Synthesis of nanoporous silica aerogel by ambient pressure drying

A crack-free silica aerogel monolith was fabricated from a cheap water glass derived silicic acid solution by adding glycerol, which served as a drying control chemical additive (DCCA). The OH surfaces of the wet gel with glycerol were modified using a TMCS/n-hexane mixture followed by solvent exchange from water to n-hexane. The obtained surface modified wet gel was dried at 75 °C under ambient pressure. The addition of glycerol appears to give the wet gel a more homogeneous microstructure (larger pore size and uniform size distribution) as well as enhanced stiffness. However, glycerol also retards surface modification and solvent exchange. The aerogel synthesized with glycerol added to the silica sol maintained a relatively low bulk density compared with the aerogels aged in a mixed ethanol (EtOH)/TEOS solution. The reproducibility of aerogel production was further improved in the aerogel synthesized with glycerol added to the silica sol and aged in a 70%EtOH/30%TEOS solution.     

Influence of thermal process on microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths

The experimental results of thermal process on the microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths are reported and discussed. With sodium silicate as precursor, ethanol/hexamethyldisiloxane/hydrochloric acid as surface modification agent, the crack-free and high hydrophobic silica aerogel monoliths was obtained possessing the properties as low density (0.096 g/cm³), high surface area (651 m²/g), high hydrophobicity (~147°) and low thermal conductivity (0.0217 Wm/K). Silica aerogels maintained hydrophobic behavior up to 430 °C. After a thermal process changing from room temperature to 300 °C, the hydrophobicity remained unchanged (~128°), of which the porosity was 95.69% and specific density about 0.094 g/cm³. After high temperature treatment (300–500 °C), the density of final product decreased from 0.094 to 0.089 g/cm³ and porosity increased to 96.33%. With surface area of 466 m²/g, porosity of 91.21% and density about 0.113 g/cm³, silica aerogels were at a good state at 800 °C. Thermal conductivities at desired temperatures were analyzed by the transient plane heat source method. Thermal conductivity coefficients of silica aerogel monoliths changed from 0.0217 to 0.0981 Wm/K as temperature increased to 800 °C, revealed an excellent heat insulation effect during thermal process.     

Preparation and characterization of ruthenium/carbon aerogel nanocomposites via a supercritical fluid route

Carbon-aerogel-supported ruthenium nanoparticles were synthesized by impregnating carbon aerogels with Ru(acac)3 or Ru(cod)(tmhd)2 from supercritical carbon dioxide (scCO2) solutions, followed by thermal reduction of these precursors. Two different carbon aerogels with pore diameters of 4 and 21 nm were synthesized. The kinetics and the thermodynamics of impregnation of carbon aerogels with the ruthenium coordination complexes were studied. The approach-to-equilibrium data indicated very fast adsorption, and the adsorption isotherms were found to follow the Langmuir model. The impregnated carbon aerogel complexes were reduced thermally at different temperatures between 300 and 1000 degrees C in the presence of nitrogen. The resulting nanocomposites were characterized using transmission electron microscopy (TEM) and hydrogen chemisorption. TEM micrographs showed that the ruthenium nanoparticles were dispersed homogeneously throughout the porous carbon aerogel matrix, and the average sizes obtained under different conditions ranged from 1.7 to 3.8 nm. Once complete decomposition of the precursor had been achieved, the mean size of the ruthenium particles increased with increasing reduction temperature.     

Synthesis and characterization of monolithic carbon aerogel nanocomposites containing double-walled carbon nanotubes

We report the synthesis and characterization for the first examples of monolithic low-density carbon aerogel (CA) nanocomposites containing double-walled carbon nanotubes. The CA nancomposites were prepared by the sol-gel polymerization of resorcinol and formaldehyde in an aqueous surfactant-stabilized suspension of double-walled carbon nanotubes (DWNTs). The composite hydrogels were then dried with supercritical CO 2 and subsequently carbonized under an inert atmosphere to yield monolithic CA structures containing uniform dispersions of DWNTs. The microstructures and electrical conductivities of these CA nanocomposites were evaluated for different DWNT loadings. These materials exhibited high BET surface areas (>500 m (2)/g) and enhanced electrical conductivities relative to pristine CAs. The details of these results are discussed in comparison with theory and literature.     

Low thermalconductive,transparent and hydrophobic ambient pressure dried silica aerogels with various preparation conditions using sodium silicate solutions

The experimental results on the preparation of low thermal conductivity and transparent ambient pressure dried silica aerogels with the sodium silicate solution, TMCS silylating agent with methanol, isopropyl alcohol, hexane and xylene solvents, are reported. This study is focussed on the effect of preparation conditions such as varying the number of preparation steps, pH of the hydrosol and hydrogel ageing temperature, for the production of the low thermal conductive silica aerogels and the results are analysed. Density, thermal conductivity, % of optical transmission and contact angle of the aerogels were measured. The Fourier Transform Infrared Spectroscopy (FTIR) studies revealed the presence of Si–C and C–H along with the Si–O–Si and OH bonds and their intensities strongly depend on the processing steps, pH of the hydrosol and gel ageing temperature. The UV–Visible spectra indicated the % of optical transmission of the aerogels decreased with increasing the number of processing steps, increase in the pH of the hydrosol from 3 to 8 and decreased for ageing temperature up to 50 °C. Further increase in temperature >50 °C, the % of optical transmission of the aerogels increased. The TGA-DTA data showed the thermal stability of the aerogels with respect to hydrophobicity is 325 °C. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) analyses revealed the nanostructure of the aerogels. The porosity of the aerogels was studied using the pore size distribution. Silica aerogels with low density (0.051 g/cc), low thermal conductivity (0.049 W/m K), optical transmission (65%), high hydrophobicity (159°) and resistance to humid atmosphere >1 year was obtained in the present studies.     

Effect of 3-glycidoxypropyltrimethoxysilane precursor on the properties of ambient pressure dried silica aerogels

The effect of an organically modified precursor, 3-glycidoxypropyltrimethoxysilane in an ambient pressure process involving aging in silane solution for silica aerogels is presented. The effect of increasing trialkoxysilane/tetraalkoxysilane precursor ratio and the influence of water to Si molar ratio on the gelation and adsorption properties were investigated. An optimum water to Si molar ratio (8) gave the fastest gelation for all precursor ratios indicating a balance between the increase in rate of hydrolysis and a decrease in concentration of the monomers. Surface area analysis proved that in the dried gel, the organic groups are largely present on the pore walls and prevent the condensation of the silanol groups during drying. This in turn prevents pore collapse and further increases the total pore volume. The inclusion of the organically functionalised silane in the process further enhances the ambient pressure drying through this effect.     

Study of thermal conductivity and effect of humidity on HMDZ modified TEOS based aerogel dried at ambient pressure

The experimental results on the study of thermal conductivity and effect of humidity on HMDZ modified TEOS based aerogels dried at ambient pressure, are reported. Silica sol was prepared by keeping the MeOH/TEOS molar ratio, Acidic water (Oxalic acid) and basic water (NH₄OH) concentrations constant at 16.5, 0.001 and 1 M, respectively throughout the experiments and the HMDZ/TEOS molar ratio (h) was varied from 0.34 to 2.1. Finally, the surface modified wet gels were dried at an ambient pressure. The thermal conductivity of the aerogel samples was measured. Further, the humidity study was carried out in 80% humid surrounding at 30 °C temperature over 80 days. The best quality aerogels in terms of low bulk density, thermal conductivity and durability (no moisture absorption) with an only 2% of weight gain were obtained for TEOS: MeOH: Acidic H₂O: Basic H₂O: HMDZ molar ratio at 1:16.5:0.81:0.50:0.681, respectively. The thermal stability and hydrophobicity of the aerogel have been confirmed with Thermo gravimetric and Differential Thermal (TG–DT) analyses and Fourier Transform Infrared Spectroscopy (FTIR), respectively. Microstructural studies were carried out by Scanning Electron microscopy (SEM).     

Synthesis of polylactide/clay nanocomposites by in situ intercalative polymerization in supercritical carbon dioxide

Polylactide (PLA)/clay nanocomposites have been prepared by in situ ring-opening polymerization in supercritical carbon dioxide. Depending on the type of organoclay used, polylactide chains can be grafted onto the clay surface, leading to an exfoliated morphology. Nanocomposites with high clay contents (30-50 wt.%), called masterbatches, have also been successfully prepared and were recovered as fine powders thanks to the unique properties of the supercritical fluid. Dilution of these masterbatches into commercial l-polylactide by melt blending has led to essentially exfoliated nanocomposites containing 3 wt.% of clay. The mechanical properties of these materials have been assessed by flexion and impact tests. Significant improvements of stiffness and toughness have been observed for the PLA/clay nanocomposites compared to the pure matrix, together with improved impact resistance.     

Synthesis and characterization of poly1-hexene/silica nanocomposites

SiO₂ nano particles, with particle size of 12 nm, were first modified by substituting surface OH groups with O-hexyl moiety. Then, poly1-hexene/modified-SiO₂ composites with various nano-SiO₂ weight fractions were prepared by three different methods: in situ, solution, and melt methods and designated as PH-SiO₂/Insitu, PH-SiO₂/Sol and PH-SiO₂/Melt, respectively. PH-SiO₂/Insitu samples showed highly uniform particle dispersion up to 30 wt. % of silica while in PH-SiO₂/Sol and PH-SiO₂/Melt samples agglomeration of the silica nanoparticles occurred for filler contents ≥5 wt. % (i.e. 5, 10, 20 and 30 wt%). In the synthesized composites, the storage modulus significantly increased as high as 20.7 times when compared with neat poly1-hexene. Maximum decomposition temperature (T_max) and char yield at 600 °C increased with increasing silica level. Rheological results showed that G^ʹ> G^ʺ over the frequency range, illustrating the elastic behavior of the composite samples. In fact, samples showed the characteristic of a non-Newtonian fluid with a strong shear thinning effect in which η* increased with increasing filler weight fraction. From the results, it can be expected that modified silica could replace silica nanoparticles in polyolefin nanocomposite reinforcement.     

Synthesis, characterisation and cytotoxicity of polyoxometalate/carboxymethyl chitosan nanocomposites

Chitosan and its derivates continue to attract considerable research interest as effective drug carriers with good biocompatibility and high cellular uptake rates. We used these versatile features to tap the considerable biomedical potential of polyoxometalates (POMs) through their encapsulation into a carboxymethyl chitosan (CMC) matrix. The nanocapsules were prepared by ionic gelification with Ca(2+); their size distribution ranges from 60 to 150 nm. Because [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) is well known for its manifold properties, such as antiviral activity, it was selected as a model POM. The resulting composites were characterised with a wide range of analytical methods, which pointed to quantitative encapsulation of intact POMs within the CMC matrix. We studied the biocompatibility of the POM/CMC nanocomposites on HeLa cells through MTT and proliferation assays. Even after prolonged incubation times at high concentrations, the composites did not display cytotoxicity, thereby drastically reducing the side effects of the pristine POMs. This opens up new avenues for designing novel inorganic drug prototypes from bioactive POMs.     

Densification of silica glass at ambient pressure

We show that densification of silica glass at ambient pressure as observed in irradiation experiments can be attributed to defect generation and subsequent structure relaxation. In our molecular dynamics simulations, defects are created by randomly removing atoms, by displacing atoms from their nominal positions in an otherwise intact glass, and by assigning certain atom excess kinetic energy (simulated ion implantation). The former forms vacancies; displacing atoms and ion implantation produce both vacancies and "interstitials." Appreciable densification is induced by these defects after equilibration of the defective glasses. The structural and vibrational properties of the densified glasses are characterized, displaying resembling features regardless of the means of densification. These results indicate that relaxation of high free-energy defects into metastable amorphous structures enriched in atomic coordination serves as a common mechanism for densification of silica glass at ambient pressure.     

Microporous carbon aerogel prepared through ambient pressure drying route as anode material for lithium ion cells

Carbon aerogel synthesized through a cost‐effective and easy method was evaluated and found to be a promising anode material for lithium ion cells. Carbon aerogel was prepared by carbonizing resorcinol–formaldehyde (RF) aerogel under inert atmosphere. Resorcinol–formaldehyde aerogel in turn was prepared through sol gel polymerization of resorcinol with formaldehyde using sodium carbonate as catalyst adopting ambient pressure drying route. The structure and the morphology of the prepared carbon aerogel are investigated using X‐ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and surface area determined using N₂–Brunauer–Emmett–Teller (BET) method. The TEM images reveal microporous morphology of the carbon aerogel particles. The evaluation of carbon aerogel as an anode material revealed promising specific capacity synergized with outstanding cyclability. The first cycle specific capacity was 288 mAh/g with an efficiency of 63% at C/10 rate. The material retained a capacity of 96.9% of the initial capacity with about 100% efficiency after 100 cycles, showing the excellent cyclability of the material. Copyright © 2017 John Wiley & Sons, Ltd.     

溶胶凝胶法制备片状氧化硅/氧化锆纳米复合材料(英文)

以氧氯化锆和二甲基二乙氧基硅烷为前驱体,在无任何表面活性剂的条件下,利用溶胶-凝胶法制备了片状SiO_2/ZrO_2复合材料.采用X射线衍射仪、扫描电镜、能谱仪、傅立叶变换红外光谱仪、透射电镜等分析了产物的结构和形貌,并探讨了SiO_2/ZrIO_2纳米复合材料的取向生长机理.结果表明,在正丁醇和水的混合溶液中制备的SiO_2/ZrO_2纳米复合材料的粒径为18nm,将干凝胶的500℃空气气氛中焙烧2h可以得到四方相片状氧化锆晶体.化合物中的硅原子通过Si—O—Zr化学键与锆原子相连;硅的引入仅影响氧化锆晶体的形貌,而不导致四方相氧化锆结构和尺寸的改变.本文所述方法可望拓展用于制备其他具有类似分级结构的无机功能纳米材料.     

Production of low-density sodium silicate-based hydrophobic silica aerogel beads by a novel fast gelation process and ambient pressure drying process

We report a method to synthesize low-density transparent mesoporous silica aerogel beads by ambient pressure drying (APD). The beads were prepared by acid–base sol–gel polymerization of sodium silicate in aqueous ammonia solution via the ball dropping method (BDM). To minimize shrinkage during drying, wet silica beads were initially prepared; their surfaces were then modified using trimethylchlorosilane (TMCS) via simultaneous solvent exchange and surface modification. The effects of the volume percentage (%V) of TMCS on the physical and textural properties of the beads were investigated. The specific surface area and cumulative pore volume of the silica aerogel beads increased with an increase in the %V of TMCS. Silica aerogel beads with low packing bed density (0.081 g/cm³), high surface area (917 m²/g), and large cumulative pore volume (2.8 cm³/g) was obtained when 10%V TMCS was used. Properties of the final product were examined by FE-SEM, TEM, BET, and TG–DT analyses. Surface chemical modifications were confirmed by FTIR spectroscopy. The hydrophobic silica aerogel beads were thermally stable up to 411 °C. We discuss our results and compare our findings for modified versus unmodified silica beads.