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Silica aerogel captures cosmic dust intact
Abstract
The mesostructure of silica aerogel resembles strings of pearls, ranging in size from 10 to 100 Å. This fine mesostructure transmits nearly 90% of incident light in the visible, while providing sufficiently gentle dissipation of the kinetic energy of hypervelocity cosmic dust particles to permit their intact capture. In 1987, silica aerogel was introduced as a capture medium to take advantage of its low density, fine mesostructure and, most importantly, its transparency, allowing optical location of captured micron sized particles. Without this feature, locating such captured particles in an opaque medium, e.g., polymer foams, is nearly impossible. The capture of hypervelocity particles has been extensively simulated in the laboratory. At the time of this symposium, more than 2.4 m2 of 20 mg/ml silica aerogel will have been flown on Space Shuttle (STS-47, STS-57, STS-60, STS-64 and STS-68). Demonstration of capturing hypervelocity particles ushers in a new, simple avenue to science in capturing intact cosmic dust from space. Since our introduction of aerogel for intact capture of cosmic dust, many useful features unique to aerogel have been identified.
... Aerogels main physical properties such as mechanical properties, permeability, transparency, insulation, etc. are mainly govern by their microstructure [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], which has been extensively studied during the past decades by different scattering techniques (SAXS, SANS, light scattering [31][32][33][34][35][36][37][38][39][40][41][42]). The silica aerogel microstructure is generally described as a fractal network at length scales ranging from 1 to 100 nm. ...
... The recent literature reports on studies describing new applications area for aerogel. They are indeed good candidate for the mitigation or removal of hazardous pollutants, such as volatile organic compounds, oils and solvents, or heavy metals [60][61][62][63][64][65] from the air and water, immobilization of radioactive wastes [15,16], or greenhouses gases [66][67][68][69][70]. All these new possibilities are the consequence of the very peculiar aerogels microstructure and the ways to control it. ...
... The fractal structure spans over almost two orders of magnitude in intensity for the lightest aerogels. The base catalysis leads to the formation of larger primary particles, with a size around [15][16][17][18][19][20] drolysis and polycondensation reaction was dissolved in various amounts (and consequently the final bulk density under neutral, basic (ammonia, 5 × 10 −2 the gram equivalent weight of a solute p 4. Aerogels were obtained by supercrit MPa) [38,39]. The samples were labeled conditions, respectively, and x referred t The density range varies between 0.02 the samples was calculated by weighin know dimensions and the standard dev [6,14]. ...
Silica aerogels are known to be materials with exceptional characteristics, such as ultra-low density, high surface area, high porosity, high adsorption, and low-thermal conductivity. In addition, these unique properties are mainly related to their specific processing. Depending on the aerogel synthesis procedure, the aerogels texture can be tailored with meso and/or macroporosity. Fractal geometry has been observed and used to describe silica aerogels at nanoscales in certain conditions. In this review paper, we describe the fractal structure of silica aerogels that can develop depending on the synthesis conditions. X-ray and neutron scattering measurements allow to show that silica aerogels can exhibit a fractal structure over one or even more than two orders of magnitude in length. The fractal dimension does not depend directly on the material density but can vary with the synthesis conditions. It ranges typically between 1.6 and 2.4. The effect of the introduction of silica particles or of further thermal treatment or compression of the silica aerogels on their microstructure and their fractal characteristics is also resumed.
... 20 The light weight of aerogels makes them appealing for space applications, where each kilogram of mass can cost tens of thousands of dollars to launch. 21 Aerogels have been critical to space missions, finding use as collectors for cosmic dust, 4,20,[22][23][24] spacesuit insulation, 25 antenna substrates, 26 and insulation for battery packs on the Mars Sojourner rover, 25,27 to name a few examples. ...
... Once the network is formed, the gel is dried by removing solvent without collapsing the solid structure, resulting in highly porous solids. 2 Primary drying methods include critical point drying or freeze drying, although some studies have demonstrated ambient pressure drying by strengthening the nanostructure 28,29 or modifying the surface to enable the nanostructure to "spring back" after drying. 1,2,[30][31][32] Aerogels have a diverse array of compositions, including silica, 1,13,20,22,[30][31][32] titania, 33 and other metal oxides, [34][35][36] along with carbon, 17,[37][38][39][40][41] chalcogenides, [42][43][44][45] polymers, 4,5,8,26,[46][47][48][49][50] and biological materials. 10,[51][52][53][54] Of these, polymer aerogels offer unique opportunities to tailor chemical composition, with polyimide aerogels being of particular interest due to their combination of good mechanical performance and high thermal stability. ...
Aerogels, which are ultralightweight and highly porous materials, are excellent insulators with applications in thermal management, acoustic impedance, and vibration mitigation. Aerogels have huge potential in the aerospace industry as a lightweight solution for thermally insulating electronic components which are confined within a small volume. However, the application of aerogels is currently hampered by the primary processing method of molding, which is costly, time-consuming, requires tooling, and limits geometric complexity. The extrusion-based 3D printing method of direct ink writing (DIW) provides an avenue to move past these constraints. However, rheology modifying additives are commonly used to make a sol printable, which may negatively impact the performance of the product aerogel. Here, we report the DIW of pure polyimide aerogels by subjecting the sol to mild heating (60 °C for 5 min) to promote gelation and produce a printable ink. This approach yielded printed aerogels with comparable microstructural, mechanical, and thermal properties to cast aerogels. The printed aerogel is stable up to 500 °C and has potential for high temperature applications relevant to the aerospace industry. We highlight the utility of this system by printing a bespoke enclosure to insulate a heating plate as a model for batteries; even at a plate temperature of 120 °C, the surface of the 8 mm thick aerogel maintained ambient temperature, indicating excellent inhibition of heat transfer. Additionally, a printed aerogel casing for a solid-state electrolyte coin cell battery resulted in a tenfold increase in ionic conductivity sustained for 100 min. This novel, simple method for 3D printing aerogels opens exciting opportunities to move beyond the geometric limitations of molding to expand the application space of these ultralightweight materials.
... Aerogels can be utilized in numerous applications and they have first been used as catalysts or insulating materials (sound and temperature) [2,[8][9][10]. More recently, aerogels are employed in the field of the environment [11][12][13], greenhouse gases sensors [16,17] pharmaceuticals [14,15] but also in exotic applications such as cosmic dust and space debris sensors [21][22][23] or nuclear wastes containment materials [18][19][20]. Like any type of material for a defined application, knowledge of their mechanical properties is necessary. ...
... Sintering is a process by which the surface of a material is reduced by mass transport [12,[22][23][24]. The aerogel network is described as an aggregates assembly (~50-200 nm). ...
In this paper, we present the different characterization techniques used to measure the mechanical properties of silica aerogels. The mechanical behavior of aerogels is generally described in terms of elastic and fragile materials (such as glasses or ceramics) but also in terms of plastic media in compression testing. Because of these very different mechanical behaviors, several types of characterization techniques are proposed in the literature. We first describe the dynamic characterization techniques such as ultrasounds, Brillouin scattering, dynamic mechanical analysis (DMA) to measure the elastic properties: Young’s modulus (E), shear modulus (G), Poisson ratio (υ) but also attenuation and internal friction. Thanks to "static" techniques such as three-point bending, uniaxial compression, compression we also access to the elastic modulus (E) and to the rupture strength (σ). The experimental results show that the values of the elastic and fracture moduli measured are several orders of magnitude lower than those of a material without porosity are. With regard to the brittleness characteristics, Weibull's analysis is used to show the statistical nature of the fracture resistance. We also present the SENB (Single Edge Notched Beam) technique to characterize toughness (KIC) and the stress corrosion mechanisms, which are studied in ambient conditions and temperature by the double cleavage drilled compression experiment (DCDC). In the last part of the paper, we show how, during the isostatic compression test, aerogels behave like plastic materials. The data allow calculating the bulk modulus (K), the amplitude of the plastic deformation and the yield strength (σel), which is the boundary between the elastic and plastic domains. These different techniques allow understanding which parameters influence the overall mechanical behavior of aerogels, such as pore volume, but also pore size, internal connectivity and silanol bounds content. It is shown that the pore size plays a very important role; pores can be considered as flaws in the terms of fracture mechanics.
... Son 15 yılda ise aerojel kategorisi oldukça genişlemiştir. Yeni birçok silika olmayan oksit aerojeller [10,11], kalkogenit aerogeller [12,13], gradient aerojeller ve diğer aerojel kompozitleri üretilmeye başlanmıştır [14,15]. Son olarak, karbon nanotüp (CNT) aerogel [16,17], grafen aerojel [18,19], silikon aerogel ve karbit (ya da karbonitrit) aerogeller de bu geniş listeye eklenmiştir [20,21]. ...
... In the last 15 years, aerogel categories have been expanded. Several non-silica oxide aerogels [10,11], chalcogenide aerogels [12,13], gradient aerogels and the other composites were produced [14,15]. Finally, carbon nanotube (CNT) aerogels [16,17], graphene aerogels [18,19], silicon aerogels and carbide (or carbonitride) aerogels were added in this list too [20,21]. ...
In recent years, aerogels attract more attentions from both academic
and industrial fields because of their excellent properties as high porosity (85–99.8%), low density (as low as 0.003 g cm−3), low thermal conductivity (as low as 0.01 W/m K), and large specific surface area (up to 1200 m2 g−1). [1, 2]. Aerogels are highly porous and light materials. These materials containing 99% air and excellent physical and chemical properties have attracted the attention of researchers in many areas of science and technology. Aerogel has the reputation as the world’s lightest solid ever produced with varying density values in the range of 0.0011-0.5 g cm-3.
... Aerogels have proven useful in a variety of applications including catalysis [18][19][20][21], membrane separation [22,23], thermal insulation [24][25][26][27], sorption [28][29][30][31][32], flame resistance [33][34][35][36], and CO 2 capture [37][38][39][40]. Aerogels are also useful for the fabrication of advanced scientific materials such as drug delivery devices [41][42][43], optics [44], cosmic dust capture [45], photonics [46][47][48], and supercapacitors [49]. The majority of applications exploit the most salient features of aerogels such as high mechanical performance, low density, and high pore volume. ...
... [21] Nevertheless, aerogels have been recognized as excellent energy absorption materials and used for hypervelocity particle capture in space. [22,23] Shear thickening fluid (STF) is a kinetic energy absorbing material (KEAM) type that finds wide applications in impact resistance and ballistic fields. [24] It comprises small nanoscale particles, such as silica, mixed with inert liquids like polyethylene glycol (PEG). ...
Aerogels, as kinetic energy absorbing materials, can find crucial applications for safeguarding in transportation, sports, buildings, construction, and aerospace. However, the highly porous structure makes it extremely fragile for endurance usage. In this study, a half‐full filled structure has been proposed, and the concept has been demonstrated based on shear thickening fluid (STF) and chemically vapor deposited carbon nanotube aerogels (CNTAs), in which the outer part of CNTA is filled with STF while the inner core keeping unfilled. Chemical vapor deposition significantly enhances the elasticity and electromagnetic shielding performance of the native CNTA. The half‐full filled aerogels (HFFA) show a 348% increase in energy absorption compared to the CNTA. At the same time, the density, electronic conductivity, and electromagnetic interference shielding effectiveness (EMI SE) of the HFFAs are 0.117 g cm⁻³, 1213 S m⁻¹, and 69.52 dB (which is a neglectable reduction of 0.63% compared to native CNTA), respectively. The HFFA strategy provides an alternative route to fabricate robust aerogels with a remarkable increase of target properties while maintaining other properties, such as low density, high pore volume, and conductivity, with limited changes.
... Aerogel has a significant history as a passive capture medium for orbital debris [2], and its energy dissipation and impact properties are informed by theoretical and experimental work [3]. Previous literature describes its use in space, including on the International Space Station as space debris collectors [4], as cosmic dust collectors on the NASA Stardust mission [5] and on the space shuttle [6]. The ISS work included silica-aerogel-based cosmic dust collectors (the Tanpopo Mission) [7]. ...
... Consequently, this approach improves the strength of reinforced silica aerogels relative to native silica aerogels [13]. The goal of such compounding with organic polymers is to generate an aerogel with good compressive strength so that it can adapt to the design of components and absorb the energy involved in shock compressions [14][15][16]. However, fabricated aerogels are not robust enough to be reshaped; thus, during the synthesis and processing stages, they must be cast into their final shapes [17]. ...
... More than two dozen particles from STS-60 and many more from other GAS canisters were found later on. [99] Recently, the panel of two aerogels has been installed on the International Space Station (Figure 19a). After 18 months of revolution and return to the earth, the aerogels have collected several debris having different impact signatures and morphologies. ...
Silica aerogels have drawn considerable attention due to their low density (almost 95% of the total volume is composed of air), hydrophobicity, optical transparency, low conductivity of heat, and large surface to volume ratio. Sol–gel processing is used to prepare aerogels from molecular precursors. To replace the pore fluid with air while retaining the solid network, a supercritical drying process (the most frequent approach) is used. However, recent technologies use atmospheric pressure to allow for liquid removal followed by chemical alteration of the gel's internal layer, which leaves only a silica network with a porous structure filled with air. This study discusses the sol–gel method for preparing silica aerogels and their various drying processes. Furthermore, various areas of applications of silica aerogels, including electronics, construction, aerospace, purification of water and air, sensing, catalyst, biomedical, absorbent, food packing, textile, etc., are also discussed. Lastly, this review provides a perception of the recent scientific progress along with the futuristic development of silica aerogel.
... Aerogels, known as the lightest solid materials, have demonstrated various applications including thermal insulation, particulates matter capturing, and precise sensors. [1][2][3][4] Combining the advantages of high porosity and green reproducibility, cellulose aerogels are ideal alternatives to conventional non-renewable aerogels derived from industrial chemical products. [5] The building blocks cellulose fibrils feature high length-to-diameter ratio, which makes cellulose-based aerogels highly deformable. ...
Cellulose aerogels are plagued by intermolecular hydrogen bond‐induced structural plasticity, otherwise rely on chemicals modification to extend service life. Here, we demonstrate a petrochemical‐free strategy to fabricate superelastic cellulose aerogels by designing hierarchical structures at multi scales. Oriented channels consolidate the whole architecture. Porous walls of dehydrated cellulose derived from thermal etching not only exhibit decreased rigidity and stickiness, but also guide the microscopic deformation and mitigate localized large strain, preventing structural collapse. The aerogels show exceptional stability, including temperature‐invariant elasticity, fatigue resistance (∼5 % plastic deformation after 10⁵ cycles), high angular recovery speed (1475.4° s⁻¹), outperforming most cellulose‐based aerogels. This benign strategy retains the biosafety of biomass and provides an alternative filter material for health‐related applications, such as face masks and air purification.
... SiO 2 aerogel [2] with excellent optical transparency has emerged as the most representative aerogel due to its inexpensive cost and straightforward preparation process [3][4][5][6]. Transparent SiO 2 aerogel [7,8] has been utilized extensively in Cherenkov detectors [9,10], thermal insulation [11][12][13][14][15], energy-saving [13,[16][17][18][19], photoelectric materials [20][21][22], wearables fields [23], thermal collectors [24][25][26][27][28], adsorbents and sensors. Thus, transparent SiO 2 aerogel has great research value and the prospects are very broad. ...
SiO2 aerogels have attracted extensive attention due to their unique structural characteristics, which exhibit many special properties, especially good optical transparency. As far as we know, the sol-gel stage during the synthesis of aerogel plays an important role in the construction of the gel skeleton. In this study, we adjusted the amount of silicon source and catalyst to explore the best scheme for preparing highly transparent SiO2 aerogels, and further clarify the effects of both on the properties of SiO2 aerogels. Results indicated that the pore size distribution was between 10 and 20 nm, the thermal conductivity was between 0.0135 and 0.021 W/(m·K), and the transmittance reached 97.78% at 800 nm of the aerogels, better than most studies. Therefore, it has the potential to be used in aerogel glass for thermal insulation.
... The primary approach to UQ revolves around Monte Carlo (MC) sampling (e.g., Robert & Casella, 2004), which is multi-query in nature and generally requires many numerical simulations to obtain estimates with acceptable accuracy (Bijl et al., 2013;Papadopoulos & Yeung, 2001). Recently, multi-fidelity Monte Carlo (MFMC) methods have been introduced to leverage low-fidelity models to Table 1 Size, Hamaker constant, charge, and coefficient of restitution (CoR) of common dust constituents reported in the literature (Bojdo & Filippone, 2019;Crosby et al., 2007;Crowe, 2019;Díaz Téllez, 2013;Dong et al., 2013;Exner et al., 2015;Faure et al., 2011;Forsyth et al., 1998;Galbreath et al., 1999;Gilbert et al., 1991;Goudarzy et al., 2016;Koper et al., 2020;Kunkel, 1950;L., 1997;Lefevre & Jolivet, 2009;Li et al., 2015;Moutinho et al., 2017;Ontiveros-Ortega et al., 2016;Reagle et al., 2013;Resurreccion et al., 2011;Sandeep et al., 2021;Singh & Tafti, 2013;Tabakoff et al., 1991;Tanaka et al., 2002;Tsai et al., 1991;Tsou, 1995;Valmacco et al., 2016;Yao et al., 2016) accelerate the uncertainty propagation by occasionally making use of a high-fidelity model (Eldred et al., 2017;Peherstorfer et al., 2016Peherstorfer et al., , 2018. The salient idea is to optimize the work distribution among models of different fidelity such that the overall MC error is minimized for a given computational budget. ...
Particle deposition in fully-developed turbulent pipe flow is quantified taking into account uncertainty in electric charge, van der Waals strength, and temperature effects. A framework is presented for obtaining variance-based sensitivity in multiphase flow systems via a multi-fidelity Monte Carlo approach that optimally manages model evaluations for a given computational budget. The approach combines a high-fidelity model based on direct numerical simulation and a lower-order model based on a one-dimensional Eulerian description of the two-phase flow. Significant speedup is obtained compared to classical Monte Carlo estimation. Deposition is found to be most sensitive to electrostatic interactions and exhibits largest uncertainty for mid-sized (i.e., moderate Stokes number) particles.
... In the military elds, SAs are used to capture hypervelocity particles and protect the primary battery pack of the Alpha Particle X-ray Spectrometer from the inuence of low temperatures. 34,35 Similarly, SAs are also used as Cherenkov detector radiators to identify the particles. 36 In the civil elds, SAs have been used in the insulating bricks, glass llers, 37,38 and adsorption of CO 2 and harmful substances. ...
Silica aerogels are three-dimensional porous materials that were initially produced in 1931. During the past nearly 90 years, silica aerogels have been applied extensively in many fields. In order to grasp the progress of silica-based aerogels, we utilize bibliometrics and visualization methods to analyze the research hotspots and the application of this important field. Firstly, we collect all the publications on silica-based aerogels and then analyze their research trends and performances by a bibliometric method regarding publication year/citation, country/institute, journals, and keywords. Following this, the major research hotspots of this area with a focus on synthesis, mechanical property regulation, and the applications for thermal insulation, adsorption, and Cherenkov detector radiators are identified and reviewed. Finally, current challenges and directions in the future regarding silica-based aerogels are also proposed.
... In addition, the composition of the same dust sample varies at different size ranges [217], which are becoming more relevant for modern helicopter turboshaft engines where large particles are filtered out during engine intakes by inertial particle separators [41]. [237,241,130,16,65,52,71,69,40,75,224,172,82,41,215,134,55,236,19,127,85,225,133,269,193,206,191,168]. demonstrating the importance of quantifying how sensitive the particle deposition rate is to these uncertainties. ...
In recent years, public awareness of the impact caused by micron-sized particles such as infectious aerosols or dust has increased drastically, ranging from severe public health concerns to various environmental issues. In addition, airborne dust and volcanic ash ingested by aircraft engines compromise the durability, performance, and safety of engine turbine components. The transport and deposition of fine (<≈O(10 μm) particulates in turbulence (e.g., dust or powder) is largely controlled by cohesive forces such as electrostatics and van der Waals. Due to their small size and cohesive nature, tracking individual particles in turbulence is challenging, and is further complicated by significant uncertainties in material properties. Although computational methods with varying levels of complexity have been developed over past decades, accurate predictive models of cohesive particle transport and deposition do not yet exist for large-scale simulations. The main objective of this work is to develop a numerical framework tailored for resolving cohesive particle interactions in turbulence. Efficient algorithms are developed to optimally resolve particle contact forces in a direct numerical simulation (DNS) framework. The framework is then used to study the effect of electrostatics on particle transport in turbulence. It is found that the short-range electric potential plays a key role in particle clustering even in dilute suspensions. A follow-up study of charged aerosols in ionized air identifies a feedback mechanism capable of generating atmospheric turbulence via an electrohydrodynamic body force. Turbulence-induced breakup of an aggregate of solid particles subject to van der Waals is also investigated. A phenomenological model of the breakup process is developed that acts as a granular counterpart to the Taylor analogy breakup (TAB) model commonly used for droplet breakup. Such a model is capable of predicting the onset of aggregate breakup in the absence of a resolved turbulent flow field. Finally, particle deposition in a turbulent pipe flow is studied in the presence of van der Waals and electrostatics. The sensitivity of deposition rate to uncertainties in cohesive forces is efficiently quantified using a multi-fidelity framework. Deposition is found more sensitive to electrostatics than van der Waals across all particle sizes and exhibits largest uncertainty for mid-sized particles.
... Aerogel is an exceptionally lightweight material that possesses high thermal insulation, ultra-high porosity, and high surface area [1,2]. The impressive thermal insulation property of aerogel lends itself to diversified applications where heat insulation is crucial, such as in aerospace [3][4][5], building retrofitting [6,7], oil and gas pipelining [8], industry for cryogenic applications [9,10], cold weather outdoor gear [11,12], etc. However, the extensive application of aerogel is restricted by its fragility [13]. ...
Aerogel-nonwoven fabrics are gradually emerging as a promising alternative in the field of thermal protective textiles. Although currently available commercial aerogel-nonwovens can provide some degree of flexibility and find their way into the conventional textiles, their flexibility has not yet reached an acceptable level essential for clothing comfort. Fabric flexibility is important for the aesthetics, comfort, and functionality of clothing. This article demonstrates the design and fabrication of an aerogel-nonwoven fabric as a thermal liner in a multilayered protective clothing system that is significantly more flexible than commercial aerogel-nonwoven fabrics. The developed aerogel-nonwoven composite possesses almost similar dry and radiant heat resistance but inferior contact heat resistance in comparison to commercial aerogel nonwovens. It has high air and moisture vapour permeability compared to its commercial counterpart and will provide satisfactory thermal comfort to the wearer in hot and humid environments. The overall thermal resistance performance, together with high flexibility and moisture management capability, make the aerogel-nonwoven fabric a potential alternative to commercial nonwovens for any thermal protective clothing system where fabric flexibility and thermal comfort are the key requirements.
... The aerogel block's surface layer was designed to have a density of 10 mg/cm 3 (Tabata et al., 2010) and was positioned on the space-exposed surface side. The surface layer with a thickness of 10 mm at maximum is expected to capture dust as intact as possible by employing an aerogel with the lowest density ever used in previous space missions (e.g., Yano and McDonnell 1994;Tsou, 1995;Tsou et al., 2003;Brownlee et al., 2006;Noguchi et al., 2011;Westphal et al., 2014). The second layer with a density of 30 mg/cm 3 formed a base under the 10 mg/cm 3 layer. ...
The Tanpopo experiment was the first Japanese astrobiology mission on board the Japanese Experiment Module Exposed Facility on the International Space Station (ISS). The experiments were designed to address two important astrobiological topics, panspermia and the chemical evolution process toward the generation of life. These experiments also tested low-density aerogel and monitored the microdebris environment around low Earth orbit. The following six subthemes were identified to address these goals: (1) Capture of microbes in space: Estimation of the upper limit of microbe density in low Earth orbit; (2) Exposure of microbes in space: Estimation of the survival time course of microbes in the space environment; (3) Capture of cosmic dust on the ISS and analysis of organics: Detection of the possible presence of organic compounds in cosmic dust; (4) Alteration of organic compounds in space environments: Evaluation of decomposition time courses of organic compounds in space; (5) Space verification of the Tanpopo hyper-low-density aerogel: Durability and particle-capturing capability of aerogel; (6) Monitoring of the number of space debris: Time-dependent change in space debris environment. Subthemes 1 and 2 address the panspermia hypothesis, whereas 3 and 4 address the chemical evolution. The last two subthemes contribute to space technology development. Some of the results have been published previously or are included in this issue. This article summarizes the current status of the Tanpopo experiments.
... The high porosity, super-hydrophobized and extremely large surface area of the aerogels made them an attractive option for catalysis, as well as template and storage media, where aerogels could find their way in applications such as gas filters, anti-corrosive, hydrogen fuel storage, liquid transport, drug delivery, Cherenkov radiation detectors, chemical absorbers, and catalyst carriers [63][64][65][66][67][68][69][70][71][72]. ...
The master thesis studies the formation and structure of ultra low density silica based aerogels using solid state NMR spectroscopy.
The aim of the present work is the detection of the main reason behind the closed pore
formation, by aging the wet gels in chemical compounds which are not present within the
gelation process.
... Since the 1990's, NASA has used density gradient aerogels for several expedition missions [9][10][11][12]. In the Stardust mission, the density gradient silica aerogel was used as a hypervelocity particle capture to collect the cometary particles and improve their integrity. ...
Density gradient aerogel is a kind of porous material with non-uniform density change in a specific direction. It has some unique properties and can be used in some extreme conditions. The resorcinol–formaldehyde (RF) gradient aerogels were prepared by the method based on double diffusion convection. We studied the influences of injection velocity and diffusion before gelation on the distribution of gel density gradient, and obtained an optimized injection velocity scheme. The RF density gradient aerogels were characterized by x-ray transmission imaging, scanning electron microscopy (SEM), and nitrogen adsorption–desorption isotherm. The density distribution and microstructure of the RF density gradient aerogels were obtained. The solution obtained by the optimized multi-velocities injection method presents a gradient change in the low-density part that is greater than the high-density part. This uneven change can offset the diffusion caused by the long gel time of the low-density part, and finally form a gel whose density changes uniformly with the thickness. The aerogel obtained by the optimized method ranges from 100 to 400 mg cm−3, and the density of the sample varies linearly. The results indicate that this is an efficient and direct method for preparing density gradient aerogels, and aerogels with specific density trends can be obtained by adjusting the injection velocities.
... Aerogels containing various species including carbon, 17 − 2 4 chalcogenides, 6 , 2 5, 2 6 metal/metal oxides, 19,20,23,27,28 and organic polymers 29,30 have been synthesized and investigated in the literature. These aerogels have a wide range of applications including being used as catalysts, 12,20,21,25,27,28,31−33 cosmic dust collectors, 34,35 electro-des, 17,18,24 nuclear waste forms, 36 sensors, 35,37 sorbents, 1,[5][6][7]26 and thermal insulators. 38,39 Studies on the applications of xerogels are mostly related to their use as catalysts or sensors. ...
Various radionuclides are released as gases during reprocessing of used nuclear fuel or during nuclear accidents including iodine-129 (¹²⁹I) and iodine-131 (¹³¹I). These isotopes are of particular concern to the environment and human health as they are environmentally mobile and can cause thyroid cancer. In this work, silver-loaded heat-treated aluminosilicate xerogels (Ag-HTX) were evaluated as sorbents for iodine [I2(g)] capture. After synthesis of the base NaAlSiO4 xerogel, a heat-treatment step was performed to help increase the mechanical integrity of the NaAlSiO4 gels (Na-HTX) prior to Ag-exchanging to create Ag-HTX xerogels. Samples were characterized by powder X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, Brunauer–Emmett–Teller analysis, gravimetric iodine loading, nanoindentation, and dynamic mechanical analysis. The structural and chemical analyses of Ag-HTX showed uniform distribution of Ag throughout the gel network after Ag-exchange. After I2(g) capture, the AgI crystallites were observed in the sorbent, verifying chemisorption as the primary iodine capture mechanism. Iodine loading of this xerogel was 0.43 g g–1 at 150 °C over 1 day and 0.52 g g–1 at 22 °C over 33 days. The specific surface area of Ag-HTX was 202 m² g–1 and decreased to 87 m² g–1 after iodine loading. The hardness of the Na-HTX was >145 times higher than that of the heat-treated aerogel of the same starting composition. The heat-treatment process increased Young’s modulus (compressive) value to 40.8 MPa from 7.0 MPa of as-made xerogel, demonstrating the need for this added step in the sample preparation process. These results show that Ag-HTX is a promising sorbent for I2(g) capture with good iodine loading capacity and mechanical stability.
... Of note are space applications, which have driven substantial research and development on both silica-and polymer-based aerogels where NASA has been a key player. A low-density silica aerogel has, for instance, been used as a capture material for hypervelocity cosmic dust particles because its mesostructure ensures a sufficiently gentle dissipation of the particles' kinetic energy and allows their intact capture, whereas its transparency enables an easy recovery of the impacted particles for the variety of chemical and physical analyses performed to understand better cosmochemical processes (Figure 8d) [17]. Other applications are based on silica aerogels' low refractive index for the detection of Çerenkov radiation [18] or exploit their large surface area and/or sorption capacity: additives for foundry applications [19], oil-spill clean-up materials [20], catalyst support materials [21], gas adsorption [22], and drug delivery [23]. ...
Silica aerogels are highly porous, predominantly mesoporous solids with an open network of amorphous silica nanoparticles produced by sol–gel processes. The materials are made from waterglass or silicon alkoxide precursors through gelation, aging, optional hydrophobization, and supercritical or ambient‐pressure drying. Silica aerogels possess a range of exceptional properties such as high vapor permeability, low density (down to 0.002 g/cm³, but much more commonly around 0.1 g/cm³), high specific surface area (500–1000 m²/g), ultralow thermal conductivity (12–18 mW/(m·K)), and tunable hydrophobicity. Potential applications include CO2 capture, oil–water separation, catalyst support, or Knudsen pumps, but by far the most established use is as thermal superinsulation for the oil‐and‐gas and construction sectors with a market size of hundreds of millions of US$. Technical innovations in production technology and the entry of new producers are expected to lead to a significant reduction in cost and accelerate an already substantial market growth in the near future.
... The SAMs have a porous structure with ultra-low density, and these dust particles will be buffered and slow down to a stop, leaving a trail through SAMs. In the meantime, the researchers can easily chase these particles following the trail due to the transparency of the aerogel [85]. In 1999, NASA launched the ''Stardust'' probe using 260 aerogel cubes to capture comet dust particles moving at high speed (6 km/s) [86]. ...
The silica aerogel monoliths (SAMs) with three-dimensional (3D) porous structures showing unique physical and chemical properties promise a new era of technologies. However, the critical challenge remains in these materials due to their poor mechanical properties in large-scale fabrication. Several attempts have been adopted to enhance the mechanical properties of SAMs to overcome this challenge and explore their applications in various communities. In this review, we introduce the general synthetic methods of SAMs and highlight the use of mechanical reinforcement strategies to improve their properties. Furthermore, the applications of SAMs in killer applications have been introduced, including heat and acoustic insulation, catalysis, and electronic devices. Critically, perspectives on the challenges and opportunities of SAMs are highlighted as potential targets for commercialization.
... Silica aerogels (SAs) are highly porous nanomaterials with extremely low densities, high specific surface areas and high porosities [1]. Due to these properties, SAs have various potential applications, e.g., drug delivery systems [2], energy storage systems [3], cosmic dust capture [4], paints [5], food packaging [6], and insulation in buildings [7]. Despite the advantages of SAs, there are still some practical limitations. ...
Here we present an economical ambient pressure drying method of preparing monolithic silica aerogels from methyltrimethoxysilane precursor while using sodium bicarbonate solution as the exchanging solvent. We prepared silica aerogels with a density and a specific surface area of 0.053 g·cm−3 and 423 m2·g−1, respectively. The average pore diameter of silica aerogels is 23 nm as the pore specific volume is 1.11 cm3·g−1. Further, the contact angle between water droplet and the surface of silica aerogels in specific condition can be as high as 166°, which indicates a super-hydrophobic surface of aerogels.
... Aerogels have many unique properties, such as high transparency in visible light, high surface area, and low thermal conductivity [1]. Utilizing these properties, applications to high-performance insulated windows, Cherenkov photodetectors, and space dust collection have been demonstrated [2][3][4][5]. In particular, silica xerogel with high thermal insulation properties are increasingly used in home appliances, housing, and automobiles [6]. ...
Silica xerogels were prepared by the sol-gel method under ultrasonic irradiation, using tetraethylorthosilicate (TEOS) as the starting material. Hexamethyldisiloxane (HMDSO) was used as the hydrophobizing agent. When preparing silica xerogel, it is necessary to perform aging and hydrophobization to suppress shrinkage during ambient pressure drying, however, such treatments are time-consuming. In this study, the semi-solid hydrogel was irradiated with ultrasonic for the first time in order to accelerate aging and hydrophobic treatment, and the effect of ultrasonic frequency on structure was investigated. Firstly, ultrasonic irradiation was performed at frequencies of 100 kHz and 500 kHz, followed by hydrophobic treatment at a frequency of 500 kHz, in order to promote aging. The results identify optimum conditions for ultrasonic irradiation to promote aging and hydrophobization reactions, and it was found to be possible to prepare silica xerogels in less than 1/5 of the conventional time. The silica xerogels had a low density and the shrinkage was suppressed. In this study, it was found that ultrasonic irradiation of semi-solid hydrogel was very effective for promoting the reaction.
... Aerogels have large surface area, low density, low thermal conductivity and high porosity, which are promising for many applications, such as the space dust collector [1], thermal insulation [2], and superfluid phase transition of 3 He [3], energy storage devices [4], pollutant treatment [5], and sensors [6] etc. In the past few decades, various kinds of aerogels have been prepared by materials including silica [7], metallic oxide [8], sulfide [9], silicon [10], metal element [11], resins [12], carbon nanotubes (CNTs) [13], graphene oxide (GO) [14], cellulose [15], and so on. ...
Inhomogeneous resorcinol-formaldehyde (RF) aerogels with continuously varying densities were prepared by a facile yet effective method. The preparation involved sol–gel technique and supercritical drying. The low-density sol was pumped into the high-density sol continuously, meanwhile the mixed sol was blended and pumped into a cylindrical container continuously for directional gelation. Then the varying-density gel was washed in hot ethanol and dried by supercritical drying. The samples were investigated by scanning electron microscope and X-ray phase contrast imaging instrument. The problems of density mutation and non-monotonicity of density in samples were studied and well solved by controlling the blending and pumping of sols. Qualified samples with thickness of ~10–28 mm and density of ~0.19–0.62 g/cm3 were obtained. Further work is focusing on the samples’ performance in the experiments of trapping high-velocity particles and impact compression.
Inhomogeneous resorcinol-formaldehyde (RF) aerogels with continuously varying densities were prepared by a facile yet effective method. The preparation involved sol–gel technique and supercritical drying. The problems of density mutation and non-monotonicity of density in samples were studied and well solved by controlling the blending and pumping of sols. The samples were investigated by scanning electron microscope and X-ray phase contrast imaging instrument. Qualified samples (~10–28 mm in thickness) with continuously varying density (~0.19–0.62 g/cm3) were obtained, and they were expected to be used in the experiments of trapping high-velocity particles and impact compression.
... Aerogels and xerogels have various applications including thermal insulation, 13−17 optical materials, 17−19 electrodes, 20,21 cosmic dust collection, 17,22 nuclear waste forms, 23 catalysis, 24−26 and gaseous iodine sorption. 27 Aerogels and xerogels are porous solids with large specific surface areas (SSA), low apparent densities, and low thermal conductivities. ...
Silver-exchanged aluminosilicate aerogels and xerogels were investigated as gaseous iodine [I2(g)] sorbents. The structures, morphologies, compositions, and pore structures of aerogels (as-made and heat-treated at 350°C) and xerogels are compared using powder X-ray diffraction (PXRD), scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, as well as specific surface area (SSA) and pore size analyses. The as-made aerogels, xerogels, and heat-treated aerogels were ion exchanged with Ag in AgNO3 solutions of deionized water and methanol (5:1 by volume), and PXRD patterns showed the presence of nanocrystalline Ag⁰ after the Ag-exchange. Gravimetric iodine loadings of Ag-aerogels and Ag-xerogels were 0.33–0.41 g g⁻1. The Ag-aerogels without heat-treatment showed an ~8 mass% higher iodine loading than Ag-impregnated xerogels and ~3 mass% higher than heat-treated Ag-impregnated aerogels. All gels after iodine uptake showed the presence of AgI, indicating chemisorption of iodine to silver. The SSA values of the as-made gels were 420–600 m² g⁻¹ but decreased significantly to 34–120 m² g⁻¹ after Ag-impregnation and iodine uptake. Overall, changes in physical and chemical properties of aerogels and xerogels after iodine uptake were similar, and the differences in iodine loading capacities of the aerogels and xerogels minimal, providing a driver for using xerogels due to their less complex synthesis process as compared to aerogels.
... Aerogels are disordered networks with high porosity and low density. 1 Due to their large specic surface area and their porous structure, they are of interest for a variety of applications, e.g. as thermal insulators, acoustic dampers or in heterogeneous catalysis. [2][3][4][5][6][7] In addition to systems based on mono-or bimetallic noble metal aerogels which are mainly used in electrocatalysis, 8,9 mixed systems for gas phase reactions like MSR or CO oxidation are also utilized. 6,10 In mixed aerogel systems one component acts as a carrier material on which the active phase is immobilized e.g. ...
In order to enable future use of aerogels in heterogeneous solid or fluidized bed catalysis a method of production of millimeter sized monolithic Au/Al2O3 aerogel spheres by a continuous flow reactor is developed. Flow velocities and synthesis parameters are optimized to produce aerogel spheres in three different sizes. The resulting aerogel spheres exhibit a porous aluminium oxide aerogel matrix with a large specific surface area of 400 m² g⁻¹ on which gold nanoparticles are evenly distributed. The aerogel spheres are compared to xerogels of the same material in contrast to their surface area, pore size distribution, morphology, crystal structure and thermal properties. The presented method allows a broad access to various mixed aerogel systems of oxidic carrier material and noble metal nanoparticles and is therefore relevant for the shaping of different aerogel catalyst systems.
... A comprehensive overview of the chemical and physical characteristics of silica aerogels has been widely discussed in several books [58,98,99]. Many research and review papers [18,43,95,[100][101][102][103][104][105][106][107][108][109][110][111][112][113] on silica aerogels, which thoroughly explain processing, properties, and applications of silica aerogels, have already been published. However, only few reports involve the different approaches to material design and processing conditions for improved mechanical properties. ...
Aerogels materials are the porous, solid materials that exhibit a different way of extreme material properties. Mainly aerogels are known for their extremely low densities (which range from 0.0011 to 0.5 g cm−3). Aerogels are nano structured materials with high specific surface area and porosity along with low density and dielectric constant with excellent heat insulation properties which have 95–99% air in volume. Aerogels materials are generally prepared by removing of the liquid substance of gel through a supercritical drying process which makes the liquid to be slowly dried out without the collapsing of the solid matrix in the gel from the capillary action. This particular paper presents a review of properties, preparation, application, and handling of aerogel materials. The total review in this paper makes ease of work and more reliable for future.
This chapter discusses the emerging and promising field of environmental applications of aerogels. Due to their large pore volume, specific surface area, and diverse range of tailorable solid-phase and surface chemistries, aerogel materials are interesting candidates for addressing many challenging environmental remediation objectives. Herein we review the use of silicate, non-silicate, and allophane-clay-based aerogels in several challenging environmental remediation applications including the removal of air pollutants, water remediation, oil spill reclamation, heavy metal capture, CO2 sequestration, trapping of pesticides, immobilization of nuclear waste, and capture of orbital space debris.
In this handbook, we explore the diverse class of porous materials called aerogels – what they are, how they are made, how they are characterized, materials properties they can exhibit, their applications, and their increasing role in commerce. In this chapter, we provide the reader with a general introduction to the topic of aerogels and an overview of their history.
NASA has used aerogel in several space exploration missions over the last two decades. Aerogel has been used as a hypervelocity particle capture medium (Stardust) and as thermal insulation for the Mars Pathfinder, Mars Exploration Rovers, and Mars Science Laboratory. Future applications of aerogel are also discussed and include the proposed use of aerogel as a sample collection medium to return upper atmosphere particles from Mars to Earth and as thermal insulation in thermal-to-electric generators for future space missions and terrestrial waste-heat recovery technology.
The present review is focused on one of the most studied aerogel materials, silica aerogels. It aims at presenting a brief overview of the elaboration steps (sol–gel synthesis, ageing, and drying), the textural and chemical characteristics (aggregation features, porosity and surface chemistry), the main physical properties (from thermal, mechanical, acoustical, and optical to biological, medical, etc.) and a rather broad panel of related potential applications of these fascinating nanostructured materials. It cannot be considered as an exhaustive synopsis but must be used as a simple tool to initiate further bibliographic studies on silica aerogels, which are amazing very light solid materials, as shown in Fig. 13.1.
Silica aerogels are important to be used as photon radiators in Cherenkov counters for high-energy physics experiments because of their optical transparency and intermediate refractive indices between those of gases and liquids or solids. Cherenkov counters that employ silica aerogels as radiators and photodetectors are often used to identify subatomic charged particles (e.g., electrons, protons, and pions) with momenta on the order of sub-GeV/c to GeV/c; they are also used to measure particle velocities in accelerator-based particle and nuclear physics experiments and in space- and balloon-borne experiments in the field of cosmic ray physics. Recent studies have demonstrated that it is important for the design of Cherenkov counters that the transparent silica aerogel tiles comprise solid material with recently improved transparency and a refractive index that can be controlled between 1.003 and 1.26 by varying the bulk density in the range of 0.01–1.0 g/cm3. Additionally, a technique for fabricating large-area silica aerogel tiles without cracking has been developed. In this chapter, we describe advances in the technologies for producing silica aerogels with high optical performances to be used in scientific instruments. We further discuss the principles underlying the operation of detectors based on the Cherenkov effect. We also review applications of silica aerogels in specific high-energy physics experiments.
Silica aerogels have piqued the interest of both scientists and industry in recent decades due to their unusual properties such as low density, high porosity, low thermal and acoustic conductivity, high optical transparency, and strong sorption activity. Aerogels may be created via two-step sol-gel synthesis from different organosilicon compounds known as precursors. Various drying processes are employed to remove the solvent from the gel pores, the most common of which is the supracritical drying method. This paper highlights the potential of silica aerogels and their modifications as adsorbents for environmental cleanup based on recent researches. Following an introduction of the characteristics of aerogels, production techniques, and different categorization possibilities, the study is organized around their potential use as adsorbents.
In recent years, aerogels attracted more attention due to their outstanding properties and potential applications in a wide variety of technological fields. Aerogels are three-dimensional porous networks or materials with a porous structure obtained from wet gels, where the solvents are replaced by air. A critical step in aerogel formation is the drying of the hydrogel. Many methods have been used to dry aerogel, the most common, safest, and cheapest method among the methods is ambient pressure drying. Due to the high-cost synthesis of monolithic aerogels, in recent years, researchers focused on the preparation of porous aerogels with modern drying methods on a large scale. In this article, aerogel, its types, history, characteristics, classification, preparation methods, properties, and applications of this interesting material are introduced. Aerogels are used in new technical applications as efficient thermal insulation, catalyst, energy storage material, water treatment adsorbent, and sound absorbent. Aerogels are also used in biomedicine and sensors. A discussion on the challenges, limitations, and urgent need to develop new technologies for aerogel production is presented.
STARDUST, a Discovery-class mission, will return intact samples of cometary dust and volatiles from comet P/Wild 2, as well as samples of the interstellar dust moving through the solar system. Dust capture utilizes aerogel, a microporous silica that is capable of intact capture of hypervelocity particles. A navigation camera, an in situ dust analyzer, and a dust flux monitor complete the payload. The Wild 2 flyby takes place in January 2004, with Earth return in January 2006.
Cellulose aerogels are plagued by intermolecular hydrogen bond‐induced structural plasticity, otherwise rely on chemicals modification to extend service life. Here, we demonstrate a petrochemical‐free strategy to fabricate superelastic cellulose aerogels by designing hierarchical structures at multi scales. Oriented channels consolidate the whole architecture. Porous walls of dehydrated cellulose derived from thermal etching not only exhibit decreased rigidity and stickiness, but also guide the microscopic deformation and mitigate localized large strain, preventing structural collapse. The aerogels show exceptional stability, including temperature‐invariant elasticity, fatigue resistance (∼5 % plastic deformation after 105 cycles), high angular recovery speed (1475.4° s−1), outperforming most cellulose‐based aerogels. This benign strategy retains the biosafety of biomass and provides an alternative filter material for health‐related applications, such as face masks and air purification. A new type of cellulose aerogels with anisotropic and hierarchical porous architecture are developed via a petrochemical‐free method. The aerogels display temperature‐invariant elasticity (∼5 % plastic deformation after 105 compressive cycles at 50 % strain), large‐strain recoverability (folding and twisting), angular recovery speed high up to 1475.4° s−1, and exceptional fatigue resistance.
This paper reported the preparation of ultra-flexible silica aerogels based on diene synthesis reaction at atmospheric pressure. Vinyl trimethoxy silane and vinyl methyl dimethoxy silane as silicon source were used to prepare vinyl aerogels by one-pot acid-base sol-gel route. The diene synthesis reaction was used to prepare ultra-flexible and super hydrophobic silica aerogels on the basis of vinyl aerogels. Maximum strain of the ultra-flexible aerogels is as high as 90% for 20 compression-decompression cycles. The aerogels can be recovered completely without structure destruction. In addition, the adsorption capacity of the samples is higher than that of the conventional silica aerogels, which can reach to 15.0 g⋅g⁻¹ of dichloromethane. It is worth noting that they also have efficient cycle performance for adsorption and filtration. Acting as core part of filtration, the organic solvents insoluble in water can be easily separated from water by the aerogels.
Particle deposition in fully-developed turbulent pipe flow is quantified taking into account uncertainty in electric charge, van der Waals strength, and temperature effects. A framework is presented for obtaining variance-based sensitivity in multiphase flow systems via a multi-fidelity Monte Carlo approach that optimally manages model evaluations for a given computational budget. The approach combines a high-fidelity model based on direct numerical simulation and a lower-order model based on a one-dimensional Eulerian description of the two-phase flow. Significant speedup is obtained compared to classical Monte Carlo estimation. Deposition is found to be most sensitive to electrostatic interactions and exhibits largest uncertainty for mid-sized (i.e., moderate Stokes number) particles.
Aerogels have been frequently reported in three-dimensional (3D) monolith bulk, granule, and two-dimensional (2D) thin-film geometries. However, their commercialization has been widely restricted due to the lack of flexibility, extensibility, and fragile network structures. Recently, a new class of aerogels, i.e. aerogel fibers has emerged. When conventional aerogels are transformed into fibers (sub-micron or nanofibers), their performance may enhance remarkably. Therefore, such aerogel fibers can exhibit improved mechanical performance, favorable flexibility, higher tensile strength, and enhanced extensibility. This will make them a suitable candidate for applications that require high mechanical flexibility and to be stretchable. In conjunction with the superior thermal insulation performance of aerogels, aerogel fibers can be extensively used in other commercial sectors where load-bearing is crucial. In this short review, we intend to assess the current technologies for the production of aerogels in fiber geometry, called “aerogel fiber.” Materials used for this purpose, along with the post-processing and functionalization methods, are also evaluated. Their recent breakthroughs in emerging applications in a variety of engineering fields are covered. A discussion of current challenges, limitations, and the urgent need for the development of new technologies for the continuous production of aerogel fibers is provided. A road map pointing to a future direction in aerogel fiber field will be discussed.
Sample return gives an added value to study the Solar System with respect to in-situ space missions, because: 1) the available laboratory analytical techniques are not limited by resources imposed by spacecraft operations; 2) the returned samples are available long after the end of the space mission, taking advantage of laboratory analytical techniques improvement. However, these advantages have to be “conquered” overcoming the technical challenges of target proximity operations and sampling mechanisms.
We present here a review of sample return scenarios describing sampling mechanisms for different sample types and mission configurations, i.e., technologies studied for future Mars and Moon surface sampling, and those used in past successful past sample return missions (as Stardust that sampled comet 81P/Wild 2, Hayabusa which returned to Earth after asteroid 25,143 Itokawa sampling) and in interplanetary and stratospheric extra-terrestrial dust sampling.
The applications of sol-gel-made materials have largely expanded during the two last decades and a significant span of these applications is the subject of this chapter. A first domain concerns direct applications in the sol or in the gel state. The various types of gel shaping come next. The techniques reviewed here comprise coatings and thin films, fibers, and monoliths directly obtained from gel monoliths, or from gel-derived powder packing. Many applications are more technical. The applications being first addressed here are filtration membranes, thermal and acoustic insulation, insulating windows optically transparent in the visible spectrum, antireflective coatings, Cherenkov counters, luminescent materials, and absorbent or confinement media. Next come the electric and energy storage applications such as electrodes or batteries, also in superconductivity and environmental applications. More involved applications which are briefly reviewed concern the electronic, dielectric, or piezoelectric domains. Catalysis and biocatalysis are other important fields, altogether with the neighbor fields of sensors and biosensors. Biomedical applications, in particular implants, have beneficiated from significant sol-gel developments and they deserve a dedicated sections. In all these domains, the newly designed hybrid or mesostructured materials brought some noteworthy progress and these are summarized all along the various sections of this chapter.
A new experimental design for directional solidification experiments with high cooling rates under microgravity conditions is presented. The aerogel-based furnace module ARTEC (AeRogel TEchnology for Cast alloys) developed at DLR extends the earlier presented sounding rocket facility ARTEX by enabling a transition from low to high solidification velocities and a simultaneous operation of five independent furnaces in the same sounding rocket module. The furnaces for directional solidification are equipped with thermally insulating aerogels as a crucible material. Their optical transparency allows the control of the solidification parameters (velocity and temperature gradient) with optical methods in the lab. In ARTEC, a drastically increased solidification velocity is achieved by contacting the sample with a movable cooling-rod during processing. Therefore, a better theoretical understanding of the influence of a sudden change in solidification velocity on microstructure formation is obtained. Carrying out experiments in microgravity gives access to purely diffusive solidification conditions. Hence, convection free-growth can be compared with growth subject to natural (earth) and/or forced-convection (earth and space). Furthermore, alloys with high density differences in their alloy components and, hence, also between the primary solidifying phase and the surrounding liquid can be studied without the negative influence of fluid-flow or macrosegregation being present.
The possibility of capturing cosmic dust at hypervelocity has been demonstrated in the laboratory and in the unintended Solar Max spacecraft. This technology will enable a comet coma sample return mission and be important for the earth orbital cosmic dust collection mission, i.e., the Space Station Cosmic Dust Collection Facility. Since the only controllable factor in an intact capture of cosmic dust is the capturing medium, characterizing the effectiveness and properties of available capture media would be very important in the development of the technique for capturing hypervelocity cosmic dust intact. We have evaluated various capture underdense media for the relative effectiveness for intact capture. 2 refs., 2 figs.
In most theoretical and experimental investigations into the shock response of underdense solid media, the influence of the medium’s mesostructure on the resulting pressure and degree of compaction has not been taken into account. In typical cases examined, shock pressures are well in excess of 1 GPa and this approach is clearly justified. However, at low pressures, calculations show that the distribution of void sizes can affect the final state achieved upon shocking the medium from a given initial porosity. This paper analyzes the response of porous aluminum to low pressure shocking and demonstrates a dependence of the final shocked state on the distribution of void sizes.
Dust from several comets can be returned to earth with minimum propulsion effort after a high-speed encounter. The collection concept involves the capture of individual dust particles by condensation in a cell after vaporization by a thin diaphragm. This collection concept has been validated in laboratory tests. To compensate for the slower laboratory velocities in a two-stage light-gas gun, higher-density metals (Zn and Cd) have been used for particles and diaphragms to simulate hypervelocity vaporization. Teflon has proved to be a suitable material for the collection cell; more than 50 percent of the vaporized projectile material has been recovered by chemical etching from such cells. Hypervelocity-impact parameters (such as diaphragm-to-projectile ratio, hole size, and plume angles) determined by tests permit the modification of existing analytical models. These new findings are necessary for flyby cometary experiments.
The ability to capture projectiles intact at hypervelocities opens new applications in science and technology that would either not be possible or would be very costly by other means. This capability has been demonstrated in the laboratory for aluminum projectiles of 1.6 mm diameter, captured at 6 km/s, in one unmelted piece, and retaining up to 95% of the original mass. Furthermore, capture was accomplished passively using microcellular underdense polymer foam. Another advantage of capturing projectiles in an underdense medium is the ability of such a medium to preserve a record of the projectile's original velocity components of speed and direction. A survey of these experimental results is described in terms of a dozen parameters which characterize the amount of capture and the effect on the projectile due to different capture media.
- P Tsou
Tsou, P., D. 1?. Brownlce & A. L. Albcc, LPSC 22,
(1991).