Improvement in optical transmission of the ambient pressure dried hydrophobic nanostructured silica aerogels with mixed silylating agents

The experimental results on the preparation of transparent and hydrophobic silica aerogels based on ion exchanged sodium silicate (Na₂SiO₃) using mixed silylating agents of trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDZ) are reported. Hydrogels were prepared with ion exchanged Na₂Si0₃ of 1.10 specific gravity with 1 N ammonium hydroxide (NH₄OH). The resulted gels were exchanged with methanol solvent followed by silylation using HMDZ + HMDSO or TMCS + HMDZ or TMCS + HMDSO mixture in methanol and hexane and dried the gels at room temperature for 24 h, at 50 and 200 °C for 1 h each. It has been observed that the percentage of the silylating agent in the mixture, time interval of addition of one agent followed by another and volume of silylating mixture have an effect on density, % of optical transmission, % of porosity, porevolume, thermalconductivity and refractive index of the aerogels. Hydrophobicity of the aerogels was studied by contact angle measurements. The TMCS + HMDSO aerogels have been found more hydrophobic (contact angle > 150°) than the other aerogels. Further, aerogels have been characterized by pore size distribution, Fourier Transform Infra Red Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and thermogravimetric analysis and differential thermal analysis (TGA–DTA) techniques. It has been found that the weight increase is the highest (325%) for HMDSO + HMDZ aerogels and lowest (1.2%) for HMDSO + TMCS aerogels by keeping the aerogels in the atmosphere over a period of 18 months. Low density (0.042 g/cc) transparent (85%), low thermal conductive (0.047 W/m K), low refractive index (1.0088) and hydrophobic (152°) silica aerogels were obtained with equivolume TMCS + HMDSO mixture of 10 ml, time interval of addition of 12 h between the two reagents prior addition of TMCS followed by HMDSO for 24 h of silylation period. It was found from the TGA–DTA of the aerogels that hydrophobicity of the aerogels remained up to the temperature of 425 °C.     

Structural study of highly porous nanocomposite aerogels

The structural properties of CoFe₂O₄–SiO₂ highly porous nanocomposite aerogels have been investigated by X-ray Absorption Spectroscopy and Transmission Electron Microscopy techniques. The aerogels are obtained by supercritical drying of composite gels obtained using a two step procedure where fast gelation is achieved using urea in the second step. The formation of CoFe₂O₄ nanocrystals in the silica matrix begins after calcination at 750 °C of the parent aerogel and is complete after calcination at 900 °C, while the high porosity of the sample is mostly retained.     

Nitridation of SiO2–B2O3 aerogels

SiO₂–B₂O₃ aerogels have been prepared by drying wet gels at a supercritical condition for ethanol in an autoclave. Aerogels have been nitrided for 6 h in flowing ammonia at the temperature of 1200 °C. It has been found that the amount of nitrogen incorporated in these aerogels always exceeds 20 wt%. This is a much higher value compared with the amount of nitrogen incorporated in a pure silica aerogel nitrided at the same conditions. The specific surface area of SiO₂–B₂O₃ aerogels has been between 312 and 359 m²/g. After nitridation some shrinkage of aerogels has been observed and the surface area decreases about 20%. In FTIR spectra of SiO₂–B₂O₃ aerogels a typical bands for SiO₂ are observed. After nitridation a shift and broadening of 1100 cm^?1 band to lower wavenumbers indicates that Si–N and B–N bonds are formed in nitrided aerogels.     

Polyaniline nanofiber–silica composite aerogels

Strong, electrically conducting aerogels were prepared by introducing polyaniline nanofibers to a silica sol just prior to gelation and drying through supercritical carbon dioxide processing. The addition of a few milligrams of polyaniline per cm³ increased the flexural strength of the cylindrical monoliths by 200%. Using preformed polymeric nanofibers avoided filling of microporosity often observed with polymer reinforcement of aerogels and allowed preparation of polyaniline–silica composite aerogels with surface areas over 900 m²/g. Despite the small amount of polyaniline nanofibers (1.3–16.5 wt.%), the composite aerogels were electrically conducting (8.0 × 10^? 8–1.83 × 10^? 5 S/cm) and it was possible to prepare chemiresistor sensors for detection of acidic (HCl) and basic (ammonia) gaseous molecules with response times similar to thin film sensors containing orders of magnitude more polyaniline.     

Silica-doped alumina cryogels with high thermal stability

Alumina cryogels with a dopant of silica in the content range of 0–10 wt% were synthesized from aqueous boehmite sol through the sol-gel technique and subsequent freeze drying. The higher thermal stability was achieved by the addition of 10 wt% silica; a γ-Al₂O₃ phase still remained after heating at 1200 °C for 5 h, and the surface area and pore volume were 47 m² g⁻¹ and 105 mm³ g⁻¹, respectively. The marked stability was ascribed to the synergetic effect of the very low bulk density (0.05 g cm⁻³) and the dopant. The thermal stability was lower for the cryogels than for the corresponding aerogels; however, it was also suggested that cryogel was highly durable in water in contrast to aerogel.     

Influence of TiO2 on proton conductivity in fuel cell electrolytes based on sol–gel derived P2O5–SiO2 glasses

P₂O₅–TiO₂–SiO₂ based glasses have been prepared by a sol–gel process. The glasses were characterized by structural, thermal, nitrogen adsorption–desorption and conductivity measurements. The structural formation has been confirmed by the FTIR and NMR analysis. The proton conductivity of the glasses increased linearly with increase in temperature. Glasses with an average pore size less than 2 nm showed higher values of proton conductivity in humid atmosphere. The conductivity value increased from 6.47 × 10⁻⁴ S/cm to 3.04 × 10⁻² S/cm at 70% RH in the temperature range 30–90 °C. We observed in fuel cell measurements that the performance of the E1 electrode is superior to that of the other electrodes at the same operating condition. The power density shows a similar pattern to current density.     

Effect of uni-axial loading on the nanostructure of silica aerogels

Aerogels are unique materials offering a combination of remarkable properties that make them useful in a wide range of applications. However, aerogel materials can be difficult to work with because they are fragile. The intent of the work presented here was to study the relationship between axial loading and pore structure in aerogel material. Silica aerogel samples with a bulk density of 0.1 g/mL were compressed by uni-axial force loads from 1 to 5 kN which resulted in stress levels up to 23 MPa. The resulting change in the pore distribution was observed using nitrogen desorption analysis and scanning electron microscopy. Uncompressed aerogel samples exhibit peak pore volume at diameters of about 20 nm. As the aerogels are subjected to increased loading, the location of the peak volume moves to smaller diameters with a reduced volume of pores occurring above this diameter. The peak diameter, the average pore diameter and pore volume all decrease and scale with increasing maximum stress while the surface area of the aerogel samples remains unaffected at about 520 m²/g. When combined with data from the literature, the relation between maximum pore diameter and applied stress suggests a failure mechanism dominated by bending induced fracture.     

Synthesis and structural characteristics of carbon aerogels with a high content of Fe,Co, Ni,Cu, and Pd

High content metal carbon aerogels have been prepared by sol–gel polymerization of formaldehyde with potassium salt of 2,4-dihydroxybenzoic acid, followed by K⁺-exchange with Fe(II), Fe(III), Co(II), Ni(II), Cu(II) and Pd(II) ions from an aqueous or acetoneous solution and subsequent supercritical drying with CO₂. Carbonization at 1050 °C, under an inert atmosphere, transforms the metal ion doped organic aerogels into metal and/or metal oxide nanoparticles-doped carbon aerogels. The resulting materials were characterized by means of elemental analysis, nitrogen adsorption, transmission electron microscopy and X-ray diffraction. The structural properties and metal concentration of the doped carbon aerogel depend on the type and valence of the precursor metal salt. The presence of some graphitic nano-ribbons was evidenced in the case of Fe-, Co-, and Ni-doped carbon aerogels.     

Experimental investigation and modeling of the creep behavior of ceramic fiber-reinforced SiO2 aerogel

Ambient creep tests at constant stresses of 0.05 and 0.1 MPa were conducted on a ceramic fiber-reinforced SiO₂ aerogel for more than 240 h. Experimental result shows that there are creep deformations even at very low stress level, and the creep strains finally reach constant values. High temperature creep experiments were also investigated at constant stresses of 0.05 and 0.2 MPa for 1800 s. The creep strains in 1800 s at 900 °C are 20–69 times higher than that at 300 °C. The creep behavior could be well described by the power-law creep model. Scanning Electron Microscope (SEM) analysis was also conducted to understand the micro mechanism of the creep behavior. Thermal softening of aerogel nanoparticles to generate larger ones was found to be the main reason for the change in mechanical properties at high temperature.     

Electrical conductivity improvement of aeronautical carbon fiber reinforced polyepoxy composites by insertion of carbon nanotubes

An increase and homogenization of electrical conductivity is essential in epoxy carbon fiber laminar aeronautical composites. Dynamic conductivity measurements have shown a very poor transversal conductivity. Double wall carbon nanotubes have been introduced into the epoxy matrix to increase the electrical conductivity. The conductivity and the degree of dispersion of carbon nanotubes in epoxy matrix were evaluated. The epoxy matrix was filled with 0.4 wt.% of CNTs to establish the percolation threshold. A very low value of carbon nanotubes is crucial to maintain the mechanical properties and avoid an overload of the composite weight. The final carbon fiber aeronautical composite realized with the carbon nanotubes epoxy filled was studied. The conductivity measurements have shown a large increase of the transversal electrical conductivity. The percolative network has been established and scanning electron microscopy images confirm the presence of the carbon nanotube conductive pathway in the carbon fiber ply. The transversal bulk conductivity has been homogenized and improved to 10^? 1 S·m^? 1 for a carbon nanotubes loading near 0.12 wt.%.     

Novel monolithic mesoporous foamed carbons prepared using micro-colloidal particles as templates

Novel mesoporous foamed carbons were synthesized from carbonization of organic gels templated by polymer micro-colloidal particles. Resorcinol and formaldehyde were allowed to gel in dilute polymethylmethacrylate (PMMA) microemulsion latex, subsequently the water in the gel was solvent exchanged with methanol and the wet gel was dried under ambient pressure. Pyrolysis was carried out at 800 °C to afford carbon xerogels with porous structures similar to those of resorcinol–formaldehyde (RF) carbon aerogels, but of higher density (>1.2 g/cm³), which provide the carbon materials with relatively higher volumetric surface area (up to 918 m²/cm³). Brunauer–Emmett–Teller (BET) adsorption results indicate that PMMA micro-colloid particles with mean diameter 25 nm contributed to the formation of mesopores of mean diameter at 5 nm.     

Temperature dependence of electric conductivity of bamboo coral skeleton and glass sponge spicules,the marine origin biomaterials

Natural structural biomaterials of marine origin including corals and sponges provide an abundant source of novel bone and cartilage replacements. Even though mechanical and optical properties of these natural biocomposites have been treated extensively in the literature, there is no available data on their electric properties. In the paper electric conductivity along with enthalpy of the process of electric charge conduction process were determined. The conductivity–temperature (σ–T) characteristics of native and deproteinated spicules of marine glass sponge and the mineral phase of bamboo corals provide information on the process of water release and phase transition in collagen. Denaturation temperature for coral was found at 203 °C. The peaks at 160° and 195 °C for the native glass sponge spicules may be explained by more complicated denaturation process where probably, less ordered regions of collagen denaturated at lower temperature and regions of high crystallinity at higher temperature. The decrease in electric conductivity observed after deproteination of glass sponge spicules showed that electric charge in the native glass sponge spicules was probably transported along the paths formed by collagen. Deproteination changed also σ–T characteristics, the kink at 140 °C is caused by glass transition of collagen, whereas the peak at 175 °C was probably caused by thermal denaturation of residual collagen of perturbed structure.     

Interfacial polarization in piezoelectric fibre–polymer composites

Polymer composites of an epoxy resin matrix filled with PZT fibres were studied by means of dielectric spectroscopy in the frequency range 0.1 Hz to 100 kHz and temperature interval from 80 °C to 170 °C. An interfacial or a Maxwell–Wagner–Sillars relaxation process was revealed in the frequency range between 0.1 Hz and 10 Hz and temperatures above the glass transition. This interfacial relaxation was found to follow the Debye law for the distribution of relaxation times.     

Zircon formation from amorphous powder and melt in the silica-rich region of the alumina–silica–zirconia system

Chemically homogeneous amorphous powders were prepared by sol–gel method from alkoxide mixtures in the silica-rich region of the alumina–silica–zirconia system. A glass with the same composition was obtained by quenching in water from the melt. The evolution with temperature in the range up to 1625 °C was studied by means of X-ray diffraction, infrared and UV-Raman spectroscopy, scanning electron microscopy and energy-dispersive X-ray analysis. Zircon crystallization and stability at temperatures higher than the liquidus temperature were confirmed in both melt-quenched and gel-glasses. Crystallization of cristobalite from the amorphous gel-glasses was observed above 1200 °C. The crystallization of zircon particles was observed after thermal treatment at 1550 °C. At 1625 °C zircon was the only stable crystalline phase due to the complete melting of the silica-rich matrix. The XRD pattern of the ternary gel treated at 1625 °C shows clearly the disappearance of the cristobalite phase, while at 1550 °C there is already partial melting. These results can help to determine the phase equilibrium in the ternary system.     

Transparent SiO2 aerogels prepared by ambient pressure drying with ternary azeotropes as components of pore fluid

Transparent SiO₂ aerogels were prepared by two-step sol–gel processing followed by ambient pressure drying at temperatures from 70 to 250 °C. The wet gels were synthesized via acid–base catalysis using tetraethyl orthosilicate as a silica precursor and isopropanol as a solvent. Isopropanol was exchanged with n-butanol, and the gel surface was modified using a trimethylchlorosilane solution in n-butanol. Next, the solvent was exchanged in several steps with saturated hydrocarbon in order to obtain pore fluids containing azeotropic mixtures of water, n-butanol and a corresponding hydrocarbon (hexane, heptane, octane, nonane). Ambient pressure drying was performed in two steps, at the boiling points of the ternary azeotropes and hydrocarbons, respectively. In this way, transparent, crack-free aerogels of different shapes, with a specific surface area of 1000 m²/g, average pore diameter of ～40–55 Å and density in the range 0.4–0.57 g/cm³ were obtained.     

Preparation of super hydrophobic silica aerogel and study on its fractal structure

The experimental results on silica aerogels with super hydrophobic property are reported. Silica alcogels were prepared via a two-step acid/base process by keeping the molar ratio of tetraethyoxysiliane (TEOS), ethanol (EtOH), water (H₂O), hydrochloric acid (HCl) and ammonia (NH₄OH) constant at 1:6:8:1.0 × 10^?3:1.1 × 10^?2, respectively, and varying the molar ratio of N,N-dimethylformamide (DMF)/TEOS (G) from 0 to 1.2. After two aging treatment steps, they were modified by isopropyl alcohol (IPA)/trimethylchlorosilane (TMCS)/n-hexane solution at 60 °C. It was found that G value at 0.8 resulted in low density (～0.2 g cm^?3) and the minimum volume shrinkage (～6%), with the total water adsorption ratio ～5.1% when exposed to water for 3 months and the contact angle θ ≈ 178°. Besides, the aerogels (G = 0.8) had higher volume fractal dimension (～1.8), which indicted that it possessed better connectivity and more uniform particle sizes.     

Performance investigation of Li2O–Al2O3–4SiO2 based glass–ceramics with B2O3, Na3AlF6 and Na2O fluxes

Influence of single fluxes (10 wt.% B₂O₃), bi-component fluxes (4 wt.% B₂O₃ + 6 wt.% Na₃AlF₆), and complex fluxes (4 wt.% B₂O₃ + 4 wt.% Na₃AlF₆ + 2 wt.% Na₂O) on the thermal kinetic parameters, microstructure, flexural strength and coefficient of thermal expansion (CTE) of Li₂O–Al₂O₃–4SiO₂ (LAS) glass–ceramics was investigated through differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscope (SEM). The results showed that complex fluxes could efficiently decrease transition temperature (T_g) and crystallization temperature (T_p), and accelerate the formation of needle-like β-spodumene crystals which benefit high flexural strength. The homogeneous LAS glass–ceramic (sample C₃) which has a high strength of 132.4 MPa and low CTE (100–650 °C) of 2.74 × 10^? 6/°C is obtained by doping of the initial LAS glass by complex fluxes of 4 wt.% B₂O₃, 4 wt.% Na₃AlF₆, and 2 wt.% Na₂O, nucleating at 630 °C/120 min and then crystallized at 780 °C/120 min. It is worthy of further investigation as a bonder of diamond composite material due to its outstanding prosperities.     

Effect of OH-content on thermal and chemical properties of SnOP2O5 glasses

Glasses with 55–60 mol% SnO and 40–45 mol% P₂O₅ have shown extremely large differences in the chemical and thermal properties depending on the temperature at which they were melted. Glasses prepared at low melting temperature, 450–550 °C, had low T_g, 150–200 °C, and low chemical stability. Glasses prepared at high melting temperature, 800–1200 °C, had much higher T_g, 250–300 °C, and much higher chemical stability. No significant differences were found by ¹¹⁹Sn Mössbauer and ³¹P Nuclear Magnetic Resonance spectroscopy. Large differences in the OH-content could be detected as the reason by infrared absorption spectroscopy, thermal analyses, and ¹H Nuclear Magnetic Resonance spectroscopy. In samples with low T_g, a broad OH – vibration band around 3000 nm with an absorption intensity >20 cm^?1, bands at 2140 nm with intensity ～5 cm^?1, at 2038 nm with intensity ～2.7 cm^?1, and at 1564 nm with intensity ～0.4 cm^?1 were measured. These samples have shown a mass loss of 3–4 wt% by thermal gravimetric analyses under argon in the temperature range 400–1000 °C. No mass loss and only one broad OH-band with a maximum at 3150 nm and low absorption intensity <4 cm^?1 could be detected in samples melted at high temperature, 1000–1200 °C, which have much higher T_g, ～300 °C, and much higher chemical stability.     

Dissolution behavior of ZnO–P2O5 glasses in water

Bulk binary ZnO–P₂O₅ glasses with 50–70 mol% ZnO were immersed in distilled water at 30–90 °C for up to 72 h. The immersed samples were characterized by weight loss, the change in solution pH, X-ray diffraction (XRD) analysis, scanning electron microscopy and Raman spectroscopy. Weight loss decreased with ZnO concentration for all immersion temperatures. Dissolution behavior was classified into two types in terms of weight loss and macroscopic appearance. Type I was primarily recognized in 50–60 mol% ZnO glasses. In type I, the weight loss for 72 h was relatively large (>1.0 × 10⁻⁷ kg mm⁻², >10% of initial sample weight). Raman spectra of the type I glasses indicated that the depolymerization of phosphate glass network occurred during the dissolution process. Crystalline Zn₂P₂O₇ · 3H₂O was precipitated in the water solution after immersion. Type II dissolution behavior was recognized in the 65 and 70 mol% ZnO glasses except for the 65ZnO–35P₂O₅ glass immersed at 90 °C. In the type II behavior, the weight loss for 72 h was relatively-small (<1.0 × 10⁻⁸ kg mm⁻², <1% of initial sample weight). The microstructure of the type II glass indicated selective dissolution. The dissolution process of the type II glass is discussed.     

Thermal expansion of glass–ceramics in the system BaO/Al2O3/B2O3

Glasses in the system BaO/Al₂O₃/B₂O₃ with and without the addition of platinum were melted. In one sample series, the BaO-concentration was varied while the ratio [Al₂O₃]/[B₂O₃] was kept constant. In another sample series, the [BaO]/[Al₂O₃]-ratio (= 0.9) was kept constant and the B₂O₃ concentration was varied. The samples were thermally treated at 720 °C for 24 h and subsequently at 780 °C for 4 h. In most thermally treated samples, the crystalline phase BaO · Al₂O₃ · B₂O₃ occurred. At some compositions, the platinum-doped samples showed larger concentrations of the crystalline phases. The most remarkable property of the obtained glass–ceramics is their zero or negative thermal expansion coefficient. Here, notable differences were observed: samples with fine grained microstructures showed thermal expansion coefficients approximately zero up to temperatures of around 80 °C. By contrast, samples with coarser microstructures and large spheroidal crystals exhibit negative expansion coefficients up to temperatures of around 280–375 °C. The thermal expansions of these samples were close to those of the mean thermal expansion of the unit cell of the BaO · Al₂O₃ · B₂O₃ phase. The thermal expansion of the fine grained samples was approximately equal to that of the crystallographic a-axis of the BaO · Al₂O₃ · B₂O₃ phase.