Method of Cross-Linking Aerogels Using a One-Pot Reaction Scheme

A document discusses a new, simplified method for cross-linking silica and other oxide aerogels, with a polymeric material to increase strength of such materials without adversely affecting porosity or low density. This innovation introduces the polymer precursor into the sol before gelation either as an agent, which co-reacts with the oxide gel, or as soluble polymer precursors, which do not interact with the oxide gel in any way. Subsequent exposure to heat, light, catalyst or other method of promoting polymerization causes cross-linking without any additional infiltration steps

Process for preparing polymer reinforced silica aerogels

Process for preparing polymer-reinforced silica aerogels which comprises a one-pot reaction of at least one alkoxy silane in the presence of effective amounts of a polymer precursor to obtain a silica reaction product, the reaction product is gelled and subsequently subjected to conditions that promotes polymerization of the precursor and then supercritically dried to obtain the polymer-reinforced monolithic silica aerogels

Effect of Processing Conditions on Chemical Makeup of Di-isocyanate Crosslinked Silica Aerogels

Sol-gel derived silica aerogels are attractive candidates for many unique thermal, optical, catalytic, and chemical applications1 because of their low density and high mesoporosity. However, their inherent fragility has restricted their use to, for example, insulation in extreme temperature environments such as Mars. We have previously reported crosslinking the mesoporous silica structure of an aerogel with di-isocyanates reacted with silanols on the surface,2,3 or epoxies reacting with an amine decorated silica surface.4 Either approach has been shown to significantly increase the strength of the aerogel with only a small effect on density or porosity. Thus, these hybrid materials may be enabling for future space exploration missions as well as advanced aeropropulsion systems which demand lighter weight, robust, dual purpose materials for insulation, radiation protection and/or structural elements of habitats, rovers, astronaut suits and cryotanks. Utilizing amine-decorated silica particles to react with di-isocyanate oligomers analogous to the epoxies will produce polyurea crosslinks, in addition to carbamates produced from reaction with silanols on the surface as shown in Scheme 1. Since it is suggested in the literature that polyureas are mechanically more robust in general than are polyurethanes5, this approach might result in yet stronger materials. Herein, we have examined the effects of four processing parameters for producing this type of polymer crosslinked aerogel on properties of the resulting monoliths. Concentration of total silane (total APTES plus TMOS in a 1 to 3 v/v ratio) from 7 to 30% by volume in acetonitrile (CH3CN) and the amount of water (7 to 25% by volume) used to catalyze gellation should determine the density of the underlying silica. The number of washes (from 0 to 4) to remove water and by-products of gellation, and concentration of diisocyanate crosslinker (7 to 34% by weight in CH3CN) used for soaking the silica gels should determine the amount/length of polymer forming the crosslinks. A statistical experimental design methodology was employed to reduce the number of experiments and to allow computation of empirical models describing the relationship between the variables and the measured responses. In all, 30 different runs using different combinations of the four variables plus 5 repeats were utilized to produce a total of 35 separate crosslinked aerogels. These were evaluated by NMR, microscopy, surface analysis, mechanical testing and skeletal and bulk density. Herein, we will focus on the results of CP-MAS NMR, giving insight to the amount of polymer crosslink present in the monoliths and relate this to microstructure

Structure-Property Relationships in Porous 3-D Nanostructures as a Function of Preparation Conditions: Isocyanate Cross-Linked Silica Aerogels

Sol-gel derived silica aerogels are attractive candidates for many unique thermal, optical, catalytic, and chemical applications because of their low density and high mesoporosity. However, their inherent fragility has restricted use of aerogel monoliths to applications where they are not subject to any load. We have previously reported cross-linking the mesoporous silica structure of aerogels with di-isocyanates, styrenes or epoxies reacting with amine decorated silica surfaces. These approaches have been shown to significantly increase the strength of aerogels with only a small effect on density or porosity. Though density is a prime predictor of properties such as strength and thermal conductivity for aerogels, it is becoming clear from previous studies that varying the silica backbone and size of the polymer cross-link independently can give rise to combinations of properties which cannot be predicted from density alone. Herein, we examine the effects of four processing parameters for producing this type of polymer cross-linked aerogel on properties of the resulting monoliths. We focus on the results of C-13 CP-MAS NMR which gives insight to the size and structure of polymer cross-link present in the monoliths, and relates the size of the cross-links to microstructure, mechanical properties and other characteristics of the materials obtained

Use of Nanofibers to Strengthen Hydrogels of Silica, Other Oxides, and Aerogels

Research has shown that including up to 5 percent w/w carbon nanofibers in a silica backbone of polymer crosslinked aerogels improves its strength, tripling compressive modulus and increasing tensile stress-at-break five-fold with no increase in density or decrease in porosity. In addition, the initial silica hydrogels, which are produced as a first step in manufacturing the aerogels, can be quite fragile and difficult to handle before cross-linking. The addition of the carbon nanofiber also improves the strength of the initial hydrogels before cross-linking, improving the manufacturing process. This can also be extended to other oxide aerogels, such as alumina or aluminosilicates, and other nanofiber types, such as silicon carbide

Carbon Nanofiber Incorporated Silica Based Aerogels with Di-Isocyanate Cross-Linking

Lightweight materials with excellent thermal insulating properties are highly sought after for a variety of aerospace and aeronautic applications. (1) Silica based aerogels with their high surface area and low relative densities are ideal for applications in extreme environments such as insulators for the Mars Rover battery. (2) However, the fragile nature of aerogel monoliths prevents their widespread use in more down to earth applications. We have shown that the fragile aerogel network can be cross-linked with a di-isocyanate via amine decorated surfaces to form a conformal coating. (3) This coating reinforces the neck regions between secondary silica particles and significantly strengthens the aerogels with only a small effect on density or porosity. Scheme 1 depicts the cross-linking reaction with the di-isocyanate and exhibits the stages that result in polymer cross-linked aerogel monoliths

Feasibility of Large High-Powered Solar Electric Propulsion Vehicles: Issues and Solutions

Human exploration beyond low Earth orbit will require the use of enabling technologies that are efficient, affordable, and reliable. Solar electric propulsion (SEP) has been proposed by NASA s Human Exploration Framework Team as an option to achieve human exploration missions to near Earth objects (NEOs) because of its favorable mass efficiency as compared to traditional chemical systems. This paper describes the unique challenges and technology hurdles associated with developing a large high-power SEP vehicle. A subsystem level breakdown of factors contributing to the feasibility of SEP as a platform for future exploration missions to NEOs is presented including overall mission feasibility, trip time variables, propellant management issues, solar array power generation, array structure issues, and other areas that warrant investment in additional technology or engineering development

Photoactivated Fluorescence from Small Silver Nanoclusters and Their Relation to Raman Spectroscopy

Photoactivated fluorescence from individual silver nanoclusters ranging in size from 2 8 atoms has been demonstrated at room temperature. The optical properties of such clusters are far superior to those of fluorescence dyes with absorption cross sections ~50 times stronger than those of even the best organic dyes. The strong oscillator strengths produced from such nanoclusters has been shown to yield comparable enhancement factors in the surface-enhanced Raman spectroscopy (SERS) process to those observed in the presence of a plasmon- supporting nanoparticle. Raman transitions are in fact so strong that antistokes scattering is also observable on a single molecule (SM) level marking the first true demonstration of SM-SERS to date. Capable of generating true scaffold specific Raman scattering on the single molecule level, the combination of fluorescence from the small nanoclusters and strong observed Raman signals in the absence of a nanoparticle strongly indicate a chemical or charge transfer SERS enhancement mechanism.Ph.D.Committee Chair: Robert M. Dickson; Committee Member: L. Andrew Lyon; Committee Member: Mohan Srinivasarao; Committee Member: Mostafa A. El-Sayed; Committee Member: Robert L. Whette

Isocyanate-crosslinked Metal Oxide-doped Silica Aerogels in Chromatic Calibration Targets for Planetary Exploration

To generate photographs from the surfaces of other planets, imaging instruments need calibration standards that can withstand harsh environmetns while maintaining accurate color. These calibration targets much have low dust adhesion, resist chaning color or degradation in the presence of intense UV radiation, have minimal spacecraft or its instruments. The panoramic camera aboard existing Mars Rovers utilizes a moderately tacky matheral (RTV 655) for such chromatic calibration targets, in which dust readily adheres. Current investigations intruduce varying metal oxides as pigmentation into silica aerogels, which in a comparison to RTV 655, are much lighter weight materials which show little tackiness or dust adhesion while maintaining the desired pigmentation

Minimizing the Density of Polymer Crosslinked Aerogels

Polymerization of a di-isocyanate with the surface amino groups of a sol-gel derived mesoporous silica network crosslinks the nanoparticles of the silica skeleton, and reinforces the otherwise fragile underlying framework. Systematically adjusting the processing variables effecting density produces aerogels whose macroscopic properties such as stress at failure and modulus could be controlled, and are attributed to changing morphology on the nanometer scale (see figure below). The lowest density crosslinked aerogels (~ 0.036 g/cm3) exhibit a forty-fold increase in strength over the corresponding non-crosslinked framework, and are flexible. Flexibility is a property that aerogels have not previously exhibited, and thus indicates that at very low silica content, the properties of the polymer crosslink begin to dominate those of the rigid silica framework