Amorphous Xerogel Research Articles

Bandgap tuning of samarium and cobalt co-doped bismuth ferrite nanoparticles

Silicon with its bandgap energy of 1.1 eV can convert one-third of solar energy for single layer junction semiconductor PV cells. However, maximum theoretical power conversion efficiency requires bandgap energy of 1.4 eV. Thus, scientific endeavor to find suitable functional PV materials has been progressing in full swing. In the realm of such efforts, perovskite BiFeO3 (BFO) is a prospective PV material, but it has an Eg of 2.67 eV. Thus, bandgap tuning of BFO by various dopants has given an impetus to researchers. In this context, samarium and cobalt co-doped BFO, Bi(1-y)SmyFe(1-x)CoxO3 (with y = 0 or 0.1 and x = 0, 0.05, 0.1 or 0.15) was synthesized by sol–gel method. The amorphous xerogel powder thus obtained was annealed to get the crystalline phase. Rietveld refinement of X-ray diffraction (XRD) revealed the structural transformation from rhombohedral to orthorhombic symmetry in single-doped and co-doped BFO nanoparticles. Fe–O bond length and Fe–O–Fe bond angle were calculated from the reconstructed crystal structure using XRD CIF data. Field emission scanning electron microscopy and transmission electron microscopy were conducted to characterize microstructural features and exhibited the decreasing trend of particle size with increasing dopant concentration. Electric impedance was found to decrease from 28 to 9.6 MΩ at 200 Hz with an increasing doping level. The bandgap energy was estimated from diffuse reflectance data obtained from UV–Vis–NIR spectrophotometry and justified in light of photoluminescence measurement. A significant reduction in bandgap energy from 2.05 to 1.40 eV would shed some light on increasing solar cell efficiency using perovskite samarium and cobalt co-doped BFO based PV cells.

Impact of reordering in structure of amorphous zirconia xerogel under fluoride modification on crystal kinetic crystallization and morphology

The influence of the amount of F-agent on the characteristics of nanoparticles (NPs), instant phase stability, the NPs shape and the process of NP crystallization are discussed. The structural reorganisation in F-modified amorphous xerogel aggregates leads to the formation of anisotropic-shaped crystallites under the crystallization of such xerogel. The kinetic parameters of crystallization are estimated. It was shown the main mechanism of crystalization is crystal growth controlled by interface reaction and bulk nucleation occurs with constant number of nuclei. The change of mechanism from 3-dimensional growth for pure zirconia to 2- or 1-dimensional growth for modified systems with the increasing of F-amounts is observed. It reflects the change of the morphology of crystal from sphere for pure zirconia to parallelopipedon and needle shapes for F-modified zirconia with high F-content. The increasing of Ea value of F-modified xerogels in comprison with an unmodified system is observed.

Preparation of Zirconium Sol by Precipitation Method

Zirconium sol was prepared using precipitation method. In the process, ZrO(OH)2precipitate was firstly obtained through the addition of strong base into the zirconium ions, and then was washed to remove the chloride ion. Lastly, the obtained ZrO(OH)2precipitate was dissolved in nitric acid solution to obtain the white mixed solution. The clear and transparent zirconium sol can be obtained after heated the white mixed solution in a water bath. In this study, the effects of three precipitating agents on the properties of zirconium sol were studied. Zirconium sol prepared by using ammonia as precipitating agent is easy to peptize, while its viscosity is very high. On the other hand, zirconium sol prepared by using urea as precipitating agent exhibits low solid content. Notably, the performance of zirconium sol prepared using ammonia and NH4Cl solution as precipitating agent is best due to its low viscosity and high solid content. In addition, zirconium sol, xerogel and zirconium dioxide powder prepared using ammonia and NH4Cl solution as precipitating agent were systematically characterized by SEM, viscometer, XRD, FTIR. The optimal zirconium sol was synthesized using a peptizing time of 8 h under a peptizing temperature was 80 °C, and the dried amorphous xerogel was converted to tetragonal ZrO2after calcined at 600 °C, while monoclinic at 1000 °C.

Synthesis of Gadolinium Scandate from a Hydroxide Hydrogel.

Gadolinium scandate (GdScO3) has been synthesized at 300 °C through the decomposition of a mixed cation hydroxide hydrogel in a humid environment. Increasing the reaction temperature produced larger particles that better adopted the Wulff shape. A lack of water vapor during the synthesis caused the solid network of the hydrogel to collapse upon heating so an amorphous xerogel was produced. Water vapor in the system imbibed the hydrogel and allowed for greater diffusion of the atomic species to allow for crystallization into the perovskite phase at temperatures lower than typical sol-gel processes. Temperatures less than 300 °C, or an excess of water vapor, promoted the formation of Gd(OH)3 and ScOOH in addition to or in lieu of GdScO3.

Effect of heat treatment time on local structure of lanthanum-aluminum–gallium-borate xerogels

The aim of this study was to investigate the effect of 850°C heat treatment time on LaAlGaB5O24 amorphous xerogel. Structural changes were evidenced by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. If xerogel remains non-crystalline after 5min treatment, nanocrystallites of LaAl2.03(B4O10)O0.54 type are developed after 15min treatment and this is the sole crystalline phase preserved by increasing the treatment time up to 24h. Boron, aluminum and gallium units from amorphous xerogel are drastically changed by crystallization, but they are only slightly affected by increasing the 850°C treatment from 15min to 24h.

Sol–gel synthesis and crystallization kinetics of dysprosium-titanate Dy2Ti2O7 for photonic applications

We present a generic sol–gel approach for the preparation of nanocrystalline Dy2Ti2O7. This approach allows the preparation of powders and highly transparent thin films having nanocrystals of a tailored size. The thermal evolution of these nanocrystals was followed by conventional structural methods and furthermore, we determine both the kinetic parameters of the nanocrystal nucleation process from amorphous xerogel as well as the crystallization mechanism. The crystallization temperature was 798 ± 2 °C. The nanocrystal growth started by homogeneous nucleation with a constant nucleation rate followed by three dimensional growth. The activation energy of the crystallization was 780 kJ mol−1, and the energy of nanocrystal growth was 24.4 kJ mol−1. We demonstrate the existence of pure pyrochlore structure of Dy2Ti2O7 with the lattice parameter a equal to 10.2 Å. The structural evolution of nanocrystalline thin films was identical to that of the evolution of xerogels. A thin film of thickness 342 nm exhibited an optical transmission of 93.39% with a refractive index of 2.138 at 935 nm. These results provide fundamental information about the crystallization behavior of Dy2Ti2O7: they can be used to prepare pure nanocrystalline xerogels and highly transparent thin films with tailored structural properties which are suitable for photonic applications.

A new approach to Y3Al5O12 transparent ceramics by vacuum sintering of spray-dried xerogels

We report fabrication of Y3Al5O12 optical ceramics via compaction of amorphous xerogel obtained by spray-drying of 5AlOOH(NO3)0.07·3Y(OH)2(CH3COO) aggregation-resistant sol, calcination to convert it into garnet phase and subsequent vacuum sintering. The spray drying conditions of the hydrosol have been optimized to ensure formation of spherical granules of xerogel with average diameter of about 330nm. The thermal decomposition of amorphous precursor and its conversion to pure garnet phase have been studied by DSC, TG and XRD methods. Pressing of the xerogel allows one to achieve quite uniform microstructure of green bodies with well-developed net channels of pores which is beneficial for achieving high sinterability. Transparent Y3Al5O12 ceramics possessing the in-line transmission of about 80% at λ=1064nm and the residual porosity of ∼5×10−3vol% have been obtained by vacuum sintering of the amorphous xerogels with the green density of 26%.

Structure Formation of a Double Sintered Nanocrystalline Silica Xerogel Converted From Sago Waste Ash

In the present study, double sintered nanocrystalline silica xerogel (NSX) bulk material has been produced from amorphous silica xerogel (ASX) powders extracted from sago waste ash. The ASX powders have been sintered at 1200°C, milled, pressed and then re-sintered at temperatures ranging from 1200° to 1600°C. The crystallization process of the NSX has been examined by thermal analysis (DSC/TGA) and X-ray diffraction (XRD), while the functional groups and the structure have been identified by infrared and Raman spectroscopy, respectively. From the DSC analysis it has been found that the tetragonal phase of α-cristobalite transforms to the stable phase of SiO2, namely cubic β-cristobalite in the samples re-sintered at 1600°C. This observation has been corroborated by an analysis of the XRD patterns, infrared and Raman spectra. The obtained data suggest that such novel material could be considered as an appropriate alternative for deposition of ceramic coatings on various machine and tool parts, e.g. components of gas-turbine engines.

Structural Characterization of Mullite-Based Ceramic Material from Al<sub>2</sub>O<sub>3</sub> and Silica Xerogel Converted from Sago Waste Ash

In this investigation, mullite ceramics were produced by mixing Al2O3 and amorphous silica xerogel (SX) extracted from sago waste ash. The composition was prepared by adding an amount from 0 to 80 mol% of Al2O3 into SX. The samples were dry pressed and sintered in the temperature range between 900°C and 1200°C. Their properties have been characterized on the basis of the experimental data obtained using thermal analysis (DSC/TGA, X-ray diffraction and scanning electron microscopy (SEM). The results show that two major differences become obvious when comparing the samples loaded by the smaller and larger amount of Al2O3. First, cristobalite formation is retarded in the sample loaded by the larger amount of Al2O3. Second, at 1200°C, the observed volume fraction of mullite is higher in the sample loaded by the larger amount of Al2O3. The DSC analysis indicates that the mullite crystallization takes place at 1200°C for 60 mol% Al2O3 loaded SX sample as confirmed by the XRD pattern. It was found that there was not a complete reaction of mulitization for sample sintered up to 1200°C.

Fabrication of zirconium oxide coatings on stainless steel by a combined laser/sol–gel technique

A new route of combining sol–gel coating technology and laser processing is developed and used for manufacturing zirconia coatings on stainless steel substrates. In this method, sol–gel zirconia is first synthesised using zirconium (IV) tert-butoxide as precursor and a mixture of isopropanol and water as the solvent. The sol is coated on the substrate by dip-coating, and then the dried sol–gel coating is irradiated by a fibre laser at different energy densities ranging from 4.35 to 10.9 J/mm 2 . Various techniques were applied to characterise the zirconia coatings on the substrate surfaces after laser treatment, including attenuated total reflectance Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and contact angle measurements. The results show that the fibre laser in continuous wave mode is effective to convert the as-dried amorphous xerogel coating on the substrate into zirconia coatings crystallised in tetragonal structure. The coating surface morphology varies with the laser energy densities and the highest coating uniformity is obtained at the lowest laser energy tested in this research. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Microstructural and Thermal Properties of Nanocrystalline Silica Xerogel Powders Converted from Sago Waste Ash Material

In the present investigation, nanocrystalline silica xerogel (NSX) powders were produced from an amorphous silica xerogel (ASX) extracted from sago waste ash. The NSX powders have been calcined at 1200oC, milled and then annealed at temperatures ranging from a room temperature to 1200oC. Their properties (and most notably the size of the particles) have been characterized on the basis of the experimental data obtained using thermal analysis (DSC/TGA), X-ray diffraction (XRD), Infrared and Raman spectroscopy. For the crystalline silica xerogel powders the results show a narrow distribution of the particle sizes centered around an average value of 636  67 nm. The DSC analysis of NSX indicates that in the temperature range from a room temperature to 300oC five distinct stages of the crystallization process take place, which are delimited by the transition temperature of 38oC, 92oC, 129oC, 168oC, and 246oC, respectively. Above 300oC, the crystalline phase is similar to an amorphous silica xerogel (ASX), i.e. cristoballite-like and tridymite-like crystalline silica phases confirmed by the XRD analysis. It has been observed that the characteristic band of cristoballite is strongly dependent on the thermal history and the NSX transforms into a stable form at a temperature of 1200oC. Both the Raman and the FTIR spectra elucidate the bonding system of the constituent atoms and groups (such as Si, O and OH) and throw light on their underlying structure. The obtained results are important for optimization of the parameters of the technological processes for production of nanocrystalline silica glass ceramics used as a host matrix for luminescence materials, each of which requires a specific porosity and structure.

Rare Earth-Doped SrTio3 Perovskite Formation from Xerogels

A synthesis process of rare earth doped SrTiO3 by modified sol-gel technique is described. Impervious strontium titanate doped with rare earth was prepared by gelification and calcination of colloidal systems. Powders of thulium substituted strontium titanate (-δ, where ; 0.02; 0.05) were obtained through cohydrolysis of titanium, strontium, and thulium precursors by sol-gel method. The xerogel obtained from the evaporation of solvents was milled and calcinated at 1100°C to give a reactive powder. Pure and doped SrTiO3 dense disks were formed by uniaxial pressing. Thermogravimetry (TGA), differential scanning calorimetry (DSC) analysis, X-ray diffractometry (XRD), and scanning electron microscopy (SEM) have been used to study the microstructural evolution of amorphous xerogel into crystalline reactive and sinterable powders. Hardness was measured for each membrane by a Vickers microindenter. Dilatometric and TGA-DSC in pure CO2 flow tests have been performed to evaluate, respectively, the thermal and chemical stability of the material. The optimized preparation route has allowed to synthesize highly reactive easy sintering powders used for fully densified, impervious ceramics with high thermal and chemical stability at high temperature.

Zirconium titanate thin film prepared by an aqueous particulate sol–gel spin coating process using carboxymethyl cellulose as dispersant

This paper deals with the preparation of ZrTiO4 thin film by a novel aqueous particulate sol–gel deposition method using carboxymethyl cellulose as dispersant. The structural characterization was performed by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. According to the results, the amorphous xerogel crystallizes to polycrystalline ZrTiO4 nanoparticles due to structural ordering conducted by calcination at 700°C. A well-covering, crack-free, and homogeneous ZrTiO4 thin film was processed, which is attributable to the efficient role of the dispersing agent in the aqueous sol. It was also found that the outermost layer of the coating surface consists of nanoparticles with 20nm in size.

Structural and Microwave Properties of Silica Xerogel Glass-Ceramic Sintered by Sub-millimeter Wave Heating using a Gyrotron

In this paper, we present and discuss experimental results from a microwave sintering of silica glass-ceramics, produced from amorphous silica xerogel extracted from sago waste ash. As a radiation source for a microwave heating a sub-millimeter wave gyrotron (Gyrotron FU CW I) with an output frequency of 300 GHz has been used. The powders of the amorphous silica xerogel have been dry pressed and then sintered at temperatures ranging from 300 °C to 1200 °C. Microwave absorbing properties of the sintered samples were investigated by measuring the dielectric constant, the dielectric loss, and the reflection loss at different frequencies in the interval from 8.2 to 12.4 GHz. Furthermore, the characteristics of the formation process for producing silica glass-ceramics were studied using a Raman Spectroscopy and a Scanning Electron Microscopy (SEM). The results indicate that the samples sintered at 1200 °C are characterized by lower reflection losses and a better transparency due to the formation of a fully crystallized silica glass- ceramic at sufficiently high temperature.

Synthesis and Structural Characterisation of a Hexanuclear GdIII Compound Gd6(μ3-OH)6(acac)12·1.25 CHCl3 as a Potential Precursor for Gd2O3 Nanomaterials

While aging the reaction mixture of gadolinium tri-isopropoxide with acetylacetone (pentane-2,4- dione, acacH) in a 1 : 2 molar ratio in toluene for crystalisation, crystals of Gd6(μ3-OH)6(acac)12・1.25 CHCl3 (1) were obtained in high yield. The crystal structure analysis reveals that 1 has a M6O12 core which is constituted of two distorted, tricapped, face-sharing cubane units (each with one missing edge) wherein the metal atoms are octacoordinated with a distorted square-antiprismatic geometry. In a sol-gel treatment the amorphous xerogel obtained from 1 was transformed into a nano-crystalline product at 600 °C while the xerogel of Gd(OPri)3 under similar conditions required a heat treatment up to 900 °C for the same development.

Spectroscopic study of ceramic precursors obtained by hydrolytic condensation of ethoxycyclotetrasiloxane

Hydrolytic polycondensation process, also called sol–gel process, leads to formation of xerogels from simple molecules e.g. most widely used tetraethoxysilane (TEOS). In the presented work 2,4,6,8-tetraethoxy-2,4,6,8-tetramethylcyclotetrasiloxane – D 4(OEt) 4 was applied as the molecular precursor. Polycondensation reactions were performed in the presence of Bu 4NF or HCl/NH 4OH as the catalysts. The obtained xerogels were studied by FT-IR, MAS-NMR and XRD methods. Depending on the condensation process conditions, structure of the resulting materials varied from octahedral cage unit (T 8) through a ladder chain to random structure. Dried xerogels were heated at the temperature of 800 and 1000 °C in argon or air atmosphere (pyrolysis or ceramization). In the atmosphere of argon SiC x O y glass materials were obtained, while in air silica glass was formed. Their properties depended on the pyrolysis conditions as well as on the xerogel structure. IR spectroscopy results allowed to conclude about the short range order in amorphous xerogel and glass materials.

Sol–Gel Synthesis of PZT–Glass Nanocomposites Using a Simple System and Characterization

A simple inorganic system has been developed for the sol–gel synthesis of piezoelectric PZT–glass nanocomposites. Nanocrystalline PZT are nucleated from the amorphous xerogel through controlled heat treatment at 600°C. The formation of the crystalline phase, particle size, morphology, kinetics, and mechanism of crystallization of PZT in the glass matrix have been studied using X‐ray diffraction, scanning electron microscopy, and differential thermal analysis. The piezoelectric characteristics of the PZT–glass compositions have been determined, and are correlated with the microstructure and the crystalline phase of PZT.

Synthesis of transparent mixed vanadia/niobia gels and their decomposition to a new metastable VNb 9O 25 phase

Controlled hydrolysis and condensation of a mixture of vanadyl-tris- n-propoxide, VO(OPr) 3, and niobium pentaethoxide, [Nb(OEt) 5] 2, at 5 °C in propanol yields clear and transparent gels in which the ratio of V:Nb is 1:1, 1:4.5 or 1:9. Oxalic acid and low temperatures are used to slow down the rate of condensation processes. At 800 °C, the thermal decomposition of a gel with the composition 1:9 forms a thermodynamically metastable, new phase of the composition VNb 9O 25. At lower temperatures, metastable solid solutions with TT-Nb 2O 5 structure (600 °C) and M-Nb 2O 5 structure (700 °C) are formed from the amorphous xerogel. The new VNb 9O 25 phase is structurally related to M-Nb 2O 5. The solid solution with M-Nb 2O 5 structure acts structure directing, leading preferentially to a monoclinic low-temperature form of VNb 9O 25. The full transformation of this metastable phase to the well known tetragonal VNb 9O 25 requires a annealing temperature of about 1000 °C.

Ba-Zr codoped sodium bismuth titanate by novel alkoxyless wet chemical route: processing and electromechanical behavior

Sodium bismuth titanate (NBT) based ceramics have been shown to be a promising lead-free alternative to commercial piezoelectrics and high capacitance materials. This article reports a novel aqueous citrate gel route for the synthesis of (Na0.5Bi0.5)xBa1-xTiyZr1-yO3 powders. Instead of alkoxides, usually used in citrate gel route, aqueous citrate solutions of titanium and bismuth were prepared from their respective inorganic salts. These solutions were used to prepare powder of (Na0.5Bi0.5)0.88Ba0.12Ti0.96Zr0.04O3. The process produced an amorphous xerogel that was readily crystallized to a pseudocubic perovskite phase when calcined at temperatures as low as 500 °C. The decomposition and oxidation of the xerogel was studied using TG-DTA, which showed complete removal of volatile components by 580 °C. The formation of crystalline phases was studied using XRD. SEM was used to evaluate the sintering behavior of the powder. The powder exhibits significant sintering at temperatures 800–900 °C. The polarization behavior of the sintered material was similar to one prepared by conventional solid state route. A maximum polarization of ∼24 μC/cm2 and peak bipolar strain ∼0.095% at 44 kV/cm was achieved in the sintered material.

Photoluminescence properties of TiO2: Eu3 + thin films deposited on different substrates

Strong photoluminescence of Eu3+ due to intra 4f transitions are obtained from amorphous xerogel TiO2: Eu3+ films prepared by sol–gel method and treated at a low temperature of 100 °C. The films are deposited on four different substrates: Si, Al, AAO (anodic alumina oxide) and porous silicon. We find that the luminescence intensity on AAO substrate increased 4 times comparing with that of Si or Al, and luminescence intensity decreases obviously on porous silicon substrate. Energy transfer mechanism from TiO2 host to Eu3+ is deduced through analysis of photoluminescence and photoluminescence excitation spectrum. Concentration quenching of Eu3+ does not appear even at high atomic concentration of 7.69%.