ceramic by hydrothermal-assisted sol-gel process and microwave sintering

During the traditional preparation of zircon, higher sintering temperatures and longer holding times are inevitable. A novel hydrothermal-assisted sol-gel process and microwave sintering (HM, microwave sintering precursors derived from hydrothermal-assisted sol-gel process) were provided to prepare zircon ceramics hereof. The influences of sintering temperature and holding time on the phase composition, microstructure, and compactness of ceramics were investigated. Data display that the properties of the obtained ceramics were highly dependent on the temperature and time, and the ZrSiO4 formation efficiency can be improved by increasing these two factors. The sample prepared by the HM process has the formation efficiency of ZrSiO4 up to 91.74 wt% (HM-1350-300), which far exceeds that of conventional microwave sintering (M-1350-300, 45.02 wt%). The results demonstrate that the HM process can be employed to obtain ZrSiO4 ceramics rapidly at low temperatures.

Durable Polyurethane/SiO2 Nanofibrous Membranes by Electrospinning for Waterproof and Breathable Textiles

Recently, waterproof and breathable materials have attracted extensive attention and have been used in a broad range of applications such as clothing, sports equipment, medical hygiene products, and other fields. In this study, an efficient chemical modification method was suggested to prepare polyurethane/SiO2 nanofibrous membranes with superhydrophobicity and durability. A polyurethane emulsion was treated with 4,4′-methylenebis(phenyl isocyanate) and 3-aminopropyltriethoxysilane. This emulsion process was combined with the emulsion electrospinning technique and a hydrothermal-assisted sol–gel process to prepare superhydrophobic polyurethane nanofibrous membranes. The final polyurethane/SiO2 nanofibrous membranes exhibited a hydrostatic pressure of 8.02 kPa, an air permeability of 3.89 mms–1, and a water vapor transmission rate of 10.12 kg m–2 d–1. Results also showed that the water contact angle of the nanofibrous membranes reached 154°, and the stress and strain were 5.3 MPa and 198%, respectively. In addition, the adhesion between SiO2 and the PU nanofibrous membranes was high, and the weight loss rate of SiO2 remained at approximately 9.72% after the ultrasonic cleaning time exceeded 24 h. These results suggested that the PU/SiO2 nanofibrous membranes were expected to provide a new perspective for waterproofing and moisture permeability.

Rapid preparation of Nd-doped zirconia ceramics for high-level radioactive waste immobilization

Rapid preparation of ceramic waste form at low temperature is of great engineering significance for immobilization of high-level radioactive waste. Herein, we demonstrated that Nd-doped zirconia ceramics can be rapidly prepared at low sintering temperature with the assistance of hydrothermal assisted sol-gel method. The effects of sintering temperature and Nd content on the phase and microstructure evolutions of the obtained ceramics were studied. With the increasing content of Nd, the phase of ZrO2 transformed from monoclinic to cubic at first, then the pyrochlore Nd2Zr2O7 phase appeared, and finally hexagonal neodymia solid solution structure formed. Moreover, the grain size and compactness of the obtained ceramics increased with the increment in sintering temperature. The successful accommodation of Nd within the ZrO2 lattice was revealed by the detected increase of cell parameters. This work demonstrated that the hydrothermal assisted sol-gel process can be employed to effectively synthesize the Nd-doped zirconia ceramics as potential hosts for HLW immobilization.

Structural, morphological, and optical properties of Co‐ substituted Zn2SiO4 nanopowders prepared by a hydrothermal-assisted sol-gel process

Cobalt-substituted zinc orthosilicate pigments were obtained by pyrolysis of precursors synthesized by a convenient hydrothermal-assisted sol-gel process without using any toxic or expensive chemical (e.g., silicon alkoxides). XRD investigation confirms that the Co2+ ions are successfully incorporated into the Zn2SiO4 crystal. Rietveld refinement data reveals that unit-cell parameters increase with Co-content in Zn2-xCoxSiO4 solid-solutions. The homogeneous pigments are composed of largely agglomerated primary particles having a size less than 70 nm. The UV–visible spectroscopy shows that the blue color of pigments is caused by the presence de Co2+ in tetrahedral sites of Zn2+. The colorimetric parameters indicate that a blue pigment could be achieved with the only x = 0.05. A content of less than 5 wt% of the prepared pigments gives a deep blue hue to ceramic glazes. Cobalt-substituted Zn2SiO4 pigments exhibit high coloring performance and a relatively less cobalt content, which make them promising pigments for ceramic tiles.

Electrical performance of fine-grained Y-TZP/TiC composites obtained through a hydrothermal-assisted sol-gel process

Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) composites with various TiC contents were successfully synthesized via a modified hydrothermal-assisted sol-gel method. Fine precursor powders can be obtained with high crystallinity, nanoscale grain size and uniform morphology. SEM images of the ceramic products show that TiC particles are distributed homogeneously in the final Y-TZP matrix, and their average grain sizes are approximately 390–670 nm and 150–230 nm for the Y-TZP and TiC phases, respectively. A higher TiC volume fraction has a negative effect on the relative density and hardness but a significant positive influence on electrical conductivity. The electrical conductivity values are increased from 115 S/m to 1.23 × 105 S/m with TiC contents varying, demonstrating that the percolation threshold is approximately as low as 11.6 vol% for the samples, which is much lower than those of previous Y-TZP/TiC ceramics. The high electrical performance is probably due to the high D (the diameters of the insulating particles)/d (the diameters of conductive particles) ratio and submicron-sized grains.

Structure evolution and aqueous durability of the Nd2O3[sbnd]CeO2[sbnd]ZrO2[sbnd]SiO2 system synthesized by hydrothermal-assisted sol-gel route: A potential route for preparing ceramics waste forms

The Nd2O3CeO2ZrO2SiO2 system represents a typical system for the simulated multivalent (tri- and tetravalent) actinides-bearing zircon ceramics waste forms. However, the preparation of such ceramics suffers from low solubility limits of Nd + Ce and tough synthetic condition. Herein, we reported a hydrothermal-assisted sol-gel route to the synthesis of this system with designed general composition of Zr1-x-y(NdxCey)SiO4-x/2 (0 ≤ x = y ≤ 0.05), which enabled a high incorporation capacity of Nd and Ce in ZrSiO4 ceramics at low sintering temperature (1350–1450 °C) and short time (6 h). The effects of the synthesis route and sintering temperature on the structure evolution, solubility limits and aqueous durability of the Nd2O3CeO2ZrO2SiO2 system were investigated. Ceramics with a single ZrSiO4-type phase were obtained for x + y < 0.08 at 1350 °C, x + y < 0.07 at 1400 °C, and x + y < 0.05 at 1450 °C in Zr1-x-y(NdxCey)SiO4-x/2 series, whereas the other compositions gave rise to the two phase structures (ZrSiO4 and (Nd,Ce)2Si2O7). The solubility limits of Nd + Ce increase from 5 at% to 7 at% and 8 at% with the decrease of sintering temperature from 1450 °C to 1400 °C and 1350 °C, respectively. These values of solubility limits are all higher than those of solid state method (4 at%). Furthermore, due to the decrease of the energy of cohesion of the structure caused by oxygen vacancies, the normalized leach rate (LRi) increases with the increase of Nd + Ce loading. In addition, the LRi decreases with the increase of sintering temperature. The as-synthesized samples exhibit excellent aqueous durability with LRNd and LRCe of ∼10−4 g m−2 d−1 after 42 days at 40 °C. Our work demonstrates that the hydrothermal-assisted sol-gel process is a potential route for preparing ceramics waste forms.

Morphological, spectroscopic and photocatalytic properties of Eu3+:TiO2 synthesized by solid-state and hydrothermal-assisted sol-gel processes

Eu3+ doped TiO2 powders were synthesized by solid-state reaction and hydrothermal-assisted sol-gel process. They were characterized by X-ray diffraction, scanning electron microscopy and photoluminescence techniques. Both synthetic procedures yield the pure anatase phase of titania. The excitation and emission spectra and the decay profiles were measured for different thermal treatments of the samples, in order to investigate the effects of the synthesis conditions on the doping mechanism. The photocatalytic activity was investigated by measuring the degradation of methylene blue under UV light irradiation and a correlation between its efficiency and the luminescence performance has been proposed.

Corrosion-resistant fluoridated Ca–Mg–P composite coating on magnesium alloys prepared via hydrothermal assisted sol–gel process

Abstract

Phase evolution and chemical stability of the Nd2O3-ZrO2-SiO2 system synthesized by a novel hydrothermal-assisted sol-gel process

A hydrothermal-assisted sol-gel process, a novel method of combining both advantages of sol-gel and hydrothermal processes, was developed to synthesize dense Nd-doped ZrSiO4 ceramics with high Nd (surrogate for trivalent actinides) loading at low sintering temperature (1723 K) and short time (6 h). The evolutions of phase and microstructure of the obtained ceramics with the varying composition, sintering temperature and pH value were investigated. The chemical durability of the ceramics was also evaluated. The results show that the properties of ceramics strongly depend on sintering temperature and pH value. The acidic condition is beneficial to phase formation and densification of ZrSiO4 ceramics. It was found that the maximum immobilization capacity of Nd in the single-phase ZrSiO4 ceramics is highly dependent upon the synthesis method and sintering temperature. With decreasing sintering temperature, the maximum immobilization capacity increases. And the immobilization capacity of Nd in ZrSiO4 ceramics was found to be enhanced by the hydrothermal-assisted sol-gel method. Furthermore, the normalized release rate of Nd in Nd-doped ZrSiO4 ceramics decreases with increasing time and remains almost unchanged (∼10−4 g m-2 d-1) after 28 days, and the fractional release of Nd in all samples is about ∼10−5 wt%, exhibiting high chemical stability.

Facile preparation of superhydrophobic silica nanoparticles by hydrothermal-assisted sol–gel process and effects of hydrothermal time on surface modification

Superhydrophobic silica nanoparticles were prepared by hydrothermal-assisted sol–gel process using a two-step synthesis route. Silica nanoparticles were obtained by hydrolysis and condensation of tetraethyl orthosilicate (TEOS). Hexadecyltrimethoxysilane (HDTMS) was added to allow the condensation reaction between HDTMS hydroxyl group and silica hydroxyl group, rendering silica particles hydrophobic. Silica nanoparticles had a HDTMS/TEOS molar ratio of 2:40 and exhibited good hydrophobic property with a high water contact angle of 156.1°. Water droplet rolled off the surface of silica powder owing to a low sliding angle of 3.6°. When hydrothermal time of TEOS decreased from 120 to 60 min, all silica powders gave rise to high water contact angles (≥154.8°) and low sliding angles (≤5.5°). However, silica powder became hydrophilic when HDTMS and TEOS were added to the precursor together. This facile preparation of superhydrophobic silica nanoparticles with short reaction time provided the advantages for potential application in industrial production.

Low temperature and rapid preparation of zirconia/zircon (ZrO2/ZrSiO4) composite ceramics by a hydrothermal-assisted sol-gel process

Hydrothermal-assisted sol-gel process, a novel method of combining both advantages of sol-gel and hydrothermal processes, was developed to synthesize 0.2ZrO2/ZrSiO4 composite ceramics. The effects of sintering temperature and pH value on the phase formation rates, microstructure and mechanical properties were investigated. The phase and microstructure evolutions of samples obtained from hydrothermal-assisted and conventional sol-gel process were compared. Dense zirconia/zircon ceramics can be obtained by hydrothermal-assisted sol-gel process followed by sintering at low temperature (1400 °C) and short time (6 h). It was found that the properties of ceramics strongly depend on sintering temperature and pH. The formation rates of zircon in samples obtained from hydrothermal-assisted sol-gel is higher than those of conventional sol-gel method. The acidity condition is beneficial to phase formation and densification of ZrO2/ZrSiO4 ceramics. SEM images show that the structure of ceramics obtained from hydrothermal-assisted sol-gel is more compact when compared with those prepared by conventional sol-gel. Furthermore, improved Vickers hardness and fracture toughness can be achieved by hydrothermal-assisted sol-gel process.

Efficient green and red up-conversion emissions in Er/Yb co-doped TiO2 nanopowders prepared by hydrothermal-assisted sol–gel process

In this work, erbium and ytterbium co-doped titanium dioxide (Er–Yb:TiO2) nanopowders have been successfully prepared by hydrothermal-assisted sol–gel method using supercritical drying of ethyl alcohol and annealing at 500°C for 1h. Nanopowders were prepared with fixed 5mol% Erbium concentration and various Ytterbium concentrations of 5 and 10mol%. The powders were characterized by studying their structural, morphology and photo-luminescent properties. The annealing treatment at 500°C was found to enhance the crystallinity of the TiO2 anatase structure and the upconversion (UC) emission of the nanopowders. UC emissions were investigated under 980nm excitation, and the Er–Yb:TiO2 nanopowders exhibited the intense green (520–570nm) and red (640–690nm) upconverted emissions of Er ions originating from an efficient Yb–Er energy transfer process. The absolute upconversion quantum yield (UC-QY) of each nanopowders was measured for the UC emissions centered at 525, 550 and 655nm at varying excitation power densities. UC-QY analysis has revealed that 5mol% Er–5mol% Yb:TiO2 nanopowders possess the highest total quantum yield of 2.8±0.1% with a power density of 16.7W/cm2. These results make these nanopowders promising materials for efficient upconversion in photonic applications.

Core–shell structured Na3V2(PO4)3/C nanocrystals embedded in multi-walled carbon nanotubes: A high-performance cathode for sodium-ion batteries

In this work, we study the synthesis and electrochemical performance of Na3V2(PO4)3/C and multi-walled carbon nanotubes (MWCNTs) hybrid nanocomposite for use as cathode material in high performance sodium-ion batteries. This nanostructured hybrid material is prepared through a hydrothermal-assisted sol–gel process followed by thermal annealing. The obtained sample has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectrum. The crosslinked MWCNTs can provide good electronic conductive pathways interconnecting Na3V2(PO4)3/C particles. Electrode fabricated from this hybrid material delivers a high reversible capacity and superior rate performance up to an extremely high rate of 10C. Moreover, ultralong cycling stability is also attained through 500cycles at 10C with 93.9% capacity retention. These results demonstrate that the addition of MWCNTs is an effective way to improve the electrochemical performance of Na3V2(PO4)3/C cathode material for sodium-ion batteries.

Hydrothermal Synthesis, Characterization and Improved Activity of a Visible-Light-Driven ZnSe-Sensitized TiO2Composite Photocatalyst

In this study, ZnSe-TiO₂ composites were synthesized by a facile hydrothermal-assisted sol-gel process and characterized by nitrogen adsorption isotherms (77 K), X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectrophotometry. The photocatalytic activity was investigated by decoloration methylene blue (MB), methyl orange (MO), and rhodamine B (Rh.B) in an aqueous solution under visible light irradiation. The results revealed that the photocatalytic activity of the ZnSe-TiO₂ photocatalyst was much higher than that of pureTiO₂. The ZnSe nanoparticles, which act as a photosensitizer, not only extend the spectral response of TiO₂ to the visible region but also reduce charge recombinations.

Cd(II)-imprinted polymer sorbents prepared by combination of surface imprinting technique with hydrothermal assisted sol–gel process for selective removal of cadmium(II) from aqueous solution

A new ion-imprinted thiocyanato-functionalized silica gel sorbents was synthesized by the combination of a surface imprinting technique with a hydrothermal-assisted sol–gel process with Cd(II) as the template, 3-thiocyanatopropyltriethoxysilane (TCPTS) as the functional monomer, and epichlorohydrin as the cross-linking agent (IIP-TCPTS/SiO2) for the selective removal of Cd(II) from aqueous solution. The Cd(II)-imprinted thiocyanato-functionalized silica sorbents was characterized by FTIR, SEM, nitrogen adsorption and the static adsorption–desorption experiment method. The results showed that the maximum static adsorption capacity of IIP-TCPTS/SiO2 sorbents prepared by hydrothermal heating method was 2.3 times as much as the conventional heating method. The IIP-TCPTS/SiO2 sorbents offered a fast kinetics for the adsorption and desorption of Cd(II). The static uptake capacity and selectivity coefficient of IIP-TCPTS/SiO2 sorbents were higher than those of the non-imprinted sorbents. IIP-TCPTS/SiO2 sorbents had a substantial binding capacity in the range of pH 4.2–8.6 and could be used repeatedly. The Langmuir adsorption model was more favorable than the Freundlich adsorption model. Kinetic studies indicated that the adsorption followed a pseudo-second-order model. Various thermodynamic parameters such as ΔG°, ΔH° and ΔS° were evaluated with results indicating that this system was a spontaneous and endothermic process. The results showed that IIP-TCPTS/SiO2 sorbents could be employed as an effective material for the selective removal of Cd(II) from aqueous solutions.

Nanostructured Li[sub 2]FeSiO[sub 4] Electrode Material Synthesized through Hydrothermal-Assisted Sol-Gel Process

A carbon-coated Li 2 FeSiO 4 material with uniform nanoparticles (approximately 40-80 nm in diameter) is synthesized by a synthesis route, i.e., a hydrothermal-assisted sol-gel process. As an electrode material for rechargeable lithium batteries, the Li 2 FeSiO 4 sample shows high rate capability and excellent capacity retention. The Li 2 FeSiO 4 electrode delivers a discharge capacity of 160 mAh g -1 at C/16 rate, corresponding to 96% of the theoretical value. A discharge capacity of about 91 and 78 mAh g -1 can be obtained at 5 and 10 C rate, respectively. No capacity loss can be observed up to 50 cycles.