Physical Properties of Silica Aerogels Prepared with Polyethoxydisiloxanes

Silica aerogels were made by sol-gel techniques using industrial silicon derivatives (polyethoxydisiloxanes, E-40), followed by supercritical drying with ethanol. The morphology and microstructure of the silica aerogels were investigated by using specific surface area, S_BET, SEM, TEM and the pore size distribution techniques. The thermal conductivity was also measured as a function of air pressure. The results show that the diameter of the silica particles is about 13 nm and the pore size of the silica aerogels is 20–80 nm. The specific surface area of the silica aerogel is about 470 m²/g and the thermal conductivity of the silica aerogel prepared with E-40 is 0.014 w m^–1 K^–1 at room temperature and 1 atm.     

Effective preparation of crack-free silica aerogels via ambient drying

Effective ambient-drying techniques for synthesizing crack-free silica aerogel bulks from the industrial waterglass have been developed. Silica wet gels were obtained from aqueous colloidal silica sols prepared by ion-exchange of waterglass solution (4–10 wt% SiO₂). Crack-free monolithic silica aerogel disks (diameter of 22 mm and thickness of 7 mm) were produced via solvent exchange/surface modification of the wet gels using isopropanol/trimethylchlorosilane/n-Hexane solution, followed by ambient drying. The effects of the silica content in sol and the molar ratio of trimethylchlorosilane/pore water on the morphology and property of final aerogel products were also investigated. The porosity, density, and specific surface area of silica aerogels were in the range of 92–94%, 0.13–0.16 g/cm³, and ∼675 m²/g, respectively. The degree of springback during the ambient drying processing of modified silica gels was 94%.     

Rapid Preparation of Mesoporous Methylsilsesquioxane Aerogels by Microwave Heating Technology

Microwave heating technology is known as an alternative to traditional gas and electric heating sources. In this work, mesoporous methylsilsesquioxane (MSQ) aerogels were prepared via a sol–gel process accompanied by microwave heating technology, and microwave heating was used in the gelation of sol and the drying of wet gels, respectively. The effects of hexadecyltrimethylammonium chloride (CTAC) as a surfactant and template, hydrochloric acid (HCl) as a catalyst, ethanol as a solvent, sodium hydroxide (NaOH) as a gelation agent, and microwave power on the pore structure of as-prepared MSQ aerogels were investigated in detail. Microwave heating at low power results in the acceleration of sol–gel transition and achieves the gelation within a few minutes. Appropriate amounts of chemical reagents and microwave heating at high power allow the preparation of mesoporous MSQ aerogels with a BET-specific surface area of 681.6 m²·g⁻¹ and a mesopore size of 19 nm, and the resultant MSQ aerogel still has a BET specific surface area as high as 134 m²·g⁻¹ after heat treatment at 600 °C for 2 h, showing high thermal stability. The MSQ aerogels/fibre composite possesses a low thermal conductivity of 0.039 W/(m·k)⁻¹, displaying good thermal insulation. Microwave heating technology is a promising heating method for the preparation of other aerogels.     

快速制备高掺杂CuO/SiO2复合气凝胶

通过环氧丙烷预反应法, 以乙腈为溶剂快速制备了高掺杂的氧化铜/二氧化硅复合气凝胶. 在典型的合成过程中, 将正硅酸甲酯(TMOS)、乙腈、去离子水和环氧丙烷混合进行预反应, 然后将该溶液与氯化铜的乙腈-水溶液混合并添加环氧丙烷, 在35℃烘箱中静置0.5 h 后转化为湿凝胶, 再经过CO₂超临界流体干燥和热处理即可获得黑色块状CuO/SiO₂复合气凝胶. 最终气凝胶样品密度约为180 mg·cm^-3, 比表面积高达625 m2·g^-1, 平均掺杂比为19.91%±2.42% (Cu:Si 摩尔比), 压缩模量为1.639 MPa, 具有成型性好、分散均匀等优点,是良好的背光源靶材料. 本论文还通过对比实验对凝胶化过程的机理进行分析, 结果表明, 通过改变溶剂和采用环氧丙烷预催化均衡了两种不同前驱体的反应速率, 实现了共凝胶的目的. 此外, 该方法还有望为其它金属氧化物/二氧化硅复合气凝胶的制备提供新思路.     

Porous structure of polybenzoxazine-based organic aerogel prepared by sol–gel process and their carbon aerogels

New organic aerogels were successfully prepared from a new class of phenolic resins called polybenzoxazines synthesized via conventional thermal curing reaction of a benzoxazine monomer using xylene as a solvent. Without the need for using supercritical conditions to remove the solvent during the process, the carbon aerogels were obtained with a much shortened time. From two different concentrations of benzoxazine solution, 20 and 40 wt%, the resulting polybenzoxazine aerogels, having densities of 260 and 590 kg/m³, respectively, were obtained after the curing process. The subsequent carbon aerogels were prepared by the carbonization of polybenzoxazine aerogels. The corresponding carbon aerogels exhibited a microporous structure with pore diameters less than 2 nm, the densities of 300 and 830 kg/m³, and surface area of 384 and 391 m²/g, respectively. The texture of the carbon aerogels was denser than that of their organic aerogel precursor, as evidenced by scanning electron microscopy. The transformation of the polybenzoxazine aerogel to the carbon aerogel was clearly observed using fourier transform infrared spectroscopy.     

Cellulose–silica composite aerogels from “one-pot” synthesis

Cellulose–silica composite aerogels were prepared via “one-pot” process: aqueous solutions of cellulose–8 wt% NaOH and sodium silicate were mixed, coagulated and dried with supercritical CO₂. The system was studied both in the fluid and solid (dry) states. Cellulose and sodium silicate solutions were mixed at different temperatures and concentrations; mixture properties were monitored using dynamic rheology. The gelation time of the mixture was strongly reduced as compared to that of cellulose–NaOH solutions; we interpret this phenomenon as cellulose self-aggregation inducing partial coagulation due to competition for the solvent with sodium silicate. The gelled cellulose/sodium silicate samples were placed in aqueous acid solution which completed cellulose coagulation and led to in situ formation of sub-micronic silica particles trapped in a porous cellulose matrix. After drying with supercritical CO₂, an organic–inorganic aerogel composite was formed. The densities obtained were in the range of 0.10–0.25 g/cm³ and the specific surface area was between 100 and 200 m²/g. The silica phase was shown to have a reinforcing effect on the cellulose aerogel, increasing its Young’s modulus.     

Preparation and characterization of silica aerogels from diatomite via ambient pressure drying

The silica aerogels were successfully fabricated under ambient pressure from diatomite. The influence of different dilution ratios of diatomite filtrate on physical properties of aerogels were studied. The microstructure, surface functional groups, thermal stability, morphology and mechanical properties of silica aerogels based on diatomite were investigated by BET adsorption, FT-IR, DTA-TG, FESEM, TEM, and nanoindentation methods. The results indicate that the filtrate diluted with distilled water in a proportion of 1: 2 could give silica aerogels in the largest size with highest transparency. The obtained aerogels with density of 0.122–0.203 g/m³ and specific surface area of 655.5–790.7 m²/g are crack free amorphous solids and exhibited a sponge-like structure. Moreover, the peak pore size resided at 9 nm. The initial aerogels were hydrophobic, when being heat-treated around 400°C, the aerogels were transformed into hydrophilic ones. The obtained aerogel has good mechanical properties.     

Silica–titania aerogel monoliths with large pore volume and surface area by ambient pressure drying

Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels show densities in the range 0.34–0.38 g/cm³. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones. The surface area for 5wt% titania aerogel has been found to be as high as 685 m²/g with a pore volume of 2.34 cm³/g and the 10wt% titania aerogel has a surface area of 620 m²/g with a pore volume of 2.36 cm³/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The surface modified aerogels possess high surface areas in the range of 540–640 m²/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity of the aerogel.     

Wettability study of surface modified silica aerogels with different silylating agents

In wettability study, surface free energy interactions are of crucial importance for silica aerogels in which absorption of organic liquids and transportation of chemicals carried out for chemical and biotechnological applications. In present study, we have used Lifshitz–van der Waals/acid–base approach for calculation of surface free energy of aerogel sample. We have investigated that the surface free energy values of aerogels are 45.95, 51.42 and 45.69 mJ/m² by modifying their surfaces using 7 % chlorotrimethylsilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ) silylating reagents with solvent, respectively. The alcogels were prepared by two step acid–base catalyzed process where the molar ratio of precursors tetraethoxysilane:methanol:oxalic acid:NH₄OH:NH₄F was kept at optimal value of 1:16.5:0.71:0.58:0.60:0.98, respectively. To modify gel surfaces, TMCS, DMDCS and HMDZ concentration have been varied from 5 to 12 % and such alcogels were dried at ambient pressure. The aerogels have been characterized by fourier transform infrared spectroscopy, scanning electron microscopy, thermo-gravimetric and differential thermal analysis and Wetting properties of silica aerogel surfaces was studied by contact angle measurements. The surface chemical composition of DMDCS modified silica aerogels was studied by using X-ray photoelectron spectroscopy. As there is not any direct method, we have used Lifshitz–van der Waals/acid–base approach which gives, polar and non-polar components of aerogels surface free energy.     

Synthesis of freestanding silica and titania-silica aerogels with ordered and disordered mesopores

Freestanding blocks of silica and titania-silica aerogels were prepared by the sol-gel method. It is possible to prepare crack-free, titania-silica aerogels with high titanium content by a careful control of the synthesis conditions. Prehydrolysis, complexation and polymer addition were used to adjust the hydrolysis and condensation rates of the silicon and titanium alkoxide precursors. Photoactive anatase TiO₂ nanocrystals with a large surface area (i.e., up to 300m²g⁻¹) were crystallized from the gel network by the high-temperature ethanol supercritical drying, and the resulting aerogel blocks were gas permeable and display a transition-regime diffusion behavior. Pore and volume shrinkages were observed in samples prepared by ethanol supercritical drying when the titanium content was increased resulting in a lower flux. Adding Pluronic P123 creates ordered mesopore domains and produces large pore aerogels even at high titanium contents. The photocatalytic oxidation reaction of trichloroethylene was performed by flowing the reactant gas mixture through the UV-irradiated aerogel blocks with excellent results.     

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.     

Poly(lactic acid)-based polymer composites with high electric and thermal conductivity and their characterization

Electrically and thermally conductive polymer composites on the basis of biodegradable poly(lactic acid) (PLA) were developed and studied in this work. Pristine single-walled carbon nanotubes (CNTs) and powder of natural graphite (G) were used as fillers in polymer composites. PLA-based composites were prepared by melt-compounding method. The volume resistivity of PLA/CNT composites can be changed by more than ten orders of magnitude compared to that for neat PLA. The thermal conductivity of PLA/G composites can be changed from 0.193 W⋅m⁻¹⋅K⁻¹ (neat PLA) up to 2.73 W⋅m⁻¹⋅K⁻¹. Loading small quantity of CNTs into PLA/G composites increases the thermal conductivity not less than by 40% of magnitude. Besides, all developed PLA-based composites are suitable for processing by injection molding, extrusion or additive manufacturing technology (3D printing).     

One-pot synthesis,characterization and properties of acid-catalyzed resorcinol/formaldehyde cross-linked silica aerogels and their conversion to hierarchical porous carbon monoliths

A rapid and facile synthesis of resorcinol/formaldehyde cross-linked silica (RF/SiO₂) aerogels was carried out in one pot based on an acid-catalyzed route, instead of the previously reported base-catalyzed ones. The gelation time was reduced to several hours at room temperature while it took several days even under heating conditions in the base-catalyzed ones. The interpenetrating network of RF/SiO₂ aerogels showed similar porous structures with those of silica aerogels or RF aerogels. Their thermal conductivity was as low as that of the typical glass wool materials. The mechanical properties are characterized by dynamic mechanical analysis and compression testing. At room temperature, the results of compression testing show that the compressive Young’s modulus or ultimate failure strength of RF/SiO₂ aerogel specimen is higher than that of native SiO₂ aerogels with a similar density. The one-pot method improves the efficiency and reduces the cost of RF/SiO₂ aerogels. The hierarchical porous carbon monoliths are also converted from carbonized RF/SiO₂ aerogels by an additional HF treatment. Hence, they could be further explored as multifunctional candidate materials for thermal, mechanical, and electrochemical applications.     

Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH₄F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N₂ adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m²/g were obtained by using NH₄F (1 M), HCl (4 M), and a molar ratio of Na₂SiO₃::H₂O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C.     

Synthesis of MWCNTs doped sodium silicate based aerogels by ambient pressure drying

The successful incorporation of multiwalled carbon nanotubes (MWCNTs) into silica aerogels prepared by sol–gel method is reported herein. Pure silica aerogels prepared using sodium silicate precursor by ambient pressure drying are so fragile that they cannot be used easily. MWCNTs were used as reinforcements to improve the mechanical properties of silica aerogels. Results show that inserting small amounts of MWCNTs in the gels causes enhanced dimensional stability of silica aerogels. The silica aerogels were prepared by doping MWCNTs in silica matrix before gelation. The influence of MWCNTs on some microstructural aspects of silica matrix has been studied using nitrogen adsorption–desorption isotherms. From SEM study it is confirmed that the silica particles get capped on the surface of MWCNTs suggesting an enhanced toughness. Further, FTIR, Raman, EDAX, thermal conductivity and hydrophobicity studies of these doped aerogels were carried out. By addition of MWCNTs, silica aerogels were formed with 706 m²/g BET and 1,200 m²/g Langmuir surface areas and 149^o contact angle. Low density (0.052 g/cc) and low thermal conductivity (0.067 W/m K) MWCNTs doped silica aerogels were obtained for the molar ratio of Na₂SiO₃::H₂O::MWCNTs::citric acid::TMCS at 1::146.67::2.5 × 10⁻³::0.54::9.46 respectively with improved mechanical strength.     

Preparation and evaluation of amphiphilic silica‐based monolithic column having surface‐bound octanoyl‐aminopropyl moieties for capillary electrochromatography

A novel amphiphilic silica‐based monolithic column having surface‐bound octanoyl‐aminopropyl moieties was successfully prepared by a one‐step in situ derivatization process. As expected, the amphiphilic monolithic column exhibited RP chromatographic behavior toward non‐polar solutes (e.g., alkyl benzenes) with high column performance. As the pH of the buffer inside the column increases, the EOF changed from −2.65×10⁻⁸m² V⁻¹s⁻¹ at pH 3.0 to 1.20×10⁻⁸ m² V⁻¹s⁻¹ at pH 8.0 with the reversion of EOF at about pH 6.4. Using acidic mobile phase, five aromatic acids can be efficiently separated in less than 6 min under co‐EOF conditions. For basic compounds, symmetrical peaks were obtained due to the existence of hydrophilic acyl amide group, which can effectively minimize the adsorption of the positively charged basic analyte to the silica‐based surface of the capillary column.     

Acidity of Alginate Aerogels Studied by FTIR Spectroscopy of Probe Molecules

Supercritical drying of alginate gels is an efficient way to prepare aerogels with high surface area (>300 m² · g⁻¹). FTIR spectroscopy allows to monitor the adsorption of NH₃ from the gas phase onto the acid sites of the alginate. Free carboxylic groups are effective Brønsted sites, whereas the divalent cations used in the ionotropic gelation present the properties of Lewis sites. The ratio between Brønsted and Lewis sites provides infomation on the role of pH in alginate gelation and suggests that non-buffered gelation by transition-metal cations is a mixed ionotropic-acid process.     

The effect of heat-treatment on the physico-chemical properties of silica aerogel prepared by sub-critical drying technique

Synthesis of transparent and crack-free monoliths of silica aerogel by sub-critical drying technique is reported in the present article. Silane ageing with 50% tetraethylorthosilicate:ethanol followed by solvent exchange using ethanol was adopted. The effect of heat-treatment on the textural and physical characteristics of silica aerogel was evaluated. The chosen composition resulted in a high surface area silica aerogel of 1,000 m² g⁻¹ and a pore volume of 1.4 cm³ g⁻¹ at room temperature. The aerogel heat-treated at 900 °C possessed a surface area of 450 m² g⁻¹ with a pore volume of 0.4 cm³ g⁻¹. The decrease in surface area and pore volume was associated with the sintering process. The present technique seems advantageous in preserving the high surface area of the material at high temperatures. The XRD studies showed that the amorphous nature of aerogel matrix was retained till 1,400 °C, beyond which it crystallized to phase pure crystoballite.     

Zirconia‐based Aerogels via Hydrolysis of Salts and Alkoxides: The Influence of the Synthesis Procedures on the Properties of the Aerogels

This contribution aims at evaluating different synthesis procedures leading to zirconia‐based aerogels. A series of undoped and yttrium‐doped zirconia aerogels have been prepared via hydrolysis and condensation reaction of different alkoxy‐ and different inorganic salt‐based precursors followed by supercritical drying. Well‐established but deleterious zirconium n‐propoxide (TPOZ) or zirconium n‐butoxide (TBOZ) were used as metal precursors in combination with acids like nitric acid and acetic acid as auxiliary agent for the generation of non‐yttrium stabilized zirconia aerogels. Yttrium‐stabilized zirconia aerogels as well as pure zirconia aerogels were obtained by the salt route starting from ZrCl₄ and crosslinking agents like propylene oxide or acetylacetone. The characteristics of the products were analyzed by nitrogen adsorption measurements, electron microscopy, and X‐ray scattering. It turned out that with respect to all relevant properties of the aerogels as well as the practicability of the synthesis procedures, approaches based on inexpensive non‐toxic salt precursors are the methods of choice. The salt‐based approaches allow not only for low‐cost, easy‐to‐handle synthesis procedures with realizable gelation times of less than 60 seconds, but also delivered the products with the highest surface area (449 m² g^?1 for ZrCl₄) within this series of syntheses.     

Partial transesterification of sunflower oil with ethanol by a silica fiber reinforced aerogel encapsulated lipase

A commercial lyophilized lipase of Burkholderia cepacia, from Amano, was encapsulated in silica aerogels reinforced with silica quartz fiber felt. This biocatalyst was applied in the direct transesterification of sunflower seed oil with ethanol, without any other solvent. When the molar ratio of ethanol to oil was two or three, the oil transformation kinetics was found to be very slow after the formation of 1 mole of fatty ethyl ester per mole of initial triglyceride. For a molar proportion of ethanol to oil ≈1, the recycling activity also decreased gradually in successive tests to reach an activity ≈7% of the initial activity, during the 5th test. Textural and structural analysis of the aerogels before and after catalytic tests showed that this deterioration was associated with a modification of the aerogel, by preferential adsorption of glycerol or possibly other transesterification products such as diglycerides. Besides, it is proposed that one of the cause for the aerogel loss of activity at an initial molar ratio of ethanol:oil of 3:1 was due to a progressive inhibition of the enzyme by excess adsorbed ethanol. The aerogel samples were also compared to a commercial product of lipase immobilized on polymer beads, from Fluka. The silica aerogels somewhat improved, to a limited extent, the activity during recycling.