Gelatin desserts, whose molecular structure is basically a kind of solid cage that traps a liquid component, are examples of gels. Some gels are wobbly, like the desserts, while others are more rigid.
A gel whose solid molecular cage structure surrounds a gas instead of a liquid is called an aerogel.[Wikipedia] Aerogels consequently have very low density as well as other properties that make many of them technologically significant. According to one recent patent[DOepatents] assigned to Sandia Corporation[Wikipedia], typical aerogels can be extremely porous (with up to 99% porosity) although their pores are extremely small (less than 10 nanometers, or just a few atoms wide). Such aerogels have extremely large surface areas per unit mass (over 1000 square meters in just one gram), and are insubstantial enough to slow down any light that passes through them by less than 10% (compared to the ~35% speed reduction of light traveling through glass[Wikipedia]). According to recent research reports available from the Department of Energy, aerogels are being used or studied for use in several things, including building insulation, electric storage batteries, solar batteries, catalyst supports, electrical insulation, microelectromechanical devices, optical displays, bright x-ray sources, and traps for radioactive gas produced during nuclear-fuel reprocessing. The reports described below each discuss different uses from this list to different degrees of detail.
High-performance thermal insulation materials have become much less expensive to produce than they used to be. The report “Cold Climate Building Enclosure Solutions”[SciTech Connect], produced for the National Renewable Energy Laboratory by the Fraunhofer Center for Sustainable Energy Systems, describes what Fraunhofer found about the cost-effectiveness of three new ways to insulate buildings in New England, two of which involved insulating with high R-value aerogels. (A material’s R-value is the inverse of the amount of heat, in British thermal units, that each square foot of the material conducts per hour for every degree Fahrenheit of temperature difference between the material’s two sides. High R-values thus represent low thermal conductivities.) The aerogels considered have the makings of effective insulators, having R-values around 10 per inch of aerogel thickness. However, these aerogels are fragile, so to make suitable building insulators out of them they are made in blanket form reinforced by fiberglass or other mechanically stronger materials. Fraunhofer found that “wall insulation retrofits using aerogel blankets could become cost competitive in certain scenarios, such as interior thermal insulation installation” (p. 1). The center also experimented with blown-in aerogels, a novel concept using shredded aerogel blanket pieces, and found that their R-values increased with the pieces’ packing density. Preliminary cost estimates show that blown-cavity aerogel insulation “could become a cost-competitive option in niche areas of building thermal insulation, such as wall cavity insulation” (ibid.).
Aerogels’ high volume of air makes them a somewhat obvious material for inhibiting thermal energy transfer. A perhaps less obvious use for aerogels, as batteries for rapid transfer (and high-density storage) of electrochemical energy, is described in a patent, “Monolithic three-dimensional electrochemical energy storage system on aerogel or nanotube scaffold”[DOepatents], issued to Lawrence Livermore National Security, LLC, the managing and operating contractor for Lawrence Livermore National Laboratory. The patent’s Background section (columns 1 and 2) states:
Batteries are traditionally energy storage devices with high energy density but low power density. Most of the energy is stored in bulk electrode material, and both chemical reaction as well as ionic transport through the porous active layer limit power output. The power density of a battery could be substantially improved if the transport resistance could be decreased while maintaining or increasing the surface area of the electrodes. ... A successful solution will require the combination of a large surface area and a thin electrolyte layer[.]
Columns 9 and 10 of the patent contain some detail about how the large surface area and thin electrolyte layer are produced.
The present invention provides a monolithic three-dimensional electrochemical energy storage system on an aerogel or nanotube scaffold. An anode, separator, cathode, and cathodic current collector are deposited on an aerogel or nanotube scaffold. ... An anode provides a negative terminal for the battery. The anode is deposited on the aerogel or nanotube scaffold. A cathode provides a positive terminal for the battery. The cathode is deposited on the aerogel or nanotube scaffold. Intermediate layers are positioned between the anode and the cathode. The intermediate layers provide an electrolyte and separator for the battery.
The arrangement of these deposited layers in the aerogel or nanotube strands is illustrated in the patent (see Figure 3).
Another recent report shows how aerogels’ low density makes them useful as intense laser-energized sources of subnanometer x-rays[Wikipedia]. When x-rays are generated from laser energy aimed at solid foil targets, the laser energy gets absorbed and converted to x-rays by the foil’s atoms. But this method of x-ray production is self-limiting. The laser beam that energizes the foil also ablates it, producing a dense plasma that inhibits further absorption of the beam, thus preventing more than a few percent of the laser energy from becoming x-rays. On the other hand, if the laser is aimed at an aerogel target, the aerogel’s low density results in less plasma production, but the low density also means that fewer atoms are in the laser beam’s path to convert the beam’s energy. If x-ray generation in hot, dense plasmas weren’t so complex, one might calculate whether the latter outweighs the former. As it is, the question was more easily answered by experiment than by a complex calculation, as reported in the Physics of Plasmas paper “Efficient laser-induced 6 - 8 keV x-ray production from iron oxide aerogel and foil-lined cavity targets”[SciTech Connect] by researchers at Lawrence Livermore National Laboratory, the French energy commission CEA, the University of Rochester’s Laboratory for Laser Energetics, and General Atomics. The result: the low density of metal-oxide aerogels does offer more of an advantage than a disadvantage with respect to solid foils for x-ray production.
The beginning of the report notes two uses for such x-ray sources: for investigations of high energy-density phenomena, like those studied in laser-driven shocks or lab experiments relevant for astrophysics, and for experiments related to inertial-confinement fusion. Aerogels may also be useful for fusion in a quite different way, as the thesis “Development of aerogel-lined targets for inertial confinement fusion experiments”[SciTech Connect] describes. This thesis, researched at Lawrence Livermore in cooperation with the Technical University of Munich, describes experiments that indicate how an aerogel can be turned into a scaffold that holds hydrogen isotopes in place on the inner surfaces of microcapsules that are imploded to induce hydrogen fusion.
Fusing low-mass atoms’ nuclei together results in a larger-mass nucleus plus other subatomic particles with slightly less total mass than the original nuclei had. The small difference in mass appears as a relatively large kinetic energy of the larger nucleus and other particles (E=mc2). To fuse, the nuclei have to collide fast enough to come in contact despite the mutual repulsion of their like positive charges. For a gas or plasma of low-mass nuclei to keep colliding so more and more of them will continue to fuse, as they do in the cores of the sun and other stars, the remaining low-mass nuclei have to be restrained from dispersing after the first fusions release their energy.
These conditions are maintained in stellar cores by the weight of the stars’ outer layers crushing the cores, which provides the pressure to force the low-mass nuclei together and keep them from dispersing. Laboratory experiments don’t have the advantage (or trouble) of a star’s enormous weight to confine nuclei, so something else has to apply and sustain the pressure. Inertial-confinement devices, like that of Lawrence Livermore’s National Ignition Facility, achieve this pressure by imploding microspherical hydrogen containers so rapidly that the hydrogen nuclei’s own inertia prevents their escape. The National Ignition Facility uses an extremely powerful set of laser beams to energize the inside of a cavity housing a microsphere, so that the cavity produces an intense blast of x-rays. The x-ray blast heats the microsphere so that its surface explodes outward, making the nuclei inside recoil inward so they collide, fuse, and release energy.
As the thesis “Development of aerogel-lined targets for inertial confinement fusion experiments” describes, the role contemplated for aerogel in this process is for its solid structure to hold the hydrogen in place on the inner surfaces of the microspheres before they are imploded by the x-ray blasts. The thesis describes how aerogels made of p-DCPD polymer[Wikipedia] were produced and applied as coatings to the inner surfaces of 2-mm diameter microspheres. In further experiments, the air in these aerogels was then displaced by liquid hydrogen-2 (deuterium), which was then frozen, giving the hydrogen the uniform distribution on the microspheres’ inner surfaces desired for inertial-confinement fusion. The experiments reported in the thesis had not progressed yet to seeing if the same uniform distribution could be achieved with the mixture of hydrogen-2 and hydrogen-3 (deuterium and tritium) replacing the aerogels’ air that would be used in fusion experiments, but the results with deuterium were encouraging.
Producing the uniform layers on microspheres’ inner surfaces is a particular instance of precise shaping of aerogels. Aerogels put to some other purposes require shapes with regular, sharply defined edges, which require different shaping methods, as discussed in the Sandia Corporation patent that was mentioned at the beginning of this article, entitled “Method of patterning an aerogel” [DOepatents]:
Aerogels have many applications including catalyst supports, electrical insulators, thermal insulators, and acoustic insulators. It is desired in many applications to employ an aerogel material as a film (e.g. a layer on the order of 1 um [micrometer] thick) on a substrate (e.g. glass, silicon, metal, ceramic, etc.) in applications ranging from the fabrication of micro-electromechanical (MEM) devices to optical display and solar energy conversion devices. In many of these applications it is required that the aerogel film be patterned on the surface of the substrate, creating sharply defined, exposed areas (e.g. aerogel removed) on the substrate and adjacent unexposed areas (e.g. areas remaining covered by the aerogel layer) on the substrate. (“Background of the invention”, column 1, op. cit.[DOepatents])
As the patent further describes, traditional photolithographic methods use photoresist to keep particular areas of solid material from being etched away during a device’s manufacture. While these methods work well to sharply define the shapes of solid-material layers left on a substrate after etching, the same methods applied to porous aerogels tend to give the aerogels irregular edges. The patent describes methods for producing sharply edged aerogel patterns by using other types of chemical besides photoresist to promote or inhibit the etching away of particular aerogel areas from their substrate.
Other reports published in the last two years describe explorations of another potential use of aerogels, with different types of aerogel being examined for their effectiveness at one task.
While a method of artificially fusing low-mass nuclei to continually release energy is a potentially useful harnessing of an important natural process that occurs throughout the universe, we already use the fission of massive nuclei to release energy. Fission fuels naturally produce volatile radioactive isotopes such as tritium, carbon-14, krypton-85, and iodine-129 that are released when the fuels are reprocessed after use[Wikipedia]. The iodine-129 “is one of the more troublesome components in the list because of its long half-life of [16 million years] and its high bioactivity,” according to the Pacific Northwest National Laboratory report “Efforts to Consolidate Chalcogels with Adsorbed Iodine”[SciTech Connect]. This report goes on to describe one approach to capturing iodine-129 and how aerogels might be applied to it:
Sorbents for capturing gaseous iodine, I2(g), have been studied for several decades and have historically focused on metal-exchanged zeolites[Wikipedia]. The primary metal of interest is silver because of the effective and spontaneous chemisorption that occurs in the presence of I2(g) to form AgI …. This chemisorption process allows the porous silver-impregnated zeolite, AgZ, to tightly hold the iodine. Once the AgZ captures iodine, it must then be placed in a secondary waste form for immobilization of the AgI that forms. A replacement for AgZ is being evaluated at Pacific Northwest National Laboratory (PNNL) in the form of silver-functionalized silica aerogels (Matyáš et al., 2011). (Page 1.)
Another iodine-capture approach, involving aerogels that don’t use silver and are based on chalcogens—chemical elements whose atoms have the outer-electron arrangement of oxygen atoms[Wikipedia]—is described in the next paragraph:
Chalcogen-based aerogels, or chalcogels, are alternative iodine sorbents that are being developed at PNNL that do not use silver to bind the iodine (Ryan et al., 2009; Strachan et al., 2010; Riley et al., 2011; Strachan et al., 2011; Riley et al., 2013). These materials capture a large fraction of iodine through what appears to be a mixture of physisorption[Wikipedia] and chemisorption[Wikipedia] processes, depending on the chemistry of the sorbent. (Ibid.)
Reports published in 2012 and 2013 describe investigations related to each approach. Radioiodine capture using aerogels that incorporate silver is discussed in “Deep Bed Adsorption Testing using Silver-Functionalized Aerogel”[SciTech Connect] from Idaho National Laboratory, and in “Characterization of Dry-Air Aged Granules of Silver-Functionalized Silica Aerogel”[SciTech Connect] and “Assessment of Methods to Consolidate Iodine-Loaded Silver-Functionalized Silica Aerogel”[SciTech Connect]” from Pacific Northwest National Laboratory. The chalcogen-based aerogels are discussed in the Pacific Northwest reports “Initial Assessment of Alternate Metals in Chalcogels”[SciTech Connect] and the just-mentioned “Efforts to Consolidate Chalcogels with Adsorbed Iodine”.
Measurements of the iodine sorption reported in “Deep Bed Adsorption Testing using Silver-Functionalized Aerogel” showed the aerogel’s maximum iodine decontamination factor[Wikipedia] was at least 10,000, which is at least in the range that previous studies had identified as necessary for US used-fuel reprocessing facilities to meet iodine-129 capture regulations. “Characterization of Dry-Air Aged Granules of Silver-Functionalized Silica Aerogel” reports on experiments to see how well such aerogels’ iodine-sorptive capacity was maintained when exposed long-term to reprocessing-plant conditions, which found a 9 mass % capacity decrease when the aerogel was aged 6 months, and hypothesizes that the decrease might be due to oxygenation of some of the silver—a hypothesis that the authors propose to test by new experiments, since the tests already performed might not have been sensitive enough.
As for the silverless chalcogen-based aerogels, the report “Initial Assessment of Alternate Metals in Chalcogels”
… provides an up-to-date account of the non-Pt [non-platinum] based chemistries available in the literature for making non-oxide, chalcogen-based aerogels, called chalcogels, for the potential application of iodine capture …. In each case, a combination of multiple precursors is required to make the chalcogels and, in most cases, the precursors are not currently commercially available and must be prepared in the laboratory. References for the preparation details of these precursors have been provided for each. An account of the chalcogels made at Pacific Northwest National Laboratory (PNNL) is also given in this report along with iodine sorption efficiencies for three very diverse chalcogel chemistries. A brief account of consolidation options is provided.
Capturing radioiodine released during fuel reprocessing keeps it from entering the atmosphere, but where does it go after that? That’s the subject of the other two reports, published the next year (2013). The “consolidation” mentioned in the titles is the consolidation of each type of aerogel, once it has absorbed its quota of radioiodine, into a glass.
The report “Efforts to Consolidate Chalcogels with Adsorbed Iodine”
… discusses the efforts taken in fiscal year (FY) 2013 to consolidate the iodine-sorbed chalcogels into a final waste form. Efforts taken in FY2012 to consolidate SnS [tin sulfide] chalcogels without adsorbed iodine showed promise with full collapse after the addition of a glass-forming additive, GeS2 [germanium disulfide] …. (P. 1.)
The report’s conclusions include the following:
This work provided details for the first consolidation experiments ever conducted at PNNL with iodine-sorbed chalcogels. These experiments showed that some work will need to be done to perfect the purity of the starting chalcogels by effectively removing the byproducts of the synthesis reactions during the rinsing process. (P. 10.)
“Assessment of Methods to Consolidate Iodine-Loaded Silver-Functionalized Silica Aerogel” also reports encouraging results so far for the possibility of consolidating aerogels of that type:
The main purpose of this report is to provide a preliminary evaluation of commercially available technologies with which iodine-loaded Ag0[silver]-functionalized silica aerogel can be rapidly consolidated to a durable SiO2-based waste form. In this study, Ag0-functionalized silica aerogel powders were consolidated/sintered with three methods: 1) hot uniaxial pressing (HUP), which involves heating samples in an open-air furnace while simultaneously applying pressure along a single axis, 2) hot isostatic pressing (HIP), which involves heating samples in enclosed metal envelopes in a sealed furnace while simultaneously applying equal pressure from all directions, and 3) spark plasma sintering (SPS), which involves heating samples via internal resistance heating under high applied voltage while simultaneously applying pressure along a single axis. Since there are no studies reported in literature on the sintering of aerogels with HIP and SPS the preliminary experiments were performed with powders containing no iodine and were focused on investigating feasibility of these methods to produce a fully dense product. This was also a conservative approach to avoid potential damage to vendor equipment, if significant quantities of iodine were have released during consolidation. (P. 1.)
The preliminary investigation of HUP, HIP, and SPS clearly shows that these sintering methods can be used effectively to consolidate powders of Ag0-functionalized silica aerogel into products of near-theoretical density. The SPS technique was found to be quite effective in producing pellets with densities >99% of theoretical. Removal of the organic moiety together with addition of small quantities of colloidal silica enhanced the sintering process and resulted in denser products. The knowledge gained from this study will be used in the next phase of the project, which focuses on identifying conditions under which the iodine-loaded aerogel is consolidated into a dense highly iodine-loaded waste form. (P. 14.)
So if further experiments show that either aerogel can be transformed into a glass after trapping iodine-129, we will have a new way to immobilize that type of radioactive waste.
Reports Available through OSTI’s SciTech Connect
Prepared by Dr. William N. Watson, Physicist
DoE Office of Scientific and Technical Information