Subcritical Drying Research Articles

Development of silica - cenosphere aerogel composites for thermal insulation applications

Novel aerogel composites were prepared by sol-gel process and dried by adding 5 to 15 weight percentage of cenosphere to silica aerogel prepared by subcritical drying process. Silica-cenosphere aerogels were calcined at 1200ºC, as α-cristobalite phase was formed in silica aerogel at 1200°C. At temperatures less than 1200°C only amorphous silica was found in silica aerogel. Microstructural analysis showed spherical shape in different sizes with micro pores in surface of cenosphere and popcorn-like structure in silica-cenosphere aerogels. Effect of cenosphere addition in silica aerogel was studied by FT-IR. The particle size and distribution of prepared aerogels and cenosphere were analysed by particle size distribution analyser. The average particle size of silica aerogel was found to reduce by increased addition of cenosphere. The nitrogen adsorption studies of cenosphere and silicacenosphere aerogels indicated type II isotherm which represents macroporous/nonporous unrestricted monolayer – multilayer adsorption. The powder flow behaviour of the cenosphere and prepared aerogels were found to be strongly dependent on the particle size and distribution. The compressive strength and thermal conductivity of all the samples were found to be dependent on the porosity and bulk density. Among all the samples, lower density and higher porosity was observed in SC15. The silica aerogel recorded the highest strength value and was reduced by the addition of cenosphere except for SC5. The thermal conductivity was also found to be lowered for silica-cenosphere aerogels than pure silica aerogel and was found suitable for thermal insulation application that do not require strength.

The Influence of the Precursor's Nature and Drying Conditions on the Structure, Morphology, and Thermal Properties of TiO2 Aerogels.

A cost-effective solution for the synthesis of high-porosity TiO2 aerogels, which can be used as a mesoporous perovskite network charge-carrier material during the manufacture of solar cells, is described. The effects of the synthesis parameters (precursor (titanium (IV) isopropoxide (TIP) and tetrabutyl orthotitanate (TBOT)), additional solvent exchange (n-hexane (nH), cyclohexane (CH), and diethyl ether (DE)), subcritical drying (800 mbar vacuum, 70 °C, 8 h), aging, and calcination on the aerogel’s structure have been investigated. Methods of XRD, FT-IR, BET, Raman, STA, SEM, UV–vis, and thermal conductivity measurements were applied to find out the relation between the synthesis conditions and the properties of the synthesized aerogels. Amorphous aerogels are polydispersed systems with the highest probability of pore diameter from 0.5 to 15 nm. An nH-exchanged, aged aerogel synthesized from the precursor TIP shows the highest diameter of pores. After calcination, the aerogels tend to crystallize into an anatase phase and the size of the crystallites depends on the precursor’s nature. Calcination leads to a significant increase in both the apparent and true density of the aerogels, and it also results in an increase in porosity and thermal conductivity.

Effect of Precursor Nature and Sol-Gel Synthesis Conditions on TiO2 Aerogel's Structure.

The aim of this investigation was to synthesize high porosity TiO2 aerogel by applying sol-gel and subcritical drying methods and to identify the influence of reagent’s nature and synthesis conditions on their structural and optical properties. Methods of XRD, FT-IR, BET, STA, SEM, and UV-vis were applied to investigate and compare the properties of synthesized TiO2 aerogels and to determine the most effective synthesis route. The structural parameters of the synthesized materials can be varied by changing the precursor type (titanium (IV), isopropoxide (TIP), or tetrabutylorthotitanate (TBOT)) and the nature of the solvent used for additional exchange (n-hexane (nH), cyclohexane (CH), or diethyl ether (DE)). All of the subcritical dried samples show the amorphous structure, which tends to crystallize into the anatase phase after calcination. The number of micro and mesopores and the specific surface area depends on the synthesis conditions. The pores with the highest diameter have been found for additionally nH exchanged and aged aerogel synthesized from precursor TIP. Despite the imperfections in the structure, the produced aerogels show structural and optical properties typical of the TiO2 structures mentioned in the literature.

Specifications study of Hydrophobic Silica Aerogel Doped with Rhodamine 6G Prepared via Sub-Critical Drying Technique

The influence of dye laser Rhodamine 6G (R6G) on the molecular structure of silica aerogel prepared by normal drying method is reported. The study also tests the effect of dye concentration on morphological and physical properties. Fourier Transform Infrared Spectroscopy (FTIR) was used to examine this effect, in addition to Field Emission Scanning Electron Microscopy (FESEM), contact angle, and surface area measurement. It was found from FTIR data that the dye laser stays with the inner structure of samples and, at high concentration, it gives a good influence by reducing (OH) band and increasing (CH) band, leading to changing the contact angle from (123á´¼) to (145á´¼). Whereas particle size varied from 22 nm to 47 nm after doping, resulting in a decrease in surface area from 673 m2/gm before doping to 357 m2/gm after doping. It was also found that the samples became lightweighted through the noticeable decrease in the amount of density from 0.28 to 0.17 gm/cm3.

Resorcinol-Formaldehyde Xerogel Derived Highly Porous Carbon Matrix: A Novel Approach for Suppressing the Shuttling Effect in High Rate Lithium-Sulfur Battery

Lithium-sulfur battery (LSB) is considered as the next generation battery with a remarkable theoretical specific capacity and energy density of sulfur electrodes as 1672 mA.h.g-1 and 2600 W.h.kg-1 respectively1. However, the bottle necks like insulating nature of sulfur, polysulfide shuttling and volume expansion of sulfur electrodes during cycling are detrimental to practically achieve such astounding electrochemical performance. Hence, the practical application of LSBs are hindered by low rate performance and capacity fading during cycling1-2. In order to address the above issues we have incorporated sulfur in a highly porous nitrogen doped carbon matrix derived from resorcinol-formaldehyde (RF) xerogel to synthesize a composite (a-N-RFC/S) cathode electrode for LSB. The high surface area and pore volume along with the nitrogen doping in RF-derived carbon xerogel imparts conductivity, accommodate volume expansion and restricts polysufide shuttling when tested electrochemically. As a result, the above synthesized composite a-N-RFC/S exhibits superior rate performance and stable cyclic performance.Nitrogen doped RF xerogel was prepared by sol-gel polycondensation reaction followed by sub-critical drying. The prepared xerogel was pyrolysed at 900 °C to obtain carbon xerogel which was subsequently activated with KOH under N2 atmosphere. The resultant a-N-RFC was subjected to melt diffusion for sulfur incorporation to yield a-N-RFC/S composite. The electrode was prepared by mixing composite, binder and carbon black in 75:10:15 ratio. As prepared electrode was used as working electrode and lithium foil as the counter and reference electrode in a coin-cell assembly.The N2 sorption studies reveal a high surface area of 1589 m2.g-1 was obtained for a-N-RFC due to the presence of micro as well as mesopores with a total pore volume of 0.96 cm3.g-1 after activation. Furthermore, the electrochemical performance of the prepared composite was carried out by subjecting it to rate capability studies at different current rates. The a-N-RFC/S composite delivered a reversible capacity of 1283, 814, 518 mAh.g-1 at 0.4, 4 and 10 C rate respectively with almost 100% coulombic efficiency. Even at current rate as high as 20 C, it delivered a specific reversible capacity of 497 mAh.g-1. Interestingly, when the current rate was swapped back, it maintained the capacity values suggesting an excellent capacity retention.Such remarkable electrochemical performance can be attributed to the high specific surface area of the host material which can accommodate the volume expansion of sulfur and confine the polysulfides in it during cycling. Moreover, nitrogen doping chemically absorb the higher order polysulfides and converts into Li2S1. Thus, polysufides are restrained in the host material by physical as well as chemical sorption leading to extraordinary performance. A detailed characterization of the prepared carbon host and composite electrode material along with their electrochemical performance will be presented during the conference meeting. 1. Chao Deng, Zhuowen Wang, Shengping Wang & Jingxian Yu, Inhibition of polysulfide diffusion in lithium–sulfur batteries: mechanism and improvement strategies. Mater. Chem. A, 7, (2019), 12381.2. Sang-Kyu Lee, Yun Jung Lee, & Yang-Kook Sun, Nanostructured lithium sulfide materials for lithium-sulfur batteries, Power Sources, 323 (2016) 174-188. Figure 1

Effect of Aromatic Amines on the Properties of Formaldehyde-Based Xerogels.

This study investigates the synthesis of formaldehyde-based xerogels using alternative aromatic precursors, with comparison to traditional resorcinol-formaldehyde analogues, in order to alter the chemical composition of the resulting gels. By replacing resorcinol with aromatic amine molecules, i.e., ammeline, melamine and melem, each expected to undergo similar reactions with formaldehyde as the substituted species, we found that for all substituted gels, at low additive contents, the gel structure was compromised and non-porous materials were formed, as opposed to the most abundant monomers, and therefore, these additives seem to act as impurities at low levels. Working towards higher additive contents, melem monomers exhibited low solubility (~5%), even at elevated temperatures, thereby limiting the range to which melem could act as a substitute, while melamine could be incorporated up to ~40% under acidic conditions, with enhanced microporosity over this range. Pure gels were successfully synthesised from ammeline, but their performance was inferior to resorcinol-formaldehyde gels, while melamine-formaldehyde analogues required acidic reaction conditions but shrank considerably on sub-critical drying, adversely affecting the gel properties and demonstrating their lack of potential as sorbents. This demonstrates the potential for the inclusion of aminated aromatics within resorcinol-based gel systems, however, only as partial substitutes and not complete replacements.

Environmental impact of the subcritical production of silica aerogels

Abstract This study evaluates for the first time the environmental impacts of three different syntheses of subcritical silica aerogels. These aerogels are produced by subcritical drying, a more economical and safer process than that of the supercritical aerogels. The environmental evaluation was performed using the standardised Life Cycle Assessment (LCA) methodology, using a cradle to gate approach. The life cycles of these materials were modelled using production data collected in laboratory and a LCA software. These aerogels are produced to be used as aggregates in wall coating mortars for buildings, conferring them thermal insulating properties. It was concluded that the most critical stages in the LCA of these aerogels are the production of raw materials and the production in the laboratory, the latter mostly due to the high energy consumption. The results presented in this paper correspond to a laboratory-scale production, which makes them expectedly higher than if the aerogels were produced at an industrial scale. It was also found that aerogels obtained using subcritical drying generally lead to lower environmental impacts than supercritical aerogels.

Experimental investigation of thermal insulating aerogel composites of hydrothermal reactor for biomass-to-hydrogen conversion

The paper presents aerogel SiO2–TiO2 composites used for thermal insulation of hydrothermal reactor for biomass-to-hydrogen conversion and the method of their production. The technology of aerogel composites production includes the following stages: ion exchange of liquid Na-glass resulting in production of silica hydrosol; hydrosol concentration; hydrogel production and its maturing; SiO2–TiO2 alcogel production; modification of its surface; subcritical drying of alcogel resulting in production of SiO2–TiO2 ambigel; its thermal treatment, granulating and classification. The influences of infrared opacifier (titanium dioxide) and thermal treatment temperature of SiO2–TiO2 composite on its structural and thermal characteristics have been investigated. Possibilities of hydrogen yield increase by reduction of power waste by means of vacuumized thermal insulation for reactor walls have been examined.

$$\hbox {TiO}_{2}$$ TiO 2 aerogel–metal organic framework nanocomposite: a new class of photoanode material for dye-sensitized solar cell applications

$$\hbox {TiO}_{2}$$ aerogel–metal organic framework (MOF) nanocomposite was synthesized using sol–gel method followed by subcritical drying technique and employed as a photoanode material in quasi-solid dye-sensitized solar cells (DSSCs). The nanocomposite material showed a BET surface area of $$250\hbox { m}^{2}\hbox { g}^{-1}$$ with an average pore size of 5 nm. Field emission scanning electron microscopic images revealed the continuous arrangement of pore-solid network structure. Energy-dispersive X-ray analysis shows the presence of MOF clusters on $$\hbox {TiO}_{2}$$ aerogel network. X-ray photoelectron spectroscopic analysis also supports the presence of MOF clusters in the aerogel network and indicates the presence of some oxygen vacancies in the nanocomposite material. The $$\hbox {TiO}_{2}$$ aerogel–MOF nanocomposite was used as photoanode in DSSC and an overall power conversion efficiency 2.34% along with a short-circuit current density 6.22 mA $$\hbox {cm}^{-2}$$ was achieved.

Resorcinol-formaldehyde derived carbon xerogels: A promising anode material for lithium-ion battery

Abstract

Ambient Pressure Hybrid Silica Monoliths with Hexamethyldisilazane: From Vitreous Hydrophilic Xerogels to Superhydrophobic Aerogels.

Hybrid silica-based monoliths were synthesized at ambient pressure, using minimum amounts of the silylating agent hexamethyldisilazane (HMDZ). Depending on the synthesis approach, the materials ranged from dense and vitreous xerogels to transparent and superhydrophobic aerogels. Emphasis was given to understanding the role of the silylating agent, its content and incorporation process on the final morphology, and properties of the xerogels/aerogels. It is proven that as a coprecursor, increasing HMDZ content contributes to increase the lipophilic/hydrophilic balance, induce high surface areas, and decrease densities, but there is a maximum usable content for producing monoliths. Conversely, as a postsynthesis modifier, there is an optimum HMDZ content that maximizes hydrophobicity (water contact angle of ∼144°) and induces high surface area (∼700 m2·g–1), keeping the density low (∼300 kg·m–3). It is proven that the aging period in the hydrophobizing solution is a crucial parameter. The most superhydrophobic xerogels were obtained using HMDZ as a postsynthesis modifier, achieving values of water contact angles as high as ∼173°, at the cost of density increase to ∼600 kg·m–3 and decrease of the surface area to ∼300 m2·g–1. The best compromise between low density, high surface area, and superhydrophobicity is obtained using HMDZ both as a coprecursor and as a postsynthesis modifier, in a low HMDZ/tetraethoxysilane total molar ratio (<0.2), with an aging period of 16–24 h. The use of subcritical drying, along with the minimization of the expensive organic modifier quantities, allows envisaging a safe and low-cost large-scale production of a variety of materials, including superhydrophobic aerogels with potential distinctive applications.

Influence of Synthesis Variables on the Textural Properties of Mesoporous Carbon Xerogels

Influence of synthesis variables on the textural properties of mesoporous carbon xerogels is studied. Variables explored in synthesizing mesoporous carbon xerogels (CX) are: (i) initial formaldehyde/resorcinol molar ratio in preparing the sol, (ii) pH of sol-gel processing, (iii) temperature of gelling and subsequent curing of the gel, and (iv) mode of subcritical drying of the wet gel. Making of sol having formaldehyde/resorcinol molar ratio little higher than 2 and sol-gel processing at pH around 6 resulted in carbon xerogel with wide pores. Curing of gel at higher temperature resulted in carbon xerogel with narrower pores. Drying of the organic gel after exchanging water with low boiling organic solvent was highly beneficial in obtaining carbon xerogel with wider pores along with larger mesoporous area.

Designing micro- and mesoporous carbon networks by chemical activation of organic resins

Carbon xerogels with ultrahigh micro- and mesopore volumes were synthesized from the activation of polymeric resins prepared by sol–gel polycondensation of resorcinol/formaldehyde mixtures in basic medium and subcritical drying. Various activating conditions (e.g., agent, temperature, impregnation conditions) were used and it was found that the textural features of the resulting carbon xerogels are linked to the experimental procedure of the activation reaction to promote the porosity development. The shrinkage and structural collapse of the fragile resins typically obtained upon annealing at high temperatures (during carbonization and/or physical activation) is suppressed when the impregnation of the chemical activating agent is performed under controlled conditions. If the alkaline reagent (either KOH or K2CO3) is put in contact with the resin by wet impregnation (liquid/solid); under such conditions, the intimate contact between both compounds allows the formation of microporosity during the activation along with the enlargement and/or preservation of the mesoporosity of the pristine resin. Furthermore, the chemical activation via wet impregnation allows the combination of high surface areas and the preservation (even higher development) of the mesoporosity created during the synthesis of the resin. The effect of the impregnation method was found highly dependent of the reagent and activation temperature, highlighting the possibility to design micro-mesoporous carbon xerogels at low temperatures with a subtle control of the activation conditions.

Heat-insulating aerogel composites for a hydrothermal reactor

The SiO2–TiO2 aerogel composites used in the heat insulation of a hydrothermal reactor and the method of their fabrication using a liquid glass technology are analyzed. The process of fabrication of the composite material includes the following stages: the ion exchange of sodium liquid glass with the formation of silica hydrosol; the concentration of hydrosol; the formation of hydrogel and its maturing; the formation of alcogel of an SiO2–TiO2 composite material; surface modification; subcritical drying of alcogel with the formation of SiO2–TiO2 composite ambigel; and its heat treatment, granulation, and classification. The influence of infrared absorber (titanium dioxide) and the temperature of heat treatment of an SiO2–TiO2 aerogel composite material on its structural and thermal characteristics is studied.

EXPERIMENTAL RESEARCH OF AEROGEL INSULATION COMPOSITES OF THE HYDROTHERMAL REACTOR FOR BIOMASS-TO-HYDROGEN CONVERSION

This paper presents aerogel SiO2–TiO2composites used for thermal protection of biomass-to-hydrogen hydrothermal reactor and the method of their production. The technology of aerogel composites production includes the following stages: ion exchange of liquid Na-glass leading to formation of hydrosol silica; hydrosol concentration; formation and maturing of hydrogel; production of SiO2-TiO2alcogel; modification of its surface; subcritical drying of alcogel leading to formation ofSiO2-TiO2 ambigel; its heat treatment, granulating and classification. The influence of the infrared opacifier (titania) and the heat treatment temperature of SiO2-TiO2composite on its structural and thermal characteristics is investigated. Possibilities of increasing the hydrogen yield by reduction of energy losses using vacuum thermal insulation for reactor walls are examined.

Catalytic graphitization of resorcinol-formaldehyde xerogel and its effect on lithium ion intercalation

Resorcinol formaldehyde (RF) derived xerogels were synthesized by polymerization of resorcinol and formaldehyde using sol–gel method followed by sub-critical drying. Thus prepared RF xerogel samples were then pyrolyzed at different temperatures ranging from 900 to 1800°C with and without catalyst to understand the role of catalyst in graphitization while varying the temperature. These RF derived carbon xerogels were structurally characterized extensively by Raman spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy and small angle X-ray scattering. Structural characterization studies confirmed that RF xerogels pyrolyzed at moderate temperatures in the presence of a catalyst were graphitized however the degree of graphitization achieved was only 10% in case of no catalyst even at elevated temperatures (2500°C). Later, these RF derived catalytically graphitized carbon xerogels were tested for their electrochemical performance as potential anode materials for rechargeable batteries. Galvanostatic charge discharge experiments were performed at 0.1C rate (37.2mA/g current density) to examine the effect of degree of graphitization on lithium ion intercalation. It was found that with increased degree of graphitization, specific capacity increased with decrease in irreversible capacity along with excellent cyclic stability and coulombic efficiency more than 97%.

Surface modification of titania aerogel films by oxygen plasma treatment for enhanced dye adsorption

Titania aerogels were synthesized by sol–gel route followed by ambient pressure subcritical drying technique. The aerogels synthesized in the present work possess a maximum surface area of 252m2/g. The pore size distribution is between 2 and 30nm which confirms their mesoporosity. The oxygen plasma treatment on titania aerogel thin films improved the surface area up to 273m2/g and produced additional hydrophilic groups on the surface. It is confirmed by BET surface area, XPS and thermal analysis in conjunction with dye adsorption studies. After plasma treatment the dye adsorption capacity was increased 2.5 times higher than that of untreated aerogel film. The increased surface area and the hydrophilic groups generated on the titania aerogel surface during plasma treatment are responsible for enhanced dye adsorption. The overall nanoporous morphology of titania aerogel is preserved after plasma treatment.

Mesochanneled hierarchically porous aluminosiloxane aerogel microspheres as a stable support for pH-responsive controlled drug release.

The molecular-scale self-assembly of a 3D aluminosiloxane (Al-O-Si) hybrid gel network was successfully performed via the cocondensation of hydrolyzed alumina (AlOOH) and (3-aminopropyl)trimethoxysilane (APS). It was transformed into a microspherical aerogel framework of Al-O-Si containing mesochannels with tunable hierarchically bimodal meso/macroporosities by a subcritical drying technique. Good homogeneity of AlOOH and APS brought during the synthesis guaranteed a uniform distribution of two metal oxides in a single body. A systematic characterization of the aerogel support was carried out using FTIR, SEM, TEM, nitrogen adsorption/desorption analysis, WAXS, SAXS, and ξ-potential measurement in order to explore the material for drug uptake and release. The drug loading and release capacity and chemical stability of an aluminosiloxane aerogel were studied using two nonsteroidal antiinflammatory drugs, ibuprofen and aspirin. A comprehensive evaluation of the aluminosiloxane aerogel with ordered mesoporous MCM-41 was also performed. Aerogel supports showed a high drug loading capacity and a pH-responsive controlled-release property compared to MCM-41. Meanwhile, kinetic modeling studies indicate that the drug releases with a zero-order profile following the Korsmeyer-Peppas model. The biocompatibility of aluminosiloxane aerogels was established via ex vivo and in vivo studies. We also outline the use of aluminosiloxane aerogel as a support for a possible 3D matrix for an osteoconductive structure for bone tissue engineering.

Investigation of the effect of sol concentration on the microstructure and morphology of Novolac hyperporous

Novolac hyperporous materials were successfully synthesized using sol–gel polymerization of Novolac resin with hexamethylenetetramine (HMTA) catalyst in the solvent vapor saturated atmosphere, followed by subcritical drying. Mercury porosimetry results showed that these materials had large pore volume, more than 1cm3/g. SEM micrographs showed that these materials formed strongly cross-linked aggregates. Therefore, these results demonstrated that these materials would be termed as hyperporous materials.Novolac hyperporous materials can be either crushed or invaded by mercury during mercury porosimetry experiments. In this work, six samples with different initial sol concentration were examined. It has been shown here that a sample with higher porosity exhibited two volume variation mechanisms successively; compaction followed by intrusion, when submitted to mercury porosimetry that real result could be obtained by applying Pirard's collapse model below the pressure of transition, Pt, and Washburn's intrusion theory above Pt. Low porosity sample showed the collapse under mercury pressure only and no transient pressure could be observed. It means mercury does not penetrate the pore network, but the whole material is densified. Microstructure of these hyperporous materials directly influences mechanism changing from collapsing to intrusion.

Quels lasers et quels paramètres pour quelles lésions : discussion intéractive de cas cliniques

Novolac hyperporous materials were successfully synthesized using sol–gel polymerization of Novolac resin with hexamethylenetetramine (HMTA) catalyst in the solvent vapor saturated atmosphere, followed by subcritical drying. Mercury porosimetry results showed that these materials had large pore volume, more than 1 cm3/g. SEM micrographs showed that these materials formed strongly cross-linked aggregates. Therefore, these results demonstrated that these materials would be termed as hyperporous materials.Novolac hyperporous materials can be either crushed or invaded by mercury during mercury porosimetry experiments. In this work, six samples with different initial sol concentration were examined. It has been shown here that a sample with higher porosity exhibited two volume variation mechanisms successively; compaction followed by intrusion, when submitted to mercury porosimetry that real result could be obtained by applying Pirard's collapse model below the pressure of transition, Pt, and Washburn's intrusion theory above Pt. Low porosity sample showed the collapse under mercury pressure only and no transient pressure could be observed. It means mercury does not penetrate the pore network, but the whole material is densified. Microstructure of these hyperporous materials directly influences mechanism changing from collapsing to intrusion.