Enhanced thermal shrinkage behavior of phenolic-derived carbon aerogel-reinforced by HNTs with superior compressive strength performance

A novel carbon/m-HNTs composite aerogel was synthesized by introducing the modified halloysite nanotubes (m-HNTs) into phenolic (PR) aerogels through chemical grafting, followed with carbonization treatment. In order to explore the best proportion of HNTs to phenolic, the micromorphology of PR/m-HNTs were investigated by SEM before carbonization, confirming 10 wt% of m-HNTs is most beneficial to the porous network of aerogels. The interaction between PR and HNTs was studied by FTIR spectra, and microstructure evolution of the target product-carbon/m-HNTs composite aerogel were illustrated by SEM and TEM techniques. SEM patterns indicated that the carbon/m-HNTs aerogels maintain a stable porous structure at 1000 °C (carbonization temperature), while a ~20 nm carbon layer was formed around m-HNTs generating an integral unit through TEM analysis. Specific surface area and pore size distribution of composite aerogels were analyzed based on mercury intrusion porosimetry and N₂ adsorption–desorption method, the obtained results stayed around 500 m²g^?1 and 1.00 cm³g^?1 (pore volume) without significant discrepancy, compared with pure aerogel, showing the uniformity of pore size. The weight loss rate (26.76%) decreased greatly compared with pure aerogel, at the same time, the best volumetric shrinkage rate was only 30.83%, contributed by the existence of HNTs supporting the neighbor structure to avoid over-shrinking. The highest compressive strength reached to 4.43 MPa, while the data of pure aerogel was only 1.52 MPa, demonstrating the excellent mechanical property of carbon/m-HNTs aerogels.     

Zirconia aerogels: Effect of acid used in the sol-gel process on morphological properties

Zirconia aerogels have been prepared from butanolic zirconium(IV) tetra-n-butoxide diluted in ethanol via stoichiometric hydrolysis with water in ethanol. Nitric acid or acetic acid were used to modify the sol-gel process. After calcination in air at 573 K, the aerogel prepared with nitric acid possesses a specific surface area of 240 m² · g^–1 and a unimodal pore size distribution with a maximum at ca. 24 nm, whereas the use of acetic acid results in an aerogel with specific surface area of 228 m² · g^–1 and bimodal pore size distribution with maxima at 3 and 65 nm. The crystalline fractions of both aerogels are predominantly tetragonal with a small contribution of monoclinic ZrO₂.     

纳米孔超级绝热材料气凝胶的制备与热学特性

以正硅酸四乙酯(TEOS)为硅源，通过溶胶–凝胶及超临界干燥过程制备了SiO2气凝胶. 同时，采用相对廉价的多聚硅(E–40)为硅源，以三甲基氯硅烷(TMCS)为表面修饰剂，硅油为干燥介质, 在常压条件下制备了同样具有纳米多孔结构的SiO2气凝胶. 用透射电镜、扫描电镜及孔径分布仪对其结构进行了表征，并用动态热线法对其热学特性进行了测试. 结果表明: 两种方法制备的气凝胶均是典型的纳米孔超级绝热材料，后者热导率略高但成本低许多，所以更具应用推广潜力.     

Preparation of silica aerogel from oil shale ash by fluidized bed drying

The silica aerogel with high specific surface area and large pore volume was successfully synthesized using oil shale ash (OSA) via ambient pressure drying. The oil shale ash was burned and leached by sulfuric acid solution, and then was extracted using sodium hydroxide solution to produce a sodium silicate solution. The solution was neutralized with sulfuric acid solution to form a silica gel. After washing with water, the solvent exchange with n-hexane, and the surface modification with hexamethyldisilazane (HMDZ), the aged gel was dried by fluidization technique and also using a furnace to yield silica aerogels. The physical and textural properties of the resultant silica aerogels were investigated and discussed. The results have been compared with silica aerogel powders dried in a furnace. From the results, it is clear that the properties of silica powders obtained in fluidized bed are superior to that of powders dried in the furnace. Using fluidization technique, it could produce silica aerogel powders with low tapping density of 0.0775 g/cm³, high specific surface area (789 m²/g) and cumulative pore volume of 2.77 cm³/g.     

有机-无机杂化柔性硅气凝胶的制备与表征

以甲基三甲氧基硅烷（MTMS）和四乙氧基硅烷（TEOS）为混合硅源、甲醇为溶剂,通过酸碱两步催化溶胶-凝胶法制备湿凝胶,经超临界流体干燥得到块状二氧化硅气凝胶。用扫描电镜、氮气吸附脱附测试以及热重分析等手段对气凝胶的微观形貌、比表面积、孔径分布、弯曲性、压缩性、热稳定性等进行研究,结果表明：MTMS/TEOS比例会影响气凝胶的微观结构、弯曲和压缩性以及热稳定性,以MTMS/TEOS=8/1制得的气凝胶密度为0.11 g·cm^-3、孔隙率为94.2%、比表面积为693.3 m²·g^-1、最大弯曲角可达92°、最大压缩比例可达41.2%、压缩回弹率为100%。     

Effect of aging temperature on the porosity characteristics of subcritically dried silica aerogels

Silica aerogels were synthesised by subcritical drying technique which involves controlled solvent exchange and aging of the wet gel in silane solution followed by drying under controlled conditions. Effect of temperature of aging in silane solution on the porosity characteristics of silica aerogels and the thermal pore stability of the resultant gels were investigated. Aging in silane solution leads to an increased degree of condensation reactions, siloxane crosslinking and the dissolution and reprecipitation of silica monomers to the gel structure and enhances the total strength of the gel. Thermal aging of the wet gel have a pronounced effect on bulk density, linear drying shrinkage, surface area and pore volume. As the temperature of aging increases the bulk density decreases whereas the surface area and pore volume were found to increase. We could achieve a surface area of 1040 m²/g, pore volume 1.2 cc/g and an average pore size of 49 Å corresponding to an aging temperature of 70 °C. Thermal pore stability of the gel was found to be up to 700 °C above which densification of SiO₂ gel starts. The novel findings will help in tailoring the process parameters to prepare mesoporous oxides from sol–gel precursors with specific pore features.     

Large size and low density SiOC aerogel monolith prepared from triethoxyvinylsilane/tetraethoxysilane

Crack-free silicon oxycarbide (SiOC) aerogel monolith was fabricated by pyrolysis of precursor aerogel prepared from triethoxyvinylsilane/tetraethoxysilane (VTES/TEOS) using sol-gel process and ambient drying. Effects of different precursors, the amount of base catalyst (NH₄OH) and the heating rate during pyrolysis on the properties such as monolithicity, bulk density, surface area and pore size distribution of aerogels were investigated. The results show that the crack-free SiOC aerogel can be easily obtained from VTES/TEOS as compared to that of methyltriethoxysilanes/tetraethoxysilane (MTES/TEOS) and phenyltriethoxysilanes/tetraethoxysilane (PhTES/TEOS) precursors. The influence of heating rate during pyrolysis process on shrinkage rate, ceramic yield and surface area of the SiOC aerogels could be ignored, while the variation in the amount of NH₄OH exerted a strong impact on the properties of SiOC aerogels. Increasing the amount of NH₄OH resulted in the decrease of bulk density and surface area of SiOC aerogels from 0.335 g/cm³ and 488 m²/g to 0.265 g/cm³ and 365 m²/g. The resultant SiOC aerogels exhibit high compressive strength (1.45–3.17 MPa). ²⁹Si MAS NMR spectra revealed the retention of Si-C bond in the SiOC aerogels after pyrolysis at 1000 °C. The present work demonstrates VTES/TEOS is a promising co-precursors to easily and low cost synthesize large size SiOC aerogel monolith.     

纳米级增强体复合硅气凝胶的研究进展

二氧化硅气凝胶的脆性大、力学强度过低,严重限制了其应用领域。常规微米级或大于微米尺寸的增强体复合硅气凝胶可以提升硅气凝胶性能,但难以在纳米尺寸范围内对凝胶孔隙增强。目前,可以采用合成聚合物纳米纤维、纤维素纳米纤维、纳米管、无机纳米纤维、石墨烯及氧化石墨烯等增强硅气凝胶,纳米级增强材料能够均匀分散在硅气凝胶纳米孔隙中,干燥收缩小、孔比表面积大、可有效提升硅气凝胶抗压强度、改进韧性。     

The relationship between the silica wet-gels relative optical transmittance at 500 nm and structural features of silica aerogels

Aerogels are usually synthesized from supercritical drying of wet-gels. High quality aerogels are usually characterized with high specific surface area (measured by BET method here) and high degree of polymerization of silicate (characterized by ²⁹Si NMR here). The quality of the aerogels depends highly on the quality of the corresponding wet-gels because the skeletal structure in the wet-gels changes only slightly in the supercritical drying process. Here we present a simple method to predict the quality of the desired silica aerogels before the supercritical drying. We found that the relative optical transmittance of the silica wet-gels at 500 nm had good linear correlation with the specific surface area and the degree of silicate polymerization in the corresponding aerogels. We can assess the quality and screen out the low-quality wet-gels from supercritical drying operation, and greatly reduce the cost for aerogel production.     

Synthesis and characterization of a xonotlite fibers–silica aerogel composite by ambient pressure drying

Xonotlite fibers (XFs) reinforced silica aerogel composites were prepared by a sol–gel method under ambient pressure drying. XFs were synthesized through a dynamic hydrothermal route and had a noodle-like structure with length of 5–10 μm and average diameter of 150–200 nm. The microstructure analysis showed that XFs were inlaid in silica aerogel matrix by physical combination which contributed to restrict the volume shrinkage of alcogels and maintain the integrality aerogels during drying process. The physical, naonporous and thermal properties of the as prepared aerogel composites were investigated and discussed in detail. The new aerogel composites possessed porous nanostructure, which exhibited typical properties of 0.126 g/cm³ density, 4.132 cm³/g pore volume, and thermal conductivity of 0.0285 W/(m K). The results indicated that the introduced XFs didn’t significantly alter the porosity, hydrophobicity or thermal conductivity of aerogel matrix. It was also found that the aerogel composites had much more outstanding porosity than that of pure aerogel upon calcinations at 800 °C. This study fabricated XFs–silica aerogel composites and explored a new way for silica aerogels to endure and remain monolithic under ambient pressure drying.     

常压干燥法制备SiO2气凝胶粉体

以工业硅溶胶为原料,通过凝胶过程与干燥条件的控制,采用常压干燥法制备了SiO2 气凝胶粉体,并考察了老化液中正硅酸乙脂(TEOS)含量和干燥控制化学添加荆甲酰胺的添加对气凝胶粉体堆积密度、比表面积和孔径分布的影响.结果表明:所得气凝胶粉体具有纳米多孔结构,组成气凝胶结构的基本粒子呈圆球形,粒径为10～25 nm,由基本粒子连接而成的网络结构具有5～50 nm的孔径分布;随着热处理温度从常温升至1 100℃,SiO2气凝胶从最初的无定形态转化为方石英;在,TEOS的醇溶液中老化,有利于增强凝胶骨架的强度;添加甲酰胺可以改善气凝胶粉体的孔径分布,提高其比表面积.     

Structural Design of Polymer‐Derived SiOC Ceramic Aerogels for High‐Rate Li Ion Storage Applications

SiOC ceramic aerogels with different porosity, pore size, and specific surface area have been synthesized through the polymer‐derived ceramic route by modifying the synthesis parameters and the pyrolysis steps. Preceramic aerogels are prepared by cross‐linking a linear polysiloxane with divinylbenzene (DVB) via hydrosilylation reaction in the presence of a Pt catalyst under highly diluted conditions. Acetone and cyclohexane are used as solvent in our study. Wet gels are subsequently supercritically dried with CO₂ to get the final preceramic aerogels. The SiOC ceramic aerogels are obtained after a pyrolysis treatment at 900°C in two different atmospheres: pure Ar and H₂ (3%)/Ar mixtures. The nature of the solvent has a profound influence of the aerogel microstructure in terms of porosity, pore size, and specific surface area. Synthesized SiOC ceramic aerogels have similar chemical compositions irrespective of processing conditions with ~40 wt% of free carbon distributed within remaining mixed SiOC matrix. The BET surface areas range from 215 m²/g for acetone samples to 80 m²/g for samples derived from cyclohexane solvent. The electrochemical characterization reveals a high specific reversible capacity of more than 900 mAh/g at a charging rate of C (360 mA/g) along with a good cycling stability. Samples pyrolyzed in H₂/Ar atmosphere show a high reversible capacity of 200 mAh/g even at a high charging/discharging rate of 20 C. Initial capacities were recovered after whole cycling procedure indicating their structural stabilities resisting any kind of exfoliations.     

In situ production of spherical aerogel microparticles

Due to their high surface area, low density, open pore structure and excellent insulation properties aerogels are intensively investigated since the past decades for a diverse range of applications. The current methods of silica aerogel production by supercritical extraction produce monolithic aerogels, where the sol is aged in molds and dried by extraction with supercritical CO₂. Aerogels in the form of spherical microparticles would be beneficial for many applications, for instance, drug delivery for respiratory route; or as insulating materials. However, because of aerogel's mechanical properties, it is difficult, rather impossible, to obtain spherical microparticles by milling or crushing of the monolithic aerogels. This work presents a new method to produce biocompatible spherical aerogel microparticles using an emulsion technique (in situ production) followed by supercritical extraction of the resulted dispersion (gel-oil). Water in oil emulsion was produced by mixing the sol (dispersed phase) with a vegetable oil (continuous phase) followed by the gelation of the dispersed phase. The size distribution of the final gel particles was found to be influenced by agitation, surfactant concentration and sol:oil volume ratios. The gel-oil dispersion was subsequently extracted with supercritical CO₂, Silica aerogel spherical microparticles with a surface area of 1100 m²g⁻¹, pore volume of 3.5 cm³/g and different mean particle diameters ranging from 200 μm to a few millimeters were produced using the presented method.     

酸碱催化剂浓度对柔性硅气凝胶性能和结构的影响

以甲基三甲氧基硅烷(MTMS)和正硅酸乙酯(TEOS)作为混合硅源,甲醇为溶剂,十六烷基三甲基溴化铵(CTAB)为表面活性剂,通过酸碱两步催化溶胶凝胶法制备醇凝胶,经超临界干燥可以制备高弹性疏水块状硅气凝胶。分别采用0.1 mol·L^-1 和0.01 mol·L^-1 的草酸作为酸催化剂,5 mol·L^-1 和10 mol·L^-1 的氨水作为碱催化剂,研究不同酸碱催化剂浓度对其网络结构的影响。发现高浓度酸和高浓度碱以及低浓度酸和低浓度碱作为催化剂合成的硅气凝胶的网络结构更加均匀,孔径分布更窄。其中,在草酸浓度为0.01 mol·L^-1、氨水浓度为5mol·L^-1 时,所得硅气凝胶密度为0.135 g·cm^-3、比表面积为807 m²·g^-1、孔隙率约为93%,凝胶最大可压缩至其起始长度的60%,压缩回弹率为100%。     

常压干燥法制备疏水性SiO2气凝胶

利用正硅酸乙酯做先躯体,盐酸与氨水做催化剂,通过二步法制备了二氧化硅气凝胶,并利用三甲基氯硅烷和正己烷做表面改性剂采用共沸法逐级对湿凝胶进行表面改性.测试分析了气凝胶的密度、表观、疏水性、比表面积、孔径分布和热稳定性.结果表明:气凝胶的物理特性及其疏水性受到三甲基氯硅烷与正己烷的体积比(ψ)的影响,其密度随着ψ增大而减小,当ψ=3%时,气凝胶的疏水性最好;所制得的SiO2在350℃仍具有良好的热稳定性,并且具有高比表面积(838.6 m2/g)和孔体积(2.10cm3/g).     

Effect of the thermal treatment on microstructure and physical properties of low-density and high transparency silica aerogels via acetonitrile supercritical drying

In this paper, we reported the experimental results about the effect of the thermal treatment on microstructure and physical properties of low-density and high transparent silica aerogels. From our results, with tetramethyl orthosilicate as precursor and via acetonitrile supercritical drying process, silica aerogel monolith was obtained possessing the properties as low-density (0.018 g/cm³), high surface area (923 m²/g), high optical transparency (87.9 %, 800 nm). It should be noted that high transparency of silica aerogel can be maintained up to 600 °C (91.5 %, 800 nm). The mechanical properties of silica aerogel decreased with increasing heat treated temperature to 600 °C, and silica aerogels still maintained crack-free monoliths completely and possessed high homogeneous density even after 600 °C thermal treatment. Furthermore, thermal conductivity of the monoliths at desired temperatures was analyzed by the transient plane heat source method. When the temperature flowed from 25 to 600 °C, thermal conductivity coefficients of silica aerogels changed from 0.021 to 0.065 W (m K)^?1, revealed an excellent heat insulation effect in high-temperature area. Currently, the specific process developed for low-density aerogels affected by thermal treatment has not been reported in previous literature.     

Preparation and performance of cobalt-doped carbon aerogel for supercapacitor

Carbon aerogels were prepared by polycondensation of resorcinol with formaldehyde in ambient conditions. The effect of resorcinol-to-catalyst ratio (R/C ratio) on volume shrinkage, BET surface area, and electrochemical property was investigated by changing R/C ratio from 50 to 2000. Carbon aerogel prepared at R/C ratio of 500 showed less than 2% of volume shrinkage and the highest BET surface area (706 m²/g). Specific capacitance of carbon aerogel prepared at R/C ratio of 500 was found to be 81 F/g in 1M H₂SO₄ electrolyte. Cobalt-doped carbon aerogels were then prepared by an impregnation method with a variation of cobalt content, and their performance was investigated. Among the samples prepared, 7 wt% cobalt-doped carbon aerogel showed the highest capacitance (100 F/g) and the most stable cyclability. The enhanced capacitance of cobalt-doped carbon aerogel was attributed to the faradaic redox reactions of cobalt oxide.     

Enhancing aerogel mechanical properties with incorporation of POSS

Silica aerogels consisting of nanoparticles and numerous nano-pores have many attractive attributes. However, the weak mechanical properties severely limited the practical application of the silica aerogel. In this work, a facile approach was employed to strengthen the silica aerogel while the multi-alkoxy polyhedral oligomeric silsesquioxane (POSS) joined with methyltriethoxysilane (MTES) as the co-precursor. Three kinds of stiff core POSS with different amount of alkoxy groups were chosen to prepare aerogels. The result attributes showed all aerogels owned mesoporous structure (10–20 nm), high specific surface area (760–877 m² g⁻¹) and good thermal stability. Moreover, with the introduction of POSS, the mechanical properties had been apparently enhanced. The suitable addition of functional groups and the adjustment of cross-linking density made the aerogel own more room to deform and the skeleton still strong enough to endure large deformation. In addition, new peaks appeared in the XRD patterns at the same time. The preparation strategy of composite aerogels may guide a facile way to regulate the aerogel attributes. Furthermore, the aerogel potential application in oil-water separation has also been investigated.     

Fabrication and thermophysical characterization of nano-porous silica-polyurethane hybrid aerogel by sol-gel processing and supercritical solvent drying technique

Thermally and mechanically enhanced nanoporous silica-polyurethane hybrid aerogel was synthesized by sol-gel processing and low temperature supercritical CO₂ drying. A partially condensed silica solution and a polymeric MDI were used as the raw materials with a tertiary amine as a catalyst and 1,4-dioxiane as a dilution solvent. After the gelation reaction was completed, aged wet hybrid aerogel was dried by a low temperature supercritical CO₂ drying technique. Also, thermophysical characteristics such as density, BET surface area, and thermal conductivity as a function of catalyst ratio and aging time of the synthesized hybrid aerogel were analyzed. It was found that, at a fixed target density, the lowest average pore size of the aerogel, 8 nm, was obtained when the catalyst ratio was 0.1 wt.%. Also, at these conditions, the BET surface area showed the highest surface area, 287.3 m²/g. It was found that with decreasing average pore size and with increasing BET surface area, thermal conductivity tends to decrease. At pressure 1 torr, the sample aerogel showed the lowest thermal conductivity, 0.0184 W/mK.     

The influence of preparation method on the physicochemical properties of titania–silica aerogels

A series of titania–silica aerogels with different compositions were prepared using four different preparation methods. The preparation steps were followed by a high temperature supercritical drying (HTSCD). It was found that application of ethanol and 2-propanol as the solvents for the titania precursor (with or without modification with acetyloacetone) resulted in materials with BET surface area up to 990 m²/g, total pore volume up to 5.6 cm³/g and density as low as 0.041 g cm⁻³. Obtained aerogels were mesoporous materials with the average pore diameter in the range 11–27 nm. It was stated that application of the I method (prehydrolysis) resulted in aerogels with higher average pore diameter than other preparation methods while aerogels with the lowest average pore diameter were obtained using the so-called IV, impregnation method. Anatase form of titania was found in all prepared samples. The prepared aerogels were being applied as catalysts in photodegradation of salicylic acid solution in water. The obtained results suggest a much higher catalytic efficiency of titania, which is present in aerogel than it is in the case of commercial P25 Degussa titanium dioxide.