SiO2 Aerogel Research Articles

One-pot synthesis and characterization of acid-catalyzed melamine formaldehyde/SiO2 aerogel via sol–gel technology

This paper reports a rapid synthesis of a novel melamine–formaldehyde (MF)/SiO2 aerogel with good transparency carried out by acid-catalyzed chemical dehydration and polycondensation of poly (melamine-co-formaldehyde) methylated and hydrolyzed tetraethoxysilane respectively in one pot via sol–gel process and subsequently supercritical drying with CO2. The one-pot method improves the efficiency and reduces the cost of MF/SiO2 aerogels. The gelation time can be as short as 13 h under the appropriate conditions, much less than that of MF aerogel obtained by traditional process. The structural properties of the aerogels were characterized by the field emission scanning electron microscopy, the Fourier transformed infrared spectroscopy and the Brunauer–Emmett–Teller methods. The results indicate the hybrid aerogel has a micro-mesoporous structure with a high surface area about 747 m2/g and the two components combine with each other by physical interaction.

A New Type of Super Hydrophilic SiO2 Aerogel for Use as a Potential NH3 Sorbent

A New Type of Super Hydrophilic SiO2 Aerogel for Use as a Potential NH3 Sorbent

Bimetallic Fe/Mo–SiO2 aerogel catalysts for catalytic carbon vapour deposition production of carbon nanotubes

Highly porous nanocomposite aerogels based on bimetallic Fe and Mo nanoparticles with a variable Fe:Mo weight ratio ranging from 5:1; 3:1; and 0.7:1 dispersed on amorphous silica were obtained. N2 physisorption, X-ray diffraction, and transmission electron microscopy indicate that the Fe/Mo–SiO2 nanocomposite aerogels as obtained by a co-gelation sol–gel route followed by supercritical drying and reduction treatment under H2 exhibit Fe and Mo nanocrystals with size in the range 4–10 and 15 nm, respectively, supported on highly porous silica. The catalytic performance of the Fe/Mo–SiO2 aerogels for the synthesis of multi wall carbon nanotubes (MWCNT) by catalytic chemical vapour deposition (CCVD) was evaluated in terms of amount and quality of the produced CNTs as assessed by gravimetric results, thermal analysis, and TEM. The effect of catalyst composition and CCVD temperature was investigated, pointing out that high reaction temperatures (800 °C) favor the formation of MWCNTs with high quality in elevated yield, the highest C uptake value being >400 %. Catalyst composition and CCVD temperature were also found to affect the homogeneity of CNT morphology, the best MWCNT quality (with outer diameter 23–25 nm) being achieved at 800 °C with the catalyst having the largest Mo content.

Design of Iosipescu Shear Test for Applying to a Bi-Modulus Ceramic-Fiber-Reinforced SiO2 Aerogel

Abstract Tension and compression tests were conducted on a ceramic-fiber-reinforced SiO2 aerogel (CFRSA). The material exhibits different tension and compression modulus, i.e., bi-modulus. The objective of present research is to establish guidelines for preparing shear tests of this CFRSA. Finite-element analysis (FEA) on various Iosipescu specimens is presented for four bi-modulus materials, including the CFRSA. It is found that the major concerns for specimen design of bi-modulus materials are: the uniformity of shear stress and the longitudinal normal stress concentration at notch tips. These concerns are all dependent on material properties. For material with Et numerically close to Ec, the normal stress concentration near notch tips is of less significance. However, for material with high tension–compression asymmetry, including this CFRSA, the normal stress concentration near notch tips is significant. The round-notch specimen with small normal stress concentration seems to be favorable for these materials.

Infrared-opacified Al2O3–SiO2 aerogel composites reinforced by SiC-coated mullite fibers for thermal insulations

To opacify infrared radiation of mullite fiber-reinforced aerogel composites, SiC coatings were prepared on surfaces of mullite fibers by soaking with polycarbosilane (PCS) solution and then pyrolysis. By impregnating SiC-coated mullite fibers with Al2O3–SiO2 sol, then aging and supercritical drying, opacified Al2O3–SiO2 aerogel thermal insulation composites were prepared. The influences of the pyrolysis process on effective specific extinction coefficient and phase composition of the SiC-coated mullite fibers were investigated by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction analyses, and their relationship was discussed. The results indicate that the outer-layer coating is composed of β-SiC. When the concentration of the PCS solution was 10wt%, the effective specific extinction coefficient increased from less than 30.4 to about 56.3m2/kg in the wavelength range of 2.5–7.5μm. The thermal conductivity of Al2O3–SiO2 aerogel composites reinforced by SiC-coated fibers at 1000°C was 0.049W/mK, which was lower than that of composites reinforced by blank mullite fibers (0.062W/mK).

Thermal stability of ZrO2–SiO2 aerogel modified by Fe(III) ion

ZrO2–SiO2 aerogel modified by Fe(III) ion was prepared and the stability of the samples under high temperature was investigated. The structure and properties of modified aerogels were characterized by N2 adsorption–desorption, FT-IR, XRD and TEM. The samples still contain a specific surface area about 228 m2/g after 1,000 °C 0.5 h calcinations. The inhibition of ZrO2 particle growth is attributed to the Fe(III) ion modified aerogel surface, which strongly retards the ZrO2 tetragonal phase transformation as well.

Nano-Structured Silicon Derived from Silica Precursors as Anode Material for Lithium Ion Batteries

Silicon is valuable as a high energy density anode material for Li-ion battery applications. However, the high lithium storage capacity leads to extreme volume expansion during lithiation and delithiation that can lead to mechanical deterioration and poor cycle life. To improve cyclability, nano-structured materials can be used to facilitate the volume change without deterioration. By using structured SiO2 precursors, the goal is to reduce SiO2 to Si while maintaining the structure and porosity of SiO2 and thus, mitigating degradation effects during cycling.Nanostructured Si has been synthesized by Mg vapor reduction from SiO2 precursors [1] (e.g. SiO2 microspheres from 3M, SiO2 aerogel, and mesoporous SiO2 [2]). Following the reduction to Si, the materials were chemically treated in HCl and HF to remove remaining MgO and SiO2 respectively. Figures 1 and 2 confirm the presence of Si in reduced samples and successful removal of MgO and SiO2. TGA/DSC has been used to optimize the reduction time and temperature. CV data (Figure 3) and cyclability tests (Figure 4) demonstrate a specific capacity of up to 2065 mAh/g with a low charge transfer resistance (Figure 3, inset).SEM, TEM, BET, EDS, XRD, and Raman spectroscopy data will be presented for SiO2 and reduced Si to describe chemical composition and morphology. Structure dependent electrochemical properties such as impedance spectroscopy, cyclic voltammetry, galvanostatic discharge, and cycle stability will be presented and discussed.

Study of Synthesis of SiO<sub>2</sub> Aerogel from Coal Gangue at an Ambient Pressure

The hydrophobic SiO2 aerogel was prepared by using coal gangue as the starting material—which is viewed as waste in most coal mining and preparation plant—via ambient pressure drying through sol-gel method. The effects of different temperature, time to keep the temperature,vary in mass ratio of coal gangue to sodium hydroxide on the leaching ratio of SiO2 from coal refuses are discussed in this paper. Results showed that the leaching ratio of SiO2 extracted from gangue will be made up to 35.23% compared with the other tests on the condition that the temperature is given 750°C, holding time 2h, the value of ratio of coal gangue to sodium hydroxide 1:0.8, concentration of NaOH 20%. Trimethylchlorosilane (TMCS) is used as modifier to obtain hydrophobicity through solvent exchange and surface modification. We achieved SiO2 aerogel with low density of 0.2559g/cm3, high porosity 88.97%,Strong hydrophobicity (contact angle with water 140.61。) respectively, which demonstrated that the final product exhibits excellent property. Differences among SiO2 aerogel properties were discussed between industrial sodium silicate as precursor and sodium silicate drew from coal gangue as precursor. Results revealed that the the latter is better than the former on properties .Our work provided a potential way to synthetize high value-added product from abandoned by-product coal gangue.

Effects of thermal efficiency in DCMD and the preparation of membranes with low thermal conductivity

The effects of the membrane characteristics and operational conditions on the vapor flux and thermal efficiency in a direct contact membrane distillation (DCMD) process were studied with a mathematical simulation. The membrane temperature, driving force of vapor transfer, membrane distillation coefficient, etc. were used to analyze the effects. The operating conditions that increased the vapor flux improved the thermal efficiency. The membrane characteristics of four microporous membranes and their performances in DCMD were compared. A polysulfone (PSf) membrane prepared via vapor-induced phase separation exhibited the lowest thermal conductivity. The PSf and polyvinylidene difluoride (PVDF) membranes were modified using SiO2 aerogel blending and coating to reduce the thermal conductivity of the membrane. The coating process was more effective than the blending process toward this end. The changes in the structure of the modified membrane were observed with a scanning electron microscope. Si was found on the modified membrane surface with an energy spectrometer. The PVDF composite and support membranes were tested during the DCMD process; the composite membrane had a higher vapor flux and a better thermal efficiency than the support. A new method based on a 3ω technique was used to measure the thermal conductivity of the membranes.

Magnesiothermic Reduction of Structured Silica As Anode for Lithium Ion Batteries

In lithium ion batteries, Si-based anodes possess the highest specific capacity from four to 10 times greater than graphite [1, 2]. However, up to 400% volume expansion of Si during lithiation results in anode pulverization due to mechanical stresses. To avoid mechanical degradation of silicon, the critical value of the silicon particle size/wall thickness should be below the required threshold of <50nm [3,4]. Thus, it is crucial to maintain the silicon particle size in this range. The second major requirement for silicon anode in lithium ion batteries is to have the second conductive phase, e.g. graphite or graphene, in the form of a flexible coating that can withstand lithiation-delithiation without deterioration of the carbon phase. The goal is to develop a silicon-based anode that can withstand lithiation-delithiation cycles without mechanical degradation.Through using structured silica precursors, the goal is to reduce the silica to silicon while maintaining its structure and porosity and thus mitigate the degradation during cycling. Two types of silicon oxide precursor materials that meet these requirements have been evaluated, among them hollow silica microspheres from 3M and silica aerogel from Aspen Aerogels. The major steps including silica synthesis and magnesium vapor reduction will be discussed.Preparation of Si precursor materials has been performed by magnesium reduction followed by the removal of unnecessary impurities such as magnesium oxide and silica by washing the material in 1M HCl followed by 1% HF. Preliminary data from XRD confirms Si formation from the magnesiothermic reduction. SEM, TEM, EDS, BET, XRD, and Raman spectroscopy will be presented to analyze the structure and morphology of the materials. Structure dependent electrochemical properties such as cyclic voltammetry, cycleability, galvanostatic discharge, and cycle stability will be discussed.Fig. 1: (A) TGA for 3M silica microspheres (iM30K) and (B) TGA for Aspen Aerogels’ SiO2 aerogel with Mg with the corresponding SEM images (C) 3M microspheres and (D) Silica aerogel precursors. (E) XRD for reduced SiO2 aerogel: top profile is fully washed reduced Si, middle profile is after reduction but before washing, and bottom profile is for a Si nanopowder reference to vindicate successful Si reduction from SiO2.

Property Changes of SiO&lt;sub&gt;2&lt;/sub&gt; Aerogel on Insulation Performance under Different Time and High Temperature Condition

To study the impacts of structural change of SiO2 aerogel on its insulating properties, the samples were treated in long time (0-12h) at 500°C, 600°C, 700°C and 800°C. The results show that with the time increased, the shrink extends in thickness, the weight loss and the density is basically unchanged under the 500°Cand 600°C. As for 700°C and 800°C, the results were treated with 0-12h, the shrink extends in thickness, the weight loss and density increases considerably. Besides, the structure and composition of SiO2 aerogel after the different temperature treaded with 0.5h were tested. The results show that when the heat treatment temperature is between 200-800°C, the small pores in SiO2 aerogel collapses and large pores increase, and the density is stil1 able to maintain the 1ow value due to the unbroken skeleton. When the heat temperature is between 800-1000°C, the secondary particle size increases and the rnutual integration of the secondary particles occurs, the density will be significantly increased.

Reusable methyltrimethoxysilane-based mesoporous water-repellent silica aerogel membranes for CO2capture

For the first time, highly mesoporous and water-repellent SiO2 aerogels were successfully coated onto a macroporous Al2O3 membrane using methyltrimethoxysilane (MTMS) precursors. The MTMS-derived hydrophobic SiO2 aerogel membranes exhibited at least 500% higher CO2 absorption flux than the uncoated MTMS-based aerogel membranes and could be reused and continuously operated for CO2 absorption for extended periods of time. As a result, MTMS-derived water-repellent silica aerogel membrane contactors are a potential candidate for large-scale CO2 absorption in industrial power plants.

Molecular Dynamics Simulation of Thermal Conductivity of Primary Particle of Silica Aerogel

The amorphous Si O2 and primary particle unit(PPU) of Si O2 aerogel are generated using the melting-quenching process based on the molecular dynamics method. Their thermal conductivities(TC) are also calculated by the non-equilibrium molecular dynamics(NEMD). The size effect on the TC caused by the heat source and heat sink has been taken into account. The predicted TC of the bulk amorphous Si O2 material is in good agreement with the available results in literature. Calculated results also show that the particle diameter has made the TC of the PPU show scale effect in the range of 2-6 nm though the effect is relative small. The diameter of the interface between particles has significant effect on the TC of the PPU which increase 171.61% as the interface diameter increases from 0.89 nm to 2.49 nm when the particle diameter is 3 nm. A temperature drop at the interface showed that there is interfacial thermal resistance between the neighboring particles. It is also shown that, the smaller the interface size, the bigger the interface thermal resistance.

Atomic Layer Deposition-Derived Ultra-Low-Density Composite Bulk Materials with Deterministic Density and Composition

A universal approach for on-demand development of monolithic metal oxide composite bulk materials with air-like densities (<5 mg/cm(3)) is reported. The materials are fabricated by atomic layer deposition of titania (TiO2) or zinc oxide (ZnO) using the nanoscale architecture of 1 mg/cm(3) SiO2 aerogels formed by self-organization as a blueprint. This approach provides deterministic control over density and composition without affecting the nanoscale architecture of the composite material that is otherwise very difficult to achieve. We found that these materials provide laser-to-X-ray conversion efficiencies of up to 5.3%, which is the highest conversion efficiency yet obtained from any foam-based target, thus opening the door to a new generation of highly efficient laser-induced nanosecond scale multi-keV X-ray sources.

Study on the Equivalent Thermal Conductivity of Superalloy Honeycomb Core Structures

The equivalent thermal conductivity of superalloy honeycomb core structures have been studied by experimental method. Test results show that the decrease in honeycomb core height and the increase in honeycomb core diameter would lead to a lower equivalent thermal conductivity for superalloy honeycomb cores. Filling adiabatic materials such as ZrO2 fibers and SiO2 aerogels into core cells would significantly reduce the equivalent thermal conductivity of honeycomb cores with a minor penalty in structure weight.

Synthesis of TiO2–SiO2 aerogel via ambient pressure drying: effects of sol pre-modification on the microstructure and pore characteristics

TiO2–SiO2 composite aerogels were prepared via ambient pressure drying by sol–gel and surface modification for both the sol and gel samples. The organosilane reagents of decamethyltetrasiloxane (DMTSO)/trimethylchlorosilane (TMCS) and hexamethyldisiloxane (HMDSO)/TMCS were introduced into the TiO2–SiO2 composite sol for pre-modification respectively, and subsequently the TMCS/hexane solution was used for surface modification of the obtained TiO2–SiO2 composite gel. The effects of sol pre-modification on the microstructure and pore characteristics of TiO2–SiO2 composite aerogels were investigated. The results indicate that HMDSO/TMCS coupling reagents is more appropriate for the pre-modification of TiO2–SiO2 composite sol than the DMTSO/TMCS reagents. The best volume ratio of HMDSO/TMCS/composite sol for preparing mesoporous TiO2–SiO2 composite aerogels is in the range of 1:0.33:10–1:1.0:10, with which the specific surface area and pore volume of the obtained TiO2–SiO2 composite aerogels are 492–645 m2/g and 2.63–2.85 m3/g, respectively. The results of adsorption and photocatalytic degradation of rhodamine B show that the as-prepared TiO2–SiO2 composite aerogels have higher adsorption/photocatalysis. Particularly, the as-prepared TiO2–SiO2 composite aerogels with HMDSO/TMCS showed prominent adsorption capability with the adsorption rate attaining to 89.4 % within 60 min.

Preparation and adsorption property of phenyltriethoxysilane modified SiO2 aerogel

The hydrophobic silica aerogel (SiO2 aerogel) was prepared by in situ polymerization sol-gel method and ethanol supercritical drying, with tetraethoxysilane (TEOS) as silica source, phenyltriethoxysilane (PTES) as modifier, ethanol as solvent and ammonia as catalyst. The effects of n(PTES)/n(TOES) were investigated on gel time, structure, and hydrophobicity. The SiO2 aerogel was measured by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The effects of n(PTES)/n(TOES) were also studied on adsorption property of pentane, hexane, heptane, octane, benzene, toluene, o-xylene, nitromethane, nitroethane, and nitrobenzene. The adsorption intensity of SiO2 aerogel was compared with that of activated carbon. The results show, with the increasing of n(PTES)/n(TOES), the surface area, pore volume, and pore size of SiO2 aerogel decreased, gel time and hydrophobicity increased, and the contact angle could be 154° with n(PTES)/n(TOES)=0.7. The adsorption intensity of SiO2 aerogel with n(PTES)/n(TOES)=0.5 was bigger than that of activated carbon with an average 5.84 times of 10 organic liquid. The adsorption intensity of aerogel with n(PTES)/n(TOES) =0.1 was the best one in all samples with the average 8.33 times compared with that of activated carbon.

Fabrication of spherical silica aerogel/magnetite nanocomposite particles

Fe3O4/SiO2 nanocomposite particles were synthesized from a hydrophobic Fe3O4 suspension, prepared by precipitating Fe (II) and Fe (III) chlorides and using a hydrophobic SiO2 wet gel from a silicic acid solution derived from a sodium silicate (water glass) solution. Fe3O4 nanoparticles having a crystallite size of ~10nm embedded in SiO2 aerogel can be successfully fabricated by mixing the Fe3O4 suspension and silica wet gel in n-hexane. It was clearly observed that the Fe3O4 nanoparticles were well crystallized and homogeneously dispersed in the SiO2 aerogel. The Fe3O4/SiO2 nanocomposite particles exhibited superparamagentic behavior. The saturation magnetization was determined to 34.5, 18.2, and 8.3emu/g for 90wt% Fe3O4, 70wt% Fe3O4, and 50wt% Fe3O4/SiO2 nanocomposite particles, respectively.

Application of Silica Aerogel-Shellac Wax Composite in Mango Preservation

In this paper, the compound shellac fruit wax was prepared by SiO2 aerogel, shellac, sodium hydroxide, fungicide. The obtained fruit wax was used to preserve the mango in tropical area. The results show that the compound shellac fruit wax with 0.2% SiO2 aerogel can suppresses water loss, adjust the respiration, reduce the rot rate, thus extending the preservation time of mango.

Preliminary Study of Activated Carbon Modified by Silica Aerogel

Activated carbon was modified with silica aerogel by the Sol-Gel. The surface characteristics and structure of activated carbon modified by the silica aerogel were characterized with N2 adsorption method, Scanning Electron Microscope and Thermal Analysis. The experiment results show that the silica aerogel can be used to modify activated carbon for the preparation of composite. SiO2 aerogel can effectively cover holes in the activated carbon, specific surface area of activated carbon composite modified by SiO2 aerogel was between activated carbon and silica aerogel. When MSiO2: MC is 1:3, specific surface area of the composite was 758.638 m2/g; When MSiO2:MC is 1:2, specific surface area of the composite was 760.38 m2/g; When MSiO2:MC is 1:1, specific surface area of the composite was 862.755 m2/g.