Thermal studies of Mn2+-doped ZnO powders formation by sol–gel method

Journal of Thermal Analysis and Calorimetry - In the present contribution, the study of nanostuctured powders of Mn2+-doped ZnO with different doping concentration (1, 2, 5 at%) was...     

Preparation and characterization of In2O3/ZnO heterostructured microbelts by sol–gel combined with electrospinning method

Long and thin In₂O₃/ZnO heterostructured microbelts were synthesized by sol–gel combined with electrospinning process. The as-prepared microbelts show the well defined one-dimensional belt structures with 1–5 μm in width and tens of millimeters in length. The polycrystalline microbelts calcined at 973 K for 1 h are still continuous and have the uniform rectangular cross sections and the thickness to width ratio is around 1:10. The crystalline phases of samples are investigated by X-ray diffraction and the morphology is examined using transmission electron microscope and scanning electron microscope. In₂O₃/ZnO heterostructured microbelts exhibit the excellent visible photocatalytic property in the photodegradation of methyl orange (MO), and over 94 % of MO was degraded within 3 h.     

Preparation of durable superhydrophobic surface by sol–gel method with water glass and citric acid

Durable superhydrophobic surface on cotton fabrics has been successfully prepared by sol–gel method. Cellulose fabric was first coated with silica sol prepared with water glass and citric acid as the acidic catalyst. The silica coated fabric was then padded with hydrolyzed hexadecyltrimethoxysilane afterwards obtaining low surface energy. Water contact angle and hydrostatic pressure were used to characterize superhydrophobicity and washing durability. Scanning electron microscopy was used to characterize the surface morphology changes after certain washing times. All results showed good durable hydrophobicity on cellulose fabrics. In addition, the influence of citric acid and sodium hypophosphite (NaH₂PO₂) on the durability of hydrophobicity was also investigated. The durability of treated cotton improved with the increase of concentration of citric acid in the presence of NaH₂PO₂. It could be concluded that citric acid acted as multi-functional heterogeneous grafting chemicals to improve washing durability of hydrophobicity by forming the ester bonds between cotton fabric and silica sol and improved the durability of hydrophobicity.     

Preparation of CuCrO2 nanoparticles with narrow size distribution by sol–gel method

Copper chromium oxide (CuCrO₂) nanoparticles were synthesized by sol–gel method. The effect of annealing temperature, duration of heat treatment and metallic ion concentration in precursor solution on the structural properties of the nanoparticles was investigated. The delafossite structure of CuCrO₂ powder was confirmed by X-ray diffractometer. It was found that the crystallite sizes as well as the size of the nanoparticles increased with annealing temperature and duration of heat treatment but decreased with metallic ion concentration. Nanoparticles’ size was obtained using particle size analyzer. The synthesized CuCrO₂ nanoparticles with 0.7 M metallic ion concentration have the lowest crystallite and particle sizes with a narrow size distribution in the range of 13.5–15.6 nm. In the presence of this metallic ion concentration, we could also produce single crystal CuCrO₂ nanoparticles. Moreover, the CuCrO₂ nanoparticles exhibit a large optical band gap that increases with metallic ion concentration. The optical band gap of the nanoparticles fabricated with 0.7 M metallic ion concentration in precursor solution is about 3.99 eV.     

Preparation of single-layer antireflective SiO2 coating with broadband transmittance using PEG-modified sol–gel method

Adding polyethylene glycol (PEG) with different molecular weights, a usual acid-catalyzed sol–gel was modified to prepare single-layer antireflective SiO₂ coatings with high and broadband transmittance and relatively better hardness. The test results of atomic force microscope and field emission scanning electron microscope show that the addition of PEG significantly affects the porosity and surface morphology of the coating layer. Due to the addition of PEG, the surface of the coatings presents groove-like and their porosity is increased, both of which contribute to the increase in transmittance. In the case of same PEG mass, PEG4000 modified coating has higher porosity and higher transmittance than PEG1000 modified one. In the present paper, the reflectance of samples for both sides was tested by ultraviolet–visible–near-infrared spectrophotometer (LAMBDA 950). The best coating’s reflectance can be decreased below 5 % from 460 to 1,740 nm. The transmittance peak value of the substrate is 90.6 % and its average value is 90.0 %, while the peak value of the best coating can reach up to 99.4 % and its average value is 95.5 % which increased by 5.5 % from wavelength of 325 to 1,000 nm. Hardness measurements show that the coatings have relatively better hardness.     

Preparation of SiO2–wood composites by an ultrasonic-assisted sol–gel technique

The vacuum impregnation assisted sol–gel technique is a promising and environmentally-friendly method for the inorganic modification of wood by the formation of wood-inorganic composites. However, vacuum impregnation is relatively cumbersome and time-consuming. In this study, SiO₂–wood composites were prepared by an ultrasonic-assisted sol–gel method, which is an innovative and simple method. Using this method, we found an increase in the degree of silicon incorporation into the cell walls of the wood. The impregnation of silica inside the cell walls were verified by Fourier-transform infrared spectra, X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Leaching test proved that the internal cross-linking silica is stably bonded to the wood cell walls. This modified method significantly reduced the hygroscopicity of the wood and consequently improved the mechanical performance of the modified wood. Thermogravimetric and differential thermal analyses showed that the incorporation of silicon retards thermal decomposition and the complete combustion of the wood matrix and it enhances the thermal stability of wood.     

Phase evolution and electrical properties of BaO–SrO–TiO2–SiO2–Al2O3-based glass ceramics prepared by sol–gel process

Crystallization of BaO–SrO–TiO₂–SiO₂–Al₂O₃-based glass ceramics, prepared by sol–gel process, was evaluated in terms of the effect of sintering temperature on phase evolution and electrical properties. The characterization of the samples was performed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) studies and impedance spectroscopy analysis. The XRD results demonstrate that fresnoite phase starts to crystallize at 700 °C and perovskite phase appears at 900 °C. The glass ceramic samples sintered at high temperatures contained three crystalline phases, including perovskite, feldspar and fresnoite. In addition, SEM observation showed that the average grain size increased and the porosity decreased with increasing sintering temperature. Furthermore, the measurement of impedance spectroscopy suggests that there is a minimum value of the activation energy associated with the sintering temperature of the glass ceramics. The possible explanation of the sintering temperature dependence was discussed.     

Preparation of ZnO nanoparticles by a surfactant-assisted complex sol–gel method using zinc nitrate

This paper presents a surfactant-assisted complex sol–gel method for the controlled preparation of Zinc Oxide (ZnO) nanoparticles using zinc nitrate and citric acid as starting material. ZnO nanoparticles with a pure wurtzite structure were obtained after calcination at 773 K. The effects of the citric acid concentration, the pH, and the surfactants on the average particle size and morphology of the ZnO nanoparticles were investigated using X-ray diffraction and scanning electron microscopy. Well dispersed ZnO nanoparticles with a uniform size distribution were obtained using polyethylene glycol (PEG) 2000 as a surfactant. During sintering, the ZnO nanoparticles revealed isotropic growth below 1,373 K and anisotropic growth above 1,473 K. The particles’ activation energy was calculated to be 140 ± 6 kJ/mol between 773 and 1,373 K.     

Preparation and characterization of CaCu3Ti4O12 powders by non-hydrolytic sol–gel method

CaCu₃Ti₄O₁₂ (CCTO) powders were prepared via a non-hydrolytic sol–gel (NHSG) method by using acetylacetone as chelating agent and ethylene glycol as solvent. The samples were characterized by TG–DSC, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscope. The dielectric properties of ceramics were also measured. The pure perovskite-like CCTO powders were obtained by heat treatment at 800 °C for 2 h. The average particle sizes of CCTO powders calcined at 800 °C were approximately 350–450 nm. The samples sintered at 1,000 °C showed the mean grain size of 2.5–4 μm. Specially, the ceramics exhibited high dielectric constant (1.19 × 10⁵–1.40 × 10⁵) and low dielectric loss (0.051–0.1) in the temperature range of 30–110 °C. Moreover, with the NHSG method the period of synthesis process was greatly shortened.     

Junction properties of In2O3:Al2O3 composite on p-type silicon by sol–gel method

Schottky barrier diode based on composite of In₂O₃ and Al₂O₃ was fabricated using sol–gel spin coating method. The electrical properties of the diode were studied using current–voltage, capacitance–voltage and resistance–voltage characteristics. The non-linear I–V characteristics suggest the formation of Schottky barrier diode. The I–V characteristics of the diode were analyzed using the thermionic emission model. The electrical properties of the diode were investigated in the temperature range of 303–243 K. It was observed that the barrier height of the diode increases with increase in temperature. The capacitance of the diode was measured at various frequencies and temperatures. It was seen that the capacitance of the diode is decreased with increase in frequency. On the other hand, the capacitance was observed to increase with increasing temperature.     

Dielectric and energy storage properties of barium strontium titanate based glass–ceramics prepared by sol–gel method

Ba_0.6Sr_0.4TiO₃ based glass–ceramics were prepared by sol–gel process. Influences of B–Si–O glass content on the microstructure, dielectric, and energy storage properties of the BST based glass–ceramics have been investigated. Perovskite barium strontium titanate phase was found at annealing temperature 800 °C. A secondary phase Ba₂TiSi₂O₈ was detected and lowered by declining the mole ratio of element Si (from 50 to 25 mol%) in glass additive. Microstructural observation indicated that the microstructure homogeneity can be improved by glass addition till 2 mol%, while worsened by excessive glass concentrations. Due to relatively homogeneous microstructure, the maximum discharged energy density and breakdown strength were also obtained in samples with 2 mol% glass additive, which were found to be 0.553 J/cm³ and 43.2 kv/mm, respectively. Microscopic observation of the breakdown area was performed and the mechanical failure, including the formation and accumulation of micro-cracks during the dielectric breakdown process, was considered to be the main cause of dielectric breakdown. Results of the charging and discharging energy densities show that the BST based glass–ceramics prepared by sol–gel method has a potential for pulse power applications.     

Preparation and characterization of magnetic TiO2/ZnFe2O4 photocatalysts by a sol–gel method

A magnetic TiO₂/ZnFe₂O₄ photocatalyst was prepared by a sol-gel method, and X-ray diffraction (XRD), magnetic and photocatalytic properties analysis were employed to characterize this photocatalyst. The XRD results show that ZnFe₂O₄ can prevent the transformation of titania from anatase to rutile. The magnetic properties analysis indicates that TiO₂/ZnFe₂O₄ is of large saturation magnetization value and low coercivity. The photocatalytic experimental results show that TiO₂/ZnFe₂O₄=3 and 4 are superior in photocatalytic reactivity to other proportions. TEM shows that TiO₂/ZnFe₂O₄ has a fine core-shell fabric. After being used for four times during the photocatalytic reaction, the TiO₂/ZnFe₂O₄ nanoparticles have good photocatalytic stability.     

Preparation of porous manganese oxide nanomaterials by one-pot synthetic sol–gel method

For the first time, the simple epoxide addition, sol–gel method has been employed to successfully prepare porous, high surface area manganese (II) aerogel nanomaterials. These uniform materials can then undergo calcination at relatively low temperature to selectively yield the mixed-valent Mn₃O₄ complex illustrating both an ease of preparation and synthesis versatility.     

Preparation of La0.67Ca0.33MnO3:Ag x polycrystalline by sol–gel method

La_0.67Ca_0.33MnO₃:mol%Ag _x (LCMO:Ag _x , x = 0, 0.04, 0.08, 0.10 and 0.20) ceramics were prepared by sol–gel method. The X-ray diffraction studies of the LCMO:Ag _x samples confirm the pure phase of the LCMO composites with Pbnm space group. With the increase of Ag doping from x = 0 to 0.20, the normalized resistivity (ρ/ρ _300K ) of the samples around the metal–insulator transition temperature (T _p ) reduced in sequence. Both T _p and the peak of temperature coefficient of resistance (T _k ) improved 5–8 K. Energy dispersive X-ray spectroscopy analysis showed that the composition of La, Ca and Mn of the obtained LCMO composites was in accordance with the composition of the precursor. The results indicated that the improvement of grain boundaries and crystallization were responsible for the enhancements of ρ, T _p and T _k of LCMO:Ag _x composites. The fitted curves of electrical resistivity for LCMO:Ag _x composites indicated that the mechanisms of grain/domain boundary, electron–electron and magnon scattering and adiabatic small polaron hopping are proposed to explain the phase transition of ferro-magnetic metallic region (T < T _p ) and the para-magnetic insulating region (T > T _p ), respectively.     

Sintering properties of sol–gel derived lithium disilicate glass ceramics

Lithium disilicate (Li₂Si₂O₅) glass-ceramics were fabricated through two sol–gel methods: the nitrate route and the alkoxide route. Thermal analysis revealed different crystallization processing of two gel-derived powders. Li₂Si₂O₅ powders were obtained after heat treatment at 800?°C. These powders were pressed and pressureless sintered under 900–1030?°C. Microstructure of sintered samples revealed the grain size and morphology of Li₂Si₂O₅ ceramics. Although grain size in both samples increased with increasing sintering temperature, samples from the alkoxide route derived powders had more uniform grain size and pore distribution. In addition, open porosity decreased in both samples with increasing sintering temperature. Unlike familiar nucleation that resulted in grain growth mechanism, the Li₂Si₂O₅ particles developed into irregular large size grains at first, and then grew into rod-shaped grains.

Open image in new window