Densification and grain growth during pressureless sintering of TiO2 nanoceramics

Anatase titania nanopowders with mean particle sizes of 7, 15, 26 and 38 nm synthesized by sol–gel method were used to sinter bulk TiO₂ nanoceramics. The relative densities and average grain sizes of the TiO₂ nanoceramics were studied as a function of the compaction pressure on green sheet, sintering temperature, and mean particle size of the starting TiO₂ nanopowders. The relative density of the TiO₂ nanoceramics increases rapidly and average grain size increases slowly with increasing sintering temperature below 800 °C. Sintering at higher temperatures above 800 °C enhances the densification of the TiO₂ nanoceramics and leads to a increase of the grain size. Bulk TiO₂ nanoceramics with an average grain size of less than 60 nm and relative density over 95% was obtained by a phase-transformation-assisted pressureless sintering at a relatively low temperature (800 °C).     

Electrochromic properties of nanocrystalline MoO3 thin films

Electrochromic MoO₃ thin films were prepared by a sol–gel spin-coating technique. The spin-coated films were initially amorphous; they were calcined, producing nanocrystalline MoO₃ thin films. The effects of annealing temperatures ranging from 100 °C to 500 °C were investigated. The electrochemical and electrochromic properties of the films were measured by cyclic voltammetry and by in-situ optical transmittance techniques in 1 M LiClO₄/propylene carbonate electrolyte. Experimental results showed that the transmittance of MoO₃ thin films heat-treated at 350 °C varied from 80% to 35% at λ = 550 nm (ΔT =  45%) and from 86% to 21% at λ ≥ 700 nm (ΔT =  65%) after coloration. Films heat-treated at 350 °C exhibited the best electrochromic properties in the present study.     

Preparation and optical properties of sol–gel derived Er-doped Al2O3–Ta2O5 films

Thin films of the system xAl₂O₃–(100 − x)Ta₂O₅–1Er₂O₃ were prepared by a sol–gel method and a dip-coating technique. The influences of the composition and the crystallization of the films on Er³⁺ optical properties were investigated. Results of X-ray diffraction indicated that the crystallization temperature of Ta₂O₅ increased from 800 to 1000 °C with increased values of x. In crystallized films, the intensities of the visible fluorescence and upconversion fluorescence tend to decrease with an increase in x values, due to the high phonon energy of Al₂O₃; the strongest fluorescence is observed in a crystallized film for x = 4 heat treated at 1000 °C. In amorphous films obtained by heat treatment at relatively low temperatures the Er³⁺ fluorescence could not be observed because strong fluorescence from organic residues remaining in the films thoroughly covered the Er³⁺ fluorescence. On the other hand, the Er³⁺ upconversion fluorescence in the amorphous films was observed to be stronger than that in the crystallized films. The strongest upconversion fluorescence is observed in an amorphous film for x = 75 heat treated at 800 °C.     

Synthesis of nanocrystalline photocatalytic TiO2 thin films and particles using sol–gel method modified with nonionic surfactants

A simple sol–gel route has been developed for the preparation of nanocrystalline photocatalytic TiO₂ thin films and particles at 500 °C. The synthesis involved a novel chemistry method employing nonionic surfactant molecules as a pore directing agent along with acetic acid-based sol–gel route without direct addition of water molecules. This study investigated the effect of surfactant type and concentration on the homogeneity, morphology, light absorption, dye adsorption and degradation, and hydrophilicity of TiO₂ films as well as on the structural properties of the corresponding TiO₂ particles. The method resulted in the synthesis of mesoporous TiO₂ material with enhanced structural and catalytic properties including high surface area, large pore volume, pore size controllability, small crystallite size, enhanced crystallinity, and active anatase crystal phase. The prepared TiO₂ thin films were super-hydrophilic and possessed thermally stable spherical bicontinuous mesopore structure with highly interconnected network. Highly porous TiO₂ films prepared with polyethylene glycol sorbitan monooleate surfactant exhibited four times higher photocatalytic activity for the decoloration of methylene blue dye than the nonporous control TiO₂ films prepared without the surfactant. This sol–gel method modified with surfactant templates is useful in the preparation of nanostructured anatase TiO₂ thin films with high photocatalytic activity and desired pore structure.     

Sol–gel thin film deposition and characterization of a new optically active compound: Er2Ti2O7

This paper reports on the first sol–gel thin film preparation of a new optically active compound: Er₂Ti₂O₇ (ETO). Optical, microstructural and spectroscopic properties of ETO films annealed in a temperature range 300–1000°C are studied. This work shows that the porosity and microstructure of ETO films depend closely on the heat-treatment temperature. Photoluminescence (PL) has been observed for films heat-treated at 600°C or more. The PL decay appears strongly influenced by quenching effects. For thin films treated at 600°C, quenching is essentially due to the presence of hydroxyl groups. After heat-treatment at 800°C or more, quenching can be explained by the high concentration of erbium atoms and by their distribution in the ETO lattice.     

Electrospun MnFe2O4 nanofibers: Preparation and morphology

The composite fibers were prepared by an electrospinning the solutions mixed manganese ferrite sol with polyvinyl acetate (PVAc). The manganese ferrite sol was obtained from manganese(II) nitrate and iron(III) nitrate nonahydrate molecular precursor based on a sol–gel procedure. Manganese ferrite (MnFe₂O₄) nanofibers were prepared by calcining the above composite fibers in air at 400, 600 and 800 °C, respectively. The obtained nanofibers were characterized by SEM, FT-IR, XRD, and extended X-ray absorption fine structure (EXAFS). After being calcined at 800 °C, the fibers became continuous crystallites as a result of the complete decomposition of PVAc. The morphology of manganese ferrite was found to be cubic spinel structure containing eight oxygens and 12 metal atoms in the unit cell, referred from the results of EXAFS. Also, the magnetic properties of the calcined samples were characterized by using a vibrating sample magnetometer (VSM).     

The properties of aluminum-doped zinc oxide thin films prepared by rf-magnetron sputtering from nanopowder targets

Aluminum-doped zinc oxide (ZnO:Al) films were deposited onto glass substrates by rf-magnetron sputtering at ambient temperature using, for the first time, doped nanocrystalline powder synthesized by the sol–gel method. The effects of aluminum on structural, electrical, morphological and optical properties were investigated. The films showed a hexagonal wurtzite structure and high preferential orientation in the (002) crystallographic direction. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the films morphology. The obtained samples have a typical columnar structure and a very smooth surface. The optical transmittance spectra showed transmittance higher than 90% within the visible wavelength region. A minimum resistivity of 5.436 · 10^− 5 Ω cm at room temperature was obtained for the 3.0 at.% Al-doped film.     

Sensitivity enhancement for H2 gas detection using a SnO2–Ag2O–PdOx nanocrystalline system

Thick film H₂ sensors were fabricated using SnO₂ loaded with Ag₂O and PdO_x. The composition that gave highest sensitivity for H₂ was in the wt.% ratio of SnO₂:Ag₂O:PdO_x as 93:5:2. The nano-crystalline powders of SnO₂–Ag₂O–PdO_x composites synthesized by sol–gel method were screen printed on alumina substrates. Fabricated sensors were tested against gases like H₂, CH₄, C₃H₈, C₂H₅OH and SO₂. The composite material was found sensitive against H₂ at the working temperature 125 °C, with minor interference of other gases. H₂ gas as low as 100 ppm can be detected by the present fabricated sensors. It was found that the sensors based on SnO₂–Ag₂O–PdO_x nanocrystalline system exhibited high performance, high selectivity and very short response time to H₂ at ppm level. These characteristics make the sensor to be a promising candidate for detecting low concentrations of H₂.     

Sol–gel alumina environmental barrier coatings for SiC grit

SiO₂ surface film is insufficient to protect SiC from the oxidation at widely varying partial pressures of oxygen, in particular in the presence of water vapor (e.g. in gas turbines) and also in other environments, e.g. during brazing for hard “tipping” of turbine blades. This work demonstrates that sol–gel alumina, coated on 0.5 mm coarse SiC grit, may form an acceptable, up to 10 μm thick “environmental barrier coating” EBC for some of these applications. The sol–gel has advantages over other methods (such as CVD) is the simplicity and low cost. We have used NH₄OH pre-treatment to hydroxylate surface of SiC prior to applying alumina coating. Such modified SiC/SiO₂ surface helped to deposit the positively charged alumina sol, and thus allowed to build thick coatings on the SiC grit. There is some indication that these coatings partially convert to mullite through reaction at the interface with the native silica on SiC. Oxidation resistance tests at 1200 °C were performed to show effectiveness of such coated SiC grit.     

Synthesis and humidity sensitive properties of nanocrystalline Ba1−xSrxTiO3 thick films

Nanocrystalline Ba_1−xSr_xTiO₃ (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) precursors were synthesized using the stearic acid gel method. After the precursors had been calcined at 600–950°C for 0.5–1 h, nanocrystalline powders with the cubic perovskite structure were obtained and these were made into thick films. The powder samples were characterized by differential thermal analysis, X-ray diffraction and transmission electron microscopy, and the thick film samples were characterized by scanning electron microscopy and X-ray diffraction. The humidity-sensitive properties of the nanocrystalline Ba_1−xSr_xTiO₃ thick films were investigated. The results show that these nanocrystalline thick films possess higher humidity sensitivity and lower resistance than those of conventional materials.     

Synthesis and characterization of polyvinylpyrrolidine assisted tantalum pentoxide films

Micron thick tantalum pentoxide (Ta₂O₅) films have been proposed as thermal insulating layers in microchemical systems, but so far it has been difficult to deposit thick enough films over complex substrates. So far sol–gel films cracked upon heating whenever the film thicknesses were above 350 nm. A 350 nm thick film is too thin for effective insulation. Other techniques are not suitable for coating the complex structures associated with microchemical systems. In this paper we report sol–gel synthesis of 1.6 μm thick tantalum pentoxide (Ta₂O₅) films. The films are almost crack free, and adhere to silicon surfaces even upon flashing to 900 °C. The key to the synthesis is the addition of Polyvinylpyrrolidine (PVP) to the sol. Films grown in the absence of PVP all show cracks upon calcination to 900 °C while few cracks are seen with PVP. X-ray diffraction and Fourier transform infra red analysis show that orthorhombic Ta₂O₅ is formed in all cases. X-ray photoelectron spectroscopy shows the O:Ta ratio to be 2.8:1. This shows that sol–gel is a viable process for making the micron thick films of Ta₂O₅ needed as insulators for microchemical systems.     

Synthesis and microstructure of boron-doped alumina membranes prepared by the sol–gel method

Pure and boron-doped γ-Al₂O₃ membranes have been synthesized by the sol–gel method. The thermal stability of the unsupported alumina membrane was studied by determining the pore structure (including average pore size, pore volume and BET surface area). The average pore size of the pure alumina membrane increased sharply after sintering at temperatures higher than 1000°C. Addition of 16% boron can considerably stabilize the pore structure of the unsupported alumina membrane. The pore diameter for the B-doped membrane was stabilized within 13 nm after sintering at 1200°C for 5 h. The substantial increase in the pore size for the pure alumina membrane at the sintering temperature of 1000–1200°C was accompanied by the phase transformation from γ-Al₂O₃ to -Al₂O₃. The addition of boron can raise the temperature of this phase transformation significantly and, thus, improves the thermal stability of the membranes.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Nanocrystalline tetragonal zirconium oxide stabilization at low temperatures by using rare earth ions: Sm and Tb

Zirconium oxide was synthesized by the sol–gel method and the tetragonal structure was stabilized up to 1000 °C by doping with different rare earth ions. The evolution of the crystalline structure as a function of the annealing temperature and rare earth concentration of doped and undoped samples were investigated by X-ray diffraction. Our experimental results show that it is possible to obtain up to 73 wt.% of tetragonal content by doping the ZrO₂ with 2 mol% of Sm₂O₃ or Tb₂O₃ and annealing at 1000 °C. Variations in the lattice parameters and nanocrystallite size were also obtained. Our results suggest that the stabilization of tetragonal structure can be obtained with other rare earth ions.     

Combustion synthesis of nanocrystalline ZrO2 powder: XRD, Raman spectroscopy and TEM studies

ZrO₂ powder was prepared by the gel combustion technique using citric acid as a fuel and nitrate as an oxidant. Calcination at 600 °C of the dried powder, obtained after sluggish combustion of the citrate–nitrate gel, produced nanocrystalline ZrO₂ powder. The Rietveld refinement of the powder XRD data clearly suggested the presence of predominantly tetragonal phase of zirconia. TEM studies showed the agglomerated powder composed of nearly spherical nanocrystals of about 10 nm. The absence of cubic phase of zirconia was conclusively inferred by Raman spectroscopy.     

Piezoelectric and dielectric properties of PZT–PZN–PMS ceramics prepared by molten salt synthesis method

Piezoelectric powders and ceramics with the composition of Pb_0.95Sr_0.05(Zr_0.52Ti_0.48)O₃–Pb(Zn_1/3Nb_2/3)O₃–Pb(Mn_1/3Sb_2/3)O₃ (PZT–PZN–PMS) were prepared by molten salt synthesis (MSS) and conventional mixed-oxide (CMO) methods, respectively. The influence of synthesis process on the properties of powders and ceramics were investigated in detail. The results show that the MSS method significantly improved the sinterability of PZT–PZN–PMS ceramics, resulting in an improvement of dielectric and piezoelectric properties compared to the CMO method. The optimum values of MSS samples are as follows: _r = 1773; tan δ = 0.0040; T_c = 280 °C; d₃₃ = 455 pC/N; k_p = 0.70; Q_m = 888; E_c = 10.3 kV/cm; and P_r = 28.2 μC/cm², at calcination of 800 °C and sintering of 1120 °C temperature.     

Consolidation behavior of L12 phase (Al + 12.5 at.% Cu)3Zr powder with nanocrystalline structure during CIP and subsequent sintering

The mechanically alloyed (Al + 12.5 at.% Cu)₃Zr powders were consolidated by cold isostatic pressing (CIP) and subsequent sintering. Effects of CIP pressure and sintering temperature on the stability of metastable L1₂ phase and nanocrystalline structure were investigated. Before sintering, the powders were CIPed at 138, 207, 276, and 414 MPa. The relative densities of the CIP compacts were not greatly affected by the CIP pressure. However, the L1₂ phase of the specimen CIPed at pressures greater than 276 MPa was partially transformed into D0₂₃. The optimum consolidation conditions for maintaining L1₂ phase and nanocrystalline microstructure were determined to be CIP at 207 MPa and sintering at 800 °C for 1 h for which the grain size was 34.2 nm and the relative density was 93.8%. Full density specimens could be prepared by sintering above 900 °C, however, these specimens consisted of L1₂ and D0₂₃ phases. The grain sizes of all the specimens were confirmed by TEM and XRD, and were found to be less than 40 nm. This is one of the smallest grain sizes ever reported in trialuminide intermetallic compounds.     

In situ synthesis mechanism and characterization of ZrB2–ZrC–SiC ultra high-temperature ceramics

The synthesis route, microstructure and properties of ZrB₂–ZrC–SiC composites prepared from a mixture of Zr–B₄C–Si powders by in situ reactive synthesis were investigated. The reactive path and synthesized mechanism of ZrB₂–ZrC–SiC composite were studied through series of pressureless heat treatments ranging from 800 °C to 1700 °C in argon. The in situ ZrB₂–ZrC–SiC composites were fabricated under different synthesis processing. The one with 88.4% relative density performed poorly in mechanical properties due to the occurring of self-propagating high-temperature synthesis (SHS). The fully dense ZrB₂–ZrC–SiC composite was obtained under the optimized synthesis processing without SHS reactions. Its Vickers hardness, flexural strength and fracture toughness were 20.22 ± 0.56 GPa, 526 ± 9 MPa and 6.70 ± 0.20 MPa m^1/2, respectively.     

Preparation of high thermal-stabile alumina by reverse microemulsion method

As a useful approach the reverse microemulsion method was used to prepare the high thermal-stabile alumina-substrate materials with high surface area at high temperature. After calcined at 1100 °C for 10 h, alumina, barium-doped alumina, and silicon-doped alumina synthesized by the reverse microemulsion method exist, still as the transition alumina phase including γ-phase, δ-phase and θ-phase; their particles are nanometer-size and rod shape, the size distribution of its particle is very uniform, and their BET surface areas remain 52, 90 and 175 m² g⁻¹, respectively. This method claims its superiority at two points: the raw materials are inorganic salts which are cheap and obtained easy, and the synthesis procedure is simple and easy to perform. As the comparative examples, three samples above prepared by the sol–gel method exist mainly as alumina phase and their BET surface areas are 20, 24 and 117 m² g⁻¹, respectively.     

Mechanical properties and microstructure of bioactive ORMOSILs containing silica particles

We synthesized organically modified silicate (ORMOSILs) gels with colloidal silica (CS) (AEROSIL®) starting from polydimethylsiloxane (PDMS), tetraethoxysilane (TEOS) and calcium nitrate (Ca(NO₃)₂·4H₂O) through sol–gel processing. Dynamic mechanical analysis indicated that relative height of the tanδ peak at about −100 °C increased with an increase in the relative content of the inorganic components. This peak growth was accounted for by the relative increase in PDMS–colloidal silica particle interactions. The colloidal silica could control the mechanical behavior of the hybrids. The gel of a specific composition could deposit apatite within 3 days of soaking in the simulated body fluid (SBF), since it included many calcium ions on the surface.