Thomas M. Tillotson

New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors

Abstract We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe3+, Al3+, In3+, Ga3+, Zr4+, Hf4+, Ta5+, Nb5+, and W6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Transparent ultralow-density silica aerogels prepared by a two-step sol-gel process

Conventional silica sol-gel chemistry is limited for the production of transparent ultralow-density aerogels because (1) gelation is either slow or unachievable, and (2) even when gelation is achieved, the large pore sizes result in loss of transparency for aerogels <.020 g/cc. We have developed a two-step sol-gel process that circumvents the limitations of the conventional process and allows the formation of ultralow-density gels in a matter of hours. we have found that the gel time is dependent on the catalyst concentration. After supercritical extraction, the aerogels are transparent, uncracked tiles with densities as low as .003 g/cc. 6 figs., 11 refs.

Nanostructured energetic materials using sol-gel methodologies

Abstract We have utilized a sol–gel synthetic approach in preparing nano-sized transition metal oxide components for new energetic nanocomposites. Nanocomposites of Fe 2 O 3 /Al(s), are readily produced from a solution of Fe(III) salt by adding an organic epoxide and a powder of the fuel metal. These materials can be processed to aerogel or xerogel monolithic composite solids. High resolution transmission electron microscopy (HRTEM) of the dried energetic nanocomposites reveal that the metal oxide component consists of small (3–10 nm) clusters of Fe 2 O 3 that are in intimate contact with ultra fine grain (UFG) ∼25 nm diameter Al metal particles. HRTEM results also indicate that the Al particles have an oxide coating ∼5 nm thick. This value agrees well with analysis of pristine UFG Al powder and indicates that the sol–gel synthetic method and processing does not significantly perturb the fuel metal. Both qualitative and quantitative characterization has shown that these materials are indeed energetic. The materials described here are relatively insensitive to standard impact, spark, and friction tests, results of which will be presented. Qualitatively, it does appear that these energetic nanocomposites burn faster and are more sensitive to thermal ignition than their conventional counterparts and that aerogel materials are more sensitive to ignition than xerogels. We believe that the sol–gel method will at the very least provide processing advantages over conventional methods in the areas of cost, purity, homogeneity, and safety and potentially yield energetic materials with interesting and special properties.

Carbon aerogels from dilute catalysis of resorcinol with formaldehyde

Abstract Aqueous polycondensation of resorcinol with formaldehyde leads to organic gels the structure of which can be controlled by the reaction parameters. The amount of catalyst controls the size of the particles constituting the gel network. We could show that for very low catalyst concentrations in a high dilution of reactants, the particle growth can be further enhanced if the reaction temperature is kept low, and the gel time is prolonged. The resultant structure is largely affected by this treatment, producing particles with sizes of about 2 μm and average pore sizes of up to 7 μm. Due to these coarse structures it is possible to dry these RF gels subcritically with very little shrinkage. The structure has been studied by nitrogen sorption, small angle X-ray scattering and acoustic sound propagation. The change of the elastic modulus caused by pyrolysis at around 1000°C has been investigated. Mechanical properties of carbon aerogels are correlated with their electrical properties. The derived carbon aerogels have large specific surface areas, very little mesopore volume however, micron-sized macropores.

Sol-gel processing of energetic materials

Abstract Traditional manufacturing of energetic materials involves processing of granular solids. One application is the production of detonators where powders of energetic material and a binder are typically mixed and compacted at high pressure to make pellets. Performance properties are strongly dependent on particle size distribution, surface area of its constituents, homogeneity of the mix, and void volume. The goal is to produce detonators with fast energy release rate that are insensitive to unintended initiation. Preparation of detonators from xerogel molding powders and aerogels, and comparison with materials produced by state-of-the-art technology are described.

Aerogel commercialization: Technology, markets and costs

Commercialization of aerogels has been slow due to several factors including cost and manufacturability issues. The technology itself is well enough developed as a result of extensive work over the past decade by an international community of researchers. Several substantial markets appear to exist for aerogels as thermal and sound insulators, if production costs can keep prices in line with competing established materials. In the present paper, the elements identified as key cost drivers are discussed, and a prognosis for the evolution of the technology leading to reduced cost aerogel production is given.

Imaging aerogels at the molecular level

Aerogels are a special class of open-cell foams that have an ultrafine cell/pore size (<50 nm), high surface area (400–1000 m2 g−1), and an ultrastructure composed of interconnected colloidal-like particles or polymeric chains with characteristic dimensions of 10 nm. The purpose of this paper is to directly image a series of resorcinol-formaldehyde (RF) and silica aerogels by high resolution transmission electron microscopy (HRTEM). A new vertical replication technique allows us to examine aerogels at the molecular level in situ so that differences in polymeric and colloidal aerogels can be visualized. Such information is crucial for nano-engineering the structure and properties of these novel materials.

Synthesis of lanthanide and lanthanide-silicate aerogels

In this paper we report on the preparation of mixed lanthanide-silicate and pure lanthanide aerogels from the chlorides of erbium, praseodymium, and neodymium. A two-step sol-gel method is described for preparing the mixed aerogels, using a sub-stoichiometric amount of water in the first step to prepare a partially condensed silica-lanthanide precursor. The pure lanthanide aerogels are prepared directly from the chlorides using propylene oxide as a scavenger for reaction generated hydrochloric acid. The aerogel microstructures vary from colloidal for the pure lanthanide and high weight percent lanthanide-silicate aerogels to polymeric for the low weight percent lanthanide-silicate aerogels. This change in microstructure is also indicated by the BET analyses, which show that the surface areas decrease with increasing lanthanide concentrations. In general, we measured reductions of lanthanide contents during the supercritical drying step due to insufficient linking and subsequent “washing out” of the lanthanides from the gels. Also, the retention efficiency for the lanthanide improves with higher silica concentrations, making quantitative doping by this method practical only for the lower lanthanide concentrations.

High resolution transmission electron microscopy nanostructure of condensed-silica aerogels☆

High resolution transmission electron microscopy (HRTEM) was used to study the morphology of ultralow-density transparent condensed-silica (CS) aerogels. Silica aerogels were synthesized by two slightly different two-step polymerization processes and they were supercritically dried in a high temperature autoclave. Aerogel CS1 had a density of 9 kg/m3 and a BET surface area of 574 m2/g; aerogel CS2 had a density of 30 kg/m3 and a specific surface area of 630 m2/g. Both samples were fractured, vertically replicated with 0.95 nm Pt-C and backed with approximately 12 nm of rotary evaporated carbon. The silica aerogel was then removed from the replica with dilute HF acid and the replicas were studied by HRTEM. The stereoscopic HRTEM images reveal that connectors in both CS aerogels are extended filaments which resemble bottlebrushes, having microporosity. This morphology results from side-chain formation on a nearly linear CS stem. The slightly different chemistry leads to different morphologies for the two aerogels. For CS1, the connectors between stems have diameters ranging from 1.7 to 14.2 nm with an average of 6.4 ± 0.5 nm and connector lengths averaged 62 ± 21 nm with some as long as 132 nm. Pore sizes ranged from 13 to 240 nm with an average of 74 ± 43 nm. The pores were slightly larger than those in CS2 which ranged from 12 to 277 nm and averaged 61 ± 56 nm. For CS2, the connectors had diameters ranging from 1.5 to 16.5 nm and averaging 9.7 ± 0.5 nm. The connector lengths in the CS2 aerogel averaged 58 ± 27 nm with some as long as 127 nm. The connector diameters in CS2 (9.7 ± 0.5 nm) were the only important aerogel feature significantly different and greater than those in CS1 (6.4 ± 0.5 nm). The CS1 and CS2 connectors had side-chain diameters of 2 ± 0.7 and 0.95 ± 0.5 nm, respectively, and a similar microporosity of approximately 2.0 ± 1 nm.

Partially hydrolized alkoxysilanes as precursors for silica aerogels

The classical sol-gel process for synthesizing SiO/sub 2/ aerogels involves the hydrolysiscondensation of tetraethyoxysilane (TEOS) andor teramethyoxysilane (TMOS) to produce a gel which can then be supercritically extracted to a low density, highly porous aerogel glass. Controlled hydrolysis of TEOS and TMOS leads to partially hydrolyzed compounds that can be subsequently water processed to form silica aerogels in the density range from .020 to .500 gcc. The partially hydrolyzed compounds are stable when sealed from moist air and can be stored for future use. We discuss the controlled conditions used to obtain these compounds and present data that characterize their structure. We detail the procedures for preparing the wide range of aerogel densities. We also report on their use as an adhesive. 4 refs., 5 figs.