Filamentous alumina–chitosan porous structures produced by gelcasting

Porous filamentous macroelements with tunable properties were developed using alumina–chitosan fibers produced by gelcasting extrusion. The initial suspension was prepared as a dilute aqueous acetic acid solution containing 13 vol% of alumina particles and 1.3 vol% of dissolved chitosan. After extrusion, coagulation in an NaOH bath and drying, 300 μm diameter continuous fibers (9 vol% of chitosan) were compacted and sintered at different temperatures (1100–1500 °C) to produce 40×40 mm² cylindrical macroelements. The effects of the thermal treatment temperature on the porosity, specific surface area, mechanical strength and microstructure of the macroelements were evaluated. It was verified that these properties can be controllably modified in a wide range, depending on the sintering conditions.     

Pressureless sintering of alumina matrix ceramic materials improved by Al–Ti–B master alloys and diopside

Al–Ti–B master alloys and diopside are simultaneously introduced in alumina matrix ceramic materials as sintering aids. Fine structural alumina matrix ceramic materials are fabricated by pressureless sintering during which liquid phase, leading to interface reactions between alumina matrix and additives, is formed. Hardness, fracture toughness and bending strength of the composites are measured. The effects of diopside on mechanical properties and fracture mechanism of fine structural alumina matrix ceramic materials are analyzed together with the microstructure observations on fracture surfaces, the polished surfaces and the indentation cracks.     

Low temperature preparation and electrical properties of sodium–potassium bismuth titanate lead-free piezoelectric thick films by screen printing

Sodium–potassium bismuth titanate (NKBT) thick films with thickness of 40 μm were prepared by screen printing. To improve the homogeneity, the sintering aids were added into the pastes as a chemical liquid-phase doping method. The results show that the addition of Bi–Li sintering aids was beneficial for both the reduction of the sintering temperature and the improvement of the electrical performance of the thick films. The thick films containing 5 wt.% Bi–Li sintering aids demonstrated optimal dielectric properties with the maximum dielectric constant of 725 and minimum dielectric loss of 2.5%. Moreover, the NKBT thick films containing 3 wt.% Bi–Li sintering aids sintered at 950 °C exhibited the remanent polarization of 19.6 μC/cm², room-temperature pyroelectric coefficient of 1.56 × 10^?4 C/(m² °C), figure of merit for specific detectivity of 0.48 × 10^?5 Pa^?0.5, and effective longitudinal piezoelectric coefficient of 88 pm/V, which are comparable to that of the high-temperature sintered thick films without sintering aids.     

Synthesis and characterization of boehmitic alumina coated graphite by sol–gel method

Non-wettability property makes graphite a good protecting material against the molten metal and/or slag. Properties like high oxidation potential between 600 and 1200 °C and non-wettability with water at room temperatures limits the usage of graphite in castable refractory applications. In this study, sol–gel method, which is a relatively cheaper process, was used. Boehmitic sol was obtained by hydrolyzing and peptiziting the alkoxide AIP (aluminum isopropoxide) used as alumina source. Then natural flake graphite was mixed with the boehmitic solution for coating of graphite. At 120 °C boehmitic sol coated graphite was dried and gelled. Then heat threaded at 550 °C for γ-Al₂O₃ transformation of boehmite. Products that obtained from the studies were characterized with FTIR and XRD tests. Alumina coated graphite samples were made by repeating the same steps and TG analysis were made to investigate the oxidation behaviour of the samples. Finally, SEM–EDS analyses were carried out to investigate the microscopic properties of the alumina coated graphite powders.     

Preparation and dielectric properties of dense and amorphous alumina film by sol–gel technology

Dense and crack-free aluminum oxide films were fabricated by sol–gel spin-coating technology. Aluminum nitrate (Al(NO₃)₃.9H₂O) was used as the precursor material. X-ray diffraction shows that the fabricated films are amorphous. X-ray photoelectron spectroscopy confirms that the thin films are alumina (Al₂O₃). Field-emission scanning electron microscopy images of the films reveal that the films are compact with a dense cross section. Dielectric measurements were carried out on samples with a metal–insulator–metal structure. The electrical characteristics of the films were affected by the thermal sintering temperature of the films. The leakage current density of the films decreases with the increase in the sintering temperature and increases with the increase in the measuring temperature. The leakage current shows a linear dependence on the voltage in the low-electric field-regime. The current density ascends to higher values due to the effect of space charges in the high-electric-field regime. The ionization energy of the top-electrode metals (Au, Pt or Ti–Au) has a strong effect on the leakage current.     

Influence of iron/aluminium ratio on the retention of lead and copper by amorphous iron–aluminium oxides

Results are presented from a study on the influence of pH on the retention of Pb and Cu by amorphous iron, aluminium and mixed iron–aluminium oxides, plus kaolinite clay. The composition (Fe/Al molar ratio) of the oxides significantly affects their surface properties. Increasing the Fe/Al ratio generally enhances the surface area although the maximum surface area is found at Fe/Al=1. The cation exchange capacity (CEC) was dependent upon pH and Fe/Al ratio, decreasing in order Fe>3FeAl>Al>Fe3Al>FeAl≫kaolinite at pH 6.3–7. The oxide points of zero charge could be divided into two groups: Fe/Al≤1 (Al, Fe3Al, FeAl; pH_ZPC=6.2–6.4) and Fe/Al>1 (3FeAl, Fe; pH_ZPC=5.3–5.8). Batch equilibrium suspension tests were conducted with Pb and Cu (single metal solutions) to assess contaminant retention capability of each oxide and kaolinite. Pb retention by mixed oxides at a particular pH increases with Fe/Al ratio in the order Fe>3FeAl>FeAl>Fe3Al>Al. The same order is observed at low to medium Cu concentrations (0.5, 2.5, 5 mmol/l) but at higher metal loading (25 mmol/l), only Fe oxide has a clearly greater retention capacity. Oxides with Fe/Al<1 (Al, Fe3Al) retain more Cu than Pb in single contaminant tests at equal metal concentration. Bulk precipitation tests (no oxides or kaolinite) combined with surface charge measurements suggest that adsorption is the dominant retention mechanism, particularly at less than pH 6. Amorphous oxides demonstrate considerably greater heavy metal accumulation capacity, specific surface area, and surface charge than kaolinite.     

Mullite–TiC–c-BN–c-ZrO2 materials Produced by Spark-Plasma Sintering and Their Properties

Refractories and Industrial Ceramics - Different c-BN/c-ZrO2 ratios are shown to affect the phase composition, microstructure, relative density, open porosity, linear shrinkage, physicomechanical...     

Fully densified zircon co-doped with iron and aluminium prepared by sol–gel processing

A sol–gel technique has been used to prepare Fe and Al doped zircon. Structural properties have been studied by X-ray diffraction, nuclear magnetic resonance, scanning electron microscopy and transmission electron microscopy (TEM). Fully densified zircon was produced with high zircon yield and promising microstructures. The presence of Fe promotes zircon formation, while Al improves densification. The zircon phase starts to form at 1215 °C, with almost single phase zircon obtained at 1400 °C when heated for 1 h. Densification increases very significantly (to 99.7% of theoretical density) when the holding time was increased to 48 h from 1 h. TEM micrographs reveal a crystalline grain boundary phase containing some Fe and Al.     

Correlation between sintering conditions and water contact angles for Ti–O thick films screen printed on an alumina substrate

TiO₂, TiO_2−x and Ti₃O₅ thick-film structures on corundum Al₂O₃ substrates were prepared using screen-printing technology. The screen-printed deposits were sintered up to 1500 °C in oxidising and reducing atmospheres to vary the Ti⁴⁺/Ti³⁺ ratio and consequently water contact angle. The structure of the thick films was studied with an X-ray powder diffractometer (XRD). The microstructural characteristics and the chemical composition were checked with a scanning electron microscope, equipped with an energy-dispersive spectrometer (EDS). The Ti–O films, up to 55 μm thick, exhibited excellent adhesion to the substrate and had uniform grain- and pore-size distributions. Ti₃O₅ and Al₂O₃ were found to be compatible phases up to 1500 °C in a reducing atmosphere. However, rutile-type TiO₂ and Al₂O₃ are not compatible compounds at temperatures up to 1400 °C, in either oxidising or reducing atmospheres. TiO₂ and TiO_2−x form two types of reaction products with Al₂O₃. These reaction products were found to have various Ti/Al ratios.     

Synthesis and optical characterization of Er-doped bismuth titanate nanoparticles grown by sol–gel hydrothermal method

The Er³⁺-doped bismuth titanate (Bi₄Ti₃O₁₂, BIT) nanoparticles were synthesized by a combined sol–gel and hydrothermal method under a partial oxygen pressure of 30 bar. The composition and morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman scattering. They showed pure and homogeneous spherical BIT nanoparticles with a size below the 30 nm. The incorporation of Er ions showed a strong decrease in the lattice parameters, as well as averaged particle size. The photoluminescence up-conversion (excitation wavelength =1480 nm) showed an enhancement of the infrared emission (980 nm) as Er concentration increased, achieving a maximum for 6% mol, while photoluminescence spectra (excitation wavelength =473 nm) showed a strong green emission (529 and 553 nm) with a maximum at 4% mol.     

Properties and estimation of mullite thermal insulators prepared by gelation–freezing with alumina nanofibers

Thermal insulators were fabricated by freezing gelatin gels containing calcined kaolinite with alumina nanofibers, followed by sintering. The resultant porosity could be varied from 81.0% to 89.8% by solid loadings in the initial slurry. The relationship among porosity, microstructure, compressive strength, and thermal conductivity was examined. The compressive strength and thermal conductivities of the insulators prepared with initial solid loadings from 3 to 6 vol% ranged from 11.6 to 56.7 MPa and from .25 to .46 W/mK, respectively. Those properties were also estimated by simulation using modeling of overall pore morphology, resulting in good agreement.     

Application of micro X-ray diffraction to investigate the reaction products formed by the alkali–silica reaction in concrete structures

Alkali–silica reaction (ASR) is one of the most important deterioration mechanisms in concrete leading to substantial damages of structures worldwide. Synchrotron-based micro-X-ray diffraction (micro-XRD) was employed to characterize the mineral phases formed in micro-cracks of concrete aggregates as a consequence of ASR. This high spatial resolution technique enables to directly gain structural information on ASR products formed in a 40-year old motorway bridge damaged due to ASR.Micro-X-ray-fluorescence was applied on thin sections to locate the reaction products formed in veins within concrete aggregates. Micro-XRD pattern were collected at selected points of interest along a vein by rotating the sample. Rietveld refinement determined the structure of the ASR product consisting of a new layered framework similar to mountainite and rhodesite.It is conceivable that understanding the structure of the ASR product may help developing new technical treatments inhibiting ASR.     

Study of molding structures and physicomechanical properties of carbon-ceramic composite materials of the SiC–C system

The microstructure of composite materials of the composition SiC–C is analyzed. It is established that they are a separate group of materials containing a ceramic matrix. The ceramic matrix experiences tensile stresses, as a result of which within the composite material a traditional internal stress field is distorted. The ceramic matrix increases strength at carbon phase boundaries of the composite material, and it reduces porosity. An excess of ceramic material reduces strength and thermal stress resistance. Requirements are provided for porosity of the structure that govern the optimum field of material composition.     

Preparation and characterization of photopolymerizable organic–inorganic hybrid materials by the sol-gel method

A series of UV-curable organic–inorganic hybrid materials were prepared by the sol-gel technique and coated onto Plexiglass^® substrate. The effects of the content of EGDMA and the content of the inorganic part on various properties of the coatings, such as tensile strength, hardness, gloss, and cross-cut adhesion, were investigated. It was found that the properties of the coating were improved by the addition of an inorganic part. The thermal properties of the hybrids were enhanced by incorporating silane sol into the organic part. Furthermore, it was found that the coating containing silica had a higher char content at 800 °C than the coating without silica. SEM studies indicated that nanosized (about 50 nm) silica particles were evenly dispersed throughout the organic matrix. A photo-DSC investigation showed that the organic coating polymerized more quickly than the hybrid coating.     

Synthesis and characterization of twinned flower–like ZnO structures grown by hydrothermal methods

Twinned flower-like ZnO structures have been synthesized by one-step CTAB assisted hydrothermal methods at a low-temperature as 90 °C. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results disclosed a twinned flower-like morphology and hexagonal wurtzite structures. The XRD pattern and temperature-dependent PL results show a mixed structure of as-grown samples, which are confirmed by the SEM results. The CL spectrum on a single twinned flower-like ZnO structures showed an excellent optical property. Based on experimental results, self-etching and regrowth are suggested as the mechanism to grow the flower-like structures.     

Ultra-stretchable thermoplastic elastomeric materials based on styrene–isoprene–styrene triblock copolymer: Influence of calcium copper titanate on mechanical and dielectric properties

Conventional charge storage devices made of ceramic materials have limited deformability and configurability due to their extreme stiffness. The high demand for compliant dielectrics has led researchers to look beyond conventional ceramics, despite their very high dielectric properties. In this work, a mechanically robust, highly-flexible and ultra-stretchable thermoplastic elastomeric material with dielectric characteristics has been fabricated by introducing calcium copper titanate [CaCu₃Ti₄O₁₂] (CCTO) dielectric material onto macromolecular chains of styrene–isoprene–styrene (SIS) triblock copolymer via a solution-based polymer processing technique. CCTO powders have been synthesized using sol–gel technique. The resulting composite is ultra-stretchable with strain at break of ~3200% and has high dielectric permittivity of ~10. High dielectric property is attributed to the well-dispersed dielectric CCTO fillers within the SIS matrix, which provide sites for interfacial polarization and space charge accumulation. The influence of CCTO on dielectric properties has also been validated using the modified Cole–Cole model.     

Evolution of interfacial structures and mechanical performance of sapphire and Sn–9Zn–2Al joints by ultrasound

A method of ultrasonic-assisted soldering was applied to join sapphire with Sn–9Zn–2Al at 250°C in air. The sapphire samples were hot-dipped in the liquid filler metal under the ultrasonic action before ultrasonic-assisted soldering. The experimental results have shown that the joints dipped for different durations had similar morphologies, while the shear strength was dependent on the dipping duration. The shear strength of the joints increased rapidly from 13 to 22 MPa within the hot-dipping duration of 50 seconds, and increased slowly to 46 MPa at 2000 seconds. The evolution of the interface structure and the fracture mode was analyzed. The interfacial strength was dominated by the deposited Al₂O₃ at the interface in the early stage. After the entire surface of sapphire was covered by deposited Al₂O₃, Zn-rich phases at the interface and the thickening of deposited Al₂O₃ layer enhanced the interfacial strength at the later stage. Our study revealed the evolution of the interface structure and the strengthening mechanism in the joint of sapphire with Sn–Zn–Al alloys.     

Effect of yttria on crystallization and microstructure of an alumina–YAG fiber prepared by aqueous sol–gel process

Continuous fiber development is needed for high performance and high temperature composites. Various methods have been used to make ceramic fibers. In this research, composite fibers (yttrium aluminum garnet (YAG)/Al₂O₃) were prepared by a sol–gel method using aqueous solution. They were synthesized from aluminum salt, aluminum metal, yttrium oxide and water used as solvent. Transparent gel fibers were obtained by immersing a thin wire into the viscous sol, then pulling it out by hand. The obtained fibers contained very fine grains with diameter ranging from 10 to 80 μm after heat treatment. When yttria content was increased, the crystallization of YAG shifted to a lower temperature, whereas the transformation temperature to α-Al₂O₃ shifted to a higher temperature. Nevertheless, the fibers with different amounts of yttria contained alumina and YAG after heat treatment at 1400 °C. The composite fibers had vermicular structure and were denser than alumina fibers. The yttria percent concerning the limits of this study (≤10 wt%) effected on fiber diameter. As the yttria content was increased, the fiber diameter increased, whereas grain size and densification of the composite fibers decreased.     

AlMgB14–TiB2 composite materials obtained by self-propagating high-temperature synthesis and spark plasma sintering

In this work, AlMgB₁₄–TiB₂ composite materials were obtained by thermochemical-coupled self-propagating high-temperature synthesis (SHS) and subsequent spark plasma sintering. The mechanism was proposed for the formation of the composite materials in the thermochemical-coupled SHS mode. The phase composition, microstructure, and properties (density and Vickers hardness) of the dense AlMgB₁₄–TiB₂ materials were investigated. At a sintering temperature of 1470 °C, AlMgB₁₄ is decomposed into AlB₁₂ and Mg. The sample sintered at 1470 °C with a holding time of 5 min had a maximum average hardness of 32.1 GPa.     

Mechanical properties and thermal stability of ambient-cured thick polysiloxane coatings prepared by a sol–gel process of organoalkoxysilanes

Ambient-curable polysiloxane coatings were prepared by pre-hydrolysis/condensation of phenyltrimethoxysilane (PTMS) and dimethyldimethoxysilane (DMDMS) in the presence of ammonia solution and subsequently mixing with aminopropyltriethoxysilane (APS). The mechanical properties of coatings were thoroughly examined at both macro- and micro-level and the thermal stability of coatings was characterized by thermogravimetic analysis, both of which were correlated with coating composition and the hydrolysis/condensation degree of polysiloxane oligomer. It was found that pro-hydrolysis step is essential for fabrication of thick crack-free coatings (18–35 μm). Higher DMDMS molar ratio, more APS dosage and lower hydrolysis/condensation degree of polysiloxane oligomer favor enhancing the hardness. Excellent impact resistance (50 cm kg) of coatings was obtained at 5% and 10% APS dosage, despite of the type and structure of polysiloxane oligomer. Whatever, the best scratch resistance of coatings was attained using the polysiloxane oligomer, prepared at PTMS-to-DMDMS molar ratio of 2:8 and water-to-precursor molar ratio of 1:1, and 5% APS dosage. The polysiloxane coatings exhibit high thermal stability, however, which strongly depends on the coating composition.