Ambient Pressure Dried Silica Aerogels Research Articles

Improvement in the characteristics of ambient pressure dried silica aerogels for thermal insulation purpose

Improvement in the characteristics of ambient pressure dried silica aerogels for thermal insulation purpose

Rapid synthesis of ambient pressure dried tetraethoxysilane based silica aerogels

Silica aerogel can be produced by using ambient pressure drying (APD) or supercritical drying depending on the required properties and kind of application. Processing time for ambient pressure dried silica aerogels are longer due to the time consuming steps such as hydrolysis and solvent exchange. Considerable decrease in time has been reported by a few research groups by employing shaking during the solvent exchange. We are reporting further reduction in processing time of tetraethoxysilane (TEOS)-based silica aerogel by carrying out the hydrolysis process at elevated temperature. The aerogel obtained in least processing time showed low density (0.066 kg/m3), low thermal conductivity (0.043 W/mK), high specific surface area, and porosity. This shows that elevated temperature hydrolysis does not hamper the desired properties of the product.

Effects of various methylchlorosilanes on physicochemical properties of ambient pressure dried silica aerogels

Methylchlorosilanes (MSCs) of the type (CH3)nSiCl4−n (n = 1, 2, 3) are usually used for surface modification of silica aerogels (SAs), in which the quantity of Si–Cl and Si–CH3 groups deeply affects the properties of prepared SAs. In this work, methyltrichlorosilane (MTCS, n = 1), dimethyldichlorosilane (DMDCS, n = 2), and trimethylchlorosilane (TMCS, n = 3) were used to modify wet gels and the effects of Si–Cl and Si–CH3 groups’ quantity on microstructures and surface properties were explored in detail. It turned out that the MTCS modified SA (MSA) possessed the most compact and nonuniform three-dimensional nanostructure among the three SAs, accompanied by the biggest density of ~ 0.14 g/cm3. Further investigations demonstrated that all the three SAs had similar BET surface area of about 900 m2/g and average pore size of 10–15 nm. Though FTIR analysis presented the discrepancies obviously among the three SAs, the thermal stability of the three SAs was similar, with the onset and peak temperatures in the exothermic reaction of methyl groups being ~ 240 °C and ~ 270 °C, respectively. The best hydrophobicity belonged to the TSA, verified by the biggest contact angles of 155°. Thus, all these indicated that the properties of the three SAs were deeply related to the quantity of Si–Cl and Si–CH3 groups in MCS.

Development of Mechanically Strong Ambient Pressure Dried Silica Aerogels with Optimized Properties

Ambient pressure dried (APD) silica aerogel-like monoliths with different underlying silica structures have been developed through a simple wet chemical approach. The improvement of the mechanical properties was accomplished by cross-linking the silica surface with triacrylate cross-linker. A solvent exchange carried out by soaking the gels in a low surface tension solvent allowed to avoid the nonsafe and costly supercritical drying process. In this context, two different sets of aerogel-like monoliths have been produced, and their main properties, namely, bulk density, mechanical strength, and thermal conductivity, were studied and modeled using the statistical Central Composite Design (CCD) approach. The empirical models derived for each property of the aerogel-like monoliths lead to further evaluation of the desirability function and optimization of silica aerogel-like properties. The key properties of the optimized APD monoliths were compared with the supercritical dried aerogels with the same synthes...

Effect of different chemical additives and heat-treatment on ambient pressure dried silica aerogels

The experimental results on the physical and super-hydrophobic properties of the ambient pressure-dried silica aerogels doped with different chemical additives (CAs) and heat-treated at 350°C for 2 h in air are reported. N,N-dimethylformamide (DMF), glycerol (GLY) and ethylene glycol (EG) were doped as CAs to the tetraethyoxysiliane (TEOS) precursor prepared aerogels. The molar ratio of CAs/TEOS (G) was varied from 0 to 1.2. DMF-doped aerogel at G = 0.8 possessed minimum shrinkage (≈6%) and low bulk density (≈0.2 g cm−3). The contact angle (θ) at G = 0.8 DMF-doped aerogel was ∼178°, which decreased a little (θ ≈ 162°) when heat-treated at 350°C in air, but the super-hydrophobic property was retained. The small angle X-ray scattering testing results showed that DMF-doped aerogel had the most uniform and smallest particles and particle size distributions compared with GLY- and EG-doped aerogels. But the CAs doping had a little influence on the degree of surface roughness of the particles.

Structural characteristics and thermal conductivity of ambient pressure dried silica aerogels with one-step solvent exchange/surface modification

The paper reports the synthesis procedures as well as structural properties and thermal conductivity of the ambient dried hydrophobic silica aerogels with ethanol (EtOH)/trimethylchlorosilane (TMCS)/n-hexane as surface modification agent. One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogels in EtOH/TMCS/n-hexane solution. When P(VTMCS/VHydrogels) ranges from 10 to 20%, the final product is hydrophilic xerogel, which is porous material with porosity of 78–89% and the specific density is over 0.24gcm−3. When P(VTMCS/VHydrogels) is at the scope of 75–100%, the specific densities of the aerogels range from 0.10 to 0.14gcm−3 and the porosity is in range of 93.5–95.8%. The silica aerogels modified with 75–100% TMCS (V/V) present good performance with the specific surface area in range of 745–770m2g−1 and the average pore sizes are about 20nm. Furthermore, using granular silica aerogels (P(VTMCS/VHydrogels)=100%) as the fillings, the thermal conductivities of granular aerogels are analyzed by guarded hot plate method. When temperature ranges from 30 to 60°C, thermal conductivity coefficients of the samples are lower than 0.03W(mK)−1.

Effect of different trimethyl silylating agents on the hydrophobic and physical properties of silica aerogels

An essential feature of the ambient pressure dried aerogel manufacturing process is the end-capping of the reactive silanol groups in the silica wet gel. In this report, we have presented the effect of two different trimethyl silylating agents viz. trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) on the hydrophobic and physical properties of ambient pressure dried silica aerogels. The hydrogels were prepared by sol–gel processing of sodium silicate precursor (Na 2SiO 3) in the presence of acetic acid catalyst followed by vapour passing treatment and different solvent exchanging steps. The silylating agent in hexane was used for end-capping of the silanols present on the silica surface of the gel. To study silylation behavior silylating agent/Na 2SiO 3 molar ratio was varied from 2.4 to 5.6. The aerogels have been characterized by density, % of volume shrinkage, porosity, % of optical transmission, Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Thermo-gravimetric and Differential Thermal Analysis (TG-DTA) and contact angle measurements.

Hydrophobic and low density silica aerogels dried at ambient pressure using TEOS precursor

Abstract In the conventional ambient pressure drying of silica aerogels, tedious repetitive gel washing and solvent exchange steps (∼6 days) are involved. Therefore, in the present studies, we intended to reduce the processing time of TEOS based ambient pressure dried silica aerogels. Solvents such as methanol, hexane and Hexamethyldisilazane (HMDZ) as surface chemical modification agents have been used. To get good quality aerogels in terms of low density, high porosity, high contact angle and low volume shrinkage in less processing time, we varied MeOH/TEOS, HMDZ/TEOS molar ratios, oxalic acid (A) and NH 4 OH (B) concentrations and stirring time from 1 to 27.7, 0.34 to 2.1, 0 to 0.1 M, 0 to 2 M and 15 to 90 min respectively. The transparent and low-density aerogels were obtained for TEOS:MeOH:acidic H 2 O:basic H 2 O:HMDZ molar ratio of 1:16.5:0.81:0.50:0.681 respectively. The thermal stability and hydrophobicity have been confirmed with Thermogravimetric and Differential Thermal (TG–DT) analyses and Fourier Transform Infrared Spectroscopy. Microstructural study was carried out by Scanning Electron Microscopy (SEM).

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.

Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents

The experimental results on the synthesis and physical properties of the ambient pressure dried hydrophobic silica aerogels in the presence of various surface modification (silylating) agents are presented. The silica aerogels were prepared with 1.12 specific gravity ion exchanged sodium silicate solution, 1N ammonium hydroxide, solvent exchanged with ethanol and hexane, and surface modification with 20% silylating agent in hexane followed by drying the modified gel up to 200 °C. The molar ratio of sodium silicate, water, ammonium hydroxide and silylating agent was kept at 1:45:4.3 × 10 −2:5, respectively. The physical properties of the aerogels such as density, % of porosity, pore volume, thermal conductivity and contact angle measurements were studied by using various mono, di and tri alkyl or aryl silylating agents (SAs). The tri alkyl silylating agents produced low % of volume shrinkage (2%), low density (0.06 g/cm 3), low refractive index (1.011), more pore volume (16.15cm 3/g), high percentage of porosity (96.9%) and hydrophobic (contact angle >150°) silica aerogels. It was found from the Fourier transform infrared spectroscopic (FTIR) studies of the aerogels that the intensity of the bands related to the Si C and C H are more and the Si OH and OH are less with the tri than mono and di alkyl SAs. It was found from the TGA-DTA studies of the aerogels with increase in temperature above 325 °C, the % of weight decrease in TGA and exothermic peak in DTA are more with tri than the mono and di alkyl SAs. The SEM studies of the aerogels showed the large pore and particle sizes in the silica network with the tri alkyl SAs. The % of optical transmission of the aerogels is less with the tri alkyl SAs than the mono and di alkyl SAs. It was found from the contact angle and water adsorption studies that the hydrophobicity of the silica aerogel is more with tri alkyl than the di and mono alkyl silylating agents.

Hydrophobic and Physical Properties of the Two Step Processed Ambient Pressure Dried Silica Aerogels with Various Exchanging Solvents

The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient pressure dried hydrophobic silica aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and condensation with NH4OH of ethanol diluted tetraethylorthosilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution and contact angle (θ) of the silica aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the silica aerogels strongly depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (≈90% optical transmission at 700 nm for 1 cm thick sample), low density (≈0.060 g/cm3), low thermal conductive (≈0.070 W/m·K), high % of porosity (97%), high surface area (750 m2/g), uniform porosity and hydrophobic (θ ≈ 160°) aerogels compared to other solvents. On the otherhand, xylene resulted in aerogels with higher hydrophobicity (θ ≈ 172°) among other solvents.

Effect of preparation conditions on the physical and hydrophobic properties of two step processed ambient pressure dried silica aerogels

The experimental results on the physical and hydrophobic properties of the ambient pressure dried silica aerogels as a function of sol-gel and drying conditions, are reported.The aerogels have been produced by a two stage (acidic and basic) catalytic sol-gel process using tetraethylorthosilicate (TEOS) precursor, oxalic acid (OXA) and ammonium hydroxide (NH4OH) catalysts, ethanol (EtOH) solvent and hexamethyldisilazane (HMDZ) silylating agent at 200∘C.The molar ratios of HMDZ/TEOS (M), OXA/TEOS (A) NH4OH/TEOS (B), acidic H2O/TEOS (Wa) basic H2O/TEOS (Wb), EtOH/TEOS (S) were varied from 0.09 to 0.9, 3.115 × 10− 5 to 3.115 × 10−3, 4 × 10− 3 to 8 × 10− 2, 2 to 9, 1.25 to 5 and 1 to 16 respectively. The physical properties such as the percentage (%) of volume shrinkage, density, thermal conductivity, percentage of porosity, the percentage of optical transmission and contact angle have been found to be strongly dependent on the sol-gel parameters. It was found from the FTIR spectra of the aerogels that with the increase of M, the bands at 3500 and 1600 cm− 1 corresponding to H-OH and Si-OH respectively decreased and the bands at 840 and 1250 cm− 1 due to Si-C and 2900 and 1450 cm−1 due to C-H increased. The best quality silica aerogels in terms of low density, low volume shrinkage, low thermal conductivity, high hydrophobicity and high optical transmission have been obtained with the molar ratio of TEOS:EtOH:acidicH2O:basicH2O:OXA:NH4OH:HMDZ at 1:8:3.75:2.25:6.23 × 10− 5: 4 × 10− 2:0.36 respectively, by ambient pressure dried method.

Thermal and Temporal Aging of Silica Gels in Monomer Solutions

In previous work we have introduced the idea of preparing ambient pressure dried silica aerogels by increasing the wet gels' stiffness by aging in a TEOS solution until shrinkage during drying is almost eliminated. The present work elucidates the possibilities for obtaining ambient pressure dried aerogels employing this idea, however, cheap water soluble sodium silicate (water glass) precursors have been used to increase the economic feasibility of the process.